S. Vogt et al., DYNAMICS OF COMPOSITION FLUCTUATIONS IN DIBLOCK COPOLYMER MELTS ABOVETHE ORDERING TRANSITION, Macromolecules, 27(15), 1994, pp. 4335-4343
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.