Block copolymers nucleation/growth kinetics

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. 

The effect of poly(methyl methacrylate), PMMA, on the crystallization kinetics of poly(ethylene oxide) has been investigated using the Avrami equation to analyze the results (80). The crystallization-rate constant, k, decreased as the concentration of PMMA increased. This and other results indicated that, in the blends, crystallization proceeds by a predetermined nucleation and this is followed with a two-dimensional growth. There has been evidence of melt compatibility for these two polymers (81-84) see Section V. Crystallization behavior of blends of poly(ethylene oxide) with poly(propylene oxide) (85) and with poly(vinyl acetate) (83) have been studied, as well as star and block copolymers of ethylene oxide and styrene (86). 

From the results given above, three cases can be considered (1) percolated systems where the crystallizable block is not in isolated MDs as most of the lamellar forming block copolymers (Avrami indices >2) (2) block copolymers that form cylinders within an amorphous matrix, which can be considered an intermediate case since it could contain a fraction of percolated cylinders and a fraction of isolated cylinders therefore, its fractionated crystallization process will be a reflection of the mixture of these two crystal populations (Avrami indices between 1 and 2) and (3) systems with isolated MDs that can be exemplified by spheres within an amorphous matrix. In this case the overall crystallization kinetics will be dominated by primary nucleation since the growth within such nano-droplets can be considered instantaneous (Avrami indices around 1 or lower). Table 12.3 shows a compilation on the Avrami index values obtained for several systems, and the data on this table are in agreement with the three cases we have just explained. 

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