Nanoindentation - Investigating Viscoelastic Properties of Polymer Thin Films at Cold Temperatures

Polymer thin films are widely used in a number of processes, including artificial skin and anti-reflective coatings. A knowledge of their viscoelastic features at multiple temperatures is crucial for guaranteeing efficient engineering performance. Unfortunately, the assessment of the viscoelastic characteristics of polymer thin films across a broad temperature spectrum is comparatively more complicated than a bulk sample.

This article investigates the viscoelastic properties of polymer thin films at temperatures ranging from -125 °C to 23 °C, through the use of dynamic nanoindentation.

Experiment Introduction

A number of polymers are viscoelastic, meaning the properties of the material possess a time-dependence as well as a temperature-dependence. Shorter times (high frequencies) correlate to low temperatures, and longer times (low frequencies) correlate to high temperatures. This principle can be quantitatively applied and is referred to as time-temperature superposition (TTS) [1]. Additionally, there can be severe alterations in the polymer's response as a result of glass transition or melting.

Figure 1. Storage modulus versus temperature for PDMS thin film.

Undertaking nanoDMA^® III frequency sweep tests at shifting temperatures enables the measurement of the glass transition (T_g) temperature of a polymer thin film. This type of experiment is not possible using a conventional dynamic mechanical analysis (DMA) instrument, as a result of the sample geometry. This information can subsequently be implemented in conjunction with TTS, to generate a master curve at a stated reference temperature.

Figure 2. Loss modulus versus temperature for PDMS thin film.

This yields frequency dependent data surpassing that which the apparatus is equipped to provide. The xSol^® temperature control stage, implemented with the cryo option, possesses an extensive temperature spectrum of -120 °C up to 800 °C, which is suitable for most applications.

Figure 3. Tangent delta versus temperature for PDMS thin film. The upper graph shows a zoomed view of the data around the Tg temperature.

Polydimethylsioxane (PDMS) is a typical polymer utilized in numerous commonplace items, such as contact lenses, lubricants, and shampoos. At room temperature and over extended time scales, PDMS operates analogously to a liquid and will therefore adapt to surface imperfections. Over shorter time scales, it operates analogously to elastic solids, like rubber, for example [2].

A 500 μm thick PDMS film was analyzed with a Hysitron® TI 980 TriboIndenter® configured with nanoDMA III,  an xSol Cryo temperature stage and a 10 μm radius conical indenter probe. Reference frequency sweep tests spanning 10 Hz to 301 Hz were carried out from -125 °C to 23 °C (room temperature) [3].

As a result of the time dependence of the majority of polymers, the strain rate of an indentation experiment can have a significant impact on the observed characteristics. In this case, the material was allowed to relax before the dynamic test. Additionally, the effective strain was held constant by indenting to 1600nm at each temperature.

Time-Temperature Results

There is a distinct alteration to both the storage and loss modulus at cooler temperatures (see Figures 1 and 2). A spike in the ratio of storage to modulus (tan delta) transpired around the T_g of PDMS. A more detailed examination of this spike establishes the frequency dependence of the modulus, as observed in the shift of the tan delta peak as the frequency is increased (see Figure 3).

The tan delta calculations utilizing nanoDMA III are congruous with calculations taken on conventional DMA apparatus, executed by a professional contract testing laboratory on the same sample (see Figure 4).

Figure 4. Tan delta comparison between nanoDMA III and macro DMA test.

A master curve possessing a reference temperature of -115 °C was generated with the use of TTS and the Williams-Landel-Ferry (WLF) equation [4]. This investigation exhibits the frequency dependence of PDMS spanning 10^-11 Hz to 10³ Hz. These frequencies far surpass the experimental capacities of both nanoDMA III and conventional macro DMA apparatus.

A correlation of master curves created by nanoDMA III and conventional DMA apparatus yield a strong correlation in storage modulus (see Figure 5).

Figure 5. Storage modulus master curve at -115 °C.

Conclusions

The synthesis of nanoDMA III with the xSol Cryo temperature stage embodies an impressive new partnership. In conjunction, they enable the investigation of viscoelastic characteristics of polymer thin films, for which macro DMA instruments have proved insufficient. This is a crucial feature of discerning the T_g of polymer thin films in comparison to their bulk equivalents, and analyzing frequency dependence at extremely low and high frequencies.

References

1. J.D. Ferry, Viscoelastic Properties of Polymers (3^rd ed.), John Wiley and Sons, New York, NY: 1980.  ISBN-13 978-047104894
2. J.E. Mark, H.R. Allcock, and R. West, Inorganic Polymers, Prentice Hall, Englewood, NJ: 1992. ISBN 0-13-465881-7
3. J. Vieregge, J. Schirer (2011). Continuous Nanoscale Dynamic Mechanical Analysis of PMMA. Retrieved from Hysitron website
4. M.L. Williams, R.F. Landel, and J.D. Ferry, "The Temperature Dependence of Relaxation Mechanisms in Amorphous Polymers and Other Glass-forming Liquids," J. Am. Chem. Soc. 77 (14): 3701–07 (1955). doi: 10.1021/ja01619a008

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

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