Microphase structure

This review article attempts to summarize and discuss recent developments in the studies of photoinduced electron transfer in functionalized polyelectrolyte systems. The rates of photoinduced forward and thermal back electron transfers are dramatically changed when photoactive chromophores are incorporated into polyelectrolytes by covalent bonding. The origins of such changes are discussed in terms of the interfacial electrostatic potential on the molecular surface of the polyelectrolyte as well as the microphase structure formed by amphiphilic polyelectrolytes. The promise of tailored amphiphilic polyelectrolytes for designing efficient photoinduced charge separation systems is afso discussed. 

Functionalized polyelectrolytes are promising candidates for photoinduced ET reaction systems. In recent years, much attention has been focused on modifying the photophysical and photochemical processes by use of polyelectrolyte systems, because dramatic effects are often brought about by the interfacial electrostatic potential and/or the existence of microphase structures in such systems [10, 11], A characteristic feature of polymers as reaction media, in general, lies in the potential that they make a wider variety of molecular designs possible than the conventional organized molecular assemblies such as surfactant micelles and vesicles. From a practical point of view, polymer systems have a potential advantage in that polymers per se can form film and may be assembled into a variety of devices and systems with ease. 

In the following chapters, we will concern ourselves with the nature of the interfacial microenvironments of some polyelectrolytes whose functionality controls photoinduced ET. Emphasis will be placed on the local electrostatic potential and also on the microphase structure of some amphiphilic polyelectrolytes in aqueous solution. 

Until recent years only a relatively few studies had been reported on the amphiphilic polyelectrolytes. However, several years ago attention began to be directed to the microphase structure as a reaction medium that modifies photophysics and photochemistry [50 — 64], redox processes [65 — 67], and chemical reactions [68, 69]. Since then the number of reports on amphiphilic polyelectrolyte systems have increased sharply. 

The formation of a microphase structure leads to a surface-active effect [31]. The surface tension of water is considerably lowered when amphiphilic copolymers are dissolved. The surface-active effect appears more significantly in the copolymers with more hydrophobic units. 

The microphase structure was clearly observed in transmission electron micrographs of the film of amphiphilic copolymers cast from aqueous solutions [29, 31]. An important finding was that no microphase structure was observed for the film cast from organic solutions. This difference indicates that a microphase structure is formed in aqueous solution, but not in organic solution. Different hydrophobic groups showed considerably different morphological features i.e. whether microphase separation leads to a secondary or higher structure depends on the type of hydrophobic units in the copolymers [31],... 

The formation of a microphase structure can be sensitively detected by using hydrophobic fluorescent probes. Hydrophobic microdomains tend to solubilize hydrophobic small molecules present together in aqueous solution. For example, diphenylhexatriene (DHT) is hydrophobically bound to the St aggregates in ASt-x in aqueous solution and, as a result, the fluorescence intensity is greatly enhanced. Figure 9 shows the fluorescence intensity of DHT in the presence of ASt-x relative to the intensity in its absence (I/I0) as a function of the ASt-x concentration [29],... 

We will be concerned in Chapter 6 with photoinduced ET of hydrophobic chromophores that are confined to the microphase structure of amphiphilic polyeletrolytes. 

The microphase structure of amphiphilic polyelectrolytes in aqueous solution provides photoinduced ET with an interesting microenvironment, where a photoactive chromophore and a donor or acceptor can be held apart at different locations. Photoinduced ET in such separated donor-acceptor systems allows an efficient charge separation to be achieved. 

As has been described in Chapter 4, random copolymers of styrene (St) and 2-(acrylamido)-2-methylpropanesulfonic acid (AMPS) form a micelle-like microphase structure in aqueous solution [29]. The intramolecular hydrophobic aggregation of the St residues occurs when the St content in the copolymer is higher than ca. 50 mol%. When a small mole fraction of the phenanthrene (Phen) residues is covalently incorporated into such an amphiphilic polyelectrolyte, the Phen residues are hydrophobically encapsulated in the aggregate of the St residues. This kind of polymer system (poly(A/St/Phen), 29) can be prepared by free radical ter-polymerization of AMPS, St, and a small mole fraction of 9-vinylphenanthrene [119]. 

Although the electrostatic potential on the surface of the polyelectrolyte effectively prevents the diffusional back electron transfer, it is unable to retard the very fast charge recombination of a geminate ion pair formed in the primary process within the photochemical cage. Compartmentalization of a photoactive chromophore in the microphase structure of the amphiphilic polyelectrolyte provides a separated donor-acceptor system, in which the charge recombination is effectively suppressed. Thus, with a compartmentalized system, it is possible to achieve efficient charge separation. 

Since the compartmentalization occurs as a result of microphase separation of an amphiphilic polyelectrolyte in aqueous solution, an aqueous system is the only possible object of study. This limitation is a disadvantage from a practical point of view. Our recent studies, however, have shown that this disadvantage can be overcome with a molecular composite of an amphiphilic polyelectrolyte with a surfactant molecule [129], This composite was dissolvable in organic solvents and dopable in polymer film, and the microphase structure was found to remain unchaged in the composite. This finding is important, because it has made it possible to extend the study on photo-systems involving the chromophore compartmentalization to organic solutions and polymer solid systems. 

A jV-Methylenebisacrylamide 163 jV-Methylolacrylamide 163 Microphase structure 55, 63 Mitochondrial matrix enzymes 159 Molecular assembly systems 52... 

Position-Selective Arrangement of Nanosize Polymer Microspheres Onto a PS-b-P4VP Diblock Copolymer Film with Nanoscale Sea-island Microphase Structure... 

Figure 12.1 AFM images of a PS-b-P4VP (301 000 19600) film (a) before and (b) after immersion in methanol for 75 min and the height profiles. S. Machida, H. Nakata, K. Yamada, A. Itaya Position-selective arrangement of nanosized polymer microsphere on diblock copolymer film with sea-island microphase structure.Jpn. /. Appl. Phys. 2006, 45, 4270—4273. Copyright Wiley InterScience. Reproduced with permission.

The equilibrium microphase structures of block copolymers result from a competition between entropic and enthalpic contributions to the free energy. The former accounts for the entropic losses due to stretching or compression of the... 

In addition to microphase structures, MDI/BDO-based polyurethane systems have exhibited spherulitic superstructure. Characterization of the birefringence of the spherulites was used to determine the orientation of the hard-segment domains (7). However, because of the sensi-... 

The random solution of defects caused by lattice dissociation and nonstoichiometry has been discussed for CeCd 4 5. In addition to this random solution in an otherwise ordered lattice, these defects can themselves order to create very many new and complex ordered lattices which we have named microphases. Structures with very similar properties appear at cadmium concentrations both greater and less than that of CeCd4 5 if the CeCd4 5 structure were the common base of all the microphases, discontinuities would be expected at CeCd4>5 where the composition shifted from excess cadmium to deficient cadmium. CeCd4, however, is a reasonable common base. The microphases are the subject of another paper (6). 

J. A. Jackson, also of this laboratory, has made room temperature nuclear magnetic resonance studies of the Knight shift of cadmium in slowly cooled CeCd, 45 alloys with different compositions and different histories. All CeCd 4 5 samples tested showed a major peak at almost the same position and shifted from that of metallic cadmium. One sample showed only this peak, while others clearly showed satellite peaks either at larger or at smaller shift. Possibly some samples had small amounts of both satellite peaks, and there was apparently some further difference in the shapes of satellite peaks and of the major peak these latter observations are tenuous, however, since they were near the resolution limit of the apparatus. The differences apparently do not correlate simply with composition however, they may correlate with differences in microphase structures. 

A metastable broad single-phase region and many additional phases, termed microphases, are indicated by the measurements. The consecutively numbered data are plotted in the figures. Each curve must represent a different structure and its continuous variation of composition with pressure. In interpreting these results it is assumed, on the basis of experimental evidence, that we are dealing with homogeneous samples and that the system is thermodynamically at equilibrium with respect to a particular microphase structure when each datum point is taken. In addition, a particular microphase structure may be, and frequently is, metastable relative to other closely related structures in the same family or to other structures in different families. 

Major factors affecting the final properties of the polymer are the composition and the degree of order within each phase, and the topology or connectivity of the microphase structure, i.e. sharpness of the domain boundaries and mixing of the phases. 

Y. Morishima, M. Tsuji, M. Kamachi, and K. Hatada, Photochromic isomerization of azobenzene moieties compartmentalized in hydrophobic microdomains in a microphase structure of amphiphilic polyelectrolytes, Macromolecules 25, 4406-4410 (1992). 

Of the microphase-structure dependent physical properties of ionomers, perhaps the most widely studied are glass transition temperatures, (Tg), and dynamic mechanical response. The contribution of the Coulombic forces acting at the ionic sites to the cohesive forces of a number of ionomeric materials has been treated by Eisenberg and coworkers (7). In cases in which the interionic cohesive force must be overcome in order for the cooperative relaxation to occur at Tg, this temperature varies with the magnitude of the force. For materials in which other relaxations are forced to occur at Tg, the correlation is less direct. 

The microphase structure and mechanical properties of the blends containing neat acrylonitrile-butadiene-styrene copolymer (ABS), styrene-acrylonitrile copolymer (SAN) and sodium sulfonated SAN ionomer have been investigated as a function of ion content of the ionomer in the blend by Park et a/.51 The interfacial adhesion was quantified by H NMR solid echo experiments. The amount of interphase for the blend containing the SAN ionomer with low ion content (3.1 mol%) was nearly the same as that of ABS, but it decreases with the ion content of the ionomer for the blend with an ion content greater than 3.1 mol%. Changing the ionomer content in the blends shows a positive deviation from the rule of mixtures in tensile properties of the blends containing the SAN ionomer with low ion content. This seems to result from the enhanced tensile properties of the SAN ionomer, interfacial adhesion between the rubber and matrix, and the stress concentration effect of the secondary particles. 

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