EFFECT OF THE ONSAGER COEFFICIENT AND INTERNAL RELAXATION MODES ON SPINODAL DECOMPOSITION IN THE HIGH-MOLECULAR ISOTOPIC BLEND POLYSTYRENE DEUTERO-POLYSTYRENE STUDIED WITH SMALL-ANGLE NEUTRON-SCATTERING

The early state of spinodal decomposition was studied by small angle n eutron scattering in the critical mixture of the isotopic blend deuter o-polystyrene/polystyrene (d-PS/PS) of equal molecular volume of 1.42 x 10(6) cm(3)/mol in a temperature range 12 K less than or equal to \T -c - T\ less than or equal to 82 K. This process can be described by t he relaxation between two static structure factors, S(Q) representing the equilibrium values of the system in the mixed state and at the tem perature where phase separation occurs. The time evolution of the rela xation process is described by the dynamical structure factor, L(Q,t) which depends on the dynamic properties of the mixture. It will be sho wn that the static structure factor of a mixed system can also be dete rmined in the unstable two-phase region during the early state of spin odal decomposition. Consistent values for the Flory-Huggins parameter were found in comparison with a lower molecular d-PS/PS sample and, th erefore, a lower critical temperature which was even smaller than the phase separation temperatures of the present system. The observed time evolution of the fluctuation modes is nonexponential. Therefore, it w as originally supposed that internal modes of the coil come into play. The analysis of the data with an ansatz by Akcasu, which takes intern al modes into account showed, however, that the phase separation in th e experimental range of wave number and time is dominated by the centr e of mass diffusion as in the C-H-C case and the nonexponential behavi or was attributed to a time dependent increase of the ''range'' of the Onsager coefficient. A range of the Onsager coefficient larger than t he radius of gyration of a single coil is predicted in case of entangl ed polymers. However, no time dependence was predicted so far. The eva luated diffusion constants follow an Arrhenius behavior and are consis tent with earlier studies. They show a D-0 proportional to N-2 scaling consistent with reptation. A further result is the observation of a s econd order peak in the structure factor already in the early times of spinodal decomposition. So far, this was only attributed to the late state of spinodal decomposition. (C) 1996 American Institute of Physic s.