Investigation of network heterogeneities in filled, trimodal, highly functional PDMS networks by 1H Multiple Quantum NMR Page: 4 of 39
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Gjersing, et al.
ABSTRACT. The segmental order and dynamics of polymer network chains in a filled, tri-modal
silicone foam network have been studied by static 1H Multiple Quantum (MQ) NMR methods to gain
insight into the structure property relationships. The foam materials were synthesized with two different
types of crosslinks, with functionalities, $, of 4 and near 60. The network chains were composed of
distributions of high, low, and medium molecular weight chains. Crosslinking was accomplished by
standard acid catalyzed reactions. MQ NMR methods have detected domains with residual dipolar
couplings (<Qd>) of near 4 kRad/s and 1 kRad/s assigned to (a) the shorter polymer chains and chains
near the multifunctional ($=60) crosslinking sites and to (b) the longer polymer chains far from these
sites. Three structural variables were systematically varied and the mechanical properties via
compression and distributions of residual dipolar couplings measured in order to gain insight in to the
network structural motifs that contribute significantly to the composite properties. The partitioning of
and the average values of the residual dipolar couplings for the two domains were observed to be
dependent on formulation variable and provided increased insight into the network structure of these
materials which are unavailable from swelling and spin-echo methods. The results of this study suggest
that the domains with high crosslink density contribute significantly to the high strain modulus, while
the low crosslink density domains do not. This is in agreement with theories and experimental studies
on silicone bimodal networks over the last 20 years. In-situ MQ-NMR of swollen sample suggests that
the networks deform heterogeneously and non-affinely. The heterogeneity of the deformation process
was observed to depend on the amount of the high functionality crosslinking site PMHS. The NMR
experiments shown here provide increased ability to characterize multimodal networks of typical
engineering silicone foam materials and to gain significant insight into structure-property relationships.
KEYWORDS. siloxanes, bi-modal networks, Multiple quantum NMR, structure-property relationships.
MQ-NMR non-stoichiometric PDMS networks
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Maxwell, R.; Gjersing, E.; Chinn, S.; Giuliani, J.; Herberg, J.; Eastwood, E. et al. Investigation of network heterogeneities in filled, trimodal, highly functional PDMS networks by 1H Multiple Quantum NMR, article, March 20, 2007; Livermore, California. (digital.library.unt.edu/ark:/67531/metadc887917/m1/4/: accessed September 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.