Crystal reorganization of poly (butylene terephthalate)

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.

Redesign of the Geometry of Parts Produced from PBT Composite to Improve Their Operational Behavior

Parts produced from PBT-GF30 (70% polybutylene terephthalate +30% fiberglass) are very often used in car construction, due to the properties of this material. The current trend is to make parts with a shape designed to be as complex as possible, to take over many functions in operation. During the research, a part that is a component of the structure of car safety systems, and that must be completely reliable in operation, was analyzed. This piece has a complex shape that involves the intersection of several walls. Thus, the research aimed at establishing the optimal radius of connection between the walls (R), the ratio between the thickness of the intersecting walls (K) and the angle of inclination of the walls (α). The composite central design method was used to design the experiments. Both new parts and parts subject to an artificial aging process were tested. All parts were subjected to shear stress, to determine the load (L) and displacement (D) at which they break. In order to observe other changes in the properties of the parts, in addition to the mechanical ones, an analysis of the color of the new and aged parts was performed, as well as a topography of the surface layer in the breaking area. The design of the parts involved changes to the parameters of the injection process. In these conditions, a PBT-GF30 viscosity analysis was performed for new and artificially aged parts.

Concepts of Nucleation in Polymer Crystallization

Nucleation plays a vital role in polymer crystallization, in which chain connectivity and thus the multiple length and time scales make crystal nucleation of polymer chains an interesting but complex subject. Though the topic has been intensively studied in the past decades, there are still many open questions to answer. The final properties of semicrystalline polymer materials are affected by all of the following: the starting melt, paths of nucleation, organization of lamellar crystals and evolution of the final crystalline structures. In this viewpoint, we attempt to discuss some of the remaining open questions and corresponding concepts: non-equilibrated polymers, self-induced nucleation, microscopic kinetics of different processes, metastability of polymer lamellar crystals, hierarchical order and cooperativity involved in nucleation, etc. Addressing these open questions through a combination of novel concepts, new theories and advanced approaches provides a deeper understanding of the multifaceted process of crystal nucleation of polymers.

Polymorphism and Multiple Melting Behavior of Bio-Based Poly(propylene 2,5-furandicarboxylate)

Furandicarboxylate-based polyesters are considered an interesting class of bio-based polymers due to their improved properties with respect to the petrol-based terephthalate homologs. An in-depth analysis of the crystal structure of poly(propylene 2,5-furandicarboxylate) (PPF), after maximum possible removal of the catalyst, was carried out. The study disclosed that purified PPF presents two different crystalline phases after crystallization from the melt. Crystallizations at temperatures lower than 120 °C lead to growth of a single crystal form (β-form), whereas two different crystal forms (α and β) were found to coexist at higher T_cs. This behavior is opposite to that previously observed for unpurified PPF. The possibility that the catalyst nucleates the α-phase, which therefore becomes the kinetically favored modification at low crystallization temperatures in the presence of a higher amount of catalyst residue, has been considered as a feasible explanation. Two concomitantly different spherulitic morphologies were observed and connected to the β- and α-phase, respectively. The association between polymorphism and melting behavior was studied. The origin of the peaks that compose the multiple melting endotherm recorded at conventional heating rates was determined by combined wide-angle X-ray scattering, differential scanning calorimetry, fast scanning chip calorimetry, and polarized light optical microscopy measurements. The higher thermal stability of the α-crystals in comparison with the β-form was thus demonstrated.

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.

Stability of Crystal Nuclei of Poly (butylene isophthalate) Formed Near the Glass Transition Temperature

Tammann’s two-stage crystal-nuclei-development method is applied for analysis of the thermal stability of homogenously formed crystal nuclei of poly(butylene isophthalate) (PBI) as well as their possible reorganization on transferring them to the growth temperature, using fast scanning chip calorimetry. Crystal nuclei were formed at 50 °C, that is, at a temperature only slightly higher than the glass transition temperature, and developed to crystals within a pre-defined time at the growth temperature of 85 °C. The number of nuclei, overcritical at the growth temperature, was detected as a function of the transfer-conditions (maximum temperature, heating rate) by evaluation of the developed crystal fraction. For different size-distributions of crystal nuclei, as controlled by the nucleation time, there is detected distinct reduction of the nuclei number on heating to maximum temperatures higher than about 90 to 110 °C, with the latter value holding for longer nucleation time. Longer nucleation allows for both increasing the absolute nuclei number and generation of an increased fraction of larger nuclei. Heating at 1000 K/s to 140–150 °C causes “melting” of even the most stable nuclei. While direct transfer of crystal nuclei from the nucleation temperature (50 °C) to the growth temperature (85 °C) reveals negligible effect of the transfer-heating rate, in-between heating to higher temperatures is connected with distinct nuclei-reorganization above 85 °C on heating slower than 1000–10.000 K/s. The performed study not only provides specific valuable information about the thermal characteristics of crystal nuclei of PBI but also highlights the importance of proper design of Tammann’s nuclei development experiment for analysis of nuclei numbers. With the evaluation of critical rates of temperature-change for suppression of non-isothermal formation of both nuclei and crystals, the kinetics of crystallization of the slow crystallizing PBI is further quantified.

Distinct dynamics of structural relaxation in the amorphous phase of poly(l-lactic acid) revealed by quiescent crystallization

Fast scanning calorimetry (FSC) experiments were performed to investigate physical aging in amorphous and semi-crystalline poly(l-lactic acid)s (PLLAs) that were thermally crystallized under conditions leading to the α'- or α-crystalline form, and either favouring or inhibiting the development of a rigid amorphous fraction (RAF). The enthalpy of recovery was calculated after two procedures of rescaling to the content of the whole amorphous phase and also to the only content of the mobile amorphous fraction (MAF), which helped in clarifying the contribution of the RAF. From the dependence of the structural relaxation rate on the aging temperature, two regimes were evidenced for all samples. In the aging temperature domain situated close to the glass transition, the structural relaxation occurs significantly faster in the MAF. Its rate is independent of the aging temperature and is not influenced by the microstructure. However, the distance to equilibrium is higher in samples for which the coupling is strong between crystal and amorphous, implying that the time to reach equilibrium is also higher. In contrast, at low aging temperatures, for which the whole amorphous phase can be considered as solid, MAF and RAF exhibit the same structrural relaxation rate. This convergence in the relaxation kinetics by decreasing the temperature of physical aging was interpreted as the evolution of relaxation dynamics in the MAF from segmental to local. This change is highlighted by the comparison between MAF and RAF relaxation kinetics, but it occurs similarly in a pure amorphous system.

Enthalpy Relaxation of Polyamide 11 of Different Morphology Far Below the Glass Transition Temperature

Polyamide 11 (PA 11) samples of different supermolecular structure, including the crystal-free glass and semi-crystalline PA 11 of largely different semi-crystalline morphology, were prepared by fast scanning chip calorimetry (FSC). These samples were then annealed at different temperatures well below the glass transition temperature T_g. The main purpose of the low-temperature annealing experiments was the calorimetric detection of mobility of chain segments at temperatures as low as −40 °C (≈T_g − 80 K) where still excellent impact resistance is predicted. It was found that annealing PA 11 at such low temperature, regardless the thermal history and supermolecular structure including crystallinity as well as crystal shape and size, permits distinct enthalpy relaxation at rather short time scale with the structural changes reverting on subsequent heating as detected with pronounced sub-T_g-enthalpy-recovery peaks. The main glass transition, associated to large-amplitude segmental mobility, as well as relaxations at temperatures only slightly below T_g are even more distinctly sensitive to the crystal morphology. In contrast to spherulitically grown lamellar crystals, presence of high-specific-surface area nanometer-sized ordered domains causes a shift of the glass transition temperature of the amorphous phase to higher temperature, proving stronger coupling of ordered and amorphous phases than in case of lamellae. In addition, the increased coupling of the crystalline and amorphous phases slows down the cooperative rearrangements on annealing the glass slightly below T_g. The performed study contributes to further understanding of the spectrum of structural relaxations in PA 11 including the effect of presence of crystals. Enthalpy relaxation and consequently the reduction of entropy at temperatures slightly below T_g strongly depends on the semi-crystalline morphology, while an only minor effect is seen on low-temperature annealing at T_g − 80 K, possibly indicating different molecular mechanisms for the processes occurring in both temperature ranges. The low-temperature process even seems proceeding in the crystalline fraction of the material.