Cure Kinetics of Epoxy-Novolac Resin Containing Tetrabromo-bisphenol-A as a Flame Retardant by Isothermal Calorimetry Method

Cure kinetics of an epoxy-novolac resin with tetrabromo-bisphenol-A, as a flame retardant, was investigated in the presence of nadic methyl anhydride as a curing agent. Different amounts of benzyl dimethyl amine were used as accelerator. The studies on kinetics were carried out using isothermal DSC experiments at 100, 110, 120 and 130°C. The conversions were calculated based on the total heat of curing reaction obtained from non-isothermal DSC analysis at a heating rate of10°C/min. Kinetic parameters were obtained from isothermal analysis and by a model especifically for autocatalytic curing reactions which have been developed by Kamal and Sourour. The results obtained illustrate that the reaction is controlled by kinetic and diffusion parameters at different reaction stages. In other words, before a glassy state is reached the reaction mainly proceeds with autocatalytic behavior and can be well explained by the equation containing k1 and k2 as the rate constants and n and m as the reaction orders. At higher conversion levels, at about 65%, at the onset of network formation the system turns to a glassy state, the curing reaction is mainly controlled by diffusion (diffusion-controlled system). We conclude therefore that, theoretical and experimental data would be confirmed by considering the diffusion parameters in the corresponding model

Structural engineering to control density, conformation, and bioactivity of the poly(ethylene glycol)-grafted poly(urethane urea) scaffolds

Poly(urethane urea) scaffolds were fabricated through combined salt leaching and solvent casting methods. The scaffolds were then functionalized via aminolysis with poly(ethylene glycol) (PEG-g-PUU). To compare its bioactivity, gelatin was also grafted onto the aminolyzed poly(urethane urea) surface (Gel-g-PUU). Chemical changes at the surface were then monitored using quantitative/qualitative methods. Grafting with both gelatin and poly(ethylene glycol) remarkably enhanced the wettability of poly(urethane urea). Proliferation of human adipose–derived mesenchymal stem cells on poly(urethane urea) and the modified poly(urethane urea)s was evaluated by 3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide assay. The cell experiment results showed that both the modified poly(urethane urea)s enhanced the attachment and proliferation of human adipose–derived mesenchymal stem cells compared to pure poly(urethane urea). Based on previous reports, while a supportive role is observed at adequate poly(ethylene glycol) graft densities, cell adhesion and proliferation are inhibited at very high grafting densities. To correlate the cell data to poly(ethylene glycol) conformations, the surface tension was measured. Data on human adipose–derived mesenchymal stem cells’ attachment/proliferation and contact angle/surface free energy together showed that the grafting density of poly(ethylene glycol) was regulated by optimizing aminolysis conditions, careful selection of poly(ethylene glycol)’s molecular weight, and bulk properties of the matrix poly(urethane urea). As a result, surface overcrowding and brush conformation of the poly(ethylene glycol) chains were avoided, and human adipose–derived mesenchymal stem cell attachment and proliferation occurred on the PEG-g-PUU scaffold at a comparable level to the Gel-g-PUU