Raman spectroscopy of polymers

Polymer blends often exhibit more desirable electrical and mechanical properties than individual homopolymers. However, most blends result in phase separation due to the system’s desire to minimize the free energy. Phase separated films can exhibit optimal characteristics, and being able to tune to resulting morphology by tailoring the preparation technique is useful. Film mixtures of polystyrene (PS) and polymethylmethacrylate (PMMA) have been attracting attention for their possible application to ultra-high-density data storage devices.

Common ways of studying film composition involve X-rays techniques such as Scanning Transmission X-ray Microscopy (STXM), X-ray Photoelectron Spectroscopy (XPS), and Resonant Soft X-ray Scattering (RSOXS). Other methods to determine the composition exist, such as molecular tracers, however, these require binding to the polymers which may alter the composition under study.

Polymers have unique Raman spectral signatures. We can use the presence or absence of Raman peaks to determine if an area contains a particular polymer species. This provides us with a non-destructive method for determining the relative concentrations of polymers in phase separated regions without costly and potentially dangerous X-ray sources.

In the present example, a PS:PMMA (1:1) thin film was prepared and annealed at a temperature above 250 °C. This resulted in spheres that could be seen in an optical microscope. To determine the composition of the spheres, a Klar Mini Pro microscope acquired Raman spectra with a 532 nm excitation laser, a scan area of 60 x 50 microns, and step size of 0.7 micron.

Raman spectra are shown on and off the spheres. Both spectra showed broad PL peaking around 4500 cm-1 and small peak near 1100 cm-1. The additional peaks seen on the sphere correspond to the Raman Peaks of Polystyrene at 3054.3 cm-1 and 1001.4 cm-1. This demonstrates that the spheres are PS-rich phase-separated regions.
Raman spectra of a polymer blend. The Raman peaks are characteristic of polystyrene.

Raman spectra of a polymer blend. The Raman peaks are characteristic of polystyrene.

Sample courtesy of Prof. Brian Collins, Washington State University.