Synthesis and rheology of pH-responsive particles with shape anisotropy
Seeded emulsion polymerization is used to produce large quantities of shape anisotropic, amphoteric particles in a size range of about 1 µm. Two different kinds of shape anisotropic particles are prepared for comparison to study the effects of incorporating pH-responsive groups on the properties of suspensions containing shape anisotropic colloidal particles. Copolymer dicolloids (CDC) containing pyridine groups are synthesized by swelling spherical, lightly cross linked polystyrene seeds with mixture of styrene and 2-vinyl pyridine (2VP) followed by secondary polymerization. Homopolymer dicolloids (HDC) are made in a similar procedure with only pure styrene swollen into seeds. To investigate the effects of weak attractions, the particles are coated with the nonionic surfactant hexaethylene glycol monododecyl ether (C12E6). As confirmed by electrophoretic mobility measurements as a function of pH, the CDC particles continue to show amphoteric properties. States of aggregation of dilute suspensions are mapped as functions of ionic strength and pH. The CDC particles show amphoteric behavior with strong attractions in medium pH range, while HDC remain stable at all pH’s studied (3<pH<9) unless ionic strength is increased to 5M showing that the C12E6 protects against aggregation into a primary van der Waals minimum as would be seen for bare polymer particles. The flow properties of glassy and gelled suspensions of CDC and HDC are studied as a function of volume fraction and pH. HDC particles display a high kinetic arrest volume fraction (ϕg>0.5) with small linear elastic modulus (G0’*) above ϕg at different pH conditions and ionic strengths up to 0.5M proving again that the particles experience repulsive or weakly-attractive conditions. The CDC particles behave in a similar manner at high or low pH at an ionic strength, [I] of 0.001M, but gel at a volume fraction of ϕg<0.3 and display anomalously large G0’* at the gel transition at intermediate pH or at pH=9 and I=0.5M. The modulus and yielding behavior of these suspensions indicate that even at pH=9 or pH= 3.7 and I=0.001 the CDC particles experience weak attractions that cannot be explained as arising from isotropic repulsions as experienced by the HDC particles. At pH=9 and I=0.5M or pH=4.6 and I=0.001M, these anomalous attractions are accentuated. These anomalous attractions are understood as arising from the chemical anisotropy of the CDC particles where the protrusion created in the second polymerization step is rich in poly-2VP while the seed is rich in negatively charged sulfate groups. These chemical differences can give rise to directional interactions due to differences in charge or due to differences in hydrophobicity. A key conclusion drawn from this work is that the signature of directional interactions comes in the way gels and glasses yield when stressed. Gel points are shifted by anisotropic interactions but the same gel points can be generated with isotropic interactions if the attractions are strong enough. Instead, gel composed of particles experiencing anisotropic chemical interactions yield with multiple constraints under conditions where this is not seen for particles with isotropic interactions.
anisotropic particles, seeded emulsion polymerization, pH-response, multiple yielding, anisotropic interaction
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign