LAYERED DOUBLE HYDROXIDES

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Chapter 3 Properties of LDH/polymer micro- and nanocomposites

Carbonate- and stearate-intercalated layered double hydroxides were used as fillers to prepare polymer micro- and nanocomposites respectively. The stearate-modified LDH starting material was a bilayer intercalated clay. During melt compounding, excess stearates were released and the clay reverted to a monolayer-intercalated form. The exuded stearate acted as a lubricant, lowering the melt viscosity of the poly(ethylene-co-vinyl acetate) and linear low-density polyethylene matrices. Strong hydrogen bond interactions between the chains of poly(ethylene-co-vinyl alcohol) and the clay platelet surfaces overwhelmed the lubrication effect and caused an increase in the melt viscosity of this matrix. The notched Charpy impact strength of this composite was almost double that of the neat polymer. It appears that this can be attributed to the ability of the highly dispersed and randomly oriented nanosized clay platelets to promote extensive internal microcavitation during impact loading. The creation of a large internal surface area provided the requisite energy-dissipation mechanism.

PROPERTIES OF LDH/POLYMER AND NANOCOMPOSITES

POLYMER COMPOSITE

Polymer composites have attracted attention due to their unique structure and enhanced properties. IUPAC defines a composite as “a multicomponent material comprising multiple different (non-gaseous) phase domains in which at least one type of phase is a continuous phase” according to specifications (Work et al., 2004). A nanocomposite refers to every type of composite materials having one of the components in the nanometre size range at least in one dimension. Manias et al., (2007) defined a nanocomposite as a “fundamentally new material (hybrid) in which the nanometre scale component/structure gives rise to intrinsically new properties not present in the respective macroscopic composites or pure components”. New properties are envisaged to originate from the interaction of the polymer and filler at the interface. A polymer composite is made of three constituents, i.e. the matrix, the filler (LDH in this study) and the interfacial region. The interfacial region is a ‘communication bridge’ between the filler and matrix and is conventionally ascribed properties different from those of the bulk matrix because of its proximity to the surface of the filler (Vaia & Wagner, 2004). These authors further expound it in terms of the radius of gyration of the matrix (Rg), which is the key spatial parameter to which the majority of the polymer’s static and dynamic properties can be ultimately related and has a value in a few tens of nanometres. How then do nanocomposites differ from conventional composites? Vaia and Wagner (2004) cited six interrelated distinguishing qualities of polymer nanocomposites:

• Low percolation threshold (~0.1–2 vol %)
• Particle-particle correlation (orientation and position) arising at low volume fraction ( _c < 0.001)
• Large number density of particles per particle volume (10⁶–10⁸ particles/µm³) Extensive interfacial area per volume of particles (10³–10⁴ m²/mL)
• Short distances between particles (10–50 nm at ф ~ 1–8 vol %)

Comparable size scales among the rigid nanoparticle inclusions, distance between particles and the relaxation volume of polymer chains.
Due to the small aspect ratio of spherical particles, the first two points do not apply to them.
Additive/filler materials are used to reduce costs or enhance properties such as tensile strength and modulus of polymer matrices (Hancock, 1995). Different types of filler are used to obtain polymer nanocomposites based on dimensionality/geometry. These include zero-dimensional nanoparticles (inorganic nanoparticles), one-dimensional nanoparticles (carbon nanotubes), two-dimensional nanoparticles (clays and LDHs), and three-dimensional nanoparticles (polyhedral oligomeric silsesquioxanes). The filler employed in this study falls under the two-dimensional category. Table 3.1 is a summary of other nanostructured layered materials that can be used as additives in polymer matrices. The two-dimensional platelet fillers lead to a lamellar microstructure. Hence the polymer composites have found application for their barrier properties such as reduced gas and vapour permeability. Depending on the aspect ratio of the platelets, they may be used to improve mechanical properties.
The pioneering work done by Toyota into clay-based polymer nanocomposites increased interest and research into clay-based polymer composites, dating back to 1986 (Kawasumi 2004). Research has focused mostly on smectite-based polymer composites. Anionic clays such as layered double hydroxides (LDHs) are a potential alternative for the preparation of polymer composites. This can be achieved since LDHs possess a layered structure similar to that of layered silicates or cationic clays. It is well established that the dispersion of particles with high aspect ratios, e.g. fibres and platelets, in polymeric matrices improves the mechanical properties. However, good interfacial adhesion and a homogeneous dispersion are prerequisites (Pradhan et al., 2008). Nanostructured clays can improve a wide range of polymer properties and are therefore ideal for polymer-clay nanocomposite preparations. The resulting polymeric hybrids exhibit improved gas barrier properties, mechanical properties (Hsuesh & Chen, 2003; Wang et al., 2006), enhanced flame retardancy (Zammarano et al., 2005; Costa et al., 2005; Chen & Qu, 2003 & 2004; Zubitur et al., 2009), UV and photo-stability (Bocchini et al., 2008) or ease of photo prodegradability (Magagula et al., 2009), etc.

POLYMER COMPOSITE STRUCTURES

Organoclay dispersion within a polymer matrix gives rise to three possible structures, i.e. phase separated, intercalated and/or delaminated/exfoliated composites, as shown in Figure 3.1. These structures are usually probed by two complementary techniques, namely X-ray diffraction (XRD) and transmission electron microscopy (TEM). The former gives the degree of separation and the latter serves as a visual confirmation of the XRD analysis.

Phase separated composites

Phase separation results from the polymer chains failing to penetrate the interlayer space of the layered material. The composite retains the same properties as conventional microcomposites. Hence the d-spacing remains the same as that of the clay.

 Intercalated composites

In these composites the polymer chain(s) is intercalated within the interlayer of the LDH. They normally exhibit a well-ordered morphology with alternating inorganic and polymeric layers or periodically stacked layers. The composite is made of alternating polymer and inorganic layers. The resulting clay-polymer hybrid exhibits increased d-spacing.

Exfoliation/delamination composites

This structure describes a case where the LDH layers are completely and uniformly dispersed in a polymer matrix. It is identified by the absence of diffraction peaks or basal reflection. This observation is thought to be due to a large increase in the layer separation 8 nm or lack of ordering or registry (Alexandre & Dubois, 2000). In some instances intercalated and exfoliated structures may co-exist; this is illustrated by the broadening of primary diffraction peaks. To eliminate ambiguous conclusions, TEM is normally used to confirm the results obtained from XRD. Some studies report the exfoliation of surfactant-intercalated LDHs (Leroux et al., 2001; Khan and O’Hare, 2002; Fischer, 2003). In general, a higher degree of exfoliation/dispersion of LDHs has been observed in polar rather than in non-polar matrices. The preparation of polymer composites from polyolefins is difficult due to their low polarity. Hence they do not interact effectively with the LDHs. Dispersion of LDHs in non-polar matrices through melt compounding has been explored using maleic anhydride grafted polyethylene (PE-g-MA) as a compatibiliser (Costa et al., 2005). It is important to note that full exfoliation and full intercalation are seldom observed in nanocomposites.
The structures that arise are related to the types of interfacial interaction that are favoured between the polymer and the clay. Vaia and Giannelis (1997) proposed three main clay-polymer interactions: polymer-surface, polymer-surfactant and surfactant-surface. They concluded that to achieve complete clay sheet dispersion, a very favourable polymer-surface interaction was necessary (Vaia and Giannelis 1997; Fischer 2003). Therefore the properties displayed by the polymer composite result from these associations.

LDH-BASED POLYMER COMPOSITE PREPARATION

Polymer-clay composites are mainly prepared in three ways, namely in situ polymerisation (Moujahid et al., 2002; Lee & Im, 2007; Huang et al., 2011), solution-intercalation methods (Ramaraj et al., 2010) and melt-processing (Zammarano et al., 2006).

In situ polymerisation

This is the first and most widely used mode of preparation of clay-based nanocomposites. It has been adopted for the preparation of LDH-based nanocomposites. It combines the basic principles of intercalation of LDHs, namely co-precipitation, regeneration and intercalation via organic/inorganic pillared LDHs (ion exchange), as shown in Figure 3.2. In the case of pillared LDHs, the pillaring agent is chemically active and hence interacts with the polymer chain (Hseuh & Chen, 2003). Usually, the first step entails the intercalation of the monomers/ionomers into the LDH. Polymerisation is initiated by thermal or radiation treatment and is also facilitated by organic initiator and catalyst (Whilton et al., 1997). Recently, polymerisation has been reported to be initiated by microwave irradiation (Herreo et al., 2011). This type of polymerisation makes thermosetting polymer-nanocomposites possible, e.g. epoxy-organoclay nanocomposites.

SYNOPSIS 
ACKNOWLEDGEMENTS
PREFACE
LIST OF FIGURES
LIST OF TABLES
LIST OF ACRONYMS, ABBREVIATIONS AND DEFINITIONS
CHAPTER 1 INTRODUCTION
1.1 LAYERED DOUBLE HYDROXIDES
1.2 LDH-BASED POLYMER COMPOSITES
1.3 LDH/JOJOBAOIL SUSPENSIONS
1.4 RESEARCH OBJECTIVE
1.4.1 Methodology
1.5 REFERENCES
CHAPTER 2  LAYERED DOUBLE HYDROXIDES
2.1 WHAT IS A LAYERED DOUBLE HYDROXIDE? .
2.2 LDH PREPARATION ROUTES.
2.2.1 Co-precipitation
2.2.2 Urea hydrolysis
2.2.3 Sol-gel
2.2.4 Post-preparation techniques
2.2.5 Texture and morphology.
2.3 INTERCALATION
2.3.1 Intercalation methods
2.3.2 Orientation of intercalated fatty acids
2.4 CHARACTERISATION OF LDH ANDMODIFIED DERIVATIVES
2.5 EXPERIMENTAL
2.5.1 Materials
2.5.2 Preparation of organo-LDH
2.5.3 Characterisation
2.6 RESULTS AND DISCUSSION
2.6.1 Composition and morphology
2.6.2 X-ray diffraction analysis
2.6.3 Fourier transform infrared analysis (FTIR)
2.6.4 Thermal analysis
2.7 CONCLUSIONS
2.8 REFERENCES
CHAPTER 3 PROPERTIES OF LDH/POLYMER AND NANOCOMPOSITES 
3.1 POLYMER COMPOSITES
3.2 POLYMER COMPOSITE STRUCTURES
3.3 LDH-BASED POLYMER COMPOSITE PREPARATION
3.4 PROPERTIES OF LDH-BASED POLYMER NANOCOMPOSITES
3.5 EXPERIMENTAL
3.6 RESULTS AND DISCUSSION
3.7 CONCLUSION
3.8 REFERENCES
CHAPTER 4  ORGANO-LDH/OIL SUSPENSIONS
4.1 INTRODUCTION
4.2 RHEOLOGY
4.3 THICKENINGMECHANISM
4.4 COLLOIDAL DISPERSIONS.
4.5 EXPERIMENTAL
4.6 RESULTS AND DISCUSSION
4.7 CONCLUSION
4.8 REFERENCES
CHAPTER 5  CONCLUSION AND RECOMMENDATIONS
APPENDIX
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