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Atomistically detailed models of free-standing thin films of random poly(ethylene-co-propylene) are produced with the help of a method recently applied to atactic polystyrene. The findings indicate that the raw experimental data obtained with an increase in ethylene fraction can be explained by the changes in the number of methyl and methylene groups on the surface rather than the increase in surface excess of the methyl groups.

With a simple thermodynamic analysis, we show that polymers do not satisfy the two major criteria that are required for establishing a unique relationship between cohesion energy density (CED) and internal pressure (IP). Since IP can take values independently of CED, we contend that the IP/CED ratio cannot be established a priori, as the ratio depends on the thermodynamic variables (temperature, pressure, etc.) and the system of interest. POLYM. ENG. SCI., 45:798-800, 2005. © 2005 Society of Plastics Engineers, INTRODUCTION The concept of solubility parameter has been frequently used for the prediction of miscibility of binary polymer blends, as the difference in solubility parameters of the blend components is [...]

We demonstrate the application of a modified form of the configurational-bias algorithm for the simulation of chain molecules on the second-nearest-neighbor-diamond lattice. Using polyethylene and poly(ethylene-oxide) as model systems we show that the present configurational-bias algorithm can increase the speed of the equilibration by at least a factor of 2–3 or more as compared to the previous method of using a combination of single-bead and pivot moves along with the Metropolis sampling scheme [N. Metropolis, A. W. Rosenbluth, M. N. Rosenbluth, A. H. Teller, and E. Teller, J. Chem. Phys. 21, 1087 (1953)]. The increase in the speed of the equilibration is found to be dependent on the interactions (i.e., the polymer being simulated) and the molecular weight of the chains. In addition, other factors not considered, such as the density, would also have a significant effect. The algorithm is an extension of the conventional configurational-bias method adapted to the regrowth of interior segments of chain molecules. Appropriate biasing probabilities for the trial moves as outlined by Jain and de Pablo for the configurational-bias scheme of chain ends, suitably modified for the interior segments, are utilized [T. S. Jain and J. J. de Pablo, in Simulation Methods for Polymers, edited by M. Kotelyanskii and D. N. Theodorou (Marcel Dekker, New York, 2004), pp. 223–255]. The biasing scheme satisfies the condition of detailed balance and produces efficient sampling with the correct equilibrium probability distribution of states. The method of interior regrowth overcomes the limitations of the original configurational-bias scheme and allows for the simulation of polymers of higher molecular weight linear chains and ring polymers which lack chain ends. [ABSTRACT FROM AUTHOR]

For a model system of polyethylene of chain lengths 40 and 100 carbon atoms, we calculated the pressure at different densities and compared them with the experimental values. The simulation was conducted on the second nearest neighbor diamond lattice, and the pressure was calculated using the virtual-volume-variation method after the system was reverse mapped to its fully atomistic form in continuous space and energy minimized. In addition, the pressure was also calculated from the virial route by conducting a short molecular dynamics simulation starting from the energy minimized structure. We show that the pressure obtained from our simulations is quite reasonable in the length of simulation time (in Monte Carlo steps) normally employed in our group. These results provide additional evidence for the equilibration of our model systems, and methodology to calculate the pressure in our lattice models. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We consider a model of living semiflexible polymers in a solution and obtain results for the liquid-liquid phase separation, which has been a subject of growing interest in the literature and has been studied in some recent experiments. The tetrahedral lattice model is solved exactly on a Husimi cactus of coordination number q = 4. The exact solution on the Husimi cactus forms the approximate theory for the original lattice. We consider the end groups to be a different species from the middle groups. This allows us to incorporate the end-group effects in our calculations, which become important at low molecular weights. We investigate the effect of chain rigidity, end-group/ middle-group interactions, solvent quality, and the chemical potential of the end-group on the liquid-liquid coexistence curve. We also calculate the average molecular weights of the coexisting liquid phases. The bending penalty ε is found to shift the phase diagram only slightly. Attractive end-group/middle-group interaction enhances the phase separation. Due to the exactness of our calculations, the theory never violates thermodynamics and is applicable in the entire parameter space. Our theory also improves upon the classical theories and hence our results should prove useful. [ABSTRACT FROM AUTHOR]

By extending a recently developed Bethe lattice theory, we calculate the cluster size distribution and average cluster size of voids in the presence of polymers. Because of the presence of interactions and because polymers have a size different from that of voids, the model we investigate is a correlated percolation model. The effects of interactions, the pressure P, the degree of polymerization (DP) M, the coordination number q, and the possibility of void percolation on the above properties are evaluated. It is found that small-sized clusters are in overwhelming majority and constitute a large fraction of the total free volume in cases of interest. Attractive monomer—monomer interactions favor the formation of larger clusters. As a function of the DP, the average cluster size shows very different behavior in two regions: with void percolation and without void percolation. The following results are valid at constant temperature and pressure. In the presence of percolation, the average cluster size increases with M, whereas in the absence of percolation it decreases with M. In the absence of void percolation, the average cluster size decreases with increasing q due to the decrease in the total free volume. We present and discuss the results and compare them with those from experiments, simulations and random percolation. We conclude that we are able to qualitatively explain experimental results if we assume that there is no void. [ABSTRACT FROM AUTHOR]

We demonstrate that the regular solution theory is qualitatively successful in describing the composition dependence of partial monomer volumes in a compressible binary mixture of polymers only when the mixture is symmetric (see text for proper definition). For asymmetric mixtures, the partial monomer volumes can be non-monotonic with composition under ordinary conditions of temperature and pressure, which contradicts the prediction of the regular solution theory. The investigation enables us to conclude that the composition dependence of the partial monomer volumes obtained from the regular solution theory can be considered trustworthy only for a low degree of asymmetry. [ABSTRACT FROM AUTHOR]