Theoretical simulation of the infrared signature of mechanically stressed polymer solids

Abstract

Mechanical stress leads to deformation of strands in polymer solids, including elongation of covalent bonds and widening of bond angles, which changes the infrared spectrum. Here, the infrared spectrum of solid polymer samples exposed to mechanical stress is simulated by density functional theory calculations. Mechanical stress is described with the external force explicitly included (EFEI) method. The uneven distribution of the external stress on individual polymer strands is accounted for by a convolution of simulated spectra with a realistic force distribution. N-Propylpropanamide and propyl propanoate are chosen as model molecules for polyamide and polyester, respectively. The effect of a specific force on the polymer backbone is a redshift of vibrational modes involving the C–N and C–O bonds in the backbone, while the free C–O stretching mode perpendicular to the backbone is largely unaffected. The convolution with a realistic force distribution shows that the dominant effect on the strongest infrared bands is not a shift of the peak position, but rather peak broadening and a characteristic change in the relative intensities of the strongest bands, which may serve for the identification and quantification of mechanical stress in polymer solids.

Chemical Composition and Strain at Interfaces between Different Morphologies in Block Copolymer Thin Films

“…Peak shifts have been related to strain in other polymer systems. Tensile strain has been found to cause a red shift as a result of increasing the bond length during strain. − Similarly, compressive strain can cause a blue shift due to the decreasing bond length. In flat areas (A), chains are in a compressed state because the film thickness, t (85 nm), is less than the equilibrium lamella period, L 0 (230 nm).…”

Mechanical Switching of Aromaticity and Homoaromaticity in Molecular Optical Force Sensors for Polymers

“…Recent advances in the incorporation of force‐responsive units, so‐called mechanophores, into the backbones of polymers have allowed the synthesis of a prototypical molecular force sensor, in which the mechanically induced isomerization of spiropyran to merocyanin is accompanied by a color change of the material from yellow to red . While several molecular force sensors based on changes in their vibrational, UV/Vis, fluorescence,, or chemiluminescence spectra have been reported, an ongoing challenge in the design of novel molecular force sensors is the need to generate a measurable spectral change well before material failure occurs. Using computational methods, it is here demonstrated that the mechanically induced switching of aromaticity and homoaromaticity in molecular force sensors are convenient approaches to achieve substantial changes in color at low stretching forces.…”

Stress-Responsive Reinforced Polymer Composites via Functionalization of Glass Fibers

“…Mechanochemistry is utilized in composites as a means of stress sensing, utilizing weak force-responsive chemical bonds to activate signals when embedded in a polymer composite material. − These signals can then be correlated with the amount of stress applied to a polymer composite and subsequent potential damage that has occurred due to the stress. While there are many different types of force-responsive moieties, the particular chemistry focused on in this paper is the cinnamoyl molecule.…”