Melt - Processable Polypeptide / Ionomer Molecular Composites

Introduction. Since Helminiak et al.' and Takayanagi2 first proposed the concept of a molecular composite, considerable remarch has been directed a t developing these materiale.3 Conceptually, molecular compitea are mixtures of a rigid rod polymer homogeneously dispersed in a flexible coil polymer matrix, thus providing fiber reinforcement at a molecular level. In practice, however, miscible blends are not obtained due to the tendency for rigid rod polymers to pack excluding the flexible coil molecule^.^ The majority of the published research involved attempts to prepare molecular composites from dilute ternary solutions by rapid coagulation or coprecipitation to overcome the tendency of phase separation. This method has serious limitations as a viable fabrication technique because of the large quantities of harsh solvents required and the fact that the materials phase separate when heated or if melt processed. An alternative approach for making thermally stable, melt-processable molecular compositea is by block copolymerization of rigid rod and flexible coil segments.&' The covalent bonds between blocks prevent macroscopic phase separation and intimately connect the reinforcement and matrix. This paper reports the preparation of melt-processable molecular composites of a rigid rod polymer, poly(benzy1 L-glutamate) (PBLG) and a flexible coil ionomer, lightly sulfonated polystyrene (SPS). Specific acid-base and/or hydrogen-bonding interactions between SPS and PBLG were sufficient to overcome the tendency for the rigid and coil polymer to phase separate, and melt-processable miscible blends were obtained. Experimental Section. Lightly sulfonated polystyrenes were prepared from a commercial polystyrene (Styron 666, Dow Chemical Co.) with M, = 106 OOO and M, = 280 000 using the procedure described by W o w s k i et al.8 Polymers with sulfonation levels of 1.9 and 5.2 mol % were prepared; hereafter these are referred to as lHSPS and 5HSPS, respectively. PBLG with M, = 20 100 was purchaaed from Sigma Chemical Co., and PBLG of M, = 3000 was synthesized by the method described by Daly and Poch6.9 These are referred to as HPBLG and LPBLG, respectively. Separate solutione of 5% (w/v) of SPS and PBLG were first prepared using a common solvent, chloroform, and the PBLG solution was then added dropwise to the SPS solution with constant stirring. Films of the blende were prepared by solution casting, followed