The ability to repair damaged mammalian axons to re-establish functional connections continues to be a goal for neuroscientists. Following axonal severance, proximal segments of mammalian axons seal themselves rapidly at the lesion site. Distal segments of severed mammalian axons undergo Wallerian degeneration within 24-72 hours. Prior to the onset of degeneration, distal axonal segments remain electrically excitable. The work described in this dissertation demonstrates that polyethylene glycol (PEG), a hydrophilic polymer, can rapidly repair severed axons by fusing the plasmalemmas of two closely apposed distal and proximal axonal segments. This plasmalemmal fusion restores morphological integrity of severed axons and their ability to conduct action potentials across the injury site. The ability to fuse proximal and distal severed axonal segments using PEG is improved when the axonal segments are exposed to antioxidants, such as melatonin and methylene blue, and also when microsutures provide additional support in transected sciatic nerves. The restoration of axonal continuity by PEG-fusion restores function, improving behavioral recovery in rats with crush-injured sciatic nerves, as well as those in which the sciatic is complete transected. / text

published_or_final_version / abstract / toc / Dentistry / Master / Master of Philosophy

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Project Completion Report / OWRT Project No. A-059-ARIZ / Development of Asphalt Cushioned Plastic and Plastic Reinforced Asphalt Membranes for Seepage Control / Agreement No. 14-31-0001-5003 / Project Dates: July 1974-June 1975. / This project was concerned with laboratory equipment development, laboratory testing, construction equipment development and field investigation of the APAC (Asphalt-Plastic-Asphalt-Chip-Coated) water seepage barrier. The laboratory equipment that was designed and fabricated for the project included hydrostatic testing vessels, slope stability apparatus, and tensile testing grips. Three testing methods were utilized and evaluated in the APAC investigation. The first test method evaluated the hydrostatic puncture resistance of the asphalt-polyethylene combination. This test confirmed the hypothesis that the asphalt effectively increases the puncture resistance of the APAC membrane over that of plain polyethylene. The second test investigated the slope stability of a protective APAC chip seal. It was found that a typical 3/8 in. (9.5 mm) cover aggregate remained stable on constructed slopes of 3:1 and 4:1 and that the 2:1 slope remained stable up to a surface temperature of 1220F (500C). The third test method evaluated adhesive materials and indicated that Presstite mastic was the best suited adhesive for sealing polyethylene overlaps. Subsequent field investigations resulted in equipment development that increased construction efficiency in the installation of the APAC membrane. Actual completed field installations were evaluated and further recommendations are included.

Rheological and processing behavior of a number of linear low-density polyethylene(LLDPE)/low-density polyethylene (LDPE) blends was studied with emphasis on the effects of long chain branching. First, a linear low-density polyethylene (LL3001.32) was blended with four LDPE's having distinctly different molecular weights. At high LDPE weight fractions, DSC melting thermograms have shown three different polymer phases; two for the pure components and a third melting peak of co-crystals. Different rheological techniques were used to check the thermo rheological behavior of all blends in the melt state and the effect of long chain branching. It was found that all blends are miscible in the melt state at small LDPE concentrations. The elongational behavior of the blends was studied using a uniaxial extensional rheometer, SER. The blends exhibit strain hardening behavior at high rates of deformation even at LDPE concentrations as low as 1%, which suggests the strong effect of branching added by the LDPE component. On the other hand, shear rheology was found to be insensitive to detect addition of small levels of LDPE up to lwt%. The second set of blends prepared and studied consisted of two Ziegler-Natta LLDPE's (LL3001.32 and Dowlex2045G) and two metallocene LLDPE's(AffinityPL1840 and Exact 3128) blended with a single LDPE. In DSC melting thermograms, it was observed that blends with metallocence LLDPE's exhibit a single melting peak at all compositions; whereas the Ziegler-Natta blends exhibit three melting peaks at certain compositions. It was found also that the metallocene LLDPE's are miscible with the LDPE at all concentrations. On the other hand, the Ziegler-Natta LLDPE's were found to be miscible with LDPE only at small LDPE concentrations. The processing behavior of all blends with emphasis on the effects of long chain branches was also studied in capillary extrusion. The critical shear stresses for the onset of sharkskin and gross melt fracture are slightly delayed with the addition of LDPE into LLDPE. Furthermore, the amplitude of the oscillations in the stick-slip flow regime, known as oscillating melt fracture, were found to scale with the weight fraction of LDPE. Amounts as low as 1 wt% LDPE have a significant effect on the amplitude of pressure oscillations. These effects are clearly due to the presence of LCB. It is suggested that the magnitude of oscillations in the oscillating melt fracture flow regime can be used as a method capable to detect low levels of LCB. Finally, the sharkskin and stick-slip polymer extrusion instabilities of a linear low-density polyethylene were studied as a function of the type of die geometry. The critical wall shear stress for the onset of flow instabilities, the pressure and flow rate oscillations, and the effects of geometry and operating conditions on the instabilities are presented for a LLDPE. It was found that sharkskin and stick-slip instabilities were present in the capillary and slit extrusion. However, stick-slip and sharkskin in annular extrusion are absent at high ratios of the inside to outside diameter of the annular die. This observation also explains the absence of these instabilities in polymer processing operations such as film blowing. These phenomena are explained in terms of the surface to volume ratio of the extrudates.

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The linear growth rates of solution grown low molecular weight polyethylene single crystals have been investigated as a function of crystallization temperature, molecular weight and solution concentration. Sharp fractions were used covering the molecular weight range from 3100 to 11600. The crystals were characterized by studying (1) the equlibrium melting/dissolution temperature as a function of the molecular weight, (2) the lamellar thickness as a function of the crystallization temperature and (3) the fusion/dissolution behavior as a function of crystallization temperature. / The most interesting results are as follows: (a) The lamellar thickness varies continuously with crystallization temperature. This behavior has been interpreted to provide information on the important question of the location of the chain ends in the crystals. (b) For the higher molecular weight fractions the growth rate decreases monotonically with the crystallization temperature. In contrast for fractions in the molecular weight range of 3000 to 4000 the growth rate versus crystallization temperature curves show a notched appearance in which two branches can be distinguished. These branches intersect at a transition temperature where the temperature coefficient of the growth rates changes discontinuously. This behavior is interpreted to mean that the transition temperatures correspond to changes in the conformation of the molecules deposited on the growth faces. The growth rate data have been analyzed using current kinetic theories of polymer crystal growth. Basal surface free energies thus obtained are compared with the estimates from the relations between lamellar thickness and supercooling and melting/dissolution temperature and lamellar thickness. These results are discussed in relation to the molecular structure of the crystals.

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