摘要:

The nuclear magnetic resonance (NMR) relaxation has been studied in polyisobutylene and polybutadiene at temperatures from −170° to +200°C and at three resonance frequencies: 20, 30, and 50 Mc/sec. In polyisobutylene, the spin‐lattice relaxation time (T1) passes through two minima with change in temperature. The low‐temperature minimum is ascribed, as in other compounds, to methyl‐group rotation, but in polyisobutylene this motion is found to encounter relatively large hindrance, presumably owing to inter‐locking among the groups. The high‐temperatureT1‐minimum is ascribed to rotational and translational motion of the segments. The extent of motion is qualitatively gauged by calculation from the Bloembergen‐Purcell‐Pound theory for NMR relaxation in simple liquids.T1is insensitive to molecular weight (M.W.) over a wide range, but the dependence ofT2upon M.W. changes abruptly when M.W.≅4×104. In polybutadiene,T1is found to depend markedly on thecis‐transcontent.

ISSN:0021-8979    

DOI:10.1063/1.1713187

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The linewidth and second moment of oriented samples of drawn PE and POM were measured as a function of temperature. from −196° to 160°C, and of the angle between draw direction and magnetic fieldH0, mostly at 0°, 45°, and 90°. Much detail in the temperature behavior of these quantities is resolved which cannot be observed with unoriented samples. On the basis of linewidth measurements of drawn PE, an interpretation of the high‐temperature narrowing of the broad line observed with PE single crystals and PE crystallized from the melt is given. The anisotropy of the decrease in second moment with rising temperature which is related to anisotropic molecular motion was determined for PE, subtracting the contribution due to lattice expansion from the total decrease measured. The ``mobile fraction'' was determined for the two polymers as a function of temperature. In both cases a reduction of mobile fraction upon drawing is observed.

ISSN:0021-8979    

DOI:10.1063/1.1713188

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

A study has been made of the morphology of fractionated linear polyethylene crystallized from solution inn‐alkanes. Crystals have been found whose habits, though markedly different from any described in this polymer, are similar to, and provide useful models for, the radial crystalline units in melt‐grown spherulites. Fold domain structures and departures from planarity in these crystals are also novel and correlate closely with interface morphology during growth. Crystals, or aggregates of crystals, grew preferentially in thebdirection under all conditions of observation; this applies equally to dendritic growth in monolayer single crystals and to spherulites of sufficiently compact texture as to exhibit cooperative twisting of orientation.

ISSN:0021-8979    

DOI:10.1063/1.1713189

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

For a large deformation, the deformation‐dependent anisotropic elastic characteristics are illustrated through the analysis of a model medium. The variations of the elastic constants of an oriented polymer are then analyzed and correlated. The analytical results compare favorably with some available experimental data reported from dynamic measurements.

ISSN:0021-8979    

DOI:10.1063/1.1713190

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The failure theory of Bueche and Halpin is generalized and expanded to obtain a prediction of the time dependence of the tensile strength and ultimate elongation. The model pictures rupture as the propagation of tears or cracks within the material. The growth of a tear or crack is viewed as an ideally simple process in which the molecular chains at the tear tip stretch viscoelastically under the influence of a high stress concentration until they rupture. As a consequence, the failure process is a nonequilibrium one, developing with time and involving the consecutive rupture of the molecular chains. Substantial support for the theory is found by comparing the theoretical prediction against experimental results obtained for SBR and EPR rubbers. In addition, it is experimentally demonstrated that delayed and forced rupture experiments can yield qualitatively identical data for viscoelastic bodies. Also discussed is the basis for the approach to the nonlinear response of a viscoelastic body required here in the development of a fracture theory.

ISSN:0021-8979    

DOI:10.1063/1.1713191

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

Although the tensile stress at break (&sgr;b) and the corresponding ultimate elongation (&agr;b) of a reinforced elastomer depend markedly on temperature and test rate, characteristic time‐to‐break curves can be obtained utilizing the Leaderman‐Tobolsky‐Ferry superpositioning techniques. In addition, a distinctive failure envelope is obtained when the log &sgr;bT0/Tis plotted against log (&agr;b−1), whereTandT0(°K) are the test temperature and an arbitrary reference temperature. Delayed ruptured experiments yield qualitatively the same results as forced rupture experiments. Furthermore, an analysis of the Leaderman‐Tobolsky‐Ferry shift parameters indicate a concomitant increase in the apparent activation energy with increasing reinforcement concentration. These observations are explicable by utilizing the failure theory of Bueche and Halpin. It is now apparent that failure is the result of the propagation of tears or cracks within the viscoelastic body and that the time and temperature dependence of the ultimate properties is directly related to the time‐temperature dependence of the modulus. A preliminary picture of the salient molecular events is presented.

ISSN:0021-8979    

DOI:10.1063/1.1713192

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The mechanical degradation of polyisobutene solutions in laminar flow under known shear stress conditions was performed in a thin‐film, high‐shear, rotational viscometer. Polyisobutenes with molecular weights from 40 000 to over a million were studied at several temperatures and concentrations inn‐hexadecane. Reduced variables, of a form similar to those used in studies of temporary viscosity losses, can be used successfully to correlate permanent viscosity losses due to degradation. The variables which superimpose are stress, temperature, and concentration. The range in which reduced variables apply corresponds to concentrations and molecular weights where degradation is commonly observed and to the region where polymer entanglements exist.

ISSN:0021-8979    

DOI:10.1063/1.1713193

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

An interpretative analysis of experimental results on the failure of synthetic fibers in fatigue under cyclic tension leads to the conclusion that rupture is of a brittle nature. Although molecular rearrangement undoubtedly takes place during longitudinal fatiguing, rupture itself cannot be regarded as a viscous, laminar flow process. Failure is localized; the undamaged sections of the specimens show relatively less important effects of the fatiguing. Lifetime cannot be predicted from measurements of fiber tenacity, modulus, or rate of creep. A theory of fatigue rupture, based on the observation of cracks in specimens strained to breakage, is formulated. The treatment involves the assumption that a crack grows by a nucleation process up to a critical size. At a slightly larger size, the crack becomes unstable and propagates spontaneously, to produce rupture. The theory leads to an expression for lifetime as a function of stress that is in substantial agreement with experimental data on a number of samples.

ISSN:0021-8979    

DOI:10.1063/1.1713194

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The permeability, diffusion, and solubility constants of helium, argon, and ethane have been measured in a series of linear polyethylene films which had been subjected (1) to various rates of cooling from the melt and (2) to subsequent annealing near the melting temperature. While solubility constants of argon and ethane showed the normal variation with amorphous content of the polymer irrespective of thermal history, helium solubility constants appeared to be anomalously high and strongly dependent on thermal history. The presence of voids or defects within the crystalline phase has been postulated to account for this behavior of helium; argon and ethane being excluded from these defects because of their size. Anomalous helium diffusion constants were also observed which have been satisfactorily explained in terms of a slow diffusion of gas into these intracrystalline defects.The monatonic decrease in geometric diffusional impedance with increasing amorphous content of the polymer has again been observed in films directly cooled from the melt. However, upon annealing these films at elevated temperatures the impedance decreases markedly, despite an increase in crystallinity, suggesting the formation of thicker lamellas with a high concentration of intralamellar imperfections in a manner similar to that observed in annealing polyethylene single crystals.The apparent activation energies for diffusion of argon and ethane were essentially constant irrespective of thermal history. This suggests the so called ``chain immobilization factor'' is not due to restricted chain mobility in the amorphous polymer, but is rather a penetrant‐size‐dependent impedance to diffusion arising from the near‐molecular dimensions of the amorphous channels in the polymer.

ISSN:0021-8979    

DOI:10.1063/1.1713195

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The set of end‐to‐end vectors (hereafter called the discrete‐state distribution) for a model polymer chain ofnbonds has been generated for a three rotational state model on the B‐5000 computer. Equally probablegauche‐right,trans, andgauche‐left states are assumed. The discrete‐state distribution is determined for a polydimethylsiloxane chain withn=6, 8, 10 and a polyethylene chain withn=6, 8. For a polyethylene chain of six links the calculated distribution is shown to differ substantially from a normalized Gaussian. For each case, a function is obtained for the discrete‐state distribution through the Gram‐Charlier technique of expanding an unknown functionf(x) in terms of a Gaussian function. The Gram‐Charlier expansion is shown to give a reasonable fit to the discrete‐state distribution. The distributionf(x) is shown to more accurately predict the equilibrium formation of octamethylcyclotetrasiloxane in a dimethylsiloxane polymer than does the Gaussian distribution. The problem of excluded volume is not considered for the short chains treated here.

ISSN:0021-8979    

DOI:10.1063/1.1713196

出版商:AIP    

年代:1964

数据来源: AIP