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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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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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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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Ziegler–Natta polymerization is another form of addition or chain‐growth polymerization used for synthesizing linear polymers over branched polymers. The catalyst used for polymerization is the Ziegler–Natta catalyst, named after Karl Ziegler and Giulio Natta, who developed it in 1953. This catalyst is an organometallic complex of titanium tetrachloride and triethyl aluminum, with the active form of the catalyst being an alkyl titanium compound. Using the Ziegler–Natta...
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The conversion of alkenes to macromolecules called polymers is a reaction of high commercial importance. The structure of the polymer is defined by a repeating unit, while the terminal groups are considered insignificant. The average degree of polymerization represents the number of repeating units in the polymer molecule and is denoted by the subscript n.
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Characteristics of Precipitation-formed Polyethylene Glycol Microgels Are Controlled by Molecular Weight of Reactants
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Homogeneous Nucleation in Polyethylene: Molecular Weight Dependence.

Gaylon S Ross1, Lois J Frolen1

  • 1Institute For Materials Research, National Bureau of Standards, Washington, D.C. 20234.

Journal of Research of the National Bureau of Standards. Section A, Physics and Chemistry
|March 19, 2020
PubMed
Summary

This study estimates crystal nucleation rates in polyethylene melts using the droplet technique. Results reveal how molecular weight influences crystal surface energies, with implications for polymer crystallization theories.

Keywords:
Chain foldsfractionshomogeneous nucleationmolecular weightnucleation theorypolyethylenesurface free energies

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Area of Science:

  • Polymer Science
  • Materials Science
  • Physical Chemistry

Background:

  • Understanding polymer crystallization is crucial for material properties.
  • Homogeneous nucleation is a fundamental process in polymer solidification.
  • Previous studies have explored nucleation in various polymers, but gaps remain for linear polyethylene fractions.

Purpose of the Study:

  • To determine the isothermal rate of homogeneous crystal nucleation in supercooled linear polyethylene melts.
  • To estimate the lateral and end-surface free energies (σ and σe) of polymer crystals as a function of molecular weight.
  • To compare experimental nucleation data with current theories and investigate the influence of molecular weight and temperature on nucleation parameters.

Main Methods:

  • Utilized the droplet technique to create highly supercooled melts of eight linear polyethylene fractions.
  • Analyzed experimental data using established theories of homogeneous nucleation for chain-folded crystals.
  • Estimated the quantity σ2σe, a product of surface free energies, across a range of molecular weights.

Main Results:

  • An anomalous nucleation behavior was observed for sample 3.2 K, suggesting potential crystallization in an extended chain form.
  • For samples 9.70 K to 23.0 K, σ2σe increased with molecular weight due to decreasing cilia per chain fold.
  • At higher molecular weights (23.0 K to 249 K), σ2σe stabilized around 19,000 ergs3/cm6.
  • Experimental nucleation frequencies (I0) deviated significantly from theoretical values, but this discrepancy reduced when assuming temperature-dependent surface free energies.

Conclusions:

  • The study provides valuable estimates of surface free energies in linear polyethylene crystallization.
  • Molecular weight significantly impacts nucleation behavior, particularly at lower ranges, due to chain folding dynamics.
  • The findings contribute to a refined understanding of homogeneous nucleation theories in polymer melts.
  • Temperature dependence of surface free energies plays a role in reconciling experimental and theoretical nucleation rates.