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It is widely known that when a polymer is heated just above its melting point and is kept at a given temperature (denoted Ts) for a short time, when it is cooled down its nucleation density increases and its peak crystallization temperature shifts to higher temperatures, as detected for instance by differential scanning calorimetry (DSC). The Ts temperature range where the described process occurs has been named Domain II self‐nucleation (SN) because the selected Ts temperatures are high enough to melt the polymer without causing detectable annealing of any remnant crystals by DSC. Experimental results obtained by DSC, polarized light optical microscopy (PLOM), and rheology indicate that these techniques are unable to detect any remaining crystal fragments in Domain II. Our kinetic results demonstrate that Domain II SN is a transient phenomenon that can even disappear if enough time at Ts is allowed. Results of the study of the time dependence of the SN effect indicates two possibilities: (a) if crystal fragments are present (even if undetected by the employed techniques) their final melting is a very slow process (in the order of hours); (b) if all crystallites have melted in Domain II, then it may be more plausible to reinterpret self‐nuclei as arising from “precursors” whose detail nature has not been the subject of this investigation but that can be regarded as either a residual segmental orientation in the melt (i.e., a melt memory effect) or a mesophase in a preordered state. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1738–1750, 2006

The confinement of crystallizable blocks within AB or ABC microphase-separated block copolymers in the nanoscopic scale can be tailored by adequate choice of composition, molecular weight, and chemical structure. In this work we have examined the crystallization behavior of a series of AB and ABC block copolymers incorporating one or two of the following crystallizable blocks:  polyethylene, poly(ε-caprolactone), and poly(ethylene oxide). The density of confined microdomain structures (MD) within block copolymers of specific compositions, in cases where the MD are dispersed as spheres, cylinders, or any other isolated morphology, is much higher than the number of heterogeneities available in each crystallizable block. Therefore, fractionated crystallization takes place with one or several crystallization steps at decreasing temperatures. In specific cases, the clear observation of exclusive crystallization from homogeneous nuclei was obtained. The results show that, regardless of the specific morphological features of the MD, it is their vast number as compared to the number of heterogeneities present in the system that determines the fractionated character of the crystallization or in extreme cases homogeneous nucleation. The self-nucleation behavior was also found to depend on the composition of the copolymers. When the crystallizable block is confined into spheres or cylinders and exhibits homogeneous nucleation, the self-nucleation domain disappears. This is a direct consequence of the extremely high density of microdomain structures that need to be self-seeded (on the order of 1015−1016/cm3). Therefore, to increase the density of self-nuclei, the self-nucleation temperature has to be decreased to values so low that extensive partial melting is achieved, and some of the unmelted crystal fragments can be annealed, in some cases even before self-nucleation takes place.

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The morphology, nucleation, and crystallization of polyethylene/carbon nanotubes nanocomposites were studied. The nanocomposites were prepared by in-situ polymerization of ethylene on carbon nanotubes (CNT) whose surface had been previously treated with a metallocene catalytic system. The effects of composition (5−22% CNT) and structure of the nanotube (single, double, or multiwall, i.e., SWNT, DWNT, and MWNT) were evaluated, and an excellent nucleating effect on polyethylene matrix was found regardless of the CNT type in comparison to neat high-density polyethylene (HDPE) prepared under identical conditions. The CNT were found to be more efficient in nucleating the HDPE than its own crystal fragments, a result obtained by self-nucleation studies. Differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) results showed that under both isothermal and dynamic crystallization conditions the crystals produced within the nanocomposite HDPE matrix were more stable than those produced in neat HDPE or in physical blends prepared by melt mixing of HDPE and untreated CNT. The remarkable stability of the crystals was reflected in melting points up to 5 °C higher than neat HDPE and concomitant thicker lamellae. The changes induced on HDPE by CNT are due to the way the nanocomposites were prepared; since the macromolecular chains grow from the surface of the nanotube where the metallocene catalyst has been deposited, this produces a remarkable nucleating effect and bottle brush morphology around the CNT. Isothermal crystallization kinetics results showed that the in-situ nanocomposites crystallize much faster at equivalent supercoolings than neat HDPE because of the nucleating effect of CNT. Wide-angle X-ray scattering studies demonstrated that the crystalline structure of the HDPE matrix within the in-situ-polymerized HDPE/CNT nanocomposites was identical to that of neat HDPE and did not change during isothermal crystallization, keeping its orthorhombic unit cell.

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