摘要:

AbstractWe use both Monte Carlo computer simulations and numerical self‐consistent field lattice calculations to determine the behavior of copolymers at penetrable and impenetrable interfaces. These computational techniques are useful as “design tools”: they allow us to systemically vary the copolymer architecture, determine optimal structures for specific applications, and establish guidelines for fabricating copolymers that yield the desired interfacial properties. We illustrate this principle with three different examples. In the first study, we combine the techniques to design copolymer compatibilizers that enhance the strength of immiscible polymer blends. These copolymers contain teeth that associate across the penetrable interface between the phase‐separated regions and form a “molecular velcro” that effectively binds the regions together. In the case of impenetrable interfaces, we determine how the copolymer sequence distribution affects the structure of a layer of copolymers grafted onto a solid surface. The results indicate how to control the morphology of the layer and the surface properties of the substrate, by varying the microstructure of the grafted copolymers. Finally, we design a polymer channel that “opens” and “closes” in response to changes in the pH and quality of the surrounding solvent. The channel is formed from polyacid chains that are anchored onto a solid surface. Due to these properties, the system can be used for controlled releas

ISSN:1022-1344    

DOI:10.1002/mats.1995.040040401

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractLight scattering experiments may be used to determine a lot of quantities characteristic of a polymer. Numeric analysis of calculated scattering intensities of ensembles of polymer model chains in dilute solution (taking into account intramolecular as well as intermolecular interferences) according to the familiar methods and comparison of the results to the respective quantities evaluated directly in real space allows a check of the liability of data extracted from light scattering experiments. It turned out that the mean‐square radius of gyration as well as the excluded volume (which may be regarded as the second moment of the pair distribution function) are nearly perfectly reproduced. Higher moments, however, are subject to some error which is due to the characteristic changes in size, shape and orientation of polymers caused by intermolecular interaction which arenotaccounted for in the theories forming the basis of the analysis of light scattering dat

ISSN:1022-1344    

DOI:10.1002/mats.1995.040040402

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractIn the self‐consistent field approximation a theory of the orientational ordering is developed for the melt of dimers containing two mesogenic fragments capable of conformational reorganization. Discrete conformations of the dimers (rotational isomers) are characterized by the values of valence angles. Both three‐dimensional and two‐dimensional melts are considered. It is shown that the presence of bent isomers in the melt reduces the temperature of the phase transition to the anisotropic phase. For a three‐dimensional system with first‐order phase transition to the ordered state, the jump of the order parameter at the transition point for dimers with conformational flexibility appears to be greater than for rigid dimers. For dimers with two isomers (a linear and a bent one) the order parameter and the statistical weight of the linear isomer at the transition point depend nonmonotonously on the statistical weight of this isomer in the isotropic phase. The effect of “supercooling” of the isotropic phase for linear conformations of flexible dimers

ISSN:1022-1344    

DOI:10.1002/mats.1995.040040403

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractA semi‐empirical formula for orientational relaxation times of worm‐like chains in dilute solutions is proposed\documentclass{article}\pagestyle{empty}\begin{document}$ \tau \cong \frac{{27}}{8}x^3 \cdot \left[ {1 - \frac{3}{4}x + \frac{3}{4}x \ {\rm exp}\left( { - \frac{{\rm 4}}{{{\rm 3}x}}} \right)} \right]^3 \tau _{{\rm rod}} $\end{document}where τrod= τ0N3is the relaxation time of a rigid rod composed ofNsegments, andx= 2a/Lis the chain rigidity, i.e. the ratio of the double persistence length to the chain contour length,L. The formula, which can be used in the entire range of molecular rigidities and chain lengths, has been tested against segment relaxation times for semi‐rigid chains calculated from the optimized Rouse‐Z

ISSN:1022-1344    

DOI:10.1002/mats.1995.040040404

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThis investigation shows that Markov‐chain copolymers can be regarded as random copolymers the segment lengths of which depend on the copolymerization parameters. It was possible to derive simple analytical formulae for the mean‐square end‐to‐end distance (〈r2〉), the Kuhn length, and the distribution ofr2under theta‐conditions. The results of these equations are in excellent agreement with data from simulations. It is shown that 〈r2〉 as well as the non‐uniformity ofr2increase strongly with increasing probabilities of homopropagation, i.e., with increasing mean homosequence lengths. Furthermore it is demonstrated by simulation that even chains of identical length and composition show a distribution ofr2because of different arrangements of the sequences inside the chains. For chains or chain segments shorter than the average homosequences, a double‐peak distribution ofr2is found. The equations derived in this paper can be applied to real copolymers as well as to chains the curvature of which is altered locally by the a

ISSN:1022-1344    

DOI:10.1002/mats.1995.040040405

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractMonte Carlo simulation of freely jointed off‐lattice chains with variable bond length is usually done with local random displacements of beads and with reptation moves (displacements of a bead along a chain). In dense systems, the acceptance ratio of reptations decreases strongly with density. We discuss versions of reptation moves, which are effective in dense systems. The idea, which comes from lattice systems, is to use a pseudovacancy (walker), which has the same size as a bead of a chain. The walker is attached to a neighbor chain and then another bead of that chain is cleaved. This is equivalent to a reptation move and a nonlocal displacement of the walker and since no free volume is needed, the move can be used with advantage in dense systems. A related technique are cooperative motions, which were introduced by T. Pakula for lattice models, where several chains change their conformation concomitantly. Such cooperative loops are implemented in the Monte Carlo algorithm by creating a temporary walker by cleaving a bead from a chain, moving it with reptations and finally annihilating the walker by attaching it to the same chain it was cleaved from. These moves and the condition of detailed balance are discussed in detail. As an example, we study the integrated autocorrelation time τintfor the radius of gyration for a two‐dimensional system. For reduced densities larger than 0,4, we find that with standard reptations and local bead displacements τintincreases strongly with density. If reptations with either a permanent or a temporary walker are used in addition to local moves, the integrated autocorrelation time changes only very little with density and very dense systems can still be simulated effici

ISSN:1022-1344    

DOI:10.1002/mats.1995.040040406

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractA novel method has been developed to generate volume fluctuations as required in isobaric‐isothermal Monte Carlo simulations of flexible (or semi‐flexible) chain molecules. While conventional global volume moves entail symmetric changes of the simulation box size, in the new method proposed here, volume changes consist of one‐dimensional fluctuations of one of two sections of the system. One of the two sections is a thin slab where all molecular rearrangements are restrained to occur. A deterministric method has been implemented to reposition the molecular segments contained within the slab after a volume change is proposed. The performance of the new method is examined by studying the evolution towards equilibrium of homopolymer hard‐core systems upon application of a step increase in the pressure. The new method is shown to be an order of magnitude faster than conventional methods in attaining the equilibrium density of our test

ISSN:1022-1344    

DOI:10.1002/mats.1995.040040407

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractA purely mathematical heuristic model is examined in order to explain the existence of an avoided crossing between two dispersion curves calculated for a certain potential energy surface designed to simulate the dynamics of a linear polyethylene chain. The model shows that a third mode coupled to a two‐mode Fermi‐resonant system is able to create a marked instability in a vibrational spectrum. It is noted that such an explanation would require a fourth‐order coupling term, which is the reason given for the failure of normal mode analysis to properly identify this dynamical e

ISSN:1022-1344    

DOI:10.1002/mats.1995.040040408

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe structure of cellulose is investigated using a recently derived force field. Published experimental data is taken only as a starting point for purely theoretical investigations. The reliability of the method is validated by calculating physical properties of the obtained geometries. In the course of the investigations, the geometries of cellulose Iα, cellulose Iβ and cellulose II are derived. For these geometries the Young's‐modulus is calculated. The structure of cellulose in aqueous solution is investigated, using cellohexaose (a hexamer of β‐D‐glucose) as a fragment of a cellulose chain. Here, the diffusion coefficient is ca

ISSN:1022-1344    

DOI:10.1002/mats.1995.040040409

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractWe have analyzed chain conformations and the existence — or otherwise — of chain overlaps and entanglements in dilute polymer solutions (at concentrationsc

ISSN:1022-1344    

DOI:10.1002/mats.1995.040040410

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY