DYNAMICS OF COMPOSITION FLUCTUATIONS IN DIBLOCK COPOLYMER MELTS ABOVETHE ORDERING TRANSITION

The synthesis of poly(ethylmethylsiloxane-block-dimethylsiloxane), EM, copolymers with two blocks that possess similar segmental mobilities, low glass transition temperatures, negligibly small optical anisotrop ies, but sufficiently different refractive indices was performed in or der to study the dynamics of composition fluctuations in disordered di block copolymer melts. Photon correlation spectroscopy was employed to measure the composition correlation functions C(q,t) for six EM sampl es with total degree of polymerization N between 240 and 1200 at diffe rent wave vectors q and temperatures. For all samples but the one with the lowest N, C(q,t) was found to display two distinct relaxation pro cesses with characteristic pertinent features, in addition to the long range density fluctuations. For the fast relaxation mode, the q-indep endent relaxation decay rate GAMMA1 varies with N-3, whereas the N-dep endent amplitude S1(q) exhibits a q2 dependence. In the low q limit, t he predictions of the random phase approximation for the internal rela xation of the copolymer chain conform well to these experimental findi ngs and, moreover, account almost quantitatively for the values of GAM MA1, and S1(q) of the symmetric EM's. The second relaxation process ha s a diffusive (q2-dependent rate) character with q-independent amplitu de S2(q) and its diffusion coefficient D, agrees well with the self di ffusion of the copolymer chain either derived by shear viscosity data or measured by pulsed-field gradient NMR. This second process can be a ttributed to additional concentration fluctuations due to the inherent composition polydispersity, and, hence, S2(q) is a measure of its deg ree, whereas D is the mutual diffusion coefficient. In the EM samples, the latter is predicted and found to be very similar to the self-diff usion coefficient.