Transparent tunable acoustic absorber membrane using inkjet printed

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Transparent tunable acoustic absorber membrane using inkjet printed PEDOT:PSS thin-film compliant electrodes Milan Shrestha, Zhenbo Lu, and Gih-Keong Lau ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12368 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 27, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

Transparent Tunable Acoustic Absorber Membrane using Inkjet Printed PEDOT:PSS Thin-lm Compliant Electrodes †,‡

Milan Shrestha,

Zhenbo Lu,

¶

and Gih-Keong Lau

∗,†

†Nanyang

Technological University, School of Mechanical and Aerospace Engineering, Singapore 639798 ‡Nanyang Technological University, Singapore Center for 3D Printing (SC3DP), Singapore 639798 ¶National University of Singapore, Temasek Laboratories, Singapore 117411 E-mail: *[email protected]

Keywords: PEDOT:PSS, inkjet printing, acoustic absorber, transparent compliant electrodes, dielectric elastomer actuators

Abstract Window glasses can block noise from outdoor, but they reverberate sound within a large indoor space. Micro-perforated glass absorbers have been developed to absorb sound over a xed but narrow bandwidth. To tune the frequency spectrum of acoustic absorption, we developed a transparent tunable acoustic absorber based on micro-perforated dielectric elastomer actuator (MPDEA) and transparent compliant electrodes. Such transparent compliant electrodes were inkjet printed from Triton-Xplasticized PEDOT:PSS ink, which shows improved wettability on the acrylate dielectric elastomer substrate. These transparent polymeric electrodes are softer with up-take 1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of moisture, while being self clearable and durable. A single layer of MPDEA using two inkjet printed electrodes is 78.64% clear, but clarity of two-layer MPDEA decreases to be 61.8%. Among the two designs, the two-layer MPDEA exhibits a broader acoustic absorption bandwidth of 444Hz for absorbing above 80% of sound energy. Inactivated resonant frequency of this MPDEA is 1170Hz; whereas the 6kV activation can reduce the resonant frequency for 15.2% by causing 9% hole-diameter contraction. This transparent tunable acoustic absorber can be tted to window glass; its acoustic performance is better than that of translucent curtains.

1

Introduction

Glass panels are widely used to cover windows for shops and oces. They are optically transparent to allow daylighting, but they reverberate sound within a large indoor space. For example, in a restaurant where windows were closed, conversation among diners can reverberate indoor and thus turn more noisy 1 than where windows were opened. There are a few solutions for sound absorption and being optically clear. Curtains are often used for interior acoustic absorption but they are usually opaque for view. A translucent woven fabric, i.e. Gerriets Absorber Light, 2 is recently developed for sound absorption while allows natural lighting. Meanwhile, micro-perforated glass panels 39 were proposed to be installed to window; their acoustic absorption spectrum is however xed and not as broad as fabric absorber. 10,11 Active tuning of a panel absorber can help shift the acoustic absorption spectrum to target varying noise. Electroactive polymers 1218 provide a silent means of distributed actuation, being much lighter than conventional servo motors. Recently, dielectric elastomer actuators (DEAs) have been used to make for tunable acoustic absorbers. 1921 A dielectric elastomer actuator is a soft capacitor which consists of a pre-stretched dielectric elastomer membrane sandwiched by a pair of compliant electrodes. It was used to reduce the membrane tension and thus shifts the acoustic resonant frequency of the membrane absorbers. 2022 In 2


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addition, the same actuation principle can control the hole size of a micro-perforated dielectric elastomer membrane for tunable bands of acoustic absorption. 23 These tunable acoustic absorbers developed so far are opaque due to the use of carbon-based 20,21 or gold-thin lm compliant electrodes 23,24 for dielectric elastomer actuators. For window use, a tunable acoustic absorber based on DEA requires transparent compliant electrodes. 25 Inkjet printing techniques promise being handy for complex patterning of conductive ink, but the uniform printing of a submicron thin lm is challenging on the slightly hydrophobic surface of acrylic elastomer substrate. Common electrode materials for dielectric elastomer actuators include conductive grease (such as carbon grease or silver grease), 12,13 conductive powders (such as carbon black or graphite powder), 26,27 and conductive nanometric networks (such as carbon nanotubes or silver nanowires). 28,29 Among them, a sparse and smooth coverage of conductive nanometric network, 29 such as carbon nanotube, is transparent on dielectric elastomer actuators. However, this initially transparent DEA turns hazy upon activation. 28,30 The haze happens because the surface of dielectric elastomer roughens when electrostatically squeezed (or indented) by the networks of conductive nanotubes or nanowires. 30 Hydrogel or ionic gel (which was swollen by ionic liquids) 31,32 can remain transparent during large-strain activation and deactivation of dielectric elastomer actuator. However, the gel is prone to dry up and the dried patch appears whitish with the left over of salt. 3335 Liquid encapsulation can help preserve moisture, 3537 but it is prone to leakage. Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) have been widely used as a transparent conductive coating of antistatic shielding bag, 38 and a transparent polymeric conductor for all-polymer capacitors, 39 transistors 40,41 or solar cells. 42 Nanometric lm of PEDOT:PSS also makes a stretchable electrode on a hyperelastic substrate of elastomer membrane. 43,44 Conductivity of PEDOT:PSS lm is higher than that required by compliant electrodes for dielectric elastomer actuators. Yet, the stretched lm of PEDOT:PSS is prone to plastic yielding beyond 2% strain. 45 When being stretched uniaxially 3



beyond 30% strain, 43 a PEDOT:PSS lm is prone to crack and lose conductivity. Being moderately stretchable, nanometric lms of PEDOT:PSS are good enough as compliant electrodes for moderate-strain actuation of dielectric elastomer. Previously, pristine PEDOT:PSS was not applied as compliant electrodes because the former's modulus of 2GPa 43,45 is 4-order higher than 0.2-0.5MPa modulus of acrylate dielectric elastomer. 46,47 This suggests that even a submicron thin lm of PEDOT:PSS is axially stier than a hundred-micron membrane of acrylate elastomer (e.g. VHB4910). Furthermore, the aqueous ink of pristine PEDOT:PSS does not spread well on the slightly hydrophobic surface of acrylate dielectric elastomer. Recently, Yoon

et. al. 48

reported the addition of surfac-

tant Triton-x100 improving the wettability of aqueous suspension of PEDOT:PSS on a polydimethylsiloxane elastomer substrate while acting as a plasticizer to soften the obtained solid lm of PEDOT:PSS. The modulus of plasticized PEDOT:PSS was not directly measured but only inferred from buckling mechanics of a thick elastomer substrate. 48,49 Motivated by this recent nding, we shall re-examine the feasibility and formulation of PEDOT:PSS for making compliant electrodes of dielectric elastomer actuators. In this paper, we developed a transparent tunable acoustic absorber (see Figure 1) based on micro-perforated dielectric elastomer actuator with inkjet printed transparent polymer electrodes. Here we formulated a plasticized and diluted ink of PEDOT:PSS with reduced modulus and improved wettability on elastomeric substrate. Such inkjet printed transparent polymeric thin lms can make compliant electrodes for driving a dielectric elastomer actuator for up to 20.6% radial strain while ductile fracture happens above 15%. In addition, the DEA with such transparent compliant electrodes are self clearable from dielectric breakdown and durable to withstand actuation cycles and humidity changes. A micro-perforated dielectric elastomer actuator (MPDEA) with the inkjet printed compliant electrode can tune the acoustic absorption spectrum while transmitting light. Such transparent acoustic absorber will suit to be t to window glass.

4


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Figure 1 (a)

(d) NI PCI-6251 Amplifier

Dielectric elastomer

2b

(b)

Holes

(c)

Mic2

T0 + ∆T

MPDEA absorber

T+ ∆T

T0 + ∆T

T0

T0

Mic1

Loudspeaker

MPDEA

T0

Trek 10/40A

Conditioner

Unit cell 2b

Back cavity

2a(V)

2a

H

T0

Unit cell T0 + ∆T

2b

V

Figure 1: Transparent tunable acoustic absorber based on micro-perforated dielectric elastomer actuator (MPDEA): (a) schematic showing the device components; (b)-(c) a unit cell at inactivated and activated states; (d) measurement setup with an acoustic impedance tube

Figure 2

Side View

Acrylic clamps L − 2mm =8mm

50mm F

(b)

Wavy PEDOT:PSS +4% Triton-X

0% Strain (L0=10mm)

1

PEDOT:PSS+4%Triton-X/VHB /PEDOT:PSS +4%Triton-X (F1)

0.8 0.6 0.4

VHB Only (F2)

0.2

L0

0 0

F

Crack

(c)

L−2mm

2

4 Extension (mm)

6

8

(e) 10 (F1 –F2)/A (MPa)

PEDOT:PSS

(d) Tensile Force (N)

(a) Tensile test setup

Front View

VHB

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Yield & crack

8 6 4 2

L0

0

15% Strain (L=15mm)

-20

0

(L−L0)/L (%)

20

40

Figure 2: Modulus determination of plasticized PEDOT:PSS nanometric lms by tensile testing: (a) schematic for a sample consisting of a uniaxially pre-stretched VHB tape (F9473PC) sandwiched by two plasticized PEDOT:PSS nanometric lms; (b)-(c) surface morphology of plasticized PEDOT:PSS nanometric lm at 0% strain (L0 =10mm) and 15%(L=15mm) respectively; (d) tension required to stretch the sample starting with a relaxed thin lm (L−20mm ) and above; (e) tensile stress as extracted from the extra force to stretch the PEDOT:PSS nanometric lms

5



2

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Methods and Materials

Figure 1a-c shows a transparent tunable acoustic absorber consisting of a microperforated dielectric elastomer actuator (MPDEA), a rigid ring frame, and a back cavity plate, all in a circular outer prole. Being a soft capacitor, a MPDEA consists of a micro-perforated membrane of dielectric elastomer sandwiched by a pair of transparent polymeric compliant electrodes. The back cavity of acoustic absorber is formed as the air spacing between the MPDEA membrane and the back plate of acrylic, which can be replaced by a window glass panel. Subsequent subsections presents the working principle and design of dielectric elastomer actuator, design of compliant electrodes, Helmholtz resonator design, device fabrication, inkjet printing, and experimental setup.

2.1

Dielectric elastomer actuator

Dielectric elastomer actuator (DEA) is a soft capacitor capable of producing deformation or tension change under electrostatic pressure (Maxwell stress). 12,13 Typically, it consists of a biaxially pre-stretched dielectric elastomer membrane sandwiched by a pair of compliant electrode. Application of high voltage V across the dielectric membrane of thickness ts induces a compressive electrostatic pressure pe = r 0 (V /ts )2 , where r is the dielectric constant and 0 is the permittivity of vacuum. 12,15 This activation reduces the bi-axial pre-tension T0 in the membrane by ∆T (V ) such that the membrane tension reduces to be:

T = T0 − ∆T (V ) = T0 −

ν pe , 1−ν

(1)

where ν is the Poisson's ratio of dielectric elastomer membrane, according to Ref. 50 on the assumption of small elastic strain. Maximum electrostatic pressure is limited to sub megapascal due to dielectric breakdown. 51,52 Figure 1a-c shows a micro-perforated dielectric elastomer actuator. Micro-perforation or puncturing a hole can locally release the pre-tension in a membrane, which was not 6


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perforated. This leads to the enlargement of initial puncture size (more than that initially drilled by laser). Activation of the annular dielectric elastomer actuator that surrounds the passive hole can reduce the hole radius by ∆a(V ) following Ref.: 23

a2 a2 ∆a(V ) = ∆T (V ) b + − 2a − ν(b − ) , b b

(2)

where b is the half pitch between holes (see Figure 1b-c).

2.2

Design of compliant electrodes

For driving a dielectric elastomer membrane for large voltage-induced deformation, compliant electrodes must be axially softer than the elastomer substrate; otherwise they will limit the deformation. 53 While wrinkled nanometric thin lm can be compliant, 24,54 thickness control for nanometric lm deposition can be dicult, except by vapor deposition method. Inkjet printing does not yield high lm uniformity due to coee ring eect. To mitigate the impact of thickness variation, we proposed to formulate the ink recipe to yield a softer solid conductive lm at submicron thickness. Let us design the modulus of plasticizer-added conductive ink to be at least 2 times softer than the elastomer substrate. Consider an acrylate elastomer substrate (3M VHB 4910) with Young's modulus Es =220kPa and membrane thickness ts =125µm, a conductive lm of thickness tf =0.3µm needs to have a Young's modulus Ef lesser than

Ef = E s ×

ts 125µm = 220kPa × = 45.84MPa. 2tf 0.3µm

(3)

A solid lm of 2% Triton-x100/98% PEDOT:PSS was reported 48 with a Young's modulus of 80-92 MPa, as calculated from the wrinkle mechanics of a PEDOT:PSS-coated elastomer substrate. 48 The modulus of the PEDOT:PSS based on this recipe is however twice stier than the required. To further soften the electrodes, we need to add more than 3% Tritonx100 (as a plasticizer) to the conductive ink of PEDOT:PSS while ensuring the uniformity 7



of printed lm on VHB substrate. Here, the conductive ink used for inkjet printing is an aqueous suspension of PEDOT:PSS. The pristine ink of PEDOT:PSS as purchased (from Clevios P Jet HC V2 from Heraeus Deutschland GmbH & Co. KG) cannot properly wet a slightly hydrophobic surface of elastomer. 44,48 O2 plasma treatment does not make the substrate of acrylate dielectric elastomer more hydrophilic as it does to that of silicone dielectric elastomer. 43,44 Hence, the ink needs to be formulated by adding a surfactant (Triton-x100) 48 to improve the wettability on elastomeric substrate. The ink was also diluted with deionized water to yield a thinner solid coating upon drying up. Later processes are based on an optimized formulation consists of 40.78% weight PEDOT:PSS ink, 4.5% Triton-X100, and 54.72% deionized water. A nanometric thin lm of plasticized PEDOT:PSS is tacky and dicult to be released from an elastomer substrate, unlike the release of pristine PEDOT:PSS lm. 49 Furthermore, its axial stiness is low relative to that of a thick elastomeric substrate. Here, we extract the modulus of PEDOT:PSS nanometric lm from its stiening eect on a soft substrate. To pronounce the stiening eect, a pair of 230nm thick plasticized PEDOT:PSS nanometric lms were applied to sandwich a thin and soft substrate of acrylic adhesive tape (VHB F9473PC) of an 200µm initial thickness. This sample of double-coated tape was bounded and clamped by two pair of 50mm-wide rigid plates. Figure S1 shows the fabrication steps for making such a tensile-test sample. The elastomeric substrate was uniaxially pre-stretched for 2 times while PEDOT:PSS lms were cast by a doctor blade and dried on both sides of the substrate through each stencil of 50mm wide and 10mm long. A tensile tester (Instron 5569) with a 10N load cell was used for measuring the tension required to stretch the sample. 52 During the tensile test, the sample was stretched from a length of 8mm to 15mm. This starting length of 8mm keeps the VHB tape (of initial 5mm length) in tension but causes surface buckling to the plasticized coatings. Figure 2 shows that the plasticized PEDOT:PSS coated VHB tape requires more tension to be stretched as compared to the none-coated VHB tape. Its rst 2mm extension requires a low tension to overcome the low elastic resistance of 8


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elastomeric tape with buckled coatings. However, its extension beyond 2mm sees a marked increase in the tension requirement to overcome the extra stiness of taut coatings. In this way, Young's modulus of the plasticized PEDOT:PSS coatings is determined to be 43.1MPa at room temperature and 57% relative humidity. Also noted is the yield and crack formation above uniaxial 15% strain in the over-stretched lm of plasticized PEDOT:PSS. Thereafter, the apparent modulus (i.e. gradient of tension with respect to stretch) of the VHB with cracked coating becomes close to that of a pristine VHB tape.

2.3

Helmholtz resonator design

A microperforated membrane acoustic absorber is an array of Helmholtz resonators which share a common back cavity (see Figure 1b,c) A Helmholtz resonator is a container or cavity of air with a neck-like open hole. 10,11,55 When disturbed by sound, mass of the air plug resonates while compliance of the back-cavity air provides a restoring force. This resonant vibration of air can dissipate acoustic energy into heat. According to Ref., 55 the fundamental resonant frequency of air-plug vibration in a Helmholtz resonator is given as

c f= 2π

r

A , SL

(4)

where c is the sound speed, e.g. 346m/s, A and L are the cross-sectional area and length of the neck-like open hole, and S is the volume space of back air cavity. A back air cavity of radius R and depth H has a total volume space of S = πR2 H . Here, a dielectric elastomer actuator is used to electrically tune the membrane thickness t = t(V ) and tension according to Equation (1). Hence, the hole cross-sectional area A = A(V ) = πa2 (V ) is electrically tunable following the hole size given in Equation (2). Consider a design of microperforated membrane (MPP) acoustic absorber with a back cavity of 2R=20mm total diameter and H =40mm depth. This membrane of t=250µm thick is micro-perforated with an array of 9 holes of 2a=540µm diameter each. For this MPP,

9



a representative unit of Helmholtz resonators has one ninth the volume of back cavity and √ the upper bound of resonant frequency is estimated to be f = ca/(6πR Ht)=1406.45Hz. Resonant frequencies of the MPP could be lower with a larger eective volume of back air cavity. For exact relationship of acoustic characteristics, please refer to the impedance analysis of a microperforated panel absorber as presented in the literature. 10,11,23,56

2.4

Device Fabrication

Figure 3 shows the fabrication steps for making a MPDEA, in a similar way to make a soft capacitor. First, an adhesive tape of acrylate dielectric elastomer (VHB 4910) was pre-stretched radially for 3 times to have a 125.0µm membrane thickness as measured by a micrometer. The measured membrane thickness is thicker than the calculated 1000µm/(3

×3) because the membrane's free edges relax and are stretched lesser than the stretcher with 9 contact points does. Later the pre-stretched membrane was transferred and adhesively bonded to a rigid ring frame of a 20.5mm internal diameter and a 28.0mm external diameter. The pre-stretched elastomer membrane was left 24 hours to relax and let the viscoelastic creep settle to the steady-state deformation. Second, aqueous conductive ink of PEDOT:PSS suspension was inkjet printed on the substrate of pre-stretched membrane. Printing on both sides of the substrate and subsequent drying yield a pair of transparent polymeric compliant electrodes sandwiching the dielectric elastomer membrane. Figure 1a shows a circular electrodes of 20mm diameter printed on a VHB substrate, except 9 uncoated minor disk areas within it. These 9 uncoated disk area were arranged in an orthogonal array with equal spacing of 2b=5mm. This distance

2b between sub-millimeter holes was designed to achieve an open ratio of less than 1%, following our previous design of opaque MPDEA. 23 This membrane DEA can be stacked up readily to another to make a multilayered membrane DEA. Finally, a laser cutting machine (Epilog Helix 24) was used to laser drill through the none-coated membrane areas to produce a micro-perforated DEA (MPDEA). The average diameter of the laser drilled holes are 10


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Figure 3

(a)

FABRICATION STEPS

(1) Pre-stretching of VHB

Travel

(b)

Top view

(2) Printhead

(2) Inkjet Printing of PEDOT:PSS thin film

Overlap

Side view

Void

Drop spacing (2)

Ink droplet

VHB substrate

Drop spacing>>D/2

Top view

(3) Repeat step (2) on other side Puddle of inks (from droplet reflow) (4) Heating in an oven (~50°C)

D

(2b)

(2b)

VHB substrate

Water evaporation upon heating @ 50°C (5) Laser drilling of through hole

VHB substrate (4) PEDOT:PSS thin film Drop spacing~D/2

Figure 3: (a) Fabrication steps for making a MPDEA: (1) substrate preparation by prestretching a VHB tape; (2) inkjet printing of PEDO:PSS thin lm on the substrate; (3) inkjet printing on the other side; (4) heating of printed ink droplets in an oven; (5) Laser drilling to make holes through the substrate membrane; (b) Evolution of print in side and top views: from (b(2)) wet droplets, (b(2b))merged puddles to (b(4)) dried solid thin lm.

2a = 447.5 ± 30.78µm for 1-layer MPDEA, and 2a = 541.0 ± 25.39µm for 2-layer MPDEA.

2.5

Inkjet printing of conductive ink

A commercial material printer (Dimatrix 2381) 57,58 was used to inkjet print the aqueous conductive ink. The printhead (cartridge) has 16 nozzles (see Figure S2); each nozzle can eject an ink droplet of 10 pL. The material printer can eject conductive ink droplets to form an isolated dot, a line out of sequential dots with partial overlaps, or an area out of sequential lines with partial overlaps. To form a continuous lm, the print drop of diameter

D needs to be properly spaced with partial overlap. Figure 3 shows that a proper overlap during printing can merge wet ink droplets by reowing into a thin puddle. However, an insucient overlap leads to a void area none-coated by the wet lm; too much overlap yields

11



a thicker puddle. Baking of the printed wet lm in an oven at 50◦ C evaporates the water content of the ink and leaves a solid nanometric lm on the substrate. Transparency of the PEDOT:PSS solid lm decreases with an increasing lm thickness. To save printing time, all 16 nozzles of the printhead were selected for droplet ejection. Typically, inkjet printing of a 20mm diameter PEDOT:PSS takes 15-20 minutes with all 16 nozzles selected. Continuity of a printed line/lm may be disrupted when some of the nozzles incidentally clog despite automated nozzle cleaning once for every printing of 5 lines. Discontinuity in printed line can be mitigated by using a closer drop spacing. A print drop spacing is controlled by tilting the printhead relative to the print direction. According to the printer manual, 57 a 10µm drop spacing is achieved by a tilt angle of 2.3 degrees; while 15µm and 20µm drop spacings can be done at the angles of 3.4 and 4.5 degrees respectively. Though this prototype with inkjet printed electrode is relatively small in area, it can be scaled up to be as large format as a window glass. Electrode printing over a large area is possible using industrial material printers, which have been developed for manufacturing of organic light-emitting diodes (OLEDs) display. 5860

2.6

Experimental setup

Figure S3 shows a high voltage supply (Trek 610E) being used to activate dielectric elastomer actuators. A computer and a NI data logger were used to monitor the voltage and current supplied during the device activation. Voltage monitor of the supply provides a signal voltage output, at a gain of one thousandth. Meanwhile, a multimeter (Agilent 34410A) was used to measure the current leaking through the DEAs and resistance of the electrode. A camera was used to take the video or pictures of the device in action, i.e. voltage induced diameter expansion. A stereoscopic microscope (Olympus SZX7) was used to have a zoom-in view of the voltage induced hole contraction. Images were taken at increasing voltage steps. Image J software was used to extract the size change from the images captured. A confocal microscope (VK-X200 Series 3D Laser Scanning Confocal Microscope) was 12


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used to measure the three-dimensional (3D) morphology of printed thin lms. This topography measurement yields the information about the ink dot thickness and line width.

Figure 1d shows an acoustic impedance tube being used to measure the acoustic absorption spectrum of a tunable absorber at normal incidence. The 500mm long and 20mm diameter tube has a loudspeaker installed at one end and the tunable absorber mounted at the other end. Two electret array microphones (PCB piezotronic, model 130E20) were used to measure the sound pressures in the tube, namely pi and pr of incident and reected sound waves respectively. According to Ref., 61 the sound absorption coecient (α) is calculated as 1 − |pr /pi |2 . At a 20mm-distance between them, the two microphones can measure the sound pressure down to 200Hz. During the acoustic testing, a data logger NI PXI 6221 was used for data recording; while a high voltage amplier (Trek model 20/20C) was used for driving a device of MPDEA.

Figure S4 shows a spectrometer from AvaSpec (USB2 Fiber Optic) being used to measure the inline transmittance of a transparent MPDEA. A halogen light source was used to generate a collimated light through a 6mm diameter collimator lens. An optical-ber photodetector with a collimator lens was used to detect the specular light transmitted through the device, which is located at a distance of 70mm from the collimated lens.

3

Results and discussions

It is well known that electrical and mechanical properties of PEDOT:PSS lm change when the conductive polymer takes up moisture. 45,62 This motivates our investigation into the durability of plasticized PEDOT:PSS nanometric lm in the presence of relative humidity change. The sample under test is a dielectric elastomer actuator sandwiched by a pair 20mm diameter circular compliant electrodes of plasticized PEDOTP:PSS cast as described above. In this test, we measure the electrode resistance and dielectric elastomer actuation while the relative humidity in a humidity chamber changes between high (80%) and low (25%), at room

13



temperature (25◦ C). Figure 4a-b shows that resistance of a plasticized PEDOT:PSS electrode rises upon drying but decreases upon moisture uptake. While this moisture-induced resistance changes for 8.3%, the resistance value of the plasticized PEDOT:PSS electrode remains sub-MΩ, which is stable enough for driving a DEA. Figure 4c-e shows cyclic activation of a dielectric elastomer actuator using square voltage pulses between 0kV and 3kV. Given a constant relative humidity, DEAs with PEDOT:PSS electrodes can repeatedly actuate. However, the plasticized PEDOT:PSS electrodes dry up and become stiened at a low relative humidity (e.g. 25%). This stiening eect of dried PEDOT:PSS greatly reduces the dielectric elastomer actuation, by close to 4 times. This observation agrees to the humidityinduced modulus change reported by Lang

et. al. 45 Interestingly, the electrodes soften and

the larger actuation restores upon re-exposure to high relative humidity (e.g. 80%). This humidity eect on dielectric elastomer actuation promises a novel kind of humidity sensor. Next, we investigated the characteristics of inkjet printed PEDOT:PSS electrodes on dielectric elastomer membrane. Figure 5a(second column) shows that an aqueous droplet of pristine PEDOT:SS ink does not spread well on a VHB substrate, at a 44.54◦ contact angle. Inkjet printing of the pristine aqueous ink cannot form a continuous wet lm but forms separated ink puddles on VHB substrate. Here, we formulated the ink in an optimized weight ratio of 40.78% PEDOT:PSS ink, 4.5% Triton-X100, and 54.72% deionized water (see Table listing in Figure 5b). This optimized ink spreads well (at an 11.5◦ contact angle) on the VHB substrate, and it can be inkjet printed to form a continuous wet lm which eventually evaporated to make a nanometric solid coating of PEDOT:PSS (see Figure 5a, third column).

Figure 6a shows a printed dot of PEDOT:PSS ink with D=38.10µm diameter and 0.27µm thick on the VHB substrate. Figure 6b-e shows a continuous line can be formed by printing at the following drop spacing: 10µm, 15µm, 20µm. A closer print drop yields a thicker and wider line, but too much ink droplets may reow to distort the `straightness' of line edges. Figure 7 shows a printed lm obtained by printing multiple lines with overlaps. 14


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2000

R

Humidity Chamber

R VHB

80%

80

Resistance

1900 1800

60 40

25%

20

Relative Humidity

PEDOT:PSS/Triton-x Top View

100

(b) Resistance (Ω)

(a)

Relative Humidity ,%

1700

0 0

Side View

5

10 Time (Hrs)

15

20

100

V (kV)

0

50

4

100

150

200

3kV

250 Time (min) D(V)

2

VHB

0kV

0 1.04

300

350

400

450

500

3kV

V

0kV

DI

Time (min)

(e) D(V)/DI

25%

25%

0

(c) (d)

80%

80%

50

Time (min)

1.03 1.02 1.01 1 105

115 Time (min)

225

230

235 240 345 Time (min)

350 355 Time (s)

360 465

470 475 Time (s)

480

16/9/2018 Figure 4: Eect of humidity on properties of plasticized PEDOT:PSS nanometric lm: (a) 4 setup for electrical resistance measurement; (b) humidity eect on electrical resistance; (c)Figure 4 (e) humidity eect on voltage-induced diameter expansion Ink-Substrate compatibility: Wetting of Aqueous ink on VHB

Contact angle

(a) Sessile drop test and print film uniformity Pristine PEDOT:PSS PEDOT:PSS+Tritonx100 drop on VHB drop on VHB 44.54°

(b) Effects of aqueous conductive ink formulation 61.3% 40.78% 26.75% / 6.8% / 4.5% / 3% / 31.8% / 54.72% / 70.25%

PEDOT:PSS /Triton-x /Water (wt%)

90% / 10% / 0%

Resistance (kΩ)

1.15

2.37

4.44

5.54

Film uniformity (on VHB)

Yes

Yes

Yes

Yes

Transmittance @550nm (%)

83.9

89.2

89.4

91.0

11.5°

Print area

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Relative Humidity (%)

Page 15 of 32

Note: Printed at 10um drop spacing using

Figure 5: Formulation aqueous PEDOT:PSS ink for improved wettability and formation 40.78% /of4.5%/ 54.72% of uniform and clear lm on VHB substrate: (a) wettability of ink droplet and uniformity of wet lm printed (at 10µm drop spacing) on VHB substrate; (b) eect of ink formulation on electrical resistance, uniform lm formation, and optical transmittance (at 10µm print drop spacing). 15

12/6/2018

ACS Paragon Plus Environment 4


Figure 6 (b) Printed line b’ b

1.2 1.1

a-a’

1 0.9

Height (μm)

Height (μm)

a

0

Line width (μm)

a’

(c) Effect of drop space 60

3.2 3 2.8 2.6

b-b’

55 50 45 40

50

25 50 75 Y-axis (μm)

0.3 Line Thickness (μm)

Printed dot

(a)

(d) 10μm drop spacing

0.2 10 15 Drop spacing (μm)

100 150 X-axis (μm)

20μm drop spacing

20

(e)

Figure 6: Topography and height measurement for (a) a dot and (b) a line (printed at 10µm drop spacing); (c) eect on the line width and thickness; (d)-(e) contour plots of printed line topography at 20µm and 10µm drop spacings respectively

Figure 7

1000μm

PEDOT: PSS Triton-X

6

10 5

Along print lines

0 10

15 20 Drop spacing (μm)

Height (μm)

Normal to print lines

15μm drop space

400

d

c

20μm drop space

Tspec (%)

15

(d) 100 80 60 40 20 0

d’

c’

R

20

(e) Topography of a film printed at 10μm drop spacing

10μm drop space

5.5 5

c-c’

4.5 4

500 600 700 Wavelength (nm)

0

100 200 x-axis (μm)

Height (μm)

1000μm

(c)

(b)

10μm drop spacing Silver grease

(a) 20μm drop spacing

R (kΩ/□)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 32

2 d-d’

1.5 0

25

50

x-axis (μm)

Figure 7: Injet printed lms of PEDOT:PSS on VHB substrate: (a) top-view micrographs showing the eect of print drop spacing; (b) a photograph showing a setup for resistance measurement; (c)-(d) Eect of drop spacing on electrical sheet resistance and specular optical transmission (Tspec ) (e) topography and height measurement of a lm of PEDOT:PSS printed at 10 µm drop spacing.

16/9/2018

16

7


Page 17 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A closer drop spacing, for example 10µm or 15µm, is required to obtain a continuous lm by allowing the reow between a freshly printed wet drop and a previously printed wet line that could have dried up a bit. A thicker lm obtained by printing at a closer drop spacing is electrically more conductive but optically less clear (see Figure 7c-d). Figure 7e shows a patch of printed lm which was measured with thicker edges due the coee ring eect 63 that happens during the drying of a ink puddle. The measured sheet resistance of a lm printed at 20µm drop spacing is less than 14.92kΩ/2 measured normal to the print direction and less than 1.34kΩ/2 measured along the print direction. This suggests a poorer electrical contact at the interface between the print lines. After all, these lms of printed PEDOT:PSS are conductive enough to make for compliant electrodes of DEAs. Stretchability of the inkjet printed PEDOT:PSS thin lm can be tested on a standard dielectric elastomer actuator. This dielectric elastomer actuator device consists of a prestretched dielectric elastomer membrane (a VHB4910 tape with 3-times radial pre-stretches) sandwiched by a pair of circular PEDOT:PSS electrodes. Two types of inkjet printed PEDOT:PSS compliant electrodes are tested, namely a thicker lm as obtained by printing at 15µm drop spacing, and a thinner lm as obtained by printing at 20µm drop spacing. A thicker lm of inkjet printed PEDOT:PSS is axially stier than a thinner lm.

Figure 8 shows the electrode area enlarging in a quadratic trend with respect to increasing voltage up to an yield point beyond which further actuation tapers. Thinner PEDOT:PSS electrodes are axially softer than thicker ones. The DEA with thinner PEDOT:PSS electrodes enlarges more than the one with thicker electrodes when the same voltage was applied to stretch the DEAs below the electrode yield point. However, beyond the yield point, the PEDOT:PSS electrodes elongate plastically and form necking. 43 Interestingly, the DEA with thicker electrodes can ultimately enlarge more beyond the yield point. In short, the thicker PEDOT:PSS lms (as obtained from 15µm drop-space printing) are more stretchable for driving the DEA. Figure 8d shows micro-ridge formation in the released electrodes post 17


Figure 8


0kV

4kV

(b)

(c)

(d)

10mm

1.25

0.2

1.15 1.1

Leakage current (μA)

D(V)/DI

(f)

Printed PEDOT:PSS (15μm drop spacing)

1.2

50μm

Printed PEDOT:PSS (15 μm drop spacing)

After 10kV activation

(d) 10mm

(e)

50μm

D(V)

DI

Electric Breakdown

(a)

Before any activation

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 32

0.1

Printed PEDOT:PSS (20 μm drop spacing)

1.05 1 0

5 Voltage (kV)

Printed PEDOT:PSS (20μm drop spacing)

0.0

10

0

5 Voltage (kV)

10

Figure 21/10/2018 8: Eect of print drop spacing on dielectric elastomer activation (DEA) and electrode 8 stretchability: (a)-(b) photographs showing deactivation and activation (see Movie S1) of a DEA with PEDOT:PSS electrodes printed at the 15µm drop spacing ; (c)-(d) Surface topography transition from smooth (prior activation) to micro-ridged (upon release from 10kV activation) (e) voltage-induced areal expansion ; (f) Leakage current across the dielectric elastomer upon activation by a step-wise voltage ramp 10kV activation (i.e. stretching) of the DEA. Figure 8f and Movie S1 shows the ultimate dielectric elastomer actuation (e.g. D(V )/DI =1.14 at 9kV) is limited by dielectric breakdown, which is marked by a current surge.

Figure 9 shows that MPDEAs with inkjet printed PEDOT:PSS electrodes are slightly bluish. While a single-layer MPDEA is optically clear with close to 78.64% optical transmittance, a two-layer MPDEA with doubling the number of electrodes are less clear (of 61.8% transmittance for 550nm wavelength). The uneven bluish tone distribution suggests thickness variation in the printed lm.

Figure 10 shows the electrical activation of MPDEAs reducing the hole diameter 2a(V ), smaller than the inactivated one 2ao . Two-layer MPDEA can sustain a higher driving voltage up to 5.5kV and thus reduces the hole diameter by close to 15% (from 541.04±25.36 µm diameter to 459.53±20.40 µm diameter). When this MPDEA was activated under 44MV/m nominal eld, the power consumed was merely 1.14mW or 3.62W/m2 . In comparison, the single-layer MPDEA can only be driven up to 4kV (for 10% hole diameter contraction) due to 18



 2-layered MP (i.e. 4 layers o PEDOT:PSS) ~80% transpa

(b) 100 Tspec (%)

(a)

80 60 40

 Hole diamete reduced by >1

1 Layer MPDEA

20

2 Layer MPDEA

0 400

500 600 Wavelength (nm)

700

 20 Figure 9: Optical clarity of micro-perforated dielectric elastomer actuators (with PE- M Note: Brightness increased DOT:PSS electrodes printed at the 15µm drop spacing): (a) a photography of a 2-layer pe Electrical Actuation of MPDEA 2 MPDEA which is placed in front of a black-and-white printed USAF target; (b) eect of the (i number of layer on specular optical transmittance Tspec P ~

Figure 8new

Results:

1

(a) a(V)/ao

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

 20 mm diame MPDEA with perforations

Results: Transmittance

(b)

0.95

1-layer MPDEA 2-layer MPDEA

0.8 0

V = 0kV

V = 5.5kV

H 1‐layer MPDEA re

0.9 0.85

Leakage current (μA)

Page 19 of 32

1

2 3 4 5 Voltage (kV) 2-layer MPDEA 1-layer MPDEA

0.2

6

0.1

(c)

0 0

1

2 3 4 5 Voltage (kV)

6

Figure 10: High-voltage activation of MPDEAs for reducing the hole size (see Movie S1): (a) a photograph of a 2-layer MPDEA prototype and micrographs showing the hole contraction upon activation; (b) A stepwise voltage ramp for activating a MPDEA and the current leak across it; (c) the voltage induced contraction of the through holes in a MPDEA pre-mature electrical breakdown of the air across shallow holes between opposite electrodes. Moreover, these MPDEA devices with plasticized PEDOT:PSS electrodes are self clearable and can survive a pre-mature breakdown upon voltage removal. As a result, the ultimate breakdown voltage was raised, for example, from 4.5V for the rst occurrence of breakdown to 5kV for subsequent occurence. This self healing of the devices happens because the PEDOT:PSS electrodes surrounding the holes can be self cleared and thus the opposite electrodes next to each hole become further apart. This self clearing eect is similar to that reported for non-perforated circular membrane DEAs using other self clearable compliant electrodes. 24,28,64 19


Figure 11


1-layer MPDEA

Absorption Coeffi.

1

5kV 4kV

(a)

0.8 0.6

0kV

Bandwidth at α 0.8 is 349Hz from 831Hz‐1180Hz

18.5% shift

0.4 0.2

860 Hz 1055 Hz

0 200

Absorption Coeff.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 32

400

600

1

1200

6kV

(b)

0.8

800 1000 Frequency (Hz) 2-layer MPDEA

1400

0kV

Bandwidth at α 0.8 is 444Hz from 846Hz‐1290Hz

5kV

0.6

15.2% shift

0.4 0.2

992 Hz

0 200

400

600

800 1000 Frequency (Hz)

1170 Hz 1200

1400

Figure 11: Acoustic absorption spectrum of MPDEAs: (a) tunable spectrums for a 1-layer MPDEA; (b) tunable spectrums for a 2-layer MPDEA

Figure 11 shows the sound absorption spectrum of MPDEA-based tunable acoustic absorbers. When not activated, a two-layer MPDEA has the peak absorption at 1170Hz resonant frequency. Then, the bandwidth for above 80% sound absorption is as broad as 444Hz (from 836 Hz to 1290 Hz). When the two-layer MPDEA is activated with 6kV, the resonant frequency is shifted down by 15.2% to 992Hz. In comparison, a single-layer MPDEA shows a narrower bandwidth and its 5kV activation reduces the resonant frequency by 18.5%.

4

Benchmark

So far, there are a few solution of transparent acoustic absorber, namely translucent curtains (e.g. Gerriets Absorber Light 2 ), clear micro-perforated glass, 3,9 and this work based on micro-perforated dielectric elastomer absorber (MPDEA). They all work on similar principle of resonant absorption. As shown in Figure 12, their sound absorption bandwidths however dier due to material, construction, and resonator design. Absorber Light curtain is a 0.83mm thick weave of polyester bres according to Gerriets. 2 This exible and translucent acoustic absorber however needs a deep back cavity, for example

20


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a 150mm depth to achieve at least 55% sound absorption over a bandwidth close to 600Hz. In comparison a 5mm thick glass panel with 0.5mm diameter perforations can achieve a similar bandwidth of sound absorption (for α >55%) even by using a slimmer back cavity of 25mm according to Fuchs and Zha. 3 This slim design of micro-perforated glass acoustic absorber can save space for installation. Last but not least, our MPDEA with exible and stretchable membrane is electrically tunable. Even when it is unpowered at the passive state, it shows 800Hz bandwidth (for α >55% ), which is close to 30% broader than those of other two and some more electrically tunable. A two-layered design of MPDEA shows higher α of up to 97.4% at 934Hz. However, it needs further optimization to be optically clearer and physically slimmer for installation to window glass. In comparison, the previous MPDEA developed by our group is larger in diameter (100mm) and opaque by using microwrinkled gold thin lms as compliant electrodes. 23 Its resonant frequency happens to be lower at 538.5Hz with a larger diameter of back cavity though the maximum peak sound absorption coecient is lower at 0.85. A 5kV activation can contract the hole size by 14.5% and this can shift down the resonant frequency by 13.1%.

5

Conclusion

This paper presented a microperforated dielectric elastomer absorber using inkjet printed electrodes of plasticized PEDOT:PSS thin lms. This transparent acoustic absorber can absorb mid-frequency sound while being optically clear. It promises to make a quiet window glass and to prevent the reverbration of indoor noises. Also formulated are soft transparent compliant electrodes based plasticized PEDOT:PSS with moderate stretchability for driving a dielectric elastomer actuator and tunable acoustic absorber membrane. These transparent compliant electrodes are self clearable and durable for repeated use. However, modulus of plasticized PEDOT:PSS is sensitive to humidity change. While being soft in humid ambience of tropical country, plasticized PEDOT:PSS becomes stiened when being dry. Further

21



(a)

Absorber light curtain

Micro-perforated glass

This work

Material and Form

Polyester Fibre weave

Micro-Perforated Glass Panel

Micro-Perforated VHB Membrane

Panel thickness

0.83mm

5mm

0.25mm

Hole size a; spacing 2b

Sub-millimeter

a=0.5mm; 2b=2.5mm

a=0.46mm–0.54mm; 2b=5mm

Back-cavity depth

150mm

25mm

40mm

Clarity

Frosted

Clear

Clear

Bandwidth above 55% sound absorption coefficient α

590 Hz (from 400Hz to 890Hz)

578Hz (from 500Hz to 1078Hz)

800 Hz (from 621 Hz to 1421Hz)

Max. α at resonant frequency

68% at 629Hz

94.8% at 780Hz

97.4% @ 934Hz

(b)

1

Absorption Coefficient α

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 32

0.8

Micro-perforated VHB

Micro-perforated glass

0.6

Absorber light curtain

0.4 0.2 0 250

450

650

850 Frequency [Hz]

1050

1250

1450

Figure 12: Performance comparison among transparent acoustic absorbers: (a) listing of material, resonator design, and optical and acoustic performances (b) sound absorption spectrums 17/9/2018

12

22


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material design will be need to keep the electrodes soft even in dry ambience. Together with dielectric elastomer, plasticized PEDOT:PSS compliant electrodes are promising to tune acoustic metasurfaces, 56 while being equally useful for transparent generator for acoustic energy harvesting, 65 and even tunable optical devices. 36,66

Acknowledgement This research was supported by Singapore Millennium Foundation managed by Temasek Foundation Innovates. The rst author M. S. is grateful to Singapore Center for 3D Printing (SC3DP) for supporting his PhD scholarship.

Supporting Information Available The following les are available free of charge. Figs. S1 to S4 Movies S1

Author information Corresponding Author

*E-mail: [email protected]; Co-corresponding author E-mail: [email protected]

Author contributions

M. S., Z. L., G.-K. L. conceived the idea and designed the study. M. S. contributed to all the material design and device fabrication. Z.L. designed the experimental setup and conducted the acoustic testing. M. S., Z. L., and G.-K. L. carried out data analysis. Z.L. and G.-K.

23



Page 24 of 32

L. derive the models. M. S. and G.-K. L. wrote the manuscript. All authors discussed the results and revised the manuscript.

Note

The authors declare no competing nancial interest.

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Graphical TOC Entry 11.5° VHB

0 kV

PEDOT:PSS +Triton-x100

5.5 kV

32


Transparent tunable acoustic absorber membrane using inkjet printed

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