Novel hydrogels for keratoprosthesis

Hydrogels may be described as cross~linked hydrophilic polymers that swell but do not dissolve in water. They have been utilised in many biomedical applications, as there is the potential to manipulate the properties for a given application by changing the chemical structure of the constituent monomers. This project is focused on the development of novel hydrogels for keratoprosthesis (KPro). The most commonly used KPro model consists of a tansparent central stem witb a porous peripheral skirt. Clear poly (methyl methacrylate) (PMMA) core material used in the Strampelli KPros prosthesis has not been the cause of failure found in other core and skirt prostheses. However, epithelialization of this kind of solid, rigid optic material is clearly impossible. The approach to the development of a hydrogeJ for potential KPro use adopted in this work is to develop soft core material to mimic the properties of the natural cornea by incorporating some hydrophilic monomers such as N, N-dimethyacrylamide (NNSMA) N~vinyl pyrrolidone (NVP) and acryloylmorpholine (AMO) with methyl methactylate (MMA). Most of these materials have been used in other ophthalmic applications, such as contact lens. However, an unavoidable limitation of simple .MMA copolymers as conventional hydrogels is poor mechanical strength. The hydrogel for use in this application must be able to withstand the stresses involved during the surgical procedure involved with KPro surgery and the in situ stresses such as the deforming force of the eyelid during the blink cycle. Thus, semi-interpenetrating polymer networks (SIPNs) based on ester polyurethane, AMO, NVP and NNDMA were examined in this work and were found to have much improved mechanical properties at water contents between 40% and 70%. Polyethylene glycol monomethacrylate (PEG MA) was successfully incorporated in order to modulate protein deposition and cell adhesion. Porous peripheral skirts were fabricated using different types of porosigen. The water content mechanical properties, surface properties and cell response of these various materials have been investigated in this thesis. These studies demonstrated that simple hydrogel SIPNs which show isotropic mechanical behaviour, are not ideal KPro materials since they do not mimic the anisotropic behaviour of natural cornea. The final stage of the work has concentrated on the study of hydrogels reinforced with mesh materials. They offer a promising approach to making a hydrogel that is very flexible but strong under tension, thereby having mechanical properties closer to the natural cornea than has been previously possible.

The chemical modification of polymer blends by reactive processing

The primary objective of this research was to examine the concepts of the chemical modification of polymer blends by reactive processing using interlinking agents (multi-functional, activated vinyl compounds; trimethylolpropane triacrylates {TRIS} and divinylbenzene {DVD}) to target in-situ interpolymer formation between immiscible polymers in PS/EPDM blends via peroxide-initiated free radical reactions during melt mixing. From a comprehensive survey of previous studies of compatibility enhancement in polystyrene blends, it was recognised that reactive processing offers opportunities for technological success that have not yet been fully realised; learning from this study is expected to assist in the development and application of this potential. In an experimental-scale operation for the simultaneous melt blending and reactive processing of both polymers, involving manual injection of precise reactive agent/free radical initiator mixtures directly into molten polymer within an internal mixer, torque changes were distinct, quantifiable and rationalised by ongoing physical and chemical effects. EPDM content of PS/EPDM blends was the prime determinant of torque increases on addition of TRIS, itself liable to self-polymerisation at high additions, with little indication of PS reaction in initial reactively processed blends with TRIS, though blend compatibility, from visual assessment of morphology by SEM, was nevertheless improved. Suitable operating windows were defined for the optimisation of reactive blending, for use once routes to encourage PS reaction could be identified. The effectiveness of PS modification by reactive processing with interlinking agents was increased by the selection of process conditions to target specific reaction routes, assessed by spectroscopy (FT-IR and NMR) and thermal analysis (DSC) coupled dichloromethane extraction and fractionation of PS. Initiator concentration was crucial in balancing desired PS modification and interlinking agent self-polymerisation, most particularly with TRIS. Pre-addition of initiator to PS was beneficial in the enhancement of TRIS binding to PS and minimisation of modifier polymerisation; believed to arise from direct formation of polystyryl radicals for addition to active unsaturation in TRIS. DVB was found to be a "compatible" modifier for PS, but its efficacy was not quantified. Application of routes for PS reaction in PS/EPDM blends was successful for in-situ formation of interpolymer (shown by sequential solvent extraction combined with FT-IR and DSC analysis); the predominant outcome depending on the degree of reaction of each component, with optimum "between-phase" interpolymer formed under conditions selected for equalisation of differing component reactivities and avoidance of competitive processes. This was achieved for combined addition of TRIS+DVB at optimum initiator concentrations with initiator pre-addition to PS. Improvements in blend compatibility (by tensiles, SEM and thermal analysis) were shown in all cases with significant interpolymer formation, though physical benefits were not; morphology and other reactive effects were also important factors. Interpolymer from specific "between-phase" reaction of blend components and interlinking agent was vital for the realisation of positive performance on compatibilisation by the chemical modification of polymer blends by reactive processing.

Novel hydrogel polymers

Hydrogels may be conveniently described as hydrophilic polymers that are swollen by, but do not dissolve in water. In this work a series of copolymer hydrogels and semi-interpenetrating polymer networks based on the monomers 2-hydroxyethyl methacrylate, N-vinyl pyrrolidone and N'N' dimethyl acrylamide, together with some less hydrophilic hydroxyalkyl acrylates and methacrylates have been synthesised. Variations in structure and composition have been correlated both with the total equilibrium water content of the resultant hydrogel and with the more detailed water binding behaviour, as revealed by differential scanning calorimetry studies. The water binding characteristics of the hydrogels were found to be primarily a function of the water structuring groups present in gel. The water binding abilities of these groups were, however, modified by steric effects. The mechanical properties of the hydrogels were also investigated. These were found to be dependent on both the polymer composition and the amount and nature of the water present in the gels. In biological systems, composite formation provides a means of producing strong, high water content materials. As an analogy with these systems hydrogel composites were prepared. In an initial study of these materials the water binding and mechanical properties of semi-interpenetrating polymer networks of N'N'dimethyl acrylamide with cellulosic type materials, with polyurethanes and with ester containing polymers were examined. A preliminary investigation of surface properties of both the copolymers and semi-interpenetrating polymer networks has been completed, using both contact angle measurements and anchorage dependent fibroblast cells. Measurable differences in surface properties attributable to structural variations in the polymers were detected by droplet techniques in the dehydrated state. However, in the hydrated state these differences were masked by the water in the gels. The use of cells enabled the underlying differences to be probed and the nature of the water structuring group was again found to be the dominant factor.

Transition metal ion coordination in hydrophilic polymer membranes

The research described within this thesis is concerned with the investigation of transition metal ion complexation within hydrophilic copolymer membranes. The membranes are copolymers of 4-methyl-4'-vinyl-2,2'-bipyridine, the 2-hydroxyethyl ester of 4,4'- dicarboxy-2,2'-bipyridine & bis-(5-vinylsalicylidene)ethylenediamine with 2-hydroxyethyl methacrylate. The effect of the polymer matrix on the formation and properties of transition metal iron complexes has been studied, specifically Cr(III) & Fe(II) salts for the bipyridyl- based copolymer membranes and Co(II), Ni(II) & Cu(II) salts for the salenH2- based copolymer membranes. The concomitant effect of complex formation on the properties of the polymer matrix have also been studied, e.g. on mechanical strength. A detailed body of work into the kinetics and thermodynamics for the formation of Cu(II) complexes in the salenH2- based copolymer membranes has been performed. The rate of complex formation is found to be very slow while the value of K for the equilibrium of complex formation is found to be unexpectedly small and shows a slight anion dependence. These phenomena are explained in terms of the effects of the heterogeneous phase provided by the polymer matrix. The transport of Cr(III) ions across uncomplexed and Cr(III)-pre-complexed bipyridyl-based membranes has been studied. In both cases, no Cr(III) coordination occurs within the time-scale of an experiment. Pre-complexation of the membrane does not lead to a change in the rate of permeation of Cr(III) ions. The transport of Co(II), Ni(II) & Cu(II) ions across salenH2- based membranes shows that there is no detectable lag-time in transport of the ions, despite independent evidence that complex formation within the membranes does occur. Finally, the synthesis of a number of functionalised ligands is described. Although they were found to be non-polymerisable by the methods employed in this research, they remain interesting ligands which provide a startmg pomt for further functionalisation.

Reactive processing of polyolefins using antioxidant systems

Various monoacrylic compounds containing a hindered phenol function (e.g.3,5-di-tert.-butyl-4-hydroxy benzyl alcohol, DBBA and vinyl-3-[3',5'-di-tert.-butyl-4-hydroxy phenyl] propionate, VDBP), and a benzophenone function (2-hydroxy-4-[beta hydroxy ethoxy] benzophenone, HAEB) were synthesised and used as reactive antioxidants (AO's) for polypropylene (PP). These compounds were reacted with PP melt in the presence of low concentration of a free radical generator such a peroxide (reactive processing) to produce bound-antioxidant concentrates. The binding reaction of these AO's onto PP was found to be low and this was shown to be mainly due to competing reactions such as homopolymerisation of the antioxidant. At high concentrations of peroxide, higher binding efficiency resulted, but, this was accompanied by melt degradation of the polymer. In a special reactive processing procedure, a di- or a trifunctional reactant (referred to as coagent), e.g.tri-methylol propane tri-acrylate, Tris, and Divinyl benzene, DVB, were used with the antioxidant and this has led to an enhanced efficiency of the grating reaction of antioxidants on the polymer in the melt. The evidence suggests that this is due to copolymerisation of the antioxidants with the coagent as well as grafting of the copolymers onto the polymer backbone. Although the 'bound' AO's containing a UV stabilising function showed lower overall stabilisation effect than the unbound analogues before extraction, they were still much more effective when subjected to exhaustive solvent extraction. Furthermore, a very effective synergistic stabilising activity when two reactive AO's containing thermal and UV stabilising functions e.g. DBBA and HAEB, were reactively processed with PP in the presence of a coagent. The stabilising effectiveness of such a synergist was much higher than that of the unbound analogues both before and after extraction. Analysis using the GPC technique of concentrates containing bound-DBBA processed in the presence of Tris coagent showed higher molecular weight (Mn), compared to that of a polymer processed without the coagent, but was still lower than that of the control processed PP with no additives. This indicates that Tris coagent may inhibit further melt degradation of the polymer. Model reactions of DBBA in liquid hydrocarbon (decalin) and analysis of the products using FTIR and NMR spectroscopy showed the formation of grafted DBBA onto decalin molecules as well as homopolymerisation of the AO. In the presence of Tris coagent, copolymerisation of DBBA with the Tris inevitably occured; which was followed by grafting of the copolymer onto the decalin, FTIR and NMR results of the polymer concentrates containing bound-DBBA processed with and without Tris, showed similar behaviour as the above model reactions. This evidence supports the effect of Tris in enhancing the efficiency of the reaction of DBBA in the polymer melt. Reactive procesing of HAEB in polymer melts exhibited crosslinking formation In the early stages of the reaction, however, in the final stage, the crosslinked structure was 'broken down' or rearranged to give an almost gel free polymer with high antioxidant binding efficiency.

Time controlled photo-oxidation of polyolefins by polymer bound additives

A variety of iron compounds containing vinyl or thiol functional groups (used as photoactivators) have been synthesised and some of these were successfully bound to both polyethylene and polypropylene backbones during processing in the presence of peroxide and interlinking agent. Concentrates (masterbatches) of the photoactivators in PP and PE were prepared and the pro-oxidant effect of the diluted masterbatches in absence and presence of an antioxidant was evaluated. An antioxidant photoactivator (FeDNC ) was found to sensitise the photoactivity of pro-oxidants (Metone A / Metone M) whereas an antioxidant (ZnDNC) was found to stabilise the polymer (PP and PE) containing both of these combinations. It was observed that the lower concentration of FeDNC sensitises the stability of the polymer containing very small concentration of NiDNC whereas higher concentration of FeDNC stabilises the polymer (LDPE) containing same amount of NiDNC compared to FeDNC alone. The photostability of unstabilised PP containing FeAc could be varied by varying the concentration of ZnDEC. Both the induction period and the UV - life time of the polymer increased by increasing concentration of ZnDEC. It is suggested that ligand exchange reaction may take place between FeAc and ZnDNC. A polymer bound UV stabiliser (HAEB) and a thermal stabiliser (DBBA) were used with a non extractable photoactivator (FeAc) in PP. Small concentrations of the stabilisers (HAEB and DBBA) in combination with the photoactivator (FeAc) sensitise the polymer. The antioxidant present in commercial polymer (LDPE and PP) was found to be of a hindered phenol type, which was found to antagonise with ZnDNC when used in combination with the photoactivators.

Tear and skin phospholipids:analysis and hydrogel analogues

This thesis is concerned with the analysis of phospholipids in the tear film and with the synthesis of phospholipids analogous to hydrogels. The work consists of two areas. The first area is the study of the phospholipids in the tear film, their nature and fate. The use of liquid chromatography mass spectrometry determined that the concentration of phospholipids in the tear film was less than previously thought. The analysis of the tear film phospholipids continued with thin layer chromatography. This showed the presence of diacylglycerides (DAGs) in the tear film at relatively high concentrations. The activity of an enzyme, phospholipase C, was found in the tear film. It was hypothesised that the low concentration of phospholipids and high concentrations of DAG in the tear film was due to the action of this enzyme. The second area of study was the synthesis of phospholipids analogous materials for use in ocular and dermal applications for use in ocular and dermal applications.For ocular applications the synthesis involved the use of the monomer N,N-dimethyl-N-(2-acryloylethyl)-N-(3-sulfopropyl) ammonium betaine (SPDA) in combination with 2-hdyroxyethyl methacrylate (HEMA). Charge-balanced membranes were also synthesised using potentially anionic monomers in conjunction with cationic monomers in stoichiometrically equivalent ratios also with HEMA as a commoner. Membranes of SPDA copolymers and charge-balanced copolymers proved to have some properties suitable for ocular applications. The dermal materials consisted of one family of partially hydrated hydrogels synthesised from SPDA in combination with ionic monomers: sodium 2-(acrylamido)-2-methyl propane sulfonate and acrylic acid-bis(3-sulfopropyl)-ester, potassium salt. A second family of partially hydrated hydrogels was synthesised from N-vinyl pyrrolidone in combination with the same ionic monomers. Both of the partially hydrated hydrogels synthesised proved to have some properties suitable for use as adhesives for the skin.

Hydrogels for dermal applications

This thesis is concerned with the development of hydrogels that adhere to skin and can be used for topical or trans dermal release of active compounds for therapeutic or cosmetic use. The suitability of a range of monomers and initiator systems for the production of skin adhesive hydro gels by photopolymerisation was explored and an approximate order of monomer reactivity in aqueous solution was determined. Most notably, the increased reactivity of N-vinyl pyrrolidone within an aqueous system, as compared to its low rate of polymerisation in organic solvents, was observed. The efficacy of a series of photoinitiator systems for the preparation of sheet hydrogels was investigated. Supplementary redox and thermal initiators were also examined. The most successful initiator system was found to be Irgacure 184, which is commonly used in commercial moving web production systems that employ photopolymerisation. The influence of ionic and non-ionic monomers, crosslinking systems, water and glycerol on the adhesive and dynamic mechanical behaviour of partially hydrated hydrogel systems was examined. The aim was to manipulate hydrogel behaviour to modify topical and transdermal delivery capability and investigated the possibility of using monomer combinations that would influence the release characteristics of gels by modifying their hydrophobic and ionic nature. The copolymerisation of neutral monomers (N-vinyl pyrrolidone, N,N-dimethyl acrylamide and N-acryloyl morpholine) with ionic monomers (2-acrylamido-2-methylpropane sulphonic acid; sodium salt, and the potassium salt of 3-sulphopropyl acrylate) formed the basis of the study. Release from fully and partially hydrated hydrogels was studied, using model compounds and a non-steroidal anti-inflammatory drug, Ibuprofen. Release followed a common 3-stage kinetic profile that includes an initial burst phase, a secondary phase of approximate first order release and a final stage of infinitesimally slow release such that the compound is effectively retained within the hydrogel. Use of partition coefficients, the pKa of the active and a knowledge of charge-based and polar interactions of polymer and drug were complementary in interpreting experimental results. In summary, drug ionisation, hydrogel composition and external release medium characteristics interact to influence release behaviour. The information generated provides the basis for the optimal design of hydrogels for specific dermal release applications and some understanding of the limitations of these systems for controlled release applications.

Solid-phase combinatorial chemistry and scintillation proximity assays

The Scintillation Proximity Assay (SPA) is a method that is frequently used to detect and quantify the strength of intermolecular interactions between a biological receptor and ligand molecule in aqueous media. This thesis describes the synthesis of scintillant-tagged-compounds for application in a novel cell-based SPA. A series of 4-functianlised-2,5-diphenyloxazole molecules were synthesised. These 4-functionalised-2,5-diphenyloxazoles were evaluated by Sense Proteomic Ltd. Accordingly, the molecules were evaluated for the ability to scintillate in the presence of ionising radiation. In addition, the molecules were incorporated into liposomal preparations which were subsequently evaluated for the ability to scintillate in the presence of ionising radiation. The optimal liposomal preparation was introduced into the membrane of HeLa cells that were used successfully in a cell-based SPA to detect and quantify the uptake of [14C]methionine. This thesis also describes the synthesis and subsequent polymerisation of novel poly(oxyethylene glycol)-based monomers to form a series of new polymer supports. These Poly(oxyethylene glycol)-polymer (POP) supports were evaluated for the ability to swell and mass-uptake in a variety of solvents, demonstrating that POP-supports exhibit enhanced solvent compatibilities over several commercial resins. The utility of POP-supports in solid-phase synthesis was also demonstrated successfully. The incorporation of (4’-vinyl)-4-benzyl-2,5-diphenyloxazole in varying mole percentage into the monomer composition resulted in the production of chemically functionalised scintillant-containing poly(oxyethylene glycol) polymer (POP-Sc) supports. These materials are compatible with both aqueous and organic solvents and scintillate efficiently in the presence of ionising radiation. The utility of POP-Sc supports in solid-phase synthesis and subsequent in-situ SPA to detect and quantify, in real-time, the kinetic progress of a solid-phase reaction was exemplified successfully.In addition, POP-Sc supports were used successfully both in solid-phase combinatorial synthesis of a peptide nucleic acid (PNA)-library and subsequent screening of this library for the ability to hybridise with DNA, which was labelled with a suitable radio-isotape. This data was used to identify the dependence of the number and position of complimentary codon pairs upon the extent of hybridisation. Finally, a further SPA was used to demonstrate the excellent compatibility of POP-Sc supports for use in the detection and quantification of enzyme assays conducted within the matrix of the POP-Sc support.

Surface coating deterioration studies

This project is concerned with the deterioration of surface coatings as a result of weathering and exposure to a pollutant gas (in this case nitric oxide). Poly(vinyl chloride) (PVC) plastisol surface coatings have been exposed to natural and artificial weathering and a comparison of the effects of these two types of weathering has been made by use of various analytical techniques. These techniques have each been assessed as to their value in providing information regarding changes taking place in the coatings during ageing, and include, goniophotometry, micro-penetrometry, surface energy measurements, weight loss measurements, thermal analysis and scanning electron microscopy. The results of each of these studies have then been combined to show the changes undergone by PVC plastisol surface coatings during ageing and to show the effects which additives to the coatings have on their behaviour and in particular the effects of plasticiser, pigment and uv and thermal stabilisers. Finally a preliminary study of the interaction between five commercial polymers and nitric oxide has been carried out, the polymers being polypropylene, cellulose acetate butyrate, polystyrene, polyethylene terephthalate and polycarbonate. Each of the samples was examined using infra-red spectroscopy in the transmission mode.

Azidoprofen as a soft anti-flammatory agent for the topical treatment of Psoriasis

Azidoprofen {2-(4-azidophenyl)propionic acid; AZP}, an azido-substituted arylalkanoic acid, was investigated as a model soft drug candidate for a potential topical non-steroidal anti-inflammatory agent (NSAIA). Reversed-phase high performance liquid chromatography (HPLC) methods were developed for the assay of AZP, a series of ester analogues and their· degradation products. 1H-NMR spectroscopy was also employed as an analytical method in selected cases. Reduction of the azido-group to the corresponding amine has been proposed as a potential detoxification mechanism for compounds bearing this substituent. An in vitro assay to measure the susceptibility of azides towards reduction was developed using dithiothreitol as a model reducing agent. The rate of reduction of AZP was found to be base-dependent, hence supporting the postulated mechanism of thiol-mediated reduction via nucleophilic attack by the thiolate anion. Prodrugs may enhance topical bioavailability through the manipulation of physico-chemical properties of the parent drug. A series of ester derivatives of AZP were investigated for their susceptibility to chemical and enzymatic hydrolysis, which regenerates the parent acid. Use of alcoholic cosolvents with differing alkyl functions to that of the ester resulted in transesterification reactions, which were found to be enzyme-mediated. The skin penetration of AZP was assessed using an in vitro hairless mouse skin model, and silastic membrane in some cases. The rate of permeation of AZP was found to be a similar magnitude to that of the well established NSAIA ibuprofen. Penetration rates were dependent on the vehicle pH and drug concentration when solutions were employed. In contrast, flux was independent of pH when suspension formulations were used. Pretreatment of the skin with various enhancer regimes, including oleic acid and azone in propylene glycol, promoted the penetration of AZP. An intense IR absorption due to the azide group serves as a highly diagnostic marker, enabling azido compounds to be detected in the outer layers of the· stratum corneum following their application to skin, using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). This novel application enabled a non-invasive examination of the percutaneous penetration enhancement of a model azido compound in vivo in man, in the presence of the enhancer oleic acid.

Lipophilic antifolates as potential antipsoriatic agents

The lipophilic dihydrofolate reductase (DHFR) inhibitor m-azidopyrimethamine (MZP) was investigated for suitability for development as a topical antipsoriatic agent. The clinical features and treatments for psoriasis were reviewed. High performance liquid chromatography (HPLC) was employed as the main analytical method, with UV spectroscopy being used in some cases. Reduction of the azido-group was proposed as a potential detoxification mechanism for MZP. The rates of reduction of a series of substituted phenyl azide compounds by dithiothreitol were investigated and found to depend on the substitution pattern of the aryl azide molecular, with electron deficient azides exhibiting faster rates of reduction in the system studied. The rates of reduction of MZP and analogous compounds were also studied using this model. The skin penetration of MZP was assessed using an in vitro hairless mouse skin model. The rate of permeation (flux) of MZP across hairless mouse skin was found to be dependent on the quantity of propylene glycol used as cosolvent in the vehicle and the pH. The use of a pretreatment regime of oleic acid in propylene glycol was shown to greatly increase the penetration of MZP through the hairless mouse skin as compared to application without pretreatment, or pretreatment with other penetration enhancers. The metabolism of MZP was studied in in vitro models comprising skin homogenates, SV-K14 human keratinocyte cell cultures and skin commensal bacterial cultures. No conversion of MZP to the corresponding amine was detected in any of the models. The growth inhibitory properties of MZP were investigated in an in vitro SV-K14 human keratinocyte cell culture model and compared with those of other DHFR inhibitors. [14C]-pyrimethamine was shown to be taken up by the SV-K14 keratinocytes.

Polymer modified cement: hydration microstructure and diffusion properties

Investigations concentrated on the styrene butadiene rubber (SBR) latex and formulations included standard carboxylated and special carboxylated latexes. The aqueous component, containing the stabilisers and antifoaming agent but not the polymer solids, was also used. For comparison, limited investigations were carried out using other polymer types e.g. acrylic, ethylene-vinyl acetate (EVA), and redispersible powders rather than emulsions. The major findings were: 1) All latex systems investigated acted as retarders for cement hydration. The extent of retardation depends on the type of polymer. The mechanism for cement hydration may be changed, and excessive retardation influences properties. 2) Polymer modified cements exhibited either similar or coarser pore structures compared with unmodified cements. Results suggest that polymer mainly exists in a mixture of cement hydrates and polymer phase. Very little evidence was found for the formation of a distinct polymer film phase. 3) During the first few days of curing the polymer solids are removed from the pore solution and concentrations of OH-, Na+ and K+ are reduced. These observations are probably a result of polymer-cement surface interactions since there was no evidence of any chemical reactions or degradation of the polymer. 4) Improved diffusional resistance of modified cements depends on the ability to achieve adequate workability at low w/c ratio, rather than modification of matrix structure.

Properties and performance of polymer modified cements and mortars

Polymer modified cements and mortars have become popular for use as patch repair materials. General evidence suggests that these materials offer considerable improvements compared to traditional mortars although the mechanisms for this are not fully understood. This work elucidates the factors which govern some properties and performance of different polymer systems. In view of the wide range of commercial systems available, investigations concentrated on the use of three of the most commonly available groups of polymers. These were: (1) Styrene Butadiene Rubber (SBR), (2) Acrylics and, (3) Ethylene Vinyl Acetates (EVA). The later two were in the form of both emulsions and redispersible powders. Experiments concentrated on: (1) Rheological behaviour of polymer modified cement pastes; (2) Workability of polymer modified mortars; (3) Influence of curing conditions on the pore size distribution and diffusion of chloride ions; (4) Bond strength of polymer modified cement and mortar patches; and (5) Microscopic examination and semi-quantitative analyses of the bulk and interfacial microstructures. The following main conclusions were reached: (1) The addition of polymer emulsions have a considerable influence on the workability of fresh cement pastes, the extent of this depending on the type of system used. (2) The rheological parameters of fresh polymer modified mortars can be established using a two-point workability test which may be used when comparing the properties of different systems at constant workability. (3) Curing conditions affect the properties of polymer modified systems and a wet/dry curing regime was essential for good adhesion of these materials to mortar substrates. (4) In contrast, the wet/dry curing regime resulted in a curing affected zone at the surface of patch materials. This can result in a much coarser pore structure and enhanced diffusion of e.g. chloride ions. (5) The microstructure of polymer modified systems was very different compared with the unmodified cement/mortar and varied depending on curing conditions.

Stabilisation of polypropylene using polymer-bound antioxidants

variety of hindered phenol and hindered piperidine antioxidants containing vinyl or vinylidine functional groups have been synthesised and some of these were successfully bound to Polypropylene backbone during processing operations in presence of a radical generator. Up to 20% concentrates were prepared using this technique. Commercially acceptable concentrates (MASTERBATCHES) can only be prepared with antioxidants that are only weakly chain breaking such as hindered piperidines. One of the antioxidants, AATP was found to polymrise as well as bind to Polypropylene. Bound antioxidants were found to be resistant to such channels of physical loss as solvent extraction. Temperature and concentration of the additive and radical generator were found to be important parameters in the preparation of the concentrates. The stabilising efficiences of the diluted bound antioxidants alone, and in combination (synergistic) with other antioxidants have been evaluated. Results of both thermal and photo-oxidative stabilities of the bound samples in Polypropylene show that the restriction on free mobility of the bound antioxidants in the polymer has virtually no effect on its stabilising efficiency. Bound AATP was found to generate nitroxyl radicals during the course of its stabilising activities, and in combination with a small amount of Irganox 1076, it was shown to be highly synergistic thermally. A mechanism of catalytic phenol regeneration by the resultant piperidine hydroxylamine was proposed. Although the mechanical properties of the masterbatches were affected by the transformation, this was not found to be carried over to the diluted samples. This work has shown that bound concentrates can be effectively prepared in saturated polymers for subsequent dilution to normal concentrates used in commercial stabilisation.