Stearate intercalated layered double hydroxides: a comparison of methods

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Materials

The layered double hydroxide used in this study had the chemical composition Mg4Al2(OH)12CO3 . 3H2O. The mole ratio of Mg:Al was determined from XRF analysis as 2.01:1 (Appendix B). The LDH was received from the local manufacturer Chamotte Holdings (Pty) Ltd. The particle size distribution as determined by a Mastersizer 2000 (Malvern Instruments) was d(0,1): 0,694 µm; d(0,5): 5,062 µm and d(0,9): 23,925 µm. The theoretical anionic exchange capacity of this LDH-CO3 was calculated to be 213 meq/100 g. 25% m/m poly(vinyl sulfonate, sodium salt) solution in water (PVS, Mw 70 000, Sigma-Aldrich), stearic acid (C18H36O2, Merck, 85% pure) and ethanol (96% rectified, Dana Chemicals) were used. Reactions were carried out in deionized water. Deionized water, tap water or solutions of CaCl2.2H2O (UNIVAR®, Saarchem) in deionized water (20 volume equivalents) were used to dilute the reacted mixtures (1 volume equivalent).

Intercalation method

8 g of the 25% m/m sodium-PVS solution in water was added to 57 ml deionised water to obtain a solution consisting of approximately 2 g of PVS in 60 ml water. This solution was heated to 78 °C. 0,7 g stearic acid (SA) was dissolved in 30 ml ethanol and heated to approximately 60 °C. The SA solution was slowly added to the PVS solution and the temperature allowed to reach 78 °C. 0,34 g of the LDH powder was added to the solution and stirred for 1 h. Itoh et al. [9] found that stearate is capable of intercalating up to about 225% of the anionic exchange capacity (AEC) of the LDH and therefore a level of 200% SA in relation to the AEC was used. The reaction mixture will be referred to as the PVS-SA-LDH mixture.

Fourier Transform Infrared spectroscopy (FTIR)

A Perkin Elmer Spectrum RX I FT-IR System was used to scan the infrared transmittance through a KBr (Uvasol, potassium bromide, Merck) pellet 32 times at a resolution of 2 cm-1. The averaged spectrum was background-corrected using a pure KBr pellet run under similar conditions. The pellets were prepared with approximately 2 mg of sample and 100 mg of KBr. The 2 mg powder samples were obtained by diluting the cooled down reaction mixture with a 20- fold excess of deionised or tap water or CaCl2 solution, in order to release (flocculate) the insoluble reaction products, which were subsequently centrifuged off, washed again (to remove any unreacted PVS or salts), dried and ground.

Powder X-ray Diffraction (XRD)

The XRD analyses were performed on a Siemens D500 X-ray system equipped with a 2.2 kW Cu long fine focus tube, variable slit and secondary graphite monochromator (to eliminate Kβ radiation and reduce fluorescent radiation). The system is computer controlled using SIEMENS DIFFRAC Plus software. The goniometer was set to reflection mode. Samples were scanned from 1 to 35° 2θ with Cu Kα radiation (1,5418 Å) at a speed of 0.02° 2θ, with a recording time of 2 s per step and generator settings of 40 kV and 30 mA. Part of the reacted mixture was cast onto an aluminium sample holder and the solvent was allowed to evaporate. This was repeated a few times to obtain enough material to mask the aluminium. One would expect the intercalated LDH phases to be highly oriented within these holders and the interlayer distances to be preferentially enhanced [10].

XRD results

The X-ray diffractogram (Fig. 3-5) of the reaction product obtained in the absence of PVS showed the presence unreacted SA (39,20 Å, 13,17 Å, 4,12 Å and 3,11 Å), unreacted LDH-CO3 (7,60 Å, 3,79 Å and 2,57 Å) and bilayer intercalated LDH-SA (49,52 Å, 25,43 Å, 17,11 Å, 10,29 Å). The product obtained in the presence of PVS was amorphous (broad, low intensity reflections, Fig. 3-6). The reflections at 13,91 Å (and 6,94 Å, second order) are due to intercalation of a bilayer of PVS [1] and it correlates well with the value of 13,1 Å observed by Oriakhi et al. [1]. The reflection at 45,17 Å is probably the remnant of bilayer intercalated LDH-SA which was too stable to allow for exchange with PVS chains because of the high degree of interpenetration of the two stearate layers. There was only a small reflection left at 7,81 Å, indicating that most of the LDH-CO3 reacted. When stirring the LDH-CO3 and PVS together under the same conditions but in the absence of SA, the diffractogram of the LDH-CO3 was unchanged (not shown). This is due to the stability of the LDH-CO3 and the slow diffusion of the polymer chains [1]. The intercalated stearate anions destabilize the layered structure and allows for the PVS chains to intercalate.

SEM results

The FTIR results showed that sodium stearate had formed. It was originally thought that the gelling was caused by formation of sodium stearate within the PVS-SA-LDH mixture. When the microstructures of sodium stearate (Fig. 3-7A) and the clear part of the gel (Fig. 3-7B) are compared, this is clearly not the case. The latter shows a spaghetti-like structure approximately 200 nm in diameter and several micrometers in length. Fig. 3-7C is the SEM micrograph of the white part of the gel, which shows large plate-like structures of the LDH (> 10 µm diameter), which are similar to the structures obtained by Itoh et al. [9] for stearate intercalated LDHs. The SA and PVS stabilize this plate-like structure above the usual sand-rose – intergrown particulates – structure of LDH-CO3 (Fig. 3-7D) [13]. The SEM micrograph (Fig. 3-7E) of the freeze-dried suspension of the mixture diluted with deionized water consisted of large thin plate-like structures (> 20 µm diameter). When the diluted mixture was allowed to air dry, the plate-like structures seemed to role up into tubes of diameter 0,5 – 1 µm (Fig. 3-7F). When freeze-dried, the rolling up could not take place and the large, thin plates remained in their original state. Dispersing these plate-like structures, of which the thickness is in the sub-micrometre region, into polymer matrices could lead to the formation of nanocomposites

Film production procedure

Wu et al. [1] attempted to lower the WVP of starch-alginate films by using, amongst others, SA. Their recipe was followed (Table 4-2) except that lecithin (emulsifier) was not used. The LDH, Mg4Al2(OH)12CO3 . 3H2O, was used additionally to the recipe of Wu et al. [1]. The ratio of SA and LDH was varied to determine its effect on WVP and Young’s modulus. The SA and LDH together are referred to as the filler, which can consist of 100% SA, 100% LDH or any ratio in between. This filler content was kept constant at 16.56% m/m of the dried film, which is similar to the amount used by Wu et al. [1]. Films are referred to in terms of the SA content of the filler in the Figures and by annotations such as 60SA/40LDH for a film where the filler consists of 60% SA and 40% LDH by mass. CO2 is released during the intercalation of stearate into LDH and this necessitated the use of a defoamer.

Table of Contents :

• Acknowledgements
• Abstract
• Samevatting
• Scripture from Corinthians
• List of Figures
• List of Tables
• List of Equations
• Nomenclature and Abbreviations as used by the Author
• References
• Preface
• Chapter 1 Introduction
  • 1.1. The evolution of this project
  • 1.2. The aims of this research
  • References
• Chapter 2 Stearate intercalated layered double hydroxides: a comparison of methods
  • Abstract
  • 2.1. Introduction
  • 2.2. Experimental procedure
  • 2.2.1. Sample preparation
  • 2.2.2. Characterization
  • 2.2.2.1. Fourier Transform Infrared spectroscopy (FTIR)
  • 2.2.2.2. Thermogravimetry
  • 2.2.2.3. Powder X-Ray Diffraction (XRD)
  • 2.2.2.4. Scanning Electron Microscopy
  • 2.3. Results and discussion
  • 2.3.1. Results and discussion of the different intercalation methods
  • 2.3.1.1. Reference sample
  • 2.3.1.2. Ethanol and Ethanol-water methods
  • 2.3.1.3. Glycerol-water and Calcined-glycerol-water methods
  • 2.3.1.4. Water, Calcined-water, SDS-water and Calcined-SDS-water
  • methods
  • 2.3.1.5. Calcined-Na stearate-water method
  • 2.3.1.6. Carlino melt method
  • 2.3.2. The effect of reaction time
  • 2.3.3. Correlation between TG and XRD data
  • 2.3.4. Recommendations for future work
  • 2.4. Conclusions
  • References
• Chapter 3 Poly(vinyl sulfonate) intercalation into stearate intercalated layered double hydroxides
  • Abstract
  • 3.1. Introduction
  • 3.2. Materials and methods
  • 3.2.1. Materials
  • 3.2.2. Intercalation method
  • 3.2.3. Characterization
  • 3.2.3.1. Fourier Transform Infrared spectroscopy (FTIR)
  • 3.2.3.2. Powder X-Ray Diffraction (XRD)
  • 3.2.3.3. Scanning Electron Microscopy (SEM)
  • 3.2.3.4. Transmission Electron Microscopy (TEM)
  • 3.3. Results and Discussion
  • 3.3.1. General
  • 3.3.2. FTIR results
  • 3.3.3. XRD results
  • 3.3.4. SEM results
  • 3.3.5. TEM results
  • 3.4. Conclusions
  • References
• Chapter 4 Stearate intercalated layered double hydroxides: effect on the physical properties of dextrin-alginate films
  • Abstract
  • 4.1. Introduction
  • 4.2. Experimental
  • 4.2.1. Materials
  • 4.2.2. Film production procedure
  • 4.2.3. Characterization
  • 4.2.3.1. Fourier Transform Infrared spectroscopy (FTIR)
  • 4.2.3.2. X-Ray Diffraction (XRD)
  • 4.2.3.3. Scanning Electron Microscopy (SEM)
  • 4.2.3.4. Optical microscopy
  • 4.2.3.5. Water Vapour Permeability (WVP)
  • 4.2.3.6. Tensile properties
  • 4.3. Results and Discussion
  • 4.3.1. Fourier Transform Infrared spectroscopy (FTIR)
  • 4.3.2. Water Vapour Permeability (WVP)
  • 4.3.3. Scanning Electron Microscopy (SEM)
  • 4.3.4. Optical microscopy
  • 4.3.5. X-Ray Diffraction (XRD)
  • 4.3.6. Tensile properties
  • 4.3.7. XRD, tensile and WVP results revisited
  • 4.4. Conclusions
  • References
• Chapter 5 Conclusions and Recommendations
  • 5.1. Conclusions
  • 5.2. Recommendations for future research
  • References
  • Appendices
  • A. List of publications, patents and conference proceedings
  • B. XRF analysis of hydrotalcite used
  • C. XRD raw data for internal standard analyses
  • D. Water vapour permeability raw data and equations
  • E. Tensile data

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