Julia Mofokeng | University of the Free State (original) (raw)

Papers by Julia Mofokeng

Polymer Testing, 2015

The morphology and thermal stability of melt-mixed poly(lactic acid) (PLA)/poly(ε-caprolactone) (... more The morphology and thermal stability of melt-mixed poly(lactic acid) (PLA)/poly(ε-caprolactone) (PCL) blend nanocomposites with small amounts of TiO 2 nanoparticles were investigated. The nanoparticles were mostly located in the PLA phase, with good dispersion of individual particles, although significant aggregation was also visible. The thermal stability and degradation behaviour of the different samples were studied using thermogravimetric analysis (TGA) and TGA-Fourier-transform infrared (FTIR) spectroscopy. Neat PCL showed better thermal stability than PLA, but the degradation kinetics revealed that PLA had a higher activation energy of degradation than PCL, indicating its degradation rate more strongly depends on temperature, probably because of a more complex degradation mechanism based on chain scission and re-formation. Blending of PLA and PCL reduced the thermal stabilities of both polymers, but the presence of TiO 2 nanoparticles improved their thermal stability. The nanoparticles also influenced the volatilization of the degradation products from the blend, acted as degradation catalyst and/or retarded the escape of volatile degradation products.

Thermochimica Acta, 2015

The effect of blending and the addition of small amounts of TiO 2 nanoparticles on the dynamic me... more The effect of blending and the addition of small amounts of TiO 2 nanoparticles on the dynamic mechanical properties of PLA/PHBV, PLA/PCL and PHBV/PCL blends is reported in this paper. The storage modulus of the PLA/PHBV blends was higher than those of both PLA and PHBV in the temperature region below the glass transition of PHBV, but the PLA/PCL and PHBV/PCL bends did not show a similar feature. The E 0 values between the glass transitions of PLA and PHBV depended on the blend compositions and morphologies. The presence of titania nanoparticles had little effect on the E 0 values of all the investigated blends. The cold crystallization transition of PLA shifted to lower temperatures in the PLA/PHBV blends, and shifts in the T g s of the two polymers indicated partial miscibility at the polymer-polymer interfaces. This partial miscibility reduced the chain mobilities of these polymers, which could be seen in a reduction in the damping during their respective glass transitions. Blending and nanoparticle addition had little influence on the glass transition temperatures of PLA and PCL in the PLA/PCL blends, but the glass transitions of PHBV and PCL in the PHBV/PCL blends were respectively at higher and lower temperatures than those of the neat polymers, which is a somewhat abnormal observation. The PCL glass transition peaks became broader as a result of blending, which was attributed to the incompatibility of the polymers in the blends, because blending had no influence on the PCL crystallinity.

Journal of Materials Science, 2015

The morphology, thermal stability and thermal degradation kinetics of melt-mixed poly(hydroxybuty... more The morphology, thermal stability and thermal degradation kinetics of melt-mixed poly(hydroxybutyrateco-valerate) (PHBV)/poly(e-caprolactone) (PCL) blends filled with small amounts of titanium(IV)oxide (TiO 2 ) nanoparticles were investigated. The nanoparticles were mostly well dispersed in both phases of the PHBV/PCL blend, which showed a co-continuous morphology at a 50/50 w/w ratio, but some large agglomerates were also observed. The equal dispersion of the TiO 2 nanoparticles in both polymers was attributed to the polymers having the same surface properties, polarities and viscosities. The

Thermochimica Acta, 2012

Phase change materials based on graphite-filled wax/polyethylene blends could find application as... more Phase change materials based on graphite-filled wax/polyethylene blends could find application as thermal energy storage materials. Such compounds, comprising wax to polyethylene in a 3:2 proportion, were prepared by twin screw compounding. Two types of graphite were used in an attempt to improve the thermal conductivity of the compounds. Expanded graphite enhanced the thermal conductivity by more than 200% at a loading of 10 wt.%, compared to a ca. 60% improvement with natural graphite flakes at the same loading. The TGA results showed that all the compounds underwent a two-step degradation. In all cases the mass % ratios of the two degradation steps were roughly 3:2 for wax:LDPE, which confirms that the wax evaporated completely before the degradation of LDPE started. The DSC results suggest that the heat energy storing capacity of the wax is not influenced by the other components as long as heating is restricted to temperatures just above the melting point of the wax. It is also apparent that the presence of both forms of graphite enhanced the rate of heat transfer to the PCMs. The DMA results show that the presence of wax had a softening effect, while the presence of graphite opposed this softening effect by reinforcing the PCM composites.

Polymer Testing, 2010

Polyamide 12 (PA12) was blended with a high content of a maleic anhydride functionalized paraffin... more Polyamide 12 (PA12) was blended with a high content of a maleic anhydride functionalized paraffin wax in an attempt to prepare shape stabilized phase change materials for energy storage. The morphology of the immiscible blends, and possible interactions between the blend components, were characterized through scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR) analyses. Although it could not be conclusively established, the obtained results indicated a strong possibility of hydrogen bonding interaction between the anhydride -C]O groups on the wax and the -N-H groups in the amide. Such an interaction was clearly evident in the thermal, thermo-mechanical, and mechanical behaviour of the blends, as investigated through differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), stress relaxation measurements, and thermogravimetric analysis (TGA).

Journal of Thermoplastic Composite Materials, 2012

Poly(lactic acid) (PLA) and polypropylene (PP) were comparatively investigated as matrices for in... more Poly(lactic acid) (PLA) and polypropylene (PP) were comparatively investigated as matrices for injection-moulded composites containing small (1-3 wt%) amounts of short sisal fibre. The morphology, thermal and dynamic mechanical properties, as well as degradation characteristics were investigated. The scanning electron microscopy (SEM) micrographs of the composites show more intimate contact and better interaction between the fibres and PLA, compared to PP. This improved interaction was confirmed by the Fourier-transform infrared (FTIR) spectroscopy results which showed the presence of hydrogen bonding interaction between PLA and the fibres. The thermal stability (as determined through thermogravimetric analysis [TGA]) of both polymers increased with increasing fibre content, with a more significant improvement in the case of PP. The differential scanning calorimetry (DSC) results showed a significant influence of the fibres on the cold crystallization and melting behaviour of PLA, even at the low fibre contents of 1-3%. The influence of the fibres on the melting characteristics of the PP was negligible, but it had a significant influence on the nonisothermal crystallization temperature range. Both the storage and loss moduli of the PLA decreased with increasing fibre content below the glass transition of PLA, but the influence on the loss modulus was more significant. The dynamic mechanical analysis (DMA) results clearly show cold crystallization of PLA around 110 C, and the presence of fibres gave rise to higher modulus values between the cold crystallization and melting of the PLA. The presence of fibres also had an influence on the dynamic mechanical properties of PP. This article further describes basic biodegradation observations for the investigated samples.