Solid-State 13C NMR Study of Thermotropic Poly(bibenzoates). 2. Poly(triethylene glycol p,p'-bibenzoate)

Abstract

Several samples of thermotropic poly(triethylene glycol p,p'-bibenzoate) (PTEB) with different thermal histories have been analyzed by DSC, X-ray scattering, and solid-state 13 C and 1 H NMR. This polymer can be quenched into the smectic LC state and becomes semicrystalline (0.39 crystalline fraction) upon annealing for 12 days at 85 °C (sample 85A). Also, annealing for 33 months at ambient temperatures (sample RTA1) yields a 31%-crystalline sample, owing to a T g of 20°C. By 13 C NMR, the spectra of the crystalline fraction of these two semicrystalline samples are isolated. These two spectra, in the aromatic region, are very similar ; however, in the aliphatic region there are significant differences which probably originate from conformational differences within the spacer. DSC data show a significant contrast in crystalline melting points for the RTA1 and 85A samples. Small-angle synchrotron and NMR spin diffusion data, combined with crystallinity values, yield a picture of relatively thin crystallites, about 3 repeat units thick in the 85A sample and about 2 units thick in the RTA1 sample so there is some question as to whether the crystallites are thick enough to express their undistorted crystalline habit in the isolated NMR spectra and whether the melting point difference can be attributed to different crystal thicknesses, as opposed to polymorphism. Synchrotron data taken in the region of the smectic layer spacing show that layer spacings persist in all samples, quenched or annealed. Yet there are definite changes in these layer spacings which accompany crystallization. In view of the thin nature of the crystallites, however, the relatively narrow width of these synchrotron peaks suggests a periodicity which must encompass both the noncrystalline (NC) and the crystalline regions. Selected NMR relaxation measurements also bear out the notion that molecular mobility within the LC regions of quenched samples and the NC regions of semicrystalline samples is similar, although not identical. Finally, the 13 C spectral observations in the region of the aliphatic carbons for the 85A sample suggest one of two possibilities : either the spacer conformation in the interior of the crystallites differs from the conformation near the crystal/NC interface or there is polymorphism. A split peak for the layer line in the synchrotron data also supports the notion of polymorphism.