Mixed polymer brushes

Motornov M, Sheparovych R, Katz E, Minko S. Chemical gating with nanostructured responsive polymer brushes mixed brush versus homopolymer brush. ACS Nano... 

Besides homogeneous and uniform SAMs or polymer brushes, systems of tailored heterogeneity such as mixed monolayers of two or more compounds, gradients, block copolymer brushes etc. are now under investigation. Especially, the development of patterned surfaces offers the exciting possibility to perform multiple parallel experiments on a single substrate or cascade reactions. 

Considering a mixed brush made of mutually incompatible grafted chains, a novel transition to a brush characterized by a lateral composition modulation was found [73]. Even more complicated spatial structures are obtained with grafted diblock copolymers [74]. Finally, we would like to mention in passing that these static brush phenomena have interesting consequences for the dynamic properties of polymer brushes [75]. 

Recently multi-component polymer brush films have been synthesized in order to produce responsive substrates. As Fig. 10 illustrates, these so-called binary brushes contain two different polymers that may, or may not, be compatible with each other. Such a mixed brush may be synthesized by two strategies First, a mixed monolayer may be deposited with two different initiators (Fig. 10a). For example, one initiator could be activated thermally, while the other photochemically. Thus, a low to medium density polymer brush could be synthesized by selectively activating initiator A. The substrate could then be cleaned and immersed into a second monomer solution to activate B and grow a second polymer interdispersed with the first brush. 

Equations (14), (15), and (17) will be used to calculate the quantities < >,-, cb lt, and z . The obtained distributions of the segment density and bond densities allow to calculate any structural property of the two interacting polymer brushes as well as the mixing free energy. The latter quantity is used to obtain the interaction force profile. 

A. Good Solvents. When two surfaces with grafted polymer brushes approach each other, the overlap of the neutral brushes generates an interaction between surfaces. In good solvents, the Flory—Huggins mixing free energy density (eq 3c) increases with the monomer concentration therefore, one expects that the overlap of the brushes would lead always to repulsion. In the present... 

Scheme 1. Uniform surface response of mixed polymer brushes.

Figure 5.21 TEM images of poly(w-butyl methacrylate)-grafted MWCNTs with azido groups (MWCNT-Az-PnBMA) (a), MWCNTs grafted with both poly( -butyl methacrylate) and poly(ethylene glycol) brushes (MWNT-PnBMA-PEG) (b, c). (d) Cartoon for the local phase separation and assembly of amphiphilic polymer brushes into Janus polymer structures on CNTs as shown in (c) (marked by arrows), (e) Photograph of MWCNT-PnBMA-PEG dispersed in a mixed solvent of water (upper layer) and chloroform (bottom layer). Reprint with permission from Zhang et al...

From a philosophical point of view, we may argue that even when large scale simulations can precisely reproduce experimental observations, we still may not understand the detailed principles at work. For this we need more deep and systematic insights in sub-problems that present themselves, e.g. with respect to the segregation of chains in the core or in the corona. For this reason, it is of considerable interest to focus here on the phase behaviour of a mixed spherical polymer brush, which mimics the behaviour of chains in a spherical corona. 

It is clear that mixed polymer brushes attached to nanopar-tides or planar supports have many properties in common. However, binary brashes anchored to spherical supports provide additional benefits similar to homopolymetic brashes, the attachment to partides creates large surface areas that can be used as carriers for nanopartides or guest molecules (see Section 6.07.5). The carrier partides can also add spedal properties (see Section 6.07.2.1). Moreover, the immobilization onto a strongly curved support might have a strong impact on the phase behavior of the mixed brash. 

It should be emphasized that these mixed polymer brushes are very similar to block copolymer micelles whose corona is composed of mixed chains, which are presented elsewhere in this comprehensive. In the following, we will focus on mixed brashes grafted to preformed colloids. There are several experimental studies on mixed and a review of mixed SPB... 

The effect of polymer brushes in different lubrication regimes is seen in Fig. 4a and b, in which the Stribeck curves obtained from pin-on-disk and MTM measurements are plotted, respectively. The coefficient of friction is plotted against speed multiplied by viscosity for all HEPES-glycerol mixtures both in the presence and the absence of the polymer. As expected, the effect of polymer in reducing the friction is predominantly seen in the boundary-lubrication regime. The effect of polymer on the friction is also extended to the mixed-lubrication regime... 

Ionov L, Minko S Mixed polymer brushes with locking switching, AGS Appl Mater Intetfaces 4(l) 483-489, 2012. 

Wang J, Mueller M Microphase separation of mixed polymer brushes dependence of the morphology on grafting density, composition, chain-length asymmetry, solvent quality, and selectivity, J Phys Chem B 113 11384—11402, 2009. 

Bernards, M. T., Cheng, G., Zhang, Z., Jiang, S. (2008). Mixed charge, overall neutral nonfouling polymer brushes formed by surface-initiated atom transfer radical polymerization. Macromolecules, 41, 4216 219. 

---