All issues / Volume 9 (2015) / Issue 3 (March)
This is an editorial article. It has no abstract.
The wear resistance of several thermoplastic polyurethanes (TPUs) having different chemical nature and micronscale arrangement of the hard and soft segments has been investigated by means of erosion and abrasion tests. The goal was correlating the erosion performances of the materials to their macroscopic mechanical properties. Unlike conventional tests, such as hardness and tensile measurements, viscoelastic analysis proved to be a valuable tool to study the erosion resistance of TPUs. In particular, a strict correlation was found between the erosion rate and the high-frequency (~107 Hz) loss modulus. The latter reflects the actual ability of TPU to dissipate the impact energy of the erodent particles.
The present paper is devoted to the verification of the capability of epoxy-SbF5 system as a healing chemistry for rapidly retarding and/or arresting fatigue cracks in epoxy materials at room temperature. Owing to the very fast curing speed of epoxy catalyzed by SbF5, epoxy monomer and the hardener (ethanol solution of SbF5–ethanol complex) are successively infiltrated into the fracture plane under cyclic loading during the tension-tension fatigue test. As a result, the mechanisms including hydrodynamic pressure crack tip shielding, polymeric wedge and adhesive bonding of the healing agent are revealed. It is found that the healing agent forms solidified wedge at the crack tip within 20 s after start of polymerization of the epoxy monomer, so that the highest healing effect is offered at the moment. The epoxy-SbF5 system proves to be effective in rapidly obstructing fatigue crack propagation (despite that its cured version has lower fracture toughness than the matrix), and satisfies the requirement of constructing fast self-healing polymeric materials.
The ductile-brittle transition behaviour of organo modified montmorillonite-based Poly(lactic acid) films (PLA/o-MMT) was analysed using the Essential Work of Fracture (EWF) methodology, Small Punch Tests (SPT) and Enthalpy relaxation analysis. While the EWF methodology could only be applied successfully to de-aged samples, small punch test (SPT) was revealed as more effective for a mechanical characterization during the transient behaviour from ductile to brittle. According to differential scanning calorimetry (DSC) results, physical aging at 30°C of PLA/o-MMT samples exhibited slower enthalpy relaxation kinetics as compared to the pristine polymer. Although all samples exhibited an equivalent thermodynamic state after being stored one week at 30°C, significant differences were observed in the mechanical performances. These changes could be attributed to the toughening mechanisms promoted by o-MMT.
Isothermal and non-isothermal curing kinetics of both N-methyl-2-pyrrolidone (NMP) and N-methylimidazole (MI) based poly(amide-imide) (PAI) resins were investigated by DSC analysis using tightly closed high-pressure crucibles. Several exothermal peaks on the non-isothermal DSC-traces were observed and attributed to the reactions of different functional groups of PAI-resin. Furthermore the final conversion (polymerization degree) of PAI was determined under isothermal conditions, simulating three programs with the post-curing temperatures set as 215, 240 and 270°C. For the MI-PAI based resin, the conversion values were found to be much higher compared to those for the NMP-PAI system. Compared to NMP-based PAI-resin, a shift of the main exothermal peaks to the lower temperatures was observed in the non-isothermal kinetic investigations when MI was used as a solvent. This was accompanied with a reduction of activation energy (Ea) values, as up to a factor of 3 determined by the Flynn-Wall-Ozawa approach for all the main formation reactions. It indicates a catalytic effect of MI on the PAI polymerization. In addition, conversion values were determined according to the Di Benedetto equation for both systems cured using open molds in the oven. Regardless the different post-curing temperatures, the conversion values were similar for all the samples. Thermal and viscoelastic properties as well as crosslink density (nc) were also investigated for these systems. It was found that the MI-based samples demonstrate lower nc values compared to the NMP-based ones at an almost two times higher storage modulus (E') at room temperature.
Polymer blend nanocomposites have been considered as a stimulating route for creating a new type of high performance material that combines the advantages of polymer blends and the merits of polymer nanocomposites. In nanocomposites with multiphase matrices, the concept of using nanofillers to improve select properties (e.g., mechanical, thermal, chemical, etc) of a polymer blend, as well as to modify and stabilize the blend morphology has received a great deal of interest. This review reports recent advances in the field of polyamide (PA) blend-based nanocomposites. Emphasis is placed on the PA-rich blends produced by blending with other thermoplastics in the presence of nanofillers. The processing and properties of PA blend-based nanocomposites with nanofillers are discussed. In addition, the mechanical properties and morphology changes of PA blends with the incorporation of nanofillers are described. The issues of compatibility and toughening of PA blend nanocomposites are discussed, and current challenges are highlighted.
The crystallization characteristics of the α-, β-and γ-phases of isotactic polypropylene were studied for welldefined and fully characterized polymers with varying amounts of stereo- and regio-defects. The specimens enabled us to study separately the influence of the type of chain defect and the concentration of defects on the parameters of interest. A combined defect fraction (CDF) was introduced to describe arbitrary iPP samples with a varying amount of stereo- and regio-defects and a combination thereof. Crystal growth rates were found to decrease linearly with the defect fraction and were substantially stronger influenced by regio-defects as compared with stereo-defects. The deceleration of the growth rate of the β-phase is higher compared to the α-phase with increasing defect fraction. We also found a critical defect fraction, (Xcrit) for which the growth rates of the α- and β-phases are equal. Analysis of the crystallization was performed using the model of Sanchez and Eby. Results of the analysis are in good agreement with the results found for the samples with a variation in the number of stereo-defects. The excess free energy for incorporating a stereo-defect into the trigonal crystal lattice of the β-phase is lower as compared with the α-phase. The theory correctly predicts the critical defect fraction, for which the growth rate of the α- and β-phase is equal.
The present study deals with thermally induced one-way and invertible two-way shape-memory effect (SME) in covalent networks on the basis of crystallizable (co)polymers and their blends and is an attempt to generalize the results of own investigation received by the authors in the last ten years. The main focus of work clearly lies on research of covalently crosslinked binary and ternary blends having two and three crystalline phases with different thermal stability, respectively. The existence of two or three crystalline phases possessing different melting and crystallization temperatures in heterogeneous polymer networks can lead to triple-shape or even quadruple-shape behavior of such networks. However, the performed investigations point to crucial effect of phase morphology of crosslinked polymer blends on multiplicity of their shapememory behavior beside the influence of blend content, crystallinity and cross-link density of blend phases as well as of processing conditions. For instance, triple-shape memory behavior in binary blends can be realized only if the continuous phase has a lower melting temperature than the dispersed phase. Cross-linked polymer blends are a facile alternative to expensive and complex synthesis of interpenetrating or block-copolymer networks used for shape memory polymers. In addition to findings of experimental investigation of SME in crystallizable covalent polymer networks, the results of modeling their shape-memory behavior on the basis of self-developed physically reasonable model have been briefly described and discussed. Thereby, good accordance between results of theory and experiment was achieved with physically justified fitting parameters.
Nanocomposite fibers of isotactic polypropylene – fumed silica AR805 were prepared by melt compounding using a two-step process: melt-spinning and hot drawing at various draw ratios up to 15. Transmission electron microscopy revealed uniform dispersion of the silica nanoparticles in polypropylene matrix, although at higher concentrations and lower draw ratios the nanoparticles showed increasing tendency to form small agglomerates. On the other hand, at low concentrations the uniform distribution of fumed silica improved mechanical properties of the composite fibers, especially at higher draw ratios. Crystallinity and melting temperature of fibers were found to significantly increase after drawing. Elastic modulus at draw ratio = 10 rose from 5.3 GPa for neat PP up to 6.2–8.1 GPa for compositions in the range 0.25–2 vol% of the filler. Moreover, higher tensile strength and creep resistance were achieved, while strain at break was rather insensitive to the filler fraction. Considering all experimental results, a failure model was proposed to explain the toughness improvement during the drawing process by the induced orientation of polymer chains and the formation of voids.
In this paper we apply and discuss some new aspects to the steric interaction of filler particles in reinforced elastomers under dynamic mechanical loading conditions. At certain concentration the filler particles (for example, carbon black, or silica) form loose clusters which themselves interact with each other and form a filler network with a significant contribution to the dynamic modulus of the rubber material. The filler concentration is relatively high, so that it is likely that the clusters undergo a ‘jamming transition’. With increasing strain amplitude under periodic mechanical deformation the disruption of the filler network resp. of finite filler cluster configurations leads to dejamming observed as softening of the rubber. As a theoretical approach we map the problem on a simple one dimensional Ising model. We present here a static model of this jamming (dejamming) and discuss the consequences on the mechanical and deformation properties of the filled rubber.
Biodegradable coronary stents have been under development for several years and a trend in biodegradable stent material development has emerged: reinforcement to enhance mechanical properties and creep resistance to improve vessel support. The aim of this work is to investigate the mechanical and viscoelastic characteristics of poly(L-lactic acid)/poly (glycolic acid) (PLLA/PGA) microfibrillar polymer-polymer composites (MFCs) at 37°C to determine the suitability of PGA fibrils as a reinforcement for polymeric, biodegradable stents. PLLA/PGA MFCs were produced via cold-drawing and subsequent compression moulding of extruded PLLA/PGA blend wires. Scanning electron microscopy revealed excellent fibril formation in the case of a 70/30 wt% PLLA/PGA MFC- the mean fibril diameter being 400 nm and aspect ratios exceeding 250. Tensile tests demonstrate Young’s modulus and strength increases of 35 and 84% over neat PLLA in the case of a 70/30 wt% PLLA/PGA MFC. Creep resistance of the PLLA/PGA MFCs is lower than that of neat PLLA, as shown via relaxation. Dynamic mechanical thermal analysis demonstrates that it is the onset of glass transition of PGA that is the underlying cause for low creep resistance of the PLLA/PGA MFCs at 37°C.
Mechanical properties and crystalline structure of isotactic polypropylene (iPP) types were studied using polymers, which were polymerized differently in order to obtain diverse molecular architectures. The objective of this work is to describe quantitative correlation between the crystalline structure and the elastic modulus in order to predict structures with expectably advantageous properties. The molecular mass was measured by gel permeation chromatography (GPC) and the regularity of molecular structure was investigated by Fourier transform infrared spectroscopy (FTIR) and stepwise isothermal segregation technique (SIST). The chain regularity of the studied samples is varying in a wide range according to the results of SIST and FTIR measurements. The crystalline structure was characterized by differential scanning calorimetry (DSC) and wide angle X-ray scattering (WAXS). The tensile properties were determined by standardized tensile tests. The results indicate clearly that the increased chain regularity is accompanied by a proportional advancement in crystallinity and consequently proportionally larger stiffness. Moreover, the results of this work were compared to those obtained on other previously produced iPP samples and it can be established that the correlations found during this work are valid generally. An empirical model was developed also, which connects the stiffness to the structural parameters of iPP and makes possible the design and prediction of materials with targeted molecular structure and properties.
This paper is aimed at studying the failure behavior of polyolefin-based self-reinforced polymer composites (SRPCs) via acoustic emission (AE). Three matrix materials (ethylene octene copolymer (EOC), polypropylene-based thermoplastic elastomer (ePP), random polypropylene copolymer (rPP), and three kinds of reinforcing structures of PP homopolymer (unidirectional (UD), cross-ply (CP) and woven fabric (WF)) were used. SRPCs were produced by compression molding using the film-stacking method. The composites were characterized by mechanical tests combined with in situ assessment of the burst-type AE events. The results showed that rPP matrix and UD reinforcement produced the greatest reinforcement, with a tensile strength more than six times as high as that of the matrix and a Young’s modulus nearly doubled compared to the neat matrix. The number of the detected AE events increased with increasing Young’s modulus of the applied matrices being associated with reduced sound damping. The AE amplitude distributions shows that failure of the SRPC structure produces AE signals in a broad amplitude range, but the highest detected amplitude range can be clearly linked to fiber fractures.