ARTICLE pubs.acs.org/Langmuir
Influence of Clay Concentration on the Gas Barrier of ClayPolymer Nanobrick Wall Thin Film Assemblies Morgan A. Priolo,†,‡ Kevin M. Holder,§ Daniel Gamboa,‡ and Jaime C. Grunlan*,†,‡,|| Materials Science & Engineering Program, ‡Department of Mechanical Engineering, §Department of Chemistry, and Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States
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bS Supporting Information ABSTRACT: The influence of the clay deposition suspension concentration on gas barrier thin films of sodium montmorillonite (MMT) clay and branched polyethylenimine (PEI), created via layer-by-layer assembly, was investigated. Films grown with MMT suspension concentrations ranging from 0.05 to 2.0 wt % were analyzed for their growth as a function of deposited polymerclay bilayers (BL) and their thickness, clay concentration, transparency, nanostructure, and oxygen barrier as a function of the suspension concentration. The film thickness doubles and the visible light transmission decreases less than 5% as a function of MMT concentration for 20-BL films. Atomic force and transmission electron microscope images reveal a highly aligned nanobrick wall structure, with quartz crystal microbalance measurements revealing a slight increase in the film clay concentration as the MMT suspension concentration increases. The oxygen transmission rate (OTR) through these 20-BL composites, deposited on a 179 μm poly(ethylene terephthalate) film, decreases exponentially as a function of the MMT clay concentration. A 24-BL film created with 2.0 wt % MMT has an OTR below the detection limit of commercial instrumentation (10 wt %) suffer from extensive platelet aggregation and a lack of alignment, which lowers the visible light transmission by 2050%.38,39 Thin Film Structure. Transmission electron micrographs of 20-BL films created with 0.05 and 2.0 wt % clay are shown in Figure 4. These images confirm the thickness disparity previously shown by ellipsometry (Figure 2A) and reveal the nanobrick wall structure (Figure 1B) created by the alternate adsorption of positively charged PEI and negatively charged MMT. The cross sections shown here are somewhat thicker than ellipsometry measures and have considerable “waviness.” These issues are primarily due to the preparation of thin sections via ultramicrotomy, where cross sections are cut at a slight angle that gives the 12109
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Figure 5. (A) Oxygen transmission rate and (B) coating oxygen permeability of 20-BL films deposited on 179 μm PET as a function of the clay suspension concentration.
impression of greater thickness and facilitates stress relief in the film. Individual clay platelets can be seen in the images as horizontally oriented dark lines, demonstrating the level of control over platelet orientation afforded by the LbL technique. From these images, the source of thicker growth demonstrated by 2.0 wt % clay suspensions is visible in which a higher level of clay layer overlap and a slightly lower level of clay exfoliation are apparent (Figure 4B). The rms surface roughness, obtained from 20 20 μm2 AFM height images (not shown), increases from 18.3 nm for (PEI/MMT0.2)20 films to 32.0 nm for (PEI/ MMT2.0)20 films. It should be noted that 0.05 wt % MMT films have a roughness of 39.1 nm, which may be due to a decrease in 2D clay packing that creates a “patchy” film surface. AFM height images (3 3 μm2) of films grown with 0.05 and 2.0 wt % MMT on silicon (Figure 4C,D) confirm this difference in film nanostructure, and these images also reveal a cobblestone path structure typical of LbL films made with highly oriented clay. This structure is a key component in a high gas barrier. With the combination of tightly packed and near perfectly aligned platelets, as revealed by AFM and TEM, these films are expected to have very good barrier performance on PET. Influence of Clay Concentration on the Oxygen Barrier. Adding clay to polymer is one of the most common methods used to enhance gas barrier performance.18,20,3845 Layer-bylayer assembly has been shown to generate higher-performance barrier films over conventional clay-filled composites because of its control of clay orientation and exfoliation. The MMT used in this study has a reported average platelet diameter of 200 nm, with a thickness of approximately 1 nm. This large aspect ratio allows clay to create an extremely tortuous pathway for permeating molecules when properly aligned and exfoliated. This tortuous path increases the diffusion length of a gas molecule by rerouting its path through the film, perpendicular to the thickness direction, resulting in a lower transmission rate through the polymer.46 This redirection is due to clay platelets acting as impermeable sheets with respect to the diffusing gas molecule. A platelet’s contribution to the barrier is maximized only when its largest dimension is oriented perpendicular to the diffusion direction, creating the largest physical barrier against permeating gas molecules. In the case of thin film coatings, this orientation is parallel to the deposition substrate and is remarkably evident in the microscopic images shown in Figure 4. The oxygen transmission rate (OTR) data for 20-BL coatings deposited on a 179 μm PET film (created with PEI at pH 10 and increasing clay suspension concentrations) is shown in Figure 5.
The OTR of the bare PET film used in this study is approximately 8.6 cm3/(m2 3 day 3 atm) under dry conditions. It can be seen that increasing the clay concentration by a factor of 40, from 0.05 to 2.0 wt %, decreases the OTR by a factor of almost 80, from 6.12 to 0.08 cm3/(m2 3 day 3 atm). Although using 0.05 wt % MMT yields the least improvement in the barrier, this 20-BL film lowers the OTR of PET by 28% while increasing its thickness by only 0.06% and retaining the transparency of the underlying film (99.9% light transmission, see Figure 3B). Even more interesting is that a 2 orders of magnitude improvement in the barrier is achieved with the same number of layers and only a factor of 2 increase in the film thickness. Figure 5B shows the thin film permeability as a function of the clay suspension concentration of these 20-BL coatings. The coating permeability is obtained by multiplying the OTR by the total thickness and decoupling from the total film permeability (LbL coating and PET film together) using a previously described method.47 This power law trend reveals that there are diminishing returns on film permeability with increasing clay suspension concentration. A film created with a 5.0 wt % clay in suspension actually exhibited such high clay aggregation and haziness that is was not considered in the present study (Supporting Information). The poor quality observed was a result of the high viscosity and lack of clay exfoliation exhibited by the 5.0 wt % clay suspension, where both of these suspension characteristics hinder the uniform deposition of clay in the film. Also, the results in Figure 5A validate the reproducibility of the LbL technique because the (PEI/MMT0.2)20 film’s OTR is within 2% of the values reported for an identical film prepared 2 years earlier for a separate study.18 With the best OTR results of these 20-BL films coming from those created with 2.0 wt % MMT suspensions, films with more than 20 bilayers were deposited to improve the barrier further. Figure 6A shows that an undetectable OTR ( 1 but have volume fractions larger than 0.6, making them more filler-saturated than the model assumes.) Although this model cannot account for contributions to the barrier from chemical interactions of the composite materials with oxygen molecules or the immobilization effects that clay may have on attached polymer chains, it does predict that a higher clay loading increases the barrier performance, a trend verified by Figure 5. Using eq 1, the permeability of PEI (Po), which is unknown, can be roughly calculated on the basis of the permeability of the 0.05 wt % thin film, which has the least amount of tortuosity and influence on the barrier. This method will most likely give a minimum value for Po but is still useful for modeling with Cussler’s equation. Montmorillonite clay platelets are assumed to have a geometric factor (μ) of 4/953,54 and an aspect ratio (α) of 100 (where α = d/2).26 Using these assumptions, the experimental and theoretical relative permeability (Po/P) of these 20-BL PEI/MMT films, as a function of the volume fraction of clay in the film, is shown in Figure 7B. Because the 0.05 wt % film’s permeability was used to calculate Po, the experimental and theoretical data points are identical. It can be seen that the experimental data shows a greater level of film permeability improvement as the MMT volume fraction increases than the level of improvement predicted by Cussler’s model. An increasing average platelet diameter as the clay concentration increases (up to 1150 nm for PEI/MMT2.0 films) must be assumed for this theory to agree with the experimental data. Assuming a larger effective aspect ratio is consistent with previous conclusions that there is more clay platelet overlap with 12111
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Table 1. Volume Fraction of MMT, Oxygen Transmission Rate, and BIF of Films Deposited on 179 μm PET oxygen permeability (1016 cm3(STP) 3 cm/cm2 3 s 3 Pa) thin film assembly
volume fraction MMT (ϕ)
179 μm PET
coatinga
OTR (cm3/m2 3 day 3 atm)
total
8.559
BIF (PS/PT)
17.50
(PEI/MMT0.05)20
0.61
6.123
0.0256
12.52
1.4
(PEI/MMT0.2)20
0.64
3.069
0.0059
6.28
2.8
(PEI/MMT1.0)20
0.80
0.574
0.0012
1.17
15.0
(PEI/MMT2.0)20
0.83
0.078
0.00019
0.16
110.0
(PEI/MMT2.0)22
0.83
0.016
0.00004
0.03
(PEI/MMT2.0)24
0.83
