Monitoring Nanoscale Deformations in a Drawn Polymer Melt with Single-Molecule Fluorescence Polarization Microscopy

Abstract

Elongating a polymer melt causes polymer segments to align and polymer coils to deform along the drawing direction. Despite the importance of this molecular response for understanding the viscoelastic properties and relaxation behavior of polymeric materials, studies on the single-molecule level are rare and were not performed in real time. Here we use single-molecule fluorescence polarization microscopy for monitoring the position and orientation of single fluorescent perylene diimide molecules embedded in a free-standing thin film of a polymethyl acrylate (PMA) melt with a time resolution of 500 ms during the film drawing and the subsequent stress relaxation period. The orientation distribution of the perylene diimide molecules is quantitatively described with a model of rod-like objects embedded in a uniaxially elongated matrix. The orientation of the fluorescent probe molecules is directly coupled to the local deformation of the PMA melt, which we derive from the distances between individual dye molecules. In turn, the fluorescence polarization monitors the shape deformation of the polymer coils on a length scale of 5 nm. During stress relaxation, the coil shape relaxes four times more slowly than the mechanical stress. This shows that stress relaxation involves processes on length scales smaller than a polymer coil. Our work demonstrates how optical spectroscopy and microscopy can be used to study the coupling of individual fluorescent probe molecules to their embedding polymeric matrix and to an external mechanical stimulus on the single-molecule level.

Dynamics of Single-Molecule Stokes Shifts: Influence of Conformation and Environment

“…[12][13][14] Since these first experiments, the obtainable information such as emission spectra, photon distributions, positions and orientations of individual emitters have become accessible in the lab as well as by commercial setups. [15][16][17] However, measuring excitation spectra of a single molecule at room temperature remains challenging. Especially for strong phonon broadening, the red shifted fluorescence signal is weak, increasing the acquisition time for an excitation spectrum.…”

Optical Microscopic Techniques for Synthetic Polymer Characterization

“…Single-molecule fluorescence microscopy was used to gain real-time nanoscale information on the viscoelastic properties of a mechanically elongated polymer film. Krause and co-workers monitored the nanoscale deformation of a free-standing polymer film induced by mechanical elongation and subsequent stress relaxation using single-molecule fluorescence polarization microscopy . The deformation was assessed from the position and orientation of individual PDI molecules doped into a free-standing PMA film, the latter of which was estimated from fluorescence intensities recorded in two orthogonal polarizations (Figure a).…”

“…(c) Individual PDI molecules and their in-plane orientation before and after elongation in the arrow direction (upper) and the distribution of orientations (bottom). Reproduced from Krause, S.; Neumann, M.; Frobe, M.; Magerle, R.; von Borczyskowski, C. ACS Nano 2016 , 10 , 1908–1917 (ref ). Copyright 2016 American Chemical Society.…”

Spatial distribution of single guest molecules along thickness of thin films of poly(2-hydroxyethyl acrylate)

“…Although a fluorescent image of each molecule detected by a CCD camera is a diffraction-limited spot as approximated as point spread function (PSF), the position of single molecule can be tracked at a very high accuracy, typically 10-20 nm in imaging (XY) plane by applying the localization method to the PSF [18][19][20][21][22]. The wide-field fluorescence microscopy and its related techniques have been applied to the investigation of a variety of polymer systems [23][24][25][26][27][28]. In addition, by using astigmatism imaging technique [29,30] the position of single molecule can be tracked along the optical (Z) axis at an accuracy of several tens of nanometers.…”