Debra A. Wrobleski

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.

Microstructures and properties of some microcellular foams

The microstructures of a variety of low-density (less than 0.1 g cm−3) polymeric foam materials are presented. Structures include the large, but well-defined, closed cells of commercially produced foams and a variety of finer, but often less well-defined, open cells of research foams produced from polymers, carbon, and silica. Other topics covered are the sizes and lowest densities of foams available, optical and X-ray opacity, and ease of handling.

Neutron capture induced radiation treatment of polymer materials

A precursor composition adapted for neutron capture induced radiation treatment of said precursor composition including a polymer matrix containing dispersed dopant material, the dispersed dopant material characterized as capable of neutron capture whereupon subsequent in situ energetic ion irradiation of the polymer matrix can occur, and further characterized as dispersed so as to provide dopant domain sizes significantly less than the energetic ion range of the dopant material is provided. Also provided is a process of in situ irradiation of bulk polymeric articles by first providing a precursor composition adapted for neutron capture induced radiation treatment of the precursor composition including a polymer matrix containing dispersed dopant material, the dispersed dopant material characterized as dispersed so as to provide dopant domain sizes significantly less than the energetic ion range of the dopant material, and then exposing the precursor composition to a source of neutrons.

Electron spin resonance study of oxidation in X-irradiated poly(ester urethane) containing nitroplasticizer

Abstract The effect of oxidation on X-irradiated Estane®5703 containing nitroplasticizer (NP) has been examined by electron spin resonance (ESR) spectroscopy, and the results are compared to similar data previously obtained on pristine Estane®5703. Although both specimens exhibit similar spectra immediately following X-irradiation, their decay upon exposure to air is quite different. The free radical concentration of the pristine specimen continuously decreases with time whereas the NP sample exhibits an initial decrease followed by a significant increase due to the growth of a newly-formed radical. Terminal species of the pristine and NP-Estane®5703 samples are identified as peroxy and nitroxide radicals, respectively. Hyperfine coupling constants and g-values are extracted for the nitroxide radical and a tentative model is proposed to explain the reaction pathway leading to its production.

Small-angle neutron scattering study of a thermally aged, segmented poly(ester urethane) binder

Abstract Small-angle neutron scattering (SANS) measurements have been performed on a thermally aged polymeric binder to understand the effects of aging on its microstructure. The binder is a 50%–50% (by weight) mixture of a segmented poly(ester urethane), known as EstaneB 5703, and a nitro-plasticizer (NP). This compound is of interest because it is used in the high-explosive (HE) PBX9501. The addition of the polymeric binder to the crystalline HE imparts both structural integrity and plasticity to the HE, allowing it to be readily machined and pressed to specific densities. In addition, the binder significantly affects the performance and the sensitivity of the HE, due in part to its influence on the propagation of microstresses between crystalline grains under shock or loading conditions. So changes in the binder over time can influence the behavior of the HE system as a whole.

Surface Modification of Poly(ether urethane) by Chemical Infusion and Graft Polymerization

The surface of a commercially available poly(ether urethane), TecoflexR, has been modified by either chemical infusion or graft polymerization techniques. The chemical infusion technique involves the physical entrapment of polymer additives in the near surface region of the sample, while graft polymerization provides chemical attachment of a polymer to the surface of the sample. The additives investigated for chemical infusion include poly(vinylpyrrolidone) (PVP) and poly(ethylene glycol) (PEG) along with iodine and silver nitrate as antibacterial agents. Graft polymerization covalently bonds polymers to the surface of the poly(ether urethane). The polymerization is initiated by photolysis of Re2(CO)10 to generate radicals on the poly(ether urethane) surface. The monomers examined for graft polymerization include N-vinyl pyrrolidone (NVP) and 2-hydroxyethylmethacrylate (HEMA), along with sulfonate containing monomers such as sodium vinylsulfonate, 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS) and its sodium salt (NaAMPS). The surface energies of these surface modified poly(ether urethane) samples were examined by contact angle measurements in water using the Wilhelmy balance technique. An increase in surface energy was observed following surface modification by both techniques, resulting in more hydrophilic surfaces than the untreated samples.

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.

Comprehensive Structural Study of Pre- and Post-Heat Treated Compression Molded Polyurethane Samples of Varying Composition Studied by Scanning Probe Techniques

Only a limited number of structural studies have been performed on polyurethanes using scanning probe techniques to determine both the microstructure and the corresponding distribution of hard and soft segments within samples. This type of information is needed to better understand the mechanical properties of these materials and to facilitate modeling. In order to address these issues, we have fabricated a series of compression molded segmented poly(ester urethane) samples with hard (HS) to soft segment ratios from 19 to 100%. Samples were examined using scanning probe phase imaging techniques to obtain the topography and corresponding distribution of hard domains before and after heating at 100 C. A number of significant differences were observed between the pre- and post-heat treated samples. Variations in structure and heat-induced morphological changes were directly related to HS content. Fine strand- or fibril-like structures were most prominent in the 23 and 19% HS sample but first appeared at 30% HS. Harder, thicker elongated structures dominated the surface of the 100% HS sample and were seen to a limited extent on all samples, especially after annealing and quenching. The 23% HS sample surface structure depended on quenching rate and time after treatment.

Surface modification of polymeric materials and its effect on blood compatibility

The surfaces of commercially available polymeric materials have been modified through the chemical infusion process and physical vapor deposition. The surfaces of poly(methylmethacrylate) (PMMA) have been modified through a chemical infusion process by treatment of the sample with a solution containing varying amounts of titanium(IV)isopropoxide and polyvinylpyrrolidone (PVP). The surfaces of silicone rubber samples have been coated with a thin coating of titanium dioxide with an ion beam sputtering technique. The treated samples were characterized by scanning electron microscopy, optical microscopy, and neutron activation analysis. The infused samples were evaluated for blood compatibility using two biological assays: an adherence assay in which the adherence of human polymorphonuclear leukocytes to the samples was determined, and a hemolysis assay using rat blood erythrocytes to determine the hemolytic activity of the samples. Based on the results of these assays, the PMMA samples treated with PVP alone resulted in an improvement in reactivity with the blood cells. 16 refs., 4 figs.