SynopsisViscoelastic properties of four linear and three very lightly crosslinked polybutadienes (microstructure about 50% trans) were studied. Of the latter, two had not reached the gel point, and their molecular weight distributions were determined by sedimentation velocity analysis; the third was crosslinked just past the gel point, with only 32% gel fraction present.Complex shear compliances were measured over a frequency range from 0.1 to 1000 cps at temperatures from -70 to 30°C. with a Fitagerald transducer and a Plazek torsion pendulum; and torsional creep measurements were made over time periods up to about three days. The creep data were converted to the corresponding dynamic viscoelastic functions at very low frequencies by conventional approximation methods. All data were reduced to 25°C. by shift factors calculated from a previously adopted equation of the WLF form. In the transition zone, the viscoelastic properties of linear samples were almost independent of molecular weight. The entanglement spacing, derived from the minimum in the loss tangent and the inflection in the storage compliance, was 130 to 160 chain atom. The maximum in the retardation spectrum attributable to motions of individual network strands was closely similar to the corresponding maxima for more highly crosslinked vulcaniaates previously studied, showing that even in the latter it is associated with entanglement network strands rather than strands between chemical crosslinks. For a linear sample with molecular weight 180,000, the retardation processes disappear at times beyond about 10 sec. a t 25°C. With crosslinking short of the gel point (i.e., branching) the slow retardation processes are enormously increased and prolonged to longer times. With further crosslinking through the gel point and beyond, the slow retardation processes decrease progressively in magnitude. Qualitatively, this behavior resembles the sharp maximum in content of highly branched and aggregated molecular species which is predicted at the gel point by crosslinking statistics; but the slow processes (or low-frequency losses) persist farther past the gel point than would be expected on this basis. The steady-state compliances of the linear samples were smaller, but for a sample crosslinked short of the gel point were much larger, than the prediction of the Rouse theory modified for molecular weight distribution.

SynopsisViscoelastic properties of uncrosslinked 1,2-polybutadiene (91.5% vinyl, 7.0% cis, 1.5% Irans, number-average molecular weight 99,000) were studied by dynamic shear measurements between 0.15 and 600 cps (torsion pendulum and Fitzgerald transducer) and shear creep measurements over time periods up to 3.7 X lo4 sec., in the temperature range from 5 to 50°C. More limited dynamic measurements were made on a sample of unvulcanized natural rubber with number-average molecular weight 350,000 a t f r e quencies from 0.4 to 400 cps and temperatures from 13 to 48°C. All datawere reduced to 25°C. by shift factors calculated from equations of the WLF form with the following coefficients: 1,2-polybutadiene, c1 = 6.23, c2 = 72.5; natural rubber, CI = 5.94, cz = 151.6. In the transition zone, the relative positions of the loss tangent curves on the logarithmic frequency scale for these and other rubbers (1,4-polybutadiene with 50% trans configuration; styrene-butadiene rubber with 23.5% styrene content; and polyisobutylene) provided relative measures of local segment mobility. At 25OC., these ranged over a factor of 3700 with 1,2-polybutadiene and polyisobutylene the lowest and 1,4-polybutadiene the highest. When the frequency scale of each rubber was reduced to a temperature 100°C. above its glass transition temperature, however, the loss tangent curves for all except polyisobutylene were nearly coincident; the latter still showed a lower mobility by a factor of about 1/800. The terminal relaxation time and steady-state compliance for the 1,2-polybutadiene calculated from the Rouse theory were larger than those observed experimentally. The level of compliance corresponding to the entanglement network of 1,2-polybutadiene, J e~, was calculated by integration over the loss compliance, J", to be 1.62 X 10-7 cm.2/dyne; integration over G" to obtain the corresponding modulus gave reasonable agreement. From such J e~ values, the average number of chain atoms between entanglement points, jZ,, was estimated as follows: 1,2-polybutadiene, 132; natural rubber, 360; 1,4-polybutadiene, 110; styrenebutadiene rubber, 186; polyisobutylene, 320. Values of jZ, were also estimated from the minimum in the loss tangent and compared with those reported from the molecular weight dependence of viscosity. The three bonrces were in generally good agreement.

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The high temperature heat content of a high-purity sample of ZrB? has been measured in a copper-block drop calorimeter over the range 410" to 1125"K., and the results correlated with low temperature heat capdcity data on a similar sample. Derived thermodynamic properties over the range 0" to 2000°K. are presented based on experimental data up to 1200" K. and on extrapolations of computer-selected fits of the experimental points. FIVE SETS OF H I G H T E M P E R A T U R E thermody-namic data ( I , 2, 7, 8, IO) and a single low temperature study ( 1 I ) have previously been available for ZrB? samples of 95-99% purity but large positive and negative discrepancies (10-20% or more) have been observed even though all the calorimeters have been calibrated in standard ways. I n a cross-check of data, Pears and coworkers (9) made runs on the same ZrB? used by Barnes and coworkers ( I ) and reproduced the results of Barnes within a few per cent, which indicates that the reported large differences between samples are real and not measurement errors. T h e AVCO results are pulse-type heat capacity measurements (2) over the temperature range 2000" to 2500' K. and seem to differ drastically from conventional extrapolations of the lower temperature results, as do the results of Prophet (IO).This report presents high temperature measurements on a sample of ZrB2 prepared by Feick ( 3 ) which is from the same batch as the sample used by Westrum and Feick ( I 1 ) for their low-temperature studies. EXPERIMENTALThe calorimeter used in this work has been described previously ( 4 ) and the samples were contained in platinum or platinum-rhodium capsules under an argon atmosphere.Temperature measurements were made with a Pt us. Pt-10% R h thermocouple calibrated against a reference which had been recently calibrated a t the National Bureau of Standards. T h e reliability of the calorimeter was checked by rerunning a standard sample of synthetic sapphire.The heat content for the empty Pt capsules was measured at several temperatures and found to check well with the equation of Kelley ( 6 ) while the heat content of the Pt-10% R h capsule was calculated from Kelley's data on the elements.

synopsisThe mechanical behavior of three kinds of organic foams, each a t two different densities, was experimentally investigated under conditions of pulsed one-dimensional strain shock loading. The input pulse width in each experiment was nominally 0.1 psec, and the input stress level (as referenced to quartz) was varied between 10 and 23 kbar. The materials studied were polyurethane foam at bulk densities of 0.33 and 0.21 g/cc, syntactic foam (phenolic microballoons dispersed in a resin binder) a t 0.66 and 0.23 g/cc, and polystyrene bead foam at 0.091 and 0.049 g/cc. Specimen thicknesses varied between 1.0 and 625 mm. It was found that the pulse duration was greatly lengthened and that the peak stress was decreased (accounting for both impedance mismatch and attenuation effects) by factors of between about 8 and 500, depending upon the type of foam, its thickness, and its density.