Plasma is a quasi-neutral gas, known as the fourth state of matter, that is, the composition of the gas molecules split to produce free electrons, free radicals, positive and negative ions, electromagnetic radiation quantum, and some molecules may still remain neutral. Plasmas contain a variety of active particles, such as electrons, ions, free radicals, transferable excitation materials, and vacuum ultraviolet radiation. According to the thermodynamic equilibrium state of the material in the plasma, the plasma can be divided into high temperature plasma and low temperature plasma.

Low-temperature plasma treatment can embed a variety of molecular chains on the surface of polymer molecules, giving them hydrophilicity, hydrophobicity and adhesion. At the same time, the low-temperature plasma only acts on the nano-scale depth of the material surface, and does not cause damage to the polymer material matrix, so it is suitable for the surface modification of polymer materials.

Polylactic acid ( PLA ) is a thermoplastic biodegradable material prepared by polymerization of lactic acid, which is produced by fermentation of potato, beet, sugarcane, corn and other agricultural by-products. PLA has good film-forming properties. Although the tensile strength of PLA film without plasticizer can reach 50 ~ 60 MPa and the elongation at break is 3 % ~ 4 %, the PLA film has poor toughness and brittleness. Although PLA film has good water vapor barrier property, its oxygen barrier property is worse than that of petroleum-based plastic packaging materials such as PE and PET, which seriously restricts the application of PLA film in the field of food packaging.

The change of hydrophilicity of PLA film treated by plasma

The contact angle is the angle formed by the intersection of the liquid-solid interface and the liquid-gas interface. When the surface contact angle of the material is θ90 °, it indicates that the material is hydrophobic. The contact angle change of PLA film surface treated by low temperature plasma is shown in figure 1. The contact angle of untreated PLA film is 90 °. With the prolongation of low temperature plasma treatment time, the contact angle of PLA film surface decreases gradually, and the hydrophilicity of material surface increases continuously. When the discharge voltage is 125 V and the discharge time is 40 s, the surface contact angle of PLA film reaches the minimum value of 42.3 °. Under different discharge voltages, the effect of low temperature plasma treatment on the hydrophilicity of PLA film surface is different. When the discharge voltage is 125 V, the contact angle of PLA film surface is the smallest and the hydrophilicity of PLA film surface is the best.This may be due to the excessive energy of the active particles generated when the discharge voltage is too large, which destroys the structure of the PLA film surface and reduces the modification effect. At a fixed voltage, the contact angle of the PLA film gradually decreases with the extension of the treatment time, and the hydrophilicity is improved. When the treatment time is more than 40 s, prolonging the plasma treatment time cannot significantly enhance the hydrophilicity of the membrane surface. It may be due to the fact that after the plasma treatment reaches a certain time, the etching of the plasma on the PLA surface causes the hydrophilic group on the new graft to be detached, and the physical etching and chemical reaction caused by the plasma tend to be saturated, so the contact angle no longer changes.

Fig.1 Change of surface contact angle of PLA film treated by plasma

X-ray photoelectron spectroscopy analysis

X-ray photoelectron spectroscopy is an effective surface analysis technique for analyzing the chemical structure of various elements on the surface of materials. The chemical structure information of the surface can be analyzed by using the binding energy of the C1s peak of the carbon element in the PLA film. The carbon atoms in the polymer can be divided into three binding forms of C1s1, C1s2 and C1s3, and three corresponding energy spectrum peaks are shown in the XPS energy spectrum of C1s. C1s1 is C-H or C-C, and its electron binding energy is low, about 285.0 eV. C1s2 is C-O, and the energy spectrum peak is located at about 286.5 eV. C1s3 is C = O, and its energy spectrum peak is located at about 288.0 eV.

The XPS full spectrum scan spectrum and XPS-C1 s spectrum of the PLA film are shown in Figure 3-4, respectively. It can be seen from Fig.3 that the oxygen content of PLA film increased after low temperature plasma treatment. It can be seen from Fig.4 that although the basic types of chemical bonds of PLA remained unchanged after plasma treatment, the relative content changed. After treatment, the content of C-C bond and C-H bond of PLA film decreased, while the content of C-O bond and C = O bond increased. The mechanism of plasma treatment on the surface of PLA film is shown in Fig.5.In the process of plasma modification of PLA film, the C-H bond in the methyl CH4 group on the surface of PLA was destroyed, resulting in the detachment of H atom, which reacted with the reactive oxygen species produced by O2 in the process of plasma treatment to form OH or HO-C = O plasma state free radicals. The free radicals recombined on the methylene CH3 group on the surface of PLA, thus forming hydrophilic groups such as hydroxyl and carboxyl groups on the surface of PLA film. The content of functional groups on the surface of PLA film is shown in Table 2. It can be seen that after plasma treatment, the mass fraction of carbon element on the surface of PLA film decreased from 77.87 % to 55.92 %, and the mass fraction of oxygen element increased from 21.63 % to 44.08 %. After low temperature plasma treatment, the content of hydrophilic groups such as hydroxyl and carboxyl groups on the surface of PLA film increased. The mass fraction of hydroxyl increased from 25.9 % to 36.0 %, and the mass fraction of carboxyl increased from 10.3 % to 28.3 %, thus improving the surface hydrophilicity of PLA film.

Fig. 2 XPS full spectrum scanning pattern of PLA film

Fig 3 XPS-C 1s spectra of PLA film

Fig. 4 Mechanism of plasma treatment on the surface of PLA film

In summary : After low-temperature plasma treatment, the oxygen content on the surface of the PLA film was increased. This is because the hydrophilic groups such as hydroxyl and carboxyl groups were grafted on the surface of the PLA film, thereby improving the hydrophilicity of the film surface.