Photochemical degradation of polystyrene

In this work the oxidative degradation of pure polystyrene, polybutadiene and butadiene-modified polystyrene (normally called high impact polystyrene or HIPS) have been studied using a variety of physical and chemical techniques. The changes in dynamic-mechanical properties occurring during the ultra-violet light accelerated weathering of these polymers were followed by a visco-elastometric technique (Rheovibron) in the solid phase over a wide temperature range. Selective cross-linking of the polybutadiene in high-impact polystyrene caused the depression of the low temperature damping peak (tan d) with a corresponding sharp peak in tan d at ambient temperature accompanied by an integral rise in complex modulus. During the same period of photoxidation, the hydroperoxide concentration and gel content increased rapidly, reaching a maximum before decomposing photolytically with the destruction of unsaturation and with the formation of stable oxidation products. Infra-red spectroscopy showed the formation of carbonyl and hydroxyl groups. a,ß-unsaturated carbonyl was also identified and was formed by decomposition of both allylic hydroperoxide and initial peroxidic gel by ß-scission of the graft between polybutadiene and polystyrene. With further photoxidation a more stable ether gel was formed involving the destruction of the conjugating double bond of a,ß-unsaturated carbonyl. Addition of saturated and unsaturated ketones which are potential sensitisers of photoxidation to high-impact polystyrene and polybutadiene failed to photo-initiate the oxygen absorption of the polymers. A prior thermal oxidative treatment on the other hand eliminated the auto- accelerating stage leading to linear kinetics as the concentration of thermally-produced hydroperoxide approached a maximum. Antioxidants which act by destroying hydroperoxide lengthened the induction period to rapid oxygen absorption, whilst a phenolic antioxidant behaved as a weak photo-activator initially and a retarder later. Prior photolysis of high-impact polystyrene photo-activated the unsaturated component and caused similar changes in dynamic-mechanical properties to those found during photoxidation although at a much lower rate. Polybutadiene behaves as a photo-pro-oxidant for the destruction of polystyrene in high-impact polystyrene.

Characterisation and modification of hydrogel polymers

The work described in this thesis can be broadly divided into two sections. The first being the characterisation of hydrogel polymers in both their hydrated and dehydrated states and the second some aspects of the structural modification of polymers. The characterisation of hydrogel polymers in their dehydrated state (xerogels) involves such techniques as elemental analysis, pyrolysis gas liquid chromatography, infra-red spectroscopy, density determination and surface characterisation by contact angle measurements. The characterisation of some commercially available hydrogel materials was undertaken using such techniques and the results obtained were compared to laboratory synthesised systems in an attempt to assess the value of the combination of techniques employed. In the characterisation of hydrated polymers the amoumt and nature of water present is the single most important factor. The most convenient method of characterising this water involves the use of differential scanning calorimetry (DSC), coupled with total equilibrium water content measurements. DSC distinguishes between non-freezing and freezing water but in addition provides some information on the continuum of states in the freezing water fraction. Two aspects of the structural modification of hydrogel polymers were studied. The first involved the incorporation of acrylamide and substituted acryamide monomers into a copolymer system and an examination of the effect of this on the amino acid interaction of the polymers. The second was the attempted synthesis of cell surface analogues by the attachment of sugar type molecules to the polymer using a variety of reaction methods.

The behaviour of antioxidants in polyolefins under conditions of U.V. exposure

The effects of antioxidants and stabilizers on the oxidative degradation of polyolefins (low density polyethylene [LDPE] and polypropylene [PPJ have been studied after subjecting to prior high temperature processing treatments. The changes in the both chemical and physical properties of unstabilized polymers occurring during processing were found to be strongly dependent on the amount of oxygen present in the mixer. Subsequent thermal and photo-oxidation showed very similar characteristics and the chromophore primarily responsible for:both thermo and photooxidative degradation of unstabilized polymers was found to be hydroperoxide formed during processing. Removal of hydroperoxide by heat treatment in an inert atmosphere although increasing ketonic carbonyl concentration, markedly decreased the rate of photo-oxidation, introducing an induction period similar to that of an unprocessed sample. It was concluded that hydroperoxides are the most important initiators in normally processed polymers during the early stages of photo-oxidation. Antioxidants such as metal dithiocarbamates which act by destroying peroxides into non-radica1 products were found to be efficient melt stabilizers for polyolefins and effective UV stabilizers during the initial photo-oxidation stage, whilst a phenolic antioxidant, n-octadecyl-3-(3',5'-di-terbutyl 4'hydroxypheny1) propionate (Irganox 1076) retarded photo-oxidation rate in the later stages. A typical 'UV absorber' 2-hydroxy-4-octyloxy-benzophenone (HOBP) has a minor thermal antioxidant action but retarded photo-oxidation at all stages. A substituated piperidine derivative, Bis [2.2.6.6-tetramethylpiperidlnyl-4] sebacate (Tinuvin 770) behaved as an pro-oxidant during thermal oxidation of polyolefins but was an effective stabilizer against UV light. The UV absorber, HOBP synergised effectively with both peroxide decomposing antioxidants (metal dithiocarbamates) and a chain-breaking antioxidant (Irganox 1076) during photo-oxidation of the poymers studed whereas the combined effect was additive during thermal oxidation. By contrast, the peroxide decornposers and chain-breaking antioxidant (Irganox 1076) which were effective synergists during thermal oxidation of LDPE· were antagonistic during photo-oxidation. The mechanisms of these processes are discussed.

The effect of certain hydrophilic polymers on suspension stability

The adsorption of nonionic surface active agents of polyoxyethylene glycol monoethers of n hexadecanols on polystyrene latex and nonionic cellulose polymers of hydroxyethyl cellulose, hydroxypropyl cellulose and hydroxypropyl methylcellulose on polystyrene latex and ibuprofen drug particles have been studied. The adsorbed layer thicknesses were determined by means of microelectrophoretic and viscometric methods. The conformation of the adsorbed molecules at the solid-liquid interface was deduced from the molecular areas and the adsorbed layer thicknesses. Comparison of the adsorption results obtained from polystyrene latex and ibuprofen particles was made to explain the conformation difference between these two adsorbates. Sedimentation volumes and redispersibility values were the main criteria used to evaluate suspension stability. At low concentrations of surface active agents, hard caked suspensions were found, probably due to the attraction between the uncoated areas or, the mutual adsorption of the adsorbed molecules on the bare surface of the particles in the sediment. At high concentrations of hydroxypropyl cellulose and hydroxypropyl methylcellulose, heavily caked sediments were attributed to network structure formation by the adsorbed molecules. An attempt was made to relate the characteristics of the suspensions to the potential energy of interaction curves. Generally, the agreement between theory and experiment was good, but for hydroxyethyl cellulose-ibuprofen systems discrepancies were found. Experimental studies showed that hydroxyethyl cellulose flocculated polystyrene latex over a rather wide range of concentrations; similarly, hydroxyethyl cellulose-ibuprofen suspensions were also flocculated. Therefore, it ls suggested that a term to account for flocculation energy of the polymer should be added to the total energy of interaction. A rheometric method was employed to study the flocculation energy of the polymer.

Novel formulations for antigen delivery using biodegradable polymers: new approaches for the use of new and established adjuvants

The use of immunological adjuvants has been established since 1924 and ever since many candidates have been extensively researched in vaccine development. The controlled release of vaccine is another area of biotechnology research, which is advancing rapidly with great potential and success. Encapsulation of peptide and protein drugs within biodegradable microspheres has been amongst the most successful of approaches within the past decade. The present studies have focused on combining the advantages of microsphere delivery systems composed of biodegradable polylactide (PLLA) and polylactide-co-glycolide (PLGA) polymers with that of safe and effective adjuvants. The research efforts were directed to the development of single-dose delivery vehicles which, can be manufactured easily, safely, under mild and favourable conditions to the encapsulated antigens. In pursuing this objective non ionic block copolymers (NIBCs) (Pluronics@ LI01 and L121) were incorporated within poly-dl-lactide (PDLA) micorospheres prepared with emulsification-diffusion method. LI0I and L121 served both as adjuvants and stabilising agents within these vaccine delivery vehicles. These formulations encapsulating the model antigens lysozyme, ovalbumin (OVA) and diphtheria toxoid (DT) resulted in high entrapment efficiency (99%), yield (96.7%) and elicited high and sustained immune response (IgG titres up to 9427) after one single administration over nine months. The structural integrity of the antigens was preserved within these formulations. In evaluating new approaches for the use of well-established adjuvants such as alum, these particles were incorporated within PLLA and PLGA microspheres at much lesser quantities (5-10 times lower) than those contained within conventional alum-adsorbed vaccines. These studies focused on the incorporation of the clinically relevant tetanus toxoid (TT) antigen within biodegradable microspheres. The encapsulation of both alum particles and TT antigen within these micropheres resulted in preparations with high encapsulation efficiency (95%) and yield (91.2%). The immune response to these particles was also investigated to evaluate the secretion of serum IgG, IgG1, IgG2a and IgG2b after a single administration of these vaccines. The Splenic cells proliferation was also investigated as an indication for the induction of cell mediated immunity. These particles resulted in high and sustained immune response over a period of 14 months. The stability of TT within particles was also investigated under dry storage over a period of several months. NIBC microspheres were also investigated as potential DNA vaccine delivery systems using hepatitis B plasmid. These particles resulted in micro spheres of 3-5 μm diameter and were shown to preserve the integrity of the encapsulated (27.7% entrapment efficiency) hepatitis B plasmid.

Interactions between organic polymers and cement hydration products

Organic substances, particularly polymers, are finding increasing use in modifying the properties of cements and concrete. Although a significant amount of research has been conducted into the modification of the mechanical properties of cements by polymers, little is known about the nature of the interface and interactions taking place between the two phases. This thesis addresses the problem of elucidating such interactions. Relevant literature is reviewed, covering the general use of polymers with cements, the chemistry of cements and polymers, adhesion and known interactions between polymers and both cements and related minerals. Although several polymer systems were studied, two in particular were selected, as being well characterized. These were: - 1) polymethyl methacrylate (PMMA), the polymer derived from methyl methacrylate (MMA), and 2) an amine-cured epoxy resin system. By this approach, a methodology was developed for the examination of other polymer/cement interactions. Experiments were conducted in five main areas:- 1) polymer-cement adhesion and the feasibility of revealing interfacial regions mechanically, 2) chemical reactions between polymers and cements, 3) characterization of cement adhesion surfaces, 4) interactions affecting overall polymerisation rates, and 5) studies of polymer impregnated cements. The following conclusions were reached:- 1) The PMMA/cement interface contains calcium methacrylate as an interfacial reaction product, water being a reactant. Calcium methacrylate is detrimental to the properties of PMMA/cement composites, being highly water-soluble. 2) The pore surface of cement accelerates the polymerisation of MMA, leading to an increased molecular weight compared to polymerisation of pure MMA, minerals in hydrated cement powders having the opposite effect. 3) The investigation of reaction products presents a number of experimental problems, selection of appropriate techniques depending upon the system studied. For the two systems examined in detail, ion chromatography proved particularly useful; DTA, IRS and XPS indicated reactions, though the data was hard to interpret; XRD proving inconclusive. 4) It is impractical to reveal interfacial regions mechanically, but may be accomplished by chemical means.

Biological interactions with synthetic polymers

Anchorage dependent cell culture is a useful model for investigating the interface that becomes established when a synthetic polymer is placed in contact with a biological system. The primary aim of this interdisciplinary study was to systematically investigate a number of properties that were already considered to have an influence on cell behaviour and thereby establish the extent of their importance. It is envisaged that investigations such as these will not only further the understanding of the mechanisms that affect cell adhesion but may ultimately lead to the development of improved biomaterials. In this study, surface analysis of materials was carried out in parallel with culture studies using fibroblast cells. Polarity, in it's ability to undergo hydrogen bonding (eg with water and proteins), had an important affect on cell behaviour, although structural arrangement and crystallinity were not found to exert any marked influence. In addition, the extent of oxidation that had occurred during the process of manufacture of substrates was also important. The treatment of polystyrene with a selected series of acids and gas plasmas confirmed the importance of polarity, structural groups and surface charge and it was shown that this polymer was not unique among `hydrophobic' materials in it's inability to support cell adhesion. The individual water structuring groups within hydrogel polymers were also observed to have controlling effects on cell behaviour. An overall view of the biological response to both hydrogel and non-hydrogel materials highlighted the importance of surface oxidation, polarity, water structuring groups and surface charge. Initial steps were also taken to analyse foetal calf serum, which is widely used to supplement cell culture media. Using an array of analytical techniques, further experiments were carried out to observe any possible differences in the amounts of lipids and calcium that become deposited to tissue culture and bacteriological grade plastic under cell culture conditions.

Reactive processing of polymers: structural characterization of products by 1H and 13C NMR spectroscopy for glycidyl methacrylate grafting onto EPR in the absence and presence of a reactive comonomer

Grafted GMA on EPR samples were prepared in a Thermo-Haake internal mixer by free radical melt grafting reactions in the absence (conventional system; EPR-g-GMA(CONV)) and presence of the reactive comonomer divinyl benzene, DVB (EPR-g-GMA(DVB)). The GMA-homopolymer (poly-GMA), a major side reaction product in the conventional system, was almost completely absent in the DVB-containing system, the latter also resulted in a much higher level of GMA grafting. A comprehensive microstructure analysis of the formed poly-GMA was performed based on one-dimensional H-1 and C-13 NMR spectroscopy and the complete spectral assignments were supported by two-dimensional NMR techniques based on long range two and three bond order carbon-proton couplings from HMBC (Heteronuclear Multiple Bond Coherence) and that of one bond carbon-proton couplings from HSQC (Heteronuclear Single Quantum Coherence), as well as the use of Distortionless Enhancement by Polarization Transfer (DEPT) NMR spectroscopy. The unambiguous analysis of the stereochemical configuration of poly-GMA was further used to help understand the microstructures of the GMA-grafts obtained in the two different free radical melt grafting reactions, the conventional and comonomer-containing systems. In the grafted GMA, in the conventional system (EPR-g-GMA(CONV)), the methylene protons of the GMA were found to be sensitive to tetrad configurational sequences and the results showed that 56% of the GMA sequence in the graft is in atactic configuration and 42% is in syndiotactic configuration whereas the poly-GMA was predominantly syndiotactic. The differences in the microstructures of the graft in the conventional EPR-g-GMA(CONV) and the DVB-containing (EPR-g-GMA(DVB)) systems is also reported (C) 2009 Elsevier Ltd. All rights reserved.

Production of renewable phenolic resins by thermochemical conversion of biomass: a review

This review covers the production and utilisation of liquids from the thermal processing of biomass and related materials to substitute for synthetic phenol and formaldehyde in phenol formaldehyde resins. These resins are primarily employed in the manufacture of wood panels such as plywood, MDF, particle-board and OSB. The most important thermal conversion methods for this purpose are fast pyrolysis and vacuum pyrolysis, pressure liquefaction and phenolysis. Many feedstocks have been tested for their suitability as sources of phenolics including hard and softwoods, bark and residual lignins. Resins have been prepared utilising either the whole liquid product, or a phenolics enriched fraction obtained after fractional condensation or further processing, such as solvent extraction. None of the phenolics production and fractionation techniques covered in this review are believed to allow substitution of 100% of the phenol content of the resin without impacting its effectiveness compared to commercial formulations based on petroleum derived phenol. This survey shows that considerable progress has been made towards reaching the goal of a price competitive renewable resin, but that further research is required to meet the twin challenges of low renewable resin cost and satisfactory quality requirements. Particular areas of concern are wood panel press times, variability of renewable resin properties, odour, lack of reactive sites compared to phenol and potential for increased emissions of volatile organic compounds.