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You are here: Home Events Dr. Martin Tress "Dielectric spectroscopy in polymers: from glassy dynamics to supramolecular associations up to isolated molecules"

Dr. Martin Tress "Dielectric spectroscopy in polymers: from glassy dynamics to supramolecular associations up to isolated molecules"

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University of Tennessee Knoxville, Department of Chemistry, 1420 Circle Dr., Knoxville, 37916, Tennessee, USA

What
  • Seminar
When May 29, 2018
from 10:30 AM to 11:30 AM
Where Amphithéâtre Henri Benoît à l’ICS
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Dielectric spectroscopy has proven a versatile tool in polymer research offering direct access to structural dynamics in a wide frequency and temperature range. In this talk I will introduce this method and present three projects where dielectric spectroscopy provided detailed insight into a variety of polymeric systems. The first part focusses on a semi-crystalline star-shaped polymer based on a central (POSS) molecule [1] which connects several strands of isotactic polystyrene (iPS). In previous studies of linear iPS, dielectric spectroscopy measurements revealed that around the crystallites the segmental dynamics is slowed down. In contrast, the POSS-iPS star-polymer exhibits faster dynamics in the semi-crystalline state [2]. This effect is presumably caused by locally reduced density resulting from the uncommon architecture of the molecule. A detailed analysis of the dielectric data reveal that more than 50% of the amorphous parts of the chains are affected by this crystallization-induced confinement effect.

In the second part I will discuss supra-molecular networks of telechelic polymers with hydrogen-bonding end-groups of different interaction strength and different backbone flexibility. The glass transition temperature (Tg) of flexible polydimethyl siloxanes (PDMS) does not vary with H-bond strength of the end-groups but differs strongly from Tg in methyl-terminated PDMS [3,4]. At the same time, Tg of the much stiffer telechelic polypropylene glycol (PPG) depends significantly on the H-bond strength [5]. In contrast, viscosity strongly depends on the H-bond strength in the PDMS series while it remains almost the same in PPG with different end-groups. Complementary measurements of shear modulus and dielectric spectroscopy indicate that these observations can be explained by competing lifetimes of the supra-molecular associations and the structural relaxations. This gives us a, yet qualitative, understanding of the extraordinary mechanical properties of this class of materials on a molecular level and eventually can help us derive design principles for these and other, related functionalities e.g. self-healing.

The third part presents a nano-structured electrode setup which enables dielectric measurements of extremely small samples like nanometer-thin polymer layers and individualized polymer chains. Remarkably, even the latter exhibit almost bulk-like glassy dynamics with only the first layer of segments at the substrate being affected by interfacial interactions [6]. This demonstrates how dielectric spectroscopy can be used to study isolated molecules (yet in averaging over large numbers of them) at surfaces in a much wider frequency range than common single molecule techniques.

[1]    Vielhauer, M.; Lutz, P. J.; Reiter, G. & Mülhaupt, R. J Polymer Sci Part A: Polymer Chem, 2013, 51, 947
[2]    Tress, M.; Vielhauer, M.; Lutz, P. J.; Mülhaupt, R. & Kremer, F. Macromolecules, 2018, 51, 501
[3]    Xing, K.; Chatterjee, S.; Saito, T.; Gainaru, C. & Sokolov, A. P. Macromolecules, 2016, 49, 3138
[4]    Xing, K.; Tress, M.; Cao, P.; Cheng, S.; Saito, T.; Novikov, V. N. & Sokolov, A. P. Soft Matter, 2018, 14, 1235
[5]    Xing, K.; Tress, M.; Cao, P.; Cheng, S.; Saito, T.; Novikov, V. N. & Sokolov, A. P. (in preparation)
[6]    Tress, M.; Mapesa, E. U.; Kossack, W.; Kipnusu, W. K.; Reiche, M. & Kremer, F. Science, 2013, 341, 1371

invited by Prof. Pierre Lutz

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