Plasma polymer layers

A core-shell structure can be seen in Fig. 16, which shows a cross-section of about 100 nm of sulfur aggregate encapsulated with poly acetylene. The plasma polymer layer is rather coherent while in other cases loose structures are also observed. 

A multi-microsensor array of potentiometric MIP chemosensors has been devised for determination of a serotonin neurotransmitter [180]. In the toluene porogenic solvent solution, the MAA functional monomer and the EGDMA cross-linker were polymerized in the presence of the serotonin hydrochloride template (Table 6). Subsequently, the resulting MIPs were immobilized on a plasma polymer layer by swelling and polymerization. Plasma polymerization was performed using styrene or ethylbenzene as the monomer. The chemosensor fabricated that way was appreciably responsive to serotonin while selectivity to serotonin analogues, like acetaminophen... 

Deposition of Functional Groups Bearing Plasma Polymer Layers... 

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. 

Table 1 Absolute and relative yields in functional groups at the surface of deposited pulsed plasma polymer layers measured with XPS after derivatization (cf. Experimental, 100 W)...

Fig. 10 Covalent bonding of fluorophores onto primary amino groups of allylamine pulsed plasma polymer layers (a - fluorescein bonding b - dansylhydrazine bonding c - dansylcadaverine bonding)...

A nanofilm of plasma polymer (up to about 100 nm) has sufficient electrical conductance as evidenced by the fact that an LCVD-coated metal plate can be coated by the electrolytic deposition of paint (E coating), i.e., plasma polymer-coated metals can be used as the cathode of the electrolytic deposition of paint (see Chapter 31). Thus, the plasma polymer layer remains in the same electrical potential of the cathode (within a limited thickness) and the work function for the secondary electron emission does not increase significantly. When the thickness of plasma polymer deposition increases beyond a certain value, the coated metal becomes eventually insulated, and DC discharge cannot be sustained. DC cathodic polymerization is primarily aimed to lay down a nanofilm (10-100 nm) on the metal surface that is used as the cathode (see Chapter 13). 

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. 

In Figure 10.5, XPS cross-sectional profiles of two plasma polymers are compared (1) that of the paint that peeled off and (2) that of a normal sample of well-adhered plasma polymer layers without primer. [The XPS data for (2) were... 

Thus, recognition of the characteristic internal stress buildup in a plasma polymer is important for estimating the upper limit of thickness of a plasma polymer for a practical application. Poor results with respect to such parameters as adhesion and barrier characteristics are often due to the application of too thick a plasma polymer layer. The tighter the network of plasma polymer, the higher is the internal stress. Consequently, the tighter the structure, the thinner is the maximal thickness... 

The expansive internal stress in a plasma polymer is a characteristic property that should be considered in general plasma polymers and is not found in most conventional polymers. It is important to recognize that the internal stress in a plasma polymer layer exists in as-deposited plasma polymer layer, i.e., the internal stress does not develop when the coated film is exposed to ambient conditions. Because of the vast differences in many characteristics (e.g., modulus and thermal expansion coefficient of two layers of materials), the coated composite materials behave like a bimetal. Of course, the extent of this behavior is largely dependent on the nature of the substrate, particularly its thickness and shape, and also on the thickness of the plasma polymer layer. This aspect may be a crucial factor in some applications of plasma polymers. It is anticipated that the same plasma coating applied on the concave surface has the lower threshold thickness than that applied on a convex surface, and its extent depends on the radius of curvature. 

Figure 21.9 The proposed seetional model of a eomposition-graded transitional buffering film layered by double-graded proeess A, pure methane plasma polymer layer B, composition-graded layer of methane plasma polymer and metal C, sputtered metal layer with earbon contamination.

The significance of LCVD is in the unique aspect of creating a new surface state that is bonded to the substrate material particularly polymeric material. The new surface state can be tailored to be surface dynamically stable. However, caution should be made that not all LCVD films fit in this category. Appropriately executed LCVD to lay down a type A plasma polymer layer creates surface dynamically stable surface state. In the domain, in which surface dynamic instability is a serious concern in the use of materials, a nanofilm by LCVD is quite effective in providing a surface dynamic stability, and other methods do not fare well in comparison to LCVD. 

Figures 33.13 shows the topography of the two plasma polymer layers deposited under different conditions on a polished iron surface. Both films show a similar topography as observed by atomic force microscopy, but the film deposited on O2 plasma-pretreated polished iron showed a little more grainy surface than (Ar + H2) plasma-pretreated sample. In Figure 33.14 the root mean square value is plotted against the film thickness. The grainy surface (O2 plasma pretreated), which showed a higher deposition rate, increased the roughness as the thickness increased as expected.

Plasma-Polymer Layers as Cushions for Lipid Membrane Architectures. 105... 

A similar phenomenon also occurs at the hyo, ophobic surface when the surface is kept in water for a prolonged period of time (1 ). This phenomenon as well as the prevention of it by application of a plasma polymer layer may be seen in the following examples. 

Surface functionalization of PP films in the O2 plasma was performed in the cw mode. PP films which were coated with plasma polymer layers of functional-group carrying monomers had been used without any additional plasma pretreatment. PTFE films were exposed first to H2 radio-frequency (RF) plasma (cw mode) for 1-1800 s at pressure p=6Pa and power P=300W, followed by the deposition of adhesion-promoting plasma polymer layers. 

By means of OH- and COOH-containing plasma polymer layers the quahfica-tion of these layers as models of single-type functionalized adhesion promoters with variable concentrations of functional groups should be proved. The plasma-initiated copolymerization of acrylic acid with ethylene or 1,3-butadiene is shown in terms of measured COOH concentration as a function of the composition of the comonomer mixture in Fig. 18.3. Depending on the co-monomer reactivity, a more linear correlation (butadiene), or a parabolic behavior (ethylene), between precursor composition and COOH groups produced was observed. For each type and concentration of functional group, its concentration was determined by chemical derivatization followed by XPS analysis as described in Section 18.2.5. 

A few variants for producing well-adhering Al-PTFE systems were tested (a) deposition of plasma polymers bearing a single type of functional group as adhesion promoters on virgin PTFE (b) H2 plasma pretreatment of PTEE followed by deposition of an adhesion-promoting plasma polymer layer and (c) H2 plasma pretreatment of PTFE alone. 

The introduction of oxygen is most likely due to post-plasma reactions of plasma-produced C-radical sites at the PTFE surface by reaction with molecular oxygen when the samples were transferred from the plasma reactor to the XPS spectrometer with transient contact with air [73, 80]. This post-plasma reaction is unavoidable, but in this case it may help to promote the adhesion between the deposited plasma polymer layer and the pretreated PTFE. 

A comparative study of the temperature dependence of the Diels-Alder reaction between [(trimethylsilyl)methyl]cyclopentadiene and dienophile groups confined in selfassembled monolayers or in pulsed plasma polymer layers has been done. The reactivity of dienophile groups confined in pulsed plasma polymer thin films is compared with the behavior of dienophile groups in monolayers, because of their well-known arrangement properties. 

Fig. 31.6 SKP potential profiles of the delamination of an epoxy adhesive (as in Fig. 31.2, about 50 pm thick) from an iron substrate (99.99% purity) coated with an HMDS/O2 plasma polymer layer about 5 nm thick. Electrolyte 0.5 M NaCI [51].

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