Cross-link point functionallity influence on elastic properties of model PDMS networks

G. A. MONTI; E. DRUETTA; R. H. ACOSTA; D. A. VEGA; M. A. VILLAR; E. M. VALLES

Tarragona, España

Congreso; EUROMAR 2007; 2007

Mechanical properties of polymeric materials have been shown to be related directly to its structure, in the particular case of polymer networks, chemical or physical cross-linking points give distinctive characteristics to these materials such as high elongation, better thermal stability and insolubility. The viscoelastic response of polymer networks has been an outstanding problem on polymer science for several decades [1]. Free chains trapped in the network and pendant chains connected to the gel by one of their end units are the most common type of defects and affect considerably the equilibrium properties of the networks. In order to study their influence model silicone networks are extensively studied. Changes in dynamics and molecular structure can be determined by changes in the residual dipolar couplings by the presence of different type of defects in the network. The determination of these dipolar couplings is carried out by excitation and detection of Double Quantum coherences (DQC). Model PDMS networks with controlled amounts of well-characterized defects have been synthesized [2,3] and used for the study of the influence of a highly controlled variation of structural parameters on mechanical properties by rheological measurements and by nuclear magnetic resonance techniques [3,4,5]. In this work we study the influence of the functionality of the cross-link points on the molecular structure. Trifunctional and tetrafunctional groups are used and the molecular weight of added chains is varied for a fixed molecular weight of elastic chains. Networks prepared with tetrafunctional cross-link points produce structures that are more rigid than the ones prepared with trifunctional groups [2]. The equivalence of a physical entanglement with a chemical cross-link point is determined by inspection of the residual dipolar couplings and elastic modulus values obtained for the different prepared networks.   [1] Doi M., Edwards S.F., The theory of Polymer Dynamics, Clarendon Press, Oxford, U.K. 1986. [2] Villar, M. A; Vallés, E. M. Macromolecules 1996, 29, 4081-4089. [3] Roth, L. E.; Vega, D. A.; Vallés, E. M.; Villar, M. A. Polymer 2004, 45, 5923-5931. [4] Vega, D. A.; Villar, M. A.; Vallés, E. M.; Steren, C. A.; Monti, G. A. Macromolecules 2001, 34, 283-288. [5] Acosta R.H., Vega D.A., Villar M.A., Monti G.A., Vallés E.M., Macromolecules 2006, 39, 4793-4801.