block copolymer (rus. блок-сополимер otherwise блоксополимер) — linear copolymer, whose macromolecule consists of regularly or statistically alternating homopolymer blocks that differ in composition or structure. Polymers that are derived from one monomer and contain alternating blocks having different spatial structures (e.g. iso-and syndiotactic configurations) are called stereoblock copolymers.

Description

Block copolymers tend to combine the properties of their constituent blocks, resulting in the modification of one polymer by the second component, and this feature differentiates them from traditional random copolymers that do not exhibit the characteristics of each of their components.

The properties of block copolymers are similar to the properties of polymer mixtures, but the presence of chemical bonds between the blocks ensures their stability and prevents their separation with the release of individual components.

Multiple reactions of synthesis of block copolymers are based on two main principles: the interaction of the macromolecular initiator with the monomer and the interaction of two or more polymers or macroradicals with each other. Synthesis of block copolymers significantly expands the possibilities for modifying the properties of polymers, as the macromolecule allows to combine regions of polymers varying in structure and properties: natural and synthetic, carbo- and heterochain, flexible and rigid, hydrophobic and hydrophilic, regular and irregular, etc. However, the correlation between the composition and structure of macromolecules and properties of block copolymers is not easy to define due to difficulties in isolating and identifying individual blocks, which explains the small number of block copolymers with a strictly defined structure among those applied in practice.

The block copolymers are characterised by microphase separation, accompanied by the formation of nano-and microregions with concentration of segments (blocks) having the same chemical structure. The presence of these microregions is the cause of a significant difference in properties between block copolymers and random copolymers of the same composition, which, in most cases, determines the additivity of structural and mechanical properties of block copolymers.

The combination of properties of homopolymers in a block copolymer manifests itself in the thermomechanical properties and transition temperatures of block copolymers consisting of incompatible or partially compatible polymeric blocks. These block copolymers have several glass transition temperatures (equal to the number of dissimilar blocks), and their temperature of flow is determined by the highest temperature of the flow of one of the components.

The properties of a block copolymer can be varied using solvents and precipitation agents, to form a globular or fibrillar form and thus obtain products with different properties.

The combination of rigid and flexible blocks of macromolecules provides for obtaining modified elastomers with enhanced strength characteristics while maintaining elasticity.

Promising directions of the block copolymerisation studies are the research into controlled synthesis of block copolymers without formation of homopolymers, determining the relationships between the properties of block copolymers and their chemical and nano-/micro-phase structure, and the development of appropriate quantitative theory defining the regularities of block copolymer synthesis.

Authors

  • Khokhlov Alexey R.
  • Govorun Elena N.
  • Nazarov Victor G.

Source

  1. Geoffrey A.O., Andre C. A., Ludovico C. Nanochemistry. A Chemical Approach To Nanomaterials. 2nd ed. — RSC, 2009. — p.782.

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