SMALL-ANGLE NEUTRON-SCATTERING OF BLENDS OF CROSS-LINKED AND LINEAR POLYSTYRENE

Small-angle neutron scattering (SANS) has been used to study the scattering function and thermodynamics of blends of linear protonated polystyrene (PSH) and cross-linked deuterated polystyrene (PSD). Two series of samples were synthesized. In both cases the samples were made by dissolving the linear PSH in deuterated (d8) styrene monomer containing a small amount of divinylbenzene, which was then polymerized to form the PSD network around the linear PSH chains. The samples were all made at a concentration of 50/50 by weight PSD/PSH. A special effort was made to keep the samples single phase so that SANS could be used to study the thermodynamics of the single-phase system and compare it with theory. This entailed working at relatively low cross-link densities (< 1 mol % cross-link units). Series 1 is a set of samples with the same cross-link density varying the length of the linear chain. Series 2 is a set of samples containing the same length linear chain varying the cross-link density systematically. By extrapolation of S(q) obtained from SANS using an Ornstein-Zernike plot (S(q)-1 versus q2) to q = 0, the zero-angle scattering, S(0), was obtained. S(0) is inversely proportional to the second derivative of the free energy with respect to composition, partial 2 (DELTA-f/kT)/ partial-phi-2. With the assumption of additivity of the free energies of mixing and elasticity, the portion of the zero-angle scattering due to elasticity is calculated from the scattering. The measured zero-angle scattering can be compared to the scattering calculated from the classical free energy of a swollen network. For the series 1 and 2 samples it was found that the samples could be driven to phase separation by increasing either the length of the linear chain or the network density. In the series 1 samples the zero-angle scattering scaled as S(0)-1 approximately 1/N(b). In the series 2 samples the zero-angle scattering scaled as S(0)-1 approximately 1/N(c). In addition, the scattering was found to increase as N(c) decreased for the series 2 samples, a behavior opposite to that predicted by combining classical rubber elasticity and mixing theories.