Softening of PMMA Brushes upon Collapse/Swelling Transition. A Combined Neutron Reflectivity and Nanomechanical Cantilever Sensor Study

Abstract

In this work, we establish a direct correlation between chain mechanics and structural properties of polymer brushes upon swelling. We present experimental results on poly(methyl methacrylate) (PMMA) brushes prepared via surface initiated atomic transfer radical polymerization. Neutron reflectivity studies gave insight into the brush thickness and volume fraction profiles of the brush, gradually swollen with solvent mixtures. Comparison of our experiments with scaling theory yielded specific polymer-solvent interaction parameters and gave insight into the desorption and adsorption behavior of bad and good solvents, respectively. Insight into the brush's chain mechanics was obtained from surface stress investigations using the nanomechanical cantilever sensor bending technique. It was shown that polymer brush swelling leads to a decrease in surface stress due to chain disentanglements and the related softening of the polymer brush under Θ-solvent conditions.

Surface-Initiated Controlled Radical Polymerization: State-of-the-Art, Opportunities, and Challenges in Surface and Interface Engineering with Polymer Brushes

“…Upon complete swelling of the PDMAEMA brushes at pH 3, this spike in density profile disappeared, due to the contribution of counterions to the osmotic pressure. The collapse-swelling transition of PMMA brushes upon gradual addition of solvent mixtures also showed that osmotic pressure dominates over the enthalpy of polymer–solvent interactions . Not only can changes in pH lead to changes in the density profile of PDMAEMA brushes, but also changes in electric fields between brush substrates and parallel electrodes .…”

“…The collapse-swelling transition of PMMA brushes upon gradual addition of solvent mixtures also showed that osmotic pressure dominates over the enthalpy of polymer−solvent interactions. 869 Not only can changes in pH lead to changes in the density profile of PDMAEMA brushes, but also changes in electric fields between brush substrates and parallel electrodes. 892 In the latter case, PDMAEMA brushes were stretched nearly to their contour length at very high positive voltage, before being detached from the substrate.…”

“…The lubrication properties of polymer brushes depend highly on their chemical structure, architecture and conformation. ,,− Most methods available to characterize the mechanical behavior of polymer brushes rely on the influence of external shearing fields . In efforts to understand the collapse-swelling transition of polymer brushes in the absence of external fields, and therefore, to make direct correlations to molecular structure, Lenz et al applied nanomechanical cantilever sensor (NCS) arrays to study PMMA mechanics during the collapse-swelling transition upon altering solvent quality. NCS bending experiments provided insight into the lateral forces within the polymer brush, e.g., interchain entanglements in bad solvents and osmotic pressure upon swelling in a good solvent.…”

Utilizing vapor swelling of surface-initiated polymer brushes to develop quantitative measurements of brush thermodynamics and grafting density

“…Molecular structure simulations conducted by Baulin and Halperin indicate monomer concentration dependence on c for thermoresponsive polymer brushes due to decreasing polymer volume fraction as the chains stretch away from the substrate [62]. Brush swelling experiments combined with surface stress measurements by Gutmann and coworkers indicate that free surface energy resulting from interfacial tension between solvent and brush significantly adds to the enthalpic and entropic energy contributions to c yielding more favorable mixing between elongated polymer brush chains and solvents than solution c values would predict [33]. This is contradictory to vapor sorption studies in polymer brushes [34] and polymer films [63], where the dominance on the entropic contribution to c makes its value much higher than predicted Hildebrand solubility parameters.…”

Challenges for nanomechanical sensors in biological detection

“…The application range of nanomechanical sensors has broadened in the last years, ranging from gas detection 4 , calorimetry 1,[5][6][7] , drug screening 8 , genetics 3,9 , proteomics [10][11][12] , microbiology 13,14 , glycomics 15 and metabolic measurements at the level of individual cells 16 , to cite just a few. A review of the chemicals and bioanalytes already detected by cantilever sensors states that nearly 50 analytes have been successfully detected 17 .…”