Preparation And Performance Of Anisotropic Composites Assisted Byexternal Field

As electronics become thinner and more flexible, the demand on the properties of their components increases particularly for their multifunctionality. Magnetic or electric field assisted alignment of particles in polymer matrix is an efficient way to prepare composites with anisotropic structure. Directional conduction occurs in anisotropic composites when chains of conductive filler are assembled during processing, unusual optical transmittance and mechanical properties caused by anisotropic structure were observed as well. In this dissertation, hence, focuses on increasing the understanding of fields(magnetic and electric) assisted assembly techniques to prepare anisotropic composites and realize roll to roll process. Novel piezoresistive material, thermal conductive material, electric conductive material were prepared by magnetic field, electric- optic smart material were prepared by electric field.The first part of this study focused on the mechanism of chains-like alignment structure of Nickel powders by analyzing the magnetization of Nickel powders and the interaction forces under an external magnetic field. Each nickel powder forms N-S magnetized structure when they are in an external field, then mutual repulsion or attraction force appears as an important role to form anisotropic chain-like structure in Polydimethylsiloxane(PDMS) matrix. A novel compress part was designed and attached to a home-made stretching machine for testing the electric conductivity and piezoresistivity. The nickel chain-like structure can improve the electric conductivity along the chain direction which is the through thickness direction of Ni/PDMS composite film. The tiny gap between the particles in nickel chains are sensitive to pressure that leads to an obviously piezoresistivity which can be adjusted by change nickel particles loading to get various sensitivity. The resistivity decreases dramatically with increasing pressure and eventually reaches plateau value as low as 101 Ohm·m under 3 MPa pressure from 108 Ohm·m without presssure. The anisotropy alignment structure formed by nickel powders also leads to an anisotropic mechanical properties, the Poisson’s ratio is 0.7 when the strain is 10% along the film plane direction while the Possion’s ratio of non-aligned samples is 0.5. Thermal conductivity increases as well because the nickel chain-like structure can be a pathway for both electric and thermal.In the second study, the mechanism of magnetization of nickel flakes and their rotation behavior and interaction force were studied. Anisotropic Ni flakes/PDMS composites were manufactured. Experiment results show that nickel flakes can form more efficient pathways than nickel powders, in another word, at the same nickel loading, the electric conductivity of Ni flakes/PDMS is higher than that of Ni powders/PDMS; the resistivity of 14vol% Ni flakes/PDMS is as low as 101 Ohm·m, the piezoresitivity is more sensitive to pressure as well. The thermal conductivity of magnetic-aligned Ni flakes/PDMS raises substantially as high as 13 times as non-aligned. Hata and Taya model was applied to analyze the experiment results, thermal conductivity of nonaligned Ni flakes/PDMS composites were lower than forecast results while the aligned ones show higher value than forecast results.Then Ni particles/PS conductive and transparent composite was processed by a magneticaligned combine solvent-vapor-shrinkage method. Nickel flakes can connect each other more efficiently since their own larger specific surface area than nickel powder. That is why nickel flakes has a lower threshold value(1.5wt%) than nickel powder. The anisotropic alignment of nickel particles in polymer matrix make the composites have excellent electric conductivity and transparency. In addition, the light transmittance varies according to incident angle. The optical transmittance of aligned Ni particles/PDMS, especially for the incident angle is 0 degree, improved obviously.A novel roll-to-roll processing line was introduced and large scale industrialization of anisotropic composites was realized. Anisotropic nickel particles/PDMS composites was manufactured by controlling the magnetic field strength and curing rate. SEM proved nickel flakes are well assembled to chain-like alignment in PDMS matrix. This composite film exhibits excellent read-through performance and dependency on incident angel. The optical transmittance of aligned 10wt% Ni flakes/PDMS decreases from 18% to 5% when the incident angle increases from 0 degree to 65 degree, however, there is no obviously change for non-aligned samples, staying at 5%. Anisotropic nickel particles/PS composites were also processed by controlling casting speed and drying rate in the roll-to-roll line. These films also have a good performance on read-through and dependency upon incident angle. The optical transmittance of 10wt% Ni flakes/PS decreases from 70% to 5% as the incident angle increases from 0 degree to 65 degree. For the non-aligned samples, the change is from 20% to 5%.In the final study, cellulose microwhiskers(CMWs) and nanowhiskers(CNWs) were prepared by acid hydrolysis method, both CMWs and CNWs show outstanding optic anisotropic properties which was observed by polarizing microscope. It was found the CNWs can be aligned or released by applying or removing electric field, and this alignment and dis-alignment endow this material shows electric-color response under polarizing microscope and CNWs has a faster electricresponse rate than CMWs.