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Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2016)

Mechanical properties of elastomeric polymers are highly sensitive to the loading rate, temperature and pressure in their transition zone between glassy state and rubbery state. This study focuses on polyurea, which is a group of elastomeric copolymer which has excellent thermal and mechanical properties. It is environmentally stable, tough, abrasion-resistant and dissipative. Both its storage and loss moduli increase significantly when the strain rate increases in the transition zone. This property gives great potential for polyurea to manage mechanical energy in various loading conditions. When applied as a coating material, it can significantly increase the structure's resistance to the shock or blast loading. Its dissipative nature can also be applied to design composite materials that have excellent properties. Its transition zone spans in a wide frequency range, which can be more than 10 decades in the logarithmic scale.

Even though various types of polyurea are widely applied, the mechanical properties of polyurea are not fully understood in such a wide frequency range. This research studies polyurea from various perspectives and provides knowledge of polyurea and its composites in terms of fabrication, characterization in various frequency ranges, modeling and the interface study of polyurea composites, as listed in the five parts:

1. Understanding polyurea, the effect of molecular structure and the stoichiometric ratio on its dynamic mechanical properties.

2. Novel polyurea-based composites.

3. A computational code to efficiently and effectively obtain constitutive models for polyurea and its composites in a wide frequency range.

4. Modification, characterization and modeling of the interfaces of polyurea composites.

5. A novel testing technique for directly characterizing the viscoelastic properties of polyurea and its composites at mid-level (kHz) frequencies, which fills the frequency gap between dynamic mechanical testing and ultrasonic wave measurement.

    Cover page: Polyurea and Polyurea Composites for Stress-wave Energy Management
    • Article
    • Peer Reviewed
    UC San Diego Previously Published Works (2016)
    Polyurea and polyurea-based composite materials are widely used due to their excellent mechanical properties. In order to facilitate large-scale computational studies for this group of materials, a robust and standard method is needed to extract their viscoelastic constitutive parameters. In this study, frequency-domain master curves which cover a wide range of frequencies are developed using the data of dynamic mechanical analysis through time-temperature superposition (TTS). The quality of the master curves is assessed both by Kramers-Kronig relations and by comparing with the ultrasonic wave testing data. Then the time-domain relaxation modulus is obtained by the high-resolution Prony series approximated from the relaxation spectrum. To reduce computational cost, 4 to 8-term Prony series are then fitted from the time-domain relaxation modulus for a limited frequency range of interest. Both the high and low-resolution Prony series are converted back to frequency domain to compare with the master curves developed by TTS and show good agreements. This method is not limited to polyurea and polyurea-based composites and it can be applied to other similar polymer systems as well.
      Cover page: Determining the Shear Relaxation Modulus and Constitutive Models for Polyurea and Polyurea-Based Composite Materials from Dynamic Mechanical Testing Data
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2016)
      Polyurea is a block copolymer that has been widely used in the coating industry as an abrasion-resistant and energy-dissipative material. Its mechanical properties can be tuned by choosing different variations of diamines and diisocyanates as well as by adding various nano- and micro-inclusions to create polyurea-based composites. Our aim here is to provide the necessary experimentally-based viscoelastic constitutive relations for polyurea and its composites in a format convenient to support computational studies. The polyurea used in this research is synthesized by the reaction of Versalink P-1000 (Air Products) and Isonate 143L (Dow Chemicals). Samples of pure polyurea and polyurea composites are fabricated and then characterized using dynamic mechanical analysis (DMA). Based on the DMA data, master curves of storage and loss moduli are developed using time–temperature superposition. The quality of the master curves is carefully assessed by comparing with the ultrasonic wave measurements and by Kramers–Kronig relations. Based on the master curves, continuous relaxation spectra are calculated, then the time-domain relaxation moduli are approximated from the relaxation spectra. Prony series of desired number of terms for the frequency ranges of interest are extracted from the relaxation modulus. This method for developing cost efficient Prony series has been proven to be effective and efficient for numerous DMA test results of many polyurea/polyurea-based material systems, including pure polyurea with various stoichiometric ratios, polyurea with milled glass inclusions, polyurea with hybrid nano-particles and polyurea with phenolic microbubbles. The resulting viscoelastic models are customized for the frequency ranges of interest, reference temperature and desired number of Prony terms, achieving both computational accuracy and low cost. The method is not limited to polyurea-based systems. It can be applied to other similar polymers systems.
        Cover page: Experimentally-based relaxation modulus of polyurea and its composites
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