No abstract

In this research paper, we investigated the impact of soft segment crystallization, the cross‐linking density (CLD) and elastic modulus of the synthesized thermoset polyurethanes on their shape memory performance (SMP). A group of tri‐block copolymers of poly(ϵ‐caprolactone) (PCL) and poly(tetramethylene glycol) (PTMG) with PCLx‐PTMG2000‐PCLx architecture were synthesized and used as polyols in a thermoset polyurethane (PU) structure. Crystallization of the soft segments was controlled through changing PCLx blocks’ length. Graphene nanoplatelets, with a fixed content of 0.50 wt.%, was used as the cross‐linker and the nano‐filler to tune elasticity of the PUs. Analysis of crystallization showed that by increasing PCLx’s length from 0 to 2000 D caused a drastic change in the crystallization behavior of the polyols. The polyols used in the thermoset PU nanocomposite led to a wide spectrum of elastic modulus at temperatures close to room temperature. The elastic modulus at room temperature ranged from 20 to 100 MPa. The changes in elastic modulus, CLD and soft segments’ crystallization resulted in a complex SMP behavior, which was studied in‐detail. The shape recovery rate was also studied, which showed the impact of crystallites’ melting, CLD and elastic modulus through a three‐stage shape recovery.

The one-pot synthesis of hybrid hollow nanoparticles of symmetric and asymmetric shapes is a challenging task and has rarely been reported. This work proposes a method for a high-yield synthesis of hybrid hollow carbon particles. In the first step, hexadecane/styrene (HD/St) is encapsulated in a silica shell. Then, by the polymerization of St, a silica/polystyrene double shell is formed. Finally, hollow carbon particles with bowl-like and crumpled shapes are obtained by carbonization. The results show that the ratio of diameter to thickness (D/H) for obtaining crumpled particles is ∼4–12, whereas this ratio is ∼7–18 for bowl-like particles. We study the effects of HD and St concentrations on the D/H ratio and the composition of hybrid particles. In contrast to suspensions of hollow carbon spheres, the suspensions of hybrid nanoparticles show shear-thinning behavior over the examined range of shear rates, which is attributed to their enhanced packing. The shape effect of hybrid particles also increases their adsorption on human mesenchymal stem cells (hMSCs) compared to the hollow carbon spheres.

The properties of colloids are considerably affected by particles’ surface morphology. In this work, for understanding the mechanism of roughness formation in polymeric core–shell (CS) particles, the surface morphology of synthesized CS particles through seeded dispersion polymerization (SDP) in the presence of poly­(methyl methacrylate) seeds was investigated. The results revealed that shell polymers with higher solubility parameters (δ) and glass transition temperatures (T g) had a rougher surface. These parameters directly affect the time needed for chain deformation, which is a critical parameter in controlling the final morphology. We suggested a relation based on these parameters to predict the surface morphology (smoothness or roughness) of CS particles synthesized through SDP in water.

The effects of several factors (i.e., nanoclay content (NC), melt temperature, screw speed, and feeding rate), on morphology, rheology, thermodynamics, and thermal stability of PA6/NC samples produced in a corotating twin-screw extruder were studied. We discuss how changes in the level of these factors can vary diffusion or imposed shear, how these variations in diffusion and/or shear can affect NC dispersion, and how the changes in the state of NC dispersion can influence several properties of the samples. Samples with low NC content, below percolation threshold, showed exfoliated/intercalated structure with negligible sensitivity to changes in the level of the factors; whereas, samples with high NC content, above percolation threshold, showed intercalated with sporadic flocculated structures and noticeable sensitivity to the changes in the level of the factors. Moreover, NC dispersion was found to be mostly diffusion-controlled: changes that resulted in higher diffusion or residence time (i.e., high melt temperature or low screw speed) also eventuated in better NC dispersion. What is more, percolation threshold was seen to move to lower NC contents as diffusion rate was increased. Moreover, as NC content increased, opposite thermal stability behaviors were observed at above and below the percolation threshold.

~ Glass reinforced polyesters (GRP), or the resins used, have to pass a wide variety of tests which vary considerably from country to country and from industry to industry. Some of these tests are carried out on a scale outside the capabilities of most laboratories and the results leave much to be desired in the matter of reproducibility. All these factors lead to problems for the formulator of fire-retardant unsaturated polyester resins.