Elsevier

Applied Surface Science

Volume 257, Issue 22, 1 September 2011, Pages 9587-9594
Applied Surface Science

Combined XPS and contact angle studies of ethylene vinyl acetate and polyvinyl acetate blends

https://doi.org/10.1016/j.apsusc.2011.06.070Get rights and content

Abstract

In this study, we prepared thin films by blending ethylene vinyl acetate copolymers (EVA) containing 12–33 (wt.%) vinyl acetate (VA) with polyvinyl acetate (PVAc) and high density polyethylene homopolymers. Large area micropatterns having controlled protrusion sizes were obtained by phase-separation especially for the PVAc/EVA-33 blends using dip coating. These surfaces were characterized by XPS and contact angle measurements. A reasonably linear relation was found between the VA content on the surface (wt.%) obtained from XPS analysis and the VA content in bulk especially for PVAc/EVA-33 blend surfaces. PE segments were more enriched on the surface than that of the bulk for pure EVA copolymer surfaces similar to previous reports and VA enrichment was found on the EVA/HDPE blend surfaces due to high molecular weight of HDPE. Water θe decreased with the increase in the VA content on the blend surface due to the polarity of VA. A good agreement was obtained between γs and atomic oxygen surface concentration with the increase of VA content. The applicability of Cassie–Baxter equation was tested and found that it gave consistent results with the experimental water contact angles for the case where VA content was lower than 55 wt.% in the bulk composition.

Highlights

► Surface properties of dip coated blends of EVA-33 with PVAc were investigated. ► Control of surface pattern protrusion size was achieved for PVAc/EVA-33 blends. ► XPS shoved a linear relation between VA content on the surface and in the bulk. ► A good agreement was found between basic surface free energy and oxygen content. ► Deviations from Cassie–Baxter equation were discussed depending on VA content.

Introduction

Polymer blending is a cheap surface modification method to obtain desired surface properties of thin polymer coatings rather than comparatively expensive methods such as plasma treatment, surface grafting, film deposition under vacuum etc. [1]. When polymers are blended, the preferential enrichment of some functional groups on the surface affects the final properties and applications of these films. Phase-separated rough or comparatively flat surfaces can be obtained by choosing convenient polymer–solvent blending systems such as homopolymer–homopolymer, homopolymer–statistical copolymer, homopolymer–block copolymer, statistical copolymer–statistical copolymer [1], [2], [3], [4], [5]. Surface free energy, miscibility, viscosity at the process temperature, and solubility of each polymer in the chosen solvent of the blend components are the most important factors which affect the blending process and the resultant films [1], [2], [3], [4], [5]. The molecular weight of these polymers, film thickness and the solvent evaporation rate are the other important parameters [1], [4]. This paper is about preparation and surface characterization of PVAc homopolymer/EVA-33 copolymer blends having different VA contents in bulk solution. We coated glass slides with the polymer blends by applying dip coating into polymer blend solutions and determined both the wettability and the surface enrichment of PE and VA contents by phase-segregation on these blend surfaces after drying in relation to the bulk VA content of the blend solution.

In a phase-segregation process, the surface free energy differences of the involved polymers are the driving force [2], [3], [5]. However, some researchers rejected this view and attributed the surface segregation with the conformational entropy differences between the surface and bulk [6], [7]. According to this group conformational entropy in the bulk is higher than in the surface and when the number average molecular weight (Mn) decreases, the conformational entropy of a chain at the film surface also decreases. Consequently, macromolecule having lower molecular weight will be at the blend surface in order to minimize the conformational entropy. This view can be disputed so that when a volatile solvent is used in casting of the polymer blend films, the solvent evaporates rapidly from the substrate and thus the system cannot be considered as an equilibrium process. For such non-equilibrium processes, polymer surface tensions and polymer–solvent interactions play much more important roles. This situation was explained by spreading coefficient concept for the polymer blends [1], [8]. Li et al. [8] studied the formation of polystyrene and polymethylmethacrylate blend films and low surface tension polystyrene was found to locate over the polymethylmethacrylate layer and spreading coefficient calculations supported this result.

Polyethylene vinyl acetate copolymer (EVA), which is a widely used thermoplastic resin, has been considered to be a good candidate to be used as a biomedical material due to its good physical properties, ease of handling and processing, and moderate biocompatibility [9]. EVA was recently used to test the removal of the sporelings of the green alga Ulva for marine fouling applications [10]. Ethylene vinyl acetate copolymers are produced by random copolymerization of ethylene and vinyl acetate monomers, which are mainly recognized for their flexibility, toughness (even at low temperatures) and adhesion characteristics [11]. Properties of EVA copolymers change mostly due to the variation of the VA content. When polar VA content is increased, the relative quantity of amorphous phase increases and the degree of crystallinity that comes from polyethylene decreases. Increasing the VA content changes the final copolymer from modified polyethylene to rubber-like products and some of the properties such as flexibility, elongation, adhesion and solubility in organic solvents improve [11], [12]. It is possible to modify EVA copolymer surfaces by blending with polyethylene (PE) and polyvinyl acetate (PVAc) homopolymers.

Contact angle measurements and surface free energy calculations are useful techniques not only for homopolymer and copolymer surfaces, but also for polymer blend surfaces to characterize film surfaces at the top layer. Surface free energy analysis of LDPE/EVA blends were previously studied by Chattopadhyay et al. [3]. Contact angle measurements and surface free energy calculations for LDPE/EVA blends were also evaluated by Ali [5] who concluded that the modification of the surface polarity occurred when the VA content of EVA copolymer increased. As a result of this increase, contact angles for water and reference liquids decreased and calculated surface free energy values raised [5]. Matsunaga and Tamai [13] and later Erbil [14] determined surface free energy values of EVA copolymers by applying contact angle method. The same method was also applied to polyethylene homopolymer by Dann [15] and Park et al. [16].

van Oss et al. [17] developed a successful approach to estimate the surface free energy of polymers. According to this theory, Lifshitz–van der Waals interactions (indicated by superscript LW) include dispersion, polar–polar, and induction interactions, and acid base interactions (indicated by superscript AB) include hydrogen-bonding interactions, in other words electron donor–acceptor interactions. Total surface free energy is the sum of these Lifshitz–van der Waals and acid–base interactions [17]. Surface free energy determination of EVA copolymers by applying van Oss–Good–Chaudhury method was studied by Grundke et al. [18]. Similarly, Michalski et al. [2] applied van Oss–Good–Chaudhury method to determine the surface free energy of EVA, PVC and their blends.

X-ray photoelectron spectroscopy (XPS) was applied to determine the surface compositions of the EVA copolymers and its blends which have varying VA contents [19], [20], [21]. Chihani et al. [19] used XPS characterization of the EVA surfaces obtained by the injection molding method and found that surface concentration of VA groups was higher than that of the bulk when perfluorinated ethylene propylene (FEP) was used as the mould. Galuska [20] studied EVA copolymer and EVA/LDPE blend surfaces by using XPS and obtained a linear relation between surface and bulk VA content according to oxygen concentration. Surface properties of EVA copolymers were modified by treatment with low pressure RF plasmas [22], UV radiation [23] and the change of its adhesion properties were determined by contact angle measurements and XPS.

In a previous study, we investigated the surface chemical structure and wetting properties of both flat and rough EVA copolymer films by varying the concentration and temperature of the dip coating solution [24]. A solution concentration of 40 mg/ml was used for the flat coatings and up to 100 mg/ml for the rough coatings and the temperatures changed from room temperature to 125 °C. XPS analysis at 0° and 60° take-off angles (approximately 10 nm and 5 nm depths, respectively) was applied and contact angle measurements were carried out by increasing the VA content of the bulk EVA copolymer. XPS results show that hydrophobic PE component was enriched on EVA surfaces around 5 nm depth for all the samples, whereas hydrophilic VA component was enriched on the surfaces when VA < 18% for only around 10 nm depth. Hydrophobic PE component was found to enrich in the near-surface region for all flat and rough EVA samples for a depth of around 5 nm. The difference between the XPS results of the flat and rough surfaces was not significant for EVA samples except EVA-33 surface where the atomic oxygen content decreased 15% for 10 nm and 20% for 5 nm depth due to its very low molecular weight [24].

In the present study, we applied dip coating of glass slides in polymer blend solutions of EVA-33 copolymer with PVAc homopolymer for the first time and determined both the wettability of dried blend surfaces and the surface enrichment of PE and VA contents by phase-segregation in relation to the VA content of the blend solution in bulk. In addition, we also blended EVA copolymers (EVA-12, EVA-18, EVA-28 and EVA-33) with HDPE homopolymer for comparison. Contact angle, surface free energy analysis and XPS measurements were done in order to investigate the wettability properties and surface compositions of these blend surfaces. The correlation of surface free energy with the XPS results was discussed and the applicability of the Cassie–Baxter equation [25], which was derived for the chemically heterogeneous surfaces; was also investigated for the blend surfaces.

Section snippets

Materials

Polyvinyl acetate and high density polyethylene (HDPE) homopolymers and ethylene-vinyl acetate copolymers with varying VA contents (EVA-12, EVA-18, EVA-28-05, EVA-28-40, EVA-28-150, EVA-33 and EVA-40) were used for the preparation of blend surfaces. The names of EVA copolymers are self-descriptive, for example that EVA-28-40 has a VA content of 28 wt.%, with a melt flow index of 40. The names of manufacturers, vinyl acetate (VA) contents and also experimentally determined melt flow index values

Optical microscopy images

Optical microscope images of PVAc/EVA-33 blends with varying VA contents at ×500 magnification are given in Fig. 1. Large area patterns having specific protrusion sizes were obtained as seen in this figure, where the size of protrusions was decreased with the increase of VA content in the bulk EVA copolymer. It can be speculated that the protrusions correspond to PE regions since their total area on the surface decreases with the increase of VA content.

XPS results

X-ray photoelectron lines of C1s and O1s

Conclusions

Large area patterns having controlled protrusion sizes were obtained for PVAc/EVA-33 blends by applying an inexpensive dip coating method. A reasonably linear relation was found between the VA content on the surface (wt.%) obtained from XPS analysis and the VA content in bulk especially for PVAc/EVA-33 blend surfaces. For pure EVA copolymer surfaces, PE segments are more enriched on the surface than that of the bulk similar to previous reports. However, we determined VA enrichment on the

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