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  • 1.
    Ahlström, Peter
    et al.
    University of Borås, School of Engineering.
    Gebäck, Tobias
    University of Borås, School of Engineering.
    Johansson, Erik
    University of Borås, School of Engineering.
    Bolton, Kim
    University of Borås, School of Engineering.
    Water absorption in polymers2010Conference paper (Other academic)
    Abstract [en]

    In this work two different examples of water absorbtion in polymers are studied by Monte Carlo simulations. Both of them are of large technical and commercial impotance. The first example is the water absorption in polyethylene cables where the water absorption plays a crucial role in the degradation of the cable insulation and thus should be as low as possible. The second example is bio-based superabsorbents made from denatured protein where water absorption capability is the prime desired property. Methods Gibbs Ensemble Monte Carlo simulations [1] were used to study the hydration of polymers. All simulations are performed with two boxes, one of which is filled with water at the start of the simulation, whereas the other contains polymer molecules and possible ions. The polymer molecules are not allowed to swap boxes whereas the water molecules are allowed to do so thus constituting an osmotic Gibbs ensemble [2]. For the polyethylene a connectivity-altering algorithm was used whereas the protein molecules were simulated using a side-chain regrowth model in addition to traditional Monte Carlo moves. For the polyethylene, the TraPPE [3] force field was used and the protein molecules, the Amber force field [4] was used. Water was modelled using simple point charge models [5]. Electrostatic interactions are treated using Ewald summation methods. The protein molecules were of different amino acid compositions and in different conformations, e.g., β-turns and random coils obtained using the amorphous cell method[6]. Studies were made with different degrees of charging on, e.g., lysine side chains mimicking different ionization states. Results The studies of polyethylene revealed the importance of ions left from the polymerisation catalyst for the absorbtion of water and the concomitant degradation of polyethylene cable insulation. Also the absorption properties of the protein molecules is strongly related to the presence of charged groups and fully charged protein molecules absorb large amounts of water. However, neither native nor denatured protein molecules show superabsorbing properties (i.e. absorbing hundreds of times their own mass) as they show in experimental studies and the reasons for this discrepancy will be discussed. References 1. A.Z. Panagiotopoulos, Mol. Phys. 61, 813 (1987). 2. E. Johansson, K. Bolton, D.N. Theodorou, P. Ahlström, J. Chem. Phys., 126, 224902 (2007). 3. M.G. Martin, and J.I. Siepmann, J. Phys. Chem. B, 103, 4508-4517 (1999). 4. W.D. Cornell, P. Cieplak, C.I. Bayly, I.R. Gould, K.M. Merz Jr, D.M. Ferguson, D.C. Spellmeyer, T. Fox, J.W. Caldwell, P.A. Kollman (1995). J. Am. Chem. Soc. 117, 5179–5197. 5. H. J. C. Berendsen, J. P. M. Postma and W. F. van Gunsteren, in Intermolecular Forces, B. Pullman, ed. (Reidel, Dordrecht, 1981) p. 331; H. J. C. Berendsen, J. R. Grigera and T. P. Straatsma, J. Phys. Chem. 91, 6269 (1987). 6. D.N. Theodorou, U.W. Suter, Macromolecules, 18, 1467 (1985).

  • 2.
    Bolton, Kim
    et al.
    University of Borås, School of Engineering.
    Johansson, Erik
    University of Borås, School of Engineering.
    Jönsson, Lennart
    University of Borås, School of Engineering.
    Ahlström, Peter
    University of Borås, School of Engineering.
    Simulation of water clusters in vapour, alkanes and polyethylenes2009In: Molecular Simulation, ISSN 0892-7022, E-ISSN 1029-0435, Vol. 35, no 10/11, p. 888-896Article in journal (Refereed)
    Abstract [en]

    The Gibbs Ensemble Monte Carlo (GEMC) technique has been used to study the clustering of water in vapour, alkanes and polyethylene, where the water clusters are in equilibrium with liquid phase water. The effect of an external electric field and ionic impurities on the clustering of water in the hydrocarbons (alkanes and polyethylene) has also been studied. The simulations of water clustering in polyethylene were made more efficient by using a connectivity altering osmotic Gibbs ensemble method. It was found that trends in the size distribution of water clusters in the hydrocarbons are similar to those found in the pure vapour, but that fewer and smaller clusters are formed as the length of the hydrocarbon chain increased. Also, large external electric fields decrease the solubility of water in hydrocarbons, whereas the presence of ionic species dramatically increases the solubility.

  • 3.
    Johansson, Erik
    University of Borås, School of Engineering.
    Simulations of water clustering in vapour, hydrocarbons and polymers2007Doctoral thesis, monograph (Other academic)
    Abstract [en]

    It is commonly known that water plays a crucial role in many natural and industrial processes. One of these processes is the formation of water trees, and the subsequent breakdown of polyethylene used for high voltage cable insulation purposes. It has been shown that the mechanism for water molecules diffusing through amorphous polyethylene includes the formation of small water clusters. Gibbs Ensemble Monte Carlo molecular simulations has been performed to study the clustering of vapour phase water under vapour - liquid equilibrium conditions at temperatures ranging from 300 K to 600 K. The increase in vapour density with increasing temperature leads to a radical increase in the fraction of molecules belonging to clusters with two or more water molecules. It is also seen that the size of the clusters increases with temperature. The topologies of the smaller clusters, up to pentamers, have also been studied. A structural transition is observed from a large percentage with cyclic topology, which is the minimum energy configuration, at lower temperatures to predominantly linear clusters, favoured by entropic effects, at higher temperatures. Similar water properties have been obseved in simulations where the vapour phase has been replaced with a hydrocarbon rich phase ( n-alkanes and polyethylene ). Application of an external electric field to the polymer system reduces the water solubility and affects the water structure. A dramatic increase in water solubility in the hydrocarbon phase is observed when two oppositely charged ions are introduced in the hydrocarbon. The structure of the water have changed from several small clusters to a single large cluster with a rod-like shape. The cluster is extremely stable during the simulation. Application of an external electric field may enhance or reduce the effect of the ions depending on the direction of the field. Based on these observations is an alternative mechanism for water tree propagation proposed.

  • 4.
    Johansson, Erik
    et al.
    University of Borås, School of Engineering.
    Ahlström, Peter
    University of Borås, School of Engineering.
    Atomistic Simulation Studies of Polymers and Water2006Conference paper (Refereed)
    Abstract [en]

    A Monte Carlo simulation study of water and hydrocarbons aiming at understanding the degradation of polyethylene cable insulation is presented. The equilibrium distributions and clustering of water in vapour and in hydrocarbons was investigated using Gibbs ensemble Monte-Carlo simulations. Different combinations of water and hydrocarbon models are investigated in order to reproduce experimental densities of water and hydrocarbons in both the water phase and the hydrocarbon phase.

  • 5.
    Johansson, Erik
    et al.
    University of Borås, School of Engineering.
    Ahlström, Peter
    University of Borås, School of Engineering.
    Bolton, Kim
    University of Borås, School of Engineering.
    Molecular simulation of the effect of ionic impurities and external electric fields on rod-like water clusters in polyethylene2008In: Polymer, ISSN 0032-3861, E-ISSN 1873-2291, Vol. 49, no 24, p. 5357-5362Article in journal (Refereed)
    Abstract [en]

    Monte Carlo methods have been combined with end-bridging methods to study the solubility and structure of water in polyethylene, where the polyethylene contains a pair of oppositely charged ionic impurities. The water in the polymer is in equilibrium with pure liquid water. Both the polymer and pure water phases are exposed to an external electric field. The ions dramatically increase the solubility of water in polyethylene and induce the formation of a stable, rod-like water cluster between the ions. The solubility, the hydrogen-bond ordering of the water molecules in the cluster and the size of the cluster increase in the presence of an external field that enhances the local electric field between the ions. When the direction of the external field is reversed, and when it has the same magnitude as the local ionic field, the rod-like structure is broken up and a smaller cluster forms around each ion. (C) 2008 Elsevier Ltd. All rights reserved.

  • 6.
    Johansson, Erik
    et al.
    University of Borås, School of Engineering.
    Bolton, Kim
    University of Borås, School of Engineering.
    Ahlström, Peter
    University of Borås, School of Engineering.
    On Polyethylene Cable Failure, Electric Fields, Water Clusters and Ions2008In: Proceedings of the 100th Annual Meeting of the American Institute for Chemical Engineering, 2008Conference paper (Refereed)
  • 7.
    Johansson, Erik
    et al.
    University of Borås, School of Engineering.
    Bolton, Kim
    University of Borås, School of Engineering.
    Ahlström, Peter
    University of Borås, School of Engineering.
    Simulations of Water Clustering in Vapour, Hydrocarbons and Polymers2008Conference paper (Other academic)
  • 8.
    Johansson, Erik
    et al.
    University of Borås, School of Engineering.
    Bolton, Kim
    University of Borås, School of Engineering.
    Ahlström, Peter
    University of Borås, School of Engineering.
    Water absorption in polyethylene under external electric fields2007In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 127, no 2Article in journal (Refereed)
  • 9.
    Johansson, Erik
    et al.
    University of Borås, School of Engineering.
    Bolton, Kim
    University of Borås, School of Engineering.
    Theodorou, Doros N.
    Ahlström, Peter
    University of Borås, School of Engineering.
    Formation of rodlike structure of water between oppositely charged ions in decane and polyethylene2007In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 127, no 19Article in journal (Refereed)
  • 10.
    Johansson, Erik
    et al.
    University of Borås, School of Engineering.
    Bolton, Kim
    University of Borås, School of Engineering.
    Theodorou, Doros N.
    Ahlström, Peter
    University of Borås, School of Engineering.
    Monte Carlo simulations of equilibrium solubilities and structure of water in n-alkanes and polyethylene2007In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 126, no 22Article in journal (Refereed)
1 - 10 of 10
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