Synthetic resin, as the film-forming substance of the paint, is the main component of the paint. Requirements for synthetic resin for coatings:
(1) Good solubility, miscibility and wetting with pigments
(2) Good adhesion, light resistance, heat resistance, water resistance, acid and alkali resistance, etc.
Physics of resin polymers is a science that studies the relationship between the structure and characteristics of a polymer and the law of molecular motion in polymers. Main content:
②The relationship between microscopic molecular structure and macroscopic physical properties is the law of molecular motion and thermal transformation.
③ Various properties;
The purpose of studying and studying the physics of resin polymers:
(1) Understand and understand the basic molecular motion of polymers
(2) Establish an inextricable link between polymer structure and performance: The structure of a polymer determines the characteristics of a polymer.
Some important rules for the relationship between the characteristics of a synthetic resin and its chemical structure;
Does not ignite quickly with benzene ring;
The fewer hydrophilic active groups, the higher the molecular weight and the greater the degree of crosslinking, the better the water resistance
The type and amount of polar groups affect paint adhesion.
(3) Leadership of polymer synthesis and molding of materials in terms of productivity,
The performance of a synthetic coating resin is closely related to its chemical structure. Learn and study the relationship between them, summarize the various influencing factors and laws, and develop and control the structure of polymer materials, so as to better design polymer structures. material design to better meet the requirements of practical applications.
1.1 Different levels of synthetic resin structure
The structure of the polymer resin is multilevel. Table 1-1 shows the different levels and content of the polymer structure. The structure of the polymer can be divided into two parts: the chain structure of the polymer and the structure of the polymer in the condensed state. Chain structure refers to the structure and shape of a single molecular chain. When studying a chain structure, the question also arises whether it should be observed in close-up or far-range. Therefore, it is divided into primary structure (near-long-range structure) and secondary structure (long-range structure). The aggregate structure of a polymer refers to the overall internal structure of polymeric materials, which is formed by the arrangement and stacking of polymer macromolecular chains. Including polymer crystal structure, amorphous structure, orientation state structure, polymer liquid crystal structure, and multi-component polymer weave structure.
Figure 1-1 Schematic diagram of the secondary and tertiary structures of polymers
Each level of structure has an impact on the properties of polymers, among which the structure of the polymer chain determines the basic properties of polymers, and the aggregate structure directly affects the characteristics of polymers.
1.2 Polymer resin chain structure
The structure of the polymer chain is mainly determined by the chemical structure of the monomer, the synthesis conditions, and the synthesis method in polymer synthesis. It is divided into primary structure (near structure) and secondary structure (remote structure).
1. Primary structure of polymer resin
The primary structure of polymers (also known as short-range structure or chemical structure) is the most basic structure of polymers, which is the most important level reflecting the characteristics of polymers, and mainly includes the chemical composition and molecular structure structure. chains.
1. The chemical composition of the structural unit
According to the type of atoms of the main chain and their arrangement, polymer compounds are divided into four categories:
(1) Carbon chain polymer
The entire backbone is covalently bonded to carbon atoms. It is obtained by addition polymerization of vinyl monomers such as PS, PE, PVC, PMMA, etc. the bond is low and the heat resistance is poor.
(2) Heterochain polymer
In addition to carbon, the main chain contains atoms of oxygen, nitrogen and sulfur. Such as polyester, PA, PC, polysulfone, PU, etc. By polycondensation or ring-opening polymerization. Features: The heat resistance and strength are greatly improved, and can be used as engineering plastics. However, the main chain contains functional groups that are prone to hydrolysis, alcoholysis and acidolysis and have poor chemical stability.
(3) Elemental polymer
The main chain does not contain carbon, but the polymer containing silicon, boron, phosphorus, chromium and other elements is called elemental polymer, and the side group is an organic substituent such as polydimethylsiloxane, which also contains inorganic materials. Heat resistance and viscoelasticity of organic polymeric materials. The disadvantage is low strength.
(4) Inorganic polymer
The backbone contains no carbon or organic substituents. It consists solely of other elements and is called an inorganic polymer. Although this type of polymer has good heat resistance, it also has the problem of low strength.
2. Molecular structure
Refers to the sequence of atoms and bonds in a molecule, regardless of their spatial arrangement
(1) Link structure
There can be various bonding methods for connecting the structural units of the polymer, which can directly affect the characteristics of the materials. In polyIn vinyl chloride, there are two possible head-to-head (or tail-to-tail) and head-to-tail connections: the head-to-tail connection is the main one, but there is also a small amount of head-to-head or tail-to-tail connection. tail":
(2) Copolymer sequence structure
Binding sequence - copolymer sequence structure
For binary copolymers of monomers A and B according to different bond sequences:
3. Polymer structure - linear, branched and crosslinked
Refers to various shapes and types of polymers, typically including: linear, comb-like, branched, star-shaped, dendritic, and cross-linked networks.
Figure 1-2 Schematic diagram of polymer chain branching and crosslinking
Linear and branched polymers, macromolecules do not have chemical bonds, can melt when appropriate solvents are added, and can melt when heated, which are called thermoplastic polymers. When thermoplastic polymers are subjected to heat and force, the molecules can slide and flow with each other, so they are easy to process and shape.
Short chain branching will greatly disrupt the regularity of the molecular chain structure, which will reduce the tendency to crystallize and reduce the crystallinity of the polymer, while long chain branching will increase the viscosity of the polymer melt.
The cross-linked polymers cannot be melted after heating, nor can they be dissolved after the addition of a solvent, but they can swell in a solvent when the degree of cross-linking is not too high, and are thermoset materials. The formation of a crosslinked structure introduces inconveniences in the synthesis and processing of polymers. Since the polymer synthesis and molding process must be completed before the formation of a crosslinked network, otherwise, after crosslinking, the shape of the material cannot be changed by heating. But, on the other hand, crosslinking also endows polymeric materials with many excellent properties, such as greatly improved heat and dimensional stability, as well as improved solvent resistance.
4. Polymer chain configuration
A single-step configuration is a spatial geometric arrangement of atoms or groups of atoms fixed by chemical bonds in a molecule. To change the configuration, chemical bonds must be broken and recombined. There are two types:
(1) Geometric heterogeneity
The main chain of the molecule contains double bonds that cannot rotate inside, forming cis- and trans-configurations. are called geometric isomers.
In the example of poly-1,4-butadiene, two methylene groups are in the cis configuration on one side of the double bond and in the trans configuration on both sides of the double bond:
The cis form has a large distance between molecular chains and is an elastic rubber at room temperature; the transform is relatively regular and crystallizes easily, so it can only be used as a plastic.
When the four groups in a saturated hydrocarbon molecule are different, the carbon atom is called an asymmetric carbon atom, denoted by the letter C*. The compound can form two optical isomers (D-configuration and L-configuration), which are mirror images of each other.
The enantiomers of the molecules in fig. 1–3
α-olefin polymer, each structural unit has an asymmetric carbon atom C*, three positions:
①The polymer molecular chain stretches into a flat zigzag shape, and the R substituent of each structural unit can be located on one side of the plane, which is called isotactic;
② Two kinds of optical isomeric units are arranged alternately, which is called syndiotactic;
③ Two kinds of optical isomeric units are arranged randomly, which is called atactic.
Figure 1-4 Schematic diagram of the stereo configuration of an ethylene polymer
Isotactic and syndiotactic polymers are called stereoregular polymers, and the degree of stereoregularity can be expressed as the total percentage of isotactic and syndiotactic components in the polymer. This is called tact or isotactic.
Stereoisomerism has a great influence on the physical properties of polymeric materials.
Atactic PP is a rubber-like elastomer with poor mechanical properties and little practical value.
Isotactic or syndiotactic polypropylene crystallizes easily and has a high melting point. It can be spun into fibers and also used as a plastic.
Secondary structure of polymers - conformation and flexibility
Takes into account the conformation (i.e. spatial shape) and size (i.e. molecular weight) of a single polymer. Also known as remote structure.
1. The size of the molecular chain (i.e. the size and distribution of molecular weight) will be presented in Chapter 4 "Properties of Polymer Solutions".
2. Conformation refers to the different geometry of the molecular chains of a polymer in space due to the internal rotation of the single bonds. Macromolecular chains represent various spatial geometric shapes, i.e., various conformations. Representative conformational states include extended chain conformation, random coil conformation, folded chain conformation, and helical chain conformation.
Conformation differs from configuration, and conformations can be transformed into each other due to the internal rotation of a single bond. The configuration is fixed by chemical bonds, and only by breaking and recombining chemical bonds can the configuration be changed.
3. BasicsKey Factors Affecting Polymer Chain Flexibility
Flexibility is the property of a polymer chain to continuously change its conformation.
A flexible chain is a molecular chain with good flexibility.
Rigid Chain ---- A molecular chain with poor flexibility.
The flexibility of a chain depends on the complexity of the internal rotation of a single bond. The main influencing factors are:
(1) Backbone structure - plays a critical role in polymer chain flexibility
A polymer made up of all the single bonds in the backbone is relatively flexible. Such as PE, PP, POM, EPDM, etc.
Si-O single bond flexibility > C-N > C-O> C-C Aliphatic polyester, polyester, polyurethane, polyamide, polysiloxane flexibility.
The backbone of diene polymers contains isolated double bonds and has good chain flexibility, resulting in the formation of polybutadiene and polyisoprene.
Conjugated double bonds in the main chain with a benzene ring structure reduce flexibility and increase rigidity (eg, polyphenylene ether, PC, PSF).
(2) Side Group Substitute
The stronger the polarity and the greater the number of side groups, the greater the interaction and the less flexible the chain.
Substituent polarity -CN>-Cl>-CH3>-H, chain flexibility PE>PP>PVC>PAN.
The more substituents, the more difficult the internal rotation and the less flexible the chain.
For example, flexible polychloroprene>PVC>polyvinyl dichloride. PMA>PMMA.
Symmetry of substituents will increase the distance between molecular chains, facilitate rotation within a single bond, and chain flexibility will become better. For example, flexible polyisobutylene>PP, polyvinylidene chloride>PVC.
The larger the volume of the non-polar side group, the greater the steric hindrance, which is unfavorable for the internal rotation of the single bond, the worse the chain flexibility and the increased chain rigidity, for example, the volume of the substituent -C6H5 > -CH3 > -H, polymer chain Flexibility PE > PP > PS.
(3) Stitching - degree of stitching
Slight cross-linking without significant change in chain flexibility. However, due to the strong crosslinking, the flexibility of the chain is greatly reduced.
Therefore, rubber can retain good elasticity after moderate crosslinking, but lose elasticity and become hard and brittle when strongly crosslinked.
(4) Molecular weight
Polymer flexibility increases with molecular weight
(5) Intermolecular force - hydrogen bond
Hydrogen bonds form within molecules or between them, increasing the rigidity of the molecular chain. Chain flexibility is reduced
As the temperature rises, the chain becomes more flexible. For example, PMMA, room temperature, hard chain (plastic), T>100°C, exhibits soft elasticity; SBR, room temperature, very flexible (rubber), T<-80°C, tough chain, hard and brittle.
(7) Influence of external force on chain compliance
When inexternal force acts slowly, it is easy to show flexibility; the external force acts quickly and the flexibility cannot be repelled.
1.3 Aggregate Structure of Polymer Resin - Tertiary Structure
Aggregate structure refers to the structure formed by the arrangement and stacking of polymer chains within a polymer material. Aggregate structures are usually formed during the molding process. Although the structure of the polymer chain plays a critical role in the basic properties of polymers, the characteristics of polymer materials are mainly dependent on the aggregate structure of polymers.
First, intermolecular forces in polymers
1. Intermolecular forces of polymers
The absence of a gaseous state in polymer resins is one of the important features that distinguish polymers from small molecules. Since the molecular weight of the polymer is large, the molecular chain is long, and there is a large intermolecular force that exceeds the binding energy of the chemical bonds that form it.
The strength of the intermolecular interaction of polymers is usually expressed as "cohesive energy" or "cohesive energy density".
Cohesive Energy (CE): The energy required to overcome intermolecular interactions and evaporate 1 mole of liquid or solid molecules:
Cohesive energy density (CED): cohesive energy per unit volume
is the molar volume.
The cohesive energy density of different types of polymers varies greatly.
Second, the amorphous state of the polymer resin
The amorphous state of the resin includes three types: a polymer body that cannot crystallize at all, an amorphous region of a partially crystalline polymer, and an amorphous solid that freezes when the crystalline polymer melt is quenched. From the structural point of view, the amorphous state of the polymer is the same as the amorphous state of small molecules, and the structural unit of the polymer chain is the object of observation. Many experiments have confirmed that the molecular chains of amorphous polymers do indeed exist as random coils, whether in the solid state or in solution.
3. Crystalline state and structure of polymer resin
1. Basic crystal shape
The aggregate structure of a polymer includes not only an amorphous structure, in which the molecular chains are randomly arranged and stacked on top of each other, but also a crystalline structure, in which the molecular chains are arranged regularly. Later, the use of electron microscopy revealed various crystalline forms of polymers, including single crystals, spherulites, dendrites, etc. The order of the amorphous state of a polymer is higher than that of the amorphous state of a small molecule.
If the polymer film is subjected to high-speed extrusion and quenching, a string crystal structure, modulus and transparency appear↑↑
In conventional crystalline polymers, there are both crystalline and amorphous regions. Bulk (or volumetric) pThe percentage of the crystal region is called crystallinity and is expressed in terms of fc (or fc).
3. Crystallization ability of polymer resin
Required conditions: Symmetry and regularity of the molecular structure, the main structural factors of the crystallization ability of the resin.
The higher the symmetry of a molecular chain, the better its regularity and the easier it is to arrange it correctly to form a highly ordered lattice.
The molecule is rigid, but the molecule is too flexible, which also does not promote crystallization.
Four, orientationPolymer state structure
1. Orientation phenomenon - the size ratio of the polymer chain is very large. Under the action of an external force field, molecular chains, segments, and crystal grains in a crystalline polymer are predominantly located along the direction of the external force. , forming an "orientation".
Orientation increases Tg, the density and crystallinity of the crystalline polymer after orientation increase, and the operating temperature increases.
5. The structure of the interweaving of polymer composites
With a simple process, two or more homopolymers, copolymers, or the same polymers with different molecular weights and different molecular weight distributions are mixed to improve certain material properties, which is called polymerization blending of materials.
The purpose of mixing: to improve the physical-mechanical properties, electrical properties and technological properties.
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