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a 6 108 104 I I I -74. I'C. 0 "c. 2 5°C. SO"C, \ 0.01 1.0 100 Time, hr. Fig. 1.-N.B.S. polyisobutylene.to the rubbery modulus value of 106.88 dynes/ Stress relaxation curves a t different temperatures can be superposed by a horizontal translation along the log time axis, and this principle of superposition permits the construction of a master curve that covers the complete time scale.Polytrifluorochloroethylene is a polycrystalline polymer whose melting temperature T m is 212'. Its glass transition temperature has not been accurately determined but should be somewhere in the neighborhood of room temperature if the approximate relation Tg = 2/3Tm is valid. Figure 2 shows stress relaxation data for polytrifluorochloroethylene in the temperature range between 30 and 193". I n this interval the modulus varies from 10'O.l dynes/cm.2 to dynes/cm.2.It is particularly interesting to contrast Figs. 1 and 2. I n Fig. 2 the log E r ( t ) versus log ( t ) curves between 30 and 144" are relatively flat; ie., the modulus change with time in the "transition region" is much less marked for the polycrystalline polymer as compared to the amorphous polymer. Also in Fig. 2 the modulus value of log Er(t) at t = 0.01 hr. changes from a value of 10'O.l dynes/ cm.2 at 30" to a value of a t 144", a very gradual change. The '(transition region," if such it can be called, for a polycrystalline polymer obviously extends over a much wider temperature range than for an amorphous polymer. The "transition region" blends into a high modulus "rubbery region," the crystallites playing the same role that entanglements or cross links do in the amorphous polymers.The relatively rapid decay of stress a t 193" is no doubt associated with a change of microcrystalline 0.001 0.01 0.1 1 10 100 1000 Time hours. Fig. 2.-Stress relaxation of polytrifluorochloroethylene. structure or texture, i e . , an orientation of crystalline material.Because there are changes with temperature in the microcrystalline structure and in the stress bearing mechanisms, it is certain that the simple time-temperature superposition that is valid for amorphous polymers in the transition region is not valid for polycrystalline polymers. There is not only a horizon displacement along the log time axis due to changing rate of molecular motions with temperature, but also an even more important vertical shift along the log Er(t) axis due to the changing structure and other factors. L I

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