Analysis of the radiative properties of insulating and conducting granular polymers and of their mixtures

Abstract

The radiative and electrical properties of an inhomogeneous material made of polyvinyl chloride (PVC, insulating polymer) in granular form on which a polypyrrole (PPy, conducting polymer) coating has been deposited by polymerization of pyrrole are described in this paper. The properties of the obtained encapsulated powders (PVC–PPy) are directly related to the nature of the substrate (composition, granulometry and surface state) and to the preparation conditions (concentration of reactants, temperature and duration of the polymerization). In the following, we report the spectral study of powders PVC, PVC–PPy and of their mixtures in the UV, visible, near (NIR)- and mid-infrared (MIR) wavelength ranges. In other respects, the study of the electrical conduction of the materials allows us, on one hand, to demonstrate the influence of the conditions of polymerization on the electrical behavior, and on the other hand, to show off the electrical threshold of the insulator–conductor mixtures.

Introduction

While olefins are traditionally used as electrical insulators, organic polymers having conjugated π electrons can show electrical conduction properties. These polymers have attracted much scientific interest since their conductivity can be increased from the insulator to the metal range upon doping. However, conductive polymers are far from having good mechanical properties unlike conventional polymers that exhibit mechanical properties suitable for applications in various fields. The idea was to overcome this limitation by producing composites prepared with a conductive polymer coating deposited on a host material such as a conventional polymer. As a result, the obtained composite combines the elasticity of polymers with the conductivity of the conductive polymer. Mixtures of the two types of polymers can also be made, thus permitting the synthesis of new materials presenting electrically conducting properties and preserving the mechanical properties of the polymers [1], [2], [3].

Polypyrrole (PPy) is one of the most widely used conducting polymers in antistatic coatings, heating and conducting textiles, electromagnetic radiation shielding, batteries, absorbing materials for missiles, the aircraft and space industry, rechargeable electrochemical cells, etc., owing to its conductivity, thermal stability and ageing behavior [1], [2], [3], [4], [5], [6].

Radiative properties take into account the energy transfer during the interaction of an incident electromagnetic wave on a surface. They are defined as the ratio of a sample flux to a relative flux. The knowledge of both the spectral and conducting properties of PPy is necessary in so far as numerous applications involve radiative and electrical transfers [7], [8], [9], [10], [11], [12]. Some of them concern, for instance, optical windows, light emitting diodes, camouflage in military domains, and also the conversion by a photocell of radiation such as solar radiation into electrical energy. The observed properties are related to chemical characteristics of the material, such as the nature and composition of the surface, and also to physical factors, such as granulometry, morphology and the surface state of the particles [1], [2], [13], [14], [15], [16], [17], [18], [19], [20].

The synthesis of polypyrrole can be made either by (1) chemical vapor deposition; (2) an electrochemical route (both are experimentally constraining and with a low yield); or (3) a chemical polymerization method, easy to handle and thereby chosen when larger quantities are required, because it can be usually done in water [1], [2], [3], [4], [5], [6], [7], [8], [13], [14], [21], [22], [23], [24]. A large variety of supports can be chemically coated with PPy; they include insulating materials such as plastics, metal oxides or glasses, or conducting materials such as metals. Materials presenting particular shapes such as porous membranes, honeycomb structures and micro- or nano-particles can also be chemically coated with PPy.

This paper reports the study of the particular radiative and conductive linked properties of polypyrrole, deposited on an insulating material, polyvinyl chloride (PVC), in granular form, and of their mixtures.