Working with Semicrystalline Polymers

Wilson, Physical Structure of Nylons, in Nylon Plastics, M. Kohan (ed.), Wiley, New York, 1973, pp. 271-305. 

Spruiell, J. E., and J. L. White, Structure Development During Polymer Processing Studies of the Melt Spinning of Polyethylene and Polypropylene Fibers, Polym. Eng. ScL, 15, 660-667, 1975. 

Polymer Single Crystals, R. E. Krieger, Huntington, NY, 1973. 

Polymer Physics, Chapman Hall, London, 1995. 

Magill, J. H., Morphogenesis of Solid Pol3mier Microstmctures, Treatise Mater. Set Technol, lOA, 1-368, 1977. 

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The coalescence of polymers is driven by the work of surface tension, which counteracts the viscous dissipation associated with the molecular diffusion within the coalescing domain. This phenomenon is often referred to in the literature as polymer sintering. In the rotational molding process, coalescence occurs at temperatures above that of the material melting point when dealing with semicrystalline polymers, or above the glass transition temperature for amorphous resins. The first analytical model describing the coalescence process was proposed by Frenkel ... 

Forced air, water mist, or water spray cool the mold after it leaves the oven. Once the mold cools sufficiently for the polymer inside to solidify, it is opened. The cooling time must take into account the resin, the wall thicknesses, and the metal from which the mold is fabricated. Very rapid cooling of the mold can introduce stresses that can weaken or distort the final part This is especially apparent when working with semicrystalline resins or parts with multiple resin layers. 

NMR work on semicrystalline polymers was started with the intention to study the difference of chain mobility in the amorphous and crystalline component. The first results were the detection of the irreversible increase of amount of amorphous component and its mobility with annealing,the mobilization of amorphous component by swelling liquids, and the substantial difference in mobility between linear and branched pe.The high orientation of crystal lattice of drawn samples allowed the investigation of angular dependence of NMR signal. Polymers with zig-zag chain conformation in the crystal lattice, i.e.,... 

This equation is not particularly useful in practice, smce it is difficult to quantify the relationship between concentration and activity. The Elory-Huggins theory does not work well with the crosslinked semicrystalline polymers that comprise an important class of pervaporation membranes. Neel (in Noble and Stern, op. cit., pp. 169-176) reviews modifications of the Stefan-Maxwell approach and other equations of state appropriate for the process. 

Researchers have examined the creep and creep recovery of textile fibers extensively (13-21). For example, Hunt and Darlington (16, 17) studied the effects of temperature, humidity, and previous thermal history on the creep properties of Nylon 6,6. They were able to explain the shift in creep curves with changes in temperature and humidity. Lead-erman (19) studied the time dependence of creep at different temperatures and humidities. Shifts in creep curves due to changes in temperature and humidity were explained with simple equations and convenient shift factors. Morton and Hearle (21) also examined the dependence of fiber creep on temperature and humidity. Meredith (20) studied many mechanical properties, including creep of several generic fiber types. Phenomenological theory of linear viscoelasticity of semicrystalline polymers has been tested with creep measurements performed on textile fibers (18). From these works one can readily appreciate that creep behavior is affected by many factors on both practical and theoretical levels. 

They have been combined with FV models and they have been applied to semicrystalline polymer-solvent systems. The results are satisfactory but they are not predictive the and / parameters should be estimated from experimental data. However, the swelling of cross-linked polymers can be estimated with such equations. In one of the very few works reported on the prediction of SLLE for polymer solutions, the Entropic-P/ and UNIFAC models have been compared for semicrystalline polymer-solvent systems. These two models are shown to yield similar results for SLLE. 

Information on how orientation during melt crystallization affects the transport properties of polymers is sparse however, increases in the permeability have been attributed to the "shish kebab" morphology (ill). Most of the work involving barrier properties of oriented semicrystalline polymers has dealt with materials drawn at temperatures well below the melting point. The transport properties of cold-drawn polyethylene (34f 42-46), polypropylene (42,42), poly(ethylene terephthalate) (12,42-4 9), and nylon 66 (22) among others have been reported. 

Semicrystalline polymers, VDC copolymer and aromatic nylon MXD-6 (Table II) showed little if any reduction in permeability at these moderate orientation levels. In fact, recent unpublished work has shown that aromatic nylon MXD-6 exhibits an initial increase in permeability up to 3X orientation followed by a significant reduction in permeability at higher orientation levels. The VDC copolymer also showed higher permeability with moderate biaxial orientation — 1.5 times the permeability of the unoriented film. This is believed to be due to orientation of the polymer after crystallinity is fully developed. If the orientation of VDC copolymers is induced prior to full development of crystallinity in the material, one would not expect to see an increased oxygen permeability. In commercial practice, therefore, forming of VDC copolymer structures is normally done on rapidly quenched polymer to orient it while still in the amorphous state at temperatures near or above the Tm of VDC copolymer. 

For the purpose of this work, processing includes three steps PVTFA sample preparation, solvolysis, and hydration. PVTFA is a semicrystalline polymer that has not been studied extensively because of its poorly defined crystallinity. Wide-angle X-ray diffraction of unoriented polymer tends to give very diffuse scattering maxima. The polymer exhibited a distinct melting endotherm with a small AH of around 13 cal/g. In the most detailed structural study published, Bohn et al. (14) suggested that the X-ray data indicated a large-pitch helical structure for the chain. PVA, on the other hand, is believed to crystallize in a planar zig-zag conformation (15) with a much more typical AH of fusion of 30 cal/g (16). 

Olson et al. [2003] carried out PALS studies of 22 poly(ethylene terephthalate) (PET) specimens with varying degrees of crystallinity 11 specimens were melt-crystallized at 210°C and 11 were made via cold crystallization at 110°C. Each sample was characterized thoroughly by DSC and WAXD in terms of Tg, Tm, and Xc, and, in fact, prior to the PALS investigation, were employed in a study of oxygen permeability at room temperature, which demonstrated that the O2 solubility in the melt-crystallized PET is higher than in the cold-crystallized PET [Lin et al., 2002]. This earlier work further established that the observed decreases in solubility and diffusion coefficient of oxygen in semicrystalline PET could be explained by the three-phase model of semicrystalline polymers, in which the RAF is considered in... 

Part of the reason for the difficulty in making hard and fast rules in these systems is that both nucleation and growth contribute to overall crystallization rate and blending has the potential to be affective either in subtle and possibly independent ways. This is illustrated by the work of Van Ende et al. [164] who found that a semicrystalline polymer formed a miscible blend with a flexible EP and this resulted in increased nucleation density and growth rate whilst the addition of a more rigid PLC of similar chemical structure but only partial miscibility was... 

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