Crystalline state mechanical behavior

Mechanical properties of plastics are invariably time-dependent. Rheology of plastics involves plastics in all possible states from the molten state to the glassy or crystalline state (Chapter 6). The rheology of solid plastics within a range of small strains, within the range of linear viscoelasticity, has shown that mechanical behavior has often been successfully related to molecular structure. Studies in this area can have two objectives (1) mechanical characterization of... 

It is a broadly accepted mechanism that in the Fischcr-Tropsch synthesis, CO dissociation into carbon and oxygen atoms occurs. l his dissociative absorption is the rate-determining step and is a prerequisite for chain growth. In the absence of CO dissociation, Ct species like methanol arc considered to be the main products. Consideration may also be given to the fluxional behavior of carbonyl clusters which has been proved by nuclear magnetic resonance studies. This could mean that the struciutcs deicmuncd in the crystalline state may be dift erent from those in solution. 

By comparing the photochemical behavior of conjugated diolefinic monomers in the crystalline state and in solution, a crystal matrix effect on four-center type photopolymerization has been revealed. It has been concluded that high molecular weigth linear polymers are produced photochemically from these monomers only by way of a crystal-lattice controlled mechanism. 

In this chapter we study the characteristics that determine the crystallinity of polymers, crystalline morphology, and the factors affecting the crystallization and melting of polymers. We describe the amorphous state, focusing on the glass transition, a fundamental property for defining the mechanical behavior of polymers. The entire description refers exclusively to synthetic polymers. 

This paper attempts to further explore the modification of ionic associations by a crystalline ionic plasticizer, such as zinc stearate, at the solid state. Mechanical properties, swelling behavior, and morphological aspects were studied in order to better understand the role of such crystalline polar additives. 

Poly(ethylene terephthalate) Poly(ethylene terephthalate) is a widely used semicrystalline polymer. The macroscopic properties of PET such as thermal, mechanical, optical, and permeation properties depend on its specific internal morphologies and microstructure arrangement. It can be quenched into the completely amorphous state, whereas thermal and thermomechanical treatments lead to partially crystallized samples with easily controlled degrees of crystallinity. The crystallization behavior of thermoplastic polymers is strongly affected by processing conditions [91-93]. 

The presence of mineral reinforcements such as talc or mica, as foreign solid particles embedded into a polypropylene matrix, usually induces a nucleation effect. A signihcant increase in the crystalline content of the polymer is evidenced if compared with the neat polymer when processed at the same setup conditions that are necessary to ensure a good accommodation of the solid particles into the amorphous phase of the polymer in order to obtain a material with a good mechanical performance (27). The comparison between PP/mica and PP/talc composites in terms of their mechanical behavior under dynamic conditions in the solid state agrees with the morphological features derived from their chemical structures of both minerals (28). 

The effect of the chemical ermstitution of the crosslinker oti the local topology of the network is the second new feature to be considered. If the crosslinker molecule is flexible it can behave like an isotropic solvent. In that case, essentially only the phase transition and phase transformatiOTi temperatures of the LC phase are affected [90]. If, however, the chemical constitution resembles that of a mesogen of the constituent polymer backbone, the history of the crosslinking process becomes important. Under these conditions the crossUnker adopts the state of order in which the final crosslink process of the network occurs and thus determines the local topology of the crosslink [120,121]. The mechanical properties and the reorientational behavior are considerably modified for networks with the same chemical constitution but crosslinked either in the isotropic or in the liquid crystalline state [122-124]. Other important aspects of the local topology at the crosslink concern the phase transformation behavior [125] as well as the positional ordering in smectic systems [126]. 

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