Investigation On The Microcellular Foaming Preparation Methods And Performances Of High-Expansion PMMA/PVDF Micro-Nanocellular Foams For Thermal Insulation

Energy conservation,emission reduction and energy efficiency improvement are key measures to achieve the "Dual Carbon" target.Insulation materials,represented by microcellular polymer foams,play important roles in implementing these initiatives.The existing polymer foams are highly flammable,toxic and harmful,and their overall performance needs to be improved.Meanwhile,traditional foam production using chlorofluorocarbons or alkane as foaming agents will cause ozone layer destruction or fire risk.In addition,a single polymer cannot meet the requirements of processing and applications at the same time,and the foaming of polymer blends often faces the problem of poor compatibility.Hence,a plan in which polymethyl methacrylate（PMMA）/polyvinylidene fluoride（PVDF）miscible blends with excellent overall properties was used as foaming matrix and green microcellular foaing technology with CO₂ as foaming agents was adopted to prepare high-performance polymer insulation foam was proposed in this thesis.Fabricating high-performance polymer foams with special cellular structures or components,such as high porosity（high expansion ratio）nanocellular foams,high expansion ratio microcellular foams,high expansion ratio open-cell foams and high expansion ratio composites foams,is a key way to achieve high-efficiency thermal insulation.However,the preparation of PMMA/PVDF foam with high performance via microcellular foaming technology is still facing severe challenges.Firstly,the phase separation and crystallization behavior of homogeneous blends under CO₂ conditions are very complicated,and the effects of phase morphology evolution,crystallization and melting behavior,rheological behavior on foaming behavior have not been systematically studied.Secondly,it is difficult to realize the cellular structure such as high expansion ratio,nano-cell or open-cell.Thirdly,the structure of nano-cell or opencell conflicts sharply with high foaming ratio.Fourthly,the influence mechanism of foam composition and cellular structure on foam properties is still unclear.To prepare high-performance and large expansion ratio polymer foams for insulation,the research on three kinds of batch foaming processes,i.e.solid foaming process,semi-molten foaming process and molten foaming process based on PMMA/PVDF blends was conducted in this work.The effects of process conditions and material components on the morphology evolution,crystallization,melting and rheological behavior of blends were systematically studied.The foaming experiments of three kinds of foaming processes were designed and carried out,and the complex relationships between the phase morphology evolution,crystallization and melting behavior,rheological behavior and foaming behavior of blends were revealed.A series of innovative methods for green and efficient preparation of high-performance polymer foams and the regulation of cellular structure was proposed and established,and the influence mechanisms of foam components and cellular structures on thermal insulation,mechanical properties and flame-retardant properties of foams were elucidated.The main research contents and results of this thesis are as follows:（1）A new method based on high-pressure heating solid-foaming process and via CO₂-induced PVDF microcrystals was proposed to prepare high-porosity nanocellular foams.It was found that with the increase of PVDF content,the modulus of the blends and CO₂ uptake decrease.A great number of uniformly dispersed PVDF micro-crystals are formed in the blends after CO₂ adsorption.The crystallization phase content andmelting point increase with adsorption pressure or PVDF content.PVDF micro-crystals promote heterogeneous cell nucleation.Cell nucleation density of blend foams is significantly increased,and the combination of cells is effectively avoided.When the pressure was 13.8 MPa and the PVDF content was 20 wt%,the nanocellular foam with porosity of more than 70%,cell size of 287 nm and nucleation density of more than 1014 nuc1eus/cm³ could be prepared in combination with the heating process.The thermal conductivity of foam is 81.56 mW·m^-1·K^-1.（2）A new method based on low-pressure solid-state foaming process to increase expansion ratio by using the plasticizing action of PVDF was proposed.It was found that with the increase of adsorption pressure,the phase separation form of PVDF changes from amorphous particles to bunched crystals,and the plasticizing effect is weakened.With the increase in PVDF content,the crystallinity,melting point and plasticizing effect of the blends increases.PVDF micro-phase or micro-crystals can promote cell nucleation.The miscible PVDF can promote the growth of cells and increase expansion ratio via plasticizing action.The foams with tiny cell size（2.14 μm）and high expansion ratio（10.7）can be obtained via this foaming method,and the microcellular foam with larger expansion ratio（25.6）can also be prepared under ultralow pressure（1.7 MPa）.The thermal conductivity of the blend foams is as low as 36.4 mW·m^-1·K^-1.（3）A new method based on semi-molten foaming process was proposed to improve PMMA foaming behavior and enhance foam performance by blending PVDF and PMMA,and light-weight,strong PMMA/PVDF foam with excellent heat insulation properties and shape memory performances was prepared.It was found that PVDF could not only reduce the viscoelasticity and curing temperature,but also crystallize during the adsorption stage.Due to the plasticizing action of PVDF,the optimal foaming temperature and the effective foaming interval decreases with PVDF content.PVDF micro-crystals promote heterogeneous cell nucleation so that the blend foam has a smaller cell size and higher cell density than PMMA foam.When the PVDF content was 30 wt%,the foam with an expansion ratio of more than 30 and cell size as low as 11.9 μm was prepared.The expansion ratio and infrared absorption of PMMA weaken the solid phase heat conduction and heat radiation so that the thermal conductivity of the foams is as low as 31.6 mW·m^-1·K^-1.With the increase of expansion ratio and PVDF content,the compressive performance of foams decreases and the hydrophobicity increases.Benefiting from PVDF micro-crystals and refined cellular structures.both the shape fixation rate and the recovery rate of foams exceed 80%.Compared to PMMA foams,the blend foams have better flame-retardant properties and higher thermal stability.（4）A new method based on the semi-molten foaming process to improve the foaming ability of PVDF and enhance the properties of PVDF foam by using PMMA melt blending with PVDF was proposed and lightweight,strong and hydrophobic PVDF/PMMA foams with wider foaming process window,excellent thermal insulation and good flame-retardant properties were successfully prepared.It was found that PMMA improves CO₂ uptake and melt viscoelasticity.PMMA inhibits the crystallization of PVDF,which is manifested as a decrease in crystal size,melting point,crystallinity and an increase in melting limit.PVDF crystals exhibit multiple melting behaviors when PMMA content was high.PMMA significantly improves the foaming ability of PVDF,such as increasing expansion ratio and foaming window,reducing the foaming temperature and refining the cellular structure,along with reduction of foam density.Compared to PVDF,the optimal expansion ratio of the blends increases from 21.9 to 31.2,the foaming window is widened from 3.6℃ to 17.4℃,the optimal foaming temperature is reduced from 158℃ to 148℃.and the foam density is reduced from 0.0810 g/cm³ to 0.0461 g/cm³.Because PMMA increases expansion ratio,enhances infrared absorption,and improves the skeleton strength of the foams,compared to PVDF foam,the thermal conductivity of blend foams reduces from 36.43 mW·m^-1·K^-1 to 33.85 mW·m^-1·K^-1,and the compression strength and modulus increase to 0.199 MPa and 3.51 MPa,respectively.The blend foams show favorable flameretardant properties and thermal stability.（5）A new method based on the molten foaming process to prepare high-expansion open-cell PVDF/PMMA foams by using PMMA to regulate the crystallization behavior of PVDF was proposed.It was found that PMMA decreases crystallization temperature and crystallization rate of PVDF,widens the crystallization interval and improves the melt viscoelasticity.Molten foaming eliminates the obstruction of crystal to foaming and broadens the foaming temperature window significantly.Low melt strength and PVDF crystallization induce open-cell structures.The blend foam with expansion ratio as high as 43.2,foam density as low as 0.0365 g/cm³,open-cell porosity of 98.6%and effective foaming window of 27.5℃ can be obtained via molten foaming process,which is superior to the PVDF foams prepared by molten foaming and the blend foams prepared by semi-molten foaming.The open-cell structure further reduces the solidphase heat transfer,and the thermal conductivity of the foam is as low as 31.07 mW·m^-1·K^-1.The blend foams have strong hydrophobic and lipophilic properties,and their contact angles of water and oil were 123.2° and 0°,respectively.The blend foams can quickly adsorb oil,and their adsorption capacity of various oil products is 8.1-23.1 g/g.Moreover,the blend foams have good cycle durability.（6）In order to reduce the heat radiation,a method that Graphene nanosheets（GNPs）were introduced into the blends to prepare high-expansion PVDF/PMMA/GNP nanocomposite foams via molten foaming process was proposed.It was found that GNP enhances the melt viscoelasticity and increases the initial crystallization temperature.As GNP content increases from 0 to 5 phr,the maximum expansion ratio decreases 44.3 from 29.4,the effective foaming window decreases and moves upward,and the optimal foaming temperature increases from 130℃to 135℃.GNP is conducive to inducing open-cell structures and can promote heterogeneous cell nucleation,thus refining cellular structure.GNP weak heat radiation so that the thermal conductivity of foams is further reduced to 30.14 mW·m^-1·K^-1.Compared to the blend foams,the nanocomposite foams have improved compressive performance,better thermal stability and flameretardant properties.