Rheo-Raman | Combining Rheology and Raman Spectroscopy: Monitoring the Crystallization and Melting of LDPE and HDPE
By combining rheology and Raman spectroscopy, changes in rheological behavior can be directly correlated to molecular changes as presented for the in-situ monitoring of polyethylene crystallization and melting facilitating interpretation of results.
Synthetic polymers such as plastics, rubbers (elastomers), or fibers play an essential role in daily life. Nowadays, more and more polymer materials are specifically tailored to the needs of the customer with respect to their physical and chemical properties. This makes a detailed analysis crucial for polymer manufacturing as well as processing.
A non-destructive method to easily and quickly assist in this task is Raman spectroscopy. The technique enables the user to identify polymers, detect remaining monomers, or also characterize the polymers with respect to tacticity, conformation, or crystal properties. Raman spectroscopy is ideally suited to monitor melting and crystallization processes of polymers as it is sensitive to sample-specific molecular vibrations, which provide a unique chemical fingerprint.
By combining Raman spectroscopy with a rheometer simultaneously the changes in the viscoelastic parameters during the melting and solidification can be monitored giving insight to physical properties of the bulk material and at the same time correlating them to the chemical structure and microenvironment.
Thus, a combination of Raman spectroscopy and rheometry is not only useful in the incoming goods inspection of polymer processing industries but can also be used in R&D departments for polymer characterization during synthesis.
Polyethylenes are semi-crystalline thermoplastics and belong to the most commonly used polymers in industry. Due to their favorable properties such as ease of processing, toughness, and chemical resistance polyethylenes have a broad range of applications, including films and packaging containers, cable insulations and household plastics. Their different forms are commonly distinguished by their density. The main forms are: HDPE (high-density PE), LDPE (low-density PE) and LLDPE (linear low-density PE). HDPE has a high molecular weight of more than 300.000 g/mol and consists of mostly unbranched polymer chains leading to a dense packing and therefore a high degree of crystallinity in the solid state. LDPE, on the other hand, exhibits large branches with non-uniform lengths. The crystallinity affects the response to deformation and is important regarding e.g. flow properties during polymer processing. The degree of crystallinity in a polymer is defined by the fractional amount of polymers in the crystalline phase present in the sample. Since the crystallinity of polyethylenes and their densities have a linear relationship, the density of the polymer is usually used.
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