During uniaxial extension, a polymer glass of high molecular weight is ductile at high temperatures (still below Tg) and turns brittle when the temperature is sufficiently lowered. Incorporation of small-molecular additives to polymer glasses can speed up segmental relaxation considerably. The effect of such plasticization should be to make the polymers more ductile. We examined the effect of blending a few weight percent of either triphenyl phosphate (TPP) or a mineral oil to commercial-grade PS, PMMA and SAN. Our Instron tests showed that the plasticized PS and SAN were less ductile. Specifically, at 70 oC, the original PS is ductile at an extensional rate of 0.02 min-1 1, whereas the PS with 4 wt. % mineral oil or TPP is brittle. Similarly, at 50 oC, the original SAN is ductile at an extensional rate of 0.02 min-1, but the SAN plasticized by 4 wt. % TPP becomes brittle. No such counterintuitive effect is observed in PMMA. This finding challenges the Eyring type idea2 of activation for flow since the mechanical spectroscopic measurements show that the alpha relaxation time of PS is shortened by more than two orders of magnitude in presence of 4 wt. % TPP. On the other hand, the effect of TPP on PS and SAN can be well anticipated from the recent molecular model 3.
1. Li, X.; Wang, S.-Q., Mapping Brittle and Ductile Behaviors of Polymeric Glasses under Large Extension. ACS Macro Letters 2015, 4 (10), 1110-1113.
2. Eyring, H., Viscosity, plasticity, and diffusion as examples of absolute reaction rates. The Journal of chemical physics 1936, 4 (4), 283-291.
3. Wang, S.-Q.; Cheng, S.; Lin, P.; Li, X., A phenomenological molecular model for yielding and brittle-ductile transition of polymer glasses. The Journal of chemical physics 2014, 141 (9), 094905.