Temperature-Dependent Order-to-Order Transition of Polystyrene-<i>block</i>-poly(ethylene-<i>co</i>-butylene)-<i>block</i>-polystyrene Triblock Copolymer under Multilayered Confinement

The order-to-order transition (OOT) plays a key role in the nanotechnological applications of block copolymer (BCP) and is dramatically dependent on the spatial environment. A multilayer-confined space has been fabricated by layer-multiplying coextrusion technology to investigate the OOT mechanism of polystyrene-<i>block</i>-poly­(ethylene-<i>co</i>-butylene)-<i>block</i>-polystyrene triblock copolymer (SEBS) under multilayered confinement. The parallel oriented ordering front, whose OOT temperature is lower than that of the bulk due to higher free energy, is induced by the “substrate surface effect” in the SEBS layers of the multilayer specimens. The OOT temperature of SEBS is mainly decided by the volume fraction of ordering front. The propagation distance maximum of the “substrate surface effect” is about 220 nm. Only when the thickness of SEBS layer is less than this critical value is the whole SEBS layer fully filled with the ordering front. As a result, the OOT temperature first decreases rapidly and then tends to be a constant value with the decrease of layer thickness. This turning point of layer thickness is found to locate around 220 nm. Finally, the change of transition temperature region with the layer thickness has been explained by the fact that the bulk, thin layer samples (less than turning point) and thick layer samples (more than turning point) have different OOT mechanisms

Ingenious Sandwich-like Adhesive Films and Controllable Introduction of Fluorine-Containing Groups toward Strong Adhesive Strength and Low Dielectric Characteristics

In the manufacturing field of high-frequency print circuit boards (HFPCBs), traditional adhesive films (such as epoxy resin and polyimide adhesive films) have attracted enormous attention because of their superior adhesive property. However, their molecular chains intrinsically contained numerous polar groups (carboxyl, carbonyl, and amino), which markedly increased the dielectric constant (Dk) and dielectric loss (Df) of the adhesive film, causing miserable deterioration of the fidelity and transmission rate of signals. In this work, a novel sandwich-like adhesive film was felicitously designed and fabricated by double-sided coating. The core layer as a supporting structure was a polytetrafluoroethylene film with a low dielectric constant and low dielectric loss, and the surface layer as a principal part of the adhesive function was polybutadiene with high vinyl content. Moreover, to further improve the interfacial adhesive strength between the adhesive film and copper foil, the fluorine-containing groups were controllably introduced onto the surface of the sandwich-like adhesive film via environment-friendly plasma treatment. Based on low polarizability and only superficial distribution of the fluorine-containing groups, the interfacial adhesive strength of the film greatly improved from 0.41 to 1.09 N/mm. Unexpectedly, the high-frequency dielectric properties slightly changed (without treatment Dk = 2.42 and Df = 0.0036 at 10 GHz, treatment Dk = 2.46 and Df = 0.0039 at 10 GHz). This work provided the key adhesive materials for the next generation of high-throughput data transmission equipment, remote sensing controllers, and unmanned aerial vehicles in high-frequency ranges

Strategy for Fabricating Multiple-Shape-Memory Polymeric Materials via the Multilayer Assembly of Co-Continuous Blends

Shape-memory polymeric materials containing alternating layers of thermoplastic polyurethane (TPU) and co-continuous poly­(butylene succinate) (PBS)/polycaprolactone (PCL) blends (denoted SLBs) were fabricated through layer-multiplying coextrusion. Because there were two well-separated phase transitions caused by the melt of PCL and PBS, both the dual- and triple-shape-memory effects were discussed. Compared with the blending specimen with the same components, the TPU/SLB multilayer system with a multicontinuous structure and a plenty of layer interfaces was demonstrated to have higher shape fixity and recovery ability. When the number of layers reached 128, both the shape fixity and recovery ratios were beyond 95 and 85% in dual- and triple-shape-memory processes, respectively, which were difficult to be achieved through conventional melt-processing methods. On the basis of the classic viscoelastic theory, the parallel-assembled TPU and SLB layers capable of maintaining the same strain along the deforming direction were regarded to possess the maximum ability to fix temporary shapes and trigger them to recover back to original ones through the interfacial shearing effect. Accordingly, the present approach provided an efficient strategy for fabricating outstanding multiple-shape-memory polymers, which may exhibit a promising application in the fields of biomedical devices, sensors and actuators, and so forth

Biocompatible Shape Memory Blend for Self-Expandable Stents with Potential Biomedical Applications

Biocompatible poly­(propylene carbonate) (PPC)/polycaprolactone (PCL) shape memory blends were fabricated using melt blending. The shape memory performance of these blends was found to depend remarkably on their components. On addition of 25 vol % PCL, one blend (PL-25) achieved an optimal shape-fixing ratio (<i>R</i>_f) and -recovery ratio (<i>R</i>_r). Specifically, its <i>R</i>_r considerably increased by 24.1 and 50.0% compared with those of pure PPC and PCL, respectively, because of the restricted irreversible deformation of the amorphous chains cross-linked by tiny crystals. After undergoing three thermomechanical cycles, <i>R</i>_f and <i>R</i>_r reached 97.0%. The PL-25 blend was further melt-processed into a stent, which showed a fast response and self-expansion at 37 °C. These results, along with those obtained from evaluating the material’s blood compatibility, in vitro degradation and drug release behavior, demonstrated the great potential of PL-25 for biomedical applications

Bioinspired Polylactide Based on the Multilayer Assembly of Shish-Kebab Structure: A Strategy for Achieving Balanced Performances

Achieving balanced mechanical performances in a polymer material has long been attractive, but it is still a significant challenge. Herein, a nonadditive strategy was proposed by tailoring the crystalline structure of neat polylactide (PLA) through a layer-multiplying extrusion. Compared with normal PLA, the layer-multiplied material exhibited a 50% increase in tensile strength, a 4-fold improvement in elongation at break, and greatly enhanced resistance to heat distortion. To comprehensively understand the origin of the balanced performances, we carefully observed and analyzed the tailored crystalline structure. It was demonstrated that the multilayer-assembled shish-kebab structure was fabricated because of the iterative extensional and laminating effects occurring in the layer-multiplying process. Inspired by that process in nacres, the layer-packed shish-kebab skeleton was regarded as the strong phase possessing the ability to offer sufficient strength for resisting mechanical and thermal deformation. Meanwhile, the tenacious interfaces played a significant role in crack deflection and termination in achieving high ductility. More significantly, because no external additives are required, the layer-multiplied PLA is capable of maintaining high transparency as well as good biodegradability and biocompatibility, which gives it competitive advantages in packaging, biomedical, and tissue engineering applications

Conformational Regulation and Crystalline Manipulation of PLLA through a Self-Assembly Nucleator

Self-assembly nucleators have been increasingly used to manipulate the crystallization of PLLA due to their strong intermolecular interaction with PLLA, while the molecular mechanism of such interaction is still unrevealed. In present work, one special self-assembly nucleator (TMC-300) with relatively high solubility in PLLA matrix, is chosen to investigate how the interaction works at molecular level to promote the crystallization of PLLA mainly through time-resolved spectroscopy. The results indicate that due to the dipole–dipole NH···OC interaction between dissolved TMC-300 and PLLA, PLLA chains are transformed into gt conformer before TMC-300 phase-separating from PLLA melt, resulting in low energy barrier to pass for the following formation of PLLA α-crystal (α-crystal is consisted of gt conformer). Once the dissolved TMC-300 starts to self-assemble into frameworks upon cooling, the transformed PLLA chains with high population of gt conformer form the primary nuclei on the surface of such self-assembling TMC-300 frameworks. For the first time, not only the heterogeneous nucleation but also the conformational regulation of PLLA chains are proved to be responsible for the high efficiency of the self-assembly nucleators (TMC-300) in promoting the crystallization of PLLA. Therefore, conformational regulation is proposed for crystalline manipulation of PLLA, and this work brings new insight on promoting the crystallization of PLLA even other polymers by regulating their molecular conformation

Structural Evolution and Toughening Mechanism of β‑Transcrystallinity of Polypropylene Induced by the Two-Dimensional Layered Interface during Uniaxial Stretching

The structure and morphology of β-crystals of isotactic polypropylene (iPP) are of great significance because β-crystals can improve the toughness and ductility of iPP. Toughening of β-spherulites, which was ascribed to phase transformation, has been extensively investigated. However, the toughening mechanism of other β-crystals with special structures and morphologies is not clear. In this study, β-transcrystallinity (β-TC), which showed a greater toughening effect than that of β-spherulite, was constructed through microlayered coextrusion. During uniaxial stretching, β-TC preferred to transform into an α-crystal, whereas β-spherulite preferred to transform to a smectic mesophase. The transformation degree of β-TC was much higher than that of β-spherulite. More importantly, the lamellar fragments from β-TC gradually rearranged along the stretching direction, accompanied by continuous absorption of energy. The special β–α phase transformation, high transformation rate, and rearrangement of lamellar fragments led to the highly improved toughness of the layered samples

Simple and Consecutive Melt Extrusion Method to Fabricate Thermally Conductive Composites with Highly Oriented Boron Nitrides

In the region of thermally conductive polymer composites, forcing anisotropic fillers into the highly oriented structure is the most effective method to improve thermal conductivity and mechanical properties simultaneously. However, up to now, such highly oriented structure was mainly achieved in low viscosity polymer matrix or solutions. For the purpose of expanding the range of applications, in the present work, a new strategy, the consecutive and powerful shear flow field, was applied to introduce highly oriented boron nitride (BN) into high viscosity polymer matrix. Results indicated that BN was almost totally oriented along the extrusion plane; as a result, the anisotropic index and thermal conductivity of the composite filled with 40 wt % BN reached as high as 480% and 3.57 W/(m K), respectively. Furthermore, compared with the samples with randomly oriented BN, elongations at break were improved more than 50-fold at the same filler content. Finite element analysis was also applied to systematically investigate the effect of the orientation direction of BN on heat dissipation property of the composites, and results indicated that orienting the longitudinal direction of BN parallel to the heat source is the best way to reduce the heat source temperature to a low level. Therefore, the simple, consecutive, and environmentally friendly melt extrusion with powerful shear flow field is an outstanding method to fabricate high efficiency thermally conductive composites, and the simulative results also have important significance on designing such composites for different applications

Enhancing the Oxygen-Barrier Properties of Polylactide by Tailoring the Arrangement of Crystalline Lamellae

The gas-barrier properties of semicrystalline polymers can be significantly adjusted by tailoring the arrangement of their impermeable crystalline lamellae. In particular, the highest barrier efficiency is achieved when the arrangement of the lamellae stacks is perpendicular to the direction of gas diffusion. The work reported on in this paper provides a strategy to achieve such a lamellar arrangement with the aid of a self-assembly nucleator and a two-dimensional (2D) interface. PT (PLA + TMC-300) and PTG (PLA + TMC-300 + graphene) were coextruded to form alternating PT/PTG multilayers with different layer numbers. During isothermal treatment at 140 °C, the dissolved TMC-300 first self-assembles into solid-state fibrils that are perpendicular to the 2D PT/PTG-layered interface due to the induced effects of the graphene. Subsequently, these TMC-300 fibrils induce the epitaxial growth of PLA lamellae with a normal parallel to the fibrillar direction of the TMC-300. In this way, a designed arrangement where the PLA lamellae stack perpendicular to the direction of gas diffusion is achieved. As expected, the resulting PLA exhibits impressively enhanced gas-barrier properties: a decrease of 85.4% in the oxygen permeability coefficient (<i>P</i>_O₂) was observed for the 16-layer sample (0.7 × 10^–19 (m³·m)/(m²·s·Pa)) compared with the sample without layer structure. Through the construction of “lamellae-barrier walls” by tailoring the arrangement of the lamellae, this work provides a route to fabricate semicrystalline polymers with superior gas-barrier properties with great potential for use in high-barrier applications such as food packing, beverage bottles and fuel tanks

Visualization of Deep Convolutional Neural Networks to Investigate Porous Nanocomposites for Electromagnetic Interference Shielding

Constructing porous structures in electromagnetic interference (EMI) shielding materials is a common strategy to decrease the secondary pollution caused by the reflection of electromagnetic waves (EMWs). However, the lack of direct analysis methods makes it difficult to fully understand the effect of porous structures on EMI, hindering EMI composites’ development. Furthermore, while deep learning techniques, such as deep convolutional neural networks (DCNNs), have significantly impacted material science, their lack of interpretability limits their applications to property predictions and defect detection tasks. Until recently, advanced visualization techniques provided an approach to reveal the relevant information behind DCNNs’ decisions. Inspired by it, a visual approach for porous EMI nanocomposite mechanism studies is proposed. This work combines DCNN visualization with experiments to investigate EMI porous nanocomposites. First, a rapid and straightforward salt-leaked cold-pressing powder sintering method is employed to prepare high-EMI CNTs/PVDF composites with various porosities and filler loadings. Notably, the solid sample with 30 wt % loading maintains an ultrahigh shielding effectiveness of 105 dB. The influence of porosity on the shielding mechanism is discussed macroscopically based on the prepared samples. To determine the shielding mechanism, a modified deep residual network (ResNet) is trained on a dataset of scanning electron microscopy (SEM) images of the samples. The Eigen-CAM visualization of the modified ResNet intuitively shows that the amount and depth of the pores impact the shielding mechanisms and that shallow pore structures contribute less to EMW absorption. This work is instructive for material mechanism studies. Besides, the visualization has the potential as a porous-like structure marking tool