Purpose. The primary objective of this study is to perform detailed and extensive rheological characterization of rheology of carbomer (Carbopol) microgels formulated using a solvent system typically used in topical gel formulations. Solvents like glycerin and propylene glycol can alter rheology and drug delivery characteristics of topical gels owing to their different viscosities and due to the change in solvent-polymer and solvent-solvent interactions. 
Methods. Aqueous gels with different pH were prepared by dissolving cross-linked Carbopol polymers in a co-solvent system comprising water, propylene glycol, and glycerol and subsequently neutralizing the carboxylic groups of the polymers with triethanolamine (TEA). Oscillatory, steady, and transient shear measurements were performed to measure viscoelastic properties, temperature dependency, yield strength, and thixotropy of carbomer pharmaceutical gels. 
Results. The topical pharmaceutical gels exhibit remarkable temperature stability. Flow curves obtained at different temperatures indicate Carbopol microgels show much more pseudoplastic behavior (lower power law index) compared to Carbopol gels dissolved only in water. Substantial yield strength is required to break the microgel network of the topical gels. The gel samples exhibit modest thixotropy at higher deformation rates. 
Conclusions. The rheological behavior of the Carbopol microgels do not change appreciably in the pH range 5.0-8.0, and the gels can be used as effective dermatological base for topical applications.

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Rheological Characterization of Topical Carbomer Gels Neutralized to Different pH

Principles of Polymer Science and Technology in Cosmetics and Personal Care

The term microgel has been used to describe a variety of particles that differ substantially in structure, physicochemical properties, preparation and application and has been used interchangeably with terms such as nanogel, microsphere and macrogel. Many excellent and wide-ranging reviews have been published on the numerous particle types considered to fall within the broad sphere of nano-/microparticles. The aim of this review is to focus on applications of microgel particles that are synthesised by polymerisation, are of colloidal size and exhibit conformational changes in response to changes in environmental conditions. It is not the intention to attempt to cover every potential microgel application; instead, a selected range of areas will be covered and the commercial implications of scaling up the production of microgels for such purposes will be discussed. A brief description of the characteristics of microgel particles is followed by discussion of applications such as enhanced oil recovery, biomaterials and catalysis, before issues of commercialising microgel production are considered.

Microgel applications and commercial considerations

Rheological measurements were performed on aqueous dispersions of two commercial crosslinked polymer microgels, Carbopol Ultrez 10 and Carbopol ETD 2050, prepared over a wide range of concentration and pH. For all concentrations studied, both the yield stress and the elastic modulus initially increased dramatically with pH and displayed broad peaks at intermediate pH. This is consistent with the onset of jamming of the Carbopol particles due to a rapid increase in particle size caused by osmotic swelling in the presence of NaOH. Scaling of both yield stress and elasticity with concentration was observed only at higher concentrations, which we believe indicates a change from a percolated structure at low volume fractions to a space filling network of compressed particles at high volume fractions. This model is supported by confocal microscopy of fluorescently dyed Carbopol dispersions.

Scaling and mesostructure of Carbopol dispersions

Rheological techniques are used to probe the behavior of hydrophobic alkali-swellable emulsion (HASE) polymers, bearing n-alkyl hydrophobes, in aqueous alkaline media. The polymers possess a comb-like architecture with a polyelectrolyte backbone (ethyl acrylate-co-methacrylic acid) and hydrophobes (∼16 per polymer chain) tethered to the backbone via polyether side chains. The size of the hydrophobes is varied from n-C8 to n-C20 in this study. It is shown that, at such a level of hydrophobic modification, and at relatively high polymer concentrations, the microstructure in these polymer systems is akin to that existing in concentrated microgels. Thus, the original polymer latex particles swell extensively in alkaline media and disintegrate to form a system of close-packed, compressible (“soft”) aggregates. This is reflected in the rheological response of the system where we observe a high steady shear viscosity with no zero-shear plateau at low shear rates followed by considerable shear thinning and, a characteristic power-law behavior (G′, G″∼ω0.4) under oscillatory shear persisting over a broad range of time scales. Concentration-independent master curves are obtained for the storage modulus, G′, with the level of G′ increasing with hydrophobe size. The similarity in the dynamic response suggests that there exists a qualitative equivalence in microstructure over the range of systems, the only difference being the “softness” or compressibility of the particles. Data from this study are also contrasted with those for a similar HASE polymer bearing a smaller number of alkylaryl hydrophobes [J. Rheol. 41, 427–444 (1997)]. In the latter case, the rheology can be interpreted in terms of hydrophobic associations and chain entanglements occurring in solution. Thus, subtle variations in molecular architecture are shown to cause significant differences in morphology and microstructure for these polymer systems.Rheological techniques are used to probe the behavior of hydrophobic alkali-swellable emulsion (HASE) polymers, bearing n-alkyl hydrophobes, in aqueous alkaline media. The polymers possess a comb-like architecture with a polyelectrolyte backbone (ethyl acrylate-co-methacrylic acid) and hydrophobes (∼16 per polymer chain) tethered to the backbone via polyether side chains. The size of the hydrophobes is varied from n-C8 to n-C20 in this study. It is shown that, at such a level of hydrophobic modification, and at relatively high polymer concentrations, the microstructure in these polymer systems is akin to that existing in concentrated microgels. Thus, the original polymer latex particles swell extensively in alkaline media and disintegrate to form a system of close-packed, compressible (“soft”) aggregates. This is reflected in the rheological response of the system where we observe a high steady shear viscosity with no zero-shear plateau at low shear rates followed by considerable shear thinning and, a cha...

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Associative polymers bearing n-alkyl hydrophobes: Rheological evidence for microgel-like behavior

The rheological properties of a series of lightly crosslinked carboxy copolymers in aqueous solutions have been evaluated in steady shear and dynamic oscillatory modes. Viscosity profiles and the behavior of storage modulus are related to the chemical composition of the copolymers and their crosslinking density. A maximum in viscosity and in storage modulus which depends on the type of crosslinking agent used is explained by a combination of a chain entanglement mechanism and a closely-packed spheres model. The recovery of viscosity and storage modulus after shearing is very fast and is related to the very fast rearrangement of the microgel structure as a function of time.

W. W. Reichert

Papers

Rheological properties of lightly crosslinked carboxy copolymers in aqueous solutions