Thermo-mechanical behavior measurement of polymer-bonded sugar under shock compression using in-situ time-resolved Raman spectroscopy

Quantitative information regarding the local behavior of interfaces in an inhomogeneous material during shock loading is limited due to challenges associated with time and spatial resolution. This paper reports the development of a novel method for in-situ measurement of the thermo-mechanical response of polymer bonded sugar composite where measurements are performed during propagagtion of shock wave in sucrose crystal through polydimethylsiloxane binder. The time-resolved measurements were performed with 5 ns resolution providing an estimation on local pressure, temperature, strain rate, and local shock viscosity. The experiments were performed at two different impact velocities to induce shock pressure of 4.26 GPa and 2.22 GPa and strain rate greater than 106/s. The results show the solid to the liquid phase transition of sucrose under shock compression. The results are discussed with the help of fractography analyses of sucrose crystal in order to obtain insights into the underlying heat generation mechanism.

To achieve this, we used a two-step process for embedding crystals in the binder. First, a 100 µm thick layer of PDMS was formed on a glass surface using spin coating which provides a mean to control the depth of crystal inside the binder. The deposited layer was cured at 80 ºC for 4 hours. Later, this layer was used as a base to place sucrose crystals inside the mold as shown in Supplementary Figure S2. The mold was prepared with adhesive putty on one side. The sucrose crystal was placed carefully with face (100) on the thin PDMS layer and touching the adhesive putty on the other side. The adhesive putty act as a barrier to the PDMS binder and provides a surface free of the binder. Finally, the PDMS binder was poured into the mold to complete the embedment followed by curing at 80 ºC for 4 hours. Once fully cured, the adhesive putty is removed and samples were imaged under a microscope to verify the quality of sample preparation.
Supplementary Figure S3

Laser-based projectile launch setup
The laser-based projectile launch system used in this work is a modified version of the system used in our previous work 2 . The principle behind such a mechanism of accelerating projectile to high speeds involves using a pulse laser beam to drive thin metal foils. A Nd: YAG laser from Continuum Lasers was used with a pulse width of 7 ns centered at 1064 nm wavelength and maximum pulse energy of 650 mJ.
The flash lamp of the laser runs continuously at 10 Hz and a single pulse was triggered using a delay streak scope, and PDV system. The beam profile of the laser was modified from Gaussian to flat-top using diffractive optics from HOLO/OR Ltd. The PDV system shown in Supplementary Figure S5 earlier is the same as our previous work 2 .
In order to obtain a planar impact profile from thin metal foils, a spatially homogenized laser spot was created at the launch assembly using a diffuser element from HOLO/OR Ltd (RD-204-I-Y-A). The beam was expanded from 8 mm to 45 mm using a system of telescopic lenses and a spatially homogenized spot of diameter 800 µm was achieved after focusing through an aspheric lens of 150 mm focal length (AL75150-C, Thorlabs Inc.). The beam profile before and after using diffuser optics is shown in

In-situ time-resolved Raman spectroscopy
Supplementary Figure

Calibration of Raman shift with temperature
The calibration of the change in Raman shift for sucrose with temperature was performed using the Raman microscope from Horiba Scientific combined with a hot stage. As shown in Supplementary Figure   S9

Analysis of Time-resolved Raman spectra under shock compression
The streak data was binned over a 5 ns window to resolve Raman shifts and obtain peak location using where ℎ is the thickness of the PDMS layer, is the shock speed in HTPB and is the time of impact on the sample. The total uncertainty for the arrival time of shock wave at the interface can be expressed as, where, ∆ = 1 ns (uncertainty from PDV analysis), ∆ℎ = 10.5 m. The uncertainty in was calculated in Engineering Equation Solver based on impedance matching between aluminum 1100 flyer 9 and PDMS binder 9 . The calculated shock speed for the flyer velocity of 1.21±0.12 km/s and 0.62±0.06 km/s are 3.28±0.15 km/s and 2.5±0.08 km/s respectively. The total uncertainty of 3.7 ns and 4.5 ns is estimated for flyer velocity of 1.21 km/s and 0.62 km/s respectively. Therefore, a maximum resolution of ~5 ns for time-resolved Raman spectroscopy can be obtained in this work.