Effect of crystallization on morphology–conductivity relationship in polypyrrole/poly(ɛ-caprolactone) blends
Introduction
The poor mechanical properties of highly conductive polymers restrict their large-scale application. In an effort to obtain thermally processable conducting blends or composites, conductive polymers have been incorporated into insulating polymeric matrices by chemical synthesis or electropolymerization [1], [2], [3], [4], [5], [6], [7]. The conductive polymer content of a blend needs to reach the percolation threshold to cause the onset of electronic conductivity [1], [3]. Conversely, it is desirable to minimize the concentration of the conductive polymer in the blend in order to avoid detrimental effect upon mechanical properties caused by incorporating rigid chains of the conductive polymer in the insulating matrix.
In some conducting polymer blends the interactions between components were found to play a strong role in determining morphology and conductivity of these materials [7]. It has been proposed that interactions between the blend components are responsible for semi-compatibilization thus promoting conducting networks at low level of conductive polymer [8]. In blends prepared by in situ polymerization, the low solubility of the monomer in the insulating matrix can make it difficult to prepare blends with sufficient conductivity for them to be useful materials. Taking into account that the level of electrical conductivity in these materials depends upon the concentration of the conductive polymer [9], specific interactions between the monomer and the matrix in which polymerization takes place would favor the solubility of the monomer in the host polymer.
Macroscopic properties in blends depend not only on the concentration and chemical structures of their components but also on microstructures developed during the blending. To obtain highly conducting polymer materials and to lower the percolation threshold, different morphologies have been developed by in situ polymerization within the lamellar microdomains of block copolymers [10], [11] as well as inside the void space of a (micro or nano) porous polymeric matrix [12], [13], [14] or inside interstitial domains of a crystallizable host matrix.
In particular, the crystallinity in the host polymer can drive the microstructure that the conductive polymer adopts in the matrix during the polymerization process. Hopkins and Reynolds [15] analyzed the electrical conductivity in blends prepared with polyaniline and an amorphous or a crystalline polyamide as insulating host. They observed that conductivity using a crystalline host is 10 times higher than when using the amorphous one. This result was related to the conducting network or pathway developed in polyaniline as a consequence of matrix crystallization. The authors propose the use of crystallization in the host as a method for controlling morphology and tuning the ultimate electrical properties of conducting polymer blends.
In spite of the technological importance of the conducting pathways upon the final conductivity of the materials, little attention has been paid to understanding the role of the crystallization process on the electrical conductivity of the resultant material. This work has attempted to observe the morphology developed in films formed with both a crystalline host and a conductive polymer. Poly(ɛ-caprolactone) (PCL) and polypyrrole (PPy) were used as the crystalline and conductive polymers, respectively. The major aim was to investigate how the electrical conductivity is related to the morphology created during film formation. Optical microscopy (TOA) and atomic force microscopy (AFM) were used to characterize the developed morphology whilst Fourier-transform infrared spectroscopy (FTIR) was used to analyze specific interactions in blends.
Section snippets
Experimental
The poly(ɛ-caprolactone) (PCL) was supplied in pellet form by Polysciences Inc., with a weight-average molecular weight () of 49,000 g/mol, as measured by gel permeation chromatography (GPC) using polystyrene standards and tetrahydrofuran (THF) as eluent and applying adequate Mark–Houwink constants. Ferric chloride hexahydrate (FeCl3·6H2O), employed as a 1 M solution in bidistilled water, was purchased from Panreac and used as the oxidant without any further purification. Methanol (CH3OH),
Determination of intermolecular interactions
In blends, molecular interactions between components control the level of miscibility. Therefore, any interaction will have an influence on the homogeneity of the distribution of Py in the PCL matrix and consequently in the final dispersion of PPy in this matrix. The most likely center for specific interactions in these blends is hydrogen bonding involving the carbonyl group in PCL and the –NH– group in Py [18]. This is confirmed from FTIR spectra studies for PCL/Py blends, prior to their
Conclusions
The morphology developed by the crystallization of the PCL/Py blend was analyzed by microscopy. It was found that the crystallization process drives pyrrole into the intra and interspherulitic regions, and when pyrrole polymerization takes place, this allows the formation of conducting PPy networks in the blend. Furthermore, the favorable intermolecular interactions, i.e. the hydrogen bonds between PCL and Py, prevent the formation of isolated domains of PPy, achieving high electrical
Acknowledgements
We are pleased to acknowledge financial support of this work from The Spanish Government (MCYT, MAT 2002-04599-C02-0-1) as well as to Diputación Foral de Gipuzkoa, dpto. para la Innovación y la Sociedad del Conocimiento en el Programa Red Guipuzcoana de Ciencia, Tecnología e Innovación 2004.
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