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Hydrogen bonds in diblock copolymer/ homopolymer melt

Periodic Reporting for period 1 - HYBOCOMIX (Hydrogen bonds in diblock copolymer/ homopolymer melt)

Reporting period: 2016-09-01 to 2018-08-31

Hydrogen bonding interactions occur very widely in nature. Although individual bonds are relatively weak, their effect on the physical properties of substances can be profound and is responsible for the anomalous properties of water and the secondary structure of proteins. However, the characteristics of hydrogen bonding, such as site specificity and cooperativity, make it difficult to build a general theoretical description of H-bonding systems.

The presence of hydrogen bonds affects both equilibrium and dynamic properties of all macromolecular systems with hydrogen bonds such as mixtures, solutions, gels, etc. However, a statistical description of hydrogen bonding in macromolecular systems is still scarce and incomplete.

If one is to truly understand, and eventually mimic, nature (and its ability to create advanced materials), we must create accurate, verifiable models that have never been used before.

Block copolymers (polymeric materials with two or more components that exhibit remarkable phase separation behaviour on the nanoscale- akin to natural systems) with one hydrogen bonding (HB) and one non-hydrogen bonding block is an important class of materials with application in nanopatterning in microelectronics. In the case when the HB block is self-associating (has both hydrogen donor and hydrogen acceptor groups like amides, alcohols and acids) [1,2], or is mixed with a complimentary acceptor/donor homopolymer [3] or even with a low molecular weight compound [4], sub-10 nm features can be achieved because of high incompatibility with the non-hydrogen bonding block. The physical reason of incompatibility lies in high energy cost of destruction of the network of hydrogen bonds upon uniform mixing. However, the understanding of the effect hydrogen bonds on microphase separation is far from complete.

This Fellowship had been designed to improve understanding of the role of hydrogen bonds on equilibrium behaviour of hydrogen bonding block copolymers. The study had theoretical and experimental parts. The theoretical part was focused on the development of an association model approach which is useful for the description of hydrogen bonding systems in general and apply it, for the first time, to block copolymer systems. Initially, it was planned to incorporate the association model approach into self-consistent field theory in order to get a versatile tool to predict equilibrium properties of block copolymer systems. The experimental part would then be aimed at the verification of the theoretical predictions. Moreover, the experimental component had the key objective of training the Fellow in synthesis and characterization of polymer systems (completely new to the Fellow who had previously only ever worked on theoretical systems and not worked in a chemistry laboratory, fabricating materials), enabling her to conduct combined theoretical and experimental research in the field of polymer materials in the future.

[1] Fabrication of Sub-3nm Feature Size Based on Block Copolymer Self-Assembly for Next-Generation Nanolithography. Kwak, J. et al., Macromolecules, 2017, 50(17), 6813-6818. [2] Realizing 5.4nm Full Pitch Lamellar Microdomains by a Solid-State Transformation, Jeong, G. et al., Macromolecules, 2017, 50(18), 7184-7154. [3] Thermodynamic and Morphological Behavior of Block Copolymer Blends with Thermal Polymer Additives, Sunday, D.F. et al., Macromolecules, 2016, 49(13), 4898-4908. [4] Facile and Efficient Modification of Polystyrene-block-poly(methyl methacrylate) for Achieving Sub-10 nm Feature Size, Yoshida, K. et al., Macromolecules, 2018, ASAP, 10.1021/acs.macromol.8b01454
The following objectives were successfully achieved:

Objective 1. The research career of a talented young Fellow was reinitiated and has been fully trained in experimental block copolymer chemistry & physics (to complement her existing expertise in theoretical block copolymer physics).

Objective 2. A new association model was created and published (Macromolecules 2018) to describe hydrogen bonding in monomers towards their application in block copolymers.

Objective 3. A theoretical study of the influence of hydrogen bond formation on diblock copolymer melt (where one monomer unit forms self-associating hydrogen bonds) was developed (preliminary findings of fitting the scattering function in a disordered state was published in Macromolecules 2018).

Objective 4. Two block copolymer families were synthesized and characterized. Their phase behavior was studied at the European Synchrotron Radiation Facility (ESRF) using x-ray beamtime (accessed via competitive proposal route).

Objective 5. Theoretical and experimental results were obtained and compared to test the predictions of the new theory.

Additional objective. Collaborative work (with the University of Warwick) was performed on the phase behavior of multiblock copolymers using x-ray scattering (published in Macromolecules 2017).

This work has been disseminated at five conferences during the period of the Fellowship.
The main progress of the project beyond the state-of-the-art is connected to the development of the association model approach and a better understanding of the physical implications behind its formalism. These findings expand applicability of the approach and show the way to develop the association approach further with the final aim of achieving an improved description and understanding of the statistical properties of hydrogen bonding both in polymers and low molecular weight systems. Development of such a theory is of interest to a large community of researchers, in particular in the field of polymer materials where there are no good tools to do it at the moment.
Our experimental and theoretical findings on block copolymer systems with one non-hydrogen bonding and one hydrogen bonding self-associating block clearly show that the system has equilibrium behaviour distinct from diblock copolymer systems without specific interactions in the region around ODT and common practice of characterising them using the theory of microphase separation developed for block copolymers without specific interactions must be revised. This conclusion is important in the field of highly incompatible block copolymers which are being actively developed and pursued at the moment for nano-patterning applications in microelectronics.
The project can be regarded as extremely successful in terms of the development of human potential and dissemination of results. The Fellow has developed experimental skills complimentary to theoretical which she hopes will be helpful in her ambition to become an independent researcher in the field of polymer materials. She transferred the part of her knowledge to the Topham group and the numerous students that she had the opportunity to supervise during the Fellowship. She also took an active role in knowledge transfer in the group by organising PhD seminars. She actively participated in workshops and conferences which were chosen on the basis of the largest chance of meeting people interested in the outputs of the work (both from academia and industry). The paper summarising the work was published in Macromolecules (2018).
Theoretical and experimental study of hydrogen-bonding molecules