Effect Of Chain Flexibility Research Articles

Electrostatic hindrance to diffusion in flexible crosslinked gels: A coarse-grained simulation study.

In this work, we study how electrostatic forces slow down the diffusion of solute in flexible gels through coarse-grained simulations. The model used explicitly considers the movement of solute particles and polyelectrolyte chains. These movements are performed by following a Brownian dynamics algorithm. The effect of three electrostatic parameters characterizing the system (solute charge, polyelectrolyte chain charge, and ionic strength) is analyzed. Our results show that the behavior of both the diffusion coefficient and the anomalous diffusion exponent changes upon the reversal of the electric charge of one of the species. In addition, the diffusion coefficient in flexible gels differs significantly from that in rigid gels if the ionic strength is low enough. However, the effect of chain flexibility on the exponent of anomalous diffusion is significant even at high ionic strength (100 mM). Our simulations also prove that varying the polyelectrolyte chain charge does not have exactly the same effect as varying the solute particle charge.

Reactive blending of bisphenol-A polycarbonate with isosorbide-based polycarbonates: Effect of chain flexibility and compatibility

In this study the effect of flexible 1,4-cyclohexanedimethanol (CHDM) units on the reactive blending of poly(isosorbide-co-CHDM carbonate) (IcC-PC) with bisphenol-A polycarbonate (BPA-PC) was investigated and the contributions of chain flexibility and compatibility to transesterification were clarified. FTIR analysis revealed the presence of specific interactions between the CHDM segments of IcC-PC and the carbonyl group and phenyl rings of BPA-PC, leading to thermodynamic miscibility of the polymer blends when the CHDM content was higher than 50 mol% in IcC-PC. Addition of organotin catalysts during melt blending triggered transesterification between IcC-PC and BPA-PC, transforming the opaque blends into transparent alloys with a single Tg when the transesterification ratio reached a critical value. It was found that the reactivity of isosorbide (ISB) segments was higher than that of CHDM segments. The increase in the CHDM/ISB ratio first accelerated and then slowed down the transesterification, demonstrating that, despite of the low reactivity of the CHDM segments, the enhanced chain flexibility and compatibility were effective for promoting transesterification. Anyway, copolymerization of CHDM with ISB improved the compatibility of IcC-PC with BPA-PC, reduced the critical transesterification ratio for transparency, and remarkably shortened the blending time essential for the large-scale fabrication of transparent ISB-based polycarbonate alloys by twin-screw reactive extrusion.

Effect of chain flexibility on cell adhesion: Semi-flexible model-based analysis of cell adhesion to hydrogels

Hydrogels have been developed and applied to various biomedical applications due to their biocompatibility. However, understanding of modulation between cells to hydrogel interface is still unclear, and parameters to explain the interaction are not sophisticated enough. In this report, we studied the effect of polymer chain flexibility on cell adhesion to various hydrogel constructs of collagen and fibrin gels. Specifically, novel method of semi-flexible model-based analysis confirmed that chain flexibility mediated microstructure of the hydrogels is a critical factor for cell adhesion on their surfaces. The proposed analysis showed possibility of more accurate prediction of biocompatibility of hydrogels, and it should be considered as one of the important criteria for polymer design and selections for enhancing both biocompatibility and biofunctionality.

Effect of Chain Flexibility and Interlayer Interactions on the Local Dynamics of Layered Polymer Systems

Layered polymeric systems are widely used in membrane separation applications; chain mobility in these layered systems is a key consideration in the design of the membranes. The transport properties of membrane polymers can be significantly altered by the perturbations in chain dynamics induced by the presence of an interface and also by the topological properties of the polymers constituting the layered systems. In this work, we use molecular dynamics (MD) simulations to determine the effects of polymer backbone flexibility and interlayer interactions on the glass transition and chain dynamics of polymer layers in the layered systems. We observed that the onset of glass transition of the entire layered system is governed by the stiffer polymer layer and is independent of the type of interactions between the layers. However, the interlayer interactions govern the strength of the glass transition of the entire layered system. Polymer mobility, on the other hand, exhibits a strong dependence on both the cha...

Chain Flexibility in Self-Assembled Monolayers Affects Protein Adsorption and Surface Hydration: A Molecular Dynamics Study.

Recent developments in the antifouling properties of Self-Assembled Monolayers (SAMs) have largely focused on increasing the enthalpic association of a hydration layer along the interface of those surfaces with water. However, an entropic penalty due to chain restriction also disfavors biomolecule-surface adsorption. To isolate the effect of this entropic penalty amid changing packing densities, molecular dynamics simulations of explicitly solvated systems of lysozyme and seven monomer length oligo (ethylene glycol) (OEG) SAMs were performed. SAM surfaces were constructed at 100%, 74%, and 53% of a maximum packing (MP) density of 4.97 Å interchain spacing and the effect of chain flexibility was isolated by selectively freezing chain monomers. The rate of protein adsorption as well as the conformation and orientation of the protein upon adsorption were examined. It was found that chain spacing was a strong determinant in adsorption properties while chain flexibility played a secondary role. Of the three packing densities, 74% of MP was the most antifouling with increased antifouling behavior at moderate chain flexibility, i.e. two to four free monomer groups.

Kinetics of Chain Exchange between Diblock Copolymer Micelles

Dissipative particle dynamics simulation is employed to study the chain exchange kinetics between micelles of diblock copolymer in aqueous solution via in silico hybridization method. One focus is placed on the effect of chain flexibility on the dynamic behavior by varying the spring constant in the bead‐spring model. The length ratio of hydrophilic to hydrophobic block is also varied. It is found that chain expulsion/insertion is the dominant mechanism in the chain exchange process. The most interesting finding is the multimodal relaxation behavior for the chain exchange and expulsion when the spring constant is small or the length ratio of hydrophilic to hydrophobic block is large. This phenomenon is due to an increase in size polydispersity of micelles with rising population of small aggregates/micelles, for which the exchange kinetics is faster. Micelles with larger aggregation numbers (>10) are found to follow single exponential relaxation kinetics. image

Effect of chain flexibility on master curve behavior for diffusion coefficient

The diffusion coefficients of simple chain models are analyzed as a function of packing fraction, η, and as a function of a parameter C that is the density raised to a power divided by temperature to look at scalar metrics to find master curves. The central feature in the analysis is the mapping onto an effective hard site diameter, d. For the molecular models lacking restrictions on dihedral angle (e.g., freely jointed), simple mappings of molecular potential to d work very well, and the reduced diffusion coefficient, D*, collapses into a single-valued function of η. Although this does not work for the dihedral angle restriction case, assuming that d is inversely proportional to temperature to a power results in collapse behavior for an empirically selected value of the power. This is equivalent to D* being a single-valued function of C. The diffusion coefficient of a single-site penetrant in the chain systems also is found to be a scalar metric that can reduce the chain diffusion data for a given system to a single master curve.

The isotropic-nematic phase transition of tangent hard-sphere chain fluids—Pure components

An extension of Onsager's second virial theory is developed to describe the isotropic-nematic phase transition of tangent hard-sphere chain fluids. Flexibility is introduced by the rod-coil model. The effect of chain-flexibility on the second virial coefficient is described using an accurate, analytical approximation for the orientation-dependent pair-excluded volume. The use of this approximation allows for an analytical treatment of intramolecular flexibility by using a single pure-component parameter. Two approaches to approximate the effect of the higher virial coefficients are considered, i.e., the Vega-Lago rescaling and Scaled Particle Theory (SPT). The Onsager trial function is employed to describe the orientational distribution function. Theoretical predictions for the equation of state and orientational order parameter are tested against the results from Monte Carlo (MC) simulations. For linear chains of length 9 and longer, theoretical results are in excellent agreement with MC data. For smaller chain lengths, small errors introduced by the approximation of the higher virial coefficients become apparent, leading to a small under- and overestimation of the pressure and density difference at the phase transition, respectively. For rod-coil fluids of reasonable rigidity, a quantitative comparison between theory and MC simulations is obtained. For more flexible chains, however, both the Vega-Lago rescaling and SPT lead to a small underestimation of the location of the phase transition.

Polymeric Michler's ketone photoinitiators: The effect of chain flexibility

Through introducing Michler's ketone (MK) moiety and diglycidyl ether monomers of different molecular chain into the same polymeric chain, polymeric photoinitiators of different chain flexibility, PMKPR, PMKPG and PMKPP were synthesized. These polymeric photoinitiators possess the similar characteristic UV–vis absorption of parent MK, and their photobleaching behavior is similar. The T g of PMKPR, PMKPG and PMKPP is 58.1 °C, −24.9 °C and −12.5 °C, respectively. Three types of monomer with different functionality, phenoxy ethyleneglycol acrylate (AMP-10G), 2,2-bis[4-(acryloxypolyethoxy) phenyl]propane (A-BPE-10) and trimethylopropane triacrylate (TMPTA), were chosen to be initiated by these photoinitiators. The result indicates the chain structure of photoinitiators has significant influence on the polymerization of monomers. PMKPG is the most efficient polymeric photoinitiator for initiating the polymerization of AMP-10G and A-BPE-10, while PMKPP is the most efficient for TMPTA.

Polyelectrolyte–macroion complexation. II. Effect of chain flexibility

A simple model with focus on the electrostatic interaction has been used to examine the complexation of a linear polyelectrolyte possessing variable flexibility with one or several oppositely charged macroions. Composition, structure, and thermodynamic properties of the complexes were obtained by using Monte Carlo simulations. Binding isotherms obtained were Langmuir-type with a quantitative binding up to a neutral complex with a plateau value corresponding to ∼50% overcharging, the largest overcharging appearing for the stiffest chain. Free energy calculations demonstrated that the complexation of the first macroion becomes less favorable as the chain stiffness is increased, whereas the opposite was found for a complexation of a macroion to a neutral complex. For a neutral complex and with a flexible chain, the repulsion between complexed macroions is strongly screened and the complexed macroions are located near each other. However, for a very stiff chain, the macroions are sequentially positioned along the chain with much fewer polyelectrolyte segments near them and with a considerable effective repulsion between the macroions. Furthermore, our results are discussed using the concept of an overcharged/undercharged complex with respect to both the polyelectrolyte and a macroion as the central species. The location of the small ions was also considered and in particular for a neutral system they are nearly homogeneously distributed in the solutions.

Microwave dielectric relaxation and molecular dynamics in binary mixtures of poly(propylene glycol) 2000 and poly(ethylene glycol)s of varying molecular weight in dilute solution

AbstractMolecular dynamics of binary mixtures of poly(propylene glycol) (PPG) and poly(ethylene glycol)s (PEGs) of varying molecular weight due to molecular interactions, chain coiling and elongation in dilute solution under various conditions, ie varying number of monomer units of PEG, method of mixing of polymers and solvent environment, has been explored using microwave dielectric relaxation times. The average relaxation time τo, relaxation time corresponding to segmental motion τ1 and group rotations τ2, of a series of binary mixtures of poly(propylene glycol) 2000 and poly(ethylene glycol) of varying molecular weight (ie PPG 2000 + PEG 200, PPG 2000 + PEG 300, PPG 2000 + PEG 400, and PPG 2000 + PEG 600 mixed by equal volume in the pure liquid states, and PPG 2000 + PEG 1500, PPG 2000 + PEG 4000 and PPG 2000 + PEG 6000 mixed equal weights in solvent) have been determined in dilute solution in benzene and carbon tetrachloride at 10.10 GHz and 35 °C. A comparison of the results of these binary systems of highly associating molecules shows that the molecular dynamics corresponding to rotation of a molecule as a whole and segmental motion in dilute solutions are governed by the solvent density when the solutes are mixed in their pure liquid state. Furthermore, the molecular motion is independent of solvent environment when the polymers are added separately in the solvent for the preparation of binary mixtures. It has also been observed that there is a systematic elongation of the dynamic network of the species formed during mixing of pure liquid polymers in lighter environment of solvent with increasing PEG monomer units, while the elongation behaviour of the same species in the heavier environment of carbon tetrachloride solvent is in contrast to the elongation behaviour of the polymeric species formed in pure PEG. The role of rotating methyl side‐groups in the PPG molecular chain has been discussed in term of the breaking and reforming of hydrogen bonds in complex polymeric species for the segmental motion. In all these mixtures, the relaxation time corresponding to group rotations is independent of the solvent environment and constituents of the binary mixtures. The effect of chain flexibility and coiling in these binary mixtures is discussed by comparing the relaxation times of the mixtures with their individual relaxation times in dilute solutions measured earlier in this laboratory.© 2001 Society of Chemical Industry

The effect of chain flexibility and chain mobility on radiation crosslinking of polymers

Radiation crosslinking of polymers mainly depends on the structure of polymer chain. The flexibility and mobility of chain directly influence the possibility of the reactive radicals recombination. Flexible chain is easier to crosslink than rigid-chain polymer. The latter must be crosslinked at high temperature, as most polymers can only crosslink above their melting point. Structural effect also influences the mechanism of radiation crosslinking of polymers. We find from the results in literature and in our laboratory that, the flexibility chain polymer mainly crosslinked with H type, but the rigid chain polymer mainly crosslinked with Y type.

Effect of chain flexibility on the nematic-smectic transition

The theory of nematic-smectic phase transition in the system of uniform semi-flexible chains with hard-core repulsion is presented. Both the general density-functional formalism the tube-model calculation show that the flexibility of the chains results in a strong first-order transition, in contrast to the common weak-cristallization scenario of the nematic-smectic transition in rigid rods. The calculated spinodal volume fraction of the uniform nematic phase and the period of the modulation instability are consistent with recent experimental results.

Intramolecular End-to-End Reactions of Photoactive Terminal Groups Linked by Poly(oxyethylene) Chains

Abstract The triplet-sensitized photochemical reaction using a series of poly(oxyethylene) chains with a pair of photoactive terminal groups, dibenz[b,f]azepine (DBA) chromophores (DBA–COCH2CH2(OCH2CH2)nOCH2CH2CO–DBA, n=0–10) was examined. The photoirradiation of bichromophoric compounds caused either intra- or intermolecular reactions. These reactions were kinetically analyzed by two different methods: the measurement of deactivation processes of the reaction intermediates (excited triplet state of DBA) by nanosecond laser photolysis and the quantitative analysis of the reaction products by GPC. The intramolecular deactivation rate constant, kintra, showed a remarkable chain-length dependence; the maximum kintra value appeared at n=5 and it was found to be 5.9×104 s−1. On the other hand, the intramolecular cyclization rate also depends on the chain length; the maximum quantum yield, φdintra, was given at n=7 (φdintra=0.51). The chain length for the maximum cyclization yield shifted slightly to the longer region than that for the maximum kintra value due to the restriction of the terminal structure (anti-configuration). The results obtained for this reaction system are compared with those obtained for the previously reported polymethylene system and the effect of chain flexibility on the intramolecular ring-closure reaction is discussed.

Effect of chain flexibility on the dynamics of rodlikk polymers in the entangled state

AbstractThe Brownian motion of semiflexible polymers in networks and in concentrated solutions is discussed. If the polymer is assumed to move only along itself by reptation, various time correlation functions are calculated rigorously. The results show that the Brownian motion is independent of the mesh size a over a wide range. It is then argued that the observed rodlike behavior of semiflexible polymers in concentrated solutions is due to the rapid bending fluctuation of the polymer.