What I've learned from the few sessions I've had on the Tecnai at Penn State's Materials Characterization Lab so far is that using a TEM is really, really confusing. And being told again and again that the instrument is very expensive and that you must be very careful only adds to the anxiety!
That being said, I've decided to write up a procedure for alignment on the Tecnai in as much detail as I could. I am not an experienced user by any stretch of the word, and I do suggest that you use what I've provided here in conjunction with your own notes from your training session, but I hope that these steps can help you in some way.
Please note that this procedure only applies to basic imaging on the Tecnai (I know, I know, these steps look anything but "basic.") Other techniques, such as electron diffraction or electron energy-loss spectroscopy, may require additional steps. I'll be sure to update this post when I eventually learn how to do those!
Research updates, literature reviews, & more from a materials science and engineering PhD student at Penn State
• paper summary
What are block copolymers and how do they self-assemble?
The following post is a modified version of a short paper I
wrote for MatSE 542 last semester. It is essentially a summary of the paper “Self-assembly of block copolymers” by Yiyong Mai and Adi Eisenberg published in The Royal Society of Chemistry in 2012.
Introduction
Block copolymers (BCPs) are a fascinating class of materials that have recently attracted significant attention due to their ability to self-assemble into a variety of morphologies, such as spheres, cylinders, gyroids, and lamellae. This post will discuss what block copolymers are, the thermodynamics behind microphase separation, and their theoretical and experimental phase diagrams.
Block copolymers (BCPs) are a fascinating class of materials that have recently attracted significant attention due to their ability to self-assemble into a variety of morphologies, such as spheres, cylinders, gyroids, and lamellae. This post will discuss what block copolymers are, the thermodynamics behind microphase separation, and their theoretical and experimental phase diagrams.
Simply put, block copolymers consist of two or more
chemically dissimilar polymer blocks that are thermodynamically immiscible yet
covalently bonded. Figure 1 below illustrates the most popularly studied
structures of block copolymers which can be formed with two types of blocks, A
and B. Such a material is referred to as a diblock copolymer, while structures
consisting of three blocks are called triblock copolymers, and so on. The
chemically distinct blocks will separate into different domains while the
covalent bonds restrict this demixing to local length scales, resulting in what
is called microphase separation and giving rise to the aforementioned myriad of
morphologies.
Figure 1: Schematic of different structures of diblock copolymers. Reproduced with permission from The Royal Society of Chemistry. |
Microphase separation
Microphase separation in BCPs is driven by a
combination of the unfavorable mixing enthalpy and a small mixing entropy, with
the covalent bonds holding blocks together to prevent macroscopic phase
separation. For a diblock copolymer consisting of blocks A
and B, microphase separation is influenced by three parameters: the volume
fractions of the A and B blocks (fA and fB), the total
degree of polymerization of the two blocks (N = NA + NB),
and the Flory-Huggins interaction parameter (χAB). The Flory-Huggins
parameter is dependent upon several factors and can be described with the
equation below:
where z is the
number of nearest neighbors per repeat unit, kB is the Boltzmann constant, T is
the temperature, and ε is the interaction energy of the respective block pairs.
The degree of microphase separation is determined by the product of the
interaction parameter χAB and the total degree of polymerization N. Thus, as χN decreases (or as temperature increases), the blocks become
increasingly miscible while the combinatorial entropy increases and the
copolymers become disordered. This behavior is referred to as an
order-to-disorder transition (ODT).
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