Polymer Coil Size Research Articles

A comparative study of polymer viscosity modifiers: Flow field challenges & alternative trends

In recent years, substantial advancements have been achieved in augmenting the energy efficiency of hydraulic fluids through the integration of polymers. This study employs a multiscale approach, encompassing an analysis of polymer coil size evolution and flow field characteristics, with the aim of investigating the behavior of both dipole and non-dipole polymers within the host solvent. The ultimate goal is to establish a comprehensive understanding of the correlation between atomic properties of additives and the macroscopic properties of the polymer solution.To achieve this, the study employs an atomistic-continuum hybrid model that combines Brownian Dynamics and Lattice Boltzmann techniques within the bead-spring concept framework. Four chains, each comprising 64 particles, are subjected to varying shear rates. The primary focus centers on the examination of alterations in the radius of gyration and the velocity field within the polymer solution. Notably, the inclusion of dipole-dipole interactions exerts a profound influence on the configuration of the polymers. The results illuminate that non-dipole polymers display a more pronounced coupling with bulk flow hydrodynamics, leading to the confinement of particle motion in directions perpendicular to the primary stream. In contrast, dipole polymers experience a slower increase in coil size when compared to their non-dipole counterparts. These findings furnish valuable insights for the enhancement of energy-efficient hydraulic fluids and contribute to a fundamental comprehension of polymer behavior in lubricants, charting the course for the development of advanced hydraulic fluids in the future.

The depletion thickness in solutions of semi-flexible polymers near colloidal surfaces: analytical approximations.

We derive a simple, yet accurate approximate mean-field expression for the depletion thickness δsf of a solution of dilute semi-flexible polymers next to a hard surface. In the case of a hard wall this equation has the simple form δsf = δ0[1 − tanh(psf/δ0)], where psf accounts for the degree of flexibility and δ0 is the depletion thickness in the case of fully flexible polymers. For fixed polymer coil size, increasing the chain stiffness leads to a decrease in the depletion thickness. The approach is also extended to include higher polymer concentrations in the semidilute regime. The analytical expressions are in quantitative agreement with numerical self-consistent field computations. A remarkable finding is that there is a maximum in the depletion thickness as a function of the chain stiffness in the semidilute concentration regime. This also means that depletion attractions between colloidal particles reach a maximum for a certain chain stiffness, which may have important implications for the phase stability of colloid–polymer mixtures. The derived equations could be useful for the description of interactions in- and phase stability of mixtures of colloids and semi-flexible polymers.

Dynamic light scattering studies on ethylene-propylene copolymers in a hydrocarbon based oil

Viscosity modifiers are inherent to many lubricating multigrade oils as they reduce the temperature dependence on the viscosity of the oil, thus increasing the viscosity index. However, direct evidence of physical mechanisms to which viscosity modifiers increase viscosity index is still lacking due to the lack of appropriate analysis tools. Here, we report a method using dynamic light scattering (DLS) to study the solution behavior and coil size change of ethylene-propylene copolymers as well as polyoctadecylmethacrylate at 1.0 wt. % in a polyalphaolefin oil. The usefulness and limitations of the DLS for monitoring polymer coil size and dynamic solution behavior in oil are discussed.

Partitioning of polymers between bulk and porous media: Monte Carlo study of the effect of pore size distribution

In this paper we investigated the partitioning of polymer chains between bulk solvent and porous stationary phase under conditions appropriate for the chromatography under critical conditions (LCCC) close to the critical adsorption point (CAP). We addressed the concentration effect and the thermodynamic effect of pore-size dispersity (PSD) and their impacts on chromatography, i.e., the topics which surprisingly escaped from the interest of scientists in spite that the hydrodynamic effect of PSD has been a subject of numerous studies. The phase equilibria in narrow pores (as compared with the size of polymer coil) with attractive pores are complex and the enthalpy-to-entropy interplay is very intricate. The chains are attracted into pores and the partition coefficients are larger than 1, which corresponds to the interaction chromatography (IC), but they are strongly confined and deformed. The entropy plays important role and the elution volumes of chains differing in molar mass correspond to the size-exclusion chromatography (SEC). The study thus reveals a new chromatography regime which could be easily overlooked without the awareness of its existence. The unexpected findings are important not only for chromatography, but for understanding the phase equilibria of polymers in porous systems in general.

Molecular Origin of Efficient Phonon Transfer in Modulated Polymer Blends: Effect of Hydrogen Bonding on Polymer Coil Size and Assembled Microstructure

Molecular level engineering of polymer or polymer blends has been recently demonstrated an effective strategy to regulate thermal conductivity. Such materials are of great interest to meet critical requirements of transparent, lightweight, flexible, and so on for thermal management in electronic applications. In this work, modulated polymer blends with poly(vinyl alcohol) (PVA) and biopolymers (lignin, gelatin) were designed and significantly enhanced thermal conductivity was achieved by tuning the intermolecular interaction among polymer components. The hydrogen bond interaction has been revealed as the major driving force that affects the polymer coil dimension in aqueous solution, the microstructure of coil–coil interaction in solid film, and thus, the thermal conduction. A solid relationship across molecular level interaction to macroscale thermal conduction is constructed via careful characterization of the coil size in liquid phase and assembled microstructure in solid phase. Appropriate integration...

Tuning Adsorption Duration To Control the Diffusion of a Nanoparticle in Adsorbing Polymers.

Controlling the nanoparticle (NP) diffusion in polymers is a prerequisite to obtain polymer nanocomposites (PNCs) with desired dynamical and rheological properties and to achieve targeted delivery of nanomedicine in biological systems. Here we determine the suppression mechanism of direct NP-polymer attraction to hamper the NP mobility in adsorbing polymers and then quantify the dependence of the effective viscosity ηeff felt by the NP on the adsorption duration τads of polymers on the NP using scaling theory analysis and molecular dynamics simulations. We propose and confirm that participation of adsorbed chains in the NP motion break up at time intervals beyond τads due to the rearrangement of polymer segments at the NP surface, which accounts for the onset of Fickian NP diffusion on a time scale of t ≈ τads. We develop a power law, ηeff ∼ (τads)ν, where ν is the scaling exponent of the dependence of polymer coil size on the chain length, which leads to a theoretical basis for the design of PNCs and nanomedicine with desired applications through tuning the polymer adsorption duration.

Sequence Control of Macromers via Iterative Sequential and Exponential Growth.

A general strategy through the use of direct azidation of alcohols allowed the sequence control of macromers via both the iterative sequential growth and iterative exponential growth methods. The chemistry was highly efficient in building polymers from a sequence of compositionally different macromers tethered together in close proximity. Using the DPPA/DBU method for near quantitative azidation of the benzyl alcohol moiety, sequence controlled polymers were made via a direct and one-step procedure for CuAAC activation. With four different macromers, spherical miktoarm star-like polymers of 50 000 molecular weight were prepared with a low dispersity, and the polymer coil size depended on the type of added macromer. Polymers made via the iterative methods opens the way for the design of advanced materials with predictable properties.

Effect of Molecular-Scale Features on the Polymer Coil Size of Model Viscosity Index Improvers

Temperature-induced changes in coil size have been proposed as a mechanism underlying the functionality of viscosity index improving polymers. Here, molecular dynamics simulations are used to characterize the effect of temperature on the coil size of model additive polymers. The simulations reproduce experimental observations, where only some polymers increase in size with increasing temperature. The results also reveal that the presence of oxygen atoms in the polymer structure is a key factor in determining whether the polymer expands or contracts. This new simulation approach provides a general methodology for investigating temperature-induced coil size changes in polymeric lubricant additives.

Distortion of Chain Conformation and Reduced Entanglement in Polymer-Graphene Oxide Nanocomposites.

We study the conformations of polymer chains in polymer-graphene oxide nanocomposites. We show that the chains have a reduced radius of gyration that is consistent with confinement at a solid interface in the melt, as is expected for well-dispersed, high aspect ratio nanoparticles that are much larger than the polymer coil size. We show that confinement of the polymer chains causes a corresponding reduction in interchain entanglements, and we calculate a contribution to the plateau modulus from the distorted polymer network via a simple scaling argument. Our results are a significant step forward in understanding how two-dimensional nanoparticles affect global material properties at low loadings.

Hofmeister effect on thermo-responsive poly(propylene oxide): Role of polymer molecular weight and concentration

Although a vast amount of research has been dedicated to investigate the Hofmeister effect on the stability of polymer solutions, a clear understanding of the role of polymer properties in this phenomenon is still missing. Here, the Hofmeister effect of NaCl (destabilizing) and NaSCN (stabilizing) salts on aqueous solutions of poly(propylene oxide) (PPO) is studied. Four different molecular weights of PPO were investigated, to determine how the variation in the polymer coil size affects the Hofmeister effect. The investigation was further conducted for different PPO concentrations, in order to understand the effect of inter-chain interactions on the response to addition of salt. The temperature-driven phase separation of the solutions was monitored by differential scanning calorimetry, which provides the precise value of the phase separation temperature, as well as the enthalpy change accompanied with the transition. It was observed that increasing the molecular weight weakens the effect of the both salts, which is interpreted in terms of a scaling law between the molecular weight and the accessible surface area of the polymers. Increasing the PPO concentration further diminished the NaCl effect, but amplified the NaSCN effect. This difference is attributed to an electrostatic stabilization mechanism in the case of NaSCN.

Scaling Behavior of Dendritic Nanoparticle Mobility in Semidilute Polymer Solutions

In our studies on particle mobility in polymer solutions, we have investigated and determined self-diffusion coefficients of nanoparticles in semidilute solutions of poly(ethylene glycol) (PEG, Mw = 6, 20, 35, and 100 kDa). Specially designed PEGylated dendrimers with well-defined sizes (dh = 3.4–11.0 nm) and with internal 19F moieties allow for background-free 19F NMR diffusion measurements. This way, we were able to assess the self-diffusion coefficients as a function of particle diameter and length scales (correlation length, tube diameter, polymer radius of gyration) with high resolution. Scaling arguments allowed us to visualize a clear crossover between particles probing a lower apparent viscosity to near macroviscosity when the nanoparticle size is comparable to the PEG polymer coil size. The same arguments are shown to correctly predict particle diffusion coefficients as a function of polymer concentration when the particles are smaller than the polymer coils.

Initial stage dynamics of bridging flocculation of polystyrene latex particles with low charge density polycation in a mixing flow near the isoelectric point

Dynamics of ortho-kintetic flocculation of PSL particles with long-chain polycation of low charge density was studied together with the adsorption process of polyelectrolyte near isoelectric point as a function of ionic strength. Within the range of investigation, the initial rate of flocculation was found to take the maximum value immediately after the onset of mixing, but the flocculation gradually slows down with the progress of adsorption. The initial rate of flocculation goes through the minimum as ionic strength increases. Without salt addition, the rate of flocculation is about three times faster than that of salt-induced rapid coagulation. With slightly increasing ionic strength the rate of flocculation slows down because of the reductions of the stiffness and the size of polymer coil in solution and also of the smooth rearrangement of adsorbed chains. The adsorption process monitored by the electrophoresis demonstrates that the zero mobility appears much earlier than the time predicted by collision process in the presence of salt (KCl 100 mM). This trend can be explained by the concept of electrokinetically stagnant layer and by smooth spreading of polyelectrolyte on the colloidal surface. The kinetics of adsorption is in good accordance with the change in the flocculation rate.

How Polymers Behave as Viscosity Index Improvers in Lubricating Oils

One of the requirements of engine lubricating oil is that it must have a low enough viscosity at low temperatures to assist in cold starting and a high enough viscosity at high temperatures to maintain its load-bearing characteristics. Viscosity Index (VI) is one approach used widely in the lubricating field to assess the variation of viscosity with temperature. The VI of both mineral and synthetic base oils can be improved by the addition of polymeric viscosity modifiers (VMs). VI improvement by VMs is widely attributed to the polymer coil size expanding with increasing temperature. However, there is very little physical data supporting this generally accepted mechanism. To address this issue, intrinsic viscosity measurements and Small-Angle Neutron Scattering (SANS) have been used to study the variation of polymer coil size with changing temperature and concentration in a selection of solvents. The results will show that coil size expansion with temperature is not necessary to achieve significant elevation of viscosity index.

The influence of quarternary salt on hyaluronan conformation and particle size in solution

Interaction of hyaluronan (NaHy) with the quaternary salt, benzalkonium chloride (BAC), was studied. Based on the DLS experiments, viscometry and surface tension measurements executed on hyaluronan samples with two molecular weights of Mw=1.8MDa and Mw=0.35MDa, the hypothesis was proposed suggesting that at certain BAC concentrations, hyaluronan can form aggregates, which lead to increase of the polymer coil size measured as z-average diameter. Moreover, it was confirmed that within the whole range of BAC concentrations, repeated variations in size and conformations of polymer coils occur, being connected with the critical micellar concentration of BAC and with hydrophobic interactions of non-polar segments of BAC with hyaluronan hydrophobic domains. Tensiometry, DLS and viscometry data support the assumption that variations of thermodynamical “favourability” of BAC–BAC or NaHy–BAC interactions take place in hyaluronan solutions, based on the BAC concentration.

Equilibration of Polymer Films Cast from Solutions with Different Solvent Qualities

We study the effect of film preparation on the equilibration and viscoelastic properties of polymer films. Polystyrene films with a thickness of 14 nm are spun-cast from decalin solutions at different temperatures near the theta temperature to produce films with different chain conformations. We find that the equilibration time of the films increases significantly near the theta temperature. We attribute this to the onset of a rapid collapse in the polymer coil size at the theta condition, along with an increase in the solvent concentration in the films and thereby interchain separation at the time of vitrification. We find that these effects also cause the plateau modulus and equilibrium viscosity of the films to decrease.

Solution thermal properties of a family of thermo-responsive N-isopropyl acrylamide-co- N-hydroxymethyl acrylamide copolymers – Aspects intrinsic to the polymers

A family of ( N-isopropyl acrylamide-co- N-hydroxymethyl acrylamide) copolymers, p(NIPAAm-co-HMAAm), was synthesised and characterised with respect to solution thermal behaviour. A notable increase in cloud point (CP) was observed as the concentration of the polymer solutions was lowered, though the effect of concentration on CP of pure p(NIPAAm), was relatively small. In all cases, the elevation of CP was most evident at low concentration between 0.1 wt.% and 0.01 wt.%. Both at fixed and decreasing concentrations of p(NIPAAm) solutions, a direct relationship between molecular weight and CP was observed, where CP shifted to higher temperatures as the molecular weight increased. The polymer coil size in solution below the CP in the concentration range studied was between 4 and 11 nm for all the polymers in this study, irrespective of the concentration and of the fraction of hydroxy monomer incorporated, but increased dramatically above the CP. CPs obtained by Dynamic Light Scattering (DLS) were lower than those obtained by UV absorbance as the latter is less sensitive. Copolymer compositions with more than 13% hydroxy monomer showed no thermal hysteresis, in contrast to pure p(NIPAAm). The rate of heating/cooling had almost no effect on the CP values recorded.

Effect of Polymer Size on Heterogeneous Catalytic Polystyrene Hydrogenation

The effect of polymer coil size on the rate of polystyrene (PS) hydrogenation was studied in a slurry reactor with mixtures of decahydronaphthalene (DHN) and carbon dioxide (CO2) as the solvent for the polymer. The PS coil size was changed by varying the polymer molecular weight from 9300 g/mol to 357 000 g/mol and by varying the CO2 concentration. Using a 5% Pd/5% Ru/SiO2 catalyst, the rate of aromatic ring hydrogenation at 150 °C was found to be strongly dependent on the size of a polymer coil relative to the average pore diameter of the catalyst. Significant pore diffusion limitations, as indicated by values of the Weisz modulus, were observed with increasing polymer molecular weight. Increasing the concentration of CO2 resulted in increased reaction rates, with an improvement of nearly 2 orders of magnitude at the highest PS molecular weight.

Investigation on extremely dilute solution of PEO by PFG-NMR

The technique of pulsed-field gradient nuclear magnetic resonance (PFG-NMR) was applied to study the solution properties of a series of low molecular weight poly(ethylene oxide). The self-diffusion coefficients of solutions from semi-concentrated to extremely diluted were measured, leading to a critical concentration. When the concentration of solution is higher than the value of critical concentration, the diffusion coefficient of the solute decreases as the concentration increases and remains the same when the concentration is lower than it. This critical concentration agrees well with the definition of dynamic contact concentration (Cs) and confirms indirectly the Flory's scaling law between the molecular weight and D0. In addition, the influences of molecular weight and terminal groups on Cs were discussed. All the diffusion coefficients determined at extremely dilute condition were equivalent to the diffusion coefficients at infinite concentration (D0), from which the polymer coil size was estimated.

Enhanced Orientational Ordering of Water Dipoles in Uniaxially Stretched Hydrogels

The electrostatics and density correlations of dipolar solvent molecules in weakly charged polyelectrolyte solutions and charged gels are studied using a field-theoretical approach. For miscible dipolar solvent mixtures, an exact expression for the effective dielectric permittivity is obtained on the mean-field level, which depends upon the individual volume of each solvent species before mixing and the final volume of the mixture. If the effect of volume change is small during mixing, the dielectric permittivity is approximately equal to the volume-averaged dielectric permittivity of the individual components of the dipolar solvent mixture. It is found that the electrostatic interaction between the dipolar solvent molecules or any charged species in the polyelectrolyte solution is screened by both counterions (and co-ions if salt is added) and polyions, and there exists a total screening length for any charged species in the system which contains the contribution from the polyions. The screening from the polyions is as important as that from the counterions on the length scale of the polymer coil size. For uniaxially stretched charged gels, it is shown that there is an enhanced orientational ordering of the dipolar solvent molecules along the stretching direction, and the correlation length (r(cor)) of this enhanced orientational ordering is about the product of the total screening length (xi(-1)) due to the counterions and polyions times the square root of the dielectric constant of the medium, that is, r(cor)approximately equal to (epsilon(r)xi(-1))1/2.

Colloid-polymer mixtures in solution with refractive index matched acrylate colloids.

Colloid-polymer (CP) mixtures extend between two limiting cases, the colloid limit with the polymer coil size small compared to the colloid radius Rcol and the protein limit with the colloidal particles much smaller in size than the radius of gyration of the polymer chains Rg. In the present work, model systems are developed for the protein limit. The colloid-solvent pairs are optimized in terms of their isorefractivity in order to facilitate the characterization of large polystyrene chains in suspensions of small colloids. The degree of isorefractivity of colloidal particles was successfully evaluated in terms of a reduced scattering intensity. Two polystyrene samples with radii of gyration of Rg = 96 nm and Rg = 78 nm, respectively, are used. The radii of the colloidal particles are close to Rcol = 12 nm, leading to size ratios of Rg/Rcol = 8 and Rg/Rcol = 6.5. Four colloid solvent systems were found to be suitable for polymer characterization by light scattering, one based on silica particles and three systems with acrylate particles. The present investigation is focused on the three acrylate systems: poly(methyl methacrylate) in ethyl benzoate (ETB) at 7 degrees C, poly(ethyl methacrylate) in toluene at 7 degrees C and poly(ethyl methacrylate) in ETB at 40 degrees C. Characterization of PS chains is for the first time performed in colloid concentrations up to 2.5% by weight. In all cases, the size and shape of the polymer chains remain unchanged. A slight mismatch of the colloid scattering or a limited colloid solubility prevented investigation of PS chains at higher colloid concentration.