Li-Rong Bao

Reversal imprinting by transferring polymer from mold to substrate

A reversal imprinting technique was developed in this study. A polymer layer was first spin coated on a patterned hard mold, and then transferred to a substrate under an elevated temperature and pressure. The reversal imprinting method offers an advantage over conventional nanoimprinting by allowing imprinting onto substrates that cannot be easily spin coated, such as flexible polymer substrates. Another unique feature of reversal imprinting is that three different pattern-transfer modes can be achieved by controlling the degree of surface planarization of the mold after spin coating the polymer resist as well as the imprinting temperature. “Embossing” occurs at temperatures well above the glass transition temperature (Tg) of a polymer; “inking” occurs at temperatures around Tg with nonplanarized polymer coating surface on the mold; and “whole-layer transfer” occurs at temperatures around Tg but with a somewhat planarized surface. These three imprinting modes have been quantitatively correlated with the s...

Nanoimprinting over topography and multilayer three-dimensional printing

We have developed a simple imprinting technique that allows patterning over a nonflat substrate without the need for planarization. In this process, a polymer film is spin coated onto the mold and then transferred to a patterned substrate by imprinting. By selecting polymers with different mechanical properties, either suspended structures over wide gaps or supported patterns on raised features of the substrate can be obtained with high uniformity. It is found that imprinting at a temperature well above the glass transition temperature (Tg) of the polymer causes the thin residue film between features to dewet from the mold, which can greatly simplify the subsequent pattern transfer process. Multilayer three-dimensional polymer structures have also been successfully fabricated using this new imprinting method. The yield and dimensional stability in the multilayer structure can both be improved when polymers with progressively lower Tg are used for different layers. Compared to existing techniques for patte...

Moisture diffusion and hygrothermal aging in bismaleimide matrix carbon fiber composites—part I: uni-weave composites

Abstract Moisture diffusion and hygrothermal aging in uni-weave bismaleimide composites were systematically studied in this research. The moisture weight gain curves of the composites were compared with those of the neat resin in order to determine the interface effect on moisture absorption. Both the neat resin and composites display similar two-stage diffusion behavior, with the first and second stages being diffusion- and relaxation-controlled, respectively. When the diffusivities of the composites were compared to theoretical predications of an appropriate mathematical model, it is found that the fiber-matrix interface has little effect on the short term moisture diffusion. However, the long term absorption is suppressed in the composites. The rigid fiber–matrix interface seems to constrain long-range segmental motions of the matrix and in so doing reduce water absorption in the relaxation-controlled second stage. Although the fiber–matrix bonding is initially very strong, prolonged water absorption at elevated temperatures eventually damage the interface and causes interfacial cracking.

Moisture absorption and hygrothermal aging in a bismaleimide resin

Moisture diffusion in a BMI resin has been investigated by submerging thin specimens in water. The weight gain of the neat resin deviates significantly from Ficks law in that the uptake continues to slowly increase over an extended time scale. A two-stage diffusion model can successfully fit the experimental data. This model considers the structural relaxation induced by absorbed moisture, which is a good plasticizer of the resin. Desorption and re-absorption experiments indicate that the structural relaxation is irreversible upon desorption of water molecules. Dynamic mechanical analysis results demonstrate the plasticizing and rejuvenating effect of water.

Moisture diffusion and hygrothermal aging in bismaleimide matrix carbon fiber composites: part II-woven and hybrid composites

Moisture absorption and hygrothermal aging in a woven and woven/uni-weave composites are studied. Different from the neat resin and uni-weave composite, the short term diffusion in the woven composite is non-Fickian, in that the uptake initially increases rather rapidly but quickly slows down. A dual-diffusivity model that can successfully describe the observed weight gain curves has been developed. This model is composed of two independent diffusion processes with very different diffusivities. The fast process is associated with diffusion in cure-induced voids and cracks, while the slow process is the intrinsic diffusion in the matrix. Two different hybrids, namely, uni-weave/woven/uni-weave (U-W-U) and woven/uni-weave/woven (W-U-W), are also investigated. The short term absorption curves of the U-W-U and W-U-W composites can be described by Fickian diffusion and the dual-difftisivity model, respectively. The resemblance between the U-W-U and uni-weave composites, as well as that between the W-U-W and woven composites suggest that the diffusion properties of a composite are controlled by the outer plies. During prolonged moisture absorption, the swelling stresses eventually cause the formation of interfacial cracks in the composite.

Polymer inking as a micro- and nanopatterning technique

A polymer inking technique was developed to form micro- and nanopatterns on a substrate. In this process, a polymer thin film is spin coated on a patterned mold. After contacting the substrate at a suitable temperature and pressure, the polymer on the protruded surfaces of the mold is transferred to the substrate and a positive image of the mold is obtained. A selective surface treatment method has been developed to improve the edge smoothness of the inked pattern. During selective surface treatment, the protruded surfaces of the mold are first treated with a flat poly(dimethylsiloxane) stamp impregnated with a silane that has medium surface energy. The mold is then immersed into the solution of another silane with very low surface energy to treat the trenches of the mold. Because the surface energy of the sidewalls is lower than that on the protrusions, polymer dewetting from the sidewalls is promoted, which makes the polymer film discontinuous along the edges of patterns. Therefore, inked polymer patter...