Jorge Ressia

Influence of polydispersity on the viscoelastic properties of linear polydimethylsiloxanes and their binary blends

Abstract Linear polydimethylsiloxanes of relatively narrow molecular weight distribution (MWD) were synthesized by anionic polymerization and characterized by different techniques. Binary blends were also prepared with some of the synthesized polymers. Linear viscoelastic parameters, such as storage (G′) and loss (G″) moduli, were obtained at different temperatures as functions of frequency (ω). The time–temperature superposition principle was applied in order to increase the frequency range measured. Zero-shear rate viscosity (η0), steady-state recoverable compliance (Je0) and zero-shear rate first normal stress coefficient (ψ1,0) were calculated from the data corresponding to the terminal relaxation zone. The molecular weight dependence of those parameters shows a good agreement with classical models and previously reported results, although Je0 and ψ1,0 are strongly affected by polydispersity. Different polydispersity factors were applied in order to fit the experimental values of the viscoelastic properties with the theory.

Hyaluronan-Itaconic Acid-Glutaraldehyde Films for Biomedical Applications: Preliminary Studies.

New hyaluronic acid–itaconic acid films were synthesized as potential materials with biomedical applications. In this work, we explored the homogeneous cross-linking reactions of hyaluronic acid using glutaraldehyde in the presence of itaconic acid and triacetin as plasticizers. Biomechanical properties were assessed in terms of stability by measuring swelling in aqueous environments, investigating wettability using contact angle tests, and evaluating bioadhesive performance. The ductility of the materials was evaluated through stress-strain measurements and the morphology was explored by scanning electron microscopy. The results show that the incorporation of itaconic acid improved most of the desirable properties, increasing adhesiveness and reducing wettability and swelling. The use of triacetin enhanced the strength, bioadhesiveness, and ductility of the material.

Linear viscoelastic relaxation modulus of polydisperse poly(dimethylsiloxane) melts containing unentangled chains

Abstract This work analyzes the relationship between the shear relaxation modulus of entangled, linear and flexible homopolymer blends and its molecular weight distribution (MWD) when a fraction of the sample contains chains with molecular weight M lower than the effective critical molecular weight between entanglements Mc eff . This effective critical parameter is defined in terms of the critical molecular weight between entanglements M c of the bulk polymer that forms the physical network and the effective mass fraction Wc eff of the unentangled chains. In the terminal zone of the linear viscoelastic response, the double reptation mixing rule for blended entangled chains and a modified law for the relaxation time of chains in a polydisperse matrix are considered, where the effect of chains with M Mc eff is included. Although chain reptation with contour length fluctuations and tube constraint release are still the relevant mechanisms of chain relaxation in the terminal zone when the polydispersity is high, it is found that the presence of a fraction of molecules with M Mc eff modifies substantially the tube constrain release mode of chain relaxation. In this sense, a modified relaxation law for polymer chains in a polydisperse entangled melt that includes the effect of the MWD of unentangled chains is proposed. This law is validated with rheometric data of linear viscoelasticity for well-characterized polydimethylsiloxane (PDMS) blends and their MWD obtained from size exclusion chromatography. The short time response of PDMS, which involves the glassy modes of relaxation, is modeled by considering Rouse diffusion between entanglement points of chains with M > Mc eff . This mechanism is independent from the MWD. The unentangled chains with M Mc eff occluded in the polymer network also follow Rouse modes of relaxation although they exhibit dependence on the MWD.

Effect of the Thermal History on the Thermal and Rheological Behavior of a Thermotropic Polyester

The biphasic behavior and phase transitions of a thermotropic main-chain liquid crystalline polyester, the poly(oxytrimetilen-etilen glycol p,p′-bibenzoate), was studied by X-ray diffractometry, differential scanning calorimetry (DSC), and rotational rheometry. The liquid crystalline mesophase structure of the polymer evolves from Smectic C to Smectic A at around 100°C and changes to the isotropic state at about 200°C. The annealing of polymer samples at different temperatures within the smectic–isotropic transition gives rise to double endotherm and exotherm peaks on the heating and cooling DSC traces. These peaks are attributed to the preferential segregation of the lower molecular weight molecules from the liquid crystalline to the isotropic phase in the biphasic zone. The rheological studies are consistent with this result. The dynamic moduli measured at constant frequency in temperature ramps using polymer samples with different thermal histories reveal that the type of annealing processes applied in the biphasic zone generates different evolution of the rheological data.