Polymer Chain Stiffness Research Articles

On the Pseudo Phase Diagram of Single Semi-Flexible Polymer Chains: A Flat-Histogram Monte Carlo Study.

Local stiffness of polymer chains is instrumental in all structure formation processes of polymers, from crystallization of synthetic polymers to protein folding and DNA compactification. We present Stochastic Approximation Monte Carlo simulations—a type of flat-histogram Monte Carlo method—determining the density of states of a model class of single semi-flexible polymer chains, and, from this, their complete thermodynamic behavior. The chains possess a rich pseudo phase diagram as a function of stiffness and temperature, displaying non-trivial ground-state morphologies. This pseudo phase diagram also depends on chain length. Differences to existing pseudo phase diagrams of semi-flexible chains in the literature emphasize the fact that the mechanism of stiffness creation matters.

Improved operational stability of Pebax-based gas separation membranes with ZIF-8: A comparative study of flat sheet and composite hollow fibre membranes

Polymer-based hollow fibre composite membranes are composed of a porous support coated with a highly permeable gutter layer, and a thin selective top layer. They are promising candidates for industrial gas separation, but there are challenges in maintaining their performance under high pressure or during long-term operation. To tackle this, we incorporated ZIF-8 into Pebax-1657 (polyamide-polyether copolymer) for gas separation membrane fabrication. To fully understand the behaviour of hollow fibre composite membranes, special attention was paid to the effect of ZIF-8 on the Pebax chain structure using dense film flat sheet membranes. The presence of porous ZIF-8 disrupted the original well-packed polymer chain structure, leading to higher fractional free volume and gas permeability. However, the introduced microvoids were non-continuous and the gas selectivity was only slightly reduced. On the other hand, the stable structure of ZIF-8, together with the hydrogen bonds between ZIF-8 organic ligands and polyamide chains, significantly improved the linear glassy polymer chain stiffness, ensuring good operational stability under elevated pressure for both flat sheet and hollow fibre membranes. Finally, good long-term operational stability was observed with the hollow fibre membrane, indicating the ZIF-8/Pebax coating overcame the aging previously observed for the poly[1-(trimethylsilyl)-1-propyne] (PTMSP) gutter layer.

Integral equation prediction of structure of nanocomposites with polymer-grafted nanoparticles near solid surface

In this work, the inhomogeneous polymer reference interaction site model (PRISM) integral equation theory is extended to investigate the microscopic structure of a new system of nanocomposites with polymer-grafted nanoparticles near solid surface. The chemical and morphologic details of polymers are described by a semiflexible chain model, which can be utilized to capture the intramolecular interactions between different segments. Based on the novel bridge functionals of different segments constructed from the corresponding chemical potentials, the modified hypernetted chain approximation is combined with the inhomogeneous PRISM equation to obtain the detailed density distributions of nanoparticle-polymer blends near substrate. The effects of particle volume fraction, nanoparticle size, attractive interaction strength between different segments and chain stiffness on the density profiles of single tethered polymer nanocomposites (PNCs) near surface are systematically investigated to capture the contribution of the packing and configurational entropies. The end effects of polymer chains and effects of solid wall confinement are also considered. It is found that the particle size and particle volume fraction play the most significant roles in the density profiles of the single tethered PNCs among the present investigated model parameters. The increase of the particle volume fraction leads to a monotonic increasing variation behavior of the density profiles of single tethered nanoparticles as the particle diameter is relatively small. With increasing the nanoparticle size, the intensity of the density profiles of nanoparticles decreases prominently. The detailed packing structure of polymer-nanoparticle blends is also influenced by the polymer chain stiffness and particle-monomer interactions, where these parameters of structure and chemistry can enable the density profiles of the single tethered PNCs to show similar variation trends. The present inhomogeneous PRISM theory can provide a detailed description of packing and conformational entropies on the density profiles of different segments in nanocomposites with polymer-grafted nanoparticles. It could also be easily extended to some real systems with more complicated architecture of polymer-grafted nanoparticles under confinement and anticipated to assist in developing some predictive approaches for design control of PNCs near substrate.

Sulfonated poly(ether ether ketone)-based hybrid membranes containing graphene oxide with acid-base pairs for direct methanol fuel cells

The graphene oxide (GO) sheets are functionalized by histidine molecules and incorporated into sulfonated poly (ether ether ketone) (SPEEK) matrix to fabricate hybrid polymer electrolyte membranes for direct methanol fuel cells (DMFCs). The loading of functionalized GO is varied to investigate its influence on cross-sectional morphology, crystalline structure, polymer chain stiffness, thermal stability and fractional free volume of membrane, etc. The acidic −SO3H groups (proton donors) in SPEEK and basic imidazole groups (proton acceptors) in histidine molecules form acid-base pairs and transport protons synergistically, thus yielding efficient proton channels inside the hybrid membranes. The maximum proton conductivity (at 100% RH) of hybrid membranes is elevated by 30.2% compared with the plain SPEEK membrane at room temperature. The functionalized GO flakes also confer the hybrid membranes low methanol permeability in the range of 1.32–3.91×10−7cm2s−1. At the filler content of 4wt%, the hybrid membrane shows a superior selectivity of 5.14×105Sscm−3 and its maximum power density of single DMFC cell (43.0mWcm−2) is 80.7% higher than that of plain SPEEK membrane.

Entropic Interactions in Semiflexible Polymer Nanocomposite Melts

By employing molecular dynamics simulations, we explored the effective depletion zone for nanoparticles (NP) immersed in semiflexible polymer melts and calculated the entropic depletion interactions between a pair of NPs in semiflexible polymer nanocomposite melts. The average depletion zone volumes rely mainly on polymer chain stiffness and increase with chain stiffness increasing. In the semiflexible polymer nanocomposite melts, the entropic depletion interactions are attractive and anisotropic, and increase with chain stiffness increasing. Meanwhile, the attractive interactions between NPs and polymers can also affect strongly the entropic depletion interactions. For the semiflexible polymer nanocomposite melts in the athermal system, the entropic depletion interactions change from anisotropic to isotropic when the NP/polymer interactions increase. For NPs in the rodlike polymer melts, a mixture structure of contact/"bridging" aggregations for NPs is formed at a strong attractive NP/polymer interaction. Our calculations can provide an effective framework to predict the morphology of NPs immersed in semiflexible polymer melts.

Assembly of Lock-and-Key Colloids Mediated by Polymeric Depletant.

Polymer-mediated lock-and-key assembly via depletion attraction is purely a shape-recognition process without any molecular bonding. Since the depletion attraction relates to osmotic pressure and excluded volume, the binding tendency in a dispersion of lock-and-key colloids can be controlled by adjusting the characteristics of polymeric depletants. In this work, dissipative particle dynamics accounting for explicit solvents, polymers, and multiple lock-key pairs are performed to investigate the influences of the polymer concentration, chain length, solvent quality, and chain stiffness. As the polymer concentration associated with osmotic pressure is increased, the binding free energy (E(b)) between a lock-key pair rises linearly and the binding fraction (θ(LK)) in the dispersion grows sigmoidally. Moreover, the increases in the chain length, solvent quality, and chain stiffness lead to the expansion of the polymer size associated with excluded volume and thus both E(b) and θ(LK) rise accordingly. However, E(b) and θ(LK) grow to be insensitive to the chain length for long enough polymer coils but still can be enhanced if the polymer becomes rod-like. As the solvent quality is varied, θ(LK) can be dramatically altered, although the radius of gyration of polymers is slightly changed.

Solid State Structure of Poly(9,9-dinonylfluorene)

We report on X-ray diffraction and grazing-incidence X-ray diffraction data of poly(9,9-dinonylfluorene) (PF9) in bulk, thin films and in the 1% methylcyclohexane gel. We denote the main crystalline phase as α phase and propose that the unit cell is monoclinic (a = 29.31 Å, b = 23.65 Å, c = 33.33 Å, and γ = 84.70°) in bulk and orthorhombic (a = 28.70 Å, b = 23.48 Å, and c = 33.23 Å) in thin films. This structure corresponds to the layered structure along the a-axis (along the elongated side chains and perpendicular to the seemingly stiff polymer chains) and to the stacking of aromatic main chain units along the b-axis. The polymer chains are aligned along the c-axis. Monoclinic structure agrees with the layer spacing of 14.6 Å, the stacking period d(040) = 5.89 Å and the monomer repeat distance of 8.33 Å. The α phase experiences an order–disorder transition at 170 °C upon heating. In the 1% methylcyclohexane gel, this structural motif is maintained but with the loss of long-range order. This is interpreted as a formation of mesomorphic β phase with an orthorhombic unit cell (a = 29.1 Å, b = 28.1 Å, and c = 16.7 Å). Structural analogues to other 9,9-di-n-alkyl-substituted polyfluorenes are discussed in terms of unit cell parameters and backbone geometry.

A catalytic oligomeric motor that walks along a filament track.

Most biological motors in the cell execute chemically powered conformational changes as they walk on biopolymer filaments in order to carry out directed transport functions. Synthetic motors that operate in a similar manner are being studied since they have the potential to perform similar tasks in a variety of applications. In this paper, a synthetic nanomotor that moves along a filament track, without invoking motor conformational changes, is constructed and its properties are studied in detail. The motor is an oligomer comprising three linked beads with specific binding properties. The filament track is a stiff polymer chain, also described by a linear chain of linked coarse-grained molecular groups modeled as beads. Reactions on the filament that are catalyzed by a motor bead and use fuel in the environment, in conjunction within the binding affinities of the motor beads to the filament beads, lead to directed motion. The system operates out of equilibrium due to the state of the filament and supply of fuel. The motor, filament, and surrounding medium are all described at microscopic level that permits a full analysis of the motor motion. A stochastic model that captures the main trends seen in the simulations is also presented. The results of this study point to some of the key features that could be used to construct nanomotors that undergo biased walks powered by chemical reactions on filaments.

Entropic competition in polymeric systems under geometrical confinement

Using molecular-dynamics simulation, we investigate the effect of confinement on a system that comprises several stiff segmented polymer chains where each chain has similar segments, but length and stiffness of the segments vary among the chains, which makes the system inhomogeneous. The translational and orientational entropy loss due to the confinement plays a crucial role in organizing the chains which can be considered as an entropy-driven segregation mechanism to differentiate the components of the system. Due to the inhomogeneity, both weak- and strong-confinement regimes show the competition in the system and we see segregation of chains as the confining volume is decreased. In the case of strong spherical confinement, a chain at the periphery shows angular mobility higher than other chains, despite being more radially constrained.

Solvatochromism and conformational changes in fully dissolved poly(3-alkylthiophene)s.

Absorption spectroscopy is commonly utilized to probe optical properties that can be related, among other things, to the conformation of single, isolated conjugated polymer chains in solution. It is frequently suggested that changes in peak positions of optical spectra result from variations in the stiffness of polymer chains in solution because this modifies the conjugation length. In this work we utilize ultraviolet-visible (UV-vis) spectroscopy, small angle neutron scattering (SANS), and all atom molecular dynamic (AA-MD) simulations to closely probe the relationship between the conformation of single-chains of poly(3-alkylthiophene)s (P3ATs) and their optical properties. SANS results show variations in the radius of gyration and Kuhn length as a function of alkyl chain length, and structure, as well as the solvent environment. Furthermore, both SANS and MD simulations show that dissolved P3HT chains are more rigid in solvents where self-assembly and crystallization are possible. Shifts in P3AT optical properties were also observed for different solvent environments. However, these changes were not correlated to the changes in polymer conformation. Furthermore, changes in optical properties could not be perfectly described by generalized solvent-solute interactions. AA-MD simulations provide new insights into specific polymer-solvent interactions not accounted for in generalized solvatochromic theory. This work highlights the need for experiments and molecular simulations that further inform the specific role of solvent molecules on local polymer conformation and on optical properties.

Cyclic tertiary amino group containing fixed carrier membranes for CO2 separation

Fixed carrier membranes containing amino groups show attractive performance in CO2 separation for clean energy supply and greenhouse gas reduction. However, the separation performance of fixed carrier membranes at high feed pressure is limited by the facilitated transport ability of membranes as well as the CO2-induced plasticization. In this work, the tertiary amino group containing fixed carrier membranes were prepared by interfacial polymerization (IP) with hexane-soluble trimesoyl chloride (TMC) and aqueous-soluble 1,4-bis (3-aminopropyl) piperazine (DAPP), N-aminoethylpiperazine (EAP) and 3,3′-diamino-N-methyldipropylamine (DNMDAm). The results show that the membrane prepared with TMC and DAPP exhibits the highest CO2 separation performance due to efficient CO2 facilitated transport and high plasticization resistance. Efficient facilitated transport of CO2 in the membrane is due to the high content of cyclic tertiary amino groups which possess high stoichiometric CO2 loading and high rate constant of amine-CO2 reaction. Moreover, high plasticization resistance of the membrane is attributed to the appropriate stiffness of polymer chains provided by IP. The membrane prepared with TMC and DAPP shows promising applications in high-pressure fuel gas purification.

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.

Watching two conjugated polymer chains breaking each other when colliding in solution

While collision theory successfully describes the kinetics of chemical reactions, very little is known about the processes at the molecular level, especially if the reacting molecules are large. In this study, using single-molecule spectroscopy, we visually observed that collision between two conjugated polymer (CP) molecules in solution leads to simultaneous rupture of both chains. In addition to opening up the possibility of monitoring chemical processes in solution at the single-molecule level, these results demonstrate that mechanical bending of two stiff conjugated backbones against each other (the effect of leverage) by Brownian motion can weaken the chemical bond and markedly accelerate photochemical oxygen-induced chain scission by at least 20 times. The catalytic effect of the chain bending is also enhanced by a prolonged interaction between the chains owing to their entanglement. These findings are important for the solution processing of CPs in their application in organic electronics, for understanding the degradation mechanisms in CPs and for the development of new catalysts based on mechanical interactions with target molecules. Mechanical forces that result from collisions between molecules play a central role in chemical reactions. The finer details of their involvement remain poorly understood, however, especially for larger molecules possessing an enormous number of degrees of freedom. Now, a team led by Ivan Scheblykin from Lund University, Sweden, has developed a fluorescence microscopy approach that enables a direct visualization of collisions between two individual polymer molecules in solution. The researchers found that the randomly moving molecules, which exhibit a rigid, double-bonded carbon backbone, broke into smaller fragments upon impact. This disintegration stems from molecular entanglement and bending stress where the stiff polymer chains touch each other. The team's single-molecule chemistry approach provides a deeper knowledge of reaction mechanisms at the molecular level and opens the door to new synthetic routes in materials chemistry. By applying single-molecule imaging technique in free solution, we visually observed polymer chains break apart when they collide with each other just because of their Brownian motion. The oxidation scission reaction is catalyzed by the leverage effect, analogous to breaking a stiff wooden stick over the knee. Our surprising results suggest that new catalysts could be designed with the idea of stiff molecules working as ‘knives’ and ‘leverages’ breaking chemical bonds. We believe that our work opens up the possibility of monitoring chemical processes in solution at the single-molecule level.

Spectroscopy of Sound Transmission in Solid Samples

These laboratory experiments are designed to familiarize students with concepts of spectroscopy by using sound waves. Topics covered in these experiments include the structure of nitinol alloys and polymer chain stiffness as a function of structure and temperature. Generally, substances that are stiffer or have higher symmetry at the molecular level transmit sound more effectively, and the higher pitched sounds that they produce upon impact can be heard and visually observed using a laptop microphone and SpectrumView freeware. This lab is designed for use in an introductory undergraduate chemistry course, although the techniques can be used for more and less advanced courses. Additional related classroom demonstrations are also presented.

Coil–helix transition of biopolymer confined in finite cylinder

An off-lattice dynamic Monte Carlo method is used to study the conformations of biopolymers confined in finite cylinders. The coil–helix transition is observed with the increase of bending energy b for the polymer chain confined in small cylinder, and helical structure becomes more perfect for larger b. Periodicity of tangent–tangent correlation function C(s) begins to disappear when the cylinder height H increases, and the influence of R and b on the ability of helix formation of polymer chain weakens significantly. Therefore, the semiflexible and stiff polymer chains can fold into helix in small cylinder.

Molecular dynamics simulation and positron annihilation lifetime spectroscopy: Pervaporation dehydration process using polyelectrolyte complex membranes

The micro-structure of various novel polyelectrolyte complex membranes (PECMs) was investigated by means of molecular dynamics (MD) simulation and positron annihilation lifetime spectroscopy (PALS). These PECMs that differed in their chemical structure design were applied to dehydrate a 90wt% ethanol aqueous solution by pervaporation, and their separation performance was correlated with their microstructure. Free-volume size and free-volume size distribution were determined both by the PALS and MD simulation techniques. To describe the free-volume shape and the polymer chain flexibility and stiffness, MD simulation analysis in terms of radial distribution function and mean square displacement was also conducted. Results obtained from PALS and those from MD simulation were in agreement with each other. These results were highly consistent with the chemical structure of the PECMs designed in this study and were demonstrated to correlate well with the membrane pervaporation separation performance.

Integral Equation Prediction of Surface-Induced Layering Transition of Polymer Nanocomposites

The polymer reference interaction site model (PRISM) integral equation for inhomogenous polymers was applied to investigate layering transitions of nanoparticle/polymer blends near solid surfaces. The equation has the advantage over other theoretical approaches in describing the chemical and morphologic details of polymers. Construction of a novel bridge function derived from a free-energy functional allowed the PRISM equation to be solved with a modified hypernetted chain approximation. Tested by the density functional theory as well as molecular simulations, the equation is quantitatively reliable for the inhomogeneous blends containing flexible or semiflexible polymer chains. Accordingly, the effects of particle size, the attractive interaction strength between particles and polymer monomers, and the stiffness of polymer chains on the layering transitions were investigated to decipher the contribution of the packing and configurational entropies.

Computer simulation of stiff-chain polymers

A review of studies on the computer simulation of the phase behavior of various stiff-chain polymer systems is presented. Methods for calculating phase diagrams of a polymer solution in a computer experiment are discussed, including the methods of extended ensembles, entropic simulation, and the Wang-Landau algorithm to obtain the density-of-states function. The authors’ original results on studying the intramolecular orientational and spatial ordering of monomer units in a single stiff-chain macromolecule in the bulk and near a planar adsorbing surface by means of the Wang-Landau algorithm and using the bond-fluctuation lattice model are presented. Corresponding state diagrams are presented for these two cases. For systems of multiple chains, the phenomenon of nematic liquid-crystalline ordering in semi-dilute solutions in the bulk and in a planar layer is considered, and the phase diagrams for these cases are presented. A survey of the published data on some other promising directions of investigation of stiff-chain polymer systems is presented.

Viscosity and dynamics of nanorod (carbon nanotubes, cellulose whiskers, stiff polymers and polymer fibers) suspensions

The zero shear viscosity and the dynamic behaviors of different nanorod dispersions (carbon nanotubes (CNTs), cellulose whiskers, polymer nanofibers, crosslinked polymer nanofibers, and stiff polymers such as poly(γ-benzyl-α-l-glutamate) (PBLG)) were compared and discussed from literature data. Their Brownian dynamic behaviors have always been discussed in the frame of the Doi–Edwards theory. In agreement with this theory, the straight rigid rods (CNTs, cellulose whisker, polymer nanofibers) obey a master curve in the reduced viscosity (or rotary diffusivity) c power laws on viscosity (η 0 ∝ φ 3) and diffusivity (D r ∝ ϕ −2). On the contrary, stiff polymer chains and crosslinked polymer fibers at temperature above T g exhibit different and two distinct dynamic behaviors. Despite their deviation from the ideal rigidity, surprisingly it can be noted that stiff polymers such as PBLG have been extremely used in the literature to verify the Doi–Edwards theory. Finally, flexible crosslinked chains at T > T g , do not obey the Doi–Edwards theory, and their dynamics are close to the physics of polymer solutions in terms of power laws.

Effect of Heat Treatment on Dilute Solution Properties of Sugar Beet Pectin

Beet Pectin (SBP) polysaccharide. Aqueous solution properties of these samples have been studied using Ubbelohde capillary viscometer in the dilute regime. Intrinsic viscosity [?] was estimated for different SBP solution samples prepared from heat-treated SBP powdered samples with different heating time. Results suggest that heat treatment has a significant effect on solution behavior of SBP samples. Intrinsic viscosities decreased with increasing heating time indicating that heat-treated SBP molecules are compact than the untreated one and the compactness increases with increasing heating time at 80°C. The intrinsic viscosities of these samples were plotted as a function of the corresponding molecular weight of the samples and can be expressed by a power law. The power law exponent was estimated as 1.4 indicating that pectin molecules takes the conformation of stiff polymer chains in dilute solution. Dhaka Univ. J. Sci. 61(1): 117-123, 2013 (January) DOI: http://dx.doi.org/10.3329/dujs.v61i1.15107