organic/inorganic hybrid star-shaped block copolymers of poly(l-lactide) and...
TRANSCRIPT
European Polymer Journal 48 (2012) 720–729
MA
CRO
MO
LECU
LAR
NA
NO
TECH
NO
LOG
Y
Contents lists available at SciVerse ScienceDirect
European Polymer Journal
journal homepage: www.elsevier .com/locate /europol j
Macromolecular Nanotechnology
Organic/inorganic hybrid star-shaped block copolymers of poly(L-lactide)and poly(N-isopropylacrylamide) with a polyhedral oligomericsilsesquioxane core: Synthesis and self-assembly
Weian Zhang a,⇑, Shaohua Wang a, Xiaohui Li a, Jiayin Yuan b, Shaolei Wang a
a Shanghai Key Laboratory of Functional Materials Chemistry, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR Chinab Makromolekulare Chemie II, Universität Bayreuth, D-95440 Bayreuth, Germany
a r t i c l e i n f o
Article history:Received 14 September 2011Received in revised form 18 January 2012Accepted 22 January 2012Available online 3 February 2012
Keywords:Poly(L-lactide)Poly(N-isopropylacrylamide)Self-assemblyBlock copolymersPolyhedral oligomeric silsesquioxane(POSS)
0014-3057/$ - see front matter � 2012 Elsevier Ltddoi:10.1016/j.eurpolymj.2012.01.019
⇑ Corresponding author.E-mail address: [email protected] (W. Zhan
a b s t r a c t
The star-shaped organic/inorganic hybrid poly(L-lactide) (PLLA) based on polyhedral oligo-meric silsesquioxane (POSS) was prepared using octa(3-hydroxypropyl) polyhedral oligo-meric silsesquioxane as initiator via ring-opening polymerization (ROP) of L-lactide(LLA). The molecular weight of POSS-containing star-shaped hybrid PLLA (POSSPLLA) canbe well controlled by the feed ratio of LLA to initiator. The POSSPLLA was further function-alized into the macromolecular reversible addition-fragmentation transfer (RAFT) agent forthe polymerization of N-isopropylacrylamide (NIPAM), leading to the POSS-containingstar-shaped organic/inorganic hybrid amphiphilic block copolymers, poly(L-lactide)–block–poly(N-isopropylacrylamide) (POSS(PLLA–b–PNIPAM)). The self-assembly behaviorof POSS(PLLA–b–PNIPAM) block copolymers in aqueous solution was investigated bydynamic light scattering (DLS), transmission electron microscopy (TEM) and atomic forcemicroscopy (AFM). DLS showed the PNIPAM block in the aggregates is temperature-responsive and its phase-transition is reversible. TEM proved that the star-shaped POS-S(PLLA–b–PNIPAM) amphiphilic block copolymers can self-assemble into the vesicles inaqueous solution. The vesicular wall and coronas are composed of the hydrophobic POSScore and PLLA, and hydrophilic PNIPAM blocks, respectively. Therefore, POSSPLLA and POS-S(PLLA–b–PNIPAM) block copolymers, as a class of novel organic–inorganic hybrid materi-als with the advantageous properties, can be potentially used in biological and medicalfields.
� 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Polyhedral oligomeric silsesquioxane (POSS), with a sizein nano-scale, has also been widely investigated, since itcan be introduced into polymer matrices to form hybridpolymers with superior mechanical and thermal proper-ties, oxidation resistance, and reduced flammability[1–10]. The typical POSS molecule has a cage-shapedframework with an inorganic silica-like core surroundedby eight organic corner groups which endow the POSS
. All rights reserved.
g).
molecule with higher solubility in organic solvents anddiversities as monomers and initiators [11,12]. POSS mole-cules can be easily incorporated into polymeric matrices toprepare polymer hybrids by physical blending or chemicalcopolymerization [13–16]. Recently, with the emergenceof new living polymerization techniques such as atomictransfer radical polymerization (ATRP) and reversibleaddition-fragmentation transfer (RAFT) polymerization,and ‘‘click chemistry’’, some novel POSS-containing poly-mers with well-defined structures have been synthesized,and the self-assembly behavior of these hybrid polymershas also attracted great interests [9,17–20]. We synthe-sized POSS-containing tadpole-shaped organic/inorganic
W. Zhang et al. / European Polymer Journal 48 (2012) 720–729 721
MA
CRO
MO
LECU
LAR
NA
NO
TECH
NO
LOG
Y
poly(N-iso propylacrylamide) (PNIPAM) and poly(acrylicacid) (PAA) hybrids using RAFT polymerization, and stud-ied the self-assembly behavior of these hybrids in water[21,22]. In this work, we continue the study of the self-assembly of POSS-containing hybrid polymers. We willexplore the self-assembly behavior of POSS-containingstar-shaped amphiphilic hybrid block copolymers, poly(L-lactide)–block–poly(N-isopropylacrylamide) (POSS(PLLA–b–PNIPAM)). We hope we can obtain some interestingself-assembly morphologies and mechanism, which wouldbe different with that of POSS-containing linear amphi-philic hybrid polymers.
POSS molecule is also a promising material in biologicalfields, due to its advantageous properties such as highmechanical property and biodegradability provided bythe Si–O–Si bonds [23–25]. The works upon POSS-contain-ing hybrid biopolymers have widely been carried out. Forexample, the star-shaped poly(e-caprolatone) (PCL) andpoly(ethylene oxide) (PEO) with POSS cores have been syn-thesized via core- and arm-first approaches, respectively[26,27]. PLLA, as a class of important biopolymers, havebeen applied in biological and medical fields, due to theirbiodegradability, biocompatibility and advantageousmechanical properties [28–32]. POSS molecules were alsoincorporated into PLLA matrices to architecture the POSS-containing PLLA hybrids using physical blending or livingopening polymerization, and the crystallization and prop-erties were further investigated [33–38].
PNIPAM is a typical temperature-responsive polymer,which has a critical phase transition temperature (32 �C)in aqueous solution. The critical phase transition tempera-ture can be adjusted close to human bodily temperature bymodifying PNIPAM. Accordingly, PNIPAM can be poten-tially applied in the biological fields such as drug delivery[39–43]. PNIPAM was also selected as a model polymerto research the physical nature of polymer chain in solu-tion [44]. For example, the fold behavior of polymer chainin solution can be obtained by studying the influence of thetemperature on the coil-to-globule phase transition of thePNIPAM chain [45]. Winnik et al. synthesized hemi-telech-elic and telechelic PNIPAMs and systemically studied theirthermal properties and self-assembly behavior [46–48]. Inaddition, PNIPAM block copolymers were also widelyinvestigated, especially in the influence of temperatureon their self-assembly behavior. For example, poly(N-iso-propylacrylamide)–block–poly(ethylene oxide) (PNIPAM–b–PEO) can self-assemble into the core-shell micelle witha PNIPAM core and a PEO shell at temperatures abovethe low critical solution temperature (LCST) [49,50]. PNI-PAM–b–PLLA diblock copolymers was synthesized byring-opening polymerization and RAFT polymerization,and their self-assembly behavior was also studied [51–53]. We have synthesized polystyrene–block–poly(N-iso-propylacrylamide) (PS–b–PNIPAM) diblock copolymers,and investigated the conformational change of PNIPAMblocks as the coronas of micelles and vesicles by a combi-nation of static and dynamic laser light scattering [54].
In this contribution, the star-shaped organic/inorganichybrid PLLA (POSSPLLA) was synthesized by ring-openingpolymerization (ROP) using a POSS molecule containingeight hydroxyl groups as an initiator (Scheme 1). The POS-
SPLLA was then functionalized into the macromolecularreversible addition-fragmentation transfer (RAFT) agentfor the polymerization of N-isopropylacrylamide (NIPAM),leading to the POSS-containing star-shaped organic/inor-ganic hybrid amphiphilic block copolymers, POSS(PLLA–b–PNIPAM). The self-assembly behavior of the star-shapedhybrid POSS(PLLA–b–PNIPAM) block copolymers wasinvestigated by dynamic laser scattering (DLS), transmis-sion electron microscopy (TEM) and atomic force micros-copy (AFM).
2. Experimental
2.1. Materials
L-Lactide (LLA) was kindly supplied by Shanghai New-genius Biotech Co. Ltd., China. Before use, it was recrystal-lized from anhydrous ethyl ester, and then dried in vacuo atroom temperature. N-isopropylacrylamide (NIPAM) wasprepared in lab according to the literature [55] and recrys-tallized from hexane three times. Stannous (II) octanoatewas purchased from Aldrich and used without furtherpurification. Other regents and solvents in analytical gradewere all obtained from Shanghai Chemical Reagents Com-pany. Thionyl chloride and carbon disulfide were distilledprior to use. Pyridine was dried over CaH2 and distilled be-fore use. Azobisisobutyronitrile (AIBN) was recrystallizedfrom ethanol. Tetrahydrofuran (THF) was distilled from apurple sodium ketyl solution. Octa(3-hydroxypropyl) poly-hedral oligomeric silsesquioxane (POSS–(OH)8) was pre-pared according to the previous literature [56].
2.2. Preparation of POSS-containing star-shaped inorganic/organic PLLA (POSSPLLA)
The POSSPLLA was prepared by ring-opening polymeri-zation (ROP) of LLA using stannous (II) octanoate [Sn(Oct)2]as catalyst. In a typical experiment, POSS–(OH)8 (0.0444 g,0.05 mmol), LLA (1.0 g, 6.94 mmol) and Sn(Oct)2 (0.0028 g,0.0069 mmol) were charged into a dried flask with a mag-netic stirring bar. The flask with the reactive mixture wasconnected to the Schlenk line and degassed via threepump–thaw–freeze cycles, and then immersed in athermostated oil bath at 130 �C for 24 h. The crude prod-ucts were dissolved with CHCl3 and precipitated into anexcessive amount of petroleum ether. This procedure wasrepeated three times to remove any unreacted monomer.The precipitates were filtered and dried in vacuo at 40 �Cfor 24 h to give 0.96 g polymer with a yield of 92 wt%. 1HNMR (ppm, CDCl3): 5.16 (m, CH in PLLA); 3.54 (t,SiCH2CH2CH2–PLLA); 1.84 (m, SiCH2CH2CH2–PLLA); 1.57(d, CH3 in PLLA); 0.79 (t, SiCH2CH2CH2–PLLA). 13C NMR(ppm, CDCl3): 170.07 (CO in PLLA); 69.22 (CH in PLLA);16.96 (CH3 in PLLA).
2.3. Preparation of S-1-dodecyl-S-(a,a-dimethyl-a00-aceticacid) trithiocarbonate (DDAT)
DDAT was prepared according to the literature [57]. 1HNMR (CDCl3, ppm): 0.90 (t, 3H, (CH2)11CH3); 1.20–1.48 (m,
Si
O
Si
Si
O
Si
Si
O
Si
SiO
O OO
OOO
O
O
Si
OH
OHHO
HO
HO
HO OH
OHO
OO
OO
H
nSi
O
Si
Si
O
Si
Si
O
Si
SiO
O OO
OOO
O
O
Si
R
RR
R
R
R
R
R
O
OO
O
On
Si
O
Si
Si
O
Si
Si
O
Si
SiO
O OO
OOO
O
O
Si
R'
R'R'
R'
R'
R'
OS S
C12H25S
(POSSPLA-DDAT)
O
OO
O
On
Si
O
Si
Si
O
Si
Si
O
Si
SiO
O OO
OOO
O
O
Si
R"
R""R
"R
"R
"R
R"
O
NHO
S SC12H25
Sm
(POSS(PLA-b-PNIPAM))
R"
R'
R'
DDAT
NIPAM
(POSSPLA)
LLA
Scheme 1. Synthesis of POSSPLLA and POSS(PLLA–b–PNIAM) block copolymers.
722 W. Zhang et al. / European Polymer Journal 48 (2012) 720–729
MA
CRO
MO
LECU
LAR
NA
NO
TECH
NO
LOG
Y
18H, (CH2)9CH3); 1.64–1.83 (m, 8H, SC(CH3)2COOH andCH2(CH2)9CH3); 3.30 (t, 2H, CH2(CH2)10CH3).
2.4. Preparation of S-1-dodecyl-S0-(a,a0-dimethyl-a00-aceticchloride) trithiocarbonate (DDACT)
DDAT (1.8 g, 4.93 mmol) and 15 mL of anhydrousdichloromethane were introduced into a dried 50 mLSchlenk flask charged with a magnetic stirring bar. ThisSchlenk flask was connected to a standard Schlenkline sys-tem and washed with highly pure nitrogen three times.Fresh thionyl chloride (10 mL) was added dropwise intothe DDAT mixture solution over thirty minutes. Then thereaction mixture was heated at reflux temperature for 1hour. The solvent was removed by first distilling at atmo-spheric pressure and then under vacuo (10�3 Torr), leavinga yellow and highly viscous liquid, which was further driedunder a high vacuum (10�3 Torr) overnight at room
temperature. The DDACT was not characterized beforeuse in the next step.
2.5. Synthesis of POSS-containing star-shaped PLLAmacromolecular RAFT agent (POSSPLLADDAT)
POSS(PLLA12)8 (3.0 g, 0.204 mmol) was dissolved in15 mL of absolute THF, and 0.4 mL of distilled pyridinewas added. A solution of above freshly prepared DDACTin 10 mL of absolute THF was added slowly into the POS-S(PLLA12)8 solution at 0 �C in half an hour. After stirringfor 12 h at room temperature, the mixture was filtered,and the filtrate was concentrated under reduced pressureby heating evaporation. The residue was further dissolvedin 5.0 mL chloroform, and precipitated into an excessiveamount of petroleum ether. This procedure was repeatedthree times. The precipitates were filtered and dried in va-cuo at 40 �C for 48 h to give 3.34 g polymer with a yield of
W. Zhang et al. / European Polymer Journal 48 (2012) 720–729 723
MA
CRO
MO
LECU
LAR
NA
NO
TECH
NO
LOG
Y
93 wt%. 1H NMR (CDCl3, ppm): 5.16 (m, CH in PLLA); 3.54(t, SiCH2CH2CH2–PLLA); 2.95 (t, CH3C10H20CH2SC(S)); 1.84(m, SiCH2CH2CH2–PLLA); 1.44–1.70 (m, CH3 in PLLA,SC(CH3)2COO); 1.13–1.36 (m, CH2C10H20CH3); 0.72–0.92(m, SiCH2CH2CH2–PLLA, CH2C10H20CH3).
2.6. Synthesis of POSS-containing star-shaped POSS(PLLA–b–PNIPAM) block copolymers
A typical polymerization procedure for the synthesis ofPOSS(PLLA–b–PNIPAM) block copolymers as following: A10 mL glass flask containing a stirring bar was chargedwith POSSPLLADDAT macromolecular RAFT agent(100.0 mg, 0.0057 mmol), NIPAM (1.0 g, 8.85 mmol), AIBN(0.31 mg, 0.0019 mmol) and 3.0 mL of 1,4-dioxane. Theglass flask was connected to a standard Schlenkline systemwith highly pure nitrogen. The solution was degassed bythree freeze-evacuate-thaw cycles, and then the glass tubewas sealed under vacuum. The polymerization was carriedout in a thermostated oil bath at 65 �C for 6 h. The poly-merization was stopped by plunging the tube into icewater. The polymerization tube was opened and 3 mL ofTHF was added, and then precipitated into 250 mL ofdiethyl ether. The precipitate was filtered, and the productwas dried at 50 �C in a vacuum oven for 24 h to give 0.53 gwith a yield of 42.7 wt%. 1H NMR (CDCl3, ppm): 5.15 (m, CHin PLLA); 3.99 (s, NHCH(CH3)2), 1.75–2.62 (m, backboneprotons of PNIPAM), 1.44–1.70 (m, CH3 in PLLA,SC(CH3)2COO); 0.86–1.33 (m, NHCH(CH3)2, CH2C10H20CH3).Mn(GPC) = 65, 241, Mw/Mn = 1.41.
2.7. Preparation of the micelles of POSS(PLLA–b–PNIPAM)block copolymers in aqueous solution
A typical micellar solution was prepared as following:POSS(PLLA–b–PNIPAM) block copolymers (100 mg) wasfirst dissolved in dioxane (25 mL), which is a good solventfor the block copolymers. The solution was stirred over-night, and gradually dialyzed against Millipore water(resistance = 18 MX) for 3 days. After gradual dialysis, themicellar solution was dialyzed at least three times usingpure Millipore water to make sure to completely removedioxane.
2.8. Characterization
2.8.1. Nuclear magnetic resonance spectroscopy (NMR)The 1H NMR and 13C NMR measurements were carried
out on a Varian Mercury Plus 400 MHz NMR spectrometerat 25 �C. The samples were dissolved with deuteratedCDCl3 and the solutions were measured with tetramethyl-silane (TMS) as an internal reference.
2.8.2. Gel permeation chromatography (GPC)The molecular weights and molecular weight distribu-
tion were measured on Waters 150C gel permeation chro-matography (GPC) equipped with refractive index detectorand Ultrastyragel columns of 100, 10,000 Å porosities. Aseries of monodisperse polystyrene standards were usedfor calibration, and tetrahydrofuran (THF) was used asthe eluent at a flow rate of 1 mL/min.
2.8.3. Differential scanning calorimetry (DSC)The calorimetric measurements were performed on a
Perkin Elmer Diamond differential scanning calorimeter ina dry nitrogen atmosphere. The instrument was calibratedwith standard Indium. The samples were first heated upto 200 �C at the heating rate of 20 �C/min and were holdat this temperature for 5 min to melt the crystals, followedby quenching to 40 �C. The second heating scans were car-ried out from 40 to 200 �C at the heating rate of 20 �C/minand the thermograms were recorded. Crystallization tem-peratures (Tc) and melting temperatures (Tm) were takenas the temperatures at minimum and maximum of bothendothermic and exothermic peaks, respectively.
2.8.4. X-ray diffraction (XRD)XRD patterns were recorded in transmission with a
Bruker D8 Advance X-ray diffractometer. The wavelengthused was CuKa (k = 0.154 nm) and spectra were recordedin the 2h range of 8–40� (step size 0.025�). The samplewas pressed between Kapton films for measurement.
2.8.5. Dynamic light scattering (DLS)Dynamic light scattering (DLS) was carried out on an
ALV DLS/SLS-SP 5022F compact goniometer system withan ALV 5000/E correlator and a He–Ne laser (k = 632.8nm). Before the light scattering measurements, the samplesolutions were filtered three times by using Millipore Tef-lon (Nylon) filters with a pore size of 0.45 lm. A CONTINanalysis was taken for the measured intensity correlationfunctions. Apparent hydrodynamic radii, Rh, of the micelleswere calculated according to the Strokes–Einstein equa-tion. All the measurements were carried out at room tem-perature. The measurements were performed at fixedscattering angle of 90�. The micelle solutions were per-formed at a concentration of 0.5 mg/mL.
2.8.6. Transmission electron microscopy (TEM)Transmission electron microscopy (TEM) images were
carried out on a Philips CM 20 TEM operated at 200 kV. A5 lL droplet of micellar solution was dropped onto a cop-per grid (300 mesh) coated with a carbon film, followedby drying at room temperature.
2.8.7. Atomic force microscopy (AFM)AFM images were obtained using Tapping Mode on a
Nanoscope IV of Digital Instruments equipped with a sili-con cantilever 125 lm and E-type vertical engage piezo-electric scanner. The AFM samples were prepared by dip-coating the solution of POSS(PLLA–b–PNIPAM) blockcopolymers onto freshly cleaved mica surfaces, and thendried naturally at room temperature for at least 24 h.
3. Results and discussion
3.1. Preparation of POSS-containing star-shaped hybrid PLLA(POSSPLLA)
The octafunctional POSS molecule has been confirmedas an effective agent to prepare the inorganic-organic hy-brid octa-armed star-shaped biopolymer [58]. Here, the
724 W. Zhang et al. / European Polymer Journal 48 (2012) 720–729
MA
CRO
MO
LECU
LAR
NA
NO
TECH
NO
LOG
Y
novel POSSPLLA was prepared using octafunctional POSS asinitiator, and POSSPLLA was further modified into the mac-romolecular reversible addition-fragmentation transfer(RAFT) agent to prepare the POSS-containing star-shapedorganic/inorganic hybrid amphiphilic POSS(PLLA–b–PNI-PAM) block copolymers using RAFT polymerization(Scheme 1). The octa(3-chloropropyl) POSS(POSS–Cl8)was prepared according to the literature [59]. The POSS–Cl8 was further used to prepare POSS–(OH)8 via the nucle-ophilic substitution of chlorine atoms by hydroxyl groupsin the mixture of tetrahydrofuran and water in the pres-ence of fresh silver oxide, which has been detailed in theprevious literature [56]. POSS–(OH)8 was used as the initi-ator to initiate the ring-opening polymerization of LLAwith stannous (II) octanoate [Sn(Oct)2] as the catalyst.The polymerization was carried out at 130 �C for 24 h.Fig. 1 shows the GPC traces of POSS-containing hybrid PLLAprepared with different amount of POSS initiator, and thepolymerization results were summarized in Table 1.Fig. 2 shows the plots of the molecular weight and polydis-persity as functions of the molar ratio of POSS to LLA. It canbe seen that the molecular weights (or arm length of PLLA)increased with increasing the molar ratio of LLA to POSS.However, the experimental molecular weight measuredby GPC is in disagreement with the theoretical molecularweight. Similar differences between experimental molecu-lar weight and theoretical molecular weight were alsofound by other groups [60,61]. Since GPC is used to evalu-ate the molecular weight of polymers relative to linearpolystyrene standards, it is assured that the hydrodynamicvolume of PLLA in THF is different from that of PS, whichresults in some discrepancy between the molecular weightof the star-shaped POSSPLLA measured by GPC and thetheoretical molecular weights. Another reason contribut-ing to the discrepancy is that the hydrodynamic volumeof the star-shaped polymer is also different with that of lin-ear polymer.
Representatively shown in Fig. 3 is the 1H NMR spectraof POSS–(OH)8 and star-shaped POSS(PLLA7)8. The signalsat 5.15 and 1.57 ppm are assigned to the resonance of
16 18 20 22 24 26 28 30 32
a: POSS(PLLA7)8b: POSS(PLLA12)8c: POSS(PLLA33)8d: POSS(PLLA46)8
dcb a
Elution Volume, ml
Fig. 1. Evolution of GPC chromatograms of polymerization of LLA withvarious amounts of POSS initiator in bulk at 130 �C for 24 h (Table 1,entries 1–4).
methine and methyl protons of PLLA chains, respectively.In the 1H NMR spectrum of POSS(PLLA7)8, the resonancesof proton from the POSS moiety still can be clearly dis-cerned. The resonance signals at 0.79 ppm and 1.84 ppmwere respectively ascribed to the methylene protons (Si–CH2CH2CH2 and Si–CH2CH2CH2) of POSS moiety. The signalat 3.53 ppm is corresponding to the methylene protons(Si–CH2CH2CH2) of POSS moiety, which is slight shift fromthe signal at 3.55 ppm in spectrum of POSS–(OH)8. TheNMR result indicates that the POSS-containing star-shapedhybrid POSSPLLA was successfully synthesized.
1H NMR was also used to evaluate the molecular weightof POSSPLLA. The ratio of integration intensity of themethine proton signal (5.15 ppm) to that of methyleneproton signal (3.53 ppm) of POSS moiety was employedto calculate the molecular weight of POSSPLLA, and the re-sults were listed in Table 1. We found that the molecularweight evaluated by 1H NMR is close to the theoretical cal-culated molecular weight. From Fig. 2, the molecularweight evaluated by 1H NMR almost linearly increaseswith the ratio of the LLA to initiator, which indicates thatthe molecular weight of POSSPLLA (or arm length of starPLLA) can be well controlled according to the feed ratios.In addition, the narrow molecular weight distributionmeasured by GPC (almost all below 1.32) was also shownin Table 1 and Fig. 2, although there is a shoulder in theGPC curve of POSSPLLA, which is perhaps due to some cou-pling reactions in preparation of star-shaped PLLA. Thus, allthe results suggest the polymerization of LLA is living/con-trolled in the presence of POSS as initiator.
3.2. Preparation of POSS-containing star-shaped hybrid PLLA–b–PNIPAM block copolymers (POSS(PLLA–b–PNIPAM))
DDAT has been confirmed to be an effective chain trans-fer agent in RAFT polymerization for many monomers suchas styrene, alkyl acrylate and acrylic acid [62,63]. In addi-tion, since it contains a carbonyl group, it can be used tomodify the other molecule and prepare many novel RAFTagents. For example, DDAT was used to modify peptideto prepare a novel oligopeptide–polymer conjugate viaRAFT polymerization [64]. Fig. 4 shows the typical 1HNMR spectrum of the POSSPLLA macromolecular RAFTagent (POSSPLLADDAT). In comparison with the 1H NMRspectrum of POSS(PLLA12)8, the signals at d = 0.84 ppm (i),1.25 ppm (h) and 2.95 ppm (g) are respectively assignedto the methyl protons and methylene protons derived fromDDAT. The signals of the protons of the methylene of POSSmoiety at 0.84 ppm (a), 1.84 ppm (b) and 3.54 ppm (c) alsocan be identified. The POSSPLLADDAT was further used toprepare POSS(PLLA–b–PNIPAM) block copolymers. Allpolymerizations of NIPAM were carried out in 1,4-dioxaneat 65 �C with AIBN as initiator. The typical 1H NMR spec-trum of POSS(PLLA12–b–PNIPAM84)8 block copolymerswas shown in Fig. 5. Beside the characteristic signals atd = 1.60 pm (a) and 5.15 ppm (b), which are respectivelyassigned to the resonance of methyl and methine protonsof PLLA chains, the characteristic signals at d = 3.99 ppm(d) and 1.14 pm (e) are respectively ascribed to the meth-ane (CONHCH(CH3)2) and methyl protons (CONHCH(CH3)2)of PNIPAM. It still can be discerned that the proton signals
Table 1Results of polymerization of LLA with various amounts of POSS initiator in bulk at 130 �C for 24 h and [LLA]/[SnOct2] = 1000.
Samples a [M]/[I]b Mn,GPCc Mn, NMR
d Mn, the Mw/Mn
c Yield (%)
POSS(PLLA7)8 69/1 21,338 8,505 10,228 1.32 94POSS(PLLA12)8 139/1 35,734 14,699 19,302 1.30 92POSS(PLLA33)8 278/1 41,692 38,890 40,119 1.30 98POSS(PLLA46)8 417/1 43,957 53,585 58,534 1.31 96
a The numbers behind PLLA correspond to the degree of polymerization, as determined by 1H NMR.b Molar ratios of monomer to initiator.c Determined by GPC with polystyrene standards.d Mn, NMR is determined by 1H NMR spectrum of POSSPLLA. Mn,NMR = [I5.16/(I3.54/2)] �MLLA � 8 + MPOSS. Where, I5.16 and I3.54 are the integral values of the
signals at 5.16 ppm (CH in PLLA) and at 3.54 ppm (SiCH2CH2CH2–PLLA), respectively; MLLA and MPOSS are respectively the molecular weights of LLA andPOSS.
e Mn, th means the theoretical number–average molecular weight, Mn, th = [LLA]/[POSS] �MLLA � x + MPOSS. Where, [LLA] and [POSS] are respectively theinitial concentrations of LLA and POSS; MLLA is the molecular weight of LLA; MPOSS is the molecular weight of POSS initiator and x is the conversion.
100 200 300 4000
10
20
30
40
50
60Mn,GPCMn,NMRMn,th
PDI
[LLA]/[POSS]
Mn(∗
103 )
1
2
3
4
PDI
Fig. 2. The plots of the molecular weight and the polydispersity for thestar-shaped PLLAs as a function of the ratio of [LLA] to [POSS].
7 6 5 4 3 2 1
O
OO
O O
H
nSi
O
Si
Si
O
Si
Si
O
SiSiO
O OO
OOO
OO
Si
R
RR
R
R
RR
R
a' b'c'
d'
e'
Si
O
Si
Si
O
Si
Si
O
SiSiO
O OO
O OOO
O
Si
OH
OHHO
HO
HO
HO OH
OH
abc d
ppm
d
b a
d'b' a'c'
e'
H2Oc
Fig. 3. The 1H NMR spectra of POSS–(OH)8 and POSS(PLLA7)8.
5 4 3 2 1
OO
O
On
OS S
S
(CH2)10CH3d
e
h
fg
i
OSi
O
Si
Si
O
Si
Si
O
SiSiO
O OO
O OOO
O
Si
R'
R'R'
R'R'
R'
R' R'
a bc
4.0 3.5 3.0 2.5
gc
ha,i
d
e,f
b
ppm
Fig. 4. The 1H NMR spectrum of POSSPLLA macromolecular RAFT agent(POSSPLLADDAT).
5 4 3 2 1
OO
O
On
O
NHO
S S
Sm
d
a
b
c
e
(CH2)10CH3f
OSi
O
Si
Si
O
Si
Si
O
SiSiO
O OO
O OOO
O
Si
R''
R''R''
R''R''
R''
R''
R''
fcd
ea
b
ppm
Fig. 5. The 1H NMR spectrum of POSS(PLLA12–b–PNIPAM84)8 blockcopolymers.
W. Zhang et al. / European Polymer Journal 48 (2012) 720–729 725
MA
CRO
MO
LECU
LAR
NA
NO
TECH
NO
LOG
Y
of DDAT at 1.24 ppm (f). Thus, 1H NMR confirmed the POS-S(PLLA12–b–PNIPAM84)8 block copolymers were success-fully synthesized.
Fig. 6 shows evolution of GPC chromatograms withreaction time. With the increasing reaction times, the
eluograms shift to lower elution volume. The molecularweight of the block copolymers were also evaluated by1H NMR spectra and were listed in Table 2. The similar dif-ference between the molecular weight measured by GPCand the theoretically predicted also occurs for blockcopolymers. The molecular weight of POSS(PLLA–b–PNI-
18 20 22 24 26 28 30
c b a
a: POSS(PLLA12-b-PNIPAM40)8b: POSS(PLLA12-b-PNIPAM84)8c: POSS(PLLA12-b-PNIPAM119)8
Elution Volume, ml
Fig. 6. Evolution of GPC chromatograms for RAFT polymerization ofNIPAM at 65 �C in 1,4-dioxane, with different reaction times in thepresence of POSS-containing macromolecular RAFT agent, POSSPLLAD-DAT (Table 2, entries 1–3).
Table 2Results of RAFT polymerization of NIPAM at 65 �C in 1,4-dioxane, with different reaction times in the presence of POSS-containing macromolecular RAFT agent,POSSPLLADDAT.
Samplesa [M]/[I]b Mn,GPCc Mn, NMR
d Mn, the Mw/Mn
c Yield (%)
POSS(PLLA12–b–PNIPAM40)8 1584/1 51,266 53,417 59,202 1.34 20.7POSS(PLLA12–b–PNIPAM84)8 1584/1 65,241 93,338 98,705 1.41 42.7POSS(PLLA12–b–PNIPAM119)8 1584/1 73,119 125,231 158,542 1.55 76.2
a The numbers behind the individual blocks correspond to the degree of polymerization, as determined by 1H NMR.b Polymerization conditions: [M]/[I] = [NIPAM]/[RAFT agent] = 1584, [RAFT agent]/[AIBN] = 3.c Evaluated based on the GPC method, where polystyrene standards were used in the calculation.d Calculated based on the 1H NMR method, Mn,NMR = [I3.99I(5.16/2)] � [I5.16/I3.54]POSSPLLA �MNIPAM � 8 + MPOSSPLLADDAT, Where, I3.99 and I5.16 are the integral
values of the signals at 3.99 ppm (NHCH(CH3)2) and at 5.16 ppm (CH in PLLA), respectively; [I5.16/I3.54]POSSPLLA is noted in Table 1. MNIPAM and MPOSSPLLADDAT
are respectively the molecular weights of NIPAM and POSSPLLA macromolecular RAFT agent (POSSPLLADDAT).e Calculated from the following equation: Mn,th = [NIPAM]/[RAFT] �MNIPAM � x + MPOSSPLLADDAT. Where, [NIPAM] and [RAFT] are respectively the initial
concentrations of NIPAM and RAFT agent; MNIPAM is the molecular weight of NIPAM; MPOSSPLLADDAT is the molecular weight of POSSPLLA macromolecularRAFT agent and x is the conversion.
40 60 80 100 120 140 160 180 200
Tg, 52 oC
T'g, 141 oC
T'g, 132 oC
T'g, 115 oCTg, 52 oC
Endo
e
d
c
b
a
a: POSS(PLLA12)8b: POSSPLLADDATc: POSS(PLLA12-b -PNIPAM40)8d: POSS(PLLA12-b -PNIPAM84)8e: POSS(PLLA12-b -PNIPAM119)8
Temperature (oC)
Fig. 7. The DSC curves of POSS(PLLA12)8, POSSPLLA macromolecular RAFTagent (POSSPLLADDAT) and POSS(PLLA–b–PNIPAM) block copolymers.
726 W. Zhang et al. / European Polymer Journal 48 (2012) 720–729
MA
CRO
MO
LECU
LAR
NA
NO
TECH
NO
LOG
Y
PAM) block copolymers evaluated by 1H NMR is also closeto the theoretical calculated molecular weight.
3.3. DSC analysis of POSS(PLLA–b–PNIPAM) block copolymers
POSSPLLA and POSS(PLLA–b–PNIPAM) block copoly-mers were also subjected to DSC analysis. After the ther-mal history was removed, the second DSC scanningcurves of the quenched samples were shown in Fig. 7. Itcan be seen that both the endothermic and exothermicpeaks are displayed on the DSC curves of POSS(PLLA12)8
and POSSPLLADDAT. The endothermic peak for POS-S(PLLA12)8 is at 109 �C, which is ascribed to the cold crys-tallization of the polymer after the glass transition duringheating scans; whereas the exothermic peak (at 170 �C)is responsible for the melting transitions. For POSSPLLAD-DAT, its DSC curve is similar with that of POSSPLLA, butits crystallization temperatures (Tc) and melting tempera-tures (Tm) are obviously lower than those of POSS(PLLA12)8,which indicates the degree of crystallization of PLLA is re-duced by the formation of POSSPLLA macromolecular RAFTagent. It is also noted that with the incorporation of PNI-PAM blocks, the crystallization transition completely
disappeared and only a small melting transition peakshows in the DSC curve of POSS(PLLA12–b–PNIPAM84)8,suggesting the crystallization of PLLA blocks is significantlyinhibited by the presence of PNIPAM blocks. This is per-haps because the incorporation of PNIPAM block dilutesthe PLLA block domain, which is not good for the crystalli-zation of PLLA block. In addition, it still can be discernedone glass transition at 52 �C for POSS(PLLA12–b–PNI-PAM40)8 block copolymers, which is corresponding to theglass transition temperature (Tg) of PLLA block, but forPOSS(PLLA12–b–PNIPAM84)8 and POSS(PLLA12–b–PNI-PAM119)8 block copolymers, their glass transition for PLLAblock is not obvious, due to the low content of PLLA blockin these star-shaped block copolymers. On the other hand,the Tg of PNIPAM block becomes more obvious and in-creases with the chain length of PNIPAM blocks.
3.4. The X-ray diffraction patterns of POSS(PLLA–b–PNIPAM)block copolymers
Fig. 8 shows the X-ray diffraction patterns of POS-S(PLLA12)8 and POSS(PLLA–b–PNIPAM) block copolymers.POSS(PLLA12)8 shows two diffraction peaks at 2h values
15 20 25 30 35
d
c
b
a
Inte
nsity
2θ (degree)
a: POSS(PLLA12)8b: POSS(PLLA12-b -PNIPAM40)8c: POSS(PLLA12-b -PNIPAM84)8d: POSS(PLLA12-b -PNIPAM119)8
Fig. 8. The XRD curves of POSS(PLLA12)8 and POSS(PLLA–b–PNIPAM)block copolymers.
W. Zhang et al. / European Polymer Journal 48 (2012) 720–729 727
MA
CRO
MO
LECU
LAR
NA
NO
TECH
NO
LOG
Y
of 16.6� and 19.0�, respectively, corresponding to the typi-cal crystal peaks of PLLA [65]. For POSS(PLLA–b–PNIPAM)block copolymers, the diffraction peak at 16.6� becomesweak, and the diffraction peak at 19.0� could not be clearlyidentified, due to the overlapping of the wide dispersionpeak of PNIPAM non-crystalline phase. The XRD is notcompletely consistent with the results of DSC, especiallyfor POSS(PLLA12–b–PNIPAM84)8 and POSS(PLLA12–b–PNI-PAM119)8 block copolymers. This is because the XRD ismuch more sensitive to detect the crystalline structure ofpolymer crystal. It can be concluded that PLLA crystal ex-ists in all the POSS(PLLA–b–PNIPAM) block copolymers.Base on the result of DSC and XRD, the PLLA crystallizationwas greatly restricted by the formation of the POSS(PLLA–b–PNIPAM) block copolymers.3.5. Self-assembly behavior of POSS-containing hybridPOSS(PLLA–b–PNIPAM) block copolymers in aqueous solution
The self-assembly of amphiphilic block copolymers insolution has attracted considerable attention in polymerscience, since it can result in the formation of a rich varietyof self-assembly morphologies such as micelles, rods andvesicles [66–70]. Star-shaped amphiphilic block copoly-mers can self-assemble into much more interesting mor-phologies in solution, which can not be obtained fromlinear block copolymers [71,72]. In this work, thestar-shaped POSS(PLLA–b–PNIPAM) amphiphilic blockcopolymers with a POSS core were prepared, and theself-assembly behavior of the block copolymers in aqueoussolution were studied (Scheme 2).
The polymeric aqueous solution was subjected to DLSanalysis at the concentration of 0.5 mg/mL. The averagehydrodynamic radius (Rh) measured by DLS in aqueoussolution at room temperature (24 �C). The Rh of POS-S(PLLA12–b–PNIPAM40)8, POSS(PLLA12–b–PNIPAM84)8 andPOSS(PLLA12–b–PNIPAM119)8 block copolymers is respec-tively 70, 85 and 119 nm. Rh has a broad distribution (smallfigure in Fig. 9). The temperature dependence of Rh for POS-S(PLLA12–b–PNIPAM119)8 block copolymers in aqueous
solution is shown in Fig. 9. Below 20 �C, the Rh is about127 nm, upon the increasing temperature range from 22to 30 �C, Rh decreases monotonically from 127 to 96 nm.At the temperature from 30 to 32 �C, the Rh sharply de-creases to 53 nm, and the scattering light intensity is alsosharply increasing. After 32 �C, the Rh tends to be stableand does not decrease with the increasing temperature.This result is disagreeable with that of the linear PNIPAMchain in aqueous solution, which only shows a sharp de-crease in its size at the narrow temperature range from30 to 32 �C [73]. In the present case, the transition of PNI-PAM is consistent with that of the PNIPAM attached onpolystyrene core or latex [54,74]. For the cooling process,with the temperature below 34 �C, PNIPAM chains beginto stretch. At the temperature decreasing from 34 to30 �C, the Rh sharply increases to 93 nm. When the temper-ature decreases below 30 �C, the Rh almost has no changein the heating and cooling process, which indicating thetransition process is reversible (Scheme 2).
The self-assembly morphology of POSS(PLLA–b–PNI-PAM) block copolymers was also investigated by TEM.The sample for TEM was prepared by dropping the poly-meric aqueous solution on the carbon-coated copper grids,and dried in air overnight. The TEM image of POSS(PLLA12–b–PNIPAM84)8 is shown in Fig. 10. The vesicular structurescan be seen in the self-assembly aggregates, but they arenot the typical perfect vesicles, since there is a broad dis-persity in the size of the vesicular aggregates and the den-sity of the vesicular wall is not uniform. The outer diameterof the vesicles was polydispersed at the range from 20 to35 nm, which should be resulted from the different aggre-gation number for the different vesicles. This size of thevesicles is obviously smaller than that measured by DLS,since DLS data directly reflects the size of self-assemblyaggregates in solution, where the chains of PNIPAM blocksare well dispersed in water, although one end of the PNI-PAM chains is attached on the surface of the vesicular wall.The similar phenomenon was also observed by Müller-Buschbaum et al. [75].
In a single vesicle (an enlarged vesicle on the top of theimage), the vesicle wall is about 8 nm, and it is composedof the hydrophobic POSS and PLLA blocks. The reason thatthe density of vesicular wall is not uniform should be re-lated to the topological structure of the star-shaped poly-mers. For POSS(PLLA–b–PNIPAM) block copolymers, theycontain eight arms, and the number of the arms whichstretch towards inner core of vesicle is difficult to be con-sistent. If the numbers for the different block copolymerswhich stretch towards inner core are all equal to four,the self-assembly morphology should be rods or worms.Of course, it is also difficult to form spherical micelle withtypical core-shell morphology, since the POSS core of thestar-shaped block copolymers is cage-shaped structurewith eight corners, and the arms, PLLA–b–PNIPAM blockcopolymers, are growing from the corners and stretchingtowards eight different directions with a similar length.Thus, for the formation of the vesicle from POSS(PLLA–b–PNIPAM) block copolymers in aqueous solution, the differ-ent number of the arms which stretch towards inner coreof vesicle might result in the different size of the vesicleand different density of the vesicle wall.
POSS PLLA PNIPAM
H O2
Scheme 2. Self-assembly process of POSS(PLLA–b–PNIPAM) block copolymers in aqueous solution.
16 20 24 28 32 36 4040
60
80
100
120
140
10 100 1000
Rh(nm)
R h(nm
)
Temperature (oC)
T increasing T decreasing
Fig. 9. Dependence of Rh for POSS(PLLA12–b–PNIPAM119)8 self-assemblyaggregates on temperature.
Fig. 10. TEM images of POSS(PLLA12–b–PNIPAM84)8 vesicular aggregatesin aqueous solution at 25 �C, C = 0.5 mg/mL.
Fig. 11. AFM images of POSS(PLLA12–b–PNIPAM84)8 block copolymers inaqueous solution.
728 W. Zhang et al. / European Polymer Journal 48 (2012) 720–729
MA
CRO
MO
LECU
LAR
NA
NO
TECH
NO
LOG
Y
The self-assembly morphology of POSS(PLLA–b–PNI-PAM) block copolymers in aqueous solution was alsoinvestigated by AFM. The samples for AFM were prepared
by dip-coating on freshly cleaved mica surface from theaqueous solution of POSS(PLLA12–b–PNIPAM84)8 at0.5 mg/ml, which can be well compared with TEM images[76,77]. Fig. 11a is the typical AFM image of the self-assem-bly morphology of POSS(PLLA12–b–PNIPAM84)8 blockcopolymers. Here, we do not clearly observe the vesicularstructure in this image, perhaps due to the size effect ofthe AFM tip. The self-assembly vesicles of POSS(PLLA12–b–PNIPAM84)8 block copolymers in AFM image also has adispersion in their size at the range from 35 to 55 nm. Inaddition, the size of the aggregates is slightly bigger thanthat measured by TEM, since the coronas of the aggregates,the chains of the hydrophilic PNIPAM blocks, cannot beclearly identified by TEM.
4. Conclusions
The star-shaped POSSPLLA was successfully synthesizedby ring-opening polymerization using octa(3-hydroxypro-pyl) polyhedral oligomeric silsesquioxane as initiator. Themolecular weight of POSSPLLA (or arm length of POSSPLLA)can be well controlled by the feed ratios of LLA to initiator.The star-shaped POSSPLLA was further modified into mac-romolecular RAFT agent to prepare the star-shaped POS-S(PLLA–b–PNIPAM) amphiphilic block copolymers. TEM
W. Zhang et al. / European Polymer Journal 48 (2012) 720–729 729
MA
CRO
MO
LECU
LAR
NA
NO
TECH
NO
LOG
Y
results show the novel self-assembly aggregates withvesicular structure can be fabricated using star-shapedPOSS(PLLA–b–PNIPAM) amphiphilic block copolymers.The vesicular wall and coronas are composed of the hydro-phobic POSS core and PLLA blocks, and hydrophilic PNI-PAM blocks, respectively. DLS revealed PNIPAM blocks ascoronas in the vesicles is temperature-sensitive and thephase transition is reversible in heating and cooling pro-cess. Thus, POSSPLLA and POSS(PLLA–b–PNIPAM) blockcopolymers, as a class of novel organic–inorganic hybridmaterials with the advantageous properties, can be poten-tially applied in biological and medical fields.
Acknowledgements
This work was supported by the National NaturalScience Foundation of China (Nos. 21074035 and 50503013). W.Z. also acknowledges support by the FundamentalResearch Funds for the Central Universities.
References
[1] Pielichowski K, Njuguna J, Janowski B, Pielichowski J. Adv Polym Sci2006;201:225.
[2] Laine RM. J Mater Chem 2005;15:3725–44.[3] Li GZ, Wang LC, Ni HL, Pittman CU. J Inorg Organomet Polym
2001;11:123.[4] Lee A, Xiao J, Feher FJ. Macromolecules 2005;38:438.[5] Cui L, Chen DY, Zhu L. ACS Nano 2008;2:921.[6] Abad MJ, Barral L, Fasce DP, Williams RJJ. Macromolecules
2003;36:3128.[7] Pyun J, Matyjaszewski K. Macromolecules 2000;33:217.[8] Ohno K, Sugiyama S, Koh K, Tsujii Y, Fukuda T, Yamahiro M, et al.
Macromolecules 2004;37:8517.[9] Drazkowski DB, Lee A, Haddad TS, Cookson DJ. Macromolecules
2006;39:1854.[10] Cordes DB, Lickiss PD, Rataboul F. Chem Rev 2010;110:2081.[11] Sprung MM, Guenther FO. J Am Chem Soc 1955;77:3990.[12] Lichtenhan JD, Schwab JJ, Feher FJ, Soulivong D. US Patent 5942638.
1999.[13] Markovic E, Matisons J, Hussain M, Simon GP. Macromolecules
2007;40:4530.[14] Hottle JR, Deng J, Kim HJ, Farmer-Creely CE, Viers BD, Esker AR.
Langmuir 2005;21:2250.[15] Xu HY, Kuo SW, Lee JSY, Chang FC. Macromolecules 2002;35:8788.[16] Chou CH, Hsu SL, Dinakaran K, Chiu MY, Wei KH. Macromolecules
2005;38:745.[17] Pyun J, Matyjaszewski K. Chem Mater 2001;13:3436.[18] Pyun J, Matyjaszewski K, Wu J, Kim GM, Chun SB, Mather PT.
Polymer 2003;44:2739.[19] Ohno K, Sugiyama S, Koh K, Tsujii Y, Fukuda T, Yamahiro M, Oikawa
H, Yamamoto Y, Ootake N, Watanabe K. Macromolecules2004;37:8517.
[20] Hirai T, Leolukman M, Hayakawa T, Kakimoto M, Gopalan P.Macromolecules 2008;41:4558.
[21] Zhang WA, Liu L, Zhuang XD, Li XH, Bai JR, Chen Y. J Polym Sci. PartA: Polym Chem. 2008;46:7049.
[22] Zhang WA, Fang B, Walther A, Muller AHE. Macromolecules2009;42:2563.
[23] Kannan RY, Salacinski HJ, Butler PE, Seifalian AM. Acc Chem Res2005;38:879.
[24] Shenhar R, Rotello VM. Acc Chem Res 2003;36:549.[25] Kannan RY, Salacinski HJ, Odlyha M, Butler PE, Seifalian AM.
Biomaterials 2006;27:1971.[26] Miao JJ, Cui L, Lau HP, Mather PT, Zhu L. Macromolecules
2007;40:5460.[27] Lee KM, Knight PT, Chung T, Mather PT. Macromolecules
2008;41:4730.
[28] Jeong B, Kim SW, Bae YH. Adv Drug Deliv Rev 2002;54:37.[29] Mi FL, Shyu SS, Lin YM, Wu YB, Peng CK. Biomaterials 2003;24:5023.[30] Pompea W, Worch H, Epple M, Friess W, Gelinsky M, Greil P, et al.
Mater Sci Eng A 2003;362:40.[31] Tsuji H, Miyauchi S. Biomacromolecules 2001;2:597.[32] Engelberg I, Kohn J. Biomaterials 1991;12:292.[33] Goffin AL, Duquesne E, Moins S, Alexandre M, Dubois P. Eur Polym J
2007;43:4103.[34] Chen GX, Shimizu H. Polymer 2008;49:943.[35] Knight PT, Lee KM, Qin H, Mather PT. Biomacromolecules
2008;9:2458.[36] Pan H, Qiu ZB. Macromolecules 2010;43:1499.[37] Guo QY, Knight PT, Wu J, Mather PT. Macromolecules 2010;43:4991.[38] Xu JW, Song J. PNAS 2010;107:7652.[39] Qiu Y, Park K. Adv Drug Delivery Rev 2001;53:321.[40] Dube D, Francis M, Lerous JC, Winnik FM. Bioconjugate Chem
2002;13:685.[41] Lo CL, Lin KM, Hsiue GH. J Control Release 2005;104:477.[42] Kohori F, Sakai K, Aoyagi T, Yokoyama M, Sakurai Y, Okano T. J
Control Release 1998;55:87.[43] Liu S, Yang Y, Liu X, Tong Y. Biomacromolecules 2003;4:1784.[44] Gil ES, Hudson SA. Prog Polym Sci 2004;29:1173.[45] Xu J, Zhu ZY, Luo SZ, Wu C, Liu SY. Phys Rev Lett 2006;96:027802.[46] Kujawa P, Segui F, Shaban S, Diab C, Okada Y, Tanaka F, et al.
Macromolecules 2006;39:341.[47] Kujawa P, Tanaka F, Winnik FM. Macromolecules 2006;39:3048.[48] Qiu XP, Winnik FM. Macromolecules 2007;40:872–8.[49] Virtanen J, Holappa S, Lemmetyinen H, Tenhu H. Macromolecules
2002;35:4763.[50] Wei H, Zhang XZ, Chen WQ, Cheng SX, Zhuo RX. J Biomed Mater Res
Part A 2007;83A:980.[51] Zhu PW, Napper DH. Macromolecules 1999;32:2068.[52] You YZ, Hong CY, Wang WP, Lu WQ, Pan CY. Macromolecules
2004;37:9761.[53] Hales M, Barner-Kowollik C, Davis TP, Stenzel MH. Langmuir
2004;20:10809.[54] Zhang WA, Zhou XC, Li H, Fang YE, Zhang GZ. Macromolecules
2005;38:909.[55] Liu F, Tao GL, Zhuo RX. Polym J 1993;25:561.[56] Liu YH, Yang XT, Zhang WA, Zheng SX. Polymer 2006;47:6814.[57] Lai JT, Filla D, Shea R. Macromolecules 2002;35:6754.[58] Chan SC, Kuo SW, Chang FC. Macromolecules 2005;38:3099.[59] Zeng FQ, Lee H, Chidiac M, Allen C. Biomacromolecules 2005;6:2140.[60] Dittmar U, Hendan JB, Floerke U, Marsmann HC. J Organomet Chem
1995;489:185.[61] Zhao Y, Shuai X, Chen C, Xi F. Macromolecules 2004;37:8854.[62] Mahanthappa MK, Bates FS, Hillmyer MA. Macromolecules
2005;38:7890.[63] Cheng C, Sun GR, Khoshdel E, Wooley KL. J Am Chem Soc
2007;129:10086.[64] Hentschel J, Bleek K, Ernst O, Lutz JF, Börner HG. Macromolecules
2008;41:1073.[65] Eling B, Gogolewski S, Pennings AJ. Polymer 1982;23:1587.[66] Zhang LF, Eisenberg A. Science 1995;268:1728.[67] Discher BM, Won YY, Ege DS, Lee JCM, Bates FS, Discher DE, et al.
Science 1999;284:1143.[68] Gohy JF, Creutz S, Garcia M, Mahltig B, Stamm M, Jerome R.
Macromolecules 2000;33:6378.[69] Jiang XZ, Ge ZS, Xu J, Liu H, Liu SY. Biomacromolecules 2007;8:3184.[70] Teng J, Zubarev ER. J Am Chem Soc 2003;125:11840.[71] Khelfallah N, Gunari N, Fischer K, Gkogkas G, Hadjichristidis H,
Schmidt M. Macromol Rapid Commun 2005;26:1693.[72] Babin J, Taton D, Brinkmann M, Lecommandoux S. Macromolecules
2008;41:1384.[73] Wu C, Wang XH. Phys Rev Lett 1998;79:4092.[74] Zhu PW, Napper DH. Langmuir 1996;12:5992.[75] Troll K, Kulkarni A, Wang W, Darko C, Bivigou Koumba AM,
Laschewsky A, Müller-Buschbaum P, Papadakis CM. Colloid PolymSci 2008;286:1079.
[76] Mahltig B, Walter H, Harrats C, Müller-Buschbaum P, Jérôme R,Stamm M. Phys Chem Chem Phys 1999;1:3853.
[77] Mahltig B, Müller-Buschbaum P, Wolkenhauer M, Wunnicke O,Wiegand S, Gohy JF, et al. J Colloid Interface Sci 2001;242:36.