Isotactic crystallization, heterogeneous

The effect of heterogeneous nucleation on the crystallization of isotactic polypropylene from the melt can be easily established as follows. A small amount of powdered polypropylene is well mixed with about 0.1 wt% of sodium benzoate in a mortar or by means of an analytical mill. Some of the mixture is transferred with a spatula to a microscope slide and melted at about 250 °C on a hot block. A cover slip is pressed on to the melt with a cork to obtain as thin a film as possible.The sample is held at 200-250 °C for some minutes and then allowed to crystallize at about 130 °C on the hot stage of the microscope an unadulterated polypropylene sample is crystallized in the same way. Both samples are observed under a polarizing microscope during crystallization,the difference in spherulite size between nucleated and untreated polypropylene can be seen very clearly. An ordinary microscope can also be used by placing polarizers on the condenser and eyepiece, and adjusting these to give maximum darkness. 

Chiral crystals generated from non-chiral molecules have served as reactants for the performance of so-called absolute asymmetric synthesis. The chiral environments of such crystals exert asymmetric induction in photochemical, thermal and heterogeneous reactions [41]. Early reports on successful absolute asymmetric synthesis include the y-ray-induced isotactic polymerization of frans-frans-l,3-pentadiene in an all-frans perhydropheny-lene crystal by Farina et al. [42] and the gas-solid asymmetric bromination ofpjp -chmethyl chalcone, yielding the chiral dibromo compound, by Penzien and Schmidt [43]. These studies were followed by the 2n + 2n photodimerization reactions of non-chiral dienes, resulting in the formation of chiral cyclobutanes [44-48]. In recent years more than a dozen such syntheses have been reported. They include unimolecular di- r-methane rearrangements and the Nourish Type II photoreactions [49] of an achiral oxo- [50] and athio-amide [51] into optically active /Mactams, photo-isomerization of alkyl-cobalt complexes [52], asymmetric synthesis of two-component molecular crystals composed from achiral molecules [53] and, more recently, the conversion of non-chiral aldehydes into homochiral alcohols [54,55]. 

The classic Ziegler-Natta heterogeneous catalysts initiate polymerization at diverse, somewhat randomly scattered active sites on modified TiCls crystals, with the result that polymer chain length is variable even though essentially complete isotacticity can be achieved. Furthermore, these catalysts offer little opportunity to manipulate the degree of stereoregularity and hence the mechanical properties of the product—the Ziegler-Natta polypropylenes are exclusively isotactic. 

The crystal form of the catalyst is also important, and the violet a, y, and 8 forms of TiClj produce a greater quantity of isotactic polypropylene when combined with an aluminum alkyl than the brown P-structure. As the active sites for heterogeneous polymerizations are believed to be situated on the crystal surfaces, the... 

In contrast to polyethylenes the crystallization of polypropylene is initiated by heterogeneous nucleation. The order in polypropylene is a result of the structural regularity of the polymer chains it s isotacticity. Normal polypropylenes are isotactic with the methyl groups lined up on the same side of the polymer chain and they crystallize in a helical configuration. TREE separation is expected to be dependent on the perfection of the stereoregularity along the chain which may be influenced by the nature of the catalyst, the polymerization conditions and also the use of comonomers, such as ethylene. The effect of comonomer is to disrupt the isotactic runs in the polypropylene chain. 

Huihui L, Liu J, Wang D, Yan S (2003) A comparison study on the homogeneity and heterogeneity fiber induced crystallization of isotactic polypropylene. Colloid Polym Sci... 

Neat isotactic polypropylene (iPP) crystallized from melt exhibits spherulitic morphology of the crystalline phase (72,73). In some cases and under very specific conditions, cylindrites, axialites, quadrites, hedrites, and dendrites may be formed of iPP (74). In general, crystallization from quiescent melts results in spherulitic morphology, whereas crystallization fi-om melts subjected to mechanical loads results in cylindrites (75). Crystalline supermolecular structure caused by oriented crystal growth from heterogeneous surfaces is commonly termed transcrystallinity (76). 

The microscopic data on primary nucleation of isotactic pol)rpropylene (iPP) for melt samples annealed at 180°C prior to crystallization and then crystallized isothermally in the temperature range from 122 to 145°C are presented in Table 1. The nucleation in iPP in these experiments was found to be heterogeneous and instantaneous. 

Figure 1 illustrates the nucleation activity of isotactic polypropylene in thin films determined in isothermal experiments. The data are differentiated to represent the contribution of new nuclei activated by the temperature decrease by 1°C. Samples in the form of thin films (20-30 g,m) were crystallized isothermally on a microscopic hot stage. It is seen that the number of nuclei increases initially as the temperature of crystallization decreases. The number of new nuclei at the temperature below 115°C levels off. Apparently all heterogeneous nuclei present in... 

Heterogeneous Nucleation of Isotactic Poly(Propylene Oxide) Crystallization... 

The crystallization of isotactic polypropylene from melt could be enhanced in the region of the temperature where heterogeneous nucleation is observed by adding some extra heterogeneous nuclei. The interest... 

Martuscelli and coworkers studied the effects of isotactic fractions, and catalyst type on the overall crystallization of iPP [154, 155). For a given crystallization temperature and catalyst type, ti/2 increased with increasing/jso- However, the effect of the choice of the catalyst was strong enough to contravene the effect of even a large change in/jsQ. The dependence of Ti/2 on catalyst type was probably due to a combination of two effects the different impurity levels left in the resin with the different catalysts, that acted as heterogeneous nucleation sites, and the different distribution... 

Such catalysts, which are of course heterogeneous in nature, are complex in their action. Some sites on the titanium trichloride crystal surface may produce isotactic chains which are stereoregular to the extent of at least 98 . What is the nature of the... 

In a continuous polymerization process, this observed activity behavior directly influences the obtained microstmcture of the polymer. Thus, a copolymerization of propene and the borane monomer leads to a copolymer, because of the preferred incorporation of propene, with long propene sequences. This copolymer forms crystalline phases similar to those obtained in pure isotactic PP. The borane groups are located in the amorphous part of the copolymer and so even under heterogeneous and mild conditions a modification of the borane groups (e.g., conversion to alcohol functionalities) can be performed quantitatively. This behavior leads to a polar modified polymer that can co-crystallize with pure isotactic PP and thus be used in polymer blends in order to optimize the PP adhesion to other materials such as aluminum and glass. 

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