Mechanics of Materials - University of California, San Diego

1 Dynamic mechanical and ultrasonic properties of polyureaJing Qiaoa,b, , Alireza V. Amirkhizib, Kristin Schaafb, Sia Nemat-Nasserb, Gaohui WuaaSchool of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, ChinabMechanical and Aerospace Engineering, Center of Excellence for Advanced Materials , University of California, San Diego , CA 92093, USAarticle infoArticle history:Received 12 April 2011 Received in revised form 9 June 2011 Available online 30 June 2011 Keywords:PolyureaMechanical propertiesTime temperature superpositionabstractDynamic mechanical analysis (DMA) and ultrasonic measurements were carried out tostudy the temperature and frequency dependences of viscoelastic properties of curves of Young s storage and loss moduli were developed from the DMA spectra were subsequently calculated by means of two approximate models,and the apparent activation energy of molecular rearrangements was also determinedbased on the temperature dependence of the time temperature shift factor.

2 Velocity andattenuation of longitudinal and shear ultrasonic waves in polyurea were measured in 2 MHz frequency range between 60 and 30 C temperatures. The complex longitudi-nal and shear moduli were computed from these measurements. Combining these resultsprovided an estimate of the complex bulk and Young s moduli at high frequencies. Theresults of the DMA and temperature and frequency shifted ultrasonic measurements arecompared and similarities and deviations are discussed. 2011 Elsevier Ltd. All rights IntroductionPolyureas are a class of segmented block copolymerderived from the chemical reaction between an isocyanateand an amine. Similar to the segmented polyurethanes,polyurea generally microphase separates into high-Tg hard domains and relatively low-Tg soft domains(Fragiadakis et al., 2010; Das et al., 2007; Yi et al., 2006;Pathak et al.)

3 , 2008). The hard segment domains, whichform thread-like, crystalline structures, are typically dis-persed in the continuous soft segment matrix (Das et al.,2007). The hard segments are extensively hydrogen-bonded and function as both reversible physical cross-linksand reinforcing fillers, thus providing good mechanicalproperties (Pathak et al., 2008). By tailoring the underlyinghard and soft domain structure through chemistry, poly-urea offers a wide range of mechanical properties, fromsoft rubber to hard plastic. Together with its rapid poly-merization and fire, abrasion, and corrosion resistance,polyurea has a myriad of applications in the coating indus-try, on transportation vehicles, pipelines, steel build-ings and marine constructions (Shime and Mohr, 2009).More recently, it has been used either as a protective coat-ing on metallic structures and buildings or an insertedlayer in a blast-tolerant sandwich structure to impart im-proved blast resistance (Tekalur et al.

4 , 2008; Mock and Bal-izer, 2005; Amini et al., 2006; Bahei-El-Din and Dvorak,2006; Amini et al., 2010a,b).As an elastomer, polyurea exhibits viscoelastic behav-ior, which depends strongly on the strain rate (or fre-quency) and temperature, as well as pressure. A numberof studies have been carried out to determine and interpretthe viscoelastic behavior of polyurea, in order to under-stand its et al. (2007)researched thedynamic mechanical properties of polyurea at low et al. (2006)andSarva et al. (2007)studiedthe stress strain behavior of polyurea in uniaxial compres-sion over a range of strain rates from 10 3s 1to 104s 1inquasistatic tests and split Hopkinson pressure bar (SHPB)tests, and found that the flow stress magnitude of polyureaincreases as the strain rate et al. (2007)reported the stress strain measurements in uniaxial0167-6636/$ - see front matter 2011 Elsevier Ltd.

5 All rights Corresponding author at: School of Materials Science and Engineer-ing, Harbin Institute of Technology, Harbin 150001, Qiao). Mechanics of Materials 43 (2011) 598 607 Contents lists available atScienceDirectMechanics of Materialsjournal at intermediate strain rates ( 573 s 1) using adrop weight test instrument, and subsequently they ex-tended the strain rate to 830 s 1(Pathak et al., 2008). Morerecently,Shime and Mohr (2009)further evaluated the ratesensitive response of polyurea in a strain rate range of 101to 103s 1, using a modified SHPB of its application as an impact-resistant coat-ing, the response of polyurea to very high strain rates (highfrequencies) is of some importance. However, the higheststrain rate provided by the SHPB system is limited to104s 1, according to the published works stated previ-ously. In order to extend the time/frequency scale,Zhaoet al.

6 (2007)determined the compressive relaxation behav-ior of polyurea using quasistatic tests in a servo-hydraulicsystem at temperatures between 49 and 22 C and thenconstructed a relaxation master curve at 0 C over areduced time range of 10 10to 107s, based on the time temperature superposition (TTS) principle. Afterwards,they demonstrated its validity with the aid of a simulationmodel and SHPB measurement. For similar work based ontensile relaxation data, seeAmirkhizi et al. (2006). On thecontrary,Fragiadakis et al. (2010), Pathak et al. (2008)opposed the applicability of the TTS principle on polyureaand they conducted dielectric spectroscopy measurements(Roland and Casalini, 2007; Bogoslovov et al., 2007), whichcould provide characterization over a wide frequencyrange (10 2 106Hz) and elevated pressure (1 GPa), toelucidate the segmental dynamics of , experimental data are still scarce at frequen-cies higher than 106Hz.

7 Characterizing rubbery polymersat high frequency is difficult, even at small dynamic mechanical analyzers are limited to fre-quencies below 100 Hz (Roland et al., 2007). When the fre-quency is higher than 1 MHz, we are not aware of anypublished literature on measuring the modulus by apply-ing known values of the stress and measuring the strain(Sinha and Buckley, 2006). Under such conditions, an ultra-sonic technique appears to be a valuable method to mea-sure response in high frequency regions. In addition, theprevious researchers have not reported the relaxationspectrum of polyurea. The relaxation spectrum is aninherent material property and dependent on molecularrelaxation times. Therefore it should be theoretically inde-pendent of the experimental technique (Alvarez et al.,2007). Bulk behavior of polyurea, which is fundamentallydifferent from the shear properties and considerably diffi-cult to measure directly at high frequencies, is also the present investigation, we report the results of aseries of tests performed to study the temperature andfrequency dependence of the viscoelastic behavior of poly-urea, including results obtained using dynamic mechanicalanalysis (DMA) and ultrasonic wave measurements.

8 Mas-ter curves are developed from the DMA data and relaxationspectra are then obtained. Ultrasonic wave measurement iscarried out over the frequency range from to 2 MHz attemperatures from 60 to 30 C. Both longitudinal andshear waves are studied. From the calculated complex lon-gitudinal and shear moduli, complex bulk and Young smoduli at high frequencies are estimated. Finally, theresults of the DMA and temperature and frequency shiftedultrasonic measurements are compared. The similarities ofthe two sets of measurements are analyzed, and the appli-cability of the theory is Experimental MaterialThe polyurea was prepared by the reaction of a polycar-bodiimide-modified diphenylmethane diisocyanate (Ison-ate 2143L, Dow Chemical) and poly(tetramethyleneoxide-di-p-aminobenzo ate) (Versalink P-1000, Air Products). Inorder to ensure that the reaction is completed and has pro-duced some cross-linking a stoichiometric ratio of :1isocyanate to amine was used herein.

9 First, the two compo-nents were degassed separately under 1 torr vacuum untilmost of the entrapped air bubbles were removed. Then,they were mixed for a few minutes while still under vac-uum. Finally, the mixture was cast into a Teflon mold toobtain the test samples. Prior to measurements, the sam-ples were cured at room temperature (25 C) in an environ-mental chamber maintained at 10% relative humidity fortwo Dynamic mechanical analysis (DMA)Dynamic mechanical analysis was conducted using a TAInstruments Dynamic Mechanical Analyzer 2980, using thecorresponding software to collect and analyze the experi-mental data. The samples measured approximately 3 mmthick by 10 mm wide and were clamped at a free lengthof mm. Both ends were cantilevered, , they wereconstrained from rotation and sliding at both ends byclamping plates and excited into a sinusoidal transversedisplacement at one end with a strain amplitude of15lm.

10 The experiments were performed over the temper-ature range from 80 to 70 C, stepping upwards in incre-ments of 3 C. At each temperature step, five frequencies of1, 2, 5, 10 and 20 Hz were tested sequentially. Thermalsoaking times of 3 min at the beginning of each step min-imized the effects of thermal gradients. Liquid nitrogenwas used to cool the system to sub-ambient Ultrasonic measurementThe ultrasonic system used in the present work wasbuilt upon a personal computer (PC) system. As illustratedinFig. 1(a), it consists of a Matec TB-1000 tone-burst sig-nal generating and receiving card, Panametrics contacttransducers, a 100:1 attenuator, and a Tektronix DPO3014 digital phosphor oscilloscope. The dashed rectangleinFig. 1(a) represents the temperature control signals of given frequencies are sent from thecard to the transmitting transducer, propagate throughthe sample to the receiving transducer, and eventually sentdirectly to the oscilloscope.