PEG-Grafted Graphene/PLLA Nanocomposites: Effect of PEG Chain Length on Crystallization Kinetics of PLLA.

Poly-l-lactic acid (PLLA) nanocomposites containing graphene oxide (GO), modified with different chain lengths of poly(ethylene glycol) (PEG) (400, 2000, and 10 000 g/mol), were prepared by solution casting. The effect of the PEG chain length and nanoparticle content (0.5, 1, and 1.5 wt %) on the nucleation, crystal growth rate, and overall crystallization rate, under isothermal conditions, was then evaluated. The results showed that, in samples containing GO modified with 400 g/mol of PEG, the nucleation density increased as a function of a modified nanoparticle concentration. In the case of the samples containing GO modified with PEG of a molar mass of either 2000 or 10 000 g/mol, the nucleation density exhibited a maximum at a concentration of 1 wt %. Furthermore, the addition of graphene oxide modified with poly(ethylene glycol) of a molar mass of 2000 g/mol resulted in the largest nucleation, fastest crystal growth, and highest overall crystallization rate, for all concentrations. The results were explained in light of the steric hindrance between the modified nanoparticles.

Scaled-Up Multi-Needle Electrospinning Process Using Parallel Plate Auxiliary Electrodes.

Electrospinning has gained much attention in recent years due to its ability to easily produce high-quality polymeric nanofibers. However, electrospinning suffers from limited production capacity and a method to readily scale up this process is needed. One obvious approach includes the use of multiple electrospinning needles operating in parallel. Nonetheless, such an implementation has remained elusive, partly due to the uneven electric field distribution resulting from the Coulombic repulsion between the charged jets and needles. In this work, the uniformization of the electric field was performed for a linear array of twenty electrospinning needles using lateral charged plates as auxiliary electrodes. The effect of the auxiliary electrodes was characterized by investigating the semi-vertical angle of the spun jets, the deposition area and diameter of the fibers, as well as the thickness of the produced membranes. Finite element simulation was also used to analyze the impact of the auxiliary electrodes on the electric field intensity below each needle. Implementing parallel lateral plates as auxiliary electrodes was shown to help achieve uniformization of the electric field, the semi-vertical angle of the spun jet, and the deposition area of the fibers for the multi-needle electrospinning process. The high-quality morphology of the polymer nanofibers obtained by this improved process was confirmed by scanning electron microscopy (SEM). These findings help resolve one of the primary challenges that have plagued the large-scale industrial adoption of this exciting polymer processing technique.

Ultra-Low Percolation Threshold Induced by Thermal Treatments in Co-Continuous Blend-Based PP/PS/MWCNTs Nanocomposites.

The effect of the crystallization of polypropylene (PP) forming an immiscible polymer blend with polystyrene (PS) containing conductive multi-wall carbon nanotubes (MWCNTs) on its electrical conductivity and electrical percolation threshold (PT) was investigated in this work. PP/PS/MWCNTs composites with a co-continuous morphology and a concentration of MWCNTs ranging from 0 to 2 wt.% were obtained. The PT was greatly reduced by a two-step approach. First, a 50% reduction in the PT was achieved by using the effect of double percolation in the blend system compared to PP/MWCNTs. Second, with the additional thermal treatments, referred to as slow-cooling treatment (with the cooling rate 0.5 °C/min), and isothermal treatment (at 135 °C for 15 min), ultra-low PT values were achieved for the PP/PS/MWCNTs system. A 0.06 wt.% of MWCNTs was attained upon the use of the slow-cooling treatment and 0.08 wt.% of MWCNTs upon the isothermal treatment. This reduction is attributed to PP crystals' volume exclusion, with no alteration in the blend morphology.

Evaluation of different solvents and solubility parameters on the morphology and diameter of electrospun pullulan nanofibers for curcumin entrapment.

In this work, the effect of different solvents and solvent binary mixtures on the morphology of electrospun pullulan (Pull) nanofibers was evaluated. The solution viscosities, interactions between solvent and polymer, as well as, solvent vapor pressure, were correlated to the morphology and diameters of the nanofibers. Water, dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) and Teas graphs that rely on the use of Hildebrand solubility parameters were used to identify the most suitable mixtures of solvents. The best binary mixture of solvents to produce Pull nanofibers without defects and with small diameters (203±32 nm) was DMF/DMSO in the ratio of 70/30 wt.%. Pull nanofibers containing 3 and 5 wt. % of curcumin (Curc) in a mixture of DMF/DMSO (70/30 wt.%) were then obtained, and the entrapment efficiency was evaluated using 1H NMR and a UV--vis spectrophotometer. The results obtained in this work create a new approach for the production of Pull nanofibers for drug delivery systems.

A rheological approach to assess the printability of thermosensitive chitosan-based biomaterial inks.

For extrusion-based bioprinting, the inks must be printable and rapidly present sufficient mechanical properties to support additional layers and provide a cohesive, manipulable structure. Thermosensitive hydrogels may be interesting candidates. However, the use of these materials is particularly challenging, since their rheological properties evolve with time and temperature. In this work, a rheological approach to characterize the printability of chitosan-based thermosensitive inks was developed. The method consists of evaluating: (1) the gelation kinetic at room temperature and at 37 °C; (2) shear-thinning behavior to estimate the shear rate applied during printing as a function of printing parameters; and (3) the viscosity after shear removal (recovery test) to simulate behaviour after biomaterial deposition. Hydrogels containing 2 and 3% w v-1 chitosan, combined with different gelling agents (sodium hydrogen carbonate (SHC), phosphate buffer, beta-glycerophosphate (BGP)) were tested, and compared with alginate/gelatin bioink as controls. To correlate the rheological studies with real printing conditions, a 3D-Discovery bioprinter was used to print hydrogels and the visual aspect of the printed structure was observed. Unconfined compressive tests were carried out to study the impact of applied shear rate during printing on the mechanical properties of printed structures. All pre-hydrogel solutions presented shear-thinning properties. The recovery of viscosity was found to depend on the hydrogel formulation, as well as the level of shear rate and the state of gelation at the time of printing. Formulations made with SHC and phosphate buffer presented too rapid gelation and phase separation, leading to poor printing results. One particularly promising formulation composed of SHC and BGP, when printed at a shear rate of 140 s-1, before its gelation time (t g ⩽ 15 min), resulted in good printability and 3D structures with rigidity comparable with the alginate/gelatin bioink. The methodology introduced in this paper could be used to evaluate the printability of other time- and temperature-dependent biomaterial inks in the future.

Ultraporous Membranes Electrospun from Nonsolvent-Induced Phase-Separated Ternary Systems.

Electrospinning of nonsolvent-induced phase-separated ternary (NIPST) systems has gained a lot of interest due to its potential to produce (nano)fibers, which are superficially and internally porous with nanoscale surface roughness. Membranes produced from such systems are expected to have a high specific surface area (SSA; e.g., more than 50 m2 g-1 ), an essential requirement for many of their applications. In spite of their advantages and potential, there are major issues regarding the electrospinning of NIPST systems that are not systematically addressed in the literature. In this paper, the most recent developments are reported and the potential and challenges associated with the electrospinning of NIPST systems are discussed. Furthermore, the essential steps to improve and optimize the electrospinning process of these systems are concisely discussed. By developing a modified time-dependent rheological model, a time range can be defined for NIPST systems as "electrospinnability window," in which fiber functionality and characteristics can be tailored through aging of the systems prior to electrospinning. Some potential post-treatment processes are also proposed based on the results of recent studies to stabilize as-electrospun membranes without damaging their highly porous fibers, which can guarantee their in-service mechanical and morphological stability.

Wetting of Hydrophilic Electrospun Mats Produced by Blending SEBS with PEO-PPO-PEO Copolymers of Different Molecular Weight.

The interaction of electrospun mats with water is critical for many possible applications, and the water contact angle on the surface is the parameter usually measured to characterize wetting. Although useful for hydrophobic surfaces, this approach is limited for hydrophilic mats, where wicking also has to be considered. In this case, it is still unclear how the fiber surface chemical composition and morphology will affect the wetting behavior of electrospun mats. In this work, wetting was studied with different hydrophilic membranes produced by blending thermoplastic elastomer poly(styrene)-b-poly(ethylene-butylene)-b-poly(styrene) (SEBS) with amphiphilic poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) molecules. Three different types of PEO-PPO-PEO, with different molar masses, PEO content, and physical form were used. The effect of these differences on the wetting behavior of the electrospun mats was evaluated by contact angle goniometry, wicking measurements, and different imaging techniques. X-ray photoelectron spectroscopy was used to characterize the surface chemical composition. The smaller molecules quickly saturated the surface at low concentrations, making the mats hydrophilic. The sheath of PEO-PPO-PEO also resulted in fast absorption of water, when comparing the saturated and nonsaturated surfaces. Longer PEO chain-ends seemed to hinder complete segregation and also led to a higher activation time when in contact with water. Liquid PEO-PPO-PEO was easily leached by water.

Blending and Morphology Control To Turn Hydrophobic SEBS Electrospun Mats Superhydrophilic.

Thermoplastic elastomer SEBS, a triblock copolymer composed of styrene (S) and ethylene-co-butylene (EB) blocks, can be dissolved and processed by electrospinning to produce flexible nonwoven mats that can be interesting for applications like filtration or separation membranes. Controlling surface properties such as hydrophobicity/hydrophilicity is critical to achieving a desired performance. In this study, hydrophobic electrospun SEBS mats were obtained, following which an amphiphilic molecule (Pluronic F127) was solution-blended with SEBS prior to electrospinning, in a bid to produce a hydrophilic membrane. The result was a fast-spreading superhydrophilic mat with thinner fibers that preserved the flexibility of the SEBS. The morphologies of nonwoven mats, flat films (prepared by dip-coating using identical solutions) and of the surface of individual fibers were characterized using different microscopy techniques (optical, scanning electron microscopy and atomic force microscopy). Chemical analysis by X-ray photoelectron spectroscopy (XPS) revealed a large F127 concentration in the outermost surface layer. In addition, an analysis of dip-coated flat films revealed that for 20 wt % of F127, there was a change in the blend morphology from dispersed F127-rich regions in the SEBS matrix to an interconnected phase homogeneously distributed across the film that resembled grain boundaries of micellar crystals. Our results indicated that this morphology change at 20 wt % of F127 also occurred to some extent in the electrospun fibers and this, combined with the large surface area of the mats, led to a drastic reduction in the contact angle and fast water absorption, turning hydrophobic electrospun mats superhydrophilic.