摘要:

We report SANS studies of bulk polyethylene (PE) samples crystallized from the melt at different rates. All samples contained both protonated and deuterated PE. Substantial isotopic concentration fluctuations are present in slow‐cooled samples. This is seen as large differences in the low‐angle (low‐Q) SANS. At largerQvalues, the SANS data from both materials is approximately identical. We interpret this in terms of an exponential radial correlation function for composition fluctuation whose correlation length is comparable to lamellar dimensions. Numerical calculations show that concentration fluctuations have minimal effect in the intermediateQregion.

ISSN:0021-8979    

DOI:10.1063/1.324442

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Polycarbonate absorbs water in two stages: in the initial period of absorption at an elevated temperature all of the water was found in an unassociated state when cooled to room temperature; and, in the second stage, most of the additional water absorbed by the polymer was identified in a separate liquid phase (clustered water). From water sorption studies on polycarbonate bars, we advance the following model for the formation of clusters. After the polymer is saturated with water at an elevated temperature and cooled, its solubility is lowered and water condenses in the form of microscopic water‐filled cavities, providing the internal pressure which is generated by the excess water exceeds the strength of the polymer. BelowTgthe clusters are formed only after the polycarbonate (Mw=26 600) has been hydrolytically degraded; whereas, aboveTg, clusters can be formed without degradation. Cluster regions were found by SEM to be spherical cavities, usually measuring from 0.1 to 1.0 &mgr; across. In addition, the glass temperature of polycarbonate was lowered as the molecular weight decreased due to hydrolysis and the amount of water in the polymer increased. Unassociated water was found to enhance the low‐temperature dielectric loss of polycarbonate. Frozen clustered water produced an additional secondary loss peak about 40 °C below the &bgr; transition. In addition, the liquid clustered water produced a loss mechanism in the kHz region that was interpreted as a Maxwell‐Wagner effect.

ISSN:0021-8979    

DOI:10.1063/1.324443

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The compressive behavior of a rigid polyurethane foam reflects its geometric structure and the physical properties of the matrix polymer. Foam properties may be modified by the inclusion of silica particles in the polymer matrix. These silica particles alter the mechanical properties of the base polymer as well as the geometrical structure of the foam. The yield stress and the modulus of the foam decrease with the initial introduction of the silica filler after which the mechanical properties of the foam increase with increasing filler content. A model is presented expressing the foam yield stress and the modulus as a function of the polymer modulus and the cell parameters. These parameters arel, a cell edge length, andd, a characteristic strut thickness. This results in the expressions,Ef=C″Ep(d/l)4and &sgr;y=‐Ep(d/l)4. This model is applicable to either filled or unfilled foams and accommodates changes in the properties of the solid phase, through the use ofEp, as well as changes induced in the cell geometry through thed/lrelationship.

ISSN:0021-8979    

DOI:10.1063/1.324444

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

A description is given of the molecular and morphological structure of polyvinylidene fluoride and from this description a classical model is proposed for calculating the piezoelectric and pyroelectric properties. The model consists of an array of crystal lamellae with a net moment from aligned dipoles in the crystals and compensating space charge on the crystal surfaces. The results for no compensation and complete compensation essentially bracket experimentally observed results and indicate that the largest contribution to the activity of this polymer arises from bulk dimensional changes rather than from changes in molecular dipole moments.

ISSN:0021-8979    

DOI:10.1063/1.324445

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The antipolar crystal form of poly(vinylidene fluoride) can be made piezoelectric and pyroelecric by the temporary application of electric fields in excess of 1 MV/cm at room temperature. Infrared and x‐ray diffraction data reveal that the polarization occurs in two stages. At fields near 1 MV/cm, a phase transition to a polar form II occurs with presumably no change in chain conformation. Fields near 5 MV/cm cause a change in conformation to produce form I. Our results indicate that at least a portion of the residual polarization occurs within the crystal phase of the polymer.

ISSN:0021-8979    

DOI:10.1063/1.324446

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The morphology of even‐even nylons has been reexamined in order to determine whether crystallographic factors can exert a dominant influence on the morphology and growth behavior of polymeric spherulites. Directional solidification of these polyamides produces long oriented spherulites at zone velocities below ∼5 &mgr;m/min, while at higher rates unusual patterns are obtained by interaction with small positive spherulites. In most cases, the stable form is that of oriented negative spherulites presenting an extremely fine fibrillar texture. Fluctuations in temperature gradient or in sample motion cause initiation of divergent fronts of spherulitic aggregates, which, however, exhibit interrupted growth patterns and variable growth rates; this exceptional growth behavior is not imposed by the characteristics of zone solidification. In addition, the aggregates appear to posess an unusually coarse texture, whereas that of positive spherulites is intermediate between the textures of the other two spherulitic species. Most aggregates have a totally mixed birefringence; however, the positive birefringence of others is inconsistent with their orientation and polarizabilities.

ISSN:0021-8979    

DOI:10.1063/1.324447

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The morphological diversity in even‐even nylons described in Paper I was found to be the result of crystallographic factors related to the high extent of hydrogen bonding. Among all spherulitic species in these polyamides, only positive spherulites crystallize in accordance with the phenomenological theory of Keith and Padden. However, as a result of their unit cell orientation which renders the hydrogen‐bonded sheets radial and the molecular chains almost tangential, folding occurs predominantlyintothe melt rather than in the usual tangential manner. The microstructure of negative spherulites departs from the norm in that long ribbonlike lamellae are absent; instead, negative fibrils are composed of short segments containing tangentially arranged stacks of hydrogen‐bonded sheets. In spherulitic aggregates the molecular chains are crystallized at the unusually acute angle of 49° to the direction of solidification. The preferred direction of fibrillar growth in these aggregates is oblique to the spherulitic radius; this could account for their variable and interrupted growth rate, their apparent coarseness of texture, as well as their extinction at 45° to the crossed polars. All spherulitic species crystallize with the &agr;‐unit cell. A possible explanation for this is offered, relating the unilateral shear of hydrogen‐bonded planes to dipole interactions between adjacent layers. Such interaction would also help explain the positive birefringence of spherulitic aggregates which is inconsistent with their polarizabilities.

ISSN:0021-8979    

DOI:10.1063/1.324448

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Rayleigh‐Brillouin light‐scattering studies have been carried out on atactic polyethylmethacrylate thermally polymerized under vacuum from extremely clean monomer. The scattered intensity ratio for theVVpolarization geometry varies from 2 (at high temperatures) to 5 and appears to be a function ofT−1with a broad discontinuity in the temperature range 40–70 °C. Depolarization intensities have also been measured and, in particular, it is found that &rgr;Hequals unity. These results agree with those previously obtained for polybutylmethacrylate and are also compared with reports on polystyrene. Other characteristics of this glass are discussed.

ISSN:0021-8979    

DOI:10.1063/1.324436

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Epoxy network systems based on DGEBA (bisphenol‐A‐diglycidyl ether) and NMA (nadic methyl anhydride) systems modified with the low molecular weight CTBN (carboxyl‐terminated butudiene‐acrylonitrile copolymer) rubbers were prepared and studied. It was found that below the glass transition of the epoxy matrix these materials displayed time‐dependent changes in their mechanical properties; specifically, the strain to break as well as the rate of stress relaxation were observed to decrease in a near‐linear behavior with the logarithm of time at sub‐Tgannealing. Calorimetric methods clearly showed a simultaneous decrease in enthalpy with time that behaved in a similar fashion as the time‐dependent mechanical properties. All the calorimetric and mechanical data are qualitatively related. The importance of this phenomena is considered in view of the widespread use of epoxys. Similar behavior is expected for other network glasses thermally quenched into a nonequilibrium state.

ISSN:0021-8979    

DOI:10.1063/1.324437

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The Brillouin spectra ofn‐dodecane,n‐hexadecane,n‐docosane,n‐octacosane, andn‐hexatriacontane are studied as a function of temperature. The Brillouin linewidths increase monotonically as the liquids are cooled toward the freezing point. The results are analyzed quantitatively in terms of frequency‐dependent volume and shear viscosities.

ISSN:0021-8979    

DOI:10.1063/1.324438

出版商:AIP    

年代:1978

数据来源: AIP