Stefano Acierno

Measurements of the rheological behavior of a crystallizing polymer by an “inverse quenching” technique

In this paper we show that a suitable thermal history can be used to produce a polymer melt in which a fixed amount of crystalline phase has been frozen. We call this novel method “inverse quenching,” since a stable amorphous/crystalline system is obtained by heating up the sample rather than cooling it down. If the inverse quenching temperature is suitably chosen, the polymer can remain stable for a long time, thus allowing different types of experimental measurements. Here we first prove the validity of the inverse quenching method in quiescent crystallization conditions, and then we use the inverse quenching method to perform rheological measurements on an isotactic polypropylene at a constant degree of crystallization. In particular, steady-state viscosity measurements in the early crystallization stages are reported for the first time, showing that the viscosity at low shear rates is much larger than that of the purely amorphous melt even for small values of crystallinity. The technique is also used to study the liquid-to-solid transitional behavior of the crystallizing polymer, which can be seen as a gelation process, at temperatures that are forbidden to traditional techniques. Such measurements are shown to provide further, robust validation of this novel method.In this paper we show that a suitable thermal history can be used to produce a polymer melt in which a fixed amount of crystalline phase has been frozen. We call this novel method “inverse quenching,” since a stable amorphous/crystalline system is obtained by heating up the sample rather than cooling it down. If the inverse quenching temperature is suitably chosen, the polymer can remain stable for a long time, thus allowing different types of experimental measurements. Here we first prove the validity of the inverse quenching method in quiescent crystallization conditions, and then we use the inverse quenching method to perform rheological measurements on an isotactic polypropylene at a constant degree of crystallization. In particular, steady-state viscosity measurements in the early crystallization stages are reported for the first time, showing that the viscosity at low shear rates is much larger than that of the purely amorphous melt even for small values of crystallinity. The technique is also used ...

Alginate-hyaluronan composite hydrogels accelerate wound healing process.

In this paper we propose polysaccharide hydrogels combining alginate (ALG) and hyaluronan (HA) as biofunctional platform for dermal wound repair. Hydrogels produced by internal gelation were homogeneous and easy to handle. Rheological evaluation of gelation kinetics of ALG/HA mixtures at different ratios allowed understanding the HA effect on ALG cross-linking process. Disk-shaped hydrogels, at different ALG/HA ratio, were characterized for morphology, homogeneity and mechanical properties. Results suggest that, although the presence of HA does significantly slow down gelation kinetics, the concentration of cross-links reached at the end of gelation is scarcely affected. The in vitro activity of ALG/HA dressings was tested on adipose derived multipotent adult stem cells (Ad-MSC) and an immortalized keratinocyte cell line (HaCaT). Hydrogels did not interfere with cell viability in both cells lines, but significantly promoted gap closure in a scratch assay at early (1 day) and late (5 days) stages as compared to hydrogels made of ALG alone (p<0.01 and 0.001 for Ad-MSC and HaCaT, respectively). In vivo wound healing studies, conducted on a rat model of excised wound indicated that after 5 days ALG/HA hydrogels significantly promoted wound closure as compared to ALG ones (p<0.001). Overall results demonstrate that the integration of HA in a physically cross-linked ALG hydrogel can be a versatile strategy to promote wound healing that can be easily translated in a clinical setting.

Coupling between kinetics and rheological parameters in the flow‐induced crystallization of thermoplastic polymers

Flow Induced Crystallization (FIC) is the common term to indicate the acceleration in polymer crystallization kinetics due to the action of flow. FIC is expected to be the result of the coupling between the intrinsic (quiescent) crystallization kinetics and the rheological response of the polymer. The choice of a suitable rheological model, therefore, is a crucial requirement for a successful FIC model. Recent work of our group [1] has demonstrated that the Doi-Edwards rheological model (DE), based on the concept of chain reptation, can be easily incorporated into classical crystallization models to successful predict the enhancement in nucleation rate under the action of a steady shear flow. In this paper, the interaction between the rheological parameters of the DE model and the crystallization kinetics parameters is investigated in more details. In particular, the effect of the crystallization temperature, which acts on both the polymer relaxation time and the free energy jump between liquid and solid phase, is determined and discussed.

Effects of molecular weight distribution on the flow-enhanced crystallization of poly(1-butene)

In this paper we analyze the crystallization kinetics under steady shear flow conditions of different samples obtained by blending two isotactic poly(1-butene)s with different average molecular weights. It is observed that the addition of a small amount of high molecular weight (MW) polymer (<6wt%) to a low MW sample does not produce any appreciable effect upon the crystallization kinetics under both quiescent and shear flow conditions. When more elevated amounts of high MW polymer are added, only mild effects upon the crystallization kinetics, under both quiescent and shear conditions, are observed. This behavior can be attributed to constraint release of high MW chains due to the relaxation of the shorter chains. Such a physical phenomenon can be described by the double reptation theory, which, indeed, allows for good quantitative predictions of the experimental results by using the relaxation times of the two blend components as the only fitting parameters.In this paper we analyze the crystallization kinetics under steady shear flow conditions of different samples obtained by blending two isotactic poly(1-butene)s with different average molecular weights. It is observed that the addition of a small amount of high molecular weight (MW) polymer (<6wt%) to a low MW sample does not produce any appreciable effect upon the crystallization kinetics under both quiescent and shear flow conditions. When more elevated amounts of high MW polymer are added, only mild effects upon the crystallization kinetics, under both quiescent and shear conditions, are observed. This behavior can be attributed to constraint release of high MW chains due to the relaxation of the shorter chains. Such a physical phenomenon can be described by the double reptation theory, which, indeed, allows for good quantitative predictions of the experimental results by using the relaxation times of the two blend components as the only fitting parameters.

Melt-spun bioactive sutures containing nanohybrids for local delivery of anti-inflammatory drugs

In this work, a novel concept is introduced in drug-eluting fibres to ensure a good control of drug delivery features and wide applicability to different bioactive compounds. Composite bioactive sutures based on fibre grade poly(ε-caprolactone) (PCL) and loaded with the anti-inflammatory drug Diclofenac (Dic) or a Dic nanohybrid where the drug is intercalated in a synthetic hydrotalcite (Mg/Al hydroxycarbonate) (HT-Dic) were developed. Fibres were prepared by melt-spinning at different PCL/HT-Dic/Dic ratios and analysed in terms of morphology, mechanical properties and drug release features. Results emphasized that tensile properties of fibres are clearly affected by Dic or HT-Dic addition, while the presence of knots has limited influence on the mechanical behaviour of the sutures. Release of Dic strongly depends on how Dic is loaded in the fibre (as free or nanohybrid) whereas the combination of free Dic and HT-Dic can allow a further tuning of release profile. In vivo experiments show a reduction of inflammatory responses associated with Dic-loaded fibers. Thus, a proof of principle is provided for a novel class of bioactive sutures integrating advanced controlled-release technologies.

Compressive and Thermal Properties of Recycled EPS Foams

Optimization of moulding process in order to maximize the recycled fraction in EPS products is a key industrial goal. To this end effects of quality and quantity of recycled EPS upon physical properties of products are studied. Results show, on the one hand, that the introduction of a recycled fraction brings to products with a reduced density. On the other hand, small modifications of the process parameters allow for a reduction in this density decrease. Furthermore, results show that mechanical and thermal behaviour of EPS depend only on the materials density apart from the possible presence of a recycled fraction.

Dispersion issues and thermal conductivity of polypropylene/multi wall carbon nanotube systems

Three types of multiwall carbon nanotubes, one non-functionalized tubes and two functionalized with polar (amino and carboxyl) groups, were used as fillers in a polypropylene resin to develop nanocomposites with improved thermal conductivity. In particular, formulations containing up to 5% in volume of carbon nanotubes, prepared by melt blending, were analyzed in terms of dynamic rheological behavior of melts and thermal conductivity. The former can give information related to the build-up of internal network structures and to the level of dispersion of the fillers. Taking into account that the properties of nanocomposites are strictly related to these aspects, the enhancement of thermal conductivity with respect to the pristine matrix are discussed as a function of the filler content, dispersion of the filler and presence of internal structures.