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扫描电子显微镜

Scanning Electron Microscope

 
 
 
 
 

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扫描电镜应用之:高分子聚合物  

2012-03-19 23:50:03|  分类: 默认分类 |  标签: |举报 |字号 订阅

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Sample preparation

The most effective SEM sample will be at least as thick as the interaction volume; depending on the image technique you are using (typically at least 2 ?m). For the best contrast, the sample must be conductive or the sample can be sputter-coated with a metal (such as Au, Pt, W, and Ti). Metals and other materials that are naturally conductive do not need to be coated and need very little sample preparation.


SEM of polymers

As previously discussed, to view features that are smaller than the wavelength of light, an electron microscope must be used. The electron beam requires extremely high vacuum to protect the filament and electrons must be able to adequately interact with the sample. Polymers are typically long chains of repeating units composed primarily of “lighter” (low atomic number) elements such as carbon, hydrogen, nitrogen, and oxygen. These lighter elements have fewer interactions with the electron beam which yields poor contrast, so often times a stain or coating is required to view polymer samples. SEM imaging requires a conductive surface, so a large majority of polymer samples are sputter coated with metals, such as gold.

The decision to view a polymer sample with an SEM (versus a TEM for example) should be evaluated based on the feature size you expect the sample to have. Generally, if you expect the polymer sample to have features, or even individual molecules, over 100 nm in size you can safely choose SEM to view your sample. For much smaller features, the TEM may yield better results, but requires much different sample preparation than will be described here.


        Polymer sample preparation techniques
        Sputter coating

A sputter coater may be purchased that deposits single layers of gold, gold-palladium, tungsten, chromium, platinum, titanium, or other metals in a very controlled thickness pattern. It is possible, and desirable, to coat only a few nm’s of metal onto the sample surface.


       Spin coating

Many polymer films are depositing via a spin coater which spins a substrate (often ITO glass) and drops of polymer liquid are dispersed an even thickness on top of the substrate.


        Staining

Another option for polymer sample preparation is staining the sample. Stains act in different ways, but typical stains for polymers are osmium tetroxide (OsO4), ruthenium tetroxide (RuO4) phosphotungstic acid (H3PW12O40), hydrazine (N2H4), and silver sulfide (Ag2S).


Examples
Comb-block copolymer (microstructure of cast film)

  • Cast polymer film (see Figure 6). 
  • To view interior structure, the film was cut with a microtome or razor blade after the film was frozen in liquid N2 and fractured. 
  • Stained with RuO4 vapor (after cutting). 
  • Structure measurements were averaged over a minimum of 25 measurements.Figure 6 (graphics6arb.jpg)
Figure 6: SEM micrograph of comb block copolymer showing spherical morphology and long range order. Adapted from M. B. Runge and N. B. Bowden, J. Am. Chem. Soc., 2007, 129, 10551. Copyright: American Chemical Society (2007).


Polystyrene-polylactide bottlebrush copolymers (lamellar spacing)

  • Pressed polymer samples into disks and annealed for 16 h at 170 °C.
  • To determine ordered morphologies, the disk was fractured (see Figure 7).
  • Used SEM to verify lamellar spacing from USAXS.
    Figure 7 (graphics7arb.jpg)
Figure 7: SEM image of a fractured piece of polymer SL-1. Adapted from J. Rzayev, Macromolecules, 2009, 42, 2135. Copyright: American Chemical Society (2009).


SWNTs in ultrahigh molecular weight polyethylene

  • Dispersed SWNTs in interactive polymer.
  • Samples were sputter-coated in gold to enhance contrast.
  • The films were solution-crystallized and the cross-section was imaged.
  • Environmental SEM (ESEM) was used to show morphologies of composite materials.
  • WD = 7 mm.
  • Study was conducted to image sample before and after drawing of film.
  • Images confirmed the uniform distribution of SWNT in PE (Figure 8).
  • MW = 10,000 Dalton.
  • Study performed to compare transparency before and after UV irradiation.
SEM of polymers - 驰奔 - -韩国专业【扫描电镜】制造商COXEM-

Figure 8: SEM images of crystallized SWNT-UHMWPE films before (left) and after (right) drawing at 120 °C. Adapted from Q. Zhang, D. R. Lippits, and S. Rastogi, Macromolecules, 2006, 39, 658. Copyright: American Chemical Society (2006).

 
Nanostructures in conjugated polymers (nanoporous films)

  • Polymer and NP were processed into thin films and heated to crosslink.
  • SEM was used to characterize morphology and crystalline structure (Figure 9).
  • SEM was used to determine porosity and pore size.
  • Magnified orders of 200 nm - 1 μm.
  • WD = 8 mm.
  • MW = 23,000 Daltons
  • Sample prep: spin coating a solution of poly-(thiophene ester) with copper NPs suspended on to ITO coated glass slides. Ziess, Supra 35
    Figure 9 (graphics9arb.jpg)
Figure 9: SEM images of thermocleaved film loaded with nanoparticles with scale bar 1 μm. Adapted from J. W. Andreasen, M. Jorgensen, and F. C. Krebs, Macromolecules, 2007, 40, 7758. Copyright: American Chemical Society (2007).


Cryo-SEM colloid polystyrene latex particles (fracture patterns)

  • Used cryogenic SEM (cryo-SEM) to visualize the microstructure of particles (Figure 10).
  • Particles were immobilized by fast-freezing in liquid N2 at –196 °C.
  • Sample is fractured (-196 °C) to expose cross section.
  • 3 nm sputter coated with platinum.
  • Shapes of the nanoparticles after fracture were evaluated as a function of crosslink density.
SEM of polymers - 驰奔 - -韩国专业【扫描电镜】制造商COXEM-

Figure 10: Cryo-SEM images of plastically drawn polystyrene and latex particles. Adapted from H. Ge, C. L. Zhao, S. Porzio, L. Zhuo, H. T. Davis, and L. E. Scriven, Macromolecules, 2006, 39, 5531. Copyright: American Chemical Society (2006). 
 
Bibliography

  • H. Ge, C. L. Zhao, S. Porzio, L. Zhuo, H. T. Davis, and L. E. Scriven, Macromolecules, 2006, 39, 5531.
  • J. Rzayev, Macromolecules, 2009, 42, 2135.
  • J. W. Andreasen, M. Jorgensen, and F. C. Krebs, Macromolecules, 2007, 40, 7758.
  • M. B. Runge and N. B. Bowden, J. Am. Chem. Soc., 2007, 129, 10551.
  • P. J. Goodhew, J. Humphreys, and R. Beanland, Electron Microscopy and Analysis, Taylor & Francis Inc., New York (2001).
  • Q. Zhang, D. R. Lippits, and S. Rastogi, Macromolecules, 2006, 39, 658.
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