Abstract
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.
Highlights
Biodegradable polymers have generated significant interest in both research and industrial communities, and degradable polyesters are the most widely studied of these
Poly(lactic acid) (PLA) undergoes changes in molar mass due to complex thermal degradation attributed to chain scission, de-polymerisation generating additional residual monomer, and recombination reactions, and these all lead to the observed changes in the linear viscoelastic response
Arrhenius behaviour was employed in studies of the kinetics of degradation of other biodegradable polymers in the literature [8,12e14,22], and the value of Ea obtained here is in agreement with the range of 77e297 kJ molÀ1 reported in the literature for similar polylactides [5,6,9,22]
Summary
Biodegradable polymers have generated significant interest in both research and industrial communities, and degradable polyesters are the most widely studied of these. Their degradation is linked to their renewable origins, and helps to close the lifecycle with reduced waste build-up and environmental strain. In comparison with traditional commodity polymers, degradable polymers degrade more readily during melt processing, and consideration of this effect is critical to both manufacture and end use. This is important since processing conditions dictate final product properties in many product types. The competitive performance, sustainability and cost of PLA have motivated efforts to develop degradable composites employing PLA as the base matrix [2e4]
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