The polymer medical device market is expanding rapidly, particular in the use of catheters and the manufacturer faces various challenges associated with the production of these devices including: resin selection, melt processing, addition ofradiopaque agents and surface finish. . Thermoplastic polyurethanes (TPUs) are widely used in the medical device industry due to their biocompatibility and unique mechanical performance, which can be attributed to the multi-block structure comprising of hard and soft segments. This programme of work begins by selecting a wide range of commercially available TPUs and the results showed marked differences in processability, mechanical an~ thermal performance due to differences in molecular weight, ratio of hard and soft segments and phase mixing. This thesis also offers an investigation into the effect of radiopaque fillers on the processability and performance of TPUs and PVCs and highlights the difficulty associated with this. The results showed addition of radiopaque fillers decreased the viscosity of these resins due to hydrophobic degradation, wall slip and thermal history effects. Nylon has been traditionally used in catheter balloon manufacture due to the excellent tensile performance, however, these polyamides lack the flexibility required for medical device applications. In this work nylon/Pebax blends were formulated and the results showed that addition of Pebax resulted in better processability and improved mechanical performance. In the latter stage of this work a range of tubes were extruded from various resins and blends used throughout the program of work and the mechanical performance including torqueability, manoeuvrability, flexibility and trackability of each tube were modelled. The results enabled the recommendations of the' materials for various medical device applications ranging from coronary angioplasty to urinary drainage. The final section ofthe thesis investigated the effect of Atmospheric Pressure Glow Discharge (APGD) treatment on the contact angle and surface functionality of the TPUs. The results showed that optimum discharge parameters are required for improved surface performance and these are related to creation ofoxygen functional groups on the surface.

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A novel thennoset polyurethane (TPU), two amorphous thennoplastics, poly(methyl methacrylate) (PMMA), high impact poly(styrene) (HIPS) and a semi-crystalline polymer, medium density poly(ethylene) (MDPE) were used in the fonnation of a range of polymer/carbon nanotube composites for commercial and development research purposes.· Composites of thennoplastic polymers were produced by melt-processing due to the immediate compliance of the melt-mixing process with current industrial practices and the scarcity of literature pertaining to the subject. In the case ofTPU, multi-walled carbon nanotubes (MWCNTs) were used as the filler material and composites were produced using a specifically designed solution based process. A true nanoscale reinforcing effect was obtained, in that stiffness, strength and relative toughness of the TPU matrix was greatly enhanced on addition of as little as 0.1wt.% MWCNTs. A range of arc discharge MWCNTs, CVD-MWCNTs and f-MWCNTs were used in the fabrication of a range of PMMAlMWCNT composites in order to determine the effect of aspect ratio, physical structure and functionalisation of MWCNTs on the morphology and end 1roperties of the composite material. Carboxyl functionalisation of the MWCNT end-caps promoted homogeneous and anisotropic dispersion and distribution off-MWCNTs within the PMMA matrix, evident from morphological analysis although reducing the intrinsic conductivity of the nanotubes. A lower screw speed than is norinally practised during the twin screw extrusion of nanocomposites achieved the highest degree of nanotube dispersion and distribution in the HIPS studied. Single screw extrusion successfully produced homogeneous MDPE/CVD-MWCNT composites with an electrical percolation threshold of 2wt.% MWCNTs. For all systems, the extent of interfacial interactions between the polymer and MWCNTs was studied using Raman spectroscopy. The formation of a percolated network of MWCNTs in the polymer matrices was also examined in tenns ofrheological and electrical behaviour.

The injection stretch blow moulding (ISBM) process is the main method used to mass-produce polyethylene terephthalate (PET) bottles for the carbonated soft drink and the mineral water industries. At present researchers have been developing numerical simulations of the process with the aim of gaining a better understanding of the process and ultimately optimize the design of new containers more efficiently. However, one of the drawbacks for these simulations is the lack of knowledge on the biaxial stress-strain behaviour of PET at processing conditions relevant to ISBM and the effect of biaxial stretching on the final properties. The main objective ofthis study was to understand and.characterize the biaxial behaviour of PET and quantify how biaxial stretching under different processing conditions influenced the final mechanical and structural properties. The results were. implemented within a numerical simulation with the aim of accurately predicting both the thickness and modulus distribution in the final container. Initially, a comprehensive stu:dy of the biaxial defonnation behaviour of three different grades of PET was carried out at conditions applicable to the ISBM process using the Queen's Biaxial Stretcher. The stress strain data generated from these biaxial tests was used to generate material constants for the Buckley-Jones-Adams material model. A new procedure for generating the constants was developed using the mathematical package MATLAB®. This semi-automated procedure was found more efficient than the original manual procedure by Buckley et al. Post-stretching analysis was carried out by using tensile and DSC methods. The modulus development of PET was found to be strongly dependent on the stretching temperature, strain rate, stretch ratio, and defonnation mode. Among those parameters, stretch ratio and defonnation mode play the most important role. A two dimensional ISBM simulation of a 1 litre PET bottle was developed for the prediction of thickness and modulus distributions. The comparison of the numerical results with the experimental measurement showed a satisfactory prediction ofbottle thickness and modulus in the side wall region.

Synthetic bioabsorbable polymers, first developed in the 1960s, are being increasingly used in the manufacture of medical devices. Some of the uses these devices have been applied to are suturing, orthopaedic fixation and drug-delivery. The principal advantage offered by bioabsorbable polymers is that a device made from these materials does not require removal. However, the degradation characteristics of these materials occasionally cause problems in medical uses, such as negative tissue reactions and poor wound healing. Therefore, a need exists to develop techniques which can modify the degradation characteristics ofbioabsorbable polymers. This study investigated the application of high-energy radiation, in the form of gamma and electron-beam (e-beam) radiation, to the bioabsorbable polymers polylactide (PLA) and polylactide-co-glycolide (pLGA). It was fqund that both forms of radiation reduce the molecular weight of these polymers in proportion to the delivered dose. Additionally, it was found that the effect of e-beam radiation is depth-dependent, with the surface material being more significantly affected than the core material. The reduction in the molecular weight ofthe bioabsorbable materials was found to have led to reduced mechanical strength and absorption time. After e-beam irradiation these characteristics were also found to be depth-dependent The work presented in this thesis suggests that e-beam radiation can be used to tailor the characteristics ofa bioabsorbable polymer that are crucial to its use in medical devices. This tailoring can be location specific, which it is believed will lead to the production ofbioabsorbable medical devices with improved efficacy and less negative response.

The thermoforming process is only one of the many manufacturing methods that converts plastic material into numerous products. As with many thermoforming companies the process is not used to its full potential. The major goal of this work was to develop a non-isothermal simulation of the plug assisted thermoforming process which has direct practical relevance with the thermoforming industry. The simulation would be used as design tool to help improve the process, enabling investigations into changes in product/tooling design and processing conditions, prior to production. Development of robust and accurate simulations has been difficult due to the interactions that exist between the key process elements, such as; highly non-linear material response, heat transfer and friction. For these reasons a fUlly coupled non-isothermal simulation of the plug assisted thermoforming process does not exist. To reduce the scope of this work a previously developed constitutive material model capable of predicting the deformation response of the material used in this work was adopted therefore allowing for more simulation and experimental investigations. . . A detailed experimental programme was carried out on a single shot experimental thermoforming machine to investigate the effect of the major thermoforming processing variables such as plug speed, sheet temperature etc. In addition a series of friction and heat transfer experiments were conducted using specially adapted test equipment in an attempt to isolate and understand each contact phenomena better. The information obtained from the experiments was used in the construction and validation of the developed simulation which was carried out using the finite element package ABAQUS®/Standard. Initial isothermal simulations were used to investigate the effects of friction only on the simulated output. The simulation capabilities were improved by incorporating heat transfer effects and a fully coupled temperature dependant simulation is presented. The peiiormance of the simulation was validated against the experimental data obtained from the thermoforming trials and the trends observed were highly comparable. The overall project goals were met with the development of a working nonisothermal simulation incorporating all aspects of the plug assisted thermoforming process such as heat transfer and friction, however further work is required on the constitutive material modelling aspects of the work to improve the simulation capabilities.

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