4.2.1 Direct synthesis

Direct synthesis by co-precipitation is the most common method used to prepare LDHs with inorganic anions and it can be used to synthesize hybrid LDHs. This method consists of the addition of the desired anion to a solution containing metal salts of the ions that will form the layers (usually chloride or nitrate salts). Control of the pH prevents co-precipitation of other phases, such as oxide impurities from the metals. To avoid the incorporation of metal salt anions used in the synthesis, it is important that the organic anions have a good affinity for the hydroxide layers. The main advantages of this method are the control of the charge density of the layers (M^2 + to M^3 + ratio) and the high purity of the synthesized LDHs.

As can be seen from Table 4.1, a wide variety of surfactant anions have been used in this method, leading to variable results in terms of structural modification efficiency. For instance, in the case of MgAl–LDHs, the best results concerning the interlayer distance have been found for hybrids with stearate anions (Liu et al., 2008) (3.37 nm interlayer distance), dodecylbenzenesulfonate (Zammarano et al., 2006; Wang et al., 2009) (interlayer distances from 2.76 to 3.15 nm) and dodecyl sulfate (Du et al., 2007; Kuila et al., 2007; Zhang et al., 2007; Martínez-Gallegos et al., 2008; Bao et al, 2006, 2008b; Liu et al, 2007, 2008; Kotal et al., 2009) (interlayer distances between 2.59 and 2.75 nm). In contrast, no significant effects were observed when using borate-like anions, dimethyl 5-sulfoisophthalate, benzoate, or adipic acid, with interlayer distances below 1.6 nm.

With ZnAl–LDH, significant increases of the interlayer distance were observed using oleate (Manzi-Nshuti et al., 2009b) (3.96 nm), 4-(12-(methacryloylamino) dodecanoylamino) benzenesulfonate (Illaik et al., 2008; Leroux et al., 2009) (3.75 nm), and dodecyl sulfate (Zhang et al., 2008a; Chen et al., 2004b) (between 2.54 and 2.85 nm). Borate (Nyambo et al., 2009a) and benzoate (He et al., 2006) anions produced the least remarkable results of all works presented in Table 4.1.

4.2.2 Anionic exchange

This procedure is very simple and consists of the dispersion of an LDH precursor in a solution containing an excess of the organic anions to be incorporated. Prior to choosing the LDH precursor, it is important to take into account the affinity of the anions with the LDH structure. Hence, the biggest difficulty lies in the carbonate anions, due to the high affinity of the layers for small divalent anions, followed by sulfate anions. On the other hand, among the most frequent monovalent anions present in LDHs, hydroxide shows the highest interchange difficulty, followed by chloride, fluoride, bromine, nitrate, and iodide (Miyata and Kumura, 1973).

This procedure was first reported by Miyata and Kumura (1973) and used to prepare an LDH hybrid with acetate anions. Meyn et al. (1990) also used ionic exchange to prepare hybrids using different organic anions.

Since then, many researchers have prepared hybrids with a wide variety of organic molecules when making LDH polymer nanocomposites, as shown in Table 4.2.

4.2.3 Hydration and reconstitution from oxides

Miyata (1980) reported that layered double hydroxides, after being transformed to oxides by calcination at temperatures between 500 and 800 °C, are able to rehydrate to their original form in the presence of anions and water, i.e., that LDHs are able to regenerate after calcination. This ability, known as a ‘memory effect,’ may be used to prepare hybrids using different anions in a rather easy way.

A wide variety of LDHs have been prepared with inorganic anions such as carbonates (Hibino and Tsunashima, 1998), organic-like naphthalene carboxylates (Tagaya et al., 1993), or carboxylates (Dimotakis and Pinnavaia, 1990) using this method. Regardless of the preparation method, the main problem when preparing high-purity hybrids lies in their high affinity for carbonates, which requires specific conditions in order to avoid the presence of this anion. For this reason, it is essential to use water free of carbonates (bi-distilled water is normally used) and an inert atmosphere (nitrogen or other inert gases are conventionally used).

As shown in Table 4.3, this method has been also used for the preparation of hybrids with a wide variety of organic molecules.

There are a few publications that compare the procedures described previously or in which a combination of more than one method is used to prepare a specific hybrid. One of the groundbreaking studies was Ulibarri et al. (1994), which compared anionic exchange and reconstitution after calcination for preparing LDH hybrids using vanadate anions. Trujillano et al. (2002) compared anionic exchange and direct synthesis for preparing nickel and aluminum hybrids with alkyl sulfate or sulfonate anions. Finally, a recent work (Dupin et al., 2004) compared the three methods previously mentioned (anionic exchange, direct synthesis, and reconstitution after calcination) in the preparation of magnesium and aluminum LDHs using dichlorophenyl amino phenyl acetate.

4.3.1 In situ polymerization

With in situ polymerization, the LDH hybrid is first mixed with a solution of the monomer or with the liquid monomer to allow intercalation into the interlayer gallery. Second, a catalyst is added to the solution and the monomer is polymerized in situ. The polymerization of the monomer in the interlayer gallery leads to rupture of the lamellar structure and promotes the homogeneous dispersion of the layers (see Fig. 4.3).

This method was the first reported in the literature to prepare a polymer-clay nanocomposite (Okada et al., 1990). It is conventionally used to prepare nanocomposites based on polar polymers or thermoset resins. In the latter, the resins are cured in the presence of the layered particles. Nonetheless, it is not the usual method for preparing nanocomposites based on commodity thermoplastics such as polyolefins.

The main advantage of this method is that it promotes high exfoliation of the layered particles. However, it also has drawbacks, as it is a somewhat complex process and is rather industrially unfeasible for commodity polymers.

This method has been used successfully to prepare LDH nanocomposites based on PA6.6 (Herrero et al., 2010; Zhu et al., 2008), poly(ethylene terephthalate) (Lee and Im, 2007; Martínez-Gallegos et al., 2008), polystyrene (PS) (Leroux et al., 2005, 2010; Manzi-Nshuti et al., 2008b, 2009b; Qiu and Qu, 2006; Ding and Qu, 2006; Marangoni et al., 2008; Illaik et al., 2008; Taviot-Gueho et al., 2007; Matusinovic et al., 2009), poly(methyl methacrylate) (Nyambo et al., 2008; Wang et al., 2005, 2006, 2009; Chen et al., 2004a; Chen and Qu, 2005; Ding et al., 2008), poly(vinyl chloride) (Bao et al., 2006, 2008b), epoxy (Hsueh and Chen, 2003a; Zammarano et al., 2005; Tseng et al., 2007; Tsai et al., 2008; Chan et al., 2008; Lv et al., 2009a), and polyimide (Hsueh and Chen, 2003b), among others.

4.3.2 Melt-mixing

The melt-mixing preparation of polymer-LDH nanocomposites consists of the dispersion of a LDH hybrid in a polymer matrix by applying high local shear stresses and using rather high temperatures in a melt-mixer dispositive, such as a twin-screw extruder or internal mixer (Fig. 4.4).

With this procedure it is possible to prepare a wide range of nanocomposites with intercalated or partially exfoliated structures, depending on the degree of intercalation of the polymer molecules in the interlayer gallery of the layered particles. It also has important advantages over other methods. First of all, from an environmental point of view, it does not require the use of organic solvents, and second, it is compatible with common plastic-processing technologies such as extrusion or injection.

The first report of this method to prepare polymer nanocomposites with cationic clays was by Giannelis (1996). Since then, it has been widely used due to its versatility. Gopakumar et al. (2002) suggest that, in the case of non-polar polymers, in order to attain good dispersion and partial exfoliation of the platelets, it is necessary, besides prior organophilization of the layered particles, to add a third component to act as a compatibilizer between the polymer and the particles. This compatibilizer needs polar groups, which should interact with the surface of the inorganic particles, promoting, alongside the high shear stresses generated during mixing, their exfoliation. In the case of polypropylene (PP), the most commonly used compatibilizer is a polypropylene copolymer with grafted groups of maleic anhydride (PP-g-MAH).

One of the first studies published describing melt-mixing was the work of Costa et al. (2005), which described the preparation and characterization of nanocomposites of low-density polyethylene (LDPE) modified with a polyethylene copolymer with methacrylic acid and hydrotalcite hybrids, prepared using dodecylbenzene sulfonate anions. Though total exfoliation of the LDH layers in the polymer was not achieved, significant changes were observed in its viscoelastic response compared with the pure polymer, even at very low LDH concentrations. It is the most widely used method nowadays. Alongside the polymers used to prepare LDH nanocomposites by means of in situ polymerization, melt-mixing has been employed with different results for many more polymers. For instance, different nanocomposites have been prepared with PA6 (Du et al., 2007; Zammarano et al.. 2006), ethylene vinyl acetate (EVA) copolymer (Du et al., 2006; Du and Qu, 2007; Zhang et al., 2007, 2008a; Nyambo and Wilkie, 2009; Nyambo et al., 2009b), poly(ε-caprolactone) (Pucciariello et al., 2007), poly(ethylene terephthalate) (Lee et al., 2006), poly(methyl methacrylate) (Manzi-Nshuti et al., 2008b; Nyambo et al., 2008, 2009a; Wang et al., 2009), poly(vinyl chloride) (Bao et al.. 2006; Chen et al.. 2010; Xu et al.. 2006), polypropylene (Ardanuy et al., 2008, 2010; Lonkar et al., 2009; Zhang et al., 2008b; Wang et al., 2010; Ding and Qu, 2006; Shi et al.. 2010), polyethylene (Manzi-Nshuti et al., 2009b; Costa et al., 2005, 2006a, 2006b, 2007; Schonhals et al., 2009; Ardanuy et al., 2009; Costantino et al., 2005; Du and Qu, 2006), among others.

4.3.3 Solution blending

With solution, the pre-expanded layered particles are dispersed in a polymer solution to promote the entry of the polymer molecules into the interlayer gallery. The remaining solvent is evaporated afterwards, resulting in the precipitation of the polymer incorporated between the inorganic composite platelets (see Fig. 4.5).

Though this procedure is quite simple, in practice it is rather complicated to find a solvent that is able to completely dissolve the polymer and fully disperse the layered particles. This method is suitable for the intercalation of polymers with a low or null polarity, facilitating the preparation of thin films with a high orientation of the inorganic particles. However, from an environmental point of view, this procedure has the inconvenience of using organic solvents, which may also be quite expensive.

In the literature, this technique has been described in the preparation of nanocomposites from water-soluble polymers such as poly(ethylene oxide) or polyvinyl alcohol (Ramaraj et al., 2010), as well as the preparation of nanocomposites in non-polar solvents like toluene (EVA: Kuila et al., 2007; PS: He et al., 2006), xylene (EVA: Ramaraj and Yoon, 2008; Zhang et al., 2008a; Kuila et al., 2008; PS: Qiu et al., 2005), linear low-density polyethylene (LLDPE): Qiu et al., 2006; Chen and Qu, 2003; Chen et al., 2004b), tetrahydrofuran (poly(vinyl chloride) (PVC): (Liu et al., 2007, 2008), or dimethylformamide (poly(vinylidene fluoride): Bao et al., 2008a), among others.

4.3.4 Others

A new procedure for preparing poly(ε-caprolactone)–LDH nanocomposites, high-energy ball milling (HEBM), was proposed by Sorrentino et al. (2005) and successfully used by other authors like Bugatti et al. (2010) and Romeo et al. (2007). This method consists in mixing the LDH particles (in this specific case a hydroxydecanoate-hybrid) with the polymer, both in powder form. Through an intensive mixing it is possible to break down the powder particles, causing the diffusion of the atoms and creating an intimate mix that results in the exfoliation of the LDH particles in the polymer. In this work, the dynamic-mechanical-thermal properties of the nanocomposites were better over the whole temperature range, with an increase of the storage modulus and tan δ decrease.

4.5.1 Rheological properties

To understand the role of the LDH particles as fillers, rheological measurements are used to reveal possible interactions and relate them with the degree of dispersion of LDH in the polymer. The linear viscoelastic regime provides important information on LDH dispersion, as well as possible particle/particle and particle/polymer interactions and, therefore, may be used to relate structure and properties.

The rheological properties of PET nanocomposites have been studied by Lee et al. (2006). PET nanocomposites containing 2.0 wt.% of MgAl–LDH with carbonate (MgAlCO₃), dodecylbenzene sulfonate (MgAlDBS), and octyl sulfate (MgAlOS) showed similar G′ vs. G″ curves, obtained from dynamic parallel plate rheometry measurements. All these nanocomposites displayed lower slopes than pure PET (1.27 vs. 2), with higher values of the storage modulus at a low loss modulus, indicative that the system was heterogeneous in terms of filler/filler interactions. However, PET nanocomposites with 2.0 wt.% dodecyl sulfate (MgAlDS) displayed a shallower slope (around 0.87) because of network structures with filler/filler and filler/matrix interactions due to the increased hydrophobic nature and exfoliation of the MgAlDS system. However, for G″ > 10³, the G′ vs. G″ slopes were steeper for all nanocomposites and approached that of the PET homopolymer. The authors argued that these results reflected the fact that some network structures in filler/filler or filler/matrix interactions were ruptured by the applied shear force, the system becoming isotropic and homogeneous.

The viscosity vs. frequency curve of PET/MgAlCO₃ indicated a drastic shear-thinning behavior due to slip between the polymer and filler caused by a low filler/matrix interaction. Additionally, although the viscosity of PET with MgAlDBS, MgAlDS, and MgAlOS has shear thinning in a low-frequency range, PET with MgAlDBS and MgAlOS showed continuous-like shear-thinning behavior, due to the breakdown of the network structure and slip between PET and the filler. PET/MgAlDS retained the viscosity at high frequencies, much like the PET homopolymer, due to improved filler/matrix interactions.

The rheological behavior of PS nanocomposites with a Zn₂Al–LDH dye (Chicago sky blue (CB), Evans blue (EB), and Niagara blue (NB)) hybrid filler was studied by Marangoni et al. (2008). The linear viscoelastic properties were studied under dynamic oscillatory shearing. The elastic G′(ω) and loss G″(ω) moduli of the unfilled PS showed in the low-ω region the usual Newtonian behavior with the expected relaxation exponent G′ ∝ ω^1.92 and G″ ∝ ω^0.98, close to the ideal slopes G′∝ ω)² and G″∝ ω¹, characteristic of the frequency-dependent liquid-like flow behavior (with a plateau in the complex viscosity master curve). For a PS nanocomposite containing either Zn₂ Al/CB or Zn₂ Al/EB, both the curvature of G′(ω) and G″(ω) and the relaxation parameters in the low-ω region remained similar to PS. In contrast, PS–Zn₂Al/NB displayed a change in slope of both moduli in the terminal zone.

This was even more pronounced in the |η*|(ω) curve, where a plateau was not observed (see Table 4.4). This effect was considered by the authors to be an incomplete relaxation of PS chains. Although this is consistent with the shift of T_g it was, however, reported as unusual for an immiscible PS nanocomposite structure. The pseudosolid-like behavior of PS–Zn₂,Al/NB was explained by attrition phenomena between PS and the filler. The changes occurring in T_g and the modulus in the low-ω domain were explained by interfacial interactions. The pseudosolid behavior of PS−Zn₂ Al/NB was considered to be a rupture of the filler's lamellar structure, in strong contradiction to the trend usually observed, where solid-like behavior is seen in intercalated or exfoliated nanocomposite structures. The authors surmised that the attrition phenomena largely developed at the interface of Zn₂Al/NB platelets and the PS chain should be attributed to the molecular arrangement of NB at the surface of the platelets.

Finally, it was noted that the molecular similarity between EB and CB was the reason for the super imposable rheological behavior, while the difference from NB was easily seen in the |η*|(ω) curve.

Leroux et al. (2009) proved that the shear-thinning exponent did not allow a quantitative discrimination between intercalated and exfoliated nanocomposite structures. The change in the shear-thinning parameter, directly correlated to the relaxation parameter, was interpreted as a modification in the interfacial interaction between the LDH filler and PS chains. In order to see if this effect a rose from the alkyl chain alone, a PS–Zn₂Al/DBS nanocomposite was studied. As with the Zn₂Al/MADABS hybrid phase, Zn₂,Al/DBS presented a well-defined lamellar structure, with several basal peaks. When incorporated into PS, the basal spacing distance remained unaltered (2.93 nm), hence defining a non-miscible PS nanocomposite structure, apparently comparable to PS-Zn₂,Al/MADABS. The glass transition temperature was similar to that of unfilled PS, as were the relaxation and shear-thinning exponents. Indeed, the presence of Zn₂ Al/DBS strongly decreased the molecular weight of PS. Also, the amplitude of the elastic modulus G′ was drastically reduced in the low-ω region. Consequently, the attrition phenomena cannot be explained solely by the length of the surfactant molecule.

The rheological behavior of hot-pressed films of LDH–PS nanocomposites was also studied. A filler percentage of 5 wt.% was used. The glass transition temperature shifted in comparison to unfilled PS, although not for the same magnitude as for a 10 wt.% content sample. PS films with 5 wt.% had intermediate rheological behavior, i.e. incomplete PS chain relaxation in the low-ω region with a shear-thinning factor different from zero. Once again, the lowering observed in the relaxation can be considered to be tethering of a polymer chain on a particle's surface and particle–particle interactions. The authors concluded that, after checking the consistency between the rheological behavior of LDH–PS nanocomposites and the state of dispersion observed by TEM, one can infer that XRD is not the most appropriate technique for elucidating the structure of polymer nanocomposites, as their morphology is much more complex than previously thought. The general trend can be explained by the existence of an LDH-percolated network. The use of lower filler loadings should induce a smaller interface, which would cause smaller attrition phenomena. These observations are to some extent similar to those for cationic clays, which form three-dimensional deformable networks in PS. Nonetheless, the effect is much more pronounced in the case of the present filler, hence the interest in the use of LDH-based materials as inorganic platelet-like fillers. Filler preparation was also reported to be important, since polymerization of the compatibilizer at the platelets' surface may give rise to aggregation when performed concomitantly with styrene bulk polymerization. Bulk polymerization acts as mechanical grinding when the platelets are pushed together in advance after a thermal pre-treatment, thus leading to a concatenated filler structure with solid-like rheological behavior.

Taviot-Gueho et al. (2007) also studied the rheological properties of several LDH–PS nanocomposites. The linear viscoelastic dynamic storage and loss modulus (G′ and G″, respectively) for PS–Zn₂Al/yellow were good matches with those of PS, whereas for PS–Zn₂Al/DBS there was a large decrease over the entire frequency domain, considerably more so for G′ than for G″, hence explaining the increase in tan δ. The increase in tan δ has to be explained by greater molecular mobility. It appears that the presence of Zn₂Al/yellow in PS, even as an intercalated structure and in contrast to Zn₂Al/DBS, does not result in worse rheological properties. The rheological behavior, expressed as η″ vs. η′, was found to be different for the three samples. At 200 °C, PS and PS–Zn₂Al/yellow curves matched well. The viscosity at zero shear (η₀), similar for the two systems, suggested similar molecular weights, which were confirmed by gel permeation chromatography (GPC). For PS–Zn₂Al/DBS, the intercept was at a much lower value, once again consistent with the GPC results. Finally, the size of the hybrid LDH aggregates was argued as a possible disadvantage during bulk polymerization. Both fillers had an intercalated structure, though it was ill-defined for PS–Zn₂Al/yellow and well-defined for PS–Zn₂Al/DBS. The presence of large Zn₂Al/DBS filler particles prevented the formation of extended polymer chains and consequently impoverished the rheological properties.

The rheological properties of LDH–polyethylene nanocomposites were studied by Costa et al. (2006a). The evolution of the complex viscosity of LDH–LDPE nanocomposites at low frequency was studied from 180 to 260 °C. The dependence of the rheological properties on temperature was weaker for the nanocomposites with an unfilled polymer, especially beyond a critical LDH content, representing a deviation of the liquid-like low-frequency flow behavior towards a pseudosolid one.

On the other hand, HDPE-g-MAH nanocomposites (HDPE is high-density polyethylene) showed the shifting behavior at much lower LDH concentrations (around 5 phr) compared with the LDPE/compatibilizer composites, where changes in viscosity with temperature were still observed at a 10 phr LDH concentration. This means that the nature of the polymer matrix strongly influences the effect of increasing the amount of LDH in terms of the rheological behavior of LDH nanocomposites. With increasing LDH nanoparticle concentration, the low-frequency response of the complex viscosity vs. frequency shifted to a shear-thinning mode, with similar behavior for both nanocomposites. Two factors could explain this situation: first of all, the polarity difference between the two matrices, and second, the presence of a low molecular weight compatibilizer in one of the composites.

It is well known that both unfilled and particulate polyolefin microcomposites conventionally display a Newtonian liquid-like behavior at low frequency. Nevertheless, LDH–LDPE nanocomposites display shear-thinning behavior with a higher shear-thinning exponent and lower relaxation exponent. Both exponents become almost independent of LDH concentration at higher contents. The shear-thinning exponent has been used as a semi-quantitative parameter of the degree of clay delamination in polymer nanocomposites (Wagner and Reisinger, 2003). Although higher negative values of this exponent are normally considered to be due to higher clay exfoliation, the higher shear-thinning exponent values of the LDH−LDPE nanocomposites were due to polymer molecule confinement, delaying the relaxation process. Higher LDH loadings enhanced this effect, with G′ increasing and being less frequency dependent. Additionally, as the average distance between the dispersed particles decreased, the tendency to interlock, which acts as an energy barrier for molecular relaxation, increased.

4.5.2 Mechanical properties

Taking into account the large surface area of layered double hydroxides, in some cases reaching 800 m²/g, a large interaction with polymer molecules is possible, and thus a large mechanical reinforcement effect is to be expected. This situation could partly explain why these layered materials may dramatically improve the modulus of polymers even when present in small amounts.

Most research into the mechanical properties of polymer–LDH nanocomposites has analyzed their tensile properties, particularly the modulus and tensile strength, as a function of LDH content. Generally speaking, the addition of low amounts of LDH increases the Young's modulus and tensile strength, mainly due to the ease of attaining better particle dispersion in the polymer matrix at low LDH contents. Higher LDH concentrations increase the probability of particle cluster formation, leading to deterioration of the mechanical properties.

Zhang et al. (2008a) reported a gradual increase in Young's modulus from 42.8 to 72.1 MPa for EVA composites containing up to 20 wt.% of ZnAl–LDH particles and an increase in tensile strength up to 5 wt.% LDH, decreasing for higher loadings. Another study (Sorrentino et al., 2005) showed how small amounts of LDH particles (up to 2.8 wt.%) significantly improved the mechanical properties of composites, with an increase of both Young's modulus and yield stress of about 100 %. However, for concentrations up to 6 wt.%, the mechanical properties, although still higher than for the pure polymer, decreased. A similar phenomenon was observed in PU/DS–LDH nanocomposites (PU is polyurethane) (Kotal et al., 2009). Kotal et al. studied the mechanical properties of nanocomposites containing 1, 3, 5, and 8 wt.%, finding that the maximum enhancement of tensile strength and elongation at break corresponded to the nanocomposite containing 3 wt.% DS– LDH. These authors suggested that the decrease observed beyond this DS–LDH content was probably due to the increasing tendency of DS–LDH to aggregate in the PU matrix at high loadings. Du et al. (2006) also studied the effect of DS/MgAl–LDH particle content (5 and 10 wt.% loadings were used) in poly(propylene carbonate) (PPC). They found that the nanocomposites displayed enhanced tensile strengths and Young's moduli compared with pure PPC. The nanocomposite containing 5 wt.% showed the highest tensile strength and Young's modulus (72 % and 57 % higher than those of pure PPC, respectively). This improvement was due to the structure found in the nanocomposites: exfoliated for the 5 wt.% LDH nanocomposite and partly exfoliated for the 10 wt.% one. A similar phenomenon was observed in aminobenzoate/MgAl–LDH/polyimide nanocomposites (Hsueh and Chen, 2003b). When LDH particle contents exceeded 5 wt.%, a decrease in the tensile strength at break was found (although still higher than for pure polyimide). Once again, this was due to the aggregation of the LDH layers.

The results of tensile tests performed by the same authors (Hsueh and Chen, 2003a) on laurate-modified MgAl–LDH epoxy nanocomposites showed how both the tensile strength and modulus increased gradually when the amount of LDH was increased up to 7 wt.%, the maximum concentration analyzed. However, these authors also found that elongation at break decreased slightly for LDH contents higher than 3 wt.%. Wang et al. (2006) also found interesting results for PMMA nanocomposites reinforced with different amounts of undecanoate-modified LDH (LDH-U). The authors found that the tensile modulus was enhanced with increasing LDH-U content (38 % and around 80 %, respectively, for LDH contents of 3 and 5 wt.%). This was due to good dispersion of the LDH layers and a strong interfacial adhesion between the layers and the PMMA matrix. A similar trend was observed for tensile strength. Nevertheless, a slight decrease was noticed for the elongation at break with increasing LDH content, due to the high resistance of the LDH layers to deformation of PMMA.

Acharya et al. (2007b) also reported an increase of both the tensile modulus and strength when increasing the amount of LDH up to 8 wt.% in EPDM/DS–LDH nanocomposites.

Other studies reported the effect of the interlamellar anion and the composition of the layers in the mechanical properties of LDH nanocomposites. For example, Manzi-Nshuti et al. (2008b) studied the effect in PMMA systems of LDH particles prepared with different combinations of NiAl, CoAl, and ZnAl layers, as well as different Zn/Al ratios (2:1 and 3:1). The authors did not find significant effects of the LDH particles on the mechanical properties, regardless of the composition and ratios of the divalent to trivalent metal cations. In another study, Bugatti et al. (2010) analyzed the effect of LDH hybrids prepared using different interlayer anions (benzoate, dichlorobenzoate, p-hydroxybenzoate, and o-hydroxybenzoate), as well as their concentration on the mechanical properties of polycaprolactone (PCL) films. Slight improvements were observed for all mechanical parameters, with some interesting differences between the differently dispersed nanohybrids. These authors showed that for a given LDH concentration, for instance 3 wt.%, the tensile modulus may vary between 218 and 300 MPa, depending on the type of interlayer anion (in this case the higher value corresponded to dichlorobenzoate-modified LDH). Wang et al. (2009) also analyzed the effect of LDH concentration and the use of different interlayer anions on the mechanical properties of PMMA and PS nanocomposites. No significant differences were found when varying the type of interlayer anion.

An interesting study was conducted by Bao et al. (2006), which analyzed the effect of LDH particles on the tensile strength and Young's modulus of PVC composites prepared by both melt-mixing and in situ polymerization. The authors found that, although the tensile strength and Young's modulus increased for both PVC/LDH–DS composites when increasing the amount of LDH–DS, this increase was higher for the ones prepared by in situ polymerization for a given LDH concentration. On the other hand, Pradhan et al. (2008) showed that, when well dispersed, LDH acted as a mechanical reinforcement in both ethylene propylene diene monomer (EPDM) and acrylonitrile butadiene carboxy monomer (XNBR) elastomers. In particular, the tensile strength increased from 1.54 and 1.94 MPa to values as high as 3.25 and 17.83 MPa for a 10 wt.% LDH content, respectively, for the EPDM and XNBR elastomers, clearly demonstrating the mechanical reinforcement effect, especially in the case of the XNBR elastomer.

Regarding some of our previous results, we studied the mechanical properties of PS and styrene-acrylonitrile resin (SAN) composites with 5 and 10 wt.% of dodecyl-sulfate-modified MgAl layered double hydroxides (o-HT) (Realinho et al., 2009). The tensile modulus and strength of PS remained almost unaltered when incorporating the o-HT particles, with only a slight increase being observed for Young's modulus. In contrast, the addition of o-HT to SAN, although decreasing its tensile strength, increased the stiffness to a higher extent than in PS and noticeably increased the ductility of the material in terms of strain at break. These results were due to better o-HT dispersion in SAN compared with PS. The decrease in tensile strength observed when incorporating o-HT into SAN was explained by the combination of a relatively weak particle–matrix interface adhesion and inherent high deformability of SAN, which promoted o-HT particle debonding at low stress levels and subsequent plastic flow.

For semi-crystalline polypropylene (Ardanuy et al., 2010), we showed that the incorporation of 10 wt.% of unmodified (HT) and dodecyl-sulfate-modified hydrotalcite (HTDS) to a PP matrix with and without PP-g-MAH and PP-g-SEBS compatibilizers resulted in nanocomposites with slightly higher Young's moduli. However, this increase was considerably lower than that observed for similar PP–montmorillonite (PP-MMT) nanocomposites, due to a higher inherent flexibility of HT layers compared with cationic clays such as MMT. Comparing HT- and HTDS-reinforced nanocomposites, the latter had higher values of Young's modulus, probably due to a higher exfoliation of the HTDS particles in PP, especially when using the PP-g-MAH compatibilizer (there were more interactions between the polymer and HT platelets). A low degree of adhesion was considered to be the reason for the slight decrease observed regarding the tensile strength for both the HT- and HTDS-reinforced PP nanocomposites, compared with the higher values obtained for the nanocomposites with compatibilizer.

4.5.3 Thermal stability

Generally speaking, the incorporation of layered particles into polymer matrices results in an enhancement of their thermal stability (Pavlidoua and Papaspyrides, 2008). The increase in thermal stability may be due to the layers hindering the diffusion of oxygen and volatile products throughout the polymer, as well as to the formation of a char layer after thermal decomposition of the organic matrix. Additionally, endothermic decomposition of the host metal hydroxide layer can provide a cooling effect, which could delay the combustion process of the organic species (surfactant anion, polymer chain segments, etc.) constrained within the interlayer gallery of the LDH particles.

As can be seen in Table 4.5, except for some specific polymer matrices like polyamide 6 (PA6) or PCL, the addition of LDH hybrids to different polymers results in an increase in the decomposition temperatures corresponding to a 50 % weight loss.

Zhang et al. (2008a) studied the thermal stability of DS/ZnAl–LDH/EVA nanocomposites as a function of LDH concentration and preparation procedure. These authors observed that the thermal degradation temperatures of the nanocomposites were between 13 and 35 °C higher than that of pure EVA, increasing with increasing LDH concentration. The authors explained this effect based on the fact that at a higher loading the LDH layers favored the formation of an obstructive char layer at the surface of the polymer, increasing the thermal stability of the nanocomposites. Moreover, samples prepared by solution blending displayed higher degradation temperatures than those prepared via melt-mixing for identical LDH contents. The authors attributed this effect to a better dispersion of the LDH particles in the nanocomposites prepared by solution blending.

Nyambo and Wilkie (2009) reported similar results for EVA-based polymer nanocomposites reinforced with borate-modified LDHs with a different composition of the metal cations. The authors found that the addition of MgAl or ZnAl-borate LDHs improved the thermal stability of EVA, more markedly with increasing LDH concentration. In particular, the best thermal stability effect was obtained for a 40 wt.% LDH loading regardless of the nature of the layer metal cations. Using a similar approach, Kuila et al. (2008) studied the thermal decomposition of EVA/LDPE-based LDH nanocomposites. They found that, although the initial weight loss for the nanocomposites was accelerated due to the early degradation of DS molecules, the thermal decomposition temperatures corresponding to a 50 % weight loss for nanocomposites with 1, 3, 5, and 8 wt.% DS-LDH were higher than for pure EVA/LDPE, increasing with increasing LDH content. This increase in thermal stability was attributed to the homogeneous dispersion of DS–LDH in the EVA/LDPE matrix. In another study, Qiu et al. (2005) analyzed the effects of LDH concentration on the thermal decomposition of PS nanocomposites containing 5, 10, and 20 wt.% ZnAlDS–LDH. They showed that the thermal decomposition temperature of the nanocomposites was between 16 and 39 °C higher than that of pure PS, with optimal concentration being 10 wt.%. Higher LDH loadings (20 wt.%) resulted in nanocomposites with lower thermal stability. These authors also analyzed the thermal behavior of the same nanocomposite systems prepared using different methods. They found that the samples prepared by rapid precipitation and solvent evaporation at 140 °C, although displaying similar thermal behavior when varying the amount of LDH, showed better thermal stability compared with samples prepared by solvent evaporation at 60 °C. This was explained by the different proportions of exfoliated and intercalated structures in these nanocomposites.

Different effects of LDH content on the thermal stability of polyolefin-based nanocomposites have been reported in the literature. For example, for PP–LDH systems, Shi et al. (2010) reported that the thermal decomposition temperatures of a series of PP-LDH nanocomposites with various LDH concentrations were higher than for pure PP (from 7 to 30 °C). Moreover, the authors found that the nanocomposites containing organo-modified MgAl–LDH particles exhibited higher decomposition temperatures than the nanocomposites containing unmodified MgAl–LDH particles. In the same way, Ardanuy and Velasco (2011) reported that the decomposition temperature of pure PP increased with organophilized LDH particles, while the thermal stability decreased when using unmodified LDH particles. These authors also found that the increase of thermal stability was more noticeable when PP-g-MAH was used as a compatibilizer. They noted that the exfoliated layers of organophilized LDH hindered the diffusion of oxygen after combustion ignition to a greater extent. Nevertheless, Wang et al. (2010) did not find any differences between the thermal decomposition of pure PP and nanocomposites with organic modified CaAl–LDH for particle loadings between 1 and 6 wt.%. These authors concluded that the particles did not improve the thermal stability of PP.

Concerning polyethylene–LDH nanocomposites, Qiu et al. (2006) compared the thermal stability of LLDPE systems with different MgAl, ZnAl–LDH, and MMT contents. They showed that the thermal stability of the nanocomposites was enhanced compared with virgin LLDPE, although the enhancement was quite different depending on the composition of the layered particles. The T_0.2 values (decomposition temperature corresponding to a 20 wt.% loss) for the LLDPE/MMT and LLDPE/MgAl–LDH samples increased gradually to 399 and 429 °C, respectively, with increasing MMT and Mg₃ Al(DS) contents from 0 to 10 wt%. However, LLDPE/ZnAl–LDH with only 2.5 wt% of Zn₃Al(DS) showed important improvements, T_0.2 increasing till 428 °C. It reached a maximum value of 434 °C for 5 wt.% Zn₃Al(DS), and decreased to 420 °C for 10 wt.% Zn₃ Al(DS). The possible reason behind these differences could be the relative extent of exfoliation in the different nanocomposites.

Costa et al. (2007) also studied LDPE–LDH nanocomposites, reporting that the onset of the decomposition temperature was delayed significantly with the addition of only 2.4 wt.% of LDH particles. Once again, the decomposition temperature increased with increasing LDH concentration, with a maximum increase of 70 °C compared with pure LDPE for a 9 wt.% LDH content. Ardanuy et al. (2009) reported similar results for HDPE nanocomposites. The onset of decomposition of HDPE was significantly delayed by the effect of the organo-modified LDH particles and, similarly, T_0.5 increased because of these particles.

However, there are results in the literature in which the incorporation of LDH particles resulted in a decrease in the decomposition temperature of the polymer. For example, Du et al. (2007) reported that the thermal degradation temperature of PA6/DS MgAl–LDH nanocomposites was lower than that of pure PA6. This effect was highlighted with increasing LDH content. These authors considered that this effect was due to a catalytic degradation phenomenon caused by the organically modified LDH layers. Herrero et al. (2010) reported similar results for PA6.6–LDH systems with filler contents higher than 1 wt.%. This behavior is because the high content of Ad-LDH could catalyze the alkaline degradation of PA6.6. Zammarano et al. (2006) also reported a reduction in the onset of the thermal decomposition temperature for LDH−PA6 nano-composites. Again, it was suggested that this effect was due to a nucleophilic attack mechanism.

4.5.4 Other properties

Besides the few works that analyze properties such as arc resistance and dielectric strength (Ramaraj and Yoon, 2008), barrier properties (Bugatti et al., 2010; Sorrentino et al., 2005), dielectric properties (Leroux et al., 2010; Schonhals et al., 2009), and optical properties (Marangoni et al., 2008; Zhang et al., 2008b; Acharya et al., 2007b), one of the most extensively studied properties in polymer-layered double hydroxide nanocomposites is fire resistance, mainly using cone calorimetry and limiting oxygen index (LOI) determination.

For instance, Du et al. (2007) reported the combustion characterization of PA6/MgAl–LDH nanocomposites. They found that the heat release rate (HRR) decreased considerably when increasing the amount of LDH in the polymer matrix. Moreover, they found that the ignition time was delayed compared with pure PA6 for low LDH contents. However, ignition times increased when the content of MgAl(H-DS) exceeded 10 wt.%. The authors suggested that this phenomenon could be due to a catalytic degradation effect of PA6 promoted by the organo-modified particles, which decreased the molecular weight of the polymer and thus caused the material to burn more easily.

Ramaraj and Yoon (2008) reported the LOI estimation and flammability rating of EVA–LDH nanocomposite films. Results showed a significant improvement in the flame retardancy of EVA nanocomposites due to the LDH particles. They showed that the minimum oxygen concentration required for a flame to appear increased from 19 to 26 % and the flammability rating improved from no rating to V1 rating with increasing LDH concentration. They reported that the flame-retardant characteristics of LDH were due to their Mg(OH)₂-like behavior, which involved endothermic decompositions with the formation of water vapor and a metal oxide residue. This residue prevented the burning process from proceeding by reducing the oxygen supply to the underlying combustible polymer.

Another study by Manzi-Nshuti et al. (2009a) gave the cone calorimetric results of PS-LDH systems. The best reduction in peak heat release rate (PHRR) (35 %) was obtained for PS filled with 10 wt.% of an organo-modified ZnAl-MgAl–LDH. Lower LDH contents did not give significant fire retardancy improvements. Nyambo et al. (2009a) also studied the fire behavior of PMMA/MgAl–LDH nanocomposites using cone calorimetry. A significant reduction was observed regarding PHRR, due to improved physical interaction and good nano-dispersion of the organically modified MgAl–LDH particles in the PMMA matrix. A lower reduction was observed when incorporating unmodified MgAl–LDH particles.

Similar improvements in fire resistance with LDH particles were found by other researchers, such as Costa et al. (2007). These authors found that the incorporation of MgAl–LDH particles resulted in a very efficient reduction of the heat release rate and total heat released during combustion. MgAl–LDH particles were also found to facilitate the formation of a carbonaceous char on the burning surface, causing a lower emission of carbon monoxide during the initial phase of burning. For LDH concentrations above 10 wt.%, the nanocomposites not only burnt at extremely slow rates, but also showed low dripping tendencies. However, they reported that LDH by itself, even at these concentrations, was not enough to obtain a high LOI value or V0 burning rate.

All of these results prove the effectiveness of LDH particles as flame retardants in polymer systems.

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