Study On Thermal Properties And Crystallization Kinetics Of Biodegradable Polylactide Based Nanocomposites

Poly(lactic acid) or Polylactide (PLA) is the most extensively researched and utilized biodegradable and biocompatible thermoplastic aliphatic polyester, produced from renewable resources, and environmentally benign material, which attracts many researchers to widely research. Because of its favorable biodegradability, high strength and stiffness, transparency, nontoxic to the human body and the environment, as well as in contrast to those conventional plastics, PLA has excellent potential for substitution of petroleum-based polymers. Although PLA is an eco-friendly bioplastic with excellent biocompatibility, processibility, and less energy dependence, it has drawbacks as well, such as its poor toughness, low heat distortion temperature, slow crystallization rate and degradation rate, hydrophobicity, unsatisfactory gas barrier properties, lack of reactive side-chain groups, which limits its development and wide practical application. Therefore, several modifications have been proposed to overcome the aforementioned problems of PLA. The main modification methods include polymer blending, copolymerization and compositing with some nano-materials. Among the several modifications, compositing with some nano-materials are regarded as a useful and convenient way. Nanoscale fillers were dispersed into the matrix of polylactide to form polylactide-based nanocomposites, which could significantly improve the mechanical properties, gas barrier properties, thermal properties and biodegradable properties of PLA. The technique had attracted great attention both in industry and in academia at home and abroad.In this article, the nanocomposites based on Poly(L-lactide) (PLLA) and nanoparticale contained reactable nano-SiO₂ (RNS), dispersible nano-SiO₂ (DNS), nano-CaCO₃ (brand name: NLY101I) and Vinyl-polyhedral oligomeric sisesquioxane (Vinyl-POSS) were prepared through ultrasound-assisted melt blending method, the thermal properties of PLLA and its nano-composites were studied; PLLA/reactable nano-SiO₂ nanocomposites were prepared by injection molding method and isothermal crystallization kinetics and non-isothermal crystallization kinetics of PLLA and PLLA/reactable nano-SiO₂ nanocomposites were studied. The main work in this paper is as follows:1. Poly (lactic acid) was modified with different nanoparticles via ultrasound-assisted melt blending method. The thermal properties of PLLA and its composites has been investigated using thermo-gravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD),Fourier-transform infrared (FTIR). The results indicted that the thermal stability of PLLA affected by same concentration (1%) of different nanoparticles were different. The thermal stability of PLLA was increased by joining nano-SiO₂ with—NH₂ (RNS) and—CH₃ (DNS) functional group. The addition of Vinyl-polyhedral oligomeric sisesquioxane (Vinyl-POSS) had little influence on the thermal stability of PLLA. While nanoCaCO₃ (brand name: NLY101I) is caused significant reduction in thermal stability of PLLA. Moreover, the additions of different nano-particles have had impact on cold crystallization, melting and crystallization.2. PLLA/reactable nano-SiO₂ nanocomposites were prepared by injection molding method. The isothermal crystallization behavior of PLLA and its composites was studied with differential scanning calorimetry (DSC) and the effect of nano-SiO₂ was explored. The crystallization Kinetics under isothermal conditions was described by the Avrami equation. It showed that reactable nano-SiO₂ (RNS) had the function of heterogeneous nucleation to PLLA matrix. With increasing RNS content, the crystallization rate of PLLA matrix was increased and the half-time crystallization of PLLA and its nanocomposites was decreased, but the Avrami exponent n had no significant change, which indicated that RNS hadn't changed the nucleation mechanism of PLLA. In addition, the Arrhenius equation and the famous Lauritzen-Hoffmann equation were applied to describe the activation energyâ–³E and the nucleation parameter Kg and the end surface energiesσe of PLLA and its nanocomposites. The results revealed that the activation energy of the nanocomposites was smaller than the activation energies of the pure PLLA. The values of Kg andσe for nanocomposites have increased slightly compared with pure PLLA. It indicated that the activation energies of PLLA nanocomposites were reduced by adding RNS, with the result that RNS promoted effectively the crystallization of PLLA matrix.3. Poly (L-lactide) (PLLA)/reactable nano-SiO₂ (RNS) nanocomposites were prepared by injection molding method. The melting crystallization and cold crystallization behavior of PLLA and its nanocomposites were characterized by Differential Scanning Calorimetry (DSC) and X-Ray Diffraction (XRD) measurements. The dynamic mechanical Properties of PLLA/RNS nanocomposites were also investigated. The microphotographs of PLLA/RNS nanocomposites were investigated with Transmission Electron Microscopy (TEM). TEM images indicated that RNS nanoparticles were homogeneously dispersed in the PLLA matrix. The nonisothermal melting crystallization and cold behavior of the PLLA and its nanocomposites was also discussed by Jeziorny model and Mo model. It shows that the addition of RNS does not change the crystalline structure of PLLA, but can influence the melting crystallization rate and crystallinity of PLLA matrix due to heterogeneous nucleation effect of RNS. Moreover, the activation energy of melting crystallization was calculated by Kissinger model. It was found that the values of the melting crystallization activation energyâ–³E for composites compared with PLLA first increased and then decreased with the increase in the amount of RNS. The results show that the addition of RNS can promote cold crystallization of PLLA matrix due to the nucleation affected by the RNS. The nonisothermal cold crystallization kinetics was analyzed by Jeziorny model and Mo model. Jeziorny model and Mo model successfully described the nonisothermal cold crystallization processes. Moreover, the activation energy was calculated by Augis-Bennett, Takhor, and Kissinger models. The results further confirmed that there is interaction between RNS particles and PLLA matrix. In the study of dynamic mechanical properties of PLLA and its nanocomposites, it was found that the addition of RNS could increase the storage modulus of PLLA matrix. For the glass transition temperature (T_g), the addition of RNS could increase the T_g of PLLA matrix.