E. Bianchi

Chitosan : chain rigidity and mesophase formation

Abstract Light-scattering measurements on samples of chitosan with degrees of acetylation (d.a.) of 42 and 15% have been performed in 0.1 m acetic acid-0.2 m sodium chloride. From the values of R G °, a persistence length q t ° of ∼ 220 A has been deduced, which is lower than that (∼ 350 A) of chitin and indicates a decrease of rigidity with decrease in d.a. The existence of a cholesteric mesophase above a critical value of [polymer] has been demonstrated.

Free radical grafting onto cellulose in homogeneous conditions 1. Modified cellulose-acrylonitrile system

Abstract Synthesis of cellulose–polyacrylonitrile copolymers has been studied in a homogeneous solution of N,N-dimethylacetamide/LiCl. The method is based on the preliminary reaction of a portion of OH cellulosic groups with acryloyl chloride to give cellulose with a certain number of pendant double bonds. Successively, acrylonitrile is grafted onto the unsaturated groups by free radical polymerization using azobisisobutyronitrile as initiator. The optimum grafting conditions are evaluated by changing reaction temperature, as well as monomer/initiator and monomer/unsaturated group molar ratios. The products are characterized by size exclusion chromatography, i.r. and 13C n.m.r. spectroscopy and a possible reaction mechanism is deduced.

Cellulose methacrylate: synthesis and liquid crystalline behaviour of solutions and gels

Abstract A new derivative of cellulose was synthesized by reaction of the polymer dissolved in dimethylacetamide (DMAc)/5% LiCl and methacryloyl chloride. The control of the molar ratio between the reagents and the temperature permits one to obtain a specific degree of substitution (DS). Cellulose methacrylate (CEMA) is a semirigid polymer with a persistence length which decreases as the DS increases. Consequently, the polymer concentration C ′ p at which the anisotropic phase becomes stable gets higher as the DS increases. CEMA solutions gellify when irradiated by UV, due to the crosslinking reaction of lateral double bonds. Swelling in DMAc/5% LiCl and water depends on the CEMA concentration and irradiation time. The first appearance of optical anisotropy may be observed at a polymer critical concentration which is always lower than the C ′ p of the uncrosslinked polymer.

On the synthesis of poly(p-benzamide) block copolymers by the phosphorylation reaction

The multistep method suggested in the literature for the synthesis of poly(p-benzamide) (PBA) block copolymers has been critically re-examined. The reaction products after each step have been isolated and characterized, and the extent of competitive processes evaluated. Under the chosen experimental conditions, almost no PBA copolymer is formed in the presence of terephthalic acid and p-aminobenzhydrazide. The increase of the limiting viscosity number has been attributed to coupling reactions between PBA chains by the difunctional reagents used in steps 2 and 3.

Optically anisotropic polymer blends: hydroxypropylcellulose/polyvinylpyrrolidone

In this paper, solid blends of HPC/PVP obtained by polymerization in situ of NVP are analyzed. These blends retain the initial organization of HPC in NVP in terms of liquid crystalline structure and banded texture. It is also demonstrated that the banded texture may be stabilized with respect to thermal cycles by crosslinking the samples.

Mesophase formation by semirigid polymers: (poly(n-hexyl isocyanate) in 1-chloronaphthalene

A study of the persistence length q and of the critical polymer volume fraction v′2 at which the mesophase appears for poly(n-hexyl isocyanate) (PHIC) in 1-chloronaphthalene is presented. The phase diagram was determined over the temperature range 80 to 188°C, corresponding to v′2 from 0.2 to 1.0. The thermal expansion of the system is accounted for in evaluating v′2 and the axial ratio of the Kuhn segment, 2qd. As reported for PHIC in toluene, and for several cellulose derivatives, the limiting value of v′2 at large molecular weight can be represented rather well by the Kuhn chain model with a temperature-dependent Kuhn segment length. However, while the thermotropic effect is adequately described by the temperature coefficient of q, the experimental v′2 is somewhat smaller than predicted.

Order Parameter in Polymer Liquid Crystal 2. Poly(γ-Benzyl-L-Glutamate) in Dioxane

Abstract We report a study of the order parameter, S, for nematic solutions of poly(γ-benzyl-L-glutamate) (PBLG) in dioxane. The homeotropic alignment evolved naturally in thin NaCl cells and was improved by a magnetic field, i.r. spectroscopy was used for determining the ratio of the band intensity in a rather wide composition range and for two different molecular weight PBLG. Bands at 3294 (amide A, N-H stretching) and at 1549 cm−1 (amide II vibration) were used for both isotropic and anisotropic solutions. Angles between the transition moment of the vibrations and the chain axis were selected from the literature. S was found to increase with both polymer concentration and molecular weight, approaching the value of 1 characteristic of perfect orientation. Comparison of the results with the predictions of virial and lattice theories indicates that the former offers the closest representation at rather low polymer volume fraction V2, while the latter is to be preferred at a somewhat larger V2.

Lyotropic block copolymers of poly(p-benzamide) and poly[2-phenylene (5-benzimidazole)]: The ternary system copolymer/poly(p-benzamide)/dimethylacetarnide—lithium chloride

New rigid-flexible copolymers based on poly( p -benzamide) (PBA) and poly[2-phenylene (5-benzimidazole)] (PABI) were synthesized, varying the molecular weight of the flexible block. Ternary solutions in dimethylacetamide—3% LiCl show the peculiar behaviour expected theoretically for this kind of system. In particular, it was demonstrated that more copolymer enters the anisotropic phase as the PABI chain length becomes shorter. Comparison with homopolymer mixtures shows the usefulness of the copolymer in improving the amount of flexible chains inside the mesophase.