ISSN:
1551-2916
Quelle:
Blackwell Publishing Journal Backfiles 1879-2005
Thema:
Maschinenbau
,
Physik
Notizen:
A water-soluble diblock copolymer, poly[(methacrylic acid)-b-(ethylene oxide)], has been used to formulate alumina slurries that are dispersed over a wide range of pH through a combination of electrostatic and steric repulsion. The anionic poly(methacrylic acid) block, a short chain (MW∼700 g/mol), interacts with the amphoteric alumina surface at pH 3 and acts as an anchor block. The nonionic poly(ethylene oxide) (PEO) block (MW= 3000 g/mol) only appears to have a steric functionality. At high pH, the polymer coated surface has a net negative charge due to the excess negative sites associated with the highly dissociated methacrylic acid (MAA) units; this provides electrostatic repulsion at, for example, pH 9, which is the isoelectric point for pure alumina. The PEO chains extend from the surface and stabilize the alumina particles between pH 5 to 9 via an entropy-driven, steric repulsion. The PEO chains provide the dominating dispersive force at low pH, where very little electrostatic repulsion is possible because the positive surface charge of alumina has been neutralized by the negative sites of the adsorbed polyacid. On consolidation through pressure filtration at low pressures (2 to 5 MPa), the saturated, consolidated bodies formulated at pH 5 have a relative density of ∼0.56. These consolidated bodies initially exhibit a flow stress, but then fluidize after an extended strain (up to 0.5). The fluidization phenomenon is believed to be due to the unraveling of the PEO chains that appear to entangle between adjacent particles during consolidation. The unraveling was directly observed with an atomic force microscope between two BaTiO3 surfaces coated with the same diblock copolymer used for the alumina powder.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1111/j.1151-2916.2000.tb01435.x
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