Molecularly designing polymer networks to control acoustic damping

The world is filled with a myriad of sounds and vibrations: the soft tones of a piano drifting down the hallway, the relaxing purr of a cat resting on your chest, the annoying buzz of office lights. Imagine being able to selectively eliminate noises of a certain frequency.

Researchers at the University of Illinois at Urbana-Champaign synthesized polymer networks with two distinct architectures and cross-linking points capable of dynamically exchanging polymer strands to understand how network connectivity and mechanisms of Link exchange governs the overall damping behavior of the network. The incorporation of dynamic bonds into the polymer network demonstrates excellent sound and vibration damping at well-defined frequencies.

“This research involves using polymers to absorb various sounds and vibrations that can occur at different frequencies,” explains Chris Evans, professor of materials science and engineering, who led this work. “We want to know how to design the chemistry of the polymer at the molecular level in a way that controls its energy absorption capacity.”

The results of this new research were recently published in Natural communications.

Being able to tailor polymers to absorb specific frequencies may be beneficial for use in earplugs and headsets intended for people near explosions and in scenarios of repeated exposure to a certain frequency of noise, such as a helicopter pilot, where such long-term exposure can lead to hearing problems.

Polymers are long-chain molecules composed of many repeating units. Some polymers are not entirely linear and have branches, like trees; and other polymers are highly cross-linked where individual polymer chains are connected by covalent bonds to other chains, like a network. The crosslink point is a bond that connects one polymer chain to another, and this is where the bonds can exchange.

The dynamic bonds within a polymer network allow it to reorganize its structure in response to a change in environment (high temperature, pH, UV exposure, etc.). Replacing a few covalent bonds in cross-linked polymer structures with dynamic bonds can improve polymer properties such as modulus (how stiff the material is) and viscosity (how easily the material flows). Dynamic bonds impart unique properties to materials such as self-healing, super-extensibility, adhesive properties, and material toughness due to changing viscoelastic properties.

“The main advance here is that we use dynamic covalent bonds,” says Evans. “These are chemical bonds but they can exchange with each other (the dynamic part) and when two different chemistries are used, they can exchange on very different time scales (the orthogonal part). We use this process to try to control what is happening.frequencies of sound and vibrations that we absorb.

The incorporation of orthogonal links, where fast links can only exchange with other fast links and slow links can only exchange with other slow links, generates multiple and well-separated relaxation modes, which gives the network excellent damping and improved mechanical properties, such as toughness.

The team made a series of polymers with controlled types of architectures and backbones and examined how the polymer chains are connected. Evans says that how the polymer chains are connected makes a big difference in order to achieve the energy dissipation processes on very specific time scales that would correspond to very specific sound waves or vibrations. If chains are only tied at the ends, this is not as effective as being tied periodically along the backbone of the chain.

However, one of the main limitations of the materials used in this research is that they eventually leak. For example, rubber bands will hold their shape, but when these dynamic links are added, they will still end up leaking, like silly putty. This is great, for example, for a soldier’s helmet where the material is enclosed in the helmet shell, but not so much for an earplug. Evans says his group is working on ways to make the polymer more of a standalone material, and in the future they would like to incorporate more dynamic bonds, so the polymer isn’t just tuned to a specific frequency , but for a much wider frequency range.

Chris Evans is also affiliated with the Materials Research Laboratory and the Beckman Institute for Advanced Science and Technology at UIUC.

Other contributors to this work include Sirui Ge (UIUC Department of Materials Science and Engineering and Materials Research Laboratory) and Yu-Hsuan Tsao (Department of Materials Science and Engineering and Materials Research Laboratory) UIUC Materials Research).