摘要:

AbstractA current theory of the tensile strength of rubbers invokes large‐scale crosslink interchange at room temperature to explain the high tensile strengths of networks crosslinked with weak linkages. This mechanical lability of certain crosslinks has been assumed from their known thermal lability at higher temperatures. Relaxation of stress experiments at various elongations have been performed at room temperature on networks crosslinked with both weak and strong linkages. Our purpose was to detect mechanically induced crosslink slippage. We have found no evidence of any mechanical lability of weak crosslinks at room temperature. A hypothesis is presented which explains the high tensile strengths of rubbers crosslinked with weak linkages as resulting from an internally relaxed network formed by the thermal lability of the crosslinks at the vulcanization temperature. This theory is shown to be consistent with some previously unexplained experimental result

ISSN:0449-2978    

DOI:10.1002/pol.1968.160060901

出版商:John Wiley&Sons, Inc.    

年代:1968

数据来源: WILEY

摘要:

AbstractThe effects of operational variables on calibration of GPC are reported. Variables examined include (1) temperature and solvent changes, (2) sample concentration, (3) injection time, and (4) flow rate. Each of these variables significantly affected molecular weight averages calculated from GPC data. The causes and methods of minimizing the effects produced by the individual variables are discussed.

ISSN:0449-2978    

DOI:10.1002/pol.1968.160060902

出版商:John Wiley&Sons, Inc.    

年代:1968

数据来源: WILEY

摘要:

AbstractGPC appearance volumes have been determined for a series of linear polyethylene, polystyrene, and polybutadiene fractions (Mw/Mn<1.1) in trichlorobenzene at 130°C. and for the latter two series in tetrahydrofuran at 23°C. A polymer‐type independent relationship between appearance volumes and the equivalent hydrodynamic radii of the polymer molecules has been demonstrated. The equivalent hydrodynamic radius is calculated from intrinsic viscosity data. It is proposed that this relationship can be used to construct a universal GPC calibration curve for polymers that assume a spherical conformation in solution. Methods for applying the universal curve to the determination of molecular weight averages and molecular weight distribution are described. In addition, a method is outlined by which the universal calibration curve can be empolyed for determining number‐average Mark‐Houwink constants from polydisperse

ISSN:0449-2978    

DOI:10.1002/pol.1968.160060903

出版商:John Wiley&Sons, Inc.    

年代:1968

数据来源: WILEY

摘要:

AbstractGum arabic was found to have an osmotic molecular weight of 250,000, in agreement with earlier determinations. A molecular weight of 365,000 was found by light scattering, somewhat higher than obtained earlier by sedimentation equilibrium analysis but lower than light‐scattering values reported by other investigators. The M̄w/Mnratio, 1.46, is quite low in gum arabic. The angular dependence of light scattering exhibited the upward curvature to be expected of a spherical molecule and a radius of gyration of about 100 A. or less, as estimated from a Zimm plot. Fractionation of the original gum arabic was done by precipitation of a 0.5% solution in aqueous 0.5% NaCl with acetone. Comparison of the curves of viscosity versus molecular weight and the estimated radius of gyration shows that the hydrodynamic volume is less than that of branched dextran of similar molecular weight. The electroviscous effects for gum arabic in aqueous solution were shown by reduced viscosity curves at various acidities and in salt. The degree of dissociation was calculated for each pH level. The minimum intrinsic viscosity was found in 0.04NHCl where the degree of dissociation at pH 1.5 was found to be 0.049. When the acidity was increased, further reduction in viscosity was found to be negligible. Routine determination of the viscosity and molecular weight of the fractions was done in 0.35MNaCl at pH 10 to which 0.25% of the sodium salt of ethylenediaminetetraacetic acid was added as a sequestrant. The intrinsic viscosity in this solvent was nearly as low as in 0.04NHCl. Light‐scattering dissymmetries in water and in 0.35MNaCl plus EDTA at pH 10 were similar, 1.13 and 1.09, respectively, which showed that actual expansion of the macroion is not the cause of the large increase in viscosity of gum arabic when the ionic strength of the solvent is reduced. Periodate oxidation of the polymer confirmed the existence of a 1–3‐linked backbone of galactose. Subsequent treatment of the oxidized polymer with alkali reduced the osmotic molecular weight to 45,000 but failed to remove oxidized side branches. The oxidized polymer was fractionated by gel permeation chromatography and the intrinsie viscosity–molecular weight relation compared with relations for fractions of the unoxidized polymer and for other branched and crosslinke

ISSN:0449-2978    

DOI:10.1002/pol.1968.160060904

出版商:John Wiley&Sons, Inc.    

年代:1968

数据来源: WILEY

摘要:

AbstractAn approximate closed expression for the excluded volume effect of linear polymer molecules is developed with the aid of a uniform expansion model of perturbed chains. The linear expansion factor α for the end‐to‐end distance is given by (α3− 1) + (3/8) (α5− α3) = (5/2)zwherezis the excluded volume parameter. This equation is numerically close to the Ptitsyn equation in the ordinary range of α; i.e., f

ISSN:0449-2978    

DOI:10.1002/pol.1968.160060905

出版商:John Wiley&Sons, Inc.    

年代:1968

数据来源: WILEY

摘要:

AbstractThis paper describes the application of gel permeation chromatography to the morphology of polymer single crystals. Various types of single crystals of polyethylene were etched with fuming nitric acid, and the molecular weight distributions of the degraded fragments determined. The crystal preparations studied were monolayer crystals grown from xylene solution at 85°C and multilayer crystals grown at 84°C and 70°C. In all cases peaks in the molecular weight distribution were observed corresponding to single and double transverses of the molecular chains through the lamellae. By using the chromatograph calibration described in a previous paper, the position of these peaks were compared and correlated with previous estimates of lamellar thickness from low‐angle x‐ray measurements. The relative positions of the peaks provide information regarding the nature of the fold surface. The results are found to be consistent with a model in which the majority of the molecules are tightly

ISSN:0449-2978    

DOI:10.1002/pol.1968.160060906

出版商:John Wiley&Sons, Inc.    

年代:1968

数据来源: WILEY

摘要:

AbstractPreliminary results are reported on the use of gel permeation chromatography in morphological studies of bulk polymers and fibres. Several samples of bulk isotropic and drawn polyethylene were analyzed by gel permeation chromatography following nitric acid oxidation. In all cases suitable samples showed two peaks in the molecular weight distribution, suggesting a qualitative similarity with results for single crystals. It is concluded that the present data are consistent with chain folding in bulk polymers, both in the isotropic and oriented states, with a less degree of regularity than exists in single crystals.

ISSN:0449-2978    

DOI:10.1002/pol.1968.160060907

出版商:John Wiley&Sons, Inc.    

年代:1968

数据来源: WILEY

摘要:

AbstractFibrous crystals of polyethylene are produced by crystallization from agitated solutions in xylene. If crystallization temperature is maintained between 97°C and 108°C these fibers are free from overgrowth effects and are seen to consist of flat, striated ribbons. This material was treated with fuming nitric acid, and the molecular weight distribution of the degradation product was analyzed by gel permeation chromatography. This distribution was found to consist of a low molecular weight peak and an exceptionally resistant high molecular weight tail over a wide range of degradation times in quantitative agreement with the serrations developed during the nitric acid attack. This is consistent with the fibers possessing a composite folded and extended chain structure. The lateral pulling out of smooth fibrils from the original fibers in the course of certain electron microscope sample preparation methods is consistent with the presence of chain‐folded material and might account for the structureless fibrils reported in the literature. Partially successful attempts to obtain the extended chain portion in pure form by selective dissolution are mentioned together with some low‐angle x‐ray

ISSN:0449-2978    

DOI:10.1002/pol.1968.160060908

出版商:John Wiley&Sons, Inc.    

年代:1968

数据来源: WILEY

摘要:

AbstractThe steady‐state current density versus applied electric field characteristics have been measured for two types of polystyrene films. Measurements were made on 1‐mil biaxially oriented film and on films produced by casting from solution. The cast films ranged from 5 to 0.5 μ in thickness. The measurements of the steady‐state current flowing through the films were done by two different methods. The first was the direct observation of current flowing in a circuit connected to the film which was under a potential stress. The second involved the observation of the decay of a static charge placed on the surface of a film. Both methods are handicapped by the fact that large transient currents flow for extended periods after any change is made in the experimental set up. The results indicate that at 25°C the current increases as the 3.5 power of the applied electric field when the field is greater than 8 × 104V/cm. At fields less than 8 × 104V/cm the current decreases more rapidly and tends to be

ISSN:0449-2978    

DOI:10.1002/pol.1968.160060909

出版商:John Wiley&Sons, Inc.    

年代:1968

数据来源: WILEY

摘要:

AbstractThe surface of thin polyethylene films (ca. 1 mil) nucleated on high‐ and low‐energy surfaces was studied. It was found that the degree of crystallinity in the surface region, as determined by transmission and reflection infrared spectroscopy, was dependent on the nature of the substrate. Relatively small and randomly oriented spherulites were observed in the surface region of thin polyethylene films nucleated on gold, a high‐energy surface. The surface region of a thin polythylene film nucleated on polytetrafluoroethylene, a low‐energy substrate, was observed to have considerably fewer spherulites, but much larger in size, with orientation restricted, in general, to the plane of the film. The relative intensity of the 720 cm−1to the 730 cm−1band in the reflectance spectra (ATR) indicates that the surface region of a polyethylene film nucleated on a high surface energy substrate is more crystalline than the surface region of a film nucleated on a low surface energ

ISSN:0449-2978    

DOI:10.1002/pol.1968.160060910

出版商:John Wiley&Sons, Inc.    

年代:1968

数据来源: WILEY