SYNOPSISThe crazing behavior of coextruded microlayer sheets consisting of alternating layers of polycarbonate (PC) and styrene-acrylonitrile copolymer ( SAN ) was investigated as a function of PC and SAN layer thicknesses. In this study, the total sheet thickness remained essentially constant 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 microspecimens were deformed under an optical microscope. Three different types of crazing behavior were identified single crazes randomly distributed in the SAN layers, doublets consisting of two aligned crazes in neighboring SAN layers, and craze arrays with many aligned crazes in neighboring SAN layers. The transition from single crazes to doublets was observed when the PC layer thickness was decreased to 6 microns. Craze array development was prevalent in composites with PC layer thickness 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.

SYNOPSISMechanisms are proposed for the two types of interactive crazing that have been observed in PC/SAN microlayer composites when the PC layer thickness is on the micron-size scale. It is demonstrated that when the PC layer is thin enough the deformation zone that forms at a craze tip can interact with the next-neighboring SAN layer. By measuring the dimensions of the craze tip in scanning electron micrographs, it was found that the craze-tip opening does not depend on the SAN layer thickness, i.e., the length of the SAN craze. Consequently, the size and shape of the resulting plastic zone in the PC layer are also independent of layer thickness. The zone that forms in the PC layer consists of a colinear plastic zone together with a pair of micro-shearbands that grow at an angle of about 45". When the PC layer is less than 6 pm, the elastic stress concentration from the colinear plastic zone increases the probability of crazing in the neighboring SAN layer with the formation of craze doublets that consist of two aligned crazes in neighboring SAN layers. By taking into consideration the Weibull distribution of crazing, a craze doublet fraction comparable to the 30% observed experimentally was predicted with a stress intensification factor in the range of 1.03-1.05. When the PC layer thickness is less than 1.3 pm, the length of the colinear plastic zone is comparable to the PC layer thickness. Formation of a craze at the point of impingement of the plastic zone on the neighboring SAN layer leads to craze arrays with many aligned crazes in neighboring SAN layers. At higher strains, the micro-shearbands grow through the PC layers and extend into several adjacent SAN and PC layers. This produces a change in deformation mechanism in the SAN layers a t the yield instability, from craze opening to shear yielding.

A comparison of glass transition temperatures measured by differential scaning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) was made using mi9crolayer composites with many alternating layers of two polymers. What appeared to be a third transition at a temperature intermediate between the glass transitions of the two components was observ ed in DMTA but not DSC. The mechanical orgin of this intermediate damping peak in the layered composites was established. The viscoelastic properties of each polymer were modelled by a spring and dashpot in series, and the composite as a parallel arrangement of two Maxwell elements. It was shown that an anomalous damping peak can appear between the glass transition temperatures when the modulus of the composite can not drop below the modulus of the glassy layers, although the viscosity of the composite continues to be defined by the viscosity of the rubbery layers in this temperature range. The intermediate damping peak may be a general feature in the dynamic mechanical behavior of co‐continuous multiphase systems including polymer blends and continuous fiber composites.

SUMMARY:Concurrent crystallization and interdiffusion of two miscible polymers, polycarbonate (PC) and a copolyester (KODAR), was studied in microlayers with 657 alternating layers. Minimal mixing of the two components during processing was indicated by separate glass transitions of the PC and KODAR layers. Changes in the glass transitions after the microlayers were annealed above the glass transition of PC were determined by DSC and DMTA. When the annealing temperature was 200 "C or higher, interdiffusion was much faster than crystallization and the glass transitions gradually shifted closer together until they merged into a single transition. An analysis based on Fick's law of diffusion described the relationship between theglass transition temperatures and the annealing time. At 195 "C, the rates of interdiffusion and crystallization were comparable. Annealing at this temperature resulted in more complex changes in the glass transitions, including the persistence of two transitions after long annealing times. This behavior was understood by considering an interlamellar amorphous phase that was trapped in the spherulites and not available for interdiffusion. Models were developed from the spherulite morphology observed in the optical microscope. l k o cases were considered a continuous layer of impinged spherulites in the PC/KODAR 20/80 (w/w) composition, and isolated spherulites aligned with the center of the KODAR layer in PC/KODAR 40/60 and 60/40. These models satisfactorily reproduced the principal features of concurrent crystallization and interdiffusion.

SUMMARYThe crystallization behavior of coextruded microlayered sheets comprised of 657 alternating layers of polycarbonate (PC) and a miscible copolyester of mainly 1 ,4-cyclohexanedimethanol and terephthalic acid (KODAR) was investigated as a function of annealing time when the KODAR was crystallized isothermally from the glass at 195 "C. Comparisons were made with crystallization of KODAR alone, and with crystallization of KODAR from melt blends with PC. The kinetics of crystallization and the morphology of the crystallized KODAR were characterized with differential scanning calorimetry, and by examination of thin sections microtomed from annealed specimens in the polarized light microscope and the transmission electron microscope. The growth rate of small, birefringent KODAR spherulites was non-linear, and was strongly affected by diffusion of PC into the KODAR layers. Diffusion of amorphous PC into the KODAR layers retarded nucleation and spherulite growth and decreased spherulite density. The effect became more pronounced as the KODAR layer thickness was reduced. Spherulites nucleated randomly throughout the KODAR layers in the PC/KODAR 20/80 (w/w) microlayer and grew rapidly to form a continuous layer of impinged spherulites. In contrast, spherulites in the PC/KODAR 40/60 and 60/40 microlayers nucleated and grew along the center of the KODAR layers where the KODAR concentration was highest.