Polymers In Good Solvent Research Articles

Translocation of polymer through nanopore: dissipative particle dynamics simulation

Dissipative particle dynamics simulations are performed to study the forced translocation of polymer through a nanopore inside which the polymer experiences a driving force F. Hydrodynamic interaction (HI) is taken into account for the polymer in good solvent. We find that the mean translocation time scales with the polymer length N as ~ N α with α = 1.26 ± 0.03 close to a theoretical prediction, and the probability distribution of τ can be described by a Gaussian function. Our results show that the dynamics of polymer translocation with the HI is different from that without the HI. However, the exponent δ in the scaling ~ F −δ is found not to be affected by the HI effect.

Self‐assembly of poly(3‐hexyl thiophene)‐b‐poly(ethylene oxide) into cylindrical micelles in binary solvent mixtures

ABSTRACTAsymmetric block copolymer based on regioregular poly(3‐hexyl thiophene) (P3HT) and poly(ethylene oxide) (PEO) was synthesized through Heck reactions. The addition of PEO block has no influence in the effective conjugation length of P3HT block and apparently provides colloidal stability for the formation of stable nanostructures. Introduction of poor solvent to good solvent containing P3HT‐b‐PEO will induce the crystallization‐driven assembly of the P3HT into cylindrical micelles with a P3HT core, owing to π–π stacking of the conjugated backbone of P3HT. The absorption spectra of the cylindrical micelles reveal a red shift as compared to the polymer in good solvent, indicating the extension of conjugation length with an improved π–π stacking of the polymer chains within the cylindrical micelles. Our results indicated that cylindrical micelles with varied diameter and length can be obtained when solvent properties were varied using several different binary solvent mixtures. More interestingly, we demonstrate that ultrasonic processing can fragment the cylindrical micelles only when the ratio of poor solvent increases. This provides a facile and effective way to fabricate cylindrical micelles for applications in the area of polymer solar cell as well as organic optoelectronics device. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41186.

Solubility and solution rheology of acrylamide-sulfobetaine copolymers

The copolymer of acrylamide and 3-[N-(2-methacryloxylethyl)-N,N-dimethylammonio]-propane sulfonate (PAM-MDMPS) was prepared via free radical copolymerization. Solubility of the copolymers was studied by turbidimetric titration method under different conditions. It was found for the first time that the critical salt concentration to dissolve the copolymer showed a plateau over one order of magnitude up to the critical overlap concentration. Rheological behavior and chain conformation of the copolymers in 1 M NaCl solution were also studied. The concentration regions according to scaling theory were found the same as neutral polymers in good solvent. The specific viscosities could be normalized by the overlap parameter. According to the Huggins relation, the copolymers adopted a more compact conformation in 1 M NaCl with increasing MDMPS content due to the hydrophobic association of the betaine unit in the macromolecular backbone, which was stabilized by the strongly hydrated dipolar pendant chains.

Molecular weight degradation of synthetic and natural polymers during inkjet print.

We compare the different modes of molecular weight degradation in DOD and CIJ of synthetic vinyl polymers in good solvent and report for the first time extension to polysaccharides This work has significance both in printer design but also in formulation of high molar mass functional and biological materials.

Viscosity properties of gelatin in solutions of monovalent and divalent Salts

The viscosity behaviors of gelatin with and without salts were examined in details by a rotational viscometer and a horizontal gravitational capillary viscometer, ranging from extremely dilute to entangled regimes. It was found that gelatin in salt free solution behaviors as neutral polymer in θ solvent. Polyelectrolyte effect can be found in extremely dilute regime. For gelatin/NaCl solution, the concentration dependence of specific viscosity showed that gelatin behaviors as neutral polymer in good solvent. The two critical concentration c* and c e of gelatin solutions with and without NaCl moved from 4.0 wt% and 14.0 wt% to 2.0 wt% and 12.0 wt%, respectively. Addition of salts can improve the gelatin viscosity. It is the characteristic of a polyampholyte. The viscosity increased more significantly in CaCl2 than that in NaCl solutions. Moreover, the effect of Ca2+ is notable in gelatin solution with high concentrations, especially in entangled solutions.

Chain Flexibility and Dynamics of Polysaccharide Hyaluronan in Entangled Solutions: A High Frequency Rheology and Diffusing Wave Spectroscopy Study

We have investigated the linear viscoelastic properties of high molecular weight hyaluronan in aqueous solution using an experimental approach combining mechanical rheometry and optical microrheology. The complex shear modulus has been measured over a broad frequency range from 10(-1) to 10(7) rad/s. Chain flexibility is characterized by the persistence length lpand this parameter has been determined for the first time in the entangled regime here from high frequency modulus data. At NaHA concentrations above the entanglement concentration ce, lp is essentially independent of polymer concentration, temperature, and ionic strength. The latter is consistent with the Odijk-Skolnick-Fixman theory. The scaling exponent describing the concentration dependence of the plateau modulus G0 agrees well with predictions for polymers in good solvents. The scaling exponents for the specific viscosity ηsp and relaxation time TR are slightly higher than theoretically predicted for polyelectrolytes in the high salt limit, indicating, that molecular aggregation occurs at higher polymer concentrations.

Comparisons of a Polymer in Confinement versus Applied Force

The similarities and differences between geometric and tensile constraints on polymers have not been fully investigated. Here we use theory (blob models) and simulations to present a comprehensive comparison between polymers in these two situations. For a polymer in good solvent, the effect of tensile force f on extension in the Pincus regime is similar to the effect of cylindrical confinement in the de Gennes regime after mapping the characteristic length kBT/f to the cylindrical diameter D, where kBT is the thermal energy. However, the comparison of the effects of tension and confinement on extension is lacking when kBT/f and D are less than the thermal blob size b, referred to as extended Pincus regime and extended de Gennes regime, respectively. In the extended Pincus regime, force can still segregate the ideal-coils with the size of ∼kBT/f, resulting in the scaling of extension L∥ ∼ (kBT/f)−1. In the extended de Gennes regime, excluded volume interaction is not sufficient to segregate the ideal-coils...

Fluctuations in the coil-stretch transition of flexible polymers in good solvents: A peak due to nonlinear force relation

Long flexible polymers undergo a coil to stretch transition (CST) in an elongational flow. Near the CST, a peak can be observed in the fluctuations of the size of a molecule (|R|). Solvent effects on the fluctuations are studied using Brownian dynamics simulations of a nonlinear spring force relation that can represent real molecules. Ignoring the influence of hydrodynamic interactions, a linear region in the spring force relation is known to cause the peak in |R| fluctuations. In contrast, we find that a peak in the fluctuations can be obtained even for the nonlinear spring force relation. We analyze the influence of hydrodynamic interactions on the fluctuations using a dumbbell model with a conformation-dependent drag coefficient.

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.

Model for the Shear Viscosity of Suspensions of Star Polymers and Other Soft Particles

AbstractA model is proposed to describe the concentration dependence of the viscosity of soft particles. The softness of the particles is incorporated in a very simple way into expressions originally developed for rigid spheres. This is done by introducing a concentration‐dependent critical packing, which is the packing at which the suspension loses fluidity. The resultant expression reproduces the experimental results for suspensions of star polymers in good solvents with high accuracy. The model allows the weak increase of the viscosity observed in the case of diblock copolymer stars to be explained, suggesting that the reason for this peculiar behavior is mainly a consequence of the softness of the particles. In the semi‐dilute regime, suspensions of star polymers are modeled using the Daoud–Cotton picture to complete the description in the whole concentration regime.

Toward a Universal Behavior of Linear Athermal Model Chains in the Limit of Infinite Chain‐length

AbstractAthermal chains of various stiffness embedded in several types of lattices were analyzed with respect to their pair distribution function G(R). Extrapolation to infinite chain‐length on a reduced distance scale made all the G(R) data coincide within extremely narrow limits, thus suggesting the existence of a universal pair distribution function for polymers in good solvents in the long‐chain limit. The ratio –ln(G(R))/Z(R) with Z(R) being the reduced distribution of overlaps is also universal in type and varies only little with distance R. Further, a likewise universal function could be established that allows the construction of Z(R) from the Flory–Krigbaum expression taking into account not only the deviations from Gaussian behavior but also the effect of intramolecular repulsion present in athermal chains. magnified image

Dual Scale Roughness Driven Perfectly Hydrophobic Surfaces Prepared by Electrospraying a Polymer in Good Solvent–Poor Solvent Systems

We demonstrated a facile method to produce perfectly hydrophobic surfaces (advancing and receding angles both 180°) via electrospraying. When a copolymer of styrene and a perfluoroacrylate monomer was electrosprayed in good solvents, surfaces composed of micrometer size beads were formed and fairly low threshold water sliding angles could be achieved. Addition of high boiling point poor solvents to the solutions resulted nanoscale roughness on the beads due to a possible phase separation that occurs in a predominantly poor solvent environment. However, sliding angles were not zero even on the nanoscale roughness dominated topographies achieved by this method. On the other hand, when the electrospraying process parameters were set such that micrometer size hills of nanoscopically rough beads were formed, 0° sliding angles were measured. Videos of droplets recorded and the adhesive forces measured during a contact and release experiment revealed that these dual scale rough surfaces were indeed perfectly hydrophobic. Application of the method with other binary good solvent-poor solvent systems also resulted in perfect hydrophobicity. Overall results showed how the differences in surface topology affected the wettability of surfaces within a very narrow range between perfect and extreme hydrophobicity (advancing and receding angles both close to 180°).

Polymer scaling laws of unfolded and intrinsically disordered proteins quantified with single-molecule spectroscopy

The dimensions of unfolded and intrinsically disordered proteins are highly dependent on their amino acid composition and solution conditions, especially salt and denaturant concentration. However, the quantitative implications of this behavior have remained unclear, largely because the effective theta-state, the central reference point for the underlying polymer collapse transition, has eluded experimental determination. Here, we used single-molecule fluorescence spectroscopy and two-focus correlation spectroscopy to determine the theta points for six different proteins. While the scaling exponents of all proteins converge to 0.62±0.03 at high denaturant concentrations, as expected for a polymer in good solvent, the scaling regime in water strongly depends on sequence composition. The resulting average scaling exponent of 0.46±0.05 for the four foldable protein sequences in our study suggests that the aqueous cellular milieu is close to effective theta conditions for unfolded proteins. In contrast, two intrinsically disordered proteins do not reach the Θ-point under any of our solvent conditions, which may reflect the optimization of their expanded state for the interactions with cellular partners. Sequence analyses based on our results imply that foldable sequences with more compact unfolded states are a more recent result of protein evolution.

Models of flexible polymers in good solvents: relaxation and coil–stretch transition

We analyze the impact of solvent quality on relaxation dynamics and the coil–stretch transition (CST) using Brownian dynamics simulations of a new bead–spring model. A new spring force relation, which can represent the polymer in theta to good solvent, is developed to obtain the force–extension (FE) behavior of molecules. Due to different regions of the FE curve, we find that the relaxation and CST behavior are altered. A simple force balance is used to explain different regions of the flow and relaxation behavior. Using a model that neglects conformation dependent drag, it is found that some molecules will have two different exponential decays in relaxation and the CST will be modified for others.

FRACTIONAL BROWNIAN POLYMERS - SOME FIRST RESULTS

Recently the Edwards model for chain polymers in good solvents has been extended to include fractional Brownian motion trajectories as a description of polymer conformations. This raises in particular the question of the corresponding Flory formula for the end-to-end length of those molecules. A generalized Flory formula has been proposed, and there are some first results of numerical validations.

Starlike Polymer Brushes

We investigate the properties of polymer brushes made of symmetric and asymmetric starlike polymers in good solvent using Langevin dynamics simulations. Two types of populations coexist in such a brush, one of them being made of highly stretched polymers and a second one made of stars that are retracted inside the lower brush regions. Scaling properties of molecular tension, brush layer thickness, and brush density are in reasonable agreement with those of linear chain brushes. The dynamics of stars switching between both the retracted and the stretched states is analyzed, and an exponential scaling with the stretching energy difference is proposed. Most of our results are found in good agreement with recent SCF calculations of Polotsky et al., with one notable exception that their proposed transition from a longest path stretching mode to a uniform stretching mode, the latter being dominant at high densities, has not been supported by our data. Instead, the longest path stretching dominates at high densi...

Force-induced desorption of self-avoiding walks on Sierpinski gasket fractals

In this work we investigate force-induced desorption of linear polymers in good solvents in non-homogeneous environment, by applying the model of self-avoiding walk on two- and three-dimensional fractal lattices, obtained as generalization of the Sierpinski gasket fractal. For each of these lattices one of its boundaries represents an adsorbing wall, whereas along one of the fractal edges, not lying in the adsorbing wall, an external force acts on the self-avoiding walk. The hierarchical nature of the lattices under study enables an exact real-space renormalization group treatment, which yields the phase diagram of polymer critical behavior. We show that for this model there is no low-temperature reentrance in the cases of two-dimensional lattices, whereas in all studied three-dimensional cases the force-temperature dependance is reentrant. We also find that in all cases the force-induced desorption transition is of first order.

Fluorescence studies of a series of monodisperse telechelic α,ω-dipyrenyl poly(N-isopropylacrylamide)s in ethanol and in water

Steady-state and time-resolved fluorescence measurements were performed on solutions in ethanol and in water (23 °C) of a series of poly(N-isopropylacrylamide)s labeled with a pyrenyl group at each chain end (Py2-PNIPAM) and ranging in molecular weight (Mn) from 5 900 to 44 500 g·mol−1. Water and ethanol are of similar solvent quality towards the PNIPAM chain. The pyrene labels, in contrast, are soluble in ethanol but not in water. The efficiency of excimer formation for Py2-PNIPAM samples in ethanol decreased with increasing chain length, a trend typical of pyrene end-labeled polymers in good solvents. The ratio IE/IM, where IE is the Py excimer emission intensity and IM is the Py monomer emission intensity, scaled as Mn−1.4, where Mn is the number-average molecular weight of the sample. The kinetics of excimer formation were more complex for aqueous Py2-PNIPAM solutions, as a consequence of pyrene–pyrene association prior to excitation. The excimer time-dependent profiles exhibited significantly faster rise times, compared with the situation in ethanol, and they could not be fitted with the traditional Birks scheme. The results are discussed in the context of the solution properties of telechelic amphiphilic PNIPAMs and are compared with data gathered previously in studies of dipyrenyl end-labeled poly(ethylene oxides), the only other polymers soluble in water and organic solvents subjected to similar studies in the past.

Rescaling of structural length scales for “soft effective segment” representations of polymers in good solvent

It is shown by simple scaling arguments that the structural length scales of semi-dilute polymer solutions, calculated from coarse-grained “soft effective segment” representations of long polymers, need to be rescaled by a single, well-defined scaling factor to match the corresponding properties of the underlying microscopic polymer model. The validity of this rescaling is illustrated by extensive Monte Carlo simulations of the density profiles and average height of stretched polymer brushes. PACS numbers: 68.47.Mn, 68.47.Pe, 61.25.Hq, 82.35.Lr

Depletion profiles for dilute solutions of linear chains, stars and H-branched molecules by self-consistent field calculations and Monte Carlo simulations

We have performed self-consistent field (SCF) calculations and Monte Carlo (MC) simulations to analyse the depletion profiles for isolated linear, star-like and H-shaped polymers in good solvent using lattice approximations in both methods. In the SCF approach the intra-molecular excluded-volume effects are accounted for using an approach that resembles Flory's method that leads to the Flory size of the chains. This gives a major improvement over the classical tanh profile, and becomes much closer to the MC results, provided that a Kuhn length of 1.5 is implemented.