【作者】 胡敏；

【导师】 张立志；

【作者基本信息】 华南理工大学 ， 化学工程与技术， 2023， 硕士

【摘要】 环境湿度过高会导致空气质量降低,细菌滋生,诱发哮喘,电路短路和材料变质等问题,同时会增加电能的消耗。因此研究优良的空气除湿技术是一个迫在眉睫的问题。本文采用基于亲疏水可逆转换的低温再生高效吸湿凝胶作为固体吸湿剂进行除湿,相比其他除湿技术,吸湿剂除湿技术具有制备成本低、装置简单、吸湿速率快、能耗低和可再生等优点。此外该吸湿凝胶可以作为空气取水的材料,有助于缓解水资源分布不均和淡水短缺的问题。聚N-异丙基丙烯酰胺（Poly（N-isopropylacrylamide）,PNIPAM）是使用最广泛的一种温敏型材料,亲疏水转换温度为25-32°C。但现有的PNIPAM基吸湿水凝胶存在制备过程复杂、原料危害性大、原料价格昂贵、吸湿量低、循环性能差和再生温度高等缺点,而柠檬酸钠（Sodium citrate,SC）具有廉价易得、吸湿性强、水溶性好、可以和Ca²⁺络合等优点。本文采用自由基聚合法制备PNIPAM聚合物,并在制备凝胶过程中将柠檬酸钠加入其中,然后浸泡CaCl₂溶液,制得干燥多孔的PNIPAM-SC-CaCl₂凝胶。该凝胶具有制备过程简单安全、成本低、吸湿量高、耐久性优良、低温再生等优点。具体研究内容如下:对五种不同原料配比的PNIPAM-SC-CaCl₂凝胶进行吸湿性能测试,确定常温下相对吸湿量最大的配料比为0.2 g NIPAM:0.4 g SC。同时将该凝胶与商用吸湿剂CaCl₂和变色硅胶在相同条件下进行吸湿实验和再生实验,发现制备的复合凝胶在25°C和86%的相对湿度（Relative Humidity,RH）下48 h内的吸湿量高达1.22 g/g（吸收水分质量/吸湿剂质量）,而且该凝胶在35°C下再生1.62 h后相对含水量迅速降至5.61%,再生速率大大优于CaCl₂和变色硅胶。通过对该复合凝胶进行SEM、XPS、FTIR、TGA、DSC、接触角测试、XRD、XPS等表征,得出以下结论:（1）该复合凝胶具有粗糙的微观表面。（2）低临界相转变温度低至34.3°C。（3）凝胶内部的SC与CaCl₂进行络合,以柠檬酸钙的形式存在。（4）凝胶内部SC通过C=O和C-O的O原子与PNIPAM的H原子形成氢键进行连接。采用分子动力学模拟计算该复合凝胶内部的径向分布函数、不同氢键的数量以及25°C和40°C时该凝胶在水环境下的构象转变。证明凝胶内部PNIPAM的酰胺基团上的H原子和SC的羧酸根和羟基的O原子通过氢键连接,而且凝胶内部只存在O-H氢键。此外,25°C时凝胶内部的PNIPAM分子链呈现伸展的线性构象,而40°C时PNIPAM分子链处于蜷缩的球形构象。制备放大3倍的PNIPAM-SC-CaCl₂凝胶,并设计空气取水装置,将其置于大气环境中进行空气取水实验。该吸湿凝胶在70%RH和25°C的夜间环境下吸湿14 h后相对吸湿量可达1.06 g/g,日间将其放置于光照强度为1000 W/m²的太阳光模拟器下,8 h的取水实验后相对取水量为0.46 g/g。本文制备了具有高吸湿量和低温再生特性的PNIPAM-SC-CaCl₂凝胶,通过表征测试从宏观和微观角度探究该凝胶的吸湿性能和材料特点,且证实了该凝胶具有低温再生的特点。分子动力学模拟结果揭示了凝胶内部的连接方式和不同温度时的构象转变,空气取水实验证明了该凝胶可以通过太阳能再生,有广阔的应用前景。更多还原

【Abstract】 High humidity in the environment can lead to decreased air quality,bacterial growth,asthma,short circuits,material degradation and increased energy consumption.Therefore,Searching excellent air dehumidification technologies is currently an urgent issue.This thesis uses a low-temperature regenerative high-efficiency hygroscopic gel based on reversible conversion between hydrophilicity and hydrophobicity as a solid desiccant for dehumidification.Compared with other dehumidification technologies,hygroscopic desiccant technology has the advantages of low preparation cost,simple fabrication,fast hygroscopic rate,low energy consumption and easy regeneration.Additionally,this hygroscopic gel can be used as a material for water harvesting from moist air,which helps to relieve the inhomogeneous distribution of water resources and the shortage of fresh water.Poly（N-isopropylacrylamide）（PNIPAM）is the most commonly used temperature-sensitive material,with a hydrophilic-hydrophobic transition temperature of25-32°C.However,existing PNIPAM-based hygroscopic gels have the disadvantages of complex preparation process,high raw material hazards,expensive raw material prices,low hygroscopic capacity,poor cyclic performance and high regeneration temperature.Sodium citrate（SC）has the advantages of being cheap and easy to obtain,strong hygroscopicity,good water solubility,and can chelate with Ca²⁺.In this article,PNIPAM polymers were prepared by free radical polymerization.Sodium citrate was added during the gel preparation process,followed by soaking in a CaCl₂solution to obtain a dry porous PNIPAM-SC-CaCl₂gel.This gel has the advantages of simple and safe preparation process,low cost,high hygroscopic capacity,excellent durability and low regeneration temperature.The detailed content is described as below:The hygroscopic performance of PNIPAM-SC-CaCl₂gels with five different raw material ratios was tested to determine the best composition with the highest adsorption capacity at room temperature.It was 0.2 g NIPAM and 0.4 g SC.At the same time,the hygroscopic and regeneration experiments of the composite gel were compared with those of commercial hygroscopic agents like CaCl₂and allochroic silica gel under the same conditions.It was found that the hygroscopic capacity of the obtained composite gel was as high as 1.22g/g（moisture quality/moisture absorbent quality）within 48 h at 25°C and 86%relative humidity（RH）,and the relative water content of the gel rapidly decreased to 5.61%after regeneration for 1.62 h at 35°C,with a regeneration rate significantly higher than that of CaCl₂and allochroic silica gel.The composite gel was characterized using SEM,XPS,FTIR,TGA,DSC,contact angle testing,XRD,XPS,etc.It was found that:（1）the composite gel has a rough microsurface.（2）Low critical phase transition temperature is as low as 34.3°C.（3）The SC inside the gel is complexed with CaCl₂,which exists in the form of calcium citrate.（4）The SC inside the gel is connected to PNIPAM through hydrogen bonds formed by the O atoms of C=O and C-O and the H atoms of PNIPAM.Molecular dynamics simulations were used to calculate the radial distribution function inside the composite gel,the number of different hydrogen bonds and the conformational transition of the gel in water at 25°C and 40°C.It was shown that the H atoms on the acrylamide groups of PNIPAM inside the gel are connected to the carboxylate and hydroxyl O atoms of SC through hydrogen bonds,and only O-H hydrogen bonds exist inside the gel.In addition,the PNIPAM molecules within the gel display an extended linear conformation at25°C,while at 40°C,the PNIPAM molecules are in a contracted spherical conformation.A PNIPAM-SC-CaCl₂gel was prepared at a 4-fold scale and an air-water collection device was designed to conduct air-water collection experiments in atmospheric conditions.The hygroscopic gel exhibited a relative moisture absorption of 1.06 g/g after 14 hours of humidification in a nighttime environment at 70%RH and 25°C,while after 8 hours of water collection under a solar simulator with an illumination intensity of 1000 W/m²during the day,the relative water uptake was 0.46 g/g.This study prepared a PNIPAM-SC-CaCl₂gel with high hygroscopicity and low temperature regeneration.The moisture absorption performance and material properties of the gel were investigated from macroscopic and microscopic perspectives,it has been confirmed that the gel can be regenerated at low temperatures.The molecular dynamics simulation results revealed the connection mode inside the gel and the conformational transition at different temperatures.The air-water collection experiment demonstrated that the gel could be regenerated using solar energy,indicating promising applications for this material.更多还原