Mechanical contrast, block copolymers

Block copolymerization was carried out in the bulk polymerization of St using 18 as the polymeric iniferter. The block copolymer was isolated with 63-72 % yield by solvent extraction. In contrast with the polymerization of MMA with 6, the St polymerization with 18 as the polymeric iniferter does not proceed via the livingradical polymerization mechanism,because the co-chain end of the block copolymer 19 in Eq. (22) has the penta-substituted ethane structure, of which the C-C bond will dissociate less frequently than the C-C bond of hexa-substituted ethanes, e.g., the co-chain end of 18. This result agrees with the fact that the polymerization of St with 6 does not proceed through a living radical polymerization mechanism. Therefore, 18 is suitably used for the block copolymerization of 1,1-diubstituted ethylenes such as methacrylonitrile and alkyl methacrylates [83]. 

In fact, even in pure block copolymer (say, diblock copolymer) solutions the self-association behavior of blocks of each type leads to very useful microstructures (see Fig. 1.7), analogous to association colloids formed by short-chain surfactants. The optical, electrical, and mechanical properties of such composites can be significantly different from those of conventional polymer blends (usually simple spherical dispersions). Conventional blends are formed by quenching processes and result in coarse composites in contrast, the above materials result from equilibrium structures and reversible phase transitions and therefore could lead to smart materials capable of responding to suitable external stimuli. 

In contrast to some theoretical predictions (23, 46, 52) aggregation or phase separation in block copolymers occurs at a slightly higher total concentration than in polymer mixtures. Covalent bonding of the two kinds of blocks thus slightly increases the mutual solubility. Polystyrene blocks with M = 2000 dissolve in polybutadiene (Af = 75000) up to concentrations of about 20% (33). Therefore, special mechanical properties are, in general, only to be expected in sequence copolymers above a certain block length (in most cases M8 > 103). 

Since the relaxation mechanisms characteristic of the constituent blocks will be associated with separate distributions of relaxation times, the simple time-temperature (or frequency-temperature) superposition applicable to most amorphous homopolymers and random copolymers cannot apply to block copolymers, even if each block separately shows thermorheologically simple behavior. Block copolymers, in contrast to the polymethacrylates studied by Ferry and co-workers, are not singlephase systems. They form, however, felicitous models for studying materials with multiple transitions because their molecular architecture can be shaped with considerable freedom. We report here on a study of time—temperature superposition in a commercially available triblock copolymer rubber determined in tensile relaxation and creep. 

In contrast to swollen homopolymer films, only a limited number of studies on thin films of block copolymers have been reported in which the degree of the film swelling has been directly accessed. In situ SE has been used to evaluate the polymer-solvent interaction parameters [144], to construct phase diagrams of surface structures [49, 51], and to control the mechanism of lamella reorientation in thick swollen films [118, 163, 164], Spectroscopic reflectometry combined with real-time GISAXS has been used to follow structural instabilities in swollen lamella films [165], Recently, it was demonstrated that swelling of diblock copolymer films in organic selective and non-selective solvents follows the same physical behavior as in thin films of homopolymers [119]. 

We note that earlier research focused on the similarities of defect interaction and their motion in block copolymers and thermotropic nematics or smectics [181, 182], Thermotropic liquid crystals, however, are one-component homogeneous systems and are characterized by a non-conserved orientational order parameter. In contrast, in block copolymers the local concentration difference between two components is essentially conserved. In this respect, the microphase-separated structures in block copolymers are anticipated to have close similarities to lyotropic systems, which are composed of a polar medium (water) and a non-polar medium (surfactant structure). The phases of the lyotropic systems (such as lamella, cylinder, or micellar phases) are determined by the surfactant concentration. Similarly to lyotropic phases, the morphology in block copolymers is ascertained by the volume fraction of the components and their interaction. Therefore, in lyotropic systems and in block copolymers, the dynamics and annihilation of structural defects require a change in the local concentration difference between components as well as a change in the orientational order. Consequently, if single defect transformations could be monitored in real time and space, block copolymers could be considered as suitable model systems for studying transport mechanisms and phase transitions in 2D fluid materials such as membranes [183], lyotropic liquid crystals [184], and microemulsions [185],... 

The method used to provide contrast in transmission electron microscopy was successful in demonstrating the presence of a two-phase structure in homopolymer blends of BR and IR (Figure 2a). The opposite situation, i.e., a clear absence of any phase separation in the block copolymers, also is demonstrated, but much less convincingly by the comparison of Figures 2b and 2c. It is necessary to consider the evidence from all of the mechanical and thermal analysis experiments, along with the evidence from microscopy. 

EXPLANATIONS. The aligning behavior of block copolymers under shear is complicated and is sensitive to block architecture, mechanical contrast between the layers, proximity of the ODT, frequency, and strain. No general theory seems to predict the alignment direction under all conditions. Theories and rationalizations are available, however, to explain some of the trends. 

A weak-segregation theory has also been developed by Fredrickson (1994) to explain the alignment behavior of diblock copolymers. In this theory, perpendicular alignment is predicted near Toot at frequencies that are high compared to rates of fluctuations (but low compared to molecular time scales) as a result of coupling of composition fluctuations to the shearing field. Well below Todt, these fluctuations are small, and mechanical contrast between the blocks, however small, dominates and favors parallel alignment. 

In contrast to precipitation procedures, nanocasting, particularly of amphiphilic block copolymer phases, ahows the fabrication of large objects (monoliths) that are macroscopically devoid of cracks and defects. Despite their mechanical robustness, the porosity of these monohths can be as high as 85%. 

The properties of the linear material 7.27 and the network copolymer 7.28 have been studied by dynamic mechanical analysis, DSC, and transmission electron microscopy. Evidence was obtained for the formation of highly ordered micro-phase-separated superstructures in the solid state from the materials 7.27. The Cu(bipy)2 moieties appear to form ordered stacks, and this leads to thermoplastic elastomer properties. In contrast, the network structure of 7.28 prevents significant microphase separation [51-53]. By means of related approaches, dinuclear Cu helical complexes have also been used to create block copolymers by functioning as cores [54], and polymer networks have also been formed by using diiron(II) triple helicates as cores for the formation of copolymers with methyl methacrylate [55]. 

FIGURE 6.4 Mechanisms of polymersome formation from block copolymers. For solvent inversion, film rehydration, and electroformation, the polymCT is first dissolved in an organic solvent and polymersomes are initiated after water is added to the system. In contrast, pH-sensitive polymers are dissolved in acidic water and polymersomes are formed by switching to basic conditions. 

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