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Rheology of Polymer Melts vs. Polymer Solutions: Important Distinctions

April 26, 2018

 

Polymer melts and polymer solutions may seem quite similar – both contain polymer chains – but their rheological characterization should be conducted with strategic differences between their methods and analyses. Mechanics behaviors change drastically depending on the solvent included in polymer solutions. Accounting for this fact requires special attention to the interactions between polymer chains and solvent, as well as polymer concentration. On the other hand, polymer melt rheometry requires high temperature testing in order to measure the molten state of the melt pertinent to processing and research. 

 

 

Although many of the same general rheometry tests can be applied to both melts and solutions, adjustments should be made to accommodate each one.  Amplitude sweeps (oscillatory strain sweeps) provide valuable data for melts and solutions, but with dissimilar interpretations. The linear viscoelastic range (LVE), the range of strain that can be tolerated before structural deformation, denotes the region of chain entanglement for melts.  However, the magnitudes and behavior of G’ and G” will vary greatly from the response seen for solutions (Figure 1). The storage modulus (G’) represents the melt strength, while the G’, G” crossover is indicative of strain or stress needed to flow. Contrastingly, LVE ranges for polymer solutions typically show a dominate G”, and if the LVE range (taken from G” plateau instead of G’) does not last through the entire range of strain, it is due to the detangling of dissolved polymer chains. Polymer solutions must be concentrated beyond dilute and semi-dilute regimes to display a LVE as an indication of chain interaction. This requirement can pose challenges for product developers who work with dilute solutions.

 

Rotational tests often do not yield meaningful information for polymer melts but work well for measuring dilute, semi-dilute, and concentrated polymer solutions. As long as a concentric cylinder or double-gap measuring system is used, viscosity differences between concentrations can be clearly delineated. Flow curves in particular demonstrate the influence of polymer interactions on the overall flow behavior of a polymer solution (Figure 2). Newtonian (straight line) behavior denotes the minimal interaction between chains, while shear thinning trends (decreasing viscosity) are evidence of temporary chain entanglement, and can be disentangled by the application of more shear. However, flow curves are not suitable for polymer melts due to their thick consistencies that do not yield insightful data when tested with an applied shear rate.

 

Polymer melt rheology contains many specialized formulae for estimating chain molecular weight, chain length distribution, and temperature transitions. These useful relations make polymer melts a focused area in which specific polymer chemistries can be easily divulged under specific settings. As tempting as it may be to apply those theories and approaches to polymer solutions, it should not be done. The concentration of the polymer in solution has such a strong impact on the rheology that polymer solution rheology cannot determine the polymer’s characteristics without the solvent. Solvents have a strong influence on the flow properties. However, rheology offers an assortment of benefits for polymer solution studies that examine the influence of solvency on flow behavior and simulation for high-shear manufacturing conditions.

 

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