This paper describes a technique for the fabrication of ordered three-dimensional hierarchical nanostructures in polymer films via nanoimprint lithography. The hierarchical structure is obtained by a series of sequential imprinting steps, where smaller structures are imprinted on top of larger imprinted structures. Higher order hierarchy is achieved by sequentially adding imprinting steps. An important feature of this fabrication technique is that the subsequent imprinting is carried out at a temperature below the glass transition temperature of the polymer film and below the imprinting temperature of the preceding imprint without the assistance of any solvents or plasticizers. Various formats of hierarchical structures can be obtained by varying the mould geometries and mould orientations in the sequential imprinting. It is found that the secondary or higher-order imprints are enabled by a reduction in the modulus of the polymer after the primary imprinting.
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