Advances in Nanoscale Infrared Spectroscopy to Explore Multiphase Polymeric Systems

Summary

This protocol describes the application of atomic force microscopy and nanoscale infrared spectroscopy to evaluate the performance of photothermal nanoscale infrared spectroscopy in the characterization of three-dimensional multi-polymeric samples.

Abstract

Multiphase polymeric systems encompass local domains with dimensions that can vary from a few tens of nanometers to several micrometers. Their composition is commonly assessed using infrared spectroscopy, which provides an average fingerprint of the various materials contained in the volume probed. However, this approach does not offer any details on the arrangement of the phases in the material. Interfacial regions between two polymeric phases, often in the nanoscale range, are also challenging to access. Photothermal nanoscale infrared spectroscopy monitors the local response of materials excited by infrared light with the sensitive probe of an atomic force microscope (AFM). While the technique is suitable for interrogating small features, such as individual proteins on pristine gold surfaces, the characterization of three-dimensional multicomponent materials is more elusive. This is due to a relatively large volume of material undergoing photothermal expansion, defined by the laser focalization onto the sample and by the thermal properties of the polymeric constituents, compared to the nanoscale region probed by the AFM tip. Using a polystyrene (PS) bead and a polyvinyl alcohol (PVA) film, we evaluate the spatial footprint of photothermal nanoscale infrared spectroscopy for surface analysis as a function of the position of PS in the PVA film. The effect of the feature position on the nanoscale infrared images is investigated, and spectra are acquired. Some perspectives on the future advances in the field of photothermal nanoscale infrared spectroscopy are provided, considering the characterization of complex systems with embedded polymeric structures.

Introduction

Atomic force microscopy (AFM) has become essential to image and characterize the morphology of a wide variety of samples with nanoscale resolution^1,2,3. By measuring the deflection of an AFM cantilever resulting from the interaction of the sharp tip with the sample surface, nanoscale functional imaging protocols for local stiffness measurements and tip-sample adhesion have been developed^4,5. For soft condensed matter and polymer analysis, AFM measurements exploring the nanomechanical and nanochemical properties of loc....

Protocol

1. Making polyvinyl alcohol (PVA) solution

1. Measure water and PVA polymer pellets (see Table of Materials) to create a 10 mL solution at a 20% PVA to water ratio by weight.
2. Heat the water in the glassware over a hot plate set to 100 °C.
3. Place the PVA polymer pellets into the heated water. Insert a magnetic stir bar.
4. Reduce the heat to 80 °C and stir until the PVA fully dissolves.
5. Cover the top of the glassware to prevent contamination.
6. Once fully dissolved, place the 20% PVA solution into an appropriate storage container for storage at room temperature.

....

Results

PS ((C₈H₈)_n) beads were deposited on a clean Si substrate (Figure 1A) and on PVA ((CH₂CHOH)_n) (Figure 1B,C). Due to the poor adhesion of the bead on Si, nanoIR imaging in contact mode could not be acquired for this sample. Instead, using the optical view of the sample on nanoIR, the gold-coated AFM probe was engaged on top of the PS bead in contact mode, with an estimated force of about 100 .......

Discussion

AFM combined with nanoIR spectroscopy can provide nanoscale chemical information using a cantilever in contact mode and a pulsed tunable IR light source. Model systems, such as embedding an absorber with finite dimensions in the volume of a polymeric material, are important to improve the understanding of image formation mechanisms and to determine the performance of the tool. In the case of the PS/PVA configuration presented here, optimization was carried out to obtain a stable PS bead positioned above or below the surf.......

Materials

Name	Company	Catalog Number	Comments
10|0 2200 Golden Taklon Round	Zem
5357-8NM Tweezers	Pelco
Adhesive Tabs	Ted Pella	16079
AFM metal specimen disks	Ted Pella	16208
Binocular	AmScope
Cantilever for nanoIR measurements	AppNano		FORTGG
Cell culture dishes	Greiner bio-one GmbH
Desiccator
Floating optical table	Newport	RS 4000
Hotplate	VWR
Isopropanol
Kimwipes	KIMTECH
Magnetic stir bar
Microparticles based on polystyrene size: 5 µm	SIGMA-ALDRICH	79633
nanoIR2 microscope	Bruker		Contact mode NanoIR2
Nitrogen Tank	Airgas
Petri dishes	Greiner bio-one GmbH
Polyvinyl Alcohol	SIGMA-ALDRICH	363170	this polymer was only 87%-89% hydrolyzed, which explains the presence of residual C=O at 1730 cm-1
Quantum Cascade Laser	Daylight Solutions		1550-1800 cm-1 range
Silicon wafer	MEMC St. Peters	#901319343000
Spin coater	Oscilla