Crystallinity rigid crystallites

Copolymers in regions 2 and 3 consist of rigid crystallites dispersed in a relatively soft, rubbery, amorphous matrix. Since the refractive indices of the crystalline and amorphous phases are in general different, materials in these regions will be translucent to opaque, depending on the size of the crystallites, the degree of crystallinity, and the thickness of the sample. Since the crystallites... 

None of the models address the question of how the main chains are packed, and details of crystallinity are neither factored into nor predicted by mathematical models of the structure and properties of Nafion. Chains packed in crystalline arrays are usually considered to be rigid within the context of certain properties for example, with regard to diffusion, crystallites are viewed as impenetrable obstacles. F NMR studies indicate otherwise. Molecular motions that do not significantly alter symmetry can in fact occur in polymer crystals. It would seem, for example, that the response of the Nafion structure to applied stress would depend on the flexibility of the polymer backbone, a certain fraction of which is incorporated in crystalline regions. On the other hand. Starkweather showed that the crystallinity and swelling of Nafion are not correlated. 

Decreased mobility of adsorbed chains has been observed and proved in many cases both in the melt and in the solid state [52-54] and changes in composite properties are very often explained by it [52,54]. Overall properties of the interphase, however, are not completely clear. Based on model calculations the formation of a soft interphase is claimed [51], while in most cases the increased stiffness of the composite is explained by the presence of a rigid interphase [55,56]. The contradiction obviously stems from two opposing effects. Imperfection of the crystallites and decreased crystallinity of the interphase should lead to lower modulus and strength and larger deformability. Adhesion and hindered mobility of adsorbed polymer chains, on the other hand, decrease deformability and increase the strength of the interlayer. 

LLDPE rapidly crystallizes from the melt with the formation of sphemlites, small spherical objects 1-5 im in diameter visible only in a microscope. The elementary structural blocks in spheruliles are lamellae, small flat crystallites formed by folded linear segments in LLDPE chains, which are interconnected by polymer chains that pass from one lamella to another (lie molecules). Crystalline lamellae within sphemlites give LLDPE articles necessary rigidity, whereas the large amorphous regions between lamellae, constituting over 60% of tile splierulile volume, provide flexibility. 

Spin-Lattice and Spin-Spin Relaxations. In order to determine the content of these crystalline and noncrystalline resonances, the longitudinal and transverse relaxations were examined in detail. It was first confirmed that the noncrystalline resonance of all samples is associated with Tic in an order of 0.45-0.57 s. Hence, the noncrystalline component of all samples comprises a monophase, in as much as judged only by Tic. However, it was found that the noncrystalline component of drawn samples generally comprises two phases with different T2C values amorphous and crystalline-amorphous interphases. The dried gel sample does not include rubbery amorphous material it comprises the crystalline and rigid noncrystalline components. However, the rubbery amorphous phase with T2C of 5.5 ms appears by annealing at 145 °C for 4 minutes. For the orthorhombic crystalline component, three different Tic values, that suggest the distribution of crystallite size, were recognized for each sample, as normal for crystalline polymers [17,54, 55]. The Tic and T2C of all samples examined are summerized in Table 6. 

It is now generally accepted that folding is universal for spontaneous, free crystallisation of flexible polymer chains. It was first of all found in crystallisation from very dilute solutions, but it is beyond doubt now, that also spherulites, the normal mode of crystallisation from the melt, are aggregates of platelike crystallites with folded chains, pervaded with amorphous material. "Extended chain crystallisation" only occurs under very special conditions in the case of flexible chains for rigid polymer chains it is the natural mode ("rigid rod-crystallisation" from the melt in case of thermotropic polymers, and from solution in case of the lyotropic liquid-crystalline polymers both of them show nematic ordering in the liquid state). 

Critical Oxygen Index (COI), 853 Critical size, 704-705 Critical spherical nucleus, 710, 711 Critical strain, 867, 868 Critical stress energy factor, 474 Critical surface tension of wetting, 232 Critical temperature, 655 Cross-linked polymers, 29 Cross-linking, 148 Cross model, 731 Cross polarisation, 376, 377 Crystallinity, 728, 732, 815 Crystallites/Crystallisation, 690, 725 of rigid macromolecules, 739 Cyclical chain length, 782... 

The stress relaxation method was employed here or determination o total crosslink density, consisting o covalent crosslink density and o crosslink density caused by physical bonding o the rigid segment crystallites. The crystalline aggregates acted as physical crosslinks at ambient and at moderately elevated temperature (5.6). 

The crystalline aggregates, probably the fringed micelle crystallites, act as physical crosslinks and decrystallize at elevated temperatures. RIM elastomers containing a higher content of rigid crystallizing segments show more resistance to thermal decay. 

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