Gas permeation of polymer blends. III. Poly(vinyl chloride) (PVC)/chlorinated polyethylene (CPE)

The permeability P, diffusivity D, and activation energy for diffusion, E_D, of He, O₂, N₂, and CO₂ were determined for blends of PVC/chlorinated polyethylene (CPE), where the chlorine content of the CPE components varied: 36 wt-% for CPE-1, 42 wt-% for CPE-2, and 48 wt-% for CPE-3. The difference in thermal expansion coefficients Δα above and below the glass transition temperature T_g of the polymers and the fractional free volume V_g of the polymers at their T_g were determined. Density and crystallinity measurements for the blends were also carried out as in the earlier work (Shur and Rånby, J. Appl. Polym. Sci., 19 , 1337 (1975)). Dynamic mechanical measurements of the blends were made using a torsion pendulum at about 1 Hz. P and D decreased, but E_D increased with increasing CI content of CPE in the blends. P and D for the blends showed no additivity. The permeability indicated phase inversion for blend compositions at about 10% of CPE-1 and CPD-2 by weight. The experimental and the calculated densities were largely the same for PVC/CPE-1 blends; but for PVC/CPE-2 and PVC/CPE-3 blends, the experimental values were higher than the calculated ones. The Δα and V_g values for PVC and the three CPE samples decreased with increasing CI content in the polymers. Dynamic mechanical measurements indicate that PVC/CPE-1 and PVC/CPE-2 blends form largely incompatible blends, while PVC/CPE-3 blends are compatible to some extent. There is some weak interaction between PVC and CPE-3 giving a low level of compatibility. The solubility of gases obtained from time-lag measurements of diffusion for 50/50 blends decreased for He, O₂, and N₂, but increased for CO₂ with increasing Cl content in CPE. The solubility of He, O₂ and N₂ shows a positive correlation with the Lennard-Jones force constant ?/k. However, a deviation from the linear relation between ?/k and In S was observed for CO₂ and the deviation became larger with increasing Cl content in CPE. The abnormally high solubility of CO₂ is probably due to the high polarizability of this gas. The heat of solution ΔH_s indicates that for He the sorption process may be a molecular slip process (endothermic), but for other gases the sorption may proceed by a dissolution process (exothermic). There is a large difference between the calculated solubility for the blends assuming incompatibility and the experimental values from time-lag measurements. This may partly be due to the uncertainty of sorption values obtained from the time-lag method and/or partly to changes of sorption modes by interaction between PVC and CPE in the blends. The resulting transport behavior of the blends is discussed on the basis of the free volume concept and of phase–phase interaction in the blends.