Structure–property relationships of low dielectric constant, nanoporous, thermally stable polyimides via grafting of poly(propylene glycol) oligomers Shahram Mehdipour-Ataei * and Samaneh Saidi Polyurethane and Advanced Polymeric Materials, Iran Polymer and Petrochemical Institute, P.O. Box 14965/115, Tehran, Iran Received 26 September 2007; Revised 26 November 2007; Accepted 3 December 2007 Synthesis of high temperature polyimide foams with pore sizes in the nanometer range was developed. Foams were prepared by casting graft copolymers comprising a thermally stable block as the matrix and a thermally labile material as the dispersed phase. The copolyimides as the matrix material were prepared via polycondensation reactions of pyromellitic dianhydride with three new diamines (4BAP, 3BAP, and BAN) through the poly(amic acid) precursors. Functionalized poly(pro- pylene glycol) (PPGBr-1000 and PPGBr-2500) as the labile oligomer was prepared via reaction of poly(propylene glycol) monobutyl ether with 2-bromoacetyl bromide. Graft copolymers were prepared by the reaction of the poly(amic acid)s with these thermally labile constituents. Upon thermal treatment the labile blocks were subsequently removed leaving pores with the size and shape of the original copolymer morphology. The polyimides and foamed polyimides were charac- terized by some conventional methods including FTIR, H-NMR, DSC, TGA, SEM, TEM, and dielectric constant. The average pore size of the polyimide nanofoams was in the range of 5–20 nm. The structure–property relationships of the prepared nanofoams were investigated based on the diamine structures and also molecular weights of labile groups. Copyright # 2008 John Wiley & Sons, Ltd. KEYWORDS: nanofoams; polyimide; phase separation; graft copolymers INTRODUCTION The need for high temperature materials with low dielectric and thermal insulating behavior for industrial applications has increased in recent years. 1 As a class of materials, polyimides have best satisfied the material requirements for these applications because they exhibit favorable balance of physical and chemical properties and show excellent thermal, mechanical, and electrical properties. 2–10 However, their dielectric constants are not low enough to meet the specifications of intermetal dielectric layers. The most common approach in modifying the dielectric properties of polyimides has been focused on the incorporation of perfluoroalkyl groups; examples include hexafluoroisopro- pylidine linkages and pendent trifluoromethyl groups. 11–13 The methodology for developing highly fluorinated poly- imides can be limited to a certain extent by synthetic difficulties associated with the fluorine-containing comono- mers. An alternative approach to lower polymer dielectric constant is to introduce nanoscopic porosity into the polymer film. 14,15 The reduction in the dielectric constant could be achieved by replacing the polymer having a dielectric constant 3.0 with air which has a dielectric constant of 1, while the desired thermal and mechanical properties were maintained. The size of the voids must be smaller than either the film thickness or any microelectronic features, i.e. much less than 1 mm. These nanoporous (nanofoam) polymers can be prepared from block or graft copolymers comprising a thermally stable and thermally labile materials, where the latter constitutes the dispersed phase. 16,17 Using a specified heat treatment, the thermally unstable block decomposes, leaving a porous structure of a size that is commensurate with the copolymer morphology, i.e. on the tens of nano- meter size scale. The graft copolymer approach, alternative to block copolymer, where the thermally labile groups are attached to a polyimide backbone as a side chain, may be considered for improving the mechanical properties of the final nanofoam; also in this system there is no limitation in growing of the molecular weight of polyimide backbone. Here we describe preparation of benzophenone- based polyimide nanofoams grafted with poly(propylene glycol) and investigation of their physical and thermal properties. POLYMERS FOR ADVANCED TECHNOLOGIES Polym. Adv. Technol. 2008; 19: 889–894 Published online 10 March 2008 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/pat.1055 *Correspondence to: S. Mehdipour-Ataei, Iran Polymer and Pet- rochemical Institute, P.O. Box 14965/115, Tehran, Iran. E-mail: s.mehdipour@ippi.ac.ir Copyright # 2008 John Wiley & Sons, Ltd.