Crystalline, semi

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. 

The aim of this chapter is to describe the micro-mechanical processes that occur close to an interface during adhesive or cohesive failure of polymers. Emphasis will be placed on both the nature of the processes that occur and the micromechanical models that have been proposed to describe these processes. The main concern will be processes that occur at size scales ranging from nanometres (molecular dimensions) to a few micrometres. Failure is most commonly controlled by mechanical process that occur within this size range as it is these small scale processes that apply stress on the chain and cause the chain scission or pull-out that is often the basic process of fracture. The situation for elastomeric adhesives on substrates such as skin, glassy polymers or steel is different and will not be considered here but is described in a chapter on tack . Multiphase materials, such as rubber-toughened or semi-crystalline polymers, will not be considered much here as they show a whole range of different micro-mechanical processes initiated by the modulus mismatch between the phases. 

An important subdivision within the thermoplastic group of materials is related to whether they have a crystalline (ordered) or an amorphous (random) structure. In practice, of course, it is not possible for a moulded plastic to have a completely crystalline structure due to the complex physical nature of the molecular chains (see Appendix A). Some plastics, such as polyethylene and nylon, can achieve a high degree of crystallinity but they are probably more accurately described as partially crystalline or semi-crystalline. Other plastics such as acrylic and polystyrene are always amorphous. The presence of crystallinity in those plastics capable of crystallising is very dependent on their thermal history and hence on the processing conditions used to produce the moulded article. In turn, the mechanical properties of the moulding are very sensitive to whether or not the plastic possesses crystallinity. 

Typical Characteristics of Some Important Plastics (a) Semi-crystalline plastics... 

Delphinine, C34H47O9N (Walz) or C33H45O9N (J. and C.). The alkaloid crystallises in rhombs, or six-sided plates, m.p. 198-200°, [a]f ° + 25° (EtOH), shows mutarotation in alcoholic solution, and forms an acid oxalate, B. H2C2O4, m.p. 168° dry), a hydrochloride, B. HCl, m.p. 208-210°, and a monobenzoyl derivative, m.p. 171-3°. On alkaline hydrolysis it yields one molecule each of acetic and benzoic acids. The basic, hydrolytic product of this action is delphonine, C24H3g07N, which is amorphous, but can be distilled at a bath temperature of 140° and a pressure of 0-001 to 0-0001 mm. The brittle, possibly semi-crystalline resin so obtained, has m.p, 76-8° and [a]f ° - - 37-5 (EtOH). 

PET fibers in final form are semi-crystalline polymeric objects of an axial orientation of structural elements, characterized by the rotational symmetry of their location in relation to the geometrical axis of the fiber. The semi-crystalline character manifests itself in the occurrence of three qualitatively different polymeric phases crystalline phase, intermediate phase (the so-called mes-ophase), and amorphous phase. When considering the fine structure, attention should be paid to its three fundamental aspects morphological structure, in other words, super- or suprastructure microstructure and preferred orientation. 

Friedrich, K. Crazes and Shear Bands in Semi-Crystalline Thermoplastics. Vol. 52/53, pp. 225-274. 

This extensive hydrogen bonding bears on several aspects of the chemistry and applications of cellulose. For instance, being a semi-crystalline polymer, cellulose cannot be processed by the techniques most frequently employed for synthetic polymers, namely, injection molding and extrusion from the melt. The reason is that its presumably lies above the temperature of its thermal... 

This effect of M can be explained as being due to the crystalline phase in the o semi-crystalline polymer. The presence of this crystalline phase reduces the molecular mobility. The crystalline structure is not something static, but it is perfected on annealing. The longer the reaction at a high temperature, the more perfect the crystalline phase, and the more the molecular mobility is restricted. After melting this starts all over again and the lower the M the faster is this crystallization process, o... 

In semi-crystalline polymers at least two effects play a role in the diffusion of the reactive endgroups. Firstly, the restriction in endgroup movement due to the lowering of the temperature, which usually follows an Arrhenius type equation. Secondly, the restriction of the molecular mobility as a result of the presence of the crystalline phase whose size and structure changes on annealing. 

This part of the chapter has shown that the relationship between and is complicated. This relationship needs to considered separately for each polymer, but can be useful for gaining an insight into the morphology of particular semi-crystalline polymers. 

BU4NF-2-3H20 (1.89 g, 6 mmol) in 20 mb abs. THF is cooled to 0°C and hexa-methyldisilazane 2 (HMDS 4.3 g, 27 mmol) is added with stirring and exclusion of humidity. After stirring for 12 h at room temperature the volatile components, including the hexamethyldisiloxane (HMDSO) 7 (b.p. 100°C), are removed in vacuo. After 3 h the semi-crystalline, colorless residue is dissolved, under argon, in 10 mb abs. THF [14] (Scheme 13.29). 

The use of Equation (22) is very general, but it is also possible, with accurate measurements and data treatment, to perform the quantitative phase analysis in semi-crystalline materials without using any internal standard. This procedure is possible only if the chemical compositions of all the phases, including the amorphous one, are known. If the composition of the amorphous phase is unknown, the quantitative analysis without using any internal standard can still be used provided that the chemical composition of the whole sample is available [51]. This approach, until now, has been developed only for the XRD with Bragg-Brentano geometry that is one of the most diffused techniques in powder diffraction laboratories. 

The true value of the chloropolymer (I) lies in its use as an intermediate for the synthesis of a wide variety of polytorgano-phosphazenes) as shown in Figure 1. The nature and size of the substituent attached to the phosphorus plays a dominant roll in determining the properties of the polyphosphazene. Homopolymers prepared from I, in which the R groups are the same or, if different, similar in molecular size, tend to be semi-crystalline thermoplastics. If two or more different substituents are introduced, the resulting polymers are generally amorphous elastomers. (See Figure 1.)... 

Many polymers solidify into a semi-crystalline morphology. Their crystallization process, driven by thermodynamic forces, is hindered due to entanglements of the macromolecules, and the crystallization kinetics is restricted by the polymer s molecular diffusion. Therefore, crystalline lamellae and amorphous regions coexist in semi-crystalline polymers. The formation of crystals during the crystallization process results in a decrease of molecular mobility, since the crystalline regions act as crosslinks which connect the molecules into a sample spanning network. 

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