Block copolymers - Their microdomain formation (in solid state) and surfactant behaviour (in solution)

Block copolymers are linear macromolecules that consist of dif ferent blocks (often incompatible) of different types of monomers. The two or more distinct and incompatible moieties provide unique solid state and solution properties to block copolymers which in turn lead to various applications. The microphase separation in block copolymers gives rise to formation of different types (e.g. spherical, cyli ndrical, lamellar, etc.) of microdomains in the solid state. In addition to this, they show micelliz ation and adsorption characteristics in suitable solvents. The present article reviews the properties of block c opolymers in the solid and solution state with special reference to ethylene oxide–propylene oxide block copolymers in solution. POLYMERS are long chain molecules and their properties like light weight, good mechanical strength and easy processibility make polymers useful materials for a variety of uses. Sometimes, the desired properties are obtained by prepa ring mixtures of two polymers (say A and B); such physical mixtures are called polyblends or polymer alloys. The polymer blends, however, tend to phase-separate, especially when the constituent pol ymers are incompatible. Another difficulty in mixing polymers at molecular level arises from the fact that polymer molecules are quite large in size. Thus to prepare a polymer blend, it is desirable to have a chem ical bond between the constituent polymer molecules so that they cannot segregate. This is achieved in block copolymers. A block copolymer is a linear arrangement where two often incompatible blocks obtained from different monomers are covalently linked together. It is possible to prepare diblock (A-B), triblock (A-B-A and B-A-B) and multi-block (or segmented) polymers as shown in Figure 1. Unlike polyblends, where the constituting polymers separate at macrosco pic scale, for block copolymers only microhomogeneous scale separation is possible, due to the covalent bond linking the blocks of different polymers, which forces them to regroup in smaller domains. The reasons for demixing of two blocks of the copolymer are the same as those for demixing of low molecular mass liquids. However, it becomes predominant for mixtures of polymers or in a block c opolymer when the chains are very long or/and the constituting polymers differ considerably. When a diblock copolymer is mixed with a h omopolymer A, the mixture may exist as an ordered or disordered single phase or undergo macrophase separation. Thus phase diagrams of mixtures containing a block copolymer and a homopolymer exhibit fascinating complexity involving macrophase or microphase separation. Two general methods used in the synthesis of block copolymers involve step and chain polymerization 1 . In the first, two polymers with functional end groups are reacted while in the later, sequential polymerization 2 involving initiation of other monomers on the active site of a macromolecular chain of one monomer is consi dered (Figure 2). Block copolymers possess unique structural features resembling those of surface active agents which have two distinct moieties in their molecul e that behave differently. Thus, block copolymers get adsorbed onto