*2.1. Arylamide Nucleators*

The addition of *<sup>N</sup>*,*N*,*<sup>N</sup>*"-tricyclohexyl-1,3,5-benzenetricarboxylamide (TCB, Scheme 1) to PLA can promote the nucleation of the polymer matrix and accelerate the overall crystallization rate [35,36]. The half-time and rate constants of non-isothermal crystallizations carried out at different cooling rates showed that TCB significantly accelerated the process at a very low content of 0.3 wt%. The crystallinity of neat PLA decreased from 47% to 6% when the cooling rate was increased from 1 to 5 ◦C/min (crystallization did not occur at 10 ◦C/min), while for PLA nucleated by TCB, it remained almost constant at 45–51% irrespectively of the cooling rates (2.5–10 ◦C/min).

**Scheme 1.** Chemical structures of *<sup>N</sup>*,*N*,*<sup>N</sup>*"-tricyclohexyl-1,3,5-benzenetricarboxylamide (TCB) and *<sup>N</sup>*,*N*-bis(2-hydroxyethyl)-terephthalamide (BHET).

Three unique crystal superstructures, including cone-like, shish-kebab, and needle-like structures, were obtained by the melt crystallization of PLLA nucleated by TCB [37]. *<sup>N</sup>*,*N*,*<sup>N</sup>*"-tricyclohexyl-1,3,5-benzenetricarboxylamide dissolved in the polymer melt self-organizes upon cooling into fine fibrils prior to PLLA crystallization. The fibrils serve, subsequently, as a "shish" to induce the epitaxial growth ("soft templating") of "kebab-like" structures approximately orthogonal to the long axis (Figure 1).

**Figure 1.** Left panel: Polarized Optical Microscopy (POM) micrographs of the crystal morphology for poly(l-lactide) (PLLA) containing different amounts of *<sup>N</sup>*,*N*,*<sup>N</sup>*"-tricyclohexyl-1,3,5-benzenetricarboxylamide (TCB): (**a**) neat PLLA, (**b**) 0.2 wt%, (**c**) 0.3 wt% and (**d**) 0.5 wt% prepared by isothermal crystallization at 130 ◦C for 55, 7, 10 and 10 min, respectively, and Atomic Force Microscope (AFM) height (**e**) and phase (**f**) images of PLLA containing 0.2 wt% TCB of a typical shish-kebab-like superstructure. Right panel: schematic representation of the evolution of crystal morphologies during the crystallization of PLLA containing TCB: (**A1**) 0.2 wt%, (**A2**) 0.3 wt% and (**A3**) 0.5 wt% [37]. Reprinted with permission from Macromolecules. Copyright (2011), American Chemical Society.

The effect of TCB on the crystallization behaviour of PLA was studied on a molecular level by time-resolved FTIR and wide angle X-ray diffraction [38]. The observed vibrational changes indicate that the arylamide molecules can accelerate the formation of a skeletal conformational-ordered structure but their presence is more favourable to the formation of a 103 helix structure that is characteristic of α and α' crystals. The value of the Avrami exponent was lower for PLA nucleated by TCB than that of neat PLA, indicating changes in the crystallization mechanism, although they had no impact on the crystal form. It was also shown that 0.3 wt% of TCB effectively increased the crystallization rate and yield upon cooling from melt at 10 ◦C/min (Figure 2).

**Figure 2.** Differential scanning calorimetry (DSC) cooling traces recorded for polylactide (PLA) and its compositions with TCB (0.1 and 0.3 wt%) [38]. Adapted from Polymer Testing. Copyright (2017), with permission from Elsevier.

The effect of the melting temperatures (ranging from 190 to 240 ◦C), concentrations of self-assembling TCB (0–0.5 wt%), and cooling rates (2.5–20 ◦C/min) was investigated for PLA/TCB mixtures [39]. The solubility of TCB was largely dependent on the processing temperature and the concentration of the compound in the polyester matrix. At 240 ◦C, TCB could dissolve completely and then self-assemble into supramolecular frameworks upon cooling. The crystallization peak temperature of PLA showed a bell-shaped dependence on the concentration of TCB and the cooling rate applied. TCB was also used as a self-assembly nucleating agen<sup>t</sup> for melt-blended PLA/poly(ethylene oxide) (PEO) [40]. Both PEO and TCB exhibited a synergistic effect on promoting PLA crystallization as well as a toughening effect on the blended material (Figure 3). Moreover, TCB prominently reinforced both neat PLA and PLA/PEO blends in the glass transition region and at T > Tg, indicating an improvement of their heat resistance. The cooperative effect on promoting PLA crystallization was explained by nucleation with TCB and plasticization with PEO chains.

*<sup>N</sup>*,*N*-Bis(2-hydroxyethyl)-terephthalamide (BHET, Scheme 1) can be a versatile additive for PLLA, operating both as a plasticizer for processing purposes and as a nucleating agen<sup>t</sup> [41]. BHET crystallizes from the PLA melt during cooling, and the formed crystals facilitate heterogeneous nucleation in the PLA matrix. The formation of BHET crystals with a high surface area-to-volume ratio is favoured at high undercooling/supersaturation. Very importantly, it was proved that the hydroxyl groups of BHET were not involved in transesterification with polylactide chains during extrusion at 200 ◦C, and the molecular weight of the polyester was not changed. When allowed to crystallize during processing, BHET induced the formation of PLA crystals oriented along the flow direction, enhancing the tensile modulus of the blend. Interestingly, a rapid cooling to T < Tg prevented the crystallization of BHET and only a plasticizing effect was indicated by a decrease in both the melt viscosity and glass-transition temperature. A characteristic suppression in the yield point of the amorphous PLA/BHET blend was also observed during mechanical tests with increasing BHET concentration.

**Figure 3.** Comparison of tensile stress–strain curves of injection-moulded neat PLA and PLA/poly(ethylene oxide) (PEO) blends with or without TCB [40]. Adapted with permission from Springer Nature: Journal of Thermal Analysis and Calorimetry. Copyright (2018).
