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Electron-beam deposition of polyethylene composite coatings at various initiators and inhibitors.

INTRODUCTION

Electron-beam deposition (EBD) is an attractive technique for rapid formation of composite coating, for example, polymeric coatings and organic--inorganic composite coatings (1-3). The fabrication of EBD coatings greatly depends upon the reaction of substrate with decomposition products formed in the radiation of electron beam (ER) to target. When EB is focused on a precursor surface, the generated secondary electrons decompose organic gas molecules forming volatile species, and thus solid carbonaceous particles are grown on the substrate. Rogachev et al. (4) have investigated EBD coatings with two elementary formation mechanisms. The first adsorption--polymerization mechanism demonstrated that the precursor gas molecules produced from initial polymeric target adsorb on substrate surface and generate second polymerization. Another aerosol mechanism proposed that the polymeric microparticles and nanoparticles generated during the dispersion of initial target adsorb on substrate surface and form polymeric coatings.

The growth mechanism of the coatings is determined to target natures, dispersion conditions, and modes. The adsorption--polymerization mechanism is performed under the dispersion conditions of target, e.g., during radiation-induced polymerization 151, the dispersion products possess a low density and concentration of active particles, which can be described by the relation [L.sub.k] < [lambda] ([L.sub.k] is the characteristic size of chamber, 2 is the average-free length of dispersion product) 161. Aerosol mechanism is characterized by the formation of dispersion products with high density and active reactivity, indicating that the quantity and size of polymeric particles depend on the relative density ratio of active particles to inactive particles in the gas phase.

Currently, EBD technique is widely used to fabricate composite coatings with varying categories of material. By suitably changing the conditions and modes of dispersion, the three-dimensional structures of formed coating with corresponding properties can be, effectively governed. Therefore the study of formation mechanisms of EBD coatings has attracted interest as a tool for scientific and practical applications.

In this study, several polyethylene (PE) composite coatings are prepared at various initiators and inhibitors by EBD technique. The influences of initiator and inhibitor on the structural and morphological properties of PE composite coatings are investigated using attenuated total reflectance-Fourier transform infrared (AIR-ETIR) spectroscopy and atomic force microscope (AFM). The characters of polymerization kinetics in the coatings formation are discussed in detail.

EXPERIMENTAL

PE composite coatings were prepared using a self-design EBD device, as shown in Fig. 1. The mechanical mixtures of low-density polyethylene (LDPE, 0.92 g/[cm.sup.-3]) with aluminum trichloride ([AlC1.sub.3]), benzoyl peroxide, hydroquinone, and diphenylamine were used as dispersion targets. The dosage of dopant and PE in the composite target was 0.02 mol dopants/g PE. The coatings were deposited on aluminum-plated PE terephthalate coating (for IR measurement) and single-crystalline silicon substrates. Prior to deposition, the silicon substrate was successively cleaned in an ultrasonic bath with acetone, alcohol, and deionized water for 15 min and then dried in air. When the chamber has been attained to a base pressure of 5 x [10.sup.-3] Pa, the coatings start to deposit. During deposition, the vacuum chamber was changed to a pressure of ~ [10.saup.-2] Pa due to the production of active gas phase from target evaporation. Working voltage and current density of EB were 0.9 ~ 1.2 kV and 0.01 ~ 0.03 A/[cm.sup.2], respectively. The deposition rate and thickness of the PE composite coatings were on-line monitored with a MICRON-5 quartz thickness gage. The thicknesses of asdeposited coatings in this experiment are ~1 [micro]m.

The structure measurements of PE coatings were carried out by FTIR spectrometer with a Bruker Optic GmbH Vertex 70 in a mode of the ATR using a special unit by "Carl Zeiss." The parallelogram-shaped plate from KRS-5 with 14 reflections and the face angle of 450 was used as a reflecting element. The compound bands of absorption were fitted by Gauss curves using OPUS program. Surface morphology of the coatings was observed by AFM using a multimode scanning microscope Solver-PRO (NT-MDT) in a tapping mode.

RESULTS AND DISCUSSION

The influences of several, compounds on the molecular structure of PE composite coatings are investigated by infrared spectra as shown in Fig. 2. All samples ([P.sub.0] ~ [P.sub.4]) show the characteristic of PE absorption band. Three strong adsorption bands appear at approximately 2926, 2853, and 1464 [cm.sup.-1], respectively, which are assigned to the vibrations C--H bond from--[CH.sub.2]--group. Meanwhile, four weak adsorption bands located at 1378, 965, 908, and 889 [cm.sup.-1] are also observed in spectra. The band at 1378 [cm.sup.-1] is assigned to the deformation vibrations C--H bond from --[CH.sub.3] group, while the other three bands at 965 [cm.sup.-1] (--CH=CH--), 908 [cm.sup.-1] (--CH--[CH.sup.2]) and 889 [cm.sup.-1] (>C--[CH.sub.2]) are responsible for the absorption peak of unsaturated groups in PE coatings (7). In addition, the band at ~ 1700 [cm.sup.-1] observed in sample [P.sub.2] can be attributed to the stretch of C=O from benzoyl peroxide (8), indicating that benzoate have reacted with free-radical groups of dispersion products during PE polymerization process.

Based on recommendations given in (7, 9), the band at 1464 [cm.sup.-1] corresponded to shear deformation vibration of C--H bond from --[CH.sub.2]-- group is selected as the internal standard for the structural study of PE coatings. The branched degree of PE coatings can be estimated by the optical density ratio of absorption band at 1378 and 1464 [cm.sup.-1] (denoted as [D.sub.1378]/[D.sub.1464]). The unsaturated degree of PE coatings is determined by the optical density ratio of 965 [cm.sup.-1]/1464 [cm.sup.-1], 908 [cm.sup.-1]/1464 [cm.sup.-1], and 889 [cm.sup.-1] /1464 [cm.sup.-1]([D.sub.965]/[D.sub.1464], D908/13,464, and [D.sub.889]/[D.sub.146). The results of optical density ratio according to PE infrared spectra are summarized in Table 1, well describing the reaction kinetics feature of composite targets during dispersion process.

TABLE 1. Optical density ratio of absorption hand of PE coatings
in FTIR spectra.

Sample number  Coatings          v([CH.sub.3])  --CH=CH--
                                 [D.sub.1378]/  [D.sub.965]/
                                 [D.sub.1464]   [D.sub.1464]

[P.sub.0]                    PE           0.22         0.078

[P.sub.1]      PE +                      0.682         0.177
               [AlCl.sub.3]

[P.sub.2]      PE +                      0.179         0.096
               benzoyl peroxide

[P.sub.3]      PE +                      0.249         0.104
               diphenylamine

[P.sub.4]      PE +                      0.176         0.062
               hydroquinone

Sample number  Coatings          --CH=[CH.sub.2]  >CH=[CH.sub.2]
                                 [D.sub.908/      [D.sub.339/
                                 [D.sub.1464]     D.sub.1464]

[P.sub.0]                    PE            0.166           0.035

[P.sub.1]      PE +                        0.067           0.038
               [AlCl.sub.3]

[P.sub.2]      PE +                         0.12           0.024
               benzoyl peroxide

[P.sub.3]      PE +                        0.139           0.033
               diphenylamine

[P.sub.4]      PE +                        0.159           0.031
               hydroquinone


From Table 1, it is obvious that different additives have significant effect on the molecular structure of PE coatings. The behavior of molecular structure in PE coatings depends greatly on which polymerization mechanism predominates in the coatings formation, i.e., the free-radical polymerization or the ionic polymerization occurs in the polymerization reaction.

In our previous study Pk the structural studies of PE coatings obtained by EBD with [AlCl.sub.3] initiator have indicated a possible cationic polymerization mechanism. Adding the [AlCl.sub.3] is thus expected to result in an increase of the trans-vinylenovoy (--CH=CH--) and vinylidene (>C=[CH.sub.2]), a decrease in unsaturated vinyl (--CH=[CH.sub.2]) and a significant increase in branch degree for the [P.sub.1] coatings. Scheme 1 illustrates the possible reaction mechanism of the [P.sub.1] coatings by EBD. Differ with conventional cationic initiating system (e.g., [H.sub.2]O/Lewis acid, halogen alkyl/Lewis acid) 110, 111, [A1Cl.sub.3] mainly interact with PE molecular fragments produced by the dispersion of ER to target and then produce carbenium ions (12). In general, carbenium ions are very instable and easily increase its stability in conjunction with the double bond of unsaturated vinyl from PE decomposition under EB irradiation. The formed macromolecular radicals continually react with double bond of unsaturated vinyl, resulting in the decrease of unsaturated vinyl and the increase of branch structure in the PE chain (Fig. 3A). Meanwhile, the carbenium ion can also present the reaction of rearrangement due to instability, leading to the formation of different unsaturated bonds (--CH=CH and >C=[CH.sub.2]) by the isomerization of the double bond and hydride and/or methide shifts (Fig. 3B).

In general, benzoyl peroxide after heating can be decomposed with the formation of bentoyloxy and then decarboxylated to form phenyl radicals, inducing the reaction of radical polymerization in the solution system (13), (14). The use of organic peroxide initiator facilitates the reaction of chain transfer and produces branched and cross-linked molecule structure, thus resulting in the increase of branched degree in PE coatings (15). Moreover, note that the EBD coatings produced under complete melting of the target present high branched and cross-linked structure, i.e., the irradiation of EB acts on polymeric materials and leads to the melting of polymer, inducing the formation of branched, and cross-linked structure. In the present study, however, the branched structure of [P.sub.2], coating shows an abnormal decrease. For the [P.sub.2] coating, the quick increase in the coating thickness is observed during initial deposition, and also the phenomena of target melting cannot be observed in the irradiation zone of EB. These indicate the sharp decomposition of benzoyl peroxide during deposition. In this case, benzoyl peroxide not only fails to initiate the reaction of chain transfer but also prevents the melting of PE target, thus leading to the reduction of branched structure. Consequently, an interesting result can be deduced that the active gas products generated in the dispersion of ER to target can not help to form the branched structure in PE coatings, i.e., high density-free radicals does not always induces the combination of chain radicals and polymer radicals in the formation of EBD coatings.

In addition, a decrease in the amount of --CH=[CH.sub.2], hands for coatings [P.sub.2] is observed as a result of the participation of unsaturated vinyl bonds in the radical polymerization. The increase of the trans-vinilenovoy (--CH=--CH--) may be related to the impact of steric hindrance in the polymerization 1151.

Diphenylamine and hydroquinone usually play an inhibited role in the radical polymerization. It is well known that the inhibiting properties of phenols compounds and aromatic amines are associated with the transfer of hydrogen atom from --NH or --OH groups (16), (17). In the case of free-radical polymerization, the reaction of diphenylamine with EB dispersion products had to accompany with the vanishing of free-radical groups, i.e., leading to a decrease in the branched-degree and tran-svinilenovoy. However, the observations from Table I are contrary to the expected results, indicating that the conventional inhibiting reactions (18) (Fig. 4) of diphenylamine can not occur in the present study. This is because the conventional inhibition reactions are performed under solution condition, but our experiment is carried out by the dispersion of EB to polymer target in vacuum. EB irradiation will change the molecular structure of inhibitor and impact its reaction activity.

Denault et al. (19) have obtained the ionization energy of aniline (7.72 eV) that is far less than EB energy. Our previous study (20) has also proved that diphenylamine is easy to oxidize at the role of high-energy electrons. Therefore, diphenylamine can be oxidized under EB irradiation and forms cation radical [([C.sub.6][H.sub.5]).sub.2][NH.sub.+]. The cation radical interacts with the free-radicals from PE molecule fragment through electron transfer that to be deoxidized, inducing the formation of carbenium ions and increasing the reaction course of cationic polymerization. The similar reaction process has been reported in Ref. (18). Figure 5 shows the possible reaction mechanism according to the above discussion. The effect of diphenylamine on the molecular structure of PE coatings is mainly related to its activity of participation in the process of oxidation--reduction.

Hydroquinone is a usual inhibitor in the free-radical polymerization. The inhibiting effectivity of hydroquinone depends on the ability of its continuous oxidation that to form quinone (21), as shown in Fig. 6. According to this mechanism, chain radical groups will react with oxygen atom or quinone ring in the formed quinone, leading to the reaction of coupling termination, or disproportionating termination in the polymerization of PE coatings. How-ever, it had to be noted that quinone is easy to sublimate and meanwhile, a decrease of unsaturated vinyl (--CH=[CH.sub.2]) in P4 coatings is observed. Under EB irradiation, so that the above-mentioned reaction fail to occur in the polymerization of [P.sub.4] coatings.

The O--H bond in hydroquinone molecule is weak as the delocalization of negative charge of oxygen atom, thus hydroquinone is decomposed under EB irradiation and generates protons and phenonium ions (Fig. 7). The phenonium ions interact with carbenium ions in a manner of electron transfer, resulting in reduction of cationic polymerization course ultimately. Therefore, a slight decrease in the branched degree and trans-vinilenovoy (--CH=CH--) of [P.sub.4] coatings can be observed in Table 1. These results indicate that the formation of [P.sub.4] coating under EB irradiation is not complete radical polymerization, simultaneously, including cationic polymerization.

According to the above results and discussions, a comprehensive polymerization model based on Rogachev's work is used to describe the formation process of PE composite coatings, as shown in Fig. 8. The dispersion products generated in the EB irradiation to composite target is composed of the free-radical fragment and carbenium fragment, resulting in that radical polymerization and cationic polymerization can be simultaneously observed in the formation of EBD coatings. Furthermore, the formation of PE composite coatings on substrate may be performed as the proposed two polymerization process (4).

Morphological characteristics of PE composite coatings prepared at various initiators and inhibitors are shown in

Fig. 9. The effect of [AlCl.sub.3] on the morphology of PE coating is manifested in the absence of spherical formations. In this case, the growth of [P.sub.1]1 coatings (Fig. 9B) is along the parallel direction to substrate surface, changing the growth characteristics of pure PE coatings. Moreover, all PE composite coatings except [P.sub.1] coatings exhibit similar spherical structure with uniform size from traditional height image of AFM. But apparently, it can not be used to explain the formation process of PE coatings at various initiators and inhibitors.

In the following, with the aim of describing the morphological changes occurring, the authors compare, the phase contrast image of different PE coatings. PE coatings obtained in the presence of benzoyl peroxide shows a clearly layered structure (Fig. 9C). The formation of layered structure is probably due to the decomposition of benzoyl peroxide under ER irradiation. This causes an increase in the pressure of reaction system and a strong etching of plasma discharge to the amorphous region of polymer coatings. In this case, coatings preferably grow according as layer-by-layer (22).

In the dispersion process of EB to polymer target, benzoyl peroxide does not induce the formation of branched structure from active products but meanwhile, can not also predicate the absence of intermolecular combination from free-radical groups. If the observed-layered structure is crystalline and finally forms folded macromolecule, the participation of free-radicals in the intermolecular combination will lead to low content of vinyl (--CH=[CH.sub.2]) bonds. The well-ordered degree of coatings surface corresponds to that of the high crystallinity, indirectly indicating low content of methyl groups, i.e., low-brached degree in coatings. The relation between the crystallinity of PE coatings and the content of methyl groups has been also investigated in the published reports (7), which is coordinated with the results observed from our experiment.

The morphology of [P.sub.3] coatings (Fig. 9D) exhibits common features between the coatings [P.sub.1] and [P.sub.2]. On one hand, it can be seen that the coatings [P.sub.3] surface shows a flat topography with vague spherical particles same as [P.sub.1] coatings. On the other hand, much fine-layered structures are observed from phase contrast image as the [P.sub.2] coating, but in comparison with the latter, the amount of layered structures in [P.sub.3] coatings is smaller and becomes elongated shape.

In addition, the morphology and phase-contrast image of coatings [P.sub.4] (Fig. 9E) have no noticeable difference in comparison with pure PE coatings. This could be good coordinated with the slight change of molecule structure for coatings [P.sub.4] in infrared spectra.

CONCLUSION

By using EBD, PE composite coatings were prepared at various initiators and inhibitors. The molecular structural and morphological characteristics of PE coatings were investigated. [AlCl.sub.3] initiator was successfully used for the cationic polymerization of PE and synthesized the PE coatings with high-branched degree by EBD. Benzoyl peroxide abnormally decreased the branched structure of the PE coatings and observed a well-ordered degree on coatings surface from AFM morphology, failing to play the role of radical initiator for the PE composite coatings by EBD. Diphenylamine inhibitor was oxidized under the irradiation of EB and generated cation radical [([C.sub.6][H.sub.5]).sub.2][NH.sup.-]. Not only failed to inhibit the radical poly-merization but also induced the formation of carbenium ion in the EB dispersion, which resulted in the increase of branched degree and trans-vinilenovoy (--CH=CH--). Though hydroquinone inhibitor using decreased-branched degree and trans-vinilenovoy of PE coatings and seems to inhibit radical polymerization, simultaneous decrease of unsaturated vinyl (--CH=[CH.sub.2]) indicated that hydroquinone had no significant inhibited effect on the radical polymerization.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (50972059), 2010-2012 Intergovernmental Cooperation Projects in Science and Technology of the Ministry of Science and Technology of PRC (No: 5 and No: 6), Science and Technology Devel-oping item of Nanjing City (200901061).

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Correspondence to: A.V. Rogachev; e-mail: rogachevav@mail.ru or Xiaohong Jiang: e-mail: jxh0668@sina.com

Published online in Wiley Online Library (wileyonlinelibrary.com).

[c] 2012 Society or Plastics Engineers

Zhubo Liu, (1) A.A. Rogachev, (1) Bing Zhou, (1), (2) M.A. Yarmolenkol (1) A.V. Rogachev, (1) D.L. Gorbachev, (1) Xiaohong Jiang (2)

(1.) Francisk Skorina Gomel State University, Gomel, Belarus

(2.) Key Lab of Soft Chemistry and Functional Materials of Ministry of Education, Nanjing University of Science and Technology, Nanjing, China

DOI 10.1002/pen.23167
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Author:Liu, Zhubo; Rogachev, A.A.; Zhou, Bing; Yarmolenko, M.A.; Rogachev, A.V.; Gorbachev, D.L.; Jiang, Xi
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:Oct 1, 2012
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