Design and direct additive manufacturing of three-dimensional surface micro-structures using material jetting technologies

Accepted Manuscript Title: Design and direct additive manufacturing of three-dimensional surface micro-structures using material jetting technologies Authors: Jessirie Dilag, Tiffany Chen, Sheng Li, Stuart A. Bateman PII: DOI: Reference:

S2214-8604(18)30748-6 https://doi.org/10.1016/j.addma.2019.01.009 ADDMA 629

To appear in: Received date: Revised date: Accepted date:

25 September 2018 14 January 2019 25 January 2019

Please cite this article as: Dilag J, Chen T, Li S, Bateman SA, Design and direct additive manufacturing of three-dimensional surface microstructures using material jetting technologies, Additive Manufacturing (2019), https://doi.org/10.1016/j.addma.2019.01.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Design and direct additive manufacturing of three-dimensional surface micro-structures using material jetting technologies

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Jessirie Dilag1, Tiffany Chen1, Sheng Li 2 and Stuart A. Bateman1*

1 School of Engineering, Royal Melbourne Institute of Technology University, Melbourne, Australia,

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3000 2 CSIRO Manufacturing, Clayton, Australia, 3168

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*corresponding author:

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Professor Stuart A. Bateman School of Engineering

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GPO Box 2476

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Melbourne VIC 3001 Australia

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[email protected]

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Authors postal and/or email address:

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School of Engineering GPO Box 2476

Melbourne VIC 3001

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Australia

[email protected] , [email protected], [email protected]

2 CSIRO Manufacturing CSIRO Private Bag 10

Clayton South Vic 3169 Australia [email protected]

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Graphical Abstract

Direct printing of microstructures using material jetting additive manufacturing (3D

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Highlights

VeroCyan™ photopolymer had enhanced wetting of the PMMA surface leading to

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printing) onto poly(methyl methacrylate) plates

greater droplet spreading compared with that deposited onto photo-cured VeroCyanTM

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integrated standalone model

Surface free energy can hence govern both microstructure design in terms of spreading

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and resolution, and ultimately the adhesion of print

Abstract

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The ability to directly print 3D microstructures across the surface of large dimension substrates opens up numerous possibilities not feasible with conventional 2D or 2.5D printing or coating techniques. Demonstrated herein is a method to print 3D microstructures onto clear poly(methyl methacrylate) (PMMA) plates using material jetting technologies. Contact angle and profilometry analysis indicated that the VeroCyan™ photopolymer had enhanced wetting of the PMMA surface leading to greater droplet spreading affecting the geometries printed compared to VeroCyanTM integrated models.

Strategies to manipulate the interfacial interactions and hence adhesion of the VeroCyan™ photopolymer were investigated by varying PMMA surface free energy through physio-chemical and chemical techniques including (i) corona discharge, followed by post-treatments with 3(trimethoxysilyl)propyl methacrylate, polyethyleneimine graft chemicals, and (ii) plasma treatments with air and plasma polymerisation of 1,7-octadiene. The surface chemistry and wetting behaviour played a crucial role in influencing interfacial interactions with the VeroCyan™ photopolymer hence its adhesion

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to the PMMA surface.

Keywords

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Additive manufacturing, material jetting, interfacial interactions, adhesion, surface chemistry

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1 Introduction

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During the last few decades additive manufacturing (AM), also referred to as 3D printing, and rapid prototyping have emerged for the rapid production of innovative, customised and sustainable products [1-3] of particular interests to a variety of fields including architecture [4, 5], aerospace [6-9] and the

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biomedical sciences [10-13]. Essentially AM is a process that involves successive addition of material to build a 3D structure. There have been many types of AM techniques developed with ISO/ASTM

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52900 (2015) categorising them into seven classes according to process and materials used; binder jetting, direct energy deposition, material extrusion, material jetting, powder bed fusion, sheet lamination and vat polymerisation [14]. Alongside growing advances in manufacturing there exists demand for

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improvements in micro- and nano-fabrication technologies and merging them in new manufacturing platforms. In this case, the ability to directly 3D print microstructures onto desired pre-formed substrates. Conventional advancing technologies for direct writing microstructures on substrates include etching [15, 16], direct patterning lithography or embossing [17, 18], surface chemical vapour deposition (CVD) [19, 20], and self-assembly such as buckling [21]. Drawbacks, however, to these techniques is that microstructures possible are only 2D or 2.5D and generally for a small surface area [22]. Complex 3D

microstructures can be realised and on a large surface area scale with additive manufacturing. This method will open a plethora of opportunities for application in scalable microstructures onto microfluidic devices [4], the transfer of functional biomimetic hierarchical structures to induce self-cleaning and drag reducing properties onto desired substrates [23-28] and towards meeting demands miniaturisation technology in areas such as micro-electrical or micro-optical products [29, 30].

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The fabrication of 3D micro-parts has been limited within each AM technique type due to their respective material properties and behaviour during the processes [31-35]. For example, resolution in the powder

bed fusion and direct energy deposition techniques will depend on the particle size distribution and its

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behaviour upon addition of the energy source [36-38]. Another technique relevant to 3D printing of

microstructures is 3D bioplotting. Although this technique offers the use of a wide range of different biomaterials, it has been specifically targeted to produce 3D scaffolds for biomedical purposes such as

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soft tissue engineering and hydrogel manufacturing [39-41]. In material extrusion techniques such as

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fused deposition modelling (FDM) efforts to improve resolution at the micron scale requires reducing

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the volume of material flowing through the die [33, 42] . This has been a challenge to achieve due to the rheological behaviour of the polymeric material through a smaller nozzle, faster feed rate and

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increased temperatures [31, 33, 34]. Direct writing AM technologies has been considered for repair particularly cold spray and laser deposition for example, the repair of metal turbine blades at one scale

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[43, 44]. We report here as we understand for the first time the printing of microstructures onto a flat

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substrate, associated with material jetting technologies. Material jetting technologies offer a higher throughput of products on a larger surface area and less

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manufacturing complexity compared with other techniques such as vat polymerisation that offer similar print resolution. With vat polymerisation technologies the substrate would need to be secured whilst in the vat and non-reflective to not interfere with the light processing. The material jetting technique is

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described whereby droplets of the liquid photopolymer are selectively deposited and cured in successive layers. Research oriented in this technology has featured in creating 3D-printed microrobots to be operated in biological fluids [45], micro pyramidal structured absorbers (PSA) for improving signals in Ultrasound applications [46] and towards design of micron sized wearable auxetic impact protection devices which rely on stiffness gradients and variable auxeticity which was realised with the multimaterial and resolution provided by the material jetting and PolyJet technologies [47]. Current studies

have directed their focus towards the behaviour of the inks, droplet size, and speed of deposition substrate [48-53]. Studies have also contributed to the accuracy and resolution of material jetting processes [53] , however the surface wetting properties will be a crucial element to direct material jetting AM onto desired substrates to achieve good or tailored adhesion [49, 54]. This paper reports methods pertaining towards the direct printing of 3D microstructures onto flat, clear-colourless poly(methyl methacrylate) (PMMA) substrates using material jetting. PMMA was considered as a neutral

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representative substrate, a universal platform thermoplastic polymeric material with endurance for UV

required during VeroCyan™ photopolymer curing in the material jetting process and its surface wetting

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can be easily tuned to change wetting properties. It also possesses high rigidity and surface flatness. The direct 3D printing methods presented focuses on PolyJet™ material jetting technique within the 3D

printer Stratasys J750 for direct printing microstructures onto untreated PMMA. Understanding the interactions at the interface of the VeroCyan™ photopolymer droplet and the surface the solid during

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printing will play a crucial role in controlling the rate spreading of the VeroCyan™ photopolymer which

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ultimately has an impact on the adhesion of the VeroCyan™ photopolymer to the surface.

2 Material and methods

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2.1 Materials

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VeroCyan™ and Transparent VeroClear™ liquid polymers (used for model and “gloss finish” respectively), SUP706 support liquid were purchased from Stratasys Ltd (Minnesota USA), via

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Objective 3D (Australia), adhesive tape and EP11HT Grey epoxy provided in BYK Chemical Company in the kits used (Cross-Cut Tester Kit Crosshatch Adhesion Kit PosiTest Model ATA-20) , polyethyleneimine (PEI, Lupasol®, WF) was obtained as samples from BASF and 3(trimethoxysilyl)propyl methacrylate (TMSPMA, 98%) from Aldrich. Poly(methyl methacrylate) (PMMA)

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clear colourless plates were cut to 76 mm x 102 mm and 2 mm plates from Acrylic Sheet (Victoria, Australia). Solvents; acetone, isopropyl alcohol (IPA), sodium hydroxide (NaOH), acetic acid, diiodomethane were purchased from Sigma Aldrich and used as is.

2.2 Experimental methods

2.2.1 Material jetting Drawings of the models, surface features and designs were sketched using CATIA V5R21. Boolean operations were performed on the final model before converting the CATIA .Part file to .stl for printing. The .stl files were checked for errors using GrabCAD, which was also used for uploading the job to the Stratasys J750 3D printer. Geometries used are length (L), spacing (s), height (h), and width will be

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referred to as thickness (t). Before printing PMMA plates were wiped with IPA and if needed, dried under nitrogen. The PMMA plates were taped to the build platform using adhesive tape on the perimeter

of the substrate. Material jetting direct printing was performed on the Stratasys J750 on ‘high quality

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mode’; by manually altering the build platform stage height and manipulating the software commands

to deposit the VeroCyan™ photopolymer correctly onto the substrate. For models with overhang structures, the support material (SUP706) was removed manually in a sodium hydroxide solution. A

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detailed discussion with respect to instrument and software configurations is provided in the ‘3.1

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Instrument method development’.

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2.2.2 Surface treatments

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2.2.2.1 Air plasma and plasma polymerisation of 1,7-octadiene Air plasma and plasma polymerisation of 1,7-octadiene surface treatments were achieved in a stainless

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steel T-piece reactor with an internal aluminium 17 cm diameter disk electrode with a 13.56 MHz radio frequency power source generator (Coaxial Power Ltd, UK) coupled to the internal disc electrode via

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an impedance matching network. A fine (CMV-VFM-2-P-KK) and medium (CMV-VFM-3-P-KK) flow needle valve (Chell Instruments Ltd, UK) were utilised to monitor the flow rate. Pressure measurements within the reactor were recorded using a Pirani Gauge (Edwards, UK). Samples were elevated within

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the chamber to a height of 4.35 cm on an aluminium stage. The PMMA panels were placed at least 3 cm away from the electrode perpendicular to the horizontal stage. Before air plasma treatment, the chamber was pumped down to a base pressure of 0.0015 mbar and the substrates were treated for 10 min at power output of 50 W and a flow rate of 2.04 sccm. For the plasma polymerisation surface treatment, 1,7-ocdatdiene monomers were purified with at least three freeze-thaw cycles. The chamber

was then pumped down to a base pressure of 0.017 mbar and the substrates were treated for 10 mins at a power output of 20 W and a flow rate of 2.22 sccm.

2.2.2.2 Corona discharge and post-treatments with TMSPMA and PEI Post-treatment surface grafting solutions were prepared. PEI was diluted to 0.25% in mixture of water

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and IPA (50:50). For TMSPMA, a 5% dilution in deionised water and isopropanol (50:50) with a small amount of acetic acid (pH 4) was pre hydrolysed overnight before being diluted further to 0.25 %. The

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pH was adjusted with 1 M NaOH (pH 12) before treatment.

PMMA panels were cleaned with IPA prior to surface treatment. Corona discharge was performed on a Tantec Corona Generator Model HV 05-2 with energy output 150 mJ/mm2. Post treatments via surface

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grafting of the corona treated panels was completed by immersing them in the prepared solutions of

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TMSPMA and PEI separately for 30 minutes with gentle agitation. Samples were then taken out from

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the grafting solution and rinsed at least three times with deionised water and dried in air.

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2.2.3 Characterisation

The DektakXT Stylus profilometer (Bruker) was used to obtain averaged profile data (n = 3 scans) of

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the 3D printed products and microscopy imaging was performed on an EyeSense, VHX-5000 digital microscope. Images were photographed at 100 x magnification and live measurements were taken

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using the built-in Eye-sense Software. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a Kratos Analytical AXIS Ultra-DLD (Kratos Analytical Ltd, Manchester, UK) fitted with a monochromatic Al Kα X-ray source (1486.6 eV). An operation power of 150 W was used. The PMMA

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samples, cut to 1 cm2 were placed in an ultrahigh vacuum chamber (10-8 mbar) with electron collection by a hemispherical analyser at a 90° angle. Elemental analysis was performed on survey spectra and high resolution spectra were collected for quantitative analysis. Using peak integration, peak and literature values, the atomic concentrations were determined. Static contact angles with liquids; water, di-iodomethane and VeroCyan™ were measured using the sessile drop method with contact angle

measurement equipment OCA20 (DataPhysics). All contact angles were reported as a mean value of six values on different parts of the substrate surface.

2.2.4 Adhesion testing Pull-off adhesion tests were carried out to measure the adhesion strengths of printed photo-polymer on

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the substrates according to the ASTM/D 4541-95 and ISO 4624 standard method. A 20 mm diameter dolly was adhered to the sample surface using the EP11HT epoxy adhesive. After complete curing of

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the adhesive of at least 24 hours at room temperature, the dolly was loaded using the pull-off testing

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equipment (PosiTest Model ATA-20, BYK Gardner) at 0.2 MPa/s until fracture.

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3 Results and Discussion

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3.1 Instrument method development

Direct printing of surface designs were performed on the Stratasys J750 which adopts the PolyJet

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(material jetting) technology. As the instruments are not designed to print on different substrates there were aspects to consider such as (i) developing a process to ‘off-set’ the print head’ to allow it to print

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onto a different substrate rather than printing a photo-curable substrate itself and then surface features on top as a unified print; (ii) ensuring no contact was made between the print head and the stage whilst

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also ensuring that the required print-head – substrate distance was maintained for accurate and precise printing and (iii) avoiding the deposition of support material to the stage prior to the build material – typically employed to enable easy removal of the printed model when the equipment is used

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conventionally. The stage height ( 𝑧𝑠𝑡𝑎𝑔𝑒 ) was changed manually to ensure a 150 µm height difference between the print head and substrate which was consistent with the manufacturers height setting between the print head and stage. The tape used to keep substrate in place did not obstruct the print head’s path. The motors are originally set with the z position at -5750 a.u where 100 a.u was equivalent to 0.2 mm and a conversion factor of 500 was used. The 𝑧𝑠𝑡𝑎𝑔𝑒 can be manually altered according to the 𝑡𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 using Equation 1.

Equation 1 𝑧𝑠𝑡𝑎𝑔𝑒 = −5750 𝑎. 𝑢 + (𝑡𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 𝑥 500 𝑎. 𝑢) The second manipulation was to restart the print at the slice number after which the initial support material is deposited (𝑧𝑖 ). This slice number to restart the print (𝑧𝑖 ) was determined by subtracting the

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number of slices used for the print overall height (ℎ𝑓 ) from the total number of slices (zf) calculated for the job. As the machine worked on a slice resolution of 14 µm, the calculation is summarised below in

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Equation 2 for a high quality printing.

ℎ𝑓 ) 14 𝜇𝑚

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𝑧𝑖 = 𝑧𝑓 − (

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Equation 2

The substrate was secured to the stage platform using adhesive tape and the print project prompted to

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start the print at slice number = 𝑧𝑖 for which the surface features began to print on the substrate surface.

3.2 Printing performance on untreated PMMA substrates

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The printing performance was assessed by interrogating the accuracy and resolution of features printed on untreated PMMA substrates compared with that observed when printed onto a VeroCyan™ substrate to form an integrated model. Squares (n=3) of L = 15000 µm, t = 15000 µm were printed with

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varying h of 14 µm, 28 µm and 100 µm which correlated to the Stratasys J750 print quality specification for the height of 1 slice, 2 slices and more than 2 slices respectively (100 m). Profilometer results are shown in Table 1.

At the dimensions explored, the designs were shown to be different (smaller) in height when printed on the untreated PMMA substrate compared with that printed on VeroCyan™. This suggests that the VeroCyan™ photopolymer’s interaction during the build may be different to that of PMMA. The interaction itself is complex when one considers the print head x-y positioning accuracy, the force of the droplet impact, its volume and the change VeroCyanTM rheology during curing of the photopolymer upon each deposition. The difference and suggested decrease in the height of features printed on untreated

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PMMA was further investigated by observing the changes in s and t as a function of h. Sets of 5 lines where L= 2000 µm, s = 300 µm, t = 300 µm with increasing height; h = 50 µm, 100 µm, 150 µm and

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200 µm were printed onto untreated PMMA substrate and also printed as an integrated VeroCyan™ model. The s and t of the lines were measured with respect to changing h. Figure 1 (a)-(c) shows an illustration of the CAD file used, stacked overlayed profiles recorded of the lines printed on untreated PMMA and integrated model, a micrograph of the lines at 5 x magnification of the lines printed on

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untreated PMMA and the changes in s and t observed as a function of h, respectively.

The profiles (Figure 1 (c)) recorded show that the lines exhibit curved edges which correlates visually

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to the micrograph shown (Figure 1 (b)), particularly at the edge-ends of the lines. The curving of the edges was a result of how the VeroCyan™ photopolymer deposits, again keeping in mind the effects

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the x-y positioning, droplet impact and rheology-curing effects upon successive layers. How the polymer spreads initially on the surface, and upon each layer (i.e. as a function of height) showed as the height

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of the lines are increased, the t increases resulting in a compromised s or resolution between features at this fixed s = 300 µm and t = 300 µm. Using a linear regression, the rate of change of t and s with respect to h, ∆t/∆h and ∆s/∆h respectively, were calculated and are shown in Table 2.

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The rate of the VeroCyanTM spreading on PMMA was greater than if it was deposited on itself, which was shown in the greater positive ∆t /∆h (0.0009) corresponding to the thickness increasing and a greater negative ∆s/∆h corresponding to the spacing compensated or decreased as a function of height. This further alluded to the significance of how the VeroCyan™ photopolymer interacts with the PMMA surface and itself (wettability) in determining the sharpness of features printable at this micron scale as was clearly observed in Figure 1 (b) and (c).

3.3 Printing 3D surface architectures To demonstrate the printing of 3D architectures a simple structure with an overhang was designed and printed onto untreated PMMA substrate. This is shown in Figure 2 with (a) a CAD file and (b) photographed micrograph. Annotated are the dimensions where L = 2000 µm, h1 = 750 µm, t1 = 750

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µm, t2 = 1500 µm, hoverhang 1 = 55 µm, hoverhang 2 = 400 µm and toverhang = 750 µm.

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As expected, the edges of the structure are curved due to the nature of the printing. The spreading of

the photopolymer at the base can be clearly observed, particularly on the bottom left where the shape is tapered, due to the interfacial interaction of the liquid VeroCyan™ photopolymer with the untreated

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PMMA surface. As the model is built it can be seen the spreading becomes less prominent. On the

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right hand side the photopolymer did not spread as much due to the deposition of the support material

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to support the overhang feature.

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The contact angles of liquids VeroCyan™, water and di-iodomethane were collected and hence surface free energy calculated for the untreated PMMA surface. These values were then compared to

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measurements taken on printed VeroCyan panels under two conditions; (i) printing of the panel was manually stopped before completion to take measurements and (ii) letting the panel print to completion

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post environmental exposure of 48 h. The contact angle of the liquid VeroCyan™ photopolymer measured on the untreated PMMA (15.1 ± 2.9 º) was less than that on interrupted printed panel of

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VeroCyan™ (39.1 ± 4.5 º) under condition (1) and cured VeroCyan™ (35.5 ± 2.3º) under condition (ii) which alludes to the increased rate of spreading on the PMMA surface. The interfacial properties governed by the surface free energy (SFE) were also calculated (using Fowkes theory). These values

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are shown in Table 3.

In general, the SFE was higher for the VeroCyan™ panels with respect to untreated PMMA; however the panel cured under condition (i) had a higher SFE due to an increase in the dispersive components indicating there is the added complexity of the cure kinetics upon layer build-up of a model which has been calibrated for printing integrated models which can also be atrributed to the proprietary formula of the photopolymer. Recent studies [71, 72] have noted that curing continues with each layer deposited

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and as such condition (i) is expected to be of lower cross-link density and hence highly mobile allowing over time strong interfacial interactions to form with subsequent layers of the photopolymer formulation

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(including wetting agents) being deposited and cured.

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3.4 Surface treatments

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PMMA was surface treated to investigate how changes in surface energy influences VeroCyan™ wetting and hence its adhesion. Oxidation of the surface was achieved using both corona discharge

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and air plasma techniques. To further explore and promote adhesion, oxidized surfaces via corona

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discharge were grafted with TMSPMA and PEI following corona discharge. To contrast, PMMA was

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are depicted in Figure 3.

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also surface modified through plasma polymerisation of 1,7-octadiene. The various surface treatments

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XPS analysis confirmed modification of the PMMA surface via these treatments. Table 4 provides a quantitative measure of the percentage surface carbon functionalities obtained from integration of the deconvoluted XPS peaks described and in addition the average surface roughness (Ra) measured by

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profilometry. The Ra of the untreated and treated PMMA substrates corresponded to the surface treatments performed showing a general increase in roughness upon each treatment.

The XPS data from survey spectra showed that C and O were detected for all samples while N detected was associated with the PEI surface treatment post corona discharge. A small peak of N was also

detected after corona treatment presumably from nitrogen in the air. As expected, a higher oxygen content was found after corona treatment, associated with decrease of C content, demonstrating surface oxidation of PMMA following corona treatment. Only traces of Si were detected on the Corona/TMSPMA treated surface suggesting low levels of surface grafting under the conditions employed with some of the silane possible hydrolysed and washed away during the extensive post treatment washing process. A small Si component was also detected in after grafting PEI which was

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attributed to possible contamination. The high nitrogen content associated with a reduction of oxygen

content clearly showing PEI was grafted to the surface and resisted well to the thorough washing

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process. The plasma treatments with air lead to a decrease in carbon content, and increase in oxygen

components as expected, and the opposite was observed with 1,7-octadiene surface plasma treatments with respect to untreated PMMA samples.

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The C1s peak was deconvoluted into four components. C-C (C1 and C2 which is due to the secondary

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shift of C* bonding to O-C=O), C-O (C3) and O-C=O (C4) [55]. The C-N bond of PEI corresponding to the amine group (-NH) mainly constituting the structure of PEI would also be present in C3 binding

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energies overlap [56]. Similarly the O1s was deconvoluted corresponding to HO − C = O (O2), C − OH

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(O3), and chemisorbed oxygen (O4) [57] and the N1s to its respective components of N-H (N1), hydrogen bonded amine (N2) and protonated amine (N3) and highly oxidated nitrogen (N4) [58]. The

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quantitative analysis of the C1s, O1s and N1s bonding are shown in the following table, Table 5

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The increase in C4 in the corona treated PMMA was associated with reduction of C1. Reduction of C4 after PEI grafting confirmed the PEI layer was present on the surface, where the C3 component

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consisted of both C-O and C-N components[56]. However an increase of C1 after PEI grafting was not explained by the PEI chemical structure and is possibly due to the surface absorption of hydrocarbons following surface covering of the highly energetically PEI layer. The reduction in C1 corresponded in

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the increase of C4 in the air plasma treated PMMA surface whereas the increase in C1 with a great decrease in C4 correlated well to the plasma polymerised 1,7-octadiene treated PMMA surface. The O1s fitting showed a general 1:1 of O=C (O2) and O-C (O3) in all cases. A small peak at the high bonding energy shoulder (O4) was added in fitting the oxygen peak of untreated PMMA, corona, corona/TMSPMA and air plasma treated PMMA. While it improved the fitting of the curve, this peak is more likely due to the surface charge. The lower bonding energy O1 appears not negligible and was

only associated with PEI treated surface. The chemical structure is not clear but oxygen presented in amide group may have shifted to this area [57]. A small amount of N was introduced on the surface after corona treatment associated with ionic bonded at N3 and high oxidated at N4. N1, N2 and N3 peaks were fitted which showed good correlation to the grafted PEI on the corona treated surface [58].

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3.5 Surface free energy and adhesion testing The contact angles of water (WCA) and di-iodomethane (DIM) were collected to calculate the SFE of

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the solid (γs) using the Fowkes method taking into count the dispersive (γd) and polar (γp) components [59, 60] shown in Table 6.

In addition, the contact angles of liquid VeroCyan™ photopolymer was collected and interrogated with

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respect to the adhesion strength measured from VeroCyan™ discs (h set to 14 µm to infer to 1 slice)

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directly printed onto untreated and treated PMMA. These values are shown in and illustrated in Figure

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4.

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After corona treatment and post grafting with TMSPMA and PEI, the contact angle of the liquid VeroCyan™ reduced, indicating higher wetting for the photopolymer to these treated surfaces with respect to untreated PMMA which was reflected in an improvement of adhesion strength from untreated

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PMMA. This was directly correlated to the water contact angle and surface free energy calculated where corona discharge treatment on the untreated PMMA resulted in an increase wetting with a decrease in

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WCA by ~10⁰ and as expected an increase

γs

due to an increased

γp,

for the oxidised surface. The

corona treatment itself is well known to promote adhesion through oxidation of the surface and surface molecule chain scission leading to the formation of water-soluble low-molecular-weight oxidised

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material (LMWOM) [61, 62]. Keeping in mind the VeroCyan™ is an acrylic based photopolymer, there is a possible interaction with both the oxygen-containing photopolymer and oxidised species on the corona-treated PMMA that has led to this improved adhesion. The post-treatment grafting with TMSPMA resulted in similar wetting and adhesion properties to the corona treated PMMA substrate which was expected as the XPS analysis revealed that the attachment

of TMSPMA was low. The methacrylate functionality was intended to promote crosslinking [63, 64]. Post treatments of grafted PEI can also promote adhesion through hydrogen bonding interactions and covalent hydrogen bonding with carboxylic acid groups and potential imine bond formation [65]. These effects were expressed with a slightly higher adhesion strength observed for the corona/PEI treated PMMA substrates compared to corona and corona/TMSPMA treated PMMA. The surface plasma treatment with air, which like corona discharge treatment, oxidised the surface also

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increased the wetting shown by the reduced WCA by ~15⁰ [66, 67]. The γp, was the greatest out of the

surface treatments and the increased wetting however, was not expressed in the adhesion with no

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significant increase in adhesion strength with respect to untreated PMMA. Air plasma works with the generation of charge states due to plasma treatment and by the ionisation or dissociation of created new functional groups (e.g., the dissociation of surface carboxylic groups) [68]. As there are multiple

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sites for initiating the oxidation on PMMA, over a prolonged exposure different oxidative reactions can

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take place simultaneously while competing with each other which has contributed to increased wetting and increased γp but can affect the orientation that can lead to diminish sites for adhesion [66, 69]. This

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reduction of sites for binding was reflected in the increase of the contact angle of the liquid VeroCyan™

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photopolymer measured on the air plasma treated surface with respect to the untreated PMMA.

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As expected the adhesion of the printed VeroCyan™ photopolymer to 1, 7-octadiene plasma polymerised PMMA surface decreased with respect to untreated PMMA due to the increase presence

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of hydrocarbons on the surface reducing the amount of available active sites for adhesion. This was reflected in a decreased surface wetting where the WCA increased by ~15⁰, and a significantly lower

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γs observed after surface treatment. The oxygen content of 15 % observed indicated post-oxidation of plasma polymerised surface when exposed to the air and could possibly contain reactive functional groups.

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Studying the interfacial interaction between the liquid VeroCyan™ photopolymer and the substrate surface treated and non-treated is only one part of understanding the practice of direct printing on substrates using material jetting. The VeroCyan™ proprietary formula would expect to inherently possess wetting agents within its proprietary formula customised for printing as an integrated structure calibrated to the specific instrument and rather than on a different substrate. The material strength will also play a role in adhesion and cohesion aspects during testing, where new studies have shown that

the material strength behavior differs at micron scale [70]. The material jetting technology process itself in terms of printing parameters; droplet size, speed of droplet, time and amount of cure occurring within each layer, would inherently affect this adhesion. The droplets are expelled at a certain speed, and are cured to an extent instantaneously (semi-cured) upon deposition, which enables subsequent layers to adhere and interlock to cure with the previous layer. Recent studies [71, 72] have been performed where they had established a representative method for measuring the kinetics and depth of curing and

IP T

estimated, using Jacob’s working curves, the critical energy to initiate polymerisation and penetration

depth of curing light of commercially available photo-resins including VeroWhite™ in which VeroCyan™

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falls in the same class of liquid photopolymers for PolyJet™ systems from Stratasys. It was also found

that the wavelength of light used to cure the resins was significant where the depth of cure can increase by more than 3-fold for the Vero-classed resins [71, 72]. The cure kinetics in particular will play an additional role in the liquid-solid and semi-cured solid-solid interaction as the structure is built on the

A

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surface.

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4 Conclusion

A method for directly 3D printing onto substrates was developed which employs material jetting

ED

technologies. The procedures developed included setting ‘off-set’ for the print head’ to allow it to print onto a different substrate rather than printing a photo-curable substrate itself and ensured no contact

PT

was made between the print head and the stage whilst also ensuring that the required print-head – substrate distance was maintained for accurate and precise printing. In addition to manipulating the

CC E

printing start station to prevent deposition of support material to the stage prior to the build material – typically employed to enable easy removal of the printed model when the equipment is used conventionally. The liquid VeroCyan™ photopolymer was found to spread at a faster rate on PMMA

A

(untreated) than in an integrated structure VeroCyan™ photopolymer which would affect the overall resolution of features, particularly at a micron scale. The interfacial interactions and adhesion of the liquid VeroCyan™ employed in the PolyJet™ system to the PMMA substrate surface was investigated with varying surface treatments of corona discharge with post-treatments of grafted TMSPMA and PEI, and air plasma with intention to promote wetting and adhesion and decrease wetting and adhesion promotion with a surface treatment of plasma polymerised 1,7-octadiene for comparison. The adhesion

of the photopolymer VeroCyan™ was improved with corona discharge and post-treatments via crosslinking and bond formation; nevertheless, understanding of how parameters additive manufacturing process affect this can be better understood with regards to how the polymer cures and species within the formulae to promote better interfacial interactions during this cure is essential. In addition the surface energies, in particular the polar contributions, for printed VeroCyan™ and hence integrated model, are relatively low compared with untreated PMMA and PMMA treated with adhesion

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promoters suggesting the liquid photopolymer itself has wetting agents to promote the interlayer adhesion during the process. This paper mainly concentrated on increasing surface wetting and

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adhesion via promoting molecular interactions between the liquid VeroCyan™ and the surface. An

extension of this work also involves printing on curved substrates by developing algorithms for the print

U

head to follow contours.

N

5 Acknowledgements

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The authors would like to acknowledge the Additive Manufacturing Precinct at RMIT University and

M

technical staff, in particular Mr. Paul Porter, technical stuff at the Micro Nano Research Facility and Dr. Karyn Jarvis for performing the plasma treatments at Biointerface Engineering Hub at Swinburne. This

ED

work was performed in part at the MicroNano Research Facility at the Royal Melbourne Institute of Technology (RMIT) and the Biointerface Engineering Hub at Swinburne, both part of the Victorian node

PT

of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy

A

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researchers.

to provide nano and micro-fabrication facilities for Australia’s

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[54]

(b)

(a)

Integrated model

0

2

4

200 µm

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(c)

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Height profile (a.u)

Printed on PMMA (untreated) substrate

6

8

10

12

14

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Lateral (mm)

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Figure 1 (a) Annotated CAD image of Array 1 (b) Surface profiles on the untreated PMMA and integrated model (c) Micrograph (5x magnification) of lines printed on untreated PMMA

(a)

(b)

L

t2

hoverhang 1

h1

toverhang hoverhang 2

1000 µm

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Figure 2 (a) Annotated CAD file and (b) side on micrograph of micro-feature

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t1

IP T SC R U N

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A

Figure 3 Schematic diagram of surface treatments performed on untreated PMMA and resultant simplified surface chemistry for illustration purposes

20

1.8

(a)

18

(b) 1.6 1.4

Adehsion strength (MPa)

VeroCyan™ CA (º)

16 14

12 10 8 6 4

1.2 1 0.8 0.6 0.4 0.2

2 0

0

Corona treated PMMA

Corona/TMSPMA

Corona/PEI

Air plasma

pp-octadiene

Untreated PMMA

Corona treated PMMA

Corona/TMSPMA

Corona/PEI

Air plasma

pp-octadiene

IP T

Untreated PMMA

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Figure 4 (a) Contact angle of the liquid VeroCyan™ and (b) Adhesion strength measured on the untreated and treated PMMA substrates

Table 1 h measurements of squares (n=3) of L = 15000 µm, t = 15000 µm printed on untreated PMMA and integrated structure 14 µm (1 slice)

28 µm (2 slices)

100 µm

Untreated PMMA

17.9 ± 0.8

32.4 ± 3.6

88.5 ± 4.2

Integrated structure

21.2 ± 0.7

33.6 ± 2.5

94.3 ± 4.3

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A

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Substrate

Table 2 Calculated rate of polymer spreading with ∆t/∆h and ∆s/∆h ∆t /∆h (µm)

∆s/∆h (µm)

Untreated PMMA

0.0009

- 0.0011

Integrated model

0.0002

- 0.0003

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A

N

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Sample

Table 3 Contact angle of water and di-iodomethane and calculated surface free energy of untreated PMMA, VeroCyan™ cured under condition (i) and (ii) SFE (mN.m-1)

CA (⁰) DIM

γp

γd

γs

Untreated PMMA

65.7 ± 6.1

24.8 ± 1.6

7.2 ± 2.5

48.4 ± 0.6

55.6 ± 2.6

Condition (i)

54.9 ± 1.1

12.4 ± 0.1

11.0 ± 2.5

52.0 ± 0.1

62.9 ± 0.6

Condition (ii)

56.6 ± 2.1

25.3 ± 1.4

11.3 ± 0.5

48.2 ± 0.5

59.5 ± 1.1

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A

N

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Water

Table 4 % Surface functional groups of the various treated PMMA surfaces and their Ra

C:O

C%

O%

N%

Si %

Ra (nm)

Untreated PMMA

2.8

73.9

26.0

0.1

0.1

9.2 ± 1.5

Corona

2.3

68.6

30.4

0.9

0.1

11.2 ± 0.7

Corona/TMSPMA

2.8

73.4

26.3

0.1

0.2

19.9 ± 3.6

Corona/PEI

3.9

73.4

19.0

7.5

0.2

21.1 ± 2.5

Air plasma

2.1

67.1

32.1

0.1

0.1

11.6 ± 1.8

1,7-octadiene

6.0

87.3

14.6

0.2

0.1

20.2 ± 2.7

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ED

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A

N

U

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Substrate

Table 5 XPS results of deconvoluted C1s, O1s and N1s peaks Substrate

C1s

O1s

C2

C3

C4

Untreated PMMA

40.7

19.1

21.1

19.1

Corona

35.3

22.3

20.0

Corona/TMSPMA

42.8

18.4

Corona/PEI

50.4

Air plasma

O2

O3

O4

N1

--

47.29

45.1

7.6

--

--

--

--

22.3

--

49.3

45.7

5.0

--

--

72.3

27.7

20.4

18.4

--

49.9

47.7

2.4

--

--

--

--

13.5

22.6

13.5

8.1

51.3

40.6

-

60.6

25.2

14.2

-

33.1

21.2

22.6

20.3

--

50.0

43.8

5.9

--

--

--

--

87.3

5.89

6.78

1.98

--

--

--

--

--

--

--

--

A

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ED

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A

N

U

SC R

1,7-octadiene

O1

N2

N3

IP T

C1

N1s N4

Table 6 CA measurements, SFE calculated for untreated and treated PMMA substrates SFE(mN/m)

Water

DIM

γp

γd

γs

Untreated PMMA

65.7 ± 6.1

24.8 ± 1.6

7.2 ± 2.5

48.4 ± 0.6

55.6 ± 2.6

Corona treated

53.9 ± 1.1

30.1 ± 2.2

13.4 ± 0.5

46.2 ± 0.9

59.6 ± 1.0

Corona/TMSPMA

56.6 ± 2.1

25.3 ± 1.4

11.3 ± 0.5

48.2 ± 0.5

59.5 ± 1.1

Corona/PEI

50.0 ± 2.4

31.3 ± 2.5

15.6 ± 1.2

45.7 ± 1.7

Air plasma

39.4 ± 4.1

23.0 ± 2.2

19.8 ± 2.4

49.0 ± 0.8

1,7-octadiene

81.7 ± 1.5

28.2 ± 0.7

1.8 ± 0.4

47.1 ± 0.3

IP T

CA (⁰)

61.3 ± 1.9 68.8 ± 1.6

A

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ED

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A

N

U

SC R

48.9 ± 1.3