Usually, the nature of surface-induced nucleation in polymer blends is not easily disclosed. A novel approach for studying surface-induced crystallization in blends of semicrystalline polymers is proposed here. It consists of detecting variations in the crystallization kinetics of the dispersed phase with changing the crystalline state of the matrix through self-nucleation. It can be used only when the dispersed phase has a lower melting temperature than the matrix phase. As a case study, the crystallization behavior of dispersed polyethylene droplets in a polypropylene matrix was investigated. An enhancement of crystallization kinetics of polyethylene was achieved when the lamellar thickness of polypropylene increased, and it was proved by the formation of a transcrystalline layer of polyethylene at the interface, as observed by scanning electron microscopy. Compared to the self-nucleated neat polyethylene, the efficiency of the nucleating effect of polypropylene toward polyethylene was estimated around 140%. This result together with a very low value for the interfacial free energy difference as obtained from isothermal crystallization measurements is evidence that such surface-induced nucleation occurs through epitaxial growth. Moreover, a mechanism of polyethylene nuclei formation through epitaxy, which was proposed in the literature, was proved to be valid for blends of the two polymers through small- and wide-angle X-ray scattering structural analysis. While epitaxy between polyethylene and polypropylene was previously shown only for ideal systems such as thin-layered films, it is hereby reported for common melt mixed blends of the two polyolefins.
This work presents the first investigation on the crystallization behavior of partially wet droplets in immiscible ternary blends. Poly(lactide), poly(ε-caprolactone), and poly(butylene succinate) (PLA, PCL, and PBS, respectively) were melt blended in a 10/45/45 weight ratio to produce a "partial wetting" morphology with droplets of the PLA minor phase located at the interface between the other two major components. The crystallization process of the higher melting PLA droplets was studied by polarized light optical microscopy, while the other components remain in the molten state. We found that neighboring partially wet droplets nucleate in close sequence. This is unexpected since partially wet droplets display points of three-phase contact and, hence, should not touch each other. Moreover, the onset of poly(lactide) crystallization is frequently observed at the interface with molten PCL or PBS, with a significant preference for the former polymer. The observed sequential droplet-to-droplet crystallization is attributed to the weak partial wetting behavior of the PCL/PLA/PBS ternary system. In fact, the contact between the interfacially confined droplets during crystallization due to their mobility can lead to a transition from a partial to a completely wet state, with the formation of thin continuous layers bridging larger partially wet droplets. This allows crystallization to spread sequentially between neighboring domains. Using a simple heterogeneous nucleation model, it is shown that the nucleation of PLA on either PCL or PBS melts is energetically feasible. This study establishes a clear relationship between the unique partial wetting morphology of ternary blends and the nucleation of the minor component, paving the way to the understanding and control of crystallization in multiphasic polymer blends for advanced applications.
International newspapers and experts have called 3D printing the industrial revolution of this century. Among all its available variants, the fused deposition modeling (FDM) technique is of greater interest since its application is possible using simple desktop printers. FDM is a complex process, characterized by a large number of parameters that influence the quality and final properties of the product. In particular, in the case of semicrystalline polymers, which afford better mechanical properties than amorphous ones, it is necessary to understand the crystallization kinetics as the processing conditions vary, in order to be able to develop models that allow having a better control over the process and consequently on the final properties of the material. In this work it was proposed to study the crystallization kinetics of two different polyamides used for FDM 3D printing and to link it to the microstructure and properties obtained during FDM. The kinetics are studied both in isothermal and fast cooling conditions, thanks to a home-built device which allows mimicking the quenching experienced during filament deposition. The temperature history of a single filament is then determined by mean of a micro-thermocouple and the final crystallinity of the sample printed in a variety of conditions is assessed by differential scanning calorimetry. It is found that the applied processing conditions always allowed for the achievement of the maximum crystallinity, although in one condition the polyamide mesomorphic phase possibly develops. Despite the degree of crystallinity is not a strong function of printing variables, the weld strength of adjacent layers shows remarkable variations. In particular, a decrease of its value with printing speed is observed, linked to the probable development of molecular anisotropy under the more extreme printing conditions.
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