Feb., 1957
MELTINGBEHAVIOR OF IRRADIATED POLYETHYLENE
137
MELTING BEHAVIOR OF IRRADIATED POLYETHYLENE BYMALCOLM DOLEAND W. H. HOWARD The Chemical Laboratory of Northwestern University, Evanston, Illinois Received July 19,.1066
The specific heat of a sample of polyethylene exposed to 50 megarep of 7-radiation as well as one exposed to approximately 336 megarep of pile radiations has been measured from -20 to 140’. In the case of the 50 megarep sample a slight depression of the maximum melting temperature of about 1.5’ was observed, although no flow of the polyethylene occurred above this temperature. In all other respects the specific heat of this sample was identical with that of unirradiated polyethylene. I n the case of the pile irradiated sample, the crystallinit decreased 5.7%. No depression of the maximum melting point could be observed, probably because of a change in the czaracter of the melting. The G-value for hydrogen evolution was estimated to be 3.75.
I. Introduction It has been stated’ that irradiation of poly-
ing a t various temperatures, the definition of the melting point has to be restricted to the temperature ethylene increases the melting point, that irradiated a t which the last bit of crystallites to melt are in polyethylene2 “no longer melts,” even a t tempera- equilibrium with the melt. The maximum melting tures of 30O0, but “loses its crystalline character a t point is the temperature at which the last detect110”” thatS the “transition temperature corre- able crystallinity disappears. Because different sponding to melting in ordinary polyethylene is methods have different sensitivities for crystallite only very slightly decreased with increasing cross- detection, the different methods may yield diflinking, so that the temperature a t which all crys- f erent results. We prefer to define the maximum melting point talline structure vanishes is little affected,” and that4 irradiated polyethylene melted a t about the as the temperature of intersection of the enthalpysame temperature as unirradiated polyethylene, temperature curve for the liquid polyethylene with “but instead of flowing, the material retains its the enthalpy-temperature curve as actually measform stability even a t temperatures considerably ured for the crystalline-amorphous solid. The latter enthalpy includes the heat required to melt the above the melting point.” I n order to clarify the question regarding the fraction of crystalline material that has melted a t effect of high energy radiations on the melting the temperature in question. Figure 2 illustrates point of polyethylene it was decided to study the such a curve. The factors that influence the melting range and specific heat and the heat of fusion of irradiated polyethylene. Previously, Charlesby3 studied the maximum melting point are the following: (a) the melting of irradiated polyethylene by taking cool- reduction in the melt of the activity of the crystaling curves from 220’. However, accurate deter- lizing segments by increase of mole fraction of nonminations of the melting point and of the range of crystallizable units such as co-polymer units or melting are not possible from cooling curve meas- units containing branch points, cross-links and urements because of super-cooling of the melt and double bonds, or irradiation degradation products, because the rapid change of temperature prevents etc.; (b) change in nature of Crystallizing units, equilibrium from being established during the cool- such as higher molecular weight cross-linked units ing. With precise colorimetric equipment much being the crystallizing substance instead of the more accurate information can be obtained. For- CH, segments of the unirradiated polyethylene; tunately, the equipment6 was a t hand for making (c) change in crystallite-melt interfacial area per such studies as many specific heat measurements of gram of material by disruption of large crystallites synthetic high polymers had previously been car- into smaller oness; (d) failure to attain equilibrium between “pockets” of impurities produced by the ried out6J in this Laboratory. irradiation and the main bulk of amorphous ma11. Theory of Melting terial ; (e) possibility of siniultaneous presence of Part of the confusion regarding the exact me1ting two kinds of crystallizing segments, either as dispoint and melting range of polyethylene arises from crete crystallites or in solid solution. Items (a) the concept of melting as a process whereby a solid and (c) would cause a lowering of the melting is converted to a liquid that will flow. Thermody- point, while as a result of (b) the melting point namically, the melting point is defined as the tem- would be increased. At low irradiation doses perature at which the solid and liquid are in equilib- items (b), (d) and (e) probably would not produce rium with each other. For a partially crystalline, detectable effects in the melting behavior. Item partially amorphous material such as polyethylene (c) might shift the melting curve in a direction to in which there are crystallites of varying sizes melt- increase the proportion that melted a t lower tem(1) E. Collinson and A. J. Swallow, Quart. Reu., 9, 311 (1955). peratures. (2) A. Charlesby, Proc. Roy. Soc. (London), 215A, 187 (1952). 111. Experimental Details (3) A. Charlesby, ibid., ZllA, 122 (1953). (4) E. J. Lawton, J. 9. Balwit and A. M. Bueche, Ind. Eng. Chem., The details of the apparatus for measuring the 46, 1704 (1954). specific heat have been described elsewhere6 and (5) A. E. Worthington, P. C . Marx and M. Dole, Rev. Sci. Instr., 26, 298 (1955). will not be repeated here. Each heating interval (6) M. Dole, W. P. Hettinger, Jr., N. R. Larson and J. A. Wethingwas about 10” a t the lower temperature but only
ton, Jr., J . Chem. Phya., 80, 781 (1952). (7) S. Alford and M. Dole, J . A m . Chem. Sac., 77, 4774 (1955) (polyvinyl chloride). This paper contains references to earlier work.
(8) M. Dole, J . Polumer Sci., 19, 347 (19513; Bee also P. J. Flory, Trans. Faraday SOC.,61,848 (1955).
MALCOLM DOLEAND W. H. HOWARD
138 5.0
4.0
3
1.
3.0
M d
I biJ
2
0
30
GO
90
120
150
T,"C. Fig. 1.-Specific
heat of irradiated polyethylene in cal. g.-l deg. -1.
about 2 to 5" in the significant melting range. -4s one hour approximately was required to regain a steady-state temperature drift of the calorimetric system and to determine this drift accurately, the rate of heating was slow enough to ensure nearly equilibrium conditions in the polyethylene. The highly irradiated pile sample required less time to come to thermal equilibrium after a heating interval than did the unirradiated sample. The polyethylene samples studied were granular "alkathene," a polyethylene9 produced commercially by I.C.I. Its maximum melting point was 113.8 h 0.4", see Fig. 2. Three irradiations were carried out a t the Argonne National Laboratory, two of them in the pure y-radiation facility and one in CP-5, the heavy water pile. The first sample was exposed to 1 X 106 roentgens of y-radiation, the second to 50 X lo6 roentgens, while the third was in the pile for 20 hours a t a thermal neutron flux of 1 X 1013thermal neutrons/sq. cm./sec. The first two irradiations using y-rays were performed with the polyethylene sealed in evacuated Pyrex tubes, while the third was carried out with the polyethylene in aluminum tubes, capped up, but open to the atmosphere. Oxidation occurred in this irradiation sufficient to produce a net weight increase of 0.8201,. In discussing quantitatively the irradiation dose it is convenient to take some significant unit. As our unit dose we have decided to take lo6rep or one megarep (one rep is defined as 93.1 ergs/g. energy absorbed in the ferrous sulfate dosimeter). One (9) Kindly supplied by Dr. R . B. Richards.
Vol. 61
megarep liberated 0.722 X g. of hydrogen per g. of polyethylene as determined from the experiment performed in which the polyethylene was exposed to 50 mega,rep. As Charlesby2quoted 7.7 X g. of hydrogen per g. of sample ,for his unit dose in the Harwell B.E.P.O., his unit dose was 107 megareps. The irradiation dose of our third sample cannot be estimated accurately because of failure to collect the hydrogen evolved. A crude guess from a comparison of the slow neutron flux in the present Argonne CP-5 pile with that of the old heavy water pile yielded the value 340 megarep.1° From the value of 0.722 X g. of hydrogen evolved per megarep the G-value was calculated to be 3.75 molecules of Hz evolved per 100 e.v. assuming that the energy absorbed in polyethylene per g. was the same as in the ferrous sulfate dosimeter used to determine the y-radiation intensity. This G-value is probably within the limits of uncertainty equal to 3.5 which Tolbert and Lemmon11 report as to the limit to which the G-value of the saturated straight chain hydrocarbons approach as the molecular weight increases. IV. Results and Discussion The specific heat of the sample of polyethylene irradiated with 1 megarep of y-radiation could not be distinguished in any way from that of the unirradiated material. It was used, therefore, as a standard to represent the unirradiated material. The 50 megarep irradiated sample had a dosage sufficient to make the polyethylene almost completely insoluble. After taking this irradiated sample through the long heating cycle, up to 135", over a heating period of 48 hours, from a visual standpoint it appeared not to have melted. Each granule had maintained its individuality and moved freely over the others when the material was shaken. Yet its specific heat-temperature curve, Fig. 1, showed definitely that it had melted to exactly the same extent as the unirradiated sample. It was the cross-linking which had produced the insolubility which also prevented the material from flowing after melting. The molten irradiated polyethylene was a gel maintaining its physical form despite being practically completely liquid. The crystallinity of the irradiated sample was calculated by the equation fraction of crystallinity = f = HL ___ -H AHr
(1)
where H L is the enthalpy of the liquid polyethylene, H the enthalpy of the sample and AHf the heat of the fusion of the 100yo crystalline m a t e d , all a t the same temperature and per gram of polyethylene. For the 50 megarep irradiated sample no change in the crystallinity a t room temperature could be detected. However, the hydrogen liberated, 3.61 X g./g. sample, was only 0.25% (10) The statement made in our previous publication, M. Dole, C. D. Keeling and D. G. Rose, J. Am. Chem. Soc., 1 6 , 4304 (19541, that Charlesby's unit dose was equivalent t o 974 hours of "goat hole" irradiation was probably incorrect. A better estimate based on a hydrogen evolution comparison yielded the value of 450 hours of goat hole irradiation. (11) B. M. Tolbert and R. M. Lemrnon, Univ. of Calif. Radiation Laboratory-Report No. UCRL-2704 (unclassified), August, 1054.
Feb., 1957
MELTINGBEHAVIOR OF IRRADIATED POLYETHYLENE
of the whole. If the crystallinity had decreased by only 0.25%, this change could not have been detected. The only detectable change in the thermal properties of the 50 megarep irradiated polyethylene was a decrease in the maximum melting point of about 1.5 i 0.5". From the enthalpy curves of Fig. 2, in which the enthalpy of the melting region is plotted, one can see that the maximum melting point has definitely decreased. The point of intersection of the curve of the liquid region with that of the solid is a t a temperature 1.5 i 0.5" lower for the irradiated sample than for the non-irradiated. From the Flory12 equation it is possible to calcu-
late the mole fraction of the crystallizing units, XA, necessary to produce a decrease of the maximum temperature of melting of 1.5". In equation 2 To is the maximum melting temperature of the pure polymer, T , is the maximum melting temperature of the polymer whose mole fraction of crystallizing units is XA,R is the gas constant and AHf is the heat of fusion per mole of crystallizing segments. Takings AHf to be 921 cal./mole of CHZunits, TO to be 387 and T,, 385.5"K., XA turns out to be 0.995 or the change in composition of the amorphous region due to the presence of irradiation products is 0.5 f 0.2%. I n the 50 megarep irradiated sample, mole of Hz the hydrogen evolution was 2.53 X per mole of CH2units. This number must be multiplied by two as each molecule of hydrogen evolved involves removing a hydrogen atom from two CHz units. The expected change in XAfrom hydrogen evolution is, therefore, 2 X 2.53 X or 0.5%. Thus, the observed lowering of the maximum melting point is the correct order of magnitude expected from Flory's equation 2. Turning now to the pile irradiated sample, the situation is less clear-cut because of the slight oxidat,ion of the sample, and because the amount of hydrogen evolved could not be measured. However, from the estimates of the latter given above, it was calculated that the expected change in XAwas 2 X 0.017 or 3.4%. Such a decrease of XAwould have produced a maximum melting point depression of 10.8". By inspection of Fig. 2 it will be seen that no such lowering of the melting point occurred, although if only the lowest points on the enthalpy (12) P. J. Flory, J . Chem. Phya., 17, 223 (1949); Trans. Faradag SI, 848 (1955).
Soc.,
139
- 10
7' -20 bh e,
5 -30 - 40
100
Fig. 2.-Enthalpy
I10
I20
130
T, "C. of irradiated polyethylene relative to the enthalpy a t 140".
curve are extrapolated to an extension of the enthalpy curve for the liquid, a freezing point lowering of 10.8" seems reasonable. The enthalpy curve for the pile irradiated sample suggests that there has been a redistribution of crystallite size and possibly molecular weight in a direction to produce a small amount of material melting a t a somewhat higher temperature and to reduce the amount melting a t the lower temperatures. The fraction of crystallinity of the pile irradiated sample was 49.9% as calculated by eq. 1, 5.7% less than the crystallinity of the unirradiated sample. This change is somewhat more than 3.4% estimated above for the decrease of XA but, because of the uncertainty iB the radiation dose, exact quantitative comparisons are meaningless. Acknowledgments.-Grateful appreciation is expressed to the Office of Ordnance Research, U. S. Army, for support of this project, t o Bernhard Wunderlich for assisting in the calculations and measurements, and to the members of the Technical Service Staff of the Argonne National Laboratory for their cooperation.
