Effect Of Solvent Quality Research Articles

Effect of Solvent Quality on Laminar Slip Flow Penetration of Poly(N-isopropylacrylamide) Films with an Exploration of the Mass Transport Mechanism.

The effect of solvent quality on the slip flow penetration of polymer films was evaluated by monitoring small-molecule mass transport under varying laminar flow rates using Förster resonance energy transfer in combination with total internal reflectance fluorescence microscopy (FRET-TIRFM). For thin films of poly(N-isopropylacrylamide) (pNIPAM), solvents with solvent quality ranging from good to poor were studied. The solvents used were composed of varying mole ratios of methanol and water in order to take advantage of the unique cononsolvency phenomenon of pNIPAM such that differences in the physicochemical properties of these solvents were insignificant for fluorescence detection. FRET quenching of a donor fluorophore covalently tethered on the substrate surface at the bottom of the pNIPAM film by a solution-confined acceptor was monitored as a function of time. Quenching curves were fit to a combined Taylor-Aris-Fickian mass transport model for the acceptor, rhodamine B (RhB) or 2-nitrobenzylaclohol (2-NBA), allowing apparent diffusion coefficients to be determined and used to assess slip flow penetration into the polymer film. An increase in the apparent diffusion coefficient of tracer molecules was observed with increasing laminar flow rate for all solvents, indicating that mass transport processes in the pNIPAM film are significantly perturbed by laminar slip flow penetration. In going from poor solvents, 31 mol % MeOH/H2O and 20 mol % MeOH/H2O, to the theta solvent, 13 mol % MeOH/H2O, and finally to a good solvent, 100% methanol, the slip length increases from 25 to 37 to 70 to 128 nm, with the corresponding percentage of the film penetrated by slip flow increasing from 19 to 27 to 42 to 57%, respectively. The apparent diffusion coefficients of the two acceptors, RhB and 2-NBA, which differ substantially in size and charge, in pNIPAM films under identical conditions were found to be of the same order of magnitude, albeit with a small difference (∼10%) due to inherently different diffusive properties. Therefore, the dominant mechanism for the mass transport of small molecules in densely grafted thin pNIPAM brush films is suggested to be linear Fickian diffusion under the chosen laminar flow conditions with linear flow velocities ranging from 192 to 2952 μm/s. High-quality fits to a Taylor-Aris-Fickian diffusion model of the experimental breakthrough curves obtained with both acceptor molecules further substantiate the proper use of this model and the validity of the FRET-TIRFM method.

Flow-induced polymer separation through a nanopore: effects of solvent quality.

Using a hybrid simulation method that combines a lattice-Boltzmann approach for the flow and a molecular dynamics model for the polymer, we investigated the effect of solvent quality on the flow-induced polymer translocation through a nanopore. We demonstrate the nontrivial dependence of the translocation dynamics of polymers on the solvent quality, i.e., the enhancement in the polymer insolubility increases the critical velocity flux and shortens the translocation time. Accordingly, we propose a new strategy to separate polymers with different solubilities via their translocations in the nanopore by adjusting the velocity flux of the flow, which appears to be promising for the design of micro-scaled polymer separation devices.

Effect of solvent quality and chain density on normal and frictional forces between electrostatically anchored thermoresponsive diblock copolymer layers

Equilibration in adsorbing polymer systems can be very slow, leading to different physical properties at a given condition depending on the pathway that was used to reach this state. Here we explore this phenomenon using a diblock copolymer consisting of a cationic anchor block and a thermoresponsive block of poly(2-isopropyl-2-oxazoline), PIPOZ. We find that at a given temperature different polymer chain densities at the silica surface are achieved depending on the previous temperature history. We explore how this affects surface and friction forces between such layers using the atomic force microscope colloidal probe technique. The surface forces are purely repulsive at temperatures <40°C. A local force minimum at short separation develops at 40°C and a strong attraction due to capillary condensation of a polymer-rich phase is observed close to the bulk phase separation temperature. The friction forces decrease in the cooling stage due to rehydration of the PIPOZ chain. A consequence of the adsorption hysteresis is that the friction forces measured at 25°C are significantly lower after exposure to a temperature of 40°C than prior to heating, which is due to higher polymer chain density on the surface after heating.

Solvent-quality-dependent contact formation dynamics in proteins

The mean time of contact formation between two ends of a protein chain shows power law dependence with respect to the number of residues, . Fluorescence quenching measurements based on triplet–triplet energy transfer show variation in the value of scaling exponent α for different protein–solvent systems. This points to the relevance of the protein–solvent interactions (solvent quality) and hydrodynamic interactions in determining the time scale of contact formation. Here, starting from a non-Markovian diffusion equation supplemented with an exponential sink term that accounts for the energy transfer reaction between the donor and acceptor groups, we calculate the mean time of contact formation using the Wilemski–Fixman closure approximation. The non-Markovian diffusion–reaction equation includes the effects of solvent quality and hydrodynamic interaction in a mean-field fashion. It shows that the contact formation dynamics is mainly governed by two time scales, the reciprocal of the intrinsic rate of quenching , and the relaxation time of the coarse-grained residue of size b. A comparison of these two time scales yields reaction-controlled and diffusion-controlled kinetic regimes with different values of the scaling exponents. In between these regimes the increase in the number of residues switches the kinetics from RC at low N to DC at large N. These general results suggest that experimental estimates of the scaling exponents reflect solvent-quality dependence of the RC kinetics.

Reconfiguration dynamics in folded and intrinsically disordered protein with internal friction: Effect of solvent quality and denaturant

We consider a flexible chain with internal friction in a harmonic confinement and extend it to include the effects of solvent quality at the mean field level by introducing a Flory type exponent ν. The strength of the harmonic confinement (kc) accounts for the denaturant concentration and connects to the internal friction of the chain (ξint) through an ansatz. Our calculated reconfiguration times falling in the range of 5–50 ns are found out to be within 10%–15% of the experimentally measured reconfiguration times of the folded cold shock protein and the intrinsically disordered protein prothymosin α. In addition, our calculations show that the reconfiguration time scales with the chain length N as ∼Nα, where α depends weakly on the internal friction but has rather stronger dependence on the solvent quality. In the absence of any internal friction, α=2ν+1 and it goes down in the presence of internal friction, but chain reconfiguration slows down in general. On the contrary, in a poorer solvent chain reconfiguration and looping become faster even though the internal friction is higher in the collapsed state.

Monte Carlo simulation of the stability and structure of polyethylene oxide nanodroplet with different solvent qualities

Monte Carlo simulation for poly(ethylene oxide), PEO, was applied to study the effect of solvent quality on the stability and structure of PEO nanodroplets in the poor solvent regime. The Hamiltonian included intrachain and non-bonded interactions. The intrachain interactions were based on an extended rotational isomeric state (RIS) model, and the non-bonded interactions were based on a discretized form of the Lennard-Jones (LJ) potential. The solvent was included implicitly into the simulations by manipulating the attractive portion of the long-range interactions. The results of the simulations clearly showed that as the solvent became poorer, the nanodroplet became denser, and their interfacial thickness decreased. These changes led to PEO chains assuming more compact conformations to enable better packing in the bulk region of the nanodroplets. The average shape of the nanodroplets changed from spherical to ellipsoidal as the solvent quality decreased. This shape anisotropy was attributed to chains assuming compact conformations while trying to pack more efficiently in the bulk region. The chain end bead distribution was strongly influenced by the solvent quality. As the solvent became poorer, a greater proportion of the end beads were excluded from the bulk region because of the need to achieve better packing. Simulations also suggested the formation of a layered structure as the solvent quality decreased.

Coarse-graining and phase behavior of model star polymer–colloid mixtures in solvents of varying quality

We study the effective interactions and phase behavior of star polymer-colloid mixtures through theory and Monte Carlo simulations. We extend previous theoretical approaches for calculating the effective star-colloid pair potential to take into account attractive contributions, which become significant at worsening solvent conditions. In order to assess the validity of our simulation and theory, we compute the effective interactions via virtual move parallel tempering Monte Carlo simulations using a microscopic bead-spring model for the star polymer and achieve excellent agreement. Finally, we perform grand canonical Monte Carlo simulations of the coarse-grained systems to study the effect of solvent quality on the phase behavior.

Conformations of molecular brushes based on polyimide and poly(methyl methacrylate) in selective solvents: Experiment and computer simulation

The effect of solvent quality with respect to main and side chains on the conformations of molecular brushes is investigated by the methods of molecular hydrodynamics and optics as well as by computer simulation. Copolymers with a polyimide backbone and poly(methyl methacrylate) side chains are studied in solvents featuring strongly different thermodynamic qualities (chloroform, ethyl acetate, and 3-heptanone). The studied samples have close total molecular masses and backbone lengths but differ in grafting densities and lengths of side chains: a brush with densely grafted and relatively short side chains versus a brush with loosely grafted but very long side chains. For both types of brushes, similar changes in hydrodynamic behavior with improvement in the solvent quality are found experimentally. Computer simulation shows that these changes have different origins. In the former case, macromolecules are elongated, and their volumes grow simultaneously, while in the latter case, the shape of the macromolecules remains close to spherical and the changes in hydrodynamic parameters are mainly due to an increase in the volume of macromolecules.

Tuning the Morphology and Performance of Low Bandgap Polymer:Fullerene Heterojunctions via Solvent Annealing in Selective Solvents

Low bandgap polymer (LBG):fullerene mixtures are some of the most promising organic photovoltaic active layers. Unfortunately, there are no post‐deposition treatments available to rationally improve the morphology and performance of as‐cast LBG:fullerene OPV active layers, where thermal annealing usually fails. Therefore, there is a glaring need to develop post‐deposition methods to guide the morphology of LBG:fullerene bulk heterojunctions towards targeted structures and performance. In this paper, the structural evolution of PCPDTBT:PCBM mixtures with solvent annealing (SA) is examined, focusing on the effect of solvent quality of the fullerene and polymer in the annealing vapor on morphological evolution and device performance. The results indicate that exposure of this active layer to the solvent vapor controls the ordering of PCPDTBT and PCBM phase separation very effectively, presumably by inducing component mobility as the solvent plasticizes the mixture. These results also unexpectedly indicate that solvent annealing in a selective solvent provides a method to invert the morphology of the LBG:fullerene mixture from a polymer aggregate dispersed in a polymer:fullerene matrix to fullerene aggregates dispersed in a polymer:fullerene matrix. The judicious choice of solvent vapor, therefore, provides a unique method to exquisitely control and optimize the morphology of LBG conjugated polymer/fullerene mixtures.

Controlling microtube permeability via grafted polymers and solvent quality

We examine pressure-driven flow through a microtube with grafted polymers using a "doubly self-consistent field" steady-state theory. Our focus is on the structure of the polymer layer, the tube permeability, and the effects of solvent quality, for different regimes of open and closed tubes. We find that, within experimentally attainable pressure gradients, the flow has very little effect on the grafted layer. However, the polymers, and in particular variations in the solvent quality and cylinder radii, can have large effects on the flow. We find that the permeability can either increase or decrease with either the radius or solvent quality, and we identify the regimes for different behaviors in terms of general parameters that can be used to generalize to other systems. This allows us to identify regimes where the systems are most sensitive to these "tuning" parameters, and we find that they correspond to the boundaries between open and closed tubes identified earlier.

Effects of Solvent Quality and Degree of Polymerization on the Critical Micelle Temperature of Poly(ethylene oxide-b-n-butyl methacrylate) in Ionic Liquids

Block copolymers are attractive building blocks for designing micelles having complex shapes and functionality, but experimental investigations into the detailed thermodynamics of block copolymer micellization have been constrained. In this work, we take advantage of the favorable solvent properties of ionic liquids to study the thermodynamics of block copolymer micelle formation. Specifically, we investigate the effects of solvent quality and degree of polymerization on the critical micelle temperature (cmt) of poly(ethylene oxide-b-n-butyl methacrylate) (PEO-b-PnBMA) in mixtures of two ionic liquids: 1-butyl-3-methylimidazolium:bis(trifluoromethylsulfonyl)imide ([BMIm][TFSI]) and 1-ethyl-3-methylimidazolium:TFSI ([EMIm][TFSI]). The solvent quality for the core-forming block of the block copolymer, PnBMA, is varied over a wide range by blending the two ionic liquids in different ratios, resulting in a large variation in the cmt of PEO-b-PnBMA. It is shown that the interaction parameter between the solvent and PnBMA at the cmt is approximately constant for all of the ionic liquid mixtures. The enthalpies and entropies of micelle formation also do not vary with ionic liquid composition, suggesting that the nature of the polymer/solvent and solvent/solvent interactions do not change much as the ionic liquid composition is varied despite the large change in solvent quality. Furthermore, the cmt is shown to depend on the degree of polymerization of PnBMA as predicted by theory.

Dendritic brushes under theta and poor solvent conditions

The effects of solvent quality on the internal stratification of polymer brushes formed by dendron polymers up to third generation were studied by means of molecular dynamics simulations with Langevin thermostat. The distributions of polymer units, of the free ends, the radii of gyration, and the back folding probabilities of the dendritic spacers were studied at the macroscopic states of theta and poor solvent. For high grafting densities we observed a small decrease in the height of the brush as the solvent quality decreases. The internal stratification in theta solvent was similar to the one we found in good solvent, with two and in some cases three kinds of populations containing short dendrons with weakly extended spacers, intermediate-height dendrons, and tall dendrons with highly stretched spacers. The differences increase as the grafting density decreases and single dendron populations were evident in theta and poor solvent. In poor solvent at low grafting densities, solvent micelles, polymeric pinned lamellae, spherical and single chain collapsed micelles were observed. The scaling dependence of the height of the dendritic brush at high density brushes for both solvents was found to be in agreement with existing analytical results.

The Effect of Solvatochromism on the Interfacial Morphology of P3HT-CdS Nanowire Nanohybrids

Increasing performance in organic/inorganic bulk heterojunctions hybrid photovoltaic systems hinges not only on the structure of the inorganic component but also on the morphology of the polymeric component. Changing the morphology of the organic component is a facile way of changing the morphology of the interface between the inorganic and organic components in the bulk heterojunction system. Engineering this interface to more efficiently split photogenerated excitons and transport these carriers to electrodes can increase the efficiency of photovoltaic devices. In this report, we investigate the effect of solvent quality on the morphology of the poly(3-hexylthiophene)-2,5-diyl (P3HT) polymer-semiconductor interface (solvatochromism). We have found that creating more order within the P3HT main chain in solution and prior to deposition has a profound effect on the nature of the P3HT-CdS nanowire interface. Solvents with a larger difference in solubility parameter, Δδ, relative to P3HT, such as methanol and isopropanol, create larger rod domains in the P3HT main chain and result in larger domains of crystalline P3HT at the interface. Solvents with similar solubility parameters as P3HT, such as pyridine and hexanol, create relatively shorter rod domains in the main chain and, as a result, nanohybrids with reduced crystallinity. The results of this paper further cement the importance of manipulating the rod-coil transition in the conducting polymers such as P3HT to improve the crystallinity at the polymer-semiconductor interface that can easily be scaled up to improve the efficiency of bulk heterojunction photovoltatic systems.

Conformation and Fluorescence Property of Poly(3-hexylthiophene) Isolated Chains Studied by Single Molecule Spectroscopy: Effects of Solvent Quality and Regioregularity

Poly(3-alkylthiophene)s (P3AT) are among the most widely used conjugated polymers for applications in polymer solar cells and thin-film transistors. In this study, the influence of the casting solvent on the conformational structure and fluorescence properties of the isolated chains of a regioregular and a regiorandom poly(3-hexylthiophene) (denoted by rr-P3HT and ra-P3HT, respectively) has been systematically investigated by single molecule spectroscopy. The isolated chains were dispersed in polystyrene matrix by casting from their ultradilute solutions with tetrahydrofuran (THF) or toluene. ra-P3HT chains were found to contain mostly high-energy emitting species with short conjugating length, while the low-energy emitting sites existed predominantly in rr-P3HT chains irrespective of the casting solvent. The measurement of the modulation depth revealed that rr-P3HT chains cast from THF formed defect cylinder structure. The regioregular chains cast from toluene exhibited relatively regular folding to form rod structure as a result of the amplification of the segmental attraction in the poorer solvent. The casting solvent had insignificant effect on the conformational structure of ra-P3HT, where the isolated chains predominantly showed defect coil conformation irrespective of the casting solvent.

Solvent effect on phase transition of lyotropic rigid-chain liquid crystal polymer studied by dissipative particle dynamics

Effect of solvent quality on phase transition of lyotropic rigid-chain liquid crystal polymer is studied by dissipative particle dynamics simulation. A rod composed of fused DPD particles is used to represent the solvated rigid polymer. The effect of solvent quality is investigated by adjusting the repulsion parameter between the rods and solvent particles. The simulation shows that the solvent quality has significant influences on the phase transition behavior of the system and the influences are also closely related with the concentrations before the solvent becomes extremely poor. The influences of the solvent quality are attributed to the interplay between the immiscibility-induced phase separation and nematic-isotropic phase transition, which can be described by the binodal lines and nematic-isotropic transition lines in the phase diagrams. If a system is located in the one phase region, it will undergo a typical nematic to isotropic phase transition as the temperature increases. If a system is located in the biphasic region, there are two different types of nematic-isotropic phase transitions depending on whether the transition temperature from the biphasic region to isotropic phase region is lower or higher than the nematic-isotropic transition temperature of the concentrated phase. The first type corresponds to the transition from the biphasic region to the isotropic one phase region and the second type is attributed to the nematic-isotropic phase transition that occurs in the concentrated phase.

Corrigendum to: Coarse-Grained Simulations of the Effects of Chain Length, Solvent Quality, and Chemical Defects on the Solution-Phase Morphology of MEH-PPV Conjugated Polymers

A mesoscale coarse-grained model of the conjugated polymer poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) in implicit solvent is developed. The model is parametrized to reproduce the local structure and dynamics of an atomistic simulation model and accounts for the effects of solvent quality and saturation chemical defects on the polymer structure. Polymers with defect concentrations of 0 to 10 % are simulated using Langevin dynamics in tetrahydrofuran (THF) and in a model poor solvent for chain lengths and solution concentrations used experimentally. The polymer chains are extended in THF and collapse into compact structures in the poor solvent. The radius of gyration decreases with defect content in THF and agrees quantitatively with experiment. The structures formed in poor solvent by chains with 300 monomer units change from toroidal to cylindrical with increasing defect content, while chains containing 1000 monomers form cylinders regardless of defect content. These results have implications for energy transfer in MEH-PPV.

Impact of Solvent Quality on the Hysteresis in the Coil–Stretch Transition of Flexible Polymers in Good Solvents

Direct observations of long flexible polymers in elongational flow show a coil to stretch transition (CST) and hysteresis in this transition. In this article, solvent effects on the hysteresis of CST were examined using Brownian dynamics simulations of a dumbbell model with conformation dependent drag. A spring force relation that is able to capture the effects of solvent quality was used. Two methods of increasing the solvent quality were analyzed: (1) by increasing the number of Kuhn steps (NK) and keeping the excluded volume parameter (v/l3) constant, and (2) by increasing v/l3 and keeping NK constant. In the first case, the width of the hysteresis (ratio of upturn to downturn strain rates) increased with increase in NK. In the second case, the width of hysteresis decreased with increase in v/l3. This implies that molecules such as ss-DNA, which have v/l3 ≈ 1 in good solvents, will have less hysteresis than a molecule with the same NK in a Θ solvent.

Interaction Between Polymer Brush-Coated Spherical Nanoparticles: Effect of Solvent Quality

The interaction between two spherical polymer brushes in solvents of variable quality is studied by molecular dynamics simulation and by self-consistent field theory, varying both the radius of the spherical particles and their distance, as well as the grafting density and the chain length of the end-grafted flexible polymer chains. Both the potential of mean force between the particles as a function of their distance is computed, for various choices of the parameters mentioned above, and the structural characteristics are discussed (density profiles, average end-to-end distance of grafted chains, etc.) It is found that for rather short chain lengths and not too large grafting densities, isolated spherical brushes in the poor solvent regime exhibit incomplete coverage of the nanoparticle, rather than a complete coverage, i.e., a core–shell structure where the nanoparticle is covered homogeneously by a dense polymeric film. This latter case occurs for long chains and/or high grafting densities. When two su...

Effect of Solvent Quality on the Solution Properties of Assemblies of Partially Fluorinated Amphiphilic Diblock Copolymers

The effect of solvent on the formation of assemblies of partially fluorinated block copolymers in solution has been examined. Two classes of materials based respectively on 2,2,2-trifluoroethyl acrylate (TFEA) and 2,2,2-trifluoroethyl methacrylate (TFEMA) were dissolved in organic solvents, and the properties on successive addition of water were studied using NMR spectroscopy, NMR imaging, AFM, and TEM. The relatively high glass transition temperature of the methacrylate blocks resulted in the formation of kinetically trapped structures that could only be resolved following heating to temperatures well above the Tg. The acrylate polymers formed loose assemblies in pure dimethylformamide, and on addition of water cylindrical micelles were observed. On the other hand, in pure acetone the partially fluorinated segments interacted more strongly with the solvent, with this structure inverting on addition of water. The NMR parameters were strongly dependent on the proposed structures in solution, and most marke...

Effect of solvent quality on the dispersibility of polymer-grafted spherical nanoparticles in polymer solutions

In this work, lattice-based self consistent field theory is used to study the structural properties of individual polymer-grafted spherical nanopartices and particle-particle interactions in polymer melts and solutions under variable solvent conditions. Our study has focused on the depth of the minimum in the potential of mean force between the two brush-coated nanoparticles, if such a minimum occurs, and we have also addressed the corresponding radial density profiles of free and grafted chains around a single nanoparticle, in an attempt to clarify the extent of correlation between the depth of the minimum, W(min), and the parameter δ characterizing the interpenetration between the profiles of free and grafted chains. Although one cannot establish a simple one-to-one correspondence between W(min) and δ, we do find common trends, in particular, if the solvent conditions for free and grafted chains differ: varying the volume fraction of the free chains, δ typically exhibits a broad minimum, corresponding to a region where the magnitude of W(min) exceeds thermal energy k(B)T, leading to particle aggregation.