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Mechanical behavior of polytetrafluoroethylene around the room‐temperature first‐order transition
Author(s) -
Koo G. P.,
Andrews R. D.
Publication year - 1969
Publication title -
polymer engineering and science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.503
H-Index - 111
eISSN - 1548-2634
pISSN - 0032-3888
DOI - 10.1002/pen.760090406
Subject(s) - materials science , crystallinity , creep , composite material , polymer , strain hardening exponent , viscoelasticity , hardening (computing) , strain rate , ultimate tensile strength , modulus , stress (linguistics) , yield (engineering) , activation energy , linguistics , philosophy , chemistry , layer (electronics) , organic chemistry
Abstract The effect of the room‐temperature first‐order transition on the plastic yield behavior of polytetrafluoroethylene (PTFE) has been investigated. Stress‐strain curves were measured at different strain rates and temperatures. Tensile creep under constant dead load was also measured as a function of temperature and stress level. The effect of degree of crystallinity was investigated by using both a rapidly quenched and slow‐cooled polymer. Observations were extended to large deformations, so that the phenomenon primarily observed was plastic yield rather than linear viscoelastic behavior. The curve of yield stress vs. temperature in the temperature range from –50 to +68°C was found to be almost identical with the curve of elastic modulus vs. temperature; the yield stress shows a marked local decrease at the first‐order transition. The yield elongation was almost constant (at about 5%) over this same range, which is in accord with the above result. The more highly crystalline polymer is always more rigid than the less crystalline polymer at small deformations, but above 19°C its stress‐strain curve shows a “cross‐over” in stress level with the curve of the less crystalline polymer as extension increases. That is, above 19°C the less crystalline polymer shows a more rapid rate of “strain hardening”, even though the strain‐hardening effect is pronounced in both polymers. Attempts to apply time‐temperature superposition to creep data at different temperatures were partially successful; the lateral shifts required corresponded to an activation energy of approximately 80 kcal. The experimental observations suggest a model of the solid‐state structure of PTFE which could be described as an “elastic‐plastic network”, in which crystalline domains are connected by elastic amorphous regions, and in which the crystalline domains can flow plastically at sufficiently high stress or temperature.