PNIPAM Chain Collapse Depends on the Molecular Weight and ...

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<strong>PNIPAM</strong> <strong>Chain</strong> <strong>Collapse</strong> <strong>Depends</strong> on the
Molecular Weight and Grafting Density
Kyle N. Plunkett, Xi Zhu, Jeffrey S. Moore, and Deborah E. Leckband
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Langmuir, 2006, 22 (9), 4259-4266• DOI: 10.1021/la0531502 • Publication Date (Web): 25 March 2006
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<strong>PNIPAM</strong> <strong>Chain</strong> <strong>Collapse</strong> <strong>Depends</strong> on the Molecular Weight and
Grafting Density
Kyle N. Plunkett, †,§ Xi Zhu, ‡,§ Jeffrey S. Moore, † and Deborah E. Leckband* ,†,‡
Department of Chemistry and Department of Chemical and Biomolecular Engineering,
UniVersity of Illinois at Urbana-Champaign, 600 South Mathews AVenue, Urbana, Illinois 61801
ReceiVed NoVember 21, 2005. In Final Form: February 7, 2006
This study demonstrates that the thermally induced collapse of end-grafted poly(N-isopropylacrylamide) (<strong>PNIPAM</strong>)
above the lower critical solution temperature (LCST) of 32°C depends on the chain grafting density and molecular
weight. The polymer was grafted from the surface of a self-assembled monolayer containing the initiator (BrC-
(CH3)2COO(CH2)11S)2, using surface-initiated atom transfer radical polymerization. Varying the reaction time and
monomer concentration controlled the molecular weight, and diluting the initiator in the monolayer altered the grafting
density. Surface force measurements of the polymer films showed that the chain collapse above the LCST decreases
with decreasing grafting density and molecular weight. At T > LCST, the advancing water contact angle increases
sharply on <strong>PNIPAM</strong> films of high molecular weight and grafting density, but the change is less pronounced with films
of low-molecular-weight chains at lower densities. Below the LCST, the force-distance profiles exhibit nonideal
polymer behavior and suggest that the brush architecture comprises dilute outer chains and much denser chains
adjacent to the surface.
Introduction
Thermally responsive polymers such as poly(N-isopropylacrylamide)
(<strong>PNIPAM</strong>) have a wide range of applications in
biotechnology and medicine. <strong>PNIPAM</strong>, in particular, is insoluble
and undergoes a conformational change above its lower critical
solution temperature (LCST) of 32 °C. Below the LCST, the
polymer chains swell in water. Above the LCST, the solvent
quality changes and the polymer segments are thought to become
more hydrophobic. Because the LCST of 32 °C is close to
physiological temperature, this polymer has enormous potential
in technology and in biomedical applications. <strong>PNIPAM</strong> coatings
have been used for the programmed adsorption and release of
proteins and cells from surfaces. 1-3 A new stationary-phase
chromatographic material used grafted <strong>PNIPAM</strong> coatings. 4 The
temperature-dependent retention time of steroids on these columns
was proposed to result from changes in the hydrophobic/
hydrophilic character of the <strong>PNIPAM</strong> above and below the
LCST. 4
Investigations of thermally induced changes in the properties
of <strong>PNIPAM</strong> coatings primarily measured the conformational
change of polymer chains5 or characterized the interfacial
properties. 6 For example, AFM measurements showed that the
swollen thickness of grafted <strong>PNIPAM</strong> films decreased by a factor
of ∼2 when the temperature increased from 25 to 40 °C. 7 The
* To whom correspondence should be addressed. Phone: 217-244-0793.
Fax: 217-333-5052. E-mail: leckband@scs.uiuc.edu.
† Department of Chemistry.
‡ Department of Chemical and Biomolecular Engineering.
§ These authors contributed equally to the work.
(1) Huber, D. L.; Manginell, R. P.; Samara, M. A.; Kim, B.-I.; Bunker, B. C.
Science 2003, 301, 352-354.
(2) Akiyama, Y.; Kikuchi, A.; Yamato, M.; Okano, T. Langmuir 2004, 20,
5506-5511.
(3) Xu, F. J.; Zhong, S. P.; Yung, L. Y. L.; Kang, E. T.; Neoh, K. G.
Biomacromolecules 2004, 5, 2392-2403.
(4) Kanazawa, H.; Yamamoto, K.; Matsushima, Y. Anal. Chem. 1996, 68,
100-105.
(5) Zhou, S.; Wu, C. Macromolecules 1996, 29, 4998-5001.
(6) Liang, L.; Feng, X.; Liu, J.; Rieke, P. C.; Fryxell, G. E. Macromolecules
1998, 31, 7845-7850.
(7) Kidoaki, S.; Ohya, S.; Nakayama, Y.; Matsuda, T. Langmuir 2001, 17,
2402-2407.
Langmuir 2006, 22, 4259-4266
10.1021/la0531502 CCC: $33.50 © 2006 American Chemical Society
Published on Web 03/25/2006
4259
kinetics of swelling and drying the <strong>PNIPAM</strong> brushes above and
below the LCST were also measured by interferometry. 5 Upon
stepping the temperature from 25 °C toT > LCST, the drying
occurred in a three-stage process: a fast initial gel shrinking
followed by a plateau and finally by another drying process. 5
Liang et al. also reported that the water meniscus height in a
capillary tube coated with <strong>PNIPAM</strong> changed by 7 mm in a 2-mm
diameter capillary tube when the temperature increased from 20
°C to about 40 °C. 6
Despite numerous reports of substantial <strong>PNIPAM</strong> changes at
the LCST, such dramatic changes are not observed in all cases.
The molecular weight and grafting density may also play
important roles. 8 Pelton and co-workers investigated the effect
of molecular weight on the kinetics of surface tension lowering
of water by soluble <strong>PNIPAM</strong> homopolymers above and below
the LCST. The kinetics were not very sensitive to temperature
for the lower-molecular-weight (13 100) <strong>PNIPAM</strong>, in contrast
to the higher-molecular-weight (547 000) polymer. 8 In a neutron
reflectivity study, Yim et al. determined the conformation of
<strong>PNIPAM</strong> chains with molecular weights of 33 000 and 220 000
end-grafted on silicon in D2O and in deuterated acetone. 9 They
found no temperature-dependent conformational change with
the lowest-molecular-weight <strong>PNIPAM</strong> (33 000), and only a slight
temperature-dependent thickness change with the higher-molecular-weight
polymer (220 000). At much higher surface density
and molecular weight, the thickness change above the LCST
was much more pronounced. 10
To describe the phase behavior of end-grafted polymers
exhibiting an LCST, Halperin developed a scaling model based
on the de Gennes n-cluster model. 11 Halperin predicted that the
phase behavior of these brushes would depend on the molecular
weight and grafting density of the chains. 12 The model also
predicts a discontinuous segment density profile normal to the
(8) Zhang, J.; Pelton, R. Colloids Surf. 1999, 156, 111-122.
(9) Yim, H.; Kent, M. S.; Huber, D. L. Macromolecules 2003, 2003, 5244-
5251.
(10) Yim, H.; Kent, M. S.; Mendez, S.; Balamurugan, S.; Balamurugan, S. S.;
Lopez, G. P.; Satija, S. Macromolecules 2004, 37, 1994-1997.
(11) Halperin, A. Eur. Phys. J. B 1998, 3, 359-364.
(12) Baulin, V.; Halperin, A. Macromol. Theory Simul. 2003, 12, 549-559.

4260 Langmuir, Vol. 22, No. 9, 2006 Plunkett et al.
Figure 1. Strategy used to graft <strong>PNIPAM</strong> from alkanethiol monolayers on gold. A mixed monolayer of SAM-Br (an ATRP initiatorterminated
alkanethiol) and SAM-OH self-assembled on gold. The ATRP initiator surface concentrations were varied between 5 and 100
mol%. The polymerization of NIPAM from the surface was accomplished using the catalyst system of CuBr and PMDETA in an aqueous
methanol solution.
surface due to the coexistence of a dense inner “phase” and a
dilute outer “phase”. A more recent study used self-consistent
field theory to describe the first-order transition of <strong>PNIPAM</strong>
brushes at the LCST. 13 The latter study predicted that the chain
collapse and transition temperature would depend on both the
grafting density and the molecular weight.
In this study, we systematically investigated the interfacial
properties of end-grafted <strong>PNIPAM</strong> as a function of the chain
grafting density, molecular weight, and temperature. The polymer
was grafted from the surface of an alkanethiol monolayer
on gold containing the initiator (BrC(CH3)2COO(CH2)11S)2
(SAM-Br). The polymer was synthesized by atom transfer radical
polymerization (ATRP). Extensive chain collapse occurred at
the highest grafting density and molecular weight, but the change
in the film thickness decreased with decreasing grafting density
and molecular weight. The LCST was the same within (1 °C
for all films. The force profiles between the <strong>PNIPAM</strong> brushes
and a second surface measured below the LCST further suggest
a one-dimensional phase separation within the polymer brush.
Materials and Methods
Materials. High-purity 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine
(DPPE) and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine
(DSPE) were purchased in powder form (purity >
99%) from Avanti Polar Lipids, Inc. (Alabaster, AL). N-Isopropylacrylamide
(NIPAM), 11-mercapto-1-undecanol, 2-bromo-2methylporpionyl
bromide, CuBr, and CuBr2 were from Aldrich.
1,1,4,7,7-Pentamethyldiethylenetriamine (PMDETA) was purchased
from Acros. NIPAM monomer was recrystallized from hexane. CuBr
was purified by dissolving in 48% HBr and precipitating with the
addition of water. All inorganic salts were high purity (>99.5%) and
were purchased from Aldrich (Milwaukee, WI). All aqueous solutions
were prepared with ultrapure water purified with a Milli-Q UV-Plus
water purification system (Millipore, Bedford, MA). The water had
a resistivity of >18 MΩ cm-1 . HPLC grade methanol and chloroform
purchased from Mallinckrodt (St. Louis, MI) were used to prepare
lipid solutions. High-purity silver shot (99.99%, Aldrich, Milwaukee.
(13) Mendez, S., Curro, J. G., McCoy, J. D., Lopez, G. P. Macromolecules
2005, 38, 174-181.
WI) used for the preparation of silver films on the mica was from
Alfa Aesar (Ward Hill, MA).
Synthesis of (HO(CH2)11S)2 (SAM-OH). 14 11-Mercapto-1undecanol
(500 mg, 2.45 mmol) was dissolved in dichloromethane
(20 mL) and 10% potassium hydrogen carbonate (3 mL). A solution
of bromine (0.2 g, 1.23 mmol) was added slowly to the well-stirred
mixture. The reaction was stirred for 20 min. The organic phase was
separated, and the resulting aqueous phase was extracted with
dichloromethane (2 × 15 mL). The organic layers were combined,
dried over MgSO4, and concentrated to give 428 mg (86.5%). The
sample was then recrystallized with 3:1 hexane/ethanol to yield 370
mg of a white solid (74.8%). 1 H NMR (400 MHz, CDCl3): δ 1.21
(bs,16 H), 1.49 (s, 1 H) 1.56 (pent, J ) 6.6, 2 H), 1.66 (pent, J )
7.0, 2H), 2.65 (t, J ) 6.7, 4 H), 3.64 (t, J ) 6.5, 4 H). m/z (FD) )
406.0
Synthesis of (BrC(CH3)2COO(CH2)11S)2 (SAM-Br). 14 2-Bromo-
2-methylpropionyl bromide (0.475 g, 0.255 mL, 2.06 mmol) was
added dropwise to a solution of the (HO(CH2)11S)2 (0.35 g, 1.72
mmol) and triethylamine (0.89 g, 1.22 mL, 8.6 mmol) in 20 mL
dichloromethane at 0°C. The reaction was stirred for1hat0°C and
2 h at room temperature. The solvent was extracted with 2 N sodium
carbonate (20 mL) saturated with NH4Cl. The organic layer was
dried with MgSO4, concentrated, and purified using silica gel
chromatography (13:1 hexane/ethyl acetate) to produce 400 mg
(66.1%). Rf (13:1 hexane/ethyl acetate) ) 0.30. 1 H NMR (400 MHz,
CDCl3): δ 1.27-1.4 (bs, 16 H), 1.67 (m, 4 H), 1.92 (s, 6 H), 2.67
(t, J ) 7.4, 2 H), 4.16 (t, J ) 6.5, 2 H). m/z (FD) ) 704.0.
Surface-Initiated Polymerization of N-Isopropylacrylamide.
Gold-coated slides were prepared by soaking microscope slides in
an acid-peroxide bath (66% HCl, 33% H2O2) for 30 min at 60 °C.
The slides were washed with water and dried under a stream of
nitrogen. They were then placed in a thermal evaporator and a 10-
Å-thick layer of chromium followed by a 400-Å-thick layer of gold
were deposited at

Poly(N-isopropylacrylamide) <strong>Chain</strong> <strong>Collapse</strong> Langmuir, Vol. 22, No. 9, 2006 4261
monomer concn and
polymerization time
Table 1. Grafting Densities and Molecular Weights of <strong>PNIPAM</strong> Brushes
grafting
density
(Å 2 /chain)
3.9 M and 20 min 229 ( 5 263 000
(n ) 2330)
480 ( 10 238 000
(n ) 2110)
1930 ( 40 207 000
(n ) 1830)
2.0 M and 1min 229 ( 5 74 000
(n ) 655)
480 ( 10 51 000
(n ) 450)
1930 ( 40 60 000
(n ) 530)
(3.75 mL) and subjected to three freeze-pump-thaw cycles. Inside
a glovebox, CuBr (48 mg, 0.33 mmol) and PMDETA (0.174 g, 210
µL, 1.0 mmol) were weighed into a jar with screw-top lid, dissolved
in the MeOH/H2O mixture containing NIPAM, and stirred for 5 min
at room temperature. Gold slides with SAMs of varying initiator
(SAM-Br) concentrations were then submerged in the solution for
the desired reaction time. To quench the reactions, the gold slides
were transferred to a second jar containing CuBr2 (44.8 mg, 0.20
mmol) and PMDETA (0.104 g, 126 µL, 0.60 mmol) in 10 mL of
MeOH. The slides were then rinsed with MeOH and H2O and dried
under a stream of nitrogen.
Gel Permeation Chromatography of <strong>PNIPAM</strong> Released from
the Surface. <strong>PNIPAM</strong> polymers were removed from 28-cm 2 samples
initially coated with <strong>PNIPAM</strong> on 5:95, 20:80, and 100:0 SAM-
Br/SAM-OH monolayers. The chains were polymerized for 20 min
in a 3.9 M solution or 1 min ina2MNIPAM solution. The removal
was accomplished by soaking the <strong>PNIPAM</strong>-coated gold slides in
5mMI2 in methylene chloride for 8 h. The resulting I2 solution
was collected and concentrated and then kept under reduced pressure
at 80 °C for 8 h. The resulting residue was dissolved in 120 µL of
a 90:10:1 THF/MeOH/TEA mixture and transferred to a 100 µL
total volume GPC vial. The injection volume for the GPC was
100 µL.
Film Thickness Measurements by Ellipsometry. Ellipsometry
measurements were obtained with a J. A. Woollam VASE ellipsometer
between 400 and 700 nm at an angle of 70°. A freshly
prepared gold slide was used to determine the optical constants of
the bare gold substrate. The polymer film data were then fit to a
three-layer model with a constant 1-mm silicon oxide base and a
40-nm gold middle layer. The refractive indices used were 1.47 for
silicon oxide and 0.166 for gold. 15 The film thickness (top layer)
was then determined by assuming a Cauchy top layer with a refractive
index of 1.47 for the <strong>PNIPAM</strong>.
Microcontact Printing. Square patterns of SAM-Br initiator were
coated on gold films by microcontact printing (µCP). To accomplish
this, hexadecanethiol was inked onto a stamp and transferred to a
gold slide. Excess hexadecanethiol was washed from the surface,
and the gold slide was submerged in a 100% SAM-Br solution for
1 h. The polymerization of NIPAM from the thus-prepared SAM-Br
regions was accomplished as described above. The resulting films
were then imaged with a Dimension 3100 AFM scanner. The growing
films were visualized at different reaction times. The thickness was
quantified, and the results were compared with the ellipsometry
data.
Surface-Grafted <strong>PNIPAM</strong> Monolayers for Force Measurements.
For the force measurements, one of the silica disks was
coated with a gold film. For each individual experiment, the gold
was incubated with a particular SAM-Br/SAM-OH composition.
The <strong>PNIPAM</strong> was then polymerized from these substrates by
incubating the monolayer in a polymerization solution for 1 and 20
(14) Shah, R. R.; Merreceyes, D.; Husemann, M.; Rees, I.; Abbott, N. L.;
Hawker, C. J.; Hedrick, J. L. Macromolecules 2000, 33, 597-605.
(15) Handbook of Optical Constants of Solids II; Palik, E. D., Ed.; Academic
Press: Boston, 1991.
molecular weight
(degree of
polymerization) RF (Å) s/2RF
chain
configuration
314 0.016 dense brush
295 0.021 dense brush
272 0.046 dense brush
146 0.033 dense brush
117 0.053 dense brush
129 0.096 dense brush
min. The concentration of NIPAM monomers in the polymerization
solution was either 3.9 or 2 M. Three ATRP initiator surface
concentrations (5:95, 20:80, and 100:0 SAM-Br/SAM-OH) were
reacted in parallel in the same solution, and their force profiles were
then compared. The corresponding grafting densities achieved were
229 ( 5, 480 ( 10, and 1930 ( 40 Å 2 /chain, respectively, at the
different initiator concentrations (Table 1). For all six cases (Table
1), the grafted polymer layers were brushes, since the distance between
polymer chains, s, is much less than twice the Flory radius, RF. 16
The Flory radius, RF, is given by
R F ) ln 3/5
where l is the effective segment length, assumed to be 3 Å, 17 and
n is the number of segments in the chain. The values of (s/2)RF are
summarized in Table 1.
Surface Force Measurements of <strong>PNIPAM</strong> Brushes above and
below the LCST. The surface force apparatus (SFA) quantifies the
force between thin films confined between two crossed cylindrical
surfaces as a function of the absolute distance between the disks.
This study used a Mark III SFA (SurForce Corp., Santa Barbara,
CA). 16 Measurements were carried out at 26 and 36 °C, which were
below and above the LCST of <strong>PNIPAM</strong>, respectively. The force,
normalized by the geometric average radii of the two disks R )
(R1R2) 1/2 is determined with a resolution of ∆F/R )(0.1 mN/m
from the deflection of a sensitive leaf spring that supports the lower
disk. 18 The normalized force Fc(D)/R is related to the corresponding
interaction energy per unit area Ef(D) between two equivalent flat
surfaces by the Derjaguin approximation: Ef ) Fc/2πR. 16 This
relationship holds when the separation distance D , R. 16 The absolute
distances are measured with a resolution of (1 Å by multiple beam
interferometry. 18,19
Forces between <strong>PNIPAM</strong> Brushes and Mica. These measurements
quantified the force between the <strong>PNIPAM</strong> brush and a mica
sheet. We did not quantify the force-distance curves between
identical <strong>PNIPAM</strong> monolayers because the gap between the
supporting gold films separated by the thin <strong>PNIPAM</strong> and water is
less than the wavelength of light. Thus, the interference fringes
transmitted through the sample cavity would not be visible by eye.
By contrast, the gap between the reflecting silver and gold films
comprising an intervening 1-µm-thick mica sheet and ∼0.1-µm
<strong>PNIPAM</strong> overlayer, respectively, is large enough that the interferometer
transmits visible light. In the sample configuration, the
<strong>PNIPAM</strong> brush in water interacts with a mica sheet with a silver
back coating. This configuration corresponds to an asymmetric “twolayer”
interferometer. 19 The resonant cavity between the gold layer
on one disk and the silver layer on the other comprises four distinct
layers: namely, the thiol (1), <strong>PNIPAM</strong> (2), water (3), and mica (4).
(16) Israelachvili, J. Intermolecular and Surface forces, 2nd ed.; Academic
Press: London, 1991.
(17) Carey, F. A. Organic Chemistry, 2nd ed.; McGraw-Hill: New York,
1992.
(18) Israelachvili, J.; McGuiggan, P. J. Mater. Res. 1990, 5, 2223-2231.
(19) Israelachvili, J. J. Colloid Interface Sci. 1973, 44, 259-272.
(1)

4262 Langmuir, Vol. 22, No. 9, 2006 Plunkett et al.
Nevertheless, the “two layer” model is still a good approximation
because the mica thickness (5-6 µm) is much larger than the thickness
of the organic overlayers (300-2000 Å). The distance between the
surfaces was therefore calculated using eq 1. 19 For a fringe of order
n in an asymmetric two-layer interferometer, the analytical solution
relating the change in the intersurface separation, D, to the shift in
the fringe wavelength, ∆λn, is 19
(1 - r
tan (4πµ 3D/λ) )
2 ) sin(2nπ∆λn /λ)
- 2r + (1 + r 2 ) cos(2nπ∆λn /λ)
Here, r ) {(µ1 - µ3)}/{(µ1 + µ3)} where µiis the refractive index
of the ith layer. In our calculation, we used 1.608 for the refractive
index of mica. 20 The value of the intervening medium (µ3) depends
on the distance between the surfaces. When surfaces are far from
contact (D . L, where L is the unperturbed polymer thickness), we
used the refractive index of water, µ3) 1.33. When the surfaces are
in contact (D < L), we used a value of 1.4 to take into account the
presence of the organic layers between the gold and mica.
Surface Plasmon Resonance (SPR). To determine whether the
<strong>PNIPAM</strong> adsorbs to the underlying alkanethiol substrate, we
measured the adsorption of soluble <strong>PNIPAM</strong> (Mw ) 3000-8000)
onto an -OH-terminated SAM. SPR measurements were carried
out with a custom-built, computer-controlled apparatus based on the
Kretchman configuration. 21 The samples consisted of several stacked
films on glass. Glass slides were thoroughly cleaned with a mixture
of concentrated hydrochloric acid, hydrogen peroxide, and water
(1:1:1 volume ratio) at 60°C. After drying, the samples were coated
with a 20-Å chromium adhesion layer and a superficial 380-400-Å
gold film by the resistive evaporation of these metals in a vacuum.
The slides were then rinsed with ethanol and blown dry with filtered
nitrogen. They were incubated in the -OH-terminated alkanethiol
(SAM-OH) solution for 2hatroom temperature.
A system of three-way valves was used to inject the <strong>PNIPAM</strong>
solution or to change the solutions without introducing any air bubbles
into the cell. All aqueous solutions used in the SPR experiments
were degassed under vacuum. All data were recorded at room
temperature.
The measured change in the plasmon resonance angle following
the <strong>PNIPAM</strong> incubation corresponded to the amount of adsorbed
polymer. The dependence of the reflected beam intensity, Ir,onthe
incident angle, Θ, was measured before and after incubation with
soluble <strong>PNIPAM</strong>. The obtained Ir vs Θ profiles were fit to theoretical
dispersion curves, which were calculated using Fresnel reflectivity
coefficients for a multilayer system. 21 A calibration slope, k, was
determined first using the refractive index, n, of the sample material
in the dry state. With this slope, the changes in the resonance angle,
Θr, can be directly related to the adsorbed <strong>PNIPAM</strong>.
Contact Angle Measurements. The water contact angle on the
different <strong>PNIPAM</strong> films was measured with a Ramè-Hart goniometer
(Model 100-00). The advancing and receding water contact angles
were measured for each water droplet at temperatures ranging from
26 to 36 °C in a temperature-controlled room. Five measurements
at different spots were taken with each substrate, and the average
of these values was determined.
Results
Polymer Thickness Determined by Ellipsometry and AFM.
To optimize the polymerization conditions and to characterize
the resulting polymer films, 14 we determined the film thickness
by ellipsometry. The dry polymer film thicknesses measured by
ellipsometry are plotted against the reaction time in Figure 2.
The polymers formed with a 3.9 M NIPAM solution yielded
thicker films than did reactions in 2.0 M NIPAM at similar reaction
(20) York, V. N. R. C. N. Handbook of Fillers and Reinforcements for Plastics;
Katz, H. S., Milewsky, J. V., Eds.; Van Nostrand Reinhold Co.: New York, 1986.
(21) Lavrik, N.; Leckband, D. E. Langmuir 2000, 16, 1842-1851.
(1)
Figure 2. <strong>PNIPAM</strong> film thickness measured by ellipsometry as a
function of polymerization time. The film thickness asymptotes at
long reaction times. The thickness of the brush formed in 3.9 M
NIPAM monomer (circles) is greater than brushes synthesized in
2.0 M NIPAM monomer (squares) at comparable reaction times.
Figure 3. AFM images of poly(NIPAM) posts synthesized from
microcontact-printed initiator. The background is hexadecanethiol.
Polymerization times were 1 (A), 2.5 (B), 5 (C), 10 (D), and 20 min
(E) in a 3.9 M NIPAM solution. Surface scans are of a 30 × 30 µm 2
area.
Table 2. Measured Thickness of Dry <strong>PNIPAM</strong> Brushes Grafted
from SAMs with Different Initiator Densities (3.9 M NIPAM)
Br/OH time (min) ellipsometry (Å) AFM (Å)
100:0 1 195 ( 3 159 ( 31
20 557 ( 6 279 ( 50
160 873 ( 11 599 ( 58
20:80 1 140 ( 1 84( 16
20 287 ( 14 212 ( 26
160 357 ( 16 237 ( 17
5:95 1 73 ( 2 -
20 79 ( 2 46( 21
160 121 ( 6 52( 13
times. Under both conditions, the film thicknesses increase rapidly
at the beginning of the reaction but level off after ca. 30 min.
Three SAM-Br/SAM-OH ratios (5:95, 20:80, and 100:0) were
compared in parallel to determine the film thickness-dependence
on the initiator densities. Polymerization reactions were carried
out in 3.9 M or in 2.0 M NIPAM solutions to vary the polymer
molecular weight and hence the film thickness. One gold slide
coated with each of the SAM compositions (each slide was also
microcontact-printed on a small portion of the surface) was
subjected to polymerization times of 1, 20, and 160 min in a 3.9
M NIPAM solution. In addition, a similar sample was reacted
for 1 and 5 min in a 2.0 M NIPAM solution. The resulting dry
film thicknesses are summarized in Table 2. The ellipsometry
data show that the dried polymer thickness decreased with
decreasing initiator surface concentrations for a given polymerization
time.
Figure 3 shows the patterns generated by µCP. The AFM
images of the thus-patterned surfaces show that the difference

Poly(N-isopropylacrylamide) <strong>Chain</strong> <strong>Collapse</strong> Langmuir, Vol. 22, No. 9, 2006 4263
Figure 4. Advancing contact angles on <strong>PNIPAM</strong> brushes measured
at different temperatures. The polymer was grafted from a 100:0
SAM-Br/SAM-OH alkanethiol monolayer. Film thicknesses were
9.6, 13.4, 19.5, 55.7, and 87.3 nm (from bottom to top).
between the hexadecanethiol (background) and <strong>PNIPAM</strong> (posts).
The polymerization reaction times were 1, 2.5, 5, 10, and 20 min
in a 3.9 M NIPAM solution. The same dependence on the initiator
density was also evident in the AFM images. However, the film
thickness values measured by AFM (Table 2) were considerably
lower than those determined by ellipsometry.
Water Contact Angle Measurements. The advancing contact
angles of sessile water droplets on various <strong>PNIPAM</strong> films were
measured between 26 and 36 °C in increments of 1 °C. Figure
4 compares the advancing contact angles on films at a grafting
density of 229 ( 5Å 2 /chain, but with varying film thickness.
There was a sharp transition at the LCST (32 °C) for all film
thicknesses between 76 and 873 Å. However, the absolute values
of contact angles above and below the LCST shifted on the
thickest polymer films. On thin (76-172 Å) films, the advancing
contact angles increased very little with increasing film thickness.
However, with films thicker than 200 Å, the advancing contact
angle increases, indicating that the surfaces of the (dry) polymer
films become more hydrophobic.
We compared the advancing and receding contact angles
measured on <strong>PNIPAM</strong> films with molecular weight Mw ) 47 000
and 19 000 at a grafting density of 480 ( 10 Å 2 /chain. With the
47 000 Mw chains, the advancing contact angle was 68.4° ( 0.4°
between 26 and 31 °C but increased to 79° ( 1° above 32 °C.
In contrast, on films of <strong>PNIPAM</strong> with Mw ) 19 000, the contact
angle was constant at 71° ( 1° up to 36 °C.
The receding contact angles were all between 25 and 28 °C.
Within experimental error, they were independent of the film
thickness or grafting density and were also similar over the entire
temperature range for all polymer films considered. This indicates
that the <strong>PNIPAM</strong> films expose similar chemical groups to the
solvent both below and above the LCST.
Surface Force Measurements between Grafted <strong>PNIPAM</strong>
Brushes and Mica. The forces between <strong>PNIPAM</strong> monolayers
and bare mica were measured in pure water at both 26 and 36
°C. This was done for two reasons. First, the volume change
caused by the temperature-dependent phase transition can be
directly quantified from changes in the force-distance profile.
Second, any significant change in the <strong>PNIPAM</strong> hydrophobicity
should change the interaction between the polymer and a
hydrophilic surface, such as mica.
Definition of D ) 0. In force measurements between grafted
polymers and mica, D ) 0 Å is defined as contact between the
SAM surface beneath the polymer chains and the mica surface
(Figure 5). We first calibrated the distance, T, between the gold
and mica. Following the monolayer assembly and polymerization,
Figure 5. Sample configuration used in direct force measurements
between the <strong>PNIPAM</strong> brushes and mica. The distance, D, reported
in the text refers to the absolute distance between the terminal groups
of the SAM layer and the mica surface.
Figure 6. (a) Normalized force versus the distance between mica
and a <strong>PNIPAM</strong> brush polymerized at a grafting density of 229 (
5Å 2 /chain and a molecular weight of 263 000 g/mol. The filled
triangles show the force measured during approach at 26 °C, and
the open triangles show the force during separation. The filled squares
show the advancing curves measured at 36 °C, and the open squares
correspond to the receding curves. The filled and open circles show
the advancing and receding curves, respectively, after the temperature
was again reduced to 26 °C. The out arrow indicates the position
of adhesive failure. (b) Fit of the force profile measured at 26 °C
to eq 2 (line).
we measured the change in the distance of closest approach
between the gold and mica. The distance between the surfaces,
D, was determined by subtracting the thickness of the SAM
layer from the total distance, T (Figure 5). Thus, D ) T - tSAM
where the tSAM ) 10 Å alkanethiol monolayer thickness was
determined by ellipsometry.
Force Measurements between High-Molecular-Weight
<strong>PNIPAM</strong> Brushes and Mica. To test whether the collapse of
<strong>PNIPAM</strong> brushes depends on the grafting architecture, we
systematically investigated the temperature-dependent behavior
of <strong>PNIPAM</strong> films with similar high molecular weights (Mw )
207 000, 238 000, and 263 000) but different grafting densities
(Table 1).
Figure 6 shows the normalized force profile between mica
and a brush of 263 000 Mw chains at a density of 229 ( 5Å 2 /
chain. The force was repulsive during approach, and the onset
of the repulsion, which corresponds to the polymer extension,
L, was 2690 ( 50 Å. This value is much larger than 1250 ( 25
Å, which is the thickness of the dry film measured by ellipsometry,
and indicates that the chains are swollen in water at 26 °C. Upon
separation, the surfaces adhered and the disks jumped out of

4264 Langmuir, Vol. 22, No. 9, 2006 Plunkett et al.
adhesive contact at 2390 ( 35 Å with a pull-off force of -0.49
( 0.07 mN/m. This adhesion is most likely due to the attraction
between the polar groups on the mica surface and polar groups
of the polymer. 22
The forces were then measured at 36 °C. The sample was kept
in the temperature-controlled room at 36 °C for 10 h before
taking measurements to allow the polymer to equilibrate. As
shown in Figure 6, during approach, the forces were still repulsive
but the range of the repulsion decreased to 2074 ( 64 Å,
confirming that these dense, high-molecular-weight chains
collapse above the LCST. The overall thickness of the brush
decreased by 30% at T > LCST (Table 3). The collapse is
reversible. Reducing the temperature to 26 °C restored the initially
measured force profiles (Figure 6). Again, the <strong>PNIPAM</strong> adhered
to the mica, and the disks jumped out of contact at 2280 ( 100
Å with a pull-off force of -0.48 ( 0.06 mN/m.
Forces were also measured at a lower grafting density of 480
( 10 Å 2 /chain, and with a <strong>PNIPAM</strong> molecular weight of 238 000.
At 26 °C, the range of the repulsive force was 2132 ( 50 Å. The
onset of the repulsion occurred at a shorter distance, compared
to the range at the higher grafting density (229 ( 5Å 2 /chain)
and slightly higher molecular weight (238 000). At high grafting
density, each chain occupies a smaller area and extends away
from the surface to form a brush. 23 Although there was some
hysteresis upon separation of the <strong>PNIPAM</strong> and mica, the surfaces
did not adhere. After increasing the temperature to 36 °C, the
range of the repulsion decreased to 1568 ( 50 Å (Table 3).
Finally, forces were measured at the third and lowest grafting
density of 1930 ( 40 Å 2 /chain at both 26 and 36 °C (Figure 7).
The molecular weight of the <strong>PNIPAM</strong> was somewhat lower at
207 000. Upon approach, the forces were repulsive at both 26
and 36 °C, with a range of the steric repulsion at 1209 ( 25 and
836 ( 25 Å, respectively (Figure 7, Table 3). There was a slight
hysteresis between advancing and receding curves at both
temperatures (Figure 7), but there was no adhesion. At this grafting
density, the film thickness decreased by 26% at 36 °C (Table 3).
Force Measurements between Low-Molecular-Weight
<strong>PNIPAM</strong> Brushes and Mica. To further investigate the
molecular weight dependence of the chain collapse, SFA
measurements were carried out at 26 and 36 °C with <strong>PNIPAM</strong>
brushes with lower-molecular-weight chains but at the same
grafting densities described above (see Table 1).
Figure 8 shows the force profile measured between mica and
a <strong>PNIPAM</strong> brush with Mw ) 74 000 chains at 229 ( 25 Å 2 /
chain. Upon approach, the surfaces repelled at both temperatures
and there was no adhesion. The onset of the repulsion was 1007
( 30Åat26°C and 830 ( 30Åat36°C. Above the LCST,
the thickness decreased by ∼18%.
A similar repulsive force profile was measured at the
intermediate grafting density of 480 ( 10 Å 2 /chain and with an
average molecular weight of 51 000. In this case, the onset of
the repulsion was at 561 ( 20Åat26°C and 492 ( 20Åat
36 °C (Table 3). At the lowest grafting density of 1930 ( 40
Å 2 /chain (Mw ) 60 000), the force profiles measured at 26 and
36 °C were very similar. The brush thickness was 383 ( 20 Å
Figure 7. (a) Normalized force versus distance between mica and
a <strong>PNIPAM</strong> brush polymerized at a grafting density of 1930 ( 40
Å 2 /chain and a molecular weight of 207 000 g/mol. The filled triangles
and open triangles indicate the force measured during approach and
separation, respectively, at 26 °C. The filled and open squares
correspond to force curves measured during approach and separation,
respectively, at 36 °C. The filled and open circles indicate the force
curves measured during approach and separation, respectively, after
the temperature was restored to 26 °C. (b) Fit of the force profile
at 26 °C to eq 2 (line).
at 26 °C and 380 ( 20Åat36°C (Table 3). There was some
hysteresis between the advancing and receding curves but no
adhesion.
Comparison of the Force Profiles Measured at 26 °C with
Simple Polymer Theory. As shown in Table 1, the values of
s/2RF in all cases were much smaller than 1, and the polymers
were in the brush regime. 16 To test whether <strong>PNIPAM</strong> behaves
as a simple polymer below the LCST, the force profiles were
compared to the Alexander-de Gennes model, which describes
the forces between end-grafted brushes in good solvent. 23-25
Equation 2 gives the theoretical normalized force-distance profile
between brushes on opposed crossed-cylinders.
The theoretical brush thickness, L, is
Here s refers to the average distance between the grafting sites,
k is the Boltzmann constant, and T is the absolute temperature.
Table 3. Measured Thickness of <strong>PNIPAM</strong> Films above and below the LCST
monomer concn
grafting density
polymer thickness (Å)
and polymerization time
(Å2 /chain) 26 °C 36 °C % change
3.9 M and 20 min 229 ( 5 2690 ( 50 2074 ( 64 30
480 ( 10 2132 ( 50 1568 ( 50 27
1930 ( 40 1209 ( 25 836 ( 25 26
2.0 M and 1 min 229 ( 5 1007 ( 30 830 ( 30 18
480 ( 10 561 ( 20 492 ( 20 12
1930 ( 40 383 ( 20 380 ( 20 0
F(R)
R
16kTπL L
≈
3 [7( 35s D) 5/4
+ 5( D
L) 7/4
- 12]
L ) s( R F
s ) 5/3
(2)
(3)

Poly(N-isopropylacrylamide) <strong>Chain</strong> <strong>Collapse</strong> Langmuir, Vol. 22, No. 9, 2006 4265
monomer concn and
polymerization time
grafting density
(Å 2 /chain) s/2RF
Since eq 2 is a scaling relationship, the prefactor is not exact.
The prefactor should, however, be of order unity. 26
In our study, forces were measured between a polymer brush
and a hard wall (mica surface) instead of between two polymer
brushes. Since the model assumes that the brushes are impenetrable,
we replaced one polymer layer with a hard wall, and 2L
(twice the polymer thickness) was replaced with L in eq 2. In
fits of the data to eq 2, both the prefactor and the brush thickness
(L) were allowed to vary. One point worth noting is that the mica
and the polymer brush adhere, and the theory does not address
this. Nevertheless, comparing the data to eq 2 gives a semiquantitative
indication of how closely <strong>PNIPAM</strong> below the LCST
approximates an ideal polymer in good solvent.
Figures 6b, 7b, and 8b show the fitting results for all force
profiles measured at 26 °C. At the highest grafting density (229
( 25 Å 2 /chain) and molecular weight (>200 000), the force
profiles agree with theory at all distances measured (Figure 6b)
(Table 4). At all other grafting densities and molecular weights
studied, there was good agreement with theory at large distances,
but the data deviated from the theory at D < 250 Å. We therefore
fit only the force profile at large distances to eq 2 and obtained
the value for L. The experimentally determined polymer thickness
was assumed to roughly coincide with the onset of steric repulsion.
The model-determined thickness was calculated with eq 3. Table
4 summarizes the results of these three different determinations
of the polymer extension.
Table 4. Polymer Brush Parameters
measured
thickness (Å)
model calcd
thickness (Å)
fitted thickness
(Å) R 2 prefactor
3.9 M, 20 min 229 ( 5 0.016 2690 ( 50 3181 2737 ( 100 0.98 5.5 ( 0.5
480 ( 10 0.021 2132 ( 50 2458 2175 ( 120 0.94 1 ( 0.1
1930 ( 40 0.046 1209 ( 25 1344 1300 ( 90 0.97 0.4 ( 0.05
2.0 M, 1 min 229 ( 5 0.033 1007 ( 30 894 1131 ( 90 0.96 1.2 ( 0.05
480 ( 10 0.053 561 ( 20 526 580 ( 80 0.95 0.8 ( 0.2
1930 ( 40 0.096 383 ( 20 390 400 ( 70 0.98 0.2 ( 0.02
Figure 8. (a) Normalized force vs distance between mica and a
<strong>PNIPAM</strong> brush polymerized at a grafting density of 299 ( 5Å 2 /
chain and Mw of 74 000. The filled and open triangles indicate the
force measured during approach and separation, respectively, at 26
°C. The filled and open squares show the force curves measured
during approach and separation, respectively, at 36 °C. The filled
and open circles indicate the force curves measured during approach
and separation, respectively, after the temperature was restored to
26 °C. (b) Fit of the force profile at 26 °C to eq 2 (line).
Adsorption of Soluble <strong>PNIPAM</strong> on OH-Terminated SAMs.
To determine whether the <strong>PNIPAM</strong> adsorbs to the underlying
SAM, the adsorption of soluble <strong>PNIPAM</strong> chains (Mw ) 100 000)
on a SAM-OH layer was measured by SPR at 26 °C. An aqueous
10-mg/mL solution was injected into SPR cell. The effective
optical thickness increased instantly to 2.6 ( 0.2 nm. After the
signal stabilized, the cell was flushed with pure water. The optical
thickness immediately dropped back to 0.0 ( 0.2 nm. The abrupt
change in the resonance angle upon changing the solution
indicated that the change is due to the refractive index change
of the solution instead of to slower polymer adsorption. This
result indicated that <strong>PNIPAM</strong> did not adhere to the -OHterminated
SAM layer at T < LCST.
Discussion
This systematic study of the temperature-dependent changes
in the interfacial properties of surface-grafted <strong>PNIPAM</strong> chains
above and below the LCST shows that the extent of chain collapse
above the LCST depends on both the grafting density and
molecular weight. <strong>Chain</strong> collapse occurred at high grafting density
and molecular weight as expected. However, the collapse and
corresponding changes in interfacial properties depend on the
grafting density and molecular weight (Table 3). These findings
are consistent with previous reports, which suggested that the
extent of the change in <strong>PNIPAM</strong> at the transition temperature
depends on the molecular weight. 8,9 They also confirm qualitatively
the predicted dependence on both chain length and grafting
density. 13
The synthetic procedures used in this study afforded good
control of the grafting density and molecular weight of the
<strong>PNIPAM</strong>. It was not possible to control the molecular weights
more tightly on account of the polymerization characteristics.
The film thicknesses increased rapidly at the beginning of
polymerization reaction but leveled off after ca. 30 min (Figure
2). This is more characteristic of a conventional redox-initiated
polymerization with chain termination via polymer coupling than
of a controlled living polymerization. Huang and co-workers
observed this polymerization behavior (asymptoting film thickness)
for the ATRP polymerization of 2-hydroxyethyl methacrylate.
27 In their system, the addition of 30 mol% CuBr2, which
is the ATRP deactivating species, provided greater control of the
film thickness over a longer reaction time. Unfortunately, the
addition of CuBr2 to the NIPAM solution yielded thin films (ca.
2-3 nm) even after long polymerization times. Such films were
not useful for these investigations of both low- and highmolecular-weight
polymers. The polymerization conditions in
the absence of CuBr2 did yield polymer brushes with controlled
thicknesses on a 100% SAM-Br surface. They were therefore
(22) Klein, J.; Luckham, P. F. Nature 1984, 308, 836-837.
(23) Alexander, S. J. Physique 1977, 38, 983-987.
(24) de Gennes, P. Macromolecules 1981, 14, 1637-1644.
(25) de Gennes, P. AdV. Colloid Interface Sci. 1987, 27, 189-209.
(26) Kuhl, T. L.; Leckband, D. E.; Lasic, D. D.; Israelachvili, J. Biophys. J.
1994, 66, 1479-1488.
(27) Huang, W.; Kim, J. B.; Bruening, M. L.; Baker, G. L. Macromolecules
2002, 35, 1175-1179.

4266 Langmuir, Vol. 22, No. 9, 2006 Plunkett et al.
used to fabricate other polymer brushes on monolayers with
varying amounts of ATRP initiator. Despite some variation in
molecular weight for given reaction conditions, the differences
are less than 15%.
AFM measurements of the dry <strong>PNIPAM</strong> film thicknesses are
considerably lower than those determined by ellipsometry (Table
1). Two other groups investigating <strong>PNIPAM</strong> on gold 28 and on
silicon 29 observed similar differences in the results obtained with
these two techniques. They suggested that the adhesion between
the <strong>PNIPAM</strong> and AFM tip dampened the tip oscillation and
increased the hysteresis in the force-distance curve. This could
introduce systematic errors in the film thickness determinations.
The steric thickness of the swollen <strong>PNIPAM</strong> films in water
measured with the SFA is nevertheless more relevant to the
brush properties in aqueous media. The latter is nearly an order
of magnitude larger than the dried films measured by either
AFM or ellipsometry.
The contact angles on <strong>PNIPAM</strong> brushes with Mw > 47 000
sharply increase above ∼32 °C. However, the contact angles on
the lower-molecular-weight brushes (