A rheology study of high-energy radiolysis of a semicrystalline ethylene-propylene copolymer containing DOP mobilizer

The ractiolysis of a poly(ethylene-co-propylene), Elpro grade P 750 J, marketed by Thai Polypropylene Co. Ltd. for the manufacture of medical goods, was investigated at ambient temperature and melt rheology measured. The roles of calcium stearate, blended with the Elpro as a processing aid, and dioctyl phthalate (DOP), added in various amounts as a radical scavenger, were assessed. Following radiolysis, G' and the viscosity of the polymer melts at 453 K both decreased with increasing radiation dose, even when the mobilizer was present. The results indicated that although the DOP did scavenge radicals, it did not protect the polymer from net chain scission in a low-dose regimen. The value of (G(S) - 4G(X)) was approximately 0.6-0.7. (c) 2006 Wiley Periodicals, Inc.

Zwitterionic and charge-balanced polyampholyte copolymer hydrogels

Zwitterionic copolymers were synthesised from N,N-dimethyl-N-(2- acryloylethyl)-N-(3-sulfopropyl) ammonium betaine (SPDA) and 2-hydroxyethyl methacrylate (HEMA) produce a series of polyzwitterion hydrogels. For the synthesis of the charge-balanced copolymer hydrogels, two cationic monomers were selected: 2-(diethylamino) ethyl methacrylate (DMAEMA) and 3-(dimethylamino) propyl methacrylamide (DMAPMA), and an anionic monomer; 2-acrylamido-2- methylpropane sulphonic acid (AMPS). Two series of charge-balanced copolymers were synthesized from stoichiometrically equivalent ratios of DMAEMA or DMAPMA and AMPS with HEMA as a termonomer. All synthesized copolymers produced clear and cohesive hydrogels. The zwitterionic and charge-balanced copolymers displayed similar equilibrium water contents together with similar mechanical and surface energy properties. The swelling of the zwitterionic and the charge-balanced copolymers shows some features of antipolyelectrolyte behavior.

Synthesis and evaluation of novel polymer modifiers for biodegradable polymers

The effects of ester plasticizers and copolymers on the mechanical properties of the natural biodegradable polymers, poly(3-hydroxybutyrate) [PHB] and poly(lactic acid) [PLA] have been studied after subjecting to melt processing conditions. Ester plasticizers were synthesized from citric, tartaric and maleic acids using various alcohols. A variety of PLA copolymers have also been prepared from poly(ethylene glycol) derivatives using stannous octanoate catalysed ring opening polymerisations of DL-lactide. A novel PLA star copolymer was also prepared from an ethoxylated pentaerythritol. The structures of these copolymers were determined by NMR spectroscopy. The plasticizing effect of the synthesised additives at various concentrations was determined. While certain additives were capable of improving the mechanical properties of PLA, none were effective in PHB. Moreover, it was found that certain combinations of additives exhibited synergistic effects. Possible mechanisms are discussed. Biotic and abiotic degradation studies showed that the plasticizers (esters and copolymers) did not inhibit the biodegradability of PHB or PLA in compost at 60°C. Simple toxicity tests carried out on compost extract and its ability to support the growth of cress seeds was established. PLA was found to be susceptible to limited thermal degradation under melt processing conditions. Conventional phenolic antioxidants showed no significant effect on this process, suggesting that degradation was not predominantly a free radical process. PLA also underwent photo-oxidative degradation with UV light and the process could be accelerated in the presence of a photoactivator such as iron (III) diisononyl dithiocarbamate. The mechanisms for the above processes are discussed. Finally, selected compounds were prepared on a pilot plant scale. Extruded and blown films were prepared containing these additives with conventional polymer processing equipment. The mechanical properties were similar to those obtained with laboratory produced compression moulded films.

Novel hydrogel copolymers and semi-interpenetraing polymer networks

Interpenetrating polymer networks (lPN's), have been defined as a combination of two polymers each in network form, at least one of which has been synthesised and / or crosslinked in the presence of the other. A semi-lPN, is formed when only one of the polymers in the system is crosslinked, the other being linear. lPN's have potential advantages over homogeneous materials presently used in biomedical applications, in that their composite nature gives them a useful combination of properties. Such materials have potential uses in the biomedical field, specifically for use in hard tissue replacements, rigid gas permeable contact lenses and dental materials. Work on simply two or three component systems in both low water containing lPN's supplemented by the study of hydrogels (water swollen hydrophilic polymers) can provide information useful in the future development of more complex systems. A range of copolymers have been synthesised using a variety of methacrylates and acrylates. Hydrogels were obtained by the addition of N-vinyl pyrrolidone to these copolymers. A selection of interpenetrants were incorporated into the samples and their effect on the copolymer properties was investigated. By studying glass transition temperatures, mechanical, surface, water binding and oxygen permeability properties samples were assessed for their suitability for use as biomaterials. In addition copolymers containing tris-(trimethylsiloxy)-y-methacryloxypropyl silane, commonly abbreviated to 'TRlS', have been investigated. This material has been shown to enhance oxygen permeability, a desirable property when considering the design of contact lenses. However, 'TRIS' has a low polar component of surface free energy and hence low wettability. Copolymerisation with a range of methacrylates has shown that significant increases in surface wettability can be obtained without a detrimental effect on oxygen permeability. To further enhance to surface wettability 4-methacryloxyethyl trimellitic anhydride was incorporated into a range of promising samples. This study has shown that by careful choice of monomers it is possible to synthesise polymers that possess a range of properties desirable in biomedical applications.

Optical fibre Bragg grating recorded in TOPAS cyclic olefin copolymer

A report is presented on the inscription of a fibre Bragg grating into a microstructured polymer optical fibre fabricated from TOPAS cyclic olefin copolymer. This material offers two important advantages over poly (methyl methacrylate), which up to now has formed the basis for polymer fibre Bragg gratings: TOPAS has a much lower water affinity and has useful properties for biosensing. The grating had a Bragg wavelength of 1569 nm and a temperature sensitivity of -36.5±0.3 pm/°C.

Polymer PCF Bragg grating sensors based on poly(methyl methacrylate) and TOPAS cyclic olefin copolymer

Fibre Bragg grating (FBG) sensors have been fabricated in polymer photonic crystal fibre (PCF). Results are presented using two different types of polymer optical fibre (POF); first multimode PCF with a core diameter of 50µm based on poly(methyl methacrylate) (PMMA) and second, endlessly single mode PCF with a core diameter of 6µm based on TOPAS cyclic olefin copolymer. Bragg grating inscription was achieved using a 30mW continuous wave 325nm helium cadmium laser. Both TOPAS and PMMA fibre have a large attenuation of around 1dB/cm in the 1550nm spectral region, limiting fibre lengths to no longer than 10cm. However, both have improved attenuation of under 10dB/m in the 800nm spectral region, thus allowing for fibre lengths to be much longer. The focus of current research is to utilise the increased fibre length, widening the range of sensor applications. The Bragg wavelength shift of a grating fabricated in PMMA fibre at 827nm has been monitored whilst the POF is thermally annealed at 80°C for 7 hours. The large length of POF enables real time monitoring of the grating, which demonstrates a permanent negative Bragg wavelength shift of 24nm during the 7 hours. This creates the possibility to manufacture multiplexed Bragg sensors in POF using a single phase mask in the UV inscription manufacturing. TOPAS holds certain advantages over PMMA including a much lower affinity for water, this should allow for the elimination of cross-sensitivity to humidity when monitoring temperature changes or axial strain, which is a significant concern when using PMMA fibre.

Block copolymer strategies for solar cell technology

A simple overview of the methods used and the expected benefits of block copolymers in organic photovoltaic devices is given in this review. The description of the photovoltaic process makes it clear how the detailed self-assembly properties of block copolymers can be exploited. Organic photovoltaic technology, an inexpensive, clean and renewable energy source, is an extremely promising option for replacing fossil fuels. It is expected to deliver printable devices processed on flexible substrates using high-volume techniques. Such devices, however, currently lack the long-term stability and efficiency to allow organic photovoltaics to surpass current technologies. Block copolymers are envisaged to help overcome these obstacles because of their long term structural stability and their solid-state morphology being of the appropriate dimensions to efficiently perform charge collection and transfer to electrodes.

Zwitterionic and charge-balanced (polyampholyte) copolymer hydrogels

Zwitterionic compounds, or zwitterions, are electrically neutral compounds having an equal number of formal unit charges of opposite sign. In common polyzwitterions the zwitterionic groups are usually located in pendent groups rather than the backbone of the macromolecule. Polyzwitterions contain both the anion and cation in the same monomeric unit, unlike polyampholytes which can contain the anion and cation in different monomeric units. The use of cationic and anionic monomers (or monomers capable of becoming charged) in stoichiometric equivalent proportions produces charge-balanced polyampholyte copolymers. Hydrogel materials produced from zwitterionic monomers have been proposed for use and are used in many biomaterial applications but synthetic charge-balanced polyampholyte are less common. Certain properties of hydrogels which are important for their successful use as biomaterials, these include the equilibrium water content, mechanical, surface energy, oxygen permeability, swelling and the coefficient of friction. The zwitterionic monomer N,N-dimethyl-N-(2-acryloylethyl)-N-(3-sulfopropyl) ammonium betaine (SPDA) was synthesized with 2-hydroxyethly acrylate (HEMA) as the comonomer to produce a series of polyzwitterion hydrogels. To produce charged-balanced copolymer hydrogels two “cationic” monomers were selected; 2-(diethylamino) ethyl methacrylate (DMAEMA) and 3-(dimethylamino) propyl methacrylamide (DMAPMA) and an anionic monomer; 2-acrylamido 2,2 methylpropane sulphonic acid (AMPS). Two series’ of charge-balanced copolymers were synthesized from stoichiometric equivalent ratios of DMAEMA or DMAPMA and AMPS with HEMA as a terpolymer. The zwitterionic copolymer and both charge-balanced copolymers produced clear, cohesive hydrogels. The zwitterionic and charge-balanced copolymers displayed similar EWC’s along with similar mechanical and surface energy properties. The swelling of the zwitterionic copolymer displayed antipolyelectrolyte behavior whereas the charge-balanced copolymers displayed behaviour somewhere between this and a typical polyelectrolyte. This work describes some aspects of the polymerisation and properties of SPDA copolymers and charge-balanced (polyampholyte) copolymers relevant to their potential as biomedical / bioresponsive materials. The biomimetic nature of SPDA together with its compatibility with other monomers makes it a useful and complimentary addition to the building blocks of biomaterials.

Charge-balanced copolymer hydrogels:potential as biomedical materials

Polyzwitterionic-containing hydrogel materials been proposed for use in biomaterial applications. Polyzwitterions contain anions and cations in the same monomeric unit, unlike polyampholytes which contain them in different monomeric units. The use of cationic and anionic monomers in stoichiometrically equivalent proportions produces charge-balanced polyampholytes (PA) copolymers. Membranes prepared using either betaine-containing (BT) polyzwitterionic copolymers or PA copolymers can share similar properties, but the range of EWCs offered by membranes incorporating BT and PA monomers is greater than that for conventional neutral hydrogels and methacrylic acid-based systems. Here we compare properties of BT-containing and PA-containing copolymer membranes, relevant to their potential as biomedical materials. Membranes of the copolymers were prepared as previously described. Surface energy was determined using a GBX Digidrop (GBX Scientific Instruments), with diidomethane and water as probes. The absorption of proteins was determined by soaking the membranes in 1mg/ml protein solutions for a predetermined time, and measuring UV absorption of the membranes at certain wavelengths. The BT and PA copolymer membranes displayed similar values for the polar components and dispersive components of total surface free energy. This was perhaps not surprising when the structures of the monomers were considered. The BT and PA copolymer membranes displayed differences in their protein absorption over time, with the PA demonstrating higher uptake of protein than the BT. In addition to the aforementioned greater EWC range, the use of BT and PA copolymer membranes also avoids some of the problems associated with net anionicity. Comparison of the BT copolymer with the “pseudo” zwitterionic PA copolymers shows that controlled molecular architecture is required to gain the benefits of balancing the charges present in the copolymers in a way that will make them beneficial to hydrogel design.

Human single-donor composite skin substitutes based on collagen and polycaprolactone copolymer

The development and characterization of an enhanced composite skin substitute based on collagen and poly(e-caprolactone) are reported. Considering the features of excellent biocompatibility, easy-manipulated property and exempt from cross-linking related toxicity observed in the 1:20 biocomposites, skin substitutes were developed by seeding human single-donor keratinocytes and fibroblasts alone on both sides of the 1:20 biocomposite to allow for separation of two cell types and preserving cell signals transmission via micro-pores with a porosity of 28.8 ± 16.1 µm. The bi-layered skin substitute exhibited both differentiated epidermis and fibrous dermis in vitro. Less Keratinocyte Growth Factor production was measured in the co-cultured skin model compared to fibroblast alone condition indicating a favorable microenvironment for epidermal homeostasis. Moreover, fast wound closure, epidermal differentiation, and abundant dermal collagen deposition were observed in composite skin in vivo. In summary, the beneficial characteristics of the new skin substitutes exploited the potential for pharmaceutical screening and clinical application.