Diblock nucleation

Since excellent reviews on block copolymer crystallization have been published recently [43,44], we have concentrated in this paper on aspects that have not been previously considered in these references. In particular, previous reviews have focused mostly on AB diblock copolymers with one crystal-lizable block, and particular emphasis has been placed in the phase behavior, crystal structure, morphology and chain orientation within MD structures. In this review, we will concentrate on aspects such as thermal properties and their relationship to the block copolymer morphology. Furthermore, the nucleation, crystallization and morphology of more complex materials like double-crystalline AB diblock copolymers and ABC triblock copolymers with one or two crystallizable blocks will be considered in detail. 

Lotz and Kovacs [93] reported n = 0.5 many years ago when their PS-b-PEO diblock copolymers crystallized at very large supercoolings (i.e., homogeneous nucleation was assumed to take place). However, the origin of this result was not explained. 

The technique of self-nucleation can be very useful to study the nucleation and crystallization of block copolymers that are able to crystallize [29,97-103]. Previous works have shown that domain II or the exclusive self-nucleation domain disappears for systems where the crystallizable block [PE, PEO or poly(e-caprolactone), PCL] was strongly confined into small isolated MDs [29,97-101]. The need for a very large number of nuclei in order to nucleate crystals in every confined MD (e.g., of the order of 1016 nuclei cm 3 in the case of confined spheres) implies that the amount of material that needs to be left unmolten is so large that domain II disappears and annealing will always occur to a fraction of the polymer when self-nucleation is finally attained at lower Ts. This is a direct result of the extremely high number density of MDs that need to be self-nucleated when the crystallizable block is confined within small isolated MDs. Although this effect has been mainly studied in ABC triblock copolymers and will be discussed in Sect. 6.3, it has also been reported in PS-fc-PEO diblock copolymers [29,99]. 

Table 5 Self-nucleation behavior for diblock and triblock copolymers ...

Chen et al. [92] also performed self-nucleation experiments by DSC in PB-fr-PEO diblock copolymers and PB/PB-b-PEO blends. The cooling scans presented in their work showed that a classical self-nucleation behavior was obtained for PEO homopolymer and for the PB/PB-b-PEO blend where the weight fraction of PEO was 0.64 and the morphology was lamellar in the melt. For PB/PB-fr-PEO blends with cylinder or sphere morphology, the crystallization temperature remained nearly constant for several self-seeding temperatures evaluated. This observation indicates that domain II or the self-nucleation domain was not observable for these systems, as expected in view of the general trend outlined earlier. 

The isothermal crystallization of PEO in a PEO-PMMA diblock was monitored by observation of the increase in radius of spherulites or the enthalpy of fusion as a function of time by Richardson etal. (1995). Comparative experiments were also made on blends of the two homopolymers. The block copolymer was observed to have a lower melting point and lower spherulitic growth rate compared to the blend with the same composition. The growth rates extracted from optical microscopy were interpreted in terms of the kinetic nucleation theory of Hoffman and co-workers (Hoffman and Miller 1989 Lauritzen and Hoffman 1960) (Section 5.3.3). The fold surface free energy obtained using this model (ere 2.5-3 kJ mol"1) was close to that obtained for PEO/PPO copolymers by Booth and co-workers (Ashman and Booth 1975 Ashman et al. 1975) using the Flory-Vrij theory. 

Lowenhaupt and Hellmann (1991) have determined whether microphase separation or macrophase separation occurs in blends of a PS-PMMA diblock with PMMA homopolymer with a < 1 and a > 1 using TEM. They found that the transition between purely microphase separation and macrophase separation occurs for a lower diblock content for blends with a smaller a, as supported by calculations of the instability limit using the random phase approximation. Blends with a < 1.4 were always initially microphase separated, although in a blend with a - 1.4 this was followed by macrophase separation. However, the macrophase-separated structure took the form of aggregates of micelles (see Fig. 6.1), suggesting a nucleation and growth mechanism for the secondary... 

Fig. 6.2 Representative micrographs showing macrophase separation (Lowenhaupt and Hellmann 1991) (a) and (b) are bicontinuous structure, typical of those for spinodal decomposition (c) and (d) show discrete domains, consistent with a nucleation and growth process of macrophase separation. The diblock details are as Fig. 6.1, the homopolymer has A/w = 161 kg mol-1. Temperatures and volume fraction of copolymer are indicated.

In search of easy dispersible nucleating agents with a high number of nucleants per volume, Spitael et al. [19] investigated the use of nanoscale diblock copolymer micelles on the batch-foaming behavior of PS. The diblock copolymers were composed of a PS block, and either PDMS, PEP, or PMMA as a second block. Several factors were identified as essential for nucleation, e.g., the size of the micelle and the surface tension of the micelle core material. 

---