1172
L. H. REYERSON AND LOWELL E. PETERSON
Vol. 60
THE SORPTION OF GASES BY SOLID POLYMERS.' I. THE SORPTION OF AMMONIA BY NYLON. 11. THE SORPTION OF HYDROGEN CHLORIDE BY NYLON BY L. H. REYERSON A N D LOWELL E. PETER SON^ Conlribdion from lhe School of Chemistry, University of Minnesota, Minneapolis, hlinn. Received Pebruarv 34, 1066
The sorption isotherms of NHa on undrawn nylon were determined at 30.4 and -31.3". The results are much like those for Ha0 on nylon and indicate that the sorption sites are the carbonyl groups which are not hydrogen bonded t o the imido groups in adjacent molecules. The sorption of HCl showed very different behavior. At -78.9" two molecules of HCI were taken up for every amide group in the nylon before the isotherm departed much from the zero pressure axis. At 0 and 20' one molecule of HC1 was sorbed per amide group a t almost zero pressure. The nylon fibers would shrink in length and swell in cross section during the early part of the HCI sorption process and then slightly elongate at higher relative pressures. The HC1 bound a t the amide groups wm difficult to remove except a t higher temperatures. After complete removal the nylon showed a greater crystallinity than before sorption. Thus it appears that hydrogen chloride penetrates the nylon, breaking all hydrogen bonds between adjacent chains and permitting a more perfect alignment on desorp tion.
The interaction of Nylon with acids and bases has been the subject of a number of investigations in recent years. However most of these studies have been done using aqueous solutions. Wall and his co-workers3 measured the sorption of small amounts of NaOH from solution and concluded that the terminal carbonyl groups bound the NaOH. Elod and Frolich4 studied the sorption of HCI and NaOH from more concentrated solutions and concluded that the sorption takes place without degradation of the Nylon with the imido groups acting as proton acceptors. They also believed that the carbonyl groups were responsible for the NaOH binding. Sorptions of water vapor, nitrogen and one or two acidic gases have been studied on Nylon. Rowen and Blaine5 compared the sorption of water vapor and nitrogen and found that the surface area calculated from the water isotherm by applying B.E.T. theory was much greater than that found from the nitrogen isotherm. They concluded that polar water molecules were able to penetrate the nylon where nitrogen cannot go. Several other investigators found that water vapor was sorbed strongly by nylon and that the isotherms were sigmoidal in character. The mode of attachment of water to nylon has usually been considered as hydrogen bonding between water and the carbonyl groups of the peptide linkage. PaulingBinterpreted the results of Bull' for water vapor sorption on nylon, by assuming that the B.E.T. monolayer point corresponds to the sorption of one water molecule on every carbonyl group which is not already saturated through hydrogen bonding with an imido group in an adjacent chain. The water sorbed a t the monolayer point would indicate that only about 6% of the carbonyl and imide groups fail to meet each other in ad(1) This material wa8 part of a thesis submitted to the Graduate School of the University of Minnesota in partial fullillment of the requirernenta for the Ph.D. degree. (2) General Mills Research Laboratories. Minneapolis. Minnesota. (3) F. Wall and T. Swoboda. THIS JOURNAL, 66, 650 (1952); F. Wall and P. Saxton, zbid.. 87, 370 (1953). ( 4 ) E. Elod and H. Frolich, Melliand Tertilber, SO, 239, 405 (1949). ( 5 ) J . W. Rowen and R. L. Blaine, I n d . Eno. Chem., 89, 1659 (1947). (0) L. Pauling, J .4m. Chern. Sne., 67, 655 (1945). (71 H. Ri111. ibid.. 66, 1499 (1044).
jacent chains. Dole* points out that a high degree of crystallinity is responsible for the fact that only about 10% of the peptide groups could adsorb water in his experiments. Recently Benson and Seehofs studied the sorption of BF3 on nylon. They found that on desorption some of the gas remained permanently bound and the amount depended on the pressure of the gas to which the polymer had been exposed. As a result of the above studies it seemed desirable t o carefully investigate the sorption of NHI and HCl on undrawn nylon fibers. Experimental Undrawn ' bright 66 nylon yarn of about 6 denier-perfilament kindly supplied by the du Pont Company, was used in this work. It contained 0.02% TiOz. Before use, the anti-static coating was removed by immersing the samples in successive portions of reagent grade CCI, for from three to six days. The sorbed solvent was removed i n v w o , and any water vapor was desorbed just before sorption isotherms were begun, by heating i n vacuo to 95" for three days. Synthetic ammonia 99.9% pure was distilled into a Aask surrounded by a Dry Iceacetone mixture. A small piece of sodium metal was admitted to the frozen ammonia, and the flask was warmed to just beyond iLq melting point. After the sodium had reacted with the trace water, the ammonia WM frozen again and any liberated hydrogen pum ed off. Then with due precaution the ammonia was distiied into receiving tubes holding 4 cc. of the liquid. These were then sealed off and stored in Dry Ice. Pure hydrogen chloride was repared by dropping reagent grade hydrochloric acid into a fPask containing reagent grade sulfuric acid. The flask was connected to a vacuum line with a trap cooled by liquid nitrogen and all stopcocks were lubricated with a fluorocarbon grease. The hydrochloric acid was allowed to flow slowly into the sulfuric acid. The dehydrated HCI gas distilled into the trap after passing through phosphorus pentoxide on glass wool. When suflicient HCI had collected in the trap, the dehydration part of the system was sealed off and the liquid HCI distilled from the trap into the sample bulbs, holding 8-10 g. each of HC1. These bulbs were stored in a Dry Iceacetone bath. When needed the tub- of both ammonia and HCI could be attached t o the sorption system through breakoffskys and by proper manipulation the liquids could be drawn for the actual sorption studies. The apparatus was a modification of the system reported by Reyewon and Honig'o having a very sensitive quartz spiral balance of the McBain type. The tuhe section of the system holding the quartz spiral waR thermostated by pumping water at a constant temperature through a surrounding jacket. The (8) M. Dole, Ann. A'. Y. Acad. Sci.. 61, 705 (1049). (9) S. Benson and J. Seehof, J . Am. Chem. Soc.. 7 6 , 3925 (1953). (IO) L. H. Ryerson and J. M. Honig. ibid.. 76, 3017 (1953).
Sept., 1956
SORPTION OF GASESBY SOLID POLYMERS
1173
moved a t a very slow rate. This retfention of one molecule of HCI per amide link seems t o check the isotherms a t 0 and 20" shown in Fig. 4. Here the uptake of HC1 follows the zero pressure axis until one molecule is adsorbed per amide link after which an isotherm was obtained which showed almost no hysteresis on desorption. I n one case a sample which still contained some HCI was removed from the gas line and examined. It had lost its tensile strength and felt sticky to the touch. The fibers broke on slight drawing. When observed under a microscope the untreated nylon was transparent and had a filament diameter of 26.1 * 0.5 y . The nylon filaments after sorbing HCI were still transparent but they had swelled to a diameter of 30.1 f 0.8 p which was a 14% increase. While the untreated fibers remained clear on contact with water, the treated one turned white and became opaque. This swelling of the nylon was . accompanied by a corresponding shrinkage in length. The shrinkage was sufficiently great to cause the crushing of one of the glass frames on which the nylon was wound. The remaining bound HC1 could be removed easily by heating the evacuated system to about 90" for three hours. The sample returned to its original weight and no longer was sticky to the touch. The tensile strength and other properties seemed to be comparable t o that of the original nylon. However, the nylon had permanently assumed whatever shape it had during the sorption-desorptioii process. For example, if it had been coiled on a glass rod it became permanently coiled. Since two moles of HCl was sorbed per mole of amide links a t -78.9" and only one mole a t 0 and 20" it was deemed desirable to measure an isobar between these temperature extremes. Figure 5 Results gives the results obtained a t 1 cm. pressure over The sorption isotherms of ammonia on nylon a t this temperature range. About a week's time was 30.4 and -31.3" are shown in Figs. 1 and 2 . The necessary to reach equilibrium a t each point. The amounts adsorbed are plotted against the relative isobar shows no sharp break in the capacity of the pressures of the gas. I n each case a hysteresis nylon t o sorb HCI under constant pressure a t the loop was observed on desorption but the loop several temperatures but a rather smooth transition, always closed a t zero pressiire indicating that no An interesting phenomenon occurred during the ammonia was permanently bound on the nylon. measurement of the rate of approach to equilibrium The sorpt,ion of HCl on nylon a t -78.9" is shown in the high pressure region a t the different temperai n Fig. 3 while the measurements a t 0 and 20" are tures. Here the nylon initially sorbed a larger given in Fig. 4. Adsorption a t -78.9" was very amount of HCl than was retained a t equilibrium. slow sometimes taking a week before equilibrium Figure G illustrates the unusual rate of approach was reached. Desorption a t this temperature pro- to equilibrium together with the corresponding ceeded more rapidly until the low pressure range changes in the closed system. Figure 7 had been reached. This isotherm shows unusual pressure illustrates the dependence of fiber length and characteristics in that the nylon takes u p two moles diameter on the uptake of HCl. of HCI per mole of amide linkage before the isoThese experiments suggested that basic structural therm departs from the zero pressure axis. Follow- changes might have occurred during the sorptioning this a rather normal sigmoidal isotherm with a small hysteresis loop is found. Desorption occurs desorption process. X-Ray diffraction studies more rapidly than adsorption until two molecules were carried out on nylon before sorption, during of HC1 remain on each amide link. The loss of sorption and after complete desorption. Four Xthis bound HC1 in a high vacuum is exceedingly ray diffraction photographs are shown in Fig. 8. slow. At the end of ten days of pumping 1.95 They were taken using copper Kcr radiation. moles of €IC1 still remained bound to each mole of Picture A was obtained by radiating untreated amide link If the sample was allowed t o warm nylon; €3 was from nylon having less than one to room temperature one mole of the gas came off mole of HC1 per mole of peptide group; the nylon rather rapidly but the remainder again was re- for C held less than half a mole of HCl per mole of peptide and D was from completely desorbed (11) R. Overarreet and W. F. Giauque, J . A m . Chem. SIJC..69, 254 nylon. The diffuse inner ring showing in pictures 1937). lower portion of the same tube surrounding the nylon sample was immersed in a low temperature thermostat (cryostat). The cryostats described in the previous work could be adjusted for the temperatures required, and they would then maintain this temperature for long periods of time. The 0.01 force constant of the spiral was found to be 59.83 =I= mg./cm. under a total load of 1.3 g. A microscope mounted on a heavy steel frame was used to measure the extensions of the spiral during sorption. The vapor pressure of the ammonia was maintained constant for a given adsorption point by keeping the U tube holding the liquid ammonia immersed in the cryostat a t a constant temperature. From this measured temperature, the vapor pressure of the liquid ammonia in the closed sorption system could be determined from the data of Overstreet and Giauque." In a given I un the temperature of the cryostat controlling the vapor pressure of the ammonia was adjusted to give the desired pressure, and the nylon was allowed to sorb the ammonia until equilibrium was reached. This required 24 hours or less for each point a t the higher temperature but at the lower temperature a much longer time was needed. The vapor pressures of ammonia varied from 0 to 986 mm. for the isotherm meyured a t 30.4' and from 0 to 747 mm. for the one a t -31.3 Even a t the highest ammonia pressure the nylon sorbed less than 6 mg.(g. Each isotherm was obtained by measuring adsorption points up to the maximum pressure followed by desorption points down to zero pressure. It was found impossible to measure the pressures of HC1 by the same method as for ammonia so that a direct manometric method was adopted. Successive increments of HCl gas were admitted to the system from a reservoir of liquid HCl which had a vapor presaure of about 1.5 atmospheres. Because large volumes of HCl were sorbed on nylon, provision had to be made for varying the volume of the system. This was done by varying the level of Hg in a large volume side tube. The actual pressures of HCl a t equilibrium were read to 5 0.01 cm. on a wide bore rnanometer. I n the low pressure range equilibrium WAS established very slowly, sometimes taking a week, while a t higher pressures from one to four d:tys were required. Desorption points required only about a day each until low pressures were reached. Several unusual happenings were observed during HCI sorptions and they will be discussed Inter.
J,. H. REYERSON AND LOWELL E. PETERSON
1174
Vol. 60
6-
x Fig. 1.-Sorption
(relative pressure) 0.05
I
b Fig. 3.-Sorption
25
0.10
isotherm ammonia on nylon at 30.4'.
50
75
isotherm hyd;ogen chloride on nylon at -78.9
.
oo-l---r o Sorption, 0' a Desorption, 0'
-
0 Sorption, 2 0 ° Q Deu~rption.20~
0.2
0.4
0.6
0.8
Fig. '2.Sorption isotherm ammonia on nylon a t -31.3'.
__
--I___
25
Fig. 4.-Sorption
5.0
Pressure (cm. Hg.)
75
isotherms HC1 on nylon a t 0 and 20°.
SORPTION OF GASESBY SOLIDPOLYMERS
Scpt., 10%
1175
Dependence of fiber length on HC1 pressure.
--Tl---l-
'1 20
30
HCI P r e s s u r e (cm.Hg) 40 50 60 70
8.01
Dependence of fiber length on HC1 uptake. o :/f;
o r i g i n o l length
(I HCI per ami Uptake (mg.6) 400
1 0
Dependence of fiber diameter on HC1 uptake. % of original diameter Sorption
HCI
.I20
leebar
. 115
on N y l o n a t I cm. Proslure
. 110 '
4
105 1
0
,0 l
3
0
Fig. 7
I
1 Temperature -20
20
-.
I .
(OC)
-40
HCI Uptake (mg./g) 1540
Pressure
Point
I
-e . (cm.Hg)
7 41.1
41.0 40.9 4o.e
Time(hours)
4
.
6
.
Point 8 68.9
Pressure --4
68.8 68.7 68.6 685
5
IO
15
20
C 84.2
'
840.
83.8 83.6 . C
20
D Fig. 8.
0
Uptake (mg./g) 400 ,
1176
L. H. REYERSON AND LOWELL E. PETERSON
A, B and C appeared only when a nickel filter was used to remove @-radiationand was attributed to its use. It is evident from these studies that the sorption-desorption of HC1 on nylon permits the reorientation of the long chain molecules, This reorientation permits the nylon system to permanently assume the shape it held during a sorptiondesorption run.
VOl. 60
cules were located a t definite intervals in the structure. I n the light of the low solubility of HCl in hydrocarbons such as hexane it did not seem probable that there was much of any HC1 sorption on the hydrocarbon chains a t low pressure, but this should not prevent diffusion between them. The rather exact stoichiometric ratios a t low pressure sorption seemed to be good evidence for the view Discussion that the HC1 was taken up a t the peptide groups. This investigation revealed two very different The observed swelling of the fibers with correspondtypes of sorption for the gases studied. Nylon ing shrinkage in length, followed by relaxation at showed no marked affinity for ammonia. At 30.4" higher partial pressures, also suggested localized less than 0.05 mmole was sorbed per gram in con- strong uptake a t the low pressures followed by more trast to 2 mmoles of water sorbed under the same general surface sorption. On the assumption that pressure a t 25" as observed by Bull. The second the HC1 breaks the hydrogen bonds a t the amide isotherm was measured at -31.3" in order to links, the improved crystallization, as observed in reach higher relative pressures. Here there was the X-ray studies, seemed reasonable. The long marked similarity to the results Bull obtained for chain molecules would be freer to assume lower water a t the same relative pressures. From energy levels so that upon removal of the HC1 by B.E.T. plots Bull reported a monolayer point cor- heating the molecules could hydrogen bond but in responding to an uptake of 1.07 mmoles of H 2 0 a more ordered fashion. Simple dipoledipole interper gram of nylon, while in this work the mono- actions between HCl and the polar groups of the layer point indicated that 0.95 mmole of NH, was polymer might also play a considerable role because sorbed per gram nylon. Thus a similar interpreta- of the low dielectric constant of the polymer. It is tion seemed logical, namely, that the ammonia interesting to consider that HC1 molecules might molecules were sorbed on those sites where the act as a lubricant for the straightening of the long carbonyl groups had not hydrogen bonded with polymer molecules. This could be happening in amide links. The case of complete removal of the the cases where the curl or shape was permanently sorbed ammonia indicated that the energies of put into the polymer. Equilibria between nylon and HCl were reached binding were weak. On the other hand nylon showed a remarkable slowly. This could be attributed to chemisorption having a considerable energy of activation capacity for taking up gaseous HC1. At -78.9' nylon sorbed about 5 moles of HCI per mole of but a slow rate of diffusion into the polymer might peptide groups a t a pressure of 919 mm. (relative well be a factor. Structural changes in the nylon pressure = 0.8G). More unusual yet was the fact during the sorption process mere thought to prothat the first two moles was taken up a t very low duce changes in uptake following each increase in pressure and then held tenaciously when desorp- the pressure of the gas as shown in Fig. 5. Since tion by evacuation was attempted. At 0 and 20" it mas proved that no foreign gas was given off a similar situation maintained except that only during the sorption process it was felt that strucone mole of HC1 was taken up a t low pressure and tural changes accounted for the slight decrease in tightly held during evacuation. Such behavior the amount sorbed as equilibrium was approached. This investigation has suggested that the sanie strongly suggested that all of the polymer was accessible to the HC1 and that in penetyating the technique be used to study other polymers having solid the gas broke the existing hydrogen bonds a t slmilar hydrogen bonding. One crystalline pruthe amide groups and probably formed a hydro- tein, insulin already has been investigated12 and chloride. The sharpening of the lines in the X-ray other studies are under way in this Laboratory. diffrnction as HC1 was sorbed up to one mole per (1'2) L. H. Iteyerson and I . E. Peterson. T H I SJ O U N N A I . , 69, 1117 mole of ninitle group also indicnt8edthat these mole- (1955).
