Semidilute Polymer Solutions Research Articles

Universal scaling of the diffusivity of dendrimers in a semidilute solution of linear polymers.

The static and dynamic properties of dendrimers in semidilute solutions of linear chains of comparable size are investigated using Brownian dynamics simulations. The radius of gyration and diffusivity of a wide variety of low generation dendrimers and linear chains in solution follow universal scaling laws independent of their topology. Analysis of the shape functions and internal density of dendrimers shows that they are more spherical than linear chains and have a dense core. At intermediate times, dendrimers become subdiffusive, with an exponent higher than that previously reported for nanoparticles in semidilute polymer solutions. The long-time diffusivity of dendrimers does not follow theoretical predictions for nanoparticles. We propose a new scaling law for the long-time diffusion coefficients of dendrimers which accounts for the fact that, unlike nanoparticles, dendrimers shrink with an increase in background solution concentration. Analysis of the properties of a special case of a higher functionality dendrimer shows a transition from polymer-like to nanoparticle-like behaviour.

Activity-induced stiffness, entanglement network and dynamic slowdown in unentangled semidilute polymer solutions.

Active polymers possess numerous unique properties that are quite different from those observed in the system of small active molecules due to the intricate interplay between their activity and topological constraints. This study focuses on the conformational changes induced by activity, impacting effective stiffness and crucially influencing entanglement and dynamics. When the two terminals of a linear chain undergo active modification through coupling to a high-temperature thermal bath, there is a substantial increase in chain size, indicating a notable enhancement in effective stiffness. Unlike in passive semiflexible chains where stiffness predominantly affects local bond angles, activity-induced stiffness manifests at the scale of tens of monomers. While activity raises the ambient temperature, it significantly decreases diffusion by over an order of magnitude. The slowdown of the dynamics observed can be attributed to increased entanglement due to chain elongation.

Rheological behavior of polysaccharide hydrogels of alginate reinforced by small amount of halloysite nanotubes for extrusion 3D printing

The rheological properties of hydrogels of a natural polysaccharide sodium alginate and small amount of clay nanotubes of halloysite were investigated. Changes of rheological properties during the transition from a semi-dilute polymer solution to a hydrogel upon cross-linking by calcium ions were shown. In the gel state, the samples have a yield stress, and their viscosity decreases with the shear rate, but the properties are quickly recovered after the load removal. It was obtained that the addition of up to 0.3 vol.% nanotubes of natural clay halloysite leads to an increase by several times of a storage modulus and an yield stress of the hydrogels. At the same time, the practically important properties of shear thinning and the rapid recovery of properties after the load removing make the nanocomposite hydrogels of alginate and halloysite nanotubes promising for use as ink for extrusion 3D printing.

Stochastic energetics of a colloidal particle trapped in a viscoelastic bath

We investigate the statistics of the fluctuations of the energy transfer between an overdamped Brownian particle, whose motion is confined by a stationary harmonic potential, and a surrounding viscoelastic fluid at constant temperature. We derive an analytical expression for the probability density function of the energy exchanged with the fluid over a finite time interval, which implicitly involves the friction memory kernel that encodes the coupling with such a non-Markovian environment, and reduces to the well known expression for the heat distribution in a viscous fluid. We show that, while the odd moments of this distribution are zero, the even moments can be explicitly expressed in terms of the autocorrelation function of the particle position, which generally exhibits a non-mono-exponential decay when the fluid bath is viscoelastic. Our results are verified by experimental measurements for an optically-trapped colloidal bead in semidilute micellar and polymer solutions, finding and excellent agreement for all time intervals over which the energy exchange takes place.

Temperature-mediated diffusion of nanoparticles in semidilute polymer solutions.

The temperature is often a critical factor affecting the diffusion of nanoparticles in complex physiological media, but its specific effects are still to be fully understood. Here, we constructed a temperature-regulated model of semidilute polymer solution and experimentally investigated the temperature-mediated diffusion of nanoparticles using the particle tracking method. By examining the ensemble-averaged mean square displacements (MSDs), we found that the MSD grows gradually as the temperature increases while the transition time from sublinear to linear stage in MSD decreases. Meanwhile, the temperature-dependent measured diffusivity of the nanoparticles shows an exponential growth. We revealed that these temperature-mediated changes are determined by the composite effect of the macroscale property of polymer solution and the microscale dynamics of polymer chain as well as nanoparticles. Furthermore, the measured non-Gaussian displacement probability distributions were found to exhibit non-Gaussian fat tails, and the tailed distribution is enhanced as the temperature increases. The non-Gaussianity was calculated and found to vary in the same trend with the tailed distribution, suggesting the occurrence of hopping events. This temperature-mediated non-Gaussian feature validates the recent theory of thermally induced activated hopping. Our results highlight the temperature-mediated changes in diffusive transport of nanoparticles in polymer solutions and may provide the possible strategy to improve drug delivery in physiological media.

Structure and Dynamics of Hybrid Colloid-Polyelectrolyte Coacervates: Insights from Molecular Simulations.

Electrostatic interactions in polymeric systems are responsible for a wide range of liquid-liquid phase transitions that are of importance for biology and materials science. Such transitions are referred to as complex coacervation, and recent studies have sought to understand the underlying physics and chemistry. Most theoretical and simulation efforts to date have focused on oppositely charged linear polyelectrolytes, which adopt nearly ideal-coil conformations in the condensed phase. However, when one of the coacervate components is a globular protein, a better model of complexation should replace one of the species with a spherical charged particle or colloid. In this work, we perform coarse-grained simulations of colloid-polyelectrolyte coacervation using a spherical model for the colloid. Simulation results indicate that the electroneutral cell of the resulting (hybrid) coacervates consists of a polyelectrolyte layer adsorbed on the colloid. Power laws for the structure and the density of the condensed phase, which are extracted from simulations, are found to be consistent with the adsorption-based scaling theory of hybrid coacervation. The coacervates remain amorphous (disordered) at a moderate colloid charge, Q, while an intra-coacervate colloidal crystal is formed above a certain threshold, at Q > Q*. In the disordered coacervate, if Q is sufficiently low, colloids diffuse as neutral nonsticky nanoparticles in the semidilute polymer solution. For higher Q, adsorption is strong and colloids become effectively sticky. Our findings are relevant for the coacervation of polyelectrolytes with proteins, spherical micelles of ionic surfactants, and solid organic or inorganic nanoparticles.

Deep Learning for Determination of Properties of Semidilute Polymer Solutions

Deep Learning for Determination of Properties of Semidilute Polymer Solutions

Scattering evidence of positional charge correlations in polyelectrolyte complexes

Polyelectrolyte complexation plays an important role in materials science and biology. The internal structure of the resultant polyelectrolyte complex (PEC) phase dictates properties such as physical state, response to external stimuli, and dynamics. Small-angle scattering experiments with X-rays and neutrons have revealed structural similarities between PECs and semidilute solutions of neutral polymers, where the total scattering function exhibits an Ornstein-Zernike form. In spite of consensus among different theoretical predictions, the existence of positional correlations between polyanion and polycation charges has not been confirmed experimentally. Here, we present small-angle neutron scattering profiles where the polycation scattering length density is matched to that of the solvent to extract positional correlations among anionic monomers. The polyanion scattering functions exhibit a peak at the inverse polymer screening radius of Coulomb interactions, q* ≈ 0.2 Å-1. This peak, attributed to Coulomb repulsions between the fragments of polyanions and their attractions to polycations, is even more pronounced in the calculated charge scattering function that quantifies positional correlations of all polymer charges within the PEC. Screening of electrostatic interactions by adding salt leads to the gradual disappearance of this correlation peak, and the scattering functions regain an Ornstein-Zernike form. Experimental scattering results are consistent with those calculated from the random phase approximation, a scaling analysis, and molecular simulations.

Effect of nanoparticles in dilute and semi-dilute polymer solutions

Effect of nanoparticles in dilute and semi-dilute polymer solutions

Theory of self-coacervation in semi-dilute and concentrated zwitterionic polymer solutions.

Based on the random phase approximation, we develop a molecular theory of self-coacervation in zwitterionic polymer solutions. We show that the interplay between the volume interactions of the monomeric units and electrostatic correlations of charged groups on a polymer backbone can result in liquid-liquid phase separation (self-coacervation). We analyse the behavior of the coacervate phase polymer concentration depending on the electrostatic interaction strength - the ratio of the Bjerrum length to the bond length of the chain. We establish that in a wide range of polymer concentration values - from a semi-dilute to a rather concentrated solution - the chain connectivity and excluded volume interaction of the monomeric units have an extremely weak effect on the contribution of the electrostatic interactions of the dipolar monomeric units to the total free energy. We show that for rather weak electrostatic interactions, the electrostatic correlations manifest themselves as Keesom interactions of point-like freely rotating dipoles (Keesom regime), while in the region of strong electrostatic interactions the electrostatic free energy is described by the Debye-Hückel limiting law (Debye regime). We show that for real zwitterionic coacervates the Keesom regime is realized only for sufficiently small polymer concentrations of the coacervate phase, while the Debye regime is approximately realized for rather dense coacervates. Using the mean-field variant of the density functional theory, we calculate the surface tension (surface free energy) of the "coacervate-solvent" interface as a function of the bulk polymer concentration. Obtained results can be used to estimate the parameters of the polymer chains needed for practical applications such as drug encapsulation and delivery, as well as the design of adhesive materials.

Hydrodynamic cavitation reduction in semidilute turbulent polymer solution flows

The hydrodynamic cavitation in semidilute solution flows of a flexible polymer additive in water was experimentally explored in a mesoscale converging–diverging nozzle to elucidate the cavitation reduction effects of polymer additives. Rheological measurements demonstrated that polymer solutions were shear-thinning, with infinite viscosities larger than pure water. The polymer additives significantly mitigated the intensity of cloud cavitation and the growth of violent cavity structures in the tested solution concentrations. Under conditions of supercavitation, the tested polymer solutions could not suppress the growth of large structures but showed a reduction in the population of cavitation bubbles. The temporal evolution and spatial variation of cavitation structures in different concentrations were captured using high-speed imaging. Statistical analysis of the images showed that polymers reduce cavitation via three main mechanisms. (1) The longitudinal expansion of cavities downstream is attenuated relative to the pure water. The streamwise distribution of vapour-ratio fluctuations was flattened, and its peak was shifted upstream in the solutions. (2) Mean collapse and growth rate of cavitating bubble pockets and their fluctuations were noticeably relaxed by polymer additives. For a 400 p.p.m. solution (parts per million (p.p.m.)), a reduction of 65 % was measured relative to pure water flow at the highest tested flow rate. (3) Spectral analysis of the downstream pressure indicated that the shedding frequency at the cavitation inception was reduced as the solution's concentration increased. This reduction was as high as 70 % for a 400 p.p.m. solution. These results highlight the strong interplay between polymer additives and the generation of cavitation-related structures.

Microrheological study of single chain dynamics in semidilute entangled flexible polymer solutions: Crossover from Rouse to Zimm modes

The dynamics of polymer chains in entangled semidilute solution have been of theoretical and experimental interest. Among a number of characteristic lengths and times of the polymer in solution, those of the correlation blob are the key to understand the applicability of the Rouse and Zimm models to rheology of the semidilute solution. Direct rheological measurements of Rouse and Zimm modes are limited as the corresponding time scale is out of the range of classical rheological techniques. We investigated the single chain dynamics of entangled poly(ethylene oxide) in semidilute aqueous solutions by high-frequency micro-rheology based on diffusing-wave spectroscopy compared to classical shear macro-rheology. Concentration dependence of the three characteristic times of the entangled polymer chains, reptation time, entanglement time, and correlation time, was studied with the help of the time-concentration superposition. At the low frequency range, dynamic moduli measured by macro-rheology and micro-rheology showed a good agreement without adjustable parameters. At the higher frequency range, we found the Rouse regime in the mean square displacement of the probe particles and in the magnitude of the complex specific viscosity of the solution. We propose a simple method to estimate the boundary of the Rouse regime. Finally, at the high frequency range, we demonstrate that the contribution of the solvent to the solution viscosity needs to be subtracted to observe the power-law behavior of the Zimm mode.

Isotropic-to-Nematic Transition in Salt-Free Polyelectrolyte Coacervates from Coarse-Grained Simulations

Recent interest in complex coacervation between oppositely charged polyelectrolytes (PEs) has been fueled by its relevance to biology in the context of membraneless organelle formation within living cells. For PEs with limited flexibility (such as double-stranded DNA), theoretical treatments and recent experiments have reported the emergence of liquid crystalline order (LCO) within the resulting coacervate phases. In this work, we study the underlying physics of this phenomenon using coarse-grained molecular dynamics simulations of symmetric semiflexible–semiflexible and asymmetric semiflexible–flexible coacervates. By comparing coacervates with the corresponding semidilute solutions of neutral polymers, we demonstrate that the presence of Coulomb interactions in coacervates facilitates orientational ordering, in agreement with theoretical predictions. Quantitative comparisons between our simulations and theory indicate that, for asymmetric nematic coacervates, the strong orientational ordering of stiff polyanions induces a weak ordering of the flexible polycations─an effect that was not anticipated by available theoretical studies. Simulations reveal that, for nematic coacervates, the preferred orientation of the PE chains at the liquid–liquid coacervate–supernatant interface is parallel, and the alignment of semiflexible PEs is homogeneous. The results presented here provide new molecular-level insights into the intra-coacervate LCO and will help motivate further experimental and theoretical activities in this area.

Effect of swelling and deswelling on the elasticity of polyacrylonitrile gels prepared by freezing and thawing techniques

. The repeating cycle of freezing and thawing brings about a physical gelation for the solutions of polyacrylonitrile (PAN) in N, N-dimethylacetamide (DMA) for appropriate concentrations. It was revealed from wide-angle-X-ray diffraction that the PAN molecules were “cross-linked” by the crystallization to form a network through the freezing and thawing steps. The profiles of small-angle-X-ray scattering for the gels showed the excess scattering from the network structure formed through gelation, which could not be observed for the pure PAN solution. A reversible swelling and shrinking were observable for the PAN gel as a function of the relative content of DMA and benzene by varying the mixing ratio. The degree of swelling measured by weight, for the swollen and shrunk gels, was in a range from 0.7 to 8. A scaling relation appeared for the gel swelling between the PAN volume fraction (ϕ) and the shear moduli (G) which was measured by tensile and indentation tests, as G = G s ×(ϕ)s; G s= 1200kPa and s = 2.26 ± 0.23. A discussion is given on the value of the scaling exponent, s, on the basis of the conformation of flexible chain molecules in solution and the analogy in the microscopic and macroscopic properties between semidilute polymer solutions and gels.

Interactions between polymers and flows in macro and micro length scales.

Polymer solution with large molecular weights show scale dependent nonlinear behavior. The dynamics of such polymer solution in a meter length scale are very different from those in a micrometer length scale. To clarify this scale dependent behavior, we have conducted several experiments that focus on interactions between polymers and flows in multiple length scales [1Hidema R. Suzuki H. Hisamatsu S. Komoda Y. Furukawa H. Effects of the extensional rate on two-dimensional turbulence of semi-dilute polymer solution flows.Rheol. Acta. 2013; 52: 949https://doi.org/10.1007/s00397-013-0733-3Crossref Scopus (13) Google Scholar, 2Hidema R. Suzuki H. Hisamatsu S. Komoda Y. Characteristic scales of two-dimensional turbulence in polymer solutions.AICHE J. 2014; 60: 1854https://doi.org/10.1002/aic.14364Crossref Scopus (15) Google Scholar, 3Hidema R. Murao I. Komoda Y. Suzuki H. Effects of the extensional rheological properties of polymer solutions on vortex shedding and turbulence characteristics in a two-dimensional turbulent flow.J. Non-Newtonian Fluid Mech. 2018; 254: 1https://doi.org/10.1016/j.jnnfm.2018.02.001Crossref Scopus (12) Google Scholar, 4Hidema R. Fukushima K. Yoshida R. Suzuki H. Vortex deformation and turbulent energy of polymer solution in a two-dimensional turbulent flow.J. Non-Newtonian Fluid Mech. 2020; 285104385https://doi.org/10.1016/j.jnnfm.2020.104385Crossref Scopus (3) Google Scholar, 5Fukushima K. Kishi H. Suzuki H. Hidema R. Modification of turbulence caused by cationic surfactant wormlike micellar structures in two-dimensional turbulent flow.J. Fluid Mech. 2022; 933: A9https://doi.org/10.1017/jfm.2021.1058Crossref Scopus (1) Google Scholar, 6Hidema R. Hayashi S. Suzuki Hiroshi Drag force of polyethyleneglycol in flow measured by a scanning probe microscope.Phys. Rev. Fluids. 2019; 4074201https://doi.org/10.1103/PhysRevFluids.4.074201Crossref Scopus (2) Google Scholar, 7Hidema Ruri Fujito Ken-ya Suzuki Hiroshi Drag force of polyethyleneglycol in flows of polymer solutions measured by a scanning probe microscope.Soft Matter. 2022; 18: 455https://doi.org/10.1039/D1SM01305JCrossref PubMed Google Scholar].In the first part of this talk, I will highlight characteristic vortex deformation and energy transfer caused by polymers in a two-dimensional turbulent flow [1Hidema R. Suzuki H. Hisamatsu S. Komoda Y. Furukawa H. Effects of the extensional rate on two-dimensional turbulence of semi-dilute polymer solution flows.Rheol. Acta. 2013; 52: 949https://doi.org/10.1007/s00397-013-0733-3Crossref Scopus (13) Google Scholar, 2Hidema R. Suzuki H. Hisamatsu S. Komoda Y. Characteristic scales of two-dimensional turbulence in polymer solutions.AICHE J. 2014; 60: 1854https://doi.org/10.1002/aic.14364Crossref Scopus (15) Google Scholar, 3Hidema R. Murao I. Komoda Y. Suzuki H. Effects of the extensional rheological properties of polymer solutions on vortex shedding and turbulence characteristics in a two-dimensional turbulent flow.J. Non-Newtonian Fluid Mech. 2018; 254: 1https://doi.org/10.1016/j.jnnfm.2018.02.001Crossref Scopus (12) Google Scholar, 4Hidema R. Fukushima K. Yoshida R. Suzuki H. Vortex deformation and turbulent energy of polymer solution in a two-dimensional turbulent flow.J. Non-Newtonian Fluid Mech. 2020; 285104385https://doi.org/10.1016/j.jnnfm.2020.104385Crossref Scopus (3) Google Scholar, 5Fukushima K. Kishi H. Suzuki H. Hidema R. Modification of turbulence caused by cationic surfactant wormlike micellar structures in two-dimensional turbulent flow.J. Fluid Mech. 2022; 933: A9https://doi.org/10.1017/jfm.2021.1058Crossref Scopus (1) Google Scholar]. The effects of the extensional rheological properties of polyethylene oxide solutions were focused to understand the vortex deformation in turbulent flow and turbulent statistics of the polymer solution. In the polymer solution, although the turbulent energy production was virtually zero, a characteristic peak appeared in turbulent energy. The results imply the extension and relaxation of polymers affect the energy transfer.In the second part, I will introduce a method to measure the drag force due to synthetic polymers in flowing fluids by using a scanning probe microscope [[6]Hidema R. Hayashi S. Suzuki Hiroshi Drag force of polyethyleneglycol in flow measured by a scanning probe microscope.Phys. Rev. Fluids. 2019; 4074201https://doi.org/10.1103/PhysRevFluids.4.074201Crossref Scopus (2) Google Scholar,[7]Hidema Ruri Fujito Ken-ya Suzuki Hiroshi Drag force of polyethyleneglycol in flows of polymer solutions measured by a scanning probe microscope.Soft Matter. 2022; 18: 455https://doi.org/10.1039/D1SM01305JCrossref PubMed Google Scholar]. The conformation of polymers attached to the cantilever probe was predicted, and the drag force due to the deformed polymers in a flow was calculated. The drag forces obtained by model calculations were compared to the force detected by experiments, and found to be reasonably close.

Characterizing purely elastic turbulent flow of a semi-dilute entangled polymer solution in a serpentine channel

Polymer solutions in the semi-dilute regime are of considerable industrial importance. The complex rheological properties of such highly viscoelastic fluids and the complexity of their flow characteristics, especially in curved geometries, necessitate a thorough experimental characterization of the dynamics of such fluid flows. We apply statistical, spectral, and structural analyses to the experimentally obtained velocity fields of a semi-dilute entangled polymer solution in a serpentine channel to fully characterize the corresponding flow. Our results show that at high Weissenberg numbers, yet vanishing Reynolds numbers, the flow resistance is significantly increased, which indicates the emergence of a purely elastic turbulent flow. Spatial flow observations and statistical analysis of temporal flow features show that this purely elastic turbulent flow is non-homogeneous, non-Gaussian, and anisotropic at all scales. Moreover, spectral analysis indicates that compared to elastic turbulence in the dilute regime, the range of present scales of the excited fluctuations is narrower. This is partly due to the entanglement of the polymers in this concentration regime, which restricts their movement, and partly due to the mixed flow type inherent in the serpentine geometry, which can reduce the extent of polymer stretching and, thus, reduce the intensity of the fluctuations in the flow. Furthermore, proper orthogonal decomposition analysis is applied to directly extract the turbulent flow structure and reveals the activity of the counter-rotating vortices associated with secondary flow, which significantly contribute to the total kinetic energy of the flow.

Probing local chromatin dynamics by tracking telomeres

Chromatin dynamics is key for cell viability and replication. In interphase, chromatin is decondensed, allowing the transcription machinery to access a plethora of DNA loci. Yet, decondensed chromatin occupies almost the entire nucleus, suggesting that DNA molecules can hardly move. Recent reports have even indicated that interphase chromatin behaves like a solid body on mesoscopic scales. To explore the local chromatin dynamics, we have performed single-particle tracking on telomeres under varying conditions. We find that mobile telomeres feature, under all conditions, a strongly subdiffusive, antipersistent motion that is consistent with the monomer motion of a Rouse polymer in viscoelastic media. In addition, telomere trajectories show intermittent accumulations in local niches at physiological conditions, suggesting that the surrounding chromatin reorganizes on these timescales. Reducing the temperature or exposing cells to osmotic stress resulted in a significant reduction of mobile telomeres and the number of visited niches. Altogether, our data indicate a vivid local chromatin dynamics, akin to a semidilute polymer solution, unless perturbations enforce a more rigid or entangled state of chromatin.

Conformation and structure of ring polymers in semidilute solutions: A molecular dynamics simulation study

Scaling is one of the most fundamental concepts in theoretical polymer physics. For linear polymers in semidilute solutions under a good solvent condition, there has been a well established scaling behavior of mean-square radius of gyration of chains with the concentration, i.e., R g 2 ~ ϕ β , with β = −0.25, which receives strong support from experiments, computer simulations and scaling arguments. However, a clear-cut conclusion about the scaling behavior of dimensions of nonconcatenated and unknotted ring polymers in the semidilute solution with the concentration has not been achieved. Values of the corresponding scaling exponent β are diversely reported in literature, ranging from −0.25 to −0.59 and in contradiction within experimental, simulation and theoretical studies. As an endeavour for clarifying this issue, we carried out extensive molecular dynamics (MD) simulations based on the Kremer-Grest model of ring polymers with chain lengths N up to 5120 in semidilute solutions, and compared our results about ring conformations with almost all of experimental, simulation and theoretical data available in literature. Our MD simulation results lead to a conclusion that the mean-square radius of gyration of ring polymers scales with the concentration in semidilute regime as R g 2 ∼ φ − 0.59 ± 0.01 . This conclusion is in a good agreement with a previous finding by a lattice Monte Carlo simulation. Surprisingly, by looking into the experimental and other simulation results reported previously with sufficient caution, we tend to conclude that in reality almost all of their data can be described reasonably well by the same scaling law, R g 2 ~ ϕ − 0.59 . Furthermore, the scaling exponent obtained (−0.59) can be successfully explained by a scaling argument based on concept of correlation length. Finally, effects of the concentration on shape properties and packing behavior of ring polymers in the semidilute solutions have also been illuminated within our MD simulations. • Scaling behavior of ring dimensions in semidilute solutions with the concentration has been revisited. • Comparison of our results with experimental, and other simulation and theoretical results has been conducted. • Effect of concentration on shape and packing behavior of ring polymers in semidilute solutions has been illuminated.

Scaling purely elastic instability of strongly shear thinning polymer solutions.

Flow of viscoelastic polymer solutions in curved channels exhibits instability caused by the elastic nature of polymers even at low Reynolds numbers. However, scaling of the onset of this purely elastic instability in semidilute polymer solutions has not been previously reported. Here we experimentally investigate the flow of highly elastic polymer solutions above their overlap concentrations using pressure measurements and particle image velocimetry. We demonstrate that the onset of instability can be scaled by including shear dependent rheological properties of the polymer solutions in the nonlinear stability analysis. As a result, a universal criterion as function of normalized polymer concentration is provided for scaling the onset of purely elastic instability in the semidilute regime regardless of the type and molecular weight of the polymer.

In Situ RheoNMR Correlation of Polymer Segmental Mobility with Mechanical Properties during Hydrogel Synthesis.

Understanding polymer gelation over multiple length‐scales is crucial to develop advanced materials. An experimental setup is developed that combines rheological measurements with simultaneous time‐domain 1H NMR relaxometry (TD‐NMR) techniques, which are used to study molecular motion (<10 nm) in soft matter. This so‐called low‐field RheoNMR setup is used to study the impact of varying degrees of crosslinking (DC) on the gelation kinetics of acrylic acid (AAc) and N,N′‐methylene bisacrylamide (MBA) free radical crosslinking copolymerization. A stretched exponential function describes the T 2 relaxation curves throughout the gelation process. The stretching exponent β decreases from 0.90 to 0.67 as a function of increasing DC, suggesting an increase in network heterogeneity with a broad T 2 distribution at higher DC. The inverse correlation of the elastic modulus G′ with T 2 relaxation times reveals a pronounced molecular rigidity for higher DC at early gelation times, indicating the formation of inelastic, rigid domains such as crosslinking clusters. The authors further correlate G′ with the polymer concentration during gelation using a T 1 filter for solvent suppression. A characteristic scaling exponent of 2.3 is found, which is in agreement with theoretical predictions of G′ based on the confining tube model in semi‐dilute entangled polymer solutions.