Coil and melt compressibility of polymer blends studied by SANS and pVT experiments

The average radius of gyration R_g for the isotopic blend deuteropolystyrene/polystyrene (d-PS/PS) and for the blends deuteriopolystyrene/poly(vinyl methyl ether) (d-PS/PVME) and deuteropolystyrene/poly(phenylmethylsiloxane) (d-PS/PPMS) has been measured by SANS experiments as a function of temperature (T) and pressure (p) up to 1.2 kbar. Furthermore, the specific volume as a function of p and T has been measured. The resulting compressibilities β_v = -(l/V)(∂V/∂ p) and the compressibility of the radius of gyration, β_R = -(3/R_g)(∂R/∂p), were compared. For d-PS/PS the following were found: (i) For T > T_g (T_g = glass temperature) R_g changes with pressure less than the macroscopic sample dimensions (β_R < β_v), whereas β_R decreases with increasing temperature, (ii) At Tg geometrical affinity is approached (βR ≈ βv). (iii) Below T_g there is still an increase of the ratio between β_R and β_v. This implies that in the glassy regime the polymer chain seems still to be mobile on a “local” scale. Thus, in general, it is concluded that the “matrix effect” in the chain is relatively small both above and below T_g. The value of the temperature coefficient K = ∂ ln(R_g²)/∂T < 0 is consistent with detailed calculations of Yoon et al. For the other blends, where the temperatures are far above Tg, the compressibilities β_v and β_R differ strongly and the change of their values with temperature is small.