**2. Organic Nucleators**

Several groups of organic compounds, such as aryl amide and hydrazide derivatives and relative compounds, can dissolve into PLA melt and then separate and self-organize into supramolecular frameworks upon cooling [31–34]. The molecular mechanism of such interactions involves hydrogen bonding between C=O in the polylactide macromolecules and the hydrogen bond donating groups (-OH, -NH, and -NH2) in the nucleating agents. Those compounds should also have relatively high thermal decomposition temperatures. Self-assembly nucleators have been increasingly used to guide the crystallization of PLA. The heterogeneous nucleation of the polymer matrix is based on crystallization of those organic compounds soluble in PLA melt. The process of organic crystal formation involves nucleation and growth. It makes the crystallite morphology highly dependent on the applied cooling rates, concentrations, and supersaturation. The nucleating efficiency of self-assembling nucleators is largely determined by their surface area-to-volume ratio after crystallization. In the following sections, the PLA nucleation ability of arylamides and hydrazides, orotic acid, and 1,3:2,4-dibenzylidene-d-sorbitol is discussed in comparison to humic acids and fulvic acids.
