Testing pendant chain dynamics in PDMS model networks by NOE measurements

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

Congreso; IV Iberoamerican NMR Meting; 2012

Portuguese Cehmical Society

Over the last 5 decades the dynamic response of entangled polymer melts has been an outstanding problem in polymer science. Most of the viscoelastic and diffusive properties of polymer melts and concentrated polymer solutions are profoundly influenced by topological interactions. The most successful model to deal with topological constrains is the tube model1. According to this model, the topological confinement exerted on a given molecule by the surrounding media can be modeled as an hypothetical tube that severely suppress the motion perpendicular to the tube?s local axis, but permits the diffusion along the tube. Polydimethylsiloxane (PDMS) networks are ideal systems to probe chain dynamics. In PDMS networks entangled and dangling chains have the same chemical structure. This means that the whole sample has the same spectroscopic components. In this work, in an attempt to probe pendant chains dynamic, we applied a pulse sequence that uses the differences between proton transverse relaxation times to generate a magnetization gradient and also to determine the magnetization transfer via Nuclear Overhauser Effect (NOE)2. Experiments were conducted on tri and tetra-functional networks with pendant chains of different molecular weight3. For selected tri and tetra-functional networks, experiments as a function of the temperature were carried out. In these cases NOE magnetization transfer responds to an Arrhenius process, enabling the determination of activation energies.