High density cationic polymer brushes from combined "click chemistry" and RAFT-mediated polymerization

Article abstract of DOI:10.1002/pola.26087

A simple method for preparing cationic poly[(ar-vinylbenzyl) trimethylammonium chloride)] [poly(VBTAC)] brushes was used by combined technology of "click chemistry" and reversible addition-fragmentation chain transfer (RAFT) polymerization. Initially, sil

Full text of DOI:10.1002/pola.26087

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High Density Cationic Polymer Brushes from Combined
‘‘Click Chemistry’’ and RAFT-Mediated Polymerization
Serkan Demirci,1 Tuncer Caykara2
1Department of Chemistry, Amasya University, Amasya 05100, Turkey
2Department of Chemistry, Gazi University, Ankara 06500, Turkey
Correspondence to: T. Caykara (E-mail: caykara@gazi.edu.tr)
Received 2 March 2012; accepted 30 March 2012; published online
DOI: 10.1002/pola.26087
ABSTRACT: A simple method for preparing cationic poly[(ar-
vinylbenzyl)trimethylammonium chloride)] [poly(VBTAC)]
infrared spectroscopy, atomic force microscopy, and ellipsom-
etry analysis. The addition of free CPAD was required for the
formation of well-defined polymer brushes, which subse-
quently resulted in the presence of free polymer chains in so-
lution. The free polymer chains were isolated and used to
estimate the molecular weights and polydispersity index of
chains attached to the surface. In addition, by varying the po-
lymerization time, we were able to obtain poly(VBTAC)
brushes with grafting density up to 0.78 chains/nm2 with
brushes was used by combined technology of ‘‘click chemis-
try’’ and reversible addition-fragmentation chain transfer
(RAFT) polymerization. Initially, silicon surfaces were modified
with RAFT chain transfer agent by using a click reaction
involving an azide-modified silicon wafer and alkyne-termi-
nated 4-cyanopentanoic acid dithiobenzoate (CPAD). A series
of poly(VBTAC) brushes on silicon surface with different mo-
lecular weights, thicknesses, and grafting densities were then
synthesized by RAFT-mediated polymerization from the sur-
face immobilized CPAD. The immobilization of CPAD on the
silicon wafer and the subsequent polymer formation were
characterized by X-ray photoelectron spectroscopy, water con-
tact angle measurements, grazing angle-Fourier transform
homogeneous distributions of apparent needle-like structures.
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KEYWORDS: click chemistry; morphology; reversible addition
fragmentation chain transfer (RAFT) polymerization; surfaces
INTRODUCTION The surface modification of silicon or gold
substrates with polymeric chains to form functional struc-
tures has attached extensive interest owing to their potential
applications in electronics, optics, catalysts, and biosensors.1–5
Among various solid substrates, silicon wafers are substrates
of choice to prepare the polymer brushes due to their chemi-
cal resistance, mechanical stability, and relatively low
costs.6,7 So far, silicon substrates coated with well-defined
polymeric chains have been achieved by various polymeriza-
tion techniques. Among them, controlled radical polymeriza-
tion approaches such as nitroxide-mediated polymerization
(NMP),8,9 atom transfer radical polymerization (ATRP),10–14
single-electron transfer living radical polymerization (SET-
LRP),15–17 and reversible addition-fragmentation chain trans-
fer (RAFT) polymerization18–27 have been widely applied to
graft polymeric chains onto solid substrates since they can
afford well-defined polymers with controlled molecular
weight, low polydispersity, and variable functionality. Among
the available controlled free radical techniques, RAFT poly-
merization has arguably the most important commercial sig-
nificance. RAFT works with a wide range of monomers com-
pared with ATRP and does not need such critical
experimental conditions as NMP and SET-LRP. However, low
grafting density and a difficulty in the fabrication of surface
immobilized RAFT chain transfer agent are two major prob-
lems, which hinder the use of RAFT-mediated polymerization
for synthesis of polymer brushes. The polymer chain density
is directly related to important surface properties such as
hydrolytic stability and fundamental dimensions such as
brush height.28,29 By taking advantage of the properties of
high density polymer brushes one can create novel surfaces
with smart or responsive systems. There is a clear need to
increase the grafting density of polymer brushes on silicon
surface and simplify the immobilization of RAFT chain trans-
fer agent.
With the development of click chemistry, it is possible that
immobilization of RAFT chain transfer agent can be simpli-
fied, defined, and modularized. Click reactions have showed
high yields and exceptional tolerance toward a wide range of
functional groups and reaction conditions in the material sci-
ence.30–34 Therefore, the combination of click reaction and
RAFT polymerization can be a powerful strategy for prepara-
tion of polymer brushes on silicon or gold substrates.
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In this study, we report the successful preparation of high
density, well-defined cationic poly[(ar-vinylbenzyl)trimethy-
lammonium chloride)] [poly(VBTAC)] brushes on silicon wa-
fer. Our strategy includes (i) synthesis of alkyne-terminated
RAFT chain transfer agent, (ii) preparation of azide-modified
silicon wafers, (iii) facile click reaction to immobilize RAFT
chain transfer agent, and (iv) RAFT-mediated polymerization
from the surface immobilized RAFT chain transfer agent. The
formation of cationic poly(VBTAC) brushes was evidenced by
using ellipsometry, X-ray photoelectron spectroscopy (XPS),
grazing angle-Fourier transform infrared spectroscopy
(GA-FTIR), atomic force microscopy (AFM), and contact-angle
goniometer. Grafting densities of the immobilized RAFT chain
transfer agent and polymer chains on silicon wafer were cal-
culated and compared with the literature values.
hydrophilic surface. The hydroxylated silicon wafers were
immersed in a solution of BPTS (1%, v/v), in dry toluene,
for 4 h at 120 ꢃC. The wafers were then washed with tolu-
ene, DCM in an ultrasonic bath, and dried in a vacuum. The
bromopropyl-modified silicon wafers were immersed inꢃ a
saturated solution of sodium azide in DMF for 18 h at 80 C.
Finally, the azide-modified silicon wafers were rinsed with
DMF, sonicated in ethanol and water for 3 min, and dried in
a stream of nitrogen.
Preparation of RAFT Chain Transfer Agent Immobilized
Silicon Wafer
Alkyne-terminated CPAD (3.0 g) were dissolved in 100 mL
of DMF and the mixture was stirred for 10 min in an inert
atmosphere. The azide-modified silicon wafers were placed
into the solution and then, a solution of CuSO4 (0.05 g) in 10
mL of water and another solution of sodium ascorbate (0.12
g) in 10 mL of water were added orderly into the solution.
The reaction mixture was reacted at 50 ꢃC for 24 h. The
RAFT chain transfer agent immobilized silicon wafers were
recovered from the reaction mixture and repeatedly washed
with DMF and acetone in an ultrasonic bath, and dried under
a stream of nitrogen.
EXPERIMENTAL
Materials and Sample Preparation
(ar-Vinylbenzyl)trimethylammonium chloride (VBTAC), N-(3-
(dimethylamino)propyl)-N0-ethylcarbodiimide hydrochloride
(EDC), 4-(N,N-dimethylamino)pyridine (DMAP), 4,40-azo-
bis(4-cyanopentanoic acid) (ACPA), di-tert-butyl dicarbonate,
dimethyl sulfoxide, dimethylformamide (DMF), tetrahydrofu-
ran, methanol, dichloromethane (DCM), hydrogen peroxide,
copper sulfate, magnesium sulfate, and sodium ascorbate
were purchased from Aldrich. ACPA was recrystallized from
methanol. 3-Bromopropyltrimethoxysilane (BPTS, 97%) was
obtained from Fluka. Propargyl alcohol, sodium azide, and
hydrofluoric acid were provided by Acros Organics.
RAFT-Mediated Polymerization Procedure
The RAFT-mediated polymerization of VBTAC (29.4 mmol) was
carried out in buffer (28 mL, pH ¼ 5.0, 0.27 mol Lꢁ1 acetic
acid, and 0.73 mol Lꢁ1 sodium acetate), initiator ACPAꢃ(0.025
mmol), and free RAFT agent CPAD (0.125 mmol) at 0 C in a
glass reactor, which was designed to hold six RAFT chain trans-
fer agent immobilized silicon wafers oriented normal to the
base of the reactor. To ensure smooth stirring and prevent
damage to the surfaces of the substrates, we isolated the mag-
netic stirring bar at the center of device from the slides by a 1-
cm-highglass O-ring. The solution was diluted to 30 mL volume
with the buffer solution and degassed by purging with nitro-
gen for 20 min. The polymerization reaction was stirred vigo-
rously at 70 ꢃC, and from time to time, small samples (ꢀ2 mL)
were removed with a syringe. The molecular weight distribu-
tion of the polymer was measured by aqueous size exclusion
chromatography (ASEC). For ellipsometric measurements, the
samples were also removed from the reactor at different times
and washed with the buffer solution and ethanol in an ultra-
sonic bath. The slides were dried with N2, and the ellipsomet-
ric thicknesses of the dry polymer films were measured at five
different spots on each sample and averaged.
Synthesis of Alkyne Terminated RAFT Chain Transfer
Agent
4-Cyanopentanoic acid dithiobenzoate (CPAD) was synthe-
sized according to the literature procedure.35 CPAD (1.14 g;
4.095 mmol), EDC (0.785 g; 4.095 mmol), DMAP (0.5 g;
4.095 mmol), and 10 mL of DCM were added to a round bot-
tomed flask and stirred for few minutes under nitrogen
atmosphere. Propargyl alcohol of 0.5 mL (8.5 mmol) was
added, and the mixture was stirred overnight at room tem-
perature. The product was washed with acidic water, water,
and brine several times and dried under reduced pressure.
Alkyne-terminated CPAD was obtained without further puri-
fication. 1H-MNR (400 MHz, CDCl3, d, ppm): 7.9 (d, 2H, o-
ArH), 7.6 (t, 1H, p-ArH), 7.4 (t, 2H, m-ArH), 4.4 (s, 2H,
OACH2A), 3.7 (s, 1H, CBCH), ꢀ2.8–2.4 (m, 4H, ACH2CH2A),
1.95 (s. 3H, ACH3). IR (ATR, cmꢁ1): 2234 (v, CBN), 2121 (v,
CBC), 1726 (vs. C¼¼O), and 1040 (s, C¼¼S).
Characterization Techniques
Synthesis of Azide-Modified Silicon Wafers
GA-FTIR spectra of the polymer brushes were carried out
using a Thermo Nicolet 6700 spectrometer coupled with a
Mercury Cadmium Telluride detector and a Smart SAGA
grazing angle (80ꢃ) attachment. The spectra were taken at a
resolution 4 cmꢁ1 after 128 scans accumulation for an ac-
ceptable signal/noise ratio. XPS spectra were recorded on a
SPECS XPS spectrometer equipped with a Mg Ka X-ray
source. After peak fitting of the C 1s spectra, all the spectra
were calibrated in reference to the aliphatic C 1s component
at a binding energy of 285.0 eV. The water contact angle
measurements were conducted at room temperature using a
The silicon (111) wafers (n-type, obtained from Shin-etsu,
Handoutai, Japan), 3 ꢂ 1 cm2, were ultrasonically cleaned
for 5 min in succession with acetone, ethanol, and water and
then etched with a 5% hydrofluoric acid solution. After being
washed with deionized water, the silicon wafers were put
into an ultrasonic bath of H2SO4:H2O2 (v/v: 70/30) for
another 30 min. The wafers were then rinsed with a large
amount of deionized water and exposed in UV/ozone cham-
ber (Irvine, CA: Model 42, Jelight Company) for 15 min prior
to modification to remove hydrocarbon and produce a
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goniometer (DSA 100, Kru¨ss) equipped with a microliter sy-
ringe. Deionized water (5 lL, 18 MX cmꢁ1 resistivity) was
used as the wetting liquid. The morphology of the surfaces
was recorded on an atomic force microscope (Park Systems
XE70 SPM Controller LSF-100 HS). A triangular shaped Si3N4
cantilever with integrated tips (Olympus) was used to ac-
quire the images in the noncontact mode. The normal spring
constant of the cantilever was 0.02 N mꢁ1. The force
between the tip and the sample was 0.87 nN. The absolute
molecular weights and polydispersities of poly(VBTAC) were
determined by ASEC at ambient temperature using Ultrahy-
drogel columns (120, 250, 500, and 1000 Å; Waters), Wyatt
Technology Optilab T-rEX RI detector (k ¼ 690 nm), a Wyatt
Technology Dawn Heleos II multi-angle laser light scattering
detector (k ¼ 658 nm), and 1 wt % acetic acid/0.1 M
Na2SO4(aq) as the eluent at a flow rate of 1.0 mL minꢁ1. The
dn/dc of poly(VBTAC) (0.172 mL gꢁ1) in the above eluent
was determined at 25 ꢃC with a Wyatt Technology Optilab
T-rEX RI detector (k ¼ 690 nm). Ellipsometric measurements
were conducted in ambient conditions using an ellipsometry
(model DRE, EL X20C) equipped with a He-Ne laser (k ¼
632.8 nm) at a constant incident angle of 75ꢃ. The average
dry thickness of polymer brushes on silicon wafer was deter-
mined by fitting the data with a three-layer model [native
silicon (refractive index, n ¼ 3.86) þ silicon oxide layer (n ¼
1.46) þ organic layer (n ¼ 1.47)].36
FIGURE 1 GA-FTIR spectra of (a) SiABPTS, (b) SiAN3, (c)
SiACPAD, and (d) poly(VBTAC) brush (Polymerization time ¼
120 min).
was covalently coupled to the SiAOH surface, and the bro-
mine end-groups were then converted into the free azide
groups by NaN3 in DMF, leading to the azide-terminated
silicon (SiAN3) surface. Finally, alkyne-terminated CPAD
molecules were chemically anchored to the azide-terminated
silicon surface by a click reaction to form a CPAD overlayer.
Attachment of CPAD molecules at every modification
steps was confirmed with GA-FTIR (Fig. 1), XPS (Fig. 2;
Table 1), ellipsometry (Table 2), AFM (Fig. 3), and water con-
tact angle measurements. The formation of the BPTS SAMs
on the silicon wafer was confirmed by the presence of the al-
iphatic ACH2A bands at 2949 cmꢁ1 [Fig. 1(a)]. XPS analysis
of the BPTS monolayer [Fig 2(a); Table 1] verifies the pres-
ence of C 1s and Br 3d. The C1s region can be curve-fitted
to two peaks with binding energies of 285.0 and 285.4 eV,
which are assigned to ACAH/CAC and ACABr groups,
respectively. More specific signal was the small bromine
peak at 71.5 eV (consisting of Br 3d5/2 and Br 3d3/2 peak
components at binding energies of about 71.1 and 72.0 eV,
respectively), indicated the formation of BPTS monolayer.39
When the bromide groups was converted to azide, analysis
of the modified silicon [Fig. 2(b)] revealed a sharp azide
band at 2123 cmꢁ1. XPS analysis also confirmed the pres-
ence of N 1s [Fig. 2(b)]. The N 1s region can be curve-fitted
to two peaks with binding energies of 401.8 and 404.5 eV,
which are assigned to AN¼¼Nþ¼¼N and AN¼¼Nþ¼¼N groups.
The grafting density (r, chains/nm2) and anticipated average
distance between grafting points, D (nm) of the poly(VBTAC)
brushes were calculated from the dry polymer thickness, h
(nm) and the number-average molar weight, Mn (g molꢁ1
;
the molecular weight of the grafted polymer chains is
assumed to be similar to that of the free polymer in solu-
tion) values using eqs 1 and 2.37,38
hqNA
r ¼
D ¼
(1)
(2)
Mn ꢂ 1021
ꢀ
ꢁ
1=2
4
pr
where q (1.05 g cmꢁ3) is the density of polymer and NA
(6.02 ꢂ 1023 molꢁ1) is Avogadro’s number. The expected
poly(VBTAC) chain conformation in good solvent was
deduced from the comparison of D with the corresponding
Flory radius (RF, nm) of the unperturbed chains calculated
from eq 338
3=5
RF ¼ bðDPnÞ
(3)
where DPn is the degree of polymerization and b, is the
effective segment length (assumed to be 0.3 nm for VBTAC
monomer).39
In the next modification step, the alkyne-terminated CPAD
was attached to the azide-terminated layer by a click reac-
tion. The CPAD immobilization was apparent from the
appearance of carbonyl band at 1720 cmꢁ1 in the GA-FTIR
spectrum [Fig. 1(c)]. The bands for CBN stretch at 2231
cmꢁ1 and C¼¼S stretch at 1040 cmꢁ1, further confirmed suc-
cessful covalent coupling of the CPAD on the azide-
RESULTS AND DISCUSSION
Immobilization of CPADB on Silicon Wafer
The SiAOH surface was functionalized via a simple two-step
procedure. First, a self assembled monolayer (SAM) of BPTS
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FIGURE 2 N 1s, C 1s, and S 2s core level XPS spectra of (a) SiABPTS (b) SiAN3 (c) SiACPAD, and (d) poly(VBTAC) brush (Poly-
merization time ¼ 120 min).
terminated layer. The core level XPS spectra of CPAD over-
layer consist of N 1s and C 1s peaks curve fitted into the
components with binding energies at about 399.8 eV (NAC)
and 400.4 eV (NBC) for N 1s and 288.4 eV (C¼¼O), 286.0 eV
(CAN), 285.4 eV (CAS), and 285.0 eV (CAC/CAH) for C 1s
[Fig. 2(c)]. The immobilization of alkyne-terminated CPAD
onto the azide-terminated layer was also confirmed from the
appearance of a S 2p peak curve fitted into two components
with binding energies at about 162.5 eV (CAS) and 160.9 eV
(C¼¼S). The thickness of SiABPTS, SiAN3, and SiACPAD
layers were measured by ellipsometry at 0.7 6 0.3, 0.9 6
0.3, and 2.0 6 0.4 nm, respectively, these values are consist-
ent with theoretical values (0.6, 0.8, and 1.8 nm) obtained by
ab initio calculations. This allows for the following two con-
clusions. First, molecules within complete monolayers are
predominantly oriented along the silicon surface normal and
prevent their extended conformation. Second, complete SAMs
are tightly packed with a refraction index very close to the
known value for bulk packing. The grafting density of RAFT
chain transfer agent calculated from molecular weight of the
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TABLE 2 Grafting Parameters Calculated for Poly(VBTAC)
with a much higher grafting density even though click chemis-
try with chain transfer agent is not 100%.
Brushes
Surface morphologies of the SiAOH substrate after BPTS, azide
and CPAD modifications are shown in Figure 3. The SiAOH
surface is rather uniform and smooth, with a root-mean-square
(rms) roughness value of about 0.32 nm. The Si-BPTS, SiAN3,
and SiACPAD surfaces remain molecularly uniform with rms
roughness values of about 0.49, 0.59, and 0.91 nm, respec-
tively. Finally, as illustrated by inset images in Figure 3, the var-
iation in static water contact angles for the functionalized sili-
con surfaces suggests that the hydrophilicity of the silicon
surfaces can be readily tuned. The static water contact angle of
the SiAOH surface is about 2.4 6 0.3ꢃ. The water contact
angles for the Si-BPTS, SiAN3, and SiACPAD surfaces are about
56.4 6 0.7, 49.7 6 0.5, and 55.4 6 0.5ꢃ, respectively.
Polymerization
Time (min)
r
D
RF
h (nm)
(chain/nm2) (nm) (nm) D/2RF
15
30
8.7 6 2.0 0.36
16.8 6 1.5 0.43
32.1 6 1.6 0.54
44.7 6 1.5 0.66
60.5 6 2.1 0.78
1.87 3.83 0.24
1.72 5.14 0.17
1.53 6.65 0.12
1.39 7.25 0.10
1.28 7.84 0.08
60
90
120
CPAD molecule and from the thickness according to eq 1 was
2.06 groups/nm2 which is higher than the literature grafting
density of RAFT chain transfer agent (0.15–0.68 groups/
nm2).40,41 The RAFT chain transfer agent group is sterically
large; hence, direct immobilization is inefficient. By an initial
silanization reaction with BPTS (2.76 group/nm2) followed by
nearly a quantitative azide displacement of bromide, we can
use click chemistry to introduce a RAFT chain transfer agent
RAFT-Mediated Polymerization of VBTAC on the Silicon
Wafer
For a high grafting density of the polymer, it is important to
introduce RAFT chain transfer agent on the substrate surface
FIGURE 3 3D (left) and 2D (right) AFM topography images (5 ꢂ 5 lm, z scale is 10 nm) in ambient conditions and photographs of
4 lL water droplets (top) on (a) SiABPTS, (b) SiAN3, (c) SiACPAD, and (d) poly(VBTAC) brush (r ¼ 0.78 chains/nm2). The cross sec-
tions corresponding to the black line shown in the AFM images are given below each image.
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FIGURE 4 Dependence of the poly(VBTAC) film thickness on
FIGURE 6 ASEC traces of poly(VBTAC) synthesized at different
the polymerization time.
times.
with a high grafting density. But not all of the RAFT chain
transfer agent sites on the substrate surface may participate
in the growth of polymer chains because the growing chains
sterically block access of the adjacent chain transfer agent
on the substrate surface. In this case, it is important to have
an efficient surface immobilized chain transfer agent to grow
a polymer chain. Hence, we attached the reactive CPAD chain
transfer agent in order to achieve better efficiency. This
chain transfer agent has been reported to have a higher
chain transfer efficiency and control over the RAFT polymer-
ization because the carbon attached to the labile sulfur atom
bears a radical stabilizing phenyl group.42
poly(VBTAC) brushes is confirmed the prominent CAH bands
recorded at 2800–3000 cmꢁ1 region. Elementary composi-
tion and values of binding energies given by XPS are sum-
marized in Table 1. The core level XPS spectra of poly
(VBTAC) consist of N 1s and C 1s peaks curve fitted into the
components with binding energies at about 402.1 eV
(CANþ) and 400.0 eV (CAN) for N 1s and 286.1 eV (CAN),
285.4 eV (CAS) and 285.0 eV (CAC/CAH) for C 1s [Fig.
2(d)]. As indicated in Table 1, similar results were obtained
for the poly(VBTAC) brushes synthesized for different times.
In all cases, atomic compositions were found to be in good
agreement with theoretical value expected for the pure pol-
y(VBTAC) layer (90% C and 10% N). In addition, the contin-
uous decrease of S 2p signal confirms that the film thickness
increases with reaction time.
The RAFT-mediated polymerization of the cationic VBTAC
monomer in aqueous media has already been reported.43 In
this study, the RAFT-mediated process was described for the
preparation of polymer brushes on the silicon substrate
modified with RAFT agent.
The thickness of the polymer brushes grown on the silicon
surface as a function of polymerization time is shown in Fig-
ure 4. As shown in this figure, the thickness of the poly(-
VBTAC) film increased almost linearly with the polymeriza-
tion time. This result indicates that the process of RAFT-
mediated polymerization of VBTAC is controlled.
GA-FTIR spectrum of poly(VBTAC) brushes synthesized for
120 min are represented in Figure 1(d). The formation of
Additional evidence on the controlled polymerization is pro-
vided by the ‘‘free’’ poly(VBTAC) formed in the solution. Fig-
ure 5 shows the linear relationship between ln([M0]/[M])
and time, where [M0] is the initial monomer concentration
and [M] is the monomer concentration. This result indicates
that the concentration of the growing species remains con-
stant and a first order kinetic is obtained.
The main difference between surface and bulk polymeriza-
tions is the probability of chain transfer between the dor-
mant and the activating chains. In surface RAFT polymeriza-
tion, the probability of chain transfer between two chains
(two chains on the surface, or one chain on the surface and
another chain in solution) is lower than that between two
chains in solution. In this study, RAFT-mediated polymeriza-
tion of the cationic VBTAC monomer in aqueous media was
FIGURE 5 Relationship of ln ([M]o/[M]t with time of RAFT-
mediated polymerization of VBTAC.
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The grafting density of the poly(VBTAC) chains was calculated
from molecular weight of the free polymer (Mn) and the dry
film thickness (h) according to eq 1 and varied between 0.36
and 0.78 chain/nm2, which means that poly(VBTAC) chains
have grown from 18 to 38% (2.06 CPAD/nm2) of the CPAD
fixed on the silicon surface (Table 2). Although the grafting
density increases with polymerization time, the increase is
nonlinear, which raises the possibility that the chain initia-
tion efficiency is dependent on the fractional coverage of
RAFT agents in the BPTS monolayer. It appears that the ini-
tiation efficiency decreases with increasing grafting density.
At high chain density, crowding within the polymer layer
will affect the local concentration of reactants and therefore
the relative reaction rates of initiation, propagation, and ter-
mination. This is likely to exert the highest effective on reac-
tions between grafted chains, such termination between
grafted radicals and degenerative chain transfer between a
grafted chain and grafted radical.
To determine whether the grafted polymers are in brush re-
gime, we calculated the distance between grafting sites (D)
relative to the Flory radius (RF), using eqs 2 and 3. It is gen-
erally accepted that when D < 2RF, end-tethered polymer
chains reside in the brush regime, and when D > 2RF the
chains are isolated and noninteracting, adopting coiled struc-
tures (often referred to as the ‘‘mushroom regime’’).38 As
shown in Table 2, base on the D/2RF calculated for poly
(VBTAC) chains, the chains are sufficiently crowded such
that they will form brushes when solvated.
FIGURE 7 Evolution of molecular weights (n) and polydisper-
sities, PDI (h) with monomer conversion.
performed in the presence of free RAFT chain transfer agent in
order to better control the polymerization. Additional RAFT
agent in solution seems to aid the efficiency of grafting process.
Moreover, introduction of free RAFT agent allows for formation
of free polymer in solution. Barsbay et al.44 reported that the
number-average molecular weight, Mn, value of polystyrene
detached from the cellulose surface by acidic hydrolysis is very
close to the value of the corresponding free polystyrene. This
finding is of high significance as detaching of the grafted poly-
mer from the silicon surface is often impossible (the detach-
ment procedure for silicon substrates might cause hydrolysis
of pendant groups) or very time-consuming. Therefore, we also
assumed that the free and the grafted polymers have the same
molecular weight and narrow polydispersity, thus proving for
the first time the control of grafting process mediated via
RAFT polymerization. The free poly(VBTAC) chains were ana-
lyzed by ASEC. Typical ASEC traces of free polymers with the
increase of polymerization time are shown in Figure 6. We can
clearly observe that the elution peaks shift to higher molecular
weight with increasing reaction time. After 120 min, the
monomer conversion is almost 93%; ASEC analysis of the free
polymers reveals a number-average molecular weight, Mn, of
49,000 g molꢁ1 and a polydispersity, PDI, of 1.19.
When the SiAOH surface was grafted with poly(VBTAC)
layer, the static water contact angle of the surface decreased
substantially to about 38.6 6 0.9ꢃ, consistent with the
hydrophilic nature of poly(VBTAC). poly(VBTAC) film mor-
phology was studied by conducting AFM measurements in
ambient conditions [Fig. 3(d)]. Poly(VBTAC) chains synthe-
sized for 120 min appear as needle-like structures heteroge-
neously distributed over entire substrate area. Similar mor-
phologies were observed for the polymeric brushes
synthesized at different times, however, with homogeneous
distributions of apparent needle-like structures. Analysis of
poly(VBTAC) brushes synthesized for 30 min indicated a rms
roughness of 12.1 nm, whereas a slight decrease of 10.6 and
7.9 nm for the brushes synthesized for 60 and 120 min,
respectively, indicating homogeneous and uniform surfaces.
CONCLUSIONS
A combinatorial approach based on click chemistry and
RAFT-mediated polymerization was used to synthesize well-
defined cationic polymeric chains grafted onto silicon wafer.
The silicon surfaces were first modified with RAFT chain
transfer agent by using ‘‘click chemistry’’ involving an azide-
modified silicon wafer and alkyne-terminated CPAD. The
click reaction produced a grafting density of 2.06 RAFT
agent/nm2, a value much higher than previously reported for
RAFT agent immobilization. Then the poly(VBTAC) brushes
were synthesized by RAFT-mediated polymerization. We
exploited the combined use of click chemistry and RAFT-
mediated polymerization to make polymer brushes with high
Figure 7 shows the relationship between molecular weight of
the ‘‘free’’ poly(VBTAC) and the conversion of the VBTAC mono-
mer. The number average molecular weight of the ‘‘free’’ poly(-
VBTAC) increased linearly with monomer conversion. The poly-
dispersities of the polymers were relatively low (1.04–1.19)
and the experimental (Mn;ASEC as determined by ASEC) molec-
ular weights were close to the theoretical (Mn;theo
¼
conversion ꢂ ½monomerꢄ0ꢂMWmonomer=½CTAꢄ0 þ MWCTA) values.
These results also indicate that the RAFT-mediated polymeriza-
tion of the VBTAC proceeded in a controlled manner.
8
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