PET-ran-PLA Partially Degradable Random Copolymers Prepared by Organocatalysis: Effect of Poly(l‑lactic acid) Incorporation on Crystallization and Morphology

Polyethylene terephthalate (PET) is a nonbiodegradable polymer whose hydrolytic degradation can take decades. Intensive research has been performed to accelerate its hydrolytic degradation without significantly affecting its properties. In this work, PET was combined with poly­(lactic acid) (PLA), a well-known biodegradable polymer, and the effect of PLA content in the crystallization of the PET component has been investigated in detail. To make the process sustainable, PET was polymerized using monomers that can be derived from PET chemical recycling (dimethyl terephthalate) and using organocatalysis (metal-free catalysts). First, low-molecular-weight telechelic PLA was prepared from the organocatalyzed ring-opening polymerization (ROP) of l-lactide followed by step-growth copolymerization with PET oligomers. The random copolymerization was confirmed by Fourier transform infrared (FTIR) and ¹H NMR. We found that PET-ran-PLA copolymers are able to crystallize up to 24 mol % of PLA. Wide-angle and small-angle X-ray scattering (WAXS and SAXS) demonstrated that PLA units interrupt the average crystallizable PET sequences, decreasing its lamellar thickness, melting point, and crystallinity. The temperature dependence of the crystallization rate remarkably switches from nucleation control to diffusion control, as the mol % of PLA approaches the maximum tolerable limit for crystallization. The copolymers exhibited a microspherulitic PET morphology that changed to axialitic at relatively high contents of PLA. Preliminary hydrolytic degradation experiments demonstrate the potential degradation character of the prepared copolymers. If we consider the degradability of the copolymers obtained together with the green synthetic route employed (using dimethyl terephthalate, a monomer that can be obtained from the chemical route for recycling PET), the copolymers produced represent a step toward revalorization of PET recycled monomers for the production of sustainable materials.