Modification of montmorillonite by cationic polyesters

Applied Clay Science 43 (2009) 420–424

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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c l a y

Modification of montmorillonite by cationic polyesters M. Huskić a,⁎, E. Žagar a, M. Žigon a, I. Brnardić b, J. Macan b, M. Ivanković b a b

National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia University of Zagreb, Faculty of Chemical Engineering and Technology, Marulićev trg 19, HR-10001 Zagreb, Croatia

a r t i c l e

i n f o

Article history: Received 10 July 2008 Received in revised form 13 January 2009 Accepted 14 January 2009 Available online 27 January 2009 Keywords: Montmorillonite Polycations Polyester hydrochlorides

a b s t r a c t Novel types of polyester hydrochlorides and polyesters were synthesized from N-octyl- or N-methyldiethanolamine and organic acid chlorides with varying chain length. Their structure, molar masses and glass transition temperatures were determined. Montmorillonite (MMT) was modified by cation exchange with the polyester hydrochlorides. The basal spacing was not significantly influenced by the length of organic acid chain but increased with the length of the alkyl group attached to nitrogen atom. Cation ratio of MMT and hydrochloride must have been at least 1:3 for quantitative cation exchange, indicating that only a part of polycation chain adheres to MMT surface. Hydrophobicity and organic content increased with increased exchange ratio, reaching approximately constant levels at ratios above 1:3. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Polymer/clay nanocomposites have been a subject of extensive research for the last 15 years. Efficiency of clay nano-filler for modification of mechanical, thermal and barrier properties of polymers depends on degree of intercalation or exfoliation of clay mineral platelets within the polymer matrix (LeBaron et al., 1999; Alexandre and Dubois, 2000; Ray and Okamoto, 2003; Ruiz-Hitzky and Van Meerbeek, 2006). Montmorillonite (MMT) is most often used in polymer nanocomposite preparation. The surface of the MMT layers is hydrophilic, and therefore has to be modified to ensure good intercalation with mostly hydrophobic commercial polymers. The most common method of MMT modification is cation exchange with organic ammonium salts containing different numbers, length and structure of organic chains (Bergaya and Lagaly, 2001). The basal spacing of intercalated nanocomposites depends on the type of modifier, which strongly influences the mechanical properties (Barber et al., 2005). Polyelectrolytes have also been used for MMT modification. The influence of polymer molar mass and cationicity (mol.% of cationic units of the polymer) has been studied. The amount of absorbed polymer increased with increasing molar mass when cationicity was low. When the molar mass was fixed, the amount of absorbed polymer decreased with increasing cationicity (Mabire et al., 1983). MMT can also be modified by polymerization of ionic organic molecules in the presence of MMT. Fournaris et al. (2001) observed that MMT dispersed in water increases the rate of polymerisation of protonated 4vinylpyridine, favouring 1,6-polyelectrolyte formation, and leads to formation of an exfoliated nanocomposite.

⁎ Corresponding author. Tel.: +386 1 4760 206; fax: +386 1 4760 420. E-mail address: [email protected] (M. Huskić). 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.01.008

Polycation-modified clay minerals have been extensively studied as catalysts (Breen and Watson, 1998a, Adams and McCabe, 2006) or adsorbents for organic pollutants (Breen and Watson, 1998b; Churchman, 2002; Churchman et al., 2006). Clay minerals has also been modified with natural polycations such as cationic starch (Wang et al., 1996) in order to substitute petro-based plastics with biodegradable materials, i.e., nanocomposites based on starch (Chivrac et al., 2008) or chitosan (Darder et al., 2004, Wang et al., 2005). To our knowledge polycation-modified clay minerals have not been used as fillers for preparation of commercial synthetic polymer/MMT nanocomposites. This is most probably due to hydrophilicity of the polycations, which makes intercalation or exfoliation of hydrophobic polymers unlikely. Polyester hydrochlorides are a new type of polycations prepared recently from alkyl derivatives of diethanolamine, R-N-(C2H4OH)2 (alkyldiethanolamine, ADEA), by reaction with organic acid chlorides, R′(COCl)2 (Huskić and Žigon, 2003). The nitrogen atom in ADEA acts as a strong acid acceptor. Thus, hydrochloride evolved during polyesterification reacts with the nitrogen atoms in the polyester. Subsequent extraction of the hydrochlorides with water yields polyesters. Until recently, only mesogenic or liquid crystalline polyesters (Chen et al., 1997; Huskić and Žigon, 2001, 2003, 2004) have been synthesized. Nitrogen atoms in polyesters based on ADEA can be partially or fully quaternized or converted back to hydrochlorides (Huskić et al., 2008). This type of polycations could be used for MMT modification and subsequent polymer nanocomposite preparation. We assume that protonated parts of the polyester penetrates into the interlayer spaces, while unprotonated parts will be present as loops close to the external surfaces. This part of macromolecule is supposed to be well miscible with polymer, leading to enhanced intercalation and improved physical and mechanical properties in comparison with the nanocomposites made of MMT modified with simple alkyl ammonium salts.

M. Huskić et al. / Applied Clay Science 43 (2009) 420–424

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Scheme 1. The synthesis of polyester hydrochloride.

In this paper, we present the synthesis and characterization of the novel type of polyester hydrochlorides and polyesters. The modification of MMT with polyester hydrochlorides was studied by FTIR, thermogravimetry and X-ray diffraction.

2. Experimental 2.1. Materials N-octyldiethanolamine (2,2′-octyliminodiethanol, ODEA) was synthesized as follows: 0.10 mol of diethanolamine (DEA), 0.95 mol of 1-iodooctane and 0.2 mol K2CO3 were mixed in 160 ml of 2-propanol. The reaction mixture was heated under reflux for 24 h. The suspension was filtered to remove K2CO3, and the filtrate was dried on rotatory evaporator. The obtained ODEA was dissolved in chloroform and purified by extracting DEA in the demineralised water until pH of water remained 6–7. The chloroform phase was separated and dried with a rotatory evaporator. N-methyldiethanolamine (MDEA, Aldrich, 99%) was used as received. Polyester hydrochlorides were synthesized according to Scheme 1 from MDEA or ODEA and various organic acid chlorides with increasing size of the organic chain: malonyl chloride, succinyl chloride, adipoyl chloride, suberoyl chloride, and 3-methyladipoyl chloride. The annotation of polyesters is given in Table 1. The polyesterification proceeded in a two-necked round bottom flask equipped with cooler and rubber septum. Dry chloroform and the reagents (0.025 mol of MDEA or ODEA and 0.025 mol of acid chloride) were added with the syringe through the septum, and then reacted at 60 °C for 4 h. The reaction mixture was transferred to round bottom flask and dried first in a rotatory evaporator and then in vacuum at 50 °C for 24 h. To determine the molar mass, the polyester hydrochlorides were converted to polyesters. Amounts of 1 g of polyester hydrochloride were dissolved in demineralised water. Saturated solution of K2CO3 was added in small portions until the pH changed from acidic to basic (7–8). The polyester precipitated because it is insoluble in water. Chloroform was added to dissolve the polyester and separate it from the aqueous phase. The chloroform phase was removed and dried in a rotatory evaporator and subsequently in vacuum at 50 °C for 24 h. Modification of MMT (Nanofil 757, Süd Chemie AG, CEC = 85 meq/100 g) was performed in water at 70 °C for 24 h, with the molar ratio of cations in MMT and polyester hydrochloride 1:1, 1:2, 1:3, 1:4 and 1:5. The ratios N1:1 were chosen to bind as much polycation molecules Table 1 Chain lengths of acid chlorides and designations of synthesized polyester hydrochlorides Acid chloride

R′(COCl)2, R′ =

MDEA

ODEA

Malonyl chloride Succinyl chloride Adipoyl chloride Suberoyl chloride 3-methyladipoyl chloride

CH2 (CH2)2 (CH2)4 (CH2)6 CH2CH2−CH(CH3)−CH2

PesMM PesMSc PesMA PesMSb PesMMA

PesOM PesOSc PesOA PesOSb PesOMA

as possible, and to increase the length of the uncharged parts of polycation chains on the OMMT surface. 2.2. Characterization Average molar masses and molar mass distributions of polyesters were determined by size exclusion chromatography combined with multi-angle light scattering (SEC-MALS), using a Hewlett Packard pump series 1100 coupled to a Dawn Heleos laser photometer (658 nm) and to an Optilab rEX refractometer. An AM Polymer Standards column with AM GPC gel with a particle size of 5 µm was used. 0.05 mol/dm3 solution of CF3COONa in tetrahydrofuran was used as an eluent, with a flow rate of 1 cm3/min. Glass transition temperatures of polyester chlorides were determined using a Perkin–Elmer Pyris 1 differential scanning calorimeter (DSC). The samples were twice heated and subsequently cooled in temperature range −60 °C to +50 °C. Heating and cooling rates were 10 °C/min. Nuclear magnetic resonance 1H NMR spectra were recorded at 25 °C on a Varian Unity Inova-300 spectrometer using DMSOd6 as the solvent and TMS as the internal standard. X-ray diffraction (XRD) patterns of MMT and OMMT were taken on a Siemens D-5000 diffractometer using Cu Kα radiation in 2θ range 0.5–15° with 0.04° steps and 1 s/step. The basal spacing was calculated according to Bragg law. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR FTIR) was performed on powder OMMT samples, using Bruker Vertex 70 FTIR spectrometer. Each spectrum from 4000 to 400 cm− 1 was averaged over 16 scans at 4 cm− 1 resolution. Thermogravimetric analysis (TGA) was performed on a Perkin Elmer thermobalance TGS-2. Sample amounts of ~ 15 mg were heated from room temperature to 1000 °C at heating rate of 10 °C/min in a synthetic air flow of 150 cm3/min. 3. Results and discussion All synthesized polyester hydrochlorides are viscous liquids at room temperature. The glass transition temperatures ranged from −25 to less than −60 °C. Molar masses determined for polyesters are shown in Table 2.

Table 2 Molar masses of polyesters Polyester

Mw (10− 4)

Mn (10− 4)

Mw/Mn

PesMM PesMSc PesMA PesMSb PesMMA PesOM PesOSc PesOA PesOSb PesOMA

1.62 1.87 2.53 1.84 2.58 2.34 3.40 1.60 1.89 2.64

1.01 1.15 1.97 1.19 1.84 1.17 1.78 1.10 1.05 1.70

1.60 1.63 1.90 1.55 1.40 2.00 1.91 1.45 1.80 1.55

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Fig. 1. 1H NMR spectra of PesMA.

The structures of ODEA, polyesters and polyester hydrochlorides were confirmed by 1H NMR spectroscopy. Spectra of MDEA and ODEA as well as their hydrochlorides (MDEA-HCl, ODEA-HCl) were used for correct assignation of polyester hydrochlorides. The 1H NMR spectrum of ODEA showed the following characteristic signals: 0.9 ppm (t, 3H, CH P 3-(CH2)6-CH2-), 1.2–1.5 ppm (m, 12H, CH3-(CH P 2)6-CH2-), 2.4 ppm (t, 2H, CH3-(CH P 2)6-CH2-N), 2.5 ppm (t,

4H, N-CH P 2- overlapping with DMSO), 3.4 ppm (t, 4H, -CH P 2-OH P ), 4.3 ppm (s, 2H,-OH P ). When HCl is bound to the nitrogen atom (ODMA-HCl), the signals of nearby protons are shifted towards lower field: 0.9 ppm (t, 3H, CH P 3-(CH2)6-CH2-), 1.2–1.8 ppm (m, 12H, CH3(CH P 2)6-CH2-), 3.0–3.5 ppm (m, 6H, N-CH P 2), 3.8 ppm (t, 4H, -CH P 2-OH), 5.3 ppm (s, 2H, -OH P ), 9.8 ppm (s, 1H, N-H P ). The 1H NMR spectrum of MDEA shows the following characteristic signals: 2.2 ppm (s, 3H, CH P 3-N), 2.4 ppm (t, 4H, N-CH P 2-), 3.4 ppm (q, 4H, -CH P 2-OH), 4.3 ppm (t, 2H, -OH P ). In MDEA-HCl the signal of the protons adjacent to the nitrogen atom was split. The signal of methyl group is a doublet at 2.8 ppm and the signal of methylene group is transformed into two multiplets at 3.0–3.4 ppm. Other signals are at the same position as in ODEA-HCl. Due to the relatively low molar mass of polyester hydrochlorides there are signals of both the main chain atoms and the end groups, which makes the 1H NMR spectra rather complex. The main signals + were assigned as: 4.5–4.4 ppm (N+-CH2-CH P 2-O-), 3.8 ppm (N -CH P 2CH2-OH end groups), 3.6–3.2 ppm (N+-CH -CH -, esters), 3.4–3.0 ppm 2 P2 + (N+-CH P 2-CH2-, end groups), 2.8 ppm (N -CH P 3), 2.4 ppm (O-CO-CH P 2-, or 3.5 ppm in the polyester prepared from malonyl chloride), 0.9– 1.8 ppm (inner -CH2- groups of the acid and octyl side chain), 0.9 ppm (-CH P 3 of the octyl side chain). There are also 3–4 signals of N+-H P at 9.8–12.2 ppm. The signal at 9.8 ppm was ascribed to the proton at the NH+ end groups, COOH end groups, inner NH+ groups and probably to NH+ group in cyclic esters which are frequently observed in polyesters synthesized from acid chlorides (Bradshaw and Thompson, 1978; Frensch and Vögtle, 1979; Huskić and Žigon, 2003). The NMR spectrum of PesMA is shown as an example in Fig. 1.

Table 3 Basal spacing of MMT modified with polyester hydrochlorides for ratio of MMT/ polyester hydrochloride 1: 3

Fig. 2. The influence of PesMA quantity on interlayer spacing of MMT: a) X-ray diffractograms, b) dependence of interlayer spacing on MMT/PesMA ratio.

Polyester hydrochloride

Basal spacing, nm

Polyester hydrochloride

Basal spacing, nm

PesMM PesMSc PesMA PesMSb PesMMA

1.38 1.39 1.40 1.42 1.41

PesOM PesOSc PesOA PesOSb PesOMA

1.73 1.75 1.78 1.80 1.83

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Fig. 3. X-ray diffractograms of MMT modified with polyester hydrochlorides based on adipic acid.

The cation exchange of MMT with polyester hydrochlorides increased basal spacing of MMT (Fig. 2a,b). The basal spacing of OMMT increases up to the ratio of 1:3, remaining constant at higher ratios. The values of basal spacings for MMT/polyester hydrochloride cation ratio 1:3 are presented in Table 3. The acid chain length practically did not affect the basal spacing which varies by at most 0.1 nm. Because of this, polyester hydrochloride based on adipic acid, PesMA, with a medium organic acid chain length, was chosen for TGA and FTIR analysis. The influence of the side chain is more significant, and the basal spacing increased from the methyl to octyl derivative by approximately 0.4 nm. (Fig. 3). Infrared spectra (Fig. 4) confirm the presence of PesMA in OMMT. Characteristic bands of PesMA at 1732 cm− 1 (CfO), 1462 cm− 1 and 1415 cm− 1 (C\H), 1398, 1375, 1253 (C\H, C\O) are present in all OMMT samples and become more pronounced with increased exchange ratio. Bands in range 1100–800 cm− 1 (not shown) characteristic of unmodified MMT do not change significantly during modification. Thermogravimetric and derivative thermogravimetric (DTG) curves are shown in Fig. 5, with curves for exchange ratios 1:2, 1:4 and 1:5 left out for clarity. Thermogravimetric curve for exchange ratio 1:2 falls between the curves for ratios 1:1 and 1:3, while curves for

Fig. 5. Thermal degradation of MMT modified with PesMA at various MMT/PesMA ratios: a) thermogravimetric curves, b) the first derivative of thermogravimetric curves.

ratios 1:4 and 1:5 almost completely overlap the curve for 1:3 indicating that cation exchange was completed at this ratio. The residue at 1000 °C is given in Table 4. The first minimum on the DTG curves (below 150 °C) is due to loss of adsorbed water. This loss is significantly lower for OMMTs, (Table 4), indicating the lower content of adsorbed water due to the reduced hydrophilicity. For unmodified MMT release of water had a maximum at 75 °C dehydroxylation of the aluminosilicate occurred at 500–700 °C. From OMMTs water was released between 60 and 100 °C. Although

Table 4 Thermogravimetric analysis of PesMA modified MMT in comparison with unmodified MMT: final weight residue at 1000 °C, w(1000 °C), weight content of adsorbed water, w (water), and mass loss due to degradation of organic phase at 200–500 °C, w(organic)

Fig. 4. FTIR spectra of MMT modified with PesMA at various MMT/PesMA ratios.

Exchange ratio MMT/PesMA

w(1000 °C)

w(water)

w(organic)

MMT 1:1 1:2 1:3 1:4 1:5

90.9% 82.1% 80.1% 76.3% 75.8% 75.4%

4.1% 2.6% 0.8% 0.5% 0.5% 0.4%

– 9% 11% 14% 14% 15%

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OMMT are considered to be hydrophobic water adsorption still occurs at the external surfaces and depends on environmental conditions, such as relative humidity. OMMT modified with 1:1 ratio still adsorbed significant amounts of water (Table 4), which is an indication of incomplete exchange of Na+ ions of MMT by the polycations. Between 200 and 500 °C, the organic constituent in the OMMT begins to decompose. All OMMTs decompose in a similar fashion: a series of overlapping events that produce a broad DTG peak between 300 and 500 °C. Dehydroxylation of aluminosilicate framework occurs from 500 to 700 °C. Degradation of residual organic components occurs between 700 and 1000 °C. Increasing the MMT/polyester hydrochloride ratio up to 1:3 increases the quantity of bound polyester as indicated by the enhanced basal spacing and. mass loss associated with degradation of the organic phase between 200 and 500 °C (Table 4). The mass loss increased significantly up to a ratio of 1:3, than increased only slightly, confirming that complete cation exchange was achieved at the ratio 1:3. Since the molar ratio MMT/hydrochloride must be at least 1:3 for complete cation exchange, it is likely that only a part of the polycation chains adhere to MMT. As HCl is removed from the non-bound segments of the polymer chain by rinsing OMMT with water, we presumme that these segments could improve the interaction of OMMT with polymers of medium and higher polarity, especially polyesters, polyurethanes or polyamides, leading to nanocomposites with improved properties compared to those prepared with OMMT modified with alkyl-amine salts. 4. Conclusions Polyester hydrochlorides are a new type of polycations, prepared from alkyl derivatives of diethanol amine and organic acid chlorides. They can be used for MMT modification by cation exchange. The length of the organic acid chain did not significantly influence the basal spacing of montmorillonite, but the length of the alkyl group attached to the nitrogen atom increased the basal spacing. For quantitative cation exchange, the molar ratio of cations in MMT and hydrochloride must be at least 1:3, indicating that only a part of polycation chains adhere to MMT while the remaining segments dangle out into the solution. Hydrophobicity and organic content of OMMT increased with increasing exchange ratio, reaching approximately constant levels for ratios above 1:3. Acknowledgments This study is a part of research Program P2-0145 supported by the Ministry of Higher Education, Science and Technology of Republic of Slovenia and Slovenian Research Agency; research project “Bioceramic, Polymer and Composite Nanostructured Materials” (1251252970-3005) supported by the Ministry of Science, Education and Sports of Republic of Croatia; and the bilateral Slovenian–Croatian

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