Brushes collapse behavior

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. 

Brush research continued to flourish after the publication of the earlier reviews. An exhaustive summary of current developments is beyond the scope of the present review. Rather, I will focus on few notable results, experimental as well as theoretical, obtained in two areas. One is the collapse of brushes in poor solvents. The other is the behavior of brushes in mixed solvents. Brush collapse was studied by Au-roy and Auvray using small angle neutron scattering (SANS). Their experiments demonstrated the existence of a sharp interface between the dense brush and the bulk. This and the characteristic thickness of the layer are in good agreement with theory. The nature of the collapse for a < is not yet fully resolved. However,... 

Fig. 26 Dry thickness of poly(acryl amide) as a function of the position on the silica substrate prepared by slow ( ) and fast ( ) removal of the polymerization solution by utilizing the method depicted in Fig. 24. The inset shows the dry poly(acryl amide) thickness as a function of the polymerization time. Note that both data sets collapse on a single curve at short polymerization times. Regardless of the drain speed, the brush thickness increases linearly at short polymerization times and levels off at longer polymerization times. The latter behavior is associated with premature termination of the growing polymers...

On the basis of our experimental results presented so far, the overall viscoelastic behavior of these triblock copolymers shows an elasticity-dominance over the viscosity. After reaching the critical mass density, where the static elasticity es reaches the maximum, these triblock copolymers collapse into the subphase and form hydrated brushes and these anchored brushes may be responsible for the result that the surface viscosities drop to around the 0 value at r. A distinctive difference between two types of polymers, sample I (PEO-PPO-PEO) and sample II (PPO-PEO-PPO), is the temperature dependence of r where both static elasticity and dilational viscosity show kinds of transitions. V of sample I increases with increasing temperature while that of sample II does not change with temperature. 

The polyelectrolyte brush shrinks strongly on addition of electrolytes. At low or moderately low salt concentrations (cs=0.01 mol L-1) the force profiles resemble those of a soft brush. At salt concentrations of cs= 0.03 mol L, however, the profile of the static force resembles more closely that of a hard surface. Interestingly, if the behavior of the PEL brush is studied close to the collapse point significantly increased compressibility can be observed. However, the compressibilty shows no bistability, which indicates that the transition between the brush and the collapsed state is not a true first-order transition, although this would be expected from mean-field theory. One possible explanation of this behavior would be that the polydisper-sity of the surface-attached chains smoothens the transition. 

Last, PMAA brushes in contact with aluminium solutions show similar behavior to the calcium case however, the collapse concentration is found to be lower by roughly two orders of magnitude. If an aluminium salt solution is added to the brush the collapse starts at concentrations of <10-5 mol L 1 (Fig. 25). Furthermore, it is interesting to note that the film still contains rather large amounts of water in the collapsed state and remains much more swollen, compared to the same film which has been collapsed by exposure to calcium. 

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. 

A mnch more recent development, however, is the drive to create responsive polymer brashes (Chen et al., 2010 Stuart et al 2010). These brushes respond with certain behavior, such as swelling, collapsing, charging to an external trigger, and allowing... 

SI-IMP has been used for synthesis of different types of stimuli-responsive polymer brushes that are responsive to several external stimuli, such as pFI, temperature, and ionic strength [28,58-65]. Because materials interact with their surroundings via their interfaces, the ability to fashion soft interfacial layers and tune the range, extent, and type of physicochemical interactions across interfaces is central to a variety of applications. Rahane et al. carried out sequential SI-IMP of two monomers to create bilevel poly(methacrylic acid)-Woc/c-poly(N-isopropylacrylamide) (PMAA-b-PNIPAM) block copolymer brushes that can respond to multiple stimuli [28]. They observed that each strata in the bilevel PMAA-b-PNIPAM brush retained its customary responsive characteristics PMAA being a "weak" polyelectrolyte swells as pH is increased and the thermoresponsive PNIPAM block collapses as temperature is raised through the volume phase transition temperature due to its lower critical solution temperature (LCST) behavior. As a result of ions added to make buffer solutions of various pH and because of the effect of surface confinement, the swollen-collapse transition of the PNIPAM layer occurs at a... 

IX - SOME RECENT DEVELOPMENTS IN BRUSH RESEARCH COLLAPSE AND MIXED SOLVENT BEHAVIOR... 

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