JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
The basic routes that are used to form polymer brushes uti-
lizing SI-RAFT polymerization include utilization of a sur-
face-bound initiator with free RAFT agent in solution and
immobilization of the RAFT agent to a surface with free ini-
tiator in solution for subsequent polymerization. The first
approach used was surface-immobilized initiators with the
addition of free RAFTagent in solution to form polymer brushes
via SI-RAFT polymerization.25–29 The disadvantage of this
approach is that for a well-defined SI-RAFT polymerization, all
chains should be initiated at the same time via the free RAFT
agent. Because of the fact that initiators dissociate and initiate
chains over a wide temperature range, use of surface-bound ini-
tiators may lead to nonuniform films.30,31 The most promising
means of formation of high grafting density, uniform polymer
brushes using SI-RAFT polymerization is via attachment of the
RAFT agent, as it is truly a ‘‘grafting from’’ method.32–34
1H (300 MHz, d, ppm, CDCl3): 1.95 (s. 3H, ACH3), ꢂ2.40–
2.80 (m, 4H, ACH2CH2A), 7.40 (t, 2H, m-ArH), 7.59 (t, 1H,
p-ArH), 7.91 (d, 2H, o-ArH).
Then the compound CPAD (10 mmol) and N-hydroxysuccini-
mide (10 mmol) were dissolved in 20 mL of anhydrous
dichloromethane. After dicyclohexylcarbodiimide (10 mmol)
was added to the solution, the mixture was stirred at 22 ꢁC
in the dark for 16 h. A white byproduct was filtrated out,
and the filtrate was concentrated. The concentrated liquid
was purified through a gel column with ethyl acetate:hexane
(1:3, v/v) as eluent. Then, the CPSE was obtained.
1H NMR (300 MHz, CDCl3, d, ppm,): 1.94 (s, 3H, ACH3),
ꢂ2.30–2.75 (m, 4H, ACH2CH2AC(CN)A(CH3)A), 2.84 (s, 4H,
AOCACH2CH2ACOA), 3.19 (m, 2H, AOCACH2CH2AC(CN)A
(CH3)A), 7.40 (t, 2H, m-ArH), 7.58 (t, 1H, p-ArH), 7.90 (d,
2H, o-ArH).
Although a number of cationic monomers or their nonionic
precursors have been polymerized in solution by RAFT tech-
nique,35–37 to our knowledge, no SI-RAFT polymerization of
a (meth)acrylamido monomer bearing an amino group has
been previously reported.
Immobilization of the RAFT Agent on the Silicon Surface
The APTS-modified silicon wafers were introduced into the
solution of CPSE (5.4 mmol) in 20 mL of anhydrous
dichloromethane. The reaction mixture was left to react at
22 ꢁC in the dark for 60 h. The silicon wafers were recov-
ered from the reaction mixture and repeatedly washed with
dichloromethane and acetone in an ultrasonic bath, and
dried under a stream of nitrogen.
In this study, we detail the facile preparation of well-defined
cationic poly(N-[3-(dimethylamino)propyl]methacrylamide)
[poly(DMAPMA)] brushes directly in aqueous media utilizing
the dithioester-immobilized silicon surface as RAFT agent.
The resulting polymer brushes were analyzed by grazing
angle-Fourier transform infrared spectroscopy (GA-FTIR),
X-ray photoelectron spectroscopy (XPS), atomic force micros-
copy (AFM), and water contact-angle measurements.
SI-RAFT Polymerization Procedure
The SI-RAFT polymerization of DMAPMA (10 mmol) was car-
ried out in buffer (1.6 mL, pH ¼ 5.0, 0.27 mol/L acetic acid,
and 0.73 mol/L sodium acetate), initiator CPA (0.01 mmol),
ꢁ
and free RAFT agent CPAD (0.08 mmol) at 0 C in a glass re-
EXPERIMENTAL
actor, which was designed to hold six RAFT agent-immobi-
lized silicon wafers oriented normal to the base of the reac-
tor. To ensure smooth stirring and prevent damage to the
surfaces of the substrates, we isolated the magnetic stirring
bar at the center of device from the slides by a 1-cm-high
glass O-ring. The solution was diluted to 10-mL volume with
the buffer solution and degassed by purging with nitrogen
for 20 min. The polymerization reaction was stirred vigo-
rously at 70 ꢁC, and from time to time, small samples
(ꢂ2 mL) were removed with a syringe. The molecular weight
distribution of the polymer was measured by gel permeation
chromatography (GPC). For ellipsometric measurements, sam-
ples 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
three different spots on each sample and averaged.
Materials
All chemicals were purchased from Aldrich at the highest
available purity and used as received unless otherwise noted.
CPA was recrystaliized from methanol. DMAPMA was distilled
over calcium hydride immediately prior to polymerization.
Wafers Cleaning and Silane Treatment
The silicon (100) wafers (n-type, obtained from Shin-etsu,
Handoutai, Japan; 3 ꢀ 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. The wafers were exposed in UV/ozone chamber (Irvine,
CA: Model 42, Jelight Company) for 15 min prior to modification
to remove hydrocarbon and produce a hydrophilic surface. The
hydroxylated waters were exposed to a solution of 3-aminopro-
pylthriethoxysilane (APTS; 1%, v/v) in dry toluene for 2 h at
60 ꢁC. Afterward, the wafers were washed with toluene, dichloro-
methane in an ultrasonic bath, and dried in a vacuum.
Characterization
GA-FTIR spectra of the polymer brushes on silicon wafer
were collected utilizing a Harrick Scientific GA-FTIR attach-
ment coupled with a Thermo Nicolet 6700 spectrometer, col-
lecting 128 sample scans, and utilizing Nicolet’s OMNIC soft-
Synthesis of RAFT Agent
4-Cyanopentanoic acid dithiobenzoate (CPAD) was synthe-
sized according to the literature procedure38 utilizing a small
amount of acetic acid (0.5%, v/v) in the chromatographic
eluent to enhance chromatographic resolution.
ware. XPS spectra were recorded on a SPECS ESCA
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
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