Free energy of conformational change in a single chain of polyvinylidene fluoride using molecular simulations

Polyvinylidene fluoride (PVDF) has several crystal forms of which the alpha-form is nonpolar, while the beta-form is polar and has the highest piezoelectric constant. alpha PVDF, when stretched, transforms into the beta form, which has wide applications in sensors and actuators. Steered molecular dynamics simulations are used to study the transformation of a single chain of PVDF from a trans-gauche conformation to an all trans one. The Helmholtz free energy change ( increment F) is estimated using Jarzynski's equality. The transformation starts at the chain ends followed by the transformation of the remaining chain. The free energy change for the transformation is found to be always positive, indicating that the TGTG' form has higher thermodynamic stability than the all trans form throughout the studied temperature range. With increasing temperature, free energy change for the transformation increases monotonically.