Obtaining Electricity from Plastics

by Jorge Mario Monsalve Guaracao

From supermarket bags to the casing of our smart phones, polymeric materials (commonly

known as “plastics”) have become ubiquitous in our everyday life. Electrically speaking, common

polymers act merely as a current insulator, thus they are only used as a protection barrier for cables.

However, some special polymers not only protect from electric current, but also generate it when they

are stretched. The mere material converts into electrical energy some of the mechanical energy that was

invested in deforming it. This is called the piezoelectric effect, and until now it has been exploited

predominantly in ceramic materials (e. g. Quartz) which, unlike polymers, are naturally brittle and thus

limited to applications with small strains. The presence of the piezoelectric effect in polymeric

materials is very rare; these materials are, in fact, among the worst candidates for developing the

complex crystalline structures that piezoceramics possess – but some do. The key to obtaining electric

currents from polymeric materials lies in an orderly yet asymmetric organization of their internal

chains.

Polymers are long chains of carbon-based molecules which, under the right conditions, form

crystalline regions. These chains resemble a backbone on which the carbon atoms take the place of the

vertebrae and side groups of other atoms branch from them. Normally these chains are randomly

oriented and entangled; however, in some sections of the material the molecules can align to one

another in an orderly fashion. If the chains of the polymer are linear – that is, not bifurcated – and their

side groups are periodic, their arrangement will result in a crystallite. These crystalline regions usually

have complex structures whose shape depends largely on the type and orientation of the side groups. A

polymer can't be completely crystalline, but with a careful processing, like cooling slowly from melt, a

high percentage of it can be made of crystallites.

Not all the possible atom arrangements within the crystallites would allow the polymer to be

piezoelectric. In a crystalline arrangement there is a group of atoms called a unit cell which is repeated

sequentially all over it – a unit cell is to a crystallite what a brick is to a wall. The material can be

piezoelectric only if the organization of the atoms within the unit cell lacks central symmetry. A cubic

cell, for instance, is perfectly symmetric and therefore non-piezoelectric. To the contrary, the cells of

crystalline cellulose have a more exotic triclinic configuration: the least symmetric of the atom

arrangements. This asymmetry results from many factors, including the presence of hydroxylmethane

side groups – a radical within cellulose's glucose units that is larger than the others. This explains why

the piezolectric effect was found in wood in the 1950's.

What allows non-centrosymmetric lattices to accumulate electrical charge when stressed is that

they possess an internal dipole moment. A dipole moment is a couple of opposite electric charges

separated at a certain distance. In the case of a crystal, the charges are set by the atoms themselves

since they lose or gain electrons when forming bonds. While in a symmetric arrangement these charges

are perfectly balanced, in non-centrosymmetric unit cells at least one of the atoms is shifted from the

plane of symmetry without a counterpart in the opposite side. This imbalance provokes the formation

of a net dipole moment within the crystal. Furthermore, this dipole is sensitive to changes in the shape

of the crystal. If the lattice of atoms is distorted, say, by applying a force, more charge will be

accumulated because the dipole has been altered.

Although all non-centrosymmetric cells allow the formation of dipoles, some structures exhibit a

stronger piezoelectric effect than others. Crystals with weak or unstable dipole moments – like wood

itself – are not suitable for most engineering applications; hence, a stronger condition is desired in

electricity-generating polymers, which is that they be ferroelectric. Ferroelectrics are a small subset of

the family of piezoelectric materials that possess permanent yet reversible electric dipoles. In an

analogous way to ferromagnetic materials (that is, permanent magnets,) ferroelectrics retain their

electrical polarization, and they also show a higher piezoelectric response. Unlike the case of wood, in

which some cells have opposite orientations and cancel out the net dipole, in ferroelectric materials the

dipoles stack in one preferred orientation along sections of the material called ferroelectric domains.

Two well-known examples of ferroelectric polymers are Polyvinilidene Fluoride (PVDF) and

odd-numbered Nylons, on which their chain structure is key to obtaining a net polarization. In PVDF,

the monomers form a small dipole from the negatively charged fluorine atoms toward the positively

charged hydrogen atoms. Under the correct processing conditions, which include mechanical stretching

and a small annealing, the fluorine ions manage to align on one side along the carbon chain, all

opposite to the the hydrogen ions; therefore, a preferred electrical orientation is achieved.

Odd-numbered Nylons achieve this kind of alignment as well. In their case, it is the amide group the

one that sets the dipoles. In even-number Nylons this group is intercalated along the chain, canceling

out the net dipole; in contrast, odd-numbered Nylons have an all-trans configuration (that is, the amide

group lies in the same side of the chain), resulting in a net piezoelectric effect.

One more step is still necessary to obtain a material capable of generating electrical signals,

which is the polarization process. Through polarization the domains of a ferroelectric material are

aligned in a single direction with the aid of a strong electric field. This straightening process involves

the displacement of charges and the rotation of the chains within the polymer; for this reason, it is

normally aided by some preheating of the material. A common means of polarization is called

“Corona”, in which a set of needles is placed slightly above a polymeric sheet at a potential difference

of thousands of volts. This considerable voltage ionizes the surrounding air, and, by attraction-repulsion

mechanisms, the sheet becomes polarized in the thickness direction. After this process, ferroelectric

polymers show a considerable improvement of their piezoelectric properties.

Apart from the internal structure of a polymer's crystallites, many factors alter its overall

piezoelectric performance. Polymers are polycrystalline, that is, their crystallites have different

orientations and sizes; as a result, many electrical and elastic interactions arise between them. Apart

from this, not all the crystallites within a polymer have the same structure: some materials crystallize

simultaneously in different phases, some of which could be non-piezoelectric. PVDF, for instance, can

crystallize in four phases and only the one known as β-PVDF is non-centrosymmetric. Furthermore,

there will always be an insulating, amorphous phase corresponding to the disorganized sections of the

polymer's chains. The exact influence of all the polymer's internal irregularities on the piezoelectric

effect is currently under research.

In summary, polymers can manifest piezoelectric behavior as long as they form crystallites with a

non-centrosymmetric structure that permits the formation of dipoles. This effect can be enhanced if the

internal dipoles are permanent and if the material is further polarized. The large number of conditions

for a polymer to exhibit a significant piezoelectric effect make such materials very rare; nonetheless,

there is a crescent interest on them. In the biomedical field, some researchers have developed PVDF

piezofilms that were attached to a lung to measure its movements. This films can also be used to build a

combined diaphragm-transducer that works like a microphone but without a casing. This same

principle can be further exploited to develop ultrasound transducers. The range of novel applications

resulting from these materials is very wide, and hopefully the ongoing research on them will make

them more accessible in the close future.

References[1] W. D. Callister & D.G Rethwisch (2008) Fundamentals of Materials Science and Engineering: An Integrated Approach. 3 ed. Chapter 12: Electrical Properties

[2] G. M. Garner & K. J. Humphrey (1995) Ferroelectric Polymers. In: Special Polymers for Electronics Engineering

[3] B. A. Newman, J. I. Scheinbeim, J. W. Lee & Y. Takase (1992) A New Class of Ferroelectric Polymers, the Odd-Numbered Nylons.

[4] L. Csoka et al (2012) Piezoelectric effect of Cellulose Nanocrystals Thin Flims. ACS MacroLetters

[5] A.L. Kholkin, N.A. Pertsev, and A.V. Goltsev (2008) Piezoelectricity and Crystal Symmetry. In A. Safari and E.K Akdoĝan (eds.) Piezoelectrics and Acoustic Materials for Transducer Applications.

[6] Wada et al (2008) The structure of Celluloses. JCPDS-International Centre for Diffraction Data

[7] Duo Mao, Bruce E. Gnade and Manuel A. Quevedo-Lope (s.f.) Ferroelectric Properties and Polarization Switching

Kinetic of Poly (vinylidene fluoride-trifluoroethylene) Copolymer. Retrieved from www.intechopen.com