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(-hydro--(4-vinylbenzyl)tetrakis(oxyethylene) ether) (PHTrEGSt). The theory relates the variations in TcpM,ϕcp for different end groups to the 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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.