Surface-initiated vapor deposition polymerization

Alternatively PAA can be obtained without solvent by vapor deposition polymerization as described first by Salem et al. [2], In this technique the dianhydride py-romellitic and the dianhydride diamine (4,4 -oxidianiline) are codeposited onto a substrate, where they react to form PAA. Again the transformation to Polyimide is obtained by subsequent heating to temperatures up to 350°C. By comparison to spun dn films, initial interaction of the polymer with the substrate occurs in the uncomplexed PAA state. The chemical interaction between PAA and the metal establishes the adhesion of the final polyimide film. This is discussed in this communication for evaporated gold cluster and bulk silver surfaces. 

Chain initiation occurs when two monomer radicals are coupled to form a dimer biradical and proceeds further." This is an endothermic reaction requiring a heat of formation of 16 kcal/mol. Because of energetic concerns, chain initiation is unlikely to happen in the gas phase at low pressure. When the monomers are adsorbed onto the surface of the substrate, it is believed that, the high local concentration of monomers promotes the formation of biradicals assisted by van der waals forces. Models developed for vapor deposition polymerization of parylene-N indicate that initiation is a third order reaction with an activation energy of 24.8 kcal/mol. 

The methods of the preparation of parylene nanofibers by oblique angle vapor deposition polymerization have been detailed [114]. Monomer vapors produced by the pyrolysis of chemically functionalized / -xylylene precursors are directed in an oblique angle toward a surface to initiate a structured polymer growth. 

The electrical conductivity, environmental stability, and surface properties of pristine polyanihne can be improved by synthesizing nanocomposite with large surface area materials such as carbon nanofibers [30]. Polyaniline/carbon nanofiber nanocomposite can be prepared by one-step vapor deposition polymerization [31]. In vapor deposition polymerization, a reactor consisting of two connecting flasks are used. In one chamber, the carbon nanofibers soaked in initiator are taken and the other chamber is equipped with a seahng apparatus. Then vacuum is appHed inside the chamber followed by the addition of liquid aniline monomer to the second chamber is carried out. The chamber is heated at low temperature in order to vaporize the anihne monomer. This process can continue... 

Cryochemical synthesis of Ag-PPX systems and their structures were studied in references 24, 34, 36, 37, and 44. The simultaneous vapor deposition of PX-, CN-PX-, and Cl-substituted PX (Cl-PX) with Ag at 77 K does not lead to complexation or the formation of any organometallic compounds [24, 36, 44]. In the case of PX and Cl-PX, such deposition proceeds without polymerization. The co-deposition of CN-PX with Ag is accompanied by the partial polymerization of monomer. The initial condensates at 77 K contain a small amount of Ag nanocrystals that can be revealed and characterized using UV-Vis spectroscopy because such crystals have the specific absorption band of surface electron plasmons of about 430nm [3] (Figure 2.5 and 2.6). UV irradiation of these condensates at 77 K leads to the total conversion of monomers to the corresponding polymers (PPX, CIPPX, and CNPPX). However, intensity (D ), maximum position (Vax), and half-width (Ai ) of the nanocrystals plasmon band do not practically change in the course of cryopolymerization (Figures 2.5 and 2.6, Table 2.1). 

As shown in Figure 12.7, an initial deposition rate that is roughly four times greater than the steady-state deposition rate is observed with styrene, which is an easily condensable vapor (high adsorption onto the surface), by method A. The gaseous monomer acetylene, on the other hand, shows an initial deposition rate identical to the steady state is obtained immediately on the inception of discharge, as shown in Figure 12.8. Whether the adsorbed monomer polymerizes at the surface or... 

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