Wax crystal modification

Leube, W. Monkenbusch, M. Schneiders, D. Richter, D. Adamson, D. Fetters, L. Dounis, P. (2000). Wax-Crystal Modification for Fuel Oils by Self-Aggregating Partially Crystallizable Hydrocarbon Block-Copolymers. Energy Fuels, Vol.l4. No.2, pp.419-430, ISSN 0887-0624... 

The overall morphology of the wax copolymer aggregates at the micrometric scale was also investigated by optical and transmission electron microscopy as complementary techniques for the USANS investigations. The results obtained allowed the clarification of the self-assembled structures in solution of the crystalhne-amorphous random copolymers. This has led to a broader understanding of wax crystal modification and control in solution than previously available. As noted, SANS is the required technique to study these hydrocarbon mixtures since contrast is easily achieved via the judicious mixture of hydrogenated and deuterated materials. As detailed experimentation has shown [7-18], this experimental capacity is vital for a complete... 

Denis, J., and Durand, J.-P. 1991. Modification of Wax Crystallization in Petroleum Products. Revenue de L Institut Franqais du Petrole, 46, 637-649. 

Despite the occasional fractionation of the crystallization or its suppression at the usual temperature, the DSC heating curves of all blends exhibit in all cases for both polymers a single melting endotherm (PA-6 one for both crystal modifications) at an almost constant temperature (variation range smaller than 6°C) of about 175°C for the PVDF and of about 217°C/223°C for the PA-6. The relative crystallinity of each component also does not change significantly with composition and extrusion cycle number, in fact, WAXS-and IR-analyses show that, in all samples, PA-6 crystallizes mainly in the l-modification at an almost constant i/a-ratio, and PVDF in the a-modification. 

Lewtas, K., Tack, R.D., Beiny, D.H.M. and Mullin, J.W. (1991) Wax crystallization in diesel fuel habit modification and growth of n-alkane crystals. In Advances in Industrial Crystallization, J. Garside, R.J. Davey and A.G. Jones (eds.) (1991), Butterworth-Heinemann, Oxford, 166-179. 

IsoL from many plant waxes incl. Candelilla wax. Major constit. of some waxes, e.g. Agave sisalana (33%), Calocephalus brownii (12%). Mp 71.8°. Four crystal modifications known. 

Information about the molecular packing of the retinoate moieties was obtained by wide-angle X-ray scattering (WAXS). The WAXS pattern of retinoic acid (curve a in Fig. 14) is characterized by a number of sharp reflections resulting from the high degree of crystallinity. Retinoic acid crystallizes in two similar crystalline modifications (triclinic and monoclinic) [149], which produce the reflex pattern observed. As shown in Fig. 14, curves b and d,... 

Table 6.1. Microhardness perpendicular to the chain orientation (strain) direction H , indentation anisotropy at room temperature (25°C) AH, X-ray crystallinity Wc(WAXS), percentage of a and modifications (according to Tashiro et al, 1980) and crystal hardness of homo-PBT He stretched at various strains, s. 

Wide-angle X-ray scattering (WAXS) studies of PL-LA revealed the existence of two crystalline modifications. De Santis [21] described the structure of the a crystalline modification as pseudoorthorhombic with the chains in a 10/3 left-handed helix. Hoogsteen [22] reported a P orthorhombic crystal structure, characterized by a 3/1 left-handed helical conformation. 

Poly-P-hydroxybutyrate (PHB) crystallizes in a low temperature modification (LTM), which is transformed into a high temperature modification (HTM) when heated above 320 °C. Both modifications exhibit distinct WAXS patterns, thus a quantitative determination of the two fractions is possible. However, measurements at the HTM are only feasible with S.R., since the molecules quickly decompose at the high temperatures. Figure 23 shows the fraction of the HTM during heating the PHB sample from 280 °C to 340 °C at a rate of 2 °C/min and subsequent cooling to 280 °C at the same rate [49], A reversible transformation is observed. But the transition from the LTM to the HTM occurs at a temperature, which lies about 20 °C higher than the transition in the opposite direction. This indicates, that each modification is formed by its own nucleation process. 

Riekel [50] investigated the irreversible transformation of ew-polyacetylene into th trans-modification. The sample was heated from room temperature to 350 °C wii a rate of 11 °C/min and the WAXS was recorded continuously. The result is shov in Fig. 24. At 150 °C the (201) crystal reflection vanishes, indicating the cisjtrans transition. Above 290 °C a decomposition takes place, leading to the observed bro peak. 

Figure 2 WAXS traces of as-prepared iPP (bottom) and of the same sample isothermally crystallized at 65°C, 75°C and 125°C. The peak assignment of the y-phase iPP (top trace) is due to Bruckner and Meille and of the a-phase iPP is due to Natta et al. The patterns of the as-prepared polymer and the sample isothermally crystallized at 65°C show exclusively or mainly the a-modification. With increasing crystallization temperature, the y-modification is promoted and at Tj = 125°C the y-modification is formed nearly exclusively.

As was shown in Section 9.2.2, the crystalline form of iPP can be correlated to isotactic block length. Based on this, the samples prepared using both MAO and borate activation (Table 9.2) should mainly crystallize in the y-form. To study the presence of the different crystalline forms in these MAO- and borate-activated low-crystalline polypropylene samples, WAXS analysis was performed as shown in Figure 9.15. Both series A and B display WAXS peaks at = 9.3°, characteristic for the presence of the a-modification, and at = 10°, characteristic for the presence of the y-modification, pointing out that lamellae will crystallize in a mixed a/y-form. 

In the WAXS diffractograms of iPP [18], many peaks of a, p, and y crystals are in similar 29 locations. However, each modification has a distinctive reflection peak, which is well defined in our experiment. Tlie a and y modifications are distinguished by their own characteristic scattering angle 29 and Miller indices (hkl), at 18.5° (130) for a, and 20.2°C (117) for y. No p phase was observed in our study. To quantify the relative amount of a and y crystals, we calculate the area under the characteristic peak for the i- crystal type, Sj (i=l, 2 for a and y, respectively). The ratio ... 

We have performed simultaneous SAXS and WAXS experiments on an m-iPP known to crystallize in a and y modifications. Our results show that ... 

The modification of crystal morphologies of 35 wax induced by the addition of PEB-7.5 and PEB-10 random copolymers are compared in Fig. 68 as a function of polymer concentration in solution a clear decrease of the crystal size with increasing the polymer amount is visible, an observation in accordance with the yield stress trend from Figs. 66 and 67. At the highest concentration (0.8%) both morphologies and yield stress effects shown by PEB-7.5 and PEB-10 are similar numerous crystallites are forming a gel at the same time the increased yield stress observed. 

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