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The crystallization and nucleation kinetics of poly(ε-caprolactones) (PCL) with molar masses between 1.4 and 6.1 kDa and negligible number of heterogeneous nuclei has been investigated by differential fast scanning calorimetry (DFSC) applying scanning rates up to 100 000 K/s. The samples were synthesized by ring-opening polymerization and chemically characterized by NMR spectroscopy, size exclusion chromatography (SEC), and multiangle laser light scattering (MALLS). For the smallest molar mass the chain length is comparable with the crystal thickness measured with small-angle X-ray scattering (SAXS), and extended chain like crystals may be formed. Because of the molar mass distribution (PDI ≈ 2), these crystals have a significant noncrystalline interface yielding nearly the same crystallinity for all molar masses. The critical cooling rate to obtain amorphous samples is below 1000 K/s and only for the lowest molar mass increased to 2000 K/s. The same trend holds for the about 1 order of magnitude higher critical heating rate to keep the samples amorphous on heating and for the analysis of isothermal nucleation and crystallization kinetics at 202 K. The samples which were shown not to contain heterogeneous nuclei active at a heating rate of >18 000 K/s were used for a study of the nucleation activity of ordered structures formed on annealing at low temperature. The analysis of the change of the thus-produced amorphous polymer samples on annealing from 202 to 272 K for times varying by a factor of more than 108 (0.1 ms to 8.3 h) revealed new details about the ordering processes (nucleation, poor crystal formation, crystallization, cold crystallization, and crystal perfection) and the accompanying changes in glass transition of the remaining amorphous phase (formation of rigid amorphous phases, RAF).

Beta-pleated-sheet crystals are among the most stable of protein secondary structures, and are responsible for the remarkable physical properties of many fibrous proteins, such as silk, or proteins forming plaques as in Alzheimer's disease. Previous thinking, and the accepted paradigm, was that beta-pleated-sheet crystals in the dry solid state were so stable they would not melt upon input of heat energy alone. Here we overturn that assumption and demonstrate that beta-pleated-sheet crystals melt directly from the solid state to become random coils, helices, and turns. We use fast scanning chip calorimetry at 2,000 K/s and report the first reversible thermal melting of protein beta-pleated-sheet crystals, exemplified by silk fibroin. The similarity between thermal melting behavior of lamellar crystals of synthetic polymers and beta-pleated-sheet crystals is confirmed. Significance for controlling beta-pleated-sheet content during thermal processing of biomaterials, as well as towards disease therapies, is envisioned based on these new findings.

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