Negative thermal expansion in crystalline linear polymers

Abstract

Molecular processes contributing to observed negative chain direction thermal expansion coefficients for linear, crystalline polymers are examined. To assess the importance of thermal contraction due to chain torsional motion below the lattice melting point, several models are proposed which approximate the dimensional changes resulting from such motion and enable relationships to be obtained between chain length contraction and root mean square torsional vibrational amplitude. Using these relationships it was found that reasonably small rms torsional rotation amplitudes could explain the entire observed negative chain direction lattice thermal expansion for polyethylene and for selenium and tellurium (respectively, time‐average planar zigzag and helical molecules). For a time‐average planar zigzag polydiacetylene, a model chosen to represent chain torsional motion related contraction indicates that significant torsional motion about a bond best represented as C=C is necessary in order for chain torsional motion to explain the entire observed chain direction macroscopic thermal expansion. Equilibrium point defect formation (interstitial or vacancy) not only contributes to both lattice and macroscopic expansion coefficients, but also can possibly result in large differences between these parameters. It is demonstrated for polyethylene single crystals close to their melting point that Reneker defect formation could result (depending upon the mechanism of defect equilibration and previously obtained formation parameters) in a negative chain direction macroscopic expansion coefficient which is several times larger in magnitude than the corresponding observed negative lattice expansion coefficient.