Visualizing Surface Phase Separation in PS-PMMA Polymer Blends at the Nanoscale

Abstract

Phase-separated polymer blend films are an important class of functional materials with numerous technological applications in solar cells, catalysis, and biotechnology. These technologies are underpinned by the precise control of phase separation at the nanometer length-scales, which is highly challenging to visualize using conventional analytical tools. Herein, we introduce tip-enhanced Raman spectroscopy (TERS), in combination with atomic force microscopy (AFM), confocal Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS), as a sensitive nanoanalytical method to determine lateral and vertical phase-separation in polystyrene (PS)-poly­(methyl methacrylate) (PMMA) polymer blend films. Correlative topographical, molecular, and elemental information reveals a vertical phase separation of the polymers within the top ca. 20 nm of the blend surface in addition to the lateral phase separation in the bulk. Furthermore, complementary TERS and XPS measurements reveal the presence of PMMA within 9.2 nm of the surface and PS at the subsurface of the polymer blend. This fundamental work establishes TERS as a powerful analytical tool for surface characterization of this important class of polymers at nanometer length scales.

Nanoscale chemical imaging of human cell membrane using Tip-enhanced Raman spectroscopy

“…[40] Since phenylalanine is a highly hydrophobic amino acid, it is most likely located inside the 6 nm-thick cell membrane. The near-field sampling depth in our TERS system was previously demonstrated to be 20 nm in the polymer blend films of polystyrene (PS) and poly(methyl methacrylate) (PMMA), [12] which have the refractive indices of 1.59 and 1.49, respectively. Given that the principle components of the cell membrane, i.e., lipids and proteins, have similar refractive indices (1.46-1.60), [41,42] a similar TERS sampling depth is expected.…”

“…This is achieved by combining SPM and surface-enhanced Raman spectroscopy (SERS) to exploit the best of both worlds, i.e., the nanoscale spatial resolution of SPM and molecular specificity and sensitivity of SERS. [7] Over the last two decades, TERS has been successfully applied to characterize a variety of different samples, including heterogenous catalysts, [8] two-dimensional (2D) materials, [9][10][11] polymer blends, [12] organic photovoltaic devices, [13] supported lipid membranes, [14] decomposition chemistry, [15] selfassembled organic monolayers, [16] solid-liquid interfaces, [17] and 2D reactive systems. [18,19] Compared to these applications, studying cell membranes is more challenging because of their chemical complexity, roughness, and the low Raman crosssection of membrane components.…”

Probing coke formation during the methanol-to-hydrocarbon reaction on zeolite ZSM-5 catalyst at the nanoscale using tip-enhanced fluorescence microscopy

“…The probe depth of a TEFL signal depends on the optical properties of the sample and is typically limited to a few tens of nm. 31,48 However, the probe depth (estimated from the axial resolution) of CFM in our system is ca. 1 μm, which is two orders of magnitude higher than the TEFL probe depth.…”

Nanoscale Chemical Imaging of Human Cell Membranes Using Tip‐Enhanced Raman Spectroscopy

“…[40] Since phenylalanine is a highly hydrophobic amino acid, it is most likely located inside the � 6 nm-thick cell membrane. The near-field sampling depth in TERS was previously demonstrated to be � 20 nm (e.g., in films of a polystyrene and poly(methyl methacrylate) blend, [12] which have refractive indices of 1.59 and 1.49, respectively). Given that the principal components of the cell membrane, i.e., lipids and proteins, have similar refractive indices (1.46-1.60), [41,42] a similar TERS sampling depth is expected.…”

“…This is achieved by combining SPM and surface-enhanced Raman spectroscopy (SERS) to exploit the best of both worlds, i.e., the nanoscale spatial resolution of SPM and molecular specificity and sensitivity of SERS. [7] Over the last two decades, TERS has been successfully applied to characterize a variety of different samples, including heterogeneous catalysts, [8] two-dimensional (2D) materials, [9][10][11] polymer blends, [12] organic photovoltaic devices, [13] supported lipid membranes, [14] decomposition chemistry, [15] self-assembled organic monolayers, [16] solid-liquid interfaces, [17] and 2D reactive systems. [18,19] Compared to these applications, studying cell membranes is more challenging because of their chemical complexity, roughness, and low Raman crosssection of membrane components.…”