Crystallization Behaviors

In the isothermal crystallization of polypropylene montmorillonite (PP-MMT) nanocomposites, it was found that the crystallinity of the composites decreased with increasing montmorillonite content, indicating the dispersion of MMT layers in the polypropylene matrices. The nanoparticles confined the polypropylene chains and hindered the polymer crystallization. The spherulites of the PP-MMT nanocomposites were greatly decreased in size as MMT was introduced. On the other hand, the crystallization rate increased dramatically with the increasing of MMT content. The interfacial free-energy per unit area perpendicular 

For further clarification, the activation energy of crystallization, E, is calculated for glasses by using the modified Ozawa equation  

SEM observations correspond well to the visible and IR transmission spectra, as shown in Fig. 1.10. When the degree of crystallization is low (Fig. 1.9a], sample GC2 shows almost an imperceptible change in its transmission spectrum. As the annealing temperature rises to 250°C (sample GC4], the growth of crystals 

Methods used to compare and contrast the crystallization behavior of water soluble crystalline polymers with dispersed silicates may include cross-polarization optical microscopy (CPOM) or atomic force microscopy (AFM), depending upon physical properties of the materials such as spherulite size and optical properties. Other methods used to study crystallization behavior of such materials include differential scanning calorimetry (DSC) and x-ray diffraction (XRD). 

1 Cross-polarized optical microscopy and atomic force microscopy 

5(f)) we see that the effeet of the MMT on the crystallite growth resulted in spherulites grown in a haphazard fashion with tortuous lamellar pathways and jagged edges. Also, the erystallite size is markedly smaller than the spherulites developed in neat PEO (Fig. 8.4(b) and (d)). 

Before we consider the differenees between neat and filled PVA systems, we shall briefly diseuss how the crystallization of PVA develops in films cast from PVA/water solutions. As cast these films are mostly amorphous, and crystallites initiate predominately in the final drying stages crystallization proceeds slowly thereafter, aided by the ambient humidity. If the ambient humidity is too low or absent, the drying polymer becomes glassy and crystal growth becomes arrested before extended crystallites can develop and impinge. Though PVA has a T  

9 The nucleation density as a function of silicate loading, as measured from cross polarization optical microscopy. Crystallization is done at 45°C (squares) and 50°C (circles). The number of nucleated spherulites per unit area Increases by more than tenfold, even at low silicate loadings. Copyright Chem. Mater. 2003,vol.15, pp. 844-849. 

Crystallization rate, nucleation, size of crystalline units, crystalline structure, crystal modification, transcrystallinity, and crystal orientation are the most relevant characteristics of crystallization behavior in the presence of fillers. Here the discussion is focused on crystallization rate. The other topics are discussed in the following sub-chapters. 

Crystallization kinetics is estimated from the Avrami equation  

The above observations are similar to those obtained for peroxide-crosslinked polyethylene. An addition of filler, such as silica, results in an increased crystallization rate and a decrease in the crystallization half-time, ti/2.  

Many problems concerning the morphology of PLCs and its effect on properties still remain unsolved. A more reasonable interpretation of the origin of hierarchical structures and molecular orientation in PLC products, the mechanisms of interfibrillar stress transfer and the relaxation process of the rigid rod molecules all need more investigation. 

Blundell [28] first demonstrated that the main features of crystallization for thermotropic longitudinal PLCs are (1) small crystallites ( 5 nm for quenched samples and about 10 nm across for annealed ones) (2) limited crystallinity (about 20% in wide angle X-ray diffraction (WAXD)) (3) low surface energy of the crystallites (4) little change of molecular conformation before and after melting and (5) low fusion entropy and enthalpy (sometimes no detectable endotherm on DSC thermograms). A model of small crystal size for extended chain polymers was proposed by Blundell [28]. 

Polymorphism is also common for thermotropic longitudinal PLCs. Two types of crystal lattice exhibit in these PLCs orthorhombic and 

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Substitution of chloropolymer is possible using a variety of nucleophiles. The most common are sodium salts of alcohols and phenols. Thermoplastics are obtained using a single substituent, whereas multiple substituents of sufficiently different size lead to elastomers (2). Liquid crystal behavior similar to polysHoxanes has been noted in most homopolymers. The homopolymer formed using trifluoroethanol as a substituent has received a fair amount of academic scmtiny (7). 

In recent years, there have been increasing reports on the study of the melting and crystallization behaviors of PE under high pressure by addition of other components. Nakafuku et al. [144-147] have reported that some high melting temperature diluents such as 1-, 2-, 4-, 5-tetrachlorobenzene I-, 3-, 5-tribromobenzene hexamethylbenzene and tetracontane affect the melting and crystallization processes, the phase transition, and... 

In carpet shampoos sulfosuccinates are used as the main ingredient, especially in powdered products. For this application products with a good crystallization behavior like DLAS and DLMS are of greater importance. The amount used is 12-30%. A typical formula is as follows ... 

The synthesis213,214, thermal215 and mechanical properties216, crystallization behavior 217,218,339) and morphology 217,218,339 of tetramethyl-p-silphenylene-... 

This is a highly polar polymer and crystalline due to the presence of amide linkages. To achieve effective intercalation and exfoliation, the nanoclay has to be modified with some functional polar group. Most commonly, amino acid treatment is done for the nanoclays. Nanocomposites have been prepared using in situ polymerization [85] and melt-intercalation methods [113-117]. Crystallization behavior [118-122], mechanical [123,124], thermal, and barrier properties, and kinetic study [125,126] have been carried out. Nylon-based nanocomposites are now being produced commercially. 

A study on the effectiveness of the E-plastomers as impact modifiers for iPP was carried out in relation to the traditional modifier EPDM. In this study, the flow properties of the E-plastomer-iPP and EPDM-PP blends were also evaluated. The blends were analyzed by solid-state 13C-nuclear magnetic resonance (NMR) spectroscopy, microscopy (SEM), and DSC. The results showed that E-plastomer-PP and EPDM-PP blends present a similar crystallization behavior, which resulted in a similar mechanical performance of the blends. However, the E-plastomer-PP blend presents lower torque values than the EPDM-PP blend, which indicates a better processibility when E-plastomer is used as an impact modifier for iPP. 

Liquid crystal behavior is a genuine supramolecular phenomenon based on the existence of extended weak interactions (dipole-dipole, dispersion forces, hydrogen bonding) between molecules. For the former two to be important enough, it is usually necessary for the molecules to have anisotropic shapes, able to pack efficiently so that these weak interactions can accumulate and co-operate, so as to keep the molecules associated in a preferred orientation, but free enough to move and slide, as they are not connected by rigid bonds. 

It is interesting to note the influence of the counteranions on the thermal behavior. Irrespective of the isocyanide used, all the nitrate gold derivatives show low thermal stability and undergo extensive decomposition at relatively low temperatures (only the low melting trialkoxyphenyl derivative shows liquid crystal behavior). In contrast. 

OC10H21)], in which rearrangement does not occur. All the mixtures studied display liquid crystal behavior with improved properties with respect to the pure components. A representative binary phase diagram and their corresponding DSCtraces are presented in Figures 8.24 and 8.25 respectively, and reveal the eutectic nature ofthese systems. 

The preparation and study of metal nanoparticles constitutes an important area of current research. Such materials display fascinating chemical and physical properties due to their size [62, 63]. In order to prevent aggregation, metal nanoparticles are often synthesized in the presence of ligands, functionalized polymers and surfactants. In this regard, much effort has focused on the properties of nanoparticles dispersed into LCs. In contrast, the number of nanoparticles reported that display liquid crystal behavior themselves is low. Most of them are based on alkanethiolate stabilized gold nanoparticles. 

Several mixtures of hexanethiol capped gold nanopartides and triphenylene based discotic LCs have been studied. These mixtures display liquid crystal behavior (columnar mesophases) and an enhancement in the DC conductivity, due to the inclusion of gold nanoparticies into the matrix of the organic LC [70]. Other studies of mixtures of gold nanoparticies with mesogens indude a series of cholesteryl phenoxy alkanoates. The inclusion of the nanopartides does not change the inherent liquid crystal properties of the cholesteryl derivative but the mesophases are thermally stabilized [71]. 

Marcos, M., Ros, M.B., Serrano, J.L., Sola, M.A., Oro, L.A. and Barbera, J. (1990) Liquid-crystal behavior in ionic complexes of silver(I) molecular structure-mesogenic activity relationship. Chemistry of Materials, 2, 748-758. 

Liquid Crystal Behavior dose to Room Temperature. EuropeanJoumalofInorganic Chemistry, (2), 133-138. 

The previous sections in this chapter have tried to stress upon the significance of distribution of sequence lengths in polyethylene-based copolymers. The sequence length of interest in a system of ethylene-octene copolymers would be the number of methylene units before a hexyl branch point. As was discussed, this parameter has a greater impact on the crystallization behavior of these polymers than any other structural feature like branch content, or the comonomer fraction. The importance of sequence length distributions is not just limited to crystallization behavior, but also determines the conformational,... 

R.G. Alamo and C. Chi, Crystallization behavior and properties of polyolefins. In Y. Morishima, T. Norisuye and K. Tashiro (Eds.), Molecular Interactions and Time-Space Organization in Macromolecular Systems, Springer, New York, 1999, p. 29. 

In what follows, we use simple mean-field theories to predict polymer phase diagrams and then use numerical simulations to study the kinetics of polymer crystallization behaviors and the morphologies of the resulting polymer crystals. More specifically, in the molecular driving forces for the crystallization of statistical copolymers, the distinction of comonomer sequences from monomer sequences can be represented by the absence (presence) of parallel attractions. We also devote considerable attention to the study of the free-energy landscape of single-chain homopolymer crystallites. For readers interested in the computational techniques that we used, we provide a detailed description in the Appendix. ... 

Fig. 3 Difference in crystallization behavior between an atomic or short chain molecular system and a polymer system, a Atoms or short chain molecules can be independently rearranged on each lattice point, while b the order of the repeating units within a polymer chain is maintained during the rearranging process. Therefore, a chain should slide along its chain axis and disentangle for rearrangement onto the lattice points...

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