Thermal And Humidity Characteristics Of Porous PDMS And Its Applications In Flexible Electronics

Flexible electronic technology is a technology that integrates electronic circuits on flexible substrates.Flexible electronics are used in displays,energy storage devices,solar films and wearable devices.Wearable devices have attracted more and more attention in clinical medicine and other fields.As a flexible electronic device for human health monitoring,it must ensure the normal material exchange(sweat emission)and energy exchange(temperature exchange)of human body,so the thermal and humidity analysis of flexible electronic device is essential.In this paper,porous PDMS(p-PDMS),which is widely used as the substrate and packaging layer of flexible electronic devices,were studied.The p-PDMS samples with different porosity were prepared by emulsion polymerization reaction,and their pore size and porosity were analyzed experimentally.At the same time,the wet-related parameters(saturated moisture content,coefficient of moisture expansion(CME)and elastic modulus)of p-PDMS samples were measured by changes in size,mass and characteristic after the moisture absorption test,and the coefficient of thermal expansion(CTE)was measured by thermo-mechanical analyzer(TMA).Then the homogenization theory is used to analyze the above parameters.Finally,on the basis of theory and experiment,the strechability of p-PDMS flexible electronic devices under the change of environmental field is theoretically analyzed through specific examples.The main research work is as follows:(1)The p-PDMS is a kind of porous material,which has the characteristics of thermal expansion and wet expansion.Its temperature and humidity change will produce thermal and humid deformation,and when deformation is constrained,thermal and humid strain and thermal and humid stress will be generated.Based on thermoelastic mechanics,an elastic constitutive of thermo-hygro-stress coupling of p-PDMS is proposed.(2)The p-PDMS samples were prepared by emulsion polymerization reaction.Scanning electron microscopy(SEM)was used to analyze the microstructure of the samples to obtain the porosity and pore size of the samples.The saturated moisture content parameters and CME parameters were obtained by the size and mass changes of the sample after the hygroscopic test,and the elastic modulus was measured by the tensile test after the hygroscopic test.At the same time,the CTE of the sample was measured by TMA.(3)the thermal and humidity parameters of p-PDMS were theoretically derived by the two-scale asymptotic homogenization theory.The theoretical and experimental values can be compared with each other,thus verifying the reliability of homogenization theory for the analysis of heat and humidity parameters of p-PDMS.Based on the theory and experiment,the prediction formula of heat and humidity parameters related to porous PDMS is obtained.A reliable fitting formula for the relationship between elastic modulus and humidity is also proposed,which lays a foundation for the analysis of stretchability of flexible electronic devices in changes of environments.(4)The calculation of the finite element modeling were carried out through the proposed thermal-moisture-stress coupling model.The change of the steady-state temperature field leading to human skin caused by devices working and the steady-state humidity field caused by human activity were analyzed.The stretchability under different environmental changes of the flexible electronic device with p-PDMS as the substrate and packaging layer were also analyzed.The research results in this paper can provide references for the optimization of thermal and wet mechanical properties of flexible electronic devices,and also provide data basis for the analysis of thermal-wet-stress coupling and damp-heat coupling of materials.