In a recent study, a block copolymer made of poly(styrene-b-isoprene) showed piezoelectric properties more than 10 times higher than a standard ceramic piezoelectric. Potentially, this material could be employed in several devices, from electronics to biomedical devices.
What are Block Copolymers?
A homopolymer – normally referred simply as polymer – is a molecule composed of a unit (monomer) repeating itself regularly; these units are bonded together to form long chains. In some cases, these chains can also be bonded together between themselves.
Copolymers, on the other hand, are formed with more monomeric units. These can be arranged in different ways. For instance, monomeric units can be alternating regularly within the copolymer chains, or they can be repeating, following a certain pattern.
Block copolymers are particular copolymers, formed by two or more types of homopolymer chain bonded together. The picture to the left gives an example of the way in which the two homopolymers can be arranged. Due to their particular structure, block copolymers can have interesting properties.
Piezoelectricity – Concept and Applications
A material is piezoelectric if it accumulates an electric charge when under a mechanical pressure. The reverse process can also take place – a mechanical strain can be caused by applying an electric voltage. With piezoelectric materials, mechanical energy can be converted into electricity and vice-versa.
Materials with piezoelectric properties are employed in several sectors, such as the automotive and electronics industries, as sensors, transducers, ultrasonic devices, sonar, etc. An example of a very common application is the electric lighter used, for instance, in some gas cookers. A pressure is applied on a small piece of piezoelectric material; this generates an electric charge, which provokes a spark. This spark, in contact with the gas, forms the flame.
Current Piezoelectric Materials and Alternatives
Today, the most efficient, and therefore most often used, piezoelectric materials are ceramics such as barium or lead titanate (BaTiO3 and PbTiO3 respectively), or lead zirconate titanate (Pb[ZrxTi1-x]O3 or simply PZT).
These ceramic materials, however, can be dangerous for health, due to the presence of toxic metals, such as barium or lead. Barium is a poisonous metal, and lead can be even more harmful both to humans and the environment. For this reason, there are strict regulations about the use and dispersion of lead in the environment. The search for lead-free piezoelectric materials is, therefore, a growing sector.
Quartz, the naturally-occurring mineral, also shows some piezoelectric properties, and it is employed for some applications. In the majority of cases, however, an appropriate, lead-free, piezoelectric material has not been found.
Block Copolymer Poly(styrene-b-isoprene)
Some significant results in the search for a better piezoelectric material were reported recently by Professor Alexander Böker and his coworkers, in the Institute of Physical Chemistry of Aachen University (Germany).
Professor Böker’s team investigated the piezoelectric properties of a block copolymer of poly(styrene-b-isoprene) (SI). This copolymer presents a lamellar structure, as shown in the figure above. It is this feature which makes SI behave as a piezoelectric material.
The team’s experiments showed that the performance of this copolymer is more than 10 times better than some of the standard materials currently in use. The piezoelectric susceptibility (i.e. the deformation caused by applying a certain voltage) of SI can be as high as 6500 pm/V, while barium titanate (BaTiO3) shows a value of only 587 pm/V.
Unexpected Behavior of poly(styrene-b-isoprene) (SI)
The piezoelectric properties of SI were better than the performance of other existing polymers. Furthermore, the main element of novelty is that SI showed very high piezoelectric susceptibility even if it has a non-polar structure.
This unexpected behavior was never reported before; it could be the beginning of new uses for a whole series of materials. It will be interesting, for instance, to find out if other non-polar copolymers, with similar lamellar structure, do behave in the same way or not.
The use of a polymer instead of a ceramic makes the material more versatile; for example, it will be easier to employ it on flexible substrates, or even textiles. Furthermore, SI does not contain poisonous elements; therefore, it could be safe to use for biomedical devices.
The article from professor Böker and his coworkers was published in Advanced Materials. The research was performed in cooperation with Oak Ridge National Laboratories and University of California Santa Barbara.
European Commission. Heavy Metals in Waste. Final Report. Accessed October 13, 2011.
C.W. Pester et al.: Piezoelectric Properties of Non-Polar Block Copolymers. Advanced Materials, 23(35), 4047, 2011. Accessed October 13, 2011.
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