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2016 | Buch

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Mechanical Properties of Polymers Measured through AFM Force-Distance Curves

verfasst von: Brunero Cappella

Verlag: Springer International Publishing

Buchreihe : Springer Laboratory

Enthalten in: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik"

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Über dieses Buch

This Springer Laboratory volume is a practical guide for scientists and students dealing with the measurement of mechanical properties of polymers at the nanoscale through AFM force-distance curves.
In the first part of the book the reader will find a theoretical introduction about atomic force microscopy, focused on force-distance curves, and mechanical properties of polymers. The discussion of several practical issues concerning the acquisition and the interpretation of force-distance curves will help scientists starting to employ this technique.
The second part of the book deals with the practical measurement of mechanical properties of polymers by means of AFM force-distance curves. Several "hands-on" examples are illustrated in a very detailed manner, with particular attention to the sample preparation, data analysis, and typical artefacts. This section gives a complete overview about the qualitative characterization and quantitative determination of the mechanical properties of homogeneous polymer samples, polymer brushes, polymer thin films, confined polymer samples, model blends and microstructured polymer blends through AFM force-distance curves.
The book also introduces to new approaches and measurement techniques, like creep compliance and force modulation measurements, pointing out approximations, limitations and issues requiring further confirmation.

Inhaltsverzeichnis

Frontmatter

Principles: Theory and Practice

Frontmatter

1. Physical Principles of Force–Distance Curves by Atomic Force Microscopy

Abstract

The atomic force microscope (AFM) is increasingly employed not only to acquire topography images of samples but also to measure force–distance curves. Such curves, beyond playing a major role in the theoretical study of surface interactions, are meanwhile a fundamental tool in surface science, nanotechnology, biology and many other fields of research.

Force–distance curves find their application in the study of numerous material properties, such as mechanical properties, surface charge densities, adhesion and Hamaker constants.

One of the most important applications of AFM force–distance curves is the study of mechanical properties of polymers. Compared to other instruments, the AFM has in this case two major advantages. First of all, elastic moduli of samples can be measured with high resolution from some GPa down to some MPa, which is the range of the elastic moduli of common polymers. Second, force–distance curves can be acquired in an array over the sample. This is a fundamental tool for the characterization of the lateral variation of sample properties and hence for the study of confined polymers and polymer blends.

The first part of this book is divided in two chapters dealing with the theoretical and practical aspects of force–distance curves. Theoretical aspects, handled in this chapter, are focused on mechanical properties of polymers.

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2. Force–Distance Curves in Practice

Abstract

In this chapter, basic experimental aspects of the use of an atomic force microscope for the acquisition of force–distance curves and the study of mechanical properties of samples are discussed.

In the first two sections, calibration issues (sensitivity, spring constant of the cantilever and radius of the cantilever tip) are treated; also, the colloidal probe technique is briefly presented, and advantages and drawbacks are discussed.

In Sect. 2.3 fundamental aspects of data analysis for force–distance curves are described. Moreover, the most common artefacts affecting the acquisition and the analysis of force–distance curves and in particular of deformation–force curves are listed.

Section 2.4 summarizes in table form the sequence of work steps of an experiment aimed to the measurement of mechanical properties of the sample through force–distance curves.

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Case Studies: Mechanical Properties of Homogeneous Polymer Films, Thin Polymer Films and Polymer Blends

Frontmatter

3. Homogeneous Polymer Films

Abstract

This chapter presents studies on the mechanical properties of homogeneous polymer samples grouped in four cathegories: basic experiments about homogeneous polymers, measurements with colloidal probes, characterisation of viscoelastic behaviour and determination of the temperature dependence of elastic moduli.

Since atomic force microscopy is a relatively recent technique for the measurement of Young’s moduli, AFM results are compared with those obtained through other techniques (instrumented nanoindentation, dynamic mechanical analysis, etc.) whenever possible.

A critical review of the results obtained, of their inherent uncertainties, mostly due to the shape of the tip and to the nanoscale topography of the sample, and of the necessary experimental effort reveals quite soon that atomic force microscopy has severe drawbacks, when compared with other well-established techniques.

Such drawbacks are mainly the fact that AFM measurements are very time-consuming and that results are affected by large uncertainties. As a matter of fact, the research field in which atomic force microscopy can be employed advantageously is the study of nonhomogeneous samples. In this case, AFM is often the only technique which can be employed.

Yet, the determination of Young’s moduli of homogeneous samples is important, because it proves that AFM indentation is a feasible technique, able to yield quantitative results.

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4. Thin Polymer Films and Polymer Brushes

Abstract

The determination of mechanical properties via force–distance curves outlined in the previous chapter, most of all through force–volume measurements, is rather complex and time-consuming. For example, for the determination of the elastic modulus of PnBMA as a function of temperature shown in Sect. 3.14, several thousand curves had to be acquired and analysed. This requires much more time than the measurement and the analysis with, e.g., DMA.

On the other hand, AFM is able to determine the mechanical properties of samples, which cannot be analysed with other techniques. The fundamental difference between AFM and other techniques such as DMA or also nanoindentation is that an AFM enables to determine the local properties of the sample, whereas other techniques give information only about the bulk properties. Hence, an AFM can be employed to study inhomogeneous samples, i.e. samples consisting of two or more phases with one or more interfaces.

There are two wide categories of inhomogeneous samples: thin films and blends. Blends are the object of Chap. 5. In this chapter, thin polymer films and a particular category of them, i.e. polymer brushes, are handled.

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5. Polymer Blends

Abstract

Polymer blends are together with polymer thin films on stiff substrate a broad category of inhomogeneous samples, whose local properties can be advantageously investigated with an AFM.

The present chapter about polymer blends is divided into two parts. In the first part, experiments on model blends and confined polymers are reviewed, whereas the second part deals with the characterisation of microstructured blends.

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6. Creep Compliance Measurement

Abstract

There are to date very few experiments reporting the measurement of creep compliance or of creep curves with AFM, since most measurements in this field are performed with instrumented nanoindenters and experimental protocols for the acquisition of creep curves are usually not implemented in commercial microscopes. Yet, AFM offers two significant advantages: a better speed performance and a lower thermal drift. For example, the stepping time of an AFM can be smaller than 1 ms, whereas the stepping time of nanoindenters is commonly limited to ca. 1000 ms (Braunsmann et al., Polymer 55:219–225, 2014).

This section presents two hands-on examples. In the first one (Moeller, J Pol Sci B Pol Phys 47:1573–1587, 2009), creep measurements are compared with force–distance curves measurements analysed with Oliver and Pharr method. Limitations of both methods, mainly due to the occurrence of plastic deformations, are surveyed.

In the second hands-on example (Braunsmann et al., Polymer 55:219–225, 2014), the unique feature of AFM, i.e. the possibility of scanning the sample with resolution in the nanometre scale while acquiring creep curves, is exploited.

Brunero Cappella

Backmatter

Titel: Mechanical Properties of Polymers Measured through AFM Force-Distance Curves
verfasst von: Brunero Cappella
Verlag: Springer International Publishing
Electronic ISBN: 978-3-319-29459-9
Print ISBN: 978-3-319-29457-5
DOI: https://doi.org/10.1007/978-3-319-29459-9

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