Martina Nevoralová

Synergistic effects in mechanical properties of PLA/PCL blends with optimized composition, processing, and morphology

Poly(lactic acid) (PLA) is a promising material for biomedical applications due to its biodegradability and high stiffness, but suffers from low toughness. We report that blending of PLA with another biodegradable polymer, poly(e-caprolactone) (PCL), can increase the impact strength above the values of the individual components, while the other important macro- and micromechanical properties remain at well-acceptable level (above the theoretical predictions based on equivalent box model). Although some previous studies indicated incompatibility of PLA and PCL polymers, we demonstrate that the melt-mixing of the polymers with optimized viscosities (PLA/PCL viscosity ratio ∼ 1), the optimized composition (PLA/PCL = 80/20 by weight), and the optimal processing (compression molding with fast cooling) leads to optimal morphology (∼0.6 μm particles of PCL in PLA matrix) and synergistic effect in the mechanical performance of the systems. In an additional set of experiments, we show that the addition of TiO2 nanoparticles slightly improves stiffness, but significantly reduces the toughness of the resulting nanocomposites. The investigated systems were characterized by electron microscopy (SEM and TEM), notched impact strength, dynamic mechanical analysis, and microindentation hardness testing.

Strong synergistic effects in PLA/PCL blends: Impact of PLA matrix viscosity

Blends of two biodegradable polymers, poly(lactic acid) (PLA) and poly(ϵ-caprolactone) (PCL), with strong synergistic improvement in mechanical performance were prepared by melt-mixing using the optimized composition (80/20) and the optimized preparation procedure (a melt-mixing followed by a compression molding) according to our previous study. Three different PLA polymers were employed, whose viscosity decreased in the following order: PLC ≈ PLA1 > PLA2 > PLA3. The blends with the highest viscosity matrix (PLA1/PCL) exhibited the smallest PCL particles (d∼0.6μm), an elastic-plastic stable fracture (as determined from instrumented impact testing) and the strongest synergistic improvement in toughness (>16× with respect to pure PLA, exceeding even the toughness of pure PCL). According to the available literature, this was the highest toughness improvement in non-compatiblized PLA/PCL blends ever achieved. The decrease in the matrix viscosity resulted in an increase in the average PCL particle size and a dramatic decrease in the overall toughness: the completely stable fracture (for PLA1/PCL) changed to the stable fracture followed by unstable crack propagation (for PLA2/PCL) and finally to the completely brittle fracture (for PLA3/PCL). The stiffness of all blends remained at well acceptable level, slightly above the theoretical predictions based on the equivalent box model. Despite several previous studies, the results confirmed that PLA and PCL could behave as compatible polymers, but the final PLA/PCL toughness is extremely sensitive to the PCL particle size distribution, which is influenced by both processing conditions and PLA viscosity. PLA/PCL blends with high stiffness (due to PLA) and toughness (due to PCL) are very promising materials for medical applications, namely for the bone tissue engineering.

Coalescence during annealing of quiescent immiscible polymer blends

Abstract Coalescence during annealing of quiescent immiscible polymer blends, containing polypropylene (PP) matrix and different amount of ethylene-propylene copolymer (EPR) as dispersed phase, was studied. Comparison of experimental results with available coalescence theories revealed that changes in the phase structure after 20 min annealing can be precisely estimated using the approximate theory of coalescence induced by van der Waals forces and considering drainage of the matrix film between spherical droplets. All results evidenced growth of the EPR droplet size with the annealing time and temperature; increase in the content of dispersed phase contributed to higher growth rate of the dispersed phase, more pronounced at the coalescence origin. Anisometry of the EPR droplets and droplet shape relaxation were perceived, both interfering the course of coalescence. Enhanced elasticity of examined blends at low frequencies and positive deviation of the complex viscosity from the linear mixing rules was observed.

Influence of clay-nanofiller geometry on the structure and properties of poly(lactic acid)/thermoplastic polyurethane nanocomposites

The effect of two clays with different geometries, viz. organophilized montmorillonite nanoplatelets (oMMT) and halloysite nanotubes (HNT), on the structure and properties of poly(lactic acid) (PLA)/thermoplastic polyurethane (TPU) 80/20 and 70/30 has been studied. The reinforcement of both the components and the blend is higher for oMMT due to its higher specific surface area and aspect ratio. The effect of both nanofillers (NFs) on the structure, reflected in a decrease of the dispersed TPU size, is comparable. A larger effect on the viscosity by oMMT is eliminated by its more marked less favourable localization in the dispersed phase in comparison with HNT. The fact that the toughness of the HNT-containing nanocomposite (NC) is markedly higher (with maximum at 1% HNT) than that with oMMT is in contradiction with PLA NC. The results indicate that the application of nanofillers can lead to a polymer system with a balanced mechanical behaviour. However, a different impact of NFs on the polymer components and interface parameters may also lead to antagonistic effects. The influence of NFs, including their geometry, on the polymer blend is more complex in comparison with single-matrix nanocomposites.

Graphite nanoplatelets-modified PLA/PCL: Effect of blend ratio and nanofiller localization on structure and properties

Structure and properties of poly(lactic acid) (PLA)/poly (ɛ-caprolactone) (PCL) influenced by graphite nanoplatelets (GNP) were studied in dependence on blend composition. Electron microscopy indicates predominant localization of GNP in PCL. GNP-induced changes in viscosity hinder refinement of PCL inclusions, support PCL continuity in the co-continuous system, and lead to reduction of PLA inclusions size without GNP being present at the interface in the PCL-matrix blend. Negligible differences in crystallinity of both phases indicate that mechanical behaviour is mainly influenced by reinforcement and GNP-induced changes in morphology. Addition of 5 parts of GNP leads to ~40% and ~25% increase of stiffness in the PCL- and PLA-matrix systems, respectively, whereas the reinforcing effect is practically eliminated in the co-continuous systems due to GNP-induced lower continuity of PLA which enhances toughness. Impact resistance of the 80/20 blend shows increase with 5 parts content due to synergistic effect of PCL/GNP stacks, whereas minor increase in the blend of the ductile PCL matrix with brittle PLA inclusions is caused by GNP-modification of the component parameters. Results indicate high potential of GNP in preparing biocompatible systems with wide range of structure and properties.

Thermoplastic starch composites with TiO2 particles: Preparation, morphology, rheology and mechanical properties

Composites of thermoplastic starch (TPS) with titanium dioxide particles (mTiO2; average size 0.1μm) with very homogeneous matrix and well-dispersed filler were prepared by a two-step method, including solution casting (SC) followed by melt mixing (MM). Light and scanning electron microscopy confirmed that only the two-step procedure (SC+MM) resulted in ideally homogeneous TPS/mTiO2 systems. The composites prepared by single-step MM contained non-plasticized starch granules and the composites prepared by single-step SC suffered from mTiO2 agglomeration. Dynamic mechanical measurements showed an increase modulus with increasing filler concentration. In TPS containing 3wt.% of mTiO2 the stiffness was enhanced by >40%. Further experiments revealed that the recommended addition of chitosan or the exchange of mTiO2 for anisometric titanate nanotubes with high aspect ratio did not improve the properties of the composites.

Quantification of structural changes of UHMWPE components in total joint replacements

BackgroundAt present time the number of implantations of joint replacements as well as their revisions increases. Higher demands are required on the quality and longevity of implants. The aim of this work was to determine the degree of oxidative degradation and the amount of free/residual radicals in selected ultra-high molecular weight polyethylene (UHMWPE) components of the joint replacements and demonstrate that the measured values are closely connected with quality and lifetime of the polymer components.MethodsWe tested both new (4 samples) and explanted (4 samples) UHMWPE polymers for total joint replacements. The samples were characterized by infrared spectroscopy (IR), electron spin resonance (ESR) and microhardness (MH) test. The IR measurements yielded the values of oxidation index and trans-vinylene index. The ESR measurements gave the free radicals concentration.ResultsIn the group of new polyethylene components, we found oxidation index values ranging from 0.00-0.03 to 0.24. The trans-vinylene index values ranged from 0.044 to 0.080. The value of free radical concentration was zero in virgin and also in sample of Beznoska Company and non-zero in the other samples. In the group of explanted components, the measured values were associated with their history, micromechanical properties and performance in vivo.ConclusionsWe demonstrated that measuring of oxidative damage may help the orthopaedic surgeon in estimating the quality of UHMWPE replacement component and thus radically to avoid early joint replacement failure due to worse polyethylene quality.

Effect of graphene oxide on structure and properties of impact modified polyamide 6

ABSTRACT Modification of polymer blends with nanofillers is an efficient way to improve material parameters. This work deals with application of neat and stearylamine-modified graphene oxide in the polyamide 6/elastomer system using different mixing protocols. Combination of ethene–propene elastomer and graphene oxide leads to a polyamide material with increased strength, stiffness, and toughness. The reason is synergistic effect of the core–shell structure (nanofiller localized at interface) on mechanical properties. The structure-directing effect of graphene oxide is comparable with that of nanoclay. The mechanism of affecting dynamic phase behavior is different as a consequence of graphene oxide nature and interactions with polymer components. GRAPHICAL ABSTRACT

Improvement of performance of a ductile/brittle polymer system by graphite nanoplatelets: effect of component coupling

Addition of a high-modulus polymer to a pseudo-ductile matrix may lead to increased strength, stiffness and toughness. This can be achieved by plastic deformation of a well-dispersed phase with higher modulus and lower Poissons ratio compared with the matrix. Recently, this system has also been successfully modified by organophilized montmorillonite. In this work, a reactively compatibilized PA6/PS system is upgraded using modified graphite nanoplatelets (GNP) in combination with their simultaneous coupling to polymer components. The best balanced mechanical properties have been obtained in the case of the combination of amine-modified GNP with a styrene maleic-anhydride copolymer. Here, coupling of both polymer phases with GNP could take place. Structure of the in situ formed adduct can be controlled by component ratio and mixing protocols. The complex effect of such modified GNP on the system behaviour, including favourable change of components parameters and modification of the interface, is discussed.

Facile preparation of biocompatible poly (lactic acid)-reinforced poly(ε-caprolactone) fibers via graphite nanoplatelets -aided melt spinning

Addition of high-aspect-ratio (AR) nanofillers can markedly influence flow behavior of polymer systems. As a result, application of graphite nanoplatelets (GNP) allows preparation of microfibrillar composites (MFC) based on PCL matrix reinforced with in-situ generated PLA fibrils. This work deals, for the first time, with preparation of analogous melt-drawn fibers. Unlike other blend-based fibers, the spinning and melt drawing leads to structure of deformed inclusions due to unfavorable ratio of rheological parameters of components. Subsequent moderate cold drawing of the system with dissimilar deformability of components causes strengthening with PLA fibrils. Unexpectedly, high velocity and extent of cold drawing leads to structure with low-AR inclusions, similar to the original melt-drawn blend. Extensive fast deformation of the soft PCL matrix does not allow sufficient stress transfer to rigid PLA. In spite of peculiarities found, the GNP-aided melt spinning allows facile preparation of biodegradable biocompatible fibers with wide range of diameters (80-400 µm) and parameters (2.35-18 cN/tex).