Miscibility, crystallization and real-time small-angle X-ray scattering investigation of the semicrystalline morphology in thermosetting polymer blends

Effect of the Simultaneous Addition of Polycaprolactone and Carbon Nanotubes on the Mechanical, Electrical, and Adhesive Properties of Epoxy Resins Cured with Ionic Liquids

Electrically-conductive epoxy nanocomposites (NCs) with improved mechanical and adhesive properties were achieved through the combined addition of poly(ε-caprolactone) (PCL) and carbon nanotubes (CNTs). Three different ionic liquids (ILs) were used as dual role agents, i.e., as both curing and dispersing agents. Regardless of the IL used, the epoxy/PCL matrix of the NCs showed a single-phase behaviour and similar glass transition (Tg) and crosslinking density (νe) values to the unfilled epoxy/PCL/IL systems. Although the CNTs were more poorly dispersed in the epoxy/PCL/CNT/IL NCs than in the reference epoxy/CNT/IL NCs, which led to slightly lower electrical conductivity values, the epoxy/PCL/CNT/IL NCs were still semiconductive. Their low-strain mechanical properties (i.e., flexural modulus and flexural strength) were similar or better than those of the reference epoxy/IL systems and their high-strain mechanical properties (i.e., deformation at break and impact strength) were significantly better. In addition, the positive effects of the PCL and the CNTs on the adhesive properties of the epoxy/IL system were combined. The substitution of ILs for traditional amine-based curing agents and biodegradable PCL for part of the epoxy resin represents an important advance on the road towards greater sustainability.

Ionic Liquid-Cured Epoxy/PCL Blends with Improved Toughness and Adhesive Properties

In this work, ionic liquid (IL)-cured epoxy resins were modified by adding poly(ε-caprolactone) (PCL). Three different ILs were used in order to study how (a) the chemical structure of the ILs and (b) the PCL content affect the phase behaviour, microstructure, mechanical and adhesive properties. Regardless of the IL used or the PCL content, the obtained materials showed a single phase. The addition of PCL to the epoxy resin resulted in plasticizing of the network blends, lower glass transition temperatures (Tg), and crosslinking densities (νe). Low PCL contents did not have a significant impact on the mechanical properties. However, the adhesive properties improved significantly at low PCL contents. Higher PCL contents led to a significant increase in toughness, especially in the case of the imidazolium-based IL. The balance achieved between the mechanical and adhesive properties of these IL-cured epoxy/PCL blends constitutes an important step towards sustainability. This is because a biodegradable polymer (PCL) was used to substitute part of the epoxy resin, and the ILs-which are non-volatile and cure effectively at much lower contents-were used instead of conventional curing agents. Given the wide use of this kind of materials in the adhesive industry, the practical significance of these results must be emphasised.

Digital Light Processing 3D Printing of Enhanced Polymers via Interlayer Welding

Digital light processing (DLP) 3D printing is advantageous in high printing efficiency and printing resolution for fabricating complex structures across various applications. However, the layer-by-layer curing manner of DLP leads to weak interlayer adhesion and the anisotropic mechanical properties of printed objects. Here, linear polymers are introduced into commercial resins to weld the interlayer by the diffusion and entanglement of linear polymers after DLP printing via heat treatment. This introduction of linear polymers not only shows a strengthening and toughening effect on the printed objects, but also has no negative impact on the DLP printability. The tensile strengths of objects containing 4.7wt% polycaprolactone can reach up to ∼500% of that of neat samples in any printing direction. This simple strategy by adding linear polymers into printing resins provides an effective access to prepare DLP printed objects with improved mechanical properties as well as ensure printing resolution and printing efficiency. This article is protected by copyright. All rights reserved.

Microstructure and Cytocompatibility of Electrospun Nanocomposites Based on Poly(ɛ-Caprolactone) and Carbon Nanostructures

Carbon nanostructures (CNSs) are attractive and promising nanomaterials for the next generation of tissue engineering scaffolds, especially in neural prosthesis. Optimizing scaffold vascularization may be an important strategy to promote the repair of damaged brain tissue. In this context, the idea was to evaluate the cell response of electrospun nanohybrid scaffolds loaded with CNSs. Fibrous composites based on poly(ɛ-caprolactone) (PCL) and CNSs were fabricated by means of electrospinning technique. High-purity carbon nanofibers (CNFs) and single-wall carbon nanotubes (SWNTs) were studied. A detailed microstructural characterization was performed to evaluate the most favorable experimental conditions for the realization of fibrous PCL/CNS fabrics. Electrospun mats comprised of rather uniform and homogeneous submicrometric fibers were obtained starting from 1:1 v/v mixture of tetrahydrofuran (THF) and N,N dimethylformamide (DMF). In vitro cytocompatibility tests were performed using rat cerebro-microvascular endothelial cells (CECs). Acquired results showed an increased cell viability for PCL/CNS nanocomposites, suggesting these materials as a suitable environment for endothelial cells. These results are indicative of the promising potential of CNS-based nanocomposites in biomedical devices for tissue engineering applications where endothelial functional properties are required.

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

We have studied the crystallization behavior of crystalline/crystalline blends of poly(ethylene oxide) (PEO) and poly(ε-caprolactone) (PCL) in electrospun nanofibers fabricated from ternary blends of polystyrene (PS), PEO, and PCL, where PS was present as the majority component. It was demonstrated previously that PEO in PS/PEO binary blend nanofibers with a low PEO weight fraction (≦0.2) crystallized predominantly through homogenous nucleation due to the small PEO domain size which excluded the presence of heterogeneities (Soft Matter, 2016, 12, 5110). Here, it was found that PCL in PS/PCL binary blend nanofibers exhibited similar behavior, but at a much lower weight fraction of PCL (≦0.1) due to the presence of an inherently higher concentration of heterogeneities in the PCL homopolymer. In the PS/PEO/PCL ternary blend nanofibers, where the combined weight fraction of PEO and PCL was kept at 0.2 or less, the crystallization of the two components took place separately through both heterogeneous and homogenous nucleation mechanisms. The phase segregated crystallization behavior was further confirmed by the melting behavior of the blend nanofibers and wide angle X-ray diffraction (WAXD) measurements. Most significantly, the homogenous nucleation of both PEO and PCL was suppressed whereas the heterogeneous nucleation was enhanced in the ternary blend nanofibers even at very low weight fraction of PEO or PCL. This was plausibly attributed to the coupling between the crystallization and the liquid-liquid phase separation (LLPS) of the PEO/PCL mixture dispersed in the PS matrix during non-isothermal cooling of the blend nanofibers. Furthermore, it was observed that thermal treatment of the PS/PEO/PCL blend nanofibers above the glass transition temperature of PS further promoted the heterogeneous nucleation-initiated crystallization of PEO because of a complex interplay between Plateau-Rayleigh instability-induced domain breakup and its further coalescence and demixing within the PEO/PCL domains embedded in the PS matrix.

Effects of dynamic vulcanization on the kinetics of isothermal crystallization in a miscible polymeric blend

Is it possible to determine the state of phase separation using the free energy of folding parameter?

Does dynamic vulcanization induce phase separation?

Immiscible and miscible blends of poly(vinylidene fluoride) (PVDF) and acrylic rubber (ACM) were subjected to dynamic vulcanization to investigate the effect of crosslinking on phase separation. As a result of different processability, mixing torque behavior of miscible and immiscible blends was significantly different from one another. Scanning electron microscopy (SEM) was used to investigate the morphology of the system. After dynamic vulcanization, submicron ACM droplets were observed in the samples near the binodal curve of the system under mixing conditions. Small angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) analysis were used to investigate the effect of dynamic vulcanization on the lamellar structure of the system. It was shown that for samples near the boundary of phase separation, increasing the crosslink density led to a decrease in the lamellar long period (L) as a sign of increment of crosslink density induced phase decomposition. Effects of shear rate on the final morphology of the system were investigated by changing the mixing temperature and by comparing the results of dynamic vulcanization at one phase and two phase regions.

Polymer powder processing of cryomilled polycaprolactone for solvent-free generation of homogeneous bioactive tissue engineering scaffolds

Synthetic polymers used in tissue engineering require functionalization with bioactive molecules to elicit specific physiological reactions. These additives must be homogeneously dispersed in order to achieve enhanced composite mechanical performance and uniform cellular response. This work demonstrates the use of a solvent-free powder processing technique to form osteoinductive scaffolds from cryomilled polycaprolactone (PCL) and tricalcium phosphate (TCP). Cryomilling is performed to achieve micrometer-sized distribution of PCL and reduce melt viscosity, thus improving TCP distribution and improving structural integrity. A breakthrough is achieved in the successful fabrication of 70 weight percentage of TCP into a continuous film structure. Following compaction and melting, PCL/TCP composite scaffolds are found to display uniform distribution of TCP throughout the PCL matrix regardless of composition. Homogeneous spatial distribution is also achieved in fabricated 3D scaffolds. When seeded onto powder-processed PCL/TCP films, mesenchymal stem cells are found to undergo robust and uniform osteogenic differentiation, indicating the potential application of this approach to biofunctionalize scaffolds for tissue engineering applications.

Initiator structure influence on thermal and rheological properties of oligo(ε-caprolactone)

Biodegradable thermoplastic oligomers have potential as biomaterials for tissue augmentation and drug-delivery applications. One means of obtaining such a biomaterial is through the ring-opening polymerization of epsilon-caprolactone using an alcohol initiator. In this paper we continue to examine the influence of the structure of the initiator used on the thermal and rheological properties of oligo(epsilon-caprolactone). Specifically, primary and secondary pentanol, and cis- and trans-pentenol were studied as initiators in the preparation of oligomers of constant molecular weight. In agreement with previous work, the secondary conformer yielded higher melt viscosities, lower degrees of crystallinity and lower glass transition temperatures. The cis conformer produced the lowest melt viscosity; however, the activation energy for flow was higher than obtained previously with oleyl alcohol. This result was attributed to the alkane chain lengths on either side of the cis double bond in the initiator. The order of melt viscosity increased with initiator conformer as follows: cis, trans, primary and secondary. The results were explained in terms of oligomer chain flexibility in the melt.

A biodegradable injectable thermoplastic for localized camptothecin delivery

Camptothecin is an example of a potent drug with a short half-life that would benefit from a localized drug depot system that maintains its stability prior to being released. For this reason, a thermoplastic, biodegradable polymer drug depot was prepared and characterized, and the in vitro release of camptothecin examined. epsilon-Caprolactone oligomers were prepared by ring-opening polymerization initiated by various alcohols. The polymers were characterized via differential scanning calorimeter (DSC) for thermal transitions, and via a parallel plate rheometer for melt viscosity. Camptothecin was loaded into the oligomers and released into PBS buffer. The viscosity of the oligomers was alterable by the initiator used. The oligomers were semi-crystalline with melting points between 37 and 45 degrees C. Camptothecin was released from the oligomers in a diffusion-controlled manner, with the release rate increasing as the melt viscosity of the oligomer decreased. The unreleased camptothecin remained in its active lactone form for a period of up to 16 weeks.