Jeffry J. Fedderly

Polyurethane networks with controlled architecture of dangling chains

Network formation from A x + B y precursors (A + B → A - B) with functionality and molecular weight distributions is described by the statistical theory of branching processes. Network formation is described in terms of sol and gel fractions, dangling chains, elastically active network chains (EANC), elastically active crosslinks, free chain ends and branch points partitioned between sol and gel. Various definitions of an EANC are considered. The general relations are applied to a special case of A1 + A2 + A3/B2 system. It is show how the size and weight fractions of dangling chains can be varied indepently by varying the functionality or molecular weight distributions. This is demonstrated experimentally analyzing corresponding polyetherurethane networks prepared from mixtures of polyoxypropylene polyols. The width of the main transition region correlates with the fraction of material in dangling chains. The dependence of the equilibrium shear modulus on the concentration of EANCs indicates much weaker intermolecular interactions for networks with many short dangling chains compared with systems having few longer dangling chains.

DYNAMIC PROPERTIES OF POLYUREA 1000

The shock response of the viscoelastic polymer material polyurea 1000 has been investigated. Sabots carrying Al or Cu metal disks were launched into target assemblies containing the polyurea material. The target consisted of a thin metal disk on the impact side, a 6.5‐mm‐thick polyurea disk, and a thick metal backup disk. 50‐Ω manganin gauges were epoxied between the metal/polymer and polymer/metal interfaces to measure the interface stresses and shock transit time. Measured longitudinal stresses ranged from 6 to 43 kbar. The measured shock velocity‐particle velocity relationship was linear over this stress range. Maximum volume compression was about 30% for the initial shock wave. Several reshock waves were also measured for each experiment.

Network structure dependence of volume and glass transition temperature

A series of polyurethanes was used to determine the molar contributions of chain ends (CE) and branch points (BP) to free volume and glass transition temperature Tg. The polyurethanes were copolymers of diphenylmethane diisocyanate and poly(propylene oxide) (PPO) with hydroxyl functionalities of one, two, and three. The equivalent weights of all the PPOs were equal, such that the chemical composition of the chain segments was essentially identical. Therefore, the only distinctions among polymers were differences in CE and BP concentration. Theory of branching processes computer simulations were used to determine the concentration of CE due to imperfect network formation. Other CE contributions were from the monofunctional PPO. Polymer volumes and Tgs were correlated to CE and BP concentrations, and the contributions of these species were determined from least squares fits. The molar volume and Tg contributions were then used to determine free volume thermal expansion coefficients. These values were compar...

THE EFFECT OF MECHANICAL DEFORMATION ON THE GLASS TRANSITION TEMPERATURE OF POLYUREA

The glass transition temperature (Tg) of a polyurea (Versathane P1000) is shown to be a function of mechanical strain and strain rate. For low strain rate (10−1/s), tensile testing, Tg values increase from −58 to −52 °C with strain. These results are interpreted as an increase in phase mixing. After annealing at 100 °C, the Tg decreases to a baseline value of −63 °C as an indication that the strain induced phase mixing is reversible. For one dimensional plate impact experiments, the Tg increases from −58 to −54 °C with strain. After annealing, the Tg value at the high strain level is about 4 °C higher than the baseline value. The impact may have caused some permanent change in the morphology. For a conical‐shaped steel impact experiment, Tg values also increase with strain from −58 to −50 °C. The Tg at the impact center after an annealing cycle is about 4 °C greater than the baseline value, indicating somewhat less than full reversal of the mixing.

Sound absorption height and width limits for polymer relaxations

An analysis was performed to relate the height and width of the peak in shear sound absorption per wavelength as a function of frequency for the glass transition of a polymer. For the single relaxation time model, the width is 1.14 decades only at small values of the ratio of relaxed to unrelaxed sound speed. For the larger values of sound-speed ratio observed in polymer glass transitions, the width increases as the height increases. For the Havriliak–Negami model, the width increases as the height decreases in a manner similar to that for the complex modulus, though in this case there is a cutoff below the absolute maximum. The curve can be described by the relation that height times width is 1.5 decades of frequency. These predictions are in good agreement with experimental data for 21 polyurethanes.

Investigation of microstructural changes in impacted polyurea coatings using small angle X-ray scattering (SAXS)

Three polyureas with decreasing soft segment molecular weights of 1000, 650, and a 250/1000 blend were molded onto circular steel plates and then impacted with a high speed (275 m/s) conical-shaped steel cylinder. The polyurea layer of the post mortem bilayers was characterized on a molecular level by small angle synchrotron X-ray scattering (SAXS) at the Advanced Photon Source at the Argonne National Laboratory. Analysis revealed that the hard domains of the polyureas with lower molecular weight soft segments reformed and oriented over a greater area of the coating, thus increasing the polymer strain hardening and resulting in visibly less out of plane bilayer deformation. This agrees with the hypothesis that polymer strain hardening is a mechanism that retards necking failure of the metal plate.

Calculation of Relaxation Time in Polyurethanes Using Additive Group Contributions

SYNOPSIS The method of additive properties was used to calculate the dynamic mechanical relaxation time for a series of polyurethanes. Calculations were also made of density and glass transition temperature. Group contributions for nine component groups were determined. With these group values, the densities of the 12 polymers used to determine the groups were calculated and found to agree with measured values within an average of 0.2%. Calculated glass transition temperatures also agreed with measured values within 0.2%. The relaxation time, defined as a parameter in the Havriliak-Negami equation, was shown to be correlated with the glass transition temperature, allowing relaxation time to also be expressed as an additive property. Calculated logarithms of relaxation times agree with measured values to within 7% over a range,of relaxation times covering many decades. 0 1996 John Wiley & Sons, Inc.

Inferring Viscoelastomer Dynamic Material Properties From Finite Element and Experimental Studies of Constrained Layer Damping Systems

Viscoelastic materials are often used to add damping to metal structures, usually via the constrained layer damping method. The added damping is strongly dependent on material temperature and frequency, as are the underlying material properties of the viscoelastomer. Several standardized test methods are available to characterize the dynamic material properties of viscoelastomers. However, they rely on limited test data which is extrapolated using the time-temperature superposition technique. The authors have found that the different testing methods typically produce significantly different material properties. A new approach to inferring viscoelastomer material properties is suggested here. Several metal bars are treated using constrained layer damping. Experimental modal analyses are conducted on the bars at different temperatures to produce sets of system resonance frequencies and loss factors. Corresponding finite element (FE) models of the treated bars are analyzed using assumed viscoelastomer material properties. The properties are adjusted by trial and error until the FE-simulated system loss factors match those of the measurements.© 2004 ASME

Effect of crosslinking on dynamic mechanical properties of polyurethanes

The dynamic mechanical properties of five polyurethane compositions of varying crosslink density were measured using a resonance technique. The five crosslinked polyurethanes are the reaction products of 4,4′‐diphenylmethane diisocyanate and various polypropylene glycol diol/triol blends. Crosslink density was controlled by varying the ratio of diol to triol. The dynamic mechanical data was fit to the Havriliak‐Negami dispersion relation. The most significant effect of increased crosslinking was to increase the low‐frequency modulus. A slight increase in relaxation time was observed as crosslink density was increased over the given range, which approximately correlates with the modest change in glass transition temperature seen in this series. Values for the high‐frequency modulus and asymmetry of the dispersion were not effected by crosslink density. Comparisons are also made with the sound absorption height and width limits predicted for uncrosslinked polymers [Hartmann et al., J. Acoust. Soc. Am. 101, ...

Sound absorption height and width limits for polymer glass transitions

At the glass transition of a polymer, the shear sound absorption per wavelength displays a relaxation covering many decades of frequency. Calculations were made of the peak height and half‐width of this relaxation based on the Havriliak–Negami dispersion relation. These calculations are an extension of our earlier study of the complex modulus loss factor height and width [B. Hartmann, G. F. Lee, and J. D. Lee, J. Acoust. Soc. Am. 95, 226–233 (1994)]. It was found that height and width are not independent: A high peak has a narrow width while broadband absorption can only be achieved for low‐peak heights. The calculation predicts that height times width is approximately constant, as expected for a relaxation for which the area under the curve is constant. These predictions are compared with published experimental data on various polymers, chiefly polyurethanes, and found to be in good agreement.