Shintaro Sasaki

Phase behavior of poly(γ-benzyl L-glutamate) solutions in benzyl alcohol

SummaryPoly(γ-benzyl l-glutamate) (PBLG) exists in the rodlike α-helical conformation and forms a thermoreversible gel in benzyl alcohol (BA). Phase diagram of the system PBLG/BA was investigated. The gel network comprises crystalline aggregates. There are two types of aggregated structures depending on the gelation temperatures; one (A) is the so-called complex phase (volume fraction of polymer is 0.76), and the other (B) is the crystal containing few solvent molecules. Melting temperatures of B type gels (in the range of 62–70°C) are slightly higher than those of A type gels. Above the gel-sol transition temperature, the solution forms a cholesteric liquid crystal, provided the polymer concentration exceeds about 15 vol%. Mechanism of the formation of A and B was discussed in the phase diagram.The solution with concentration between 10 and 20 vol % (depending on molecular weight) contains the isotropic and anisotropic phases at equilibrium. The composition and fractionation effect within conjugate phases were investigated at 80 °C for solutions containing two PBLG samples differing in molecular weights. The biphasic region was widened by mixing the two PBLG samples, and the isotropic phase showed a tendency to exclude the high-molecular-weight fraction.

Thermotropic cholesteric mesophases in copoly(γ-n-alkyl l-glutamate)s

SummaryThermotropic cholesteric mesophases were found for three types of α-helical copoly(γ-n-alkyl L-glutamate)s; the combination of the n-alkyl groups was methyl-hexyl, methyl-octyl, and propyl-octyl. Cholesteric colors due to the selective reflection were observed in the temperature range from 110 to 190°C for the films with copolymer compositions of about 50/50 in mol%. These thermotropically mesomorphic copolymers had disordered structures in the molecular packing as inferred from the x-ray diffraction patterns. The cholesteric sense was right-handed, since the negative circular dichroism associated with the cholesteric pitch was observed for all the mesomorphic films. The temperature dependence of the pitch was discussed in comparison with that of lyotropic liquid crystals of these polymers.

Side-chain relaxation behavior of racemic mixtures of ?-helical polypeptides having phenyl groups at the end of the side chains

SummarySide-chain interactions in racemic mixtures of α-helical polypeptides were investigated by means of dielectric measurements in relation to the regular stacks of phenyl groups in the racemic mixture of poly (γ-benzyl glutamate) (PBG). The samples used in this work were poly(β-phenethyl aspartate) (PPA), poly(γ-phenethyl glutamate) (PPG), and poly[γ-(3-phenylpropyl) glutamate] (PPPG). A dielectric dispersion due to the side-chain motion was observed around room temperature for all the samples. For PPA and PPPG, the dispersion shifted to lower frequencies by racemization, and the relaxation strength decreased in a similar manner as PBG. The racemic mixture of PPPG showed a thermally reversible transition at about 80°C, which was attributed to the breakdown and the formation of stacks of phenyl groups. Racemization had no effect for PPG. The conformational change from the right-handed α-helix to the left-handed π-helix which occured at 140°C in the L isomer of PPA was not observed in the racemic mixture.

Phase behavior of the ternary system solvent: poly(γ-benzyl L-glutemate) and polystyrene

SummaryPhase equilibria were investigated at 80°C for a ternary system involving a solvent benzyl alcohol, a rodlike polymer poly(γ-benzyl L-glutamate) (PBLG), and a flexible polymer polystyrene (PS). The fairly dilute solution of the two polymers was isotropic and monophasic. At greater concentrations, it was separated into two isotropic phases. Above an even higher critical concentration of the two polymers, three phases, a cholesteric (A1) and two isotropic phases (I1 and I2), coexisted at equilibrium. PBLG was contained exclusively in the phases A1 and I1, while PS was in the phase I2. The more concentrated solution was separated into a cholesteric phase and an isotropic phase. The latter contained PS but excluded PBLG, These features were discussed on the ground of the Flory theory by considering the polymer compatibility.

Molecular conformations ofd,l-alternating polypeptides

SummaryD,L-Alternating poly(γ-benzyl glutamate) (PBDLG) and poly(γ-phenethyl glutamate) (PPDLG) were prepared by the racemization-free method proposed by Caille et al. (1) for the synthesis of PBDLG. Both polymers were fractionated into two components, (A) and (B), using solubility differences in DMF, methanol, and ethanol. Infrared and X-ray data support the α-helical molecular conformation for the (A) films obtained from chloroform solutions, as was reported by Heitz et al. (2) for PBDLG. The molecular conformation of PPDLG-(B) is essentially the same as that of PBDLG-(B), which was assigned to the antiparallel-chain polar pleated-sheet structure by Lotz et al. (3). The ω-helical conformation was found for PBDLG by exposing the (A) film to the vapor from DMF with a small amount of water, but not for PPDLG.

Molecular aggregation of poly(γ-phenacyl L-glutamate) in N,N-dimethylformamide

SummaryGel formation and the melting of solutions of poly (γ-phenacyl L-glutamate) in DMP were investigated up to 40 wt% of polymer concentration. Two thermally reversible transitions were observed around 24°C and 55°C. The transition temperatures were almost independent of the concentration. The transition at 24°C was considered to be due to the collapse of the order in side-chain structures. The transition from gel into Isotropie or cholesteric liquid-crystal phase occured in the 50–60°C range, above which the gel rigidity and an X-ray reflection of about 40 A suggesting a complex phase disappeared.