Effect of viscoelasticity of PDMS on transfer printing

An increasing number of technologies for MEMS require large-scale integration of heterogeneous components from a donor substrate to a wide range of surfaces. These routes require the ability to direct the placement of devices or components that are too small, too fragile, and too numerous to economically handle by conventional pick-and-place technologies. Possible solutions therefore include self-assembly and new approaches based on adhesion to elastomeric stamps. We introduce a commercially available PDMS (polydimethylsiloxane) rubber (Sylgard 184, Dow Corning). It can grab micro objects from donor substrate and print them to a variety of substrates without specially designed surface chemistry or separate adhesive layers, such as cured surface or flexible substrate. Based on fracture mechanics, whether the PDMS stamp can pick up the component or not depends on the strain energy release rate and interface toughness. On the other hand, viscoelasticity of PDMS plays an important role in the process of transfer printing. Dynamic Measurment Analysis (DMA) method is a main tool to study viscoelasticity of a variety of materials. As is well-known, polymers tend to show elasticity and viscocity at the same time and PDMS is no exception as a kind of rubber. Through DMA experiment we can observe the relationship between storage modulus, loss modulus, etc. and frequency at which the force is loaded on the material. Further more, we can obtain the material constitutive relation. PDMS is a bi-component rubber, prepolymer and curing agent mixed in varying proportions owning different capabilities, accordingly they have different effects in the process of transfer printing. In this article, we have prepared two different proportions of PDMS to conduct DMA experiment to study their viscoelasticity and their effect on transfer printing. According to what have been mentioned above, adhesion between the PDMS stamp and the components (i.e., Van der waals forces ) depends on strain en- rgy release rate and interface toughness. Therefore, we adopt simulation method to study the relationship between the two different proportions of PDMS and their effect on transfer printing. Different proportions of PDMS have different viscoelastic parameters, so they have different constitutive laws. Through the results obtained from simulation we can obviously observe their different properties. The combination of DMA experiment and simulation helps us to conduct the process of transfer printing successfully. Material possess characteristics of viscoelasticity has complex mechanical behaviors, and making their constitutive relation clear will play a more and more important role in variety of micro processes apart from transfer printing.