Civil Engineering Reference
In-Depth Information
Polarized light microscopy (POM) is the easiest method to identify thermotropic
liquid crystals [63, 64] by recording an image known as the optical texture, due to ori-
entation of the director and defects formed in the sample. h e area over which the
vector abruptly changes direction represents a defect. h e liquid crystals present three
elementary types (point, line and plane), and two major types (disclination and disloca-
tion) of defects. For each type of mesophase, several, [65, 66], more or less characteris-
tic, textures are known. Identii cation of mesophase type by polarized light microscopy
is dii cult, because the same type of texture may appear for several mesophases, along
with polymorphism. For this reason, a number of specii c rules should be observed.
First, to produce mesophase and to obtain clear textures, the sample must have a high
degree of purity. Another aspect refers to the orientation of mesogene groups, which
may be parallel (homeotropic alignment) or vertical (planar alignment) in the direc-
tion of polarized light. In this last case, the following tests are required: mild heating of
the table tilts, so that the mesogene groups will not be parallel with the polarized light,
or shearing of the sample (by sliding the upper blade on the bottom), which induces
a temporarily homogeneous arrangement over time in which the sample appears as a
l ash, for a short time becoming birefrigent [4]. h e nematic mesophase can be easily
evidenced by shearing the samples, because of their lower ordering degree and high
l uidity in comparison to the smectic mesophase. Frequently, smectic mesophase tex-
tures are mixed with the crystalline state so that, in addition to checking the l ow of the
sample by shearing, heating/cooling multiple cycles of the sample at dif erent speeds
is recommended. In the case of mesophases, transitions are maintained at about the
same temperature. If a sample has several smectic mesophases, the likelihood of non-
specii c textures increases because the type of defects generated by a specii c mesophase
is inl uenced by the previous mesophase; thus, it is recommended to observe these
mesophases by cooling and not by heating procedures [5, 6].
Selective miscibility [67] represents an easy method for the identii cation and clas-
sii cation of liquid crystalline mesophases. Liquid crystalline substances with a high
degree of polymorphism are used as test components, playing an important role in
identifying liquid crystals. For mesophase production, the liquid crystalline poly-
morphism shows a dependence of the ordering degree on temperature [6]. h us, the
nematic mesophase is situated at the highest temperature, while smectic polymorphic
substances generally occur in the following order: smectic F mesophase - smectic I
mesophase - smectic B mesophase - smectic C mesophase - smectic D mesophase -
smectic A mesophase (where the alphabetical order of sui xes indicates the order of
their discovery).
Identii cation of the nematic mesophase through polarized light microscopy is easier
than that of the smectic mesophase, because specii c defects take a linear form. h e
nematic mesophase is characterized by a large number of textures: Schilieren texture
is one of the most common nematic textures with defect centers with two arms, nem-
atic droplets ot en occur from the isotropic liquid in the form of drops, string texture
consists of a disclination type
- line and appears as thin lines, and marble texture con-
sists of several zones with dif erent orientations of the director, inducing dif erent color
interferences [5] .
Assigning the corresponding texture to the smectic mesophase is more dii cult, due
to the defects appearing in these mesophases. Smectic A mesophase may adopt a focal
