Yawo-Kuo Twu

Thermal degradation of MDI-based segmented polyurethanes

Thermal degradations of 4,4′-diphenylmethane diisocyanate-based thermoplastic polyurethane elastomers were conducted and investigated as functions of heating conditions by using thermogravimetric analysis, ultraviolet-visible (UV-vis) spectroscopy, gel permeation chromatography (GPC), and Fourier transform infrared (FTIR) spectroscopy. The extent of degradation increased with increasing temperatures and times. The degradation was accompanied by crosslinking and was more significant under air than under nitrogen, indicating that a free-radical mechanism was involved. The degradation mainly was due to unstable hard segments and gave a red shift in the UV-vis spectra. The degradation, leading to considerable discoloration, was demonstrated by UV-vis spectroscopy, starting from 240 °C in air for 10 min. Heated in nitrogen for the same period of time, the samples did not show considerable discoloration until 280 °C. The UV-vis data suggested that the degradation occurred through cleavages of NH bonds and CH bonds on the hard segments. Chain scission of polymer main chains, as demonstrated by GPC data, occurred at a temperature as low as 200 °C in nitrogen, although cleavage of NH bonds was not detectable by UV-vis and FTIR spectroscopy at these conditions. FTIR spectroscopy also provided evidence of cleavage of NH bonds and depolymerization of urethane linkages. Irganox 1010 was found to be an efficient antioxidant.

Optimized enzymatic synthesis of caffeic acid phenethyl ester by RSM.

In this study, optimization of enzymatic synthesis of caffeic acid phenethyl ester (CAPE), catalyzed by immobilized lipase (Novozym 435) from Candida antarctica was investigated. Novozym 435 was used to catalyze caffeic acid and 2-phenylethanol in an isooctane system. Response surface methodology (RSM) and 5-level-4-factor central-composite rotatable design (CCRD) were employed to evaluate the effects of synthesis parameters, such as reaction temperature (30-70 degrees C), reaction time (24-72 hours), substrate molar ratio of caffeic acid to 2-phenylethanol (1:10-1:90) and enzyme amounts (100-500 PLU) on percentage conversion of CAPE by direct esterification. Reaction temperature and time had significant effects on percent conversion. On the basis of ridge max analysis, the optimum conditions for synthesis were: reaction time 59 hours, reaction temperature 69 degrees C, substrate molar ratio 1:72 and enzyme amount 351 PLU. The molar conversion of predicted values and actual experimental values were 91.86+/-5.35% and 91.65+/-0.66%, respectively.

Highly Efficient Synthesis of an Emerging Lipophilic Antioxidant: 2-Ethylhexyl Ferulate

Ferulic acid in ester form has shown a stronger ability in ameliorating certain pathological conditions and inhibiting lipid oxidation. In present study, a solvent-free and reduced pressure evaporation system was developed for lipase-catalyzed synthesis of 2-ethylhexyl ferulate (2-EF) from ferulic acid and 2-ethylhexanol. A Box-Behnken design with response surface methodology (RSM) and artificial neural network (ANN) was selected to model and optimize the process. Based on the yields of 2-EF, reaction temperature was shown to be the most important process factor on the molar conversion among all variables. The residual values and the coefficient of determination (R2) calculated from the design data indicated that ANN was better than RSM in data fitting. Overall, the present lipase-catalyzed approach for 2-EF synthesis at low reaction temperature in a reduced pressure evaporation system shows high 2-EF production efficiency. Notably, this approach can reduce the enzyme denaturation and ferulic acid oxidation that usually occur during long-term biosynthetic operations at high temperature.

An Investigation on Chitosan/Carbon Nanotube Composite Used as a Sensing Material

In this study, the functionalized carbon nanotube (CNT) was blended with chitosan (CS), as a matrix, via solvent casting and self-assembly methods to fabricate films on ITO electrode for sensing. These films were investigated on their electrocatalytic properties by cyclic voltammetry. This study aimed to find the optimal formula and film formation conditions to develop an optimal sensing material. Cyclic voltammetry investigation found that CNT in the CS/CNT cast film exhibited electrocatalytic capacity in oxidation of [Fe(CN) 3-]. The 60 °C-3 h-acid-treated CNT in the cast CS/CNT film exhibited a better electrocatalytic capacity than the pristine CNT and the 80 °C-2 h-acid-treated CNT. Among the three acids used to prepare CS/CNT solutions, HCOOH exhibited a better dissolution of CS and dispersion of CNT in CS/CNT solution leading to a better electrocatalytic capacity in oxidation of [Fe(CN)63] than CH3COOH and HCl. The CS/CNT films prepared by casting from CH3COOH aqueous solutions were durable for sensing.