No description available.

Thesis (Ph. D.)--Virginia Polytechnic Institute and State University, 1991. / Vita. Abstract. Includes bibliographical references (leaves 192-196). Also available via the Internet.

Thesis (Ph. D.)--Lehigh University, 2001. / Includes bibliographical references and vita.

Thesis (Ph. D.)--Lehigh University, 2003. / Includes vita. Includes bibliographical references (leaves 215-216).

An investigation of the effects of various water-soluble polymers upon some properties of natural rubber latex has been carried out. All the polymers, which were of two principal types, distinguished according to the rate of addition of polymerisation reactants, were prepared by free-radical initiated solution polymerisation. Two types of monomers, namely, ionisable hydrophilic monomers and hydrophobic monomers were used in the preparation of polymers. Both types of polymers, referred to as Type 1 and Type 2, were principally characterised by viscometry, hydrophilicity assessment and infra-red analyses. It has been found that such characteristics of these polymers as molecular weight, composition distribution and hydrophilicity are dependent upon the method used for their preparation. Explanations which have been suggested for this dependence accord with theoretical predictions. It has also been found that certain properties of natural rubber (NR) latex such as mechanical stability (MST), viscosity, zinc-oxide thickening, heat- sensitivity and creaming can be modified with the addition of watersoluble polymers and that the extent to which each of these properties is modified depends upon the characteristics of the added polymer. The extent of the modification of MST and viscosity of NR la tex greatly depended upon both the molecular weight and composition of the added polymer. The influence of composition of the added polymer in modifying zinc-oxide thickening was found to be more important than that of molecular weight. Even though the effects of polymers of Type 1 and Type 2 upon the properties of NR latex were generally similar it has not been able to make proper comparisons between the effects of these two types of polymers as their characteristics were different.

A comparison has been made of the vulcanization of NR in the latex form and in the dry form. In latex form, NR wa s found to vu Lcani ze at a much f asterra t ethan in the dry form. The faster rate of reaction in latex prevulcanization is attributed to the presence of the aqueous phase which acts as a medium for the dissolution of ZDBC and sulphur, and provides a suitable environment for the two vulcanizing ingredients to react to form the active sulphurating agent. A comparison has also been made of the prevulcanization behaviour of NR latex and synthetic cis-polyisoprene latex. It has been suggested that the indigenous vulcanization accelerators and activators present in NR latex act as additional vulcanizing accelerators and therefore increase the rate of prevulcanization. Experiments have been carried out to determine the solubilities of sulphur and ZDBC in the aqueous phase of the latex during prevulcanization. Sulphur was found to dissolve to an appreciable extent; ZDBC was found to have a solubility of ca. 0.011%. The effects of varying the levels of sulphur and ZDBC upon NR latex prevulcanization was investigated. The variations of the following properties with time of prevulcanization were investigated: free and network-combined sulphur, disappearance of ZDBC, crosslink insertion, and tensile strengths of cast films. Information was also obtained concerning the polymer-solvent interacticn parameter for the cast films swollen in n-decane at 23°C. Investigations of the different types of sulphidic crosslinks as a function of prevulcanization time for vulcanizate obtained from prevulcanized latex have also been carried out. Tne crosslinks were found to be mainly of the polysulphidic type. The effect of varying the dialkyl chain-length and metal counterion of the dithiocarbamate accelerator upon NR latex prevulcanization have been studied. The molal effectiveness of the zinc dialkyl dithiocarbamates appears to pass through an optimum as tbe dialkyl chain-length of the accelerator is increased. PbDEC was found to be of very low activity as an accelerator and the water soluble SDEC has poor. accelerating properties when compared to ZDEC. Plausible hypotheses to account for the observations have been postulated.

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Surface modification of polydimethylsiloxane (PDMS) based vulcanizate rubber by C02-pulsed laser as the excitation source, without a photosensitizer, was studied at room temperature. The modified surfaces were characterized using a variety of techniques including scanning electron microscopy (SEM) combined with energy dispersive X-ray analysis (EDXA), attenuated total reflectance infrared (ATR-IR) spectroscopy and water drop contact angle analysis. EDXA showed that all of the treated PDMS surfaces contained a higher ratio of O/Si than the base PDMS. SEM micrographs and water drop contact angle variations showed the uniform porosity and high decrease in the wettability of the surface of PDMS respectively. The bulk mechanical properties of PDMS after laser-treatment did not change, as shown by dynamic mechanical thermal analysis (DMTA). The friction coefficient of the surface of the modified silicone decreased drastically, even after only one pulse was delivered to it. Data from in vitro blood compatibility experiments indicated a significant reduction of platelet adhesion and aggregation for the modified surfaces and those platelets which were adherent remained unspread (no activation). The extent of reduction of platelet adhesion was correlated to the number of laser pulses. The attachment of anchorage dependent cells, namely Baby Hamster Kidney (BILK) fibroblastic cells was investigated under stationary culture conditions. The laser treated surfaces showed little adhesion, no spreading and growth properties. This technique can be employed to prepare PDMS samples in which one surface is laser treated and the other is untreated. Such materials may be useful in, for example, medical implants in which one (the treated) surface is in contact with a blood supply and hence does not cause cell aggregation (clotting) and the other side is tissue compatible (allows adhesion of tissue). Acrylamide (AAm), 2-hydroxyethylmethacrylate (HEMA) and hydroxyethylmethacrylate phosphatidyl choline (HEMAPC) were grafted onto preirradiated PDMS. Platelet adhesion and cell attachment studies show that the biocompatibility of the AAm and HEMA grafted PDMS are intermediate between that of untreated PDMS and either the HEMAPC grafted PDMS surface, or PDMS surfaces that had been treated with 10 CO. laser pulses.

No description available.