Structural features amorphous polymers

Kargin and his school in a large number of papers, have discussed the structural features of polymers as revealed by examination under the electron microscope. It is suggested that, even in amorphous glasses, aggregation into long, thin bundles of chains is possible. 

Taking into account that the only structural feature of polymers in amorphous state is the existence of the network of intermolecular entanglements, one can suppose that the formation of the surface layers, on filler introduction, can only be caused by the changes in the initial network of entanglements. Consequently, the density of this network, in the surface layers, should differ from the same value for the bulk phase. The average molecular mass. Me, between two adjacent junction points of the network can be used as a measure of network density. It is thus evident that the effect of a solid surface should extend by the distance from the surface of at least equal A6 in relation to Me. Using the empirical relationship 2Me Me (where Me is critical molecular mass for the entanglements) and available data on Me for some flexible chains, it may be shown that the expected values of A6 should be in the limits of 40-100 This prediction coincides with... 

From the study of disordered chain conformations, while threedimensional order of some feature of the structure is maintained, we went into the consideration of model building of the "structure" of amorphous polymers. 

Amorphous stereotactic polymers can crystallise, in which condition neighbouring chains are parallel. Because of the unavoidable chain entanglement in the amorphous state, only modest alignment of amorphous polymer chains is usually feasible, and moreover complete crystallisation is impossible under most circumstances, and thus many polymers are semi-crystalline. It is this feature, semicrystallinity, which distinguished polymers most sharply from other kinds of materials. Crystallisation can be from solution or from the melt, to form spherulites, or alternatively (as in a rubber or in high-strength fibres) it can be induced by mechanical means. This last is another crucial difference between polymers and other materials. Unit cells in crystals are much smaller than polymer chain lengths, which leads to a unique structural feature which is further discussed below. 

As increasing amounts of NaOH are added to the Cr(N03) solution, the hydrolyzed Cr forms dimeric, polymeric and three-dimensional species. Gelled, amorphous and colloidal Cr(OH) is eventually formed. E. Matijevic reported the preparation of a monodispersed Cr(0H)g sol by forced hydrolysis of Cr(III) salt at 90° C (7). Because of the differences in structural features, each olated species (n=l,2,3) should react differently with polymers and form gels of different properties. 

Local structural features have been postulated for amorphous polymer systems, based on the asymmetry of chain-like molecules. Flory (56) has shown that molecular asymmetry in itself is no barrier to a dense random packing of the chains are sufficiently flexible. Robertson (57) suggests, however, that some degree of local alignment is required simply to accomodate linearly connected sequences in the rather limited space available. Unfortunately, Calculations of local cooperative effects are extremely difficult and sensitive to specific assumptions about available packing arrangements. 

X-ray diffraction measurements and structural calculations on murein [22-25] and pseudomurein [26-28] have revealed several common structural features in both polymers. Murein and pseudomurein sacculi possess a density of p= 1. 39-1.46g/cm which is characteristic of highly ordered material. A much lower density, in the range of p= 1.24-1.32g/cm is to be expected for amorphous polymers [26]. X-ray diffraction showed diffuse Debye-Scherer rings with Bragg periodicities of about 0.45 nm and 0.94 nm in the planes and of 4.3-4.5nm vertically to the planes of both types of cell walls. These data have been interpreted in two different ways ... 

In this section we will consider polydimethylsiloxane (PDMS) as an example of the type of work that is possible with amorphous polymers. The structure and INS spectrum of PDMS are shown in Fig. 10.21a [40]. The repeat unit shown in Fig. 10.21b was used to model the spectrum using the Wilson GF matrix method [41]. The major features are reproduced skeletal bending modes below 100 cm", the methyl torsion and its overtone at 180 and 360 cm respectively, the coupled methyl rocking modes and Si-0 and Si-C stretches at 700-1000 cm and the unresolved methyl deformation modes 1250-1500 cm. The last are not clearly seen because the intensity of the methyl torsion results in a large Debye-Waller factor, so above 1000 em or so, most of the intensity occurs in the phonon wings. 

As another (and perhaps more important for making reasonable practical predictions in a very simple manner) improvement in the ability to predict the thermal conductivities of polymers, a direct correlation was developed between the i(298K) of amorphous polymers and some of their most important structural features. This correlation will be presented in Section 14.B. While the indirect relationship (Equation 14.6) describes the physics of thermal... 

A dataset of twenty X(exp) values, all measured at room temperature (298 5K), for amorphous polymers and for the amorphous phases of scmicrystalline polymers, was prepared by combining information provided in several sources [2,4,6]. This dataset was used to develop a direct correlation between the stmctural features of polymers, as encoded into the zeroth-order and first-order connectivity indices, and the thennal conductivity at room temperature. This correlation is a first step towards quantifying the relationships between the structures and thermal conductivities of amorphous polymers. Its value does not lie in its absolute accuracy but in the clues it provides on the stmctural factors determining thennal conductivity. 

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