Anindita Ghosh

Solubility improvements in aromatic polyimides by macromolecular engineering

Polyimides are proved to be an important class of polymers due to their outstanding set of physical and chemical properties. However, this class of polymers suffers from poor processability, which limits their scope of application. This review article deals with approaches that have been undertaken by different researchers to improve the processability of this class of polymers. Mostly, these are synthetic approaches using new diamine and dianhydride monomers that reduce the interchain interaction of the final poyimide by flexibilizing the polymer chain or increasing the fraction free volume. Accordingly, the structural factors that are responsible for better processability are discussed and representative diamine and diahydride structures are tabulated under different categories. Major efforts towards development of soluble polyimides but maintaining excellent mechanical and thermal properties have been done by our group and are also covered in this article.

New Fluorinated Poly(imide siloxane) Random and Block Copolymers with Variation of Siloxane Loading

Several new random and block copoly(imide siloxane)s have been prepared by the solution polycondensation of commercially available 4,4′-oxydianiline (ODA) and amino-propyl terminated polydimethylsiloxane (APPS) with 4,4′-(hexafluoro-isopropylidene)diphthalic anhydride (6FDA). The siloxane loading was kept to 10, 20, 30, 40 and 50 wt% in the copolymers. The random copolymers were prepared by a one pot solution imidization technique, and two pot solution imidization technique was adopted for the synthesis of the block copolymers. The diamine ODA and the dianhydride 6FDA composed the hard block segment, while APPS and 6FDA composed the soft block segment. The hard block length was kept constant while the soft block lengths were varied by varying the siloxane loading. Accordingly, block copoly(imide siloxane)s were prepared on increasing the soft block lengths (DP) from 3 to 6, 10, 18 and 36 for fixed hard block length of 22. The resulting polymers have been well characterized by IR, NMR and GPC techniques. Thermal and mechanical properties of the random and block copolymers were compared with the already reported homopolyimide without siloxane moiety.

Sulfonated fluorinated-aromatic polymers as proton exchange membranes

Abstract In recent years, extensive research on the preparation and properties of proton exchange membranes (PEMs) has been realized. This article focusses on the recent studies on new PEM materials based on aromatic hydrocarbon polymers with sulfonated groups as hydrophilic domains and fluorinated groups as hydrophobic domains as alternatives to conventional perfluorinated polymers. It is necessary to improve the proton conductivity especially under low-humidity conditions and at high operating temperatures to break through the current aromatic PEM system. Hence, there is a need to develop new high-conductivity fuel cell ionomers with improved thermal, chemical, and electrochemical stability by designing a suitable polymer structure for PEM application.

Synthesis, Characterization and Comparison of Properties of Novel Fluorinated Polyimides Derived from Bisphenol-A-di(phthaleic anhydride)

Four new poly(etherimide)s have been synthesized by reaction with commercially available bisphenol‐A‐(diphthaleic anhydride) (BPADA) with four different kinds of diamines, namely 4,4′‐bis(p‐aminophenoxy‐3,3″‐trifluoromethyl) terphenyl,4,4′‐bis(3″‐trifluoromethyl‐p‐aminobiphenyl ether)biphenyl,2,6‐bis(3′‐trifluoromethyl‐p‐aminobiphenyl ether)pyridine, 2,5‐bis(3′‐trifluoromethyl‐p‐aminobiphenylether)thiopene. The poly(etherimide)s are named as 1a, 1b, 1c and 1d, respectively. The synthesized polyimides show good solubility in various organic solvents. The polyimide films had low water absorption of 0.19–0.30% and low dielectric constant of 2.79–3.1 at 1 MHz. These polyimides showed very high thermal stability with decomposition temperature (5% wt loss) up to 522°C in nitrogen. Transparent thin films of these polyimides exhibited tensile strength up to 97 MPa, a modulus of elasticity up to 1.56 GPa and elongation at break up to 20%.

Synthesis and Characterization of Novel Poly(Arylene Ether)s from 4,4'-Thiodiphenol

Four novel perfluoroalkylated poly(arylene ether)s have been synthesized successfully using four perfluoroalkyl‐activated bisfluoro monomers. These polymers are synthesized through nucleophilic displacement of the fluorine atoms on the benzene ring with 4,4′‐thiodiphenol and are named as 1a, 1b, 1c and 1d, respectively. The polymers obtained by displacement of the fluorine atoms exhibit weight‐average molar masses up to 3.9×104 g · mol−1 in Gel permeation chromatography. These poly(arylene ether)s showed very high thermal stability up to 548°C for 10% weight loss in TGA under nitrogen and high glass transition temperature (Tg) up to 178°C in DSC depending on the repeat unit structures. The glass transition temperatures taken as peak in tan δ in DMA measurements are in good agreement with the DSC Tg values. All the polymers synthesized are soluble in a wide range of organic solvent such as CHCl3, CHCl2, THF, NMP, DMF and toluene. Transparent thin films of these polymers cast from THF exhibited tensile strengths up to 72 MPa, modulus up to 1.69 GPa with low elongation at break depending on their exact repeating unit structures. Rheological properties showed ease of processability of these polymers with no change in melt viscosity with temperature.

New Silicone Grafted Copoly(ether imide) from 4,4′-(hexafluoro-isopropylidene)diphthalic Anhydride

A new copoly(ether imide) 1a was synthesized from 4,4′-(hexafluoro-isopropylidene)diphthalic anhydride (6FDA) and 4,4′- bis [3′-trifluoromethyl-p-aminobiphenyl ether)biphenyl (TFBB) with 20 wt% loading of 3,5-diaminobenzoate terminated polydimethylsiloxane (DBPDMS) by solution imidization technique. The copoly(ether imide) showed a glass transition temperature of 272°C by differential scanning calorimetry and of 300°C by dynamic mechanical analysis, respectively. The polymer showed thermal stability up to 371°C for 5% wt loss in synthetic air. Dynamic mechanical analysis showed storage modulus as high as 1.62 GPa at 40°C. In the DMA plot a distinct sub Tg relaxation was observed at 130°C. The copolyimide formed tough transparent film, with tensile strength up to 47 MPa, a modulus of elasticity up to 1.83 GPa and elongation at break up to 65%. Thermal, mechanical and surface properties of the copoly(ether imide) have been evaluated and compared with the already reported analogous random copoly(ether imide) containing siloxane unit in the main chain and with the homopolyimide without siloxane unit.

Synthesis, Characterization and Properties of New Fluorinated Poly(imide siloxane) Co-polymers from 4,4′-(Hexafluoro-isopropylidene)diphthalic Anhydride

Several new poly(imide siloxane)s co-polymers have been prepared by the reaction of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) with commercially available 4,4′-oxydianiline (ODA) and with five different novel trifluoromethyl-substituted diamines, each with 20 wt% aminopropyl-terminated polydimethylsiloxane (APPS). The poly(imide siloxane)s are well characterized by different spectroscopic, thermal, mechanical and electrical techniques. The synthesized polymers exhibit good solubility in different organic solvents. The 1H-NMR indicates that the siloxane incorporation is about 17–19% for polymers 1a–f. These poly(imide siloxane) films show low water absorption rate (0.88–0.09%) and a low dielectric constant (2.43–2.58) at 1 MHz. The polymers show very good thermal stability, even up to 419°C for 5% weight loss in synthetic air and a glass transition temperature of up to 230°C. All poly(imide siloxane)s formed tough transparent films, with a tensile strength of up to 79 MPa, a modulus of elasticity of up to 1.38 GPa and elongation-at-break of up to 30%. Thermal, mechanical and dielectric properties of these polymers have been evaluated and compared with their non-siloxane analogues.

Linear and Hyperbranched Poly(arylene ether)s from a New Semifluorinated AB Monomer

A new trifluoromethyl-activated AB monomer has been successfully synthesized by Pd-initiated coupling of 4-bromo anisole with 4-fluoro-3-trifluoromethylphenylboronic acid followed by demethylation. The monomer leads to a semifluorinated poly(arylene ether) by nucleophilic displacement polymerization reaction. The AB monomer has been further copolymerized with a corresponding AB 2 monomer to form the corresponding semifluorinated hyperbranched (hb) poly(arylene ether). The resulting linear and hb poly(arylene ether)s exhibited weight average molecular weight of 75700 and 144100 g/mol, respectively. The hb copolymer exhibited better solubility in different organic solvents compared to the linear poly(arylene ether). The polymers showed excellent thermal stability up to 522°C at 10% wt loss in air and glass transition temperatures as high as 187°C. The mechanical properties of the linear poly(arylene ether) film 1a exhibited tensile strength at break of 89 MPa, elongation at break of up to 3% and a Young’s modulus value of 2.66 GPa. The films of the polymers were hydrophobic in nature and showed water contact angle as high as 93.6°.

Fluorinated Polyimides: Synthesis, Properties, and Applications

Abstract Aromatic polyimides are a class of high-performance polymers. They have been of great technological importance because of their outstanding thermal, mechanical, and electrical properties. However, this class of polymers is generally insoluble in organic solvents and has extremely high glass transition or melting temperatures that preclude melt processing. A great deal of effort has been made to improve the processing characteristics of these intractable polyimides. One successful approach is the incorporation of bulky trifluoromethyl (–CF 3 ) groups, which help increase the free volume, thereby improving several properties of polyimides such as increased solubility and lowered dielectric constant without forfeiture of thermal stability. This chapter deals first with the general synthetic procedure for aromatic polyimides, followed by tailoring of structures incorporating –CF 3 groups to improve various properties. Finally, the use of these fluorinated polyimides in different membrane-based applications such as gas separation ad pervaporation and as a proton exchange membrane in fuel cell application is discussed.

Fluorinated Poly(Arylene Ether)s: Synthesis, Properties, and Applications

Abstract Poly(arylene ether)s consisting of aromatic rings and ether linkages form an important class of high-performance polymeric material with an excellent combination of thermal and mechanical properties. Poly(ary1ene ether)s are generally synthesized via nucleophilic aromatic substitution of an activated dihalide with an aromatic bisphenol. Fluorinated poly(arylene ether)s containing bulky –CF 3 groups are especially interesting because they enhance the solubility of the polymers and increase the glass transition temperature without reducing thermal stability. The bulky –CF 3 group also serves to increase the free volume of the polymer, thereby improving various properties of the polymer including its application as a membrane material and in microelectronic applications by endowing a low dielectric constant. The first part of the chapter deals with the synthetic strategies, characterization, and general properties of fluorinated poly(arylene ether)s. The second part deals with the application of fluorinated poly(arylene ether)s in gas separation and pervaporation and as proton exchange membranes for fuel cell application.