Tailoring Mechanical Properties of Polymers Through Localisation of Polymeric Phases by Raman Microscopy Mapping

Using polymers in new applications involves matching the material properties with the application requirements. In most cases the polymer will be engineered to suit the application. This involves selection of the monomers and designing the manufacturing processes in order to tailor mechanical properties such as toughness, hardness, elasticity and electrical properties.

When more than one type of polymer is used to leverage the properties of the two (or more) chemical types, synergism can be achieved in the combination. The physical properties of the finished materials will depend on the chemical composition as well as the polymer’s morphology. Two methods are used to combine the chemical phases. Polymerising all monomers simultaneously will make random co-polymers. Blending, an alternative method for engineering products that combines the properties of polymer types is a physical mixing. This is not just economic and simple but also enables re-cycling used material.

The first section of this article is concerned with the dispersion of the two components in a polyethylene-polybutylene terephthalate blend. The second part deals with the depth analysis of laminated films made of different polymer layers.

Experiments – PBT Blend

Figure 1 shows a 0.5µm thick sample of the PE/PBT film illuminated in transmission that indicates heterogeneities. The image shows contrast, however it was only when the Raman spectra were recorded, one could say that the more transparent material contained PBT while the more opaque material contained higher concentrations of PE.

Phase identification

Figure 1. Phase identification

Using the microprobing capability of the LabRAM, the two phases can be fingerprinted within seconds. The upper spectrum was recorded from the clear phase, and the lower spectrum was recorded from the darker material. The clear material was identified as PBT. The dark material spectrum had some residual bands from PBT, but most of the intensity was attributed to PE.

As seen in Figure 2, PBT has bands at 1615 and 1735cm-1 that are diagnostic of the aromatic ring and carbonyl groups; it also has CH bands above 3100 cm-1 indicating non-saturated organics.

Polyethylene Polybutylene Terephtalate (PBT) blend

Figure 2. Polyethylene Polybutylene Terephtalate (PBT) blend

The diagnostic spectral features can be summarized as follows:

  • Fingerprint bands at 1060, 1130, and 1300cm-1 and the CH band between 2800 and 3000cm-1 are typical for PE.
  • The difficulty in obtaining a pure spectrum of PE could be interpreted to indicate a certain degree of miscibility of PBT in PE.
  • These spectra were recorded in a matter of seconds on the LabRAM, making characterisation of these polymer blends easy enough for routine measurements.

Chemical Mapping of Phases

A Raman map is used to further confirm the chemical identity of the two phases with the opaque and transparent material in the TV image. A confocal map can be achieved on the LabRAM with the unique patented confocalline scan.

With this system, the line-illumination on the sample is multiplexed by focusing a spectrum from each point on that line on to a track on the CCD. The full spectrum from each sample point is stored. After the spectral acquisition, one can examine the full file, and select the analytical band in the spectrum from which an image can be reconstructed.

By selecting these Raman bands with cursors, the LabSpec software creates a map showing the PBT in red and the PE in blue as shown in Figure 3.

Map showing PBT in red and PE in blue

Figure 3. Map showing PBT in red and PE in blue

By comparing the TV image and the Raman map these is a less than perfect correspondence between the two images. The Raman map created by the LabRAM provides a more comprehensive chemical distribution of the polymer species in this blend than could be achieved with a TV image and a Raman microprobe without mapping capabilities. One sees that the fields with the purest PBT (red) are larger than the regions with pure PE (blue). Furthermore, the largest domains show mixed spectra, but there are many abrupt frontiers.

Raman Chemical Depth Profiling With Automated Z Focus

A confocal Raman microscope can be used to depth profile a multi-layered structure. In this experiment several layers of pressure sensitive adhesive tape were stacked on a microscope slide that produced a multilayer structure for test purposes. The spectra of the polymer film and the adhesive layer are shown Figure 4.

Spectra of polymer film and adhesive layer

Figure 4. Spectra of polymer film and adhesive layer

By comparing the spectrum of the film to reference polymer spectra, the film is identified as isotactic polypropylene of medium crystallinity. Similar comparisons indicate that the glue layer consists of low-crystallinity, atactic polypropylene with some short polyethylene sequences (1305cm-1), and an ester carbonyl (1727cm-1).

Hence the glue layer most likely consists of a partially esterified ethylene-propylene copolymer. Most of the intensity of the CH band at 2836cm-1 is attributed to the polymer film. Figure 5 shows the analytical regions of the spectra showing more detail.

Analytical regions of the spectra

Figure 5. Analytical regions of the spectra

The multilayer sample structure is represented in Figure 6.

Multilayer sample structure

Figure 6. Multilayer sample structure

In the LabRAM spatial filtering is achieved by closing a computer controlled, variable aperture, confocal pinhole. Depth profiles are automatically acquired with the piezo focus objective.

Two profiles are displayed for demonstration purposes, one with the pinhole open to 1000µm diameter and one with the pinhole set to 100mm diameter (for good spatial filtering).When spectra are acquired with the 100x microscope objective, the depth resolution has been shown to be 2-3µm.

The profiles shown in Figure 7a represent the integrated intensities of the Raman analytical bands of the respective components and in Figure 7b show the ratio of the Raman band areas.

a. Polymer film-top, adhesive bottom, b. Adhesive/Polymer

a. Polymer film-top, adhesive bottom, b. Adhesive/Polymer

Figure 7. a. Polymer film-top, adhesive bottom, b. Adhesive/Polymer

Depth Profile with Confocal Hole Set to 100µm

These depth profiles were acquired with the confocal hole set to 100µm for better definition of the layers. The reduction in the FWHM (full width at half maximum) of the profile of the adhesive signal divided by the polymer signal is very essential.

Its value is improved from about 9µm in the first layer when the confocal is open, to about 6 µm in the first layer when the confocal hole is set to 100µm.

Cross-Sectioning of a Laminar sample

The following results concern the analysis of a 75µm-laminar film made of 2 polyethylene layers sandwiching a middle nylon layer. XZ plane mapping was done by recording spectra by point imaging along a line in the X direction and repeating that at different depths separated by 5µm. The results shown in figure 8 illustrates the confocal approach when high depth spatial resolution is required. Both depth profiles and cross-sectioning of polymer laminated films have demonstrated the confocality and high Z-discrimination of the LabRAM’s Raman microprobe.

Confocal approach

Figure 8. Confocal approach

This information has been sourced, reviewed and adapted from materials provided by HORIBA Scientific.

For more information on this source, please visit HORIBA Scientific.

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