Differential Thermal Analysis of Linear PolyethyleneHigh Pressure Polyethylene Blends BERT
H. CLAMPlll
Research and Developmenf Department, Spencer Chemical Co., Merriam, Kan.
b A number of blends of a linear polyethylene, Alathon 7 0 4 0 and a high pressure polyethylene, Poly-Eth 1 61 7, are characterized by differential thermal analysis. By proper annealing the thermograms of these blends may b e resolved into three endothermic peaks coming at 115', 124', and 1 3 4 " C. From the relative areas under the various peaks, it is concluded that the 115' C. peak is associated with the fusion of high pressure polyethylene crystals while the 134" C. peak is associated with the fusion of linear polyethylene. The area under the 134' C. peak is proportional to the linear content of the system, thereby giving a convenient analytical method for estimating the composition of linear high pressure polyethylene blends.
A
of differential thermal analysis (DTX) to polymer systems is relatively new (1, 3) and its potentialities in this field have not been fully explored. One area of polymer physics in which DTA appears particularly useful is in the characterixation of polymer blends. Previous DTA investigations of polymer blends have been confined to linear polyethyleneisotactic polypropylene blends @), and nylon 66-nylon 610 blends (4). While both linear polyethylene and high pressure polyethylene have been characterized by DTX (21, blends of PPLICATION
KEITHLEY MODEL 151 MICROVOLTMETER
WEST CIRCULAR CHART PROGRAMMER
n
C
T
ICE BATH
n
_IHREE 400 WATT OJARTZ HEATERS
JIJ T
DTA BLOCK
Figure 1.
OIL BATH
DTA electrical system
the two types of polyethylene have not been studied-probably because previous workers felt that the two types of polyethylene would form homogenous crystals to give only one DTA peak corresponding to the fusion of this crystal. This paper will show that the two types of polyethylene do not completely form isomorphous crystals and indeed it is possible to resolve the DTA curve into peaks corresponding to the fusion of the various types of crystals. EXPERIMENTAL
The experimental apparatus is similar to the one described by Ke (2); however, the apparatus was modsed so that the heating to the aluminum DTA block was accomplished by circulating oil. This manner of heating allows the DTA block to be electrically insulated so that the only electrical circuits in the block are the measuring thermocouple e.m.f.s. Schematic hydraulic and electrical diagrams of the apparatus are given in Figures 1 and 2. The actual DTA block is pictured in Figure 3, and is constructed of a large aluminum casting having a diameter of 5 inches and a length of 8 inches; while the surrounding asbestos insulation gives the over-all dimensions of the block as 8l/2 inches in diameter and 13 inches in height. The sample holders, also pictured in Figure 3, were made of S/8-inch aluminum tubing having a wall thickness of inch and length of 4l/2 inches. The bottom end of the sample tubes was threaded for a short aluminum plug, thereby allowing good thermal contact between the sample holder and the DTA block, and yet the polymer samples could easily be admitted and removed from the apparatus. The programmed heating rate of the oil bath was such m to give a linear heating rate of 2.4" C. per minute to the aluminum block. The circulating oil was General Electric Silicon Oil SF81-50, and the linear rate could be maintained up to about 170" C. Ironconstantan thermocouples were used in these experiments with aluminum oxide being used as the reference material. Sample preparation consisted of weighing about 0.70 gram of polymer and placing it in the sample holder. It was then melted a t 190" C. for 10 minutes and compressed by hand with a l/r-inch-diameter Teflon plunger-Le.,
PUMP-EASTERN DTA BLOCK ICE BATH
OIL BATH c
Figure 2.
DTA hydraulic system
the plunger was the same diameter as the inside diameter of the sample holder. This gave a compact, bubblefree sample which was then annealed for 30 minutes a t 120" C. (as discussed subsequently). A warmed thermocouple was then placed in the semimolten polymer and the sample holder placed in the DTA block where it rapidly cooled to room temperature. Sample blends were prepared from B linear polyethylene, Alathon 7040 by Du Pont, having a melt index of 6.0, and a high pressure polyethylene, Spencer Poly-Eth 1617 (TD-1521-Lot l ) , having a melt index of 6.0 and possessing about 14 methyls per 1000 carbon atoms. Blends of 5, 10, 25, and 40% linear were prepared by Banbury Mixing the components, extruding, and cube cutting the product, thereby producing a uniform composition. RESULTS AND DISCUSSION
Careful examination of DTA curve9 of linear high pressure polyethylene blends prepared without annealing revealed they were not smooth curves but had shoulders on the main endothermic peak as Figure 4,A indicates. These shoulders appeared to be unique to these blends, and various methods were tried better to resolve the peaks with the best annealing procedure producing the thermogram depicted in Figure 4,B. Annealing of the samples for 15 minutes at 120' C. gave some, but not complete, resolution of the peaks. Samples annealed at this temperature for l/?, 1, 2, and 16 hours gave well resolved peaks with DTA curves identical to those depicted in Figure 4,B in all cases. Annealing a t 100' or 150' C. for 30 minutes gave no resolution of the peaks, while annealing a t 128' C.-Le., in the region between the second and third peak in Figure VOL. 35, NO. 4, APRIL 1963
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4,B-produced only two endothermic peaks with the area under the second peak being added to the 118' C. peak in the resulting thermogram. As a result of these annealing experiments, it is concluded that for analyzing linear high pressure polyethylene blends, the 120' C. annealing for 30 minutes is the best annealing procedure. The annealing procedure did not alter the D T d curve of the pure components and identical results were obtained whether the samples were annealed or not. Figure 5 shows thermograms of various blend compositions annealed by the above procedure. Thermograms of all of the blends contain three endothermic peaks, a t 115O, 124', and 134' C.; however, the pure components
show only one peak with the high pressure polyethylene peak coming a t 118' C. and the linear polyethylene peak coming a t 136' C. The area under the 115' C. peak decreases as the amount of linear increases while the area under the 134' C. increases as the linear composition increases. Both the temperature of these peaks and their relative areas in the various blends appear to indicate that the DTA peaks correspond in some manner to the melting of crystals of various types of polyethylene. The 115' C. peaks can be associated with the presence of crystals of high pressure polyethylene, while the 134' C. peak presumably is due to crystals of linear polyethylene. The melting points
AMPLE WELL ALUMINUM BLOCK
THERMON
of the components in the mixture are slightly lower than those observed with the pure substances. Similar results were obtained by Ke in polyamide systems (3). The nature of the 124' C. peak is purely speculative but could correspond to a "co-crystal" of the components having only a small range of compatibility for molecules of the two types. Some support for this is inferred from the fact that the area associated with the 124' C. peak is relatively independent of the composition of the blend whereas the area associated with the other peaks varies with composition. It has also recently been reported by Swan (6) on the basis of a detailed x-ray analysis of linear polyethylene that the unit cell of linear polyethylene could accommodate a limited amount of branched polyethylene which indicates that a limited range of isomorphous crystal. is possible for the system. The fact that the linear and high pressure polyethylene crystals segregate into crystals of two different types despite the great similarity of the molecules is somewhat surprising. I t has, however, been previously observed
SAMPLE HOLDER
CEMENT
5% LINEAR
E A T I N G -COOLING C O I L S
A
!-
a Figure
3. DTA block and sample holder 10% LINEAR
25% LINEAR
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D
I-
< 134'
134.
100% LINEAR
UNANNEALED A
Figure 4.
578
ANNEALED 30 MINUTES AT 12OoC
Effect of annealing on
ANALYTICAL CHEMISTRY
F
E
B
2570 linear blends
T
Figure 5. ene blends
Thermograms of linear-high pressure polyethyl-
that blends. of structurally similar moleciiles of linear polyethylene isotactic polypropylene crystallize into two distinct crystal types (9), and also that blends of nylon 66 and nylon 610 crystallize a=. two distinct crystals ( 4 ) . I t would appear, therefore, that structurally similar polymer molecules often do not give isomorphous crystals and that DTA offers real promise as a method of differentiating between homopolymers and polymer blends of very similar molecular types. Ke has pointed out that the area under the DTX peak of polyethylene is proportional to the crystallinity of the sample ( 2 ) . The proportionality constant selected by Ke was based on the knon-n heat of fusion of dotriacontane; lionever, for this paper the proportionality constant was selected on the basis of the area under the DTA curve of the pure linear polyethylene. This means that the crystallinity values reported in this paper are relative crystallinity values. On this basis the total crystallinities of the various blends were calculated from the total area under all the DT-1 peaks with the result. being given in Table I. As one would expect, the total crystallinity of the system increases as the concentration of linear increaqes. More significant, however, are the crystallinities associated R ith the areas under the 134” C. peak nhich are also computed in Table I.
DTA Total Crystallinity Values Relative Calcd. Composition, crystallinity, linear yo linear % content, yo 0 62 ... 5 64 6 10 65 14 25 74 34 40 ’79 41 100 100 100 Table 1.
have been investigated and in all cases this method estimating the linear content shows good agreement with the actual blend composition. In view of the increasing use of linear high pressure polyethylene blends in industry, a method of analyzing for the linear content of such blends should find considerable use. ACKNOWLEDGMENT
If one assumes all of the linear polyethylene is crystallized, regardless of the blend composition, then on the basis of the selection of the proportionality constant for calculating crystallinity, the area under the 134’ C. peak is a measure of the linear content of the system. Table I indicates fairly good agreement between the true blend composition and the calculated linear content. Considering the difficulties of extrapolating the linear peaks to the base line (straight line extensions of the peaks to the base line were used in computing the areas), the agreement is quite good. -4 revien- of the literature has not revealed that other methods are available for the estimation of the linear content of blends and, therefore, the DTA method appears valuable as an analytical tool in this particular case. I n addition to the system reported in this paper, a number of other systems
Special appreciation is expressed by the author to Nathan Scarritt, Jr., for the actual construction of the DTA apparatus and to J. E. Barney for help in design of the apparatus. Thanks are also expressed to C. F. Mosier, W. A. Pavelich, C. F. Feldman, and H. D. Anspon for helpful discussions about the various thermograms. LITERATURE CITED
(1) Clampitt, B. H., German, D. E., Galli, J. R., J . Polymer Sci. 27, 515 (1958). ( 2 ) Ke, B., J . Polymer Sci. 42, 15 (1960). (3) Ice, B., “Organic Analysis,” Chap. 6, Vol. 4, Interscience, New York (1960). (4) Ke, B., Sisko, A. W., J . Polymer Sci. 5 0 , s (1961). (5) Swan, P. R., J . Polymer Sci. 5 6 , 409 (1961). RECEIVED for review Sovember 23, 1962. Accepted January 21,1963. Southwestern Regional Meeting, ACS, Dallas, Tex., December 6-8, 1962.
Determination of Oxygen in Glasses, Refractories, and Refractory Oxides by the Inert Gas Fusion Method H. L.
MACDONELL, R. J. PROSMAN, and J.
P. WILLIAMS
Corning Glass Works, Corning, N. Y.
b An inert gas fusion method has been employed for the direct determination of oxygen in glasses, refractories, and refractory oxides. Modification of commercial instrumentation has permitted analysis of samples containing up to 55% oxygen with a relative precision of 2 2y0. Following calibration an oxygen analysis can b e completed in about 15 minutes. Results of several oxygen analyses are given for a number of glasses, refractories, and refractory oxides.
T
ISF RT gas fusion technique dcwloped by Singer (7’) with irnprovcments by Smiley (8) has been modified and adapted to the determination of oxygen in various. substance?. HL
Equipment commercially available from Laboratory Equipment Corp. was slightly modified by the authors to permit determination of oxygen in glasses and refractories containing up to 55% oxygen. Other researchers who used this technique have been mainly concerned with the determination of minor amounts of oxygen in metals and alloys. Elbling and Goward (4)determined oxygen in the 900- to 3700-p.p.m. range in zirconium and zircaloy ; Banks, O’Laughlin, and Kamin ( 1 ) found 0.01 to 0.5y0oxygen in yttrium metal and yttrium fluoride; and Kallmann and Collier ( 5 ) analyzed beryllium metal containing 0.2 to 1.1% oxygen. Nore recently Beck and Clark ( 2 ) have employed a graphite encapsulation modification to determine up to 407, oxygen
in such materials as U&, TiOl, and B203,as well as metals and nitrides. The need for information concerning oxygen content of fluoride-containing glasses as well as glasses manufactured under various conditions of oxidation or reduction led to this investigation of the inert gas fusion method for the oxygen analysis of glasses and refractory materials. EXPERIMENTAL
Apparatus and Reagents. A Leco Oxygen Analyzer, catalog S o . 537300, was modified by the addition of two heliopots as desciibed later under Modifications. Calibration of plate current us. induction furnace temperature was achieved by measureVOL 35, NO. 4, APRIL 1963
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