Crystallization, recrystallization, and melting of polymer crystals on heating and cooling examined with fast scanning calorimetry

Drop-Cast Hybrid Poly(styrene)-b-Poly(ethylene oxide) Metal Salt Films: Solvent Evaporation and Crystallinity-Dependent Evolution of Film Morphology

Morphology templates of solution-based diblock copolymer (DBC) films with loading metal salts are widely applied in photocatalysts, photovoltaics, and sensors due to their adjustable characteristics based on surface (de-)wetting and microphase separation. The present work investigates the morphologies of drop-cast hybrid films based on poly(styrene)-b-poly(ethylene oxide) (PS-b-PEO) and the metal salts titanium isopropoxide (TTIP) and zinc acetate dehydrate (ZAD) in comparison to the pure DBC. By utilizing scanning electron microscopy, grazing-incidence small- and wide-angle X-ray scattering, and differential scanning calorimetry, we find that the resulting film morphologies depend not only on the presence of metal salts but also on solvent evaporation and crystalline formation. At 20 °C, additional TTIP and ZAD in the polymer template cause the morphology to change from packed globular structures to separated wormlike structures attributed to the changed polymer environment. Furthermore, additional tetrahydrofuran causes irregular structures at the precursor film part and the overlapped wormlike structures to transition into close-packed globular structures at the cap film parts of the pure DBC. In contrast, at 50 °C, the globular structures transit to fingerprint patterns due to the thermal behavior of the crystallizable PEO blocks, and the metal salt additives suppress crystalline structure formation in the PEO domains.

A Naphthalenetetracarboxdiimide-Containing Covalent Organic Polymer: Preparation, Single Crystal Structure and Battery Application

Inspired by dative boron-nitrogen (B←N) bonds proven to be the promising dynamic linkage for the construction of crystalline covalent organic polymers/frameworks (COPs/COFs), we employed 1,4-bis(benzodioxaborole) benzene (BACT) and N,N'-Di(4-pyridyl)-1,4,5,8-naphthalenetetracarboxdiimide (DPNTCDI) as the corresponding building blocks to construct a functional COP (named as CityU-25), which had been employed as an anode in rechargeable lithium ion batteries. CityU-25 displayed an excellent reversible lithium storage capability of 455 mAh/g after 170 cycles at 0.1 A/g, and an impressive one of 673 mAh/g after 720 cycles at 0.5 A/g. These findings suggest that CityU-25 is a standout candidate for advanced battery technologies, highlighting the potential application of this type of materials.

Melting Is Well-Known, but Is It Also Well-Understood?

Contrary to continuous phase transitions, where renormalization group theory provides a general framework, for discontinuous phase transitions such a framework seems to be absent. Although the thermodynamics of the latter type of transitions is well-known and requires input from two phases, for melting a variety of one-phase theories and models based on solids has been proposed, as a generally accepted theory for liquids is (yet) missing. Each theory or model deals with a specific mechanism using typically one of the various defects (vacancies, interstitials, dislocations, interstitialcies) present in solids. Furthermore, recognizing that surfaces are often present, one distinguishes between mechanical or bulk melting and thermodynamic or surface-mediated melting. After providing the necessary preliminaries, we discuss both types of melting in relation to the various defects. Thereafter we deal with the effect of pressure on the melting process, followed by a discussion along the line of type of materials. Subsequently, some other aspects and approaches are dealt with. An attempt to put melting in perspective concludes this review.

Unraveling the Complex Polymorphic Crystallization Behavior of the Alternating Copolymer DMDS-alt-DVE

A complex crystallization behavior was observed for the alternating copolymer DMDS-alt-DVE synthesized via thiol-ene step-growth polymerization. Understanding the underlying complex crystallization processes of such innovative polythioethers is critical for their application, for example, in polymer coating technologies. These alternating copolymers have polymorphic traits, resulting in different phases that may display distinct crystalline structures. The copolymer DMDS-alt-DVE was studied in an earlier work, where only two crystalline phases were reported: a low melting, L - T_m, and high melting, H - T_m phase. Remarkably, the H - T_m form was only achieved by the previous formation and melting of the L - T_m form. We applied calorimetric techniques encompassing seven orders of magnitude in scanning rates to further explore this complex polymorphic behavior. Most importantly, by rapidly quenching the sample to temperatures well below room temperature, we detected an additional polymorphic form (characterized by a very low melting phase, denoted VL - T_m). Moreover, through tailored thermal protocols, we successfully produced samples containing only one, two, or all three polymorphs, providing insights into their interrelationships. Understanding polymorphism, crystallization, and the resulting morphological differences can have significant implications and potential impact on mechanical resistance and barrier properties.

Application of Atmospheric-Pressure Jet Plasma in the Presence of Acrylic Acid for Joining Polymers without Adhesives

This study investigates the treatment of surfaces with jet plasma at atmospheric pressure in the presence of acrylic acid as a resource-saving and efficient approach to joining polymers on polystyrene (PS) and polyamide 12 (PA 12) surfaces. Acrylic acid was added in order to introduce functional groups to the polymer surfaces. XPS analysis revealed a high density of oxygen-containing groups, e.g., carboxylic acid groups, on the polymer surfaces, the detailed composition depending on the polymer. The AFM measurements indicated that the modification of polyamide resulted in morphological changes and an increase in surface roughness due to polymer recrystallization. When the surface-modified polymers were brought in contact under a load, significant adhesion between the polymer surfaces was measured. In particular, PS and PA 12, which are otherwise difficult to join by gluing, could readily be connected in this way. The joint polymers could be separated intentionally by immersion in water, thus enabling the recycling of the materials. The resistance of the joint to water depends on the polymer system, with polyamide providing strikingly higher resistance than polystyrene. Accordingly, treating the joint polymers with water allows debonding on demand, particularly when PS is involved. Exposure of modified polymer surfaces to solutions of metal ions increased the resistance of joint polymers to water.

Development and Characterization of Fully Renewable and Biodegradable Polyhydroxyalkanoate Blends with Improved Thermoformability

Poly(3-hydroxybutyrate-co-3-valerate) (PHBV), being one of the most studied and commercially available polyhydroxyalkanoates (PHAs), presents an intrinsic brittleness and narrow processing window that currently hinders its use in several plastic applications. The aim of this study was to develop a biodegradable PHA-based blend by combining PHBV with poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH), another copolyester of the PHA family that shows a more ductile behavior. Blends of PHBV with 20% wt., 30% wt., and 40% wt. of PHBH were obtained by melt mixing, processed by cast extrusion in the form of films, and characterized in terms of their morphology, crystallization behavior, thermal stability, mechanical properties, and thermoformability. Full miscibility of both biopolymers was observed in the amorphous phase due to the presence of a single delta peak, ranging from 4.5 °C to 13.7 °C. Moreover, the incorporation of PHBH hindered the crystallization process of PHBV by decreasing the spherulite growth rate from 1.0 µm/min to 0.3 µm/min. However, for the entire composition range studied, the high brittleness of the resulting materials remained since the presence of PHBH did not prevent the PHBV crystalline phase from governing the mechanical behavior of the blend. Interestingly, the addition of PHBH greatly improved the thermoformability by widening the processing window of PHBV by 7 s, as a result of the increase in the melt strength of the blends even for the lowest PHBH content.

Additive Manufacturing of α-Amino Acid Based Poly(ester amide)s for Biomedical Applications

α-Amino acid based polyester amides (PEAs) are promising candidates for additive manufacturing (AM), as they unite the flexibility and degradability of polyesters and good thermomechanical properties of polyamides in one structure. Introducing α-amino acids in the PEA structure brings additional advantages such as (i) good cytocompatibility and biodegradability, (ii) providing strong amide bonds, enhancing the hydrogen-bonding network, (iii) the introduction of pendant reactive functional groups, and (iv) providing good cell-polymer interactions. However, the application of α-amino acid based PEAs for AM via fused deposition modeling (FDM), an important manufacturing technique with unique processing characteristics and requirements, is still lacking. With the aim to exploit the combination of these advantages in the creation, design, and function of additively manufactured scaffolds using FDM, we report the structure-function relationship of a series of α-amino acid based PEAs. The PEAs with three different molecular weights were synthesized via the active solution polycondensation, and their performance for AM applications was studied in comparison with a commercial biomedical grade copolymer of l-lactide and glycolide (PLGA). The PEAs, in addition to good thermal stability, showed semicrystalline behavior with proper mechanical properties, which were different depending on their molecular weight and crystallinity. They showed more ductility due to their lower glass transition temperature (T_g; 18-20 °C) compared with PLGA (57 °C). The rheology studies revealed that the end-capping of PEAs is of high importance for preventing cross-linking and further polymerization during the melt extrusion and for the steadiness and reproducibility of FDM. Furthermore, our data regarding the steady 3D printing performance, good polymer-cell interactions, and low cytotoxicity suggest that α-amino acid based PEAs can be introduced as favorable polymers for future AM applications in tissue engineering. In addition, their ability for formation of bonelike apatite in the simulated body fluid (SBF) indicates their potential for bone tissue engineering applications.

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.

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.