engleski

Thermal properties of poly(L -lactide)/calcium carbonate nanocomposites

Most of biodegradable polymers have two major applications: biomedical and ecological and poly(L-lactide) (PLLA) is one of that type of polymer. Although synthetic, PLLA is prepared from renewable agriculture-based feed stocks which are fermented to lactic acid and than polymerized. The properties of PLLA can be modified and application field broadened by addition of different additives, such as fillers and/or plasticizers. The common fillers may be inorganic (calcite, talc, silica, mica, clay) or organic (different polysaccharides). Polymer-based nanocomposites are rapidly expanding area of investigation resulting in polymeric materials with enhanced properties. Calcium carbonate (CaCO3) is known for the years as the cost-reduction filler, but nano-sized calcite, due to the cube like shape and isotropic properties as well, plays an important role as the functional filler. In this work the PLLA/CaCO3 composite material was prepared by extrusion and the thermal properties were determined by DSC. The thermal degradation was followed by TG and the kinetic parameters of thermal degradation in nitrogen were calculated according Kissinger’ s equation. Homogenized mixtures of PLLA and 5, 10 and 20 % CaCO3 were extruded on Dynisco laboratory extruder (Qualitest, Canada) at 170°C and 150 rpm. According to DSC measurements addition of CaCO3 has no influence on glass transition, pre-melting crystallization and melting temperatures of PLLA. Addition of CaCO3 slightly decreases crystallization temperature on heating, suggesting that CaCO3 particles enhance the nucleation of PLLA crystallites during heating. That phenomenon is accompanied with increase in crystalline fraction of PLLA from 13% up to 20%. In the investigated temperature range PLLA decomposes in one step and the residual weight is proportional to CaCO3 content. Characteristic temperatures of the thermal degradation, such as temperature of 5% mass loss (T5%), onset degradation temperature (Tons) and temperature at the highest degradation rate (Tmax) strongly decrease when CaCO3 was added. It is evident that CaCO3 thermally destabilize PLLA, so it might be concluded that the PLLA/CaCO3 interfaces are the possible weak points of the composite. Activation energy of PLLA is about 160 kJ/mol. Determined kinetic doublet (Ea and A) confirms the presumption that CaCO3 lowers the thermal stability of PLLA.

Poly(L-lactide); nanocomposites; DSC; TGA; activation energy.

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