Computer Simulation Studies on the Polymer-Induced Modification of Water Properties in Polyacrylamide Hydrogels

Dissipative Particle Dynamics Simulation of Nanoparticle Diffusion in a Crosslinked Polymer Network

Dissipative particle dynamics simulation is conducted to investigate the diffusion of a nanoparticle in a crosslinked polymer network based on a bead-spring model. Focusing on cases where the particle is comparable in size to the network mesh, we find from rigid networks that the excluded-volume and hydrodynamic interaction effects associated with solvent beads lead to lubricity, which assists the particle to slip through an opening into the adjacent cell. For flexible networks, the hopping mechanism for particle escape becomes less pronounced with higher network flexibility due to either a smaller spring constant or slacker strands, each consisting of more beads. This behavior could be explained by the larger cell size fluctuation and its slower relaxation, whereby large enough openings temporarily formed are longer-lived, increasing the chance for particle hopping.

Identical Water Dynamics in Acrylamide Hydrogels, Polymers, and Monomers in Solution: Ultrafast IR Spectroscopy and Molecular Dynamics Simulations

The dynamics and structure of water in polyacrylamide hydrogels (PAAm-HG), polyacrylamide, and acrylamide solutions are investigated using ultrafast infrared experiments on the OD stretch of dilute HOD/H₂O and molecular dynamics simulations. The amide moiety of the monomer/polymers interacts strongly with water through hydrogen bonding (H-bonding). The FT-IR spectra of the three systems indicate that the range of H-bond strengths is relatively unchanged from bulk water. Vibrational population relaxation measurements show that the amide/water H-bonds are somewhat weaker but fall within the range of water/water H-bond strengths. A previous study of water dynamics in PAAm-HG suggested that the slowing observed was due to increasing confinement with concentration. Here, for the same concentrations of the amide moiety, the experimental results demonstrate that the reorientational dynamics (infrared pump-probe experiments) and structural dynamics (two-dimensional infrared spectroscopy) are identical in the three acrylamide systems studied. Molecular dynamics simulations of the water orientational relaxation in aqueous solutions of the acrylamide monomer, trimer, and pentamer are in good agreement with the experimental results and are essentially chain length independent. The simulations show that there is a slower, low-amplitude (<7%) decay component not accessible by the experiments. The simulations examine the dynamics and structure of water H-bonded to acrylamide, in the first solvent shell, and beyond for acrylamide monomers and short chains. The experiments and simulations show that the slowing of water dynamics in PAAm-HG is not caused by confinement in the polymer network but rather by interactions with individual acrylamide moieties.

Decelerated Liquid Dynamics Induced by Component-Dependent Supercooling in Hydrogen and Deuterium Quantum Mixtures

Isotopic mixtures of p-H₂ and o-D₂ molecules have been an attractive binary system because they include two kinds of purely isotopic molecules which possess the same electronic potential but the twice different mass inducing differently pronounced nuclear quantum effects (NQEs). Accessing details of structures and dynamics in such quantum mixtures combining complex molecular dynamics with NQEs of different strengths remains a challenging problem. Taking advantage of the nonempirical molecular dynamics method which describes p-H₂ and o-D₂ molecules, we found that the liquid dynamics slows down at a specific mixing ratio, which can be connected to the observed anomalous slowdown of crystallization in the quantum mixtures. We attributed the decelerated dynamics to the component-dependent supercooling of p-H₂ taking place in the mixtures, demonstrating that there is an optimal mixing ratio to hinder crystallization. The obtained physical insights will help in experimentally controlling and achieving unknown quantum mixtures including superfluid.

Solvation dynamics of N-substituted acrylamide polymers and the importance for phase transition behavior

Functional groups present in thermo-responsive polymers are known to play an important role in aqueous solutions by manifesting their coil-to-globule conformational transition in a specific temperature range. Understanding the role of these functional groups and their interactions with water is of great interest as it may allow us to control both the nature and temperature of this coil-to-globule transition. In this work, polyacrylamide (PAAm), poly(N-isopropylacrylamide) (PNIPAm), and poly(N-isopropylmethacrylamide) (PNIPMAm) solvated in water are studied with the goal of discovering the structure of the solvent and its interaction with these polymers in determining the polymer conformations. Specifically, all-atom molecular dynamics (MD) simulations were performed on polymer chains with 30 monomer units (30-mers) at 295 K, 310 K and 320 K, which is below and above the lower critical solution temperature (LCST) of PNIPAm (LCST = 305 K) and PNIPMAm (LCST = 315 K), respectively. The MD simulation trajectories suggest that changes in the functional groups in the backbone and side-chains alter the water solvation shell around the polymer. This results in a change in the residence time probability and hydrogen bond characteristics of water at simulated temperatures. Specifically, water molecules reside for longer times near PAAm (no LCST) and PNIPMAm (LCST = 315 K) chains as compared to PNIPAm. This might be one of the possible causes for the higher LCST of PNIPMAm as compared to that of PNIPAm. These results can guide experimentalists and theoreticians to design new polymer structures with tailor-made LCST transitions while controlling the water solvation shell around the functional group.

Understanding hydrogelation processes through molecular dynamics

Molecular dynamics (MD) is currently one of the preferred techniques employed to understand hydrogelation processes for its ability to include large amounts of atoms in computational calculations, since substantial amounts of solvent molecules are involved in gel formation.

Coarse-Grained Molecular Dynamics Force-Field for Polyacrylamide in Infinite Dilution Derived from Iterative Boltzmann Inversion and MARTINI Force-Field

We present a mesoscale model of aqueous polyacrylamide in the infinitely dilute concentration regime, by combining an extant coarse-grained (CG) force-field, MARTINI, and the Iterative Boltzmann Inversion protocol (IBI). MARTINI force-field was used to retain the thermodynamics of solvation of the polymer in water, whereas the structural properties and intrapolymer interactions were optimized by IBI. Atomistic molecular dynamics simulations of polymer in water were performed to benchmark the mesoscale simulations. Our results from the CG model show excellent agreement in structure with the atomistic system. We also studied the dynamical behavior of our CG system by computing the shear viscosity and compared it with the standard IBI model. The viscosity trends of our model were similar to the atomistic system, whereas the standard IBI model was highly dissimilar as expected. In summary, our hybrid CG model sufficiently mimics an infinitely dilute system, and is superior to both MARTINI and IBI in representing the structure and thermodynamics of the atomistic system, respectively. Our hybrid coarse-graining strategy promises applicability in large-scale simulations of polymeric/biological systems where the structure needs to be replicated accurately while preserving the thermodynamics of a smoother surrounding.

Molecular dynamics study of wetting behavior of grafted thermo-responsive PNIPAAm brushes

In this work, the effect of temperature on the contact angle of a water droplet on grafted thermo-responsive poly-(N-isopropylacrylamide) (PNIPAAm) polymer brushes is studied using all-atom molecular dynamics simulations in the temperature range of 270-330 K. A shift from 55° to 65° in contact angle values is observed as the temperature increases from 300 K to 310 K, which is in line with the experimental reports. The behavior of a water droplet on PNIPAAm brushes is analyzed using hydrogen bond analysis, water diffusion, radial distribution functions, the potential of mean force, excess entropy and the second virial coefficient (B2). The thermo-responsive behavior of PNIPAAm brushes, quantified using the excess entropy and B2 of PNIPAAm-water and water-water interactions, is mainly governed by polymer-water interactions. In particular, the excess entropy and B2 of PNIPAAm resulting from the PNIPAAm-water interactions are found to increase with increasing temperature. The dehydration of PNIPAAm brushes and the increase in the contact angle of water were confirmed to be entropy driven processes.

The coexisting phase behavior of thermo-responsive copolymer solutions

Using a molecular theory for dilute PEO-b-PNIPAm solutions, we first take the formation of hydrogen bonds between copolymer monomers and water molecules into account, which enables us to study the impact of temperature on PEO-b-PNIPAm self-assembly effectively by quantitatively describing the different changes in water affinities of two blocks. With the increase of temperature, hydrogen bonds between PNIPAm and water break down dramatically, resulting in the hydrophobic character of PNIPAm while PEO remains hydrophilic. Amphiphilic copolymers in the aqueous surrounding can aggregate into various structures: micelles and vesicles. According to the equilibrium criterion of the excess grand potential under the conditions of the grand canonical ensemble, we find that both structures are stable and can coexist. Theoretically calculated potentials of mean force of aggregates further verify the coexistence of micelles and vesicles, although the low critical solution temperatures of different aggregates are different under these conditions. A phase diagram as functions of temperature and the weight fraction of PEO (fPEO) is obtained, which shows different regions of micelles, vesicles and their coexistence. It implies the appearance of two types of micelle-vesicle transition: spontaneous and temperature-induced. Since PEO-b-PNIPAm as a thermoresponsive material has a broad range of applications, a systematic investigation of the phase behavior is very useful not only for the scientific interest but also for the practical applications.

Vibrational spectra of proximal water in a thermo-sensitive polymer undergoing conformational transition across the lower critical solution temperature

The vibrational spectrum of water near a thermo-sensitive polymer poly(N-isopropylacrylamide) (PNIPAM) undergoing conformational transition through the lower critical solution temperature (LCST) is calculated using molecular dynamics simulations. The characteristic structural features observed at the atomic scale for these proximal water molecules in a solvated polymer chain while undergoing the conformational transition are strongly correlated to their vibrational densities of states. Comparison of the vibrational spectrum below LCST for the proximal water with the vibrational spectrum obtained for bulk water reveals a significant fraction of the hydrogen bonding between the proximal water molecules and the polymer side groups. Hydrogen-bonded bridges of water molecules are formed between two adjacent and alternate monomers. This network of hydrogen bonding results in formation of locally ordered water molecules at temperatures below the LCST. Analysis of the simulation trajectories confirms the presence of a quasi-stable solvation structure near the PNIPAM. The calculated vibrational spectra for proximal water above the LCST suggest significantly reduced hydrogen bonding with the polymer and indicate a reduction in the structural stability of proximal water around a collapsed polymer chain. Systematic trends in the observed peak intensities and frequency shifts at the low- and high-frequency ends of the spectrum can be correlated with the structural and dynamical changes of water molecules below and above the LCST transition, respectively, for various polymer chain lengths. The simulations reveal that, compared to bulk water, the libration bands are blue shifted and OH stretch bands red shifted for water in proximity to PNIPAM with 30 monomer units below the LCST. The simulations suggest that vibrational spectra can be used as a predictive tool for quantifying atomic-scale structural transitions in solvation of thermo-sensitive polymers such as PNIPAM.

Brownian dynamics simulation of tracer diffusion in a cross-linked network

Brownian dynamics simulation is employed to study the self-diffusion of tracer particles in a cross-linked gel network based on a coarse-grained bead-spring lattice model. Several effects are investigated including the network porosity, flexibility, degree of cross linking, and electrostatic interaction. For uncharged systems, the tracer long-time diffusivity is found to decrease with deceasing porosity and flexibility, but with increasing degree of cross linking. For charged systems, the diffusion is further hindered by the electrostatic interaction, regardless of whether the tracer particle and the network are oppositely or similarly charged. However, there exists a difference in the hindrance mechanism between the two cases. For the former, a substantial decrease in the diffusivity can occur at high network porosities due to electrostatic entrapment.

Water and Polymer Dynamics in Chemically Cross-Linked Hydrogels of Poly(vinyl alcohol): A Molecular Dynamics Simulation Study

A topologically extended model of a chemically cross-linked hydrogel of poly(vinyl alcohol) (PVA) at high hydration degree has been developed for a molecular dynamics simulation with atomic detail at 323 K. The analysis of the 5 ns trajectory discloses structural and dynamic aspects of polymer solvation and elucidates the water hydrogen bonding and diffusion in the network. The features of local polymer dynamics indicate that PVA mobility is not affected by structural constraints of chemical junctions at the investigated cross-linking density, with a prevailing dumping effect due to water interaction. Simulation results are validated by a favorable comparison with findings of an incoherent quasi-elastic neutron scattering study of the same hydrogel system.

Supercooled Water in PVA Matrixes. II. A Molecular Dynamics Simulation Study and Comparison with QENS Results

Molecular dynamics (MD) simulations were carried out to elucidate the dynamic behavior of water confined in poly(vinyl alcohol), PVA, hydrogels. Model topology is supported by experimental network parameters, and simulation results are compared to an incoherent quasielastic neutron scattering (QENS) investigation carried out on PVA hydrogels. From the QENS dynamic scattering law (part I), a random jump model was adopted for the description of water diffusion to extract a microscopic diffusion coefficient and a residence time between two "jumps". In the present work, consistently with this framework, water diffusion parameters as diffusion coefficients and residence times have been evaluated using the mean square displacement of water in a time window of 10 ps and the time autocorrelation function of water hydrogen bonds. The calculated parameters are in good agreement with the experimental ones, giving confidence to this approach. Further developments are in progress to take into account a more realistic description of hydrogel structure in the molecular dynamics simulations.

Ultrafast dynamics in aqueous polyacrylamide solutions

We have investigated the ultrafast dynamics of aqueous polyacrylamide ([-CH(2)CH(CONH(2))-](n), or PAAm) solutions using femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The observed aqueous PAAm dynamics are nearly identical for both M(w) = 1500 and 10 000. Aqueous propionamide (CH(3)CH(2)CONH(2), or PrAm) solutions were also studied, because PrAm is an exact model for the PAAm constitutional repeat unit (CRU). The longest time scale dynamics observed for both aqueous PAAm and PrAm solutions occur in the 4-10 ps range. Over the range of concentrations from 0 to 40 wt %, the picosecond reorientation time constants for the aqueous PAAm and PrAm solutions scale linearly with the solution concentration, despite the fact that the solution shear viscosities vary exponentially from 1 to 264 cP. For a given value of solution concentration in weight percent, constant ratios of measured reorientation time constants for PAAm to PrAm are obtained. This ratio of PAAm to PrAm reorientation time constants is equal to the ratio of the volume for the PAAm constitutional repeat unit (-CH(2)CHCONH(2)-) to the molecular volume of PrAm. For these reasons, we assign the polymer reorientation dynamics to motions of the entire constitutional repeat unit, not only side group motions. Simple molecular dynamics simulations of H[-CH(2)CH(CONH(2))-](7)H in a periodic box with 180 water molecules support this assignment. Amide-amide and amide-water hydrogen-bonding interactions lead to strongly oscillatory femtosecond dynamics in the Kerr transients, peaking at 80, 410, and 750 fs.