Toughened commercial poly(l-lactide) (PLLA) using degradable and recyclable poly(ester-alt-ether)-b-PLLA

Abstract

A more sustainable future for plastics relies on the development of high performance materials that are renewably sourced, recycled without suffering losses in performance, and which are, ultimately, degradable to small molecules. Poly(L-lactide) (PLLA) is the largest scale commercial bio-derived plastic, and fulfills many of the above criteria, but is too brittle. Tackling this limitation could allow it to become a substitute for some engineering petrochemical plastics like high impact polystyrene (HIPS) or poly(acrylonitrile-butadiene-styrene) (ABS) which are not recyclable and cannot be easily defossilised. This study focusses on a series of new block polymers as rubber tougheners enabling such PLLA ductility. These block polymers are efficiently synthesised using controlled polymerizations. They are also fully chemically recyclable and biodegradable. The series of new poly(ester-alt-ethers)-b-PLLA show controllable monomer compositions, block ratios and molar mass. They are synthesised using a one-pot switchable catalysis from epoxides, anhydrides and L-lactide, using a well-controlled Zr(IV) catalyst, which selectively forms the poly(ester-alt-ether)-b-PLLA in high yield. The block polymers are blended, using systematically controlled weight percentages, with commercial, semi-crystalline PLLA (M_n = 103 kg mol⁻¹, Đ = 1.81). The PLLA blends are comprehensively evaluated using thermal analyses, melt rheology, dynamic mechanical analyses and by tensile mechanical analyses – all techniques show the promise of the new rubber tougheners in improving PLLA properties. The best performing material, featuring 15 wt% block polymer (11 wt% poly(ester-alt-ether)), combines the beneficial high modulus (E = 3.1 ± 0.1 GPa) and high tensile strength (σ = 48.7 ± 1.2 MPa) of PLLA with higher ductility (7× higher than PLLA, ε_B = 24.5 ± 4.6%) and greater tensile toughness (8× PLLA, U_T = 10.8 ± 2.2 MJ m⁻³). Its mechanical properties are improved without compromise to the PLLA thermal properties, as evidenced by very similar glass transition temperature, crystallinity and melt temperature. The PLLA/block polymer blend (15 wt%) shows a lower melt viscosity (3789 Pa s⁻¹ vs. 10 335 Pa s⁻¹ for PLLA) and earlier onset of shear thinning, facilitating its processing. The PLLA blends are efficiently chemically recycled, using a solid state catalysed process, to L-lactide (87% yield, 100% L-LA selectivity) and the starting poly(ester-alt-ethers)-b-PLLA, facilitating its reuse in blending. The blend components, including the block polymer, are enzymatically degraded, at 37 °C, using Humicola insolens Cutinase over 25 days (HiC, trademark name Novozyme 51032). The properties of these toughened PLLA samples are discussed as replacements for poly(acrylonitrile butadiene styrene) (ABS) and high impact polystyrene (HIPS). In contrast to these petrochemicals, the PLLA blends are bio-derived, fully recyclable and enzymatically degradable after use.