Dynamic structure factor of diblock copolymer solutions in the disordered state. 1. Far from the ordering transition

The dynamic structure factor of entangled solutions of very high molecular weight styrene-isoprene (SI) diblock copolymers in the common solvent tolue ne is studied by photon correlation spectroscopy. The intermediate scatteri ng function S(q,t) with wave vector q spanning the peak position q* of the static structure factor S(q) displays three relaxation processes in the hom ogeneous disordered state far away from the order-to-disorder (ODT) transit ion. In this regime, the systems investigated are marginally entangled. To examine the sensitivity of the shape of S(q,t) with regard to polydispersit y and composition we report experimental S(q,t) of one symmetric and two as ymmetric SI with different distributions. In all cases, chain self-diffusio n and two chain conformational motions describe the experimental S(q,t). Th e main contribution to S(q approximate to q*,t) originates from the slow (r eptation-like) chain motion with rate Gamma(1) = O(tau(1)(-1)) where the lo ngest relaxation time sl is obtained by dynamic mechanical shear measuremen ts. The unexpected findings relate to the stronger contribution of fast (Ro use-like) to the S(q approximate to q*, t) of the asymmetric SI rich in I ( f(s) = 0.26) and the increase of the S(q similar or equal to 0) due to poly dispersity with concentration in the homogeneous state.