Does Flory-Rehner theory quantitatively describe the swelling of thermoresponsive microgels (original) (raw)

Swelling behavior in multi-responsive microgels

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2017

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 empirically find that sharper is the transition higher is the order of the χ(φ) relation which has to be considered. Moreover the VPT can be experimentally controlled by tuning the polymer/solvent interactions through pH and solvent allowing to directly modify the delicate balance between energetic and entropic contributions and to explore the swelling behavior in a wide range of environmental conditions. In particular we find that the most advantageous condition for swelling is in water at acidic pH.

Structure and Dynamics of a Thermoresponsive Microgel around Its Volume Phase Transition Temperature

The Journal of Physical Chemistry B, 2010

Sustained drug delivery requires the use of multifunctional devices with enhanced properties. These properties include responsiveness to external stimuli (such as temperature, pH, ionic strength), ability to deliver suitably designed ligands to specific receptors, enhanced bioadhesion to cells, and cytocompatibility. Microgels represent one of such multifunctional drug delivery devices. Recently, we described the fabrication of a stable colloidal aqueous suspension of cytocompatible microgel spheres based on a poly(vinyl alcohol)/poly(methacrylateco-N-isopropylacrylamide) network (Ghugare, S.; Mozetic, P.; Paradossi, G. Biomacromolecules 2009, 10, 1589). These microgel spheres undergo an entropy-driven volume phase transition around the physiological temperature, this phase transition being driven by the incorporation of NiPAAm residues in the network. In that study, the microgel was loaded with the anticancer drug doxorubicin. As the microgel shrank, a marked increase in the amount of doxorubicin released was noted. Indeed, dynamic light scattering measurements showed the diameter reduction to be about 50%. In the present paper, we focus on some fundamental issues regarding modifications of the hydrogel architecture at a nanoscopic level as well as of the diffusive behavior of water associated with the polymer network around the volume phase transition temperature (VPTT). Sieving and size exclusion effects were studied by laser scanning confocal microscopy with the microgel exposed to fluorescent probes with different molecular weights. Confocal microscopy observations at room temperature and at 40°C (i.e., below and above the VPTT) provided an evaluation of the variation of the average pore size (from 5 nm to less than 3 nm). Using quasielastic neutron scattering (QENS) with the IRIS spectrometer at ISIS, UK, the diffusive behavior of water molecules closely associated to the polymer network around the VPTT was investigated. A clear change in the values of diffusion coefficient of bound water was observed at the transition temperature. In addition, the local dynamics of the polymer itself was probed using the QENS spectrometer SPHERES at FRM II, Germany. For this study, the microgel was swollen in D 2 O. An average characteristic distance of about 5 Å for the localized chain motions was evaluated from the elastic incoherent structure factor (EISF) and from the Q-dependence of the Lorentzian width.

Effect of Polymer Molecular Weight and Synthesis Temperature on Structure and Dynamics of Microgels

Bulletin of the American Physical Society, 2012

to study the dependences on polymer molecular weight (M W) and synthesis temperature (T syn). The dynamics and structure of the synthesized microgels below and above the LCST of the polymer (T c ∼41 o C) were studied using dynamic and static light scattering spectroscopy. All microgels exhibit a volume phase transition above the LCST of the polymer and undergo a reversible 15-50-fold volume shrinkage. The size distribution, structure, deswelling ability, and temperature response of microgels strongly depend on synthesis conditions. T syn dependence was studied with 1000kDa polymer. Increasing ∆T = T syn-T C yields smaller microgels with a smaller swelling ratio up to ∆T = 8.5 o C, after which the trend is reversed. The amphiphilic nature of the polymer may explain this trend. T syn also affects the structure of microgels; low T syn yields elongated particles, while high T syn microgels are more spherical. Polymer M W directly effects microgel polydispersity and temperature response. While microgels synthesized with 1000kDa polymer are relatively monodisperse, synthesis with low M W polymers (80-370kDa) yields systems with a large population (R h ∼1000nm) precipitating out of solution and a smaller population (R h ∼300nm) staying in suspension. M W also influences the temperature response of microgels; high M W microgels show a gradual shrinkage with increasing temperature while low M W microgels display a delayed and sudden shrinkage at high temperatures.

Mechanics versus Thermodynamics: Swelling in Multiple-Temperature-Sensitive Core–Shell Microgels

Angewandte Chemie International Edition, 2006

We report on synthesis and characterisation of doubly temperature sensitive coreshell microgels. These core-shell microgels are composed of a core of chemically cross-linked poly-(N-isopropylacrylamide) (PNIPAM) and a shell of cross-linked poly-(Nisopropylmethacrylamide) (PNIPMAM). PNIPAM exhibits in water a lower critical solution temperature (LCST) of ca. 34 °C, the LCST of PNIPMAM is ca. 45 °C. Solution properties were investigated by means of dynamic light scattering (DLS), optical transmission, and small-angle neutron scattering (SANS). Core-shell microgels of this composition display a temperature dependent two-step shrinking behavior. The influences of the content of the cross-linking agent N,N'-methylenbisacrylamide (BIS) and of the thickness of the PNIPMAM shell on the thermosensitive response of the PNIPAM/PNIPMAM core-shell microgels were investigated. Core-shell microgels with a thick shell do not show a size transition at the PNIPAM LCST anymore. The volume transition is adjustable by varying the cross-link density of the shell. The swelling behaviour of the core-shell microgels is compared to that of pure PNIPAM and PNIPMAM. Additionally, an inverse system consisting of a PNIPMAM core and a PNIPAM shell was prepared and investigated by DLS.

Brushlike Interactions between Thermoresponsive Microgel Particles

Physical Review Letters, 2010

Using a simplified microstructural picture we show that interactions between thermosensitive microgel particles can be described by a polymer brushlike corona decorating the dense core. The softness of the potential is set by the relative thickness L0 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 to be in excellent agreement with diffusing wave spectroscopy data and mechanical rheometry. Our model thus provides design rules for the microgel architecture and opens a route to tailor rheological properties of pasty materials.

Functionalized Microgel Swelling: Comparing Theory and Experiment

The Journal of Physical Chemistry B, 2007

A comprehensive gel swelling model accounting for the effects of added salt, counterion/polyelectrolyte charge condensation, inter-cross-link chain length distribution, polyelectrolyte chain stiffness, and direct chargecharge repulsion between fixed polymer network charges has been applied to predict water fraction profiles in-COOH-functionalized microgels based on poly(N-isopropylacrylamide). The model can successfully order the microgels according to their rheologically measured water fractions and explains key differences in observed microgel swelling according to the different functional group and cross-linker distributions in the microgels. The cross-linking efficiency is used as an adjustable variable to match the magnitude of the different model predictions with the experimental water contents from rheological measurements. The resulting cross-linking efficiency predictions are correlated with the ability of the different comonomers to facilitate chain transfer and/or radical termination in the polymerization environment. The model can capture the differing responses of the microgels in the presence of different salt concentrations and can account for the impact of many key physical parameters and heterogeneities in microgel swelling which the Flory-Huggins model cannot directly address.

Micromechanics of temperature sensitive microgels: dip in the Poisson ratio near the LCST

Soft Matter, 2013

Microgels of poly-N-isopropylacrylamide (pNIPAM) exhibit a remarkable sensitivity to environmental conditions, most strikingly a pronounced deswelling that occurs close to the lower critical solution temperature (LCST) of the polymer at ≈ 32 • C. This transition has been widely studied and exploited in a range of applications. Along with changes in size, significant changes are also expected for the mechanical response of the particles. However, the full elastic properties of these particles as a function of temperature, T , have not yet been accessed at the single-particle level.

Relaxation Dynamics, Softness, and Fragility of Microgels with Interpenetrated Polymer Networks

Macromolecules, 2020

Microgels are elastic and deformable particles with a hybrid nature between that of polymers and colloids and unconventional behaviours with respect to hard colloids. We investigated the dynamics of a soft microgel made of interpenetrated polymer networks of PNIPAM and PAAc by means of coherent X-ray and light scattering techniques. By varying the particle softness through PAAc content we can tune at wish the fragility of IPN microgels. Interestingly we find the occurrence of a dynamical crossover at a critical packing fraction which leads to an evolution of the structural relaxation time from super-Arrhenius to Ar-1 rhenius, a minimum for the shape of the intensity autocorrelation function and the emerging of distinct anomalous mechanisms for particle motion. This complex phenomenology can be modelled by a Fickian diffusion at very low concentrations, an effective non Fickian anomalous diffusion at intermediate values and a ballistic motion well described within the Mode Coupling Theory on approaching the glassy state.

Volume transition and internal structures of small poly( N -isopropylacrylamide) microgels

Journal of Polymer Science Part B: Polymer Physics, 2005

Monodispersed poly(N-isopropylacrylamide) (PNIPAM) nanoparticles, with hydrodynamic radius less than 50 nm at room temperature, have been synthesized in the presence of a large amount of emulsifiers. These microgel particles undergo a swollen-collapsed volume transition in an aqueous solution when the temperature is raised to around 34°C. The volume transition and structure changes of the microgel particles as a function of temperature are probed using laser light scattering and small angle neutron scattering (SANS) with the objective of determining the small particle internal structure and particle-particle interactions. Apart from random fluctuations in the crosslinker density below the transition temperature, we find that, within the resolution of the experiments, these particles have a uniform radial crosslinker density on either side of the transition temperature. This result is in contrast to previous reports on the heterogeneous structures of larger PNIPAM microgel particles, but in good agreement with recent reports based on computer simulations of smaller microgels. The particle interactions change across the transition temperature. At temperatures below the transition, the interactions are described by a repulsive interaction far larger than that expected for a hard sphere contact potential. Above the volume transition temperature, the potential is best described by a small, attractive interaction. Comparison of the osmotic second virial coefficient from static laser light scattering at low concentrations with similar values determined from SANS at 250-time greater concentration suggests a strong concentration dependence of the interaction potential.