Synthesis and self-assembly in bulk of linear and Mikto-arm star block copolymers based on polystyrene and poly(glutamic acid)

Both AB linear-type and AB(2) miktoarm star-type amphiphilic block copolymers based on polystyrene (PS) as A block and poly(glutamic acid) (PGA) as B block were obtained by a four-step synthetic approach combining (i) atom transfer radical polymerization of styrene and (ii) chemical modification of the bromo end groups of ATRP-derived precursors into many or twice as many primary amino groups, followed by (iii) ring-opening polymerization of gamma-benzyl-L-glutamate N-carboxyanhydride and (iv) a final step of hydrolysis. The self-assembly properties in bulk of these linear PS-b-PGA and miktoarm star PS-b-(PGA)(2) block copolymers were subsequently investigated by different analytical means, including Fourier transform infrared spectroscopy (FTIR), wide and small-angle X-ray scattering (WAXS and SAXS) and atomic force microscopy (AFM). FTIR analysis revealed that the PGA block systematically adopted a rodlike alpha-helix conformation, even for degree of polymerization as low as 18. The very high immiscibility between PS and PGA blocks (conformational asymmetry) drove all of these rod-coil block copolymers to self-assemble in a hexagonal in lamellar (HQ morphology in the nanometer size range, whatever their composition and architecture. In addition, WAXS and SAXS results evidenced the effect of the macromolecular architecture on the local organization of PGA helices: the linear block copolymers showed an internal lamellar structure with PGA helices stacked, inter-digitated or folded depending on the weight fraction of the rodlike PGA blocks, whereas miktoarm stars exhibited a stacked microstructure, independently of the PGA content.