Polymer fluids: Uncovering the amazing science behind everyday products

summary:

As a special state of existence of matter, polymer fluids are everywhere in our production and life. They exhibit both viscous flow characteristics and elastic deformation capabilities, so polymer fluids have complex and diverse rheological behaviors. This article will fully reveal the wonderful world of polymer fluids, starting from basic concepts and classifications, deeply exploring the molecular mechanisms of a series of rheological phenomena, and finally looking forward to its prospects in the field of application, so that readers can appreciate the latest progress and important achievements in this field.

Written by Lu Yuyuan (Researcher at Changchun Institute of Applied Chemistry, Chinese Academy of Sciences) and An Lijia (Researcher at Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Academician of the Chinese Academy of Sciences)

Polymer fluids sound a bit abstract. However, whether it is the plastics, rubbers, and fibers we use in our daily lives, or the various polymers and their composite materials in industrial production, their production, processing, and molding all require the understanding and application of the flow and deformation properties of polymer fluids, that is, rheological properties. Polymer fluids will exhibit amazingly complex and diverse rheological behaviors under different experimental or processing conditions. So, what scientific principles are hidden behind these phenomena? This article will use a series of interesting examples to take you to a deeper understanding of the mysteries of polymer fluids and reveal the magical science behind polymer fluids.

Types of polymer fluids

Polymers are long-chain compounds composed of many repeating units (monomers), also commonly referred to as polymers. For example, we can link ethylene molecules together to form a very long molecule, which is polyethylene. Polymer fluids are a specific state of polymers when the temperature is much higher than their glass transition temperature or solidified, including polymer melts and solutions. Because polymer fluids have both viscous and elastic characteristics and exhibit complex and diverse rheological behaviors, they have become a classic model system for basic research in polymer physics and even polymer science; polymer fluid rheology has also become the disciplinary basis for polymer material processing and molding.

According to the different topological structures of polymer chains (as shown in Figure 1), polymers can be divided into linear polymers, circular polymers, branched polymers, hyperbranched polymers, etc. [1-3]. Each type of polymer has unique rheological and physical properties, which makes them have their own applications in different fields.

Figure 1 Schematic diagram of polymers with different topological structures: (a) linear, (b) circular, (c) branched and (d) hyperbranched polymers.

(1) Linear polymers are compounds composed of repeating units connected linearly. They are the most common type of polymers and have good processability. For example, linear polyethylene has very high flexibility and plasticity, so it is widely used in industrial and daily necessities such as engineering pipes, plastic bags, and plastic wrap.

(2) Annular polymers are polymers with a closed ring structure formed by repeating units and have no ends. The flow behavior of annular polymers at the microscopic scale is very sensitive to changes in the external environment, that is, it has the characteristics of "small stimulus, large response". At the same time, it also has unique solution properties (such as intrinsic viscosity), which makes annular polymers have important application value in micro- and nanoscale fluid dynamics research.

(3) Branched polymers are a special class of polymers with many side chains. Compared with general linear polymers, branched polymers have a series of unique advantages. We can flexibly adjust their properties by adjusting the number and position of side chains, thereby preparing a variety of materials with different properties to meet the needs of different applications. When the side chains are short, branched polymers have higher melt fluidity, which makes it easier to plastically deform during processing, thereby creating complex shapes and structures. Therefore, branched polymers are widely used in the production of plastic products such as children's toys. On the other hand, if the side chains are longer, the molecular structure of the branched polymer will become intricate, making it better able to resist corrosion and dissolution by acids, alkalis and other chemicals, and show good chemical corrosion resistance, which makes long-chain branched polymers an ideal packaging material.

(4) Hyperbranched polymers are a type of polymer with a higher degree of branching, with a more complex spatial structure and more molecular chain branching points [4-6]. Dendrimers are a special type of hyperbranched polymer with a perfect structure. This complex chain topology gives hyperbranched polymers better properties, such as higher strength, elasticity, friction resistance, and excellent transport properties. This makes hyperbranched polymers widely used in lubricants, adhesives, coatings, drug carriers, and even tire tread rubber.

Structure determines properties, properties determine use

“Structure determines properties, and properties determine uses” is the principle that materials scientists usually follow. In order to further improve and expand the performance of polymer materials, scientists mainly use copolymerization and blending methods (as shown in Figure 2) [2, 7] .

Figure 2 Schematic diagram of copolymerization and polymer blending. [7]

Copolymerization refers to the polymerization reaction of two or more repeating units under certain fluid conditions to form a copolymer with complex properties. For example, acrylonitrile-butadiene-styrene copolymer (ABS plastic) is a commonly used high-performance engineering plastic. Among them, acrylonitrile (A) gives the material excellent heat resistance and chemical corrosion resistance, the addition of butadiene (B) makes the material have good impact toughness, and styrene (S) increases the hardness and rigidity of the material, making the material widely used in automobiles, electronics, home appliances, construction and other fields.

Blending is the mixing of two or more different polymers in a fluid state to form a material with excellent comprehensive properties. This blended material is easy to prepare and can combine the performance characteristics of different polymers, so it has a wider range of applications and is relatively cheap.

From the above introduction, we can see that different types and structures of polymer fluids give polymer materials their own unique rheological characteristics and physical properties. Through reasonable design and modification, scientists continue to explore and develop new polymer materials, providing more possibilities for our production, life and scientific and technological progress.

Typical rheological phenomena

In basic research, researchers will design various experiments to characterize the rheological phenomena of polymer fluids in order to gain a deeper understanding of the nonlinear rheological behavior and mechanism of polymer fluids. Taking plastics, the most common type of polymer material in daily life, as an example, let's take a look at the typical rheological phenomena of polymer fluids.

Plastic is a plastic material made of polymers. It becomes soft and easy to shape when heated, and becomes tough after cooling [1, 2]. This plasticity is due to the different rheological behaviors of polymer fluids at different temperatures. After heating, when the soft plastic is subjected to external force, the polymer chains in it will move rapidly, causing the material to undergo plastic deformation as a whole; and when the external force is removed, it will partially rebound (or even completely rebound), and the polymer chains will return to their original state. If the plastic is cooled quickly before the external force is removed, it will maintain its current shape and become tough; if it is heated quickly again, the plastic will rebound, giving it a strong "memory effect".

Another common polymer material in daily life is rubber. It has special properties such as excellent elasticity and durability, and is called an "elastomer" [2] . On the one hand, because of its low glass transition temperature, it can be regarded as a special polymer fluid at room temperature; on the other hand, because of its unique polymer structure and cross-linking properties, it can be regarded as a special polymer solid. The cross-linking property refers to the three-dimensional network structure formed by the rubber molecular chains connected to each other through chemical bonds or physical cross-linking points. This cross-linking structure gives the rubber material the ability to quickly return to its original shape and makes it have high tensile, compressive and wear resistance. It is precisely because of the existence of the cross-linking property that rubber materials can adapt to various complex stress environments, such as playing an important role in tires, sports shoe soles, rubber tubes and seals.

The most common rubber band is a highly cross-linked polymer fluid. Although it is usually regarded as a solid, the molecular segments inside it can still undergo relatively free thermal motion like liquid water molecules at room temperature, which is also a significant difference between rubber materials and small molecule materials. When we stretch a rubber band quickly, an interesting phenomenon will occur: we can see many fuzzes on the rubber band with the naked eye. This phenomenon can be explained by the lamellar slip of the polymer chain: when the external force stretches the rubber band, the polymer chain will be stretched; at the same time, the cross-linking points between the polymer chains will also be affected by the tensile force. However, due to the unevenness of the cross-linking points, some cross-linking points are easier to move than other cross-linking points, and then some chain segments will slide in the direction perpendicular to the tension, forming a fuzzy structure. This lamellar slip phenomenon is caused by the redistribution of energy during the stretching process. However, as long as the rubber band is not stretched too long, that is, the polymer chain is not broken, then when the external force disappears, the polymer chain will return to its original state, and the rubber band will return to its original state. In fact, whether it is the resilience of the rubber soles of sports shoes or the grip of car tires, they are inseparable from the special movement and deformation of the polymer chains in the rubber.

Polymer fluids have great application potential

The study of polymer fluids is not only of great significance to the development of basic science, but also shows great potential in many application fields. In the field of materials science, studying the rheological behavior of polymer fluids is of great significance for improving the preparation methods of materials and achieving performance regulation. By deeply understanding the behavior of polymer fluids, people can optimize the process of synthesizing materials and improve the strength, toughness, weather resistance, thermal and electrical properties of materials. For example, researchers from Jilin University and Soochow University have used dynamic reversible bonds to give materials and devices excellent mechanical properties, repairability, and chemical corrosion resistance [8-10].

Polymer fluids also have important application significance in the field of biomedicine. For example, researchers at Stanford University have developed a series of biomimetic materials for use in fields such as tissue engineering and medical devices. Among them, they have developed an artificial skin that can simulate natural skin to a greater extent [11, 12]. This artificial skin can rebound or heal quickly under the action of external forces and can better sense changes in the surrounding environment. In the medical field, it can be used to treat burns, trauma, and skin transplant surgery, etc., to accelerate the patient's healing process and relieve their pain.

Polymer fluids have also shown amazing potential in many other applications, such as 3D printing, nanotechnology, flexible electronics, etc. The research results of polymer fluids will promote industrial progress and the development of new technologies, thus bringing more convenience and well-being to people's production and life.

Challenges in polymer fluid research

Due to the complex chain structure and chain motion as well as the nonlinear response under flow conditions, the basic research of polymer fluids also faces some severe challenges. For example, the "strain localization" phenomenon of polymer fluids is an issue that has attracted widespread attention and debate in the international academic community. The so-called strain localization refers to the phenomenon that non-uniform strain or even fracture occurs in a macroscopically uniform structure; under certain conditions, strain localization will cause an "avalanche-like" attenuation of the mechanical properties of polymer materials. Therefore, confirming its existence and revealing its mechanism at the molecular level are of great significance to scientific research and material development.

In recent years, large-scale computer simulation has become an important means to reveal the complex rheological behavior and molecular mechanism of polymer fluids. The Changchun Institute of Applied Chemistry of the Chinese Academy of Sciences, in collaboration with the California Institute of Technology, successfully confirmed the existence of typical strain localization phenomena in polymer fluids - "macroscopic flow (melt fracture)" and "shear band" (see Figure 3), and revealed the corresponding molecular mechanism [13, 14] . Jilin University has developed the GPU-accelerated molecular dynamics simulation software GALAMOST, which can quickly simulate the motion process of molecular chains in polymer fluids, providing researchers with a powerful and effective tool [15] . In addition to computer simulation, some research is dedicated to developing simulation platforms independent of commercial software to solve challenges faced in specific fields. For example, Jilin University and the Changchun Institute of Applied Chemistry of the Chinese Academy of Sciences have cooperated to develop an independent, low-level technology digital design platform for comprehensive performance simulation of aviation tires [16-18], which can quickly and accurately solve the constitutive relationship of tires under complex working conditions. This digital design software can provide key technical support for aviation tire design.

Figure 3 Typical strain localization phenomena in polymer fluids—“macroscopic flow” and “shear band”.

summary

As a special state of matter, polymer fluids exhibit amazing rheological properties. From plastic bags, rubber bands to synthetic fibers, polymer materials provide us with a variety of practical solutions and bring many conveniences to life. In fact, the application prospects of polymer fluids are very broad, such as: providing new ideas for solving energy and environmental problems, opening up new possibilities for bionic materials and drug delivery, and so on. All of these are inseparable from the mastery of the rheological properties of polymer fluids. In terms of research mechanism, computer simulation and simulation software will become a powerful tool to solve the complex rheological behavior of polymer fluids. With the continuous development and application of these technologies, people's understanding of polymer fluids will become more and more in-depth, and they will definitely play a higher value in the fields of science and engineering.

References

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[18] Zuo Wenjie; Zhao Cunwei; Zhang Ran; Lu Yuyuan; An Lijia; Tang Tao. Aviation tire structure finite element process automation modeling software V1.0. 2023, China, National Copyright Administration, 2023SR0874126.

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

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