The solid-state structure of comblike poly(R-n-alkyl γ-glutamate)s derived from bacterial poly(γ-glutamic acid) (PAAG-n, n being the number of carbons contained in the linear alkyl side chain) was investigated for even values of n between 12 and 22. Two series, one with a D:L enantiomeric ratio of approximately 9:1 and the other with a nearly racemic composition, were examined. The thermal transitions and the structures involved therein were characterized by DSC, polarizing infrared, X-ray and electron diffraction, and 13 C CP/MAS NMR. In all cases and within the whole examined range of temperature, the polypeptide main chain was arranged in an R-helix-like conformation. A first-order transition occurring at a temperature T1 between 20 and 80 °C and interconverting two well-defined phases, A and B, was characterized for PAAG-n with n g 14. Phase A is a biphasic structure consisting of layers of polypeptide helices separated by a paraffinic pool of alkyl side chains. The layer periodicity was found to increase steadily from 2.7 to 3.7 nm according to a structure in which the alkyl side groups are partially crystallized, oriented nearly normal to the layers, and extensively interdigitized. Phase B retains the layered structure present in phase A but with side chains being in a disordered state. A second first-order transition, leading to a third phase C at a temperature T 2 near 100 °C, was characterized for PAAG-n with n g 18. Phase C is thought to be a nematic structure consisting of a quasi-hexagonal packing of helical polypeptide rods lacking axial register. Although no clear differences in the structure of any of the three phases were detected between the two investigated series, the response to the thermal treatment was significantly affected by the stereochemistry of the PAAG-n. Crystallization from the melt was favored in the DL series, the dimensional changes involved in the A-B transition showed opposite sign for D and DL polymers, and the B-C transition was much more noticeable in the DL series.
Two series of comblike poly(R-n-alkyl γ-glutamate)s (PAAG-n, n being the number of carbons contained in the linear alkyl side chain), one with a D:L enantiomeric ratio approximately 9:1 and the other with a nearly racemic composition, were prepared and characterized for even values of n ranging from 12 to 22. Esterification of bacterial poly(γ-glutamic acid) to poly(R-ethyl γ-glutamate) followed by transesterification of the latter with long linear 1-alkanols afforded pure PAAG-n in almost full conversions and high yields. Thermal decomposition temperatures of PAAG-n were above 300 °C, and they increased slightly with the length of the alkyl side group. 1 H NMR studies in chloroform solution revealed the presence of regularly folded conformations that were disrupted upon addition of strong acids. In the solid state, PAAG-n with n g 14 displayed melting of the alkyl side chains at temperatures increasing from 20 up to 80 °C for increasing values of n. Preliminary X-ray diffraction showed that PAAG-n adopt the typical layered structure of comblike polypeptides with alkyl groups crystallized in a paraffinic interlayer phase. The layer periodicity was found to steadily increase from 2.7 to 3.7 nm in agreement with a structure made of polypeptide chains in helical conformation with the alkyl side groups oriented nearly normal to the layers and extensively interdigitized. No significant differences either in structure or in thermal behavior were observed between the two investigated series.
Comb‐like poly(β‐aspartate)s that adopt layered arrangements with well‐defined phase transitions are revealed here. Outstanding thermochromic effects are displayed by these materials under polarized light, with a rapid, fully reversible color response that is highly sensitive to temperature (see Figure) and that can be adjusted by copolymerization.
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