Collapse, polymer brushes

In cases when the two surfaces are non-equivalent (e.g., an attractive substrate on one side, an air on the other side), similar to the problem of a semi-infinite system in contact with a wall, wetting can also occur (the term dewetting appHes if the homogeneous film breaks up upon cooHng into droplets). We consider adsorption of chains only in the case where all monomers experience the same interaction energy with the surface. An important alternative case occurs for chains that are end-grafted at the walls polymer brushes which may also undergo collapse transition when the solvent quality deteriorates. Simulation of polymer brushes has been reviewed recently [9,29] and will not be considered here. 

The collapse of a polymer gel in response to a change in environment can be scaled down to the single chain level. A layer of polymer chains grafted to a surface forms a polymer brush, for which the collapse transition can be nicely observed using neutron reflectivity. A pore lined with a responsive polymer brush will form a selective valve for example if the grafted polymer is a weak polybase, in aqueous acidic conditions the brush will be charged and will expand to close the pore, while in basic conditions the brush will be neutral. This principle has been used to create a selective membrane, which shows greatly reduced permeability in acidic conditions. 

Increasing the salt concentration one expects the lateral interactions to be reduced and the polyelectrolyte to be less stretched. This is indeed observed. Also as typical for the salted brush the brush length increases upon compression [44]. For this case one theoretically expects the dependence on density as depicted by the full line describing the measurements for 0.1 mol L-1 salt. At still higher salt contents one expects counter-ion condensation [45], hence part of the counter-ions do not contribute to the osmotic pressure. In this case one expects a further shrinking of the brush with salt content. In the extreme of a collapsed polymer one expects the length to be inversely proportional to the molecular area which corresponds to the steepest slope at highest salt content. 

Amphiphilic diblock copolymers act as a surfactant and stabilize free-standing films. They are assumed to adsorb at the interface by analogy with low-molecular-weight surfactants The hydrophobic part is collapsed at the interfaces and the hydrophilic part is directed towards the film core (Fig. 2a). Investigations of the structure at a single liquid interface (air/water) show that the amphiphilic diblock copolymers present polymer brushes which are anchored by the hydrophobic block at the interface [22, 23], This structure is also assumed at the film surfaces. Fig. 3 shows the disjoining pressure iso-... 

Poly(iV-isopropylacrylaniide) (PNIPAAm) is by far the most prominent example of a thermally responsive polymer. It undergoes a phase transition at the lower critical solution temperature (LCST), resulting in a strong decrease in hydration of the polymer. For polymer brushes, this behavior is reflected in a collapse of the structure above the LCST. Because of the proximity of the LCST (32°C) to the body temperature, PNIPAAm is considered an interesting candidate for drug release systems. 

In contrast to organosoluble polymers, for most known water-based nonionic polymers, the quaUty of water as a solvent decreases upon an increase in temperature. This is known as LCST (lower critical solution temperature) behavior [144], Experimental observations of LCST behavior (thermoinduced collapse) of neutral stars or spherical polymer brushes in water are rare [145, 146], and do not yet provide systematic relationships between the LCST and the degree of branching. 

Due to the strong hydrophobicity of the blocks B, the interface between the collapsed hydrophobic domain and the surrounding aqueous environment is narrow compared to the size of the core. Therefore, the coronal blocks A can be envisioned as tethered to the interface to form a polymer brush [33, 39, 40]. The hydration of the corona and the repulsion between different coronae ensure the solubility (aggregative stability) of the micelles in water. 

The collapse of a polymer brush as the temperature is dropped through theta conditions is nicely illustrated by a neutron reflection study of end-grafted polystyrene in cyclohexane (Karim et al. 1994). The results are shown in figure 6.9 note that the brush height varies in a smooth way through the theta temperature, with no sign of a sharp transition. 

FIGURE 12.12 Schematic of mouse embryonic stem cells (mESC) attachment (above LCST) and detachment (below LCST) to/from thermo-responsive polymer substrates. Poly(ME02MA-co-OEGMA) polymer brushes are shown as chain extended at temperatures below the LCST forming a cell-resistant surface, whereas collapse of the bmshes above the LCST results in a more favorable surface for cell attachment [192]. 

Jonas, A. M. GUnel, K. Oren, R. Nysten, B. Huck, W. T. S. Thermo-responsive polymer brushes with tunable collapse temperatures in the physiological range. Macromolecules 2007,40, 4403 1405. 

Milk is a natural colloidal dispersion that contains casein micelles, self-assembled protein associates with a diameter of about 200 nm [20]. The casein micelles are protected against flocculation by an assembly of dense hairs (often called a brush ) at their surfaces. Polymer brushes can thus provide steric stabilization of colloids. For millennia, man used the fact that milk flocculates and gels when it is acidified, as in yogurt production. Below pH = 5 macroscopic flocculation of the casein micelles in milk is observed [21]. This means that the interactions between casein micelles change from repulsive to attractive. The explanation is that acidification leads to collapse of the casein brushes [22]. In cheese-making the steric stabilization is removed by enzymes, which induce gelation into cheese curd. 

Auroy, P., Auvray, L. Collapse-stretching transition for polymer brushes—preferential solvation. Macromolecules 25, 4134-4141 (1992). doi 10.1021/ma00042a014... 

Refers, A., Schimmel, M., Ruhe, J., Johannsmann, D. Collapse of a polymer brush in a poor solvent probed by noise analysis of a scanning force microscope cantilever. Langmuir 14, 3999-4004 (1998). doi 10.1021/la971409d... 

Xu and Liu recently reported the syntheses of the well-defined 7-arm and 21-arm PiPAAm stars with a P-cyclodextrin core [278, 568] and presented a thorough analysis of the literature on thermoresponsive stars and polymer brushes tethered to curved surfaces, such as latex particles [279, 280, 569], gold nanoparticles [282] and microgels [570], A unique feature of these architectures is that they form a densely packed spherical core and a less-dense outer shell [159]. As a result of such a non-uniform density distribution, two temperature-induced phase transitions have been observed experimentally in several systems based on PiPAAm [279, 280, 282, 568, 569], One transition has been ascribed to the phase transition of the inner segments of PiPAAm, whereas the other transition, which is concentration dependent, was assigned to the collapse of the outer PiPAAm segments [282],... 

In a sense, all polymer brashes are responsive to solvent. For example, a simple PEO polymer brush will swell in water, but will be collapsed when exposed to an apo-lar solvent such as hexane. StiU, what we discuss here as solvent-resistant brashes... 

Malham, I. B., Bureau L. 2010. Density effects on collapse, compression and adhesion of thermoresponsive polymer brushes, n muir 26 4762-4768. 

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