The microstructure of polymers controls their chemical reactivity as well as their mechanical and transport properties. However, quantitative characterization of the microstructure of soft polymers is not often possible or practical with the few analytical methods available to study it at the sub-micrometer scale. Freeze-fracturing is one such method, but often the surfaces of samples prepared with the process are too rough, leaving only a very small area appropriate for quantitative SEM investigation.
The results demonstrate that cryo-BIB-SEM enables microstructure characterization of soft polymers with high resolution over large planar areas of damage-free sample cross sections. Only minor artifacts from the cryo-BIB-SEM sample preparation process were seen at the scale of observation, none of which appear to be a consequence of freezing.
Precise characterization of a polymer’s microstructure is important, because it influences the chemical reactivity and mechanical and transport properties of the polymer and, thus, governs durability and robustness to weathering, heat, cold, and light exposure.
Moreover, this fact applies to any type of polymer, whether soft, hard, natural, or synthetic.
Cryo-BIB-SEM was developed to enable high resolution imaging and chemical analysis of micropores over relatively large, cryogenically stabilized samples with damage-free areas. In a previous report, cryo-BIB-SEM was used to study the microstructure of lithium-ion battery electrodes during drying . Here, cryo-BIB-SEM is applied to characterize natural polymers. Examples of natural polymers are wood and the skin of fruits and vegetables.
As mentioned above, an accurate determination of the soft polymer microstructure with sub-micrometer resolution is necessary. Unfortunately, few methods can deliver such a high resolution for such soft samples. Furthermore, quantitatively characterizing the microstructure of freeze-fractured soft polymer samples is often difficult, because the surfaces roduced are too rough making analysis with SEM or energy dispersive spectroscopy (EDS) challenging as only small areas can be accurately investigated. Additionally, for organic materials which can swell and shrink, sample preparation for and measurement with SEM, especially materials with a high moisture content, are difficult to perform with the sample in its native state. Alternative methods, such as micro-computed tomography (μCT) and cryogenic focused ion beam milling with SEM (cryo-FIB-SEM), are often too low in resolution or the results obtained are not representative.
This application note describes the use of cryo-BIB-SEM to characterize at high resolution (sub-micrometer level) the microstructure of soft, delicate natural polymers (wood and tomato skin).
Cryo-BIB enables the preparation of cryogenically stabilized cross sections with a large planar area (as much as 4 mm²). The sample preparation uses a rapid cooling (quenching) step which allows a clean cut of the polymer with much less risk of damage, i.e., fracture, deformation, etc. The sample is quenched in a liquid nitrogen (