Evgeny Zhuravlev

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.

Nonisothermal crystallization of polytetrafluoroethylene in a wide range of cooling rates.

Compared to other semicrystalline polymers, PTFE demonstrates a very fast crystallization process on cooling. This study explores for the first time the nonisothermal PTFE ultrafast crystallization under tremendously fast cooling rates (up to 800,000 K·s(-1)) achieved by using fast scanning calorimetry (FSC) and ultra-fast scanning calorimetry (UFSC). Regular DSC was also used to get crystallization at slower rates. The data obtained on a wide range of cooling rates (over 8 orders of magnitudes) help to get new knowledge about crystallization kinetics of PTFE. Both FSC and UFSC data show that it is impossible to bypass the crystallization and thus to reach a metastable glassy state even for the fastest cooling rate employed (800,000 K·s(-1)). The crystals formed under such conditions are slightly less stable than those produced under slower cooling rates, as reflected by a shift of the melting peak to lower temperature. The difference in crystal morphologies was confirmed by SEM observations. The variation of the effective activation energy (Eα) with the relative extent of crystallization reveals that PTFE crystallization follows a transition from regime II to regime III around 315-312 °C. Corroborated temperature dependences of Eα obtained respectively for crystallizations under slow and fast cooling rates were combined and fitted to the theoretical dependence of the growth rate derived from the Hoffman-Lauritzen theory.

Characterization of the copolymer poly(ethyleneglycol-g-vinylalcohol) as a potential carrier in the formulation of solid dispersions

In order to fully exploit the graft copolymer poly(ethyleneglycol-g-vinylalcohol) (EG/VA) in the formulation of solid dispersions, a characterization of its phase behavior before, during and after spray-drying and hot-melt extrusion is performed. Solid state characterization was performed using MDSC and XRPD. The effect of heating/cooling rate on the degree of crystallinity was studied using HPer DSC and ultra-fast chip calorimetry. EG/VA consists of two semi-crystalline fractions, one corresponding to the polyethyleneglycol (PEG) fraction (T(g)=-57 degrees C, T(m)=15 degrees C) and one corresponding to the polyvinylalcohol (PVA) fraction (T(g)=45 degrees C, T(m)=212 degrees C). XRPD analysis confirmed its semi-crystallinity, and EG/VA showed Bragg reflections comparable to those of PVA. Spray-drying at a temperature lower than 170 degrees C resulted in amorphization of the PVA fraction, while after hot-melt extrusion at different temperatures, the crystallinity of this fraction increases. In both cases, the PEG fraction is not influenced. Plasticization of the amorphous domains of the PEG or PVA fraction of the copolymer was dependent on the type and concentration of plasticizer, suggesting that also other small organic molecules like drugs may not homogeneously mix with both amorphous domains. A controlled cooling rate of 3000 degrees C/s was necessary to make the copolymer completely amorphous.

Size and rate dependence of crystal nucleation in single tin drops by fast scanning calorimetry

The experimentally accessible degree of undercooling of single micron-sized liquid pure tin drops has been studied via differential fast scanning calorimetry. The cooling rates employed ranged from 100 to 14,000 K/s. The diameter of the investigated tin drops varied in the range from 7 to 40 μm. The influence of the drop shape on the solidification process could be eliminated due to the nearly spherical shape of the single drop upon heating and cooling and the resultant geometric stability. As a result it became possible to study the effect of both drop size and cooling rate in rapid solidification experimentally. A theoretical description of the experimental results is given by assuming the existence of two different heterogeneous nucleation mechanisms leading to crystal nucleation of the single tin drop. In agreement with the experiment these mechanisms yield a shelf-like dependence of crystal nucleation on undercooling. A dependence of crystal nucleation on the size of the tin drop was observed and is discussed in terms of the mentioned theoretical model, which can possibly also describe the nucleation for other related rapid solidification processes.

Competitive crystallization of a propylene/ethylene random copolymer filled with a β-nucleating agent and multi-walled carbon nanotubes. Conventional and ultrafast DSC study.

A propylene/ethylene polymeric matrix was reinforced by the simultaneous addition of a β-nucleating agent (calcium pimelate) and multi-walled carbon nanotubes (MWCNTs) in various concentrations. The present manuscript explores the competitive crystallization tendency that is caused by the presence of the two fillers. On the one hand, calcium pimelate forces the material to crystallize predominantly in the β-crystalline form, while, on the other, the strong α-nucleating ability of MWCNTs compels the material to develop higher α-crystalline content. An in-depth study has been performed on the nanocomposite samples by means of conventional, temperature-modulated, and differential fast scanning calorimetry (DFSC) under various dynamic and isothermal conditions. The results showed that β-crystals are predominant at low MWCNT content (<2.5 wt %), while, at high MWCNT content, the material crystallizes mainly in the α-form. The recrystallization phenomenon during melting was confirmed with step-scan DSC, and the use of very high cooling rates by UFDSC made it possible to achieve and study the nucleation of the samples. The presence of MWCNTs enabled the nanocomposites to crystallize faster under both isothermal and dynamic conditions. The activation energy of the samples was also calculated according to Friedmans theory.

Silk I and Silk II studied by fast scanning calorimetry

Using fast scanning calorimetry (FSC), we investigated the glass transition and crystal melting of samples of B. mori silk fibroin containing Silk I and/or Silk II crystals. Due to the very short residence times at high temperatures during such measurements, thermal decomposition of silk protein can be significantly suppressed. FSC was performed at 2000K/s using the Mettler Flash DSC1 on fibroin films with masses around 130-270ng. Films were prepared with different crystalline fractions (ranging from 0.26 to 0.50) and with different crystal structures (Silk I, Silk II, or mixed) by varying the processing conditions. These included water annealing at different temperatures, exposure to 50%MeOH in water, or autoclaving. The resulting crystal structure was examined using wide angle X-ray scattering. Degree of crystallinity was evaluated from Fourier transform infrared (FTIR) spectroscopy and from analysis of the heat capacity increment at the glass transition temperature. Silk fibroin films prepared by water annealing at 25°C were the least crystalline and had Silk I structure. FTIR and FSC studies showed that films prepared by autoclaving or 50%MeOH exposure were the most crystalline and had Silk II structure. Intermediate crystalline fraction and mixed Silk I/Silk II structures were found in films prepared by water annealing at 37°C. FSC results indicate that Silk II crystals exhibit endotherms of narrower width and have higher mean melting temperature Tm(II)=351±2.6°C, compared to Silk I crystals which melt at Tm(I)=292±3.8°C. Films containing mixed Silk I/Silk II structure showed two clearly separated endothermic peaks. Evidence suggests that the two types of crystals melt separately and do not thermally interconvert on the extremely short time scale (0.065s between onset and end of melting) of the FSC experiment. STATEMENT OF SIGNIFICANCE Silkworm silk is a naturally occurring biomaterial. The fibroin component of silk forms two types of crystals. Silk properties depend upon the amount and type of crystals, and their stability. One measure of stability is crystal melting temperature. Crystals which are more stable have a higher melting temperature. Until now, it has been challenging to study thermal behavior of silk crystals because they degrade at high temperature. To avoid degradation, and study the melting properties of silk biomaterial, we heated silk at a very fast rate of 2000K/s using a special calorimeter. We have shown that the two crystal types have very different melting temperatures, indicating that one crystal type is much more stable than the other.

A transient polymorph transition of 4-cyano-4′-octyloxybiphenyl (8OCB) revealed by ultrafast differential scanning calorimetry (UFDSC)

In this work, ultrafast differential scanning calorimetry (UFDSC) with heating and cooling rates up to 20 000 K s−1 is used to study the transient polymorph transition of the liquid crystal 8OCB. The square plate form (SP), which was reported to grow only from the solution in binary solvent mixtures at low temperature, is found to be the only form growing from a deeply quenched smectic glass during very rapid heating. If the heating rate is slower than 8000 K s−1, reorganization to the parallelepiped form will start, and if the heating rate is slower than 1000 K s−1, the square plate form will reorganize to the parallelepiped form completely, and only melting of the parallelepiped form is observed. The capacity of the UFDSC to capture the unstable polymorphs of low molar mass organic molecules and to follow their rapid transition is demonstrated in this work.

Formation and Reorganization of the Mesophase of Isotactic Polypropylene

A short review about the structure, condition of formation, and reorganization behavior of the mesophase of isotactic polypropylene, summarizing recent work of the authors, is provided. Emphasis is put on the presentation of data collected by novel analysis techniques like fast scanning chip calorimetry (FSC), temperature-resolved X-ray analysis, or temperature-resolved atomic force microscopy (AFM), for quantitative characterization of the kinetics of the liquid—mesophase transition and of the conversion of the mesophase into crystals. In addition, the impact of crystallization of polypropylene via intermediate formation of a mesophase on engineering properties will be highlighted.

Fast Scanning Calorimetry of Silk Fibroin Protein: Sample Mass and Specific Heat Capacity Determination

Our work over the past decade has involved thermal studies of fibrous proteins, especially those produced by silkworms and spiders, as well as genetically modified variants such as copolymers that maintain some of the important properties of the fibrous proteins [1–9]. The goals of our research include quantifying the thermal properties of crystallizable fibrous proteins, establishing connections between bio-derived fibrous proteins and synthetic polymers, and developing a knowledge base to enable use of fibrous proteins like silk, inside the human body in novel ways [10–13].

Non-Adiabatic Scanning Calorimeter for Controlled Fast Cooling and Heating

This chapter describes a power-compensated differential fast scanning calorimeter, which allows heat capacity determination of nanogram samples on both controlled heating and cooling in the range from 100 to 10,000,000 K/s. A submikron SiNx membrane sensor was developed together with Xensor Integration as a basis of the calorimeter. Minimizing addenda heat capacity and aiming particularly on fast cooling, the active measuring area of the sensor was embedded into the central part of the membrane and has dimensions down to 5 × 5 μm2. The differential power-compensated temperature control scheme was designed for precise temperature control and heat capacity determination. Software programmable temperature scans allow transitions from controlled heating and cooling up to 5 MK/s to isotherm with over/undershoot less than 1 K and within 2 ms. Though the absolute values of sample temperature and heat capacity determination is still complicated due to the free-standing sample configuration, they can be measured with reproducibility ±1 K and 1 pJ/K sensitivity, respectively.