tailieunhanh - Encyclopedia of Smart Materials (Vols 1 and 2) - M. Schwartz (2002) WW Part 7

Tham khảo tài liệu 'encyclopedia of smart materials (vols 1 and 2) - m. schwartz (2002) ww part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Figure 3. Crystal structures of a 1-2 complex and b 1-3 complex solvent molecules have been omitted for clarity . concentration-dependent. These assemblies range from a rigid-rod character at very dilute concentrations to a lyotropic liquid-crystalline gel at higher concentrations 5 . Most interestingly Meijer developed supramolecular polymers of type 5 that are held together by quadruple hydrogen bonding between the ureidopyrimidone units. This material displays most if not all properties of macroscopic polymers based only on non-covalent connections Fig. 4 6 . Materials based principally on hydrogen bonding and other intermolecular interactions have been generated using low molecular weight organogelators 7 . Recently mi-crocellular organic materials have been prepared by drying of organogels in supercritical CO2 8 . The field of organogelation has evolved from molecules that have different structural and recognition properties to the rational design and fine-tuning of materials. In the following section we describe advances in organogelation and the use of intermolecular interactions in developing these new supramolecular structures. INTERMOLECULAR INTERACTIONS Hydrogen Bonding Hydrogen bonds are usually formed when a donor D that has an available acidic hydrogen is brought into close contact with an acceptor A that possesses a lone pair of electrons Fig. 5 . Hydrogen bonding has been the subject of statistical investigations 9 10 X-ray diffraction analysis 11-13 and theoretical studies. The hydrogen bond can vary in strength from 1 kcal mol for C-H O hydrogen bonding 14 15 to 40 kcal mol for the HF- ion in the gas phase 16 17 . Hydrogen bonding has played a c the development of areas such as self-asse crystal engineering 19 20 . Solid-state and ies of the hydrogen bond have provided evid interaction is a highly ordered phenomenon event. In the solid state Zaworotko demonst inorganic complex 6 that has four hydroge oriented in a tetrahedral geometry can f bonds