*2.2. Hydrazide Nucleators*

Aryl hydrazides used as nucleators for the enhancement of polylactide crystallization have two characteristic structural features—a carbohydrazide linkage (-C(O)NH-NHC(O)-) that can take part as an acceptor/donor during the formation of hydrogen bonds, and at least one aryl group providing a system of π-electrons for π-π interactions (Scheme 2). The combination of these two functions allows for an effective supramolecular organization of hydrazide derivatives within polymer melts. Even the small molecule of phthalhydrazide (PH, Scheme 2) can be an efficient nucleating agen<sup>t</sup> that enhances the crystallization of polylactide. It was reported that the non-isothermal melt crystallization of PLA started much earlier in the presence of even a low content of PH (0.1 wt%) [42]. The analysis of Avrami plots indicated that the crystallization mechanism was not changed. The overall effect was ascribed to the increased number of nucleation sites.

However, larger hydrazide derivatives built of at least two aryl groups linked by two carbohydrazide systems separated by an alkyl or aryl spacer are much more effective. They can crystallize within the polymer melt and form structures that can serve as templates for growing polymer crystals. Tetramethylenedicarboxylic dibenzoylhydrazide (TMC, *n* = 2, Scheme 2)—the shortest dihydrazide—exhibited an excellent nucleating effect on PLA. With the addition of 0.05 wt% of TMC, the crystallization half-time of PLA decreased from 26.06 to 6.13 min at 130 ◦C [43]. TMC did not change the crystallization mechanism in the PLA matrix, as indicated by comparison of the Avrami values. However, the density of nuclei in the PLA was increased, while no discernible effect on the crystalline structure was noted. Non-isothermal crystallization studies indicated that *<sup>N</sup>*,*N*-bis(benzoyl)suberic acid dihydrazide (BSDH, *n* = 4, Scheme 2) can accelerate the overall PLLA crystallization rate due to a heterogeneous nucleation effect. With the incorporation of BSDH, the PLLA crystallization peak became sharper and was shifted to a higher temperature range as the sample was cooled down from the melt at a rate of 1 ◦C/min [44]. The presence of BSDH also affected the isothermal crystalline behaviours (a shorter crystallization time and a faster overall crystallization due to an increased number of small spherulites).

Octamethylenedicarboxylic dibenzoylhydrazide (OMBH, *n* = 6, Scheme 2) was found to be very effective for the acceleration of PLA crystallization under a high cooling rate (50 ◦C/min with 1 wt% of OMBH) [33]. Very short isothermal crystallization half-times were recorded within 90–130 ◦C. The moulding cycle time of PLA/OMBH was <3 min. The physical and mechanical properties of the blend were improved as well (a heat distortion temperature of 124 ◦C, flexural modulus of 4.1 GPa, and Izod impact strength of 7.9 kJ/m2). Decamethylenedicarboxylic dibenzoylhydrazide (DMBH, *n* = 8, Scheme 2) also increased the crystallization temperature (Tc) of PLA [33]. The overall effect of larger dihydrazide molecules is rather complicated and dependent on the crystallization temperature and the crystalline structure. The nucleation efficiency of OMBH significantly depended on its solubility during the thermal annealing (generally increasing with temperature and decreasing with a concentration increase) [45]. However, under certain conditions, the crystallization temperature and nucleation efficiency of OMBH increased with the concentration of the additive, resulting in higher crystallization enthalpy.

**Scheme 2.** Chemical structures of phthalhydrazide and symmetrical or branched di-, tri-, and tetra-hydrazides.

Time-resolved spectroscopic studies on the interactions between OMBH and polylactide elucidated the nucleation process at the molecular level [46]. The results showed that the crystallization of PLLA in the presence of the dihydrazide nucleator involves not only heterogeneous nucleation with OMBH but also the conformational regulation of polyester chains by hydrogen bonding between the two components of the blend (Scheme 3). The results indicated that due to NH···O=C interactions between the dissolved nucleator and PLLA, the building blocks of the PLLA chains were transformed into trans-gauche conformers before the self-assembling of OMBH into nanocrystals and their phase-separation from the PLLA melt. It resulted in a decrease in the energy barrier to the formation of α-crystals of polylactide. Once the dissolved molecules of OMBH start to self-assemble into nanostructures upon cooling, the polylactide chains with an increased population of trans-gauche conformers begin to form primary nuclei on the surface of OMBH nanofibrils and crystals. Therefore, conformational regulation was proposed for the crystalline manipulation of PLLA by hydrazide nucleators.

**Scheme 3.** Crystallization of neat PLLA (**<sup>a</sup>**–**d**) compared to the formation of α-crystals due to "soft templating", and conformational regulation of PLLA chains through supramolecular interactions with octamethylenedicarboxylic dibenzoylhydrazide (OMBH) (**<sup>e</sup>**–**h**) [46]. Adapted with permission from Biomacromolecules. Copyright (2017), American Chemical Society.

It was also shown that OMBH can self-assemble into diverse frameworks that induce the formation of various crystalline superstructures of polylactide, depending on their content in the polymer melt and the processing conditions (Figure 4). Larger amounts of OMBH (1 wt%) first self-assembled into star multiarm frameworks (at 170 ◦C), and then each of the resultant arms served as a "shish" to induce the growth of PLLA lamellae after cooling down to 150 ◦C. The nucleating sites on the surface of such structures are scarce, and thus, the lamellae grew as branched "calabash" structures. A slightly lower amount of OMBH (0.5 wt%) self-assembled at 125.3 ◦C into short fibril-like frameworks with a sufficient number of available nucleating sites. It resulted in a transcrystalline superstructure with PLLA crystals growing epitaxially orthogonally to the long axis of the OMBH fibrils. An interesting sunflower-like superstructure with big PLLA spherulite centres, surrounded by hybrid fibril-like trans-crystalline superstructures at the boundary between the spherulite and the amorphous region, was obtained in the presence of a very small amount of OMBH (0.3 wt%).

**Figure 4.** POM (**A**1–**C**1) and SEM micrographs (**A**2–**C**2 and **A3**–**C**3) of the crystalline superstructures formed by PLLA in the presence of OMBH (A: 1 wt%; B: 0.5 wt%; C: 0.3 wt%) after different thermal treatments [47]. Reprinted with permission from Biomacromolecules. Copyright (2017), American Chemical Society.

The crystallization of polylactive was also studied in the presence of more structurally complicated dihydrazides. The studies revealed that both the cooling rate and the melting temperature affected the non-isothermal crystallization behaviour of PLLA in the presence of *N*,*N*-succinic bis(hydrocinnamic acid) dihydrazide (BHSH, Scheme 2) [47] and *N*,*N*-sebacic bis(hydrocinnamic acid) dihydrazide (HAD, Scheme 2) [48]. BHSH accelerated both the melt crystallization and the non-isothermal crystallization of PLLA. At a content of 2 wt%, the half time of crystallization at 100 ◦C decreased by twelve times, compared to that of neat PLLA (48.6 vs. 575.7 s). HAD (1 wt%) increased both the crystallization temperature and non-isothermal crystallization enthalpy from 94.5 ◦C and 0.1 J/g (neat PLLA) to 131.6 ◦C and 48.5 J/g, respectively. It was also found that the cold crystallization behaviour of PLLA/HAD was almost independent of the HAD concentration, when it was larger than 2 wt%. Although those compounds vastly improved the crystallization of PLLA, their presence decreased the thermal stability and light transmittance of the PLLA films (Figure 5). This effect was ascribed to the increased crystallinity of the blends as well as the colour of the additives.

**Figure 5.** Light transmittance of PLLA and its composites with bis(hydrocinnamic acid) dihydrazide (BHSH) [47]. Reprinted with permission from Springer Nature: Polymer Science, Series A. Copyright (2019).

The non-isothermal crystallization of PLLA was also significantly promoted by the presence of 1 wt% of *N*,*N*-bis(1H-benzotriazole) adipic acid acethydrazide (BA, Scheme 2) [49], 0.5 wt% of *N*,*N*,*N*-tris(1H-benzotriazole) trimesinic acid acethydrazide (BD, Scheme 2) [50], and 3 wt% of *<sup>N</sup>*,*N*,*N*,*N*-salicylic tetra(1,2,4,5-benzenetetracarboxylic acid) hydrazide (BAS, Scheme 2) [51]. Upon cooling at a rate of 1 ◦C/min, the onset of crystallization temperature was shifted from 101.4 to 111.3 ◦C upon the addition of 0.5 wt% BD to the PLLA. The enthalpy of non-isothermal crystallization increased from 0.1 to 38.6 J/g. The isothermal crystallization half-time decreased as well. An enhanced nucleation density was indicated by double-melting peaks of PLLA/0.5% BD, assigned to melting-recrystallization. The equilibrium melting point of the PLLA/0.5% BD blend was set at lower temperatures than those of neat PLLA. A similar effect was observed for blends of PLLA and BAS. This additive is a highly efficient nucleating agen<sup>t</sup> that can significantly promote the crystallization of PLLA upon cooling at a rate of 20 ◦C/min. The best acceleration of the melt crystallization of PLLA carried out upon cooling at 1 ◦C/min was achieved with 3 wt% of BAS. The final melting temperature was another important factor. The best temperature for the melt-crystallization of the blend was 200 ◦C. Despite the improvement of the thermal stability and fluidity compared with the neat PLLA, a decrease in the light transmittance through the PLLA/BAS blends was observed.

Hydrazide nucleating species were also obtained by an in situ reaction between 4,4-diphenylmethane diisocyanate (MDI) and benzohydrazine (P) in the polylactide matrix (Scheme 4) [52]. The rate of PLLA crystallization was enhanced by blending those reagents with the polyester upon melting. The reaction between the components was confirmed by NMR spectroscopy. This procedure increased the compatibility between the nucleating agents and the polylactide matrix. The crystallinity of the PLLA increased from 10.3% to 42.1% upon adding 0.25% (MDI + P) and melting them for 8 min. The PLLA crystallization half-time at 130 ◦C decreased from 42.0 to 1.1 min. Moreover, the heat resistance of the PLA was enhanced and good mechanical properties were achieved.

**Scheme 4.** Postulated pathway of the reaction between 4,4-diphenylmethane diisocyanate, benzohydrazine, and PLLA.

## *2.3. PLA*/*Orotic Acid Blends*

Orotic acid (2,6-dioxo-1,2,3,6-tetrahydropyrimidine-4-carboxylic acid, OA) (Scheme 5) is a compound synthesized by living organisms as a key substance in the biosynthesis of naturally occurring pyrimidines [53,54]. Owing to its hydrogen-bond donating/accepting properties, it may also serve as an excellent biobased nucleating agen<sup>t</sup> for polylactide. The temperature of the melting of OA (345–346 ◦C) is high enough to allow melt compounding with PLA.

**Scheme 5.** Chemical structure of 2,6-dioxo-1,2,3,6-tetrahydropyrimidine-4-carboxylic acid (orotic acid).

Both the non-isothermal and isothermal melt crystallization kinetics of PLLA were enhanced significantly by orotic acid, even at a very low content (0.3 wt%) [55]. The presence of OA e ffectively induced the crystallization of PLLA upon fast cooling (10 ◦C/min) (Figure 6). The crystallization peak temperature was 123.9 ◦C and the crystallization enthalpy was equal to 34.1 J/g when the sample was crystallized non-isothermally from melt at a cooling rate of 10 ◦C/min. The half-time of isothermal crystallization at 120 ◦C was 0.64 min.

**Figure 6.** A comparison of DSC cooling (10 ◦C/min) traces of neat PLLA and the composite containing 0.3 wt% of OA (OA-0.3) [55]. Reprinted with permission from Industrial & Engineering Chemistry Research. Copyright (2011), American Chemical Society.

Tests with anhydrous (OA-a) and monohydrated (OA-m) orotic acid were carried out to gain an insight into the role that water molecules play in the activity of orotic acid as a nucleating agen<sup>t</sup> in PLA crystallization [56]. The compounds were mixed with PLLA melt, and their nucleation e ffectiveness in the non-isothermal and isothermal melt crystallization of PLLA was investigated. Although OA-a showed more prominent nucleation e fficiency than OA-m, both forms of orotic acid improved the nucleation density, the degree of crystallinity of the PLLA, and the overall crystallization rate. The data derived from Avrami plots (Figure 7) sugges<sup>t</sup> a simultaneous nucleation and a three-dimensional crystal growth. The number of spherulite nucleation sites increases with increasing OA content, while their size decreases. The Avrami exponent *n*, indicating the nature of nucleation and the dimensionality of crystal growth, is very close to 3 regardless of the degree of orotic acid hydration. Nevertheless, the molecules of water bound to OA-m and its dehydration transition seem to decrease the nucleation e ffect. The nucleation density is higher and the spherulite size distribution is more uniform in PLLA/OA-a blends. This can be linked to the better dispersion and more uniform distribution of active nucleating agents in the blends.

The isothermal crystallization behaviour of PLLA/OA blends was investigated with time-resolved FTIR spectroscopy [56]. The spectral evolution of conformational changes and chain packing during thermal annealing at 140 ◦C showed that for both PLLA admixed with OA-a and OA-m, the crystallization proceeded with changes in the interchain interactions that were followed by the formation of the 103 helix. However, the induction time and crystallization half-times were much shorter for the sample containing anhydrous OA. The formation of crystal structure is similar to that observed for pure PLLA at, for example, 120 ◦C, despite the fact that without OA, PLLA hardly crystallized at 140 ◦C. The photodegradation and biodegradation of PLA in the presence of orotic acid

were also studied [57]. Rheological measurements showed that OA slightly enhanced the UV-induced cleavage of polyester chains. Photodegradation was shown to accelerate the subsequent biodegradation, as macromolecules of smaller molecular weight are more prone to decomposition. Orotic acid itself promoted biodegradation both in native and photodegraded samples, by shortening the lag phase and increasing the rate of the process.

**Figure 7.** Avrami plots for blends of PLLA and (**a**) anhydrous or (**b**) monohydrate orotic acid isothermally melt-crystallized at 130 ◦C [56]. Published by The Royal Society of Chemistry (2017).
