Article pubs.acs.org/Macromolecules
Combined Ionic and Hydrogen Bonding in Polymer Multilayer Thin Film for High Gas Barrier and Stretchiness Chungyeon Cho, Fangming Xiang, Kevin L. Wallace, and Jaime C. Grunlan* Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843, United States S Supporting Information *
ABSTRACT: A unique polymer multilayer thin film with high gas barrier at high strain (>10%) was achieved with a combination of ionic and hydrogen bonding. Layer-by-layer assembly was used to deposit quadlayers (QL) of polyethylenimine (PEI), poly(acrylic acid) (PAA), poly(ethylene oxide) (PEO), and PAA. Altering the deposition pH of the various layers resulted in different physical and mechanical properties. PEI/PAA/PEO/PAA quadlayers assembled at pH 10/4/2.5/2.5 grow much thicker than the same film with all components deposited at pH 3, which is due to a porosity transition during assembly and in-and-out diffusion of the partially charged polyelectrolytes with high chain mobility (PEI at pH 10 and PAA at pH 4). The change in pH during the film assembly induces a porous structure in the 10/4/2.5/2.5 film that results in poor gas barrier. Films deposited on 1 mm thick polyurethane rubber at pH 3 have a densely packed structure with no pores. A 20 QL film (∼1 μm thick) achieves an oxygen transmission rate 15 times lower than uncoated rubber due to the synergistic effect of the interdigitated layers of ionic and hydrogen bonding. When stretched 10%, the barrier improves by a factor of 28 relative to uncoated polyurethane. This combination of stretchability and high gas barrier is unprecedented and offers the opportunity to produce relatively high barrier elastomers.
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INTRODUCTION There is growing demand for low-cost thin films that are flexible and/or stretchable, for protection of paper-like displays, as well as electronic textiles, flexible sensors, and numerous food items.1−7 Conventional gas barrier films are typically fabricated with inorganic oxide thin films or clay-filled polymer composites. Each of these barrier layers has its own advantages and drawbacks. Inorganic oxide films, such as SiOx or Al2O3, are vapor deposited and exhibit reasonably low oxygen transmission rate (OTR), high transparency, and microwaveability.8,9 These oxide thin films also suffer from inherent pinholes and poor adhesion, and they crack upon flexing due to their brittle nature. Flexible clay-filled polymer composites, where clay platelets are randomly dispersed in a polymer matrix, have been shown to modestly improve gas barrier, but they suffer from poor optical clarity and low barrier relative to oxide films.10−12 More recently, polymer and composite thin films with barrier exceeding inorganic oxides have been prepared using layer-by-layer assembly.13,14 Layer-by-layer (LbL) self-assembly is a simple method to create polyelectrolyte multilayer (PEM) thin films by alternately depositing polymers, quantum dots, nanoparticles, dendrimers, and biological molecules with complementary interactions.15−18 Electrostatic interaction, with entropy gain as the driving force for multilayer buildup (due to the release of counterions during deposition), is the most common means of assembly, but other complementary interactions are possible (e.g., hydrogen bonding, coordination bonding, covalent bonding, and hydrophobic interactions).19−21 Growth and © XXXX American Chemical Society
physicochemical properties of LbL assemblies can be simply tailored by adjusting deposition cycles, pH, temperature, molecular weight, ionic strength, and exposure times.22−25 The morphology of these films can be altered by manipulating the pH or salt concentration of the assembly solutions or with postassembly treatments. In the case of pH, it is charge density that alters growth, with low charge density polyelectrolytes producing thick layers.26,27 In regard to polymeric substrates, LbL assembly has been used to impart flame retardant, antimicrobial, and gas barrier behavior.28−30 Assembly of polymers and inorganic platelets has been used to produce a brick wall nanostructure in which clay nanoparticles are highly aligned and exfoliated in a polymer matrix, exhibiting super gas barrier properties (OTR < 0.005 cm3/(m2 day atm)).31−33 This high barrier performance is attributed to near-perfectly oriented clay platelets that force permeating gases to travel long distances perpendicular to the direction of diffusion due to the highly tortuous path created.34 Unlike traditional clay-filled gas barrier films, which suffer from aggregation and random platelet alignment, these multilayer thin films are easy to process, exhibit high transparency, and are very flexible. Most of these assemblies are based on electrostatic interactions (akin to ionic cross-links) and contain a high concentration of inorganic clay particles, so they are prone to crack upon straining. More recently, a stretchable gas barrier Received: June 12, 2015 Revised: August 4, 2015
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DOI: 10.1021/acs.macromol.5b01279 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
dangerous, handle with extreme care. Polyurethane rubber (1 mm thick, McMaster-Carr, Atlanta, GA) was used as the substrate for oxygen transmission rate and strain testing. The PU rubber was rinsed with DI water and methanol before use. Cleaned polyurethane rubber was then plasma-treated in order to improve adhesion of the first PEI layer by oxidizing the film surface. Polished Ti/Au crystals with a resonance frequency of 5 MHz were purchased from Maxtek, Inc. (Cypress, CA), and used to characterize deposited mass per layer with a quartz crystal microbalance (QCM). Assembly of Polyelectrolyte Multilayers. For bilayer (BL) deposition, a given substrate was first dipped into either PEI or PEO for 5 min and rinsed with DI water. This procedure was followed by an identical dipping and rinsing process in the PAA solution, resulting in one deposition sequence of either PEI/PAA or PEO/PAA. After this initial BL was deposited, the same procedure was followed with only 1 min dip times for subsequent layers. The longer immersion time for the first two layers is to ensure the best possible surface coverage of the substrate by the polymers. This process was carried out using homebuilt robot systems.40 For quadlayer (QL) deposition, each treated substrate was first alternately dipped into the PEI and PAA solutions for 5 min, with rinsing steps in between. The substrate was then submerged in PEO and PAA solutions for 1 min, with DI water rinsing steps in between, which results in one quadlayer of PEI/PAA/PEO/ PAA, as shown in Figure 1a. After this initial QL was deposited, the
was produced by introducing hydrogen-bonding layers between electrostatically bonded clay−polymer layers.35 This LbL assembly withstood 10% strain without cracking, but the OTR was only 4 times lower than the neat PET substrate. Fully hydrogen-bonded assemblies of poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA) were able to withstand 100% strain and reduce the oxygen permeability of rubber by a factor of more than 5.36 In that case, 1.5 mm natural rubber OTR was reduced from 840 to 150 cm3/(m2 day atm) with a 20 BL PEO/PAA film assembled at pH 3. In an effort to obtain the best combination of barrier and stretchability, all-polymer LbL thin films were assembled with electrostatic and hydrogen bonding layers. The physical properties of this unique film, such as thickness and mass and the film’s morphology, are shown to depend on the assembly pH conditions. pH is one of the key factors that alters growth and microstructure in LbL films containing weak polyelectrolytes (polyethylenimine (PEI) and PAA in the present case).26,37−39 Of special significance in this study is the intricate control of weak polyelectrolyte-based LbL films that can be induced by changes in the assembly pH. Two different pH conditions, with the same polymers deposited in the same sequence, show the structural extremes that these multilayer films are able to achieve. PEI/PAA/PEO/PAA quadlayer (QL) films assembled at pH 10/4/2.5/2.5 exhibit a porous structure, which is not a good gas barrier, but this high surface area material could be useful for applications such as electrode separators, low refractive index layers, or filtration membranes. The same multilayer assembled at pH 3 results in a dense film (i.e., no observable pores). A 20 PEI/PAA/PEO/PAA QL pH 3 assembly reduced OTR of 1 mm polyurethane (PU) rubber from 138 to 5.5 cm3/(m2 day atm) after a 10% strain. Even with defects appearing on the film’s surface beyond 15% strain, the coated polyurethane maintained OTR 6−8 times below the uncoated substrate, up to a strain of 50%. This excellent combination of gas barrier and stretchiness is thought to be caused by the synergy between the individual bilayer systems: hydrogen-bonded PEO/PAA layers provide the film’s stretchability, while ionically bound PEI/PAA layers provide relatively high gas barrier.
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Figure 1. (a) Schematic of layer-by-layer assembly sequence for PEI/ PAA/PEO/PAA quadlayer thin films and (b) growth profiles of the two QL systems deposited with different pH conditions. remaining quadlayers were deposited using 1 min deposition steps. Unless otherwise stated, the outermost layer of the LbL films was always PAA. PEI/PAA or PEO/PAA LbL films are denoted as (X/Y)Z, where X and Y are the deposition pH of each solution and Z is the number of bilayers assembled. For the QL system, PEI/PAA/PEO/ PAA are denoted as (A/B/C/D)Z, where A, B, C, and D are the deposition pH of the PEI, PAA, PEO, and PAA solutions, respectively, and Z is the number of the QL deposited. Characterization of Thin Films. Assembly thickness on Si wafers was measured with a P-6 profilometer (KLA-Tencor, Milpitas, CA), and reported values represent an average of at least 10 measurements on each film. Deposited mass of each layer was measured with a research QCM (Inficon, East Sycrase, NY). The 5 MHz quartz crystal was plasma cleaned, inserted in a holder, and dipped into each solution. After each deposition, crystals were rinsed with DI water and then left on the microbalance to stabilize for 5 min. Top-view images of LbL assemblies on Si wafers or PU rubber were taken with a JEOL JSM-7500F SEM. For imaging a cross section in SEM, the coated films were immersed in liquid nitrogen and cut with a diamond cutter. The surface structure of films was captured with AFM (Digital Instruments Nanoscope, Plainview, NY) using tapping mode at a scan rate of 0.5 Hz. Root-mean-square (rms) surface roughness of LbL films was averaged from five different AFM images, using a scan size of 20 × 20 μm. OTR testing was performed by MOCON (Minneapolis, MN), in accordance with ASTM-3985, using an Oxtran 2/21 ML instrument at 23 °C and 0% RH. Thin film assemblies with ∼1 μm in thickness were strained up to 100% using an Instron model 4411 tensile tester (Norwood, MA) before measuring the OTR. Elastic modulus was obtained using peak force quantitative nanomechanics (PF-QNM)
EXPERIMENTAL SECTION
Materials. Branched polyethylenimine (Mw = 25 000 g/mol) and poly(acrylic acid) (Mw = 100 000 g/mol, 35 wt % aqueous solution) were purchased from Sigma-Aldrich (Milwaukee, WI). Poly(ethylene oxide) (PEO, Mw = 400 0000 g/mol) was purchased from Polysciences (Warrington, PA). All chemicals were used as received without further purification. Ultrapure water (Milli-Q, Billerica, MA) with a specific resistance greater than 18 MΩ was used in all aqueous solutions and rinses. Prior to deposition, the 0.1 wt % PEI solution’s pH was adjusted from its unaltered value (∼10.5) to 10 and 3 by adding 1.0 M hydrochloric acid (HCl). The 0.2 wt % PAA solution’s pH was adjusted to 2.5, 3, or 4 from its unaltered value (∼3.1) by adding either 1.0 M HCl or 1.0 M sodium hydroxide (NaOH), as needed, prior to multilayer assembly. Aqueous solutions of 0.1 wt % PEO were pH-adjusted to 2.5 or 3 with 1.0 M HCl. Substrates. Single-side-polished, 500 μm thick silicon wafers (University Wafer, South Boston, MA) were used as deposition substrates for profilometry, atomic force microscopy (AFM), and scanning electron microscopy (SEM). Fused quartz glass slides (1 mm thick, Structure Probe, Inc., West Chester, PA) were used as a substrate for visible light transmission measurement. Si wafers and quartz slides were piranha treated for 30 min with a 3:7 ratio of hydrogen peroxide (30%) to sulfuric acid (99%), and stored in deionized water prior to use. Caution: Piranha treatment is very B
DOI: 10.1021/acs.macromol.5b01279 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Bruker Dimension Icon AFM (Billerica, MA) and calculated using the retraction curve near the peak force in conjunction with the Hertz model.41 Values reported represent an average of at least 10 separate measurements on each sample.
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RESULTS AND DISCUSSION
Two different PEI/PAA/PEO/PAA QL systems were assembled, differentiated by deposition pH conditions (10/4/ 2.5/2.5 and 3/3/3/3), using the procedure shown in Figure 1a. This deposition sequence involves alternate immersion of a given substrate into cationic polyethylenimine and anionic poly(acrylic acid), which are attached to each other through electrostatic interactions. The chains of PAA are only partially charged at pH 3 and 4,37 so there are some protonated carboxylic acid groups not participating in the ionic linkages with the amines of PEI. These protonated acid groups are available for hydrogen bonding with the ether oxygen in the deposition of PEO. The adsorbed PEO layer also participates in hydrogen bonding with the next PAA layer, in which free (unpaired) COO− groups at the surface enable ionic bonding with the subsequent PEI layer to start a new quadlayer assembly cycle.42 Quadlayer Film Growth. Figure 1b shows thickness as a function of PEI/PAA/PEO/PAA quadlayers deposited on Si wafers with two different pH conditions. Although both films are exponentially growing, the growth rate of the 10/4/2.5/2.5 system is much faster than 3/3/3/3, achieving a thickness of 1 μm at 10 QL (3/3/3/3 is 300 nm at 10 QL). This difference in growth is attributed to two factors: (1) low charge density of the weak polyelectrolytes (PEI and PAA) in the ionic-bonding bilayers and (2) the influence of pH on the hydrogen-bonded PEO/PAA system. The growth behavior of the individual bilayer systems (PEI/PAA and PEO/PAA) was evaluated to provide some additional insight. PEI and PAA chains are both loopier and less extended at low charge density (pH ≥10 for PEI and ≤4 for PAA) due to intramolecular van der Waals attractions and lack of electrostatic repulsion.38,43 During deposition, these coiled polymers highly interdiffuse to compensate for the charge reversal of the film’s surface, resulting in a dramatic increase in thickness (see Figure S1a in Supporting Information). When depositing at pH 3, where PEI (pKa ∼ 4.5, 6.7, and 11.6 for primary, secondary, and tertiary amine groups, respectively)44 and PAA (pKa ∼ 5.5−6.5)37 are far from their pKa, PEI is fully ionized and assumes a flattened conformation due to a self-repulsion, consequently resulting in relatively thin assemblies.45 In the PEO/PAA system, pH only influences PAA. When the pH of the PAA solution is above 4, the repulsion among the ionized COO− groups is strong enough to prevent hydrogen-bond formation between PEO and PAA, preventing the buildup of layers.46 At lower pH (
