Ultrahigh molecular weight techniques

The open FFF channel is especially suited to fragile materials, and thermal FFF has found a definite niche in its application to ultrahigh-molecular-weight polymers. Furthermore, because samples need not be filtered, thermal FFF is the technique of choice for analyzing gels, rubbers, and other materials that tend to plug SEC columns [7]. Even particles can be analyzed... 

The wear behaviour of polytetrafluoroethylene (PTFE), carbon-filled PTFE, high density polyethylene (HDPE), ultrahigh molecular weight polyethylene (UHMPE), low density polyethylene (LDPE) and polymethyl methacrylate (PMMA) was studied. To ensure consistent and controlled properties of the samples, many of the materials were processed in the authors laboratory. The details of sample preparation and processing techniques are reported elsewhere ( ). ... 

Misheva, M., Djourelov, N., Dimitrova, A., and Zamfirova, G., Ultrahigh molecular weight polyethylene free volume hole structure studied by positron annihilation lifetime technique, Macromol. Chem. Phys., 201, 2348-2353 (2000). 

Kanamoto, T., Tsuruta, A., Tanaka, K., Takeda, M. and Porter, R. S. Super-drawing of ultrahigh molecular weight polyethylene. 1. Effect of techniques on drawing of single crystal mats. Macromolecules 1988 21 470-7. 

In 1978 Shen and Bell [36] developed a technique called plasma-initiated polymerization in which the plasma was used to initiate conventional polymerization of liquid vinyl monomers, resulting in soluble linear polymers with high or even ultrahigh molecular weights [37]. Moreover, we found that if the polymerization conditions were set up appropriately, the polymer molecules would be aligned and packed more regularly, even leading to the formation of crystallized products [38]. 

More recently, however, a process for the manufacture of ultrahigh molecular weight cellulose esters has been developed by reaction of nonactivated, secondary cellulose with trifiuoroacetic acid, trifiuoroacetic anhydride, and either an organic acid or acid chloride (23). This process is amenable to a larger variety of organic esters not normally available through conventional means. The technique requires less reaction time and less excess solvent, and it is easier to control the extent of the reaction than conventional sulfuric acid activation. Unfortimately, the handling and toxic nature of trifiuoroacetic acid and the anhydride currently limit its utility. 

There is another beautiful experiment which is even more convincing in its support of the reptation model. Fluorescence microscopy enables the motion of fluorescently stained single chains to be directly observed. The technique was applied to a concentrated solution of monodisperse DNA-molecules with ultrahigh molecular weight corresponding to a contour length in the 100 /xm-range. On one end of the chain, an especially coated 1 /xm-diameter... 

Section 3. Properties of Oriented Ultrahigh Molecular Weight Polyethylene Prepared using the Powder and Coextrusion Techniques... 

This section deals with semicrystalline thermoplastics that cannot be processed by the techniques described earlier. Perkins and Porter (22) have reviewed the solid state deformation of polymers in detail and describe the numerous reports of solid state extrusion. Aharoni (23) has reported that a number of polymers may be solid state extruded to high draw ratio (> 10) by the conventional process. These include HOPE, poly(ethylene oxide), poly(4-methyl pentene-1). Polypropylene is also readily extrudable (24). However, there are many other polymers that would be attractive if they could be obtained in high draw, particularly the established fiber-forming polymers such as the nylons and poly(ethylene terephthalate). The ma,ximum extrusion draw ratio that has been reported for nylon 6 is 5 (25). This has been attributed to the onset of strain hardening at much lower extensions than polyethylene (10). Ultrahigh molecular weight polyethylene is also of interest as a way of improving the mechanical properties. 

IJe have attempted to extrude nylon 11, poly(ethylene tereph-thalate) (PET) and ultrahigh molecular weight polyethylene (UHflWPE). The billets were prepared by slow cooling from the melt. The results are summarized below. Nylon 11 was extruded up to draw ratio 5 in the presence of silicone oil lubricant. Both high molecular weight PET and UHMWPE could not be extruded at this draw ratio. Use of the split billet and push-pull techniques did not assist the extrusion. For the purposes of this discussion these polymers will be described as "unprocessable". 

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