All relevant C-13 NMR signals of a series of 19 homogeneous ethylene-propylene copolymers were used to fit the first-order Markov reactivity ratios of the catalyst and the theoretical feeds. The copolymers cover a broad range of comonomer incorporations, from 17.0 to 56.5%, and show both primary (1,2) and secondary (2,1) insertions. As expected, two solutions are found, the normal solution showing a better fit and reliability >> 99.5%. The reactivity ratios, r(12) = 20.0, r(13) = 162.0, r(21) = 0.015, r(23) = 1.3, and r(31) = 0.060, provide direct information about the vanadium-based catalyst, the kinetics, and the chain microstructure. The values also explain the comonomer content-dependent inversion and even predict a 100% secondary insertion PP homopolymer. With these results, the direct peak method shows that the use of all relevant C-13 NMR peaks improves the accuracy of first-order Markov modeling. (c) 2006 Wiley Periodicals, Inc.

To describe the detailed microstructure of homogeneous ethylene-1-alkene copolymer chains and to study the characteristics of single site catalysts, Markov statistics are used to fit peak intensities of all relevant C-13 NMR signals of series of copolymers. In the case of the occurrence of inverted comonomer units, a first-order Markov terpolymer is applied, otherwise a second-order Markov copolymer model. Chain propagation probabilities are obtained via modeling of the entire NMR spectrum. This procedure results in an accurate reproduction of the chain microstructure, including ethylene, 1-alkene, and methylene sequence length distributions. If the experimental (co)monomer feeds are known, the reactivity ratios and the theoretical (co)monomer feeds are also found providing information about the copolymerization kinetics and the characteristics of the catalyst. (c) 2006 Wiley Periodicals, Inc.

Several series of polyamide-6 (PA-6) nanocomposites, differing in montmorillonite (MMT) type and content and PA-6 matrix molecular weight, were prepared by melt-extrusion and the associated PA-6 crystallization behavior and morphology was evaluated using (synchrotron) X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The nucleating ability of silicate layers is poor in PA-6 nanocomposites made by melt-extrusion because highly active, stable PA-6 crystallization precursors are generated during melt-extrusion. In most of the studied PA-6/MMT nanocomposites the dispersed silicate layers act as impurities and decrease rather than increase the overall crystallization kinetics of PA-6, especially at high MMT contents. Furthermore, at a given MMT concentration, the crystal growth retardation inflates with increasing degree of exfoliation, which dependents on the MMT type and which increases with increasing PA-6 molecular weight. One of the considered MMT types leads to a poorly exfoliated nanomorphology and as a result no retardation of crystal growth is observed. Furthermore, the disturbed crystal growth does not alter the PA-6 semicrystalline stack morphology. Moderate nucleation effects due to the presence of MMT can be observed when the particle load is low (low amount of MMT and/or poor degree of exfoliation) and provided the supercooling is sufficiently large.

It is shown that the application of a chip calorimeter, allowing very fast cooling and heating rates up to 10,000 K/s. can be successfully applied to study reorganization phenomena in crystallizable polymers. In this research both bulk Polyamide 6 (PA6) as well as an immiscible (polystyrene/styrene-maleic anhydride copolyrner)/Polyamide 6 blend with dispersed Polyamide 6 droplets of sub-micrometer size have been studied. The blends with sub-micrometer PA6 droplets have been shown to crystallize at low temperature via a homogeneous nucleation mechanism, due to a lack of heterogeneities in the small droplets. Upon fast cooling with more than 500 K/s crystallization of PA6 could be totally prevented. No cold crystallization takes place upon subsequent heating with 500 K/s. Upon fast heating of 2000 K/s after isothermal crystallization, the 'real', initial melting of the crystallites formed at very low temperature could be obtained, which was not possible with standard DSC apparatus or HPer DSC. However, even heating, with 5000 K/s was not fast enough to completely avoid reorganization. Annealing experiments in the melt-range clearly show the fast reorganization of PA6 crystallites, resulting in improved stability within a timescale in the order of 0.01-0.1 s. Most probably reorganization takes place mainly in the crystalline state, and only to a lesser extent via a melting-recrystallization-remelting process. The reorganization process is not hindered by the confined dimensions of the Polyamide 6 droplets, because both bulk PA6 as well as the confined (PS/SMA2)/PA6 blend give rise to identical reorganization phenomena. (c) 2006 Elsevier Ltd. All rights reserved.

The genesis and stability of different PA6 crystalline polymorphs, dispersed as micro- and submicrometer sized droplets inside an amorphous polymer matrix, are discussed over a very broad temperature range. Different PA6 droplet sizes lead to different PA6 crystallization events in a 100°C wide temperature window that extends down to 85°C. Static WAXD and DSC experiments on micrometer sized PA6 droplets indicate the formation of a stable γ-crystal phase in the region between 175 and 130°C. Sub-micrometer sized PA6 droplets only crystallize at 85°C in the β-phase. Upon heating above the PA6 glass transition, these crystals progressively increase their perfection and ultimately transform into the α-phase around 170°C.

In this paper the relation between the blend phase morphology and the fractionated crystallization behavior of PA6 in reactively compatibilized immiscible PS/PA6 and (PPE/PS)/PA6 immiscible blends is studied. Reactive compatibilization is used as an effective tool for controlling the blend phase morphology, and to reduce the PA6 dispersed droplet size. As reactive compatibilizers, SMA2 and SMA17 are used, which differ in their level of miscibility with the amorphous PS and (PPE/PS) components. With SMA2 a strong shift of PA6 crystallization to much higher supercoolings than before is found after compatibilization resulting in crystallization at temperatures as low as 85 °C. This is ascribed to the strong decrease of the droplet sizes down to 100–150 nm. Nucleation experiments show that heterogeneous bulk nucleation can be reintroduced in the submicron-sized PA6 droplets by adding enough nucleating agents of sufficient small size. The degree of fractionated crystallization is found to depend on the interface between PA6 droplets and surrounding medium, as it is influenced by vitrification of the matrix polymer and by the location of the compatibilizers SMA2 and SMA17. The method used for mixing the reactive compatibilizer with the blend components also affects the fractionated crystallization process.

Three series of ethylene-propylene copolymers are made using the catalysts (1) rac-[Me-2-Si(2-Me-4-(1-Naph)Ind)(2)]ZrCl2/MAO, (2) [Me2Si(Ind)(Flu)]ZrCl2/MAO, and (3) a 1:5 mixture of 1 and 2, all with a broad range of (co)monomer mole fractions in the reactor and copolymers. All three series are analyzed by C-13 NMR, DSC, and SEC. The C-13 NMR spectra of the single-site series are the input of the direct peak method to determine the second-order Markov reactivity ratios of the catalysts used. The activities and reactivity ratios of the catalysts in the single-site experiments are used to model the dual-site series based on C-13 NMR data. The results of this modeling are used to interpretate the DSC and SEC data of the dual-site series: most of the melting peak temperatures and molar masses of the dual-site can be attributed to one of the two catalysts.

The original, first-order Kakugo method is extended to find second-order reactivity ratios of homogeneous catalysts in ethene/propene copolymerizations based on triad fractions. Subsequently, the first- and second-order reactivity ratios of eight Si-bridged C-2-symmetric ansa-zirconocenes or hafhocenes, as obtained by the Direct Peak method, are used to compare with the Kakugo methods. From the results it is clear that the second-order methods give better descriptions than the first-order methods. Only the results of the second-order Direct Peak method are statistically justified, reflecting reliabilities >99.5%. Though the Direct Peak method in all cases gives the most reliable reactivity ratios, the second-order Kakugo method turns out to be a practical tool to find estimates.

The crystallinity development in heterogeneous ethylene-1-butene copolymers is compared with that in ethylene copolymers, with more bulky 1-heptene as a comonomer. The thermal transitions of the 1-heptene based copolymers persistently occur at higher temperatures than of the corresponding I-butene copolymers. The earlier crystallization onset is reflected in thicker primary crystals, which in turn are associated with the presence of longer ethylene sequences because of the inaccessibility of 1-heptene to sterically shielded catalytic sites. In addition, the 1-heptene based copolymers are characterized by a higher degree of primary crystallinity, whereas the 1-butene copolymers exhibit more prominent secondary crystallization. The I-butene based copolymers thus have a less heterogeneous chemical composition distribution. At high comonomer contents, the highly heterogeneous nature of the 1-heptene copolymers is emphasized by a more pronounced presence of low crystalline spherulite inclusions accomplished by the liquid-liquid phase separation of dissimilar polymeric chains before crystallization. (c) 2005 Wiley Periodicals, Inc.

The advent of synchrotron radiation has revolutionised the structural studies of hard and soft condensed matter In this paper we discuss the future potential of synchrotron radiation for the study of soft condensed matter with the emphasis on Polymer Science. The brilliance of third-generation synchrotron sources is such that time-resolved studies are feasible in the millisecond range, hence allowing studies of structural changes in polymer systems under processing conditions that involve fast kinetics. Microfocusing devices, the potential of nano-sized beams and the parallel development of fast counting devices in conjunction with adequate software offer new, unprecedented challenges for the structural polymer scientist.

Crystallization kinetics and crystallinity development of PA6 droplets having sizes from 0.1 to 20μm dispersed in immiscible uncompatibilized PS/PA6 and reactively compatibilized (PS/Styrene-maleic anhydride copolymer=SMA2)/PA6 blends are reported. These blend systems show fractionated crystallization, leading to several separate crystallization events at different lowered temperatures. Isothermal DSC experiments show that micrometer-sized PA6 droplets crystallizing in an intermediate temperature range (Tc∼175°C) below the bulk crystallization show a different dependency on cooling rate compared to bulk crystallization, and an athermal crystallization mechanism is suggested for PA6 in this crystallization temperature region. The crystallinity in these blends decreases with PA6 droplet size. Random nucleation, characteristic for a homogeneous nucleation process, is found for sub-micrometer sized PA6 droplets crystallizing between Tc 85 and 110°C using isothermal DSC experiments. However, crystallization in the PA6 droplets is most likely initiated at the PA6-PS interface due to vitrification of the PS matrix during crystallization. Very imperfect PA6 crystals are formed in this low temperature crystallization region, leading to a strongly reduced crystallinity. These crystals show strong reorganization effects upon heating.

The paper addresses the crystallization behavior of homogeneous branched polyethylenes, where the branches cannot be incorporated within the lattice. The polymer was chosen to investigate the morphology achievable by polymers where the chains cannot extend, which is considered to be a requisite to minimize the surface free energy. Pressure-temperature conditions similar to those necessary to form extended chain crystals in linear polyethylenes are applied. In-situ wide-angle X-ray diffraction and Raman spectroscopy are used to follow the structural and conformational changes during crystallization. The hexagonal phase is not observed in these polymers unlike in linear polyethylene. However, crystallization at elevated pressures results in a structural organization of the interphase and the fold surface; this provides adjacent reentry, where the branches will also possess structural order. Crystallization of these components leads to the formation of an incompressible open-orthorhombic phase (alpha = 7.56 angstrom, b = 5.03 angstrom, c = 2.55 angstrom, density = 960 kg/m(3)) in addition to the existing orthorhombic crystalline domain. With the crystallization of the chains at the interphase and on the fold surface, a contraction in the parent orthorhombic phase or its transformation into the monoclinic phase is observed. Our experimental data suggest that in such a class of polymers the thermodynamically stable state will be crystals with an ordered interphase that can be achieved ultimately by disentanglement of the chains in the amorphous region. The disentangled nature of the amorphous component is further supported by solid-state mechanical deformation of samples.

Liquid and vapour n-hexane sorption/desorption were studied on homogeneous poly(ethylene-co-octene)s produced by metallocene-catalyzed polymerisation covering a crystallinity range from 3.5 to 72.4%. Crystal core contents determined by Raman spectroscopy were lower than those determined by density assessments, particularly at low degrees of crystallinity. The solubility showed deviation from Henry's law. The solubilities of n-hexane in the homogeneous copolymers depended in a non-linear manner on the content of penetrable polymer component and were lower than those earlier reported for heterogeneous copolymers at the same contents of penetrable component. The concentration dependence of the thermodynamic diffusivity predicted by the Cohen–Turnbull–Fujita free volume theory was confirmed by the data obtained by the differential method and the differences between the results obtained from the integral and differential methods were within the margins of experimental error. The fractional free volume of the penetrable polymer fraction increased with increasing fraction of penetrable polymer and with relative proportion of liquid-like component in the penetrable polymer fraction. The homogeneous copolymers showed a decreasing trend in the geometrical impedance factor with increasing degree of crystallinity.

The melting of a homogeneous ethylene-1-octene copolymer after isothermal crystallization is discussed based on DSC and time-resolved SALS, SAXS and WAXD data. Two melting peaks appear in DSC suggesting the presence of two crystal fractions. All crystals grow in a lamellar habit and there is no evidence for fringed micellar or isolated block-like crystals. The high melting fraction crystallizes while segregating comonomer-rich parts into separate regions where in a later stage the low melting fraction crystallizes. The data support the view of lamellae that grow via the secondary nucleation of crystalline blocks from a preexisting layer-like mesomorphic phase with preservation of the mesomorphic layer thickness. The stability of these blocks increases due to sintering, forming lamellae that melt slightly above the crystallization temperature. The high melting fraction is generated from those lamellae that are able to reduce the crystalline-amorphous interfacial tension.

In this paper the crystallization behavior of PA6, dispersed as droplets in various immiscible amorphous polymer matrices, is reported. PA6 was melt-mixed at various compositions with PS, (PPE/PS 50/50 wt/wt) and PPE using twin-screw extrusion. The phase morphologies of the obtained blends were analysed using SEM, etching experiments and image analysis. The crystallization behavior of PA6 was investigated by dynamic and isothermal DSC experiments. In case PA6 is dispersed as droplets, fractionated crystallization behavior occurs, characterized by several crystallization events at different, lowered crystallization temperatures. It is found to depend on the blend morphology (size of the droplets) and the thermal history of the samples (heterogeneous nucleation density). The PA6 droplet size distribution is shown to strongly influence the crystallization behavior of the droplets. Vitrification of the matrix appears to cause nucleation in the droplets at the interface. Decreasing the PA6 droplet size results in slower overall crystallization rates.

We report Monte Carlo simulations of a lattice-polymer model that can account for both polymer crystallization and liquid-liquid demixing in solutions of semiflexible homopolymers. In our model, neighboring polymer segments can have isotropic interactions that affect demixing, and anisotropic interactions that are responsible for freezing. However, our simulations show that the isotropic interactions also have a noticeable effect on the freezing curve, as do the anisotropic interactions on demixing. As the relative strength of the isotropic interactions is reduced, the liquid-liquid demixing transition disappears below the freezing curve. A simple, extended Flory-Huggins theory accounts quite well for the phase behavior observed in the simulations.

DSC & simultaneous real-time SAXS and WAXS studies of the isothermal crystallisation, melting and morphology of homogeneous polyethylene-1-octene copolymers are presented. The multiple melting behaviour of the copolymers observed after isothermal crystallisation indicates that two kinds of crystals of different stability are formed. Two melting endotherms appear after different periods of isothermal crystallisation, testifying that the melting-recrystallisation-remelting process is not responsible for the effects observed. The crystals which melt at higher temperatures are probably formed from the longest ethylene sequences and are of the lamellar type, while the crystals which melt at lower temperatures are formed from the shorter extended sequences and are of a fringed micelle type.

A review describing the latest advances in the mesoscopic and molecular modelling of polyolefins is presented. Mesoscopic investigation into the effects of sequence length and sequence length distribution on the reinforcement of stereoblock-stereoregular polyolefins has been performed. These polymers consist of alternating atactic sequences, which are amorphous and act as elastomeric chains, and isotactic sequences which, if long enough, will crystallise, and act as physical reinforcing cross-links. According to simulated morphology, the degrees of crystallinity of the different samples have been predicted. Mechanical properties such as Young's modulus at small extensions are also predicted in terms of the block size of the alternating isotactic and atactic sequences. Molecular simulation investigation into the influence of the chain microstructure on the conformational behaviour of these polymers has been detailed. Characteristic ratios, calculated on the basis of the rotational isomeric state model, have indicated the increased extension of the polymer backbone with the increase in the side chain length. The lower characteristic ratio calculated for octene polymers may explain the experimental observation that polyoctene has a lower melting point than other polyolefins. Probability distribution surfaces constructed by the integration of the molecular dynamics trajectories indicated an increase in the probability of g(+/-)t joint states on the expense of g(+/-)g(+/-) pairs with the increase in the side chain length.

Laboratory measurements on polymer raw materials and products are often carried out (quasi-) isothermally, or at best at relatively slow cooling and heating rates. Such conditions differ greatly from those occurring during processing and during the productis use in real life. During processing, high cooling rates and high pressures are common, the parameters of which have a†major influence on the most important forming processes, namely vitrification and crystallisation. In addition, polymer systems are highly metastable: all kinds of changes are possible as a function of time and temperature, such as reorganisation (via recrystallisation and annealing), cold crystallisation, solidsolid transitions, and superheating. Reorganisation phenomena can be hindered by applying high heating rates. Therefore, not only high cooling rates are required, but also techniques enabling high heating rates would be very welcome. In this review article, we will comment on the metastability of polymers under extreme conditions: high-pressure DSC and X-ray at hundreds of MPais; scan-iso temperature (time ramps by DSC), and X-ray & high scanning rates as made possible by the recently developed High Performance DSC (HPer DSC). This is a generic name that encompasses quantitative measurement using controlled, linear scanning rates at hundreds of ∞C/min (high-rate DSC) in both the cooling and heating of (sub) milligram amounts of material. As such, it also facilitates high-throughput DSC.

We report Monte Carlo simulations of the melting of a single-polymer crystallite. We find that, unlike most atomic and molecular crystals, such crystallites can be heated appreciably above their melting temperature before they transform to the disordered "coil" state. The surface of the superheated crystallite is found to be disordered. The thickness of the disordered layer increases with superheating. However, the order-disorder transition is not gradual but sudden. Free-energy calculations reveal the presence of a large free-energy barrier to melting.