L. H. Sperling

Department of Chemical Engineering, Department of Materials Science and Engineering, Materials Research Center, Center for Polymer Science and Engineering, and Polymer Interfaces Center, Lehigh University, Bethlehem, PA 18015-3194

The nomenclature related to the chemical structure of block copolymers is relatively well established(1,2). Thus, a diblock copolymer of polybutadiene and polystyrene is written:

polybutadiene-block-polystyrene . . . . . . . . . . . . (1)

(The reader should note that the use of -b- to indicate a block copolymer is out of date; nomenclature such as poly(butadiene-b-styrene) should be discouraged.)

Block copolymers come in three main categories: diblock copolymers, as delineated by structure (1) above and sometimes referred to as AB block copolymers, triblock copolymers of the ABA type, and multiblock copolymers of the (AB)_n type. There are several other types, such as ABC block copolymers containing three different blocks, and the star block copolymers. This last has a central point, with three or more arms, each of which is in the form of a block copolymer, usually of the AB type. Some novel types of block copolymers are discussed by Hadjichristidis and coworkers(3,4), including the 4-miktoarrn star quaterpolymer of the ABCD type, and a super-H block copolymer of the B₃AB₃ type. There is also the inverse 4-miktoarm block copolymer of the (AB)₂(BA)₂, a type of star block copolymer. These latter, newly named by Hadjichristidis, have not been discussed yet by the IUPAC polymer nomenclature committee.

However, there is no formal nomenclature at all relative to phase separation and concomitant morphologies. This back page of PMSE will be mostly devoted to a description of current practice, with a few suggestions for use.

Most polymer blends exhibit lower critical solution temperatures (LCST). This means that the two polymers become more miscible as the temperature is lowered, see Figure 1. Note that the ordinary tie lines of physical chemistry apply here. In Figure 1, the solid line is called the binodal and the dashed line the spinodal(5). These two lines demarcate regions of different kinetics of phase separation. Liquid-liquid phase separation is assumed. If one of the components crystallizes, quite different phase diagrams emerge(6). Occasionally, upper critical solution temperatures (UCM) are observed for blends at some lower temperature range. Typical data may be obtained by raising the temperature rapidly from some temperature in the totally miscible range to some higher temperature, and observing changes via optical clarity and microscopy(7). Figure I assumes particular molecular weights for the two polymers in question. Different molecular weights result in different positions of the phase diagram, and different maximum temperatures of mutual miscibility. Here, the temperature and the composition of a particular polymer pair is varied.

The case for block copolymers is significantly different. The molecular weights of the blocks and the overall composition of the block copolymer cannot be varied independently. Therefore, a classical phase diagram, such as shown in Figure 1, cannot exist, The result has been a plethora of new notation.

Some of the terms used for the temperature of phase separation in block copolymers include the order-disorder (ODT) or the microphase separation transition (MST) see Figure 2. In Figure 2, c Z represents the heat of mixing per chain. Other notation in common use is the narrow interphase approximation (NIA) which means that one is in the strong segregation regime, i.e., far from the MST(8). Closer to the MST, one speaks of the weak segregation regime(9). At the MST, other people talk about an upper critical ordering temperature (UCOT) and a lower critical ordering temperature (LCOT)(10). While most polymer blends exhibit a LCST, most block polymers exhibit an UCOT. An UCOT implies a nucleation and growth mechanism for the kinetics of phase separation. Whether one speaks of a LCST and an UCST or a LCOT and an UCOT, it must be noted that the UCST and UCOT appear at lower temperatures than the corresponding LCST and LCOT, respectively.

Most block copolymers are phase separated, that is, the two kinds of blocks are immiscible. (Most of the important properties of block copolymers, including thermoplastic elastomers, elastic fibers, and surfactant applications, depend on phase separation.) The three most common morphologies observed are spheres, cylinders, and lamellae. The appearance of each of these depends on the relative block lengths, see Figure 2(11). Some of the more exotic morphologies include the ordered, bicontinuous double-diamond (OBDD)(12), consisting of an interconnected tetrahedral arrangement of short rods, in the form of a double diamond kind of structure. There is also an ABCB block copolymer that forms an ordered tricontinuous double-diamond (OTDD) morphology(13), and a ripple or perforated-lamellar morphology of an ill-defined type(14). Semi-crystalline block copolymers often possess a lamellar morphology(15).

A major restriction on all block copolymer morphologies is that the interfaces must be placed relative to one another such that the junction between the blocks can fit easily. There are several other possible morphologies that fit this restriction, the subject of future research. Overall, the whole area of block copolymer phase separation and morphology lacks a coherent and accepted nomenclature, although its great importance is leading to continuous and numerous research programs.

1. IUPAC, Compendium of Macromolecular Nomenclature, W. V. Metanomski, Ed., Blackwell Scientific Publications, Oxford, 1991.
2. IUPAC, Pure Appl. Chem., 57, 1427 (1985).
3. H. Iatrou, A. Avgeropoulos, and N. Hadjichristidis, Macromolecules, 27, 6232 (1994).
4. H. Iatrou and N. Hadjichristidis, Macromolecules, 26, 2479 (1993).
5. O. Olabisi, L. M. Robeson, and M. T. Shaw, Polymer-Polymer Miscibility, Academic Press, Orlando, FL 1979.
6. D. R. Paul, in Multicomponent Polymer Materials, Adv. Chem. Ser. 211, ACS Books, Washington DC, 1986.
7. T. Nishi, T. T. Wang, and T. K. Kwei, Macromolecules, 8, 227 (1975).
8. E. Helfand and Z. R. Wasserman, Macromolecules, 13, 994 (1980).
9. M. Mayes and M. Olvera de la Cruz, J. Chem. Phys., 95, 4670 (1991).
10. T. P. Russell, T. E. Karis, Y. Gallot, and A. M. Mayes, Nature, 368, 729 (1994).
11. S. Sakurai, Trends Polym. Sci. (TRIP), 3, 90 (1995).
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13. Y. Mogi, H. Kotsuji, Y. Kaneko, K. Mori, Y. Matsushita, and I. Noda, Macromolecules, 25, 5409 (1992).
14. K. Almdal, K. A. Koppi, F. S. Bates, and K. Mortensen, Macromolecules, 25, 1743 (1992).
15. Rangarajan, R. A. Register, D. H. Adamson, L. J. Fetters, W. Bras, S. Naylor, and A. J. Ryan, Macromolecules, 28, 1422 (1995).