Showing papers by "Alan J. Lesser published in 2010"

Aspects of network formation in glassy thermosets

Abstract: Aspects of network formation and the physical properties of stoichiometric amine-cured epoxy resins are investigated. The state of network formation is systematically controlled by using isothermal cure conditions and a linear temperature ramp condition. Affine deformation in compression at a constant true strain rate is used to measure the true stress–strain response from small strains to large strains. Beginning with the low-strain responses and continuing through yielding, partially cured samples demonstrate characteristics of physical ageing that diminish as the networks approach full conversion. The postyield properties are characterized by a strong dependence on network connectivity with little influence of the curing method. The mechanical responses are also applied to necking and shear banding analyses. The resins are unable to achieve stable necking in tension. However, shear bands are able to be stabilized, as predicted by a model developed in this work. These results suggest that physical ageing is inherent in epoxy network formation, and the resulting strain localization significantly degrades the mechanical integrity of epoxy resins in the partially cured state, regardless of the cure conditions. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Mechanical and thermo‐mechanical studies of double networks based on thermoplastic elastomers

Abstract: A new approach to prepare and characterize double network elastomeric systems was investigated. A styrene-ethyl- ene-co-butylene-styrene (SEBS) triblock copolymer system con- taining physical crosslinks was used to achieve a double network by additional crosslinking using ultra-violet (UV) light. An ethylene-propylene-diene monomer (EPDM) terpolymer system containing chemical crosslinks was used to achieve a conventional double network using UV crosslinking. Properties from conventional monotonic tensile tests, dynamic mechani- cal analysis, and thermomechanical properties were investigated. These double network elastomers show a transition between competitive and collaborative behavior in their mechanical proper- ties and lower coefficients of thermal expansion arising from a competition of the networks. V C 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 778-789, 2010

Quantitative Poly(vinyl alcohol) Modification in Ionic Liquids: Esterification and Urethanation with Low Surface Tension Producing Reagents

Abstract: A method for rapid and quantitative chemical modification of poly(vinyl alcohol) (PVOH) is described using two different ionic liquids as reaction solvents. The quantitative modification of alcohol-containing polymers such as PVOH, cellulose, and other polysaccharides has been elusive due to solvent incompatibility with respect to alcohol-reactive reagents and to the limited solvent options for these polymers. Ionic liquids are ideally suited to address this problem as they are a diverse and extremely tunable class of solvents. Two ionic liquids that differ in both their cation and anion are demonstrated to dissolve and facilitate quantitative modification of PVOH under simple reaction conditions. Rapid esterification of PVOH using four different acid chlorides is demonstrated in both 1-butyl-3-methylimidazolium chloride (BMIM-Cl) and tri-n-butylethylphosphonium diethylphosphate (TBEP-DEP). Urethanation of PVOH was also demonstrated to be quantitative with the appropriate ionic liquid solvent. The reagent...

Reinforcing thermosets with crystallizable solvents

Abstract: This study demonstrates an approach to generate reinforcement in thermosetting polymers through crystal growth of crystallizable solvents. Emphasis is to identify the reaction conditions, which lead to suitable reinforcement in selected compounds. Crystallization behavior and miscibility of dimethylsulfone (DMS) in diglycidylether of bisphenol-A epoxy monomer was investigated. Small angle laser scattering and optical microscopy were utilized to monitor phase separation and crystallization of DMS at different isothermal conditions during the cure process. It is shown that DMS crystals grow anisotropically to form faceted geometries and demonstrate possible structures to anchor into the epoxy matrix. The growth mechanism and the agility of crystals are shown to be affected by the cure reaction as well as depth of supercooling. A completely cured sample with 15 wt % DMS shows a broad map of rich morphologies from nanoscale particles to uniformly distributed macroscale, discontinuous fiber-like crystals generated only by altering the curing conditions. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 840–849, 2010