Nano Level Thermal Analysis of Multilayer Biaxially Oriented Polypropylene (BOPP) Films Using the nano-TA Thermal Probe from Anasys Instruments

Topics Covered

Background
Objectives
nano-TA Thermal Probe Local Thermal Analysis Technique
Experimental Setup
Results and Discussion
Top Down Configuration
Cross Sectional Analysis
Conclusions
Acknowledgements

Background

Biaxially Oriented polypropylene (BOPP) films, both heat sealable and non-heat sealable are extensively used in the packaging industry. These films are uni or multi-layered structures having a typical total thickness of only 15-25 µm. The simplest multilayer films correspond to three-layer structures: one thick core layer of polypropylene homopolymer sandwiched between two thin (usually close to 1 µm) skin layers. Each layer has its own contribution to the properties of the film. In the standard three-layer structures, the core layer mainly provides the rigidity of the film, whereas the skin layers provide sealing and/or surface properties.

Objectives

The aim of this work was twofold:

  • To investigate the thermal properties of three-layer BOPP films and in particular to use nano-TA thermal probe to investigate the transition temperature of the 1 µm skin layer in cross-section for the first time.
  • To investigate the effect of ageing on the thermal properties of BOPP films. An additional goal was to compare the differences in measurement with a Wollaston wire probe (probe radius of around 2.5 micron by 25 microns) versus the nanoscale probe (probe radius of around 20 nm) in nano-TA thermal probe.

nano-TA Thermal Probe Local Thermal Analysis Technique

Nano-TA thermal probe is a local thermal analysis technique which combines the high spatial resolution imaging capabilities of atomic force microscopy with the ability to obtain understanding of the thermal behaviour of materials with a spatial resolution of sub-100nm. (a breakthrough in spatial resolution ~50x better than the state of the art, with profound implications for the fields of Polymers and Pharmaceuticals). The conventional AFM tip is replaced by a special nano-TA thermal probe probe that has an embedded miniature heater and is controlled by the specially designed nano-TA thermal probe hardware and software. This nano-TA thermal probe probe enables a surface to be visualised at nanoscale resolution with the AFM's routine imaging modes which enables the user to select the spatial locations at which they would like to investigate the thermal properties of the surface. The user then obtains this information by applying heat locally via the probe tip and measuring the thermomechanical response.

Experimental Setup

The results were obtained using an Explorer AFM equipped with an Anasys Instruments (AI) nano-thermal analysis (nano-TA thermal probe) accessory and AI micro-machined thermal probe. The nano-TA thermal probe system is compatible with a number of commercially available Scanning Probe Microscopes. The sample was a BOPP film manufactured by Solvay. The "fresh" version of the sample corresponds to the BOPP film as produced while the "aged" version corresponds to the same film annealed at 60°C.

The nano-TA thermal probe data presented are of the probe cantilever deflection (whilst in contact with the sample surface) plotted against probe tip temperature. This measurement is analogous to the well established technique of thermo-mechanical analysis (TMA) and is known as nano-TA thermal probe. Events such as melting or glass transitions that result in the softening of the material beneath the tip, produce a downward deflection of the cantilever. Further information on the technique can be obtained at www.anasysinstruments.com.

Results and Discussion

The films were analysed in two configurations: The vertical or top-down configuration and the cross-section configuration.

Top Down Configuration

The uppermost layer (the one that the thermal probe is placed on) of the BOPP film is the skin layer and it has a thickness of around 1£gm or less. Below it is the core layer with a thickness of 15-25 µm and this is again followed by the skin layer.

Figure 1 shows the results of a micro-TA experiment (with a Wollaston wire probe) performed on the sample. The figure contain the results for both the "fresh" and the "annealed" or "aged" film and show the phase transition measurement on the skin layer.

Figure 2 shows the results of a nano-TA thermal probe experiment (with the nanoscale probe) performed on the sample. The figure shows the differences between the "fresh" and "aged" sample in terms of the phase transition measurement of the skin layer.

Figure 1. micro-TA on the sample

Figure 2. nano-TA thermal probe on the sample

There are 2 aspects that clearly standout when we compare Figures 1 and 2:

The main penetration of the skin layer is lower with nano-TA thermal probe, around 80oC versus 120°C with micro-TA. The micro-TA does show a gradual penetration starting at approximately the same temperature as the main penetration of the nano-TA thermal probe. This difference is most probably due to the difference in end radius and aspect ratio of the two probes. The micro-TA probe is significantly larger and lower aspect ratio and so requires more material to melt and move out of the way of the probe. Due to this, the nano-TA thermal probe has a much higher sensitivity to smaller reductions in crystallinity (less material needs to melt for the probe to penetrate). It is known that the skin layer has a broad melting endotherm with the first onset of a small melting peak at around 50°C and a larger melting peak at around 110°C. The micro-TA measurement is showing the onset of this larger melting peak while the nano-TA thermal probe is sensitive to the smaller initial peak.

The measured Tm of the "fresh" layer is lower than that of the "aged" layer in the case of nano-TA thermal probe while this distinction is not so clear in the micro-TA measurement. By annealing, the crystal lamellae become thicker and this increases their melting point. Again the higher sensitivity of nano-TA thermal probe catches this distinction more clearly than the micro-TA.

Cross Sectional Analysis

For this portion of the work, the BOPP films were embedded in epoxy resin and a cross-section was made. Figure 3 below shows the cross-section of the BOPP film in the epoxy matrix.

Figure 3. Cross section of an embedded BOPP Film (Left Topography and Right Sensor Signal)

In the past, Barrel and co-workers (2) have attempted to measure transition temperatures of the skin layer in cross section but the lack of spatial resolution of the micro-TA technique prevented this from happening. The 100x improvement in spatial resolution of the nano-TA thermal probe now allows this as shown in Figure 4 and Figure 5 below.

Figure 4. Zoom in of the epoxy, skin and core layers showing the nano-TA thermal probe indents in the skin layer and a nano-TA thermal probe indent in the core layer.

Figure 5 below shows Local Thermal Analysis performed using nano-TA thermal probe on the epoxy, core and skin layers. The melting temperature of the skin layer correlates well with the top-down measurements.

Figure 5. Transition temperatures on the 3 layers measured using nano-TA thermal probe.

Conclusions

The sub-100nm thermal analysis capability of the nano-TA thermal probe system has enabled transition temperature measurements of the skin layer in the cross section of BOPP films for the first time. It is clearly demonstrated that the nano-TA thermal probe system is more sensitive than the micro-TA system in measuring onset temperatures and showed more distinct differences in transition temperatures for the fresh and the annealed BOPP films.

Acknowledgements

Dr. Antoine Ghanem of Solvay is kindly acknowledged for providing the BOPP samples.

Source: Anasys Instruments

For more information on this source please visit Anasys Instruments

Date Added: Feb 13, 2008 | Updated: Jun 11, 2013
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