The effects of block copolymer molecular weight (MW) and composition on the critical micelle concentration (CMC) were studied using ionic liquids (ILs) as model solvents. Pyrene fluorescence was used to measure CMCs as a function of block MW for three polystyrene–poly(ethylene oxide) (PS–PEO) samples and three PS–poly(methyl methacrylate) (PS–PMMA) samples in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide. The CMC decreased by a modest factor of 1.5 in the PS–PEO series, in which the solvophobic PS block MW remained unchanged (20 000) while the PEO block MW was decreased from 13 000 to 5000. This result correlated reasonably well with calculations from self-consistent-field (SCF) theory. A greater decrease (factor of 5) was seen in the PS–PMMA series, where the solvophobic PS block MW was varied from 3000 to 11 000 while maintaining a constant overall MW (ca. 15 000). However, this decrease was much weaker than that predicted by SCF calculations. A compilation of literature CMC data for amphiphilic block copolymers in water generally reveals a strong dependence on solvophobic block degree of polymerization N for low N, but a much weaker dependence for longer solvophobic blocks. From master plots of the compiled data, a scaling parameter shift from CMC ∼ exp(−cN) to CMC ∼ exp(−cN 1/3) was found above a critical solvophobic block N. The parameter c correlates with the χ parameter between the solvophobic block and the solvent. The weaker N dependence was found to fit the IL data very well. While such a change in MW dependence has previously been attributed to the collapse of unimer solvophobic blocks, we also discuss the potential role of kinetic limitations.

Self-consistent field theory is used to calculate free energy barriers and reaction rates for the spontaneous association and dissociation of micelles formed of block copolymers in a homopolymer matrix. The barriers are prohibitively large for copolymers of typical molecular weights when the unimer (free surfactant) concentration is near the equilibrium critical micelle concentration (CMC). As a result, polymeric micelles normally cannot reach true thermodynamic equilibrium. The rates of association and dissociation are, however, sensitive to unimer concentration, making it possible to form or destroy micelles at observable rates in sufficiently highly supersaturated or subsaturated solutions, respectively, even when both reactions are suppressed near the equilibrium CMC. The barrier to dissociation is particularly sensitive to unimer concentration, and vanishes when the unimer concentration is only slightly (for example, a percentage of a few tens) below the equilibrium CMC.

SynopsisCobalt (111) acetylacetonate serves as an accelerator for anhydride curable epoxy resin system and the rate of curing is found to increase with enhanced concentrations of the metal chelate. There is also an appreciable reduction in the cure gel time. Kinetic studies based on thermal analytical techniques reveal that the overall curing process follows first order kinetics. Based on the kinetic results a cure schedule has been proposed. It is also observed that the electrical, mechanical, and thermal properties of the cured epoxy system are not altered by the presence of the metal chelate at the concentration studied.

No abstract

The dynamic DSC method, which utilizes the variation in peak exotherm temperature with heating rate, is used to measure the energy of activation for anhydride cured bisphenol-A epoxy and epoxidized novolak systems. Kinetic parameters are also determined for Bstage glass fabric reinforced epoxy novolak hardened with precursor novolak. Using the expression developed for life estimation of wire enamels by Toop, probable cure schedules are evaluated. The results are compared with glass transition temperature measured under isothermal conditions. It is inferred that the "loop's equation of thermal index can be used to derive reliable cure schedules of epoxies with relative improvement in speed and ease.Differential Scanning Calorimeter (DSC) which measures the heat flow to the sample as a function of temperature directly, rapidly and accurately has been extensively used to study the cure kinetics of various thermosets. This method requires small quantity of sample and the measured value of heat of polymerzation is uncomplicated by complex changes in physical state (liquid-solid) which accompany curing and therefore correlates directly with progress of chemical reaction [1][2][3][4][5][6][7][8][9].Ozawa's method [10] based on the variable programme rate has been commonly used to derive kinetic parameters for many industrial materials and forms the basis of the ASTM method on Arrhenius Kinetic Constants for Thermally Unstable Materials [11]. The present paper examines the possibilities of extracting data on cure schedule of epoxies by fitting the kinetic data into the mathematical expression developed by Toop [12] that relates activation energy to time by temperature.John Wiley & Sons, Limited, Chichester Akad~miai Kiad6, Budapest