Hydrogen-Bonding Bottlebrush Networks: Self-Healing Materials from Super-Soft to Stiff

Abstract

The impact of polymer architecture on network dynamics and self-healing is presented using bottlebrushes containing side chains that are end-functionalized with 2-ureido-4[1H]-pyrimidinone (UPy). The synthesis of these materials is straightforward through a three-step process: (1) synthesizing rubbery poly(4-methylcaprolactone) macromonomers (p4MCL–OH) with a norbornene-based initiator, (2) functionalizing the terminal hydroxyl group with UPy–isocyanate (p4MCL–UPy), and (3) statistically copolymerizing p4MCL–OH and p4MCL–UPy via ring-opening metathesis polymerization (ROMP) to form hydrogen-bonding bottlebrushes having a fraction (p) of side chains functionalized with UPy. Attaching UPy to the free end of bottlebrush side chains dilutes the impact of friction from complementary UPy interactions on segmental dynamics, leading to a much weaker dependence of the glass-transition temperature (T g) on p than observed in linear analogues, while the activation energy to dissociate UPy–UPy bonds (41–47 kJ/mol) remains mostly unchanged. Longer side chains result in a competition between reducing T g and inducing entanglements that influence hydrogen-bonded network dynamics. Increasing the backbone length extends the sticky Rouse region without affecting the network modulus (G x) or UPy–UPy dissociation time (τs). G x scales linearly with p and ranges from 27 kPa to 1.6 MPa, while τs remains nearly constant in contrast to linear telechelic ionomers, implying a similar self-healability across bottlebrush networks containing different amounts of UPy. These stretchable networks with p ≤ 0.25 undergo self-healing upon repeated rupture and melt pressing at ≤100 °C while retaining similar tensile properties. In summary, decorating bottlebrush polymers with hydrogen bonds creates opportunities to independently manipulate associative network dynamics and mechanical moduli.

Cyclic vs Linear Bottlebrush Polymers in Solution: Side-Chain Length Effect

“…Bottlebrush polymers (BBPs) have attracted considerable interest due to their unique properties originating from chain architecture − and have found a wide range of applications, such as super-soft elastomers, nanomedicine, , electronics, and self-healing materials . Recently, cyclic BBPs have emerged as a very interesting material.…”

“…Bottlebrush polymers (BBPs) have attracted considerable interest due to their unique properties originating from chain architecture 1−3 and have found a wide range of applications, such as super-soft elastomers, 4 nanomedicine, 5,6 electronics, 7 and self-healing materials. 8 Recently, cyclic BBPs have emerged as a very interesting material. Experiments have established that the difference in backbone topology results in different molecular properties, e.g., cyclic BBPs exhibit a more compact shape and less interchain association 9,10 than their linear analogues.…”

A super‐stretched self‐healing silicone elastomer based on high molecular weight polydimethylsiloxanes through intermolecular quadruple hydrogen bonding

“…In the reversible hydrogen bond design system, 2‐ureido‐4[1H]‐pyrimidone (UPy) is one of the most interesting groups due to its ability to form self‐complementary quadruple hydrogen bonds. The dissociation activation energy of UPy‐UPy is 41–47 kJ/mol 24 . UPy units can be introduced into polysiloxane networks to improve the mechanical properties and obtain self‐healing properties of materials 25–27 .…”

“…The dissociation activation energy of UPy-UPy is 41-47 kJ/mol. 24 UPy units can be introduced into polysiloxane networks to improve the mechanical properties and obtain self-healing properties of materials. [25][26][27] Because of the relationship between structure and properties, the distribution of UPy units on the polymer backbone will inevitably affect the properties of materials.…”

Urea as a Hydrogen Bond Producer for Fabricating Mechanically Very Strong Hydrogels

“…Hydrogen bonding also plays a very important role in the construction and stabilization of natural and synthetic polymeric materials. − As a typical example, cellulose, the most abundant natural biomacromolecule with huge potential applications, is cross-linked by very strong cooperative hydrogen bonds between the hydroxyl groups of neighboring chains . However, on the other hand, the presence of strong hydrogen bonds in natural polymers strongly hinders their practical applications.…”