Effect of long chain branching on linear-viscoelastic melt properties of polyolefins

Abstract

Abstract:The aim of this review is to provide evidence that rheological testing is a potent tool for characterising polymers in the melt. An effort has been made in order to gather results in conventional and model polyolefins, and correlating them with phenomena occurring at the molecular level. We have focused our interest on long chain branching (LCB). In the case of materials containing long side-chain branches, strong effects on viscosity, elastic character and activation energy of flow are general features. Literature results mostly indicate that the effect of polydispersity on these parameters could be very similar to that expected due to the presence of LCB -notwithstanding that the effects of LCB seem to be stronger than those due to polydispersity for a given molecular weight. Different relaxation processes appear as a consequence of the presence of LCB: slower terminal relaxation behaviour than of linear counterparts, and faster additional branch relaxation at higher frequencies, clearly distinguishable from polydispersity effects. To measure the amount of LCB from limited viscoelastic data and molecular properties seems to be a suitable instrument to explain the rheological features of the different polymers, but it fails when the results are compared with measured values of LCB density in model polymers. The actual framework leads us to say that the number of branches is less important than the topology itself. Therefore, the position and architecture of the branches along the polymer main chain are the main factors that control the rheology of the material.

A Comprehensive Review on Controlled Synthesis of Long‐Chain Branched Polyolefins: Part 3, Characterization of Long‐Chain Branched Polymers

“…However, the theoretical explanations for LCB detection via rheological techniques from molecular dynamics and models are not discussed here, as this information can already be found in other excellent reviews. [79][80][81] There are also early reviews that report on the aspects of melt rheology for POs, [71] structure and rheology of molten polymers, [81] effects of LCB on linear-viscoelastic melt properties of POs [82] and rheological characterization of LCB. [83] …”

“…[82] Such phenomenon becomes more obvious with increased LCBD. Newtonian viscosity regime thus becomes smaller.…”

Analytical Rheology of Polymer Melts: State of the Art

“…However, LCB has an enormous impact of rheological properties. It has long been known that the addition of trace amounts of long-chain branching (LCB) produces dramatic changes in the linear and nonlinear rheology [40][41][42][43][44][45]. These well-documented effects include a departure from the "3.4 power law" relating the zero shear viscosity η 0 to the weightaveraged molecular weight, unusually large sensitivity of η 0 to temperature or higher flow activation energies [46], enhanced shear thinning and strain hardening that lead to improved tear resistance, processability, and so forth.…”

Using viscoelastic properties to quantitatively estimate the amount of modified poly(lactic acid) chains through reactive extrusion

“…Obtaining molecular information about LCB using only rheology-based methods is not trivial. Numerous studies reported that increasing the MW, broadening the MW distribution (MWD), or increasing the content of LCB have similar effects on the shear viscosity function [Shroff and Mavridis (1999); Vega et al (2002); Doerpinghaus and Baird (2003); Shroff and Mavridis (1999); van Ruymbeke et al (2005); Stadler and Munstedt (2008b)]. That is, the distribution of relaxation times is not a unique function of chain architecture.…”