Elsevier

Polymer

Volume 47, Issue 19, 7 September 2006, Pages 6759-6764
Polymer

Effect of crystallization on morphology–conductivity relationship in polypyrrole/poly(ɛ-caprolactone) blends

https://doi.org/10.1016/j.polymer.2006.07.042Get rights and content

Abstract

Electrically conducting blends, based on polypyrrole (PPy) as the conductive polymer and poly(ɛ-caprolactone) (PCL) as an insulating polymeric matrix, were prepared by polymerizing pyrrole (Py) in its vapor state inside the PCL matrix. The roles of specific interactions between blend components as well as the crystallization of PCL matrix in the resulting morphology have been analyzed by Fourier-transform infrared spectroscopy (FTIR), thermo-optical analysis (TOA) and atomic force microscopy (AFM). The results indicate that PPy is located within both the intra and interspherulitic regions of the PCL matrix achieving a well-developed connected network. Compared with amorphous matrices, considerable conductivity (around 1 S/cm) was raised with the crystalline PCL matrix with only a relatively low level of the conductive polymer (∼5%) in the blend.

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 (M¯w) 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.

References (29)

  • P. Dyreklev et al.

    Polymer

    (1996)
  • C. Cassignol et al.

    Polymer

    (1999)
  • D. Mecerreyes et al.

    Synth Met

    (2002)
  • J. Duchet et al.

    Synth Met

    (1998)
  • M. Chakraborty et al.

    Synth Met

    (1999)
  • J. Kim et al.

    Synth Met

    (2003)
  • D.V. Andreeva et al.

    Thin Solid Films

    (2002)
  • V. Mano et al.

    Polymer

    (1996)
  • P. Chandrasekhar

    Conducting polymers, fundamental and applications: a practical approach

    (1999)
  • Skotheim TA, Elsembaumer RL, Reynolds JR. Handbook of conductive polymer, 2nd ed. New York;...
  • M.D. Paoli et al.

    Macromol Symp

    (2002)
  • W. Wang et al.

    React Funct Polym

    (2001)
  • M.C. de Jesús et al.

    Polym Eng Sci

    (1997)
  • Cited by (23)

    • Synthesis and characterization of a conductive and self-healing composite

      2018, Synthetic Metals
      Citation Excerpt :

      A slight decrease in hydroxyl groups peak in the PGS/PPy composite spectra, and appearance of a peak associate to amine bond, can also be seen in Fig. 4, indicating the formation of hydrogen bonds between the polymers chains [25]. Corres et al [28] cited this hydrogen bonding interactions as playing a considerable role on the morphology of the polymer matrix, providing a more homogeneous distribution of PPy in the crystallizable matrix and, consequently, favoring the conductivity of the composite. The bound rubber theory states that the adsorption of macromolecules on a filler surface proceeds by bond formation [29].

    • In situ surface selective functionalization of honeycomb patterned porous poly(ε-caprolactone) films using reactive substrate

      2018, Polymer
      Citation Excerpt :

      Breath figure (BF) method is considered as a simple and convenient method for fabrication of HCP films [6]. Among various polymers used for the fabrication of HCP films, poly (ε-caprolactone) (PCL) has been attracted because of its biodegradability, biocompatibility, and non-toxicity [7,8]. PCL is successfully used for the fabrication of HCP films by several researchers [7–9].

    • Impact of carbon nanotube prelocalization on the ultra-low electrical percolation threshold and on the mechanical behavior of sintered UHMWPE-based nanocomposites

      2017, Polymer
      Citation Excerpt :

      A well-known approach for improving physical and electrical properties of an insulating polymer matrix is to introduce conducting nanofillers. For this purpose, carbonaceous fillers such as carbon black particles [8,9], carbon fibers [10], graphite [11], graphene [12,13], metallic fillers [14,15], or intrinsically conducting polymers [16] have received considerable attention. The electrical conductivity of the composite material is then controlled by the intrinsic properties of the conductive fillers (shape, size, conductivity), the filler content and the microstructure of the composite (homogeneity and quality of the filler dispersion within the polymer matrix).

    • Synergic effect in electrical conductivity using a combination of two fillers in PVDF hybrids composites

      2013, European Polymer Journal
      Citation Excerpt :

      PPy can be doped with different anions [29–33]: perchlorate, tosylate, nitrate, phosphate, organic sulfonic acids, glycine, iron chloride, among others. To facilitate the processing, therefore, PPy has been mixed with other polymers to form blends, for example, with thermoplastic polyurethanes [34], poly(aniline) [35,36], poly(methylmethacrylate) [37,38], poly(vinlychloride) [39], polycaprolactone [38,40], elastomers [41] or even with inorganic fillers to form composites [42,43]. The percolation threshold varies between 2 and 14 wt%, depending mainly on the polymer and processing conditions.

    • High-pressure solution blending of poly(ε-caprolactone) with poly(methyl methacrylate) in acetone + carbon dioxide

      2008, Polymer
      Citation Excerpt :

      It is used to lower the Tg of another polymer or enhance its degradation for environmental remediation. Indeed, blends of PCL have been reported at ambient pressures with poly(vinyl methyl ether), poly(styrene-co-acrylonitrile) [14], polypropylene [15], poly(l-lactic acid) [16], poly(butylene terephthalate) [17], tetramethyl polycarbonate [18] and polypyrrole [19]. PCL has also been explored as compatibilizer for polymer blends such as bisphenol polycarbonate with styrene–acrylonitrile copolymer [20].

    View all citing articles on Scopus
    View full text