EVOLUTION OF STRUCTURE IN A GRAFT COPOLYMER HOMOPOLYMER BLEND UNDER STRAIN

Two-dimensional light scattering and digital imaging studies of the st ructural evolution in a graft copolymer-homopolymer blend under quasi- static elongation are reported. The graft copolymer consists of a poly disperse elastomeric poly(ethyl acrylate) (PEA) backbone onto which mo nodisperse thermoplastic polystyrene (PS) chains are grafted. The homo polymer poly(ethyl acrylate) is produced in situ since grafts are not incorporated into every chain. The 'quasi-equilibrium'' structure of t he graft copolymer blend cast from a good solvent exhibits isotropic s cattering (i.e., appearance of a spinodal ring) with an inverse charac teristic length q(m) almost-equal-to 4 mum-1. When the blend is subjec t to a quasi-static elongation ratio lambda (=final length/initial len gth), three regimes are observed: (i) A transition regime at very low elongation ratios where the system behaves reversibly. (ii) A hyperela stic regime where the spinodal ring deforms in an anisotropic manner, i.e., ellipsoid-shaped scattering elongated in the direction perpendic ular to the applied strain. The intensity I(parallel-to) (q(z)) parall el to the elongation increases in an exponential manner due to coopera tive alignment of the hard PS-rich phase, and its peak position shifts in an affine manner to smaller values (i.e., q(zm) approximately lamb da-1). Moreover, the scattered light perpendicular to the elongation, I(perpendicular-to) (q(y)), decreases in intensity as its peak positio n q(ym) diverges to infinity. (iii) A plastic regime where I(parallel- to)(q(z)) remains nearly constant as its position q(zm) converges to a finite value at q(zm) almost-equal-to 0.9 mum-1. In the three regimes , the structure factor parallel to the elongation, S(parallel-to)(q(z) ), remains self-similar. The underlying mechanism of the deformation w ill be discussed within the framework of concentration fluctuations in ''soft'' elastic two-component solids.