Tuning the Thermal Properties and Morphology of Isodimorphic Poly[(butylene succinate)-ran-(ε-caprolactone)] Copolyesters by Changing Composition, Molecular Weight, and Thermal History

The Hidden Isodimorphic Crystallization of Poly(ε-Caprolactone-Ran-ω-Pentadecalactone) Copolymers

Poly(ε-caprolactone-ran-ω-pentadecalactone) (PCLx-PPDLy) copolymers were synthesized by using ring-opening polymerization with Candida antarctica lipase B as a catalyst across various compositions. The aim was to study their crystallization behavior and ascertain whether they are isomorphic or isodimorphic. Differential scanning calorimetry, polarized light optical microscopy, in situ wide- and small-angle X-ray scattering, and Fourier-transform infrared spectroscopy were employed to assess the crystallization mode. Various crystallization conditions were used to investigate their influence on the comonomer inclusion/exclusion balance. The copolymers exhibited pseudoeutectic behavior across all compositions, crystallizing in either PPDL-type or PCL-type unit cells and conformations, independent of crystallization conditions. This indicates that they are isodimorphic, contrary to previous reports. Self-nucleation tests showed that the Domain II width decreases with increasing comonomer content, supporting isodimorphism. The pseudoeutectic point was observed at CL contents above 83%, which explains the previously unrecognized isodimorphic character of these copolyesters.

How the Aliphatic Glycol Chain Length Determines the Pseudoeutectic Composition in Biodegradable Isodimorphic poly(alkylene succinate-ran-caprolactone) Random Copolyesters

We synthesize four series of novel biodegradable poly(alkylene succinate-ran-caprolactone) random copolyesters using a two-step ring-opening/transesterification and polycondensation process with ε-caprolactone (PCL) as a common comonomer. The second comonomers are succinic acid derivatives, with variations in the number of methylene groups (nCH2) in the glycol segment, nCH2 = 2, 4, 8, and 12. The obtained copolyesters were poly(ethylene succinate-ran-PCL) (ESxCLy), poly(butylene succinate-ran-PCL) (BSxCLy), poly(octamethylene succinate-ran-PCL) (OSxCLy), and poly(dodecylene succinate-ran-PCL) (DSxCLy). We discovered a new mixed isodimorphic/comonomer exclusion crystallization in ESxCLy copolymers. The BSxCLy, OSxCLy, and DSxCLy copolymers display isodimorphic behavior. Our findings revealed a significant variation in the pseudoeutectic point position, from mixed isodimorphism/comonomer exclusion crystallization to isodimorphism with pseudoeutectic point variation from 54% to up to 90%. Moreover, we established a link between the melting temperature depression slope variation and the comonomer inclusion/exclusion balance, providing valuable insights into the complex topic of isodimorphic random copolymers.

Non-monotonic Information and Shape Evolution of Polymers Enabled by Spatially Programmed Crystallization and Melting

Stimuli-responsive polymer materials are a kind of intelligent material which can sense and respond to external stimuli. However, most current stimuli-responsive polymers only exhibit a monotonic response to a single constant stimulus but cannot achieve dynamic evolution. Herein, we report a method to achieve a non-monotonic response under a single stimulus by regionalizing the crystallization and melting kinetics in semicrystalline polymers. Based on the influence of cross-linking on the crystallization and melting kinetics of polymers, we employ light to spatially regulate the cross-linking degree in polymers. The prepared material can realize the self-evolved encryption of pattern information and the non-monotonic shape evolution without complex multiple stimuli. By combination of pattern and shape evolution, the coupled encryption of shape and pattern can be achieved. This approach empowers polymers with the ability to evolve under constant stimulus, offering insights into the functional design of polymer materials.

Crystal Polymorphism of Isodimorphic Polyesters Tuned by cis- and trans-C═C Comonomer Units

Polymorphism is ubiquitous in polymer crystallization due to the diversified chain conformations and interchain packings in polymer crystals. Controlling chain conformation is effective in tailoring the crystal polymorphism of polymers, which, however, is challenging at the molecular level. Herein, we have synthesized poly(butylene adipate) (PBA)-based copolymers containing C═C units and demonstrated the important role of trans/cis-C═C units in tuning the chain conformation and crystal polymorphism of polymers. Both PBA-based trans- and cis-copolymers show isodimorphic crystallization behavior with the partial inclusion of C═C units in PBA crystals. The presence of trans-C═C units favors the formation of metastable β-crystals of PBA and retards the β-to-α crystal transition upon heating due to the highly conformational matching between trans-C═C units and β-crystals. Conversely, the incorporation of cis-C═C units destroys the regularity of the trans conformation and favors the growth of α-crystals of PBA. This work has elucidated the crucial role of local chain conformation in the crystal polymorphism of polymers.

Challenging Isodimorphism Concepts: Formation of Three Crystalline Phases in Poly(hexamethylene-ran-octamethylene carbonate) Copolymers

In this work, poly(hexamethylene-ran-octamethylene carbonate) copolycarbonates were synthesized by melt polycondensation in a wide range of compositions. The copolymers displayed some of the characteristic isodimorphic thermal behavior, such as crystallization for all the compositions and a pseudoeutectic behavior of the melting temperature (Tm) versus composition. The pseudoeutectic point was located at 33 mol % poly(octamethylene carbonate) (POC) content (i.e., corresponding to the PH67O33C copolymer). Surprisingly, the crystallinities (Xc) for a wide range of copolymer compositions were higher than those of the parent components, a phenomenon that has not been observed before in isodimorphic random copolymers. The structural characterization, performed by wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering experiments, revealed unexpected results depending on composition. On the one hand, the poly(hexamethylene carbonate) (PHC)- and POC-rich copolymers crystallize in PHC- and POC-type crystals, as expected. Moreover, upon cooling and heating, in situ WAXS experiments evidenced that these materials undergo reversible solid-solid transitions [δ-α (PHC) and δ-α-β (POC)] present in the parent components but at lower temperatures. On the other hand, a novel behavior was found for copolymers with 33-73 mol % POC (including the pseudoeutectic point), which are those with higher crystallinities than the parent components. For these copolymers, a new crystalline phase that is different from that of both homopolymers was observed. The in situ WAXS results for these copolymers confirmed that this novel phase is stable upon cooling and heating and does not show any crystallographic feature of the parent components or their solid-solid transitions. FTIR experiments confirmed this behavior, revealing that the new phase adopts a polyethylene-like chain conformation that differs from the trans-dominant ones exhibited by the parent components. This finding challenges the established concepts of isodimorphism and questions whether a combination of crystallization modes (isodimorphism and isomorphism) is possible in the same family of random copolymers just by changing the composition.

Disappearance of Melt Memory Effect with Comonomer Incorporation in Isodimorphic Random Copolyesters

Melt memory effects in polymer crystallization have attracted much attention in the past few years. Although progress has been made in understanding how the chemical structure of polymers can affect melt memory, there are still some knowledge gaps. In this work, we study how incorporating a second comonomer unit that is partially included in the crystalline unit cell affects the melt memory effect of the major component in a random isodimorphic copolymer for the first time. This second comonomer unit depresses the melting temperature of the homopolymer, reduces the crystallinity, and distorts the crystalline unit cell. However, its effect on the stability of self-nuclei and the production of melt memory has not been studied so far. To this aim, we have selected poly[(butylene succinate)-ran-(ε-caprolactone)] random copolyesters PBS-ran-PCL that are isodimorphic, i.e., they exhibit a pseudoeutectic point. This point separates the formation of BS-rich crystals from CL-rich crystals as a function of composition. The results reveal that the melt memory effect of these isodimorphic copolymers is strongly reduced with the incorporation of even very small amounts of comonomer unit (i.e., 1 molar %). This indicates that the incorporation of a second comonomer unit in the polymer chain disrupts the intermolecular interactions present between the chain segments in the crystal lattice of the major component and reduces the capacity of the material to produce self-nuclei. This reduction is more drastic for copolymers in which the second comonomer unit is mostly rejected from the crystalline phase. Contrary to olefin-based copolymers, for copolyesters, the second comonomer unit eases the process to reach an isotropic melt state upon melting. This work reveals the impact of introducing comonomer units on the melt memory effect in isodimorphic random copolyesters.

Differential Scanning Calorimetry Material Studies: Benzil Melting Point Method for Eliminating the Thermal History of DSC

How to eliminate the thermal history present in differential scanning calorimetry, this question has been widely concerned. Benzil has a serious thermal history that is not well eliminated by conventional thermal history methods. Herein, using benzil as a target, we developed a freeze-gradient temperature rate heating up method to eliminate the thermal history of DSC. Compared with the conventional method, this method avoids the introduction of new thermal histories by new crystalline forms that may appear during the cooling crystallization process. The results show that the peak shape of the melting peak is sharper and the peak emergence position is closer to the theoretical melting point after the elimination of the thermal history by the freeze-gradient heating up method. Based on this method, we optimized other factors to establish a complete method for the determination of melting point by DSC, analyzed the uncertainty of the method, and obtained an extended uncertainty of 0.19°C for DSC in this method. The validation results show that the p-nitrotoluene of the melting point reference substance was 52.64°C, which is within its reference value of (52.53 ± 0.20) °C, showing that this method is reliable. This study provides a reference for other thermal analysis methods to eliminate thermal history.

A Review on Current Strategies for the Modulation of Thermomechanical, Barrier, and Biodegradation Properties of Poly (Butylene Succinate) (PBS) and Its Random Copolymers

The impact of plastics on the environment can be mitigated by employing biobased and/or biodegradable materials (i.e., bioplastics) instead of the traditional "commodities". In this context, poly (butylene succinate) (PBS) emerges as one of the most promising alternatives due to its good mechanical, thermal, and barrier properties, making it suitable for use in a wide range of applications. Still, the PBS has some drawbacks, such as its high crystallinity, which must be overcome to position it as a real and viable alternative to "commodities". This contribution covers the actual state-of-the-art of the PBS through different sections. The first section reviews the different synthesis routes, providing a complete picture regarding the obtained molecular weights and the greener alternatives. Afterward, we examine how different strategies such as random copolymerization and the incorporation of fillers can effectively modulate PBS properties to satisfy the needs for different applications. The impact of these strategies is evaluated in the crystallization behavior, crystallinity, mechanical and barrier properties, and biodegradation. The biodegradation is carefully analyzed, highlighting the wide variety of methodologies existing in the literature to measure PBS degradation through different routes (hydrolytic, enzymatic, and soil).

Rheology and Tack Properties of Biodegradable Isodimorphic Poly(Butylene Succinate)-Ran-Poly(e-Caprolactone) Random Copolyesters and Their Potential Use as Adhesives

The sole effect of the microstructure of biodegradable isodimorphic poly(butylene succinate)-ran-poly(ε-caprolactone) random copolyesters on their rheological properties is investigated. To avoid the effect of molecular weight and temperature, two rheological procedures are considered: the activation energy of flow, E_a, and the phase angle versus complex modulus plots. An unexpected variation of both parameters with copolyester composition is observed, with respective maximum and minimum values for the 50/50 composition. This might be due to the peculiar chain configurations of the copolymers that vary as a function of comonomer distribution within the chains. The same chain configuration variations are responsible for the isodimorphic character of the copolymers in the crystalline state. Tack tests, performed to study the viability of the copolyesters as environmentally friendly hot melt adhesives (HMA), reveal a correlation with rheological results. Tackiness parameters, particularly the energy of adhesion obtained from stress-strain curves during debonding experiments, are enhanced as melt elasticity increases. Based on the carried-out analysis, the link microstructure-rheology-tackiness is established, allowing selecting the best performing HMA sample considering the polymer chemistry of the system.

Polybutylene Succinate, A potential bio-degradable polymer: Synthesis, copolymerization And Bio-degradation

Poly(butylene succinate) is one of the emerging bio-degradable polymer, which has huge potential to be employed in a wide range of applications. Further, it is also recognized as one of...

Poly(butylene succinate-co-ε-caprolactone) Copolyesters: Enzymatic Synthesis in Bulk and Thermal Properties

This work explores for the first time the enzymatic synthesis of poly(butylene-co-ε-caprolactone) (PBSCL) copolyesters in bulk using commercially available monomers (dimethyl succinate (DMS), 1,4-butanediol (BD), and ε-caprolactone (CL)). A preliminary kinetic study was carried out which demonstrated the higher reactivity of DMS over CL in the condensation/ring opening polymerization reaction, catalyzed by Candida antarctica lipase B. PBSCL copolyesters were obtained with high molecular weights and a random microstructure, as determined by ¹³C NMR. They were thermally stable up to 300 °C, with thermal stability increasing with the content of CL in the copolyester. All of them were semicrystalline, with melting temperatures and enthalpies decreasing up to the eutectic point observed at intermediate compositions, and glass transition temperatures decreasing with the content of CL in the copolyester. The use of CALB provided copolyesters free from toxic metallic catalyst, which is very useful if the polymer is intended to be used for biomedical applications.

Confined Crystallization of Polymers within Nanopores

Crystallization of polymeric materials under nanoscopic confinement is highly relevant for nanotechnology applications. When a polymer is confined within rigid nanoporous anodic aluminum oxide (AAO) templates, the crystallization behavior experiences dramatic changes as the pore size is reduced, including nucleation mechanism, crystal orientation, crystallization kinetics, and polymorphic transition, etc. As an experimental prerequisite, exhaustive cleaning procedures after infiltrations of polymers in AAO pores must be performed to ensure producing an ensemble of isolated polymer-filled nanopores. Layers of residual polymers on the AAO surface percolate nanopores and lead to the so-called "fractionated crystallization", i.e., multiple crystallization peaks during cooling.Because the density of isolated nanopores in a typical AAO template exceeds the density of heterogeneities in bulk polymers, the majority of nanopores will be heterogeneity-free. This means that the nucleation will proceed by surface or homogeneous nucleation. As a consequence, a very large supercooling is necessary for crystallization, and its kinetics is reduced to a first-order process that is dominated by nucleation. Self-nucleation is a powerful method to exponentially increase nucleation density. However, when the diameter of the nanopores is lower than a critical value, confinement prevents the possibility to self-nucleate the material.Because of the anisotropic nature of AAO pores, polymer crystals inside AAO also exhibit anisotropy, which is determined by thermodynamic stability and kinetic selection rules. For low molecular weight poly(ethylene oxide) (PEO) with extended chain crystals, the orientation of polymer crystals changes from the "chain perpendicular to" to the "chain parallel to" the AAO pore axis, when the diameter of AAO decreases to the contour length of the PEO, indicating the effect of thermodynamic stability. When the thermodynamic requirement is satisfied, the orientation is determined by kinetics including crystal growth direction, nucleation, and crystal growth rate. An orientation diagram has been established for the PEO/AAO system, considering the cooling condition and pore size.The interfacial polymer layer has different physical properties as compared to the bulk. In poly(l-lactic acid), the relationship between the segmental mobility of the interfacial layer and crystallization rate is established. For the investigation of polymorphic transition of poly(butane-1), the results indicate that a 12 nm interfacial layer hinders the transition of Form II to Form I. Block and random copolymers have also been infiltrated into AAO nanopores, and their crystallization behavior is analogously affected as pore size is reduced.

A Biodegradable Copolyester, Poly(butylene succinate-co-ε-caprolactone), as a High Efficiency Matrix Former for Controlled Release of Drugs

A biodegradable copolyester, poly(butylene succinate-co-ε-caprolactone) (PBS_CL), was used for first time as an excipient for pharmaceutical dosage forms using direct compression and hot processing techniques (ultrasound-assisted compression (USAC) and hot melt extrusion (HME)). Robust binary systems were achieved with hot processing techniques, allowing a controlled release of the drug. With only 12% v/v of PBS_CL, controlled release forms were obtained using USAC whereas in HME over 34% v/v of excipient is necessary. Amounts over 23% v/v allowed a long-extended release for more than 72 h following diffusional kinetic. Thanks to the high melting point of theophylline and the physicochemical properties of PBS_CL selected and synthesized, the structure of the excipient inside the USAC tablets and HME filaments corresponds to a continuum medium. A percolation threshold around 23% v/v was estimated, which agrees with a continuum percolation model. The polymer shows a high excipient efficiency value using HME and USAC. A nanostructured matrix with wall thicknesses lower than 0.1 µm was obtained. This leads to a very effective coating of the drug particles by the excipient, providing a slow and reproducible release. The present study therefore supports the use of PBS_CL, for the preparation of controlled release dosage forms using hot processing techniques.

Synthesis, Structure, Crystallization and Mechanical Properties of Isodimorphic PBS-ran-PCL Copolyesters

Isodimorphic behavior is determined by partial inclusion of comonomer segments within the crystalline structure and arises from the comparatively similar repeating chain units of the parental homopolymers. Isodimorphic random copolymers are able to crystallize irrespective of their composition and exhibit a pseudo-eutectic behavior when their melting point values are plotted as a function of comonomer content. At the pseudo-eutectic point or region, two crystalline phases can coexist. On the right-hand and the left-hand side of the pseudo-eutectic point or region, only one single crystalline phase can form which is very similar to the crystalline structures of the parent homopolymers. This article aims to study the synthesis method, structure, crystallization behavior and mechanical properties of isodimorphic random PBS-ran-PCL copolyesters. Moreover, this study provides a comprehensive analysis of our main recent results on PBS-ran-PCL random copolyesters with three different molecular weights. The results show that the comonomer composition and crystallization conditions are the major factors responsible for the crystalline morphology, crystallization kinetics and mechanical performance of isodimorphic random copolyesters. Our studies demonstrate that in the pseudo-eutectic region, where both crystalline phases can coexist, the crystallization conditions determine the crystalline phase or phases of the copolymer. The relationships between the comonomer composition and mechanical properties are also addressed in this work.

Effect of the Crystallization Conditions on the Exclusion/Inclusion Balance in Biodegradable Poly(butylene succinate-ran-butylene adipate) Copolymers

Biomedical applications of polymers require precise control of the solid-state structure, which is of particular interest for biodegradable copolymers. In this work, we evaluated the influence of crystallization conditions on the comonomer exclusion/inclusion balance of biodegradable poly(butylene succinate-ran-butylene adipate) (PBSA) isodimorphic random copolymers. Regardless of the crystallization conditions, the copolymers retain their isodimorphic character, displaying a pseudo-eutectic behavior with crystallization in the entire composition range. This illustrates the thermodynamic nature of the isodimorphic behavior for PBSA random copolymers. However, depending on the composition, the crystallization conditions affect the exclusion/inclusion balance of the comonomers. Fast cooling favors butylene adipate (BA) inclusion inside the poly(butylene succinate) (PBS) crystals, whereas isothermal crystallization strongly limits it. PBA-rich compositions behave differently. Both fast and slow crystallization formed the β-phase, whereas BS unit inclusion is favored independently of the cooling conditions. During successive self-nucleation and annealing, the BA inclusion is intermediate between non-isothermal and isothermal conditions, while the crystalline structure of the PBA phase changes from the β-phase to the more stable α-phase. We propose a simple crystallographic model to explain the changes in the unit cell dimension of the copolymers.

Controlling the Isothermal Crystallization of Isodimorphic PBS-ran-PCL Random Copolymers by Varying Composition and Supercooling

In this work, we study for the first time, the isothermal crystallization behavior of isodimorphic random poly(butylene succinate)-ran-poly(ε-caprolactone) copolyesters, PBS-ran-PCL, previously synthesized by us. We perform nucleation and spherulitic growth kinetics by polarized light optical microscopy (PLOM) and overall isothermal crystallization kinetics by differential scanning calorimetry (DSC). Selected samples were also studied by real-time wide angle X-ray diffraction (WAXS). Under isothermal conditions, only the PBS-rich phase or the PCL-rich phase could crystallize as long as the composition was away from the pseudo-eutectic point. In comparison with the parent homopolymers, as comonomer content increased, both PBS-rich and PCL-rich phases nucleated much faster, but their spherulitic growth rates were much slower. Therefore, the overall crystallization kinetics was a strong function of composition and supercooling. The only copolymer with the eutectic composition exhibited a remarkable behavior. By tuning the crystallization temperature, this copolyester could form either a single crystalline phase or both phases, with remarkably different thermal properties.