Rotational Dynamics of Naphthalene-Labeled Cross-link Junctions in Poly(dimethylsiloxane) Elastomers

Abstract

A series of end-linked poly(dimethylsiloxane) (PDMS) networks were prepared with different cross-link functionalities and molecular weights. This was achieved by simultaneous end-linking and self-condensation of a trifunctional silane cross-link precursor. These networks had a nonpolar naphthalene chromophore covalently attached to a fraction of the cross-link junctions. We probe the time-dependent reorientation of the naphthalene, and infer reorientation of the cross-links, by determining the time-dependence of the fluorescence depolarization in the picosecond time domain. A two-step relaxation model describes the orientational dynamics. Fast, partial depolarization in a restricted geometry is superimposed on a slower relaxation that completely depolarizes the fluorescence. We determine the two rotational diffusion constants at temperatures varying from 235 to 298 K, while we vary network parameters such as cross-link density, molecular weight, and macroscopic strain. These diffusion constants have an Arrhenius activation energy of 11.4 ( 0.8 kJ/mol. The fast relaxation is driven by motions of a few chain segments; this process is dominated by the density of the network polymer around the labeled cross-links. The slower, complete reorientation is driven by cooperative motions of a larger number of chain segments connected to the cross-link that are insensitive to steric constraints in the immediate vicinity of the cross-links. I. Introduction Model networks of known molecular weight between crosslinks, Mc, and cross-link functionality, , are used to test molecular theories of rubber elasticity. These networks can be prepared by end-linking telechelic polymers using a-functional cross-linking precursor. For this study, we prepared PDMS elastomers through the condensation reaction of hydroxyterminated PDMS with hydrolyzed trifunctional alkoxysilanes. In the presence of water and a catalyst this reaction is not quantitative, since the hydrolyzed alkoxysilanes will react with each other in a self-condensation reaction, forming silane clusters with an effective average functionality avg > , resulting in an elastically active cross-link density, Ia. The relative rates of the end-linking and homopolymerization reactions will determine the size and functionality distribution of the cross-link junctions and other network structure parameters. By manipulation of the stoichiometry, the resulting PDMS network structure and, hence, the mechanical, transport, and swelling properties can be controlled. We determine the swelling behavior of the various PDMS networks and use swelling theory to make assumptions about the network structure.