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Towards a long-chain perfluoroalkyl replacement: Water and oil repellency of polyethylene terephthalate (PET) films modified with perfluoropolyether-based polyesters Tugba Demir, Liying Wei, Naoki Nitta, Gleb Yushin, Philip J. Brown, and Igor Luzinov ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05799 • Publication Date (Web): 28 Jun 2017 Downloaded from http://pubs.acs.org on July 4, 2017
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ACS Applied Materials & Interfaces
Towards a long-chain perfluoroalkyl replacement: Water and oil repellency of polyethylene terephthalate (PET) films modified with perfluoropolyether-based polyesters Tugba Demir1, Liying Wei1, Naoki Nitta2, Gleb Yushin2, Philip J. Brown1, Igor Luzinov1* 1
Department of Materials Science and Engineering, Clemson University, Clemson, South Carolina
29634 2
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia,
30332 *To whom correspondence should be addressed: e-mail:
[email protected] ABSTRACT Original perfluoropolyethers (PFPE)-based oligomeric polyesters (FOPs) of different macromolecular architecture were synthesized via polycondensation as low surface energy additives to engineering thermoplastics. The oligomers do not contain long-chain perfluoroalkyl segments, which are known to yield environmentally unsafe perfluoroalkyl carboxylic acids. To improve the compatibility of the materials with polyethylene terephthalate (PET) we introduced isophthalate segments into the polyesters and targeted the synthesis of lower molecular weight oligomeric macromolecules. The surface properties such as morphology, composition, and wettability of PET/FOP films fabricated from solution were investigated using atomic force microscopy, X-ray photoelectron spectroscopy, and contact angle measurements. It was demonstrated that FOPs, when added to PET film, readily migrate to the film surface and bring significant water and oil repellency to the thermoplastic boundary. We have established that the wettability of PET/FOP films depends on three main parameters: (i) end-groups of fluorinated polyesters, (ii) the concentration of fluorinated polyesters in the films, and (iii) equilibration via annealing. The most effective water/oil repellency FOP has two C4F9-PFPE- tails. The addition of this oligomeric polyester to PET allows (even at relatively low concentrations) reaching a level of oil repellency and surface energy comparable to that of polytetrafluorethylene (PTFE/Teflon). Therefore, the materials can be considered suitable replacements for additives containing longchain perfluoroalkyl substances.
Keywords: oil repellency, water repellency, polyethylene terephthalate, perfluoropolyethers, oleophobicity, hydrophobicity.
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ACS Applied Materials & Interfaces
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INTRODUCTION This paper concentrates on the surface modification of polyethylene terephthalate (PET) with the addition of perfluoropolyethers-based polyesters to improve the water and oil repellency of engineering thermoplastic. Employed in this work, perfluoropolyethers (PFPEs) are macromolecules possessing in their backbone -CF2-, -CF2-CF2-, and -CF(CF3)-CF2- units, which are separated by oxygen atoms.1 PFPEs were first reported in the early 1960s, and since then have proven to be a unique class of macromolecules with low volatility, high chemical inertness and radiation resistance, nonflamability, good oxidative/thermal stability, low coefficients of friction, and low surface tension.2-3 The surface tension of linear PFPE is, indeed, quite low (20–22 mN/m)3 and approaching the surface tension of polytetrafluorethylene (PTFE/Teflon, 18.5 mN/m)4. Owing to their low surface energy, PFPE-based (macro)molecules are considered as a safer replacement for perfluoroalkyl substances containing CnF2n+1 with n ≥ 7 functional groups, which have been used widely for decades to obtain materials with hydro- and oleophobicity.1 However, according to the US Environmental Protection Agency (EPA) carboxylic acids originating from the longchain perfluoroalkyls are “persistent in the environment, bioaccumulative in wildlife and humans, and are toxic to laboratory animals and wildlife, producing reproductive, developmental, and systemic effects in laboratory tests.”5 In general, surfaces with limited wettability by water and oils have received significant attention for a number of applications, including optics, textiles, membranes, solar cells, anti-fouling and self-cleaning boundaries.6-10 In this respect, an extensive portfolio of (low-surface energy) polymers demonstrating low wettability by water is available for use.11 In contrast to hydrophobic surfaces, the preparation of olephobic surfaces is more challenging, as it is connected to the low surface tension of oils (23-40 mN/m), which is much lower than that of water (72 mN/m).12 Since fluorocarbon functional groups possess lower surface energy than hydrocarbon ones (CF3
