In recent years, the synthesis of polymer brushes has received significant attention. Inthis presentation we talked about several different aspects of polymer brushes andsynthetic strategies for the generation of polymer brushes . Finally, example isprovided that highlight some recent developments aimed at strategies for thefunctionalization of surfaces with polymer brushes, at ways of realizing smartsurfaces with switchable properties.

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Polymer brushes

A polymer brush consists of end-tethered (grafted, anchored) polymer chainsstretched away from the substrate so that in the given solvent the brush height (h) islarger as compared to the end-to-end distance (<r2>1/2) of the same non-graftedchains dissolved in the same solvent. In polymer brushes the distance betweengrafting points (d) is smaller than the chain end-to-end distance. Polymer brushes canbe introduced as thin films of end-grafted polymer molecules when the followingconditions are satisfied: h> <r2>1/2, d < <r2>1/2. Outside these conditions the graftedlayers are in the “mushroom regime” (see next slide).

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Polymer Brushes: General Features

When polymer molecules are tethered (grafted) to a surface, two basic cases must bedistinguished depending on the graft density of the attached chains.

1. If the distance between two anchoring sites is larger than the size of the surface-attached polymers, the segments of the individual chains do not “feel” each otherand behave more or less like single chains “nailed” down onto the surface by oneend. Depending on the strength of interaction of the polymer segments with thesurface, again two cases must be distinguished. If the interaction between thepolymer and the surface is weak (or even repulsive), the chains form a typical randomcoil that is linked to the surface through a “stem” of varying size. For such a situation,the term “mushroom” conformation has been coined (Slide). However, if thesegments of the surface attached chains adsorb strongly to the underlying surface,the polymer molecules obtain a flat, “pancake”-like conformation (Slide)

2. A completely different picture is obtained if the chains are attached to the surfaceat such short distances between the anchor points that the polymer moleculesoverlap. In this case, the segments of the chains try to avoid each other as much aspossible and minimize segment–segment interactions by stretching away from thesurface (slide). This chain stretching, however, reduces the number of possiblepolymer conformations, which is equivalent to a reduction in the entropy of thechains.

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This loss of entropy gives rise to a retracting force trying to keep the chains coiled, asoccurs in a stretched piece of rubber. Thus, a new equilibrium at a higher energy levelis obtained in which the chains are stretched perpendicular to the surface.The structure of a surface-immobilized polymer can be evaluated by the inverse valueof the distance between grafting points (D). As the size of grafted polymer chainsapproaches the distance between grafting points, the grafted chains overlap. Thispoint is a transition point between a single grafted chain (mushroom) regime andbrush regime. A commonly used literature parameter for quantitativecharacterization of this transition is the reduced tethered density

where Rg is radius of gyration of a tethered chain at specific experimental conditionsof solvent and temperature. The definition of grafting density (σ) is determined by

Where h is the brush thickness; ρ, bulk density of the brush composition; and NA,Avogadro’s number. It is generally recognized that three regimes occur in brushformation: (1) the ‘‘mushroom’’ or weakly interacting regime (∑ < 1), (2) thecrossover regime (∑ ~ 1), and (3) the highly stretched regime (∑ >1). However, in realsystems, the transition between single grafted chains and a polymer brush is lesssharp because of the statistical characteristic of grafting and polydispersity of thetethered chains

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The term “responsive behavior” is rather a term which reflects applications, andconsequently, there is no universal definition of responsiveness. For many applicationswe suppose to obtain a steep and well noticeable change (switching) of the givenproperty, thus, transitions from the state which can be characterized by some property tothe state with the contra property. Responsiveness of polymer brushes to external stimulirefers to changes of polymer molecule conformations. The size of polymer chains issensitive to its environment. In Θ solvents (attraction and repulsion are compensated)isolated polymer chains of the degree of polymerization N possess ideal coilconformation when <r2>1/2 ~ N1/2. The size of the isolated chain is a function of solventquality (that may be expressed in terms of the χ-Flory-Huggins interaction parameter orΘ temperature).In good solvents and at high polymer concentrations the excluded volume effect (Chains

cannot take the position of other chains) modifies chain conformations substantially. Insemi-diluted polymer solutions the chain size decreases with the 1/8th power of thepolymer volume fraction in a good solvent. In theta solvents the polymer chain size isconcentration independent. In poor solvents bulk polymer solutions undergo a phaseseparation into two phases: almost pure solvent and concentrated polymer solution ofoverlapping Gaussian coils. Both the scaling exponent and the prefactor are sensitive tosolvent quality. Constraints due to the end grafting of the polymer chains introduce aspecific character of the response which is somewhat different from the response ofisolated chains in solution or melt. In the crowded grafted layers (polymer brushes) thechains stretch out of the grafting surface until the excluded volume effect is compensatedby elastic energy (stretching entropy) of polymer coils. Polymer brushes expand in goodsolvents and collapse in poor solvents. The change of characteristic size between goodand poor solvents is much larger for polymer brushes as compared to the polymer chainin solution.

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Mechanism of ResponsivenessThe general idea behind the theoretical description of polymer brushes is that thefree energy F of the chains is obtained from a balance between the interaction energybetween the statistical segments Fint and energy difference between stretched andunstretched polymer chains Fel (elastic free energy) caused by the entropy loss of thechains:

F = Fint + Fel

The most important parameters, which are of interest for a description of brushsystems, are the segment density profile (ϕ(z)) of the surface-attached chains and/orthe brush height h as a function of the graft density σ, the molecular weight (/degreeof polymerization) of the surface-attached chains, and the solvent quality of thecontacting medium (Fig.1).

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(Fig.1)Two hundred chains of a polymer brush (chain length N = 100) under good solvent conditions.

The first description of such a brush system has been attempted by Alexander formonodisperse chains consisting of N segments, which are attached to a flat, non-adsorbing surface with an average distance of the anchor points d much smaller thanthe radius of gyration of the same unperturbed chains not in contact with the surface(Fig.2).

If both the interaction energy resulting from binary monomer– monomer interactionsand the elastic energy of a Gaussian chain are calculated and minimized in respect tothe brush height h, the following equation is obtained for brushes in a good solvent:

In a poor solvent – that is, close to Θ conditions – the exponent describing theinfluence of the grafting density is slightly different and is obtained.

It should be noted, that in both cases the brush height scales linearly with the degreeof polymerization/molecular weight of the polymer molecules, which is a muchstronger dependency than that of the size of a polymer coil in solution on themolecular weight, where the radius of gyration Rg, scales with Rg ~ N0.59 for apolymer in a good solvent and Rg ~ N0.50 for solutions close to Θ conditions.

In addition to these somewhat straightforward calculations, more complicatedsituations have also been tackled where the polymer chains have a distinctpolydispersity, which exhibit a significant curvature also on the molecular scale, andto brushes which carry charges along the polymer chain. In particular, the latter casecan become very complicated if the polymer chains interact specifically with ions inthe surrounding medium, as under these circumstances the situation can no longerbe described by simple mean field approaches, but specific complex formation and(local) changes in the solubility of the polymer play a key role in describing theswelling behavior of such brushes.

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(Fig.2).Schematic illustration of the Alexander model for the

theoretic description of polymer brushes. The chainsegments with the “blobs” (indicated by the circles) behaveas random (“Gaussian”) coils. (d represents the averagedistance between anchor points.)

This slide presents the density profiles vs. distance from the grafting surface atdifferent β-stretching parameter values (1/β = d = <r2>1/2 /h). As the grafting densityincreases from the mushroom regime (β < 1) to the strong stretching limit (β = 100)the profile changes dramatically. The impenetrable grafting surface causes a decreaseof the polymer density close to the grafting surface when the grafting densitymaximum is located in some distance from the grafting surface. This distanceincreases as the grafting density decreases. The grafting density profile is much moresensitive to the brush characteristic at moderate grafting densities as compared to avery high stretching regime.

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Types of polymer brushesThere are many different criteria to classify polymer brushes but based on theconstitution we have following types of polymer brushes:

Homopolymer brushesPolyelectrolyte brushesBlock copolymer brushesMolecular brushesReversible Self assembled brush

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Typically there are three main methods for synthesising polymer brushes:

•Grafting onto (grafting to)•Grafting from•Grafting through

Amonge these three, the first two one are more important ,so some of the advantageand disadvantages have been mentioned above.

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In Grafting onto method a polymer chain which has a functional group at the enddiffuses through the surface, on the surface there are other functional groups , whichcan react and therefore chain will graft to the surface.It has to be metioned that due to the stereochemical hinderence , density of graftingin this method is not high.

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In Grafting from method a proper initiater is attach to the surface first, and thensurface will encounter with the monomer at approprate condition for polymerizationin side the reactor. At the polymerization media chains will grow on the surface,while they have been grafted to it from the begining.

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The uniformed polymer chain growth, which leads to low polydispersity, stems fromthe transition metal based catalyst. This catalyst provides an equilibrium betweenactive, and therefore propagating, polymer and an inactive form of the polymer;known as the dormant form. Since the dormant state of the polymer is vastlypreferred in this equilibrium, side reactions are suppressed.This equilibrium in turn lowers the concentration of propagating radicals, thereforesuppressing unintentional termination and controlling molecular weights.There are five important variable components of Atom Transfer RadicalPolymerizations. They are the monomer, initiator, catalyst, solvent and temperature.The following section breaks down the contributions of each component to theoverall polymerization.

MonomerMonomers that are typically used in ATRP are molecules with substituents that canstabilize the propagating radicals; for example, styrenes, (meth)acrylates,(meth)acrylamides, and acrylonitrile. ATRP are successful at leading to polymers ofhigh number average molecular weight and a narrow polydispersity index when theconcentration of the propagating radical balances the rate of radical termination. Yet,the propagating rate is unique to each individual monomer. Therefore, it is importantthat the other components of the polymerization (initiator, catalysts, ligands andsolvents) are optimized in order for the concentration of the dormant species to begreater than the concentration of the propagating radical and yet not too great toslow down or halt the reaction.

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InitiatorThe number of growing polymer chains is determined by the initiator. The faster theinitiation, the fewer terminations and transfers, the more consistent the number ofpropagating chains leading to narrow molecular weight distributions. Organic halidesthat are similar in the organic framework as the propagating radical are often chosenas initiators.[Alkyl halides such as alkyl bromides are more reactive than alkylchlorides and both have good molecular weight control.

CatalystThe catalyst is the most important component of ATRP because it determines theequilibrium constant between the active and dormant species. This equilibriumdetermines the polymerization rate and an equilibrium constant too small may inhibitor slow the polymerization while an equilibrium constant too large leads to a highdistribution of chain lengths.There are several requirements for the metal catalyst:there needs to be two accessible oxidation states that are separated by one electronthe metal center needs to have a reasonable affinity for halogensthe coordination sphere of the metal needs to be expandable when its oxidized so tobe able to accommodate the halogena strong ligand complexation.The most studied catalysts are those that polymerizations involving copper, which hasshown the most versatility, showing successful polymerizations regardless of themonomer.

SolventToluene,1,4-dioxane

Temperature

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ATRP has been conducted from a range of surfaces since the concept was firstdisclosed, Because of their appearance these materials have been called polymerbrushes. The two most common types of polymer brushes are illustrated above andhave been formed by both "grafting from" and "grafting to" inorganic particles andflat surfaces. The synthesis of organic/inorganic hybrid materials is an area of growinginterest as the useful properties of disparate components can be combined into asingle material.Spherical particles:

Organic/inorganic hybrid nanoparticles containing an inorganic core and tetheredglassy or rubbery homopolymers or copolymers have been prepared by the ATRP ofstyrene and (meth)acrylates from colloidal initiators.Flat Surfaces:

Modification of surfaces with thin polymeric films allows one to tailor surfaceproperties such as wetability, biocompatibility, biocidal activity, adhesion, adsorption,corrosion resistance and friction. Polymers with reactive groups or segments can beprepared for "grafting onto" surfaces or functional groups can be attached to thesurface for a more efficient "grafting from" approach. The properties of surfaces areaddressed elsewhere on this site in this section we primarily address "grafting from"surface tethered initiators. It is also possible to prepare block copolymers where oneor more segments of the block copolymer had been prepared by a non-CRPprocedure. The only requirement is to ensure the terminal functional groups presenton the initial functional polymer can be converted into radically transferable atom(s)for the second controlled ATRP step

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Diffrent products with diffrent morphology, for diffrent sufesticated applycations canbe produced by ATRP.Many researching groups are working in this area, therefore just for making animpression aboat visatility of ATRP the following slides have made.In each photo the mode of grafts are diffrent.

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The major objective for the application of responsive polymer brushes is to regulate,adjust, and switch interaction forces between the brush and its environment constitutedof liquid, vapor, solid, another brush, particles, etc. The simplest formulation of theResponsive Polymer Brushes problem is switching between attraction and repulsion. Forexample, the polymer brush like layer stabilizes colloidal dispersion, however, uponchange of its environment the colloid coagulates because the repulsive forces of thebrush have been “switched off.” This simple effect has numerous important applicationsin various technologies and it is not fully explored and engineered yet. The same simpleproblem is important if the friction coefficient, adhesion, or wetting could be rapidlychanged to switch off and on capillary flow, cell adhesion, protein adsorption, cellgrowth, membrane permeability, and drug release.One of the targets is the application of the responsive brushes for smart devices such asdrug delivery devices, microfludic analytical devices, and sensors. Smart drug deliverydevices are seen as a drug loaded capsule coated with a brush-like shell. Expansion andshrinking of responsive polymer brushes can be used to fabricate mechanical actuators.The effect of switching of wetting behavior of the mixed weak PEL brushes upon achange of pH was recently explored for the fabrication of “smart” microfluidic devices.The passage of liquids through the microfluidic channels was regulated by responsivenessof the mixed brushes of different compositions. Reversible changes of mixed brushmorphologies in solvents of different thermodynamic quality were used for the motion ofnanoparticles deposited on the brush surface. The simplest device which explorespolymer brush responsiveness is a sensor working on the principle of the brushexpansion–collapse transitions upon changes in its environment. Currently, research isfocused on how the interactions with polymer brushes may be precisely tuned andmonitored in a controlled environment.

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Reversible Cantilever actuation by PEL-Brushes

The bending of microcantilevers upon adsorption of polymers (DNA, proteins) orsmall molecules has great potential for the development of highly sensitive sensorsand efficient nanoactuators. For microcantilevers to be useful as actuators, precisepositioning, reversibility, and large-scale bending are prerequisites. Conventionalmodification by self-assembled monolayers (SAMs) usually generates small cantileverdeflections. By grafting polymers to the cantilever surface, a much wider range ofresponses can be achieved due to conformational changes in the polymer backbones,and recently, the bending of pH responsive copolymer brush-coated AFM cantileverswas studied under different conditions. However, reversible and multi-stage actuationof cantilevers remains a significant challenge. The use of polyelectrolytes and theircollapse in response to salt has recently emerged as a promising potential syntheticequivalent of one of the most powerful biological motors: the spasmoneme spring.

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Polyelectrolyte brushes

Polyelectrolytes are polymers whose repeating units bear an electrolyte group. Thesegroups will dissociate in aqueous solutions (water), making the polymers charged.Polyelectrolyte properties are thus similar to both electrolytes (salts) and polymers(high molecular weight compounds), and are sometimes called polysalts. PEL inaqueous solutions attract great interest because of their relevance to many biologicalsystems. Interactions which involve charged macromolecules are strongly modified byCoulomb forces. The charge density on a polymer chain in a polar solvent depends onthe chain constitution and degree of dissociation (f) of ionizable groups. If ionizablegroups are strong acids or bases (strong PE) f is equal to 1 and is not affected by theenvironment. If ionizable groups are weak acids or bases (weak PEL) f depends onlocal pH. For the latter case charges are mobile within the polymer chain.

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For the dense strong PEL brush (high f and grafting density) all counterions aretrapped inside the brush (Slide.b). The brush height is determined by the balancebetween osmotic pressure of the trapped counter-ions and the stretching entropy ofthe chains (so called osmotic brush regime). The contribution of the excluded volumeeffect depends on the grafting density. At very high densities the excluded volumeeffect may dominate while at moderate densities the electrostatic nature will have amajor contribution. The latter will be reflected in the prefactor in the scalingrelationship h ~ N. These brushes are insensitive to local pH. Added salt does notaffect the brush unless the ionic strength of the solution approaches the level of theionic strength inside the brush (Slide.d). In that case the prefactor is an inverse cubicroot function of the external salt concentration and the grafting density (so calledsalted brush regime). Thus, in terms of responsive applications strong PEL areinteresting for design of responsiveness to humid and aqueous environments whenthe high swelling of the brush in water or a humid atmosphere is resulted from strongosmotic pressure of trapped counterions (Slide. a,b). Weak PEL brushes representone of the most interesting responsive behaviors. They demonstrate responsivenessto changes in external pH and ionic strength. Weak PEL brushes carrying basicfunctionalities expand upon a decrease of pH, while acidic PEL brushes expand uponan increase of pH (Slide. c, d). At a high salt concentration weak PEL brushes shrinkdue to the same mechanism as strong PEL brushes. However, it is noteworthy, that insome range of pH values they shrink also at no salt added or at very small saltconcentrations, thus, expressing non-monotonous dependence of the brush heightvs. salt concentration. This behavior originates from the sensitivity of f for weakPEL(s) to the local electric field.

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Slide shows the experimental setup to measure the deflection of cantilever whileswitching between different environments.

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Behaviour of PMEP brush modified cantilever

The conformational changes of the brushes in response to salt solution or pH areschematically shown in Scheme 1.

Scheme 1. Schematic of Reversible Swollen/Collapse of PMEP Brush

PMEP can be switched between three ionic states: fully protonated,monoprotonated/ monobasic, and dipotassium salt/dibasic states, dependingdepending on pH. Slide (above) displays the bending of brush-coated cantileverswhen varying the pH of the solution between 1 and 13. In region I, the brushes arefully protonated, while in region III, they are fully deprotonated, and compressivestress is generated in both strongly acidic (pH < 2) and basic (pH > 8) environments.At pH < 2, the protonated brushes are no longer soluble and will collapse, generatinga compressive surface stress since the “footprint” of the polymers is too small toaccommodate the collapsing chain. This effect is consistent with previous reports thatpolymer brushes generate a compressive surface stress upon polymer collapse.

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At pH > 8, the PMEP brushes are fully deprotonated, and the electrostatic repulsionbetween charged polymer chains leads to the development of a large compressivestress. The maximum deflection of the cantilevers, up to micrometer scale(approximate 1300 nm), is found in this fully charged state. It should be noted thatthe cantilever deflections are highly reversible, and that the brushes can be cycledthrough a number of pH cycles. The magnitude and sensitivity of the response to saltdepend strongly on the length of the brushes, the grafting density, and the degree ofcharging of the polymer. Generally there is no or very small deflection for low (<10%initiator) grafting densities of brushes.

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Slide shows the reversible bending and return to equilibrium position of the brush-coated cantilever when switching between a 100 mM KCl solution and pure water,respectively. The response of the cantilever to changes of solution is very fast (30 s).The return to zero deflection upon addition of water is slow due to the slow diffusionof excess salt away from the brush layer. The compressive stress is generated by thebrushes collapsing under the influence of the high salt environment; this situation issimilar to the compressive stress generated at low pH (see above). The controlexperiments (black line) show that non-brush-modified cantilevers show no responseto changes in salt concentration.

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Control over the actual position of cantilever can be achieved by exposing the brush-coated cantilevers to different salt concentrations between 0 and 100 mM. Bygradually increasing or decreasing the salt concentration, the actuation can beprecisely manipulated in discrete multiple steps. By plotting the bending of thecantilever versus the salt concentration (slide), or to the logarithm of the saltconcentration (inset), one can distinguish two distinct response regimes. At saltconcentrations below 1 mM, the response is small (remaining below 10% ofmaximum bending amplitude), whereas at higher concentrations, a much largerresponse is observed. Conversely, when lowering the salt concentration, we can seean approximately linear dependence of log[salt] versus normalized bendingamplitude. Polyelectrolyte brush theory predicts that for annealed brushes at low saltconcentrations the brush heights (and therefore surface stress) first increase slightlydue to the exchange between external cations and associated protons. At higherconcentrations, charge screening (removal) is the dominant effect, leading to collapseof the brushes and generation of much more significant compressive stress. It shouldbe noted that the chemical nature of the ions (valency, lipophilicity, etc.) alsoinfluences the collapse process, opening up possibilities for selectivity.

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Conclusion

The field of responsive polymer brushes is a continuously expanding area of research.The expansion is not very fast because of the complexity of the systems for thefabrication as well as for investigations. Nevertheless, the continuous and successfuldevelopment of the field is predetermined by the fact that the polymer brushes arethe most effective structures to regulate complex interactions in synthetic colloidaland natural living systems. The potential for the design of the interactions is veryhigh. Mimicking natural systems and designing new structures will accompany thedevelopment of the field of polymer brushes. That will stimulate expansion oftheoretical and experimental investigations. We may also benefit from thecombination of polymer brushes and gels in complex responsive devices.

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