Syndiotactic polymers polymorphism

Figure 2.12 Maps of conformational energy of various syndiotactic polymers as function of backbone torsion angles 0 and 0227 (a) syndiotactic polystyrene, (b) polypropylene, (c) poly (1-butene), and (d) poly(4-methyl-l-pentene). Succession of torsion angles. .. 0i 0i 0202 - - -[s(M/N)2 symmetry] has been assumed. Isoenergetic curves are reported every 5 kJ/mol of monomeric units with respect to absolute minimum of each map assumed as zero. Values of energies corresponding to minima (x) are also indicated. Experimental conformations observed for different polymorphic forms of polymers are indicated by triangles. (Reproduced with permission from Ref. 27. Copyright 1992 by the Socicta Chimica Italiana.)...

Models of the chain conformations found in the various polymorphic forms of syndiotactic polymers are reported in Figure 2.13. 

Whereas most of the early work on crystallization, etc., were concerned with predominantly isotactic chains, the recent developments in synthetic methodologies have enabled the preparation of highly syndiotactic polymers [13,14]. Since the high stereoregularity of these syndiotactic polymers facilitates their crystallization, several papers have been published on the x-ray crystal structure and polymorphism of syndiotactic polystyrene [15-18]. The chain conformation in the crystalline state has also been analyzed using NMR [19]. Similarly, the crystal structure of syndiotactic polypropylene has also been studied by a number of authors [20-22]. 

Wang, C., Liao, W-P., Cheng, Y-W. Strong diffuse scattering and lamellar morphologies of syndiotactic polystyrene Polymorphic effects. J. Polym. Sci. Part B Polym. Phys., 41,2457-2469 (2003). 

Syndiotactic polymers also show polymorphism due to the fact that the chains may assume different low energy conformations that crystallize in different polymorphic forms. Calculations of the conformational energy under the constraints of the equivalence principle and, hence, assuming a sequence of the backbone torsion angles. .. i,01,02, 2,, allow for an easy inter-... 

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

Syndiotactic PP is a thermoplastic with a slightly lower melting point (150-155 °C), lower crystallization rate, and higher flexibility (elastic modulus ca. 500) than iPP. It also has lower density, lower heat sealing temperature, but also better impact properties and better transparency. One of the difficult aspects of sPP as a material, but a great playground for polymer chemists and physicists, is its polymorphic behavior sPP has complex thermal properties and... 

Polarized IR spectroscopy was employed by Wu et al. to investigate crystallinity, the formation of different crystal orientations and the crystal texture of syndiotactic PS (sPS) crystallized inside AAO hard templates while in contact with a bulk reservoir of the same polymer [101]. The 6-polymorph was obtained by cooling from the melt to 260 °C and crystallizing at this temperature for 2 h, while heating amorphous samples quenched from the molten state to 240 °C and heating to this temperature for 2h resulted in the formation of the or-polymorph. A comparison of the areas of peaks... 

Syndiotactic polystyrene, s-PS, is a highly stereoregular, semicrystalline vinyl polymer that normally melts at 270 °C [45,46], S-PS shows a large number of crystalline polymorphs [47-50] obtained by melt crystallization and solvent-exposure techniques. However, s-PS assumes only two distinct crystalline conformations [all trans, planar zigzag (... tttttttt...) and 2 -helical (... ttggttgg. ..) t= trans and g = gauche], which are characterized by fiber repeats of... 

De Rosa C, Petraccone V, Guerra G, Manfredi C (1996) Polymorphism of syndiotactic poly(p-methylstyrene) oriented samples. Polymer 37 5247-5253... 

Finally, a few comments about the uniqueness of polymer crystal structures and phase space localization are warranted. Almost all crystallizable polymers exhibit polymorphism, the ability to form different crystal structures as a result of changes in thermodynamic conditions (e.g., temperature or pressure) or process history (e.g., crystallization conditions) [12]. Two or more polymorphs of a given polymer result when their crystal structures are nearly iso-energetic, such that small changes in thermodynamic conditions or kinetic factors cause one or another, or both, to form. Polymorphism may arise as a result of competitive conformations of the chain, as in the case of syndiotactic polystyrene, or as a result of competitive packing modes of molecules with similar conformations, as in the case of isotactic polypropylene. In some instances, the conformational change may be quite subtle isotactic polybutene, for example, exhibits... 

The ordered structures of some polymers are governed by the influence of specific diluents. This involves a specific type of polymorphism, the more general aspects of which will be discussed in the chapter concerning thermodynamic quantities. Syndiotactic poly(styrene) is a polymer that is rich in compound formation with solvent mediated polymorphic behavior.( 126-130) The polymer can crystallize in four major crystalline modifications that involve two different chain conformations. In the a and p modifications the chains adopt an all trans planar zigzag conformation. These two modifications are formed by crystallization from the melt and, under special conditions, from solution. In contrast the y and 5 modifications are characterized by a helical conformation. The 5 polymorph can only be prepared in the presence of solvent. Its exact crystal structure depends on the nature of the solvent. Compound formation between the 5 form of the polymer and the solvent has been demonstrated. Complete elimination of the solvent results in the pure, helical y form. 

Chatani, Y., Shitnane, Y, Inoue, Y, Inagaki, T., Ishioka, T, Ijitsu, T, Yukinari, T. Structural study of syndiotactic polystyrene. 1. Polymorphism. Polymer, 33(3),... 

Rizzo, E, Albania, A. R., Guerra, G. Polymorphism of syndiotactic polystyrene Gamma phase crystallization induced by bulky uou-guest solvents. Polymer, 46(23), 9549-9554 (2005). 

Woo EM, Sun YS, Yang CP. Polymorphism, thermal behavior, and crystal stability in syndiotactic polystyrene vs. its miscible blends. Prog Polym Sci 2001 26 945-83. 

Fully syndiotactic polystyrene is a relatively recent discovery and polymers are now manufactured commercially. Only limited structural information is available [40-47]. It has four main crystalline polymorphic forms a, P, y and 8. The attractive and interesting physical characteristics of this polymer can be summarised as follows (1)... 

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