Hydrogen bonding in acrylamide and its role in the scattering behavior of acrylamide-based block copolymers

TL;DR

This study combines theoretical modeling and experiments to understand hydrogen bonding in acrylamide, revealing its impact on the scattering behavior of acrylamide-based block copolymers and improving predictive models of microphase separation.

Contribution

It develops association models with one or two constants for hydrogen bonding in polymers and integrates them into scattering function calculations, advancing the theory of hydrogen bonding in block copolymers.

Findings

Two-constant model predicts bond energies more accurately.

Hydrogen bonding modifies the Flory-Huggins parameter to an effective value.

Theoretical predictions align with IR spectroscopy data.

Abstract

Hydrogen bonding plays a role in the microphase separation behavior of many block copolymers, such as those used in lithography, where the stronger interactions due to H-bonding can lead to a smaller period for the self-assembled structures, allowing the production of higher resolution templates. However, current statistical thermodynamic models used in descriptions of microphase separation, such as the Flory-Huggins approach, do not take into account some important properties of hydrogen bonding, such as site specificity and cooperativity. In this combined theoretical and experimental study, a step is taken toward the development of a more complete theory of hydrogen bonding in polymers, using polyacrylamide as a model system. We begin by developing a set of association models to describe hydrogen bonding in amides. Both models with one association constant and two association constants are considered. This theory is used to fit IR spectroscopy data from acrylamide solutions in chloroform, thereby determining the model parameters. These parameters are then employed to calculate the scattering function of the disordered state of a diblock copolymer with one polyacrylamide block and one non-hydrogen-bonding block in the random phase approximation. It is then shown that the expression for the inverse scattering function with hydrogen bonding is the same as that without hydrogen bonding, but with the Flory-Huggins parameter $\chi$ replaced by an effective value $\chi_\text{eff}=\chi+\delta\chi_\text{HB}\left(f\right)$, where the hydrogen-bonding contribution $\delta\chi_\text{HB}$ depends on the volume fraction $f$ of the hydrogen-bonding block. We find that models with two constants give better predictions of bond energy in the acrylamide dimer and more realistic asymptotic behavior of the association constants and $\delta\chi_\text{HB}$ in the limit of high temperatures.