Potential of polymer–clay nanocomposites as barrier materials

Early in the development of polymer–clay nanocomposites it was recognized that, due to the platy morphology of the smectic nanoparticles, the gas permeability of the composite would be altered considerably from that of the pure polymer. This improved barrier has major applications potential in the food and pharmaceutical industries. These composites have the additional advantage of maintaining clarity of display of packaged foods or medicines. The fundamental origin of the barrier properties exhibited by polymer–clay nanocomposites appears to derive largely from the physical morphology of the nanocomposites, but in some notable cases, this cannot be explained by the physical barrier of the nanoparticles.

The number and types of applications utilizing the barrier properties of polymer–clay nanocomposites are significant. In general terms the majority of applications involve the protection of food or drugs from the ingress of either oxygen or water vapor. In the area of flexible food packaging, the nanocomposites will not only protect the food from spoilage and improve shelf life, but also should allow down-gauging in applications where the existing packaging barrier is sufficient. Because of the size and refractive index of the clay nanoparticles, the packaging will also be transparent.

Can one imagine the utility of a dispersed-phase reinforcement for polymers that has a thickness of 1 nm, a platelike morphology with minimal dimensions of 150 to 200 nm, robust with a modulus of 180 GPa, nontoxic (FDA classification of GRAS; generally regarded as safe for a majority of applications), a surface area in excess of 750 m2/g, a charge suitable for altering its hydrophobic–hydrophilic balance at will, and a refractive index similar to polymer so that the nanoparticles will appear transparent in the polymer composite? How difficult would it be to prepare such a particle?

This particle is naturally occurring and found around the world. It is easily mined and purified. The reactor for the particle was a volcano. The ash from many volcanoes was spread around the earth during an intense period of activity many millions of years ago. This ash was transformed into clay (montmorillonoids or smectites) by natural processes, into uncharged species (talc and pyrophyllite) and charged species through isomorphic substitution of the crystal structure (hectorite, montmorillonite, saponite, suconite, volchonskoite, vermiculite, and nontronite).

Montmorillonite serves as the principle mineral for the development of polymer–clay nanocomposites discussed in this book. A misunderstanding of the terms bentonite (the ore or rock) and montmorillonite (the mineral) are pervasive in the literature. We will focus on utilizing the mineral name. The composition of montmorillonite can be described by imagining a sandwich structure with the top and bottom layers composed of silica dioxide tetrahedral structures.

In polymer–clay nanocomposites, to truly reach the ultimate in property improvements requires full exfoliation. A fully exfoliate composite yields the maximum interfacial interaction between the nanoparticle and polymer matrix. In order to produce optimally exfoliated systems requires that direct methods be available to measure the level of exfoliation. The ideal analytical method should be rapid, nondestructive, applicable to many sample matrices, low cost, and should require minimal sample preparation. The only method that fits these criteria is wide-angle X-ray diffraction (WAXD). This method, however, has some major drawbacks that will be discussed in detail in this chapter.

The other analytical methods for confirming the level of exfoliation include scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The utility and limitations of these three microscopy techniques for measuring exfoliation in nanocomposites will be discussed in detail with specific examples in this chapter.

There are also a number of indirect methods to measure the level of exfoliation but all of them require a direct method with which to standardize them. As examples, two methods, melt viscosity and tensile modulus, will illustrate the indirect methods. Unfortunately, the overall area has not received a great deal of attention, with limited numbers of publications on the subject [1–3].

An early observation by Blumstein [1] indicated that montmorillonite present in the polymerization of methyl methacrylate to produce polymer–clay composites significantly increased the thermal stability of the methyl methacrylate polymer in relation to polymethyl methacrylate prepared without the montmorillonite present. The polymer within the galleries of the montmorillonite was reported to have significantly higher thermal stability. Speculation on the cause of this enhanced thermal stability focused on restricted polymer chain mobility in the galleries and the prevention of oxygen diffusion into the galleries. The presence of oxygen during the thermal degradation of polymer–clay nanocomposites will be demonstrated to be a significant independent variable relating to the thermal degradation.

Little further activity is found in the literature until the advent of the importance of exfoliated layered clays in the dramatic enhancement of polymer mechanical performance at low concentrations was reported [2]. Subsequent systematic evaluations of the thermal stability of polymer–clay nanocomposites were initiated by Jeff Gilman's group at NIST and Emmanuel Giannelis' group at Cornell, with remarkable results. This work led to a dramatic increase in scientific investigations focused on the structure–property relationships of polymer–clay nanocomposites to thermal stability and flame retardancy.

An excellent review of the work on the flame retardancy of polymer nanocomposites was published in 2007 [3]. This chapter will focus on the evaluation of the proposed mechanisms for enhanced thermal stability of polymer–clay nanocomposites, the proposed relationships between enhanced thermal stability of polymer–clay nanocomposites and flame retardancy, and the synergies that develop between traditional flame retardants for polymers and polymer–clay nanocomposites.

The polymer as a significant independent variable in the mechanical performance of polymer–clay nanocomposites

Chavarria and Paul [1] performed a complete evaluation of a comparison of the significant variables that relate to the successful exfoliation of organomontmorillonite in nylon 6 with the utility of these variables in the preparation of organomontmorillonite–nylon 6,6 polymer nanocomposites. The equipment and protocol for these evaluations were identical to those found in reference [1] with nylon 11 and 12. The same nylon 6 was evaluated (B135WP). The molecular weight of the nylon 6 was measured to be Mn = 29 300 by intrinsic viscosity. This is slightly different from the reported viscosity, Mn = 31 100, in reference [1]. The same organomontmorillonite (montmorillonite exchanged with octadecytrimethyl quaternary ammonium ion at 95 meq/100 g of montmorillonite) was employed in both studies. The nylon 6,6 was produced by DuPont, Zytel 42A. The molecular weight was not reported.

The production of nylon 6,6 is significantly different from the ring opening polymerization of ε-caprolactam to produce nylon 6. Hexanedioic acid (adipic acid) is neutralized with hexamethylenediamine in a 50% aqueous solution. The pH is carefully monitored in order to ensure the proper stoichiometry of dicarboxylic acid and diamine.

In order for nanocomposites to be useful, they must be thermodynamically stable. It is therefore critical to ensure that clay nanoparticles have surfaces that interact with polymer in a way that yields exfoliated structures that do not spontaneously phase separate. Although some intercalated–exfoliated systems may yield useful improvements in properties, the exfoliated state is still the ultimate goal in producing a nanocomposite with the ultimate property enhancements.

The rate at which intercalation/exfoliation occurs is also of some importance in ensuring that a nanocomposite can be made on a timescale that is commercially viable. Since the level of exfoliation is critical in order that the maximum change in properties in nanocomposites is reached, the ability to measure the level of exfoliation is of paramount importance.

In this chapter, the thermodynamics of intercalation/exfoliation will be discussed in detail, including surface modification of clays, processing strategies, and the enthalpic and entropic components of the intercalation/exfoliation process. In addition, the kinetics related to intercalation/exfoliation will be presented. Finally, a critical evaluation of the analytical methods utilized commonly to determine the level of intercalation/exfoliation will be given.

Clay surface compatibility with polymers

Smectite clay structure

The discussion of clay surface compatibility with polymers in this section will focus primarily on montmorillonite as the example clay. The characteristics discussed will only vary by degree for other smectic clays.

Complexity of polyolefin–montmorillonite nanocomposites

Preparing polyolefin–montmorillonite nanocomposites presents another challenge in relation to the preparation of block copolymer–montmorillonite nanocomposites found in Chapter 6. An excellent example of the complexity of exfoliating organomontmorillonite into a pure hydrocarbon polymer is found in the work by Hotta and Paul [1]. Linear low-density polyethylene (LLDPE; Dowlex 2032 manufactured by Dow Chemical) was melt blended with two different organomontmorillonites (Cloisite 20A and montmorillonite exchanged with trimethyl hydrogenated tallow quaternary ammonium ion). The importance of blending maleic anhydride grafted LLDPE (LLDPE–g–MA; 0.9 wt. % MA content; Fusabond MB266D produced by DuPont, Canada) with LLDPE as regards achieving exfoliation was determined in this study.

The procedures and equipment that were employed in this work were identical to those utilized by Fornes and Paul in the preparation of melt-blended nylon 6–montmorillonite nanocomposites described in Chapter 5. As one may anticipate from the studies in Chapters 5 and 6, the more hydrophobic Cloisite 20A was more efficient in producing exfoliated composites. The presence of the LLDPE–g–MA in the polymer blend further encouraged the exfoliation of Cloisite 20A. When the weight ratio of LLDPE–g–MA to Cloisite 20A is increased to 4 and subsequently to 11, the WAXS indicated good exfoliation with a loading of 4.6 and 4.9 wt.%, respectively, of montmorillonite (determined by incineration of the polymer composite in an oven). The TEM for the composite with a ratio of 11 at 4.9% montmorillonite indicated good exfoliation.