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In this paper, the synergistic effects that carbon nanotubes (CNTs) produce on the basic rheological properties and crystallization of polyethylenes with different branch contents and molecular weights was investigated. Multiwalled carbon nanotubes coated with polyethylene (as produced by in situ polymerization) were blended in the melt (in a 1% wt. ratio) with three polyethylene matrices of different molecular weights and branch contents. Transmission electron micrographs demonstrated excellent carbon nanotube dispersion in all samples and the existence of a geometrical percolation network. The rheological and calorimetric properties of the nanocomposites were determined and the results compared to those obtained for neat polyethylene resins. Both Newtonian viscosity and steady-state shear recoverable compliance increased with the addition of CNTs in all cases. However, the increase was strongly dependent on the molecular weight (and dispersity index) of the matrices regardless of the branch content. A novel screening effect of the CNTs network due to the high relaxation times of the matrix with the highest molecular weight was detected. This important result demonstrates that viscoelasticity can hinder the measurement of the rheological percolation threshold of CNTs network depending on the scale of relaxation times involved. Additionally, it was found that in relative terms (comparing each nanocomposite with its neat polyethylene matrix), the Mw values also play a vital role in CNT nucleation besides chain branching content. Both nonisothermal and isothermal nucleation effects caused by CNTs increased as the Mw of the polyethylene matrix decreased in spite of the role played by short chain branches in decelerating their overall crystallization kinetics. The capability for producing more stable lamellae through successive annealing of the nanocomposites as compared to their neat matrices also followed a decreasing trend with molecular weight increases, as indicated by SSA thermal fractionation results. Nevertheless, the presence of branches played a major role, since fractionation quality improved greatly as the branch content increased in the samples, as expected on the basis of the sensitivity of thermal fractionation to the presence of defects along crystallizable sequences. © 2014 American Chemical Society., The authors acknowledge funding support from the Spanish Ministerio de Economia y Competitividad (MINECO) for the ́ Project: MAT2012-36341.

In this work, the nucleation and crystallization behavior of melt mixed PCL/CNT nanocomposites has been studied. The mixtures of PCL and pristine MWNTs were prepared by extrusion with different nanofiller contents: 0.3, 0.5, 0.7, 1 and 3%. Standard DSC measurements demonstrated pronounced nucleation effects as well as increases in PCL crystallinity. The nucleation effect saturates at only 0.5% (a value much lower than those previously reported in the literature for similar nanocomposites) indicating that the dispersions obtained were excellent. This was corroborated by both TEM observations and by the determination of a very low dielectric percolation threshold (i.e., 0.3%). In self-nucleation experiments, supernucleation effects were obtained up to a maximum of approximately 200% efficiency. This is the first time that supernucleation effects of this order have been reported for PCL filled with untreated MWNTs, a result that we attribute to the excellent dispersion achieved. Isothermal crystallization experiments performed by DSC showed an increase in the crystallization kinetics of PCL with increases in MWNT content as a consequence of the supernucleation effect. The Avrami equation successfully described the overall crystallization kinetics and while neat PCL exhibited Avrami indexes close to 3, indicating that instantaneously nucleated spherulites were formed, the nanocomposite yielded mostly Avrami index values close to 2, as expected for axialites instantaneously nucleated on the surface of the MWNTs. Remarkably, the temperature dependence of the overall crystallization rate exhibited a dramatic change with MWNT content. This novel effect was described as a crystallization regime change (i.e., from Regime II to Regime III) induced by the presence of the MWNTs in terms of the Lauritzen and Hoffman theory.

Dc and ac conductivities in a broad angular frequency, 2π × 10−3 s−1 < ω < 2π × 107 s−1, and temperature range, 133 K < T < 323 K, were measured in poly(e-caprolactone)/multiwalled carbon nanotube nanocomposites as a function of weight concentration (0 wt % ≤ p ≤ 3 wt %) . The universal dynamic response was observed at low p values, and the parameters were adjusted by working in several domains. Scaling laws predicted by the variation of dc conductivity as a function of p or characteristic frequency in percolation theory were followed and showed the formation of a 3D percolative cluster at concentration values as low as 0.3 wt %. The existence of tunneling conduction was proved, and its independence on temperature indicates that we are in the presence of a conventional tunnel effect among nanoparticles not yet in physical contact, through energy barriers made by the polymer chains. All these combined facts allowed construction of a master curve evidencing the time−temperature−composition superposition exi...

A nanocomposite sample was prepared by melt mixing a high density polyethylene (HDPE) with an in situ polymerized HDPE/multi wall carbon nanotube (MWNT) masterbatch. The nanocomposite had an approximate content of 0.52 wt % MWNT. Rheological, thermal, and mechanical properties were investigated for both neat HDPE and nanocomposite. The nanocomposite, when compared to the neat polymer, exhibits lower values of viscosity, shear modulus and shear stress in extrusion and a concurrent delay of the distortion regimes to higher shear stresses and rates. The nanocomposite presents also improved dimensional stability after processing, and lower values of the melt strength, draw ratio and viscosity in elongational flow. This behavior has been observed in composites in which an adsorption of a fraction (that with the highest molecular weight or relaxation time) of the polymer chains is considered. Furthermore, the enhancement in the crystallization kinetics, probed by rheometry and DSC, suggests that the carbon nano...

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.

Nanocomposites of high-density polyethylene (HDPE) and carbon nanotubes (CNT) of different geometries (single wall, double wall, and multiwall; SWNT, DWNT, and MWNT) were prepared by in situ polymerization of ethylene on CNT whose surface had been previously treated with a metallocene catalytic system. In this work, we have studied the effects of applying the successive self-nucleation and annealing thermal fractionation technique (SSA) to the nanocomposites and have also determined the influence of composition and type of CNT on the isothermal crystallization behavior of the HDPE. SSA results indicate that all types of CNT induce the formation of a population of thicker lamellar crystals that melt at higher temperatures as compared to the crystals formed in neat HDPE prepared under the same catalytic and polymerization conditions and subjected to the same SSA treatment. Furthermore, the peculiar morphology induced by the CNT on the HDPE matrix allows the resolution of thermal fractionation to be much better. The isothermal crystallization results indicated that the strong nucleation effect caused by CNT reduced the supercooling needed for crystallization. The interaction between the HDPE chains and the surface of the CNT is probably very strong as judged by the results obtained, even though it is only physical in nature. When the total crystallinity achieved during isothermal crystallization is considered as a function of CNT content, it was found that a competition between nucleation and topological confinement could account for the results. At low CNT content the crystallinity increases (because of the nucleating effect of CNT on HDPE), however, at higher CNT content there is a dramatic reduction in crystallinity reflecting the increased confinement experienced by the HDPE chains at the interfaces which are extremely large in these nanocomposites. Another consequence of these strong interactions is the remarkable decrease in Avrami index as CNT content increases. When the Avrami index reduces to 1 or lower, nucleation dominates the overall kinetics as a consequence of confinement effects. Wide-angle X-ray experiments were performed at a high-energy synchrotron source and demonstrated that no change in the orthorhombic unit cell of HDPE occurred during crystallization with or without CNT.

The confined crystallization of poly(ethylene oxide) (PEO) in predominantly spherical microdomains formed by several diblock copolymers was studied and compared. Two polybutadiene-b-poly(ethylene oxide) diblock copolymers were prepared by sequential anionic polymerization (with approximately 90 and 80 wt % polybutadiene (PB)). These were compared to equivalent samples after catalytic hydrogenation that produced double crystalline polyethylene-b-poly(ethylene oxide) diblock copolymers. Both systems are segregated into microdomains as indicated by small-angle X-ray scattering (SAXS) experiments performed in the melt and at lower temperatures. However, the PB-b-PEO systems exhibited a higher degree of order in the melt. A predominantly spherical morphology of PEO in a PB or a PE matrix was observed by both SAXS and transmission electron microscopy, although a possibly mixed morphology (spheres and cylinders) was formed when the PEO composition was close to the cylinder-sphere domain transitional composition as indicated by SAXS. Differential scanning calorimetry experiments showed that a fractionated crystallization process for the PEO occurred in all samples, indicating that the PE cannot nucleate PEO in these diblock copolymers. A novel result was the observation of a subsequent fractionated melting that reflected the crystallization process. Sequential isothermal crystallization experiments allowed us to thermally separate at least three different crystallization and melting peaks for the PEO microdomains. The lowest melting point fraction was the most important in terms of quantity and corresponded to the crystallization of isolated PEO spheres (or cylinders) that were either superficially or homogeneously nucleated. This was confirmed by Avrami index values of approximately 1. The isothermal crystallization results indicate that the PE matrix restricts the crystallization of the covalently bonded PEO to a higher degree compared to PB.

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...

The crystallization kinetics of the polyethylene block within polyethylene-b-polystyrene (PE-b-PS) and polyethylene-b-poly(ethylene-alt-propylene) (PE-b-PEP) diblock copolymers has been investigated in a wide composition range by differential scanning calorimetry (DSC) and compared to an equivalent homopolymer. The morphology of the copolymers in the melt and its changes after crystallization were explored by small-angle X-ray scattering (SAXS) experiments. SAXS experiments demonstrated that PE-b-PS formed microdomain ordered structures in the melt. On the basis of the lamellar morphology that was developed by PE-b-PEP (demonstrated by transmission electron microscopy, TEM) regardless of composition, it was deduced that these diblocks crystallize from either a homogeneous or a weakly segregated melt. Also, the effects of the neighboring block on thermal fractionation of the PE block were obtained by successive self-nucleation and annealing (SSA). Classical kinetics theories of polymer nucleation and cryst...

In this paper we offer guidelines to adequately fit isothermal polymer crystallization kinetics data obtained by differential scanning calorimetry (DSC) employing the widely used Avrami equation. A methodology on how the experimental DSC data should be measured and later analyzed in order to minimize the possible errors associated with data manipulation is provided by a thorough evaluation of: (i) the determination of the onset of crystallization or induction time, (ii) the establishment of the baseline and incomplete isothermal crystallization data, (iii) the effect of the cooling rate from the melt to the isothermal crystallization temperature and (iv) the conversion range employed for the fitting. Therefore, this paper provides a practical guide to the fitting of the Avrami equation along with error assessments.

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.

Well defined polyethylene-b-polystyrene (PE-b-PS) diblock copolymers have been synthesized by anionic polymerization in a wide composition range, keeping the length and the microstructure of the PE block constant. These copolymers provide a model system to study the confined crystallization of PE. A fractionated crystallization behavior (i.e., multiple exotherms are observed upon cooling from the melt) was observed for the PE block within block copolymers with 26% and 11% PE (i.e., E 26 S 74 105 and E 11 S 89 244 diblock copolymers, where the subscripts indicate the composition in wt% and the superscript the molecular weight in kg/mol). For confined PE, annealing is always observed when the material is self-nucleated. In the case of E 11 S 89 244 , most of the PE spheres crystallize at very large supercoolings, a behavior previously reported in the literature and associated with homogeneous nucleation. However, in our case, the peak crystallization temperature (i.e., Tc = 46.6 °C) is much lower (16 °C) than that reported for similar size PE nano-droplets but in diblock copolymer whose second block is chemically different to PS. We therefore conclude that the nature of the interphase between the two neighboring blocks may be responsible for such low temperature nucleation, since this Tc is still quite high with respect to the vitrification temperature of PE (in comparison with other polymers whose homogeneous nucleation temperature has been found close to Tg), a fact that could indicate that the homogeneous nucleation temperature has not been reached because surface (or interfacial) effects can dominate. Thermal fractionation takes advantage of the distribution of methyl sequence length in hydrogenated polybutadiene in order to produce by successive thermal annealing a distribution of lamellar thickness within each PE block. Such a distribution of thermal fractions is affected by confinement and therefore these experiments demonstrate the influence of morphological restrictions on the crystallization of the PE block within the PE-b-PS diblock copolymers. As the PE content in the copolymer decreases, topological confinement effects limit the size of the lamellar crystals that can be formed within the reduced dimensions of the microdomains (MD). By the use of the Gibbs-Thomson equation and the thermal fractionation results, a distribution of crystalline lamellar thickness within each MD was obtained and the orientation of the chains within the MD was deduced.

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.

Thermal fractionation techniques offer quick and practical ways to evaluate chain heretogeneities in semicrystalline thermoplastic materials by employing carefully designed thermal cycles in a Differential Scanning Calorimeter (DSC). They are particularly useful to study the degree and distribution of short chain branches produced by the copolymerization of ethylene with α-olefins, however, other materials have also been recently examined by these techniques. Thermal fractionation provides an alternative to experimentally more time consuming and complicated fractionation techniques that involve preparative or analytical fractionation in solution. In this review, a particular emphasis is made on the two techniques most commonly applied in the literature: step crystallization from the melt (SC) and successive self-nucleation and annealing (SSA). The numerous applications that have been recently developed are also reviewed.

In this work, the behavior of the crystallizable blocks within polystyrene- b -poly(ethylene oxide)- b -poly(e-caprolactone) linear triblock copolymers is studied. The apparent crystallization and melting temperatures of PEO and PCL can be coincident as a result of their close values, their dependence on the molecular weight of the blocks and on the relative amount of each component within the copolymers. When the content of both PEO and PCL is low, typically 20% or less, both blocks crystallized coincidentally upon cooling from the melt at very large supercoolings ( T c =−40 °C), suggesting that homogeneous nucleation has taken place. A S 63 EO 16 C 20 copolymer (where subscripts denote the weight fraction of the components) exhibited a small angle X-ray scattering (SAXS) pattern at room temperature that suggests a cylindrical morphology for the minor components, where the crystallization of PEO and PCL blocks can take place subsequently near −40 °C in a hard confinement fashion within the vitreous PS matrix. We have employed differential scanning calorimetry (DSC) to study the self-nucleation process and the results indicate that the difficulty in generating enough self-seeds to nucleate every isolated microdomain causes the disappearance of Domain II (or exclusive self-nucleation Domain ) and therefore a direct transition from complete melting ( Domain I ) to self-nucleation and annealing ( Domain III ) is observed. When the content of both PEO and PCL blocks is larger than 35%, a coincident crystallization can be observed as well as coincident melting. For a S 15 EO 37 C 48 triblock copolymer, PS is a minor phase and mixed spherulites composed of both PCL and PEO blocks were observed by Polarized Optical Microscope (POM) at appropriate supercoolings. In this case, the self-nucleation behavior is standard, and through its use the crystallization and melting process of both blocks can be separated. Wide angle X-ray scattering (WAXS) at different temperatures was used to corroborate the sequential melting of both semicrystalline blocks deduced by self-nucleation. Isothermal crystallizations were also followed by WAXS as a function of time and the results showed that depending on the crystallization temperature ( T c ), only the PCL block crystallizes (at high T c ), or the PCL block crystallizes first followed by the PEO at later times (at lower T c ).

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.

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 ).

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(e-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 morphologica...

The radical polymerization of methyl n-octadecyl itaconate initiated with AIBN, produces poly(methyl n-octadecyl itaconate) with a yield of 47% and a molecular weight of about 105 g/mol. 13C-NMR studies point out that this polymer is mainly syndiotactic. The combined studies of DSC and solid state 13C CP/MAS NMR indicate that the polymer side chains are able to crystallize in a paraffinic phase, presumably in an hexagonal form which melts at 44°C with a ΔH of 4.8Kcal/mol.

The preparation of polystyrene-b-poly(ethylene oxide)-b-poly(-caprolactone) triblock co- polymers using three different strategies is described. Experimentally, a method we have called "thermal" is the simplest one of the three employed. However, it presents serious limitations for the preparation of high molecular weight triblock copolymers with low polydispersity. The titration method is a more elegant alternative to prepare the PS-b-PEO-b-PCL triblock copolymers. Nevertheless, the stoichiometric titration of the precursor terminal hydroxyl groups requires much expertise, and it becomes more difficult as the concentration of such terminal groups decreases when the precursor molecular weight increases. Finally, the sequential method may be regarded as the most convenient one. However, the experimental steps involved a solvent exchange during polymerization that must be performed under conditions of strict purity and high vacuum. Several triblock and pentablock copolymers of varying compositions and molecular weights were successfully prepared. The fact that these copolymers contain two crystallizable blocks, i.e., PEO and PCL, makes them very interesting from a structural point of view. The copolymers were found to be phase segregated according to evidence provided by DSC and WAXS. The crystallization and melting behavior of both PEO and PCL blocks is greatly affected by composition. Fractionated crystallization phenomena were detected when either one or both blocks constituted the minor components within the block copolymers.

A new procedure to fractionate ethylene/α-olefin copolymers using DSC is presented. This procedure allows melt/melt and melt/solid segregation to occur during thermal cycles that promote self-nucleation, crystallization and annealing processes (Successive Self-Nucleation/ Annealing, SSA). The SSA has been compared with the Step-Crystallization (SC) method proposed earlier in the literature to qualitatively characterize chain branching distribution in a faster and easier way than Temperature Rising Elution Fractionation (TREF). In general, SSA produces better fractionation than SC and the DSC derived chain branching distribution by SSA can be qualitatively comparable to that obtained by TREF. The SSA technique could have important applications for the characterization of polymers that crystallize over a broad temperature range.

Differential Scanning Calorimetry (DSC) can provide a qualitative measure of the state of dispersion of an immiscible blend if the minor phase exhibits fractionated crystallization when dispersed into fine particles. The technique is only sensitive to the volume of the dispersed particle and not to its shape and can only be used when the exotherms of interest do not overlap with other thermal transitions present in the multicomponent system. Selfnucleation is a valuable tool to ascertain the presence of fractionated crystallization. The morphology induced by fractionated crystallization in immiscible blends could lead to enhanced plastic deformation during yielding of the matrix.

Recent results on the crystallization of block copolymers with more than one crystallizable block are reviewed. The effect that each block has on the nucle- ation, crystallization kinetics and location of thermal transitions of the other blocks has been considered in detail. Depending on the thermodynamic repulsion between the blocks, the initial melt morphology in weakly segregated double crystalline di- block copolymers can be sequentially transformed by the crystallization of the dif- ferent blocks. The crystallization kinetics of each block can be dramatically affected by the presence of the other, and by the crystallization temperature; the magni- tude of the effect is a function of thermodynamic repulsion. Also the morphology has been investigated and peculiar double crystalline spherulites with intercalated semi-crystalline lamellae of each component have been observed in weakly segre- gated diblock copolymers. In the case of ABC triblock copolymers with more than one crystallizable block, many interesting effects have been found; among them, self-nucleation, sequential or coincident crystallization, and fractionated crystalliza- tion can be mentioned. Additionally, the effect of the topological constrains due to the number of free ends has been studied. Factors like chemical structure, molec- ular weight, molecular architecture and number of crystallizable blocks provide a very large number of possibilities to tailor the morphology and properties of these interesting novel materials.

In this paper we reexamine recent results obtained by our group on the crystallization of nanocomposites and linear and miktoarm star copolymers in order to obtain some general features of their crystallization properties. Different nanocomposites have been prepared where a close interaction between the polymer matrix and the nano-filler has been achieved: in situ polymerized high density polyethylene (HDPE) on carbon nanotubes (CNT); and polycaprolactone (PCL) and poly(ethylene oxide) (PEO) covalently bonded to carbon nanotubes. In all these nanocomposites a “super-nucleation” effect was detected where the CNTs perform a more efficient nucleating action than the self-nuclei of the polymer matrix. It is believed that such a super-nucleation effect stems from the fact that the polymer chains are tethered to the surface of the CNT and can easily form nuclei. For polystyrene (PS) and PCL block copolymers, miktoarm star copolymers (with two arms of PS and two arms of PCL) were found to display more compact morphologies for equivalent compositions than linear PS-b-PCL diblock copolymers. As a consequence, the crystallization of the PCL component always experienced much higher confinement in the miktoarm stars case than in the linear diblock copolymer case. The consequences of the topological confinement of the chains in block copolymers and nanocomposites on the crystallization were the same even though the origin of the effect is different in each case. For nanocomposites a competition between super-nucleation and confinement was detected and the behavior was dominated by one or the other depending on the nano-filler content. At low contents the super-nucleation effect dominates. In both cases, the confinement increases as the nano-filler content increases or the second block content increases (in this case a non-crystallizable block such as PS). The consequences of confinement are: a reduction of both crystallization and melting temperatures, a strong reduction of the crystallinity degree, an increase in the supercooling needed for isothermal crystallization, a depression of the overall crystallization rate and a decrease in the Avrami index until values of one or lower are achieved indicating a nucleation control on the overall crystallization kinetics.

In this work, isotactic poly(propylene) (iPP) was finely dispersed in immiscible atactic polystyrene (PS) matrices. When the dispersion obtained is fine enough (droplet size of approximately 1-2 μm), the iPP crystallizes in a fractionated fashion as temperatures between 104 and 42°C. By applying a self-nucleation procedure, we were able to corroborate that what causes the fractionated crystallization in most droplets is the lack of highly active heterogeneous nuclei (i.e. those normally active at low supercoolings in the bulk polymer) in every droplet. When a sulficient amount of a compatibilizer is used to obtain very small particle sizes and more homogenerous dispersions, the iPP crystallizes exclusively at a low temperature exotherm that exbibits an onset at 51°C and peaks at 46°C. Wide-angle X-ray diffraction measurements indicated that both the iPP in the bulk and in dispersed droplets crystallized in the monoclinic α-phase, this evidence may rule out the possibility that the crystallization observed at 46°C is due to the formation of another crystal modification or a mesomorphic phase as previously suggested in literature. The results presented in this work indicate that this low temperature exotherm may represent the dynamic crystallization during cooling of heterogeneity-free droplets that nucleate homogeneously at temperatures close to 51°C.