Properties of amorphous and crystallizable hydrocarbon polymers. I. Melt rheology of fractions of linear polyethylene

In this paper we have carried out molecular dynamics simulations (MD) for model systems of liquid n-alkane oligomers (C 12~C80 ) at high temperatures (~2300 K) in a canonical ensemble to calculate viscosity η, selfdiffusion constants D, and monomeric friction constant ζ. We found that the long chains of these n-alkanes at high temperatures show an abnormality in density and in monomeric friction constant. The behavior of both activation energies, E η and E D , and the mass and temperature dependence of η, D, and ζ are discussed.

supporting

confidence: 88%

Molecular Dynamics Simulation Studies of Viscosity and Diffusion of n-Alkane Oligomers at High Temperatures

Lee

2011

“…The activation energies obtained from the slope of the least square fit are 2.33, 3.29, 4.63, and 5.46 kcal/mol for C 12 , C 20 , C 32 , and C 44 , respectively and are plotted in Figure 4. It was experimentally reported for n-alkanes and linear polyethylene 6 that the activation energy increases with chain length and at the highest molecular weight tested (M ≈ 4.4 kg/mol) the activation energy reaches 6.6 kcal/mol, which is similar to the average value found for NBS 1482-4 (6.7 kcal/ mol) and the values reported by others 36,37 for high molecular weight linear polyethylene (6.1-6.9 kcal/mol). The molecular weights of our n-alkanes considered in this study are in the range of 170-620 g/mol.…”

Section: 34supporting

confidence: 78%

Viscosity and Diffusion Constants Calculation of n-Alkanes by Molecular Dynamics Simulations

2003

In this paper we have presented the results for viscosity and self-diffusion constants of model systems for four liquid n-alkanes (C 12 , C 20 , C 32 , and C 44 ) in a canonical ensemble at several temperatures using molecular dynamics (MD) simulations. The small chains of these n-alkanes are clearly /6>1, which leads to the conclusion that the liquid n-alkanes over the whole temperatures considered are far away from the Rouse regime. Calculated viscosity η and self-diffusion constants D are comparable with experimental results and the temperature dependence of both η and D is suitably described by the Arrhenius plot. The behavior of both activation energies, E η and E D , with increasing chain length indicates that the activation energies approach asymptotic values as n increases to the higher value, which is experimentally observed. Two calculated monomeric friction constants ζ and ζ D give a correct qualitative trend: decrease with increasing temperature and increase with increasing chain length n. Comparison of the time auto-correlation functions of the end-toend vector calculated from the Rouse model for n-dodecane (C 12 ) at 273 K and for n-tetratetracontane (C 44 ) at 473 K with those extracted directly from our MD simulations confirms that the short chain n-alkanes considered in this study are far away from the Rouse regime.

“…E η is also small for small alkanes. It was experimentally reported for n-alkanes and linear polyethylene 17 that the activation energy increases with chain length and at the highest molecular weight tested (M~4.4 kg/mol) the activation energy reaches 6.6 kcal/mol, which is similar to the average value found for NBS (National Bureau Standard) 1482-4 (6.7 kcal/mol) and the values reported by others 21,22 for high molecular weight linear polyethylene (6.1-6.9 kcal/mol). As chain length n increases the increment of E η decreases, and it is expected to approach an asymptotic value as n increases to the higher values.…”

Section: At High Molecular Weight (M > 5 Kg/supporting

confidence: 78%

Viscosity and Diffusion of Small Normal and Isomeric Alkanes: An Equilibrium Molecular Dynamics Simulation Study

Yoo¹,

Kim²,

Lee

2008

Constitutional isomers1 have different physical properties that have the same carbon number but different structures. The difference may not be large, but they are found to have different melting points, boiling points, densities, indexes of refraction, and so forth. For example, a branched chain isomer has a lower boiling point than a straight chain isomer. Thus normal pentane has a boiling point of 36 o C, isopentane with a single branch 28 o C, and neopentane with two branches 9.5 o C. This effect of branching on boiling point is observed within all families of organic compounds. It is reasonable that branching should lower the boiling point: with branching the molecular shape tends to approach to that of a sphere and as this happens the surface area decreases, as the result of that the intermolecular forces become weaker and are overcome at a lower temperature.The branching effect on the dynamic properties of liquid alkanes, such as self-diffusion constant, viscosity, and thermal conductivity, is one of the most interesting phenomena. For liquid butane, the experimentally observed behavior 2 tells us that viscosity increases with branching. For liquid pentane, hexane, and heptane, however, branching decreases the viscosity, for example, 0.289 and 0.273 cp at 273.15 K, and 0.240 and 0.223 cp at 293.15 K for normal pentane and isopentane, 0.326 and 0.306 cp at 293.15 K for normal hexane and isohexane, and 0.409 and 0.384 cp at 293.15 K for normal heptane and isoheptane, 3 respectively. These experimental results, except for liquid butane, indicate that as the molecular shape tends to approach that of a sphere and the surface area tends to decrease with branching, the intermolecular forces becomes weaker and the viscosity of alkane isomers decreases.In the present note, we report equilibrium molecular dynamics (MD) simulations for the systems of small normal and isomeric alkanes -normal butane and isobutane, normal pentane and isopentane, and normal hexane and isohexane. The primary study goal is to analyze the diffusion and viscosity dynamics of small normal and isomeric alkanes at different temperatures. Molecular Models and MD Simulation MethodsFor small normal alkanes, we have chosen 3 systemsnormal butane (C 4 H 10 ), normal pentane (C 5 H 12 ), and normal hexane (C 6 H 14 ), and the corresponding isomeric alkane systems are isobutane (C 4 H 10 ), isopentane (C 5 H 12 ), and isohexane (C 6 H 14 ). Each simulation was carried out in the NVT ensemble; the number of n-alkane was N=100 and the lengths of cubic simulation boxes were obtained from the experimental densities 4 for given temperatures of 248, 273, 293 and 298 K. The usual periodic boundary condition in the x-, y-, and z-directions and the minimum image convention for pair potential were applied. Gaussian isokinetics was used to keep the temperature of the system constant. 5 We used a united atom (UA) model for n-alkanes, that is, methyl and methylene groups are considered as spherical interaction sites centered at each carbon atom. This model was u...