Physical Aging Behavior of the Side Chain of a Conjugated Polymer PBTTT.

This paper provides a viewpoint of the technology of the fast-scanning calorimetry with the relaxation behavior of disordered side chains of poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT-C12) around the glass transition temperature of the side chains (Tg,γ). PBTTT is an ideal model of the high-performance copolymer of poly(alkylthiophenes) with side chains. The γ1 relaxation process of the disordered side chains of PBTTT was detected as a small endothermic peak that emerges before the γ2 relaxation process. It shows an increase with increasing temperature as it approaches the glass transition temperature of the disordered side chains of PBTTT. The ductile-brittle transition of PBTTT in low temperatures originating from the thermal relaxation process is probed and illustrated by physical aging experiments. The signature is shown that the relaxation process of the disordered side chain of PBTTT at low temperatures varies from Arrhenius temperature dependence to super Arrhenius temperature dependence at high temperatures. These observations could have significant consequences for the stability of devices based on conjugated polymers, especially those utilized for stretchable or flexible applications, or those demanding mechanical robustness during tensile fabrication or use in a low-temperature environment.

Correlation between Differential Fast Scanning Calorimetry and Additive Manufacturing Results of Aluminium Alloys.

High-strength aluminium alloy powders modified with different nanoparticles by ball milling (7075/TiC, 2024/CaB6, 6061/YSZ) have been investigated in-situ during rapid solidification by differential fast scanning calorimetry (DFSC). Solidification undercooling has been evaluated and was found to decrease with an increasing number of nanoparticles, as the particles act as nuclei for solidification. Lower solidification undercooling of individual powder particles correlates with less hot cracking and smaller grains in the material produced by powder bed fusion of metals by a laser beam (PBF-LB/M). Quantitatively, solidification undercooling less than about 10-15 K correlates with almost crack-free PBF-LB/M components and grain sizes less than about 3 µm. This correlation shall be used for future purposeful powder material design on small quantities before performing extensive PBF-LB/M studies.

Requirements for Processing High-Strength AlZnMgCu Alloys with PBF-LB/M to Achieve Crack-Free and Dense Parts.

Processing aluminum alloys employing powder bed fusion of metals (PBF-LB/M) is becoming more attractive for the industry, especially if lightweight applications are needed. Unfortunately, high-strength aluminum alloys such as AA7075 are prone to hot cracking during PBF-LB/M, as well as welding. Both a large solidification range promoted by the alloying elements zinc and copper and a high thermal gradient accompanied with the manufacturing process conditions lead to or favor hot cracking. In the present study, a simple method for modifying the powder surface with titanium carbide nanoparticles (NPs) as a nucleating agent is aimed. The effect on the microstructure with different amounts of the nucleating agent is shown. For the aluminum alloy 7075 with 2.5 ma% titanium carbide nanoparticles, manufactured via PBF-LB/M, crack-free samples with a refined microstructure having no discernible melt pool boundaries and columnar grains are observed. After using a two-step ageing heat treatment, ultimate tensile strengths up to 465 MPa and an 8.9% elongation at break are achieved. Furthermore, it is demonstrated that not all nanoparticles used remain in the melt pool during PBF-LB/M.

Surface Crystal Nucleation and Growth in Poly (ε-caprolactone): Atomic Force Microscopy Combined with Fast Scanning Chip Calorimetry.

By using an atomic force microscope (AFM) coupled to a fast scanning chip calorimeter (FSC), AFM-tip induced crystal nucleation/crystallization in poly (ε-caprolactone) (PCL) has been studied at low melt-supercooling, that is, at a temperature typically not assessable for melt-crystallization studies. Nanogram-sized PCL was placed on the active/heatable area of the FSC chip, melted, and then rapidly cooled to 330 K, which is 13 K below the equilibrium melting temperature. Subsequent isothermal crystallization at this temperature was initiated by a soft-tapping AFM-tip nucleation event. Crystallization starting at such surface nucleus led to formation of a single spherulite within the FSC sample, as concluded from the radial symmetry of the observed morphology. The observed growth rate in the sub-micron thin FSC sample, nucleated at its surface, was found being much higher than in the case of bulk crystallization, emphasizing a different growth mechanism. Moreover, distinct banding/ring-like structures are observed, with the band period being less than 1 µm. After crystallization, the sample was melted for gaining information about the achieved crystallinity and the temperature range of melting, both being similar compared to much slower bulk crystallization at the same temperature but for a much longer time.

Visualization of Polymer Crystallization by In Situ Combination of Atomic Force Microscopy and Fast Scanning Calorimetry.

A chip-based fast scanning calorimeter (FSC) is used as a fast hot-stage in an atomic force microscope (AFM). This way, the morphology of materials with a resolution from micrometers to nanometers after fast thermal treatments becomes accessible. An FSC can treat the sample isothermally or at heating and cooling rates up to 1 MK/s. The short response time of the FSC in the order of milliseconds enables rapid changes from scanning to isothermal modes and vice versa. Additionally, FSC provides crystallization/melting curves of the sample just imaged by AFM. We describe a combined AFM-FSC device, where the AFM sample holder is replaced by the FSC chip-sensor. The sample can be repeatedly annealed at pre-defined temperatures and times and the AFM images can be taken from exactly the same spot of the sample. The AFM-FSC combination is used for the investigation of crystallization of polyamide 66 (PA 66), poly(ether ether ketone) (PEEK), poly(butylene terephthalate) (PBT) and poly(ε-caprolactone) (PCL).

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.

Crystallization of Polyethylene at Large Undercooling.

Extremely fast crystallization of high-density polyethylene and random copolymers of ethylene with up to 16 mol % 1-octene was observed for the first time by ultrafast scanning calorimetry. In order to account for the inherently high crystallization rate of polyethylenes, in nonisothermal and isothermal crystallization experiments cooling rates up to 1 000 000 K/s and crystallization times as short as 10 µs, respectively, were employed. It was possible to supercool the melt of high-density polyethylene down to 57 °C and the melt of a random ethylene/1-octene copolymer with 16 mol % 1-octene down to -33 °C, without prior crystallization. At these temperatures, the characteristic time of the primary crystallization process is of the order of magnitude of 100 µs. Complete vitrification of the liquid would require cooling even faster than 1 000 000 K/s. Compared to the homopolymer, the cooling-rate dependence of the crystallization temperatures and the temperature dependence of the characteristic time of primary crystallization of random ethylene/1-octene copolymers both are nearly parallel shifted to lower temperatures. Fast crystallization under conditions of reduced linear crystal growth rate is possibly caused by boosting homogeneous nuclei density up to 1027 m-3 and urgently requires further investigation.

Competitive crystallization of a propylene/ethylene random copolymer filled with a ß-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 ß-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 ß-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 ß-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 Friedman's theory.

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.

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 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.

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.