Free Volume Determination in Polymers and Polymer Blends
The free volume of a polymer, which corresponds to the unoccupied regions accessible to segmental motions, is an important parameter for understanding and predicting many of its characteristic properties. The concept of free volume is the basis for understanding the molecular mobility in polymers. A non-destructive technique that can be used to obtain free volume parameters of polymers is positron annihilation lifetime spectroscopy (PALS).
The aim of the present work was to explore the possibilities and limitations in using PALS in free volume studies of polymers and polymer blends. Chemical and physical influences on the measured free volume parameters were studied in homopolymers, copolymers and polymer blends. The possibility of extracting free volume hole size distributions from positron lifetime spectra was investigated.
An inter-laboratory comparison of positron and positronium lifetimes and intensities in polymers measured with PALS showed that the ortho-positronium lifetimes and intensities can be reproduced well under a large variety of experimental conditions. These two parameters essentially contain the free volume information. It is shown that free volume hole size distributions can be extracted from positron lifetime spectra by applying the maximum entropy principle. Experimentally obtained hole size distributions were compared with theoretically calculated distributions from molecular simulations. The theoretically obtained distributions contain a considerable fraction of small holes not accessible to ortho-positronium. The measured free volume parameters can be affected by the chemical constitution of the polymer. Copolymers of styrene with maleic anhydride and acrylonitrile were found to inhibit positronium formation. The inhibition is strongly dependent on the amount of the polar constituents that are part of the polymer main chain. Large variations in average free volume hole size and hole size distributions were found in blends of poly(ethylene oxide) and poly(methyl methacrylate) as a function of composition. Moreover, the hole size was found to be time and temperature-dependent, which behavior is attributed to phase separation and crystallization in the blends. Investigations of miscible and immiscible blends of styrene-maleic anhydride and styrene-acrylonitrile show that positron lifetime measurements are promising for miscibility studies of polymer blends.