PolytetrahydrofuranClay Nanocomposites by In Situ Polymerization

PolytetrahydrofuranClay Nanocomposites by In Situ Polymerization

(Parte 1 de 2)

Polytetrahydrofuran/Clay Nanocomposites by In Situ Polymerization and “Click” Chemistry Processes

Mehmet Atilla Tasdelen,†,‡ Wim Van Camp,‡ Eric Goethals,‡ Philippe Dubois,§ Filip Du Prez,*,‡ and Yusuf Yagci*,†

Department of Chemistry, Istanbul Technical UniVersity, Maslak, TR-34469, Istanbul, Turkey, Department of Organic Chemistry, Polymer Chemistry Research Group, Ghent UniVersity, Krijgslaan 281 S-4, B-9000 Ghent, Belgium, and Center of InnoVation and Research in Materials and Polymers (CIRMAP), Laboratory of Polymeric and Composite Materials (LPCM), UniVersity of Mons-Hainaut, Place du Parc 20, B-7000 Mons, Belgium

ReceiVed May 23, 2008; ReVised Manuscript ReceiVed July 1, 2008

ABSTRACT: Polytetrahydrofuran (PTHF)/clay nanocomposites were prepared by two routes: in situ cationic ring opening polymerization (CROP) and a method involving “click” chemistry. In the first method, PTHF chains were grown from the surface of the organo-modified montmorillonite clay by CROP of tetrahydrofuran (THF) through the hydroxyl functions of the clay by using trifluoromethanesulfonic anhydride, in the presence of 2,6- di-tert-butylpyridine as proton trap and dichloromethane as solvent. The polymerizations were affected by the clay content ratios. The living characteristics of the polymerization were demonstrated by the semilogarithmic first order kinetic plot. In the second method, CROP of THF has been performed independently to produce alkynefunctionalized PTHF and the obtained polymers were subsequently anchored to azide-modified clay layers by a “click” reaction. The exfoliated polymer/clay nanocomposites obtained by both methods were characterized and compared by X-ray diffraction spectroscopy, thermogravimetric analysis, and transmission electron microscopy. Compared to the virgin polymer, the nanocomposites exhibited improved thermal stabilities regardless of the preparation method. However, the nanocomposites prepared by the “click” chemistry approach appeared to be thermally more stable than those prepared by in situ polymerization. Moreover, the “click” chemistry method also provided better exfoliation.

Introduction

Polymer/clay nanocomposites represent a new class of materials, which have attracted much attention because of their excellent physical properties such as high dimensional stability, gas barrier performance, flame retardancy, and mechanical strength when compared to the pure polymer or conventional composites (micro- and macro composites).1–3 Three methods have been developed over time for the synthesis of polymer/ clay nanocomposites: solution exfoliation, melt intercalation, and in situ intercalative polymerization.4 Solution exfoliation has been used with water-soluble polymers to produce mostly intercalated nanocomposites because of the need for large amounts of the solvent to ensure a good clay dispersion.5 Melt intercalationis a method to enable mixing of the layered silicate with the polymer matrix in the molten state. This solvent-free method requires the polymer to be compatible with the clay layer surfaces. In the intercalativepolymerizationtechnique, the monomer, together with the initiator and catalyst, is intercalated withinthe silicatelayersand the polymerizationis initiatedeither thermally or chemically in situ. The chain growth in the clay galleries triggers the clay exfoliation and, hence, the nanocomposite formation. In recent years, polymer/clay nanocomposites have been prepared by various in situ polymerization methods such as ring-opening polymerization,6–9 controlled radical polymerization,9–20 conventional free radical polymerization,21–26 cationic polymerization,26,27 and living anionic polymerization.28

Polytetrahydrofuran (PTHF) is an important soft segment for producing thermoplastic elastomers such as polyester and polyurethane materials.29 In these applications, PTHF is valued as a precursor leading to products with outstanding hydrolytic stabilityat elevatedtemperatures,high fungalresistance,superior abrasion resistance, excellent resiliency, and attractive dynamic properties. PTHF is obtained by cationic ring opening polymerization (CROP) of tetrahydrofuran (THF) by using a wide range of conventional and photo initiators. The living character of the polymerization makes it possible to prepare polymers with well-defined end group functional and a narrow molecular weight distribution by using a functional initiator or termination agent. To the best of our knowledge, no examples of the synthesis of PTHF/clay nanocomposites have been reported so far.

The Huisgen 1,3-dipolar cycloaddition reaction of azides and terminal alkynes, catalyzed by copper(I) complexes is known as one of most efficient “click” reactions because of its high yield, short reaction time, mild reaction conditions, and a high tolerance toward other functional groups.30,31 This “click” reaction has been applied to macromolecular engineering,32–37 surface modification38 of both nanoparticles39,40 and silica,41 functionalization of carbon nanotubes,42 generation of nanostructured semiconductors,43 construction of degradable networks,4 and so on. However, “click” chemistry has never been applied for the synthesis of polymer/clay nanocomposites.

In this work, we report two effective routes for the synthesis of PTHF/clay nanocomposites, namely, (1) in situ CROP and (2) a method involving “click” chemistry. In the first approach, the nanocomposites have been prepared by the CROP of THF that is initiated in the intercalated layers of the clay. In the second approach,CROP of THF has been performedto produce alkyne-functionalized PTHF and the obtained polymers were subsequentlyanchoredto azide-modifiedclay layersby a “click” reaction. In principle, this approach can be extended to the

* To whom correspondence should be addressed. Tel.: +90-212- 2853241(Y.Y.); +32-9-2644503(F.D.P.). Fax: +90-212-2856386 (Y.Y.); +32-9-2644972 (F.D.P.). E-mail: yusuf@itu.edu.tr (Y.Y.); filip.duprez@ ugent.be (F.D.P.).

† Istanbul Technical University. ‡ Ghent University. § University of Mons-Hainaut.

10.1021/ma801149x C: $40.75 2008 American Chemical Society Published on Web 07/23/2008

combination of azide-modified clay with any other alkyne containing polymers.

The resulting exfoliated polymer/clay nanocomposites have been characterized by X-ray diffraction (XRD) spectroscopy, thermogravimetric analysis (TGA), and transmission electron microscopy (TEM).

Experimental Section

Materials. Organo-modified clay, Cloisite 30B (MMT-

(CH2CH2OH)2) was kindly supplied by Southern Clay Products (Gonzales, TX). The clay is a montmorillonite modified by methyl bis(2-hydroxyethyl) (tallow alkyl) ammonium ions. The organic content of the organo-modified montmorillonite, determined by TGA, was 21 wt %. Before use, the clay was dried under vacuum at 110 °C for 1 h. Tetrahydrofuran (THF; Aldrich HPLC grade) was dried on sodium wire under reflux in the presence of traces of benzophenone until a blue color persisted and was used directly after distillation.Dichloromethane(Acros, HPLC grade) was stored on calcium hydride and used after distillation. Trifluoromethane- sulfonic anhydride (Tf2O; Acros 98%), methyl trifluoromethanesulfonate (MeOTf; Acros 98%), and triethylamine (Aldrich, HPLC grade) were purified by distillation just before use. Sodium azide

(NaN3, Acros 9%), copper(I) sulfate·5H2O (CuSO4, Acros 9%), L-ascorbic acid sodium salt (NaAsc, Acros 9%), 2,6-di-tert- butylpyridine (DTBP; Maybridge Chemicals >97%), methanesulfonyl chloride (Acros 9.5%), lithium bromide (LiBr, Fluka 98%), propargyl alcohol (Aldrich, 9%), propargyl methacrylate (Lancaster,98%), dimethylsulfoxide(DMSO, Acros HPLC grade), methanol (Acros HPLC grade), ethanol (Acros 96%), and pentane (technical) were used as received. In Situ Polymerization of THF. A typical polymerization procedure is as follows (Table 1, entry 1). MMT-(CH2CH2OH)2 clay (0.18 g, 1 wt % with respect to THF) was added in a two- necked 50 mL flask and dried under vacuum at 110 °Cf or1h .

Initiator Tf2O (0.50 mL, 2.97 mmol), DTBP (1 mL, 4.46 mmol), and CH2Cl2 (7 mL) were added to the flask under a nitrogen atmosphere. The mixture was stirred for1ha t0 °C. The initiator solution was brought to 20 °C, after which a large amount of THF (20 mL, 244 mmol) was introduced. After a polymerization time of, respectively, 15, 30, and 60 min at 20 °C, the polymerization was terminated by adding 1 mL of methanol. The polymer was then precipitated in cold pentane (-20 °C), filtered off on a cold glass filter, washed with cold pentane, and finally dried in vacuum.

Synthesis of Alkyne-Functionalized PTHF (PTHF-Alkyne).

In a two-necked flask (flame-dried) fitted with a magnetic stirrer, an inlet for dry nitrogen, and a rubber septum, THF (40.0 mL, 488 mmol) was introduced. The polymerization was initiated with methyl triflate (0.1 mL, 0.97 mmol) at 20 °C. After a polymerization time of, respectively, 15, 30, and 60 min at 20 °C, with respect to PTHF samples 1, 2, and 3 in Table 2, propargyl alcohol (1 mL, 17 mmol) was added as a terminating agent. The polymer was then precipitated in cold pentane (-20 °C), filtered off on a cold glass filter, washed with cold pentane, and finally dried in vacuum.

Synthesis of Azide-Functionalized Montmorillonite Clay

(MMT-N3). Methyl bis(2-hydroxyethyl)(tallow alkyl) ammonium- organomodified clay (MMT-(CH2CH2OH)2, 4.50 g, 5.3 mmol, OH content)and triethylamine(3.7 mL, 26.5 mmol) were added in THF

(200 mL) and cooled to 0 °C. Methanesulfonyl chloride (2.1 mL, 26.5 mmol)was addeddropwisewhilestirring.The reactionmixture was allowed to heat up to room temperature and stirred overnight. Solvent was removed by rotary evaporation, and ethanol (200 mL) was added to the reaction mixture. Sodium azide (1.72 g, 26.5 mmol) was added, and the reaction mixture was refluxed overnight. After cooling to room temperature and removing the solvent by rotary evaporation, ether (200 mL) was added to the crude reaction mixture and washed three times with a saturated NaCl aqueous solution.The clay was then filtered off on a cold glass filter, washed with water, and finally dried in vacuum.

“Click” Coupling Reaction between Propargyl Methacrylate and Azide-Functionalized Montmorillonite Clay. Azidefunctionalized montmorillonite clay (0.5 g, 6 mmol), propargyl methacrylate (0.4 mL, 3 mmol), and DMSO (20 mL) were added in a round-bottomed flask and stirred. A solution of CuSO4 (0.02 g, 0.12 mmol) in 1 mL of water was added to the mixture, followed by addition of a solution of sodium ascorbate (0.09 g, 0.45 mmol) in 1 mL of water. The mixture was heated in an oil bath at 70 °C overnight. The particles were recovered by centrifugation at 3000 rpm for 30 min. Then they are redispersed in water and the mixture was centrifuged; this cycle was repeated four times. Finally, the particles were placed in a Soxhlet extractor and extracted with THF for 18 h and dried in vacuum.

“Click” Coupling Reaction between Alkyne-Functionalized

PTHF and azide-Functionalized Montmorillonite Clay. Azidemontmorilloniteclay (0.25 g, 3 mmol), alkyne functionalizedPTHF (0.5 g, 1.5 mmol), and DMSO (20 mL) were added in a round- bottomedflask and stirred.A solutionof CuSO4 (0.02 g, 0.12 mmol) in 1 mL of water was added to the mixture, followed by addition of a solution of sodium ascorbate (0.09 g, 0.45 mmol) in 1 mL of water. The mixture was heated overnight in an oil bath at 70 °C. The particles were recovered by the same procedure as described above.

Polymer Separation. Polymer bound on the clay was cleaved off by refluxing the polymer/clay nanocomposite with LiBr in tetrahydrofuran (THF) with overnight stirring.7 The cleaved polymers were separated from the solid clay nanoparticles by centrifugation. 1H NMR, GPC, and TGA analyses were performed on the polymer clay nanocomposite and the detached polymer.

Techniques. 1H NMR spectra were recorded in CDCl3 at room temperaturewith a Bruker AM500 spectrometer.GPC analysis was performed on a Waters instrument using a refractive index detector (2410 Waters) equipped with Waters Styragel 103 - 104-105 Å serial columns (5 µm particle size) at 35 °C. PS standards were used for calibration and CHCl3 as eluent at a flow rate of 1.5 mL/ min. Fourier transform attenuated total-reflectance infrared (FT-

ATR-IR) spectra of the original and modified TM were recorded on a BIO-RAD FT-ATR-IR spectrometer 575C using 64 scans at a resolution of 4 cm-1. Thermogravimetric analysis (TGA) was performedwith a MettlerToledoTGA/SDTA851einstrumentunder air atmosphere at a heating rate of 20 °C/min between 25 and 800

Table 1. Polymerization of Tetrahydrofuran in the Presence of 1, 3, and 5% of Clay (MMT-(CH2CH2OH)2) and TfO2 at 20 °C samples clay content

(wt %) time(min) conv.a(%)

Mn,th b

Mn c and hydroxyl groups of MMT-(CH2CH2OH)2 are active. c Polystyrene was used as a standard and a correction factor, measured for linear polytetrahy- drofuran, of 0.4 was used.

Table 2. Cationic Ring Opening Polymerization of Tetrahydrofuran Initiated by Propargyl Alcohol in the Presence samples time(min) conv.a(%)

Mn,th b

Mn,NMR c

Mn d conversion + Mpropargyl alcohol. c Calculated from 1H NMR spectroscopy. d Polystyrene was used as a standard and a correction factor, measured for linear polytetrahydrofuran, of 0.4 was used.

6036 Tasdelen et al. Macromolecules, Vol. 41, No. 16, 2008

°C. The powder X-ray diffraction (XRD) measurements were performedon a SiemensD5000 X-ray diffractometerequippedwith graphite-monochromatized Cu KR radiation (λ ) 1.5405 Å). TEM micrographs were obtained with a Philips CM100 apparatus using an acceleration voltage of 100 kV. Ultrathin sections (ca. 80 nm thick) were cut at -100 °C from 3 m thick hot-pressed plates by using a Reichert-Jung Ultracut FC4E microtome equipped with a diamond knife. Because of the large difference in electron density between silicate and polymer matrix, no selective staining was required.

Results and Discussion

For the preparation of PTHF/clay nanocomposites by the in situ polymerization method, the hydroxyl functions of the modified intercalated montmorillonite clay (MMT-(CH2CH2- OH)2) are reacted with trifluoromethanesulfonic anhydride

(TfO2), in the presence of DTBP as proton trap and dichloromethane as solvent, to produce the corresponding triflate ester that is known to be an initiator for the CROP of THF. Then, a large amount of THF monomer is added.45 After the prescribed reaction time, the process directly leads to the formation of the PTHF/clay nanocomposites (Scheme 1).

A series of polymerizations were conducted using different clay contents while reaction times were also varied to gain more insight in the CROP initiated on the clay surface. The results are shown in Table 1 and Figures 1 and 2. As can be seen from Table 1, the molecular weights of the polymers, which are obtained after cleaving them off from the clay by LiBr, are higher than the expected molecular weight. A possible reason for this discrepancy may be ascribed to the complex nature of the intercalated initiating sites and to the fact that a fraction of the hydroxyl groups may not be activated by triflic anhydride. However, the polydispersity indices are quite low and remain below 1.4, even at high conversions. The corresponding semilogarithmic first order kinetic plot is depicted in Figure 1a. The linear relationship indicates that the living characteristics of CROP also apply to the present system. The polymerizations exhibit different rate constants, which show that the polymerization is affected by the clay content ratios. Furthermore, the molecular weights of the polymers (Mn) increase linearly with monomer conversion as expected for a living polymerization

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