Effect of End Groups on the Cloud Point Temperature of Aqueous Solutions of Thermoresponsive Polymers: An Inside View by Flory-Huggins Theory.

In an effort to gain insight into the origin of the effects of end groups on the cloud point temperature (Tcp) as a function of the polymer molar mass of thermoresponsive polymers with lower critical solution behavior in dilute aqueous solutions, we use the Flory-Huggins (FH) theory amended for end groups. The theory was applied to available experimental data sets of poly(N-isopropylacrylamide) (PNIPAM), poly(4-vinylbenzyl methoxytris(oxyethylene) ether) (PTEGSt), and poly(α-hydro-ω-(4-vinylbenzyl)tetrakis(oxyethylene) ether) (PHTrEGSt). The theory relates the variations in TcpM,ϕcp for different end groups to the effective FH χ parameter of the end groups and explains the qualitative notion that the influence of the end groups is related to the hydrophobicity/hydrophilicity of the end groups relative to that of the so called intrinsic TcpM,ϕcp response of a polymer without end groups. The limits to the applicability of the FH theory are established, and a set of possible theoretical improvements is considered. The ultimate scrutiny of the simple FH theory and suggested improved theories must await the measurement of truly thermodynamic cloud points; the available cloud points are merely estimations of the thermodynamic cloud point, for which the deviation to the true cloud point cannot be established with sufficient accuracy.

Enhancing Metal Separations by Liquid-Liquid Extraction Using Polar Solvents.

The less polar phase of liquid-liquid extraction systems has been studied extensively for improving metal separations; however, the role of the more polar phase has been overlooked for far too long. Herein, we investigate the extraction of metals from a variety of polar solvents and demonstrate that, the influence of polar solvents on metal extraction is so significant that extraction of many metals can be largely tuned, and the metal separations can be significantly enhanced by selecting suitable polar solvents. Furthermore, a mechanism on how the polar solvents affect metal extraction is proposed based on comprehensive characterizations. The method of using suitable polar solvents in liquid-liquid extraction paves a new and versatile way to enhance metal separations.

Mesoscopic simulations of temperature-dependent anchoring and wetting behavior at aqueous-liquid crystal interfaces in the presence of a rod-coil amphiphilic monolayer.

Dissipative particle dynamics simulations have been applied to study the temperature dependent anchoring and wetting behavior of thermotropic liquid crystals (LCs) in the presence of a rod-coil amphiphilic monolayer at the aqueous-LC interface. Upon cooling in the nematic phase, a thermally-induced anchoring transition from homeotropic through tilted to planar has been observed. The growth and propagation of smectic order from the interfaces to the bulk nematic LCs are demonstrated to be mainly responsible for this novel transition sequence. In particular, when a complete smectic layer in the amphiphile monolayer is induced around the bulk transition of nematic-smectic-A, the propagation of homeotropic alignment fails instantly and a unique planar anchoring configuration is formed instead. While heating towards the isotropic phase, simulation results show that the nematic-isotropic transition of confined LCs is slightly shifted to a higher temperature, and a nematic wetting layer with homeotropic alignment appears in the rod block monolayer when the bulk LCs is isotropic. Our systematic simulations throughout the whole phase regimes of LCs provide important molecular-level insight into how the coupling between the ordering of LCs and adsorbents and their temperature dependencies affect the anchoring behavior in this complex system, which should be instrumental in the rational design and application of advanced LC-based biosensors with optimal operating temperature range.

A parallel algorithm for step- and chain-growth polymerization in molecular dynamics.

Classical Molecular Dynamics (MD) simulations provide insight into the properties of many soft-matter systems. In some situations, it is interesting to model the creation of chemical bonds, a process that is not part of the MD framework. In this context, we propose a parallel algorithm for step- and chain-growth polymerization that is based on a generic reaction scheme, works at a given intrinsic rate and produces continuous trajectories. We present an implementation in the ESPResSo++ simulation software and compare it with the corresponding feature in LAMMPS. For chain growth, our results are compared to the existing simulation literature. For step growth, a rate equation is proposed for the evolution of the crosslinker population that compares well to the simulations for low crosslinker functionality or for short times.

Relating hydrogen-bonding interactions with the phase behavior of naproxen/PVP K 25 solid dispersions: evaluation of solution-cast and quench-cooled films.

In this work, we investigated the relationship between various intermolecular hydrogen-bonding (H-bonding) interactions and the miscibility of the model hydrophobic drug naproxen with the hydrophilic polymer polyvinylpyrrolidone (PVP) across an entire composition range of solid dispersions prepared by quasi-equilibrium film casting and nonequilibrium melt quench cooling. The binary phase behavior in solid dispersions exhibited substantial processing method dependence. The solid state solubility of crystalline naproxen in PVP to form amorphous solid dispersions was 35% and 70% w/w naproxen in solution-cast films and quench-cooled films, respectively. However, the presence of a single mixed phase glass transition indicated the amorphous miscibility to be 20% w/w naproxen for the films, beyond which amorphous-amorphous and/or crystalline phase separations were apparent. This was further supported by the solution state interactions data such as PVP globular size distribution and solution infrared spectral profiles. The borderline melt composition showed cooling rate dependence of amorphization. The glass transition and melting point depression profiles of the system were treated with the analytical expressions based on Flory-Huggins mixing theory to interpolate the equilibrium solid solubility. FTIR analysis and subsequent spectral deconvolution revealed composition and miscibility dependent variations in the strength of drug-polymer intermolecular H-bonding. Two types of H-bonded populations were evidenced from 25% w/w and 35% w/w naproxen in solution-cast films and quench-cooled films, respectively, with the higher fraction of strongly H-bonded population in the drug rich domains of phase separated amorphous film compositions and highly drug loaded amorphous quench-cooled dispersions.

A Replica Exchange Molecular Dynamics Simulation of a Single Polyethylene Chain: Temperature Dependence of Structural Properties and Chain Conformational Study at the Equilibrium Melting Temperature.

The conformational properties of a finite length polyethylene chain were explored over a wide range of temperatures using a replica exchange molecular dynamics simulation providing high quality simulation data representative for the equilibrium behavior of the chain molecule. The radial distribution function (RDF) and the structure factor S(q) of the chain as a function of temperature are analyzed in detail. The different characteristic peaks in the RDF and S(q) were assigned to specific distances in the chain and structural changes occurring with the temperature. In S(q), a peak characteristic for the order in the solid state was found and used to determine the equilibrium melting temperature. A detailed scaling analysis of the structure factor covering the full q range was performed according to the work of Hammouda. In the Θ region, a quantitative analysis of the full structure factor was done using the equivalent Kuhn chain, which enabled us to assign the Θ region of our chain and to demonstrate, in our particular case, the failure of the Gaussian chain approach. The chain conformational properties at the equilibrium melting temperature are discussed using conformational distribution functions, using the largest principal component of the radius of gyration and shape parameters as order parameters. We demonstrate that for the system studied here, the Landau free energy expression based on this conformational distribution information leads to erroneous conclusions concerning the thermodynamic transition behavior. Finally, we focus on the instantaneous conformational properties at the equilibrium melting temperature and give a detailed analysis of the conformational shapes using different shape parameters and a simulation snapshot. We show that the chain does not only take the lamellar rod-like and globular conformational shapes, typical of the solid and liquid states, but can also explore many other conformational states, including the toroidal conformational state. It is the first demonstration that a flexible molecule like PE can also take a toroidal conformational state, which is normally linked to stiffer chains.

Dynamics of the crystal to plastic crystal transition in the hydrogen bonded N-isopropylpropionamide.

N-Isopropylpropionamide (NiPPA), which can self-associate via hydrogen bonds, was found to undergo a solid-solid transition as identified by DSC and X-ray diffraction. Below the melting temperature of 51 °C NIPPA adopts a plastic crystalline state with a tetragonal unit cell until it transforms into an ordered crystal with a monoclinic structure at temperatures ≤10 °C. Dielectric spectroscopy was used to characterize the dynamics of the system, determining the activation parameters for the plastic to crystalline phase transition. The activation enthalpy is relatively high, as expected for a system that involves hydrogen bonds. However, most of the activation energy as the plastic phase assumes a more crystalline state is due to the activation entropy, suggesting that the increased cooperativity observed in the relaxation processes is due to a steric locking of the molecules.

Energy-driven asymmetric partitioning of a semiflexible polymer between interconnected cavities.

The distribution of a semiflexible chain in the volume of two interconnected spherical cavities of equal size has been investigated by using Monte Carlo simulations. The chain possessed an extension exceeding that of the cavity, leading to large probabilities of translocated states despite the entropic penalty of passing the narrow passage. Furthermore, an asymmetric state with unequal subchain lengths in the two cavities was more favorable than the symmetric state. The preference for the asymmetric state is driven by the bending energy. Basically, in the symmetric state both subchains are forced to be bent, whereas in the asymmetric case only one of the subchains must bend, leading to an overall smaller bending penalty and overall smaller free energy of the asymmetric state. These results are in contrast to the entropy-controlled partitioning of polymers into confinement and the symmetric translocation state appearing for flexible polymers.

Nascent crystallization of a growing chain on a catalyst surface: a nonequilibrium molecular dynamics simulation study.

The growing chain molecular dynamics (GCMD) simulation method, a new nonequilibrium molecular dynamics code, is proposed to simulate the polymer chain aggregation behavior during polymerization on a catalyst surface. We found that the growing chain crystallizes on the surface in two stages: the nucleation stage and the crystal growth stage. In the first part of the nucleation period, the short polymerizing chain first absorbs on the surface and can be in either an ordered or disordered structure. Still in the nucleation period, when the chain reaches a degree of polymerization, about 100 bonds, the chain folds into a stable nucleus on the substrate with 3-5 stems. In the crystal growth stage where the polymerization also proceeds, we observed a stem elongation process in combination with a chain folding process. In the stem elongation step, the number of stems in the nucleus remains constant, and all the stems expand together to a length of ca. 5-25 ns. In the subsequent chain folding step, the stem length decreases about 20 bonds within a period of ca. 0.1-0.5 ns. During chain growth, the elongation process and the folding process occur in an alternating and repeated fashion. The crystallization mechanism of the polymerizing chain was discussed.

Isothermal crystallization kinetics study on aqueous solution of poly(vinyl methyl ether) by FTIR and optical microscopy method.

A study on the isothermal crystallization kinetics of aqueous solution of poly (vinyl methyl ether) (PVME) was carried out by Fourier transform infrared (FTIR) and optical microscopy respectively. IR spectra of PVME solution were measured as a function of time under the isothermal crystallization conditions. With the process of crystallization, the phase of solution changes from transparent state to opaque one within around 1-2 min for 40 or 45 wt % PVME sample, the C-H symmetric stretching bands (nus(CH3)) shift to lower wave number 2823 cm(-1). The red shift of nus(CH3) absorption band was not observed in the transparent noncrystallization area. Using the temperature jump method, we determined the growth rate of ice crystal between the glass transition temperature Tg and the melting temperature Tm. At the different crystallization temperatures Tc, the different morphologies and dimension of ice crystal are also observed.

Phase behavior of N-(Isopropyl)propionamide in aqueous solution and changes in hydration observed by FTIR spectroscopy.

The phase behavior of N-(isopropyl)propionamide (NiPPA), which is the repeat unit of poly(N-isopropyl-acrylamide) (PNiPA), in deuterated aqueous solution was investigated. Temperature induces a phase separation of NiPPA in aqueous solution above the lower critical solution temperature (LCST), as shown by optical microscopy. The phase behavior of NiPPA resembles that of PNiPA, but the demixing domain is much narrower. Monitoring the liquid-liquid phase separation by Fourier transform infrared (FTIR) spectroscopy reveals that a fraction of the NiPPA molecules becomes dehydrated above the LCST. Our findings therefore shed new light on the results of a recent dielectric relaxation experiment in which the behavior of NiPPA was found to be "completely contrary" to that of PNiPA. It is argued that the differences in the spectroscopic results of polymer and repeat unit solutions can be easily understood from the phase behavior of NiPPA and PNiPA.

Coarse-grained molecular dynamics modeling of strongly associating fluids: thermodynamics, liquid structure, and dynamics of symmetric binary mixture fluids.

Symmetric binary mixtures capable of strong association via a highly directional and saturable specific interaction between unlike molecules are investigated by canonical molecular dynamics simulations. The specific interaction of the molecules is defined in a new coarse-grained pair potential that can be applied in continuous molecular dynamics as well as in Monte Carlo simulations. The thermodynamic, structural, and dynamic properties of the associating mixture fluids are investigated as a function of density, temperature, and association strength of the specific interaction. Detailed analysis of the simulation data confirms a two-stage mechanism in the formation of specific bonds with increasing interaction strength, including a fast dimerization process and a subsequent stage of perfecting the bonds. A large heat capacity peak is found during the formation or breaking of the bonds, reflecting the large energy fluctuation introduced by the strong association. The fractions of nonbonded molecules obtained from the simulations as a function of density, temperature, and interaction strength are in excellent agreement with the predictions of Wertheim's thermodynamic perturbation theory. The translational and rotational dynamics of the Tmer mixture are effectively retarded with increasing association strength and are analyzed in terms of autocorrelation functions and a non-Gaussian parameter for the translational dynamics. The lifetimes of molecules in bonded and nonbonded states provide detailed information about the transformation of molecules between the bonded state and the nonbonded state. Finally, simulation sampling problems inherent to strongly interacting systems are easily overcome using the parallel tempering simulation technique. This latter result confirms that with the new continuous coarse-grained simulation potential we have a versatile and flexible interaction potential that can be used with many available molecular dynamics and Monte Carlo algorithms under various ensembles.

Coarse-grained molecular dynamics modeling of associating fluids: thermodynamics, liquid structure, and dynamics in the limit of zero association strength.

A continuous coarse-grained potential model for associating fluids, consisting of an off-center specific site bonded with a harmonic potential to a center particle, has been developed and used in canonical molecular dynamics simulations. The thermodynamic, structural, and dynamic properties of the limiting nonassociating reference coarse-grained fluid are investigated as functions of the mass distribution and bond strength between center and site particles. It is theoretically shown and confirmed by simulation that in this limit variations in these potential parameters do not alter the equation of state of the reference coarse-grained fluid but that they have profound influences on both the translational and the rotational dynamics. From the simulation results we arrive at some guidelines that should be kept in mind in the selection of appropriate values for the model parameters. This work provides the precursory knowledge for the study of coarse-grained associating fluids using the conventional molecular dynamics method.

Phase transformations in aqueous low molar mass poly(vinyl methyl ether) solutions: theoretical prediction and experimental validation of the peculiar solvent melting line, bimodal LCST, and (adjacent) UCST miscibility gaps.

Supported by theoretical predictions based on the Wertheim Lattice Thermodynamic Perturbation Theory, modulated temperature differential scanning calorimetry (MTDSC) was used to further the knowledge of the phase behavior of aqueous poly(vinyl methyl ether) (PVME) solutions. Using a narrowly dispersed low molar mass PVME, we determined the following phase boundaries: (i) a bimodal lower critical solution temperature (LCST) miscibility gap at physiological temperature (around 37 degrees C), (ii) an upper critical solution temperature (UCST) two-phase area at sub-zero temperatures and high polymer concentration, and (iii) the melting line of the solvent across the entire concentration range, showing a peculiar stepwise decrease with composition. The location of the glass transition region and its influence on the crystallization/melting behavior of the solvent is discussed.

The influence of pressure on the lower critical solution temperature miscibility behavior of aqueous solutions of poly(vinyl methyl ether) and the relation to the compositional curvature of the volume of mixing.

In mixtures of PVME and water, the influence of pressure on the LCST miscibility gap is determined covering the whole composition range and pressures from atmospheric pressure up to 900 MPa. The cloud point curve at atmospheric pressure has the characteristic bimodal shape in agreement with literature data. Upon increasing pressure the cloud point curve at the low concentration side decreases with pressure, whereas at the high concentrations the cloud point curve increases with pressure. The overall influence of pressure results in a less pronounced bimodality and ultimately the bimodal shape disappears. In addition to the pressure dependence of the miscibility behavior, the density of mixtures of water and PVME are determined at atmospheric pressure. The experimental excess specific volumes are negative for all measured compositions, but the compositional curvature varies with composition. The curvature of the excess specific volume is positive for the higher concentrations but it is negative in the lower composition range. The density measurements are linked to the pressure dependence of the LCST miscibility behavior using exact thermodynamic relationships. The excess specific volume and miscibility results are shown to be in good agreement. Moreover, it is shown that the Clapeyron equation, which is exact for pure components and also frequently assumed to apply to mixtures, is not valid in the system PVME/water. The system PVME/water is an example where the usual approximation of one-to-one correspondence between curvature and excess volume does not apply. Finally, the molecular origins for the observed excess volume and miscibility behavior are briefly discussed from theoretical and molecular simulation points of view.

Upper critical solution temperature phase behavior, composition fluctuations, and complex formation in poly (vinyl methyl ether)/D2O solutions: small-angle neutron-scattering experiments and wertheim lattice thermodynamic perturbation theory predictions.

Small-angle neutron-scattering measurements are presented for homogeneous mixtures of poly (methyl vinyl ether) (PVME) and deuterium oxide (D(2)O) at high polymer concentrations and for temperatures lower than the equilibrium melting point of the solvent. The experimental data are analyzed to give values for the second-order compositional derivative of the Gibbs energy and the Ornstein-Zernike correlation length. The experimental data together with earlier SANS data determined at higher temperatures cannot be represented with an extended Flory-Huggins (F-H) interaction function depending on composition and temperatures. The experimental data confirm the existence of a narrow upper critical solution temperature (UCST) miscibility gap at high concentrations in agreement with theoretical predictions of the Wertheim lattice thermodynamic perturbation theory (LTPT). The Wertheim LTPT incorporates the influence of hydrogen bonding and predicts not only the existence of bimodal lower critical solution temperature (LCST) phase behavior but also the occurrence of highly unconventional two narrow adjacent UCST miscibility gaps. Finally, the experimental data do not support the existence of a stable molecular complex at the investigated temperatures and compositions. Even at the lowest investigated temperature, the energy required to induce typical Ornstein-Zernike-like concentration fluctuations is smaller than the thermal energy. Also, in this case, the Wertheim LTPT provides a theoretical basis to understand the formation of polymer solvent associations in PVME/water.