Preparation And Properties Of Octadecane/Polydimethylsiloxane Composite Films With Reversible Thermochromism

Polydimethylsiloxane（PDMS）has emerged in the field of light modulation due to its high optical transparency,good thermal,excellent mechanical strength,non-toxicity,mild processing conditions,and low cost.PDMS can be reversibly changed in transparency and/or color by altering its surface and internal microstructure or by doping the medium.However,due to the high cost,low efficiency and complex fabrication process of the current PDMS-based smart optical materials,the practical application of smart optical materials still faces great challenges.Therefore,there is an urgent need to develop simple and effective strategies to design novel smart optical materials with fast response time,large transmittance modulation,and good repeatability.Therefore,in this thesis,using PDMS as the matrix,and firstly selecting small molecule octadecane as the thermochromic filler,a C₁₈H₃₈/PDMS composite film with excellent color-changing properties was prepared.Then,the effect of montmorillonite addition on the surface migration of small molecule octadecane in C₁₈H₃₈/PDMS composite films was studied by using lamellar montmorillonite as a blocking agent.In addition,the composite material system was systematically studied in terms of thermal properties,discoloration properties,mechanical properties,and discoloration mechanisms.The research content is divided into the following two parts:Using C₁₈H₃₈ and PDMS as raw materials,a C₁₈H₃₈/PDMS composite film with excellent color-changing properties was prepared by one-step method.The discoloration properties of the composites were studied by changing the different contents of C₁₈H₃₈,the thickness and temperature of the composite films,and the effects of different contents of C₁₈H₃₈ on the thermal properties,mechanical properties,and crystalline properties of the composites were investigated.The color change speed of the electrical response is achieved by using electrical heating to achieve a fast response.The results show that as the C₁₈H₃₈ content in the samples increased,the degree of octadecane subcooling decreased.This is because the three-dimensional network structure of PDMS has a certain hindering effect on the crystallization of C₁₈H₃₈.Using the phenomenon of reduced subcooling,different contents and types of alkanes can be used to prepare composite membranes with different switching temperatures.Mechanical properties characterization found that with the increase of C₁₈H₃₈content,the mechanical properties of the material deteriorated due to the plasticization of the small molecule C₁₈H₃₈.In addition,the experimental characterization of color change performance found that only when the content of C₁₈H₃₈ was higher than 15%,the C₁₈H₃₈/PDMS composite film could achieve reversible light transmittance near the C₁₈H₃₈ phase transition temperature（28°C）.The 15%C₁₈H₃₈/PDMS composite film has excellent color changing ability,the transmittance adjustment range in the visible region can reach 90%in the temperature range of 20℃to 30℃,and it has excellent cycle stability.At the same time,we found that the thickness of the sample is inversely proportional to the transmittance at low temperature.Electric heating experiments also show that the color change response time of the samples can be controlled within 10 seconds by means of electric heating.On the basis of the research of 15%C₁₈H₃₈/PDMS composite film,lamellar montmorillonite was added as a barrier agent,and the effect of montmorillonite content on the surface precipitation of small molecule C₁₈H₃₈ was studied.The effects on the thermal properties and discoloration properties of the composites were also explored.The composite film with 4%MMT content has a good effect of slowing the migration and precipitation of C₁₈H₃₈ from the surface.Since the montmorillonite sheet layer in the sample provides a part of the interface for the precipitation of small molecules of octadecane,the small molecules are precipitated on these interfaces and the precipitation on the surface is reduced.In addition,the existence of the lamellar structure of montmorillonite makes the migration route of small molecules tortuous and long,thus slowing down the precipitation of small molecule octadecane to a certain extent.At the same time,with the increase of montmorillonite content,the crystallization peaks and melting peaks of the composite samples changed from two to one,because montmorillonite provided enough interface for the precipitation of alkanes,so that the original samples needed low temperature to crystallize.Small molecules of octadecane are easily crystallized at these interfaces,resulting in the disappearance of the internal crystallization peaks and the fusion of the crystallization peaks into one peak.In addition,with the increase of montmorillonite content,the high temperature transmittance of the composite samples decreased.However,the content of montmorillonite had no effect on the low temperature transmittance.