Engineering the properties of polymers and polymer composites requires the ability to control molecular orientation through the processing steps and in the final product. Molecular orientation can affect mechanical properties such as strength and flexibility as well as physical characteristics such as optical and thermal properties.
Isotactic polypropylene (iPP) is a good example to illustrate the analysis of molecular orientation in polymers. It is a classic example of a stereo-regular polymer. iPP has a simple hydrocarbon backbone consisting of carbon-carbon single bonds with pendant methyl groups (-CH3) arranged along one side of the polymer chain. This regular repeating structure leads to a high degree of crystallinity. iPP can be found in a variety of different polymorphic forms.
Polarized Raman spectroscopy is a quick and easy-to-use analytical method that readily provides detailed information on molecular structure and orientation. This information can be used to supplement or replace results from other techniques. Observing Molecular Orientation in Isotactic Polypropylene Films with Polarized Raman Spectroscopy describes a study in which two different samples of isotactic polypropylene films were analyzed to illustrate how polarized Raman spectroscopy can be used to evaluate molecular orientation. While visually similar, these two iPP films displayed differences in their polarized Raman spectra as well as differences in their mechanical properties.
Both of the iPP samples were thin transparent films of similar thickness. However the tensile strengths of the two films were decidedly different. When a section of the first film was clamped in place between two jaws on the sample holder and stretched, it continued to elongate to the limits of the sample holder. The second film, when subjected to the same conditions, required more force to move the jaws apart, elongating the polymer film. The second film also reached a point where the film did not elongate any further and instead it broke when additional stress was applied.
The polarized Raman spectra from the two isotactic polypropylene films showed some distinct differences. The first iPP film showed no evidence of a preferred orientation. Equivalent spectra were obtained with the incident polarization and analyzer both oriented either parallel or perpendicular to the x axis. Rotating the film did not change the results. The same type of analysis with the second iPP film showed definite evidence of a preferred orientation. There were significant variations in the relative intensities of certain peaks when the polarization of the incident light and the analyzer were both oriented either parallel or perpendicular to the x axis. When the film was rotated 90° and the analysis repeated, the opposite results were obtained, where the directions were just reversed. This is consistent with a distinct molecular orientation in the film. The observed orientation was very similar to what was observed for samples when they are stretched along the x axis.
While this study used isotactic polypropylene films to illustrate the concepts, this same type of analysis could be applied to many different types of polymer films. For examples, study details, Raman microscopes used, and spectra, read Observing Molecular Orientation in Isotactic Polypropylene Films with Polarized Raman Spectroscopy.