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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 T g data, is relatively weak in all of the blends. However, the fractional free volume quantity measured by PALS (τ 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.

SYNOPSISA free volume probing technique, positron annihilation lifetime spectroscopy (PALS) is used to characterize miscible blends of amorphous polycarbonate with a semicrystalline copolyester at room temperature. The paper describes the analysis required to use the PALS technique to probe the amorphous regions in which the miscibility of the blends occurs. Analysis of the raw PALS data in this system involves taking account of the influence of crystallinity on the PALS results. The results demonstrate that the PALS technique is able to discriminate between free volume in the amorphous regions of as-molded and annealed samples. This difference in free volume between samples of differing thermal history is assigned to the influence of polymer crystals on the stress state of the miscible amorphous regions.