I N D U S T R I A L A N D E W G I N E E R I N G CHEMISTRY
272
A t 184" F. and 300 pounds per square inch absolute the components have the following K values: K
h 7
Thus the iiiininiiiiii rcfliix ratio is calculated as follo\~-s:
1
+ 26 + 9
(25) (2.065)
11 11 + 12 + 17 + 100 (136
- ~- -
0 3 1.36
'1.36 0.141 = 1 (51.6
The value of 0.955 agrees with the calculations of ,Jenny, who found that a reflux ratio of 1.0 was too high and a reflux ratio of 0.9 was too low, and concluded that the true minimum reflux ratio was about 0.95. The method of Colburn (2) gave a value of 0.96 for the minimum reflux ratio. Thus the empirical equation developed in this paper applies to this problem with extreme accuracy. LITERATURE CITED
r R M = 25
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~
-
0.68
+ 1.8 + 3.9) = 0.933
(1)
Brown, G. G., and Martin, H. Z., Trans. Am. Inst. Chem. Enur..,
35,679-708 (1939). (2) Colburn, A. P., Ibid., 37, 805-25 (1941). (3) Fenske, -M.P., IXD.ENG.CHEM.,24, 482-5 (1932). (4) Gilliland. E. R., I h i d . , 32, 1220-3 (1 940). (6) Hogan, J . J.,Ihid., 33, 1132-8 (1911). (6) Jenny, F. J., Trans. -4m. Inat. Chem. Engrs., 35, 635-77 (1939,. (7) Robinson, C. S., and Gilliland, E. R., "Elements of Fractional Distillation", New York, McGraw-Hill Book Go., 1939. (8) Underwood, A. J. V., Trans. Inst. Chem. Engrs. (London), 1 0 , 112-58 (1932).
Thermal Evidence of rvstallinitv olymers b
d
vc'. 0. BAKER
AND C. S . FULLER Bell Telephone Laboratories, Inc., Murray H i l l , S. J .
Unusually sharp differences between physical states at molding or fabricating temperatures and those at use temperatures are striking properties of newer thermoplastics such as polyethylene and polyamides. This report shows that such polymers, unlike polystyrene, methacrylates, and many cellulose derivatives, show relatively sharp phase transitions on cooling from a fluid state. Timetemperature studies indicate that these transitions resemble the setting of an ordinary crystalline organic material. Correspondingly, the large shrinkage after molding of these compounds is explained. However, the cooling curves also exhibit some striking differences from the freezing of usual small molecules. Supercooling can be extensively induced and controlled and utilized technically for appreciable times, w-hereas it is very unstable with small molecules. Further, it depends on the molecular weight and polar structure of the long chains. Con\ersely, the ordering denoted by the phase transitions is never complete; i.e., some 'Gsupercooling"is always present in the solid and affects physical properties.
heat capacity (sa), heat of fusion (6), density ( S T ) , and stressstrain dependence on temperature; ( d ) x-ray estimation of crystallinity in unorient'ed, unstressed polymers (16, $3). While these other researches have usually treated final crystallinity as special cases for different' polymers (mostly natural), the present report investigates the existence of spontaneous local ordering over a narrow temperature range, which xould account for the final states reported in the preceding references. That is, a phase change whose thermal effects are closely analogous to those observed in the crystallization of ordinary molecules has been sought in the solidification of polymers containing a wide range of macromolecular weights. From such phase changes may be inferred the presence of ext,ensivc, cooperative, local ordering in a fashion supplementing the usual x-ray or electron diffraction patterns. Further, i t was desired t o see if the sharp shrinkage and change in physical properties known to occur on cooling these polymers from molding or fabricating temperatures were associated x i t l i evidence of latent heat evolution, marking a phase chango.
T
POLTCTHTLESE. Material with electrical properties indicating freedom from polar impurities, and with a high-temperature solubility (toluene a t 100" C.), indicating freedom from cross linkage, was obtained from Imperial Chemical Industries (111 qr/c = 0.775, for c = 0.7000 gram/100 cc. in Tetralin a t 80" C.). POLYESTEI~S were prepared from purified ingredients, as previously described (2). The average molecular n-eight,s of theso compounds were accurately determined, and unless othorwise noted lay in the range -Ifw(n-eight average) 18,000to 22,000. P o L Y A h i I D E s were ohta.ined from the Du Pont Company or prepared experimentally; they vere pure, linear polymers having negligible ash content and average molecular weights of M , 20,000 to 25,000 and In ~ ~ in/ cresol c of 0.9 to 1.2, except for the low-viscosity sample.
MATERIALS
HE concept of crystallinity or local order in high polymers has been studied in this investigation by means of timetemperature curves (35). The object was t o seek phase changes nhich would reveal further the significance of the following earlier investigations of molecular order in high polymers: (a)x-ray detection of crystallinity in natural, highly oriented fibers such as cotton (19, $6) and silk (18); ( b ) x-ray detection of crystallization from externally applied stress, as in stretched rubber (ai?) and relation of this crystallinity to the physical properties of the stressed polymer ($0) ; (c) thermodynamic or mechanical determination of crystallinity, TTith or without orientation, from
March, 1946
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
273
connected to a gas and vacuum line so that the pressure and atmosphere around the sample could be accurately controlled. (The effect of this reduced pressure on the melting point was insignificant.) The temperature Gradient provided by the furnace, into which the sample jacket fitted snugly, was regulated by a Wheelco Capacitrol. The sample temperature was continuously lotted by a Leeds & Northrup recor&r. RESULTS WITH POLYTHENE E .
I
Since nearly all thermoplastics are derived from substitution on or in a paraffin carbon chain, polyethylene represents the basic structure. The cooling curve of Figure 1 agrees with the x-ray results (6, 13) which show t h a t a definite first-order phase transition should exist on solidification of high-molecular-weight polyethylene. The halt occurring at T I M E I N MINUTES about 96’ C. i n the graph may be Figure 1. Cooling Curve for Polyethylene, Showing First-Order Phase shifted up or down, depending on Transition after Supercooling the rate of cooling. Thus, the incidence of this effect during molding or extrusion will depend on The cresol was freshly purified and dried, since water changes cooling conditions. However, in the experimental observaSupertions it~is reproducible, if ~rate of cooling is controlled. ~ ~ ep~~~~~ ~ ~ ~ cooling is universal in high polymers, and the thermal flucform, (77) = K M: tuations leading t o nuclei formation and hence to crvstalliza.., tion are favored by the same factors discovered by Tamman where M , = “viscosity average” molecular weight. (36)for melts such as glycerol. A large temperature gradient below the melting point drives toward crystallization because of CELLULOSEESTERS were obtained principally from Eastman Kodak Company and Tennessee Eastman Corporation. C e h the increasing free energy of the metastable, uncrystallized state; lose triesters and certain of the cellulose acetate butyrates &ere this holds &Iso for transitions witkin the crystalline state characterized ( 3 ) previously. The triesters. showed less than 0.1 mole Der unit of unsubstituted hvdroxvls and thus analveed But the ViSCOSity of the supercooled melt also increases steeply with decreasing temperature so that the intermolecular rearas accept’able “triesters”. All cellufose eiters were from {heroughl washed flake and had very low ash content. Stability rangements necessary for crystallization are greatly retarded analysis of gases evolved a t 240” C. indicated low comtests crystallization rate obtains a7ith poly(36). Hence, a bined sulfate and high purity for the acetate butyrates. I n general, this test produced less than 50 x 10-6 gram of replaceable mer% 8s with other supercooled melts, a t some optimum temperature below the true melting point (4,SO). I n addition, the “true hydrogen per gram of polymer per hour. melting point” is more probably a melting range (10,11, 99,34) in high polymers containing a relatively broad tlistribution of moTEMPERATURE-TIME MEASUREMENTS lecular species. Nevertheless, essentially linear polymers showThe experimental determination of time-temperature curves ing transformations like t h a t a t the halt on Figure 1 exhibit only followed accepted techniques. However, controls possible with limited “plastic” behavior, but are either highly viscous liquids the usual organic compounds of low molecular wei ht could not all be applied t o polymers, Stirring was never feasi%le; the melt or relatively rigid solids. Such halts support the theories of coviscosity (a), several degrees above the melting point, was over 500 poises even for the compounds of lower molecular weight. A true phase equilibrium thus probably did not exist an where along the cooling curves and, probably, only over very fmited sections of the heating curve. The basic feature of the timetemperature curves was, however, accurately preservednamely, t h a t definite proof of phase transitions was obtained. The cooling and heating curves for polymers melting below 125” C. were run in a transparent, vacuum-jacketed apparatus. *\ The sample was flooded with purified nitrogen, although polymers studied in this temperature range were relatively insensitive $ 7714 t o oxidation. The platinum resistance thermometer was replaced in most cases b y a totally immersed Anschiitz type thert ,?I mometer, calibrated by the National Bureau of Standards. The h +? 70 sample tube was of dimensions such that the thickness of polymer between the thermometer and the tube wall was 2.0 t o 2.5 mm. a 66 The oil bath surrounding this vacuum-lagged tube was careful1 z controlled t o ive a smoothly varying temperature head, whicc 66 was practicalfy constant near the time-temperature plateau as shown, for example, in Figure 1. 6 4 The measurements above 125” C., principally on polyamides 0 10 20 30 40 60 60 ?O and cellulose esters, were made in tubes in an electric furnace riur /N Mtwrcs whose temperature was re ulated t o *1.5O C. The polymer Figure 2. Cooling Curve for Poly-w-hydroxydecanoate samples were melted around calibrated thermocouples in 2 X 6 of Mw 10,000 cm. Pyrex inserts. These inserts were centered in a glass jacket
$?;$):
-
k
‘
INDUSTRIAL AND ENGINEERING CHEMISTRY
274
20
30
40
50 60 TIME IN MINUTES
70
80
eo
Figure 3. Cooling Curve for Poly-w-hydroxydecanoate of Mw 20,800, Showing Increased Supercooling
T I M E IN M I N U T E S
Figure 4.
Cooling Curve for Poly-w-hydroxyundecanoate of Mw 22,000
operative (9,68) setting up of the whole mass. Hence, individual molecular segments can no longer be displaced past one another, even one at a time, stepwise, which mould result in fairly ready flow, according to recent concepts of this process (34). By contrast, molecular segments of polymers which do not a t least partially transform t o a crystalline phase lose their mobility only gradually over a wide temperature interval. This rate of stiffening is reflected in the “second-order transition” (5, 21, SB), a kind of “rigidification” distinct from the sharper process of Figure 1. RESULTS WITH POLYESTERS
One simple modification of the paraffin chain of polyethylene is obtained by occasional substitution of methylene groups by polar linkages, i n pairs for polyesters, polyamides, etc., or singly as with the polyglycols, polyamines, etc. A profound new influence, the production of coordinated or associated dipole layers (1, 88) then arises. When these polar groups are introduced regularly into the polyethylene chain, as in the simple polyesters rather than i n copolymers, the strong crystallizing tendency of polyethylene persists or may even be enhanced. The polar linkages cause an over-all increase in interchain forces, as shown by physical properties. The effect on the melting point, as compared t o that of polyethylene, may, however, actually be a decrease, depending on restriction of chain motion in the solid (leading t o decreased entropy in the solid, 4). Phase transitions, indicating t h a t a t least part of the sample crystallizes spontaneously, are readily found for a variety of polyesters. Figure 2 shows the halt, following supercooling, for poly-w-hydroxydecanoate of M, 10,000. The true melting point of this compound is about 80’ C., so supercooling was only 8” C. although the subsequent latent heat evolution merely returned the sample temperature to about 74’ C. (Presumably, although not certainly, the freezing and melting points are identical.) A slightly slower rate of cooling produced.(Figure 3) somewhat more supercooling for another poly-w-hydroxydecanoate of about twice the weight-average molecular weight (20,800) of the sample
Vol. 38, No. 3
of Figure 2. This effect of more sluggish crystallization for polymers of higher molecular weight is general and will be observed in their technical utilization. The relatively flat portion of the curve of Figure 3 again indicates widespread, fairly sharp, phase transition, and certainly does not suggest the selective formation of crystallites by individual species of entire chains, as has been sometimes proposed. The poly-w-hydroxydecanoates have nine methylene groups per polar linkage. Increasing this number by one converts the poIymer t o poly-a-hydroxyundecanoate. The polar groups have been diluted, but the more general consequence of the change is that the dipole vectors from these groups are no longer arranged the same along the chains and, therefore, do not interact with the same energy between the chains (1, 38). Figure 4 shows that a poly-w-hydroxyundecanoate, cooled under approximately the same conditions as the poly-w-hydroxydecanoate of Figure 3 and of about the same average molecular weight (22,000), shows a sharp setting point some 6 ” higher. These are accurate comparable values and display the sensitivity of polymer properties to delicate changes in microstructure of the chains. As the dilution of polar groups by methylene groups in linear polyesters is diminished, chains approaching those of vinyl polymers result. Thus, while polyvinylidene chloride, polyvinyl chloride, and polyvinyl alcohol have one methylene group per polar group on the chain, polyethylene succinate has two methylene groups per polar linkage or one per polar atom in the chain, each arranged in pairs. The importanre of the analogy is that with such a high concentration of polar forces, the polymers become far harder (higher elastic modulus) and less waxy then polyethylene. Their liquids, for the same average chain length, are more viscous than the melts of the less polar chains. They can be supercooled to a vastly greater extent, with consequent retention of the plastic state ( I S ) . This is especially striking for polyvinylidene chloride (90) which resembles polyethylene succinate in many ways. (The forces in so intensely polar a chain as polyvinyl alcohol are, however, so high through hydroxyl bond association that all phases a t start of decomposition tcmperaturcs appear t o have Bn unusually orderly arrangement. Hence, while it would be easy to supercool a disordered state if it existed, we have $0 far had difficulty producing a quenched sample of low crystallinity.) For these highly polar polymers, Figure 5 shows the crystallization of polyethylene succinate of M , 12,000 approximately. This sample actually freezes a t about 99” C. so there is over 30’ supercooling, as contrasted t o Figures 2, 3, and 4,although the cooling rate for Figure 5 was even slightly less than that for Figure 4. The sample temperature in Figure 5 xyas so low a t the time of the halt that the heat evolution was in-
16 w 14
