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E. A . HAUSER AND D. 9. LE BEAU
2. The amount of sorption a t 50°C. is not appreciably different from that a t 30°C. 3. The microgel fraction in rubber is not or is hardly sorbed by Graphon; the sol fraction is sorbed completely. REFERENCES
(1) BAKER,W. 0.:Ind. Eng. Chem. 41, 511 (1949). (2) FREUNDLICH, H.: Colloid and CapilZary Chemistry, p. 224. Methuen and Co. Ltd., London (1926). (3) MEDALIA, A. I., AND KOLTHOFF, I. M.: Unpublished work from this laboratory. (4) SHEPARD,N. A , : “Carbon Black in the Rubber Industry,” in Colloid Chemistry, edited by Jerome Alexander, Vol. IV. The Chemical Catalog Company, Inc., New York (1932). (5) STAMBERGER, P . : Kautschuk 7, 182 (1931). (6) WIEGAND, W. B.: Can. Chem. Process Ind. 28, 151 (1944).
STRUCTURE OF LYOGELS. V1
A STUDYOF POLYMER FRACTIONS PREPARED FROM SMOKED SHEET,STANDARD GR-S, LOW-TEMPERATURE GR-S (X-435),AND
~ E O P R E N E10
E . A. HAUSER
Department of Chemical Engineerzng, Massachusetts Institute of Technology, Cambridge, Massachusetts, and Department of Chemistry and Chemical Engineering, Worcester Polytechnic Institute, Worcester, Massachusetts AND
D. S. LE BEAU Midwest Rubber Reclaiming Company, East St. Louis, Illinois
Received August 88, 194.9
Previous experiments had shown that the structure of the polymer film as observed in the ultramicroscope using incident light depended on the molecular weight of the polymer (molecular weight distribution) and on the physical structure of its chains. The effect of time and temperature on the structure of natural isoprene polymer films had been correlated with the folding and the internal structure of thenatural polymer chains (6, 7 ) .The effect of the molecular weight on the flow of isobutylene polymers had also been studied (8). Because flow can be considered a phenomenon closely related to molecular weight and its distribution as well as to the physical and chemical nature of the polymer, it was felt that it deserved more attention. Therefore smoked sheet, standard GR-S, low-temperature GR-S, and ?;eoprene were studied in greater detail by subjecting each polymer to fractionation 1 Presented a t the Twenty-third National Colloid Symposium, which was held under the auspices of the Division of Colloid Chemistry of the American Chemical Society a t Minneapolis, Minnesota, June 6-8, 1949.
257
STRL'CTURE O F LYOGELS. Y
by precipitation. The viscosity of each of the various fractions was determined. Polymer film preparations of each of the fractions for observation in the incident light microscope were made and studied at various temperatures and over a period of time. Some of the fractions were also subjected to infrared spectroscopic studies for the purpose of evaluating the differences in molecular configuration of the high polymer. 40
20
IO
eo 60
p 3 s g
4o
20
c,
a $-
l o 08 C
06
-
LOW T E M P
0 4
0 2
0
01
02
03
0 4
0 5
0 6
CONCENTRATION. G / L
FIQ.1. Plot of ?,,/concentration zleraus concentration for the various fract ions of each polymer. FRACTIOK.iTIOX
The polymer was first subjected t o either acetone or ethyl alcohol extraction t o remove nonrubber constituents. Acetone was used for this purpose for smoked sheet; ethyl alcohol was used for the synthetic high polymers. To eliminate any differences in the microscopic structure of these films which might possibly be ascribed t o the use of different solvents it was decided to use the same solvent for all microscopic preparations and fractionations. Benzene was used to dissolve the polymers and only that part of the polymer which was soluble in benzene
258
E. A. H4USER AND D. S. LE BEAU
was considered for further study. Fractionation was carried out a t 25OC., using acetone as precipitant. The precipitated sample was stored overnight to obtain better separation. The supernatant liquid was decanted and each fraction was redissolved in benzene and again precipitated. This procedure proved adequate for the high-molecular-weight fraction but became more and more difficult as the polymer size decreased. The precipitate became of colloidal size and settling of it by such means as freezing out or even centrifuging (up to 10,000 R.P.M.) was not successful. The first fractions of each of the polymers were tough and elastic upon drying. The toughness of the fractionated polymer decreased as its molecular size decreased until the fractions of very low molecular weight resembled highly viscous liquids. Each fraction was dried in vacuum t o weight constancy before viscosity measurements or microscopic studies were attempted. VISCOSITY
Each of the fractions was dissolved in benzene and the viscosity was determined by means of an Ostwald viscosimeter at 25OC. Figure 1 shows the relationship between T.,/concentration (in grams per liter X 10-l) and concentration
d
b
FIQ.2. Film spread from high-molecular-weight fractions of elastomers: (a) Hevea rubber; (b) cold butadiene-styrene copolymer.
(in grams per liter X 10-l). Except for Keoprene it can be seen that this relationship was found t o be quite constant. I n the case of Neoprene the greatest deviation from constancy was observed for the fraction of highest molecular weight. FILM-FORMIKG PROPERTIES
The special technique developed for the preparation of microscopic film specimens from high-polymer solutions has been described in full detail (3). To study the macroscopic film formation of each high-polymer fraction a drop of a 0.5 per cent benzene solution of each of the fractions was placed on a clean water surface preparatory t o microscoplc manipulations and was observed for its filmforming characteristics. All smoked sheet fractions gave smooth, round, soft, and pliable films (figure 2a). Microscopic observation as usual showed a greater amount of netting for the fractions of higher molecular weight than for those of lower molecular weight (figure 3a, b, c). The macroscopic appearance of films prepared from the various fractions of X-435 and standard GR-S varied considerably n ~ t hthe molecular weight of these fractions. The very low-molecular-weight, T'ISCOUS fractions of X-435 as well as of standard GR-S form very soft and weak films on mater. It was difficult
260
H l X D D.
s. LI:
HLAC
for 1 . 5 hr. arid films preparcd from these solotioris w e then subjected to micro. scopie observation no change c m he ohserved i n their sl.riiclnre. This is interesting to note, since it had been found previously that, contrary 10 tllis behavior, smoked &et, ivoiild hreak d o m in its film structure when subjected wen to lower tempwatiires (9).Also, if film preparations of llie high& fractions of cither standard GR-S or X-435 are hcated to 60°C. whilc under microscopic ohserva-
Pr6. 5. Stnndrrd GR-S: (a) fraction 1; (b) fiactioii 2; (e) irnction 3; (d) fraction 4
FIG.7. Senyienr:
(a) frneiion I ; (b) iiactioii 2; ( c ) imction 3;
(d) fraction 4
tion 00w does not uppear to occur and the rnicroxopic preparations do not change during an obsrrvatiorr period of tis long as 2 hr. at that, temperature. 'The same holds truc for ihe sisndard GR-S fraction whcn lieatcd to 75%. In cont,mL t,herdo i t w&s found that lire X-43.5 fraction will show How onder thcse conditions \vit,hin half an hour. If Xeoprmr 10 is fract,ionrrt,edand the fracl.ions are subjected to microscopic observations, the same general trend in the striicrrirc of the films prepared from thrso fract,ioirs can be becn. The fraction of lowest moleiwlar weight, s h o w tho formation of single throads only (figure 7d). More and more net formation eitn hc
263
STRUCTC-RE OF LYOOELS. V
extinction coeffcient,s of the 911 m-' band characteristic for butadiene with t h s t of thv io1 cm.? band characteristic fur styrene. Also the ratios of the ext i n d o n coefficients at the 1488 cm:.' band (aliphatic CH bonding) and that at the 1446 cm.-' band (phcnyl CH honding) were determined.
Fio. 10. Seopieno 10, fraction 1 st 70'C.: (a)immediately, (b) after 15 min , (01 a f t w
M niin.; (d) sftor 1 h i .
Fro. 11. Neoprene 10, fraction 3 at 40'C.: (a) immcdintoly; (b: after I3 min.;
(0)
after
25 min.
Fie. 12. Noaprenc 10. fraction 3 st i W C . : (a)niter 2 mi".; (b) Niter 1 min
Those ratios wcre alike within experinrental error for emh iraction arid for bot,h poiyrriars. Minor differences in the amount oi 1 , 2 addition between not fractionated standard GH-S and lowtempcrilturc GR-S polyrnem have been reported ( 5 ) . However, t,he major change so far reported in the structure of the law-temperaiore GR-S polyrnem has been an increase in ihe amount, of trans
262
E. .A. HAT;SEH AND
n. s.
LE REAU
enee in the styrene content or in the amvunt oi 1 , 2 addil.ion ai butadiene had occurred which could mell explain the difference iii rigidity and fragility OS films prepared from these iractions. The frsctions were dissolved in carbon tetrachloride and spread onto a rock salt p h t e (1). The amount of polymer deposited on thrse plates was in each case determined. After drying, the polymers were subjected to infrnrcd analysis.
Wo. 9. Nooprcne 10, frnetion 1 at, 40°C.:(a) immediately; (b) after I hr.; (d) after 2.5 hi.; ( e ) after 4 hr.; (0 after 3 hr.
(0)
after 2 hr.;
One fraction each of the standard GR-S and X-435 was studied over the whole region of 650 cm.? to 3600 crn;-'without detecting m y obvious differtmces betwoeii them. The relative arnot~ntsoi 1,2 and 1 , 4 addition were studied by determining the ratio oi the extinction coefficicnth (K = In $/", l' being the per cent, t.ransrnission) of the band represcnting terminal double bonds (912 em.->) t o thc hand represent.inginternal double bonds (066 cm-9. S a difference in this ratio could he found bet.ween any oi the high-moleeular-weight.iraelions of standard GR-S and X.-435. The relative mioimt OS styrcne present, in the highest-mol~:ular-\i~~t, fr~ction of each of the two polymers w&s determined hy comparing the ratio8 of the
264
5:. .A. HAUSER AND D . 8. LE BEAli
polymer (4). Gltr;Linic.roscopic ohservat,iona on natural isoprpne polymers lieve shown that balata, the tm,cs-isoprene polymer, mill give less smuot,h and somewhat sliffrr films than nnliirirl rubber, which corresponds to the cia stnietiirc. If the lrflns eoniigiirat,ion of the G R d polymer w r e responsible for the film hehavirir, w? should expect greater film rigidity from t,he lowtempem1,ure Glt-S polymers. This, howcver, was not apparent. Yet t,he sharp break iii the flow behavior of X-435 a? a definite temperature 5 s observed under the microseopc indicates changes in the m&ng point, which should occur as the irans roinponerit of the polymer increases. Therefore no explanation for the peerilisr lilm drliciency of the high-molee,ilar-\reigh( fractions of standard Glt-S chn he ofirred. I-Iowever, it should he rememBered that, IL very sinall amount of netting ( 2 ) could change the physical configurat,ion of the polymer chriins enough to account for the differences observed in film formation without neee~sarilychanging !he resuks obtained from infrared spectrogmphy. SI,.IIM.,RY .AND c"zcI.IrcIo'ys Smoked sheet, standard GK-S, lowtempriature GI. S.:J. Phye. 6 Colloid Chon,. 62. 2 i (lN8). (9) €l~i.sr:n,E. A , , ~m LE B E A ~ T ID., S.: J. Pliys. 6 Colloid Cheni. 68, 274 (19.8).
264
E . A . HdUSER A S D D. S. LE BEAU
polymer (4). Ultramicroscopic observations on natural isoprene polymers have shown that balata, the trans-isoprene polymer, will give less smooth and somewhat stiffer films than natural rubber, which corresponds to the cis structure. If the trans configuration of the GR-S polymer were responsible for the film behavior, we should expect greater film rigidity from the low-temperature GR-S polymers. This, however, mas not apparent. Yet the sharp break in the flow behavior of X-435 at a definite temperature as observed under the microscope indicates changes in the melting point which should occur as the trans component of the polymer increases. Therefore no explanation for the peculiar film deficiency of the high-molecular-weight fractions of standard GR-S can be offered.However, it should be remembered that a very small amount of netting (2) could change the physical configuration of the polymer chains enough to account for the differences observed in film formation without necessarily changing the results obtained from infrared spectrography. SUMMARY A N D COSCLUSIOBS
Smoked sheet, standard GR-S, low-temperature GR-S (X-435), and Seoprene 10 were subjected to fractionation. The ratios of gsp/concentration (in grams per liter X 10-l) were determined over a range of concentrations. The fractions were submitted to microscopic observation by incident light. On the basis of this and previous work, it can be concluded that the structure of the polymer fractions as observed in the microscope and previously observed in the electron microscope does not depend on the chemical composition but must be considered to be dependent on the physical structure of the polymer molecules, including their length and size distribution, as well as on their configuration. Net-like structures can result from an increase in the forces acting between the chains as well as from an increase in actual chemical bonds between them. I n polymers of very low molecular weight, be they synthetic or natural, surface forces predominate during film formation and film breaking; hence the microscopic structures of such polymers show the characteristics of viscous liquids. REFERENCES (1) (2) (3) (4) (5)
(6)
(7) (8) (9)
DINSMORE, H. L., ABD SMITH,D . C.: Anal. Chem. 20, 11 (1948). FLORY,P. J.: J. Am. Chem. SOC.69, 2893 (1947). HALL,E., et ai.:Ind. Eng. Chem. 36, 634 (1944). HIMPTON, ,R, R . : Paper presented before the Division of Rubber Chemistry of the American Chemical Society a t a meeting held in Boston, Massachusetts, hlay 23-25, 1949. HART,E . J., AND MEYER,A . W.: Paper presented before the Division of Rubber Chemistry of the American Chemical Society at a meeting held in Detroit, Michigan, November 8-10, 1948. HACSER,E. A , , AND LE BEAU,D . S.: India-Rubber J. 111,453 (1946). HAUSER,E. A,, A N D LE BEAU,D . S . : J. Phys. & Colloid Chem. 61, 2% (1947). HAUSER,E . A,, ASD LE BEAU,D. S.: J. Phys. & Colloid Chem. 62, 27 (1948). HAUSER, E . A , , ASD LE BEAU,D. S.: J. Phys. & Colloid Chem. 63, 274 (1949).
