ISSN:0032-3888    

DOI:10.1002/pen.760180602

出版商:Society of Plastics Engineers, Inc.    

年代:1978

数据来源: WILEY

摘要:

AbstractThe extensive literature on equilibrium melting of flexible linear macromolecules has been reviewed during the last year. The discussion in this paper is based on experiments on actual, large, extended chain crystals and extrapolations on metastable crystals. A series of 32 complete sets of melting temperatures, heats of fusion, volume changes on fusion, and entropies of fusion have been collected. These data are compared to melting data on rigid macromolecules and small macromolecules. Only a relatively simple model of rotational isomerism is needed to understand the entropy of fusion of flexible linear macromolecules which is 8‐12 J/K/mole of rigid backbone units whenever there is no mobility in the crystal at the time of melting. This is much less information on detailed conformational analysis than is generally assumed to be necessary to interpret melting. Instead, it is of importance to consider for the discussion of melting also the change in packing fraction on fusion, which is high for crystals with planar zig‐zag conformation, and low for helices. Both types of crystals lead to similar packing fractions in the melt. Higher packing fractions, both in the melt and crystal, are found for polyesters, polyamides, and polyoxides. The heats of fusion which make up the third group of melting parameters considered are more connected with molecular size (surface area) and cohesive energy density than mobility, so that larger mobile backbone units and higher cohesive energy densities lead to higher melting temperatures. A further addition to the heat of fusion comes from the presence of high energy rotational isomers in the melt, which again increases the melting temperature. Further refinement of this initial discussion is possible, but all aspects of melting must be taken into account rather than concentrating on single aspects as has been done in the p

ISSN:0032-3888    

DOI:10.1002/pen.760180603

出版商:Society of Plastics Engineers, Inc.    

年代:1978

数据来源: WILEY

摘要:

AbstractCooling is a critical step in any crystalline polymer molding or extrusion process. A simulation is proposed which will predict the transient temperature and crystallinity profiles developed when a finite polymeric slab comes in contact with a cooling fluid. A generalized, phenomenological model of the crystallization kinetics of polymers is incorporated to account for the effect of the latent heat of crystallization on the thermal history as well as on the crystalline structure at any point in the slab. The model assumes heterogeneous nucleation and temperature‐dependent radial growth of spherulites. DSC cooling thermograms for the polymer are used to verify the kinetic model for comparing experimental measurements against simulated results. Observed spherulite sizes should also be matched by the simulation. Kinetic data have been obtained for two grades of poly(ethylene terephthalate) which have the same growth rate expression but different nucleation characteristics. Crystallinity of these two polymers decreases rapidly as either quench temperatures or nucleation densities are decreased independently. Calculations have been carried out for 1/16 in. thick sheets of polymer exposed to a cooling medium with a heat transfer coefficient of 100 Btu/hr/ft2/°F. Temperature gradients are also presented. The simulation can be used for optimizing quench conditions in polyester film extrusi

ISSN:0032-3888    

DOI:10.1002/pen.760180604

出版商:Society of Plastics Engineers, Inc.    

年代:1978

数据来源: WILEY

摘要:

AbstractData are presented for the longitudinal and radial growth rates of polyethylene fibers grown from high molecular weight solutions using a seeding method in Poiseuille tube flow. Results indicate growth is a two‐step process with the first step the formation of very thin untapered filaments ∼25 μm in diameter which grow very rapidly the full length of the capillary tube. The second step involves radial thickening of the filament into a tapered shape, the kinetics of which have been measured. The discussion includes an analysis of the rapid longitudinal growth and a critical comparison to earlier experiments in both Poiseuille and Couette flow. A possible growth mechanism is suggested to explain the high growth rates found in these experiments as well as the unusual growth observed earlier for Couette rotor growth. A discussion of some kinetic data for the radial growth process is also given along with preliminary results which indicate thickening may only occur in flow of extensional kinema

ISSN:0032-3888    

DOI:10.1002/pen.760180605

出版商:Society of Plastics Engineers, Inc.    

年代:1978

数据来源: WILEY

摘要:

AbstractCrystallization of glassy nylon 6 has been investigated with differential scanning calorimetry and wide angle X‐ray diffraction in order to determine the effect of average molecular weight. Samples were prepared by quenching molten films between platens: chilled with liquid nitrogen. These films had number average molecular weights ranging from 10,000 to 42,000 and had polydispersity indices ranging from 2.0 to 3.1. Crystallization kinetics at large undercoolings were measured in terms of the systematic dependence of conversion half time, Avrami exponent and heat of crystallization. Number average molecular weight was observed to influence crystallization rate at large undercoolings through its effect on glass transition temperature. At intermediate undercoolings, molecular weight affected crystallization rate through terms other than the glass transition temperature. In addition, a dependence on the degree of order of the macromolecules, both prior to and after crystallization, was observe

ISSN:0032-3888    

DOI:10.1002/pen.760180606

出版商:Society of Plastics Engineers, Inc.    

年代:1978

数据来源: WILEY

摘要:

AbstractTen linear polyethylenes ranging fromMw= 4.9 × 104to 4.6 × 106were crystallized in a dilatometer at 0.51 GPa and 242°C and then cooled slowly. Volume vs time data were used to follow the kinetics of the crystallization. The dilatometer data for the isothermal part of the crystallization were fitted to the Avrami equation. The time exponent was independent of molecular weight and the average wasn= 2.2. Electron microscopy of fracture surfaces showed that all of the polyethylenes crystallized in extended chain morphology. The crystalline order and maximum extended chain crystallite thickness decreased with increasing molecular weight. The dominant morphological feature of the crystallized high molecular weight samples was a strand‐like network superstructure. Attempts to stabilize the hexagonal structure formed in the isothermal part of the crystallization failed, and all specimens had only the usual orthorhombic crystal struc

ISSN:0032-3888    

DOI:10.1002/pen.760180607

出版商:Society of Plastics Engineers, Inc.    

年代:1978

数据来源: WILEY

摘要:

AbstractThe high pressure crystallized specimens described in the preceding paper (1) were characterized by X‐ray diffraction, density, and differential scanning calorimetry, Nitric acid etching, followed by gel permeation chromatography and DSC on the residues provided further characterization of the morphology. The crystallinity decreased from 100 to 80 percent with increasing molecular weight, over the rangeMw= 4.9 × 104to 4.6 × 106. The crystallites were predominately in extended chain morphology, but the percent in extended chain form decreased from 98 to 85 percent over the indicated molecular weight range. In all cases, there was a wide, bimodal or multi‐modal, distribution of lamellar thickness. The physical characterization results were consistent with and complementary to the results from electron microscopy. The large differences in crystallinity and gross morphology readily account for observed molecular weight dependent differences in mechanical prope

ISSN:0032-3888    

DOI:10.1002/pen.760180608

出版商:Society of Plastics Engineers, Inc.    

年代:1978

数据来源: WILEY

摘要:

AbstractA quantitative analysis of the deformation of thin spherulitic polyethylene films was undertaken at the microscopic level with the aid of a gold marking technique. The deformation microstructure which developed was further characterized through the use of a gold decoration technique. The microfibrillar morphology was found to agree quite well with Peterlin's model of the microfibril. Changes in the sample morphology as delineated by the gold decoration, support the proposed mechanisms of the transformation of the folded chain spherulitic morphology into the microfibriliar morphology and the subsequent deformation of the microfibrillai structure.

ISSN:0032-3888    

DOI:10.1002/pen.760180609

出版商:Society of Plastics Engineers, Inc.    

年代:1978

数据来源: WILEY

摘要:

AbstractNylon 6 fibers which had been relaxed to different extents by annealing were examined at fixed strains by small angle and wide angle X‐ray techniques. It was found that the strain of the long period of the semicrystalline microfibrils is identical to the macroscopic fiber strain. Approximately 1/3 of the tensile deformation results from molecular shear of imperfectly oriented crystalline chains. Virtually no evidence for intercrystalline slip is found; the orientation of the intercrystalline amorphous regions results in a low compliance for the shear of crystals past one another. The majority of the microfibril deformation occurs by stretching these intercrystalline amorphous regions, accompanied by the flow of extrafibrillar amorphous material to maintain constant volume. In highly annealed fibers this “filling” mechanism is less efficient, as the amount of extrafibrillar material has been reduced during shrinkage. This effect leads to a decrease in Poisson's ratio after increasingly severe annealing. A related result of annealing is the dehomogenization of the microstructure, leading to the presence of more stress‐induced “microcracks” during the stretching of anne

ISSN:0032-3888    

DOI:10.1002/pen.760180610

出版商:Society of Plastics Engineers, Inc.    

年代:1978

数据来源: WILEY

摘要:

AbstractThe annealing of drawn samples mobilizes the almost fully extended amorphous tie molecules which try to assume the thermodynamically required random conformations. The sample shrinks if annealed with free ends which permits the crystal blocks on different microfibrils and connected by almost fully extended taut tie molecules to move towards the position they had before plastic deformation. Hence the annealed sample has irretrievably lost most of its high axial elastic modulus which in the sample as drawn was caused by the high fraction of taut tie molecules. With fixed ends no shrinkage is possible so that the partial relaxation of interfibrillar taut tie molecules still lets them connect far away blocks. If their fraction is large enough so that in spite of the high surface to volume ratio which drastically depresses the crystallization temperature they can crystallize they do so after cooling to room temperature. The new axial crystalline bridges restore the high elastic modulus of the material before annealing, partially stabilize the sample against shrinkage during a new annealing, but also cause the dead bend effect which is the consequence of the replacement of flexible taut tie molecules in still amorphous conformation by rigid crystalline bridges. The drawing or extrusion at high temperature produces some annealing effects comparable with those of cold drawn material annealed with fixed ends.

ISSN:0032-3888    

DOI:10.1002/pen.760180611

出版商:Society of Plastics Engineers, Inc.    

年代:1978

数据来源: WILEY