The Influence of Nucleating Agent Positional Isomer Structure on the Nucleation Efficiency of Poly(lactic acid): Interaction-Induced Conformational Transitions under High Lattice Matching

The “lattice mismatch” and “intermolecular
interactions” are two critical parameters influencing the effectiveness
of nucleating agents, significantly impacting the enhancement of polymer
crystallization performance. However, for a long time, we have overlooked
the contribution of “intermolecular interactions” to
polymer crystallization. In this study, a series of nucleating agents
(ODPTA) with different hydroxyl substitution positions (ortho, meta,
and para) were designed, synthesized, and subsequently used to prepare
PLA-ODPTA composites. The objective was to investigate the influence
of lattice matching and intermolecular interactions on the crystallization
behavior of PLA. The results demonstrate that 2ODPTA (with hydroxyl
groups at the ortho position) exhibits the highest nucleating activity.
At a loading level of 0.5 wt %, 2ODPTA significantly increases the
crystallization peak temperature to 121.5 °C, whereas as the
hydroxyl substitution shifts from the ortho position to the para position,
the temperature decreases to 108.2 and 105.8 °C, respectively.
The single crystal diffraction results indicate that there is a favorable
lattice matching relationship between ODPTA and PLA, with 2ODPTA exhibiting
a lattice mismatch of 1.6% along the c-axis, while
3ODPTA (with hydroxyl groups at the meta position) shows a mismatch
of 0.8% along the a-axis. In situ FTIR measurements
and molecular docking simulations further reveal that 2ODPTA forms
a strong binding energy with PLA (−4.5 kcal/mol), significantly
promoting the formation of gt conformational isomers
in the molten state (with a content of 41.9%). Although the lattice
mismatch between the a-axis of 3ODPTA and the a-axis of PLA is only 0.8%, which is the lowest among the
three isomers and theoretically most favorable for the nucleation
process, the stronger intermolecular binding energy between 2ODPTA
and PLA can significantly enhance the crystallization rate of PLA
through the transition of the gt conformation during
the accelerated cooling process, thereby compensating for the disadvantages
associated with its relatively higher lattice mismatch. These findings
offer new insights for the design of efficient nucleating agents for
PLA.