Nucleation Mechanism of α-Cyclodextrin-Enhanced Crystallization of Some Semicrystalline Aliphatic Polymers

The nucleation effects of α-cyclodextrin (α-CD) and its inclusion complexes (ICs) with poly(ε-caprolactone) (PCL), poly(ethylene glycol) (PEG), and poly(butylene succinate) (PBS) on the crystallization of PCL, PEG, and PBS were investigated by differential scanning calorimetry (DSC) and polarized optical microscopy. The formations of ICs of α-CD with PCL, PEG, and PBS were characterized by DSC, Fourier transform infrared spectroscopy, wide-angle X-ray diffraction (WAXD), and solution 1H NMR. The WAXD studies showed that all of the α-CD-ICs adopt a channel structure. Both DSC and 1H NMR results suggested that the polymer chains were partially complexed by α-CDs. Thus, the polymer chains in the bulk phase can contact not only the exterior of the α-CD macrocycle but also the end parts of polymer chains protruding from the α-CD cavity and/or the free chain segments uncovered by α-CD, when the α-CD-IC as a nucleating agent is added into the polymers. Therefore, the difference in the interfacial interaction between bulk polymers and the nucleating agents should be responsible for the difference in the nucleation effects. α-CD-PCL-IC particles have little nuclear formation effect on the crystallization of bulk PBS; however, they can greatly accelerate the nucleation and crystallization of the bulk PCL. The same results are also observed during the crystallization of PBS and PEG; that is, PBS-IC and PEG-IC particles can greatly accelerate the nucleation and crystallization of PBS and PEG, respectively. It seemed that IC of a given polymer could greatly enhance the nucleation and crystallization of the polymer itself. This result may be attributable to the limited mobility of the uncovered part of the polymer segments constrained by its interior part resided in the α-CD cavity, leading to the nucleation of polymer crystallization. The effect of fine talc powder was also examined. The experimental results suggested that the nucleation ability of α-CD on the crystallization of the polymer is comparable to that of the talc.