Rubberized Thermoforming Method Based On PMMA Cross-linked Microspheres For Fabrication Of Non-oriented Optical Transparent Devices

Polymer optical devices are widely used in the national pillar industries,national defense,military,aerospace industry and other pivotal fields.However,during the traditional forming process,the flow shear stress of molten polymer leads to the polymer chain orientation,resulting in the drawbacks such as optical distortion and birefringence and other defects of polymer devices.Although the orientation of molecular chains can be decreased by optimizing the forming process parameters and post-processing methods,and so on,it is difficult to meet the increasingly stringent requirements of polymer optical devices,which is one of the main bottlenecks of the existing flow forming methods.To this end,inspired by colloidal microsphere template structure of photonic crystal,this paper proposed rubberized forming method of polymerized and cross-linked polymethyl methacrylate（PMMA）microspheres for non-oriented transparent optical devices.The main achievements are as follows.Non-oriented PMMA and cross-linked microspheres were designed and prepared by emulsion polymerization method.The effects of various parameters（type and content of crosslinking agents,stirring speed,emulsifier,and reaction temperature）on the microspheres’morphology and physico-chemical properties were investigated.The monodispersed PMMA and PMMA crosslinked microspheres ranging from 160 nm to 300nm with different components and uniform size distribution were obtained,of whose microstructure and physical and chemical properties were systematically studied.The molecular chains of the microspheres exhibited no orientation,which solved the orientation drawback of traditional raw materials caused by extrusion shear.The non-oriented PMMA microspheres provided non-oriented structural units for subsequent forming.A discrete element flow model concerning submicron microspheres flow was built.Based on the surface effect and electrostatic effect,the packing characteristics of submicron microspheres under the action of van der Waals force,electrostatic force and gravity were studied.The peak frequencies of packing angles of submicron microspheres were 60^o and110^o～125^o,respectively.The correlation law of van der Waals force,electrostatic force,porosity and microsphere size was expounded.Comparing with macroscopic particles,the force effect of submicron microspheres increased,which was conducive to the compact assembly of the microspheres to form three-dimensional structures.An innovative rubberized forming method for polymer microspheres was proposed.Based on the high surface activity of the non-oriented microsphere and the rubberized state of the polymer,viscosity of the microspheres varying with time characteristics was investigated.The influence mechanism of forming process parameters to microsphere arrangement was revealed,and the forming process window of polymer microspheres was established.And the theory and method of rubberized thermoforming were explored,at rubbery state,the thermoforming of microspheres avoided the flow shear without causing the orientation of molecular chains.The precision devices of non-oriented polymer microspheres were prepared.Polymer transparent devices such as precision lenses are formed.The optical path difference and birefringence of non-orientated PMMA optical devices was as low as 8.3 nm and 6.67×10^-6,respectively,which was an order of magnitude lower than the best level ever reported.The molecular chain inside the device presented isotropy,overcoming the defects of orientation and birefringence existing in traditional optical devices.Moreover,the mechanical properties（hardness and modulus,etc.）were more than 20%better than that of traditional devices.The relationship and regulation between microstructure and macroscopic optical and mechanical properties of PMMA optical devices was elucidated.The regulation rules and mechanisms of the type,size and properties of polymer microspheres were established.The corresponding process test and demonstration platform was set up.This work illustrated the polymerization and forming mechanism of PMMA molecular chain,clarified the rubberized thermoforming mechanism of non-oriented devices,established the rubberized thermoforming process of polymer microspheres,and formed polymer optical products without optical distortion and birefringence,which can be used in the field of high precision and harsh optics.It is of great significance to promote engineering application of rubberized thermoforming technology.