Molecular stiffness and thermal properties of polymers

Because it merely reflects rotational hindrance, the parameter σ alone does not convey a measure of chain stiffness. Stiffness can arise from short links joined by rigid joints, or from long, stiff links with flexible joints. Flexibility results from the absence of rigid joints and stiff, long links. The slope of the product σΔT versus TR indicates stiffness; the higher the slope, the stiffer the chain. The polymers whose thermal data are presented in this paper are divisible into two major groups. Those whose d(σΔT)/dTR = 1.37 are flexible, and those whose d(σΔT)/dTR = 3.44 are stiff. The indication is that there exists a direct relationship between the polymer expansivity above Tg and the stiffness parameter of the chain.