Polymer, semi-crystalline

If a modulus is plotted as a function of temperature, a very characteristic curve is obtained which is different in shape for the different types of polymer amorphous (glassy) polymers, semi crystalline polymers and elastomers (cross-linked amorphous polymers). 

Liquid diffusion in polymers is generally slower than gas diffusion, with diffusivities of the order of lO m s. The equilibrium solubility of liquids can be much larger than that of gases, and the liquid content can change the diffusion constant or even the physical state of the polymer. Semi-crystalline polymers are in general more resistant to organic liquids than glassy polymers, so the former are preferred. 

As a conclusion to our discussion of the mechanisms of fracture discussed in Section 12.7, the fracture toughnesses Kic of a selection of industrial polymers consisting of glassy polymers, semi-crystalline polymers, and thermosetting polymers are assembled and presented in Table 12.3. 

Table 12.3 Fracture toughnesses /(Jc at 300 K of a selection of prominent glassy polymers, semi-crystalline polymers, and epoxy resins (Williams 1984)...

To retain continuity with the earlier report, this review will be split into five sections Amorphous and Molten Polymers, Semi-crystalline Polymers, Solutions and Polyelectrolytes, Biological Materials, and Dynamical Studies of Macromolecules. 

The plot in Storage and loss properties for unfilled nylon 6 shows a DMA plot for nylon 6, a semi-crystalline polymer. Semi-crystalline polymers are so named because... 

This standard linear solid can be used to model the behavior of amorphorus polymers. Semi-crystalline polymers probably would require a more complex model to distinguish the deformation of the crystalline material from die amorphorus. The primary limitation of the standard linear solid to model amorphorus polymers is the assumption of a single viscosity p , whereas the local viscosity of a real polymer would be quite heterogeneous. 

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. 

Friedrich, K. Crazes and Shear Bands in Semi-Crystalline Thermoplastics. Vol. 52/53, pp. 225-274. Fujita, H. Diffusion in Polymer-Diluent Systems. Vol. 3, pp. 1-47. 

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. 

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. 

In semi-crystalline polymers, ordered structures appear at different dimensional levels... 

Figure 1 Illustration showing the increase in HDT at a fixed modulus by the addition of glass fibres to amorphous and semi-crystalline polymers.

Table 3 Comparison of HDTs for amorphous and semi-crystalline polymers... 

Polymers with blocks containing different tactcities can be produced, e.g., atactic PP (amorphous)/isotactic PP (semi-crystalline) can be made using metallocene catalysts. They behave in a manner similar to SBS thermoplastic elastomers. 

Volume and mass-based expressions for the degree of crystallinity are easily derived from the experimentally measured density (p) of a semi-crystalline polymer. The method is based on an ideal crystalline and liquid-like two-phase model and assumes additivity of the volume corresponding to each phase... 

Vc crystalline Va, amorphous). The densities of the pure crystalline (pc) and pure amorphous (pa) polymer must be known at the temperature and pressure used to measure p. The value of pc can be obtained from the unit cell dimensions when the crystal structure is known. The value of pa can be obtained directly for polymers that can be quenched without crystallization, polyfethylene terephtha-late) is one example. However, for most semi-crystalline polymers the value of pa is extrapolated from the variation of the specific volume of the melt with temperature [16,63]. 

The enthalpic change from the solid to the liquid-like phase of a semi-crystalline polymer can be obtained from DSC [16]. The mass-based degree of crystallinity (XI)SC) is calculated as the ratio of the heat of fusion of the sample (AH) and the value per mole of purely crystalline polymer (AHc). 

Both vibrational spectroscopies are valuable tools in the characterization of crystalline polymers. The degree of crystallinity is calculated from the ratio of isolated vibrational modes, specific to the crystalline regions, and a mode whose intensity is not influenced by degree of crystallinity and serves as internal standard. A significant number of studies have used both types of spectroscopy for quantitative crystallinity determination in the polyethylenes [38,74-82] and other semi-crystalline polymers such as polyfethylene terephthalate) [83-85], isotactic poly(propylene) [86,87], polyfaryl ether ether ketone) [88], polyftetra-fluoroethylene) [89,90] and bisphenol A polycarbonate [91]. 

In addition to quantitative crystallinity data, IR and Raman have been proven valuable tools to extract information on chain conformation in the three major phases [112-114], local order in amorphous polymers [115,116] high throughput characterization [117] and structural and polymorphic changes on heating and cooling semi-crystalline polymers [118-120]. 

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