Configurations block copolymers

The first report of ROMP activity by a well-characterized Mo or W species was polymerization of norbornene initiated by W(CH-t-Bu)(NAr)(0-f-Bu)2 [122]. In the studies that followed, functionality tolerance, the synthesis of block copolymers, and ring-opening of other monomers were explored [30, 123]. Two important issues in ROMP concern the cis or trans nature of the double bond formed in the polymer and the polymer s tacticity. Tacticity is a consequence of the presence of two asymmetric carbons with opposite configuration in each monomer unit. The four ROMP polymers (using polynorbornene as an example) that have a regular structure are shown in Scheme 3. 

A block polymer is a polymer comprising molecules in which there is a linear arrangement of blocks, a block being defined as a portion of a polymer molecule in which the monomeric units have at least one constitutional or configurational feature absent from the adjacent portions [4]. In a block copolymer, the distinguishing feature is constitutional, i.e. each of the blocks comprises units derived from a characteristic species of monomer. 

In contrast, crystallization of one or both components of a block copolymer is accompanied by profound structural and dynamic changes. The fundamental process in crystallization of chains in a crystallizable block copolymer is the change in block conformation, i.e. the adoption of an extended or a folded structure rather than a coiled configuration found in the melt or solution (see Fig. 1.5). 

There is no comprehensive theory for crystallization in block copolymers that can account for the configuration of the polymer chain, i.e. extent of chain folding, whether tilted or oriented parallel or perpendicular to the lamellar interface. The self-consistent field theory that has been applied in a restricted model seems to be the most promising approach, if it is as successful for crystallizable block copolymers as it has been for block copolymer melts. The structure of crystallizable block copolymers and the kinetics of crystallization are the subject of Chapter 5. 

Fig. 6.9 Schematic showing the effect of addition of low-molecular-weight homopolymer on block copolymer chain configuration (Hasegawa and Hashimoto 1996). (a) A symmetric diblock forms a lamellar phase, (b) On addition of homopolymer, swelling induced by solubilized homopolymer causes stretching of the corresponding block chain/and or contraction of the other block, resulting in a decrease in conformational entropy, (c) Alternatively, a curved interface is formed to attain a uniform packing density.

In graft copolymers the chain backbone is composed of one kind of monomer and the branches are made up of another kind of monomer. The structure of a block copolymer consists of a homopolymer attached to chains of another homopolymer. In either case, cis or trans (Z or E) double bond configurations around any double bond not involved in the polymerization will normally be unaltered. 

Tsarkova L, Knoll A, Magerle R (2006) Rapid transitions between defect configurations in a block copolymer melt. Nano Lett 6 1574—1577... 

Detailed analysis of defect configurations in the cylinder phase and of their evolution allowed us to conclude that representative defect configurations provide connectivity of the minority phase in the form of dislocations with a closed cylinder end or of classical disclinations with incorporated alternative, non-bulk structures with planar symmetry. Further, block copolymers show a strong correlation between the defect structure and chain mobility on both short- and long-term time scales. 

Table I. Block Configurations and Maximum Retardation Times of Block Copolymers (26)...

The basic driving force for microdomain formation in block copolymers is the reduction in the positive surface free energy of the system resulting from the increase of the domain size. This domain size increase gives rise to a decrease in the volume fraction of interfacial region in which junction points of the copolymers must be distributed. In addition, configurations of the block chains must also change in order to even-up the density deficiency in the interior of the domains. 

Finally, we draw attention to a topic somewhat neglected in this review, namely the interplay between concentration inhomogeneities ( > p,z near the surfaces and interfaces and the local configurational properties of the polymer coils (enrichment of chain ends, orientation and possibly distortion of polymer coils, etc.). The reason for this omission was that not so much general features are known about these questions. Clearly, the subject of phase transitions of polymer blends and block copolymer melts in thin film geometry will remain a challenge in the future. 

Another example is that of lattice chain models. Simple square lattice models were established by Flory as a vehicle for calculating configurational entropies etc., and used later in the simulations of the qualitative behaviour, e.g. of block copolymer phase separation. More sophisticated models such as the bond fluctuation model, and the face centred cubic lattice chain modeP ... 

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