Acetyl tri-n-butyl citrate (ATBC) and poly-(ethyleneglycol)s (PEGs) with different molecular weights (from 400 to 10000) were used in this study to plasticize poly(L-lactic acid) (PLA). The thermal and mechanical properties of the plasticized polymer are reported. Both ATBC and PEG are effective in lowering the glass transition (T g ) of PLA up to a given concentration, where the plasticizer reaches its solubility limit in the polymer (50 wt % in the case of ATBC; 15-30 wt %, depending on molecular weight, in the case of PEG). The range of applicability of PEGs as PLA plasticizers is given in terms of PEG molecular weight and concentration. The mechanical properties of plasticized PLA change with increasing plasticizer concentration. In all PLA/plasticizer systems investigated, when the blend T g approaches room temperature, a stepwise change in the mechanical properties of the system is observed. The elongation at break drastically increases, whereas tensile strength and modulus decrease. This behavior occurs at a plasticizer concentration that depends on the T g -depressing efficiency of the plasticizer.
Simple esterification and etherification reactions were applied to steamexploded Flax (Linum usitatissimum) with the aim of changing the surface properties through modification of fiber surface chemistry. Native and chemically modified cellulose fibers were characterized in terms of thermal stability, surface chemistry, morphology, and crystal structure. Independent of the substituent nature, chemically modified fibers exhibited a thermal stability comparable to that of native cellulose. Introduction of the desired chemical groups at the fiber surface was demonstrated by TOF-SIMS analysis, whereas FTIR showed that the substitution reaction involved only a small fraction of the cellulose hydroxyls. No change of the native crystalline structure of cellulose fibers was caused by chemical modification, except in the case where ether substitution was carried out in water-isopropanol medium. Cellulose fibers with unchanged structure and morphology and carrying at the surface the desired chemical groups were obtained for reinforcing applications in polymer composites.
Steam-exploded fibers from flax (Linum usitatissimum) are heterogeneously acetylated using acetic anhydride and sulfuric acid as catalyst, with the aim to modify the surface properties without changing fiber structure and morphology. The acetylation reaction follows first-order kinetics up to a reaction time that depends on catalyst concentration (15 h when using 0.4 vol % of H(2)SO(4) or 50 h with 0.1 vol %). The fibers undergo no structural and/or morphological changes under either reaction condition. On the contrary, surface damage and structural modifications appear after longer reaction times, when the reaction kinetics change. The extent of biodegradation of acetylated fibers, evaluated from the weight percent remaining after 13 days of exposure to previously isolated cellulolytic bacteria Cellvibrio sp., decreases with increasing acetylation degree. After biodegradation the fibers show a higher acetyl content than before the experiment, indicating that the bacteria preferentially biodegrade unsubstituted cellulose, though also acetylated chains are cleaved. Biodegradable acetylated cellulose fibers with modified surface chemistry and unchanged structure are obtained for applications as polymer composite reinforcements.
Copolymers of octanediol adipate and sorbitol adipate, P(OA-SA), copolymers of octanediol adipate and glycerol adipate, P(OA-GA), poly(octamethylene adipate), POA, and poly(sorbitol adipate), PSorA, were synthesized using immobilized Lipase B from Candida antarctica (Novozyme-435) as catalyst. The molecular weights and polydispersity indices of these polyesters were determined. The physical properties of these polyesters were investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), and dynamic mechanical analysis (DMA). These polymers are semicrystalline except for PSorA, which is amorphous. With increasing sorbitol or glycerol content in the polyesters, both melting and crystallization temperatures decreased. The melting temperature depression is well described by Baur's equation for random copolymers where the second comonomer is completely excluded from the crystal phase of the crystallizable first monomer. Also, the melting enthalpy decreases, and WAXS measurements confirm that the degree of crystallinity decreases upon copolymerization with either sorbitol or glycerol. Furthermore, the crystal phase is that of POA.
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