Solubility of crystalline polymers

The solubility of crystalline polymers is rather less than for amorphous polymers. Polyethylene and isotactic polypropylene have much less miscibility in potential solvents than that exhibited by amorphous aliphatic polymers such as polyisobutylene or ethylene-propylene copolymer. This is associated with the necessity of the heat of mixing overcoming the heat of fusion of the crystalline lattice. The aliphatic hydrocarbon solvents, which dissolve amorphous polymers such as polyisobutylene, have small heats of mixing which carmot disrupt the crystalline lattices of polyolefins. The amorphous regions of the semi-crystaUine polyolefins are swollen by aliphatic hydrocarbon solvents and their mechanical properties are lowered, but they do not dissolve. It is only when the temperature approaches the polymer s melting point that dissolution occurs. 

The situation can be different with polar polymers. Formic acid and sulfuric acid are well able to overcome the lattice energies of a polar crystalline polymer such as polyamide 6 and dissolve it. These acids are not, however, effective with non-polar crystalline polyolefins, though they may help induce chemical degradation. 

For a polymer with a low level of crystallinity such as polyvinyl chloride, there is quite extensive swelling (in polar solvents) and a rubbery plasticized material is obtained, which is held together by a crystalline network (see Section 3.11) 

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Cost effectiveness of PHA isolation does not only depend on equipment and chemicals needed, but, most of all, on the yields for product recovery and the possibility to reutilize the compounds needed for the isolation. For direct extraction of PHA from biomass extraction solvents that can easily be recycled will be of interest [69]. Solubility of PHAs, especially pure crystalline PHB, is a complex topic. Unlike most low molecular mass compounds and noncrystaUizing amorphous polymers, the solubility of crystalline polymers such as PHB cannot be predicted... 

Liquid/solid Equilibria. The solubility of crystalline polymers is normally considerably lower than that of amorphous polymers because they require an additional energy, namely, the heat of fusion, in order for the bulk polymer to mix with solvent. Fig. 6 shows as an example the behavior of semi crystalline polyethylene in two different solvents(20). The solvent xylene is favorable in the temperature range of interest (no liquid/liquid demixing) up to the melting temperature T. o of the pure polymer a saturated solution coexists with the crystalline polyethylene and the components are completely miscible once T has surpassed Tm,o- Nitrobenzene on the other hand, is thermod5mamically less favorable and exhibits liquid/liquid demixing in addition to the solid/liquid phase separation. In this case one observes the coexistence of three phases at a characteristic temperature (broken line in Fig. 6) and concentration. 

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. 

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. 

The trans-poly-1,4-butadiene isomer is a harder and less soluble rigid crystalline polymer than the cis isomer. As shown by the skeletal structures for the trans isomer (Figure 1.11), chain extensions on opposite sides of the double bonds allow good fitting of adjacent polymer chains, and this, results in a rigid structure. In contrast, the os-poly-1,4-butadiene isomeric polymer units do not permit such interlocking of alternate units. Even so, chain... 

Imidisation of PCA based on 4,4 -di-(/7-aminophenoxy)-benzophenone was accompanied by the precipitation of the polymers from the reaction solutions, which is partially due to crystallinity in the polyimides formed. This appears to hinder the solubility of the polymers in amide and phenolic solvents. Polyimides based on l,l-dichloro-2,2-di-(/7-aminophenoxyphenyl)-ethylene are more soluble. The polymer formed from this diamine and the dianhydride of benzophenone-3,3 -4,4 -tetracarboxylic acid is soluble in a TCE/phenol (3 1) mixture. 

Besides the chemical structure, also the physical state of a polymer is important for its solubility properties. Crystalline polymers are relatively insoluble and often dissolve only at temperatures slightly below their crystalline melting points. 

In the heterophase region LC -f- C the system goes over to the solid state typical of crystalline substances. Usually this solidification of the system is treated as a limited solubility of a polymer. For para-aromatic polyamides the solid state lying... 

In order to dissolve crystalline polymers, it is necessary to consider the Gibbs energy of fusion. This additional energy expenditure is not taken into account in the concept of the solubility parameter. Crystalline polymers therefore often dissolve only above their melting temperatures and in solvents with roughly the same solubility parameter. Unbranched, highly crystalline poly(ethylene) (62 = 8.0) only dissolves in decane (61 = 7.8) at temperatures close to the melting point of 135°C. 

Block copolymer surfactants show a qualitatively similar phase behaviour with temperature to conventional materials. However, solubility requires the presence of branched chains and/or ether links to reduce the occurrence of crystalline polymer with a high melting point. It must be emphasized that commercial polymeric surfactants are polydisperse in both alkyl chain and EO blocks. 

The solubility of a polymer depends on the solvent-solute (polymer) interactions, which must be greater than the solute-solute and solvent-solvent interactions. A polymer can be solubilised by a solvent with similar solubility parameters if certain polymer-solvent interactions are present between them. Polymers with flexible chemical linkages such as -0-/-S- or linear structures, have a better solubility than polymers with rigid linkages such as -N=N-, -C=C-, -C=N-, aromatic, heterocyclic, ladder or cross-linked structures. Similarly, amorphous and flexible polymers have better solubility than crystalline or rigid polymers. 

Apolar substances have low solubility parameters, whereas those of polar substances are high, since the heat of vaporization is higher for the latter. Apolar, noncrystalline polymers will therefore dissolve well in solvents with low di values. Predictions about solubility on the basis of the solubility parameter are still quite permissible for polar, noncrystalline polymers in polar solvents (see Table 6-3). It is more difficult in the case of crystalline polymers or apolar polymers in polar solvents, and vice versa, since equation (6-13), which was derived for pure dispersion forces, no longer applies in these cases. 

Solvent bonding is most effective with polymers having low intermolecu-lar forces. Amorphous polymers or polymers with low crystallinity are more soluble in most solvents. Lower molecular weight polymers and polymer molecules with less cross-linking and more branching structures are more easily dissolved in solvents. Elevated temperatures increase the solubility of all polymers. Polymers dissolve most easily in solvents of the same polarity polar polymers generally dissolve in polar solvents, and non-polar... 

Of these featores, the pressure-dependence of SCF properties dominates or influences virtually every process conducted on polymers. Pressure governs such properties as density, solubility parameter, and dielectric constant changes of more than an order of magnitude are common when pressure is sufficiently increased to transform a gas into a supercritical fluid. This chapter primarily compiles experimental data on the pressure dependence of physical properties of fluid phase polymer-SCF mixtures. Phase equilibria are addressed, including the solubility of polymers in SCFs, the solubility of SCFs in liquid polymers, and the three-phase solid-fluid-fluid equilibria of crystalline polymers saturated with SCFs. Additional thermodynamic properties include glass transition temperature depressions of polymers, and interfacial tension between SCF-swollen polymers and the SCF. The viscosity of fluid phase polymer-SCF mixtures is also treated. 

The poly(alkyl/arylphosphazene)s prepared by the above method have moleeular weights between 50,000 to 200,000 with a polydispersity index of 2.0. The solubility of these polymers seems to depend on the type of substituents present on the phosphorus. Thus, [NPMe2]n is soluble in di-chloromethane, chloroform, ethanol as well as a 1 1 mixture of THF and water. In contrast, [NPEtiJn is virtually insoluble in any organic solvent. However, upon protonation with weak acids [NPEt2] becomes soluble. It has been speculated that protonation destroys the crystallinity of this polymer and enables its solubility. Poly(methylphenylphosphazene), [NP(Me)(Ph)]n is also soluble in a large number of organic solvents such as chlorinated hydrocarbons as well as THF [33]. 

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