Enhanced Separation Performance of Hierarchically Porous Membranes Fabricated via the Combination of Crystallization Template and Foaming.

In this work, poly (vinylidene fluoride) (PVDF) hierarchically porous membranes (HPMs) with isolated large pores and continuous narrow nano-pores have been fabricated from its blend with poly (methyl methacrylate) (PMMA) based on the combination of crystallization template with chemical or supercritical CO2 foaming. On the one hand, the decomposition of azodicarbonamide (ADC, chemical foaming agent) or the release of CO2 can produce isolated large pores. On the other hand, PMMA is expelled during the isothermal crystallization of PVDF in their miscible blend, yielding narrow nano-pores upon etching with a selective solvent. In the case of supercritical CO2, the attained PVDF HPMs fail to improve separation performance because of the compact wall of isolated-large-pore and consequent poor connectivity of hierarchical pores. In the case of ADC, the optimal HPM exhibits much higher flux (up to 20 times) without any loss of selectivity compared with the reference only with nano-pores. The enhanced permeability can be attributed to the shorter diffusion length and lower diffusion barrier from isolated large pores, while the comparable selectivity is determined by narrow nano-pores in THE matrix.

Panther chameleon-inspired, continuously-regulated, high-saturation structural color of a reflective grating on the nano-patterned surface of a shape memory polymer.

In this work, surface nano-stripes and a reflective grating have been fabricated on shape memory polymers (SMPs) to simulate the active color change of chameleons. The structural color resulting from the interference of reflected light exhibits high saturation and it can be regulated continuously based on the shape memory effect. In addition to the viewing angle, the attained color is sensitive to the deformation at the macroscale. Uniaxial tension along stripes at high temperature produces a remarkable blueshift of the resultant color (from red to green and blue) which can switch back to red after shape recovery upon heating. The evolution of structural color can be attributed to the lower and higher magnitudes of nano-structure periods in temporary (deformed) and permanent (recovery) states respectively. Based on the combination of angle and deformation dependences of structural color, a "colorful" product code has been fabricated. It exhibits enhanced ability to hide and display information which plays an important role in anti-counterfeiting.

Inclusion/Exclusion Behaviors of Small Molecules during Crystallization of Polymers in Miscible PLLA/TAIC Blend.

In this work, PLLA/TAIC has been taken as a model system to investigate the inclusion and exclusion of small molecules during the crystallization of polymers in their miscible blend. Our results indicate that it is the growth rate and diameter of PLLA spherulites that dominate the localization of TAIC. On the one hand, crystallization temperature plays an important role. Crystallization at higher temperature corresponds to higher growth rates and a greater diameter of PLLA spherulites. The former improves the ability of PLLA crystals to trap TAIC while the latter leads to a lower volume fraction of space among neighboring PLLA spherulites. The combination of the two contributes to the enhanced inclusion behaviors. On the other hand, when compared to melt crystallization, cold crystallization results in much smaller spherulites (from higher nucleation density) and sufficient space among spherulites, which accounts for the enrichment of TAIC in interspherulitic regions and for its enhanced exclusion. In the adopted polymer-small molecule blend, TAIC can enrich in interspherulitic regions even in its miscible blend with PLLA, which can be attributed to its stronger diffusion ability.

Enhancement of strength and toughness of bio-nanocomposites with good transparency and heat resistance by reactive processing.

Growing concerns in addressing environmental challenges are driving the rapid advancement of both bio-based and environmental friendly materials. Biodegradable polymers have been compounded with various nanofillers to fulfill the multiple requirements in real applications. However, current technologies remain to be improved in terms of the intrinsic inferior performance and the lack of interfacial interactions. In this work, we employed a facile route to develop bio-nanocomposites integrating multiple functionalities by reactive processing of polylactide and reactive boehmite nanorods. The grafting of polymer chains onto the surface of the nanorods encourages fully homogeneous dispersion of nanofillers with even 30 wt% loadings. Such nanocomposites exhibit simultaneously enhanced tensile strength, modulus, ductility, and impact strength. Moreover, the bio-based nanocomposites present promising features such as high transparency, improved flame resistance, and heat resistance. This work demonstrates exciting opportunities to produce bio-plastics with diverse functionalities in versatile applications of sustainable packaging industry and engineering plastics.

Wrinkled CNTs@PLLA Composite Membranes for Enhanced Separation Performance.

To break the trade-off effect between permeability and selectivity in separation, wrinkled carbon nanotubes@polylactic acid (CNTs@PLLA) composite membranes were successfully fabricated in this work. On pre-deformed PLLA membranes, CNTs were loaded by filtrating their suspension, followed by releasing the PLLA upon heating based on its shape memory effect. The asynchronous deformations of CNTs and PLLA layers produced wrinkled CNTs@PLLA composite membranes. Relative to the reference without wrinkles, the attained wrinkled composite membranes exhibit much higher flux (~12 times) without any loss of rejection ratio during the separation of water-in-hexadecane emulsion. The significant improvement of separation performance can be attributed to the following issues: Firstly, the existence of wrinkles results in higher surface roughness, providing an additional driving force for separation resulting from the enlarged contact-angle difference between water and oil; Secondly, the shrinkage of the supporting PLLA layer during recovery induces the preferred alignment of CNTs along the wrinkle direction, which is the reason for the orientated slit pores with enhanced overlap of neighboring pores in the film-thickness direction; Finally, a wrinkled surface significantly increases the available area for separation. The synergism of the effects discussed above contributes to much higher permeability and comparable selectivity relative to the reference.

Programmable Transition between Adhesive/Anti-Adhesive Performances on Porous PVDF Spheres Supported by Shape Memory PLLA.

Superhydrophobic surfaces with switchable adhesive/anti-adhesive performances are highly desired but still challenging. Herein, by loading porous poly (vinylidene fluoride) (PVDF) spheres on a shape memory polylactic acid (PLLA) film, a quasi-superhydrophobic surface of composite film (PVDF@PLLA) with the ability to tailor its surface structures/composition and related adhesive behaviors was fabricated. The as-prepared surface is covered by porous PVDF spheres. The combination of hydrophobicity of PVDF and hierarchical roughness resulted from porous spheres contributing to the high contact angle and low sliding angle, corresponding to Cassie state and lotus leaves effect. Upon uniaxial or biaxial tension, the distance among hydrophobic spheres is so high that more and more hydrophilic defects (PLLA film) have been exposed to water droplets, accounting for the quasi-superhydrophobic surface with a higher sliding angle. This is the reason for the Wenzel state and rose petals effect. After heating, PLLA film recovers to its original state. The porous PVDF spheres cover the whole film again, leading to the enhanced mobility of water droplets on the surface. The transition between the rose petals effect and the lotus leaves effect is programmable and reversible. Our result provides a novel strategy to tailor adhesive behaviors by combining (quasi-)superhydrophobic surface with shape memory effect.

Crystallinity Dependence of PLLA Hydrophilic Modification during Alkali Hydrolysis.

Poly(L-lactic acid) (PLLA) has been extensively used in tissue engineering, in which its surface hydrophilicity plays an important role. In this work, an efficient and green strategy has been developed to tailor surface hydrophilicity via alkali hydrolysis. On one hand, the ester bond in PLLA has been cleaved and generates carboxyl and hydroxyl groups, both of which are beneficial to the improvement of hydrophilicity. On the other hand, the degradation of PLLA increases the roughness on the film surface. The resultant surface wettability of PLLA exhibits crucial dependence on its crystallinity. In the specimen with high crystallinity, the local enrichment of terminal carboxyl and hydroxyl groups in amorphous regions accelerates the degradation of ester group, producing more hydrophilic groups and slit valleys on film surface. The enhanced contact between PLLA and water in aqueous solution (i.e., the Wenzel state) contributes to the synergistic effect between generated hydrophilic groups and surface roughness, facilitating further degradation. Consequently, the hydrophilicity has been improved significantly in the high crystalline case. On the contrary, the competition effect between them leads to the failure of this strategy in the case of low crystallinity.

Effect of PMMA Molecular Weight on Its Localization during Crystallization of PVDF in Their Blends.

In miscible crystalline/amorphous polymer blends, the exclusion behaviors of the latter with various molecular weights during the crystallization of the former were investigated by the combination of SAXS and DSC by taking a PVDF/PMMA blend as an example. The ratio between internal crystallinity from SAXS and overall crystallinity of the entire blend from DSC was employed to characterize the exclusion of PMMA. Our results indicate that the molecular weight of the amorphous component produces a remarkable influence on the diffusion coefficient (D) and the crystal growth rate (G) of the crystalline component. There are both inter-lamellar and inter-fibrillar structures when PVDF blended with lower-molecular-weight PMMA. With increasing molecular weight of PMMA, the decrease in crystal growth rate (G) dominates the enhanced exclusion behaviors of PMMA, resulting in bigger pores after extraction. Our results are significant not only for the basic understanding of crystallization in polymer blends, but also for the fabrication and structure control of porous structures based on crystallization templates.

Improvement of PLLA Ductility by Blending with PVDF: Localization of Compatibilizers at Interface and Its Glycidyl Methacrylate Content Dependency.

In this work, the localization of reactive compatibilizer (RC, containing poly(methyl methacrylate) (PMMA) backbone with randomly distributed glycidyl methacrylate (GMA) on it) at the polyvinylidene fluoride/poly(l-lactic acid) (PVDF/PLLA) interface has been manipulated by means of GMA contents. At the very beginning of mixing, RC tends to stay in the PVDF phase due to the miscibility between PVDF and PMMA. Upon further shearing, more and more PLLA chains have been grafted on PMMA backbone, producing PLLA-g-PMMA copolymer. The balanced stress on two sides accounts for the localization of compatibilizers at the PVDF/PLLA interface. Finally, the stress of the PLLA side has been enhanced remarkably due to the higher graft density of PLLA, resulting in the enrichment of the copolymer in the PLLA matrix. The migration of RC from the PVDF phase to the immiscible interface and PLLA matrix can be accelerated by employing RC with higher GMA content. Furthermore, the compatibilizer localization produces a significant influence on the morphology and ductility of the PVDF/PLLA blend. Only when the compatibilizers precisely localize at the interface, the blend exhibits the smallest domain and highest elongation at break. Our results are of great significance for not only the fabrication of PLLA with high ductility, but also the precise localization of compatibilizers at the interface of the immiscible blend.

Switchable Isotropic/Anisotropic Wettability and Programmable Droplet Transportation on a Shape-Memory Honeycomb.

Programmable droplet transportation is required urgently but is still challenging. In this work, breath figure was employed to fabricate shape-memory poly(lactic acid) (PLLA) honeycombs in which tiny crystals and an amorphous network act as the shape-fixed phase and recovery phase, respectively. Upon uniaxial tension, circle pores from the breath figure were deformed to elliptical pores, producing contact angle differences and anisotropic wetting behaviors in two directions. Both pore geometry and anisotropic wettability can be tailored via the draw ratio. On the PLLA honeycomb surface with a lower draw ratio, the contact angle difference is too small to induce droplet transportation along the desired direction. In the case of a higher draw ratio, however, the movement of water droplets has been controlled absolutely along the tension direction. The transition between them can be achieved reversibly during uniaxial tension and recovery processes based on the shape-memory effect. The enhanced flow control, which can be attributed to the synergism between optimal hydrophobicity and enlarged anisotropic wetting behaviors, endows water droplets with the ability to turn a corner spontaneously on a V-shaped surface including two regions exhibiting different oriented directions.

Stabilizing Polymeric Interface by Janus Nanosheet.

A strategy is proposed to stabilize the polymeric interface by using the irregular Janus nanosheet (JNS). The poly(vinylidene fluoride) (PVDF)/poly(l-lactic acid) (PLLA) at 60/40 (wt/wt) with a bi-continuous structure is selected as the model melt blend, and the PMMA/epoxy JNS is synthesized and used as the compatibilizer. The JNS is preferentially located at the interface. The interfacial coverage by the JNS reaches a saturated state forming the interconnected jamming structure at 0.5 wt% of the JNS. The interface is thus stabilized which is well preserved after annealing at high temperature. After selectively etching PLLA, the robust PVDF porous material is derived with the JNS armored at the pore skeleton surface. The porous material provides a universal scaffold to achieve stable functional materials after filling the pores.

Multiple-Step Melting/Irradiation: A Strategy to Fabricate Thermoplastic Polymers with Improved Mechanical Performance.

To fabricate thermoplastic polymers exhibiting improved ductility without the loss of strength, a novel multiple-step melting/irradiation (MUSMI) strategy was developed by taking poly(vinylidene fluoride)/triallyl isocyanate (PVDF/TAIC) as an example, in which alternate melting and irradiation were adopted and repeated for several times. The initial irradiation with a low dose produced some local crosslinked points (not 3-dimensional network). When the specimen was reheated above the melting temperature, they redistributed in the PVDF matrix, which is an efficient way to avoid the high crosslinking density at certain positions and the disappearance of thermoplastic properties. During the subsequent cooling process, the crosslinked domains in the thermoplastic polymer matrix is expected to play double roles in turning crystal structures for enhancing the ductility without reducing strength. On one hand, they can act as heterogeneous nucleation agents, resulting in higher nucleation density and smaller spherulites; on the other hand, the existence of crosslinked structures restricts the lamellar thickening, accounting for the thinner crystal lamellae. Both smaller spherulites and thinner lamellae contribute to better ductility. At the same time, these local crosslinked points enhance the connectivity of crystal structures (including lamellae and spherulites), which is beneficial to the improvement of strength. Based on the influence of local crosslinked points on the ductility and strength, thermoplastic PVDF with much higher elongation at break and comparable yielding stress (relative to the reference specimen upon strong irradiation only once) was prepared via MUSMI successfully.

Micropore Geometry Manipulation by Macroscopic Deformation Based on Shape Memory Effect in Porous PLLA Membrane and its Enhanced Separation Performance.

An effective strategy to tailor the microporous structures has been developed based on the shape memory effect in porous poly(l-lactic acid) membranes in which tiny crystals and amorphous matrix play the roles of shape-fixed phase and reversible-phase, respectively. Our results indicate that not only PLLA membranes but micropores exhibit shape memory properties. The proportional deformations on two scales have been achieved by uniaxial or biaxial tension, providing a facile way to manipulate continuously the size and the orientation degree of pores on microscale. The enhanced separation performance has been validated by taking polystyrene colloids with varying diameters as an example.

TiO2 Nanotube Arrays: Fabricated by Soft-Hard Template and the Grain Size Dependence of Field Emission Performance.

Highly ordered TiO2 nanotube (TNT) arrays were successfully synthesized by the combination of soft and hard templates. In the fabrication of them, anodic aluminum oxide membranes act as the hard template while the self-assembly of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) complexed with titanium-tetraisopropoxide (TTIP, the precursor of TiO2) provides the soft template to control the grain size of TiO2 nanotubes. Our results indicate that the field emission (FE) performance depends crucially on the grain size of the calcinated TiO2 which is dominated by the PS-b-PEO and its blending ratio with TTIP. The optimized sample (with the TTIP/PEO ratio of 3.87) exhibits excellent FE performances involving both a low turn-on field of 3.3 V/um and a high current density of 7.6 mA/cm2 at 12.7 V/µm. The enhanced FE properties can be attributed to the low effective work function (1.2 eV) resulted from the smaller grain size of TiO2.

Fabrication of Superhydrophobic Surfaces with Controllable Electrical Conductivity and Water Adhesion.

A facile and versatile strategy for fabricating superhydrophobic surfaces with controllable electrical conductivity and water adhesion is reported. "Vine-on-fence"-structured and cerebral cortex-like superhydrophobic surfaces are constructed by filtering a suspension of multiwalled carbon nanotubes (MWCNTs), using polyoxymethylene nonwovens as the filter paper. The nonwovens with micro- and nanoporous two-tier structures act as the skeleton, introducing a microscale structure. The MWCNTs act as nanoscale structures, creating hierarchical surface roughness. The surface topography and the electrical conductivity of the superhydrophobic surfaces are controlled by varying the MWCNT loading. The vine-on-fence-structured surfaces exhibit "sticky" superhydrophobicity with high water adhesion. The cerebral cortex-like surfaces exhibit self-cleaning properties with low water adhesion. The as-prepared superhydrophobic surfaces are chemically resistant to acidic and alkaline environments of pH 2-12. They therefore have potential in applications such as droplet-based microreactors and thin-film microextraction. These findings aid our understanding of the role that surface topography plays in the design and fabrication of superhydrophobic surfaces with different water-adhesion properties.

Crystallization-modulated nanoporous polymeric materials with hierarchical patterned surfaces and 3D interpenetrated internal channels.

Poly(oxymethylene)/poly(L-lactic acid) (POM/PLLA) blends are typical melt-miscible binary systems. During isothermal crystallization at various temperatures, in the presence of amorphous PLLA chains, POM crystallizes into banded spherulites with different band spaces, which forms a continuous crystalline phase and serves as a sturdy frame in the final porous materials. On the other hand, the amorphous PLLA chains are simultaneously expelled out from POM crystal lamellae to generate the other continuous phase during the crystallization of POM. Consequently, the interpenetration of the POM lamellae and the amorphous PLLA phase construct a cocontinuous phase structure. All the PLLA constituents are fully included in the interlamellar or interfibrillar of POM crystals. Thus, nanoporous POM materials with hierarchical patterned surface and 3D interpenetrated internal channels have been successfully obtained by extracting the amorphous PLLA phase. It is further found that the POM crystal morphologies in the blends are much dependent on the crystallization conditions. Therefore, the hierarchical patterned structure and the size of internal channels (pore size) can be modulated by adjusting the crystallization conditions.

Solvent annealing induced phase separation and dewetting in PMMA∕SAN blend film: film thickness and solvent dependence.

The competition between "dewetting" and "phase separation" behaviors in polymer blend films attracts significant attention in the last decade. The simultaneous phase separation and dewetting in PMMA∕SAN [poly(methyl methacrylate) and poly(styrene-ran-acrylonitrile)] blend ultrathin films upon solvent annealing have been observed for the first time in our previous work. In this work, film thickness and annealing solvent dependence of phase behaviors in this system has been investigated using atomic force microscopy and grazing incidence small-angle X-ray scattering (GISAXS). On one hand, both vertical phase separation and dewetting take place upon selective solvent vapor annealing, leading to the formation of droplet∕mimic-film structures with various sizes (depending on original film thickness). On the other hand, the whole blend film dewets the substrate and produces dispersed droplets on the silicon oxide upon common solvent annealing. GISAXS results demonstrate the phase separation in the big dewetted droplets resulted from the thicker film (39.8 nm). In contrast, no period structure is detected in small droplets from the thinner film (5.1 nm and 9.7 nm). This investigation indicates that dewetting and phase separation in PMMA∕SAN blend film upon solvent annealing depend crucially on the film thickness and the atmosphere during annealing.

Novel cigarlike TiO2 nanofibers: fabrication, improved mechanical, and electrochemical performances.

By coupling the self-assembly of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) containing titanium precursors with the electrospinning technique, novel cigarlike nanofibers with an outer-shell and inner-continuous-pore structure and resultant fabrics were fabricated. Different from typical porous metal oxides, the prepared high-surface-area nonwoven fabrics show excellent mechanical properties. Not only are these fabrics self-supporting over a large area, but they can also be cut using scissors, which is important for large-scale applications. Furthermore, as electrode materials in Li-ion batteries, these fabrics exhibit much higher charge/discharge capacity and cycle stability compared with the commercially available nanosized TiO2 (P25). The improved mechanical and electrochemical performances are attributed to the presence of an outer-shell, inner-bicontinuous structures (including continuous TiO2 frame and continuous nanopores) and hierarchical pores from the cigarlike nanofibers.

Mechanical and thermal properties of eco-friendly poly(propylene carbonate)/cellulose acetate butyrate blends.

The eco-friendly poly(propylene carbonate) (PPC)/cellulose acetate butyrate (CAB) blends were prepared by melt-blending in a batch mixer for the first time. PPC and CAB were partially miscible because of the drastically shifted glass transition temperatures of both PPC and CAB, which originated from the specific interactions between carbonyl groups and hydroxyl groups. The incorporation of CAB into PPC matrix enhanced not only tensile strength and modulus of PPC dramatically, but also improved heat resistance and thermal stability of PPC significantly. The tensile strength and the modulus of PPC/CAB=50/50 blend are 27.7 MPa and 1.24 GPa, which are 21 times and 28 times higher than those of the unmodified PPC, respectively. Moreover, the elongation at break of PPC/CAB=50/50 blend is as high as 117%. In addition, the obtained blends exhibited good transparency, which is very important for the package materials. The results in this work pave new possibility for the massive application of eco-friendly polymer materials.

Reactive bonding mediated high mass loading of individualized single-walled carbon nanotubes in an elastomeric polymer.

A reactive chemical bonding strategy was developed for the incorporation of a high mass loading of individual single-wall carbon nanotubes (SWCNTs) into an elastomeric matrix using a reactive ionic liquid as a linker. This method simultaneously prevented the agglomeration of SWCNTs and caused strong interfacial bonding, while the electronic properties of the SWCNTs remained intact. As a result, the high conductivity of the carbon nanotubes (CNTs) and the flexibility of the elastomeric matrix were retained, producing optimum electrical and mechanical properties. A composite material with a loading of 20 wt% SWCNTs was fabricated with excellent mechanical properties and a high conductivity (9500 S m(-1)). The method could be used to form transparent thin conductive films that could tolerate over 800 bend cycles at a bending angle of 180° while maintaining a constant sheet resistance.