RESEARCH NOTE pubs.acs.org/IECR
Mg Al Layered Double Hydroxides Intercalated with Polyetheramidoacids and Exhibiting a pH-Responsive Releasing Property Chih-Wei Chiu, Yi-Lin Liao, and Jiang-Jen Lin* Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan ABSTRACT: Mg Al layered double hydroxides (LDH) are prepared by coprecipitation of Mg(NO3)2 and Al(NO3)3 and subsequently incorporated with poly(oxypropylene)-bis-amidocarboxylic acid (POP). When characterized by X-ray diffraction, the interlayer spacing of the synthesized POP/LDH hybrids were found to be spatially expanded, with basal spacing ranging from 33 94 Å, proportionately correlated to the POP molecular weight of 400 4000 g/mol. With increasing POP molecular weight incorporation, a critical d-spacing is reached when the POP/LDH hybrids become hydrophobic. The organo-LDH units possessed an amphiphilic property and ability of dispersing in water, ethanol, and toluene. Furthermore, the hybrid was actually stable under basic conditions but structurally disintegrating at pH 2 through the release of POP organics into aqueous solution. The understanding of the mechanism involving basal spacing shrinkage and structure disintegration allows the future development of utilizing the POP-modified clay as pH-sensitive and self-destructive materials.
1. INTRODUCTION In recent years, there has been considerable interest in polymer/layered silicate nanocomposite applications because of their advanced properties such as thermal characteristics,1,2 mechanical properties,3,4 increased biodegradability of biodegradable polymers,5 increased strength and heat resistance,6 flammability,7 and decreased gas permeability.8,9 Most work has been performed on the cationic clays. For example, the cationic montmorillonite (MMT) and synthetic fluorinated mica (Mica) clays, consisting of sodium ions on the silicate surface (tSiO Na+), can be intercalated with organic onium salts including the quaternary alkyl ammonium (R4N+X )10,11 or alkyl phosphonium (R4P+X )12,13 salts. The alkyl ammonium quaternary cations of amino acid have resulted in an enlarged X-ray basal spacing in the range of 19 40 Å.14 Recently, we reported the uses of poly(oxypropylene) diamines (POP amine) for expanding the basal spacing of MMT and Mica up to 92 Å basal spacing.15,16 The polyamine salt modification could ultimately alter the hydrophilic clay to become have a hydrophobic property and be dispersible in organic mediums. Layered double hydroxide (LDH) is a family of anionic clays that are oppositely charged from the natural smectite aluminosilicate clays. With the presence of positive ion charges on the LDH surface, the synthesized LDH can be described by the ideal formula of [M2+1‑x M3+x (OH)2]An x/n 3 mH2O where the metal ions can be Mg2+, Ni2+, Cu2+, or Mn2+ for divalent and Al3+, Fe3+, or Cr3+ for trivalent cations, while A is an anion such as OH , F , Cl , Br , NO3 , CO32 , SO42 , etc.17 19 Partial M2+ to M3+ substitution induces positive charge for the layers and the counteranions in the interlayer galleries. These anions are exchangeable with organic substances including carboxylic acids,20 aniline sulfonic acid,21 anionic polymers,22 cyclodextrin,23 organic phosphoric acids,24 fullerene,25 and sulfonate,26 etc. Research efforts on using organically modified LDH for various applications such as DNA reservoirs,27,28 catalysts,29 nanocomposites,30 optical materials,31 and drug release32 were reported. r 2011 American Chemical Society
The series of anionic poly(oxypropylene)-bis-amidoacids (POP acid) that were derived from the reaction of POP amine and succinic anhydride or maleic anhydride were found to be effective for ionic exchange intercalation with both cationic MMT and anionic LDH.33,34 Here, we report that the same intercalated POP organics, both clays, were stimuliresponsive under varying pH values or self-destructive and organic releasing.
2. MATERIALS AND METHODS 2.1. Materials. Mg(NO3)2 3 6H2O, Al(NO3)3 3 9H2O, and NaOH were obtained from SHOWA Chemicals. Sodium montmorillonite (Na+-MMT), from Nanocor Co., is a Na+ form of smectite clay with a cationic exchange capacity (CEC) of 120 mequiv/100 g. The synthetic fluorinated mica (Na+-Mica) (trade name of SOMASIF ME-100) with a CEC 120 mequiv/ 100 g was obtained from CO-OP Chemical Co., Japan. A series of poly(oxyalkylene)-diamines (POA) including hydrophobic poly(oxypropylene)-diamines (POP) and hydrophilic poly(oxyethylene)-diamines (POE) of different molecular weights are commercial products from Huntsman Chemical Co. The POP amines of 400 Mw are backboned with low molecularweight POP segments and water-soluble, while their high Mw analogues such as 2000 and 4000 Mw are hydrophobic and water insoluble. These POP amines with 400, 2000, and 4000 Mw, are abbreviated as POP400, POP2000, and POP4000, respectively. POE2000 is a hydrophilic and water-soluble diamines with a POE middle block of 2000 Mw. Maleic anhydride (MA) Received: July 15, 2011 Accepted: December 18, 2011 Revised: December 9, 2011 Published: December 18, 2011 581
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Industrial & Engineering Chemistry Research Table 1. Basal Spacing and Property of POP POE 2MA-Na+ Intercalated LDH
RESEARCH NOTE
and
Scheme 1. Chemical Structures of Poly(oxyalkylene)bis-amidoacid as the Intercalating Agents
weight fraction (w/w)c
dispersion f
intercalating
d spacing
agenta
(Å)b
AECd
TGAe water ethanol toluene
none POP400 2MA-Na+
7.8 33
37/63
41/59 64/36
POP2000 2MA-Na+
65
87/13
90/10
+
+
POP4000 2MA-Na+
94
93/07
87/11
+
+
POE2000 2MA-Na+
13
87/13
72/28
+ +
+
+
+
a Equivalent ratio of POA Na+/AEC (300 mequiv/100 g LDH) = 300/ 300. b Basal spacing of X-ray diffraction calculated by Bragg equation (n = 1). c Weight fraction: organic/inorganic composition. d Calculated by anionic exchange capacity (AEC). e Measured by TGA. f +, dispersible; , aggregate.
was purchased from Aldrich Chemical Co. and purified by sublimation. 2.2. Preparation of Mg2Al-NO3-LDH. Mg2Al-NO3-LDH was prepared by a coprecipitation method according to previously reported procedures.35 A mixture of Mg(NO3)2 3 6H2O (201 g, 0.784 mol) and Al(NO3)3 3 9H2O (147 g, 0.392 mol) was dissolved in 500 mL of deionized water (at the Mg/Al molar ratio of 2.0). The aqueous solution was vigorously stirred at 60 °C under nitrogen purge in order to minimize contamination with atmospheric CO2, while maintaining the pH at 10 ( 0.2 by adding 2 N NaOH. The resulting suspension was isolated by filtration and washed thoroughly with deionized water until the filtrate pH = 7 ( 0.2. The X-ray powder diffraction pattern indicated a basal spacing of 7.8 Å. 2.3. Synthesis of Poly(oxyalkylene)-bis-amidoacids (POA) as Intercalating Agent. The adducts of poly(oxyalkylene)diamine and maleic anhydride at 1:2 molar ratio were prepared according to Scheme 1.33,34 The general procedures for preparing these POE- and POP-amidoacids are described below. To a 1 L three-necked and round-bottomed flask, equipped with a mechanical stirrer, nitrogen inlet outlet lines, and a thermometer, was placed maleic anhydride (15 g, 150 mmol) in THF (15 g), followed by POP- or POE-amines of 75 mmol. The mixtures were stirred vigorously and the temperature was maintained below 30 °C over 3 h. THF was removed under a reduced pressure at 60 °C. The product was obtained as a viscous lightcolored liquid. The progress of the reaction was monitored by taking analytical samples periodically and checking the maximal appearance of 1660 cm 1 for the amide formation and 1720 cm 1 for the carboxylic acid absorbance in FT-IR spectrum. In addition, the amine titrations indicated a complete consumption of the amine basicity during the reaction. The products of amine to maleic anhydride at 1:2 molar ratio or NH2/MA at 1:1 were prepared and abbreviated as POP400 2MA, POP2000 2MA, POP4000 2MA, and POE2000 2MA, for various POP and POE diamines. Their corresponding sodium salts were prepared by treating with equivalent amounts of 0.1 N NaOH. 2.4. Intercalation of LDH with POP and POE acid Salts. An example of ionic exchanging experiment for preparing POP/ LDH hybrid is followed. The prepared Mg 2 Al-NO 3 -LDH (1 wt %, 50 g of aqueous solution, anionic exchange capacity
(AEC) = 300 mequiv/100 g)36 38 was placed in a 100 mL roundbottomed flask and dispersed vigorously at 100 °C with reflux system. To the slurry was added the POP2000 2MA-Na+ (3.3 g, 1.5 mequiv) that was previously prepared by neutralizing the amidoacid with an equal equivalent of 0.1 N NaOH (15 mL, 1.5 mmol). The solution was then poured into the flask containing the swelled LDH slurry. The mixture was stirred for 8 h and then allowed to cool to room temperature. The resultant crude product was filtered through a filter paper (Whatman1 Qualitative Circles, 90 mm), washed thoroughly with an equal volume of water/ethanol mixture (300 mL for three times), collected the solid on a Petri dish, and dried under vacuum at 80 °C. 2.5. Intercalation of Layered Silicate by POA Amidoacids. An example of intercalation experiment for preparing POA/ layered silicate hybrid is followed. The prepared MMT (1 wt %, 50 g of aqueous solution, CEC = 120 mequiv/100 g) was placed in a 100 mL round-bottomed flask and dispersed vigorously at 100 °C with a reflux system. To the slurry was added the POP2000 2MA (1.3 g, 0.6 mequiv) which was mixed with equivalent weight of ethanol. The solution was then poured into the flask containing the swelled MMT slurry. The mixture was stirred for 3 h and then allowed to cool to room temperature. The resulting agglomerate was collected, washed with water/ethanol, and dried under vacuum at 80 °C. The preparation of POA/Mica hybrid is in the same manner. 2.6. Characterization. The interlayer basal spacing or d spacing was analyzed by using an X-ray powder diffractometer (Rigaku D/DAX-II B using a Cu target, λ = 1.5418 Å at 30 kV, 20 mA) with a scanning rate of 2°/min from 2θ = 2° to 14°. The organoclays generally exhibit a series of multiple peaks with a pattern following the Bragg’s equation. The value of d spacing for n = 1 was calculated from the observed values of n = 2, 3, 4, etc., according to the Bragg’s equation (nλ = 2d sin θ). The thermal stability was analyzed by using thermal gravimetric analyzer (TGA), on a Perkin-Elmer Pyris 1 model. The organic weight was estimated from the weight losses by ramping the temperature from 100 to 850 °C at the rate of 20 °C/min in air. Fourier transform infrared spectroscopy (FT-IR) was carried out using a Perkin-Elmer Spectrum One FT-IR spectrometer in the range of 400 4000 cm 1. Samples were prepared by dissolution of the 582
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phase in the galleries. As a result, although the hybrid was constituted of high POE organic content at 72 wt % POE, it demonstrated a hydrophilic character or forming stable dispersion in water and ethanol, but not in toluene. The differences between POP and POE acid salts in clay galleries are conceptually described in Scheme 2. The hydrophobic POP is suitable for altering the LDH in layer spacing and solvent dispersion. 3.2. Critical Conformation Change of POP Intercalation to LDH. It is noteworthy that the incorporated POP may generate a hydrophobic phase to support the interlayer distance in a critical manner.39 The generation of a critical phase separation is elucidated by determining the intercalation profile. We first investigated the representative POP2000 2MA-Na+ agents for ionic exchange reaction with LDH. The water-insoluble POP2000 2MA precursor was treated with NaOH to generate the soluble form of POP2000 2MA-Na+ as the anionic species. The change in solubility by ionization actually implied the amphiphilic nature for the hydrophobic POP backbone in the POP-acid structure. With the control equivalent ratio of POP salt to LDH anionic exchange capacity (AEC = 300 mequiv/100 g), a series of experiments were described in Table 2. At 1.0 equiv, the intercalation of POP2000 salt afforded the hybrid with a 65 Å basal spacing (XRD) and high organic content of 84 wt %, near the theoretically calculated 77 wt % based on the AEC value. The experiments with a different equivalent ratio of POP/LDH from 50/ 300 to 600/300 revealed the actual intercalation profile and the correlation between the POP amount and clay interlayer spacing. The basal spacing from 7.8 to 31 Å was observed throughout the POP addition from 50/300 to 150/300 AEC equivalents. Although the LDH was actually incorporated with an incremental amount of the organics, as shown by TGA from 41 to 76 wt %, the d spacing remained the same distance. As the addition was beyond 200/300 AEC equivalents, there was observed a sudden increase of d spacing to 59 Å. The basal space enlargement is expressed by plotting the basal spacing against the intercalant as illustrated in Figure 2. There was a critical transition in which the LDH basal spacing increases sharply from 31 to 59 Å and then to 65 Å. It appears to have three stages for the clay gallery saturation, 50 100/300, 150/300, and g200/300 of POP equiv to the AEC. In the first stage, the POP2000 2MA-Na+ molecules were adsorbed on outside LDH, followed by a random-coil shape which existed in the confined spacing of the LDH. In the third stage, the transformation of the organics from coil into a stretched array occurred, and the basal spacing reached a maximum. A conceptual representation of the conformation change with respect to the relative concentration in the gallery is described in Scheme 2. 3.3. Comparison between Hydrophobic POP2000 and Hydrophilic POE2000 /LDH in Interfacial Tension Measurements. The amphiphilic nature of two classes of POP and POE intercalating agents is investigated by calculating their toluene/water interfacial surface activity. Since the POP/LDH hybrids consist of POP hydrophobic embedment and ionic clay in a multilayer structure, they are amphiphilic in nature and expected to show similar property as a surfactant in solutions. The concept of hydrophilic/hydrophobic balance in the hybrid structures is demonstrated by a comparison of the abilities of the POP2000 and POE2000-intercalated LDH hybrids to reduce the toluene/water interfacial tension. As shown in Figure 3, the POP/LDH at a concentration of 0.01 wt % is capable of lowering the interfacial tension from 36
Figure 1. X-ray diffraction patterns of (a) pristine LDH, (b) POP400 acid/LDH, (c) POP2000 acid/LDH, (d) POP4000 acid/LDH, and (e) POE2000 acid/LDH.
organoclays in THF followed by evaporation of the solvent to a thin film on a KBr plate. The interfacial tension was measured by the Wilhelmy method with a Kruss-K10 digital tensiometer equipped with a spherical ring. The critical micelle concentration (CMC) values were the transition points obtained by the extrapolation of the surfactant concentrations versus the surface tension measurements.
3. RESULTS AND DISCUSSION 3.1. Intercalation of LDH with POP and POE Amidoacid Salts. The anionic exchange reaction of LDH with the POP
amidoacid sodium salts (POP 2MA-Na+) was performed. The amidoacid intercalating agents were prepared from POP diamine and 2 equiv of maleic anhydride (MA) according to the previously reported procedures.33 As summarized in Table 1, three different POP acids with molecular weights of 400, 2000, and 4000 g/mol were prepared for the LDH intercalation to afford the POP/LDH hybrids with the basal spacing at 33, 65, and 94 Å, respectively. The basal spacing increased proportionally to the POP molecular length. Furthermore, their XRD patterns generally exhibited a series of Bragg peaks from n = 2 to 6 or 7 when using the high Mw POP intercalating agents such as POP2000 and POP4000 2MA-Na+ (Figure 1), indicating a highly ordered gallery structure. The organoclays generally exhibit a series of multiple peaks with a pattern following the Bragg’s equation. The value of d spacing for n = 1 was calculated from the observed values of n = 2, 3, 4, 5, 6, 7, etc., according to the Bragg’s equation (nλ = 2d sin θ). With a high POP content in the gallery, the LDH hybrids became hydrophobic; for example, the POP2000 and POP4000 2MA-Na+/LDHs are organophilic and dispersible in organic solvents such as ethanol and toluene. In contrast, the low Mw of POP400 derived hybrid was still hydrophilic and dispersible in water and polar organic mediums such as ethanol. These dispersion behaviors indicate that the incorporated organics largely affect the pristine LDH hydrophilic and swelling property. For comparison, the hydrophilic poly(oxyethylene)backbonded POE2000 2MA-Na+, an analogue to POP2000 2MA-Na+, proceeded with intercalation to afford a hybrid of only 13 Å d-spacing due to the lack of generating hydrophobic 583
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Scheme 2. Two Different Modes of LDH Intercalation by Hydrophilic POE2000- and Hydrophobic POP2000-Derived Intercalating Agents and the LDH Gallery Expansion at Different Equivalent Ratios
Table 2. Basal Spacing Expansion by Various Equivalents of POP2000 2MA-Na+ Intercalation weight fraction (w/w)c
dispersion f
POP2000 2MA-Na+/LDH
d spacing
(equivalent ratio)a
(Å)b
AECd
TGAe
water ethanol toluene
0/300
7.8
--
41/59
+
50/300 100/300
7.8 7.8
35/65 52/48
62/38 71/29
+ +
150/300
31
62/38
76/24
+
+
200/300
59
69/31
79/21
+
+
250/300
67
73/27
82/18
+
+
300/300
65
77/23
84/16
+
+
600/300
63
87/13
86/14
+
+
Figure 2. Critical increase in basal spacing under the increasing amount of POP2000 acid intercalation to LDH.
a
Equivalent ratio of POA 2Na+/AEC (300 mequiv/100 g LDH). Basal spacing of X-ray diffraction calculated by Bragg equation (n = 1). c Weight fraction: organic/inorganic composition. d Calculated by anionic exchange capacity (AEC). e Measured by TGA. f +, dispersible; , aggregate. b
demonstrated a high structural stability in the range of pH 7 to 2, with the maintenance of its basal spacing, as shown in Figure 4. Under the treatment of HCl solution reaching pH of 2, the hybrid maintained a constant 60 Å d spacing. However, at pH = 1, the hybrid dropped the d spacing to 7.8 Å and then disintegrated into Mg2+ and Al3+ salts which were dissolved in water as the pH reached zero. For the cationic clays, the POP2000 2MA intercalated MMT and Mica hybrids, their layered basal spacing and the embedded organics were considerably stable until pH 2. At pH = 0, the basal spacing still remained at 17.7 Å for MMT and 26.6 Å for Mica, implying a release of POP organics from the layered embedment. Hence, it was observed that by controlling the pH values the intercalating organic salts could be released from all types of clays but by
to 4.5 mN/m. It implies that the hybrid has a strong tendency to accumulate in the toluene/water interface. In contrast, the POE2000 intercalation incorporates a hydrophilic POE backbone that makes the LDH too hydrophilic to interact with the toluene phase. 3.4. Property of pH-Sensitive and Structural Instability. The structural stability of the synthesized LDH was found to be sensitive to the environmental pH value. When treating the LDH with an acidic solution, the dissolution of the LDH plates did occur. The representative POP2000/LDH hybrid 584
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aqueous medium. The pH-controllable LDH hybrid could be developed for use as drug delivery devices through the mechanism of basal spacing shrinkage and structure disintegration.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: +886-2-3366-5312. Fax: +886-2-8369-1384. E-mail: jianglin@ ntu.edu.tw.
’ ACKNOWLEDGMENT We acknowledge financial supports from the Ministry of Economic Affairs and the National Science Council (NSC) of Taiwan. ’ REFERENCES (1) Pavlidou, S.; Papaspyrides, C. D. A Review on Polymer-Layered Silicate Nanocomposites. Prog. Polym. Sci. 2008, 33, 1119. (2) Kumar, A. P.; Depan, D.; Tomer, N. S.; Singh, R. P. Nanoscale Particles for Polymer Degradation and Stabilization-Trends and Future Perspectives. Prog. Polym. Sci. 2009, 34, 479. (3) Kang, S. G.; Hong, J. H.; Kim, C. K. Morphology and Mechanical Properties of Nanocomposites Fabricated from Organoclays and a Novolac Phenolic Resin via Melt Mixing. Ind. Eng. Chem. Res. 2010, 49, 11954. (4) Schaefer, D. W.; Justice, R. S. How Nano Are Nanocomposites. Macromolecules 2007, 40, 8501. (5) Bordes, P.; Pollet, E.; Averous, L. Nano-biocomposites: Biodegradable Polyester/Nanoclay Systems. Prog. Polym. Sci. 2009, 34, 125. (6) Chiu, C. W.; Cheng, W. T.; Wang, Y. P.; Lin, J. J. Fine Dispersion of Hydrophobic Silicate Platelets in Anhydride-Cured Epoxy Nanocomposites. Ind. Eng. Chem. Res. 2007, 46, 7384. (7) Kiliaris, P.; Papaspyrides, C. D. Polymer/Layered Silicate (Clay) Nanocomposites: An Overview of Flame Retardancy. Prog. Polym. Sci. 2010, 35, 902. (8) Ray, S. S.; Okamoto, M. Polymer/Layered Silicate Nanocomposites: A Review from Preparation to Processing. Prog. Polym. Sci. 2003, 28, 1539. (9) Chiu, C. W.; Lin, J. J. Self-Assembly Behavior of PolymerAssisted Clays. Prog. Polym. Sci. 2011, published online July 29, 2011. DOI: 10.1016/j.progpolymsci.2011.07.007. (10) Chiu, C. W.; Chu, C. C.; Cheng, W. T.; Lin, J. J. Exfoliation of Smectite Clays by Branched Polyamines Consisting of Multiple Ionic Sites. Eur. Polym. J. 2008, 44, 628. (11) Chiu, C. W.; Chu, C. C.; Dai, S. A.; Lin, J. J. Self-Piling Silicate Rods and Dendrites from High Aspect-Ratio Clay Platelets. J. Phys. Chem. C 2008, 112, 17940. (12) Imai, Y.; Nishimura, S.; Abe, E.; Tateyama, H.; Abiko, A.; Yamaguchi, A; Aoyama, T.; Taguchi, H. High-Modulus Poly(ethyleneterephthalate)/Expandable Fluorine Mica Nanocomposites with a Novel Reactive Compatibilizer. Chem. Mater. 2002, 14, 477. (13) Xie, W.; Xie, R.; Pan, W. P.; Hunter, D.; Koene, B.; Tan, L. S.; Vaia, R. Thermal Stability of Quaternary Phosphonium Modified Montmorillonites. Chem. Mater. 2002, 14, 4837. (14) Usuki, A.; Kawasumi, M.; Kojima, Y.; Okada, A.; Kurauchi, T.; Kamigaito, O. Swelling Behavior of Montmorillonite Cation Exchanged for Amino Acids by Caprolactam. J. Mater. Res. 1993, 8, 1174. (15) Lin, J. J.; Cheng, I. J.; Wang, R. C.; Lee, R. J. Tailoring Basal Spacings of Montmorillonite by Poly(oxyalkylene)diamine Intercalation. Macromolecules 2001, 34, 8832. (16) Lin, J. J.; Chen, Y. M. Amphiphilic Properties of Poly(oxyalkylene)amine-Intercalated Smectite Aluminosilicates. Langmuir 2004, 20, 4261. (17) Rives, V.; Ulibarri, M. A. Layered Double Hydroxides (LDH) Intercalated with Metal Coordination Compounds and Oxometalates. Coord. Chem. Rev. 1999, 181, 61.
Figure 3. Interfacial tensions of POP2000 acid/LDH and POE2000 acid/ LDH in toluene/water.
Figure 4. Relative shrinkage and disruption of the layered structure of LDH, MMT, and Mica with intercalated POP organics.
different manners. For the LDH, the process is particularly labile, disintegrating the layered structure.
4. CONCLUSION The use of POP2000 2MA sodium salts, prepared from POP diamine and 2 equiv of maleic anhydride, allowed the anionic exchanging intercalation of the organics into the LDH galleries. The incorporation of hydrophobic POP of different Mw enlarged the LDH layered structure with the d spacing in the range of 33 94 Å, and also altered the LDH dispersion in organic mediums such as toluene. By comparison, the hydrophilic POE intercalation failed to generate the interlayer space enlargement but did render the material dispersible in water. Their differences in amphiphilic nature were demonstrated by the ability of lowering the toluene/water interfacial tension. In contrast to cationic MMT and Mica clays, the POP-modified LDH is pH-sensitive and self-destructive. At a low pH environment, the POP salts are releasable from the clay units first and then the layered structure eventually disintegrates into the 585
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