A free volume approach to the mechanical behaviour of miscible polycarbonate blends

Composition-dependent mechanical properties and free volumes are compared for miscible, amorphous blends of bisphenol-A polycarbonate (PC) with (a) polyaryloxysiloxane (PAS), (b) a copolyester of 1,4-cyclohexanedimethanol and a mixture of isophthalic and terephthalic acids (EASTAR) and (c) an experimental polyester of 1,4-cyclohexane dicarboxylic acid and 1,4-cyclohexanedimethanol (CDACD). The free volumes were measured by the positron annihilation lifetime spectroscopy (PALS) technique. The strength of specific interactions, as indicated by Tg data, is relatively weak in all of the blends. However, the fractional free volume quantity measured by PALS (tau(3)(3)I(3)) is less than additive in the polyester blends and is additive, or greater than additive, in the PC-PAS blends. The mechanical behaviour of the blends can be rationalized in terms of the free volume behaviour. The polyester blends which lose free volume (contract) on mixing exhibit higher than averaged yield strengths and brittle impact responses. The PC-PAS blends which retain or gain free volume on mixing exhibit averaged yield strengths and averaged ductile impact responses of the constituent polymers.