STRUCTURE AND THERMAL-PROPERTIES OF NEW COMB-LIKE POLYAMIDES - HELICAL POLY(BETA-L-ASPARTATE)S CONTAINING LINEAR ALKYL SIDE-CHAINS

The structure and thermal properties of a series of novel comblike pol yamides derived from nylon 3, poly(alpha-n-alkyl beta-L-aspartate)s (n being the number of carbons in the linear alkyl side group) with n = 6, 8, 12, 18, and 22, were investigated. Polarizing infrared and solid -state C-13 CP-MAS NMR measurements revealed that these polyamides ado pt alpha-helix-like conformations and that these structures are retain ed at high temperatures. Two first-order transitions at temperatures T -1 and T-2 separating three structurally distinct phases, namely, A, B , and C, were characterized for polymers with n greater than or equal to 12 by DSC and X-ray and electron diffraction methods. T-1 and T-2 r anged from -15 to +75 degrees C and from 50 to 129 degrees C, respecti vely, for n increasing from 12 to 22. Phase A (T < T-1) consisted of a layered structure of main chain helices with side chains crystallized in a separated hexagonal lattice. Phase B (T-1 < T < T-2) was found t o be substantially like phase A but with side chains in the molten sta te. Phase C (T > T-2) was interpreted as a uniaxial arrangement of ind ependent helices embedded in the amorphous side chain matrix. Microsco pe optical observations suggested that a cholesteric-nematic rearrange ment is probably implied in the B-C transition. Members with n = 6 and 8 displayed a peculiar behavior. The octyl derivative crystallized by annealing in a three-dimensional structure composed of 13/4 helices o f the type reported for poly(beta-L-aspartate)s bearing short side cha ins while the uncrystallized polymer was arranged as in phase B. On th e contrary, the hexyl derivative could neither crystallize nor organiz e in a layered structure. It was concluded from this study that the ti tle compounds follow closely the general pattern of behavior described for poly(gamma-n-alkyl alpha-L-glutamate)s, a family of thermotropic polypeptides that has received great attention in recent years.