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SYNOPSISThe cure kinetics of a diglycidyl ether of bisphenol A (DGEBA)-based epoxy resin with methyltetrahydrophthalic anhydride and an accelerator was studied by nonisothermal DSC data. The systems were uncured resin and partially cured with the following extents of cure measured by the residual heat method (aDsc): 0.37,0.63,0.81, and 0.90. The activation energy calculated by the Kissinger method increases from 63 kJ/mol for the uncured epoxy to 77 kJ/mol for the partially cured with aDsC = 0.90. Additionally, the activation energy calculated by the isoconversional method shows a dependence on the conversion degree a. The activation energy tends to decreases initially with the conversion degree, possibly due to the autocatalytic effect; then, it passes through a minimum about a = 0.4 and, finally, increases slightly due to the increase of crosslinks which reduce the mobility of the unreacted groups. A simple, consistent method of kinetic analysis was applied. This method enables one to select the most convenient model and the calculation of kinetic parameters. A twoparameter (m, n) autocatalytic model (Sesthk-Berggren equation) was found to be the most convenient model to study the curing of epoxy systems. The results show a dependence of the kinetic parameters on the initial degree of crosslinking of the partially cured epoxy. The exponent m tends to decrease with the extent of cure, while the exponent n remains practically invariable. These results show a change of the kinetic when the initial extent of cure of the epoxy system increases. The In A data, A being the preexponential factor in the Arrhenius dependence of the temperature on the rate of conversion, increase with the extent of cure, showing a correlation with the calculated activation energy values. The nonisothermal DSC curves theoretically calculated show a very good agreement with the experimental data. The two-parameter (m, n) autocatalytic model gives a good description of the curing kinetics of epoxy resins with different extents of cure. 0 1995 John Wiley & Sons, Inc.

SYNOPSISThe viscoelastic response of some vinylic copolymers of cellulose prepared with vinyl acetatemethyl acrylate mixtures and with Ce( IV) ion as initiator, and native cellulose, were studied a t 110 Hz in a range of temperatures from -120-100°C.The viscoelastic spectrum of cellulose shows the @-relaxation that is not shown in its vinylic copolymers. We observed the same effect in the dielectric fi-relaxation.For the vinylic copolymers of cellulose, one viscoelastic relaxation attributed to the arelaxation of the grafted vinylic chains is observed. Some differences in the characteristics of this relaxation may be related to the composition of PVA/PMA vinylic side chains and to the ratio of cellulose in the copolymer. The plots of the Argand diagrams give us a better understanding of the viscoelastic behaviour of these materials. The results seem to indicate that the cellulose hinders the large-scale motions of the vinylic chains grafted onto it. The glass transition temperature ( T,) determined by differential scanning calorimetry (DSC ) also shows the same fact: the T, of the vinylic copolymers of cellulose are higher than both the T, of polyvinyl acetate-polymethyl acrylate copolymers ( PVA-PMA) without cellulose and the T, of some blends of cellulose and the PVA-PMA whose composition was as similar as possible to the cellulosic copolymers. The importance of the covalent bonds between cellulose and the vinylic side chains in the structural transitions are revealed. The present results are compared with the dielectric a-relaxation that we described elsewhere. 0 1993

ABSTRACT:Cotton cellulose with different % NaOH treatments and graft copolymers of cellulose prepared with vinyl acetate (AV) and methyl acrylate (MA), and Ce(IV) ion as an initiator were submitted to biodegradation conditions. Cellulose is a biopolymer consisting solely of glucose units, and, consequently, is also easily biodegradable. Nevertheless, modified cellulose, for example, by graft copolymerization, shows an increased resistance to biodegradation. The aim of this work was to study by calorimetric and dynamic-mechanical analysis how the chemical modification of cellulose affects its biodegradability. From the obtained results some information has also been deduced about the composition and mechanical behavior of the vinylic grafted chains.

SYNOPSISThe real and imaginary parts of the complex dielectric permittivity, d and t", for some vinylic copolymers of cellulose [prepared with vinyl acetate (VA) and methyl acrylate (MA) and Ce (IV) ions as initiator] and for cellulose were measured over a frequency band of 0.1-10' kHz and a temperature range from -40 to 100OC. In vinylic copolymers of cellulose, we observed one dielectric relaxation attributed to the a-relaxation of the vinylic side chain grafted on cellulose. In cellulose dielectric spectra, this relaxation did not appear, but we detected one relaxation that may correspond to the p-relaxation. For these vinylic copolymers of cellulose, the t" against t' plot gives a skewed arc that closely resembles that of the Davidson-Cole model, with a broader distribution for high frequencies that shows the overlap of several relaxations in the process considered. Some differences observed between the vinylic copolymers of cellulose may be due to the composition and the length of the vinylic side chains and to the frequency of grafting on the cellulose.

SYNOPSISGraft copolymerization of vinyl acetate (VA) and methyl acrylate ( M A ) on cotton cellulose was initiated by the Ce (IV) ion, and ungrafted vinylic polymer was separated from the graft copolymer by acetone extraction. The influence of the ratio aqueous initiator solution volume/monomeric volume ( Vaq/VmOn), vinyl acetate volume/methyl acrylate volume ( VVA/ VMA), and the cellulose crystallinity index (CI ) on the grafting reaction were studied. To modify the crystallinity of cellulose, native cotton was treated with NaOH in the concentrations 10, 15, and 20% (mercerized). The viscosimetric average molecular weight ( M u ) , the polymerization degree (PD) , and the crystallinity index proposed by Nelson and O'Connor (CI) were determined for native and NaOH-treated cotton. The polymeric side chains grafted were separated from the cellulose backbone by acid hydrolysis in 72% H,SO,. The viscosimetric average molecular weight ( M u ) was determined, and the number of vinylic chains per cellulosic chain (graft frequency, G F ) were calculated. The grafting percentage, %G, was higher for most amorphous cellulose and for a higher methyl acrylate percentage (%MA) in monomeric reaction mixtures (VA-MA). The Vaq/Vmon ratio that yields the highest %G was 70/30. The increase of the %G with the %MA in the VA-MA monomeric mixture seems to be due to both an increase in the length of vinylic grafted chains (as shown by its M u ) and the number of grafted chains ( G F ) . The increase in the %G when the crystallinity index (CI) of the cellulosic substrate decreases seems to be due to an increase in the length of the vinylic grafted chains, but not to an increase in the number of grafted chains, since the Mu increases and GF decreases when the CI of cellulose decreases.

SYNOPSISCotton cellulose with different % NaOH treatments (mercerized), graft copolymers of cellulose prepared with vinyl acetate (VA) and methyl acrylate (MA) and Ce( IV) ions as initiator, and some blends of cellulose-copolymer PVA-PMA were submitted to differential scanning calorimetry (DSC) analysis in nitrogen atmosphere. Two aspects were considered ( a ) moisture loss of celluloses with different % NaOH treatments, which showed differences attributed to structural changes in the amorphous region of cellulose-I and cellulose-11; ( b ) thermal degradation analysis of celluloses, their copolymers, and their blends. Experimental results show that thermal stability of celluloses decreases when % NaOH in mercerization increases. For cellulosic vinylic copolymers, the thermal stability increases with the grafting frequency ( G F ) . The blends of cellulose-copolymer PVA-PMA were found to have lower thermal stability than the cellulosic copolymers and the cellulose alone, which was attributed to the acetic acid eliminated in the thermal decomposition of PVA-PMA. The present results seem to be in agreement with the thermal degradation mechanism of cellulose proposed by Pate1 et al. and provide useful confirmation that the free OH group content is a very important factor in the thermal stability of cellulose.

Thermal degradation of unmercerized and mercerized cotton cellulose with different % NaOH solutions and grafted vinylic copolymers with different mixtures of vinyl acetate-methylacrylate 1 have been studied by thermogravimetric analysis (TGA) in nitrogen between 25 and 600°C at different heating rates. The differences between unmercerized and mercerized samples are related to structural differences between cellulose-I (native) and cellulose-II. The grafted cellulosic vinylic copolymers have shown that their thermal stability depends upon the cellulosic substrate and the grafting percentage. From our results, it can be deduced that it is possible to prepare the cellulosic materials with good thermal stability, short degradation temperature interval, and various residues at the end of degradation.