ISSN:0032-3888    

DOI:10.1002/pen.760190602

出版商:Society of Plastics Engineers, Inc.    

年代:1979

数据来源: WILEY

摘要:

AbstractPolymer crystallization is recognized as a non‐equilibrium process and the postulate of local equilibrium is combined with the formalism of small systems thermodynamics to provide a general method of relating structure, kinetics and thermodynamics. In this method a single polymer molecule is assumed to be in equilibrium with its environment and the appropriate partition function is factored into the bulk free energy and the remaining factors which are important only in small system

ISSN:0032-3888    

DOI:10.1002/pen.760190603

出版商:Society of Plastics Engineers, Inc.    

年代:1979

数据来源: WILEY

摘要:

AbstractThe strain‐induced crystallization phenomenon of a crosslinked polymer network was studied using statistical, thermodynamics. The basic approach was essentially that used by Flory (1), but in this work Flory's assumption of only a single crystalline phase (the extended‐chain crystalline structure with chain parallel to stretch direction) was abandoned. The dimension of the crystalline vector‐which is assumed to be parallel to the stretch direction and the percent crystallinity is taken as two independent variables instead of being treated as one single variable. A single variable treatment is inherent in the assumption of a single extended‐chain crystallite. By use of the present approach, either the folded‐chain crystallite or the extended‐chain one is found to be thermodynamically stable depending on temperature, stretch ratio and molecular weight. The retractive force of a crystallized polymer network has been calculated. It is shown theoretically that the formation of an extended‐chain crystallite will cause the retractive force to decrease, and that the formation of a folded‐chain crystallite will cause the retractive force to increase in a reversible crystall

ISSN:0032-3888    

DOI:10.1002/pen.760190604

出版商:Society of Plastics Engineers, Inc.    

年代:1979

数据来源: WILEY

摘要:

AbstractA nucleation theory for strain‐induced crystallization is formulated to explain and to predict the effects ofmolecular strainon crystallization kinetics and crystallite size. Unlike any current theories that have based their formulations on some assumed extended‐chain line nuclei or folded‐chain crystals, the present theory avoids all assumptions concerning the crystal morphology. It is based on experimental findings which indicate limited crystal growth in the strain direction, following a reciprocal dependence of crystal thickness on supercooling ΔT. (ΔT =T mo, −T, where the equilibrium melting temperature,T mo, is a variable dependent on degree of molecular strain prior to strain‐induced crystallization.) It is predicted that the logarithm of the nucleation rate,No, is dependent on (T mo)2/T(ΔT) orT mo/T(ΔT), and that the critical nucleus thicknessl*ois shown to be proportional toT mo/ΔT. In addition, expressions are also presented, including examples, to show the dependence ofNo,l*oandTomon degree of molecular strain, ϵ, or melt entropy reduction, Δs′. Our analysis predicts that, on comparing a polyethylene crystallized in the presence of strain to one crystallized in the absence of strain at 130°C, an increase in “coil” dimension of less than about 50 percent can bring about a 104fold increase in heterogeneous nucleation rate, a 30–40 percent reduction in critical nucleus thickness and a 10°C increase in equilibrium melting temperature. These results will be discussed and compared

ISSN:0032-3888    

DOI:10.1002/pen.760190605

出版商:Society of Plastics Engineers, Inc.    

年代:1979

数据来源: WILEY

摘要:

AbstractThe empirical equation, 1/ti=AeEi/RT, which expresses the exponential dependence of the reciprocal of crystallization induction time,ti, has been analyzed and shown to be equivalent to the nucleation rate equations derived earlier in Part III (1). Consequently we have used thetimeasurements obtained earlier by Krueger and Yeh to calculate not only the nucleation rate enhancements but also the melting point elevations, the relative crystal thickness changes and molecular coil extension ratios of shear‐crystallization polyethylene. It is shown that polyethylene when crystallized between 129 and 131°C at shear rates between 1.56 and 9.70 sec−1can have melting point increases of 4.2 to 7.2°C and crystal thickness decreases of 20 to 25 percent, when compared to those crystallized at 130°C in the quiescent state. The predicted “coil” extension in the melt just prior to shear‐induced crystallization ranges between 21 and 36 percent. The results of these analyses as well as those on nucleation rates of polyethylene oxide are discuss

ISSN:0032-3888    

DOI:10.1002/pen.760190606

出版商:Society of Plastics Engineers, Inc.    

年代:1979

数据来源: WILEY

摘要:

AbstractAn experimental study of the crystallization of chemically crosslinked polyethylene at constant strain under isothermal conditions was carried out. Changes in the stress and birefringence were measured simultaneously as a function of time. Both the stress and the birefringence behavior are consistent with previous work carried out under constant rate of cooling conditions. This indicates that the cooling conditions do not affect the structural features of the crystallization process. A comparison of the stress and birefringence curves shows that there are periods off time during which both stress and birefringence increase; stress increases and birefringence decreases; stress decreases and birefringence increases; and both stress and birefringence decrease. None of the crystal morphological models proposed to date for the strain‐induced crystallization of crosslinked polyethylene appear to be able to account fully for this behavio

ISSN:0032-3888    

DOI:10.1002/pen.760190607

出版商:Society of Plastics Engineers, Inc.    

年代:1979

数据来源: WILEY

摘要:

AbstractElastomeric networks made up of chain molecules of sufficient structural regularity generally exhibit strain‐induced crystallization. Crystallites thus formed have a pronounced reinforcing effect within the network, and thus increase its ultimate properties (ultimate strength and maximum extensibility), Increase in temperature or addition of diluent (plasticizer) suppresses the strain‐induced crystallization and thus diminishes the ultimate properties. These effects are demonstrated using stress‐strain isotherms obtained in elongation for crystallizable networks ofcis‐1, 4‐polybutadiene and of polyisobutylene. The magnitude of the effects of strain‐induced crystallization are determintud by comparisons of the ultimate properties of these two crystallizable networks with the corresponding ultimate properties of noncrysrallizable networks of polydimet

ISSN:0032-3888    

DOI:10.1002/pen.760190608

出版商:Society of Plastics Engineers, Inc.    

年代:1979

数据来源: WILEY

摘要:

AbstractData are presented for the seeded growth of polyethylene fibers from solutions undergoing laminar flow in tubular geometry. Radial growth rates are reported for solutions of high and low molecular weight fractions for the second stage fiber thickening process which occurs in the tube entrance region. Results are also shown for high molecular weight growth from fiberglass seeds which indicate an enhanced growth rate at elevated temperatures. A two‐stage growth pattern documented earlier for the high molecular weight fraction is shown to occur for the lower molecular weight material. The Discussion includes an analysis for the tapered fiber geometry in the second stage of growth and calculations for stress‐induced, diffusion‐limited growth in the first stage downstream from the tube ent

ISSN:0032-3888    

DOI:10.1002/pen.760190609

出版商:Society of Plastics Engineers, Inc.    

年代:1979

数据来源: WILEY

摘要:

AbstractThe reaction of fibrinogen with thrombin to produce fibrin goes through a multistep sequence in which up to four peptides are cleaved off. When this reaction takes place at pH 6.3 in a shear flow field, the formation of fibrous product is retarded as compared with stagnant conditions. The retardation is greater at 50 s−1than at 135 s−1. The morphology of the coagulated product appears to be different when produced at 50 s−1than at either stagnant or 135 s−1, with the material at 50 s−1having a sheetlike appearance in contrast to a fibrous appearance for the material at 135 s−1and stagnant conditions: The explanation of the retardation may lie in the separation of thrombin from fibrinogen during the sheared reactions. Shear rate varies more in the vascular bed than does pH or ionic strength and probably is a more important factor in controlling physiological coagulation than these more frequently studie

ISSN:0032-3888    

DOI:10.1002/pen.760190610

出版商:Society of Plastics Engineers, Inc.    

年代:1979

数据来源: WILEY

摘要:

AbstractFactors affecting the formation of hard elastic polypropylene and nylon 6,6 are described. In the case of polypropylene, it is shown that the morphology of the stress spin‐oriented fibers depends on the molecular weight, the molecular weight distribution, and the cooling rate. Narrow molecular weight distribution and/or fast cooling rate can lead to a smectic, essentially fibrillar morphology in contrast to the normally observed crystalline, essentially lamellar structure. Hard elastic nylon 6,6 fibers are produced by drawing regularly spun nylon 6,6 yarn to draw ratios in the range of 1.6‐3 and annealing the resulting fibers in phenol solutions (cone. 1‐5 percent). By this method, a highly increased crystalline order is created, simultaneously transforming the original morphology to a lamellare one or superimposing a lamellar morphology on the poorly organized fibrillar base. Some tensile and elastic characteristics of polypropylene and nylon 6,6 yarns as. functions of spinning and annealing variables are pres

ISSN:0032-3888    

DOI:10.1002/pen.760190611

出版商:Society of Plastics Engineers, Inc.    

年代:1979

数据来源: WILEY