Qinglong Zhang

Core–shell-like structured graphene aerogel encapsulating paraffin: shape-stable phase change material for thermal energy storage

The development of energy storage materials is critical to the growth of sustainable energy infrastructures in the coming years. Here, a composite phase change material (PCM) based on graphene and paraffin was designed and prepared through a modified hydrothermal method. Graphene oxide sheets were reduced and self-assembled into three-dimensional graphene aerogels consisting of numerous hollow graphene cells, and paraffin was simultaneously encapsulated into the cells in the form of micrometer-scale droplets during the hydrothermal process. The resulting core–shell-like structured, composite PCM exhibits a high encapsulation ratio of paraffin, large phase change enthalpy, and excellent cycling performance. Due to the unique encapsulated structure and continuous graphene network in the matrix, such a composite PCM holds a good shape-stable property, which prevents the leakage of paraffin above its melting point. In addition, it inherits the intrinsic thermally and electrically conductive nature of the embedded graphene, and thus shows enhanced thermal and electrical conductivity compared to pure paraffin. This novel composite PCM can realize efficient thermal energy storage and demonstrates the potential to be directly used as an actual thermal storage device without containers.

A new strategy to prepare polymer composites with versatile shape memory properties

A new strategy to prepare multi-shape memory polymers, which utilizes the confined co-crystallization characteristics of small molecules to construct tunable switch, has been proposed. On the basis of this strategy, a novel shape memory polymer comprising paraffin molecules with different chain lengths and styrene-b-(ethylene-co-butylene)-b-styrene (SEBS) was prepared. As paraffin molecules tend to form continuous series of solid solution over a wide range of chain lengths, deemed as co-crystallization, a broad thermal transition may be obtained through mixing paraffin molecules with distinct melting points, and for our shape memory composites, a broad transition is essential for tunable abilities. Thermal properties of the SEBS–paraffin composites have been characterized by differential scanning calorimetry, suggesting excellent confined co-crystallization characteristics, and such characteristics have been further verified by wide angle X-ray diffraction and successive self-nucleation and annealing thermal fractionation analysis. The results of a series of shape memory testing demonstrate that the composites possess versatile shape memory properties, such as temperature memory and multi-shape memory effect. Compared with the reported methods to prepare multi-shape memory polymers, this method not only simplifies the fabrication procedure from raw materials to processing, paving the way for large-scale fabrication, but also offers a controllable approach for adjusting the mechanical properties as well as tailoring the transition temperature.

A Facile Strategy to Fabricate Multishape Memory Polymers with Controllable Mechanical Properties.

A facile blending strategy to fabricate multishape memory polymers (SMPs) with only one sort of phase transition material has been reported. In this work, olefin block copolymer (OBC) and styrene-b-(ethylene-co-butylene)-b-styrene (SEBS), which are both physically crosslinked, are blended with crystalline paraffin together. Due to the different interactions between polymer matrices and paraffin, the paraffin penetrated in OBC and SEBS exhibit separated melting transitions. It is quite interesting that merely paraffin distributed in OBC also shows two distinct melting transitions with enough OBC content in composites. Therefore, excellent quadruple shape memory effect can be achieved with a maximum of three melting transitions. Furthermore, through adjusting the polymer species and content, the mechanical and rheological properties can be conveniently tuned to a great extent. Compared with the reported strategies, this simple and controllable method sheds light on rapid design of multi-SMPs using inexpensive raw materials, which greatly paves the way for multi-SMPs from laboratory to factory.

Formation of banded spherulites and the temperature dependence of the band space in olefin block copolymer

The formation of banded spherulites, which is a representative morphological feature for polymer crystalline aggregates, has attracted great interest during the past few decades. In this study, the crystalline morphologies of a type of olefin block copolymer (OBC) at different crystallization temperatures were observed systematically. It was found that banded spherulites formed at comparatively higher temperatures and the temperature dependence of the band space in OBC-banded spherulites could be divided into two regions: it firstly increased continuously with crystallization temperature between 115 and 119 °C; while beyond 120 °C, the changing tendency of the band space became unapparent and irregular. Scanning electron microscopy and atomic force microscopy confirmed that the alternative negative and positive bands could be attributed to the alternative flat-on and edge-on lamellae in the spherulites. Through analyzing the change of lamellar thickness and long period with temperatures, we speculated that the formation of the intriguing change of band space might be ascribed to the unbalanced surface stress, which was closely correlated to the amorphous layers of the OBC lamellae. We believe that this study contributes to understanding the relationship between the crystalline structure and banding phenomenon for semi-crystalline block copolymers.

Exploring the crystallization-induced mesophase evolution in an olefin block copolymer through a rationally designed two-step isothermal crystallization strategy

The competition between crystallization and phase separation has always been a major concern for semi-crystalline block copolymers. In this work, in order to examine the mesophase evolution during crystallization in an olefin block copolymer (OBC), a “two-step isothermal crystallization” strategy was designed elaborately: the OBC samples were first isothermally crystallized at different temperatures (Tc) to create diverse crystallization histories; after melting, the second crystallization process was carried out to detect the degree of mesophase separation change during the first crystallization. It was quite interesting that the first crystallization at a lower Tc displayed little influence on the second crystallization process, while the first crystallization at a higher Tc could obviously accelerate the subsequent crystallization. In combination with the observation of the crystalline morphologies and phase morphologies in the melt, we speculated that hard blocks mainly crystallized separately during relatively rapid crystallization with a large quantity of nuclei; while crystallization-induced aggregation of hard blocks occurred during slow crystallization with a limited number of nuclei, changing the distribution of the hard blocks and accelerating the subsequent crystallization as a consequence. This work contributes to the understanding of the phase evolution during crystallization of OBC and such multi-block copolymers under different circumstances.