Andreas Wurm

Beating the Heat - Fast Scanning Melts Silk Beta Sheet Crystals

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 Alzheimers 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.

Formation and disappearance of the rigid amorphous fraction in semicrystalline polymers revealed from frequency dependent heat capacity

For semicrystalline polymers the observed relaxation strength at glass transition is often significantly smaller than expected from the non-crystalline fraction. This observation leads to the introduction of a rigid amorphous fraction (RAF) which does not contribute to the heat of fusion or X-ray crystallinity nor to the relaxation strength at glass transition. The RAF is non-crystalline and in a glassy state at temperatures above the common glass transition. Complex heat capacity in the high frequency limit allows for the measurement of base-line heat capacity also at temperatures above the glass transition. From that the temperature and time dependence of the RAF can be obtained. For PC, PHB and syndiotactic polypropylene (sPP) it is possible to study the creation and disappearance of the RAF in situ during isothermal crystallization and on stepwise melting. If crystallization is not limited by the stability (melting point) of the crystals to be formed the total RAF is created during the isothermal crystallization. Simultaneously with the melting of the smallest crystals the RAF disappears. For these polymers vitrification and devitrification of the non-crystalline material detected as the RAF at glass transition is structural (conformational) and not temperature induced. The formation of the last growing crystals, which melt first, are responsible for the vitrification of the amorphous material around them and, consequently, by that they limit their own growth.

Vitrification and devitrification of the rigid amorphous fraction of semicrystalline polymers revealed from frequency-dependent heat capacity

The relaxation strength at the glass transition shows significant deviations from a two-phase model for semicrystalline polymers. The introduction of a rigid amorphous fraction (RAF), which is noncrystalline but does not participate in the glass transition, allows a description of the relaxation behavior. The question arises when does this amorphous material vitrify. Temperature-modulated differential scanning calorimetry measurements allow the online study of heat capacity changes during isothermal crystallization. For bisphenol-A polycarbonate (PC) and poly(3-hydroxybutyrate) (PHB) at reasonably high modulation frequencies (10 mHz), no contribution from reversing melting to the measured heat capacity was detected at the crystallization temperature; therefore, changes in the baseline heat capacity can be studied. The amount of RAF obtained at the crystallization temperature was compared with that obtained from the step in heat capacity at the glass transition at lower temperatures. No changes in the amount of the RAF occur in the temperature range between crystallization and the glass transition. Consequently, the rigid amorphous material is totally established during the isothermal crystallization of PC and PHB. The reason for the vitrification of the RAF is the immobilization of cooperative motions owing to the fixation of parts of the molecules in the crystallites, which is favorable at the fold surfaces. In this way, crystallization in PC and PHB limits itself by vitrifying the crystallizable material next to the growing crystals. On heating, devitrification of the RAF occurs when the crystals, which were formed last, melt in the temperature range of the lowest endotherm.

Dynamics of reversible melting revealed from frequency dependent heat capacity

Abstract Heat capacity of semi-crystalline polymers shows frequency dependence not only in the glass transition range. Also above glass transition and below melting temperature a frequency dependent heat capacity can be observed. The asymptotic value of heat capacity at high frequencies equals base-line heat capacity while the asymptotic value at low frequencies yield information about reversing melting. For polycarbonate (PC), poly(3-hydroxybutyrate) (PHB) and syndiotactic polypropylene (sPP) the asymptotic value at high frequencies can be measured by temperature-modulated DSC (TMDSC). For polycaprolactone (PCL) and sPP the frequency dependence of heat capacity can be studied in quasi-isothermal TMDSC experiments. The heat capacity spectra were obtained from single measurements applying multi-frequency perturbations (spikes in heating rate) like in StepScan™ DSC or rectangular temperature–time profiles. Actually, the dynamic range of commercial TMDSC apparatuses is limited and only a small part of the heat capacity spectrum can be measured by TMDSC. Nevertheless, comparison of measured base-line heat capacity with expected values from mixing rules for semi-crystalline polymers yield information about the formation (vitrification) and disappearance (devitrification) of the rigid amorphous fraction (RAF). For PC and PHB the RAF is established during isothermal crystallization while for sPP only a part of the RAF is vitrified during crystallization. Devitrification of the RAF seems to be related to the lowest endotherm.

Separation of components of different molecular mobility by calorimetry, dynamic mechanical and dielectric spectroscopy

The relaxation strength at the glass transition for semi-crystalline polymers observed by different experimental methods shows significant deviations from a simple two-phase model. Introduction of a rigid amorphous fraction, which is non-crystalline but does not participate in the glass transition, allows a description of the relaxation behavior of such systems. The question arises when does this amorphous material vitrify. Our measurements on PET identify no separate glass transition and no devitrification over a broad temperature range. Measurements on a low molecular weight compound which partly crystallizes supports the idea that vitrification of the rigid amorphous material occurs during formation of crystallites. The reason for vitrification is the immobilization of co-operative motions due to the fixation of parts of the molecules in the crystallites. Local movements (Β-relaxation) are only slightly influenced by the crystallites and occur in the whole non-crystalline fraction.

CRYSTALLIZATION OF POLYMERS STUDIED BY TEMPERATURE MODULATED CALORIMETRIC MEASUREMENTS AT DIFFERENT FREQUENCIES

Quasi-isothermal temperature modulated DSC (TMDSC) were performed during crystallization to determine heat capacity as function of time and frequency. Non-reversible and reversible phenomena in the crystallization region of polymers were distinguished. TMDSC yields new information about the dynamics of local processes at the surface of polymer crystals, like reversible melting. The fraction of material involved in reversible melting, which is established during main crystallization, keeps constant during secondary crystallization for polycaprolactone (PCL). This shows that also after long crystallization times the surfaces of the individual crystallites are in equilibrium with the surrounding melt. Simply speaking, polymer crystals are ‘living crystals’. A strong frequency dependence of complex heat capacity can be observed during and after crystallization of polymers.

Isothermal Crystallisation of PCL Studied by Temperature Modulated Dynamic Mechanical and TMDSC Analysis

Temperature modulated dynamic mechanical analysis (TMDMA) was performed in the same way as temperature modulated DSC (TMDSC) measurements. As in TMDSC TMDMA allows the investigation of reversible and non-reversible phenomena during crystallisation of polymers. The advantage of TMDMA compared to TMDSC is the high sensitivity for small and slow changes in crystallinity, e.g. during re-crystallisation. The combination of TMDMA and TMDSC yields new information about local processes at the surface of polymer crystallites. It is shown that during and after isothermal crystallisation the surface of the individual crystallites is in equilibrium with the surrounding melt.

Crystallization and Melting of Polycarbonate Studied by Temperature-Modulated DSC (TMDSC)

Temperature-modulated DSC (TMDSC) measurements at reasonably high frequencies allow for the determination of base-line heat capacity. In this particular case vitrification and devitrification of the rigid amorphous fraction (RAF) can be directly observed. 0.01 Hz seems to be a reasonably high frequency for bisphenol-A polycarbonate (PC). The RAF of PC is established during isothermal crystallization. Devitrification of the RAF seems to be related to the lowest endotherm. For PC the melting of small crystals between the lamellae is expected to yield the lowest endotherm.

Crystallization of polymers studied by temperature-modulated techniques (TMDSC, TMDMA)

Abstract Quasi-isothermal temperature-modulated differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) measurements (TMDSC and TMDMA, respectively) permit determination of heat capacity and shear modulus as a function of time during crystallization. Nonreversible and reversible phenomena in the crystallization region of polymers can be observed. The combination of TMDSC and TMDMA yields new information about local processes at the surface of polymer crystals, like reversible melting. Reversible melting can be observed in the complex heat capacity and in the amplitude of sheer modulus in response to temperature perturbation. The fraction of material involved in reversible melting, which is established during primary crystallization, remains constant during secondary crystallization for polycaprolactone (PCL) and polyether ether ketone (PEEK). This shows that, also after long crystallization times, the surfaces of the individual crystallites are in equilibrium with the surrounding mel...

Reversible Melting During Crystallization of Polymers Studied by Temperature Modulated Techniques (TMDSC, TMDMA)

Quasi-isothermal temperature modulated DSC and DMA measurements (TMDSC and TMDMA, respectively) were performed to determine heat capacity and shear modulus as a function of time during crystallization. Non-reversible and reversible phenomena in the crystallization region of polymers can be observed. The combination of TMDSC and TMDMA yields new information about local processes at the surface of polymer crystals, like reversible melting. Reversible melting can be observed in complex heat capacity and in the amplitude of shear modulus in response to temperature perturbation. The fraction of material involved in reversible melting, which is established during main crystallization, keeps constant during secondary crystallization for PCL PET and PEEK. This shows that also after long crystallization times the surfaces of the individual polymer crystallites are in equilibrium with the surrounding melt. Simply speaking, polymer crystals are ‘living crystals’.