The phase behavior of poly(ethylene-alt-propylene)-block-polydimethylsiloxane (PEP-PDMS) diblock copolymers at low temperatures reveals a complex interplay between thermodynamics, kinetics, and structural stability. Despite the FK phase being the equilibrium packing below 110 °C, experimental observations show that cooling the system to sub-ambient temperatures leads to significant deviations from this expected state. Quenching from disorder into the low-temperature regime results in a nonergodic liquid-like packing (LLP), characterized by a broad, intense scattering peak and solid-like mechanical response—indicative of a supercooled soft glass.
Extended annealing at temperatures below 80 °C is required for the FK phase to fully develop, with ordering times exceeding one year at 40 °C. This extreme kinetic retardation arises from suppressed chain exchange below the ergodicity temperature (Terg), which arrests mass redistribution and prevents the system from reaching its thermodynamic minimum. As a result, the material becomes trapped in metastable configurations such as the DDQC or LLP, even though these are not the lowest-energy states.
Detailed SAXS analysis during stepwise cooling confirms that the FK phase remains intact up to 40 °C but begins to degrade upon further cooling to −15 °C. The scattering pattern exhibits pronounced peak broadening and a shift to lower q-values, corresponding to a 7% increase in mean particle radius and a 33% rise in aggregation number (Nagg). These changes are consistent with an ongoing structural reorganization driven by the need to minimize interfacial area per chain. However, the loss of translational order and increased defect density suggest that the process involves lattice disruption rather than simple growth.Acid phosphatase/ACP1 Antibody supplier
To resolve this issue, we performed repeated thermal cycles: after reheating the sample to room temperature and annealing for 115 days, the well-resolved FK phase was fully recovered. A second cooling cycle to −15 °C and subsequent annealing for 243 days retained the FK structure, albeit with slightly broader peaks, indicating reduced grain size. This demonstrates that the observed distortions are not due to irreversible damage but stem from dynamic adjustments in micelle size and distribution during cooling.
The reduction in optical birefringence reported in earlier studies below 80 °C can now be explained: structural degradation and grain size reduction disrupt long-range orientational order, diminishing the birefringent signal.CK II alpha Antibody Technical Information Moreover, the persistence of LLP at low temperatures highlights the role of particle size distribution in stabilizing disordered states.PMID:34106300 The presence of a multimodal Nagg distribution, necessary for space-filling in the FK phase, likely contributes to kinetic frustration, preventing rapid equilibration.
Importantly, the absence of a clear transition from BCC to FK under shear at low temperatures suggests that the system’s response is dominated by local rearrangements rather than bulk phase transformations. The high resistance of the FK phase to shear-induced disorder reflects its intricate internal structure—comprising 30 particles per unit cell with five distinct geometries and volumes—making it inherently more stable against deformation than the simpler BCC phase.
These findings underscore a fundamental principle: in systems with topologically close-packed structures, the balance between thermodynamic stability and kinetic accessibility determines the observed morphology. At low temperatures, where chain mobility is severely restricted, the system cannot overcome energy barriers to achieve full crystallization, leading to prolonged arrest in metastable states. This phenomenon has implications beyond block copolymers, extending to other self-assembling soft materials where slow dynamics govern phase evolution.
In summary, this work reveals that low-temperature phase behavior in PEP-PDMS is governed not by thermodynamic preference alone, but by the competition between structural complexity and kinetic limitations. Understanding these dynamics is essential for designing materials with predictable self-assembly pathways and for avoiding misinterpretations in phase identification based on incomplete equilibration.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com