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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.

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...

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

Polymers

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...

Polymers

Polymers

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Accurate Force Field Development for Modeling Conjugated Polymers.

Kateri H DuBay¹, Michelle Lynn Hall¹, Thomas F Hughes¹

¹Department of Chemistry, Columbia University in the City of New York, New York, New York, United States.

Journal of Chemical Theory and Computation

|November 26, 2015

Summary

This summary is machine-generated.

Classical force fields struggle to model conjugated polymers due to steric hindrances. Adjustments to the OPLS-2005 force field improve simulations of poly(phenylene vinylene) and polythiophene, enhancing accuracy in modeling these important materials.

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Area of Science:

Computational chemistry
Materials science
Polymer physics

Background:

Conjugated polymers present unique modeling challenges for classical force fields.
Strong constraints from conjugation favor planar configurations, leading to steric repulsions and strained geometries.

Purpose of the Study:

To address inaccuracies in modeling conjugated polymer conformations using classical force fields.
To improve the OPLS-2005 force field for better simulation of these systems.

Main Methods:

Comparison of LMP2-calculated torsion potentials with standard classical force fields.
Implementation of adjustments to the OPLS-2005 force field.
Molecular dynamics simulations of MEH-PPV and P3HT.

Main Results:

Identified significant inaccuracies in standard force fields for conjugated polymer torsion potentials.
Developed and implemented force field adjustments to improve modeling.
Observed impact of adjustments on dihedral angle, persistence, and conjugation length distributions.

Conclusions:

Adjusted OPLS-2005 force field provides more accurate conformational modeling for conjugated polymers.
Improved simulations are crucial for understanding structure-property relationships in materials like MEH-PPV and P3HT.