| Summary: | 碩士 === 國立成功大學 === 化學工程學系 === 102 === SUMMARY
The preparation and the investigation of suspensions of layered double hydroxid in poly(N-vinyl pyrrolidone) (PVP) and poly (vinyl pyrrolidone-co-acrylic acid) (PVP-co-PAA) have been studied in detail.
Mg-Al layered double hydroxides (Mg-Al LDH) and organically modified LDH using sodium dodecyl sulfate (SDS-LDH) were synthesized by co-precipitation method. The structure properties have been characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Transmission electron microscope (TEM).
The effects of interactions between LDH particles and polymers on viscous behavior were studied for suspensions of LDH particles with different concentration. For both particles (LDH and SDS-LDH), the suspensions show an increase in viscosity as compare to the polymer solutions.
The suspensions of LDH/PVP-co-PAA are flocculated by bridging and charge neutralization mechanisms, while depletion flocculation occurs in the suspensions of LDH/PVP. Furthermore, we have concluded that hydrophobic interactions dominate the increment of viscosity in both SDS-LDH/PVP and SDS-LDH/PVP-co-PAA systems.
Key words: Suspensions of LDH/polymer, viscosity.
INTRODUCTION
We live through the courtesy of plants grown on clayey soil; we eat our food from ceramic dinnerware; we live in buildings partly made of clay bricks which rest on clay-containing soils into which they sometimes tend to disappear.
Layered double hydroxides (LDHs), also known as hydrotalcite clays, are also laminar minerals with anions sandwiched between hydroxide layers. Contrary to cationic clay widely discovering in nature, LDHs can be synthesized in the laboratory; synthetic LDHs have better-defined geometry and crystal structure than their natural counterparts. LDH have the general formula [M1-x2+Mx3+(OH)2]x+[Ax/nn- • mH2O]x- , where M2+ and M3+ are di- and tri-valent metallic cations and An- represent the anions in the interlayer space.
In almost every field of clay study, one has to deal at one time or another with dispersions of clay in water or in another fluid (e.g., polymer solution).Such dispersions, which are characterized by large interfacial area between the extremely small clay particles and the surrounding liquid, are colloidal systems.
Colloid chemist looks primarily at a system on a microscopic scale.
We are well aware that the bulk properties of the dispersion of clay system depend on the concentration as well as on the type of clay (e.g., anionic clay, LDH in our research); but above of all, we are familiar with─and often puzzled by─the remarkably strong dependence of these properties on the composition of the fluid phase. Comparatively minor change in the composition of the liquid often have surprisingly large effect on the behavior of the system.
The effect of small amount of dissolved chemicals in the clay system are governed by the rules of colloid chemistry because the size of particles involved usually under micrometer or even smaller.
MATERIALS AND METHODS
Synthesis of LDH platelets
SDS-LDH particles were synthesized by co-precipitation method with DS- as the interlayer anions. SDS (3.46g) was dissolved in 20 mL boiled deionized water in a three-neck flask. Magnesium nitrate (5.28g) and aluminum nitrate (3.86g) were dissolved in 20 mL boiled deionized water. The nitrate solution was then slowly dropped into the vigorously stirred SDS solution and the pH of the reaction solution was maintained at 10 by adding 2 M NaOH solution. The reaction was proceeded about 1 day with nitrogen purging.
Preparation of suspensions for viscosity study
1%, 3%, and 5% LDH or SDS-LDH powders were add to 1%, 2%, and 3% PVP or PVP-co-PAA directly. The unit of percentage was weight percentage concentration.
RESULT and DISCUSSION
Characterization LDH and its modified forms.
The Mg-Al-LDH and SDS-LDH powders were firstly characterized by XRD. The XRD patterns are shown in Fig. 1 and Fig. 2, the basal spacing was calculated according to the Bragg diffraction law. The sharps peaks observed at 2θ=10.2° and 3.4° correspond to the d003 peak with the basal spacing of 0.87 nm and 2.6 nm respectively.
FTIR for pristine LDH and SDS-LDH were recorded for wave numbers 400-4000 cm-1. The broad band around 3500 cm-1 is due to the O-H stretching vibration of metal hydroxide layer and interlayer water molecules. The bands locate at 1220 cm-1 and 1060 cm-1 are symmetric and asymmetric S=O vibration respectly. The absorbance present around 2920 cm-1 and 2845 cm-1 are caused by stretching vibration of CH2. The FTIR result are shown in Fig. 3.
Viscosity study of LDH/polymer suspensions
Rheological measurements were carried out on Brookfield LVDV-Ⅲcone-plate rheometer. The viscos behavior were investigated by plot of apparent viscosity (cP) vs. shear rate (1/s) .we have observed conspicuous shear thinning behavior of LDH/water suspension. These results due to the breakdown of the agglomerated structure when larger shear stress was applied.
We have found that the suspensions of LDH/PVP show an increase in viscosity as compare to the PVP solutions result from depletion flocculation in Fig. 4. When PVP is added to the colloidal suspensions, because the depletion layers are impenetrable to the polymer, the polymer concentration varies from zero (around the depletion layer) to the value of the bulk polymer, there is a polymer concentration gradient resulting in unbalanced osmotic pressure which pushes the LDH plates together.
Because attractive force induce by depletion are stronger than van der waal force in LDH/water suspensions, adding PVP result in gently decrease of viscosity with increasing shear rate imply shear thinning behavior is much more unobvious.
As shown in Fig. 5, the suspensions of LDH/PVP-co-PAA show an increase in viscosity as compare to the PVP-co-PAA solutions. It’s thought to be flocculate by mechanism of bridging or charge neutralization flocculation. As adsorption of PVP-co-PAA at surface of LDH by electrostatic interaction between negative segment of polymer and positive LDH platelet, LDH particles were push together via each end of PVP-co-PAA. In addition, the adsorption of a anionic PVP-co-PAA on a positive particle would reduce the surface charge of later, and this charge neutralization could be an important factor in flocculating the particles.
In Fig. 6 and Fig. 7, we have found that elevation of viscosity in both of SDS-LDH/PVP and SDS-LDH/PVP-co-PAA suspensions and concluded that hydrophobic interactions dominate the increment of viscosity in both systems.
CONCULSION
The LDH-surfactant hybrid synthesized by co-precipitation method and characterized by XRD and FTIR. The modified Mg-Al-LDH show an increase in interlayer distance as compared to the unmodified Mg-Al-LDH.
Depletion flocculation occurs in the suspensions of LDH/PVP resulting in viscosity increment. Increase of viscosity in LDH/PVP-co-PAA systems is result from bridging and charge neutralization flocculatation.Finally, elevation of viscosity in S-LDH/PVP and S-LDH/PVP-co-PAA system are both due to hydrophobic interactions between S-LDH particles and alkyl chain of polymer.
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