AhmedSheriff-Research+Paper

__**Mixed polymer brushes**__

Ahmed Bulhassan Drexel University Department of Chemistry Drexel University, Philadelphia, PA 19108

(I) Abstract (II) Introduction (III) Grafting unlike polymers on a substrate (IV) Selective/nonselective solvent on a binary (mixed) brushes (V) Memory effect on mixed polymer brushes (VI) Conclusion (VII) References

This research project focuses on research carried on mixed polymer brushes, that are based on the characteristics of two or more unlike polymers. An example such as polystyrene (mw= 36400 g/mol) and poly(methyl methacrylate) (mw = 787000 g/mol) were grafted on the same substrate or surface. We would focus on the mutual relationship between the "phase segregated morphology" of these two different polymers and their distinct modes of attachment by changing or switching selectively good and bad solvent. We would also focus on their memory effect on how this polymer "brushes" behave in respond to environmental conditions.
 * Abstract**

Polymer molecule is a generic term used to describe repeating units, called monomers, which are connected by covalent bonds. The process by which these subunits are linked together to form a polymer molecule is by a chemical reaction known as polymerization. Now we understand how polymers are formed but what is it meant by the phrase "polymer brush?" The notion of polymer brush can be modified as "long-chain polymer molecules attached by one end to a surface of interface by some means, with a density of attachment points high enough so that the chains are obliged to stretch away from the interface, sometimes much farther than the typical unstretched size of a chain (1). Furthermore, we know the simplest idea of what polymer brushes are and this paper would focus on mixed polymer brushes, that are described as two homopolymers being covalently attached with one end to a solid substrate. These polymer brushes have attracted abiding interest because of their ability to switch properties such as the surface energy (hydrophilicity, hydrophobicity) in which the bonds between these brushes and the surface are disrupted and or their surface topography (formation of rough or smooth surface) in response to changes of their environment (2). Polymer brushes play a key role in solving problems in polymer science and are also relevant in biophysics. These brushes have variations among them and their variations can be observed only if solvent is present or not. When solvent is present, these brushes could lead to chains stretching away from the interface to which they are attached, is their affinity for the solvent or dislike of each other (1). The van der waals forces acting on these polymer brushes could not hold these polymers close enough when in the solvent. The presence of these solvent leads to these polymers stretching away from the surface due to the low forces acting on them. In addition, the absence of solvent could also lead to stretching because of melt conditions (no solvent is present), the chains must stretch away from the interface to avoid overfilling space since the matter of which the chains are made is approximately incompressible (1). However, there are various interfaces in which these brushes are attached and these interfaces may be a solid substrate or the interface between two solvents, between solvent and air, or between melts or solutions of homopolymers. Since there are variations in interface the mechanism by which these chains are attached also varies and these could lead to several physical systems such as "copolymer microphases; colloid stabilization by end-grafted chain; polymer compatibilizer; and polymer surfactants." The absence of solvent (melts condition) and concentrated solution in copolymer (unlike polymer) brushes would produce different microphases when varying the relative length, bulk and flexibility of the two dissimilar blocks of the molecule (3). Another physical system that applies is colloid stabilization by end-grafted chain in which particles are in suspension to protect against flocculation (process where a solute comes out of a solution in the form of flakes) due to the brush and the presence of van der waals attraction. These solid particles are in suspension to prevent flocculation because these mixed polymer brushes that has been attracted to the end-grafted chains which are compatible with the suspension, that overlapping cannot occur because the van der waals forces is too weak to keep these particles close enough to overlap. The physical system polymer surfactants which has a polar end-group that do attached to a chain of hydrocarbon, and that made it insoluble in polar solvents. Finally, polymer compatibilizer a diblock copolymer, formed by chemically joining two dissimilar polymers end to end, adsorbs at interfaces between its constituent polymers. Here the 'solvents" are homopolymer solutions or melts, and may in fact be excluded from the copolymers at the interface (1). Other modifications in which a mixed polymer brushes are grafted to other polymer surfaces in which they can regulate adhesion, wettability, friction, and adsorption.
 * Introduction**


 * Grafting unlike polymers on a substrate**

Polymer brushes can be prepared following two main strategies: (I) the grafting to and (II) the grafting from strategies. The grafting to strategy involves the attachment of prefabricated polymers via either physisorption or covalent bond formation. The grafting to have limitations, which make it difficult to make or produce thick or more dense polymer brushes. Also, an increase in molecular weight of the polymer, the end-group of the polymer would be less efficient with the substrate. In the grafting from approach, the polymerization is directly initiated from the initiator-fuctionalized surfaces. This is a controlled "living" polymerization techniques that are particularly more interested to researchers to make polymer brushes (4).

Block copolymers or grafted copolymers occurs in the presence of selective solvents or selective surfaces, giving rise to selective solvation and selective adsorption. The detailed polymer brush structure depends on the selectivities of these media and the nature of the copolymers, the architecture of copolymers, the length of each block and the interactions between blocks and surface (5) To attach an azo-initiator on a polyamide substrate, the surface of the polyamide has to be treated with low pressure ammonia plama (6) but before treatment begins, diluents (ease movement of fluid) must be matched with the initiator. Therefore, using initiators and diluent of different sizes show that the steric demands of the initiator and diluent, as well as their reactivities, must be matched to ensure a homogeneous initiator layer on the surface (7). This procedure was done before a functional group can be introduced on a polyamide surface before radical polymerization can be initiated. The ammonia plasma "introduces N-containing functionality such as amino, imino, and in addition oxygen containing groups such as amido and hydroxyl groups (6). These functional groups were used to covalently bond the azo-initiator to polyamide surfaces. After these functional groups were introduced on a polyamide surface, these researchers succeeded in applying these two binary polymer brushes (PS/PMMA) on a polyamide surface. They proposed that grafting of unlike polymer on a polyamide surface, an ammonia plasma would be a good method to introduce functional groups such as hydroxyl and amino before radical polymerization can be initiated. Figure 1 below show the process of which these unlike polymers were grafted on a polyamide surface.

A research was conducted by using the same binary brushes (PS/PMMA) but the surface was on a gold and silicon substrate using the "grafting-from" method with surface bound initiators. They found out that the binary brushes (PS/PMMA) mixed brushes show rapid switching in selective and non-selective solvents. The surface can be changed from a hydrophobic to hydrophilic state within a minute by simply rinsing with neutral water (8). Though these binary brushes do attached to different substrate, they do posses different behavior when it comes in selecting solvents that are compatible with these brushes. The length or height of a binary brush can be decide upon the conformation of the tethered chains, which rely on the quality of the solvent used or the grafting density. The application of solvent to binary brush is highly selective because the addition of a poor solvent to a mixed polymer brush can cause swelling on the end-grafted and can also cause a delamination (layers separate) from the substrate used in grafting these brushes. However, applying a good solvent to these brushes would show no effect of swelling or chain contraction. A research conducted by Gallagher found that the addition of a poor solvent or good solvent to a mixed polymer (PS/PMMA), with these solvent having a very small amount of surface active impurity can have a large impact on the dimensions of polymer brush layers and the adhesion stability of polymer layers which would cause the surface to change. This brush swelling effect is unexpected from the perspective of ideal brush layers, where little sensitivity to polymer-surface interaction has been presumed to exist. Apparently, real grafted layers with physically accessible grafting densities can be stimulated to expand or contract through the addition of surface active molecules (9). A research carried out by (10), were able to distinguished regimes of low and high grafting density and the incompatibility parameter with scaling arguments. For the brush regime, they found a homogeneous phase at low incompatibility, a periodical laterally segregated phase at higher incompatibility, and a segregated phase with diminished periodicity at very high incompatibility (10). This same observation we can see in figure 2 proposed by (3) showing the effect on polymer being absorbed by a nonselective solvent. Figure 2 diagram shows the two polymers (PS/PMMA) an increase in elevation with a nonselective solvent, a laterally segregation also an alternation of these binary polymers domains. These observations can further be explained as a strong influence on the adhesive stability of these polymer layers and their active surface impurities. In addition, the expansion of these brushes can also be observed if the surface segregating solvent has a very strong repulsive interaction with these binary polymers. Now let consider selective solvent. How would these binary polymers behave in a selective solvent. Figure 3 shows that these brushes show very good efficient dynamic properties of surface on end-grafted polymers in good solvent. Research have shown that when two mica (group of sheet silicate minerals) surfaces densely covered with polystyrene brushes immersed in toluene (good solvent) were sheared above a critical shear rate and sheared induced swelling of the brush occurred, but when neutron reflectivity measurement and other theoretical and simulation studies on steady shear past a single brush layer in a good solvent, the brush do not show such swelling. In a selective solvent, as shown in figure 3, segregation into layered phases of these binary polymers are also seen. The cluster polymer is polystyrene (PS) and the matrix form of polymer is poly(methyl methacrylate) (PMMA) (3) A research conducted by (11) proposed the same idea of morphology and segregation in which they predicted that a selective solvent (good solvent for one of the grafted polymers) stabilize a dimple like morphology, an unfavorable polymer forms clusters that are segregated to the grafting surface while the favorite polymer forms a discontinuous phase preferentially segregated to the brush surface (a top layer).

In a nonselective solvents, lateral segregation dominates and the mixed brushes form a ripple like morphology: lamellar -like stripes are formed by alternating microphases of both polymers (11). The morphology of these binary polymers in the above diagrams does have similarities and differences. The similarity here is both selective and nonselective solvent in a mixed brush would show segregation. The differences are, in a nonselective solvent, an alternating of PS/PMMA domains would be observed while in a selective solvent, they observed a cluster rich PS and a domination of PMMA on the top layer. The degree of swelling of polymer brushes are largely determined by the solvent quality in the ambient medium. In a bad solvents (nonselective), brushes collapse to form a dense layer on the substrate. This now well established that this proceeds continuously as a function of solvent quality (12). In addition, if mixed brushed were exposed to tetrahydrofuran (THF) (good solvent) for PS, the polymer chain is swollen in the favorable solvent with the structure lock in place as THF evaporates. In the dry film under these conditions the polymers are in their glassy state providing a stable long-term structure (13)  We can say that the choices of solvent do affect the way these brushes behave when in solution. However, segregation in morphology can take place upon the adding of a small amount of a surface active impurities, however, a good solvent is observed for that or those polymers. The switching of the mechanism is caused by these phases segregation, and these brushes can always switch their mechanism when a selective or nonselective solvent is applied because of their good domain memory that they possesses. When these mixed brushes are covalently bound to a surface, a stable conditioned is obtained. However, they do respond quickly to external stimulation. Such behavior had encouraged researchers to look deeply in these characteristics behavior bu changing surface properties in which they introduced such characteristics as "domain memory" which is of important for the basic understanding of the structure formation in binary brushes and also in nanoscience. These mixed polymer brushes can induce motion of nano-objects adsorbed on their top, that is, motion driven by changes in the spatially varying force between brush and particles upon topography switching. If these brushes pattern completely recover their initial conformation after topography (complete domain memory), then the brushes are not capable of relocating adsorbed objects (14). The dependence of the domain memory has been investigated by (15) in which they study how the domain memory measures depends on the composition fluctuations of the grafting points by comparing two types of PS-PMMA mixed brushes that are synthesized from a conventional initiator and from a Y initiator. The Y-shape polystyrene-poly(methyl methacrylate) mixed brushes were prepared from an asymmetric difunctional initiator-terminated self assembled monolayer (Y-SAM) by combining two different living radical polymerization techniques such as atom-transfer radical polymerization (ATRP) and nitroxide-mediated radical polymerization (NMRP) (15). These Y-shaped binary brushes were used to know what extent domains form at the same position with the same size and shape when the morphology of these brushes are change in exposing them to a solvent of different quality.
 * Memory effect in mixed polymer brushes**

The domain memory of these Y-shaped binary brushes on the grafting above has been study by comparing these two types of mixed brushes, the conventional and Y-shaped PS-PMMA mixed brushes. In the case of conventional mixed brushes, there are significant fluctuations in both the grafting density and local composition of the attached chains. Composition fluctuations of the grafted chains result in a strong domain memory effect (i.e, there is a high probability that a specific domain re-forms at the same position and with the same shape after switching the morphology from a laterally structured to a laterally homogeneous one back and forth).In the case of Y-shaped PS-PMMA mixed brushes, in contrast, exhibit a significantly weaker domain memory because fluctuation in the composition of the grafting points are eliminated (15). They succeeded in showing that microphase-separated morphology is similar in both PS and PMMA, but for the Y-shaped brushes exhibit a significantly weaker domain memory than do conventional PS-PMMA mixed brushes. It was found that PS-PMMA brushes possesses a partial domain memory effect. This means that locations on the brush surface exhibit complete recovery, that is the pattern on the brushes appear at the sample place and with the same shape and size after each cycle of topography switching (16).These results obtained on domain memory of binary polymer brush with different size and shape by comparing the single-chain-in-main-field simulations (SCMF) and experiments to demonstrate the importance of local fluctuations in the positions of the grafting points for the nucleation of the domain structure in mixed polymer brushes (15). **Conclusion** The potential of mixed polymer brushes and mixed brush-grafted particles has certainly not been fully tapped, though they have been used in the fabrication of switchable (super)hydrophobic/(super)hydrophilic smart surfaces, pickering emulsions, environmentally responsive lithography, transport of particles across liquid-liquid interface etc. (17) Mixed polymer brushes have very unique characteristics upon responding to external stimuli or environmental conditions. In case of environmental conditions such as applying selective and nonselective solvent a change in morphology would be observed as a result of forming ripple structure and dimple morphology. The following observations can be concluded that the solvent quality has a strong influence on the pattern formation (18). The surface active impurities can cause a delamination of polymer layers. The properties of binary brushes have been studied for the past decades in which conclusions are made with general agreements between theory and experiment. **References** 1. Milner, S. T. "Polymer Brushes. "__Science (Washington, DC, United States)__ 251.4996 (1991): 905-14. DOI: __http://dx.doi.org/__ __10.1126/science.251.4996.905__  Web: __http://www.sciencemag.org/content/251/4996/905.abstract__ 2. Svetlana Santer; Alexey Kopyshev; Jorn Donges; Hyun-Kwan Yan; Jurgen Ruhe: "Domain Memory of Mixed Polymer Brushes." __Langmuir__ 22.10 (2006): 4660-7. DOI: http://dx.doi.org/10.1021/la060134b 3. Denys Usov, Viacheslav Gruzdev, Mirko Nitschke, Manfred Stamm,* Olha Hoy, Igor Luzinov,*Ihor Tokarev, and Sergiy Minko: "Three-Dimensional Analysis of Switching Mechanism of Mixed Polymer Brushes." __Macromolecules (Washington, DC, United States)__ 40.24 (2007): 8774-83. DOI: http://dx.doi.org/10.1021/ma071090w 4. Raphael Barbey; Laurent Lavanant; Dusko Paripovic; Nicolas Schuwer; Caroline Sugnaux; Stefano Tugulu; Harm-Anton Klok "Polymer brushes via Surface-Initiated Controlled Radical Polymerization: Synthesis, Characterization, Properties and Applications." DOI: http://dx.doi.org/10.1021/cr900045a 5. B. Zhao, W. J. Brittain "Polymer brushes: surface-immobilized" __macromolecules__ Prog. Polym. Sci. 25 (2000) 677–710. web: http://www.sciencedirect.com/science/article/pii/S0079670000000125 6. Mikhail Motornov; Sergij Minko; Mirko Nitschke, Karina Grundke; Manfred Stamm: "Mixed Polymer Brushes on Polyamide Substrates." __PMSE Preprints__ 88 (2003): 264-5. Web: http://people.clarkson.edu/~sminko/nanostructured/papers/PMSE2003NOMotornov.pdf

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