摘要:

The viscosity of carboxylic‐acid‐terminated polybutadiene increases markedly upon neutralization with mono‐ and divalent bases. These viscosity increases are related to apparent molecular weight, considering the association of the terminal salts as ``polymerizing'' the difunctional prepolymer. For the monovalent alkali‐metal salts, simple end‐group dimerization seems to occur with an average &Dgr;Hof ∼−25 kcal/mole. Neutralization with ions of higher valency leads to materials with higher viscosities.

ISSN:0021-8979    

DOI:10.1063/1.1657179

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

An apparatus has been designed and constructed that will allow determination of the mechanical behavior of polymer specimens subject to applied tensile and compressive loading, while the sample is simultaneously subject to a hydrostatic pressure environment. The moduli of high‐density polyethylene and polypropylene determined from compressive tests show a significant increase with pressure. Tensile nominal stress‐strain curves have been obtained at various pressures up to 100 000 psi. These show that the yield stress also increases significantly with increasing pressure for both materials. The nature of yielding and fracture is found to be quite different for the two polymers studied. Polyethylene tends to deform more by shear, and the necked region at high pressures reduces to a fine point before separation. In polypropylene, fracture occurs by plastic tearing across the cross section. An attempt is made to account for the experimental results by use of yield criteria that includes a hydrostatic pressure component. Consideration is also given to the effect of finite deformation theory on the increase in modulus of elasticity under a high pressure environment. The specific nature of the effects produced by the high pressure is found to depend strongly on the molecular structure of the polymer.

ISSN:0021-8979    

DOI:10.1063/1.1657180

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The small‐angle x‐ray scattering (SAXS) pattern of polypropylene (PP) drawn at various temperatures (Td= 20°, 100°, and 135°C) was investigated in a range of very closely spaced draw ratio (&lgr;) values through the neck region. In the zone before the neck starts (&lgr; = 1.05–1.1) the uniform diffraction ring due to microspherulitic material with a long periodL0changes into a four‐point diagram superposed onto an elliptical halo. At the beginning of the neck (&lgr; = 1.3–2.5) two meridional maxima arising from a new fibrous structure with a limiting long periodLTappear. Irrespective of whetherL0>LTorL0<LTthe transition betweenL0andLTis discontinuous. The four‐point pattern reminiscent of the spherulitic structure remains observable up to the end of the neck. In the same manner as in the case of PE, stacks of lamellae exhibiting the most favorable orientation for chain tilt and slip start being plastically deformed and finally break into folded‐chain blocks which are incorporated into microfibrils. The fracture occurs in a thin destruction zone which separates the new microfibrils from the not yet transformed stack of original lamellae. Because of the morphological inhomogeneity of the spherulitic sample, the destruction zones and, hence, the starting points of the microfibrils are rather randomly scattered in the neck region and do not merge into one single layer extending over the whole cross section of the sample.

ISSN:0021-8979    

DOI:10.1063/1.1657181

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Brillouin scattering of laser light has been used to measure the frequency of hypersonic sound waves in the range of 1010Hz in PMMA as a function of temperature through the glass‐transition region. A discontinuity in the temperature coefficient of sound velocity is observed at the glass‐transition temperature; this is explained as a consequence of a corresponding discontinuity in the temperature coefficient of the specific volume (thermal expansion coefficient). The ratio of the light scattered by isothermal density fluctuations to that scattered by adiabatic density fluctuations was also measured. This ratio was large and did not change appreciably near the glass‐transition temperature. The value of the Landau‐Placzek ratio is approximately what one would expect from previously observed ultrasonic‐velocity‐dispersion data as a function of temperature well above the glass‐transition temperature. Both the velocity and intensity ratio data indicate that no velocity‐dispersion effects are present for the hypersonic sound waves up to temperatures 35° above the glass‐transition temperature. These results also indicate that the glass transition is not a classical second‐order phase transition.

ISSN:0021-8979    

DOI:10.1063/1.1657182

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The infrared spectra of single‐crystal mats of physical mixtures of polyethylene and perdeuteropolyethylene show splittings of CH2and CD2bending and rocking modes which are smaller than those for the pure species. This is believed to be a result of the penetration of cilia from the surface of one crystal into the lattice of an adjacent crystal. Calculations of the concentration of cilia are in good agreement with percentages derived from the study of a mixed‐crystal paraffin system, and these concentrations change with crystal thickness as is predicted. Such cilia can account for most of the amorphous component of polyethylene single crystals.

ISSN:0021-8979    

DOI:10.1063/1.1657183

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

A modification of the Primak analysis for activation energy distributions is presented and compared with the well‐known analysis for relaxation time distributions. Mathematical and physical interrelationships between the two analyses are discussed. Activation energy spectra are derived from the data of Kovacs for volume relaxation in polystyrene and polyvinyl acetate. The energy spectra are bell shaped and cover the range from 5 to 20 kcal/mole. The concept of a shift factor is introduced to account for changing structure in the glass transition region. The shapes and locations of the spectra are discussed and compared with previous interpretations of volume relaxation data.

ISSN:0021-8979    

DOI:10.1063/1.1657184

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Poly(tetramethyl‐p‐silphenylene siloxanes) fractions ranging in molecular weight from below 104to above 106, have been crystallized isothermally from the melt. Samples have also been prepared under a wide range of annealing conditions varying from minutes to hundreds of hours. The morphology of these fractured specimens has been studied by electron microscopy. The ease of fracture increases as the molecular weight decreases. The dimension of the lamellar structures ascertained from Pt shadowing and Au decoration techniques increase as the annealing or crystallization temperature is raised. The values are consistent with the crystallite sizes estimated from small‐angle x‐ray measurements made on these same samples, and with the single crystal thickness obtained under comparable crystallization temperatures. The topography of the fracture surfaces depends on the molecular weight and on the annealing conditions. The lower molecular weight samples (below 40 000) exhibit a much higher degree of regularity than the fractions with longer chain lengths.

ISSN:0021-8979    

DOI:10.1063/1.1657185

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Studies of the photoelastic behavior of a PVC homopolymer film have been carried out in the rubbery state above the glass transition (up to 160°C) to complement studies already carried out in the glassy state. Some measurements were also made on the same samples in the glassy state for completeness. Step‐loading experiments were used, as in previous experiments in the glassy state. The results obtained are somewhat complex, indicating that this polymer is considerably different in nature from a simple amorphous polymer such as polystyrene. There is a progressive shift in the zero load values of birefringence (when the sample is unloaded from increasing stress levels), indicating that a progressive structure change is taking place as load level is increased. This could be a result of progressive plastic yield or breaking and reforming of stress‐sensitive crosslinks. Below the glass transition this shift takes place in two stages, which may be related to the secondary beta transition and major glass transition of the polymer. The stress‐optical coefficient changes from small negative values to large positive values as temperature is increased through the glass transition; a zero value is observed at 80°C. In addition to the abrupt change of SOC at the glass transition, a second abrupt change is observed at 145°C which is the temperature of annealing of the film; annealing conditions may therefore be playing a very important role in determining the solid‐state structure of the polymer.

ISSN:0021-8979    

DOI:10.1063/1.1657186

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

For application in un‐thermostated delay lines a delay medium possessing a low acoustic loss, a zero temperature coefficient of delay and a zero or low aging rate is required. Present isopaustic glasses meet these requirements satisfactorily with the exception of aging which is found to be dependent upon thermal history to a degree that causes difficulty in some applications. This paper reports a new approach to forming a zero temperature coefficient delay medium consisting of the combination in a hot pressed ceramic, with grain size much less than the acoustic wavelength, of two individually stable materials possessing temperature coefficients opposite in sign. Three mixtures have been prepared with different mass ratios of vitreous SiO2and powdered crystalline MgF2by pressing at temperatures of 900°–1120°C and at pressures of 7000–20 000 psi. A zero temperature coefficient is found at 41°C for a mixture with a mass ratio (SiO2/MgF2) of 0.360, a positive coefficient is found for a mixture with mass ratio of 0.306 and a negative coefficient for a mass ratio of 0.419. The internal friction of the mixtures near 1.5 MHz is comparable to and even lower than that of typical isopaustic glasses, though the frequency dependence of the loss is higher. For the mixture with mass ratio of 0.360 the attenuation coefficient depends on frequency asf2.3. Tests have shown no measurable room temperature aging over an eight week period following a mild heating to 110°C.

ISSN:0021-8979    

DOI:10.1063/1.1657187

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

A theory, based on Chambers' solution to the classical Boltzmann equation, is developed for the acoustic gain in a high‐mobility (&mgr;), nondegenerate, extrinsic semiconductor subjected to crossed dc electric (E0) and magnetic (B0) fields with the wave vector k in theE0×B0direction. The theory does not restrict the cyclotron radiusRcto a value much less than acoustic wavelength; previous theory based on the phenomenological semiconductor equations requireskRc≪1 for validity. Simple approximations to the general result are developed for the cases ofkRc≲1andkRc≫1, whereRcis the cyclotron radius. The theory is found to explain recent experimental measurements in the regimekRc≲1where the simpler theory has failed. A lowering of the maximum attainable gain at a given frequency occurs with decreasingB0. Cyclotron resonance peaks in the acoustic gain occur in the regimekRc≫1 when the Doppler‐shifted frequency is an integral multiple of the cyclotron frequency. The anomalous behavior in the threshold curve of acoustoelectric current oscillation in InSb is qualitatively explained on the basis of the theory.

ISSN:0021-8979    

DOI:10.1063/1.1657188

出版商:AIP    

年代:1969

数据来源: AIP