Clay based polymeric composites: Preparation and quality characterization

Abstract

Commercial clays Cloisites^®Na⁺, 30B and 20A were labelled with the fluorescent dye Rhodamine B and used as fillers of polypropylene in order to prepare composites to be studied with confocal fluorescence microscopy. The dye uptake by clays was monitored by X-ray powder diffraction and spectroscopic analyses and clear evidences of intercalated dye in the organically modified montmorillonites Clo30B and Clo20A were obtained. Clay–Rhodamine B hybrids were investigated by steady-state absorption and emission spectroscopy to explore the effect of dye arrangement on the optical properties. The obtained information was used to rationalize fluorescence behaviour of composites. Confocal fluorescence imaging gave rise to bright fluorescent images of Cloisite aggregated labelled with the dye allowing to easily and directly visualize the 3-D dispersion of the labelled fillers in the polymer matrix in a non-invasive manner. The images were analyzed in terms of size distribution of the fluorescence grains to quantify the dispersion degree. The data indicate that Clo20A is able to homogeneously distribute in the polymer matrix forming a composite material.

Introduction

Over the past decade nanocomposites obtained by dispersion of inorganic nanoparticles in polymeric matrices have attracted great interest, both in industry and in academia, because the presence of nanofillers affords a remarkable improvement of the material properties when compared to those of the virgin polymer or of conventional micro and macro-composites. The improvements can include mechanical properties, heat resistance, flammability, gas permeability and biodegradability of biodegradable polymers [1], [2], [3], [4], [5], [6]. Moreover, the composites may show additional specific properties if the fillers are properly modified, i.e. with organic species having magnetic, non-linear optics [7], [8], [9] or pharmaceutical [10], [11], [12] properties.

To ascertain the degree of dispersion of the filler and the type of the polymeric composites obtained, techniques as X-ray diffraction, transmission electron microscopy and rheology in the molten state of composites are generally used.

Confocal fluorescence microscopy is an emerging and complementary technique to study the degree of dispersion of inorganic or organo-inorganic fillers onto polymers when fluorescent dyes or chromophores are anchored to the filler particles and act as probes of their dispersion. This technique, compared with those above mentioned, offers the advantage to analyze different focal planes of the composites, to deeply explore the materials and to obtain a wider representative sampling. The space-resolved fluorescence images and fluorescence spectra obtained give in fact valuable information on the size and distribution of the filler inside the polymer.

In the past years, in our research groups, the confocal fluorescence microscopy has been used to investigate hydrotalcite-like compounds loaded with fluorescence dyes and a direct view of the dye distribution on the micro-crystals was obtained. In general, the hydrotalcite crystals presented dimensions in the range of few μm in agreement with the morphologic analysis obtained with scanning electron microscopy. Furthermore, the inhomogeneous fluorescence distribution on the micro-crystals and the lack of overlap of the fluorescence spectra recorded from different areas of the sample (presenting differences in intensities) led to the conclusion that the anionic dyes are not homogeneously located in the lamellar matrices [13], [14]. These interesting results stimulated the use of confocal microscopy in the investigation of polymeric composites since the internal areas of the composites can be visualized in a non-invasive way. In a preliminary study [15] polymeric composites obtained by dispersion of Cloisites^®, commercial cationic clays, into polypropylene, were equilibrated for one day in n-propanol containing 10⁻² mol dm⁻³ of Rhodamine B, chosen as a fluorescent dye probe. The dye migrated in the polymer and was adsorbed on the surface of the filler. The analysis of the composites with the confocal fluorescence microscope gave rise to bright fluorescent images of Cloisite particles labelled with the dye. The dimensions of these particles gave an indication on the dispersion degree of the clay into the polymer. However there was no clear evidence that all the particles in the composite were reached by the diffusing dye and interferences due to dye salt solubilized in the polymer could not be excluded. Therefore, a study to label the Cloisite fillers before the compounding process was undertaken and it is the object of the present paper.

In particular, three different Cloisites have been chosen, Cloisite^®Na⁺, Cloisite^®30B, Cloisite^®20A, and their ability to adsorb the dye Rhodamine B has been studied. Moreover, the obtained Cloisite–Rhodamine hybrid materials have been characterized by absorption and emission spectroscopy to select the filler containing the proper amount of dye in order to prepare a suitable polypropylene composite to be studied with confocal fluorescence microscopy.