Article pubs.acs.org/Macromolecules
Functional Thin Films Resulting from Parylene−Vinyl Copolymerization Maciej Bobrowski,* Sylwia Freza, and Piotr Skurski Department of Chemistry, Univeristy of Gdańsk, Sobieskiego 18, 80-952 Gdańsk, Poland Department of Technical Physics and Applied Mathematics, Gdańsk University of Technology, Narutowicza 11/12, 80-233 Gdańsk, Poland ABSTRACT: Quantum and kinetic studies of novel materials based on parylene polymers and variously substituted vinyl molecules are presented and discussed. It is demonstrated that the thin films which are the products of the copolymerization reactions can be prepared (i) within the parylene CVD process on active vinyl substrates or (ii) employing the same process but involving an additional source of vinyl molecules transported to the reaction chamber. Both methods lead to differently functionalized microstructures, primarily dominated by the parylene units, while the vinyl molecules appear relatively seldom (depending on the nature of the substituents used). The copolymerization is shown to be independent of penultimate monomers in linear chains and to undergo composition drift within certain scope. Namely, it is found that even a small amount of p-xylylene monomers in feed leads to the formation of long blocks of pure parylene while the substituted vinyl molecules are expected to appear occasionally in chains. Finally, it is predicted that the two types of parylene functionalization result in specific structures and thus behavior of the corresponding thin films. and other areas of novel complex materials.6−14 Such coatings also demonstrate excellent barrier properties, low dielectric constant, and high solvent resistance. It seems important to recall that the parylene CVD can be used with respect to liquid substrates which significantly extends the area of possible applications (especially due to the stability of the final products).15 The layer thickness ranges from a few hundred nanometers to micrometers, while the polymer itself possesses relatively large molecular weight (200 000−400 000 g/mol) with a typical chain length of 2000−4000 units. Typical CVD processes are performed according to the original procedure established by Gorham in 1966 in which cyclo-di-p-xylylene (CDPX, [2,2]paracyclophane) is cracked into monomers which next take part in polymerization. The polymerization initialization engages two monomers (possessing sufficiently high kinetic energy) that combine with each other to produce biradical di-p-xylylene (DPX). Next, the DPX can react with another monomer which leads to biradical trimer (tri-pxylylene, TPX), and the reaction further proceeds in a similar way with the formation of longer polymeric chains. The parylene coating itself is generally inert in a sense that it forms a thin, transparent, and nonreactive layer; hence, its usage is limited only to precise preventive coatings. The advantage of current parylene applications is the simplicity of technological process. An accurate covering undertaken at room temperature is with no doubt very attractive and useful, although this is the functionalization that makes this material
1. INTRODUCTION It has been demonstrated recently that chemical vapor deposition of parylene C (poly[2-chloroxylylene]) over a series of unsaturated fluorenes evokes chemical binding between the polymer and fluorenes, as explicitly proven by the FT-IR and FT-Raman spectra.1 It was also found that the analogous chemical binding between parylene and acrylates might be accomplished using the same technique2 (assuming liquid substrates in both cases). These observations were followed by theoretical studies revealing that the reactions likely involve double CC bonds (in fluorenes and acrylates) and parylene radical chains, and two possible mechanisms were proposed.2−4 An important conclusion was formulated on the basis of those experimental and theoretical considerations performed over a relatively short time period; namely, it became apparent that various novel functional parylene thin layers might be easily produced and applied (e.g., to sophisticated materials in many types of devices) using practically the same or slightly modified facile technological process. As those reactions involve radical active chains of functional materials, the copolymerization processes resulting in formation of new materials seem likely. Therefore, in the present contribution, we discuss the parylene functionality issue as well as the possible types of copolymers formed by combining parylene and selected vinyl molecules. Chemical vapor deposition (CVD) of [2,2]paracyclophanes leading to parylene polymers requires neither catalysts nor solvents to result in inherently clean products.5 The obtained polymer layers are extremely adjusted, tailored, and tight and possess desirable mechanical and optical characteristics which make them promising not only in current applications in electronics, medicine, or aviation but also in optics, acoustics, © 2012 American Chemical Society
Received: September 11, 2012 Revised: October 11, 2012 Published: October 24, 2012 8532
dx.doi.org/10.1021/ma301902d | Macromolecules 2012, 45, 8532−8546
Macromolecules
Article
where k is the Boltzmann constant, T is the temperature (298.15 K), and h stands for Planck’s constant. For all reactions, the electronic energies obtained from B3LYP were corrected by the thermal and zero-point energy contributions to obtain the enthalpies and entropies of the processes. The rate constants were calculated for the temperature T of 298.15 K and for the pressure p of 1013 hPa (1 atm). In addition, some of the calculations were repeated for the same temperature T and the reduced pressure of p = 10 Pa. All calculations were performed with the Gaussian0325 package. 2.2. Kinetics of the Copolymerization. The equilibrium thermodynamics resulting from the preceding quantum calculations was applied in two separate copolymerization kinetic models, and the relationships between the rates and concentrations of the reactants were determined for one of them. These considerations were followed by the statistical analysis in order to determine the sequences (i.e., the first-order structures) of the resulting copolymers. The kinetic models employed are commonly used techniques, and their usefulness in studying copolymerization processes was confirmed in the past for various reaction schemes.26−29 2.2.1. Terminal Model. Initiation of parylene polymerization as well as a possible role of monomer in parylene−vinyl copolymerization was extensively studied elsewhere.4,5 In the terminal model it is assumed that the rate constants depend on the nature of terminal units only, and thus we considered four possible propagation routes in the case of p-xylylene/vinyl copolymerization, namely (1) ∼∼X• + X → ∼∼XX• (kxx), (2) ∼∼X• + V → ∼∼XV• (kxv), (3) ∼∼V• + X → ∼∼VX• (kvx), and (4) ∼∼V• + V → ∼∼VV• (kvv), where k (in parentheses) indicates the corresponding rate constants for each route, whereas X and V stand for p-xylylene monomer and substituted vinyl molecule, respectively. The basic equations for consumption of p-xylylene monomer (X) and substituted ethylene (V) are the following:
and the technology underneath indeed truly accomplished. Chemical functionalization of parylene layer can be achieved basically by two different methods. 16−19 One is the modification of [2,2]paracyclophane before the polymerization, and after vapor deposition one can obtain functionalized chains of parylene. However, this method requires modification of technological parameters, and hence the resulting polymers usually possess different properties. Also, the chains are strongly modified due to different structure of the constituting monomers, which in turn causes a different morphology of the resulting new materials. The second method involves the use of suitable substrates which are capable of reacting with nonmodified parylene during the CVD polymerization (as mentioned before); alternatively, one can use a separate source of molecules reacting with parylene which results in a copolymer-like structure. In the present contribution we apply theoretical methods to determine the molecular structure of functionalized parylene layer where the agents are unsaturated organic molecules. These agents might be either introduced as liquid substrates or added from an adapted diffuser to the reactive chamber together with p-xylylene monomers. In both cases the resulting thin layer of parylene is expected to be effectively chemically functionalized while the process remains basically the same.
2. METHODS Step-growth polymerization usually requires substantial time to achieve good CVD-produced tiny parylene layer. We applied the kinetic theory followed by statistical tools to determine the first-order structure of the parylene−vinyl copolymers. Kinetic growth of polymer chains was first investigated by means of quantum methods which provided the extensive conformation and energetic data. Experimental measurements of copolymer composition (at low degrees of conversion,
