Amorphous polymers chemical structure

Irradiation (e-beam or y-irradiation) is classified among the high-energy solid-state processes. Some similarities between mechanical milling and irradiation to compatibilize nonmiscible blends have been observed by different authors. 1- Smith et al. considered that these processes lead to the same phenomena (chain scission, cross-linking, amorphization), and they claimed that the factors influencing these phenomena (polymer chemical structure and temperature) are not dependent on the process. ... 

Polymers can exhibit a hierarchical organization of structure at four successive levels, the molecular, nano-, micro-, and macrolevel [33, 34], On the scale of tens of microns, semicrystalline polymers contain spherulites, the spherulites have a lamellar texture, and the molecules within the lamellae are organized in crystals and amorphous domains. Amorphous polymers are structured on the molecular and macroscopic scale only [34]. Thermoplastic SMPs are usually phase-segregated materials, i.e., they consist of at least two different domains, which are related to different thermal transition temperatures (Tinms)- Therein hard domains have a TJrans (glass transition temperature Tg or melting temperature 7]n) usually much higher than room temperature and determine the permanent shape, while switching domains show a lower thermal transition (Tg or 7]n). SMP networks contain chemical crosslinks instead of hard domains to fix the permanent shape. 

This chapter is concerned with aspects of the structure of polymeric materials outside those of simple chemical composition. The main topics covered are polymer stereochemistry, crystallinity, and the character of amorphous polymers including the glass transition. These may be thought of as arising from the primary structure of the constituent molecules in ways that will become clearer as the chapter progresses. 

As we discussed in the section on the structural properties of amorphous polymers, the relative size of the bond length and the Lennard-Jones scale is very different when comparing coarse-grained models with real polymers or chemically realistic models, which leads to observable differences in the packing. Furthermore, the dynamics in real polymer melts is, to a large extent, determined by the presence of dihedral angle barriers that inhibit free rotation. We will examine the consequences of these differences for the glass transition in the next section. 

Thermoplastic polymers can be heated and cooled reversibly with no change to their chemical structure. Thermosets are processed or cured by a chemical reaction which is irreversible they can be softened by heating but do not return to their uncured state. The polymer type will dictate whether the compound is completely amorphous or partly crystalline at the operating temperature, and its intrinsic resistance to chemicals, mechanical stress and electrical stress. Degradation of the basic polymer, and, in particular, rupture of the main polymer chain or backbone, is the principal cause of reduction of tensile strength. 

It is important to mention that the structure/properties relationships which will be discussed in the following section are valid for many polymer classes and not only for one specific macromolecule. In addition, the properties of polymers are influenced by the morphology of the liquid or solid state. For example, they can be amorphous or crystalline and the crystalline shape can be varied. Multiphase compositions like block copolymers and polymer blends exhibit very often unusual meso- and nano-morphologies. But in contrast to the synthesis of a special chemical structure, the controlled modification of the morphology is mostly much more difficult and results and rules found with one polymer are often not transferable to a second polymer. 

The chemical structure determines a given polymer s tendency toward being crystalline or amorphous, or being a mixture of crystalline and amor-... 

Many polymers, after irradiation at low temperature, give off light when allowed to warm. This phenomenon of thermoluminescence depends not only on the chemical structure but also on crystal morphology. In polyethylene, for example, peaks in the thermoluminescence glow curve correspond, respectively, to the crystalline and the amorphous regions (9, 19, 22) (Figure 2). 

Some representative backbone structures of PQs and PPQs and their T data are given in Table 1. As in other amorphous polymers, the T s of PQs and PPQs are controlled essentially by the chemical structure, molecular weight, and thermal history. Several synthetic routes have been investigated to increase the T and also to improve the processibility of PPQ (71). Some properties of PPQ based on 2,3-di(3,4-diaminophenyl)quinoxaline and those of l,l-dichloro-2,2-bis(3,4-diaminophenyl)ethylene are summarized in Table 2. 

Amorphous polymers, as the name implies, are structureless except at the molecular level where we shall propose a suitable RVE. Semicrystaliine polymers exhibit a wide variety of structures depending upon their chemical nature, the degree of polymerization, the form and size of crystals and their assembly into spherulites, lamellae, fibrils etc. 

In this paper, the investigation of solid-state transitions has been performed on quite a large series of amorphous polymers. Various types of chemical structures have been considered ... 

In another paper in this issue [1], the molecular motions involved in secondary transitions of many amorphous polymers of quite different chemical structures have been analysed in detail by using a large set of experimental techniques (dynamic mechanical measurements, dielectric relaxation, H, 2H and 13C solid state NMR), as well as atomistic modelling. 

The purpose of this paper is to investigate the mechanical properties (plastic deformation, micromechanisms of deformation, fracture) of several amorphous polymers considered in [1], i.e. poly(methyl methacrylate) and its maleimide and glutarimide copolymers, bisphenol A polycarbonate, aryl-aliphatic copolyamides. Then to analyse, in each polymer series, the effect of chemical structure on mechanical properties and, finally, to relate the latter to the motions involved in the secondary transitions identified in [ 1] (in most cases, the p transition). 

Bisphenol A polycarbonate (BPA-PC), whose the chemical structure is shown in Fig. 66a, has very interesting fracture properties, exhibiting quite a high toughness for a pure amorphous polymer. At a very low temperature (- 100 °C at 1 Hz) it presents a secondary fi transition, shown in Fig. 67, which has been analysed in detail in [1] (Sect. 5). 

A first approach was to consider amorphous polymers with quite different chemical structures, either containing side groups responsible for the /> transition (PMMA and its copolymers), or only a chain backbone without side chains (BPA-PC, aryl-aliphatic copolyamides) in which the ft transition results from motions of some of the main-chain units (typically phenyl ring, carbonate, or amide groups). 

---