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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers
Transparent Polyelectrolyte Complex Thin Films with Ultralow Oxygen Transmission Rate Ryan J. Smith, Carolyn T. Long, and Jaime C. Grunlan Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02391 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018
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Transparent Polyelectrolyte Complex Thin Films with Ultralow Oxygen Transmission Rate Ryan J. Smitha, Carolyn T. Longb, Jaime C. Grunlana,b,c* a
Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, Texas,
USA 77843 b
Department of Mechanical Engineering, Texas A&M University, 3123 TAMU, College
Station, Texas, USA 77843 c
Department of Materials Science and Engineering, Texas A&M University, 3003 TAMU,
College Station, Texas, USA 77843 *Corresponding author. Email:
[email protected] 1 ACS Paragon Plus Environment
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ABSTRACT Limiting oxygen permeation through plastic films is important for extending the shelf-life of food and flexible electronic devices. Polyelectrolyte complex (PEC) thin films can be used to reduce small molecule diffusion through commodity plastic films. PEC thin films are frequently applied using layer-by-layer assembly, which often requires many processing cycles to deposit a film with desired thickness. An aqueous solution of poly(diallydimethylammonium chloride) and poly(acrylic acid) can be deposited in a single-step to quickly fabricate a high oxygen barrier thin film. These films have an ionically bonded network that forms after polyelectrolyte deposition and exposure to buffer. Increasing buffer concentration and adding salt increases film cohesion and improves transparency by reducing surface roughness. When deposited onto a 178 µm poly(ethylene terephthalate) film, a ~1.9 µm thick PEC coating imparts a two order of magnitude reduction in oxygen transmission rate. Achieving this level of gas barrier with a single thin coating layer creates numerous opportunities for the protection of sensitive food, pharmaceuticals, and electronics.
Keywords: poly(acrylic acid), poly(diallyldimethylammonium chloride), polyelectrolyte complex, oxygen barrier, poly(ethylene terephthalate)
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Introduction Electrostatic complexation between oppositely-charged polyelectrolytes can occur along a spectrum, ranging from a homogeneous solution to a liquid-like coacervate, or a dense precipitate.1-2 The nature of these complexes is dependent on the salt concentration, pH of the polyelectrolyte mixture, and the chemistry of the mixed polyelectrolytes.3-5 Since polyelectrolyte complex precipitates (PEC) are insoluble in water, thin films are difficult to deposit from aqueous solutions. Layer-by-layer (LbL) assembly has been used to deposit films with multiple functionalities, to various substrates, by sequentially building a PEC thin film on a surface through alternating exposures to oppositely-charged polyelectrolytes.6-7 To reduce the numerous processing steps that LbL processing typically requires, other methods of depositing PEC coatings from aqueous solutions in a single step have been developed. Sedimentation of complexes has been shown to produce coalesced and uniform thin films.8-9 PEC thin films have also been deposited by simultaneously spraying oppositely charged polyelectrolytes onto a substrate.10-11 Films have also been cast by controlling the pH of the polyelectrolyte mixture solutions in an effort to lower charge density and limit electrostatic interactions.12-14 Coacervates have also been deposited through electrospinning and spin-coating.15-16 Thin, transparent, and flexible films are required for food packaging to improve shelf-life and minimize food waste.17-18 This is often achieved through constructing thick (tens to hundreds of µm) multilayered plastic films or by metalizing plastics.19-20 Metalized plastic films are typically opaque and both types of barrier films are difficult to recycle. SiOx and AlxOy thin films provide excellent oxygen barrier, but suffer from poor adhesion to plastic substrates, are very inflexible (i.e. they crack under very low strain), and are relatively expensive.21-22 Transparent, flexible thin films made of polyelectrolyte complexes have been developed and applied to a 3 ACS Paragon Plus Environment
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myriad of substrates using the LbL assembly technique.23 Clay nanoplatelet-containing composites have been deposited to impart significant tortuosity, slowing the transport of small molecules through a film.24-26 Transparent all-polymer thin films have also been deposited layerby-layer to achieve high barrier and selectivity towards small molecules due to increased cohesion energy and low free volume within the PEC.27-30 These coatings are very effective, but the number of processing steps (often > 10) inherently required to construct them diminishes commercial feasibility. Recently, a branched polyethylenimine and poly(acrylic acid) coacervate was deposited in a single step (using a Meyer rod), significantly reducing the oxygen transmission rate of poly(ethylene terephthalate) (PET) film.31 Despite exhibiting high oxygen barrier, this coacervate did not wet the PET evenly and a post-cure humidity treatment was required to fill in pinholes and improve film clarity. In the present study, transparent poly(diallydimethylammonium chloride) (PDDA) and poly(acrylic acid) (PAA) PEC thin films (~1.9 µm) were quickly fabricated by dipping 178 µm PET film into a polyelectrolyte mixture, followed by curing in a citric acid buffer. These films did not require any post-cure annealing to achieve high transparency or gas barrier.
Experimental Section Materials. Poly(diallyldimethylammonium chloride) (PDDA;400-500 kg/mol; 20 wt% aqueous solution), poly(acrylic acid) (PAA; 250 kg/mol; 35 wt% aqueous solution), and citric acid monohydrate (CA; 99%) were purchased from Sigma Aldrich (St. Louis MO) and used as received. 18 MΩ Deionized (DI) water was used for all aqueous solutions and rinsing procedures. CA was prepared as 25, 100, 200, and 300 mM solutions. Solutions at the same concentration were also prepared at constant ionic strength (~150 mM) by adding the appropiate 4 ACS Paragon Plus Environment
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amount of sodium chloride. All CA solutions were adjusted to pH 3. Poly(ethylene terephthalate) film (PET, 178 µm thick, ST505, Dupont-Teijin) was purchased from Tekra (New Berlin, WI). The PET was rinsed with a sequence of DI water, methanol, and DI water and dried with filtered compressed air. Clean PET was exposed to plasma cleaning for five minutes using a PDC-32G plasma cleaner (Harrick Plasma Inc. Ithica, NY). Glass slides for atomic force microsopy, UV-Vis light transmission, nanoindentation, aluminum foil for FTIR, and silicon wafers for ellipsometry were prepared the same way. Deposition of Polyelectrolyte Complex Thin Films. PDDA and PAA were diluted with DI water (5 and 8.75 wt%, respectively). PAA was slowly added to PDDA until a 1:3 molar ratio (based on repeat units) was reached. The polyelectrolyte mixture was then diluted to 1.5, 3, 4.5 or 6 wt% dissolved solids with DI water. The pH was adjusted to 2 using 5 M HCl. The solution was allowed to stir (magnetic) overnight to dissolve any complex formed during intial mixing. The final solution was non-turbid and homogeneous. Cleaned PET film was immersed in the polyelectroltye solution for five minutes, after which the excess polymer solution was wicked away, and the sample was placed in an oven at 150 °C for 20 minutes. The sample was then placed in a dry box (~ 11% RH) for 3 hours, before being immersed in citric acid buffer at pH 3 for 20 minutes to cure the coating (i.e. form ionic crosslinks). The sample was then dip rinsed in DI water for 20 seconds three times, and placed in an oven for 20 minutes at 150 °C. The finished samples were stored in a dry box (~11% RH) prior to testing. This process was carried out in an identical fashion on glass slides and Al foil. Film Characterization. Thickness of PEC films deposited on glass slides was measured using a DektakXT Surface Profiler (Bruker, Billerica, MA), with a stylus force of 2 mg and stylus radius of 12.5 µm. Surface morphology was evaluated using a Dimension Icon atomic force microscope 5 ACS Paragon Plus Environment
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(Bruker, Billerica, MA) in tapping mode. Changes in film thickness during curing were measured using an α-SE ellipsometer (J.A. Wollam Co. Lincoln, NE) on PEC films deposited on silicon wafters. Changes in mass during curing were measured using a Q-50 thermogravimetric analyzer (TA Instruments, New Castle, DE). Polyelectrolyte solution was deposited in an aluminum pan and placed in an isothermal hold at 150 °C for 20 minutes to simulate oven drying. The mass was recorded and the pan was then immersed in a citric acid buffer for 20 minutes, dip-rinsed for 20 seconds in DI water three times and then placed back in the TGA for 20 minutes (isothermal 150 °C), where the mass was recorded again. Film surfaces were characterized using a Dimension Icon atomic force microscope (Bruker, Billerica, MA) in tapping mode. AFM probes (HQ:NSC35/Al BS, Micromasch USA Watsonville, CA) had a force constant of 5.5-16 N/m and a tip radius of ~8 nm. Reduced modulus (Er) of PEC coatings deposited on glass slides was evaluated using a TI 950 Triboindenter (Hysitron, Inc, Minneapolis, MN). A Berkovich tip with a radius of curvature of ~150 nm was used with a loading force of 200 µN (to keep indentation depth ~10% of coating thickness) and was calibrated against a fused quartz standard to generate the area function. A loading profile of 10 s of loading, followed by 5 seconds at a stationary position and 2 seconds of unloading was used. Infrared spectra were taken on PEC coatings deposited on aluminum foil,32 using an Alpha Platinum-ATR FTIR spectrometer (Bruker, Billerica, MA), taking 30 scans from 400-4000 cm-1 with a resolution of 4 cm-1. Light transmission through gas barrier coatings deposited on glass slides was measured using a USB2000 UV-Vis spectrometer (Ocean Optics Inc., Largo, FL) at 550 nm. Data was normalized so uncoated glass slides measured ~100 % transmission. Oxygen transmission rate measurements were performed by Ametek-Mocon
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(Minneapolis, MN) using an Oxtran 2/21 ML oxygen permeability instrument (in accordance with ASTM Standard D-3985) at 23 °C and at 0% RH. Results and Discussion In order to simultaneously deposit oppositely-charged polyelectrolytes as a uniform thin film from water, the electrostatic interactions must be inhibited to prevent complexation in solution. This can typically be achieved through the introduction of salt and/or pH adjustment, depending on polyelectrolyte chemistry.3-5 Poly(diallyldimethylammonium chloride) and poly(acrylic acid) can be mixed at pH ~2 to form a homogeneous mixture suitable for deposition. PDDA is a strong polyelectrolyte whose charge density is independent of pH. PAA on the other hand, has a pKa of ~4.5 and charge density can be decreased by reducing pH through protonation of the carboxylic acid.33 The polyelectrolyte mixture (PM) at pH 2 is suitable for coating a wide variety of substrates (e.g. glass, PET film, and Al foil). Deposition of the polyelectrolyte mixture onto a substrate was carried out through immersion (i.e. dipping) of the substrate into the polymer solution. The polyelectrolytes likely adsorb through a combination of van der Waals and dipole interactions. After initial deposition of the PM, the coating is dried at 150 °C, thereby immobilizing the polymers onto the substrate surface (Figure 1a). The films are then cured by exposure to a citric acid (CA) buffer solution, which causes deprotonation of PAA and subsequent formation of ionic crosslinks with PDDA. FTIR spectroscopy shows that the PAA within the film is completely protonated (strong asymmetric stretch at ~ 1700 cm1) (Fig. 1b) before curing. When the coating is cured in 300 mM CA buffer (pH 3), the intensity of the absorbance at 1700 cm-1 decreases, while a new absorbance at ~1550 cm-1 appears (from the carboxylate).34 This suggests that an ionic network is formed, leading to an insoluble polyelectrolyte complex film (coating). 7 ACS Paragon Plus Environment
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Figure 1. Schematic of the polyelectrolyte complex thin film deposition process (a). FTIR of PDDA/PAA PEC thin film deposited on aluminum foil before (green) and after (blue) curing in 300 mM citric acid buffer at pH 3 (a).
The thickness of PEC coatings is influenced by the concentration of dissolved polyelectrolytes in the solution used for deposition. Thickness of the coatings deposited on glass slides was measured using profilometry at varying pH and polyelectrolyte concentration. Prior to curing with buffer, coating thickness increases with the concentration of polymers in solution (Fig. 2a). As this process utilizes solvent evaporation to deposit the initial coating, having more dissolved polyelectrolyte in solution leads to more polymer deposited.35 The influence of citric acid concentration and ionic strength during the curing step was also evaluated. Increasing buffer concentration increases the ionic strength of the curing solution, which is well known to influence the formation of polyelectrolyte complexes.3-5 In order to account for this, samples were also cured in citric acid buffer solutions that were held at a constant ionic strength (~150 mM) by adding NaCl. Figure 2b shows the coating thickness as a function of buffer and salt 8 ACS Paragon Plus Environment
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concentration at pH 3. Increasing buffer concentration, with and without added salt, does not appear to influence the final thickness of the coatings. On average, these coatings are ~ 1900 µm thick. The uncured coatings are also ~1900 nm thick, suggesting that the thickness is determined by the initial amount of polymer mixture deposited rather than curing conditions (pH, buffer concentration, and ionic strength), as evidenced by the increase in thickness observed with greater polymer concentration in the deposition solution (Fig. 2a).
Figure 2. Thickness of PDDA/PAA thin films as a function of dissolved solids and pH (with 200 mM citric acid buffer held constant) (a). Thickness of 6 wt% PDDA/PAA thin films cured at pH 3, as a function of citric acid concentration, with and without added salt (b).
PDDA/PAA PEC polyelectrolyte complexes were deposited and cured on glass slides to observe any difference in film transparency due to variations in buffer concentration and ionic strength. Data was normalized so that uncoated glass slides exhibited ~ 99.2 ± 0.6% light transmittance. The uncured polyelectrolyte mixture has a visible light transmittance of ~ 99.3 ± 1.0%. Curing of a PEC coating causes some rearrangement of the polymers, as evidenced by the reduction of light transmission. Table 1 shows light transmission as a function of curing conditions. Increasing the buffer concentration appears to increase transparency. Coating uniformity is much better when NaCl is included in the curing solutions at 25 mM and 100 mM 9 ACS Paragon Plus Environment
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CA. This can be seen by the considerably smaller standard deviation in transparency for these films. At 200 mM CA (with and without NaCl) and 300 mM CA, almost all light is transmitted (> 95%). Salt is known to plasticize PEC through electrostatic charge shielding,36-38 and adding salt to the curing solutions likely increases polymer mobility that creates a more coalesced (or inter-diffused) structure. When the ionic strength is held constant with NaCl, an increase in transparency is observed, indicating that ionic strength is not the only determining factor. It is believed that increasing buffer concentration increases the citrate concentration, which increases deprotonation (up to ~3 %) and leads to more interpolymer ionic crosslinks and a more uniform coating. These assumptions are reinforced by images of surface topography.
Table 1. Transparency, roughness and reduced modulus of PDDA/PAA PEC films as a function of curing pH and added salt. Curing Conditions*
Transparency [%T]
Roughness (Rq) [nm]
Reduced Modulus [GPa]
No Cure
99.2 ± 0.2
