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The application of a new DTA technique to the measurement of the heats of fusion of bulk crystallized polyethylenes and polyethylene single crystals.

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Die Angewandte Makromolekulare Chemie 27 (1972) 165-173 ( N r . 394)
From the University of Cambridge, Department of Chemical Engineering,
Cambridge, England
The Application of a New DTA Technique to the
Measurement of the Heats of Fusion of Bulk
Crystallized Polyethylenes and Polyethylene Single
Crystals
By DAVIDA. BLACKADDER
and TREVORL. ROBERTS
(Eingegangen am 20. Oktober 1971)
(Neufassung eingegangen am 24. April 1972)
SUMMARY:
DTA has been widely used for many years and it has always been assumed that
the technique is intrinsically secondary in that the determination of enthalpy
changes requires calibration of the equipment. Recently the present authors have
shown that this is not so, and under certain conditions DTA becomes a n absolute
method. A modification of the new technique has now been applied t o a polymer
and the results are presented here. The heats of dissolution and hence of fusion
of various polyethylene samples have been measured, together with their densities.
The results are shown t o be consistent with those of other workers. No special
accuracy or importance is claimed for the actual results, however, because the
purpose of the paper is to introduce a new technique in a familiar polymer context.
There is scope for improvement and extension of the procedures described here,
and the status of DTA in polymer science will be increased in consequence. The use
of a liquid system greatly facilitates the preparation and handling of the material
under test.
ZUSAMMENFAS SUNG :
Die in den letzten Jahren vie1 angewandte MeDmethode der DTA wird allgemein
als eine Methode fur Relativmessungen angesehen, da die Messung von Enthalpieanderungen stets eine Eichung der Apparatur voraussetzt. I n einer kurzlich veroffentlichten Arbeit konnten wir jedoch zeigen, da13 unter bestimmten Bedingungen
auch Absolutmessungen moglich sind. Hier wird jetzt uber solche Messungen a n
verschiedenen Polyathylenproben berichtet . Es wurden die Losungs- und damit
Schmelzwarmen bestimmt. Die Ergebnisse sind in sich konsistent und stimmen mit
denen anderer Autoren uberein. Auf eine hohe Genauigkeit wurde kein besonderer
Wert gelegt, d a die Arbeit im wesentlichen nur der Einfuhrung der neuen Technik
dienen sollte.
165
D. A. BLACKADDER
and T. L.
ROBleRTs
In.troduction
Previous experiments1 have shown that dilute suspensions of polyoxymethylene single crystals in organic liquids provide ideal media for differential
thermal analysis (DTA) when used both as diluent and as reference material.
(The d i l u e n t and the s p e c i m e n jointly constitute the s a m p l e . ) Convection
currents are suppressed and the diluent medium, which behaves as a BINGHAM
Plastic Fluidz, can support fine particles of the solid specimen under test, even
when the solid has a relatively high specific gravity. It is therefore possible to
regard the whole sample as though it were a rigid solid. This fact, together
with the dominance of the thermal properties of the organic liquid, has important consequences for the theory of DTA, and equipment need not be calibrated. An extended theory of operation was verified by comparing measured
heats of fusion of two inorganic salt hydrates with literature valuesl.
The purpose of this paper is to illustrate the use of the new technique, in
studying the dissolution and fusion of polymers. Specimens of bulk crystallized
polyethylenes and polyethylene single crystal mats were used. To avoid confusion it must be stressed that there are two polymers present in each sample
for DTA. The suspension of polyoxymethylene crystals gives the medium its
desirable properties and is thermally inert over the temperature range covered
by the enthalpic process involving the other polymer. The polyethylene constitutes the specimen, and is suitably dispersed in the diluent medium. The
reference consists of the diluent medium only.
The general advantages of liquid systems for DTA have been described
previously', but there are additional considerations relating specifically to
polymers. The fusion of dry powdered polymer leaves a hard intractable mass3,
and the sample holders may become fouled with decomposition products4
unless special precautions are taken. With a liquid system no fouling occurs
and the sample holders are easily cleaned. I n addition the thermocouples are
easily withdrawn and cleaned, so they need not be particularly robust and
require minimal support. Dry samples must either be prepared by grinding
the specimen, with a consequent risk of damage, or by fusion in the holder
prior to the run, an unacceptable procedure if the effect of thermal history is
to be investigated. With a liquid diluent the preparation of the sample is much
easier, and the depression of the melting point is helpful in avoiding oxidation
or decomposition.
Experimental
Three polyethylenes were supplied by I.C.I. and some properties of each are
given below.
Rigidex Type 50:
166
a,
=
80.000;
a,= 16.000; density as supplied 0.972 g/cm3.
New D T A Technique
Rigidex Type 3 : Mw about 160 000;
M n about
8.500; density as supplied
0.947 g/cm3.
Alkathene V 14114:Bwabout 1.500.000;density as supplied 0.925 g/cm3.
Sticks of each polyethylene were scraped with a scalpel to obtain tiny flakes.
On average, each particle weighed about 0.2 mg, so that 100 mg of polymer dispersed in 5 cm3 of diluent provided about 100 particles per cm3. The samples were
made up as described previouslyl, using dekalin as the organic liquid.
Results
1. Bulk Crystallized Polyethylene
Fig. 1 displays thermograms for the dissolution of one of the polyethylenes,
the heating rate, @, being varied systematically. T R is the temperature recorded
by the reference thermocouple. The traces are similar to fusion thermograms.
The technique provides an extremely steady baseline, the portions before and
after dissolution being easily connected except a t the very highest heating
rates. Earlier work on the DTA of polymer dissolution5$6 suffered from the
crippling disadvantage that the baseline collapsed towards the end of the
dissolution process. This was inevitable in the absence of the inert supporting
medium described here.
The breadth of a DTA peak is influenced partly by the inherent dissolution
range of the specimen, and partly by the parabolic temperature distribution
within the sample. The latter effect is more pronounced a t higher heating ratesRiyidex Type 50
0.5O C
1
70
80
90
TR
Fig. 1 .
120
110
100
("C)
Thermograms of dissolution of Rigidex 50 in dekalin at various heating
rates p (0.1g polymer in 5 cm3 medium).
167
D. A . BLACKADDER
and T. L. ROBERTS
Table 1 shows the results, calculated as described in the earlier paperl. Ten
runs were performed on each polyethylene, and the value of the standard
deviation for each set of runs is probably related to the state of subdivision
achieved in preparing the specimens.
I n the formula
AH, is the heat of solution, A the area of the peak on the DTA trace, 1the
thermal conductivity of the sample at the peak temperature, V the volume of
the sample a t room temperature (20 "C), ez0 and e the densities of dekalin a t
20 "C and a t the peak temperature, 9, the rate of heating, R the radius of the
sample in the apparatus, and m the mass of specimen present in the sample.
The thermal conductivity of the sample a t the peak dissolution temperature
was obtained from the separate values for the medium and the polyethylene.
The thermal conductivity of each polyethylene was found by extrapolating
the data of SHELDON
and LANE^, knowing the density a t 25 "C. The thermal
conductivity ?f dekalin was measured by using the DTA technique in reverse,
with the heat of fusion of sodium thiosulphate pentahydrate as referencel. The
value of the thermal conductivity of dekalin at any temperature other than the
peak temperature of the hydrate fusion process was calculated from an empirical relationships.
The heat of mixing is required in order t o calculate the heat of fusion from
the heat of dissolution of the polymer. According to F L O R Y ~the enthalpy of
mixing a t low polymer concentrations is given by
x
Here R is the gas constant, T the peak temperature (370 OK), the interaction
parameter (-0.35)10, M, the molecular weight of the solvent (138.2), es the
solvent density (0.81 g/cm3) and ep the polymer density (0.97 g/cm3), each
density being appropriate to the temperature of the dissolution peak. With
these values equation ( 2 )gives the heat of mixing of dekalin with polyethylene
as - 1.6 cal/g of polymer.
2 . Mats Composed of Agglomerated Polyethylene Crystals
The mats, kindly supplied by Dr. M. J. RICHARDSON,
were composed of
crystals formed from fairly dilute solution in xylene a t three crystallization
temperatures. (There is reason to suppose that the crystallization temperature
given as 70 "C is not correct, and should be higher.) A scalpel was again used
to subdivide the polymer into small flakes.
168
101.4
100.9
276 x 10-6
283 x
1.075
55.7
4.5
- 1.6
57.3
Rigidex type 50
93.7
93.5
279 x 10-6
283 x 10-6
1.067
43.0
1.8
- 1.6
44.6
Rigidex type 3
I
77.1
76.7
284 x
289 x
1.051
33.9
2.4
- 1.6
35.5
Alkathene
Radius of sample wells was 0.496 cm, so R2 was 0.246 cm2.
About 5 01113 of medium contained 0.1 g polymer.
* Used for all calculations on a given polyethylene where slight dependence of T, on heating rate could be ignored.
(cal g-1)
(cal g-1)
(cal g-1)
(cal g-1)
(cal cm-1 "C-1 s-1 1
(cal cm-1 "C-1 s-1 )
("C)
("C)
~~
DTA of the dissolution of bulk crystallized polyethylenes.
Dissolution peak temperature, T,, average*
T, extrapolated to zero heating rate
Thermal conductivity of dekalin at T,
Thermal conductivity, A, of sample at T,
Ratio ezo/e, see equation (2)
Heat of dissolution, AH,, average of 10 runs
Standard deviation of AH,
Heat of mixing, AH,
Heat of fusion, AH1
Table 1.
f
is.
3
2
b
T
("C)
58.8
* Average of
OC,
88.3, 94.1
1.580
See footnotes to Table 1.
centre of gravity temperatures viz 93.3
Density of polymer (g cm-3)
Heating rate
("C min-1)
Dissolution peaks
("C)
temperature, T,
Average T,
("C)
Ratio ezo/p, see eqn. (2)
Thermal conductivit y of dekalin at T, (cal cm-1 "C-1 s- 1)
Thermal conductivityof specimenat T, (cal cm-1 O C - 1 s- 1)
Heat of dissolution,
A Hs
(cal g-1)
(cal g-1)
Average AH,
Heat of mixing, AH,
(cal g-1)
this work
Heat of fusion, AH! (cal g-1)
ref.11
Crystallization
temperature
94.0 "C and 92.5
- 1.6
59.1
54.1
54.8
57.5
286
56.3
OC.
- 1.6
56.9
56.2
54.8
55.3
286
54.9
x 10-6
93.5
x 10-6
1.067
93.6
93.6
278.5 x 10-6
58.9
88.0, 93.7 93.7
278.5 x 10-6
1.067
89.2, 95.2
93.3*
x 10-6
60.6
57.6
- 1.6
56.1 59.7 60.7
58.9
286
278.0 x 10-6
1.069
94.8 94.9 94.9
94.9
0.974
0.978
1.496 1.458 1.550 1.480 1.535 1.520
1.535
0.971
1.434
90
I
70
1
z
M
gm
F
a
5
P
d
!
s
FU
%
P
P?
New D T A Technique
0.5O C
80
100
90
TR
("c)
110
Fig. 2. Thermograms of dissolution in dekalin of finely-dividedpolyethylene single
crystal mats prepared at different crystallization temperatures (0.1 g
polymer in 5 01113 medium).
Fig. 2 shows typical thermograms for the three specimens, and all the data
appear in Table 2. The specimen crystallized a t 70 "C shows unmistakeable
signs of recrystallization having occurred in the course of the experiment. The
heats of fusion of these specimens have been determined at the National
Physical Laboratory using precision adiabatic calorimetry and the results11
are shown in Table 2 for comparison.
Discussion
1. Correlation of Heats of Fusion with Specific Volumes
Two important intercepts may be evaluated from a plot of heat of fusion
against specific volume (Fig. 3): a zero value of the heat of fusion should
correspond t o the specific volume for perfectly non-crystalline material
(1.17 cm3/g a t 25 "C)12 while for a specific volume of unity the heat of fusion
is that of perfectly crystalline material. (Surface enthalpy effects would make
the plot gently concave upwards, but there is no evidence of this on Fig. 3 and
only a straight line plot can be justified). The intercepts are in harmony with
current values, and lend support to the view that these are reliable t o between
171
D. A. BLACKADDER
and T. L. ROBERTS
1 and 2 per cent. MANDELKERN13 and others
matters a t some length.
11J4J5J6
have considered these
70
60
50
40
30
20
10
0
-
+ Single crystal mats:
T, = 70,80and 90°C
1.00
1-05
1-10
1.15
(cm?/grarn I
Specific Volume
Fig. 3. Heat of fusion of various polyethylene samples as a function of specific
volume.
2. Dissolution temperatures
Extrapolation to zero heating rate, illustrated in Fig. 4 for one of the bulk
polyethylenes, eliminates the slight dependence of dissolution temperature on
heating rate. It must not be assumed that this extrapolated value is necessarily
the same as a t infinite dilutionl7. Unlike small molecules, dissolving polymer
molecules do not diffuse away rapidly from the vicinity of the solid, and this
may serve to extend the peak temperature to an over-high value.
Dissolution Temperature ("C)
103
r
101
102
100
c
n
o
-
0
t
"1
I
I
0
1
"
;
I
1.
3
Rate of Heating (OClmin.1
Fig. 4. The effect of heating rate on the temperature of dissolution of Rigidex 50
in dekalin.
One of the authors (T. L. ROBERTS)
acknowledges an award from the UNITED
KINQDOM
SCIENCE
RESEARCH
COUNCIL.
172
New D T A Technique
1
D.A. BLACKADDER
and T. L. ROBERTS,
Talanta 18 (1971) 287.
D. A. BLACKADDER
and T. L. ROBERTS,Rheol. Acta 9 (1970) 409.
J. S. DOUBLE,Trans. J. Plastics Institute, April 1966, p. 73.
4 P. E. SLADEand L. T. JENKINS
(Eds), Techniques and Methods of Polymer
Evaluation, Thermal Analysis, Edward Arnold, London 1966, Vol. 1.
5 D. A. BLACKADDER
and H. M. SCHLEINITZ,
Polymer 7 (1966) 603.
6 J. L. KOENIGand A. J. CARRANO,
Polymer 9 (1968) 359.
7 R . P. SHELDON
and K . LANE,Polymer 6 (1965) 205.
8 J. H. PERRY,Chemical Engineers Handbook, 4th Edition, McGraw-Hill, 1963.
9 P. J. FLORY,
Principles of Polymer Chemistry, Cornell University Press, Ithaca,
New York 1953.
1 0 A. Y.CORANand C. E. ANAGNOSTOPOULOS,
J. Polym. Sci. 57 (1962) 13.
11 C. M. L. ATKINSON
and M. J. RICHARDSON,
Trans. Faraday SOC.65 (1969) 1774.
12 M. J. RICHARDSON,
P. J. FLORY,and J. B. JACKSON,
Polymer 4 (1963) 221.
13 L. MANDELEERN,Polym. Eng. Sci. 9 (1969) 255.
1 4 M. J. RICHARDSON,
Brit. Polym. J. 1 (1969) 132.
1 5 H. HENDUS
and K. H. ILLERS,Kunststoffe 57 (1967) 193.
l6 C. M. L. ATKINSON
and M. J. RICHARDSON,
Trans. Faraday SOC.65 (1969) 1764.
1 7 R. K . SHARMA
and L. MANDELKERN,
Macromolecules 3 (1970) 758.
2
3
173
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