Effect of triblock copolymers based on liquid natural rubber and low molecular weight poly(lactic acid) on physical properties of poly(lactic acid)/natural rubber blend

Triblock copolymers of poly(lactic acid) (PLA) and natural rubber (NR) (PLA–NR–PLA) with different lengths of PLA end blocks were produced from hydroxyl-terminated liquid natural rubber (HTLNR) and low molecular weight PLA (pre-PLA). The HTLNR with a number-average molecular weight \((\bar{M}_{\text{n}} )\) ~ 28,000 g/mol obtained from photochemical degradation of NR and pre-PLA with varying molecular weight (\(\bar{M}_{\text{n}}\) ~ 3000, ~ 6500, ~ 9700 g/mol) was subjected to condensation polymerization. The compatibilizing effect of copolymers on the physical properties of the PLA/NR blend was studied using PLA/NR/PLA–NR–PLA blend ratios of 90/10/0, 90/9/1, 90/8/2 and 90/7/3 percent by weight (wt%). From the tensile testing results, elongation at break of the blends increased with an increase in the amount of triblock copolymer and with the length of the PLA end block. The blend without the copolymer showed elongation at break of 60.54%, whereas the blend with 3 wt% of PLA–NR–PLA prepared from pre-PLA with \(\bar{M}_{\text{n}}\) of ~ 9700 g/mol showed elongation at break of 199.38%. This was about a 300% increase. The highest impact strength of 79.58 kJ/m² (400% higher than neat PLA) was also found for the blend containing 3 wt% of PLA–NR–PLA with the longest PLA end block. A reduction in the diameter and well dispersion of the rubber particles in the PLA matrix were seen in micrographs taken with a scanning electron microscope. In general, after the addition of PLA–NR–PLA into a PLA/NR blend, the T_g of NR phase shifted to a higher temperature, whereas the T_g of the PLA phase decreased. The PLA–NR–PLA triblock copolymer with the longest PLA end block is possibly used as an effective compatibilizer for the PLA/NR blend.