MECHANISM of . SOLVENT ACTION Arthur
K. Doolittle
CARBIDE AND C A R B O N C H E M I C A L S C O R P O R A T I O N , S O U T H C H A R L E S T O N , W. VA.
il
Two principal equilibria are simultaneously operative in the solution of resinous substances-solvation-desolvation and aggregation-disgregation. The rate of desolvation is substantially fixed a t constant temperature, but the rate of solvation is a function principally of solvent concentration. The threshold concentration required to initiate the solution process diminishes to a constant minimum value as w e ascend a homologous series of solvents. This minimum value, called “class threshold concentration”, is independent of the diluent and therefore serves as an absolute comparison of different classes of solvents for a given resinous substance. The aggregation-disgregation equilibrium of the solute macromolecules depends on the solvation-desolvation equilibrium, since disgregation results when the “active centers” are solvated. In fluid solu-
tions the extent of aggregation of the solute macromolecules increases linearly with decrease in solvent concentration] but as solvent concentration approaches the threshold value, desolvation permits aggregation a t multiple points of contact. The result is either gelation or precipitation. In film formation, the high viscosity, resulting from aggregation by means of gradual two- and three-dimensional growth as the resin desolvates during evaporation of the solvent, obstructs the tendency of the macromolecules to unite with one another at the maximum number of points o f contact. A rather extended structure results. Plasticization prevents complete desolvation of the resin, and therefore diminishes the opportunity of the macromolecules to unite with one another a t multiple points of contact. In this manner a less rigid structure is provided.
THE
whereas the molecular motions resulting from thermal agitation tend to break them apart. When solvent and solute molecules are joined in this manner, the process is called “solvation” (6), and when solute molecules are joined to one another, the process is called %ggregation”. The reverse processes are desolvation and disgregation ( 4 ) . Aggregation of solvent molecules likewise takes place, but in case of solutions of resinous substances in simple organic solvents, the number of active centers per resin macromolecule so far exceeds the number per solvent molecule that the abnormal viscosity behavior of such solutions is largely governed by the nature of the aggregation of the solute molecules alone. We are therefore justified in neglecting the aggregation of the solvent molecules in our interpretation of viscosity data as indicating changes in the nature of the aggregation of the resin macromolecules. The loose type of aggregation postulated in the above discussion is not to be confused with the more definite association observed in the case of certain highly polar substances such as acetic acid, methanol, and water. The two conceptions probably differ only in degree, but since our experiments indicate that the aggregation-disgregation behavior of macromolecular substances in ‘ solution is definitely a reversible process, we have chosen the term “aggregation” to differentiate this situation from the more familiar conception of “association”. From this preliminary discussion it should be evident that, in the process of the solution of macromolecular substances, two important equilibria are simultaneously operative : the solvationdesolvation equilibrium between solvent and solute molecules, and the aggregation-disgregation equilibrium of the solute macromolecules with one another. To appraise this mechanism more rigorously, consider a series of exhibits each containing a fixed weight of a resinous substance in a given volume of a solventdiluent mixture. Since we may prepare an unlimited number of solvent-diluent mixtures between the limits of pure diluent and pure solvent, we may say that, by making up a sufficient number of exhibits, we continuously vary the concentration of solvent from 0 to 100% of the solvent-diluent mixture. Obviously the solvent-lean exhibits will contain undissolved resin, whereas the solvent-rich exhibits will become homogeneous solutions when equilibrium has been established. Thus, as we pass continuously from the solvent-lean to the solvent-rich mixtures, a point will
purpose of this paper is to offer a mechanical picture of the action of solvents on macromolecular substances, such as nitrocellulose, Vinylite resins, and the like, in order to afford a better understanding of the solution process and especially of the reverse of this process (film formation) which is of great technical importance. The oversimplified picture to be discussed by no means represents the entire mechanism of solvent action; while we believe that the proposed mechanism is of more general application than is indicated by the examples chosen for the purpose of this paper, nevertheless one should proceed with caution in applying these ideas t o situations altogether different from those with which this study is concerned. S O L U T I O N PROCESS
The point of view taken in regard to the solution mechanism will be outlined, and the terminology clarified before the experimental work is described. Lewis, Squires, m d Broughton (4), in discussing emulsoidal solutions, state that highly solvated macromolecules do not agglomerate on contact because the solvent molecules held on their surfaces act as buffers. This point of view coincides with our conception of the function of bound solvent molecules in masking, so to speak, the cohesive forces (3) which originate on the solute macromolecules. We may say, therefore, that the process of the dissolution of a macromolecular substance by a solvent involves rendering inoperative many of the cohesive bonds uniting the solute macromolecules with one another. These cohesive forces are thought to originate at d e h i t e points or areas on the solute macromolecules (6) which, for the purpose of this discussion, will be called “active centers”. Thus, the active centers may be due to the influence of polar groups or clusters of polar groups, or they may represent the centers of force fields that are not localized about any particular groups on the molecule. According to this interpretation we should expect t o have active centers on both solvent and solute molecules; but owing to the much greater complexity of the resin macromolecules, we should expect that there would be many more such centers per molecule o n the solute molecules than on the solvent molecules. The cohesive forces originating a t the active centers tend to draw together such pairs as are within the effective range of these forces,
239
INDUSTRIAL AND E N G INEERING CHEMISTRY
240
TABLE I. CRITERIAOF PURITY OF COMPOUNDS USED Ester,
Alcohol,
%
%
Acid,
%
Acetates 0 . 006a
Methyl Ethyl n-Propyl n-Butyl n-Amyl n-Hexyl . n-Heptyl n-Octyl n-Dodecyl
0.014a 0.01oa
O.Olaa 0.020a 0.024O 0.007“ 0 . 005n 0.012a
..
Water, Freeaing Refractive % Point,OC. I n d e x , n g
0.05 0.07 0.06 0.08 0.08 0.05 0.01 0.03
..
Ketones Acetone Methyl ethyl Methyl n- ropy1 Methyl n-{utyl Methyl n-amyl Methyl n-hexyl Methyl n-nonyl Dimethyl Diethyl Di-n-butyl Di-n-hexyl Di-n-octyl Di-n-dodecyl
99.8 100.1 100.2 100.5 99.5 99.8 99.9 100.3 100.2 100.3 98.9
Nitromethane Nitroethane 1-Nitro ropane 1-NitroLane 1-Nitroheptane a As acetic. b As phthalic. C As succinic.
0.002a 0 . 003a
0.04 0.18 0.05 0.03
0,002a 0,0025 0.004a
Nil
100.0
Dimethyl Diethyl Di-n-butyl Di-n-hex yl Di-n-oct 1 Di-n-do& yl
0.10 0.21
.. ... ... ..
.. ..
.. I
.
.. ..
.. .... .. .. .
I
0.006a
0 . 004a
Phthalates
0.0126 0.0076 0.0076
0.0256 0.004b
0.016b
..
*.
Succinates
.. ..
..
.. ..
..
.. .*
..
.. .. ..
.. ..
0.003c 0.011c 0.011c
.. ..
0.016C
0.023C Nil
Nitroparaffins
.. *... ..
.. .. .. .. ..
.... ..* . ..
.... ..
*.
..
- 29.3 - 89.8 -104.5 - 81.6 - 31.5
1.3817 1.3918 1.4014 1.4105 1.4285
shortly be reached where the rate of solvation’ just exceeds the initial rate of desolvation. At this point a portion of the solvent molecules t h a t penetrate the resin aggregates will, on the average, become bound to a certain fraction of the active centers on the resin macromolecules. The active centers solvated in this manner are thus eliminated (statistically) as potential linkage points for uniting the resin macromolecules with one another, and a corresponding amount of disgregation results. As we proceed farther toward the solvent-rich end of the series, the rate of solvation increases. The solvation-desolvation equilibrium continues to shift, resulting in a corresponding shift in the aggregation-disgregation equilibrium toward more complete disgregation of the solute macromolecules. Although it appears from these considerations that the two equilibria are wholly dependent upon each other, it is possible, by a proper experimental approach, to study each of these equilibria individually.
stantially the same a t fked temperature, since the binding force (in so far as it depends on the dipole moment) is practically the same for all members of the series ( 2 ), and the energy required for desolvation is a function of temperature alone. The rate of solvation, however, depends primarily on the concentration of effective solvent molecules and hence may be varied a t will by changing the relative proportion of solvent to nonsolvent in each case. This operation is carried out in practice by titrating a solution of the resinous substance in the solvent with diluent (nonsolvent). Addition of diluent reverses the process described in the previous section; that is, reducing the concentration of solvent shifts the solvation-desolvation equilibrium toward more complete desolvation, which is accompanied by a corresponding shift in the aggregation-disgregation equilibrium toward greater aggregation of the solute macromolecules. The composition a t which the rate of solvation no longer equals the rate of desolvation is the end point in the dilution ratio titration; at this point there is a marked change in the physical appearance of the solution, which is often accompanied by precipitation of the resin. The procedure involved in determining dilution rat,ios is sufficiently familiar to solvent users so that the operation need not be more fully described here. I n the work reported in this paper, the A.S.T.M. procedure ( 1 ) was used throughout with the single exception that the end point was taken a t 0.5 gram of resin per 100 cc. of solvent-diluent mixture instead of the usual 8 grams per 100 cc., in order t o avoid gel end points as far as possible. The results obtained from a study of the folloffing systems will be considered in this discussion: half-second nitrocellulose (PXP) in mixtures of n-acetic esters with n-heptane and toluene as diluents; half-second nitrocellulose in mixtures of di-n-alkyl phthalates with n-heptane, toluene, and carbon tetrachloride; half-second nitrocellulose in mixtures of di-n-alkyl succinates with toluene; Vinylite resin VYNS in mixtures of 1-nitroparaffins with n-butanol or n-heptane. The degree of purity of the solvents used in this study may be appraised by reference to Table I. The concentration of solvent in moles per liter of the solventdiluent mixture a t the dilution ratio end point, called the “threshold concentration”, nDRJis plotted on semilog paper against the molecular weight of the solvent in each series. These results are shown on Figures 1 to 4, plotted from the data of Tables I1 to V. All of these curves are similar; that is, the threshold concentrations of the low-molecular-weight members of each series are high, but the values decrease to a constant minimum level as the molecular weight increases. The value of this horizontal asymptote is called the “class threshold concentration”, nB,and is a n indication of the relative effectiveness of each class as solvents for the
TABLE 11. NITROCELLULOSE THRESHOLD CONCENTRATIONS FOR n-ACETIC ESTERB AND WZ-KETONES AT 20’ c,‘ Toluene
m
The use of the expressions “ r a t e of solvation” and “rate of desolvation” assumes potential rates in the solvent-lean region. (Compare solution tension of metals.) As one proceeds toward the solvent-rich end, actual rates are encountered a s soon as the threshold concentration is passed. The rate of desolvation per unit area of exposed solute surface is thought t o be independent of solvent concentration, whereas the total rate of solvation depends on the mole concentration of solvent. The process of dissolution exposes additional solute surface as it proceeds, and equilibrium is reached in each instance when the total rate of desolvation (number of active centers desolvated per second) equals the total rate of solvation (number of active centers solvated per second). 1
PI0
l/v
n-Heptane
DR
“DR
DR
nDR
2.32 2.58 2.78 2.70 2.52 2.22 1.98 1.74 1.04
3.80 2.85 2.30 2.05 1.Y1 1.88 1.85 1.84 1.85
0.88 0.89 0.90 0.87 0.86 0.82 0.79 0.75 0.63
6.70 5.40 4.58 4.05 3.62 3.32 3.08 2.88 2.46
4.29 4.23 4.11 3.93 3.64 3.30 2.36
2.57 2.15 1.83 1.64 1.54 1.48 1.44
0.68 0.79 0.88 0.95
n-Acetates
SOLVATION-DESOLVATION EQUILIBRIUM
The use of homologous series of solvents is most illuminating in the study of the solvation-desolvation equilibrium because ascending or descending the series affords opportunity to change the mass of the solvent without altering the character of its polar group. Thus, for all members of a given class of solvents, the rate of desolvation under identical conditions should be sub-
Vol. 36, No. 3
Methyl Ethyl n-Propyl n-Butyl n-Amyl n-Hexyl n-Heptyl n-Octyl n-Dodecyl
74.1 88.1 102.1 116.2 130.2 144.2 158.2 172.3 228.4
0.9337 0.9005 0.8885 0.8815 0.8770 0,8737 0.8706 0.8682 0.8632
12.6 10.2 8.70 7.59 6.74 6.05 5.50 5.04 3.78
n-2-Ketones Acetone 58.1 0.7897 13.6 72.1 0.8053 11.2 Methyl ethyl Methyln-propyl 86.1 0.8065 9.36 Methyln-butyl 100.2 0.8110 8.10 Methyln-amyl 114.2 0.8154 7.15 Methyln-hexyl 128.2 0.8182 6.39 Methyln-nonyl 170.3 0.8265 4.85 Key: m = moleculaf weight; p = density; mole. D R = dilution ratio. n~= k threshold concn., or moles solvent/liter
= A DR + 1
v
-
8.10 6.26 4.98 4.15 1.00 3.57 1.00 3.19 0.92 2.52
molar volume, litem/
a t dilution ratio end point
INDUSTRIAL AND ENGINEERING CHEMISTRY
March, 1944
resinous substances used. Figure 2 (di-n-alkyl phthalates with n-heptane, toluene, or carbon tetrachloride) shows that the class threshold concentration is independent of the diluent used (provided only that the diluent is truly a nonsolvent for the resin); hence nx is probably determined primarily by the force binding the solvent to the solute. The following table summarizes these results, and some others not included in the examples chosen for the figures, for nitrocellulose at 20' C. : Class (Pure Compounds) " E (Approx.) n-Acetic esters 1.84= n-2-Ketones ~~. ..... 1.44a Di-n-alkyl succinates 1.10 Di-n-alkyl phthalates 0.77 Tri-malkyl phosphates 0.10 e Extrapolated from Figure 1. I t is assumed that, if higher-molecularweight n-acetic esters and nd-ketones had been included, the n-heptane and toluene curyes would have approached a common asymptote in each case just as in Figure 2.
241
TABLE 111. NITROCELLULOSE THRESHOLD CONCENTRATIONS FOR DI-n-ALKYL PHTHALATES AT 20 c. Phthalate Dimethyl Diethyl Di-n-butyl Di-n-hexyl Dl-n-octyl n..-ut-,*dodecyl
m 194.2 222 2 278'3 334:4 890.5
1/v 1.191 6.13 1 118 5 01 1:047 3'76 1.006 3101 0.977 2.50
Toluene n-Heptane nDR D R nDR 2.63 1.69 4.48 0.91 3 90 0.77 1'35 1'60 2:87 0.78 2:OO 1 : O O 2.22 0 . 7 8 2.13 0.80
DR
"DR
2.05 3.05 3 4 2:9 2.3
2.0 1.24 0.85 0.77 0.76
602.8
0.943
1.44
1.5
0.75
DR
P1.a
1.88
..
0.77
..
1.45 0.77
CClr
~
TABLEIV. NITROCELLULOSE THRESHOLD CONCENTRATIONS FOR DI-n-ALKYL SUCCINATES AT 20' C. Succinate Dimethyl Diethyl Di-n-butyl Di-n-hexyl Di-n-oct 1 Di-n-do&cyl
112
146.1 174.2 230.3 286.4 342.5 454.5
P2
1.120 1.042 0.976 0.944 0.926 0.905
l/v
7.66 5.98 4.24 3.30 2.70 1.99
Toluene nDR 1.72 2.82 2.30 1.81 2.52 1.20 1.96 1.11 1.45 1.10 0.80 1.10
DR
Ether-alcohols are not included in this tabulation because they are two-type solvents which cannot properly be compared with the others. A study of these TABLEV. VINYLITERESINTHRBSHOLD CONCENTRATIONS FOR curves contributes n-1-N ITROPARAFFINS IO materially to the n-Butanol n-Heptane e n-ACETIC ESTERS AT 20%. p i c t u r e of t h e Nitroparaffina m Pio I/v DR nDR DR nDR mechanism of the Nitromethane 61.0 1.1357 18.6 n.HEPTANE Nitroethane 7 5 . 1 1.0499 14.0 O'h5 8:kO '' solution process de1-Njiropropane 89.1 1.0008 11.2 0:65 6.80 0:67 6:iO 1-Nitrobutane 103.1 0.9731 0.4 0.65 5.70 0 . 6 5 5 . 7 0 velopedearlier. For 145.2 0.9287 6.4 0.49 4.30 0.48 4.33 1-Nitroheptaneb example, the class TOLUENE a Solvents obtained through courtesy Commercial Solvents Corporation threshold concenand further urihed in this laboratory b Preparezand purified in this laborkory. tration, n ~may , be considered to repre10 sent the minimum 8n-2 KETONFS AT 2OoC, Vinylite resins give such poor end points in the dilution number of effective 6ratio titration that the results are not trustworthy. With molecules of the 4 this particular grade of Vinylite, however, the nitroparafgiven class that fins give satisfactory end points, and the results in Figure 4 must be present are quite reproducible. Dilution ratio values were deter(per liter) to initiate mined at 20' C. using both n-butanol and n-heptane as diluthe solution process ents. The results are identical in the case of I-nitropropane, 1a t 0.5 g r a m nitrobutane, and I-nitroheptane, but since the lower nitroresin per 100 paraffins are not miscible with n-heptane a t 20" C., only the ncc. at 20' C. butanol value is shown for nitroethane. It is apparent from FigTo explain the high Figure 1. Nitrocelluose Threshold Conure 4 that nitromethane is not a solvent for Vinylite VYNS a t initial level of the centrations VI. Molecular Weight for 20" C. because pure nitromethane does not afford a sufficiently curves, perhaps n-Acetic Esters and n-2-Ketones at PO" C. high concentration of solvent molecules to initiate the solution only part of the process a t this temperature. A somewhat simjlar situation exists molecules of the in the case of the solubility of Vinylite VYNW in n-2-ketones. lower molecular weight members of the series are effective Methyl ethyl ketone and the immediately higher members disin solvating, whereas the proportion of effective molecules imsolve this resin readily, but acetone will dissolve Vinylite VYNW proves as we ascend the molecular weight scale. Thus, if we only after prolonged heating and agitation. tentatively accept this point of view as an explanation of the It is equally possible to cross the boundary representing the shape of these curves, it appears that the nature of the diluent maximum possible concentration of solvent by going the other influences the distribution of the smaller molecules into the effective and noneffective categories. This influence diminishes as I I I , we ascend the series, and disCARBOW TETRACHLORIDE appears altogether with molecules above a certain size in each class. The fact that the lower-molecular-weight members of homologous series may dissolve certain substances poorly or not a t all, whereas higher-molecular200 300 400 500 weight members are quite satisM M factory, is often observed in Figure 2. Nitrocellulose Threshold ConFigure 3. Nitrocellulose Threshold practical work. An excellent centrations VI. Molecular Weight for Concentrations vs. Molecular illustration is afforded in the Di-n-alkyl Phthalates in Toluene, n-HepWeight for the Di-n-alkyl case of the nitroparaffins as tane, and Carbon Tetrachloride at 90" C. Succinates in Toluene at 20" C. solvents for Vinylite resin 3. DI-n-butyl 5 Di-n-oc I 1. Dimethyl VYNS (Figure 4). As a rule, 4. Di-n-hexyl 6: Di-n-doabcyl 2. Diethyl "
I
'
INDUSTRIAL AND ENGINEERING CHEMISTRY
242
way-that is, ascending the molecular-weight-scale until the molecule becomes too large to be a solverit. Thus, in the case of Figure 4,it would appear that 1-nitroparaffins of molecular iveight in excess of about 260 would not dissolve Vinylite VYNS at, 20" C. because the mole concentration of pure compounds of this size would not be so great as the threshold concentration required t o initiate solvation. AGGREGATION-DISGREGATION EQUILIBRIUM
The experimental evidence so far considered applies only to the initial solvation-desolvation equilibrium; that is, we have determined the threshold concentraLion that will just initiate the solu%ionprocess. We do not know how many active centers must
20
'j 4
3
60
Fi ure 4.
V5NS-91
00
140 M
180
220
Threshold Concentrations of Vinylite VI. Molecular Weight for n-1-
Nitroparaffins in n-Butanol or n-Heptane at I , Nitromethane 2. Nitroethane (butanol end point only, n-heptane not miscible)
PO" C.
3. n.1 -Nitropropane 4. n-I-Nitrobutane 5. n-1-Nitroheptane
Vol. 36, No. 3
with an equal volume of toluene. As the curves recede, therefore, the solvent mixtures become leaner. The curves are essentially linear for a considerable portion, then rise abruptly to approach vertical asymptotes a t values of l/n = 1/1.84 and 1/1.44, respectively. These are the same values of n as the threshold concentrations, ~ D R for , octyl acetate-toluene and methyl n-nonyl ketone-toluene determined previously by the dilution ratio method. Thus, we arrive at the solvent noncentration that initiates the solution process by two independent methods. Considering the changes that take place as we pass from the solvent-rich to the solvent-lean portion of this curve, we may concliide that in the linear portion the aggregation of the solute molecules is essentially regular. That is, a small decrease in solvent concentration occasions a corresponding desolvation of active centers, which results in a small increase in aggregation by means of branching or two-dimensional growth. As we approach the vertical portion of the curve, however, the concentration of solvent is no longer sufficient to maintain most of the active centers solvated; opportunity is therefore afforded for aggregation a t multiple points of contact, resulting in cross linking or threedimensional gromth which rapidly increases the extent of the aggregation and finally leads to a stiff gel. While the mechanism just described leads to gel formation, we visualize that precipitation likewise results from the three-dimensional growth of the resin macromolecules. In this case, however, a closer packing arrangement must be provided. Such a situation is possible only with macromolecular substances when the transition from the essentially solvated to the essentially desolvated condition is abrupt. When the free mobility of the resin macromolecules is hindered by the gradual growth of a ramifying three-dimensional structure, it is not possible for the resin macromolecules to arrange themselves in the denser packing arrangement necessary for precipitation. Thus, in the first illustration cited (27.5 grams of nitrocellulose per liter of n-octyl acetate-toluene mixtures) a gel is formed as we approach an n value of 1.84. At considerably lower solids content the resin would precipitate a t n = 1.84 instead of gelling the solution, since the lower concentration would not be adequate to build up a gel structure, and the tendency of the solute molecules to unite with one another a t the maximum number of points of contact would not be so greatly obstructed. An example of the dependence of the equilibria involved i n the solution process on the mobility of the resin macromolecules may
be solvated to initiate solution, but there is ample evidence to indicate that only a fraction of them are involved a t this point. If, however, we increase the coiicentration of solvent, the initial solvation-desolvation equilibrium will be shifted. More active centers will become solvated until a new equilibrium is established, and the solute aggregates will become further disgregated. Experimental evidence bearing on the aggregation-disgregation equilibrium of the solute macromolecules may best be considered in the form of viscosity data involving a series of solutions, all made up t o the same solids concentration per liter of volatiles, in which the concentration of solvent in the volatile portion is varied. Extensive viscosity studies (not yet published) TABLE VI. VISCOSITY O F HALF-SECOXD KITKOCELLULOSE SOLUTIONS AT 20" c. lead us to believe that the viscosity of w.o:u. = 0 Wsolv. = 27.5 liquids is related to the extent of agSolvent m pzo nzo l/nto TI In 1/q q In 1 / q In qr gregation of the molecules, of which the Ethyl acetate 88.1 0.9005 10.20 0.0978 0.00453 5.40 0.0252 3.68 1.72 liquid is composed, in such a manner that n-Propyl acetate 102.1 0,8885 3.43 1.71 8.70 0.1149 0,00586 5.14 0.0324 n-Butyl acetate 3.09 1.83 116.2 0.8815 7.59 0,1319 0.00730 4.92 0.0458 the logarithm of the viscosity bears a n-Amyl acctate 2.72 1.90 130.2 0,8770 6.74 0.1485 0,00983 4.62 0,0659 2.48 1.97 %-Hexyl acetate practically linear relation to the average 144.2 0,8737 6.05 0.1651 0.01171 4.45 0.0837 2.17 2.05 n-Heptyl acctate 158.2 0,8706 5.50 0,1819 0,01472 4.22 0,1142 size of the molecular aggregates. From 1.82 2.19 n-Octyl acetate 172.3 0.8682 5.04 0.1984 0.01836 4 . 0 1 0.1629 this point of view, then, a plot of the n-Octyl acetatetoluene ,.. . , , 4 . 0 0.250 0.01345 4 . 3 1 0.1393 1.97 2.34 mixtures .logarithm of the relative viscosity of a 1.96 2.60 0,01050 4.56 0.1412 ... 3.0 0.333 1.76 2.92 ... 2.5 0.400 0,00931 4.68 0.1726 resin solution against the reciprocal of 1.15 3.58 . . . 0.456 0.00880 4.73 0.316 2.2 the concentration of solvent would be ... 0,500 0,00843 4.78 2.000 0.69 6.47 2.0 interpreted as the dependence of the Acetone 58.1 0.7887 13.60 0.0736 0.00313 5.76 0.01371 4.29 1.47 72.1 0.8083 11.16 0.0898 0.00400 5.52 0.01780 4.02 1.50 Methyl ethyl ketone extent of aggregation of the solute on 86.1 0.8065 9.36 0.1068 0,00496 5 . 3 1 0.02480 3.70 1.61 Methyl n-propyl ketone 3.10 1.73 114.2 0.8154 reciprocal solvent concentration. Such 7.14 0 . 1 4 0 1 0,007%3 4.83 0.0450 Methyl n-amyl ketone 2.76 1.83 6.38 0.1668 0.01016 4.59 0.0631 128.2 0.8182 Methyl n-hexyl ketone plots are provided in Figure 5 (Table 1.85 2.04 4.84 0.2063 0.02040 3.89 0.1591 170.3 0.8255 Methyl n-nonyl ketone VI), taken from typical studies in which 1.01504 4.20 0.1384 1.98 2.22 . .. .., 4.0 0.250 C Methyl n-nonyl ketone2.09 2.41 0,01114 4.50 0.1232 toluengB mixtures ... 3.0 0 . 333 -all solutions were made up to 27.5 grams 2.06 2.57 0,1275 I , 00977 4.63 ... 0.400 a 1.90 2.85 ... . .. .. 2.5 2.0 0 . 5 00 0.00869 4.75 0.1491 of half-second nitrocellulose per liter of 1.77 3 . 0 0 ... . 1.9 0.526 0l.00850 4.77 0.1702 volatiles. The concentration of solvent 0.588 cI. 00820 4 . 8 1 0.2300 1.47 3.34 ... ... ... 1.6 1.7 0.96 3.86 ... 0.625 01.00810 4.82 0.3815 i n the volatile portion was varied first by a Key: m = molecular weight. p = density. n = concentration of solvent, moles/liter of volatiles; ascending the homologous series of pure absolute viscosity, poises; I)* = relative Wsoiu. = concentration of solute,'grams/liter volatiles; 4 viscosity = (viscosity of solution/viscosity of solvent). solvents, then by replacing part of the pure solvent of highest molecular weight '
I
.
.
I . .
-
INDUSTRIAL AND ENGINEERING CHEMISTRY
March, 1944
be had in the relative ease of preparing concentrated as compared with dilute resin solutions, I n lacquer work it is always easier t o prepare a concentrated solution than a dilute one, provided proper equipment and adequate power are available to handle the heavy solutions. An important factor is the fact that the viscous resistance of the heavy solutions permits greater mechanical work to be done on the resin, which helps to break down the aggregates. Aside from the question of greater shear, however, the hindered
5
8
1
I
I
w
243
ber of points of contact, and thus form a hard and sometimes brittle substance; whereas any means that will provide a more gradual transition will favor the growth of a ramifying structure of maximum strength and toughness in the final film. An illustration of these two extremes may be observed in the case of Vinylite VYHH films cast from solution in hot toluene in one case and from the usual ketone-toluene mixtures at room temperature in the other. Toluene, as indicated above, becomes a
8
-
I I
w
6 -
O
0.1
0.2
0.3
0.4
0.5
0.2
0.3
0.4
0.5
0.6
0.7
V" Figure 5. Relative Viscosity at PO" C. of Solutions of Half-Second Nitrocellulose in Pure n-Acetic Esters and in n-Octyl Acetate-Toluene Mixtures (left), and in Pure n-%Ketones and in M e t h y l n-Nonyl Ketone-Toluene Mixtures In,, = log, of relative vlrcositv, I / n = reciprocal concentration of solvent, liton/rnole
mobility of the resin macromolecules in the heavy solution obstructs the tendency toward reorientation in the closer packing arrangement necessary for precipitation, and this factor alone likewise favors the solution process. A specific illustration of the latter situation is to be had in the case of solutions of Vinylite VYHH in hot toluene. A 0.5% solution of this resin may be made in toluene simpIy by heating the mixture to 125' C. On cooling this solution to 100" C , however, the resin precipitates. A 10% solution in hot toluene remains stable at 100" C., whereas a 20y0 solution may be readily prepared which is stable at about 80" C. No mechanical work is involved since the mixtures were simply heated and then cooled to the temperatures indicated. Therefore the essential difference between a fluid solution and a gel appears to be that, in the latter case, the three-dimensional structure now possible through aggregation a t multiple points of contact immobilizes the whole solution. At low solids content a gel cannot form, and the greater mobility of the resin macromolecules permits aggregation in a closer packing arrangement, resulting in precipitation of solid resin. The region intermediate between a fluid solution and a gel exhibits the characteristics of both types and affords the situation that is ordinarily referred to as thixotropism. That is, there is a tendency to gel which is counterbalanced by a tendency to be fluid such that the condition of rest favors the former, whereas agitation favors the latter. F I L M F O R M A T I O N AND P L A S T I C I Z A T I O N
The picture of the mechanism of the solution process presented above affords the basis for explaining many phenomena observed in the use of thermoplastic resins. For example, evaporation of a solvent from a resin solution parallels in many respects the course of the curves of Figure 5. At the start the solution is essentially fluid. As solvent evaporates, the concentration of solvent per mole of solute decreases and the process of going into solution is reversed. Thus, more and more active centers on the solute become desolvated, and aggregation by branching and cross linking increases as the solution passes through the gel phase to the final film. Any means that will serve to make the transition more abrupt from the substantially solvated to the substantially desolvated condition will favor the tendency of the resin molecules to unite with one another at the maximum num-
good solvent for this grade of resin only when it is hot (near i t s boiling point). As soon as the hot solution is poured out on a cold glass or metal surface, the temperature rapidly falls and t h e resin passes from the solvated to the desolvated condition so abruptly that little opportunity is afforded for further growth of a, ramifying structure in the process of film formation. The resulting film, though clear and transparent, is hard and brittle and will shatter with a blow. On the other hand, a film of the same resin, properly deposited by evaporation of a suitably balanced solventdiluent mixture, is tough and flexible. The presence of a plasticizer (nonvolatile solvent) prevents complete desolvation of the resin and, in proportion to its amount and solvent strength, reduces the tendency of the resin macromolecules to aggregate at multiple points of contact. Plasticization may be thought of, therefore, as a means of limiting the extent to which three-dimensional aggregation of the resin macromolecules can take place. EXPERIMENTAL S U M M A R Y
1. I n homologous series of solvents the mole concentration of solvent at the dilution ratio end point, called the "threshold concentration" n D R , approaches a minimum value as the molecular weight of the solvent increases. This minimum is independent of the diluent used, provided only that the diluent is truly a nonsolvent and is an indication of the relative effectiveness of each class of solvents for a given macromolecular substance. 2. Since lower-molecular-weight solvents of a given class a r e less effective per mole of solvent than higher-molecularweight members, it is possible in certain instances t o pass out of the solvent region on the low-molecular-weight side of the series. It is always possible, however, to pass out of the solvent region on the high-molecular-weight side because the mole concentration of solvent required to initiate the solution process approaches a fixed minimum value whereas the mole concentration of the pure solvent diminishes continuously as the molecular weight increases. 3. The logarithm of the relative viscosity of resin solutions of fixed solids content is approximately linear with the reci rocal concentration of solvent except in the neighborhood of t t e reciprocal threshold concentration, l / n ~ ~I n. this region the viscosity increases rapidly, the curve becoming asymptotic to l/noR. Thus, the threshold concentration may be experimentally determined b either dilution ratio or viscosity methods, 4. Tile experimental evidence is interpreted in the light of the author's point of view in regard to the mechanism of solvent action, and further conclusions are drawn in regard t o film formation and plasticization.
244
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
Vol. 36, No. 3
ACKNOWLEDGMENT
LITERATURE CITED
The author desires to acknowledge the assistance of the staff of the Coatings Division, Research and Development Departmerit, Carbide and Carbon Chemicals Corporation, all of whom contributed materially to the work reported in this paper. He is also indebted to R. W. Quarles of the Mellon Institute, and t o H. L. Bender, ~ l l v.~E.~&Teharg, , v, H. Turkington, and W. J. Jebens of the Bakelite Corporation, for critical review of the manuscript before presentation a t the Detroit meeting of the AMERICAN CHEMICAL SOCIETY. Prior to publication, this _paper _ was discussed with Charles Bogin of Commercial Solvents Corporation and p. J. Flory of Esse Laboratories. The author is particularly indebted to them for constructive suggestions.
(1) Am. SOC. for Testing Materials, Designation D-288-T (1939). P h ~ s99 *~ 251-7 (lg41)* (2) Colei R *H.9 (3) Gloor and Gilbert, IND. ENG.CHEM., 33, 597-601 (1941). (4) Lewis, Squires, and Broughton, “Industrial Chemistry of ColUP. 2, 170, New York. loidal and Amorphous hlaterials”, _. Macmillan Co., 1942. ( 5 ) Mark, H.9 “Physical Chemistry of High Polymeric Systems”, P. 254, New York, Interscience Publishers, 1940. (6) M ~ K. H,, ~
