On the Selective Enzymatic Recycling of Poly(pentamethylene 2,5-furanoate)/Poly(lactic acid) Blends and Multiblock Copolymers

Abstract

Among novel renewable furanoate-based polyesters, poly(pentamethylene 2,5-furandicarboxylate) (PPeF) shows outstanding gas barrier properties and high flexibility. PPeF blending/copolymerization with another well-known bio-based polymer, poly(lactic acid) (PLA), leads to considerably better mechanical and gas barrier properties of the latter, making it suitable for flexible food packaging applications. In this work, enzymatic depolymerization of PLA/PPeF blends with different compositions (1, 3, 5, 20, 30, and 50 wt % PPeF) and a PLA-PPeF block copolymer (50 wt % PPeF) by cutinase 1 from Thermobifida cellulositilytica (Thc_Cut1) was investigated as a possible recycling strategy. Based on quantification of weight loss and high-performance liquid chromatography (HPLC) analysis of released molecules, faster hydrolysis was seen for PLA/PPeF blends with increasing PPeF content when compared to neat PLA, while the block copolymer (P(LA50PeF50)) was significantly less susceptible to hydrolysis. Surface morphology analysis (via scanning electron microscopy), Fourier transform infrared spectroscopy, and NMR analysis confirmed preferential hydrolysis of the PPeF component. Through crystallization, 2,5-furandicarboxylic acid was selectively recovered from the depolymerized films and used for the resynthesis of the PPeF homopolymer, demonstrating the potential of enzymes for novel recycling concepts. The possibility of selective recovery of 2,5-furandicarboxylic acid from the completely depolymerized films with a 75% yield could bring further evidence of the high value of these materials, both in the form of blends and copolymers, for a sustainable whole packaging life cycle, where PPeF is potentially enzymatically recycled and PLA is mechanically recycled.

Scheme 1. Representation of the PLA/PPeF Physical Blend (Top) and Block Copolymer (Bottom) Chemical Structure and Architecture

Figure 1

Weight loss of samples under investigation hydrolyzed with Thc_Cut1 after 3, 6, and 7 days of reaction time. (A) Physical blends with different PPeF content (ranging from 1 to 50 wt %). (B) Neat polymers and the P(LA50PeF50) block copolymer. The data shown are the average of triplicates. Physical blends indicated as PP1: PLA-PPeF 1; PP3: PLA-PPeF 3; PP5: PLA-PPeF 5; PP20: PLA-PPeF 20; PP30: PLA-PPeF 30; PP50: PLA-PPeF 50. Block copolymer P(La50PeF50).

Figure 2

FT-IR ATR spectra of the P(LA50PeF50) block copolymer after enzymatic hydrolysis compared with the control. A: Zoom in on the 3200–2690 cm^–1 region, B: zoom in on the 1925–1415 cm^–1 region, and C: zoom in on the 1160–650 cm^–1 region.

Figure 3

SEM imaging of PP30 (top) and PP50 (bottom) film blends enzymatically hydrolyzed for 3 days (right) and the respective controls (buffer only, left) at 1000× magnification.

Figure 4

SEM imaging of P(LA50PeF50) enzymatically hydrolyzed for various time intervals and the respective controls (buffer only) at 1000× magnification.

Figure 5

HPLC analysis of acids released from PLA/PPeF blends and the P(LA50PeF50) copolymer. Dark gray bars: 2,5-furandicarboxylic acid (FDCA); light gray bars: lactic acid (LA). (A) PP20; (B) PP30; (C) PP50; (D) P(LA50PeF50). The quantification of FDCA was the average of triplicate measurements and LA of duplicates.

Figure 6

¹H NMR spectra of PP30 at different stages of the enzymatic hydrolysis reaction when compared with the starting material (start, green) and the control reaction (blank, light blue). Integrated areas are reported in the SI, Tables S1 and S2.