Synthesis and single molecule force spectroscopy of graft copolymers of poly(2-hydroxyethyl methacrylate-g-ethylene glycol)

Article abstract of DOI:10.1021/ma035065b

In this study, an end-functionalized graft copolymer, thiol-terminated poly(2-hydroxyethyl methacrylate-g-ethylene glycol) or SH-poly(HEMA-g-EG), was synthesized by the atom transfer radical polymerization (ATRP) method and then characterized by ¹

Full text of DOI:10.1021/ma035065b

Macromolecules 2004, 37, 4271-4282
4271
Synthesis and Single Molecule Force Spectroscopy of Graft Copolymers
of Poly(2-hydroxyethyl methacrylate-g-ethylene glycol)
Don g Zh a n g a n d Ch r istin e Or tiz*
Department of Materials Science and Engineering, Massachusetts Institute of Technology,
RM 13-4022, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139
Received J uly 23, 2003; Revised Manuscript Received December 10, 2003
ABSTRACT: In this study, an end-functionalized graft copolymer, thiol-terminated poly(2-hydroxyethyl
methacrylate-g-ethylene glycol) or SH-poly(HEMA-g-EG), was synthesized by the atom transfer radical
1
polymerization (ATRP) method and then characterized by H nuclear magnetic resonance (NMR), gel
permeation chromatography (GPC), and light scattering (LS). The polymer was covalently end-grafted
to an Au substrate at low enough grafting densities to produce well-separated individual polymer
“mushrooms” and single molecule force spectroscopy (SMFS) was then employed to measure the single
polymer extensional elastic properties in phosphate buffered saline solution (PBS, ionic strength (IS) )
-
5
0
.15 M, pH ) 7.4) and deionized H
as an extensible freely jointed chain with a statistical segment length, a ) 0.52 ( 0.09 nm, and a segment
elasticity, ksegment ) 10.5 ( 3.3 N/m, in PBS and a ) 1.03 ( 0.01 nm and ksegment ) 4.2 ( 0.5 N/m in H O.
2
O (IS ∼ 6 × 10 M, pH ) 5). In both solvents, the polymer behaved
2
Comparison to the known single molecule elasticity behavior for the PEG homopolymer suggests a smaller
number and/or smaller magnitude of the noncovalent inter- and intramolecular interactions along the
HEMA backbone and that even at this low PEG side chain grafting density, the side chains are still
quite effective in causing local expansion of the PHEMA backbone by overcoming hydrophobic collapsing
forces relative to the PHEMA homopolymer. Since the PEG chains are still well-solvated in PBS, one
explanation for the small variation in nanomechanical properties with solvent is that in PBS the salt
ions compete for and weaken the inter- and intramolecular H-bonding that does exist along the PHEMA
backbone, causing a local contraction and an increased segmental stiffness due to the reduced noncovalent
nature of the segments.
In tr od u ction
covalent intramolecular interactions between the side
chains and main chain backbone and /or solvent result-
ing in different equilibrium conformations, where sys-
tematic variation and control of the nanomechanical
properties can potentially be achieved through the
length of side chain, the side chain grafting density, and
the type of side chain-main chain interactions. One
may think of this as the nanotechnological analogue of
understanding structure-property relationships and
controlling the stress vs strain curve of a macroscopic
material. In addition, stimulus-responsive properties
may be achieved by using polyelectrolyte side chain or
main chains or by using any interactions that are
sensitive to environmental conditions such as solution
pH and ionic strength.
Functional nanometer-scale devices have now become
a reality; for example, carbon nanotube “nanotweezers”,
molecular junctions consisting of an individual organic
conducting molecule between two gold leads, and
1
2
3
metallic wires consisting of a single row of gold atoms.
Individual or a few polymer chains will soon be able to
be incorporated into such devices to function as molec-
ular springs, switches, sensors, shock absorbers, and
motors to impart adhesive control and serve as the basic
components for dynamic assembliessjust as they do in
biological systems. In biological systems, macromolecu-
lar elastic “spring” properties, as well as other proper-
ties, are controlled through the precise incorporation
and organization of multiple, noncovalent intramolecu-
lar interactions (e.g., electrostatic, hydrophobic, H-
bonds, van der Waals forces, etc.). For synthetic sys-
tems, a comprehensive knowledge of the molecular-level
forces and deformations involved will be needed for
appropriate design and maximum efficiency and reli-
ability. With the advent of single molecule force spec-
troscopy (SMFS),4 which is the direct measurement
of the elasticity of individual macromolecules, single
macromolecule nanomechanical design is fast becoming
a reality.
Our first model system chosen was poly(2-hydroxy-
ethyl methacrylate-g-ethylene glycol) or poly(HEMA-g-
EG). Both PHEMA and PEG homopolymers, copoly-
mers, and cross-linked networks (e.g., hydrogels) are
well-studied, technologically important materials used
extensively for biomedical applications such as intra-
molecular and soft contact lenses, coatings for blood-
contacting devices and surgical sutures, artificial soft
tissue replacements such as postsurgical construction
-6
of female breasts and nasal cartilages, wound (burn)
In this paper, we explore the possibility of using comb-
type graft copolymers to control single molecule nano-
mechanical properties, i.e., the force vs chain end-to-
end separation distance profile, spring constant, extensi-
bility, heterogeneity, nonlinearity, strain hardening, etc.
The general concept is to impart differences in non-
7-13
dressings, and drug delivery.
PEG is neutral, highly
mobile, hydrophilic, and water-soluble at room temper-
ature due to its strong tendency to form H-bonds with
water via the -O- groups and has a large excluded
volume, low van der Waals interactions, and lower
critical solution temperature (LCST) or inverse temper-
1
4
ature solubility. It is also known to contain some
*
Corresponding author: e-mail cortiz@mit.edu.
degree of hydrophobicity due to the -CH2-CH2- groups
1
0.1021/ma035065b CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/04/2004

4
272 Zhang and Ortiz
Macromolecules, Vol. 37, No. 11, 2004
since it is soluble in some organic solvents,14 adsorbs to
1
5-19
hydrophobic surfaces and the air-water interface,
and is used as a surfactant in a number of applica-
2
0-22
tions.
PHEMA is also neutral, and although higher
molecular weight PHEMA is known to be somewhat
hydrophilic and has a relatively high degree of hydra-
tion.2 PHEMA with degrees of polymerization DPn g
4a
4
0 starts to exhibit insoluble fractions and also a
2
4b
LCST. PHEMA can form H-bonds with water or itself
through the -OH or -O-CdO groups. PHEMA is also
expected to have some hydrophobic interactions between
-
CH2- groups and a high van der Waals attractive
interaction, since it is reported to have a high refractive
2
3
index of 1.512.
The technique of atom transfer radical polymerization
ATRP)2
6-35
was chosen to synthesize the poly(HEMA-
(
g-EG) model system described in this paper for a
number of reasons. ATRP is an excellent general method
to synthesize statistical, gradient, block, and graft
copolymers as well as star and hyperbranched polymers
due to the facile availability of functional monomers,
halogen initiators, and catalysts. ATRP is able to
synthesize well-defined polymers with low polydisper-
sities and controlled molecular weights, partially be-
cause a living polymerization is easily controlled, and
a radical process is very tolerant of impurities and other
functionalities in the monomer. In addition, ATRP was
employed here to synthesize the target polymer in
(mono)thiol-terminated form, SH-poly(HEMA-g-EG), so
that a strong tether could be formed between the
polymer chain end and an Au surface, facilitating the
ease of SMFS experiments. Our long-term plans are to
synthesize graft copolymers with different graft density,
graft length, and side chains as well as the introduction
of functional groups to the polymers such as -COOH
groups in poly(MAA-g-EG) to create stimulus responsive
architectures which vary as a function of pH and
solution ionic strength.
Exp er im en ta l Section
Ma ter ia ls. 2-Mercaptoethanol (98%), 2,4-dinitrofluoroben-
zene (99%), 2-bromoisobutyryl bromide (98%), 2-hydroxyethyl
methacrylate (97%), poly(ethylene glycol) methyl ether meth-
acrylate, 2,2′-dipyridyl (bpy, 99%), triethylamine (99.5%),
tetramethylsilane (TMS, 99.9%), chloroform-d, toluene (99.8%,
HPLC grade), N,N-dimethylformamide (DMF, 99.9%, HPLC
grade), and copper(I) bromide (99.999%) were purchased from
Aldrich. Methanol-d
ratories was used as received. De-Hibit 200 resin was obtained
from Polysciences. Polystyrene (PS) standards with M of
82 100, 44 000, 29 300, 13 700, and 2430 were obtained from
Aldrich. Poly(methyl methacrylate) (PMMA) standards with
of 350 000 from Polysciences and 143 000, 75 000, 68 000,
0 100, and 6540 from Polymer Laboratory were obtained.
Poly(ethylene glycol) (PEG) standards with M of 30 800,
3 500, and 2100 and M of 93 750, 73 400, and 18 300 were
4
(99.8%) from Cambridge Isotope Labo-
w
3
F igu r e 1. Synthesis of DPEBM (a) and SH-poly(HEMA-g-
EG) (b).
M
5
w
sized according to a procedure described elsewhere with some
modifications as shown in Figure 1a.3
6-38
In a 250 mL, three-
w
5
w
necked round-bottom flask equipped with a condenser and
paddle stirrer, 2.10 g of mercaptoethanol dissolved in 20 mL
obtained from Polymer Source and Polymer Laboratory,
respectively. All water used for solutions, rinsing, and storage
was deionized (18 MΩ‚cm resistance, Purelab Plus UV/UF, US
filter) and are known to have pH ∼ 5 and an ionic strength IS
of CHCl
3
was slowly added to a mixture of 5.00 g of 2,4-
N in 30 mL of CHCl
dinitrofluorobenzene and 8.14 g of Et
3
3
.
The reaction flask was loosely stoppered to restrict the
exposure to atmospheric moisture. The reaction mixture was
stirred for 15 h at room temperature and poured into 2 M HCl.
A yellow crystal was separated from the water phase and was
washed with distilled water. The product was filtered, washed
with water, and then dried at 25 °C in a vacuum oven (5
mmHg) overnight to yield yellow crystals. For further purifica-
tion, chloroform was used to extract the product. The extrac-
tion solvent was isolated by filtration and held in air overnight
to allow crystallization. The crystal was filtered and dried at
25 °C in a vacuum overnight to yield 4.20 g of 2-(2,4-
dinitrophenylthio)ethanol as a yellow crystalline phase.
-
5
48
∼
6 × 10 M. Phosphate buffered saline tablets (PBS) were
purchased from Sigma, and PBS solutions were prepared by
dissolving one tablet in 200 mL of water to obtain 0.01 M
phosphate buffer, 0.003 M KCl, and 0.137 M NaCl (0.15 M
total), pH 7.4 at 25 °C. Silicon wafers with N type and 100
orientation were purchased from Crystaltek. Chromium from
R. D. Mathis and gold (99.99%) from J & J Materials were
used as received. All the other chemicals were received from
commercial suppliers and used without further purification.
Syn th esis of 2-(2,4-Din itr op h en ylth io)eth yl 2-Br om o-
2
-m eth ylp r op ion a te (DP EBM). The DPEBM was synthe-

Macromolecules, Vol. 37, No. 11, 2004
Graft Copolymers of Poly(HEMA-g-EG) 4273
Ta ble 1. Ch a r a cter iza tion of SH-P oly(HEMA-g-EG)
In a 250 mL, three-neck flask fitted with an addition funnel,
condenser, and paddle stirrer, 4.26 g of 2-bromoisobutyryl
poly(HEMA-g-
poly(HEMA-g-
bromide dissolved in 60 mL of CHCl
stirring to a solution of 3.0 g of 2-(2,4-dinitrophenylthio)ethanol
and 4.26 g of Et N in 60 mL of CHCl . The solution was
3
was added dropwise with
EG) 16K
EG) 120K
1H NMR (methanol-d4)
Mn
3
3
16500
0.4
119700
0.5
maintained at room temperature for 12 h and extracted twice
by 1 M HCl and distilled water. The product was isolated by
suction filtration and dried at 25 °C in a vacuum overnight to
yield yellow crystals. For further purification, methanol was
used to extract the product, and the extraction solvent was
held in air to crystallize. The crystal was filtered and dried at
EG/HEMA (mole ratio)
EG graft density (%)
Mn,PHEMA
0.9
1.0
14400
111
103200
794
NHEMA
Lcontour,PHEMA (nm)
Lmax,PHEMA (nm)
Mn,PEG
34.2
27.9
2080
45
19.8
16.2
3.6
244.6
199.5
16500
360
2
5 °C in a vacuum overnight to yield DPEBM as a yellow
NEG
crystalline phase.
Lcontour,PEG (nm)
Lmax,PEG (nm)
19.8
Syn th esis of SH-P oly(HEMA-g-EG). In a typical polym-
erization (Figure 1b), a 250 mL, three-neck round-bottom flask
was equipped with a condenser, argon inlet, and paddle stirrer.
Inhibitors in monomers were removed by the De-Hibit 200
resin. To 19.1 g of 2-hydroxyethyl methacrylate and 6.8 g of
poly(ethylene glycol) methyl ether methacrylate were added
16.2
3.6
56
Rg,PEG (nm)
LS (DMF)
Mw
Rg (nm)
23400
18.5
51300
26.2
GPC (DMF)
Mw
31600
104000
1.2
0
.158 g of bpy ligand, 0.057 g of CuBr, and 12.5 g of methanol.
Mw/Mn
1.3
Then 0.157 g of DPEBM was added to the stirred solution at
room temperature, followed by purging with argon to remove
oxygen. The polymer molecular weight could be controlled by
adding different concentrations of the catalyst, initiator, and
ligand. To produce lower molecular weight polymer, the
catalyst, initiator, and ligand were 0.057, 0.157, and 0.158 g,
respectively. To produce higher molecular weight polymer, the
catalyst, initiator, and ligand were 0.021, 0.057, and 0.057 g,
respectively. The mixture was heated at 65 °C in a silicone oil
bath and was refluxed with continuous stirring under argon
over 17 h. The reaction mixture was then cooled, and the
polymer was precipitated into cyclohexane. The solid was then
washed with THF, followed by toluene. Then the solid was
dried at 50 °C in a vacuum oven overnight. For further
purification of the polymer, methanol was used to dissolve the
polymer and was fractionated by adding toluene into the
solution. The polymer was washed by cyclohexane several
times and was dried at 50 °C in a vacuum oven overnight to
yield the protected SH-poly(HEMA-g-EG).
Ccrit (mg/mL)
θa (deg, in H2O)
θr (deg, in H2O)
0.36
0.92
89.5 ( 1.8
89.3 ( 2.2
29.0 ( 2.9
31.4 ( 3.5
calculated to be 0.162 and 0.147, respectively. dn/dc for poly-
HEMA-g-EG) was calculated to be 0.160. The molecular
(
1
weight and graft density of the polymer from a H NMR
spectrum were calculated by comparing the integrals of the
protons from the protecting group, PHEMA, and PEG of the
poly(HEMA-g-EG). The contour length, Lcontour, was calculated
for a fully stretched chain using bond lengths of 0.154 nm for
the C-C bond and 0.143 nm for the C-O bond, while the
maximum possible chain length Lmax was calculated using a
planar zigzag chain with tetrahedral bond angles of 109.5°.
P r ep a r a tion of Gold -Coa ted Silicon Wa fer s. Polycrys-
talline gold-coated silicon wafers were prepared by thermal
evaporation. The silicon wafers were clipped to a holder which
was placed in a vacuum chamber of a diffusion-pumped
thermal evaporator. The chamber was pumped to a pressure
In a 20 mL borosilicate glass scintillation vial, 1 g of the
polymer, 0.6 g of 2-mercaptoethanol, 0.02 g of triethylamine,
and 0.2 g of distilled water were added and stirred by a
magnetic stirrer at room temperature for 17 h. The solution
was then precipitated from toluene and dried in a vacuum oven
at 50 °C overnight. For further purification, methanol was used
to dissolve the polymer and was fractionated by adding
petroleum ether into the solution. The polymer was separated
and dried at 50 °C in a vacuum oven overnight to yield SH-
poly(HEMA-g-EG).
-6
below 4.0 × 10 Torr with liquid nitrogen to cool the diffusion
pump. Then chromium was evaporated from a resistively
heated tungsten boat to the silicon wafer surface at a rate of
0
.1 nm/s up to the total thickness of 2 nm to promote adhesion
between silicon oxide and gold. Immediately after gold was
evaporated from another resistively heated tungsten boat at
a rate of 0.3 nm/s up to the total thickness of 50 nm. The
thickness was monitored with a quartz crystal oscillator from
Inficon.
P r ep a r a tion of P olym er Mu sh r oom s on Gold -Coa ted
Silicon Wa fer s. Gold-coated silicon wafers were cleaned by
first placing them in a H
2 4 2 2
SO /H O (3:1) bath at room tem-
Ch a r a cter iza tion . 1H NMR spectra were obtained on
Varian Inova-501 in chloroform-d or methanol-d solvents.
4
Chemical shifts (δ) were in ppm concentrations downfield from
internal TMS reference. GPC spectra were carried out using
DAWN from Wyatt Technology, a Waters 510 HPLC pump,
and a Waters 410 differential refractometer. The GPC calibra-
tion curves for PS, PMMA, and PEG in DMF were then
determined using GPC. A BI-200SM light scattering goniom-
eter from Brookhaven Instruments Corp. was used to build
Zimm static light scattering plots. Different concentrations of
the polymer solutions in the range 1-10 mg/mL in DMF were
prepared first and then were filtered using the 5 mL syringe
from Becton Dickinson with the sterilizing 0.22 µm filter from
Millipore. The calibration was made in toluene. The wave-
length of the laser was set at 514 nm, and the measurement
angles were in the range 60°-130°. The refractive index
increments, dn/dc, of the polymers were calculated on the basis
of an equation that assumes that dn/dc is an additive function
of the composition of a copolymer composing monomers A and
perature for 10 min. The wafers were then extensively rinsed
with water and alcohol. Each of them was placed in a vial
which was extensively rinsed with water and acetone and then
was incubated in a polymer solution for 1 h at room temper-
ature. The concentration of the polymer solution was kept
much lower than the polymer critical concentration, calculated
40
by
-
25
M
Ccrit ) 1.38 × 10
(2)
3
(
)
Rg
where M is the molecular weight of the polymer (g/mol) and
g
R the radius of gyration of the polymer (nm). DMF and
methanol could be used as solvents, and the concentration for
the 16K and 120K polymers was 0.05 mg/mL, which was much
less than their critical concentrations listed in Table 1. The
wafers were then washed with water. The precleaned surfaces
were conditioned by successive rinses in methanol, acetone,
DMF, methanol, and water. They were then immersed in
water or PBS buffer solution to equilibrate for 12 h.
Ch a r a cter iza tion of P olym er Mu sh r oom s on Gold -
Coa ted Silicon Wa fer s. A NanoScope IIIA system controller
and Multimode (Digital Instruments) atomic force microscope
3
9
B, expressed in terms of weight fraction w which is obtained
1
from H NMR
dn
dc
dn
dc A
dn
dc B
)
wA(
+ wB(
(1)
)
)
dn/dc for PHEMA and PEG in methanol at 25 °C were

4
274 Zhang and Ortiz
Macromolecules, Vol. 37, No. 11, 2004
measured directly by SEM. RTIP was found to be equal to ∼26
nm for the probe tip employed in this study.
Resu lts
In itia tor a n d P olym er Ch a r a cter iza tion . The
ATRP method using DPEBM initiator was used to
prepare protected thiol-terminated poly(HEMA-g-EG),
and then the end-protecting group was removed with
mercaptoethanol. The success of the synthesis was
F igu r e 2. SEM images of the Si
nanomechanical experiments reported in this paper.
3 4
N probe tip used in the
1
verified by H NMR with the DPEBM intermediate
2
9
-(2,4-dinitrophenylthio)ethanol (chloroform-d, δ): 9.09-
(
AFM) were used to image the surface of the gold-coated silicon
.08 (d, 1H, J ) 2.5 Hz), 8.40-8.37 (dd, 1H, J 1 ) 2.6,
wafers and end-grafted polymer mushroom surfaces. Tapping
mode AFM (TMAFM) was used under ambient conditions
using the 4197 J V scanner. The typical scan size, scan rate,
scan angle, and image line number were 100-300 nm, 4 Hz,
J 2 ) 8.9 Hz), 7.71-7.68 (d, 1H, J ) 9.1 Hz), 4.08-4.02
(q, 2H, J ) 5.9 Hz), 3.34-3.30 (t, 2H, J ) 6.1 Hz), 1.89
(t, 1H, J ) 5.6 Hz), and the DPEBM initiator (chloroform-
d, δ): 9.12-9.11 (d, 1H, J ) 2.5 Hz), 8.49-8.45 (dd, 1H,
J 1 ) 2.5, J 2 ) 9.1 Hz), 7.85-7.82 (d, 1H, J ) 8.8 Hz),
0
°, and 512, respectively. Basic parameters optimized to obtain
the highest quality of images were the integral gain, propor-
tional gain, amplitude set point, drive frequency, and drive
amplitude. The cantilever used was model TESP from Digital
Instruments, with cantilever length of 125 µm, resonant
frequency of 200-400 kHz, force constant of 20-100 N/m, and
nominal tip radius of curvature of 5-10 nm. The distance
between two polymer chains was calculated using the “Section
Analysis” command in the image analysis software package
of the AFM. A line was drawn between the centers of two
nearest-neighbor polymer chains across the image and the
4
.50-4.46 (t, 2H, J ) 6.0 Hz), 3.43-3.38 (t, 2H, J ) 6.0
1
Hz), and 1.96 (s, 6H). Figure 3a shows the H NMR
spectrum for the protected SH-poly(HEMA-g-EG)16K in
methanol-d4 with peaks at δ 9.08, 8.54, and 8.03 with
equivalent integrated areas that were attributed to the
protons at positions 1, 2, and 3 (chemical structure
shown as inset of Figure 3a) from the 2,4-dinitrophenyl
protecting group. The peaks at δ 4.05 and 3.79 were
attributed to the protons at positions 4 and 5, respec-
tively, from the backbone of PHEMA, and the peak at
δ 3.65 was attributed to the protons at positions 6 and
2
-dimensional topography mapped along this line. Cursors
were then placed on the two points of interest, and the distance
between them was recorded. Contact angle measurements
were performed using deionized water by a VCA 2000 video
contact angle system from ASC.
7
from the PEG graft side chains. Once the protecting
group was removed with mercaptoethanol, the charac-
teristic end group peaks at positions 1, 2, and 3 vanished
as shown in Figure 3b.
Sin gle Molecu le F or ce Sp ectr oscop y (SMF S). SMFS
experiments were conducted using a new cantilever-based
instrument, the Molecular Force Probe (MFP, Asylum Re-
search, Inc) set on a Halcyonics MOD-1 active vibration
isolation system, to measure force, F (nN), vs tip-sample
separation distance, D (nm) (henceforth referred to and labeled
on graphs as “distance”), on “approach” (i.e., probe tip advanc-
ing toward surface) and “retract” (i.e., probe tip moving away
from surface). A full description of this instrument, its limits
of force and displacement detection in fluids, procedures for
spring constant calibration and conversion of raw data, details
of measurement errors, and description of typical force vs
distance curves including the mechanical instabilities of the
1
As shown in Table 1, the H NMR spectra were also
employed to provide further information including the
PEG side chain graft density and the molecular weight
of the polymers. The graft density of PEG to PHEMA
was obtained by the ratio of the integrated area at peaks
6
and 7 to the integrated area at peaks 4 or 5. By setting
the integrated area at peak 1, 2, or 3 as a unit, the
integrated area at peak 4 or 5 could give half of the
number of 2-hydroxyethyl groups. Thus, the corre-
sponding molecular weight of the backbone of the
polymer could be obtained. This combined with the graft
density could give the total molecular weight of the
polymer (Table 1). From these MWs, the HEMA polymer
contour lengths, Lcontour, were calculated as detailed in
the Experimental Section to be ∼34 and ∼245 nm for
the 16K and 120K MWs, respectively, and the PEG
Lcontour was ∼20 nm. Figure 4 is a schematic summariz-
ing these data of the sizes of the polymers synthesized
4
1,42
cantilever, are given in our previous work.
distance (nm) curves were measured at room temperature
using Thermomicroscopes microfabricated V-shaped Si N
4
cantilevers with the square-pyramidal probe tip. The cantile-
vers were with the following dimensions: cantilever length of
Force (nN) vs
3
3
20 µm, cantilever width of 22 µm, cantilever thickness of 0.6
µm, force constant of 0.01 N/m, and resonant frequency of 7
kHz. The cantilevers were at a constant z-piezo displacement
rate ) 1 µm/s reversing immediately between approach and
retract cycles (no dwell time on the surface), z-piezo range )
1
1
µm (slow enough to minimize hydrodynamic effects), and rate
drawn approximately to scale based on H NMR data.
of data acquisition ) 4000 points/s, in PBS solution or
deionized water. On approach, nanomechanical data were
averaged over many experiments and presented always with
standard deviations and normalized by the end probe tip
radius, RTIP, giving a representative effective energy of inter-
action. On retract, the adhesive forces and distances were
analyzed statistically.
Figure 5 shows the light scattering Zimm plot of SH-
poly(HEMA-g-EG)120K in DMF. Since no standard poly-
(
HEMA-g-EG) was available, the MWs obtained by
NMR should be more accurate.
Ch ar acter ization of En d-Gr afted P olym er Mu sh -
r oom s on Gold -Coa ted Silicon Wa fer s by AF M.
TMAFM was applied to characterize the bare gold-
coated silicon wafer surfaces (Figure 6a) and the gold
substrates that had been end-grafted with poly(HEMA-
g-EG) (Figure 6b,c). The gold root-mean-square surface
roughness, island size, and island peak-to-valley height
were found to be ∼1.2 nm, 32.4 ( 3.7 nm, and 2.0 ( 0.6
nm, respectively. The raised white, globular spots on
top of the gold substrate in Figure 6b,c were attributed
to individual separated poly(HEMA-g-EG) chains. As
shown in Figure 6b, the end-grafted polymers were
R
TIP for the probe tip used for nanomechanical testing was
measured via SEM (Figure 2) using a J EOL 6320 SEM with
an operating voltage of 5 kV and magnitude of 64 000×. RTIP
was calculated by eq 3
d/2
sin(90° - R)
RTIP
)
(3)
where R () 35°) was the tip half angle and d was the distance
between two points of tangency obtained by two side lines of
the triangle tip image tangential to the tip end, which was

Macromolecules, Vol. 37, No. 11, 2004
Graft Copolymers of Poly(HEMA-g-EG) 4275
F igu r e 3. 1H NMR spectra in methanol-d
of protected SH-poly(HEMA-g-EG)16K (a) and SH-poly(HEMA-g-EG)16K (b).
4
found to be exhibit a wide variety of conformations,
including collapsed and expanded spherical coils and
partially to fully extended wormlike conformations. This
broad distribution of conformations observed is most
likely due to deformation of the polymer chains by
compressive and lateral forces applied by the probe tip
during imaging and polymer-substrate intermolecular
interactions (e.g., hydrophobic interactions). The dis-
tance between neighboring polymer chains, s, was
measured to be 33.4 ( 6.5 and 71.8 ( 9.0 nm for the
found to be 2.3 ( 0.6 nm, and the maximum polymer
dimension was 30.2 ( 8.2 nm. The heights are quite
small since the polymer chains are unavoidably com-
pressed and dragged laterally by the probe tip during
imaging.
Advancing contact angle (θa) and receding contact
angle (θr) measurements for the end-grafted poly-
(HEMA-g-EG) mushroom gold substrates in DI water
are listed in Table 1 and were ∼89 ( 2° and ∼30 ( 3°,
respectively, for both MW polymers. As a control, θa and
θr for the bare gold-coated silicon wafer surfaces were
85.7 ( 0.7° and 57.1 ( 2.1°, respectively.
1
6K and 120K polymers, respectively. For the 16K
polymer, the height was found to be 1.0 ( 0.4 nm, and
the maximum polymer dimension was measured to be
Na n osca le Su r fa ce In ter a ction s of En d -Gr a fted
P oly(HEMA-g-EG)Mu sh r oom s on Ap p r oa ch . Parts
1
2.5 ( 10.3 nm. For the 120K polymer the height was

4
276 Zhang and Ortiz
Macromolecules, Vol. 37, No. 11, 2004
F igu r e 4. Schematic of sizes of polymers synthesized drawn
approximately to scale.
F igu r e 6. Tapping mode AFM amplitude images in air of bare
Au (a) and end-grafted mushrooms of poly(HEMA-g-EG)16K on
Au: 200 nm scan (b) and 85 nm scan (c).
F igu r e 5. LS Zimm plot of SH-poly(HEMA-g-EG)120K in DMF.
average value of ∼-5 pN, corresponding to a force/
radius of ∼-0.2 mN/m at a distance of ∼3 nm. As shown
in Figure 7b, the poly(HEMA-g-EG)120K exhibited simi-
lar behavior compared to the poly(HEMA-g-EG)16K,
including a long-range, purely repulsive force for Dmax
a and b of Figure 7 show average approach force (pN)
and force/radius (mN/m) vs distance (nm) curves, along
with standard deviations, between a Si3N4 probe tip and
an Au substrate end-grafted with poly(HEMA-g-EG)
<
∼40 nm reaching a larger maximum magnitude of
∼
61 pN, corresponding to a force/radius ∼ 2.3 mN/m at
1
6K and 120K mushrooms, respectively, in H2O (b) and
D ∼ 3 nm. In PBS, the range of the interaction dropped
significantly to Dmax < 4.5 nm but still remained
repulsive reaching a maximum magnitude of ∼20 pN,
corresponding to a force/radius of ∼0.07 mN/m.
PBS (O). As shown in Figure 7a, the poly(HEMA-g-
EG)16K in water exhibited a long-range, purely repulsive
force starting at Dmax ∼ 40 nm and reaching a maximum
magnitude of ∼33 pN, corresponding to a force/radius
∼
1.22 mN/m at D ∼ 3 nm. For D < 3 nm, a drop in the
The relevant approach parameters were calculated as
4
3
force with decreasing distance occurs due to attractive
surface interactions until reaching the constant compli-
ance regime at D ) 0. An interesting force sign reversal
is observed when the nanomechanical experiments are
carried out in PBS, and the interaction becomes purely
attractive for Dmax < 10 nm reaching a maximum
described previously and are summarized in Table 2.
The approximate number of polymer chains present in
the maximum projected surface interaction area at D
) 0 was found to be <1.9 (based on the probe tip radius
and polymer grafting density measured by AFM imag-
ing), and the polymer chains covered <14% of the total

Macromolecules, Vol. 37, No. 11, 2004
Graft Copolymers of Poly(HEMA-g-EG) 4277
Ta ble 2. Releva n t P a r a m eter s for Ap p r oa ch
Na n om ech a n ica l Da ta a
poly(HEMA-g- poly(HEMA-g-
sample
EG)16K
EG)120K
Solvent ) PBS
Asubstrate (nm2)
s (nm)
1320
33
1320
72
area per polymer chain
including Au, s (nm )
1089
5184
2
2
no. polymer chains in Asubstrate
Apolymer (nm )
1.3
110
10
0.27
700
14
2
%
of surface area taken up by
polymer
Solvent ) DI Water
Asubstrate (nm2)
s (nm)
2123
33
2123
72
area per polymer chain
including Au, s (nm )
1089
5184
2
2
no. polymer chains in Asubstrate
Apolymer (nm )
1.9
110
10
0.41
700
14
2
%
of surface area taken up by
polymer
a
Asubstrate is the approximate maximum projected substrate
interaction area. For the experiments conducted in HOH
maximum interaction distance of D ) 27 nm was employed due
a
4
3
to the fact that this calculation is only valid for D < Rtip. Hence,
Asubstrate is expected to be slightly underestimated. Apolymer is the
approximate area taken up by a single polymer chain as calculated
from the measured dimensions by AFM imaging.
modeled using a constant surface charge approximation
based on the numerical solution to the nonlinear Pois-
son-Boltzmann (PB) equation including divalent
4
9-51
ions,
and these simulated curves are shown in
Figure 8 as solid lines. The Si3N4 vs Au EDL interaction
in H2O was not presented here due to the fact that the
hemispherical probe tip approximation used to ap-
5
2
proximate the actual probe tip geometry is expected
to greatly underestimate the magnitude of the force for
-
1
RTIP ∼ 27 nm and interactions that take place up to 5κ
)
195 nm; i.e., much more surface area up the pyrami-
dal sides of the probe tip will contribute to the force and
is not taken into account by the approximation.
Nan om ech an ical P r oper ties of En d-Gr afted P oly-
F igu r e 7. Average force (nN) and force/radius (mN/m) vs
(
HEMA-g-EG)Mu sh r oom s on Retr a ct. When the
distance (nm) curves on approach between Si
end-grafted poly(HEMA-g-EG)16K (a) and poly(HEMA-g-EG)120K
b) on Au in PBS (O) and DI H O (b); smaller symbols
3 4
N probe tip and
Si3N4 probe tip was retracted away from the poly-
(HEMA-g-EG) substrates, it almost always exhibited a
small, short-range pull-off due to surface adhesion. A
small percentage of the total force curves for all four
data sets (<2%) showed an individual attractive peak
in which the magnitude of the force increased nonlin-
early with distance while the majority of the rest (∼98%)
did not exhibit any long-range interaction. These at-
tractive peaks were attributed to the stretching of an
individual polymer chain where some chain segments
had become physically adsorbed to the probe tip on
approach and act as a tether enabling bridging and
extension of the polymer chain between the substrate
and tip. Such typical single macromolecule elasticity
force profiles are given in Figure 8, which shows
individual approach and retract force (nN) and force/
fadius (mN/m) vs distance (nm) curves between the
Si3N4 probe tip and an Au substrate end-grafted with
poly(HEMA-g-EG) mushrooms in H2O and PBS.
(
2
represent standard deviations and solid red lines represent
EDL theoretical simulations.
surface area while the remaining >86% was bare Au
(calculated on the basis of the largest polymer dimension
measured by AFM imaging). Previously, we approxi-
mated the net approach surface interaction of polymer
mushrooms end-grafted on Au as a linear addition of
4
4
the polymer and Au interactions taken separately. The
bare polycrystalline Au used in this study has been
found previously43 to have a net negative charge under
these solution conditions due to adsorbed anions with
2
a surface charge per unit area of σ(Au) ) 0.0144 C/m ,
and the Si3N4 probe tip is known4
5-47
to also have a net
-
negative charge due to ionized silanol (SiO ) and
-
2
silylamine (SiN ) groups of σ(Si3N4) ) -0.032 C/m ,
yielding an electrostatic double-layer (EDL) repulsion
with an electrical interaction Debye length (calculated
At high enough extensions, the elastic restoring force
of the chain becomes equal to and just exceeds the
physisorption force tethering the chain segments to the
tip and the tethered chain segments detach from the
probe tip. Upon detachment, the cantilever exhibits a
mechanical instability and is returned back to its
-
1
from the solution IS ) 0.15 M), κ of ∼1 nm in PBS
4
8
and ∼39 nm in DI H2O. The EDL repulsion solely due
to the Si3N4 vs Au interaction in PBS (i.e., for 90% and
8
6% of total surface area for the poly(HEMA-g-EG)16K
and poly(HEMA-g-EG)120K samples, respectively) was

4
278 Zhang and Ortiz
Macromolecules, Vol. 37, No. 11, 2004
F igu r e 8. Individual force (nN) and force/radius (mN/m) vs distance (nm) curves on approach (thin black lines) and retract
(
(
thick black lines) using a Si
HEMA-g-EG)120K in DI H O, (c) poly(HEMA-g-EG)16K in PBS, and (d) poly(HEMA-g-EG)120K in PBS all fitted to the extensible
3 4 2
N probe tip showing extension of a single chain of (a) poly(HEMA-g-EG)16K in DI H O, (b) poly-
2
FJ C model (solid lines).
undeflected position corresponding to zero force. By
comparing the poly(HEMA-g-EG)16K (Figure 8a,c) to the
poly(HEMA-g-EG)120K (Figure 8b,d), one can immedi-
ately see that, as expected, the extensional distances
were significantly greater for the higher MW polymer
(
a full statistical analysis of these distances will be
presented in the following section) and consistent with
the contour lengths estimated from the known molec-
ular weights (Table 1). A minute percentage (0.03%) of
the total force curves exhibited multiple peaks on retract
as shown in Figure 9.
The solid lines shown in the plots in Figure 8 and
similar SMFS data were fits using the extensible freely
5
3
jointed chain model (extFJ C) which takes into account
configurational entropy as well as high strain enthalpic
extension of the statistical segments. The text insets in
the plots of Figure 8 give the three extFJ C fitting
parameters used for those particular fits in the corre-
sponding graph; i.e., a is the statistical segment (Kuhn)
length and reflects the resistance to bending of the
polymer backbone which dominates entropic elasticity
in the low-to-intermediate strain regime, ksegment is the
statistical segment elasticity which reflects the resis-
tance of the polymer chain backbone to extensional
deformations and contributes to the total molecular
elasticity profile via enthalpic disruptions of supramo-
lecular structure in the intermediate to high strain
regime, and n is the number of statistical segments. The
F igu r e 9. The 0.03% of total force curves exhibited a force
vs distance curve with multiple attractive peaks on retract
similar to that shown here for poly(HEMA-g-EG)120K in PBS.
two parameters that represent the physical nanoscale
properties of the chain (a and ksegment) are independent
of the MW or length of the bridging chain segment. The
extFJ C model fits for the poly(HEMA-g-EG)120K single
molecule elasticity peaks yielded a ) 0.52 ( 0.09 nm,
ksegment ) 10.5 ( 3.3 N/m in PBS and a ) 1.03 ( 0.01
nm, ksegment ) 4.2 ( 0.5 N/m for the H2O. The poly-

Macromolecules, Vol. 37, No. 11, 2004
Graft Copolymers of Poly(HEMA-g-EG) 4279
F igu r e 10. Probability distributions of single molecule detachment forces and distances for (a) poly(HEMA-g-EG)16K and (b)
poly(HEMA-g-EG)120K in DI H O. Inset numerical values are the calculated mean and standard deviation for that dataset.
2
Ta ble 3. Sta tistica l An a lysis of Ad h esion Da ta on
nN and 〈Dadhesion〉 ) 35.98 ( 11.86 nm for the 16K and
Fadhesion〉 ) 0.12 ( 0.11 nN and 〈Dadhesion〉 ) 137.49 (
a
Retr a ct
〈
poly(HEMA-g-
poly(HEMA-g-
64.72 nm for the 120K. The distance distributions were
generally less than Lcontour calculated from the known
MW and greater than the known Rg measured by LS in
DMF (shown as vertical solid bars in Figure 10). An
interesting distinct separation of the distribution of
Dadhesion into two peaks was observed for the poly-
sample
EG)16K
EG)120K
solvent ) PBS
n ) 30, 1.3% of
data
n ) 41, 1.3% of
data
〈
〈
〈
Fadhesion〉 (nN)
Fadhesion/Rtip〉 (mN/m)
Dadhesion〉 (nm)
a (nm)
ksegment (N/m)
solvent ) DI water
0.39 ( 0.18
15.04 ( 6.95
42.31 ( 6.64
0.16 ( 0.13
6.14 ( 4.99
77.17 ( 27.79
0.52 ( 0.09
10.5 ( 3.3
n ) 33, 8.7% of
data
(HEMA-g-EG)120K in H2O with a smaller peak centered
at ∼60 nm and another larger peak at ∼190 nm.
n ) 27, 2.1% of
data
The probability distributions of Fadhesion and Dadhesion
for single molecule poly(HEMA-g-EG) on retract in PBS
are shown in Figure 11 and Table 3. The means values
were as follows: 〈Fadhesion〉 ) 0.39 ( 0.18 nN and
〈Dadhesion〉 ) 42.31 ( 6.64 nm for the 16K, and 〈Fadhesion〉
〈
〈
〈
Fadhesion〉 (nN)
Fadhesion/Rtip〉 (mN/m)
Dadhesion〉 (nm)
a (nm)
ksegment (N/m)
0.15 ( 0.14
5.83 ( 5.25
35.98 ( 11.86
0.12 ( 0.11
4.67 ( 4.11
137.49 ( 65.72
1.03 ( 0.01
4.2 ( 0.5
)
0.16 ( 0.13 nN and 〈Dadhesion〉 ) 77.17 ( 27.79 nm for
a
the 120K. Once again, the distance distributions were
generally less than Lcontour calculated from the known
MW and greater than the known Rg measured by LS in
DMF. A bimodal distribution of Dadhesion was observed
for the poly(HEMA-g-EG)120K in PBS with one larger
peak centered at ∼60 nm and another smaller and
broader peak at ∼125 nm, suggesting two specific
regions of preferred binding along the polymer chain.
Even though the polymer-probe tip binding is expected
to be nonspecific, this phenomenon could arise if the
polymers were initially in conformations on the sub-
strate (due to polymer intramolecular interactions or
polymer-substrate interactions) that would facilitate
binding to these specific regions of the polymer chains.
n is the number of force curves with single molecule peaks
used in the calculation.
(
HEMA-g-EG)16K single molecule elasticity peaks were
generally too short to produce reasonably accurate
estimates of the fitting parameters.
Ad h esion P r ob a b ilit y Dist r ib u t ion s b et w een
Si3N4 P r obe Tip a n d En d -Gr a fted P oly(HEMA-g-
EG) Mu sh r oom s on Retr a ct. The probability distri-
butions of the single molecule poly(HEMA-g-EG) de-
tachment or desorption forces from the probe tip,
Fadhesion, and corresponding distances, Dadhesion, on re-
tract in H2O are shown in Figure 10 and Table 3. The
mean values were as follows: 〈Fadhesion〉 ) 0.15 ( 0.14

4
280 Zhang and Ortiz
Macromolecules, Vol. 37, No. 11, 2004
F igu r e 11. Probability distributions of single molecule detachment forces and distances for (a) poly(HEMA-g-EG)16K and (b)
poly(HEMA-g-EG)120K in PBS. Inset numerical values are the calculated mean and standard deviation for that dataset.
Unfortunately, as mentioned previously, it is suspected
that the AFM imaging of the chains on the substrate
does not yield accurate information on the equilibrium
conformation being probed in the nanomechanical mea-
surements.
similar to, but more expanded than, that found for the
crystalline form. Figure 12 is a schematic that sum-
marizes some of the possible noncovalent interactions
for the poly(HEMA-g-EG) in aqueous solution which
ultimately will determine the equilibrium conformation
of the copolymer and include intramolecular H-bonding,
intermolecular H-bonding with HOH, and hydrophobic
interactions.
Discu ssion
The combination of PHEMA and PEG into a block
copolymer form has just recently started to be explored
in the context of improved aqueous, solubility, hydro-
philicity, and biocompatibility54 and for applications
Na n osca le Su r fa ce In ter a ction s of En d -Gr a fted
P oly(HEMA-g-EG)Mu sh r oom s on Ap p r oa ch . Prior
work reported in the literature related to the approach
data presented in this study include nanomechanical
experiments on thiol end-grafted PEG2K brushes on Au
using a hydrophobically modified C16 probe tip which
show a purely repulsive interaction on approach for D
5
5
such as nanoreactors for nanoparticle preparation.
Unique equilibrium conformations of poly(HEMA-g-EG)
mushrooms are expected due to the amphiphilic nature
of each of the monomers in aqueous solution. Reorienta-
tion of segments on the surface to maximize exposure
of hydrophilic groups to water (in particular the EG side
chains) and hydrophobic groups to the underlying
hydrophobic Au substrate is expected. The advancing
contact angles for both the 16K and 120K polymers were
similar to the Au substrate, which indicated the poly-
mers were initially oriented in such a way that some
hydrophobic groups were exposed to air. However, the
receding contact angles for the polymer mushroom
substrates were much smaller than that of the Au
substrate, so the polymer conformations reoriented in
a way that the hydrophilic groups were exposed to
water. For PEG, it has been postulated that the majority
of chain segments exist in a trans-trans-gauche (ttg)
conformation that is stabilized by water binding, leading
to an (11/2) helical supramolecular structure that is
5
6
< 12 nm. In our experiments, in addition to the EDL
repulsion imparted by the 86-90% surface area taken
up by the Au substrate, the 10%-14% of the surface area
covered by the graft copolymer mushrooms may con-
tribute configurational entropy, enthalpic deformability
due to noncovalent intramolecular interactions and/or
polymer-solvent interactions, and hydration repulsion
as well as attractive van der Waals and hydrophobic
interactions depending on the polymer surface confor-
mation. In PBS the poly(HEMA-g-EG)16K shows a net
attractive force for D < ∼10 nm (Rg(DMF) ) 18.5 nm
5
6
with Rg(PEG2K side chains) ) 3.6 nm), thus suggest-
ing that the polymer is contributing enough van der
Waals and/or hydrophobic interactions from exposed
functional groups to outweigh the EDL repulsion im-
parted by the Au or the PEG2K. Since EDL simulations

Macromolecules, Vol. 37, No. 11, 2004
Graft Copolymers of Poly(HEMA-g-EG) 4281
F igu r e 12. Possible noncovalent interactions for poly(HEMA-g-EG) in aqueous solution.
for the Si3N4 vs Au interaction could not be performed
for the low IS conditions of the H2O, it is unclear
whether the dramatic increase in the magnitude of the
repulsion compared to that of PBS is primarily due to
the increased EDL repulsion of the Au or a conforma-
tional transition of the graft copolymer to a more
expanded form. It is noted that there is a slight increase
in the magnitude of the repulsion for the 120K poly-
the salt ions compete for and weaken the inter- and/or
intramolecular H-bonding that is present along the
PHEMA backbone, shown schematically in Figure 12,
causing a local contraction and an increased segmental
stiffness due to the reduced noncovalent nature of the
segments.
Con clu sion s
(HEMA-g-EG) compared to that for the 16K, suggesting
In this study, a unique graft copolymer, SH-poly-
that the polymer was contributing a component of the
net interaction.
(
HEMA-g-EG), was synthesized by the ATRP method
1
and then characterized by H NMR, GPC, and LS. The
polymer was covalently end-grafted to an Au substrate
at low enough grafting densities to produce well-
separated individual “mushrooms” which were charac-
terized by TMAFM imaging and contact angle measure-
ments. SMFS was then employed to measure the single
polymer extensional elastic properties in PBS and H2O.
In both solvents, the polymer behaved as an extensible
FJ C with a ) 0.52 ( 0.09 nm, ksegment ) 10.5 ( 3.3 N/m
in PBS and a ) 1.03 ( 0.01 nm, ksegment ) 4.2 ( 0.5
N/m for the H2O. Comparison to the known single
molecule elasticity behavior for the PEG homopolymer
suggests a smaller number and/or smaller magnitude
of the noncovalent inter- and intramolecular interac-
tions along the HEMA backbone and that even at this
low PEG side chain grafting density, the side chains are
still quite effective in causing local expansion of the
PHEMA backbone by overcoming hydrophobic collaps-
ing forces relative to the PHEMA homopolymer. Since
the PEG chains are still well-solvated in PBS, one
explanation for the variation in nanomechanical proper-
ties with solvent is that in PBS the salt ions compete
for and weaken the intermolecular H-bonding present
along the PHEMA backbone, causing a local contraction
and an increased segmental stiffness due to the reduced
noncovalent nature of the segments. Ongoing studies
include preparation SMFS of PHEMA homopolymer,
poly(HEMA-g-EG) block copolymers with different side
chain density and different PEG lengths, and other side
chains with varying hydrophilicity/phobicity such as
introduction of -N), -COOH, and -CH2- groups.
Molecu lar Elasticity of En d-Gr afted P oly(HEMA-
g-EG)Mu sh r oom s on Retr a ct. As mentioned previ-
ously, the poly(HEMA-g-EG)120K data are more ame-
nable to interpretation due to the longer Lcontour of ∼264
nm, which enables single macromolecule extensions out
to distances beyond the range of nonspecific tip-surface
interactions (∼40 nm). Clearly, for experiments that
showed extensions out to hundreds of nanometers, such
as those examples given in Figure 8b,d, what is being
measured is primarily the PHEMA backbone since
Lcontour(PEG2K) ∼ 20 nm. The single molecule elasticity
profiles for the poly(HEMA-g-EG) are observed to be
very different from the known unique nanomechanical
behavior of PEG in PBS, which exhibits increased force
in aqueous solution (relative to nonpolar solvents) and
deviation from standard FJ C models due to the presence
of a fully reversible, strain-induced conformational
transition from a tightly H-bonded, water-bound, con-
tracted ttg state to the more extended ttt state.57 Hence,
even though the known PEG Kuhn length is similar (a
0.7 nm)57 to that observed for the poly(HEMA-g-EG),
)
the graft copolymer is in a conformation that requires
much less force for extension than PEG homopolymer,
suggesting a smaller number and/or smaller magnitude
of the noncovalent inter- and intramolecular interac-
tions along the HEMA backbone relative to PEG, which
is quite interesting given the large number of hydro-
phobic groups present on the HEMA backbone. Given
this relatively low value of the resisting force to exten-
sion, it seems that in PBS the PEG side chains are quite
effective in causing local expansion of the PHEMA
backbone by overcoming hydrophobic collapsing forces
relative to the PHEMA homopolymer. Comparison of
the single molecule elasticity profiles for a single poly-
Ack n ow led gm en t. The authors thank the NSF
(PECASE, 0094194) and 3M for financial support and
Dr. T. Alan Hatton’s Group for providing LS help in the
Department of Chemical Engineering, Dr. Anne M.
Mayes’ Group for providing GPC help in the Depart-
ment of Materials Science and Engineering at MIT, and
Delphine Dean for providing EDL simulations.
(HEMA-g-EG)120K in different solvents indicated that
the Kuhn length decreased by ∼50% and the segmental
spring constant increased by ∼60% from the H2O to PBS
solvent. Since the PEG chains are still well-solvated in
PBS, one explanation for this behavior is that in PBS

4
282 Zhang and Ortiz
Macromolecules, Vol. 37, No. 11, 2004
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