R. B. Blumstein

Differential scanning calorimetry investigation of phase transitions in water/chitosan systems

Transition temperatures of water/chitosan systems with water contents ranging from 8 to 300% (weight per cent of water per dry polymer weight) are measured by differential scanning calorimetry (d.s.c.). Upon heating, three kinds of phases, which vary with the water content, are observed for the system: cold crystallization, melting of freezable water, and a birefringent to isotropic phase which occurs for samples containing 44–190% water with a transition temperature that does not vary significantly with the water content. The glass transition temperature (Tg) of chitosan is observed at approximately 303 K for water contents ranging from 8 to 30%. From the d.s.c. data, a transition map for the water/chitosan system is compiled.

Phase behavior study of chitosan/polyamide blends

Blends of chitosan with strongly crystalline polyamides (nylon-4 and nylon-6) and weakly crystalline polyamides (caprolactam/laurolactam and Zytel®) were investigated. Phase behavior, morphology, interactions with water, mechanical properties, and catalytic reactivity were studied. Films were made from formic acid solutions with the chitosan concentrations ranging from 5% to 95% (w/w). The 80% deacetylated chitosan is in the salt, neutral, or copper chelate form. All the blends have higher relative water contents than does the pure chitosan. Dry neutral chitosan shows a relaxation centered at approximately 90°C which is attributed to local motion. The phase behavior of the blends is influenced by preparation conditions such as the drying temperature. Characterization of blends by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) suggests partial miscibility of chitosan with nylon-4 and lack of miscibility in the remaining cases. Blending with nylon-4 enhances mechanical properties with marked antiplasticization in blends containing 90% chitosan. Catalytic activity of the chitosan is enhanced by blending with nylon-4. Salt and neutral forms of chitosan appear to be equally effective.

Synthesis and properties of some polyesters with mesogenic groups and flexible spacers in the main chain

Abstract Polymers containing, in the main chain, various mesogenic moieties separated by flexible spacers have been prepared by condensation of the corresponding diphenol or diol with an aliphatic diacid chloride. The polymers have been characterized by X-ray, d.s.c. and polarizing microscopy. The diacids used were sebacic (SEB), succinic (SUC) and 3-methyladipic (MAA). The biphenols were 4-hydroxyl- N -benzylidene-4′-hydroxyaniline(1), 4.4′-biphenol(2), 4,4′-bis (2-hydroxyethoxy)-biphenyl(5) and 3,3′-dimethyl-4,4′-di(2-hydroxyethoxy)-stilbene(7). The polymers SEB-1 and SEB-2 were classified as giving thermotropic lamellar mesophases; SEB-5, SEB-7 and SUC-5 as potentially nematic; MAA-2 and SUC-2 gave crystalline polymers.

Influence of Molecular Weight on Phase Transition Entropies of a Thermotropic Nematic Polyester

Abstract We report the values of isotropisation enthalpies and entropies of some model compounds, oligomers, and polymers of poly(2,2′-methyl-4,4′-oxyazoxybenzene dodecanedioyl), a thermo-tropic nematic polymer with moderately low transition temperatures and regularly alternating rigid-flexible units in the main chain. The values of ΔHIN and ΔIN increase rapidly with increasing molecular weight, to reach a plateau at M n > 3,000 g/mole. Results are interpreted in terms of increasing participation of the flexible aliphatic spacer units in the ordering process of the nematic phase. In the polymer, the entire repeating unit is aligned by the nematic potential.

Inherently flexible thermotropic main chain polymeric liquid crystals

Abstract Phase behavior (isotropic—nematic transition, disorder in the crystal) and order of chains in the mesophase (orientational and conformational) are investigated in the homologous series of polymers and corresponding Siamese twin models. Results from the following experiments are presented: DSC (normal and high pressure), polarizing microscopy, broad line NMR (PMR and DMR), small angle X-ray and neutron scattering, induced magnetic birefringence and melt rheology. Influence of chain length, spacer length and parity, temperature and concentration in nematic solutions on nature and level of order is discussed. Molecular segregation by chain length in the nematic—isotropic biphase is considered in relation to its impact on morphology of the isotropic, nematic and solid phases. This series provides a model system for characterization of inherently flexible main chain PLCs formed by alternating mesogens and spacers. Such PLCs are easily processable materials with potentially high levels of micro and mac...

Small Angle Neutron Scattering from Liquid Crystal-Line Main Chain Polyesters

Abstract Small angle scattering studies were carried out on a spacer-mesogen polyester family. A strong anisotropy was observed in the nematic phase. Studies of the anisotropy in isotropic-nematic biphasic, nematic, and nematic vitrous state and the temperature variation in melts and in dilute solutions in a low molecular weight liquid crystal are presented here. The role of spacer parity is emphasized.

The Existence of Two Distinct Levels of Order in Thermotropic Nematic Polyesters

Abstract It is shown by means of x-ray diffraction and PMR broad line spectroscopy, that the structure of the flexible spacer drastically influences the order within the nematic phase of thermotropic polyesters based on the 4,4′ - dioxy- 2, 2′-dimethyl azoxybenzene moiety. The existence of two distinct molecular arrangements is proposed: one is characterized by an unusually high degree alignment of macromolecules, the other by a lower degree of alignment. The former is displayed by spacers with an even number of methylene units in the alkanedioic acid moiety of the spacer (odd number of bonds); the latter by a certain number of flexible spacer moieties with an odd number of methylene groups (even number of bonds), as well as in mixed spacers irregularly alternating along the macromolecular chain (copolyesters).

Synthesis of Model Compounds of The Nematic Polyester Poly(2,2′-Dimethyl-4,4′-Dioxyazoxy-Benzenedodecanedioyl)

Abstract We describe synthesis and phase characterization of model compounds of the thermotropic nematic polymer poly(2,2′-dimethyl-4,4′-dioxyazoxybenzene dodecanedioyl), which consists of regularly alternating rigid mesogenic and flexible aliphatic moieties. Models were composed of the following sequences: flexible-rigid-flexible; rigid-flexible-rigid (9-DDA-9); rigid-flexible; flexible-rigid-flexible-rigid-flexible. Only 9-DDA-9 displays a mesophase (monotropic nematic). The appearance of an enantiotropic nematic phase is associated with the development of a polymeric structure (approximately six repeating units are required).

NMR study of segregation by molecular mass in a polydisperse nematic polymer

Abstract Using proton and deuterium NMR we show that in polydisperse nematic polyesters with mesogenic units in the main chain: (i) in the nematicisotropic biphase, the nematic phase is enriched with the longest species and (ii) in the nematic phase itself there is additional segregation by molecular mass into nematic domains.

Model Compounds of Main-Chain Thermotropic Nematic Polyesters

Abstract Investigation of precursors of thermotropic nematic polyesters with flexible spacers ((CH2)10 and (CH2)7) and mesogens in the main chain is reported. The mesogens are Mixtures of structural isomers of 2,2′-dimethylazoxybenzene derivatives display broad multiple melting. Several types of sequencing of mesogen and spacer were investigated. Compounds formed by the sequence mesogen-central spacer-mesogen appear to provide an adequate model of polymeric behavior. Influence of mesogen and spacer on phase transition entropies and mesogen nematic order parameter, as well as temperature dependence of alkyl chain flexibility were investigated in selected compounds. These can also serve as models for study of mesophase glasses and the semicrystalline state of nematic polymers.