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Successive self-nucleation and annealing (SSA) is applied to thermally fractionate a model hydrogenated polybutadiene. SSA produces discrete and well-separated thermal fractions with distinct melting peaks. The samples are studied by SAXS as a function of temperature. The SAXS profile displays only one scattering peak associated with the interlamellar correlation and is unable to discriminate the multimodal distribution of lamellar thicknesses in the samples. The average lamellar thickness associated with each melting fraction can be estimated from SAXS data at different temperatures. A modified Gibbs-Thomson equation is employed to predict the lamellar thickness from DSC data, giving results consistent with those derived from SAXS.

The Successive Self-nucleation and Annealing (SSA) thermal fractionation technique is briefly reviewed to highlight recent advances such as the use of high speed DSC concepts to perform faster fractionation. Additionally, recent applications of SSA to characterize confined semi-crystalline model polyethylenes in block copolymer nanophases and in situ prepared nanocomposites with carbon nanotubes are also reviewed. A novel result presented here deals with the use of a macromolecular plasticizer to improve the quality of SSA thermal fractionation.

Summary: The crystallization behavior of four types of polyethylene (PE) containing diblock copolymers with an approximate 50/50 composition has been investigated and compared with their respective PE homopolymers. A glassy block of polystyrene (PS), a rubbery block of poly (D,L-lactide) (PDLA), a semicrystalline block of poly (L-lactide) (PLLA) and a miscible block of (poly(ethylene-alt-propylene) (PEP) were used to asses the influence of the degree of confinement and miscibility on the crystallization kinetics of PE. PEP had the largest effect on the crystallization kinetics of the PE block in view of its miscibility. For the strongly segregated systems larger restrictions were imposed by vitreous PS due to hard confinement as compared with the soft confinement of the rubbery PDLA block. A nucleation effect of previously crystallized PLLA on the PE block was detected which offset its depression of the crystallization kinetics of PE.

A series of well defined polyethylene-b-polystyrene diblock copolymers (E x S y z , where x and y represent the composition in weight % and z the molecular weight in Kg/mol) has been synthesized in a wide composition range by sequential anionic polymerization. The molecular weight of the PE block was kept constant. A fractionated crystallization behavior was observed for the PE block within E 26 S 74 105 (PE cylinders) and E 11 S 89 244 (PE spheres). When the PE blocks form a continuous or percolated phase (PE, E 79 S 21 41 and E 53 5 47 51 ), a classic self-nucleation behavior (where the usual three self-nucleation domains are obtained) was observed. When the PE block is located within isolated microphases (having dimensions on the nanometer scale) and a fractionated crystallization was detected (E 26 5 74 105 and E 11 S 89 244 ), the fraction of crystals formed at higher temperatures exhibits a classic self-nucleation behavior, while those crystals that crystallized at the largest supercooling (lower exotherms) can only be self-nucleated at lower temperatures where annealing of unmolten material has already started. An unusual fractionated crystallization behavior for isolated, spherical PE microphases (E 11 S 89 244 ) is reported.

A first attempt was made to produce novel ABC triblock terpolymers with three potentially crystallisable blocks: polyethylene (PE), poly(ethylene oxide) (PEO), and poly(e-caprolactone) (PCL). Polybutadiene-b-poly(ethylene oxide) diblock copolymers were synthesized by living anionic polymerization. Then, a non-catalyzed thermal polymerization of e-caprolactone from the hydroxyl end group of the PB-b-PEO diblock precursors was performed. Finally, hydrogenation by Wilkinson catalyst produced PE-b-PEO-b-PCL triblock terpolymers. Side reactions were detected that lead to the formation of undesired PCL-b-PEO diblock copolymers, however, these impurities were successfully removed by purification. A range of triblock terpolymers with PCL and PEO minor components were prepared. Topological restrictions on the PEO middle block prevented this block from crystallizing while the complex crystallization behavior of the PE and PCL blocks was documented by DSC and WAXS measurements.

Summary: The technique of successive self-nucleation and annealing (SSA) has been applied using differential scanning calorimetry (DSC) to a model hydrogenated polybutadiene prepared by anionic polymerization and to a commercial Ziegler-Natta ethylene/1-butene copolymer. Here, it is shown that the use of high scanning rates (50 °C · min−1) in the SSA protocol can reduce the thermal fractionation time to only 78 min in length (if 6 fractions are produced with a fractionation window of 5 °C), taking into consideration the need to compensate the increment in heating rates by reducing the sample mass. This time is much shorter than those previously achieved by thermal fractionation in the literature, where fractionation times of 12 or 24 h are common. Potentially, much higher rates could be employed by further reducing the fractionation times by SSA. For the first time, a distribution of lamellar thicknesses has been obtained by transmission electron microscopy after SSA fractionation and compared to distributions calculated by the Thomson-Gibbs equation. It is shown that the results are highly dependent on the equilibrium melting temperatures employed in the calculations, and it is recommended that the values of lamellar thicknesses obtained are checked by comparing with methylene sequence lengths derived from calibration measurements available in the literature. The Thomson-Gibbs equation is useful for comparing lamellar thickness distributions obtained by SSA with different polymers only on a semi-quantitative basis. Rate and mass compensation as a way to reduce fractionation time employing SSA.

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

Blends of copolymers of ethylene/hexene (9 CH 3 /1000 C) and ethylene/butene (77 CH 3 /1000 C) synthesized with metallocene catalysts were prepared by co-precipitation from solution. A phase diagram for this blend had been obtained in a preceding work, where the blends were found to be miscible in the melt with a characteristic upper critical solution temperature (UCST). In this work, the successive self-nucleation and annealing (SSA) thermal fractionation method revealed that both pure copolymers, even though they had been prepared by metallocene catalysis, displayed a very broad distribution of short chain branching. The thermal behavior of the pure copolymers and their blends revealed that co-crystallization effects were present within each phase and that molecular fractionation during crystallization can prevail when the materials are slow cooled from the melt. A crystallization kinetics study was performed by differential scanning calorimetry (DSC) and it revealed a strong competition between crystallization and phase segregation. Upon cooling from a one phase melt, phase segregation precedes crystallization if the crystallization temperature (T g ) is high. On the other hand when low crystalliation temperatures are employed, the amount of phase segregation that can be achieved before crystallization is limited. TEM allowed the observation of the lamellar morphology and their aggregation state. In specially prepared samples, the relative lamellar frequency of isothermally crystallized material was employed to ascertain whether the blend under consideration was within the one or the two phase region of the blends phase diagram.

Blends of very low density polyethylene (VLDPE) and high density polyethylene (HDPE) were prepared by melt extrusion. These blends exhibit a tendency to phase segregate when they are slow cooled from the melt. If they are cooled at increasingly faster rates, a finite population of co-crystals can be isolated from the rest of the phase segregated material, indicating that this system is probably miscible in the melt but phase separates during cooling. Transmission electron microscopy observations are consistent with the blend melt miscibility since inter-lamellar mixing was clearly appreciated in the samples examined. Other effects arising from interactions between the polymers were the nucleation of VLDPE rich phase by HDPE rich phase, and a melting point depression of HDPE rich phase caused by a dilution effect exerted by molten VLDPE rich phase. After a successive self-nucleation and annealing thermal fractionation procedure is applied to the blends, phase separation dominates the behavior, although some small fraction of co-crystals was still present.

We have recently prepared a series of Polystyrene-b-Poly(ethylene oxide)-b-Polycaprolactone (PS-b-PEO-b-PCL or SEOCL) triblock copolymers of varying compositions and molecular weights. These ABC triblock copolymers present the peculiarity that two of the three blocks are able to crystallize upon cooling from an already phase segregated melt. When either of the crystallizable blocks or both are a minor phase, a fractionated crystallization process develops. The confinement of crystallizable blocks in the nanoscopic scale enables the clear observation in some cases of exclusive crystallization from homogeneous nuclei of two components within the triblock copolymer. The homogeneous nature of the nucleation was deduced since the supercooling attained is the maximum possible before vitrification of the material takes place. The self-nucleation domains were also found to depend on the composition and molecular weight of the copolymers. The block copolymers exhibited a marked decrease in crystalline memory and when the crystallizable blocks constitute minor phases, the self-nucleation domain disappears. The reason behind this behavior is that only at lower self-nucleation temperatures the density of self-nuclei becomes high enough to include at least one crystal fragment per confined microdomain in view of their vast numbers (e.g., 10 16 /cm 3 ).

In this work, several PS/HDPE (high density polyethylene), PS/LLDPE (linear low density polyethylene) and PS/ULDPE (ultra low density polyethylene) blends were prepared in a composition range were atactic polystyrene (PS) was always the matrix component. All the types of polyethylenes employed, when dispersed into droplets, exhibited fractionated crystallisation exotherms in the temperature range between 67 and 70 °C. These low crystallisation temperatures are probably closer to the homogeneous nucleation temperature of linear polyethylene than any previously reported value, since they occur at higher supercoolings. The higher values of crystallisation temperatures previously reported could be explained by the crystallisation from heterogeneous nuclei of relatively low nucleation efficiency or by a weak nucleation capacity of the droplets interface. By applying a self-nucleation procedure we were able to corroborate that what causes the fractionated crystallisation is the lack of highly active heterogeneous nuclei (i. e., those normally active at low supercoolings in the bulk polymer) in every droplet. When polyethylene/α-olefin copolymers are finely dispersed in a PS matrix, a molecular segregation process can be induced during mixing facilitated by the heterogeneous distribution of short chain branches in the copolymers. The crystallisation of droplets that contained mostly these highly branched chains occurs at temperatures lower than 50 °C, thereby producing another low temperature exotherm which may be the lowest present in the blend and could be mistaken by the crystallisation of homogeneously nucleated crystals. We have shown that the self-nucleation technique can help to distinguish these low temperature exotherms from those originated by differences in nucleation behaviour; therefore, a plausible interpretation of all the possible fractionated crystallisation exotherms of ethylene/α-olefin copolymer droplets was made possible.

When crystallisable polymers like isotactic poly(propylene) (iPP) or linear low density polyethylene (LLDPE) are finely dispersed in an incompatible matrix like atactic polystyrene (PS) a fractionated crystallisation process will develop if the number of dispersed droplets is greater than the number of active heterogeneities originally present in the bulk polymer. In this work, several PS/iPP and PS/LLDPE blends were prepared in a composition range where PS was always the matrix component and in some cases compatibilizers were used to enhance dispersion. By applying a self-nucleation procedure we were able to corroborate that what causes the fractionated crystallisation is the lack of highly active heterogeneous nuclei (i.e., those normally active at low supercoolings in the bulk polymer) in every droplet. A detailed characterisation of the particle size distribution was carried out by SEM and the validity of using a Poisson distribution to calculate the concentration of heterogeneities present in one blend system was examined. The calculation of the concentration of heterogeneities can qualitatively explain the presence or absence of particular exotherms in the complex DSC cooling behaviour of some compositions of the PS/LLDPE/SEBS blends. However, the effect of the dispersity of the particle size distribution was found to greatly influence the results. When sufficient amount of a compatibilizer is used to obtain the minimum possible particle size the iPP crystallises exclusively at 45 C. The origin of such crystallisation and the possibility that it may be interpreted as arising from homogeneous nucleation is discussed along with analogous data for dispersed LLDPE.