Study on the Synthesis of Poly(diglycidyl maleate-co-stearyl methacrylate) and MorphologyConversion of Their Self-Assembly Systems

Haojie Yu, Li Wang,* Junfeng Zhou, Guohua Jiang, and Zhenrong ZhaoState Key Laboratory of Polymer Reaction Engineering, College of Materials Science and ChemicalEngineering, Zhejiang UniVersity, Hangzhou 310027, China

ReceiVed: September 29, 2005; In Final Form: NoVember 19, 2005

A novel polymer of poly(diglycidyl maleate-co-stearyl methacrylate) (P(DGMA-co-SMA)) was synthesizedby reaction between poly(maleic anhydride-co-stearyl methacrylate) (P(MA-co-SMA)) and epichlorohydrin.The self-assembly behavior of the resultant copolymer was investigated. It was found that the spheral aggregatescould converse to nanorods after being aged for 2.5 days and nanolines composed of the nanorods wereobtained after being aged for an additional 5.5 days. The mechanism of their self-assembly behavior andmorphology conversion of self-assembly systems is discussed.

Introduction

Self-assembly is a promising way to prepare well-definednanostructures for drug delivery, microreactors, microcapsules,and encapsulation of various kinds of guest molecules.1-7

Amphiphilic molecules are important optional materials for self-assembly for their unique solution and associative properties.8-10

Various factors that affect the morphologies of amphiphilicmolecules, such as molecule composition,11-13 concentration,14

solvents,15 temperature,16 and pH,17 have been widely investi-gated. A noteworthy phenomenon had been found that morphol-ogy conversion occurred in some self-assembly systems. Burkeand Eisenberg investigated kinetics and the mechanism of thevesicle-to-rod transition of aggregate systems prepared from theternary system of polystyrene-b-poly(acrylic acid)/dioxane/water.18 They also reported morphological conversion in ag-gregates of polystyrene-b-poly(acrylic acid) induced by thesmall-molecule surfactant sodium dodecyl sulfate.19 The mor-phology conversion of aggregates provides a method to preparesome materials with specific shapes.

Compounds containing epoxy groups have caught consider-able attention for the presence of easily transformable epoxygroups. They have been applied in protein and enzyme im-mobilization,20,21 biological tissue fixation,22 and biomoleculebinding.23 Preparation of amphiphilic molecules containingepoxy pendant chains can provide significant materials used inthe fields of nanotechnology, medicine, and biology. Can˜ameroet al.24 have synthesized a novel poly(glycidyl methacrylate)-b-poly(butyl acrylate) diblock copolymer through atom transferradical polymerization (ATRP).

A self-assembly system with a specific shape and the stabilityof the resultant micells are required in some applications.Amphiphilic copolymers with epoxy groups have such meritsfor the epoxy group can cross-link after self-assembly. In thispaper, we report a synthesis route of P(DGMA-co-SMA)through grafting the epichlorohydrin to the P(MA-co-SMA)copolymer obtained by ATRP. The resulting copolymer can self-assemble in THF solution, using deionized water as a precipitant.The morphology conversion of the resulting aggregates was

found and investigated. The mechanism of morphology conver-sion is discussed.

Experimental Section

Materials. Maleic anhydride (MA) from Shanghai No. 1Chemical reagent factory was recrystallized in chloroform beforeuse. Stearyl methacrylate (SMA) obtained from Shanghai No.1 Chemical reagent factory was washed with 5 wt % of NaOHsolution. Di-tert-butyl peroxide (DTBP) purchased from Shang-hai No. 1 Chemical reagent factory was treated according tothe literature method.25 Epichlorohydrin, an analytical reagent,was used as obtained from Wulian Chemical Plant in Shanghai.Tetrabutylammonium bromide (TBAB) was purchased fromSinopharm Chemical Reagent Ltd. Co. Tetrahydronfuran (THF),NaOH, and CH3CH2OH were purchased from East of ChinaChem. Ltd. Co. THF was purified by fluxing over a K-Na alloyunder nitrogen and other reagents were used as received.

Synthesis of P(DGMA-co-SMA). P(MA-co-SMA) wassynthesized according to ref 26.1H NMR (500 MHz, CDCl3):δ 0.88 (3H, inCH3(CH2)15-), 0.86 and 0.90 (3H, inCH3C-),1.20-1.40 (30H, in-(CH2)15CH3), 1.58-1.78 (2H, in-CH2-(CH2)15CH3, 2H, in -CH2C-), 2.2-3.8 (2H, in -CHCH-),4.13 (2H, in-OCH2-).27 The molecular weight and molecularweight distribution of P(MA-co-SMA) were Mn ) 3371 andPDI (Mw/Mn) ) 1.73. The MA segment content was 15.34 wt%.

P(DGMA-co-SMA) was prepared by the reaction of P(MA-co-SMA) and epichlorohydrin. The typical synthesis process isas follows: 0.8171 g of P(MA-co-SMA), 4.8 mL of epichlo-rohydrin, 0.0102 g of TBAB, and 0.15 mL of H2O were mixedin a 25-mL three-necked flask with a magnetic stirrer and areflux condenser and reacted at 120°C for 2.5 h. The excessiveepichlorohydrin was removed under vacuum after reaction. Theproduct was dissolved in 10 mL of THF at room temperature,after which 0.51 mL of 30 wt % of NaOH solution was addeddropwise in 15 min. The reaction was continued for another 2h. Then the product was purified by precipitation in 200 mL ofdeionized water 2 times and dehydrated in a vacuum at roomtemperature.1H NMR (500 MHz, CDCl3): δ 0.88 (3H, inCH3-(CH2)15-), 0.86 and 0.90 (3H, inCH3C-), 1.20-1.40 (30H,in -(CH2)15CH3), 1.58-1.78 (2H, in-CH2(CH2)15CH3, 2H,

* Address correspondence to this author. E-mail: opl_wl@dial.zju.edu.cn.Phone: +86-571-87953200. Fax:+86-571-87951612.

837J. Phys. Chem. B2006,110,837-841

10.1021/jp055557l CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 12/21/2005

in -CH2C-), 2.2-3.8 (2H, in-CHCH-), 2.61 and 2.81 (4H,in -CHCH2O-), 3.17-3.27 (2H, in-OCH-), 3.36 and 3.86(4H, in -OCH2CH<), 4.13 (2H, in-OCH2-).

Preparation of a Self-Assembly System.P(DGMA-co-SMA) was dissolved in THF at 25°C. Then deionized wateras a precipitant, with a volume ratio of 1:9 to THF, was addeddropwise to the solution to induce association of the long alkylchains. Samples for TEM observation were prepared after theself-assembly system was aged for different time.

Characterization. 1H NMR spectra of polymers in CDCl3

were recorded with a 500 MHz AVANCE NMR spectrometer(Model DMX500), using TMS as a standard. TEM images wereobtained on a JEOL model 1200EX instrument operated at anaccelerating voltage at 160 kV. The samples for TEM observa-tion were prepared as follows: 10µL of solution was droppedonto 200-mesh gilder copper TEM grids and air dried at roomtemperature before TEM observation. The gel permeationchromatography (GPC) measurements were carried out on aWaters 201 with aµ-styragel column and THF as an eluent,and the molecular weight was calibrated with standard poly-styrene (PS). The MA segment content in the copolymer wasdetermined by nonaqueous direct titration.28

Results and Discussion

Characterization of P(DGMA-co-SMA). Figure 1 showsthe1H NMR spectrum of P(DGMA-co-SMA). Compared to the1H NMR data of P(MA-co-SMA), new peaks of glycidyl esterappeared at 2.61 and 2.81 ppm for Hj and 3.17-3.27 for Hi ofthe oxirane ring indicating that epichlorohydrin have beengrafted onto P(MA-co-SMA). The conversion ratio of MAsegments to glycidyl ester was calculated from the1H NMRspectrum. If the MA segments completely converted to glycidylester, the theory value of (peak area of Ha and Hf)/(peak areaof Hi) should be 4.8/1; however, the calculated value of the1HNMR spectrum was 5/1, which indicates that about 96% of MAsegments converted to glycidyl ester.

Self-Assembly Behavior of P(DGMA-co-SMA). The self-assembly behavior of block polymer is mainly influenced bythe copolymer composition, solution concentration, and solventsproperties.11-15 As for P(DGMA-co-SMA), both of the segmentscan dissolve in THF and the pendant chain with epoxy groupare more hydrophilic than the stearyl pendant group in deionizedwater for the oxide atom of the oxirane ring may form hydrogenbonds with the H2O molecule. The copolymer spheral aggregateswere obtained by adding deionized water dropwise, as aprecipitant, to the solutions of P(DGMA-co-SMA) in THF atdifferent copolymer concentrations. Figure 2 shows the TEMimages of the obtained aggregates of P(DGMA-co-SMA). Itcan be seen that the copolymer formed spheral aggregates andthe diameter of the aggregates became bigger with the increaseof concentration. Figure 2B is the magnification of Figure 2A,from which we can see that the transmission around theperiphery of the aggregates is higher than that in the center.This phenomenon can be attributed to the better hydrophilicityof the pendant chain with the epoxy group, which formed theloose shell of the spheral aggregates.

The aggregates of P(DGMA-co-SMA) have a loose structureof epoxy pendant chains in the periphery, and a similarcomposition of the aggregate shell may lead to the amalgamationof the aggregates in contact with each other. To investigate theamalgamation of the aggregates, the self-assembly systems were

Figure 1. 1H NMR spectrum of P(DGMA-co-SMA).

Figure 2. Aggregates of P(DGMA-co-SMA) formed in the THF/H2O (with a volume ratio of 9:1) solvent system. The concentrations of thepolymer were (A) 0.1, (B) 0.1, (C) 0.2, and (D) 0.3 wt %, respectively.T ) 25 °C.

838 J. Phys. Chem. B, Vol. 110, No. 2, 2006 Yu et al.

Figure 3. Aggregates of P(DGMA-co-SMA) formed in THF/H2O (volume ratio of 9:1) mixed solvent after being aged for 2.5 days. The concentrationsof the polymer were (A) 0.1, (B) 0.1, (C) 0.2, and (D) 0.2 wt %, respectively.T ) 25 °C.

Figure 4. Aggregates of P(DGMA-co-SMA) formed in THF/H2O (volume ratio of 9:1) mixed solvent after being aged for 8 days. The concentrationsof the polymer were(A) 0.1, (B) 0.1, (C) 0.1, (D) 0.1, (E) 0.1, and (F) 0.2 wt %, respectively.T ) 25 °C.

Poly(diglycidyl maleate-co-stearyl methacrylate) J. Phys. Chem. B, Vol. 110, No. 2, 2006839

aged and then observed by TEM. Figure 3 shows the TEMimages of the self-assembly systems aged for 2.5 days, fromwhich the amalgamated aggregates can be observed. Panels Aand B of Figure 3 are the aggregates with concentration of 0.1wt % of copolymer while panels C and D of Figure 3 are the0.2 wt % aggregates. The number of aggregates increased withthe increase of concentration.

As the amalgamation of the aggregates which formed longaggregates has been observed after the self-assembly systemswere aged for 2.5 days, longer aggregates may be formed withthe increase of aging time. With this in mind, the self-assemblysystems were aged for another 5.5 days. The TEM images areshowed in Figure 4. We can see that the long aggregates werefurther amalgamated to form nanolines after being aged for 8days. As the nanolines were made of many aggregates and alsomany aggregates was deposited after the long aging time, therewere only several nanolines found in the observation area andonly one nanoline can be shown in a TEM image for eachnanoline that was separated. Panels B and D in Figure 4 arethe magnifications of panels A and C, respectively. Theperiphery of the nanolines was made up of a loose structurethat was similar to the original spheral aggregates. And therealso were some intertwisted nanolines as Figure 4E shows atsome aggregate-rich areas. However, in the self-assembly systemwith 0.2 wt % of copolymer, the probability of the aggregatesconnecting is greater and many neighborhood nanolines ag-glomerated as Figure 4F shows, and the separated nanolinescan hardly be found.

Mechanism of Self-Assembly of P(DGMA-co-SMA).Through TEM observation, we have found that P(DGMA-co-SMA) can form spheral aggregates with deionized water as aprecipitant in THF solution, and the obtained spheral aggregatesamalgamated to form long aggregates when the self-assemblysystem was aged for 2.5 days and the long aggregates werefurther amalgamated to form nanolines after being aged foranother 5.5 days. A possible mechanism of the self-assemblybehaviors of P(DGMA-co-SMA) is shown in Figure 5. In theself-assembly process, first, the copolymer formed spheralaggregates with a diameter of about 300 nm (as showed inFigure 5A) and the epoxy pendant chain formed a looseperiphery of the aggregates for their relatively better hydrophi-licity in water. The Brownian motion of nanosized aggregatesin solution leads to the collision of the aggregates with eachother; unlike block ionomer complexes which can form stableaggregates for the steric stabilization of the corona,29 the

P(DGMA-co-SMA) aggregates connected together because theepoxy group of the periphery of the aggregates was lesshydrophilic than ionomer complexes and results in a lowerstability of the aggregates (as shown in Figure 5B). Theconcentration of segments bearing an epoxy pendant is higherat the interface between the connected aggregates30 and thesegments migrating to the surface under the tensile force of thesolvent result in the amalgamation of the aggregates as shownin Figure 5C. For the same mechanism, the long aggregateswere further amalgamated to form nanolines during the agingtime.

In this self-assembly system, aggregates with differentmorphology can be obtained by cross-linking of the epoxy groupat different self-assembly processes, and can supply specificallyshaped aggregates for different applications.

Conclusions

A novel copolymer of P(DGMA-co-SMA) was synthesizedby reaction of P(MA-co-SMA) and epichlorohydrin. Thecopolymer aggregates were obtained by adding deionized waterdropwise to the THF solutions of P(DGMA-co-SMA). It wasfound that the aggregates amalgamated after being aged for 2.5days and the nanolines were obtained with 8 days of aging. Sospecifically shaped aggregates for different applications can beobtained by cross-linking of the epoxy group at different self-assembly processes.

Acknowledgment. Financial support by the Science &Technology Commission of Zhejiang Province (2004C34005)is gratefully acknowledged.

References and Notes

(1) Rosler, A.; Vandermeuler, G. W. M.; Klok, H. A.AdV. Drug.DeliVery ReV. 2001, 53, 95.

(2) Allen, C.; Maysinger, D.; Eisenberg, A.Colloids. Surf. B1999,16, 3.

(3) Borisov, O. V.; Zhulina, E. B.Macromolecules2002, 35, 4472.(4) Moffitt, M.; Eisenberg, A.Macromolecules1997, 30, 4363.(5) Lee, M.; Jang, C. J.; Ryu, J. H.J. Am. Chem. Soc.2004, 126, 8082.(6) Zhao, H.; Douglas, E. P.; Harrison, B. S.; Schanze, K. S.Langmuir

2001, 17, 8428.(7) Jenekhe, S. A.; Chen X. L.Science1998, 279, 1903.(8) Huang, W.; Zhou, Y. F.; Yan, D. Y.J. Polym. Sci. A2005, 43,

2038.(9) Riegel, I. C.; Samios, D.; Petzhold, C. L.; Eisenberg, A.Polymer

2003, 44, 2117.(10) Jain, S.; Bates F. S.Science2003, 300, 460.(11) Huang, X.; Li, C.; Jiang, S.; Wang, X.; Zhang, B.; Liu, M.J. Am.

Chem. Soc.2004, 126, 1322.(12) Discher, D. E.; Eisenberg, A.Science2002, 297, 967.(13) Discher, B. M.; Won, Y. Y.; Ege, D. S.; Lee, J. C.; Bates, F. S.;

Discher, D. E.; Hammer, D. A.Science1999, 284, 1143.(14) Zhang, L.; Eisenberg, A.Macromolecules1999, 32, 2239.(15) Soo, P. L.; Eisenberg, A.J. Polym. Sci. Part B: Polym. Phys.2004,

42, 923.(16) Steinbeck, C. A.; Hedin, N.; Chmelka, B. F.Langmuir2004, 20,

10399.(17) Luo, L.; Eisenberg, A.Angew. Chem., Int. Ed. 2002, 41, 1001.(18) Burke, S. E.; Eisenberg, A.Polymer2001, 42, 9111.(19) Burke, S. E.; Eisenberg, A.Langmuir2001, 17, 8341.(20) Mateo, C.; Torres, R.; Ferna´ndez-Lorente, G.; Ortiz, C.; Fuentes,

M.; Hidalgo, A.; Lopez-Gallego, F.; Abian, O.; Palomo, J. M.; Betancor,L.; Pessela, B. C. C.; Guisan, J. M.; Ferna´ndez-Lafuente, R.Biomacro-molecules2003, 4, 772.

(21) Fujita, A.; Kawakita, H.; Saito, K.; Sugita, K.Biotechnol. Prog.2003, 19, 1365.

(22) Sung, H. W.; Hsu, H. L.; Hsu, C. S.J. Biomed. Mater. Res.1997,37, 376.

(23) Shanthi, C.; Rao, K. P.Carbohydr. Polym.2001, 44, 123.(24) Canamero, P. F.; Fuente, J. L.; Madruga, E. L.; Ferna´ndez-Garcı´a,

M. Macromol. Chem. Phys.2004, 205, 2221.(25) Jackson, R. A.; Ingold, K. U.; Griller, D.; Nazran, A. S.J. Am.

Chem. Soc.1985, 107, 208.

Figure 5. Mechanism of self-assembly of P(DGMA-co-SMA).

840 J. Phys. Chem. B, Vol. 110, No. 2, 2006 Yu et al.

(26) Stewart, T.; Lesko, P. M.; El A’mma, A. G. U.S. Patent 5316860,1994.

(27) Zhou, J. F.; Wang, L.; Wang, C. L.; Chen, T.; Yu, H. J.; Yang, Q.Polymer2005, 46, 11157.

(28) Brown, A.; Fujimori, K.Polym. Bull.1986, 16, 441.

(29) Solomatin, S. V.; Bronich, T. K.; Eisenberg, A.; Kabanov, V. A.;Kabanov, A. V.Langmuir2004, 20, 2066.

(30) Shen, Z.; Zhao, Z. G.; Wang, G. T.Colloid and Surface Chemistry;Chemical Industry Press: Beijing, China, 2004; p 105.A

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