(39b) Crystallization in Ordered Polydisperse Polyolefin Diblock Copolymers

In semicrystalline block copolymers, the formation of solid-state structure can be driven either by block incompatibility or by crystallization of one or more blocks [1-3]. Depending on the block interaction strength, represented by the quantity cN, where c is the Flory-Huggins interaction parameter and N is the degree of polymerization, a wide array of solid-state morphologies may be observed. Here, we aim to explore both the melt and solid-state morphologies of polydisperse ethylene-octene semicrystalline diblock copolymers, consisting of one crystallizable "hard" block of low 1-octene content and one non-crystalline "soft" block of high 1-octene content, synthesized by a coordinative chain transfer polymerization recently developed at Dow Chemical [4]. When the difference in octene content between hard and soft blocks (i.e., c) and the block molecular weight (i.e., N) are sufficiently high, the block copolymers exhibit suppressed spherulite growth and some rather unusual optical properties, appearing blue in reflection and yellow in transmission [5]. These observations suggest confined or templated crystallization from an ordered melt, where the domain periodicity exceeds 100 nm. Because the diblocks are prepared in two cascaded continuous-flow stirred reactors, each block ideally exhibits the most probable distribution of chain lengths [4,5]. This provides an excellent opportunity to investigate the effect of polydispersity on block copolymer morphology, particularly in the weakly-segregated regime.

Diblock copolymer morphologies were examined using two-dimensional synchrotron small-angle and wide-angle x-ray scattering on flow-aligned specimens. At near-symmetric compositions, polyolefin diblocks with an octene differential between the soft and hard blocks exceeding 25 mol% and number-average molecular weights of 69-93 kg/mol, self-assembled in the melt into well-ordered lamellar structures with domain spacings well above 100 nm. These lamellar domains exhibited an extraordinarily long structural equilibration time, exceeding 10 hours. The lamellar structures were also preserved in the solid state, with no significant dependence on crystallization history. However, the resulting crystals were isotropic in orientation, even when the block copolymer lamellae were highly aligned. All polymers showed sigmoidal isothermal crystallization kinetics with an Avrami exponent n = 3, indicating interconnected crystal growth, which traverses the domain interfaces.

This combination -- near-isotropic crystal growth for any crystallization conditions, with retention of the domain structure present in the melt -- falls outside the breakout / templated / confined scheme which effectively classifies more conventional near-monodisperse block copolymers [6]. We believe that this surprising crystallization behavior in these polydisperse olefin block copolymers is a result of soft / hard block phase mixing: specifically, the dissolution of short hard segments in the soft domain. Due to the polydisperse nature of these polymers, the hard and soft blocks in an individual chain may be greatly mismatched in length. When this mismatch is sufficiently large, the shorter chain segments may be pulled away from the domain interface and dissolved in the domain rich in the unlike block [7]. Such phase mixing could then lead to isotropic crystal orientation in the solid state, since the hard blocks dissolved in the soft domain can still crystallize upon cooling; a crystal nucleated in a hard-block-rich domain need not stop growing when it reaches the lamellar interface, but instead may continue to grow through the soft domain via the dissolved hard blocks, without requiring the transport of blocks between domains. Consequently, even though the lamellar structure is preserved upon crystallization, it exerts no "templating" or "confining" effect on the growing crystals, leading to a new mode of crystallization within heterogeneous block copolymers, which we term "pass-through".

Comparing the lamellar domain spacings of these polydisperse high-octene diblock copolymers to those of monodisperse block copolymers with similar molecular weight and chemical structure [8-10], we observed a nearly threefold increase in periodicity. An increase in domain spacing with polydispersity is in agreement with previously reported experimental and theoretical results [11], but the very large swelling factor observed here reflects the high polydispersity in both blocks and the modest segregation strength in these materials, such that short blocks of either type are largely dissolved in domains rich in blocks of the other type. These results indicate that controlled polydispersity in both blocks offers a route to generating lamellar domain structures with an exceptionally large period, while preserving the long-range order commonly associated with near-monodisperse block copolymers.

We are especially grateful to Brian G. Landes, Phillip D. Hustad, and Jeffrey D. Weinhold of The Dow Chemical Company, Core R&D, without whose contributions this work would not have been possible, and for financial support from Dow and from the National Science Foundation, Polymers Program (DMR-0505940 and -1003942).

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