Plasma polymer gases

A Perkin-Elmer instrument (TGA 7) was used for the thermal analysis. The heating temperature was varied from 50 to 600°C at a rate of 10°C/min, and air was used as purging gas. These measurements provide information about the weight of the plasma-polymer coating per unit weight of the substrate. 

A great deal of research has been focused on the evaluation of plasma polymers and plasma treated materials for blood and soft tissue contacting-applications (2,3). A number of studies have involved the physical adsorption or covalent attachment of a variety of biomolecules to various gas plasma-treated polymer surfaces (4,5). In such studies, however, the covalent immobilization is often assumed to take place through precursor groups formed at the biomaterial surface from ill-defined oxygen and nitrogen functionalities obtained directly from the plasma. 

Plasma polymer layers were deposited in the same reactor as described before. However, in this case, the pulsed plasma mode was applied. The duty cycle of pulsing was adjusted generally to 0.1 and the pulse frequency to 103Hz. The power input was varied between P 100 ()() V. Mass flow controllers for gases and vapours, a heated gas/vapour distribution in the chamber, and control of pressure and monomer flow by vaiying the speed of the turbomolecular pump were used. The gas flow was adjusted to 75-125 seem and the pressure was varied between 10 to 26 Pa depending on the respective polymerization or copolymerization process. The deposition rate was measured by a quartz microbalance. 

The gases, obtained from Matheson Gas,were passed through a sodium fluoride column to remove trace amounts of hydrogen fluoride. No effort was made to remove small amounts of water and oxygen that may have been present in the incoming gases. The polymer films were treated at the plasma bulk gas temperature which was close to ambient for all reactions. Visual observation of the films did not shew any melting or deformation and hence it was assumed that if the surface temperatures did vary, the fluctuations were small. 

In the middle of a spectrum, the gain in one feature is attained on sacrifice of another feature. Therefore, one must choose a plasma polymerization process, including type of reactor, reaction conditions, and type of monomer (starting gas or vapor), aimed at a specific type of plasma polymer i.e., type A or type B, suitable for an application. 

Low-temperature plasma processes, such as gas plasma treatment and plasma polymerization, have unique advantages in that active (depositing) species strongly interact with the surface of the substrate and modify the surface state. An ultrathin layer of plasma polymer, e.g., thickness less than 50 nm, can be viewed as a new surface state because such a thin layer does not develop a characteristic bulk... 

No deposition of materials occurs in most cases however, the deposition of plasma polymer could occur depending on the nature of substrate polymer. Such a deposition of materials can be viewed as PP of organic vapors, which emanated from the substrate, by the interaction with plasma. Because the major player is the luminous gas phase, the surface treatment is included in this book under the term luminous chemical vapor treatment (LCVT). 

TMS deposition on PE showed only substrate signals with no detectable TMS signal (Fig. 6.12b). The absence of the TMS signal in this system could be due to the fast reaction of TMS radicals with the surface radicals generated from PE. The more likely explanation is that the number of free radicals in the plasma polymer layer is too small in comparison with the free radicals created in the bulk of the substrate, PE. What we see in Figure 6.13 is the decay of PE polymer free radicals, which were created by the luminous gas of TMS. With substantial decay of the PE free radicals, TMS dangling bonds, which decay much slower, became discernible. 

Acetylene (HCsCH) and benzene are very similar in their LCVD characteristics. Both compounds form plasma polymers with the least amount of hydrogen production (type I monomer), and their characteristics of copolymerization with N2 and/or H2O are nearly identical if we consider that one molecule of benzene is equivalent to three molecules of acetylene. Analysis of the gas phase in both closed and flow systems are given in Tables 7.5 and 7.6. 

Although numerous kinds of reactions could occur in the luminous gas phase, as far as the dissipation of vapor phase molecules (LCVD deposition) is concerned, one benzene molecule behaves as three acetylene molecules. Consequently, the final polymers formed (from acetylene and benzene) under the condition of relatively high WjFM are very similar. The transport characteristics of ultrathin films of plasma polymers and copolymers (with N2 and/or H2O) of acetylene and benzene are nearly identical. 

Figure 7.7 depicts type of plasma polymer of TFE depending on the location in a small tube reactor [7]. In the tubular reactor shown, the formation of F would occur at the upstream side of the reactor, where the monomer flow makes contact with the luminous gas phase of TFE. Then, the — CF3 could be used as a labeled species or an indicator of the change in the chemical nature of the polymer due to the kinetic pathlength of a growing species. The XPS data obtained with polymers... 

Another important and unique feature of plasma polymerization is the incorporation of gases that do not form polymer or solid deposits in plasma by themselves during polymer formation of organic molecules in plasma. This incorporation of gases is plasma copolymerization and not the trapping of gas molecules in plasma polymers. 

Figure 7.12. The partial pressure of N2 in each experiment is shown as a horizontal line crossing the pressure-decay curve. As can be seen, the system pressure decreases beyond the partial pressure of N2 in a mixture, indicating that N2 is incorporated into the plasma polymer (and thus disappears from the gas phase). Such a copolymerization of an unusual monomer has been observed for N2, CO, and H2O and is particularly efficient with monomers containing triple bond(s), double bond(s), or aromatic structure(s) (type I and type II monomers).

Since the dissociation glow can be considered to be the major medium in which polymerizable species are created, the location of the dissociation glow, i.e., whether on the electrode surface or in the gas phase, has the most significant influence on where most of the LCVD occurs. The deposition of plasma polymer could be divided into the following major categories (1) the deposition that occurs to the substrate placed in the luminous gas phase (deposition G) and (2) the deposition onto the electrode surface (deposition E). The partition between deposition G and deposition E is an important factor in practical use of LCVD that depends on the mode of operation. 

In 1972, Liepins and Sakaoku [7] reported that polymeric powders were formed nearly exclusively in the radio frequency reactors shown in Figures 8.12 and 8.13, in which an organic vapor was introduced into the glow discharge of a carrier gas. The monomers that formed powders nearly exclusively and the yield of powder formation are summarized in Table 8.1. Monomers that did not form powders exclusively (i.e., formed plasma polymer in the form of a film or a film with powders) are shown in Table 8.2. The significant points about these experiments are as follows ... 

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. 

The way in which a plasma polymer is formed has been explained by the rapid step growth polymerization mechanism, which is depicted in Figure 5.3. The essential elementary reactions are stepwise recombination of reactive species (free radicals) and stepwise addition of or intrusion via hydrogen abstraction by impinging free radicals. It is important to recognize that these elementary reactions are essentially oligomerization reactions, which do not form polymers by themselves on each cycle. In order to form a polymeric deposition, a certain number of steps (cycle) must be repeated in gas phase and more importantly at the surface. The number of steps is collectively termed the kinetic pathlength. 

Some oligomers formed in LCVD process of F-containing gas, which adhere to some surfaces (metal oxides) by chemisorption, cannot be pumped out under vacuum, even under the high vacuum used for XPS. Consequently, XPS analysis depicts plasma oligomers as indistinguishable from plasma polymers unless detailed analysis of the interface is performed. 

These findings clearly show that the principle of CAP also applies to glow discharge treatment, which is not intended to deposit plasma polymers, in a similar manner with respect to the interaction of luminous gas with materials. 

The internal stress of plasma polymers is dependent not only on the chemical nature of monomer but also on the conditions of plasma polymerization. In the plasma polymerizations of acetylene and acrylonitrile, apparent correlations are found between and the rate at which the plasma polymer is deposited on the substrate [2], as depicted in Figure 11.3. The effect of copolymerization of N2 and water with acetylene on the internal stress is shown in Figures 11.4 and 11.5. The copolymerization with a non-polymer-forming gas decreases the deposition rate. These figures merely indicate that the internal stress in plasma polymers prepared by radio frequency discharge varies with many factors. The apparent correlation to the parameter plotted could be misleading because these parameters do not necessarily represent the key operational parameter. 

In general plasma polymerization processes it has been established that the deposition rate and properties of a plasma polymer primarily depend on the value of the normalized energy input parameter WjFM, as described in Chapter 8. In LPCAT polymerization processes, as described in Chapter 16, the deposition rate of a plasma polymer primarily depends on the value of the normalized energy input parameter, which is given by W FM)J FM). In this composite parameter, W is the power input applied to arc column, FM) is the mass flow rate of carrier gas (argon), and FM) is the mass flow rate of monomer that is injected into the cascade arc torch. The quantity of W FM)J FM) can be considered as the energy, which is transported by carrier gas plasma, applied to per mass unit of monomers. 

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