Semicrystalhne polymers

Copolymentation of CTFE and ethylene yields linear, semicrystalhne polymers known as ECTFE resins. They have a highly altemahng structure... 

Poly(vinyl fluonde) is a linear semicrystalhne polymer of the following general structure... 

In reality, the morphology of a polycrystalline thermoplastic consists of spherulites which holds for common PP, PE, PA 6, PA 6,6 and PEEK crystalHzed under common conditions. Some semicrystalhne polymers as weU as the above mentioned moderately filled ones may exhibit lameUar crystahine morphology without any spherulitic order. As a result of random orientation of individual crystallites in spheruhtes and the manner of their connectivity, the elastic modulus of about 10 GPa has been extrapolated for a hypothetical ideal polycrystalline PE containing no amorphous phase from the dependence of the elastic modulus of PE on the degree of crystallinity. The presence of an amorphous phase which reduces the content of the crystalline phase results in a further reduction of the overaU elastic modulus of the semicrystalhne polymers compared to ideal mono crystals. 

From a commercial point of view, semicrystalline polymers are of prime importance. Among the four mostly used commodity plastics (PE, PS, PVC and PP), only PS is completely amorphous. The three semicrystalhne polymers account for the largest volume of the commercial polymer blends. A majority of the polymer blends contains at least one crystalline component. Most polymer blends are immiscible. 

Figure 16.18 Schematic DTA thermal curves for the totally amorphous polymer structure and the semicrystalline polymer structure shown in Fig. 16.17. Both show only the semicrystalhne polymer has a crystallization exotherm.

Gj is a pre-exponential constant assitmed to be constant or proportional to 7, AF is the activation energy responsible for the transport of molecnlar chains across liquid-sohd interface. A is the free energy of formation of a snrface nucleus with critical size. R and is gas constant and Boltzman constant, respectively. The quantity G is influenced by the type, molecular weight, composition, and so on of the semicrystalhne polymer. 

As well known, nanofillers often have significant influence on the crystal morphology of a semicrystalhne polymer. As shown in Figure 5.5, various spheruHte morphologies of PET and PET /clay nanocomposites have been characterized with polarized optical microscopy [21]. Neat PET demonstrates a well-defined spheruHte texture with a fibril pattern and a maltese cross at 200 °C. However, the maltese cross is not observed at higher crystallization temperature. PET/clay nanocomposites exhibited typical crystalline morphologies indicating that day exhibits a... 

For semicrystalhne polymers the same workers identified relevant absorptions for crystalhne and amorphous bands and concluded that, for blends containing 70 wt % or more phenoxy, there was no PCL crystalhnity and blends were totally amorphous [85]. This limitation is consistent with the data of Harris et al. [83]. 

The conclusion based on these very limited dynamical studies is that existing models are adequate for understanding the spinline dynamics of amorphous polymers, such as PET at takeup speeds below 4,000 m/min, but they must be used with caution for semicrystalhne polymers. It is not possible to draw a firm conclusion about the importance of the treatment of the solidification process in the frequency response analysis, although stress-induced crystallization and the effect of the crystalline phase on the mechanics are likely to be important and should be included since the theoretical framework is available. 

The preceding discussion allows one to address the problem of the phase state of sixrface and interfacial layers of pol3rmers in composites. Despite the non-uniformity, they can be characterized by their intrinsic dimensions, thermodynamic fimctions (entropy, enthalpy, specific volume), and the distinctions of mean local properties from the properties of the polymer in the brdk. In a number of instances these distinctions may be similar to the difference in the properties between amorphous and crystalhne regions in semicrystalhne polymers. The redistribution of fractions of different molecular mass in a smface layer, taking account of hmited thermodynamic immiscibility of polymer homo-logues, provides a basis to consider the transition layer as an independent phase. However, whether the surface and interfacial layers can be considered as an independent phase in the thermod5mamical meaning or not is a very important question. 

PVDF is a semicrystalhne polymer (35-70% crystallinity) with an extended zigzag chain [65,66,72]. Head-to-tail addition of VDF dominates, but there are head-to-head or tail-to-tail defects that affect the crystallinity and properties of PVDF. 

Gvozdic, N. V., Meier, D. J. On the melting temperature of syndiotactic polystyrene 2 enhancement of the melting temperature of semicrystalhne polymers by a novel anneaUng procedure. Po/ym. Cotmtmmtz., 32(16), 493-494 (1991). 

SPS is a semicrystalhne polymer with a glass transition temperature (Tg) of 100°C and a crystaUine melting point (Tm) of 21Q°C. SPS readily accepts fillers, and by compounding with short glass fibers, the heat deflection temperature can be increased to close to the crystalline melting point. A comparison of the Tg and Tm of SPS with other engineering thermoplastic materials is shown in Figure 15.1. 

Poly trimethylene terephthalate A semicrystalhne polymer that is similar to PBT and has many of the same characterishcs. First synthesized in 1941, it has seen little commercial applicahon because it was more expensive to produce than either PBT or PET. However, recent developments— including the ability to generate the intermediate propanediol from com sugar— have helped make it more cost effechve. This is also a significant advance in the use of renewable materials. 

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