Block copolymers lamellar structure

Light microscopy has been used in a number of contexts to characterize block copolymer morphology. For crystalline block copolymers, spherulitic structures that result from organization of crystalline lamellae can be examined using microscopy. In solutions, polarized light microscopy can reveal the presence of lamellar and hexagonal-packed cylindrical micellar phases. Cubic micellar phases are optically isotropic, and consequently cannot be distinguished from sols only on the basis of microscopy. 

Table 5a. Structure of copolymers SV2P in solution in a preferential solvent for the PS block (toluene). Following Grosius etal.211 the cylinders are filled with the insoluhle PV2P blocks L Lamellar structure.

Five fundamental domain structures are possible for block copolymers consisting of two types of blocks. Generally lamellar structures will form at compositions with approximately equal proportions of the two components. As the proportion of one component increases at the expense of the other, cylindrical morphologies will result. The matrix phase will... 

Cakmak M. and Wang M.D., Structure development in the tubular blown film of PP/EPDM thermoplastic elastomer, Antec 89, 47th Annual Tech. Conference of SPE, New York, May 1, 1989, 1756. Hashimoto T., Todo A., Itoi H., and Kawai H. Domain boundary structure of styrene-isoprene block copolymer films cast from solution. 2. Quantitative estimation of the interfacial thickness of lamellar microphase systems. Macromolecules, 10, 377, 1977. 

As an example of blends with attractive interactions, Fig. 65 shows a superstructure in which interactions between methacrylic acid groups and pyridine side groups of a polystyrene-fc-polybutadiene-fo-poly(f-butyl methacry-late-staf-methacrylic acid) (PS-b-PB-b-P(MAA-sfaf-fBMA)) triblock quater-polymer and a PS- -P2VP diblock copolymer lead to a wavy lamellar structure with cylinders from mixed P2VP and P(MAA-sfaf-fBMA) blocks [194],... 

The phase morphology of block copolymers can also be visualized by transmission electron microscopy. Figure 10.8 shows the lamellar structure of Fluoro-PSB-IX. From diblock copolymers it is well known that the resulting microphase morphology depends on the volume fraction (< >) of the two phases. By simple adjustment of the relative block lengths we are able to synthesize block copolymers with specific structures.1718... 

One can have the same type of situation in a blend of two mutually immiscible polymers (e.g., polymethylbutene [PMB], polyethylbutene [PEB]). When mixed, such homopolymers form coarse blends that are nonequilibrium structures (i.e., only kinetically stable, although the time scale for phase separation is extremely large). If we add the corresponding (PEB-PMB) diblock copolymer (i.e., a polymer that has a chain of PEB attached to a chain of PMB) to the mixture, we can produce a rich variety of microstructures of colloidal dimensions. Theoretical predictions show that cylindrical, lamellar, and bicontinuous microstructures can be achieved by manipulating the molecular architecture of block copolymer additives. 

A system with clearly defined disperse (A) and continuous (B) component phases is afforded by copolymers of styrene (A) grafted onto a polydimethyl siloxane matrix (B)101 Lack of appreciable interaction between the components was indicated by gas solubility and Tg measurements. The permeability coefficient of propane and other paraffins over a composition range vA = 0 — 0.55 followed the trend described by Eqs. (30)—(33) (with PA = 0, in view of the fact that the polystyrene phase is practically impermeable). Of particular relevance to the present discussion is the close agreement with the Bruggeman, and definite deviation from the Bottcher, equations at higher vA (cf. Fig. 11). Corresponding block copolymer membranes with vA = 0.34 also fitted into this pattern, except in one case where the structure was found to be lamellar and P was considerably lower. 

Crystallization from the melt often leads to a distinct (usually lamellar) structure, with a different periodicity from the melt. Crystallization from solution can lead to non-lamellar crystalline structures, although these may often be trapped non-equilibrium morphologies. In addition to the formation of extended or folded chains, crystallization may also lead to gross orientational changes of chains. For example, chain folding with stems parallel to the lamellar interface has been observed for block copolymers containing poly(ethylene), whilst tilted structures may be formed by other crystalline block copolymers. The kinetics of crystallization have been studied in some detail, and appear to be largely similar to the crystallization dynamics of homopolymers. 

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. 

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