POLYMER LETTERS EDITION VOL. 10, PP. 777-779 (1972)

EFFECT OF LIQUID NITROGEN ON THE TENSILE STRENGTH OF POLYETHYLENE AND

POLYTETRAFLUOROETHY LENE

Parrish and Brown (1) first discovered that having a polymer in contact with liquid nitrogen would effect its intrinsic tensile stress-strain behavior. They found that amorphous polymers crazed in liquid nitrogen, but not in a helium or vacuum environment at about 78'K. The strength in helium was generally equal or greater than in a nitrogen environment. Whereas in nitrogen the effect of strain rate was large, in helium, the effect of strain rate on the frac- ture stress was very small. Both liquid argon and gaseous nitrogen acted like liquid nitrogen in that they produce crazing and caused a large strain rate effect. In the prior work (1) PMMA, PET, and PC were found to behave similarly. In this letter the behavior of crystalline polymers, PTFE and PE, when tensile tested in liquid nitrogen and helium at 78"K, is reported.

The PTFE was obtained from DuPont as compression molded film 0.03 in. thick. The PTFE sheet was cooled at 0.2OC per min so that its density was 2.1904 and crystallinity 66.3%. The PE was Marlex which was obtained from Phillips Petroleum Company as compression molded film 0.030 in. thick; the density was 0.964, crystallinity, 94%, weight average molecular weight, 92,000, with 0.04 weight per cent antioxidant additives.

The tensile stress-strain curves for the PTFE in liquid nitrogen and helium at 78°K are shown in Figure 1. The specimen fractured in liquid nitrogen after the stress had reached a maximum. The stress-strain curve is similar to that obtained by Speerschneider and Li (2) who immersed polymer in liquid nitrogen. The fracture stress in helium is 33% greater than in liquid nitrogen. Both specimens showed completely brittle fracture. Thus far no difference in fracture morphology has been observed.

linear stress-strain behavior up to the fracture stress. The only difference be- tween the two environments was exhibited by the fracture stress. In liquid nitrogen at a strain rate of 0.004/min the average fracture stress was 14.3 X lo3 psi with a standard deviation of 0.5 X lo3 psi. In helium the average fracture stress was 17.3 X lo3 psi with a standard deviation of 0.5 X lo3 psi. Thus, the difference between the liquid nitrogen and helium is 3.0 X lo3 psi or 6 standard deviations. This difference is certainly significant.

We have shown that linear polymers, both amorphous and crystalline, will usually exhibit a lower tensile strength in liquid nitrogen than in helium or under vacuum. We have not tested crosslinked material. Preliminary results with one way oriented PET indicated that the effect is greatest if the stress is normal to the draw direction and may disappear if the stress is parallel to the draw direction. Since the previous experiments (1) showed that liquid nitro-

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The PE specimens, both in liquid nitrogen and helium, exhibited nearly

0 1972 by John Wiley & Sons, Inc.

778 POLYMER LETTERS EDITION

STRAIN

Fig. 1. Tensile stress-strain curves of PTFE in liquid nitrogen and gaseous helium at 78°K.

gen causes crazing, there is the implication that crazing fracture may also be produced in the crystalline polymers by liquid nitrogen. Thus far, no direct evidence of crazing has been observed in the PTFE or PE.

The general explanation for the effect of liquid nitrogen in reducing the tensile strength is based on the adsorption of nitrogen which reduces the sur- face free-energy. It has been shown by Graham (3,4) that nitrogen and argon at temperatures near their boiling point reduced the surface free-energy of F'TFE and PP by about 1 1 ergs/cm2 by means of adsorption. Braught et al. (5) reported that nitrogen and argon at low temperatures reduce the surface free-energy of PE, PMMA, PS, and PVC by about 20 - 40 ergs cm2. Since the typical surface tension for all linear polymers is about 40 ergs/cm2, it is seen that liquid nitrogen makes a significant decrease in the surface energy. Thus far no difference between nitrogen and helium atmospheres has been observed at 200" and 300°K, presumably, because the amount of adsorbed nitrogen is negligible at these temperatures. The details of the mechanism whereby the liquid nitrogen changes the tensile strength will vary from one type polymer to another. The brittle behavior of PE should require a differ- ent explanation than that for the amorphous polymers which yield and craze.

This work was supported by the Advanced Research Projects Agency of the Department of Defense and the National Science Foundation. Mr. S. Fischer aided in the experimental work.

POLYMER LETTERS EDITION 779

References

(1) M. Parrish and N. Brown, Nature, (Physical Sc.). 22(77), 122 (1972). (2) C. J. Speerschneider and C. H. Li, J. Appl. Phys., 34, 3004 (1963). (3) D. Graham, J. Phys. Chem., 66, 1815 (1962). (4) D. Graham, J. Phys. Chem., 68, 2788 (1964). (5) D. C. Braught, D. D. Bruning, and J. J. Scholz, J. Colloid Interface

Sci., 2, 263 (1969).

Norman Brown Mark F. Parrish

School of Metallurgy and Materials Science Laboratory for Research on the Structure of Matter University of Pennsylvania Philadelphia, Pennsylvania 19 104

Received July 12, 1972 Revised September 1, 1972