Plasma refers to ionized gases, which are aggregates composed of particles such as electrons, ions, atoms, molecules, or free radicals. Low temperature plasma refers to ionized gases with temperatures below tens of electron volts. The energy of particles in low-temperature plasma is generally several to tens of electron volts, which is greater than the binding energy of polymer materials (several to tens of electron volts), and can completely break the chemical bonds of organic macromolecules to form new bonds; It only involves the surface of the material and can be treated on the surface of various shapes of materials without affecting the properties of the substrate material. Therefore, low-temperature plasma has the characteristics of environmental protection, low reaction temperature, fast speed, and high particle energy. Nowadays, it has been widely applied in industry.

Analysis of Surface Hydrophilicity and Timeliness

Plasma treatment can activate the surface of polyethylene (PE), thereby improving its hydrophilicity. The relationship between different plasma treatment times and contact angles is shown in Figure 1. The contact angle of the untreated sample surface is 100.7 °. In a short period of time, as the processing time increases, the contact angle sharply decreases. At 10 seconds, the contact angle drops to 66.3 °, and at 180 seconds of plasma activation time, the contact angle drops to 49.4 °. This is because when the polymer surface is subjected to plasma treatment for a short period of time, hydrogen peroxide is mainly formed on the surface; As processing time increases, these hydrogen peroxides further decompose to form highly reactive hydroxyl radicals and alkanes; Finally, due to the hydrogen extraction on the polymer molecular chain, various polar groups such as carbonyl (C=O), carboxyl (- COOH), hydroxyl (- OH), etc. are formed on the polymer surface, making the surface increasingly hydrophilic. The contact angle did not show significant changes within the processing time of 120s~600s, with values ranging from 51 °~58 °. This may be due to a relative equilibrium between the free radicals generated on the polymer surface and those consumed after processing for more than 120s.

Figure 1a) Schematic diagram of plasma activation; b) The relationship between plasma treatment time and contact angle; c) The variation of contact angle with time after plasma activation

In order to investigate the stability of the plasma treatment method itself, the experiment examined the change of contact angle with time after plasma treatment. Due to the fact that the depth of plasma treatment on the material surface can only reach a range of a few nanometers to a few hundred nanometers from the material surface, this modification method cannot obtain a stable modified surface, and the number of polar groups generated on the material surface is also relatively high. From Figure 1, it can be seen that with the passage of time, the contact angle gradually increases, and the modification effect will be partially lost. The main reason for this phenomenon is that: (1) the surface of the processed material is still in a high-energy metastable state due to electron collisions and energy accumulation. This unstable high-energy state will inevitably lead to the release of energy, resulting in natural recovery; (2) The surface of the material after plasma activation treatment is exposed to air, and the surface energy decreases due to the adsorption of small molecules in the air; (3) The polar groups introduced into the macromolecular chain penetrate into the body from the surface as the macromolecular chain rotates freely, that is, the modified surface is buried.