Low-cost plastic memories from a ‘forgotten’ commodity polymer

March 18, 2013

Scientists from the Zernike Institute for Advanced Materials of the University of Groningen and the German Max Planck Institute for Polymer Research have discovered a way to make a plastic memory from a commodity polymer. They have published their discovery in the journal Nature Materials.

Mainz/Groningen. The polymer they used is PVDF – polyvinylidene fluoride. This low-cost material is often used for membrane filters and packaging foils. It is well-known that PVDF is thermally and chemically extremely stable, but for application in a memory cell ferroelectric properties are essential. In other words, the material must exhibit an electric polarization, comparable to the north and south poles of a magnet. The polarization of ferroelectric materials can be switched by application of an electric field. The bistable polarization state can be used to store information.

Challenge accepted

Making a functional electric switch from neat PVDF is notoriously difficult. “There are two reasons for this”, says professor of physics Dago de Leeuw, one of the authors of the Nature Materials article. “First, it was very challenging to make a smooth, thin film from PVDF.” Comprising films were rough, resembling microscopic sand paper. “In addition, conventional processing yields non-ferroelectric thin films because the PVDF crystallises in a non-polar phase.”

Mengyuan Li, with two thin PVDF samples: Opaque rough PVDF film via conventional processing on the right, and transparent smooth PVDF film via improved processing on the left.

“PVDF has four different crystal phases”, De Leeuw continues. With her work, Li has formed the delta phase, which is ferroelectric and also stable at high temperatures. The existence of delta-PVDF was predicted in the 1980s, but had never experimentally been proven in a thin film.

In the article, the researchers show that a plastic memory can be made of delta-PVDF. Plastic memories already exist, but they are made of a specialty copolymer of PVDF with trifluoroethylene. “That material is difficult to make, very expensive, and it also loses its ferroelectric properties at temperatures above 80 degrees Celsius. When that happens you lose all stored data.” PVDF, on the other hand, costs next to nothing and the films preserve the stored information up to temperatures of about 170 degrees Celsius. As a result, delta-PVDF is the ideal candidate for data storage in plastic electronics.

Promising fields of application

Plastic electronics has developed rapidly and is now on the verge of commercialization, promising new applications from smart food packaging that keeps track of the expiration date, to wearable health monitors. All these applications require programmable non-volatile memory. The University of Groningen has already made several breakthroughs in the field of plastic electronics. De Leeuw: “Both the plastic ferroelectric transistor and the plastic ferroelectric diode were invented at ZIAM. We can now add a usable plastic memory to that list.” A European project (MOMA) is presently upscaling this new technology.

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