Double Crystalline AB and ABA Copolymers

In 1972, Ferret and SkouUos [6,7] pubUshed the first reports on double crys-taUine diblock copolymers with PEO and PCL semicrystaUine components, i.e., poly (ethylene oxide)-6-poly(e-caprolactone) (PEO-6-PCL or EOC). Later on, several authors have prepared and characterized this kind of materials or their ABA analogs, i.e., PCL-6-PEO-6-PCL [8-19]. Recently, PEO as well as PCL have also been incorporated in linear [20-23] and star shaped ABC triblock terpolymers [24-27]. 

Based on X-ray measurements, Nojima et al. [9], Piao et al. [17] and He et al. [18,19] found that poly(ethylene oxide)-6-poly(e-caprolactone) diblock 

In the literature many differences can be found in the temperature range used for the study of the crystalUzation and melting processes of PEO and PCL based AB diblock and ABA triblock copolymers. When the studies are performed above room temperature, an important fraction of the blocks may remain amorphous [8-11, 14, 16] however, most authors report that when the study is extended at temperatures below Tg, both blocks can crystallize [13-15,17]. In the case of ABA triblock copolymers, it has been found that the B-block remains amorphous when its content is lower than 10%, or its molecular weight is very low. Piao et al. [17] and He et al. [18,19] synthesized either poly(e-caprolactone)-6-poly(ethylene oxide)-6-poly(e-caprolactone) ABA triblock copolymers, as well as poly(ethylene oxide)-6-poly(e-caprolactone) AB diblock copolymers. They used poly(ethylene glycol) (PEG) as precursor and a calcium catalyst. Then, they characterized the materials by using NMR, DSC, WAXS and Polarized Optical Microscopy (POM). Cooling DSC scans carried out by He et al. [18] in AB diblock copolymers of different compositions are presented in Fig. 13.1. 

Although almost all the copolymers examined exhibited two crystaUization exotherms, in EOg Cg/ (the subcripts and superscripts indicate the wt.% and molecular weight, in Kg/mol, of each block respectively) the PEO does not crystalUze. EOgQ C5o also exhibits a single crystalUzation exotherm, but in this case, this is due to coincident crystalUzation of both blocks as demonstrated by WAXS. He et al. previously evaluated the crystaUization and melting behaviour of the corresponding homopolymers they obtained values of T, = 31.6°C and Tm = 63.5°C for a PCL , and = 42.6°C and 

6°C for a PEO [18], The crystallization process is more complicated in the double crystalline diblock copolymers than in the homopolymeis. In the specific case of PEO-6-PCL diblocks, the crystallization of the first block occurs at supercoolings higher than in the correspondmg PEO homopolymer as a consequence of the diluent effect caused by the homogeneous melt. This effect becomes evident when comparing the crystallization temperature of PEO (Tc = 42.6°C) with that of the PEO block within = 36.2°C)  

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