Study On Automatic Fabrication Of PDMS-based Microstructured Conductive Composites Via Differential Temperature Hot Embossing And Its Application

As the most commonly used silica gel,polydimethylsiloxane(PDMS)showed incomparable superiority in applications of structure transfer(e.g.im-printing lithography)according to its outstanding fluidity.The advantages in-cluding high elasticity,stretchability,transparency,and biocompatibility of cured PDMS matrix attracted more and more attentions from the researchers around the world and made it be applied in many fields.For the preparation of polymer conductive composites with superior electrical conductivity,the key point was to form a continuous and compact conductive network in polymer matrix.Till now,three types of methods were widely applied for the formation of conductive network in polymer matrix:self-assembly method,dual continu-ous phase method and phase separation method.All of these three methods had their own advantages and disadvantages.Among them,only the self-assembly method had already been used in mass production attributed to its easy opera-tion and high efficiency.However,in most cases,the high viscosity of polymer matrix and the addition of filling contents made it extremely difficult to obtain products with high conductivity using the traditional self-assembly method.To solve the above problems,we proposed a new method of spatial confining forced network assembly(SCFNA)in our previous studies for the preparation of conductive composites with compact conductive network.Comparing with the products prepared with traditional methods,conductivity of products pre-pared with SCFNA can be multiple increased or even increased by several or-ders of magnitude.Up to now,the size and manufacturing precision of conduc-tive products were seriously limited by the traditional method and equipment.The preparation of PDMS-based conductive composites by traditional method and equipment contained many processes including batching,mixing,metering,drying,curing,mold moving,mold heating and cooling,product taking out,and so on.Moreover,the whole processing efficiency and repeatability would be relatively low if all the processes were performed by hands.Thus,in order to improve the fabrication efficiency,and product quality and repeatability,the method of differential temperature hot embossing was proposed in this paper based on the plate to plate isothermal hot embossing we studied before to realize the independent temperature control of upper and lower molds.Comparing with isothermal hot embossing,the application of differential temperature hot em-bossing can independently balance the differences of optimized temperatures needed between microstructured and flat products,and significantly reduce the risks of demolding defects and other possible defects caused by inappropriate mold temperatures.Furtherly,an automatic hot embossing equipment was de-signed and developed for the realization of fully automatic fabrication of PDMS-based conductive composites with and without microstructures.In view of the problems existing in the preparation of functional compo-sites by SCFNA at this stage,combined with liquid silicone molding technology and microstructure hot stamping technology,to improve production efficiency and shorten production cycle,flexible composite sheets for flexible sensors can be prepared.For the purpose of this material,this paper has done the following research work and obtained research results.(1)The rheological properties of PDMS-based composite materials were tested by a rotating rheometer.The viscous rheological behavior of the material under the production process conditions was studied,and the constitutive equa-tion of the material at normal temperature(30 0C)was fitted by the nonlinear fitting function in Origin software,which is the optimization and molding equipment for the hot press forming process.The development provided data.(2)Based on the SCFNA method,this paper proposes a process for prepar-ing PDMS-based conductive composites by differential temperature plate hot stamping method,and studies and optimizes the processes of mixing stage,in-jection stage and hot stamping stage.And the best process parameters were ob-tained.The problem that the molding die needs repeated heating and cooling during the molding process is avoided,the molding cycle is shortened,and a large area of imprinting is realized.(3)This paper designed a double-station automatic flat plate hot stamping equipment.In view of the problems of thicker homogeneous materials and dif-ficult material feeding,the paper also designed the "bullet-filled" precision con-veying system for the accurate feeding of fully automatic imprinting equipment.Decentralized molds and semi-fixed semi-overflow compression molds are de-signed,and independent temperature control is adopted for the upper and lower molds.The production cycle is shortened and the production cost is saved.(4)The fully developed flat hot stamping equipment developed by the company has been successfully applied to the preparation of flexible sensor sub-strates,which greatly improves the preparation efficiency and product precision.The feasibility of the differential temperature plate hot stamping method and the designed equipment was verified by experiments.The PDMS-based con-ductive composite sheets with different sizes were prepared,and the filling in-tegrity,thickness uniformity,uniformity and conductive consistency were all obtained.It can meet the requirements and has been successfully applied to the production preparation of flexible sensors.