Dynamic response of polymer networks with dangling molecules

AGUDELO C; EM VALLES; VILLAR MA; DA VEGA

Córdoba

Workshop; XIII LATIN AMERICAN WORKSHOP ON NONLINEAR PHENOMENA; 2013

LAWNP

The equilibrium and dynamic properties of cross-linked polymer networks are highly sensitiveto the presence of dangling molecules. These imperfections that are unavoidable producedduring a cross-linking reaction, alter the network connectivity and also aect the damping responseof elastomers. Here the dynamics of pendant chains trapped in a cross-linked networkis investigated using end-linked poly(dimethyl-siloxane networks with well dened structure.Model networks with linear bimodal pendant chains were prepared by the end-linking technique.These networks were characterized by the extraction of soluble material and rheology.While the equilibrium behavior of the network can be well described by a mean eld theory forrubber elasticity, the terminal region follows a power law dependence in time. This dynamicresponse resembles the behavior random networks, where the relaxation modulus follows theempirical Chasset-Thirion equation. It was found that the long-time dynamic response of thenetworks was nearly insensitive the content of the pendant material but deeply inuenced bythe average molecular mass of these defects. The parameters in the Chasset-Thirion equationare related to the molecular structure of the network and the Rouse time of the pendant chainswhile the exponent in the power law is dictated by the weight average molecular weigh of thependant material. In entangled linear polymers, the slow relaxational dynamics is dictated bythe diusion of the molecules along its own contour, a process known as reptation [Milner andMcLeish, Phys. Rev. Lett., 81, 725, 1998]. Dierently from linear chains, here the danglingmolecules cannot reptate to recover equilibrium congurations and renew their congurationsthrough a dierent mechanism, named arm retraction, in which the end of each arm independentlyretracts partway down its conning tube and then loose the memory of its earlyconguration reemerging along a dierent path [Vega et al., Phys. Rev. Lett., 95, 166002,2005]. This process is entropically unfavorable and in the entangled regime the time scale forcomplete retraction increases roughly exponentially with the size of the branch.