Melting and recrystallization kinetics of poly(butylene terephthalate)

Critical Cooling Rate of Fast-Crystallizing Polyesters: The Example of Poly(alkylene trans-1,4-cyclohexanedicarboxylate)

Controlling the cooling rate experienced by a material during a manufacturing process is a challenge and a major issue. Industrial processing techniques are very diverse and may involve a whole range of cooling rates, which are sometimes extremely high for small and/or thin manufactured parts. For polymers, the cooling rate has consequences on both the microstructure and the time-dependent properties. The common cooling rates associated with conventional calorimetric measurements are generally limited to a few tens of degrees per minute. This work combines several calorimetric techniques (DSC, modulated-temperature DSC, stochastically-modulated DSC and Fast Scanning Calorimetry) to estimate the critical cooling rate required to melt-quench fast-crystallizing polyesters to their fully amorphous state, based on the example of a series of poly(alkylene trans-1,4-cyclohexanedicarboxylate) (PCHs) with a number of methylene groups in the main structure of the repeating unit nCH2 varying from 3 to 6. The even-numbered ones require faster cooling rates (about 3000 K s-1 for nCH2 = 4, between 500 and 1000 K s-1 for nCH2 = 6) compared to the odd-numbered ones (between 50 K min-1 and 100 K s-1 for nCH2 = 3, between 10 and 30 K min-1 for nCH2 = 5).

Reorganization of Poly(Butylene Succinate) Containing Crystals of Low Stability

Poly(butylene succinate) (PBS) forms small and imperfect crystals of low melting temperature at high supercooling of the melt. Slow heating allows reorganization of the obtained semicrystalline structure with the changes of the crystallinity and of the size and perfection of crystals analyzed by differential scanning calorimetry (DSC) and temperature-resolved X-ray scattering techniques. Crystals generated at 20 °C begin to melt and reorganize at a few K higher temperature with their initial imperfection and thickness unchanged upon heating to 70-80 °C. Slow heating to temperatures higher than 70-80 °C yields a distinct exothermic peak in the DSC scan, paralleled by detection of crystals of larger size/higher perfection, beginning to melt at ≈100 °C. These observations suggest that below 70-80 °C, reorganization of the semicrystalline morphology is constrained such that only minor and local improvement of the structure of crystals are possible. The formation of both perfect and thicker crystal lamellae at higher temperature proceeds via melting of imperfect crystals followed by melt-recrystallization as for PBS solid-state thickening is impossible. The study shows the limit of low-temperature reorganization processes when not involving both complete melting of crystals and rearrangement of the lamellar-stack structure.

Transparent and Flexible Graphene Pressure Sensor with Self-Assembled Topological Crystalline Ionic Gel

This study demonstrates transparent and flexible capacitive pressure sensors using a high-k ionic gel composed of an insulating polymer (poly(vinylidene fluoride-co-trifluoroethylene-co-chlorofluoroethylene), P(VDF-TrFE-CFE)) blended with an ionic liquid (IL; 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide, [EMI][TFSA]). The thermal melt recrystallization of the P(VDF-TrFE-CFE):[EMI][TFSA] blend films develops the characteristic topological semicrystalline surface of the films, making them highly sensitive to pressure. Using optically transparent and mechanically flexible graphene electrodes, a novel pressure sensor is realized with the topological ionic gel. The sensor exhibits a sufficiently large air dielectric gap between graphene and the topological ionic gel, resulting in a large variation in capacitance before and after the application of various pressures owing to the pressure-sensitive reduction of the air gap. The developed graphene pressure sensor exhibits a high sensitivity of 10.14 kPa^-1 at 20 kPa, rapid response times of <30 ms, and durable device operation with 4000 repeated ON/OFF cycles. Furthermore, broad-range detections from lightweight objects to human motion are successfully achieved, demonstrating that the developed pressure sensor with a self-assembled crystalline topology is potentially suitable for a variety of cost-effective wearable applications.

Influence of Cooling Rate on Crystallization Behavior of Semi-Crystalline Polypropylene: Experiments and Mathematical Modeling

As crystallization behavior has a great effect on the injection molding process, the flash differential scanning calorimetry (FSC) method was employed to study the influence of cooling rate on the crystallization behavior of a semi-crystalline polypropylene (PP). As the experimental results show, crystallization temperatures (onset crystallization temperature and maximum crystallization temperature) and crystallinity decrease as the cooling rate increases. In addition, the corresponding mathematical models were established to describe the relationship between the crystallization temperatures/crystallinity and the cooling rate. A revised Tait equation was also carried out based on the mathematical models.

The 3-Phase Structure of Polyesters (PBT, PET) after Isothermal and Non-Isothermal Crystallization

According to the 3-phase model, semi-crystalline thermoplastics consist of a mobile amorphous fraction (MAF), a rigid amorphous fraction (RAF), and a crystalline fraction (CF). For the two polyesters Polybutylene Terephthalate (PBT) and Polyethylene Terephthalate (PET), the composition of these phases was investigated using the largest possible variation in the isothermal and non-isothermal boundary conditions. This was performed by combining the conventional Differential Scanning Calorimetry (DSC) with the Fast Scanning Calorimetry (FSC). From the results it can be deduced that the structural composition of both polymers is characterised by a large fraction of the rigid amorphous phase. This is mainly formed either during the primary crystallization in the low temperature range or during the subsequent secondary crystallization that follows primary crystallization in the high temperature range. Depending on the thermal history, the fraction of the mobile amorphous phase of both polymers approaches a minimum, which does not appear to be undercut.

Temperature-Modulated Scanning Calorimetry of Melting-Recrystallization of Poly(butylene terephthalate)

The melting and recrystallization behaviors of poly(butylene terephthalate) (PBT) were investigated using temperature-modulated scanning calorimetry in both fast- and conventional slow-scan modes. With this method, the response of multiple transition kinetics, such as melting and recrystallization, can be differentiated by utilizing the difference in the time constants of the kinetics. In addition to the previous result of temperature-modulated fast-scan calorimetry of polyethylene terephthalate (PET), the supporting evidence of another aromatic polyester, PBT, confirmed the behavior of the exothermic process of recrystallization, which proceeds simultaneously with melting on heating scan in the temperature range of double melting peaks starting just above the crystallization temperature up to the main melting peak. Because the crystallization of PBT is much more pronounced than that of PET, similar behavior of recrystallization was obtained by the conventional temperature-modulated differential scanning calorimetry at a slow-scan rate.

Competition between Structural Relaxation and Crystallization in the Glass Transition Range of Random Copolymers

Structural relaxation in polymers occurs at temperatures in the glass transition range and below. At these temperatures, crystallization is controlled by diffusion and nucleation. A sequential occurrence of structural relaxation, nucleation, and crystallization was observed for several homopolymers during annealing in the range of the glass transition. It is known from the literature that all of these processes are strongly influenced by geometrical confinements. The focus of our work is copolymers, in which the confinements are caused by the random sequence of monomer units in the polymer chain. We characterize the influence of these confinements on structure formation and relaxation in the vicinity of the glass transition. The measurements were performed with a hydrogenated nitrile-butadiene copolymer (HNBR). The kinetics of the structural relaxation and the crystallization was measured using fast differential scanning calorimetry (FDSC). This technique was selected because of the high sensitivity, the fast cooling rates, and the high time resolution. Crystallization in HNBR causes a segregation of non-crystallizable segments in the macromolecule. This yields a reduction in mobility in the vicinity of the formed crystals and as a consequence an increased amount of so-called "rigid amorphous fraction" (RAF). The RAF can be interpreted as self-assembled confinements, which limit and control the crystallization. An analysis of the crystallization and the relaxation shows that the kinetic of both is identical. This means that the Kohlrausch exponent of relaxation and the Avrami exponent of crystallization are identical. Therefore, the crystallization is not controlled by nucleation but by diffusion and is terminated by the formation of RAF.

Supermolecular Structure of Poly(butylene terephthalate) Fibers Formed with the Addition of Reduced Graphene Oxide

Nanocomposite fibers based on poly(butylene terephthalate) (PBT) and reduced graphene oxide (rGO) were prepared using a method able to disperse graphene in one step into a polymer matrix. The studies were performed for fibers containing four different concentrations of rGO at different take-up velocities. The supermolecular structures of the fibers at the crystallographic and lamellar levels were examined by means of calorimetric and X-ray scattering methods (DSC, WAXS, and SAXS). It was found that the fiber structure is mainly influenced by the take-up velocity. Fibers spun at low and medium take-up velocities contained a crystalline α-form, whereas the fibers spun at a high take-up velocity contained a smectic mesophase. During annealing, the smectic phase transformed into its α-form. The degree of transformation depended on the rGO content. Reduced graphene mainly hindered the crystallization of PBT by introducing steric obstacles confining the ordering of the macromolecules of PBT.

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).

Homogeneous crystal nucleation in polymers

The pathway of crystal nucleation significantly influences the structure and properties of semi-crystalline polymers. Crystal nucleation is normally heterogeneous at low supercooling, and homogeneous at high supercooling, of the polymer melt. Homogeneous nucleation in bulk polymers has been, so far, hardly accessible experimentally, and was even doubted to occur at all. This topical review summarizes experimental findings on homogeneous crystal nucleation in polymers. Recently developed fast scanning calorimetry, with cooling and heating rates up to 10⁶ K s^-1, allows for detailed investigations of nucleation near and even below the glass transition temperature, including analysis of nuclei stability. As for other materials, the maximum homogeneous nucleation rate for polymers is located close to the glass transition temperature. In the experiments discussed here, it is shown that polymer nucleation is homogeneous at such temperatures. Homogeneous nucleation in polymers is discussed in the framework of the classical nucleation theory. The majority of our observations are consistent with the theory. The discrepancies may guide further research, particularly experiments to progress theoretical development. Progress in the understanding of homogeneous nucleation is much needed, since most of the modelling approaches dealing with polymer crystallization exclusively consider homogeneous nucleation. This is also the basis for advancing theoretical approaches to the much more complex phenomena governing heterogeneous nucleation.

Glass transition of anhydrous starch by fast scanning calorimetry

By means of fast scanning calorimetry, the glass transition of anhydrous amorphous starch has been measured. With a scanning rate of 2000Ks^-1, thermal degradation of starch prior to the glass transition has been inhibited. To certify the glass transition measurement, structural relaxation of the glassy state has been investigated through physical aging as well as the concept of limiting fictive temperature. In both cases, characteristic enthalpy recovery peaks related to the structural relaxation of the glass have been observed. Thermal lag corrections based on the comparison of glass transition temperatures measured by means of differential and fast scanning calorimetry have been proposed. The complementary investigations give an anhydrous amorphous starch glass transition temperature of 312±7°C. This estimation correlates with previous extrapolation performed on hydrated starches.