Semicrystalline polymers melting range

The melting range of a semicrystalline polymer may be very broad. Branched (low-density) polyethylene is an extreme example of this behavior. Softening is first noticeable at about 75°C although the last traces of crystallinity do not disappear until about 115°C. Other polymers, like nylon-6,6, have much narrower melting ranges. 

Poly(vinyl fluoride) is a semicrystalline polymer with a planar, zigzag conformation.The degree of crystallinity can vary significantly from 20-60% and is a function of defect structures. Commercial PVF is atactic, contains approximately 12%o head-to-head linkages, and displays a peak melting point of about 190°C (52,53,62,63). Poly(vinyl flouride) displays several transitions below the melting temperature. Lower Tg occurs at — 15 to —20°C and upper Tg is in the 40-50°C range. Two other transitions at 80 and 150°C have been reported. 

The amorphous phase of a semicrystalline polymer will follow Cp T), while the crystalline phase will follow Cps(T), between the glass transition temperature Tg and melting temperature Tm. Measurement of Cp(T) as a function of percent crystallinity can therefore enable the extrapolation of both Cps(T) and Cp (T), as limits at 100% and 0% crystallinity, respectively, in this temperature range. Experimental values of both Cps and Cp1 can thus be determined between Tg and Tm if measurements are performed on samples of different percent crystallinity. 

A wide variety of chemical catalysts is nowadays available to polymerize monomers into well-defined polymers and polymer architectures that are applicable in advanced materials for example, as biomedical applications and nanotechnology. However, synthetic polymers rarely possess well-defined stereochemistries in their backbones. This sharply contrasts with the polymers made by nature where perfect stereocontrol is the norm. An interesting exception is poly-L-lactide, a polyester that is used in a variety of biomedical applications [1]. By simply playing with the stereochemistry of the backbone, properties ranging from a semicrystalline, high melting polymer (poly-L-lactide) to an amorphous high Tg polymer (poly-meso-lactide) have been achieved [2]. 

Perfectly crystalline polymers are, however, rarely seen in practice and real polymers may instead contain varying proportions of ordered and disordered regions in the sample. These semicrystalline polymers usually exhibit both Tg and T i (not r, ) corresponding to the disordered and ordered regions, respectively, and follow curves similar to E-H-D-A. Tm is lower than and more often represents a melting range, because the semicrystalline polymer contains crystallites of various sizes with many defects which act to depress the melting temperature. 

Both Tg and Tm are important parameters that serve to characterize a given polymer. While Tg sets an upper temperature limit for the use of amorphous thermoplastics like poly(methyl methacrylate) or polystyrene and a lower temperature limit for rubbery behavior of an elastomer-like SBR rubber or 1,4-cw-polybutadiene, Tm or the onset of the melting range determines the upper service temperature for semicrystalline thermoplastics. Between T,n and Tg, these polymers tend to behave as a tough and leathery material. They are generally used at temperatures between Tg and a practical softening temperature that lies above Tg and below Tm-... 

The stmctural dependence of the crystalline melting temperature is essentially the same as that for the glass transition temperature. The only dilTerence is the effect of structural regularity, which has a profound influence on crystallizability of a polymer. T is virtually unaffected by structural regularity. From a close examination of data for semicrystalline polymers it has been established that the ratio Tg/T , (K) ranged from 0.5 to 0.75. The ratio is formd to be closer to 0.5 in symmetrical polymers (e.g., polyethylene and polybutadiene) and closer to 0.75 in unsymmetrical polymers (e.g., polystyrene and polychloro-prene). This behavior is shown in Figure 4.9. 

Some polymers are essentially amorphous (e.g., polystyrene, acrylonitrile butadiene styrene copolymer, polycarbonate, and polymethyl methacrylate) while others are semicrystalline (e.g., polyolefins and polyamides). The former tend to have a wide melting temperature range with a comparatively high melt strength, while semicrystalline polymers tend to have a narrow melting temperature range and frequently a low melt strength. 

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