Physically Cross-Linked Polybutadiene by Quadruple
Hydrogen Bonding through Side-Chain Incorporation of Ureidopyrimidinone
with Branched Alkyl Side Chains
posted on 2022-01-19, 15:09authored byJente Verjans, Alexis André, Evelyne Van Ruymbeke, Richard Hoogenboom
The
synthesis of a supramolecular polymer network based on ureidopyrimidinone
(UPy) is demonstrated starting from a low-molecular weight polybutadiene
(PB) polymer, which was modified with UPy quadruple hydrogen-bonding
motifs through UV-initiated thiol–ene chemistry under ambient
conditions. Here, the UPy units contain a branched alkyl side chain
rather than a methyl group, as is the case in most literature examples.
The sterically demanding aliphatic 2-ethylpentyl side chain was introduced
to prevent stacking of the UPy dimers and to enhance the compatibility
of the UPy unit with the apolar PB polymer matrix. The UPy units were
grafted onto PB with different functionalization degrees yielding
materials with a relatively broad range of glass transition temperatures
(Tg) and material properties, as evaluated
by thermogravimetric analysis, differential scanning calorimetry (DSC),
and dynamic mechanical thermoanalysis. Importantly, only the system
with 13 mol % UPy functionalization showed some macroscopic phase
separation as indicated by its partial opaque appearance, even though
this was not detected by DSC. Previous reports on PB end functionalized
with methyl-functional UPy revealed phase separation through DSC,
indicating that the branched side chains indeed suppress the phase
separation. The network dynamics were assessed by rheological measurements
at different temperatures, which were subsequently fitted to the sticky
Rouse model and creep experiments. We found that the sticky Rouse
model satisfactorily fits the linear response of the systems in the
terminal regime. However, significant discrepancy from theory still
persists in the rubbery plateau regime. This deviation is attributed
to the presence of structural defects in the systems. Altogether,
this work demonstrates the importance of designing the supramolecular
physical cross-linker unit with a branched aliphatic side chain to
enhance the compatibility with the polymer matrix for the formation
of supramolecular polymer networks.