Relationship between rheological properties and structure of poly(vinyl chloride)

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Abstract
The viscosity and extrudate swell as functions of shear rate and temperature as well as flow profiles and complex shear modulus were measured on the melt of three suspension poly(vinyl chlorides) of different molecular weights. For all the specimens the primary particles, about 1 μ in diameter, had been previously destroyed by suitable milling. Below a distinct critical temperature the samples show a rheological behavior similar to that of slightly crosslinked amorphous polymers. Above this critical temperature the rheological properties are comparable to those of melts of amorphous polymers. The unusual rheological behavior of poly(vinyl chloride) is explained by the assumption of a network structure formed by crystallites in the melt. Because of the gradual melting of the crystallites the network becomes ineffective above temperatures of about 200°C. This temperature increases with increasing molecular weight. The assumption of crystallites acting as crosslinks in the melt is experimentally confirmed by the reversibility of the measured rheological effects, the wide melting range determined by differential scanning calorimetry, and electron micrographs which show particles of about 0.02 μm in size.