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The objective of this research program is to understand the implications of block-selective polydispersity on the equilibrium microstructure formation and the order-disorder transition of diblock copolymers. The key hypotheses to be tested in this proposal are that asymmetric block polydispersity affects the stability regions of the traditional microdomain structures but also stabilizes morphologies that are otherwise only observed as ‘metastable phases' in monodisperse block copolymers through mitigation of packing frustration.

Statement of work for 09/01/08 – 08/31/09

Within the first year the project has been focused on

(1)   Synthesis of block copolymer model systems

(2)   Characterization of hexagonal perforated morphology in block copolymers with block-selective polydispersity

(3)   Understanding the effect of residual macroinitiator on the structure formation process (ongoing)

1.      Block Copolymer Synthesis

The synthesis of block copolymer model systems based on poly(styrene-b-methacrylate) by activators regenerated by electron transfer atom transfer radical polymerization (ARGET-ATRP). Using ARGET-ATRP the polydispersity of the respective blocks can be controlled by adjusting the amount of Cu²⁺ that is present during the synthesis. Table 1 summarizes the polymers that were synthesized during the first year of the project.  

Table 1. Composition, molecular weights and block selective block polydispersities of PS-PMA systems used in the present study. Numbers in the subscript correspond to volume percent of composition.

The microstructure formation of all PS-PMA systems was elucidated using combined small-angle X-ray scattering (SAXS) and electron microscopy (TEM). In agreement with previous theoretical predictions the introduction of block-selective polydispersity was found to shift the stability regions of the block copolymer equilibrium morphologies in such as way that the interface bends towards the polydisperse domain. However, while most polymer systems were found to follow the predicted trends several samples were observed to exhibit irregular structure formation, for example, the formation of a hexagonal perforated (HPL) morphology was observed for a composition PS₆₅-PMA₃₅ with block selective polydispersity in the PMA domain (PDI ~ 1.7).

2.      Understanding the formation of HPL in polydisperse block copolymers – relevance of the skewness of molecular weight distributions

Using combined SAXS and TEM studies the stabilization of a HPL morphology could be demonstrated for PS₆₅-PMA₃₅ with PDI_PMA ~ 1.7. The size exclusion chromatographic characterization of both blocks along with an electron micrograph of the HPL microstructure is shown in Figure 1a and 1b.

Figure 1. Panel a: GPC analysis of molecular weight distribution of PMA synthesized using ARGET-ATRP with varying concentration Cu(II) (5 and 50 ppm), respectively. Decreasing the amount of Cu(II) results in increase of polydispersity, molecular weight distributions are found to exhibit weak positive skew. Panel b: TEM picture of HPL morphology along with distinct projections normal (upper right inset) and parallel (upper left inset) to the layer orientation. Panel c: Summary of the observed microdomain structures observed for the samples shown in Table 1.

This unexpected finding (HPL was predicted to be unstable in block polydisperse systems) was interpreted to be a consequence of the near-symmetric molecular weight distribution that is characteristic of polymers synthesized using the ARGET-ATRP approach. Next to the PDI itself, the present study suggests that the symmetry of the molecular weight distribution is an important parameter in controlling the structure formation process. The stabilization of the HPL morphology in the case of a PS-PMA copolymer with a block polydispersity of the polydisperse MA domain of PDI between 1.5 and 2 is interpreted as a consequence of the efficient alleviation of packing frustration that is facilitated by the balanced presence of small and large chains of the minority component. This result points to the relevance of developing novel synthetic techniques such as ARGET ATRP which facilitate the preparation of block

copolymers with varying PDI and near-symmetric molecular weight distributions and thus potentially provide a route toward the targeted synthesis of nonregular microstructures with particular topological characteristics (such as combined 2D/3D continuity in the case of the HPL morphology) that might be of future technological interest.

It is noted that since the publication of this result (see reference 1) several other groups have reported similar observations thus supporting the hypothesis that block polydispersity does not only affect the stability region but also the nature of equilibrium microstructures in block copolymer materials.

3.      Understanding the effect of residual macroinitiator on the structure formation process

Our ongoing work focuses on elucidating the relevance of residual macroinitiator on the structure formation in block copolymers with block selective polydispersity. In particular, we are interested in the effect of added macroinitiator on the characteristic block copolymer domain size as well as the potential stabilization of non-regular morphologies such as the HPL and OBDD (ordered bicontinuous double diamond) that exhibit a significant deviation of mean curvature.

Student Training and Education

Funding was used to provide partial support for one graduate student (Jessica Listak) and one undergraduate student (Alexander Hansen) involved in this project.

Project Outcomes

During the first year of support of this project one paper was published (see reference 1 below), also the results were presented as invited talk at the ACS National Meeting in Philadelphia [2]. Several publications are planned in the context of polydispersity in binary blends of homo and block copolymers.

Acknowledgement

The author thanks the Donors of the American Chemical Society Petroleum Research Fund for support of this research. Help of the Matyjaszewski group in synthesizing the PS-PMA systems is gratefully acknowledged.

References

[1] Jessica Listak, Wojciech Jakubowski, Laura Mueller, Andrzej Plichta, Krzysztof Matyjaszewski, and Michael R. Bockstaller “Effect of Symmetry of Molecular Weight Distribution in Block Copolymers on the Stabilization of Metastable Morphologies”, Macromolecules 41, (2008), 5919-5927.

[2] M. R. Bockstaller “Polydispersity Effects on the Structure Formation in Block Copolymers”, Invited Talk, ACS National Meeting, Philadelphia, 2008.