Responsive Polymer Brushes

SERGIY MINKO

Department of Chemistry and Biomolecular Science, Clarkson University,

Potsdam, NY, USA

The review focuses on responsive/switching behavior of polymer brushes. Thestructure of the polymer brushes is analyzed in terms of the constitution and confor-mation of polymer chains. Responsive properties are widely discussed based onphase transition mechanisms in a controlled environment when changes are inducedby solvent quality, temperature, concentration of ions, and interactions with liquidsand solids. Potential applications of polymer brushes are also discussed.

Keywords response, grafted layers, phase transition, interactions, applications

Introduction

A polymer brush1 – 4 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 (kr2l1/2) 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 . kr2l1/2, d , kr2l1/2. Outside these conditions the grafted layers are in the

“mushroom regime”.

The chains can be grafted to different substrates that may result in planar brushes

(Fig. 1) for planar or “reasonably flat” substrates (root mean square roughness should be

much smaller than end-to-end distance of grafted polymer chains), cylindrical brushes

(Fig. 2) for fiber- or rod-like substrates (the substrate in this cases can be as small in

radius R as a polymer chain backbone. In this case the brushes are called “molecular

brushes”), and spherical brushes for substrates of spherical shape (Fig. 3). For the cylind-

rical and spherical brushes two different regimes can be selected: (1) h . R and (2) h , R

(R is the radius of curvature of the grafting surface). The substrates can be in any physical

state: solid, liquid, or gas (for example foams stabilized by an amphiphilic block-

copolymer). The grafting points may diffuse laterally on the substrate surface. The

grafting can be realized through covalent bonds, electrostatic interactions, hydrogen

bonds, or strong van der Waals interactions. However, weak interactions are less interest-

ing. The elastic energy component accumulated in the polymer brush due to the stretching

may destroy weak bonds. Soft substrates may be deformed by the brush.5

Received 14 March 2006; Accepted 31 May 2006.Address correspondence to Sergiy Minko, Department of Chemistry and Biomolecular Science,

Clarkson University, 8 Clarkson Ave, Potsdam, NY 13699, USA. E-mail: sminko@clarkson.edu

Journal of Macromolecular Sciencew, Part C: Polymer Reviews, 46:397–420, 2006

Copyright # Taylor & Francis Group, LLC

ISSN 1558-3724 print/1558-3716 online

DOI: 10.1080/15583720600945402

397

Responsiveness of polymer brushes to external stimuli refer to changes of polymer

molecule conformations.6 Most research reports on responses affected by changes of

macroconformations in solutions and melts when the segmental mobility is reasonably

high. The conformational changes in specially tailored polymer brush systems are used

in materials and devices to approach demanded effects of switching material properties

upon external signals. Theory predicts very large potential to regulate and tune complex

interactions between brush-coated surfaces.7

It is noteworthy that polymer brushes represent well-defined grafted polymer chain

architecture when the structure (end-grafted chains) is not very complex as compared to

other grafted polymer layers (randomly grafted chains, LBL thin polymer films, grafted

composite/hybrid films, etc). Polymer brushes can be analyzed theoretically more easily.

Interpretations of the experimental study of polymer brushes are based on the well-

developed theoretical background that facilitates intensive investigations of these responsive

systems. The exponential increase of the number of publications on polymer brushes (around

400 papers in 2005 vs. 150 in 2000 and 70 in 1995) is good evidence for that.

Responsiveness in Polymer Systems

As in any polymer system, polymer brushes respond to their environment by a change of

the polymer chain conformations. The size of polymer chains is sensitive to its

Figure 1. Planar homopolymer brush: a homogeneous smooth layer of stretched chains in good

solvent (a), pinned micelles (b) and a layer of collapsed chains (c) in poor solvent.

Figure 2. Cylindrical brushes and their responsive conformational changes: bending, contraction,

compacting, and coiling. (Reproduced from Ref. 50, Copyright 2006, with permission from Black-

well Publishing).

S. Minko398

environment. In Q solvents (attraction and repulsion are compensated) isolated

polymer chains of the degree of polymerization N possess ideal coil conformation when

kr2l1/2 � N1/2. The size of the isolated chain is a function of solvent quality (that may

be expressed in terms of the x-Flory-Huggins interaction parameter or Q-

temperature). In a good solvent the extended chains have self-avoiding coil statistics:

kr2l1/2 � N0.588. The isolated chain in a poor solvent collapses into a globule:

kr2l1/2 � N1/3. In good solvents and at high polymer concentrations the excluded

volume effect 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.8 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

Figure 3. Spherical brushes: collapsed and stretched chains of a homopolymer brush in poor and

good solvents, respectively (a); polyelectrolyte (PEL) brush swells when ionizable groups dissociate

in a polar solvent (b); depending on solvent selectivity a mixed two-component brush switches

between the state when chains of the polymer A are stretched and chains of the polymer B are

collapsed (left) and the state when the chains of the polymer B are stretched and the chains of the

polymer A are collapsed (right) (c).

Responsive Polymer Brushes 399

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. For some

applications it is important that the grafting points cannot diffuse laterally.

The specific goal of this review is to analyze responsive brushes. There is not an

obvious definition which changes of polymer brush properties should be recognized as a

responsive behavior. Any material is responsive since changes of its environment always

cause some changes of material properties. 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. For example, we

may mention the transitions from wetting to non wetting, from swollen to collapsed

brush, from adhesive to non adhesive, from adsorbing to non adsorbing, from attractive

to repulsive, etc. Typically (but not necessarily), such changes of properties are

accompanied by phase transitions in polymeric systems. These transitions are assigned as

responsive properties of polymer brushes. For some applications even a small change of

brush properties may have a dramatic effect. This change may be also assigned as a respon-

sive property of the brush. Consequently, we may agree that the response of polymer

brushes is a noticeable effect for the given application of the brush if this effect appears

as a change of the brush properties introduced by a change of the brush environment.

Mechanisms of Responses

Homopolymer Brushes

In scaling theory and self-consistent field theory8,9 the thickness of the polymer brush is

linearly proportional to N: h � N (but not obviously in a bad solvent). Experiments

proved that this relationship retains unchanged for solvents of different quality.10

However, the prefactor depends on solvent quality and grafting density of polymer

chains. Besides the chain constitution, the grafting density is the parameter which

affects conformational responsiveness of polymer brushes. At small grafting densities

(so called mushroom regime) the response of grafted chains is very similar to that of

the bulk polymer solution.8,9 At high grafting densities the collapse is weak and the

brush forms a homogeneous layer which is slightly thinner in a poor solvent than in a

good solvent or in the Q regime. That is because of the very crowded layer of strongly

stretched polymer chains when there is no free space for conformational changes. In the

grafting density regime which lays between very low and high surface coverage

regimes the brushes are unstable in poor solvents and form clusters (pinned micelles)

on the surface (Fig. 1b) to avoid unfavorable interactions with the poor solvents.11,12 In

a good solvent the brush is swollen and it forms a homogeneous layer of stretched

tethered chains (Fig. 1a). In this grafting density regime (cross-over regime) the

polymer brush demonstrates the most pronounced response to solvent quality as it is

obvious from the graph in Fig. 4.13 Thus, we may conclude that the largest conformational

response in a polymer brush is in the very beginning of the brush regime when chains start

to overlap. Switching a range of the thin polymer film properties relevant to density and

thickness of the film can be easily approached with the brushes of moderate grafting

densities. It is noteworthy, that the optimal grafting density may be affected by a concen-

tration dependency of x-parameter for some polymer-solvent systems. For example, the

high responsiveness (collapse in water above a lower critical solution temperature

S. Minko400

308C) was reported for 0.25 nm22 dense poly(N-isopropylacrylamide) (PNIPAM)

brushes,14 while for one order less dense brushes no collapse was detected.15

Many properties of brushes can be referred not only to the brush thickness but also to

the density profile. Careful considerations of the problem using self-consistent field theory

(SCFT) revealed a non-monotonous dependence of the polymer density as a function of

the distance from the grafting surface. In the limit of strong stretching (h� kr2l1/2) a

density profile is parabolic according to the so called classical ISL SCFT of

brushes16,17 (refers to the infinite stretching limit (ISL) approximation when the

fluctuations around the most probable classical pass can be ignored).18 At a moderate

stretching density the profiles obtained with SCFT19 and simulations20 deviate from the

classical parabolic profile substantially. Figure 5 presents the density profiles (SCFT

results) vs. distance from the grafting surface at different b-stretching parameter values

(1/b ¼ d ¼ kr2l1/2/h).21 As the grafting density increases from the mushroom regime

(b , 1) to the strong stretching limit (b ¼ 100) the profile changes dramatically.

Figure 4. Height of the brush layer, scaled by the bulk radius of gyration of the chain in good solvent,

as a function of the scaled surface coverage for a variety of qualities of solvents. The thin line marks the

onset of instability, that is, it corresponds, for each T, to the surface coverage at which the isothermal

compressibility of the grafted chains is zero. (Reproduced from Ref. 12, Copyright 1996, with per-

mission from John Wiley & Sons, Inc.).

Figure 5. Self-consistent-field results for the rescaled density profiles as a function of the rescaled

distance z from the grafting surface. Shown are results for b ¼ 0.1, 1, 10, and 100 (dotted, dash-

dotted, dashed, and solid lines, respectively), progressively approaching the asymptotic result

(valid for b! 1), shown as a thick dashed line. (Reproduced from Ref. 21, Copyright 1998,

with permission from American Chemical Society).

Responsive Polymer Brushes 401

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.

Concluding this paragraph, both brush thickness and density profile are sensitive to

grafting density and solvent quality. The responsive behavior of the homopolymer

brush is referred to changes of free energy of the brush in its environment due to the

change of solvent quality. Any application of the responsive homopolymer brushes will

be grounded on this principle.

Polyelectrolyte (PEL) Brushes (Fig. 6)

Macromolecules with ionizable groups carry electrical charges in polar solvents. 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.

For the dense strong PEL brush (high f and grafting density) all counterions are trapped

inside the brush (Fig. 6b). The brush height is determined by the balance between osmotic

Figure 6. Planar polyelectrolyte (PEL) brush with ionizable groups (a) swells when the groups

dissociate a polar solvent (b); the dissociation is pH dependent for weak polyelectrolytes (c);

addition of salt leads to the shrinkage of the PEL brush (d).

S. Minko402

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 insen-

sitive 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 (Fig. 6d). 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).22,23 Thus, in terms of responsive appli-

cations 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 (Fig. 6 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 (Fig. 6 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.24 At a low ionic strength electrostatic interactions

result in the condensation of counterions in the brush (the neutral polymer brush

regime). If ionic strength is increased the screening of the electrostatic interactions

affects an increase of f and brings the system in the osmotic regime. The graph of the

brush thickness vs. salt concentration has a maximum.25

In mixed brushes (Fig. 7) two or more different polymers grafted to the same substrate

constitutes the brush. The mechanism of responsiveness of the mixed brushes is very

different from homopolymer brushes. Unlike polymers in the mixed brush segregate

into nanoscopic phases. The size of the nanophases scales with kr2l1/2 value. The phase

Figure 7. Mixed two-component brush: lateral phase segregation of chains in non-selective solvent

(a), and combined lateral and layered segregation in a selective solvent (b, c).

Responsive Polymer Brushes 403

segregation is a lateral segregation in a nonselective solvent when unlike polymers form

spherical or elongated clusters. Both the polymers are exposed on the top of the brush

(Fig. 7a). In selective solvents the mixed brush structure may be seen as a combination

of lateral and layered segregation mechanisms.26 In the latter case one polymer preferen-

tially segregates to the top of the brush, while another polymer forms clusters segregated to

the grafting surface (Fig. 7b, c). The most important difference of the mixed brush

as compared to the homopolymer brush is that not only the height and density profile

but also the composition profile depends on solvent quality. In other words, the surface

composition of the brush is switched by a change in its environment.

If two unlike polymers are of the same chemical constitution but they differ by

molecular weight the brush stratifies.27 – 29 The polymer of the largest molecular weight

will occupy the top of the brush. If two unlike polymers are of different chemical

compositions the lateral and layered segregation takes place. A combination of both

kinds of heterogeneity (by chemical composition and molecular weight) results in the

more complex phase diagram when the whole range of parameters of the system can be

used to design the brush responsiveness.30 The further modification of the responsiveness

can be obtained for mixed PEL brushes (Fig. 8). In this case electrostatic interactions are

used to regulate the mechanism of the phase segregation. Weak mixed PEL brushes are of

special interest. At isoelectric points they form a neutral PEL complex (Fig. 8a), while

outside the isolectric point one PEL dominates in the topmost layer and affects the elec-

trical charge of the brush (Fig. 8 b, c). Thus, the surface composition of the mixed PEL

brush can be switched just by a change of external pH.31 – 33

Block-Copolymer Brushes (Fig. 9)

In this case two or more chemically different polymers constitute a polymer brush

with block-copolymer architecture. The responsiveness of these brushes is determined

Figure 8. Mixed weak polyelectrolyte brush forms a neutral complex in the isoelectric point (a); in

acidic or basic solutions positively or negatively charged polymers, respectively, are stretched while

a counterpart chains are collapsed and segregated to the grafting surface (b, c).

S. Minko404

by phase segregation of unlike polymers, however, the mechanism depends on whether the

AB block copolymer is tethered by the more (A) or the less (B) soluble block.34,35 In poor

solvents the brushes grafted by the B blocks form dumbbell-like micelles with the

B-polymer segregated to the grafting surface and the A-polymer micelles on the top

(Fig. 9b). The brushes grafted by the A blocks form B-polymer micelles shielded by

A-polymers (Fig. 9a).

If the solvent quality is improved (increased selectivity of solvent) for the A-polymers

the dumbbell-like micelles of the B-grafted brushes are transformed to the structures which

are similar to the mixed brushes in selective solvents. The B-polymers form pinned micelles

segregated to the grafted surfaces and shielded by the brush of the A-polymers. In general,

the block-copolymer brushes have a complex phase diagram affected by the ratio between

the molecular weights of the A and B blocks and the grafting density. The major difference

between mixed and block-copolymer brushes can be found for the case of the A-grafted

brushes in a selective solvent for the A-blocks. The mixed brushes of the A and B

polymers form pinned B-micelles segregated to the grafting surface. The AB block-

copolymer brushes form B-micelles located at the top of the brush (Fig. 9a). Consequently,

block-copolymer brushes introduce unique mechanisms of the phase segregation.

The experimental studies36 – 39 have demonstrated reach responsive behavior of block-

copolymer brushes based on the rearrangement of segregated phases in good agreement

with theoretical predictions. PEL block-copolymer brushes are less investigated. The

A-B PEL block-copolymer brushes grafted by the neutral blocks A demonstrated no

rearrangement upon treatment by different solvents.40 However, it seems to be a

specific case of the brush structure. Generally, electrostatic interaction can modify inter-

actions in the block copolymer brushes resulting in additional responsive mechanisms that

could be a subject of future investigations.

Figure 9. The block-copolymer brush constituted of more soluble A blocks and less soluble

B blocks. The brushes grafted by the A-blocks (a) form B-polymer micelles shielded by the

A-polymers (a, right). In good solvent for the A-polymers B-micelles are located at the top of the

brush (a, left). The brushes grafted by the B-polymers (b) form dumbbell-like micelles (b, right).

In good solvent for the A-polymers the A-blocks are stretched and the B-blocks form micelles

segregated to the grafting surface (b, left).

Responsive Polymer Brushes 405

Non-Planar Brushes

In many important applications the grafting surface is curved. Most brush applications

are for colloidal objects of spherical and cylindrical shapes. The traditional application

of the brushes against flocculation of colloidal particles is well known.41 However,

responsive brushes for colloidal systems are considered to be an under-investigated

field which promises numerous exciting applications for a new generation of functional

materials: smart coatings and detergents, drug delivery systems, smart containers for

biomolecules, elements of miniaturized sensors, etc. for textiles, paints, cosmetics,

microscopic devices, medicine, and biotechnology. A curved grafting surface may

strongly modify properties of the brushes. First of all, the volume available for the

grafted chains is a function of distance from the grafting surface (z) and changes with

curvature (Fig. 10) In terms of theoretical analysis the boundary conditions imposed

by curved grafting surfaces are different than those for a planar grafting surface.13 It

seems that these “difficulties” have postponed intensive studies in the area of non-

planar polymer brushes, although industrial needs and published research reports give

well- justified prospects for success.25,42 – 45 Fabrication of mixed brushes on the

curved surfaces46 – 48 sounds very promising for the design of a range of various inter-

action mechanisms in colloidal systems.49

Molecular Brushes

Molecular brushes are of special interest because they represent a tailored responsiveness

on the level of single molecule events.50 Brush-like molecules are formed by grafting

polymer arms to a molecular (polymer) backbone. They change conformations in

response to their environment. Changes of solvent quality induce switching between

rodlike and globule conformations. The basic mechanism of responsiveness is the same

as for planar brushes: solvent quality and entropic elasticity balance the conformation.

The flexible backbone also contributes to the balance and in turn changes its conformation.

Figure 10. Density profiles for chains tethered at spherical surfaces for a variety of radii R. z is the

radial distance from the tethered surface. The full line with filled circles is for a planar film, dot-

dashed line R ¼ 100l, long dashed line R ¼ 20l, dashed line R ¼ 10l, dotted line R ¼ 5l, and a

full line R ¼ 2l, where l is the bond length of a chain. (Reproduced from Ref. 13, Copyright

1996, with permission from John Wiley & Sons, Inc.)

S. Minko406

Thus, the cylindrical brushes change length and stiffness with changes in solvent quality. If

the polymer arms shrink in a poor solvent the cylindrical brush undergoes rod-to-globule

phase transition.51 Adsorbed brush-like molecules undergo conformational transitions

strongly modified by interactions with the solid substrate52 (Fig. 2).

Hierarchically Structured Brushes

These brushes are a combination of different architectures. Discovery and manufacturing

of complex brush architectures appear in response to the demand of the development of

hybrid functional nanomaterials with a complex response mimicking interactions in life

systems. To assemble nanoscale building blocks into responsive hierarchical nanostruc-

tures, new synthetic techniques are developing that are capable of linking a wide range

of length and response-time scales. An important challenge is to precisely manipulate

and quantify the response (switching) of a given property in the material based on

external stimuli. Multiple levels of functionality can be integrated into complex brushes

by carefully designing their architecture.

Recent research attempts to combine several architectural approaches in polymer

brushes. Y-shaped amphiphilic brushes combine two dissimilar polymer chains attached

to a single focal point or a leg. These brushes represent the first step to a more complex

architecture (Fig. 11).53,54 Although the responsive behavior of mixed and Y-shaped

brushes can be explained by very similar mechanisms, the difference between randomly

mixed polymer brushes and Y-shaped brushes is in the polymer distribution on the

surface. Incompatibility between two unlike polymers may affect the distribution

between grafting points for mixed brushes and for Y-shaped brushes differently. The

Y-shaped brushes could potentially form more regular structures because the grafting

of two different polymers is always coupled due to the Y-shaped architecture. This

morphology may be promising for the precise design of complex interactions.

Tightly grafted to the planar solid substrate spherical,47 cylindrical brushes (bottle-

brush brushes),55 or hyperbranched polymers56 represent new hierarchically structured

brush architectures.

Figure 11. Molecular graphics representation of the proposed structural rearrangements in

the Y-shaped brushes. (a) Internally segregated pinned micelle composed of seven grafted

polystyrene — poly(acrylic acid) (PS-PAA) molecules spaced 3.5-nm apart. Upon treatment

with toluene and subsequent drying, the PS arms (gray) form a corona covering the micelle’s

core consisting of seven PAA (dark gray) arms. (b) Representation of the same seven molecules

swelled in a nonselective solvent (light gray). (c) Top-open craterlike structure containing seven

collapsed PS arms partially covered by seven PAA chains. (Reproduced from Ref. 54, Copyright

2003, with permission from American Chemical Society).

Responsive Polymer Brushes 407

Reversibly Self-Assembled Brushes

These brushes are formed

(1) if polymer arms are tethered to the grafting surface via non-covalent bonds and

(2) if a grafted interface is formed just as a result of self-assembling of polymer chains.

Examples of these brushes encounter but are not limited to: end adsorbed polymer

chains, adsorbed block-copolymer layers, thin block-copolymer films, block-copolymer

micelles and vesicles, lipid-bilayers and vesicles, etc. These brushes demonstrate at

least three levels of responsiveness if assembling forces are strong enough to sustain

conformational changes of the polymer arms. The first level is the same as for

“classical” planar polymer brushes: balance between entropic spring and excluded

volume effect. The second level evolves bending of the grafting surface similar to

cylindrical brushes. Finally, the third level of response is an entire reorganization of the

structure when the brush may completely rearrange the supra-structure: for example the

transitions between micelles and vesicles of different shapes,57,58 and the inversion of

block-copolymer micelles.59,60 These structures possess a very complex response. Self-

assembled structures of amphiphilic molecules share many characteristics with brushes.

These systems are the building blocks of synthetic membranes (e.g. polymersomes61) or

cell membranes that are found in many living systems resulting in them being one of

the major targets of modern synthetic polymer chemistry.

Synthesis

Synthesis of polymer brushes of various structures was thoroughly introduced and

described in several reviews4,6,23 and in the book on polymer brushes.2 Thus, only the

major concepts are listed in this paragraph.

Three well known approaches are broadly used for synthesis of polymer brushes:

“grafting from,” “grafting to,” and “grafting through.” The major advantage of the

“grafting from” approach is that the tightly tethered chains may be grown from the

grafting surfaces. The active end-centers of the growing chains are easily accessible for

monomer units when small monomers swell the brush. Thus, the highest grafting

density can be approached with the “grafting from” approach. Although, almost all

possible polymerization mechanisms were employed to grow tethered chains from

different substrates, the most popular mechanisms (by number of publications) are

classical and controlled radical polymerizations. The controlled radical polymerization

seems to be the most technically simple method. However, the “grafting from”

approach is a synthesis of the polymer at a high local concentration of polymer chains.

In these conditions side reactions (chain transfer to polymer) should be considered as

a source of defective molecules. This problem seems to be underinvestigated yet. An

important step of the “grafting from” approach is surface immobilization of the initiator

of the polymerization. In the “grafting to” approach end-functionalized polymer chains

are grafted directly to a solid substrate via a chemical reaction of the end-groups with com-

plimentary groups on the grafting surface. In this case the functionalization of the grafting

surface is an important step. The “grafting to” approach is limited by diffusion kinetics of

end-functionalized polymer chains through the crowed grafted brush-like layer. The

brushes of a moderate grafting density can be obtained by the “grafting to” methods.

Nevertheless, the major conformational response of the brushes was indeed observed

for the brushes with moderate grafting densities. Thus, the “grafting to” is considered

S. Minko408

by the author of this review as the best method for the fabrication of responsive brushes for

many applications because of two major advantages:

(1) the grafted polymer can be thoroughly synthesized and characterized by traditional

methods in solutions, many commercially available polymers can be used,

(2) the “grafting to” procedure is very simple technically.

The “grafting through” approach is the polymerization of macromonomers. This

method is used for the fabrication of cylindrical brushes.

Two new tendencies have appeared in the field of synthesis of responsive brushes:

patterned surfaces and combinatorial approach.

Surfaces which are patterned on the nano- and microscale with responsive polymer

brushes can be used in sensing and actuation devices. These applications have stimulated

the development of synthetic approaches to nanopatterning of stimuli-responsive

brushes.62 Several methods were suggested for the fabrication of the patterned brushes.

The template methods refers to the fabrication of the patterns of an initiator on solid

substrates using photolithography63 or soft lithography methods.64 Alternative methods

explore scanning probe lithography: nanoshaving, anodization lithography, and dip-pen

lithography. In the nanoshaving approach a resist is mechanically removed by the AFM

tip and the obtained patterned are backfield with the initiator for the “grafting from”

process.65 AFM anodization lithography was used to generate silicon oxide nanopatterns

on silicon substrates by electrochemical anodic oxidation with the conductive AFM can-

tilever and using a resist layer on the surface of the silicon substrate. Local electrochemical

processes near the AFM tip results in the decomposition of the resist and generation of

silicon oxide patterns. The patterns are treated by the initiator for grafting polymer

brushes.62 Dip-pen lithography was used for direct patterning of the thiol initiator.66

Lift-off electron beam lithography was used to initially fabricate gold nanopatterns on

Si-wafers which later were surface modified with the initiator for “grafting from”

polymerization.67 The combination of the common lithography methods and surface-

initiated polymerization resulted in the development of the method for the fabrication

of 70 nm wide polymer brushes.

Responsive properties of brushes strongly depend on polymerization degree, grafting

density, and brush composition (for mixed and block-copolymer brushes). The precise

design of responsiveness can thus be approached by optimization of the brush structure.

This request was realized through the development of combinatorial methods to study

and optimize responsive polymer brushes. The combinatorial approaches explored the fab-

rication of the samples of planar brushes with a 1D or 2D gradual change of the brush

characteristics such as grafting density, molecular weight, and composition. Both the

“grafting to” and “grafting from” methods were used for the fabrication of the gradient

brushes. The brush grafting density gradient was used to study the effect of grafting

density on the brush structure in solvents.68 – 70 The mixed brushes71 – 73 and block-

copolymer brushes74 with composition gradients were used for the study of the brush

rearrangement in different solvents.

Applications of the Responsive Brushes: So Far

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 409

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. The complexity of the problem, however, is substantially increasing if

we have in mind the design of complex responsive systems which mimic functions of

life systems. Such artificially created systems will be capable of: reporting on toxins, diag-

nosing cancer cells, monitoring important parameters of organs, and targeting the release

of drugs. Currently, research is focused on how the interactions with polymer brushes may

be precisely tuned and monitored in a controlled environment.

Wetting by Solvents

Wetting of responsive brushes by solvents is the well-explored field of research. The most

practical case relevant to many applications is wetting by water. Spreading of paints,

adhesives, deposition of proteins and microorganisms, cleaning and self-cleaning

surfaces—all these processes are frequently used in different technologies and in

everyday life. Switching between wetting and nonwetting, thus, may help to improve effi-

ciency of many technological processes and may result in the fabrication of smart

materials with switchable wetting behavior.

Generally, wetting of polymer brush includes the interaction of the liquid with solid

substrate (grafting surface) and the interaction of the liquid with the tethered polymer

chains. The interaction with the brush is the most important component to approach

complete wetting of the substrate. A good solvent swells the brush and the complete

wetting is expected in this case. However, experiments show that complete wetting can

not be easily approached. Many studies report on finite contact angle for water on the

planar polymer brushes prepared from water soluble grafted polymer layers. Thus,

wetting of the brush is strongly affected by the structure of the brush-water-gas interfaces.

One of the possible explanations for this effect is the surface activity of water soluble

polymers which results in polymer bridges between the substrate and the solvent-vapor

interface. This effect may cause partial wetting.75

If the solvent quality is tuned toward a poor solvent the water contact angle will

increase due to the increased polymer brush–solvent interfacial tension. This tendency,

however, is not obvious. The scenario depends on the constitution of the grafted chains

and their microconformations in the topmost layer of the brush. A rearrangement of

polymer chains way results in “self-adaptive” behavior of the brush. The microconforma-

tion of the polymer chain at interface and the location of end-functional groups will adapt

the lowest polymer brush–solvent interfacial energy. The changes of contact angle,

therefore, will not be big. For example, a transition between good and poor solvent

conditions induced by temperature change for PNIPAM brushes has almost no effect on

the wetting behavior of the brush.76

In weak PEL brushes switching of wetting can be approached by a change of pH.23,32

For example, weak PEL brushes prepared from poly(2-vinylpyridine) (hydrophobic

backbone) are much better wettable by acidic water than by neutral or basic aqueous

solutions. Ionization of PEL brushes results in their strong swelling in aqueous

solutions77 and, therefore, in increased wettability. If polyvalent groups are explored in

S. Minko410

weak PEL brushes several wetting regimes can be obtained for the brush based on an

association-dissociation equilibrium. A three-stage switching of surface wetting was

demonstrated using phosphate-bearing polymer brushes.78

Strong PEL brushes are less sensitive to pH, however, they were found to be

responsive to the nature of counterions. The brushes prepared from poly(2-(methacry-

loyloxy)ethyltrimethylammonium chloride) have shown the collapse and transition

from 378 to 798 water contact angle if Cl2 counterions were replaced by ClO42 less

hydrated ions.79

Surface texture may strongly amplify wetting and non-wetting behavior. If the brush

is grafted on the textured surface the switching between ultrahydrophobic and hydrophilic

states can be approached.80

Mixed and block-copolymer brushes offer an interesting opportunity to freeze wetting

behavior in a non-equilibrium state. If the brush was treated by a selective solvent for

hydrophobic chains they will occupy the topmost layer of the brush. As soon as the

solvent evaporates the hydrophobic state will be “frozen.” Water will not wet the layer

and will not switch the brush because the topmost hydrophobic layer is not permeable

for water.6

Wetting by Polymers

Responsiveness of polymer brushes to polymer liquids of the same chemical structure as

the polymer brush or in other words wettability of polymer brushes by thin liquid polymer

films was in a focus of theoretical81 – 84 and experimental85 – 87 works because this problem

is very important for thin film technologies. The stability of a molten homopolymer film on

a polymer brush of identical chemical composition is of particular interest. It was found

that the stability of the film depends on the grafting density of the brush. In the case of

the brush grafted to the non-wettable solid substrate the polymer film demonstrates two

wetting/dewetting transitions. At low grafting densities the grafted polymer chains do

not change significantly the interfacial tension between the polymer melt and the solid

substrate (gsm). The film tends to dewet. As the grafting density rises, gsm decreases to

the level, when the spreading parameter (S ¼ gs 2 gsm 2 gm, where gs and gm are the

surface tensions at the vacuum/brush and vacuum/homopolymer melt interfaces,

respectively) becomes positive and the film begins to wet the substrate. In this region of

the film stability the grafted chains are totally or partially intermixed with the free

chains of the melt. The width of the brush/homopolymer interface is determined by a

balance of two effects.82 The polymer melt is a solvent. Therefore, its penetration into

the brush is entropically favored. However, that implies an extra stretching of the

grafted chains with an entropic cost of the elastic free energy. Thus, the interface width

decreases with an increase of the grafting density because the densely grafted chains

create an entropic barrier for the penetration of free polymer chains of the melt. The

decrease in the interface width is accompanied by the growth of the interfacial tension

gsm. The same effect on gsm has the increase of temperature.87 As gsm increases the

spreading parameter again becomes negative and the polymer film dewets. This

phenomenon has been called autodewetting or autophobicity. This behavior was

observed experimentally.85 – 87

Interesting wetting behavior has been reported for diblock copolymer films on

random copolymer brushes.88,89 The grafting density of the brush was adjusted to

obtain a complete wetting of the brush by the block-copolymer film. Block copolymers

are known to exhibit a variety of ordered microdomain morphologies produced by the

Responsive Polymer Brushes 411

phase separated polymer blocks. The important examples are lamellar and cylindrical

morphologies, because the orientation of lamellae and cylinders in thin films is affected

by the film interfaces. Even a small difference in interfacial tensions between the

substrate and each polymer (constituting the block copolymer) leads to the parallel orien-

tation of lamellar and cylindrical microdomains with respect to the substrate plane. In this

state the polymer block that has the lowest interfacial tension wets a substrate surface thus

allowing for minimization of the interfacial energy. The perpendicular orientation,

however, is more preferable in terms of a potential application of block copolymer

films as nanolithographic templates. This orientation implies that both phases wet the

substrate surface, therefore, the substrate has no preference with respect to any block.

Such a neutral surface can be prepared by modification of a substrate surface with a

random copolymer brush. A random copolymer brush allows for precise control over

the interfacial interactions simply by changing the copolymer composition. That has

been demonstrated for films of a polysterene-block-poly(methyl methacrylate)

deposited on silicon substrates modified with brushes of a random copolymer of styrene

and methyl methacrylate.88,89 The perpendicular orientation of the lamellar and cylindrical

microdomains was observed for the certain copolymer compositions.

Brush-Brush, Brush-Solid, and Brush-Particle Interactions

These interactions represent a classical field of colloid science.90,91 Polymer brushes

stabilize colloids because of repulsive forces (originated from the excluded volume

effect) which oppose the brush compression. At a moderate grafting density the interpene-

tration of the two interacting brushes may lead to an attraction if the interaction of grafted

chains with the interacting surface dominates. Both repulsive and attractive interactions

were well documented in experiments (for example).92 – 94 For PEL brushes electrostatic

interactions may strongly modify the interactions resulting in strong repulsion between

similarly charged brushes.95 Therefore, the interactions between two brushes or the

brush and solid surface is a tiny balance between volume excluded effect, attractive

interactions (van der Waals) and Coulomb interactions. The balance can be tuned by a

change of environmental conditions (temperature, pH, ionic strength). In the case of

mixed polymer brushes or the block-copolymer brushes the balance between attractive

and repulsive interactions can be switched depending on which polymer preferentially

segregates to the topmost layer of the brush or on how the interaction with the solid

surface will change the phase segregation in mixed/block-copolymer brushes.7,49,96

It is difficult to overestimate the importance of the interaction regulation between

polymer-coated solids.

The responsiveness of a polymer brush to the insertion of a small impenetrable object

is the problem of a special and very important interest. These phenomena are critical for

the regulation/control of particle, dust, protein, and cell adsorption on polymer brushes.

Theoretical analysis shows that the insertion of a small object in the polymer brush

creates a repulsive pressure field which decays exponentially away from the region of

insertion.97,98 Interaction with the brush also affects the spatial organization of particles

in the brush. Small particles that strongly interact with the polymer brush disperse

freely within the polymer brush, while polymer “insoluble” particles tend to aggregate

in the brush. The big aggregates are expelled towards the brush-air interface.99

Therefore, switching particle-polymer interactions via the change of solvent quality will

affect the particle organization in polymer brushes100 and can be used for a controlled

release of particles from the surfaces.101

S. Minko412

The interaction between particles/proteins and a brush in real systems is much more

complex since many different forces are involved in the phenomenon. The amount of

protein adsorbed on the surfaces depends on the surface density of the polymer, the

protein-surface interaction, the polymer-surface interaction, the size of the polymer

segments, and the ability of the proteins to undergo conformational changes upon adsorp-

tion.102 – 105 For the example of polyethyleneoxide (PEO) brushes, the adsorption of

proteins is governed by the interplay between the attractive interaction of the protein

with the surface, the attractive interaction of the adsorbed PEO segments with the

grafting surface, and the entropic changes associated with the deformation of the PEO

layer due to the adsorbing proteins. In the case of the grafting surface which is not attrac-

tive for PEO the amount of protein adsorbed depends on how the adsorbing protein

deforms the PEO brush. The mechanism responsible for the prevention of protein adsorp-

tion by grafted PEO results from the ability of the grafted polymer to block the adsorption

sites on the grafting surface. Therefore, the fact that the PEO is attracted to the surface

results in more effective control of nonspecific protein adsorption. It was found exper-

imentally104 that the surface concentration of adsorbed proteins decreased as the surface

density of the grafted PEO increased, but the surface concentration of the adsorbed

protein never reached zero. An analysis of the protein adsorption dynamics shows that

brushes of the same constitution express a different efficiency in the prevention of

protein adsorption at different time scales. The polymers that are not attracted to the

surface are effective for kinetic control but not good for equilibrium reduction of

protein adsorption, while polymers with attractions to the surface show exactly the

opposite behavior.106 This result demonstrates the importance of the brush response in

dynamics. These problems are still very poorly investigated.

If considering PEL brushes, the electrostatic interactions between proteins and the

brush should be also taken into account.107 The electrostatic interactions are easily

tuned by pH change since proteins are polyampholytic molecules constituted of weak

acids and bases. Ionic strength also strongly contributes to protein adsorption on

polyelectrolyte brushes that opens really broad possibilities to regulate protein

adsorption/desorption processes.108

Interaction with biological molecules is a very complex phenomena. Precisely

tailored materials for controlled interactions with proteins and cells will explore a

complex architecture when a wide spectrum of forces will be used. To this end the func-

tional polymer brushes bearing ionizable groups and other type of functional groups

located at different distances from the grafting surfaces are promising candidates to

design and tune the brush-layer response.109

Application in Devices

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. The first

approach to drug delivery devices comprised the fabrication of stealth particles coated

with PEO brushes. The brush masks the particles from macrophages and results in an

increase in the blood circulation half-life of the particles by several orders of

magnitude.110 – 112 The next step could be seen as a fabrication of the particle shell

from a mixed spherical brush constituted of polymer chains (which control nonspecific

adsorption) and embedded selective centers for the specific adsorption and targeting the

drug release. Intensive investigations in this field for controlled drug release are outside

Responsive Polymer Brushes 413

of the frames of this review. The precise design of the capsule will need intensive research

on the development of the fundamental backgrounds for understanding the interactions of

responsive brushes with biological objects. The first experiments show that responsive

polymer brushes can switch the protein adsorption upon external signal.113,114

Expansion and shrinking of responsive polymer brushes can be used to fabricate

mechanical actuators.115,116 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.117

Reversible changes of mixed brush morphologies in solvents of different thermodynamic

quality were used for the motion of nanoparticles deposited on the brush surface.100

The simplest device which explores a polymer brush responsiveness is a sensor

working on the principle of the brush expansion–collapse transitions upon changes in

its environment. For example, the strong PEL brush expands in a humid atmosphere.

That can be detected as a change of a bending of the micromechanical cantilever (with

a single-sided brush layer)118,119 or as a change of optical characteristics of the brush

layer using ellipsometry,32 surface plasmon resonance spectroscopy23 or fluorescence

intensity.120 The brush expansion-collapse was also detected as a shift of an absorption

band in visual spectra based on the principle of the transmission plasmon spectroscopy

when the responsive brush was synthesized on the top of gold nanoislands deposited on

the glass substrate. The brush was coated by a monolayer of gold nanoparticles. The

change in the distance between gold nanoparticles and nanoislands upon expansion and

collapse of the polymer brush resulted in the shift of the absorbance band in visual

spectra.121

Conclusions

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. A number of reports on the application of polymer brushes for

devices is still very small. Further expansion of the field will appear in the near future.

Acknowledgement

Work described in this review was supported in part by the NSF award CTS 0456548 and

US ARO grant W911NF-05–1–0339.

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