Polypropylene chain conformations

Fig. 2. (a) Chain conformation of isotactic polypropylene, and (b) model of a polypropylene spheruHte. 

Figure 1.3 Chain conformation of isotactic polypropylene in its crystalline phases.

Chauve et al. [253] utilized the same technique to examine the reinforcing effects of cellulose whiskers in EVA copolymer nanocomposites. It was shown that larger energy is needed to separate polar EVA copolymers from cellulose than for the nonpolar ethylene homopolymer. The elastomeric properties in the presence of spherical nanoparticles were studied by Sen et al. [254] utilizing Monte Carlo simulations on polypropylene matrix. They found that the presence of the nanofillers, due to their effect on chain conformation, significantly affected the elastomeric properties of nanocomposites. 

Rather recently, we have studied the solid-state structure of various polymers, such as polyethylene crystallized under different conditions [17-21], poly (tetramethylene oxide) [22], polyvinyl alcohol [23], isotactic and syndiotactic polypropylene [24,25],cellulose [26-30],and amylose [31] with solid-state high-resolution X3C NMR with supplementary use of other methods, such as X-ray diffraction and IR spectroscopy. Through these studies, the high resolution solid-state X3C NMR has proved very powerful for elucidating the solid-state structure of polymers in order of molecules, that is, in terms of molecular chain conformation and dynamics, not only on the crystalline component but also on the noncrystalline components via the chemical shift and magnetic relaxation. In this chapter we will review briefly these studies, focusing particular attention on the molecular chain conformation and dynamics in the crystalline-amorphous interfacial region. 

In this section we will discuss the molecular structure of this polymer based on our results mainly from the solid-state 13C NMR, paying particular attention to the phase structure [24]. This polymer has somewhat different character when compared to the crystalline polymers such as polyethylene and poly(tetrameth-ylene) oxide discussed previously. Isotactic polypropylene has a helical molecular chain conformation as the most stable conformation and its amorphous component is in a glassy state at room temperature, while the most stable molecular chain conformation of the polymers examined in the previous sections is planar zig-zag form and their amorphous phase is in the rubbery state at room temperature. This difference will reflect on their phase structure. 

Syndiotactic polypropylene exists in various crystal modifications. The most stable chain conformation is helical and involves a succession of the type TTGG or TTG G . The helix conformation is very nearly a rectangular stair-... 

CHCHj-) where all the methyl (CH3) substituents are on the same side are shown schematically in Figure 2-12. (The methyl carbon atoms are shown in red to make them easier to see.) We call this arrangement isotactic, so this polymer is isotactic polypropylene. Because of steric repulsions between CH3 groups on adjacent units, the chain would not want to sit in this planar zigzag shape or conformation, but would fold into a different shape by rotating the polymer backbone bonds. More on this when we discuss conformations. Also shown in Figures 2-13 and 2-14 are two more polypropylene chains. One of these consists of units that are all racemic to one another and is called syndio-tactic. The other has a random arrangement of units and we call such chains atactic. 

The technology of gel spinning has not been successful with polypropylene. Unlike polyethylene with a simple linear chain conformation, the hehcal chain conformation of polypropylene can conceivably forfeit a tight crystal packing required for a high-strength,... 

Fig. 2 (A) Chain conformation of isotactic polypropylene in the crystalline state. Symbols R and L identify right- and left-handed helices, respectively, in 3/1 conformations. Subscripts up and dw ( dw standing for down ) identify chains with opposite orientation of C - C bonds connecting tertiary carbon atoms to the methyl groups along the z-axis (B) Limit-ordered model structure (a2 modification, space group P2 /c) [113] (C) Limit-disordered model structure (otl modification, space group Cl/c) [114]. In the a2 modification up and down chains follow each other according to a well-defined pattern. The al modification presents a complete disorder correspon ng to a statistical substitution of up and down isomorphic helices...

Gregoriou, V. G.,Kandilioti, G., and Bellas, S. T. Chain conformational transformations in syndiotactic polypropylene/layered silicate nanocomposites during mechanical elongation and thermal treatment. Polymer, 46, 11340-11350 (2005). 

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]. 

The and can be used to cause coarse-grained chains to mimic the conformational properties of specific real chains, because these probabilities enforce the proper distribution function for r for the entire coarse-grained chain, as well as all of its subchains [150,151]. This feature facilitates the recovery of atomistically detailed models from equilibrated ensembles of coarse-grained chains [152]. It also causes the coarse-grained chains to be sensitive to subtleties such as the dependence of the miscibility of polypropylene chains in the melt on their stereochemical composition [153,154]. 

SCHEME 7.1 Stereochemical and regiochemical conformational alternatives for a representative ansa-metallocene catalyst shown with growing polypropylene chain. 

FIG. 3 Different chain conformations of atactic polypropylene as a methane penetrant (sphere labeled p) jumps between two sorption states. Text labels indicate the extent along the reaction coordinate. Differences between the positions of dashed and solid triangles indicate the motion of chain segments to create a more open channel for diffusion. Conformations are adapted from Movie 1 in [90]. 

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