V. La Carrubba

An experimental methodology to study polymer crystallization under processing conditions. The influence of high cooling rates

Abstract A new experimental route for investigating polymer crystallization under very high cooling rates (up to 2000°C/s) is described. A complete and exhaustive description of the apparatus employed for preparing thin quenched samples (100– 200 μm thick) is reported, the cooling mechanism and the temperature distribution across sample thickness is also analysed, showing that the final structure is determined only by the thermal history imposed by the fast quench apparatus. Details concerning the characterization techniques used to probe the final structure are reported, including density measurements and wide angle X-ray diffraction patterns. Experimental results concerning isotactic polypropylene, polyethylenetherephthalate and polyamide 6 are reported, showing the reliability of this experimental route to assess not only a quantitative information but also a qualitative description of the crystallization behaviour of different classes of semi-crystalline polymers.

Polymer Solidification under Pressure and High Cooling Rates

Abstract Polymer solidification under processing conditions is a complex phenomenon in which the kinetics of flow, high thermal gradients and high pressures determine the product morphology. The study of polymer structure formed under pressure has been mainly made using conventional techniques such as dilatometry and differential scanning calorimetry under isothermal conditions or non isothermal conditions but at cooling rates several orders of magnitude lower than those experienced in industrial processes. A new equipment has been recently developed and improved to study the crystallization of polypropylene when subjected to pressure and cooled rapidly. An experimental apparatus essentially constituted of a special injection mould has been employed. Polymer samples can be cooled at a known cooling rate and under a known pressure. Micro Hardness (MH), Wide angle x-ray diffraction (WAXD), Polarised Optical Microscopy (POM) and density measurements are then used to characterize the sample morphology. The results of rapid cooling experiments under pressure on an iPP sample display a lower density and a lower density dependence on cooling rate for increasing pressure. Micro hardness confirms the trend. A deconvolution technique of WAXD patterns is used to evaluate the final phase content of samples and to assess a crystallization kinetics behaviour. Phase distribution results indicate that the decrease of alpha phase with pressure is balanced by an increase of the mesomorphic phase leaving unaffected the amorphous phase content. This peculiar behaviour can be easily related to a negative influence of pressure on the kinetics of the crystallization of alpha phase.

Measurement of cloud point temperature in polymer solutions

A temperature-controlled turbidity measurement apparatus for the characterization of polymer solutions has been instrumented and set up. The main features are the coupled temperature-light transmittance measurement and the accurate temperature control, achieved by means of peltier cells. The apparatus allows to measure cloud point temperatures by adopting different cooling protocols: low rate for quasi-equilibrium measurements and high rate for detect kinetic effects. A ternary polymeric solution was adopted as case study system showing that cooling rate affects the measured cloud point temperature.

Evaluation of mechanical and morphologic features of PLLA membranes as supports for perfusion cells culture systems

Porous biodegradable PLLA membranes, which can be used as supports for perfusion cell culture systems were designed, developed and characterized. PLLA membranes were prepared via diffusion induced phase separation (DIPS). A glass slab was coated with a binary PLLA-dioxane solution (8wt.% PLLA) via dip coating, then pool immersed in two subsequent coagulation baths, and finally dried in a humidity-controlled environment. Surface and mechanical properties were evaluated by measuring pore size, porosity via scanning electron microscopy, storage modulus, loss modulus and loss angle by using a dynamic mechanical analysis (DMA). Cell adhesion assays on different membrane surfaces were also performed by using a standard count method. Results provide new insights into the foaming methods for producing polymeric membranes and supply indications on how to optimise the fabrication parameters to design membranes for tissue cultures and regeneration.

Preparation of Poly(l-lactic acid) Scaffolds by Thermally Induced Phase Separation: Role of Thermal History

Abstract Poly-L-Lactic Acid (PLLA) scaffolds for tissue engineering were prepared via thermally induced phase separation of a ternary system PLLA/dioxane/tetrahydrofurane. An extension to solution of a previously developed method for solidification from the melt was adopted, the technique being based on a Continuous Cooling Transformation (CCT) approach, consisting in recording the thermal history of rapidly cooled samples and analysing the resulting morphology. Different foams were produced by changing the thermal history, the dioxane to THF ratio (50/50, 70/30, 90/10 v/v) and the polymer concentration (2, 2.5, 4 ° wt) in the starting ternary solution. Pore size, porosity, melting and crystallization behavior were studied, together with a morphological and kinetic analysis of the foams produced. A large variety of morphologies was achieved, the largest pore size (20 μm) was achieved at the highest polymer concentration (4 ° wt) and the lowest dioxane concentration (50/50 dioxane/THF v/v), whereas the largest porosity (90 °) was attained at the highest dioxane concentration (90/10). The average pore size is related to cooling rate, with a 1/3 power law exponent at low polymer concentrations and low dioxane content for thermal histories driven by low undercoolings. At high undercoolings, the growth of the demixed domains significantly departs from the diffusive-like regime.

A poly-L-lactic acid/ collagen/glycosaminoglycan matrix for tissue engineering applications

Adhesion of tissue cells to biomaterials is a prerequisite of paramount importance for the effectiveness of a tissue engineering construct (cell and scaffolds). Functionalization of polymeric scaffolds with organic polymers, such as collagen or proteoglycans, is a promising approach in order to improve the cytocompatibility. As a matter of fact, organic polymers, isolated directly from the extracellular matrix, contain a multitude of surface ligand (fibronectin, laminin, vitronectin) and arginine–glycine–aspartic acid-containing peptides that promote cell adhesion. In tissue engineering, the combination of organic and synthetic polymers gives rise to scaffolds characterized simultaneously by the mechanical strength of synthetic materials and the biocompatibility of natural materials. In this work, porous poly-L-lactide acid scaffolds were functionalized with a synthetic collagen–glycosaminoglycans matrix in order to improve cell adhesion. For this purpose, a protocol for collagen–glycosaminoglycans conjugation into the pores of the scaffolds was set up. Moreover, an innovative protocol for the quantification of the conjugated glycosaminoglycans inside the scaffolds was created and adopted. The results have confirmed the effectiveness of the developed protocol: a collagen–glycosaminoglycans conjugation, with an efficiency of about 21% was obtained inside the scaffold. Moreover, SEM analysis highlighted the presence of the homogeneous synthetic matrix into the bulk of porous scaffolds. Finally, cell culture assays carried out by utilizing mouse embryonic fibroblasts showed that cell proliferation on poly-L-lactide acid-collagen–glycosaminoglycans scaffold is higher than on poly-L-lactide acid collagen scaffold (utilized as control). Therefore, it can be stated that the presence of glycosaminoglycans not only increases the mechanical strength of the matrix, thanks to their cross-linking effect, but also it seems to lead to a more significant cell growth. Overall, it is reasonable to state that the concerned protocol may be proposed as a reliable route to increase the rate of proliferation and in some cases to stimulate the cell differentiation in tissue engineering devices.

Biological evaluation of PLLA membranes, with different pore diameters, to stimulate cell adhesion and growth in vitro

Polymeric membranes prepared via DIPS (Diffusion Induced Phase Separation) are widely studied and utilized as scaffolds for the regeneration of tissue. In this work, poly (L)-lactide membrane are prepared through a DIPS protocol starting from a ternary solution made of polymer, dioxane (solvent) and water (non-solvent). A three-dimensional, porous and mechanically stable membrane is desirable for ingrowth of human bronchial epithelial cells.

Design, build-up and optimization of a fast quenching device for polymeric thin film

In this work an innovative apparatus for the characterization of polymer solidification under very high cooling rates (up to thousand of K/s) is described, according to the continuous cooling transformation approach adopted in metallurgy for studying structure development in metals. The proposed model experiment is addressed to design a method for the characterization of non-isothermal solidification behaviour, encompassing typical cooling conditions of polymer processing. Only temperature history determines the structure formed, as melt solidification takes place in quiescent conditions. With respect to the device previously developed by the authors [1, 3] the present equipment presents the following features: - reduced size (30*30*20 cm); - higher heating rates, by using two electrical resistances supported on ceramic plates; - higher cooling rates, carried out through water sprays at very high pressures (120 bar); - thermal insulation of the sample from the sample holder via the application of two wave...

No‐flow temperature and solidification in injection molding simulation

The no‐flow temperature (NFT) is a parameter representing the rheological solidification temperature of a polymer. A polymer, during injection molding filling stage, can stop its flow because of its high viscosity, although it is not yet fully solidified by means of glass transition or crystallization. The NFT is used in most of injection molding simulation packages: with this simple parameter it is possible to reduce the errors deriving from viscosity extrapolation at relatively low temperatures. The viscosity measurements for polymers are usually carried out at high temperatures, and the viscosity models can fail in prediction at temperatures close to the glass transition or crystallization temperature.The NFT is still a parameter not well defined and a standard method to measure it is still lacking. Nevertheless, a simple correlation for NFT estimation, derived from Cross‐WLF equation, is proposed for both amorphous and semicrystalline polymers. The presented correlation takes into account the changes ...

Water Fluxes in Polymeric Membranes for Desalination via Membrane Distillation

Membrane distillation is an emerging technique for seawater desalination. Hydrophobic polymeric membranes are used to separate the solute‐free water vapour from the hot solution. Vapour fluxes of commercial polymeric membranes were measured in various conditions, i.e. natural and forced convection and vacuum. Vapour fluxes were also predicted with models and compared with experimentals. Higher fluxes were recorded in vacuum conditions.