Characterization of a Thermo-Responsive Polymer Film in Air and in Water

Topics Covered

Abstract
Introduction
Experimental Method
Materials
Instrumentation
Results
Characterisation in Air
Characterisation in Water
Conclusion
Advantages of SARFUS

Abstract

A dynamic study of a thermo-responsive nanometer-thin polymer film is performed with the Sarfus 3D-IMM equipment. The ability to carry out an analysis in air as well as in liquid and to easily characterize a thermal transition with a live visualization of the layer quality and morphology is demonstrated.

Introduction

Responsive coatings have attracted considerable attention over the last 20 years because they are a powerful approach to tailor dynamically surface properties. Responsive polymer brushes, which are dense assemblies of grafted responsive polymer chains, can adapt their configuration to external stimuli such as light, temperature, pH or/and salt concentration, which proves useful for applications such as sensors, actuators, drug delivery, affinity control or stimuli-gated filtration.


Figure 1. Swollen and collapsed configurations of a thermoresponsive polymer brush in water.

Thermo-responsive brushes use polymers displaying a lower critical solution temperature (LCST) in a suitable solvent, which means that, at low temperature, grafted polymer chains are swollen and stretch away from the surface. When temperature increases, the brush switches to a collapsed state of different thickness, density and viscoelasticity (Fig.1).

The collapse transition of polymer brushes has been previously monitored by neutron reflectometry, quartz crystal microbalance with dissipation monitoring (QCM-D), ellipsometry or atomic force microscopy. These characterization methods suffer from a series of drawbacks such as a high cost, a long time of analysis, sample degradation, or the need to use complex modelling to extract physically meaningful parameters.

In contrast, the SARFUS technique provides an innovative and comparatively easier methodology to monitor the collapse transition of responsive polymer brushes. The aim of this note is to demonstrate that the SARFUS technique is indeed relevant for the real-time study of stimuli-responsive films both in the dry state and in solution.

Experimental Method

Materials

A P(MEO2MA)-co-(OEGMA) random copolymer brush was grown by atom-transfer radical polymerization from an initiator-silanized immersion Surf (top layer: SiO2), in a solution of 90 mol% di(ethylene glycol) methyl ether methacrylate (MEO2MA, 188g /mol) and 10 mol% (ethylene glycol) methyl ether methacrylate (OEGMA, 475 g/mol) in water/methanol. Films of ~35 nm thickness were grown by selecting the appropriate polymerization time. A collapse transition around 35 °C +/-10 °C is expected from previous results obtained by QCM-D for the selected composition. The refractive indexes of MEO2MA and OEGMA given by Sigma Aldrich are 1.44 and 1.49, respectively.

The ratio of thickness between the swollen and collapsed configurations was estimated from a previous study to be ~2d and ~1.2d, respectively, where d is the thickness of the dry layer in air.

Instrumentation

Visualizations were done with immersion Surfs in the dry state and in solution. Thickness measurements were performed with a SARFUS 3D-IMM, which includes the dry and the immersion version of the same instrument. A scratch was made in the brush in order to observe simultaneously the background and the brush.

The layer was first characterized in air at room temperature (T<<40 °C). Then the Surf was placed in a Petri dish and covered with deionized water (18.2 MΩ.cm). The characterization of the layer thermal collapse was then performed in water from 35 °C to 60 °C by steps of 5 °C (stabilization time at each temperature: 45 min) and steps of 3 °C in the range 35 to 45 °C.

For each temperature two SARFUS images of the layer were taken as well as one image of the calibration standard and one image of the background. The four images were recorded in less than 1 minute.

Results

Characterization in air

Due to the direct, real-time visualisation capability of SARFUS, the quality control of the layer, especially its complete coverage, was easily performed. On the prepared sample, only very few defects and uncovered area were observed. The average thickness of the layer was measured to be 32.5 nm (Ra ~1.1 nm).

Characterization in water

The layer was immersed in water to study the thermal collapse. The evolution of the layer thickness versus temperature is shown in Figure 2.

Figure 2. Apparent thickness of a P(MEO2MA)-co-(OEGMA) copolymer brush in water, versus temperature

A marked variation of thickness is observed around Tt=43 °C, which is in accordance with the expected thermal transition (35 °C+/-10 °C). During the temperature increase, the topography and behaviour of the nanometerthin layer were also observed in real-time. Typical 2D and 3D SARFUS images recorded before and after the thermal transition are shown in Figure 3.

Figure 3. 2D and 3D P(MEO2MA)-co-(OEGMA) copolymer film in water at two different temperatures

In addition to the thickness variation, we also observed an evolution of the surface roughness (Ra (Tt) ~ 0.6 nm; Ra (T>Tt) ~ 2.2 nm) for both configurations.

From real thickness measurements and Sarfus measurements, it becomes possible to compute the refractive index of the layer (see Table 1).

Table 1. The refractive index computed from SARFUS for the two different configurations of the layer. The real thickness in water was obtained from the thickness measured by ellipsometry in air multiplied by the swelling coefficients obtained by QCM-D.

Configuration of the Layer Real Thickness
nm
Sarfus (apparent) Thickness
(nm)
Refractive index*
Dry 35 32.5 1.45
Extended (T 71 32.9 1.405
Collapsed (T>Tt) 42 36.9 1.44

*index correction (included as plug-in in Sarfusoft) is applied regarding the refractive index of the calibration standard (n=1.465).

The refractive index determined for the dry sample is in agreement with the composition of the material and the indices of refraction of the monomers. As for the refractive index of the wet layers, they should be related to their water content. Since the index of refraction of water is 1.33, the decrease of the refractive index of the extended layer is compatible with about ~50% water in the film, as expected from the QCM-determined swelling. As for the collapsed layer, which contains about 15% water, the obtained value of index is again in agreement with expectations. The determined values display the proper evolution: n (dry) > n (collapsed) > n (extended).

Conclusion

In this note, we have illustrated the capacities of the SARFUS technique for the study of a thermo-responsive nanometer-thin polymer film. We have demonstrated the ability to perform an analysis in air as well as in liquid, and to easily characterize a thermal transition with a live visualization of the layer quality and morphology.

Advantages of SARFUS

The advantages of SARFUS include:

  • Work in air or in immersion
  • Ability to perform thermal studies
  • Fast technique (one day for this study)
  • Direct visualization of the nanometric sample
  • Ability for real-time study
  • Large field of view (from 70x70μm² to few mm²)
  • Non-invasive/non-contact technique
  • No labeling/no pretreatment of the sample
  • 3D representation of the sample

Source: Nanolane

For more information on this source please visit Nanolane

Date Added: Jun 3, 2012 | Updated: Jun 11, 2013
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