Creep-induced anisotropy in covalent adaptable network polymers.

Area of Science:

• Polymer Science and Engineering
• Materials Science
• Mechanical Engineering

Background:

• Anisotropic polymers with aligned macromolecule chains offer enhanced directional mechanical properties.
• Achieving polymer chain orientation in permanently crosslinked thermosets is challenging.

Purpose of the Study:

• To demonstrate the use of rearrangeable networks with bond exchange reactions (BERs) for tailoring anisotropic mechanical properties in thermosetting polymers.
• To investigate the relationship between creep-induced chain alignment and resulting network anisotropy.

Main Methods:

• Utilized thermosetting polymers capable of bond exchange reactions (BERs).
• Applied constant force at BER-activated temperatures to induce creep and chain alignment.
• Developed a multi-length scale constitutive model to analyze creep kinetics, chain orientation, and mechanical anisotropy.
• Verified chain orientation predictions using molecular dynamics (MD) simulations.

Main Results:

• Creep under constant force at elevated temperatures leads to macromolecule chain alignment in the direction of stress.
• Aligned chains create anisotropic networks with increased stiffness in the creep direction and compliance in the transverse direction.
• The degree of anisotropy is directly proportional to the creep strain.
• The constitutive model accurately predicts creep strain evolution and anisotropic stress-strain behavior without fitting parameters.
• Time-temperature superposition principle (TTSP) governs the evolution of directional modulus.

Conclusions:

• Rearrangeable networks with BERs offer a facile method to create anisotropic thermosets from isotropic precursors.
• Processing conditions (creep time, temperature) can be precisely controlled to tailor the degree of network anisotropy.
• This approach provides a simple heating-based method to imbue thermosets with direction-dependent mechanical properties.