Influence of Sonication on the Molecular Characteristics of Carbopol® and Its Rheological Behavior in Aqueous Dispersions and Microgels

Abstract

The effect of sonication on the molecular characteristics of poliacrylic acid (Carbopol® Ultrez 10) and its rheological behavior in aqueous dispersions and microgels containing 0.25 wt. % of the polymer was analyzed in this work by rheometry, weight-average molecular weight (Mw) measurements via static light scattering (SLS), Fourier transform infrared (FTIR) spectroscopy and confocal microscopy. For this, the precursor dispersion and the microgels were sonicated in a commercial ultrasound bath at constant power and different times. We observed a softening of the microgel microstructure consisting of a systematic decrease in its shear modulus, yield stress and viscosity with increasing sonication time, while their overall Herschel-Bulkley (H-B) behavior was preserved. SLS measurements evidenced a reduction of Mw of polyacrylic acid with sonication time. Separately, FTIR measurements indicate that sonication produces scission in the C-C links of the Carbopol® backbone, which results in chains with the same chemistry but lower molecular weight. Finally, confocal microscopy measurements revealed a concomitant diminution of the size of the microsponge domains with sonication time, which is reflected in a softer microstructure resulting from reduction of the molecular weight of polyacrylic acid. The present results indicate that both the microstructure and the rheological behavior of Carbopol® microgels, in particular, and complex fluids in general, may be manipulated or tailored by high-power ultrasonication.

Introduction

Sonication or ultrasonication is the application of ultrasound energy to a sample, which most often consists of a fluid with dispersed particles. There are two main ways for sonication, namely, by using an ultrasonic bath or a probe sonicator. In the first mode, the fluid in a vessel is set in a water containing ultrasonic bath; in the second, the ultrasonic probe is directly immersed into the fluid of interest. Applications of ultrasound may be roughly divided into low-power (<1 W/cm2) and high frequency (>100 kHz) regime, as well as in high-power (>1 W/cm2) and low-frequency (20–100 kHz) regime, being the first mainly used for non-destructive analysis or materials characterization, while the second is used in industrial processes as well as to produce chemical reactions and changes in the microstructure of materials [1,2].

According to the Royal Society of Chemistry [3], propagation of ultrasonic waves (typically >20 kHz) in a liquid medium results in agitation along with alternating high-pressure (compression) and low-pressure (rarefaction) cycles. During rarefaction, high-intensity sonic waves create small vacuum bubbles or voids in the liquid, which then collapse violently (cavitation) during compression, creating very high local temperatures. Thus, prolonged high-intensity sonication may produce chemical reactions in a sample. This fact, which was known since the early decades of the last century, resulted in the establishment of sonochemistry as a field, after the first international symposium on sonochemistry organized by the Royal Society of Chemistry in 1986 and the influencing reviews by Lorimer and Mason [1] and Lindley and Mason [2], respectively. Afterward, high-intensity sonication has been exploited in many applications, including ultrasonic cleaning, drilling, soldering, chemical processes, emulsification, deagglomeration, extraction, cell disruption [4], and many others as those found in food science and processing [5–7].

An application of sonication of particular interest to this work is the possible manipulation or tailoring of the microstructure of complex fluids, including gels. Gels appear in many everyday products such as cosmetics, pharmaceuticals, detergents, coatings, and foods, among many others. Then, tuning the flow or rheological properties of gels using ultrasound may be of practical relevance. Interestingly, scarce work has been done to understand the gel structural changes and its concomitant rheological behavior arising from sonication. In this regard, Seshadri et al. [8] studied the effect of high-intensity ultrasound (40 W) at various times on the rheological and optical properties of high-methoxyl pectin (HMP) dispersions. These authors found that ultrasonically pretreated pectin dispersions formed weaker gels with increasing sonication power and time and resulted in more transparent gels. The results were attributed to an overall reduction in the average molecular weight of pectin due to cavitational effects. Zheng et al. [9] also analyzed the effects of sonication at different powers (120, 240, 360, and 480 W) on the rheological properties of HMP dispersions and showed that their viscosity was reduced significantly with increasing sonication power and time; meanwhile, the overall pseudoplastic behavior of the gel, was retained. Zheng et al. [9] suggested that the cavitation
effect damaged the structure of HMP as ultrasonic power increased, leading to a significantly decreased strength of the gel.

Recently, Gibaud et al. [10] introduced what they called “rheoacoustic” gels, that is, colloidal gels sensitive to ultrasonic vibrations. These authors used a combination of rheological and structural characterization to evidence and quantify a strong softening, including decreased yield stress and accelerated shear-induced fluidization, in three different colloidal gels submitted to ultrasonic vibrations (with submicron amplitude and frequencies in the range between 20–500 kHz). The softening was attributed to micron-sized cracks within the gel network, which could or could not fully heal, depending on the acoustic intensity, once vibrations are turned off. In this work, the effects of sonication time at a fixed power on the molecular characteristics of Ultrez 10 and its rheological behavior in a 0.25 wt. % dispersion in bi-distilled water and the resulting microgel after neutralization, were analyzed by rheometry, molecular weight measurements via
static light scattering (SLS), Fourier transform infrared (FTIR) spectroscopy and confocal microscopy. For this, the precursor dispersion and the microgel were sonicated in a commercial ultrasound bath at constant power for different times. We observed a softening of the microgel microstructure consisting of a systematic decrease in its shear modulus, yield stress, and viscosity with increasing sonication time, while the Herschel-Bulkley behavior was maintained. SLS measurements evidenced a reduction of Mw of polyacrylic acid with sonication time. Separately, FTIR measurements indicate that sonication produces scission in the C-C links of the Ultrez 10 backbone, which results in chains with the same chemistry but lower molecular weight. Finally, confocal microscopic measurements revealed a concomitant diminution of the size of the microsponge domains, resulting from reduction of the molecular weight of polyacrylic acid with sonication time. Overall, results in this work indicate that both the microstructure and rheological behavior of microgels, in particular, and complex fluids in general, may be manipulated or tailored by high-power ultrasonication.

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Materials

The polymer utilized in this work was a poly(acrylic acid), Carbopol® Ultrez 10 (Lubrizol Corporation). Carbopol® resins are hydrophilic cross-linked acrylic acid polymers differing in cross-link density. The more highly cross-linked members of the Carbopol® family are rigid particles, while the more lightly cross-linked members are delivered as micron-sized powder particles, which can swell to a large extent, being these last best representatives of microgels [33]. When the resin is mixed with water, an acid dispersion is obtained. Upon neutralization with a suitable base the protons in the carboxylate groups are substituted by the cation of the base and the molecules adopt a highly expanded configuration. The as-formed highly swollen and deformable particles resemble individual sponges that give rise to elastoviscoplastic microgels [31]. Carbopol® polymers are used in a variety of applications encompassing the cosmetics, pharmaceutical, paint, and food industries as a thickening, suspending, dispersing, and stabilizing agent. In particular, Carbopol® Ultrez 10 is a multi-purpose polymer for a wide range of applications, such as hair gels and skin care emulsion products.

Source: Pérez-González, J.; Muñoz-Castro, Y.; Rodríguez-González, F.; Marín-Santibáñez, B. M.; Medina-Bañuelos, E. F. Influence of Sonication on the Molecular Characteristics of Carbopol® and Its Rheological Behavior in Aqueous Dispersions and Microgels. Preprints 2024, 2024060259. https://doi.org/10.20944/preprints202406.0259.v1