The structure, morphology and thermal properties of biobased and biodegradable poly(butylene succinate-ran-butylene azelate) random copolyesters with a wide composition range were studied. These PBS-ran-PBAz copolymers can crystallize in the entire composition range despite being random, displaying a eutectic point when their melting point is plotted as a function of composition. Wide Angle Xray Scattering (WAXS) studies confirmed isodimorphic behavior where PBS-rich copolymers crystallize with PBS type unit cells with some PBAz repeating units inclusions and vice versa for PBAz-rich copolymers. Away from eutectic compositions the copolymers exhibit only one crystalline phase (PBS-rich or PBAzrich crystalline phases) while at the eutectic point both phases can crystallize. The only double crystalline copolymer amongst those prepared had a composition close to the eutectic point of 45 mol% PBS (and 55 mol% PBAz). The crystallization of the two phases occurred in the same temperature range upon cooling from the melt at 10ºC/min in a DSC (i.e., coincident crystallization). Self-nucleation (SN) studies of the PBS-rich phase were performed. After SN, the separate crystallization of each phase was clearly observed during cooling from the self-nucleation temperature (i.e., PBS and PBAz-rich phases). Small Angle X-ray Scattering (SAXS) experiments were performed for the first time for this type of isodimorphic copolymers. The results show that the lamellar long period is a strong function of composition. While limited inclusions of PBAz units within the crystal lattice only cause a slight expansion of the PBS component unit cell, the increase of comonomer content produces an unexpected synergistic increase in long periods and lamellar thickness values. In the case of the only double crystalline copolymer examined, the PBS-rich phase forms space filling spherulites (observed by Polarized Light Optical Microscopy, PLOM) at higher temperatures that template the superstructural morphology of the copolymer. These PBS-rich phase spherulites contain radial lamellar stacks whose long period was determined by SAXS. Upon further cooling, the PBAz-rich phase crystallizes in the intra-spherulitic amorphous regions with newly form lamellae that have their own distinct long period according to SAXS results. AFM observations of the PBS-rich crystalline lamellae confirmed the lamellar thickness and long spacings determined by SAXS. A schematic morphological model of the mixed spherulites produced by this double crystalline diblock copolymer is proposed based on the experimental evidences collected by SAXS, PLOM and AFM.

Isodimorphic random copolyesters are intriguing polymeric materials that can crystallize in their entire composition range, despite the random distribution of comonomer units along their chains. This behavior stems from the relatively similar chemical repeating units of the parent homopolymers. In this feature article, we review our recent works on isodimorphic aliphatic copolyesters, and extract general trends in the framework of the literature. Isodimorphic behavior is a complex phenomenon driven by comonomer partition within the crystalline unit cells formed. These copolyesters crystallize in the entire composition range displaying a pseudo-eutectic behavior when their melting points are plotted as a function of composition. Two crystalline phases, which resembled the crystalline structures of the parent homopolymers, are formed, depending on the considered composition range. The unit cell dimensions of the parent homopolymers change, as a consequence of the inclusion of co-units. At the pseudo eutectic point or pseudo-eutectic region, two crystalline phases can co-exist and their formation strongly depends on thermal history. In this case, double crystalline random copolyesters with two melting points and mixed double-crystalline spherulites can be obtained. The exact composition of the pseudo-eutectic point, the level of comonomer inclusion and the crystallinity degrees cannot be easily predicted by the copolyester chemical structure and composition. These are important issues for further future studies, as well as the quantitative determination of comonomer inclusion in the generated crystalline phases. The extraordinary variation of thermal properties, morphology and crystallinity that isodimorphic random copolyesters display as a function of composition, allows to conveniently tailor their biodegradation, permeability to gases and mechanical properties.

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Poly(butylene succinate‐ran‐butylene azelate) random copolyesters were thermally fractionated by successive self‐nucleation and annealing (SSA). The samples before and after SSA were analyzed by differential scanning calorimetry (DSC) and X‐ray diffraction (WAXS and SAXS). WAXS results indicate that a small degree of comonomer inclusion is present in the crystalline phases that are formed in the copolymers depending on composition: a PBS‐like unit cell or/and a PBAz‐like unit cell, thus confirming the isodimorphic behavior of the samples. SSA on the other hand demonstrated that the degree of comonomer exclusion during crystallization is far larger than comonomer inclusion, as judged by the increase in fractionation degree with compositions leading to the pseudo‐eutectic point. Furthermore, WAXS, SAXS, and SSA results show that the isodimorphic behavior is not highly dependent on kinetic factors, as the degree of comonomer inclusion or exclusion in the samples was not significantly altered by SSA thermal fractionation, a thermal treatment that promotes annealing and molecular segregation of defects to the amorphous regions of the material. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 2346–2358

Three series of aliphatic random copolycarbonates, poly(heptane-co-dodecane carbonate) P7C-P12C, poly(butane-co-dodecane carbonate) P4C-P12C and poly(butane-co-heptane carbonate) P4C-P7C, were synthesized by two-step polycondensation process. The organocatalyst 4-(dimethylamino) pyridine (DMAP) was used for the first time to prepare copolycarbonates, as an alternative to metal catalysts, to avoid the toxicity of the remaining catalysts impurities that are difficult to remove after synthesis. Differential scanning calorimeter studies demonstrated the isodimorphic character of the copolycarbonates showing pseudoeutectic points and the crystallization in a wide composition range. Wide angle X-ray scattering (WAXS) results displayed changes in crystallographic plane spacings possibly due to the isodimorphic behavior of the systems. Two double crystalline copolymers were obtained, i.e., 85/15 P4C-P12C and 80/20 P7C-P12C, as they correspond to pseudo-eutectic compositions. Remarkably, for the 80/20 P7C-P12C copolycarbonate, we found a novel behavior. This copolymer exhibits both coincident crystallization and coincident melting during non-isothermal DSC runs. However, WAXS revealed that the material is double crystalline as it contains crystals from P7C-rich and P12C-rich phases. This is the first example of a double crystalline polymeric material that exhibits a single crystallization and a single melting peak, in spite of being double crystalline. Comparing the results obtained for the 3 series of copolycarbonates, we can conclude that it is easier to incorporate a shorter repeating unit chain segment in a crystal formed by a larger repeating unit chain segment.

Poly(butylene succinate-ran-butylene azelate) random copolyesters (PBS-ran PBAz) were studied using Broadband Dielectric Spectroscopy (BDS) and Differential Scanning Calorimetry (DSC). These copolymers are characterized by being isodimorphic and by displaying a eutectic behavior. Depending on their composition, the PBS rich phase, the PBAz rich phase or both phases can crystallize. The complex development of the amorphous phase, especially at compositions around the eutectic point was demonstrated by BDS results. The comonomer fraction included in the amorphous phase was quantified from the relaxation strength of the dielectric β relaxations and thus the crystalline fraction can be calculated. The good agreement between the values determined from BDS and the crystallinity obtained from DSC experiments demonstrate that the degree of comonomer exclusion in these random copolymers during crystallization is far larger than comonomer inclusion. However, the glass transition temperatures determined from the dielectric α relaxation behavior are lower in most of the copolymers than those measured by DSC, whereas they are about the same in the homopolymers. This result is probably caused by a significant amount of amorphous phase in the copolymers that is not influenced by crystallinity. Furthermore, the crystallization behavior of these copolymers at different cooling protocols showed that the α relaxation not only depends on the crystalline fraction but also on the details of the crystallization process in these complex multiphasic copolymers.

Films with shape-memory properties were obtained after photocuring blends of diglycidyl ether of bisphenol A (DGEBA) with previously crystallized poly(ε-caprolactone) (PCL). The effect of the epoxy photopolymerization on the semicrystalline PCL was studied, and transesterification reactions between epoxy and PCL were confirmed to occur in the amorphous regions of the DGEBA/PCL samples by Fourier transform infrared (FTIR) and gel content determination. Polarized light optical microscopy (PLOM) observations showed that because curing and transesterification reactions were confined to the amorphous regions of the sample, a spherulitic template was created with remarkable reversibility upon melting and recrystallization. To analyze the effect of this template on PCL crystallization, self-nucleation experiments were performed by differential scanning calorimetry (DSC). Within the spherulitic templates of photocured samples, in the amorphous regions, chains are confined near cross-linking points. The crystalline memory vanishes upon melting because the cross-link points induce an elastic entropy-driven response that changes the direction of chain orientation in the melt, as revealed by changes observed in the birefringence sign of the sample. The effect of the DGEBA/PCL composition on the shape-memory properties was analyzed, and better fixation was obtained at higher contents of epoxy resin. Instead, better recovery could be obtained for samples with higher amounts of PCL.