Optical microscopy to observe the crystalline morphology of polymers
The structure and principle of polarized light microscope, the use of polarized light microscope.
The polymer spherulites were prepared by melting method, the morphology of the spherulites obtained at different crystallization temperatures was observed, and the radius of the polymer spherulites was measured.
Crystals and amorphous are the two basic forms of polymer aggregates, and many polymers can crystallize. The practical performance of crystalline polymer materials (such as optical transparency, impact strength, etc.) is closely related to the crystalline morphology, grain size and degree of perfection inside the material. Therefore, the study of polymer crystal morphology has important theoretical and practical significance. Polymers form different crystals under different conditions, such as single crystals, spherulites, fiber crystals, etc. When the polymer is cooled from the molten state, spherulites are mainly formed, which is the most common form of polymer crystallization. Performance has a big impact.
Spherulites are named after the crystal nucleus grows radially to form a spherical shape, which is a "three-dimensional structure". But it can also be regarded as a disc-shaped "two-dimensional structure" in an extremely thin test piece, and the spherulite is a polyhedron. The unit cell is composed of molecular chains, the stacking of the unit cell constitutes a wafer, the wafer stacking constitutes a microfiber bundle, and the microfiber bundle grows along the radial direction to form a spherulite. There are crystal defects between the wafers and amorphous inclusions between the microfiber bundles. The size of spherulites depends on the molecular structure of the polymer and the crystallization conditions. Therefore, the size of spherulites varies greatly depending on the type of polymer and crystallization conditions. The diameter can range from micrometers to millimeters, or even as large as centimeters. The spherulites are dispersed in the amorphous polymer. Generally speaking, the amorphous is a continuous phase, and the peripheries of the spherulites can intersect to form an irregular polygon. Spherulites have optical anisotropy and refract light, so they can be observed with a polarizing microscope. Polymer spherulites exhibit a characteristic black cross extinction image between the crossed polarizers of a polarizing microscope. When some polymers form spherulites, the helical distortion of the wafer as it grows along the radius allows concentric extinction images to be seen under a polarizing microscope.
The optimal resolution of the polarized light microscope is 200 nm, and the effective magnification exceeds 500 to 1000 times. Combined with electron microscope and X-ray diffraction method, it can provide more comprehensive crystal structure information.
Light is an electromagnetic wave, or transverse wave, and its propagation direction is perpendicular to the direction of vibration. But for natural light, its vibration directions are evenly distributed, and no direction prevails. But after reflection, refraction or selective absorption, natural light can be transformed into light waves that vibrate in only one direction, namely polarized light. A beam of natural light passes through two polarizers. If the two polarization axes are perpendicular to each other, the light cannot pass through. When a light wave propagates in an anisotropic medium, its propagation speed changes with the vibration direction, and the refractive index value also changes accordingly. Generally, birefringence occurs, and it is decomposed into two parts with mutually perpendicular vibration directions, different propagation speeds, and different refractive indices. strips of polarized light. When the two polarized lights pass through the second polarizer, only the light in the direction parallel to the second polarization axis can pass. The two passing beams will interfere due to the optical path difference.
Observed under a crossed polarizing microscope, the amorphous polymer does not have birefringence because of its isotropy, the light is blocked by the orthogonal polarizer, and the field of view is dark. Spherulites will exhibit a unique black cross extinction phenomenon, and the two arms of the black cross are parallel to the directions of the two polarization axes. Except for the vibration direction of the polarizer, the rest of the light appears due to refraction. Figures 2-7 are photographs of spherulites of isotactic polypropylene.
Under polarized light conditions, the morphology of crystals can also be observed, the size of crystallites can be determined and the pleochroism of crystals can be studied.
1) Cut a small piece of polypropylene film or 1/5 to 1/4 pellet, put it on a clean glass slide, keep it away from the edge of the glass slide, and cover the sample with a cover glass.
2) Preheat the tablet press to 240°C, melt the polypropylene sample on a hot plate (the sample is completely transparent), press to form a film for 2 minutes, and then quickly transfer it to a 50°C hot stage to crystallize it. The same samples were crystallized at 100°C and 0°C after melting.
2) Adjust the microscope
1) Turn on the mercury arc lamp for 10 min in advance to obtain a stable light intensity, and insert a monochromatic filter.
2) Remove the microscope eyepiece, and place the polarizer and analyzer at 90°. While viewing the microscope tube, adjust the position of the lamp and mirror, and adjust the analyzer if necessary to achieve complete extinction (the field of view is as dark as possible).
3) Measure the spherulite diameter
The polymer crystal flakes are observed under an orthogonal microscope, and the diameter of the spherulites is measured with a microscope eyepiece scale. The determination steps are as follows:
1) Insert the eyepiece with a graduated ruler into the lens barrel, and place the stage microruler on the stage, so that two rulers can be seen in the viewing area at the same time.
2) Adjust the focal length so that the two feet are arranged in parallel, the scale is clear, and the two zero points are coincident with each other, and the value of the eyepiece scale can be calculated.
3) Remove the stage micro-ruler, place the predicted sample in the center of the stage's field of view, observe and record the crystal shape, read the scale of the spherulite on the eyepiece scale, and then calculate the diameter of the spherulite.
