Small angle X-ray scattering in polymers and polymer composites
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摘要:
目的 在聚合物及其复合材料的合成过程中有时会涉及复杂的结构变化机制,实时、动态的观察材料的结构演变并以此为基础进行性能预测提供启示是当前研究的热点与难点,面临着不小的挑战,需要引入更为优秀的技术,小角X射线散射(Small Angle X-ray Scattering,SAXS)技术就是其中之一。本文总结SAXS在聚合物材料中的应用,以此促进SAXS在聚合物研究中解决更多实际问题。 方法 SAXS技术可以研究物质的微观、亚微观结构,能够反应出独特的微观构象信息,比如表面分形、多晶构象尺寸等。同时,SAXS技术可以系统的研究链状、网状、层状等各类形状的高聚物的形貌特征以及它们的形成过程,这对聚合物材料聚集态结构形成的机制解析、宏观性能预测分析至关重要。SAXS技术在聚合物材料中常见的研究方法有三种,分别是通过观察SAXS一维曲线走向与峰值的峰值观测法,通过建立或选择合适模型拟合SAXS一维曲线的模型拟合法以及针对各向异性材料的不均匀的散射图像从各个角度进行积分的环形积分法.。在实际研究中,常通过联用这几种方法,最大程度的分析SAXS测试数据。 结果 根据SAXS的应用方法与检测目的,又可以区分出SAXS在聚合物材料中的几个应用领域:① SAXS在聚合物材料形成过程中结构的动态演变和形成机制上的应用,这类应用主要针对的是聚合物聚合的过程或是聚合物受到刺激发生相应的变化过程,本文主要介绍了脂质高分子聚合膜的优先取向过程,嵌段共聚物在不同聚集程度下形成产物的中间状态,水乳液聚合过程中产物形成过程,多级介晶结构和天然高分子聚合物(纤维素)的表征研究,在演变过程或形成机制的研究过程中,SAXS全面的结构变化信息丰富研究者对复杂聚合反应过程的认识,促进研究者提出合理、准确的反应机制。② SAXS在聚集体结构和性能研究上的应用,在经历聚合过程复杂的结构变化后,聚合物材料最终会形成固定的宏观形态,这对应了聚合物稳定的微观结构,如结晶结构、孔隙网络等,进而决定聚合物材料的物理、力学、化学性能。本文介绍了SAXS用于分析聚合物结晶区与非结晶区的分布与占比,天然材料的微观组织构造的分布,材料表面的分形结构,含孔隙结构材料的孔隙占比与分布。通过SAXS研究聚合物材料的结构特征与结构变化,获得的使具有统计学意义的参数,这使研究者可以从整体角度审视材料的散射体分布、微观结构、形貌特征变化,进而对聚合物材料的物化性能进行准确预测。 结论 SAXS技术可以从动态与静态、宏观与微观的角度在理论与实验间建立联系,它丰富的信息量、多角度的应用方向,可以全面地反映研究聚合物材料的结构、性能,亦或是结构演变过程。但SAXS技术也存在局限,较为复杂的理论、对设备和模型的要求将部分研究者拒之门外,因此需要更多的聚合物材料与SAXS技术的实践,在降低SAXS技术门槛的同时促进聚合物研究向更深层次发展。 -
关键词:
- 小角X射线散射(SAXS) /
- 聚合物及其复合材料 /
- 演变过程 /
- 形成机制 /
- 微观构象
Abstract: In the synthesis of polymers and polymer composites, it is still a challenge to observe the real-time and dynamic evolution of material structure and provide implications for property prediction. As one of the methods to characterize the microscopic and submicroscopic structures of substances, small angle X-ray Scattering (SAXS) technology can reflect unique microscopic conformational information, and can systematically study the morphological characteristics and formation process of chain-like, network-like, and layered polymers. The analysis of the formation mechanism of the aggregated structure of molecular materials, and their macroscopic performance prediction are very important. In this paper, three common methods for SAXS applications in current polymer materials research were presented, i.e., the peak observation, the model fitting, and the annular integration. Based on the above three methods, this paper introduced the practical functions of SAXS in studying different polymer materials, such as dynamic observation of the microstructural evolution process, and obtaining large-scale and statistically significant microstructural parameters. After comparing and evaluating the application methods and influences of SAXS in different polymer materials, it was concluded that SAXS plays a comprehensive role that is difficult to replicate in the study of complex polymer materials. It was hoped that this paper could serve as a primer to attract researchers' attention to understand SAXS technology, provide alternative research methods for the investigation of complex polymers, and expand the application of SAXS in wider fields to solve more problems. -
图 5 峰值观测法观察的SAXS一维曲线:(a)不同温度下HDPE的SAXS一维曲线;(b)不同温度下HDPE的SAXS一维曲线的洛伦兹校正图[31];(c)低浓度纤维素乙醇凝胶珠干燥过程的SAXS曲线;(d)低浓度纤维素水凝胶珠干燥过程的SAXS曲线[32]
Figure 5. SAXS 1D curves observed by the peak observation method: (a) SAXS 1D curves of HDPE at different temperatures; (b) Lorentz correction of SAXS measurements of HDPE at different temperatures[31]; (c) SAXS 1D curve of low concentration cellulose ethanol gel during the drying process; (d) SAXS 1D curve of low concentration cellulose hydrogel bead gel during the drying process[32]
图 7 脂质聚合膜示意图:(a)水相中脂质膜的形成过程;(b)脂质膜与底物接触时的状态(绿色形状为底物);(c)水相中脂质膜的三维结构示意[49]
Figure 7. Schematic diagram of lipid polymeric membrane: (a) Formation of a lipid membrane in an aqueous phase; (b) State of the lipid membrane when in contact with the substrate (Green stuff is the substrate); (c) 3D structure of lipid membrane in the aqueous phase[49]
图 9 嵌段共聚物研究中所涉及的概念及设备示意图:(a)临界堆积参数P的计算方法及P的大小与嵌段共聚物微观形貌间的关系;(b)原位SAXS测试实验装置[63];(c)不同聚合程度的嵌段共聚物微观形貌模拟示意图[62]
Figure 9. Concepts and equipment diagram of block copolymer part: (a) Critical packing parameter P calculation method, relationship between P and morphology of block copolymer; (b) In situ SAXS experimental device[63]; (c) Schematic diagram of the microstructure of block copolymer at different degrees of polymerization[62]
图 8 水乳液聚合和胶体聚合机制示意图:(a)水乳液聚合过程中疏水相单体聚合的3个阶段[52];(b)氧化铈胶体形成团聚体的形成机制[53]
Figure 8. Schematic diagram of water emulsion polymerization and colloid polymerization mechanism: (a) Representation of the three main intervals (I, II, and III) that occur during the aqueous emulsion polymerization of a water-immiscible monomer[52]; (b) Formation mechanism of colloidal aggregates of cerium oxide[53]
图 10 多级介晶各级结构示意图:(a) Bi3+、EA和NMP合成的基本结构;(b)由基础结构经过偶极-偶极相互作用所形成的细长纳米丝;(c)纳米细丝间通过静电作用形成的Bi-EA介晶结构[64]
Figure 10. Schematic diagram of each level of multistage structure monocrystals: (a) Basic structure of Bi3+, EA, and NMP synthesis; (b) Nanofilaments formed by dipole-dipole interaction of the infrastructure; (c) Bi-EA mesocrystals formed by electrostatic action of the filaments[64]
图 11 CNF模型示意图、模拟和实际测试所得到的SAXS数据:(a)符合理想假设的CNF模型;(b) CNF横切面上主平面与晶体平面示意图;(c)实测的单根CNF的SAXS一维曲线(蓝色)、模拟CNF的SAXS曲线(黑色)及三者对强度的贡献曲线;(d) 实验测量3种组分CNF在WAXD中对散射强度的贡献情况[66]
Figure 11. Schematic diagram of CNF ideal model with simulation and actual experimental data: (a) CNF model schematic diagram; (b) Relation between the principal plane and crystal plane on the CNF cross-section; (c) Measured SAXS 1D curve (blue dots), the simulated SAXS 1D curve (black), and the contribution of the three parts to the strength; (d) Experimentally measured WAXD curve and the contribution of scattering intensity in three-part WAXD[66]
图 12 部分天然材料中的组织结构分布图:(a)毛竹显微CT重建切片影像,1区域为维管束区,2区域为薄壁组织区;(b) 毛竹的SAXS二维散射图,其中Ⅰ区域的散射强度主要由维管束贡献,Ⅱ区域的散射强度主要由薄壁细胞贡献;由区域Ⅰ(c)和区域Ⅱ(d)重构的散射分布图[75];((e)~(h))在冻融干燥法制备泡沫结构材料中CNF的实际取向情况,色轮的颜色代表CNF的实际取向,虚线箭头代表了冻结过程开始的主方向[76]
Figure 12. Tissue structure distribution of some natural materials: (a) Microscopic CT reconstruction section of phyllostachys edulis, 1 represented the vascular bundle region, 2 represented the parenchyma region; (b) SAXS 2D scattering diagram of Phyllostachys edulis, region Ⅰ was contributed by the vascular bundle and region Ⅱ was contributed by parenchyma cells; Slice of scattering distribution reconstructed from region Ⅰ (c) and region Ⅱ (d)[75]; ((e)-(h)) True orientation of CNF in foam structure materials prepared by freeze-thawing-drying, the colors of the color wheel represents the true orientation of the CNF, the dotted arrows indicate the main direction of the freezing front[76]
图 13 聚烯烃催化剂与钙钛矿纳米片层水凝胶:(a)聚烯烃催化剂的SAXS一维曲线;(b)从总散射曲线中分离得到的关于催化剂孔隙的SAXS一维曲线;通过截断高斯场法对催化剂C1 (c)与催化剂C2 (d)的三维模型重构[81];(e) 2wt%和3wt%钙钛矿纳米片凝胶溶液;(f)橙色和蓝色钙钛矿纳米片水凝胶[82]
Figure 13. Polyolefin catalyst and perovskite nanosheet hydrogels: (a) SAXS 1D curve of catalyst; (b) SAXS 1D curve of catalyst pores; 3D reconstruction model of catalysts C1 (c) and C2 (d) by correlated Gaussian random filed method[81]; (e) 2wt% and 3wt% perovskite nanosheet gel solution; (f) Orange and blue perovskite nanosheet hydrogels[82]
图 14 HDPE横截面上的空洞:((a), (b))当
${ \varepsilon }\text{H} $ 小于0.5时HDPE内部电镜显示空洞的数量较少且尺寸较小;((c), (d))当$ { \varepsilon }\text{H} $ 增大时HDPE内部空洞的数量和尺寸同步增加与扩大[37]Figure 14. Cavities on HDPE fracture surface: ((a), (b))When
$ { \varepsilon }\text{H} $ was less than 0.5, the HDPE internal electron microscope showed that the number and size of cavities were small; ((c), (d)) When$ { \varepsilon }\text{H} $ increased, the number and size of HDPE holes increased synchronously[37] -
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