Probing the Collapse Dynamics of Poly(N-isopropylacrylamide) Brushes by AFM: Effects of Co-nonsolvency and Grafting Densities

• Published in: Small (Weinheim an der Bergstrasse, Germany) Vol. 7; no. 10; pp. 1440 - 1447
• Main Authors: Sui, Xiaofeng, Chen, Qi, Hempenius, Mark A., Vancso, G. Julius
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 23.05.2011; WILEY‐VCH Verlag
• Subjects: Acrylamides - chemistry; Acrylic Resins; Atomic force microscopy; Brushes; Collapse; collapse dynamics; Colloids; Density; Dynamics; Elastic Modulus; grafting density; Methyl alcohol; Microscopy, Atomic Force; polymer brushes; Polymers - chemistry; solvent effects; Solvents; Solvents - chemistry; Tin
• ISSN: 1613-6810; 1613-6829; 1613-6829
• DOI: 10.1002/smll.201002229

Collapse of poly(N‐isopropylacrylamide) (PNIPAM) brushes in the mixed solvent system (water/methanol 50% v/v) is studied by in‐situ atomic‐force microscopy (AFM). PNIPAM brushes with three different grafting densities and similar chain lengths are synthesized via surface‐initiated atom‐transfer radical polymerization. By changing the solvent from water to a water/methanol (50% v/v) mixture, the polymer brushes switch from a swollen to collapsed state. AFM force measurements using a silica colloidal probe attached to the tip are employed to obtain the Young’s moduli of the polymer brushes in different solvation states. The collapse dynamics of the brush is followed by monitoring the pull‐off force (adherence) in situ. The modulus of the swollen high‐density polymer brush is four times lower than that of the same brush in the collapsed state. It is shown that in the case of the high‐density polymer brush with a thickness (tin water) of 900 nm, the collapse takes place in a time scale of ~25 s, whereas the collapse occurs faster for the medium‐density brush (tin water = 630 nm) and much more rapidly for the low‐density brush (tin water = 80 nm). This difference in the response kinetics is primarily ascribed to the time needed for solvent exchange in the polymer brushes. Poly(N‐isopropylacrylamide) brushes are switched from swollen to collapsed states by introducing a co‐nonsolvent consisting of water and methanol. Polymer brushes under different solvent conditions and grafting densities exhibit different mechanical properties according to colloidal probe atomic force microscopy measurements. The collapse dynamics are obtained by monitoring the change of adherence between the probe and the surface.