A new foldable “bottlebrush” polymer network is both stiff and stretchy – two properties that have been difficult to combine in polymers until now. The material, which has a Young’s modulus of 30 kPa even when stretched up to 800% of its original length, could be used in biomedical devices, wearable electronics and soft robotics systems, according to its developers at the University of Virginia School of Engineering and Applied Science in the US.
Polymers are made by linking together building blocks of monomers into chains. To make polymers elastic, these chains are crosslinked by covalent chemical bonds. The crosslinks connect the polymer chains so that when a force is applied to stretch the polymer, it recovers its shape when the force is removed.
A polymer can be made stiffer by adding more crosslinks, to shorten the polymer chain. The stiffness increases because the crosslinks supress the thermal fluctuations of network strands, but this has the effect of making it brittle. This limitation has held back the development of materials that need both stiffness and stretchability, says materials scientist and engineer Liheng Cai, who led this new research effort.
Foldable bottlebrush polymers
In their new work, the researchers hypothesized that foldable bottlebrush-like polymers might not suffer from this problem. These polymers consist of many densely packed linear side chains randomly separated by small spacer monomers. There is a prerequisite, however: the side chains need to have a relatively high molecular weight (MW) and a low glass transition temperature (Tg) while the spacer monomer needs to be low MW and incompatible with the side chains. Achieving this requires control over the incompatibility between backbones and side chain chemistries, explains Baiqiang Huang, who is a PhD student in Cai’s group.
The researchers discovered that two polymers, poly(dimethyl siloxane) (PDMS) and benzyl methacrylate (BnMA) fit the bill here. PDMS is used as the side chain material and BnMA as the spacer monomer. The two are highly incompatible and have very different Tg values of −100°C and 54°C, respectively.
When stretched, the collapsed backbone in the polymer unfolds to release the stored length, so allowing it to be “remarkably extensible”, write the researchers in Science Advances. In contrast, the stiffness of the material changes little thanks to the molecular properties of the side chains in the polymer, says Huang. “Indeed, in our experiments, we demonstrated a significant enhancement in mechanical performance, achieving a constant Young’s modulus of 30 kPa and a tensile breaking strain that increased 40-fold, from 20% to 800%, compared to standard polymers.”
And that is not all: the design of the new foldable bottlebrush polymer means that stiffness and stretchability can be controlled independently in a material for the first time.
Potential applications
The work will be important for when it comes to developing next-generation materials with tailored mechanical properties. According to the researchers, potential applications include durable and flexible prosthetics, high-performance wearable electronics and stretchable materials for soft robotics and medical implants.
Looking forward, the researchers say they will now be focusing on optimizing the molecular structure of their polymer network to fine-tune its mechanical properties for specific applications. They also aim to incorporate functional metallic nanoparticles into the networks, so creating multifunctional materials with specific electrical, magnetic or optical properties. “These efforts will extend the utility of foldable bottlebrush polymer networks to a broader range of applications,” says Cai.
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