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Knowledge of the solidification behaviour of isotactic polypropylene (iPP) composites with reduced graphene oxide nanoplatelets is key to open the possibility to widespread use of these high performing nanocomposites. The crystallization behaviour of these systems, with filler content in the range of 0.2–3.5 wt% has been investigated by differential scanning calorimetry (up to 100 °C/min) and ex-situ structural and morphological characterization of samples prepared at processing-relevant cooling conditions (up to 2000 °C/s). Compared to the self-nucleated neat iPP, the nucleation efficiency was estimated to vary from 44 to 93% when increasing the filler content. Such a high nucleating efficiency has not been reported yet for a nanocomposite with iPP matrix. This result is due to the very good dispersion of the filler in these melt-mixed graphene-based polypropylene systems. The nucleation ability of the graphene filler does not reach a saturation in the concentration range studied. The gap between laboratory and industrial cooling rate scale is here reduced thanks to the information achieved from the fast-cooling experiments. By varying the filler amount in the investigated range, a significant shift of the cooling rate window in which the transition from α-to mesophase dominated crystallization takes place was detected. Notably, with 3.5 wt% graphene platelets, α-phase crystals are predominantly present in the material even after quenching at 1000 °C/s, similar to highly nucleated commercial iPP grades.

A significant portion of the global plastics market encompasses the production of polyolefin materials and especially polypropylene (PP) and polyethylene (PE), as commodity polymers with a wide range of applications. However, the increase in the generation of unsustainable plastic waste requires a close technological look-up to address this challenge adequately. In this context, mechanical recycling is part of the strategies expected to contribute to the solution. Nevertheless, the melt blending process presents a challenge due to the immiscibility between PP and PE. Therefore, compatibilization strategies are meant to solve the problem effectively. In this paper, we employ a commercial ethylene-ran-methyl acrylate random copolymer as a compatibilizer for PP/PE blends. With the addition of the compatibilizer, it was possible to obtain a 44% reduction in PE domain size, while ductility increased by around similar to 40% with respect to uncompatibilized blends. Interesting results were obtained concerning the crystallization behavior of the blends. The overall isothermal crystallization kinetics of the different blend components was studied, and a synergistic nucleation effect of the PP and the compatibilizer toward the PE phase was found. For the first time, the effect of the compatibilizer on the surface nucleation of PE in a self-nucleated PP matrix phase is reported. An enhancement in the crystallization rate of PE was found when the self-nucleation protocol was applied to the polypropylene matrix phase for neat and compatibilized blends. The nucleation efficiency was in the range of 120-124%, indicating a supernucleation behavior. The induced crystallization at the interface by the self-nucleated polypropylene is the reason for such high nucleation efficiencies. Surprisingly, a higher amount of compatibilizer decreases the overall crystallization rate of PE droplets. The compatibilizer segregates at the interface between both polymers, reducing the surface nucleation of the PE droplets on the PP matrix phase. The results presented in this paper lead the way toward improving the use of post-consumer recycled materials. All of the sources of funding for the work described in this publication are acknowledged below: This work has received funding from the European Union's Horizon 2020 research and innovation program under Grant Agreement No 860221 under the name of the REPOL project.

The routine analysis of polymer blends at the nanoscale is usually carried out using electron microscopy techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM), which often require several sample preparation steps including staining with heavy metals and/or etching. Atomic force microscopy (AFM) is also commonly used, but provides no direct chemical information about the samples analyzed. AFM-IR, a recent technique which combines the AFM's nanoscale resolution with the chemical information provided by IR spectroscopy, is a valuable complement to the already established techniques. Resonance enhanced AFM-IR (contact mode) is the most commonly used measurement mode, due to its signal enhancement and relative ease of use. However, it has severe drawbacks when used in highly heterogenous samples with changing mechanical properties, such as polymer recyclates. In this work, we use the recently developed tapping mode AFM-IR to chemically image the distribution of rubber in a real-world commercially available polyethylene/polypropylene (PE/PP) recycled blend derived from municipal and household waste. Furthermore, the outstanding IR resolution of AFM-IR allowed for the detection of small PP droplets inside the PE phase. The presence of micro and nanoscale particles of other polymers in the blend was also established, and the polymers identified.

The complete understanding of polymers crystallization kinetic in real industrial processes is fundamental for designing materials towards specific applications. Hereby, we propose a method for fast cooling preparation of polymers by means of air and water sprays, which presents several enhancements with respect to previous similar devices. The new device was built in order to fulfill safety and cleaning standards required in an industrial environment. Furthermore, the small dimensions and the limited costs enable its implementation in any laboratory. Moreover, preliminary results on selected isotactic-polypropylenes (iPP) were obtained in a cooling range from 0.01 to a few thousand °C/s, in order to validate the present method by comparison with previous literature results and to highlight the large range of studies that can be carried out with this set-up. Ex-situ analysis with WAXS, DSC and AFM techniques were performed to characterize such prepared samples.

Single-screw extruders are usually operated with the screw fully filled (flood-fed mode) and not partially filled (starve-fed mode). These modes result in completely different processing characteristics, and although starve-fed mode has been shown to have significant advantages, such as improved mixing and melting performance, it is rarely used, and experimental studies are scarce. Here, we present extensive experimental research into starve-fed extrusion at feeding rates as low as 25%. We compared various operating parameters (e.g., residence time, pressure build-up, and melting performance) at various feeding rates and screw speeds. The results show a first insight into the performance of starve-fed extruders compared to flood-fed extruders. We explored starve-fed extrusion of a polyethylene material which contains a Very High Molecular Weight Polyethylene fraction (VHMWPE). VHMWPE offers several advantages in terms of mechanical properties, but its high viscosity renders common continuous melt processes, such as compression molding, ram extrusion and sintering, ineffective. This work shows that operating single-screw extruders in extreme starve-fed mode significantly increases residence time, melt temperature, and improves melting and that-in combination—this results in significant elongation of VHMWPE particles.

Crystallization kinetics under processing-relevant cooling conditions of polyethylene/isotactic-polypropylene (PE/PP) blends, with compositions similar to real recycled blends, was investigated through an in situ temperature monitoring technique. Continuous Cooling Curve (CCC) diagrams in a large crystallization temperature range were obtained for the two phases, thanks to a new apparatus allowing fast quenching of polymer samples. Using this technique, we found an "inversion point" in the crystallization order of the two polymers, which arises from the difference in crystallization rates between PP and PE with an increasing cooling rate. The order of crystallization of the two polymers at low cooling rates, that is, before the inversion point, can be tuned by employing non-nucleated or nucleated PP. In particular, non-nucleated PP crystallizes after PE above 0.5 degrees C/s, while nucleated PP solidifies before PE until 80 degrees C/s. Interestingly, mutual nucleating effects, previously shown only for ideal systems such as thin-layered films, were found at the interface between the phases, in correlation with the inversion point. During cooling, the phase which crystallizes first can enhance the onset of crystallization of the second lower crystallizing phase through nucleation at the interface. Ex situ morphological and wide-angle X-ray diffraction experiments confirmed the validity of the results shown by CCC diagrams. Our approach to characterize semicrystalline polymer blends with CCC diagrams facilitates tailoring their properties at cooling rates relevant for polymer processing. Moreover, the importance of knowing and controlling the type of components in blends obtained from recycling is demonstrated, given the variety of crystallization behaviors attainable.

The mechanical properties of novel low percolation melt-mixed 3D hierarchical graphene/polypropylene nanocomposites are analyzed in this study. The analysis spans a broad range of techniques and time scales, from impact to tensile, dynamic mechanical behavior, and creep. The applicability of the time&ndash, temperature superposition principle and its limitations in the construction of the master curve for the isotactic polypropylene (iPP)-based graphene nanocomposites has been verified and presented. The Williams&ndash, Landel&ndash, Ferry method has been used to evaluate the dynamics and also Cole&ndash, Cole curves were presented to verify the thermorheological character of the nanocomposites. Short term (quasi-static) tensile tests, creep, and impact strength measurements were used to evaluate the load transfer efficiency. A significant increase of Young&rsquo, s modulus with increasing filler content indicates reasonably good dispersion and adhesion between the iPP and the filler. The Young&rsquo, s modulus results were compared with predicted modulus values using Halpin&ndash, Tsai model. An increase in brittleness resulting in lower impact strength values has also been recorded.

A fast and easy-to-implement technique able to investigate the complex chemical composition of polyethylene/polypropylene (PE/PP) blends derived from recycling is required. Thermal fractionation, in particular the Successive Self-nucleation and Annealing (SSA) method, proves to be suitable for this purpose. Therefore, a new tailor-made SSA protocol, able to fractionate both PE and PP in the same temperature program, was developed. Furthermore, the fractionation of model blends with known composition allowed the assessment of the temperature regions where co-crystallization among the blend components does not occur, enabling the development of a method for the quantitative evaluation of the amount of the high melting PE component and of the PP phase in a blend. Finally, the composition of two recycled blends was successfully assessed by means of the novel quantitative method and the results were compared with those achieved by a solution-based fractionation technique on the same materials.

The efficiency of the degree of cooling and shear-induced crystallization on the formation of different crystal polymorphs and morphology development of isotactic polypropylene (iPP) and random propylene–ethylene copolymer was analysed in cast films. Wide-angle X-ray scattering was applied to evaluate the crystal structure formed in films solidified at different chill roll temperatures (15, 40, and 70 °C) and throughputs of the line (60 and 120 kg/h). The crystal geometry and morphology were visualized by atomic force microscopy. For films generated at 15 °C or at large supercooling together with the nodular domain additional grained round-shaped structures composed of mesophase were detected on the film surface. Increase of the chill roll temperature or lower supercooling allowed formation of the monoclinic phase of iPP. Higher drawing rate triggered crystal formation preferably on the surface of films quenched at 15 °C. The crystal structure and morphology were correlated to selected optical and mechanical properties. At close-to-identical crystallinity, the crystal aspect ratio and film roughness play a major role in controlling the film transparency and stiffness.

This work presents a new empirical method to analyse the non-isothermal crystallization kinetics of polypropylene. The Kissinger analysis, widely used in the literature to calculate an activation energy of the crystallization mechanism, was critically examined regarding its applicability to the crystallization process of semicrystalline polymers. The analogy between the systems modelled by Kissinger and polymer crystallization applies only in very restrictive cases, and the activation energy, as commonly used, is an ill-defined quantity. Nevertheless, it was shown that the Kissinger analysis yields a phenomenological parameter, here called REact, related to the crystallization kinetics at moderately high cooling rates (up to ~50 °C s−1). This parameter, created with data obtained on more than 20 poly(propylene) compounds based on four different base polymers of different chain structure and composition, correctly estimates the crystallization performance of different systems at higher cooling rates, demonstrated by a good correlation between REact and the morphology formed in cable extrusion and cast film extrusion processes.

Graphene-based materials are a family of carbonaceous structures that can be produced using a variety of processes either from graphite or other precursors. These materials are typically a few layered sheets of graphene in the form of platelets and maintain some of the properties of pristine graphene (such as two-dimensional platelet shape, aspect ratio, and graphitic bonding). In this work we present melt mixed graphene-based polypropylene systems with significantly reduced percolation threshold. Traditionally melt-mixed systems suffer from poor dispersion that leads to high electrical percolation values. In contrast in our work, graphene was added into an isotactic polypropylene matrix, achieving an electrical percolation threshold of ~1 wt.%. This indicates that the filler dispersion process has been highly efficient, something that leads to the suppression of the &beta, phase that have a strong influence on the crystallization behavior and subsequent thermal and mechanical performance. The electrical percolation values obtained are comparable with reported solution mixed systems, despite the use of simple melt mixing protocols and the lack of any pre or post-treatment of the final compositions. The latter is of particular importance as the preparation method used in this work is industrially relevant and is readily scalable.

Crystallization of isotactic polypropylene (iPP) at the interface with crystalline films of two commercially employed nucleating agents (sodium benzoate (NaBz) and sodium 2,2&rsquo, methylene bis-(4,6-di-tert-butylphenyl)phosphate (NA-11)) and with a glass fiber (GF) was investigated using a polarized optical microscope. The analysis of the light intensity evolution during the crystallization process enabled the successful estimation of the time at which the crystal growth began, i.e., the induction time (ti), at various crystallization temperatures. Meaningful differences in the ti values were observed between the investigated systems. Moreover, the ti data have been analyzed according to different nucleation models proposed in the literature, which consider either the time to form the first crystalline layer in contact with the substrate or the time required to grow a cluster of critical size. It has been found that the two models are applicable in different temperature ranges depending on the efficiency of the given substrate. Therefore, in order to obtain the value of the surface free energy difference function, &Delta, &sigma, which is directly related to the nucleation energy barrier and useful for the definition of a universal nucleating efficiency scale, a model that considers both the above-mentioned times was fitted to the overall data. The values of &Delta, for the nucleation of iPP on the surface of the different substrates are thus obtained and discussed in the framework of the literature results.

The polymorphism of isotactic polypropylene (iPP) in combination with the strong response of this polymer to nucleation can be utilized for expanding the application range of this versatile polymer. Based on three “case studies” related to β-iPP pressure pipes, ethylene-propylene (EP) random copolymers for thin-wall injection molding and transparency and sterilization resistance of cast films we demonstrate ways of combining polymer composition, nucleation and process settings to achieve the desired application performance. The importance of considering interactions between polymer design, nucleation and processing parameters for designing application properties is highlighted.

One possible way to modify the properties of isotactic polypropylene (iPP) is to use the material’s polymorphism which in case of film casting means to balance the content of α-crystalline and mesomorphic phase in the product. A cast film study was performed with several iPP homopolymers of different isotacticity and an ethylene-propylene (EP) random copolymer at two different film thickness levels (50 and 200 μm), varying the cooling rate in solidification by adapting the processing conditions. The results from film characterization can be correlated to findings from a parallel investigation with a combination of DSC and FSC techniques in a cooling rate range of 0,01 to 1.000°C/s. A similar effect of crystallinity on tensile modulus was found for both iPP grades studied and at both levels of film thickness. The dominant processing parameter is the chill roll temperature. Output and line speed have a limited effect on crystallinity, but a stronger one on transparency (haze & clarity), where microscopy revealed surface roughness modifications as significant contributing factor.One possible way to modify the properties of isotactic polypropylene (iPP) is to use the material’s polymorphism which in case of film casting means to balance the content of α-crystalline and mesomorphic phase in the product. A cast film study was performed with several iPP homopolymers of different isotacticity and an ethylene-propylene (EP) random copolymer at two different film thickness levels (50 and 200 μm), varying the cooling rate in solidification by adapting the processing conditions. The results from film characterization can be correlated to findings from a parallel investigation with a combination of DSC and FSC techniques in a cooling rate range of 0,01 to 1.000°C/s. A similar effect of crystallinity on tensile modulus was found for both iPP grades studied and at both levels of film thickness. The dominant processing parameter is the chill roll temperature. Output and line speed have a limited effect on crystallinity, but a stronger one on transparency (haze & clarity), where microscopy rev...

Carbon black (CB) based polymer nanocomposites exhibit percolation-type conductive behaviour. In this work, the electrical performance of a nanocomposite based on poly(ethylene vinyl acetate) (EVA) was studied. By introducing a second polymer and creating a system with three phases, the filler is localised in a single phase. This localisation has been seen to affect the overall morphology of both phases, indicating that despite the phase separation there is interaction between the two systems. Furthermore, by keeping the filler to polymer ratio constant on the EVA phase, we have managed to achieve improved electrical performance by reducing the overall percolation threshold while increasing the overall amount of the second polymer. Atomic Force Microscope (AFM) and Scanning Electron Microscope (SEM) imaging has been used to verify the preferential location of the conductive particles and to reveal the complex morphology developed. The results presented in this study show the possibility of specially-designed polymer compositions for conductive applications.

Summary Polymer brush synthesis by surface initiated polymerization (SIP) comprises a powerful strategy to tailor interfacial properties. Among others, the passivation of solid substrates against non-specific protein adsorption or the implementation of stimuli responsive properties can be achieved. In this context controlled radical polymerization approaches are frequently employed. Here we report on our efforts in the synthesis of oligo and di(ethylene glycol) methylether methacrylate (OEGMA and DEGMA)-based brushes with (i) controlled grafting density and (ii) new diblock copolymer structures. We assembled mixed self-assembled monolayers (SAMs) comprising octadecanethiol (ODT), 16-mercaptohexadecanoic (MHDA) and ω-mercaptoundecyl bromoisobutyrate (MUBiB) on gold to initiate the controlled radical polymerization of OEGMA and DEGMA. The SAMs as well as the polymer layers obtained by atom transfer radical polymerization for a constant polymerization time were characterized by contact angle measurements with water, FTIR spectroscopy, ellipsometry and atomic force microscopy (AFM). It was found that ODT and MHDA assemble in the mixed SAMs preferentially and despite the different endgroups very similar brushes were obtained. SAMs with fractional surface coverages of the initiator ω-mercaptoundecyl bromoisobutyrate of > 0.45 showed no further increase in brush thickness, which is in line with a mushroom to brush transition below this coverage. In addition, diblock copolymer brushes comprising POEGMA and poly(tert-butyl acrylate) (PtBA) blocks were obtained for the first time. The PtBA block was successfully hydrolyzed to yield POEGMA-block-poly acrylic acid (PAA) brushes.

We report on the successful replication of the smallest pores in anodized aluminum oxide (AAO) via the layer-by-layer (LBL) deposition of polyelectrolytes to date to yield free-standing, open nanotubes with inner and outer diameters (±2σ) down to 37 ± 4 and 52 ± 19 nm, respectively. This work is based on the fabrication of defined arrays of highly regular nanopores by anodic oxidation of aluminum. Pores with pore diameters between 53 ± 9 and 356 ± 14 nm and interpore distances between 110 ± 3 and 500 ± 17 nm were obtained using an optimized two-step anodization procedure. 3-(Ethoxydimethylsilyl)propylamine-coated pores were replicated by alternating LBL deposition of poly(styrenesulfonate) and poly(allylamine). The detrimental adsorption of polyelectrolyte on the top surface of the template that typically results in partial pore blocking was eliminated by controlling the surface energy of the top surface via deposition of an ultrathin gold layer. The thickness of the deposited LBL multilayer assembly at the pore orifice agreed to within the experimental error with the thicknesses measured by variable angle spectroscopic ellipsometry and atomic force microscopy (AFM) for layers assembled on flat substrates. The selective dissolution of the alumina template afforded free-standing, open polymer nanotubes that were stable without any cross-linking procedure. The nanotubes thus obtained possessed mean outer diameters as small as 52 nm, limited by the size of the AAO template.

We show that both the morphology and crystal structure of regioregular poly(3-hexylthiophene) (RR-P3HT) films in organic field effect transistor (OFET) devices with top surface contacts are altered, when an electric field is applied. Grazing incidence X-ray diffraction (GIXD) analysis reveals a dramatic decrease in structural order in the non-linear current regime and, by contrast, much less pronounced structural changes of the ( h 0 0) peaks in the linear current regime. Using for the first time resonance soft X-ray reflectivity (RSoXR) on RR-P3HT to avoid radiation damaged and to enhance the scope of the attained depth resolved information, we show that these changes are located in a several tens of nanometer thin sub-surface layer of the active polymer in the device. Furthermore, the RSoXR data reveal that the changes are strongly related to the C 1s → π ∗ and C 1s → σ ∗ -resonances. In situ AFM measurements of the RR-P3HT device surface support the reciprocal space analysis results by showing a substantial softening and lateral expansion of the RR-P3HT films, when 20 V are applied between the contacts. These morphological and structural changes are partially reversible in nature.

In this study the effects of crystal habit, crystallinity and superstructure on the modulus of elasticity of isotactic polypropylene films were analyzed by atomic force microscope (AFM) nanoindentation. The modulus of elasticity, evaluated on the nanometer scale by AFM, showed a qualitatively similar dependence on the crystal habit, crystallinity and superstructure as the modulus of elasticity measured by dynamic-mechanical analysis and tensile testing. The observed values of both the surface stiffness measured by AFM and the macroscopic/bulk stiffness were distinctly larger in the presence of non-isometric lamellae organized in spherulite than in the presence of isometric nodular crystals, not organized in a spherulitic superstructure. The experimental data showed that the modulus of elasticity is not primarily influenced by the presence or absence of spherulite but by the molecular-deformation constraint associated to the crystal habit.

The analysis of the mechanical properties of thin films by nanoindentation has been recently subject of numerous theoretical and phenomenological studies as well as numerical simulations. In this work, we report on the application of the Winkler elastic foundation theory to analyze atomic force microscope nanoindentations of poly(n-butyl methacrylate) films with thicknesses of 90, 196, and 485 nm. The elastic moduli of the samples were found to be 1.13 ± 0.43 GPa, 1.34 ± 0.32 GPa and 1.23 ± 0.24 GPa, respectively, after indentations of at least 50% of the film thickness. These data rely on the independent determination of the mechanical properties of 485-nm thick films, using Sneddon's model at low penetration depth (yielding 1.27 ± 0.37 GPa). Our data show that the substrate effects begins to be noticeable only after indenting to a depth of more than 40% of the film thickness. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers

Protein resistant polymer brushes of poly(diethylene glycol methylether methacrylate) (DEGMA) are synthesized by surface-initiated polymerization (SIP) on gold and are thoroughly characterized with particular attention to the nanomechanical properties. Compared to the well known brushes of poly(oligoethylene glycol methylether methacrylate) (POEGMA) on gold, PDEGMA brushes grow faster and yield higher final brush heights. In confocal fluorescence microscopy experiments it is shown that both types of brushes possess similar low non-specific adsorption levels of labeled bovine serum albumin (BSA) from buffer. While the surface stiffness evaluated by depth sensing indentation in the dry state suggests that side chain crystallization imparts high stiffness to the POEGMA brush, PDEGMA brushes behave similar to poly(methyl methacrylate) (PMMA) brushes of comparable thickness. In the wet state PDEGMA brushes are shown to possess an elastic modulus of 800 kPa, as estimated by AFM nanoindentation, which is smaller than that of POEGMA (3 000 kPa). Expanding these brushes to end-functional polymers with recently developed block copolymer brush approaches, tailored soft biointerfaces can be generated for cell - surface interaction and cell culture studies.

We report on the dependence of the mechanical properties of confined polymers on the film architecture. Compared to spin-coated films, grafted polymer brushes showed higher elastic moduli, which is attributed to entropy effects, and required higher loads to achieve the same residual indentation depth when compared to spin-coated samples. The contribution of entropy changes to the mechanical response of brushes was evaluated. Although qualitative, this study tries for the first time to evaluate this entropic contribution. Therefore, the role of morphology and architecture of the polymer films during mechanical deformation is highlighted, showing that the change of surface thermodynamic properties (entropy in particular) changes accordingly the nanometer scale mechanical properties.

The coadsorption of alkanethiols on noble metals has been recognized for a long time as a suitable means of affording surfaces with systematically varied wettability and other properties. In this article, we report on a comparative study of the composition of the mixed self-assembled monolayers (SAMs) obtained (i) by the coadsorption of octadecanethiol (ODT) and 16-mercaptohexadecanoic acid (MHDA) from ethanol and chloroform onto gold substrates and (ii) by microcontact printing using poly(dimethyl siloxane) (PDMS) stamps. SAMs prepared by coadsorption from solution showed a preferential adsorption of ODT for both solvents, but this trend was reversed in microcontact-printed SAMs when using chloroform as a solvent, as evidenced by contact angle and Fourier transform infrared (FTIR) spectroscopy measurements. An approximately linear relationship between the static contact angle and the degree of swelling with different solvents was observed, which suggests that the surface composition can be controlled by the interaction of the solvent and the PDMS elastomer. The altered preference is attributed to the different partitioning of the two thiols into solvent-swelled PDMS, as shown by (1)H NMR spectroscopy. Finally, molecularly mixed binary SAMs on ODT and MHDA on template-stripped gold were applied to study the effect of surface nanobubbles on wettability by atomic force microscopy (AFM). With a decreasing macroscopic contact angle measured through water, the nanoscopic contact angle was found to decrease as well.

Several poly(ethylene) samples with a broad range of morphologies were studied in this work using nanoindentations. The samples had degrees of crystallinity ranging from 30 to 100% while their Young's modulus ranged from few tens of MPa up to several GPa. Experimental conditions for the correct evaluation of Young's modulus were at first identified, choosing a suitable loading rate in order to minimize viscoelastic effects on the unloading. The force curves, i.e., plots of applied load vs. penetration depth, were then analyzed following two common procedures available in the literature. None of these procedures leads to satisfying results when compared to other experimental techniques. However, it was found that a recently proposed correction factor to the Oliver and Pharr procedure allows to evaluate reliably Young's modulus of the poly(ethylene) samples exhibiting very different, fine-tuned morphologies.

Polyethylenes of various macromolecular and supermolecular structures were studied from the point of view of their susceptibility to an ion beam treatment. An influence of molecular weight (Mw), molecular weight distribution (Mw/Mn) and the degree of branching were compared within the set of low-density polyethylenes (LDPE) studied. An influence of the length of branches was compared between LDPE, linear low-density (LLDPE) and high-density (HDPE) polyethylenes. An influence of the degree of crystallinity and the morphology of a crystalline phase were compared for HDPE samples solidified under various thermal conditions and ultra-high molecular weight polyethylene (UHMWPE). Plate polymer targets ∼2 mm were bombarded with 100 keV He+ or 130 keV Ar+ ions (dose of 1014–1016 ions/cm2; ion energy stream density Ar+ ion beam bombardment generally lowers micromechanical properties of the polyethylenes, whereas He+ ion beam treatment makes them higher. The effect is the stronger the higher the molecular weight of polyethylene. However, a long chain branching adversely affects the modification. The degree of crystallinity facilitates an ion beam bombardment from the point of view of micromechanical properties of the materials, however, also the morphology of a crystalline phase was found to play a role.

The analysis of nanoindentation force curves collected on polymers through the common Oliver and Pharr procedure does not lead to a correct evaluation of Young’s modulus. In particular, the estimated elastic modulus is several times larger than the correct one, thus compromising the possibility of a nanomechanical characterization of polymers. Pile-up or viscoelasticity is usually blamed for this failure, and a deep analysis of their influences is attempted in this work. Piling-up can be minimized by indenting on a true nanometer scale, i.e., at penetration depth smaller than 200 nm. On the other side, it is common knowledge that fast indentations minimize the effect of viscoelasticity. However, changing the indentation time in a broad range of contact time (fractions of second up to hundreds of seconds) did not allow the correct estimation of Young’s modulus for the polymers used in this work. The final result is that the Oliver and Pharr procedure as well as any other procedure analyzing the unloading curve with elastic contact mechanics models cannot be employed to measure Young’s modulus of polymers because its application is incorrect from a theoretical point of view, unless the analysis is limited to the very first nanometers of penetration depth when the contact is perfectly elastic. Viscoelastic contact mechanics models should instead be employed to characterize these materials.

Summary: The nanoindentation test is a fundamental tool to assess the link between morphology and mechanical properties. The preliminary results of a more exhaustive study about the applicability to polymers of the most used procedure to determine elastic modulus by indentation are reported in this short communication. A departure of the experimental conditions from the theoretical assumptions and results that give rise to the Oliver and Pharr analysis is shown to occur under a wide range of experimental conditions, with applied loads and penetration depths covering several orders of magnitude and using different indenter geometries. Unloading curves with exponents significantly larger than 2 are observed in disagreement with the contact mechanics approach used by Oliver and Pharr. An AFM image obtained in non contact mode of an indentation induced by a sharp AFM tip with a maximum applied load of ca 1.2 µN on amorphous PET.

A nanoindentation technique using an atomic force microscope (AFM) was applied to characterize the mechanical behaviour of several isotactic polypropylene (iPP) samples. The samples were solidified from the melt with a CCT (continuous cooling transformation) procedure spanning a wide range of cooling rates thanks to a fast quenching apparatus developed by the authors. The influence of instrumental parameters on the nanoscale mechanical properties (indentation depth, Young's modulus) shows that for modulus determination one has to rely on simpler methods of force curve analysis based on trace curve alone. Structure homogeneity up to the scale of macroscopic samples used to evaluate elastic moduli allowed a successful comparison of these values with those determined by AFM, which showed that an increase in cooling rate leads to a significant decrease in the material's mechanical response. AFM can thus provide correlations between operating conditions and mechanical properties and can be used for analysing the structure distribution and for mapping properties on a sub-micron scale.

The mechanical response of extruded and injection molded semicrystalline materials, in which an oriented microstructure is commonly observed, depends on the direction of loading. Plastic deformation and failure are, therefore, both anisotropic. The ability of a micromechanical model, including the characterization of the kinetics of crystallographic slip and amorphous yield, to describe the yield kinetics of oriented high-density polyethylene tapes with different draw ratios is evaluated here. The initial morphology of the material is characterized with wide-angle X-ray diffraction experiments, which evidence a strong alignment of molecular chains with the drawing direction for specimens produced with a large draw ratio. Anisotropic crystal plasticity alone in a two-phase framework proves not able to quantitatively describe the macroscopic mechanical response in the solid state hot drawn samples. Most likely, these deviations are related to process-induced orientation within the amorphous domains. Therefore, the influence of loading angle dependent yield kinetics for the amorphous phase is evaluated, and indeed the description improves considerably. Finally, the possibilities for characterizing the properties of different crystallographic slip systems are discussed.

The residual imprint left behind by the AFM nanoindentation of polymers has been seldom studied in the past. In this work, the evolution of indentations at room temperature performed on a semicrystalline poly- (ethylene) in a broad range of experimental conditions is presented. The study shows that the recovery after 24 h is substantial, although not complete. Moreover, the dynamics of the recovery process is not seen to depend on the magnitude of the applied load for the nanoindentation, but instead on the rate of the indentation used. This points out that viscoelastic processes are minimized when performing fast nanoindentations, while at low loading rates there seems to be a residual viscoelastic energy stored in the system. Although, the recovery process involves complex displacements and bending of crystalline lamellae, which is difficult to model theoretically, the present work attempts to quantify the magnitudes as well as the directions of such displacements for indentations involving from two to seven lamellae.

The mechanical properties (hardness and elastic modulus) of organically modified silicate thin films can be finely tuned by varying the degree of alkylation and thus the fraction of six- and four-membered siloxane rings in the organosilica matrix. This opens the way to large tunability of parameters that are of crucial practical importance for films that are finding increasing application in numerous fields ranging from microelectronics to chemical sensing.

In this paper we report on the unprecedented deformation behavior of stratified ultrathin polymer films. The mechanical behavior of layered nanoscale films composed of 8-12 nm thin plasma polymerized hexamethyldisiloxane (ppHMDSO) films on a 70 nm thick film of polystyrene was unveiled by atomic force microscopy nanoindentation. In particular, we observed transitions from the deformation of a thin plate under point load to an elastic contact of a paraboloid of revolution, followed by an elastic-plastic contact for polystyrene and finally an elastic contact for silicon. The different deformation modes were identified on the basis of force-penetration data and atomic force microscopy images of residual indents. A clear threshold was observed for the onset of plastic deformation of the films at loads larger than 2 μN. The measured force curves are in agreement with an elastic and elastic-plastic contact mechanics model, taking the amount of deformation and the geometry of the layer that presumably contributed more to the overall deformation into account. This study shows that the complex deformation behavior of advanced soft matter systems with nanoscale dimensions can be successfully unraveled.

Tethering macromolecules to surfaces represents a versatile approach for functionalizing, protecting, and structuring both organic and inorganic materials. In this study, thin films of poly(acrylamide) (PAAm) brushes and covalently cross-linked hydrogel brushes were grown from iniferter-functionalized silicon substrates by UVLED-initiated photopolymerization and their properties subsequently studied by means of a variety of analytical methods. The employed photografting method allowed the controlled fabrication of very thick films (up to 1 μm) in an aqueous environment, over a period of less than 1 h of polymerization and in the absence of side reactions. PAAm covalently cross-linked hydrogel brushes were prepared by feeding trace amounts of the cross-linker bis(acrylamide) (up to 1.0 wt % of monomer solution) into the reaction vessel. Both bulk and interfacial properties of these polymer films were found to be strongly influenced by lateral cross-linking of the grafted polymer chains. In agreement with t...

We introduce a simple Scanning Near-Field Ellipsometer Microscopy (SNEM) setup to address the rapidly increasing need for simple, routine optical imaging techniques with resolution well below the diffraction limit. Our setup is based on the combination of commercially available atomic force microscope (AFM) and ellipsometry equipment with gold-coated AFM tips to obtain near-field optical images with a demonstrated resolution below λ/10. AFM topographical data, obtained in contact mode, and near-field optical data were acquired simultaneously using a combined AFM-ellipsometer. The highly enhanced field due to lightning-rod effects and localized surface plasmons excited at the end of the gold-coated tip allowed us to resolve and identify metallic nanoparticles embedded in poly(methyl methacrylate) as well as microphases in microphase-separated block copolymer films.

Complex materials exhibit a hierarchical structure where a gradient of features on nanometer scale is induced by the synthetic route eventually enhanced by the loading condition. The nanometer scale at which individual components arrange, determining their properties, is a current challenge of mechanical testing. In this work, a survey on nanoindentation is outlined based on the comparison of results obtained by Atomic Force Microscopy and Depth-Sensing Instruments and their combination. An Atomic Force Microscope equipped with a Force Transducer gives indeed the possibility to scan the sample surface in contact mode, thereby allowing one to choose a suitable position for the nanoindentation, as well as imaging the residual imprint left on the sample. The analysis of the applied load vs. penetration depth curve, also called force curve, shows the limitations of current approaches to determine elastic moduli of compliant viscoelastic materials. Significant deviations from the expected values are observed even after optimizing testing conditions, so as to minimize the artifacts like viscoelastic effects or pile-up. As rigorous approaches are yet to be applied to the interpretation of force curves accounting also of viscoelastic material behavior, an empirical calibration recently proposed by the authors is verified against a set of data on model samples spanning a range of moduli, typical of compliant materials and close to each other, so as to challenge the resolution potential of this method, as well as others in use in the literature.

The analysis of mechanical properties on a nanometer scale is a useful tool for combining information concerning texture organization obtained by microscopy with the properties of individual components- Moreover, this technique promotes the understanding of the hierarchical arrangement in complex natural materials as well in the case of simpler morphologies arising from industrial processes. Atomic Force Microscopy, AFM, can bridge morphological information, obtained with outstanding resolution, to local mechanical properties. When performing an AFM nanoindentation, the rough force curve, i.e., the plot of the voltage output from the photodiode vs. the voltage applied to the piezo-scanner, can be translated into a curve of the applied load vs. the penetration depth after a series of preliminary determinations and calibrations. However, the analysis of the unloading portion of the force curves collected for polymers does not lead to a correct evaluation of Young’s modulus. The high slope of the unloading curves is not linked to an elastic behavior, as would be expected, but rather to a viscoelastic effect. This can be argued on the basis that the unloading curves are superimposed on the loading curves in the case of an ideal elastic behavior, as for rubbers, or generally in the case of materials with very short relaxation times. In contrast, when the relaxation time of the sample is close to or even much larger than the indentation time scale, very high slopes are recorded. Where AFM nanoindentations are concerned, one observes a dependence of the penetration, i.e., the relative motion between the sample and the tip (indenter), on the elastic properties of a material when using equivalent loads. This relationship becomes visible on samples that are homogeneous down to the scale of nanoindentation. The elastic modulus can be obtained by applying Sneddon’s elastic contact mechanics approach, since the contact between the tip and the sample is dominated by an elastic behavior with negligible plastic deformation. Under such circumstances, the dependence of the penetration on the load follows an exponent of 1.5, consistent with elastic contact mechanics and justified on the basis of the large elastic range exhibited by polymers, on the constraints due to the geometry of the deformation during indentation and to the critical yielding volume needed in order to induce plasticity. As a result, elastic moduli taken from AFM force curves show a very good agreement with bulk values obtained by macroscopic tensile testing. This is true for a broad range of polymers, from materials with rubbery to semicrystalline, or even glassy behaviors. This result confirms that AFM nanoindentations in polymers take place mostly in the elastic range and opens the possibility of characterizing the mechanical behavior of polymers on an unparalleled small scale as compared to commercial depth sensing instruments (DSIs), which use much blunter indenters. A further application is discussed where, upon decreasing the load, and consequently the penetration depth, the scale becomes comparable to that of the underlying texture which is probed as opposed to the bulk material. Although this apparently presents a limitation on the resolution of the scale that can be mapped, this feature is discussed and shown to open the possibility of identifying properties of individual phases with their surroundings as well as the role of the connectivity among the phases.

AFM nanoindentations show a dependence of penetration, i.e., the relative motion between the sample and the tip (indenter), on material elastic properties when using the same load. This elationship becomes visible by using of samples being homogeneous down to the scale of nanoindentation. They were prepared from materials covering a broad range of mechanical behavior: from rubbery networks to glassy and semicrystalline polymers. The elastic modulus can be obtained applying Sneddon’s elastic contact mechanics approach. To do this, some calibrations and instrumental features have to be measured accurately. All the polymers tested show that the contact between the tip and the sample is dominated by elastic behavior with negligible plastic deformation. In contrast to a standard metallic material like lead, the penetration dependence on lad follows an exponent of 1.5, consistent with elastic contact mechanics. This can be justified on the basis of the large elastic range polymers exhibit, on the constraints due to the geometry of the deformation during indentation and to the critical yielding volume needed in order to induce plasticity. For the polymers studied, this volume is chosen in such a way that a significant material volume is irreversibly deformed. Elastic moduli taken from AFM force curves show a very good agreement with bulk values obtained by macroscopic tensile testing, on all the polymers tested. This result confirms that AFM nanoindentations in polymers take place mostly in the elastic range and opens the possibility to characterize the mechanical behavior of polymers on an unparalleled small scale compared to commercial DSI (depth sensing instruments), which use a much blunter indenter.

The Oliver and Pharr [J. Mater. Res. 7, 1564 (1992)] procedure is a widely used tool to analyze nanoindentation force curves obtained on metals or ceramics. Its application to polymers is, however, difficult, as Young’s moduli are commonly overestimated mainly because of viscoelastic effects and pileup. However, polymers spanning a large range of morphologies have been used in this work to introduce a phenomenological correction factor. It depends on indenter geometry: sets of calibration indentations have to be performed on some polymers with known elastic moduli to characterize each indenter.

A previous study carried out on PET has shown that this polymer undergoes a continuous structural modification over a wide cooling rate interval when solidified from the melt[1] assuming a semi-crystalline structure below 2 K s 1 and a completely amorphous one above 100 K s 1. Most important was the existence of a state of intermediate order between the above cooling rates which was evidenced by the absence of crystalline reflections in the WAXS patterns and the occurrence of SAXS maxima[2] and exothermic peak areas (DSC) in the cooling rate range above 2 K s 1. Microhardness (MH) measurements revealed that this phase affects the mechanical properties[3] plausible if one thinks of crystalline clusters of intermediate order, dispersed in the amorphous polymer