A MODEL FOR ANTIPLASTICIZATION IN POLYSTYRENE

Antiplasticization can occur when small quantities of a known ''plasti cizer'' have been blended into a glassy polymer. Commonly, the T-g of the polymer and its free volume decrease. However, the mechanical prop erties of the antiplasticized polymer are altered significantly, causi ng the polymer to become stiffer and more brittle. Experimental result s from flexural tests of polystyrene/mineral oil blends conducted at r oom temperature showed that antiplasticization is molecular weight dep endent, thus supporting a hypothesis that the phenomenon can be attrib uted to a chain-end effect. A high molecular weight polystyrene (M(W) = 270 000 D) exhibited plasticization only, whereas a low molecular we ight (M(W) = 40 000 D) exhibited both antiplasticization and plasticiz ation effects. The 40 000 MW sample showed a 2-fold increase in flexur al moduli and flexural strengths as mineral oil concentration increase d up to 6 vol %. These moduli and strengths decreased rapidly at highe r concentrations of mineral oil. Positron annihilation spectroscopy (P AS) data showed a 10% decrease in fractional free volume up to 6% mine ral oil. C-13 NMR experiments showed that there was no change in the p olymer backbone dynamics during antiplasticization. H-1 NMR Goldman-Sh en experiments showed that antiplasticization occurs when the average diameter of the mineral oil domains is less than the average size of t he free volume voids. It was determined that one mineral oil molecule was associated with a polystyrene chain end during antiplasticization. These results are consistent with the hypothesis that antiplasticizat ion is due to a decrease in fractional free volume at the chain ends.