(692c) DFT Study of the Effects of Polymer Solvation on the Rotational Modes of (L)-Lactic Acid Oligomers

An important trend in the development of new polymeric materials has been a movement toward polymers that are derived from renewable resources. One of the first bioderived polymers to be economically viable as a commodity polymer is poly(lactic acid), also called PLA or polylactide. This polymer is readily synthesized from low-cost renewable feedstocks, such as corn, and has received widespread attention because its physical properties resemble those of several widely used petroleum-derived polymers.

In polymeric materials, most physical and transport properties of interest are strongly influenced by the underlying rotational energy landscape of the chain molecules. Because molecular motion occurs primarily by the rotation of backbone bonds, a knowledge of the potential energy barriers to such rotation is indispensible for an understanding of the overall dynamics. This information has wide applicability in modeling material properties, ranging from simple rotational isomeric states (RIS) calculations to complex molecular mechanics simulations. Hence, we have decided to employ the methods of quantum chemistry to establish the valuable relationship between energy and conformation.

In the present study, computations were performed on model lactic acid oligomers, with particular attention given to the variation of molecular properties with rotational degrees of freedom. Density functional theory computations at the 6-31G**/B3LYP level were performed on several oligomeric surrogates of PLA, both in vacuo and within several different simulated solvent mediums. In all cases, the potential energy and free energy landscapes were charted, and electrostatic charge densities were also estimated. This information is important when considering the conformational behavior of PLA, and in the development of predictive models for the polymer, including the development of improved force-field parameters for molecular mechanics simulation.

The rotational energy landscape was traversed by starting with 144 unique conformations that were derived via the rotation of a single conformer in 30 degree increments. Each of these were subject to unconstrained energy minimization at the 6-31G**/B3LYP level. The 144 initial configurations settled each into one of seven unique minima, and from these minima sequential grid searches were performed in two dimensions. A diagonal square grid was used with a spacing of 5(°î2)¢ª on the hypotenuse, over the full rotational range in the (¥õ, ¥÷) parameter space. Upon completion of these sequences, a representative minimum-energy surface was constructed by selecting the lowest energy at each grid point from the seven data sets. Several potential energy surfaces were obtained in the (¥õ, ¥÷) space, including one in vacuo and one under simulated ¥è-conditions. A survey of local minima was performed on each surface, and the thermodynamic stability was estimated for each local basin using the eigenmode analysis implemented in Jaguar. Each local minimum was refined in an unconstrained minimization at the B3LYP/6-31G** level using tighter convergence criteria before calculation of the normal modes. In this case, the reaction-field method was used during the optimization of the condensed-phase minima. The entropy and free energy were then calculated using the harmonic approximation for each potential energy basin.