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Shape-controlled metal-metal and metal-polymer Janus structures by thermoplastic embossing Molla Hasan, Niloofar Kahler, and Golden Kumar ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12365 • Publication Date (Web): 11 Apr 2016 Downloaded from http://pubs.acs.org on April 12, 2016

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ACS Applied Materials & Interfaces

Shape-controlled metal-metal and metal-polymer Janus structures by thermoplastic embossing Molla Hasan, Niloofar Kahler, and Golden Kumar* Department of Mechanical Engineering, Texas Tech University, Lubbock 79409, TX

Abstract: We report fabrication of metal-metal and metal-polymer Janus structures by embossing of thermoplastic metallic glasses and polymers. Hybrid structures with controllable shapes and interfaces are synthesized by template-assisted embossing. Different manufacturing strategies such as co-embossing and additive embossing are demonstrated for joining the materials with diverse compositions and functionalities. Structures with distinct combinations of properties such as hydrophobic-hydrophilic, opaque-transparent, insulator-conductor, and nonmagnetic-ferromagnetic are produced using this approach. These anisotropic properties are further utilized for selective functionalization of Janus structures. Keywords: Janus particles, hybrid structures, metallic glasses, embossing, functionalization

*Corresponding author: Golden Kumar ([email protected])

1 ACS Paragon Plus Environment


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Introduction Janus structures are composed of two regions with distinct chemical or physical properties1-5. This anisotropic distribution of properties in Janus structures makes them particularly attractive for applications in surfactants2, 6-8, self-assembly9-12, sensing13-15, targeted drug delivery16-18, bioimaging19, and catalysts20-23. For example, amphiphilic Janus particles which have both hydrophobic and hydrophilic regions, can be used in stabilization of Pickering emulsions10, 24-25. Interaction of different regions in Janus particles can be tuned to direct their assembly into controllable colloidal structures9, 26. The dual functionalization of Janus structures can be harnessed for targeting and delivering of drugs to desired cells18. Catalytic activity can be enhanced by controlling the shape and interfacial effects in Janus structures23. Owing to these intriguing opportunities, there is increasing interest in fabrication and characterization of Janus and multiphasic structures from a broad range of materials2-5, 10, 12, 27-31. In the last two decades, many processing routes have been developed to produce Janus particles with controllable shapes and properties. Janus particles have been synthesized by surface modification32-33, template directed self-assembly34, phase separation35, controlled surface nucleation36, and microfluidic methods10, 27, 29. Microfluidic techniques offer unique advantages with respect to controlling the composition and geometry of particles37. However, these techniques are primarily applicable to polymers that can be cross-linked from their monomer solutions at room temperature. Some metals with low melting temperatures can also be integrated in Janus structures using microfluidic methods38. Janus structures based on inorganic materials are studied to a much lesser extent because they cannot be fabricated as conveniently as polymers. Microfluidic and soft lithography techniques used in polymers are not directly applicable to metals and ceramics which remain as 2 ACS Paragon Plus Environment

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rigid solids till very high temperatures. Integration of multi-element alloys in Janus architectures is even more challenging due to difficulty in controlling the composition and the shape of different regions concomitantly. The majority of metal and oxide containing Janus structures has been fabricated by using multiple lithographic and thin film deposition steps39-40. Therefore, the existing fabrication techniques for Janus structures are either complex or limited to organic materials and simple geometries. This processing disparity between organic and inorganic materials limits access to a variety of potential functionalities that can be realized from Janus structures. Metallic glasses (MGs) are a unique class of alloys that can combine the properties of metals and the processing of thermoplastic polymers41-46. Here, we utilize the thermoplastic flow of MGs to demonstrate the synthesis of Janus structures consisting of metal-metal and metalpolymer pairs with independent control over the shape, the chemistry, and the fraction of distinct regions. MGs exhibit a broad range of functional properties due to their diverse compositions spanning elements such as Au, Co, Cu, Fe, La, Mg, Nd, Ni, Pd, Pt, Ti, and Zr41-44. As a result of their glassy structure, MGs transform into metastable supercooled liquid state above the glass transition temperature (Tg) and become thermoplastically moldable45,

47-48

. Thermoplastic

processing can be carried out over a range of temperature and viscosity values in the supercooled liquid state of MGs45, 49-50. The upper limit for the processing temperature is the devitrification temperature where the supercooled liquid irreversibly orders into a solid crystalline state. The viscosity and processing temperature of some MG supercooled liquids are comparable to thermoplastic polymers such as PMMA (poly-methyl methacrylate), PE (polyethylene), and PP (polypropylene)46,

49-50

. Figure 1 shows the thermoplastic processing range and the viscosity

values of five MG formers considered here: Mg54Cu28Ag7Y11 (Mg-based), Pt57.5Cu14.7Ni5.3P22.5



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(Pt-based), Pd43Cu27Ni10P20 (Pd-based), Ni60Pd20P17B3 (Ni-based), and Zr35Ti30Cu8.25Be26.75 (Zrbased). Among these alloys, the supercooled liquid states of Pd-based, Ni-based, and Zr-based MGs overlap over a temperature range whereas the Pt-based and Mg-based MGs exhibit distinct thermoplastic processing temperatures. There are several other MGs with lower as well as higher thermoplastic processing temperatures than shown here45. The thermoplastic capability of MGs has been utilized in a wide range of fabrication methods such as nanoimprinting extrusion

59-60

51-54

, surface patterning55-56, net-shaping

46, 57

, rolling

58

,

, blow molding61-62, and joining63. These techniques are typically applied to a

single MG resulting in a homogeneous product with isotropic properties. A recent study showed that hot-pressing of two dissimilar MG discs together can result in chemical joining if the processing temperature is in the supercooled liquid range of both MGs64. The joint strength exceeding 50% of bulk shear strength of MGs could be achieved by varying the joining conditions64. Here, we utilize this joining ability of MGs to develop a new fabrication technique for metal-based Janus structures. Metal-metal Janus structures with controllable shapes are fabricated by embossing and joining different MGs in microscale cavities etched in silicon. The same strategy is applied to MGs and thermoplastic polymers to enable fabrication of metalpolymer Janus structures. In addition, we also explore mechanical joining through surface nanostructures to generate Janus structures from materials that cannot be thermoplastically processed at a common temperature. We use parallel plate embossing to prevent the premature failure of silicon templates. The experimental setup, parameters, and procedure are described in detail in the Supporting Information document.


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Results and Discussion The thermoplastic fabrication of Janus structures is based on two approaches: coembossing (Figure 2a) and additive embossing (Figure 2b). The co-embossing is applicable to materials with an overlapping thermoplastic processing temperature range, whereas additive embossing can be applied to any combination of thermoplastic materials. During co-embossing, the joining of distinct materials is facilitated by enhanced diffusion at the liquid-liquid interface64. In contrast, additive embossing relies on mechanical interlocking through surface asperities and thermal expansion mismatch. Long-range diffusive mixing and convective transport are unlikely in both approaches because of the low Reynold numbers (Re TgB). Pulling of mechanically joined Pd-based and Pt-based MGs results in fracture of MG nanorods suggesting the formation of a durable joint.


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Figure 3: Fabrication of core-shell Janus structures by co-embossing of thermoplastic materials. Two thermoplastic materials (A & B) are stacked and embossed together at a temperature above the Tg of both materials (a). The material in contact with the template takes the shape of a shell and the other forms core of the structure. Subsequently, the template can be planarized from both sides to release the particles with core-shell interface. Examples of metal-metal (b) and metalpolymer (c) core-shell Janus structures fabricated by this approach are shown. Metal-metal Janus structures consist of Ni-based and Pd-based MGs, whereas the metal-polymer Janus structures are fabricated from Pt-based MG and polyethylene. The formation of core-shell interface is confirmed by a combination of backscattered SEM images and the corresponding elemental maps.



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Figure 4: Fabrication of rod-shaped Janus structures using co-embossing in through-etched templates. Two thermoplastic materials simultaneously fill the template cavities from opposite ends to form Janus structures (a). The SEM images show examples of metal-metal and metalpolymer structures fabricated using various combinations of MGs and thermoplastic polymers (b).


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Figure 5: Fabrication of Janus structures by mechanical joining through surface nanostructures. A through-etched template is partially filled with a thermoplastic material decorated with nanostructures (a). The second thermoplastic material fills the remaining cavity while mechanically anchoring to the surface nanostructures of the former material. Feasibility of this approach is demonstrated by examples of mechanically joined Janus structures of two sets of MGs (b). A close view reveals no interface delamination after multiple processing steps.



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Figure 6: Examples of free Janus structures with different shapes and interfaces. Rod-shaped Janus structures were fabricated using co-embossing and additive embossing of MGs and polymers (a). Gear-shaped Janus structures were produced using co-embossing of two MGs in side-by-side or core-shell arrangement (b). Free Janus structures were achieved by two-side planarization and etching of templates after embossing.


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Figure 7: Functionalization of metal-metal Janus structures by using their distinct magnetic and chemical properties. Ferromagnetic beads selectively adhered to crystallized Ni-based MG in Janus pillars, while no beads attached to a non-Janus pillar which is composed of only Zr-based MG (a). Preferential de-alloying of Zr-based MG in Janus pillars results in formation of Cu dendrites on Ni-based MG (b). The de-alloying was carried out in KOH solution.



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Shape-Controlled Metal–Metal and Metal–Polymer ... - ACS Publications

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