Why Do Polymeric Glass-Forming Liquids Tend to Have a Relatively High Segmental Fragility?

Recently, there has been great interest in the prediction of the glass transition temperature T_g and fragility m_s of polymer materials in connection with the need for utilizing these material parameters to guide materials design. One of the least understood properties of polymeric liquids in comparison to atomic and small molecule liquids is their often relatively high m_s. To theoretically address this problem, we utilize the generalized entropy theory (GET), a synthesis of the lattice cluster theory of polymer thermodynamics with the Adam−Gibbs model relating the structural relaxation time to the fluid configurational entropy density S_c to estimate the variation of T_g and m_s of polymer materials having variable chain monomer structure, stiffness, cohesive interaction strength and molecular mass (M). Validating molecular dynamics simulations of a coarse-grained polymer melt were performed for these and other molecular variables. An examination of GET calculations of T_g and m_s performed over many years revealed that these fundamental parameters characterizing respectively the location and relative breadth of the glass transition, respectively, are largely determined by the magnitude of the configurational entropy S_c of fluids in their high temperature athermal state S_c^* where relaxation is nearly Arrhenius. We conclude based in the GET framework that the often relatively high fragility, and the associated relatively large extent of cooperative motion in polymer materials, which can be precisely quantified in molecular dynamics simulation, derive from the often relatively large "packing frustration" in this class of glass-forming liquids, defined quantitively in terms of the dimensionless thermal expansion coefficient and isothermal compressibility. Apart from the practical matter of understanding trends in T_g and m_s with varying M, molecular topology, chain stiffness and cohesive interaction strength for various practical applications, the GET offers a new theoretical perspective for understanding variations in T_g and m_s in glass-forming liquids broadly.