Low temperature plasma has the advantages of low reaction temperature, simple and convenient operation, economic practicality, high efficiency, and no pollution. More importantly, the effect of low-temperature plasma treatment is limited to the surface of the material and does not affect the physical and chemical properties of the substrate. Since the late 1960s, the application of plasma technology in the field of polymers has focused on three main areas: plasma polymerization, plasma induced polymerization, and plasma surface modification and treatment of polymer materials.

Plasma polymerization

Plasma polymerization is one of the main applications of plasma technology in the field of polymers. It uses discharge to ionize organic gaseous monomers, producing various active species. The addition reaction between these active species or between active species and monomers forms a polymer film. It is a polymerization reaction that occurs when monomers are in a plasma state and is a new method for depositing polymer thin films. This process is also known as plasma vapor deposition technology (PPCVD).

There are several issues with plasma polymerization, including:

1. The basic reactions in plasma polymerization are extremely complex, and the polymerization mechanism is not clear.

2. Structural analysis of plasma polymerization products is difficult. The results of testing soluble/molten oily or powdery products will vary greatly depending on the structure of the film.

3. The structure and properties of plasma polymerization products are heavily dependent on the form of the reaction apparatus and polymerization process conditions.

Plasma polymerization has the following characteristics:

(1) Plasma polymerization does not require monomers to contain unsaturated units, nor does it require the presence of two or more functional groups. Even reactions that cannot be carried out under normal circumstances or are very difficult to carry out can be conveniently, simply, and smoothly carried out in plasma polymerization systems, greatly expanding the types and application scope of substances.

(2) The obtained polymer is allowed to have no repeating structural units and can only have branched and high-density network structures, while the network structure and branching degree are controllable.

(3) Plasma polymerization, as a "dry" process technology, is simple, convenient, flexible, and efficient to operate, and can obtain polymerization products that are difficult to obtain by conventional methods.

(4) The plasma polymerization device can be a glow discharge, corona discharge, or other types of discharge methods. But there is a limitation - the generated polymer cannot be decomposed due to high-energy discharge. Due to the fact that the main form formed by plasma during discharge is electrons, and the energy distribution and electron density determine the state of electrons. When electrons collide with molecules attached to solid surfaces or molecules present in the gas phase, they can form active groups such as excited atoms, free radicals, molecules, ions, and so on. There are currently five representative mechanisms for plasma polymerization reactions: free radical mechanism, ion polymerization mechanism, RSGP stepwise growth model, CAP polymerization and ablation competition model, and AGM activation growth model.

Plasma initiated polymerization

Plasma initiated polymerization is a new polymerization method that uses plasma as an energy source to irradiate different monomers for a short period of time (seconds to minutes). Low temperature plasma technology uses vapor phase reactions of monomers to form active centers, which are then placed at an appropriate temperature to initiate monomer polymerization. It is a new polymerization method that does not require initiators.

One of the advantages of plasma induced polymerization is that it can generate pure polymers with a molecular weight exceeding 1000000 in a simple, efficient, and convenient manner. However, conventional free radical induced polymerization is difficult to achieve this. For example, after glow plasma discharge for a certain period of time, methyl methacrylate (MMA) can undergo polymerization reaction in a constant temperature water bath (25 ℃) in the absence of light for several days. When the conversion rate reaches 20%, it can generate polymer polymethyl methacrylate (PMMA) with a molecular weight of 2.7 × 107, and the molecular weight of PMMA gradually increases with the increase of monomer conversion rate.

The main characteristics of plasma induced polymerization (PIP) are as follows:

Firstly, compared with conventional free radical polymerization methods, plasma induced polymerization uses plasma as the energy source and does not require external initiators to prepare products that are harmless to living organisms, environmentally friendly, and of high quality and efficiency.

Secondly, the PIP method has high selectivity for monomers. Currently, only three types of monomers can undergo plasma induced polymerization: water-soluble olefins, styrene and its derivatives, and cyclic compounds. The structure and properties of the monomer itself have a significant impact on whether it can undergo plasma induced polymerization.

Thirdly, only chain initiation occurs in the gas phase of the plasma during the reaction process, while other elementary reactions occur in the condensed phase. The obtained polymer product is generally a linear ultra-high molecular weight polymer, which maintains the monomer structure and has excellent and stable polymer properties.

Fourth, unique aggregation mechanism

Although plasma induced polymerization is generally considered a free radical process, certain phenomena that occur during the polymerization process, such as strong solvent effects, extremely high selectivity and harshness towards monomers, and the generation of ultra-high molecular weight polymer composites, cannot be explained and detailed by the mechanism of free radical polymerization.

Fifthly, solvents have a significant impact on the process of plasma induced polymerization due to the presence of solvation effects. In plasma, solid-state monomers are difficult to directly initiate polymerization, but after specific conditions of plasma radiation, the free radical active sites present on the monomers can be preserved for a long time. When solvent water is added, polymerization reactions occur rapidly, resulting in ultra-high molecular weight polymer materials.

Sixth: The aggregation process is mainly characterized by active polymerization, and the active sites generated can last for weeks or even months.

Plasma surface modification

Plasma has the characteristics of high efficiency, cleanliness, time-saving, environmental protection, and no damage to the properties of the material matrix, and is widely used in the field of surface modification of various materials. Low temperature plasma can smoothly modify the surface of polymer materials in both reactive and non reactive gas atmospheres.

The energy of particles in low-temperature plasma is generally several eV to tens of eV, which is greater than the binding energy of polymer materials (several eV to tens of eV), and can completely break the chemical bonds of organic macromolecules to form new bonds; But its energy is much lower than high-energy radioactive rays, so it only involves the surface of the material and does not affect the performance of the substrate. By utilizing the characteristic of low-temperature plasma, surface modification of materials can be carried out. Through low-temperature plasma surface treatment, the material surface undergoes multiple physical and chemical changes, or becomes rough due to etching, or forms a dense cross-linked layer, or introduces oxygen-containing polar groups, resulting in improved hydrophilicity, adhesion, dyeability, biocompatibility, and electrical properties. Due to the advantages of simple process, convenient operation, fast processing speed, good treatment effect, low environmental pollution, and energy saving, the research and application of low-temperature plasma in material surface modification have shown strong vitality in recent years and are in a period of vigorous development.