Jaroslav Kratochvíl

Crystalline Character and Microhardness of Gamma‐Irradiated and Thermally Treated UHMWPE

Relationships between Vickers microhardness, x‐ray and differential scanning calorimetry (DSC) crystallinity, x‐ray long period, and melting points were determined for ultrahigh molecular‐weight polyethylene (UHMWPE) of various histories (as‐produced, irradiated, annealed, and remelted). It was shown that the microhardness responds very sensitively to both the irradiation conditions (total radiation dose, radiation dose rate) and the thermal treatment (annealing below the melting temperature, remelting). As microhardness reflects the yield point parameters, the results show that not only the total dose, but also the irradiation dose rate has a considerable influence on mechanical properties of UHMWPE. It was demonstrated that neither x‐ray nor DSC results are so sensitive to treatment as the microhardness results. The most important differences in properties were found between remelted samples and those thermally untreated or annealed.

Microdomain structure and chain orientation in polypropylene/polyethylene blends investigated by micro-Raman confocal imaging spectroscopy

Compositional domain structure of blends of isotactic polypropylene with linear polyethylene and chain orientation of neat polymers and of the blends were assessed by micro-Raman confocal imaging spectroscopy and FT Raman spectroscopy. The results were correlated with polarised photoacoustic FTIR and with DSC. The surface and inner parts of compression-moulded, injection-moulded and drawn specimens were compared. Polymer domains in the blends and domains of different orientation were identified. The larger size of the polymer domains and lower chain orientation in the core of the specimens prepared by injection-moulding were explained by different cooling conditions of the melt. Possibilities and limitations of the micro-Raman confocal spectroscopy method were discussed.

Effect of layered silicates and reactive compatibilization on structure and properties of melt-drawn HDPE/PA6 microfibrillar composites

Microfibrillar composites (MFCs) of HDPE matrix with PA6 reinforcing fibrils formed in situ by melt drawing were modified by ethylene/glycidyl methacrylate copolymer (PEGMA) and addition of layered silicates using different mixing protocols. The goal was enhancement of adhesion between fibrils and matrix, their reinforcement by clay, and evaluation of the effect of clay on the MFC morphology, especially the fibril dimensions. Improved mechanical properties, including toughness, were found in the case of pre-blending Cloisite 15A (C15) with both polymeric components. Pre-blending of Cloisite C30B (C30) in PA6 is effective at low draw ratios only, whereas simultaneous addition of both clays leads to significant worsening of properties. In many cases, low stiffness increase indicates existence of clay-induced controversial effects which may eliminate reinforcement induced by fibrils and clay. The results indicate that these effects are significantly affected by the extent and course of clay migration between polymer phases during extrusion-mixing and melt drawing. Possible explanation of affecting the properties due to low-modulus interface supported by finite element analysis (FEA) is presented.

Crystallization and thermal properties of melt-drawn PCL/PLA microfibrillar composites

Abstract Microfibrillar composites (MFC) are advanced systems with reinforcement formed in situ by melt or cold drawing of suitable polymer blends. In the case of biodegradable poly (ɛ-caprolactone)/poly (lactic acid) (PCL/PLA) system, formation of microfibrillar structure by melt drawing is only possible by modification of the polymer components by clay. The effect of clay is quite complex; this study is focused on the effect of in situ formed fibrillar structure of PLA and clay in MFC with the PCL matrix on crystallinity and glass transition of both polymer components. The DSC analysis shows that clay addition to neat PCL, its blending with PLA, and addition of clay to the PCL/PLA blend increase PCL crystallinity. Clay addition and blending with PCL dramatically increase crystallinity of originally almost amorphous PLA. The non-isothermal PCL crystallization kinetics evaluated by the new recently proposed method reveals pronounced nucleation effect of clay addition and blending on PCL crystallization. An indication has been found of a negative correlation between maximum crystallization rate of PCL and its crystallinity. The glass transition temperature Tg of both components evaluated by DMA in the undrawn system mostly decreases with clay addition and increases with drawing. These effects are more significant for PLA due to its fibrillar structure. Tg is affected by the mixing protocol, as a result of different course of clay migration between components and localization, especially in the interfacial area.

Structure of polypropylene/ polyethylene blends assessed by polarised PA-FTIR spectroscopy, polarised FT Raman spectroscopy and confocal Raman microscopy

Physical structure and morphology of differently prepared and treated bulk specimens of isotactic polypropylene, linear polyethylene and their blend was studied by polarised FT Raman spectroscopy, polarised FTIR spectroscopy with photoacoustic detection, and by confocal Raman microscopy. The specimens differed in preparation technique, mechanical and thermal history. The orientation and reorientation of molecular chains of individual blend components, their crystallinity and the size and structure of the microdomains both in the skin and in the core of the specimens were correlated with the preparation conditions, mechanical treatment and thermal history. The possibilities and limitations of each method were discussed.

The effect of urea and urea-modified halloysite on performance of PCL

The effects of urea and urea-modified halloysite nanotubes (HNT) on structure and properties of poly(ɛ-caprolactone) (PCL) were evaluated using mechanical testing combined with FTIR, DSC, DMA, and various microscopic techniques. The results indicate important changes in mechanical behavior by urea-mediated interchain hydrogen bonding in PCL, whereas no linking between PCL and HNT in the related nanocomposite was found. As a result, the improved mechanical behavior of nanocomposites with urea-modified HNT was caused by combination of the matrix modification and urea-aided enhanced dispersion of HNT. The additives do not have any marked effect on PCL crystallinity. HNT increases and urea reduces the overall rate of crystallization. Both additives show a moderate nucleating effect.

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.

Non-isothermal kinetics of cold crystallization in multicomponent PLA/thermoplastic polyurethane/nanofiller system

The effect of platy and tubular nanofillers (NF) in combination with thermoplastic polyurethane (TPU) on cold non-isothermal crystallization (NIC) of poly(lactic acid) (PLA) was studied by differential scanning calorimetry. The data were processed using a recently proposed method of evaluating NIC kinetics. The results indicate that the NF/TPU combination leads to crystallization behaviour of the PLA matrix which is dissimilar from that of the NF-free blend and neat PLA. Addition of organophilized montmorillonite (C30) results in a dual effect on PLA crystallinity—stimulation at lower concentrations and suppression at higher loadings. In the case of halloysite nanotubes (HNT), this effect is practically absent. Crystallinity of injection-moulded samples is significantly increased by cold NIC. This indicates a synergy originating in a complex effect of NF/TPU, e.g. NF-induced changes in morphology and interphase parameters supporting the solid annealing process. TPU partially eliminates negative effect of high NF contents on cold crystallization rate. This effect is more marked in the remelted samples where the rate increases for all NF loadings. This is probably caused by absence of as-prepared crystallites in combination with higher mobility of PLA chains due to presence of TPU. The observed differences between C30 and HNT could be attributed to differences in size, shape, and specific surface. The results indicate that synergistic and antagonistic effects of NF/polymeric modifiers on mechanical and other material parameters are also significantly determined by the effects on crystallization of the matrix.

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