Poly(N-isopropylacrylamide) (pNIPAM) microgels are renowned for their thermoresponsive behavior, exhibiting a distinct volume phase transition (VPT) upon temperature changes. This study investigates the influence of microgel softness, controlled by varying the crosslinking density during synthesis via free radical polymerization (FRP), on the difference between the volume phase transition temperature (VPTT) and the electrokinetic transition temperature (ETT). These transition temperatures mark the points at which the microgel size and surface charge, respectively, undergo significant alterations in response to temperature. Here, we investigate this phenomenon, employing dynamic light scattering (DLS) and electrophoretic light scattering (ELS) measurements to characterize the size and electrophoretic mobility response of pNIPAM microgels with different crosslinking densities as a function of temperature. By analyzing the observed trends in the difference between the transition temperatures, we aim to develop a hypothesis that provides a deeper physical understanding of the microgel structure and its relationship to transition temperatures. This investigation thus sheds light on the intricate interplay between microgel structure and its thermoresponsive behavior, offering insights for the design and optimization of pNIPAM microgels for future applications.
PNIPAm microgels synthesized via free radical polymerization (FRP) are often considered as neutral colloids in aqueous media, although it is well known, since the pioneering work of Pelton and coworkers [Langmuir 1989, 5, 816-818], that the vanishing electrophoretic mobility characterizing swollen microgels largely increases above the lower critical solution temperature (LCST) of PNIPAm, at which microgels partially collapse. The presence of an electric charge has been attributed to the ionic initiators that are employed when FRP is performed in water and that stay anchored to microgel particl…
Core-shell Au@pNIPAM nanocomposites are assembled through a simple and inexpensive approach where drops of bulk nanoparticle solution are left to dry on ITO surfaces at fixed and finely tuned temperatures. Interparticle distances are determined by the length of the thermoresponsive PNiPAM shell which acts as mechanical spacer. Structural characterization is performed through microscope imaging techniques and we quantify the pair and orientational correlation functions characteristic to each array at each deposition temperature. The elasticity of the polymer shell at each temperature is conside…
Responsive microgels based on poly(N-isopropylacrylamide) (PNIPAM) exhibit peculiar behaviours due to the competition between the hydrophilic and hydrophobic interactions of the constituent networks. The interpenetration of poly-acrilic acid (PAAc), a pH-sensitive polymer, within the PNIPAM network, to form Interpenetrated Polymer Network (IPN) microgels, affects this delicate balance and the typical Volume-Phase Transition (VPT) leading to complex behaviours whose molecular nature is still completely unexplored. Here we investigate the molecular mechanism driving the VPT and its influence on …
Micron-sized hard core-soft shell hybrid microgels are promising model systems for studies of soft matter as they enable in-situ optical investigations and their structures/morphologies can be engineered with a great variety. Yet, protocols that yield micron-sized core-shell microgels with a tailorable shell-to-core size ratio are rarely available. In this work, we report on the one-pot synthesis protocol for micron-sized silica-poly(N-isopropylacrylamide) core-shell microgels that has excellent control over the shell-to-core ratio. Small-angle light scattering and microscopy of 2- and 3-dimen…
Combining elastic incoherent neutron scattering and differential scanning calorimetry, we investigate the occurrence of the volume phase transition (VPT) in very concentrated PNIPAM microgel suspensions, from a polymer weight fraction of 30~wt\% up to dry conditions. Although samples are arrested at the macroscopic scale, atomic degrees of freedom are equilibrated and can be probed in a reproducible way. A clear signature of the VPT is present as a sharp drop of the mean square displacement of PNIPAM hydrogen atoms obtained by neutron scattering. As a function of concentration, the VPT gets sm…
Hierarchical polymer structures such as pNIPAM microgels have been extensively studied for their ability to undergo significant structural and physical transformations that can be controlled by external stimuli such as temperature, pH or solvent composition. However, direct three-dimensional visualization of individual particles in situ have so far been hindered by insufficient resolution, with optical microscopy, or contrast, with electron microscopy. In recent years superresolution microscopy techniques have emerged that in principle can provide nanoscopic optical resolution. Here we report …
The volume phase transition of microgels is one of the most paradigmatic examples of stimuli-responsiveness, enabling a collapse from a highly swollen microgel state into a densely coiled state by an external stimulus. Although well characterized in bulk, it remains unclear how the phase transition is affected by the presence of a confining interface. Here, we demonstrate that the temperature-induced volume phase transition of poly(N-isopropylacrylamide) microgels, conventionally considered an intrinsic molecular property of the polymer, is in fact largely suppressed when the microgel is adsor…
Hypotheses: Additives are commonly used to tune macromolecular conformational transitions. Among additives, trehalose is an excellent bioprotectant and among responsive polymers, PNIPAM is the most studied material. Nevertheless, their interaction mechanism so far has only been hinted without direct investigation, and, crucially, never elucidated in comparison to proteins. Detailed insights would help understand to what extent PNIPAM microgels can effectively be used as synthetic biomimetic materials, to reproduce and study, at the colloidal scale, isolated protein behavior and its sensitivity…
Hypothesis: The peculiar swelling behaviour of poly(N-isopropylacrylamide) (PNIPAM)-based responsive microgels provides the possibility to tune both softness and volume fraction with temperature, making these systems of great interest for technological applications and theoretical implications. Their intriguing phase diagram can be even more complex if poly(acrylic acid) (PAAc) is interpenetrated within PNIPAM network to form Interpenetrating Polymer Network (IPN) microgels that exhibit an additional pH-sensitivity. The effect of the PAAc/PNIPAM polymeric ratio on both swelling capability and …
The long debated protein dynamical transition was recently found also in non-biological macromolecules, such as poly-N-isopropylacrylamide (PNIPAM) microgels. Here, by using atomistic molecular dynamics simulations, we report a description of the molecular origin of the dynamical transition in these systems. We show that PNIPAM and water dynamics below the dynamical transition temperature Td are dominated by methyl group rotations and hydrogen bonding, respectively. By comparing with bulk water, we unambiguously identify PNIPAM-water hydrogen bonding as the main responsible for the occurrence …
We combine small angle scattering experiments and simulations to investigate the internal structure and interactions of composite Poly(N-isopropylacrylamide)-Poly(ethylene glycol) (PNIPAM-PEG) microgels. At low temperatures the experimentally determined form factors and the simulated density profiles indicate a loose internal particle structure with an extended corona, that can be modeled as a star-like object. Increasing temperature across the volumetric phase transition, the form factor develops an inflection which, using simulations, is interpreted as arising from a configuration in which P…
Microgel suspensions of Interpenetrated Polymer Network (IPN) of PNIPAM and PAAc in D$_2$O, have been investigated through dynamic light scattering as a function of temperature, pH and concentration across the Volume Phase Transition (VPT). The dynamics of the system is slowed down under H/D isotopic substitution due to the different balance between polymer/polymer and polymer/solvent interactions suggesting the crucial role played by H-bondings. The swelling behavior, reduced with respect to PNIPAM and water, has been described by the Flory-Rehner theory, tested for PNIPAM microgel and succes…
Interpenetrated Polymer Network (IPN) microgels of PNIPAM and PAAc have been investigated and the experimental data have been compared with theoretical models from the Flory-Rehner theory. We confirm that the swelling behavior of PNIPAM microgels is well described by this theory by considering the second order approximation for the volume fraction $ϕ$ dependence of the Flory parameter $χ(ϕ)$. Indeed the Volume-Phase Transition (VPT) of the PNIPAM-PAAc IPN microgel at neutral conditions and in D$_2$O solvents can be well described only considering a third-order approximation. Interestingly we e…
A low-temperature dynamical transition has been reported in several proteins. We provide the first observation of a `protein-like' dynamical transition in nonbiological aqueous environments. To this aim we exploit the popular colloidal system of poly-N-isopropylacrylamide (PNIPAM) microgels, extending their investigation to unprecedentedly high concentrations. Owing to the heterogeneous architecture of the microgels, water crystallization is avoided in concentrated samples, allowing us to monitor atomic dynamics at low temperatures. By elastic incoherent neutron scattering and molecular dynami…
Microgel particles composed of Interpenetrated Polymer Networks (IPN) of poly(N-isopropylacrylamide) (PNIPAM) and poly(acrylic acid) (PAAc) dispersed in water have been investigated through dynamic light scattering. The study of the temperature, concentration and pH dependence of the relaxation time has highlighted the existence of a thermoreversible transition corresponding to the swollen-shrunken volume phase transition. The presence of PAAc introduces an additional pH-sensitivity with respect to the temperature-sensitivity due to PNIPAM and leads to interesting differences in the transition…
Thermo-responsive microgel particles can exhibit a drastic volume shrinkage upon increasing the solvent temperature. Recently we found that the spreading of poly(N-isopropylacrylamide)(PNiPAm) microgels at a liquid interface under the influence of surface tension hinders the temperature-induced volume phase transition. In addition, we observed a hysteresis behavior upon temperature cycling, i.e. a different evolution in microgel size and shape depending on whether the microgel was initially adsorbed to the interface in expanded or collapsed state. Here, we model the volume phase transition of …
Interpenetrated polymer network microgels, composed of crosslinked networks of poly(N-isopropylacrylamide) and polyacrylic acid (PAAc), have been investigated through rheological measurements at four different amounts of polyacrylic acid. Both PAAc content and crosslinking degree modify particle dimensions, mass and softness, thereby strongly affecting the volume fraction and the system viscosity. Here the volume fraction is derived from the flow curves at low concentrations by fitting the zero-shear viscosity with the Einstein-Batchelor equation which provides a parameter k to shift weight co…
Unraveling the Mechanisms of Ultrasound-Induced Mechanical Degradation of Microgels: Effects of Mechanoresponsive Crosslinks, Softness, and Core-Shell Architecture
Heinrich-Heine University（海因里希·海涅大学）；DWI – Leibniz Institute for Interactive Materials（DWI–莱布尼茨交互材料研究所）；RWTH Aachen University（亚琛工业大学）；Lund University（隆德大学）
We report temperature-responsive soft composites of semiflexible biopolymer networks (fibrin) containing dispersed microgel colloidal particles of poly(N-isopropylacrylamide) (pNIPAM) that undergo a thermodynamically driven de-swelling transition above a Lower Critical Solution Temperature (LCST). Unlike standard polymer-particle composites, decreasing the inclusion volume of the particles (by increasing temperature)is concomitant with a striking increase of the overall elastic stiffness of the composite. We observe such a behavior over a wide composition space. The composite elastic shear mod…
The influence of shape fluctuations on deformable thermosensitive microgels in aqueous solution is investigated by dynamic light scattering (DLS) and depolarized dynamic light scattering (DDLS). The systems under study consist of a solid core of polystyrene and a thermosensitive shell of cross-linked poly(N-isopropylacrylamide) (PNIPA) without and with embedded palladium nanoparticles. PNIPA is soluble in water, but has a lower critical solution temperature at 32 C (LCST). Below the LCST the PNIPA shell is swollen. Here we find that besides translational and rotational diffusion, the particles…
Poly(N-isopropylacrylamide) (PNIPAM) is a synthetic polymer that is widely studied for its thermoresponsive character. However, recent works also reported evidence of a low temperature (protein-like) dynamical transition around 225 K in concentrated PNIPAM suspensions, independently of the polymer architecture, i.e., both for linear chains and for microgels. In this work, we investigate water-polymer interactions by extensive differential scanning calorimetry (DSC) measurements of both systems, in order to understand the effect of the different topological structures on the solution behaviour,…
The structural characterization of microgels at interfaces is fundamental to understand both their 2D phase behavior and their role as stabilizers that enable emulsions to be broken on demand. However, this characterization is usually limited by available experimental techniques, which do not allow a direct investigation at interfaces. To overcome this difficulty, here we employ neutron reflectometry, which allows us to probe the structure and responsiveness of the microgels in-situ at the air-water interface. We investigate two types of microgels with different cross-link density, thus having…
We investigate the temperature dependent phase behaviour of mixtures of poly(Nisopropylacrylamide) (pNIPAM) microgel colloids and a triblock copolymer (PEO-PPO-PEO) surfactant, which undergoes micellisation. Gelation in these systems results from an increase in temperature. Here we alter the heating rate and observe that the mechanism for aggregation changes from one of depletion of the microgels by the micelles at low temperatures to association of the two species at high temperatures. We use this to access multiple structures at one microgel concentration. Samples with a micelle concentratio…
We investigated the complexation of thermoresponsive anionic poly(N-isopropylacrylamide) (PNiPAM) microgels and cationic $ε$-polylysine ($ε$-PLL) chains. By combining electrophoresis, light scattering, transmission electron microscopy (TEM) and dielectric spectroscopy (DS) we studied the adsorption of $ε$-PLL onto the microgel networks and its effect on the stability of the suspensions. We show that the volume phase transition (VPT) of the microgels triggers a large polyion adsorption. Two interesting phenomena with unique features occur: a temperature-dependent microgel overcharging and a com…
Using a simplified microstructural picture we show that interactions between thermosenstive microgel particles can be described by a polymer brush like corona decorating the dense core. The softness of the potential is set by the relative thickness $L_0$ of the compliant corona with respect to the overall size of the swollen particle $R$. The elastic modulus in quenched solid phases derived from the potential is found in excellent agreement with diffusing wave spectroscopy (DWS) data and mechanical rheometry. Our model thus provides design rules for the microgel architecture and opens a route …
The formation of smart emulsions or foams whose stability can be controlled on-demand by switching external parameters is of great interest for basic research and applications. An emerging group of smart stabilizers are microgels, which are nano- and micro-sized, three-dimensional polymer networks that are swollen by a good solvent. In the last decades, the influence of various external stimuli on the two-dimensional phase behavior of microgels at air- and oil-water interfaces has been studied. However, the impact of the top-phase itself has been barely considered. Here, we present data that d…
This is an integrated experimental and theoretical study of the dynamics and rheology of self-crosslinked, slightly charged, temperature responsive soft Poly(N-isopropylacrylamide) (pNIPAM) microgels over a wide range of concentration and temperature spanning the sharp change in particle size and intermolecular interactions across the lower critical solution temperature (LCST). Dramatic, non-monotonic changes in viscoelasticity are observed with temperature, with distinctive concentration dependences in the dense fluid, glassy, and soft-jammed states. Motivated by our experimental observations…
We explore the influence of particle softness and internal structure on both the bulk and interfacial rheological properties of colloidal suspensions. We probe bulk stresses by conventional rheology, by measuring the flow curves, shear stress vs strain rate, for suspensions of soft, deformable microgel particles and suspensions of near hard-sphere-like silica particles. A similar behavior is seen for both kind of particles in suspensions at concentrations up to the random close packing volume fraction, in agreement with recent theoretical predictions for sub-micron colloids. Transient interfac…
Hierarchical Dynamics and Time-Length Scale Superposition in Glassy Suspensions of Ultra-Low Crosslinked Microgels
Univ Montpellier（蒙彼利埃大学）；CNRS（法国国家科学研究中心）；Institut Charles Sadron UPR22, Université de Strasbourg（斯特拉斯堡大学查尔斯·萨德兰研究所）；Lund University（隆德大学）；RWTH Aachen University（亚琛工业大学）；Institute of Physical Chemistry, RWTH Aachen University（亚琛工业大学物理化学研究所）；Institut Universitaire de France（法兰西学院）
Poly(N-isopropylacrylamide), PNIPAM, is a widely studied polymer, which serves as a key constituent of nanostructured soft materials with responsive properties. Upon increasing temperature the PNIPAM polymer chain undergoes a reversible coil-to-globule transition at ~305K, which is reflected by a volume phase transition in cross-linked architectures, such as microgels, valuable for many practical applications. The addition of a cosolvent is a simple method to tune the transition temperature according to the specific purpose. In this study, we use atomistic molecular dynamics simulations to exp…
A solvent-flux theory for nonequilibrium swelling dynamics of thermoresponsive microgels
University of Granada（格拉纳达大学）；Institute Carlos I of Theoretical and Computational Physics, University of Granada（格拉纳达大学卡洛斯一世理论与计算物理研究所）；Faculty of Engineering, University of Deusto（德乌斯托大学工程学院）；Center for Nanoscience and Sustainable Technologies (CNATS), and Department of Physical, Chemical and Natural Systems, Universidad Pablo de Olavide（巴勃罗·德·奥拉维德大学纳米科学与可持续技术中心及物理、化学与自然系统系）
