Crystallinity mechanical properties

Lin and Isayev (2006) prepared blends of PA-6 and PP by treatment with high-intensity ultrasound during extrusion. Mechanical properties, crystallinity, and morphology were investigated. The competition between polymer degradation and partial in situ compatibilization was assessed. 

Recycle PET/recycle HDPE/ PE-g-MA/SEBS Morphology/mechanical properties/crystallinity level/optionally with addition of methylene di(phenyl isocyanate) coupling agent Lei et al. 2009... 

For some mechanical properties, crystalline polymers have distinct advantages over the polymeric glasses which do not crystallize. This is particularly true in respect of toughness and ductility amorphous polymers will not form useful fibres. As an example, consider polyethylene, which at room temperature is partially crystalline. The crystals are extremely thin and have the form of lamellae (see Fig. 1.7). Those portions of the molecule which have not... 

Note the mechanical properties, crystallinity, molecular weight, and Tg/m.p. of the biodegradable polymer depends on various factors such as the monomer/initiator structure, synthetic and postprocessing conditions, and the presence of additives. In particular, excessively high processing temperatures will shift the equilibrium toward monomer formation, which will affect the mechanical... 

The good mechanical properties of this homopolymer result from the ability of the oxymethylene chains to pack together into a highly ordered crystalline configuration as the polymers change from the molten to the solid state. 

No polymer is ever 100% crystalline at best, patches of crystallinity are present in an otherwise amorphous matrix. In some ways, the presence of these domains of crystallinity is equivalent to cross-links, since different chains loop in and out of the same crystal. Although there are similarities in the mechanical behavior of chemically cross-linked and partially crystalline polymers, a significant difference is that the former are irreversibly bonded while the latter are reversible through changes of temperature. Materials in which chemical cross-linking is responsible for the mechanical properties are called thermosetting those in which this kind of physical cross-linking operates, thermoplastic. 

Below Tg the material is hard and rigid with a coefficient of thermal expansion equal to roughly half that of the liquid. With respect to mechanical properties, the glass is closer in behavior to a crystalline solid than to a... 

In the last three chapters we have examined the mechanical properties of bulk polymers. Although the structure of individual molecules has not been our primary concern, we have sought to understand the influence of molecular properties on the mechanical behavior of polymeric materials. We have seen, for example, how the viscosity of a liquid polymer depends on the substituents along the chain backbone, how the elasticity depends on crosslinking, and how the crystallinity depends on the stereoregularity of the polymer. In the preceding chapters we took the existence of these polymers for granted and focused attention on their bulk behavior. In the next three chapters these priorities are reversed Our main concern is some of the reactions which produce polymers and the structures of the products formed. 

Mechanical Properties. Polyester fibers are formed by melt spinning generally followed by hot drawing and heat setting to the final fiber form. The molecular orientation and crystalline fine stmcture developed depend on key process parameters in all fiber formation steps and are critical to the end use appHcation of the fibers. 

Commercial production of PVA fiber was thus started in Japan, at as early a period as that for nylon. However, compared with various other synthetic fibers which appeared after that period, the properties of which have continuously been improved, PVA fiber is not very well suited for clothing and interior uses because of its characteristic properties. The fiber, however, is widely used in the world because of unique features such as high affinity for water due to the —OH groups present in PVA, excellent mechanical properties because of high crystallinity, and high resistance to chemicals including alkah and natural conditions. 

The dynamic mechanical properties of PTFE have been measured at frequencies from 0.033 to 90 Uz. Abmpt changes in the distribution of relaxation times are associated with the crystalline transitions at 19 and 30°C (75). The activation energies are 102.5 kj/mol (24.5 kcal/mol) below 19°C, 510.4 kJ/mol (122 kcal/mol) between the transitions, and 31.4 kJ/mol (7.5 kcal/mol) above 30°C. 

Mechanical Properties. The principal mechanical properties are Hsted in Table 1. The features of HDPE that have the strongest influence on its mechanical behavior are molecular weight, MWD, orientation, morphology, and the degree of branching, which determines resin crystallinity and density. 

Polypropylene molecules repeatedly fold upon themselves to form lamellae, the sizes of which ate a function of the crystallisa tion conditions. Higher degrees of order are obtained upon formation of crystalline aggregates, or spheruHtes. The presence of a central crystallisation nucleus from which the lamellae radiate is clearly evident in these stmctures. Observations using cross-polarized light illustrates the characteristic Maltese cross model (Fig. 2b). The optical and mechanical properties ate a function of the size and number of spheruHtes and can be modified by nucleating agents. Crystallinity can also be inferred from thermal analysis (28) and density measurements (29). 

Similady, hquid-crystal polymers exhibit considerable order in the hquid state, either in solution (lyotropic) or melt (thermotropic). When crystallized from solution or melt, they have a high degree of extended-chain crystallinity, and thus have superior mechanical properties. Kevlar (Du Pont) is an aromatic polyamide (atamid) with the repeating unit designated as (2). It is spun into... 

It must be kept in mind that mechanical properties are influenced by factors other than the degree of crystallinity (molecular weight, in particular). 

Copolymerisation also affects morphology under other crystallisation conditions. Copolymers ia the form of cast or molded sheets are much more transparent because of the small spheruHte size. In extreme cases, crystallinity cannot be detected optically, but its effect on mechanical properties is pronounced. Before crystallisation, films are soft and mbbery, with low modulus and high elongation. After crystallisation, they are leathery and tough, with higher modulus and lower elongation. 

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