Crystalline polymers, high-resolution

The incidence of these defects is best determined by high resolution F nmr (111,112) infrared (113) and laser mass spectrometry (114) are alternative methods. Typical commercial polymers show 3—6 mol % defect content. Polymerization methods have a particularly strong effect on the sequence of these defects. In contrast to suspension polymerized PVDF, emulsion polymerized PVDF forms a higher fraction of head-to-head defects that are not followed by tail-to-tail addition (115,116). Crystallinity and other properties of PVDF or copolymers of VDF are influenced by these defect stmctures (117). 

Thin polymer films may also be investigated by TEM and high resolution images are obtained for e.g. thin films of liquid crystalline polymers [64]. Usually thin microtome cuts from bulk samples are investigated, but also epitaxial growth of polyoxymethylene on NaCl [152], chain folding of polyethylene crystals [153], epitaxial crystallization of polypropylene on polystyrene [154] or monomolecular polystyrene particles [155] are observed. The resolution is, however, in most cases not comparable to STM. 

D.E. Axelson, Carbon—13 Solid State NMR of Crystalline Polymers, In R.A. Komoroski (Ed.), High Resolution NMR Spectroscopy of Synthetic Polymers in Bulk, VCH, E>eerfield Beach, Florida, USA, 1986. 

Operation procedures for SAED, dark-field (DF) imaging and high-resolution observation [5,6] are briefly described here, which are utilized in our laboratory for structural investigation of crystalline polymers . As for conventional bright-field (BF) imaging, any text books [5,6] can be referred to. 

In the presence of chiral lithium alkoxides as initiators, chloral forms completely isotactic, crystalline polymers that are insoluble in all solvents (Scheme 71) (166). The polychloral film displays rotation values as high as 3000-5000°. The stereoregular helicity comes about at the stage of the trimer (167). A pure enantiomer of a tert-butoxy-initiated, acetate end-capped pentamer of chloral, which has 4r-helical conformation in both chloroform solution at 35 °C and in the crystalline state, can be obtained by HPLC resolution on a chiral stationary phase (168). 

CP/MAS NMR, with C-13, P-31, and a variety of spin-one-half nuclides can yield high-resolution NMR data that provide badly needed bridges between the liquid and solid states. These bridges can be used to estimate the extent to which diffraction-determined structural results on crystalline solids can be extrapolated into the liquid state. A substantial effort in this direction is underway in our laboratory. The potential for CP/MAS NMR in characterizing polymer-supported catalysts is also evident. 

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 chapter we review the study of the solid structure of many crystalline polymers mainly with high-resolution solid-state 13C NMR, so we will briefly summarize here its principles for the convenience of the reader. For this purpose we first consider the Hamiltonian of an ensemble of nuclei possessing spin in a static field B0. The Hamiltonian to be considered of this spin system can be written as ... 

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. 

Wide-line and 2H NMR spectra and T2 relaxation experiments have been used to determine the composition of the phases in semi-crystalline polymers [133, 136, 138-144]. The experiments were also used to obtain real-time information on the kinetics of crystallisation and melting [143-148]. The use of high-resolution NMR methods to characterise semi-crystalline polymers is reviewed elsewhere [17, 18, 30, 34, 149]. 

The structure of the crystalline polymer was thoroughly examined and documented (158—160). High resolution nuclear magnetic resonance spectra showed only two sharp peaks characteristic for the CH3- and... 

During the initial polymerization of trioxane with (C4H9)2OBF3 in melt or solution, no solid polymer is formed, and the reaction medium remains clear. Using a high resolution NMR spectroscope, C. S. H. Chen and A. Di Edwardo observed the appearance of soluble linear polyoxy-methylene chains. In the cationic copolymerization of trioxane with 1,3-dioxolane, V. Jaacks found also that a soluble copolymer forms first and turns later into a crystalline copolymer of different composition. Crystallization and polymerization proceed simultaneously in the solid phase. 

Fig. 9. The NMR spectra observed after a HqiiM crystalline sdiition is placed in tl magnetic field of a high resolution NMR spectrometer (14 kG) for MILG (DP 1500) in CH2CI2. Polymer concentration, 14.0 vol%. The numbers near the curves give the numbor in minutes...

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