Plasma ablation polymerization

The polymer formation and properties of polymers formed by glow discharge polymerization are controlled by the balance among plasma-induced-polymerization, plasma state polymerization, and ablation. 

Because polymer formation can proceed through more than one major type of reaction, i. e., plasma-induced polymerization and plasma state polymerization, depending on the chemical structure of the monomer and also on the conditions of discharge, such as discharge wattage, flow rate, type of discharge, and geometrical factors of the reactor, the balance between polymer formation (polymerization) and ablation is for most cases extremely complicated. 

The monomer chosen is hexafluoroethane, which cannot be polymerized by plasma-induced polymerization and which cannot be polymerized in a glow discharge imder ordinary conditions presumably because the ablation process associated with the glow discharge is excessive. Attempts have been made to supress the ablation process and to shift the balance between plasma state polymerization, which is assumed to be present, and ablation. However, it has been observed that polymer formation for hexafluoroethane does occur when polyethylene is used as substrate. On the other hand, no polymer formation can be observed either with ESCA or by surface energy analysis when glass is used as a substrate. 

Plasma-Induced Plasma State Polymerization Polymerization Ablation... 

This indicates that ablation is no loiter dominant, and polymer formation prevails. This phenomenon can be explained by postulating that Hg reacts with F atoms emanating from the fluorine containing compound in the glow discharge and forms the more stable HF, which reduces the ablation in a dramatic manner. Because hexafluoroethane does not form a polymer by plasma-induced polymerization, the overall effect can be explained by the balance between plasma state polymerization and ablation. 

Figure 5. Competitive ablation and polymerization (CAP) and plasma induced polymerization (PIP) mechanisms.

The parameters of treatment were chosen since these led to the most pronounced changes of polymer surface in our previous experiments [70-74]. It was observed elsewhere that plasma treatment of polymer macromolecules results in their cleavage, ablation, alterations of chemical structure and thus affects surface properties e g. solubility [75]. The chemical structure of modified polyethylene (PE) was characterized by FTIR and XPS spectroscopy. Exposition to discharge leads to cleavage of polymeric chains and C-H bonds followed by generation of free radicals which easily oxidize [10,76]. By FTIR spectroscopy the presence of new oxidized structures within whole specimen volume can be detected. IR spectra in the 1710-1745 cm" interval [71,77] from PE, exposed to... 

Unexpected elements in a plasma polymer often are due to the redeposition of ablated materials. The presence of nitrogen found in a plasma polymer of a monomer that does not contain nitrogen can be traced to contamination of the reactor, which has been used for plasma polymerization of nitrogen-containing monomers [1]. The ablation of electrode material has been utilized to create a graded metal-polymer and polymer-metal interfaces to obtain an excellent adhesion [2,3]. Ablation, therefore, could be utilized in a beneficial way in the engineering of interfaces if we know the nature of ablation and how to control it. 

The creation of reactive species that cause ablation is essentially the same process as that occurs in LCVD, except that the final result is completely opposite, i.e., ablation vs. deposition. In this context, ablation by luminous gas could be described as luminous chemical vapor treatment (LCVT). Therefore, the dependence of ablation on operational parameters in LCVT is very similar to that of LCVD, which is discussed in more detail in Chapter 4. The chemical ablation of polymeric materials by O2 plasma [6] is described here to demonstrate how oxidative ablation is influenced by the operational parameters of discharge. 

Because solid materials must maintain the vacuum, the reactor wall always exists in an LCVD reactor. The plasma also interacts with wall materials as well as any other materials that exist in the plasma, such as substrate and support. Therefore, polymer-forming intermediates and gaseous by-products may also originate from solid materials with which plasma interacts by virtue of the ablation caused by the luminous gas. In this sense, any material that interacts with plasma becomes a source of monomer for plasma polymerization. 

Because polymer formation and ablation are competitive and opposing processes, polymer-forming plasma has the least ablative effect however, ablation in such plasmas cannot be completely ruled out. Sputtering of metals used as the internal electrodes for plasma polymerization has been recognized as a contamination of plasma polymers. Under certain conditions, the sputtering of the electrode materials becomes significant and plays an important role in the engineering of interface as described in Chapter 9. 

What happens in a low-pressure plasma process cannot be determined in an a priori manner based only on the nature of the plasma gas or on the objective of the process. The plasma sensitivity series of elements involved, in both the luminous gas phase and the solids, that make contact with the luminous gas phase seems to determine the balance between ablation and polymerization by influencing the fragmentation pattern of molecules in the luminous gas environment. 

Material deposition occurs via plasma formation of reactive species however, it is not a simple step of forming a polymeric material from a set of reactive species. The reactive species do not necessarily originate from the monomer because the ablation process can and does contribute. Gaseous reactive species can originate from once-deposited material (plasma polymer) and also from the reactor wall or any other solid surfaces that are in contact with the luminous gas. How these complex reactive species lead to the material deposition is described in Chapter 5. 

Figure 10.1 Schematic diagram of the Competitive Ablation and Polymerization (CAP) principle (1) dissociation (ablation) of monomer to form reactive species, (2) deposition of plasma polymer and ablation of solid including plasma polymer deposition, (3) deposition to and ablation from nonsubstrate surfaces, and (4) removal of stable molecules from the system.

The effect of gold particles on plasma-polymerized fluorocarbon films was examined by Creasy ef al. Laser ablation of gold-containing films produced ions that were similar to those formed in pyrolysis. This observation suggests that gold particles absorb energy, conductively heat the polymer, and affect the mechanism of the ablation. 

Creasy, W. R., Zimmerman, J. A., Jacob, W., and Kay, E., "Pyrolysis and Laser Ablation of Plasma-Polymerized Fluorocarbon Films Effects of Gold Particles," /. Appl. Phys., 6, 2462-2471,1992. 

This work reports the development of a polymeric/sol-gel route for the deposition of silicon carbide and silicon oxycarbide thin films for applications such as heat-, corrosion-, and wear-resistant coatings, coatings on fibers for controlling the interaction with the matrix in ceramic matrix composites, or films in electronic and optoelectronic devices. This method, in which the pre-ceramic films are converted to a ceramic coating either by a conventional high temperature annealing or by ion irradiation, is alternative to conventional methods such as chemical or physical vapor deposition (CVD, PVD), molecular beam epitaxy, sputtering, plasma spray, or laser ablation, which are not always practical or cost efficient. 

Plasma treatment of microchannels can be useful for improving the functionality of microdevices. For example, previous studies have shown that PDMS microchannels can be made hydrophilic by the addition of silane molecules with polar head groups [6]. In this process (3-mercaptopropyl)trimethoxysilane (3-MPS) was absorbed to PDMS to increase the hydrophilic properties of microchannels. Additionally, plasma polymerization has been used to induce in the long-term hydrophilic surface modification by covalently bonding a polymer layer to the surface. Barbier et al. [7] describe a method based on plasma polymerization modification with acrylic acid coatings. First, argon plasma pretreatment was used to activate trace oxygen molecules in the chamber, which partially oxidize the top layer of the substrate. This step cross-linked the surface to reduce ablation of silicon... 

---