MICRO AND NANO LAYERED POLYMERS

The crazing behavior of coextruded microlayer sheets consisting of alt ernating layers of polycarbonate (PC) and styreneacrylonitrile copolym er (SAN) was investigated as a function of PC and SAN layer thicknesse s. In this study, the total sheet thickness remained essentially const ant and the PC and SAN layer thicknesses were changed by varying both the total number of layers from 49 to 1857 and the PC/SAN volume ratio . Photographs of the deformation processes were obtained when microspe cimens were deformed under an optical microscope. Three different type s of crazing behavior were identified: single crazes randomly distribu ted in the SAN layers, doublets consisting of two aligned crazes in ne ighboring SAN layers, and craze arrays with many aligned crazes in nei ghboring SAN layers. The transition from single crazes to doublets was observed when the PC layer thickness was decreased to 6 microns. Craz e array development was prevalent in composites with PC layer thicknes s less than 1.3 microns. It was concluded that SAN layer thickness was not a factor in formation of arrays and doublets; formation of craze doublets and craze arrays was dependent only upon PC layer thickness. Subsequently, the mode delamination failure, and the corresponding del amination toughness, of coextruded microlayer sheets consisting of alt ernating layers of PC and SAN, were studied with the T-peel test. Four delamination modes were observed; the mode depended on both the SAN l ayer thickness and the PC layer thickness. The SAN layer thickness det ermined whether the crack propagated along a PC-SAN interface (interfa cial delamination) or through crazes within a SAN layer (crazing delam ination). Only interfacial delamination was observed if the SAN layers were thinner than 1.5 mu m. For SAN layers thicker than 1.5 mu m, the amount of crazing delamination increased as the thickness of the SAN layers increased. The corresponding toughness, measured as the critica l load, was lowest for interfacial delamination failure and increased with the amount of crazing delamination. With both interfacial and cra zing delamination, the crack could either propagate along a single lay er or could jump from one layer to the next. This depended on the PC l ayer thickness. If the PC layers were thin enough, the crack jumped fr om one layer to another with tearing of the PC layer. This significant ly increased the delamination toughness. The interdiffusion of two mis cible polymers, polycarbonate (PC) and a copolyester (KODAR) was studi ed at temperatures from 200 to 230 degrees C. The two polymers were co extruded as microlayer composites with up to 3713 alternating layers. The microlayer structure provided a large area of intimate contact bet ween the two polymers with minimal mixing. Initially, two glass transi tion temperatures were observed by DSC that were intermediate between the glass transition temperatures of the pure components. Upon anneali ng, the glass transition temperatures shifted closer together, reflect ing the extent to which interdiffusion had occurred. After no more tha n 2 h of annealing, a single glass transition temperature was observed . A model was formulated based on Pick's law of diffusion that related the mutual diffusion coefficient, D, to the change in the glass trans ition temperatures. The model. also incorporated an ''equivalent'' res idence time to account for diffusion that occurred during the coextrus ion process. I was not necessary to consider the concentration depende nce of D to satisfactorily describe the data with this model. For the temperature range from 200 to 230 degrees C, the value of D varied fro m 4.0 x 10(-16) to 1.6 x 10(-15) m2/s. The activation energy of interd iffusion was determined to be 95 kJ/mol.