John H. Daly

Permeability of N2, Ar, He, O2 and CO2 through biaxially oriented polyester films : dependence on free volume

Permeability, diffusion and solubility coefficients are reported for biaxially orientated polyester films based on poly(ethylene terephthalate) [PET], poly(ethylene naphthalate) [PEN] and copolymers containing PET and PEN moieties. Data for cast amorphous sheets and materials produced with different biaxial draw ratios are compared. The crystallinity of the samples was assessed using differential scanning calorimetry and density measurements. The changes in the void structure at a molecular level were investigated using positron annihilation lifetime spectroscopy (PALS). The variation of the gas diffusion behaviour with the gas used (carbon dioxide, nitrogen, argon, helium and oxygen) reflects the effects of change in morphology on the solubility and diffusivity components of the permeability. The diffusivity of the gas is influenced not only by both the changes in the void size and content at a molecular level, but also by the effects of crystallinity on the percolation behaviour of the gas through the matrix. Changes in the extent of chain alignment also have a profound affect on the solubility of the gas in the matrix. The observed behaviour for the gas permeation can be interpreted as being the result of the complex interplay of changes in the crystalline content, the polymer chain alignment and the void structure of the amorphous phase.

In vitro and in vivo response to nanotopographically-modified surfaces of poly (3 -hydroxybutyrate -co -3 -hydroxyvalerate) and polycaprolactone

Colloidal lithography and embossing master are new techniques of producing nanotopography, which have been recently applied to improve tissue response to biomaterials by modifying the surface topography on a nano-scale dimension. A natural polyester (Biopol™), 8% 3-hydroxyvalerate-component (D400G) and a conventional biodegradable polycaprolactone (PCL) were studied, both nanostructured and native forms, in vitro and in vivo. Nanopits (100-nm deep, 120-nm diameter) on the D400G surface were produced by the embossing master technique (Nano-D400G), while nanocylinders (160-nm height, 100-nm diameter) on the PCL surface were made by the colloidal lithography technique (Nano-PCL). L929 fibroblasts were seeded on polyesters, and cell proliferation, cytotoxic effect, synthetic and cytokine production were assessed after 72 h and 7 days. Then, under general anesthesia, 3 Sprague–Dawley rats received dorsal subcutaneous implants of nanostructured and native polyesters. At 1, 4 and 12 weeks the animals were pharmacologically euthanized and implants with surrounding tissue studied histologically and histomorphometrically. In vitro results showed significant differences between D400G and PCL in Interleukin-6 production at 72 h. At 7 days, significant (P < 0.05) differences were found in Interleukin-1 β and tumor necrosis factor-α release for Nano-PCL when compared to Nano-D400G, and for PCL in comparison with D400G. In vivo results indicated that Nano-D400G implants produced a greater extent of inflammatory tissue than Nano-PCL at 4 weeks. The highest vascular densities were observed for Nano-PCL at 4 and 12 weeks. Chemical and topographical factors seem to be responsible for the different behaviour, and from the obtained results a prevalence of chemistry on in vitro data and nanotopography on soft tissue response in vivo are hypothesized, although more detailed investigations are necessary in this field.

Physical ageing in poly(ethylene terephthalate) : Its influence on cold crystallisation

The relationship between the crystallisation behaviour and physical ageing history of poly(ethylene terephthalate) is reported. It is shown that short term ageing can enhance the subsequent rate of crystallisation, but longer ageing times can retard the crystallisation process. It is suggested that physical ageing favours the formation of ordered domains.

Rubber-modified epoxy resins: 1. Equilibrium physical properties

Abstract Dilatometry, thermal mechanical analysis, differential scanning calorimetry, electron microscopy and mechanical measurements are reported on a series of rubber-modified epoxy resins. The data obtained is discussed in terms of the interplay of molecular mobility, network and domain structure of the resins. Three transitions were identified; two being assigned respectively to the onset of motion of the acrylonitrile/butadiene units and epoxy units. The higher temperature transition is more difficult to assign, but appears to be associated with specific interactions involving the interface between the epoxy and acrylonitrile/butadiene interfaces or the crosslinked regions of the epoxy resin.

Rubber-modified epoxy resins: 2. Dielectric and ultrasonic relaxation studies

Abstract Dielectric and ultrasonic relaxation measurements are reported on a series of rubber-modified epoxy resins over a temperature range from 190 to 423K. The resins were prepared by the reaction of the diglycidyl ether of bisphenol A with a chain-extender formed from carboxyl-terminated acrylonitrile-butadiene copolymer and cured with either triethylene tetramine or diethylene glycol bis-propyl amine. At low temperature, a dielectric relaxation was detected which was also active ultrasonically and can be associated with molecular motion of the acrylonitrile-butadiene chain extender. Changes in the concentration of the rubbery phase and type of curing agent used have marked effects on both the activation energy for molecular relaxation and in the breadth of the relaxation curves. Extensive studies of the dielectric properties at higher temperatures failed to reveal a resolvable relaxation associated with either of the higher temperature transitions observed using equilibrium measurements. Electrical conductivity, permittivity and acoustic velocity measurements performed as a function of temperature, however, indicate the existence of a change in slope at approximately the value of the glass transition of the epoxy phase. A further change in slope at approximately the temperature of the highest temperature transition was detected. The observed relaxation properties are compatible with the resins being phase-separated at low rubber concentrations and changing to a semicontinuous interpenetrating network structure at higher rubber concentrations.

The effect of cure on thermally stimulated discharge' measurements in epoxy resin coatings

Thermally stimulated discharge (TSD) measurements are reported as a function of the degree of cure for a series of dicyandiamide cured powder epoxy resins. Data are presented on the effects of ageing and water uptake on the TSD traces. The observed effects are interpretated in terms of the generation of traps, creation of voids and changes in the bulk conductivity on the charge mobility. The TSD data are compared with dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) observations on these systems. These studies indicate that the method is very sensitive to changes in the state of the coating and has potential as a test method for the characterization of epoxy resin coatings.

The effect of processing on the properties of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymers

Semi-crystalline poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymers are biodegradable systems with potential as substrates for use in tissue regeneration. Previous studies have shown that severe embrittlement occurs on storage at room temperature restricting their application possibilities. Concepts such as secondary, advancing crystallisation causing changes in the amorphous/crystalline ratio have been mooted as the cause of the embrittlement. Using films prepared by extrusion and compression moulding procedures we have attempted to probe not only the pure amorphous and crystalline phases but also the interfacial region. Interpretation of dynamic mechanical and dielectric data highlights the changes in the nature of the interfacial region on processing. Moreover, the use of the Thermally Stimulated Discharge technique is a powerful probe for highlighting the morphological changes induced in multiphase systems by the processing step.

Studies of dielectric constant measurements and tribo-electric charging of pigmented polymer systems

Data are presented on the variation of the dielectric constant and the tribo-electric coefficient for a series of pigmented, melt mixed, polymer materials. Dielectric spectra are also presented for other pigmented polymeric materials. On the basis of these data it is shown that in certain situations a correlation may exist between the dielectric constant and the observed tribo-electric coefficient. A comparison of the values obtained with those predicted on the basis of the simple Wagner Model indicates that the dielectric increment can only be explained if a significant proportion of the material is present in a chained form. Model studies of carbon fibres are presented and a model for the prediction of the dielectric properties of chained carbon particles is also described.

Rubber-modified epoxy resins: 3. Influence of filler on the dielectric relaxation properties

Abstract Dielectric measurements are reported on the effect of addition of glass (wool and beads) to a rubber modified epoxy resin. A parallel study of the influence of the addition of polydimethylsiloxane is also reported. It is found that there are significant shifts in the position of the dielectric relaxation associated with the acrylonitrile/butadiene phase. Analysis of the data indicates that the activation energy of the relaxation is little affected by the additives and the observed effects can be explained by the change in internal field brought about by the addition of filler through its effect on the permittivity of the material.

Natural and synthetic polyesters for musculoskeletal tissue repair: experimental in vitro and in vivo evaluations.

Two natural Biopol™ polyesters, containing 8% (D400G) and 12% (D600G) of hydroxyvalerate component, and a synthetic polyester based on 1,4 cyclohexanediol [Poly(cyclohexyl-sebacate) - PCS] were studied to investigate their in vitro and in vivo behavior for application in musculoskeletal tissue repair. The polyesters were placed in direct contact with L929 fibroblasts and cell proliferation (WST-1), cytotoxic effect (LDH), synthetic activity (total proteins) and cytokine production (IL-1β, IL-6, TNFα) were assessed after an incubation period of 72 hours and 7 days. Then, 12 Sprague-Dawley rats underwent dorsal subcutaneous implants of tested polyesters under general anesthesia. After 1 and 4 weeks from surgery, the animals were pharmacologically euthanized and the implants retrieved with surrounding tissue for histologic and histomorphometric investigations. In vitro results showed that D600G behaved a little worse in comparison to other tested polyesters in terms of cell proliferation and TNFα at 7 days. PCS presented the lowest total protein value at 7 days. In vivo results indicated that PCS implants produced a higher (p < 0.01) extent of inflammatory tissue in comparison to D600G at 1 week and to D400G at 4 weeks, and the lowest vascular densities at both experimental times. D400G seems to be the most suitable material for biomedical application when tested in fibroblast cultures and in the subcutaneous tissue of rats.