The thermal stability of a series of dialkylimidazolium carboxylate ionic liquids has been investigated using a broad range of experimental and computational techniques. Ionic liquids incorporating fluoroalkyl carboxylate anions were found to have profoundly differing thermal stabilities and decomposition mechanisms compared with their non-fluorinated analogues. 1-Ethyl-3-methylimidazolium acetate was observed to largely decompose via an S(N)2 nucleophilic substitution reaction when under inert gas conditions, predominantly at the imidazolium methyl substituent. The Arrhenius equations for thermal decomposition of 1-ethyl-3-methylimidazolium acetate, and the C(2)-methylated analogue 1-ethyl-2,3-dimethylimidazolium acetate, were determined from isothermal Thermogravimetric Analysis experiments. The low thermal stability of 1-ethyl-3-methylimidazolium acetate has important implications for biomass experiments employing this ionic liquid. For these two ionic liquids, ion pair and transition state structures were optimised using Density Functional Theory. The activation barriers for the S(N)2 nucleophilic substitution mechanisms are in good agreement with the experimentally determined values.

SynopsisA generalized theory for the glass transition temperature of crosslinked and uncrosslinked polymers has been developed, which takes into account the influences of end groups, branching, and crosslinking, and their functionality distribution. DiBenedetto's theory was found to correctly characterize the influence of crosslinks on the glass temperature. Normalized to constant crosslink functionality, the crosslink constant is a universal parameter suggesting that the entropic theory of glasses is applicable to crosslinked systems. Data on linear polymers and networks from the crosslinking of polymer chains, vinyl /divinyl-copolymers and step-growth polymers, such as polyurethanes, amine-cured epoxies, or inorganic glasses, are presented.

SYNOPSISA theoretical approach to thermoset cure kinetics based on Arrhenius kinetics and mobility was developed by considering the activation of the reacting group and chain mobility as elementary steps for reaction. This extended kinetic equation was successfully applied to the curing of an epoxy by a n amine, the trimerization of a cyanate, and to the polymerization of methyl methacrylate. Full agreement between theory and experimental data was obtained in all cases. The activation energies for chain mobility were exceptionally low (0.3-1 k J / mol for bisphenol-A-based epoxy and cyanate) which indicates that the structural units must undergo only small-angle rotational oscillations to allow a reaction. A theoretical time-temperature-transformation (TTT ) diagram is also presented. 0 1993 John Wiley & Sons, Inc.

The main basis of structure-property-relationships of polymeric materials is a good characterization of the structure under study. Such is the case for cross linked polyurethanes such as flexible foams. To overcome the problems of solubility of these materials, a selective degradation process was used, leaving the hard segments unaffected. These were then analyzed by MALDI-TOF (Matrix Assisted Laser Desorption-Ionization Time of Flight mass spectrometry). Using these techniques, the length distribution of the hard segments was examined. It was found that the length of the hard segments decreases much slower for polyester foams than for polyether foams. This fact was attributed to a lower solubility of the hard segments in polyethers, which leads to an earlier onset of phase separation. Thus, both mobility and reactivity of the segments in the segregated hard domains are reduced. This fact also explains why the length of the hard segments is only slightly dependent on the water content, or the index, at least in polyether foams. Chemical differences such as the incorporation of polymeric MDI or reactive catalysts could also be shown. Furthermore, this work shows for the first time that presence of cyclic hard segments could be established in a foam.

The influence of molecular structure on cure kinetics was studied using a new approach for characterizing the cure kinetics of thermosets in the glass transition region. Reactions in or near the glassy state are controlled by chain mobility, where the whole structural unit undergoes small‐angle oscillation motions around the network link. All structural features affecting the mobility such as stiffness, bulky structures, substitution, symmetry effects, and functionality lead to high segment activation energies and slow final curing. The influence of the segment activation energy on the kinetic curves is also discussed. © 1993 John Wiley & Sons, Inc.