IPS-iPP blends

The recent successful synthesis in our laboratory of styrene-propylene diblock copolymer (iPS-fc-iPP), with crystalline isotactic structure in each block (20, 21), makes it possible to attempt toughening of the iPS-iPP blend by the method of block copolymer emulsification of the diblock copolymer. This... 

In the transmission electron microscopic (TEM) studies, we found that it was exceedingly difficult to obtain ultramicrotomed sections of iPS-iPP blends, whereas the iPS-fo-iPP diblock copolymer could be cut with relative ease. This result exhibits one major difference between the diblock copolymer and the corresponding homopolymer blend. Unfortunately, owing to the difficulty of finding a selective staining technique, the sample of diblock copolymer did not display visible contrast or obvious structural features in the TEM studies. However, the results of SEM studies do reveal a clear difference between the blend and the diblock copolymer the macrophase separation is revealed on the etched surface of the blend and is not present in the copolymer (Figure 3). The diblock copolymer exhibits only a finely dispersed and continuous submicron structure throughout the field of view, as expected. 

Figure 3. SEM images of cast films of (a) iPS-b-iPP copolymer (40/60), and (b) iPS-iPP blend (40/60). The surface of the castingfilms was etched with allylamine vapor at room temperature for 1 h. The scale is 20 pm.

Taking all the fact presented in this section into account, together with the synthesis method and fractionation results, we conclude that the purified copolymer separated from reaction products is an iPS-fo-iPP diblock copolymer consisting of iPS and iPP blocks it is definitely not a simple blend of homopolymers. On the other hand, the distinctive characteristics of the copolymers crystallization kinetics also indicate that, compared with homopolymers and the iPS-iPP blend, the purified copolymer is a true iPS-fo-iPP diblock copolymer (23). 

Figure 5. Triangular phase diagram showing the composition of the iPS-b-iPP-iPS-iPP blends examined.

Enhancement of mechanical properties is of interest only if it is not accompanied by a loss of other important properties of the blend. Of particular concern for such polymer blends is stiffness, because most means of increasing impact strength also reduce stiffness (14-19). But this is not the case for the iPS-fc-iPP-iPS-iPP blends studied here as seen in Table II. It is clear that the enhancement in toughness just described is not accompanied by a loss of stiffness, but it is essentially unaffected by the compatibilizer. And the stiffness of iPS-fc-iPP-iPS-iPP is higher than that of iPP and HIPS. The impact-modulus behavior seems to be due to the tough (or rigid) characteristics, morphologies of phases, and semicrystalline isotactic structure of each block in the iPS-b-iPP diblock copolymer. 

Of considerable interest is the fact that added iPS in PP resin can improve the heat resistance of the materials and thus upgrade their endurance at elevated temperature. Also, as seen in Table II, the HDT of iPS-iPP blends is, as expected, higher than that of common HIPS and iPP. The results seem to be interrelated with the isotactic structure and crystallinity of the iPS component. The iPS-fc-iPP that is added contributes to a further improvement in the HDT of the iPS-iPP binary blend. Figure 6 shows that, at lower measured pressure, the HDTs increase slowly with increasing iPS-fo-iPP copolymer. The HDTs of the blends, however, increase rapidly at higher measured pressure as iPS-fc-iPP copolymer is increased. 

Table III. DSC Melting and Ciystallization Results for Homopolymers and iPS-b-iPP-iPS-iPP Blends...

Enhanced interphase interactions, deduced from thermal and dynamic mechanical properties and morphology observed by SEM, demonstrate the efficient compatibilizing effect of iPS-fo-iPP copolymer on iPS-iPP blends. Each sequence of the iPS-fc-iPP diblock copolymer can probably penetrate or easily anchor its homopolymer phase and provide important entanglements, improving the miscibility and interaction between the iPS and iPP phases. This is in good agreement with what is inferred from the mechanical properties of the iPS-fo-iPP-iPS-iPP polyblends. 

Preparation of iPS-b-iPP-iPS-iPP Polyblends. The blends of iPS-fo-iPP, iPS, and iPP were prepared by completely dissolving all polymers and an antioxidant into o-dichlorobenzene at 165 °C, and using a 1 1 mixture of acetone and methanol as a precipitant. After being thoroughly washed and dried under vacuum at 60 °C for 20 h, the precipitated powders were compression-molded at 300 °C into sheets or plates suitable for cutting specimens for mechanical testing and morphologic study. 

It is well known that the third components, although small in quantity, might also bring about other significant changes such as changes in crystalline behavior, morphology, and dynamic mechanical properties, which influence the blend properties for end use. Table III lists the data obtained by DSC for iPS-fc-iPP-iPS-iPP ternary blends. The results indicate that PP crystallization... 

Thus Xu and Lin (154) successfully compatibilized a high molecular weight blend of iPS and iPP with a iPS-iPP diblock copolymer, where the molecular weight of both blocks amoimted to 150,000. Feng (165) has shown that for PS/polyolefin blends, even PE-g-PS graft copolymers can be suitable compatibihzers. 

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