Crystallization rate of SPS

Takebe et al. [28] studied the effect of temperature and molecular weight on the crystallization rate of SPS by DSC. When SPS was melted at 300 °C, then rapidly cooled to the crystallization temperature, T, the evolution of crystallization showed a sigmoidal curve with reference to crystallization time (Figure 18.6). The crystallization rate becomes slower as T approaches close to melting point. When the crystallization rate of SPS is analyzed based on Avrami theory, the Avrami index, n, is equal to 3, which suggests that crystallization of SPS proceeds via three-dimensional heterogeneous growth [28,29]. 

Figure 18.8 Analysis of crystallization rate of SPS according to Lauritzen-Hoffman nucleation theory [Equation (1)] (AT = 7, -7), (a) Data plotted for various Mw O 4.70 X 104 1.26 x 105 tx, 3.78 x 105 A, 6.91 x 105 o, 9.23 x 105 , 2.00 x106. (b) Superposed line by horizontal shift with reference to 7°/TAT...

Figure 18.9 Molecular weight dependence of crystallization rate of SPS shift factor aM plotted against Mw. The crystallization rate of SPS has an M 08 dependence...

Whereas atactic PS is an amorphous polymer with a Tg of 100 CC, syndio-tactic PS is semicrystalline with a Tg similar to aPS and a Tm in the range 255-275 °C. The crystallization rate of sPS is comparable to that of polyethylene terephthalate). sPS exhibits a polymorphic crystalline behavior which is relevant for blend properties. In fact, it can crystallize in four main forms, a, (3, -y and 8. Several studies [8] based on FTIR, Raman and solid-state NMR spectroscopy and WAXD, led the a and (3 forms to be assigned to a trans-planar zig-zag molecular chain having a (TTTT) conformation, whereas the y and 8 forms contain a helical chain with (TTG G )2 or (G+G+TT)2 conformations. In turn, on the basis of WAXD results, the a form is said to comply with a unitary hexagonal cell [9] or with a rhombohedral cell [10]. Furthermore, two distinct modifications called a and a" were devised, and assigned to two limiting disordered and ordered forms, respectively [10]. 

Polystyrene (PS) now on the market is atactic PS (APS), but there is a problem of low heat resistance. Isotactic PS (IPS) is also known, but there is a problem of low crystallization rate. A homogeneous Ti/metal-locene and MAO system is an effective catalyst for syndiotactic polystyrene (SPS). Advantages of SPS are heat resistance 7 110°C) and chemical resistance like engineering plastics, which are derived from its high crystallinity compared with APS produced by radical polymerization. Furthermore, the crystallization rate of SPS is faster than that of APS or IPS. 

The crystallization rate of SPS is much faster than that of IPS [10] however, a fast cooling below Tg does not allow SPS to fully crystallize, resulting in halfway crystallization. When SPS is injection molded, the degree of crystallinity and the morphology of SPS is greatly affected by the mold temperature [11], When neat SPS is molded at a high mold temperature, a well-developed lamellar structure is observed in the specimen and Xc is as high as about 50% across the whole cross section of the molded specimen, from the surface to... 

The crystallization rate is dependent on the crystallization temperature, T, and molecular weight of the polymer. It is important to study the effect of T and molecular weight on the crystallization nature of SPS. 

The C—C backbone of SPS takes a planar zig-zag conformation in the crystal whereas IPS takes a 3/1 helical conformation in crystal. This difference in chain conformation in the crystal affects the crystallization rate of these polystyrenes. Table 18.1 [32] compares the crystallization parameters between SPS and IPS. The work of chain folding, Q, of SPS is smaller than that of IPS, which suggests that the chain folding of SPS can take place more easily than that of IPS. Also, SPS has a smaller surface free energy. These differences are related to the crystallization rate difference between SPS and IPS, which have different stereoregularities. 

Li et al. [20] found that the cooling rate from the melt should not be the intrinsic factor controlling the formation of the different phases. The crystallization temperature seems to have a more important influence. The crystallization rate of the a-form crystal is much faster than that of the P form, but the respective maxima are shifted in two different positions. Li et al. concluded that the SPS samples present a transition temperature (dependent on their molecular structures) [20], above which the crystallization rate of the p form is higher than that of the a form, and thus the a form can only be obtained below this temperature. 

The rapid crystallization rate of homopolymer SPS reduces the practical benefit of cast film production. However, copolymerization of the SPS structures helps to delay the onset of crystallization during cast film production until a secondary biaxial orientation operation. SPS crystallites improve the dimensional stability and chemical resistance of films and have also been investigated for unique optical properties. 

It is necessary to carry out the preparation under a hood. About 200 ml. of fuming nitric acidt (sp. gr. 1.60) contained in a 350-ml. tube (test-tube shape) is cooled with ice and salt, and sulfur dioxide is passed into it fairly rapidly. The reaction is exothermic, and the rate of addition should be such that the temperature of the solution does not rise above 5°C. In 6 or 8 hours, the crystals of nitrosylsulfuric acid fill the space of the original liquid, forming a compact mass with a layer of dark fuming liquid 2 to 3 cm. 

Properties Yellow crystals. Sp. gr. 1.69 at 20°. M. P. 121.2°. More powerful and having higher hri-sance than TNT, but producing smaller shell fragments. Less sensitive to impact and more toxic than TNT. Rate of detonation (d = 1.66) 7350 m/sec., compared with 7140 m/sec. for TNT (d = 1.60) when tested under similar conditions. 

Properties Yellowish, nearly white crystals. M. P. 128°. Explodes when heated to 235°. Sp. gr. 1.82. Soluble in nitroglycerine. Sensitivity to impact similar to, and sensitivity to friction somewhat greater than, that of Tetryl hut the shattering power is much greater for the Pentryl. The disruptive power of Pen-tryl as measured in the small Trauzl block is 30 per cent greater than that of TNT, 15 per cent greater than that of Tetryl, and 27 per cent greater than that of picric acid.5 Rate of detonation (d = 0.8) 5000 m/sec. 

In a 2-1. three-necked, round-bottomed flask fitted with a thermometer, mechanical stirrer, and 1-1. dropping funnel is placed 200 g. (1.33 moles) of powdered d-tartaric acid (Note 1). To this are added, successively, 432 cc. of nitric acid (sp. gr. 1.42) and 432 cc. of fuming nitric acid (sp. gr. 1.50). The mixture is stirred until the tartaric acid is all, or nearly all, in solution (five to ten minutes) (Note 2). Then 800 cc. of concentrated sulfuric acid (sp. gr. 1.84) is slowly added from the dropping funnel. As soon as the temperature of the reaction mixture reaches 38°, the flask is surrounded by a vessel of ice water, and the rate of addition of acid is regulated so that the temperature is maintained at 38-43° (Note 3). Near the end of the addition (about fifteen minutes), tartaric acid dinitrate sometimes begins to crystallize. After all the sulfuric acid has been added the mixture is allowed to stand in a cool (20-25°) place for three hours. 

Figure 18.7 shows the time evolution of the crystallinity of SPS of various weight-average molecular weight, Mw. The crystallization rate becomes slower as Mw becomes larger at a fixed T. 

When SPS is compared with isotactic polystyrene (IPS), a substantial difference is seen in the crystallization. IPS is also crystalline PS, but it has not been commercialized because of its very slow crystallization rate. In contrast, SPS... 

A limited number of patents concern sPS blend which are not compatible. In these cases the properties that are described are functional ones and not related to the poor toughness of sPS. For instance, blends of sPS and partially saponified ethylene-vinyl acetate copolymers exhibit improved gas barrier properties (entry 11) small amounts of sPS added to polyethylene terephthalate) (although the patent actually claims a wide range of compositions) are useful to increase the polyester crystallization rate (entry 5). 

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