C-13 AND H-1 AMORPHOUS LINEWIDTHS IN SEMICRYSTALLINE POLY(4-HYDROXYBUTYRATE) AND POLY(3-CO-4-HYDROXYBUTYRATE) ABOVE T-G BY HIGH-RESOLUTION AND SOLID-STATE NMR EXPERIMENTS

The high-resolution C-13 and H-1 nuclear magnetic resonance (NMR) line widths of semicrystalline poly(4-hydroxybutyrate), P4HB, and poly(3-hy droxybutyrate-co-4-hydroxybutyrate), (P3/4HB-18, 18% 4HB units) in the amorphous phase and in the melt are studied as a function of temperat ure and magnetic field strength. Measurements of the C-13 spin-spin re laxation times under the same experimental conditions show that the na tural linewidth is a minor contributor to the line-broadening observed in the C-13 spectra of the solid polymers. A variety of coherent aver aging solid-state NMR methods are used to examine possible contributio ns from various line-broadening mechanisms. It is shown that magnetic susceptibility and chemical shift dispersion are the major factors for the broadening of the proton and carbon resonances of P4HB in the amo rphous phase and the melt, respectively. Incomplete motional narrowing due to a slow motional mode restricted in amplitude by the presence o f crystallites and/or chain constraints was found to be the major line -broadening factor for P3/4HB-18 in the amorphous phase. Correlations between crystalline morphology, physical and mechanical properties, an d polymer chain dynamics are discussed, along with the way these facto rs affect the NMR linewidth data presented. (C) 1996 John Wiley & Sons , Inc.