Synthesis, characterization and properties of N-maleimide side-chain liquid crystalline copolymers

Article abstract of DOI:10.1002/pola.24463

A homologous series of side-chain liquid crystalline (SCLC) poly{[N-[10-((4-(((4′-n-hexyloxy)benzoyl)oxy)phenoxy)carbonyl)-n-decyl] maleimide]-co-[N-(n-octadecyl)maleimide]} [(ME6)-co-(MI-18)] random copolymers with various MI-18 contents have been synthesized and their properties studied. The high content in threo-disyndiotactic sequences of the maleimide main chain seems responsible for the stability of the highly ordered smectic mesophase. The relationship between structure and composition on thermotropic mesophase was investigated by polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction. For copolymers with mesogenic unit contents less than ～0.655 molar fraction the transition from (S_A) texture to isotropic (I) is maintained, as shown by the T_Cl, ΔH _Cl and ΔS_Cl amounts and intermolecular spacing 4.42-4.53 A and intralayer correlation lengths of 44.2-45.2 A. The layer thickness does not appreciably depend on copolymer composition. However, copolymers with non-mesogenic comonomer MI-18 molar contents larger than >0.655 molar fraction X(M), are no longer liquid crystalline materials, despite its packing is preserved without any detectable appearance of birefringence. Thermodynamic boundaries of the liquid crystalline state have been established through a phase diagram. The properties of this n-hexyloxy pendant group-based series are compared to those of the analogous materials containing methoxy pendant groups (ME1), and differences are accounted for in terms of the local side-chain packing within the mesophase.

Full text of DOI:10.1002/pola.24463

Synthesis, Characterization and Properties of N-Maleimide
Side-Chain Liquid Crystalline Copolymers
J. M. BAUTISTA DE OJEDA, I. QUIJADA-GARRIDO, J. M. BARRALES-RIENDA
Departamento de Quı´mica-F´ısica de Polı´meros, Instituto de Ciencia y Tecnologı´a de Pol´ımeros (ICTP),
Consejo Superior de Investigaciones Cientı´ficas (CSIC), c/. Juan de la Cierva, 3. E-28006 Madrid, Spain
Received 13 August 2010; accepted 27 October 2010
DOI: 10.1002/pola.24463
Published online 29 November 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: A homologous series of side-chain liquid crystalline
(SCLC) poly{[N-[10-((4-(((40-n-hexyloxy)benzoyl)oxy)phenoxy)-
carbonyl)-n-decyl]maleimide]-co-[N-(n-octadecyl)maleimide]}
[(ME6)-co-(MI-18)] random copolymers with various MI-18 con-
tents have been synthesized and their properties studied. The
high content in threo-disyndiotactic sequences of the malei-
mide main chain seems responsible for the stability of the
highly ordered smectic mesophase. The relationship between
structure and composition on thermotropic mesophase was inves-
tigated by polarizing optical microscopy, differential scanning calo-
rimetry, and X-ray diffraction. For copolymers with mesogenic
unit contents less than ꢀ0.655 molar fraction the transition from
(SA) texture to isotropic (I) is maintained, as shown by the TCl,
ness does not appreciably depend on copolymer composition.
However, copolymers with non-mesogenic comonomer MI-18
molar contents larger than >0.655 molar fraction X(M), are no
longer liquid crystalline materials, despite its packing is preserved
without any detectable appearance of birefringence. Thermody-
namic boundaries of the liquid crystalline state have been estab-
lished through a phase diagram. The properties of this n-hexyloxy
pendant group-based series are compared to those of the ana-
logous materials containing methoxy pendant groups (ME1), and
differences are accounted for in terms of the local side-chain pack-
C
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ing within the mesophase. 2010 Wiley Periodicals, Inc. J Polym
Sci Part A: Polym Chem 49: 495–510, 2011
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DHCl and DSCl amounts and intermolecular spacing 4.42–4.53 A
KEYWORDS: liquid-crystalline polymers; microstructure; phase
˚
and intralayer correlation lengths of 44.2–45.2 A. The layer thick-
diagrams
INTRODUCTION Side chain liquid crystalline polymers
(SCLCP) are formed when mesogenic units are attached to
the polymer backbone as side chains by a flexible spacer.
Mesogens are able to orient because the spacer allows for
independent movement.
SCLC copolymers may also exhibit a different behavior
depending on the stiffness of the main chain. The backbone
chains give the material polymeric properties and the flexible
spacers groups to decouple the mesogenic units and allow
them to form layer structures essentially unaffected by the
motions of the backbone.12 In addition to what has been
said above, when the main chains are very flexible as may
be poly(siloxanes), liquid crystal copolymers are obtained
where the lateral mesogen groups and the main chain form
separated or segregated microphases2,13–25 depending on
spacer length.
The properties of SCLCPs may be affected by varying the
chemical structures of these components. To modify the
properties in a wide range, one logic approach is to co-
polymerize the mesogenic monomer with a non-mesogenic
comonomer.1–3 By changing the ratio of mesogenic to non-
mesogenic units in the copolymer, it is possible to carry out
an effective control over the phase transition. In some cases,
it has been found that the introduction of non-mesogenic
units in SCLCPs did not change the type of mesophase. How-
ever, in some cases, an increase of the number of phases
was reported, that is, polymorphic mesophases, specially in
the case of poly(siloxanes).4–8 The incorporation of non-
mesogenic units can also hinder progressively the formation
of a mesophase in a range, which depends on their chemical
nature and length. In some cases the length of the non-
mesogenic comonomeric units may produce the complete
disappearance of the mesophase.9–11
Most of the studies on SCLC polymers have been concerned
with materials in which the mesogenic groups are attached
to a more or less flexible main chain. However, to our knowl-
edge, there are only a relatively small number of reports in
the current literature concerning the use of a stiff polymer
backbone for SCLC polymers.26 Thus, systematic studies to
prepare SCLC polymers with a stiff or rigid backbone have
been nowadays restricted to a few of them such as poly(1,6-
heptadiyne),27 poly(acetylenes) or poly(1-alkynes),28–32 poly
(dipropargylamine),33 poly(norbornenes),34–43 and poly(N-
maleimides).38,44–47 Poly(N-maleimides) with a liquid crystalline
Correspondence to: J. M. Barrales-Rienda (E-mail: jmbarrales@ictp.csic.es)
C
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 495–510 (2011) 2010 Wiley Periodicals, Inc.
N-MALEIMIDE SCLC COPOLYMERS, BAUTISTA DE OJEDA, QUIJADA-GARRIDO, AND BARRALES-RIENDA
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 1 A simplified chemical structural unit of poly{N-[10-((4-(((40-n-hexyloxy)benzoyl)oxy)phenoxy)carbonyl)-n-decyl)male-
imide]-co-[N-(n-octadecyl)maleimide} [(ME6)-co-(MI-18)] random copolymers. Ordinal numbers are included in each of the two
monomers are required for the localization of proton and carbon atoms in 1H- and 13C-NMR resonance spectra of copolymers,
monomers and intermediates as indicated in the text.
moiety in the side group are very interesting, not only because
of the stiffness of their (2,5-dioxopyrrolidine-3,4-diyl) residue in
the backbone but also of the alignment of the side-chain liquid
crystalline (SCLC) moieties, which would cause an improvement
of orientation and effective ordering in the main chain (micro-
tacticity). It is also very well-known that the high dipole
moment of the above ring at right angle to the backbone mostly
in the threo-disyndiotactic conformation (ladder polymer) and
its cyclic structure confers a great stiffness to the polymer
main-chain thus resulting in high Tg as well as in thermal
stability.48–53 The structural unit of a poly{[N-[10-((4-(((40-n-
hexyloxy)benzoyl)oxy)phenoxy)carbonyl)-n-decyl)maleimide]-
co-[N-(n-octadecyl)maleimide]} [(ME6)-co-(MI-18)] random
copolymers is shown in Figure 1. The phase behavior of
[(ME6)1ꢁx-co-(MI-18)] copolymers and those of their respec-
tive homopolymers is discussed in terms of composition. The
homopolymer PME6 (x ¼ 0) is the SCLCP. The non-mesogenic
homopolymer PMI-18 (x ¼ 1) may be crystalline with a paraf-
fin-like structure in the pendant n-octadecyl side-chains.
for a week on molecular sieves (Merck, A. G., Darmstadt) of
2 mm diameter and 3 Å of pore size to keep anhydrous
before use. Tetrahydrofuran (THF) (Scharlau Chemie, S. A.
Barcelona) used as eluent for SEC experiments was freshly
collected by distillation from copper (I) chloride and potas-
sium hydroxide under a high purity nitrogen atmosphere.
The initiator 2,20-azobisisobutyronitrile (AIBN) (Fluka, A. G.
Buchs) was purified twice by means of two successive frac-
tional crystallizations in a closed heating and cooling and
crystals isolation system under nitrogen atmosphere using
methanol as solvent.
Syntheses
Intermediates and monomers were synthesized using multi-
step procedures. Detailed schematic synthetic chemical route
to the liquid crystalline comonomer [N-[10-((4-(((40-n-hexyl-
oxy)benzoyl)oxy)phenoxy)carbonyl)-n-decyl)maleimide] (ME6) is
given in Figure 2.
Characterization Techniques of Intermediates
and Monomers
EXPERIMENTAL
Intermediates and monomers were characterized according
to the following experimental techniques: the degree of
purity of the monomers and intermediates were verified
using thin layer chromatography (TLC). Bidimensional-TLC
was used when required. TLC chromatograms were obtained
by using Merck silica gel 60 F254 precoated aluminum plates.
Molecular structures of intermediates and monomers were
confirmed by means of organic elemental analysis as well as
by techniques such as IR, 1H- and 13C-NMR spectroscopies.
Elemental analyses were performed on a HERAEUS CHN-OS
RAPID elemental analyzer (W. C. Heraeus GmbH, Hanau).
Materials
Chemicals and solvents were reagent grades and purchased
as follows: hydroquinone, 4-hydroxybenzoic acid, 11-amino-
undecanoic acid, and dicyclohexylcarbodiimide (DCC), (Fluka,
A. G. Buchs). Benzene, 1-bromohexane, dimethylaminopyri-
dine (DMAP), chloroform, boric acid, and potassium iodide
(Merck, A. G., Darmstadt). Glacial acetic acid and diethyl
ether (Jaber, S. A., Madrid). Toluene, ethanol, isopropanol,
potassium hydroxide, sodium carbonate, sodium bicarbonate,
n-hexane, and sulfuric acid (98%) (Panreac Momplet and
Esteban S. A., Barcelona). Maleic anhydride (Scharlau
Chemie, S. A., Barcelona). All were used as received without
any previous purification. Benzene for polymerization was
purified by refluxing it over sodium wires for a day and col-
lected and stored for a week on molecular sieves (Merck,
A. G., Darmstadt) of 2 mm diameter and 3 Å of pore size,
then distilled under vacuum to the polymerization ampoules,
which previously were charged with monomers and initiator,
in a high vacuum line when required. Chloroform was stored
Infrared (IR) absorption spectra were recorded in transmis-
sion mode by means of IR double beam Perkin-Elmer spec-
trometers models 457 and 783. KBr disks technique (2–
3 mg/100 mg KBr) was used. IR spectra have been used to
characterize chemical structures. 1H-NMR spectra have been
used to characterize structures as well as to estimate copoly-
mers molar fraction X(M) composition. They were recorded
at 200 MHz on a Bruker AM-200 spectrometer. All spectra
were recorded in CDCl3 solution with TMS as the internal
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water pump, and suspended by vigorous stirring in n-hexane
and filtered off at room temperature to eliminate the
unreacted n-hexyl bromide. The white solid is recrystallized
twice from glacial acetic acid, dried in vacuum, yielding to
20.15 g (0.091 mol) of 4-n-hexyloxybenzoic acid (I) (yield ¼
57.2 %). C13H18O3 (222.3) Calculated: % C 70.24; H 8.16.
Found % C 70.23; H 8.38. IR (KBr) cmꢁ1: 3000 and 2500 (t
ACOOH), 2920 (t CAH, asymmetric, including ACH3 and
ACH2A), 2850 (t CAH, symmetric, including ACH3 and
ACH2A), 1695 (t C¼¼O), 1610, 1580, and 1520 (t CAC),
1460 (da ACH3 and ACH2A), 1390 (ds ACH3), 1300 (t
CAOAC, dimers), 1250 (ts CAOAC, ester), 930 (dip OCAOH,
dimers), 840 and 780 (doop CAH) and 720 (q
A(ACH2A)nA).
4-Hydroxyphenyl-40-n-hexyloxybenzoate (II)
In a three necks 1000 mL round-bottom glass flask equipped
with mechanical stirrer, a vacuum isolated Vigreux column
coupled to a Dean-Stark distilling trap, and an inlet of dry
nitrogen 5.7 g of (I) (0.0256 mol), 11 g hydroquinone
(0.1004 mol), 200 mL toluene, 0.1 mL sulfuric acid, and 0.04
g (6.4 mol ꢂ 10ꢁ4 mol) finely milled boric acid are intro-
duced and heated until toluene reflux. After 7 h refluxing
and elimination of water by the Dean-Stark trap, toluene is
stripped off to dryness and the remaining solid left to cool
at room temperature. Then the solid is dissolved in 200 mL
of dried diethyl ether. The solution is successively washed
up with 0.1 N sodium bicarbonate water solution, 0.1 N
hydrochloric acid and finally with distilled water until
neutral pH is achieved. The resulting solution is dried over
anhydrous calcium chloride overnight and concentrated by
vacuum distillation. The solid is recrystallized from ethyl
alcohol/water (80/20) and finally it is purified by column
chromatography on silica gel using a gradient of elution of
chloroform/n-hexane/acetone-diethyl ether (40/40/20). It
gave 3.86 g (0.0123 mol) of monoester (II) (yield ¼ 48 %).
White crystals of mp ¼ 380.8 K. C19H22O4 (314.4) Calcu-
lated: % C 72.59; H 7.05. Found: % C 72.67; H 6.96. IR
(KBr) cmꢁ1: 3400 (m AOH), 3080 (m CAH aromatic), 2920 (m
CAH, asymmetric), 2860 (m CAH, symmetric), 1720 (m C¼¼O,
ester), 1605, 1580, 1510, and 1440 (m CAC, aromatic), 1460
(da ACH3 and ACH2), 1350 (ds ACH3), 1260 (m CAOAC
ether), 1245 (ma CAOAC ester), 1195 (m CAO phenol), 1080
(ms CAOAC, esther), 1030 (ms CAOAC, ether), 855, 780, and
770 (doop CAH) and 730 (q A (ACH2A)4A). 1H-NMR
(200 MHz, CDCl3, TMS, d ppm): 13C-NMR (200 MHz, CDCl3,
d ppm): 153.63 (C-14), 116.21 (C-15), 122.50 (C-16), 144.17
(C-17), 166.08 (C-18), 121.38 (C-19), 132.3 (C-20), 114.32
(C-21), 163.63 (C-22), 68.38 (C-23), 29.02 (C-24), 25.81(C-
25), 31.51(C-26), 22.54 (C-27), and 13.97 (C-28).
FIGURE 2 Simplified synthetic chemical route to [N-[10-((4-(((40-
n-hexyloxy)benzoyl)oxy)phenoxy)carbonyl)-n-decyl)maleimide]
(ME6) mesogenic comonomer.
standard. 13C-NMR spectra were carried out on a Gemini
200 (Varian) spectrometer operating at 50.28 MHz and a
Brucker WP-80 operating at 20.15 MHz both at room tem-
perature (296.2 K) on undegassified 20 % (w/v CDCl3 or
DMSO-d6 solutions in 5 mm i.d. tubes. Chemical shifts are
referenced to the central resonance of CDCl3 or DMSO-d6
[77.00 ppm or 39.50 ppm, respectively, from (CH3)4Si]. They
were also employed to characterize the chemical structure
as well as the microtacticity of the macromolecular main
chain.
Syntheses of Intermediates and Monomers
4-n-Hexyloxybenzoic acid (I)
In a 1000 mL four-neck round-bottom glass flask equipped
with mechanical stirrer, dropping funnel, nitrogen inlet, and
a bulbs condenser, 30 g (0.536 mol) of potassium hydroxide
in 100 mL of distilled water are dissolved. Then 300 mL of
ethanol and 22 g (0.159 mol) of 4-hydroxybenzoic acid are
added to react and purged with N2 for half an hour. After
complete dissolution, potassium iodide traces are added,
then the contain is heated up and at the same time 32.7 g
(0.198 mol) of n-hexyl bromide dissolved in 200 mL of
warm ethanol are added by dropwise and vigorous stirring
and heating up to reflux for 12 h. Then, it is cooled down to
room temperature, filtered off, and the solid washed up with
distilled water until neutral pH. It is dried in vacuum by a
N-(10-carboxy-n-decyl)maleamic acid (III)
In a 3000 mL three-neck round-bottom glass flask equipped
with mechanical stirrer, bulb condenser and thermometer,
252.6 g (2.576 mol) of very fine grinded maleic anhydride
(dihydro-2,5-dioxofuran), 451 g (2.240 mol) of 11-amino-
undecanoic acid and 1500 mL of glacial acetic acid are mixed
to react and maintained under vigorous stirring during 3 h
at room temperature.
N-MALEIMIDE SCLC COPOLYMERS, BAUTISTA DE OJEDA, QUIJADA-GARRIDO, AND BARRALES-RIENDA
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
N-(10-carboxy-n-decyl)maleimide (IV)
C
34H43NO7 (577.7) Calculated: % C 70.68; % H 7.50; % N
Then the solution in glacial acetic acid of (III) is refluxed for
9 h. The intramolecular cyclation of (III) has been controlled
by IR following the variation of the ratio between two absor-
bances at 840 cmꢁ1 and 880 cmꢁ1 since each one of these
signals are characteristics of the N-maleimide and N-malea-
mic acid, respectively (out-of-plane deformation vibration of
CAH conjugated with C¼¼O). After the ratio had got a steady
constant value, 50 % of the acetic acid from the solution is
stripped off and the solution cooled down to room tempera-
ture. It was poured by dropwise on an ice/water mixture
with vigorous stirring until a white precipitate is formed.
Then the precipitate was filtered off and water extracted
until free from acetic acid, dried at room temperature and
extracted with warm n-hexane in a solid-liquid extractor
yielding to 325 g (1.1551 mol) (yield ¼ 45.0 %). White crys-
tals of mp ¼ 360.4 K. C15H23NO4 (281.35) Calculated: % C
64.03; % H 8.24; % N 4.98; % O 22.75. Found: % C 63.94;
% H 8.35; %N 5.13; %O 22.48. IR (KBr) cmꢁ1: 3500–2500
(t AOH hydrogen bonded), 3090 (t CAH, AHC¼¼CHA), 2920
and 2850 (ta and ts CAH), 1720–1690 (t C¼¼O, ACOOH and
AOCAN¼¼), 1590 (t C¼¼C), 1470 (ds CAH), 1420 (t CANAC
and d CAH, ACH2ACOA), 1185 (t CAO), 1125 (t CANAC),
940 (dip OCAOH), 845 (doop CH conjugated with C¼¼O), 720
2.42. Found: % C 70.96; % H 7.37; % N 2.46. IR (KBr) cmꢁ1
:
3110 (m ¼¼CAH), 3080 (t CAH, aromatic), 2930 and 2860
(m CAH asymmetric and symmetric), 1760, 1730, and 1700
(m C¼¼O, RACOOAØ, ØACOOAØ, N-maleimide ring), 1610,
1580, and 1510 (t, CAC, aromatic), 1470 (da, ACH3, ACH2),
1380 (ds, ACH3), 1420 (m, CANAC, d ACH2 from ACH2A
C¼¼O included), 1260 (ma, ØACOOAØ, m CAOAC and from
ether), 1180 and 1170 (ma, CAOAC, RACOOAØ), 1070 (ms,
CAOAC, ØACOOAØ), 1020 and 1010 (dip, CAH), 690 (d,
C¼¼O), 840, 770, and 700 (doop, CAH and doop, CAH and
from the N-maleimide ring) and 720 (q, (ACH2A)nꢁ). 1H-
NMR (200 MHz, CDCl3, TMS, d ppm): d 2.55 (t, 2H, H-2,
ACH2ACOAOA), 1.4–1.9 (q, 2H, H-3), 1.29 (s, 12H, from H-4
to H-9), 1.1.4–1.9 (2H, H-10), 3.51 (t, 2H, H-11, ¼¼NACH2A),
6.66 (s, 2H, H-13, AHC¼¼CHA), 7.12 (d, 2H, H-15), 7.12 (d,
2H, H-15), 7.21 (d, 2H, H-16), 8.12 (d, 2H, H-20), 6.97 d, 2H, H-
21), 4.04 (t, 2H, H-23, AØAOACH2A), 1.4–1.9 (m, 24H, from
H-24 to H-27) and 0.92 (t, 3H, H-28, ACH3). 13C-NMR (200
MHz, CDCl3, d ppm): d 172.06 (C-1, ACOAOAØA), 34.28 (C-2,
ACH2ACOA-OAØA), 24.83 (C-3), 29.29, 29.23, 29.11, and
28.99 (from C-4 to C-8), 28.44 (C-9), 26.63 (C-10), 37.83 (C-11,
¼¼NACH2A), 170.60 (C-12, ¼¼C¼¼O, from N-maleimide ring),
133.96 (C-13, AHC¼¼CHA), 148.34 (C-14), 122.33 (C-15),
122.52 (C-16), 148.05 (C-17), 164.71 (C-18, AOACOAØA),
121.33 (C-19), 132.21 (C-20), 114.27 (C-21), 163.54 (C-22),
68.28 (C-23, AØAOACH2A), 29.47 (C-24), 25.58 (C-25), 31,47
(C-26), 22.50 (C-27), and 13.93 (C-28, ACH3).
1
(q A(ACH2A)4ꢁ for n ꢃ 4) and 700 (d CAO). H-NMR (200
MHz, CDCl3, TMS, d ppm): d 1.27 (s 12H, H4–H9), 1.59 (m
4H, H3, and H10), 2.34 (t 2H, H2, ACH2ACOOH), 3.50 (t 2H
H11, ¼¼NACH2A), 6.68 (s 2H H13, AHC¼¼CHA). 13C-NMR
(200 MHz, CDCl3, d ppm): d 24.62 (C-3), 26.65 (C-10), 28.46
(C-9), 29.31, 29.22, 29.11, and 28.98 (C-4 to C-8), 33.92 (C-
2, ACH2ACOOH), 37.88 (C-11, ¼¼NACH2A), 134.01 (C-13,
AHC¼¼CHA), 170.79 (C-12, ACOAN¼¼) and 179.69 (C-1,
ACOOH).
N-(n-octadecyl)maleimide (MI-18)
Its synthesis, purification, and characterization have been
described elsewhere.54
Polymerization and Copolymerizations
Copolymers and their respective homopolymers were syn-
thesized by free radical polymerization in benzene solution
up to high conversion at 353.0 K, using [I] ¼ 2.0 M AIBN as
initiator. Total monomers concentration was [M] ¼ 0.20 M
and polymerization time 48 h and 80 % conversion. All the
polymerizations were performed under high vacuum in
sealed glass ampoules. The product of polymerization was
terminated and precipitated by pouring dropwise the reac-
tion benzene mixture under a vigorous stirring into a large
volume of isopropanol as precipitant. The polymer produced
as a fine white precipitate was filtered off, washed with
isopropanol, dried, and dissolved again in benzene and
repeated once again the isolation procedure, and then dried
in a hot vacuum plate (320.0 K). The resulting copolymers
and homopolymers were completely soluble in various
organic solvents such as CH3Cl, CH2Cl2, THF, C6H6, and so
forth. and films could be very easily cast on glass slides to
give clear-brownish, shiny thin films. They are, however in-
soluble in methanol, ethanol, isopropanol, diethyl ether, n-
hexane, and so forth.
[N-[10-((4-(((40-n-hexyloxy)benzoyl)oxy)phenoxy)
carbonyl)-n-decyl]maleimide] (V) (ME6) Monomer
In a 200 mL double-wall thermostatized glass vessel 4.56 g
(0.0145 mol) of (II) and 4.22 g (0.0145 mol) of (IV) in
100 mL of dry chloroform are dissolved. Then the solution is
vigorous stirred and cooled down to 263.2 K adding a
spatula of 4-dimethylaminopyridine (DMAP) and 3.095 g
(0.0150 mol) of dicyclohexylcarbodiimide (DCC) in 100 mL
of dry chloroform. The solution is stirred at 263.2 K for
30 min. and left to stand to room temperature for 2 h. The
1,3-dicyclohexyl urea (DCU) precipitate is filtered off.
The procedure is repeated once again after 2 h. to remove
the last traces of DCU. The solution is washed off few times
with 0.1 N hydrochloric acid, distilled water until neutral pH,
0.1 N sodium carbonate, and water to neutral pH. The
remaining solution is dried on anhydrous sodium sulfate and
concentrated under vacuum. The solid material is again dis-
solved in chloroform and it is filtered off to remove the last
traces of DCU and concentrated and washed again. The
crude product is purified by column chromatography using a
neutral silica/alumina as stationary phase and chloroform as
eluent to obtain 3.35 g (0.0058 mol), yield 40 % of mp ¼
350.9 K. Subsequently (V) was crystallized from n-hexane
and stored in a brownish glass bottle in the refrigerator.
Characterization Techniques
Poly[N-(n-octadecyl)maleimide] (PMI-18)
C22H39NO2 (349.0) Calculated: % C 76.02; % H 10.73; % N
4.03. Found: % C 75.75; % H 10.83; % N 3.99. IR (KBr)
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7.52; N 2.71. IR (KBr, cmꢁ1): 3080 (t CAH, aromatic), 2930
(t CAH, asymmetric), 2860 (t CAH, symmetric), 1760 (t
C¼¼O, RACOOAØ), 1730 (t CAO, ØACOOAØ), 1690 (t C¼¼O,
N-maleimide ring), 1610, 1580, 1510, and 1440 (t CAC, aro-
matic), 1465 (da ACH3 and ACH2A), 1405 (t CANAC includ-
ing d ACH2A, from ACH2A (C¼¼O) A), 1380 (ds ACH3),
1260 (ta CAOAC, ØACOOAØ), 1185 and 1170 (ta CAOAC,
RACOOAØ), 1070 (ts CAOAC, ØACOOAØ), 1020 and 1010
(dip CAH), 850, 765 and 690 (doop CAH and d C¼¼O and
ØACOOAØ) 725(q, (ACH2A)nꢁ) and 655 (d C¼¼O). 13C-NMR
(200 MHz, CDCl3, d ppm): d 176.75 and 176.35 (two broad
superimposed gaussian bands, 2C, C-2, ¼¼C¼¼O, N-maleimide
ring, threo-disyndiotactic and threo-diisotactic conforma-
tions), 171.89 (1C, C-13, ACOAOA), 164.62 (1C, C-18,
AOAOCA), 163.59 (1C, C-22, ¼¼C¼¼, aromatic), 148.43 (1C,
CA17, ¼¼C¼¼, aromatic), 148.18 (1C, C-14, ¼¼C¼¼, aromatic),
132.24 (2C, C-20, ¼¼CHA, aromatic), 122.50 (2C, C-15,
¼¼CHA, aromatic), 122.34 (2C, C-16, ¼¼CHA, aromatic),
121.58 (1C, C-19, ¼¼C¼¼, aromatic), 114.36 (2C, C-21, ¼¼C¼¼,
aromatic), 68.36 (1C, C-23, AOACH2A), 44.11 and 39.56
(two broad superimposed gaussian bands, 2C, C-1, ¼¼CHA,
ring, threo-disyndiotactic and threo-diisotactic conforma-
tions), 39.33 (1C, C-3, ¼¼NACH2A), 34.35 (1C, C-12,
ACH2AOA), 31.87 (1C, C-26, ACH2A), 29.53 (3C, C-7, C-8,
and C-9, ACH2A), 29.35 (1C, C-6, ACH2A), 29.27 (1C, C-24
ACH2A), 29.14 (1C, C-10, ACH2A), 27.49 and 26.93 (two
broad superimposed gaussian bands, 2C, C-4, and C-5,
respectively, ACH2A), 25.99 (1C, C-25, ACH2A), 24.93 (1C,
C-11, ACH2A), 22.63 (1C, C-27, ACH2A), 14.01 (1C, C-28,
ACH3). In Figures 3(a,b) enlargements of the methyne car-
bon part of PMI-18 and PME6 13C-NMR spectra are shown.
The deconvolution and integration of the chemical shifts
appearing in these Figures 3(a,b) as two broad super-
imposed gaussian signals centered at ꢀ44.0 ppm and ꢀ39.5
ppm yield to 63 6 5 % threo-disyndiotactic and 37 6 5 %
threo-diisotactic sequences for PMI-18 and 62 6 5 % threo-
disyndiotactic and 38 6 5 % threo-diisotactic sequences for
PME6. Two regular planar conformation for poly(N-male-
imides) main chain are shown in Figure 4. Figure 4(a) shows
a multi-ladder planar 100 % threo-disyndiotactic (structural
units are located in successive step parallel planes). That
one shown in Figure 4(b) represents a (50 %) threo-disyn-
diotactic/(50 %) threo-diisotactic configuration. The con-
figurations of the main chain in our copolymers and two
homopolymers may be represented for such a type but with
only a little different composition, namely with a 62 6 5 %
FIGURE 3 Enlargement of the methyne carbon part of the 13C-
NMR (200 MHz) spectra of homopolymers: (a) poly[N-(n-octa-
decyl)maleimide] (PMI-18) and (b) poly{N-[10-((4-(((40-n-hexy-
loxy)benzoyl)oxy)phenoxy)carbonyl)-n-decyl]maleimide} (PME6).
The deconvolution and integration of the chemical shifts
appearing as two broad superimposed gaussian signals cen-
tered at ꢀ44.0 ppm and ꢀ39.5 ppm have been used to estimate
the microtacticity. The chemical shift appearing at 39.24 ppm in
the PMI-18 spectrum (a) corresponds to the ¼NACH2A methyl-
ene carbon. It has been substracted to the gaussian signal cen-
tered at ꢀ39.5 ppm.
cmꢁ1: 1770 (m C¼¼O), 1700 (m C¼¼O), 660 (d C¼¼O), 2920 and
2850 (m ACHA in ACH2A and ACH3), 1440 (c ACHA),
1405 (m CANAC). 13C-NMR (200 MHz, CDCl3, d ppm): d
176.77 and 176.24 (two broad superimposed gaussian
bands, 2C, C-2, ¼¼C¼¼O, threo-disyndiotactic and threo-diiso-
tactic conformations), 44.11 and 39.45 (two broad super-
imposed gaussian bands, 2C, C-1, ¼¼CHA, threo-disyndi-
otactic and threo-diisotactic conformations), 39.24 (s, C-3,
¼¼NACH2A), 35.75 (s, C-4, ¼¼NACH2ACH2A), 31.94 (s, C-18,
ACH2A), 29.77 (m, C-7 to C-16, ACH2A), 29.68 (s, C-6,
ACH2A), 29.36 (s, C-17, ACH2A), 22.67 (s, C-19, ACH2A),
14.03 (s, C-20, ACH3).
threo-disyndiotactic and
sequences.
a 38 6 5 % threo-diisotactic
Copolymers Chemical Composition
The actual copolymer molar fraction compositions X(M)
were evaluated from the 1H-NMR spectra integrals of pro-
tons as follows: from methyl groups ACH3 or methylene
groups ¼¼NACH2A (for MI-18 and ME6 units) and from phe-
nylen groups AØAOACOAØA or methylene groups
AØAOACH2A (for ME6 units), where AØA is equal to
AC6H4A. A 1H-NMR spectrum for a 0.301 X(M) MI-18 co-
polymer is shown in Figure 5. Each one of the pairs of
Poly{N-[10-((4-(((40-n-hexyloxy)benzoyl)oxy)phenoxy)
carbonyl)-n-decyl]maleimide} (PME6)
C34H43NO7 (577.7) Calculated: % C 70.69; H 7.50; N 2.42.
Found: % C 70.93; H
N-MALEIMIDE SCLC COPOLYMERS, BAUTISTA DE OJEDA, QUIJADA-GARRIDO, AND BARRALES-RIENDA
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
chromatograms showed a unique peak in the high molecular
weight region and were free of tails in the low molecular
weight region, which is characteristic for the absence of
oligomers and monomers. More experimental details have
been given elsewhere.55,56 Molecular weight averages and
polydispersity index of the series are listed in Table 1. The
number-average molecular weights of the polymers and
copolymers range from 1.6 ꢂ 104 to 3.2 ꢂ 104 daltons with
a corresponding degree of polymerization range of 28–92.
These values are sufficiently high to ensure that the thermal
properties of the samples lie outside the molecular weight
dependent regime, allowing meaningful comparisons to be
made of the thermal behavior of members within the series
and with other SCLC poly(N-maleimides). This crude
approach to obtain molecular weight distribution is per-
formed very frequently in the case of SCLCP.57–61 For these
reasons, values gathered in Table 1 do not represent abso-
lute values but estimated ones. They may be useful only in
those cases in which data are discussed for comparative
purposes. The molecular weights were high enough to be in
the range where their thermal properties are supposed to be
molecular weight independent.
Differential Scanning Calorimetry (DSC)
The transition temperatures for the series of copolymers and
the two homopolymers were studied with a Perkin-Elmer
Model DSC-7 differential scanning calorimeter, connected to
a temperature controller and interfaced to a thermal analysis
data station and sealed aluminum pans using about 10 mg
of sample. Temperature and enthalpy calibrations were
FIGURE 4 Molecular models: (a) Two possible enchainment
conformation modes Cis and Trans of the N-maleimide ring
units (2,5-dioxopyrrolidine-3,4-diyl residues) and possible chain
configuration and spatial arrangements of the N-maleimide
ring units: (b) Threo di-syndiotactic (100 %) and (c) Threo di-
syndiotactic/Threo di-isotactic (50 %/50 %) configurations.
checked with Indium (Tm ¼ 429.8 K, DHm ¼ 28.55 J gꢁ1
)
and Gallium (Tm ¼ 303.1 K, DHm ¼ 80.14 J gꢁ1). In all cases
the heating rates were 10 K minꢁ1 for clearing temperature
TCl and melting temperatures Tm, and 20 K minꢁ1 for the
glass transition Tg. TCl and Tm were taken to be at the max-
ima of the endotherms. Glass transition temperatures were
located at the middle of the change in the heat capacity
trace.62 All copolymers and two homopolymers samples
chemical shifts led to the same values for the chemical com-
position. Comonomeric MI-18 molar fraction X(M) com-
position of copolymers are given in the second column of
Table 1. In the text, the homopolymers and copolymers will
be referred as PME6, PMI-18, and CPME6-X, respectively.
Physical Characterization of Homopolymers
and Copolymers
Random copolymers with various contents of n-octadecyl
side chains as non-mesogenic units and their respective
homopolymers were studied by SEC, DSC, WAXS and SAXS,
and POM techniques.
Size Exclusion Chromatography (SEC)
Number- and weight-averaged molecular weights Mn and Mw
of the copolymers and homopolymers were determined by
SEC. The experiments were performed at 296.2 K on a
Waters Associates Model 200 gel permeation chromatograph
with THF as the eluent. A series arrangement of six styragel
columns with upper porosity ratings of 103, 3 ꢂ 103, 3 ꢂ
104, 4 ꢂ 104, 105, and 106 Å (Waters description) was used.
A calibration curve was obtained using poly(styrene) stand-
ards (Waters Associates and Pressure Chemical). All the SEC
FIGURE 5 1H-NMR (200 MHz spectrum) of a 0.301 molar frac-
tion X(M) of MI-18 (CPE6-0.301) SCLC poly[(ME6)-co-(MI-18)]
random copolymer used to calculate its chemical composition.
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TABLE 1 Characterization of a Series of Copolymers, [(ME6)-co-(MI-18)] and Their Respective
Homopolymers, by Chemical Composition from 1H-NMR as MI-18 Molar Fraction X(M),
Weight-, and Number-Average Molecular Weights (Mw and Mn), Polydispersity Index (Mw/
Mn) and Number-Average Degree of Polymerization (DPn) Estimated as It Has Been Indicated
as a Foot Note and Data Obtained by Size Exclusion Chromatography (SEC)
X(M) (Molar
Mw ꢂ 10ꢁ4
Mn ꢂ 10ꢁ4
a
Sample
Fraction)
(Daltons)
(Daltons)
Mw/Mn
DPn
PME6
0
3.6
11.9
4.1
1.6
2.7
1.7
3.2
2.4
2.5
1.9
2.4
2.4
3.1
2.7
2.5
3.2
2.2
4.4
2.4
3.5
2.8
4.3
3.9
3.5
5.2
3.3
4.3
3.9
4.0
27.7
48.3
31.4
61.5
47.2
50.7
41.0
53.9
56.0
77.2
70.9
68.0
91.5
CPME6–0.08
CPME6–0.16
CPME6–0.252
CPME6–0.301
CPME6–0.375
CPME6–0.500
CPME6–0.580
CPME6–0.655
CPME6–0.773
CPME6–0.862
CPME6–0.921
PMI-18
0.08
0.16
0.252
0.301
0.375
0.500
0.580
0.655
0.773
0.862
0.921
1.0
11.2
6.8
10.8
7.4
8.5
12.4
10.2
11.6
9.8
12.8
a
Molecular mass of the equivalent structural unit for copolymers as a function of composition needed to
estimate DPn was calculated according to the following expression: M ¼ 577.7 ꢄ (1 ꢁ x) þ 349.6 ꢄ x, where x
is the MI-18 molar fraction, X(M).
were submitted to the same thermal history, that is, after the
first heating scan, the samples were then annealed at 10 K
above the isotropic temperature for 10–15 min. Under these
experimental conditions the samples were submitted to suc-
cessive cooling and heating cycles. After the first cycle the
experimental traces results were always reproducible.
Results on the thermal transitions were recorded during the
second heating scan. Details of the thermal transitions values
and the liquid crystalline phase transition temperatures for
each of the samples are summarized in Table 2, listed data
TABLE 2 Phase Transition Temperatures, Enthalpies, and Entropies of [(ME6)-co-(MI-18)] Copolymers and Their Respective
Homopolymers
a
b
X(M) (Molar
Tg
TCl
DHCl
DHCl
DSCl
Tm
DHm
DHm
DSm
Tg
Sample
Fraction)
(K)
(K)
(cal/g)
(kJ/mol)
(J/mol.K)
(K)
(cal/g)
(kJ/mol)
(J/mol.K)
(K)
PME6
0
299.2
284.2
276.2
277.5
283.2
382.0
274.2
273.1
272.2
–
439.3
430.6
416.6
409.1
392.0
379.5
354.6
349.2
344.6
–
1.9
2.0
2.0
1.7
1.5
1.3
1.1
0.7
0.5
–
4.6
4.4
4.1
3.1
2.5
2.0
1.3
0.71
0.41
–
10.5
10.3
9.7
7.5
6.5
5.2
3.7
2.0
1.2
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
CPME6–0.08
CPME6–0.16
CPME6–0.252
CPME6–0.301
CPME6–0.375
CPME6–0.500
CPME6–0.580
CPME6–0.655
CPME6–0.773
CPME6–0.862
CPME6–0.921
PMI-18
0.08
–
0.16
–
0.252
0.301
0.375
0.500
0.580
0.655
0.773
0.862
0.921
1.0
–
–
–
–
–
–
c
–
c
–
c
–
c
285.2
287.4
287.0
281.5
299.2
3.0
4.2
3.39
5.30
8.49
18.4
11.8
18.5
30.2
61.6
338.0
342.3
356.8
371.1
–
–
–
–
–
–
–
–
–
–
6.3
–
–
–
–
–
12.6
DHCl in kJ/(mol ME6) and DSCl in J/(mol MI-18)ꢄK.
DHm in kJ/(mol MI-18) and DSm in J/(mol MI-18)ꢄK.
b
a
Molecular mass of the equivalent structural unit for copolymers as a
function of composition needed to transform DHCl (cal/g) into DHCl (kJ/
mol) was calculated according to the following expression: M ¼ 577.7 ꢄ
(1 ꢁ x), where x is the MI-18 comonomer molar fraction composition
X(M).
Molecular mass of the equivalent structural unit for copolymers as a
function of composition needed to transform DHm (cal/g) into DHm (kJ/
mol) was calculated according to the following expression: M ¼ 349.6 ꢄ
x, where x is the MI-18 comonomer molar fraction composition X(M).
c
They have not been possible to be determined.
N-MALEIMIDE SCLC COPOLYMERS, BAUTISTA DE OJEDA, QUIJADA-GARRIDO, AND BARRALES-RIENDA
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
have been extracted from the second heating cycle of the
DSC heat capacity-temperature Cp-T profiles. For this reason,
they represent uncorrected results to zero rate of heating.
Thermal transitions and characteristics for the non-meso-
genic homopolymer have been very well established by
some of the present authors.63
Polarized Optical Microscopy (POM)
A
Carl-Zeiss (Jena) Jenaval-Constrast model polarizing
optical microscope (POM) (with Planachromat LD lens of
ꢂ16 (/0.20) and ꢂ25 (/0.50), and a ꢂ10 ocular, that
is, ꢂ160 and ꢂ250 magnification, ꢂ10 eyepiece and ꢂ2 and
ꢂ4 photographic lens. It is equipped with Mettler models
FP82HT and FP84 hot stages and a Mettler FP800 micro-
processor were used to check and characterize the aniso-
tropic textures. The samples were prepared and observed
between untreated glass slides. Figure 6 shows microphoto-
graphs of smectic structures of PME6 cooled down from the
isotropic state and then annealed at several temperatures.
Small- and Wide X-Ray Diffraction
To elucidate the structures of the mesophases, wide (WAXS)
and small (SAXS) X-ray diffraction (XRD) measurements
were carried out for copolymers and homopolymers. Three
different kinds of XRD experiments were carried out: (i)
SAXS and (ii) WAXS diffractograms of an unoriented PM6
sample at room temperature and (iii) SAXS and WAXS
experiments of well oriented PM6 fibres. The experimental
conditions were as follow:
i. SAXS diffractograms of unoriented PM6 sample were
recorded oh a Rigaku camera Rotaflox RU200 model
using Ni filtered Cu/Ka radiation (40 kV and 40 mA) over
2h angles between 0.5 and 8ꢅ. The experimental slits
conditions: 1/6ꢅ divergency, 0.1 mm reception and 1/6ꢅ
antiscatter. Figure 7(a) shows the diffraction pattern at
room temperature.
ii. WAXS diffractograms of unoriented PM6 sample, were
recorded on a Philips PW 1710-30 X-ray diffractometer
using Ni filtered Cu/Ka radiation (40 kV and 40 mA) over
2h angles between 2 and 30ꢅ. A WAXS diffractogram for
the SCLC homopolymer PME6 is given in Figure 7(b).
Prior to the SAXS- and WAXS-ray runs, the samples were
cast on a glass slide, from a polymer solution in CHCl3,
dried in vacuum at 343.0 K, annealed at temperatures
above their respective glass transitions for 5 h, and then
slowly cooled to room temperature.
iii. In a more detailed study where well oriented samples of
the mesogenic homopolymer PME6 for SAXS and WAXS
experiments were produced by drawing fibers out of the
isotropic melt with a pair of iron steel tweezers, namely,
upon quenching into liquid nitrogen from the isotropic
phase (460.0 K) and then left to stand to room tempera-
ture (298.0 K) for 2 h. The X-ray experiments were per-
formed using a flat camera and a Philips diffractometer
by using Ni filtered Cu/Ka radiation (40 kV and 40 mA)
with a distance from the fiber axis to the photographic
film of D ¼ 60 or 80 mm. Figure 8(a) shows a diffraction
pattern at room temperature of a oriented PME6 fiber. X-
FIGURE 6 Polarizing optical microscopy microphotographies
of: poly{N-[10-((4-(((40-n-hexyloxy)benzoyl)oxy)phenoxy)carbonyl)-
n-decyl]maleimide} (PME6); (a) Mixture of isotropic phase and SA
mesophase with baˆtonetts and brocken fonical conic fan-shaped
texture obtained upon cooling from very above (459.3 K) the TCl
¼ 439.3 K to 448.2 K; (b) Mixture of isotropic phase and SA meso-
phase with baˆtonetts of brocken focal conic fan-shaped texture
obtained upon cooling from very above (459.3 K) the TCl ¼ 439.3K
to 436.2 K. And (c) SA mesophases with baˆtonetts and brocken
focal conic fan-shaped texture obtained upon cooling from very
above (459.3 K) the TCl ¼ 439.3 to 428.2 K.
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ARTICLE
molar fraction X(M) of non-mesogenic comonomer MI-18 are
not only mesomorphic but also show a very peculiar Tg-like
trace. Copolymers containing between >0.655 and 1.0 molar
fraction X(M) of non-mesogenic comonomer MI-18 did not
exhibit any more liquid crystalline structure.
First of all we are going to discuss the thermal behavior of
high content MI-18 copolymers. Let have a look at the right-
hand side of Figure 9. As it can be seen there exists a soft
but very noticeable influence of the copolymer composition
on the thermal transitions, Tg and Tm of the enriched MI-18
copolymers, especially on the Tg of the non-mesogenic group
rich copolymers It is very well established that Tg in this
type in poly(N-maleimides) with long pendant n-alkyl chains
is due to localize cooperative motion among main and side
FIGURE 7 Characteristics X-ray diffraction diagrams obtained
with a side-chain liquid crystal homopolymer PME6 sample at
room temperature. (a) SAXS diffraction pattern and (b) WAXS
diffraction pattern.
ray measurements show no tilted or hexagonal patterns,
but only two narrow non-split wide-angle halos and six
non-split small-angle reflection corresponding to a smec-
tic layer distance of 45.0 Å with a nontilted arrangement
of the mesogen within the layers. All these observations
confirm that the investigated system exhibit a SA phase.
Its contour map diagram is shown in Figure 8(b). All the
X-ray experimental results have been collected in Table 3.
RESULTS AND DISCUSSION
FIGURE 8 (a) Fiber X–ray diffraction pattern of the SA meso-
phase for an oriented poly{N-[10-((4-(((40-n-hexyloxy)benzoyl)oxy)
phenoxy)carbonyl)-n-decyl]maleimide} sample at room tempera-
ture. The primary beam extends perpendicular to the fiber axis.
The arrow indicates the direction of the fiber axis. The sample-
to-film distance was 60 mm. (b) Schematic diagram of its
intensity contour map.
DSC experimental results for all the samples studied includ-
ing transition temperatures as well as their thermodynamic
magnitudes are listed in Table 2. The dependence of the
transition temperatures on the MI-18 molar fraction content
copolymer composition X(M) as a phase diagram is shown in
Figure 9. Copolymers containing between 0 and <0.655
N-MALEIMIDE SCLC COPOLYMERS, BAUTISTA DE OJEDA, QUIJADA-GARRIDO, AND BARRALES-RIENDA
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 3 X-Ray Diffraction Data for a Series of Copolymers and Their Respective Homopolymers
X(M) (Molar
Fraction)
n
d1 6 1
d2 6 1
d3 6 0.5
d0 6 0.05
?
˚
˚
˚
˚
˚
Sample
(A)
k ¼ b1/b2
(A)
(A)
(A)
(A)
PME6 (oriented)
PME6
0
45.0
22.2
22.9
23.2
22.1
23.2
22.6
22.9
23.5
----
14.8
15.0
15.2
14.7
15.2
14.8
14.6
----
4.35
4.38
4.42
4.37
4.44
4.46
4.48
4.53
4.57
4.55
4.47
4.43
4.46
4.33
0
147.3
126.2
42.1
28.5
20.3
18.8
18.0
18.0
17.3
16.7
16.9
17.5
17.8
0.71
1.33
0.56
0.63
0.81
0.90
0.88
1.04
1.08
1.00
0.95
1.01
0.62
CPME6–0.08
CPME6–0.16
CPME6–0.252
CPME6–0.301
CPME6–0.375
CPME6–0.500
CPME6–0.580
CPME6–0.655
CPME6–0.773
CPME6–0.862
CPME6–0.921
PM1–18
0.08
0.16
0.252
0.301
0.375
0.500
0.580
0.655
0.773
0.862
0.921
1.0
----
----
----
18.8
18.4
18.3
15.8
----
----
----
----
PME6 stretched samples for oriented patterns were produced by drawing fibers out the isotropic melt with a pair of iron steel tweezers and then
cooled down to room temperature.
chains.63,64 The decreasing of the Tg with increasing the
mesogenic comonomer content ought to be explained only
because an internal plastizisation effect on N-(n-octadecyl)-
maleimide] units by the [(4-((40-benzoyl)oxy)phenoxy)-
carbonyl)] moiety in the mesogenic units, that is,
ACOAOAØAOACOAØAOA. At this point it is also worth a
mention on what we have assigned to a glass transition tem-
perature between the smectic glass (zone A) and the SA
phase (zone B) in Figure 9. Values are given in the third col-
umn of Table 2. Its dependence on non-mesogenic X(M) com-
position is not very significant or almost independent. At a
glance, this transition seems to be very low since it is due to
localize cooperative motion among the backbone (2,5-dioxo-
pyrrolidine-3,4-diyl residue) and mesogenic side chains [((4-
(((40-n-hexyloxy)benzoyl)oxy)phenoxy)carbonyl)]
substitu-
ents. Romero-Colomer et al.45 have studied this SCLC homo-
polymer by dielectric spectroscopy. Three main relaxation
processes were found. Two relaxations b1 (Ea ¼ 59 kJ
molꢁ1) and b2 (Ea ¼ 40 kJ molꢁ1) appearing at temperatures
below Tg and attributed respectively to rotational motions
and torsional oscillations of the mesogen. The origin can be
the reorientation of the perpendicular component to the
major axis of the permanent dipolar moment located at the
center of the mesogen. And a third one, a (Ea ¼ 320 kJ
molꢁ1), occurring at temperature above Tg, which is assigned
to dipole reorientation of the main chain motions.
A peculiar jump of Tg vs X(M) of the mesogenic moiety may
be observed. The same behavior has been also observed for
the [(ME1)-co-(MI-18)] random copolymers system as can be
it seen in Figures 10(a,b) respectively, where data for both
series of copolymers have been plotted separately to see
more clearly the aforementioned effect. The jump may be
tentatively rationalized in terms of: (i) an artefact; it is inter-
FIGURE 9 Phase transition temperatures diagram as a function
of TCl, Tg, and Tm of [(ME6)-co-(MI-18)] random copolymers
against the molar fraction composition X(M) of the non-meso-
genic MI-18 content in the copolymers.
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ARTICLE
therefore to a higher glass transition temperature.66 Local-
ized motions of mesogenic moieties in this case are similar
to those of small molecules in a confined system.67
The magnitude of the associated entropy change at clearing
point DSCl as shown in the seventh column of Table 2 is con-
sisting with a smectic-isotropic transition. And as we will
see later on, copolymers containing between 0 and <0.655
mol fraction X(M) of non-mesogenic comonomer MI-18
exhibited baˆtonetts and brocken focal conic fan-shaped
textures, which support the assignments as SA phase and
the magnitude of the entropy change associated with the
clearing temperature.
The nature of the phase behavior (molecular textures) of
SCLC copolymers may depends on the following factors: (i)
the length and chemical structure of the mesogenic core
(anisodiametric moiety), (ii) interactions due not only to the
pure chemical composition but also to the molecular geome-
try, (iii) the chemical structure of the main chain, (iv) the
flexible spacer, and (v) the copolymer composition. The
mesogen is normally attached to the main chain through a
flexible spacer. The main chain and mesogen have opposite
tendencies, that is, the main-chain has tendency to form a
random-coil structure or configuration, whereas the meso-
gens promote long range orientational order. The flexible
spacer decouples the interactions of the mesogenic side-
groups from the polymer main-chain and renders the meso-
gens to orientational order. Optical textures of liquid crystal-
line phases, of poly{N-[10-((4-(((40-n-hexyloxy)benzoyl)oxy)-
phenoxy)carbonyl)-n-decyl]maleimide} homopolymer viewed
between crossed polarizers after cooling from the isotropic
state are shown as microphotographs in Figure 6. Figure
6(a) shows a mixture of isotropic state and a mesophase
with a baˆtonetts and brocken focal conic fan-shaped texture
FIGURE 10 Comparison of the glass transition temperature Tg
of the mesogenic group for two series of [(ME6)-co-(MI-18)]
and [(ME1)-co-(MI-18)] SCLC poly(N-maleimide) random
copolymers as a function of the non-mesogenic (MI-18) molar
fraction content X(M): (a) [(ME6)-co-(MI-18)] data are repre-
sented by full circles (l) and (b) [(ME1)-co-(MI-18)] by open
circles (*).
obtained upon cooling from 459.3 K to very above the TCl
¼
439.3 K to 448.2 K and then annealed at this temperature
for 63 min. and no dramatic change of the texture was visi-
ble until the room temperature (298.2 K). It is typical of a SA
mesophase. Figure 6(b) shows, that on reheating the same
sample and cooling down then from very above the clearing
point (459.3–436.2 K) and then annealed for 3 h, a mixture
of isotropic and mesophase with baˆtonetts and focal conic
fan-shaped texture was observed, which confirms once again
the SA phase nature. And finally Figure 6(c) shows that on
reheating and cooling again the same sample from 459.2 K
to 428.2 and then cooled down to room temperature and
annealed for 2 h, a mesophase with baˆtonetts and brocken
focal conic fan-shaped texture typical of SA mesophases
was also observed. This texture is characteristic of SCLC
compounds with layered structures.
esting to note that the width of the trace associated to the Tg
increases with decreasing mesogen content. However, we
have used the same protocol62 through all the series to
define Tg, therefore it would only produce an additional
but continuous drift and not a jump. (ii) An abrupt change
in the smectic field (SF) as a result of a change in concentra-
tion of the n-octadecyl side-chains within the non-mesogenic
comonomer in the neighborhood of the mesogenic moieties.
(iii) It may also due to a progressive change of the surface
fields among mesogens. The self-alignment of liquid crystal
in their nematic or SFs may be influenced by surfaces, elec-
tric and magnetic fields, stress, and flow history. Findlay
et al.65 have found evidence in copolymers of SCLC and non-
mesogenic backbone units for a significant degree of pre-
transitional alignment due to surface fields, and mesogen
ordering perpendicular to the flow-induced backbone align-
ment. It may affect in a very small but detectable amount
the localized motions of the mesogen and therefore its glass
transition. And (iv) to a diluent effect of n-octadecyl side-
chains which could produce a transition between a SA with
normal packing to a less compact SA ordered phase with
more free volume, thus increasing the free volume and
As it was mentioned in the experimental part, in Figure 7
SAXS and WAXS diffraction patterns of a PME6 homopolymer
at room temperature are shown. Figure 7(a) shows the SAXS
diffraction pattern at room temperature. It displays several
sharp reflections in the small angle range. It has also the
characteristic of presenting, a second order of diffraction,
which is very strong when it is compared with the first and
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
third ones. This behavior is attributed to the electronic
density distribution along the normal to the layers68 for very
ordered mesophases. This behavior or the absence of some
orders of diffraction from the layer structure of SCLCPs, has
been observed in some other cases.69–72 This very peculiar
behavior seems to be due to these specific electron density
distributions, which produce the extinction of the diffraction
spacings. The corresponding lattice dimensions are also char-
acterized by a ratio of 1:2:3, which is indicative of the pres-
ence of an ordered layer structure. Figure 7(b) shows a
WAXS diagram obtained with a PME6 homopolymer. Experi-
mental results for this PME6 homopolymer, the pristine
comb-like homopolymer PMI-18 and copolymers of various
MI-18 molar fraction X(M) compositions have been collected
in Table 3.
angle, the intralayer correlation length, n may be estimated
by using the relation n ¼ 57.3 k/(b ꢁ?bo), where k is the
?
wavelength of the X-ray radiation, b is the halfwidth of the
diffraction maximum, and bo is the halfwidth of the instru-
mental broadening. The numerical factor 57.3 is used to con-
vert degrees into radian units; for Ni filtered Cu/Ka radiation
k ¼ 1.542 Å. The instrumental corrections for the instrumen-
tal broadening, bo, of the diffraction peaks, necessary for the
calculation, were made by using powdered naphthalene crys-
tals as a standard material. It is very probable that n may
?
be related to the order of the mesogenic domains in the
side-chain, as well as their size, concentration and distribu-
tion along the smectic layer. This method, although not rigor-
ous, gives an internally consistent measured of n and rela-
?
tive changes in its value. The numerical value of n obtained
?
in this way is only an approximation to the actual value and
should not be taken too seriously. Another important struc-
tural feature of the wide angle maxima can be their asymme-
try, which is characterized by the asymmetry parameter k ¼
b1/b2, where b1 and b2 are defined as b ¼ b1 þ b2, and rep-
resent the two parts in which b is divided by a perpendicu-
lar line traced through the maximum. For symmetrical pro-
files k ¼ 1, and asymmetries of the wide angle maxima are
due to the presence of two different intramolecular distances
in the sample. For the samples investigated, k values lie
around ꢀ1. This means that n-octadecyl units do not intro-
duce a detectable distortion on the mode of packing of
the side chains with composition. Results are collected in
Table 3.
Figure 8(a) shows a fiber X-ray diffraction pattern of the SA of
PME6 obtained from a stretched fiber produced by drawing a
fiber out of the isotropic melt with a pair of iron steel tweez-
ers, namely, upon quenching into liquid nitrogen from the
isotropic phase (460.0 K) and then left to stand to room tem-
perature (298.0 K) for 2 h. It shows six equatorial arcs of dif-
fraction in the direction perpendicular to the fiber axis. They
correspond to the first (45.0 Å), second (22.2 Å), and third
(14.7 Å) (1:2:3) orders of diffraction from the planes of a smec-
tic structure with periodicity ꢀ44.5 Å. The fact that they were
aligned on the equator is indicative that the smectic layers and,
consequently, the polymer chains, are parallel to the fiber axis.
Six sharp wide-angle maxima indicate a very ordered interlayer
hexagonal packing of the mesogenic groups. A rough estimation
of the possible mode of arrangement of the side mesogenic
groups of this SCLCP using conventional molecular models and
nominal values for the standard bond lengths and valence
angles gives us a value of L ¼ 42 6 1 Å, which is almost iden-
tical with the observed layer spacing. It corresponds to the
spacer and the side-chain in their most extended conforma-
tions. Therefore, a monolayer structure of this polymer at room
temperature can be assumed in which these side-chain groups
(spacer, mesogen, and pendant groups) are arranged antiparal-
lel, so that the n-hexyloxy pendant group of one chain is sur-
rounded by methylene groups from the flexible spacers of
another chain. Small angle data confirm the assumption of the
existence of this type of structure. Diffraction arcs at high
angles correspond to a spacing of d0 ¼ 4.40 Å. The average
intermolecular distance in intralayer packing of mesogenic
groups, calculated according to De Vries73 formula as D0 ¼ d0
ꢂ 1.117 Å, is D0 ¼ 4.91 Å, which is typical of highly ordered
smectic phases and correspond to dense hexagonal packing of
benzene (p-phenylene) ring74,75 in oblong molecules. The dif-
fraction pattern obtained from the stretched fiber implies an
orthogonal array between the backbone and side-chains of
macromolecules; that is the side-chains are perpendicular to
the smectic layers, these being parallel to the direction of
stretching. Figure 8(b) shows a scheme of its intensity contour
map. All the XRD primary data are summarized in Table 3.
There exist several additional differences among the X-ray
diffraction diagrams of copolymers series of [(ME6)-co-(MI-
18)] studied in this work and of [(ME1)-co-(MI-18)] studied
elsewhere.91 Diffraction patterns of copolymers from [(ME6)-
co-(MI-18)] series show a progressive modification in the
parameters derived from the spacing at high angles, which
furnish information about the lateral order in the smectic
layers. Figure 11 shows the dependence of the lateral corre-
lation parameter n (Å) on non-mesogenic copolymer molar
?
fraction X(M) composition in the copolymers series of
[(ME6)-co-(MI-18)] studied in this work and of [(ME1)-co-
(MI-18)] studied elsewhere.76 Whereas in the [(ME1)-co-(MI-
18)] series, this parameter remains almost constant through
the series, in the [(ME6)-co-(MI-18)] series an initial expo-
nential sudden decreasing is produced. It is indicative of a
reduction in the ‘‘microdomains’’ size of the smectic struc-
tures inside the glass matrix of the polymer. As it can be
observed the diminution of the smectic domains size is very
drastic. For a composition of 0.200 molar fraction X(M) non-
mesogenic comonomer content this parameter is reduced
about to 0.600 molar fraction X(M) and for contents higher
than 0.25 X(M) it remains constant and with a value almost
the same that in the [(ME1)-co-(MI-18)] series.
The ‘‘microdomains’’ or SF of the ordered or aligned meso-
gens (anisodiametric moiety) is found dispersed inside a
glass matrix, which is composed by the spacers and n-alky-
loxy pendant groups of the mesogenic units and the paraf-
finic-like n-alkyl side-chains of the non-mesogenic units. For
The half-widths at half-maximum (HWHM) diffraction b of
2h are directly related to the degree of order. Assuming a
Lorentzian profile for the maximum at the high diffraction
506
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
tems, we must say that a prerequisite may be that the length
of both comonomers must be similar or comparable.
The incorporation of n-hexyloxy pendant groups as p-substitu-
ents to the mesogens may result in a more ordered structure
of the mesogens. Validity of this alternative assumption may
be confirmed as follows. As it can be seen in Figure 12, where
together with TCl [Fig. 12(a)], the thermodynamic properies
DHCl [Fig. 12(b)] as well as DSCl [Fig. 12(c)] have also been
plotted for [(ME6)-co-(MI-18)] copolymers, as full circles (l),
and [(ME1)-co-(MI-18)] copolymers, as open circles (*), for
comparative purposes. In this Figure 12, a decrease on going
from the n-hexyloxy to the methoxy members of the series is
observed in these three thermodynamic properties. Further-
more, among copolymers one with X(M) ¼ 0. 655 of MI-18
exhibited liquid crystalline structure, which is another piece
of evidence that the n-hexyloxy pendant group strengthens
interaction between phenyl benzoate mesogens, obviously for
the same n-alkyl spacer length linking them to the main chain.
In addition, this effect in possible because poly(N-maleimide)
backbone stiffness exerts a strong influence on its SCLC poly-
mers and copolymers. It governs the structural influence on
the stereochemical configuration of the own backbone order
(microtacticity). Thus these backbones with a ꢀ65 % threo-
disyndiotactic content promote more ordered mesomorphic
structures as well as it also retains the mesomorphism in
FIGURE 11 Comparison of the intralayer correlation length, n
?
for two series of [(ME6)-co-(MI-18)] and [(ME1)-co-(MI-18)]
SCLC poly(N-maleimide) random copolymers as a function of
the non-mesogenic MI-18 molar fraction X(M): [(ME6)-co-(MI-
18)] data are represented by full circles (l) and [(ME1)-co-(MI-
18)] by open circles (*).
this reason, mesogen ‘‘microdomains’’ or the SF play the
same role that crystallites in semicrystalline polymers. This
interpretation is also coherent with the rising observed
experimentally in the clearing enthalpy DHCl as it can be
seen in Figure 12. Owing to a dilution effect upon cooling, a
more ordered structure is formed and, therefore, a small but
detectable increment in the clearing enthalpy DHCl is
observed. This effect is quite similar to that one taking place
in the crystallization from the melt or from solution (single
crystals) for semicrystalline polymers. A subsequent increase
of the non comonomeric units content, starts to reduce the
attractive interactions among the mesogen groups (DHCl),
and to increase the flexibility of the structure and therefore
a depression of Tg, in this case, non mesogenic units act as a
plasticizer. The difference between these n-alkyl side chains
as diluents and plasticizers with conventional one is that
they are anchored to one of its ends, thus hindering some of
their motions. The transition between diluent and plasticizer
effects of the non mesogenic n-alkyl side chain units, namely,
the crossover, will be produced at a critical concentration,
which will depend on the length of the n-alkyl side-chain in
the non mesogenic comonomer and the lengths of the spacer
and n-alkyl pendant groups in the mesogenic comonomer. As
far we are concerned this effect has been only observed in
the present system [(ME6)-co-(MI-18)] copolymers series
and that one of [(ME1)-co-(MI-18)] copolymers series stud-
ied elsewhere.76 Therefore as a general conclusion, and
keeping in mind the experimental results for these two sys-
FIGURE 12 Comparison of thermodynamic properties at the
clearing point (SA !I) for two series of [(ME6)-co-(MI-18)] and
[(ME1)-co-(MI-18)] SCLC poly(N-maleimide) random copoly-
mers as a function of the non-mesogenic comonomer (MI-18)
molar fraction X(M): (a) Clearing temperature TCl; (b) Clearing
enthalpy DHCl and (c) Clearing entropy DSCl [(ME6)-co-(MI-18)]
data are represented by full circles (l) and [(ME1)-co-(MI-18)]
by open circles (*).
N-MALEIMIDE SCLC COPOLYMERS, BAUTISTA DE OJEDA, QUIJADA-GARRIDO, AND BARRALES-RIENDA
507

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
within the side-chains and non mesogenic units in relation
to their thermal transitions, mesophase textures and degree
of order in the smectic layers, a series of poly[N-[10-((4-
(((40-n-hexyloxy)benzoyl)oxy)phenoxy)carbonyl)-n-decyl)ma-
leimide]-co-[N-(n-octadecyl)maleimide] [(ME6)-co-(MI-18)]
random copolymers with various contents of n-octadecyl
side chains as non-mesogenic were studied by DSC, WAXS
and SAXS and POM techniques.
The results confirm the advanced hypothesis that poly(N-male-
imide) backbone stiffness exerts a strong influence on its SCLC
polymers and copolymers. On one hand, it governs the struc-
tural influence on the stereochemical configuration of the own
backbone order (microtacticity), which is thought to be a factor
controlling the microstructure, and thus the mesomorphic
behavior of these polymers and copolymers. In this case, back-
bones with ꢀ65 % threo-disyndiotactic content promote more
ordered mesomorphic structures as well as an increment in
the transition temperatures and thermal stability. However, the
stiffness and regularity of the backbone retain the mesomor-
phism in case of random copolymers with a higher content of
the non-mesogenic comonomer.The properties of this [(ME6)-
co-(MI-18)] random copolymers series are compared to those
of the analogous materials [(ME1)-co-(MI-18)] random copoly-
mers series and differences are accounted for in terms of
the local side-chain effect within the same mesogenic moiety.
The replacement of a methoxy by a n-hexyloxy pendant group
as p-substituent of the mesogen may result in a more ordered
structure of the phenyl benzoate moieties. Validity of this alter-
native assumption has been clearly confirmed. Thermodynamic
properties such as clearing temperature TCl, clearing enthalpy
DHCl, as well as the clearing entropy DSCl show a noticeable
decrease on going from the n-hexyloxy to the methoxy mem-
bers of the series. Furthermore, among copolymers with como-
nomer ME6 one with a composition X(M) ¼ 0.655 MI-18 still
exhibited liquid crystalline structure, which is another piece of
evidence that the n-hexyloxy pendant group strengthens inter-
action between phenyl benzoate mesogens, obviously for the
same n-alkyl spacer length linking them to the main chain. This
copolymer has the most disordered structure among the SCLC
polymers investigated in this article.
FIGURE 13 Plot of the apparent melting enthalpy DHm against
the MI-18 molar fraction X(M) for a homopolymer PMI-18 and
two series of copolymers. DHm have been corrected by taking
into account the number of ME6 and MI-18 or ME6 and MI-18
units in order that the enthalpic data may be comparable in
the case of the copolymers rich in pendant n-alkyl side-chains
[>0.500 X(M) MI-18)]. Thus, plotted values refer non-mesogenic
units contents. Data from [(ME6)-co-(MI-18)] copolymers are
represented by full circles (l) and from [(ME1)-co-(MI-18)] by
open circles (*).
case of random copolymers with a higher content of the non-
mesogenic comonomer.
In Figure 12 the apparent melting enthalpy DHm of the para-
ffin-like crystals against the MI-18 molar fraction X(M), is
plotted for copolymers rich in n-alkyl side-chains [>0.50 X(M)
MI-18] of the two series: [(ME6)-co-(MI-18)] and [(ME1)-co-
(MI-18)]. DHm have been corrected by taking into account the
number of ME6 or ME1 and MI-18 units in order that the
enthalpic data may be comparable in the case of the copoly-
mers rich in pendant n-alkyl side-chains [>0.50 X(M) MI-18].
Therefore plotted values refer only to non-mesogenic units
contents. Data for [(ME6)-co-(MI-18)] copolymers are repre-
sented by full circles (l) and [(ME1)-co-(MI-18)] by open
circles (*). It is interesting to note that the nature of the n-
alkyloxy pendant groups in the mesogenic phenyl benzoate
moieties (n-hexyl for ME6 or methyl for ME1) does not exert
a remarkable significant influence on the crystallization of the
MI-18 comonomer n-alkyl side-chains (Fig. 13).
This article is dedicated to A. Horta with occasion of his 70th
birthday. Thanks are given to C. Marco Rocha (Instituto de Cien-
cia y Tecnologı´a de Polı´meros, Madrid, C.S.I.C.) for running
some of the X-ray diffraction diagrams. J.M.B.de Ojeda gratefully
acknowledges support to this work by a grant from the Plan de
Formacio´n del Personal Investigador of the Ministerio de Edu-
cacio´n y Ciencia. J.M.Barrales-Rienda is grateful also to the
Comisio´n Asesora de Investigacio´n Cientı´fica y Te´cnica of
the Ministerio de Educacio´n y Ciencia for partial support
(Grant-in-Aid for Scientific Research 87-051) of this research.
Thanks are also given to the Ministerio de Ciencia e Innovacio´n
for a Research Project (CTQ2008-03229/BQU).
CONCLUSIONS
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