M a s s a c h u s e tts I n s t i t u t e o f T e c h n o lo g y , C a m b r id g e , M a s s .
D. S.
LEBEAU
M id w e s t R u b b e r R e c l a im in g C o m p a n y , E a s t S t . L o u is , M o .
A new technique is described which perm its th e m icro
scopic exam ination of lyogels, particularly of n a tu ra l and synthetic rubber, with ultra-illu m in atio n by incident light.
The morphology of th e original lyogel and th a t of its frac
tions, separated by solvent extraction, can be studied, as well as th e changes it undergoes when subjected to chem i
cal reactions or physical forces. R esults so far obtained by th is technique in th e study o f n a tu ra l and synthetic rubbers, soap, and o th er lyogels are discussed. Several photom icrographs are reproduced as examples of th e appli
cability and versatility of th is sim ple technique for study
ing th e morphology of lyogels.
T
H E introduction of dark-field illum ination in th e m icroscopic studies of colloidal system s usually called u ltra m icroscopy m u st still be considered one of th e m ost im p o rtan t advances tow ard a b e tte r u n d erstanding of m a tte r present in th e colloidal range of dim ensions. H ow ever, th e ultram icro
scope has its lim itations. T h e slit ultram icroscope, for exam ple, will n o t reveal th e presence of fibrillar particles or aggregates if th ey are sta tio n a ry a n d if th eir longitudinal axes lie parallel to th e beam of lig h t entering th e p rep aratio n th rough th e slit
Figure 1. P a th of Light th ro u g h Lens System o f Ultropak Microscope for U ltra-illum ination w ith
In cident Light
L e n s e s t h r o u g h w h i c h l i g h t i s f o c u s e d o n t o o b j e c t a r e m o v a b l e .
a t rig h t angles to th e optical axis of th e m icroscope. T h e m od
em dark-field condensers, on th e o th er h an d , illu m in ate th e p rep aratio n from all angles an d therefore will n o t p erm it one to ascertain w ith accuracy if an y preferential o rien ta tio n of m a t
te r is p resen t in th e p re p ara tio n unless th e condenser is fitted w ith an A zim uth sto p (7). E v en th en a n o th e r d raw b ack of th e custom ary dark-field condensers lim its th e ir use. T h ey are applicable only if th e p rep aratio n is p resen t in an extrem ely th in lay er an d if th e continuous p hase of th e p re p ara tio n is tra n s p a ren t. T hese lim itations are prim arily responsible for th e fact th a t th e use of th e ultram icroscope has been largely lim ited to th e stu d y of dispersions of lyophobic colloids (lyophobic sols o r em ulsions), or to those lyophilic system s w hich h a v e become optically heterogeneous due eith er to low te m p e ratu re (for exam ple, soap gels, 18, 19), to desolvation of th e colloid (14), or to th e use of higher concentrations th a n th o se w hich p e rm it com plete solvation (2, 3, 4, 12, 15, 16). All a tte m p ts to apply ultra-m icroscopy to solvated lyogels h av e, th erefore, m e t w ith little success.
R ecently th e electron m icroscope w ith its high resolving pow er and th e technique developed for th e stu d y of lyophilic colloids, such as soaps (1, 15) a n d n a tu ra l an d sy n th e tic ru b b e r (5), has offered a new approach to th e m orphology of lyogels.
O rdinary m icroscopes or ultram icroscopes using w h ite or even u ltrav io let lig h t, in accordance w ith th e fu n d am en tal law s of optics, can n ev er approach th e resolving pow er of th e electron m icroscope; nevertheless, it seem ed of in te re st to ascertain w hether th e m ethod u sed in prep arin g sam ples of lyogels su it
able for electron m icroscopy could also be successfully applied to ultram icroscopic studies. Success in such a n a tte m p t w ould m ake available a m eth o d based on eq u ip m en t w ithin m ore rea
sonable financial reach th a n a n electron m icroscope, n o t to m en
tio n th e com plexity of in stallatio n an d difficulties encountered in operating it. Such a technique w ould also m ak e it possible to follow visually, u n d er an ultram icroscope, th e changes gels undergo if exposed to th e influence of various chem ical or physical reac
tions, such as desolvation, aging, tension, or pressure. A ll th is is n o t possible w ith th e electron microscope. I n w orking w ith lyogels, one alfio m ay n o t overlook th e fa ct t h a t exposure to th e electron beam can cause certain changes in th e condition of som e specim ens, such as em b rittlem en t of ru b b er sam ples (5),
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
788 I N D U S T R I A L A N D E N G
Figure 9. Fibers from Palmolive Soap (X 3000)
Figure 10. Fiber from Methylsilicon, “ Bouncing P u tty ” (X 900)
Figure 11. Fibers from California B entonite (X 2100)
Figure 12. Hevea Rubber Latex Coagulated with Aeetie Acid and Then Stretched (X 1750)
T H E ULTROPAK
Of all known ultram icroscope construction, only one seemed to fulfill th e desired requirem ents. T his is th e U ltropak, originally designed by H eine (10, 11). A lthough th e im portance of th is in
stru m en t in th e scientific an d technical studies has frequently been pointed o u t (6, 7, 8), i t has n o t y e t found th e recognition it deserves. T he U ltro p ak differs from th e stan d ard ty p e of dark- field optics b y m aking use of ultra-illum ination w ith indirect light. T h is m eans circular illum ination of th e preparation from above; i t causes a dark-field effect, b u t th e light does n o t pass th rough th e m agnifying lens system before it is reflected b y th e specim en th rough it in to th e observer’s eye or onto th e photo
graphic film. T h e p a th th e lig h t tak es is shown schem atically in Figure 1.
T h e leDS system th rough w hich th e lig h t passes before it illum inates th e p rep aratio n can be lowered or raised independently of th e objective which has been b rought in to th e correct focal dis
tance from th e preparation. T hus, one can form a steep or
flat-angled cone of illum inating light. B y th is arrangem ent diffusion or scatterin g of lig h t caused b y reflections from uneven surfaces o r from opaque specim ens can be avoided; th e resulting clear ultra-illum ination can be ad ju sted to su it th e condition of th e investigated sam ple.
T h e preparations stu d ied were m ade b y dispersing or dissolv
ing th e colloid to be investigated in an ap p ro p riate solvent—
ether, petroleum ether, benzene, hexane, xylene, toluene, etc.
for ru b b er (5) and plastics, an d w ater for proteins, soap, clay, etc.— an d spreading th is solution on a nonm iscible liquid. T h en a sm all piece of v ery fine w ire gauze (18 m icrons p e r m esh) w hich has been inserted below th e solution is lifted up w ard th ro u g h it.
T h e coated wire gauze is th e n placed on a m icroscope slide, and th e p rep aratio n is ready for observation.
PH O TO M IC R O G R A PH S
Figures 2 to 9 show results obtained w ith n a tu ra l ru b b er as.
well as its sol and gel fraction, th e effect of vulcanization, a n d
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 786 th e m orphology of B u ty l rubber and soap. Considering th a t
these photom icrographs were tak en w ith dark-field illum ination and rem em bering th e difference in resolving power of th e in stru m ent, th e sim ilarity of th e illustrations to those obtained w ith th e electron m icroscope (1, 5 , 14) is evident.
In th e case of a p u tty lik e d ila tan t m aterial such as some of the recently developed silicon resins, a small piece of th e substance can be placed on th e wire gauze and be expanded u n til fine fibers form (Figure 10). In th e stu d y of clay gels th e thixotropic gel is m erely spread on a glass slide (9) and allowed to dry. One can th ereb y observe th e form ation of a coherent film consisting of a netw ork (Figure 11) caused by filam entous aggregation of the clay particles.
T his ty p e of ultra-illum ination by incident lig h t m akes pos
sible various m anipulations under th e microscope w ithout en
hancing th e clearness of th e observation by blurring th e picture with undesired reflections or th e like. F o r exam ple, it is possible to stu d y elastic deform ations b y form ing a blob of elastom er on the points of tw o m icrom anipulator needles and th en carefully moving them a p a rt; or a coagulum of rubber latex can be formed on th e ends of a m icropincette, and th e pincette is th en allowed to open, as Figure 12 shows.
T he results so far obtained w ith th is technique substantiate the assum ption th a t th e m orphology of lyogels of quite different ehemieal composition is very sim ilar (17, 18) ; this finding offers an explanation for th e analogy of some of th eir properties.
In conclusion, th e au th o rs would like to stress th a t th is con
tribution offers only a condensed discussion of th eir findings and
a limited number of illustrations. They hope to add more de
tailed results as soon as possible, but believe that the information presented here should not be withheld any longer.
L IT E R A T U R E C IT E D
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