Chains In Solvent Research Articles

Megasupramolecules for safer, cleaner fuel by end association of long telechelic polymers.

We used statistical mechanics to design polymers that defy conventional wisdom by self-assembling into "megasupramolecules" (≥5000 kg/mol) at low concentration (≤0.3 weight percent). Theoretical treatment of the distribution of individual subunits—end-functional polymers—among cyclic and linear supramolecules (ring-chain equilibrium) predicts that megasupramolecules can form at low total polymer concentration if, and only if, the backbones are long (>400 kg/mol) and end-association strength is optimal. Viscometry and scattering measurements of long telechelic polymers having polycyclooctadiene backbones and acid or amine end groups verify the formation of megasupramolecules. They control misting and reduce drag in the same manner as ultralong covalent polymers. With individual building blocks short enough to avoid hydrodynamic chain scission (weight-average molecular weights of 400 to 1000 kg/mol) and reversible linkages that protect covalent bonds, these megasupramolecules overcome the obstacles of shear degradation and engine incompatibility.

Transient polymer links are better

Polymer Chemistry Very long polymer molecules elongate in shearing flows. This ordering of the chains lowers the viscosity of small-molecule solvents. The chains also reduce the dangers of misting during explosive spreading of the solvents. However, the long polymer chains degrade during normal fuel handling and clog pumping equipment. Wei et al. used telechelic polymers—short chains with reactive end groups—to form extremely long chains in organic solvents (see the Perspective by Jaffe and Allam). These reformable polymers offer the potential for better fuel safety without the drawbacks of covalently bonded long-chain polymers. Science , this issue p. [72][1]; see also p. [32][2] [1]: /lookup/doi/10.1126/science.aab0642 [2]: /lookup/doi/10.1126/science.aac9827

Branched conformational properties of macromolecules in close relation to chemical synthesis. II. Influence of excluded volume interactions.

The description of perturbed particle conformations needs as a prerequisite the algorithm of unperturbed chains which is outlined in Paper I [J. Chem. Phys. 143, 114906 (2015)]. The mean square segment length ⟨r(2)(n)⟩=b(2)n(2ν) with ν = 0.588 for linear chains in a good solvent is used as an approximation also for branched samples. The mean square radius of gyration is easily derived, but for the hydrodynamic, the segment distribution by Domb et al. [Proc. Phys. Soc., London 85, 624 (1965)] is required. Both radii can analytically be expressed by Gamma functions. For the angular dependence of scattered light, the Fourier transform of the Domb distribution for self-avoiding random walk is needed, which cannot be obtained as an analytical function and was derived by numerical integration. The summation over all segment length in the particle was performed with an analytic fit-curve for the Fourier transform and was carried out numerically. Results were derived (i) for uniform and polydisperse linear chains, (ii) or f-functional randomly branched polymers and their monodisperse fractions, (iii) for random A3B2 co-polymers, and (iv) for AB2 hyper-branched samples. The deviation of the Gaussian approximation with the variance of ⟨r(2)(n)⟩=b(2)n(2ν) slightly overestimates the excluded volume interaction but still remains a fairly good approximation for region of qR(g) < 10.

Rotational Diffusion of a New Large Non Polar Dye Molecule in Alkanes.

Rotational reorientation times of a newly synthesized 2,5-bis(phenylethynyl)1,4-bis(dodecyloxy) benzene (DDPE) are experimentally determined in series of n-alkanes by employing steady state and time resolved fluorescence depolarization technique with a view to understand rotational dynamics of large non-polar solute molecule in non-polar solvents and few general solvents of different sizes and varying viscosity. It is observed that rotational reorientation times vary linearly as function of viscosity. The hydrodynamic stick condition describes the experimental results at low viscosities while the results tend to deviate significantly from it at higher viscosities. This is attributed to the possibility of long chains in solvents hosting a variety of chain defects (end-gauche, double-gauche, all-trans, kink, etc.) thereby reducing the effective length of the molecule, leading to a slightly reduced friction. The experimental results are compared with the predictions of Stokes-Einstein-Debye (SED) hydrodynamic theory as well as the quasi-hydrodynamic theories of Gierer-Wirtz (GW) and Dote-Kivelson-Shwartz (DKS). The predictions from these theories underestimate τr in the solvents employed in the study.

Conjugated foldamers with unusually high space-charge-limited current hole mobilities.

Charge carrier mobility and its optimization play a critical role in the development of cutting-edge organic electronic and optoelectronic devices. Even though space-charge-limited current (SCLC) hole mobilities as high as 1.4 cm(2) V(-1) s(-1) have been reported for microscopically sized highly ordered liquid-crystalline conjugated small molecules, the SCLC hole mobility of device-sized thin films of conjugated polymers is still much lower, ranging from 10(-6) to 10(-3) cm(2) V(-1) s(-1). Herein, we report the synthesis, characterizations, and thin-film SCLC mobility of three discotic conjugated polymers, INDT-TT, INDT-BT, and INDT-NDT. Optical studies indicate that polymer INDT-NDT adopts a folded conformation in solutions of good or poor solvents, whereas polymer INDT-TT stays as random monomeric chains in good solvents and interchain aggregates in poor solvents. INDT-BT polymer chains, however, stay as foldamers in dilute solutions of good solvents but interchain aggregates in concentrated solutions or poor solvents. Circular dichroism spectroscopy provides clear evidence for the helical folding of INDT-NDT in solutions. Thin films spin-coated from 1,2-dichlorobenzene solutions of the polymers show SCLC hole mobility of 2.20 × 10(-6), 8.79 × 10(-5), and 2.77 × 10(-2) cm(2) V(-1) s(-1) for INDT-TT, INDT-BT, and INDT-NDT, respectively. HRTEM and powder XRD measurements show that INDT-NDT pristine thin films contain nanocrystalline domains, whereas the INDT-TT and INDT-BT films are amorphous. Thin films of INDT-NDT:PC71BM blends show increased crystallinity and further improved SCLC hole mobility up to 1.29 × 10(-1) cm(2) V(-1) s(-1), one of the highest SCLC mobility values ever recorded on solution-processed organic semiconducting thin films. The persistent folding conformation of INDT-NDT is believed to be responsible for the high crystallinity of its thin films and its high SCLC mobilities.

Viscoelasticity of Poly(ethylene glycol) Solutions on Supported Lipid Bilayers via Quartz Crystal Microbalance with Dissipation

Supported lipid bilayers of 1,2-dioleoyl-sn-glycero-3-phosphocholine were formed on a silicon oxide substrate, and the viscoelasticity of poly(ethylene glycol) (PEG) solutions above the bilayer was subsequently studied by quartz crystal microbalance with dissipation monitoring. No detectable adsorption of PEG molecules to the bilayer was found over a broad range of PEG molecular weight at various concentrations. The viscoelastic properties of the PEG solutions were obtained from the shifts in the resonance frequency and the energy dissipation factor of the polymer solution in contact with the resonator-supported lipid bilayer. The resulting viscoelastic properties of PEG solutions were found to be in excellent agreement with the Zimm model for linear polymer chains in a good solvent. The excluded volume scaling exponent ν for PEG in water shows an ideal-to-real crossover with increasing molecular weight. The exponent adopts a value of 0.50 for short chains and gradually increases to 0.565 for long chains. The onset of the excluded volume effect of PEG in water, a good solvent, lies in the molecular weight range between 4000 and 8000.

A dielectric study of interpolymer complexes of polyaniline and DNA

Abstract When positively charged conjugated polymer chains are jointly dispersed with (single-stranded or double stranded) DNA chains in solvents of low polarity, complex hybrid structures can result. Interpolymer complexes (IC) can be formed due to the mutual Coulomb interactions between the positive charges in the doped conjugated polymer and the negatively charged phosphate groups along the external ridge of the nucleotide chains. In fact, some of these IC have been reported as potential candidates for several biotechnological applications as, for instance, the development of reliable methods for determining the precise concentration of DNA, a still acute necessity for a number of applications. In this work, we measure the dielectric response of polyaniline/DNA complexes as a convenient procedure to investigate the dominating mechanisms involved in their process of aggregation when dispersed in chloroform. As the ratio of concentrations [PANI]/[DNA] is progressively increased, the corresponding charge transfer resistance varies in a characteristic way, according to the nature of the microscopic reactions taking place in the solution. In fact, the patterns of variation of the charge transfer resistance and optical absorbance of the solution corresponding to the polaron band transition are quite similar, but exhibiting opposite trends. We take this as strong evidence that the electrical characterization of such IC systems represents an alternative method to conventional spectroscopic techniques for investigating the dominant interaction mechanisms DNA and PANI chains in solutions.

Simulation of graphene nanoribbon aggregation and its mediation by edge decoration.

Large polyaromatic molecules, including synthetic graphene nanoribbons (GNRs), are the subject of considerable interest for a variety of electronic applications. For GNRs in particular, functional groups can be bonded along the ribbon edges to modify their dispersibility, self-assembly behavior, and electronic properties. However, these side chains are usually chosen in a "trial and error" fashion, without an underlying molecular-scale picture of the conformations they will adopt in solution and the resulting influence of such structures on macroscopically observable phenomena, particularly aggregation. In this study, we use molecular dynamics (MD) to predict the behavior of various side chains in different solvents as a means to understand how this influences aggregate morphologies and binding energies. Specifically, oligomeric PEG and n-alkoxy chains of varying lengths and grafting densities are examined in vacuum, water, and N-methylpyrrolidone. Examining the binding energies and side chain dispositions that occur with different sets of parameters allows us to suggest a combination of these variables that will minimize aggregational tendencies for the GNRs. The results underscore the value of molecular-scale computational techniques to understand the aggregational tendencies of 2D materials and guide the design of future polyaromatic edge modifications.

Characterization of chain dimensions of poly(ε-caprolactone) diols in THF by size-exclusion chromatography coupled with multi-angle light scattering (SEC-MALS)

In this work, we studied the chain dimensions, shape, and thermodynamic characteristics of poly(e-caprolactone) diols (HO-PCL-OH) in solution. PCL diol samples of different molecular weights and architectures were synthesized using immobilized Yarrowia lipolytica lipase as catalyst. Gel permeation chromatography (GPC) with online right-angle laser-light scattering (RALLS), differential viscometer (DV), and interferometric refractometer IRS detectors offered a proper way to obtain information on thermodynamic characteristics and chain flexibility. The weight-average molecular weights of the PCL diol samples (M w ) ranged from 2,750 to 13,120 uma. The z-average radius of hydrodynamic volume (<R h 2 > z 1/2 ) vs. z-average molecular weight curve (M z ) could be satisfactorily fitted to a power-law equation. Mark–Houwink–Sakurada parameters (K = 2.74 × 10−3 dL/g and a = 0.64) were derived from the plots of [η] w against M w . The overall results clearly suggest that a flexible geometry is present in tetrahydrofuran (THF) solution of PCL at 33 °C. Unlike other PCL systems, PCL diols are more hydrophilic and expected to form associated species. They behave differently from common hydrophobic polymers and do not strictly conform to thermodynamic relationships generally used in Polymer Science. A more compact geometry is present for Mn lower than 4,000 Da. For higher molecular weights, the chain expands and become more elongated. Plots of ln [η] w against ln M w (MHS equation) and of log R hz against log M z suggest that PCL diols behave as flexible chains in a good solvent.

A flexible polymer chain in a critical solvent: Coil or globule?

We study the behavior of a flexible polymer chain in the presence of a low-molecular weight solvent in the vicinity of a liquid-gas critical point within the framework of a self-consistent field theory. The total free energy of the dilute polymer solution is expressed as a function of the radius of gyration of the polymer and the average solvent number density within the gyration volume at the level of the mean-field approximation. Varying the strength of attraction between polymer and solvent we show that two qualitatively different regimes occur at the liquid-gas critical point. In case of weak polymer-solvent interactions the polymer chain is in a globular state. On the contrary, in case of strong polymer-solvent interactions the polymer chain attains an expanded conformation. We discuss the influence of the critical solvent density fluctuations on the polymer conformation. The reported effect could be used to excert control on the polymer conformation by changing the thermodynamic state of the solvent. It could also be helpful to estimate the solvent density within the gyration volume of the polymer for drug delivery and molecular imprinting applications.

Stretching of Anchored to the Surface a Real Polymer Chain in a Good Solvent: The Massive Field Theory Approach

SummaryApplication of the massive field theory approach in a fixed space dimensions d = 3 up to one loop order for investigation of the elastic properties of a real polymer chain immersed in a good solvent and anchored by one end to the surface and with other end being under tension of the external applied force as it is usually takes place in the single ‐ molecule force spectroscopy experiments. The obtained results include calculations of the dependence of the applied force upon stretching of a real polymer chain in a good solvent on the distance from the repulsive and inert surfaces. Besides, the obtained results are in good qualitative and quantitative agreement with previous theoretical investigations, results of Density Functional Theory (DFT) and Monte Carlo (MC) simulations and have important practical applications for understanding of the elastic properties of the individual macromolecules, networks, gels and brush layers.

Polymer Chain Conformation on Deuterated Polystyrene Nanoparticles Investigated by SANS

We previously reported that grafted polystyrene (PS) chains on silica nanoparticles at a low grafting density show similar conformations to free PS chains in the same solvent, THF (diameter ˜50 nm, Colloid.poly.Sci. (2013), 291, 9, 2087–2099). As an extension of our previous study we choose an organic nanoparticle (deuterated polystyrene, dPS) instead of inorganic nanoparticle to see the impact of the substrate material on chain conformation. Additionally, a wider range of molecular weights were prepared to investigate the conformation feature of grafted PS chains more in detail. Small angle neutron scattering (SANS) experiments were performed to characterize PS grafted dPS particles in good solvent condition, with deuterated toluene and deuterated THF as solvent. To get insight into the conformation of the grafted PS layer we apply a scaling law describing the dimension of free PS polymer in good solvent condition to the obtained thickness of the grafted PS layer. We find an overall agreement with the scaling law where the thickness of the grafted PS layer is slightly larger than 2Rg of the free polymer chains in the respective solvent giving hint for semi dilute polymer brush (SDPB) situation.

Effects of implicit solvent and relaxed amino acid side chains on the MP2 and DFT calculations of ligand–protein structure and electronic interaction energies of dopaminergic ligands in the SULT1A3 enzyme active site

The electronic interaction energies between eight dopaminergic ligands and resveratrol and the SULT1A3 active site have been calculated using MP2 and M062X with the 6-311+G∗ basis set. The SULT1A3 active site was isolated from the crystal structure with dopamine bound (PBD ID 2A3R). Structures for the nine ligands bound in the active site were obtained by three different optimization approaches with M062X/6-31G: by optimization in vacuo with rigid amino acid residue side chains (i.e. using the side chain structures from the crystal structure; the ligand and all protons were optimized), with implicit solvation by water with rigid amino acid residue side chains, and with implicit solvation by water and relaxed amino acid residue side chains (i.e. the ligand and all amino acid residue side chains were optimized). The calculations show that the use of implicit solvent and a relaxed active site offer considerable improvement in ligand interaction energies (more strongly binding), and that the solvated-relaxed model shows less variability in ligand interaction than the other two models. Finally, 6-carboxydopamine was shown to be the most strongly binding of the nine ligands, with an interaction energy over 100kcal/mol stronger than its nearest neighbor. Four of the eight non-dopamine ligands studied interact with SULT1A3 more strongly than the endogenous ligand, dopamine, in the solvated-relaxed model.

Thermal annealing: a facile way of conferring responsivity to inert alkyl-chain-passivated nanoparticle arrays.

This work demonstrates a facile post-treatment strategy, vacuum thermal annealing, to fabricate a dodecanethiol-passivated gold nanoparticle (Au NP) array with organic solvent sensitivity. Through investigating the structure change of the Au NP array, it was found that the interparticle distance decreased during vacuum heat treatment, which meant a closer arrangement of the particles and a more dense packing of the dodecanethiol ligands in the interparticle region. The condensation would increase the interaction of the alkyl chain and enhance their interdigitation. Furthermore, on the basis of the stretching of the alkyl chains in organic solvents, the thermally treated Au NP array showed a good response to organic solvent or vapor by using the interdigitated dodecanethiol network as its responsive unit. The alkyl chains stretch to different extents in different organic solvents, leading to differences in interparticle distance, which provided a distinct blue shift of maximum wavelength upon exposure to various organic solvents or vapors. All of these results indicated that thermal annealing was an efficient way to confer responsivity to inert Au NP arrays. Together with the cost-effectiveness of such NP arrays, this study has potential in the development of economical sensors for medical diagnostics, food safety screening, and environmental pollution monitoring.

Conformational transitions of a weak polyampholyte.

Using grand canonical Monte Carlo simulations of a flexible polyelectrolyte where the charges are in contact with a reservoir of constant chemical potential given by the solution pH, we study the behavior of weak polyelectrolytes in poor and good solvent conditions for polymer backbone. We address the titration behavior and conformational properties of a flexible diblock polyampholyte chain formed of two oppositely charged weak polyelectrolyte blocks, each containing equal number of identical monomers. The change of solution pH induces charge asymmetry in a diblock polyampholyte. For diblock polyampholyte chains in poor solvents, we demonstrate that a discontinuous transition between extended (tadpole) and collapsed (globular) conformational states is attainable by varying the solution pH. The double-minima structure in the probability distribution of the free energy provides direct evidence for the first-order like nature of this transition. At the isoelectric point electrostatically driven coil-globule transition of diblock polyampholytes in good solvents is found to consist of different regimes identified with increasing electrostatic interaction strength. At pH values above or below the isoelectric point diblock chains are found to have polyelectrolyte-like behavior due to repulsion between uncompensated charges along the chain.

Crumpled globule formation during collapse of a long flexible and semiflexible polymer in poor solvent.

By introducing explicit solvent particles and hydrodynamic interactions we demonstrate that crumpled globules are formed after the collapse of long polymer chains (N = 10(4)) in a poor solvent. During the collapse crumples of all sizes form sequentially, but small crumples are not stable and convert to blobs with Gaussian statistics. The observed effective mean squared distance R(2)(n) ∼ n(0.38) at n > Ne and contact probability index p(n) ∼ n(-0.5) at n ≫ Ne, which is not following either the model of a fractal globule, or the predictions for an equilibrium globule. Polymer chain stiffness pushes the system to form globular crystallite, and this freezes crumpled structure with R(2)(n) ∼ n(0.33) at n > Ne as a stable state. We note that there is some similarity to crumple globule formation and crystallization of polymer melt.

Transitions of tethered chain molecules under tension.

An applied tension force changes the equilibrium conformations of a polymer chain tethered to a planar substrate and thus affects the adsorption transition as well as the coil-globule and crystallization transitions. Conversely, solvent quality and surface attraction are reflected in equilibrium force-extension curves that can be measured in experiments. To investigate these effects theoretically, we study tethered chains under tension with Wang-Landau simulations of a bond-fluctuation lattice model. Applying our model to pulling experiments on biological molecules we obtain a good description of experimental data in the intermediate force range, where universal features dominate and finite size effects are small. For tethered chains in poor solvent, we observe the predicted two-phase coexistence at transitions from the globule to stretched conformations and also discover direct transitions from crystalline to stretched conformations. A phase portrait for finite chains constructed by evaluating the density of states for a broad range of solvent conditions and tensions shows how increasing tension leads to a disappearance of the globular phase. For chains in good solvents tethered to hard and attractive surfaces we find the predicted scaling with the chain length in the low-force regime and show that our results are well described by an analytical, independent-bond approximation for the bond-fluctuation model for the highest tensions. Finally, for a hard or slightly attractive surface the stretching of a tethered chain is a conformational change that does not correspond to a phase transition. However, when the surface attraction is sufficient to adsorb a chain it will undergo a desorption transition at a critical value of the applied force. Our results for force-induced desorption show the transition to be discontinuous with partially desorbed conformations in the coexistence region.

Polymeric Assembly of Gluten Proteins in an Aqueous Ethanol Solvent

The supramolecular organization of wheat gluten proteins is largely unknown due to the intrinsic complexity of this family of proteins and their insolubility in water. We fractionate gluten in a water/ethanol mixture (50/50 v/v) and obtain a protein extract which is depleted in gliadin, the monomeric part of wheat gluten proteins, and enriched in glutenin, the polymeric part of wheat gluten proteins. We investigate the structure of the proteins in the solvent used for extraction over a wide range of concentration, by combining X-ray scattering and multiangle static and dynamic light scattering. Our data show that, in the ethanol/water mixture, the proteins display features characteristic of flexible polymer chains in a good solvent. In the dilute regime, the proteins form very loose structures of characteristic size 150 nm, with an internal dynamics which is quantitatively similar to that of branched polymer coils. In more concentrated regimes, data highlight a hierarchical structure with one characteristic length scale of the order of a few nm, which displays the scaling with concentration expected for a semidilute polymer in good solvent, and a fractal arrangement at a much larger length scale. This structure is strikingly similar to that of polymeric gels, thus providing some factual knowledge to rationalize the viscoelastic properties of wheat gluten proteins and their assemblies.

Structural and mechanical properties of advanced polymer gels with rigid side-chains using coarse-grained molecular dynamics

Computational modeling was utilized to design complex polymer networks and gels which display enhanced and tunable mechanical properties. Our approach focuses on overcoming traditional design limitations often encountered in the formulation of simple, single polymer networks. Here, we use a coarse-grained model to study an end-linked flexible polymer network diluted with branched polymer solvent chains, where the latter chains are composed of rigid side-chains or “spikes” attached to a flexible backbone. In order to reduce the entropy penalty of the flexible polymer chains these rigid “spikes” will aggregate into clusters, but the extent of aggregation was shown to depend on the size and distribution of the rigid side-chains. When the “spikes” are short, we observe a lower degree of aggregation, while long “spikes” will aggregate to form an additional secondary network. As a result, the tensile relaxation modulus of the latter system is considerably greater than the modulus of conventional gels and is approximately constant, forming an equilibrium zone for a broad range of time. In this system, the attached long “spikes” create a continuous phase that contributes to a simultaneous increase in tensile stress, relaxation modulus and fracture resistance. Elastic properties and deformation mechanisms of these branched polymers were also studied under tensile deformation at various strain rates. Through this study we show that the architecture of this branched polymer can be optimized and thus the elastic properties of these advanced polymer networks can be tuned for specific applications.

Unusual swelling of a polymer in a bacterial bath.

The equilibrium structure and dynamics of a single polymer chain in a thermal solvent is by now well-understood in terms of scaling laws. Here, we consider a polymer in a bacterial bath, i.e., in a solvent consisting of active particles which bring in nonequilibrium fluctuations. Using computer simulations of a self-avoiding polymer chain in two dimensions which is exposed to a dilute bath of active particles, we show that the Flory-scaling exponent is unaffected by the bath activity provided the chain is very long. Conversely, for shorter chains, there is a nontrivial coupling between the bacteria intruding into the chain which may stiffen and expand the chain in a nonuniversal way. As a function of the molecular weight, the swelling first scales faster than described by the Flory exponent, then an unusual plateau-like behaviour is reached and finally a crossover to the universal Flory behaviour is observed. As a function of bacterial activity, the chain end-to-end distance exhibits a pronounced non-monotonicity. Moreover, the mean-square displacement of the center of mass of the chain shows a ballistic behaviour at intermediate times as induced by the active solvent. Our predictions are verifiable in two-dimensional bacterial suspensions and for colloidal model chains exposed to artificial colloidal microswimmers.