R ESUMEN DE ÍNDICES FLUVIOMÉTRICOS Y BALANCE HÍDRICO

The clay used in polymer nanocomposites is actually a layered silicate, which are natural or synthetic minerals. The pristine layered silicates have a dioctahedral or

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trioctahedral structure, and the basic building blocks are tetrahedral sheets. Negative charges on the layers are generated due to the substitution of Si4+ by Al3+ in tetrahedral positions and Al3+, or Fe3+ by Mg2+ or Fe2+ in octahedral positions, and the excessive negative charges generated are balanced by alkali or alkali earth cations in the interlayer space, which are usually hydrated Na+, K+, Ca2+ and Mg2+.65 Obviously, the thin sheets of layered silicates are bound together with counter-ions by electrostatic forces. In actual fact, a wide variety of organic cations, such as alkylammonium ions, can be introduced by exchange reaction, which can increase the compatibility between the clay and the hydrophobic polymer matrix such as the Cloisite series from Southern Clay.66

Figure ‎2.1-4 The structure of layered silicates67

There are different types of clay, as the structure/composition of layered silicates varies slightly, montmorillonite and bentonite are the most widely used clay minerals. Montmorillonite, named after Montmorillon in France, is a very soft phyllosilicate mineral and typically forms in microscopic crystals. Bentonite is named by Wilbur C. Knight and is essentially a impure clay consisting mainly of montmorillonite.68 According to the dispersion of clay in polymer matrix, polymer composites can be classified into three main types. They are (conventional) micro-composites, intercalated nanocomposites and exfoliated nanocomposites.

(A) In micro-composites, phase separation occurs due to the poor compatibility between the clay and the polymer matrix. In this case, the addition of clay is unlikely to improve any mechanical properties of the polymer, because the weak

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interface cannot transfer the load from the matrix to the filler effectively. Furthermore, the existence of weak interface may function as defects in the continuous polymer matrix, which weakens the mechanical properties of the composites. Conventional polymer/clay composites are typical micro-composites. (B) Intercalated structures are formed when the polymer chains diffuse into the clay

interlayer galleries. In this case, the d-spacing of the clay can be increased by around 2-3 nm or more due to the intercalation of polymer chains, and a well ordered multilayer structure of alternating polymeric and inorganic layers is formed. 67,69–72

(C) Exfoliated structures are obtained when the layered silicates are completely broken down into individual platelets and homogeneously dispersed in the continuous polymer matrix.67,69,71,72 In this case, the interspacing of clay platelets are separated by 8-10 nm or more.70

Figure ‎2.1-5 Clay dispersion in polymer matrix: A) Immiscible, B) Intercalated and C) Exfoliated structures

The exfoliated or intercalated structures are of particular interest, because they bring polymer/clay composites into the nano-scale category, hence the term polymer/clay nanocomposites. It makes the entire clay surface available for attraction to the polymer and, also, maximises the polymer-clay interactions, resulting in the most significant improvement in mechanical and physical properties.65,73 Normally, it is believed that exfoliated structures can provide further enhancement in properties relative to intercalated structures, due to increased polymer-clay interaction hence increased interfacial area.72,74,75

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There are six characteristics distinguishing polymer/clay nanocomposites from conventional ones, and they are attributed to the nanoscopic dimensions and the extreme aspect ratios of layered silicates. These characteristics are:76

 Low percolation threshold (ca. 0.1-2 vol.%)

 Particle-particle correlation (orientation and position) arising at low volume fractions.

 Large number 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 ~ 1-8 vol.%).

 Comparable size scales among the rigid nanoparticle inclusions, distance between particles, and the relaxation volume of polymer chains.

To obtain effective polymer/clay nanocomposites, at least two significant issues need to be solved first. The first issue is to separate the clay platelets within tactoids by polymer migration and to broaden their interlayer spacing, and eventually separate individual clay sheets apart. The second one is to enhance the interfacial interactions and improve the compatibility between the clay sheets and the polymer matrix, because intercalation/exfoliation of clay is significantly affected by the compatibility between clay particles and the polymer matrix. Pristine clay platelets have hydrophilic surfaces while many of polymers are hydrophobic e.g. PP, thus, their compatibility is very poor, leading to unsatisfactory clay dispersion. To improve the compatibility, clay surface can be chemically modified with organic cations, alkylammonium ions are most commonly used. 67,77 As a result of chemical modification, the dispersion of clay in polymer matrix such as PP, is significantly improved.55,78–80

In actual fact, for non-polar polymers like PP, the modification of clay is far from sufficient to a high level of dispersion, because the difference in polarity of molecules restricts the compatibility. In order to further improve the compatibility and the dispersion of clay in polymers, compatibilisers and co-intercalants can be used.11,12,56,81,82 For PP/clay nanocomposites, clay modification, incorporation of compatibilisers and co-intercalants, and mixing effects are thought to be the most significant factors that determine the dispersion of clay,65 and they are thoroughly discussed in the a later section (2.1.6). As a matter of fact, it is difficult to achieve

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complete exfoliation for polymer/clay nanocomposites especially when the polymer matrices are non-polar. The majority of the polymer nanocomposites reported in the literature were found to be partially exfoliated, which indicates that the clay particles are in a state that is between intercalation and complete exfoliation.72

2.1.4 Characterisation of Polymer/Clay Nanocomposites