Responsive brushes and gels as components of soft nanotechnology

Abstract

Progress in the development of generic molecular devices based on responsive polymers is discussed. Characterisation of specially synthesised polyelectrolyte gels, "grafted from" brushes and triblock copolymers is reported. A Landolt pH-oscillator, based on bromate/ sulfite/ferrocyanide, with a room temperature period of 20 min and a range of 3.1 < pH < 7.0, has been used to drive periodic oscillations in volume in a pH responsive hydrogel. The gel is coupled to the reaction and changes volume by a factor of at least 6. A continuously stirred, constant volume, tank reactor was set-up on an optical microscope and the reaction pH and gel size monitored. The cyclic force generation of this system has been measured directly in a modified JKR experiment. The responsive nature of polyelectrolyte brushes, grown by surface initiated ATRP, have been characterised by scanning force microscopy, neutron reflectometry and single molecule force measurements. Triblock copolymers, based on hydrophobic end-blocks and either polyacid or polybase mid-block, have been used to produce polymer gels where the deformation of the molecules can be followed directly by SAXS and a correlation between molecular shape change and macroscopic deformation has been established. The three systems studied allow both the macroscopic and a molecular response to be investigated independently for the crosslinked gels and the brushes. The triblock copolymers demonstrate that the individual response of the polyelectrolyte molecules scale-up to give the macroscopic response of the system in an oscillating chemical reaction.

Chemo-responsive, self-oscillating gels that undergo biomimetic communication

“…Notably, there are various examples of polymer gels that can be driven to pulsate through a periodic change in the local environment, which is accomplished through the use of a continuously stirred tank reactor or controlled variations in external cues. [10][11][12][13][14][15][16][17] The BZ gels, however, are the only gel system that can swell and deswell autonomously, with these mechanical oscillations being driven by the chemical oscillations of the BZ reaction. In fact, millimeter sized pieces of the BZ gels can pulsate autonomously for hours.…”

Direct Imaging of Nanoscopic Plastic Deformation below Bulk Tg and Chain Stretching in Temperature-Responsive Block Copolymer Hydrogels by Cryo-TEM

“…[6][7][8][9] Alternatively, block copolymers containing a hydrophilic midblock and hydrophobic end blocks can also be used to form physically cross-linked systems with thermoresponsive behavior. 10,11 Hydrogels have also been designed by combining two types of polymers capable of interacting via mutual hydrogen bonding into interpenetrating networks 12,13 or by using water-soluble ABA triblock copolymers where the solubility of the end blocks is suitably controlled by external conditions. [14][15][16][17] Recently, also protein-based materials, [18][19][20] polypeptides, 21,22 and block copolypeptides [23][24][25] have been exploited to design stimuli-responsive gels.…”

“…Small-angle X-ray scattering can be used to explain the changes in stimuli-responsive structure in block copolymer gels in bulk. 11,27 For responsive micelles in solution, light scattering studies can provide information on the change of micelle size with varying environmental stimuli. 28,29 However, these methods give only statistically averaged information on the transition behavior, and they only provide information on the reciprocal space, so that models have often to be used in order to account for morphology changes.…”

Phase Behavior and Temperature-Responsive Molecular Filters Based on Self-Assembly of Polystyrene-block-poly(N-isopropylacrylamide)-block-polystyrene

“…33,35 Hydrogels and gelators are relevant in the field biomaterials, since they can be used for targeted drug delivery, sensors, membranes capable of releasing or separating selectively specific substances, or actuators. [36][37][38][39][40][41][42][43][44] Even autonomous self-beating systems have been demonstrated using random copolymers, based on swelling/deswelling oscillations due to changes of the ion concentration in the gel, as a consequence of reversible enzymatic reactions. 13 Stimuli-responsive hydrogels have also been used as components of composite materials.…”