Temperature-Dependent Adsorption and Adsorption Hysteresis of a Thermoresponsive Diblock Copolymer

Abstract

A nonionic-cationic diblock copolymer, poly(2-isopropyl-2-oxazoline)60-b-poly((3-acrylamidopropyl)trimethylammonium chloride)17, (PIPOZ60-b-PAMPTMA17), was utilized to electrostatically tether temperature-responsive PIPOZ chains to silica surfaces by physisorption. The effects of polymer concentration, pH, and temperature on adsorption were investigated using quartz crystal microbalance with dissipation monitoring and ellipsometry. The combination of these two techniques allows thorough characterization of the adsorbed layer in terms of surface excess, thickness, and water content. The high affinity of the cationic PAMPTMA17 block to the negatively charged silica surface gives rise to a high affinity adsorption isotherm, leading to (nearly) irreversible adsorption with respect to dilution. An increase in solution pH lowers the affinity of PIPOZ to silica but enhances the adsorption of the cationic block due to increasing silica surface charge density, which leads to higher adsorption of the cationic diblock copolymer. Higher surface excess is also achieved at higher temperatures due to the worsening of the solvent quality of water for the PIPOZ block. Interestingly, a large hysteresis in adsorbed mass and other layer properties was observed when the temperature was cycled from 25 to 45 °C and then back to 25 °C. Possible causes for this temperature hysteresis are discussed.

Thermoresponsive poly(2-oxazoline)s, polypeptoids, and polypeptides

“…Claesson et al demonstrated that the adsorption of a block copolymer of PiPrOx as themoresponsive block and poly (3-acrylamidopropyltrimethylammonium chloride) as charged block to adsorb onto a negatively charged silica substrate is indeed temperature dependent. 97 Upon increasing the temperature, the PiPrOx is partially dehydrated and becomes more hydrophobic which favors the adsorption process leading to an increase in adsorbed mass. This process was found to be reversible and decreasing the temperature led to partial desorption of the block copolymer.…”

Poly(2-isopropyl-2-oxazoline)-b-poly(lactide) (PiPOx-b-PLA) Nanoparticles in Water: Interblock van der Waals Attraction Opposes Amphiphilic Phase Separation

“…Examples of hydrophobic blocks employed include poly­(ethylene), poly­(2-(4-( tert -butoxycarbonyl)­amino)­butyl-2-oxazoline), and poly­(ferrocenyl­dimethylsilane). , Stoichiometric mixtures in water of two oppositely charged diblock ionomers, such as the PiPOx- b -poly­( l -lysine)/PiPOx- b -poly­(aspartic acid) pair, also assemble in water, forming core–corona polyion complex (PIC) micelles first reported by Kataoka . Dihydrophilic block copolymers are also of interest, in which the PiPOx block is linked to another water-soluble block, − which may be thermoresponsive. − …”

Thermo-responsive diblock and triblock cationic copolymers at the silica/aqueous interface: A QCM-D and AFM study

“…The PAMPTMA block of the copolymers is positively charged in aqueous solution, thus can electrostatically adsorb onto negatively charged surfaces such as silica. [44][45][46] This feature can be employed to produce stable polymer layers, which can be used in various practical applications, e.g., to prevent adhesion of biomolecules or conversely to promote specific attachment of cell types to surfaces. [47][48][49] Dynamic light scattering (DLS) is employed to inquire the thermo-responsive behavior and hydrodynamic size of the copolymers in bulk aqueous solution.…”