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Research Papers

Effects of Ultrasound and Strain Rate on Tensile Mechanical Behavior of Thermoplastic Poly Urethane Thin Films

[+] Author and Article Information
Anandh Balakrishnan

 School of Aerospace and Mechanical Engineering, University of Oklahoma, Norman, OK 73019

Mrinal C. Saha1

 School of Aerospace and Mechanical Engineering, University of Oklahoma, Norman, OK 73019msaha@ou.edu

1

Corresponding author.

J. Eng. Mater. Technol 133(4), 041008 (Oct 14, 2011) (7 pages) doi:10.1115/1.4004695 History: Received March 16, 2011; Revised July 14, 2011; Accepted July 22, 2011; Published October 14, 2011; Online October 14, 2011

Thermoplastic poly urethane (TPU) is a diblock copolymer which naturally phase segregates into thermodynamically incompatible hard (H) and soft (S) segments. The size of the segments and their spatial distribution can significantly affect the microstructure and mechanical properties of the material. In this paper, we investigated the effect of duration of exposure to ultrasound on the solution prior to film formation on the final film properties. The response variable for the study was primarily mechanical properties of the TPU thin films fabricated via a solution route utilizing tetra hydro furan as a solvent. The times of sonication were varied between 30 min and 90 min, while all films were fabricated at average thickness of 20 ± 5 μm. The mechanical tests have been conducted at two different displacement rates of 5 and 10 mm/min. Our results indicated that (relative to untreated TPU) ultrasound tends not to deteriorate the fracture strength, strain and yet improve the fracture toughness. We attribute these results to subtle events at the H and S segment/domain levels. To further understand these microstructural variations, we conducted differential scanning calorimetry scan tests between 25 °C and 200 °C at 5 °C/min on untested and tested TPU samples of all types. This data showed a delicate sonication time dependent trend and has been interpreted in conjunction with our mechanical test data. Transition temperatures, enthalpies, and specific heat capacities have been computed for these cases.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Photographs showing TPU film fabrication steps (a) solution preparation on magnetic hot plate, (b) glass mold and individual glass substrate that forms the bottom of the mold, (c) drying of films in a covered petridish, and (d) ultrasound bath sonication setup

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Figure 2

Schematic showing different steps in thin films sample placement procedure on mica rig

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Figure 3

Sample testing in a material testing machine (photograph on top) and cutting the mica edges during testing (schematic in bottom)

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Figure 4

Solvent evaporation datasets (setup is shown in the inset) in normal scale (a) and in logarithmic scale (b)

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Figure 5

Typical tensile responses for one of the unsonicated TPU sample tested at 5 mm/min

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Figure 6

Instantaneous modulus as a function of strain showing strain hardening of unsonicated neat TPU sample tested at 5 mm/min

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Figure 7

Effect of sonication on fracture strains for the TPU films tested at different strain rates

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Figure 8

Effect of sonication on fracture strengths for the TPU films tested at different strain rates

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Figure 9

Effect of sonication on fracture toughness for the TPU films tested at different strain rates

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Figure 10

Schematic showing different microstructures of the TPU due to processing or applied loading

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Figure 11

DSC datasets for mechanically tested and untested samples

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Figure 12

Transition behavior of the TPU samples due to sonication

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