Richard L. Lehman

Multiphase blends from poly(L-lactide) and poly(methyl mathacrylate)

Melt processing of poly(L-lactide) (PLLA) and poly(methyl methacrylate) (PMMA) was conducted over a targeted range of compositions with PLLAs of 118 and 316 kDa in molecular mass to identify morphologies and the phase relationships in these blends. These blends are of interest for use in biomaterials and the morphologies are critical for tissue-engineering studies where biodegradability, pore connectivity and surface texture control tissue viability and adhesion. Simple extrusion of the two polymers produced multiphase blends with an average domain size near 25 μm. Scanning electron microscopy and dynamic mechanical analysis demonstrated that these blends are immiscible, at least in a metastable sense, and regions of co-continuous structures were identified. Such co-continuous, which occurred generally in accordance with rheology prediction models, exhibit a fine interconnected structure that appears effective for fabricating certain biomaterials. A broad and unexpected transition appears in these blends, as measured by modulated differential scanning calorimetry between 70 and 100°C, which may be the glass transition temperature of an alloy phase. The magnitude of this transition is greatest in the fine-structured co-continuous composition region of blends, suggesting the presence of a complex or other derivative of the two primary phases.

Carbon coated alumina fiber/glass matrix composites

Abstract Carbon coated alumina fiber reinforced borosilicate glass composites were fabricated by slurry infiltration and hot-pressing. For fiber contents of 20 vol.%, composite strengths of 118 and 263 MPa were observed for uncoated and carbon coated composites respectively. Fiber pull-out was observed in fracture surfaces only for carbon coated composites, and pull-out lengths were approximately 30–60 μm. The fiber/matrix interface was studied to characterize the interfacial frictional stress. Direct measurements by indentation were compared with calculated values based on pull-out lengths and fiber strengths. Direct indentation measurements gave interfacial frictional stress values of 100 MPa, which were reduced by as much as 40% after a correction for Poissons ratio effects. Values calculated from pull-out lengths suggested a value of 63–84 MPa, in good agreement with direct measurements. Elastic modulus and thermal expansion measurements confirmed the debonded character of the carbon coated fiber/matrix interface.

Drawing‐enhanced defect precursors in low‐OH content, oxygen‐deficient synthetic silica optical fibers

The 74 and 13 G electron paramagnetic resonance hydrogen hyperfine doublets of the Si E’ center were observed in low‐OH content, oxygen‐deficient silica optical fibers exposed to hydrogen at room temperature. These spectra were also found in hydrogen treated bulk samples but at negligible intensities compared to those in fiber samples. The significant enhancement of these doublets in the fiber samples indicates that the precursors of the 74 and 13 G hydrogen hyperfine doublets of Si E’ are substantially produced in the fiber drawing process. Possible structures of the two precursors are discussed.

Electron paramagnetic resonance hyperfine spectrum of the Si E’ defect associated with weakly bonded hydrogen molecules in synthetic silica optical fibers

Electron paramagnetic resonance (EPR) studies of low hydroxyl synthetic silica optical fibers have revealed the presence of a 13 G hyperfine splitting of the Si E’ center which is associated with weakly bonded hydrogen molecules in the silica structure. The fibers were previously exposed to hydrogen at room temperature. During an annealing of the silica fibers at 100 °C in air, the 13 G doublet was observed to decline and disappear while the 74 G doublet and the Si E’ signal were growing. The result correlates with the redistribution of hydrogen molecules and is relevant to the environmental aging that is encountered in fiber optical waveguides under certain conditions.

Excimer-laser-induced spatially variant luminescence in pure-silica core fibers with fluorine-doped silica cladding

Optical fibers with pure silica cores of both low and high water content and fluorine-doped silica claddings were irradiated with 248 nm (KrF) excimer-laser radiation. In addition to the differences in the spectra of luminescent emissions of the respective cores, spatial variance was observed in the luminescence behavior across cross sections of the cores. Photographic evidence of this phenomenon is presented along with the corresponding luminescence spectra. Spectral correlations of the spatial variances are made, and the defects that are responsible for the luminescence are discussed.

Insight into the Molecular Arrangement of High-Density Polyethylene Polymer Chains in Blends of Polystyrene/High-Density Polyethylene from Differential Scanning Calorimetry and Raman Techniques:

Polystyrene/high-density polyethylene (PS/HDPE) blends were synthesized by melt blending in a single screw extruder. Co-continuity measurements using solvent extraction and scanning electron (SEM) micrographs showed that co-continuity was obtained around 35% PS. Thermal analyses measurements revealed a reduction in crystallinity of the HDPE phase around the co-continuous composition. Raman analyses across the entire composition range of these blends showed an increase in the normalized integral intensities of the 1128 cm−1 and 1061 cm−1 stretching vibrations of the HDPE molecules. The presence of all-trans HDPE chains that are not packed into an orthorhombic structure is used to explain the simultaneous occurrence of reduced crystallinity and increased intensity of all-trans HDPE stretch vibrations.

Transient electron paramagnetic resonance hydrogen hyperfine doublet in low‐OH content synthetic silica optical fibers during isothermal annealing

A transient 75 G electron paramagnetic resonance (EPR) hydrogen hyperfine doublet was observed for the first time in low‐OH content silica optical fibers during isothermal annealing in the temperature range between 250 and 400 °C in air. The results are consistent with the diffusion of hydrogen molecules into the glass with subsequent interaction with E’ centers to form an intermediate E’/H and, in turn, an EPR‐inactive end product. The response of the doublet to trace amounts of hydrogen was used to monitor hydrogen diffusion and the reaction between E’ and hydrogen in the glass.

Sustainable polymer composites: immiscible blends prepared by extrusion of poly(trimethylene terephthalate) and polyamide6,10 with high bio-based content

Components for binary polymer blends were sought to produce an immiscible blend of improved renewable character and with good structural properties. The poly(trimethylene terephthalate) and polyamide6,10 system was selected based on the molecular structure of the molecules and the bio-based origin of the feedstocks. A preliminary study of three compositions in this system demonstrated the similar thermal properties of the two polymers as measured by differential scanning calorimetry (DSC) and the ability of these polymers to be processed together in conventional extrusion equipment to produce blends with micrometer-scale domains. Dispersed phases were observed by electron microscopy near the end members. Available viscosity data and the appearance of columnar blends at the 50/50 composition suggest the possibility of co-continuous blends in close proximity to this composition.

Viscosity and domain morphology in binary immiscible blends of poly(trimethylene terephthalate) and polyamide6,10

Abstract Poly(trimethylene terephthalate) (PTT) and polyamide6,10 (PA6,10) are polymers with significant bio-based content that forms an immiscible blend system of high-value engineering materials with enhanced sustainability. Knowledge of the melt viscosity of these thermoplastic materials is critical, when processing blends to achieve optimum morphologies. We measured the viscosities of five extruded blends near the phase inversion composition, over a range of shear rates using both parallel plate and capillary methods. Based on the viscosities of the end-members, Jordhamo co-continuity should be observed in the 21–44 volume percent PTT range, depending on the processing shear rate. Extruded blend viscosities were lower than the linear rule of mixtures and the phase inversion composition was identified near 55 volume percent PTT. SEM images showed clear indications of developing co-continuity in the 55 volume percent PTT. We conclude that the viscosities of immiscible polymer blends in this system do not follow the rule of mixtures, due to slip between immiscible domain interfaces, but that the viscosity and the power law index are useful in locating the phase inversion composition. The empirical Jordhamo relationship, although generally useful in immiscible polymer systems, is not as valuable in this system.

Electron paramagnetic resonance characterization of nonuniform distribution of hydrogen in silica optical fibers

The hydrogen distribution within low‐OH silica optical fibers has been characterized by electron paramagnetic resonance in combination with a chemical etch‐back technique. The 74 G doublet, the signal generated by a hydrogen hyperfine structure of the Si E′ center, was used to investigate the spatial distribution of hydrogen reaction products in silica optical fiber and, in particular, the reaction between the twofold coordinated silicon and hydrogen. A nonuniform distribution of the 74 G species was observed with the highest concentration near the fiber surface, indicative of a corresponding nonuniform distribution of hydrogen. The polymer fiber coating appears to be the primary source of the trace amounts of hydrogen observed in the silica fibers.