Reversible cross-linking of aliphatic polyamides bearing thermo- and photoresponsive cinnamoyl moieties

Article abstract of DOI:10.1021/ma502083p

Photosensitive polymers have gained strong attention in development of different polymeric systems for diverse applications. Particular attention is paid to polymers with cinnamoyl groups either in polymer backbone or in pendent position owing to their excellent photoreactivity and thermoreactivity upon UV light irradiation or heating. This work aims at synthesizing a novel photo- and thermosensitive aliphatic polyamide by anionic ring-opening copolymerization of ε-caprolactam and α-cinnamoylamido-ε-caprolactam. The photochemical reversible cross-linking of solutions and thermal cross-linking of spin-coated films made up of these new cinnamoyl-functionalized polyamides were monitored by UV-vis spectroscopy. These aliphatic polyamide copolymers were characterized by magic angle spinning solid-state NMR, and their thermal behaviors were studied by thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis.

Full text of DOI:10.1021/ma502083p

Article
pubs.acs.org/Macromolecules
Reversible Cross-Linking of Aliphatic Polyamides Bearing Thermo-
and Photoresponsive Cinnamoyl Moieties
Deniz Tunc,†,‡,§ Cedric Le Coz,†,‡ Michael Alexandre,§ Philippe Desbois,∥ Philippe Lecomte,*,§
and Stephane Carlotti*,†,‡
†Laboratoire de Chimie des Polymer
̀
es Organiques, UMR 5629, IPB/ENSCBP, Univ. Bordeaux, 16 avenue Pey-Berland, F-33607
Pessac, Cedex, France
‡Laboratoire de Chimie des Polymer
Cedex, France
̀
es Organiques, UMR 5629, Centre National de la Recherche Scientifique, F-33607 Pessac,
§Center for Education and Research on Macromolecules, University of Lieg
̀ ̀
e, Sart-Tilman, B6a, 4000 Liege, Belgium
∥Advanced Material and System Research, BASF SE, D-67056 Ludwigshafen, Germany
ABSTRACT: Photosensitive polymers have gained strong attention in development
of different polymeric systems for diverse applications. Particular attention is paid to
polymers with cinnamoyl groups either in polymer backbone or in pendent position
owing to their excellent photoreactivity and thermoreactivity upon UV light
irradiation or heating. This work aims at synthesizing a novel photo- and
thermosensitive aliphatic polyamide by anionic ring-opening copolymerization of ε-
caprolactam and α-cinnamoylamido-ε-caprolactam. The photochemical reversible
cross-linking of solutions and thermal cross-linking of spin-coated films made up of
these new cinnamoyl-functionalized polyamides were monitored by UV−vis
spectroscopy. These aliphatic polyamide copolymers were characterized by magic
angle spinning solid-state NMR, and their thermal behaviors were studied by
thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical
analysis.
thermoresponsive systems have gained great interest for the
synthesis of reversible cross-linking polymers. As an example,
the preparation of reversibly cross-linking polyurethane based
on Diels−Alder reaction of furan and maleimide moieties were
studied by Du Prez and co-workers.23
INTRODUCTION
■
In recent decades, polymers containing photosensitive moieties
proved a remarkable interest because of their potential use in a
broad range of application fields such as photolithography,1,2
photocurable coatings,3 and photoresists.4
Cinnamoyl,5,6 coumarin,7−10 anthracene,11,12 and azoben-
zene13,14 are the mostly used chromophore groups for
photoresponsive systems by dimerization or isomerization
reaction. Photoreversibility of these groups can be set in the
main chain or as pendant groups to enable controlling of some
physical and chemical properties by cross-linking. Among
photosensitive moieties, the photoreactive cinnamoyl group
dimerization, occurring through the [2 + 2] thermal or
photochemical cycloaddition reaction,15 was implemented for
polymerization reactions and for cross-linking purposes.16 The
so-obtained cyclobutane dimer combines thermal stability17,18
and photosensitivity.19 The photochemical reversibility of the
cross-linking was, for instance, nicely exploited for the synthesis
of self-healing polymers20,21 and tunable shape-memory22
materials. Recently, Oriol, Barner-Kowollik, and co-workers
studied on miktoarm star polymer bearing azobenzene
photoresponsive units and reported the self-assembly proper-
ties of this novel polymer in water.13 Quite recently, Yagci and
co-workers reported self-healing strategy for poly(propylene
oxide)s bearing coumarine−benzoxazine units based on light-
induced coumarine dimerization reactions.9 On the other hand,
Experimental conditions as well as the nature of polymer
chains have a significant influence on the efficiency of the
dimerization reaction. Polymers bearing photosensitive groups
show a higher photoreactivity in solution than in a solid-state
coated film because the photosensitive groups are not in close
vicinity enough in the solid state.24 Besides, the efficiency of the
dimerization reaction of pendant cinnamoyl groups decreases
when the chain length increases owing to a lower mobility.25
Polyamide 6 (PA6) is a high performance semicrystalline
thermoplastic with large industrial applications. It is produced
through (1) the bulk anionic ring-opening polymerization
(AROP) of ε-caprolactam (CL)26−30 and (2) the hydrolytic
ring-opening polymerization.26,27,31,32 PA6 has attained interest
owing to its high strength, good fatigue resistance, moderate
water absorption (about 8−10%), and good resistance to most
common solvents and weak acids.27 Besides those attractive
properties, polyamides are difficult to process due to poor
Received: October 10, 2014
Revised: November 20, 2014
© XXXX American Chemical Society
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(Specialty Coating Systems, P67080 spin coater) on glass plates from
1% solutions in HFIP at 2000 rpm for 5 min and then dried at 25 °C
overnight under vacuum. The UV irradiation of the spin-coated film
and solution was performed by using a UV lamp TQ 150 (150 W)
between λ = 200−280 nm and 450HHL halogen (325−380 nm, 400
W), respectively. UV absorbances of de-cross-linked samples were
followed by a Hitachi U-3300/130-0614 model UV−vis spectropho-
tometer.
Synthesis of α-Cinnamoylamido-ε-caprolactam [(2E)-N-(2-
Oxoazepan-3-yl)-3-phenylprop-2-enamide]. To the solution of
cinnamoyl chloride (5.2 g, 30 mmol) and triethylamine (5.05 g, 50
mmol) in 50 mL of tetrahyrofuran (THF) cooled in an ice bath, α-
amino-ε-caprolactam (4 g, 30 mmol) was added dropwise while
purging the reaction flask with argon. After 15 min, the ice bath was
removed, and the reaction was allowed to proceed overnight at room
temperature. The precipitate was removed by filtration, then THF was
evaporated, and the product was resolubilized in chloroform. The
solution was transferred to an extraction funnel and washed with dilute
HCl(aq) (1 × 50 mL), distilled water (1 × 50 mL), and brine solution
(1 × 50 mL). The organic phase was separated, dried over anhydrous
Na2SO4, filtered, and evaporated. The product was further purified by
washing with diethyl ether to give a white solid (80% yield). 1H NMR
(CDCl3) δ (ppm) = 7.26−7.49 (m, 5H), 7.64 (d, 1H), 7.07 (s, 1H),
6.5 (d, 1H), 4.6 (m, 1H), 3.3 (m, 2H), 2.3−1.2 (m, 6H). 13C NMR
(CDCl3) δ (ppm) = 175.8, 164.5, 140.8, 135.1, 129.9, 135.1, 129.9,
129.0, 127.8, 120.5, 52.7, 41.7, 31.7, 29.0, 27.7.
Synthesis of Polyamide 6. Polyamide 6 was prepared in bulk by
anionic ring-opening polymerization. ε-Caprolactam (8.35 g, 73.8
mmol) and C10 initiator (1.15 g) (or 1.53 mmol of sodium ε-
caprolactamate) were added in a glass reactor at 140 °C under a
nitrogen atmosphere. After C20 activator (0.5 g) (or 0.21 mmol of
reactive functions) was added to the molten mixture, the polymer-
ization started with solidification. After reaction, the polymer obtained
was crushed and refluxed in water for 24 h and then dried in an oven
overnight at 90 °C under vacuum before analysis. Conversion of the
monomer was determined gravimetrically. 1H NMR (CDCl3) δ (ppm)
= 6.0 (s, 1H), 3.1 (m, 2H), 2.0 (m, 2H), 1.25−1.75 (m, 6H). 13C
NMR (solid-state) δ (ppm) = 169.8, 39.7, 32.9, 26.6, 22.7.
Synthesis of Polyamide 6 Bearing Pendant Cinnamoyl
Groups. Polyamide 6 with pendant cinnamoyl groups was prepared
by a one-step pathway by anionic ring-opening polymerization. As a
typical example, α-cinnamoylamido-ε-caprolactam (0.2 g, 0.77 mmol),
ε-caprolactam (8.15 g, 72 mmol), and C10 initiator (1.15 g) (or 1.53
mmol of sodium ε-caprolactamate) were added to a glass reactor
under a nitrogen atmosphere and were molten at 140 °C with
continuous stirring after hexamethylene-1,6-dicarbamoylcaprolactam
(C20) (0.5 g) (or 0.21 mmol of reactive functions) was added to the
mixture to start the polymerization. After reaction, all polymers were
refluxed in water first and followed by washing with dichloromethane
for 24 h each. The polymers were dried in an oven overnight at 90 °C
under vacuum before any analysis. Conversions of the monomers were
determined gravimetrically. 13C NMR (solid-state) δ (ppm) = 169.9,
131.9, 125.7, 39.8, 36.6, 33.0, 26.6, 18.4.
solubility and to high softening and melting temperatures
caused by high crystallinity. On the other hand, adding a
functional groups to tune PA6 properties is a key issue. To date,
scarce examples of functionalization and copolymerization of
CL with its derivatives were reported either by anionic or
hydrolytic ring-opening polymerization. For example, Reim-
schuessel et al. reported on the copolymerization of CL and β-
carboxymethylcaprolactam33 with linear polyimide structure.
Overberger et al.34 studied the ring-opening polymerization of
γ-tert-butoxymethylcaprolactam by obtaining soluble copolya-
mide owing to hydroxymethyl pendant groups. The hydrolytic
ring-opening polymerization γ-phenylsulfonatecaprolactam was
successfully reported by Nijenhuis et al.35 γ-Carboxyethylcap-
rolactam and γ-aminoethylcaprolactam were used for the
synthesis of hyperbranched aliphatic polyamides.36 It is worth
noting that the synthesis of these monomers requires several or
complex reaction steps with moderate yields. Recently, Mathias
et al. reported the AROP of γ-ethylene ketal-ε-caprolactam.37
After deprotection of pendant ketal groups into ketones, the
polyamide was successfully thermally and photochemically
cross-linked. In our previous work, a controlled cross-linking of
polyamide 6 was proposed by AROP of CL and a new bis-
monomer synthesized from α-amino-ε-caprolactam for differ-
ent rheological behaviors of such a material.38
The aim of the present work is to synthesize novel reversibly
cross-linked aliphatic polyamide by implementing the photo-
and thermodimerization reactions of pendant cinnamoyl units.
Toward this end and starting from cyclic lysine, the one-pot
synthesis of α-cinnamoylamido-ε-caprolactam was carried out
followed by its anionic ring-opening copolymerization with CL.
EXPERIMENTAL SECTION
■
Materials. DL-α-Amino-ε-caprolactam (3-aminoazepan-2-one)
(>98%, BASF) was purified by solubilization in toluene and filtration.
After evaporation of toluene a white product was obtained. Cinnamoyl
chloride [(2E)-3-phenylprop-2-enoyl chloride)] (98%) and triethyl-
amine (N,N-diethylethanamine) (>99.5%) were purchased from
Sigma-Aldrich and used without further purification. ε-Caprolactam
(azepan-2-one) (CL) (BASF, 99%) was recrystallized from dry
cyclohexane prior to use. Bruggolen C20 (hexamethylene-1,6-
̈
dicarbamoylcaprolactam [N,N′-hexane-1,6-diylbis(2-oxoazepane-1-car-
boxamide] in CL, 17% w/w of isocyanate in CL, Bruggemann
̈
Chemical) and Bruggolen C10 (∼18% w/w of sodium ε-
̈
caprolactamate [sodium 2-oxoazepan-1-ide] in CL, Bruggemann
̈
Chemical) were used as received. Tetrahydrofuran, chloroform,
acetone, heptane, and diethyl ether were purchased from Sigma-
Aldrich. 1,1,1,3,3,3-Hexafluoropropan-2-ol (HFIP) (Acros Organics,
≥99.7%) was used without any further purification.
1
Characterization. H NMR spectra of reactants were recorded by
Synthesis of α-Truxillic Acid.39 trans-Cinnamic acid (Aldrich,
99%) was recrystallized from acetone (Aldrich) to obtain the α-
polymorph. The crystals (2 g, 13.5 mmol) were put in a reactor and
distributed in 8 mL of heptane. The powder sample in solid state was
exposed to 325−380 nm irradiation using a light source which is a
450HHL halogen (400 W) for 24 h to obtain truxillic acid. 13C NMR
(solid-state) δ (ppm) = 178.9, 175.5, 134.7, 125.6, 123.4, 121.9, 47.8,
42.3, 40.9, 37.4.
using a Bruker AC250 instrument at a proton frequency of 250 MHz
at room temperature. 13C solid state NMR spectra were recorded on a
Bruker 400 Avance I 400 MHz spectrometer at room temperature.
During experiments, the magic angle spinning (MAS) rate was 30 kHz
using a Bruker 2.5 mm HX probe. The number of scans was 256, and
recycle delay was 5 s. Single-pulse experiment (90° pulse of duration)
was 1.8 μs, and FID acquisition time was 41 ms. The FTIR spectra
were recorded using a Nicolet Nexus with an ATR, accessory by
continuum microscope provided with an ATR germanium (Ge)
crystal. Differential scanning calorimetry (DSC) was performed on a
PerkinElmer Diamond DSC with a heating rate of 10 °C min−1 under
nitrogen flow (10 mL min−1). Thermogravimetric analysis (TGA) was
performed on a PerkinElmer Diamond TA/TGA with a heating rate of
10 °C min−1 under nitrogen flow. The storage modulus (E′) and loss
factor (tan δ) for each sample were automatically recorded by a
computer throughout the test. For surface re-cross-linking by heating
and UV irradiation, thin films of PA6 precursors were spin-coated
De-Cross-Linking of Polyamide 6. The 1.2 × 10−4 mol L−1 gel−
solution of cinnamoyl-containing cross-linked polyamide 6 (10% Cin-
PA6) in HFIP was prepared before irradiation. To induce the de-cross-
linking reaction, the sample was irradiated with a UV lamp TQ 150
(150 W, λ = 200−280 nm) at 25 °C for 7 h, and the distance between
the samples and the light source was 4 cm. The increasing absorbance
of the cinnamoyl units was followed by UV absorbance.
Re-Cross-Linking of Polyamide 6. After 7 h, de-cross-linking of
cinnamoyl-pendant polyamide 6 solution (10% Cin-PA6), solvent was
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evaporated, and 1% w/v of HFIP solutions of cinnamoyl-containing
de-cross-linked polyamide 6 was prepared and spin-coated on a quartz
plate. Irradiations under re-cross-linking conditions were carried out
placing the samples at 10 cm from the light source which is a 450HHL
halogen (325−380 nm, 400 W). Following 7 h irradiation, re-cross-
linking was completed with 1 h heating at 140 °C. Absorbance
decreasing of cinnamoyl unit double bonds was followed by UV
absorbance. The coated sample was not soluble in HFIP anymore.
are 1:1:1:2 and support the synthesis and purification of the α-
cinnamolyamido-ε-caprolactam.
First of all, neat PA6 was prepared by AROP in a reactor
system which provides inert atmosphere and easy way to follow
the rate of polymerization (temperature/time diagram). The
polymerization was carried out in bulk at 140 °C. ε-
Caprolactam (CL, monomer) and sodium caprolactamate
(C10, initiator) were added to a glass reactor at 140 °C
under a nitrogen atmosphere to activate the monomer and
were molten with continuous stirring after hexamethylene-1,6-
dicarbamoylcaprolactam (C20, activator) was added to mixture
to start the polymerization. In 15 min, solidification completed
and exothermy slowed down; PA6 was refluxed in water to
remove unreacted reagents and then dried in an oven overnight
at 90 °C under vacuum before any analysis. Conversion of the
monomer (98%) was determined gravimetrically.
RESULTS AND DISCUSSION
■
A successful synthesis of α-cinnamoylamido-ε-caprolactam
(Cin-CL) monomer was achieved by a one-step amidation
reaction (Scheme 1) from α-amino-ε-caprolactam (ACL)
Scheme 1. One-Step Amidation Reaction of Cinnamoyl
Chloride with α-Amino-ε-caprolactam
Syntheses of PA6 bearing different amount of cinnamoyl
units were conducted exactly under the same polymerization
conditions as the one of PA6 was carried out. Addition to that,
Cin−CL comonomer was added to the reactor at the same time
with CL. Results are summarized in Table 1, and the proposed
reaction is depicted in Scheme 2. After the polymerization step,
all obtained copolymers were washed by refluxing first from
water and then with dichloromethane (CH2Cl2) to remove
unreacted Cin−CL comonomer and other reagents.
Different molar ratios of α-cinnamoylamido-ε-caprolactam
were used in each copolymerization with the same amount of
activator and initiator at the same reaction temperature of 140
°C. As the relative amount of cinnamoyl unit was increased
from 2% to 10%, the obtained polymers turned to gel formation
in hexafluoroisopropanol (HFIP) for 6% and 10% of Cin-PA6
(runs 3 and 4). Those polymers, which have higher quantity of
cinnamate units, were thermally cross-linked because of the
reduced distance and mobility of dimerizable groups. It has
already been shown that the dimerization reaction has a
favorable encounter of the cinnamoyl groups due to polymer
chain mobility through heating and irradiation.15,24,25
which can be obtained from ring closure reaction of L-lysine.
After 24 h reaction in an inert atmosphere and then purification
steps, a white solid product was obtained with 80% yield.
The structure of the α-cinnamoylamido-ε-caprolactam was
1
confirmed by H NMR analysis, and Figure 1a exhibits the
During the PA6 synthesis, an exothermic reaction, polymer-
ization and crystallization occur almost simultaneously (Figure
2). These temperature/time data were followed by a
thermocouple which is installed in the calorimeter to measure
the reaction temperature. On the other hand, the copoly-
merization of CL with its derivative leads to a decrease of
polymerization and crystallization exothermic temperatures
(run 2, Table 1, and Figure 2). The flat evolution of
temperature with time and limited conversion were observed
by using 10 wt % of the cinnamated monomer (run 4, Table 1).
Figure 2 indicates clearly a decreased reactivity of the overall
polymerization process and the introduction limit of cinna-
mated groups into PA6. The copolymerization of an increasing
amount of α-cinnamoylamido-ε-caprolactam slows down the
polymerization time and decreases the monomer conversion.
Trying to introduce much more comonomer was not
successful. This is an expected behavior as ring-opening
polymerization of acyl derivative of CL is concerned. The
presence of amide N anions along the polymer chain may
produce imide groups and lead to polymer branching in
strongly basic medium.40 A similar effect can be a concern for
this anionic copolymerization. In addition, a competitive
hydrogen abstraction, due to difference of hydrogen acidities,
from the exocyclic and endocyclic amides by C10 initiator may
occur on Cin-CL (Scheme 3). The deprotonation of the
exocyclic amide consumes C10 amount, decreasing its
1
Figure 1. H NMR spectra of (a) α-amino-ε-caprolactam and (b) α-
cinnamolyamido-ε-caprolactam in CDCl3.
typical proton signals attributed to ethylene groups of the α-
amino-ε-caprolactam structure between 1.25 and 2.30 ppm
(−CH2), primary amine at 2.10 ppm, and amide proton (CO−
NH−C) at 6.75 ppm. After reaction (Figure 1b), endocyclic
and exocylic protons were observed at 6.10 and 7.10 ppm,
respectively. Cinnamoyl aromatic protons appeared between
7.2 and 7.5 ppm. At the same time cinnamoyl alkene protons
were shown at 6.5 and 7.6 ppm. Indeed, integral ratios of i:h:c:b
C
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Table 1. Synthesis of Polyamide 6 and Polyamide 6 Bearing Pendant Cinnamoyl Groups at 140 °C in Bulk and Their Solubility
in HFIP
a
b
b
c
d
run
1, PA6
activator (mol/L)
initiator (mol/L)
CL (mol/L)
Cin-CL (mol %)
Cin-CL (wt %)
time (min)
conv (%)
sol
0.021
0.021
0.021
0.021
0.153
0.153
0.153
7.38
7.20
6.84
6.50
15
15
30
24
98
90
60
60
S
S
2, 2% Cin-PA6
3, 6% Cin-PA6
4, 10% Cin-PA6
1.03
3.19
5.45
2
6
NS
NS
0.153
10
a
b
c
d
ε-Caprolactam. α-Cinnamoylamido-ε-caprolactam. Determined gravimetrically; overall conversion. Solubility in HFIP (hexafluoroisopropanol);
S = soluble, NS = not soluble.
concentration in the reaction medium, and leads to a new
amidate able to react with lactam chain ends. The new chain
end is then a nonreactive one and stops the propagation.
Cross-linking of 2% Cin-PA6 did not occur during the AROP
as the final polymer was soluble in HFIP. It is mostly probable
that a low amount of the cinnamated monomer (2 wt %) leads
to a high distance between the reactive double bonds as well as
faster AROP than the dimerization reaction. On the other hand,
6% Cin-PA6 synthesis and cross-linking, owing to higher
amount of cinnamated pendant units, were completed in a
short polymerization time (15 min.). Only temperature/time
diagrams of 2% and 6% of cinnamoyl pendant PA6 (runs 2 and
3, Table 1) were able to be observed and compared with PA6
(Figure 2). 10% Cin-PA6 (run 4, Table 1) has a flat
temperature/time plot with a quite longer polymerization
time (24 h) to reach 60% yield. After the copolymerization
reaction, 6% Cin-PA6 and 10% Cin-PA6 were not soluble (gel
formed) in HFIP due to the cross-linking occurrence. The
decelerating effect of the comonomer on the copolymerization
process also reflects a lower monomer conversion compared to
homopolymerization of CL (Table 1). The exact amount of
comonomer incorporated in the copolymer could not be
determined by molecular characterizations due to its non-
solubility in any organic compounds. The soluble 2% sample
was not significant enough.
Scheme 2. Anionic Ring-Opening Copolymerization and
Thermal Dimerization of ε-Caprolactam and α-
Cinnamoylamido-ε-caprolactam
Solid-state 13C MAS NMR was helpful to confirm the
structure of PA6 bearing cinnamoyl groups (Figure 3). The
dimerization−cross-linking (for more than 2% of α-cinnamoy-
lamido-ε-caprolactam, Table 1) which occurs during AROP of
CL and Cin-CL was attributed to the cyclobutane ring
formation. To further explore and compare possible influences
of dimerized cinnamoyl units on polyamides, α-truxillic acid
was prepared from cinnamic acid dimerization by UV
irradiation.41 As the dimerization proceeds during the
copolymerization, the new carbon signals attributed to the
aromatic carbons indicate the successful copolymerization. As
expected, the new chemical shift of aromatic carbon peaks
appeared between 115 and 140 ppm, as shown elsewhere.41
The solid-state chemical shifts of PA6 and PA6 with pendant
cinnamoyl groups were similar for aliphatic carbon and
carbonyl groups. One signal in the carbonyl region around
170 ppm and four different signals of aliphatic carbons of PA6
appeared and overlap the cyclobutane ring (around 40 ppm)
and its carbonyl functions. In addition to PA6 aliphatic carbon
peaks, there is one more signal which may belong to
overlapping cyclobutane ring carbons.
Thermal and thermomechanical characterizations of PA6
bearing cinnamoyl units and neat PA6 were investigated by
differential scanning calorimetry (DSC), thermogravimetry
(TGA), and dynamic mechanical analysis (DMA). DSC
measurements of PA6 copolymers having 2, 6, and 10 wt %
of cinnamoyl units were performed in nitrogen (Table 2). The
Figure 2. Temperature−time diagrams of PA6 (solid), 2% Cin-PA6
(dashed), 6% Cin-PA6 (dotted), and 10% Cin-PA6 (dashed-dotted).
D
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Scheme 3. Competitive Proton Abstractions from Exo- and Endocyclic Amide Protons of α-Cinnamoylamido-ε-caprolactam
Figure 3. 13C MAS NMR spectra of 10% Cin-PA6 (a), PA6 (b), truxillic acid (c), and cinnamic acid (d).
copolymers of PA6 bearing bulky cinnamoyl pendant groups,
either cross-linked PA6 (10% Cin-PA6 and 6% Cin-PA6) or
non-cross-linked (2% Cin-PA6), demonstrated significant
increase in Tg up to 62 °C as compared to neat PA6 with a
Tg of 53 °C. It is due to a decrease of chain mobility in the
amorphous phase and related to the pendant cinnamoyl
moieties. This trend was also confirmed by the measurement
of the maximum of tan δ by DMA with an increase from 72 to
84 °C. Melting temperatures (Tm) and melting enthalpies
(ΔHm) of the copolymers measured during the second analysis
E
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Table 2. Thermal and Thermomechanical Properties PA6 and PA6S Containing Pendant Cinnamoyl Moieties
DSC 2nd run
DMA
TGA
a
b
c
sample
PA6
Tg (°C)
Tm (°C)
ΔHm (J/g)
tan δ (°C)
E′ (Pa)
T5 (°C)
T10 (°C)
Td (°C)
53
59
62
61
216
214
204
201
70
60
47
44
72
74
78
84
2.1 × 109
7.4 × 108
7.8 × 108
6.2 × 108
290
307
308
300
321
336
390
308
330
346
407
2% Cin-PA6
6% Cin-PA6
10% Cin-PA6
361
a
b
c
Decomposition temperature at a 5% weight loss. Decomposition temperature at a 10% weight loss. Maximum of the peak decomposition
temperature.
by DSC were shown to decrease with increasing cinnamoyl
content, as compared to neat PA6, due to chain reorganizations.
It is worth noting that those values were not changed during
the first run, i.e., right after the polymerization. The single
melting endotherms tend to conclude on a statistical
copolymerization. The elastic modulus (E′) measured below
Tg (at 25 °C) by DMA exhibited a decrease down to 6 × 108 or
7 × 108 Pa, instead of 2 × 109 Pa for neat PA6, in agreement
with a reduced crystallinity.
Thermal stability of the polyamides was investigated by
thermogravimetric analysis under a nitrogen atmosphere. The
TGA thermograms of PA6 and PA6 bearing cinnamoyl units
are shown in Figure 4, and the results are summarized in Table
were monitored at different wavelengths. After exposing the
10% Cin-PA6 gel formed in HFIP to 220−280 nm light
irradiation for de-cross-linking (decomposition of the dimer),
the gel became a solution as represented in Scheme 4.
Polymers having cinnamoyl groups are known to be
insolubilized16 after photoirradiation between 325 and 380
nm or thermal treatment due to the [2 + 2] cycloaddition to
form cross-linkages. As depicted in Figure 6, the cinnamate
residues, after cleavage, reached to their highest absorption
state in 6 h irradiation at 295 nm. The double signal at t = 0 is
characteristic of the truxillic acid moiety but shifted to slightly
higher wavelength probably due to its polyamide environment.
Possible degradation of polyamide may occur upon irradiation,
but it was not observed by TGA, DMA, or size exclusion
chromatography in HFIP in the conditions used.
After de-cross-linking, spectral changes of the spin-coated
thin films upon UV irradiation at 325−380 nm or following
heating at 140 °C for re-cross-linking are shown in Figure 7.
Absorption spectra of spin-coated polymer film decreased
gradually upon further irradiation with time. A more efficient
decrease was also observed by heating at 140 °C for 1 h after
following the irradiation at 25 °C. Re-cross-linking was also
confirmed from the insolubility of the thin film after irradiation
and heating at 140 °C. A difference in terms of efficiencies of
re-cross-linking by UV irradiation and heating was clearly
observed. The reason for this difference in efficiency relies in
the much higher mobility degree43 of chains bearing pendant
cinnamoyl units at high temperature during the thermal
treatment compared to the lower temperature used during
irradiation. It was already reported that, whenever photo-
sensible groups are in close proximity in the condensed state
compared to solution,24 heating increases the mobility of chains
and thus favors the diffusion of cinnamoyl units to each one
another. Restricted solubility of cinnamoyl pendant PA6 allows
only spin-coated film irradiation which is also required due to
polar solvent effect of HFIP on dimerization. It is also noted for
another system24 that the UV spectrum of the re-cross-linked
content on the glass plate is broader than de-cross-linked
samples in HFIP solutions. The percentages of reacted
chromophore groups are related to the accessibility. Once the
network starts to form, the encounter between two
chromophores on different molecules is increasingly hin-
dered.44
Figure 4. TGA thermograms of PA6 (dashed-dotted), 2% Cin-PA6
(dotted), 6% Cin-PA6 (dashed), and 10% Cin-PA6 (solid).
2. Thermal stabilities of the cross-linked (10% Cin-PA6 and 6%
Cin-PA6) and non-cross-linked (2% Cin-PA6) polyamides are
higher than neat PA6. For PA6, the initial decomposition
temperatures of 5 and 10% weight losses (T5 and T10) are
measured at 290 and 300 °C, respectively. For 10% Cin-PA6,
T5 and T10 are increased until 361 and 390 °C. The
improvement of the thermal stability of modified PA6s could
be explained by minimizing the depolymerization process,
which is known to occur by different scission or elimination
reactions.42 During the process, the electronic effect of the
cinnamoyl group attached to CL chain end may be different
than the one in CL chain end. The bond scissions and
formation of cyclic compounds are certainly disfavored at 300−
350 °C.
Cross-linking rate of 10% Cin-PA6 was measured by
following UV absorbance intensity of polymer film after UV
irradiation and 1 h heating at 140 °C, according to following
expression: rate of cross-linking (%) = [(A0 − AT)/A0] × 100,
where A0 and AT are absorption intensities of the CC
chromophore after irradiation or heating at times t = 0 and t =
T, respectively. The rate of cross-linking of cinnamated PA6 is
33% after 6 h UV irradiation and 42% after 1 h heating. After
UV irradiation and heating of 10% Cin-PA6, the polymer
Photosensitivity of PA6 bearing cinnamoyl groups were
studied by means of UV absorbance spectroscopy. Both
photocleavage reactions and photo- or thermodimerization
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Macromolecules
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Scheme 4. From Gel to Solution Form of 1.23 × 10−4 mol L−1 of PA6 Bearing Pendant Cinnamoyl Groups in HFIP Solution
after Irradiation at 254 nm for 6 h
Figure 6. UV absorption spectra of 1.23 × 10−4 mol L−1 of 10% Cin-
PA6 bearing pendant cinnamoyl group in HFIP solution at irradiation
Figure 7. UV absorption of 1% w/v spin-coated PA6 bearing pendant
times from 1 to 6 h between 200 and 280 nm.
cinnamoyl groups (10% Cin-PA6) film before UV irradiation (solid),
after UV irradiation 6 h (dashed) between 325 and 380 nm, and after
heating at 140 °C for 60 min (dotted).
became insoluble in HFIP due to the efficient but not complete
cross-linking.
observed. Crystallinity was shown to decrease after heat
CONCLUSION
treatment. The cross-linking reaction was shown to be
reversible, and the functionalized aliphatic polyamides can be
cross-linked thermally (at 140 °C) or photochemically (at 364
nm) and de-cross-linked photochemically (at 254 nm). This
work highlight also the potential of α-amino-ε-caprolactam as
an interesting precursor of comonomers in the anionic ring-
opening polymerization of ε-caprolactam.
■
In this work, a reversible thermal and photochemical cross-
linked polyamide was synthesized. The strategy was based on
the anionic copolymerization of ε-caprolactam and α-
cinnamoylamido-ε-caprolactam, a monomer prepared from
biosourced L-lysine. During the anionic ring-opening polymer-
ization at 140 °C in bulk, the thermal dimerization of
cinnamoyl pendant groups took place to end-up with cross-
linked polyamides, whenever the amount of functionalized
comonomer is 6 or 10 wt %. The thermal and thermomechan-
ical properties of these cross-linked polyamides were
investigated. An increase of both Tg and thermal stability was
AUTHOR INFORMATION
Corresponding Authors
*E-mail carlotti@enscbp.fr, tel +33540002734, fax
+33540008487 (S.C.).
■
G
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Article
Reference; Matyjaszewski, K., Moller, M., Eds.; Elsevier: Amsterdam,
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*E-mail philippe.lecomte@ulg.ac.be, tel +3243663466, fax
2012; pp 331−396.
+3243663497 (P.L.).
(27) Roda, J. Polyamides. In Handbook of Ring-Opening Polymer-
ization; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2009; pp
165−195.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
The authors thank the Erasmus Mundus−IDSFunMat program
for the financial support of this work. CREMAN Laboratory
(Liege/Belgium) and BASF (Germany) are also acknowledged
for conducting the solid state NMR measurements and
supplying the materials, respectively.
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