Measuring the Spatial Distribution of Dielectric Constants in Polymers through Quasi-Single Molecule Microscopy

Abstract

The variation in dielectric constant is measured for thin films of poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVDF) using confocal fluorescence microscopy. Spatial variation in the local dielectric constant of the polymer films on the ~250 nm length scale is measured using the solvochromatic emission from incorporated nile red (NR) at “quasi-single molecule” (10−7 M) and true single molecule (SM) concentrations (10−9 M). Correlation of the NR fluorescence wavelength maximum with dielectric constant is used to transform images of NR’s emission maxima to spatial variation in local dielectric constant. We demonstrate that the distributions of dielectric environments measured in the quasi- and true SM approaches are equivalent; however, the enhanced signal rates present in the quasi-SM approach results in this technique being more efficient. In addition, the quasi-SM technique reports directly on the continuous spatial variation in dielectric constant, information that is difficult to obtain in true SM studies. With regards to the polymers of interest, the results presented here demonstrate that a limited distribution of dielectric environments is present in PMMA; however, a broad distribution of environments exists in PVDF consistent with this polymer existing as a distribution of structural phases.

Dielectric Dependence of Single-Molecule Photoluminescence Intermittency: Nile Red in Poly(vinylidene fluoride)

“…Thin films of nile red (NR, Aldrich, 99+% pure by LC-MS) embedded in poly(vinylidene fluoride) (PVDF, Sigma-Aldrich, MW ≈ 534 000 by GPC) were prepared as described previously. 18 Films were ∼300 nm thick as determined by ellipsometry, with sample preparation tailored to express the ferroelectric β phase of PVDF, as described in the literature. 19 Heavily dyed samples demonstrated no degradation in fluorescence intensity or optical density over periods of months.…”

“…SM emission was collected on an inverted scanning fluorescence confocal microscope described elsewhere. 18 The 488 nm (Novalux, Protera) excitation field was circularly polarized using a λ/4 waveplate to excite all dye orientations within the films. An excitation power of 3 μW, as measured at the entrance port of the microscope, was employed, and SMs were located by raster scanning the film across the objective (Nikon, Plan-Fluor) focal volume in 100 nm steps.…”

Imaging the Effects of Annealing on the Polymorphic Phases of Poly(vinylidene fluoride)

“…Similar to our previous studies on NR embedded in PVDF, the solvatochromism of NR was used to determine the distribution of ε present within annealed (γ-phase) and non-annealed (primarily β-phase) films. 9,11 In these studies the annealing time was varied as follows: 0.625 hours, 1.25 hours, 1.825 hours, 2.5 hours and 5 hours. By measuring the variation in emission wavelength max of 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 NR across several 10 µm x 10 µm images of annealed and non-annealed films at quasi-SM concentrations (i.e.…”

“…luminophore number density of ~10 molecules/µm 2 ), we were able to spatially resolve this variation versus position in the film. 9 Figure 3( 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 …”

“…5 In this manuscript, we explore the effect of annealing on the dielectric properties of PVDF using our developed quasi-single molecule (quasi-SM) technique that measures the spatial variation in the dielectric constant (ε) in polymer films. 9 By employing the solvatochromic properties of NR embedded in non-annealed and annealed PVDF films, we are able to spatially map the dielectric environments present within a polymer before and after annealing. The results presented here demonstrate that PVDF non-annealed films contain much larger values of ε as compared to that of annealed films, with a continuous decrease in ε with annealing time.…”