Thermo-Active Behavior of Ethylene-Vinyl Acetate | Multiwall Carbon Nanotube Composites Examined by in Situ near-Edge X-ray Absorption Fine-Structure Spectroscopy

Abstract

NEXAFS spectroscopy was used to investigate the temperature dependence of thermally active ethylene-vinyl acetate | multiwall carbon nanotube (EVA|MWCNT) films. The data shows systematic variations of intensities with increasing temperature. Molecular orbital assignment of interplaying intensities identified the 1s → π*C=C and 1s → π*C=O transitions as the main actors during temperature variation. Furthermore, enhanced near-edge interplay was observed in prestrained composites. Because macroscopic observations confirmed enhanced thermal-mechanical actuation in prestrained composites, our findings suggest that the interplay of C=C and C=O π orbitals may be instrumental to actuation.

Prestrain relaxation in non-covalently modified ethylene-vinyl acetate | PyChol | multiwall carbon nanotube nanocomposites

“…Decreased π * 2 C=C resonances and increased π * C=O has also been attributed to straininduced MWCNT alignment and highlights the interconnectivity of π * C=O from EVA and π * 2 C=C from the PyChol-CNT ensemble (Figure 2(b)). 12 In fact, we have reported improved thermal response from (fresh) prestrained composites, which we tentatively attribute to improved interfacial connectivity, i.e., stronger π -π interactions that have been facilitated upon straining (Figure 2(b)). We propose the externally provided elastic energy could be stored in a modified metastable π -π system.…”

“…We had observed an increased conformational response from strained composites during temperature-resolved NEXAFS altogether, 12 suggesting stronger π -π interactions. Incidentally, the notion of stronger π -π interactions upon mechanical straining bears reminiscence to catch bonds in biophysics.…”

“…We have recently reported temperature resolved in situ Near Edge X-Ray Absorption Fine Structure (NEXAFS) on thermo-active EVA|MWCNT composites, in an effort to correlate macroscopic actuation to spectral variations. 12 By probing unoccupied states, NEXAFS spectroscopy offers invaluable information about a system's chemical environment and structural composition. 13 Temperature-variable NEXAFS spectra had revealed synchronous interplay between π * C=C and π * C=O resonances, suggesting conformational variations between polymer chains and MWCNTs as a result of latching through CH-π interactions.…”

“…In this work, we address effects of aging by conducting in situ temperature-resolved NEXAFS spectroscopy and Atomic Force Microscopy (AFM) imaging on identically processed composites as those reported earlier. 12 Description of experimental procedures can be found in the supplementary material. 15 Figure 1 shows room temperature C K-edge NEXAFS spectra of EVA, PyChol, MWCNT, fresh (shortly after manufacturing), and aged (a year after manufacturing) composites, which have either been subjected to prestrain or not.…”

“…Further, π * 2 C=C and π * C=O intensities increase and decrease, respectively, reversing the conformational trends originally seen upon straining, also pointing at relaxation. 12 A possible mechanism conducive to non-conforming EVA chains is through PyChol unlatching. In this scheme, π -π interactions are no longer viable and connectivity between MWCNTs and EVA is lost (Figure 2(c)).…”