Synthesis, thermal, and photocrosslinking studies of thermotropic liquid crystalline poly(benzylidene-ether)esters containing α,β-unsaturated ketone moiety in the main chain

Article abstract of DOI:10.1002/pola.26560

A series of poly(benzylidene-ether)esters containing a photoreactive benzylidene chromophore in the main chain were synthesized from 2,6-bis(4-hydroxy-3-methoxybenzylidene)cyclohexanone (BHMBCH) with various aliphatic and aromatic diacid chlorides by an interfacial polycondensation technique. The intrinsic viscosity of the synthesized homo and copolymers determined by Ubbelohde viscometer was found to be 0.12 to 0.17 dL/g. The molecular structure of the monomer and polymers was confirmed by FT-IR, ¹H NMR, and ¹³C NMR spectral analyses. These polymers were studied for their thermal stability and photochemical properties. Thermal properties were evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). It was found that the polymers were stable up to 280 °C and start degrading thereafter. Increase in acid methylene spacer length decreased the thermal stability. The self-extinguishing property of the synthesized polymers was studied by calculating the limiting oxygen index (LOI) value using a Van Krevelen's equation. The influence of the length of methylene spacer on phase transition was investigated using DSC and odd-even effect has been observed. Hot-stage optical polarizing microscopic (HOPM) study showed that most of the polymers exhibited birefringence and opalescence properties. The photolysis of liquid crystalline poly(benzylidene-ether)esters revealed that α,β-unsaturated ketone moiety in the main chain dimerises through 2π + 2π cycloaddition reaction to form a cyclobutane derivative and leads to crosslinking.

Full text of DOI:10.1002/pola.26560

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Synthesis, Thermal, and Photocrosslinking Studies of Thermotropic
Liquid Crystalline Poly(benzylidene-ether)esters Containing
a,b-Unsaturated Ketone Moiety in the Main Chain
Athianna Muthusamy,1 Krishnasamy Balaji,2 Salem Chandrasekaran Murugavel2
1Department of Chemistry, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641020, India
2Polymer Research Laboratory, Department of Basic Sciences—Chemistry, PSG College of Technology, Coimbatore 641004, India
Correspondence to: S. C. Murugavel (E-mail: psgmvel@yahoo.co.in)
Received 29 November 2012; accepted 8 January 2013; published online 6 February 2013
DOI: 10.1002/pola.26560
ABSTRACT: A series of poly(benzylidene-ether)esters containing
a photoreactive benzylidene chromophore in the main chain
were synthesized from 2,6-bis(4-hydroxy-3-methoxybenzylide-
ne)cyclohexanone (BHMBCH) with various aliphatic and aro-
matic diacid chlorides by an interfacial polycondensation
technique. The intrinsic viscosity of the synthesized homo and
copolymers determined by Ubbelohde viscometer was found
to be 0.12 to 0.17 dL/g. The molecular structure of the mono-
mer and polymers was confirmed by FT-IR, 1H NMR, and 13C
NMR spectral analyses. These polymers were studied for their
thermal stability and photochemical properties. Thermal prop-
erties were evaluated by thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC). It was found that the
polymers were stable up to 280 ꢀC and start degrading there-
after. Increase in acid methylene spacer length decreased the
thermal stability. The self-extinguishing property of the synthe-
sized polymers was studied by calculating the limiting oxygen
index (LOI) value using a Van Krevelen’s equation. The influ-
ence of the length of methylene spacer on phase transition
was investigated using DSC and odd-even effect has been
observed. Hot-stage optical polarizing microscopic (HOPM)
study showed that most of the polymers exhibited birefrin-
gence and opalescence properties. The photolysis of liquid
crystalline poly(benzylidene-ether)esters revealed that a,b-un-
saturated ketone moiety in the main chain dimerises through
2p þ 2p cycloaddition reaction to form a cyclobutane derivative
C
V
and leads to crosslinking.
2013 Wiley Periodicals, Inc. J.
Polym. Sci., Part A: Polym. Chem. 2013, 51, 1707–1715
KEYWORDS: photocrosslinking;
poly(benzylidene-ether)esters;
spectral studies; synthesis; thermal properties; thermogravi-
metric analysis
INTRODUCTION Photocrosslinkable polymers are currently
attracting significant attention in a wide area of applications
such as photoresists to make integrated circuits, printing
plates, printing inks, photocurable coatings and energy
exchange systems, because of their excellent curing charac-
teristics, thermal stability and chemical resistance.1–3 Photo-
crosslinkable liquid crystalline polymers find applications in
anisotropic network systems like LC elastomers and LC ther-
mosets, information storage devices, and thin films with a
controlled orientation of functional groups, which are of in-
terest in microelectronics and optoelectronics.4 Photosensi-
tivity, solubility, and thermal stability are the prime require-
ments for the use of these polymers in this field.
sociation of the polymer chains under the influence of UV
irradiation.6 Polymers containing a,b-unsaturated carbonyl
groups such as cinnomoyl,7 benzylidene groups8 are well
studied for their phototransformation phenomena, occurring
upon irradiation under UV light and named as negative type
photoresists.9 The photosensitivity of these materials is based
on the p-electron density of the chromophores present in the
polymer backbone.10 The literature reveals that the chalcone
groups possess higher sensitivity to UV irradiation than that
of the cinnamate groups.11 The molecularly designed poly-
mers12 with narrow polydispersities can be used for further
functionalization or cross-linking reactions are required for
applications in areas as diverse as optical data storage, non-
linear optical systems and electro-optic displays.13 The poly-
mers possessing these properties were prepared from block
copolymerization of alkyl vinyl polyethers,14 macrocylic LC
polyethers,15 crown ethers,16 and so on.
Main chain liquid crystalline polymers with photosensitive
groups have attained special attention owing to the presence
of dual functionalities. The mesogenic groups contribute to
the LC properties and the photoactive groups in the polymer
backbone,5 may facilitates photoisomerization, photocros-
slinking, photodimerization, photodissociation, and photoas-
Several researchers have been actively engaged in the study
of main chain thermotropic poly(ether-ester)s containing
C
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rod-like mesogens.17 However, LC poly(ether-ester)s have
not been receiving the deserved focus and attention. After
performing extensive work on these polymers, Shaffer and
Percec18 observed that the main advantages of LC polyethers
over LC polyesters are lower melting temperatures, broader
mesophase stability range, and higher solubility. A pioneer-
ing work by Rosen et al.19 in the field of liquid crystalline
polyethers, polyesters, and dendrons covers all aspects
related in this field. The rigid polyether unit does not have
to exhibit liquid crystallinity itself in order to be used as a
mesogenic unit in LC polymers.20 It must, however, possess
an appreciable amount of stiffness, non-coplanar structure
units,21 asymmetrical shape,22 and complex architectures.23
Such a novel class of macromolecules with conformational
isomerization was provided by dendrimers or hyperbranched
polymers with tree like architecture.24,25 This concept of LC
based on conformational isomerization26,27 in polyethers
based on rigid and flexible mesogenic groups28 and in hyper-
branched and dendritic29 polyethers were thoroughly
reviewed by Percec et al.12–18,23–29
have been reported to possess interesting semiconducting
and mechanical properties, attractive morphology, and liquid
crystalline behavior.
In this study, we dealt with the synthesis of photoreactive
bisbenzylidene diol with a,b-unsaturated ketone moiety in
the main chain and polymerized with various aliphatic and
aromatic diacid chlorides by interfacial polycondensation
technique. Two copolymers containing bisphenol-A were also
synthesized for comparing the thermal properties. Spectral,
thermal, liquid crystalline, and photocrosslinking studies
were also carried out in this investigation.
EXPERIMENTAL
Materials
Vanillin (Sd fine chemicals, India), chloroform (Sd fine chemi-
cals, India), adipic acid (Sd fine chemicals, India), sebacic
acid (Sd fine chemicals, India), terephthalic acid (Sd fine
chemicals, India), isophthalic acid (Sd fine chemicals, India),
cyclohexanone (Merck, India), n-hexane (Merck, India), thi-
onyl chloride (Ranbaxy, India), methanol (Loba Chemie,
India), and other solvents were purified by the procedure
reported in the literature.37 Boron trifluoride diethyletherate
(Fluka), hexadecyltrimethylammonium bromide (HDTMAB)
(Fluka), bisphenol-A (Fluka), subaric acid (Fluka), and azelaic
acid (Aldrich, Germany) were used as received.
Among them, some aliphatic and wholly aromatic thermo-
tropic polyesters generally have high crystal-to-isotropic
transitions Tm, and are insoluble in most of the organic sol-
vents. To obtain polymers with considerably low Tms, and
increased solubility several chemical modifications can be
made: (i) the introduction of bulky, or non-symmetrical sub-
stituents; (ii) introducing long flexible spacer into the meso-
gen backbones; (iii) the use of non-linear or bent monomers;
and (iv) copolymerization of different monomers.30 Most of
the researchers paid attention to semiflexible polymers, in
which the hard mesogenic groups are connected by flexible
spacer groups. Until now, enormous semiflexible type ther-
motropic polymers have been reported. The typical examples
are polyesters, poly(ether-ester)s, poly(ester-amide)s, poly-
ethers,31 etc. Recently, various thermotropic poly(ether-
ester)s with flexible spacers have been synthesized.32 The
poly(ether-ester)s having the regular sequence of flexible
spacer and aromatic group showed the great difference in
polarity between the aliphatic spacer and ester linkage to
increase the solubility and reduce the transition temperature.
Liquid crystalline poly(ether-ester)s has captured the excite-
ment and imagination of contemporary polymer scientists
and engineers. These materials exhibit many unique proper-
ties that present not only challenges for basic research, but
also numerous technological opportunities. Liquid crystalline
poly(ether-ester)s have provided a number of potential
applications in photonics and optoelectronics based technol-
ogies such as speed data storage devices and optical fiber
communications.33 The general drawback of this kind of pol-
yesters are poor thermal resistance and fire behavior.34 This
can be minimized by the addition of flame retardants.35
Measurements
The solubility of the polymers was examined using 0.3 to
0.5 mg of polymer in 5 mL of solvent at room temperature.
The intrinsic viscosity of the synthesized polymers was
measured in DMSO at 30 ꢀC using Ubbelohde viscometer.
The number average, weight average molecular weights, and
polydispersity indices (PDI) of the polymers were estimated
using gel permeation chromatography (GPC) in THF with a
Shimadzu LC-20AD GPC. Polystyrene standards of known
molecular weight were used for calibration. The infrared
spectra were recorded on Shimadzu Fourier transform infra
red spectrophotometer using KBr pellet. 1H and 13C NMR
spectra were recorded on 400 MHz Brucker AV-III 400 NMR
spectrometer in CDCl3 using TMS as an internal standard.
The DSC analysis was carried out on a Perkin Elmer Pyris 6
DSC for all polymers using empty aluminium pan as refer-
ꢀ
ence with heating rate of 10 C/min in nitrogen atmosphere.
Temperature and heat flow scale of the instrument was cali-
brated using precrimped In and Zn as standard references.
Thermogravimetric analysis was performed on Perkin Elmer,
Diamond TG/DTA in nitrogen atmosphere with heating rate
of 10 ꢀC/min. The photocrosslinking studies of the synthe-
sized polymers were performed in the solution and film
state using UV spectrophotometer. The polymer was dis-
solved in DMSO in a quartz cuvette and irradiated in UV cur-
ing reactor with a medium pressure Hg lamp (Heber Scien-
tific Photo reactor, 300–420 nm) exposed at a distance of
10 cm from the sample. Subsequently the irradiated solution
was subjected for UV spectral analysis on a Systronics 119
UV spectrophotometer.
A series of articles have been published on the synthesis and
characterization of arylidene polyethers and polyesters, con-
taining rigid cycloalkanone moieties. Incorporation of meth-
ylene spacer plays a significant role in determining the rela-
tionship between thermomechanical history, structural, and
morphological organization of polymers.36 Such polymers
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Synthesis of Monomer
Synthesis of Polymers
2,6-Bis(4-hydroxy-3-methoxybenzylidene)cyclohexanone
(BHMBCH)
All the polymers were prepared by the interfacial polycon-
densation method using HDTMAB as a phase transfer cata-
lyst. A typical procedure for the synthesis of polymer P1 is
as follows.
A mixture of cyclohexanone (25 mmol) and 4-hydroxy-3-
methoxybenzaldehyde (50 mmol) was dissolved in 25 mL of
absolute ethanol with three drops of boron trifluoride diethyl
etherate. The solution was stirred for 4 h at 80 ꢀC and
cooled to room temperature. The solid product formed was
filtered off, washed with cold ethanol, and dried in vacuum
oven. Recrystallization from hot ethanol gave fine yellow
Poly[2,6-bis(4-hydroxy-3-methoxybenzylidene)cyclohexano-
ne]adipate (P1)
One millimole of BHMBCH was dissolved in aqueous sodium
hydroxide (1N) solution containing HDTMAB (2 wt % of the
diol). The solution turned wine red in color. To this solution
1 mmol of adipoyl chloride in CHCl3 was added with vigor-
ous stirring. After 30 min of stirring the solid polymer was
formed in between organic and aqueous layer. It was filtered,
washed with ethanol, and dried to constant weight in
vacuum.
color product with more than 75% yield of the title com-
38
ꢀ
ꢀ
pound. Melting point 181 C. (lit.179 C).
FT-IR (KBr): 1637 cmꢁ1 (cC¼O), 1579 cmꢁ1(cC¼C exocyclic),
1513 cmꢁ1 (cC¼C aromatic), 1254 cmꢁ1 (-OCH3), 3367 cmꢁ1
(cOH).
1H NMR (CDCl3, TMS) ppm: 9.50 (s, 2H, AOH), 6.80-7.20 (m,
6H, aromatic protons), 7.67 (s, 2H, ACH¼), 3.82 (s, 6H,
AOCH3), 2.92 (s, 4H, bCH2 of cyclohexanone), 1.75 (s, 2H,
cCH2 of cyclohexanone). 13C NMR (CDCl3) ppm: 188.55
(C¼O, ketonic), 133.6 (¼CH-), 130.0 (a carbon of cyclo-
hexanone), 115–125 (aromatic carbons), 55.77 (—OCH3),
22.59 (b carbon of cyclohexanone), 27.94 (c carbon of
cyclohexanone).
All the other polymers namely, poly[2,6-bis(4-hydroxy-3-
methoxybenzylidene)cyclohexanone]subarate (P2), poly[2,6-
bis(4-hydroxy-3-methoxybenzylidene)cyclohexanone]azelate
(P3), poly[2,6-bis(4-hydroxy-3-methoxybenzylidene)cyclohex-
anone]sebacate (P4), poly[2,6-bis(4-hydroxy-3-methoxyben-
zylidene)cyclohexanone]terephthalate (P5), poly[2,6-bis(4-
hydroxy-3-methoxybenzylidene)cyclohexanone]isophthalate
(P6), were also prepared in a similar manner.
Two co-polymers (COP1 and COP2) were synthesized from
equimolar quantities of photosensitive diol (BHMBCH), acid
chlorides (subaryl/sebacyl), and non-photosensitive diol
(bisphenol-A) by adopting the procedure used to synthesize
the homo polymers.
Preparation of Acid Chlorides
Recrystallized adipic acid (100 mmol) and distilled thionyl
chloride (240 mmol) were taken in a 250-mL round bottom
flask and refluxed in an oil bath for 4 h. After refluxing,
excess thionyl chloride was removed under reduced pres-
sure. Sebacyl chloride, subaryl chloride, and azeloyl chloride
were also prepared by adopting similar procedure used in
the preparation of adipoyl chloride.38 Terephthaloyl chloride
and isophthaloyl chloride were prepared by refluxing the re-
spective diacids with thionyl chloride with the addition of
two drops of pyridine as a catalyst for 12 h. The solid prod-
uct obtained was recrystallized from n-hexane.38
RESULTS AND DISCUSSION
Synthesis
The monomer, BHMBCH, was prepared by Claisen-Schmidt
condensation of 4-hydroxy-3-methoxybenzaldehyde with
cyclohexanone using BF3 diethyletherate as a catalyst in
ethanol medium (Scheme 1). It was then polymerized using
various aliphatic and aromatic acid chlorides. All the homo
and copolymers were prepared by interfacial polycondensa-
tion technique and the synthetic route for the preparation is
depicted in Scheme 2.
Polymerization
Polycondensation is usually carried out by melt, solution or
interfacial methods. Since the synthesized monomers and the
polymers are sensitive to heat, the room temperature interfa-
cial polycondensation route was selected for the synthesis of
polymers in this study. This technique possesses several
advantages over solution polycondensation like using of inex-
pensive solvents, rapid formation of polymers, and good
yield. This system by nature is non-equilibrium with reac-
tants and products distributed among various phases. All the
polyesters have been prepared by interfacial polycondensa-
tion technique using a phase transfer catalyst (PTC).
All the polymers were obtained fairly in good yield and yel-
low in color. All the aliphatic polyesters were soluble in po-
lar solvents like DMSO, DMF, CHCl3 in room temperature and
insoluble in ethanol, methanol, toluene, and n-hexane. The
solubility of the polymer increases in polar aprotic solvents
with the increase of polarity index. This may be due to the
intermolecular interactions of polar solvents with ester link-
age of the polymer molecules.39 The aromatic polyesters
were insoluble in most of the organic solvents and soluble in
H2SO4. Molecular weights of the resulting polymers were
SCHEME 1 Synthesis of monomer, BHMBCH.
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SCHEME 2 Synthesis of polymers.
determined by GPC. Number and weight average molecular
weights of polymers were in the range of 9667 to 9964 g/
mol and 10,538 to 11,742 g/mol, respectively with polydis-
persity of 1.08 to 1.20. The intrinsic viscosities of the homo
and copolymers were found to be in the range of 0.12 to
0.17 dL/g.
methoxy protons attached to the phenyl ring appeared as a
singlet at 3.85 ppm. Whereas, the strongly shielded aliphatic
acid spacer protons appeared as multiplet at around 0.80 to
2.90 ppm and confirmed the presence of the acid spacer pro-
tons in polymers P1 to P4. These data strongly confirmed
the formation of the polyesters.
The proton decoupled 13C NMR spectra of polyester P4 is
shown in Figure 2. The 13C NMR spectrum exhibited the
expected peaks for every carbon atoms of the polymers. The
chemical shift assignments were carried out using normal
additive parameter of the substituent in the benzene ring
and the considerations of the off resonance spectra in conju-
gation with the difference in the intensities among the qua-
ternary, methane, methylene, and methyl carbons.41 The a,b-
unsaturation in the carbonyl compound makes the carbonyl
carbon to resonate in the upfield. The ketonic carbonyl car-
bon of the polymer appeared around 191 ppm. The b and c
methylenic carbons of cyclohexanone resonate in the region
Spectral Studies
The structure of the polyesters was studied by FT-IR and
NMR spectroscopic techniques. All the polyesters exhibited
characteristic medium absorption bands in the region of
1650 to 1689 cmꢁ1 corresponding to carbonyl (C¼O)
stretching of the cyclohexanone group. The lowering of car-
bonyl stretching vibration can be attributed to the meso-
meric effect and the p-orbital conjugation induced by the
unsaturation in the a-carbon atoms and co-planarity of the
(–CH¼C–(C¼O)–C¼CH–) group.40
A stretching peak at
around 1595 cmꢁ1 is attributed to the olefinic double bonds
of benzylidene unit.41 All the polymers showed a strong
absorption at around 1716 to 1751 cmꢁ1 characteristic of
>C¼O stretching vibrations of ester carbonyl group which is
absent in FT-IR spectrum of monomer, BHMBCH. This con-
firmed the formation of the polyester. Asymmetric aryl alkyl
ether stretchings of methoxy group appeared at 1249 to
1259 cmꢁ1. The C–O stretching due to the ester groups
appeared at around 1240 cmꢁ1. The prominent bands
occurred in the low frequency region of 900 to 650 cmꢁ1
are due to the C–H out of plane bending of the benzene ring.
1
The representative H NMR spectrum of the polymer (P3) is
shown in Figure 1. The olefinic protons appeared as a singlet
around 7.7 ppm in all the polymers as in the monomer. This
indicated the existence of E configuration38,42,43 in the back-
bone of the polymers. The singlet peaks appearing at 2.85 to
2.95 ppm and 1.72 to 1.90 ppm correspond to b and c meth-
ylene protons of cyclohexanone respectively.41 The multiplet
at 6.89 to 7.14 ppm was attributed to aromatic protons. The
FIGURE 1 1H NMR spectrum of polyester (P3).
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uncured copolymer. This may be due to the increase in rigid-
ity imparted by the photocrosslinking of the polymer chain.
Among the aromatic polyesters, the polymer P6 containing
isophthaloyl units showed a higher stability and char yield
compared to the polyester containing terephthaloyl units.
This may be ascribed to the higher crystallinity inside the
hard domain of the polyester derived from terephthalic acid
compared with that of the polyester derived from isophthalic
acid which is reflected in the DSC results also.
Limiting oxygen index for all the polymers was determined
using Van Krevelen equation47 and are in the range of 16 to
45. According to Van Krevelen, polymers having LOI above
threshold value of 26 possess self extinguishing property.48
LOI data indicates that all the polyesters except P4 have self
extinguishing property. The homo polymer, P4 derived from
sebacic acid and copolymer, COP1 derived from suberyl chlo-
ride exhibited LOI below the threshold value. This may be
due to the more number of methylene units in the homo
polymer P4 and the presence of flexible bisphenol-A moiety
in the main chain of copolymer COP1.
FIGURE 2 13C NMR spectrum of polymer (P4).
of 24.14 to 28.43 ppm and 21.84 to 24.90 ppm, respectively.
The exocyclic olefinic a and b carbons are appearing in the
range of 127.41 to 134.81 ppm and 135.09 to 139.58 ppm,
respectively. The resonance peaks appearing at around 112
to 124 ppm were attributed to the aromatic carbons present
in the polymer. A peak appeared at around 173.5 ppm in all
the polymers signified the presence of ester carbon. The
methoxy carbon attached to the phenyl ring resonated at
around 55 to 57 ppm in all the polymers. The aliphatic poly-
esters showed a resonance signal at around 22.2 to 34.0
ppm corresponding to methylenic acid spacer.
The DSC data of the synthesized polymers are given in
Table 1. Most of the synthesized polymers showed double
endotherms (except P2 and P3) and the phase transitions
are irreversible during cooling cycle; they are monotropic in
nature. Double melting endotherms, however, are not a suffi-
cient criterion for the detection of LC phases in polymers.
Such behavior can also be observed with non-liquid crystal-
line polymers.49 There are examples in literature citing the
observation of more than one melting peaks due to the phe-
nomenon of annealing (crystal thickening) during the heating
scan.50 The annealing effect is heating rate dependent,
whereas, single crystal melting peaks are observed at very
low and very high heating rates. Thus, multiple peaks are
found at intermediate heating rates.51 In the present case,
crystal thickening is unlikely to occur in polymers with rigid
chains, and hence the two principal endotherms in DSC
traces are likely to be due to two separate phase transitions.
Thermogravimetric Analysis
The TGA data revealed that all the homo polymers are stable
up to 300 ꢀC and that 10% weight loss occurred around
312 ꢀC. The 50% weight loss was observed around 415 to
600 ꢀC for all the polymers which may be attributed to the
decomposition of rigid cyclohexanone segment.44 Most of the
synthesꢀized polyesters except P4 showed a higher char yield
at 650 C. The char yield of the polymers was around 21 to
45%. The data revealed that the char yields were quite high
for these polymers. A polymer providing high char yield in
thermogravimetric analysis under nitrogen atmosphere indi-
cates that the polymer can be a fire retardant. Thick char
becomes a better thermal insulating layer and undergoes
slow oxidative degradation, protecting the remaining poly-
mer from heat radiation.4
The melting transition temperature (Tm) of the polymers
ꢀ
was obtained in the range of 115 to 241 C and the isotropic
transition temperature (Ti) was observed in the region of
189 to 405 ꢀC. Except P2 and P3 all the polymers showed
well-defined mesophase separation. The polymers (P1 and
P2) which contain lesser number of methylene spacer units
showed a glass transition temperature (Tg) of 112 ꢀC and
102 ꢀC, respectively. It was observed that aromatic polyest-
ers containing terephthaloyl and isophthaloyl units showed
higher isotropization temperature than aliphatic polyesters.
This may be due to increased rigidity inside the hard
domain.38
The thermal stability of the polymers decreases as the acid
methylene spacer length increases.45 The presence of ether
linkage in the spacer units is expected to introduce flexibility
to the polymer chain and bring down thermal stability.46 The
stability among the aliphatic polyesters is given as P1 > P2
> P3 > P4. A copolymer, COP1 derived from suberyl chlo-
ride showed considerably less thermal stability than its
homo polymer, P2. This due to the flexibility of the non-pho-
tosensitive diol (Bisphenol-A) which decreased the thermal
stability. The 10%ꢀ weight loss occurred around 281 ꢀC and
char yield at 650 C was found to be 18.22%. The UV cured
copolymer COP1 exhibited higher thermal stability than the
The copolymer COP1 showed double melting at 119 ꢀC and
ꢀ
173 C and COP2 showed first melting at 90 ꢀC and on fur-
ther heatꢀing it exhibited multiple meltings in addition to that
Tg at 83 C. On comparing the copolymers with their respec-
tive homo polymers we observed that the presence of
bisphenol-A in the copolymers enhanced the crystallinity
which resulted double and multiple meltings. The UV
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TABLE 1 Transition Behavior of Polymers
DSC (ꢀC)a
Ti
292
HOPM (ꢀC)b
Polymer Code
Tg
Tm
DT
Tm
Ti
DT
Stirred Opalescence
P1
112
102
–
175
–
117
–
180
295
115
Strong
None
P2
189
–
c
191
c
–
c
P3
–
305
–
None
P4
–
115
137
241
116
119
90
158
43
258
164
226
54
–
120
140
245
120
120
92
162
c
142
c
Strong
Strong
Strong
Strong
None
P5
118
–
395
c
c
c
c
P6
405
342
COP1
89
–
COP1 UV cured
173
–
c
–
c
COP2
83
Multiple melting
Strong
a
Transition temperature identified using DSC (at heating rate of 10 ꢀC/min under N2 atmosphere).
Transition temperature identified with HOPM at a heating rate of 5 ꢀC/min.
Unable to determine since HOPM can be operatable only below 300 ꢀC.
b
c
unexposed polymer has shown Tg whereas no Tg was
revealed that most of these polymers (P1, P4, P5, P6, COP1,
and COP2) exhibited birefringence and stirred opalescence
propꢀerties. The polymer, P1 exhibited globular texture at
180 C (Fig. 4) and on further heating it will changed to iso-
tropic state. The polymer, P4 formed staggered aggregates
when it was cooled from the isotropic state to 120 ꢀC. For
the polymer P3, P5, and P6 we were unable to perform the
HOPM study beyond the 400 ꢀC due to the lack in cooling
provision. The opalescence property was well observed
within the Tm limit.
observed in UV exposed polymer. Further, Tm has increased
in UV exposed polymer. As a result of photocrosslinking the
crystallinity of the polymer get increased and exhibited dou-
ble melting. A change in sharpness of the endotherms was
noticed in UV treated sample when compared with that of
non-irradiated sample, which is slightly broad. This difference
in sharpness may be ascribed that during photoreaction poly-
meric molecules absorb energy and organized themselves in
a particular order to bring down charge transfer interaction
in benzylidene chromophore. This charge transfer interaction
reduces double bond character and increases single bond
order thereby increases rotational barrier. This charge trans-
fer interaction may be the reason for providing sharpness in
peak compared with that of non-irradiated sample.52
Photocrosslinking Studies
Polymers having olefinic groups are sensitive to ultraviolet
irradiation. The peculiar property of these types of polymers
is the tendency to proceed rapid photocrosslinking without
the addition of any photoinitiators.56 The scope of the pres-
ent investigation is to study the photoreactivity of the poly-
mers in solution and film states by UV spectrophotometry.
Figures 5 and 6 show the changes in the UV spectral pattern
Odd-Even Effect
Figure 3 shows the correlation between isotropic melting
temperature (Ti) of aliphatic polyesters and the number of
acid spacer carbon atoms. The effect of carbon chain length
on the isotropic temperature (Ti) of the synthesized polymer
was also studied by DSC. A plot was drawn between number
of carbon atoms (acid spacer) of aliphatic polyesters and
melting temperature. Figure 3 reveals that, the odd-even
effect has been well observed in the aliphatic polyesters.
This type of odd-even effect has been studied in detail by
Rovielo and Sirigu,53 Blumstein and Thomas,54 and Iimura
et al.55 The observation of the odd-even zig-zag variations in
Ti was common in all of these studies for polymers contain-
ing methylene spacers, but the cause of this effect is not well
understood and it needs further investigation.
Birefringence Texture
Hot-stage optical polarizing microscopic (HOPM) observa-
tions were performed to study the liquid crystal texture of
the polymers. Table 1 shows the transition temperatures and
their differences. The phase transition temperature deter-
mined by DSC are consistent with those observed by HOPM.
Observation of polyesters under a polarizing microscope
FIGURE 3 Correlation between isotropic temperature of ali-
phatic polyesters and number of acid spacer carbon atoms.
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FIGURE 4 HOPM photographs of polymers (i) P1 at 180 ꢀC, (ii) P4 at 120 ꢀC.
during the photolysis of polymer P1 in DMSO solution and
film states at various intervals of time. The absorption bands
observed at around 315 to 345 nm for all the polymers cor-
respond to p!p* transition of the olefinic double bond pres-
ent in the main chain of the polymer.38 During the successive
UV irradiation of polymer a decrease in intensity of the
absorption was noted. This may be attributed to the dimeri-
zation of the olefinic double bond of the polymer chain,
which involves the 2p þ 2p cycloaddition reaction of the
same leading to the formation of cyclobutane ring57 (Fig. 7).
The rate of photocrosslinking in film state is faster than the
solution state. This may be due to the restricted chain mobil-
ity of polymer in film state, it takes more time than the solu-
tion state for crosslinking.
The photocrosslinking ability of a polymer was also studied
by monitoring with FT-IR spectroscopy. There are significant
changes in the FTIR spectra of a,b-unsaturated carbonyl
group containing polymers occurred upon the UV exposure.
The >C¼C< stretching vibration at 1603 cmꢁ1 was observed
to decrease after UV exposure and a new absorption band
corresponding to a saturated carbonyl stretching vibration
was appeared at 1660 to 1665 cmꢁ1
.
In order to understand this type of polymers to be used as a
negative photoresist compounds, the polymer samples were
irradiated with UV light for 3 h and the crosslinked polymer
was obtained by evaporating the solvents.58 The residue
obtained was tested for solubility in various solvents. It was
found that the UV irradiated polymer becomes insoluble in
the solvents in which they were soluble before irradiation.
The irradiated samples were further subjected for HOPM
and DSC analysis and found that they lost their LC properties
with increase in transition temperatures. It was noticed that
for an irradiated polymer P1, the Tg was shifted to higher
temperature. In addition to that, Tm observed for the same
sample before irradiation was disappeared and Ti was
observed at a higher temperature with decomposition. This
study confirmed the formation of crosslinked network struc-
ture upon UV irradiation.
The relative reactivity rate [(A0 ꢁ At)/A0 where A0 is the ab-
sorbance before irradiation and At is the absorbance after irra-
diation time t] was plotted against the time of irradiation.
From Figure 8, it is observed that the rate of photocrosslinking
decreases significantly on increasing methylene carbon chain
length of acid spacer. In 30 min, 40% of crosslinking was
observed for the polymer P1 and 28%, 26%, and 24% of cross-
linking has been observed for P2, P3, and P4, respectively. It is
clear that the rate of photocrosslinking decreases significantly
on increasing methylene carbon chain length of acid spacer.
FIGURE 5 Photocrosslinking of polymer P1 in DMSO solution
FIGURE 6 Photocrosslinking of polymer P1 in film state (0, 5,
(0, 1, 2, 4, 10, 30, 45, 55 min).
10, 15, 20, 25, 30, 35, 40 min).
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ACKNOWLEDGMENTS
Authors gratefully acknowledge the Principal and Management,
PSG College of Technology, Coimbatore for providing the neces-
sary facilities. One of the authors K.B. sincerely acknowledges
the Council of Scientific and Industrial Research (CSIR), New
Delhi, India, for the award of Senior Research Fellowship. Sin-
cere thanks are also due to SAIF, Indian Institute of Science,
Bangalore, for NMR spectral studies and STIC - Cochin Univer-
sity for TGA analysis.
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FIGURE 7 Representation of the chemical structure of the poly-
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