In Situ X-Ray Scattering Methods for Probing Polymer Deconstruction

Plastics are highly versatile materials that can be found everywhere around us. However, the increased production poses an environmental threat since plastics do not degrade readily. Current recycling techniques such as pyrolysis or mechanical recycling result in lower value materials. This is where chemical deconstruction is important, as it can lead to value-added starting materials.<br/>One main thrust in the BOTTLE (Bio-Optimized Technologies to keep Thermoplastics out of Landfills and the Environment) consortium is related to creating pathways for deconstructing thermoplastics into materials with added value. One such pathway is the VolCat (Volatile Catalyst) process for deconstructing polyethylene terephthalate (PET) using ethylene glycol (EG) and triethylamine (TEA). While this process has been studied extensively, questions still remain related to both the kinetics and mechanism of the deconstruction process which is performed at high temperature and elevated pressure in a heterogeneous mixture of phases. The impact of the reaction conditions (e.g. reaction temperature) and reaction components (e.g. polymer structure, catalyst, and solvent) need to be understand in order to understand the deconstruction kinetics and mechanism.<br/>This talk will focus on small-angle and wide-angle X-ray scattering (SAXS/WAXS) techniques at SSRL (Stanford Synchrotron Radiation Lightsource). Simultaneous SAXS/WAXS can be used to probe length scales from the nano- (~1-100 nm) to the atomic- scale (&lt;1 nm) for SAXS and WAXS, respectively. These are the length scale that are relevant for understanding a semi-crystalline polymer system like PET with crystalline regions embedded in an amorphous matrix. Information that can be extracted includes the degree of crystallinity, crystallite length scale, and polymer conformation.<br/>We used SAXS/WAXS to disentangle the role of the solvent, catalyst, and process temperature on the evolution of the polymer structure. We designed a sample environment that allowed us to perform the deconstruction reaction at the synchrotron beamline. These <i>in situ</i> SAXS and WAXS experiments were particularly powerful tools for monitoring the reaction kinetics by characterizing the structural evolution of the reactants and products over the entire course of the reaction without altering the sample or taking aliquots. When coupled to microkinetic modelling we were able to differentiate the reaction rate constants for the amorphous and crystalline fractions, and explain the average observed reaction rates in terms of interchange between amorphous and crystalline fractions throughout the deconstruction process. The understanding gained from these studies will enable expansion to other polymer systems and deconstruction reactions.