Double-Crystalline Diblock Copolymers

There have been relatively few reports dealing with double-crystalline diblock copolymers [102-110,197-200]. The particular case of ABC triblock copolymers with two semicrystalline blocks will be presented in a separate section. Works pertaining to one of the most studied systems PCL-fo-PEO have already been previously reviewed [43]. Recently, probably the most comprehensive studies on double-crystalline diblock copolymer systems were performed on poly(p-dioxanone)-fc-PCL diblock copolymers, PPDX-fo-PCL, and therefore several important aspects of these works [102,103,107] will be summarized in this section. 

The PPDX-fr-PCL diblock copolymers were recently synthesized [111] and apart from the references already mentioned, only the contribution of Lendlein and Langer [112] deals with chemically similar materials, although structurally quite different since they employed multiblock copolymers of PPDX and PCL with very low molecular weights to prepare shape memory polymers for biomedical applications. 

PPDX crystallization, reaching a plateau prior to a further increase in integrated area upon PCL crystallization. The PCL crystallization is clearly seen in the growth of the 26 = 21.5° peak, which can be ascribed to an exclusive 

In a related double-crystalline PE-fr-poly(L-lactide), PE-fo-PLLA diblock copolymer (with an approximate 50/50 weight ratio), similar behavior was observed [103]. When the diblock copolymer is cooled from the melt at 10 °C min 1 only one crystallization exotherm is observed, indicating that the L-lactide block is also affected by the molten PE block that is covalently bonded to it [103]. However in this case, the crystallization exotherms can be separated by reducing the cooling rate to 2 °C min-1 [ 106]. 

In order to study the overall crystallization kinetics of the PCL block within PPDX-fo-PCL diblock copolymers, Muller et al. [103] first crystallized the PPDX block until saturation by performing a special thermal procedure (it consisted of first cooling from the melt as in Fig. 6 to allow both blocks to crystallize, then the sample was heated to 62 °C and annealed at that temperature for 70 min, a temperature at which the PCL is molten, before quenching to a Tc where the PCL block isothermal crystallization was followed by DSC). With the use of such a procedure the overall isothermal crystallization of only the PCL block was determined in the diblock copolymers where the PPDX block was already crystallized. 

The results of Muller et al. [103] on PPDX-fo-PCL diblock copolymers differ from those obtained previously by Bogdanov et al. [105] when they studied by DSC the crystallization kinetics of 80/20 PCL-fo-PEO diblock copolymers. In their case, the PCL block crystallized first from a homogeneous melt and the Avrami parameters K and n were found to be similar to the kinetics parameters of the isothermal crystallization of a corresponding PCL homopolymer. Significant crystallization retardation was found for the PEG block that crystallized second. The retardation was attributed to the mutual influence between the PEG constituent and the PCL crystal phase which fixes (hardened) the total copolymer structure [ 105]. In the PPDX-fo-PCL diblock copolymer case, conversely, when the PPDX block crystallizes it does so at a slower rate than a comparable PPDX homopolymer. The crystallization of the PCL block, on the other hand, strongly depends on the composition of the diblock copolymer as shown in Fig. 9. However, for D77 C23 , the overall kinetics is retarded, which could be regarded as similar to the retardation experienced by the PEG block in the 80/20 PCL-fi-PEG diblock copolymer studied by Bogdanov et al. [105] the Avrami index in both cases was also of the order of 2. 

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T, = 66.6°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) ... 

As can be seen from the results presented above, the study of PEO-6-PCL has made evident the complexity of the crystallization and melting processes in double crystalline diblock copolymers that have the following characteristics their crystallization starts from a homogenous melt, crystallization and melting points of the blocks are very near to each other, and there is an absence of a nucleating effect of each block on the other. 

In this section, two examples of the use of self-nucleation are presented for block copolymers and nanocomposites. The first example shows how self-nucleation can help separate the coincident crystallization of a double crystalline diblock copolymer. In the second example, the efficiency scale based on self-nucleation helps to define supernucleation. 

Hamley IW, Parras P, Castelletto V, Castillo RV, Muller AJ, PoUet E, Dubois P, Martin CM. Melt structure and its transformation by sequential crystallization of the two blocks within poly(L-lactide)-WocA -poly(e-caprolactone) double crystalline diblock copolymers. Macromol ChemPhys 2006 207 941-953. 

Li S, Myers SB, Register RA. Sohd-state structure and crystallization in double-crystalline diblock copolymers of linear polyethylene and hydrogenated polynorbomene. Macromolecules 2011 44 8835-8844. 

Salim NV, Hanley T, Guo Q. Microphase separation through competitive hydrogen bonding in double crystalline diblock copolymer/homopolymer blends. Macromolecules 2010 43(18) 7695-7704. 

Crystallization within Strongly Segregated Double-Crystalline Diblock Copolymers and Triblock Copolymers, 355... 

Polyethylene-fi-poly(ethylene oxide) diblock copolymers (PE-6-PEO) represent an interesting double-crystalline diblock copolymer system since both blocks are strongly segregated and at the same time they have a marked tendency to crystallize in view of their highly flexible linear chains [20]. Hillmyer et al. [271] synthesized PE-6-PEO block copolymers of 2.1 and 2.8 kg/mol from hydroxyl terminated poly(l,4-butadiene) catalytically... 

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