Chemists strive to extend the life of crystalline plastics

August 24, 1999

(Blacksburg, Va., August 24, 1999) Semicrystalline plastics, such as polyethylene, polypropylene,nylons, and polyesters, are abundant materials in modern society. These important materials are used in the form of films for packaging applications, fibers for clothing or carpets, containers for milk and sodas, and even in the make up of soles for running shoes.

A memorial symposium for Andrew Keller, a pioneer in the field of semicrystalline polymers since the late 1940s, will be offered at the 218th American Chemical Society National Meeting in New Orleans Aug. 22-26, as part of the Division of Polymeric Material Science and Enginering program. Symposium organizer Hervé Marand says that some 100 researchers from around the world will discuss semicrystalline plastics, particularly how such plastics crystallize and how the way in which they crystallize governs their end-use properties.

Now that semicrystalline plastics have been around for several decades, a significant consideration is -- what happens to them over time?

Chemistry Professor Marand's Virginia Tech research group is studying secondary crystallization, and its role in governing the evolution of plastics' properties over long periods of time. Secondary crystallization is a slow process which takes place in crystallizable polymers after processing and over extended periods of time (months and even years) and which gives rise to changes, often detrimental, in their properties.

"Frequently, crystalline plastics become brittle. The plastic garbage bags you've been keeping in your picnic basket or the trunk of your car may tear instead of stretch, for instance. The soles of your running shoes may lose their bounce or cushion attributes," Marand says. A material that originally remained pliable to very cold temperatures can become less resilient.

The research group is 1) looking a the material and environmental parameters that give rise to change in the material characteristics, and 2) creating models relating the changes in the plastics structure to the evolution of their properties. Such models will hopefully in the end help chemists design new materials with better long term properties.

"Once you've created a material, you want to have minimum changes over time and you want the material to be able to withstand a wide range of environments -- maybe even extreme environments," he explains. "You wouldn't want packaging of medical or safety supplies to breakdown, for instance."

The talk by Marand and Assistant Research Professor Azar Alizadeh, "Polymer secondary crystallization: A universal model (PMSE 120)," will be presented Tuesday, Aug. 24, at 2:40 p.m. in the Hilton Riverside Melrose Room.

In the process of studying the crystallization of plastics, the researchers discovered a new method for determining the equilibrium melting temperature for semicrystalline polymers. The precise knowledge of this material characteristic is very important especially in modeling their crystallization during processing, Marand says.

"We all know that water freezes -- makes the transition from liquid to solid -- at zero degrees C or 32 degrees F. The transition point for polymers has been less certain," Marand explains. Virginia Tech Graduate Student Jiannong Xu, Marand, and Srivatsan Srinivas of Exxon Chemical Co., present a new theory that defines the transition point for semicrystalline polymers. Their poster,

"Determination of the equilibrium melting temperature of semicrystalline polymers by the nonlinear Hoffman-Weeks extrapolation" (PMSE 140), will be available at 6 p.m. Tuesday in Convention Center Exhibit Hall A. Posters from the Virginia Tech research group also present applications of their universal secondary crystallization model to a wide variety of crystallizable polymers.
Contact for more information, Dr. Hervé Marand, or 540-231-8227. He will be out of his office between August 21 and 26.

Virginia Tech

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