Temperature controlled surface hydrophobicity and interaction forces induced by poly (N-isopropylacrylamide)

Area of Science:

• Materials Science
• Surface Chemistry
• Polymer Science

Background:

• Poly (N-isopropylacrylamide) (PNIPAM) is a thermoresponsive polymer exhibiting a lower critical solution temperature (LCST) around 32°C.
• Above its LCST, PNIPAM transitions from a hydrophilic to a hydrophobic state, altering its solubility and surface interactions.
• This thermoresponsive behavior has implications for mineral processing, where PNIPAM can induce flotation of hydrophilic minerals.

Purpose of the Study:

• To investigate the interaction forces between bare silica surfaces in PNIPAM solutions.
• To examine the influence of PNIPAM molecular weight and three-phase contact angles on these interactions.
• To elucidate the mechanism behind PNIPAM-induced hydrophobicity and adhesion on silica surfaces.

Main Methods:

• Atomic Force Microscopy (AFM) was employed to measure surface forces.
• Experiments were conducted at temperatures both below and above the LCST of PNIPAM.
• Surface hydrophobicity was assessed by measuring three-phase contact angles.

Main Results:

• PNIPAM significantly increases the hydrophobicity of silica surfaces at temperatures above the LCST.
• AFM measurements revealed strong adhesion between PNIPAM-coated silica surfaces at elevated temperatures, an effect absent without the polymer.
• The degree of hydrophobicity and adhesion was influenced by PNIPAM molecular weight.

Conclusions:

• The observed attractive forces and adhesion between silica surfaces are attributed to hydrophobic interactions induced by PNIPAM adsorption above its LCST.
• PNIPAM's ability to modify surface properties makes it a promising agent for applications requiring controlled surface hydrophobicity, such as mineral flotation.