rheological properties and structures - Springer Link

Appl Phys A (2011) 102: 501–507 DOI 10.1007/s00339-010-5955-y

Direct-writing construction of layered meshes from nanoparticles-vaseline composite inks: rheological properties and structures Kunpeng Cai · Jingbo Sun · Qi Li · Rui Wang · Bo Li · Ji Zhou

Received: 25 March 2010 / Accepted: 29 June 2010 / Published online: 24 July 2010 © Springer-Verlag 2010

Abstract Direct-writing is superior in the construction of arbitrarily designed three-dimensional (3D) structures. In this work, we develop a series of organic inks doped with nanoparticles to fabricate 3D meshes of interpenetrating rods. The effects of nanoparticle addition on the rheological properties of organic inks were analyzed. The results revealed intelligible relationship between the ink’s formability and rheological properties, which could be beneficial to the construction of 3D structures from organic inks by direct writing.

1 Introduction Fine construction of soft materials is highly desirable in many fields, including tissue engineering [1–3], cell growth guide [4], bionics [5] and bandgap structures [6, 7]. With the ability of forming arbitrarily designed 3D structures [8], the direct-writing technique is one of the promising approaches K. Cai · J. Sun · R. Wang · J. Zhou () Department of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, China e-mail: [email protected] Fax: +86-10-62772975 Q. Li Materials Center for Water Purification, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China e-mail: [email protected] B. Li Advanced Materials Institute, Shenzhen Graduate School, Tsinghua University, Shenzhen 518055, China e-mail: [email protected]

to pattern soft materials at micro- and mesoscale. Structures assembled by direct writing are formed with extruded ink rods in a layer by layer way, and could be utilized in structure building [9–11], flexible electronics [12, 13], memristor [14, 15] etc. To meet the requirement of arbitrary structure design for various applications, inks used in this technique must satisfy several criteria, as follows. Firstly, the ink must be extruded fluently through the deposition nozzle without clogging or fracture. Secondly, the ink should change from fluid-like to solid-like right after its extrusion out of the nozzle to facilitate the shape retention of the deposited features. Thirdly, the formed structure must maintain its shape and strength in the treatment processes following. As a result, attention should be concentrated on rheological and mechanical properties during the ink design. Several ink designs have been developed which successfully get over these challenges, including binary organic inks [9, 10], nanoparticle inks [16] and sol-gel inks [17]. Here, we developed a series of organic vaseline inks doped with TiO2 nanoparticles as an example of nanoparticle-doped organic composite inks. Structures assembled by these organic composite inks show a superior performance to retain their shapes after repeated bending and stretching on a flexible substrate. Rheological properties of these inks were systematically investigated to establish connections between ink’s formability and internal structure change caused by the addition of nanoparticles. As a mixture of hydrocarbon (with carbon numbers mainly higher than 25), whose branches interlace with one another, vaseline shows a semi-solid behavior at room temperature (Fig. 1(a)). The addition of nanoparticles causes agglomeration between nanoparticles and hydrocarbon branches (Fig. 1(b)), therefore, exerting influence on ink’s

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rheological properties. Besides the capability to modulate the rheological properties of organic inks, nanoparticle doping could also bring special functions to organic inks for various applications, which is very useful in functioning composite materials. Thus, this kind of ink design could be relevant to a broad range of technological applications.

2 Experimental 2.1 Materials and organic inks’ preparation Organic composite inks were prepared by first melting vaseline (Medicinal White Vaseline, Baiyun Pharmaceutical Co. Ltd., Nanchang) in a beaker at 50◦ C for 15 min, then a certain amount of liquid dispersant (Triton X-100, Xudong Chemical Plant, Beijing) was added through magnetic stirring to improve its compatibility with inorganic nanoparticles. Next, commercially available TiO2 nanoparticle powder (primary particle diameter, d = 21 nm, specific surface area of 50 m2 g−1 ; AEROXIDE® TIO2 P 25) was added into the mixture part by part. The final obtained suspension was then homogenized through magnetic stirring for 1 h and cooled down to room temperature. Inks with nanoparticle content of 0 wt.-%, 2.5 wt.-%, 5 wt.-%, 7.5 wt.-% and 10 wt.-% were prepared for the following studies. 2.2 Rheological characterization

Fig. 1 Schematic illustrations of ink structures: (a) vaseline molecule branches, (b) ink structure doped with agglomerated nanoparticles, (c) ink’s network structure formed under low shear stress, (d) ink’s structure under high shear stress

Table 1 Rheological measurements of inks for the investigation of corresponding properties

Several measurements were undertaken to analyze the effects of nanoparticle addition on the rheological properties of inks, as is shown in Table 1. The rheological properties of these inks were measured using a rheometer (HAAKE MARS, Thermo Electron GMBH, Karlsruhe, Germany). The inks’ viscosity (η) was acquired in controlled rate mode using a C35/2 cone plate (diameter of 35 mm, 2 deg and gap size of 0.105 mm). Data were collected as a function of shear rate (γ˙ , 0–0.2 s−1 ) in a linear ascending order during 60 s under controlled rate mode. Viscosity test results were verified by another rheometer (Physica MCR300 Modular Compact Rheometer) under similar conditions. Inks’ yield stress (τy ) was measured as a function of shear rate in a logarithmically ascending series of discrete steps (0–5 s−1 ) during 30 s under controlled rate mode. The creep and recovery properties study of the inks was performed in controlled stress mode, where deformation (γ ) of inks was obtained as a function of

Measurements for the inks

Properties to investigate

Matters relating to the corresponding properties of inks

Viscosity test

Viscosity change of inks during a linear shear rate sweep

Shear rate control during rod extrusion process

Yield stress and thixotropy

Yield stress and thixotropy of inks

Stress and energy needed for extrusion of inks

Creep and recovery test

Time response of concentrated inks

Capability of formed structures to retain shape under external force and to recover after the force is unloaded

Viscoelastic properties

Viscoelastic response of inks under frequency sweep oscillation

Energy transformation between formed structures and external force with different frequencies

Direct-writing construction of layered meshes from nanoparticles-vaseline composite inks: rheological

time (t) during 30 s under a constant shear stress of 100 Pa and 60 s after stress unloading. Then, inks’ thixotropy was measured by collecting shear stress (τ ) data as a function of shear rate (γ˙ ) between 0 and 5 s−1 in a series of logarithmical ascending steps during 30 s, followed by another 30 s logarithmical descending steps down to 0 s−1 . In order to determine the linear viscoelastic regime, oscillatory shear stress sweep experiments were performed at frequencies of 0.1, 1 and 10 Hz. Subsequently, elastic (G ) and viscous (G ) moduli were measured in upward oscillation frequency sweep experiments at constant shear stress within the linear regime (75 Pa). To eliminate the influence of temperature, all the rheological measurements were carried out at 23◦ C. 2.3 Three-dimensional scaffold fabrication The 3D scaffold structures were created layer by layer on a computer controlled robotic stage (Ultra TT series, EFD Inc., East Providence, RI). The procedure started with controlled deposition of ink rods on a moving x–y platform, yielding a designed two-dimensional (2D) pattern. Square patterns composed of parallel array of rods with designed diameter, length and inter-rod distance were created by deposition of inks through nozzles attached to a syringe pump (barrel diameter = 22.5 mm, EFD Inc., East Providence, RI) at a constant volumetric flow rate (=0.25πD 2 υ) required to maintain a constant x–y speed (υ). After one layer was accomplished, the writing direction was rotated by 90◦ to form another perpendicular upper layer on the first layer. Then a third layer was deposited onto the second one in another 90◦ rotated direction. Procedure would be continued until the designed scaffold was wholly built. Besides inflexible substrates usually used in direct writing, flexible substrates were also used in this study to demonstrate the good bending performance of structures formed with this composite ink. 2.4 Morphology analysis Scaffolds’ surfaces were scanned using micro-computed tomography (AXio Imager.Z1m, Carl Zeiss Shanghai Co. Ltd., Shanghai, China) to determine the agglomeration of nanoparticles.

3 Results and discussion 3.1 Viscosity test The viscosity of these composite inks showed a sharp increase in the low shear rate region, followed by an exponential decay to an approximately constant value (Fig. 2). During this process, these inks experienced a shear-thickening,

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Fig. 2 Plot of viscosity as a function of shear rate for inks with varying nanoparticle mass fraction. Upper inset: enlarged view for low shear rate region marked by the left frame

shear-thinning, and Newtonian behavior conversion, which could be suitably modeled by the power-law relationship: η = K × γ˙ n

(1)

where η is the inks’ viscosity, γ˙ is the shear rate, K and n are fitting parameters (n = 1 for low shear rate region, and n < 0 for high shear rate region). Dispersion of agglomerated nanoparticles in the organic ink (Fig. 1(c)) made it difficult to destroy the ink’s original structure and accordingly caused the increase of viscosity in the low shear rate region (0 to 10−4 s−1 approximately). Deformation of the formerly developed inner structure contributed to the decrease of viscosity at a higher shear rate (10−4 to 0.2 s−1 approximately). As the shear rate increased to high enough value (>0.2 s−1 ), all inner structures were destroyed, while the organic molecules and nanoparticles rearranged in a stress-determined direction (Fig. 1(d)), generating fluid-like inks with constant viscosity. The viscosity test results were verified with two different rheometers. Similar results were observed for both tests, which indicates that the viscosity or other rheological properties change of inks is due to the addition of nanoparticles, not to instrumental error. At the very beginning of ink extrusion, the applied shear rate on inks increases from zero to a constant value. As discussed above, a change in ink’s viscosity will take place, so the addition of a lead wire to the designed structure is necessary for the adjustment of shear stress and depositing speed. In addition, the inks’ viscosity increases with the increase of nanoparticle content (Fig. 2). As a result, the applied extrusion pressure should accordingly be increased to overcome the increasing viscous resistance. 3.2 Yield stress and thixotropy Appropriate shear stress should be loaded to overcome the ink’s viscous resistance for fluent extrusion from nozzles.

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in distinguishing the inks’ elastic and viscosity response. Tested inks showed two kinds of deformation-time curve in Fig. 4(a) and (b). Deformation of inks with low nanoparticle content (0%, 2.5% and 5%) experienced an increase in the creep phase and maintained constant in the recovery phase. After the decrease in viscosity, these inks’ shear behavior is similar to that of ideal Newtonian liquid and can be approximately characterized with the solution of state equation for a dashpot (Fig. 4(c)): γ=

Fig. 3 Flow curve (shear stress as a function of shear rate) for inks with different nanoparticle weight content (0%, 2.5%, 5% and 7.5%): (a) flow curve for a increasing shear rate procedure, (b) flow curve for a combined procedure, in which the shear rate increased from 0 to 5 s−1 (step 1) then decreased down to 0 s−1 (step 2)

Yield stress (τy ) is an index that reflects the inks’ strength. Deformation caused by shear stress lower than τy is recoverable, while inks under higher shear stress will flow and show viscoelastic properties. Figure 3(a) showed that addition of nanoparticles increased ink’s yield stress by a large degree, especially for high nanoparticle content region. Hence, higher shear stress should be applied during the squeezing out procedure for inks with higher nanoparticle content. When inks went through a round process described in Fig. 3(b), it takes time for inner destroyed cross-linked networks to recover. Therefore, the ink’s viscosity of step 2 was lower than that of step 1, generating a fusiform loop, whose area is a measure of inks’ thixotropy. With a dimension of energy, the loop area denotes the energy needed to damage the ink’s structure. As is shown in Fig. 3(b), more energy is needed for deposition of inks with higher nanoparticle content. 3.3 Creep and recovery test Creep and recovery test was taken out to determine the time response of concentrated inks. And, it is also helpful

τ ·t η

(2)

where τ is the shear stress, η is viscosity, γ is ink’s deformation and t is test time. Structures formed of low nanoparticle content inks were easy to be damaged and would never recover because of the destruction and dislocation of inner molecules and particles (Fig. 4(a)). Deformation of inks with high nanoparticle content (7.5% and 10%) showed none-zero initial value then increased with creep time, followed by a sharp drop at the beginning of recovery phase and down to a stable value (Fig. 4(b)). This behavior can be described by state equation of Burgers model (Fig. 4(f)), which is composed of a Maxwell model (Fig. 4(d)) and a Kelvin model (Fig. 4(e)) arranged in series [18]: G1 τ dγ 1 dτ τ= (3) − η1 · + + η1 · · γ1 dt G0 dt η0 where τ is the total shear stress (τ = τ0 = τ1 ), γ is the total deformation (γ = γ0 + γ1 ), t is test time, G0 , γ0 , η0 and τ0 are, respectively, the shear modulus, deformation, damper’s viscosity and shear stress of Maxwell model, G1 , γ1 , η1 and τ1 are the corresponding parameters of Kelvin model. The solution of (3) for the creep phase is presented as follows: τ0 · t τ0 τ0 − t γ (t) = (4) + + · 1 − e λ1 η0 G0 G1 where λ1 is the relaxation time of the Kelvin model, which is defined as λ1 =

η1 G1

(5)

The second term of (4) shows an instantaneous leap of deformation at the moment when shear stress is loaded, because of the elastic response of organic molecular chain. Then the inks begin to show viscous properties, and γ increase with time. After the applied stress is removed at time t1 the solution of (3) for the recovery phase could be written as −t τ 0 · t1 τ0 λt1 γ (t) = + · e 1 − 1 e λ1 (6) η0 G1


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Fig. 4 Deformation of concentrated inks as a function of time for inks with nanoparticle weight content of: (a) 0%, 2.5% and 5%, (b) 7.5% and 10%. Schematic illusions of different models: (c) Newtonian liquid model, (d) Maxwell model, (e) Kelvin model, (f) Burgers model

It is indicated by (6) that γ will decrease with time, but the first term will remain unrecoverable as time tends to infinity, which can be described by the irreversible viscous flow of inks. The great drop of deformation from t1 is due to the inks’ elastic recovery. Organic inks with higher nanoparticle content showed lower deformation tendency, more elastic properties and better shape preserving ability than inks with low nanoparticle content (Fig. 4(a) and (b)). It is reasonable to infer that transformation of inks from Newtonian fluid to viscoelastic fluid takes place at a critical adding amount of nanoparticles, which is beneficial to the shape keeping and strength recovery of inks after unloading of shear stress. 3.4 Viscoelastic properties Measurement of elastic (G ) and viscous (G ) moduli can be used to illustrate inks’ viscoelastic properties. As the relationship between moduli and the amount of added ingredient has already been discussed in many previous works [9, 10, 16], attention was paid to the dependency of moduli on oscillation frequency in this research. The inks showed linear viscoelastic behavior with a constant elastic modulus at low shear stress region ( fi ). Accordingly, the applied energy was partially conserved into elastic energy of molecules’ elastic deformation and was recoverable to some extent. Therefore, inks under high frequency external force showed solid-like behaviors. Meanwhile, a position shift of fi was also detected with the nanoparticle content change in our research.

a superior performance to retain their shapes after repeated bending and stretching, due to the recoverable property

3.5 Assembled structures

Fig. 6 Images of assembled structures on different scale: (a) a four-layer scaffold piled by perpendicular rods with diameter of 500 μm and inter-rod distance of 2 mm in a 20 mm × 20 mm square, (b) bending of the formed structure, (c) agglomeration of nanoparticles at the rod’s surface, (d) magnified image of nanoparticles doped ink

The assembled scaffold and rods exhibited excellent potential capability to build complex 3D layered structure (Fig. 6(a)). Scaffolds assembled on a flexible substrate show

Fig. 7 Images of scaffolds: (a) top view, (b) side view. (c) Images of the extruded filaments. (d) Images of filament surface. All the images were taken for structures formed by filaments with different nanoparticle weight content of 0%, 2.5%, 5%, 7.5% and 10% (from left to right), diameter of 500 μm and inter-rod distance of 2 mm in a 20 mm × 20 mm square


caused by the addition of nanoparticles (Fig. 6(b)). Agglomeration and distribution of nanoparticles in these inks were observed in Fig. 6(c) and (d). A series of scaffolds were assembled by direct writing with inks of various nanoparticle doping concentration. Increase of nanoparticle doping content changed the ink’s rheological properties a lot as discussed above, thus reduced rod bending (Fig. 7(a) and (b)), and caused rod shrinking (Fig. 7(c)) and surface folding (Fig. 7(d)). Scaffolds with different rod diameters, inter-rod distances and other shapes were also assembled for wider applications in our research.

4 Conclusions TiO2 nanoparticles were added into molten vaseline to develop organic composite inks for direct-writing assembly of 3D layered structures, which show a superior performance to retain their shapes after repeated bending and stretching on a flexible substrate. Rheological tests were taken to reveal the influence of the nanoparticle addition on the ink’s microstructure, extruding property, and shape keeping capability. We anticipate that our findings here are also applicable for other nanoparticle-doped organic composite inks, for example, low melting point thermoplastic polymer inks doped with metal nanoparticles. This kind of ink design could be relevant to a broad range of technological applications, such as structure construction for tissue engineering, template for micro- and mesostructures, and the realization of flexible electronics and memristors. Acknowledgements This work is supported by National Science Foundation of China under grants No. 90922025, 50632030, 50921061, and 10774087.

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