Decreasing the Glass Transition Temperature (T_g) of Poly(Ethylene Terephthalate) Films at the Air-Water Interface by Reducing Sample Dimensions

PET degradation through enzymatic hydrolysis was shown to be promoted at temperatures close to or above the <i>T</i>_g of the PET material, which falls around 70 °C. Given the low thermal stability of relevant enzymes, decreasing <i>T_g</i> of PET by physical means would be an effective strategy to enhance its degradation. Given the interfacial nature of the enzymatic hydrolysis of PET, polymer films with significantly low thicknesses and high surface area could in addition improve the catalysis condition. Langmuir Monolayer Degradation studies (LMD) provide a method for examining the chemical and physical changes in ultrathin polymer films when exposed to the water surface. Various properties can be measured over time: surface tension, rheological properties, infrared molecular fingerprints, and reduction of the area covered by the polymer film.<br/>In this study, PET ultrathin amorphous layers were prepared and characterized at the air-water interface. The films had a thickness of ca. 2.5 nm (at 21°C), and a porous-like structure with heterogeneous micro- and nano-holes (50-250 nm diameter) as evidenced by TEM and AFM. PET films were subjected to heating cycles from 21°C to ~45°C to determine the thermal properties of PET floating on an aqueous subphase. With our system, it was possible to determine the <i>T</i>_g of the films, calculated as the intersection of two tangent lines from the plot of the logarithm of G′ vs. linear temperature as measured by interfacial rheology. It was shown that thin films have a <i>T</i>_g between 40-44 °C, 30 °C lower than that of bulk PET. Furthermore, films were prepared at different initial water temperatures and transferred to solid substrates to measure their thickness by ellipsometer. It was found that films were formed with different thickness, in a temperature-dependent manner, with thicknesses values of 3.22 ± 0.2, 5.22 ± 0.43, and 6.58 ± 0.86 nm at 10, 21, and 42°C, respectively.<br/>Altogether, our results show a direct correlation between temperature, thickness, and the onset of <i>T</i>_g in ultrathin films solely in contact with an aqueous subphase. The reduced <i>T</i>_gof PET films is expected to allow for faster enzymatic degradation and at temperatures lower than those used in traditional bulk PET or micrometric film studies (~70 °C), potentially improving biocatalysis in plastic biorefineries.