Polymer nucleation

Fig. 20 Chain sliding diffusion model of primary nucleation. Polymer chains are rearranged from Gaussian shape within the melt into a nucleus through chain sliding diffusion within the nucleus and disentanglement within the interface. Bottom graph indicates change in free energy of the nucleus against N...

Polyacrylonitrile, which is a semicrystalline polymer, can be used for many engineering applications, such as fiber spinning or for housing and package applications. A peculiarity of polyacrylonitrile is that it is insoluble in its monomer. This makes it very difficult to homopolymerize acrylonitrile in an emulsion polymerization process since nucleated polymer particles cannot grow by monomer swelling. 

Shifting the site of nucleation to the droplets greatly enhances the robustness of the nucleation process to recipe variations, inhibition levels, and changes in operating procedure (initiation rate and/or agitation rate). As a result of droplet nucleation, polymer-stabilized miniemulsion polymerizations are far less sen-... 

With vinyl acetate the situation is more complex since transfer to polymer and terminal double bond reactions are significant at all conversion levels. If one calculates the and fci a nucleated polymer... 

If polymerization is carried out under conditions of high nucleation, polymer chains are deposited rapidly and indiscriminately and the active sites necessary for growth are occluded. If growth is carried out near the equilibrium concentration, depolymerization becomes important and again the active sites become occluded. 

The frequency with which two reactive species encounter one another in solution represents an upper bound on the bimolecular reaction rate. When this encounter frequency is rate limiting, the reaction is said to be diffusion controlled. Diffusion controlled reactions play an important role in a number of areas of chemistry, including nucleation, polymer and colloid growth, ionic and free radical reactions, DNA recognition and binding, and enzyme catalysis. 

Particle nucleation. Polymer particles can be formed by several mechanisms. Homogeneous nucleation, a term used by Fitch and coworkers (1 ), can occur in the water phase by precipitation of the growing oligomeric radicals. 

GOE Goel, S.K. and Beckman, E.J., Generation of microcellular polymeric foams using supercritical carbon dioxide. I. Effect of pressirre and temperature on nucleation, Polym. Eng. Sci., 34, 1137, 1994. 

In order to obtain an ideally nucleated polymer, it is heated to just above its melting point so that large number of residual crystal fragments exist in the melt and act as nuclei. This method is referred to as self-seeding or self-nucleation (Blundell 1966). Zhao et al. have evaluated crystallization behavior of propylene/ethylene copolymer by self-seeding approach (Zhao et al. 2001), which was found in good agreement with the earlier published DSC data (Laihonen et al. 1970,1997). 

M. Imai, K. Mori, T. Mizukami, K. Kaji and T. Kanaya, Structural formation of poly (ethylene terephthalate) during the induction period of crystaUization 1. Ordered structure appearing before crystal nucleation. Polymer, 33, 4451-4456 (1992). 

All the above effects related to concentration of nucleating agents produce many essential improvements in physico-mechaiucal performance of nucleated polymers. Here are some randomly selected examples. 

Figure 5.13 shows the effect of concentration of nucleating agent on flexural modulus of isotactic polypropylene." Similar to data for other properties presented above, smaller additions of nucleating agent cause the biggest impact on physico-mechanical properties of nucleated polymers. 

B experimental parameter for nucleated polymer B experimental parameter for neat polymer... 

The nucleation constant, Kg is related to the product of lateral and folding surface free energy and it represents the free energy which is needed in order to form a nucleirs of a critical size." For neat PP, nucleation constant was 1.95x10 and 0.433x10 for P-nucleated polymer with proprietary nucleating agent." ... 

After primary nucleation, polymer chains difiuse to the growth front and crystalhze fur-... 

Low-shrink polypropylene fibers requires presence of additive that nucleates polymer crystals within the target polypropylene after exposure to sufficient heat to melt the initial pelletized polymer and allowing such a melt to cool." The preferred rigidifying compounds include dibenzylidene sorbitol based compounds, as well as less preferred compounds, such as sodium benzoate, certain sodium and lithium phosphate salts (such as sodium 2,2 -methylene-bis-(4,6-di-tert-butylphenyl)phosphate, otherwise known as NA-11)."... 

Propylene polymers having polydispersity index greater than or equal to 15 are used for the nucleation of commercial polypropylene resins. The 0.1 wt% of nucleating polymer is dispersed in the molten state in commercial polypropylene. "... 

Hu W, Cai T (2008) Regime transitions of polymer crystal growth rates molecular simulations and interpretation beyond Lauritzen-Hoffman model. Macromolecules 41(6) 2049-2061 Hu W, Frenkel D (2004) Effect of metastable liquid-liquid demixing on the morphology of nucleated polymer crystals. Macromolecules 37(12) 4336-4338 Hu W, Frenkel D (2005) Polymer crystallization driven by anisotropic interactions. Adv Polym Sci 191 1-35... 

Lee, S. T. 1993. Shear effects on thermoplastic foam nucleation. Polym Eng Sci 33 418-22. 

Baldwin, D. R, Park, C. B., and Suh, N. P. 1996a. A microcellular processing study of poly(ethylene terephthalate) in the amorphous and semicrystalhne states. Part I. Microcell nucleation. Polymer Engineering and Science 36 1437-1445. 

Hu W, Frenkel D. Effect of metastable liquid-liquid demixing on the morphology of nucleated polymer crystals. Macromolecules 2004 37 4336 338. 

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