Cooling isotacticity

The syndiotactic polymer configuration is not obtained in pure form from polymerizations carried out above 20°C and, thus has not been a serious concern to most propylene polymerization catalyst designers. Eor most commercial appHcations of polypropylene, a resin with 96+% isotacticity is desired. Carbon-13 nmr can be used to estimate the isotactic fraction in a polypropylene sample. Another common analytical method is to dissolve the sample in boiling xylene and measure the amount of isotactic polymer that precipitates on cooling. 

Figure 11 Left Spherulites of a Ziegler-Natta isotactic poly(propylene) with Mw = 271,500 g/mol and mmmm — 0.95, isothermally crystallized at 148°C. Right Banded spherulites of a linear polyethylene with Mw = 53,600 g/mol slowly cooled from the melt.

In order to illustrate the fractionated crystallization behavior we will present here previous results on immiscible atactic PS and isotactic polypropylene blends (iPP) [68]. The cooling behavior of PS, iPP and an 80/20 PS/iPP blend is presented in Fig. 1, as well as that of an unmixed blend , labeled... 

Fig.l Differential scanning calorimetry (DSC) cooling scans from the melt, at 10°Cmin 1, of the following materials (from top to bottom) Isotactic polypropylene (iPP) iPP after self-nucleation treatment at TS = 162°C 80/20 polystyrene (PS)/iPP melt mixed blend 80/20 PS/iPP melt mixed blend after self nucleation treatment at Ts = 161 °C 80/20 PS/iPP unmixed blend (UB), see text and atactic PS homopolymer. (From [68] with permission)... 

Polymer glasses are formed best when the macromolecular chains are irregular in structure (atactic, branched, crosslinked) so that crystallization is prevented. Regular (isotactic, syndiotactic unbranched) polymer chains form glasses only if they are cooled down so fast that crystallization is prevented such a quenching procedure freezes the material in the glassy state even if the polymer is able to crystallize. 

Thin films of isotactic or elastomeric [51] polypropylene can be prepared from pellets by melt pressing. For this purpose several iPP or ePP pellets are placed between two sheets of thick aluminum foil and are subjected to pressure in in a hot press (T - 175°C pressure ca. 1-10 tons). Careful Wear appropriate safety shields and gloves After cooling the sample down to ambient temperature, the foil is unwrapped. If the polymer sticks to the foil, the film can be removed after cooling it down below the glass transition temperature e.g. in liquid nitrogen. Careful For... 

Isotactic polystyrene. Isotactic polystyrene was prepared by heating a 10% solution of styrene in benzene at 60° for 24 hours in the presence of a catalyst prepared in the presence of monomer from equimolar amounts of (C2H5)3A1 (heptane solution) and T1CI3-AA (heptane suspension). The reaction mixture was diluted with benzene and poured into an excess of isopropanol. The resulting precipitate was dissolved in warm methylene chloride. The solution was filtered and added to hot methyl ethyl ketone. The solution was concentrated and cooled to obtain the isotactic polymer in the form of a white powder that was washed with methyl ethyl ketone and dried under vacuum. Final purification was achieved by reprecipitation of the polymer from benzene solution into methanol, followed by drying at 60° under vacuum. 

To study cooling rate effects which might affect the supramolecular self-assembly of trisamides in i-PP, we investigated the influence of the processing conditions on the electret performance. Isotactic polypropylene containing 0.09 wt% of compound 1 was chosen for these experiments. After compression molding, the films... 

We also note that certain polymer melts crystallize partially upon cooling. The transition occurs at a well-defined temperature rf. Crystallization takes place only if the polymer has a perfect linear structure for instance, it must not contain any asymmetric carbon. However, this tacticity condition is not sufficient, since polydimethylsiloxane, which is perfectly periodical, does not crystallize under normal conditions. On the contrary, polyethylene and isotactic polyethylene crystallize easily. In general, these polymers contain a large amorphous fraction. This is why they are called semi-crystalline. In certain conditions, it is possible to prepare polymer samples that are perfect crystals, in particular by polymerization in situ of a crystal made of monomers (polydiacetylene and polyoxyethylene). 

Isotactic polypropylene crystallizes below 350 K on quick cooling from the melt increasingly as condis glass This conformationally disordered polypropylene had first been described as bdng smectic mesomorphic or paracrystalline ... 

Hendra PJ, Vile J, Willis HA, Zicfay V (1984) The effect of cooling rate upon the morphology erf quenched melts of isotactic polypropylenes. Polymer, 25 785... 

Isotactic PP has extremely good flow properties at a wide range of flow rates, and therefore good processing behavior. The melt flow index typically ranges from 0.5 to 50 g/10 min. Films, which can be produced by both blown and cast methods, can be oriented to provide improved optical characteristics and better strength. Because of the rapid crystallization of PP, blown films must be produced by either water quench or mandrel quench processes, unlike PE, which is cooled by air. 

Choi, R, and Mattice, W. L. 2004. Molecular origin of demixing, prior to crystallization, of atactic polypropylene/isotactic polypropylene blends upon cooling from the melt. The Journal of Chemical Physics 121 8647-8651. 

The purpose of this chapter is to provide a general experimental route for studying the crystallization behaviour of isotactic polypropylene under high cooling rates and pressure. 

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