An investigation of polyamides based on isoidide-2,5-dimethyleneamine as a green rigid building block with enhanced reactivity

Article abstract of DOI:10.1021/ma302126b

Novel, semicrystalline polyamides and copolyamides were synthesized from a new carbohydrate-based diamine, namely isoidide-2,5-dimethyleneamine (IIDMA). In combination with 1,6-hexamethylene diamine (1,6-HDA) as well as the biobased sebacic acid (SA) or brassylic acid (BrA), the desired copolyamides were obtained via melt polymerization of the nylon salts followed by a solid-state polycondensation (SSPC) process. Depending on the chemical compositions, the number average molecular weights (M_n) of the polyamides were in the range of 4000-49000 g/mol. With increasing IIDMA content in the synthesized copolyamides, their corresponding glass transition temperatures (T_g) increased from 50 °C to approximately 60-67 °C while the melting temperatures (T_m) decreased from 220 to 160 °C. The chemical structures of the polyamides were analyzed by NMR and FT-IR spectroscopy. Both differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) analyses revealed the semicrystalline character of these novel copolyamides. Variable-temperature (VT) ¹³C{¹H} cross-polarization/ magic-angle spinning (CP/MAS) NMR and FT-IR techniques were employed to study the crystal structures as well as the distribution of IIDMA moieties over the crystalline and amorphous phases of the copolyamides. The performed ab initio calculations reveal that the stability of the IIDMA moieties is due to a pronounced boat conformation of the bicyclic rings. The incorporation of methylene segments in between the isohexide group and the amide groups enables the hydrogen bonds formation and organization of the polymer chain fragments. Given the sufficiently high T_m values (～200 °C) of the copolyamides containing less than 50% of IIDMA, these biobased semicrystalline copolyamides can be useful for engineering plastic applications.

Full text of DOI:10.1021/ma302126b

Article
pubs.acs.org/Macromolecules
An Investigation of Polyamides Based on Isoidide-2,5-
dimethyleneamine as a Green Rigid Building Block with Enhanced
Reactivity
Jing Wu,†,○,⊥ Lidia Jasinska-Walc,*,†,⊥,§ Dmytro Dudenko,∥ Artur Rozanski,‡ Michael Ryan Hansen,*,∥
Daan van Es,○,⊥ and Cor E. Koning†,⊥,#
†Laboratory of Polymer Materials, Eindhoven University of Technology, Den Dolech 2, P.O. Box 513, 5600 MB Eindhoven, The
Netherlands
⊥Dutch Polymer Institute DPI, PO Box 902, 5600 AX Eindhoven, The Netherlands
§Department of Polymer Technology, Chemical Faculty, Gdansk University of Technology, G. Narutowicza Str. 11/12, 80-952
Gdansk, Poland
∥Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
‡Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, Poland
○Food & Biobased Research, Wageningen University and Research Center, Bornse Weilanden 9, 6708 WG Wageningen, The
Netherlands
#DSM Coating Resins, Ceintuurbaan 5, Zwolle, The Netherlands
S
* Supporting Information
ABSTRACT: Novel, semicrystalline polyamides and copolya-
mides were synthesized from a new carbohydrate-based
diamine, namely isoidide-2,5-dimethyleneamine (IIDMA). In
combination with 1,6-hexamethylene diamine (1,6-HDA) as
well as the biobased sebacic acid (SA) or brassylic acid (BrA),
the desired copolyamides were obtained via melt polymer-
ization of the nylon salts followed by a solid-state
polycondensation (SSPC) process. Depending on the chemical
compositions, the number average molecular weights (Mn) of
the polyamides were in the range of 4000−49000 g/mol. With
increasing IIDMA content in the synthesized copolyamides, their corresponding glass transition temperatures (Tg) increased
from 50 °C to approximately 60−67 °C while the melting temperatures (Tm) decreased from 220 to 160 °C. The chemical
structures of the polyamides were analyzed by NMR and FT-IR spectroscopy. Both differential scanning calorimetry (DSC) and
wide-angle X-ray diffraction (WAXD) analyses revealed the semicrystalline character of these novel copolyamides. Variable-
temperature (VT) 13C{1H} cross-polarization/magic-angle spinning (CP/MAS) NMR and FT-IR techniques were employed to
study the crystal structures as well as the distribution of IIDMA moieties over the crystalline and amorphous phases of the
copolyamides. The performed ab initio calculations reveal that the stability of the IIDMA moieties is due to a pronounced “boat”
conformation of the bicyclic rings. The incorporation of methylene segments in between the isohexide group and the amide
groups enables the hydrogen bonds formation and organization of the polymer chain fragments. Given the sufficiently high Tm
values (∼200 °C) of the copolyamides containing less than 50% of IIDMA, these biobased semicrystalline copolyamides can be
useful for engineering plastic applications.
form one of the most important groups of step-growth
polymers, and are extensively used in industry for injection
molding, extrusion and film or fiber applications. Nevertheless,
a fundamental fact one should realize is that all developments in
the polyamide industry cannot be continued and not even be
maintained without the abundant availability of raw materials,
which are typically fossil resource based.
INTRODUCTION
■
The various commercially available polyamides (PAs) belong to
one of the most important classes of semicrystalline polymers
owing to their good thermal and mechanical properties.
Concerning the versatile applications of PAs, often in
demanding conditions, some naturally occurring polyamides
formed from amino acids, e.g., wool or silk, can be used for
textile production. Recently, PAs such as PA 6.6, PA 6, and PA
6.10 have gradually substituted wool and silk and have been
applied in a wide range of applications. Nowadays, owing to
their excellent physicochemical and mechanical properties, PAs
Received: October 10, 2012
Revised: November 13, 2012
Published: November 19, 2012
© 2012 American Chemical Society
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Scheme 1. Strategies for Generating Novel Isohexide-Based Diamines from the Parent Isohexides
With the growing concern of the fossil feed stock shortage
and the relating environmental issues of fossil feed stock
consumption, biomass is regarded as a promising resource for
producing sustainable chemicals, fuels and energy. Among
numerous existing biomass monomers, the carbohydrate-
derived 1,4:3,6-dianhydrohexitols (isohexides) are highly
interesting building blocks for step-growth polymerization.1−3
Isohexides are a group of rigid secondary diols derived from
C6-sugars, which are found in three major isomeric forms,
namely isosorbide (1,4:3,6-dianhydro-D-glucitol, IS, with 2-
endo, 5-exo oriented hydroxyl groups), isomannide (1,4:3,6-
dianhydro-D-mannitol, IM, with 2-endo, 5-endo oriented
hydroxyl groups) and isoidide (1,4:3,6-dianhydro-L-iditol, II,
with 2-exo, 5-exo oriented hydroxyl groups).4−7 These
molecules can be obtained from starch or cellulose via few
(bio)organic transformations and therefore are considered as
biogenic. In recent years, extensive interests raised from both
academia and industry focusing on the utilization of isohexides
for various types of step-growth polymers, such as polyesters,
polyamides, polycarbonates and polyurethanes.1−3 One of the
most appealing features of the isohexides is their intrinsic
rigidity originating from the bicyclic rings, which can
significantly enhance the glass transition temperature (Tg)
and thereby broaden the application window of polymers. Such
effects have been demonstrated by several authors for the
isohexide-modified poly(ethylene terephthalate) (PET)8−11
and poly(butylene terephthalate) (PBT)12 as well for
succinic-acid based powder coatings.13−16 The potential
industrial applications of the isohexide-based polymers
encompass packaging, (powder) coating resins, and perform-
ance polymers as well as optical or biomedical materi-
als.1−3,17−21
Despite these promising and unique properties of isohexides,
the relatively poor reactivity of their secondary hydroxyl groups
at the 2- and 5-positions (see Scheme 1) has been recognized as
a major drawback, which often results in low molecular weights
(MW) and severely colored polymers.11,15,22 Moreover, as
bifunctional alcohols, the parent isohexide apparently is limited
to be directly used as monomer for the synthesis of certain
types of step-growth polymers, e.g., PAs. Hence, a number of
isohexide-derivatives have been reported for better reactivity
and/or diverse functionalities.1−3,20
In order to synthesize PAs, the parent isohexide has been
transformed into amino-derivatives through chain extension on
the oxygen atom of the 2/5-hydroxyl groups (−OH) by, e.g.,
propylamine (−(CH2)3NH2), phenylamine (−PhNH2) or
benzylamine (−PhCH2NH2) (strategy 1, Scheme 1),23−26 or
through the direct replacement of the 2/5-hydroxyl groups by
amine (−NH2) groups (strategy 2, Scheme 1).27−30 Since the
functional groups are directly attached to the bicyclic skeleton
of isohexide, the latter strategy has been proven to be able to
retain the structural rigidity to a large extent.31 Following such a
strategy, thus far three isomeric isohexide-based diamines have
been generated, namely diaminoisosorbide (DAIS), diaminoi-
somannide (DAIM), and diaminoisoidide (DAII). Thiem et
al.28 reported the first PAs derived from these isohexide-based
diamines in combination with several aromatic or aliphatic
diacyl chlorides via interfacial polymerization.28 These PAs
show high degrees of polymerization and Tg values in the range
of 50−70 °C, which are significantly higher than those of
commercially available PA 6 or PA 6.6. Recently, our studies on
isohexide-derived PAs17−19 presented the comprehensive
analyses concerning the preparation of series of fully biobased
copolyamides (co-PAs) by incorporating DAII or DAIS into PA
4.10 or PA 4.13. The synthetic protocol, starting with the
preparation of the corresponding nylon salts, combines a
conventional melt-polymerization with a solid-state postpoly-
condensation (SSPC), affording the desired co-PAs with
number-average molecular weights (Mn) above 20 000 g/mol.
In comparison to PA 4.10 or PA 4.13, these isohexide-derived
co-PAs exhibit lower degrees of hydrogen bond density and
thus have lower melting points.
We are interested in developing versatile isohexide-based
building blocks as well as in their applications in step-growth
polymerization. Recently, with the aim to develop novel
isohexide derivatives with improved reactivity and preserved
rigidity, we reported a new family of isohexide-based bifunc-
tional monomers following the second strategy presented in
Scheme 1.31 The original hydroxyl groups were replaced by
several 1-carbon extended functionalities including carboxylates
(−COOH, −COOCH3), methylenehydroxyl (−CH2OH) and
methyleneamine (−CH2NH2).31 Subsequently, several partially
or fully biobased polyesters from these building blocks,
including isoidide dicarboxylic acid (IIDCA) and isoidide
dimethanol (IIDML), were synthesized.21 The sufficiently high
molecular weights (Mn = 10 000−30 000 g/mol) as well as the
appropriate thermal properties of the resulting polyesters
revealed the improved reactivity and largely preserved structural
rigidity of the novel monomers.
In this work, we present a series of copolyamides based on a
novel one-carbon extended isohexide diamine, namely isoidide-
2,5-dimethylene amine (IIDMA), which can be synthesized
from isomannide via a dinitrile intermediate (IIDN) (Scheme
2).31 Compared to the known secondary isohexide-based
diamines, like diaminoisoidide (DAII) or diaminoisosorbide
(DAIS), IIDMA is a primary diamine and therefore is expected
to exhibit higher reactivity. In this work, the synthesis, chemical
structure characterization, thermal properties and crystal
structure analysis of the PAs based on IIDMA, 1,6-hexam-
ethylene diamine (1,6-HDA) and the biobased dicarboxylic
acids, including sebacic acid (SA) and brassylic acid (BrA)
produced from castor oil and erucic acid, will be described. The
chemical structure of the synthesized PAs was characterized by
nuclear magnetic resonance (NMR) spectroscopy. In addition,
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magnetic stirrer, internal thermocouple, pressure-equalizing dropping
funnel, and a reflux condenser, was charged with potassium cyanide
(1.43 g, 22 mmol), 18-crown-6 (5.81 g, 22 mmol), and THF (50 mL).
The suspension was cooled down below 0 °C, and then a solution of
IIBTf (8.2 g, 20 mmol) in THF (20 mL) was added dropwise over 1 h
under stirring and continuous flow of nitrogen. During the course of
the reaction, the internal temperature increased rapidly. After addition
was complete, the reaction was stirred below 0 °C for 3 h under
nitrogen. When the reaction reached completion as indicated by TLC,
the reaction mixture was poured onto cold water (200 mL) and
extracted with chloroform (5 × 30 mL). The combined organic layers
were dried over MgSO4 and decolorized with activated carbon. After
filtration over a glass filter containing Celite, the resulting solution was
evaporated under reduced pressure using a rotary evaporator to give
the crude product as a light brown solid. Finally, pure dinitrile (IIDN)
was obtained as a white solid by flash column chromatography (ethyl
acetate: petroleum ether, 1: 2). Yield: 2.63 g, 80% (purity: 100%,
Scheme 2. Chemical Structure of Isoidide Dimethylene
Amine (IIDMA) Prepared from Isomannide via the Dinitrile
Intermediate
a
a
For detailed reaction conditions and yields, see Experimental Section.
FT-IR spectroscopy, wide-angle X-ray scattering (WAXD) and
solid-state NMR spectroscopy were employed to elucidate the
hydrogen bonding nature and chain packing capabilities as well
as the distribution of IIDMA over the crystalline and
amorphous phases of the PAs.
1
GLC). Mp: 123−125 °C. H NMR (CDCl3, ppm): δ = 5.02 (s, 2H),
4.14 (s, 4H), 3.19 (m, 2H). 13C NMR (CDCl3, ppm): 117.22, 86.20,
70.34, 36.70. FT-IR: 2492, 2901, 2248 (CN), 1487, 1117, 1080,
1025, 947, 755 cm−1. HR-MS (Q-Tof), m/z [M + Na]+: calcd for
C8H8N2O2, 187.0478; found, 187.0474.
EXPERIMENTAL SECTION
■
Materials. ((3S,6S)-Hexahydrofuro[3,2-b]furan-3,6-diyl)-
dimethanamine (IIDMA) was prepared according to the published
method.31 The following chemicals and solvents were used as received
unless stated otherwise: isomannide (Sigma-Aldrich), trifluorometha-
nesulfonic anhydride (≥99%, Aldrich), dichloromethane (Merck, p.a.),
pyridine (Merck, p.a.), tetrahydrofuran (anhydrous, ≥99.9%, Sigma-
Aldrich), potassium cyanide (extra pure, Merck), 18-crown-6 (≥99%,
Fluka), chloroform (Merck, p.a.), borane−tetrahydrofuran complex
(BH3-THF) in THF solution (1.0 M, Sigma-Aldrich), hydrochloride
in diethyl ether solution (2.0 M, Sigma-Aldrich), 1,6-hexamethylene
diamine (98%, Sigma-Aldrich), sebacic acid (99%, Sigma-Aldrich),
brassylic acid (1,11-undecanedicarboxylic acid, Sigma-Aldrich, 94%,
purified by recrystallization from toluene), chloroform-d (99.8 atom %
D), trifluoroacetic acid-d, (99.5 atom % D, Sigma-Aldrich, TFAA),
DMSO-d6 (99.8 atom % D, Sigma-Aldrich) D2O (>99.8 atom %,
Merck), methanol-d4 (99.8 atom % D, contains 0.05% (v/v) TMS,
Aldrich), 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP, Biosolve), activated
carbon (Norit, CN1), magnesium sulfate (Acros Organics, 99% extra
pure, dried, contains 3−4 mol of water), Celite 545 coarse (Fluka),
and Amberlyst A26 (Aldrich) hydroxide form (strongly basic,
macroreticular resin with quaternary ammonium functionality from
Rohm and Haas Co; prior to use, the resin was washed with
demineralized water by sonication in an ultrasonic bath at room
temperature for 10 min. The water layer was subsequently removed by
decantation. This procedure was repeated 5 times, until the water layer
remained colorless).
((3S,6S)-Hexahydrofuro[3,2-b]furan-3,6-diyl)dimethanamine
Hydrochloride Salt (IIDMA Salt).31 A 250 mL three-necked round-
bottom flask, equipped with a magnetic stirrer, pressure-equalizing
dropping funnel, and a reflux condenser, was charged with BH3−THF
complex in THF solution (1.0 M, 50 mL). Next, a solution of IIDN
(0.8 g, 4.88 mmol) in THF (20 mL) was added dropwise at room
temperature under a nitrogen flow in 20 min. After addition was
complete, the reaction was stirred for another 16 h. Next, methanol
(30 mL) was added carefully to quench the reaction, during which
hydrogen gas was released vigorously. When no more gas evolved,
hydrogen chloride in diethyl ether solution (2.0 M, 30 mL) was added
dropwise and a white precipitate formed immediately. The suspension
was then filtered over a glass filter (G-3), and the collected gum-like
solid was dried in an oven (70 °C, 1 atm.) for 1 h to give the crude
diamine HCl salt as white hard solid. Finally, the pure white diamine
HCl salt was obtained after repeated washing with ethanol (3 × 20
mL) and drying under vacuum (35 °C, <1 mbar). Yield: 1.19 g, 66%.
1H NMR (D2O, ppm): δ = 4.45 (s, 2H), 4.01 (m, 2H), 3.67 (m, 2H),
3.54 (d, 4H), 2.42 (m, 2H). 13C NMR (D2O, ppm): δ = 85.24, 69.79,
61.70, 49.38. FT-IR: 3364, 2917, 2883, 2851, 1601, 1213, 1060 cm−1.
HR-MS (Q-Tof), m/z [M-2(HCl) + H]+: calcd for C8H18N2O2Cl2,
173.1285; found, 173.1282.
((3S,6S)-Hexahydrofuro[3,2-b]furan-3,6-diyl)dimethanamine
(IIDMA).31 Diamine HCl salt (0.19 g, 0.78 mmol) was dissolved in
demineralized water (20 mL) giving a colorless solution. To this
solution was added freshly washed Amberlyst A 26-OH (0.75 g, 3.2
mmol). The resulting suspension was sonicated in an ultrasonic bath
for 1 h at 30 °C. After the reaction was complete, the suspension was
filtered over a glass filter (G-3) containing Celite, and the IEX-resin
was washed thoroughly with water (3 × 15 mL) and methanol (3 × 15
mL). The combined clear colorless solutions were evaporated to
dryness using a rotary film evaporator to afford the pure diamine
(3R,6R)-Hexahydrofuro[3,2-b]furan-3,6-diyl Bis-
(trifluoromethanesulfonate) (Isoidide Bistriflate, IIBTf).31
A
500 mL 3-necked round-bottom flask, equipped with a mechanical
stirrer and a dropping funnel, was charged with isomannide (36.5 g,
0.25 mol), pyridine (49 mL), and dichloromethane (150 mL). The
colorless solution was cooled down to −10 °C. Next, trifluorometha-
nesulfonic anhydride (0.6 mol, 100 mL) was added dropwise over 1 h.
After stirring at room temperature for an additional 3 h, the reaction
mixture was poured onto ice−water (0.5 L) and stirred. The organic
layer was separated, and the water layer was extracted with chloroform
(3 × 150 mL). The combined organic layers were subsequently
washed with aqueous HCl (1.0 M, 3 × 150 mL), water (2 × 150 mL),
dried over MgSO4, and decolorized with activated carbon. After
filtration over a glass filter containing Celite, the resulting clear
solution was evaporated under reduced pressure using a rotary
evaporator. Finally, the pure product IIBTf was obtained by
recrystallization from ethanol as colorless needles. Yield: 95 g, 93%
1
(IIDMA) as a white solid. Yield: 0.1 g, 74.6% (purity: 99.0%, H
1
NMR). H NMR (CD3OD, ppm): δ = 4.37 (s, 2H), 3.97 (m, 2H),
3.60 (m, 2H), 2.74 (m, 4H), 2.32 (m, 2H). 13C NMR (CD3OD,
ppm): δ = 87.28, 71.51, 49.94, 42.63. FT-IR: 3342, 3275, 3174, 2938,
2878, 1605, 1093 cm−1. HR-MS (Q-Tof), m/z [M + H]+: calcd for
C8H16N2O2, 173.1285; found, 173.1281.
Isoidide-2,5-dimethylamine (IIDMA) Sebacic Acid Salt. To a
solution of sebacic acid (1.2 g, 0.006 mol) in ethanol (10 mL) at 50 °C
was added a solution of IIDMA (1.0 g, 0.006 mol) in an ethanol/water
mixture (3 mL, 2:1, v/v) dropwise. During the addition, a precipitate
was formed. The mixture was stirred at 80 °C for 1 h and then at 50
°C for 1 h. The crude product was filtered and recrystallized from an
ethanol/water mixture (10:1, v/v) to afford the salt as white crystals.
1
(purity: 100%, GLC). Mp: 62−63 °C. H NMR (CDCl3, ppm): δ =
5.22 (dd, 2H), 4.77 (m, 2H), 4.15 (m, 4H). 13C NMR (CDCl3, ppm):
δ = 118.48 (q, 1JFC = 319 Hz), 83.43, 80.32, 70.86. FT-IR: 2943, 2901,
1415, 1248, 1197, 987 cm−1. HR-MS (Q-Tof), m/z [M + Na]+: calcd
for C8H8F6O8S2, 432.9457; found, 432.9463.
1
Yield: 1.9 g, 85%. H NMR (D2O, ppm): δ = 4.62 (s, 2H), 4.10 (m,
2H), 3.75 (m, 2H), 3.10 (m, 4H), 2.60 (m, 2H), 2.17 (t, 4H), 1.55 (m,
4H), 1.30 (m, 8H).
(3S,6S)-Hexahydrofuro[3,2-b]furan-3,6-dicarbonitrile
(IIDN).31 A 250 mL three-necked round-bottom flask, equipped with a
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1,6-Hexamethylene Diamine Sebacic Acid Salt. To a solution
of sebacic acid (4.6 g, 0.04 mol) in ethanol (200 mL) at 50 °C was
added a solution of 1,6-hexamethylene diamine (8.1 g, 0.04 mol) in
ethanol (10 mL) dropwise. During the addition, a precipitate was
formed. The mixture was stirred at 80 °C for 1 h and then at 50 °C for
1 h. The crude product was filtered and recrystallized from ethanol/
water mixture (10:1, v/v) to afford the salt as white crystals. Yield: 11.8
°C. The transitions were deduced from the second heating and cooling
curves.
Variable-temperature (VT) 13C{1H} cross-polarization/magic-angle
spinning (CP/MAS) NMR experiments were carried out on a Bruker
ASX-500 spectrometer employing a double-resonance probe for rotors
with 4.0 mm outside diameter. These experiments utilized 10.0 kHz
1
MAS and a 4 μs π/2 pulse for H. All VT 13C{1H} CP/MAS NMR
1
g, 93%. H NMR (D2O, ppm): δ = 2.85 (s, 4H), 2.02 (m, 4H), 1.54
spectra were recorded using a CP contact time of 3.0 ms and TPPM
decoupling during acquisition.32 The temperature was controlled using
a Bruker temperature control unit in the range from 30 to 180 °C. The
VT 13C{1H} CP/MAS NMR spectra were recorded under isothermal
conditions at intervals of 10 °C. A heating rate of 2 °C/min was
employed between temperatures. Reported temperatures are corrected
for friction- induced heating due to spinning using 207Pb MAS NMR of
Pb(NO3)2 as a NMR thermometer.33 The 2D 1H−1H double-
quantum single-quantum (DQ-SQ) correlation and 13C{1H} fre-
quency-switched Lee−Goldburg hetero-nuclear correlation (FSLG-
HETCOR) experiments34,35 were recorded on a Bruker AVANCE-III
850 spectrometer using a double-resonance probe for rotors with 2.5
mm outside diameter. These experiments employed spinning
frequencies of 29762 and 15000 Hz, respectively. DQ excitation was
performed using the BaBa sequence and the FSLG-HETCOR
(m, 4H), 1.40 (m, 4H), 1.28 (m, 4H), 1.15 (m, 8H).
Isoidide-2,5-dimethylamine (IIDMA) Brassylic Acid Salt. To a
solution of brassylic acid (1.5 g, 0.006 mol) in ethanol (20 mL) at 50
°C, a solution of IIDMA (1.0 g, 0.006 mol) in an ethanol/water
mixture (3 mL, 2:1, v/v) was added dropwise. During the addition, a
precipitate was formed. The mixture was stirred at 80 °C for 1 h and
then at 50 °C for 1 h. The crude product was filtered and recrystallized
from ethanol/water mixture (10:1, v/v) to afford the salt as white
1
crystals. Yield: 2.0 g, 81%. H NMR (D2O, ppm): δ = 4.47 (s, 2H),
3.96 (m, 2H), 3.59 (m, 2H), 2.93 (m, 4H), 2.46 (m, 2H), 2.03 (t, 4H),
1.40 (m, 4H), 1.13 (m, 16H).
1,6-Hexamethylene Diamine Brassylic Acid Salt. To a solution
of brassylic acid (1.5 g, 0.006 mol) in ethanol (20 mL) at 50 °C, a
solution of 1,6-hexamethylene diamine (0.69 g, 0.006 mol) in ethanol
(3 mL) was added dropwise. During the addition, a precipitate was
formed. The mixture was stirred at 80 °C for 1 h and then at 50 °C for
1 h. The crude product was filtered and recrystallized from ethanol/
water mixture (10:1, v/v) to afford the salt as white crystals. Yield: 2.1
g, 96%. 1H NMR (D2O, ppm): δ = 2.84 (t, 4H), 2.01 (t, 4H), 1.53 (m,
4H), 1.39 (m, 4H), 1.27 (m, 4H), 1.13 (m, 16H).
experiment used a CP time of 2.0 ms.34,35 Chemical shifts for H
1
and 13C MAS NMR are reported relative to TMS using solid
adamantane as an external reference.36,37 CP conditions were
optimized using L-alanine. All samples were annealed just below
their melting temperature for 5 min under a flow of nitrogen gas
before NMR analysis to remove precipitation-induced structural
conformations.
Synthesis of the Polyamides. A three-necked round-bottom
flask, equipped with an overhead mechanical stirrer, a Dean−Stark
type condenser and vigreux column, was charged with 1,6-hexam-
ethylene diamine-sebacic acid salt (0.5 g), isoidide-2,5-dimethylamine-
sebacic acid salt (0.5 g), 1,6- hexamethylene diamine (0.05 g, 0.43
mmol), isoidide-2,5-dimethylamine (0.05 g, 0.29 mmol), and Irganox
1330 (0.01 g) and the mixture was stirred at 170−175 °C for 30 min
under argon atmosphere. Then the temperature was raised to 190 °C
and the polycondensation process was continued for 2 h. After cooling
down to room temperature, the synthesized prepolymer was isolated
from the flask and ground into powder, washed with demineralized
water at 80 °C, filtered and dried under reduced pressure at 80 °C.
The resulting product was subsequently submitted to solid-state
polymerization carried out about 20−30 °C below the melting point of
the prepolymer for 5 h under Ar2 atmosphere. Analogous conditions of
the polymerization process were applied for all the investigated
polyamides.
Geometry optimization and 13C NMR chemical shift calculations
were performed within the Gaussian03 program package. The
geometries of the conformations obtained by merging results from a
set of 1D potential energy surface (1D-PES) scanning procedure via
B97-D/6-311G**38 were further fully optimized at the same level. The
most abundant conformer with respect to energy has been proceeded
for further NMR chemical shifts calculations at the B97-D/6-311G**
level of theory. Further details about information on the 1D-PES
procedure, comparison of B97-D and MP2 results, and the NMR
chemical shift calculations are given in the Supporting Information.
Fourier transform infrared spectra (FT-IR) were obtained using a
Varian 610-IR spectrometer equipped with a FT-IR microscope. The
spectra were recorded in a transmission mode with a resolution of 2
cm−1. PA films obtained from 1,1,1,3,3,3-hexafluoroisopropanol were
analyzed on a zinc selenium disk and heated from 30 °C to
temperatures slightly above the melting points of the polyamides. For
this purpose a Linkam TMS94 hotstage and controller were used. The
samples were cooled in 10 °C steps and reheated with the same
heating steps. The spectra from the second heating run were collected.
Varian Resolution Pro software version 4.0.5.009 was used for the
analysis of the spectra.
Analysis of the crystalline structure of the materials was performed
using wide-angle X-ray scattering measurements by means of a
computer-controlled goniometer coupled to a sealed-tube source of
CuKα radiation (Philips), operating at 50 kV and 30 mA. The Cu Kα
line was filtered using electronic filtering and the usual thin Ni filter.
The data were collected at room temperature. The 1D profiles were
subsequently background-corrected and normalized. Since reflections
from the different crystallographic phases frequently overlap each
other, it was necessary to separate them by deconvolution. Analysis of
diffraction profiles of the examined samples and peak deconvolution
were performed using WAXSFit software designed by M. Rabiej at the
University of Bielsko-Biała (AHT).39 The software allows to
approximate the shape of the peaks with a linear combination of
Gauss and Lorentz or Gauss and Cauchy functions and adjusts their
settings and magnitudes to the experimental curve with a “genetic”
minimizing algorithm.
1
Characterization. H NMR and 13C NMR spectra were recorded
at room temperature using a Varian Mercury Vx spectrometer
operating at frequencies of 400 and 100.62 MHz for H and 13C,
1
respectively. The NMR analysis of the polyamides was carried out in
1
TFAA-d or DMSO-d6. For H NMR experiments, the spectral width
was 6402.0 Hz, acquisition time 1.998 s and the number of recorded
scans equal to 64. 13C NMR spectra were recorded with a spectral
width of 24154.6 Hz, an acquisition time of 1.300 s, and 256 scans.
Size exclusion chromatography (SEC) in hexafluoroisopropanol
(HFIP) was performed on a system equipped with a Waters 1515
Isocratic HPLC pump, a Waters 2414 refractive index detector (35
°C), a Waters 2707 auto sampler, and a PSS PFG guard column
followed by 2 PFG-linear-XL (7 μm, 8 × 300 mm) columns in series at
40 °C. HFIP with potassium trifluoroacetate (3 g/L) was used as
eluent at a flow rate of 0.8 mL/min. The molecular weights were
calculated against poly(methyl methacrylate) standards (Polymer
Laboratories, Mp = 580 Da up to Mp = 7.1 × 106 Da).
The thermal stability of the polymers was determined by
thermogravimetric analysis (TGA) with a TGA Q500 apparatus
from TA Instruments. The samples were heated from 30 to 600 °C at
a heating rate of 10 °C/min under a nitrogen flow of 60 mL/min.
Glass transition temperatures (Tg) and melting temperatures (Tm)
were measured by differential scanning calorimetry (DSC) using a
DSC Q100 from TA Instruments. The measurements were carried out
at a heating and cooling rate of 10 °C/min from −60 °C to 180−250
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tion process in the solid state, SSPC did not result in further
enhancement of their Mn values. Therefore, regardless of the
diamine configuration, the procedure consisting of melt
polymerization followed by SSPC can be considered as the
most suitable route toward the family of isohexide-derived
products. However, as presented in Table 1, the attempt to
enhance the molecular weight of the homopolymer derived
from 100% IIDMA and sebacic acid (PA 5) was not successful.
Due to the relatively low melting temperature of this product
(157.7 °C), the SSPC process carried out at around 140 °C
proved to be ineffective. Despite the limited thermal stability of
the sugar-derived monomers for high temperature polymer-
ization processes, the applied short melt polymerization in
combination with SSPC allowed the successful preparation of
the PAs with minor degrees of discoloration and high molecular
weights.17−19 When comparing the weighted-in and the
incorporated molar ratio of the diamines, a slight preferential
incorporation of 1,6-HDA residues over IIDMA was observed.
The reasons can be a lower reactivity of IIDMA and/or a lower
thermal stability of IIDMA than 1,6-HDA. Nevertheless, as
shown in Table 1, the applied two-step polymerization
conditions lead to the successful preparation of the desired
products with close to expected chemical compositions.
RESULTS AND DISCUSSION
■
Synthesis and Molecular Characterization of the
IIDMA-Derived (Co)polyamides. Two series of novel
(co)polyamides were synthesized from isoidide-2,5-dimethyle-
neamine (IIDMA) and/or 1,6-hexamethlyene diamine (1,6-
HDA), in combination with either sebacic acid (SA) or
brassylic acid (BrA). The synthetic protocol comprises, after
making the nylon salts of both diamines with both dicarboxylic
acids, a solvent-free melt polycondensation process followed by
an optional solid-state postcondensation (SSPC), as described
previously for the diaminioisoidide (DAII)- or diaminoisosor-
bide (DAIS)-derived PAs.17,18 When the IIDMA content was
below 75 mol %, the two series of PA prepolymers were
obtained with Mn values ranging from 6,700−21,500 g/mol,
which were further enhanced to 12 400−49 000 g/mol by
SSPC (Table 1, entries 2, 3, 4, 7, 8). The SSPC temperatures
Table 1. Chemical Compositions, Molecular Weights, and
Polydispersity Indices of the Homo- and Copolyamides
Synthesized from Isoidide-2,5-dimethyleneamine (IIDMA),
1,6-Hexamethylenediamine (1,6-HDA), Sebacic Acid (SA),
and Brassylic Acid (BrA)
polymer after
The molecular structure of the synthesized IIDMA-based
polyamides was analyzed by 1H and 2D 1H−1H COSY nuclear
magnetic resonance (NMR) spectroscopy. As exemplified by
the 1H NMR spectrum of the polyamide based on IIDMA and
SA (PA5) in Figure 1, all proton signals from the repeating unit
of the polymer are found at the expected chemical shift values
with matching multiplicities. The stereochemistry of the
IIDMA moieties in PA5 can be clearly identified by the
prepolymer
SSPC
feed molar
ratio
built-in
molar ratio
(IIDMA/
(IIDMA/
1,6-HDA)
Mn
c
Mn
c
a
b
c
c
symbol 1,6-HDA)
(g/mol)
PDI (g/mol) PDI
SA/IIDMA/1,6-HDA-Based Polyamides
PA1
PA2
PA3
PA4
PA5
0/100
25/75
50/50
75/25
100/0
0/100
26/74
49/51
68/32
100/0
13400
21500
13300
9200
2.0
2.8
2.1
2.0
1.2
−
−
48900
38500
12400
−
4.2
3.8
2.3
−
1
resonances of the isohexide unit: based on the 2D H−1H
1
COSY NMR spectrum there is no H−1H dihedral coupling
between the βIIDMA bridge protons with the neighboring αIIDMA
protons (see Supporting Information Figure S1). Thus the
bridge βIIDMA protons appear as a singlet at 4.25 ppm in the 1H
NMR spectrum (Figure 1). Therefore, the adjacent αIIDMA
protons have no dihedral coupling with the βIIDMA bridge
protons and adopt an endo-orientation. Logically, the
methyleneamine groups should be in an −exo orientation.
This feature has been frequently illustrated in other isohexide-
based polymers.9,15,17,18,21 Given the fact that the molecular
weight of PA5 is in a low range (Mn = 3900 g/mol), the low
intensity signals as indicated by dots are assumed to come from
the IIDMA end-groups. Overall, we conclude that the
stereoconfiguration of IIDMA moieties were preserved during
the applied polymerization conditions.
3900
BrA/IIDMA/1,6-HDA-Based Polyamides
PA6
PA7
PA8
0/100
25/75
50/50
0/100
16/84
42/58
4900
13060
6700
1.6
2.3
1.7
17400
47300
13900
2.3
4.1
2.3
a
b
Ratio determined by weighed-in monomers. Ratio determined by
c
1H NMR; Values determined using SEC against PMMA standards in
HFIP.
are approximately 20−30 °C lower than the corresponding
melting points of the prepolymers.40 In comparison with the
PAs based on the secondary diamines, viz. diaminoisoidide
(DAII, exoexo oriented diamine) or diaminoisosorbide (DAIS,
exo/endo oriented diamine),17−19 the impact of the chemical
structure of the primary diamine (IIDMA) on its reactivity was
significant, since higher molecular weight prepolymers based on
IIDMA were obtained and additionally the time of SSPC
reaction was shorter than for secondary diamines-based
copolymers. However, it should be taken into account that in
the synthesis of DAII- and DAIS-derived copolymers 1,4-
diaminobutane, instead of 1,6-hexamethylene diamine, was
used. Nevertheless, the differences in the molecular weight of
the prepolymers obtained using similar reaction conditions and
comparable contents of the bicyclic compounds were
significant. As also presented for DAII- and DAIS-based
polyamides,17,18 interfacial polymerization of the diamines
and sebacoyl chloride afforded lower molecular weight
materials and due to the relatively low melting point of the
homopolymers, which were too low for proper polycondensa-
Conformational Analysis of the Copolymers from
Solid-State NMR and Quantum-Chemical Calculations.
Compared to other step-growth polymers, e.g., polyesters, PAs
often have higher melting temperatures mainly due to the
strong interchain hydrogen bonds formed between the
recurring amide groups. In the solid-state form, the crystal
packing of the polymer chain fragments is highly related to the
construction of a hydrogen-bonded network. As shown
previously, the introduction of DAII or DAIS into the linear
polyamides PA 4.10 or PA 4.13 affects the ordered crystal
packing and therefore lowers the melting temperatures of the
co-PAs.17−19 The solution 1H NMR spectroscopy, as described
above, has been effective in elucidating the molecular structure
of the synthesized PAs. In order to study the effect of IIDMA
incorporation on the hydrogen-bonding network of the
obtained co-PAs, we have employed variable-temperature
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Figure 1. 1H NMR spectrum of the polyamide synthesized from SA and IIDMA (PA5) and the chemical structure of the repeating unit of the homo-
and copolyamides. Proton and carbon labels shown in the scheme were used for the analysis of the liquid-state and solid-state NMR spectra. The
symbol • indicates the resonances of the IIDMA end-groups.
(VT) 13C{1H} cross-polarization/magic-angle spinning (CP/
MAS) NMR spectroscopy. Upon heating from 40 °C to the
temperatures close to their corresponding melting points, the
hydrogen bonding density of the specimen can be affected,
which will be reflected in a change of the NMR resonance
signals. Moreover, through careful examination of the
resonance signal changes over a wide range of temperatures,
additional information concerning the distribution of isohexide-
based monomers over crystalline and amorphous phases of the
PAs, as well as on the mobility of the polymer chain fragments
can be identified. In this work, a particular emphasis is given to
the investigation of the existence of different potential IIDMA
conformers and their influences on the formation and/or
regularity of hydrogen bonding structures by using solid-state
NMR spectroscopy techniques. Subsequently quantum-chem-
ical calculations were performed to reveal the most energeti-
cally stable IIDMA conformations. Two co-PAs samples, PA3
and PA8 based on SA and BrA, respectively and both having
approximately a 50/50 IIDMA/1,6-HDA molar ratio, were
characterized for this purpose.
ature range. At high temperature the reduced 13C line width
suggests a decrease in hydrogen bonding density of the
polyamides upon heating. Furthermore, the characteristic
carbonyl resonances located at ∼173 ppm also appear as a
rather broad signal, which gradually decreases in intensity with
increasing temperature. In the melt two carbonyl signals with
similar intensity appear that can be assigned to the different
monomer parts of the copolymer. As reported for PAs
synthesized from DAII and DAIS17−19 the broad and bimodal
CO resonance signals may indicate that the carbonyl groups
are involved in two types of hydrogen-bonding environments,
most probably referring to the hydrogen bonding occurring in
the crystalline and amorphous regions of the sample. However,
for PA3 it is not possible to make such an assignment due to
similar intensity decay of the both types of signals (Figure 2a)
and the occurrence of the carbonyl signals in the melt at the
same position.18,19 Moreover, over the investigated temperature
range, the αN and αC methylene groups attached to each side of
the amide bonds are present as well-defined and narrow signals
at 40.5 and 36.7 ppm, respectively, without any visible
shoulders (see Figure 2a, Table 2 and Figure 1 for assignment).
Interestingly, based on the chemical shift for αN and αC below
and above the melting temperature, and a comparison to our
previous work on PAs synthesized from DAII and DAIS, the
number of trans conformers in the linear chain fragments of the
copolymer PA3 appears to be very limited, i.e., the signals from
trans conformers should be visible as well resolved resonances
at higher frequency.18,19 Thus, the (VT) 13C{1H} CP/MAS
NMR spectra of the PA3 in Figure 2a illustrate that the
incorporation of IIDMA significantly increases the transition
between trans and gauche conformers in the methylene moieties
of SA and 1,6-HDA, causing the appearance of more gauche
conformations and a visible reduction in the fraction of trans
conformers. These observations are in line with FT-IR analysis
of PA3 where signals of gauche CH2 scissor vibrations were
During the heating course of PA3, the resonances from the
IIDMA moieties appear as well resolved, albeit broad signals at
86.4, 71.6, and 48.2 ppm for βIIDMA, β′IIDMA, and γIIDMA
,
respectively (Figure 2a, Table 2, see Figure 1 for signal
assignment). The resonance signals of the methylene proton
from the cyclic skeleton (αIIDMA) are difficult to recognize due
to overlapping with signals of the alkylene protons from 1,6-
HDA units (αN). All the β′IIDMA, βIIDMA, and γIIDMA signals
show additional minor shoulders that gradually disappear with
increasing the temperature, i.e., above ∼150 °C these signals
have disappeared. Meanwhile, the remaining major signals from
the IIDMA groups become narrower and shift only slightly to
lower frequency even when going to the molten state compared
to the situation at low temperature. This shows that the IIDMA
groups adopt similar conformations over the whole temper-
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2b the presence of two different hydrogen bond environments
can be identified via the correlations of the amide groups to the
aliphatic moieties. The non-hydrogen bonded fraction does not
show any correlation to the carbonyl groups whereas the
hydrogen bonded fraction does. This observation combined
with the isotropic chemical shift position of the amide protons
at ∼8.3 ppm and its correlations to the aliphatic moieties in
Figure 2c show that the formed hydrogen bonds in PA3 based
on IIDMA are stronger than in PAs synthesized from DAII and
DAIS. A possible explanation for the increase in chemical shift,
and thereby an increase in hydrogen bond strength,41 could be
that the incorporation of a methylene segment in between the
isohexide group and the amide groups enables the hydrogen
bonds formation to become organized more perfectly due to
improved spatial arrangements, although this occurs at the
expense of the occurrence of more gauche conformers in the
methylene segments as discussed above. The increases in
hydrogen bonding strength in turn also explain the higher
melting temperatures for the samples studied in this work.
An enhanced mobility of the polymer chain fragments,
caused by the increase in gauche conformers, is also visible for
PA8 synthesized from BrA, 1,6-HDA, and IIDMA. As shown in
Figure 3a, the αN and αC resonances associated with gauche
conformers of the linear methylene groups attached to NH and
CO motifs at 40.3 ppm and 37.0 ppm are clearly visible
throughout the whole temperature range. As also observed for
PA3, the intensity of these signals upon heating decreases with
only minor displacements visible. Meanwhile, a gradual shift of
the relatively broad carbonyl resonance at 173.4 ppm to higher
ppm values is observed throughout the temperature range in
Figure 3a. This is most likely caused by a reduction in hydrogen
bonding efficiency leading to an enhanced mobility of the non-
hydrogen bonded amide domains during the course of heating.
The 13C{1H} CP/MAS NMR spectra of PA8 also includes
Figure 2. (a) Variable-temperature solid-state 13C{1H} CP/MAS
NMR spectra recorded at 11.75 T (500 MHz for 1H) of copolyamide
PA3 synthesized from SA, IIDMA, 1,6-HDA with a IIDMA/1,6-HDA
molar ratio of 49/51. (b) 2D 13C{1H} FSLG-HETCOR spectrum
acquired using a 2.0 ms CP step and five FLSG blocks per t1 increment
to improve the 1H resolution. (c) 2D 1H−1H DQ-SQ spectrum
recorded using a BaBa recoupling period of 67.2 μs. Both 2D spectra
̀
broad γIIDMA, βIIDMA and βIIDMA signals between 47−90 ppm
and the αIIDMA resonance located at 40.3 ppm, overlapping with
the αN signal. Again, the narrow 13C line widths of the IIDMA
resonances point toward a reduced number of IIDMA
conformers over the whole temperature range. From the 2D
NMR correlation spectra shown in Figure 3, parts b and c,
similar conclusions about the polymer chain organization can
be derived for PA8 as for PA3. This includes that the non-
hydrogen bonded fraction does not show any correlation to the
carbonyl groups and that strong hydrogen bonds are formed,
since the amide protons resonate at ∼8.6 ppm.
1
were recorded at 20.0 T corresponding to 850.27 MHz for H. The
dashed lines in parts b and c illustrate selected cross-peaks and
autocorrelation peaks, including hydrogen bonded (HB) and non-
hydrogen bonded (non-HB) amide fragments. Assignment of the
signals is performed according to Figure 1 and the asterisk in part a
indicates the position of a spinning sideband (ssb) from the carbonyl
resonance.
The IIDMA moiety in both PA3 and PA8 show quite narrow
13C resonances suggesting that only a single IIDMA conformer
is present. To reveal if this is the case we have performed a 1D
potential energy scan (PES). In our previous study on DAII
and DAIS fragments, we have shown that the cheaper DFT
method B97-D reproduces the potential energy curve obtained
emphasized (see below). Further information about the local
organization of polymer chains in PA3 can be achieved from
the 2D NMR correlation spectra shown in Figure 2, parts b and
c. From the 2D 13C{1H} FSLG-HETCOR spectrum in Figure
Table 2. 13C Chemical Shifts for the Copolyamides PA3 and PA8 at Different Temperatures
13C chemical shift (ppm)
CO
αN
αC
βC
γC
δC
PA 3
βN
γN
γIIDMA βIIDMA β′IIDMA αIIDMA
41.0 °C
173.6 °C
173.7
174.0
40.5
40.2
36.7
37.1
22.5−34.5
22.5−34.5
22.5−34.5
22.5−34.5
22.5−34.5
22.5−34.5
PA 8
22.5−34.5
22.5−34.5
22.5−34.5
22.5−34.5
48.2
48.4
86.4
87.5
71.6
71.8
40.5
41.3
41.0 °C
164.1 °C
173.4
174.0
40.3
41.0
37.0
37.6
22.3−35.0
22.3−35.0
22.3−35.0
22.3−35.0
22.3−35.0
22.3−35.0
22.3−35.0
22.3−35.0
22.3−35.0
22.3−35.0
47.8
48.4
86.4
87.2
72.2
71.6
40.3
41.0
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Figure 4. (a) Top and side views of the most stable conformers for the
primary diamine (IIDMA) studied in this work, and the
diaminoisosorbide (DAIS or exoendo oriented diamine) and
diaminoisoidide (DAII, exoexo oriented diamine) studied in our
previous work17−19 All conformers were found via gas phase MP2/6-
311G** level of theory (see Supporting Information for details). (b)
Graphical representation of the calculated 13C chemical shifts for all
three conformers shown in part a. Note that we have used the
nomenclature 4−6 to assign the 13C resonances for nonsymmetric
isohexide fragments as shown for DAIS in part a.
“chair” conformation for DAIS.42 These conformational
differences come as a result of the specific stereo configuration
of the different isohexide groups. To compare the NMR
spectroscopic signatures of all three isohexides we have
performed 13C chemical shift calculations at the B97-D/6-
311G** level of theory and summarized these as shown in
Figure 4b. For the IIDMA unit studied in this work, the
calculated 13C chemical shifts are in good agreement with the
experimental results in Figures 2 and 3. Clearly, the different
isohexide units can be distinguished by their characteristic
signals for the anchoring carbons. These carbons are inherently
sensitive to a specific conformation of the isohexide moiety
(“boat” or “chair”), resulting in a 13C chemical shift spread of
14 ppm in the region from 45 to 65 ppm.
Figure 3. (a) Variable-temperature solid-state 13C{1H} CP/MAS
NMR spectra recorded at 11.75 T (500 MHz for 1H) of copolyamide
PA8 synthesized from BrA, IIDMA, 1,6-HDA with a IIDMA/1,6-HDA
molar ratio 42/58. (b) 2D 13C{1H} FSLG-HETCOR and (c) 2D
1H−1H DQ-SQ spectra recorded as described in the caption of Figure
2. The dashed lines in parts b and c illustrate selected cross-peaks and
autocorrelation peaks, including hydrogen bonded (HB) and non-
hydrogen bonded (non-HB) amide fragments. Assignment of the
signals is performed according to Figure 1 and the asterisk indicates
the position of a spinning sideband (ssb) from the carbonyl resonance.
FT-IR Investigation of the Polyamides upon Heating.
The hydrogen bonding between the amide moieties of the
IIDMA-based polyamides was further investigated by temper-
ature-dependent Fourier transformed infrared (FT-IR) spec-
troscopy. The applied temperature ranges from 30 °C to
slightly above the melting point of the PAs. Three polyamides
samples were characterized for this purpose; they are PA3, PA8,
and PA6 (PA6.13) with their FT-IR spectra presented in
Figures 5−7.
Special attention was given to analyze the intensities of the
signals appearing at around 3300 cm−1 for all the three PAs,
since these signals can be assigned to N−H stretching
vibrations. At elevated temperatures, these signals were
observed to broaden and their intensities decreased significantly
as well. As pointed out by Schroeder43 and Skrovanek,44,45 for
even- and odd-type nylons, the wavenumber of such bands
reflect the average absorptive strength of the hydrogen-bonded
N−H groups. The broadening and weakening of these signals
upon heating are related to the diminishing of the hydrogen
bond density and the formation of non-hydrogen bonded N−H
groups. As can be seen from the FT-IR spectra recorded above
180 °C in Figure 5−7, additional signals can be observed at
by the more expensive MP2 method.18,19 For this reason, we
have chosen to perform the current study of the conformational
profile for the dihedral angle between the amide group and the
isohexide fragment in IIDMA at the B97-D level of theory.
From the 1D PES it is clear that three conformational minima
exist with comparable energies (see Figure S4, Supporting
Information). On the basis of this result, a full set of possible
conformations have been generated and further optimized at
the same level of theory. Contrary to our previous studies of
other isohexide derivatives (DAIS and DAII), which showed a
large spread of conformers, the incorporation of a methylene
segment in between the isohexide and amide groups enables
the isohexide fragment to adopt a unique conformation as
illustrated in Figure 4a. This conformation is ∼10 kJ/mol lower
in energy compared to the second most abundant conformer
found. In Figure 4a the most abundant conformers of DAII and
DAIS fragments are also shown for comparison with the
current IIDMA unit. Inspection of these conformers show that
the stability of the IIDMA and DAII moieties is due to a
pronounced “boat” conformation, which is distorted into the
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Figure 5. Variable-temperature FT-IR spectra of the copolyamide PA3 recorded from 30−220 °C. PA3 was synthesized from SA, IIDMA, 1,6-HDA
with a built-in IIDMA/1,6-HDA molar ratio of 0.49/0.51. The spectra show wave numbers ranging from 3500−2700 cm−1 (a) and 1600−800 cm−1
(b).
Figure 6. Variable-temperature FT-IR spectra of the copolyamide PA8 recorded from 30−220 °C. PA8 was synthesized from BrA, IIDMA, 1,6-HDA
with a built-in IIDMA/1,6-HDA molar ratio of 0.42/0.58. The spectra show wave numbers ranging from 3500−2700 cm−1 (a) and 1600−800 cm−1
(b).
Figure 7. Variable-temperature FT-IR spectra of PA6 (PA 6.13) recorded from 30−240 °C. The spectra show wave numbers ranging from 3500−
2700 cm−1 (a) and 1600−800 cm−1 (b).
around 3444 cm−1. They are the typical indication of the
presence of the non-hydrogen bonded N−H groups.
Interesting information about the population of trans and
gauche conformers in the backbone of the polyamides is
provided by the signals of C−H vibration from the methylene
units attached to each side of the amide groups (-HN-C(O)−).
According to literature and our previous findings on the DAII/
DAIS-based copolyamides, the C−H vibration from the trans-
methylene units attached to amine (NH) and carbonyl (CO)
groups show absorptions at wave numbers of 1475 and 1417−
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1419 cm−1, respectively.46,47 In the case of PA 6.13, given the
pronounced absorptions at 1474 and 1419 cm−1, both the
methylene units attached to each side of the amide groups
(−HN−C(O)−) are believed to adopt trans-conformations
(Figure 7b). When IIDMA is incorporated, as shown in Figure
6b for the BrA-based PA8, the presence of the absorption at
1419 cm−1 indicates the trans-type of methylene units next to
CO; while the methylene units next to N−H in contrast
adopt the gauche-conformation by showing absorptions at
around 1440−1460 cm−1 instead of 1474 cm−1. According to
Yoshioka et al,46 due to the lower torsion energy barrier, the
conformational changes of the methylene groups next to the
CH2−NH motifs can be easier compared to those attached to
the carbonyl group. Similarly, the SA-based PA3 should also
adopt gauche conformation, as shown in Figure 5.
Interestingly, as we reported for the DAII-based polyamides,
the presence of exo/exo oriented isohexide diamines in the
backbone of the macromolecules more readily enhances the
molecular motion and the induction of gauche type conformers
in the linear fragments than their exo/endo oriented
analogues.19 IIDMA is a one-carbon extended isohexide
diamine, however, due to a similar configuration as DAII the
vanishing of the bands corresponding to CH2 scissoring next to
N−H and CO groups adopting trans conformation was
already observed for the copolyamides containing around 20
mol % of the bicyclic comonomer (see Supporting Information,
Figure S2 and Figure S3).
With regard to the resonances from the isohexide bicyclic
skeleton, the FT-IR bands located at 1093 cm−1, 1055−1057
cm−1 and 907−910 cm−1 are assigned to the asymmetric C−
O−C stretching, skeletal C−C stretching and C−CO stretching
signals, respectively. It was noticed that these bands exhibit
lower intensity when the applied temperatures are close to the
melting points, suggesting that IIDMA motifs distribute over
both the crystalline and amorphous phases of the materials.
Similar phenomena have also been observed for the DAII and
DAIS-derived polyamides.18,19 Further, the signals at 1552 and
1537 cm−1 represent the Amide II in-plane N−H deformation
with CN and CO stretch vibration, while the signals
corresponding to Amide III and Amide II + III coupled with
the hydrocarbon skeleton appear at 1360−1370 and 1180−
1190 cm−1, respectively. Since the latter vibrations display a
significant decay in their intensity when the recorded
temperatures are close to the melting points of the materials,
they most probably originate from the temperature-sensitive
crystalline phase. Besides, the FT-IR bands corresponding to
the methylene motifs of the PAs and related to CH2 wagging or
twisting and skeletal C−C stretch vibrations were found at
1280−1290, 1230−1250, 1218, 1050−1060, and 1019 cm−1,
respectively. These modes, excluding signals at 1055−1057
cm−1, display significant changes in their line width at elevated
temperature and can be assigned to the crystalline phase of the
polyamides.
Table 3. Thermal Stability, Glass Transition Temperatures
(Tg), Melt and Crystallization Temperatures (Tm, Tc), and
Enthalpy of the Transitions upon Heating (m) and Cooling
a
(c) of the Novel IIDMA-Based Polyamides
T5%
(°C)
Tmax
(°C)
Tg
(°C)
Tm
(°C)
ΔHm
ΔHc
entry
(J/g)
Tc (°C)
(J/g)
SA/IIDMA/1,6-HDA
PA1
PA2
PA3
PA4
PA5
399.6
390.2
403.1
356.7
323.6
456.7
460.5
449.7
446.5
443.8
−
−
59.5
66.8
62.6
222.6
210.4
204.9
181.6
157.7
63.1
49.5
36.7
33.7
26.1
196.42
174.4
160.4
123.4
120.7
55.0
52.2
40.9
14.1
22.7
BrA/IIDMA/1,6-HDA
PA 6
PA7
PA8
408.7
413.1
401.8
460.8
455.3
450.9
−
−
−
206.7
201.8
197.1
71.6
53.8
54.1
184.3
175.3
173.9
59.7
45.5
37.3
a
T5% = temperature of 5% mass loss, Tmax = temperature of maximal
rate of decomposition, Tg = glass transition temperature, Tm = melting
point, Tc = crystallization temperature, and ΔH = enthalpy of the
transition during melting (m) and crystallization (c).
6.13. However, for the PAs having a IIDMA content greater
than 50 mol %, as indicated for the SA-based series, the thermal
stability of these materials decreased significantly due to the
presence of ether linkages in the isohexide unit. Especially the
5% of weight loss temperature of SA/IIDMA-derived
homopolyamide is as low as 323 °C (entry 5, Table 3). A
similar trend has been observed by us when proportionally
introducing diaminoisoidide (DAII) into PA4,10,17 as well as
upon introducing diaminoisosorbide (DAIS) into PA 4.13.18
Such an adverse effect on thermal stability of IIDMA
incorporation on the copolyamides is assumed to be caused
by the generation of crystal structural irregularity of this cyclic
moiety into the original linear chains and therefore a reduction
of hydrogen bonding density, as shown earlier by the solid-state
NMR and FT-IR analyses. Moreover, the relatively low
molecular weight of the PA5 (Mn = 3900 g/mol) can also be
a factor contributing to the observed sharp decay of thermal
stability with increasing IIDMA content.
To study the melting (Tm), glass transition (Tg), and
crystallization (Tc) temperatures of the synthesized polyamides,
differential scanning calorimetry (DSC) analysis was con-
ducted. According to the data presented in Table 3, the
presence of IIDMA in the linear PA4.10 and PA4.13 also affect
the corresponding melting points of the investigated materials.
As the IIDMA content increases from 0 to 100 mol %, the Tm
values of the SA-based polyamides progressively decrease from
223 to 158 °C (PA1−5, Table 3, Figure 8). A similar trend was
also observed for the BrA-based polyamides (PA6−8, Table 3,
Figure 8). An approximate 10 °C reduction of the Tm was
observed when 42% of 1,6-HDA was replaced by IIDMA (PA8,
Table 3). In the meanwhile, upon increasing the IIDMA
content, the melting enthalpies of both series of the
copolyamides were reduced accordingly. Compared to PA
6.10 (PA 1, Table 3), the melting enthalpy of the PA IIDMA.10
(PA 5, Table 3) was reduced by more than half. Such trend is in
agreement with our previous observation for DAII- and DAIS-
derived homo- and copolyamides.17−19 Such phenomena were
explained previously in terms of the formation of less perfect
chain packing as well as the reduction of the hydrogen bond
density of these isohexide-based copolyamides.17,48 Addition-
ally, bimodal melting endotherms combined with the presence
Thermal Properties of the Polyamides. The thermal
stability of the polyamides was investigated by thermogravi-
metric analysis (TGA). As presented in Table 3, it is evident
that the thermal stability of these new biobased materials is
dependent on the chemical composition, viz. the molar ratio of
IIDMA and 1,6-HDA of the copolyamide, as well as the
molecular weight. For the SA- or BrA-based PAs containing less
than 50 mol % of IIDMA, the 5% weight loss temperatures of
these PAs are in the range of 390−410 °C, which are
comparable to those of the reference polyamides PA 6.10 or PA
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polyamides. As presented in Figure 9, all the synthesized SA-
based polyamides are semicrystalline materials, as evidenced by
Figure 8. DSC thermograms of the homopolyamides PA 6.10 (PA1),
PA IIDMA.10 (PA5), PA 6.13 (PA6), SA, and 1,6-HDA, the IIDMA-
based copolyamides PA2, PA3, PA4, BrA, and 1,6-HDA, and the
IIDMA-based copolyamides PA7 and PA8. The graph shows the
second DSC heating runs with heating rate of 10 °C/min.
Figure 9. X-ray powder diffraction profiles of 1,6-hexamethylene
diamine-, isoidide-2,5-dimethylamine-, and sebacic acid-based poly-
amides. For compositions of the copolyamides see Table 1.
of cold crystallization exotherms were observed for the
copolyamides synthesized from BrA. This is a typical indication
of the existence of imperfect crystals (low melting temperature
peak), their melting upon heating and recrystallization into the
more perfect crystals (high melting temperature peak). Similar
bimodal melting endotherms have also been observed for the
DAII-based copolyamides containing more than 40 mol % of
DAII, as well for all the copolyamides based on DAIS and BrA-
derived copolyamides.17,18 In contrast, the shapes of the second
DSC melting peaks of the SA-based (co)polyamides containing
similar percentages of IIDMA appear to be unimodal. This may
imply that the crystal structure of SA-based copolyamides is
more regularly formed from the melt than the BrA-based
copolyamides. Concerning the number of carbon atoms
existing in the used comonomers, it is speculated that the
combination of brassylic acid and IIDMA/1,6-HDA, viz. the
combination of odd (=13) and even (=6) number of carbon
atoms, is less favorable for a regular crystal packing than the
combination of two monomers with odd numbers of carbon
atoms, viz. SA and IIDMA/1,6-HDA.
Closely related to the decrease of Tm and the melting
enthalpies, the crystallization temperatures (Tc) and crystal-
lization enthalpies were reduced as well with increasing
percentage of IIDMA in the copolyamides. Nevertheless,
when the IIDMA content was below 50 mol %, such adverse
effect on Tc and crystallization enthalpy is considerably less
pronounced than in the cases where the IIDMA content is
greater than 50%.
the sharp scattering signals appearing in their diffractograms
recorded at room temperature. The crystallographic structure
of these polyamides show strong dependence on the chemical
compositions, viz. the molar ratio of IIDMA/1,6-HDA in the
copolyamides. When a relatively low content of IIDMA (≤50
mol %, PA2 and PA3) is incorporated, the diffraction profiles of
these copolyamides are rather similar to that of the linear PA
6.10 (PA1, Figure 9). Given the presence of four characteristic
reflections in the 2θ ranges of 3−7, 10−12, and 18−25°
corresponding to the crystallographic planes 001, 002, 100, and
010/110 (Figure 9, Table 4), the crystallographic structures of
these polyamides are believed to adopt triclinic forms.49 The
100 diffraction peak corresponds to the interchain distance, i.e.,
the distance between two polymer chains lying within the same
hydrogen-bonded sheet, while the 010/110 peak corresponds
to the intersheet distance, i.e., the distance between two
adjacent hydrogen-bonded sheets. The 001 and 002 diffraction
peaks give information on the length of the chemical repeat
unit. With increasing content of IIDMA in the copolyamides,
different trends in the four types of diffraction signals were
observed: the 001 and 002 diffraction peaks tend to move
toward a region with lower 2θ values, meaning an increase of
the c-axis dimension of the copolymers due to cocrystallization
phenomenon of the comonomers in the same crystallographic
lattice; 100 diffraction peaks move toward higher 2θ regions,
meaning a decrease of interchain distance within the same
hydrogen-bonded sheets; 010/110 diffraction peaks move
toward lower 2θ regions, indicating an increase of the intersheet
spacing. Additionally, in the diffractogram of PA 6.10 (PA1)
shown in Figure 9, we can observe a weak signal with a 2θ value
of 21.6°, which is indicative of the presence of a small amount
of the pseudohexagonal type of crystals.49 Comparing to PA1 to
PA3, further increasing the IIDMA content to >50 mol % in the
SA-based copolymer leads to significant changes of the
diffractograms. The diffraction profile of PA4 containing
about 68% IIDAM (PA4) displays a comparable diffraction
pattern as that of PA IIDMA.10 (PA5). In both cases, there are
only two major strong scattering signals appearing in the 2θ
regions of 3−7° and 18−25°, respectively. Thus, with
In addition, during the cooling DSC runs at a cooling rate of
10 °C/min (Table 3), pronounced glass transitions were
observed for the SA-based copolyamides containing more than
50 mol % IIDMA. From PA3 to PA5, the glass transition
temperature of the (co)polyamides increases with the
percentage of IIDMA. Compared to the commercialized PA
6.10 (Tg = 50 °C), introduction of IIDMA into PA6.10 raises
Tg by 10−17 °C. As extensively shown for other types of
isohexide-derived monomers,1−3 such a Tg-enhancing effect can
be understood by the intrinsic rigidity of the bicyclic cis-fused
tetrahydrofuran rings in IIDMA.
WAXD Analysis of the Polyamides. Wide-angle X-ray
scattering measurements were conducted on all the obtained
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Table 4. X-ray Diffraction Spacings of the Copolyamides Based on 1,6-Hexamethylene Diamine, Isoidide-2,5-Dimethylamine,
and Sebacic Acid
2θ (deg)
X-ray diffraction (nm)
001
5.15
002
100
010/110
001
1.715
002
100
010/110
PA1
PA2
PA3
PA4
PA5
10.33
9.89
9.62
−
20.22
20.42
20.55
20.30
19.99
24.02
22.83
21.56
−
0.856
0.894
0.919
−
0.439
0.435
0.432
0.437
0.444
0.370
0.389
0.412
−
4.88
1.810
4.83
1.828
a
a
4.78, 5.13
4.32, 5.03
1.848, 1.722
2.044, 1.756
−
−
−
−
a
Due to overlapping of the signals coming from different crystallographic phases, data were derived by deconvolution using WAXSFit software to
indicate the possible positions of the diffraction signals.
increasing IDMA content in the compolymides, the crystal
structures transform from triclinic (when IIDMA% ≤ 50 mol
%, PA1, PA2, and PA3) to a close to hexagonal form (PA4 and
PA5). Moreover, as a result of overlapping of the signals
coming from different potential crystallographic phases, the
scattering signals of PA4 and PA5 in the 2θ range of 3−7°
appear to be asymmetric. Hence, two data are given in Table 4
by deconvolution using WAXSFit software to indicate the
possible positions of these two peaks for samples PA5 and PA4.
In addition, a few low intensity signals were observed to be
present at 2θ values of 18°, 22.5°, and 23.6°, respectively, which
together with the previous phenomena may indicate the
presence of a small percentage of the triclinic crystals as well. In
comparison with PA5, the scattering signals of 001 and 100
reflections in the diffraction profile of PA4 were observed to be
located at regions of slightly higher 2θ values, suggesting the
slightly smaller intersheet and interplane spacings. Never-
theless, given the potential displacements that may exist for the
reflection signals of triclinic phase in terms of 2θ values as well
as in view of the low amount of this crystallographic form,
determining the precise location and displacement of these
signals is difficult.
signals of the BrA-based polyamides should be the same as in
the case of SA-based analogous polyamides. In this regard, we
assume that the reflection signal located at the 2θ range of 3−
7° arises from the 001 plane, while the signals appearing in the
2θ range of 18−25° most probably come from 100 and 010/
110 planes of the triclinic crystallographic form.49 To indicate
the uncertainty of the assigned planes for further discussion,
individual signals of BrA-based polyamides are marked by
asterisks (∗) as shown in Figure 10. In the diffractogram of
PA6, several weak signals in the 2θ range of 6−14° were
observed for all the three PAs, which might come from the
crystallographic planes indexed as 00l. Again, the presence of
IIDMA strongly influences the crystallographic structures of the
BrA-based polyamides (PA7, PA8). When 18 mol % 1,6-HDA
is replaced by IIDMA (PA7, Figure 10), the diffraction signal
indexed as 010/110* planes was observed to shift from 24° to a
significantly lower 2θ value of 21.8°, suggesting the increase of
the intersheet distances (Table 5). Incorporation of 42 mol %
Table 5. X-ray Diffraction Spacings of the (Co)polyamides
Based on 1,6-Hexamethylene Diamine, Isoidide-2,5-
Dimethylamine, and Brassylic Acid
Little knowledge has been available concerning the crystallo-
graphic structure of the BrA-based polyamides, e.g. PA 6.13
(PA6). As shown in Figure 10, the diffraction profile of PA 6.13
shows a reflection signal of medium intensity in the 2θ range of
3−7°, as well as two strong reflections in the 2θ range of 18−
25°. Such a diffraction pattern resembles those of several linear
aliphatic PAs, such as PA 6.10,49−52 PA4.13,18 PA 6.1249,51 or
PA 6.18.49 Therefore, we assume that indexing of individual
2θ (deg)
100*
X-ray diffraction (nm)
001*
010/110*
001*
100*
010/110*
PA6
PA7
PA8
3.67
3.61
3.68
20.57
20.85
21.11
23.49
21.83
−
2.406
2.446
2.399
0.432
0.426
0.421
0.378
0.407
−
of IIDMA completely changed the diffractogram as evidenced
by the diminishing of the 010/110* signal (in case of the SA-
based polyamides, the complete change of the crystallographic
structure has taken place only when the IIDMA content is
higher than 49 mol %). Therefore, we observe a single
reflection in the 2θ range 18−25°, meaning that the IIDMA-
rich material crystallized into a close to hexagonal form.
Furthermore, with the introduction of IIDMA, the 100* signals
show a slight increase in 2θ values, which indicates a decrease of
spacing between planes of particular population.
CONCLUSIONS
■
A combination of melt polymerization and solid state
postcondensation (SSPC) has been used as a suitable route
to synthesize two novel series of homo- and copolyamides
based on the newly developed isoidide-2,5-dimethyleneamine
(IIDMA), 1,6-hexamethylenediamine (1,6-HDA), and the
renewable sebacic acid (SA) or brassylic acid (BA). The
number-average molecular weights (Mn) of the synthesized
polyamides are in the range 3900−49000 g/mol. 1D and 2D
NMR spectroscopy confirmed the exo/exo configuration of the
Figure 10. X-ray powder diffraction profiles of the (co)polyamides
based on 1,6-hexamethylene diamine, isoidide-2,5-dimethylamine, and
brassylic acid.
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combination of (VT) 13C{1H} CP/MAS NMR experiments
with temperature dependent FT-IR analyses revealed that, as a
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ASSOCIATED CONTENT
* Supporting Information
NMR and FT-IR spectra and conformational analysis of the
polyamides. This material is available free of charge via the
Internet at http://pubs.acs.org/.
■
S
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AUTHOR INFORMATION
Corresponding Author
*E-mail: (L.J.W.) l.jasinska@tue.nl; (M.R.H.) mrh@mpip-
mainz.mpg.de. Telephone: +31-40-2472527. Fax: +31-40-
2463966.
■
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge Prof. H. W. Spiess for helpful
discussions. This work forms part of the research program of
the Dutch Polymer Institute (DPI, Project Nos. 656 and 685).
The financial support from DPI is gratefully acknowledged.
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