Polymer Nanocomposite Technology, Fundamentals of Barrier

The term “nanocomposite” is widely employed to describe an extremely broad range of materials, where one of the components has a dimension in the sub-micron scale. A better –and far more restrictive– definition would require that a true nanocomposite should be a fundamentally new material (hybrid) in which the nanometer scale component or structure gives rise to intrinsically new properties, which are not present in the respective macroscopic composites or the pure components. This latter definition necessitates that the nanostructure has dimensions smaller than a characteristic scale that underlies a physical property of the material. For example, for the electronic properties of a conductor or semi-conductor this scale would relate to the de Broglie wavelength of the electron (ranging from a few nanometers for a metal to hundreds of nanometers for a semiconductor), for the mechanical properties of a polymer it would relate to the size of the polymer coil or crystal (again ranging from a few nanometers to hundreds of nanometers), and for the thermodynamic properties of a polymer glass it would relate to the cooperativity length (a few nanometers). Here we shall restrict our discussion even further, focussing on one subclass of polymer/inorganic nanocomposites, where the inorganic component is a high aspectratio nanoscale filler. Particular emphasis will be given to principles that apply to pseudo-two-dimensional layered inorganic fillers (such as 2:1 alumino-silicates , from where most of our examples will be derived, and layered double hydroxides). In these systems, concurrent improvements across multiple properties are typically achieved –with simultaneous enhancement of barrier, mechanical, thermal, and thermomechanical response– in addition to ‘new’ properties –such as improved flammability and biodegradability, or antimicrobial activity– compared to the unfilled polymer. Consequently, the resulting nanocomposite material is better described by the term “hybrid” (denoting the large scale changes in multiple material characters) rather than polymer “composite” (a term traditionally associated with an incremental improvement of one or two key properties). ∗email: [email protected], fax: +1-814-865-2917, tel: +1-814-863-2980.