Solid-State NMR Analysis of Chain Packing and Dynamics in Polycarbonates

Washington University, Saint Louis MO

Investigators

Abstract

Rotational-echo double resonance (REDOR) solid-state NMR will be used to determine the local packing of chains in polycarbonates typical of those currently used in engineering applications. Ten polycarbonates are on hand with differently positioned 2H, 13C, and 19F labels. REDOR will be used to measure specific heteronuclear distances in a variety of homogeneous blends of these labeled polycarbonates. These REDOR experiments will allow the rings and sidechains of one polycarbonate to be located relative to the rings, isopropylidenes, and carbonates of its neighbors. Preliminary REDOR experiments have confirmed the feasibility of this approach for the determination of local order in polycarbonate chain packing. Centerband-only detection of exchange (CODEX) will be used to characterize the molecular details of motion in the mechanically important 1-100 Hz frequency regime at 300 K for the same polycarbonates examined by REDOR. In the standard CODEX experiment, spin coherence is created by rotor-synchronized pulses acting on a 13C chemical shift tensor. If the coherence is instead created by pulses acting on a heteronuclear dipolar tensor, then the results of REDOR and CODEX experiments are directly correlated. Intellectual Merit. Rotational-echo double resonance (REDOR) measures accurately heteronuclear distances between dipolar-coupled spin pairs under magic-angle spinning conditions. If two miscible polymers are specifically labeled with different stable isotopes (2H and 13C, for example), then 13C{2H} REDOR can be used to determine interchain distances and orientation, which define local chain packing. The blending is done with the observed nucleus at high concentration and the dephasing nucleus at low concentration so that the REDOR results are interpreted in terms of simple sums of pair-wise couplings. Distributions of distances are required but not specific packing models. The mechanically important slow motions in a polycarbonate glass are identified as belonging to a specific type of packing arrangement using a combination of REDOR and CODEX in the same experiment. This new approach will be an important step in reaching the long-term goal of connecting the molecular properties of glassy polymers with their macroscopic behavior. Broader Impact. High-Tg amorphous polyesters containing azobenzene sidechains are being examined in several materials laboratories for use in (eraseable) holographic optical data storage. These polyesters have dichroic (anisotropic absorption) and birefringent (anisotropic refraction) properties when illuminated by linearly polarized light. The ease of ordering of sidechains and the observation of biaxial birefringence suggest that prior local order in mainchain packing is important. Polycarbonates are good candidates for data-storage thin films. They are mechanically tough, have high Tg's, can be synthesized with photosensitive sidechains, and show local order in mainchain packing. The characterization of their packing and dynamics by REDOR and CODEX should help in the development of new data-storage materials. Polymers and materials science are good topics to use in reaching out to inform the community about modern technology. Polymers are intuitive and do not require the background necessary to understand structural biology, for example. The PI is an active participant in a formal education training program for St. Louis K-8 science teachers and plans to continue this effort for the next four years. He applies the basic ideas of mass spectrometry and nuclear magnetic resonance to familiar objects like plastics (especially impact-resistant polycarbonates!). Materials science is also attractive to undergraduates and graduates. The PI will include undergraduates in the proposed polymer work, and will train both graduate students and postdoctoral students using polycarbonate-NMR problems to provide a rich mix of instrumentation, theory, and experience in practical applications of modern solid-state NMR.