Search

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.

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.

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...

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.

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.

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.

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

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 ).

A series of poly(mono n-alkyl itaconate)s, poly(methyl n-alkyl itaconate)s and poly(di n-alkyl itaconate)s with n ¼ 12; 14, 16, 18 and 22 have been prepared by radical polymerization. NMR studies point out that poly(methyl n-alkyl itaconate)s and poly(di n-alkyl itaconate)s are mainly syndiotactic polymers whereas poly(mono n-alkyl itaconate)s are obtained as almost atactic polymers. The characterization performed by DSC, solid state 13 C CP/MAS NMR and X-ray diffraction, indicates that the side chains of poly(mono n-alkyl itaconate)s and poly(methyl n-alkyl itaconate)s derivatives with more than 12 carbon atoms are able to crystallize in hexagonal lattices. In the case of poly(di n-alkyl itaconate)s, when the side chains contain 12 or more carbon atoms, they are able to crystallize also in hexagonal lattices. q 2003 Elsevier Science Ltd. All rights reserved.

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 ).

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.

In the present work, the recently developed thermal fractionation technique successive self-nucleation and annealing (SSA) was applied to blends of branched and linear polyethylenes (PEs), as a method to evaluate the phenomena of miscibility and segregation in PE blends. The melting behavior of the systems, after a controlled cooling and after the application of SSA was compared. The DSC scans corresponding to cooling of blend samples from the melt or subsequent heating displayed an overlap of the exotherms and endotherms, respectively, of the blend components that could lead to interpretations of partial miscibility. These effects were interpreted as arising from: a nucleation effect of the crystals formed by the more linear PE on the crystals of branched PE, a dilution effect of the molten branched chains on the crystals formed by the more linear PE and finally possible partial miscibility effects. However, after SSA, even when the nucleation and dilution effects are still present for some of the fractions produced, the thermal fractionation procedure helps to distinguish them from co-crystallization effects. This is mainly achieved by observing how the number of thermal fractions generated by SSA in the blends varies with composition and by comparing the relative amounts of the thermal fractions produced by SSA. The results indicate that only those PE fractions that are similar in chemical structure, as regards to content and distribution of short chain branches, are probably miscible in the melt and can oppose molecular segregation during SSA and therefore produce stable co-crystals in the solid state. The application of SSA to the study of PE blends can be a quick and convenient tool for making comparisons and ascertain miscibility of different types of PE blends.

Copoly(α-alkyl-β,l-aspartate)s containing n-octadecyl and n-butyl side groups at different ratios were prepared by anionic ring-opening polymerization of the corresponding optically pure (S)-4-alkoxycarbonyl-2-azetidinones. Random copolymers were obtained by polymerization of comonomer mixtures. Diblock copolymers were achieved by sequential copolymerization using the living poly(α-n-octadecyl-β,l-aspartate) block to initiate the polymerization of (s)-4-n-butoxycarbonyl-2-azetidinone as a second block. Composition and sequence distribution were characterized by NMR with the help of a model copoly(β,l-aspartate) made of α-n-dodecyl and α-benzyl β,l-aspartate units. All the copolymers were found to adopt the layered structure made of 13/4 helices described previously for comb-like poly(α-n-alkyl-β,l-aspartate) homopolymers. Copolymers containing at least 50% of n-octadecyl side groups have these groups crystallized in an interlayer microphase and aligned normal to the main helical chain. Melting of the paraffinic crystallites happened within the 40–60°C temperature range with formation of a liquid-crystal phase in which side chains are molten but retain the alignment of the low temperature phase. Different from what is known to happen to poly(α-n-octadecyl-β,l-aspartate), no indications on the occurrence of a second structural transition were observed at higher temperatures.

A new technique to thermally fractionate polymers using DSC has been recently developed in our laboratory. The applications of the novel successive self-nucleation and annealing (SSA) technique to characterize polyolefins with very dissimilar molecular structures are presented as well as the optimum conditions to thermally fractionate any suitable polymer sample with SSA. For ethylene/α-olefin copolymers, the SSA technique can give information on the distribution of short chain branching and lamellar thickness. In the case of functionalized polyolefins, detailed examinations of SSA results can help to establish possible insertion sites of grafted molecules. The application of the technique to characterize crosslinked polyethylene and crystallizable blocks within ABC triblock copolymers is also presented.

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.

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.

Graft copolymers of atactic polypropylene (aPP) and polystyrene (PS) were synthesized and characterized by 13C NMR analysis. The 13C NMR spectra of the grafts exhibited changes with respect to physical blends of identical compositions. The most important occurred in the meso sequences of the aPP blocks of the copolymer. These changes suggest that some grafting took place during the synthesis. SEM micrographs indicate greater interfacial interaction between phases in the copolymers than in the physical blends. Thermal Analysis showed that the Tg of the copolymers is higher than that of the PS homopolymers prepared under the same conditions. This could be the result of an apparent increased in molecular weight caused by the grafting of the aPS into the aPP chains. TGA results indicated that the thermal stability of the copolymer decreases as the aPP content in the copolymer increases.

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.