Synthesis, characterization and photocrosslinking properties of polyacrylamides having bromo substituted pendant cinnamoyl moieties

EUROPEAN POLYMER JOURNAL

European Polymer Journal 41 (2005) 35–45

www.elsevier.com/locate/europolj

Synthesis, characterization and photocrosslinking properties of polyacrylamides having bromo substituted pendant cinnamoyl moieties P. Selvam a, K. Victor Babu b, S. Nanjundan a

a,*

Department of Chemistry, College of Engineering, Anna University, Sardar Patel Road, Chennai 600 025, India b Department of Chemical Physics, Central Leather Research Institute, Chennai 600 020, India Received 8 July 2004; accepted 30 August 2004 Available online 14 October 2004

Abstract New 3- and 4-bromocinnamoyl aniline were synthesized condensing 4-aminoacetophenone and the respective bromobenzaldehydes in the presence of sodium hydroxide. The monomers, 4-(3 0 -bromocinnamoyl) phenyl acrylamide (4,3 0 -BCPA) and 4-(4 0 -bromocinnamoyl) phenyl acrylamide (4,4 0 -BCPA) were prepared by reacting the respective chalcones and acryloyl chloride in the presence of triethylamine at 0–5
C. Homopolymers of 4,3 0 -BCPA and 4,4 0 -BCPA was carried out in methyl ethyl ketone using benzoyl peroxide (BPO) under nitrogen atmosphere at 70
C. The prepared polymers were characterized by UV, IR, 1H-NMR and 13C-NMR techniques. The molecular weights (Mw and Mn) of the polymers were determined by gel permeation chromatography. The thermogravimetric analysis (TGA) of the polymers in nitrogen atmosphere reveals that they possess very good thermal stability required of a negative type photoresist. The glass transition temperature of poly(4,3 0 -BCPA) and poly (4,4 0 -BCPA) were found to be 55 and 64
C respectively. The solubility of the polymers was tested in various polar and non-polar solvents. Photocrosslinking nature of the polymer samples was carried out in the presence and absence of various triplet photosensitizers in solution phase using chloroform solvent under medium frequency UV light. For using the polymers as negative photoresist materials the rate of photocrosslinking of the polymers was measured under the influence of different solvents, concentrations and position of the substituent.  2004 Elsevier Ltd. All rights reserved. Keywords: Poly[4-(3 0 -bromocinnamoyl)phenyl acrylamide]; Poly[4-(4 0 -bromo cinnamoyl)phenyl acrylamide]; Photocrosslinking; Cinnamoyl group; Negative photoresist

1. Introduction The synthesis of new polymeric materials and especially functionalized polymers are important in the * Corresponding author: Tel.: +91 44 22203155; fax: +91 44 22200660. E-mail address: [email protected] (S. Nanjundan).

development of new applications such as photoresist materials, smart polymers, molecular imprinting, smart gels and DNA microchips. Functional polymers in the crosslinked form are also useful in a number of applications such as column materials in chromatography, in combinational synthesis, and as simple chemical reagents [1]. Photosensitive polymers with photocrosslinkable groups have gained a considerable interest in resent

0014-3057/$ - see front matter  2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.08.015

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P. Selvam et al. / European Polymer Journal 41 (2005) 35–45

years owing to a wide variety of applications in the field of microlithography [2,3], printing materials [4], liquid crystalline display [5], and non-linear optical materials [6,7]. Polymers having a, b-unsaturated carbonyl groups either in the backbone or in pendant position undergo crosslinking upon UV irradiation and such polymers are regarded as negative-type photoresists. These polymers with the properties of high photosensitivity, filmforming ability, good thermal stability, good solubility before irradiation, resistance towards solvents, plasmas and etching agents after crosslinking are very important for commercial photoresist applications. UV radiation curing has been revealed as a powerful tool to crosslink rapidly heat sensitive polymers and to modify, selectively in the illuminated areas, their physico-chemical characteristics. Phototunable technology has found major openings in various industrial applications where its distinct advantages such as fast and selective curing at ambient temperature have allowed this environmentally friendly technology to outclass more conventional processing techniques. Several polymers with pendant photofunctional groups have been reported [8–10]. The synthesis and photosensitive properties of polymers such as cinnamoylated poly(vinylamide) [11], N-phenylamide [12], Azo functional polymers [13], poly (2-vinyloxy carbonyl chalcone) [14], poly(cinnamoyloxy ethyl methacrylate) [15] and other systems have been reported [16–20]. In this article, the synthesis, characterization, thermal stability and photocrosslinking properties of new acrylamide polymers bearing bromo substituted chalcone groups are reported. This paper also reports the effect of different solvents and different concentrations on the rate of photocrosslinking.

2. Experimental 2.1. Materials 4-Aminoacetophenone (Merck) was used as received. 3-Bromobenzaldehyde and 4-bromobenzaldehyde (Fluka) were distilled before use. Benzoyl peroxide (BPO) was recrystallized from a chloroform/methanol (1:1) mixture. Acryloyl chloride was prepared according to the method of stampel et al. [21]. All the other solvents were distilled by standard procedure. 2.2. Synthesis of 4-(3 0 -bromocinnamoyl) aniline (4,3 0 BCA) 4-Aminoacetophenone (8.3 g, 0.0614 mol) in 60 ml of ethanol and a solution of sodium hydroxide (2 g) in distilled water (20 ml) were placed in a three necked flask equipped with a mechanical stirrer, thermometer and a dropping funnel and were cooled in an ice bath.

3-Bromobenzaldehyde (11.4 g, 0.0614 mol) dissolved in 30 ml of ethanol was added drop wise with constant stirring such that the temperature was not exceeded 20
C. After stirring the reaction mixture at room temperature for 12 h the precipitated solid product was filtered, washed with ice-cold water, dried and recrystallized from ethanol to get yellow crystals. Yield: 12.4 g (67%), m.p.: 172–173
C. The structure of the compound was analyzed by elemental analysis, IR, 1H-NMR and 13C-NMR spectral techniques. Elemental analysis (%): C = 59.89 (Found), 59.94 (Calcd); H = 4.29 (Found), 4.32 (Calcd); N = 4.58 (Found), 4.62 (Calcd). IR (KBr, cm1): 3417 and 3330 (N–H stretching), 3052 (@C–H stretching); 2930 and 2875 (C–H stretching); 1652 (>C@O stretching), 1627 (NH bending); 1608 (olefinic >CH@CH< stretching), 1580, 1557, 1515 and 1442 (aromatic C@C stretching), 1341 (C–N stretching), 830 and 779 (@C–H out of plane bending), 662 (N– H wagging); 567 (C–Br); 523 (C@C out of plane bending). 1 H-NMR (DMSO, ppm): 7.87–7.24 (m, 8H, Ar–H), 6.54 (d, 1H, @CH–Ar), 6.09 (d, 1H, –CO–CH@), 3.26 (s, 2H, NH2). 13 C-NMR (DMSO, ppm): 186.60 (>C@O), 154.89, 140.50, 138.63, 133.25, 132.15, 131.70, 131.25, 128.68, 126.30, 125.00 and 113.64 (aromatic carbons), 142.35 (@CH–Ar), 123.23 (–CO–CH@). 2.3. Synthesis of 4-(4 0 -bromocinnamoyl) aniline (4,4 0 BCA) 4,4 0 -BCA was prepared by adopting the classical procedure for the preparation of 4,3 0 -BCA. 4-Aminoacetophenone (9.5 g, 0.0703 mol) was reacted with 4-bromo benzaldehyde (13 g, 0.0703 mol) in the presence of sodium hydroxide (2 g) in ethanol–water mixture. The crude product was recrystallized from ethanol to get yellow crystals. Yield: 15.3 g (72%), m.p.: 160–161
C. The formation of 4,4 0 -BCA was confirmed by elemental analysis, IR, 1H-NMR and 13C-NMR spectral techniques. Elemental analysis (%): C = 59.85 (Found), 59.94 (Calcd); H = 4.27 (Found) 4.32 (Calcd); N = 4.53 (Found), 4.60 s (Calcd). IR (KBr, cm1): 3461 and 3341 (N–H stretching), 3050 (@C–H stretching); 2958 and 2869 (C–H stretching); 1647 (>C@O stretching), 1628 (NH bending), 1606 (olefinic >CH@CH< stretching), 1548, 1486 and 1443 (aromatic C@C stretching), 1313 (C–N stretching), 814, 786 (@C–H out of plane bending), 671 (N–H wagging); 591 (C–Br); 505 (C@C out of plane bending). 1 H-NMR (DMSO, ppm): 7.84–7.45 (m, 8H, Ar–H); 6.53 (d, 1H, @CH–Ar); 6.08 (d, 1H, –CO–CH@); 3.27 (s, 2H, NH2).

P. Selvam et al. / European Polymer Journal 41 (2005) 35–45 13

C-NMR (DMSO; ppm): 186.64 (>C@O); 154.84, 135.38, 132.64, 132.04, 131.64, 126.30, 124.04 and 113.65 (aromatic carbons); 140.84 (@CH–Ar); 124.26 (@CH–CO). 2.4. Synthesis of 4(3 0 -bromocinnamoyl) phenyl acrylamide (4,3 0 -BCPA) The chalcone, 4,3 0 -BCA (10.6 g; 0.0363 mol) and triethylamine (3.7 g; 0.0363 mol) were dissolved in 150 ml of ethyl methyl ketone (EMK) in a 250 ml three necked flask and cooled between 0–20
C. Acryloyl chloride (3.7 g; 0.0363 mol) in 30 ml of EMK was then added drop wise in to the reaction mixture with constant stirring. After stirring for 1 h at 0–5
C and 2 h at room temperature, the precipitated quaternary ammonium salt was filtered off. The organic layer was evaporated with a rotary vacuum evaporator to get the crude monomer. It was recrystallized from ethyl acetate to give pure yellow crystals. Yield: 7.6 g (60%), m.p.: 126–127
C. The structure of the monomer, 4,3 0 -BCPA was identified by elemental analysis, IR, 1H-NMR and 13CNMR spectra and found to be consistent with the assigned structure. Elemental analysis (%): C = 60.64 (Found), 60.69 (Calcd); H = 3.93 (Found), 3.96 (Calcd); N = 3.88 (Found), 3.93 (Calcd). IR (KBr, cm1): 3307 (N–H stretching); 3052 (@C–H stretching); 2985 and 2864 (C–H stretching); 1660 (amide band I); 1599 (olefinic >C@C<); 1599, and 1473 (aromatic >C@C< stretching); 1529 (amide band II); 1410 (CH2@ scissoring);1313 (C–N stretching), 980, 833 and 787 (@C–H out of plane bending); 666(N–H wagging); 572 (C–Br); 512 (C@C out of plane bending). 1 H-NMR (CDCl3, ppm): 8.81 (1H, NH); 8.03–7.24 (10H, Ar–H and –CH@CH–); 6.69–6.34 and 5.76–5.73 (2H/1H, CH2@CH–). 13 C-NMR (DMSO, ppm): 189.43 (keto, >C@O); 165.13 (amide >C@O); 143.79, 138.15, 131.99, 131.61, 131.15, 130.56, 128.90, 127.84, 123.81 and 120.30 (ArC); 143.31 (@CH–Ar); 124.15 (–CO–CH@); 131.61 (@C–H) and 128.89 (CH2@). 2.5. Synthesis of 4-(4 0 -bromocinnamoyl) phenyl acrylamide (4,4 0 -BCPA) 4,4 0 -BCPA was prepared from 4,4 0 -BCA (11.4 g; 0.039 mol) and acryloyl chloride (3.5 g; 0.039 mol) in EMK (200 ml) adopting the procedure described earlier for the preparation of 4,3 0 -BCPA. The product was recrystallized from absolute ethanol to give yellow crystals. Yield: 8.4 g (62%), m.p.: 120–121
C. The formation of the 4,4 0 -BCPA was confirmed by elemental analysis, IR, 1H-NMR and 13C-NMR spectroscopy.

37

Elemental analysis (%): C = 60.67 (Found), 60.69 (Calcd); H = 3.90 (Found), 3.96 (Calcd); N = 3.86 (Found), 3.93 (Calcd). IR (KBr, cm1): 3280 (s, N–H stretching); 3040 (@C– H stretching); 2979 and 2935 (C–H stretching); 1660 (amide band I); 1598 (olefinic >C@C<); 1598 and 1486 (C@C aromatic stretching); 1531 (amide band II); 1406 (CH2@scissoring); 1337 (C–N stretching); 908 and 812 (@C–H out of plane bending); 670(N–H wagging); 601 (C–Br stretching); 497(C@C out of plane bending). 1 H-NMR (CDCl3, ppm): 8.76 (1H, NH); 8.05–7.21 (m, 10H, Ar–H and –CH@CH–); 6.69–6.37 and 5.77– 5.74 (2H/1H, CH2@CH–). 13 C-NMR (DMSO, ppm): 189.51 (keto; >C@O); 164.92 (amide >C@O); 143.69, 134.17, 132.90, 131.85, 130.49, 128.87, 123.16 and 120.19 (Ar–C); and 142.10 (@CH–Ar); 123.44(–CO–CH@); 131.63 (@C–H) and 130.30(CH2@). 2.6. Polymerization Homopolymerization of new acrylamide monomer, 4,3 0 -BCPA was carried out as 2 M solution in ethyl methyl ketone in the presence of benzoyl peroxide (BPO) (0.5 wt% monomer) as an initiator at 70
C. Suitable amounts of the monomer, initiator, and solvent were mixed in a polymerization tube, flushed with oxygen free nitrogen for 20 min, and kept in a thermostat at 70
C. After a predetermined time (18 h), the reaction mixture was poured into excess methanol to isolate the polymer. The crude polymer, poly(4,3 0 -BCPA) was purified by reprecipitation by methanol from an ethyl methyl ketone solution and finally dried in a vacuum at 40
C for 24 h. The yield of the polymer was 44%. Similarly, poly(4,4 0 BCPA) was synthesized from 4,4 0 -BCPA by employing the same procedure. The yield of the polymer was 47%. 2.7. Instruments UV spectra were recorded with a Hitachi UV-2000 spectrophotometer. IR spectra were obtained with a Hitachi 270-50 spectrophotometer with KBr pellets. 1 H-NMR spectra were run on a JOEL 400 MHz spectrometer at room temperature using 15 wt% solution in CDCl3 and tetramethylsilane (TMS) as the internal standard. 13C-NMR spectra were run on a Bruker 270 MHz spectrometer. Thermogravimetric analysis (TGA) was performed in air with a Mettler TA 3000 thermal analyzer at a heating rate of 10
C min1. Glass transition temperature (Tg) of the polymers was determined with a Perkin Elmer DSC-7 at a heating rate of 15
C min1 in air. The molecular weights (Mw and Mn) were determined using the Waters 501 gel permeation chromatograph, tetrahydrofuran was used as an eluent and the polystyrene standards were employed for calibration.

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P. Selvam et al. / European Polymer Journal 41 (2005) 35–45

by adopting the standard Claisen–Schmide condensation reaction. The new monomers, 4,3 0 -BCPA and 4,4 0 -BCPA were prepared by amidification reaction (Scheme 1). The formation of chalcones, as well as the monomers is evidenced by elemental analysis and IR, 1 H-NMR and 13C-NMR spectra. Photopolymers, poly(4,3 0 -BCPA) and poly(4,4 0 BCPA) containing bromo substituents at para or meta position on the aromatic ring of the cinnamoyl group were prepared by the free radical polymerization in a ethyl methyl ketone solution with BPO as the initiator at 70
C (Scheme 2). The polymerization conversion has been restricted to 45–50% in order to avoid any possibility of crosslinking at higher conversions.

2.8. Photoreactivity of the polymers The photoreactivity of the polymers was measured in chloroform solution and irradiated at room temperature in air with a medium pressure mercury lamp (Haber Scientific photoreactor-UV, 6 W, 254 nm) for selected time intervals at a distance of 10 cm. After each exposure, the UV spectrum of the polymer solution was recorded at definite intervals and the rate of disappearance of >C@C< of the pendant cinnamoyl unit was calculated using the following expression Extent of conversionð%Þ ¼

A0  AT  100 A0  A1

where A0, AT and A1 and the absorption intensities due to the >C@C< group after the irradiation times t = 0, t = T and t = 1 (maximum irradiation time), respectively.

3.2. Solubility As solubility is one of the most important requirements for a photosensitive polymer, the solubility of the homopolymers were tested in various organic solvents. The polymers were easily soluble in aprotic polar solvents such as N-methyl-2-pyrolidone, dimethylformamide, dimethylacetamide, dimethylsulphoxide and tetrahydrofuran and in chlorinated solvents such as chloroform and methylene dichloride when the conversion was below 50%. They were insoluble in hydrocarbons such as benzene, toluene and xylene and in

3. Results and discussion 3.1. Synthesis The photosensitive bromo substituted chalcone units, 4,3 0 -BCA and 4,4 0 -BCA were derived from 4-aminoacetophenone and the corresponding bromobenzaldehyde NH2

Y X +

H2N

Y

H

Ethanol / NaOH

+ H-O-H

0 - 20ºC

X O

O

CH3

O

H

H

1. X = Br, Y = H ; 4,3'-BCA 2. X = H, Y = Br ; 4,4'-BCA H NH2

H +

H

H

EMK / Et3 N

H

H HN

+

Et3 N.HCl

O

0 - 20 ºC H

O H

Cl

O

X H

O Y H 1. X = Br, Y = H ; 4,3'-BCPA 2. X = H, Y = Br ; 4,4'-BCPA

Scheme 1. Synthesis of 4,3 0 -BCPA and 4,4 0 -BCPA.

X Y

P. Selvam et al. / European Polymer Journal 41 (2005) 35–45 H H

H

EMK, BPO

n

0 - 70 ± 1 ºC

HN

HN

O

H

O

H

O X

H

O

H

X Y

Y

1. X = Br, Y = H ; Poly(4,3'-BCPA) 2. X = H, Y = Br; Poly(4,4'-BCPA)

Scheme 2. Synthesis of poly(4,3 0 -BCPA) and poly(4,4 0 -BCPA).

hydroxyl group containing solvents such as methanol, ethanol and 2-propanol. The polymer was insoluble when the conversion was above 50%. This might have been due to the crosslinking of the olefinic group present in the pendant chalcone moiety of the polymer. 3.3. IR spectra Fig. 1 shows the FT-IR spectra of poly(4,3 0 -BCPA) and poly(4,4 0 -BCPA). The IR spectrum of poly(4,3 0 -

39

BCPA) shows a peak at 3430 cm1 due to the N–H stretching. The symmetrical and unsymmetrical C–H stretching of methylene and methyne groups is found at 2927 cm1 and 2855 cm1. The amide band I is overlapped with the keto >C@O stretching and is observed at 1658 cm1. The strong absorption band at 1598 cm1 is attributed to the >C@C< stretching vibration of olefinic double bond present in the pendant chalcone units as well as that of the aromatic rings. The relative intensity of this peak in the IR spectra of the polymer, when compared to that of the monomer (4,3 0 -BCPA) is very much reduced this indicates that the vinyl group is involved in polymerization. The amide band II due to the N–H bending is observed at 1527 cm1. The aromatic C@C stretching are observed at 1598 and 1473 cm1. The strong absorption band at 1312 cm1 is due to C–N stretching. The peaks at 783 cm1 and 667 cm1 are attributed to the C–H out of plane bending vibrations of the aromatic nuclei. The C-Br stretching of the polymers exhibit at 571 cm1. IR spectrum of poly(4,4 0 -BCPA) shows a peak at 3399 cm1 due to the N–H stretching. The symmetrical and unsymmetrical C–H stretching of methylene and methyne groups is found at 2930 cm1 and 2863 cm1. The amide band I is overlapped with the keto >C@O stretching and is observed at 1657 cm1. The strong absorption band at 1599 cm1 is attributed to the >C@C< stretching vibration of olefinic and the aromatic double bond. The amide band II due to the N– H bending is observed at 1527 cm1. The aromatic

Fig. 1. IR spectra of: (a) poly(4,3 0 -BCPA) and (b) poly(4,4 0 -BCPA).

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P. Selvam et al. / European Polymer Journal 41 (2005) 35–45

C@C stretching are observed at 1599 and 1486 cm1. The strong absorption band at 1327 cm1 is due to C– N stretching. The peaks at 810 cm1 and 705 cm1 are attributed to the C–H out of plane bending vibrations of the aromatic nuclei. The C–Br stretching of the polymers exhibit at 576 cm1. 3.4. 1H-NMR spectra The proton NMR spectrum of poly(4,3 0 -BCPA) and poly(4,4 0 -BCPA) is given in Fig. 2. Poly(4,3 0 -BCPA) shows a broad peak at 8.31–6.12 ppm due to the aromatic protons which is overlapped with the signals of the pendant olefinic protons and N–H protons. The broad resonance signal obtained between 2.96 and 1.34 ppm is due to the methyne and methylene protons of the backbone of the polymer. The 1H-NMR spectrum of poly(4,4 0 -BCPA) shows a broad peak at 8.02–6.28 ppm due to the aromatic protons which is overlapped with the signals of the pendant olefinic protons and N–H protons. The methyne and methylene protons of the backbone of the polymer give a broad resonance signal between 2.94 and 1.52 ppm. 3.5.

13

C-NMR spectra

The proton decoupled 13C-NMR spectrum of poly(4,3 0 -BCPA) is shown in Fig. 3. The chemical shift assignments were made from the off-resonance decoupled spectra of the polymer. The ketonic carbonyl carbon resonance of the pendant cinnamoyl unit appears at 188.62 ppm. The amide carbonyl carbons of the polymers

Fig. 3.

13

Fig. 2. 1H-NMR spectra of (a) poly(4,3 0 -BCPA) (b) poly(4,4 0 BCPA).

C-NMR spectrum of poly(4,3 0 -BCPA).

P. Selvam et al. / European Polymer Journal 41 (2005) 35–45

can be observed at 175.18 ppm. The aromatic carbon that is attached to the nitrogen atom gave resonance signals at 143.74 ppm. The signal between 143.32 and 120.25 ppm is due to the other aromatic and olefinic carbons. Resonance signals of backbone methyne and methylene carbons appear between 41.43 and 36.52 ppm. The proton decoupled 13C-NMR spectra of poly(4,4 0 -BCPA) is presented in Fig. 4. The ketone and amide carbonyl carbon are observed at 188.64 and 175.04 ppm, respectively. The resonance signals at 144.23 and 123.54 ppm may be assigned to the olefinic carbons attached to the aromatic ring and keto groups respectively. The aromatic carbon that is attached to the nitrogen atom gave resonance signal at 142.47 ppm. The other aromatic carbons are observed at 142.47–120.23 ppm. The signal due to backbone methyne and methylene carbons is observed at 41.93 and 36.33 ppm respectively. 3.6. Molecular weights by GPC The molecular weights of the polymers were determined by gel permeation chromatography (GPC). The weight average molecular weights (Mw) of poly(4,3 0 BCPA) and poly(4,4 0 -BCPA) are 5.34 · 104 and 5.12 · 104 respectively and their number average molecular weights (Mn) are 3.14 · 104 and 3.18 · 104. The polydispersity index of poly(4,3 0 -BCPA) and poly(4,4 0 BCPA) are 1.70 and 1.61 respectively. The theoretical value of Mw/Mn of polymer produced via radical combination and disproportionation are 1.5 and 2.0 respectively [22]. In free radical homopolymerization of acrylate monomers, the polymeric radicals undergo termination mainly by recombination [23]. The polydispersity value of polymers suggests a greater tendency for

Fig. 4.

13

41

chain termination by recombination than disproportionation. 3.7. Glass transition temperature Glass transition temperature (Tg) of the polymers were determined by differential scanning calorimetry (DSC). The Tg values for poly(4,3 0 -BCPA) and poly(4,4 0 -BCPA) are 55
C and 64
C respectively. The polymers exhibit a single Tg, thereby indicating the absences of microphase separation at a length scale of tens of nanometers. The higher Tg value of the polymers when compared to poly(methyl acrylate) may be attributed to the presence of stiff and bulk aromatic pendant groups (chalcone moieties) introduced into the system. 3.8. Thermogravimetric analysis Thermogravimetric analysis was used to study the thermal stability of poly(4,3 0 -BCPA) and poly(4,4 0 BCPA). The thermogravimetric (TG) and differential thermogravimetric (DTG) traces of the polymers are shown in Fig. 5. The initial decomposition temperature (IDT) of poly(4,3 0 -BCPA) is 211
C and that of poly(4,4 0 -BCPA) is about 216
C and 50% weight loss of these polymers occur at 326 and 384
C, respectively. These thermal studies reveal that the polymers have very good thermal stability required for negative type photoresists. Both poly(4,3 0 -BCPA) and poly(4,4 0 -BCPA) exhibit two stage decomposition. The first stage and second stage decomposition of poly(4,3 0 -BCPA) was observed in the range 211–353
C and 353–476
C respectively and that of poly (4,4 0 -BCPA) in the range 216–416
C and 416–542
C, respectively. The first stage decomposition

C-NMR spectrum of poly(4,4 0 -BCPA).

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P. Selvam et al. / European Polymer Journal 41 (2005) 35–45

Fig. 6. UV absorption spectra of poly (4,3 0 -BCPA) before and after irradiation.

Fig. 5. TGA traces of: (a) poly(4,3 0 -BCPA) and (b) poly(4,4 0 BCPA).

may be attributed to the cleaving of the pendant group and the second stage decomposition may be due to the cleavage of the main chain bonds. 3.9. Photosensitivity properties The photosensitivity of the polymer was measured by the irradiation of the polymer solution with a mediumpressure mercury lamp and by the measurement of the UV absorption intensity due to >C@C< of the pendant bromocinnamoyl group. Typical changes in the UV spectral pattern of the poly(4,3 0 -BCPA) and poly(4,4 0 BCPA) in chloroform solution for different time intervals of irradiation at room temperature in the absence of photosensitizer are presented in Figs. 6 and 7 respectively. The polymers, poly(4,3 0 -BCPA) and poly(4,4 0 BCPA) show an absorption band at 318 and 325 nm respectively due to the p–p* transitions of the pendant cinnamoyl unit of the polymer. It is clear from Fig. 6 and Fig. 7 that the absorbance values were getting reduced as the time of exposure of the polymer solution to UV radiation increases. The >C@C< bond absorption decreases drastically upon irradiation, where as that of the –C–C– group increases gradually. As a result an isobestic point appears at 272 and 283 nm respectively for poly(4,3 0 -BCPA) and poly(4,4 0 -BCPA) systems which can be attributed to the decrease in the conjugated system upon irradiation,

Fig. 7. UV absorption spectra of poly (4,4 0 -BCPA) before and after irradiation.

with a concomitant increase in the absorbance at a shorter wavelength at 272 and 283 nm due to single bond formation in the cyclobutane ring via 2p + 2p cycloaddition [24,25] of the pendant chalcone units as shown in Scheme 3. The absorption band disappears almost completely within 85 min of irradiation. The homopolymers become insoluble in polar aprotic and chlorinated solvents, in which it was easily soluble before irradiation. The decrease in the UV absorption intensity due to the pendant chalcone unit and the insoluble nature of the homopolymers are due to the crosslinking of polymers through 2p + 2p cycloaddition of the >C@C< group of the pendant bromo cinnamoyl unit [26,27] as shown in Scheme 3.

P. Selvam et al. / European Polymer Journal 41 (2005) 35–45 O

H

100

Br

O

80

H

N H

+

H N

H

Conversion (%)

P

P O

Br

H

O

60

40

20

0

UV-light

0

P HN

NH Br

O

O

Br O

HN

HN

P

O

P

P=

CH2

CH

60

80

100

Fig. 8. Disappearance rate associated with photoactive >C@C of poly(4,3 0 -BCPA) (d) and poly(4,4 0 -BCPA) (s) with irradiation time in chloroform solution.

Br

OR

Br

40

O

O O

20

Irradiation time (Sec)

n

Scheme 3. Photocycloaddition reaction of poly(4,3 0 -BCPA) and poly(4,4 0 -BCPA) upon UV irradiation.

The rate of photocrosslinking of polymers, poly(4,3 0 BCPA) and poly(4,4 0 -BCPA) was observed in terms of the rate of disappearance of the photoactive –CH@CH– bond as seen from Fig. 8. It compares these rates associated with polymers, which have bromo substituent at 3 0 or 4 0 positions of the pendant cinnamoyl group. In chloroform solution, poly(4,3 0 -BCPA) shows photoconversion of 10%, 48%, 70% and 87% after 1, 19, 45 and 70 s of irradiation time, respectively. About 95% was achieved within 85 s of irradiation. In the case of poly(4,4 0 -BCPA) photoconversion of 23%, 58%, 78% and 89% was observed after 2, 23, 43 and 58 s of irradiation time respectively. About 95% was achieved within 65 s of irradiation. It was found that photoconversion was higher for poly(4,4 0 -BCPA) with the value of 92%, while poly(4,3 0 -BCPA) showed 76% for the same time of irradiation (60 s). This is due to the fact that electron releasing nature of the bromo group at para position strengthen the electron density of –CH@CH– double

bond through an extended conjugation. On the other hand, the bromo group at the meta position does not increase the electron density of –CH@CH– double bond but increases the steric repulsion due to its bulky nature. The FT-IR spectra of the irradiated polymer do not show the absorption band at 1598 cm1 and also the carbonyl peak is shifted to higher wave length (1715 cm1) due to the loss of conjugation during the crosslinking reaction. The effect of various solvents such as CHCl3, CH2Cl2, dioxane, THF and DMF on the rate of photocrosslinking poly(4,3 0 -BCPA) and poly(4,4 0 -BCPA) is shown in Figs. 9 and 10, respectively. These results indicate that there is clear difference on the rate disappearance of >C@C< double bond and the crosslinking rate was followed in the order CHCl3 > CH2Cl2 > dioxane > THF > DMF. This indicates that the type of sol-

100

80

Conversion(%)

P O

43

60

40

20

0

0

20

40

60

80

100

Irradiation time (Sec)

Fig. 9. Rate of disappearance of >C@C< of the chalcone unit of poly(4,3 0 -BCPA) with irradiation time in different solvents: (n) CHCl3, (d) 1,4-dioxane, (m) THF, (h) DMF and () DMSO.

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P. Selvam et al. / European Polymer Journal 41 (2005) 35–45

state (T) electron, but by the singlet state electron, leading to one step concerted 2p + 2p cycloaddition [31]. From the results described above, we were able to conclude that the degree of photosensitivity is higher for poly(4,4 0 -BCPA) than poly(4,3 0 -BCPA). These polymers having bromo substituted cinnamoyl group have a higher rate of photocuring even in the absence of photosensitizer. We believe that these polymers can be effectively used as negative photoresist materials.

100

Conversion (%)

80 60

40 20

0 0

20

40

60

80

Irradiation time (Sec)

Fig. 10. Rate of disappearance of >C@C< of the chalcone unit of poly(4,4 0 -BCPA) with irradiation time in different solvents: (n) CHCl3, (d) 1,4-dioxane, (m) THF, (h) DMF and () DMSO.

vents used also have a significant effect in the rate of photocrosslinking. The effect of different concentration of photosensitive polymer, poly(4,4 0 -BCPA) (Fig. 11) was examined in chloroform solution with concentration range of 21– 166 mg1. In both cases, rate of crosslinking increases with increase in the concentration of photomonomer, because of the availability of more photosensitive units. The photocrosslinking reaction of the polymers was studied in the presence of various triplet sensitizers like MichlerÕs ketone, benzoin and p-nitroaniline but there was no sensitizing effect on the rate of disappearance of >C@C< of the pendant chalcone unit of the polymer which is similar to that reported for some other photoresists [28–30]. This indicates that the photocrosslinking of these polymers does not take place through the triplet

100

Conversion(%)

80

4. Conclusions Photocrosslinkable polymers, poly(4,3 0 -BCPA) and poly(4,4 0 -BCPA) were synthesized and characterized by UV, IR, 1H-NMR and 13C-NMR spectral techniques. Molecular weights of these polymers have been determined by Gel permeation chromatography and the polydispersity index value indicate that the chain termination by radical recombination is predominant than disproportionation. The prepared polymers were soluble in chlorinated solvents and aprotic solvents and insoluble in alcohols and hydrocarbons. These polymers exhibit higher Tg value due to the presence of bulky and inflexible pendant groups (cinnamoyl unit). The polymer possesses good thermooxidative stabilities, with IDT greater than 200
C. The photosensitivity studies by UV revealed that poly(4,4 0 -BCPA) has higher photosensitivity when compared with poly(4,3 0 -BCPA). The rate of disappearance of >C@C< of the chalcone unit in the polymers was studied in various solvents, and at different concentration. The rate of photocrosslinking was faster at higher concentration because of the availability of more photoresposive group for cycloaddition reaction. The crosslinking reaction carried out in presence of various triplet sensitizers showed that there is no significant changes in the rate of disappearance of >C@C< double bond. These polymers having bromo substituted pendant chalcone unit possess higher rate of photocuring even in the absence of sensitizers and hence it might be expected that these polymers can be used for negative photoresist application.

60

Acknowledgment

40

This work was financially supported by the Department of Science and Technology, New Delhi, India.

20

0 0

20

40

60

80

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References

Irradiation time (Sec)

Fig. 11. Rate of disappearance of >C@C< of the chalcone unit of poly(4,4 0 -BCPA)in chloroform solution at different concentrations: (n) 166 mg l1; (d) 124.4 mg l1; (m) 92 mg l1; (h) 44.4 mg l1; and () 21 mg l1.

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