Solid State NMR Detection of Molecular-Level Mixing Phenomena in Strongly Interacting Polymer Blends and Phase-Separated Copolymers

Abstract

This research contribution addresses mixing phenomena in polymer blends that exhibit strong intermolecular association. Molecular-level observations of specific interactions between dissimilar blend components have been obtained from high-resolution solid state proton and carbon-13 NMR experiments at ambient temperature. Results illustrate mixing effects on the isotropic chemical shifts of the critical component in a completely or partially phase-mixed blend. Perturbations in the NMR spectra result from conformational changes, hydrogen bonding, molecular complexation, or altered packing geometries that occur concomitantly with the mixing process. More convincing evidence that two components of a strongly interacting blend reside in a near-neighbor environment is obtained from the measurement of proton spin diffusion between dissimilar species. Proton spin diffusion is measured directly via the high-resolution CRAMPS experiment (Combined Rotation and Multiple Pulse Spectroscopy) in a molecular complex of poly(ethylene oxide) and resorcinol, and indirectly via the carbon-13 spin system in a modified version of the Goldman-Shen experiment for microphase-separated copolymers. One of the primary objectives of this research endeavor is to bridge the gap between macroscopic and molecular-level probes of phase behavior and intermolecular association in mixtures that form molecular complexes. Hence, in addition to the molecular-level solid state NMR results, implications from a macroscopic phase of the polymer-blend research are discussed. In summary, we have measured the thermal and mechanical properties of strongly interacting systems, whose interaction sites have been characterized via solid state NMR. The success of this proposed structure-property relationship scheme, which bridges microscopic fundamentals (via NMR) with practical macroscopic (DSC and stress-strain) measurements, depends on our ability to understand material properties at a level where continuum hypotheses are no longer valid.