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Polymers: Molecular Weight Distribution01:10

Polymers: Molecular Weight Distribution

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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.
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Polymers: Defining Molecular Weight01:01

Polymers: Defining Molecular Weight

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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...
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Debye–Huckel–Onsager Conductance Equation01:28

Debye–Huckel–Onsager Conductance Equation

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The Debye-Hückel-Onsager equation is a cornerstone of physical chemistry, providing a method to determine the molar conductance (Λm) and molar conductance at infinite dilution (Λ°m) for uni-univalent electrolytes.Uni-univalent electrolytes are electrolytes that dissociate in solution to produce one cation with a +1 charge and one anion with a –1 charge per formula unit.This equation addresses two crucial phenomena: the asymmetry effect and the electrophoretic effect.
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Polymer Classification: Stereospecificity01:26

Polymer Classification: Stereospecificity

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Polymerization generates chiral centers along the entire backbone of a polymer chain. Accordingly, the stereochemistry of the substituent group has a significant effect on polymer properties. Polymers formed from monosubstituted alkene monomers feature chiral carbons at every alternate position in the polymer backbone. Relative to the predominant orientation of substituents at the adjacent chiral carbons, the polymer can exist in three different configurations: isotactic, syndiotactic, and...
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Molecular Weight of Step-Growth Polymers01:08

Molecular Weight of Step-Growth Polymers

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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...
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IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration01:16

IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration

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A covalently bonded heteronuclear diatomic molecule can be modeled as two vibrating masses connected by a spring. The vibrational frequency of the bond can be expressed using an equation derived from Hooke's law, which describes how the force applied to stretch or compress a spring is proportional to the displacement of the spring. In this case, the atoms behave like masses, and the bond acts like a spring.
According to Hooke's law, the vibrational frequency is directly proportional to...
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Excitonic Hamiltonians for Calculating Optical Absorption Spectra and Optoelectronic Properties of Molecular Aggregates and Solids
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Polymer Optical Constants from Long-Range Corrected DFT Calculations.

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This study introduces a new method to calculate plastic refractive indices using the Lorentz-Lorenz equation. Accurate predictions are achieved by refining polymer models and selecting appropriate computational functionals for different plastic types.

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Area of Science:

  • Materials Science
  • Computational Chemistry
  • Polymer Physics

Background:

  • Accurate refractive index prediction is crucial for polymer characterization and optical applications.
  • Existing methods often struggle with the complexity of polymer structures and electronic properties.

Purpose of the Study:

  • To develop a robust computational methodology for calculating the refractive indices of polymers.
  • To assess the accuracy of long-range corrected functionals in predicting polymer polarizability.
  • To investigate the impact of end-group corrections on model accuracy.

Main Methods:

  • Utilizing the Lorentz-Lorenz equation for refractive index calculations.
  • Employing long-range corrected functionals to predict polarizability of polymer repeat units.
  • Implementing end-effect corrections in polymer models.
  • Comparing different exchange-correlation functionals (e.g., 100% Hartree-Fock, CAM-B3LYP).

Main Results:

  • The proposed methodology achieves accuracy comparable to molecular liquids (approx. 1% mean absolute deviation).
  • Functionals with 100% Hartree-Fock exchange are optimal for aromatic/hydrogen-deficient polymers.
  • CAM-B3LYP performs well for hydrogen-rich polymers.
  • Excellent agreement was found for polystyrene, poly(methyl methacrylate), and CYTOP, with low root-mean-square deviations.

Conclusions:

  • The developed methodology provides a reliable approach for predicting polymer refractive indices.
  • The choice of computational functional significantly impacts prediction accuracy based on polymer composition.
  • End-effect corrections are vital for accurate modeling of polymer repeat units.