A positive temperature coefficient is the term which has been used to indicate that an increase in solubility occurs as the temperature is raised, whereas a negative coefficient indicates a decrease in solubility with rise in temperature.

Effect of Temperature

Reactive extrusion of PLA, PBAT with a multi-functional epoxide: Physico-chemical and rheological properties

A family of racemic and enantiopure indium complexes 1–11 bearing bulky chiral diaminoaryloxy ligands, H(NNOR), were synthesized and fully characterized. Investigation of both the mono- and the bis-alkoxy-bridged complexes [(NNOR)InX]2[μ-Y][μ-OEt] (5, R = tBu, X = Y = Cl; 8, R = Me, X = I, Y = OEt) by variable temperature, 2D NOESY, and PGSE NMR spectroscopy confirmed dinuclear structures in solution analogous to those obtained by single-crystal X-ray crystallography. The dinuclear complexes in the family were highly active catalysts for the ring-opening polymerization (ROP) of lactide (LA) to form poly(lactic acid) (PLA) at room temperature. In particular, complex 5 showed living polymerization behavior over a large molecular weight range. A detailed investigation of catalyst stereoselectivity showed that, although (R,R/R,R)-5 is highly selective for l-LA, only atactic PLA is obtained in the polymerization of racemic LA. No such selectivity was observed for complex 8. Importantly, the selectivities obtai...

Mechanism of living lactide polymerization by dinuclear indium catalysts and its impact on isoselectivity.

The classical no-slip boundary condition of fluid mechanics is not always a valid assumption for the flow of several classes of complex fluids including polymer melts, their blends, polymer solutions, microgels, glasses, suspensions and pastes. In fact, it appears that slip effect in these systems is the rule and not the exemption. The occurrence of slip complicates the analysis of rheological data, although it provides new opportunities to understand their behavior in restricted environments delineating additional molecular mechanisms i.e. entropic restrictions due to limitations in the number of molecular conformations. This article discusses these complexities and provides future research opportunities.

Slip mechanisms in complex fluid flows

We measure the flow kinetics of a polyethylene extruded through the exit of a sapphire capillary tube in order to understand the nature of sharkskin, a surface roughness in the extruded material. Optical velocimetry shows that sharkskin can occur under a variety of polymer/wall boundary conditions; stick, slip, or oscillating stick/slip, demonstrating that the flow boundary condition is not the direct cause of sharkskin. Downstream of the exit, high-speed video microscopy reveals two distinct material failures during each sharkskin cycle, the first is cohesive and splits the material into two regions, the second one occurs at the polymer–wall interface. Upon modification of the surface with a polymer processing additive (PPA), we confirm strong slip at the wall and a suppression of sharkskin, but find that sharkskin does return at sufficiently high flow rates. The extensional strain rate at the onset of sharkskin is significantly higher in the case with PPA than that without. We then empirically define a ...

Extensional deformation, cohesive failure, and boundary conditions during sharkskin melt fracture

The wall slip and melt fracture behaviour of several commercial polylactides (PLAs) as well as their rheological properties under shear and extensional have been investigated. The PLAs have had weight-average molecular weights in the range of 104–105 g/mol and studied in the temperature range of 160–200°C. The solution properties and linear viscoelastic behaviour of melts indicate linear microstructure behaviour. PLAs with molecular weights greater than a certain value were found to slip, with the slip velocity to increase with decrease of molecular weight. The capillary data were found to agree well with linear viscoelastic envelope once correction for slip effects was applied. The onset of melt fracture for the high molecular weight PLAs was found to occur at about 0.2 to 0.3 MPa, depending on the geometrical characteristics of the dies and independent of temperature. Addition of 0.5 wt.% of a polycaprolactone (PCL) into the PLA that exhibits melt fracture was found to be effective in eliminating and delaying the onset of melt fracture to higher shear rates. This is due to significant interfacial slip that occurs in the presence of PCL.

Wall slip and melt fracture of poly(lactides)

Processing aids for biodegradable polymers

In this paper, the flow-induced crystallization (FIC) behavior of various polypropylenes with different molecular characteristics was investigated using a capillary rheometer. The Cogswell analysis was applied on the capillary data to obtain the apparent extensional strain rate and strain as well as the apparent extensional viscosity. The extensional viscosity obtained using this method was in good agreement with the zero shear viscosity obtained using a cone-and-plate rotational rheometer (Anton Paar MCR-502). Extensional flow parameters did not influence crystallization kinetics in the capillary die. FIC was found to depend strongly on the length-to-diameter (L/D) ratio of the capillary die that is directly related to the residence time. It was also found that the crystallization kinetics were enhanced with increasing molecular weight, indicating the importance of high-end tail of molecular weight distribution (MWD) on FIC. Finally, temperature impacted the FIC behavior significantly since it alters the activation energy needed for the formation of macroscopic structures.

Flow-induced crystallization of polypropylenes in capillary flow

The wall slip and melt fracture behaviour of several commercial polylactides (PLAs) and poly(e-caprolactone), (PCLs) have been investigated. PLAs with molecular weights greater than a certain value were found to slip, with the slip velocity to increase with decrease of molecular weight consistent with wall slip data reported in the literature for other systems. The onset of melt fracture for the high molecular weight PLAs was found to occur at about 0.2 to 0.3 MPa, depending on the geometrical characteristics of the dies and independent of temperature. Similarly, sharkskin and gross melt fracture was observed for the case of PCLs depending on the molecular characteristics of the resins and the geometrical details of the capillary dies. It was also found that the addition of a small amount of PCL (typically 0.5 wt.%) into the PLA and vice versa is effective in eliminating and delaying the onset of melt fracture to higher shear rates in the capillary extrusion of PLA and PCL respectively. This is due to significant interfacial slip that occurs in the presence of PCL or PLA as well as to the immiscibility of the PLA/PCL blend system at all compositions.

Bashar Jazrawi

Papers

Wall slip and melt fracture of poly(lactides)

Processing aids for biodegradable polymers

Flow-induced crystallization of polypropylenes in capillary flow

Biodegradable polymers: Wall slip, melt fracture, and processing aids