The use of the indentation test for studying the solidification behaviour of different semicrystalline polymers during injection moulding

An in-line method for monitoring the solidification process during injection molding of semicrystalline polymers (demonstrated previously in J. Appl. Polym. Sci.2003, 89, 3713) is based on a simple device, where an additional ejector pin is pushed on the injection molded part at different times during the solidification phase. The ‘indentation depth profile’, i.e., residual deformation as a function of time, was obtained and allowed to determine the evolution of the solidification front in the mold as a function of the cooling time. The present work shows the reliability and the powerfulness of the aforementioned method for a large variety of different semicrystalline polymers (PET, PBT, polyamide-6 PA6, isotactic poly(propylene) iPP) characterized also by different molecular weight and/or nucleating agents. The results show that the indentation test may be considered as a ‘predictive’ tool to qualitatively and quantitatively compare the solidification process of different polymers and polymer grades during injection molding

PLLA biodegradable scaffolds for angiogenesis via Diffusion Induced Phase Separation (DIPS)

A critical obstacle in tissue engineering is the inability to maintain large masses of living cells upon transfer from the in vitro culture conditions into the host in vivo. Capillaries, and the vascular system, are required to supply essential nutrients, including oxygen, remove waste products and provide a biochemical communication “highway”. For this reason it is mandatory to manufacture an implantable structure where the process of vessel formation – the angiogenesis – can take place. In this work PLLA scaffolds for vascular tissue engineering were produced by dip-coating via Diffusion Induced Phase Separation (DIPS) technique. The scaffolds, with a vessel-like shape, were obtained by performing a DIPS process around a nylon fibre whose diameter was 700 μm. The fibre was first immersed into a 4% PLLA dioxane solution and subsequently immersed into a second bath containing distilled water. The covered fibre was then rinsed in order to remove the excess of dioxane and dried; finally the internal nylon fibre was pulled out so as to obtain a hollow biodegradable PLLA fiber. SEM analysis revealed that the scaffolds have a lumen of ca. 700 μm. The internal surface is homogeneous with micropores 1–2 μm large. Moreover, a cross section analysis showed an open structure across the thickness of the scaffold walls. A cell culture of endothelial cells was carried out into the as-prepared scaffolds. The result showed that cells are able to grow within the scaffolds and after 3 weeks they begin to form a “primordial” vessel-like structure

Ageing of isotactic polypropylene due to morphology evolution, experimental limitations of realtime density measurements with a gradient column

Ageing in crystalline polymers is responsible for the deterioration of physical properties leading, for example, to a decrease in toughness and to dimensional changes that are to some extent responsible for warpage and scrap production in injection molding. Since, it depends on the mutual transformation of stable and metastable phases, being always related to changes in morphological organization, it is here preferred to call it ‘Morphological ageing’. Although, one would expect the ageing regime to be determined by the complex morphology with amorphous phases of different mobility and eventually multiple crystalline phases, transformed into each other at an associated transition, existing literature always shows a more trivial linear with the logarithm of time dependence of every probe used to describe ageing. Existing literature always overlooks the initial morphology, often complex, or, adopts well equilibrated samples, not representative of processing conditions. In this work, ageing of iPP was addressed producing samples characterized by an homogeneous morphology even at the largest cooling rates adopted using a CCT approach. This paper describes few of the many attempts undertaken to quantitatively relate ageing to initial morphology in iPP melt solidified under conditions emulating polymer processing. Ageing was monitored by measuring the density time dependence offline, i.e. separately applying the ageing protocol, and online in a gradient column conditioned at the ageing temperature following samples’ apparent density evolution. The offline method can give hints about the role of temperature and initial morphology but the results so obtained suffer from significant data scattering. The online method on the other hand, can provide a more accurate interpretation of ageing if due account is made of all the features of the fluid dynamic transient superimposed on the densification due to morphological ageing. Densification during ageing of the iPP samples used, solidified between 1 and 100 8C/s, takes place as a superposition of two phenomena: one is the linear increase of density with the logarithm of time, which is quantitatively related to the initial morphology, i.e. the cooling rate, and the other is a density jump that takes place at times much smaller than the available resolution and thus not quantitatively accessible

Local mechanical properties by Atomic Force Microscopy nanoindentations

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