The science of studying the physical properties of polymer materials
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Polymer PhysicsIs researchHigh molecular substancesThe science of physical properties.Its main research directions include polymer morphology, polymer mechanical properties,Polymer solution, PolymercrystalSuch disciplines as thermodynamics and statistical mechanics, and such dynamics disciplines as polymer diffusion.
In the past 100 years, the polymer material industry has made rapid development.Now polymer materials are no longer metal, wood, cotton, hempNatural rubberetc.Traditional materialsIt is one of the basic materials in the national economy and national defense construction.At the same time, the three major components of polymer science - polymer chemistry, polymer physics and polymer engineering are also maturing.
Research objectives
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Polymer physics is to study the structureperformanceIt has a very close internal relationship with the synthesis, processing, modification and application of polymer materials.Because only by mastering the internal relationship between polymer structure and performance and its laws, can we guide the design and synthesis of polymers with a clear aim, reasonably select and modify polymer materials, and correctly process and shape various polymer products.The course of polymer physics is based on physical chemistry, polymer chemistry, solid physicsMechanics of MaterialsIt is also the foundation of polymer materials, polymer molding and processing and other courses. It is one of the most important professional basic courses for polymer materials science and engineering in colleges and universities.
Material Science
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In recent years, polymer materials with various special functions (including biologyMedical polymer, OptoelectromagneticFunctional polymer、Polymer separation membrane, adsorption and separation resinSmart gel) and its devices have been developed rapidly.This is due to people's profound understanding of the relationship between polymer structure and performance, and also provides new content for polymer physics.However, the basic research of functional polymers has not yet formed a relatively complete system.
Structural characteristics
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The difference between macromolecules and low molecules lies in the formerrelative molecular massIt is very high. Generally, those with a relative molecular weight higher than about 10000 are calledhigh polymer, those with a relative molecular weight less than about 1000 are called low molecules.The relative molecular weight between high molecular weight and low molecular weight is calledOligomer(also known asOligomer）。Generally, the relative molecular weight of high polymers is 10 ^ 4~10 ^ 6, and those whose relative molecular weight is greater than this range are also called ultra-high relative molecular weight polymers.
The structure of polymer is very complexPolymer structureIt is composed of different levels and can be divided into three main structural levels.
Due to the characteristics of polymer structure, polymer has the following basic properties: small specific gravity, high specific strength, elasticity, plasticity, abrasion resistance, insulation, corrosion resistance, and radiation resistance.
In addition, macromolecules cannot be vaporized and are often insoluble,viscosityLarge and equal characteristics are also closely related to structural characteristics.
Chinese and English
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In English, "polymer" or "polymer compound" mainly has two words, namely polymers andMacromolecules。The former can also be translated as polymer or polymer;The latter can also be translatedmacromolecule。Although the two words are often mixed, there are still some differencesDuplicate unitSynthetic products of, generally excludingnatural polymerThe latter refers to a class of compounds with high relative molecular weight, including natural andSynthetic polymerIt also includes complex macromolecules without certain repeating units.
The crystal morphology is composed of three basic structural units, namelyRandom coilAmorphous structureFolded chain chipAnd extended chain crystals.Therefore, the amorphous part is contained in the crystalline form, because it is impossible for the polymer crystallization to reach 100% crystallization.
Characteristics of polymer crystals
⑴ Polymer crystals are essentially molecular crystals.
The folded chain model is suitable for explaining the structure of single crystals, while the other two models are more suitable for explaining thecrystal structure。
In short, different views of the model are still in dispute.yesAmorphous state, the focus of the argument is completedisorderOr partial order;For the crystalline state, the focus is the degree of order, whether it is a large number of neighbor orders or a few neighbor orders.
Polymer inUnit cellThere are two conformations in, namely, plane zigzag conformation (PZ, for example, PE) and spiral conformation (H, for example, PP).adoptCell parameterThe density of complete crystallization can be calculated:
A polymer may have different crystal cells due to different crystallization conditions, which is called homogeneous polycrystal phenomenon.
Crystallization ability and crystallization process of polymers
In general, the internal factor affecting the structural process is that the polymer must have the regularity of chemical structure and the regularity of geometric structure to crystallize.Typical examples are as follows:
In addition, good flexibility andIntermolecular forceStrength is also a factor to improve the crystallization capacity. The former improves theChain segmentThe activity ability of diffusing and arranging toward crystallization, which makes the crystal structure stable and thus conducive to crystallization. Typical examples arenylon(due to stronghydrogen bond）。
The external factors affecting the crystallization process mainly include:
(1) Temperature (understood as providing heat energy);
(2) Solvent (providing chemical energy), called solvent induced crystallization;
(3) Stress or pressure (providing mechanical energy), called stress induced crystallization;
(4) Impurities (nucleation or dilution).
crystallinity
Crystalline polymers and crystalline polymers are two different concepts. Polymers with the ability to crystallize are called crystalline polymers. However, due to limited conditions (such as quenching), crystalline polymers may not be crystalline polymers, butAmorphous polymerHowever, it can form crystalline polymers under certain conditions.
The crystallization of polymers is always incomplete, so crystalline polymers are actually semi crystalline polymers.usecrystallinityTo describe this state, its definition is:
Both crystallinity and crystalline size have important effects on the properties of polymers.
(1) Mechanical properties:
Crystallization makes plastics brittle（impact strength But increase the tensile strength of rubber.
(2) Optical properties
Crystallization makes the polymer opaque because light scattering occurs at the interface between the crystalline and amorphous regions.
reduceSpheruliteWhen the size reaches a certain extent, it not only improves the strength (reduces the intergranular defects) but also improves the transparency (when the size is smaller than the light wavelength, there will be no scattering).
(3) Thermal performance
Crystallization increases the use temperature of plastics from to.
Quenching or addingNucleating agentCan reduce spherulite size, while annealing is used to increasecrystallinity, improve crystallization perfection and eliminateinternal stress。
Crystallization thermodynamics
Unlike small molecules, the crystallization and melting of macromolecules occur in a wide temperature range“Melting limit”。However, polymer crystallization and crystal melting are stillThermodynamic phase transitionProcess, when reaching balance:
Orientation Structure of Polymer
Whether crystalline orAmorphous polymer, orientation can occur under the action of external field (such as tensile force), and the degree of orientation is expressed by orientation function.
Polymer liquid crystal(liquid crystal) state isMolten stateOr the general name of ordered fluid formed in the solution state, which is the intermediate state between the liquid state and the crystalline state.
(1) According to the molecular arrangement, it can be divided into smectic type, nematic type and cholesteric typeOrdered structure。
(2) It can be divided into thermotropic liquid crystal and lyotropic liquid crystal according to the formation mode. The former can obtain ordered melt by heating in a certain temperature range (from Tm to clear point), while the latter can only be obtained in a solution higher than a certain concentration without liquid crystal phase in pure substances.
(3) According to the position of the mesogen in the molecular chain, it can be divided into main chain liquid crystal and side chain liquid crystal.Liquid crystal has a special viscosity property, which still has a low viscosity at high concentrations“Liquid crystal spinning”, which not only greatly improves the spinning process, but also has ultra-high strength and ultra-high modulusKevlarAroma of (kevlar) fibernylon。
The electro-optic effect of polymer side chain liquid crystals is also used for display.
Fabric Structure of Blended Polymers
The actual polymer material is usually a multi-component polymer system or composite material. Here, only the mixture of polymer and polymer is discussedBlendingPolymers or blends, which are the products of mixing different kinds of polymers together through physical methods, are also called figuratively because of their many similarities with alloys“Polymer alloy”。
The purpose of blending is to learn from each other and improve performance. The most typical blending is rubberModified plasticsAn example ofHigh impact polystyreneAnd ABS (blended orgraftType).
Polymer mixtureIt is difficult to achieve molecular level mixing, because according to thermodynamic compatibility conditions.
