Speaker
Reidar Lund
(University of Oslo)
Description
Telechelic polymers, that is, hydrophilic polymers with hydrophobic end-groups,
spontaneously form hydrogels consisting of interconnected micelles[1,2]. Here we investigate
the relation between the microscopic dynamics determining the connectivity, that is, the lifetime
of the physical bonds and the resulting rheological properties. This is achieved by quantitatively
relating the chain exchange kinetics measured by time-resolved small-angle neutron scattering
(TR-SANS) and the mechanical response obtained from linear oscillatory shear
measurements. The results show that the characteristic relaxation time obtained from rheology
coincides exactly with TR-SANS at intermediate concentrations. The activation energy, Ea, is
concentration- independent and remain exactly the same as for TR-SANS. Upon crossing the
melting point of the micellar n-alkane cores, a discrete change in activation energy is observed
showing the contribution from the enthalpy of fusion to the release/debridging process.
Moreover we will show preliminary results from neutron spin-echo (NSE) spectroscopy where
we have used selective deuteration to reveal the dynamics of either, i) the micellar cores or
ii), the single-chain relaxations within the network.
The results clearly show that the mechanical response and connectivity indeed are controlled
by slow diffusional processes leading to debridging events.[3] The relaxation time at the lowest
concentration is found to be faster in rheology as compared to TR-SANS, which can be
quantitatively attributed to entropic forces arising from conformational deformation of bridging
chains.
References:
[1] Winnik, M. A.; Yekta, A. Curr. Opin. Colloid In. 1997, 2, 424.
[2] Annable, T.; Buscall, R.; Ettelaie, R. Colloids Surfaces A 1996, 112, 97.
[3] T. Zinn, L. Willner and R. Lund, ACS Macro Letters, 2016, 5, 1353–1356.
