Micro- and nano-mechanical characterization of polyamide 11 and its composites containing cellulose nanofibers

Nanocomposites from polyamide 11 and dried cellulose nanofibers (CNs), 16–30 nm in thickness and 50–400 nm in length, were prepared via direct melt mixing and their micro- and nano-mechanical properties were studied. (PF) QNM (Quantitative Nanomechanical Mapping) method was used to map nanomechanical properties at the surface of polyamide 11 and nanocomposites. This new AFM method emphasized both the increased modulus in nanocomposites as compared to the matrix and the microstructure on different levels in polyamide 11 and its nanocomposites. PF QNM showed that their crystalline structure consists of bundles of lamellar stacks, 200–350 nm in width and 20–40 nm wide lamellar stacks. Moreover, PF QNM study emphasized higher structural order in nanocomposites with 3 and 5 wt.% CNs and lower in the nanocomposite with 8 wt.% CNs as compared to the reference. These observations were verified and are consistent with both crystallinity values determined by DSC and micro-mechanical test results. The oriented bundles of lamellar stacks, observed by PF QNM, could be considered as the main blocks determining high mechanical properties for the studied nanomaterials.     

Tailoring the properties of electrospun PHBV mats: Co-solution blending and selective removal of PEO

Microstructure, surface topography, thermal and mechanical features of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) electrospun non-woven mats were modified, modulated and tailored through blending with different polyethylene oxide (PEO) amounts (20, 30 and 50% wt/wt). The optimal parameters of the soaking protocol for the selective removal of the sacrificial polymer were accurately identified by means of scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy, simultaneous thermogravimetric and differential analyses (TG-DTA) and differential scanning calorimetry (DSC). The complete PEO removal after soaking in H₂O for 7 days with daily refreshment was confirmed. The resulting samples were only comprised of PHBV fibers characterized by a remarkable decrease of the average size with respect to the respective blends. Their surface topography was corrugated and rough and presented nodules, pits, nanopores, shallow and elongated nanostructured indents/grooves along the fiber axis. A remarkable reduction (>75%) of the tensile modulus (E) of electrospun PHBV mats (15–20 MPa) was obtained, maintaining comparable elongation at break (ε_max) values (20–30%).     

Micromechanics of polymers

Polymers possess a very large inherent capacity for property modifications. The bridge between structure or morphology and mechanical properties is created by the micromechanical processes of deformation and fracture, the “micromechanics.” Developments mainly in electron microscopy (EM) (scanning, transmission, and high-voltage electron microscopy) and scanning force microscopy (SFM) opened up a wide range of experiments previously impossible, including the in situ study of micromechanical processes. These new techniques are reviewed and used to study micromechanical properties of amorphous and semi-crystalline polymers and several toughened polymers. On the basis of the detailed knowledge of micromechanical mechanisms, a new method of polymer modification becomes a realistic possibility, a method of micromechanical construction of new polymeric systems.     

Mixed mode fracture of a piezoelectric–piezomagnetic bi-layer structure with two un-coaxial cracks parallel to the interface and each in a layer

The purpose of the present work is to study the mixed mode fracture of a piezoelectric–piezomagnetic composite with two un-coaxial cracks parallel to the interface and each in a layer. Methods of generalized dislocation simulation, Green’s function, Cauchy singular integral equation and Lobatto–Chebyshev collocation are combined together to get the numerical results of mechanical strain energy release rate (MSERR). Three kinds of effects are revealed by parametric studies, i.e., the free-surface effect, the shielding effect and the interference effect, and they are used to interpret the characteristics of COD and MSERR curves. In addition, the effects of shear loading, magnetic loading and electric loading on MSERR are also disclosed, respectively, by varying the corresponding loading factor.     

REACTIONS IN SOLID STATE WITHIN POLYURETHANES. KINETICS AND POSTCURE REACTION MECHANISM IN CASTING POLYURETHANE ELASTOMERS

The solid state postcure reaction mechanism of polyurethane elastomers (PU) synthesized using a relatively small excess (up to 10%) of isocyanate was studied. The postcure process succeeds especially with the assistance of atmospheric humidity and, its process velocity depends on PU sample thickness. The polymer network is consolidated mainly by the formation of a new urea group. The formation of allophanate, uretidinedione, and isocyanurate groups and possible reticulations by the intermediary amine groups formed, play only a secondary role in the studied conditions. Kinetic equations regarding the postcure evolution were followed by means of the changes in mechanical properties. The evolution of the process was correlated to different kinetic measurements regarding the elementary processes involved like the consumption of NCO groups, absorption of water from the atmospheric humidity, and desorption of CO₂ resulted during the formation of urea group. The CO₂ desorption appears to be the slowest dynamic process.     

The reinforcing effect of polydopamine functionalized graphene nanoplatelets on the mechanical properties of epoxy resins at cryogenic temperature

In order to obtain epoxy nanocomposites with excellent mechanical properties at cryogenic temperature, an efficient method to functionalize graphene nanoplatelets (GNPs) is proposed. Through a simple dip-coating procedure, the GNPs were first functionalized with deposition of polydopamine coating (PDA@GNPs). Then, using polydopamine as a bridge, the PDA@GNPs were modified with amine groups after polyetheramine T403 grafting (T403-PDA@GNPs). Fourier transform infrared spectroscopy, thermogravimetric analysis and X-ray photoelectron spectroscopy analyses proved the successful functionalization of PDA and polyetheramine T403 on the surface of GNPs. Adding 0.1 wt% T403-PDA@GNPs significantly improved the cryogenic tensile strength and impact strength of the epoxy nanocomposites by 34.5% and 64.5%, which showed greater reinforcing effect than the pristine GNPs (12.6% and 19.1%) and PDA@GNPs (26.3% and 50.1%). The results of dynamic mechanical analysis and scanning electron microscopy observations indicated that the PDA and further polyetheramine T403 functionalization improved the interfacial interactions between GNPs and matrix, which ensured the much improved mechanical properties.     

The role of microstructure on the mechanical properties of polyurethane foams containing thermoregulating microcapsules

Rigid polyurethane foams with up to 50 wt% of microcapsules from LDPE-EVA containing Rubitherm^®RT27 were synthesized. The influence of microcapsules on the foams density, microstructure and mechanical resistance was studied. Cell size and strut and wall thicknesses were analyzed by SEM. The relationships between densities and foam microstructures with their Young's moduli and collapse stress were found by the Gibson and Ashby formulations and the Kerner equation for mechanical properties of composites. It was found a cell structure change from polyhedral closed-cells to spherical or amorphous open-cells. A good agreement between the experimental and theoretical data was observed but requiring a cell form factor. Thus, Fitting parameters confirmed the high trend of these microcapsules to be incorporated into the foam cell walls and the form factors depicted the abrupt change of cell morphology. Thus, these equations are suitable for predicting the mechanical properties of foams containing fillers of low mechanical resistance.     

Mechanical recyclability of technical biopolymers: Potential and limits

The mechanical recyclability of the technical biopolymers polytrimethylene terephthalate (PTT), cellulose acetate butyrate (CAB), polybutylene succinate (PBS) and polyhydroxy alkanoate blend (PHBV/PBAT) was evaluated by assessing the effect of repeated polymer processing (extrusion without further compounding with virgin material or additives) on the material structure and mechanical properties. Reprocessing-induced hydrolytic degradation was found to be the prevalent aging mechanism of the investigated biopolymer grades. However, susceptibility to hydrolysis, and thus maintenance of the performance characteristics, differed strongly between the biopolymer types. To that effect, PTT and CAB especially exhibit a high potential for mechanical recycling. By taking advantage of appropriate additivation, the mechanical recyclability of PBS and PHBV/PBAT is also assumed to be high.     

Effect of melt viscosity and surface tension of polymers on the percolation threshold of conductive-particle-filled polymeric composites

The electrical conductivities of carbon-black-filled low-density polyethylene (LDPE), poly(methyl methacrylate) (PMMA), and poly(vinyl chloride)-vinyl acetate (PVC/ VAc) copolymer were measured as functions of carbon content and melt viscosity of the matrix at the temperatures at which the composites were prepared. Sharp breaks in the relationship between the carbon filler content and the conductivity of composites were observed in all specimens at some content of the carbon filler. The conductivity jumps as much as 10 orders of magnitude at the break point. This phenomenon has been known as the “percolation threshold”. The critical carbon content corresponding to the break point