Fabrication and viscoelastic properties of biodegradable polymer/organophilic clay nanocomposites

Abstract

Compared to the conventional polymer blends in which the internal structure of the clay is retained without intercalation of polymer, polymer/clay nanocomposites are a new class of composites that show the characteristic incorporation on the order of nanometer (nm) range in-between the consisting components. As a result they usually exhibit physical and mechanical properties superior to those of conventional composites such as increased moduli and strengths [1, 2], high heat distortion temperatures [3], electrical conductivity [4, 5], anticorrosion [6] and electrorheological properties [7, 8], due to a unique morphology and improved interfacial properties [9–11]. There are two types of internal structures: intercalation, in which extended polymer chains are inserted between clay layers [12, 13], and exfoliation, in which the clay single layers are dispersed into a continuous polymer matrix [14]. Although the success of synthetic polymeric materials lies in their strength, low cost, and resistance to chemical and biological attack, these properties also have created environmental problems [15, 16]. As a result, there is a need for biodegradable polymers such as poly(hydroxybutyrate) (PHB) [17, 18], poly(ecaprolactone), chitosan derivatives, and polylactide. Among various biodegradable polymers, much attention has been focused on synthetic biodegradable aliphatic polyesters (BAP), which are usually synthesized from a diol and dicarboxylic acid through condensation polymerization [19]. Despite their biodegradability, they possess some difficulties in engineering applications due to their low melting temperature, weak thermal stability, and low molecular weight. To improve the applicability, various blends of BAP with other polymers including linear low density polyethylene [18] and poly(epichlorohydrin) [20] have been examined. In this study, we investigated the synthesis and rheological properties of nanocomposites fabricated from a biodegradable polymer and organophilic montmorillonite (OMMT). Even though BAP can compete with PHB in cost, applications of BAP are limited because it is still expensive when compared to conventional polymers. Therefore, BAP/clay nanocomposites can lower cost and increase the range of applicability of biodegradable polymers. Recently, biodegradable polymers have been used to produce poly(l-lactide) (PLLA)-montmorillonite (MMT) [21], poly(e-caprolactone) (PCL)-MMT [22] and BAP [23] intercalated nanocomposites. A synthetic BAP (Skygreen 2109) sample obtained from synthesis of diols and dicarboxlic acids with a weight-average molecular weight of 6.0 × 104 g/mol was supplied from SKI, Korea. The Cloisite 25A, supplied by Southern Clay Products (USA), was used as an OMMT. The pristine hydrophilic Na+-MMT was engineered by the manufacturer to be organophilic by cation exchange reaction between the Na+ and a dimethyl hydrogenated tallow 2-ethylhexyl quaternary ammonium ions. The replacement of inorganic exchange cations by surfactant on the MMT surface not only serves to match the surface polarity with that of the polymer, but it also expands the interlayers. The structures of BAP and organic modifier of Na+-MMT are shown below: