Combining Spectroscopy and Rheology to Study Polypropylene Melts and Phase Transitions

Polypropylene (PP), highly crystalline thermoplastic resin built up by the chain-growth polymerization of propylene (CH2=CHCH3), became a prominent material three years after Italian chemist, Professor Giulio Natta, first synthesized it in 1954. At its advent, polypropylene’s ability to crystallize created a lot of interest in its possible uses. By 1957, its popularity exploded and widespread commercial production began across Europe. Today, it is one of the most commonly produced plastics in the world.

Plastic food containers

Stiffer than polyethylene (PE), this polymer has a high melting temperature and more oxidation sensitivity. It has very low moisture-absorption characteristics and a low softening point. Not only is it difficult to dye or paint, but it is also difficult to bold with other material surfaces. It can be flexed frequently without fatigue and is therefore useful in applications with frequent bending such as hinges. Often manufactured as fibers, it is found in fabrics for furniture, rope, cordage, and indoor/outdoor carpets.

Blow-molded into bottles for food and household items, it is also injection-molded into products such as appliance housings, dishwasher-proof food containers, toys, automobile battery casings, and outdoor furniture. Polypropylene has a relatively slippery surface, useful in low friction applications like gears. 

The physicochemical and morphological relationships during polymer crystallization are of critical importance in polypropylene manufacturing. Scientists at Thermo Fisher Scientific used the newly developed analytical tool that combines Raman spectroscopy with rheology to investigate the temperature-dependent melting and crystallization of PP, as well as the isothermal crystallization process. The melt and crystalline phase transitions of polymeric materials are commonly correlated with variations in viscous and elastic behavior during rheological analysis. Because Raman spectroscopy and rheometry are combined in one multimodal tool, a more descriptive in situ analysis of any many other material processes such as polymerization, gelation, curing, and other shear-related characteristics is now possible.