Two-Stage Melting in Dilute Gels of Poly(γ-benzyl l-glutamate)

Abstract

Two-stage melting of the dilute gels of poly(γ-benzyl l-glutamate) in benzene has been studied by differential scanning calorimetry, wide-angle X-ray diffraction, and small-angle X-ray scattering using synchrotron radiation. Differential scanning calorimetry measurements show that the traces on heating exhibit two endothermic peaks at about 25 °C and at about 31 °C, thereby indicating the melting of two types of aggregates, while those on cooling exhibit no exotherm even at 0 °C, thereby indicating a large thermal hysteresis. Wide-angle X-ray diffraction measurements show that the profiles yield little of interest other than diffuse halos from the solvent except a diffuse small-angle scattering, thereby indicating that the aggregates do not contain a solid crystalline phase but a crystal−solvate phase. Small-angle X-ray scattering measurements show that the two-stage melting of the aggregates can be attributed to a melting first of the bundles of three rods and then of the bundles of two rods and that, above the two-stage melting temperature, the gels transform into the dilute and isotropic solutions, in which the poly(γ-benzyl l-glutamate) stiff chains are randomly dispersed and do not interact with each other. We suggest that the intermolecular interactions causing the bundles to aggregate are induced by the solvent.

Elucidation of the structure of poly(γ-benzyl-l-glutamate) nanofibers and gel networks in a helicogenic solvent

“…This interface can be visualized as a sharp interface between aligned PBLG helices and open space in between the fibers. These results are in line with the interpretation of SAXS measurements on nascent gels by Izumi et al [30] who have suggested that the aggregates can be approximated as isolated assemblies of parallel rods. Quantitative information about this system was gained using a Teubner-Strey fitting [36, 37].…”

“…As pointed out by Shukla et al [6], bundles of four helices with approximately 2–2.5-nm thickness are formed as primary aggregates, while Korenaga et al [28] suggested three helices in an aggregate. These bundles then may undergo a further level of aggregation forming nanofibers as already proposed by Izumi et al [30], resulting in relatively uniform thickness of the nanofibers. In the case the concentration is low, the nanofibers form spherical particles with tentacles (Fig.…”

“…An alternative gelation mechanism of PBLG solutions is based on “nucleation and growth” of a crystalline phase [4, 15, 29]. Measurements of enthalpic endotherms during gel melting support the existence of crystals in the gel structure [4, 30]. Cohen and coworkers [31] propose the basis for the formation of gel microfibrils to be the formation of crystal solvate phases.…”

Fibrillar gels via the self-assembly of poly(l-glutamate)-based statistical copolymers

“…17 None of the aforementioned gels result from the selfassembly of PLGA α-helices. Unlike PLGA, PBLG α-helices can assemble in a head-to-tail and side-by-side fashion 28,43,44 and generate long range order in helicogenic organic solvents. At high concentrations (over 10 wt%), PBLG self-assembly yields lyotropic phases.…”

“…45,46 Although contrasting mechanisms have been proposed for the gelation of PBLG, including (i) phase separation by spinodal decomposition, (ii) nucleation growth, and (iii) a combination of both mechanisms, the result is the formation of a percolated network of fibers. 44,47,48 The stability of PBLG helices and fibers is often attributed to a combination of the following features: intra-molecular hydrogen bonding (especially in aprotic solvents), 11 dipole-dipole interaction of the PBLG helices (especially in non-polar solvents), 43,49,50 and π-π stacking of the outward-pointing pendant benzyl groups. 51 Such organogels present attractive features, including a robust fibrous network and a high porosity, 46 which would be valuable in the context of biomedical hydrogels.…”