Deformation Mechanisms in Block Copolymers

A deformation of a lamellar block copolymer in the direction perpendicular to the lamellae orientation results in processes of kinking, breaking, and twisting of lamellae, yielding a fish-bone structure or chevron morphoiogy. 

If materials consist of regions (domains, stacks) of parallel lamellae, where the domains themselves are situated at different angles with respect to the deformation direction, four local deformation modes can take place in the lamellar stacks, as schematically shown in Fig. 3.12. [6j  

These local deformation modes are similar to the situation in semicrystalline polymers with a parallel arrangement of crystalline lamellae (stiff) and amorphous layers (soft) compare Figs. 2.20 and 2.21. Due to the larger number of molecular defects (chain ends, weak entanglements) in the amorphous phase of semicrystalline polymers, cavitation and interlamellar separation often occur. Contrary to this, block copolymers with less molecular defects in the soft layers (PB) can easily deform with chevron formation (6,25). 

In hlends of block copolymers with homopolymers, often the homopolymer forms a separate phase, like the homopolymer PS particles in the example of Fig. 3.5(a). 

In this case, the PS particles act as stiff particles embedded in the soft block copolymer matrix. Due to the maximum stress concentration at particle poles (areas in the loading direction), intensified plastic deformation and microvoid formation take place as shown in Fig. 3.13. Both PS and PB lamellae turn to fibrillar structures with a parallel array of fibrils lying normal to the lamellae axis (details 

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Pioneering works on the micromechanical deformation mechanisms in block copolymers date back to the mid-eighties when cavitation mechanism in styrene-butadiene (SB) diblock copolymers containing PB cylinders in a PS matrix was proposed (52,53). Based mainly on tern investigations, a two-step craze growth mechanism was proposed ... 

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