Modification of Statistical Threading in Two-Component Pseudorotaxane Melts Using the Amphiphilic Approach and Variations in the Confinement Geometry

Recently we described a coarse-grained model of poly(ethylene oxide) and then employed that model to study the amount of spontaneous threading of cyclic molecules by linear chains in the melt [C. A. Helfer, G. Xu, W. L. Mattice, and C. Pugh, Macromolecules 36, 10071 (2003)]. Since the amount of statistical threading at equilibrium is small, there is interest in identifying physical changes in the system that will increase the threading. We now use that coarse-grained model to investigate the effect on threading of various hypothetical (but feasible) modifications to the two-component system of macrocycles and linear chains in the melt, and different confinement geometries, that can bring about correlations in the arrangement of the rings. Our work follows on the concept of an amphiphilic approach [C. Pugh, J.-Y. Bae, J. R. Scott, and C. L. Wilkins, Macromolecules 30, 8139 (1997)] for increasing the statistical threading in homopolyrotaxane melts. We investigate whether introducing such correlations in the macrocycles can increase the spontaneous threading. This paper shows that some of our modifications can yield more than double the amount of threading seen in purely statistical mixing. (C) 2004 American Institute of Physics

Inclusion Complexes of Chain Molecules with Cycloamyloses. 2. Molecular Dynamics Simulations of Polyrotaxanes Formed by Poly(ethylene glycol) and α-Cyclodextrins

Molecular dynamics (MD) simulations were performed for “channel type” polyrotaxanes with α-cyclodextrins (α-CDs) threaded onto monodisperse chains of poly(ethylene glycol) (PEG). The polymer captures as much α-CD as its length permits, forming a close-packed structure from the one end to the other. The van der Waals interactions are the main source of the stabilization of these polyrotaxanes. Hydrogen bonds between successive α-CDs slightly favor head-to-head, tail-to-tail sequences over head-to-tail sequences. The α-CDs in polyrotaxanes are more symmetric and less distorted than the isolated α-CDs. The PEG in the polyrotaxane is more extended than an unperturbed chain, because it has a larger population of trans states at internal bondsThis research was supported by UAH-017/95 and DGICYT PB94-0364 and by National Science Foundation grant DMR 9523278

Intramolecular excimer formation in naphthalene-containig polyesters. Bichromophoric model compounds derived from phathalic, siccinic or malonic acid and 2-hydroxynaphthalene or 2-hydroxy-methylnaphthalene.

Steady state fluorescence measurements in dilute solutions are performed for bis(2-naphthyl) phthalate, bis(2-naphthylmethyl) phthalate, bis(2-naphthyl) succinate, bis(2-naphthylmethyl) succinate, bis(2-naphthyl) malonate and bis(2-naphthylmethyl) malonate, as models for polyesters containing naphthalene in their rigid units and flexible spacers that impose differences in the types of interactions between successive naphthalene units. The amount of intramolecular excimer in dilute solution depends on the type of flexible spacer, and also whether the compound is derived from 2-naphthol or 2-hydroxymethylnaphthalene. No excimer is detected from the compounds derived from 2-naphthol, but all compounds derived from 2-hydroxymethylnaphthalene showed excimer formation. These results are rationalized with a theoretical analysis of the conformations of the flexible spacers.This research was supported by DGICYT PB91-0166 (FM) and by National Science Foundation grant DMR 9220369 (WLM)

Inclusion Complexes of Chain Molecules with Cycloamyloses III. Molecular Dynamics Simulations of Polyrotaxanes Formed by Poly(propylene glycol) and beta-Cyclodextrins

Molecular dynamics simulations were performed in vacuo on “channel type” polyrotaxanes composed of β-cyclodextrins (βCDs) threaded onto isotactic and syndiotactic poly(propylene glycol) (PPG). In the most stable complex, the βCDs form a close-packed structure from one end of the PPG chain to the other. Non-bonded van der Waals interactions between βCD and PPG are the main source of stabilization of the complex. Head-to-head and tail-to-tail orientation of successive βCDs in the complex is more favorable than a head-to-tail orientation, due to intermolecular hydrogen bonding between head-to-head βCD units. βCDs in polyrotaxanes adopt a more rigid and symmetrical macroring conformation than does an isolated βCD. Formation of the polyrotaxane is accompanied by an increase in the number of trans states at the bonds in the backbone of PPG. For this reason, the PPG chain in the polyrotaxane is much more extended than the unperturbed chain.This work was supported by DGICYT PG94-0364 and by National Science Foundation grant DMR 952327

Intramolecular energy transfer in naphthalene-containing polyesters: Experiment and simulation for model compounds derived from five aliphatic dicarboxylic acids and 2-hydroxynaphthalene

Steady-state fluorescence depolarization measurements and molecular dynamics simulations have been used to study the efficiency of nonradiative intramolecular singlet energy transfer between 2-naphthoxy groups, denoted N, in model compounds for polyesters derived from 2,6-dihydroxy-naphthalene and aliphatic dicarboxylic acids. The five bichromophoric compounds studied are the diesters abbreviated as N-OOC-(CH₂)n-COO-N;n = 2–6, which are condensation products obtained from 2-naphthol and aliphatic dicarboxylic acids. The anisotropy of the fluorescence of these compounds dispersed in a solid matrix of glassy poly(methyl methacrylate) indicates that there is nonradiative singlet energy transfer between naphthoxy groups. The efficiency of this transfer depends on. A theoretical treatment using molecular dynamics simulations for the conformations of the five model compounds has been performed in order to evaluate the parameters related to the efficiency of the transfer. The experimental and theoretical variation of such parameters withn is consistent with the estimated Förster radius of 9–10 Å for this system.This research was supported by Grants DGICYT PB94-0364 and UAH 017/95 (A.S.-C., J.P., and F.M.) and by NSF Grant DMR 9523278 (W.L.M.

Molecular Mechanics Study of the Inclusion Complexes of 2-Methyl Naphthoate with α- and β-Cyclodextrins

Molecular mechanics calculations were employed to study the inclusion of 2-methyl naphthoate in α- and β-cyclodextrinin vacuoand in the presence of water as a solvent. The driving forces for complexation are dominated by nonbonded van der Waals host:guest interactions in both environments. The 2-methyl naphthoate penetrates completely into the cavity of β-cyclodextrin, but there is only partial penetration by the same molecule into the smaller cavity of α-cyclodextrin.This research was supported by CICYT Grant PB94-0364, by the University of Alcalá, (UAH 017/95), and by National Science Foundation Grant DMR 9523278. We express our thanks to Mrs. Heijnen for assistance with the preparation of the manuscript

Simulation of the adsorption of symmetric diblock copolymers at the interface of the two monomeric homopolymers

A lattice model is presented to study a block copolymer compatibilizing two immiscible monomeric homopolymer blends. Each monomeric homopolymer occupies a single site on the lattice. The symmetric diblock copolymer AB is modeled by a self-avoiding walk on a cubic lattice. The two monomeric homopolymer fields interacting with the diblock copolymer are modeled by introducing two different kinds of vacancies in the lattice. These vacancies have the reduced pairwise interaction with A or B segments representing the homopolymer fields, namely εAS1 = 0, εBS2 = 0, εAS2 = εBS1 = εAB = ε ≥ 0. The two kinds of vacancies each occupy half of the lattice sites divided along the z direction, and forming a sharp interface at z = Lz/2. The equilibrium of copolymers dispersed in either phases vs localization at the interface is studied for a range of copolymer concentrations and different interaction energies. The number of chains per surface area, σ, was found to depend on the product of φb exp(εNc), where φb is the equilibrium concentration of the symmetric diblock copolymers in two phases, and Nc is the degree of polymerization of the copolymers. The dependence of σ on φb exp(εN c) is close to the Langmuir type adsorption. These results hold both at the cases that the copolymer chains in the bulk do not form micelles, and do form micelles. Micellization processes do not modify the overall function very much. That is partly due to the fact there is a large entropy loss when forming the micelles compared to the localization of the AB copolymer chains at the flat interface between the two monomeric homopolymer fields. © 1993 American Institute of Physics

Intramolecular vs intermolecular formation of bityrosine upon photoreaction of poly(l-tyrosine) in dilute aqueous solution

The molecular weight dependence of the rate of the formation of bityrosine upon irradiation of poly(L-tyrosine) at 300 nm has been investigated in dilute aqueous solution, pH 12. The samples studied were the monodisperse monomer, dimer, trimer, and hexamer, as well as two polydisperse polymers with average degrees of polymerization of ∼ 102 and ∼ 103. The reaction is predominantly intermolecular in the monodisperse oligomers, but it is predominantly intramolecular in the two polymers. The intramolecular reaction in the polymers involves phenolic rings on monomers i and i+j, with j\u3e5. The steric restraints imposed by the backbone of the chain make it unlikely that two rings will approach one another with the appropriate geometry for reaction if j\u3c5. © 1992 Springer-Verlag

Adsorption of Homopolymers on a Solid Surface: A Comparison between Monte Carlo Simulation and the Scheutjens‒Fleer Mean-Field Lattice Theory

A comparison of a multichain Monte Carlo simulation of homopolymer adsorption on a solid surface with the mean-field lattice theory of Scheutjens‒Fleer is presented. The comparison reveals certain systematic deviations between the theory and the simulation, which are reflected in the bound fraction of the chain, surface coverage, and the adsorbed amount. Such deviations can be attributed to two approximations adopted in the theory. One approximation is the allowance of direct back-fold of the chain, and the other approximation is the random-mixing within each layer. The allowance of direct back-fold of the chain is a result of treating the chain as a Markovian chain. It gives rise to a difference in the number of allowed conformations compared to the Monte Carlo simulation. However such differences do not affect the distribution of chain segments in homogeneous solution. It would only cause a difference when the chain is in an inhomogeneous solution or when it encounters an impenetrable solid surface. The study reveals that the deviation in bound fraction introduced due to the allowance of direct chain back-fold persists throughout the whole range of concentration. It is more pronounced at weak adsorption. On the other hand, the random mixing approximation works better in moderate concentration under weak adsorption since the adsorbed chains can more easily penetrate each other. In the strong adsorption limit, the adsorbed chains are confined to two dimensions and they resist interpenetration. Thus the deviation in surface coverage and adsorbed amount caused by the random mixing between the theory and simulation is more pronounced under strong adsorption. © 1994, American Chemical Society. All rights reserved