Morphology, Barrier, and Mechanical Properties of Biaxially Deformed Poly(ethylene terephthalate)-Mica Nanocomposites Kok Soon, Eileen Harkin-Jones, Rajvihar S. Rajeev, Gary Menary, Peter J. Martin, Cecil G. Armstrong School of Mechanical and Aerospace Engineering, Queen’s University Belfast, BT9 5AH, UK Nanocomposites of poly(ethylene terephthalate) PET with a partially synthetic fluoromica were prepared by melt mixing and extruded into sheet and subjected to large-scale biaxial stretching. Transmission electron microscopy (TEM) analysis of the mica tactoids showed that biaxial stretching had caused the tactoids to be more orientated and with improved exfoliation. The moduli of the nanocomposites were enhanced with increasing mica loading and the reinforcement effect was higher when the stretch ratio was 2 or 2.5, accom- modated by having more aligned tactoids and reduced agglomeration. Enhancement in modulus was less pro- nounced for a stretch ratio of 3. Storage modulus was enhanced more significantly above the glass transition temperature. The barrier properties were enhanced by addition of mica before and after stretching. The Hal- pin-Tsai theory underpredicted the relative modulus of the PET nanocomposites, whereas the Nielsen model over-predicted the relative permeability. POLYM. ENG. SCI., 52:532–548, 2012. ª 2011 Society of Plastics Engineers INTRODUCTION Poly(ethylene terephthalate) (PET) is an engineering thermoplastic which finds many applications in industries such as food and beverage packaging, and textiles. It is generally desirable to enhance the mechanical and barrier properties of PET so that lighter products can be manu- factured and the shelf life lengthened when food and bev- erage packaging are concerned. One promising method to achieve these enhancements is by adding nanoclays to the PET matrix. Nanoclays are readily expandable and dis- persed in a polymer matrix, provided it is compatible with the matrix and varying exfoliation levels can be achieved depending on the processing conditions. Since, unlike conventional composite materials, the filler has dimen- sions on a nanometer scale reinforcement can be achieved without significant loss of transparency which is a major advantage in packaging applications. The morphology of polymer-clay nanocomposites is usually characterized as either intercalated or exfoliated. An intercalated structure has polymer chains diffused into the clay layer galleries, expanding them whereas an exfoliated structure has its clay layers completely delaminated and the layered struc- ture is destroyed. Complete exfoliation is generally desira- ble due to the high aspect ratio of dispersed nanoparticles which is believed to give superior mechanical and barrier properties. However, complete exfoliation is notoriously difficult to achieve and normally nanocomposites reported in literature are only partially exfoliated [1]. Recently, numerous studies on PET-clay nanocompo- sites have been reported in the literature. Kim and Kim processed PET nanocomposites by in situ polymerization using self-prepared montmorillonite (MMT) based organo- clay. It was pointed out that the nanocomposites synthe- sized had better exfoliation compared to nanocomposites based on unmodified MMT. The tensile strength was improved by 78% for a 0.5 wt% clay loading and the bar- rier to oxygen improved by 52% at 1 wt% clay loading [2]. Hwang et al. attempted to process PET nanocompo- sites by in situ polymerization and arrived at a similar conclusion in that organically modified clay achieved bet- ter exfoliation than the unmodified counterpart. The improved exfoliation with organoclay was ascribed to the increased clay basal spacing and improved compatibility with the PET matrix. The modulus of PET was increased by 33% at 5 wt% clay loading, however, the tensile strength and elongation at break reached a maximum at 0.5 wt% clay loading and decreased with a further increase in clay loading, which the authors attributed to an increased agglomeration level [3]. Their studies showed the significance of the organic surfactant in the nanoclay dispersion. Also, they pointed out that agglomer- ates of nanoclay particles and delaminated clay sheets can coexist, hence it is necessary to analyze the nanocompo- site morphology at both micron and submicron scales. Also, the incorporation of nanoclay could cause degrada- tion of the matrix through side reaction between the sur- factant and the polymer chains [4]. This degradation can Correspondence to: K. Soon; e-mail: Ksoon01@qub.ac.uk Contract grant sponsor: Engineering and Physical Sciences Research Council (EPSRC), UK. DOI 10.1002/pen.22114 Published online in Wiley Online Library (wileyonlinelibrary.com). V V C 2011 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—-2012