(748c) Nanoscale Structure and Crystallization in Double-Crystalline Diblock Copolymers

Crystalline-crystalline block copolymers, containing two or more chemically distinct crystallizable blocks, are fascinating because their solid-state structures can be set either by block incompatibility or by crystallization of one or more blocks, depending on macromolecular design and processing history [1]. In the solid state, semicrystalline homopolymers typically show a quasi-periodic structure of alternating crystalline and amorphous material, with a period of a few tens of nanometers, and crystal thicknesses of ~10 nm. When two such crystalline polymers are combined in a double-crystalline diblock copolymer, this basic crystalline-amorphous motif is retained, but the nanoscale structure which is formed, and its formation process, can be quite complex. In many of the studies reported thus far, the two components of the block copolymers have crystallization temperatures that are far apart, so that one block always crystallizes ahead of the other block [2,3]. In the present work, double-crystalline diblock copolymers of linear polyethylene (LPE) and hydrogenated polynorbornene (hPN) were synthesized, and their crystallization behavior and morphology were examined using two-dimensional small-angle and wide-angle x-ray scattering. In addition, homopolymers of both LPE [4] and hPN [5] are highly crystalline and have similar melting temperatures (T_m,LPE^o = 142^oC and T_m,hPN^o = 156^oC), thus diblock copolymers of hPN and LPE represent a unique opportunity to access a wide range of nanoscale morphologies within a single block copolymer system.

In previous work, we showed that in symmetric diblock copolymers of hPN and LPE, with molecular weights above 50 kg/mol, the hPN block crystallized first and set the solid-state microstructure [6]. The crystallization behavior was also tunable by changing the block composition: in hPN-rich diblocks, hPN crystallized first, and the crystallization processes of hPN and LPE were well-separated in temperature; in LPE-rich diblocks, LPE crystallized first, which then ?triggered? the crystallization of hPN, resulting in the observation of only a single crystallization process [6]. Here, we extend these studies to lower molecular weights, and more importantly, we examine the structural relationship between the nanoscale crystals formed by the two blocks under different conditions of confinement.

When the hPN/LPE diblock molecular weight is reduced, the melting and crystallization processes of the two blocks are not resolvable by differential scanning calorimetry. However, melting and crystallization of each block may be individually tracked via synchrotron-based time- and temperature-resolved wide-angle x-ray scattering. In symmetric hPN/LPE diblock copolymers, we found that lowering the molecular weight to 20 kg/mol led to a reversal in the crystallization order of the two blocks: LPE crystallized first and became the solid-state structure-directing block. The melting temperature of hPN homopolymer [7] is known to exhibit a stronger dependence on molecular weight than the melting temperature of LPE homopolymer. Assuming that a similar trend applies to crystallization, then at sufficiently low molecular weights, the crystallization temperature of the hPN block drops below that of the LPE block, resulting in the observed reversal in crystallization order. Similar to the higher molecular weight diblocks previously examined, the crystallization behavior in the lower molecular weight diblocks was also tunable by adjusting the block ratio.

In addition to the order of crystallization, the crystal morphology and orientation in hPN/LPE diblock copolymers were examined using two-dimensional wide-angle x-ray scattering on hand-drawn fiber specimens. Since the block that is second to crystallize must do so under confined conditions created by the first block, the resulting crystallites are expected to exhibit preferred orientation [8-10]. We examined a wide range of hPN/LPE diblocks of varying molecular weight and composition, including cases where either hPN or LPE is the first-to-crystallize block. Drawn fibers of each hPN/LPE were reheated to a temperature where the lower-melting-temperature block is molten while the higher-melting-temperature block remains crystalline. The partially-melted diblock fibers were then cooled such that the lower-melting-temperature block recrystallizes between the crystalline lamellae established by the higher-melting-temperature block. In all diblock copolymers examined thus far, the room-temperature fiber scattering patterns show that the hPN and LPE crystals have their stems oriented parallel to each other, meaning that the second-to-crystallize block has its crystal stems perpendicular to the confining lamellar interfaces established by crystallization of the first block. This directly contrasts with confinement by amorphous blocks, where the confined crystal stems lie parallel to the domain interface [8,9]. The origin of this qualitative difference between confinement in crystalline-crystalline and crystalline-amorphous diblocks is currently being investigated and will be discussed in the presentation.

References

[1] A.J. Muller, M.L. Arnal, V. Balsamo, Lect. Notes Phys., 714, 229 (2007).

[2] J. Albuerne, L. Marquez, A.J. Muller, J.M. Raquex, P. Degee, P. Dubois, V. Castelletto, I.W. Hamley, Macromolecules, 36, 1633 (2003).

[3] I.W. Hamley, V. Castelletto, R.V. Castillo, A.J. Muller, C.M. Martin, E. Pollet, P. Dubois, Macromolecules, 38, 463 (2005).

[4] S.T. Trzaska, L.-B.W. Lee, R.A. Register, Macromolecules, 33, 9215 (2000).

[5] L.-B.W. Lee, R.A. Register, Macromolecules, 38, 1216 (2005).

[6] S.B. Myers, R.A. Register, Macromolecules, 41, 6773 (2008).

[7] L.-B.W. Lee, R.A. Register, Macromolecules, 37, 7278 (2004).

[8] K.C. Douzinas, R.E. Cohen, Macromolecules, 25, 5030 (1992).

[9] I.W. Hamley, J.P.A. Fairclough, N.J. Terrill, A.J. Ryan, P.M. Lipic, F.S. Bates, E. Towns-Andrews, Macromolecules, 29, 8835 (1996).

[10] L. Zhu, S.Z.D. Cheng, B.H. Calhoun, Q. Ge, R.P. Quirk, E.L. Thomas, B.S. Hsiao, F.J. Yeh, B. Lotz, J. Am. Chem. Soc., 122, 5957 (2000).