Michael Niaounakis

Water effects in polyurethane block copolymers

Equilibrium and dynamic sorption isotherm measurements, differential scanning calorimetry (DSC) measurements, and, mainly, dielectric relaxation spectroscopy (DRS) measurements by means of the thermally stimulated depolarization currents (TSDC) method were used to investigate the hydration properties of linear segmented polyurethane copolymers. Three types of samples were investigated with various fractions of hard and soft block segments. They were based on polyethylene adipate (PEA), 4,4′-diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BDO). At 20°C the water content h of the samples at various values of relative humidity rh increases in proportion to the weight fraction of soft block segments phase. At saturation (rh = 100%) the ratio of sorbed water molecules to polar carbonyl polyester groups is 0.13. At saturation at 20°C there is no fraction of freezable water. The glass transition temperature, Tg, measured by DSC and by TSDC, shifts to lower temperature with increasing h by about 8–10 K at saturation at 20°C. A dielectric relaxation mechanism related to interfacial polarization in the phase-separated morphology is also plasticized by water in a way similar to that observed for the main (α) relaxation.

Physical and chemical cross-linking effects in polyurethane elastomers

A series of polyester-urethane block copolymers of various molecular weights was prepared via a two-step polymerization process. The prepolymer composition was kept constant in all the samples, while the NCO/OH ratio during the chain extension was varied from 0.9 to 1.2. Chemical and physical cross-linking effects were studied by means of F.T.I.R spectroscopy, swelling, and elastic behavior. Equilibrium stress-strain measurements and tensile-retraction tests were carried out to examine the elastomeric behavior of the materials tested. The extent of agreement between microscopic and macroscopic behavior was then evaluated.

Thermomechanical behavior of metallocene ethylene-α-olefin copolymers

Abstract The thermal, dynamic mechanical and stress–strain properties of a set of commercial LLDPEs prepared by metallocene catalysts were studied and compared with two LLDPEs obtained with traditional Ziegler–Natta catalysts. The type and amount of comonomer strongly affects the degree of crystallinity and branching, resulting in different material morphology and macroscopic thermomechanical behavior. Within each set of ethylene-α-olefin copolymers a gradual decrease in the percentage crystallinity and position and intensity of β- and γ-transition is observed, with respect to the comonomer content. In addition, the studied materials are characterized by the absence of an α-transition. This behavior is attributed to the high comonomer content (5–24)%. Tensile behavior changes from the typical necking, cold drawing and strain hardening to the uniform elastomeric deformation, as the comonomer content increases. The experimental methods used, in combination with scanning electron microscopy, lead to converged results, while tensile testing proved to be sensitive to the crystallite size and distribution.

Comparing Nanofillers in Polylactide Nanocomposites

The present study compares the effects of two different types of nanosized fillers (silica and montmorillonite) at three different weight fractions as well their mixtures on the thermomechanical properties of polylactide (PLA). The role of aggregation and interphase was investigated with several experimental techniques including DSC, WAXS, SEM and tensile measurements. The experimental results clearly suggest that silica and montmorilonite have different reinforcing and toughnening effects on PLA, while the combination of the two different nanofillers seems to have a detrimental effect on the total interphase volume of the material. Four micromechanics models (Mori‐Tanaka, Chen, Taya‐Chu and Odegard), describing the Young’s modulus of the nanocomposites, were used to study the different matrix‐nanofiller interactions.