A comparison of the network structure and inner dynamics of homogeneously and heterogeneously crosslinked PNIPAM microgels with high crosslinker content

Witte, Judith; Kyrey, Tetyana; Lutzki, Jana; Dahl, Anna Margarethe; Houston, Judith E.; Rădulescu, Aurel; Pipich, Vitaliy; Stingaciu, Laura R.; Kühnhammer, Matthias; Witt, Marcus U.; Klitzing, Regine von; Holderer, Olaf; Wellert, Stefan

doi:10.1039/c8sm02141d

Cited by 46 publications

(44 citation statements)

References 74 publications

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“…Two other methods that provide nanoscale, polymer-level, information on microgels, averaged over a large volume, are neutron spin-echo (NSE) and nuclear magnetic resonance (NMR). The methods can provide valuable information about the local dynamics and fluctuations that can then be linked to the heterogeneity of the gel network 35,36 .…”

Section: Microgel Characterization With Nanoscale Resolutionmentioning

confidence: 99%

“… a Sketch of a macroscopic polymer gel with a statistically homogeneous distribution of permanent or reversible cross-links. In a good solvent, the gel is swollen, and the degree of swelling is controlled by a balance between the mixing entropy, favoring swelling, and the elastic penalty when stretching the polymer chains between cross-links 21 , 36 . b Volume-phase transition (VPT) of stimuli-responsive microgel particles.…”

Section: Introductionmentioning

confidence: 99%

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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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Section: Microgel Characterization With Nanoscale Resolutionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Pathways and challenges towards a complete characterization of microgels

Scheffold

2020

Nat Commun

103

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“…Interestingly, a core-shell structure was observed in all the polymers. This might be due to an uneven distribution of cross-linking density caused by the reactivity difference between MAA and the cross-linkers [ 22 , 23 ].…”

Section: Resultsmentioning

confidence: 99%

Development of Neutral pH-Responsive Microgels by Tuning Cross-Linking Conditions

Okada

Takayasu

Tomita

et al. 2020

Sensors

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Polymer microgels that respond in a range of neutral pH can be useful for the development of molecular imaging tools and drug-delivery carriers. Here, we describe a simple approach in developing microgels that undergo volume phase transitions and substantial nuclear magnetic resonance (NMR) relaxometric changes within a narrow pH range of 6.4 to 7.4. The pH-responsive microgels were synthesized using methacrylic acid and a series of ethylene glycol dimethacrylate cross-linkers with repeating units of ethylene glycol that range from one to four. NMR relaxometry demonstrated that the transverse relaxation time (T2) of a suspension containing microgels that were cross-linked with diethylene glycol dimethacrylate sharply decreases at the pH where volume phase transition occurs. The polymer microgels cross-linked with 40 and 45 mol% of diethylene glycol dimethacrylate caused about 50% T2 reduction with decreasing pH from 6.8 to 6.4. These results demonstrated that responses of microgels to a range of neutral pH can be easily tuned by using appropriate cross-linkers with certain cross-linking degree. This approach can be useful in developing highly sensitive molecular sensors for magnetic resonance imaging (MRI) of tissue pH values.

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“…Single-compartment microgels (“core-only“) can be described in first approximation by homogeneous spheres 30 , but due to the different chemical reactivities of monomers and cross-linker, the particles have soon been recognized to have a fuzzy structure: dense inside, and progressively less dense towards the outside 31 . Different feeding strategies during synthesis have been used to control the degree of fuzziness 32 . Two different “fuzzy sphere” form factors have been proposed in the past to describe such profiles, either by convoluting with a Gaussian 9 , or by adding a polynomial decay to the homogeneous sphere 27 .…”

Section: Introductionmentioning

confidence: 99%

Spatial distribution of core monomers in acrylamide-based core-shell microgels with linear swelling behaviour

Cors

Wrede

Wiehemeier

et al. 2019

Sci Rep

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The peculiar linear temperature-dependent swelling of core-shell microgels has been conjectured to be linked to the core-shell architecture combining materials of different transition temperatures. Here the structure of pNIPMAM-core and pNNPAM-shell microgels in water is studied as a function of temperature using small-angle neutron scattering with selective deuteration. Photon correlation spectroscopy is used to scrutinize the swelling behaviour of the colloidal particles and reveals linear swelling. Moreover, these experiments are also employed to check the influence of deuteration on swelling. Using a form-free multi-shell reverse Monte Carlo approach, the small-angle scattering data are converted into radial monomer density profiles. The comparison of ‘core-only’ particles consisting of identical cores to fully hydrogenated core-shell microgels, and finally to H-core/D-shell architectures unambiguously shows that core and shell monomers display gradient profiles with strong interpenetration, leading to cores embedded in shells which are bigger than their isolated ‘core-only’ precursor particles. This surprising result is further generalized to different core cross-linker contents, for temperature ranges encompassing both transitions. Our analysis demonstrates that the internal structure of pNIPMAM-core and pNNPAM-shell microgels is heterogeneous and strongly interpenetrated, presumably allowing only progressive core swelling at temperatures intermediate to both transition temperatures, thus promoting linear swelling behaviour.

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A comparison of the network structure and inner dynamics of homogeneously and heterogeneously crosslinked PNIPAM microgels with high crosslinker content

Cited by 46 publications

References 74 publications

Pathways and challenges towards a complete characterization of microgels

Pathways and challenges towards a complete characterization of microgels

Development of Neutral pH-Responsive Microgels by Tuning Cross-Linking Conditions

Spatial distribution of core monomers in acrylamide-based core-shell microgels with linear swelling behaviour

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