摘要:

AbstractStretching of flexible macromolecules by an external force, acting on the chain ends, in solvents of variable quality was simulated by the Monte Carlo method on a tetrahedral lattice. The forcelength relations and the stress‐induced changes in the population of thegaucheconformers were calculated for coils in theta and athermal solvents. In poor solvents, the stretching of a collapsed coil involves a rather abrupt upturn of the force‐length curve explained as a transition of a globular coil into an extended coil. In conformity with this interpretation, a bimodal shape of the chain vector distribution functionP(R) was found in the intermediate range of deformation of collapsed coils. The significance of the results to the stress‐strain behavior of networks with pronounced interaction (such as collapsed gels) are disc

ISSN:1022-1344    

DOI:10.1002/mats.1995.040040201

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe exponent γ in the expression for the partition functionZ≅ μN·Nγ−1is derived for a Gaussian chain. The method of Ullman is adopted. Results of calculations show that for 1, 2, 3 and 4 dimensions, γ = 1, 1,25, 1,2 and 1, respectively. It is also shown that γ remains unity for all dimensions higher than four. These results are in better agreement with exact computer simulation results than the first‐order renomalization group theory calculations. The self consistency of the present method and possible improvements along the lines of the present study ar

ISSN:1022-1344    

DOI:10.1002/mats.1995.040040202

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe rotational relaxation time for different models of the spectrin αβ‐heterodimer and spectrin subunits were obtained through numerical simulation making use of the so‐called rigid‐body approximation. Based on information on the helix structure of the 106 aminoacids in the repeating motif of spectrin, three different models with varying degree of refinement were set up to represent the spectrin subunits: the two‐bead, the many‐bead and the shell model. For one‐ and two‐subunit spectrin the relaxation time was found to be 20 ± 4 ns and 92 ± 5 ns, respectively. The last result conforms well with available experimental data from transient electric birefringence measurements. For the αβ‐heterodimer two different models were applied: a chain of beads (pearl necklace model) and a more refined so‐called San necklace model. Using the San necklace model, the relaxation time was found to be in the range of 9 to 18 μs (depending on the flexibility of the joint), which is in accordance with what has been obtained from intrinsic viscosity measurements, but considerably higher than experimental data from measurements of transien

ISSN:1022-1344    

DOI:10.1002/mats.1995.040040203

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe unperturbed chain dimensions of poly(3‐ethylthiophene)s have been calculated using the full‐relaxation optimization of conformational energies and the chain statistical mechanics with a rotational isomeric state model of two states. The computation indicates that the conjugated polymer with side groups is more flexible than without, and that the orientation of the ethyl group and the regioirregular sequences perturb the chain conformation to a certain extent. Specially, the calculated chain conformations under θ‐conditions are more rigid than those observed in good solvent. This supports experiments showing that the chain dimensions of conjugated polymers are very sensitive to variations of intermolecular intera

ISSN:1022-1344    

DOI:10.1002/mats.1995.040040204

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractA range of polyamides has been modelled using Monte Carlo routines based on carefully determined bond‐rotation potentials, bond length and bond angle. For each polymer type, a large number of chains has been built for different simulated temperatures, and the persistence lengths determined as an average over all models for a given set of conditions. The simulations predict mesogenicity of aromatic polyamides to be smaller than that of equivalent aromatic polyesters, while the experimental values of persistence length obtained from polyamides in solution are large on account of the strong bonding between polymer and solven

ISSN:1022-1344    

DOI:10.1002/mats.1995.040040205

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe Stockmayer bivariate distribution is used to qualitatively and quantitatively interpret TREF (temperature rising elution fractionation) spectra of polyolefins made by multiple site type catalysts. TREF spectra simulated by the proposed method can adequately fit experimental TREF data presented in the literature and can be used to help understand the mechanism of TREF separation and the nature of multiple site types catalysts.

ISSN:1022-1344    

DOI:10.1002/mats.1995.040040206

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe Monte Carlo method has been used for numerically simulating pulsed‐laser polymerization (PLP) in microemulsion, in order to establish if a shift from inflection point to peak maximum as the best measure of the propagation rate constant,kp, will occur theoretically. Termination is assumed to be instantaneous in the simulations as droplet sizes can be very small in microemulsions. From the results of the simulations it is found that instantaneous termination indeed causes the peak maximum to become the best measure ofkp. From these results it can be deduced that in bulk it is not simply the Poisson‐broadening that causes the peak maximum to yield an overestimation ofkp. This overestimation is rather caused by the fact that the termination rate is finite leading to an asymmetrical peak in the molecular weight distribution. In combination with broadening this yields the inflection point to be the best measure ofkpin the b

ISSN:1022-1344    

DOI:10.1002/mats.1995.040040207

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractComputer simulations show that ion pair aggregation can be responsible for the unusual dependence of the initial rate of polymerization on the concentration of added salt in the cationic polymerization of styrene initiated by RCl/SnCl4/NR 4+Cl−. Addition of small amounts of tetraalkylammonium chloride to the system reduces the rate of polymerization due to the decrease of the concentration of propagating free cations. Subsequent salt addition leads to a small rate increase, and then the rate decreases at higher [salt]0/[SnCl4]0ratio. The simulations show that the rate increase can be ascribed to the formation of aggregates of ion pairs and thus to a higher overall proportion of carbocations resulting in faster polymerization. The decrease of the polymerization rate at higher concentrations of added salt can be explained by the conversion of free SnCl4to SnCl 5−anions which are weaker Lewis acids. The effect of various equilibrium constants on the total concentration of carbocations as a function of added salt is si

ISSN:1022-1344    

DOI:10.1002/mats.1995.040040208

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractA theoretical consideration of molecular weights and molecular weight distribution (MWD) of polymers formed in anionic polymerization proceeding via active centres of two different types under conditions of chain transfer to solvent with a fast exchange between propagating species is presented. Analytical expressions for number‐and weight‐average degrees of polymerization are obtained. Expressions forPnandPware shown to be the same as in a one‐centre process with the apparent intensity of chain transfer proportional to the weight fraction of the polymer formed via “transferring” centres. The polymers formed possess a moderately wide unimodal MWD. The dependence of the polydispersity index on the effective intensity of chain transfer goes through a maximum; forM0/I0= 103the maximum value ofPw/Pnis ca. 4,6. The method is suggested for the estimation of the relative reactivity in chain propagation of two active centres from the dependence of molecular weight on initiator mixture composition. The effects of association of active centres on the average molecular weights are analyzed. The case when one of the centres is dormant is also c

ISSN:1022-1344    

DOI:10.1002/mats.1995.040040209

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe effect of slow exchange between growing chains on the average degree of polymerization (DP) and the molecular weight distribution (MWD) of polymers formed in a two‐state polymerization accompanied by chain transfer to solvent is considered. For the case when centres of only one type are capable of chain transfer it is shown that the exchange can be regarded as fast or slow, depending on the ratio of characteristic times of exchange (τex) and chain transfer (τtr) for these centres. The conditions for fast exchange, when the MWD of the polymer becomes independent of the rate of exchange, are derived. Under otherwise the same conditions, polymer of the narrowest MWD is formed if the exchange is fast, the number‐average DP being independent of the exchange rate. With increasing ratio τex/τtr, the weight‐average DP of the polymer increases and the MWD changes from unimodal to bimodal. For the case when “non‐transferring” centres are dormant, it was also found that at slow exchange the number‐average DP is greater and the MWD is less broadened than

ISSN:1022-1344    

DOI:10.1002/mats.1995.040040210

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY