- K. Saalwächter, P. Ziegler, O. Spyckerelle, B. Haidar, A. Vidal, J.-U. Sommer.
1H Multiple-Quantum Nuclear Magnetic Resonance
Investigations of Molecular Order Distributions in Poly(dimethylsiloxane)
Networks: Evidence for a Linear Mixing Law in Bimodal Systems.
J. Chem. Phys. 119, 3468-3482 (2003).
We present proton-proton multiple-quantum investigations on a series of
monomodal and strongly bimodal end-linked poly(dimethylsiloxane) model networks.
A robust pulse sequence characterized by a well-defined double-quantum
Hamiltonian along with a specific normalization approach is used to obtain
double-quantum build-up curves. These curves are analyzed in terms of the spin
dynamics of a local subsystem of monomer-fixed spins, where analytical fitting
functions yielding residual dipole-dipole coupling constants are derived on the
basis of exact solutions provided by simulations. Further employing the novel
experimental strategy of double-quantum pre-selection of elastically active
network chains, it is shown that the network response is purely heterogeneous,
and that the data can be analyzed in terms of distributions of local dynamic
order parameters using different models. The results yield consistent proof that
local chain order in bimodal networks obeys a linear mixing law of short- and
long-chain components. The order parameter distribution in a long-chain
monomodal network is found to be surprisingly narrow, with a rather high average
order parameter. Implications on the validity of present theories used to
explain order and dynamics in networks are discussed.