The Swelling of Poly(Isopropylacrylamide) Near the θ Temperature: A Comparison between Linear and Cross‐Linked Chains

Lopez, Carlos G.; Scotti, Andrea; Brugnoni, Monia; Richtering, Walter

doi:10.1002/macp.201800421

Cited by 17 publications

(17 citation statements)

References 122 publications

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“…This can be considered as a lower estimate of σ, according to different types of measurements for linear PNIPAM chains. 44 Note that the use of a larger value of σ would significantly decrease λ B , thus resulting in a very small effect of the Debye-Hückel repulsion as compared to the neutral case.…”

Section: Monomer Interactionsmentioning

confidence: 99%

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Numerical insights on ionic microgels: structure and swelling behaviour

et al. 2019

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Recent progress has been made in the numerical modelling of neutral microgel particles with a realistic, disordered structure. In this work we extend this approach to the case of co-polymerised microgels where a thermoresponsive polymer is mixed with acidic groups. We compare the cases where counterions directly interact with microgel charges or are modelled implicitly through a Debye-Hückel description. We do so by performing extensive numerical simulations of single microgels across the volume phase transition varying the temperature and the fraction of charged monomers. We find that the presence of charges considerably alters the microgel structure, quantified by the monomer density profiles and by the form factors of the microgels, particularly close to the volume phase transition (VPT). We observe significant deviations between the implicit and explicit models, with the latter comparing more favourably to available experiments. In particular, we observe a shift of the VPT temperature to larger values as the amount of charged monomers increases. We also find that below the VPT the microgel-counterion complex is almost neutral, while it develops a net charge above the VPT. Interestingly, under these conditions the collapsed microgel still retains a large amount of counterions inside its structure. Since these interesting features cannot be captured by the implicit model, our results show that it is crucial to explicitly include the counterions in order to realistically model ionic thermoresponsive microgels.

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Section: Monomer Interactionsmentioning

confidence: 99%

“…Kuhn length of both neutral NIPAM and charged AAc monomers. This can be considered as a lower estimate of σ, according to different types of measurements for linear PNIPAM chains 44. …”

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confidence: 99%

Numerical insights on ionic microgels: structure and swelling behaviour

et al. 2019

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“…Since the NIPAAm and NAPMAm monomers have roughly the same backbone length of l ≈ 0.25 nm, we can estimate the strand size in a gel composed of these monomers using the Kuhn length l k found in linear poly(isopropylacrylamide) (PNIPAM) in aqueous solution. Recently, Lopez et al reported an estimated Kuhn length l k ≈ 4 ± 1 nm for linear PNIPAM by fitting static and dynamic light scattering (SLS/DLS) data on the chain hydrodynamic radius and R g to the ideal worm-like chain model, while previous single-molecule force spectroscopy (SMFS) experiments rendered a much smaller average value of l k ≈ 0.7 nm, see ref and references therein. Taking the SLS/DLS value of l k ≈ 4 nm, there are l k / l ≈ 16 NIPAAm monomers per Kuhn segment in a linear PNIPAM.…”

Section: Resultsmentioning

confidence: 99%

“…In the 20% gel samples we used, the average number of 36.25 NIPAAm/NAPMAm monomers per strand corresponds to N k ≈ 2.26 Kuhn segments. The average network strand sizes in the absence of Ru(bpy) 3 groups can then be estimated as R e ≈ N k 1/2 l k ≈ 6.02 nm and R g ≈ 2.46 nm using the Gaussian chain model and R e ≈ 5.32 nm and R g ≈ 1.85 nm using the ideal worm-like chain model. , If the SMFS value of l k ≈ 0.7 nm is used instead, each strand would have N k ≈ 12.95 Kuhn segments, giving estimated strand sizes R e ≈ 2.52 nm and R g ≈ 1.03 nm using the Gaussian chain model and R e ≈ 2.47 nm and R g ≈ 0.97 nm using the ideal worm-like chain model. On the other hand, the Ru(bpy) 3 complex takes a spherical shape with an estimated diameter of approximately 1.3 nm, which is of comparable order to the gel strand size.…”

Section: Resultsmentioning

confidence: 99%

Delayed Mechanical Response to Chemical Kinetics in Self-Oscillating Hydrogels Driven by the Belousov–Zhabotinsky Reaction

et al. 2021

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We show experimentally that chemical and mechanical self-oscillations in Belousov–Zhabotinsky hydrogels are inherently asynchronous, that is, there is a detectable delay in swelling–deswelling response after a change in the chemical redox state. This phenomenon is observable in many previous experimental studies and potentially has far-reaching implications for the functionality and response time of the material in future applications; however, so far, it has not been quantified or reported systematically. Here, we provide a comprehensive qualitative and quantitative description of the chemical-to-mechanical delay, and we propose to explain it as a consequence of the slow nonequilibrium swelling–deswelling dynamics of the polymer material. Specifically, standard hydrogel pieces are large enough that transport processes, for example, counterion migration and water diffusion, cannot occur instantaneously throughout the entire gel piece, as opposed to previous theoretical considerations. As a result, the volume response of the polymer to a chemical change may be governed by a characteristic response time, which leads to the emergence of delay in mechanical oscillation. This is supported by our theoretical calculations.

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“…58 In all cases, increasing temperature results in a significant decrease in the liquid viscosity since it reduces the particle size. 59 In this work, we deal with small linear PNIPAM microgels in comparatively low solution concentrations. Their viscosity does not change substantially as a function of the shear rate, and the assumption of being a Newtonian liquid may be valid (Figure S17 in the Supporting Information).…”

Section: ■ Experimental Sectionmentioning

confidence: 99%

Visualization of Acoustic Energy Absorption in Confined Aqueous Solutions by PNIPAM Microgels: Effects of Bulk Viscosity

et al. 2021

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Ultrasound propagation in liquids is highly influenced by its attenuation due to viscous damping. The dissipated energy will be partially absorbed by the liquid due to its dynamic viscosity as well as its bulk viscosity. The former results in the generation of a flow that is called acoustic streaming, and the latter is associated with the vibrational and rotational relaxation of liquid molecules. Measuring the ultrasonic wave attenuation due to the bulk viscosity is presented as a novel method in this article. Poly(Nisopropylacrylamide) (PNIPAM) microgels, which are soluble in several solvents such as water, were used as acousto-responsive markers in water, which upon absorption of ultrasonic energy undergo a volume phase transition due to the breakage of their hydrogen bonds. Thus, they become insoluble in water, and due to shrinking, their optical density increases. As a result, their agglomeration can be seen as a turbid medium. We managed to visualize the ultrasonic energy absorption due to the bulk viscosity using the turbidity since the excess acoustic energy on top of the absorbed energy for the translational motion of liquid is spent to break the hydrogen bonds between PNIPAM and water. In addition, to quantify the turbidity phenomenon, the total energy required for breaking hydrogen bonds in the solution is calculated, and its evolution, according to the input power intensity, is quantified by image processing. The effect of viscosity by changing the microgel concentration was investigated, and it is shown that an increasing microgel concentration increases the acoustic energy absorption rate much greater than its dynamic viscosity. Therefore, the bulk viscosity, as the responsible parameter for this increase, is measured directly from the energy of broken hydrogen bonds. The results show that at low solution concentration (0.2 wt %) the bulk viscosity is in the same order of magnitude as its dynamic viscosity. Increasing the concentrations to 1 and 5 wt % increases the bulk viscosity and consequently the structural relaxation time by 1 and 2 orders of magnitude, respectively.

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The Swelling of Poly(Isopropylacrylamide) Near the θ Temperature: A Comparison between Linear and Cross‐Linked Chains

Cited by 17 publications

References 122 publications

Numerical insights on ionic microgels: structure and swelling behaviour

Numerical insights on ionic microgels: structure and swelling behaviour

Delayed Mechanical Response to Chemical Kinetics in Self-Oscillating Hydrogels Driven by the Belousov–Zhabotinsky Reaction

Visualization of Acoustic Energy Absorption in Confined Aqueous Solutions by PNIPAM Microgels: Effects of Bulk Viscosity

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