An adaptable flexural test fixture for miniaturised polymer specimens

An adaptable flexural test fixture is proposed to characterise the mechanical properties of miniature beam specimens (≤10 mg) at ambient conditions or in the presence of fluids at elevated temperatures. The fixture is validated using representative amorphous and semi-crystalline polymers. The response of miniature specimens is compared against that of medium-sized specimens (≤1 g) on the same fixture and on conventional test equipment. Good agreement is found between the specimen sizes for all materials, but the comparisons highlight small differences attributed to factors such as specimen dimensional accuracy, crystallinity and span-to-thickness ratios. Flexural tests in water at 37 o C using both specimen sizes were performed to investigate the evolution of mechanical behaviour of hydrolytically degraded polylactides. Here, specimen size influences the diffusion timescale of acidic by-products which can reduce or enhance autocatalysis

Modeling non-linear rheology of PLLA: comparison of Giesekus and Rolie-Poly constitutive models

Rheological models for biobased plastics can assist in predicting optimum processing parameters in industrial forming processes for biobased plastics and their composites such as film blowing, or injection stretch-blow molding in the packaging industry. Mathematical descriptions of polymer behavior during these forming processes are challenging, as they involve highly nonlinear, time-, temperature-, and strain-dependent physical deformation processes in the material, and have not been sufficiently tested against experimental data in those regimes. Therefore, the predictive capability of two polymer models, a classical Giesekus and a physically-based Rolie-Poly, is compared here for extensional and shear rheology data obtained on a poly(L-lactide) (PLLA) across a wide range of strain rates of relevance to those forming processes. Generally, elongational and shear melt flow behavior of PLLA was predicted to a satisfactory degree by both models across a wide range of strain rates (for strain rates 0.05–10.0 s−1), within the strain window up to 1.0. Both models show a better predictive capability for smaller strain rates, and no significant differences between their predictions were found. Hence, as the Giesekus model generally needs a smaller number of parameters, this class of models is more attractive when considering their use in computationally demanding forming simulations of biobased thermoplastics

Rheological techniques for determining degradation of polylactic acid in bioresorbable medical polymer systems

© 2015 AIP Publishing LLC. A method developed in the 1980s for the conversion of linear rheological data to molar mab distribution is revisited in the context of degradable polymers. The method is first applied using linear rheology for a linear polystyrene, for which all conversion parameters are known. A proof of principle is then carried out on four polycarbonate grades. Finally, preliminary results are shown on degradable polylactides. The application of this method to degrading polymer systems, and to systems containing nanofillers, is also discubed. This work forms part of a wider study of bioresorbable nanocomposites using polylactides, novel hydroxyapatite nanoparticles and tailored dispersants for medical applications

Compounding and rheometry of PLA nanocomposites with coated and uncoated hydroxyapatite nanoplatelets

Polylactic acid and novel nanoplatelets of hydroxyapatite (HANP) were compounded in a laboratory scale twin-screw extruder and injection moulded to shape. The effect of HANP loading content, between 1 wt% and 10 wt%, and of HANP surface coating with tailored molecular dispersants, on the processability and rheological behaviour were investigated. Dispersion of HANP within the matrix system was determined qualitatively using transmission electron micrographs. Surface coating of HANP with dispersants was observed to change the state of HANP dispersion in the nanocomposites. This was also reflected in the changes of the nanocomposites’ rheological response with the moduli of coated HANP systems increasing at lower frequencies

Characterisation of tack for uni-directional prepreg tape employing a continuous application-and-peel test method

Employing a test method with coupled application and peel phases, tack was characterised for a UD prepreg tape. Different aspects of tack were explored by varying test parameters and material condition. In addition, different surface combinations were studied. In general, the test parameters, feed rate and temperature, affect the balance between cohesion within the resin and adhesion between resin and substrate. Exploring a range of parameters is required to understand the effect of viscoelastic resin properties on tack. The application pressure determines the true contact area between prepreg and substrate and hence affects tack. Changes in molecular mobility in the resin related to specimen conditioning, i.e. ageing or moisture uptake, result in maximum tack to occur at lower or higher feed rates, respectively. Differences in tack for different material combinations can be attributed to different molecular interactions at the contact interfaces and different resin distributions on the prepreg surfaces

Melt-processed PLA/HA platelet nanoparticle composites produced using tailored dispersants

Hydroxyapatite (HA) nanoparticles, similar to those seen in the structure of human bone, have been produced via hydrothermal synthesisand used to produce nanocomposite materials via melt blending with poly(lactic acid)(PLA). Both of these processes are scalable and commercially relevant. Tailored dispersants were developed and used to improve the dispersion of the HA. Modest improvements in flexural properties were observed (max increases 30% of dry modulus, 13% of wet strength). Rheometry is not suggestive of achieving percolation,so there is potential to improve mechanical properties further. It was established that very dry processing conditions are essential to maintaining the molecular weight of the PLA during processing and that the use of the tailored dispersants can also help to mitigateprocess-induced degradation.MicroCT has proved to be a useful quality control tool to support TEM analysis

Viscoelastic melt rheology and time-temperature superposition of polycarbonate – multi-walled carbon nanotube nanocomposites

This work investigates the linear and non-linear viscoelastic melt rheology of four grades of polycarbonate melt-compounded with 3 wt% Nanocyl NC7000 multi-walled carbon nanotubes and of the matching matrix polymers. Amplitude sweeps reveal an earlier onset of non-linearity and a strain overshoot in the nanocomposites. Mastercurves are constructed from isothermal frequency sweeps using vertical and horizontal shifting. Although all nanocomposites exhibit a second plateau at ~105 Pa, the relaxation times estimated from the peak in loss tangent are not statistically different from those of pure melts estimated from cross-over frequencies: all relaxation times scale with molar mass in the same way, evidence that relaxation of the polymer network is the dominant mechanism in both filled and unfilled materials. Non-linear rheology is also measured in large amplitude oscillatory shear. A comparison of the responses from frequency and amplitude sweep experiments reveals the importance of strain and temperature history on the response of such nanocomposites

Exploiting Generative Design for 3D Printing of Bacterial Biofilm Resistant Composite Devices

As the understanding of disease grows, so does the opportunity for personalization of therapies targeted to the needs of the individual. To bring about a step change in the personalization of medical devices it is shown that multi-material inkjet-based 3D printing can meet this demand by combining functional materials, voxelated manufacturing, and algorithmic design. In this paper composite structures designed with both controlled deformation and reduced biofilm formation are manufactured using two formulations that are deposited selectively and separately. The bacterial biofilm coverage of the resulting composites is reduced by up to 75% compared to commonly used silicone rubbers, without the need for incorporating bioactives. Meanwhile, the composites can be tuned to meet user defined mechanical performance with ±10% deviation. Device manufacture is coupled to finite element modelling and a genetic algorithm that takes the user-specified mechanical deformation and computes the distribution of materials needed to meet this under given load constraints through a generative design process. Manufactured products are assessed against the mechanical and bacterial cell-instructive specifications and illustrate how multifunctional personalization can be achieved using generative design driven multi-material inkjet based 3D printing.Engineering and Physical Sciences Research Council EP/I033335/2, EP/N024818/1, EP/P031684/

Melt-processed PLA/HA platelet nanoparticle composites produced using tailored dispersants

Hydroxyapatite (HA) nanoparticles, similar to those seen in the structure of human bone, have been produced via hydrothermal synthesisand used to produce nanocomposite materials via melt blending with poly(lactic acid)(PLA). Both of these processes are scalable and commercially relevant. Tailored dispersants were developed and used to improve the dispersion of the HA. Modest improvements in flexural properties were observed (max increases 30% of dry modulus, 13% of wet strength). Rheometry is not suggestive of achieving percolation,so there is potential to improve mechanical properties further. It was established that very dry processing conditions are essential to maintaining the molecular weight of the PLA during processing and that the use of the tailored dispersants can also help to mitigateprocess-induced degradation.MicroCT has proved to be a useful quality control tool to support TEM analysis