E. Bruce Orler

Brillouin-scattering determination of the acoustic properties and their pressure dependence for three polymeric elastomers

The acoustic properties of three polymer elastomers, a cross-linked poly(dimethylsiloxane) (Sylgard 184), a cross-linked terpolymer poly(ethylene-vinyl acetate-vinyl alcohol), and a segmented thermoplastic poly(ester urethane) copolymer (Estane 5703), have been measured from ambient pressure to approximately 12 GPa by using Brillouin scattering in high-pressure diamond anvil cells. The Brillouin-scattering technique is a powerful tool for aiding in the determination of equations of state for a variety of materials, but to date has not been applied to polymers at pressures exceeding a few kilobars. For the three elastomers, both transverse and longitudinal acoustic modes were observed, though the transverse modes were observed only at elevated pressures (>0.7 GPa) in all cases. From the Brillouin frequency shifts, longitudinal and transverse sound speeds were calculated, as were the C(11) and C(12) elastic constants, bulk, shear, and Youngs moduli, and Poissons ratios, and their respective pressure dependencies. P-V isotherms were then constructed, and fit to several empirical/semiempirical equations of state to extract the isothermal bulk modulus and its pressure derivative for each material. Finally, the lack of shear waves observed for any polymer at ambient pressure, and the pressure dependency of their appearance is discussed with regard to instrumental and material considerations.

Infrared linear dichroism study of a hydrolytically degraded poly(ester urethane)

Static and dynamic infrared linear dichroism data have been used to supply additional insight into changes in tensile properties as a consequence of hydrolytic degradation of a segmented poly(ester urethane). Unaged material responds to tensile deformation with the soft (polyester) segments supplying the elasticity and the hard (polyurethane) segments supplying strength. Upon hydrolytic degradation, the static and dynamic data indicate altered orientational responses at the molecular-level, which are interpreted as resulting from cleavage of the soft segment chains and altered hydrogen-bonding interactions for both segments.

Carbon nanofibre reinforcement of soft materials

In elastomeric matrices carbon nanofibres are found to be twenty times more effective than carbon black as a reinforcing filler. In hard matrices, by contrast, reinforcement is minimal. Tensile and dynamic mechanical tests were performed to elucidate the mechanism of reinforcement in order to explain the superior performance in soft matrices. Small-angle neutron scattering and ultra-small-angle X-ray scattering were used to quantify filler morphology, which turns out to be the key factor that limits reinforcement potential. The presence of fractal cluster formed by agglomeration of the nanofibres reduces the effective aspect ratio of the nanotubes. Clustering, however, introduces a new reinforcement mechanism based on elastic deformation of the fibre clusters. This mechanism is operative in soft matrices but not in hard matrices, thus explaining the enhanced performance in soft matrices.

Structural Characterization of Segmented Polyurethanes by Small Angle Neutron Scattering

The beneficial mechanical properties of segmented polyurethanes derive from microphase separation of immiscible hard and soft segment-rich domains at room temperature. We are interested in the structure of the domains, how these are affected by hydrolytic aging, and how the structure is modified by low molecular weight plasticizers. To assessed the distribution of the plasticizer in polyurethane, we did small-angle neutron scattering measurements on mixtures of 23% hard segment poly(esterurethane) with different amounts of either non-deuterated or deuterated plasticizer. We analyzed the results using a simple model in which the contrast, Δ=H-, between the hard and soft segment-rich domains is varied by the amount of deuterated or hydrogenated plasticizer, using the fact that I(Q) ∼ Δ 2 . The result demonstrated that the plasticizer is largely associated with the soft segment rich domains. The structure of PESU with the chain extender of the hard segment was assessed after aging under hydrolytic conditions. The results show that the microphase structure coarsens and segregates and that the hard and soft segments segregated as a result of the loss of constraints from hydrolytic soft segment chain scission. The results on plasticizer distribution and the effects of hydrolytic aging give insight on the loss of mechanical properties that occur in each case.

Infrared and Raman Spectral Signatures of Aromatic Nitration in Thermoplastic Urethanes

The spectral signatures of nitro attack of the aromatic portion of thermoplastic urethanes (TPU) were determined. Eight fragment molecules were synthesized that represent the nitrated and pristine methylenediphenyl section common to many TPUs. Infrared (IR) and Raman (785 nm illumination) spectra were collected and modeled using the B3LYP/6-31G(d)//B3LYP/6-31G(d) model chemistry. Normal mode animations were used to fully assign the vibrational spectra of each fragment. The vibrational assignment was used to develop a diagnostic method for aromatic nitro attack in thermoplastic urethanes. The symmetric NO2 stretch coupled out of phase with the C–NO2 stretch (1330 cm−1) was found to be free from spectral interferences. Spectral reference regions that enable correction for physical differences between samples were determined. The carbonyl stretch at 1700 cm−1 was the best IR reference region, yielding a limit of quantitation (LOQ) of 0.66 ± 0.02 g N/100 g Estane. Secondary IR reference regions were the N–H stretch at 3330 cm−1 or the urethane nitrogen deformation at 1065 cm−1. The reference region in the Raman was a ring stretching mode at 1590 cm−1, giving an LOQ of 0.69 ± 0.02 g N/100 g Estane. Raman spectroscopy displayed a larger calibration sensitivity (slope = 0.110 ± 0.004) than IR spectroscopy (slope = 0.043 ± 0.001) for nitration determination due to the large nitro Raman cross-section. The full spectral assignment of all eight molecules in the infrared and Raman is presented as supplemental material.

A Study of Polymer Materials Subjected to Isentropic Compression Loading

This work applies a ramped, quasi‐isentropic compression loading technique (ICE) to investigate the mechanical behavior of polymers that are often used in energetic composites. The focus of this effort is the determination of appropriate constitutive and EOS property data at high stress states and moderate strain rates that is needed for detailed mesoscale modeling. Several thicknesses of samples were subjected to a ramp load of ∼45 Kbar over 500 ns duration using the Sandia Z‐machine. Profiles of transmitted ramp waves were measured at window interfaces using conventional VISAR. Shock physics analysis is then used to determine the nonlinear material response of the binder materials.