Steric Effects on the Rheology of Nanocomposite Genls of Organoclay in Dicarboxyl-Terminated Polybutadiene
Nanocomposite gels were formed by mixing organically modified clay into a linear, end-functionalized polymer (dicarboxyl-terminated polybutadiene). Two differently sized but otherwise similar counterions were chosen for preparing the organoclay. Hydrogen bonding between polymer and clay causes the polymer/clay interface to grow by splitting the clay aggregates into smaller clay particles, then swelling these particles, exfoliating the clay sheets, and eventually assuming a stable dispersion in the polymer matrix. The clay with the larger counterion exfoliates faster, but does not form the stronger network (lower modulus, lower yield stress), and it needs more clay to reach its gel point (percolation threshold [curly or open phi]c). These seemingly contradictory observations (fast exfoliation but weak gel and later gel point) are attributed to steric effects of the larger macro-counterion. Parameters of the study are clay concentration [curly or open phi] and distance from the gel point. The low frequency linear viscoelastic behavior was analyzed using a percolation model (near [curly or open phi]c) and a power law in concentration (far above [curly or open phi]c). The use of two different organoclays allows comparison of the observed phenomena. The extent of agreement between experimental data and known models was used to theorize that the particle–polymer interactions are the controlling factor in the increasing solid-like behavior with increasing clay content.
H Henning Winter, M. Siebenburger, D. Hajnal, O. Henrich, and M. Fuchs. "Steric Effects on the Rheology of Nanocomposite Genls of Organoclay in Dicarboxyl-Terminated Polybutadiene" Soft Matter 6 (2010): 2442-2448.
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