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Thermal expansion coefficient of polytetrafluoroethylene in the vicinity of its glass transition at about 400°K
Author(s) -
Araki Yoshio
Publication year - 1965
Publication title -
journal of applied polymer science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.575
H-Index - 166
eISSN - 1097-4628
pISSN - 0021-8995
DOI - 10.1002/app.1965.070090203
Subject(s) - glass transition , amorphous solid , thermal expansion , inflection point , materials science , thermodynamics , transition point , transition temperature , polymer , analytical chemistry (journal) , composite material , condensed matter physics , chemistry , physics , crystallography , mathematics , organic chemistry , geometry , superconductivity
The linear thermal expansion of molded PTFE samples of various crystallinities was measured in the vicinity of the transition at about 400°K. In the plot of length of the specimen versus temperature, an inflection of the straight line was observed in the temperature range from 388 to 402°K., the mean value being 396°K. (123°C.) The change of the coefficient of linear thermal expansion at this temperature represents a typical feature of the second‐order transition or the glass transition, designated as T g . The ratio T g / T m is equal to 0.66, in accordance with the so‐called ⅔‐rule between the glass transition temperature ( T g ) and the melting point ( T m ) of many polymers. The coefficients of linear thermal expansion, below and above T g , designated α 1 and α 2 respectively, increased with increasing amorphous fraction, A . The extrapolated values of α 1 and α 2 at A = 0 were both 0.6 × 10 −4 deg. −1 , and those at A = 1 were 2 × 10 −4 deg. −1 and 3 × 10 −4 deg. −1 , respectively. The value of T g Δβ/ A , where Δβ = 3(α 2 – α 1 ), was approximately 0.12 which was closely in agreement with the value of 0.08 or 0.113 predicted by the theories for amorphous high polymers. From these experimental facts, it may be concluded that the transition of PTFE at about 400°K. is the glass transition which occurs in the amorphous region.