Surface modification of thermoplastics by atomic layer deposition of Al2O3 and TiO2 thin films
Introduction
Synthetic high-performance thermoplastics such as polymethylmethacrylate (PMMA), polyetheretherketone (PEEK), polytetrafluoroethylene (PTFE) and ethylene-tetrafluoroethylene copolymer (ETFE) are indispensable in modern-day world. Their applications include packaging, bearings, tubing, process equipment, chemical ware, electrical insulation, non-stick coatings, etc. [1], [2].
For some applications, it would be useful if surfaces of these thermoplastics could be coated with well-adherent functional thin films. The coatings could act for example as protective coatings against mechanical wear or UV light, as self-cleaning coatings, or as gas diffusion barriers [3]. These coatings could also serve as modifying interlayers to improve (bio)compatibility [4], or adhesion of subsequent (metallic) layers which could then be used for electrical contacting etc. [5]. The preparation of optical or electronic thin film devices on plastics [6] is another fascinating possibility. One limiting issue is the difficulty of forming good interfaces between some polymers and inorganic deposited layers, which is manifested by, for example, the poor adhesion of sputtered thin films on the surfaces of hydrophobic fluoropolymers [7].
Atomic layer deposition (ALD) [8], [9] is a chemical thin film deposition method where the film grows via self-limiting surface reactions. The deposition occurs (sub)monolayer by (sub)monolayer, giving an accurate control over film thickness and composition. Furthermore, the self-limiting growth mechanism enables the deposition of uniform films on large areas and complex-shaped surfaces.
ALD nucleation usually requires that the substrate has reactive surface groups with which the precursor molecules can react to initiate ALD growth. Hydroxyl groups on oxide surfaces are typical examples of such reactive groups. In that case, the deposited thin film is chemically bonded to the underlying substrate, and will therefore usually have a good adhesion. However, ALD growth on substrates that lack reactive surface groups has also been reported. George et al. [10], [11] studied ALD growth of Al2O3 from trimethylaluminum (TMA) and H2O at 85 °C on several polymers, with and without reactive surface groups. They explained the initial nucleation on polymers lacking reactive surface groups by strong absorption and retention of TMA in the surface layers of the polymer, and the reaction of the subsequent pulse of water vapor with this absorbed TMA [10], [11]. After the initial nucleation period of 10–20 ALD cycles, the polymer becomes covered by Al2O3 and the process continues as a normal ALD growth with a rate similar to those observed on other surfaces [10].
Nucleation and growth on polymers depends naturally not only on the nature of the polymer, but also on the ALD precursors used. Färm et al. [12] have studied the use of PMMA thin films as masking layers for selective area ALD of several thin film materials from various kinds of precursors. Depending on the precursors and the deposition temperature, some ALD processes resulted in nucleation and film growth on PMMA, whereas some did not [12]. Earlier, Sinha et al. [13] studied ALD of TiO2 at 160 °C from two titanium precursors, TiCl4 and Ti(OiPr)4. TiCl4 as the metal precursor led to some nucleation of TiO2 on PMMA, whereas Ti(OiPr)4 did not. This was attributed to the ability of the strong electron acceptor TiCl4 to coordinate to the carbonyl bonds of PMMA [13].
As summarized in Table 1, ALD growth of thin films has been studied on a variety of polymeric substrates [3], [5], [6], [10], [11], [13], [16], [17], [18], [19], [20], [21], [22], [23]. In addition, also highly porous poly(styrene-divinylbenzene) (PS-DVB) particles [14] and polystyrene/poly-4-vinylpyridine (PS/P4VP) nano-objects [15] have been coated. By now, the materials deposited on polymers by ALD include only a few oxides, metals and nitrides.
In this work, we have used atomic layer deposition to grow Al2O3 and TiO2 thin films at temperatures 80–250 °C on various polymeric substrates including PMMA, PEEK and two different fluoropolymers, i.e., PTFE and ETFE. As far as the authors know, no studies on ALD growth on PEEK or ETFE have been published to date. Growth rates and properties of the films on different substrates are compared.
Section snippets
Experimental
Four kinds of polymeric materials, i.e., polymethylmethacrylate (PMMA; CAS # 9011-14-7), polyetheretherketone (PEEK; CAS # 84137-36-0), polytetrafluoroethylene (PTFE; CAS # 9002-84-0) and ethylene-tetrafluoroethylene (ETFE; CAS # 65324-12-1), were used as substrates in this work. PMMA (average Mw = 350,000, Alfa Aesar) was spin-coated on Si substrates from 2% PMMA in toluene solution, and annealed at 100 °C for 60 min, forming approximately 70–100 nm thick PMMA films. PEEK and PTFE (Goodfellow
Results and discussion
The deposition parameters and film thicknesses measured on the different thermoplastics are presented in Table 2. Within the accuracy limits of EDX, the growth rates on the polymers were roughly in agreement with those measured on reference Si samples processed in the same runs. There are some differences, however. The Al2O3 film deposited on PMMA by 1000 cycles of AlCl3 and H2O had a thickness of 100 nm, whereas the thickness on reference Si substrate was 79 nm. It is possible that the
Conclusions
Al2O3 and TiO2 thin films were deposited on PMMA, PEEK, PTFE and ETFE substrates at 80–250 °C using TMA, AlCl3, TiCl4 and H2O as the precursors. The growth rates on the thermoplastics were roughly in agreement with the growth rates measured on Si substrates. The oxide films had good adhesion on PEEK, whereas the adhesion on PTFE was poor, which may be an indication of different surface reactions during the initial stages of film growth. Al2O3 coatings decreased the water contact angles on all
Acknowledgments
The authors thank Dr. Antti Rahtu for the TiO2 deposition on the ETFE film. Funding from the Academy of Finland (Projects 115601, 201564, 209739, 123248) is gratefully acknowledged.
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