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Polymers: Molecular Weight Distribution

Polymers: Molecular Weight Distribution

For any given polymer, the weight average molecular weight (Mw) is higher than, if not equal to, the number average molecular weight (Mn). The only situation in which the weight average molecular weight and the number average molecular weight are equal is when a polymer consists only of chains with equal molecular weight. However, this never happens in a synthetic polymer, since it is difficult to control the polymerization process up to a molecular level with accuracy to a hundred percent.

Molecular Weight of Step-Growth Polymers

Molecular Weight of Step-Growth Polymers

Step growth polymerization involves bi or multifunctional monomers. Bifunctional monomers react to form linear step growth polymers, whereas multifunctional monomers react to form non-linear or branched polymers.
As the step-growth polymerization involves step-wise condensation of monomers, the molecular weight also builds up eventually. Consequently, high molecular weight polymers are obtained at the late stages of the polymerization, where 99% of monomers have been consumed.
The extent of the...

Polymers: Defining Molecular Weight

Polymers: Defining Molecular Weight

Unlike small molecules with definite molecular weights, polymers are a mixture of individual polymer chains of varying lengths, each with a unique molecular weight. So, the molecular weight of a polymer is expressed as an average value based on the average size of the polymer chains. The two most common forms of averages used for polymers are the number average molecular weight and weight average molecular weight.
The number average molecular weight (Mn) is the summation of the number...

Radical Chain-Growth Polymerization: Chain Branching

Radical Chain-Growth Polymerization: Chain Branching

The skeletal structure of polymers synthesized via radical polymerization is always branched. For example, the polymerization of ethylene by radical polymerization results in a low-density grade of polyethylene with a heavily branched skeletal structure. Here, the radical site abstracts hydrogen from the growing chain, and the radical site shifts from the end (a primary carbon center) to anywhere within the growing chain (a secondary carbon center). Consequently, the part of the chain from the...

Polymers

Polymers

The word polymer is derived from the Greek words “poly” which means “many” and “mer” which means “parts”. Polymers are long chains of molecules composed of repeating units of smaller molecules, known as monomers. They either occur naturally, such as DNA and proteins, or can be constructed synthetically, like plastics. They have varied structural characteristics, such as linear chains, branched chains, or complex networks, that contribute to the...

Predicting Molecular Geometry

Predicting Molecular Geometry

VSEPR Theory for Determination of Electron Pair Geometries

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Polymer Microarrays for High Throughput Discovery of Biomaterials

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Published on: January 25, 2012

Multiscale Strategy for Predicting Radiation Chemistry in Polymers.

Matthew P Kroonblawd¹, Anthony Yoshimura¹, Nir Goldman^1,2

¹Lawrence Livermore National Laboratory, Livermore, California 94550, United States.

Journal of Chemical Theory and Computation

|August 12, 2022

Summary

Ballistic electrons cause polymer radiation damage through electronic excitations. Our multiscale simulations reveal polyethylene

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Area of Science:

Polymer science
Materials science
Physical chemistry

Background:

Radiation damage in polymers is often initiated by energetic electrons.
The precise chemical pathways following electronic excitation are not well understood.
Understanding these mechanisms is crucial for predicting material lifetime and performance.

Purpose of the Study:

To develop a predictive multiscale computational strategy for radiation-induced polymer chemistry.
To elucidate the specific chemical mechanisms governing radiation damage in polymers.
To explain the observed differences in cross-linking behavior in crystalline polymers.

Main Methods:

A multiscale approach combining subatomic scattering calculations with nonadiabatic molecular dynamics (NAMD).
NAMD simulations to capture initial bond-breaking events after electronic excitation.
Semiempirical simulations to model subsequent chemical reactions and approach equilibrium.
Application to polyethylene (PE) as a model polymer system.

Main Results:

The study establishes a computational framework to predict radiation-induced chemical changes in polymers.
Simulations identified key bond-breaking events initiated by electronic excitations in polyethylene.
A specific mechanism was revealed that explains the low cross-linking propensity in crystalline polyethylene samples.

Conclusions:

The developed multiscale strategy provides a powerful tool for understanding polymer radiation chemistry.
The findings offer insights into the fundamental processes governing material degradation under irradiation.
The results contribute to the design of more radiation-resistant polymers, particularly in crystalline forms.