Shockwave Energy Dissipation by Mechanoresponsive Materials

SHOCKWAVE ENERGY DISSIPATION BY MECHANORESPONSIVE MATERIALS

JAEJUN LEE

DISSERTATION Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Materials Science and Engineering in the Graduate College of the University of Illinois at Urbana-Champaign, 2018

Urbana, Illinois

Doctoral Committee: Professor Nancy R. Sottos, Chair and Director of Research Professor Kenneth S. Schweizer Assistant Professor Cecilia Leal Assistant Professor Christopher M. Evans ABSTRACT

Shockwaves are characterized by high pressure and strain rate with extremely short duration about nanoseconds. Blast-induced traumatic brain injury sustained by personnel exposed to shockwave-generating high-energy explosives results in costly and debilitating health effects.

Novel approaches for shockwave energy dissipation (SWED) are demanded since conventional impact absorbing materials do not provide sufficient shockwave attenuation. This research seeks to develop new approaches for efficient SWED in materials designed based on the availability of shock responsive chemical and physical reactions.

A laser-induced shockwave test is developed to evaluate the energy dissipation performance of testing films. For a glass substrate, the input pressure of the shockwave with a Gaussian shaped pressure profile is tuned from 1 to 2 GPa by adjusting launch laser fluence. A fused silica substrate develops an initial stress wave to a triangle shaped pressure profile due to the nonlinear wave propagation properties, which allows the exploration of the duration time effects on energy dissipation. Benchmark energy dissipation values are obtained from polyurea films to provide a standard of comparison.

The availability of energy dissipation by shock-induced microstructural changes and chemical reactions is investigated. In a series of network-forming ionic liquids (NILs) synthesized, each

NIL possesses a different nano-scale structure consisting of charged clusters and alkyl domains.

Irreversible shock-induced structural ordering changes with the creation of new nano-segregated domains in the NIL with the longest alkyl chain spacer enhances the shockwave absorption performance. The more ordered domains result in an 11.2 K increase of Tg due to the extra spatial restriction. In another set of experiments, the energy dissipation properties of the dynamic PDMS networks (PDMS-B-DR) containing boronic ester bonds, which allows self-healing via reversible

ii bond exchange reactions are studied. By increasing the density of boronic ester bonds in PDMS-

B-DR, lower peak pressures are measured, and its energy absorption capability outperforms

covalent PDMS and polyurea. PDMS7-B-DR with the highest density of boronic ester bonds reduces the input peak pressure to 19.9 % at the highest launch laser fluence. The dissociation of boronic ester bonds in PDMS-B-DR is assumed as the main mechanism for energy dissipation.

Finally, the potential of using a laser-induced shockwave to facilitate the phase transformation of

supercooled liquids is explored. The accelerated nucleation of 1,2-bis(phenylethynyl)benzene

(PEB) from its supercooled state is induced by shockwave impact with 1.2 GPa peak pressure and

15 ns duration.

The nano-scale structural effects on shockwave energy dissipation is explored using a series

of polymerized ionic liquids (PILs) with varying alkyl spacer length between imidazolium cations.

X-ray scattering analysis reveals that each of the amorphous PILs exhibit distinct nanoscale

structural heterogeneity, depending on the length of the chain spacer. Although the morphology is

different, the PILs are designed and synthesized to have similar glass transition temperature.

Increased structural heterogeneity in the PILs, corresponding to more ordered charged clusters,

leads to greater energy dissipation. In addition, we observe the amorphous phase is more effective

at attenuating energy than the crystalline phase due to close packed morphology and slow kinetics.

The shock-induced chemical reactions and nano-scale structural changes observed from the

mechanoresponsive materials in this dissertation provide new insights for the development of

efficient shockwave absorption materials that prevent traumatic brain injuries. The nano-scale

heterogeneous structure consisting of two different phases is critical for designing soft materials

for effective dissipation of shockwave loading.

iii

ACKNOWLEDGMENTS

First, I want to express my sincere appreciation to my research advisor, Professor Nancy Sottos,

who has guided me to finish my dissertation study throughout the last six years. Her passion on

the fields of science and engineering has influenced me to pursue a career in academic research

and driven me to become a real scientist. I would also like to thank my thesis committee, specifically Professors Kenneth Schweizer, Cecilia Leal, and Christopher Evans for their advices and discussions, which have allowed me to succeed in graduate studies.

I would like to thank all of AMS and Sottos/White research group colleagues for the helps and discussions. They taught me how sharing ideas is valuable for the successful research. My collaborators, Ke Yang, Yi Ren, and Vivian Lau, in the field of chemistry provided me materials and presented knowledge to drive my research forward.

I could not have finished the dissertation study without love from my parents. They always

encourage me to embrace challenges and support my life decisions. I also want to thank all my

friends in Champaign/Urbana who made my life healthy and fruitful during the six years.

I would like to acknowledge the Office of Naval Research for the funding of my graduate

studies.

TABLE OF CONTENTS

CHAPTER 1 INTRODUCTION ...... 1 1.1 Blast-induced Traumatic Brain Injuries ...... 1 1.2 Shockwave Dissipation Mechanisms ...... 3 1.2.1 Impedance mismatch technique ...... 3 1.2.2 Shockwave-induced chemical reaction ...... 6 1.2.3 Shockwave-induced physical reaction ...... 6 1.3 Polyurea for Shockwave Mitigation ...... 8 1.4 Measurement Techniques for Shockwave Energy Dissipation ...... 12 1.5 Overview of Thesis Research ...... 14 1.6 References ...... 16 CHAPTER 2 LASER-INUDCED SHOCKWAVE TEST ...... 21 2.1 Introduction ...... 21 2.2 Experimental Methods ...... 22 2.2.1 Laser-induced shockwave test setup and protocols ...... 22 2.2.2 Input shockwave pressure measurement ...... 25 2.2.3 Preparation of polyurea specimen ...... 26 2.3 Results and Discussions ...... 26 2.3.1 Tunable input shockwave pressure and duration time ...... 26 2.3.2 Shockwave energy dissipation by polyurea ...... 29 2.4 Conclusions ...... 30 2.5 References ...... 31 CHAPTER 3 SHOCK-INDUCED ORDERING OF NETWORK-FORMING IONIC LIQUIDS ..33 3.1 Introduction ...... 33 3.2 Experimental Methods ...... 34 3.2.1 Materials and synthesis ...... 34 3.2.2 Sample preparation for laser-induced shockwave test ...... 35 3.2.3 Protocol for repeated shockwave testing ...... 36 3.2.4 Characterization of NILs...... 36 3.3 Results and Discussions ...... 37 3.3.1 Shockwave energy dissipation by NILs ...... 37

3.3.2 Shock-induced ordering in NILs ...... 39 3.4 Conclusions ...... 45 3.5 References ...... 46 CHAPTER 4 SHOCKWAVE ENERGY DISSIPATION VIA DYNAMIC BORON ESTER BONDS ...... 49 4.1 Introduction ...... 49 4.2 Experimental Methods ...... 49 4.2.1 Materials and synthesis ...... 49 4.2.2 Sample preparation for laser-induced shockwave test ...... 50 4.2.3 Rheology ...... 52 4.2.4 Confocal Raman characterization ...... 52 4.3 Results and Discussion ...... 53 4.3.1 Reversible bond exchange reactions in the dynamic PDMS networks ...... 53 4.3.2 Molecular weight effects of control PDMS on energy dissipation ...... 54 4.3.3 Shockwave energy dissipation by dynamic covalent bonds ...... 55 4.4 Conclusions ...... 58 4.5 References ...... 59 CHAPTER 5 SHOCK-INDUCED NUCLEATION OF SUPERCOOLED LIQUIDS ...... 61 5.1 Introduction ...... 61 5.2 Experimental Methods ...... 62 5.2.1 Materials ...... 62 5.2.2 Shockwave-induced nucleation setup and protocols ...... 63 5.2.3 Theoretical calculation of rotational energy barrier ...... 65 5.3 Results and Discussion ...... 65 5.3.1 Shockwave-induced nucleation of 1,2-Bis(Phenylethylnyl)benzene (PEB) ...... 65 5.3.2 Microstructural characterization of PEB before and after shock impact ...... 69 5.3.3 Rotational energy barriers for shock-induced nucleation ...... 71 5.4 Conclusions ...... 74 5.5 References ...... 74 CHAPTER 6 EFFECT OF POLYMERIZED IONIC LIQUID STRUCTURE AND MORPHOLOGY ON SHOCKWAVE ENERGY DISSIPATION ...... 78 6.1 Introduction ...... 78 6.2 Experimental Methods ...... 79

6.2.1 Materials ...... 79 6.2.2 Sample preparation for laser-induced shockwave test ...... 80 6.2.3 Material characterization of PILs ...... 81 6.3 Results and Discussion ...... 81 6.3.1 Phase behavior of PILs depending on the length of alkyl spacer ...... 81 6.3.2 Nano-scale heterogeneity of amorphous PILs ...... 83 6.3.3 Structural effects of PILs on energy dissipation ...... 85 6.4 Conclusions ...... 87 6.5 References ...... 87 CHAPTER 7 SUMMARY AND FUTURE WORK ...... 91 7.1 Summary of Thesis Research ...... 91 7.2 Future Work ...... 92 7.2.1 Effects of relaxation time on energy dissipation using supercooled liquids ...... 92 7.2.2 Soft composites ...... 97 7.2.3 Variation of relaxation time ...... 98 7.2.4 In-situ spectroscopy studies and high-speed imaging under laser-induced shockwave loading ...... 99 7.3 References ...... 101

vii

CHAPTER 1

INTRODUCTION

1.1 Blast-induced Traumatic Brain Injuries

Shockwaves are a non-linear, non-continuous, high amplitude acoustic pressure waves of

extremely short duration. Depending on the source of shockwave, peak pressures reach 100 GPa

with a rise time from nanoseconds to milliseconds in an almost discontinuous jump.[1] Figure 1.1

shows a typical shockwave pressure profile with compressive pressure followed by temporary

tensile pressure.[2] The positive and negative peak pressure, energy transferred, the rise time, and

the pulse width are important parameters for analysis.[3] In a condensed medium, shockwaves

create greater pressure, travel speed, mass particle speed, and internal energy changes in

comparison to acoustic sound waves, which produce low pressure, constant acoustic sound speed,

and a negligible change of internal energy.

Figure 1.1. The schematic of typical shockwave pressure profile. [2]

Shockwave-induced traumatic brain injury (TBI) has become a serious issue for soldiers in

recent years.[4] As shown in Figure 1.2, there are three major causes for TBI triggered by explosive or blast events.[5] The primary cause is the shockwave from the explosive, which penetrates brain

tissues with a faster speed than sound and a rapid increase in pressure. The secondary and tertiary 1

causes are projectiles and head acceleration/deceleration forces, respectively. The primary cause

of TBI attracts more attention in the literature as it causes severe and complex damage.[6] The

shockwave-body/head interaction and relative motion may delaminate the interfaces between

tissues with different acoustic impedances, resulting in distinct pathobiological consequences in

the whole body.

Figure 1.2. Brain injury mechanisms caused by explosions: Primary blast effects directly associated with shockwaves, secondary blast effects caused by projectiles, and tertiary blast effects affected by acceleration/deceleration forces.[7]

During shockwave propagation, the brain is especially susceptible to shockwave overpressure.

Previous studies have revealed that when brain tissues are exposed to high-intensity shockwaves greater than 10 MPa, severe hemorrhage is possible. Exposure to low-intensity shockwaves less than 1MPa also cause minor morphological changes in neurons, leading to mild-to-moderate traumatic brain injury (mTBI).[2] The human resource loss from mTBI have significant direct economic impact and indirect costs due to loss of earning ability and the burden of care.[8]

Therefore, there are urgent needs to develop materials that effectively absorb low-intensity

shockwaves.

1.2 Shockwave Dissipation Mechanisms

1.2.1 Impedance mismatch technique

The most common approach for shockwave mitigation is to adopt multi-layer structures

causing the shockwave to scatter at the interfaces. As shown in Figure 1.3, multi-layer armor

improves penetration resistance and shock attenuation by impedance mismatch compared to

single-layer armor. According to Zhuang et al., the scattering effect at the interface between

periodically aligned metal and polycarbonate layers reduces the shockwave velocity based on both

experimental and numerical studies.[9] In a multi-layered composite with different thicknesses,

Petel et al. reported that thinner layers are more effective for the dispersion of shockwaves and demonstrated the equivalent time scale for stress equilibrium across the interfaces plays an important role for attenuation of shockwave.[10] Simulations reveal that a gradient change of

impedance also dissipates shockwave energy, which can prevent failures of interfacial

delamination.[11]

Figure 1.3. Schematic depiction of stress propagation through a multi-layered structure via transmissions and reflections at the interfaces.[11]

Elder et al. introduced the impedance mismatch technique and used molecular dynamic simulations to investigate shockwave attenuation in semi-crystalline polyethylene made up of nanoscale segregated domains.[12] They found that energy reflection at the crystalline/amorphous interfaces is enhanced due to an increase in an acoustic impedance mismatch between two phases.

When the size of the amorphous domain decreases, the shockwave attenuation also decreases as a result of two mechanisms: pre-compression of amorphous regions under faster moving elastic wave in two-wave elastic-plastic shockwave structure and/or a decrease in impedance mismatch by nano-confinement effect of the amorphous domain surrounded by stiff crystalline domains

(Figure 1.4).

Figure 1.4. Shockwave reflection at the interfaces in semi-crystalline polyethylene compared to pure crystalline polyethylene based on molecular dynamic simulations. In semi-crystalline polyethylene, amorphous domain with different thicknesses is sandwiched by crystalline domains as a shape of lamellar. (a) Normalized total energy as a function of time at the left, middle, and right sections of pure crystalline and semi-crystalline models upon a piston impact with velocity Up = 0.5 km/s. The open arrow indicates when the velocity of piston is 0. The filled arrows indicate when shockwave reaches the first and second interface of crystalline/amorphous. (b) Fraction of energy reflected in the semi-crystalline polyethylene model as a function of width of amorphous region at three different piston velocities, which indicates polyethylene with a bigger amorphous domain reflects more energy.[12]

Depending on the selection and sequence of the layered materials, shockwave pressure can be

amplified. According to Grujicic et al., when the shockwave impacts a target surface with a higher

impedance, a reflection wave is generated and the shockwave pressure is amplified compared to

the input pressure due to conservation of linear momentum.[13] For shockwaves propagating from

a layer consisting of weakly-bound molecules, such as gas or Aerogel®, the intensity can be

amplified multiple times.[14] In porous composites, Nesterenko reported that pores with an

insufficient size enhance intensity of shockwave pressure.[15] Youssef et al. reported that resonance

of shockwaves in polyurea layers sandwiched between acrylic and polycarbonate layers can

amplify amplitude up to 16 times greater than input value.[16] Hence, the impedance mismatch technique for the energy dissipation must be designed accurately as considering a sequence of layers, an impedance/density of each layer, and porosity.

1.2.2 Shockwave-induced chemical reaction

Chemical reactions with large negative volumes of reactions and activation have been proposed as a new paradigm for shockwave energy dissipation.[17-18] Upon exposure to a

shockwave, these materials must react at a high strain rate with an irreversible reduction of volume

accompanied by creating metastable higher-energy chemical bonds. Using a particle-based

mesoscale model incorporating chemical reactions under shockwave loading conditions. Strachan

et al. discovered that the amount of volume collapse and the velocity of the chemical wave created

behind the leading shockwave are both critical parameters for shockwave dissipation.[17] Hambir

et al.[19] introduced a viscoplastic model in which viscoplastic collapse generates rarefraction

chemical waves, which attenuates the shock front (Figure 1.5). Although the concept of shockwave

energy dissipation by chemical reactions with volume collapse has been introduced, experimental

results demonstrating shockwave attenuation with such chemical reactions have not been reported

yet.

Figure 1.5. Shockwave energy dissipation by volume collapse. (a) A planar shockwave moving at Us is created by pushing a medium at pressure P, volume V, and temperature T with speed of sound c. (b) The volume collapses behind the front, launching rarefraction waves. (c) Rarefraction waves catch up and attenuate the shockwave and increase its rise time.

1.2.3 Shockwave-induced physical reaction

Shockwave-induced physical reactions in materials are another novel approach for shockwave

energy dissipation.[20-22] Physical changes under shockwave loading include phase transformations,

nucleation and melting, ordering changes, conformational changes, and microstructural changes

in materials. Shockwave-induced physical reactions have been investigated more than 70 years but

only more recent work has focused on energy dissipation.[23]

Phase transformations of metals at extremely high shockwave pressure > 10 GPa have been of interest for researchers who aim to synthesize new material using shockwaves.[24] Shockey et al.

proposed that the phase change due to hypervelocity impact has a considerable effect on the stress

mitigation and fracture damage.[25] In phase-transforming materials, the shockwave-induced phase change leaves a fingerprint, splitting the shockwave into a wave structure with three parts: elastic, plastic, and a phase-transformed wave.[26] Boettger et al. proposed that the shockwave amplitude

is attenuated by the phase-transformed wave through an α to ε phase transition in iron, since

shockwave energy is trapped in the higher energy ε phase. Figure 1.6 indicates the reduced free

surface velocity by shockwave dissipation associated with α to ε phase transition in iron. The

dynamic of the phase transition may play an important role in shock-induced phase transition.

Simulations of iron showed that the α to ε phase transition proceeds within tens of nanoseconds,

and varies depending on shockwave strength.[27] When the shockwave loading occurs in a shorter

time than the phase transition, phase transformation may neither occur nor dissipate shockwave

energy.

Figure 1.6. Free surface velocity vs time showing shockwave dissipation by phase change. The results of circles, solid, and dashed line involving reverse phase transition from ε to α reach the impact velocity, 1.292 km/s. Slower free surface velocity of dotted line is recorded due to shockwave energy dissipation by α to ε forward phase transition. [27]

1.3 Polyurea for Shockwave Mitigation

Polyurea films exhibit remarkable shockwave absorption properties. Synthesized from a multifunctional amine and a difunctional isocyanate, the microstructure of polyurea consists of hard segments containing a high glass transition temperature hydrogen-bonded phase, and soft segments comprising a low glass transition temperature phase (Figure 1.7).[28] Nanoscale segregated microstructure of polyurea is promoted by hydrogen bond formation between urea linkages in dissimilar hard segments, which result in physical crosslinking sites. As the ratio of soft segments increases, polyurea becomes more flexible and malleable. Castagna et al. studied the effects of soft segment molecular weight on the microstructure of polyurea, demonstrating that shorter soft segments with reduced flexiblity in polyurea favor mixing of hard/soft segments to form a more homogeneous phase (Figure 1.8).[29] When the molecular weight of soft segments

8 decreases, the dynamics of segmental relaxation process are slowed down, confirmed by broadband dielectric relaxation measurements.

Figure 1.7. Molecular structure of polyurea consisting of hard and soft segments.[28]

Figure 1.8. Nano-segregated microstructure of polyurea. An example of a typical tapping-mode AFM image of a polyurea (a) P1000 and (b) P650. A rod-like morphology is the hard segments. (c) Small angle X-ray scattering (SAXS) data of polyurea with different molecular weights in a soft segment. Each number X in PX, indicates the molecular weight (g/mol) of soft segment.[29]

The mechanism by which polyurea absorbs shockwaves is still under investigation.[30-32] Both experimental data and computational models (mesoscale, all-atom, and coarse-grained molecular level) have offered insights into polyurea’s shockwave attenuation capability.[33-34] Grujicic et al. stated that the shock-induced breakage of hydrogen-bond in hard domains plays an important role

in the shock-impact mitigation capacity of polyurea.[33, 35] They also proposed shock-induced

ordering/densification within the hard domains and viscoelastic relaxation within the hard/soft

interfacial regions as another possible mechanism for reducing shock impact.[21, 36] Arman et al.

suggested that hard domain motion coupling due to the networked structure between hard and soft

domains may enhance shockwave energy dissipation in polyurea through resonant motions based

on computational studies using the hard and soft bead model.[28] Figure 1.9 shows the smaller hard beads in multiblock copolymer, (H2S8)4, are more easily deformed compared to larger ones in

diblock copolymer. Smaller hard domains are dissociated more easily under shock loading due to

weak binding forces between hard segments, which also contributes to enhancing dissipation.

Figure 1.9. The maximum relative displacement between beads of (a) diblock copolymer H2S8 and (b) multiblock copolymer (H2S8)4 under the unshocked (left, label U) and the shocked (right, label S) state. The corresponding colors according to bead types are shown in (b) and (d), respectively. Red and green spheres are the hard (H) and soft (S) beads, respectively. (e) The maximum relative displacement profiles between beads as a function z, distance of shock propagation and (f) the profiles for each bead type in diblock and multiblock copolymers. Shock front is located around 780 along z axis.[28]

The viscoelastic rate-dependence of polyurea properties is another possible mechanism for the excellent shock attenuation performance. Bogoslovov et al. reported energy dissipation by high- speed impact induced glass transition in a polyurea coating.[20] Polybutadiene with the faster segmental dynamics remains in the rubbery state upon impact, while polybutadiene-based polyurea undergoes a rubbery-to-glassy transition (Figure 1.10). According to dielectric relaxation measurements, an impact with a faster mechanical strain rate than the segmental dynamics of polymer can induce a glass transition, accompanied by brittle failure and large energy dissipation.

Qiao et al. constructed master curves of polyurea using dynamic mechanical analysis and ultrasonic measurement up to frequency ~ 1015 Hz at 1oC, pointing out maximum loss modulus at high frequency.[37] They also suggested that the hard domains in polyurea contribute to high loss modulus at high frequency via resonance motions and tailoring the size and distribution of hard domains can enhance shockwave energy performance of polyurea.

Figure 1.10. Images of (a) the polybutadiene and (b) the polyurea specimen upon impact by the projectile. The projectile travels from the left to the right with a speed 900 m/s. Both specimens are composed of rubber-coated side (left) and 5.1 mm thick steel plate. Each grid is 25 mm apart.[20]

Even though an explicit shockwave absorption mechanism is still unknown, these groups along with other researchers have reached the agreement that the micro-phase segregation in polyurea plays an important role in its high shockwave absorption performance.

1.4 Measurement Techniques for Shockwave Energy Dissipation

Dynamic loading techniques to investigate material properties under high pressure at high strain rate have expanded from the initial powder and gas gun systems to laser systems.[38] An early approach was to generate planar shock waves using in-contact explosives creating a pressure pulse that varies in time.[39] However, a narrow range of induced stresses is available for this

explosive design. Flat ended projectiles propelled by gas gun and explosive powder-driven guns were used to increase the range of stress amplitudes available for shockwave experiments, especially to lower stress values. Unfortunately, this technique is not suited to investigate materials of small dimensions.[40] Dlott et al. have developed a novel laser-driven technique to generate microscale thin film “flyer-plates”. The resulting strain-rates and pressures developed are very high and the reproducibility must be improved.[41]

In 1963, Askaryon and Morez reported stress wave generation from impingement of a high- energy, pulsed laser beam on metal surfaces.[42] Since visible and near infrared laser light can

penetrate <1µm into metals due to the high attenuation coefficient, only a very thin absorbing layer

is heated by laser light. Metal vaporizes and achieves extremely high temperature >1000 oC, where

the ionization of electrons is followed by the formation of plasma. Vaporization rather than melting

of the metals occurs due to the limited thermal diffusion associated with an exceptionally short

energy deposition time. The expansion of plasma produces a compressive shockwave. Shockwave

pressure is determined by various factors including energy of laser, spot size, laser wavelength,

the existence of a confining layer, and reflection by a impedance mismatch.[43] Higher and faster

shockwaves can be generated by changing the confinement of the metal surface, the types of metal

absorber, the source of the laser, and by employing the impedance mismatch technique.[44-50] These advancements allow the investigation of material behavior under extremely high pressure (>1 GPa) and strain rate (>107 s-1).[51-52] Novel experimental techniques to study interaction between high- strain rate stress waves and materials have been developed. The strength of planar interfaces between thin films and substrates has been determined using tensile stress generated by laser spallation (Figure 1.11).[53-54] Interfacial strength of thin films under mixed-mode conditions also

has been characterized by using laser-induced shockwave test.

(a) (b)

Figure 1.11. (a) Schematic of laser spallation technique and (b) schematic of laser spallation experiment and interferometric displacement measurement. [54]

Several experimental techniques have been introduced to measure the velocity or pressure

profile of shockwaves, including piezoelectric crystal pins, quartz gauges, streak cameras,

electromagnetic inductions, and interferometric systems.[55-57] Since the rise time of laser-induced

shockwaves is less than a few nanoseconds, sub-nanosecond resolution measurement techniques

are required. For example, Barker and Hollenbach introduced the velocity interferometer system

for any reflector (VISAR),[58] which records velocity information with high accuracy and

nanosecond time resolution. VISAR records velocity directly from a simple count of the fringes.

Wang et al.[54] utilized a simple Michelson interferometer to measure displacements on highly reflective surfaces, shown in Figure 1.8b. A continuous laser beam is split into two beams, one beam being reflected by a stationary mirror and the other beam being reflected by the moving surface of the sample. The split beams are combined at the detector. A similar system is adapted in this work.

1.5 Overview of Thesis Research

The work presented in this dissertation aims to investigate new strategies for efficient shockwave energy dissipation using mechanoresponsive materials. As described in Chapter 2, the

14 laser-induced shockwave test has been developed to precisely evaluate the energy dissipation performance of testing materials. The energy dissipation properties of polyurea were also measured as a benchmark to provide a standard of comparison.

Several different approaches for the energy dissipation are examined in this dissertation. First, the ability to efficiently dissipate energy through shock-induced physical/chemical reactions is investigated. In Chapter 3, the ability of network-forming ionic liquids (NILs) to undergo a shock- induced ordering transition is demonstrated and the resulting reduction of peak pressure quantified.

In Chapter 4, dynamic bonds subjected to exchange reactions in polydimethylsiloxane (PDMS) networks are shown to dissipate energy under shockwave loading. In Chapter 5, the potential of using a laser-induced shockwave to induce the nucleation of supercooled liquids is explored.

Finally in Chapter 6, the microstructural effects on shockwave energy dissipation were studied using a series of polymerized ionic liquids which exhibits distinct nano-scale structural heterogeneity depending on the length of chain spacer.

The result of this research provides new guidelines for the development of shockwave dissipating materials. Accommodating shockwave pressure in chemical and physical reactions offers a novel approach attenuating more energy than polyurea. Moreover, the microstructural study using polymerized ionic liquids on the peak pressure attenuation provides a solution to an understanding of shockwave energy dissipation (SWED) mechanisms at the molecular and mesoscale levels.

1.6 References

(1) Cauble, R.; Phillion, D. W.; Hoover, T. J.; Holmes, N. C.; Kilkenny, J. D.; Lee, R. W., Demonstration of 0.75 Gbar planar shocks in x-ray driven colliding foils. Physical Review Letters 1993, 70, 2102-2105. (2) Nakagawa, A.; Manley, G. T.; Gean, A. D.; Ohtani, K.; Armonda, R.; Tsukamoto, A.; Yamamoto, H.; Takayama, K.; Tominaga, T., Mechanisms of Primary Blast-Induced Traumatic Brain Injury: Insights from Shock-Wave Research. Journal of Neurotrauma 2011, 28, 1101- 1119. (3) White, C. S., THE SCOPE OF BLAST AND SHOCK BIOLOGY AND PROBLEM AREAS IN RELATING PHYSICAL AND BIOLOGICAL PARAMETERS*. Annals of the New York Academy of Sciences 1968, 152, 89-102. (4) Bhattacharjee, Y., Shell Shock Revisited: Solving the Puzzle of Blast Trauma. Science 2008, 319, 406-408. (5) Chen, Y. C.; Smith, D. H.; Meaney, D. F., In-Vitro Approaches for Studying Blast-Induced Traumatic Brain Injury. Journal of Neurotrauma 2009, 26, 861-876. (6) Cernak, I., Understanding blast-induced neurotrauma: how far have we come? Concussion 2017, 2, CNC42. (7) Cernak, I.; Noble-Haeusslein, L. J., Traumatic Brain Injury: An Overview of Pathobiology with Emphasis on Military Populations. Journal of Cerebral Blood Flow & Metabolism 2010, 30, 255-266. (8) Courtney, A. C.; Courtney, M. W., A thoracic mechanism of mild traumatic brain injury due to blast pressure waves. Medical Hypotheses 2009, 72, 76-83. (9) Zhuang, S.; Ravichandran, G.; Grady, D. E., An experimental investigation of shock wave propagation in periodically layered composites. Journal of the Mechanics and Physics of Solids 2003, 51, 245-265. (10) Petel, O. E.; Jetté, F. X.; Goroshin, S.; Frost, D. L.; Ouellet, S., Blast wave attenuation through a composite of varying layer distribution. Shock Waves 2011, 21, 215-224. (11) Hui, D.; Dutta, P. K., A new concept of shock mitigation by impedance-graded materials. Composites Part B: Engineering 2011, 42, 2181-2184.

(12) Elder, R. M.; O’Connor, T. C.; Chantawansri, T. L.; Sliozberg, Y. R.; Sirk, T. W.; Yeh, I.- C.; Robbins, M. O.; Andzelm, J. W., Shock-wave propagation and reflection in semicrystalline polyethylene: A molecular-level investigation. Physical Review Materials 2017, 1, 043606. (13) Grujicic, M.; Bell, W. C.; Pandurangan, B.; He, T., Blast-wave impact-mitigation capability of polyurea when used as helmet suspension-pad material. Materials & Design 2010, 31, 4050-4065. (14) Schimizze, B.; Son, S. F.; Goel, R.; Vechart, A. P.; Young, L., An experimental and numerical study of blast induced shock wave mitigation in sandwich structures. Applied Acoustics 2013, 74, 1-9. (15) Nesterenko, V. F., Shock (Blast) Mitigation by “Soft” Condensed Matter. MRS Proceedings 2002, 759, MM4.3. (16) Youssef, G.; Gupta, V., Resonance in Polyurea-Based Multilayer Structures Subjected to Laser-Generated Stress Waves. Experimental Mechanics 2013, 53, 145-154. (17) Antillon, E.; Strachan, A., Mesoscale simulations of shockwave energy dissipation via chemical reactions. The Journal of Chemical Physics 2015, 142, 084108. (18) Grujicic, M.; Snipes, J.; Ramaswami, S., A computational analysis of the utility of chemical reactions within protective structures in mitigating shockwave-impact effects. Multidiscipline Modeling in Materials and Structures 2016, 12, 438-472. (19) Hambir, S. A.; Kim, H.; Dlott, D. D.; Frey, R. B., Real time ultrafast spectroscopy of shock front pore collapse. Journal of Applied Physics 2001, 90, 5139-5146. (20) Bogoslovov, R. B.; Roland, C. M.; Gamache, R. M., Impact-induced glass transition in elastomeric coatings. Applied Physics Letters 2007, 90, 221910. (21) Grujicic, M.; Snipes, J. S.; Ramaswami, S.; Yavari, R.; Runt, J.; Tarter, J.; Dillon, G., Coarse-grained Molecular-level Analysis of Polyurea Properties and Shock-mitigation Potential. Journal of Materials Engineering and Performance 2013, 22, 1964-1981. (22) Chen, G. Q.; Ahrens, T. J.; Yang, W.; Knowles, J. K., Effect of irreversible phase change on shock-wave propagation. Journal of the Mechanics and Physics of Solids 1999, 47, 763-783. (23) Duvall, G. E.; Graham, R. A., Phase transitions under shock-wave loading. Reviews of Modern Physics 1977, 49, 523-579. (24) Johnson, P. C.; Stein, B. A.; Davis, R. S., Temperature Dependence of Shock‐Induced Phase Transformations in Iron. Journal of Applied Physics 1962, 33, 557-561.

(25) Shockey, D. A.; Curran, D. R.; De Carli, P. S., Damage in steel plates from hypervelocity impact. I. Physical changes and effects of projectile material. Journal of Applied Physics 1975, 46, 3766-3775. (26) Nina, G.; Diego, R. T.; Eduardo, M. B.; Herbert, M. U., Interplay of plasticity and phase transformation in shock wave propagation in nanocrystalline iron. New Journal of Physics 2014, 16, 093032. (27) Boettger, J. C.; Wallace, D. C., Metastability and dynamics of the shock-induced phase transition in iron. Physical Review B 1997, 55, 2840-2849. (28) Arman, B.; Reddy, A. S.; Arya, G., Viscoelastic Properties and Shock Response of Coarse-Grained Models of Multiblock versus Diblock Copolymers: Insights into Dissipative Properties of Polyurea. Macromolecules 2012, 45, 3247-3255. (29) Castagna, A. M.; Pangon, A.; Choi, T.; Dillon, G. P.; Runt, J., The Role of Soft Segment Molecular Weight on Microphase Separation and Dynamics of Bulk Polymerized Polyureas. Macromolecules 2012, 45, 8438-8444. (30) Bahei-El-Din, Y. A.; Dvorak, G. J.; Fredricksen, O. J., A blast-tolerant sandwich plate design with a polyurea interlayer. International Journal of Solids and Structures 2006, 43, 7644- 7658. (31) Grujicic, M.; d’Entremont, B. P.; Pandurangan, B.; Runt, J.; Tarter, J.; Dillon, G., Concept-Level Analysis and Design of Polyurea for Enhanced Blast-Mitigation Performance. Journal of Materials Engineering and Performance 2012, 21, 2024-2037. (32) Gardner, N.; Wang, E.; Kumar, P.; Shukla, A., Blast Mitigation in a Sandwich Composite Using Graded Core and Polyurea Interlayer. Experimental Mechanics 2012, 52, 119-133. (33) Grujicic, M.; Pandurangan, B.; Bell, W. C.; Cheeseman, B. A.; Yen, C. F.; Randow, C. L., Molecular-level simulations of shock generation and propagation in polyurea. Materials Science and Engineering: A 2011, 528, 3799-3808. (34) Grujicic, M.; Pandurangan, B., Mesoscale analysis of segmental dynamics in microphase- segregated polyurea. Journal of Materials Science 2012, 47, 3876-3889. (35) Grujicic, M.; Yavari, R.; Snipes, J. S.; Ramaswami, S.; Runt, J.; Tarter, J.; Dillon, G., Molecular-level computational investigation of shock-wave mitigation capability of polyurea. Journal of Materials Science 2012, 47, 8197-8215.

(36) Grujicic, M.; Snipes, J. S.; Ramaswami, S.; Yavari, R.; Ramasubramanian, M. K., Meso- scale Computational Investigation of Shock-Wave Attenuation by Trailing Release Wave in Different Grades of Polyurea. Journal of Materials Engineering and Performance 2014, 23, 49- 64. (37) Qiao, J.; Amirkhizi, A. V.; Schaaf, K.; Nemat-Nasser, S.; Wu, G., Dynamic mechanical and ultrasonic properties of polyurea. Mechanics of Materials 2011, 43, 598-607. (38) McQueen, R. G.; Marsh, S. P.; Taylor, J. W.; Fritz, J. N.; Carter, W. J., CHAPTER VII - THE EQUATION OF STATE OF SOLIDS FROM SHOCK WAVE STUDIES A2 - KINSLOW, RAY. High-Velocity Impact Phenomena, Academic Press: 1970; pp 293-417. (39) Murr, L. E., Shock Waves for Industrial Applications. William Andrew Publishing/Noyes. (40) Trott, W. M.; Setchell, R. E.; Farnsworth, A. V., Development of Laser‐Driven Flyer Techniques for Equation‐of‐State Studies of Microscale Materials. AIP Conference Proceedings 2002, 620, 1347-1350. (41) Banishev, A. A.; Shaw, W. L.; Bassett, W. P.; Dlott, D. D., High-Speed Laser-Launched Flyer Impacts Studied with Ultrafast Photography and Velocimetry. Journal of Dynamic Behavior of Materials 2016, 2, 194-206. (42) Askar'yan, G. A., Moroz, E. M., Pressure on Evaporation of Matter in a Radiation Beam. JETP letters 1963, 06, 1638. (43) Clauer, A. H.; Holbrook, J. H.; Fairand, B. P., Effects of Laser Induced Shock Waves on Metals. Shock Waves and High-Strain-Rate Phenomena in Metals: Concepts and Applications, Springer US: Boston, MA, 1981; pp 675-702. (44) Anderholm, N. C., LASER‐GENERATED STRESS WAVES. Applied Physics Letters 1970, 16, 113-115. (45) Yang, L. C., Stress waves generated in thin metallic films by a Q ‐switched ruby laser. Journal of Applied Physics 1974, 45, 2601-2608. (46) Ready, J. F., Effects Due to Absorption of Laser Radiation. Journal of Applied Physics 1965, 36, 462-468. (47) Fox, J. A., Effect of pulse shaping on laser‐induced spallation. Applied Physics Letters 1974, 24, 340-343. (48) apos; Keefe, J. D.; Skeen, C. H.; York, C. M., Laser‐induced deformation modes in thin metal targets. Journal of Applied Physics 1973, 44, 4622-4626.

(49) White, R. M., Elastic Wave Generation by Electron Bombardment or Electromagnetic Wave Absorption. Journal of Applied Physics 1963, 34, 2123-2124. (50) Fairand, B. P.; Clauer, A. H., Laser generation of high‐amplitude stress waves in materials. Journal of Applied Physics 1979, 50, 1497-1502. (51) Youssef, G.; Crum, R.; Prikhodko, S. V.; Seif, D.; Po, G.; Ghoniem, N.; Kodambaka, S.; Gupta, V., The influence of laser-induced nanosecond rise-time stress waves on the microstructure and surface chemical activity of single crystal Cu nanopillars. Journal of Applied Physics 2013, 113, 084309. (52) Yang, K.; Lee, J.; Sottos, N. R.; Moore, J. S., Shock-Induced Ordering in a Nano- segregated Network-Forming Ionic Liquid. Journal of the American Chemical Society 2015, 137, 16000-16003. (53) Gupta, V.; Argon, A. S.; Parks, D. M.; Cornie, J. A., Measurement of interface strength by a laser spallation technique. Journal of the Mechanics and Physics of Solids 1992, 40, 141-180. (54) Wang, J.; Weaver, R. L.; Sottos, N. R., A parametric study of laser induced thin film spallation. Experimental Mechanics 2002, 42, 74-83. (55) Bell, C. E.; Landt, J. A., LASER‐INDUCED HIGH‐PRESSURE SHOCK WAVES IN WATER. Applied Physics Letters 1967, 10, 46-48. (56) Dlott, D. D., ULTRAFAST SPECTROSCOPY OF SHOCK WAVES IN MOLECULAR MATERIALS. Annual Review of Physical Chemistry 1999, 50, 251-278. (57) Sheffield, S. A.; Gustavsen, R. L.; Alcon, R. R., In-situ magnetic gauging technique used at LANL-method and shock information obtained. AIP Conference Proceedings 2000, 505, 1043-1048. (58) Barker, L. M.; Hollenbach, R. E., Laser interferometer for measuring high velocities of any reflecting surface. Journal of Applied Physics 1972, 43, 4669-4675.

CHAPTER 2

LASER-INUDCED SHOCKWAVE TEST

2.1 Introduction

Several experimental techniques to measure shockwave energy dissipation were discussed in

Chapter 1. Although weak shockwaves < 1 GPa are responsible for most traumatic brain injuries

(TBI)[1-5], most of the shockwave studies in the literature focus on generation of high pressures

(10-100 GPa) associated with enormous energy explosions/collisions. New metrologies capable

of generating weaker shockwaves are needed to characterize material response in the range

associated with TBI. Vossen[6] introduced a laser spallation technique and Gupta et al.[7] further

developed this technique to measure the interfacial strength of thin-film with a generation of

compressive stress wave up to 1 GPa with nanoseconds duration. Wang et al. analyzed the

propagation of stress wave in different substrates using a theoretical analysis and extended this

technique to mixed-mode loadings.[8-11] They used a Michelson interferometer with a continuous

laser to detect the displacement of sample. These techniques originally aim to explore the interfacial debonding phenomenon of thin-film using a tensile stress wave generated when a compressive stress wave reflects at a free surface. More recently, Grady et al.[12] and Sung et al.[13]

utilized the spallation technique to study mechanochemical activity at a free surface and a solid

interface. In this chapter, we describe a laser-induced shockwave test setup adapted from the laser

spallation technique to characterize the shockwave absorption properties of mechanoresponsive

materials with several modifications. First, a weak compressive stress wave from hundreds MPa

to a few GPa is generated to quantify the attenuation of shockwaves at low pressures. Second, the

impedance mismatch between the substrate and testing film is minimized to measure the energy

dissipation by the testing film on its own. Third, shockwave pressures and duration times are tuned

to explore material responses under different circumstances. Lastly, pressure measurements are

performed at sub-nanoseconds resolution using a laser-based interferometric system.

2.2 Experimental Methods

2.2.1 Laser-induced shockwave test setup and protocols

The set-up to generate laser-induced stress waves is shown schematically in Figure 2.1a.

Shockwaves are generated by impingement of a high-energy Nd:YAG pulsed laser on a 400 nm

thick Al energy-absorbing layer. Transfer of energy from the laser pulse leads to rapid expansion

of the Al layer. The presence of the confining layer on top of the Al film causes a high amplitude

compressive shockwave to propagate through the specimen. The YAG laser power and beam

diameter, differed by the position of the Nd:YAG focusing lens, were varied to systematically

control the launch laser fluence. The out-of-plane displacement of the specimen’s surface was

measured using a Michelson interferometer with a 532 nm laser diagnostic beam (Figure 2.1b).

photodetector connected to 40 GHz oscilloscope recorded the interference signal as a voltage trace,

which was converted to displacement, velocity, pressure and energy per area history. Before running test, collinear alignment of both lasers is performed to detect a sufficiently measurable voltage intensity higher than 100 mV reflected from the surface of the specimen.

(a) (b)

Figure 2.1. Schematic depiction of (a) laser-induced shockwave experimental test setup and specimen structure and (b) the photograph of optical table setup with 1064 nm Nd:YAG pulsed (red arrow) and 532 nm continuous beam path (green solid line) for Michelson interferometer.

The voltage output, V(t), is given by

VVVV++ V() t =+∗max min max min sin(2π nt ()) (1) 22

where and are the maximum and minimum voltage output respectively of each fringe.

𝑚𝑚𝑚𝑚𝑚𝑚 𝑚𝑚𝑚𝑚𝑚𝑚 After the𝑉𝑉 interference𝑉𝑉 fringe number, n(t), was obtained from equation 1, the displacement can be

determined as

λ nt() ut()= 0 (2) 2

where the wavelength of diagnostic beam, , is 532 nm. The free surface velocity, u , can be

0 𝐹𝐹𝐹𝐹𝐹𝐹 calculated as 𝜆𝜆

ut(+∆ t ) − u (t) u = (3) FSV ∆t

where the interval time, , is 25 ps. Representative photodiode output, surface displacement,

surface velocity, and totalΔ energy𝑡𝑡 per unit area are shown in Figure 2.2.

The pressure profile, Pt(), was obtained from the velocity history using conversation of momentum,

Pt()=ρρ00 ( Utsp ()) ∗=+ Ut () ( scUtpp ()) ∗ Ut () (4) where is the initial material density and ( ) is the particle velocity obtained from the

𝜌𝜌0 𝑈𝑈𝑝𝑝 𝑡𝑡 measurement. Shock velocity, Uts (), is given by s+ cUp () t , where and are fitted parameters

𝑠𝑠 𝑐𝑐 from the UUsp− Hugoniot relation of the substrate. The energy per area, i.e., total transmitted energy, was calculated from the velocity history using conservation of energy and momentum as previously described by Forbes.[14]

1 t J( t )=ρ ( U (t))2 ∗+ ( s cU ( t )) dt (5) 2 0 ∫0 pp

Figure 2.2. Representative interferometric data obtained from laser-induced shockwave test of a network-forming ionic liquid (NIL 5-6) sample: (a) photodetector voltage fringe data captured by the oscilloscope, (b) displacement as a function of time from photodetector voltage fringe, (c) free surface velocity calculated from displacement, (d) pressure profile obtained from Eq. 4, and (e) energy/area calculated from Eq. 5.

2.2.2 Input shockwave pressure measurement

Input shockwave peak pressures were measured using specimens fabricated without a test film or top substrate as shown in Figure 2.3. The laser fluence was controlled by changing the Nd:YAG laser power and/or the beam diameter as described in the prior section.

Figure 2.3. (a) Schematic depiction of laser-induced shockwave experimental test set-up and specimen structure for input shockwave pressure measurement.

2.2.3 Preparation of polyurea specimen

The precursor mixture solution was prepared by mixing 80 wt% of an oligomeric amine

(Versalink P-1000, Air Product and Chemicals) and 20 wt% of a multi-functional isocyanate precursor (Isonate 143L, Dow Chemical) in a vial, and sonicate the mixture in a bath for 5 minutes to remove air bubbles. Polyurea testing films were prepared by drop casting the 20 mg mixture onto a glass substrate with a 50μm thick polyimide spacer. A second glass substrate was then

placed on top of the specimen and the mixture was cured 24 hours at room temperature and another

24 hours at 60 °C. The polyurea sandwich specimens were prepared for laser-induced shockwave testing by electron beam deposition of a 400 nm thick Al layer (400 nm) on the outer surface of one glass substrate, followed by spin coat deposition of a 6 µm thick sodium silicate layer on the

top of the Al layer. Another Al layer (200 nm) was deposited on the surface of the glass substrate

on the opposite side of the specimen via electron beam deposition.

2.3 Results and Discussions

2.3.1 Tunable input shockwave pressure and duration time

The average input peak pressures and energy transferred were recorded as a function of launch laser fluence (Figure 2.4a,b) using the specimen type in Figure 2.3 with a glass substrate. .

Representative input pressure profiles as a function of time at different laser fluences are shown in

Figure 2.4c. A higher peak pressure and a skewed Gaussian shaped pressure profile are achieved at a higher input laser fluence.

Figure 2.4. (a) Input shockwave peak pressure as a function of launch laser fluence using a glass substrate. (b) Representative figure of input pressure profile as a function of time at various launch laser fluences. (c) Representative pressure profile at different launch laer fluences.

Shock wave development was also investigated for fused silica substrates, which has significant nonlinear elastic properties. Figure 2.5 shows that both average peak pressure and energy transferred increase with an increase in a launch laser fluence for fused silica substrates of different thickness. Lower average peak pressures were measured at all launch laser fluences for the 1.5 mm thick substrate (Figure 2.5c). Barker and Wang et al. analyzed shockwave development in fused silica based on a stress-strain relation.[9, 15] In elastic substrates like glass, the Nd:YAG laser produces a stress wave with a shape similar to the input laser pulse, which is a near Gaussian distribution.[16] Due to the negative nonlinearity of the fused silica substrate, the Gaussian shaped stress pulse develops into, classic decompression shock. According to Wang et al.[9], the speed of wave propagation , c()σ , is written as,

1 ∂σ c()σ = (6) ρε0 ∂

where, ρ0 is the initial density, σ and ε are the compressive stress and the strain, respectively.

The tangent modulus-strain relation of fused silica, which is given by,

∂σ E ==+++α23 βε γε23 4 ζε t ∂ε , (7) where α = 77.6 GPa, β = -415.9 GPa, γ = 3034.0 GPa, and ζ = -6926.0 GPa, causes the lower amplitude stress at the front and tail to travel faster, while the peak stress is delayed since the tangent modulus is proportional to a speed of wave propagation. Representative input pressure profiles from two different thickness, shown in Figure 2.5c, indicate how the stress pulse evolves differently depending on the distance of travel. The thick fused silica results in a broader stress profile due to a longer path. By adjusting the thickness of substrate and the laser fluence, a wide range of shockwave duration times can be achieved from 3 ns pulse width of Nd:YAG laser, which provides the possibility of exploring duration time effects on the shockwave energy dissipation.

Figure 2.5. (a) Input shockwave peak pressure as a function of launch laser fluence using a fused silica substrate. (b) Representative figure of input pressure profile as a function of time at various launch laser fluences.

2.3.2 Shockwave energy dissipation by polyurea

The energy dissipation properties of polyurea were investigated using the laser-induced shockwave test setup. As shown in Figure. 2.6a, energy transferred is measured in the range of 50 to 320 J/m2 for the 50 µm polyurea film. Figure 2.6b shows 400 - 920 MPa average peak pressures

are measured at all launch laser fluences . In a comparison with input pressures and energy

transferred, the percentage of reduction in pressures and energy by polyurea are presented in Figure

2.6c. The reduction in energy transferred reveals that polyurea is an effective energy absorption

material. This film reduces input energy to 90.3% and 80.1% at the lowest and the highest launch

laser fluence, respectively. The energy dissipation performance of polyurea is better at lower

launch laser fluences. The percentage reduction in peak pressure is lower than 68% at all launch

laser fluences, indicating that polyurea is less effective at attenuating pressure. We hypothesized

that the energy dissipation performance of polyurea is associated with the dissipation mechanisms

including dissociation of hard domains under shock loading and fast dynamics as described in

Chapter 1.

Figure 2.6. (a) Energy transferred and (b) average peak pressures at different input laser fluences for 50 µm polyurea film in a comparison with input energy transferred and average peak pressures. (c) The percentage decrease in energy and pressure by 50 µm polyurea film compared to input values.

2.4 Conclusions

A laser-induced shockwave test was developed to measure the shockwave energy dissipation properties of mechanoresponsive materials. For specimens with a glass substrate, this setup generates Gaussian shaped pressure profile and tunable input peak pressures ranging from 1 to 2

GPa controlled by launch laser fluences. For specimens with fused silica substrates, decompression shockwaves (linear ramp) were generated with peak pressures of 300 MPa to 1.2 GPa. We also demonstrated that shockwave profiles can be adjusted by a thickness and a launch laser fluence,

which allows the investigation of duration time effects on shockwave energy dissipation. The

energy dissipation of polyurea was investigated by this laser-induced shockwave technique. The

average peak pressures and the percentage of decreases in peak pressure of polyurea film will be

used as a benchmark value to evaluate the energy dissipation properties of testing materials in

subsequent chapters.

2.5 References

(1) Nakagawa, A.; Manley, G. T.; Gean, A. D.; Ohtani, K.; Armonda, R.; Tsukamoto, A.; Yamamoto, H.; Takayama, K.; Tominaga, T., Mechanisms of Primary Blast-Induced Traumatic Brain Injury: Insights from Shock-Wave Research. Journal of Neurotrauma 2011, 28, 1101- 1119. (2) Nakagawa, A.; Ohtani, K.; Goda, K.; Kudo, D.; Arafune, T.; Washio, T.; Tominaga, T., Mechanism of Traumatic Brain Injury at Distant Locations After Exposure to Blast Waves: Preliminary Results from Animal and Phantom Experiments. Intracranial Pressure and Brain Monitoring XV, Springer International Publishing: Cham, 2016; pp 3-7. (3) Masel, B. E.; DeWitt, D. S., Traumatic Brain Injury: A Disease Process, Not an Event. Journal of Neurotrauma 2010, 27, 1529-1540. (4) Cernak, I., Understanding blast-induced neurotrauma: how far have we come? Concussion 2017, 2, CNC42. (5) Mishra, V.; Skotak, M.; Schuetz, H.; Heller, A.; Haorah, J.; Chandra, N., Primary blast causes mild, moderate, severe and lethal TBI with increasing blast overpressures: Experimental rat injury model. Scientific Reports 2016, 6, 26992. (6) Vossen, J., Measurements of Film-Substrate Bond Strength by Laser Spallation. Measurements of Film-Substrate Bond Strength by Laser Spallation, 1978. (7) Gupta, V.; Argon, A. S.; Parks, D. M.; Cornie, J. A., Measurement of interface strength by a laser spallation technique. Journal of the Mechanics and Physics of Solids 1992, 40, 141-180.

(8) Wang, J.; R. Sottos, N.; L. Weaver, R., Tensile and mixed-mode strength of a thin film- substrate interface under laser induced pulse loading. Journal of the Mechanics and Physics of Solids 2004, 52, 999-1022. (9) Wang, J.; Weaver, R. L.; Sottos, N. R., Laser-induced decompression shock development in fused silica. Journal of Applied Physics 2003, 93, 9529-9536. (10) Wang, J.; Weaver, R. L.; Sottos, N. R., A parametric study of laser induced thin film spallation. Experimental Mechanics 2002, 42, 74-83. (11) Wang, J.; Sottos, N. R.; Weaver, R. L., Mixed-mode failure of thin films using laser- generated shear waves. Experimental Mechanics 2003, 43, 323-330. (12) Grady, M. E.; Beiermann, B. A.; Moore, J. S.; Sottos, N. R., Shockwave Loading of Mechanochemically Active Polymer Coatings. ACS Applied Materials & Interfaces 2014, 6, 5350-5355. (13) Sung, J.; Robb, M. J.; White, S. R.; Moore, J. S.; Sottos, N. R., Interfacial Mechanophore Activation Using Laser-Induced Stress Waves. Journal of the American Chemical Society 2018, 140, 5000-5003. (14) Forbes, J. W.; SpringerLink (Online service), Shock wave compression of condensed matter a primer. In Shock wave and high pressure phenomena [Online] Springer,: Berlin ; London, 2012; p. 1 online resource. (15) Barker, L. M.; Hollenbach, R. E., Shock‐Wave Studies of PMMA, Fused Silica, and Sapphire. Journal of Applied Physics 1970, 41, 4208-4226. (16) Wang, J. Thin-film Adhesion Measurement by Laser-induced Stress Waves. University of Illinois at Urbana-Champaign, 2002.

CHAPTER 3 SHOCK-INDUCED ORDERING OF NETWORK-FORMING IONIC LIQUIDS 1

3.1 Introduction

Polyurea (PU) exhibits excellent shockwave absorption properties [1-3]. In spite of many years

of study, the mechanism by which polyurea dissipates shockwave energy is still under debate.[1, 3-

5] Both experimental data and computational models (mesoscale, all-atom, and coarse-grained molecular level) have offered insights into polyurea’s shockwave attenuation capability.[4, 6-7]

Roland et al. rationalized that impact-induced, rubbery-to-glassy second-order transition within

the soft matrix of PU is responsible for the shock attenuation. The authors further suggested that

hydrogen bond-abundant, hard domains of PU have a small or negligible role in shockwave

absorption.[8] In contrast, Grujicic et al. stated that the shock-induced hydrogen bond breaking in

hard domains plays an important role in the shock-impact mitigation capacity of polyurea.[4] They

also proposed shockwave induced ordering within the hard domains and viscoelastic relaxation

within the hard/soft interfacial regions as another mechanism for reducing shock impact.[9] Even

though an explicit shockwave absorption mechanism is absent, both groups along with other

researchers reached the agreement that the micro-phase segregation in polyurea plays an important

role for the high shockwave absorption performance.

Similar to the microphase segregation observed in polyurea, amphiphilic ionic liquids with alkyl tails also display structural heterogeneities on the nanometer spatial scale that may serve as an effective candidate for shockwave energy dissipation.[10-15] Evidence from both computer

1 The results presented in this chapter are published in: Yang, K., Lee, J., Sottos, R. N., Moore, S. J., Shock-induced ordering in a Nano-segregated Network-forming Ionic Liquids, Journal of American Chemical Society, 2015, 137, 16000-16003 33

simulation and neutron/X-ray diffraction suggested that the alkyl chains in ionic liquids pack into

a "soft, oily" matrix while the charged head groups tend to segregate into "hard" domains.[16-17]

Yang et al. studied a class of network-forming ionic liquids (NILs), which are composed of alkyl- diammonium cations and citrate anions.[18] The long alkyl side chains of cations are used to

frustrate the crystallization so that amorphous glassy solids form upon cooling. Peaks in the low

Q (Q ≈ 0.4-0.7 Å-1) regime, corresponding to the nanometer spatial scale, provide the signature of

structural heterogeneities in NILs.

3.2 Experimental Methods

3.2.1 Materials and synthesis

A series of NIL as depicted in Figure 3.1, which are composed of alkyl-diammonium cations

and citrate anions, is explored for the availability of shock-induced ordering for energy dissipation.

Figure 3.1. Chemical scheme for the network-forming ionic liquids (NILs) under investigation: naming of these ionic liquids is “NIL #A-#B”, where #A is the backbone methylene unit number and #B is the side chain methylene/methyl unit number.

All chemicals were purchased from Aldrich as highest purity grade and used without further

purification. The NILs studied in this work were prepared based on established procedure.[18] The

diammonium bromide salt2 was dissolved in methanol. The solution was added into an anion

exchange column (Dowex® Monosphere® 550A UPW type 1 strong base anion exchange resin,

preliminary elution and wash was carried out using methanol). In order to maximize the conversion

of bromide anion into hydroxide anion, the column was run carefully and the eluent was protected

under argon atmosphere. The eluent was reacted directly (in situ) with citric acid in methanol in

ice bath. After the anion exchange column, the solution was evaporated. The sample was freeze-

dried for 48h. Seradyn Aquatest CMA Karl-Fisher titrator was used to determine the water content

in the final product. All the products have water content below 1800 ppm. For elemental analysis,

air sensitive capsules were used to avoid the effect from moisture in air. The bromide analysis was

done to confirm the conversion of ion exchange. For all samples, bromide content is less than

100ppm.

3.2.2 Sample preparation for laser-induced shockwave test

NIL test specimens, shown schematically in Figure 2.1a, were prepared by drop casting 20mg of NIL on a glass substrate (2.5mm x 2.5mm square, 1 mm thick) with a 50μm thick polyimide

spacer to control the thickness of the NIL layer. A second glass substrate was then placed on top

of the specimen with a pressure of 55 kPa. A NIL layer with 50μm thickness was confirmed by

scanning electron microscope. NIL sandwich specimens were prepared for laser-induced

shockwave testing by electron beam deposition of a 400 nm thick Al layer (400 nm) on the outer

surface of one glass substrate, followed by spin coat deposition of a 6 µm thick sodium silicate

layer on the top of the Al layer. Another Al layer (200 nm) was deposited on the surface of the

glass substrate on the opposite side of the specimen via electron beam deposition.

2 Diammonium bromide salt was synthesized by Ke Yang, University of Illinois at Urbana-Champaign Department of Materials Science and Engineering 35

3.2.3 Protocol for repeated shockwave testing

NIL sandwich specimens were also subjected to repeated shockwave impacts. For these experiments, the entire area of the energy absorbing layer of a NIL specimen was impinged by the

Nd:YAG pulsed laser (2 mm spot size, laser fluence of 91 mJ/mm2 corresponding to 1.8 GPa input

pressure). The sample was rastered through the beam such that a 2mm impact spot was created with a 2.5mm center to center distance from adjacent impact spots across the entire specimen.

After the energy absorbing aluminum layer was fully consumed, the shocked NIL layer was transferred to a new set of glass substrates with a pristine Al energy absorbing layer for a subsequent round of shock testing.

3.2.4 Characterization of NILs

X-ray diffraction experiment was conducted using Rigaku Miniflex 600 X-ray diffractometer with Cu Kα radiation. A thin layer of sample was pasted on a glass sample holder, which was then tested in the measurement chamber. Grazing-incidence wide angle X-ray scattering (GIWAXS) and small-angle X-ray scattering (SAXS) experiments were performed using custom-built X-ray setup in MRL at University of Illinois at Urbana-Champaign.3 Xenocs GeniX3D Cu Kα ultralow

divergence X-ray source (8 keV) was used for both GIWAXS and SAXS. PerkinElmer XRPad

4343F detectors were used for GIWAXS. A Pilatus 300 K 20 Hz hybrid pixel Detector (Dectris)

was used to collect beams scattered by the specimen. 1D spectrum data were calculated by radially

averaging the 2D diffraction data.

The DSC measurement was performed using TA Instrument Q20 Differential Scanning

Calorimeter equipped with a Liquid Nitrogen Cooling System (LNCS). Aluminum pan and lids

3 GIWAXS and SAXS measurements were carried out by Minji Kang, UIUC department of materials science and engineering 36

were used as sample testing containers. Nitrogen was used as sample purge gas. Typical DSC

measurement procedure includes 3 cyclic scans. One cyclic scan includes one heating and cooling

process. To minimize the aging effect of NIL at higher temperature, temperature range for each

scan is -100~60 °C with heating/cooling rate 10°C/min. The glass transition temperatures were

determined at the inflection point of the step from the second heating scan. For post-shock samples, we used sample that is untested by DSC to avoid any aging effect from the heating process in DSC runs.

3.3 Results and Discussions

3.3.1 Shockwave energy dissipation by NILs

Laser induced stress waves were used to characterize the shockwave absorption property of

NILs (see Figure 2.1), and the laser fluence and pressure profiles were recorded for all NIL samples

using polyurea as a reference. Shockwave loading resulted in a characteristic pressure profile

displaying an abrupt rise on the nanosecond time scale. Representative pressure profiles for the

different NILs are compared to the input and the benchmark PU pressure profiles at 48 mJ/mm2

laser fluence in Figure 3.2a. All of the materials tested caused a desirable reduction in peak

pressure. As shown in Figure 3.2a, the absorption of shockwave energy by NILs and PU also

resulted in a shift of peak pressure over time. The total transferred energy is plotted in Figure 3.2b.

NIL 5-4 and NIL 5-6 dissipated 82.7% and 87.6% of the total input energy at 48 mJ/mm2 fluence, respectively. Both the reduction in peak pressure and the reduction in total energy demonstrate that NILs are effective shockwave absorption materials. In addition, the NILs with longer side chains exhibited superior shockwave absorption performance. Average peak pressures of pristine

NILs and PU obtained from multiple pressure profile data at each laser fluence are plotted in Figure

3.2c. The NILs with longer alkyl chains attenuated more shockwave peak pressure than NILs with

shorter alkyl chains at all fluences.

The peak pressures obtained from pristine and post-shock NILs correspond to those from first-

shock and second-shock NILs in Figure 3.2c,d. Plotting the average peak pressures of all pristine

NIL samples against laser fluences revealed that the shockwave energy dissipation performance

of NIL 5-6 is the best in the series, followed by NIL 5-4, NIL 5-3, and PU at all input fluences

(Figure 3.2c). Furthermore, the peak pressure differences between NILs increased with fluence since higher input laser fluences generated stronger shockwaves. For the post-shock NILs samples, the peak pressures of NIL 5-3 and NIL 5-4 remained unchanged compared to the peak pressures from pristine samples. In contrast, the peak pressure of post-shock NIL 5-6 increased and became comparable to that of NIL 5-4. Given that NIL 5-6 lost shockwave absorption ability after the first shockwave impact, we hypothesized that pristine NIL 5-6 attenuated the impact via an irreversible alteration of material structure.

(a) (b)

Figure 3.2. (a) Representative pressure profiles of NIL samples and PU obtained during laser- induced shockwave test at 48 mJ/mm2 laser fluence. (b) Representative total transferred energy profiles of NIL and benchmark PU specimens at 48 mJ/mm2 laser fluence. (c) Average peak pressures at different laser fluences for pristine samples including PU. (d) Average peak pressures at different laser fluences for post-shock NIL samples. Error bars represent standard error.

3.3.2 Shock-induced ordering in NILs

PXRD patterns of all NIL samples reflect their amorphous nature (Figure 3.3). There are three major diffraction features in the XRD plots. With a rough calculation based on the Q value at each peak’s position, the correlation lengths for features resulting in peaks I, II, and III are 11-13, 7-8, and 3.8-4.4 Å, respectively. In particular, peak I has been observed in various IL systems, including alkyl-ammonium/phosphonium-based salts, imidazolium salts and other protic ILs, and detailed neutron and X-ray scattering data show that it represents features associated with the structural

heterogeneities on the nanometer spatial scale.[17] A previous study also demonstrated that even

short alkyl chains, such as ethyl or propyl groups, cause such heterogeneity.[16] The solvophobic

interaction between alkyl chains and charged heads likely plays an important role in leading to this

structural heterogeneity. Moreover, as alkyl side chain length increases, the nonpolar domains

become interconnected and cause “swelling” of the entire ionic network, resulting in a “sponge- like” structure consisting of nonpolar domains and charged clusters.[19] Comparing pristine

samples of NIL 5-3 and NIL 5-6, it is evident that peak I in XRD shifts to lower Q values,

indicating an increase in not only the size of the alkyl domain but also the separation between

charged clusters. This result, along with the trend of shockwave dissipation, suggests that

shockwave attenuation performance correlates positively with side-chain length. After multiple

shock impacts (up to three), the XRD patterns of NIL 5-3 and 5-4 remain the same, indicating little

change in the microstructure. In contrast, there is peak sharpening, with almost a 2-fold increase

of the amplitude of peak I for NIL 5-6 after the initial impact, suggesting that the segregation

related to peak I becomes better defined. The unchanged peak position indicates that the

shockwave impact does not affect the size of the domains. We assume that the shockwave causes

the polar heads in NIL 5-6, which has the largest structural heterogeneity, to rearrange into a more correlated configuration. This rearrangement is responsible for the increase of NIL 5-6’s peak pressure after first-shock impact.

(a) (b) (c)

Figure 3.3. XRD patterns before and after shockwave impact for samples (a) NIL 5-3, (b) NIL 5-4, (c) NIL 5-6. For NIL 5-3 and NIL 5-4, multiple shockwave impacts did not change the microstructure significantly, while for NIL 5-6, the amplitude of the low-Q peak (peak I) increased.

The average alkyl domain sizes of NILs are compared in Figure 3.4 using full width at half

maximum (FWHM) values. FWHM values in pristine NIL 5-3, 5-4, and 5-6 are 0.22, 0.20, and

0.11 Å-1, respectively, which demonstrates a growth of the average nonpolar domain size with

increasing the length of alkyl chain. The difference of FWHM values before and after shockwave

impact in each NIL corresponds to the variation in the size of alkyl domains and the separation

between charged clusters affected by shockwave impact. Figure 3.4 reveals FWHM values of each

NIL remain unperturbed regardless of shockwave impact.

Figure 3.4. Full width at half maximum (FHWM) before and after shockwave impact for NIL samples. FWHM values are independent on shockwave impact.

Grazing-incidence wide angle X-ray scattering (GIWAXS) with a two-dimensional detector was used to investigate any anisotropic alignment of the microstructure induced by shockwave.

Figure 3.5 shows the amplitude data for both pristine and shocked NIL (1.8 GPa) are independent of the incidence angle, supporting the hypothesis that the charged clusters are randomly aligned.

(a) (b)

Figure 3.5. GIWAXS two-dimensional raw data of (a) pristine and (b) shocked NIL 5-6 sample.

Small angle X-ray scattering (SAXS) was used to characterize the structure of the pristine and shocked NIL 5-6 at larger dimensions. As shown in Figure 3.6, SAXS patterns obtained after 1 hr from both pristine and shocked NIL 5-6 are identical and have no clear peak, providing no evidence of shock-induced microstructural change and long-range correlation. Ideally, data collection over a longer time frame should be carried out to fully confirm the lack of long-range correlation.

Figure 3.6. Small angle X-ray scattering (SAXS) measurement of NIL 5-6 pristine and shocked. Both SAXS patterns show no distinct peak and are identical in the Q value 0.025- 0.25 Å-1.

Based on these findings, we conclude that the sharpening of peak I after shockwave impact is associated with an addition rather than the rearrangement of charged clusters. Pristine NIL 5-6 samples contain localized heterogeneous phases, composed of the charged domains surrounded by the alkyl domains, and the remaining sample area consists of a neutral phase, in which the charged and the nonpolar domains are homogeneously mixed. We hypothesize that shockwave impact transfers energy to the neutral regions, and new charged domains are formed in the vicinity of the

pre-existing heterogeneous regions via the growth mechanism. New heterogeneous phases are

created without any long-range correlation with other heterogeneous phases after shockwave

impact.

To further support this mechanism of shock-induced ordering, we examined the differential

scanning calorimetry (DSC) data of pristine NIL 5-6 and re-recorded the DSC data immediately after shocking on pristine samples (Figure 3.7). Pristine NIL 5-6 has a glass transition temperature

(Tg) of 229.2 K. After the first shock, the Tg NIL 5-6 increased to 240.4 K. NMR and mass

spectroscopy on post-shock samples ruled out the possibility of any shock-induced chemical

changes. These results are consistent with our hypothesis of shock-induced ordering in the heterogeneous domains. The 11.2 K increase of Tg may be due to the extra spatial hindrance from the more ordered heterogeneous domain. To examine whether this rearrangement relaxes after shockwave impact, we kept the post-shock NIL 5-6 sample at room temperature and recorded the time evolution of the DSC curves of the post-shock sample over a period of 11 days. The Tg of

NIL 5-6 increased by another 11.2 K after 7 days and reached a stable value of 251.6K. This result

indicates that the ordering process continues for days after the shockwave impacts. The relaxation

dynamics is rather slow due to the high viscosity of NILs at room temperature. For comparison,

the Tg of pristine NIL 5-6 is rather stable for months at room temperature.

(a) (b) (c)

Figure 3.7. (a) differential scanning calorimetry (DSC) measurements of three batches of NIL 5-6 pristine samples and post-shock samples. Glass transition temperature (Tg) value is marked. (b) DSC curves’ time evolution for post-shock NIL 5-6 samples. Over 7 days at room temperature, the Tg of post-shock samples increased by 11.2 K from day 7 to day 11, Tg did not change. (c) Plot of Tg as a function of time for pristine NIL 5-6 sample and post-shock sample. The 0 day point is when samples were freshly prepared. The samples were freeze-dried for 2 days prior to DSC measurement and shock impacts.

We hypothesize that new heterogeneous phases consisting of charged clusters and nonpolar domains, can be created by overcoming an energy barrier. Similar effects have been observed under high hydrostatic pressures. For example, high pressure can cause configurational changes in the alkyl groups of imidazolium ILs.[20-21] Apparently, NILs with longer alkyl chains, such as NIL

5-6, are more easily reorganized because there is less restriction from the charged headgroup. The major structural change occurs at the first shock impact because the more correlated conformations are more stable. We also hypothesize that the molecular conformation does not reach its local energy minimum immediately after the shockwave impacts, so the ordering process slowly continues over time.

3.4 Conclusions

Combining these findings with the multiple shock experiments, the relationship between the microstructures of NILs and their shockwave absorption performances is evident. In NIL 5-3 and

NIL 5-4, the microstructure and shockwave absorption performance do not change through

multiple shocks. In NIL 5-6, subsequent shockwave absorption performance is reduced by

irreversible shock-induced structural evolution and the creation of new nano-segregated domains,

as schematically depicted in Figure 3.8, from the first shockwave impact. We conclude that the

observed shock-induced ordering contributes to the better shockwave absorption performance in

the initial shock of NIL 5-6. We hypothesize that at least two mechanisms of shockwave absorption

exist in the NIL system. In the case of NIL 5-3 and NIL 5-4, the nano-segregated ionic network in

the NIL dissipates shockwave kinetic energy without causing noticeable structural change. In the

case of NIL 5-6, an irreversible change in spatial ordering within the ionic network also plays a

key role in extra shockwave energy-absorbing capability.

Figure 3.8. Schematic depiction of creation of heterogeneous domains in nano-segregated NIL. Grey background indicates homogeneously mixed region in NIL 5-6. Orange circles represent alkyl domains. Blue columns are charged clusters.

3.5 References

(1) Bahei-El-Din, Y. A.; Dvorak, G. J.; Fredricksen, O. J., A blast-tolerant sandwich plate design with a polyurea interlayer. International Journal of Solids and Structures 2006, 43, 7644- 7658. (2) Grujicic, M.; d’Entremont, B. P.; Pandurangan, B.; Runt, J.; Tarter, J.; Dillon, G., Concept- Level Analysis and Design of Polyurea for Enhanced Blast-Mitigation Performance. Journal of Materials Engineering and Performance 2012, 21, 2024-2037.

(3) Gardner, N.; Wang, E.; Kumar, P.; Shukla, A., Blast Mitigation in a Sandwich Composite Using Graded Core and Polyurea Interlayer. Experimental Mechanics 2012, 52, 119-133. (4) Grujicic, M.; Pandurangan, B.; Bell, W. C.; Cheeseman, B. A.; Yen, C. F.; Randow, C. L., Molecular-level simulations of shock generation and propagation in polyurea. Materials Science and Engineering: A 2011, 528, 3799-3808. (5) Grujicic, M.; Bell, W. C.; Pandurangan, B.; Cheeseman, B. A.; Fountzoulas, C.; Patel, P., Molecular-Level Simulations of Shock Generation and Propagation in Soda-Lime Glass. Journal of Materials Engineering and Performance 2012, 21, 1580-1590. (6) Grujicic, M.; Pandurangan, B., Mesoscale analysis of segmental dynamics in microphase- segregated polyurea. Journal of Materials Science 2012, 47, 3876-3889. (7) Arman, B.; Reddy, A. S.; Arya, G., Viscoelastic Properties and Shock Response of Coarse- Grained Models of Multiblock versus Diblock Copolymers: Insights into Dissipative Properties of Polyurea. Macromolecules 2012, 45, 3247-3255. (8) Bogoslovov, R. B.; Roland, C. M.; Gamache, R. M., Impact-induced glass transition in elastomeric coatings. Applied Physics Letters 2007, 90, 221910. (9) Grujicic, M.; Snipes, J. S.; Ramaswami, S.; Yavari, R.; Runt, J.; Tarter, J.; Dillon, G., Coarse-grained Molecular-level Analysis of Polyurea Properties and Shock-mitigation Potential. Journal of Materials Engineering and Performance 2013, 22, 1964-1981. (10) Zhao, Y.; Hu, Z., Structural origin of energetic heterogeneity in ionic liquids. Chemical Communications 2012, 48, 2231-2233. (11) Song, X.; Hamano, H.; Minofar, B.; Kanzaki, R.; Fujii, K.; Kameda, Y.; Kohara, S.; Watanabe, M.; Ishiguro, S.-i.; Umebayashi, Y., Structural Heterogeneity and Unique Distorted Hydrogen Bonding in Primary Ammonium Nitrate Ionic Liquids Studied by High-Energy X-ray Diffraction Experiments and MD Simulations. The Journal of Physical Chemistry B 2012, 116, 2801-2813. (12) Ji, Y.; Shi, R.; Wang, Y.; Saielli, G., Effect of the Chain Length on the Structure of Ionic Liquids: from Spatial Heterogeneity to Ionic Liquid Crystals. The Journal of Physical Chemistry B 2013, 117, 1104-1109. (13) Canongia Lopes, J. N. A.; Pádua, A. A. H., Nanostructural Organization in Ionic Liquids. The Journal of Physical Chemistry B 2006, 110, 3330-3335.

(14) Hettige, J. J.; Araque, J. C.; Margulis, C. J., Bicontinuity and Multiple Length Scale Ordering in Triphilic Hydrogen-Bonding Ionic Liquids. The Journal of Physical Chemistry B 2014, 118, 12706-12716. (15) Li, S.; Banuelos, J. L.; Zhang, P.; Feng, G.; Dai, S.; Rother, G.; Cummings, P. T., Toward understanding the structural heterogeneity and ion pair stability in dicationic ionic liquids. Soft Matter 2014, 10, 9193-9200. (16) Atkin, R.; Warr, G. G., The Smallest Amphiphiles: Nanostructure in Protic Room- Temperature Ionic Liquids with Short Alkyl Groups. The Journal of Physical Chemistry B 2008, 112, 4164-4166. (17) Zheng, W.; Mohammed, A.; Hines, L. G.; Xiao, D.; Martinez, O. J.; Bartsch, R. A.; Simon, S. L.; Russina, O.; Triolo, A.; Quitevis, E. L., Effect of Cation Symmetry on the Morphology and Physicochemical Properties of Imidazolium Ionic Liquids. The Journal of Physical Chemistry B 2011, 115, 6572-6584. (18) Yang, K.; Tyagi, M.; Moore, J. S.; Zhang, Y., Odd–Even Glass Transition Temperatures in Network-Forming Ionic Glass Homologue. Journal of the American Chemical Society 2014, 136, 1268-1271. (19) Hayes, R.; Imberti, S.; Warr, G. G.; Atkin, R., Pronounced sponge-like nanostructure in propylammonium nitrate. Physical Chemistry Chemical Physics 2011, 13, 13544-13551. (20) Zhao, Y.; Liu, X.; Lu, X.; Zhang, S.; Wang, J.; Wang, H.; Gurau, G.; Rogers, R. D.; Su, L.; Li, H., The Behavior of Ionic Liquids under High Pressure: A Molecular Dynamics Simulation. The Journal of Physical Chemistry B 2012, 116, 10876-10884. (21) Gardas, R. L.; Freire, M. G.; Carvalho, P. J.; Marrucho, I. M.; Fonseca, I. M. A.; Ferreira, A. G. M.; Coutinho, J. A. P., High-Pressure Densities and Derived Thermodynamic Properties of Imidazolium-Based Ionic Liquids. Journal of Chemical & Engineering Data 2007, 52, 80-88.

CHAPTER 4

SHOCKWAVE ENERGY DISSIPATION VIA DYNAMIC BORON ESTER BONDS

4.1 Introduction

In the past decade, there has been a substantial interest in placing transient crosslinks into

polymer networks to yield materials which behave as thermosets at their use temperature but can

be heated, typically in the presence of a catalyst, and flow.[1-2] Some of the schemes which have

been implemented in dynamic networks include transamination,[3] transesterification,[4] disulfide

exchange,[5] hindered urea exchange,[6] and boronic-esterification.[7-9] All of these transient

network motifs have energy stored in chemical bonds and thus require an input of energy to

undergo bond exchange. We hypothesize that dynamic bond breakage can be used as an effective

mechanism for dissipating energy, in this case from a shockwave input. In this chapter, we

investigate polymer networks with controlled molecular weights between crosslinks synthesized

from boric acid and commercially available dimethylsiloxane diols of varying length. The density

of dynamic boronic ester linkages is systematically controlled, while the network chemistry

remains unchanged. Using the laser induced shockwave technique introduced in Chapter 2,[10-11]

we demonstrate superior energy dissipation in a PDMS boronic ester dynamic rubber (PDMS-B-

DR) compared to the benchmark polyurea.

4.2 Experimental Methods

4.2.1 Materials and synthesis 4

The PDMSn-B-DR networks were synthesized by the reaction of boric acid with diols

containing an average of n repeat units (Figure 4.1a). This well-known reaction[12] produces neutral,

trigonal boron sites, which serve as trifunctional network junctions. The final products were all

4 Synthesis of PDMS-Bn-DR and NMR characterization were performed by Brian Jing in University of Illinois at the Department of Materials Science and Engineering. 49

transparent, rubbery networks which became more flexible as the PDMS linker molecular weight

increased. Solid state boron NMR (Figure 4.1b) revealed a single, broad site consistent with

trigonal boron and a signal intensity that decreased with decreasing boron density (higher

molecular weight diols).

Figure 4.1. (a) Synthesis scheme for dynamic PDMS networks from boric acid and dimethylsiloxane diol oligomers. (b) Solid state 11B NMR reveals the presence of trigonal boron sites in the networks. Peak intensity is proportional to the density of boron in dynamic PDMS rubbers.

4.2.2 Sample preparation for laser-induced shockwave test

A sandwich specimen configuration similar to that described in Chapter 2 was utilized to investigate the shockwave energy dissipation properties of dynamic and control PDMS networks.

PDMSn-B-DR films were prepared and placed between two glass substrates with a 50 µm

polyimide spacer as shown in Figure 4.2. As described in Chapter 2, one glass substrate had a 400

nm Al film deposited on the outer surface (absorbing layer), while the other substrate had a 200nm

Al film (reflective layer). Each PDMSn-B-DR test film was made via different preparation protocols due to varying hardness and elastic modulus. For PDMS7-B-DR films, 20 mg pellets were placed between two Al plates with a 50 µm polyimide spacer in a hydraulic compression press and subject to 2 kN at 180 ºC for 10 min. The pressed film was then sandwiched between

the glass substrates and was annealed at 60 ºC for 1 hour under 55 kPa to ensure a tight contact

between the film and glass substrates. For the PDMS57-B-DR and PDMS216-B-DR, 20 mg pellets

o of each PDMSn-B-DR were placed between the glass substrates and then was annealed at 60 C

for 24 hours under 55 kPa. The final film thickness was measured by either scanning electron

microscope and stylus surface profiler.

Figure 4.2. Schematic depiction of laser-induced shockwave test specimen for PDMS-B-DR sandwiched between two glass substrates.

For the preparation of control PDMS specimens with nondynamic crosslinks (PDMSn), the

mixture of vinyl-terminated poly(dimethylsiloxane) (VDSn) as a chain spacer and

(mercaptopropyl)methylsiloxane – dimethylsiloxane copolymer (MDS) as a crosslinker was

prepared in an equivalent ratio with 0.2 wt % of 2,2-dimethoxy-2-phenylacetophenone (DMPA) as a photoinitiator. VDS11, VDS232, and VDS847 containing different average n dimethylsiloxane

units were used. 20 mg of the solution was dropcast on a bare glass substrate with a 50 µm

polyimide spacer, followed by placing a top glass substrate with a 400 nm thick Al layer to

sandwich the solution between two glasses with 55 kPa pressure. The PDMSn film was prepared

by photo-curing under 354 nm UV light via thiol-ene reaction. A 200 nm Al thickness was

deposited via electron beam deposition on the outer surface of the bare glass substrate.

4.2.3 Rheology

The stress relaxation behavior of dynamic and control PDMS networks was measured using a

rheometer. For dynamic PDMS7-B-DR, a test specimen was prepared by a hydraulic compression

press under 1.96 kN for 10 min at room temperature with a 25.4 mm diameter and 1.5 mm thick

metal gasket. For control PDMS11, the pre-mixed PDMS precursor solution was poured into the gasket, topped with a bare glass substrate, and photo-cured under 354 nm UV light. The prepared specimens were placed between 25.4 mm parallel cylindrical plates. The shear modulus of dynamic PDMS7-B-DR and control PDMS11 was recorded with 1 % and 0.1 % initial strain,

respectively at room temperature over time.

4.2.4 Confocal Raman characterization

Raman spectra of 50 µm thick control PDMS11 before and after 1.8 GPa shockwave impact were obtained by the confocal Raman microscope. A 532 nm excitation laser was tightly focused

in the shocked spot area to achieve signal from the region subject to the shockwave impact.

4.3 Results and Discussion

4.3.1 Reversible bond exchange reactions in the dynamic PDMS networks 5

The dynamic boronic ester bonds endow polymer networks with self-healing and reprocessing capabilities via reversible bond exchange reactions. The rapid self-healing capability of PDMS7-

B-DR at room temperature under pressure is demonstrated in Figure 4.3. Many carbon-based self- healing systems require high temperature/pressure[4] or catalysts[8] to invoke and accelerate healing,

while this dynamic PDMS network has flexible siloxane chains that facilitate recovery kinetics at

modest temperatures without the need of any additives.[13]

Figure 4.3. Photos of the self-healing process for the PDMS216-B-DR sample at room temperature under pressure. Self-healing process takes less than 5 min under 3.5 MPa pressure.

To further confirm that the dynamic exchange of boronic ester bonds, which is associated with

the malleability of dynamic PDMS polymer networks, we performed stress relaxation

measurements for dynamic network PDMS7-B-DR and control PDMS11 with non-dynamic

5 The demonstration of self-healing dynamic PDMS was by Brian Jing in University of Illinois at the Department of Materials Science and Engineering. 53

crosslinks at room temperature using a rheometer. The samples were prepared between 25 mm

cylindrical parallel plates with a thickness of about 1.5 mm. As shown in Figure 4.4, the shear modulus of PDMS11 slowly decrease to 72 % of its initial value at 2000 s, while PDMS7-B-DR

exhibited much faster stress relaxation and completely relaxed within 200 s. This relaxation response indicates that the boronic ester bonds act as crosslinks on short timescales but can break on longer time scales, allowing polymer network to reconfigure and flow. We hypothesized that the dynamic bonds are potent shock-responsive sites for efficient energy dissipation through the dissociation of the bonds.

Figure 4.4. Stress relaxation of control PDMS (PDMS11) and dynamic PDMS (PDMS7-B- DR) measured by rheometer at room temperature with 0.1% strain and 1% strain, respectively. Normalized shear modulus was calculated based on the initial modulus value.

4.3.2 Molecular weight effects of control PDMS on energy dissipation

A series of nondynamic control PDMS with varying crosslinking density were prepared via

photo-thiol-ene chemistry. Figure 4.5 shows peak pressures measured at various launch laser

fluences for a series of control PDMS networks. All nondynamic PDMS networks exhibited very

similar shockwave energy dissipation, illustrating that the chain dynamics between crosslinks do

not play a significant role in energy dissipation performance.

Figure 4.5. Peak pressure as a function of launch laser fluence for control PDMS with varying crosslink density, PDMS with n (number of repeat units)=11 (black triangle), 232 (red triangle), 847 (blue triangle).

4.3.3 Shockwave energy dissipation by dynamic covalent bonds

Laser-induced shockwave tests were performed on a series of dynamic PDMS networks with varying boronic ester crosslinking density. As shown in Figure 4.6, we observed a systematic

decrease in peak pressure with increasing crosslinking. PDMS7-B-DR was the most effective at

shockwave energy dissipation. At the highest input laser fluence the peak pressure was reduced by

80 % of the input value. As the molecular weight of network linkers increases, the performance

approaches that of the control PDMS networks. Furthermore, both nondynamic and dynamic

PDMS networks dissipate energy more effectively than polyurea as a benchmark, with PDMS7-B-

DR performing the best.

Figure 4.6. Peak pressure as a function of input laser fluence for polyurea (PUrea, black squares), control PDMS (red triangles) and dynamic systems PDMSn-B-DR with n = 7 (black circles), 57 (green inverted triangles), 216 (blue diamonds). Lower peak pressure corresponds to greater energy dissipation.

Confocal Raman spectra for the control PDMS before and after shockwave loading are shown

in Figure 4.7. The lack of change in spectra indicates that C-S bond in control PDMS is not

dissociated under 2 GPa impact, which supports that the shockwave energy is attenuated via non-

destructive interactions. Armstrong et al. reported that shockwave pressures ranging from 1 to 2

GPa are not sufficient to trigger chemical reactions including a bond cleavage in PDMS.[14] They also stated that the faster relaxation of chain segments in rubbery polymers under shock loading

may contribute to energy dissipation. Champion proposed that an attenuation of shockwave

pressure results from viscoelasticity of rubbery polymers, which is associated with chain segmental

dynamics.[15] Although both polymers are rubbery at room temperature,[16] the lower glass transition temperature of PDMS endows more flexibility at high-strain rate compared to polyurea.

This is rationalized by time-scale independent shock response of PDMS from sub-nanoseconds to

hundred nanoseconds.[14]

Figure 4.7. Confocal Raman spectra of pristine and shocked control PDMS11 after 1.8 GPa shockwave pressure was applied. Identical spectra were detected from both pristine and shocked PDMS11, demonstrating C-S bonds are not dissociated by shockwave impact.

From these results, we infer that the dissociation of boronic ester bonds in dynamic PDMS

networks contribute to energy dissipation is demonstrated, but we do not have a full understanding

of the exact mechanisms occurring in the networks. Parris et al. reported that the viscosity of gels

containing boron crosslinkers drops significantly under the static pressure associated with the pressure-induced breakage of B-O-C bonds, while invariant viscosity was observed from gels with other metal crosslinkers.[17] Wulff et al. showed that the rates of transesterifications (s-1) vary eight orders of magnitude depending on neighboring groups in a small molecule system.[18] Cromwell

et al. also observed the transesterification rate up to 103 /s using small molecules and demonstrated the kinetics of boronic ester transesterification at the molecular-scale influence on the rate of stress relaxation at the macroscopic scale.[9] These kinetics studies support the possibility of chemical

changes in boronic ester bonds within nanoseconds of shockwave loading.

4.4 Conclusions

We demonstrated superior energy dissipation in the dynamic PDMS networks containing boronic ester bonds compared to the benchmark polyurea. The dynamic PDMS also outperforms covalent PDMS and shows dissipation performance which scales with the density of dynamic bonds. We hypothesized that the dissociation of boronic ester bonds in dynamic PDMS rubber is the main mechanism for energy dissipation. Furthermore, reversible bond exchange reactions enable polymer networks to self-heal damage without chemical additives at room temperature

under gentle pressure. These results indicate that dynamic networks are a promising route to

engineering improved SWED materials which are flexible and able to withstand repeated shocks

via reversible exchange reactions.

4.5 References

(1) Rowan, S. J.; Cantrill, S. J.; Cousins, G. R. L.; Sanders, J. K. M.; Stoddart, J. F., Dynamic covalent chemistry. Angew Chem Int Edit 2002, 41, 898-952. (2) Kloxin, C. J.; Scott, T. F.; Adzima, B. J.; Bowman, C. N., Covalent Adaptable Networks (CANS): A Unique Paradigm in Cross-Linked Polymers. Macromolecules 2010, 43, 2643-2653. (3) Denissen, W.; Droesbeke, M.; Nicolay, R.; Leibler, L.; Winne, J. M.; Du Prez, F. E., Chemical control of the viscoelastic properties of vinylogous urethane vitrimers. Nature Communications 2017, 8. (4) Montarnal, D.; Capelot, M.; Tournilhac, F.; Leibler, L., Silica-Like Malleable Materials from Permanent Organic Networks. Science 2011, 334, 965-968. (5) Fairbanks, B. D.; Singh, S. P.; Bowman, C. N.; Anseth, K. S., Photodegradable, Photoadaptable Hydrogels via Radical-Mediated Disulfide Fragmentation Reaction. Macromolecules 2011, 44, 2444-2450. (6) Ying, H. Z.; Zhang, Y. F.; Cheng, J. J., Dynamic urea bond for the design of reversible and self-healing polymers. Nature Communications 2014, 5. (7) Rottger, M.; Domenech, T.; van der Weegen, R.; Nicolay, A. B. R.; Leibler, L., High- performance vitrimers from commodity thermoplastics through dioxaborolane metathesis. Science 2017, 356, 62-65. (8) Cash, J. J.; Kubo, T.; Bapat, A. P.; Sumerlin, B. S., Room-Temperature Self-Healing Polymers Based on Dynamic-Covalent Boronic Esters. Macromolecules 2015, 48, 2098-2106. (9) Cromwell, O. R.; Chung, J.; Guan, Z. B., Malleable and Self-Healing Covalent Polymer Networks through Tunable Dynamic Boronic Ester Bonds. Journal of the American Chemical Society 2015, 137, 6492-6495. (10) Rahimzadeh, T.; Arruda, E. M.; Thouless, M. D., Design of armor for protection against blast and impact. Journal of the Mechanics and Physics of Solids 2015, 85, 98-111. (11) Petel, O. E.; Jette, F. X.; Goroshin, S.; Frost, D. L.; Ouellet, S., Blast wave attenuation through a composite of varying layer distribution. Shock Waves 2011, 21, 215-224. (12) Springsteen, G.; Wang, B. H., A detailed examination of boronic acid-diol complexation. Tetrahedron 2002, 58, 5291-5300.

(13) Schmidtke, B.; Hofmann, M.; Lichtinger, A.; Rössler, E. A., Temperature Dependence of the Segmental Relaxation Time of Polymers Revisited. Macromolecules 2015, 48, 3005-3013. (14) R., A. M.; V., G. P.; M., S. A.; P., L. J.; C., C. J.; M., Z. J.; B., R. H.; Elissaios, S.; T., A. C.; Julie, H.; S., M. R., Ultrafast shock compression of PDMS‐based polymers. Journal of Polymer Science Part B: Polymer Physics 2018, 56, 827-832. (15) Champion, A. R., Shock Compression of Teflon from 2.5 to 25 kbar‐Evidence for a Shock‐Induced Transition. Journal of Applied Physics 1971, 42, 5546-5550. (16) Holzworth, K.; Jia, Z.; Amirkhizi, A. V.; Qiao, J.; Nemat-Nasser, S., Effect of isocyanate content on thermal and mechanical properties of polyurea. Polymer 2013, 54, 3079-3085. (17) Parris, M. D.; MacKay, B. A.; Rathke, J. W.; Klingler, R. J.; Gerald, R. E., Influence of Pressure on Boron Cross-Linked Polymer Gels. Macromolecules 2008, 41, 8181-8186. (18) Günter, W.; Manfred, L.; Helmut, B., Rapid Proton Transfer as Cause of an Unusually Large Neighboring Group Effect. Angewandte Chemie International Edition in English 1984, 23, 741-742.

CHAPTER 5

SHOCK-INDUCED NUCLEATION OF SUPERCOOLED LIQUIDS 1

5.1 Introduction

Controlling crystallization of organic molecules is of great interest for the development of

pharmaceuticals,[1-2] organic electronics,[3] and gas-storage materials.[4] Upon cooling, some

molten crystalline materials are capable of forming thermodynamically unstable supercooled

liquids. The effect of pressure in the form of compression on crystal nucleation and growth in

supercooled liquids has been explored;[5-7] however, contradictory examples of pressure-promoted

or -inhibited crystallization are found in literature.[5, 8-9] Other external forces such as

ultrasonication,[10] mechanical grinding,[11] and shear[12] are often utilized to assist in

crystallization, but the complex mechanisms of crystallization, especially in the presence of such

external triggers, still lacks a thorough understanding. In chapter 3, we demonstrated that a shock

wave is capable of inducing structural ordering in an amorphous ionic liquid.[13] A shock wave

imparts a transient pressure jump of ns duration, potentially long enough for nuclei to form[14] and

may serve as a useful probe of crystal nucleation in supercooled liquids.

While studying the reactivity of aryl-substituted arenediynes,[15] we discovered that 1,2- bis(phenylethylnyl)benzene (PEB) forms a stable supercooled liquid at room temperature.

Although PEB has been extensively studied to probe the Bergman cyclization reaction and to investigate its optical properties,[10, 16-18] to the best of our knowledge, its crystal structure is not

reported.

1 The results presented in this chapter are published in: Ren, Y., Lee, J., Hutchins, M. K., Sottos, N. R., Moore, S. J., Crystal structure, Thermal properties and Shock Wave-induced Nucleation of 1,2-bis(phenylethynyl)benzene, Crystal Growth & Design, 2016, 16, 6148-6151 61

Herein, we present the single crystal structure and thermal properties of PEB, along with

crystallization of the supercooled liquid. We demonstrate that a shockwave with 1.2 GPa peak

pressure and 15 ns duration accelerates nucleation of supercooled PEB (Scheme 5.1). Systematic studies of shock wave peak pressures were conducted to correlate pressure effects and frequency of nucleation. The generality of shock-induced nucleation was probed with other supercooled liquids. However, accelerated nucleation after shock wave impact was only observed for PEB.

Several hypotheses are discussed to provide insight into the mechanism of shock-accelerated crystal nucleation of supercooled liquids bearing phenyl rings.

Scheme 5.1. Depiction of shock-wave-induced crystallization of supercooled PEB

5.2 Experimental Methods

5.2.1 Materials 2

PEB was synthesized via a Sonogashira coupling reaction following literature procedure.[19]

Other supercooled liquids including diphenyl phthalate (DP), benzophenone (BP), salol, and 2-

biphenylmethanol (PM) were purchased from Aldrich. Chemical structures of supercooled liquids

used in this work are graphically presented in Scheme 5.2. These supercooled liquids are already

known for being capable of supercooling below the melting temperature.[20-21]

2 PEB was synthesized by Yi Ren, University of Illinois at Urbana-Champaign Department of Chemistry 62

Scheme 5.2. Chemical structures of supercooled liquids used in this study and their corresponding melting points (mp) as determined by differential scanning calorimetry (DSC).

5.2.2 Shockwave-induced nucleation setup and protocols

The specimen configuration for observing shockwave-induced nucleation is shown schematically in Figure 5.1. Supercooled liquids were sandwiched between two glass substrates.

The glass substrates (1 mm thick, 1” x 1” square) were cleaned by a piranha solution (a mixture of sulfuric acid and hydrogen peroxide). A 50 µm thick polyimide film as a spacer was applied on the glass substrate. A 2 cm diameter circle was laser-cut and removed to allow space for the supercooled liquids between the two glass substrates. Each powder sample of PEB, DP, BP, salol, and PM (15 mg per sample) was placed in the circle where the polyimide was removed. The sample was then heated above its melting temperature on a hot plate for 5 min and subsequently cooled to room temperature to form a supercooled liquid. A 400 nm thick Al film was deposited by electron beam deposition on the other glass substrate. A 6 µm thick transparent water-glass confining layer was deposited on the Al film by spin-coating of a sodium silicate solution. This glass substrate was then placed on top of the supercooled specimen with a pressure of 55 kPa such that the water glass layer was on the outer surface.

Figure 5.1. Schematic of specimen configuration for shockwave-induced nucleation

The input pressures were characterized in a series of calibration experiments as described in

Chapter 2 and shown schematically in Figure 2.3. Figure 5.2 shows the resulting pressure profiles

as a function of laser fluence used to induce crystallization in the supercooled liquids. After the supercooled liquid specimens were subjected to shock wave loading, time-lapse images of nucleation and crystal growth were recorded using a QImaging micropublisher 3.3 with AF Micro-

Nikkor 60mm f/2.8D lens.

Figure 5.2. Shockwave pressure profiles at 28.9, 40.1, and 47.6 mJ/mm2 laser fluence with peak pressure at 0.4, 0.8, and 1.2 GPa, respectively.

5.2.3 Theoretical calculation of rotational energy barrier

All calculations were performed with the B3LYP/6-31G(d) basis set. Geometry optimizations

were carried out using Gaussian 09.[22] Vibrational frequencies were computed for each optimized

structure to confirm that each geometry was a stationary point. Calculations on rotational energy

of the terminal phenyl ring were carried out using Spartan 10.[23] Both the optimized structures and crystal structures were used as the starting structures and all atoms except the terminal phenyl ring were frozen. The relative energy profile was generated for each molecule as the unfrozen terminal phenyl ring rotated 360° with a step size of 5°.

5.3 Results and Discussion

5.3.1 Shockwave-induced nucleation of 1,2-Bis(Phenylethylnyl)benzene (PEB)

While crystalline PEB melts at 55 °C as seen in the DSC trace (Figure 5.3), subsequent cooling

of the molten PEB results in a supercooled liquid phase that is stable down to ca. 0 °C. The

supercooled liquid phase persists during the second heating cycle without showing any

crystallization peaks, indicating it forms a supercooled liquid with good thermal stability.[12] Using

the Hoffman equation,[12]

−2 ∆=∆G Hfm( T − TT )* T m (1)

where ∆G is the Gibbs free energy difference between supercooled liquid and solid, and the heat

of fusion (∆Hf) and the melting point (Tm)of each molecule are estimated from DSC curves in

Figure 5.3. These values are listed in Table 5.1. The Gibbs free energy difference (∆G) between

supercooled and crystalline PEB at 25 °C was estimated as -1.84 kJ/mol, which is comparable to

the ∆G between tolfenamic acid polymorphs and is over one order of magnitude smaller than the

∆G between cis- and trans-azobenzene.[24-25] The relatively small ∆G of PEB suggests that PEB

has a relatively large critical radius of nucleation (r*), the minimum nucleus size that can grow

spontaneously.[12] Thus, the large r* of PEB suppresses nucleation, which potentially explains the

formation of supercooled PEB.

Fig. 5.3. DSC curves of supercooled PEB, DP, PM, BP, and salol. Upon cooling at 10 °C/min, no recrystallization was observed after melting.

Table 5.1. Thermodynamic parameters of PEB, salol, BP, DP, and PM. The melting point (Tm) and the heat of fusion (∆Hf) were measured by DSC (Figure 5.3). The Gibbs free energy o difference (∆G) between supercooled liquid and crystalline solid at 25 C was calculated using Hoffman equation (equation 1).

Since external forces often trigger the crystallization of amorphous materials and crystal nucleation in liquids is known to take place within nanoseconds,[13-14] we investigated the

possibility of using a laser-induced shock wave with GPa peak pressure and ns duration to induce

the nucleation of supercooled PEB.[13] The peak pressure of a shock wave was controlled by

systematic variation of the laser fluence (Figure 5.2). Interestingly, immediately after shock wave

exposure, a nucleation site within the PEB liquid was observed with optical imaging (Figure 5.4),

whereas no nucleation site was observed after one week for liquid PEB without impact.

Figure 5.4. Shock induced crystallization of supercooled PEB after a) 0 hr; b) 7 hrs; c) 14 hrs; d) 22 hrs; e) 29 hrs; f) 36 hrs.

These findings suggest that the nucleation rate of supercooled PEB is significantly increased

( ) [ ] = upon shock wave impact. Using the relationship 0, the change in natural log of the 𝜕𝜕 𝑙𝑙𝑙𝑙𝑙𝑙 Δ𝑉𝑉 𝜕𝜕𝜕𝜕 𝑇𝑇 𝑅𝑅𝑅𝑅 equilibrium constant (K) is proportional to the change in pressure (P), since ∆V0 is fixed for the transformation from supercooled liquid to crystal. Thus, crystallization of PEB is preferred under high pressure (large ∆P). Shockwave-induced nucleation experiments with varying peak pressure were conducted. As the peak pressure of the shockwave increased, the frequency of shock-induced nucleation also increased (Figure 5.5). Since and = ,[5, 12] it follows ∗ 1 𝑟𝑟 ∝ − ∆𝐺𝐺 ∆𝐺𝐺 𝑉𝑉∆𝑃𝑃 − 𝑆𝑆∆𝑇𝑇 that r* decreases as ∆P increases. At higher pressure, the shock-accelerated nucleation of PEB is therefore promoted by a reduction in r*.[5-6]

Figure 5.5. Investigation of shock wave peak pressure on accelerated nucleation of PEB.

To explore the scope and generality of shock-induced nucleation of supercooled liquids, we performed similar experiments on DP, BP, salol, and PM, which are also capable of forming supercooled liquids with good thermal stability (Figure 5.3).[26-28] No accelerated nucleation rate was observed with any of these four substances upon shocking their supercooled liquids under 1.2

GPa pressure.

5.3.2 Microstructural characterization of PEB before and after shock impact

To understand shock-induced nucleation phenomenon, the crystal structure of PEB was explored. Solution grown, diffraction quality crystals of PEB were attempted from common organic solvents such as cyclohexane, benzene, tetrahydrofuran, and chloroform at different temperatures. In spite of extensive experimentation with typical crystallization conditions, PEB generally remained in solution or oiled-out after solvent evaporation. We were able to successfully obtain single crystals of PEB suitable for X-ray analysis through slow evaporation of a saturated hexanes solution at -20 °C. A single-crystal X-ray analysis revealed that PEB crystallizes in the

acentric space group Pca21, with two crystallographically unique molecular conformations (PEB1

and PEB2) in the asymmetric unit (Figure 5.6a). The terminal phenyl rings of PEB1 lie twisted from the plane of the central benzene ring at dihedral angles of 28.6° and 74.2°. In PEB2, however, one terminal phenyl ring lies nearly coplanar with the central benzene ring (4.4° twist), while the second terminal phenyl ring is twisted significantly from the plane of the central benzene at 71.0°.

The dihedral angle between the two terminal phenyl rings is 87.9° for PEB1 and 73.0° for PEB2, confirming that PEB crystallizes in a nonplanar geometry. The two PEB rotamers pack in an alternating PEB1 | PEB2 pattern along the a-axis (Figure 5.6b), and neighboring central benzene

rings engage in π-facial interactions (3.32 Å separation). Conformers PEB1 and PEB2 pack into alternating columns[29] that extend along the b-axis (Figure 5.6c), sustained by edge-to-face C-

H···π interactions (ca. 3.5 Å separation).

Figure 5.6. Single-crystal X-ray structures involving PEB: (a) PEB1 and PEB2 rotamers, (b) alternating packing along a-axis, (c) column packing, and (d) cocrystal with OFN highlighting phenyl-perfluorophenyl interactions.

The powder X-ray diffraction (PXRD) patterns of the solidified PEB after shockwave impact are consistent with the predicted pattern from the single crystal X-ray data (Figure 5.7). No chemical change in the PEB sample was observed by NMR, UV-Vis, or IR following shock wave impact (Figure 5.8).

Figure 5.7. PXRD pattern of crystallized PEB after shock wave impact (blue) and predicted pattern from single crystal of PEB (black).

Figure 5.8. (a) UV-Vis; (b) IR; and (c) 1H NMR spectra of PEB after shock wave impact (PEB shocked) and PEB crystallized without an external agitation (PEB pristine).

5.3.3 Rotational energy barriers for shock-induced nucleation

Although it is known that the effect of pressure on crystallization of supercooled liquids is not universal,[5, 7, 9, 30-31] we propose several hypotheses to explain our observations. We originally assumed that compounds with high crystal packing efficiencies crystallize more readily upon shock wave impact due to shock-induced densification. However, crystal packing efficiencies among these five tested compounds are similar, lying in the normal range of 65-68% (Table 5.2).

We then hypothesized that shock-induced nucleation is more successful for compounds possessing

smaller r* values. The free energy of crystallization at 25 °C was calculated for each tested

compound using the Hoffman equation (Table 5.1).[12] Based on the ∆G values, r* of PEB is smaller than that of BP, PM, salol, but larger than that of DP.

Table 5.2. Crystal packing efficiencies of PEB, salol, BP DP, and PM. Molecular Unit cell Packing Number of molecules in a Compounds volume Volume Efficiency unit cell (Å3) (Å3) (%) PEB 8 252.8 3043.7 66.4 salol 4 178.1 1051.9 67.7 BP 4 165.7 992.3 66.8 DP 4 261.5 1604.3 65.2 PM 8 165.3 2035.2 65.0

We also determined the viscosity and glass transition temperature of each tested compound

(Table 5.3) to test the hypothesis that nucleation is transport limited. Since the kinetics of nucleation also depend on rotational energy barrier,[32] i.e. the energy required for molecules to go

through an intermediate arrangement and achieve structural alignment, we calculated the energy

profile of each molecule as its terminal phenyl ring rotates 360° using the optimized structure as the starting geometry (Figure 5.9 and Table 5.4). Among all the compounds tested, the only unique characteristic of PEB is that its rotational barrier is at least one order of magnitude lower than the other molecules. Whether this is a coincidental finding or has consequences for shock-induced crystallization remains unresolved at this time.

Table 5.3. Viscosities and glass transition temperatures (Tg) of PEB, salol, BP DP, and PM. PM crystallized upon cooling from 80 °C to 25 °C using the rheometer’s parallel plate geometry; a - the viscosity of each tested compound was determined as a function of frequency (1 to 1000 s 1 b ) at 25°C using a rheometer; the glass transition temperatures were determined at the inflection point of the step from the second heating scan with heating/cooling/heating rate of 10 °C/min using DSC.

a b Compounds Viscosity at 25 °C (Pa·s) Tg (K) PEB 0.2 247 salol 0.019 224 BP 0.013 214 DP 13.7 257 PM N/A* 242

Figure 5.9. (a) Energy profiles of supercooled PEB as the terminal phenyl ring rotates 360° while other atoms are frozen using optimized geometry in vacuum (black), optimized geometry in dichloromethane (blue), crystal structure in vacuum (green) as the starting geometry. (b) Optimized PEB geometry (black) vs. crystal (green) PEB geometry. The difference in geometry likely explains the difference in relative energy.

Table 5.4. Rotational energy barriers of supercooled liquids under different conditions.

5.4 Conclusions

we successfully obtained the single crystal structure of PEB, as well as a cocrystal, to demonstrate its propensity to pack in nonplanar arrangements. The DSC curve of PEB indicated that it has a relatively low ∆G, which leads to the formation of a stable supercooled liquid at room temperature. A shockwave with ns duration induced nucleation of supercooled PEB, confirming that nucleation in liquids can occur within a very short time frame. The nucleation behavior of additional supercooled liquids under shockwave impact was studied; however, the facilitated crystallization was observed only for PEB. The low rotational energy barrier of the terminal phenyl rings potentially explains the unique behavior of PEB.

5.5 References

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(4) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M., The Chemistry and Applications of Metal-Organic Frameworks. Science 2013, 341. (5) Adrjanowicz, K.; Grzybowski, A.; Grzybowska, K.; Pionteck, J.; Paluch, M., Toward Better Understanding Crystallization of Supercooled Liquids under Compression: Isochronal Crystallization Kinetics Approach. Cryst. Growth. Des. 2013, 13, 4648-4654. (6) Adrjanowicz, K.; Kaminski, K.; Paluch, M.; Niss, K., Crystallization Behavior and Relaxation Dynamics of Supercooled S-Ketoprofen and the Racemic Mixture along an lsochrone. Cryst. Growth Des. 2015, 15, 3257-3263. (7) Adrjanowicz, K.; Grzybowski, A.; Grzybowska, K.; Pionteck, J.; Paluch, M., Correction to Effect of High Pressure on Crystallization Kinetics of van der Waals Liquid: An Experimental and Theoretical Study. Crystal Growth & Design 2014, 14, 4226-4226. (8) Mierzwa, M.; Paluch, M.; Rzoska, S. J.; Ziolo, J., The liquid-glass and liquid-liquid transitions of TPP at elevated pressure. J. Phys. Chem. B 2008, 112, 10383-10385. (9) Adrjanowicz, K.; Kaminski, K.; Wojnarowska, Z.; Dulski, M.; Hawelek, L.; Pawlus, S.; Paluch, M.; Sawicki, W., Dielectric Relaxation and Crystallization Kinetics of Ibuprofen at Ambient and Elevated Pressure. J. Phys. Chem. B 2010, 114, 6579-6593. (10) Evenzahav, A.; Turro, N. J., Photochemical Rearrangement of Enediynes: Is a “Photo- Bergman” Cyclization a Possibility? J. Am. Chem. Soc. 1998, 120, 1835-1841. (11) James, S. L.; Adams, C. J.; Bolm, C.; Braga, D.; Collier, P.; Friscic, T.; Grepioni, F.; Harris, K. D. M.; Hyett, G.; Jones, W.; Krebs, A.; Mack, J.; Maini, L.; Orpen, A. G.; Parkin, I. P.; Shearouse, W. C.; Steed, J. W.; Waddell, D. C., Mechanochemistry: opportunities for new and cleaner synthesis. Chem. Soc. Rev. 2012, 41, 413-447. (12) Chung, K.; Kwon, M. S.; Leung, B. M.; Wong-Foy, A. G.; Kim, M. S.; Kim, J.; Takayama, S.; Gierschner, J.; Matzger, A. J.; Kim, J., Shear-Triggered Crystallization and Light Emission of a Thermally Stable Organic Supercooled Liquid. ACS Cent. Sci. 2015, 1, 94-102. (13) Yang, K.; Lee, J.; Sottos, N. R.; Moore, J. S., Shock-Induced Ordering in a Nano- segregated Network-Forming Ionic Liquid. J. Am. Chem. Soc. 2015, 137, 16000-16003. (14) Sosso, G. C.; Chen, J.; Cox, S. J.; Fitzner, M.; Pedevilla, P.; Zen, A.; Michaelides, A., Crystal Nucleation in Liquids: Open Questions and Future Challenges in Molecular Dynamics Simulations. Chem. Rev. ASAP, DOI: 10.1021/acs.chemrev.5b00744.

(15) Nagarjuna, G.; Ren, Y.; Moore, J. S., Synthesis and reactivity of anthracenyl-substituted arenediynes. Tetrahedron Lett. 2015, 56, 3155-3159. (16) Jana, S.; Anoop, A., Effect of [small pi]-[small pi] interaction in Bergman cyclisation. Phys. Chem. Chem. Phys. 2015, 17, 29793-29802. (17) Melinger, J. S.; Pan, Y.; Kleiman, V. D.; Peng, Z.; Davis, B. L.; McMorrow, D.; Lu, M., Optical and Photophysical Properties of Light-Harvesting Phenylacetylene Monodendrons Based on Unsymmetrical Branching. J. Am. Chem. Soc. 2002, 124, 12002-12012. (18) Zhang, W.; Huang, P. C., Effect of the substituting groups and their positions on the optical properties of phenylacetylenic compounds. Mater. Chem. Phys. 2006, 96, 283-288. (19) Kovalenko, S. V.; Peabody, S.; Manoharan, M.; Clark, R. J.; Alabugin, I. V., 5-Exo-dig radical cyclization of enediynes: The first synthesis of tin-substituted benzofulvenes. Org. Lett. 2004, 6, 2457-2460. (20) Dixon, P. K.; Wu, L.; Nagel, S. R.; Williams, B. D.; Carini, J. P., Scaling in the relaxation of supercooled liquids. Physical Review Letters 1990, 65, 1108-1111. (21) Schmidtke, B.; Petzold, N.; Kahlau, R.; Hofmann, M.; Rössler, E. A., From boiling point to glass transition temperature: Transport coefficients in molecular liquids follow three- parameter scaling. Physical Review E 2012, 86, 041507. (22) Frisch, M. J. T., G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M. J.; Heyd, J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.; Gaussian, Inc.: Wallingford, CT, USA, 2009. (23) Sawyer, T. W.; Ritzel, D. V.; Wang, Y.; Josey, T.; Villanueva, M.; Nelson, P.; Song, Y.; Shei, Y.; Hennes, G.; Vair, C.; Parks, S.; Fan, C.; McLaws, L., Primary Blast Causes Delayed

Effects without Cell Death in Shell-Encased Brain Cell Aggregates. Journal of Neurotrauma 2017, 35, 174-186. (24) Lopez-Mejias, V.; Kampf, J. W.; Matzger, A. J., Polymer-Induced Heteronucleation of Tolfenamic Acid: Structural Investigation of a Pentamorph. J. Am. Chem. Soc. 2009, 131, 4554- +. (25) Cembran, A.; Bernardi, F.; Garavelli, M.; Gagliardi, L.; Orlandi, G., On the mechanism of the cis-trans isomerization in the lowest electronic states of azobenzene: S-0, S-1, and T-1. J. Am. Chem. Soc. 2004, 126, 3234-3243. (26) Graham, D. J.; Magdolinos, P.; Tosi, M., INVESTIGATION OF SOLIDIFICATION OF BENZOPHENONE IN THE SUPERCOOLED LIQUID-STATE. J. Phys. Chem. 1995, 99, 4757-4762. (27) Hatase, M.; Hanaya, M.; Oguni, M., Studies of homogeneous-nucleation-based crystal growth: significant role of phenyl ring in the structure formation. J. Non-Cryst. Solids 2004, 333, 129-136. (28) Diogo, H. P.; Pinto, S. S.; Ramos, J. J. M., Thermal behaviour and slow molecular mobility in two isomers of biphenylmethanol - DSC and TSDC study. J. Therm. Anal. Calorim. 2006, 83, 361-366. (29) Zhu, N.; Hu, W.; Han, S.; Wang, O.; Zhao, D., Folding a conjugated chain: Oligo(o- phenyleneethynylene-alt-p-phenyleneethynylene). Org. Lett. 2008, 10, 4283-4286. (30) Polsky, C. H.; Martinez, L. M.; Leinenweber, K.; VerHelst, M. A.; Angell, C. A.; Wolf, G. H., Pressure-induced crystallization of vitreous ZnCl2. Phys. Rev. B 2000, 61, 5934-5938. (31) Yoshino, T.; Maruyama, K.; Kagi, H.; Nara, M.; Kim, J. C., Pressure-Induced Crystallization from Amorphous Calcium Carbonate. Cryst. Growth Des. 2012, 12, 3357-3361. (32) Shah, N.; Sandhu, H.; Choi, D. S.; Chokshi, H.; Malick, A. W., Amorphous Solid Dispersions : Theory and Practice. Springer: New York, NY, 2014.

CHAPTER 6

EFFECT OF POLYMERIZED IONIC LIQUID STRUCTURE AND MORPHOLOGY ON

SHOCKWAVE ENERGY DISSIPATION

6.1 Introduction

Several materials-based energy absorbing strategies have been introduced to provide

protection from high- and low-intensity shockwaves, including ordering transitions.[1-2], chemical

reactions[3-4], and plastic deformations[5-6]. Most of these strategies result in one-time irreversible changes and are unsuitable for attenuation of multiple shockwave events. Alternative approaches for shockwave energy attenuation utilize viscoelastic molecular relaxations[7] and impedance mismatch of multi-layered structures consisting of alternating layers of high and low impedance

materials.[8-10] Recent investigations of polyurea layers emphasize the importance of polyurea

structure on shockwave energy dissipation.[11-13] Molecular dynamic simulations by Grujicic et al.,

predict that the deformation of soft domains associated with hard domain densification in nano-

segregated polyurea contributes to energy dissipation.[14] Arman et al. also predict that well-

dispersed and easily pliable hard domains in microphase-separated polyurea play an important role

for energy dissipation.[11] Surprisingly, there are few experimental investigations of how polymer

structure effects shockwave energy dissipation. In this chapter, we vary the structure of polymeric

ionic liquids (PILs) and characterize the corresponding change in energy dissipation, providing

insight into mechanisms for shock wave absorption.

Schroder et al. first proposed a nano-segregated morphology of imidazolium-based ionic

liquids consisting of polar and non-polar regions.[15] Subsequent studies explored the diverse

microstructures of ionic liquids depending on cations, anions, and non-polar parts.[16-19] In Chapter

3, we investigated a series of network-forming ionic liquids (NILs) that possess an intriguing

shockwave absorption property upon laser- induced shockwave. The nano-segregated

microstructure of quaternary ammonium network-forming ionic liquids (NILs) is tunable and

depends on the length of the side alkyl chains. The soft alkyl domain in NILs played an important

role in absorbing shockwaves. A shock-induced ordering in the NIL with the longest (hexyl) side

chain was observed, indicating that both nano-segregated structure and shock-induced ordering contribute to NIL’s shockwave absorption performance. The glass transition temperature of the

NILs also varied with alkyl chain length, indicating that the molecular structure influences the relaxation behavior. For NILs, determining the contribution of structure to energy dissipation is complex since the dynamics of segmental motion in polymers are an important factor in determining energy absorbing performance.[20-22] In this chapter, we more systematically explore

the effects of polymer structure on shockwave energy dissipation through the selection of PILs

that exhibit similar dynamics regardless of chain size. Charged domains and non-polar domains in

PILs are also nano-segregated and the morphology can be tuned by the molecular structure of the

polymer.[23-24]

6.2 Experimental Methods

6.2.1 Materials 1

A series of polymerized ionic liquids was synthesized via simple step-growth polymerization

between alkyl bis(imidazole) and dibromide starting materials as depicted in Figure 6.1. A

stoichiometric mixture of bis(imidazole) and dibromide were dissolved in DMF and heated to 80°

C for 48 hours to yield the corresponding PIL with Br– as the counterion. The bromide anion was

exchanged for bis(triflimide) by stirring the resulting PILs in methanol with excess lithium

1 Synthesis was performed by Vivian Lau in University of Illinois at Urbana-Champaign at the Department of Chemistry. 79

bis(triflimide) for 24 hours, then precipitated into deionized water, and freeze-dried to yield thick

syrup-like polymers. The backbone methylene unit number, X, designates a name of each PIL as

PIL-CX. The PILs were characterized by 1H NMR and SEC-MALLS, giving Mn values of 15.8,

16.7, 17.3, and 15.9 kg/mol for PIL-C6, C8, C12 and C16, respectively.

Figure 6.1. Synthesis of polymerized ionic liquids with different chain length spacers

6.2.2 Sample preparation for laser-induced shockwave test

The specimen configuration for shockwave test is shown schematically in the prior chapter in

Figure 2.1. The two glass substrates were cleaned by a piranha solution (a mixture of sulfuric acid and hydrogen peroxide). The energy absorbing layer was prepared by depositing a 400 nm aluminum layer on the glass substrate by electron beam deposition, and a 200 nm aluminum layer was deposited on the other glass substrate for reflection. A sodium silicate solution was spin coated on the 400 nm aluminum layer to provide a 6 µm thick transparent water glass confining layer.

PIL-C8, -C12, and -C16 test specimens were prepared by drop casting 25 mg of each PIL on the glass substrate and sandwiching between the two glass substrates with a 50 µm thick polyimide spacer to keep the thickness of PIL layer. Gentle pressure was applied on both glass substrates using binder clips at least for 30 minutes before laser-induced shockwave testing. In a similar manner, melted PIL-C6 was drop cast on the glass substrate and sandwiched between the two glass substrates with a 50 µm thick polyimide spacer. The amorphous PIL-C6 sample was tested by the laser-induced shockwave test setup within an hour of preparation, and the crystalline PIL-C6 was tested after 24 hours of sample preparation, which offers enough time for crystallization. 80

6.2.3 Material characterization of PILs

For crystalline samples, X-ray scattering experiment was conducted using Rigaku Miniflex

600 X-ray diffractometer with Cu Kα radiation. A thin layer of crystalline sample was prepared by drop-casting small amount of fresh melted PILs on the quartz substrate and keep them at the room temperature until being crystallized. Amorphous PIL-C6 sample was prepared via heating up above the melting temperature before filling into a capillary tube. Other amorphous PIL samples were prepared in a capillary tube for wide-angle X-ray scattering. Wide-angle X-ray measurement was performed at a benchtop X-ray scattering setup in the MRL at UIUC using 8 keV Xenocs

GeniX3D Cu Kα ultralow divergence X-ray source and PerkinElmer XRPad 4343F detectors.

A TA Instrument Q20 Differential Scanning Calorimeter equipped with a Liquid Nitrogen

Cooling System (LNCS) was used to measure glass transition temperatures of PILs. 2 cycles were run for each DSC measurement. Each cycle includes single heating and cooling process

Temperature range -100~100oC with heating/cooling rate 10oC/min. The inflection point of the

step from the second heating scan was used to determine the glass transition temperatures. Melted

PIL-C6 was placed in the desiccator at the room temperature over 24 hours before running DSC measurement to detect the melting of PIL-C6.

6.3 Results and Discussion

6.3.1 Phase behavior of PILs depending on the length of alkyl spacer

The melting temperature of PIL-C6 measured by DSC trace is 321 K (Figure 6.2). Solid PIL-

C6 melts upon heating, and subsequent cooling of the molten PIL-C6 results in an amorphous

phase as shown in Figure 6.3. Nucleation starts within a few hours, followed by transforming into

a crystalline phase. X-ray diffraction data supports the amorphous-to-crystalline phase transformation at room temperature. A slow crystallization process with a few nuclei creates large grains, which minimizes shockwave energy dissipation from grain boundaries, and allows us to examine shockwave energy dissipation properties for each phase individually.

Figure 6.2. DSC experiments for the PIL-C4 and PIL-C6 crystallized in the first heating cycle. o Both were run at 10 C/min rate. Endothermic peaks indicate the melting of crystalline phase. PIL-C4 has higher melting temperature, 358 K as a peak maximum, than PIL-C6, 321 K as a peak maximum.

Figure 6.3. Amorphous and crystalline characteristics of PIL-C6. Optical images of (a) amorphous and (b) crystalline PIL-C6. (c) X-ray diffraction data of PIL-C6 indicates that the crystalline phase is formed after 24 hours at room temperature.

PILs with a shorter alkyl chain favor a highly ordered microstructure, while random

morphology is preferred for PILs with a longer alkyl chain at room temperature. Both PIL-C4 and

-C6 prefer a crystalline phase at room temperature with the high melting temperature (Figure 6.2).

In contrast, the PILs with a longer alkyl side chain than PIL-C6 form an amorphous phase at room temperature. Melting temperatures of PIL-C8, -C12, and -C16 are not found in the temperature range 173.15 K to 373.15 K by differential scanning calorimeter.

6.3.2 Nano-scale heterogeneity of amorphous PILs

The microstructural features of PILs were analyzed using powder X-ray diffraction (PXRD) and correlated with stress wave energy dissipation. Figure 6.4a shows the X-ray scattering intensity as a function of Q for amorphous PIL-C6, -C8, -C12, and -C16, indicating three main peaks, peak

I at Q ≈ 0.3-0.5 Å-1, peak II at Q ≈ 0.8-0.9 Å-1, and peak III at Q ≈ 1.4 Å-1. As the alkyl backbone spacer length increases, peak II slightly shifts to a lower Q with weaker intensity, representing a somewhat larger separation between anions.[25] Peak III, the amorphous halo,

remains unchanged regardless of the length of the alkyl spacer. Only two peaks are observed in

PIL-C6, while for PIL-C8, an additional small peak appears on the lower Q shoulder of the peak

II. For monomeric ionic liquids, peak I is generally considered to correspond to the separation

between charged aggregates surrounded by non-polar regions.[16, 25] Identical nanoscale structural

heterogeneity associated with peak I is also found in polymeric ionic liquids.[26-27] As the number of methylene groups in an alkyl chain spacer increases up to 16, backbone-to-backbone separation increases, causing the growth of non-polar domains along with the development of structural heterogeneity. Lopes et al. reported that for imidazolium-based ionic liquids, homogeneous

domains are formed with a short alkyl chain.[18] As the length of alkyl chain increases, non-polar domains are formed and grow due to aggregation of the alkyl chains, which develops phase

separated nanostructure with the creation of charged clusters. Based on the intensity of peak I for

PIL-C12, the non-polar domains start to separate the charged aggregates. As shown in Figure 6.4a

for PIL-C16, there is a corresponding increase in intensity and a large shift of peak I toward lower

Q.[18, 28] Comparing the peak I of PIL-C12 and PIL-C16, the correlation length shifts from 13.6 Å

to 17.4 Å with a significant increase in the intensity, supporting not only the formation of more

charged clusters but also a larger separation.

The glass transition temperatures (Tg) of the PILs were measured by differential scanning

calorimetry (DSC) (Figure 6.4b). In contrast to the NILs reported previously by Yang et al.[29], the

PILs have similar glass transition temperatures of approximately 237 K, independent of the alkyl

[30] spacer length. Linear polyethylene has a Tg=231 ± 9 K , which is comparable to that of the

PILs in this study. Since Tg is associated with the α process characteristic relaxation time, we infer

that the dynamics of the PILs are strongly influenced by the α relaxation dynamics of the

polyethylene-like chain spacers.[31] The dynamics of the NILs studied previously are more

84 strongly influenced by the ionic strength and Van der Waals force since Tg decreased with a decrease in the charge density and an increase in the length of the alkyl chain spacer.

Figure 6.4. Microstructural and thermal analysis of PILs. (a) X-ray diffraction data of a series of PILs shows each PIL has distinct microstructure. (b) Differential scanning calorimetry (DSC) measurements of PILs indicate all PILs have a similar glass transition temperature around 237 K.

6.3.3 Structural effects of PILs on energy dissipation

The shockwave energy dissipation performance of the PILs was examined using a laser- induced shockwave protocol developed previously and described in Chapter 2.[1] As shown in

Figure 6.5b, amorphous PIL-C6 attenuates more pressure than crystalline PIL-C6 across all input laser fluences. The amorphous phase consisting of entangled chains with weak inter-chain Van der

Waals forces is more easily densified by the shockwave, converting shock wave energy to irreversible heat.[32] We hypothesize that the difference in energy dissipation between the amorphous and crystalline morphologies is due to the difference in the mobility of the alkyl chain segments. The amorphous region with higher atomic-scale mobility can have more degrees of freedom to attenuate shockwave energy via various molecular motions.[33]

Figure 6.5. Shockwave energy dissipation properties of PILs. (a) Average energy transferred and (b) average peak pressures at different input laser fluences for PILs.

Amorphous PIL-C6 and PIL-C8 dissipate almost identical amounts of pressure at all fluences

(Figure 6.5b). For both PIL-C6 and PIL-C8, no peak I is observed in the XRD pattern (Figure 6.4a),

indicating that the structural heterogeneity is not well developed. Although the shorter alkyl

spacers in PIL-C6 should result in a stronger correlation between anions due to stronger ionic

strength than PIL-C8, the anion-anion correlations in PILs do not appear to have a strong effect on

energy dissipation.

As the length of the alkyl side chain increases for PIL-C12 and PIL-C16, shockwave energy

dissipation also improves dramatically (Figure 6.5b). At the highest fluence, PIL-C16 shows a 26%

decrease in peak pressure and 42% decrease in energy transferred over amorphous PIL-C6. The

higher intensity of the XRD peak I for the PILs (Figure 6.4a) is strongly correlated to greater

energy dissipation and lower average peak pressures at all input laser fluences. Hence, more

developed structural heterogeneity and charged clusters lead to enhanced shockwave energy

dissipation properties. This result is consistent with a prior simulation of shockwave attenuation

in polyurea with a nano-segregated microstructure that suggests a dissociation of hard domains

under high pressure absorbs shockwave energy.[11] Similarly in the PIL materials, a more ordered

morphology of charged clusters absorbs more energy.

6.4 Conclusions

Nanoscale structure and ordering have a significant effect on the shockwave energy dissipation

of PILs. As the length of the alkyl chain spacer between cationic imidazolium groups increases,

the degree of microstructural heterogeneity increases. However, the dynamics of the PILs as

characterized by Tg, remain similar for the different spacer lengths. We conclude that increased

order of charged clusters in amorphous PILs favor effective energy dissipation, yet the crystalline

phase is much less effective at dissipating energy. Hence, the structure and morphology are critical

for designing soft materials for effective dissipation of shockwave loading.

6.5 References

(1) Yang, K.; Lee, J.; Sottos, N. R.; Moore, J. S., Shock-Induced Ordering in a Nano- segregated Network-Forming Ionic Liquid. Journal of the American Chemical Society 2015, 137, 16000-16003. (2) Grujicic, M.; Snipes, J. S.; Ramaswami, S.; Yavari, R.; Runt, J.; Tarter, J.; Dillon, G., Coarse-grained Molecular-level Analysis of Polyurea Properties and Shock-mitigation Potential. Journal of Materials Engineering and Performance 2013, 22, 1964-1981. (3) Antillon, E.; Strachan, A., Mesoscale simulations of shockwave energy dissipation via chemical reactions. The Journal of Chemical Physics 2015, 142, 084108. (4) Grujicic, M.; Snipes, J.; Ramaswami, S., A computational analysis of the utility of chemical reactions within protective structures in mitigating shockwave-impact effects. Multidiscipline Modeling in Materials and Structures 2016, 12, 438-472.

(5) Banlusan, K.; Strachan, A., Shockwave Energy Dissipation in Metal–Organic Framework MOF-5. The Journal of Physical Chemistry C 2016, 120, 12463-12471. (6) Su, Z.; Shaw, W. L.; Miao, Y.-R.; You, S.; Dlott, D. D.; Suslick, K. S., Shock Wave Chemistry in a Metal–Organic Framework. Journal of the American Chemical Society 2017, 139, 4619-4622. (7) Hatanaka, K.; Saito, T., Numerical analysis of weak shock attenuation resulting from molecular vibrational relaxation. Shock Waves 2011, 21, 121-129. (8) Petel, O. E.; Jetté, F. X.; Goroshin, S.; Frost, D. L.; Ouellet, S., Blast wave attenuation through a composite of varying layer distribution. Shock Waves 2011, 21, 215-224. (9) Zhuang, S.; Ravichandran, G.; Grady, D. E., An experimental investigation of shock wave propagation in periodically layered composites. Journal of the Mechanics and Physics of Solids 2003, 51, 245-265. (10) Tsai, L.; Prakash, V., Structure of weak shock waves in 2-D layered material systems. International Journal of Solids and Structures 2005, 42, 727-750. (11) Arman, B.; Reddy, A. S.; Arya, G., Viscoelastic Properties and Shock Response of Coarse-Grained Models of Multiblock versus Diblock Copolymers: Insights into Dissipative Properties of Polyurea. Macromolecules 2012, 45, 3247-3255. (12) Grujicic, M.; Pandurangan, B., Mesoscale analysis of segmental dynamics in microphase- segregated polyurea. Journal of Materials Science 2012, 47, 3876-3889. (13) Castagna, A. M.; Pangon, A.; Choi, T.; Dillon, G. P.; Runt, J., The Role of Soft Segment Molecular Weight on Microphase Separation and Dynamics of Bulk Polymerized Polyureas. Macromolecules 2012, 45, 8438-8444. (14) Grujicic, M.; Snipes, J. S.; Ramaswami, S.; Yavari, R.; Ramasubramanian, M. K., Meso- scale Computational Investigation of Shock-Wave Attenuation by Trailing Release Wave in Different Grades of Polyurea. Journal of Materials Engineering and Performance 2014, 23, 49- 64. (15) Schroder, U.; Wadhawan, J. D.; Compton, R. G.; Marken, F.; Suarez, P. A. Z.; Consorti, C. S.; de Souza, R. F.; Dupont, J., Water-induced accelerated ion diffusion: voltammetric studies in 1-methyl-3-[2,6-(S)-dimethylocten-2-yl]imidazolium tetrafluoroborate, 1-butyl-3- methylimidazolium tetrafluoroborate and hexafluorophosphate ionic liquids. New Journal of Chemistry 2000, 24, 1009-1015.

(16) Araque, J. C.; Hettige, J. J.; Margulis, C. J., Modern Room Temperature Ionic Liquids, a Simple Guide to Understanding Their Structure and How It May Relate to Dynamics. The Journal of Physical Chemistry B 2015, 119, 12727-12740. (17) Kapoor, U.; Shah, J. K., Globular, Sponge-like to Layer-like Morphological Transition in 1-n-Alkyl-3-methylimidazolium Octylsulfate Ionic Liquid Homologous Series. The Journal of Physical Chemistry B 2018, 122, 213-228. (18) Canongia Lopes, J. N. A.; Pádua, A. A. H., Nanostructural Organization in Ionic Liquids. The Journal of Physical Chemistry B 2006, 110, 3330-3335. (19) Zheng, W.; Mohammed, A.; Hines, L. G.; Xiao, D.; Martinez, O. J.; Bartsch, R. A.; Simon, S. L.; Russina, O.; Triolo, A.; Quitevis, E. L., Effect of Cation Symmetry on the Morphology and Physicochemical Properties of Imidazolium Ionic Liquids. The Journal of Physical Chemistry B 2011, 115, 6572-6584. (20) Bogoslovov, R. B.; Roland, C. M.; Gamache, R. M., Impact-induced glass transition in elastomeric coatings. Applied Physics Letters 2007, 90, 221910. (21) Kim, H.; Hambir, S. A.; Dlott, D. D., Shock Compression of Organic Polymers and Proteins: Ultrafast Structural Relaxation Dynamics and Energy Landscapes. The Journal of Physical Chemistry B 2000, 104, 4239-4252. (22) Gao, Y.; Williams, D. R. M.; Sevick, E. M., Dynamics of molecular shock-absorbers: energy dissipation and the Fluctuation Theorem. Soft Matter 2011, 7, 5739-5744. (23) Liu, H.; Paddison, S. J., Direct Comparison of Atomistic Molecular Dynamics Simulations and X-ray Scattering of Polymerized Ionic Liquids. ACS Macro Letters 2016, 5, 537-543. (24) Weber, R. L.; Ye, Y.; Schmitt, A. L.; Banik, S. M.; Elabd, Y. A.; Mahanthappa, M. K., Effect of Nanoscale Morphology on the Conductivity of Polymerized Ionic Liquid Block Copolymers. Macromolecules 2011, 44, 5727-5735. (25) Hettige, J. J.; Araque, J. C.; Margulis, C. J., Bicontinuity and Multiple Length Scale Ordering in Triphilic Hydrogen-Bonding Ionic Liquids. The Journal of Physical Chemistry B 2014, 118, 12706-12716. (26) Evans, C. M.; Bridges, C. R.; Sanoja, G. E.; Bartels, J.; Segalman, R. A., Role of Tethered Ion Placement on Polymerized Ionic Liquid Structure and Conductivity: Pendant versus Backbone Charge Placement. ACS Macro Letters 2016, 5, 925-930.

(27) Choi, U. H.; Lee, M.; Wang, S.; Liu, W.; Winey, K. I.; Gibson, H. W.; Colby, R. H., Ionic Conduction and Dielectric Response of Poly(imidazolium acrylate) Ionomers. Macromolecules 2012, 45, 3974-3985. (28) Tamami, M.; Salas-de la Cruz, D.; Winey, K. I.; Long, T. E., Structure–Property Relationships of Water-Soluble Ammonium–Ionene Copolymers. Macromolecular Chemistry and Physics 2012, 213, 965-972. (29) Yang, K.; Tyagi, M.; Moore, J. S.; Zhang, Y., Odd–Even Glass Transition Temperatures in Network-Forming Ionic Glass Homologue. Journal of the American Chemical Society 2014, 136, 1268-1271. (30) Davis, G. T.; Eby, R. K., Glass transition of polyethylene: Volume relaxation. Journal of Applied Physics 1973, 44, 4274-4281. (31) Priestley, R. D.; Broadbelt, L. J.; Torkelson, J. M.; Fukao, K., Glass transition and $\ensuremath{\alpha}$-relaxation dynamics of thin films of labeled polystyrene. Physical Review E 2007, 75, 061806. (32) Bourne, N. K., On the Shock Response of Polymers to Extreme Loading. Journal of Dynamic Behavior of Materials 2016, 2, 33-42. (33) Elder, R. M.; O’Connor, T. C.; Chantawansri, T. L.; Sliozberg, Y. R.; Sirk, T. W.; Yeh, I.- C.; Robbins, M. O.; Andzelm, J. W., Shock-wave propagation and reflection in semicrystalline polyethylene: A molecular-level investigation. Physical Review Materials 2017, 1, 043606.

CHAPTER 7

SUMMARY AND FUTURE WORK

7.1 Summary of Thesis Research

New strategies for effective shockwave energy dissipation using mechanoresponsive materials

were explored in this dissertation. Network-forming ionic liquids (NILs) and dynamic PDMS

networks were synthesized to investigate energy dissipation mechanisms by both shock-induced

physical and chemical reactions. Polymerized ionic liquids (PILs) were examined to understand

the effects of structure at nanoscale on energy dissipation.

A laser-induced shockwave test was developed to evaluate the shockwave energy dissipation

properties of mechanoresponsive materials. Compression shockwaves with a wide range of the

peak pressure and duration time were generated by varying the thickness of fused silica substrates

and launch laser fluence. The energy dissipation performance of polyurea was measured by the

laser-induced shockwave technique, which offers a benchmark value to evaluate the energy

dissipation properties of testing material.

Chemical and physical reactions in mechanoresponsive materials caused by the shockwave

impact were explored to find the possibility of efficient energy dissipation. The formation of new

heterogeneous phases consisting of charged clusters and nonpolar domains was observed in NIL

5-6 under shock loading. We demonstrated this shock-induced ordering change contributes to the

better shockwave absorption performance in the initial shock of NIL 5-6.

The dynamic PDMS networks containing boronic ester bonds, which allows polymer networks to self-heal damages due to reversible bond exchange reactions, exhibited superior energy dissipation performance compared to the benchmark polyurea and the covalently crosslinked

PDMS. We hypothesized that the dissociation of boronic ester bonds in dynamic PDMS networks

is the main mechanism for its energy dissipation performance.

The potential of using a laser-induced shockwave to induce the nucleation of supercooled liquids is explored. The facilitated crystallization was observed only for PEB with the lowest rotational energy barrier of the terminal phenyl rings among supercooled liquids.

The ability of nano-segregated PILs to dissipate shockwave energy is investigated for a series of imidazolium-based PILs with varying alkyl spacer length. We observed that the crystalline phase is less effective at dissipation energy than the amorphous phase at dissipating energy due to slower kinetics. The PILs are designed to have similar glass transition temperature but different nano-scale morphologies depending on the length of alkyl spacer. Laser-induced shockwave test of the PILs revealed that greater structural heterogeneity, associated with more charged clusters, results in greater energy dissipation.

7.2 Future Work

7.2.1 Effects of relaxation time on energy dissipation using supercooled liquids

Development of effective shockwave energy dissipation materials have been explored in this dissertation and we observed that polymer dynamics play an important role to determine energy

dissipation performance under shockwave impact with a nanosecond duration. We found that NILs with faster relaxation attenuated more energy (Chapter 3). Bogoslovov et al. also demonstrated that the dynamics are important to determine the energy dissipation performance upon high-strain rate impact.[1] Kim et al. reported that a short duration shock pulse causes structural relaxation by

ultrafast plastic deformation and a small shock viscosity.[2] According to Dlott, ultrafast nanoshock spectroscopy is realizable by detecting the molecular response to picosecond time scale

shockwave.[3] If the time scale of a shockwave is commensurate with molecular relaxation time

and a large tan δ, the system can dissipate much of the force and energy via molecular relaxations.

Supercooled liquids are good candidate materials since for exploring energy dissipation by

molecular relaxations these liquids have fast dynamics and a wide range of relaxation times.[4-5]

Figure 7.1a contains the molecular structures of several supercooled liquids. When liquid is rapidly

cooled down below its melting temperature, a rearrangement of molecules occurs slowly such that

they cannot form the thermodynamically stable microstructure which holds the lowest free energy within a time frame allowed by the quenching rate, resulting in a supercooled liquid. Two classes of supercooled liquids, strong and fragile, were proposed by Angell depending on liquid behavior during cooling.[6] As depicted in Figure 7.1b, viscosities of strong supercooled liquids follow an

Arrhenius relationship over all temperatures. In contrast, fragile liquids show a different trend. At

high temperature where the reorientation of molecules is independent of interaction with neighbor

molecules, the viscosity follows an Arrhenius temperature dependence. After further cooling, there

is a deviation from Arrhenius behavior at some temperature, called Arrhenius temperature (TA),

where super-Arrhenius behavior begins. At this onset temperature, the transition from non- cooperative to cooperative relaxation dynamics commences and then viscosity of liquids increases

13 [7] rapidly upon approaching to Tg, reaching 10 poise.

Figure 7.1. (a) Chemical structure of common fragile supercooled liquids including n-butyl benzene (BB), propylene carbonate (PC), salol, toluene, ethanol, ortho-t-phenyl (OTP), and benzophenone (BP). (b) Viscosities of supercooled liquids as a function Tg-scaled temperature. Some of supercooled liquids (strong) exhibit linearity directly associated with Arrhenius behavior with a temperature-invariant activation energy. Organic supercooled liquids (fragile) show non-linearity, super-Arrhenius behavior with a temperature-dependent activation energy at a low temperature range, while as temperature increases, Arrhenius behavior becomes dominant. [8]

At high temperature, the relaxation time can be predicted from the viscosity according to the

Maxwell relation indicating the relaxation time (τ) is proportional to viscosity (η),

η τ = (1) G∞

[9] where G∞ is glassy modulus. However, liquid molecules start to cooperatively interact as the temperature decreases and at some point this relation breaks down, which requires the experimental measurement of relaxation time for each supercooled liquid molecule.[8] The Rössler

group has developed the depolarized the dynamics light scattering technique combined with a

tandem-Fabry-Perot interferometer and a double monochromator to probe the relaxation time of

molecular reorientation down to picoseconds.[10-12] Figure 7.2 shows experimentally measured

relaxation times for supercooled liquids as a function of temperature, exhibiting a wide range of

relaxation time up to 16 decades. At room temperature, many of supercooled liquids possess

94 relaxation times as short as 10-8 – 10-12 s, which is comparable with shockwave nanoseconds duration time.

Figure 7.2. Relaxation time of supercooled liquids as a function of (a) inverse temperature and (b) temperature. [4]

In preliminary studies, the shockwave energy dissipation performance of supercooled liquids was evaluated. A wide spectrum of energy dissipation properties of supercooled liquids was measured, exhibiting that butylbenzene has the best performance among them, while OTP dissipates the least amount of energy as depicted in Figure 7.3a. Figure 7.3b shows superior energy dissipation performance of butylbenzene as same as PDMS7-B-DR and better than polyurea as a benchmark and NIL 5-6.

Figure 7.3. Average peak pressure plot as a function of launch laser fluence for 50 µm thick (a) supercooled liquids and (b) polyurea, PDMS7-B-DR, butylbenzene, and NIL 5-6.

The mechanism of mitigating shockwave energy by supercooled liquids is unclear although these exhibit a potential for promising shock absorbing materials. Figure 7.4a shows average peak pressures of supercooled liquids at 69 mJ/mm2 launch laser fluence. The overall trend that

supercooled liquids with a faster relaxation dissipate more energy appears and these relaxation

time-scales are shorter than the duration of shockwave (10 – 20 ns). Similarly, Figure 7.4b reveals

that average peak pressure decreases with TA, where the cooperative interaction begins. However, butylbenzene has the lowest average peak pressure, butan intermediate TA among them.

Interestingly, average peak pressures of salol, benzophenone, propylene carbonate, and toluene

reduce linearly as a log-scale relaxation time increases, and butylbenzene, ethanol, and OTP also

exhibit a linear relation with relaxation time. Except for butylbenzene, the supercooled liquids

show a linear reduction of average peak pressure as TA decreases. We hypothesized that the

shockwave energy dissipation properties of supercooled liquids may be strongly associated with

their dynamics although detailed mechanism still needs to be clarified. Chen et al. proposed that

the movement and displacement of liquid molecules contribute to mitigate the peak pressure of

shockwave for water.[13] The reorientation and displacement of liquid molecules can occur under

the shockwave impact, provided that the relaxation time of the liquids are shorter than the duration of a shockwave. However, the proposed explanation fails to describe energy dissipation performance of butylbenzene and ethanol because of their relatively slower relaxation compared to toluene and propylene carbonate, respectively.

Figure 7.4. Average peak pressure at 69 mJ/mm2 launch laser fluence for 50 µm thick supercooled liquids as a function of (a) relaxation time at room temperature and (b) Arrhenius temperature (TA). Relaxation times of supercooled liquids are extracted from the data by Rössler [4] [4, 7, 14] group. TA values are obtained from the references.

7.2.2 Soft composites

Butylbenzene and other supercooled liquids possess promising energy dissipation properties,

but is difficult to incorporate in a material. Our area for further investigation is the fabrication of

composites containing supercooled liquid for energy dissipation. Supercooled liquids with a low

viscosity and a small molecular volume favor dissolving in polymer matrix precursors, which

limits producing the phase-separated liquid inclusions in a solid matrix. Use of a porous polymer

matrix or gel is one potential solution to accommodate supercooled liquids, schematically depicted

in Figure 7.5.

Figure 7.5. The schematic of composite containing supercooled liquids for shockwave energy dissipation.

7.2.3 Variation of relaxation time

Varying duration time of a shockwave is another approach to explore effects of relaxation time

on energy dissipation. Several techniques including tuning of the laser cavity, optical laser-pulse-

stretching, and altering substrate thickness have been introduced to change duration time of

shockwave in laser-induced shockwave test. [15-16] We have already found out substrate thickness

changes the duration time of a shockwave in a non-linear substrate, such as fused silica as

described in Chapter 2. By adjusting the thickness of substrate and the laser fluence, 15 -60 ns

shockwave durations can be achieved from a 3 ns input laser pulse width. Other techniques can be

applied to further expand duration time over the limit by substrate thickness technique. In the ring

cavity laser-pulse-stretching system as depicted in Figure 7.6, optical components split the incident

laser pulse into two or more pulses, and recombine them with optical delay, which results in a

stretched pulse. [15] The percentage of energy transferred and peak pressure attenuation as a function of duration time will be compared to determine duration-time dependent shockwave

dissipation for test materials.

Figure 7.6. Schematic of laser-pulse stretching system based on a triangular ring cavity. M indicates mirror, and BS is beam splitter. [15]

7.2.4 In-situ spectroscopy studies and high-speed imaging under laser-induced shockwave

Real-time probing the behavior of materials under shockwave loading is important to understand the fundamentals of energy dissipation and develop efficient impact-absorbing materials. This dissertation focused on characterizing permanent chemical or physical changes after the impact, while transient responses and dynamics of materials were not analyzed due to technical constraints. The Dlott group has developed an in-situ fluorescent measurement technique to characterize the dynamics of polymers under shockwave loading combined with laser-launched flyer plates (Figure 7.7).[17] Transient red shifts of organic dyes dispersed in a polymer matrix

under shock compression were detected with a nanosecond time interval. The dynamic local

density changes induced by a shockwave pressure cause redshifted dye emission, offering clues to elucidate dynamic relaxation processes within nanoseconds. Compared to Raman real-time

measurement setup, fluorescent emission is a great deal more sensitive such that a higher degree

of time resolution can be studied. In future work, the transient responses of materials under

shockwave by adding the time-resolved fluorescent measurement system to the laser-induced

shockwave test setup. The simultaneous energy dissipation and spectroscopy measurements will provide better understanding of shock-induced ordering changes in NILs (Chapter 3), dynamic exchange bonds (Chapter 4), microstructural changes in PILs (Chapter 6), and dynamics of supercooled liquids (Chapter 7.2.1), associated with their shockwave energy dissipation performance.

Figure 7.7. (a) Schematic of experimental apparatus of the real-time fluorescent measurement technique with laser-induced flyer plates. Dichroic beam splitter (DBS), objective lens (OBJ), and photonic doppler velocimeter (PDV). (b) Schematic of sample structure and emission probing. [17]

High-speed imaging of shockwave propagation in materials would offer insights to understand the behavior of materials under high-strain rate stress wave lasting only a few nanoseconds. The

Nelson group in MIT developed the imaging technique to capture high-speed impact dynamics

using a high-speed camera as shown in Figure 7.8.[18-19] The high-frame rate camera under the

illumination by a pulsed laser with a µs duration records multiple images with an interval as short

as 35 ns. The high-frame rate camera consists of several CCDs that each can be triggered independently.

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In the laser-induced shockwave system, an interval time between each image needs to be shorter than a few ns because the duration time-scale of shockwave is only 10 – 20 ns. By employing a higher amplitude pulsed imaging laser and a shorter interval time between frames, high-frame-rate images of shockwave propagation in a specimen can be captured, which allows to obtain the more accurate Hugoniot data and understand transient elastic/plastic deformations under shockwave in real-time.

Figure 7.8. High-speed impact real-time imaging configuration to capture particle dynamics. Two pulsed lasers are used in this system: 800 nm wavelength laser pulse with a 300 ps duration for launching silica particles and 527 nm wavelength laser imaging pulse with a 1.5 µs duration.[19]

7.3 References

(1) Bogoslovov, R. B.; Roland, C. M.; Gamache, R. M., Impact-induced glass transition in elastomeric coatings. Applied Physics Letters 2007, 90, 221910. (2) Kim, H.; Hambir, S. A.; Dlott, D. D., Shock Compression of Organic Polymers and Proteins: Ultrafast Structural Relaxation Dynamics and Energy Landscapes. The Journal of Physical Chemistry B 2000, 104, 4239-4252.

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(3) Dlott, D. D., Nanoshocks in Molecular Materials. Accounts of Chemical Research 2000, 33, 37-45. (4) Schmidtke, B.; Petzold, N.; Kahlau, R.; Hofmann, M.; Rössler, E. A., From boiling point to glass transition temperature: Transport coefficients in molecular liquids follow three-parameter scaling. Physical Review E 2012, 86, 041507. (5) Mallamace, F.; Branca, C.; Corsaro, C.; Leone, N.; Spooren, J.; Chen, S.-H.; Stanley, H. E., Transport properties of glass-forming liquids suggest that dynamic crossover temperature is as important as the glass transition temperature. Proceedings of the National Academy of Sciences 2010, 107, 22457. (6) Angell, C. A., Formation of Glasses from Liquids and Biopolymers. Science 1995, 267, 1924. (7) Novikov, V. N., Connection between the glass transition temperature Tg and the Arrhenius temperature TA in supercooled liquids. Chemical Physics Letters 2016, 659, 133-136. (8) Debenedetti, P. G.; Stillinger, F. H., Supercooled liquids and the glass transition. Nature 2001, 410, 259. (9) Roland, C. M.; Ngai, K. L., Short-time viscous and density relaxation in glycerol and ortho- terphenyl. The Journal of Chemical Physics 1997, 106, 1187-1190. (10) Cummins, H. Z.; Li, G.; Hwang, Y. H.; Shen, G. Q.; Du, W. M.; Hernandez, J.; Tao, N. J., Dynamics of supercooled liquids and glasses: comparison of experiments with theoretical predictions. Zeitschrift für Physik B Condensed Matter 1997, 103, 501-519. (11) Adichtchev, S. V.; Benkhof, S.; Blochowicz, T.; Novikov, V. N.; Rössler, E.; Tschirwitz, C.; Wiedersich, J., Anomaly of the Nonergodicity Parameter and Crossover to White Noise in the Fast Relaxation Spectrum of a Simple Glass Former. Physical Review Letters 2002, 88, 055703. (12) Petzold, N.; Rössler, E. A., Light scattering study on the glass former o-terphenyl. The Journal of Chemical Physics 2010, 133, 124512. (13) Chen, Y.; Huang, W.; Constantini, S., Blast Shock Wave Mitigation Using the Hydraulic Energy Redirection and Release Technology. PLOS ONE 2012, 7, e39353. (14) Roland, C. M., Characteristic relaxation times and their invariance to thermodynamic conditions. Soft Matter 2008, 4, 2316-2322.

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(15) Wang, T.; Kumavor, P. D.; Zhu, Q., Application of laser pulse stretching scheme for efficiently delivering laser energy in photoacoustic imaging. Journal of Biomedical Optics 2012, 17, 061218. (16) Wang, J.; Weaver, R. L.; Sottos, N. R., A parametric study of laser induced thin film spallation. Experimental Mechanics 2002, 42, 74-83. (17) Banishev, A. A.; Shaw, W. L.; Dlott, D. D., Dynamics of polymer response to nanosecond shock compression. Applied Physics Letters 2014, 104, 101914. (18) Hassani-Gangaraj, M.; Veysset, D.; Nelson, K. A.; Schuh, C. A., Melting Can Hinder Impact-Induced Adhesion. Physical Review Letters 2017, 119, 175701. (19) Veysset, D.; Hsieh, A. J.; Kooi, S.; Maznev, A. A.; Masser, K. A.; Nelson, K. A., Dynamics of supersonic microparticle impact on elastomers revealed by real–time multi–frame imaging. Scientific Reports 2016, 6, 25577.

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