RESEARCH ARTICLE

Molecular dynamics study of water diffusion in an amphiphilic block copolymer with large difference in the blocks’ glass transition temperatures

  • Yang Zhou ,
  • Phillip Choi
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  • Department of Chemical and Materials Engineering, University of Alberta Edmonton, Edmonton, AB, T6G 1H9, Canada

Received date: 06 Oct 2016

Accepted date: 27 Nov 2016

Published date: 23 Aug 2017

Copyright

2017 Higher Education Press and Springer-Verlag Berlin Heidelberg

Abstract

Isothermal-isobaric molecular dynamics simulation was used to study the diffusion mechanism of water in polyurethane-block-poly(N-isopropyl acrylamide) (PU-block-PNIPAm) with a hydrophobic PU/hydrophilic PNIPAm mass ratio of 1.4 to 1 at 298 K and 450 K. Here, the experimental glass transition temperature (Tg) of PU is 243 K while that of PNIPAm is 383 K. Different amounts of water up to 15 wt-% were added to PU-block-PNIPAm. We were able to reproduce the specific volumes and glass transition temperatures (250 K and 390 K) of PU-block-PNIPAm. The computed self-diffusion coefficient of water increased exponentially with increasing water concentration at both temperatures (i.e., following the free volume model of Fujita). It suggested that water diffusion in PU-block-PNIPAm depends only on its fractional free volume despite the free volume inhomogeneity. It is noted that at 298 K, PU is rubbery while PNIPAm is glassy. Regardless of temperature, radial distribution functions showed that water formed clusters with sizes in the range of 0.2–0.4 nm in PU-block-PNIPAm. At low water concentrations, more clusters were found in the PU domain but at high water concentrations, more in the PNIPAm domain. It is believed that water molecules diffuse as clusters rather than as individual molecules.

Cite this article

Yang Zhou , Phillip Choi . Molecular dynamics study of water diffusion in an amphiphilic block copolymer with large difference in the blocks’ glass transition temperatures[J]. Frontiers of Chemical Science and Engineering, 2017 , 11(3) : 440 -447 . DOI: 10.1007/s11705-017-1626-2

Acknowledgments

The authors would like to thank WestGrid and Compute/Calcul Canada for providing the computational facility and support for this research. Funding from Agriculture Funding Consortium was greatly appreciated.
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