Dynamic Response of Polymer Networks with Dangling Molecules

D.C. AGUDELO; E.M. VALLÉS; M.A VILLAR; D.A. VEGA

Córdoba

Workshop; XIII Latin American Workshop on Nonlinear Phenomena, LAWNP 2013; 2013

Universidad Nacional de Córdoba

The equilibrium and dynamic properties of cross-linked polymer networks are highly sensitive to the presence of dangling molecules. These imperfections that are unavoidable produced during a cross-linking reaction, alter the network connectivity and also aect the damping response of elastomers. Here the dynamics of pendant chains trapped in a cross-linked network is 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 for rubber elasticity, the terminal region follows a power law dependence in time. This dynamic response resembles the behavior random networks, where the relaxation modulus follows the empirical Chasset-Thirion equation. It was found that the long-time dynamic response of the networks was nearly insensitive the content of the pendant material but deeply influenced by the average molecular mass of these defects. The parameters in the Chasset-Thirion equation are related to the molecular structure of the network and the Rouse time of the pendant chains while the exponent in the power law is dictated by the weight average molecular weigh of the pendant material. In entangled linear polymers, the slow relaxational dynamics is dictated by the dilusion of the molecules along its own contour, a process known as reptation [Milner and McLeish, Phys. Rev. Lett., 81, 725, 1998]. Dierently from linear chains, here the dangling molecules cannot reptate to recover equilibrium congurations and renew their congurations through a dierent mechanism, named arm retraction, in which the end of each arm independently retracts partway down its conning tube and then loose the memory of its early conguration 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 for complete retraction increases roughly exponentially with the size of the branch.