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Project A1: Transient self-assembled networks: network restructuring and mechanical behavior under shear

Principal Investigators: C. Friedrich (Freiburg) / J. Wittmer (Strasbourg)
Collaborator: A. Blumen (Freiburg)

PhD Students: Carina Gillig, Patrycja Polinska

Current state of the research

Transient self-assembled networks constitute a class of complex materials spontaneously forming reversible three-dimensional networks at thermodynamic equilibrium. Depending on the molecular weight, degree of branching and distribution of functional groups of their building blocks, a wide range of well defined assemblies (crystals, supramolecular networks and block copolymers) can be formed. Recent publications ([1-2] and references therein) highlight the synthesis and linear viscoelastic properties of such polymers with H-bond functionalities and resultant networks. Materials with superior property performance can be expected. The investigation of the relationships between structure and rheological properties is currently under way. An imposed (shear) flow may generate macroscopic deformation patterns and novel self-assembled structures (onions, shear-bands, ...) [3] in these materials and may in turn become unstable [4]. The investigation of the failure behaviour of these materials is of vital interest and of great fundamental as well as theoretical importance.

Contributions of the participating groups

The Friedrich group accomplished the synthesis of a first series of hyperbranched polyols (in collaboration with Prof. Frey, Uni Mainz), and the measurement of melt rheological properties is currently under way. The development of structure/rheological-property relationships for network systems of different chemical nature (fibre networks, hydrogels, organogels) has been the subject of the group's work over years [5]. To gain deeper insight into the ultimate properties of polymeric systems, the group developed a method that allows the experimental determination of critical conditions in shear in form of a critical frequency and a critical deformation [6]. Moreover, the dynamics of microemulsions, cross-linked by telechelic molecules, was probed by rheology and modelled by rheological constitutive equations with fractional derivatives. Its relation to generalized Gaussian (fractal) structures was explored in cooperation with the group of A. Blumen [7,8].

The Strasbourg theory group has investigated analytically in the past various aspects of self-assembled transient networks [9]. In collaboration with the experimental group in Montpellier (Ligoure, Porte) equilibrium polymers, bridged by telechelic chains, have been simulated recently (using the LAMMPS code [10] and dissipative particle dynamics [11]) without addressing, however, the non-linear response. Preliminary (unpublished) results suggest that even more simplified models are needed to overcome the large relaxation times.

Research project and collaborations

We want to study generic rheological and ultimate properties of transient selfassembled networks formed by (A) hyperbranched polyol polymer chains and (B) telechelic polymers (see the figure below). This project combines experimental, theoretical and computational techniques addressing the large-scale rheological response (“stickiness”, viscosity, brittle failure of the network at high shear rates). Within the project we hope to answer the following open questions: How do the thermo-rheological properties of the networks depend on structural features of the constituting polymers? What is an adequate theoretical description of these materials and how are the chemical structures mapped to model parameters? How is the failure of these materials under shear flow correctly described and how are the critical conditions related to the Griffith’s criterion?

a1We offer two PhD stipends, one focusing on each system class. The PhD thesis on rheological and ultimate properties of hyperbranched polyols, directed by C. Friedrich (Freiburg), will be more experimentally oriented, but will also address the theoretical calculation of the relaxation processes in generic hyperbranched objects in collaboration with A. Blumen (Freiburg) and the numerical simulation of coarse-grained models in collaboration with the Strasbourg theory and simulation group (J. Wittmer).

The second PhD thesis directed by J. Wittmer, addresses the rheological properties of branched transient networks from a more theoretical and computational point of view. We will start with a simple toy model of branched microemulsion droplets where spheres are bridged by virtual springs which are added and removed according to a Monte Carlo scheme with an attempt frequency w. The restructuring and ultimate failure of this simple network under strong shear will be investigated as a function of the frequency w. At low w one expects a brittle fracture, while at large w the response might be more ductile. We will use the “observed” behavior to formulate a more general a rheological model bridging between brittle and ductile response. At a second stage of the project the springs will be replaced by coarse-grained telechelic polymer chains with end groups adsorbing on the colloids. Hydrodynamic interactions will be included by means of soft solvent particles [11].


[1] D. Montarnal et al., JACS 131, 7966 (2009).
[2] K. E. Feldman et al., Macromolecules, in press.
[3] W. Richtering, Curr. Opinion in Colloid & Interface Sci. 6, 446 (2001) and references therein.
[4] E. Michel, et al., J. Rheol. 45, 1465 (2001).
[5] M. Fahrländer, K. Fuchs, R. Mülhaupt, and C. Friedrich, Macromolecules 36, 3749 (2003).
[6] K.M. Mattes, R. Vogt, and C. Friedrich, Rheol. Acta 47, 929 (2008).
[7] A. Jurjiu, et al., Chem. Phys. 284, 221 (2002).
[8] A. Blumen, A. Jurjiu, Th. Koslowski, C. Friedrich, Macromol. Symp. 237, 53 (2006).
[9] F. Clement, A. Johner, J. F. Joanny, A.N. Semenov, Macromolecules 33, 6148 (2000).
[10] J. Plimpton, J. Comp. Phys. 117, 1 (1995).
[11] R. D. Groot and P. B. Warren, J. Chem. Phys. 107, 4423 (1997).

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