【Basic Knowledge of Glossary Definitions for “Polymers and Materials”-021】Glossary definitions related to polymers

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Reconsidering orientation: Orientation represents a state; what is its impact on the material?

For low-molecular substances, if their structure is spherically symmetric, then there is no orientation. When the molecular structure is non-spherical (such as in polymer chains), or when its charge distribution is asymmetric, the molecule will align itself in the direction of the electric or force field under the influence of these fields. It’s the polymer orientation! In real life, polypropylene packing ropes can be found everywhere; due to uniaxial stretching, they cannot be torn apart even with great force in the direction of orientation, but can be easily ripped into microfibers in the perpendicular direction ； If a small cut is made in the rubber first and then it is stretched, the cut will quickly extend deeper after a short while, and it can be torn apart with little effort. But if the rubber is stretched out first so that its molecular bonds are aligned, and then a cut is made with a knife, the cut will expand in the direction of the stretching force rather than going deeper; in this case, it will take more force to tear it apart ； Nanocarbon tubes have a wide range of applications, but many of them rely on their oriented arrangement. Therefore, studying the orientation structure of polymers is of great significance; it provides a scientific basis for obtaining materials with oriented structures and properties by controlling the processing conditions. Orientation refers to the fact that when linear polymers are fully stretched, their length is several hundred, thousands, or even tens of thousands of times their width. This special geometric asymmetry enables them to be easily arranged in a manner dominated by an external force field when under the influence of such a force field. For unoriented polymer materials, the chain segments are randomly oriented; there are as many segments pointing in one direction as those pointing in any other direction. Therefore, unoriented polymer materials are isotropic, whereas in oriented polymer materials, the chain segments are preferentially oriented in certain directions. Since the bonds are covalent along the molecular chain direction, while the inter-chain van der Waals forces act perpendicular to it, the oriented material exhibits anisotropy. The orientation results in significant changes in the mechanical properties, optical properties, thermal conductivity, and sound propagation speed of polymers. Among the mechanical properties, tensile strength and flexural fatigue strength increase significantly in the orientation direction, while they decrease in the direction perpendicular to the orientation direction ； In terms of optical properties, the orientation of polymer chains leads to the occurrence of birefringence ； In terms of thermal properties, the coefficient of thermal expansion differs in the oriented and unoriented directions. There are two ways in which polymer materials can be oriented under external forces: uniaxial orientation and biaxial orientation. Uniaxial orientation occurs when a polymer is stretched in a single direction by an external force; as a result, the length of the polymer increases while its thickness and width decrease, and the molecular chains align in the direction of the applied force.

The most common example of uniaxial orientation is the drawing of synthetic fibers. Generally, during the spinning of synthetic fibers, the molecular chains in the filaments emerging from the spinnerets are already somewhat oriented; further drawing by a certain factor increases the degree of orientation of these molecular chains along the fiber direction. Biaxial orientation occurs when an external force stretches a polymer in two perpendicular directions; as a result, the length of the polymer increases in the direction of the force while its thickness decreases. The polymer chains arrange themselves parallel to the plane of stretching, with a random arrangement on the side facing the stretch. It can be seen that after biaxial orientation, the properties of the polymer in the stretching plane become isotropic. The most common example of biaxial orientation is film biaxial orientation, which aligns the molecular chains in any direction parallel to the film plane, thereby resulting in a film that is isotropic within that plane. To compare the degree of orientation of materials, the concept of orientation degree is introduced; it refers to the extent to which the molecular chains in a polymer are arranged in a specific direction. The degree of orientation is generally represented by the orientation function F, where F = 0.5(3cos2θ – 1). θ is the angle between the principal axis of the molecular chain and the orientation direction. For actual polymer materials, θ is not a constant value but rather varies in a certain manner; therefore, in the equation for the orientation function, θ is usually replaced by the average value of the actual orientation angles. There are many methods used to determine the degree of orientation, including optical refractometry, infrared dichroism, wide-angle X-ray diffraction, acoustic wave propagation, and polarized fluorescence. The first three of these methods will be briefly introduced below. Optical birefringence method: Typically, the difference in refractive indices in two perpendicular directions is used directly as an indicator of the degree of orientation. Randomly oriented samples are optically isotropic, with △n = 0, whereas in fully oriented samples, △n can reach its maximum value. It should be noted that for a given sample, different directions will yield different values; for example, in uniaxially oriented films, the largest △n is observed between two directions parallel to the film plane. In biaxially oriented films, however, △n between the two directions parallel to the film plane is small or zero, with the largest △n occurring only between the two directions parallel and perpendicular to the film plane. This property can be used to distinguish between different types of orientation. Infrared dichroism: When infrared polarized light passes through the sample under test, the absorption intensity A of a certain group in the sample is related to the variation in the vibrational dipole moment M. Infrared absorption is greatest when the direction of the electric vector is parallel to the direction of change in the dipole moment; no absorption occurs when these two directions are perpendicular to each other. This phenomenon is known as infrared dichroism. The direction of change in M for unoriented polymer materials shows a uniform distribution, while M in oriented polymer materials also becomes oriented. Therefore, the degree of orientation of polymer materials can be characterized by infrared dichroism. Depending on the selected infrared spectral bands, it is possible to determine the orientation of the crystalline and amorphous regions separately, as well as the average orientation of the entire material. Based on whether the vibration spectrum corresponds to groups in the side chain or the main chain, the orientation of the main chain and side chains can be distinguished, and infrared dichroism can yield a wide range of orientation parameters. In actual production and daily life, the application of polymer orientation is becoming increasingly widespread. In liquid crystal spinning, thanks to the orientation properties of liquid crystal molecules, it is possible to achieve a high degree of orientation under relatively low drawing conditions, thereby preventing stress and damage to the fibers during high-level stretching. In the production process of liquid crystal displays (LCDs), the technology for controlling the orientation of liquid crystal molecules is very important; it not only affects the response speed of these molecules but also has a direct impact on the display quality of the LCDs. Good orientation effects can increase the display capacity and improve the display quality. Organic polymer materials exhibit little change in their properties as a function of liquid crystals, and they are suitable for mass production, which has led to their gradual use as materials for orientation films. People are constantly researching how to control the orientation of liquid crystal molecules so that they can respond rapidly to an external electric field. The orientation control techniques widely used in industry mainly include the traditional friction method and the non-friction methods that have been developed in recent years. Polymer materials with predetermined orientation hold great potential for application, and it is believed that in the coming years, as research on polymer orientation and related technologies continue to advance, they will bring even more benefits to humanity.