Mechanism And Control Of Polymer Micro-/Nano-structuring Based On Spatially Modulated Electric Field

Due to the unique properties in physics and chemistry,polymer micro-/nano-structures have attracted considerable attention,with a wide applications in microfluidics,organic photoelectron,bioassays,etc.Among the existing approaches of fabricating polymeric structures on micro-/nano-scale,polymer micro-/nano-structuring by spatially modulated electric field(SMEF)has unique advantages of material versatility,high uniformity,etc,which is usually implemented by applying a voltage to an electrode pair composed of a patterned template and a polymer-coated substrate,separated in parallel by an air gap.With an external voltage,a modulated electric field is generated on the air-polymer interface,producing the Maxwell stress tensor to drive the polymer to move upwards to the template,and resulting in a micro-/nano-structure.2The major work covered in this dissertation includes:1)The polymer morphology is fully determined by the modulated electric field without other constraints,leading to the urgent requirements for understanding the mechanism of the polymer rheology under SMEF.However,the existing linear stability analysis cannot be adopted to describe the polymer rheology under SMEF,especially for the polymer with a small viscosity coefficien.For this issue,a numerical model based on two-phase flow is proposed with a considertaion of electrohydrodynamics and scale effects in this dissertation,by which the mechanism of the polymer deformation under SMEF is analyzed from the viewpoint of mechanical behavior.Furthermore,according to the flow field and polymer morphology,the whole process is divided into three stages: generating stage,quasi-stability stage and coalescence stage.And the quasi-stability stage should be adopted to cure the polymer to obtain the anticipated structure.2)In the published literatures,the numerical or experimental deformation was identical to the template pattern,ignoring the difference between the polymer structure and the template pattern.In practice,this difference may appear due to the unreasonable process variables,and need to be studied in depth for duplicating a polymer structure identical to the template pattern.As for the difference between the polymer structure and the template pattern,a strong/weak modulation is defined in this dissertation.The direct cause of the weak modulation is analyzed by the numerical analysis and experimental study.Moreover,the modulating intensity is defined to study the influence of the process parameters on the strong/weak modulation.3)Up to now,the polymer structure duplicated through structuring by SMEF is presented with the aspect ratio usually smaller than 1,which can be attributed to the conflict of the high fidelity with the air gap.In other words,a high aspect ratio implies a large air gap,leading to an insufficient electrostatic force to drive the polymer to overcome the surface tension and viscous force for generating a structure identical to the template pattern.Due to phenonmenon of the polymer deformation by SMEF with a low aspect ratio,two novel techniques of the electrically induced structuring are proposed based on the material property or process operation,consisting of the conductive polymer deformation by SMEF and a deformation on a prestructured polymer under SMEF.Furthermore,the difference between the novel techniques and conventional method is studied from the viewpoint of modulating intensity,demonstrating the advantages of the former one in fabricating a structure with a high-aspect-ratio structure.4)The published literatures only demonstrated the duplicate of regular or irregular structures with comparatively uniform size features,ignoring the function of SMEF on the producing irregular structures with features of significantly varying size,which have potential applications in many aspects such as microfluidics,bioassays,etc.Aiming at the issue for the fabrication of polymer structures with comparatively uniform size features by SMEF,this dissertation explores the capability of the electrically induced structuring in duplicating a structure with features of significantly varying size from the viewpoint of the strong/weak modulation.According to the comparison between the template pattern and the duplicating structure,the strong/weak modulation is divided into three status,fine-modualtion,under-modulation and over-modulation,by which the influence of process parameters is investigated.Finally,a structure with features of varying size is fabricated experimentally.5)Based on the mechanism of polymer deformation by SMEF,the potential applications were explored in fabricating the hierarchical structure and microlens array by polymer structuring under SMEF.(a)The hierarchical structure is fabricated on flat polymer under weak modulation and on prepatterned polymer under strong modulation,which has a static contact angle larger than 150° and sliding angle smaller than 10°.(b)The microlens array is fabricated under strong modulation induced by a template with largely separated features,with the maximum fill factor of 98.8%.