Synthesis and self-assembly behavior of organic-inorganic macrocyclic molecular brushes composed of macrocyclic oligomeric silsesquioxane and poly(N-isopropylacrylamide)

Article abstract of DOI:10.1039/c4ra02624a

The novel organic-inorganic molecular brush composed of macrocyclic oligomeric silsesquioxane (MOSS) and poly(N-isopropylacrylamide) (PNIPAAm) (denoted by PNIPAAm@MOSS) was synthesized via the atom transfer radical polymerization (ATRP) approach. In bulk, the organic-inorganic molecular brushes were microphase-separated; the spherical MOSS microdomains with the diameter of 10-50 nm were dispersed into a continuous PNIPAAm matrix. Depending on the lengths of PNIPAAm chains, the PNIPAAm@MOSS molecular brushes were capable of self-assembling into cylindrical or spherical nano-objects in aqueous solutions as evidenced by transmission election microscopy (TEM) and dynamic light scattering (DLS). Both micro-differential scanning calorimetry (Micro-DSC) and ultraviolet-visible spectroscopy showed that the MOSS backbones exerted significant restriction of coil-to-globule transition of PNIPAAm chains.

Full text of DOI:10.1039/c4ra02624a

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Synthesis and self-assembly behavior of organic–
inorganic macrocyclic molecular brushes
composed of macrocyclic oligomeric
Cite this: RSC Adv., 2014, 4, 28439
silsesquioxane and poly(N-isopropylacrylamide)
*
Yulin Yi and Sixun Zheng
The novel organic–inorganic molecular brush composed of macrocyclic oligomeric silsesquioxane (MOSS)
and poly(N-isopropylacrylamide) (PNIPAAm) (denoted by PNIPAAm@MOSS) was synthesized via the atom
transfer radical polymerization (ATRP) approach. In bulk, the organic–inorganic molecular brushes were
microphase-separated; the spherical MOSS microdomains with the diameter of 10–50 nm were
dispersed into a continuous PNIPAAm matrix. Depending on the lengths of PNIPAAm chains, the
PNIPAAm@MOSS molecular brushes were capable of self-assembling into cylindrical or spherical nano-
objects in aqueous solutions as evidenced by transmission election microscopy (TEM) and dynamic light
scattering (DLS). Both micro-differential scanning calorimetry (Micro-DSC) and ultraviolet-visible
spectroscopy showed that the MOSS backbones exerted significant restriction of coil-to-globule
transition of PNIPAAm chains.
Received 25th March 2014
Accepted 3rd June 2014
DOI: 10.1039/c4ra02624a
www.rsc.org/advances
structural units, because these PNIPAAm-containing amphi-
philes are capable of self-assembling into a variety of thermor-
Introduction
esponsive nano-objects in aqueous solutions. The self-assembly
behavior of the PNIPAAm amphiphiles can exert a profound
inuence on the thermoresponsive behaviors of this poly-
mer.8,19,20 For instance, Winnik et al. investigated the effect of
the hydrophobic n-octadecyl termini on the merging of PNI-
PAAm mesoglobules and proposed that the rigidity and partial
vitrication of mesoglobules may be the predominant cause of
their stability against aggregation.8 Zhang et al.21 investigated
the self-assembly of PS-b-PNIPAAm-b-PS triblock copolymers in
aqueous solutions and reported the formation of ower-like
aggregates with PNIPAAm interlocking rings and associated PS
blocks as the core and PNIPAM rings as the coronas. It was
found that the morphologies of micelle-like aggregates can
signicantly affect the collapse transition of PNIPAAm coronas.
More recently, the PNIPAAm amphiphiles containing bulky and
hydrophobic inorganic groups have attracted considerable
interest owing to the specic architectures and self-assembly
behavior of this class of organic–inorganic hybrids.22–28 Poly-
hedral oligomeric silsesquioxane (POSS)-capped semi-telechelic
PNIPAAm hybrids have been synthesized via reversible addi-
tion-fragmentation chain transfer (RAFT) or atom transfer
radical polymerization (ATRP);24,25 these organic–inorganic
PNIPAAm amphiphiles can be self-assembled into thermores-
ponsive nano-objects. Wang et al.26 reported synthesis of a,u-
diPOSS-capped PNIPAAm telechelics with a “POSS-spreading”
strategy. In this approach, a diPOSS-bearing trithiocarbonate
was synthesized and used as the chain transfer agent, with
Responsive or “smart” materials have attracted considerable
interest owing to their promising potential applications in
biomedical elds.1–6 Poly(N-isopropylacrylamide) (PNIPAAm) is
among the most frequently investigated thermoresponsive
polymers. In aqueous solutions, PNIPAAm can display a lower
critical solution temperature (LCST) of about 32 ꢀC.7–11 Below
this temperature, individual PNIPAAm chains adopt a random
coil conformation, and the random coils collapse into globules
upon heating the aqueous solutions up to about 32 ꢀC; i.e., a so-
called coil-to-globule transition occurs. It is realized that there
are intermolecular hydrogen bonding interactions between
amide groups of PNIPAAm and water molecular at lower
temperatures, which promote the water solubility of this poly-
mer. Upon heating the solution up to 32 ꢀC or above, the
intermolecular hydrogen bonding interactions are interrupted
owing to the conformational changes in the PNIPAAm chains.
In past decades, the thermoresponsive behavior of PNIPAAm
was extensively investigated. These studies have revealed that
some structural factors such as co-monomers, tacticity, cross-
linking, graing, molecular weights and end groups all affect
the LCST behavior of PNIPAAm in aqueous solutions.12–18
Recently, considerable attention has been paid to the prep-
aration of PNIPAAm amphiphiles containing hydrophobic
Department of Polymer Science and Engineering, The State Key Laboratory of Metal
Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China.
E-mail: szheng@sjtu.edu.cn; Fax: +86-21-54741297; Tel: +86-21-54743278
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 28439–28450 | 28439

View Article Online
RSC Advances
Paper
which the RAFT polymerization of NIPAAm was carried out and received. Copper(II) chloride (CuCl2) was obtained from Ruan-
two ends of PNIPAAm chain were capped with hepta(3,3,3-tri- shi Chemical Co, Jiangsu, China. Sodium hydroxide (NaOH),
uropropyl) POSS. Owing to the high hydrophobicity of the silver nitrate (AgNO3), pyridine and organic solvents such as
POSS end groups, these POSS-capped PNIPAAm telechelics can anhydrous ethanol, methanol and toluene were purchased from
form organic–inorganic physical hydrogels in water. These Shanghai Reagent Co, Shanghai, China. Before use, toluene and
physical hydrogels displayed fast deswelling and reswelling pyridine were distilled over calcium hydride (CaH2) and then
˚
properties compared with traditional chemically crosslinked stored in a sealed vessel in the presence of a 4 A molecular sieve.
hydrogels. Kuo et al.27 reported the synthesis of octa-armed N-Isopropylacrylamide (NIPAAm) was prepared in this lab via
PNIPAAm stars with POSS cores. It was found that these the reaction between isopropylamine and acryloyl chloride.
organic–inorganic PNIPAAm stars undergo a sharp coil-to- Copper(I) chloride (CuCl) was of chemically pure grade,
ꢀ
globule transition in water at about 32 C, which is similar to supplied by Shanghai Reagent Co. China. Tris(2-(dimethyla-
linear PNIPAAm homopolymers. More recently, He et al.28 mino)ethyl)amine (Me6TREN) was prepared by following the
investigated the thermoresponsive and pH dual-responsive methods reported by Matyjaszewski et al.42 All other solvents
behaviors of a star-like block copolymer with a POSS core and used were obtained from commercial sources. Before use,
poly(N-isopropylacrylamide)-block-poly(acrylic acid) diblock tetrahydrofuran (THF) was reuxed above sodium and distilled;
copolymer arms. The results showed that with the star-block triethylamine (TEA) and isopropyl alcohol (IPA) were dried over
architecture, the high local chain density near the POSS core CaH2 and then distilled; N,N-dimethylformamide (DMF) was
may lead to enhanced intramolecular interactions between dried over anhydrous magnesium sulfate and distilled under
different types of units and hence signicantly alter the critical reduced pressure.
phase transition behavior.
Polyhedral oligoalkylmetallasiloxanes are a class of novel
organometallic compounds that consist of stereoregular alkylsi-
loxane macrocycles coordinated to alkaline and/or transition
Synthesis of dodecamethyldodeca(dimethylhydro)
cyclododecasilsesquioxane (1)
metals (e.g., Mn, Co, Ni, Cu and trivalent lanthanide metals).29–37 First, the coordinate of the metals (i.e., alkaline and copper)
These metallasiloxane coordinates can conveniently be converted with methylsiloxanolates was prepared via the reactions
into macrocyclic oligomeric silsesquioxanes (MOSS) with satis- between methyltriethoxysilane [MeSi(OEt)3] and copper(II)
factory yields, which have potential for use as a new family of chloride (CuCl2) in a basic media (e.g., NaOH) by following the
building blocks for organic–inorganic hybrids.38–41 In the present methods reported by Shchegolikhina et al.43,44 Second, the
work, we explored the synthesis of the organic–inorganic macro- coordinate (8.480 g, 4.99 mol) was added to the mixture
cyclic molecular brushes composed of a MOSS backbone and composed of anhydrous toluene (160 mL), dimethyl-
PNIPAAm arms. Toward this end, we rst synthesized a twenty- chlorosilane [(CH3)2SiHCl] (42.000 g, 442.1 mmol) and pyridine
four membered MOSS macromer bearing twelve 2-chlor- (28.400 g, 359.1 mmol) at room temperature. The reaction was
opropionate moieties; the functionalized MOSS was then used as carried out at 25 ꢀC for 24 hours with vigorous stirring. The
a macroinitiator for the atom transfer radical polymerization of mixture was ltered to isolate the precipitates, and the solution
N-isopropylacrylamide (NIPAAm) to afford the organic–inorganic was washed with deionized water until no chlorine ions were
macrocyclic molecular brushes. The purpose of this work is detected using an aqueous solution of silver nitrate (AgNO3).
twofold: (i) to explore the synthesis of organic–inorganic macro- Aer drying with anhydrous MgSO4, all the solvents were
cyclic molecular brushes with an inorganic macrocyclic molecule eliminated with rotary evaporation to afford 7.36 g viscous
as the backbone and PNIPAAm as side chains; and (ii) to inves- liquid (i.e., compound 1) with the yield of 98.0%. 1H NMR
tigate the effect of organic–inorganic macrocyclic brush archi- (CDCl3): 4.72 (m, 1H, SiH), 0.205–0.194 [d, 6H, Si(CH3)2], 0.10
tectures on the thermoresponsive and self-assembly behavior of (s, 3H, O3SiCH3). 29Si NMR (ppm, CDCl3): ꢁ66.2 (d, SiO3/2), ꢁ6.4
PNIPAAm. In the past years, the thermoresponsive and self- [s, Si(CH3)2H].
assembly behavior of a variety PNIPAAm amphiphiles has been
extensively investigated. To the best of our knowledge, nonethe-
less, there has been no previous report on organic–inorganic
Synthesis of allyl 2-chloropropionate (2)
PNIPAAm macrocyclic brushes. In this work, the thermores- To a ask equipped with a magnetic stirrer, dichloromethane
ponsive and self-assembly behavior of organic–inorganic macro- (80 mL), triethylamine (18.200 g, 180 mmol) and allyl alcohol
cyclic molecular brushes were addressed on the basis of the (8.350 g, 144 mmol) were charged with vigorous stirring and
results of transmission electronic microscopy (TEM), micro- thenꢀ2-chloropropionyl chloride (18.0 g, 141.7 mmol) was added
ꢀ
differential scanning calorimetry (Micro-DSC), UV-vis spectros- at 0 C. The reaction was performed at 0 C for 2 hours and at
copy and dynamic laser scattering (DLS).
room temperature for 24 hours. The mixture was washed with
deionized water, and the solvent was eliminated via rotary
evaporation. The product (19.400 g) was obtained via distilla-
tion at reduced pressure with the yield of 92.1%. 1H NMR
(ppm, CDCl3): 5.96–5.87 (m, 1H, CH2]CHCH2OOCCHClCH3),
Experimental
Materials
Organic silanes (i.e., methyltriethoxysilane and dimethyl- 5.38–5.26 (m, 2H, CH2 ¼ CHCH2OOCCHClCH3), 4.68–4.66
chlorosilane) were purchased from Gelest Co, USA and used as (m, 2H, CH2]CHCH2OOCCHClCH3), 4.45–4.39 (q, 1H,
28440 | RSC Adv., 2014, 4, 28439–28450
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
CH2]CHCH2OOCCHClCH3), 1.71–1.69 (d, 3H, CH2] dichloromethane as the solvent. The MALDI-TOF-MS experi-
CHCH2OOCCHClCH3).
ment was carried out on an IonSpec HiResMALDI mass spec-
trometer equipped with a pulsed nitrogen laser (l ¼ 337 nm;
pulse with ¼ 3 ns). This instrument operated at an accelerating
potential of 20 kV in reector mode. Sodium was used as the
cationizing agent; all data shown are for positive ions.
Synthesis of MOSS macromolecular initiator (3)
To a ask equipped with a dried magnetic stirrer, anhydrous
toluene (24 mL), compound
1
[i.e., dodecamethyl-
Gel permeation chromatography (GPC). The molecular
weights and molecular weight distribution of polymers were
determined on a Waters 717 Plus autosampler gel permeation
chromatography apparatus equipped with Waters RH columns
and a Dawn Eos (Wyatt Technology) multiangle laser light
scattering detector; the measurements were carried out at 25 ꢀC
with dimethylformamide (DMF) as the eluent, at the rate of
dodeca(dimethylhydro)cyclododecasilsesquioxane] (2.400 g,
1.492 mmol) and compound 2 (viz. allyl 2-chloropropionate)
(4.000 g, 26.88 mmol) were charged with vigorous stirring. The
ask was connected onto a Schlenk line to degas via three
freeze–pump–thaw cycles, and Karstedt catalyst (80 mL) was
ꢀ
added with a syringe. The reaction was carried out at 85 C for
10 hours. The solvent and unreacted allyl 2-chloropropionate
were eliminated via rotary evaporation, and the product
(4.600 g) (i.e., compound 3) was obtained with the yield of
1.0 mL minꢁ1
.
Micro-differential scanning calorimetry (Micro-DSC). The
Micro-DSC measurements were performed with a SETARAM
micro-DSC III system under a dry nitrogen atmosphere and
circulating water. Before the measurement, the specimens were
maintained at 18 ꢀC for 15 min and were then heated to 60 ꢀC at
the heating rate of 1 ꢀC minꢁ1. The temperature at the
minimum of the exothermic peak was taken as the lower critical
solution temperature (LCST).
Determination of cloud point. Each polymer (0.01 g) was
dissolved in 0.5 mL tetrahydrofuran (THF) and then the solu-
tion was added dropwise to 50 mL ultrapure water by a dropping
funnel with vigorous stirring. Aer that, the suspension was
1
92.0%. H NMR (ppm, CDCl3): 4.42–4.30 (q, 1H, CHCl), 4.16–
4.06 (s, 2H, COOCH2), 1.75–1.61 (d, 5H, ClCHCH3, CH2CH2Si),
0.63–0.54 [t, 2H, CH2CH2Si], 0.20–0.03 [t, 9H, O3SiCH3,
O2Si(CH3)2]. 29Si NMR (ppm, CDCl3): ꢁ66.2 (d, SiO3/2), ꢁ6.4 [s,
Si(CH3)2CH2CH2CH2OOCCHClCH3].
Synthesis of organic–inorganic molecular brushes
Typically, to a ask equipped with a magnetic stirrer, the above
MOSS macromolecular initiator (i.e., compound 3) (0.18 g,
0.64 mmol with respect of allyl 2-chloropropionate group),
NIPAAm (2.70 g, 23.8 mmol) and isopropyl alcohol (6 mL) were
charged with vigorous stirring. The ask was connected onto
the Schlenk line to degas via three freeze–pump–thaw cycles,
and then copper(I) chloride (29.60 mg) and Me6Tren (87 mL)
were added. The system was purged with highly pure nitrogen
for 45 min and the polymerization was performed at room
temperature for 24 hours. Aer that, 20 mL of tetrahydrofuran
was added to dissolve the reacted product, and the solution was
passed through a neutral alumina column to remove the cata-
lyst. The solution was concentrated via rotary evaporation and
then dropped into 200 mL of petroleum ether to afford the
precipitates. The precipitates were re-dissolved in tetrahydro-
furan and the as-obtained solution was re-dropped into petro-
leum ether to obtain the precipitates. Aer dried in vacuo at
ꢀ
30 C for 24 hours, the polymer (2.1 g) was obtained with the
conversion of PNIPAAm to be 71%. 1H NMR (400 MHz, CDCl3):
6.06–6.57 [m, –NHCH(CH3)2], 4.02 [s, NHCH(CH3)2], 2.4–1.2 [m,
CH2CH], 1.14 (d, CH(CH3)2), 0.02–0.18 [m, O3SiCH3 and
O2Si(CH3)2]. GPC: Mn ¼ 36 200 with Mw/Mn ¼ 1.46.
Measurement and techniques
Nuclear magnetic resonance spectroscopy (NMR). The 1H
NMR measurement was carried out on a Varian Mercury Plus
400 MHz NMR spectrometer at 25 ꢀC and the 29Si NMR spectra
were obtained on a Bruker Avance III 400 MHz NMR spec-
trometer. The samples were dissolved with deuterated chloro-
form (CDCl3) and the solutions were measured with
tetramethylsilane (TMS) as an internal reference.
Matrix-assisted ultraviolet laser desorption/ionization time-
of-ight mass spectroscopy (MALDI-TOF-MS). Gentisic acid
Scheme
1 Synthesis of organic–inorganic molecular brushes
(2,5-dihydroxybenzoic acid, DHB) was used as the matrix with composed of MOSS and PNIPAAm.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 28439–28450 | 28441

View Article Online
RSC Advances
Paper
stirred for additional 30 min, and a transparent emulsion was coated copper grids, and the solvent was slowly evaporated at
formed with the concentration of the polymer being 0.2 g Lꢁ1
.
room temperature and then in vacuo at 30 C for 2 hours. The
ꢀ
The solvent (i.e., THF) in the suspension was eliminated via specimens were directly observed without an additional stain-
rotary evaporation for 2 hours. The cloud points were deter- ing step. To investigate the self-assembly behavior of the
mined with an Agilent Technologies Cary 60 Ultraviolet-visible samples in aqueous solutions, the specimens were prepared by
spectrometer at the wavelength of light l ¼ 550 nm. In the dropping the suspensions of the polymer in water (about 10 mL
measurements, a thermostatically controlled cuvette holder was at 0.2 g ꢂ Lꢁ1) onto carbon-coated copper grids; the water was
employed and the plots of light transmittance as a function of eliminated via freeze drying.
temperature were obtained.
Dynamic laser scattering (DLS). The above suspensions of
Results and discussion
Synthesis of organic–inorganic molecular brushes
PNIPAAm@MOSS at the concentration of 0.2 g Lꢁ1 were sub-
jected to dynamic laser scattering (DLS). The laser light scat-
tering experiments were conducted on a Malvern Nano ZS90
equipped with a He–Ne laser operated at the wavelength of l ¼
633 nm; the data were collected at a xed scattering angle of 90ꢀ.
Transmission electron microscopy (TEM). The morpholog-
ical observations were conducted on a JEOL JEM 2100F electron
The route of synthesis for the organic–inorganic macromolec-
ular brushes composed of macrocyclic oligomeric silsesquiox-
ane (MOSS) and poly(N-isopropylacrylamide) (PNIPAAm)
(denoted by PNIPAAm@MOSS) is shown in Scheme 1. First, the
MOSS macromolecular initiator bearing 2-chloropropionate
moieties was synthesized via the hydrosilylation reactions
of dodecamethyldodeca (dimethylhydro)cyclododecasilses-
quioxane (compound 1) with allyl 2-chloropropionate
microscope at
a voltage of 200 kV. To investigate the
morphology of the samples in bulk, the THF solutions of the
polymers at the concentration of 1 wt% were cast on carbon-
Fig. 1 29Si NMR spectrum of dodecamethyldodeca(dimethylhydro) cyclododecasilsesquioxane (1).
28442 | RSC Adv., 2014, 4, 28439–28450
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
(compound 2). Compound 1 was synthesized via the silylation baseline upliing of the spectral line in the range of ꢁ75 to
reaction of the coordinate of copper and sodium with ꢁ100 ppm resulted from the resonance of 29Si nucleus in the
methysiloxanolate with dimethylchlorosilane. The metal- NMR tube. The expanded 29Si NMR spectrum shows that the
lasilsesquioxanes used in this work were prepared via the signal of resonance at ꢁ66.2 ppm was composed of two splitting
reaction of methyltriethoxylsiloxane [CH3Si(OCH2CH)3] with peaks with very close chemical shis; the ratio of integral
sodium hydroxide (NaOH) and copper(II) chloride (CuCl2) in the intensity of the peak at ꢁ66.1 ppm to that at ꢁ66.4 ppm was
presence of water and alcohol by following the method reported 2 : 1. The splitting of silicon resonance signal was related to the
by Sergienko et al.43,44 It was reported that the crystal of the conguration of the MOSS macromer. It is proposed that tri-
coordinate possessed a globular structure, in which the twelve- methylsiloxyl groups appeared in the 24-membered macrocyclic
SiO3/2-membered organosiloxanolate ligand [i.e., vinylSi(O) silsesquioxane with the tris-cis-tris-trans conguration.
Oꢁ1
]
12
in the tris-cis-tris-trans conguration constituted a Compound 1 was subjected to MALDI-TOF mass spectroscopy
saddle conformation, which was xed by four Cu(II) ions. The to measure its molecular weight; its mass spectrum is shown in
metallacyclosiloxanolate was allowed to react with the dime- Fig. 2. A group of intense peaks centered at the value of m/z
thylchlorosilane to afford the macrocyclic oligomeric silses- equal to 1633.3 were detected, implying that the compound
quioxane bearing twelve Si–H bonds (compound 1). The 29Si possessed the molecular weight of 1610 Da. The molecular
NMR spectrum of compound 1 is shown in Fig. 1. In the 29Si weight is identical to the value estimated according to the
NMR spectrum, two resonance peaks were detected at ꢁ6.4 and formula of compound 1. The 29Si NMR and MALD-TOF mass
ꢁ66.2 ppm, respectively. The former is assignable to the silicon spectroscopy results indicate that the MOSS macromer bearing
nucleus bonded with hydrogen atom (i.e., that in the Si–H twelve Si–H bonds was successfully obtained.
bond), whereas the latter is assigned to the nucleus of silicon of
macrocyclic backbones. The appearance of the two resonance compound 1) with allyl 2-chloropropionate (2) was carried out
peaks indicates that the compound had a macrocyclic struc- to afford dodecamethyldodeca(dimethylpropyl-2-chlor-
Hydrosilylation reaction of the MOSS macromer (viz.
tures with silsesquioxane as the backbone and hydro- opropionate)cyclododecasilsesquioxane (compound 3), i.e., a
dimethylsiloxyl as side groups. It should be pointed out that the MOSS macromolecular initiator bearing 2-chloropropionate
Fig. 2 MALDI-TOF mass spectrum of dodecamethyldodeca(dimethylhydro) cyclododecasilsesquioxane (1).
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 28439–28450 | 28443

View Article Online
RSC Advances
Paper
Fig. 3 1H NMR spectra of dodecamethyldodeca(dimethylhydro) cyclododecasilsesquioxane (1), MOSS macromolecular initiator (3) and
PNIPAAm2K@MOSS.
moieties. Also shown in Fig. 1 is the 29Si NMR spectrum of are assignable to the protons of methyl groups connected to
compound 3. Compared with the 29Si NMR spectrum of silsesquioxane backbone and the methyl in dimethylhy-
compound 1, the positions of the resonance peaks assignable to drosiloxayl groups, respectively. The resonance at 4.73 ppm is
the silicon nucleus of MOSS backbone (at ꢁ66.2 ppm) and the attributable to the protons in Si–H bonds. The ratio of integral
side groups (at ꢁ6.4 ppm) remained almost invariant, indi- intensity for these protons was measured to be 3 : 6 : 1, which is
cating that the hydrosilylation reaction did not alter the struc- in good agreement with the value calculated according to the
ture of the macrocyclic silsesquioxane. Shown in Fig. 3 are the structural formula of compound 1. Compared with the 1H NMR
1H NMR spectra of compounds 1 and 3. For compound 1, the spectrum of compound 1, the signal of resonance at 4.73 ppm
resonance peaks of methyl and Si–H protons were detected at assignable to the protons of Si–H bonds completely disappeared
1
0.09, 0.203 and 4.73 ppm, respectively. The former two peaks in the H NMR spectrum of compound 3, suggesting that the
28444 | RSC Adv., 2014, 4, 28439–28450
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Table 1 Results of polymerization for the preparation of organic–
inorganic molecular brushes composed of MOSS and PNIPAAm
hydrosilylation has been performed to completion. Concur-
rently, several new signals of resonance appeared at 0.57, 1.69,
2.45, 1.98, 4.11 and 4.38 ppm, which are assignable to the
protons of propyl 2-chloropropionate moiety as indicated in this
gure. The 1H and 29Si NMR spectroscopy results indicate that
the MOSS macromer bearing 2-chloropropionate moiety
(compound 3) was successfully obtained.
Sample
Larm (PNIPAAm) Mna (Da) Mw/Mn Mnb (Da)
PNIPAAm10K@MOSS 10 100
124 800 1.30
126 200
113 560
72 800
66 720
42 460
—
PNIPAAm8K@MOSS
PNIPAAm6K@MOSS
PNIPAAm4K@MOSS
PNIPAAm2K@MOSS
Macroinitiator
7700
5600
4300
2700
—
95 600
70 200
54 900
36 200
3400
1.41
1.43
1.45
1.46
1.12
The MOSS macromer bearing 2-chloropropionate moieties
was used as the macromolecular initiator, and the atom transfer
radical polymerization (ATRP) was carried out to afford the
organic–inorganic macromolecular brushes with MOSS back-
bone and with poly(N-isopropylacrylamide) (PNIPAAm) arms
(see Scheme 1). By controlling the molar ratio of the macro-
molecular initiator to NIPAAm and the conversion of the
monomer, the organic–inorganic molecular brushes with vari-
able lengths of PNIPAAm were obtained. Representatively
a
The values measured by means of gel permeation chromatography
(GPC). b The values estimated according to thermogravimetric analysis
(TGA).
macromolecular initiator was detected in the resulting samples.
Shown in Fig. 5 are the plots of number-average molecular
weights and the values of polydispersity index as functions of
the molar ratio of NIPAAm to the MOSS macromolecular initi-
ator. The molecular weights increased almost linearly with
increase of the molar ratio of NIPAAm to the MOSS macromo-
lecular initiator; the polydispersity indices of PNIPAAm@MOSS
were in the range of 1.30–1.46. The unimodal and fairly narrow
distribution of molecular weights indicates that the MOSS
macroinitiator (compound 3) can be used to obtain the PNI-
PAAm@MOSS samples, i.e., the organic–inorganic molecular
brushes were successfully obtained, and that the polymeriza-
tions of NIPAAm with the MOSS macromolecular imitator pro-
ceeded in a living and controlled manner. It is noted that while
the PNIPAAm arms were short (e.g., PNIPAAm2K@MOSS and
PNIPAAm4K@MOSS), the distribution of the molecular weights
was relatively broad. This observation could account for the
restriction of the MOSS macrocycle on the ATRP of NIPAAm. In
shown in Fig.
3
is the 1H NMR spectrum of PNI-
PAAm2K@MOSS. The signals of resonance at 1.14, 1.64, 2.02
and 4.01 ppm are assigned to the proton resonance of methyl,
methylene, methine of PNIPAAm main chain, and methine of
isopropyl groups of PNIPAAm, respectively; the broad signals of
resonance at 6.0–7.0 ppm are assignable to the protons of N–H
moiety. The signal of proton resonance at 0.10 ppm was
discernable and is assignable to the protons of methyl groups of
MOSS backbone. The 1H NMR spectra indicate that this product
combined the structural features of PNIPAAm and MOSS. All
PNIPAAm@MOSS samples with variable lengths of PNIPAAm
chains together with compound 3 (i.e. macroinitiator) were
subjected to gel permeation chromatography (GPC) to measure
the molecular weights; the GPC proles are presented in Fig. 4.
The results of the molecular weights are summarized in Table 1.
In all cases, unimodal peaks were displayed and no MOSS
Fig. 5 Plots of number-average molecular weights and polydispersity
Fig. 4 GPC profiles of the organic–inorganic macrocyclic molecular indices as functions of the molar ratio of NIPAAm to the MOSS
brushes.
macromolecular initiator (3).
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 28439–28450 | 28445

View Article Online
RSC Advances
Paper
this work, the lengths of PNIPAAm arms were also estimated by exothermic transition stemmed from the coil-to-globule tran-
thermogravimetric analysis (TGA). Assuming that the ceramic sition of the hydrated PNIPAAm chains. Notably, all PNI-
residues of the degradation were from MOSS, the lengths of PAAm@MOSS samples displayed similar exothermic
PNIPAAm arms were thus estimated according to the yields of transitions. These exothermic peaks were asymmetrical with
degradation (the TGA curves are not shown for brevity), and the steep leading regions before approaching the minima. It is
results were also incorporated into Table 1. The values of noted that the LCSTs of all organic–inorganic molecular
molecular weights by GPC were lower than those estimated by brushes were signicantly lower than that of plain PNIPAAm.
TGA. This observation could be ascribed to the relatively low The decreased LCSTs are attributable to the inclusion of the
hydrodynamic volumes of the brush-like PNIPAAm samples.
hydrophobic backbones (viz. MOSS), which reduced the affinity
of PNIPAAm chains with water molecules. It is seen that the
LCSTs of the hybrid molecular brushes decreased with
decreasing length of the PNIPAAm chains. Also shown in Fig. 6
is the plot of the exothermic enthalpy (DHLCST) as a function of
mass percentage of MOSS (see the inset). Assuming that the
value of LCST enthalpy (denoted by DHLCST) is proportional to
the quantity of the PNIPAAm chains which underwent the coil-
to-globule transition, a straight line should be drawn according
to the mass fraction of PNIPAAm chains if the LCST behavior of
PNIPAAm chains were not affected by MOSS. However, the
experimental DHLCST deviated negatively from this straight line.
This observation suggests that the fractions of the PNIPAAm
chains which can undergo the coil-to-globule transition in the
organic–inorganic molecular brushes were signicantly smaller
than plain PNIPAAm. It is proposed that owing to the restriction
of MOSS those PNIPAAm segments closely connected to MOSS
macrocycles were unable to undergo the coil-to-globule transi-
tion: these PNIPAAm segments remained hydrophilic even
while the solutions were heated to above the LCST. This spec-
ulation can be conrmed by means of cloud-point analysis and
DLS (see infra).
LCST behavior
PNIPAAm chains are capable of undergoing a coil-to-globule
transition in aqueous solution in the vicinity of its lower critical
solution temperature (LCST) at ꢃ32 ꢀC.7–11 A single PNIPAAm
chain adopts a random coil conformation in aqueous solution
below the LCST. Upon heating up to the LCST or above, the
random coil collapses into a globule owing to dehydration. In
the present case, each PNIPAAm chain was graed onto a bulky
and hydrophobic MOSS backbone and possessed only one free
end. It is of interest to examine the effect of the macrocyclic
brush architecture on the LCST behavior of PNIPAAm chains.
The thermoresponsive behavior was investigated by means of
micro-differential scanning calorimetry (Micro-DSC) and UV-vis
spectroscopy. Shown in Fig. 6 are the Micro-DSC curves of the
aqueous solutions of the organic–inorganic molecular brushes.
In the Micro-DSC curves, plain PNIPAAm displayed a single
ꢀ
exothermic transition at 33.3 C, attributable to the lower crit-
ical solution temperature (LCST) of this polymer. The
Fig. 6 Micro-DSC curves of the aqueous solutions of plain PNIPAAm Fig. 7 Plots of light transmittance (l ¼ 550 nm) as functions of
(Mn ¼ 12 400 with Mw/Mn ¼ 1.26) and the organic–inorganic macro- temperature for the aqueous solution (0.2 g Lꢁ1) of plain PNIPAAm and
cyclic molecular brushes.
PNIPAAm@MOSS samples at a heating rate of 0.2 ꢀC minꢁ1
.
28446 | RSC Adv., 2014, 4, 28439–28450
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
The LCST behavior of the organic–inorganic PNIPAAm DSC results. Second, the coil-to-globule transition ranges of the
molecular brushes was further investigated in terms of cloud- PNIPAAm@MOSS samples were signicantly broadened, espe-
point analysis by means of UV-vis spectroscopy. Shown in Fig. 7 cially for the sample with shorter PNIPAAm chains (e.g., PNI-
are the plots of visible light transmittance for the organic–inor- PAAm2K@MOSS). The broadened coil-to-globule transition
ganic molecular brushes as function of temperature. For ranges could be associated with the restriction of MOSS on the
comparison, the curve of plain PNIPAAm (Mn ¼ 12 400 Mw/Mn ¼ coil-to-globule transitions of PNIPAAm chains. It is plausible to
1.26) aqueous solution was included. At low temperatures, the propose that the conformational alteration of PNIPAAm chains
optical transmittance of all the aqueous solutions was as high as was hindered by the bulky MOSS backbone while the coil-to-
97%. It is noted that the optical transmittance of the PNI- globule transition of PNIPAAm side chains occurred. Third, the
PAAm@MOSS suspension was slightly lower than that of plain optical transmittance of all the organic–inorganic molecular
PNIPAAm. Upon heating the aqueous solutions to specic brushes at elevated temperatures was signicantly higher than
temperatures, the optical transmittance decreased signicantly, that of the plain PNIPAAm. For instance, the optical transmittance
indicating the occurrence of coil-to-globule transitions. For plain of PNIPAAm10k@MOSS at 44 ꢀC was still as high as 93% (see the
PNIPAAm, the LCST was measured to be ꢃ32 ꢀC, and the transi- inset, the optical images of the aqueous solution at 24 and 44 ꢀC),
tion range of temperature was as narrow as 2–3 ꢀC. Aer the LCST suggesting that there was a considerable portion of the PNIPAAm
the optical transmittance were decreased to ꢃ50% at 44 ꢀC. chain which remained hydrated even at the temperature above
Nonetheless, the PNIPAAm@MOSS suspensions displayed the LCSTs. This observation can account for the restriction of
features quite different from those of plain PNIPAAm. First, the MOSS on the coil-to-globule transition of PNIPAAm. This restric-
LCSTs of the PNIPAAm@MOSS samples were much lower than tion resulted in a failure that the PNIPAAm segments became
those of plain PNIPAAm; the LCSTs decreased with decreasing dehydrated above the LCST. The similar case has been theoreti-
lengths of the PNIPAAm chains in the organic–organic molecular cally rationalized by some investigators45,46 and has been found in
brushes. This observation was in good agreement with the Micro- organic PNIPAAm brushes.47–52
Fig. 8 TEM micrographs of the organic–inorganic macrocyclic molecular brushes in bulk: (A) PNIPAAm10K@MOSS, (B) PNIPAAm8K@MOSS, (C)
PNIPAAm6K@MOSS, (D) PNIPAAm4K@MOSS and (E) PNIPAAm2K@MOSS.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 28439–28450 | 28447

View Article Online
RSC Advances
Paper
assembly behavior in aqueous solutions. The self-assembly
behavior of the organic–inorganic molecular brushes was
investigated by transmission electron microscopy (TEM) and
dynamic light scattering (DLS). Shown in Fig. 9 are the TEM
micrographs of the aqueous solutions aer freeze drying. In the
aqueous solutions, PNIPAAm2K@MOSS and PNIPAAm4K@
MOSS were self-assembled into worm-like and interconnected
nano-objects (Fig. 9A and B). With increasing lengths of the
PNIPAAm chains, the organic–inorganic molecular brushes
were gradually self-assembled into the spherical nano-objects
100–150 nm in size (see Fig. 9C–E). Shown in Fig. 10 are the
plots of hydrodynamic radius distribution as function of
hydrodynamic radius (Rh) for the aqueous solutions of the
PNIPAAm@MOSS samples at 24 and 48 ꢀC. Notably, all the
organic–inorganic macrocyclic molecular brushes were capable
of self-organizing into nano-objects in aqueous solution. Apart
from PNIPAAm2K@MOSS, the intensity-averaged hydrody-
namic radius (Rh) of all other PNIPAAm@MOSS samples dis-
played unimodal distribution dependent on the length of the
PNIPAAm arms at 24 ꢀC. Depending on the lengths of the
PNIPAAm arms, the values of intensity-averaged hydrodynamic
radius (Rh) were 239, 231, 226, 194 and 162 nm for
Self-assembly behavior in bulk and aqueous solutions
The morphologies of the organic–inorganic molecular brushes
in bulk were investigated by means of transmission electron
microscopy (TEM). Shown in Fig. 8 are the TEM micrographs of
PNIPAAm2K@MOSS, PNIPAAm4K@MOSS, PNIPAAm6K@
MOSS, PNIPAAm8K@MOSS and PNIPAAm10K@MOSS.
Notably, microphase-separated morphologies were exhibited in
all cases. Spherical microdomains with the diameter of 10–50
nm were dispersed in a continuous matrix depending on the
length of PNIPAAm chains. According to the difference in
electron density between MOSS and PNIPAAm chains, the
spherical microdomains are assignable to MOSS aggregates,
whereas the continuous matrix is assignable to PNIPAAm.
Notably, the size of MOSS microdomains decreased with
increase in the lengths of PNIPAAm. The formation of the
microphase-separated morphologies is attributable to the
immiscibility of the inorganic backbone (viz. MOSS) with PNI-
PAAm chains; the spherical MOSS microdomains were formed
via MOSS–MOSS interactions.
In view of the hydrophobicity of MOSS backbone and the
water solubility of PNIPAAm chains, it is expected that the
organic–inorganic molecular brushes would exhibit self-
Fig. 9 TEM micrographs of the organic–inorganic macrocyclic molecular brushes in aqueous solutions at 0.2 g Lꢁ1: (A) PNIPAAm2K@MOSS, (B)
PNIPAAm4K@MOSS, (C) PNIPAAm6K@MOSS, (D) PNIPAAm8K@MOSS and (E) PNIPAAm10K@MOSS.
28448 | RSC Adv., 2014, 4, 28439–28450
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
molecular brushes were self-organized into bigger nano-objects.
For PNIPAAm6K@MOSS, PNIPAAm4K@MOSS and PNI-
PAAm2K@MOSS, however, the Rh values remained unchanged
or even decreased, suggesting that in these systems, a consid-
erable amount of PNIPAAm remains hydrated at this tempera-
ture. Owing to the presence of these hydrated PNIPAAm chains,
the structural arrangement of the original aggregates resulted
in the decreased size of the nano-objects under the new
hydrophilic/hydrophobic balance. Notably, although decreased
Rh values were observed for PNIPAAm4K@MOSS and PNI-
ꢀ
PAAm2K@MOSS at 48 C, the distribution of Rh became fairly
broad, possibly due to the restriction of MOSS on PNIPAAm
chains.
Conclusions
Organic–inorganic molecular brushes composed of macrocyclic
oligomeric
silsesquioxanes
(MOSS)
and
poly(N-iso-
propylacrylamide) (PNIPAAm) chains were synthesized via atom
transfer radical polymerization (ATRP) with a 24-membered
MOSS macromer bearing twelve 2-chloropropionate moieties.
Gel permeation chromatography (GPC) showed that the poly-
merization of N-isopropylacrylamide (NIPAAm) mediated by
copper(I) chloride and tris(2-(dimethylamino)ethyl)amine
(Me6TREN) occurred in a living and controlled manner. Trans-
mission electron microscopy (TEM) showed that the organic–
Fig. 10 Hydrodynamic radius (Rh) of PNIPAAm@MOSS samples in
aqueous solutions (0.2 g Lꢁ1) at 24 (solid lines) and 48 ꢀC (dashed lines).
PNIPAAm2K@MOSS, PNIPAAm4K@MOSS, PNIPAAm6K@ inorganic molecular brushes were microphase-separated. The
MOSS, PNIPAAm8K@MOSS and PNIPAAm10K@MOSS at 24 ꢀC, self-assembly of the organic–inorganic molecular brushes in
respectively; the Rh values decreased with increasing PNIPAAm bulk resulted in the formation of a morphology in which
arm length. It is proposed that the self-assembled nano-objects spherical MOSS microdomains with the diameter of 10–50 nm
are micelle-like aggregates composed of hydrophobic cores via were dispersed into a continuous PNIPAAm matrix. All the
the MOSS–MOSS interactions and hydrated PNIPAAm coronas. organic–inorganic molecular brushes can be dispersed in water.
It should be pointed out that for the aqueous solution con- The PNIPAAm@MOSS samples were capable of self-organizing
taining PNIPAAm2K@MOSS, there were nano-objects with into nano-objects in aqueous solutions, as evidenced by
smaller size (i.e., Rh ¼ 32 nm). The bimodal distribution of the dynamic light scattering (DLS) and transmission election
hydrodynamic radius could result from the hydrophilicity/ microscopy (TEM). The size of the nano-objects was quite
hydrophobicity balance not being fully established under the dependent on the percentage of MOSS in the organic–inorganic
present conditions when the percentage of the inorganic molecular brushes. Both micro-differential scanning calorim-
hydrophobic component (viz. MOSS–MOSS) was too high. etry (Micro-DSC) and ultraviolet-visible spectroscopy showed
Furthermore, the results of DLS were in good agreement with that the organic–inorganic molecular brushes can exhibit the
those of TEM. It is should be pointed out that the sizes of thermoresponsive properties in aqueous solution as a plain
aggregates measured with TEM (Fig. 9) were not necessarily PNIPAAm homopolymer. Nonetheless, the MOSS cages in the
identical to the Rh values by means of DLS. The former were molecular brushes exerted a signicant restriction on coil-to-
obtained in the dry state and were much lower than the latter globule transition of PNIPAAm chains.
obtained in solution, which contains the dominant contribu-
tion of the solvated coronas (viz. PNIPAAm).
Acknowledgements
Returning to Fig. 10, the coil-to-globule transition restriction
of MOSS on PNIPAAm chains can be accounted for by the The nancial support from Natural Science Foundation of
hydrodynamic radii at a tꢀemperature higher than that of the China (no. 21274091 and 51133003) was gratefully
ꢀ
LCSTs (e.g., 48 C). At 48 C, the Rh values of PNIPAAm10K@ acknowledged.
MOSS, PNIPAAm8K@MOSS and PNIPAAm6K@MOSS were
measured to be 412, 246 and 224 nm, respectively. The
increased Rh values are readily interpretable on the basis of the
References
occurrence of the coil-to-globule transition of PNIPAAm chains.
With this transition, a portion of PNIPAAm chains became
1 Y. C. Wang, L. Tang, Y. Li and J. Wang, Biomacromolecules,
2008, 10, 66.
dehydrated. To establish
balance even at the elevated temperature, the organic–inorganic
a
new hydrophobic/hydrophilic
2 A. W. York, S. E. Kirkland and C. L. McCormick, Adv. Drug
Delivery Rev., 2008, 60, 1018.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 28439–28450 | 28449

View Article Online
RSC Advances
Paper
3 Y. Hu, Y. Chen, Q. Chen, L. Y. Zhang, X. Q. Jiang and 32 N. V. Sergienko, E. S. Trankina, V. I. Pavlov, A. A. Zhdanov,
C. Z. Yang, Polymer, 2005, 46, 12703.
4 Z. L. Ding, R. B. Fong, C. J. Long, P. S. Stayton and
A. S. Hoffman, Nature, 2001, 411, 59.
K. A. Lyssenko and M. Y. Antipin, Russ. Chem. Bull., Int.
Ed., 2004, 53, 351.
33 Y. A. Molodtsova, Y. A. Pozdnyakova, K. A. Lyssenko,
5 Y. F. Zhang, S. Z. Luo and S. Y. Liu, Macromolecules, 2005, 38,
9813.
I.
V.
Blagodatskikh,
D.
E.
Katsoulis
and
O. I. Shchegolikhina, J. Organomet. Chem., 1998, 571, 31.
6 P. De, S. R. Gondi, D. Roy and B. S. Sumerlin, 34 O. I. Shchegolikhina, Y. A. Pozdnyakova, S. D. Molodtsova
Macromolecules, 2009, 42, 5614. and B. Korkin, Organometallics, 2000, 19, 1077.
7 M. Heskins and J. E. Guillet, J. Macromol. Sci., Part A, 1968, 2, 35 O. I. Shchegolikhina, Y. A. Pozdnyakova, Y. A. Molodtsova,
441.
S. D. Korkin, S. S. Bukalov, L. A. Leites, K. A. Lyssenko,
A. S. Peregudov, N. Auner and D. E. Katsoulis, Inorg.
Chem., 2002, 41, 6892.
8 P. Kujawa, F. Tanaka and F. M. Winnik, Macromolecules,
2006, 39, 3048.
9 H. G. Schild, Prog. Polym. Sci., 1992, 17, 163.
10 C. Wu, Macromolecules, 1997, 30, 574.
11 P. Kujawa and F. M. Winnik, Macromolecules, 2001, 34, 4130.
12 Y. Hirokawa and T. Tanaka, J. Chem. Phys., 1984, 81, 6379.
13 E. S. Matsuo and T. Tanaka, J. Chem. Phys., 1988, 89, 1695.
14 A. Suzuki and T. Tanaka, Nature, 1990, 346, 345.
36 Y. A. Pozdnyakova, K. A. Lyssenko, I. V. Blagodatskikh,
N. Auner, D. E. Katsoulis and O. I. Shchegolikhina, Eur.
J. Inorg. Chem., 2004, 2004, 1253.
37 O. I. Shchegolikhina, Y. A. Pozdnyakova, M. Y. Antipin,
D. E. Katsoulis, N. Auner and B. Herrscha,
Organometallics, 2000, 19, 1077.
15 K. Otake, H. Inomata, M. Konno and S. Saito, 38 J. Han and S. Zheng, Macromolecules, 2008, 41, 4561.
Macromolecules, 1990, 23, 283.
16 H. Inomata, S. Goto, K. Otake and S. Saito, Langmuir, 1992, 8,
39 J. Han and S. Zheng, J. Polym. Sci., Part A: Polym. Chem., 2009,
47, 6894.
687.
40 J. Han, L. Zhu and S. Zheng, Eur. Polym. J., 2012, 48, 730.
17 M. Shibayama, M. Morimoto and S. Nomura, 41 L. Zhu, C. Zhang, J. Han and S. Zheng, So Matter, 2012, 8,
Macromolecules, 1994, 27, 5060. 7062.
18 L. Liang, P. C. Rieke, J. Liu, G. E. Fryxell, J. S. Young, 42 J. S. Wang and K. Matyjaszewski, J. Am. Chem. Soc., 1995, 117,
M. H. Engelhard and K. L. Alford, Langmuir, 2000, 16, 8016. 5614.
19 J. Shan, J. Chen, M. Nuopponen and H. Tenhu, Langmuir, 43 Y. A. Molodtsova, K. A. Lyssenko, I. V. Blagodatskikh,
2004, 20, 4671.
E. V. Matukhina, A. S. Peregudov, M. I. Buzin,
V. G. Vasil’ev, D. E. Katsoulis and O. I. Shchegolikhina, J.
Organomet. Chem., 2008, 693, 1797.
20 M. Nuopponen, J. Ojala and H. Tenhu, Polymer, 2004, 45,
3643.
21 X. Zhou, X. Ye and G. Zhang, J. Phys. Chem. B, 2007, 111, 44 Y. A. Molodtsova, Y. A. Pozdnyakova, I. V. Blagodatskikh,
5111.
A. S. Peregudov and O. I. Shchegolikhina, Russ. Chem.
Bull., Int. Ed., 2003, 52, 2722.
22 J. Mu and S. Zheng, J. Colloid Interface Sci., 2007, 307, 377.
23 K. Zeng, Y. Fang and S. Zheng, J. Polym. Sci., Part B: Polym. 45 E. B. Zhulina, O. V. Borisov, V. A. Pryamitsyn and
Phys., 2009, 47, 504. T. M. Birshtein, Macromolecules, 1991, 24, 140.
24 W. Zhang, L. Liu, X. Zhuang, X. Li, J. Bai and Y. Chen, J. 46 G. S. Grest and M. Murat, in Monte Carlo and Molecular
Polym. Sci., Part A: Polym. Chem., 2008, 46, 7049.
Dynamics Simulations in Polymer Science, ed. K. Binder,
25 Y. Zheng, L. Wang and S. Zheng, Eur. Polym. J., 2012, 48, 945.
Clarendon, Oxford, 1994.
¨
26 L. Wang, K. Zeng and S. Zheng, ACS Appl. Mater. Interfaces, 47 G. E. Yakubov, B. Loppinet, H. Zhang, J. Ruhe, R. Sigel and
2011, 3, 898. G. Fytas, Phys. Rev. Lett., 2004, 92, 115501.
27 S. W. Kuo, J. L. Hong, Y. C. Huang, J. K. Chen, S. K. Fan, 48 A. Domack, S. Prucker, J. Ruhe and D. Johannsmann, Phys.
F. H. Ko, C. W. Chu and F. C. Chang, J. Nanomater., 2012,
749732, DOI: 10.1155/2012/749732.
Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top.,
1997, 56, 680.
28 Y. Bai, J. Wei, L. Yang, C. He and X. Lu, Colloid Polym. Sci., 49 A. Karim, S. K. Satija, J. F. Douglas, J. F. Ankner and
2012, 290, 507. L. J. Fetters, Phys. Rev. Lett., 1994, 73, 3407.
29 G. Li, L. Wang, H. Ni and C. U. Pittman, J. Inorg. Organomet. 50 S. Balamurugan, S. Mendez, S. S. Balamurugan, M. J. I. I. O’
Polym., 2001, 11, 123. Brien and G. P. Lopez, Langmuir, 2003, 19, 2545.
30 O. I. Shchegolikhina, V. A. Igonin, Y. A. Molodtsova and 51 H. Yim, M. S. Kent, S. Mendez, S. S. Balamurugan,
Y. A. Pozdnyakova, J. Organomet. Chem., 1998, 562, 141.
31 A. A. Zhdanov and T. V. Strelkova, J. Organomet. Chem., 1998,
562, 141.
S. Balamurugan, G. P. Lopez and S. Satija, Macromolecules,
2004, 37, 1994.
52 P. W. Zhu and D. H. Napper, Langmuir, 1996, 12, 5992.
28450 | RSC Adv., 2014, 4, 28439–28450
This journal is © The Royal Society of Chemistry 2014