Degradation of Polymers: Mechanical
Course Module for ME 5XX - The Degradation of Polymers
Types of Degradation - Mechanical
A polymer can be degraded mechanically by two processes; commonly, by machining processes, and less commonly, by ultrasonic processes.
Machining processes take advantage of the visco-elastic behavior of most polymers. The range includes five regions. Familiarity with the regions is the best way to prevent entering defomation ranges.
- Glassy region - chain segments are frozen in place and movement is minimal.
- Transition region - chain segments undergo short-range diffusional motion, and alterations occur as a function of applied forces.
- Rubbery plateau - short-range diffusional motion is rapid, but entanglements act as crosslinks, inhibiting motion.
- Rubbery flow - molecules as a whole are now moving, long-range entangle ments are slipping, and applied stress leads to permanent deformation.
- Liquid flow - long-range changes in the chain occur instantaneously, and deformation is completely irreversible.
Two elementary chemical reactions occur during mechanical degradation. Primarily, the whole polymer is divided into two radicals, purely by virtue of the mechanical forces placed upon it. Subsequently, the radicals that are formed can undergo any of a multitude of reactions, as outlined by Reich and Stavala.
Ultrasonic processes induce main-chain rupture by cavitation. Cavitation occurs under the influence of ultrasound, in which bubbles are formed and collapse in the materials. The rapid collapse sends out shock waves into the material and the main-chain rupture is believed to stem from this rapid motion of solvent molecules, to which the macromolecules cannot adjust.
This page prepared by Kimberly Farrell for Professor Sisson of WPI, for use in ME XXX, Corrosion and assorted fun times.