摘要:

Void formation is shown to be a significant response of styrene‐isoprene‐styrene block copolymers under conditions of tensile stress relaxation. When brought to zero stress, these voids are observed to close and heal, restoring the original mechanical properties of the undamaged material. Dark‐field optical microscopy and light‐transmission photometry have been used to study void formation and healing in the size regime appropriate to optical techniques. As meaured from a reference state of damage, healing was found to increase with time and temperature in the zero‐stress state, with an apparent activation energy of 9 kcal/mole. The Mie theory of particle scattering was used to analyze changes in void size and size distribution during the healing process. The results are discussed in terms of possible mechanisms for healing.

ISSN:0021-8979    

DOI:10.1063/1.327423

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

Polyethylene with a low concentration of deuterated methylene groups displays an infrared band in the frequency region 646–651 cm−1. This band is attributed to a rocking normal mode of a CD2group with one of the adjacent dihedral angles approximatelytransand the other approximatelygauche(tg). This mode vibrates at 620 cm−1when the dihedral angles adjacent to the CD2group are bothtrans(tt). In polyethylene crystalstgsequences can occur only in defects where constraints cause some of the dihedral angles to be approximatelytransorgauche. Calculations of the rocking mode vibrational frequencies of CD2groups in model chains which incorporate some of the typical defects showed that the bands were broadened but not completely disrupted by the distorted dihedral angles. Measurements of the relative intensities of the CD2rocking bands show an increase in the concentration oftgsequences consistent with the thermal generation of defects which may be involved in transport of the polymer chain through the crystal. Confidence in the particular values of the concentration ratio is limited by uncertainties in the determination of the base line for the infrared bands of interest. Quantitative measurements of the concentration ratiotg/(tg+tt), determined from the integrated band intensities, fall between limits set by reasonable independent estimates of the concentration of folds and point dislocations.

ISSN:0021-8979    

DOI:10.1063/1.327424

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

The crystal structure of phase IV (polar &agr; or &dgr; phase) poly(vinylidene fluoride) was studied by x‐ray diffraction. Oriented phase IV was obtained by orienting phase II (&agr; phase) and then corona poling at approximately 2 MV/cm. The resulting material is a mixture of phases I, II, and IV. Infrared spectra indicate that phase IV has the same chain conformation (TGTG′) as phase II. The reflections of phase IV can be indexed to an orthorhombic unit cell with lattice constants similar to the phase II lattice constants,a=0.496 nm,b=0.964 nm, andc(chain axis)=0.462 nm. There are two chains in this unit cell. The two chains pack with the chain dipole moments parallel. Three different ways of packing the chains were investigated: both chains up, one up and one down, and a statistical up‐down packing. It was found that to within a 97% confidence level, the chains pack with a statistical up‐down packing.

ISSN:0021-8979    

DOI:10.1063/1.327425

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

Several studies have been carried out on a polyurethane elastomer (Solithane 113). At atmospheric pressure, dielectric methods were used to study &egr;″over the temperature range from −180 to +20 °C. Three peaks in &egr;″were observed: an &agr; peak associated with the glass transition and two low temperatures peaks ( &bgr; and &ggr;). Mechanical tests were peformed to osberve the effect of the pressure‐induced glass transitionPgon the Young’s modulus. The glass transition was studied of pressure from 1 bar to 6.5 k bar by observing the shift as a function of the dielectric &agr; peak with pressure and also by volumetric methods. A densification of the glass and a resultant shift in the glass‐transition temperature could be achieved by forming the glass at high pressures (Tg) as opposed to pressuring the glass formed at 1 bar (T*g). It was found that &Dgr;&bgr;/&Dgr;&agr;≳dTg /dP, but &Dgr;&bgr;/&Dgr;&agr;?dT*g /dP. At high pressures,dTg /dPreached a limiting value of 10.4 °C/kbar.

ISSN:0021-8979    

DOI:10.1063/1.327426

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

The pressure dependence of the pyroelectric coefficientPywas determined from atmospheric pressure to 7 kbar over a temperature range from −80 to +48 °C for poled uniaxially oriented poly (vinylidene fluoride) film. The effect of the glass transition onPyas a function of pressure is compared to the pressure dependence of the glass‐transition temperature obtained from dielectric studies. The ratio ofdp, the hydrostatic pressure piezoelectric coefficient, toPyis presented as a function of both temperature and pressure. The details of a high‐pressure system design for these measurements are also presented.

ISSN:0021-8979    

DOI:10.1063/1.327427

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

A proper tensorial logarithmic strain measure and its associated time rate of change is developed using an equivalence between the expressions for an isotropic tensor function and the Sylvester‐Lagrange formalism. It is shown that the specific Cauchy stressT/&rgr; is derivable from a potential with respect to this strain measure. A modified stressTFis shown to be derivable from a potential with respect to the left polar decomposition of the deformation gradient. Advantages and disadvantages of this formulation are discussed.

ISSN:0021-8979    

DOI:10.1063/1.327428

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

Molecular deformation of polypropylene (PP) was investigated by stress infrared experiments on ultraoriented isotactic PP samples, and the results were compared with single chain deformation calculations. A nonlinear stress‐strain curve of PP was determined in a strain range of 8% which resulted in an average chain modulus,Ec?40 GPa, and an initialEc?30 GPa. PP chain stretching was accompanied by considerable C‐C bond stretching and valence angle bending in addition to torsional angle changes. Vibrational frequency shifts &Dgr;&ngr; with stress were calculated using both harmonic and anharmonic potential energy functions for C‐C stretching. The harmonic results incorrectly predicted a considerable number of &Dgr;&ngr;’s to higher frequencies in addition to lower frequencies. Experimentally, only one band shifted to higher frequencies, and shifts to lower frequencies generally increased with increasing temperature. These results are in agreement with the anharmonic calculation in which the majority of vibrations shifted to lower frequencies. Detailed valence coordinate changes and vibrational frequency shifts with stress and temperature are presented and discussed.

ISSN:0021-8979    

DOI:10.1063/1.327429

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

The stress‐relaxation modulusE(t) is defined in terms of an experiment involving an instantaneously applied strain; such experiments are, in practice, impossible. We have critically investigated several methods which have been suggested in the literature to extractE(t) from experimental stress values used to bring about a constant‐strain rate followed by a constant strain; such strain histories are easy to realize in the laboratory. Strengths and weakness of the various methods are analyzed in terms of data generated via an appropriate analytical expression. In addition, an alternative technique for converting experimental data toE(t), which involves a direct fit of this analytical expression to experimental data, is suggested. The accuracy of this last method has been assessed using stress‐relaxation data for polystyrene in the primary (glass to rubber) transition region.

ISSN:0021-8979    

DOI:10.1063/1.327430

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

As a part of a program to understand the mechanisms of failure in time‐dependent adhesion and film bonding, the creep compliance of polyvinyl acetate (PVAc) in shear has been determined both as a function of temperature and absorbed moisture. Volumetric expansion as a function of temperature or moisture takeup was also measured. We find that practically realizable changes in moisture content affect both the creep compliance and the swelling of PVAc to a degree comparable to that resulting from realistic changes in temperature. For example, the creep rates (histories) at corresponding times for PVAc subjected to 92% relative humidity storage are accelerated by approximately four orders of magnitude over those found for the dry material. Moreover, we find within reasonable experimental error that water concentration affects the time scale of creep like temperature through a concentration‐dependent shift factor. An attempt is made at discussing the interrelation of temperature‐ and moisture‐induced volume changes.

ISSN:0021-8979    

DOI:10.1063/1.327458

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

The recovery of polyvinyl chloride glasses prepared at high temperature and pressure (120 °C, 7 kbar) have been observed using Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). Changes in absorbance associated with conformational reequilibration occur above 80 °C, the normal glass temperature of polyvinyl chloride (PVC). Peaks are observed inCp15–20 °C belowTgwhich are attributed to relaxation of volume strain or holes. Since the enthalpy and the conformational recovery occur at different temperatures and time scales, we conclude that neither free volume models nor flexed bond models are adequate to explain the properties of glasses prepared at high pressure.

ISSN:0021-8979    

DOI:10.1063/1.327459

出版商:AIP    

年代:1980

数据来源: AIP