Competing crystallization pathways and cold crystallization kinetics in 10OS5 liquid crystal

This study investigates the competing crystallization pathways and cold crystallization kinetics of the liquid crystal 10OS5, revealing that its thermal history can be manipulated to tune the energy released during phase transitions, thereby highlighting its potential for thermal energy storage applications.

The Gist

Imagine a special liquid called 10OS5. Think of it not just as a liquid, but as a crowd of tiny, long molecules that love to organize themselves. Sometimes they line up neatly like soldiers (a crystal), sometimes they flow like a messy crowd (a liquid), and sometimes they form a middle ground where they are ordered but still flowing (a liquid crystal).

This paper is like a detective story about how this crowd behaves when we heat it up or cool it down, and how we can "trick" it into storing and releasing energy.

The Main Characters: The "Dance Floors"

The molecules in 10OS5 can stand on different "dance floors" (phases):

• The Liquid Floor: Totally messy and free.
• The Liquid Crystal Floors: They start lining up in rows, but can still slide past each other.
• The Crystal Floors (Cr1 and Cr2): The ultimate party where everyone is frozen in a perfect grid.

The researchers found that there are two types of "frozen" dance floors: Cr1 and Cr2. Both are a bit messy inside (like a messy room where the furniture is arranged but the items are tilted), which is why they are called "conformationally disordered."

The Plot: Cooling Down (The Freeze)

When you cool this liquid down, what happens depends entirely on how fast you turn down the thermostat:

1. Slow Cooling (The Patient Freezer): If you cool it slowly (like 2 degrees per minute), the molecules have plenty of time to find their perfect spots. They form the Cr2 phase first. It's like a crowd slowly finding their seats in a theater.
2. Fast Cooling (The Shock Freezer): If you cool it very quickly (25–30 degrees per minute), the molecules don't have time to organize. They get "frozen" in a messy, jumbled state called a glass. It's like pouring water into a mold and instantly freezing it so the ice crystals never form. The paper calls this the "SmY glass."

The Twist: Heating Up (The Thaw and the Surprise)

Now, here is the magic trick. If you take that "frozen mess" (the glass) or the "messy crystal" (Cr2) and start heating it up, something surprising happens.

Instead of just melting back into a liquid, the molecules suddenly decide to reorganize into a new, better-ordered crystal (Cr1) before they melt. This is called Cold Crystallization.

• The Energy Release: When these molecules snap into their new, organized positions, they release a burst of energy (heat). Think of it like a spring-loaded toy snapping shut; it releases energy when it locks into place.
• The Control Knob: The researchers discovered that by changing how fast they cooled the sample initially, they could control how much energy was released later.
  • If you cool it super fast, you trap a lot of energy in the "glass." When you heat it up, it releases a huge burst of energy as it tries to organize.
  • If you cool it slowly, it organizes a bit on its own, so there is less energy left to release later.

The "Energy Storage" Analogy

Imagine you have a backpack.

• Cooling it fast is like stuffing the backpack with heavy rocks and zipping it up tight. It's unstable and tense.
• Heating it up is like opening the zipper. The rocks (energy) fall out all at once.
• The paper shows that 10OS5 is a backpack you can tune. You can decide exactly how heavy the rocks are and when they fall out, just by changing the speed of your cooling and heating.

The Tools Used

To figure this out, the scientists used two main tools:

1. DSC (The Thermometer): This measures how much heat is absorbed or released. It told them exactly when the molecules were organizing and how much energy was involved.
2. BDS (The Radio): This sends radio waves through the material to see how the molecules are wiggling. It helped them understand if the molecules were just spinning in place or if they were completely stuck. They found that even in the "frozen" crystal states, the molecules were still wiggling a bit (conformational disorder), which explains why they can turn into glass.

The Conclusion

The paper concludes that 10OS5 is a very special material because its behavior is tunable. By simply changing the speed of cooling and heating, scientists can control:

• Which "dance floor" the molecules end up on.
• How much energy is released when they reorganize.
• The temperature at which this energy is released.

The authors suggest that because you can control this energy release so precisely, this material is a great candidate for thermal energy storage. It's like a rechargeable battery, but instead of electricity, it stores and releases heat.

Technical Summary

Technical Summary: Competing Crystallization Pathways and Cold Crystallization Kinetics in 10OS5 Liquid Crystal

Problem Statement
The study investigates the complex phase behavior and crystallization kinetics of 4-pentylphenyl-4'-decyloxythiobenzoate (10OS5), a liquid crystal known for exhibiting nematic, smectic, and tilted hexagonal phases. While previous literature has characterized its mesophases, the interplay between melt crystallization, cold crystallization, and the metastability of intermediate phases (specifically the tilted smectic Y phase and the metastable Cr2 crystal phase) remains insufficiently explored. A primary objective is to determine how thermal history influences the formation of conformationally disordered crystal (CONDIS) phases and to quantify the energy released during cold crystallization, assessing the material's potential for thermal energy storage applications.

Methodology
The research employs a multi-technique approach combining Differential Scanning Calorimetry (DSC) and Broadband Dielectric Spectroscopy (BDS), supported by Density Functional Theory (DFT) calculations.

• Thermal Analysis: DSC was used to analyze non-isothermal and isothermal crystallization processes across cooling and heating rates of 2–30 K/min. Kinetic parameters were extracted using the Avrami model, the Augis-Bennett method, and the isoconversional method to determine activation energies ($E_{cr}$) and the Avrami exponent ($n$).
• Dielectric Spectroscopy: BDS measurements (0.1–10$^7$ Hz) were conducted to distinguish between conformational and orientational disorder in the crystal phases by analyzing relaxation processes and activation energies.
• Computational Modeling: DFT calculations (B3LYP-D3(BJ)/def2TZVPP) were performed to map potential energy surfaces for specific torsional angles, aiding in the interpretation of dielectric relaxation mechanisms.

Key Results

• Thermal History Dependence: The final phase state is highly sensitive to cooling rates.
  • Fast Cooling (25–30 K/min): Suppresses crystallization, resulting in a vitrified tilted smectic Y (SmY) phase with herring-bone order.
  • Slow Cooling (2 K/min): Leads to complete crystallization into a metastable Cr2 phase.
  • Intermediate Rates: Yield mixtures of SmY and Cr2.
• Crystallization Pathways:
  • Melt Crystallization: Upon cooling, the SmY phase transforms into Cr2. Subsequent heating induces a transition from Cr2 to a more stable Cr1 phase.
  • Cold Crystallization: Heating vitrified SmY results in cold crystallization. Depending on the heating rate, this manifests as a direct formation of pure Cr1 or a mixture of Cr1 and Cr2.
  • Kinetics: Isothermal studies reveal that the SmY $\to$ Cr2 transition is rapid ($\tau_{cr} \approx 94-112$ s) with a low activation energy ($E_{cr} \approx -7$ kJ/mol), indicating thermodynamic driving force control. Conversely, the Cr2 $\to$ Cr1 transition is significantly slower ($\tau_{cr} \approx 980-5500$ s) with a high activation energy ($E_{cr} \approx -162$ kJ/mol), suggesting a strong dependence on thermodynamic stability differences.
• Nature of Crystal Phases: Both Cr1 and Cr2 are identified as conformationally disordered crystal (CONDIS) phases. This is evidenced by melting entropy values ($\Delta S_m = 109$ J/(mol·K) for Cr1 and $88$ J/(mol·K) for Cr2) which are lower than the theoretical value for a perfectly ordered crystal, and by dielectric spectra showing relaxation processes consistent with conformational changes rather than pure orientational disorder.
• Glass Formation: The Cr1 phase exhibits a glass transition ($T_g \approx 228$ K) with a high fragility index ($m_f \approx 153$), characteristic of a fragile glassformer. The Cr2 phase also shows glass-forming tendencies, though its $T_g$ is less distinct in DSC thermograms.
• Energy Release: The enthalpy released during cold crystallization ($\Delta H_{cr}$) is tunable. The maximum energy release ($\approx 34.9$ kJ/mol) was observed during isothermal cold crystallization at 248 K. Under non-isothermal conditions, cooling at 30 K/min followed by heating at 6 K/min yielded $\Delta H_{cr} \approx 25.2$ kJ/mol.

Significance and Claims
The paper demonstrates that the energy released during the cold crystallization of 10OS5 can be effectively tuned by manipulating the thermal history (cooling and heating rates). By controlling the fraction of metastable SmY and Cr2 phases, researchers can adjust both the temperature range and the magnitude of energy release. The authors posit that 10OS5 serves as a promising model system for thermal energy storage applications based on metastable phases, offering a mechanism to store energy in a supercooled state and release it upon controlled heating. The study further clarifies the kinetic competition between nucleation and diffusion in liquid crystal crystallization and provides a detailed dielectric characterization of CONDIS phases.