The relationship between the structure and the optical properties of isotactic polypropylene (PP) containing 1,3:2,4-di-p-methylbenzylidene sorbitol (MDBS) has been studied. It is found that thinner injection-molded plaques of PP/MDBS show higher levels of transparency than compression-molded plaques. The enhanced molecular orientation in the skin layer is responsible for the depression of light scattering because of polarizability fluctuation, i.e., orientation fluctuation in the crystalline phase, because the size of the fluctuation becomes larger than the wavelength of visible light. Further, the crowded network structure of the MDBS fibers generated in molten PP prohibits spherulite formation and, as a result, depresses the spatial size of polarizability fluctuation in the core, which is smaller than the wavelength of visible light. Consequently, light scattering from the core layer is also reduced. FIG. 7. Optical micrographs of the core layer for (left) PP and (right) PP/MDBS. FIG. 8. Oscillatory shear moduli, such as (closed symbols) storage modulus G 0 and (open symbols) loss modulus G
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
The effects of neutralizer species on the transparency of injection-molded plates were studied using isotactic polypropylene (PP) containing a crystal nucleating agent—i.e., 1,3:2,4-bis-o-(4-methylbenzylidene)-d-sorbitol (MDBS). A plate containing lithium stearate (StLi) was more transparent than one containing calcium stearate (StCa) when the MDBS content was 0.1 wt. %. The addition of StLi accelerated the formation of MDBS fibers and the crystallization of PP. However, when the MDBS content was 1.0 wt. %, StCa improved the transparency more effectively than StLi. These results indicate that the combination of an appropriate amount of MDBS and the correct neutralizer species is critical for enhancing the transparency of injection-molded PP plates.
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