Polymer/layered silicate nanocomposites: a review from preparation to processing

Abstract

A review is given of the academic and industrial aspects of the preparation, characterization, materials properties, crystallization behavior, melt rheology, and processing of polymer/layered silicate nanocomposites. These materials are attracting considerable interest in polymer science research. Hectorite and montmorillonite are among the most commonly used smectite-type layered silicates for the preparation of nanocomposites. Smectites are a valuable mineral class for industrial applications because of their high cation exchange capacities, surface area, surface reactivity, adsorptive properties, and, in the case of hectorite, high viscosity and transparency in solution. In their pristine form they are hydrophilic in nature, and this property makes them very difficult to disperse into a polymer matrix. The most common way to remove this difficulty is to replace interlayer cations with quarternized ammonium or phosphonium cations, preferably with long alkyl chains.

A wide range of polymer matrices is covered in this review, with special emphasis on biodegradable polymers. In general, polymer/layered silicate nanocomposites are of three different types, namely (1) intercalated nanocomposites, for which insertion of polymer chains into a layered silicate structure occurs in a crystallographically regular fashion, with a repeat distance of few nanometers, regardless of polymer to clay ratio, (2) flocculated nanocomposites, for which intercalated and stacked silicate layers flocculated to some extent due to the hydroxylated edge–edge interactions of the silicate layers, and (3) exfoliated nanocomposites, for which the individual silicate layers are separated in the polymer matrix by average distances that depend only on the clay loading. This new family of composite materials frequently exhibits remarkable improvements of material properties when compared with the matrix polymers alone or conventional micro- and macro-composite materials. Improvements can include a high storage modulus, both in solid and melt states, increased tensile and flexural properties, a decrease in gas permeability and flammability, increased heat distortion temperature, an increase in the biodegradability rate of biodegradable polymers, and so forth.

Introduction

In recent years polymer/layered silicate (PLS) nanocomposites have attracted great interest, both in industry and in academia, because they often exhibit remarkable improvement in materials properties when compared with virgin polymer or conventional micro- and macro-composites. These improvements can include high moduli [1], [2], [3], [4], [5], [6], increased strength and heat resistance [7], decreased gas permeability [8], [9], [10], [11], [12] and flammability [13], [14], [15], [16], [17], and increased biodegradability of biodegradable polymers [18]. On the other hand, there has been considerable interest in theory and simulations addressing the preparation and properties of these materials [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], and they are also considered to be unique model systems to study the structure and dynamics of polymers in confined environments [35], [36], [37], [38], [39], [40], [41].

Although the intercalation chemistry of polymers when mixed with appropriately modified layered silicate and synthetic layered silicates has long been known [42], [43], the field of PLS nanocomposites has gained momentum recently. Two major findings have stimulated the revival of interest in these materials: first, the report from the Toyota research group of a Nylon-6 (N6)/montmorillonite (MMT) nanocomposite [1], for which very small amounts of layered silicate loadings resulted in pronounced improvements of thermal and mechanical properties; and second, the observation by Vaia et al. [44] that it is possible to melt-mix polymers with layered silicates, without the use of organic solvents. Today, efforts are being conducted globally, using almost all types of polymer matrices.

This review highlights the major developments in this area during the last decade. The different techniques used to prepare PLS nanocomposites, their physicochemical characterization, and the improved materials properties that those materials can display; their processing and probable application of PLS nanocomposites will be discussed.

The commonly used layered silicates for the preparation of PLS nanocomposites belong to the same general family of 2:1 layered or phyllosilicates. Their crystal structure consists of layers made up of two tetrahedrally coordinated silicon atoms fused to an edge-shared octahedral sheet of either aluminum or magnesium hydroxide. The layer thickness is around 1 nm, and the lateral dimensions of these layers may vary from 30 nm to several microns or larger, depending on the particular layered silicate. Stacking of the layers leads to a regular van der Waals gap between the layers called the interlayer or gallery. Isomorphic substitution within the layers (for example, Al³⁺ replaced by Mg²⁺ or Fe²⁺, or Mg²⁺ replaced by Li¹⁺) generates negative charges that are counterbalanced by alkali and alkaline earth cations situated inside the galleries. This type of layered silicate is characterized by a moderate surface charge known as the cation exchange capacity (CEC), and generally expressed as mequiv/100 gm. This charge is not locally constant, but varies from layer to layer, and must be considered as an average value over the whole crystal.

MMT, hectorite, and saponite are the most commonly used layered silicates. Layered silicates have two types of structure: tetrahedral-substituted and octahedral substituted. In the case of tetrahedrally substituted layered silicates the negative charge is located on the surface of silicate layers, and hence, the polymer matrices can react interact more readily with these than with octahedrally-substituted material. Details regarding the structure and chemistry for these layered silicates are provided in Fig. 1 and Table 1, respectively.

Two particular characteristics of layered silicates that are generally considered for PLS nanocomposites. The first is the ability of the silicate particles to disperse into individual layers. The second characteristic is the ability to fine-tune their surface chemistry through ion exchange reactions with organic and inorganic cations. These two characteristics are, of course, interrelated since the degree of dispersion of layered silicate in a particular polymer matrix depends on the interlayer cation.

The physical mixture of a polymer and layered silicate may not form a nanocomposite. This situation is analogous to polymer blends, and in most cases separation into discrete phases takes place. In immiscible systems, which typically correspond to the more conventionally filled polymers, the poor physical interaction between the organic and the inorganic components leads to poor mechanical and thermal properties. In contrast, strong interactions between the polymer and the layered silicate in PLS nanocomposites lead to the organic and inorganic phases being dispersed at the nanometer level. As a result, nanocomposites exhibit unique properties not shared by their micro counterparts or conventionally filled polymers [1], [2], [3], [4], [5], [6].

Pristine layered silicates usually contain hydrated Na⁺ or K⁺ ions [45]. Obviously, in this pristine state, layered silicates are only miscible with hydrophilic polymers, such as poly(ethylene oxide) (PEO) [46], or poly(vinyl alcohol) (PVA) [47]. To render layered silicates miscible with other polymer matrices, one must convert the normally hydrophilic silicate surface to an organophilic one, making the intercalation of many engineering polymers possible. Generally, this can be done by ion-exchange reactions with cationic surfactants including primary, secondary, tertiary, and quaternary alkylammonium or alkylphosphonium cations. Alkylammonium or alkylphosphonium cations in the organosilicates lower the surface energy of the inorganic host and improve the wetting characteristics of the polymer matrix, and result in a larger interlayer spacing. Additionally, the alkylammonium or alkylphosphonium cations can provide functional groups that can react with the polymer matrix, or in some cases initiate the polymerization of monomers to improve the strength of the interface between the inorganic and the polymer matrix [42], [48].

Traditional structural characterization to determine the orientation and arrangement of the alkyl chain was performed using wide angle X-ray diffraction (WAXD). Depending on the packing density, temperature and alkyl chain length, the chains were thought to lie either parallel to the silicate layers forming mono or bilayers, or radiate away from the silicate layers forming mono or bimolecular arrangements (see Fig. 2) [49]. However, these idealized structures have been shown to be unrealistic by Vaia et al. [50] using FTIR experiments. They showed that alkyl chains can vary from liquid-like to solid-like, with the liquid-like structure dominating as the interlayer density or chain length decreases (see Fig. 3), or as the temperature increases. This occurs because of the relatively small energy differences between the trans and gauche conformers; the idealized models described earlier assume all trans conformations. In addition, for longer chain length surfactants, the surfactants in the layered silicate can show thermal transition akin to melting or liquid-crystalline to liquid-like transitions upon heating.

In general, layered silicates have layer thickness on the order of 1 nm and a very high aspect ratio (e.g. 10–1000). A few weight percent of layered silicates that are properly dispersed throughout the polymer matrix thus create much higher surface area for polymer/filler interaction as compared to conventional composites. Depending on the strength of interfacial interactions between the polymer matrix and layered silicate (modified or not), three different types of PLS nanocomposites are thermodynamically achievable (see Fig. 4):

• Intercalated nanocomposites: in intercalated nanocomposites, the insertion of a polymer matrix into the layered silicate structure occurs in a crystallographically regular fashion, regardless of the clay to polymer ratio. Intercalated nanocomposites are normally interlayer by a few molecular layers of polymer. Properties of the composites typically resemble those of ceramic materials.

• Flocculated nanocomposites: conceptually this is same as intercalated nanocomposites. However, silicate layers are some times flocculated due to hydroxylated edge–edge interaction of the silicate layers.

• Exfoliated nanocomposites: in an exfoliated nanocomposite, the individual clay layers are separated in a continuous polymer matrix by an average distances that depends on clay loading. Usually, the clay content of an exfoliated nanocomposite is much lower than that of an intercalated nanocomposite.

The large variety of polymer systems used in nanocomposites preparation with layered silicate can be conventionally classified as below.

These include the vinyl addition polymers derived from common monomers like methyl methacrylate [42], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], methyl methacrylate copolymers [65], [66], [67], [68], other acrylates [69], [70], [71], acrylic acid [72], [73], acrylonitrile [74], [75], [76], [77], styrene (S) [38], [44], [78], [79], [80], [81], [82], [83], [84], [85], [86], [87], [88], [89], [90], [91], [92], [93], [94], [95], [96], [97], [98], [99], [100], [101], [102], [103], 4-vinylpyridine [104], acrylamide [105], [106], poly(N-isopropylacrylamide) [107] and tetrafluoro ethylene [108]. In addition, selective polymers like PVA [47], [109], [110], [111], [112], poly(N-vinyl pyrrolidone) [113], [114], [115], [116], [117], poly(vinyl pyrrolidinone) [118], [119], poly(vinyl pyridine) [120], poly(ethylene glycol) [121], poly(ethylene vinyl alcohol) [122], poly(vinylidene fluoride) [123], poly(p-phenylenevinylene) [124], polybenzoxazole [125], poly(styrene-co-acrylonitrile) [126], ethyl vinyl alcohol copolymer [127], polystyrene–polyisoprene diblock copolymer [128], [129] and others [130] have been used.

Several technologically important polycondensates have been used in nanocomposites preparation with layered silicate. These include N6 [1], [12], [131], [132], [133], [134], [135], [136], [137], [138], [139], [140], [141], [142], [143], [144], [145], [146], [147], [148], [149], [150], [151], [152], [153], [154], [155], several other polyamides [15], [16], [19], [156], [157], [158], [159], [160], [161], [162], poly(ε-caprolactone) (PCL) [10], [163], [164], [165], [166], [167], [168], [169], [170], [171], [172], [173], [174], [175], poly(ethylene terephthalate) [176], [177], [178], [179], [180], [181], [182], poly(trimethylene terephthalate) [183], [184], poly(butylene terephthalate) [185], polycarbonate (PC) [186], [187], PEO [36], [40], [46], [188], [189], [190], [191], [192], [193], [194], [195], [196], [197], [198], [199], [200], [201], [202], [203], [204], [205], [206], [207], [208], ethylene oxide copolymers [209], [210], poly(ethylene imine) [211], poly(dimethyl siloxane) [212], [213], [214], [215], [216], [217], polybutadiene [218], butadiene copolymers [219], [220], [221], epoxidized natural rubber [222], [223], epoxy polymer resins (EPR) [224], [225], [226], [227], [228], [229], [230], [231], [232], [233], [234], [235], [236], [237], [238], [239], [240], [241], [242], [243], [244], [245], [246], [247], phenolic resins [248], [249], polyurethanes (PU) [250], [251], [252], [253], [254], polyurethane urea [8], polyimides [255], [256], [257], [258], [259], [260], [261], [262], [263], [264], [265], [266], [267], [268], [269], [270], [271], [272], poly(amic acid) [273], [274], polysulfone [275], polyetherimide [276], [277], and fluoropoly(ether-imide) [278].

Polyolefins such as polypropylene (PP) [279], [280], [281], [282], [283], [284], [285], [286], [287], [288], [289], [290], [291], [292], [293], [294], [295], [296], [297], [298], [299], [300], [301], [302], [303], [304], [305], [306], [307], [308], [309], [310], [311], [312], [313], [314], [315], [316], [317], [318], [319], polyethylene (PE) [320], [321], [322], [323], [324], [325], [326], [327], [328], [329], polyethylene oligomers [330], copolymers such as poly(ethylene-co-vinyl acetate) (EVA) [331], ethylene propylene diene methylene linkage rubber (EPDM) [332] and poly(1-butene) [333] have been used.

In addition to the above mentioned conventional polymers, several interesting developments occurred in the preparation of nanocomposites of layered silicates with specialty polymers including the N-heterocyclic polymers like polypyrrole (PPY) [334], [335], [336], [337], [338], [339], poly(N-vinylcarbazole) (PNVC) [340], [341], and polyaromatics such as polyaniline (PANI) [342], [343], [344], [345], [346], [347], [348], [349], [350], [351], [352], [353], [354], [355], [356], poly(p-phenylene vinylene) [357] and related polymers [358]. PPY and PANI are known to display electric conductivity [359], and PNVC is well known for its high thermal stability and characteristic optoelectronic properties [360], [361], [362], [363]. Research has also been initiated with liquid crystalline polymer (LCP)-based nanocomposites [364], [365], [366], [367], [368], hyperbranched polymers [369], cyanate ester [370], Nafion^® [371], and aryl-ethanyl-terminated coPoss imide oligomers [372].

Today, tremendous amounts and varieties of plastics, notably polyolefins, polystyrene and poly(vinyl chloride) produced mostly from fossil fuels, are consumed and discarded into the environment, ending up as wastes that do not degrade spontaneously. Their disposal by incineration produces large amounts of carbon dioxide, and contributes to global warming, some even releasing toxic gases. For these reasons, there is an urgent need for the development of green polymeric materials that would not involve the use of toxic or noxious components in their manufacture, and could allow degradation via a natural composting process. Accordingly, polylactide (PLA) is of increasing commercial interest because it is made from renewable resources and readily biodegradable.

Recently, our group has started the preparation, characterization, and materials properties of various kinds of biodegradable polymers/layered silicate nanocomposites having properties suitable for a wide range of applications. So far reported biodegradable polymers for the preparation of nanocomposites are PLA [18], [373], [374], [375], [376], [377], [378], [379], [380], [381], [382], [383], [384], [385], [386], [387], [388], poly(butylene succinate) (PBS) [389], [390], [391], [392], [393], PCL [163], [164], [165], [166], [167], [168], [169], [170], [171], [172], [173], [174], [175], unsaturated polyester [394], polyhydroxy butyrate [395], [396], [397], aliphatic polyester [398], [399], [400], [401], etc.