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Polymer Brushes


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Inrecentyears,thesynthesisofpolymerbrusheshasreceivedsignificantattention.Inthispresentationwetalkedaboutseveraldifferentaspectsofpolymerbrushesandsyntheticstrategiesforthegenerationofpolymerbrushes.F …


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  • 1. In recent years, the synthesis of polymer brushes has received significant attention. In this presentation we talked about several different aspects of polymer brushes and synthetic strategies for the generation of polymer brushes . Finally, example is provided that highlight some recent developments aimed at strategies for the functionalization of surfaces with polymer brushes, at ways of realizing smart surfaces with switchable properties. 1
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  • 3. Polymer brushes A polymer brush consists of end-tethered (grafted, anchored) polymer chains stretched away from the substrate so that in the given solvent the brush height (h) is larger as compared to the end-to-end distance (<r2>1/2) of the same non-grafted chains dissolved in the same solvent. In polymer brushes the distance between grafting points (d) is smaller than the chain end-to-end distance. Polymer brushes can be introduced as thin films of end-grafted polymer molecules when the following conditions are satisfied: h> <r2>1/2, d < <r2>1/2. Outside these conditions the grafted layers are in the “mushroom regime” (see next slide). 3
  • 4. Polymer Brushes: General Features When polymer molecules are tethered (grafted) to a surface, two basic cases must be distinguished 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 other and behave more or less like single chains “nailed” down onto the surface by one end. Depending on the strength of interaction of the polymer segments with the surface, again two cases must be distinguished. If the interaction between the polymer and the surface is weak (or even repulsive), the chains form a typical random coil 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 the segments 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 surface at such short distances between the anchor points that the polymer molecules overlap. In this case, the segments of the chains try to avoid each other as much as possible and minimize segment–segment interactions by stretching away from the surface (slide). This chain stretching, however, reduces the number of possible polymer conformations, which is equivalent to a reduction in the entropy of the chains. 4
  • 5. This loss of entropy gives rise to a retracting force trying to keep the chains coiled, as occurs in a stretched piece of rubber. Thus, a new equilibrium at a higher energy level is obtained in which the chains are stretched perpendicular to the surface. The structure of a surface-immobilized polymer can be evaluated by the inverse value of the distance between grafting points (D). As the size of grafted polymer chains approaches the distance between grafting points, the grafted chains overlap. This point is a transition point between a single grafted chain (mushroom) regime and brush regime. A commonly used literature parameter for quantitative characterization of this transition is the reduced tethered density where Rg is radius of gyration of a tethered chain at specific experimental conditions of 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 brush formation: (1) the ‘‘mushroom’’ or weakly interacting regime (∑ < 1), (2) the crossover regime (∑ ~ 1), and (3) the highly stretched regime (∑ >1). However, in real systems, the transition between single grafted chains and a polymer brush is less sharp because of the statistical characteristic of grafting and polydispersity of the tethered chains 5
  • 6. The term “responsive behavior” is rather a term which reflects applications, and consequently, there is no universal definition of responsiveness. For many applications we suppose to obtain a steep and well noticeable change (switching) of the given property, thus, transitions from the state which can be characterized by some property to the state with the contra property. Responsiveness of polymer brushes to external stimuli refers to changes of polymer molecule conformations. The size of polymer chains is sensitive to its environment. In Θ solvents (attraction and repulsion are compensated) isolated polymer chains of the degree of polymerization N possess ideal coil conformation when <r2>1/2 ~ N1/2. The size of the isolated chain is a function of solvent quality (that may be expressed in terms of the χ-Flory-Huggins interaction parameter or Θ 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. In semi-diluted polymer solutions the chain size decreases with the 1/8th power of the polymer volume fraction in a good solvent. In theta solvents the polymer chain size is concentration independent. In poor solvents bulk polymer solutions undergo a phase separation into two phases: almost pure solvent and concentrated polymer solution of overlapping Gaussian coils. Both the scaling exponent and the prefactor are sensitive to solvent quality. Constraints due to the end grafting of the polymer chains introduce a specific character of the response which is somewhat different from the response of isolated chains in solution or melt. In the crowded grafted layers (polymer brushes) the chains stretch out of the grafting surface until the excluded volume effect is compensated by elastic energy (stretching entropy) of polymer coils. Polymer brushes expand in good solvents and collapse in poor solvents. The change of characteristic size between good and poor solvents is much larger for polymer brushes as compared to the polymer chain in solution. 6
  • 7. Mechanism of Responsiveness The general idea behind the theoretical description of polymer brushes is that the free energy F of the chains is obtained from a balance between the interaction energy between the statistical segments Fint and energy difference between stretched and unstretched polymer chains Fel (elastic free energy) caused by the entropy loss of the chains: F = Fint + Fel The most important parameters, which are of interest for a description of brush systems, are the segment density profile (ϕ(z)) of the surface-attached chains and/or the brush height h as a function of the graft density σ, the molecular weight (/degree of polymerization) of the surface-attached chains, and the solvent quality of the contacting medium (Fig.1). (Fig.1)Two hundred chains of a polymer brush (chain length N = 100) under good solvent conditions. 7
  • 8. The first description of such a brush system has been attempted by Alexander for monodisperse 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 than the radius of gyration of the same unperturbed chains not in contact with the surface (Fig.2). (Fig.2).Schematic illustration of the Alexander model for the theoretic description of polymer brushes. The chain segments with the “blobs” (indicated by the circles) behave as random (“Gaussian”) coils. (d represents the average distance between anchor points.) If both the interaction energy resulting from binary monomer– monomer interactions and the elastic energy of a Gaussian chain are calculated and minimized in respect to the 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 the influence 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 degree of polymerization/molecular weight of the polymer molecules, which is a much stronger dependency than that of the size of a polymer coil in solution on the molecular weight, where the radius of gyration Rg, scales with Rg ~ N0.59 for a polymer in a good solvent and Rg ~ N0.50 for solutions close to Θ conditions. In addition to these somewhat straightforward calculations, more complicated situations have also been tackled where the polymer chains have a distinct polydispersity, which exhibit a significant curvature also on the molecular scale, and to brushes which carry charges along the polymer chain. In particular, the latter case can become very complicated if the polymer chains interact specifically with ions in the surrounding medium, as under these circumstances the situation can no longer be 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 the swelling behavior of such brushes. 8
  • 9. This slide presents the density profiles vs. distance from the grafting surface at different β-stretching parameter values (1/β = d = <r2>1/2 /h). As the grafting density increases from the mushroom regime (β < 1) to the strong stretching limit (β = 100) the profile changes dramatically. The impenetrable grafting surface causes a decrease of the polymer density close to the grafting surface when the grafting density maximum is located in some distance from the grafting surface. This distance increases as the grafting density decreases. The grafting density profile is much more sensitive to the brush characteristic at moderate grafting densities as compared to a very high stretching regime. 9
  • 10. Types of polymer brushes There are many different criteria to classify polymer brushes but based on the constitution we have following types of polymer brushes: Homopolymer brushes Polyelectrolyte brushes Block copolymer brushes Molecular brushes Reversible Self assembled brush 10
  • 11. 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 advantage and disadvantages have been mentioned above. 11
  • 12. In Grafting onto method a polymer chain which has a functional group at the end diffuses through the surface, on the surface there are other functional groups , which can react and therefore chain will graft to the surface. It has to be metioned that due to the stereochemical hinderence , density of grafting in this method is not high. 12
  • 13. In Grafting from method a proper initiater is attach to the surface first, and then surface will encounter with the monomer at approprate condition for polymerization in side the reactor. At the polymerization media chains will grow on the surface, while they have been grafted to it from the begining. 13
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  • 15. The uniformed polymer chain growth, which leads to low polydispersity, stems from the transition metal based catalyst. This catalyst provides an equilibrium between active, and therefore propagating, polymer and an inactive form of the polymer; known as the dormant form. Since the dormant state of the polymer is vastly preferred in this equilibrium, side reactions are suppressed. This equilibrium in turn lowers the concentration of propagating radicals, therefore suppressing unintentional termination and controlling molecular weights. There are five important variable components of Atom Transfer Radical Polymerizations. They are the monomer, initiator, catalyst, solvent and temperature. The following section breaks down the contributions of each component to the overall polymerization. Monomer Monomers that are typically used in ATRP are molecules with substituents that can stabilize the propagating radicals; for example, styrenes, (meth)acrylates, (meth)acrylamides, and acrylonitrile. ATRP are successful at leading to polymers of high number average molecular weight and a narrow polydispersity index when the concentration of the propagating radical balances the rate of radical termination. Yet, the propagating rate is unique to each individual monomer. Therefore, it is important that the other components of the polymerization (initiator, catalysts, ligands and solvents) are optimized in order for the concentration of the dormant species to be greater than the concentration of the propagating radical and yet not too great to slow down or halt the reaction. 15
  • 16. Initiator The number of growing polymer chains is determined by the initiator. The faster the initiation, the fewer terminations and transfers, the more consistent the number of propagating chains leading to narrow molecular weight distributions. Organic halides that are similar in the organic framework as the propagating radical are often chosen as initiators.[Alkyl halides such as alkyl bromides are more reactive than alkyl chlorides and both have good molecular weight control. Catalyst The catalyst is the most important component of ATRP because it determines the equilibrium constant between the active and dormant species. This equilibrium determines the polymerization rate and an equilibrium constant too small may inhibit or slow the polymerization while an equilibrium constant too large leads to a high distribution of chain lengths. There are several requirements for the metal catalyst: there needs to be two accessible oxidation states that are separated by one electron the metal center needs to have a reasonable affinity for halogens the coordination sphere of the metal needs to be expandable when its oxidized so to be able to accommodate the halogen a strong ligand complexation. The most studied catalysts are those that polymerizations involving copper, which has shown the most versatility, showing successful polymerizations regardless of the monomer. Solvent Toluene,1,4-dioxane Temperature 16
  • 17. ATRP has been conducted from a range of surfaces since the concept was first disclosed, Because of their appearance these materials have been called polymer brushes. The two most common types of polymer brushes are illustrated above and have been formed by both "grafting from" and "grafting to" inorganic particles and flat surfaces. The synthesis of organic/inorganic hybrid materials is an area of growing interest as the useful properties of disparate components can be combined into a single material. Spherical particles: Organic/inorganic hybrid nanoparticles containing an inorganic core and tethered glassy or rubbery homopolymers or copolymers have been prepared by the ATRP of styrene and (meth)acrylates from colloidal initiators. Flat Surfaces: Modification of surfaces with thin polymeric films allows one to tailor surface properties such as wetability, biocompatibility, biocidal activity, adhesion, adsorption, corrosion resistance and friction. Polymers with reactive groups or segments can be prepared for "grafting onto" surfaces or functional groups can be attached to the surface for a more efficient "grafting from" approach. The properties of surfaces are addressed elsewhere on this site in this section we primarily address "grafting from" surface tethered initiators. It is also possible to prepare block copolymers where one or more segments of the block copolymer had been prepared by a non-CRP procedure. The only requirement is to ensure the terminal functional groups present on the initial functional polymer can be converted into radically transferable atom(s) for the second controlled ATRP step 17
  • 18. 18
  • 19. Diffrent products with diffrent morphology, for diffrent sufesticated applycations can be produced by ATRP. Many researching groups are working in this area, therefore just for making an impression aboat visatility of ATRP the following slides have made. In each photo the mode of grafts are diffrent. 19
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  • 22. The major objective for the application of responsive polymer brushes is to regulate, adjust, and switch interaction forces between the brush and its environment constituted of liquid, vapor, solid, another brush, particles, etc. The simplest formulation of the Responsive Polymer Brushes problem is switching between attraction and repulsion. For example, the polymer brush like layer stabilizes colloidal dispersion, however, upon change of its environment the colloid coagulates because the repulsive forces of the brush have been “switched off.” This simple effect has numerous important applications in various technologies and it is not fully explored and engineered yet. The same simple problem is important if the friction coefficient, adhesion, or wetting could be rapidly changed to switch off and on capillary flow, cell adhesion, protein adsorption, cell growth, membrane permeability, and drug release. One of the targets is the application of the responsive brushes for smart devices such as drug delivery devices, microfludic analytical devices, and sensors. Smart drug delivery devices are seen as a drug loaded capsule coated with a brush-like shell. Expansion and shrinking 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 a change of pH was recently explored for the fabrication of “smart” microfluidic devices. The passage of liquids through the microfluidic channels was regulated by responsiveness of the mixed brushes of different compositions. Reversible changes of mixed brush morphologies in solvents of different thermodynamic quality were used for the motion of nanoparticles deposited on the brush surface. The simplest device which explores polymer brush responsiveness is a sensor working on the principle of the brush expansion–collapse transitions upon changes in its environment. Currently, research is focused on how the interactions with polymer brushes may be precisely tuned and monitored in a controlled environment. 22
  • 23. Reversible Cantilever actuation by PEL-Brushes The bending of microcantilevers upon adsorption of polymers (DNA, proteins) or small molecules has great potential for the development of highly sensitive sensors and efficient nanoactuators. For microcantilevers to be useful as actuators, precise positioning, reversibility, and large-scale bending are prerequisites. Conventional modification by self-assembled monolayers (SAMs) usually generates small cantilever deflections. By grafting polymers to the cantilever surface, a much wider range of responses can be achieved due to conformational changes in the polymer backbones, and recently, the bending of pH responsive copolymer brush-coated AFM cantilevers was studied under different conditions. However, reversible and multi-stage actuation of cantilevers remains a significant challenge. The use of polyelectrolytes and their collapse in response to salt has recently emerged as a promising potential synthetic equivalent of one of the most powerful biological motors: the spasmoneme spring. 23
  • 24. Polyelectrolyte brushes Polyelectrolytes are polymers whose repeating units bear an electrolyte group. These groups 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 in aqueous solutions attract great interest because of their relevance to many biological systems. Interactions which involve charged macromolecules are strongly modified by Coulomb forces. The charge density on a polymer chain in a polar solvent depends on the chain constitution and degree of dissociation (f) of ionizable groups. If ionizable groups are strong acids or bases (strong PE) f is equal to 1 and is not affected by the environment. If ionizable groups are weak acids or bases (weak PEL) f depends on local pH. For the latter case charges are mobile within the polymer chain. 24
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  • 26. For the dense strong PEL brush (high f and grafting density) all counterions are trapped inside the brush (Slide.b). The brush height is determined by the balance between osmotic pressure of the trapped counter-ions and the stretching entropy of the chains (so called osmotic brush regime). The contribution of the excluded volume effect depends on the grafting density. At very high densities the excluded volume effect may dominate while at moderate densities the electrostatic nature will have a major contribution. The latter will be reflected in the prefactor in the scaling relationship h ~ N. These brushes are insensitive to local pH. Added salt does not affect the brush unless the ionic strength of the solution approaches the level of the ionic strength inside the brush (Slide.d). In that case the prefactor is an inverse cubic root function of the external salt concentration and the grafting density (so called salted brush regime). Thus, in terms of responsive applications strong PEL are interesting for design of responsiveness to humid and aqueous environments when the high swelling of the brush in water or a humid atmosphere is resulted from strong osmotic pressure of trapped counterions (Slide. a,b). Weak PEL brushes represent one of the most interesting responsive behaviors. They demonstrate responsiveness to changes in external pH and ionic strength. Weak PEL brushes carrying basic functionalities expand upon a decrease of pH, while acidic PEL brushes expand upon an increase of pH (Slide. c, d). At a high salt concentration weak PEL brushes shrink due to the same mechanism as strong PEL brushes. However, it is noteworthy, that in some range of pH values they shrink also at no salt added or at very small salt concentrations, thus, expressing non-monotonous dependence of the brush height vs. salt concentration. This behavior originates from the sensitivity of f for weak PEL(s) to the local electric field. 26
  • 27. Slide shows the experimental setup to measure the deflection of cantilever while switching between different environments. 27
  • 28. Behaviour of PMEP brush modified cantilever The conformational changes of the brushes in response to salt solution or pH are schematically 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, depending depending on pH. Slide (above) displays the bending of brush-coated cantilevers when varying the pH of the solution between 1 and 13. In region I, the brushes are fully protonated, while in region III, they are fully deprotonated, and compressive stress 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, generating a compressive surface stress since the “footprint” of the polymers is too small to accommodate the collapsing chain. This effect is consistent with previous reports that polymer brushes generate a compressive surface stress upon polymer collapse. 28
  • 29. At pH > 8, the PMEP brushes are fully deprotonated, and the electrostatic repulsion between charged polymer chains leads to the development of a large compressive stress. The maximum deflection of the cantilevers, up to micrometer scale (approximate 1300 nm), is found in this fully charged state. It should be noted that the cantilever deflections are highly reversible, and that the brushes can be cycled through a number of pH cycles. The magnitude and sensitivity of the response to salt depend strongly on the length of the brushes, the grafting density, and the degree of charging of the polymer. Generally there is no or very small deflection for low (<10% initiator) grafting densities of brushes. 29
  • 30. 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 diffusion of excess salt away from the brush layer. The compressive stress is generated by the brushes collapsing under the influence of the high salt environment; this situation is similar to the compressive stress generated at low pH (see above). The control experiments (black line) show that non-brush-modified cantilevers show no response to changes in salt concentration. 30
  • 31. 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. By gradually increasing or decreasing the salt concentration, the actuation can be precisely manipulated in discrete multiple steps. By plotting the bending of the cantilever versus the salt concentration (slide), or to the logarithm of the salt concentration (inset), one can distinguish two distinct response regimes. At salt concentrations below 1 mM, the response is small (remaining below 10% of maximum bending amplitude), whereas at higher concentrations, a much larger response is observed. Conversely, when lowering the salt concentration, we can see an approximately linear dependence of log[salt] versus normalized bending amplitude. Polyelectrolyte brush theory predicts that for annealed brushes at low salt concentrations the brush heights (and therefore surface stress) first increase slightly due to the exchange between external cations and associated protons. At higher concentrations, charge screening (removal) is the dominant effect, leading to collapse of the brushes and generation of much more significant compressive stress. It should be noted that the chemical nature of the ions (valency, lipophilicity, etc.) also influences the collapse process, opening up possibilities for selectivity. 31
  • 32. 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 the fabrication as well as for investigations. Nevertheless, the continuous and successful development of the field is predetermined by the fact that the polymer brushes are the most effective structures to regulate complex interactions in synthetic colloidal and natural living systems. The potential for the design of the interactions is very high. Mimicking natural systems and designing new structures will accompany the development of the field of polymer brushes. That will stimulate expansion of theoretical and experimental investigations. We may also benefit from the combination of polymer brushes and gels in complex responsive devices. 32
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