Fractionation of crystalline polymers

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...

Here, Da is diffusion coefficient in the amorphous phase alone, oc is the volume fraction of crystalline polymer, and t is a scalar quantity that denotes the tortuosity of diffusional path of the solute. The value of Da may be estimated by the Peppas-Reinhart model if the amorphous regions of the polymer are highly swollen. This substitution yields... 

As is well established, fractionation of crystalline polymers can only be effective if the phase separations are of the liquid-liquid type. Since the amylose concentration in the precipitates obtained from the salt systems is at least 25%, Figure 4 indicates that liquid-liquid phase-separation has a chance to occur only if the critical temperature of precipitation is higher than 100°. In order to reach precipitation temperatures above 100°, the salt concentration of the system has to be higher than 12 % (see Figure 3). This conclusion is confirmed by the experimental results, as practically... 

The reader will have noted that although we have made precise statements about the molecular conformation in the crystal, our description of molecular conformations in the amorphous fraction of crystalline polymers has been vague. There are two reasons for this. First, there is no precise physical technique available for determining the amorphous conformation. Second, statistical methods are inapplicable for the short length of the molecule which is constrained between the crystals for the long macromolecule in the completely amorphous state the position is quite different, as we now show. 

Curves 2 and 3 in Figure 5.5 show the shapes obtained for retention diagrams when Xc = 0.33 (curve 2) and 0.66 (curve 3) [171]. They were drawn using the following assumptions the vaporization enthalpy in the amorphous regions below is the same as that in the melt above Tb, that the fraction of crystalline polymer remains constant below a temperature and that no new retention mechanisms arise, such as adsorption on crystalline surfaces. Thus curves 1, 2 and 3 are parallel at temperatures below Curve 4 shows the behaviour of the completely crystalline polymer whose 7 = 0 below... 

The general rule that like dissolves like holds well with polymers for example, polymers absorb considerable quantities of their own monomers. If the polymer is linear it may well completely dissolve in its own monomer or other good solvent. If the solubility is lower then the polymer, when immersed in solvent, will absorb in equilibrium a fraction of its own mass and will not dissolve the lower the solubility the lower the fraction. In (1.N.9) the equilibrium water contents of nylons are given for SO per cent and lOO per cent RH. This is the behaviour observed for vapours equilibrium absorption increases with vapour pressure. The absorbed molecules are absorbed only into the amorphous fraction of crystalline polymers such as the nylons. Cross-linking in rubbers lowers the absorption the swelling of the network is impeded by the crosslinks. As the network expands because of the absorption of diluent, the entropy forces between tie points increase (Figs 3.3 and 3.4) and this limits the swelling. 

Furukawa and coworkers demonstrated that the effectiveness of diethylzinc increased with controlled addition of water, methanol, or oxygen (81, 82) (Tables 6 and 7). These polymerization systems with propylene oxide produced a substantial fraction of crystalline polymer (16 percent in the case of water/diethylzinc). In some cases, poly(propylene oxide) was prepared with molecular weights estimated to be tens of millions (84). Efforts to understand these polymerization systems centered on identifying the coordination sites available as initiation sites on different metal alkoxides and on differentiating cationic initiation systems from coordinate initiation systems. In some cases, very similar systems led to quite different polymerization reactions and sometimes very different polymer products. 

In order to carry out an experimental study of the kinetics of crystallization, it is first necessary to be able to measure the fraction d of polymer crystallized. While this is necessary, it is not sufficient we must also be able to follow changes in the fraction of crystallinity with time. So far in this chapter we have said nothing about the experimental aspects of determining 6. We shall now briefly rectify this situation by citing some of the methods for determining 6. It must be remembered that not all of these techniques will be suitable for kinetic studies. 

Since the fractions of crystalline (subscript c) and amorphous (subscript a) polymer account for the entire sample, it follows that we may measure whichever of the two is easiest to determine and obtain the other by difference. Generally, it is some property P, of the crystalline phase that we are able to... 

The plastic deformation of such polymers is a major research area and has a triennial series of conferences entirely devoted to it. The process seems to be drastically different from that familiar from metals. A review some years ago (Young 1988) surveyed the available information about polyethylene the yield stress is linearly related to the fraction of crystallinity, and it increases sharply as the thickness... 

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. 

In most cases, however, polymers crystallize neither completely nor perfectly. Instead, they give semicrystalline materials, containing crystalline regions separated by adjacent amorphous phases. Moreover, the ordered crystalline regions may be disturbed to some extent by lattice defects. The crystalline regions thus embedded in an amorphous matrix typically extend over average distances of 10-40 nm. The fraction of crystalline material is termed the degree of crystallinity. This is an important parameter of semicrystalline materials. 

What explanation can be given for this apparently contradictory result that both yield and molecular weight of crystalline polymer increase with the increase in the amount of catalyst It seems reasonable to assume that the experimental result is due to a cocatalytic action of some substance contaminating the polymerizing system. This substance may activate a fraction of the catalyst by its cocatalytic action and the unactivated catalyst may suppress effectively the termination or chain transfer reaction. If this assumption is correct, some correlations should exist between the amount of catalyst and that of cocatalyst. The most plausible candidate for such a cocatalyst is water and/or oxygen. 

Polymerization of propylene oxide-a-d was carried out by the EtZnNBu ZnEt catalyst in benzene solution in the presence of varying amounts of added water at 70° C, and was terminated after 7 days. The microstructure of the crude polymer was determined by the 1H-NMR method and the yields of amorphous and crystalline polymers were determined by a fractionation method (Fig. 16). When the amount of added water was increased up to 0.3 mole per mole of catalyst, the yield of crystalline polymer increased remarkably, whereas that of amorphous one remained nearly constant, and the isotactic dyad content (I) increased remarkably while syndiotactic one (S) remained almost constant. Thus, the striking parallel was observed between the yield of crystalline polymer and the isotactic dyad content, and between the yield of amorphous polymer and the syndiotactic dyad content. It is therefore concluded that water contributes more remarkably to the formation... 

In principle, the calorimetric determination of the fraction of crystallinity of a semicrystalline polymer is simple. One needs only to know (1) the enthalpy change required to convert the sample to the completely... 

Nevertheless, when we carry out x-ray crystallinity measurements on textile fibers, we must consider distortions that always affect crystalline material. Even in a completely crystalline material, the scattered x-ray intensity is not located exclusively in the diffraction peaks. That is because the atoms move away from their ideal positions, owing to thermal motion and distortions. Therefore, some of scattered x-rays are distributed over reciprocal space. Because of this distribution, determinations of crystallinity that separate crystalline peaks and background lead to an underestimation of the crystalline fraction of the polymer. In this paper, we attempt to calculate the real crystallinity for textile fibers from apparent values measured on the x-ray pattern. This is done by taking into account the factor of disorder following Ruland s method (3). 

The narrow component corresponds to the amorphous fraction of the polymer and the axially symmetric powder pattern to the crystalline fraction of the polymer. Moreover, the amorphous as well as the crystalline line shapes extracted from samples of different crystallinity are essentially identical, which indicates that the structure and dynamics in the crystalline and amorphous fractions are independent of the degree of crystallinity. 

Phillips catalysts are active in the polymerisation of propylene and higher a-olefins, yielding tacky polymers with irregular structure and small amounts of crystalline polymers in the case of polypropylene, a small amount of crystalline fraction appeared to constitute the isotactic polymer [236]. 

It is to be recalled that effective fractionation is hard to achieve especially in the case of crystalline polymers. However, we also recall that special difficulties with fluorescence occur in light scattering measurements with polyacrylonitrile [see Cleland and Stockmayer (68 ) and Krigbatjm and Kotliar (750)]. 

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