How and Why Polymer Glasses Lose Their Ductility Due to Plasticizers

Tensile extension tests have been carried out to examine mechanical responses of polymer glasses under the influence of small molecule additives that plasticize the polymers by lowering their glass transition temperatures T g and speeding up their α relaxation times. It is shown that incorporation of 4% triphenyl phosphate (TPP) into poly(methyl methacrylate) turns a brittle PMMA at 70 °C into a ductile glass, capable of significant cold drawing, as expected according to the conventional wisdom. However, despite the evidence that the same TPP also reduces polystyrene’s T g and increases the segmental mobility, the ductile PS turns brittle at 70 °C upon addition of 4% TPP at the same low drawing speed. This counterintuitive behavior is also found in the pair of TPP and poly(styrene–acrylonitrile). Thus, the present observations require us to incorporate new ingredients into the classical Eyring-type interpretation of plastic flow in a glassy polymer.