大空形态的控制相分离.docx

1、大空形态的控制相分离Macroporous Morphology Control by Phase SeparationINTRODUCTIONIn ordinary solgel processing, starting compositions as well as reaction conditions are selected so as to maintain the mixture in a homogeneous state throughout the processes including mixing of starting compounds, gelation, agi

2、ng, drying and heat-treatment. High homogeneity of a precursor solution is especially important for the fabrication of fibers and coatings. The apparent disadvantage of the formation of microscopic (sometimes macroscopic) heterogeneity in gels, however, can be utilized to control the pore structure

3、of the gels. Macropores with precisely controlled size and size distribution are especially important when various functionalized gel materials are to be used in contact with liquid solutions in that they increase the contact probability of external substances onto the surface sites.The present chap

4、ter describes the method of macroporous morphology control through the solgel reaction accompanied by the concurrent phase separation. General principles of the method are explained adopting typical experimental systems as examples. The formation of macroporous morphology in a small confined space i

5、s also shown for an intriguing example of miniaturization of the material. Another important feature of the gels with controlled macropores is that the mesopore structure can be tailored independently of the macroporous characteristics. Sharply distributed mesopores are formed via basic or hydrother

6、mal aging process. Examples of supramolecularly templated mesopores in monolithic macroporous gels by use of amphiphilic additives are also presented (Nakanishi, 1997).POLYMERIZATION-INDUCED PHASE SEPARATIONLet us first consider a typical hydrolysispolycondensation of alkoxysilanes under acidic cond

7、itions which gives relatively narrow distribution of the molecular weight of the polymerizing oligomers. The average molecular weight of the polymerizing species in a solution increases with reaction time by virtually irreversible polycondensation reactions among the monomers/oligomers. The thermody

8、namics of a solution containing polymerizing species tells us that mutual solubility among the constituents becomes lower as the average molecular weight of the polymerizing species increases (Flory, 1971). This is mainly due to the loss of entropy of mixing among the constituents which leads to the

9、 increase of the free energy of mixing, G. (23-1)The reduction in mutual solubility caused by polymerization can be contrasted with that by physical cooling of the system (de Gennes, 1979; see Fig. 23-1). In thelattercase, the free energy of mixing is increased by lowering temperature. In both cases

10、, a multi-component system becomes less stable as the absolute value of the TS term decreases. In some cases, changes in the polarity of oligomers with the generation and/or consumption of silanol groups may contribute to increase in H term, which also destabilize the system against homogeneous mixi

11、ng. In any case, when the sign of free energy of mixing of the system becomes positive, the thermodynamic driving force for phase separation is generated.Figure 23-1. Physical vs. chemical cooling.In real experimental systems, poor solvents of the oligomers, several kinds of water-soluble polymers,

12、and cationic or nonionic surfactants can be used as an additive component to induce the phase separation in the course of a solgel reaction. Typical examples follow:(a) Low-water hydrolysis of tetraalkoxysilane or alkyltrialkoxysilaneWhen hydrolyzed with understoichiometric amount of water (H2O/Si 2

13、 and 1.5 in the cases of tetraalkoxysilane and alkyltrialkoxysilane, respectively), the siloxane oligomers retain a considerable amount of unreacted alkoxy groups. These oligomers with relatively low polarity tend to phase separate against a highly polar solvent mixture. Addition of an extremely hig

14、h concentration of mineral acid or formamide is preferable to induce phase separation in the solution derived from tetraalkoxysilanes (Kaji et al., 1993). With alkyltrialkoxysilanes, the generated oligomers have inherent hydrophobic groups and thus exhibit higher phase separation tendency even again

15、st the mixture of water and alcohol with a dilute acid catalyst.A series of exception has been found recently with bridged alkoxysilanes. Bis(trialkoxysilyl)alkanes with C6 or C8 bridging alkylene chains typically phase separate against 5070 fold molar amount of water relative to silicon under acidi

16、c conditions (Nakanishi et al., 2002). Relatively long alkylene chains buried in the siloxane network only moderately contribute to enhance the phase separation tendency of the polymerizing oligomers.(b) High-water hydrolysis of tetraalkoxysilane in the presence of weakly-interacting additivesWith a

17、 sufficient amount of water, almost all the alkoxy groups are hydolyzed into silanol groups. The polarity of resultant siloxane oligomers is high enough to be dissolved in alcoholwater solvent mixture containing ionic catalysts. An addition of water-soluble polymer such as poly(acrylic acid) or poly

18、(sodium p-styrenesulfonate) to this system can induce the phase separation mainly based on the incompatibility between the polymer and siloxane oligomers (Nakanishi and Soga, 1991, 1992). The added polymer is preferentially distributed to the phase containing minor amount of siloxane oligomers, and

19、thus constitutes the “fluid phase” in contrast to the “gel phase” rich in siloxane oligomers. In this case, the additive component just plays an assisting role to induce the phase separation to form micrometer-range heterogeneous structures.(c) High-Water hydrolysis of tetraalkoxysilane or alkylene-

20、bridged alkoxysilane in the presence of hydrogen-bonding additivesSeveral surfactants and water-soluble polymers are known to exhibit strong hydrogen-bonding interaction between silanol groups on the surfaces of silica colloids and in siloxane oligomers. Among others, polyoxyethylene chains specific

21、ally form strong hydrogen bonds with silanols by their ether oxygens. When alkoxysilanes undergo hydrolysis/polycondensation in the presence of the poly(ethylene oxide) or surfactant containing polyoxyethylene units, the polymer or surfactant forms hydrogen-bonded amorphous complex as soon as suffic

22、ient amounts of continuous silanol sites are generated as a result of polycondensation of hydrolyzed alkoxysilanes in the solution (Fig. 23-2). In the case that surfactants and polymers cover silanols so strongly that any further polycondensation is inhibited by the adsorbed molecules, only low mole

23、cular weight oligomers will segregate to form a dispersed, non-gelling phase. By an appropriate choice of the HLB (hydrophilelypophile balance) value or the molecular weight, on the other hand, the phase separation can be concurrently induced with the homogeneous solgel transition of the reaction sy

24、stem (Nakanishi et al., 1994). Being different from the cases with weakly-interacting polymers, most of the additive surfactants or polymers are distributed to the phase to which majority of the siloxane oligomers are also distributed, and form a gel phase together. The fluid phase is then composed

25、mainly of the solvent mixture.Figure 23-2. Hydrogen-bonding of PEO chains on surface silanols.The system containing hydrogen-bonding additives has an advantage in controlling the pore structure of the resultant gels. As will be explained in detail below, the size of the pores (to be more exact, the

26、size of separated phase domains) primarily depends on the phase separation tendency of the polymerizing siloxane oligomer solution. The pore volume is determined mainly by the volume fraction of the fluid phase, and thus roughly proportional to the concentration of water and solvent in the starting

27、composition. The pore size and the pore volume of a gel sample can be independently controlled by adjusting the concentrations of the additive and the solvent, respectively. In the system (b) described above, the phase separation tendency and the volume fraction of the pore-forming phase are interde

28、pendent, which makes it difficult to design a wide variety of pore structure.(d) Morphology development by spinodal decompositionIn a phase diagram with a miscibility window, the two-phase region is divided into two sub-regions. One is that between binodal and spinodal, called metastable region. In

29、the metastable region, any infinitesimal fluctuation of the composition is energy consuming, that is, finite activation energy is required to develop phase-separated domains. The typical phase separation mechanism in this region is the “nucleation and growth” where dispersed small regions called nuc

30、lei grow accompanied by an addition of constituents diffusing from the bulk (not yet separated) regions of the system. The natural consequence of this mechanism is a morphology with “dispersed A” and “matrix B” phase domains (Fig. 23-3). The other region is that within a spinodal line, called unstab

31、le region. In the unstable region, any infinitesimal fluctuation gains energy so that the fluctuation spontaneously develops with time without requiring the activation energy. Depending mainly on the depth of quench (the difference between the critical temperature and the actually quenched temperatu

32、re) and the mobility of the constituents (more precisely, that of diffusing units), only a single Fourier component among the various fluctuation wavelengths survives and dominates the characteristic size of the domains. A clear difference can be seen between the nucleation and growth mechanism that

33、 the phase domains have no distinct interface in the initial stages of the phase separation. The contrast in chemical composition develops continuously with time until the equilibrium phase compositions are reached. Under comparable volume fractions of conjugate phase domains without anisotropy, the sponge-like structure called co-conti