Vol. 74
T-I. TRACY HALL
613 [CONTRIBUTIOS FROM THE
RESEARCH LABORATORY, GENERAL ELECTRIC COMPANY ]
The Solubility of Polytrifluorochloroethylene RY H. TRACY HALL A physico-chemical study of the solubility of the semi-crystalline polymer of trifluorochloroethylene has been made. Curves are given from which it is possible to determine whether a particular liquid will be a solvent and if so the solution temperature. The information necessary for the utilization of these curves is (a) the solubility parameter of the liquid a t 25", (b) the molar volume of the liquid at 25' and (c) the coefficient of cubical expansion of the liquid. Several thermodynamic constants have been determined from solubility and crystal melting temperature data for this polymer among which are the heat of fusion of the crystallites (18.1 cal./g.), the solubility parameter of the amorphous polymer (8; = 8.05 e--O.m9ST where T is in "C.), and the interaction parameter, p, for various polymer-solvent systems. For this particular polymer the interaction parameter p. is given by the expression pa = V,.,./V~T~where t'*." is the volume of the repeating unit in the amorphous polymer and V P is the molar volume of the solvent a t the solution temperature T,.
This report presents the results of a physicochemical study of the solubility behavior of the semi-crystalline polymer trifluorochloroethylene in normal solvents. The principal problems considered are as follows: (1) the prediction of solution temperature when certain fundamental data are known, (2) the lowest solution temperature to be expected, and (3) the determination of thermodynamic quantities for the polymersolvent systems. Because the temperature of solution for this polymer is much higher than that previously encountered in studies of polymer solubility it was found necessary to introduce the temperature dependence of solubility parameters and molar volumes. This has not been done before. Satisfactory interpretation of the solubility of non-polar polymers in non-polar solvents has been made with the use of the equation' p
= PI
+ pg
= pa
+
VOY80T
- S$)Z/RT
(1)
where p is a free energy parameter expressing the interaction of the polymer and solvent as the sum of a temperature independent entrop term ps and a temperature dependent heat term pH. F IroT is the molar volume of the solvent, 6oT the solubility parameter of the solvent and SpT the solubility parameter of the polymer all at the temp. T,OK. R is the Solvent
Tm, "C.
Cyclohexane Methylchloroform Carbon tetrachloride p-Xylene Toluene Benzene Mesitylene SnCl4 Tic14 GeCL BClr Cyclohexene None
145 120 114 133 128 118 127 158
... 143
>92
138 208*
Ta, 'C.
>235 120 114 140 142 200 140 > 15s > 165 > 180 >92 > 150
-
6; expj
- Kn + 1)/21apTl
The polymer used in this investigation was made by the M. W. Kellogg Company and designated as Kel-F #W. Before use it was micropulverized in a colloid mill. After this treatment the particle size was 1 to 10 microns. Three cc. of solvent plus 0.150 g. of this micropulverized polymer was placed in a heavy-walled glass tube, cooled in liquid air and sealed off with a torch. The tube was heated with shaking in a silicone oil-bath, the temperature of which was raised about one degree per minute. The polymer solvent mixture appears milky in the tube as it is shaken in the bath. Suddenly, however, when the proper temperature is reached, the fine polymer particles stick together and form a viscous ball. This phenomenon is easy to observe and occurs sharply. This is the temperature a t which the crystallites melt. (As mentioned, this polymer is partially crystalline and hence contains both amorphous and crystalline regions. ) On further heating a temperature is reached a t which the ball of polymer is completely dissolved. The following table gives the crystallite melting temperature T,, the solution temperature T. and the solubility parameters and molar volumes of the solvents at these temperatures where data were available for calculating the latter.
TABLEI 8,'" 6.32 7.05 7.35 7.71 7.62 7.78
V p
6p
130 114 109.5 135 122 101.2
..
...
7.05 7.35 7.62 7.46 6.27
114 109.5 136 124 118.3
. , .
...
V,'.
&a98
VPa
8.20 8.5 8.60 8.75 8.90 9.15 8 80
109 100 97 124 107 89 140
8.7
118 111
9 0 8.1
..
115
...
8.58
molar gas constant. 6$ and V,T are all functions of temperature while ps is essentially independent of the temperature. The change of sohbility parameter with temperature for the amorPhous Polymer can be expressed by the relationship Sg
where 6J is the solubility parameter of the polymer at the temperature T , "C., 6; is the solubility parameter of the polymer a t O'C., ap is the cubical coefficient of expansion of the amorphous polymer and n = 1.5. :6 for the solvents considered here was calculated from heat of vaporization and density data available from the "International Critical Tables." Experimental
(2)
(1) %e "Sohbility of Non-Blectmlytu," Rildebrand and Scott, Ed. 8 , Rdnhold Publirhing Corp., New York, N.Y.. 1950.
Derivations from the Data.-The data from Table I when used with (1) and (2) above, and (3) and (4) below are sufficient to determine the heat of fusion per gram of crystalline polymer (for aelting the crystallites) hi, the solubility parameter of the polymer a t Oo, ,a; the interaction paramk and and the Of the idvent (a)
Unpublished data of F. P. Price, this L&boratory.
SOLUBILITY OF POLYTRIFLUOROCHLOROETHYLENE
Jan. 5, 1952
69
TABLE I1 THERMODYNAMIC DATAFOR POLYMER IN VARIOUSSOLVENTS For n
-
1.6: 8%
Substance
#-Xylene Toluene Benzene Methylchloroform Carbon tetrachloride
-
0.450 .476 .478 .497 ,499
8.05, hf = 18.1 cal./g., 85 VI
VD’.
0.030 .034 .051 -0 -0
136 124 118.3 114 109.5
and polymer present in the swollen gel at the crystal melting temperature. Note that solvent in these experiments is always present in excess of that imbibed at the crystal melting temperature T,. When the molecular weight is large, as it is in the case under consideration In (1
- v2) + vz + ~ ( o t =) ~0s
-
8.06 exp(-e.sx l O - W ( T m ,
61.2 59.1 56.6 54.7
- (vi)2(SpTm - s p T m ) 2 / T m ] I / ~ ( 1 / T-m l / T & )
(4) = heat of fusion/g. of crystalline polymer (to melt crystallites) p = density of the polymer = 2.0 for this case ol = (1 ol) = volume fraction of solvent in swollen polymer at the melting temperature T& = melting temperature of crystallites in the absence of any solvent GoTm = solubility parameter of the solvent a t the crystallite melting temperature 135, = solubility parameter of the polymer a t the crystallite melting temperature.
sp
bpT8
0.510 .511 .517 .500 .500
7.07 7.06 6.69 7.20 7.25
7.62 7.46 6.27 7.05 7.35
and n. A plot of hi vs. Sfl a t fixed n = 1.5 is shown in Fig. 1. I
e2lBENLfNE TOLUENE P-XYLENE
(3)
hr = [Rvl/VoTm
-
NTm
27.6 28.0 27.0 28.2 27.3
56.6
ARBON TETRACHLOR4DO METHYL CHLOROFORM
The quantity v2 is the volume fraction of polymer in the solvent-swollen polymer (at the crystal melting temperature for this work). Equation (4) below, is obtained by rearrangement of and substitution in (33) of Flory’s4 paper “Thermodynamics of Crystallization in High Polymers. IV.” The substitutions are: ZIZ, = VIVO, ht = h u X / M , V = MIpX where the symbols have the meaning given in Flory’s paper.
hf
OC.)
a:E
anvor, ‘
I
60
I
I
70
I
9.0
BO
Fig. 1.-Plot
I
100
SOLUBILITY PARAMETER
8;
of heat of fusion h os. assumed value of solubility parameter :6 for n = 1.50.
The average crossover point in Fig. 1 gives the correct value of Sg for the assumed n. Expressed simply as average f average deviation from the mean 6s = 8.05 f 0.08 and hr = 18.1 f 0.3 for n = 1.5. Similar plots to that of Fig. 1 for n = 1.0 give Sg = 7.95 0.09 and hr = 18.1 0.4. For n = 2.0, 89 = 8.23 0.09 and hr = 18.2 f 0.5. Changing n within these reasonable limits does not have tqo great an effect on S9 and ht. For further discussion it will be assumed that n has the usual value of 1.50 that holds for liquids. Table I1 gives data derived by using n = 1.5. Note from the table that ps increases regularly as VoT0 decreases. The product psVoro is nearly 1.8 expressed as a constant with a value of 57.6 average f average deviation from the mean. More nearly constant is the product pLfV0~8. Ex ressed as average average deviation, p s 2 . V O=~27.6 f 0.4 for the five polymer-solvent systems of Table 11. It is of interest to note that the molar volume of the repeating unit in the amorphous polymer is 58.2 cc. Thus ps approximately equals the ratio of repeating unit volume to solvent molecule volume at the solution temperature. A better relationship is po = 0.688 ( VoTs)’/2= 5.26/( V?s)’/: (5) where Vr.u. = molar volume of the repeating unit in the amorphous polymer. Assuming that this relationship holds for other non-polar solvents, the solution temperature of this polymer is included in the expression
*
*
*
A direct algebraic solution of (1) through (4) is not possible. Consequently, the following ap roach was used : pa) values of 8; of 6.0, 6.5, 7.0.. . l O . O were substituted in (2) (first with n = 1, then with n = 1.5 and finally with n = 2). The temperature used was the solution temperature of the polymer in the particular solvent being considered. The cubical coefficient of expansion apof the amorphous polymer has the value 7.4 X lo-*.% (b) At the solution temperature p = 0.5.l Equation (1) was then solved for pS for each assumed SO, and n. Note that ps is temperature independent. (c) When ps was known (in terms of assumed Sg and n) for each polymer solvent system, pTm a t the crystallite melting temperature T, was obtained by again applying (1) with data at T,. (d) A graph of pTm vs. v2 was constructed by using (3). From this graph vz was obtained corresponding to the above values of p T m . (e) When v 2 was known, v.1 was known since V I v2 = 1. All the data needed to solve (4) (at 298)]]’/$ Voeg8[1 assumed 69’s and n’s) were now available. Equa- 0.5 = 5.26/{ Vo’OlSs[l mo(T. ao(T1 - 298)][8oW8(exp- aoko(Tn - 298)) 8.05 (exp tion (4) was solved for hr for each assumed 6% 9.25 X lO“(Tn 273))l2/RT1
*
*
+
(D) IL8fWWCE 1, Ch. XX, Bq. (U). (4) P. J. mow,J . Chrm. P ~ Y I . IT, , a m (1040).
+
-
-
where
T,is in degrees Kelvin,
uo
-
+ -
+
-
(6)
coefficient of
70
H. TRACY HALL
cubical expansion of the solvent, VOzes= molar volume of the solvent at 298'K. and ko = (n 1)/2 where n is characteristic of the solvent and is approximately equal to 1.50. Equation (6) is obtained by utilizin the above expression for ps and substituting 6$ = 8.05 (exp - 9.25 X (T - 273)), 6oT = 6 0 (exp ~ ~ -~ C Y O ~ O ( T 298)) and VT = VOzg3 [l ao(T - 298)] in (1). Equation (6) has been solved for 602g3as a function of T, for 010 = 0.0010 and various values of VOz9*. The results are displayed in Fig. 2. If the polymer were completely amorphous the solution temperature as a function of 6029sat fixed CYO and VOzQs would follow both the solid and dashed parts of the curves. However, the solution temperature must equal or be greater than the melting temperature so the melting temperature curves must be obtained. The melting temperature as a function of 602gs for fixed CYO and Vozgs is shown as the shallow curve connecting the limbs of the corresponding solution curve above the dashed portions in Fig. 2.
+
+
11
,
,ae;o;l;
1701.
74
minimum T, = T , and hence vl = 1. The above procedure by giving the intersections of the T m and T, curves as well as the minimum is sufficient to define the T , curve where it falls within the limbs of the T, curve. To establish the T , curve outside this region is much more difficult. The author has carried out the computations only for the case of CYO = 0.0010 and VOzg8 = 110 as shown in Fig. 2. The following procedure was used: (1) A value of T, was picked. This fixes bozg8 and k8. For example, if (YO = 0.001, VoZg8= 110 and T, = 180, 60298= 8.87 (from Fig. 2), VoTs = 127 and ps = 0.466 from (5). (2) Equation (3) is solved for p T m and substituted in (7). (3) Equation (7) is equated to (8) and the resultant expression solved for T,. This gives a quadratic in T m as a function of VI when a0 and V0298are fixed. (4) Various values of v l are substituted in this
I
60
/
1-4'
20 72
76
80
84
88
92
96
D O
a?.
Fig. 2.-Solution temperature of polytrifluorochloroethylene as a function of the solubility parameter and molar volume of the solvent when the cubical coefficient of expansion for the solvent is 0.0010.
To obtain T , the following equations must be simultaneously satisfied along with (3), ( 5 ) and (6) 60'08
60% =
= { *[(pTm
(A{[RvdV?m
Fig. 3.-Solution temperature of polytrhorochloroethylene as a function of the solubility parameter and molar volume of the solvent when the cubical coefficient of expansion for the solvent is 0.0005.
300 k
1
1
1
/
/
/
/
- ~)RTm/VoTm]'/: + &"ml/b (7) - p h t ( U T m - l/Tn.)l Trnlw2I1/1+ hTm)/b
(8)
where b = (exp - aOko(T, - 298)) and and SSm have the temperature dependence shown previously. Equations (7) and (8) are obtained from equations (1) and (4). w When T. = T,, vl+l, pTm = 0.5 and the terms raised to the one-half power in (7) and (8) can be // equated. This gives a quadratic in T, that is 60 I I , 106 l I10I I 114, l o o ~ l readily solvable in terms of VO and (YO. This 82 06 98 94 98 IO2 determines the intersection of the T, and T m .'a: c w e s on the Fig.2 plot. Fig. 4.--SoIution temperature of polytrifluorochloroethyWhen T , is greater than the corresponding minimum in the T,CuNe in Fig. 2 the minimum lene as a functioh of the solubility parameter and molar wlvalue of T, is found by equating the term raised ame of the lolvent when the cubical coefficient of &od to the one-half power in (8) to zero. A i this for the solvent is 0.0018.
,
Jan. 5, 1952
SOLUBILITY OF POLYTRIFLUOROCHLOROETHYLENE
71
equation and the corresponding T, found. (5) with the polymer and cause solution to occur a t The above VI and corresponding T m are now sub- temperatures lower than 115'. Indeed, this restituted in equation (8) and (8) is solved for 6 0 ~ . search indicates that any attempt to find solvents (6) The 6oZes obtained is plotted vs. T m and the that will dissolve polytrifluorochloroethyleneof this correct Tm corresponding to the 60288 of step (1) molecular weight a t lower temperatures should be directed toward finding solvents with specific above obtained from the graph. Figure 3 gives the solubility curves for (YO = interactions for the polymer. 0.0005 and Fig. 4 for (YO = 0.0015. The Minimum Solution Temperature.-Figures 2,3 and 4 show that as Vogets smaller the minimum in T, rises and the minimum in T m decreases. Since crystals must melt before solution can take place the lowest solution temperature obtainable will occur when Tmmin= Tamin,This value is obtained by eliminating (S,T - bT)* from (1) and (4). The resultant equation is L I I l l 1 I I I I I I I I I I I I I I I T m = T. = h i / p ( h f p / R T m 5.26/( VP)*/* 0 . 5 / V P ) (9) 6.2 6.4 6.6 66 7.0 7.2 7.4 26 7.8 According to this equation T, decreases as VoTa decreases. However 6oT# is fixed and must have a Fig. 5.-Minimum solution temperature and correspondreal value when this e uation is satisfied. This ing molar volume as a function of the solubility parameter puts a lower limit on Vo 8 that is not obvious from of the solvent a t the solution temperature for the condition the above equation. If various VoT~are placed in that = TmmDJ". (9) and corresponding T,obtained these may then Supercooling.-It should be mentioned that be substituted in (7) and T, as a function of a O T a once the polymer is in solution it will supercool. obtained. This plot shows that Tsmin= 115' when 6 0 ~ 8 = 7.24 and b70T0 = 110 cc. Thus the Solvents that dissolve the polymer near the minilowest solution temperature attainable is 115' mum solution temperature of 115' will supercool and the solvent that dissolves the polymer a t this to approximately 90". Solvents with higher temperature will have a molar volume of 110 cc. solution temperatures supercool only a few deand a solubility parameter of 7.24 a t 115'. Carbon grees. Conclusion.-The information as to whether tetrachloride comes very close to satisfying these requirements. Figure 5 is a pIot of Tsd4 vs. 60~. or not a non-polar liquid will dissolve polytrifor the condition that Tsmin= Tmmin.The corres- fluorochloroethylene such as Kel-F #240 and the ponding VoTsis also given. The plot shows that temperature of solution can be obtained from Figs. T, changes slowly near 6oT~ = 7.24 so that there 2, 3 and 4 if the solubility parameter and molar is some latitude in obtaining a solvent with the volume of the solvent are known a t 25' and if the proper characteristics to give the lowest solution coefficient of expansion of the solvent is known. temperature. Note that 115' is the lowest solu- Solubility parameters are not generally available tion temperature that can be expected for non- but can be estimated by use of the Hildebrand polar solvents. It is entirely possible that highly rule.' polar solvents exist that would interact strongly SCHENECTADY, NEW YORK RECEIVED JUNE 6, 1951
+
+
s
