One of the unique features of the chemistry of carbon (and, to some extent, silicon) is its ability to form long chains of atoms. Polymers are substances that have macromolecules composed of many repeating units (known as ‘mers’). Many naturally occurring substances are polymers, including rubber and many substances based on glucose, such as the polysaccharides cellulose and starch (in plants) and glycogen (in animals). Proteins, nucleic acids, and inorganic macromolecular substances (like polysiloxanes, commonly called silicones, or silicates) are other examples.
An important area of industrial chemistry is concerned with the manufacture of polymeric materials with a variety of properties. Plastics are some of the best known of these polymers and are everywhere; if humanity is committed to make a rational use of them and find substitutes wherever possible, understanding the physical behaviour of polymers is paramount. One of these physical aspects important in any industrial process is how the material moves; the science that focuses on how materials flow is called rheology. The rheology of polymer melts is, thus, fundamental in many ways.
Today, it is well accepted that the unique rheological properties of high molecular weight polymer melts are controlled by entanglements: topological constraints imposed by the mutually interpenetrating and uncrossable polymer chains. Regarding the entangled state as a set of chains between cross-links it is possible to regard the chain as being in a ‘tube’, with the tube being formed by these topological constraints. The chain is longer than the tube so that the ‘slack’ of the chain moves through the tube, which causes the tube itself to change with time. This motion was called reptation (from the Latin reptare, to creep) by Pierre Gilles de Gennes in 1971. Many experiments indicate that reptation dominates the dynamics of polymer chains when they are entangled.
However, in reality, other mechanisms compete with reptation, mainly, contour length fluctuations (CLF)—fluctuations of the length of the tube—and constraint release (CR), referring to probe motion driven by the motions of the surrounding matrix chains. A simplified version of the many-body CR mechanism is the dynamic tube dilation (DTD), where CR effects are represented by an increase with time of the tube diameter. The idea of DTD was first introduced by Marrucci, who proposed that even in a homopolymer melt the portion of the chain that has escaped from the original tube acts as a solvent for the remaining tube sections. CR in general becomes very important in the case of polydisperse melts and binary blends, especially where long polymer chains are mixed with short additives of the same chemistry (iso-frictional blends) but sometimes of different polymer architecture.
A direct experimental microscopic observation of the time dependence of the CR or DTD mechanisms remains elusive. Basically, because it is assumed that the time scale of these processes is beyond the accessible times by neutron spin echo (NSE)—the microscopic technique often used to address reptation problems in polymers at molecular level.
Now, a team of researchers reports 1 a microscopic observation of the time-dependent dynamic tube dilation process on iso-frictional bidisperse melts. They combine dielectric spectroscopy (DS) and NSE to address the dynamics of long polyisoprene (PI) chains in iso-frictional blends with “additives” of two different topologies: short PI linear chains and star-branched PI.
By means of DS in the terminal range the researchers confirm a dilated tube mechanism for long chains relaxation. They also demonstrate that the connected time-dependent tube dilation can be directly accessed by NSE. The characteristic time for CR is identified as the terminal time of the short component.
The team proposes a simple model inspired by DTD ideas, which nicely describes the NSE data with the model parameters fixed from dielectric results and from the NSE data of bulk long-PI chains. The comparison of some results from this model with available simulation results give additional support to the approach.
Author: César Tomé López is a science writer and the editor of Mapping Ignorance
Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper.
- Paula Malo de Molina, Angel Alegría, Jürgen Allgaier, Margarita Kruteva, Ingo Hoffmann, Sylvain Prévost, Michael Monkenbusch, Dieter Richter, Arantxa Arbe, and Juan Colmenero (2019) Direct Observation of Dynamic Tube Dilation in Entangled Polymer Blends: A Combination of Neutron Scattering and Dielectric Techniques Phys. Rev. Lett. doi: 10.1103/PhysRevLett.123.187802 ↩