Synthesis of polyfarnesene based on aqueous free-radical polymerization and molecular dynamics simulation of its properties

Farnesene monomer, obtained through the bio-fermentation, offered a unique long side chain structure that served as an excellent platform for the synthesis of “bottlebrush-like” polymers with exceptional thermal properties. Such polymers had immense potential in the production of polyurethane elastomers and adhesives. Hydroxyl-terminated polyfarnesene (PFD), which was safe, environmentally-friendly and pollution-free, was synthesized with farnesene as the monomer and industrial grade product hydrogen peroxide as the initiator by aqueous free-radical polymerization. Molecular dynamics (MD) simulations of PFD and polyfarnesene (PF) were performed using Materials Studio to calculate pivotal performance parameters, such as glass transition temperature (Tg), cohesive energy density (CED) and free volume. Tg was verified by comparing the results from the simulations with measured values obtained through experimental tests. Additionally, PFD was cured with multi-isocyanate (brand N 100) crosslinking agent, using a crosslinking script, to evaluate its mechanical characteristics. As showed in Table 1, the results from the simulation of trans-1,4-PFD exhibited a Tg of 231 K, which was in close agreement with Tg (226 K) obtained from differential scanning calorimetry measurements. These findings demonstrated the usefulness of MD simulations in providing a thorough qualitative and quantitative understanding of the intricate connection between the properties and structure of polymers. The lower Tg of PFD indicated its excellent low-temperature performance. The trans-structure in the PFD system had good symmetry, stable structure and tight arrangement, which hindered the movement of its molecular chains. The Tg of 3,4-PFD was significantly higher than those of cis-1,4- and trans-1,4- structures, this was because the main chain of 3,4-PFD was shorter, and there was no isolated double bond in the molecular chain, reducing the flexibility of the molecular chain. The cohesive energy of a material was closely related to its intermolecular interaction. In the case of PF, the non-polar alkyl side chain provided a flexible sheath around the long-chain molecules, reducing the obstruction caused by the surrounding medium to the long-range movement of the molecular chain, weakening the intermolecular interaction, and lowering the CED. In polymers, the parameter free volume represented the volume expansion property and described the molecular motion amplitude. The simulation results presented in table 1 demonstrated that the free volume of 3,4-PFD was significantly smaller than 1,4-PFD. This could be attributed to the relatively low flexibility of the 3,4-structure that limited the molecular motion amplitude, which resulted in a smaller free volume. Table 1 Simulation parameters of different PF and PFD structures ■ Moreover, the mechanical properties of PFD were significantly improved through the crosslinking with the N 100 vulcanization system. In the absence of crosslinking reaction, the elastic modulus of PFD was 1.03 GPa and shear modulus was 0.39 GPa. When the crosslinking reaction occured with N 100 and the crosslinking density reached up to 73%, the elastic modulus of the crosslinked three-dimensional network of PFD increased to 21.12 GPa, while the shear modulus rised to 8.19 GPa, representing the increases by 1 950% and 2 000%, respectively.