Polymer foam materials are widely used in production and life. They are a large class of important porous materials. At present, the polymer foam is mainly prepared by foaming agent directly inside the resin. It is like making bread. First, mix the yeast and flour evenly to form the dough. The yeast produced by CO2 expands under the action of heat, making the solid dough become a fluffy body. However, the inevitable and difficult control of bubble nucleation, merging and breakage in the process will cause greater damage to the structure and morphology of the foams, thereby affecting the service performance. How to control the micro bubble morphology and macro foam structure accurately is still a difficult point. In recent years, the computer-aided additive manufacturing technology (3D printing) has developed rapidly. Compared with the traditional molding technology, its biggest advantage is that it can accurately and rapidly shape the three-dimensional complex structure as small as micro nano size. In order to solve some problems in the preparation of polymer foam by direct expansion foaming method, Claas Willem Visser (Communication writer) and Jennifer A. from Harvard University, USA Professor Lewis (co-author) and other researchers have applied 3D printing technology to the preparation of polymer foam. For the first time, a polymer (polyethylene glycol diacrylate) bubble ink, which can be written directly, has been developed for the first time. The micro bubble morphology, distribution and polymer foam with precisely adjustable macrofoam structure are prepared by direct writing UV curing method. And by changing the composition of the "ink", it can also give the foam new function. The research results are published in advanced materials with a paper entitled "architected polymer foams via direct bubble writing". Direct writing method is used to prepare foam principle. In order to achieve direct writing of "ink" as a single micro bubble forming macro bubble, as shown in Figure 1, the researchers designed special nozzle for inner and outer double layer structure: the inner tube conveyed polymer precursor solution containing polymer monomer, initiator and surfactant, and the outer pipe conveyed gas, which was mixed at nozzle mouth to form writable. Bubble "ink", a single bubble falling from the nozzle to the substrate, and after UV initiated polymerization, the bubbles form a macro foam block. Among them, the "ink" dripping flow and gas pressure are the key to the preparation of different microbubbles. According to theoretical calculation and experimental results, the authors have drawn the phase diagram of the influence of the liquid flow rate and pressure on the ink morphology of the bubble. In the follow-up study, the author chose the liquid flow rate Q = 10 mL At constant rate of min? 1, the effects of parameters such as liquid composition, gas composition and pressure, and nozzle moving speed on the micro and macro structure of foam were studied. Figure 1.a) foam writing and curing device, white arrow for bubble, "ink" nozzle outlet, transparent pipe for gas delivery pipe, white pipe for liquid delivery pipe, 4 golden tubes for ultraviolet source; b) foam preparation schematic diagram; C, D, e) nozzle surface and section enlargement and schematic diagram; F, g) nozzle external and internal hole image; H K) The shape of bubbles ejected from the mixture of precursor liquid and air at different pressures increases from left to right. L) the phase diagram of the influence of liquid velocity and pressure on the ink morphology of the bubble; m) the foam block prepared by direct writing method; n (o) is not identical with the pore size map of the foam under the microscope multiple. Compared with the traditional foam preparation methods, the direct writing method has the following 4 advantages: advantage one: the morphology and distribution of the foams and the distribution of controllable foam are directly related to the performance of the foams. The best way to create foam with excellent performance is to make the foam directly. The cell morphology mainly includes the open / closed cell structure, pore size and distribution. The author controls the structure of open and closed pores by the type of gases: when the O2 with polymerization inhibition is used as gas, the monomer can be stopped or delayed polymerized in the range of O2 to about 40um, so that the bubble wall becomes thinner and eventually becomes open cell foam. When inert N2 is used as the gas source, the polymerization occurs normally, and the bubble wall is thicker, forming closed cell foam. The pore size and distribution are regulated by air pressure. At lower air pressure, a single dispersed (2 ≤ P ≤ 2.7 kPa) bubble "ink" will be produced. The bubble diameter is about 0.5 mm, and the distribution is very uniform. When the air pressure increases, the bubble becomes double dispersed (2.8 ≤ P ≤ 3.4 kPa) or even triple dispersed (P > 3.6 kPa), and the bubble pore size is between 0.3 - 0.7 mm, and the distribution is also wide. Fig. 2. A and b) are the preparation process of open cell and closed cell foam and X tomography images; c) the relationship between pressure and foam density; D, e) monodisperse and doubly dispersed foamed structure and F are the statistical results of their pore size distribution; g) the influence of gas pressure on the pore size. Advantage two: the preparation of multi-layered foam in the nozzle writing while changing the location of the nozzle and the size of the air pressure, can be made of different 3D framework of multi-layered structural materials, such as the triangular grid in Figure 3. By increasing the air pressure or slowing down the movement speed of the nozzle, the single component of the grid will be widened, and vice versa. Fig. 3. A) from left to right are the line width after writing curing under the conditions of P = 2.4 kPa, respectively; b) the influence of air pressure and nozzle moving speed on the line width during writing; C - F) the macro and micro structure photos of multi-level triangular grid constructed. Advantage three: preparation of gradient gradient foam from the structural gradient in the continuous writing process to adjust the process parameters in order to change the density and modulus of the local foam and other parameters, so that the final foam will show several orders of magnitude in the overall mechanical performance. The macro body of. This structure of foam is difficult to be prepared by conventional foaming method. Fig. 4. d) schematic diagram of preparation and structural composition of hemispherical gradient structure foams; E) changes in the shape of the compression process of gradient structure materials; F) the variation of stress in the compression process with deformation, (I) - (IV) corresponding to (I) - (IV) structure in e. Advantage three: preparation of functional foam to change the composition of precursor solution, such as adding some functional material, after the foam formation, the retention of the material can give the foam new function. For example, the AgNO3 was added to the precursor solution in advance. After curing, it was changed into a flexible foam containing nano silver. When the pressure was reduced, the resistance became smaller, and had a good linear relationship with the pressure and high sensitivity. It was expected to be used in pressure sensors. Fig. 5. a) nano Ag foam image; b) resistance test schematic diagram for compression; c) variation of foam resistance with pressure under different air pressures; d) sensitivity of foam prepared under different air pressures with pressure. It is concluded that the direct writing method can control the micro bubble morphology and macro foam structure accurately, regulate the mechanical properties of foam, regulate the foam's mechanical properties, and give new functions to foam easily. It will play a positive role in improving the foam preparation method. Full text link: https://onlinelibrary.wiley.xilesou.top/doi/10.1002/adma.201904668 the forefront of polymer science has established "foaming", "printing" and other communication groups, added small editors as friends (micro signal: polymer Xiang, please note: name unit Title Research direction), invited to the group.
Source: polymer science frontier
Statement: only on behalf of the author's personal point of view, the author's level is limited, if there is any unscientific, please leave a message below for correction!
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