Self-assembly behavior of polymer-assisted clays
Introduction
Self-assembly is an important step in bottom-up nanotechnology and often involves the use of surfactants or block copolymers as soft templates for shaping ordered structures [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Copolymers with chemically distinct blocks may undergo non-equilibrium transformation from random coils into globules or may self-organize into ordered structures. Di- and tri-block copolymers are recognized for their ability to use non-covalent bonding interactions to form various geometric shapes of nanometer dimensions [11], [12], [13], [14], [15], [16]. A wide range of applications such as in patterning inorganic nanoparticles, interacting with biomaterials, and fabricating electronic devices have been reported [17], [18], [19], [20]. The monolayer self-assembly technique can afford thin films with tailored surface properties [21], [22], [23]. The morphology can be controlled by varying the copolymer structure and process parameters such as concentration [23], temperature [24], [25], pH [26], and choice of medium [27].
In addition to the above, inorganic nanomaterials are considered basic building blocks in nanotechnology. Inorganic nanomaterials may be classified according to their geometric shape and have at least one dimension in the range of 1–100 nm: spherical (e.g., metal and metal oxide nanoparticles), fibril-shape (e.g., carbon nanotube and metal wires), and platelet-like (e.g., natural smectite clays, graphite and graphene sheets), as illustrated in Fig. 1. These nanometer-scale units have inherent van der Waal's interactions that facilitate self-aggregation and, under certain controlled environments, can also lead to self-assembly and thus the formation of ordered secondary and tertiary structures with different morphologies. For example, spherical nanoparticles [28] and quantum dots [29] undergo controllable self-aggregation into ordered arrays for applications such as optics [30], [31], electrical sensors [32], [33], [34], and magnetic devices [35], [36]. This approach of building up the dimensional scale from nanosized units, called bottom-up synthesis, has been an important research topic in recent years because of its wide range of applications. Self-assembly of tiny particles such as inorganic quantum dots for optoelectronic devices [37] and the synthesis of silver nanoparticles of various shapes [38] are examples of applications that employ the aforementioned bottom-up approach. Other geometric shapes and morphologies that have been reported include inorganic nanoboxes [39], nanowires [40], [41], nanospheres [42], [43], nanotubes [40], [44], nanocubes [45], [46], and nanorods [47], [48].
Non-covalent bonds, including electrostatic charge attraction, hydrogen bonds, van der Waal's forces, metal coordination, aromatic π–π stacking interactions, and hydrophobic effects, are the fundamental driving forces for the self-assembly of copolymers and nanomaterials [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [18], [19], [20], [49], [50], [51], [52], [53], [54]. Cooperative interaction among these non-covalent bonds in a particular system controls biological reactions and kinetic transformations. A simple self-assembly process may lead to selective recognition for guest materials or hierarchical transformation into higher-order structures from primary to quaternary microstructures in a sequence. In nature, self-organized structures such as double-helix DNA [55], [56], [57]; folded protein globules [58], [59]; and bio-mineralization such as seashells, bones, and nacres are commonplace [60], [61], [62], [63], [64]. Biomaterials and biomimetic complexes [65], [66] self-assemble from basic organic and inorganic building blocks through non-covalent interactions in aqueous media. The hierarchy of molecular self-assembly from polymeric surfactants with a random coil structure to molecular bundles with highly ordered morphologies [67], [68], [69] and the unique phase separation of chiral copolymers resembling biomaterials [70] are examples of synthetic structures mimicking the diversity of nature.
Among the naturally occurring inorganic minerals, phyllosilicate and smectite clays with layered structures are among the most abundant and have found many industrial applications. Their generic aluminosilicate structure is composed of multiple silicate plates stacked in layers and crystalline defects with divalent metal species (counter ions) in the interlayer galleries. For the commonly utilized smectites, the fundamental units are comprised of two tetrahedral sheets sandwiched with edge-shared octahedral sheet at 2:1 structure. Smectites have been well characterized with regards to chemical composition, lamellar structure with high aspect ratio, geometric shape, surface area, and counter-ion exchange capacity. Perhaps owing to their polydispersed dimensions and contamination with amorphous impurities, the natural clays are less known for their ability to self-assemble into ordered patterns. There are only few reports regarding superstructures of mesoscopic orientation [71] and the properties of sol–gel and isotropic–nematic phase transition [72].
In this review, we summarize the recent developments in the study of the intensive interactions between silicate clay and organic polymers as well as clay self-assembly behaviors. Since the use of layered silicate clays in nanocomposites has been extensively documented, this review instead places emphasis on their chemical interactions with polymers and on clay self-assembly with organic involvement. The intercalation and the exfoliation of layered silicates are discussed with respect to modification of the structures through interlayer spacing enlargement and randomization into individual platelets. New findings on clay self-assembly from both types of units, organically intercalated and exfoliated silicates, for the hierarchical formation of various microstructures are reviewed. The mechanism involving the geometric shape, ionic charge attraction, and platelet pilling direction is also discussed.
Section snippets
Generic multilayered structures and conventional uses
Among many different clay species, smectic clays are naturally abundant with a well-characterized lamellar structure of multiple inorganic plates, high surface area, and ionic charges on the surface [73]. The phyllosilicate clays of the 2:1 type, such as montmorillonite (MMT), bentonite, saponite, and hectorite, have conventionally been employed as catalysts [74], [75], [76], [77], [78], adsorbents [79], [80], metal chelating agents [81], and polymer nanocomposites [82], [83], [84], [85], [86],
Intercalation with alkyl quaternary and polymeric amine-salts
Intercalation of the layered silicate clay can be performed by the ionic exchange reaction of the counter ions in the interlayer with low organic quaternary ammonium salts. As a result, the hydrophilic and water-swelling clays are converted into hydrophobic organoclays. When intercalating organics are employed, the clay layer basal spacing is expanded and can be easily accessed by hydrophobic monomers or polymers. Thus, the organoclays become compatible with the subsequent steps of exfoliation
Microsphere and rod-like self-assemblies
Organoclays from the incorporation of organic ions into MMT layered structure were shown to be capable of forming self-organized microstructures (Fig. 25) [184]. Hollow microstructures 3–9 μm in diameter were prepared from self-organization of MMT organoclay units. The direct fabrication of opened hollow microspheres was first reported in 2008. Under the common spray-drying conditions, the formation of hollow spheres from sodium alkylsulfonate intercalated MMT was observed. The mechanism for the
Dendritic microstructures from platelet self-piling
The clay layered structure was possibly exfoliated by polyamine-salts via several different mechanisms. After aqueous exfoliation, the randomized silicate platelets could be isolated by toluene extraction out of the organics leaving the silicates dispersible in water without any organic contamination. The isolated thin-platelets with high-aspect-ratio geometric shape and ionic character tended to self-pile and aggregate into ordered stacks due to their high surface charge attraction [168]. When
From intercalated clay stacks
In the past decade, the intercalation of layered silicate clays by organic salts affording a number of organoclays has been reported. Their ability for self-assembly was subsequently disclosed. Through the ionic charge exchange intercalation with hydrophobic polymer amine-salts, the clay layered structures can be incorporated with hydrophobic organics and assume an amphiphilic character because of the existence of both surface ionic charges and hydrophobic organics. Under the methods of
Conclusions and outlook
Recent developments in clay chemistry involving organic polymer-assisted intercalation and exfoliation have shown that thin-platelet geometric shapes are an important factor for self-assembly. Literature reports have revealed extensive clay–polymer interactions for fabrication of nanocomposite clay gels, hydrophilic polymer–clay films, layered structure encapsulation for biomaterials, quantum dots, and controlling super hydrophobic surface. Previous efforts at modifying clays by organic
Acknowledgments
We acknowledge financial support from the Ministry of Economic Affairs and the National Science Council (NSC) of Taiwan.
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