title: Effect of solvent quality and chain density on normal and frictional forces between electrostatically anchored thermoresponsive diblock copolymer layers.
abstract: Equilibration in adsorbing polymer systems can be very slow, leading to different physical properties at a given condition depending on the pathway that was used to reach this state. Here we explore this phenomenon using a diblock copolymer consisting of a cationic anchor block and a thermoresponsive block of poly(2-isopropyl-2-oxazoline), PIPOZ. We find that at a given temperature different polymer chain densities at the silica surface are achieved depending on the previous temperature history. We explore how this affects surface and friction forces between such layers using the atomic force microscope colloidal probe technique. The surface forces are purely repulsive at temperatures <40°C. A local force minimum at short separation develops at 40°C and a strong attraction due to capillary condensation of a polymer-rich phase is observed close to the bulk phase separation temperature. The friction forces decrease in the cooling stage due to rehydration of the PIPOZ chain. A consequence of the adsorption hysteresis is that the friction forces measured at 25°C are significantly lower after exposure to a temperature of 40°C than prior to heating, which is due to higher polymer chain density on the surface after heating.
authors.affiliation: KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Chemistry, Division of Surface and Corrosion Science, Drottning Kristinas väg 51, SE-100 44 Stockholm, Sweden. Electronic address: junxue@kth.se.
authors.forename: Junxue
authors.initials: J
authors.lastname: An
authors.affiliation: KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Chemistry, Division of Surface and Corrosion Science, Drottning Kristinas väg 51, SE-100 44 Stockholm, Sweden. Electronic address: xialiu@kemi.dtu.dk.
authors.forename: Xiaoyan
authors.initials: X
authors.lastname: Liu
authors.affiliation: KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Chemistry, Division of Surface and Corrosion Science, Drottning Kristinas väg 51, SE-100 44 Stockholm, Sweden; SP Technical Research Institute of Sweden, Chemistry, Materials and Surfaces, Box 5607, SE-114 86 Stockholm, Sweden. Electronic address: andra@kth.se.
authors.forename: Andra
authors.initials: A
authors.lastname: Dedinaite
authors.affiliation: Department of Chemistry and Faculty of Pharmacy, University of Montreal, CP 6128, Succursale Centre Ville, Montreal, QC H3C3J7, Canada. Electronic address: korchagina@polly.phys.msu.ru.
authors.forename: Evgeniya
authors.initials: E
authors.lastname: Korchagina
authors.affiliation: Department of Chemistry and Faculty of Pharmacy, University of Montreal, CP 6128, Succursale Centre Ville, Montreal, QC H3C3J7, Canada; WPI International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan; Department of Chemistry and Faculty of Pharmacy, University of Helsinki, Helsinki, Finland. Electronic address: francoise.winnik@umontreal.ca.
authors.forename: Francoise M
authors.initials: FM
authors.lastname: Winnik
authors.affiliation: KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Chemistry, Division of Surface and Corrosion Science, Drottning Kristinas väg 51, SE-100 44 Stockholm, Sweden; SP Technical Research Institute of Sweden, Chemistry, Materials and Surfaces, Box 5607, SE-114 86 Stockholm, Sweden. Electronic address: percl@kth.se.
authors.forename: Per M
authors.initials: PM
authors.lastname: Claesson
keywords: Adsorption hysteresis; Boundary lubrication; Friction; PIPOZ; Poly(2-isopropyl-2-oxazoline); Surface forces; Thermoresponsive polymer