Semicrystalline engineering

Novel Composites from Blends of Amorphous and Semicrystalline Engineering Thermoplastics with Liquid-Crystalline Polymers... 

Poly(vinylidene fluoride) (PVDF) is a semicrystalline engineering polymer with very good resistance to chemicals, oxidation, and UV radiation (J. H. Yen, 2006). PVDF is known for its polymorphism crystalline structure and complicated microstructure. It is one of the most widely studied polymers due to its non-linearity, piezo- and pyro-electricity (L. T. Vo, 2007 K. Pramoda, 2005). PVDF can crystallize in at least five well-known crystalline phases (E. Giannetti, 2001, A. Lovinger, 1982 N. S. Nalwa, 1995) ... 

Polyacetals, also referred to as polyoxymethylenes (POMs) or polyfonnalde-hydes, are a semicrystalline engineering thermoplastic polymerized as a homopolymer and copolymer. The homopolymer and copolymer have somewhat different molecular structures and performance values. The difference between performance values is narrowing with new formulations (compounds). Polyacetal engineering thermoplastics were introduced to the world in 1956 with the potential of replacing metals, aluminum, brass, and cast zinc, which polyacetals continue to do. 

TABLE 7.2 Properties of PBT Compared with Other Semicrystalline Engineering Thermoplastics... 

MDI Azelaic acid Semicrystalline engineering plastic poly( amide)... 

Homopolymers derived from MDI and azelaic acid are semicrystalline engineering plastics with a Tg of 135°C and a Tm of 290°C (88). Copoljrmers of MDI with azelaic acid, containing 20-30 mol% of adipic acid show a eutectic Tm of approximately 240°C. These amorphous or slightly crystalline copolymers have mechanical properties comparable to transparent nylons or polycarbonates. Although injection molded samples are transparent, they will crystallize and turn opaque. Copolyamides derived from MDI and aromatic dicarboxylic acids are more difficult to prepare. Because of the very high Tm (420°C) of the isophthalic acid/MDI block it was necessary to prevent the formation of any appreciable ciystalline blocks, which was accomplished by prereacting a portion of the isophthalic acid (15-20 mol%) with 2,4-TDI. In this manner crystallization of the isophthalic acid/MDI blocks was surpressed (89). Thus, copolyamides containing IPA/azelaic acid (50 50) are obtained with thermal and mechanical properties similar to poly-sulfone. 

While there are subtle differences within the family, all of these materials are semicrystalline engineering thermoplastics, with high strength and toughness, high chemical resistance, and tiigh-temper-ature resistance. 

Aromatic-aliphatic polyesters, in which either R1 or R2 is aromatic, are generally high-melting (150-270°C) semicrystalline materials that find applications as engineering thermoplastics, films, or fibers. 

This class of polyesters consists of four major commercial polymers and their copolymers, namely PET, PTT, PBT, and PEN (see Table 2.1). They compete for engineering thermoplastics, films, and fibers markets with other semicrystalline polymers, such as aliphatic polyamides, and for some other applications with amorphous engineering plastics such as polycarbonate. The syntheses of PET and PBT, detailed in numerous reviews and books,2-5 are described in Sections 23.2.2 and 2.3.2.1. 

Thermomechanical behaviour is most probably the most widely exploited property of engineering thermoplastics. Figure 3.1 shows the behaviour of two types of thermoplastic, one amorphous and the other semicrystalline, versus temperature. We can see several steps moving from low to high temperatures ... 

Fig. 5.20 The PED for PP is apparently smaller in magnitude than that for PS it is also more temperature dependent, decreasing with increasing initial temperature. Semicrystalline plastics are weaker and their amorphous phase in the region T > Tg becomes more mobile, rapidly lowering the needed deformation stresses. [Reprinted by permission from M.H. Kim, Ph.D. Thesis, Department of Chemical Engineering, Stevens Institute of Technology, Hoboken, NJ (1999).]...

Polyacrylonitrile, which is a semicrystalline polymer, can be used for many engineering applications, such as fiber spinning or for housing and package applications. A peculiarity of polyacrylonitrile is that it is insoluble in its monomer. This makes it very difficult to homopolymerize acrylonitrile in an emulsion polymerization process since nucleated polymer particles cannot grow by monomer swelling. 

Several liquid cryatalline polymers were melt blended with an amorphous (Ultem) and two semicrystalline (PEEK and PPS) engineering thermoplastics in a single screw extruder. Flat film was processed with different degrees of stretch imparted while the film was being cooled. In the case of Vectra, which was thought to be ideally suited to be blended with PPS based on thermal and rheological... 

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