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Tate, Thomas John (1984) Piezo-optical study of the behaviour of short chain liquid polymers. Doctoral thesis, City of London Polytechnic.
The molecules of short chain liquid polymers exist in a dynamic mixture of conformations. Intermolecular interactions are important, rendering statistical mechanical models inappropriate: instead, free volume concepts are used.
Evidence of an ordered liquid state in the n-alkanes is disputed, and high-pressure Raman spectroscopy has shown the shorter homologues to become more globular with increasing pressure.
Brillouin laser light scattering is a powerful probe of molecular dynamics, and has been shown to be applicable to liquid polymers at high pressures. The present work has been concerned with relating molecular parameters, obtained by light scattering, to bulk properties of short chain homologous liquid series. To avoid thermal effects, pressure has been used to obtain pure volume-changes. Brillouin scattering, refractive index, density and viscosity data have been obtained at high pressure (up to 240 MPa) for members of four related homologous series, including some n-alkanes. Refractive index and density-cell piston movement were measured interferometrically; viscosity by falling ball and falling slug methods.
The refractive index, and density, data, both fit cubic polynomials in pressure, and the Lorentz-Lorenz equation applies with constant molar refractivity. Viscosity is exponentially, and hypersonic phonon velocity linearly, dependent on density, though Schaaff's equation is incorrect. A relaxation of 10(to the power of -11) was observed for the polyisobutenes.
In all cases, the important parameter is the density. A model of specific volume comprising contributions from chain segment- and chain end- volumes is proposed. This shows that packing is principally achieved by loss of free volume associated with chain ends for n-alkanes and polyisobutenes, and with chain segments for polyethylene glycols and polypropylene glycols, consistent with the freezing behaviour. The n-alkane data may be interpreted as showing some degree of molecular ordering above 150 MPa, analogous to the thermal observations of a mesomorphic liquid state.
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