Applied Polymer Science - ACS Publications - American Chemical

30 Electron Microscopy in Coatings and Plastics Research

Downloaded by SUFFOLK UNIV on January 22, 2018 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch030

L. H. PRINCEN Northern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, Peoria, IL 61604

Optical Microscopy Transmission Electron Microscopy Scanning Electron Microscopy Peripheral Techniques Trends and Future Projections

Almost all morphological and many phenomenological studies of coatings and plastics require some form of microscopy because their building blocks are so small in size. The dimensions of pigments, fibers, latex particles, film thicknesses, surface undulations, or emulsion droplets are often below the resolution of the naked eye or a hand lens, and need a good deal of magnification to become visible, either i n d i v i d u a l l y or collectively. During the early periods of modern and more scientific approach to coatings research and development, the only magnifying techniques available involved the various modes of o p t i c a l microscopy. Although these modes included transmission, reflection, polarization, dark f i e l d , UV, and ultramicroscopy, there were many drawbacks to their use. Resolution and depth-of-focus especially limited their usefulness. In 1932, the electron microscope was developed i n Europe. I t promised to become the microscopist's dream, with predicted magnifications i n the order of 200,000X, and resolutions i n the order of 10 Å. Still, another 12 years went by before the electron microscope was used occasionally in problems related to coatings and plastics. However, with the development of shadowing and replica techniques and with the introduction of commercial instruments, electron microscopy became commonplace i n polymer research. Although specimen p r e p a r a t i o n remained tedious and image interpretation was often d i f f i c u l t , transmission electron microscopy increased our understanding in the f i e l d of polymers a great deal. This chapter not subject to U.S. copyright. Published 1985, American Chemical Society

Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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In 1965, the scanning e l e c t r o n microscope became commercially a v a i l a b l e , and almost overnight i t became so popular that pictures of insects, moon dust, p r e h i s t o r i c p o l l e n , and other materials could be admired a l m o s t d a i l y i n newspapers, magazines, and s e m i s c i e n t i f i c j o u r n a l s . Also the impact on s c i e n t i f i c l i t e r a t u r e has been tremendous. The a v a i l a b i l i t y of r e s e a r c h funds to purchase s c i e n t i f i c equipment i n the 1960s, the s i m p l i c i t y of sample p r e p a r a t i o n , and the s p e c t a c u l a r , e a s i l y i n t e r p r e t e d images were p r o b a b l y the main s t i m u l i t h a t i n c r e a s e d the p o p u l a r i t y of the scanning e l e c t r o n microscope. Despite t h i s o v e r a l l popularity and s u i t a b i l i t y , i t s use i n c o a t i n g s and polymer r e s e a r c h was r a t h e r l i m i t e d for some time. In addition to these major forms of microscopy, there have been s e v e r a l developments of peripheral techniques that are important for the polymer s c i e n t i s t . They i n c l u d e X-ray spectroscopy, e l e c t r o n spectroscopy, and the e l e c t r o n microprobe. Their present and future impact has been evaluated. A l s o some predictions have been made on some expected or d e s i r e d f u r t h e r developments i n the f i e l d of microscopy. O p t i c a l Microscopy In the e a r l y 1900s, o p t i c a l microscopy was w e l l developed and was a l r e a d y being used r e g u l a r l y i n the r e s e a r c h l a b o r a t o r y and i n i n d u s t r i a l p r o d u c t i o n . V a r i o u s modes of imaging were a v a i l a b l e , such as transmission, r e f l e c t i o n , p o l a r i z i n g , and ultramicroscopy, and a l l of them were s u c c e s s f u l l y applied by the research s c i e n t i s t . U n f o r t u n a t e l y , methods f o r r e c o r d i n g the image on f i l m and subsequent p r o d u c t i o n of h i g h - q u a l i t y p r i n t s for p u b l i c a t i o n were not w e l l d e v e l o p e d at t h a t time. Often the r e s e a r c h e r might have been tempted to apologize for the q u a l i t y of the images appearing i n p r i n t by s a y i n g , "I wish you c o u l d have been t h e r e to see f o r yourself what I mean." In the m i d d l e 1920s were the years of improved a r t i f i c i a l l i g h t i n g (1-3), micromanipulation (4), microtoming and s e c t i o n i n g (5.-7), u l t r a m i c r o s c o p y (8-11), and pigment s t u d i e s (12-14). The f i r s t books on i n d u s t r i a l microscopy appeared at t h i s time (15-17) and indicated a greater demand for information i n t h i s area than one might suspect from the s m a l l number of research a r t i c l e s published on the s u b j e c t . In the e a r l y 1930s, o p t i c a l microscopy was used r e g u l a r l y and s u c c e s s f u l l y i n both rubber and f i b e r research (1821), but i n the c o a t i n g s f i e l d most work was r e s t r i c t e d to pigment powders. Of course, the o p t i c a l microscope was not so w e l l s u i t e d f o r p a i n t r e s e a r c h because of i n h e r e n t r e s o l u t i o n and d e p t h - o f - f i e l d limitations. For example, pigment p a r t i c l e s are often w e l l below lym i n s i z e ; t h e r e f o r e , the most i n t e r e s t i n g f e a t u r e s of t h e i r s u r f a c e s are below the r e s o l v i n g power a t t a i n a b l e . Optical s c i e n t i s t s have c o n t i n u a l l y pushed for greater r e s o l u t i o n through oil-immersion objectives (14), shorter wavelength UV r a d i a t i o n (22), or d e s i g n of an instrument based on a new concept, namely, the scanning o p t i c a l microscope (23). The l i m i t e d d e p t h - o f - f o c u s , however, i s not a f e a t u r e t h a t can be improved e a s i l y , and u n t i l today, has remained a s e v e r e handicap i n o p t i c a l microscopy.


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Another r e s t r i c t i o n to microscopy of c o a t i n g s i s r e l a t e d to t h e i r o p t i c a l nature. Pigmented f i l m s are opaque to l i g h t , and c l e a r f i l m s are often too transparent to r e v e a l any d e t a i l s . In the mid-1930s when both photographic and p r i n t i n g techniques f o r photomicrographs were g r e a t l y improved, U.S. r e s e a r c h was not d i r e c t e d toward microscopy, but r a t h e r to development of heavy p r o d u c t i o n equipment and manufacturing f a c i l i t i e s . At t h i s same t i m e , a c o m p l e t e l y new instrument—the t r a n s m i s s i o n e l e c t r o n microscope—was i n t r o d u c e d i n Europe and went almost unnoticed i n the United States. I t took many years before t h i s development gap was overcome.


Transmission Electron Microscopy Although the discovery i n 1928 of the d e f l e c t i o n of an e l e c t r o n beam i n a magnetic f i e l d i s seen by some as the birthdate of the e l e c t r o n microscope, the f i r s t working transmission e l e c t r o n microscopes were d e s c r i b e d by German i n v e s t i g a t o r s i n 1932 ( 2 4 , 2 5 ) . This development was soon followed by other manuscripts i n Russian (26), E n g l i s h (27), French (28), and I t a l i a n (29). However, r a p i d developments and improvements were coming almost e n t i r e l y from Germany. For example, by the time t h a t the f i r s t U.S. e l e c t r o n microscope, w i t h a m a g n i f i c a t i o n of 40X, was d e s c r i b e d i n the l i t e r a t u r e (30), German s c i e n t i s t s were already thinking i n terms of 200,000X (31), and the new technique was being a p p l i e d to a c t u a l problems i n c o l l o i d r e s e a r c h (32), b i o l o g y (33), and a e r o s o l and pigment p a r t i c l e i n v e s t i g a t i o n s (34). D e s p i t e these r a p i d d e v e l o p m e n t s , t r a n s m i s s i o n e l e c t r o n microscopy could not p o s s i b l y contribute s i g n i f i c a n t l y tro coatings and polymer research u n t i l commercial machines became a v a i l a b l e . The f i r s t Siemens microscope was marketed i n 1938 (35), followed the next year by a more advanced u n i t . In t h i s c o u n t r y , RCA marketed i t s f i r s t u n i t i n 1941 (36). Even w i t h commercial equipment a v a i l a b l e , many other problems had to be overcome. The main d i f f i c u l t y was t h a t of sample p r e p a r a t i o n . S e c t i o n s had to be extremely t h i n to be penetrated s u f f i c i e n t l y by the e l e c t r o n beam. Often c o n t r a s t was not s u f f i c i e n t to form a s u i t a b l e image. In 1939, the shadowing technique was d e v e l o p e d to enhance c o n t r a s t (37). In 1941, the r e p l i c a technique was i n t r o d u c e d to s i m u l a t e surfaces of materials normally opaque to electrons (38). Radiation damage t o the s p e c i m e n became r e c o g n i z e d (39), and s p e c i a l techniques for viewing pigment powders were devised (40). During the 1940s, t r a n s m i s s i o n e l e c t r o n microscopes became g e n e r a l l y a v a i l a b l e , and t h e i r h a n d l i n g and specimen p r e p a r a t i o n techniques were g r e a t l y improved. S c i e n t i f i c research i n general increased r a p i d l y , and synthetic polymers appeared at a record pace for f i b e r s , p l a s t i c s , and coatings. These changes expanded the use of both o p t i c a l and e l e c t r o n microscopy as evidenced by the large number of p u b l i c a t i o n s . No major changes i n microscopy technology took p l a c e d u r i n g the 1950s and the f i r s t h a l f of the 1960s. The major e f f o r t d u r i n g these years was toward i n c r e a s e d r e s o l u t i o n . More powerful machines were b u i l t , with a c c e l e r a t i o n p o t e n t i a l s as h i g h as 1,000,000 k V . During these 25 y e a r s , many s c i e n t i s t s were working on ion microscopy, by which magnifications of 1,000,000X can


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be a c h i e v e d and i n d i v i d u a l atoms of heavy m e t a l s can be seen. U n f o r t u n a t e l y , the l i m i t e d u s e f u l n e s s of t h i s form of microscopy excludes the study of polymers and coatings. During the e a r l y years of e l e c t r o n microscopy, two s c i e n t i s t s stand out because of t h e i r unorthodox approach to problem s o l v i n g and instrument d e s i g n . T h e i r names are Max K n o l l and Manfred von Ardenne. Only one year after he developed one of the f i r s t e l e c t r o n microscopes (25), K n o l l was able to form images from the secondary electrons created by e l e c t r o n i r r a d i a t i o n of metals and i n s u l a t o r s (41). K n o l l ' s main i n t e r e s t was i n e l e c t r o n e m i s s i o n and cathode ray tubes during these years. In contrast, von Ardenne was mainly i n t e r e s t e d i n e l e c t r o n microscopy at t h a t time. In 1938, he c a l c u l a t e d that the l i m i t of r e s o l u t i o n i n the e l e c t r o n microscope i s 10" mm at 50 kV, and that t h i s l i m i t may not be reached because of l e s s than i d e a l conditions (42). That same year he constructed what he c a l l e d an "electron scanning microscope" and obtained images from a crude t e l e v i s i o n tube (43, 44). K n o l l d e s c r i b e d a s i m i l a r instrument soon afterwards (45). Von Ardenne r e c o g n i z e d image imperfections i n conventional transmission e l e c t r o n microscopy due to e l e c t r o n s c a t t e r i n g i n the o b j e c t (43). He p r o p o s e d a combination e l e c t r o n - X - r a y microscope to e l i m i n a t e t h i s problem (46). He was e x t r e m e l y a c t i v e i n a p p l y i n g h i s m i c r o s c o p i c techniques to polymer and c o l l o i d research (47, 48). In t h e U n i t e d S t a t e s , t h e f i r s t t r a n s m i s s i o n e l e c t r o n micrographs on coatings and r e l a t e d materials appeared i n 1944 (40, 49). Although s e v e r a l researchers have contributed to coatings and polymer e l e c t r o n microscopy s i n c e then, E. G. Bobalek and h i s r e s e a r c h s t a f f s h o u l d be m e n t i o n e d e s p e c i a l l y . They were p a r t i c u l a r l y a c t i v e i n t h i s f i e l d during the mid-1950s (50-54).


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Scanning Electron Microscopy Although the concept of the scanning microscope was introduced e a r l y (23, 43), i t was not u n t i l the e a r l y 1960s t h a t s c i e n t i s t s s t a r t e d looking s e r i o u s l y i n t o t h i s mode of imaging. Presumably, the main reason f o r t h i s d e l a y was caused by the s t r o n g emphasis p l a c e d on increased r e s o l u t i o n that appeared to be the main goal i n instrument development during the f i r s t 25 years. However, i n the e a r l y 1960s a research group at Cambridge U n i v e r s i t y , England, started b u i l d i n g on the i n i t i a l studies of K n o l l and von Ardenne. Coupled with the improved technology of e l e c t r o n optics and e l e c t r o n i c s , t h i s work c u l m i n a t e d i n 1965 i n the f i r s t c o m m e r c i a l l y a v a i l a b l e scanning e l e c t r o n microscope, the Cambridge Stereoscan. Whereas b a s i c a l l y image formation i n the transmission e l e c t r o n microscope i s s i m i l a r to that i n o p t i c a l microscopy, complete with condensor, o b j e c t i v e , f i e l d , and p r o j e c t o r l e n s e s , i n scanning e l e c t r o n microscopy the image i s formed i n an e n t i r e l y d i f f e r e n t fashion (55). Images can be produced from the r e f l e c t e d , secondary, conducted, or transmitted e l e c t r o n s , as w e l l as from the X-rays and o p t i c a l r a d i a t i o n t h a t are produced by the t a r g e t area upon r a d i a t i o n by t h e p r i m a r y e l e c t r o n beam. Images a r e o f t e n s p e c t a c u l a r , e s p e c i a l l y those made i n the secondary mode. They often appear as i f the o b j e c t was i n s p e c t e d at c l o s e range under normal l i g h t i n g conditions. Consequently, such images normally are e a s i l y interpreted. Sample preparation i s simple: a s m a l l piece of



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the m a t e r i a l to be s t u d i e d i s mounted on a p l a t f o r m , coated w i t h a t h i n layer of metal, and placed i n the microscope for examination. Because only the surface l a y e r can be observed, no r e p l i c a s have to be made. I n t e r n a l s t r u c t u r e s can be observed by c u t t i n g or fracturing the sample. Magnifications from 10X to 200,00QX can now be reached r o u t i n e l y , r e s o l u t i o n i s i n the order of 30 A, and the depth-of-focus can be more than 500 times that attained i n o p t i c a l microscopy. The f i r s t nine years of a c t i v e scanning e l e c t r o n microscopy have seen tremendous progress i n instrument d e s i g n . Solid state e l e c t r o n i c s has helped reduce the equipment from console to tabletop models; improved vacuum techniques and e l e c t r o n gun d e s i g n have r e s u l t e d i n b e t t e r r e s o l u t i o n and a d d i t i o n a l modes of o p e r a t i o n . Scanning e l e c t r o n microscopy became popular almost overnight, and many instruments were purchased and used worldwide to examine almost any kind of l i v i n g or n o n l i v i n g m a t e r i a l . As many as 10 different brands of scanning e l e c t r o n microscopes became a v a i l a b l e i n a few years. A l t h o u g h the technique was a p p l i e d to c o a t i n g s and polymers e a r l y (56, 57), i t s use i n these f i e l d s has not been so prevalent as would be expected. At p r e s e n t , the o n l y drawback to scanning e l e c t r o n microscopy appears to be the c o s t of equipment and operation. The b i b l i o g r a p h y on scanning e l e c t r o n microscopy, compiled by 0. C. W e l l s (58), includes l e s s than 20 manuscripts that can be l i n k e d d i r e c t l y to the study of polymers, c o a t i n g s , and p l a s t i c s . To promote such use, a symposium on t h i s s u b j e c t was expressly organized and presented before the j o i n t conference of the Chemical I n s t i t u t e of Canada and the American Chemical Society (ACS) at Toronto, Canada, i n May 1970, w i t h the ACS D i v i s i o n of Organic Coatings and P l a s t i c s Chemistry as prime sponsor (59). A follow-up symposium on the same topic was h e l d at D a l l a s , Texas, i n A p r i l 1973 (60). These symposium papers were representative of the work done i n t h i s f i e l d . Some a d d i t i o n a l recent techniques were described at the ACS meeting i n 1981 at A t l a n t a (61). A good example of the c a p a b i l i t y and u s e f u l n e s s of a l l t h r e e modes of microscopy i s i l l u s t r a t e d i n Figure 1 with micrographs of r u t i l e titanium dioxide pigment. Figure l a i s an o p t i c a l micrograph at nearly maximum magnification. Often such an image i s s u f f i c i e n t , f o r example, to determine degree of d i s p e r s i o n , and no t i m e consuming procedures and e x p e n s i v e equipment have to be used. I f p a r t i c l e shape and s i z e become i m p o r t a n t , such as i n p r o d u c t i o n processes, the scanning e l e c t r o n microscope i s often the most s u i t a b l e instrument, as exemplified i n Figure l b . I f more surface d e t a i l i s required, the t r a n s m i s s i o n e l e c t r o n microscope w i t h i t s increased r e s o l u t i o n i s the preferred instrument. Figure l c shows such d e t a i l at an i d e n t i c a l m a g n i f i c a t i o n used i n F i g u r e l b . The example i s j u s t one of many t h a t c o u l d be taken from the c o a t i n g s and p l a s t i c s research and production f i e l d s . Peripheral Techniques Although the an e l e c t r o n 1950s was an for a n a l y s i s

production of X-rays upon bombardment of a target with beam was known i n the 1930s (46), not u n t i l the e a r l y instrument s p e c i f i c a l l y designed to use t h i s p r i n c i p l e of materials at the microscopic l e v e l . The e l e c t r o n


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Figure 1.

O p t i c a l (a), scanning (b), and transmission (c) e l e c t r o n micrographs of r u t i l e titanium dioxide pigment show the differences i n information that can be obtained with the three modes of microscopy.



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microprobe has an e l e c t r o n beam t h a t can be moved f o r mapping an area or kept stationary for spot a n a l y s i s . Upon e x c i t a t i o n with an e l e c t r o n beam, a l l c h e m i c a l elements emit c h a r a c t e r i s t i c X-rays, which then can be a n a l y z e d a c c o r d i n g to t h e i r w a v e l e n g t h or t h e i r energy. When the e l e c t r o n beam i s scanned over the target surface, the X-ray s i g n a l may be converted i n t o an image s i m i l a r to that made i n the scanning e l e c t r o n microscope. I f the image i s produced from the X - r a y s of a g i v e n w a v e l e n g t h or energy, i t w i l l show the l o c a t i o n s of a given chemical element i n the target area examined. Images can be made at different wavelengths or energies to obtain a complete m i c r o a n a l y s i s of the t a r g e t . A l s o , the beam may be kept s t a t i o n a r y at any g i v e n spot i n the t a r g e t a r e a , and an X - r a y spectrum can be taken i n a s h o r t time f o r a complete c h e m i c a l a n a l y s i s of t h a t spot. A good r e v i e w of the use of the microprobe i n coatings research has been presented by Labana (62). Upon development of the scanning e l e c t r o n microscope, i t was soon combined with features of the microprobe through attachment of an X - r a y d e t e c t o r to the new i n s t r u m e n t . U n f o r t u n a t e l y , the beam i n t e n s i t y and spot s i z e on the t a r g e t are both much l e s s i n the scanning e l e c t r o n microscope than i n the microprobe; consequently, the generation of X-rays i n the scanning e l e c t r o n microscope i s l e s s intense, and both elemental and s p a c i a l r e s o l u t i o n are reduced. New developments i n d e t e c t o r t e c h n o l o g y have improved the e l e m e n t a l r e s o l u t i o n , and i t has become p o s s i b l e to detect low elements, such as f l u o r i n e or oxygen; even l i t h i u m can be detected without problems i n the microprobe. In c o a t i n g s and polymer r e s e a r c h , the most important elements are often carbon, n i t r o g e n , and oxygen. A l s o , the p h y s i c a l f e a t u r e s of c o a t i n g s and p l a s t i c s are often i n the submicron s i z e range, and again, r e s o l u t i o n i s too poor to produce good r e s u l t s . D e s p i t e these shortcomings of present-day X - r a y attachments to scanning e l e c t r o n microscopes, t h i s instrument combination has already produced e x c e l l e n t r e s u l t s i n selected areas of coatings research (63). A l s o the scanning transmission e l e c t r o n microscope (STEM) can be regarded as a p e r i p h e r a l instrument (64) t h a t u n t i l now has had l i t t l e impact on polymer r e s e a r c h . T h i s form of microscopy may become more important w i t h g r e a t e r a v a i l a b i l i t y . R e s o l u t i o n of better than 15 A i s already a t t a i n a b l e i n the STEM mode. Trends and Future Projections Although o p t i c a l microscopes become more sophisticated every year, b a s i c a l l y no improvements can be expected i n magnifying power, r e s o l u t i o n , or d e p t h - o f - f o c u s i n the conventional o p t i c a l system. Perhaps, o p t i c a l microscopy could be r e v o l u t i o n i z e d by combining the scanning p r i n c i p l e w i t h l a s e r o p t i c s . Even where e l e c t r o n microscopes are a v a i l a b l e , low-power b i n o c u l a r or high-power standard microscopes s t i l l are e x c e l l e n t supporting instruments i n the c o a t i n g s , f i b e r , or polymer l a b o r a t o r y . A q u i c k l o o k at l o w power magnification or under s p e c i a l l i g h t i n g may r e v e a l features that might otherwise be overlooked. Developments i n transmission e l e c t r o n microscopy w i l l not affect c o a t i n g s and p l a s t i c s r e s e a r c h much i n the near f u t u r e . New i n s t r u m e n t s may have scanning and X - r a y f e a t u r e s i n c l u d e d as options. New guns, lower vacuum, and s o l i d state e l e c t r o n i c s have



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i m p r o v e d s t a b i l i t y and r e s o l u t i o n w i t h o u t going to h i g h e r a c c e l e r a t i n g v o l t a g e s , but a g a i n , d r a s t i c changes cannot be expected. However, these same developments w i l l probably further r e v o l u t i o n i z e scanning e l e c t r o n microscopy. Although t h i s technique became commercially a v a i l a b l e only 16 years ago, we have already seen three generations of instruments, and many more w i l l f o l l o w i n s h o r t o r d e r . S o l i d s t a t e e l e c t r o n i c s , computer-assisted imaging, new high-vacuum t e c h n i q u e s , new h i g h - i n t e n s i t y e l e c t r o n guns, improved detectors of emitted r a d i a t i o n , and new presentation modes have a l r e a d y been i n t r o d u c e d i n a s h o r t t i m e . TJiese improvements have a l r e a d y r e s u l t e d i n r e s o l u t i o n s below 30 A. Improved X - r a y detectors and data processors have a l s o improved the elemental and s p a c i a l r e s o l u t i o n of t h i s research mode to a range needed for most polymer r e s e a r c h . A l s o , e l e c t r o n spectroscopy w i l l become more prominent w i t h these new developments. Mass p r o d u c t i o n and m i n i a t u r i z a t i o n have already reduced the p r i c e of basic equipment i n e l e c t r o n microscopy. In turn, independent l a b o r a t o r i e s are now able to charge r e l a t i v e l y low rates for contract work to those s m a l l companies that cannot afford to maintain t h e i r own microscopy f a c i l i t y .

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