Browsing by Author "Ophir, Zohar H."

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    Sub-Tg annealing studies of rubber-modified and unmodified epoxy systems
    Ophir, Zohar H.; Emerson, John A.; Wilkes, Garth L. (American Institute of Physics, 1978)
    Epoxy network systems based on DGEBA (bisphenol_A_diglycidyl ether) and NMA (nadic methyl anhydride) systems modified with the low molecular weight CTBN (carboxyl_terminated butudiene_acrylonitrile copolymer)rubbers were prepared and studied. It was found that below the glass transition of the epoxy matrix these materials displayed time_dependent changes in their mechanical properties; specifically, the strain to break as well as the rate of stress relaxation were observed to decrease in a near_linear behavior with the logarithm of time at sub_T g annealing. Calorimetric methods clearly showed a simultaneous decrease in enthalpy with time that behaved in a similar fashion as the time_dependent mechanical properties. All the calorimetric and mechanical data are qualitatively related. The importance of this phenomena is considered in view of the widespread use of epoxys. Similar behavior is expected for other network glasses thermally quenched into a nonequilibrium state.
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    A systematic study of the effect of casting solvent on the bulk properties of linear SBS block copolymers
    Wilkes, Garth L.; Bagrodia, Shiriam; Ophir, Zohar H.; Emerson, John A. (American Institute of Physics, 1978)
    Using a well_characterized series of styrene_butadiene triblock copolymers, a systematic study was carried out to determine the influence of casting solvent, used for film preparation, on the final mechanical properties. The results clearly indicate that in the range of styrene contents of 30–40 mole%, the modulus behavior varies greatly as the solubility parameter _ of the casting solvent is varied. Specifically, the modulus increases as _ increases. At lower or higher styrene content, the properties are influenced but to a much lesser degree. Interpretation of the measured mechanical behavior is given in light of morphological data obtained by rheo_optical methods.