Johannes Gerardus Petrus Goossens

In this research, poly(ε-caprolactone) (PCL) was melt-mixed with sepiolite nanoclays in a twin-screw extruder. In a subsequent step, the extruded films were drawn in the solid state to highly oriented nanocomposite films or tapes. A twin-screw extruder equipped with a Sultzer mixer for improved mixing in combination with a bench top drawing unit was used to prepare oriented nanocomposite tapes of different sepiolite loading and draw ratios. In order to study the influence of the solidification step on the drawability of the materials, different cooling procedures were applied prior to drawing. Optical microscopy images showed that slow or fast solidification using different chill rolls settings (open or closed) for the cast films resulted in different morphological conditions for subsequent drawing. The addition of sepiolite nanofillers led to nucleation and faster crystallization kinetics and oriented tapes which deformed by homogenous deformation rather than necking. The addition of sepiolite significantly improved the mechanical properties of both undrawn and drawn PCL tapes and Young’s modulus (1.5 GPa) and tensile strength (360 MPa) for composites based on 4 wt% sepiolite were among the highest ever reported for PCL nanocomposites. Interestingly, samples cooled with open chill rolls (slow crystallization) showed the highest modulus while solidification with closed rolls (fast crystallization) showed the highest tensile strength after drawing.

Different techniques of grafting polymer chains to filler surfaces are often employed to compatibilise filler and polymer matrices. In this paper the influence of graft length and graft density on the state of dispersion, crystallisation and rheological properties of poly(ε-caprolactone) (PCL)/silica (SiO₂) nanocomposites are reported. Grafted silica nanoparticles were prepared through polymerisation of PCL from the nanoparticle surface. Graft length was controlled by the reaction time, while the grafting density was controlled by the monomer-to-initiator ratio. Grafted nanoparticles were mixed with PCL of different molecular weights and the state of dispersion was assessed. Different matrix-to-graft molecular weight ratios resulted in different states of dispersion. Composites based on the higher molecular weight matrix exhibited small spherical agglomerates while the lower molecular weight matrix revealed more sheet-like microstructures. The state of dispersion was found to be relatively independent of graft length and density. Under quiescent conditions the grafts showed increased nucleation ability in the higher molecular weight PCL, while in the lower molecular weight matrix the effect was less pronounced. Rheological experiments showed an increase in viscosity with increased filler content, which was beneficial for the formation of oriented structures in shear-induced crystallisation.

Vitrimers, which form a bridge between thermosets and thermoplastics, are a class of materials with promising opportunities for modern material innovations. Poly(butylene terephthalate) (PBT) vitrimers that combine the properties of neat PBT and those of adaptable networks are expected to greatly extend the potential applications of this industrially important engineering plastic. The current study aims at building up a tailor-made semi-crystalline vitrimer through understanding the effect of the dynamics and density of the adaptable network on the physical properties of PBT vitrimers. We show that the rubber plateau modulus of PBT vitrimers is almost exclusively governed by the cross-linker (glycerol) content, whereas the ratio of the glycerol to the Zn(II) catalyst content strongly influences both the elastic and stress relaxation properties. This enables independent tuning of the tensile storage modulus (E′) and rubber plateau modulus. The PBT vitrimer exhibits a better creep resistance than neat PBT at service temperature while a similar crystallinity is maintained.

Although the network dynamics and mechanical properties of poly(butylene terephthalate) vitrimers can to some extent be controlled via chemical and physical approaches, it remains a challenge to be able to process PBT vitrimers with the same processing conditions via, for example, injection molding as neat PBT. Here, it is shown that the use of protected pentaerythritol as a latent cross‐linker and the use of a Zn(II) transesterification catalyst allows for the in situ dynamic network formation in PBT during processing, with a delayed onset of gelation. This process can be controlled by adjusting the processing temperature, (protected) cross‐linker content, and the type of protection group. This solvent‐free deprotection strategy opens the way to high production rates of PBT vitrimer products via injection molding with the combination of low viscosity during processing and vitrimer characteristics in the final product.

Molecular precatalysts complexes (nBuCp)₂ZrCl₂ (Zr) and (η1:η5‐Me₂NCH₂CH₂C₅Me₄)CrCl₂ (Cr) have been successfully supported on silica nanoparticles, via a single support (SS) or a double support (DS) strategy. These catalyst systems have been successfully used to produce bimodal polyethylenes with an ultrahigh molecular weight polyethylene content in a single reactor. The SS and DS catalyst systems have been fully evaluated under an identical polymerization condition to assess the challenges in tailoring the molecular weight distribution. The results show that a detrimental interaction exists between Zr and Cr catalysts, part of the Cr catalyst species is deactivated during polymerization in the both DS and SS systems. The detrimental interaction in the DS system is reduced because the catalysts are supported on separate nanoparticles. But, surprisingly the two catalysts in the DS system are still able to “communicate” to each other via cocatalyst‐induced catalyst leaching or other potential reasons. image

The crystallization behavior of poly(lactic acid) (PLA) has been studied extensively, and this has resulted in different reported values for the nucleation densities (Ns) and crystal growth rates (Gs) for similar grades. These inconsistencies may be magnified when they are used in subsequent modeling studies. Therefore, the quiescent crystallization behaviors of three PLA grades were studied with polarized optical microscopy and small‐angle light‐scattering experiments. The Gs and Ns were determined at several isothermal crystallization temperatures with a device that provided near‐instantaneous cooling to the isothermal crystallization temperature. Two growth rate regimes, which were attributed to α and α′ crystallization with a transition around 120 °C, were observed. Avrami analysis revealed that the poly(l‐lactic acid) homopolymer crystal growth was three‐dimensional and was unaffected by the presence of stereocomplex PLA. The PLA copolymer crystals had a transition from an initial sheaflike conformation to three‐dimensional growth. Furthermore, the lamellar twisting of the homopolymer was observed at the isothermal crystallization temperature around 144 °C. These findings can be used for future modeling studies to predict material behavior in various industrial processes. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44566.