Shao-Long Li

Chitin Whiskers: An Overview

Chitin is the second most abundant semicrystalline polysaccharide. Like cellulose, the amorphous domains of chitin can also be removed under certain conditions such as acidolysis to give rise to crystallites in nanoscale, which are the so-called chitin nanocrystals or chitin whiskers (CHWs). CHW together with other organic nanoparticles such as cellulose whisker (CW) and starch nanocrystal show many advantages over traditional inorganic nanoparticles such as easy availability, nontoxicity, biodegradability, low density, and easy modification. They have been widely used as substitutes for inorganic nanoparticles in reinforcing polymer nanocomposites. The research and development of CHW related areas are much slower than those of CW. However, CHWs are still of strategic importance in the resource scarcity periods because of their abundant availability and special properties. During the past decade, increasing studies have been done on preparation of CHWs and their application in reinforcing polymer nanocomposites. Some other applications such as being used as feedstock to prepare chitosan nanoscaffolds have also been investigated. This Article is to review the recent development on CHW related studies.

Composition dependence of physical properties of biodegradable poly(ethylene succinate) urethane ionenes

To obtain an excellent comprehensive performance of poly(ethylene succinate) (PES), we have synthesized a series of poly(ethylene succinate) (PES) urethane ionenes (PESUIs) with various content of urethane ionic group by the chain extension reaction of dihydroxyl-terminated poly(ethylene succinate) and diethanolamine hydrochloride with hexamethylene diisocyanate as a chain extender, and we systematically investigated the composition dependence of the physico-chemical properties of PESUI through a series of characteristic techniques. The results of thermal and crystallization behaviors suggest that the incorporation of urethane ionic group slightly affects the glass transition temperature, melting temperature, and thermal stability, and significantly accelerates the crystallization rate of PES without changing the crystallization mechanism. The fastest crystallization rate was reached with the incorporation of 4 mol% urethane ionic groups. Spherulitic morphology observation indicates that nucleation density significantly increased, while spherulitic growth rate gradually decreased with increase in urethane ionic group content. Both complex viscosity and storage modulus initially increased and then decreased with increase in urethane ionic group content, and their maximum values were observed for the sample with 4 mol% of urethane ionic group. Mechanical properties slightly varied with urethane ionic group content.

Thermal and Thermo-Oxidative Degradation of Biodegradable Poly(Ester Urethane) Containing Poly(L-Lactic Acid) and Poly(Butylene Succinate) Blocks

A novel biodegradable poly(ester urethane; PEU) was synthesized by chain extension reaction of dihydroxylated poly(L-lactic acid; PLLA) and poly(butylene succinate; PBS) using diisocyanate as a chain extender. The kinetics of thermal and thermo-oxidative degradation of PEU containing PLLA and PBS blocks were studied by thermogravimetric analysis (TGA). TGA results indicated that PEU was more stable in air than in nitrogen and went through a two-stage degradation process irrespective of the experimental atmosphere. Activation energy of each stage was calculated by means of Kissinger, Kim-Park, Friedman, Flynn-Wall-Ozawa, and Kissinger-Akahira-Sunose methods. For the first stage, the activation energy value obtained in air was slightly higher than the corresponding value obtained in nitrogen; and for the second stage, the activation energy showed a much higher value in air than in nitrogen. The Coats-Redfern method was employed to study the degradation mechanism of each stage. The results indicated that the degradation of the first stage follows the P3/4 mechanism irrespective of the experimental atmosphere; the degradation of the second stage of PEU obeys the P1 mechanism in nitrogen while P3/2 in air.