Crystallization behavior of crystalline/crystalline polymer blends under confinement in electrospun nanofibers of polystyrene/poly(ethylene oxide)/poly(ε-caprolactone) ternary mixtures

Thermomechanical Properties of Virgin and Recycled Polypropylene-High-Density Polyethylene Blends

This paper provides evidence and discusses the variability in the thermomechanical behaviour of virgin and recycled polypropylene/high-density polyethylene blends without the addition of other components, which is sparse in the literature. Understanding the performance variability in recycled polymer blends is of critical importance in order to facilitate the re-entering of recycled materials to the consumer market and, thus, contribute towards a circular economy. This is an area that requires further research due to the inhomogeneity of recycled materials. Therefore, the thermal and mechanical properties of virgin and recycled polypropylene/high-density polyethylene blends were investigated systematically. Differential scanning calorimetry concludes that both the recycled and virgin blends are immiscible. Generally, recycled blends have lower overall crystallinity and melting temperatures compared with virgin blends while, remarkably, their crystallisation temperatures are compared favourably. Dynamical mechanical analysis showed little variation in the storage modulus of recycled and virgin blends. However, the alpha and beta relaxation temperatures are lower in recycled blends due to structural deterioration. Deterioration in the thermal and mechanical properties of recycled blends is thought to be caused by the presence of contaminants and structural degradation during reprocessing, resulting in shorter polymeric chains and the formation of imperfect crystallites. The tensile properties of recycled blends are also affected by the recycling process. The Young's modulus and yield strength of the recycled blends are inferior to those of virgin blends due to the deterioration during the recycling process. However, the elongation at break of the recycled blends is higher compared with the virgin blends, possibly due to the plasticity effect of the low-molecular-weight chain fragments.

Air-jet spinning corn zein protein nanofibers for drug delivery: Effect of biomaterial structure and shape on release properties

Nanofiber materials are commonly used as delivery vehicles for dermatological drugs due to their high surface-area-to-volume ratio, porosity, flexibility, and reproducibility. In this study air-jet spinning was used as a novel and economic method to fabricate corn zein nanofiber meshes with model drugs of varying solubility, molecular weight and charge. The release profiles of these drugs were compared to their release from corn zein films to elucidate the effect of geometry and structure on drug delivery kinetics. In film samples, over 50% of drug was released after only 2 h. However, fiber samples exhibited more sustained release, releasing less than 50% after one day. FTIR, SEM, and DSC were performed on nanofibers and films before and after release of the drugs. Structural analysis revealed that the incorporation of model drugs into the fibers would transform the zein proteins from a random coil network to a more alpha helical structure. Upon release, the protein fiber reverted to its original random coil network. In addition, thermal analysis indicated that fibers can protect the drug molecules in high temperature above 160 °C, while drugs within films will degrade below 130 °C. These findings can likely be attributed to the mechanical infiltration of the drug molecules into the ordered structure of the zein fibers during their solution fabrication. The slow release from fiber samples can be attributed to this biophysical interaction, illustrating that release is dictated by more than diffusion in protein-based carriers. The controlled release of a wide variety of drugs from the air-jet spun corn zein nanofiber meshes demonstrates their success as drug delivery vehicles that can potentially be incorporated into different biological materials in the future.

Synthesis, crystallization, and molecular mobility in poly(ε-caprolactone) copolyesters of different architectures for biomedical applications studied by calorimetry and dielectric spectroscopy

In this work, we synthesized poly(ε-caprolactone) (PCL) and three copolyesters of different architectures based on three different alcohols, namely a three arm-copolymer based on 1% glycerol (PCL_Gly), a four arm-copolymer based on 1% pentaerythrytol (PCL_PE), and a linear block copolymer based on ∼50% methoxy-poly(ethylene glycol) (PCL_mPEG), all simultaneously with the ring opening polymerization (ROP) of PCL. Due to their biocompatibility and low toxicity, these systems are envisaged for use in drug delivery and tissue engineering applications. Due to the in situ ROP during the copolyesters synthesis, the molecular weight of PCL, Wm initially ∼62 kg mol-1, drops in the copolymers from ∼60k down to ∼5k. For the structure-properties investigation we employed differential scanning calorimetry (DSC and TMDSC), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), Fourier transform infra red (FTIR) spectroscopy, polarized optical microscopy (POM), broadband dielectric spectroscopy (BDS) and isothermal water sorption. DSC revealed that the crystalline fraction of PCL increases whereas the crystallization rate drops in the copolymers in the order PCL ∼ PCL_Gly > PCL_PE ≫ PCL_mPEG, which coincides with that of decreasing Wm. In PCL_mPEG the major amount of PCL (87%) was found to crystallize while the majority of mPEG (92%) was found amorphous exhibiting constrained amorphous mobility and severely slower/weaker crystallization as compared to neat mPEG. Segmental dynamics in BDS, in agreement with DSC, is similar and in general slow for the samples of star-like structure for Wm ≥ 30k arising from PCL, whereas it is severely faster and enhanced in strength for the linear PCL_mPEG (lower Wm) copolymer arising from mPEG. For the latter system, the data provide indications for the formation of complex structures consisting of many small PCL crystallites surrounded by amorphous mPEG segments with constrained dynamics and severely suppressed hydrophilicity. These effects cannot be easily assessed by conventional XRD and POM, confirming the power of the dielectric technique. The overall recordings indicated that the different polymer architecture results in severe changes in the semicrystalline morphology, which demonstrates the potential for tuning the final product performance (permeability, mechanical).

Well-Blended PCL/PEO Electrospun Nanofibers with Functional Properties Enhanced by Plasma Processing

Biodegradable composite nanofibers were electrospun from poly(ε-caprolactone) (PCL) and poly(ethylene oxide) (PEO) mixtures dissolved in acetic and formic acids. The variation of PCL:PEO concentration in the polymer blend, from 5:95 to 75:25, revealed the tunability of the hydrolytic stability and mechanical properties of the nanofibrous mats. The degradation rate of PCL/PEO nanofibers can be increased compared to pure PCL, and the mechanical properties can be improved compared to pure PEO. Although PCL and PEO have been previously reported as immiscible, the electrospinning into nanofibers having restricted dimensions (250–450 nm) led to a microscopically mixed PCL/PEO blend. However, the hydrolytic stability and tensile tests revealed the segregation of PCL into few-nanometers-thin fibrils in the PEO matrix of each nanofiber. A synergy phenomenon of increased stiffness appeared for the high concentration of PCL in PCL/PEO nanofibrous mats. The pure PCL and PEO mats had a Young’s modulus of about 12 MPa, but the mats made of high concentration PCL in PCL/PEO solution exhibited 2.5-fold higher values. The increase in the PEO content led to faster degradation of mats in water and up to a 20-fold decrease in the nanofibers’ ductility. The surface of the PCL/PEO nanofibers was functionalized by an amine plasma polymer thin film that is known to increase the hydrophilicity and attach proteins efficiently to the surface. The combination of different PCL/PEO blends and amine plasma polymer coating enabled us to tune the surface functionality, the hydrolytic stability, and the mechanical properties of biodegradable nanofibrous mats.

Block copolymer compatibilization driven frustrated crystallization in electrospun nanofibers of polystyrene/poly(ethylene oxide) blends

The confined crystallization behaviour of poly(ethylene oxide) (PEO) has been studied in electrospun nanofibers of the phase-separated blends of polystyrene (PS) and PEO compatibilized with polystyrene-block-poly(ethylene oxide) (PS-b-PEO) block copolymer. The PS was present as the majority component such that the electrospun nanofibers consisted of PEO domains dispersed in the PS matrix. The phase separation in the blend occurred under the radial constraint of the nanofibers which led to the formation of small-sized fibrillar PEO domains. The use of block copolymer compatibilizer resulted in a noticeable decrease in the PEO domain size in the as-spun nanofibers. Moreover, the decrease in the domain size and domain connectivity was more substantial in the thermally annealed blend nanofibers due to the suppression of the domain coalescence mechanism resulting from the localization of the PS-b-PEO block copolymer at the interface. Consequently, the fraction of PEO domains crystallizing via homogeneous nucleation increased in the compatibilized blend nanofibers due to the presence of higher number of heterogeneity free PEO domains and disruption in their spatial connectivity. Interestingly, in the compatibilized blend nanofibers consisting of low molecular weight PEO, additional crystallization event attributed to surface nucleation was observed. The surface nucleation, plausibly, resulted from the formation of wet-brush structures where the PEO homopolymers homogeneously wet the PEO blocks present at the interface. In such a scenario, the PEO crystallization occurred via surface nucleation at the domain interface. The surface nucleated crystallization was absent in the compatibilized blend nanofibers composed of high molecular weight PEO presumably due to the formation of morphology with dry-brush structures.

Confined crystallization behaviour of poly(ethylene oxide) (PEO) was studied in electrospun nanofibers of the phase-separated blends of polystyrene (PS) and PEO compatibilized with polystyrene-block-poly(ethylene oxide) (PS-b-PEO) block copolymer.