Dynamic Molecular Behavior on Thermoresponsive Polymer Brushes

Abstract

The surface dynamics of individual surfactant and polymer molecules on thermally responsive polymer brushes (poly­(N-isopropylacrylamide), PNIPAAM) were studied using high throughput single molecule tracking microscopy. The probe molecules universally exhibited intermittent hopping motion, in which the diffusion switched between mobility and confinement with a broad distribution of waiting times; this was analyzed in the context of a continuous time random walk (CTRW) model described using “waiting time” and “flight length” distributions. We found that the surface mobility, which was affected by waiting times and flight lengths, of both probe molecules increased abruptly with temperature above the 32 °C lower critical solution temperature (LCST) transition of the PNIPAAM brush. In particular, above the LCST, where the polymer brush collapsed into a more hydrophobic dense polymer film, the effective diffusion coefficients and mobile fraction of probe molecule increased, suggesting that mobility was inhibited by penetration into the brush at lower temperatures. Waiting times at lower temperature were twice as long as at higher temperatures, and the longest flight length increased from 0.9 to 1.8 μm. Moreover, we found that the high density of strong binding sites available on the swollen PNIPAAM brush led to long waiting times and a high probability of readsorption, which resulted in short flight lengths, while the absence of strong binding sites on collapsed PNIPAAM films led to short waiting times and long flights.

Polymer Surface Transport Is a Combination of in-Plane Diffusion and Desorption-Mediated Flights

“…18,22 The remainder of the adsorbed polymers exhibit continuous time random walks (CTRW) in which diffusion switches between flights and apparently immobile waiting-time periods. 23−26 Interestingly, the diffusion coefficient of the flying mode is similar to that reported by FCS, 10,27 suggesting that these seemingly diverse results could potentially be reconciled if the apparently immobile CTRW waiting-times actually comprise slow in-plane diffusion as measured using FRAP and HS-AFM.…”

Effect of Architecture of Thermoresponsive Copolymer Brushes on Switching of Their Adsorption Properties

“…[32,33] This example includes the switching of the adhesion of cells on thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) brushes. [34][35][36][37][38][39] In this case, the architecture of the brushes and the grafting density play a less important role because the responsive properties are determined by the chemical nature of the polymer itself. In the second approach, responsive properties are provided by switching the conformation of polymer chains, which regulates the accessibility of the functional groups either on the substrate or on the chains.…”

“…In one approach for the design of stimuli‐responsive brushes, the switching is provided by changing the properties (charge and hydrophobicity) of the polymer themselves . This example includes the switching of the adhesion of cells on thermoresponsive poly(N‐isopropylacrylamide) (PNIPAM) brushes . In this case, the architecture of the brushes and the grafting density play a less important role because the responsive properties are determined by the chemical nature of the polymer itself.…”

Optical Microscopic Techniques for Synthetic Polymer Characterization

“…In addition, Chin and co-workers measured the effects of temperature on the desorption-mediated surface diffusion of small molecules and polymers at aqueous interfaces with thermoresponsive polymer brushes grown via radical polymerization . SMT was employed to measure the surface diffusion of fluorescently labeled fatty acid and dextran ( M W = 10 kg/mol) on a PNIPAM brush below and above the lower critical solution temperature (LCST) (∼32 °C).…”

“…In addition, Chin and co-workers measured the effects of temperature on the desorption-mediated surface diffusion of small molecules and polymers at aqueous interfaces with thermoresponsive polymer brushes grown via radical polymerization. 102 SMT was employed to measure the surface diffusion of fluorescently labeled fatty acid and dextran (M W = 10 kg/ mol) on a PNIPAM brush below and above the lower critical solution temperature (LCST) (∼32 °C). Regardless of molecular size, molecular surface mobility was distinctly higher above the LCST, reflecting the collapse of the brush to give a more hydrophobic dense polymer surface that was less permeable for molecules.…”