JOUIiNAI, OF I’OLYMElt SCIISNCK: I’AIVr A-2 VOL. 7 , 2051-205Y (1969)

Extended-Chain Crystals. 11. Crystallization of Polyethylene under Elevated Pressure*

THEODORE DAVIDSOXt and BERNHARD WUNDERLICH, Department of Chemistry, Elensselaer Polytechnic Institute, Troy,

New York, 12181

Synopsis

A report on crystallization of polyethylene at elevated piessures to an extended-chain morphology is presented. The crystals have been characterized by electron microscopy and density determinntion. Pressure, supercooling (temperature), and crystallization time have been varied to find t.he best conditions for production of perfect crystals. At 10-30°C supercooling complet,ely crystallized polyethylene was obtained between 4.5 and 7 kb crystallization pressure in 1-8 hr. Analysis of fracture surfaces of samples Crystallized for different lengths of time shows an increase in size and number of crystal lamellne and an improvement of extended chain crystals in the early stages of crystal- 1iznt.ion. A further improvement of the less well crystallized material between the lamellae occurs after 15 min of crystallization time.

INTRODUCTION

I n the preceding paper of this series’ it was indicated that crystallization of polyethylene under elevated pressure is to date the best method for the production of extended-chain polyethylene crystals. The present paper will deal with crystallization of polyethylene under elevated pressure. Previous work in this area has been reviewed also in the foregoing paper. New data on crystallization under controlled conditions in a new pressure system will be described. In particular, the effect of up to 7 kb pressure on crystallization, the effect of temperature a t a constant pressure of 5 kb, and the time effect on crystallization a t 5 kb and 200°C was studied.

The new set of experiments was designed to test the idea that crystalli- zation of polyethylene under pressure, which yields extended-chain crystals, is not different in principle from crystallization a t atmospheric pressure. Also, it was considered necessary to delineate more precisely the limits of extended-chain crystal growth.

* This paper was presented in part a t the IUPAC hlacromolecular Symposium,

t Present address: L)epart.nient of Materials Science, Northwestern University, Toronto 1968.

Evaiiston, Illinois 60201. 2051

2052 DAVIDSON AND WUNDERLICH

EXPERIMENTAL

Materials

For all crystallizations a linear polyethylene of the Marlex type was used. The number-average and weight-average molecular weights were 8530 and 153,000, respectively. A more detailed molecular weight analysis of the polymer is given in the third paper of this series.2 The polymer is identical to polymer A described p r e v i o ~ s l y . ~ , ~ The somewhat different average weights reported here are the result of additional effort spent on characterization of the sample.

Methods

Density measurements were made in a gradient column of toluene and monochlorobenzene at 23°C. Different pieces of the same sample showed a spread 0.0018 g/cm3, so that the reported densities are accurate to f0.001 g/cm3.

Electron microscopy was carried out on two-stage replicas of fracture surfaces. Well crystallized samples showed brittle fracture a t room tem- perature. More poorly crystallized samples were tougher and had to be fractured a t liquid nitrogen temperature.

Samples for optical microscopy were cut to 5-7 p thickness on a micro- tome before observation with a polarizing microscope.

The crystallizations under pressure were carried out in the newly de- veloped pressurestat.* Its design is a further development of the pre- viously used e q ~ i p m e n t . ~ Details are available from the author on request. In principle, pressure is generated by pumping hydraulic fluid into the thermostatically controlled pressure vessel, which has a capacity of about 50 cm3. Pressure is maintained to f l % by a reversible automatic pump. Temperature calibration showed fluctuations of less than f 1°C. The absolute temperature is known to somewhat less accuracy because of the temperature gradients within the steel assembly. It is estimated that a constant error of 2-3°C is possible in all quoted temperatures.

RESULTS

A first series of six samples was prepared by a technique similar to the one used previou~ly .~ .~ The polymer, in bronze bellows, was heated to the chosen crystallization temperature and, after equilibration, pressurized. After 8 hr under crystallization conditions, the sample was cooled a t a rate of 5"C/hr to room temperature. At room temperature the pressure was released arid the sample removed for analysis. Figure 1 shows an optical micrograph of a slice of a well crystallized polyethylene. Figure 2 is an electron micrograph of a replica of the fracture surface of a well

* The design was realized in cooperation with the a!ipplier of the equipmelit, Pressure Products Industries, Hatsboro, Pa.

EXTENDED-CHAIN CRYSTALS. 11. 2053

Fig. 1. Optical micrograph of a well crystallized polyethylene viewed betm-een crossed The scale bar represents polarizers (crystallization conditions: 201"C, 5.01 kh, 8 hr).

20 p.

Fig. 2. Electron micrograph of a replica of the fracture surface of a well crystallized The scale bar represents polyethylene (crystallization conditions: 241"c, 6.98 kb, 8 hr.)

2 M.

2054 DAVIDSON AND WUNDEIILICH

TABLE I Density as a Functioii of Crystallization Pressure at Similar Supercooling and 8 hr

Crystallization Time

Ilensity, g!cm3 PI, bars TZ, "C

0.980 0.979 0.980 0.981 0. 983 0. !I80 0.979 0.986 0.980 0.984 0.991 0.991 0.989 0.994 0.991 0.990 0.993 0.993

1 486 908

15.50 2026 23:w 2614 2796 2897 3100 3586 3860 3980 4562 5030 5070 6120 6979

1 3 0 a 1408 150- 160" 170% 176 a

181" 186" 186a 1918 201 a

206 a

199 226a 217 207 22 I 241

a Samples from previous ~ t u d i e s . ~ . ~

crystallized sample. The densities of all samples are listed together with previous data3s6 in Table I. All samples were crystallized a t 10-30°C supercooling and 8 hr crystallization time. The present experiments ex- tend the available data to 7 kb. The electron micrographs revealed no further chain extension. Melting experiments were within experimental error identical to previously reported data.3*6.7

I n a second series of pressure crystallizations, the crystallization temper- ature was varied between 181 and 267°C while the time and pressure were kept constant at 8 hr and 5 kb, respectively. I n an attempt to quickly terminate the crystallization a t the end of 8 hr, the pressure was increased to 6.8 kb before cooling to room temperature by shutting off the heaters. The results are displayed in Table 11. Between approximately 195 and 230°C samples were obtained of consistently high density.

A third series of crystallizations varied the crystallization time at con- stant temperature and pressure. Again 5 kb was chosen as the crystalli- zation pressure while the crystallization temperature was fixed a t 201°C. Less than 1 min was required to increase the pressure to 5 lib. The pressure was applied after the sample had been held for several hours at the crystallization temperature. When the desired crystallization time had elapsed, the pressure was increased to 6.8 kb to quench crystallization. The AP of 1.8 kb is equivalent to rapidly lowering the temperature by

The initial cooling rate under this condition was l"C/min.

EXTENDED-CHAIN CRYSTALS. 11. 2055

TABLE IT Density as a Function of Crystallization Temperature a t 8 hr. Crystallizalioii Time

and 5 kb Pressure

Density, g/cmS Tz, "C P,, bars

0.983 0.989 0.991 0.993 0.993 0.992 0.992 0.993 0.992 0.994 0.991 0.990 0.988 0.981

170 177 192 20 1 207 215 218 218 225 226 24 1 2;i8 265 267

5240. 4820 5000 5010 50.50 4970 4980 4930 5000 4860. 4920 5230 5010 .5060

a These two points are taken from ref. 6 and represent crystallizations terminated by slow cooling, rather than by presslire jump.

TABLE IT1 Density as a Function of Crystallization Time at 5 kb and 201OC

Time, min Density, g/cm3 P,, bars

Quenched 5

15 30

480

0.9827 0.9902 0.9910 0.9899 0.9930

5070 5030 .5070 5010

48°C. Shutting off of heaters after reaching quenching pressure again achieved an in- itial cooling rate of l"C/min. The results are displayed in Table 111. Figures 3-5 show the change in appearance of the fracture surfaces with increasing crystallization time. Less information could be gained from optical microscopy. All samples showed incipient spherulitic character as illustrated in Figure 1.

The zero time sample was raised to 6.8 kb immediately.

DISCUSSION

The data of Table I show that only a minor increase in density occurs on crystallization under pressures below 2 kb, as was found earlier by Matsuoka.8 The previously described3v6 sharp increase in density between 3 and 4 kb, which parallels the formation of increasingly extended chain

2056 DAVIDSON AND WUNDERLICH

Fig. 3. Electron micrograph of a replica of the fracture surface of a pressire-quenched polyethyelene (crystallization conditions: 201 "C, 6.8 kb, immediate cooling). The scale bar represents 2 p.

crystals, levels off a t about 4.5 kb. The new data in Table I indicate no further increase in density upon raising the crystallization pressure to 7 kb at approximately constant supercooling. Also, no further chain extension was detected on inspection of fracture surfaces. The density of crystalline polyethylene a t 23°C calculated from the unit cell dimension is 0.9998 g/cm3. To compare this density with the experimental data presented here, a correction must be made for the lower density of the fractions of lower molecular weight. Assuming complete separation of all fractions on crystallization, one can calculate a completely crystalline density of the sample used in this research of 0.9970 g/cm3. The detailed calculations are presented in paper IV of this ~ e r i e s . ~ A somewhat lower actual density is expected. As will be shown in the next two papers of this such eutectic separation is possible only below molecular weight 10,000 under the chosen crystallization condition, since a minor amount, of triclinic component was found in the pressure crystallized

EXTENDED-CHAIN CRYSTALS. 11. 2057

Fig. 4. Electron micrograph of a replica of the fracture surface of a 5 min pressure crystallized polyethylene (crystallization conditions: 201"C, 5 kb, 6 min). The scale bar represents 2 p.

samples, and since a small number of branches, double bonds, and carbonyl groups is present in the sample. The best samples of Table I can thus be looked upon as practically completely crystalline. Figure 2 supports this conclusion. No poorly crystallized polymer can be seen and clean interfaces between lamellar crystals are shown. Increasingly poorer crys- tallization yields less well defined fracture surfaces and interfaces out of which fibrils of polymer are drawn on fracture. The details of the extended- chain morphology of the samples crystallized here are identical to those reported previously.10-12

The experiments a t different crystallization temperatures a t 5 kb and 8 hr crystallization time resulted in a region of complete crystallization between 195 and 230°C (Table 11). The upper temperature limit is close to the melting point of extended-chain crystals of polyethylene a t 5 kb.4 The initially slow decrease in density a t higher crystallization pressure is due to the inefficiency of quenching in this temperature region. As soon as the melting point a t the quenching pressure is reached the density drops sharply. The best crystallization conditions are fixed by this experi-

2058 DAVIDSON AND WUNDERLICH

Fig. 5. Electron micrograph of a replica of the fracture surface of a pressure-crystal- lized polyethylene (crystallization conditions: 20loC, 5 kb, 30 min). The scale bar- represents 2 /r.

ment at a supercooling of 10 to 30°C. The region of best crystallization at elevated pressure is somewhat larger than a t atmospheric pressure.

The development of high perfection with time becomes clear from the results in Table 111. Relatively quickly (ca. 2Ck30 min.) high crystallinity is achieved. Similarly high densities could only be obtained a t atmos- pheric pressure on fractions of relatively low molecular weight crystallized at low supercooling for many days.13 One to eight hours a t elevated pres- sure seems to be a convenient compromise for crystallization. Occasional extension of crystallization times to 20 and 49 hr3*6 did not yield significant density increase. Slow cooling after crystallization, however, is felt to improve the density slightly. Probably the low molecular weight fraction left uncrystallized a t the lower degrees of supercooling then has a chance to achieve better crystallization.

Some information on the development of high crystallinity can be gained from the study of fracture surfaces. The sample quenched to

EXTENDED-CHAIN CRYSTALS. 11. 2059

6.8 kb without stopping at 5 kb is shown in Figure 3 (zero crystallization time a t 5 kb in Table 111). Only few and poorly fracturing extended- chain crystal lamellae are present. Although the degree of chain extension is only slightly less than in the perfectly crystallized samples (Fig. a) , the lamellae lack the sharp edges on fracture. In addition, a large number of fibrils are pulled out between the lamellae on fracture. The sample held for 5 minutes a t 5 kb already shows (Fig. 4) much improved lamellar fracture surfaces. The drawn out fibrils, however, remain. Figure 5, which shows a fracture surface of the sample crystallized for 30 min still indicates some fibrils. Etching with nitric acid after fracture removes the fibrils and indicates otherwise complete crystallization. Figure 2 finally represents a fully crystallized sample. From these observations it is concluded that in the rapid early stage of crystallization the large extended- chain lamellae form. They increase in size and number, but also in per- fection parallel with the large increase in density. A much slower increase is noted beyond 15 min. This is linked with an improvement of the interlamellar material. After about 8 hr, almost complete crystallization has been achieved. Further improvement is most likely restricted to interlamellar low molecular weight polymer above its respective melting point, which crystallizes according to the cooling conditions.

This research was supportpd by the Office of Naval Research. One of us (T. D.) held a Union Carbide Corporation fpllowship during a portiori of this work.

References 1. B. Wunderlich and T. Davidson, J . Polym. Sci. A-2,7,2043 (1969). 2. R. B. Prime and B. Wunderlich, J . Polym. Sci. A-2,7,2061 (1969). 3. T. Arakawa, and B. Kunderlich, in Macromolecular Chemistry. Prague 1965, J.

Po2ym. Sci. C, 16, D. Wichterle and B. SedlLEek, Ed., Interscience, New York, 1967, p. 653.

4. T. Davidson and B. Wunderlich, J . Polym. Sci. A-2,7,377 (1969). 5. B. Wunderlich, Rev. Sci. Instr., 32.1424 (1961). 6. B. Wunderlich and T. Arakawa, J. Polym. Sci. A, 2,3697 (1964). 7. E. Hellmuth and B. Wunderlich, J . Appl. Phys., 36,3039 (1965). 8. S. Matsuoka, J. Appl. Polym. Sci., 4,115 (1960). 9. R. B. Prime and B. Wunderlich, J . Polym. Sci. A-2, 7,2073 (1969).

10. P. €1. Geil, F. R. Anderson, B. Wunderlich, and T. Arakawa, J. Polym. Sci. A, 2,

11. B. Kunderlich and L. hlelillo, Makromol. Chem., 118,250 (1968). 12. B. Wunderlich, L. Melillo, C. M. Corniier, T. Davidson, and G. Snyder, J. Mac-

13. L. Mandelkern, J. M. Price, A t . Gopalan, and J. G. Fatou, J . Polym. Sci., A-2,

3707 (1964).

T O ~ O ~ . Sci.-Phys., B1,485 (1967).

4, 385 (1966).

Received March 20, 1969 Revised June 23, 1969