SOUT-JH 72-0447
JOURNAL OF POLYMER SCIENCE: PART A-2
VOL. 10, 1135- 1143 (1972)
Melting Behavior of Polyethylene Crystallized in a
Pressure Capillary Viscometer
JOHN H. SOUTHERN* and ROGER S. PORTER, Polymer Science and
Engineering, University of Massachusetts, Amherst, JIll assachusetts 01002
aud H. E. BAlR, Bell1'elephone Laboratories, Murray Hill, New Jersey
07974
Synopsis
The melting of various polyethylene structures is compared by using data obtained on
the Perkin-Elmer differential scanning calorimeter (DSC). Transparent, high-density
samples crystallized under both orientation and pressure in the Instron capillary rheometer are compared with samples crystallized from dilute solution by stirring and with
samples crystallized under high pressure. The latter two structures are assumed to
contain extended-chain crystallites. By comparison, the melting points and the superheatability of the Instron samples are consistent with the presence of an extended-chain
crystal component. The melting of irradiated samples crystallized in the rheometer is
also observed to be consistent with this conclusion. In addition, DSC data are compared
with the melting points defined with a polarized light microscope equipped with a hot
stage.
Introduction
Transparent, orystalline strands of high-density polyethylene have been
produced in the Instron capillary rheometer under the combined effeots of
pressure and orientation. 1 - 5 At temperatures in the vicinity of the atmospheric melt transition, the polyethylene is crystallized in the rheometer
barrel and capillary entranoe region and subsequently forced into the relatively narrow capillary under the maximum pressure available, approximately 1900 atm. This forced extrusion of the semicrystalline mass results in a reorganization of the structure into one having a high degree of
both crystalline perfection and orientation. Recent microscopy and electron-diffraction studies4 indicate the presence of both chain-folded lamellae
and extended-chain crystal structures in the strands produced in the rheometer capillary. The primary subject presented herein is the analysis of
the thermal data obtained on the strands with the Perkin-Elmer differential
scanning calorimeter (DSC), Modell-b.
The polyethylene used to form the strands is, in all cases, duPont Alathon 7050 having weight-average and number-average molecular weights of
52,500 and 18,400, respeotively. However, other commercial high-density
* Presellt address :
Nylou Research, Monsallto Company, Pell~ acola, Florida 23502.
1135
© 1972 by Johu Wiley & :::ions, Iuc.
AUG 281972
j
SOUTHEH , POHTER , AND BAlli
1136
TABLE I
Conditions for the Formation of the Instron Strands
Samples used
in figure:
Temperature,OC
Plunger
velocity,
em/min
1
2 and 5
136
132
5. 0
0.5
Capillary
dimen~ions
Diameter, em Length, em
0.0508
0 .0762
2 . .5.5
2 .34
Entrance
angle
90°
90°
polyethylene samples produced similar results, so that the phenomena apparently apply to high-density polyethylenes in general. The relevant
sample formation conditions for the Instron capillary rheometer can be
found in Table I. The data obtained include the melting points as well as
the melting point values as a function of the heating rate. These melting
points are compared with literature values obtained on specimens of pressure-crystallized extended-chain crystallites6 •7 and stirrer-crystallized
"shishkabob" structures. 8 - 10 The comparative evidence is consistent with
the presence of an extended-chain component in the Instron strands. DSC
results obtained on irradiated strands are presented in order to define the
possible existence of multiple structures in the strands. In addition, birefringence measurements are obtained, by use of a Mettler hot stage
mounted on a polarized light microscope, in order to study the final melting
of the most perfect crystallites in the strands. From the birefringence
information and a supporting DSC study on paraffins, conclusions are made
concerning the definition of the melting point from fusion curves observed
on the DSC.
Melting Behavior
A series of DSC fusion curves on separate samples provides the melting
point as a function of heating rate for the Instron strands. The high melting points (defined from the fusion curve peak values) as well as the tendency to superheat, can pe observed in Figure 1. This latter effect is
indicated by the increase in the melting point as the heating rate is elevated.
All DSC values are corrected for instrumental thermal lag. 11 The samples
produced at 136°C2 have the highest melting points so far observed for the
structure resulting from the Instron procedure. The high melting point of
the Instron-prepared samples as well as their tendency to superheat ar~
properties that have been previously attributed for other samples to the
presence of extended chains in a polyethylene crystal structure. 12
It is relevant to compare the data in Figure 1 with those obtained with
the DSC on various well-defined polyethylene structures. The melting
point values shown in Figure 1 are found to be distincly higher than those
predicted by a chain-folded model, as is indicated by the following comparison of these Instron samples with a known chain-folded structure. Singlecrystal mats having a 278 ± 30° A low-angle x-ray spacing have been grown
by crystallizing at 90°C from a dilute solution of polyethylene in xylene,
1
MELTING OF POL YETHYLE E
1137
14 5
.u
~14 0
~
t9
z
f-
---.l
~ 135
13~------~5
~D~----~1~
O~O------1~5~O----~20
~.O
DSC HE ATING RAT E:C/MIN.
Fig. 1. Effeet of heating rate on peak melting point of transparent portion of strands
formed at 5 em/ min and 136°C.
followed by annealing for a period of 570 hr at 128 ± l°C.13 Within instrumental precision, this x-ray spacing is comparable to the rather small
average spacing of 230 it obtained 14 for the Instron strands. The singlecrystal structure has a melting point of 136.9°C, as determined from the
peak value of the fusion curve measured at a DSC heating rate of 10°C/min,
whereas the Instron sample has an apparent melting point of 142.8 ± O.4°C
at the same heating rate, as can be seen by the appropriate point in Figure
1. It should be noted that the single-crystal sample, crystallized by means
of thermal treatments alone, has a greater crystalline content (94% versus
83% for the Instron sample), a higher weight-average molecular weight
(153,000 versus 52,500), and is crystallized directly from dilute solution,
rather than from the melt. All of these factors would result in a higher
melting point for the single-crystal mats 13 ,15,16 relative to the strands crystallized in the Instron rheometer, assuming that the measured low-angle
spacings can be considered as an indication of similar crystal fold periods.
Thus it is necessary to explain the 142.8°C melting point observed for the
Instron sample in terms of a crystal structure different from tpe conventional chain-folded model that definitely holds for the single-crystal samples. The 5.9°C increment of the Instron strand relative to a purely chainfolded structure having approximately the same low-angle spacing is a
primary reason for the assertion of an extended-chain crystalline component in the Instron strands in addition to the chain-folded component
that produces the 230 it low-angle x-ray spacing.
A DSC trace of a pressure-crystallized extended-chain structure, having
a peak melting point of 138°C at a heating rate of 8°C/min, has also been
reported.6 It is of interest to compare this with the melting point for the
structure crystallized in the Instron apparatus. The maximum pressure
1138
SOUTHERN, POHTER, A D BAIR
used in the Instron procedure is less than 2000 atms, whereas the static
pressure required to produce an extended-chain crystal is at least 3000
atms when pressure is the only driving force. 7 A melting point value for
the Instron strand is interpolated from Figure 1 to be 142.4°C at the same
heating rate. Hence, the magnitude of the melting point for the Instron
sample is some 4°C greater than that of a known extended-chain structure.
This increment is not significantly attributable to the liability of the DSC
to maintain the sample at the programmed temperature due to the difficulty of conducting heat into the cylindrical sample at the relatively rapid
heating rates. Presumedly, an extended-chain crystal structure can also
be formed from crystallization of the polyethylene by shearing a dilute
solution. 9,17 A shishkabob structure crystallizes which is believed to contain an extended-chain crystallite backbone that has nucleated epitaxial
chain-folded lamellae. A DSC trace of such a polyethylene structure yields
a peak melting point of approximately 128°C with a high-melting tail that
returns to the base line at 135°C at a heating rate of 5.0°Cjmin.8 The
high-melting tail is believed to represent the melting of the extended-chain
crystallites. Other researchers have observed high-melting-point tails for
the shishkabob structures at temperatures as high as 147°C on the DSC at
a heating rate of 5 b Cjmin. 10 This melting behavior can be compared with
the single relatively sharp fusion curve that peaks at 14O.8°C as obtained
from the Instton strand melting at the same heating rate. The lack of a
high-melting-point tail for the DSC fusion curves of the Instron strands
may be significant; however, no explanation is proposed at this time.
Conditions providing a sufficient amount of time to remove the superheating effect are obtained on the Mettler hot stage, where final melting is
observed at a temperature of 134.5°C for Instron strands. This is comparable to a 134.1°C final melting point for the shishkabob structure under
equilibrium conditions. 10 The low melting point under equilibrium conditions for a presumedly extended-chain crystal structure has been attributed
to imperfections within the structure. 10 Thus the comparative studies of
melting point values of the Instron strands with those of purely chainfolded samples, pressure-crystallized extended-chain structures, and the
shear-crystallized shishkabobs (supposedly containing both extended-chain
and chain-folded crystallites) have consistently implied that a model
having an extended-chain crystal component provides a likely explanation
for the melting behavior of the Instron samples.
To define further the melting behavior of the Instron skands, the shape
of a typical fusion curve is shown in Figure 2. The lowest DSC heating
rate of 0.625°Cjmin is chosen in order to minimize superheating. The
fusion curve initially departs from the base line at 131.6°C and returns at
136.4°C. This melting range of less than 5°C indicates an unusually narrow distribution of crystallite perfection for a polyethylene crystal structure; however, this may be an artifact resulting from the highly constrained
nature of the crystal structure. 4 Furthermore, it is believed that the electronics of the DSC are so arranged and the standard calibration is so per-
MELTI G OF POLYETHYLE E
1139
~r
dt
I
HEATING RATE =062 S"ClMIN
TEMPERATURE SCAL E. ·C
I
132
134
I
I
I
I
DSC
Tp _ 135.4"C
136
1
BIREFRINGENCE
TFINAL=1370 ·C
Fig. 2. Fusion cmve of transparent segment obtained on differential scanning calorimeter
calibrated by optical procedmes.
formed that the melting point defined from the peak value of the fusion
curve for a structUl'e having a graded degree of crystal perfection, such as
OCCUl'S in all polymers, more nearly approximates the temperature at which
the majority of the crystallites melt rather than the temperatUl'e at which
the final crystallites melt. In support of this hypothesis, the temperature
scale of the Mettler hot stage is calibrated with the same standard samples
(Fisher thermetric standard adipic and benzoic acid) used to calibrate the
DSC trace obtained in Figure 2. The temperature at which birefringence
due to the crystal value of the DSC trace obtained on the same standards.
With such a correlation, it is possible to place the melting point determined
from birefringence (Tfina l) on the same scale as the DSC fusion curve.
This is shown in FigUl'e 2 by using a duplicate . ample of that melted in
obtaining the fusion curve. T final is 137.0°C, significantly higher than the
melting point of 135.4°C defined from the DSC fusion curve. The conclusion from this data is that the Instron. strands have a small amount of
relatively perfect crystallites that have not melted at 135.4°C, the temperature at which the CUl've returns to the baseline. It is of interest that electron-diffraction studies indicate a nearly perfect cry. tal structure only in
the inner core of the Instron strands. 4 Indeed, it is possible that this
structure constitutes the high-melting material detected only by the birefringence measurement of the melting process.
Paraffin Study
With the DSC, fillal melting is usually assumed to occur at the peak
value of the fusion curve. However, the results of the above birefringence
measurement, as well as an accompanying study of a paraffin hydrocarbon
indicate that melting occurs after the peak temperature. An explanation
is provided in the results of the paraffin study.
During fusion, a thermal lag develops in the differential scanning calorimeter between the sample and the sample container. In Figure 3, the
proper melting temperature for the n-paraffin C94H190, is located by correcting the peak temperature, point B, for thermal lag by drawing a line through
B with the slope determined from the melting of a high pUl'ity Indium
standard, and reading the temperature at the point A, where line AB inter-
1140
SOUTHERN, PORTER, AND BAIR
B
I
,
,,
2mcal/sec
1
/
1
/
1
/.
/
/
--~=---=-:-::-
105
- - ----
I
I
,
I
--i -C-o-""':E=-----
110
115
TEMPERATURE °C
120
Fig. 3. Fusion curve of n-Cg<H\vo.
sects the baseline. u .l3 Point C is the projection of B onto the bl;l.seline.
In contrast to this typical melting behavior, it can be shown that melting
occurs beyond the melting peak for chain-folded cyrstals. The region
BCE under the curve in Figure 3 normally represents the energy required
by the sample to catch up to the programmed temperature of the calorimeter after termination of melting. The area under this curve is theoretically the same for samples of similar heat capacity and mass. However, in
Figure 4 the area BCE' under the trailing edge of the melting curve for
solution-grown polyethylene single crystals extends beyond the terminating
edge of that of the n-paraffin which has been depicted by the dashed line
BD in Figure 4. Points A and C are defined as in Figure 3. Since equal
amounts of material (1.20 m~) are melted at lOoe/min in each case, and
both samples have similar specific heats and apparent heats of fusion, it is
concluded that the polymer crystals melt over a wider temperature interval,
including temperatures as high as D', rather than up to the point C (typical
for paraffins). Point D' is given by the projection of the linear portion of
BE' onto the baseline. On the basis of the area BDD', the fraction of
crystals which melt beyond the peak temperature is 18%. Therefore, in
MELTING OF POLYETHYLE E
1141
POL YE THYLENE SINGLE CRYSTALS
8
T , 105·C n-HEXADECANE
I
2mcal/se
1
I
________________ _
__ ,_
A
\
110
120
TEMPERATURE °C
_
: I
.JI _ _
....1 -""I
CDD'E'
130
Fig. 4. Fusion curve of polyethylene single crystals grown from solution in n-hexadecane
at 105°C.
bulk-crystallized samples, where larger variations in lamellar thicknesses
are probable, a substantial amount of melting may occur after the melting
peak. This phenomenon will influence the slope of the trailing edge of the
melting curve and should be remembered in defining the melting behavior
from DSC traces. Because of its reproducibility, the fusion-curve peak
value (corrected for thermal lag) is arbitrarily defined for the data presented here as the melting point.
Irradiated Samples
Previous researchers have found that irradiation of polyethylene results
in crosslinks that effectively prevent the reorganization of the crystal structure during melting. For example, the multipeak fusion curve often obtained for single-crystal samples is reduced to a single peak on irradiation
of the samples; furthermore, this single peak is usually in the vicinity of
the initial peak of the un irradiated specimen. 13 The melting behavior of
the Instron strands is compared for samples that have been exposed to 0,
50, and 80 Mrad (Fig. 5). In contrast to the results obtained with single
crystals, the fusion curves of the Instron strands are resolved from a single
peak into at least two peaks. Such a phenomenon has also been observed
for the melting of irradiated samples of the shishkabob structure and definitely indicates a discontinuity in the structure produced in the Instron
rheometer. The lower melting peak of the irradiated strands may result
from a less oriented crystalline component. It is of interest to note that a
SOUTHERN, PORTER, AND BArn
1142
~i
dt
I
~.o
MRAD
T =1390·C
p
L>H=495caLlg.
5.0 MRAD -------7 i'
Tp~ 138.5 DC
'
L>H =49.6 caUg
,
.
: :
HEATING R ATE~5.o°C/MIN .
.. ;,"
TEMPERATURE , ·C
.'
,/
8 .0 MRAD
Tp =139.5°C
~ :
,:
,.,:
\ :
I
'
I
·.
L> H ~ 47.ocal /g
I ··
I ··
I
I
I
132
136
14.0
Fig. 5. ElTect of radiation on fusion curves of transparent segments produced in the
Instron rheometer.
rise in melting point as a function of orientation has been previously discussed in studies relating to natural rubber.16 Alternatively, the appearance of the t.wo fusion peaks during the melting of the strands may be the
direct result of different degrees of crystalline order, the existence of which
has been indicated by electron diffraction data. 14 Research in other laboratories17 has shown that dual-peak endotherms can be obtained from irra~
diated polyethylene samples held at a fixed strain while melting. The
higher peak value so obtained is attributed to a fibrillar structure consisting, at least in part, of extended-chain crystals. The dual pea,k of the fusion
curves of the irradiated strands is a function of the crystal structure existing prior to irradiation and is not due to the destruction of crystallites.
Tbis is borne Qut by the approximately equal heats of fusion for the unirradiated sample and those irradiated at the doses as high as 50 Mrad
(see Fig. 5 for confirmation). The lack of temperature drop in the high
melting peak of the irradiated relative to the unirradiated samples is also
consistent with this observation. There may be some crystallite destruction at the higher irradiation levels, as evidenced by the slight drop in the
heat of fusion of the samples irradiated at 80 Mrad (Fig. 5).
Conclusion
The thermal data have been collected in order to emphasize the unusual
melting behavior of the Instron strandsj that is, the relatively high, sharp
melting points, the apparent superheatability, and the multipeak fusion
curves on irradiation. The comparison of these properties with those of
known structures in the literature is considered to be of definite use ~
developing a structural model for the Instron strands. This comparison
implies the existence of an extended-chain crystal component in the transparent Instroll strands. Tbe presence of an extended-chain crystalline
component has recently been confirmed by using electron-diffraotion
techniques. 4
I
I
MELTING OF POLYETHYLENE
1143
References
\
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2. J. H. Southern and R. S. Porter, J . Appl. Polym. Sci., 14,2305 (1970).
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11. Perkin-Elmer Corporation, Thermal Analysis Newsletter, No.5, Norwalk, Connecticut.
12. E. Hellmuth and B. Wunderlich, J. Appl. Phys., 36, 3039 (1965).
13. H. E. Bair, T. W. Huseby, and R. Salovey, in Analytical Calorimetry, R. S. Porter
and J. F. Johnson, Eds. Plenum Press, New York, 1968, p. 31.
14. C. R. Desper, private communication (1970).
15. T. W. Huseby and H. E. Bai.r, J. Polym. Sci. B, 5, 265 (1967).
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The contribution from the University of Massachusetts by J.H.S. and R.S.P. was
supported by National Science Foundation Grant. GK 22837.
Received August 20, 1971
Revised December 9,1971