Solid-state polycondensation crystallinity

In industrial PET synthesis, two or three phases are involved in every reaction step and mass transport within and between the phases plays a dominant role. The solubility of TPA in the complex mixture within the esterification reactor is critical. Esterification and melt-phase polycondensation take place in the liquid phase and volatile by-products have to be transferred to the gas phase. The effective removal of the volatile by-products from the reaction zone is essential to ensure high reaction rates and low concentrations of undesirable side products. This process includes diffusion of molecules through the bulk phase, as well as mass transfer through the liquid/gas interface. In solid-state polycondensation (SSP), the volatile by-products diffuse through the solid and traverse the solid/gas interface. The situation is further complicated by the co-existence of amorphous and crystalline phases within the solid particles. 

The papers of Mallon and Ray [98, 123] can be regarded as the state of the art in understanding and modelling solid-state polycondensation. They assumed that chain ends, catalysts and by-products exist solely in the amorphous phase of the polymer. Because of the very low mobility of functional groups in the crystalline phase, the chemical reactions are modelled as occurring only in the amorphous phase. Additionally, the diffusion of by-products is hindered by the presence of crystallites. The diffusivity of small molecules was assumed to be proportional to the amorphous fraction. Figure 2.32 shows the diffusion coefficients for the diffusion of EG and water in solid PET. 

For the solubility of TPA in prepolymer, no data are available and the polymer-solvent interaction parameter X of the Flory-Huggins relationship is not accurately known. No experimental data are available for the vapour pressures of dimer or trimer. The published values for the diffusion coefficient of EG in solid and molten PET vary by orders of magnitude. For the diffusion of water, acetaldehyde and DEG in polymer, no reliable data are available. It is not even agreed upon if the mutual diffusion coefficients depend on the polymer molecular weight or on the melt viscosity, and if they are linear or exponential functions of temperature. Molecular modelling, accompanied by the rapid growth of computer performance, will hopefully help to solve this problem in the near future. The mass-transfer mechanisms for by-products in solid PET are not established, and the dependency of the solid-state polycondensation rate on crystallinity is still a matter of assumptions. 

Figure 5.9 Effect of crystallinity on the solid-state polycondensation of PET, shown as the number-average molecular weight as a function of time. Conditions fluidized bed polymerization at 230°C particle size, 35-48 mesh superficial velocity of nitrogen, 43cm/s [6]. From Chang, T. M., Polym. Eng. Sci., 10, 364 (1970), and reproduced with permission of the Society of Plastics Engineers...

Certain polyamides and polyesters can be prepared by solid-state polycondensation step polymerization, usually by heating the appropriate monomer or monomer salt. For example, nylon 11 can be prepared by heating crystals of 11-aminoundecanoic acid (melting point =188 C) at 160°C under vacuum. Polyaddition step polymerization of conjugated dialkene monomers can be induced in the solid state by exposure of the monomer to ultra-violet radiation. For example, irradiation of crystalline 2,5-distyrylpyrazine yields quantitatively a highly crystalline linear cyclobutane polymer... 

Polycondensations of aminoacids and oligoamids have led only to successive pol5mierization and crystallization with a loss of the initial orientation. A seeming exception, the poljonerization of e-aminocaproic acid from the solid state which produced oriented pol5mier crystals was shown to be an epitactic pol5nnerization and crystallization (75). Some of the reactions were topochemical since they followed a reaction path only possible from the crystalline monomer, but none of the investigated cases demonstrated a solid state path from the monomer to the polymer crystal. 

As discussed earlier, solid-state polymerization reactions are used to increase the degree of polymerization in the production of nylon-6 and nylon-6,6. The solid-state polymerization process has been studied by process simulation. Mallon and Ray [208] developed a comprehensive model to handle the reactions in polymers undergoing polycondensation reactions in the solid state. The polymer crystalline fraction is modeled as containing only repeat units,... 

The preparation of wholly aromatic sterns in halogenated hydrocarbon-salt mixtures, poly(l,4-benzamide) synthesis by thermal and suspension methods and extended chain systems, and their liquid crystal solutions have similarly been reviewed. The polycondensation of poIy(p-benzamide) chain ends has been shown to occur in the solid and liquid-crystalline state and the reactivity in the former exceeds that in the latter. Poly(l,4-phenyleneterephthalamide) and poly(chloro-l, 4-phenyleneterephthalamide) have been synthesized in hexamethyl-phosphoramide-iV-methyl-2-pyrollidooe and conditions established for the formation of liquid crystalline solutions. ... 

This is the preferred method for the synthesis of both, aliphatic and aromatic polyesters derived from CHDM. The reaction is usually accomplished in two steps. The first step is carried out under a pressure which depends on the diacid or the dimethyl ester derivative used for the synthesis. Usually an excess of diols to diacid (1.2-2.2 1) is employed so a mixture of short oligomers is produced. In the second polycondensation step, which is carried out at higher temperatures and under vacuum, the oligomers react to generate the polymer and the excess of diol is removed. If the resulting polyester is crystalline it may be subjected to solid state postpolycondensation (SSP) in order to increase the molecular weight. 

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