Semicrystalline structure continuous

Fig. 3. Differential thermal analysis of linear low density polyethylene on cooling (continuous lines), followed by heating (broken lines), showing a high content of reversing crystallization and melting. Standard DSC thin lines TMDSC thick lines. The overall supercooling contrasts the partially reversible crystallization and melting after an overall metastable, semicrystalline structure has been set up on the initial cooling. The modulation amplitude on TMDSC is given by the letter A, and the modulation period by p.

One might think at first that the formation of the semicrystalline structure is essentially completed when the crystallization at the first chosen temperature is finished. This is not the case. Crystallization continues on cooling to room temperature, proceeding by two different modes of secondary crystallization. 

In semicrystalline polymers such as polyethylene, yielding involves significant disruption of the crystal structure. Slip occurs between the crystal lamellae, which slide by each other, and within the individual lamellae by a process comparable to glide in metallic crystals. The slip within the individual lamellae is the dominant process, and leads to molecular orientation, since the slip direction within the crystal is along the axis of the polymer molecule. As plastic flow continues, the slip direction rotates toward the tensile axis. Ultimately, the slip direction (molecular axis) coincides with the tensile axis, and the polymer is then oriented and resists further flow. The two slip processes continue to occur during plastic flow, but the lamellae and spherullites increasingly lose their identity and a new fibrillar structure is formed (see Figure 5.69). 

Table 4.3 shows the permselectivity characteristics of pure, semicrystalline PEO films [76]. The selectivity characteristics for 02/N2 are rather similar to those for silicone rubber and natural rubber shown in Table 4.2. However, the values of permselectivity for C02 relative to the various light gases shown are all much higher than Table 4.2 shows for the rubbery polymers listed there and even for polysulfone except for C02/CH4. Comparison of the data in Tables 4.2 and 4.3 makes it clear that this high permselectivity of PEO stems from its high solubility selectivity for C02 versus other gases this is augmented by modest values of diffusivity selectivity. Data in Table 4.4 for the C02/N2 pair illustrate that this effect can be translated into various block-copolymer structures when the PEO content is high enough to ensure it is the continuous phase. In fact, nearly all these materials have higher permselectivity and solubility selectivity for C02/N2 than does pure PEO (see Table 4.3) however, the diffusion selectivity for these copolymers is much closer to, or even less than, unity than seen for pure PEO. Furthermore, the copolymers all have much higher absolute permeability coefficients than does PEO.

This subblock of the Heat Capacity Data Bank contains empirical as well as theoretical prediction calculations of the heat capacity which are continuously updated. Based on the chemical structure (back-bone, side-chain) as well as on the physical state (glass, crystal, mesophase, liquid, semicrystalline, equilibrium, history), a heat capacity is retrieved. The Prediction Scheme subblock can also be searched for its base, i.e., the precise assumptions which go into the prediction, with documentation to the 1iterature. 

Position Dependence. In polymers with heterogeneous structures— for example, semicrystalline polymers and filled elastomers— the transport process is complicated by the generally impermeable dispersed phase. Not only does the crystallite or filler particle create a larger path for the diffusing molecule to traverse, but also the presence of a high area interface within the polymer changes the nature of the continuous phase from that of the pure homogeneous state. These effects are related by the expression (4) ... 

Finally, the left curves of Fig. 2.45 show that above about 260 K, melting of small, metastable crystals causes abnormal, nonlinear deviations in the heat capacity versus crystallinity plots. The measured data are indicated by the heavy lines in the figure. The thin lines indicate the continued additivity. The points for the amorphous polyethylene at the left ordinate represent the extrapolation of the measured heat capacities from the melt. All heat capacity contributions above the thin lines must thus be assigned to latent heats. Details of these apparent heat capacities yield information on the defect structure of semicrystalline polymers as is discussed in Chaps. 4-7. 

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. 

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