Synthesis and characterization of soluble and transparent co-polyimides with controlled glass transition temperature

Indian Journal of Chemical Technology Vol. 19, July 2012, pp. 271-277 Synthesis and characterization of soluble and transparent co-polyimides with controlled glass transition temperature Xiuzhi Tian, Xue Jiang *, Hui Lu & Dan Huang Key Laboratory of Eco-Textiles of Ministry of Education, Jiangnan University, Wuxi, Jiangsu 214122, China Received 22 June 2011; accepted 10 April 2012 Two dianhydrides, namely 4,4'-oxydiphthalic dianhydride and 2,2 -Bis[4-(3,4-dicarboxyphenoxy)phenyl] propane dianhydride, and one diamine (4,4 -Diaminodiphenyl sulphone), have been used for the synthesis of a novel co-polyimide with good solubility, high visible light transparency and high thermal stability. The solubility in several solvents of polyimides with different proportion of dianhydrides has been tested, and the glass transition temperature is determined by differential scanning calorimetry. The polyimide film has been prepared and the optical transmittance is mensurated. The polyimides solubility and light transmittance improved by introducing flexible monomer, and the glass transition temperature of the polyimides is well controlled by the content of 2,2 -Bis[4-(3,4-dicarboxyphenoxy)phenyl] propane dianhydride that has the flexible isopropylidene structure. Keywords: Co-polyimide, Differential scanning calorimetry, Glass transition temperature, Infrared spectrum Polyimides (PIs) are a class of representative high-performance polymers that have been widely used in flexible displays 1, polymer electronic memories 2, pervaporation, biofuels separation 3 and many other fields of microelectronics, optics, aerospace industries and biomedical engineering 4,5. They are extensively used not only for their considerably excellent thermal stability but also for their good mechanical properties, low dielectric constant, low coefficient of thermal expansion and high radiation resistance. However, polyimide materials are usually difficult to be processed because of their infusibility at high temperature and insolubility in most organic solvents. In order to improve the solubility and melting ability of polyimides, many studies have been focused on introducing the fluoro-containing groups 6-11 or flexible groups 12-22 into the polymer backbone. The polyimides with the fluoric aromatic diamine or dianhydride has good solublity and are studied widely, nevertheless, the fluorous monomers high cost makes them difficult to be industrialized. Isopropylidene groups are proved to be good for improvement of the solubility of polyimide and the polyimide with isopropylidene groups has been synthesized by Hamciuc et al. 15 and Chen and Yin 22. Moreover, as one kind of important optical materials, light transmittance and refractive index * Corresponding author. E-mail: jiangx@jiangnan.edu.cn play an active role in polyimide materials Unfortunately, most current polyimides are deep in color, and hence the light transmittance of polyimides is usually not high. Thus, the improvement in light transmittance of polyimides is one of the important problems when polyimides are used as optical materials. Many studies have already discussed the transparency of polyimides 23-27, but most of them are focused on the fluorinated polyimides 23-26. High transparency and refractive index of polyimide have been obtained by introducing chlorine, as reported by Choi et al 27. Transparent polyimides (PIs) that have a high refractive index and low birefringence as well as good thermal and mechanical stability have been synthesized by the judicious introduction of di- and tetrachlorinated aromatic diamines with aromatic/aliphatic dianhydrides. A high transmittance of 82% at 400 nm with an average transmittance > 90% in the visible region and strong fluorescent emission is achieved in a fully aromatic PI (BPDA/TCDB). For the use in optical materials, refractive index and birefringence are another two important parameters. Terui and Ando 28 investigated the controlling of refractive index and birefringence, and the results show that the introduction of sulfonyl group could change the refractive index and birefringence. However, the polyimides with sulfonyl groups usually show very low solubility, deep color (weak transparency) and high T g, which cannot be widely used. Therefore, the properties

272 INDIAN J. CHEM TECHNOL., JULY 2012 of the polyimides with sulfonyl groups should be optimized. The structure of the monomer containing sulfonyl groups should be kept unchanged for the good refractive index and birefringence, so that the copolymerization is a good solution for improving the transparency and processing ability of the polyimide with sulphonyl groups. This paper reports a pilot study of the design and synthesis of a soluble and transparent co-polyimide with good thermal stability. A dianhydride with flexible isopropylidene group (BPADA) and a diamine containing sulfonyl group have been introduced into poly (ether) imides copolymer with different proportions in order to improve the solubility and the visible light transmittance of polyimide materials without affecting the T g of polyimide. The optical transmittance and thermal property have also been characterized. Experimental Procedure Materials Two dianhydrides (99%) namely 2,2 -Bis[4-(3,4- dicarboxyphenoxy)phenyl] propane dianhydride (BPADA) and 4,4'-oxydiphthalic dianhydride (ODPA) were purchased from Shanghai Synthetic Resin Manufacturer, China, and used after drying at 150 o C; 4,4 - Diaminodiphenyl sulphone was purchased from Aldrich and used as received; chloroform, N,N - dimethylacetamide (DMAc), N,N -dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), tetrahydrogen furan (THF), toluene, acetone were obtained from Sinopharm Medicine Holding Chemical Reagent of China and used after distillation in reduced pressure. Synthesis of co-polyimide containing sulfonyl group Polycondensation of dianhydrides and diamine was adopted to prepare co-polyimide using the reaction mechanism as shown in Scheme 1. Poly (amic acid) Scheme 1 Synthesis process of polyimide copolymers

TIAN et al.: SYNTHESIS & CHARACTERISATION OF SOLUBLE & TRANSPARENT COPOLYIMIDE 273 was prepared firstly in solvent by the reaction of dianhydride and diamine, and then either chemical imidization or thermal imidization was adopted. In the case of chemical imidization, the catalysts and dehydrate agent were added, the imide ring was formed, and the polyimide was prepared after a few hours of reactions at room temperature or a little heating. In the case of thermal imidization, the poly (amic acid) solution was poured on a glass plate spreading to a film, and then the film was heated to the preconcert temperature. After scheculed heating for a few hours, the thermal imidized polyimide was prepared. In this work, polyimides with different BPADA contents were synthesized and the mol percentage of BPADA was determined using the following equation: n BPADA B = 100% (1) n + n BPADA ODPA where B is the mol percentage of BPADA in two dianhydrides (mol%); n BPADA, the amount of substance of BPADA (mol); and n ODPA, the amount of substance of ODPA (mol). Preparation of polyimide films About 2 g polyimide sample (chemical imidized) was dissolved in DMAc or NMP and then filtered through a 0.5 mm syringe filter to remove the insoluble materials and other probable dust particles. The filtered solution was poured on a leveled clean glass plate, spreaded homogeneously, and then put into an oven at 60 o C. The polyimide films thus formed were taken out after 6 h. On the other hand, thermal imidization was adopted on the insoluble polyimide samples. The poly (amic acid) solution was diluted, poured on a leveled clean glass plate, spreaded homogeneously, and then put into an oven. The temperature was increased as follows: 60 o C for 2 h, 80 o C for 1 h, 100 o C for 1 h and 200 o C for 3 h. Finally, the thermal imidized polyimide film was prepared. Measurements of co-polyimide Nexus Fourier transform infrared (FTIR, Thermo Company, USA) was used for the analysis of polyimides structures, in order to evaluate whether the imidization was complete. The number average molecular weight and weight average molecular weight of copolyimides were measured by gel permeation chromatography in a Waters 150 C GPC instrument (Waters Co. Ltd, USA) at 30 o C in THF. The weight concentration of the polyimide in THF solution was 0.3%. Two mixed Columns (10 2 10 6 ) were used. Universal calibration against narrow polystyrene standards was adopted. Solubility of polyimides was tested in different solvents. A series of polyimide samples with different proportions of dianhydride was dissolved in several kinds of solvents, in order to test the polymers solubility. The most usual solvents were chosen for the test, such as N, N-dimethylacetamide (DMAc); N,N-dimethylformamide(DMF); N-methy l- 2-pyrrolidone (NMP); tetrahydrofuran (THF); toluene and acetone. The glass transition temperature (T g ) of polyimide was measured on differential scanning calorimetric apparatus (DSC7 style, Perikin Elmer Co. Ltd. USA). The sample was protected by nitrogen during the course of scanning, and the temperature was raised at the rate of 20 o C/min. The light transmittance of polyimide film was tested by an ultraviolet-visible-near infrared spectrophotometer (Shimazi, Japan). Results and Discussion Characterization of imidization by FTIR BPADA and ODPA were adopted as dianhydrides and DDS was used as diamine in the step-wise copolymerization to synthesize poly (amic acid). The chemical imidization or thermal imidization was adopted to prepare co-polyimides. The chemical structures of synthesized polyimides were characterized by FTIR. The data of FTIR of polyimide and the relevant poly (amic acid) are shown in Table 1. The aryl imide absorption on FTIR are mainly at about 1780 (C=O), 1720 (C=O), 1380 (C-N), and 725 cm -1 (C=O), moreover, the absorption at 1780, 1720, and 725 cm -1 are easily confused by the probable absorption of acid anhydride 29. An internal standard absorption should be chosen to evaluate the degree of Table 1 IR data of main functional groups in poly (amic acid) and polyimide In references Wavenumber, cm -1 In experiment Functional group 1780 1778 1780 C=O, asymmetric stretching 1720 1722 1726 C=O, symmetric stretching 1380 1367 1385 C-N, stretching 725 723 730 C=O, flexural

274 INDIAN J. CHEM TECHNOL., JULY 2012 the imidization. The characteristic absorption of aryl accessed at about 1500cm -1 is usually chosen as the internal standard absorption 29. Thus, the ratio of absorption at 1380 cm -1 and at 1500 cm -1 is usually utilized as an indication of imidization. The four main characteristics bands of polyimide are shown in Table 1, especially the absorption at 1380 cm -1, where the difference of poly (amic acid) and polyimide is defined. The polyimide shows the imide characteristic absorptions at 1778 1780 cm 1 due to the asymmetric carbonyl stretching vibrations and at 1722 1726 cm -1 due to the symmetric carbonyl stretching vibrations. The absorption bands at 1367 1385 cm -1 are assigned to C-N stretching vibrations, and the C=O bending absorption bands are detected in the range 723 730 cm -1. Apparently, the absorption of polyimide sample at 1380 cm -1 is far more than that of poly (amic acid) at the same wavenumber, as compared to FTIR of the polyimide with that of poly (amic acid). Therefore, an almost complete imidization of co-polyimide has been proved. Molecular weight of co-polyimides The gel permeation chromatography (GPC) results are shown in Table 2. The molecular weight of polyimide increases with the content of BPADA, and the M M w / n values of all the polyimides are found to be about 2. The homo-polyimide HPI-i that is synthesized with ODPA and DDS has the lowest molecular weight with number average molecular weight and weight average molecular weight of 22.3 10 3 and 43.9 10 3 respectively. The molecular weight of co-polyimides is higher than that of HPI-i, and it increases with the content of BPADA, based on the molecular weight of the co-polyimides that are from CPI-1 to CPI-6. The homo-polyimide HPI-ii that is synthesized with BPADA and DDS has the highest molecular weight of 30.7 10 3 and 62.5 10 3 respectively. Solubility of co-polyimides The solubility of novel co-polyimides with sulfonyl structure is tested with several usual organic solvents such as DMAc, NMP, DMF, chloroform, THF, toluene and acetone (Table 3). The symbols (++) represent that the polyimide sample is soluble (the percentage of the dissolved polymer is over 10%), (+) represents that the polyimide sample is partially soluble (the percentage of the dissolved polymer is 1% 10%), and (-) represents that the polyimide sample was insoluble (the percentage of the dissolved polymer is less than 1%). The polyimide samples in Polyimide No. Table 2 Number average molecular weight and weight average molecular weight of polyimide BPADA content, mol% M 10 3 n M w 10 3 M / M w n HPI-i 0 22.3 43.9 1.97 CPI-1 20 25.2 55.1 2.19 CPI-2 40 26.9 54.8 2.04 CPI-3 50 27.1 56.3 2.08 CPI-4 60 29.2 61.8 2.12 CPI-5 70 28.8 59.5 2.07 CPI-6 80 31.0 63.2 2.04 HPI-ii 100 30.7 62.5 2.04 Table 3 Solubility of polyimide in organic solvents PI BPADAcontent, mol% DMAc NMP DMF Chloroform THF Toluene Acetone No. HPI-i 0 + + + - - - - CPI-1 20 + + + - - - - CPI-2 40 ++ ++ + + - - - CPI-3 50 ++ ++ ++ + - - - CPI-4 60 ++ ++ ++ + + - - CPI-5 70 ++ ++ ++ + + - - CPI-6 80 ++ ++ ++ + + - - HPI-ii 100 ++ ++ ++ ++ + - - (++) Soluble (over 10%), (+) Partly soluble (1 10%), and (-) Insoluble (less than 1%).

TIAN et al.: SYNTHESIS & CHARACTERISATION OF SOLUBLE & TRANSPARENT COPOLYIMIDE 275 Table 3 are two types of homopolymers and six types ofcopolymers. The homopolymers are shown as HPI-i and HPI-ii that are polyimide based on ODPA-DDS and BPADA-DDS respectively. The co-polyimides obtained by the copolymerization of BPADA/ODPA- DDS have different mol percentage of BPADA (B) in the two dianhydrides. In this work, co-polyimides with B of 20, 40, 50, 60, 70 and 80% are shown as CPI-1 to CPI-6 respectively The results demonstrate that HPI-i is only partially dissolved in DMAc, NMP and DMF, while HPI-ii is well dissolved in DMAc, NMP, DMF, chloroform and partially dissolved in THF. The solubility of the co-polyimides, which depends on the percentage of BPADA, becomes better with the increase in percentage of BPADA. Thus, HPI-i has the smallest solubility and HPI-ii has the best solubility. The solubility is changed little when the percentage of BPADA is above 60%, indicating that the co-polyimide with 60% of BPADA might be good for the improvement in solubility of polyimide with sulfonyl group. The cause of solubility may be explained by introducing the flexible group, the polarizability of solvents, the structural comparability of the polymers and the solvents, and the solvency of the solvents. The dependence of the solubility on the percentage of BPADA of co-polyimide is reasonably attributed to the flexible group in BPADA. The new sulfonylcontaining co-polyimides with high content (about more than 50%) of BPADA (from CPI-3 to CPI-6) and the homo-polyimide of BPADA and DDS (HPI-ii) are easily dissolved in polar solvents, such as DMF, DMAc and NMP, and partially dissolved in less polar solvents, such as THF and chloroform at room temperature. Co-polyimides with low BPADA content (less than 50%, CPI-1 and CPI-2) are dissolved in polar solvents, such as DMF, DMAc and NMP, upon heating, or partially dissolved at room temperature. But co-polyimides CPI-1 and CPI-2 are difficult to be dissolved in common organic less polar solvents, such as THF and chloroform. The homo-polyimide of ODPA and DDS (HPI-i) shows the smallest degree of dissolving; it is only partially dissolved in polar organic solvents such as DMF, DMAc and NMP upon heating, and insoluble in less polar solvents such as chloroform and THF even upon heating. Moreover, in more weak polar solvent such as toluene, neither co-polyimides nor homo-polyimides are dissolved. The reasonable explanation of the good solubility of the polyimides containing BPADA is the presence of flexible aryl ether linkages, and bulky flexible isosubpropyl group, and moreover, the metaposition substituted ether linkages resulting serrated chain structure. The reasonable explanation of the poor solubility of HPI-i is the rigid and symmetric nature of the polymer. The poor solubility of all polyimides in acetone may not only be due to the difference in the polarizability between solvent and polymer, but also due to the structure difference and the weak solvency of acetone. Therefore, the results demonstrate that the flexibility of polymer chain is the most important factor that affects the solubility of polyimides, and several other factors such as the polarizability of solvents, the structural comparability of the polymers and the solvents. The solvency of the solvents also affects the solubility of polyimides to some extent. Transparency of polyimide films It can be observed that the colorlessness of three series polyimides (homopolymers and copolymers) is in the following order: HPI-i>CPI>HPI-ii. The light transmittance of polyimide films has been tested on UV-visible light spectrophotometer. Here, three samples such as HPI-i, CPI-4 and HPI-ii are selected for the transmittance test (Fig. 1). The curves a, b and c in Fig. 1 represent the homopolyimide with ODPA & DDS (HPI-i), co-polyimide with ODPA/BPADA (B is 60mol%) & DDS (CPI-4) and homopolyimide with BPADA & DDS (HPI-ii) respectively. The three curves show that the two homo-polyimides and the co-polyimide give excellent light transmittance at the band above 500 nm of visible light. The light transmittance is diversified with the different compositions, as the wavelength of the light is lower than 500nm. The transmittance of HPI-i decreases apparently as the wavelength is lower than 500nm, as shown in curve a of Fig. 1. In fact, the Fig. 1 Transmittance of the polyimide films

276 INDIAN J. CHEM TECHNOL., JULY 2012 film of HPI-i shows deeper color than both of the other films namely CPI-4 and HPI-ii, as shown in curves b and c in Fig. 1 respectively. From the structural point of view, the phenoxy-containing in meta-structure reduces the electron-conjugation on the imide ring and thus these results are attributed to the reduction of the intermolecular charge transfer complex (CTC) effect. A meta-positioned phenoxy in BPADA is an important reason of the light colour of the polyimide containing BPADA. The polyimides with CH 3 groups in their dianhydride moieties show light colour of the decreased intermolecular interactions. The bulky CH 3 group in dianhydride is effective in decreasing CTC formation between polymer chains through steric hindrance and the inductive effect (by decreasing the electron-donating property of diamine moieties). The meta-structure also reduces the degree of resonance and colour intensities. Therefore, introducing BPADA into polyimide increases the light transmittance at the band range from 400 nm to 500 nm of the films of the polyimides containing sulfonyl. Heat resistance characterization of co-polyimide The homopolyimides that are based on ODPA & DDS (HPI-i) and BPADA & DDS (HPI-ii) and the co-polyimides having different proportions of BPADA (from CPI-1 to CPI-6) are analyzed by differential scanning calorimetric (DSC). The glass transition temperature (T g ) of the samples is tested and the results are shown in Fig. 2. It is found that the polyimide with only ODPA & DDS (HPI-i) has the highest T g (305 o C), while several co-polyimides containing BPADA has the T g ranging from 230 C to 270 C. The T g of the copolymers of ODPA, BPADA and DDS (CPI) decreases with the increase in proportion of BPADA, and the T g of homopolyimide of BPADA & DDS (HPI-ii) is found to be 236 C that was far lower than that of HPI-i. Figure 2 shows that the T g of co-polyimide decreases rapidly as the content of BPADA increases from 0% to 60%. When compared to the homo-polyimides with co-polyimides, HPI-ii synthesized by BPADA and DDS shows the lowest thermal stability due to the presence of flexible isopropylidene. The flexibility of the isopropylidene increases the mobile ability of the chain segment, and the large steric space weakened the interaction of molecular chain. On the other hand, polymers containing isopropylidene group has larger Fig. 2 Relationship of T g and BPADA percentage in two dianhydrides Fig. 3 DSC thermogram of BPADA-ODPA-DDS copolyimide (CPI-4), in which BPADA is 60%

TIAN et al.: SYNTHESIS & CHARACTERISATION OF SOLUBLE & TRANSPARENT COPOLYIMIDE 277 free volume and more types of free rotation of conformations, because the bulky CH 3 groups rotate freely. Thus, the polymer backbone has higher flexibility, and the T g of BPADA-based polyimides is controlled by the content of isopropylidene group. The T g decreases slowly as the amount of BPADA is higher than 60% based on Fig. 2. It may because, the flexibility of PI is not changed so much as the BPADA is higher than 60%. Thus, 60% of BPADA might be the best content for the improvement of the property of co-polyimide, which is in accordance with solubility test. The DSC profile of one copolyimide sample that has 60% of BPADA (CPI-4) is shown in Fig. 3. The T g of CPI-4 is about 250.8 o C. Conclusion A class of soluble and transparent co-polyimides with controlled glass transition temperature was synthesized in this stuty. The introduced flexible isopropylidene through BPADA improves the solubility of polyimide containing sulfonyl group and lowers its glass transition temperature to some extent. The lowering of glass transition temperature is propitious to processing and makes no difference in the use of polyimides. The flexible co-monomer could also weaken the color of co-polyimide films. The results show that the glass transition temperature of the co-polyimides could be controlled by the content of BPADA that has flexible structure. Thus, the processing ability of the co-polyimide materials improves with copolymerization of the two dianhydrides (BPADA and ODPA) and a diamine (DDS) without any loss of thermal stability of the polyimides. The best mole content of BPADA in the two dianhydrides is 60%. Acknowledgement The authors greatfully acknowledge the supports from the National Natural Science Foundation of China (Grant No. 20704018) and supports from the Fundamental Research Funds for the Central Universities (Grant No. JUSRP10902 and JUSRP10904). References 1 Choi M C, Kim Y & Ha C S, Prog Polym Sci, 33 (2008) 581. 2 Ling Q D, Liaw D J, Zhu C X, Chan D S H, Kang E T & Neoh KG, Prog Polym Sci, 33 (2008) 917. 3 Jiang L Y, Wang Y, Chung T S, Qiao X Y & Lai J Y, Prog Polym Sci, 34 (11) (2009) 1135. 4 Chung T S, Jiang L Y, Li Y & Kulprathipanja S, Prog Polym Sci, 32 (2007) 483. 5 Ding M X, Prog Polym Sci, 32 (2007) 623. 6 Yang C P, Chen Y C, Hsiao S-H, Guo W J & Wang H-M, J Polym Res, 17 (2010) 779. 7 Li Z, Liu J G, Gao Z Q, Yin Z H, Fan L & Yang S Y, Eur Polym J, 45 (2009) 1139. 8 Wang K L, Liou W T, Liaw D J & Huang S T, Polymer, 49 (2008) 1538. 9 Han X T, Tian Y, Wang L H & Xiao C F, J Appl Polym Sci, 107 (2008) 618. 10 Chen J S, Yang S Y, Fan L, Li Z X, Zuo H J & Tao ZQ, Acta Polymerica Sinica, 1 (2007) 15. 11 Yang C P, Su Y Y & Wu K L, J Polym Res, 12 (2005) 257. 12 Behniafar H & Boland P, J Polym Res, 17 (2010) 511. 13 Shah S, Tian R, Shi Z & Liao Y, J Appl Polym Sci, 112 (2009) 2953. 14 Köytepe S, Paşahan A, Ekinci E, Alıcı B & Seçkin T, J Polym Res, 15 (2008) 249. 15 Hamciuc E, Cazacu M & Okrasa L, Polym Bull, 59 (2008) 825. 16 Shin M H, Huang J W, Huang M C, Kang C C, Chen W C & Yeh M Y, Polym Bull, 60 (2008) 597. 17 Shin G J, Jung J C, Chi J H, Oh T H & Kim J B, J Polym Sci Part A: Polym Chem, 45 (2007) 776. 18 Xu S G, Yang M J & Cao S K, Polymer, 48 (2007) 2241. 19 Zhang Q Y, Chen G & Zhang S B, Polymer, 48 (2007) 2250. 20 Han S S, Im S S, Won J C, Lee J H, Choi K Y & Kim Y S, Eur Polym J, 43 (2007) 1541. 21 Kim Y H, Kim H S & Kwon S K, Macromolecules, 38 (2005) 7950. 22 Chen H & Yin J, Polym Bull, 49 (2003) 313. 23 Chern Y T, Twu J T & Chen J C, Polymer, 47 (2006) 7021. 24 Yang C P, Hsiao S H, Tsai C Y & Liou G S, J Polym Sci Part A: Polym Chem, 42 (2004) 2416. 25 Chen Y Y, Yang C P & Hsiao S H, Eur Polym J, 42 (2006) 1705. 26 Jang W B, Lee H S, Lee S, Choi S, Shin D & Han H, Mater Chem Phys, 104 (2007) 342. 27 Choi M C, Wakita J J, Ha C S & Ando S, Macromolecules, 42 (2009) 5112. 28 Terui Y & Ando S J, Polym Sci Part B: Polym Phys, 42 (2004) 2354. 29 Ding M X, Polyimide Chemistry, Relationship of Structure Property and Materials (Scientific Publishing House, Beijing) 2006.