A method for the determination and correction of the effect of thermal degradation on the viscoelastic properties of degradable polymers

Small amplitude oscillatory shear is carried out during isothermal degradation of poly(lactic acid) (PLA) in order to determine the evolution of the characteristic relaxation time with degradation time and temperature. After reducing the relaxation time data to a single mastercurve, a 4-parameter function is fitted to the data to allow prediction of the change in relaxation time following an arbitrary thermal history. The method enables separation of the effects of temperature and of degradation on the relaxation time, both of which lead to a horizontal shift of dynamic data along the frequency axis, and hence enable a correction for thermal degradation during rheometry to be carried out. To validate the method, two isothermal frequency sweeps were measured with different temperature histories, producing different mastercurves due to dissimilar in-test thermal degradation. After correcting for thermal degradation using the function and the thermal histories, the two frequency sweeps reduce to the same viscoelastic mastercurve in the undegraded pre-test state

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

A study of melt-compounded nanocomposites of polycarbonate and carbon nanotubes in the melt and solid states

Polycarbonate-carbon nanotube nanocomposites are promising materials for electrostatic shielding and conductive packaging applications. The nanotubes impart electrical conductivity and increases thermal conductivity and stiffness of the matrix. However, the nanofiller also affects the rheology, and hence the evolution of a filler network during processing. This thesis examines the effects of matrix molar mass and of compounding temperature on the thermal, rheological, electrical and mechanical properties of these materials. Thermal analysis demonstrated that the glass transition decreased as a consequence of the nanotubes. Degradation of the matrix was ruled out as a possible cause, and the decrease was attributed to a poor interface between matrix and filler. Thermal conductivity of the matrix increased with the addition of nanotubes, in line with model predictions. Furthermore, the nanofillers also marginally increased thermal stability of the matrix in atmospheric conditions. Oscillatory shear rheology showed that the nanocomposites deviate from linearity earlier than the matrix polymers. A technique was developed to assemble mastercurves over an extended frequency range. The nanocomposites exhibit a low frequency plateau at ∼10^5 Pa, identified as the stiffness of the nanotube network. Relaxation times estimated from the peak in loss tangent scale with matrix molar mass in the same way as terminal relaxation times in pure matrix materials, providing evidence that relaxation of the polymer network is the dominant relaxation mechanism in filled and unfilled polymers. The effects of melt processing on electrical and mechanical properties were investigated using nanocomposites melt-compounded at a range of extrusion temperatures, but subsequently produced by either injection moulding or compression moulding. Electrical resistivity measurements obtained using a two-terminal method revealed that the resistivity of compression moulded specimens was an order of magnitude lower than that of injection moulded specimens. The compounding temperature had only a mild effect on resistivity. Compression moulded specimens also exhibited greater surface hardness and lower modulus than injection moulded specimens. The elastic modulus recorded is in line with expectation due to nanotube orientation, demonstrated using a modified Halpin-Tsai model. The model can explain much of the observed effects, but suggests that nanotubes may be considerably shortened by melt processing

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

A study of melt-compounded nanocomposites of polycarbonate and carbon nanotubes in the melt and solid states

Polycarbonate-carbon nanotube nanocomposites are promising materials for electrostatic shielding and conductive packaging applications. The nanotubes impart electrical conductivity and increases thermal conductivity and stiffness of the matrix. However, the nanofiller also affects the rheology, and hence the evolution of a filler network during processing. This thesis examines the effects of matrix molar mass and of compounding temperature on the thermal, rheological, electrical and mechanical properties of these materials. Thermal analysis demonstrated that the glass transition decreased as a consequence of the nanotubes. Degradation of the matrix was ruled out as a possible cause, and the decrease was attributed to a poor interface between matrix and filler. Thermal conductivity of the matrix increased with the addition of nanotubes, in line with model predictions. Furthermore, the nanofillers also marginally increased thermal stability of the matrix in atmospheric conditions. Oscillatory shear rheology showed that the nanocomposites deviate from linearity earlier than the matrix polymers. A technique was developed to assemble mastercurves over an extended frequency range. The nanocomposites exhibit a low frequency plateau at ∼10^5 Pa, identified as the stiffness of the nanotube network. Relaxation times estimated from the peak in loss tangent scale with matrix molar mass in the same way as terminal relaxation times in pure matrix materials, providing evidence that relaxation of the polymer network is the dominant relaxation mechanism in filled and unfilled polymers. The effects of melt processing on electrical and mechanical properties were investigated using nanocomposites melt-compounded at a range of extrusion temperatures, but subsequently produced by either injection moulding or compression moulding. Electrical resistivity measurements obtained using a two-terminal method revealed that the resistivity of compression moulded specimens was an order of magnitude lower than that of injection moulded specimens. The compounding temperature had only a mild effect on resistivity. Compression moulded specimens also exhibited greater surface hardness and lower modulus than injection moulded specimens. The elastic modulus recorded is in line with expectation due to nanotube orientation, demonstrated using a modified Halpin-Tsai model. The model can explain much of the observed effects, but suggests that nanotubes may be considerably shortened by melt processing

Activity profiles in adolescent netball: A combination of global positioning system technology and time-motion analysis

Purpose: This study sought to understand the activity profiles of adolescent netball players in match play conditions. To date there has been no published research in this area and such findings would give important insights to inform the training and preparation of adolescent netball players. Methods: Twenty-two adolescent netball players (age 14.5 ± 0.5 y, stature 165 ± 5 cm, body mass 58.32 ± 7.43 kg) were analysed while competing in a specially arranged outdoors match of 6 x 10 min periods. The players were categorised into one of three positional groups. To track the players’ movement GPS units were worn located between the scapulas in a purpose built harness. The match was filmed so that post-match time-motion analysis could be carried-out. Both the GPS and time-motion analysis data was classified into one of six movement categories for analysis. Results: Midcourt players covered the greatest distance per quarter (37.73 ± 17.39 m/min) with the Attacking players covering the least distance (35.42 ±11.66 m/min). The Attacking players covered the greatest distance per quarter through sprinting (8.5 ±16.2 m/qtr), travelling nearly twice the distance of the other two groups. When comparing the mean frequency and duration of movements from the GPS and time-motion analysis, there was a discrepancy between the two methods. Conclusions: The results from this study suggest that in adolescent netball Midcourt players cover the greatest distance per quarter while Attacking players cover the greatest distance through sprinting. A comparison between data from the GPS and time-motion analysis suggest that the GPS units used in this study under report the frequency and duration of high intensity movements

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

Challenges to the industrial melt-processing of conductive plastics

In this work, we investigate the relationship between the timescales available for polymer mobility during processing and post-processing and the electrical resistivity of melt-processed thermoplastics filled with carbon nanoparticles. Post-process annealing below the glass transition temperature was one avenue explored to uplift electrical conductivity. Detailed analysis of available literature on thermoplastics filled with either graphite nanoplatelets or carbon nanotubes, and of relevant processing data suggests that the required timescale for shaping process or post-processing to obtain conductive material needs to be sufficiently longer than that of the base polymer characteristic relaxation time τd. Four factors have been identified that promote the formation of a conductive filler network in thermoplastics: filler loading content, polymer molar mass, processing temperature and processing timescales

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