Inverse Vulcanized Polymers with Shape Memory,Enhanced Mechanical Properties,and Vitrimer Behavior

The invention of inverse vulcanization provides great opportunities for generating functional polymers directly from elemental sulfur, an industrial by-product. However, unsatisfactory mechanical properties have limited the scope for wider applications of these exciting materials. Here, we report an effective synthesis method that significantly improves mechanical properties of sulfur-polymers and allows control of performance. A linear pre-polymer containing hydroxyl functional group was produced, which could be stored at room temperature for long periods of time. This pre-polymer was then further crosslinked by difunctional isocyanate secondary crosslinker. By adjusting the molar ratio of crosslinking functional groups, the tensile strength was controlled, ranging from 0.14±0.01 MPa to 20.17±2.18 MPa, and strain was varied from 11.85±0.88 % to 51.20±5.75 %. Control of hardness, flexibility, solubility and function of the material were also demonstrated. We were able to produce materials with suitable combination of flexibility and strength, with excellent shape memory function. Combined with the unique dynamic property of S−S bonds, these polymer networks have an attractive, vitrimer-like ability for being reshaped and recycled, despite their crosslinked structures. This new synthesis method could open the door for wider applications of sustainable sulfur-polymers.     

Inverse Vulcanized Polymers with Shape Memory,Enhanced Mechanical Properties,and Vitrimer Behavior

The invention of inverse vulcanization provides great opportunities for generating functional polymers directly from elemental sulfur, an industrial by‐product. However, unsatisfactory mechanical properties have limited the scope for wider applications of these exciting materials. Here, we report an effective synthesis method that significantly improves mechanical properties of sulfur‐polymers and allows control of performance. A linear pre‐polymer containing hydroxyl functional group was produced, which could be stored at room temperature for long periods of time. This pre‐polymer was then further crosslinked by difunctional isocyanate secondary crosslinker. By adjusting the molar ratio of crosslinking functional groups, the tensile strength was controlled, ranging from 0.14±0.01 MPa to 20.17±2.18 MPa, and strain was varied from 11.85±0.88 % to 51.20±5.75 %. Control of hardness, flexibility, solubility and function of the material were also demonstrated. We were able to produce materials with suitable combination of flexibility and strength, with excellent shape memory function. Combined with the unique dynamic property of S?S bonds, these polymer networks have an attractive, vitrimer‐like ability for being reshaped and recycled, despite their crosslinked structures. This new synthesis method could open the door for wider applications of sustainable sulfur‐polymers.     

A Double Network Hydrogel with High Mechanical Strength and Shape Memory Properties

Double network (DN) hydrogels as one kind of tough gels have attracted extensive attention for their potential applications in biomedical and load-bearing fields. Herein, we import more functions like shape memory into the conventional tough DN hydrogel system. We synthesize the PEG-PDAC/P(AAm-co-AAc) DN hydrogels, of which the first network is a well-defined PEG (polyethylene glycol) network loaded with PDAC (poly(acryloyloxyethyltrimethyl ammonium chloride)) strands, while the second network is formed by copolymerizing AAm (acrylamide) with AAc (acrylic acid) and cross-linker MBAA (N, N'-methylenebisacrylamide). The PEG-PDAC/P(AAm-co-AAc) DN gels exhibits high mechanical strength. The fracture stress and toughness of the DN gels reach up to 0.9 MPa and 3.8 MJ/m³, respectively. Compared with the conventional double network hydrogels with neutral polymers as the soft and ductile second network, the PEG-PDAC/P(AAm-coAAc) DN hydrogels use P(AAm-co-AAc), a weak polyelectrolyte, as the second network. The AAc units serve as the coordination points with Fe³⁺ ions and physically crosslink the second network, which realizes the shape memory property activated by the reducing ability of ascorbic acid. Our results indicate that the high mechanical strength and shape memory properties, probably the two most important characters related to the potential application of the hydrogels, can be introduced simultaneously into the DN hydrogels if the functional monomer has been integrated into the network of DN hydrogels smartly.     

PCL-based Shape Memory Polymers with Variable PDMS Soft Segment Lengths

Thermoresponsive shape memory polymers (SMPs) are stimuli-responsive materials that return to their permanent shape from a temporary shape in response to heating. The design of new SMPs which obtain a broader range of properties including mechanical behavior is critical to realize their potential in biomedical as well as industrial and aerospace applications. To tailor the properties of SMPs, "AB networks" comprised of two distinct polymer components have been investigated but are overwhelmingly limited to those in which both components are organic. In this present work, we prepared inorganic-organic SMPs comprised of inorganic polydimethyl-siloxane (PDMS) segments of varying lengths and organic poly(ε-caprolactone) (PCL) segments. PDMS has a particularly low T(g) (-125 °C) which makes it a particularly effective soft segment to tailor the mechanical properties of PCL-based SMPs. The SMPs were prepared via the rapid photocure of solutions of diacrylated PCL(40)-block-PDMS(m)-block-PCL(40) macromers (m = 20, 37, 66 and 130). The resulting inorganic-organic SMP networks exhibited excellent shape fixity and recovery. By changing the PDMS segment length, the thermal, mechanical, and surface properties were systematically altered.     

Thermosetting Shape Memory Polymers and Composites Based on Polybenzoxazine Blends,Alloys and Copolymers

When dealing with smart polymers, in particular with shape memory polymers, the polymer type and composition specify the overall material properties and in particular the extent of the shape memory effect. Polybenzoxazines as a polymer with high potential for structural applications represent a promising component for materials with both shape memory effect and structurally interesting material properties. This minireview gives insight into how the shape memory effect, in particular the shape recovery event, is influenced by internal factors such as polymer structure, morphology and external factors such as filler addition.     

A Facile Strategy to Fabricate Thermoresponsive Polymer Functionalized CdTe/ZnS Quantum Dots: Assemblies and Optical Properties

Novel thermoresponsive CdTe/ZnS quantum dots (QDs) decorated with a copolymer ligand (CPL) containing 8‐hydroxyquinoline and NIPAM units are prepared through coordinate bonding in aqueous solution. The dependence of the morphology and optical properties of the QDs/CPL assemblies formed via coordinate bonding on the experimental conditions is studied. The coordinate induced self‐assemblies are observed by controlling the molar ratio of QDs and CPL. The self‐organized structure of QDs/CPL proceeds through a first step of QDs‐chains, followed by a necklace‐like single annular chain, and subsequently increases its annular chain structure, forming a network. The CPL functionalized QDs can emit multiple colors from the cooperating interaction between the inherent emission (606 nm) of the QDs and the surface‐coordinated emission (517 nm) of the CPL complex formed on the QD surface. For QDs‐CPL systems, both Förster resonance energy transfer (FRET) and a high rate of photoinduced electron transfer (PET) are simultaneous, the latter mainly contributing to PL quenching. The thermoresponsive QDs/CPL assemblies also exhibit dual reversible PL properties between the inherent emission of QDs and surface‐coordinated emission.

     

新型光、 热双响应形状记忆聚合物的制备与性能

通过多巴胺表面原位聚合反应修饰玻璃微珠, 利用X光电子能谱仪(XPS)和傅里叶变换红外光谱仪 (FTIR)对修饰前后玻璃微珠表面的化学组成进行了表征, 用热失重分析仪(TGA)对其热稳定性进行了测试, 并利用透射电子显微镜(TEM)和扫描电子显微镜(SEM)对其形貌进行了观察; 研究了改性玻璃微珠对形状记忆共混物聚己内酯和聚氨酯(PCL/TPU)的热性能、 力学性能和形状记忆性能的影响. 结果表明, 成功制备了表面包覆聚多巴胺的玻璃微珠(PHGM), 改性玻璃微珠的加入不仅增强了复合材料的力学性能(当改性玻璃微珠含量为3%时, 材料的拉伸强度提高到53.3 MPa, 杨氏模量提高到178.4 MPa), 还赋予了复合材料优异的光热效应. 所制备的形状记忆复合材料在808 nm近红外光的照射下, 可以在短时间内(7 s)升高到材料的开关温度并回复到初始形状.     

合成不同拓扑结构聚合物的一个简便策略

随着高分子化学的进步, 特别是可控自由基聚合反应的发现, 不同拓扑结构聚合物的合成己获得了巨大的进展. 为合成这些复杂结构的聚合物, 人们成功地研究了不同的聚合方法. 其中, 不同功能团引发剂方法是较为方便和可行的方法之一, 并且已广泛用于不同结构聚合物的合成. 本文综述了用该方法合成不同拓扑结构聚合物, 包括嵌段、杂臂星形、H-形、π-形、接枝和环状聚合物所取得的进展, 特别强调的是这篇论文没有覆盖这方面的所有研究工作. 这一合成方法的思路是基于将不同聚合反应, 尤其是不同可控聚合反应相结合, 或者将可控聚合与不同的偶合反应, 例如与点击反应、Diel-Alder反应和活性酯反应等相结合. 所讨论的多功能引发剂不仅包括小分子化合物, 也包括带有两个和两个以上引发基团或链转移基团的聚合物. 后者称为大分子多功能团引发剂, 可以独立引发不同的聚合反应.     

Enhancing the Mechanical Properties of Electrospun Nanofiber Mats through Controllable Welding at the Cross Points

This communication describes a simple and effective method for welding electrospun nanofibers at the cross points to enhance the mechanical properties of their nonwoven mats. The welding is achieved by placing a nonwoven mat of the nanofibers in a capped vial with the vapor of a proper solvent. For polycaprolactone (PCL) nanofibers, the solvent is dichloromethane (DCM). The welding can be managed in a controllable fashion by simply varying the partial pressure of DCM and/or the exposure time. Relative to the pristine nanofiber mat, the mechanical strength of the welded PCL nanofiber mat can be increased by as much as 200%. Meanwhile, such a treatment does not cause any major structural changes, including morphology, fiber diameter, and pore size. This study provides a generic method for improving the mechanical properties of nonwoven nanofiber mats, holding great potential in various applications.

     

A Mechanistic Perspective on Plastically Flexible Coordination Polymers

Mechanical flexibility in single crystals of covalently bound materials is a fascinating and poorly understood phenomenon. We present here the first example of a plastically flexible one‐dimensional (1D) coordination polymer. The compound [Zn(μ‐Cl)₂(3,5‐dichloropyridine)₂]_n is flexible over two crystallographic faces. Remarkably, the single crystal remains intact when bent to 180°. A combination of microscopy, diffraction, and spectroscopic studies have been used to probe the structural response of the crystal lattice to mechanical bending. Deformation of the covalent polymer chains does not appear to be responsible for the observed macroscopic bending. Instead, our results suggest that mechanical bending occurs by displacement of the coordination polymer chains. Based on experimental and theoretical evidence, we propose a new model for mechanical flexibility in 1D coordination polymers. Moreover, our calculations propose a cause of the different mechanical properties of this compound and a structurally similar elastic material.     

Mechanical and thermal properties of green polylactide composites with natural fillers

Green composites of PLA with micropowders derived from agricultural by-products such as oat husks, cocoa shells, and apple solids that remain after pressing have been prepared by melt mixing. The thermal and mechanical properties of the composites, including the effect of matrix crystallization and plasticization with poly(propylene glycol), have been studied. All fillers nucleated PLA crystallization and decreased the cold-crystallization temperature. They also affected the mechanical properties of the compositions, increasing the modulus of elasticity but decreasing the elongation at break and tensile impact strength although with few exceptions. Plasticization of the PLA matrix improved the ductility of the composites.     

Microstructure,Mechanical and Electrical Properties of Hybrid Copper Matrix Composites with Fe Microspheres and rGO Nanosheets

Copper matrix composites have a wide application as magnetic conductive materials, electromagnetic materials, electrical discharge machining materials, etc. Such materials are expected to have a good combination of excellent electrical conductivity and good mechanical strength. In this work, micro/nano hybrid reinforcements with Fe microspheres and reduced graphene oxide (rGO) nanosheets were developed for copper matrix composites. The rGO/Fe/Cu powders were firstly wet-mixed and then densified by the vacuum hot-pressing sintering to obtain the bulk compacts. Microstructure, electrical conductivity and mechanical properties of such compacts were investigated. Microstructural result of as-sintered compacts shows that the Fe microspheres could distribute in the matrix uniformly, and rGO nanosheets exhibit both agglomerated and dispersed states. The grain size of Cu matrix decreased with the increase of the rGO content. Hardness, compression and tensile 0.2% yield strength of the as-sintered compacts were improved evidently by the addition of the hybrid Fe/rGO, comparing with pure Cu and single Fe-added composites. However, a lower electrical conductivity appeared in the more rGO-added composites, but still reached more than 33.0% international annealing copper standard (IACS). These performance change could be sought in the spatially geometrical distribution and characteristics of such micro/nano Fe/rGO hybrid addition, and the relevant mechanisms were discussed.     

Semi‐IPNs with Moisture‐Triggered Shape Memory and Self‐Healing Properties

Moisture or water has the advantages of being green, inexpensive, and moderate. However, it is challenging to endow water‐induced shape memory property and self‐healing capability to one single polymer because of the conflicting structural requirement of the two types of materials. In this study, this problem is solved through introducing two kinds of supramolecular interactions into semi‐interpenetrating polymer networks (semi‐IPNs). The hydrogen bonds function as water‐sensitive switches, making the materials show moisture‐induced shape memory effect. The host–guest interactions (β‐cyclodextrin‐adamantane) serve as both permanent phases and self‐healing motifs, enabling further increased chain mobility at the cracks and self‐healing function. In addition, these polyvinylpyrrolidone/poly(hydroxyethyl methacrylate‐co‐butyl acrylate) semi‐IPNs also show thermosensitive triple‐shape memory effect.

     

Mechanical Properties and Dynamic Mechanical Analysis of Thermoplastic-Natural Fiber/Glass Reinforced Composites

The mechanical properties and dynamic behavior of thermoplastic composites based on polypropylene/glass fibers and polypropylene/natural fibers (i.e. kenaf, hemp, flax) are presented. A survey is given on some aspects, crucial for the use of these composites in structural and non-structural components such as their vibration-damping response, in relation to the composite compaction level and the manufacturing procedure. In order to investigate a wide vibration frequency range, including acoustic frequencies, different testing techniques, both with forced and free vibrations, were applied. A comparison between natural fiber and glass fiber reinforced laminates is presented. Compaction levels, allowing to obtain the best compromise between mechanical performance and damping response, are investigated.     

橡胶木粉填充LLDPE复合材料的结构和力学性能

用橡胶木粉填充线性低密度聚乙烯(LLDPE),研究了酸、碱溶液预处理木粉的效果和硅烷偶联荆(KH-570)、MMA接枝的天然橡胶胶乳(MGL-30)两种改性剂对橡胶木粉表面改性的效果,以及未粉粒径和填充量等对木粉／LLDPE复合材料力学性能的影响,并用SEM对复合材料拉伸断面的形态结构进行了分析。结果表明：木粉的拉径、木粉填充量和改性荆用量对复合材料的力学性能有较大的影响,经碱溶液预处理再用改性荆改性后的木粉能有效地改善木粉与LLDPE的界面粘结强度,提高橡胶木粉／LLDPE复合材料的力学性能。     

A Facile Approach Toward Scalable Fabrication of Reversible Shape‐Memory Polymers with Bonded Elastomer Microphases as Internal Stress Provider

The present communication reports a novel strategy to fabricate reversible shape‐memory polymer that operates without the aid of external force on the basis of a two‐phase structure design. The proof‐of‐concept material, crosslinked styrene‐butadiene‐styrene block copolymer (SBS, dispersed phase)/polycaprolactone‐based polyurethane (PU, continuous phase) blend, possesses a closely connected microphase separation structure. That is, SBS phases are chemically bonded to crosslinked PU by means of a single crosslinking agent and two‐step crosslinking process for increasing integrity of the system. Miscibility between components in the blend is no longer critical by taking advantage of the reactive blending technique. It is found that a suitable programming leads to compressed SBS, which serves as internal expansion stress provider as a result. The desired two‐way shape‐memory effect is realized by the joint action of the temperature‐induced reversible opposite directional deformabilities of the crystalline phase of PU and compressed SBS, accompanying melting and orientated recrystallization of the former. Owing to the broadness of material selection and manufacturing convenience, the proposed approach opens an avenue toward mass production and application of the smart polymer.