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Related Concept Videos

Polymer Classification: Architecture01:14

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Polymers are classified as linear or branched on the basis of their chain architecture. The polymer chains in linear polymers have a long chain-like structure with minimal to no branching at all. Even if a polymer features large substituent groups on the monomer, which appear as branches to the skeleton, it is not considered a branched polymer. A branched polymer contains secondary polymer chains that arise from the main polymer chain. The branching occurs when the polymer growth shifts from...
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Unlike ionic or small covalent molecules, polymers do not form crystalline solids due to the diffusion limitations of their long-chain structures. However, polymers contain microscopic crystalline domains separated by amorphous domains.
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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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Copolymers are the products obtained from the polymerization of multiple monomer species. So, in a polymer chain itself, there can be multiple repeating units that come from different monomers. The process of synthesizing a polymer from different monomer species is called copolymerization. When two monomers are involved, the polymer is known as a bipolymer. Polymers with three and four monomers are termed terpolymers and quaterpolymers, respectively. Figure 1 depicts the copolymerization of...
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Polymorphic microstructure of MDI/BD-block polyurethane as determined by temperature-sensitive conformation

Zeyu Wang1, Xuke Li1, Elmar Pöselt2

  • 1Ningbo Key Laboratory of Specialty Polymers, School of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, P. R. China. lixuke@nbu.edu.cn.

Soft Matter
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Thermoplastic polyurethanes (TPUs) form distinct crystal structures (form I and form II) based on crystallization temperature. Form II, formed at higher temperatures, exhibits superior elastic modulus and extended chain conformation compared to form I.

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Area of Science:

  • Materials Science
  • Polymer Chemistry
  • Crystallography

Background:

  • Thermoplastic polyurethanes (TPUs) are versatile polymers with tunable properties.
  • Understanding the crystallization behavior of TPUs is crucial for optimizing their performance.
  • MDI/BD-block TPUs are a specific class with unique structural characteristics.

Purpose of the Study:

  • To investigate the influence of isothermal crystallization temperature and cooling rates on MDI/BD-block TPUs.
  • To characterize the different crystalline forms (form I and form II) generated.
  • To elucidate the relationship between crystalline structure and mechanical properties.

Main Methods:

  • Differential Scanning Calorimetry (DSC) for thermal analysis.
  • Fourier Transform Infrared (FTIR) spectroscopy for conformational analysis.
  • Mechanical testing to determine elastic modulus.
  • X-ray diffraction (XRD) for crystal structure determination.

Main Results:

  • MDI/BD blocks crystallized into form II at isothermal temperatures ≥ 150 °C and form I at lower temperatures.
  • Form II exhibited a significantly higher crystal elastic modulus (6.75 GPa) than form I (1.31 GPa).
  • Conformational analysis revealed extended chains in form II and contracted chains in form I along the c-axis.

Conclusions:

  • Crystallization temperature dictates the formation of distinct crystalline structures (form I and form II) in MDI/BD TPUs.
  • The higher elastic modulus of form II is attributed to its extended chain conformation.
  • Physicochemical changes, including urethane group rotation and bond angle adjustments, drive the observed crystallization behavior at elevated temperatures.