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Fabrication of Poly(methyl methacrylate)-block-poly(N-isopropylacrylamide) Amphiphilic Diblock Copolymer on Silicon Substrates via Surface-Initiated Reverse Iodine Transfer Polymerization.

Amphiphilic diblock copolymer on silicon substrates were synthesized via surface-initiated reverse iodine transfer polymerization (RITP) technique. The silicon substrates (Si (111) surface) were modified with the azo groups, which were introduced by the treatment of Si (111) surface with 4,4' -azobis (4-cyanopentanoic acid). The poly(methyl methacrylate) (PMMA) were then prepared under RITP conditions from the Si (111) wafer. The synthesis of amphiphilic diblock copolymer was carried out on Si-g-PMMA substrate by sequential addition of monomer W-isopropylacrylamide (NIPAM). The observed narrow molecular weight distributions ([M.sub.w]/[M.sub.n]),linear kinetic plots, and linear plots of molecular weight (M") versus monomer conversion indicate that the chain growth from the silicon substrates is a controlled process with a "living" characteristic. The elltpsometry and contact angle results indicated that the MMA bad grafted from the surface of the silicon substrates successfully and the graft layer was well defined. The structure of the polymer and the ability to extend the chains were characterized and confirmed with the surface sensitive techniques such as X-ray photoelectron spectroscopy and atomic force microscope. POLYM. ENG. SCI., 54:925-931. 2014. 2013 [c] Society of Plastics Engineers

INTRODUCTION

Grafting polymer onto solid surface has various application such as electronic devices 111. hiocompaiibilily (2), colloidal stabilization J3], and so on [4-6J. However, conventional surface-initiated free radical polymerization cannot control the molecular weight, polydispersity. and the lenninal chain of the graft polymers. These limitations can be overcome by living radical polymeri/alion techniques. The most prominent living free radical polymerization techniques are iniferter (iniliation-transfcr-terminatioo) method (7), nitroxidc mediated polymerization (NMP) (8), atom transfer radical polymerization (ATRP)(9), (10), reversible addition-fragmentation chain transfer (RAKT)(11), (12), iodine transfer polymerization (1TP) (13). or reverse iodine transfer polymerization (RITP) 114-16). ITP was discovered by Talemoto ami Nakagawa in lale 1970s 117]. and studied wilh much interest since the mid- 1990s. By using an initialing radical, iodofluor-ocompounds could enter in a controlled pmeess. based on a degenerative transfer iDT). However. ITP exhibits two im-portanl limitations! 18. 19j. hirst, in the initiation system, the chain transfer agenls used in ITP are unstable which is due to the weak C-I bond ami thus lead to inconvenience upon storage. Second. ITP of monomers involving tertiary propagating radicals (such as melhacrylates with l-phenylethyliodide as transfer agenl I was not successful because il would require more activated iodoalkyl compounds such as ethyl 2-iodo-2-methylpropionate, which are inherently even more unstable. To overcome these drawbacks, Lacroix-Desmazes et al. 118) reported a new process called RITP. which is based on a direct reaction of radicals with molecular iodine. In this way. the reversible chain transfer agent is generated in situ in the reaction mixture, thus avoiding the problems of synthesis and storage. At present, the research about RITP is mainly

concentrated in the mechanism.

In this work, we wish lo report the first successful use of RHP technique lo synihesize well-control led amphiphilic diblock copolymer on silicon substrate. The molecular weight and polydispersity data, kinetics regarding chain growth in solution, and thickness increase on surfaces was analyzed and discussed. The preliminary results in this work should be of theoretical and practical importance in investigating surface-initialed polymerization (SIP) and RITP mechanism.

EXPERIMENTAL

Materials

Si (III) wafers (one side polished) were purchased from Semiconductor Processing Inc. These wafers were sliced into rectangular strips of about 1 * 2 cm (2) in size. The strips were washed in piranha solution, a mixture of 91 WfS sulfuric acid (70 vol%) and hydrogen peroxide solution (30 voWc) at 90[degrees]C for 2 h to remove the surface organic residues. Methyl methacrylate (MMA) (AR. Shanghai Chemical Reagent Plant) was washed with l()9r NaOH and ion-free water, stirred over CaH? and distilled under reduced pressure prior to use. AMsopropylaeryla-mide (NIPAM, 98%) and 3-aminopropyltriethoxysilane (KH550. 99%) were purchased from Shanghai Aladdin Chemical Reagent Co. Ltd. 4.4'-Azobis (4-cyanopenlanoic acid) (ACPA) was purchased from Sigumas Co and the 4.4'-azobis (4-cyanopentanoic acid chloride) (ACPAC) was synthesized following a previously described procedure respectively (20). All other reagents, including iodine, dichloromethane. lelrahydrofuran (THF), toluene, methanol, and cyclohcxanonc. were of analytical grade and used as received.

Measurement

The water contact angles were measured in ambient air (relative humidity 50*7*) using a KRUSS DSA100 contact angle goniometer widi a drop size of 4.0 pi al room lemperaiure. The values showed here are the averaged values of at least five measurements on different locations. The thickness of molecular and polymeric layers was meas-ured with a spectroscopic ellipsometei (Gaertner model LI I6C). The molecular weights and polydispersities (PDI) of the polymers were detennined via an alliance GPC 1515 (Waters. USA) gel permeation chromatogra-pher (GPC) using THF as the eluem at a flow rale of 0.6 mL min (-1) and operated al 40[degrees]C. Atomic force microscope (AFM) measurements were made by using a Veeco NanoScope Ilia scanning probe microscope, operating in tapping mode. X-ray phoioeleclron spectra (XPS) were performed on a PHI-5702 multifunction X-ray phoioeleclron spectrometer using MgKo radiation with pass energy of 29.35 ev.

Immobilization of AZO Groups on Silicon Substrate

The target compound was synthesized according lo our previous work (21). Typically, silicon (a piece). (3-amino-propyl) iriethoxysilane (KH-550) (I mL). and toluene 10 mL were mixed and heated al 80[degrees]C for 11 h under a niirogen atmosphere. Then the wafer was removed and cleaned by an ultrasonic bath in toluene to remove excess compound KH-550. The resulting amino-functionalized silicon substrate (Si-N[H.sub.2]) was dried in a stream of nitrogen.

A piece of Si-N[H.sub.2], 3 mL of dichloromethane and 0.1 g of ACPAC were introduced into a dried 25 mL round-bottom flask. The reaction mixture was stirred slowly al room temperature for 6 h and the wafer was cleaned by an ulirasonic balh in mixed solvent of ethanol and waler (vol/vol = 1:1). ethanol. ether and dried in a stream of nitrogen to afford azo group-functional tzed silicon substrate.

PMMA Growing From Silicon Substrate by RITP Method

PMMA were prepared by placing a piece of azo groups immobilized silicon substrate inlo a reaction flask with cyclohexanone (3 mL). iodine (0.004 g. 0.016 mmol). and degassed MMA (3 mL. 0.028 mol). After three iVee/.e-pump-thaw cycles, the ftask "as heated at SO (' in .in oil balh. I'he polymerization was conducted in the dark under nitrogen atmosphere with magnetic stirring. Afler reaction, the flask was cooled in an ice bath and the mixture was diluted with telrahydrofuran (THF) and precipitated inlo methanol. The silicon substrate with grafted PMMA (Si-.e-PMMA) was extracted with THF to remove ungrafted PMMA and dried in a stream of nitrogen.

PMMA-b-PNIPAM Diblock Copolymer Grafted Silicon Substrate (Si-g-PMMA-b-PNIPAM)

As shown in Fig. I. the synthesis of diblock copolymer was carried oul on the Si-g-PMMA substrale. The procedures were similar to those used for synthesis of the Si-g-PMMA hybrid malerial. except using Si-g-PMMA hybrid material as macro chain transfer agent and NIPAM as the second monomer via a conventional ITP process. A general procedure is as follows: The mixture of Si-e-PMMA hybrid malerial (a piece) and 5 mL cyclohexanone in flask was ullrasonically agitated for .30 min and then the azo-bis-isobulryonitrile (AIBN) (0.0066 g, 0.04 mmoll and NIPAM (0.1130 g. I mmol) were added. The flask was suhsequenlly evacuated and Hushed with nitrogen. The polymerization was carried oul under nitrogen in 80[degrees]C oil bath for 10 h. stopped by diluting with THF and precipitated into ether. The wafer was washed with excess THF to remove ungrafted poly(A/-isoprop\ lacry lamidc) (PNIPAM) and dried in a stream of nitrogen.

RESULTS AND DISCUSSION

Table 1 summarizes the contact angles of the silicon substrates. The contact angle of silicon wafer (Si-OH) was 3.0[degrees]. Alter RITP of MMA. the contact angle increased largely to about 70[degrees], which indicated the PMMA had grafted from the silicon wafer. As for the Si-e-PMMA-/i-PNIPAM hybrid material, the contact angles changed to 74 and 83, respectively, at different temperature owing lo the PNIPAM grafted from silicon wafer. As the thennoresponsive PNIPAM can change their hydrophilic or hydrophobic behavior around the lower critical solution lemperaiure (LCST).

TABLE 1. Reverse iodine transfer
polymerization (RITP) of MMA on
silicon substrates and the chain
extension process.

Sample        Time  [M.sub.n](a)   PDI(b)  Conversion(c)
              (h)   * [10.sup.-4]               (%)
                        (g/mol)
Si-OH          --         --           --          --
Si-g-PMMA(A)    4           5.5         1.38       27.42
Si-g-PMMA(B)    6           10.8          1.41       53.32
Si-g-PMMA(C)    8           14.4          1.35       67.55
Si-g-PMMA(D)   10         16.4          1.32       74.31
Si-g-PMMA(E)   12         17.6          1.33       80.22
Si-g-PMMA-b    10         0.41          1.26       31.30
-PNIPAM-10
h(e)
Sample        Contact angle(d)    Film
                ([theta])     thickness
                                  (nm)
Si-OH          3.0 [+ or -]      2.8
               0.5[degrees]
Si-g-PMMA(A)  68.8 [+ or -]       13
               0.5[degrees]
Si-g-PMMA(B)  70.1 [+ or -]       19
               0.5[degrees]
Si-g-PMMA(C)  71.4 [+ or -]       21
               0.5[degrees]
Si-g-PMMA(D)  71.9 [+ or -]       34
               0.5[degrees]
Si-g-PMMA(E)  73.2 [+ or -]       40
               0.5[degrees]
Si-g-PMMA-b   74.1 [+ or -]       55
               0.5[degrees]
              (25[degrees]C)
-PNIPAM-10    83.0 [+ or -]
h(e)           0.5[degrees]
              (40[degrees]C)

(a) Free polymer formed in the solution, obtained by GPC with standard polystyrene as reference.
(b) PDI: molecular weight distribution is calculated from [M.sub.w]/[M.sub.n]
(c) The monomer conversion values were determined by gnivimetric analysis.
(d) contact angle of water at mom temperature on the substrate.
(e) Silicon-g-PMMA-b-PNIPAM-10 h: using Silicon-c-PMMAID) as macroinitiator.


The surface of the silicon wafer was further investigated by the spectroscopic ellipsometer is also shown in Table I. The ellipsomelry measurement results exhibited a large increase in film thickness after the growth of the PMMA layer on silicon substrates. This result is another confirmation lhal the PMMA has been grafted from the silicon wafer. Figure 2 shows that the film thickness increased linearly with polymerization time which is consistent with a conirolledriiving" process.

A living polymerization is also characterized by narrow polydispersity products, linear increase in molecular weighl wiih conversion, and the ability lo extend the chains by sequential addition of monomer. Table 1 summarizes the results of all polymerization under various ex-perimenlal conditions. The data showed lhal narrow polydispersity was achieved afler polymerization and the molecular weight increased with the polymerization lime. To further prove the living nature of RITP process, the relationship between the molecular weighl and monomer conversion, as well as the kinetic behavior of the process were plotted according lo the values from Table I.

In Fig. 3, the linear relationship between the molecular weight of the free polymer formed in solution and the monomer conversion was observed. Although the exact molecular weight of polymer grafted on the inorganic substrate is not known, its molecular weight is expected to be proportional to that of the polymer formed in the solution (22). The closely linear increase in molecular weight with monomer conversion indicated that the process of surface-initiated RITP of MMA is controlled.

The kinetic plot of Ln ([[M].sub.0]/[M]) (where [M] is the concentration of monomer) versus polymerization time is shown in Fig. 4. It could be noted that the kinetic plot always showed straight line, indicating that the kinetics is first order in monomer and that the concentration of propagating radicals remains constant during the polymerization. Moreover, there exist about a I h induction period.

To verify the RITP mechanism reported previously (18), (1) H-NMR spectra of PMMA and PNIAPM formed in solution was performed. Figure 5 shows the (1) H-NMR spectra of PMMA fonned in solution. The signals at 0.84. 1.81. and 3.60 ppm are assigned lo the methyl -C[H.sub.2]- (a) the methylene -C[H.sub.2]- (a), and the melhoxy -OC[H.sub.3] (d) groups of the monomer units in PMMA chain respectively. The signal al about 2.9 ppm is attributed to the methylene in The [BETA] position of iodine (c) at the chain end. Owing to the end functionality of alkyl iodine group, the PMMA chains on silicon substrate can serve as macroiniiialor for subsequent block polymerization.

(1) H-NMR spectra of PNIPAM fonned in solution is shown in Fig. 6. The peaks at around 6.10 ppm. 4.15 ppm, and 1.18 ppm are attributed lo the proton signals of PNIPAM. However, the methine in the [alpha] position of iodine at chain end COUU not be delected because of the lower iodine conlenl.

Tapping mode AFM was carried out lo investigate the surface topography of these hybrid materials after Soxhlel extraction in THF. Figure 7 shows a typical topography image of (a) Si-c-PMMA-10 h and (b) Si-e-PM.MA-/>-PNIPAM hybrid materials. As shown in Fig. 7a. bare particles formed closely packed arrays, wilh clearly discernible individual nanopanicles. Protrusions are clearly visible on height image. The mean film thickness of the PMMA layer detennined by AFM topography imaging in the vicinity of the scratch was about 30 nm. The mean diameter of the protrusions detennined by AFM is about 135 nm. Figure 7b depicts the AFM image of PMMA-/>-PNIPAM copolymer graft on silicon wafer. The section analysis of AFM images gives the mean film thickness of the PMMA-fc-PNIPAM copolymer layer in the range of 50-55 nm. The mean diameler of the copolymer-grafted silica nanoparticles determined by AFM is about 220 nm. Meanwhile in AFM image of Si-y-PMMA-/>-PNIPAM. the interparticle spacing was considerably larger than that of The PMMA-grafted silicon wafer. This is because the PMMA chains were densely arrayed and some tails of the chains were not exposed lo extend the chains by sequential addition of monomer NIPAM. causing steric crowding which prevented the growth of some PNIPAM chains from the silicon substrate.

To qualitatively verily the presence of the grafted polymer from the modified Si (III) surface. XPS was used lo investigate the composition. Figure 8 shows the lypical XPS survey spectrum (left) and high-resolution elemental scan of 13d (right), respectively. In Fig. 8a, the peaks of CIs (285.3 eV). Ols (532.7 eV). I3d (620.9 eV). and Si2p (103.0 eV) indicated the successful grafting of PMMA on silicon surface. According to the RITP mechanism, the peak of 13d ascribes to the iodine of C-I bond that exists, at the end of the polymer chains. Furthermore, the peak of Si2p was also observed which can be attributed to the exposure of silicon underneath. Afler grafting PNIPAM on Si-e-PMMA substrate (Fig. 8b). the peak of 13d become weak obviously. The atomic concentration (At. %) of C increased from 55% to 64% and the atomic concentration (At. %) of Si decreased from 15% to 14% respectively according lo the XPS quantitative analysis data, which are inev itable results of the chain extension. In addition, the atomic concentration (Al. %) of N decreased from 3.2% to 2.6%. the possible reason is: with the chain extension of PNIPAM on hollow spheres surface, the N in KH550 and ACPA on hollow spheres surface was enveloped more deeper and a pan of N could not be detected by XPS owing to its restricted testing deepness.

CONCLUSION

The amphiphilic diblock copolymer grafted from silicon substrates were prepared by surface-initiated RITP. Afler immobilization of the azo initiator. PMMA chains were successfully grafted from the surface of silicon substrates. The GPC analysis revealed that the [M.sub.n] of the polymer formed in solution approximately increased linearly with increasing monomer conversion, keeping the polydipersity ([M.sub.w]/[M.sub.n]) from 1.32 lo 1.41. Moreover. AFM images and XPS results clearly revealed that the PMMA grafted from silicon substrates have the ability lo extend the chains by sequential addition of NIPAM. illustrating the living nature of the polymerization process. This work presented a new melhod lo synthesize amphiphilic diblock copolymer on silicon substrates with controlled molecular weights and "well-defined" structures, which will open up a new route to a vast range of surface modification and extend potential applications of RITP.

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Correspondence to: Li-Ping Wang: e-mail: wangliping5@ I 63.com

Contract grant sponsor: Natural Science Foundation of China; contract grant number 21203085; contract grant sponsor Promotive Research Fund for Young and Middle-Aged Scientists of Shandong Province (doctor fund); contract grant number: BS201ICL011. BS2011CL012. 8S2012CL009.

DOI 10.1002/pen.23626

Published online in Wiley Online Library twileyonlinelihrary.conti.

[C] 2013 Society of Plastics Engineers

Li-Ping Wang, Xin-Hu Lv, Guang Li, Yu-Chao Li

College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, China
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Author:Wang, Li-Ping; Lv, Xin-Hu; Li, Guang; Li, Yu-Chao
Publication:Polymer Engineering and Science
Article Type:Report
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Date:Apr 1, 2014
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