Polyethylene with cross-polarization

Figure 2 Spherulites in a thin film of high-density polyethylene in crossed polars with the first-order red plate. The pattern of blue and yellow quadrants shows that the spherulites are negative. The circumferential dark rings are the loci of low birefringence, where the twisting lamellar crystals have the molecular chain axis parallel to the axis of the microscope. Scale bar=20pm.

Figure 6 Spherulites of isotactic poly-l-butene (a, during growth) and of polyethylene (b, after completion) by optical microscopy (OM) under crossed polars. Reproduced from Ref. [3] with permission of John Wiley Sons, Inc.

All but the polyurethane are characterized by methylene backbones with ligands that are sufficiently polar to make them water soluble. Thus, upon dissolution in water, the polarity of the water molecule associates with the polarity of the acrylic or acrylamide groups to form a shell. We discussed hydrophilic polyurethanes that are typically cross-linked and are not (but could be) considered effective thickeners. Nevertheless they too have hydration shells developed due to the influence of the polyethylene glycol backbone. The extent of that shell is determined by the hydro-philicity of the ligand the acrylic > acrylamide > alcohol > polyurethane. The volume... 

Fio. 3.3.3 Selectivity of cross-polarization demonstrated for polyethylene terephthalate at room temperature and a spinning frequency of = 4090 Hz. (a) Polarization of protonated and unprotonated C nuclei with tcp = 5 ms. (b) Polarization of protonated C nuclei for cp = 50 xs. (c) Polarization of unprotonated nuclei for fcp = 5 ms and subsequent dephasing for fa = 100 p.s under the influence of the dipole-dipole coupling. Adapted... 

The polyethylene films were viewed under cross polarizers in a Leitz Ortholux microscope with transmitted light to determine the crystallite size. 

Solid state NMR spectra were recorded at 50.3 MHz with a Bruker CXP-200 spectrometer equipped with a Doty probe. Proton 90 pulse widths were 4 (is, and cross-polarization times were 1 ms. Matching conditions were checked with an adamanteme standard. The magic angle was set by monitoring the sr NMR spectrum of KBr (28). Sample containers were made of aluminum oxide with Kel-F caps and were spun at 2.5-3.5 KHz. Characteristically, 200-10000 acquisitions were obtained per spectrum, with recycle time of 4 s. All peaks in the spectra were referenced to the peak of linear polyethylene (33.6 ppm). A small amount of polyethylene was added to each sample (29). 

In this paper, we use SALS under cross-polarized Hy) alignment and USAXS to probe structural changes during of polymers. During early stage crystallization of polyethylene/olefin copolymers, fractal objects with diffuse interfaces are formed initially, and the interfaces of these domains sharpen with time. This compaction process is observed for both polyethylene/olefin copolymers, and the rates and pathways depend on both crystallization temperature and the chemical structure of the polymer. 

Fig. 1. A series of images showing the growth of spherulites of polyethylene. The uniformity of spherulite size, and the linear intersection of the spherulites once crystallization is complete, can be clearly seen. Exposures taken after the marked times. Optical micrographs with the sample between crossed polars. Reprinted from Ref 19. Copyright (1982), with permission from John Wiley Sons, Inc.

The random arrangement of adjacent crystals in the fringed-micelle model (see Fig. 2.4), does not accord with evidence obtained from microscopy. The electron microscope shows the crystals in polyethylene to be very thin twisted lamellae laid one upon another (Fig. 1.7). The lamellae are too small to be observed with the light microscope. Light microscopic examination of thin films or sections between crossed polarizers reveals complex polyhedral objects known as spherulites (see Fig. 2.5). They are in fact a complex ordered aggregation of the sub-microscopic crystals. In polyethylene the crystal lamellae are about 10 nm thick. They are separated one from another by thin lamellae of amorphous polymer of about the same thickness. The lateral... 

Figures 3.15 and 3.16 show spherulites in crossed polars and with the sensitive tint plate. In crossed polars, the spherulites are bright except at the four perpendicular radial directions where the crystals are in the extinction position. High density polyethylene (see Fig. 3.15) produces spherulites that are unusually perfect, and also have circumferential dark bands. The bands are regions of apparent isotropy where the optic axis is perpendicular to the specimen plane. With the first order red plate placed so that its slow direction is at -l45° (top right to bottom left of the image), the...

The polyethylene glycols or Carbowaxes and cyanopropylpolysiloxanes are the most common of the polar stationary phases. These phases possess permanent dipoles, and acid-base interaction with solutes is common. These phases are very retentive toward solutes with polar or polarizable functionality. They have lower upper temperature stabilities and higher lower minimum operational temperatures compared to the nonpolar phases. When these phases are cross-linked, the operational upper temperature limits are about 220-270°C and the lower limits about 40-60°C. 

The cross-axis CPC produces a unique mode of planetary motion, such that the column holder rotates about its horizontal axis while revolving around the vertical axis of the centrifuge.This motion provides satisfactory retention of the stationary phase for viscous, low-interfacial tension, two-phase solvent systems, such as aqueous-aqueous polymer phase systems. Our previous studies demonstrated that the cross-axis CPC equipped with a pair of multiplayer coils or eccentric coil assemblies in the off-center position was very useful for the separation of proteins with polyethylene glycol-potassium phosphate solvent systems.The apparatus is also useful for the separation of highly polar compounds such as sugars,hippuric acid, and related compounds, which require the use of polar two-phase solvent systems. 

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