Observing Molecular Orientation in Isotactic Polypropylene

It is important to control molecular orientations when designing the properties of polymers and polymer composites because such orientations can significantly impact physical properties like optical and thermal properties as well as mechanical characteristics like flexibility and strength. Molecular orientations can be done via the processing steps as well as in the end product. Details about the chemical makeup of a polymer can be obtained through Raman spectroscopy. Polarized Raman spectroscopy is suitable to define molecular orientation.

The analysis of molecular orientation in polymers can be better described with isotactic polypropylene (iPP), which presents an excellent example of a stereo-regular polymer. iPP is seen in different polymorphic forms and possesses a simple hydrocarbon backbone, which includes single carbon-carbon bonds with pendant methyl groups organized along the side of the polymer chain. A high degree of crystallinity is imparted through this regular repeating structure. While the crystalline structures possess a standard helical conformation of the polymer chains, they vary in terms of positioning and ordering of the helical chains. Differences in the molecular structure of a simple repeating unit are what determine the material’s properties. Several analytical methods are available that can be employed to study the orientation and molecular structure of polymers. Some of the common methods include:

• Wide angle and small angle X-ray scattering
• X-ray diffraction
• Birefringence
• Solid state NMR
• infrared and Raman spectroscopy

Polarized Raman spectroscopy is a fast and simple analytical technique that gives complete data about orientation and molecular structure. This data can be applied to replace the results obtained from other methods.

Experimental Procedure

In this analysis, two different Raman microscopes were used to make Raman measurements. A Thermo Scientific™ DXR™2 Raman microscope was used to collect single point data and a Thermo Scientific™ DXR™2xi Raman imaging microscope was used to acquire the imaging data. Both of these Raman microscopes are capable of collecting single point data as well as Raman images, and each instrument can be used in a wide range of applications.

Software control, provided by the polarization option, helps to set the orientation of the plane polarized laser excitation, which is centered on the sample. This polarization option even provides control across the orientation of a polarization analyzer, which is utilized to study the Raman light scattered from the sample.

The orientation of the plane polarized laser excitation can be made parallel to the right and left movement of the microscope stage, or parallel to the front and back motion of the stage or perpendicular to this direction (Figure 1). The z-axis is the axis parallel to the incident laser beam. The analyzer can also be oriented either perpendicular or parallel to the incident plane polarized laser excitation, or can be set at a user-defined angle.

Figure 1. Defining the instrumental axes and the relationship to the mounted sample.

The different polarized states can be described with the Porto notation. Z(XX)Z defines the state where the analyzer orientation and the plane polarized laser excitation orientation are aligned parallel to the x-axis, and Z(YY)Z defines the state where the analyzer orientation and the plane polarized laser excitation orientation are aligned parallel to the front and back movement of the microscope stage.

In this analysis, two samples of isotactic polypropylene films were studied to demonstrate how polarized Raman spectroscopy can be effectively utilized to assess molecular orientation. The two iPP films, while visually similar, show differences in both their mechanical properties and polarized Raman spectra. While the thin, transparent iPP films had a similar thickness, they had different tensile strengths.

S.T. Japan’s MicroVice sample holder was utilized to hold and stretch the films. These were later mounted on a rotatable stage insert, which helped to vary the sample orientation with regard to the coordinate system as defined by the instrument (Figure 1).

Results and Discussion

Distinguishing Oriented and Non-Oriented iPP Polymer Films

Clear differences were seen in the polarized Raman spectra acquired from the two isotactic polypropylene films. Sample 1 (first iPP film) did not show any evidence of the desired orientation, but similar spectra were acquired with the analyzer and incident polarization, both oriented perpendicular or parallel to the x-axis, as shown in Figure 2. The results did not change even after rotating the film. However, the same kind of analysis with Sample 2 (second iPP film) showed a clear evidence of the desired orientation (Figure 3).

Figure 2. Polarized Raman spectra from Sample 1, the unoriented iPP film. There are no appreciable differences in the spectra.

Figure 3. Polarized Raman spectra from Sample 2, the oriented iPP Film. Colored areas (yellow) indicate 3 sets of peaks that are used to represent the differences.

When the polarization of the analyzer and the incident light were oriented perpendicular or parallel to the x-axis, major differences were seen in the relative intensities of specific peaks. However, opposite results were acquired when the analysis was repeated and the film was rotated to 90 degrees. Here, the directions were simply reversed. This is in line with the distinct molecular orientation seen in the film. The orientation observed was quite similar to what was seen for samples when they were stretched along the x-axis. This study emphasizes the benefits of utilizing polarized Raman for polymer analysis.

As previously noted, a clear difference was seen in the mechanical properties of both iPP films, and this is distinctly reflected in the variations in the polarized Raman spectra. Slight variations in the unpolarized spectra may have given some indication of the variations in these materials, but it became more apparent when polarized Raman spectra were utilized.

Establishing Sample Orientation using Polarization and Sample Rotation

As soon as it was shown that some molecular orientation is present in the iPP Sample 2 film, the next step would be to define the directionality of the molecular orienta-tion with regard to the orientation of the sample.

In most polarization measurements, it would be useful to align or relate the polarization directions of the instrument with sample axes. Also, the mechanical properties would probably differ along the directions of the sample related to molecular orientation. Iit would be useful to have a means for defining this direction. Here, polarized Raman spectra is quite useful. Figure 4 depicts the equivalent Raman spectra acquired from the orientated sample (Sample 2), irrespective of the orientation of the sample when polarized light is not used.

Figure 4. Unpolarized Raman spectra from Sample 2 show no sample orientation dependence

Conversely, the desired sample axis or sample orientation can be defined by utilizing polarized Raman spectra. Figure 5 depicts a plot of the ratio of the peak heights at 841 cm^-1 and 808 cm^-1 from Sample 2 (the oriented iPP film) as the sample is rotated with respect to the laser polarization. The highest point is close to the orientation of the sample defined as zero degrees.

Figure 5. Determining Sample Orientation – Plot of the peak height ratio (808/841 cm^-1) from Z(XX)Z– polarized Raman spectra as a function of sample orientation (rotation).

Inducing Changes in Molecular Orientation – Stretching the Film

Polarized Raman spectroscopy can be utilized to assess the molecular orientation in iPP films. This would help to evaluate whether a certain sample displays a desired orientation or not. This is essential as it can correlate with mechanical characteristics. Polarized Raman spectra acquired at varied sample orientations can also be used to define an axis of desired orientation. Knowing which peaks will modify with molecular orientation can provide a better understanding of what is happening at a molecular level. Sample 1 did not display any desired molecular orientation, however upon stretching the same film along the x-axis, it develops a desired orientation that is similar to that seen in Sample 2. Figure 6 displays the changes occurring in the three sets of peaks from polarized (Z(XX)Z) Raman spectra acquired from Sample 2, as the film is stretched successively. Figure 7 displays how the peak area ratios alter when stress is successively applied.

Figure 6. The effect that stretching the iPP film (Sample 2) has on the Raman polarized (Z(XX)Z) spectra (a) 2962 and 2953 cm^-1, (b) 998 and 973 cm^-1, (c) 841 and 808 cm^-1.

Figure 7. The effect of stress on the peak height ratios (808/841 cm^-1 and 973 and 998 cm^-1) from the Raman polarized (Z(XX)Z) spectra.

Instead of using peak heights, peak areas were utilized because small shifts in peak positions occurred on stressing the films. While quantitative measurement of the amount of stress was not done, the jaws of the sample holder were moved further apart with each and every step. This indicates that when stress is applied to the film, the molecular orientation can be changed even in a sample with a desired orientation. It would be useful to apply polarized Raman spectra during mechanical testing of polymers, as more data can be obtained on the changes occurring at the molecular level during the testing process.

Imaging Stress with Polarized Raman

Polarized Raman imaging can be applied to give views of distributions or differences of molecular orientations over a sample. This approach would make it possible observe the various aspects of molecular structure, including molecular orientation. Figure 8 depicts the results of Raman imaging on Sample 1 when it is effectively stretched.

Figure 8. Raman images of stretched iPP film (Sample 1). The images are based on the peak height ratio, 808/840 cm^-1, where the red color indicates a higher ratio (greater molecular orientation) and the blue color a lower ratio (lower molecular orientation). (a) image derived from polarized (Z(XX)Z) Raman spectra. (b) image derived from unpolarized Raman spectra.

No orientation effects were seen in Sample 1 until it was stretched. A 0.2 seconds exposure time was utilized along with five averaged scans of the area. The resultant Raman image is based on the peak height ratio of 808/841 cm^-1. The blue color denotes areas with a lower peak ratio and thus likely to have lower molecular orientation. The red color denotes a higher peak ratio that is more or less related to greater molecular orientation.

Although the unpolarized spectra indicate the variations in molecular orientation, such variations are relatively smaller, and the image lacks a good contrast. In contrast, the image created through the polarized Raman spectra display more details and contrast. This shows the potential of using polarized Raman imaging for such kinds of applications. Also, Polarized Raman imaging can considerably enhance Raman images that show variations in molecular orientation over large sample areas.

Conclusion

Polarized Raman micro-spectroscopy can be effectively employed to assess molecular orientation, and this was shown by using isotactic polypropylene films. It is important to detect a material’s molecular orientation, as it correlates with physical characteristics and sheds light on the results of processing steps utilized for engineering preferred physical properties into the product. Polarized Raman spectroscopy can also be utilized to explore molecular orientation that is caused due to applied stress, and for observing what changes take place at the molecular level when the sample is subject to stress.

Apart from using isotactic polypropylene films, the same kind of analysis can be used on other types of polymer films. The Thermo Scientific DXR2xi Raman imaging microscope and Thermo Scientific DXR2 Raman microscope, equipped with the polarization options, facilitate acquiring molecular orientation and structural data, and can be extended beyond Raman imaging or single point analysis.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.

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