VOC-Induced Flexing of Single and Multilayer Polyethylene Films As

Mar 29, 2016 - Single-, 24-, and 288- layer films displayed no motion when uniformly subjected to. VOCs, but they could display simple curving modes w...
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VOC-Induced Flexing of Single and Multilayer Polyethylene Films As Gas Sensors Nazanin Alipour, Richard L. Andersson, Richard T. Olsson, Ulf W. Gedde, and Mikael S. Hedenqvist* KTH Royal Institute of Technology, School of Chemical Science and Engineering, Fibre and Polymer Technology SE-100 44 Stockholm, Sweden S Supporting Information *

ABSTRACT: The differential swelling and bending of multilayer polymeric films due to the dissimilar uptake of volatile organic compounds (VOCs; n-hexane, limonene) in the different layers was studied. Motions of thin polyethylene films triggered by the penetrant were investigated to learn more about how their deformation is related to VOC absorption. Single layers of metallocene or low-density polyethylene, and multilayers (2−288 layers) of these in alternating positions were considered. Single-, 24-, and 288layer films displayed no motion when uniformly subjected to VOCs, but they could display simple curving modes when only one side of the film was wetted with a liquid VOC. Two-layer films displayed simple bending when uniformly subjected to VOCs due to the different swelling in the two layers, but when the VOC was applied to only one side of the film, more complex modes of motion as well as dynamic oscillations were observed (e.g., constant amplitude wagging at 2 Hz for ca. 50 s until all the VOC had evaporated). Diffusion modeling was used to study the transport behavior of VOCs inside the films and the different bending modes. Finally a prototype VOC sensor was developed, where the reproducible curving of the two-layer film was calibrated with n-hexane. The sensor is simple, cost-efficient, and nondestructive and requires no electricity. KEYWORDS: VOC sensor, multilayer films, modeling, flexing, polyethylene

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INTRODUCTION The handling of organic solvents requires strict safety requirements, especially in an industrial setting, where large quantities of toxic volatile organic compounds (VOCs) may be released.1,2 Regulations limiting the concentrations of these vapors are enforced by law and robust, accurate, and cheap gas sensor devices are therefore required.3 Due to the inert and insulating properties of many organic compounds under ambient conditions, traditional sensing devices have previously relied on semiconducting ceramics coated with catalysts (e.g., SnO2 or Y2O3−Al2O3),4,5 or infrared spectroscopy techniques. The catalyst-based sensors often rely on the catalytic properties of noble metals, and function by detecting a decrease in oxygen concentration inside the ceramic material with increasing concentration of VOC. Hence, they work only in atmospheres with 21% O2 and require a long stabilization time to allow for the diffusion of the gas into the ceramic material. Another major drawback is that these sensors work efficiently only at elevated temperatures (typically 200 °C) and require a relatively large continuous supply of electrical power for heating, which makes them unsuitable for portable use. Catalyst poisoning or contamination also affects these sensors, which require frequent calibration.6 Some of these problems are avoided by using infrared-based techniques, with the drawback of their being significantly more expensive to manufacture and © XXXX American Chemical Society

specific for only one type of hydrocarbon, because the characteristic infrared-absorption of other compounds can give false readings.6 Additionally more complex sensing techniques also exists, however these are primarily aimed for laboratory use or high end in situ monitoring. Examples include mass or ion mobility spectrometers, photoionization detectors (PID), flame ionization detectors (FID), thermal conductivity detectors (TCD), electron capture detectors (ECD), flame photometric detectors (FPD) or far UV absorbance detectors (FUV) often combined with chromatographic techniques for the separation of different VOCs.7 Another emerging group of sensors are based on the use of microcantilevers.8−10 Typically, an inorganic cantilever is coated with a polymer film able to absorb vapors like VOCs, a process which leads to bending or a change in the resonance frequency of the cantilever. This response can then be measured externally with an electromechanical control system often using lasers and piezoelectric actuators. The drawback of this kind of sensor is thus the complexity of the control system, adding cost and the fragility of the brittle cantilever. Received: January 28, 2016 Accepted: March 29, 2016

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DOI: 10.1021/acsami.6b01178 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

M̅ w = 130 kDa). The multilayer films were 37−38 μm thick and were produced with a multiplication technique.26 The films were extruded onto a PET support film from which it could be removed later. The films were extruded into 2, 24, and 288 layer films, hereafter referred to as 2L, 24L, and 288L, respectively. The multilayer films consisted of alternating mPE and LDPE layers (...mPE/LDPE/mPE/LDPE...). The thicknesses of the individual layers were 18−20 μm (2L), 0.5−2.6 μm (24L) and 0.15−1.4 (288L) μm. The 24L and 288L films were used only in the first part of the paper dealing with the flexing behavior of the films and not in the second part dealing with the sensor properties. The 50 μm single-layer films were produced by compression molding. The details of the production are described in ref 26. The mass crystallinity at 25 °C according to DSC was 51% for 2L and 288L and 50% for 24L, whereas the mass crystallinity for the single-layers of mPE and LDPE was 50 and 58.5% respectively. n-Hexane (purity 99%, HPLC grade; Scharlau), n-heptane (purity 99%, VWR) limonene (purity 97%, Alfa Aesar), methanol (purity 99.9%, HPLC grade; Sigma-Aldrich) and acetone (purity >95%, Fisher Scientific, analytical reagent grade) were used as the swelling media. The density of the paperboard used for n-hexane impregnation was 780 kg/m3. The penetrant solubility in the different polymers was assessed by keeping the films in desiccators above the liquid penetrant, the films being removed intermittently to measure the mass of the films on a Precisa XR205SM-DR balance (10 μg resolution) until constant mass was reached. Mechanical properties of the films were measured according to ISO 527 with an Instron 5944. The bending curvature (measured from still images from a recorded video using a calibrated scale) of the 2L films was evaluated in detail at different concentrations of n-hexane in air under ambient conditions using a TGS 822 hydrocarbon sensor (Figaro Engineering, Inc., Osaka, Japan). The curvature of the film and the measured value of the TGS 822 sensor were allowed to stabilize for 1 min before each reading was taken. This made it possible to calibrate the curvature for each concentration of n-hexane.

Herein, we present a novel, cost-efficient and simple approach to a VOC sensor based on the differential expansion and curvature of polymeric bilayer films exposed to hydrocarbon vapors. The sensor is based on a mechanism similar to that of traditional bimetallic thermostats or coil-type hygrometers, where two layers of different materials are bonded to each other and exhibit different expansion/swelling characteristics. It contains no moving parts except for the single deforming element, and it can be manufactured at competitive prices with tunable accuracy based only on the purity and predictability of the two materials. A clear advantage is the passive nature of this type of sensor (e.g., no electricity is needed during operation). Hence, they can be used in portable devices for the detection of, for example, hydrocarbon gases, and may be monitored visually by the human eye or equivalent digital optics of its radius of curvature. Similar spontaneous curving can also be observed in nature, where even a monolayer film or membrane can curve when triggered by a differential wetting by the “penetrant”, a property often referred to as a curling instability.11 The curving of articular cartilage, deformations relating to seed dispersal, and pine cones opening and closing with humidity changes are examples in nature.12 Aizoaceae protects seeds against drought and release them when the humidity is appropriate for them to grow, where the folding and unfolding typically occur within a time from several minutes to up to an hour.13,14 The motions can also be fast; the trapping motion of the Venus flytrap can occur within less than 100 ms.14,15 The external impulse, such as humidity, temperature, or even light generates a mechanical stress, which leads to elastic deformation.16 The rapid deformation (within ca. 3 ms) of liquid crystalline polymer films triggered by light has also been reported.17 Polymer films, with a gradient in properties, and penetrants are able to form complex structures by bending, wrinkling, creasing, and folding.18−21 Self-folding polymer films have a broad range of potential applications in e.g. the biological, biomedical and medical fields.18,19,22,23 There are a few reports describing the fabrication of self-folding films with different responsive properties.18,19,24 Applications include pH-responsive systems based on polyelectrolytes; thermoresponsive systems based on a gradient in thermal expansion and systems triggered by an electric signal or enzymes. The freedom to tailor the deformation increases if materials are combined as in bilayers.25 The purpose of the present study was to investigate how layered and thin polyethylene (PE) films deform when exposed to VOCs and if it is possible to predict the deformations/ motions based on simulated penetrant diffusion within the films. The results obtained for coextruded films were compared with those for single-layer films of metallocene (mPE) and lowdensity polyethylene (LDPE). The question of whether penetrant-induced deformations/motions of the thin layered PE films could be used as a sensor to measure atmospheric VOC concentrations was also evaluated. The results are promising and show that the swelling/deswelling behavior and kinetics can be described with diffusion simulations. It is suggested that these thin layered films can, with further development, be used as sensors for detecting VOCs on a large industrial scale inexpensively.

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RESULTS AND DISCUSSION When the 2L film was exposed to n-hexane vapor, the film bent solely due to a higher uptake of n-hexane in the mPE layer than in the LDPE layer (Figure 1). The process was reversible,

Figure 1. 2L films (a) before and (b) during exposure to n-hexane vapor (liquid n-hexane at the bottom of the beaker). The arrow indicates the machine direction, and the LDPE layer was facing the camera. The beaker had a diameter of 7 cm.

repeated exposure and drying yielded the same deformation. The curving occurred only for the 2L film; neither the multilayers nor the monolayer films showed any deformation when exposed to the n-hexane vapor (Supporting Information, Figure S1). This was because the mPE and LDPE components were more uniformly distributed in the multilayered films than in the 2L film. This confirmed that the curving effect was due to

EXPERIMENTAL SECTION

Single and multilayer films consisted of low-density polyethylene (LDPE, density = 920 kg/m3, M̅ n = 16 kDa, M̅ w = 250 kDa,) and metallocene polyethylene (mPE, density = 913 kg/m3, M̅ n = 16 kDa, B

DOI: 10.1021/acsami.6b01178 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces the difference in expansion as a result of differences in swelling of the two materials due to n-hexane uptake. Figure 2 shows a scanning electron micrograph of the border between the two mPE and LDPE layers in the 2L film. The

Figure 2. Scanning electron micrograph showing the internal morphology of the 2L film.

difference in the surface texture of the two layers reflects differences in the supermolecular structure of the two polymers. It is clear that the two layers were firmly bonded to each other, with no visible defects resulting from the manufacturing process. Curving similar to that displayed in Figure 1 was observed in films consisting of only one material (monolayer films), provided that the liquid penetrant or vapor was applied to only one side of the film. When n-hexane was applied to a paperboard surface and a film of just one material was placed over the wetted area, the initially flat film immediately bent into a U-shape and occasionally stood up on one of its edges almost vertically due to the swelling/expansion of the polymer on the underside of the film. The motion was faster and the extent of the U-shape was greater for mPE than for LDPE, reflecting both its higher n-hexane solubility and its lower modulus due to its lower crystallinity.26 By adding more n-hexane to the paperboard just in front of and at the base of the film, the time period when the film was standing could be extended (Figure 3a and Movie 1 (mPE left, LDPE right)). The bending mode for a monolayer film, when wetted differentially with a penetrant is analogous to that of a 2L film when subjected uniformly on both sides to a VOC, in that one of the layers swells more than the other, but, when the multilayered 2L, 24L, and 288L films were subjected to a differential wetting by a penetrant, more complex and dynamic modes of motion were discovered. When a dry 2L mPE/LDPE film was placed on the wet region of the paperboard with the LDPE on the lower side, the film bent into a U shape along the extrusion machine direction (MD), with the ends partly in contact (Figure 3b). In this form, the film flipped a few times and then performed a tumbling walk out from the puddle of nhexane, that is, the penetrant induced a displacement of the film of about 30 mm (Figure 3c,i and Movie 2). The layer closest to the wet region was always the one that swelled the most and, in this case, after the first roll partly out of the wet region, the mPE was at the bottom and its swelling led to the formation if a U, in this case perpendicular to the MD (Figure 3c,ii). A similar

Figure 3. Illustration of the different flexing modes of polyethylene films when exposed to VOCs; (a) standing film, (b) film forming a U shape, (c) film displacements, (d) oscillating film, (e) wing-like shape, and (f) a reorienting U-shaped film.

2L film was immersed in n-hexane for a few seconds and then placed on the dry paperboard surface, which led to a wet area around and beneath the film. The film formed a U shape and then began to wag/oscillate slowly (Figure 3d). After some time, the oscillation became faster and lasted for 50 s with a frequency of about 2 Hz (Movie 3). Note that the bending occurred in a diagonal direction across the film and not exactly in the MD. We refer to this film as the oscillating film. The observed oscillation was explained as follows: (1) Part of the film bent upward due to swelling as a result of the uptake of nhexane on the lower side facing the wet region. (2) Due to a mass imbalance in the partly bent film, the bent part fell down. A contributing factor to this behavior was that the swelling decreased in the part of the film that was exposed to the air. (3) The low yet finite rigidity of the film forced the opposite side, which also experienced swelling on the lower side, to rise, and steps 1, 2, and 3 were repeated about 100 times without any detectable change in amplitude or frequency. The oscillation was clearly driven by the presence of the penetrant and the motion ceased first after the liquid had dried out (no n-hexane left). It is worth pointing out here that many, but not all, films forming U-shapes were bent in the MD. With an external stimulus, the 24L films could be reoriented to bend in the TD, but it flexed back to bend in the MD after a few seconds (Figure 3f). This suggests that, if there is a molecular C

DOI: 10.1021/acsami.6b01178 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces orientation, the penetrant-induced deformation occurs preferentially along the orientation direction. We reported a small molecular orientation preferentially in the MD for all the layered films, as is commonly observed in extruded thin films.26 To increase the understanding of this process, we modeled the sorption and desorption kinetics. The n-hexane-concentration-dependent diffusion coefficient in the polymer was described by20

D(C) = DC 0e αC

(1)

where DC0 is the zero concentration diffusivity, and α is the plasticization power. DC0 was higher for mPE (2.5 ± 0.06 × 10−8 cm2/s) than for LDPE (1.3 ± 0.06 × 10−8 cm2/s).26 On the other hand, the plasticization power was lower for mPE (α = 0.13 ± 0.01 (hg/g)) than for LDPE (α = 0.26 ± 0.02 (hg/ g)), leading to an average diffusivity D̅ determined by 1 D̅ = Cs

∫0

Cs

Figure 4. Modeled average n-hexane concentration (g penetrant/hg polymer) through thickness of the 2L film, starting with the wetting stage and through the initial oscillating stage. The arrow points to the start of the rapid oscillation.

αc

DC 0e dc

(2) −8

which was only slightly higher for mPE, (5.5 ± 0.5 × 10 cm2/ s) than for LDPE (5.4 ± 0.5 × 10−8 cm2/s). The n-hexane saturation concentrations (Cs) were 11 g penetrant/hg polymer (mPE) and 9 g penetrant/hg polymer (LDPE) (Table 1). The n-hexane diffusivity and solubility were

shows the history of the film prior to the rapid oscillation and includes the initial immersion in the liquid n-hexane and subsequent slow initial wagging on top of the wet paperboard. The saturation concentrations and diffusion coefficients for the single films were input data. The modeling indicated that the average content of n-hexane in the film did not decrease during the modeled period and most importantly not during the rapid oscillation. This is consistent with the observation that the oscillation did not cease until the n-hexane had completely evaporated. Hence the oscillation was driven by the penetrant, and could probably continue “forever” if the process were continuously fed with n-hexane. Figure 5 shows two modeled concentration profiles in the oscillating 2L film during the oscillating stage (beyond the

Table 1. Saturation Concentrations for Different VOCs in the Two Polymers mPE (g penetrant/ hg polymer)

LDPE (g penetrant/ hg polymer)

16 12 11 0.05 1.4 0a

15 10 9 0a 1.3 0a

limonene n-heptane n-hexane methanol acetone water a

Below detection limit.

moderately higher in mPE than in LDPE, which reflected the higher crystallinity of LDPE. The n-hexane diffusion between the two layers (mPE and LDPE, named a and b, respectively) was given by27 Da(Ca)

∂Ca ∂C = Db(Cb) b ∂xa ∂xb

(3)

which yielded the boundary concentration in layer a Ca , k − 1 + Ca , k =

1+

Db(ξCa , k + 0.5) Da(Ca , k − 0.5)

ξCa , k + 1

Db(ξCa , k + 0.5) Da(Ca , k − 0.5)

ξ

(4) Figure 5. n-Hexane concentration profiles during the oscillating stage for the 2L film (beyond the arrow in Figure 4, black and red curves correspond to 19 and 19.25 s in Figure 4, respectively).

where ξ is the partition coefficient (i.e., the ratio of Cs in layer a to that in layer b), and k is the x coordinate at the boundary. In the modeling, it was assumed that the vapor pressure of nhexane decreased rapidly with increasing distance from the wet region (justified by empirical testing of the n-hexane content adjacent to the wet region). Figure 4 shows the modeled n-hexane concentration (average value through the film) as a function of time for the oscillating 2L film. The simulated data referred to a section of the film that was alternately in the air and on the wetted paperboard (an ideal on and off situation). The time period between 0 and 16 s

arrow in Figure 4), one profile for when the film was in the down position (high surrounding n-hexane concentration) and one when the film was bent upward (low surrounding n-hexane concentration). In this rapid oscillating region the concentration profiles were, in contrast to the initial period (0−16 s, Supporting Information, S2), almost constant with time, except for the region close to the surface of the LDPE side, facing the D

DOI: 10.1021/acsami.6b01178 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces wet region (Figure 5, close to origo). After drying, the films reverted to a flat shape without any permanent deformation. During drying, the films sometimes adopted a wing-like or an upside down wing-like shape due to the complex differential evaporation of penetrant combined with the different swellings in the two materials (Figure 3e). In the presence of n-hexane on one side, the 24L films experienced motions and deformations similar to those observed for the 2L films. The film with 288 layers behaved, in principle, as the 2L and 24L films. However, the clearest case of a “tumbling move” from the wet area occurred here (Figure 3c,iii and Movie 4). Part of the film initially bent upward because of the swelling on the lower side. The film subsequently fell down causing a wagging to such an extent that the film made a “somersault” ending up as a partly curved/ partly planar film, where the planar part was outside the wet region. The curved part straightened out because of the swelling on the lower side and the same part then rose and flipped over so that the whole film was then outside the wet region. Sensor for Volatile Organic Compounds. The curvature of the 2L film (Γ, reciprocal of the radius of curvature) was shown to depend on the n-hexane content in the surrounding atmosphere (Figure 6). A logarithmic dependence of the film-

where εi = linear swelling fraction, h = thickness of the 2L film, Ei = Young’s modulus, ti = layer thickness, and Ii = area moment of inertia (square cross-section: Ii = ti3/12), where i = a and b, correspond to mPE or LDPE, respectively. By expressing the Young’s modulus for the layers as a ratio; n = Eb/Ea and considering that the thicknesses of the layers in the VOC sensor were the same, the following expression was obtained: Γ=

24(εa − εb)

(

h 14 + n +

1 n

)

(7)

Hence, the modulus ratio (n), the thicknesses of the layers, and the difference in swelling of the two layers are the only parameters that have an impact on the final curvature. The measured Young’s moduli for the monolayer films yielded stiffness values of 118 and 156 MPa for mPE and LDPE respectively, resulting in n = 1.32. It should be noted however that the curvature (Γ) is essentially insensitive to changes in the modulus ratio (n) for the demonstrated films due to the larger factor 14 (eq 7) and also since any change in n cancels out the change in 1/n when n is close to 1 (n = 1.32 compared to n = 1.0 yields only a 0.5% difference in Γ). If the modulus ratio (n) is 1 instead of 1/2 or 2, and all the other parameters are unchanged, the change in curvature is a mere 3%. The described VOC sensor consisting of mPE and LDPE layers is thus very insensitive to changes in the modulus ratio, which might arise because of a change in temperature or the uptake of penetrant during operation. With a curvature of 0.098 mm−1 (when the film is subjected to 100 μg/cm3 of n-hexane, see Figure 6) and a layer thickness of 19 μm, the Timoshenko formula predicts a difference in expansion/swelling of the two layers (εa − εb) of 2.5 × 10−4. The reason why such a very small amount of swelling can be detected is attributed to the very thin nature of the bilayer film. This predictable behavior could therefore be used to design an inexpensive passive sensor for the detection of the n-hexane concentration in the surrounding atmosphere. Figure 7 illustrates how such a sensing film could be implemented in a readable device.

Figure 6. Measured curvature (Γ) of the 2L film as a function of nhexane concentration in the surrounding atmosphere.

curvature on the n-hexane concentration (linear with respect to log hexane concentration) was observed in the range of 8−300 μg/cm3. Thus, an expression to predict the n-hexane concentration (Chexane) in the surrounding air from the curvature (Γ) of the film could be derived from the measured data (R2 = 0.997): C hexane = −17.3 + 19.6e18.3Γ

(5)

−1

where Γ is expressed in mm and the n-hexane concentration (Chexane) is given in μg/cm3. The mechanism by which the bilayer film curves due to the expansion of the different layers was studied according to the classic Timoshenko formula:28 εa − εb Γ= 2(EaIa + EbIb) h 1 1 + + tE 2 h tE

(

a a

b b

)

Figure 7. Schematic representation of a proposed passive sensor/ indicator for the presence of n-hexane, consisting of a bilayer polymeric film with a completely reversible and predictable behavior. The mPE expands more than LDPE when exposed, and hence, the upward bend when the bilayer is as shown.

(6)

E

DOI: 10.1021/acsami.6b01178 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces The influence of other penetrants on the curving of the film was also investigated. The 2L films responded rapidly to nheptane and more slowly to limonene, whereas no curving occurred in water, acetone or methanol vapor. As shown in Table 1, the reason for the absence of curving in the latter three penetrant vapors was the negligible uptake or the similarity in uptake (acetone) by the two layers. To increase the understanding of the curving behavior of the 2L layer films, the radius of curvature was measured while the sensor film was cycled inside and outside an atmosphere of saturated n-hexane vapors. As seen in the top panel of Figure 8, the average

penetrant uptake and thus the difference in swelling between the two layers. The modeled concentration profiles of n-hexane and limonene in the 2L films (4−25 s) are shown in Figure 9.

Figure 9. Simulated concentration profiles of n-hexane (top) and limonene (bottom) as a function of distance from the surface of the 2L films. The arrows indicate how the profiles changes with time (4−25 s) when subjected to n-hexane or limonene vapor.

The n-hexane diffusivity data used were those given above, and those for limonene were obtained from ref 21: (Dco = 1.1 × 10−8 cm2/s and α = 0.1 (hg/g)). It is shown here that the swelling was far from complete after these times (no penetrant had diffused to the center of the film). The response times could be much faster than the diffusion because of the similar diffusion rates in the two layers, combined with the fact that any swelling (strain) at the surface of the film and, thus, also the curvature depend on the stiffness of the entire structure (as in sandwich structured materials). The choice of polymers (mPE and LDPE) was therefore essential for the repeatability, accuracy, chemical resistance, and fast response time of the VOC sensor described in this work.

Figure 8. (Top) The change in curvature with repeated insertions (start) and removal (stop) of the 2L film into a saturated n-hexane atmosphere. The first, second, and third cycles are represented by the black, red, and green lines, respectively. (Bottom) The much slower response of the curvature when instead inserted into a limonene atmosphere.

response time (in order to reach 90% of the final value) for the detection of n-hexane was 3.7 s, while a time of 5.4 s was required for the film to recover in an ambient atmosphere without n-hexane. For comparison the same response time when subjected to limonene was ca. 25 s. In particular, the response time for n-hexane for the presented sensor is much quicker than for other absorption-based sensors, e.g. Dong et.al.10 presented a microcantilever-based VOC sensor which had a response time on the order of 1 min. The reason for the smaller curvature with limonene than with n-hexane in Figure 8, even though the total uptake of penetrant was greater for limonene, is the smaller difference in uptake between the two layers (Table 1). It is not the total uptake of penetrant that determines the curvature, but the difference in

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CONCLUSIONS Self-folding films/objects, triggered by moisture, for example, usually consist of two layers with very different properties. This study has shown that the combination of similar materials with only minor molecular differences can result in films that exhibit large and rapid deformations in the presence of VOCs. The mPE layer with a lower crystallinity, with only slightly higher nhexane solubility, next to the LDPE layer led to a film that curved with the mPE on the outside. The fact that the two layers were both of polyethylene simplified the coextrusion F

DOI: 10.1021/acsami.6b01178 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Author Contributions

process. No tie layers were needed. It was shown that the extent and the kinetics of the curving depended on the penetrant. As shown in the tests where films were either pre-immersed in the penetrant or placed directly over a paperboard wet with nhexane, the films, regardless of the type of film (1, 2, 24, and 288 layers), deformed in such a way that there was little contact between the wet part and the film, a consequence of the greater swelling in the film layer closest to the penetrant source. Several striking deformations and motions were observed and they were all a consequence of and fueled by the VOC. Films were observed to stand up on their own, a consequence of the balance around the center of gravity and the degree of swelling. A long-term oscillation/wagging (100 cycles) with unchanged frequency and amplitude was observed for two-layer films. This occurred as a consequence of the repeated uptake and loss of nhexane as the film bent closer and further away from the nhexane source, as verified by diffusion modeling. The oscillation ceased first after the penetrant source beneath the film had evaporated. In some cases, films placed on the paperboard wet with n-hexane were tumbling/rolling away from the wet source. In essence, it was possible to move the film without touching it with the help of the penetrant, as a consequence of greater swelling/expansion in the lower part of the film. The curvature of the 2L film was calibrated against the atmospheric concentration of n-hexane, demonstrating a completely new type of passive sensor for detecting VOC in the surrounding air. The detection limit and workable range for n-hexane was from 8 μg/cm3 and up to a fully saturated atmosphere. The average response time (90% of final value) was 3.7 s for the absorption of n-hexane in a saturated atmosphere and 5.4 s for desorption in ambient conditions. The curvature was fitted with good accuracy (R2 = 0.997) and high repeatability to an exponential equation, making possible a direct correlation of the atmospheric content of n-hexane solely from the curvature observed in the film.

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The manuscript was written through the contributions of all the authors. All the authors have given their approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. All funding was provided from KTH-Royal Institute of Technology, TK5600 − MAT.

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ABBREVIATIONS VOC volatile organic compound mPE metallocene polyethylene LDPE low-density polyethylene

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01178. Photographs of different films exposed to n-hexane and calculated n-hexane concentration profiles. Figure S1 shows that the 24 layer and the 288 layer mPE/LDPE films do not curve or change shape when treated uniformly in a VOC-rich atmosphere (beaker with 1 cm of n-hexane at the bottom). Figure S2 shows the simulated n-hexane concentration profiles for the twolayer films during the initial stages prior to the observed rapid oscillating behavior. (PDF) Movie 1: films standing up above the paperboard wet with n-hexane. (MOV) Movie 2: a film wagging slowly above the wet paperboard and moving out of the wet area. (MOV) Movie 3: a rapidly wagging film. (MOV) Movie 4: a film tumbling out of the n-hexane wet area. (MOV)

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REFERENCES

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DOI: 10.1021/acsami.6b01178 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.6b01178 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX