(543h) Development of Structure-Property Relationships for Biobased Polymers Using Quantum and Molecular Mechanics Simulations

With today's growing environmental and economic concerns surrounding the use of petroleum, several biobased alternative plastics are beginning to enter consumer markets. Many of these polymeric materials have the additional benefit that the can be composted to form ecologically benign degradation products. Among these materials are the polyester classes of polylactides (PLAs) and polyhydroxy alkanoates (PHAs). While many potential energy functions, or 'force fields', have been developed extensively for other biopolymers such as proteins, there is currently no force field that has been parameterized specifically for modeling these new materials. In this work, we have developed a set of force field parameters specifically for polylactide. The force field is based on the widely-used OPLS model, but provides newly developed intramolecular (especially, bond rotational) parameters, which were fit to reproduce extensive quantum-mechanical potential energy data for solvated polymer systems at the B3LYP/6-31G** level and results from wide-angle X-ray diffraction (WAXD) studies of crystalline PLA. These simulations represent the first time that solvated quantum models specifically optimized to mimic a bulk polymer environment have been used in force field development for polymers. We also demonstrate the suitability of the optimized force field in modeling crystalline and amorphous PLA systems at the atomistic level. These molecular mechanics models examined the vibrational, diffusive and rheological behavior of PLA and are the basis for a set of structure-property relationships for PLA and other biobased polyesters.