Mechanics of polymer brush

Problem description and significance: 

Polymer brushes are densely packed, surface-tethered polymer chains. In the swelled state of a polymer brush, the monomers in the brush repel each other due to excluded volume interaction, and the repulsion is relieved by chain extension away from the surface, resulting in brush like shape. These interactions cause modifiable residual stress in a stimulus responsive polymer brush leading to tuneable deformation of the substrate. This allows application of brush is sensing and actuation. To enable these applications, we are trying to understand the mechanics of a polymer brush and the key parameters that control its working using analytical and semianalytical theory, and molecular dynamics simulation.

 Three primary molecular scale parameters governing mechanics of a polymer brush are: average number of effective monomers in a polymer chain (N), graft density (number of polymer chains grafted to unit area of a substrate) ρ, and polydispersity (signifying distribution of chains of varying length in a polymer brush). We show that in a monodisperse polymer brush (having chains of constant length) of lower graft density (ρg<0.1), the normal stress aligned parallel to the substrate in a brush varies quartically with the distance from the grafting surface with the maximum at the grafting surface. Also, the stress varies as ρg4/3N.

We also study the large deformation of a soft thin substrate due to a polymer brush coating. A mechanics model for large deformation (but small strain) of a beam with polymer brush coating is developed and equations governing the deformation is derived by accounting for Young-Laplace correction and curvature elasticity corrections. The generalized equation yields governing equations in the literature for deformation of a beam with the surface layer as special cases.

Selected publications:

  1. Manav, M., Anilkumar, P., and Phani, A. S., 2018, “Mechanics of polymer brush based soft active materials– theory and experiments,” Journal of the Mechanics and Physics of Solids, 121, pp. 296-312.
  2. Manav, M., and Phani, A. S., 2017, “Polymer brush: molecular parameters – mechanical properties relation,” Society of Engineering Sciences Conf., Boston, USA.
  3. Manav, M., Bajgai, M. P., Phani, A. S., and Kizhakkedathu, J. N., 2016, “Mechanics of thermoresponsive polymer brush based soft materials: theory and experiments”, International Congress of Theoretical and Applied Mechanics, Montreal, QC, Canada.
  4. Manav, M., Bajgai, M. P., Kizhakkedathu, J. N. , and Phani, A. S. , 2015, “Mechanics of temperature sensitive polymer brush based soft actuator,” BAMN, Vancouver, Canada.

Funding:

The work is funded by the Canadian Institute of Health Research (CIHR) and the Natural Sciences and Engineering Research Council of Canada (NSERC) NanoMat Create program. This work is carried in collaboration with Prof. Kizhakkedathu‘s group at Centre for Blood Research, UBC.