Segregation of mass at the periphery of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>N</mml:mi></mml:math>-isopropylacrylamide-co-acrylic-acid microgels at high temperatures

Hyatt, John S.; Do, Changwoo; Choi, Hong Sung; Kim, Jinwoong; Lyon, L. Andrew; Fernández‐Nieves, Alberto

doi:10.1103/physreve.92.030302

Cited by 11 publications

(10 citation statements)

References 16 publications

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“…For crosslinked pNIPAM microgels synthesized without AAc or another comonomer, R g /R h typically increases with T , approaching the homogeneous sphere value at high T [4]. Previous experiments performed on the same crosslinked microgels used in this study found R g /R h to increase above 3/5 at high T and low pH, where the AAc groups are not ionized [6]. Thus, since the only difference in the pH 5 suspensions of crosslinked microgels in this study is the presence of partially ionized AAc, the decreasing and unusually low R g /R h must be a direct consequence of charge.…”

Section: Crosslinked Microgelssupporting

confidence: 53%

“…Using microgels composed of NIPAM copolymerized with acrylic acid (AAc) and crosslinked with poly(ethylene glycol) diacrylate (pEG-d), we recently showed that at pH 3, where the AAc is uncharged, this copolymer can frustrate the collapse of pNIPAM at high temperatures, leading to heterogeneous clumping of polymer within the particle and concentration of mass at the periphery [6]. In this work, we observe a segregation of mass at the particle center of the same particles due to a completely different mechanism when the AAc groups are partially charged.…”

Section: Introductionmentioning

confidence: 99%

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Charge segregation in weakly ionized microgels

et al. 2017

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We investigate microgels synthesized from N-isopropylacrylamide (NIPAM) copolymerized with a large mol% of acrylic acid, finding that when the acid groups are partially ionized at high temperatures, competition between ion-induced swelling and hydrophobic deswelling of poly(NIPAM) chains results in microphase separation. In cross-linked microgels, this manifests as a dramatic decrease in the ratio between the radius of gyration and the hydrodynamic radius to ∼0.2, indicating that almost all the mass of the microgel is concentrated near the particle center. We also observe a concurrent decrease of the polymer network length scale via small-angle neutron scattering, confirming the presence of a dense, deswollen core surrounded by a diffuse, charged periphery. We compare these results to those obtained for a system of charged ultralow-cross-linked microgels; the form factor shows a distinct peak at high q when the temperature exceeds a threshold value. We successfully fit the form factor to theory developed to describe scattering from weakly charged gels in poor solvents, and we tie this behavior to charge segregation in the case of the cross-linked microgels.

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Section: Crosslinked Microgelssupporting

confidence: 53%

Section: Introductionmentioning

confidence: 99%

Charge segregation in weakly ionized microgels

et al. 2017

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“…During the microgel synthesis, the degrees of freedom for the polymer strands are intentionally reduced by forming covalent cross-links between polymer chains. The chemical cross-linking itself is a nonequilibrium process, and it does not necessarily progress uniformly, which may lead to spatial fluctuations, gradients, and heterogeneities in the microgel particle architecture 20 . For a complete understanding of a microgel colloid as a material building block or a carrier, it is imperative to know the spatial distribution of polymer and cross-link densities on the nanoscale.…”

Section: Introductionmentioning

confidence: 99%

Pathways and challenges towards a complete characterization of microgels

Scheffold

2020

Nat Commun

103

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Due to their controlled size, sensitivity to external stimuli, and ease-of-use, microgel colloids are unique building blocks for soft materials made by crosslinking polymers on the micrometer scale. Despite the plethora of work published, many questions about their internal structure, interactions, and phase behavior are still open. The reasons for this lack of understanding are the challenges arising from the small size of the microgel particles, complex pairwise interactions, and their solvent permeability. Here we describe pathways toward a complete understanding of microgel colloids based on recent experimental advances in nanoscale characterization, such as super-resolution microscopy, scattering methods, and modeling. T he term "microgel" usually refers to a cross-linked polymer network of colloidal size that is swollen in a good solvent and has a diameter between~100 nm and several micrometers 1-3 (Fig. 1). Colloidal hydrogels are a common subtype where the continuous phase is water. Often, but not always, these networks exhibit conformational changes in response to changes in their environment, such as the solvent composition or temperature 1,4,5. The sensitivity to external stimuli, such as temperature, pH, or solvent conditions, is an essential feature of many microgels, in particular for the most widely studied type of microgels, based on pNIPAM (poly-N-isopropylacrylamide) and its derivatives 1,6 (Fig. 1b). Here, we consider homogeneous suspensions of these particles at different densities from the dilute to the highly packed regime, as well as microgels adsorbed at surfaces. It is also possible to synthesize larger, millimeter-sized, gel particles 7 , or to study two-dimensional assemblies at an interface 8,9. Additional functions, for example, for chemical transformation or sensing applications 10,11 , can be added using core-shell particles or by decorating the microgels with inorganic nanoparticles 12,13. Microgels are already used in several applications as viscosity modifiers and lubricants 14 , for CO 2 capturing 15 or 3D bioprinting 16 , as biocompatible additives and delivery vehicles 17 , sensors, or stimuli-responsive color-changing systems 18,19. Despite their popularity and widespread application, the internal structure of microgels is not well-understood. During the microgel synthesis, the degrees of freedom for the polymer strands are intentionally reduced by forming covalent cross-links between polymer chains. The chemical crosslinking itself is a nonequilibrium process, and it does not necessarily progress uniformly, which may lead to spatial fluctuations, gradients, and heterogeneities in the microgel particle architecture 20. For a complete understanding of a microgel colloid as a material building block or a carrier, it is imperative to know the spatial distribution of polymer and cross-link densities on the nanoscale. Initially, many studies assumed that microgels could be described simply as spherical, homogeneous gel particles, as discussed in ref. 1. Based on this assumption, t...

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“…Notably, these are not synonymous with the model‐inferred mesh sizes (Figure 3D) which are on the order of nanometers, consistent with the expectations of a gel [ 41,42 ] including P(AAc‐co‐NIPAm) networks. [ 43 ] As such, what is observed in Figure 4 is the hierarchical mesh of the gels, indicating that (at least in the dry state) chains cluster together. Note that for both gels, heterogeneity may be exacerbated by the fact that the scans depicted in Figure 4 were taken from the cross‐sectional area of the fracture plane, where significant damage must have occurred.…”

Section: Mechanical Effects Of Compositionmentioning

confidence: 82%

Thermosensitive P(AAc‐co‐NIPAm) Hydrogels Display Enhanced Toughness and Self‐Healing via Ion–Ligand Interactions

Wagner

et al. 2022

Macromol. Rapid Commun.

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Hydrogels containing thermosensitive polymers such as poly(N‐isopropylacrylamide) (P(NIPAm)) may contract during heating and show great promise in fields ranging from soft robotics to thermosensitive biosensors. However, these gels often exhibit low stiffness, tensile strength, and mechanical toughness, limiting their applicability. Through copolymerization of P(NIPAm) with poly(Acrylic acid) (P(AAc)) and introduction of ferric ions (Fe3+) that coordinate with functional groups along the P(AAc) chains, here a thermoresponsive hydrogel with enhanced mechanical extensibility, strength, and toughness is introduced. Using both experimentation and constitutive modeling, it is found that increasing the ratio of m(AAc):m(NIPAm) in the prepolymer decreases strength and toughness but improves extensibility. In contrast, increasing Fe3+ concentration generally improves strength and toughness with little decrease in extensibility. Due to reversible coordination of the Fe3+ bonds, these gels display excellent recovery of mechanical strength during cyclic loading and self‐healing ability. While thermosensitive contraction imbued by the underlying P(NIPAm) decreases slightly with increased Fe3+ concentration, the temperature transition range is widened and shifted upward toward that of human body temperature (between 30 and 40 °C), perhaps rendering these gels suitable as in vivo biosensors. Finally, these gels display excellent adsorptive properties with a variety of materials, rendering them possible candidates in adhesive applications.

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Segregation of mass at the periphery ofN-isopropylacrylamide-co-acrylic-acid microgels at high temperatures

Cited by 11 publications

References 16 publications

Charge segregation in weakly ionized microgels

Charge segregation in weakly ionized microgels

Pathways and challenges towards a complete characterization of microgels

Thermosensitive P(AAc‐co‐NIPAm) Hydrogels Display Enhanced Toughness and Self‐Healing via Ion–Ligand Interactions

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