Polylactide (PLA) films with an excellent balance of toughness and stiffness were realized by extensional stress efficiently. For the relatively low extensional stress, gauche− gauche conformers that originated from the oriented amorphous chains lead to super-toughening behavior. Among higher extensional stress, strain-induced orientation and crystallization act as the driving force of reinforcement. This mechanism is evidenced by the pronounced enhancement in the elongation at break from 16.9 up to 294.9% accompanying the variation yield strength from 45.3 up to 135.5 MPa. The highest elongation at break results from the early stretching stages, whereas the highest yield strength is obtained from a high draw ratio. More impressively, PLA films show temperature-invariant super-ductility and reinforcement at low temperatures (0 and −20 °C). This work provides a preferable and scalable method to fabricate competitive PLA materials, expanding the practical application of sustainable polymers served at a wide temperature range or in harsh environments.

With the modern development of power electrification, polymer nanocomposite dielectrics (or nanodielectrics) have attracted significant research attention. The idea is to combine the high dielectric constant of inorganic nanofillers and the high breakdown strength/low loss of a polymer matrix for higher energy density polymer film capacitors. Although impressively high energy density has been achieved at the laboratory scale, there is still a large gap from the eventual goal of polymer nanodielectric capacitors. In this review, we focus on essential material issues for two types of polymer nanodielectrics, polymer/conductive nanoparticle and polymer/ceramic nanoparticle composites. Various material design parameters, including dielectric constant, dielectric loss, breakdown strength, high temperature rating, and discharged energy density will be discussed from both fundamental science and high-voltage capacitor application points of view. The objective is to identify advantages and disadvantages of the polymer nanodielectric approach against other approaches utilizing neat dielectric polymers and ceramics. Given the state-of-the-art understanding, future research directions are outlined for the continued development of polymer nanodielectrics for electric energy storage applications.

The crystallization behavior and crystalline structure of poly(vinylidene fluororide) (PVDF) in the presence of graphene oxide (GO) platelets were investigated using time-resolved Fourier transformation infrared spectroscopy (FTIR), wide-angle X-ray diffraction (WAXD), as well as differential scanning calorimetry (DSC). It is shown that GO platelets induce the formation of γ phase when crystallizing from solution, but only α phase forms from melt crystallization. The crystallization kinetics of α phase is promoted due to heterogeneous nucleation ability of GO, which is probably originated from a weak π-dipole interaction between GO and PVDF. Intriguingly, after introduction of strong ion-dipole interactions between GO and PVDF by addition of an ionic surfactant (cetyltrimethylammonium bromide, CTAB), a significant amount of γ crystals are obtained during isothermal melt crystallization. Time-resolved FTIR results further provide a detailed evolution of the γ phase formation, and there are two distinct stages during the melt crystallization in the PVDF/GO composites in the presence of CTAB, i.e., a simultaneous growth of γ and α phases in the first stage, and a solid α to γ transition in the second stage. These results may provide a facile routine to manipulate the crystalline structure in PVDF/GO composites, and thus to gain desirable properties.

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Manipulating polymorphism in extended chain-crystals (ECCs), which are commonly achieved by crystallization under high pressures, is important for enriching our understanding of basic polymer crystallization as well as for achieving high performance materials. In this study, the influence of high pressure and ion−dipole interaction on the polymorphism was investigated by comparing neat poly(vinylidene fluoride) (PVDF) and PVDF with 1 wt % cetyltrimethylammonium bromide (CTAB) nonisothermally crystallized from the melt at 210 °C. Under low pressures (≤10 MPa), γ folded-chain crystals (FCCs), rather than α FCCs, were obtained for PVDF/1 wt % CTAB because of the ion−dipole interaction. Under a moderate pressure (100 MPa), pure β FCCs were formed in PVDF/1 wt % CTAB, owing to the synergistic effect of both high pressure and ion−dipole interaction. Under high pressures (≥200 MPa), mixtures of β/γ FCCs and ECCs were obtained for PVDF/1 wt % CTAB. This was different from the neat PVDF, where mixtures of α FCCs and α/γ/β ECCs coexisted when the pressure was between 200 and 400 MPa. The formation mechanisms of various crystalline forms and FCCs versus ECCs during the nonisothermal crystallization are discussed using the T−P phase diagram for PVDF.

The structural manipulation of the electroactive β phase of poly(vinylidene fluoride) (PVDF) is particularly important in sensor and actuator applications. Herein, an efficient way to enhance dielectric and ferroelectric properties of PVDF films by annealing preoriented PVDF films through thermal treatment combined with the pressure field is proposed. During annealing processing, an appropriate pressure is attributed to the efficient dipole rotation and results in complete phase transformation of the nonpolar α phase (TGTG′) into the polar β phase (TTTT). Moreover, the appropriate pressure and temperature fields synergistically promote a more perfect alignment of the main chain in β-crystallites along the stretching direction. Thus, the pure β phase with ultrahigh orientation (Herman's orientation factor >0.97) is successfully obtained. Moreover, the enhanced mobility of molecular chains with the increase of temperature contributes to the perfection of β-crystallites with high crystallinity. The relaxation of oriented chains in the amorphous region at high temperature during the annealing process is obviously inhibited by high pressure, leading to increased density of dipoles capable of efficient rotation under an electric field. The unique structure obtained imparts a distinctly enhanced dielectric and ferroelectric properties to the PVDF films. The highest dielectric constant at room temperature is observed in preoriented films annealed at 160 °C due to the optimal chain orientation. Moreover, the film with more perfect and tightly packed β-crystallites annealed at 180 °C shows clearly ferroelectric switching and the maximum remnant polarization (5.8 μC/cm 2 ). The outcomes of this work indicate that a rational combination of pressure and temperature fields could effectively achieve optimal dielectric and ferroelectric properties of PVDF oriented films.

Polymeric films with high electromagnetic interference (EMI) shielding effectiveness (SE) and polyfunctionality are highly desirable for wearable electronic devices. Herein, a sandwich-structured EMI shielding film with Joule heating performance composed of a poly­(vinylidene fluoride) (PVDF) layer and a conductive filler layer (silver nanowire (AgNW) and MXene) was constructed by electrostatic spinning, vacuum-assisted filtration (VAF), and hot compression. An independent AgNW layer endowed the film with predominant EMI shielding performance at a low conductive filler content. A high EMI SE of 45.4 dB was obtained in the X band at a AgNW fraction of 1.28 wt %. The introduction of MXene improved the connection of the AgNW networks, thereby further enhancing the electrical conductivity and imparting the film with long-term stability. The as-obtained PVDF-AgNW/MXene film exhibited an outstanding EMI SE of 47.8 dB and only a slight decrease in EMI SE was detected after 2000 bending cycles. Besides, the PVDF-AgNW/MXene film exhibits excellent Joule heating performance. The surface temperature of the film could exceed 77 °C under an applied voltage of 2.5 V. Therefore, our sandwich-structured film with enhanced EMI shielding and Joule heating performance can be used for flexible electronic device applications in extreme conditions.

Relaxor ferroelectric (RFE) polymers exhibiting narrow hysteresis loops are attractive for a broad range of potential applications such as electric energy storage, artificial muscles, electrocaloric cooling, and printable electronics. However, current state-of-the-art RFE polymers are primarily poly(vinylidene fluoride-co-trifluoroethylene-co-X) [P(VDF-TrFE-X)] random terpolymers with X being 1,1-chlorofluoroethylene (CFE) or chlorotrifluoroethylene (CTFE). Potential dehydrochlorination at elevated temperatures can prevent the melt-processing of these Cl-containing terpolymers. It is desirable to achieve the RFE behavior for Cl-free terpolymers such as P(VDF-TrFE-HFP), where HFP stands for hexafluoropropylene. Nonetheless, HFP units were mostly excluded from the crystalline structure because of their large size, and thus no RFE behavior was observed when crystallized from the quiescent melt. Intriguingly, mechanical stretching could effectively pull the HFP units into the P(VDF-TrFE) crystals, forming nanosized ferroelectric (FE) domains with a strong physical pinning effect. Consequently, the RFE behavior was observed for the uniaxially stretched P(VDF-TrFE-HFP) film. Thermal annealing above the Curie temperature (ca. 50 °C) without tension led to the return of the normal FE behavior with broad hysteresis loops. However, thermal annealing above Curie temperature under tension prevented the exclusion of HFP units from the crystalline structure, and thus relatively stable RFE behavior was achieved. Various characterization techniques were utilized to unravel the structure−property relationships for these P(VDF-TrFE-HFP) films. In addition, the RFE behavior of P(VDF-TrFE-HFP) was compared to those of other terpolymers. This study provides a unique and simple strategy solely based on film processing to achieve the RFE behavior for P(VDF-TrFE)-based terpolymers.