Processing technologies for poly(lactic acid)

Chitin and chitosan, typical marine polysaccharides as well as abundant biomass resources, are attracting a great deal of attention because of their distinctive biological and physicochemical characteristics. To fully explore the high potential of these specialty biopolymers, basic and application researches are being made extensively. This review deals with the fundamental aspects of chitin and chitosan such as the preparation of chitin and chitosan, crystallography, extent of N-acetylation, and some properties. Recent progress of their chemistry is then discussed, focusing on elemental modification reactions including acylation, alkylation, Schiff base formation and reductive alkylation, carboxyalkylation, phthaloylation, silylation, tosylation, quaternary salt formation, and sulfation and thiolation.

Chitin and Chitosan: Functional Biopolymers from Marine Crustaceans

Cylindrical molecular brushes: Synthesis, characterization, and properties

Of late, the most bountiful natural biopolymer chitin and chitosan have become cynosure of all party because of an unusual combination of biological activities plus mechanical and physical properties. However applications of chitin are limited due to its inherent insoluble and intractable nature. Chitosan, alkaline hydrolytic derivative of chitin has better solubility profile, less crystallinity and is amenable to chemical modifications due to presence of functional groups as hydroxyl, acetamido, and amine. The chemical modification of chitosan is of interest because the modification would not change the fundamental skeleton of chitosan, would keep the original physicochemical and biochemical properties and finally would bring new or improved properties. In view of rapidly growing interest in chitosan its chemical aspects and chemical modification studies is reviewed. The several chemical modifications such as oligomerization, alkylation, acylation, quternization, hydroxyalkylation, carboxyalkylation, thiolation, sulfation, phosphorylation, enzymatic modifications and graft copolymerization along with many assorted modifications have been carried out. The chemical modification affords a wide range of derivatives with modified properties for specific end use applications in diversified areas mainly of pharmaceutical, biomedical and biotechnological fields. Assorted modifications including chitosan hybrids with sugars, cyclodextrin, dendrimers, and crown ethers have also emerged as interesting multifunctional macromolecules. The versatility in possible modifications and applications of chitosan derivatives presents a great challenge to scientific community and to industry. The successful acceptance of this challenge will change the role of chitosan from being a molecule in waiting to a lead player.

Chitosan-modifications and applications: Opportunities galore

Structure and interactions in chitosan hydrogels formed by complexation or aggregation for biomedical applications

Characterization of chemical and solid state structures of acylated chitosans

Biaxial extensional flow behavior of polystyrene and polypropylene melts was measured utilizing the lubricated squeezing flow method under the condition of constant strain rate. The uniaxial extensional flow behavior of these samples was also measured using a Meissner‐type rheometer for comparison with the biaxial behavior. The biaxial stress growth coefficient η B + grows more slowly than the uniaxial stress growth coefficient η E + at long times. Deviation of η B + from the low strain rate asymptote is not so prominent as that of η E +. The deviation of η B + and the upturn of η E + from the low strain rate asymptotes are more apparent in broad distribution samples than in a narrow distribution sample. It is found that η B + decreases with strain rate at long times near steady state. The upturn behavior of η E + at high strain rates in a narrow distribution sample is discussed based on the Doi–Edwards theory and stress relaxation data. The upturn is clearly observed when the strain rate exceeds the contraction (retraction) rate τeq −1, where the contraction time τeq is evaluated from the relaxation data. Consideration on chain stretching in various multi‐axial extensional flows gives a prediction for deviation of stress growth coefficients from the low strain rate asymptotes in uniaxial, biaxial, planar, and ellipsoidal extensional flows.

Measurement of biaxial and uniaxial extensional flow behavior of polymer melts at constant strain rates

The deformation and recovery of a poly(isobutylene) droplet with a lower viscosity embedded in a poly(dimethyl siloxane) matrix are directly observed from two directions after application of a large step shear strain. The droplet shape and recovery time strongly depended on the magnitude of the applied strain. Just after application of a large strain, a droplet deforms almost affinely to a flat ellipsoid. It then changes into a rodlike shape, a dumbbell, and to an ellipsoid of revolution, and finally to a sphere. Model calculations show that the primary driving force for the shape recovery is the interfacial energy in order to reduce the surface area of the deformed droplet.

Observation of deformation and recovery of poly(isobutylene) droplet in a poly(isobutylene)/poly(dimethyl siloxane) blend after application of step shear strain

We measured dynamic shear moduli of poly(macromonomer)s of an increased backbone chain length and those bearing branch chains of increased branch length up to the critical molecular weight for the intermolecular chain entanglement, Mc, of a linear polystyrene as functions of frequency and temperature using a parallel-plate rheometer. The poly(macromonomer)s were prepared from ω-methacryloyloxyethyl polystyrene macromonomers (MA-PSt)s or ω-vinylbenzyl polystyrene macromonomers (VB-PSt)s. It was revealed that a great increase in the backbone length caused the poly(macromonomer)s to show a weak rubbery plateau region in the master curves of the storage modulus G‘, where the molecular weights of the branch chains were smaller than Mc. The plateau modulus was much lower than that observed for the linear polystyrenes. On the other hand, the poly(macromonomer)s with polystyrene branches are nearly equal to the Mc, and a very short backbone chain showed a clear rubbery plateau region. The glass transition tempera...

Masaoki Takahashi

Papers

Characterization of chemical and solid state structures of acylated chitosans

Measurement of biaxial and uniaxial extensional flow behavior of polymer melts at constant strain rates

Observation of deformation and recovery of poly(isobutylene) droplet in a poly(isobutylene)/poly(dimethyl siloxane) blend after application of step shear strain

Bulk Properties of Poly(macromonomer)s of Increased Backbone and Branch Lengths

Surface curvatures of trabecular bone microarchitecture.