Langmuir 2003, 19, 5851-5860
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Surface Characterization of Linear Low-Density Polyethylene Films Modified with Fluorinated Additives Keisha B. Walters,† Dwight W. Schwark,‡ and Douglas E. Hirt*,† Department of Chemical Engineering and Center for Advanced Engineering Fibers and Films, Clemson University, Clemson, South Carolina 29634-0909, and Packaging Research, Cryovac Division of Sealed Air Corporation, Duncan, South Carolina 29334-0464 Received July 24, 2002. In Final Form: April 4, 2003 The modification of a polymer surface is valuable in applications where specific chemical and physical properties must be presented to other contacting materials. The surface concentration and orientation of linear and branched fluorosurfactant additives and the corresponding surface properties were investigated in fluorosurfactant-modified films of linear low-density polyethylene (LLDPE). The results from surfacesegregated samples were compared to those from samples that were solution-coated with the same additives to achieve surface coverages similar to those attainable with complete surface segregation. The chemical composition and physical structure of the modified film surfaces were characterized using attenuated total reflectance FTIR (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and static water contact angle measurements. Results from ATR-FTIR, XPS, and contact angle measurements indicated that, for the fluorinated additives studied, the linear, lower molecular weight additive (DuPont Zonyl FTS, (F(CF2CF2)nCH2CH2OCOC17H35), n ∼ 3.7) segregated more prevalently to the polymer surface than the branched, higher molecular weight additive (DuPont Zonyl TBC, ((F(CF2CF2)nCH2CH2OCO)3C3H5O), n ∼ 4.1). For both additives studied, ATR-FTIR indicated that the effective equilibrium surface concentrations via segregation in bulk-loaded films were lower than those achieved by solution coating additive in amounts equivalent to fully surface-segregated, or one-half the loading of, bulk-loaded films. Topography and phase imaging from AFM showed distinct differences in the physical form of the additives at the polymer surface. The surface-segregated FTS and TBC exhibited grainy, finer scale surface structures than the solution-coated samples. Solution-coated FTS showed overlaid strands of material in the plane of the surface, and solution-coated TBC showed clumplike structures. It is suspected that the difference in surface structure between the two solution-coated samples is due to micellar aggregation behavior of these fluorosurfactants. Angle-resolved XPS (ARXPS) and contact angle results indicated that the perfluoroalkyl end groups of the fluorosurfactants orient at the polymer surface with the CF3 group toward the polymer-air interface. This orientation behavior was clearly seen for bulk-loaded FTS samples and also in solution-coated FTS and TBC samples. For bulk-loaded films, the FTS fluorosurfactant was able to reduce the surface energy of the polymer film at low bulk loadings while TBC had little to no effect at the same loadings due to ineffective surface segregation of the TBC.
Introduction Many polymer films do not have the necessary surface characteristics for particular applications. Modification of the surfaces of polymer films can change the surface properties while leaving key bulk polymer properties intact. This dramatically increases the value of polymers for use in applications such as specialty films, adhesive technologies, biomedical materials, membranes, and packaging. The ability to functionalize the surface of relatively inexpensive polymers, such as polyethylene, would allow them to be used as “tailored” polymers for new applications. There are many physical and chemical techniques that can be used to modify polymer surfaces, including plasma, corona, wet-chemical, etching, and other treatments. The primary goal of the work presented here is to examine the creation of low-energy surfaces for end-use films via surface segregation of fluorinated additives. It has been found that the presence of even very low levels of a fluorinated species can substantially decrease polymer * To whom correspondence should be addressed. E-mail: hirtd@ clemson.edu. † Department of Chemical Engineering and Center for Advanced Engineering Fibers and Films, Clemson University. ‡ Packaging Research, Cryovac Division of Sealed Air Corporation.
surface wettability.1-3 This is commercially important, since maintaining low levels of additive can prevent potential processing problems and also keep the cost of the end product low. Researchers have studied the influence of fluorine concentration on surface segregation and found that additives with higher fluorine content migrate to the surface at a faster rate and are more likely to remain at the surface (i.e., there is a lower potential for redistribution) as long as there are no subsequent changes at the interface.4 For low-energy (e.g., fluorinated) additives, the effective reduction in the polymer surface tension due to the surface localization of the low-energy species can drive the surface segregation of the additives.5 The factors that influence the ability of an additive to segregate to a given polymer-air interface include molecular weight and crystallinity of the additive, chemical functionality of the additive backbone and end groups, architecture of the additive, molecular weight and crys(1) Park, I. J.; Lee, S.; Choi, C. K. J. Appl. Polym. Sci. 1994, 54, 1449-1454. (2) Mason, R. B.; Jalbert, C. A.; Elman, J. F.; Long, T. E.; Gunesin, B.; Koberstein, J. T. Polym. Prepr. 1998, 39, 910-911. (3) Mason, R. B.; Jalbert, C. A.; Muisener, P.; Koberstein, J. T.; Elman, J. F.; Long, T. E.; Gunesin, B. Adv. Colloid Interface Sci. 2001, 94, 1-19. (4) Su, Z.; Wu, D.; Hsu, S. L.; McCarthy, T. J. Macromolecules 1997, 30, 840-845. (5) Schaub, T. F.; Kellog, G. J.; Mayes, A. M.; Kulasekere, R.; Ankner, J. F.; Kaiser, H. Macromolecules 1996, 29, 3982-3990.
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tallinity of the polymer matrix, processing conditions (time and temperature), and environmental conditions at the polymer-air interface. Investigators have modeled the behavior of blends of linear and branched polymers to examine the importance of entropic versus enthalpic contributions to surface segregation.5,6 Walton and Mayes demonstrated that, for a mixture of branched and linear polymers of equivalent molecular weight, the former should segregate preferentially to the surface.7 In their model, it was assumed that the polymers were chemically identical. Preferential surface segregation of the branched material was rationalized upon the basis of the lower entropic penalty associated with placing chain ends at the surface. In general, for a blend of polymeric materials, the drive to reduce the interfacial free energy will cause the component with the lowest surface tension to segregate to the polymer-air interface. The extent of this segregation will be influenced by the miscibility and the mobility of the blend components. Many different systems of graft polymers, block copolymers, and polymer blends involving various degrees of fluorination have been studied.8-11 A particularly thorough series of experimental and theoretical results for the surface segregation and rearrangement of endfluorinated polystyrenes has been reported by Koberstein et al.2,3,12-15 They have demonstrated quite clearly that the surface fraction of fluorinated end groups depends strongly on relative fluorine concentration and molecular weight. In this work, the emphasis will be on the quantitative and qualitative comparison of the surface segregation of linear and branched perfluoroalkyl-terminated additives in a dissimilar polymer matrix, linear low-density polyethylene (LLDPE). The research focuses on two additives in bulk-loaded films, that is, where an additive is blended into a polymer film and allowed to segregate to the surface. By way of comparison, we also characterize solution-coated films, where additive is deposited on a film surface from solution with subsequent evaporation of the solvent. Experimental Section Materials. The two fluorosurfactant additives, one linear and one branched, were examined in Exxon Escorene LLDPE (LL3003.32; F ) 0.9175 g/cm3). The fluorinated additives were obtained from DuPont as waxy solids and ground with a mortar and pestle into a fine powder before being compounded with the polymer. These fluorosurfactant additives are known commercially as Zonyl FTS (MW ) 703) and TBC (MW ) 1563). They are commercially used as processing aids and have the following structures:
FTS RfCH2CH2OCOC17H35
(linear)
TBC (RfCH2CH2OCO)3C3H5O (branched) where Rf ) F(CF2CF2)n and n is approximately 3.7 for FTS and 4.1 for TBC.16 (6) Mayes, A. M.; Kumar, S. K. MRS Bull. 1997, 22, 43-47. (7) Walton, D. G.; Mayes, A. M. Am. Phys. Soc., Phys. Rev. E 1996, 54, 2811-2815. (8) Jannasch, P. Macromolecules 1998, 31, 1341-1347. (9) Thomas, R. R.; Lloyd, K. G.; Stika, K. M.; Stephans, L. E.; Magallanes, G. S.; Dimonie, V. L.; Sudol, E. D.; El-Aasser, M. S. Macromolecules 2000, 33, 8828-8841. (10) Grundke, K.; Pospiech, D.; Kollig, W.; Simon, F.; Janke, A. Colloid Polym. Sci. 2001, 279, 727-735. (11) Sivaniah, E.; Genzer, J.; Fredrickson, G. H.; Kramer, E. J.; Xiang, M.; Li, X.; Ober, C.; Magonov, S. Langmuir 2001, 17, 4342-4346. (12) Balaji, R.; Koberstein, J. T.; Bhatia, Q. S. Polym. Mater. Sci. Eng. 1991, 62, 876-880. (13) Elman, J. F.; Johs, B. D.; Long, T. E.; Koberstein, J. T. Macromolecules 1994, 27, 5341-5349.
Walters et al. Preparation of Bulk-Loaded Films. Films containing 0.5 and 1 wt % additive were extruded using an 18 mm Leistritz twin-screw extruder with 15 cm flex lip die by the Cryovac Division of Sealed Air Corporation in Duncan, SC. The films were nominally 75 µm thick. For extrusion of all films, the stages were set at 190 °C (including the die). Only the feed stage temperature deviated from the set points and was measured to be 174 °C. The corotating twin-screw extruder was operated at 100 rpm, a pressure of 1.45 MPa (210 psi), and a melt temperature of 165 °C. Control films (without any additive present) were also extruded under identical conditions. All films were stored in a freezer to minimize additive migration until room-temperature aging or further experiments and surface characterization were performed. Preparation of Solution-Coated Films. Neat polymer films were coated with additive by first preparing solutions of FTS and TBC in acetone. The solutions were then sonicated to ensure complete mixing and allowed to cool to room temperature. Several concentrations of each additive were prepared, ranging from approximately 0.00026 to 0.00176 g/mL. This concentration range was selected because it provided a range of additive surface coverage that mimics the surface additive concentration attained when the additive in a 1% bulk-loaded, 75 µm thick film completely segregates to the surface (approximately 30 µg/cm2 on each surface). The solutions were applied to neat LLDPE films at a known liquid-layer thickness of 375 µm (15 mils) using a doctor blade, and the solvent was allowed to evaporate completely at room temperature. IR Spectroscopy Measurements. The attenuated total reflectance FTIR (ATR-FTIR) and transmission-FTIR analyses were performed in dry air purged chambers with a Nicolet Avatar 360 at room temperature. The ATR-FTIR analysis was performed under constant clamping load with a horizontal multibounce accessory and Germanium crystal at a 45° incidence angle,17 which provides a penetration depth of approximately 500 nm.18 The peaks at 1150 and 1210 cm-1 were used as the characteristic peaks for the stretching of the CF2/CF3 groups. The peak at 1465 cm-1 (corresponding to CH2 scissoring)19,20 was used for hydrocarbon characterization. The peak at 1735 cm-1, corresponding to the CdO stretch, was used to characterize the carboxylate group. The results are reported as a peak area ratio, namely, the CF2/CF3 peak areas (1150 and 1210 cm-1) divided by the sum of the CF2/CF3 peak areas and the hydrocarbon peak area (1465 cm-1). Two overlapping peaks are evident for these additives within the accepted wavenumber range (1100-1350 cm-1) for the infrared (IR) absorption of the CF2 and CF3 groups.19-23 Therefore, the areas under the two peaks were added and used to represent the fluorine content. The 1150 and 1210 cm-1 peaks were integrated separately with integration limits of 1099-1164 and 1168-1280 cm-1, respectively. The integration limits of the 1465 cm-1 hydrocarbon peak were 1407-1488 cm-1. XPS Measurements. XPS data were obtained using a Kratos AXIS 165 XPS Spectrometer equipped with a monochromated Al KR (1486.6 eV) X-ray source and hemispherical analyzer. The (14) Jalbert, C. A.; Koberstein, J. T. SPE ANTEC Conf. 1995, 2, 2571. (15) Koberstein, J. T. MRS Bull. 1996, 21, 19-23. (16) Dupont Performance Chemicals Technical Information Bulletin, Zonyl Fluorochemical Intermediates, http://www.dupont.com/zonyl/pdf/ genbrochure.pdf, 2001. (17) Muire, L. B.; Hirt, D. E. SPE ANTEC Proceedings, New York City, 1999. (18) Laot, C. M.; Marand, E.; Oyama, H. T. Polymer 1999, 40, 10951108. (19) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. In The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: San Diego, CA, 1991; pp 13-14. (20) Kuptsov, A. H.; Zhizhin, G. N. Handbook of Fourier Transform Raman and Infrared Spectra of Polymers; Elsevier: Amsterdam, 1998. (21) Gordon, A. J.; Ford, R. A. In The Chemist’s Companion. A Handbook of Practical Data, Techniques, and References; John Wiley & Sons: New York, 1972; p 195. (22) Nakanishi, K.; Solomon, P. H. In Infrared Absorption Spectroscopy; Holden-Day Inc.: Oakland, CA, 1977; p 55. (23) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. In Spectrometric Identification of Organic Compounds; John Wiley & Sons: New York, 1991; p 162.
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Figure 2. Representative ATR-FTIR spectra for an LLDPE control film (curve a), an equilibrated 1% FTS film (curve b), and a solution-coated (17 µg/cm2) FTS film (curve c). Figure 1. Plot of ATR-FTIR peak area ratio versus sample age at ambient conditions (a) and effect of sample age on fluorine content as determined by XPS (b). Error bars represent 95% confidence intervals. X-ray source was operated at 15 kV (45 W), and fixed angle XPS measurements were taken at a 45° takeoff angle (TOA), defined as the angle between the sample surface and the axis of the analyzer. Angle-resolved XPS (ARXPS) measurements were performed at TOAs of 0, 15, 30, 45, 60, 75, and 90°. The TOA range used in the ARXPS experiments gives sample probe depths from approximately 1 to 10 nm. The analysis area diameter was maintained at roughly 200 µm. Quantitative analysis was performed by analyzing XPS peak areas using atomic sensitivity factors. The binding energy analysis was referenced to the C 1s signal at 284.8 eV. Sample charging was neutralized using the dual approach of an electron flood gun and low-energy Ar+ ions. AFM Measurements. AFM was used to gather information on the morphology of the film surfaces. These data were obtained with a Digital Instruments Dimension 3100 in “tapping mode”. The height (topography) and phase images were both captured at 10 µm × 10 µm and 2 µm × 2 µm scan sizes using a frequency of 1.0 Hz and 256 scans per image. Surface roughness was determined using the NanoScope Software Version 4.23 rootmean-square (rms) roughness calculation. Contact Angle Measurements. Static contact angle measurements were made at room temperature using a Kruss G10 instrument with a digital photoanalyzer. A constant sessile droplet volume was used and was chosen so that the Bond number (Bo) was
