Construction,Performance And Applications Of Pomelo Peel-Inspired Gradient Porous Materials

In nature,organisms have evolved various structures to meet the requirements of specific properties and functions for their survival.In addition to the function of transporting substances,gradient porous structures also endow the materials with excellent mechanical properties.As a representative of gradient porous materials,pomelo peel has the advantage of strong energy absorption,which could provide new conception for designing lightweight energy-absorbing materials.However,the lack of systematic quantification of porous structure of pomelo peel severely limits the development of pomelo peel-like porous materials.Therefore,the work here firstly analyzes and quantifies the porous structure in pomelo peel,clarifies its influence on the energy-absorbing capacity,and establishes a pomelo peel-like model of gradient porous structure.By using the 3D network of cellulose in pomelo peel,composite hydrogel with good mechanical properties and high structural stability is prepared and its application as pressure sensors is explored.Furthermore,under the guidance of gradient porous model extracted from pomelo peel,gradient porous materials are designed and additively manufactured,and their mechanical and thermal insulation properties are explored.The main contents of the thesis are as follows:First,three kinds of pomelos originated from production areas with a large difference in latitude are selected as the study objects.Combining the sampling with frozen section and characterization with scanning electron microscope,the distribution of size,density and porosity of pores along the radial direction in pomelo peels in the upper,middle and lower parts of a pomelo fruit are investigated in detail.It is found that the pore structures of the three parts in a pomelo fruit have similar pore structures and distributions,which are also quite similar among the three kinds of pomelos.Endocarp(the part close to the pulp with relative position（RP）ranging between RP_0%and RP_20%)and exocarp(RP_80%～RP_100%)have relative smaller pores,higher pore density,and lower porosity（about 32%）;mesocarp(RP_20%～RP_80%)has relative larger pores,lower pore density and the highest porosity（about 50%）.Furthermore,the hydrostatic deformation behavior of pomelo peel along the thickness direction is studied by digital image correlation technology.The deformation first starts from the mesocarp,which shows a Poisson’s ratio close to 0.In contrast,the Poisson’s ratio of exocarp is the largest,around 0.2.Combining the porous structure and the deformation behavior,a model of"low porosity-high porosity-low porosity"of gradient porosity in pomelo peel is established.The influence of gradient porosity on the deformation behavior and the energy absorption capacity of pomelo peel is thus revealed.By in-situ polymerization of polyacrylamide precursor in the 3D network of cellulose from pomelo peel,a composite of pomelo peel hydrogel（PPH）is prepared.The compressive strength of PPH reaches 330 k Pa,which is 2.9 times of polyacrylamide hydrogel（PAM）.The compressive strength of PPH prefabricated with1 defect decreases by 7%,while that of PAM prefabricated with 1 defect decreases by44%.Moreover,the compressive strength of PPH prefabricated with 10 defects remains higher than that of PAM.The 3D network of cellulose and the strong interfacial interaction between cellulose and PAM are considered to be responsible for the high compressive strength,high mechanical stability and excellent resistance to crack growth of PPH.Meanwhile,PPH has a gradient modulus,which allows it to possess a larger deformation than the hydrogel with a uniform modulus under the same pressure.By anchoring silver nanowires onto the 3D network of cellulose from pomelo peel,a pomelo peel hydrogel sensor（PPHS）is constructed.The cellulose network greatly improves the structural stability and impact resistance of PPHS.The sensitivity of PPHS under pressure of 0～0.6 k Pa reaches 2.033 k Pa^-1,which is 4 times of the conductive PAM.PPHS realizes real-time monitoring of writing,finger bending,walking gait and free fall of objects,indicating that PPHS has the potential to serve as a sensor with high sensitivity to compression,bending and impact.Based on the in-depth understanding of the influence of porous structure on the mechanical properties of pomelo peel and its composite hydrogels,a pomelo peel-inspired polyetheretherketon gradient porous structure with cubic lattice（PP_i）is designed and fabricated by fused deposition 3D printing technology.When PP_i is compressed,the middle part with higher porosity deforms first.The deformation under compression is relatively smooth,and the specific energy absorption is 1.27 times of the uniform porous structure with the same density.The effective improvement in the stability of compressive deformation and the energy-absorbing capacity by the gradient porous design in PP_i is further confirmed by numerical simulations.Furthermore,by mimicking the gradient pore structure and the pattern of cell connection in pomelo peel,a gradient porous structure with sinusoidal pore walls is constructed.Under compression,the middle region with higher porosity deforms first,and drives the adjacent regions to deform cooperatively,which further improves the stability of compressive deformation of the porous structure.Meanwhile,the structure also has good shape memory effect and thermal insulation performance.Experiments and simulations together thus confirm the effectiveness of the pomelo peel-inspired gradient porosity model in improving the stability of structure deformation and the energy absorption capacity,which provides a new idea for the designing of bioinspired gradient porous materials.