Melting of crystalline polymers

In addition large crystallites have a higher melting temperature than smaU ones. The relationship between the thickness, /, of a crystal and its melting temperature, T, is known as the Thomson-Gibbs equation  

The crystallisation temperature determines the thickness of the lamellae, and then the melting temperature depends on the thickness of the lamellae. So by programming the crystallisation temperature, one can obtain materials with quite peculiar melting patterns, such as multiple melting peaks. Phenomena like these, of course, are not observed in small molecules. 

Some polymers can undergo crystallisation in what is apparently the solid state (for example cold crystaUisation in poly(ethylene terephthalate)). Molecular motion of specific parts of the polymer chains occurs and molecules can then 

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Mandelkern, L. The melting of crystalline polymers. Rubber Chem. and Technol. 32, 1392-1451 (1959). 

Tanaka, K. Uchiyama, Y., "Friction, Wear and Surface Melting of Crystalline Polymers," Advances in Polymer Friction and Wear, L. H. Lee, Ed., Polymer Sci. Techn., Vol. 5B, Plenum Press, New York, 1974, pp. 499-531. 

Melts of crystalline polymers and amorphous polymers characterized by an orientation related to liquid crystalline ordering ... 

The profile of the DSC curves for the melting of crystalline polymers is closely related to the conditions under which the crystal formed. For example, heat polyethylene to 120 °C and maintain it for a definite period of time. After the sample has completely melted, allow it to cool to... 

The many commercially attractive properties of acetal resins are due in large part to the inherent high crystallinity of the base polymers. Values reported for percentage crystallinity (x ray, density) range from 60 to 77%. The lower values are typical of copolymer. Poly oxymethylene most commonly crystallizes in a hexagonal unit cell (9) with the polymer chains in a 9/5 helix (10,11). An orthorhombic unit cell has also been reported (9). The oxyethylene units in copolymers of trioxane and ethylene oxide can be incorporated in the crystal lattice (12). The nominal value of the melting point of homopolymer is 175°C, that of the copolymer is 165°C. Other thermal properties, which depend substantially on the crystallization or melting of the polymer, are Hsted in Table 1. See also reference 13. 

In the case of crystalline polymers better results are obtained using an amorphous density which can be extrapolated from data above the melting point, or from other sources. In the case of polyethylene the apparent amorphous density is in the range 0.84-0.86 at 25°C. This gives a calculated value of about 8.1 for the solubility parameter which is still slightly higher than observed values obtained by swelling experiments. 

In principle the heat required to bring the material up to its processing temperature may be calculated in the case of amorphous polymers by multiplying the mass of the material (IP) by the specific heat s) and the difference between the required melt temperature and ambient temperature (AT). In the case of crystalline polymers it is also necessary to add the product of mass times latent heat of melting of crystalline structures (L). Thus if the density of the material is D then the enthalpy or heat required ( ) to raise volume V to its processing temperature will be given by ... 

In the case of crystalline polymers such as types E and F the situation is somewhat more complicated. There is some change in modulus around the which decreases with increasing crystallinity and a catastrophic change around the. Furthermore there are many polymers that soften progressively between the Tg and the due to the wide melting range of the crystalline structures, and the value determined for the softening point can depend very considerably on the test method used. 

These opposing tendencies may defeat the purpose of the fractional precipitation process. The fractional precipitation of crystalline polymers such as nitrocellulose, cellulose acetate, high-melting polyamides, and polyvinylidene chloride consequently is notoriously inefficient, unless conditions are so chosen as to avoid the separation of the polymer in semicrystalline form. Intermediate fractions removed in the course of fractional precipitation may even exceed in molecular weight those removed earlier. Separation by fractional extraction should be more appropriate for crystalline polymers inasmuch as both equilibrium solubility and rate of solution favor dissolution of the components of lowest molecular weight remaining in the sample. 

Fig. 16. Schematic of a typical temperature profile in a crystallization experiment and the resulting evolution of the fraction of crystalline polymer and dynamic moduli with time. The preheating temperature Tp is above the melting temperature Tm...

Plasticizers and Copolymerization also shift the glass transition responses of the amorphous phase of crystalline polymers. In addition, the degree of Crystallinity and melting point are lowered. The resulting effects on the... 

Crystallinity—about.i to 15% (213,232). The creep of plasticized poly(vinyl chloride) polymers as a function of temperature, concentration, and kind of plasticizer has been studied by many workers, including Aiken et ai. (232), Neilscn ct ai. (234), and Sabia and Eirich (243). These last workers also studied stress relaxation (244). In the case of crystalline polymers, plasticizers and Copolymerization reduce the melting point and the degree of Crystallinity. These factors tend to increase the creep and stress relaxation, especially at temperatures approaching the melting point. 

As the temperature is increased there is available sufficient energy to melt the crystalline polymer, the Tm, and before this for the amorphous polymer sufficient energy so that in both cases ready wholesale movement of polymer chains occurs. The entire polymer now behaves as a viscous liquid such as molasses. For the cross-linked material wholesale mobility is not possible, so it remains in the rubbery region until the temperature is sufficient to degrade the material. 

The prediction of the chemical thermostability is based on the rules on the thermal stability and the reactivity of chemical bonds known for low-molecular-weight compounds. Instead, the physical thermostability depends on the transition points of the macromolecules, i.e., the glass transition temperature Tg in case of amorphous polymers, and additionally the crystalline melting point in case of crystalline polymers. 

The T of crystalline polymers may be determined by observing the first-order transition (change in heat capacity value) by DTA or by DSC (ASTM-D3418). Some comparative information on thermal properties of polyolefins may be obtained from the melt index. To determine the melt index, the weight of extrudate or strand under a specified load and at a specified temperature is measured. Melt index values are inversely related to the melt viscosity. 

The second group involves polymers with three-dimensional ordering of side branches (e.g., those forming Mj-phaseXTable 5). On X-ray patterns of these polymers 3-4 narrow reflexes at wide angles are observed. As a rule, the authors define this type of structure as crystalline, or ascribe a smectic type of structure, characteristic for ordered smectics in SE or SH phases. The heats of transition from anisotropic state to isotropic melt are usually small and do not exceed the heats of transition smectic liquid crystal — isotropic melt . The similarity of structural parameters of three-dimensionally ordered smectics and that of crystalline polymers of the type here considered, make their correct identification quite a difficult task. 

The equilibrium melting temperature, T°m, can be obtained from data for crystals of finite thickness using the Thompson-Gibbs equation. The melting point of crystalline polymers with a well-defined crystal thickness (/c) can be measured and the data extrapolated to 41 = 0 using the Thompson Gibbs equation (Gedde 1995) ... 

White, T. R. Melting behavior of crystalline polymer fibres. Nature (London) 175, 895 (1955). 

The higher thermal conductivity of inorganic fillers increases the thermal conductivity of filled polymers. Nevertheless, a sharp decrease in thermal conductivity around the melting temperature of crystalline polymers can still be seen with filled materials. The effect of filler on thermal conductivity for PE-LD is shown in Fig. 2.5 [22], This figure shows the effect of fiber orientation as well as the effect of quartz powder on the thermal conductivity of low density polyethylene. 

Liquid-crystalline solutions and melts of cellulosic polymers are often colored due to the selective reflection of visible fight, originating from the cholesteric helical periodicity. As a typical example, hydroxypropyl cellulose (HPC) is known to exhibit this optical property in aqueous solutions at polymer concentrations of 50-70 wt%. The aqueous solution system is also known to show an LCST-type of phase diagram and therefore becomes turbid at an elevated temperature [184]. 

Fig. E5.6 Rate of melting of a 2 x 2-in block of HDPE on a hot rotating drum, (a) Drum temperature at 154°C. (b) Drum temperature at 168°C. Rate of melting measured in volume of displaced solid. [Reprinted by permission from D. H. Sundstrom and C. Young, Melting Rates of Crystalline Polymers under Shear Conditions, Polym. Eng. Sci., 12, 59 (1972).]...

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