title: Improvement of ductility and crystallization properties of poly (lactic acid) (PLA)
Creators: name: Gokhale-Stec, Maria
Publisher: name: UNSPECIFIED
Year: 2021

description: The objective of the present thesis was the improvement of the tensile properties and crystallization properties of PLA. Considering that the tensile properties of PLA is a vast field of research, the focus was laid on the enhancing of the elongation at break of PLA without deteriorating the degradation property of PLA. Another objective linked to the tensile property improvement was to develop a synthetic route to functionalize a commercially available hyperbranched polymer to enhance the compatibility of PLA/PE blends. The strategy to improve the elongation at break was based on the theory of polymer blends. First, the binary blends comprising of PLA and different PE-types - HDPE, LDPE, LLDPE and mPE - were prepared in a twin-screw extruder. PLA-HDPE and PLA-LDPE blends exhibited a low elongation at break; whereas PLA/LLDPE and PLA/mPE blends exhibited a very high elongation at break. The addition of PE to PLA dropped the elasticity modulus of the PLA-PE blends, which could be due to the plasticizing effect induced by PE into the blend system. The morphology of the blends was thoroughly investigated, and correlations were drawn between the blend morphology and the elongation at break. The low elongation at break of PLA/HDPE and PLA/LDPE blends was related to weakly-adhered large globules of HDPE and LDPE in the PLA matrix. On the other hand, a very high elongation at break of PLA/LLDPE and PLA/mPE was related to elongated microfibrils of the dispersed phase, interpenetrated in the PLA matrix. The impact of the drawing method on the morphology of PLA/PE blends was evaluated with a calendar roller system at the die exit, that imparted an elongation flow field on the extruded material. PLA/HDPE and PLA/mPE blends were prepared under three different conditions. In one case, the blends were cooled directly in a water bath, and in the other two cases, the blends were drawn continuously at a draw ratio of 4.5 and 9. The blends cooled directly in a water bath exhibited a globular morphology, which indicated poor interfacial adhesion properties. Upon drawing, PLA/mPE blend exhibited fine fibrillar morphology of mPE dispersed well in PLA matrix with very few cavities present. Similar observations were made at both draw ratios. In case of drawn PLA/HDPE blends, the diameter of HDPE droplets reduced progressively with an increasing draw ratio because of intensive stretching deformation. The second strategy to improve the elongation was based on the use of compatibilizers in the blend composition. The topic of ternary blends consisted of two parts, functionalization of a hyperbranched polymer towards a potential new class of compatibilizers for PLA/PE blends and PLA/PE blends comprising of a compatibilizer such as E-GMA. Boltorn™ HB20 (H0), a commercially available hyperbranched polyester polyol was selected as a scaffold and was functionalized with stearic acid (SA) and PLA in a two-step process. Prior to the functionalization of H0 with SA and PLA, a model study was conducted to determine the degree of functionalized -OH groups. The so-called ‘TAI-model’ was based on trichloracetylisocyanate (TAI) functionalized H0, where the degree of functionalization was estimated via 1H-NMR spectroscopy. The TAI-model was a very important tool that enabled quantification of the residual -OH groups during the functionalization of H0 with SA and PLA. In the first step, SA was added to H0 in two molar ratios; H0:SA 1:8 (H08) and 1:12 (H12). A stearyl substitution of 48 % in H08 and 78 % in H12 was confirmed via 1H-NMR spectroscopy. In addition, TGA analysis confirmed that the functionalization of SA onto H0 improved the thermal stability of H0. In the second step, PLA:H08 / 5:1 and PLA:H12 / 5:1 were blended with 0.02 % BTA catalyst in a reactive extrusion process. It was observed that BTA supported the reaction but BTA also affected thermal stability of PLA. 1H-NMR spectroscopy confirmed functionalization of PLA on H08 in presence of BTA. However, the complete functionalization of PLA on H12 could not be confirmed owing to weak signal intensities. Finally, a blend comprising of PLA/mPE and PLA-modified H08 or PLA-modified H12 was prepared in a twin-screw extruder. The extrudate was highly deformed, and it was not possible to investigate the tensile properties of the resulting material. Ternary blends comprising of PLA, different PE-types (HDPE, LDPE, LLDPE and mPE), and E-GMA were prepared in a twin-screw extruder. PLA/HDPE and PLA/LDPE blends compatibilized with 5 % E-GMA showed a major improvement in the elongation at break and nearly matched the elongation at break of PLA/mPE and PLA/LLDPE binary blends. Addition of E-GMA to PLA/LLDPE and PLA/mPE gave a slight boost to the elongation break of PLA/LLDPE and PLA/mPE binary blends. In case of all-ternary blends, the morphology of the dispersed phase was much finer, with a very few cavities on the fractured surface. The size of PE globules in HDPE and LDPE ternary blends dropped substantially and facilitated a better adhesion between the matrix and the dispersed phase, which was related to much improved elongation at break. The impact of the dispersed phase and the compatibilizer on the weathering properties of PLA was investigated under hot and humid natural environmental conditions in Queensland, Australia. Blown film extrusion samples of PLA, PLA/mPE and PLA/mPE/E-GMA were placed above and underneath the soil over a period of 5 months. The weathering properties were correlated to the thermal stability data from TGA, as the low molecular weight species that originate from the degradation of long polymer chains can volatilize easily. Amongst the samples placed above the soil and with an exposure to direct sunlight, the thermal stability of PLA/mPE/E-GMA was better than that of PLA/mPE and was related to interfacial stability that E-GMA incorporates in the ternary blend system. FT-IR spectroscopy confirmed that E-GMA and mPE did not hinder the degradation behavior of the PLA placed over the soil. In addition, GPC analysis confirmed that the molecular weight of PLA extracted from the PLA/mPE and PLA/mPE/E-GMA samples above the soil after five months of weathering period decreased compared to the molecular weight of PLA before weathering. This confirmed that mPE and E-GMA accelerate the degradation process of PLA in the blend. In the case of the sample specimens buried under the soil, PLA/mPE showed minor degradation in contrast to PLA and PLA/mPE/E-GMA that showed no signs of degradation. The degradation of PLA/mPE was attributed to the presence of micro voids that promoted water intake and subsequent PLA hydrolysis. A novel nucleating agent, terephthaloyl-bis-N, N’-naphthalimidester (TN) was introduced in the present thesis. The nucleating ability of TN and different commercially-available nucleating agents, such as tetramethylenedicarboxylic dibenzoylhydrazide (TMC) and orotic acid (OA), on the crystallization capability of PLA, PLA/mPE and PLA/mPE/E-GMA was evaluated. First, the nucleating potential of TN, TMC, and OA under non-isothermal conditions was measured in two different PLA types - PLA 4032D (L/D; 98/2) compared to PLA 4043D (L/D; 95/5). All the nucleating agents significantly improved the crystallization kinetics of PLA 4032D compared to PLA 4043D. The nucleation efficiency of TN was comparable to OA and TMC, where only 0.3% of each nucleating agent improved the crystallization of PLA 4032D. The nucleating efficiency of TMC, TN, and OA on the crystallization of PLA was investigated under isothermal conditions. An addition of 0.3% TMC, TN, and OA to PLA completed the crystallization process in less than 3 minutes. PLA nucleated with OA exhibits the highest crystallization rate, followed by PLA nucleated with TN, and PLA nucleated with TMC. It was found that crystallization is improved with an increase in crystallization temperature and the concentration of the nucleating agent. The nucleation ability of TMC, TN, and OA in PLA/mPE binary blends and PLA/mPE/E-GMA ternary blend was evaluated under non-isothermal conditions. The blends were prepared in a twin-screw extruder and the blends were nucleated with 1 % of TMC, TN, and OA. The addition of TMC and TN had a positive impact on the crystallization behavior of binary blends, whereas the OA-nucleated ternary blend did not show any improvement in the crystallization behavior of PLA. The addition of TMC, TN, or OA caused a substantial drop in elongation at break of the binary blends. However, the addition of E-GMA to the blend composition improved the elongation at break. The ternary blend comprising of TN showed the maximum improvement, followed by the blend comprising of TMC. Addition of OA to the ternary blend substantially deteriorated the elongation at break.