Mechanistic Insights into Bending of Soft Molecular Crystals: Crystal Engineering Approach for Property Tuning

Das, Susobhan (2022) Mechanistic Insights into Bending of Soft Molecular Crystals: Crystal Engineering Approach for Property Tuning. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Susobhan Das (16RS051)) 16RS051.pdf - Submitted Version Restricted to Repository staff only Download (15MB)

Abstract

Brittle fracture of crystals under mechanical stress greatly limits their use in devices, particularly in flexible devices. It remains a challenge to combine crystallinity and flexibility in materials as crystallinity leads to brittle nature. Hence, preparing mechanically flexible organic crystals is not only a challenge in materials chemistry but also an important aspect from the fundamental understanding point of view. Chapter 2 presents insights into the elastic bending in cocrystal solvate of caffeine:4-chloro-3-nitrobenzoic acid:methanol (1:1:<1), from careful investigation of the structural changes using high resolution micro-focus X-ray diffraction, μ-IR and μ-Raman studies. Structure-property correlation in a wide variety of elastic molecular crystals indicates that presence of isotropic corrugated structures dominated by weak dispersive interactions would allow elastic flexibility as such structures not only prevent slippage but also act as structural buffers to absorb stress. Countering these propositions, Worthy et al. claimed that these criteria are “incorrect” as the crystals of [Cu(acac)2] are anisotropic with no interlocking, but show elastic bending (within 1% strain). My detailed experiments on crystals of [Cu(acac)2] revealed that these crystals readily undergo plastic bending deformation on (101) face, similar to other well-known anisotropic plastically bendable crystals. These observations contradict Worthy et al.‘s own conclusions. Therefore, I conclude that the [Cu(acac)2] crystal is not an ideal model system for rejecting the prevailing—and widely accepted—molecular mechanisms for exceptional elastic flexibility of organic crystals. The Chapter 4 covers a unique dynamic crystal, involving a heavy transition metal element rhenium, namely, dimethylammoniumperrhenate. At ambient conditions, the crystals show an initial elasticity (within ~ 1 % strain), followed by elasto-plastic deformation at higher external stresses, but when heated, undergo an ordered-to-disordered phase transition, forming a rotator or plastic crystalline phase, thus allowing mechanical thinning under compressive stress. I have shown a systematic approach for designing a range of known deformation responses, such as brittle, elastic, plastic and shearing type using basic principles/art of crystal engineering, in Chapter 5. This paves the way not only for designing mechanically responsive molecular crystals but also helps to access the next generation materials for technological applications, like flexible electronics, soft robotics, 3D printing, solid state coolants, etc.

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