Solid state NMR and wide angle X-ray diffraction studies of supercritical fluid CO2-treated poly(ethylene terephthalate)

Poly(ethylene terephthalate) (PET) was treated with supercritical fluid (SC F) carbon dioxide to study crystallization of this polymer under SCF condit ions. Solid state NMR spectroscopy and wide angle X-ray diffraction; (WAXD) detected dramatic changes in the PET molecular structure and motion after treatment. To fit WAXD data, the powder diffraction pattern for crystalline PET was calculated and is reported here for the first time. The WAXD resul ts indicate an increase in crystallinity from essentially zero in the as-re ceived sample to 62% in the SCF-treated sample. This large increase in the crystallinity of the SCF-treated PET was verified by the NMR relaxation mea surements. The X-ray crystallite size, obtained from WAXD, was compared wit h those obtained from NMR proton spin diffusion measurements. The C-13 sign als for aliphatic carbons (centered at 61.5 ppm) in the PET crystalline dom ain were resolved for the first time in a C-13 cross polarization/magic ang le spinning spectrum (CP/MAS) due to the SCF treatment. The C-13 chemical s hift tensors for PET were determined experimentally and were also compared to theoretical ab initio calculations. The anisotropic chemical shift data were then interpreted in terms of changes in the molecular conformation in PET as a result of SCF CO2 treatment. It was found that the SCF CO2 treatme nt is an effective method for enhancing the crystallinity of PET. These res ults in PET strongly support the previously proposed model for the existenc e of three motional regimes; crystalline, rigid amorphous, and mobile amorp hous.