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Langmuir 1998, 14, 5586-5593
Surface Modification of Poly(ethylene terephthalate) To Prepare Surfaces with Silica-Like Reactivity Alexander Y. Fadeev1 and Thomas J. McCarthy* Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003 Received May 1, 1998. In Final Form: July 14, 1998 Reactions of semicrystalline poly(ethylene terephthalate) (PET) film with 3-aminopropyltrialkoxysilanes at the film-solution interface and subsequent hydrolysis render silanol (Si-OH) functionality that is attached to the PET surface by amide linkages (PET-CONH(CH2)3Si(OH)3). Toluene was found to be the preferred solvent for the initial amidation reaction, rendering a higher surface concentration of silanol groups than that for other solvents. Polycondensation of tetraethyl orthosilicate on the surface of PETCONH(CH2)3Si(OH)3 produces thin silica films, the thickness of which can be controlled with reaction time. The surface of this composite film (PET-(SiO2)X-OH) also contains reactive silanol groups. These two reactive surfaces were further modified using other reactive silanes to introduce alkyl, perfluoroalkyl, bromoalkyl, and aminoalkyl functionality to the PET film surface. The surface density of attached groups was assessed by X-ray photoelectron spectroscopy, and wettability of the surfaces was determined using contact angle analysis. The reactivity of these surfaces was compared to that of oxidized silicon wafers, and we conclude that these PET modification procedures (amidation with 3-aminopropyltrialkoxysilanes and polycondensation of tetraethyl orthosilicate) produce surfaces that react with the versatility of oxidized silicon wafers.
Introduction We have developed a number of synthetic routes to chemically modified polymer surfaces with the objective of preparing substrates that we can use to rationally address the control of macroscopic surface properties (adhesion, adsorption, friction, and wettability). Our approach is to introduce a discrete covalently attached functional group (e.g., alcohol, -OH) that can be further derivatized to a series of chemically different surfaces through well-controlled chemical reactions of this functional group. We have focused primarily on chemically resistant polymers, including poly(tetrafluoroethylene),2,3 poly(tetrafluoroethylene-co-hexafluoropropylene),4,5 poly(chlorotrifluoroethylene),5-12 poly(vinylidene fluoride) (PVF2),5,13 polypropylene,14 and poly(ether ether ketone) (PEEK).15 These materials have a number of advantages as substrates for surface chemistry (that have been discussed in these papers), but in particular they function as inert supports for chemical reactions that require a range of harsh conditions. Carrying out similar modifications with less chemically resistant (more reactive) polymers is often of practical interest, but this is more difficult because conditions have
to be chosen that affect only the functionality introduced and not the reactive substrate. This can severely restrict the choices of reaction conditions. We have recently turned our attention to more reactive polymers and have reported16-18 studies on the chemical surface modification of poly(ethylene terephthalate) (PET). PET is used in various forms for an enormous range of applications;18 the fact that it can be (and is being) recycled practically suggests that its use will increase. Its intrinsic low surface energy, however, results in poor adhesion, wettability, and biocompatibility, and it has to be surface-modified for many applications. One of the objectives of our work16,17 was to prepare “pure” (containing no other reactive functionality) alcohol surfaces and assess their reactivity. This was accomplished by reduction and glycolysis reactions (eq 1), and we point out several problems with these
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* To whom correspondence should be addressed. (1) On leave from the Chemistry Department, M. V. Lomonosov Moscow State University, 119899 Moscow, Vorob. Gory, Russia. (2) Costello, C. A.; McCarthy, T. J. Macromolecules 1987, 20, 2819. (3) Costello, C. A.; McCarthy, T. J. Macromolecules 1990, 23, 2648. (4) Bening, R. C.; McCarthy, T. J. Macromolecules 1990, 23, 2648. (5) Shoichet, M. S.; McCarthy, T. J. Macromolecules 1991, 24, 982. (6) Dias, A. J.; McCarthy, T. J. Macromolecules 1985, 18, 1826. (7) Dias, A. J.; McCarthy, T. J. Macromolecules 1987, 20, 2068. (8) Lee, K.-W.; McCarthy, T. J. Macromolecules 1988, 21, 2318. (9) Lee, K.-W.; McCarthy, T. J. Macromolecules 1988, 21, 3353. (10) Kolb, B. U.; Patton, P. A.; McCarthy, T. J. Macromolecules 1990, 23, 366. (11) Cross, E. M.; McCarthy, T. J. Macromolecules 1990, 23, 3916. (12) Bee, T. G.; McCarthy, T. J. Macromolecules 1992, 25, 2093. (13) Dias, A. J.; McCarthy, T. J. Macromolecules 1984, 17, 2529. (14) Lee, K.-W.; McCarthy, T. J. Macromolecules 1988, 21, 309. (15) Franchina, N. L.; McCarthy, T. J. Macromolecules 1991, 24, 3045.
systems: (1) Chemistry at the ester functionality causes chain cleavage that leads to surface functionality at new chain ends, but also to polymer degradation; surface functionalization has to be balanced against sample erosion. (2) The surface functional group concentration depends on the orientation of chains relative to the plane (16) Chen, W. Ph.D. Dissertation, University of Massachusetts, Amherst, MA, 1997. (17) Chen, W.; McCarthy, T. J. Macromolecules 1998, 31, 3648. (18) Chen, W.; McCarthy, T. J. Macromolecules 1997, 30, 78.
S0743-7463(98)00512-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/21/1998
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of the surface and this depends on sample history.16,17 If chains are oriented perpendicular to the plane of the surface, then surface functional group density is higher than if chains are oriented parallel. (3) The surfacefunctionalized PET samples contain reactive ester functionality as well as alcohols, and this restricts conditions for further modification reactions of the alcohols. There are numerous reports from other groups of the chemical modification of PET using ester chemistry, which range in objectives from chemical recycling20 to surface modification (by hydrolysis,21-23 reduction,23,24 and aminolysis23,25). PET has also been surface-modified by other techniques including plasma,26 corona discharge,27 ion beam treatment,28 laser treatment,29 photoinitiated graft polymerization,30 entrapment of poly(ethylene oxide),31 and activation of alcohol chain ends with p-toluenesulfonyl chloride and subsequent chemistry.32,33 The degradation that accompanies surface modification has been appreciated, and polyamines that give rise to cross-linking have been used to minimize degradation.25 Thompson et al.23 showed that reaction of PET with 3-aminopropyltriethoxysilane (APTES) forms a cross-linked multilayer siloxane network (eq 2)
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with a thickness in excess of the sampling depth of X-ray photoelectron spectroscopy. This research showed that no degradation of the PET occurs and offers evidence that the principal mode of interaction between PET and APTES is a covalent amide bond. Another mode of interaction between PET and APTES (through a siloxane bond) has also been (erroneously, in our opinion) proposed.34 Thompson et al.23 proposed the insertion of APTES into the PET chain (eq 3) to account for the absence of polymer degradation. (19) Werner, E.; Janocha, S.; Hopper, M. J.; Mackenzie, K. J. Enclyclopedia of Polymer Science and Engineering, 2nd ed.; Mark, H. F., Bikales, N. M., Overberger, C. G., Mengers, G., Kroschwitz, J. I., Eds.; John Wiley and Sons: New York, 1989; Vol. 12, p 193. (20) Paszun, D.; Spychaj, T. Ind. Eng. Chem. Res. 1997, 36 (6), 1373. (21) Kumar, D. J.; Srivastava, H. C. J. Appl. Polym. Sci. 1987, 33, 455. (22) Solbrig, C. M.; Obendorf, S. K. J. Appl. Polym. Sci.: Appl. Polym. Symp. 1991, 47, 437. (23) Bu`i, L. N.; Thompson, M.; McKeown, N. B.; Romaschin, A. D.; Kalman, P. G. Analyst 1993, 118, 463. (24) Collin, R. J. U.S. Patent 2,955,954, 1964. (25) Avny, Y.; Reubenfeld, L. J. Appl. Polym. Sci. 1986, 32, 4009. (26) Wang, J.; Feng, D.; Wang, H.; Rembold, M.; Fritz, T. J. Appl. Polym. Sci. 1993, 50, 585. (27) Strobel, M.; Lyons, C. S.; Strobel, J. M.; Kapaun, R. S. J. Adhes. Sci. Technol. 1992, 6, 429. (28) Bertrand, P.; DePuydt, Y.; Beuken, J. M.; Lutgen, P.; Feyder, G. Nucl. Instrum. Methods Phys. Res. 1987, B19-20, 887. (29) Arenolz, E.; Heitz, J.; Wagner, M.; Baeuerle, D.; Hibst, H.; Hagemeyer, A. Appl. Surf. Sci. 1993, 69, 16. (30) Yao, Z. P.; Rånby, B. J. Appl. Polym. Sci. 1990, 41, 1459. (31) Desai, N. P.; Hubbell, J. A. Macromolecules 1992, 25, 226. (32) Mougenot, P.; Marchand-Brynaert, J. Macromolecules 1996, 29, 3552. (33) Mougenot, P.; Koch, M.; Dupont, I.; Schneider, Y.-J.; MarchandBrynaert, J. J. Colloid Interface Sci. 1996, 177, 162. (34) Kurematsu, K.; Wada, M.; Koishi, M. J. Colloid Interface Sci. 1986, 109, 531. (35) Iler, R. K. The Chemistry of Silica; John Wiley & Sons: New York, 1979. (36) Leyden, D. E., Ed.; Silanes, Surfaces, and Interfaces; Gordon and Breach: New York, 1986.
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With the chemistry of eq 3, we began the research described here. Our objectives were to carry out this transformation without the accompanying APTES multilayer formation and to hydrolyze the insertion product to yield covalently attached silanol functionality (PETCONH(CH2)3Si(OH)3). Silanol surface chemistry is highly evolved for inorganic surfaces,35,36 and a wide range of silane coupling chemistry is feasible that will not affect the ester functionality in PET. We report here the synthesis of PET-CONH(CH2)3Si(OH)3 as well as a PETsupported thin silica film (PET-(SiO2)X-OH) prepared by condensing tetraethyl orthosilicate (TEOS) on the surface of PET-CONH(CH2)3Si(OH)3 and demonstrate that both of these surfaces react with silane coupling agents in a fashion similar to oxidized silicon wafers. Scheme 1 is a summary of the transformations that we have carried out on PET-CONH(CH2)3Si(OH)3 and PET(SiO2)X-OH surfaces. Experimental Section General Information. PET film (duPont Mylar, 125 µm) was cleaned by rinsing with distilled water, drying, and rinsing with hexane. Toluene, methanol, ethanol, 2-propanol, and heptane (all Fisher HPLC grade) were used as received. House purified water (reverse osmosis) was further purified using a Millipore Milli-Q system that involves reverse osmosis, ion exchange, and filtration steps (1018 Ω/cm). 3-Aminopropyltrimethoxysilane (APTMS), 3-aminopropyltriethoxysilane (APTES), tetraethyl orthosilicate (TEOS), ethyldiisopropylamine, bromomethyldimethylchlorosilane (BrCH2Me2SiCl), trimethylchlorosilane (Me3SiCl), and bromopropyltrichlorosilane (Br(CH2)3SiCl3) were obtained from Aldrich and used as received. 1,1′,2,2′Tetrahydroperfluorodecyldimethylchlorosilane (RfMe2SiCl) and 1,1′,2,2′-tetrahydroperfluorodecyltrichlorosilane (RfSiCl3) were obtained from Gelest and used as received. X-ray photoelectron spectra (XPS) were recorded with a Perkin-Elmer Physical Electronics 5100 with Mg Ka excitation (400 W). Spectra were obtained at two different takeoff angles, 15° and 75° (between the plane of the surface and the entrance lens of the detector optics). Contact angle measurements were made with a Rame`Hart telescopic goniometer and a Gilmont syringe with a 24gauge flat-tipped needle. Probe fluids were water, purified as described above, and hexadecane, purified by vacuum distillation. Dynamic advancing (θA) and receding angles (θR) were recorded while the probe fluid was added to and withdrawn from the drop, respectively. The values reported are averages of 5-10 measurements made on different areas of the film sample surface. The modified surfaces reported here showed very homogeneous surfaces as evidenced by contact angle measurements that were within (2° of the average. Preparation of PET-CONH(CH2)3Si(OH)3. APTES or APTMS (0.3 mL) was added by syringe to a flask containing 30 mL of toluene and 7 samples of PET film (1.5 × 1.5 cm) that were held in a custom sample holder. The resulting solution was stored at room temperature or heated at 55 °C for 24 or 48 h under nitrogen. The film samples were isolated, rinsed with 4 × 30 mL of toluene, and dried at reduced pressure overnight. These film samples were hydrolyzed by submersion in water overnight, then were rinsed with water, and dried at reduced pressure for greater than 2 h. Preparation of PET-(SiO2)X-OH. PET-CONH(CH2)3Si(OH)3 film samples were placed in a suspension of 0.3 mL of TEOS in 30 mL of water and stirred at room temperature for 45 min to 6 h. After this time, samples were isolated, washed with water, and dried at reduced pressure for greater than 2 h.
5588 Langmuir, Vol. 14, No. 19, 1998
Fadeev and McCarthy Scheme 1
Reactions of PET-CONH(CH2)3Si(OH)3 and PET(SiO2)X-OH with Organosilanes. Modified PET samples were placed in 10 mL of toluene containing 1 × 10-3 M (∼0.15 mL) ethyldiisopropylamine, and 0.2-0.5 mL (to render a concentration of ∼1 × 10-3 M) of organosilane was added by syringe. Reaction mixtures were stored overnight at room temperature, and then samples were isolated, washed (in this order) with toluene, ethanol or methanol, water, and then ethanol or methanol, and dried at reduced pressure for greater than 2 h. The organosilanes listed in the General Information section above were used in these reactions. No ethyldiisopropylamine was added in the reaction of PET-(SiO2)X-OH with APTES.
Results and Discussion Preparation of Silanol-Functionalized PET. PET samples were exposed to APTES (or in one case APTMS) using a range of reaction conditions that is summarized in Table 1 along with XPS atomic composition data for the products of the reactions. The presence of N(1s) (401 eV), Si(2s) (153 eV), and Si(2p) (103 eV) in the XPS spectra of
the products (Figure 1) indicates the incorporation of amidopropylsilane functionality into the PET surface. The data in Table 1 indicate that the reaction yield (determined from the nitrogen atomic concentration) depends on solvent, temperature, and reaction time. The highest degree of surface substitution occurs with toluene; heating the reaction mixture increases the rate of reaction, but the same degree of substitution occurs at lower temperatures with longer reaction times (see entries 6-8). APTMS produces surfaces that are indistinguishable from those prepared with APTES (see entries 4 and 5). The atomic composition data in Table 1 provide insight into the structure of the modified products. The PET substrate has a theoretical carbon:oxygen ratio of 10:4, and this is confirmed by XPS. A small amount of silicon is present in the virgin PET that we believe is due to a silica filler that is added to the film as an antiblocking agent; micron-size subsurface spherical particles are observed in secondary electron images of this film.16 This
Surface Modification of Poly(ethylene terephthalate) Table 1. XPS Atomic Composition Data for PET Modified with APTES entry
reaction conditions
Ca
Oa
Sia
Na
composition
1
virgin PET
70.5 28.1 69.1 30.4
1.1 0.5
C10O4
2
2-propanol (25 °C, 24 h)
71.8 25.8 71.7 27.3
2.3 (1.2)b
