Polymeric Amorphous Materials

In the last few decades, the importance of friction between polymeric materials has rapidly increased. While the coefficients of friction for such materials are usually found to fall in the normal range, their behavior as a function of load indicates that the deformations occurring at the points of contact are elastic rather than plastic in nature. 

The effect of such a transfer mechanism between two polymer surfaces might be expected to be small. For metal-polymer interfaces, on the other hand, the effect may be significant, since one is now going from a situation of metal-polymer sliding contact to one of polymer-polymer contact, which would be expected to have a much smaller inherent coefficient of friction. Because polymers are relatively soft materials, the questions of plowing contributions and elastic and viscoelastic work loss must be considered. 

FIGURE 18.6. The discovery of new forms of carbon, the spherical bucky balls, opens the possibility of new and more effective lubrication in critical areas. One can imagine the presence of a layer of bucky ball bearings lubricating moving surfaces by rolling along with the flow. 

FIGURE 18.7. For some polymers in contact with a harder surface, small sections of the surface material may detach and slide along the surface with the harder material. The apparent coefficient of friction will then be that of polymer against polymer. 

Obviously, the question of friction and its various contributing factors can become quite complicated, which explains the large volume of scientific and technical literature on the subject. So far, the discussion has been limited to unlubricated (formally, at least) surfaces. From a surface chemical point of view, the more interesting subject, perhaps, is not friction but how to combat it, or perhaps in a few cases increase it. With that in mind, we now turn to the subject of lubrication. 

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In most polymeric as well as non-polymeric amorphous materials, the ability to undergo large-scale molecular motions implies the freedom to flow, so that the material becomes a fluid above Tg. However, in the special class of polymers commonly described as thermosets , covalent crosslinks limit the ability to undergo large-scale deformation. Consequently, above Tg, thennosets become elastomers (also known as crosslinked rubbers ). 

Condensed phosphates are derived by dehydration of acid orthophosphates. The resulting polymeric stmctures are based on a backbone of P—O—P linkages where PO tetrahedra are joined by shared oxygen atoms. The range of materials within this classification is extremely broad, extending from the simple diphosphate, also known as pyrophosphate, to indefinitely long-chain polyphosphates and ultraphosphates (see Table 1). Both weU-defined crystalline and amorphous materials occur among the condensed phosphates. 

C), and is much less reactive it is therefore safer and easier to handle, and is essentially non-toxic. The amorphous material can be transformed into various crystalline red modifications by suitable heat treatment, as summarized on the right hand side of Fig. 12.3. It seems likely that all are highly polymeric and contain three-dimensional networks formed by breaking one P-P bond in each P4 tetrahedron and then linking the remaining P4 units into chains or rings of P atoms each of which is pyramidal and 3 coordinate as shown schematically below ... 

In contrast wdth pyrrole, the polymerization does not appear to go beyond the trimer stage, any amorphous material produced being the product of autoxidation. 

One of the most important areas of application of the solid-state NMR technique is the investigation of the structures of cross-linked amorphous materials in cases where X-ray diffraction technqiues are not applicable. Polymeric resins are one such important class of materials. A lot of work has been done in this area by several investigators 36,37 38 since the beginning of the 80. Some solid-state NMR data of phenolic resins are presented in Fig. 10. Comparison with liquid state data for... 

Even within a particular class of polymers made by step-growth polymerization, monomer composition can be varied to produce a wide range of polymer properties. For example, polyesters and polyamides can be low-Tg, amorphous materials or high-Tg, liquid crystalline materials depending on the monomer composition. 

X-ray crystallographic techniques when extended to polymeric solids some interesting features of the internal structure of these substances. It was found that good majority of polymers diffract X-rays like any crystalline substance but many behave like amorphous materials giving very broad and diffuse X-ray diffraction patterns. This is seen in following figure. 

In theory, almost any comonomer diacid or dialcohol could lead to amorphous copolymers of PET. For example, incorporation of 20-80% of 2,6-naphthalate, or greater than 30% of isophthalate, will generate amorphous materials [9], Amorphous copolymers of PET, produced by the wholesale substitution of other monomers into the polymeric backbone, rarely possess desirable thermomechanical properties, unlike the Eastman PETG compositions. 

Various a-methylenemacrolides were enzymatically polymerized to polyesters having polymerizable methacrylic methylene groups in the main chain (Fig. 3, left). The free-radical polymerization of these materials produced crosslinked polymer gels [10, 12]. A different chemoenzymatic approach to crosslinked polymers was recently introduced by van der Meulen et al. for novel biomedical materials [11]. Unsaturated macrolactones like globalide and ambrettolide were polymerized by enzymatic ROP. The clear advantage of the enzymatic process is that polymerizations of macrolactones occur very fast as compared to the chemically catalyzed reactions [13]. Thermal crosslinking of the unsaturated polymers in the melt yielded insoluble and fully amorphous materials (Fig. 3, right). 

To obtain as much information as possible on a material, an empirical technique known as time-temperature superposition (TTS) is sometimes performed. This technique is applicable to polymeric (primarily amorphous) materials and is achieved by performing frequency sweeps at temperatures that differ by a few degrees. Each frequency sweep can then be shifted using software routines to form a single curve called a master curve. The usual method involves horizontal shifting, but a vertical shift may be employed as well. This method will not... 

Most MIPs show a heterogeneous distribution of binding sites and can be considered as polyclonal in their nature. In non-covalent imprinting, the amorphous material contains binding sites which are not identical because they may have different cross-linking density or accessibility. Moreover, the monomer (M) and the template molecule (A) may form complexes of different stoichiometry (MnA) in the pre-polymerization mixture [5]... 

The calculation of residual stresses in the polymerization process during the formation of an amorphous material was formulated earlier.12 The theory was based on a model of a linear viscoelastic material with properties dependent on temperature T and the degree of conversion p. In this model the effect of the degree of conversion was treated by a new "polymerization-time" superposition method, which is analogous to the temperature-time superposition discussed earlier. 

Figure 10.5 shows scanning electron micrographs of blend samples that were prepared as described in the Experimental Section . The etchant preferentially attacks polyethylene, producing a topography in which the polystyrene-rich domains are raised above the polyethylene domains. The interlamellar amorphous material provides a location for styrene to penetrate and polymerize. A considerable amount of polystyrene is present in the center of the spherulites. This is due either to amorphous polyethylene that is present in these locations or to voids that develop during crystallization... 

Polymeric PR materials show advantages in mechanical strength and in forming amorphous films. However, if one considers the fact that most of the PR measurements and applications will be in the form of films sandwiched between two... 

Among the supports that have been used in the preparation of supported transition metal nanoparticles are carbon, silica, alumina, titanium dioxide, and polymeric supports [57], and the most frequently used support is alumina [56], These supports normally produce an effect on the catalytic activity of the metallic nanoparticles supported on the amorphous material [60], In Chapter 3, different methods for the preparation of metallic catalysts supported on amorphous solids were described [61-71],... 

Another possible evidence of grafted organic/polymeric molecules onto CNT surface can be achieved by microscopy analyses using both principal types of electron microscopy - transition electron microscopy, TEM, or scanning electron microscopy, SEM (15,19,24,44,45,47). Such analyses are usually performed after careful extraction of the polymer from tubes by polymer solvents, performed several times by a reflux procedure with an excess of solvent therefore it is supposed that only covalently attached molecules remain fixed at CNT surface. High Resolution mode of TEM analysis shows the evidence of amorphous material on nanotubes surface (15). 

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