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Journal of Materials Chemistry b924392e
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FEATURE ARTICLE
grafted polymer brushes across the length scales by atomic 10

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Characterization and molecular engineering of surface-

force microscopy

Xiaofeng Sui, Szczepan Zapotoczny, Edmondo M. Benetti,Peter Sch€on and G. Julius Vancso*

Atomic force microscopy (AFM) is a powerful analytical tool forthe characterization of polymer brushes, as well as for thefabrication of brush structures across the length scales.

FEA � B924392E
_GRABS
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Characterization and molecular engineebrushes across the length scales by atom1

Xiaofeng Sui, Szczepan Zapotoczny,† Edmondo M. Bene

Received 20th November 2009, Accepted 23rd January 2010

First published as an Advance Article on the web ?????

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ring brush thickness, estimating the

ive behavior and probing surface

FEATURE ARTICLE www.rsc.org/materials | Journal of Materials Chemistry

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DOI: 10.1039/b924392e

With the advent of regulated, surface initiated polymerizations, s

approaches, the choice of polymerizable compounds and the cont

tremendous advancement. New analysis techniques and approach

these brushes with molecular precision. In addition, spatial struc

tuning of thickness as well as composition of the brushes, have b

developed enabling molecular nanofabrication approaches. Atom

a powerful analytical tool for the characterization of polymer bru

brush structures across the length scales. AFM has been used to

a number of ways including imaging surface morphologies, measu

value of number average molar mass, observing stimulus respons

mechanical properties. In addition, AFM based methods such as na

nanolithography (DPN) and scanning probe oxidation (SPO) have b

nanofabrication of patterned polymer brushes. This feature article g

and highlights recent advances.

Materials Science and Technology of Polymers, MESA+ Institute forNanotechnology, University of Twente, Enschede, NL-7500, TheNetherlands. E-mail: [email protected]; Fax: +31 (0)53 489 3823;Tel: +31 (0)53 4892967

† Permanent address: Faculty of Chemistry, Jagiellonian University,Ingardena 3, PL-30-060 Cracow, Poland.

Xiaofeng Sui

Xiaofeng Sui was born in 1983 in

Shandong, China. After obtain-

ing his BSc degree in 2006 and

MSc degree in 2008 from

Tsinghua University, he joined

the group of Prof. Julius Vancso

at the University of Twente. He

is currently pursuing his doctoral

studies in the field of macromo-

lecular nanotechnology with

stimulus responsive polymers.

FEA � B924

This journal is ª The Royal Society of Chemistry 2010

ring of surface-grafted polymeric force microscopy

tti, Peter Sch€on and G. Julius Vancso*

ifically using controlled radical

over grafting chemistry have seen

are now needed to characterize

control at the nanoscale, and

me feasible by utilizing recently

force microscopy (AFM) is

s, as well as for the fabrication of

estigate polymer brushes in

noscratching, dip-pen

een also employed for the

ives a short account of this field

Introduction

Polymer brushes consist of polymer chains densely attached by

one end to a surface or interface.1–6 The density of the anchoring

sites should be high enough to ensure an extended conformation

of the crowded chains (in the swollen state) with end-to-end

distance larger than for the free chains in the same solvent. At

lower grafting densities of the tethered chains, they adopt

conformations with reduced macromolecular stretching,

Szczepan Zapotoczny

Szczepan Zapotoczny is Asso-

ciate Professor at Jagiellonian

University (JU) in Cracow,

Poland, where he obtained his

habilitation (2009). After

completing his PhD on poly-

meric photosensitizers under the

supervision of Professor M.

Nowakowska he joined the

group of Professor G. Julius

Vancso as a postdoctoral

researcher (1999–2001). His

work focused on force spectros-

copy and surface chemistry of

self-assembled systems. Then he

returned to Poland and joined the group of Nanotechnology of

Polymers and Biomaterials at JU. In the period 2003–2009 he

visited the groups of G. Julius Vancso and C. Gooijer (VU

Amsterdam) working on collaborative projects. His current

interests focus on nanostructural polymeric materials (films,

brushes) for photochemical and biomedical applications.

392E

J. Mater. Chem., 2010, xx, 1–14 | 1

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2

G: Julius Vancso

G. Julius Vancso (PhD in solid

state physics) studied physics

(E€otv€os Lor�and University,

Budapest) and materials science

(ETH Z€urich). After holding an

Associate Professorship at the

University of Toronto in Can-

ada, he took on an appointment

in 1995 as chairholder and full

professor at the University of

Twente in the MESA+ Institute

for Nanotechnology, in The

Netherlands. He also holds

a Visiting Principal Scientist

position at the Institute of

Materials Research and Engineering of A*STAR in Singapore.

His research interests include macromolecular nanotechnology,

nanofabrication, single molecule studies (AFM, photonics),

surface engineering, stimulus responsive polymers, and materials

chemistry of organometallic polymers.

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exhibiting so-called ‘‘pancake’’ or ‘‘mushroom’’ shapes (Fig. 1),

while for higher grafting densities a polymer brush layer is

formed. In view of recent enhancements in preparing polymer

brushes with molecular control, characterization approaches

that provide information at the molecular (nanometre) length

scale are needed. In addition, due to the brush thickness values

being in the range of single chain dimensions, the amount of

surface grafted polymers available for analysis and character-

ization is very limited. Consequently, molecular characterization

of polymer brushes remains a challenging experimental task.

Polymer brushes can be formed on a variety of solid substrates

including metals, semiconductors and polymeric supports, and

Fig. 1 Scheme of the conformations of the surface tethered polymer

chains: (a) ‘‘pancake’’, (b) ‘‘mushroom’’ and (c) ‘‘brush’’.

Edmondo M: Benetti

Edmondo M. Benetti studied

Industrial Chemistry at the

University of Padova (Italy)

where he obtained his degree in

2004. He subsequently carried

out his PhD at the University of

Twente in the group of Mate-

rials Science and Technology of

Polymers headed by Prof. G.

Julius Vancso. Since 2009 he has

been working as a postdoctoral

researcher in the Laboratory of

Surface Science and Technology

at the ETH Z€urich concen-

trating his research activities in

the groups of Prof. Marcus Textor and Prof. Nicholas Spencer.

His research focuses on polymer chemistry and surface science.

Peter Sch€on

Peter Sch€on studied chemical

engineering and chemistry at the

Universities of Darmstadt and

Osnabr€uck, Germany. After

completing his PhD in chemistry

he did his postdoctoral research

at the Radboud University in

Nijmegen, in The Netherlands.

Subsequently he accepted

a position in industry working in

the field of development of AFM

methodologies in close collabo-

ration with the University of

M€unster, Germany. Since

February 2009 he is Assistant

Professor in the Materials Science and Technology of Polymers

group (MTP) of Prof. G. Julius Vancso at the University of

Twente, in The Netherlands.

FEA � B924

2 | J. Mater. Chem., 2010, xx, 1–14

have thicknesses typically ranging from a few to several hundred

nanometres.7 Chemical functionality can be incorporated in

specific positions, allowing a precise embedding of desired

molecules in the brush layer. The stretched conformation of the

tethered chains leads to unique properties of the brushes as well

as to specific applications. Among others, the brushes can be

used to efficiently stabilize colloids, to significantly reduce fric-

tion between the sliding surfaces, and to control and switch

surface wetting.

Polymer brushes can be prepared by either of two main

approaches: ‘‘grafting to’’ and ‘‘grafting from’’ (Fig. 2).8–10 In

the ‘‘grafting to’’ technique, end-functional macromolecules are

attached to a surface via physical adsorption or chemical reac-

tions. Due to steric hindrance (slow diffusion of the macro-

molecules through the already grafted chains to the surface),

only low grafting densities can be achieved. ‘‘Grafting from’’

utilizes surface-tethered initiating sites from which polymeric

chains may be grown. This method renders it possible to control

the surface concentration of the active sites (e.g. by using mixed,

Fig. 2 Scheme of (a) ‘‘grafting to’’ and (b) ‘‘grafting from’’ techniques for

fabricating polymer brushes.

392E


self-assembled monolayers (SAMs) of surface active initiators

mixed with inactive molecules) and can yield high grafting

densities. Recent advances in controlled free radical polymeri-

zations enable the use of a wide variety of monomers to obtain

precisely tailored, grafted surfaces with near-molecular

control.11–19

Analytical techniques routinely used for brush characteriza-

tion encompass the usual approaches employed for surface and

thin film analysis including the various imaging modes of

AFM.3 In the past 20 years, AFM has evolved as a general

probing technique for imaging, measuring (molecular) forces,

assessing surface properties, and monitoring processes across

the length scales from nanometres to several hundred micro-

metres, with a very favorable signal-to-noise ratio. AFM has

also been successfully used for (nano)fabrication of surfaces,

and as a functioning component in devices. Hence, the tech-

nique has become a true enabling platform for the analysis and

characterization, as well as for the fabrication and engineering

of polymer brushes. AFM, its physical principles, modes of

operation, and application examples in polymer science and

technology have been extensively reviewed in the literature.

The interested reader may find a reference list and a short

account on the state of the art in one of our recent ‘‘mini’’

reviews.20

AFM has been used to investigate polymer brushes in

a number of ways including imaging surface morphologies,

measuring brush thickness, estimating the value of the

number average molar mass and the graft density of surface-

tethered chains, observing stimulus responsive behavior and

probing surface mechanical properties. Furthermore, AFM

measurements can be performed in varying media (solvents;images of these surfaces showed some very interesting structures.

Fig. 4 shows the influence of film thickness on brush

morphology. Very thin (5 nm) brushes show a moss-like

appearance (root-mean-squared (rms) roughness of 1.2 nm),

whereas the 19 nm thick brushes show featherlike bundles (rms

of 4.4 nm). This was the first time that such stiff polymers have

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controlled pH and ionic strength; controlled atmosphere) or

under a wide range of external conditions (e.g. varying

temperature).

The structural control of polymer brushes is of central

importance for advanced molecular surface engineering.

Patterned brush nanostructures with controlled structural

motifs, feature dimensions and controlled chemical functionality

offer great opportunities in molecular engineering of ‘‘designer’’

surfaces, sensing, microfluidics, biomedical applications, as

nanoreactors for particle synthesis, etc. due to the precise control

of physical and chemical surface properties, and so on. Nano-

patterned polymer brushes with defined features become espe-

cially interesting when the pattern dimensions are close to the

length of the grafted macromolecules.21–24 Several AFM-based

methods, such as nanoscratching, dip-pen nanolithography

(DPN) and scanning probe oxidation (SPO) which enable

nanofabrication of polymer brushes, have attracted rapidly

growing attention.

This feature article offers a selection of examples to illus-

trate the opportunities and limitations of AFM in polymer

brush characterization and nano-fabrication. Due to limita-

tions of the length and scope of this article, we cannot

provide a full coverage of the literature; and we thus apolo-

gize for any omissions and for the subjective selection of

examples. This present review is divided in two major

sections. First, examples of AFM characterization are

provided, followed by a description of AFM-assisted nano-

fabrication approaches.

FEA � B924


Characterization

Surface morphology

The surface morphology of polymer brushes is of fundamental

importance for determining brush properties. AFM was initially

developed for imaging, thus it is not surprising that it has been

used for visualizing the surface morphology of polymer

brushes.25 Brushes in both dry and wet states can be imaged

across the usual AFM length scales from nanometres to

hundreds of micrometres. AFM topography imaging can

provide information regarding the surface morphologies and

transitions as functions of graft density.26–31 As a first example,

we discuss poly (N-isopropylacrylamide) (PNIPAM) brushes,

which were prepared with a surface gradient in grafting densities

by varying the initiator coverage across the grafted substrates.

This ‘‘high throughput’’ approach allowed the imaging of the

morphology variation at one substrate in the different grafting

regimes. The images demonstrated that variation in chain density

is accompanied by a concomitant change in surface morphology,

gradually evolving from discontinuous mushroom structures at

low grafting densities to heterogeneous patchy structures at

intermediate grafting densities. The size of the patch structures

gradually increased with increasing initiator coverage, until at

high grafting density region the morphology evolved to

a smoother, presumably more extended, structure encompassing

more extended chains (Fig. 3).28

As next example, morphologies of helical polyisocyanopeptide

brushes were characterized using AFM as a function of reaction

time, monomer concentration, and growth conditions.32 AFM

Fig. 3 The relationship of PNIPAM film morphology to local grafting

density as monitored based on the initiator density. The solid line gives

the initiator coverage as a function of position. Individual insets show 5

mm � 5 mm tapping mode AFM topography images and section analysis

line scans (above the respective images) at x¼ 1, 3, 5, 7, and 9 mm of a 10

mm along PNIPAM density gradient. Reprinted with permission from

ref. 28.

392E

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been grown from the surface, and unusual, featherlike

morphologies in the brush films were very efficiently visualized

by AFM. The thickness of the brush layer is less than the

persistence length of the polymer chains, which indicates that the

observed featherlike morphologies most likely consist of stiff

bundles of polyisocyanide chains that are more or less aligned in

the x,y-plane.

Fig. 4 AFM images showing the morphology of helical poly-

isocyanopeptide brushes (a) 5 nm and (b) 19 nm thick. Reprinted with

permission from ref. 32.

AFM has been used to study the morphological changes

re a

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accompanying photochromism of polymer brushes containing

spiropyran moieties.33 These photoresponsive polymer brushes

Fig. 5 AFM images of photochromic spiropyran polymer brushes (a) befo

from ref. 33.

FEA � B924

4 | J. Mater. Chem., 2010, xx, 1–14

exhibit reversible photocontrol of surface wetting and show large

changes in surface morphology when irradiated in polar solvents

that stabilize the isomeric ring-opened merocyanine form. After

irradiation in (polar) DMF, as a consequence of photo-

isomerization of spiropyran groups, large pillars of aggregates

were formed in the brush layer (Fig. 5). On the other hand, when

irradiated in the solid state or in nonpolar solvents, the brushes

did not exhibit any morphological changes.

Thickness

The thickness of polymer brushes represents a fundamental

structural parameter and its knowledge is essential for the eval-

uation of several properties. Although optical methods such as

ellipsometry are traditionally employed for the determination of

thickness, AFM has emerged as a fast and accurate tool for

investigating this parameter in combination with various surface

preparation approaches prior to e.g. optical thickness measure-

ments.

The first approach involves the selective removal of the brush

layer all the way down to the surface of the (hard) substrate.34,35

Polymer brushes can be for example carefully scratched and

locally removed with a ‘‘blade’’ tip. Control experiments have

demonstrated that on bare, clean silicon wafers, the underlying

substrate was not damaged by gentle scratching off of the

polymer brushes. However, for gold-coated surfaces, the Au/Cr

layer may be easily damaged and removed thus such experiments

must be performed with great care. Brush thickness is calculated

from the cross-sectional analysis between unscratched (brush

surface) and scratched regions (bare substrate) (Fig. 6).35

nd (b) after irradiation with UV light in DMF. Reprinted with permission

Fig. 6 PNIPAM brush grown by surface-initiated polymerization on

gold: (a) ‘‘three dimensional’’ AFM height image and (b) corresponding

cross-section profile showing determination of the brush height.

Reprinted with permission from ref. 35.

392E


As an alternative to the mechanical removal of a brush layer,

several ‘‘bottom-up’’ approaches have been exploited for the

fabrication of patterned samples which exhibit uncovered areas

of the substrate for height determination. Such methods are

based on the decoration of the substrate’s surface with patterned

brushes which is accomplished by spatially limiting polymeriza-

tion in given positions. The various techniques that can be

employed for this purpose include microcontact printing,36–38

electron beam-assisted fabrication,39–42 UV irradiation-assisted

methods,43,44 AFM-assisted methods,21 as well as nanoimprint

and contact lithography.45

Several other studies have shown that the brush thickness

could also be estimated through the compression profiles

measured by force spectroscopy.29,46

By preparing a series of different patterned samples following

increasing polymerization times and subsequently measuring the

brush thickness with AFM, the growth kinetics for the specific

brush system could be investigated. Lego et al. demonstrated the

control of grafting density and polymer chain length of poly(tert-

estimated using the monomer length l. Despite the potential

influence of confinement effects, the estimated value of number

average molar mass often exhibits the same order of magnitude

as the values obtained by GPC measurements of bulk systems (in

solution) with the same initiator.52

AFM can also be used to directly obtain information on Mn of

polymer brushes by employing single chain stretching

studies.29,53–58 This approach is based on extending individual

macromolecules from the grafting surface with an AFM tip.

Typically, a tethered polymer that is adsorbed onto the tip by the

free end will, under moderate tension, exhibit a restoring force

that varies nonlinearly with distance. The stretching force that is

measured by a calibrated cantilever increases with chain exten-

sion until the force gradient eventually exceeds the spring

constant of the cantilever, thus causing rupture of contact at the

weakest point (most often adhesion contact between the chain

and the AFM tip). The molar mass of the individual chain is

estimated using the chain contour length at rupture in addition to

the size and molar mass of the monomer. It is assumed that the

by studying a copolymer brush that consisted of a long non-

adsorbing poly(methyl methacrylate) (PMMA) block and a short

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butyl acrylate) (PtBA) on OH-activated mica substrates.47 These

authors found a linear relationship of the polymer thickness with

respect to the molar mass of the free polymer and with respect to

the monomer conversion, which suggested that Atom Transfer

Radical Polymerization (ATRP) was well controlled and rela-

tively densely end-grafted layers were obtained.

Possible drawbacks of measuring the thickness and generally

the dimensions of brush structures by using AFM techniques

arise when, through the application of high loads, the deforma-

tion of the ‘‘soft’’ polymeric features causes errors in the esti-

mation of the height.48 This problem can be reduced by

decreasing the force applied to the quasi-non-contact condition

or by minimizing tip–surface interaction using tapping mode

AFM. However, the possibility of AFM tip–brush interactions

must always be critically assessed (chain entropy pressure on the

tip). In specific studies the mechanical compression of the brush

structure with an AFM tip can be used to assess mechanical

properties of the brush itself. This powerful approach in the

characterization of tethered polymer chains will be discussed

later in the review.

Number average molar mass

Characterization of the number average molar mass (Mn) (degree

of polymerization, chain length) of tethered polymer brushes is

very difficult to achieve by standard analytical methods such as

gel permeation chromatography (GPC) or multiangle light

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scattering. This is due to the low amount of grafted material and

the difficulties involved in removing the grafted layer from the

substrate without destroying it. Brush molar masses are usually

estimated using aliquots of polymers synthesized with a sacrifi-

cial initiator in the solution medium. However, surface-initiated

polymerization may result in different molar mass distributions

from bulk polymerization. In addition, untethered polymer

chains in the brush may cause additional complications.49,50

Taking into account that, in the presence of a good solvent the

height of the stretched brushes h is comparable to the length of

the polymer chains L,51 we can estimate the chain length from

brush height h measurements performed by AFM as described

earlier. Assuming a stretched conformation, Mn can also be

FEA � B924


entire chain is tethered between tip and substrate. Collection of

a large number of force curves is required to obtain an accurate

statistical distribution of the contour lengths so that the Mn and

polydispersity index (PDI) can be determined. This approach is

scientifically interesting as it allows one to estimate single chain

parameters. However, due to the complicated and time-

consuming experiments, it has little potential to become a routine

characterization approach for brushes. It should be noted at this

point that the elastic properties of the chains in polymer brushes

differ from those of a single isolated polymer chain grafted to

a substrate because of interchain interactions. In addition, when

the chain is ‘‘picked up’’ by the tip (physisorption) then there is

no guarantee that contact is established between the tip and the

grafted macromolecule at its chain’s end. This would result in

rupture length values less than (and only occasionally equal to)

the contour length. Multiple chains pose another difficulty for

this single chain approach.

Yamamoto et al. were the first to investigate the elastic

properties of a grafted chain in a dense polymer brush.53 They

circumvented the above mentioned difficulties to a large extent

adsorbing poly(4-vinylpyridine) (P4VP) block on the free end.

Since predominantly contacts from the short P4VP at the free

end contributed to the extension profile, the rupture length

values could be successfully used to estimate the Mn and PDI

values.

Goodman et al. studied the extension profile from poly(N,N-

dimethylacrylamide) (PDMA) and PNIPAM brushes.54,55 For

the high density brushes, the shape of the decompression profile

was closely related to the cumulative mass fraction of chains of

each length determined by GPC (the grafted polymer was cleaved

from the surface for analysis by hydrolyzing an ester linkage at

the point of grafting) (Fig. 7). The separation at maximal

attractive force, corresponding to the maximum fraction of

adsorbed chains, approximated the average contour length. The

weight fraction as a function of the molar mass was obtained

from the derivative of the force profile. A fit to log-normal

distribution was then performed, enabling the estimation of the

value of the polydispersity index.

392E

J. Mater. Chem., 2010, xx, 1–14 | 5

Fig. 7 Comparison of contour length distributions obtained by AFM

and GPC for high (s ¼ 0.171 chains nm�2) grafting densities of polymer

brushes. Reprinted with permission from ref. 54.

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The grafting density, that is the number of chains per unit

surface area, is a very important property that determines the

surface’s performance in various applications.59,60 It may be

determined using the following equation:

s ¼ hrNA

Mn

where s is the graft density (chains nm�2), h is the layer thickness,

r is the bulk density of the polymer layer, NA is Avogadro’s

number, and Mn is the number average molar mass. Thus, from

the value of number average molar mass (chain length), the graft

density values can be derived.47,58,61–63 As an example, the graft

density of poly(methacrylic acid) (PMAA) brushes on silicon

substrate characterized by Mn value of 106 000 g mol�1 (as

determined by single chain stretching) was calculated to be 0.12

chains nm�2.58

The so-called Alexander-de Gennes polymer brush model

provided a theoretical framework to look at polymer brush

systems.64–67 An important result is that the height of a brush (in

contact with solvent) grows linearly with the degree of poly-

merization N, while the radius of gyration of a single chain only

scales (approximately) with the square root of N. The grafting

density can also be estimated by directly fitting the compression

profiles measured by AFM with the Alexander-de Gennes

model.29,46,68,69

Compressibility

As it was described in the previous paragraph, AFM imaging has

been successfully used to evaluate the thickness of brush struc-

tures. Generally, during imaging, a light compression force

should be used to achieve a stable AFM feedback necessary for

monitoring the surface topography. In response to this force, the

brush surface, especially when it is swollen in a good solvent,

becomes deformed. This deformation can be measured as

a decrease in the recorded height/volume of the polymeric

structures.70 Following this procedure, as it was recently reported

by our group,26 the effective ‘‘compressibility’’ (indicated as

height compression) for a grafted polymer layer can be tested by

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6 | J. Mater. Chem., 2010, xx, 1–14

Fig. 8 Representative cross-sections of micro-patterned samples

showing the step height of PMAA films. The data were recorded by AFM

in PBS solution at pH 7.4 by applying loads of increasing magnitude

(from 12 to 43 nN). The corresponding inset displays the compressibility

values measured as the relative height decrease in a function of the load.


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applying varying loads with the AFM tip while measuring the

step-height of brush patterns. This parameter allows an estima-

tion of the brush compressibility which was shown to be

dependent on the swelling properties of the grafted chains

(Fig. 8).

The estimation of brush compressibility by AFM is very useful

for evaluating the mechanical properties of brush-gels and for

comparing their mechanical performance with similar not cross-

linked brushes.71 Due to the presence of cross-links, brush-gels

should show different compressibilities if compared to the cor-

responding free brushes. To evaluate this notion, micro-

patterned samples were analyzed with AFM, by recording the

volume occupied by the polymeric structures by scanning the

features at different loads. The obtained compression values

exhibited a decrease of volume (%) of the brushes as a function of

the load applied. The results of several samples were compared.

The values thus obtained for free brushes and brush-gels were

fitted by a linear relationship in order to estimate trends in the

brush compressibility values. As expected, the freely grafted

brush showed the highest slope (e.g. the highest compressibility).

Stimulus responsive behavior

The thickness, surface morphology and phase separation of

polymer brushes encompassing stimuli-responsive polymers can

be altered in response to changes in external stimuli (such as pH,

temperature, ionic strength, solvent quality, redox potential,

etc.).13,72 AFM turns out to be very useful to monitor and

characterize these reversible conformational changes in situ, as

well as changes of the related properties (such as adhesion,

wetting, mechanical performance, friction, etc.).34,48,52,73–75

The morphology of stimuli-responsive brushes can be in situ

monitored by AFM, while the stimulus is switched on and off.

An example is given by our study of thermo-responsive PNI-

PAM brushes which across a relatively narrow range of

temperatures (30–34 �C) in water undergo a hydrophilic–

hydrophobic transition. When the temperature is raised to above

the lower critical solution temperature (LCST), the brush

structure becomes transformed from a swollen state to

392E


soluble segments to the interface and resulting in a collapse of the

3

forces at defined normal load values. The measurements can be

performed in a variety of environments and over a relatively

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a collapsed one, showing a precipitation (collapse) coupled with

a lateral aggregation of the chains. Above LCST, in the collapsed

state, hydrophobic polymeric aggregates appear at the brush

surface and this phenomenon caused an abrupt change in the

average film thickness and a dramatic increase in the values of

RMS roughness (Fig. 9).52

AFM can be used to monitor the properties of brushes

whereby chains undergo a morphological/physical rearrange-

ment in response to multiple stimuli. An example of such

responsive brush architecture was given by PNIPAM brushes

copolymerized with weak polyacids–poly(2-succinyloxyethyl

methacrylate) (PSEMA).76 The so-formed brushes responded to

an increase of temperature above the LCST of the PNIPAM

block, undergoing aggregation of the corresponding segments.

Simultaneously, the charge density of the PSEMA block could be

varied by changing the pH of the medium. By performing AFM

pull-off force measurements and quantifying the differences in

adhesion values as a function of the degree of swelling, the

Fig. 9 AFM images of the patterned surface with the grafted PNIPAM

chains in water at: (a) 31.0 �C and (b) 36.0 �C. The respective cross-

sections are displayed below each image. Reprinted with permission from

ref. 52.

Fig. 10 Representative morphologies for PMMA/PS mixed brushes.

The ripple (a and b) and dimple (c and d) morphologies adopted after 5

min of exposure to toluene and acetone, respectively, and were recorded

in the AFM repulsive tapping mode (2 � 2 mm: topography (a and c),

phase contrast (b and d)). Cross-sections are shown in the panel (e).

White arrows in panels a and c mark the locations of the cross-sections. Z

ranges are (a) 13 nm, (b) 3.2�, (c) 78.4 nm, and (d) 27.2�. Reprinted with


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transitions could be recorded. This study demonstrated that it

was possible to tune the swelling properties of the ionizable

segments, and then monitor them by AFM as the pH of the

medium was varied. In addition, a change in the degree of

protonation of the polyelectrolyte was also found to influence the

LCST of PNIPAM.

AFM was also successfully employed to investigate mixed

brush platforms. For example, binary mixed polymer brushes,

composed of two distinct polymer chains randomly immobilized

at a solid substrate, were studied by AFM. In this case, the

exposure to a selective solvent induced a clear morphological

rearrangement in the architecture of the grafts by turning the

non-soluble ones.19,77–80 Minko et al. investigated binary poly-

(methyl methacrylate)/polystyrene (PMMA/PS) brushes and

found surface features with two distinct AFM phase shifts,

indicating lateral phase separation of the polymers and the

presence of PS- and PMMA-rich surface regions (Fig. 10).79

Friction and adhesion

Surface forces that control adhesion and friction play an

important role in the assembly, manipulation, and operation of

nano-scale devices, as well as biological objects. (Strictly

speaking, adhesion should refer to reversible, thermodynamic

adhesion. Despite the use of the term ‘‘adhesion’’, what is meant

most frequently is ‘‘adherence’’, adhesion energy in combination

with mechanically dissipated energy.81) AFM enables the quan-

titative probing of friction and adhesion forces on the nanometre

scale by measuring force–tip distance curves, or lateral (friction)82

40

broad range of applied pressure/force by utilizing either

conventional AFM tips or colloidal probes attached to the AFM

cantilevers.56,83–88

Polyelectrolyte brushes have been reported to exhibit superior

lubrication properties in an aqueous medium with friction coef-

ficients below 0.001.89,90 This was attributed to their exceptional

resistance to mutual interpenetration of counterion-swollen

polymer chains, as well as to the fluidity of the hydration layers

surrounding the charged, ‘‘rubbing’’ polymer surfaces. The

surface force balance (SFB) apparatus was used in the nano-

tribological studies mentioned above and the applied pressure

was limited to p z 7.5 MPa (similar to the conditions observed in

human joints). While AFM has been used for friction measure-

ments even on an atomic scale, the pressure exerted by the

scanning tip is usually much higher (due to lower contact area)

and the sensitivity of the lateral force measurements is lower than

that of SFB. While some of the mentioned problems may be

partially overcome by for instance, using e.g. the colloidal probe

approach, AFM in its classical set-up is not the method of choice

for quantitative measurements of ultralow friction surfaces.

AFM has nevertheless been employed to investigate the

solvent-dependent friction force response of PS brushes prepared

by surface-initiated polymerization.91 The lowest friction forces

were found for a good solvent (such as toluene), which was

attributed to the extended conformation of the chains in the

swollen brushes. Systematic studies on adhesion and friction

properties of mixed polymer brushes, polystyrene–poly-

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(2-vinylpyridine) (PS–P2VP), grafted-to a silicon surface were

also carried out by varying brush composition, grafting density,

solvents used as well as size and coatings of the AFM probes.74 It

is possible to obtain switching in the adhesion and friction forces

stripes separated by bare non-functionalized gold substrates, the

tip–sample interactions at 31.0 �C were dominated by repulsive

forces whereas adhesive interactions were operative with the bare

Au surface (Fig. 12).52 At 32.5 �C the contrast between Au and

the polymer disappeared as a result of the transition of the brush

to collapsed chain aggregates above the LCST. This gave rise to

attractive tip–brush interactions and strong adhesive pull-off

forces upon withdrawal of the tip. The force–distance curves

observed at the two temperatures confirm that the repulsive

forces upon compression of the swollen chains at 31.0 �C, which

are typical for packed brushes, disappear at 32.5 �C subsequent

to the LCST being crossed.

Nano-fabrication

The fabrication of patterned polymer brushes with control over

chemical functionality, pattern shape and feature dimensions is

attracting growing attention.21 AFM-based techniques, such as

nanoscratching, dip-pen nanolithography (DPN) and scanning

probe oxidation (SPO) offer versatile patterning techniques

across the length scales. Advantages of AFM based methods for

the nano-fabrication include high resolution, the ability to

generate features with nearly arbitrary geometries, and a precise

position control. However, AFM related nano-fabrication

approaches are relatively slow (mostly serial) and are presently

not very suitable for large-scale and high-throughput pattern

formation. Efforts have been made to increase the speed of these

techniques by incorporating integrated arrays of the tips capable

of writing in parallel as well as incorporating piezoelectric

elements with higher resonance frequencies.93–96

Fig. 11 The variation in friction coefficient as a function of P2VP

fraction (in percent) for a gradient PS/P2VP brush treated with toluene,

ethanol and acidic water. Reprinted with permission from ref. 74.

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up to a factor of 4.5 by the conformation of mixed brushes being

altered upon treatment with selective solvents. The PS–P2VP

gradient polymer brush displayed a gradual change of the fric-

tion coefficient with respect to the brush composition. Fig. 11

shows an increase in the friction coefficient when the P2VP

content is raised as was observed for the gradient brush surfaces

treated with ethanol and acidic water. For these surfaces,

hydrophilic P2VP was on the top, and hence the friction coeffi-

cient increased with an increase in the P2VP content due to

higher interaction forces between the brush surface and the

hydrophilic silicon nitride tip.

A number of reports in the literature discuss exclusively

adhesion force measurements on polymer brushes in view of the

advantages of AFM. These advantages include a relatively low

invasive nature and, when using a proper calibration, the possi-

bility to obtain quantitative results. Adhesion force measure-

ments have been performed in order to probe the effects of phase

transitions e.g. LCST for temperature responsive brushes.35,52,92

For PNIPAM brushes exhibiting a pattern consisting of brush

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Fig. 12 AFM of a patterned PNIPAM brush. The area captured shows:

(a) a 40 mm �40 mm section exhibiting a schematic of the structure of the

surface-grafted polymer platform. The bottom right is an experimental

AFM force–volume image (20 mm �20 mm) of the grafted PNIPAM

chains in water. The grey scale in the AFM image corresponds to areas of

high (dark) and low (bright) adherence. The temperature was cycled

between 31.0 and 32.5 �C. (b) Cantilever deflection (i.e. force–displace-

ment curves) for the two regimes, captured on the polymeric features.


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Nanoscratching

Approaches including ‘‘bottom-up’’ and ‘‘top-down’’ encom-

passing tip assisted mechanical manipulation are conceptually

simple but powerful methods for fabricating patterned polymer

brushes. The basic premise of these techniques is that sufficiently

large contact pressures during patterning create shear forces that

exceed the displacement threshold of chemisorbed molecules.

Fig. 13 (i) A schematic representation of the fabrication of ‘‘scratched’’

polymer brushes; (ii) PS polymer brushes after lithography, before (left

column) and after (right column) sonication of 20 s in chloroform.


392E


Nanoscratching can be used directly on polymer brushes as

a ‘‘top-down’’ method.97 Polymer brushes with a thickness of 20–

30 nm can be reproducibly structured and scratched down to the

substrate (silicon oxide layer of the wafer) by means of high-

Fig. 14 Preparation of surface-confined PNIPAM polymer brush

nanopatterns by combining ‘‘nanoscratching’’ and surface-initiated

ATRP using a surface-tethered thiol initiator; AFM height images of

a PNIPAM brush line nanopattern imaged at room temperature in air.


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loading-force AFM lithography. After lithography, the sample
can be easily cleaned by sonication (Fig. 13).

Regarding ‘‘top-down’’ approaches, a self-assembled mono-

layer (SAM) (featuring for instance polymerization initiating

head groups) can be mechanically removed from the substrate

with an AFM tip that is ‘‘dragged’’ over the surface (under

mechanical pressure). This leaves behind bare patches on the

substrate that can subsequently be modified through backfilling

with a different SAM. The resulting SAM patterns can be used as

templates for surface-initiated polymerization.35,98 As an

example, stimulus-responsive PNIPAM brush nanopatterns

were prepared by nanoscratching gold substrates and backfilling

with an ATRP initiator SAM. Subsequent polymerization led to

structured polymer brushes (Fig. 14).98

Dip-pen nanolithography

AFM tip-assisted deposition of molecules at surfaces, commonly

referred to as ‘‘dip-pen’’ nanolithography (DPN), is a powerful

tool for molecular nanofabrication. In DPN, physisorbed

molecules at an AFM tip in contact with a substrate are trans-

ported from the tip to the surface through the water meniscus

formed through capillary forces.99 DPN deposition of e.g.

polymerization initiators has been used to create nanosized

features on a variety of substrates.100 Combining DPN and

‘‘grafting from’’ methods would render it possible to obtain

polymer brush arrays on the nanometre length scale with great

control over feature size and shape. For example, Ma et al. used

DPN and surface initiated ATRP (SI-ATRP) to produce poly(-

oligoethyleneglycol methacrylate)–poly(OEGMA) nano-

structures.101 In brief, a thiol-functionalized ATRP initiator was

patterned on a gold substrate by DPN, the unpatterned region

was backfilled with 1-undecanenthiol, and surface initiated

ATRP was subsequently carried out to get the patterned polymer

brushes.

FEA � B924


Fig. 15 (i) The preparation of polymer brushes grafted from immobi-

lized precursors on gold nanowires: (a) and (b) tip-assisted deposition of

gold nanowires on hydride-terminated silicon; (c) selective immobiliza-

tion of functional adsorbates on the gold structures; and (d) UV-initiated

grafting of PMAA brushes using the functionalized nanowires as plat-

forms. (ii) Height images (vertical scale from black to white 10 nm) from

Similarly, Liu et al. combined DPN and ring-opening

metathesis polymerization (ROMP) to fabricate polymer brush

arrays.102 In this approach, a tip coated with norbornenylthiol

molecules was brought in contact with the gold substrate to

obtain the pattern, after which the substrate was passivated in

a dodecanethiol solution and subsequently reacted with Grubbs’

first generation catalyst and norbenyl monomers, yielding poly-

norbornene brush arrays.

Our group used DPN to deposit tetrachloroauric acid onto the

H-terminated silicon substrates, thus yielding gold wires upon

reduction in contact with the substrate.103 Subsequently, disulfide

iniferters were immobilized on these gold wires. PMAA polymer

brushes were then grafted from the functionalized nanopatterns

by means of controlled photopolymerization. The height and

width of the polymer brush nanostructures were controlled by

the preparation conditions (Fig. 15).

AFM tip assisted oxidation

AFM-assisted oxidation is a promising method for the generation

of structures with chemical functionalities.104 This technique

encompasses an electrochemical lithography process in which

a voltage bias applied to a tip establishes a strong, localized electric

field between the tip and the surface of the substrate. Interest has

been focused toward substrates that are covered with additional

organic monolayers. Depending on the patterning conditions, one

of several oxidation processes can occur. For a monolayer of n-

tapping-mode AFM measurements: (a) 240 (�30) nm wide gold wires

deposited on hydride terminated silicon and (b) the subsequently grafted

PMAA brushes. Reprinted with permission from ref. 103.

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octadecyltrichlorosilane (OTS) self-assembled on silicon

substrates, under mild oxidation conditions, an electrochemical

oxidation of the surface hydrophobic end groups converts these

into hydrophilic, chemically active surface –COOH groups.

Harsher oxidation conditions, on the other hand, resulted in the

monolayer breaking down, and initiated the local growth of silicon

oxide (Fig. 16).105 Both of these processes can form precursor

platforms for the subsequent grafting of polymer brushes.

Becer et al. converted surface terminal –CH3 groups of an OTS

monolayer into –COOH by sufficient voltage pulses.106 This

oxidation process is mediated by water and the voltage is applied

via a conductive AFM tip to a copper transmission electron

microscope (TEM) grid placed on the monolayer surface. The

method allowed one to transfer a pattern structure to the substrate

exhibiting features with different dimensions. Parallel patterning

of the surface (due to the presence of the TEM grid) is reliably fast

(a grid structure of 7 mm2 can be oxidized in 10 s and larger

modified areas become accessible). The –COOH groups are then

turned into a bromine-functionalized surface by the site-selective

self-assembly of a bromo-undecyltrichlorosilane precursor

attached to the –COOH functions. This precursor provides the

necessary initiator function for the subsequent ATRP polymeri-

zation of styrene on the surface template (Fig. 17).

Lee et al. combined AFM-assisted oxidation on silicon

substrates with ROMP to fabricate nanopatterned polymer

structures.107 Silicon oxide patterns acted as platforms toward

the selective chemisorption of Ru-based metathesis catalysts

through the reaction of organosilane linkers. After the subse-

quent ring opening polymerization in the presence of cyclo-

octatetraene in solution or 5-ethylidene-norborene in vapor

phase, polyconjugated brushes were successfully grafted forming

regular patterns reaching sub-100 nm widths.

Fig. 16 A schematic representation of the electrochemical oxidation of

OTS-covered silicon substrates. (A) At small negative bias voltages and

short interaction times a selective oxidation of the surface terminal –CH3

end groups to –COOH functions takes place. (B) Under harsher oxida-

tion conditions the monolayer breaks down and a selective anodization of

the silicon/silicon oxide substrate occurs. Within this process, silicon

oxide is locally grown in the vicinity of the AFM tip. Reprinted with


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Fig. 17 (i) A schematic outline of grafting polymer brushes from surface t

electrochemical oxidation of the surface terminal –CH3 groups to –COOH

transferred onto a bromine-functionalized surface; and (d) ATRP polyme

grafted from the surface template. Reprinted with permission from ref. 1

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10 | J. Mater. Chem., 2010, xx, 1–14

lates: (a) self-assembled monolayers of n-octadecyltrichlorosilane; (b) the

nctions assisted by a conducting Cu TEM grid; (c) the COOH groups are

tion of styrene on the surface template. (ii) AFM images of the polymers

392E


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Our group demonstrated the fabrication of precursor silicon

oxide lines and dots through AFM-assisted oxidation. These

then served as site-specific anchoring platforms for the immo-

bilization of initiators and subsequent grafting of hydroxyethyl

methacrylate (HEMA) by ATRP.108 The optimization process

allowed for controlled synthesis of only a few grafted macro-

molecules forming sub-40 nm stimulus-responsive circular

structures (‘‘dot’’ brush) or grafted line structures (‘‘hedge’’

brush). The width of the latter was on the order of merely a few

polymer chains (Fig. 18).

Conclusions

AFM provides a multifunctional analytical platform for the

characterization of surface grafted polymer brushes, as well as

Fig. 18 (i) Preparation of pH-responsive polymer brushes from function

selective functionalization of silicon oxide nanopatterns with ATRP ini

PHEMA ‘‘dot’’ brushes with succinic anhydride. (ii) AFM tapping-mode h

on OTS resist by SPO at different voltages; (b) following surface initiated A

sectional profiles relative to (d) the largest and (e) the thinnest silicon oxide

taken in air. Reprinted with permission from ref. 108.

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for the fabrication of brush nanostructures. AFM can be used to

investigate a wide range of properties of the polymer brushes,

e.g., surface morphology, brush thickness, stimulus responsive

behavior, surface mechanical properties, number average molar

mass and distribution of the polymer as well as grafting density

can be studied by AFM. The technique enables a mapping of

those properties with resolutions on a nanometre range and with

a high signal-to-noise ratio. AFM has become an irreplaceable

method for studying the behavior of stimuli-responsive brushes

in situ, as measurements in varying solvents, at several temper-

atures and pH and so on can easily be performed. It is a powerful

characterization method also for gradient brushes for which it

enables simultaneous visualization of gradients in brush height,

grafting density, copolymer block length, etc. with very high

resolution spanning over hundreds of micrometres dimensions.

ed silicon oxide patterns: (a) formation of silicon oxide nanopatterns; (b)

or; (c) surface-initiated ATRP of HEMA; and (d) functionalization of

ht images of (a) silicon oxide nanodots with variable lateral size deposited

P of PHEMA; and (c) after succinic anhydride functionalization. Cross-

t recorded following every fabrication step are reported. The images were

392E

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Compression of the swollen brushes under the scanning tip

cannot be neglected and it may lead to underestimation of the

brush height (molar mass) measured by AFM. The tip pene-

trating the brushes may change the conformation of the tethered

chains as well as their response, especially in tribological

measurements. This drawback may be partially overcome by

colloidal probe approach or usage of ‘‘blunt’’ tips.

AFM-based nanopatterning methods, such as nanoscratching,

dip-pen nanolithography (DPN) and scanning probe oxidation

(SPO) in combination with other micropatterning techniques,

offer great potential for patterning functional brushes across the

length scales; and this despite the fact that they are at present

relatively slow (mostly serial) and not very suitable for large-scale

and high-throughput pattern formation.

Due to the versatility of AFM as a platform in surface analysis

and nanofabrication, a rapid continuing growth of the use of

AFM in materials chemistry of surface grafted polymer brushes

is expected.

Acknowledgements

The MESA+ institute for the Nanotechnology, and The Neth-

erlands Organization for Scientific Research (NWO, TOP Grant

700.56.322, Macromolecular Nanotechnology with Stimulus

Responsive Polymers) are gratefully acknowledged for financial

support.

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