摘要:

AbstractSimulations of the stability of the secondary and tertiary structure of the α‐keratin intermediate filament (IF) monomeric unit of wool are reported. Based on the assumed secondary structure three segments of the primary structure were selected: 1A, L12, and a part of 2B. Starting with an ideal α‐helical conformation for each IF‐segment, molecular dynamics simulations were carried out on the atomistic level at various temperatures in vaccum using the CFF91 force field. In either simulation the expected destabilization of the helical structure with increasing simulation temperature was observed. By use of different procedures of analysis, transition temperatures for the α‐helical denaturation were determined that are significantly higher for the supposedly α‐helical segments 1A and 2B than for the linker segment L12. The different stabilities of segments 1A and L12 were further verified through simulations in water environment that show the linker segment to be non‐helical at room temperature. The lower transition temperature of segment L12 confirms the expectation that its amino acid sequence leads to increased conformational flexibility. The mobility of the water molecules surrounding the IF‐segment is found to be significantly decreased by protein/w

ISSN:1022-1344    

DOI:10.1002/mats.1997.040060101

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe hydrodynamically induced orientation dynamics of charged polymer molecules during electric dichroism has been studied by Brownian dynamics simulations. The effect of a locally changed flexibility or bending angle has been of special interest. The results show that simultaneously changing the bending angles and the corresponding force constants so as to keep the persistence length constant does not produce any large changes in the orientation dynamics or molecular shape although, quantitatively, a larger number of kinks in the chain will give a smaller steady‐state orientation. Releasing the constraint of a fixed persistence length when randomly increasing or decreasing the flexibility at different positions in the molecule drastically changes the dynamics, and the corresponding results are discussed and also compared with the orientation obtained when introducing the inhomogeneities in a cooperative manne

ISSN:1022-1344    

DOI:10.1002/mats.1997.040060102

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractIt has been found that a method allowing the calculation of an anisotropic‐isotropic transition volume fraction for a given semirigid polymer can be derived from the equations recently proposed by Blonski and coworkers to evaluate the equilibrium compositions of isotropic and anisotropic phases coexisting within ternary systems semirigid polymer/flexible polymer/solvent. By using the term anisotropic‐isotropic transition volume fraction of a semirigid polymer we will refer to its smallest volume fraction giving an anisotropic character to liquid ternary phases also including a flexible polymer and a solvent. Its evaluation simplifies the construction of the corresponding ternary diagram and can also be seen as a fast and quantitative way of estimating the influence exerted by the semirigid polymer, through its intrinsic anisotropy, on ternary pha

ISSN:1022-1344    

DOI:10.1002/mats.1997.040060103

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe swelling of a nematogenic elastomer by an isotropic solvent was earlier predicted (Macromolecules25, 445 (1992)) to be more complex than that of a classical elastomer. We extend here to the swelling by solvents having a nematic‐isotropic transition in the pure phase in the region of that of the pure elastomer. The phase diagrams are qualitatively different from the first case, but still with the novel features of a triple point and a gel‐gel biphasic gap reported in the initial study. Where the cross‐coupling is stronger than the self‐couplings we have an azeotropi

ISSN:1022-1344    

DOI:10.1002/mats.1997.040060104

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractA procedure is developed that allows the calculation of chain‐length distributions of polymers prepared by periodic modulation of the termination rate constant, considering termination by disproportionation. Applying some (hypothetical) field which suppresses — or at least diminishes — the termination rate constant periodically, results in step‐function‐like chain‐length distributions. In principle, the same information may be obtained as in the case of periodical variation of the rate of

ISSN:1022-1344    

DOI:10.1002/mats.1997.040060105

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThis work deals with the kinetics of the living polymerization initiated by a bifunctional initiator with different rate constants of the first and the second step of initiation. The expressions of the molecular weight distribution (MWD) function, the number‐ and weight‐average degrees of polymerization and the polydispersity index were derived rigorously. Numerical results show that the nonequal initiation rate constants often lead to a bimodal MWD except the ratio ofkitoki′ being very large (>500, for instance) or relatively small (<1, for example). The MWD of the resultant polymer is rather narrow even in the presence of double peaks. With decreasing initiator concentration, the MWD at full conversion becomes narrower, and the shoulder peak of the MWD curve dec

ISSN:1022-1344    

DOI:10.1002/mats.1997.040060106

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractConstant pressure constant temperature molecular dynamics method is employed to investigate the atomistic scale dynamics of a model Bisphenol A polycarbonate in the vicinity of its glass transition temperature. First, the glass transition temperature and the thermal expansion coefficients of the polymer are predicted by performing simulations at different temperatures. To explore the significance of different modes of motion, various types of time correlation functions are utilized in analyzing the trajectories. In these nanosecond scale simulations, the motion of the chain segments is found to be highly localized with little reorientation of the vectors representing these segments. Detailed analysis of trajectories and the correlation functions of the backbone dihedrals and side methyl groups indicates that they exhibit numerous conformational transitions. The activation energies of the conformational transitions obtained from the simulation are generally larger than the potential barriers for the rotations of these dihedrals, however, both show the same trend. We also have estimated the phenylene ring flip activation energy as 12.6 kcal/mol and the flip frequency as 0.77 MHz at 300 K. These values fall either fall within the range determined by various NMR spectroscopy experiments or slightly out of the range. The study shows that the conformational transitions between the adjacent dihedrals are strongly correlated. Three basic cooperative modes are identified from the simulation. They are: a positive synchronous rotation of two phenylene rings, a negative synchronous rotation of two phenylene rings, and a carbonate group rotation. Above the glass transition temperature, the large scale cooperative motions become much more significant.

ISSN:1022-1344    

DOI:10.1002/mats.1997.040060107

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractIn this work we display recent findings on the conformations and dynamics of polyampholytes (PAs; polymers with positively and negatively charged monomers) in external electrical fields. We consider the case in which the interactions between the charges are less important (weak coupling limit) and also the case in which they are fundamental (strong coupling case). In the weak coupling limit we present analytical results for Gaussian and also for freely‐jointed chains. Through scaling arguments we discuss the influence of the excluded volume on the PA's configurations. Furthermore we evaluate the dynamics of PAs in the framework of the Rouse and of the Zimm models. In the strong‐coupling regime PAs with vanishing total charge form spherical globules. Using a droplet analogy we examine the response of PAs to external fields and show the onset of an instability reminiscent of a first‐order phase trans

ISSN:1022-1344    

DOI:10.1002/mats.1997.040060108

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractA novel type of pulsed laser polymerization (PLP)‐size exclusion chromatography (SEC) experiment is presented in which laser pulse sequences are applied at considerable dark time intervals between each pulse package. The pulse sequences give rise to the characteristically structured molecular weight distribution (MWD) from whichkpmay be derived. Polymer produced during the extended dark time intervals determines the MWD at high molecular weights and allows for the measurement of chain transfer rates. The method by which propagation and transfer rates are accessible from one experiment is illustrated by simulations using the program package PREDIC

ISSN:1022-1344    

DOI:10.1002/mats.1997.040060109

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe topology of networks formed by radical crosslinking copolymerization is strongly dependent on the type of crosslinker chosen. By a series of computer simulations, the influence of the relative reactivities of crosslinker and monomer on the network properties was investigated. Basic condition of the simulation is the postulation of a diffusion process control of the polymerization reaction. The resulting networks are analyzed with respect to cycle rank, loop formation, dangling ends, sol fraction and crosslinker molecules reacted only by one double bond. The results of the computer simulations indicate that the reactivity of the crosslinker decisively affects the properties of the network. Particularly the cycle rank, which determines the elastic properties of the network, is influenced strongly by the reactivity of the crosslinker. A change of the cycle rank of approximately one decade was observed. The influence of the reactivity was found to be most important in the region where the reactivity of the crosslinker is less than the reactivity of the monomer.

ISSN:1022-1344    

DOI:10.1002/mats.1997.040060110

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY