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Gas exchange, the intake of molecular oxygen (O2) from the environment and the outflow of carbon dioxide (CO2) into the environment, is necessary for cellular function. Gas exchange during respiration occurs largely via the movement of gas molecules along pressure gradients. Gas travels from areas of higher partial pressure to areas of lower partial pressure. In mammals, gas exchange occurs in the alveoli of the lungs, which are adjacent to capillaries and share a membrane with them.
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Kinetic Molecular Theory and Gas Laws Explain Properties of Gas Molecules02:34

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The test of the kinetic molecular theory (KMT) and its postulates is its ability to explain and describe the behavior of a gas. The various gas laws (Boyle’s, Charles’s, Gay-Lussac’s, Avogadro’s, and Dalton’s laws) can be derived from the assumptions of the KMT, which have led chemists to believe that the assumptions of the theory accurately represent the properties of gas molecules.
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Although gaseous molecules travel at tremendous speeds (hundreds of meters per second), they collide with other gaseous molecules and travel in many different directions before reaching the desired target. At room temperature, a gaseous molecule will experience billions of collisions per second. The mean free path is the average distance a molecule travels between collisions. The mean free path increases with decreasing pressure; in general, the mean free path for a gaseous molecule will be...
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Polymers: Defining Molecular Weight01:01

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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.
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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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Molecular Weight of Step-Growth Polymers01:08

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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.
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Analyzing Melts and Fluids from Ab Initio Molecular Dynamics Simulations with the UMD Package
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Comparing gas transport in three polymers via molecular dynamics simulation.

Luke R Anderson1, Quan Yang, Andrew M Ediger

  • 1Department of Material Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA. lukea@vt.edu.

Physical Chemistry Chemical Physics : PCCP
|August 17, 2018
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Molecular dynamics simulations reveal how methane and n-butane transport through polymers like polyethylene (PE), poly(4-methyl-2-pentyne) (PMP), and polydimethylsiloxane (PDMS). Gas transport properties correlate with polymer chain flexibility and cavity size.

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Area of Science:

  • Materials Science
  • Polymer Science
  • Computational Chemistry

Background:

  • Polymers are crucial for separation media and containers, necessitating a deep understanding of gas transport at the molecular level.
  • Limited research exists on the molecular-level behavior of gases in various non-homologous polymers, particularly concerning bulk and interface transport properties.
  • Developing correlations between gas transport and polymer characteristics like cavity size distribution and chain flexibility is lacking.

Purpose of the Study:

  • To conduct a molecular-level study of methane and n-butane transport in the bulk and interface regions of polyethylene (PE), poly(4-methyl-2-pentyne) (PMP), and polydimethylsiloxane (PDMS).
  • To investigate the relationship between gas transport properties (diffusivity, solubility, permeability) and polymer structural features (accessible cavity fraction, chain oscillation amplitude).
  • To develop predictive models for gas transport properties based on polymer characteristics.

Main Methods:

  • Employed molecular dynamics (MD) simulations to study gas transport in bulk and interface regions of PE, PMP, and PDMS.
  • Analyzed penetrant diffusivity, solubility, and permeability in the bulk phase.
  • Utilized nonequilibrium MD simulations to investigate interface properties and gas transport in the interface region.

Main Results:

  • Gas molecules exhibited subdiffusion behavior, with transport mechanisms explored.
  • Polyethylene (PE) showed significantly lower diffusivities, solubilities, and permeabilities compared to PMP and PDMS, attributed to lower accessible cavity fraction (ACF) and average oscillation amplitudes (AOAs).
  • Developed successful equations correlating penetrant diffusivity and permeability with ACF and AOAs, showing proportionality to ACF^(1/3) and ACF^(4/3), respectively.

Conclusions:

  • Molecular dynamics simulations provide profound insights into gas transport mechanisms in different polymers.
  • Polymer structure, specifically ACF and AOAs, significantly influences gas transport properties.
  • The developed predictive equations offer a valuable tool for designing polymer-based separation materials.