Exploration of the transition temperatures and crystal structure of highly crystalline poly(1,3-cyclohexadiene): An experimental and computational investigation

Experimental determination of transition temperatures for highly crystalline polymers such as poly-1,3-cyclohexadiene (PCHD) can be difficult due to reduced solubility and thermalization processes which occur during data acquisition. In order to facilitate further understanding of these processes for PCHD, density functional theory (DFT) and molecular dynamics (MD) were used in conjunction with differential scanning calorimetry (DSC) and powder X-ray diffraction (XRD) to explore the oligomer microstructures, the crystal structure, and the temperature dependence of the specific volume (1/ρ). DFT geometry minimizations on isolated oligomers were used to identify the lowest energy confirmer; revealing that alternating R,R and S,S chiral bonds between monomer units afford the lowest energy structure. MD simulations of crystalline PCHD were constructed so as to replicate the experimental XRD pattern of crystalline PCHD, with the best fit producing a monoclinic crystal structure. The temperature dependence of the specific volume derived from MD simulations provided insight into the glass/vitrification (T_g) and melting (T_m) transition temperatures. Comparison of the simulation transition temperatures with differential scanning calorimetry data of PCHD polymerized with Ni(acac)₂/MAO shows good agreement and solidifies the fidelity of the newly defined PCHD crystalline structure.