Crystalline thermoplastics

Crystalline polymers undergo a discontinuous decrease in volume when cooled through (Fig. 4). This can lead to nonuniform shrinkage and warping in molded objects. On the other hand, it also causes the polymer to "lock on" to reinforcing fibers, eg, glass (qv), so that crystalline thermoplastics benefit much more than amorphous thermoplastics from fiber reinforcement. 

The pseudocross-links, generated by the hard-segment interactions, are reversed by heating or dissolution. Without the domain crystallinity, thermoplastic polyurethanes would lack elastic character and be more gum-like in nature. In view of the outlined morphology, it is not surprising that many products develop their ultimate properties only on curing at elevated temperature, which allows the soft- and hard-phase segments to separate. 

Polycarbonates. Polycarbonates (qv) are pardy crystalline thermoplastics with some disorder in the crystalline part and considerable order in the amorphous part. This disorder conveys high impact strength which, combined with its good transparency and outdoor exposure resistance, makes polycarbonates usefiil for vandal-resistant glazing and outdoor lighting. It is easily processed by extmsion and injection mol ding. Various uv and dame-retardant agents are often added. 

Advanced Thermoplastics Materials. Thermoplastics and linear plastics of finite molecular weight that can be fabricated into very complex stmctures by hot melt or injection mol ding are different from the thermoset materials that require cross-linking to build up infinite molecular weight to form network (cross-link) stmctures. Advances in thermoplastic engineering materials include amorphous thermoplastics, crystalline thermoplastics, Hquid crystal thermoplastics, and fluorinated thermoplastics (see Engineering plastics). 

As is common for crystalline thermoplastics the deflection temperature of unfilled grades is similar to Tg (quoted as being in the range 45-70°C) while for glass-filled grades it is much closer to the T of 225°C. 

Thermoplastics may themselves be considered in four sub-classes (a) amorphous thermoplastics, (b) rubber-modified amorphous thermoplastics, (c) plasticised amorphous thermoplastics and (d) crystalline thermoplastics. 

Friedrich, K. Crazes and Shear Bands in Semi-Crystalline Thermoplastics. Vol. 52/53, pp. 225-274. 

For the past century one successful approach is to plot a secant modulus that is at 1% strain or 0.85% of the initial tangent modulus and noting where they intersect the stress-strain curve (Fig. 2-2). However for many plastics, particularly the crystalline thermoplastics, this method is too restrictive. So in most practical applications the limiting strain is decided based on experience and/or in consultation between the designer and the plastic material manufacturer. Once the limiting strain is known, design methods based on its creep curves become rather straightforward (additional information to follow). 

The carbon atom which carries the methyl group is chiral, but biosynthesis is stereoselective, and gives rise to a natural polymer with the R configuration. The polymer is a partially crystalline thermoplastic which melts at about 80 °C. 

Crystalline thermoplastics—HDPE, linear low-density polyethylene (LLDPE)... 

The true value of the chloropolymer (I) lies in its use as an intermediate for the synthesis of a wide variety of polytorgano-phosphazenes) as shown in Figure 1. The nature and size of the substituent attached to the phosphorus plays a dominant roll in determining the properties of the polyphosphazene. Homopolymers prepared from I, in which the R groups are the same or, if different, similar in molecular size, tend to be semi-crystalline thermoplastics. If two or more different substituents are introduced, the resulting polymers are generally amorphous elastomers. (See Figure 1.)... 

Solvent Resistance. One of the distinct advantages of a crystalline thermoplastic elastomer over an amorphous one should be its superior solvent resistance, since the latter types are generally soluble. Table III shows the swelling behavior of the H2-BIB triblocks in toluene at 25°C. It can be seen that the maximum swelling obtained was in the case of the H2-BIB-34, which had the lowest end-block content. Furthermore, the equilibrium swelling ratio of 3-26 obtained for this polymer is considerably less than the value of 5 or 6 generally exhibited by a well-vulcanized natural rubber. 

The properties of PHAs are dependent on their monomer composition and therefore it is of great interest that recent research has revealed that, in addition to PHB, a large variety of PHAs can be synthesized microbially. The monomer composition of PHAs depends on the nature of the carbon source and microorganism used. PHB is a typical highly crystalline thermoplastic whereas medium chain length PHAs are elastomers with low melting points and a relatively lower degree of crystallinity. By (chemical) modification of the PHAs, the ultimate properties of the materials can be adjusted even further, when necessary. 

Ogata, N., Yasumoto, H., Yamasaki, K., Yu. H., Ogihara, T., Yanagawa, T., Yoshida, K. and Yamada, Y. (1992). Evaluation of interfacial properties between carbon fibers and semi-crystalline thermoplastic matrices in single fiber composites. J. Mater. Sci. 27, 5108-5112. 

Friedrich, K. and Karger-Kocsis, J. (1989). Unfilled and short fiber reinforced semi-crystalline thermoplastics. In Fractography and Failure Mechanisms of Polymers and Composites, (A.C. Roulin-Moloney ed.), Elsevier Appl. Science, London, pp. 437-494. 

All three transitions are shown by a semi-crystalline thermoplastic with a chain length long enough to extend the rubbery region to above the melting point. 

Only Tg and are present in an amorphous thermoplastic and in a non-vulcanised rubber. A semi-crystalline thermoplast with short chains only shows Tg and Tm- In a (very exceptional) fully crystalline polymer Tg is not present... 

These moldable thermoplastic polyesters are marketed under the trade names Celanex, Tenite, and Valox. These highly crystalline thermoplastics may be extruded or injection molded and may also be reinforced by fiberglass. 

APC-2 is a semi-crystalline thermoplastic prepreg. The surface roughness characterization for two different batches of APC-2 prepreg are reported in Table 7.2. Batch I is a 152.4 mm (6in.)-wide prepreg sheet, whereas batch II is 304.8-mm wide. In addition, the... 

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