Conformation polypropylene solid-state

Although many stereoregular polymers have a helical conformation in the solid state (5,96], the conformation is lost in solution in most cases, except in the case of some polyolefins with optically active side groups [12], because the dynamics of the polymer chain are extremely fast in solution. Therefore, isotactic polystyrene [15,16] and polypropylene [17] prepared with an optically active catalyst do not show optical activity due to a helical conformation. However, a helical conformation can be maintained in solution for some polymers having a rigid main chain or bulky side groups that prevent mutation to random conformation, and the conformation may... 

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. 

On the other hand, in the solid-state high resolution 13C NMR, elementary line shape of each phase could be plausibly determined using magnetic relaxation phenomenon generally for crystalline polymers. When the amorphous phase is in a glassy state, such as isotactic or syndiotactic polypropylene at room temperature, the determination of the elementary line shapes of the amorphous and crystalline-amorphous interphases was not so easy because of the very broad line width of both the elementary line shapes. However, the line-decomposition analysis could plausibly be carried out referring to that at higher temperatures where the amorphous phase is in the rubbery state. Thus, the component analysis of the spectrum could be performed and the information about each phase structure such as the mass fraction, molecular conformation and mobility could be obtained for various polymers, whose character differs widely. 

The conformation and molecular mobility of the noncrystalline chains for syndiotactic polypropylene (sPP) samples well crystallized have been characterized at different temperatures by high-resolution solid-state NMR. The purposes are to investigate the cause inducing the high trans fraction of sPP chains in the noncrystalline state just after quenching at 0 °C from the melt and to know some correlation with the crystallization of form III with the planar zigzag conformation around 0 °C. 

Figure 5.13 Solid-state CNMR spectra of polypropylene, (a) Solid-state CNMR spectrum of isotatic polypropylene (reproduced with modification from Fleming et al. [23] by permission of the American Chemical Society) plus representation of the helical conformation of isotactic polypropylene, (b) Solid-state NMR spectrum of syndiotactic polypropylene, plus represent-...

Table 2.5 clearly Indicates that the mechanical properties of solid-state extruded HOPE are much superior to the melt extruded HDPE. In fact, the tensile strength of solid-state extruded HDPE is about the same as carbon steel There are some other interesting benefits associated with solidstate extrusion of polymers. There is essentially no die swell at high extrusion ratios (extrusion ratio is the ratio of the area in the cylinder to the area in the die). Thus, the dimensions of the extrudate closely conform to those of the die exit. The surface of the extrudate produced by hydrostatic extrusion has a lower coefficient of friction than that of the un-oriented polymer. Above a certain extrusion ratio (about ten), polyethylene and polypropylene become transparent. Further, solid-state extruded polymers maintain their tensile properties at elevated temperatures. Polyethylene maintains its modulus up to 120°C when it is extruded in the solid state at a high extrusion ratio. The thermal... 

The conformational sensitivity of the solid-state chemical shifts is illustrated in Figure 3.16, which shows the solid-state spectra of isotactic and syndiotactic polypropylene [16]. While both polymers are crystalline, the isotactic polymer crystallises in a 2i. .. ggttggtt.. . conformation. One consequence of this conformational difference is that the methylene carbons are in a symmetric environment in the isotactic polymer and are non-equivalent in the syndiotactic polymer. The methylene carbons are separated by 8.7 ppm in the CPMAS spectrum of syndiotactic polypropylene. 

The CP-MAS C-NMR spectrum of syndiotactic polypropylene (sPP) is compared with the spectrum of iPP in Fig. 9.6 [31]. The solid state NMR spectra are substantially different in spite of the common chemical structure because of the differences in the conformations in the solid state. The sPP has a 2 helix with a (gg)(tt)(gg)(tt) structure. This structure involves external (outside the helix) and in-... 

Because of familiarity with x-ray crystallography and IR spectroscopy, Natta was able to show that solid polypropylene was a crystalline stereoregular polymer with a three fold helical conformation. As stated in Floiys book, which was read by Natta, C. Schildknecht had... 

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]. 

---