SOLID-STATE C-13 NMR-STUDY OF THERMOTROPIC POLY(BIBENZOATES) .2. POLY(TRIETHYLENE GLYCOL P,P'-BIBENZOATE)

Several samples of thermotropic poly(triethylene glycol p,p'-bibenzoat e) (PTEB) with different thermal histories have been analyzed by DSC, X-ray scattering, and solid-state C-13 and H-1 NMR. This polymer can b e quenched into the smectic LC state and becomes semicrystalline (appr oximate to 0.39 crystalline fraction) upon annealing for 12 days at 85 degrees C (sample 85A). Also, annealing for 33 months at ambient temp eratures (sample RTA1) yields a approximate to 31%-crystalline sample, owing to a T-g of 20 degrees C. By C-13 NMR, the spectra of the cryst alline fraction of these two semicrystalline samples are isolated. The se 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 thi n 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 whet her the crystallites are thick enough to express their undistorted cry stalline habit in the isolated NMR spectra and whether the melting poi nt difference can be attributed to different crystal thicknesses, as o pposed to polymorphism. Synchrotron data taken in the region of the sm ectic layer spacing show that layer spacings persist in all samples, q uenched or annealed. Yet there are definite changes in these layer spa cings which accompany crystallization. In view of the thin nature of t he crystallites, however, the relatively narrow width of these synchro tron peaks suggests a;periodicity which must encompass both the noncry stalline (NC) and the crystalline regions. Selected NMR relaxation mea surements also bear out the notion that molecular mobility within the LC regions of quenched samples and the NC regions of semicrystalline s amples is similar, although not identical. Finally, the C-13 spectral observations in the region of the aliphatic carbons for the 85A sample suggest one of two possibilities: either the spacer conformation in t he interior of the crystallites differs from the conformation near the crystal/NC interface or there is polymorphism. A split peak for the l ayer line in the synchrotron data also supports the notion of polymorp hism.