The Amorphous Polymer State

An amorphous polymer does not exhibit a crystalline X-ray diffraction pattern, and it does not have a flrst-order melting transition. If the structure of crystalline polymers is taken to be regular or ordered, then by difference, the structure of amorphous polymers contains greater or lesser amounts of disorder. 

The older literature often referred to the amorphous state as a liquid state. Water is a noncrystaUine (amorphous) condensed substance and is surely a liquid. However, polymers such as polystyrene or poly(methyl methacrylate) at room temperature are glassy, taking months or years for significant creep or flow. By contrast, skyscrapers are also undergoing creep (or flow), becoming measurably shorter as the years pass, as the steel girders creep (or flow). Today, amorphous polymers in the glassy state are better called amorphous solids. 

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We hypothesized that this polymerization proceeds by a single-site catalyst under different morphological conditions and variable monomer concentrations, i.e. polymerization in the crystalline polymer and in the amorphous polymer state. The frequency factor of the polymerization in the crystalline polymer should be lower than that in the amorphous polymer. The effect of monomer concentration on the polymerization rate is shown in Figure 17.14 using toluene as a solvent. The reaction rate is proportional to the monomer concentration. From these results, the polymerization reaction can be described by the following equations ... 

Table 5.8 Major models of the amorphous polymer state...

While the amorphous polymer state is liquid-like in the classical sense, if the polymer is glassy, a better term would be amorphous solid, since measurable flow takes years or centuries. Ordinary glass, an inorganic polymer, is such a glassy polymer. The chains are rigidly interlocked in glassy polymers, motion being restricted to vibrational modes. 

As-polymerized PVDC does not have a well-defined glass-transition temperature because of its high crystallinity. However, a sample can be melted at 210°C and quenched rapidly to an amorphous state at <—20°C. The amorphous polymer has a glass-transition temperature of — 17°C as shown by dilatometry (70). Glass-transition temperature values of —19 to — 11°C, depending on both method of measurement and sample preparation, have been determined. 

For all the cases cited above, which represent those data for which a comparison can be presently made, there is a direct connection between the critical molecular weight representing the influence of entanglements on the bulk viscosity and other properties, and the NMR linewidths, or spin-spin relaxation parameters of the amorphous polymers. Thus the entanglements must modulate the segmental motions so that even in the amorphous state they are a major reason for the incomplete motional narrowing, as has been postulated by Schaefer. ( ) This effect would then be further accentuated with crystallization. 

Photoluminescence intensity of the amorphous polymers was generally much larger than that of the more crystalline polymers. The energy level of the lowest singlet excited state Es was evaluated to be 2.5-2.7 eV for the amorphous polymer pristine films, and 2.0 eV for the more crystalline polymers. The Stokes shifts were also observed to be much larger for the amorphous polymer films compared with those of the more crystalline polymer films. This indicates a larger structural relaxation of the amorphous polymers following photoexcitation. 

Most of the amorphous polymers are dissolved when they are in the glassy solid state. In this case the surface layer is "fully developed". The solid state of the polymer permits the existence of all four layers. The gel layer S2 is very important because it heals the cracks and holes, which have been created by the penetrating front of dissolving macromolecules. 

Estimate the free enthalpy of polymerisation of 1,3-butadiene to polybutadiene (1 4) when the monomer is in the liquid state and the polymer is in the amorphous solid state. 

An interesting example of the difference in drawing behaviour between amorphous and crystalline yam is the drawing of crystalline poly(ethylene terephthalate). It is often stated that crystalline PETP cannot be drawn. It is tme that the material breaks if drawn at a temperature of 80 °C, which is a drawing temperature normal for the amorphous polymer. Mitsuishi and Domae (1965), however, were able to draw crystalline PETP to a draw ratio of 5.5 at a temperature of 180 °C. 

IR spectra of the cis-1,4 isotactic or syndiotactic polymers, in the molten state or in solution, and of the amorphous polymers are practically identical. All are characterized by an intense band at 751.8 cm.-1 (cis double bonds). In the spectra of the solid isotactic or syndiotactic polymers, however, new bands appear which are typical of the crystallinity of the polymers (Figure 1 and 2). The positions of the most intense of these bands are as follows (1) isotactic polymers 746.2 843.8 925.9 1005 cm.-1 (2) syndiotactic polymers 757.5 854.7 925.9 1000 1136 cm."1 It is interesting to observe that the band of the... 

Figure 10 presents the kinetic trans-cis photoisomerization process, under UV irradiation in the solid state, hi this case, significant differences appear between samples behaviour, as a function of the nucleobase chemical structures. It is interesting to note that, in the case of azo-polysiloxane substituted with adeiune (sample 2 -Table 1), the behaviours in the solid state and in solution are similar. That means that the polysiloxane chain flexibility, combined with the amorphous polymer ordering assure enough free volume for the trans-cis isomerization process. 

A radius of gyration in general is the distance from the center of mass of a body at which the whole mass could be concentrated without changing its moment of rotational inertia about an axis through the center of mass. For a polymer chain, this is also the root-mean-square distance of the segments of the molecule from its center of mass. The radius of gyration is one measure of the size of the random coil shape which many synthetic polymers adopt in solution or in the amorphous bulk state. (The radius of gyration and other measures of macromolecular size and shape are considered in more detail in Chapter 4.)... 

In previous examples of substituted polysiloxanes, the relaxation of the side chains with the rigid backbone was assumed to describe the polymer chain in the amorphous glassy state. This assumption, relaxation of side chains only, can be used to study the crystalline states of PDES. Certainly, this simplification is extreme, but it can be usefiil to understand the available orientation of the pendant groups when the polymer chains undergo transition from one crystalline form to another. The present approach does not address the chain reorientation or the interchain interactions in the crystalline state. 

Most crystalline polymers with metylenic groups in their structure and with a degree of crystallinity below 50% present a sub-glass relaxation whose intensity and location scarcely differ from those observed for the amorphous polymer in the glassy state. The temperature dependence of this relaxation follows Arrhenius behavior, and its activation energy is of the same order as that found for secondary processes in amorphous polymers. 

In the lamellar crystals of semicrystalline materials and the extended chain structure of oriented polymers, chain packing is usually much more efficient than in the amorphous, isotropic state. The efficiency of chain packing in the crystalline phase reduces the free volume available for transport to such an extent that, as a first approximation, the crystalline phase may be regarded as impermeable relative to the amorphous phase. 

The effect of orientation on oxygen permeability of the medium and high barrier resins is seen to be dependent upon the morphological nature of the barrier resin prior to orientation. A plot of the oxygen transmission rates as a function of the overall draw ratio (figure 3) illustrates this clearly. While the semicrystalline polymers, VDC copolymer, and aromatic nylon MXD-6, show little change in the permeability with moderate amounts of orientation in the solid state, orientation of the amorphous polymers SELAR PA 3426 and XHTA-50A causes reduction in the permeability by 5-30% in both resins, depending upon the overall level of orientation. 

Sometimes (18) the water molecules that fail to freeze on lowering the temperature are denoted as bound. This notion is open to criticism, however. It is true that collagen shares with other polymers the property that a considerable fraction of water remains unfrozen on lowering the temperature. On the basis of the number of grams of water per g of polymer the values are 0.5, 0.3 and 0.3 for collagen, elastin, and methyl-cellulose, respectively. For different reasons the polymer chains are essentially immobile. For collagen the crystalline, rodlike, molecules are apparently in close contact with each other and in elastin and methylcellulose the amorphous polymers are in the glassy state. At temperatures below 0 C ice is the stable phase in bulk. In the narrow, fixed, polymer interstices, however, space requirements are insufficient to form three-dimensional ice crystals. Other options available to the water molecules are to remain in the interstices in liquid form, or to form ice outside the polymer as a separate phase. 

Polyethylene terepthalate (PET) differs from PE mainly by a benzene ring, which is incorporated into every monomer unit of the chain molecule. Compared to PE, the chain becomes bulkier, which influences the crystallization behaviour of this polymer. PET can be obtained perfectly amorphous when quenched from the melt. Thus it can be crystallized from the amorphous solid state, in contrast to PE, which is always obtained with a high degree of crystallinity. The degree of crystallinity that is reached in PET depends on many material parameters and crystallization conditions, but more than 60% is unusual. 

The amorphous polymers can easily be dissolved in non-polar solvents. The situation is different if the polymers are semi-crystalline, and in this case, to dissolve the polymer in the solvent, it is necessary to raise their temperature to a high level (for instance, 110°C for polyethylene in xylene). Therefore, if a polymer is amorphous in its solid state, it is easier to study its solutions. 

Many crystallizable polymers can be prepared in the amorphous glassy state by rapid quenching as films. Measurements of Aglass transition temperature determined. Such results are shown for amorphous polyethylene terephthalate (PET) in Figure 13 (17). The Brillouin splittings change slope at 70°C. If both Aa>(i) and Awt can be measured, the Poisson ratio (T can be determined according to ... 

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