Strain-Induced Optical and Molecular Transformations in PET Films for Organic Electronic Applications

TL;DR

The study addresses how mechanical strain affects the optical and molecular properties of PET films at room temperature, using UV-Vis and Raman spectroscopy to cover 0–30% strain. A custom stretcher enables controlled tensile loading, with ex-situ UV-Vis and in-situ Raman measurements revealing increased UV-Vis absorbance and pronounced, often irreversible Raman changes at higher strains, including shifts, broadening, and intensity variations in multiple vibrational modes. The analysis links specific vibrational features to gauche/trans conformations and amorphous/crystalline state transitions, including proto-TX phase formation and C=O sensitivity, showing strain-driven crystallinity and optical anisotropy. Practically, the results define strain thresholds for PET in flexible devices and suggest avenues for strain sensing via Raman, informing the design of OSCs, OLEDs, and flexible sensors with improved optical and mechanical resilience.

Abstract

Poly(ethylene terephthalate) (PET) films are widely used in flexible electronics and optoelectronics, where their mechanical durability and optical performance under strain are essential for device reliability. This study investigates the impact of applied mechanical strain on the optical and molecular properties of PET at room temperature,using UV-Vis absorption and Raman spectroscopy. The work explores how varying strain levels, from 0% (unstretched) to 30%, affect the transparency, vibrational modes, and molecular reorganization within PET films. UV-Vis absorbance measurements reveal that strain induces significant changes in the light transmission properties of PET, particularly in the visible range, and increases absorption in the UVA and visible region by up to 100%. Raman spectra indicate that strain levels higher than 5% lead to irreversible shifts of vibrational lines, accompanied by an increase of their full width at half maximum (FWHM), suggesting molecular reorientation and crystallinity changes. The phonon mode coupled with C-O stretching [O-CH2] shows the strongest response to applied mechanical stress. This study provides a comprehensive understanding of strain-induced optical and structural alterations in PET, with implications for improving the mechanical and optical performance of PET-based devices in strainsensitive applications, such as organic solar cells (OSCs), organic light-emitting diodes (OLEDs), and flexible sensors.

Figures (9)

• Figure 1: Custom-built stretching device; the stretcher is controlled by an Arduino board, via a motor controller.
• Figure 2: The PET sample's thickness was measured at different positions. (a) without stretching and (b) after 20% stretching; the thickness deduction resulting from the strain exposure is observed. The images were captured using a Carl Zeiss Axiotech 100HD light microscope equipped with an Axiocam 105 color camera.
• Figure 3: UV-Vis absorbance plots of PET samples: (a) unstretched PET and (b) strained PET.
• Figure 4: Normalized UV-Vis absorbance of PET films across different visible and UV spectral regions in response to varying residual applied strain. The bar graph highlights the absorbance contributions of specific wavelength ranges (UV-A, purple, blue, green, yellow, orange, and red; see also main text) at strain levels from 0 to 30%, showing a trend of increasing absorbance with higher strain.
• Figure 5: Raman spectrum of an unstretched PET film.