Electronic structure basis of strength and toughness in fluoropolymers

Using density functional theory calculations, we investigate and report the electronic structure basis of symmetry-breaking in atomic structures and variations in extreme mechanical properties of a number of widely used polymers, namely, polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyethylene, and three structural variants of PVDF. All of these polymers consist of C-C bonds in the backbone and a combination of C-H and C-F bonds attached to the backbone. Our results show that the relative proportion and arrangement of the C-H and C-F bonds, hereafter called the chemical heterogeneity, lead to two distinct sets of structural symmetries and mechanical properties. The set with the lower chemical heterogeneity possesses higher structural symmetry and exhibits higher strength and toughness. Electronic population analysis shows that chemical heterogeneity breaks the structural symmetry and alters deformation induced electron redistribution patterns in the C-C backbone. Strong variations in electron redistribution serve as the basis for distinctive mechanical behavior among the polymers: the higher the structural variation and electron redistribution, the lower the bond rupture force and toughness of the polymer. The bond rupture process is initiated by a decrease in the electronic population in the backbone, while the electronic interactions at the C-H and C-F bonds dictate where, how, and which bonds rupture. Furthermore, in the linear regime of mechanical deformation, the force-strain behavior is insensitive to the chemical heterogeneity and controlled by the strong covalent interactions of the C-C bonds. In the nonlinear regime, the chemical heterogeneity dramatically reduces the strength and toughness of the polymer chain.