We investigated the effect of gelation solvent, monomer type, and monomer concentration on the physical properties of freeze-dried poly(urethane)-poly(isocyanurate) (PUR-PIR) aerogels, with particular emphasis on their thermal conductivity. It was found that the gelation solvent considerably affects aerogel morphology and physical properties. Aerogels with the lowest thermal conductivity were obtained using a mixture of tetrahydrofuran (THF) and acetonitrile, in a 50% volume ratio. The influence on thermal conductivity of polyol and isocyanate structure and of their concentration was also investigated. Rigid precursors, phloroglucinol (POL), and an aromatic polyisocyanate based on toluene diisocyanate (Desmodur RC) yielded the lowest thermal conductivity. Our results were compared with recent work reporting on parameters that could be used as predictors of thermal conductivity and other physical properties of organic aerogels. None of these parameters were found to be satisfactory predictors of aerogel properties. For example, no systematic correlation between solvent solubility parameters and aerogel properties was observed. We also examined the role of the K-index. This index, defined as the ratio between porosity and contact angle, was shown recently to be a good predictor of the properties of polyurea aerogels. While the thermal conductivity scaled with the K-index, the scaling was different for each of the isocyanate monomers considered in our experiments. Thermal conductivity, instead, scaled well with the product of density and shrinkage of aerogels, independent of monomer type. The reasons of this dependence on shrinkage and density are discussed, and the use of these parameters to guide experimentation on other systems is discussed. Physical properties such as static and dynamic compression modulus and thermal stability of the most promising formulations were also examined.

中文翻译：

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密度和收缩率是优化聚（氨基甲酸酯）类气凝胶导热系数的指导标准

我们研究了凝胶溶剂，单体类型和单体浓度对冷冻干燥的聚（氨基甲酸酯）-聚（异氰脲酸酯）（PUR-PIR）气凝胶的物理性能的影响，尤其着重于它们的导热性。发现胶凝溶剂显着影响气凝胶的形态和物理性质。使用四氢呋喃（THF）和乙腈的混合物（体积比为50％）可获得导热系数最低的气凝胶。还研究了多元醇和异氰酸酯结构及其浓度对导热系数的影响。刚性前体，间苯三酚（POL）和基于甲苯二异氰酸酯的芳族多异氰酸酯（Desmodur RC）的导热系数最低。我们的结果与最近的工作报告进行了比较，该工作报告中的参数可用作有机气凝胶的热导率和其他物理特性的预测指标。这些参数都没有找到令人满意的气凝胶特性的预测指标。例如，没有观察到溶剂溶解度参数和气凝胶性质之间的系统相关性。我们还研究了K指数的作用。该指数被定义为孔隙率和接触角之间的比率，最近被证明可以很好地预测聚脲气凝胶的性能。尽管热导率随K指数成比例变化，但对于我们实验中考虑的每种异氰酸酯单体，其变化比例却有所不同。相反，热导率与气凝胶的密度和收缩率的乘积很好地缩放，而与单体类型无关。讨论了这种依赖于收缩率和密度的原因，并讨论了使用这些参数指导其他系统的实验。还检查了最有希望的配方的物理性能，例如静态和动态压缩模量以及热稳定性。