Influence of the Main-Chain Configuration on the Mechanical Properties of Linear Aliphatic Polyesters

Abstract

Linear aliphatic polyesters have been investigated extensively over the last decades. However, the relation between the configuration of the ester groups in the main chain and the mechanical properties is only poorly understood. Therefore, in this work the influence of the composition of these polymers on the morphology, mechanical properties and relaxation processes for a set of random copolymers of ε-caprolactone (CL) and ω-pentadecalactone (PDL) is explored. For these isomorphic copolymers, the crystallinity and lamellar thickness was shown to be independent of the composition over the largest part of the composition range. However, the yield stress does decrease significantly when comonomers were introduced in either of the homopolymer chains. Dynamic mechanical analysis revealed an additional high-temperature relaxation process for the copolymers, which was attributed to mobility of the crystalline phase (α c -mobility) and is responsible for the decrease in yield stress relative to the homopolymers. The origin of this mobility was related to the stacking of the ester groups in the crystal lamellae, which occurs less regular in copolymers and therefore gives rise to lower energy barriers for defect propagation (responsible for α c -mobility). Furthermore, the yield-kinetics of polypentadecalactone and two copolymers were accurately captured using the Ree−Eyring theory, showing the relation between α c -mobility and the contribution of both interlamellar and intralamellar shear to the yield stress. ■ INTRODUCTIONAliphatic long-chain polyesters (ALCPEs) are interesting alternatives for polyethylenes in high-end applications, since their structure can be tuned to include additional functionalities, such as degradability, which is difficult to achieve for polyethylene. 1−3 The "PE-like" character of this class of polymers originates from their long linear methylene backbone dominating the crystallization behavior and crystal structure, as well as the resulting mechanical properties. 4−6 An example is polypentadecalactone, which crystallizes in an orthorhombic unit cell with dimensions very similar as for PE. 7 The ester groups fit well in the orthorhombic unit cell, leading to the inclusion of these groups with only a minor associated expansion of the unit cell dimensions compared to PE. 7 The most profound effect of the inclusion of ester groups is an enthalpic penalty reducing the heat of fusion of the crystal, which simultaneously decreases the melting temperature. 8,9 Nevertheless, the crystallinity remains high (>50%) independent of the amount of ester groups in the backbone of the polymer.Even though there are many reports on the synthesis and thermal properties of ALCPEs, 10−15 a systematic investigation into the influence of the amount and configuration of the ester groups on the mechanical properties has to our knowledge not been performed. Copolymers of ω-pentadecalactone (PDL) and ε-caprolactone (CL) are ideal candidates for such an investigation, since the random copolymers are easily synthesized in ...

Deformation and failure kinetics of iPP polymorphs

“…Concerning the high temperature/low strain rate process I, plastic deformation of the crystalline lamellae through a crystallographic slip mechanism was previously proposed as the underlying mechanism . Remarkably, however, the results presented here for the different polymorphs seem to disagree with this interpretation.…”

“…The yield stress of polymers depends strongly on loading conditions and, in the case of semicrystalline polymers, it depends on structural features like crystallinity or lamellar thickness as well . Furthermore, imperfections in the crystal lattices can have a pronounced effect . It is difficult to draw clear conclusions on the effect of these distinct structural features, since it is hard to vary single features without affecting others.…”

Synthesis of statistical PET/PEN random block copolymers and their crystallizability in the bulk and at the surface

“…A later study with AFM directly imaged the lamella thicknesses, in good agreement with the SAXS measurements. 49 In contrast, a study of linear aliphatic polyesters consisting of E-caprolactone (CL) and x-pentadecalactone (PDL) 50 indicated that, up to 80% CL units could be incorporated within the PDL lattice without loss of crystallinity and with constant lamellar thickness. Contrary to the PET/PEN system however, the authors considered that in the CL/PDL system the CL units are incorporated within the PDL crystal structure without the need for sequence matching.…”

Tuning the thermal properties of poly(ethylene)‐like poly(esters) by copolymerization of ε‐caprolactone with macrolactones, in the presence of a pyridylamidozinc(II) complex

“…34 In the framework of our interest in the ROP of macrolides, we wish to report the achievement of the homo-and copolymerization of 6HDL with ω-PDL and CL in the presence of a pyridylamidozinc(II) complex, which has been recently described by some of us for the copolymerization of CO 2 and epoxides 35 and for the block copolymerization of CL and lactides. 36 The copolymerization of ω-PDL with other lactones, having conventional ring-size, has been already described in the literature 27,[37][38][39][40][41][42][43][44] to give both block and random copolymers. Conversely, the copolymerization of other macrolactones, such as ethylene brassilate, with smaller lactones was also described in the literature.…”