Relation between structure and properties is studied for polypropylene (PP) containing 1,3:2,4‐di‐benzylidene sorbitol (DBS) that forms network structure composed of nanofibrils in a molten PP. It is found that the aggregation state of DBS, which can be controlled by the applied flow field and thermal history, affects the spherulite texture of PP and thus the transparency. When injection‐molded products, in which the nanofibrils of DBS orient to the flow direction, are reprocessed at 180 °C, that is, lower temperature than the melting point of DBS, the obtained material shows high level of transparency, although it has been believed that sorbitol‐derivatives have to be melted and dissolved into the molten PP at high temperature, for example, 240 °C. Further, it is found that the particle flow occurs in the blend at 180 °C. The applied shear force generates the particles, that is, flow units, by the fragmentation of the network structure. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 41–47, 2008

ABSTRACT:The rheological properties are studied for binary blends composed of a low-density polyethylene (LDPE) and a linear polyethylene. It is found that some blends exhibit higher oscillatory shear moduli and drawdown force than the individual pure components, demonstrating that relaxation mechanism with longer characteristic time appears in the blend. The anomalous rheological behavior is detected more clearly for the blends with autoclave-LDPE than those with tubular-LDPE. Furthermore, the number of short-chain branches in a linear polyethylene has no influence on the rheological properties of the blends, suggesting that the phase separation will not be responsible for the anomalous behavior. It is also found that blends of autoclave-LDPE and tubular-LDPE show no synergetic effect.

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Effect of applied processing history on flow instability at capillary extrusion is studied using a commercially available low-density polyethylene (LDPE) having long-chain branches. It is found that processing history in an internal mixer in a molten state depresses long-time relaxation mechanism associated with long-chain branches, which is known as ''shear modification.'' Consequently, the onset of output rate for melt fracture increases greatly. Furthermore, it should be noted that the sample having intense shear history shows shark-skin failure without volumetric distortion, although it has been believed that LDPE exhibits gross melt fracture at capillary extrusion. The reduction of elongational viscosity by the alignment of long-chain branches along to the main chain is responsible for the anomalous rheological response. As a result, the sample shows shark-skin failure like a linear polyethylene at a lower output rate than the critical one for gross melt fracture.