The melting point of polyethylene has been determined at high pressures up to 30, 000 atm by the volume-discontinuity method and differential thermal analysis. The effect of increasing the pressure up to 5, 000 atm is to reduce the volume of fusion. Suitable values for the parameters of the Simon equation, a and c, have been obtained from the melting curve

Hara, Kimihiko

Osugi, Jiro

Kyoto University Research Information Repository

Title High-pressure melting in polyethyleneAuthor(s)Osugi, Jiro; Hara, KimihikoCitationThe Review of Physical Chemistry of Japan (1966), 36(1): 28-34Issue Date1966-10-30URL http://hdl.handle.net/2433/46876RightType Departmental Bulletin PaperTextversionpublisherKyoto UniversityThe Review of Physical Chemistry of Japan Vol. 36 No. 1 (1966)THE REVIEW OP PHYSICAL CHEMISSRY OP JAPAN, VOL. 36, Na. 1, 1966HIGH-PRESSURE MELTING IN POLYETHYLENEBY )IRO QSliCI ARD KIMIHIRU IIARA   The melting point of polyethylene has hem determined a[ high pressures up to 30,000 atm by the volume-discontinuity method and di6erential thermal analysis. The effect of increasing the pressure up to 1,000 atm is to reduce the volume of fusion. Suitable values for the parameters of the Simon equation, a and c, have been obtained tram Iht melting curve.IMroduttion   Knowledge of melting properties at high pressures i important because many thermodynamic infor-mations, which experiments atatmospheric pressuredo not yield directly, can be obtained. Most previous investigations of the effect of pressure on the melting points have largely been concerned with elements, semiconductors, ionic compounds, and simple organic compounds. In early works elements, especially metals were widely studied and they Were reported by Strongll, Bundy and S[rongzl, and others, Recently there are many investigations about inorganic compounds, uch as alkali halides (Clark, 1959)31, II-VI compounds (Jayaraman, Kennedy et a1., 1963. 1964}[1, nitrates (Oven, 1963, 19G5)s>, potassium halides (Pistorius. 1965)61, silver halides (Deacon, 1966)73, and so on. Nelson et al.e1 have studied the melting of n-paraRlns under pressure up to 10 kb. But few works about polymers have been done because of many difficulties.    Polyethylene has high crystallinity among various polymers and shows sharp melting phenomena analogous to that of the substances which have low mo]ecular weight. The melting cun•e of polyethylene as a function of pressure was reported over the pressure range of 0 to 3,000 atm3>, but no other previous investigation of the effect of high pressure on the melting point has been reported. Since polyethylene is Widely employed as a solid pressure container. the determination of the pressure dependence of its(Recei.ed Augnst 10, 1966) Q H. 3i. Strong, in "Progress in Very High Pressure Research" (F. P. Bundy, N. R. Hibbard, Jr., and    H. \S. Strong, eds.). Niley, '9ew York, (1961) 2) F. P. Bundy and H. hi. Strong, "Solid State Physics" Yol 13, (F. Seitz and D. Turnhulh eds.). Academic    Press, New York, (1962) 3) S. P. Clark, Jr.. !. CAem. Ph}s., 31. 1526 (1959) 4) A. Jayaraman, 1V. Klement Jr., R. C. Newton. and G. C. Kennedy, J. Phyt. Chem. Solids, 24, 7(1961)    A. Jayaraman, SV. Klement Jr., and G. G Kennedy, Phys. Rev.. 130, 1177 (1963) 5) B. B. Ovens, !. Chem. Phys., 39, 1053 (1963)    B. B. Ovens, J. Chem. Phys„ 42, 2219(1965) b) Carl 1V. F. T. Pistorius, J. Phys. Chem. SoIidt, 26, 1543 (1965) 7) B. C. Deacon, J. Appl. Phys.. 36, 1500 (1965)    B. C. Deacon and F. A. Blum, Jr., Phys. Rev., 137, AI13] (1965) 3} R. R. Nelson, R'. N'ebb, and J. A. Dixan, J. Chern. Phys., 33, 1736 (1960) 9) 5. \fatsuoka, 1. Polymer Sci., 57, 569 (1962)The Review of Physical Chemistry of Japan Vol. 36 No. 1 (1966)                           High-Pressure \Selting in Polyethylene 29 melting is of special importance. The present work was carried out to study the high pressure melting of polyethylene up to 30.000 atm. The melting curve has been analysed if it can be fit with the Simon equation and by comparing the heat of fusion calculated by using the Clapeyron equation with the value reported in literature. The melting poinu estimated from the change of folded chain length of single cry-stal in the previous paperro~ were found to agree wellwith those of the direct measurement o[ this work.    The density has been measured for the samples Crystallized from the melt under pressure.                                    Experimental    The sample used for this investigation a•as high density linear polyethylene, Aiarlex 50. The melting point at atmospheric pressure agreed with [he best value obtained from literature, i, e. 136`C. The phase transition up to 5,000 atm was investigated with Che volume-discontinuity method. The piston-cylinder type-high pressure vessel and high pressure press used are the same as those in the previous paperm>. Only the vessel was illustrated in Fig. 1. Heating was carried out with an electric furnace, whose temperature was constant within ~OS`C. The sample leakage was prevented by the use of Bridgman's unsupported area gaskets of fluorinated rubber and Teflon. The back-up-ring o[ copper was used at high temperature. The vertical piston displacement n•as measured with a dialmicrome[er, the sensitivity of which was 10-~ mm. The values of the changes of volume on melting were obtained from the compres-sing curves at constant temperature. The isothermal compressing curve at 1 iS°C is shown in Fig. 2.SmdeGavM=,aa~,     carhiCe o~a.~"iaaLnE vJL'I910a1m           1 ~ 11 1 IRJ ,fN Re yn YD                ,e~ Pressure fa m) Fig. 1 Piston-Cylinder type high pressure vessel Fig. 2 Isothemal compressing curve a[ lig`C    Pressures up to 30,000 atm were generated in a compact cubic anvil device, which consists of six carboloy anvils. The sample is held insidea cubic block of pyrophyllite whose sides are initially 2i per cent larger than [he sides of the square anvil faces. As the anvils advance to compress the sample volume,      10) J. Osugi, R. Hara, :F. Hirai, and J. Hikasa, Tbis lourual, 3s, 59 (196x)The Review of Physical Chemistry of Japan Vol. 36 No. 1 (1966)30 j. Osugi and Ii. 73ara some of the pyrophyllite is extended into the gap between the anvil sides, and this extended material provides compressive support for the high pressure nds. The pressure calibration techniques have been already described is detailtt~. A cylindrical specimen, 3mm in diameter and 6mm fn length, was placed inside the pyrophclli[e cube. As shown in Fig. 3, the specimen was enclosed in a pyrophyllite tube and placed inside a carbon healer tube. The phase transition was detected by diEerential thermal analysis (DTA). Iron-constantan thermocouples were used, where one junction s•as placed near the beater and another was kept at the center of the specimen. This technique yielded barely detectable_ignals for the melting. Thermocouple l ads were placed between the anvils. The carbon served as a heat source due to [he passage of high alternating current at low voltage from the ta•o opposing anvils..~i!•, f.;. 'li~:~„~~ • .~' .~.en.,.,®~~Fig. 3 Schematic view showing the internal construction      of the pyrophyllite pressure-transmitting cube for       DT ~ measurementr                           csmnen.    A density gradient column of ethanol-water mis[ure was used for the density measurement. This column covered a density range of 0.9001.OOOg/cros with an accuracy of -1-0.003g/cm'. The specimen cut into small pellets, 3mm in diameter. ss•as investigated.                              Results and Discussion    The melting P-T curve o[ polyethylene is shown in Fig. 4. Pig. 4(a) was obtained from the volume discontinuity method and Fig. 4 (b) was obtained from DTA measurement. It has as initial slope of 0.025'K/atm and shows aconcavity oward the pressure azis; it means (dT,,,/dP)~0 and (d2T,„/dln)<0.  do polymocphiC transition teas observed in the pressure ange of [his work, and there is no indication of irregularity- in the melting cur.•e.     As shown in Fig. 5, dV decreases with pressure, and it amounts o dVaomarv+=0.06cc/g, while    The melting points are predicted from the Clapeyron equationIt) J. Osugi, Ii. Shimizu. 1:. moue, and P.. Yasuoami, This Jourual. 3a, 1 (1964)The Review of Physical Chemistry of Japan Vol. 36 No. 1 (1966)High-Pressure Melting in Polyethylene 313 F s .` z,~    .~ m ,~ ,~       Pressure (atm)  Melting curve of polyethylene  up to 5,000 atm              dP dSU v v 2 E Ec~ c .a v,nFig. 4. (a).i                     dT> dV where T,,, and P are the temperature and enthalpy and entropy on melting respect (dPfdT,,,)P_o and the value o(dV estrapo I p comparison.                           Table I Melting parameters at zero pressure              rm                 o a s                           Pressure( x10'atm)     Fig. 4. (b) Mdting curve of polyethylene up to 30.000 atm.             Small points show the results in Fig. 4(a) __ dl! pressure at the melting, and dH and dS are the changes of ively. Table 1 contains the enthalpy on melting obtained Prom laced to zero ~ressurc, and the value re orted in literature PordV III40 atm/'F 0:17 cc/g 69 cal/gdH (literature valueL~7)66.i cal/g (calorimetrically)(») M. Dole and B. R'underlich, .Ifakrornol. Chzm., 3d, Z9 ([959)a.na Aeo >m .m         Pressure (atm) Variation of dl' with pressure     ~m w 5 Ir n tFig. 5 Fig.u• J  o ~uu mm aeo aru m. m voo           Pressure (atm) 6 Variation of dP/dTm with pressureiThe Review of Physical Chemistry of Japan Vol. 36 No. 1 (1966)32 J. Osugiand Ti. Haraa e am m_ ~,; s ~~ Qa              o wm mo xm .m .uo o m                                                                  m sm w taao                       pressure ;atml Pressure (a[m)        Fig. 7 Variation of dA with pressure Fig. 8 Variation of dSwit6 pressure    The slope of the melting curve increases with pressure (Fig. 6). Thus knowledgeofdP/dT„ and dV permits the calculation of the change of dAand dS with pressure from the egation (1). They are shown in Figs. 7 and 8. Both tend to decrease slightly with pressure.    The melting Curve can be represented quite well by the Simon equation where P,,, and T,,, are the pressure and absolute temperature atthe melting point. To is the melting point at pressure Po, and a and c are adjustable constants. The Simon parameters have been determined so that Sin the following equation may be minimized. where x~=(Tr/Ta)'-1. The parameters of the Simon equation are given in Table 2.                          Tahle 2 Parameters of the Simon equationTa c a409'& 4.5 3.1 x IW a[mThey agree well with the melting curve over all the studied pressure ange. The initial slope of the melt-ing curve, (dT„/dP)p~o, is Ta/ac b}• differentiation of the Simon equation. Substituting a and t with the obtained values, a~e have 0.028`R/atm, which agrees well with [he value described above, 0.025`6/atm. This is also an evidence that these parameters xpress well the experimental results. Considerable effort has been made in 6t[ing high pressure melting tunes [o [he Simon equation, and Babb1zl has estimated [he values of the parameters formany substances. The values obtained above seem favorable in com-parison with those found for other materials. For comparison the Simon parameters of the substances from recent papers are given in Table 3.:1t present no satisfactory theoretical explanation f [he Simon parameters xists, but it is probable that the quantity ais related to the internal pressure. As for the value of a. organic rystals have smaller values than most of [he inorganic onic crystals.    The folded chain length in polyethylene single crystal increases rapidly as it approaches themelting      12) S. E. Babb. Jr„ !. Cherr+. Pys., 38, 2743 (1963)          S. E. Babb, ]c, Rev. Stod. PGys..35,400(1953)The Review of Physical Chemistry of Japan Vol. 36 No. 1 (1966)Table             High-Pressure Melting in Polyethylene 3 Parameters of the Simon equation for various substances from recent literatures33Substance a (kb) c LiteratureLiCI \aF \aCl 1~aBr NaI KClKBr AgCI agar agI n-Clq (Ocladecane) n-Czr (Tetrarosane) 14.5  19.3  16.7  12.2  10.1   6.9  5.98   3.57 55.0  28.0   9.5   3.45   4.51.5 i.i 2.7 2.9 2.fi 5.7 5.99 3.57 1.7 2.i ss 3.39 2.84(3) (~) (6) (3) {3) (3) (6) (6) (i) (7) l%) (s) (a)paint. This is correct at elevated pressureslo>. Themelting points estimated from the changes offolded period (161°C a[ 1,010 atm, 1g5`C at 1,970 atm) are round to be suitable in the present paper.    The density tr}•stallinity was determined by use of the average of Nielson'stay nd Gubler and Rovacs'1+1 value for Va (1.173 cm3/g) and Swan'stsl calve for Y'~ (1.001 tm3/g)..4s een in Table 4, the crystallinity of polyethylene crystallized from the melt under high pressure increases. But in the case of non-melting no appreciable change was observed even under 30.000 atm. {Vhen polyethylene was crystallized from the melt under high pressure, the extended-chain l mellae were observedlo>, This in-crease incrystallinity means the effect of the occurrence of the structure. It is conceivsble that his quan-tity exceeds the possible change ofvoids. X-ray diffraction i dicates no change of the structure inevery case, as may be expected.                             Table 4 Density and crystallinityystallizatioa conditions    Pressure(alm)DensitY (R/ctns)Crystalliniq-   (.°6)1 1,010  1.900 3,100 30,000(Cryst. from melting) (Cryst. from melting) (Cryst. from melting) (Non-melting)0.971 0.973 0.990 0.990 0.97787 91 95 95 89   It is possible to conclude from the results given above that the extendedthain crystallized from the melt under high pressure is not the change of crystal structure, morphology of the molecular chain.      13) L. E. Nielson, L Appl.Phys., ii, ]209 (1954)      14) lf. G. Gubler and A. J, Kovacs, J. Polymer Sci., 35, (1959)      15) P. R. Swaa, !. Polymer Sei., 56, 403 (1967)lamella structure but the change ofThe Review of Physical Chemistry of Japan Vol. 36 No. 1 (1966)34 J• Osugi and %. HaraThe authors wish to [hank Dr. K. ShAcknowledgement imizu for his helpful advice throughout this work. Laboratory of Physic! Cbemfslry Department of Chemistry Fatally of Science Kyoto Unixrsfty Kyoto, Japan

High-pressure melting in polyethylene

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High-pressure melting in polyethylene

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