“…a comparison with a reliable NMR measure holds large potential to advance our understanding of dynamics of polymer melts”
Entangled polymers are very long linear macromolecules with chains that are heavily overlapping. A prime example is polymer melts, which are polymers that have become fluid due to the temperature rising above their point of crystallization. These have become commercially important compounds, providing the main ingredients for many modern materials.
The popularity of polymer melts is built largely on the discovery that they possess a broad range of unusual viscoelastic properties that can be manipulated by changing the production environment. The structural basis underlying the elastic behaviors of polymer melts has long been the focus of research.
Initially, the elastic properties were thought to be achieved by virtue of the polymer chains becoming entangled. It was proposed that, during such transient entanglement, cross-linking could occur between the chains or friction may arise between chains to drawn the chains back to their original form, However, the tube model is now the most widely accepted explanation for describing the physics of polymer chain dynamics.
The tube model states that the transverse motion of a polymer is restricted to a tube-like region as more extensive movements are prevented by the surrounding chains1. Although the validity of this model has been theoretically proven, obtaining obtain direct evidence within a microscopic framework has proved immensely challenging. The widely used course grained models are limited by the fact that the interactions are truncated at very short distances and it is usual for only repulsive interactions to be considered. Imaging technologies, such as nuclear magnetic resonance (NMR) spectroscopy, have the disadvantage that they are directly sensitive to orientation fluctuations, and so the entangled chains appear solid.
More recently, time domain NMR, one of several distinct NMR methodologies, has made a comeback and has shown efficacy in the study of chain dynamics and the cross-linking of polymer networks. In contrast to NMR spectroscopy, which measures the energy released as nuclei excited by exposure to a magnetic field are restored to their base energy level, time domain NMR measures the time required for nuclei to return to equilibrium after excitation. Time domain NMR can be performed easily using compact bench-top spectrometers and achieves significant resolving power with high reproducibility, although it does not have the power of atomic or spatial resolution seen with NMR spectroscopy.
Study of the entangled linear polymers, poly(butadiene), poly(isoprene), and poly(dimethylsiloxane), using an advanced time domain NMR technique (multiple quantum NMR) has recently brought the tube model under scrutiny2,3. The time domain NMR data, obtained with a Bruker benchtop minispec mq20 NMR spectrometer, revealed that the ‘tube’ constraints to chain mobility are actually dynamic, rather than constant as previously believed. Furthermore, these constraints to transverse motion were shown to be governed by the matrix rather than by the chain itself.
This study confirms that time domain NMR is a powerful and readily available technique for the quantitative study of the dynamics of highly entangled polymers.
Contact Bruker for more information about the minispec TD-NMR.
- McLeish TCB. Tube theory of entangled polymer dynamics. Adv. Phys. 2002;51:1379–1527.
- Vaca Chávez F.and Saalwächter K. NMR Observation of Entangled Polymer Dynamics: Tube Model Predictions and Constraint Release. Phys. Rev. Lett. 2010, 104, 198305.
- Vaca Chávez F and Saalwächter K. Time-Domain NMR Observation of Entangled Polymer Dynamics: Universal Behavior of Flexible Homopolymers and Applicability of the Tube Model. Macromolecules 2011; 44:1549–1559.