Spherulitic crystallites

It is suggested that the plasticization of a low density polyethylene, LDPE, occurs inside the spherulitic crystallite in the interlamellar and interfibrillar regions. DOP-plasticized LDPE melts at lower temperatures as a result of plasticization. ... 

Undeformed NR forms spherulitic crystallites below 0°C. The tempera-ture/time induced crystallites generally form as folded chain lamellae, whereas the morphology of the strain/stress induced crystallites has been reported to be various fibrils, fibrils and folded lamellae, and shish-kebabs. " Strain-induced crystallites and their orientation have been indicated as the key reasons for the sharp increase in modulus that accompanies strain-induced crystallization. 

As expected, the creep compliance master curves indicated a lower compliance for all the iiaiiocomposites at shorter times (up to 10" s) as compared to neat PP. However, it is interesting to note that the creep compliance curves for Iiaiiocomposites with 0.5, 1, and 2.5 wt% of MWNT showed a cross over at about 10 s. The iianocomposite with 5 wt% of MWNTs showed the lowest compliance over the entire time range when compared to all the other compositions. This peculiar behavior was tentatively explained by the authors considering that two competing factors may concur in determining the overall creep response (i) the size of the spherulites/crystallites (which is a consequence of incorporation of nanotubes and hence inter-phase effects), (ii) the thermal expansion coefficient (which depends on the amount of nanotubes). 

The presence of spherulites or smaller crystallites is comparable to cross-linking and affects not only the moduli and compliances, but also the ultimate properties such as yield strength and ultimate elongation. 

Figure 3.6). This theory known as the fringed mieelle theory or fringed crystallite theory helped to explain many properties of crystalline polymers but it was difficult to explain the formation of certain larger structures such as spherulites which could possess a diameter as large as 0.1 mm. 

A further increase in extension leads to irreversible changes which immediately precede the transition of the polymer into the oriented state. During this transition, the spherulites undergo considerable structural changes and are thus converted qualitatively into different structural elements i.e. macrofibrils4). After a certain critical elongation has been attained, the initial crystallites collapse and melt and a new oriented structure is formed in which the c axes of crystals are oriented in the direction of extension. 

Explain the hierarchy of crystalline regions using the words unit cell, crystallite, lamellae, cylindrites and spherulites. 

Nylon crystallites consist of sheets of chains that are hydrogen-bonded to their neighbors. On a supermolecular scale, crystallites have a lamellar structure, that is they are many times longer and broader than they are thick. When nylon crystallizes from an isotropic molten state, it generally forms spherulites, which consist of ribbon-like lamellae radiating in all directions... 

All of these parameters are not under control. To deliver a more uniform distribution of crystallites, specific nucleating agents are added whilst processing. It has also been shown that many of the desirable properties are the result of small regular spherulites (or nascent crystals). 

The volume inside the semicrystalline polymers can be divided between the crystallized and amorphous parts of the polymer. The crystalline part usually forms a complicated network in the matrix of the amorphous polymer. A visualization of a single-polymer crystallite done [111] by the Atomic Force Microscopy (AFM) is shown in Fig. 9. The most common morphology observable in the semicrystalline polymer is that of a spherulitic microstructure [112], where the crystalline lamellae grows more or less radially from the central nucleus in all directions. The different crystal lamellae can nucleate separately... 

These concepts have to be correlated with the inherent helical form of the starch molecule, and attempts which have been made to determine the orientation of the portions of the molecules in the crystallites will be dealt with later (in the Section on x-ray diffraction studies—see p. 376). In this connection, the work on the orientation of synthetic crystalline polymers into spherulites containing helically arranged molecules23 may be important. 

Silicate minerals that usually occur as spherulitic aggregates of fibers have formed as a result of the alteration of the many minerals subsumed within the category of biopyriboles. Alteration of the micas under hydrothermal conditions produces compositional variants on recrystallization such as hydrous muscovite. Some of these samples have been labeled asbestiform, probably because they are found in veins that criss-cross rock masses. Fibrous micaceous minerals also occur as discrete disseminated particles, although few detailed analyses of crystallites from the disperse occurrences have been made. Fibrous mica found in veins usually grades (composition-ally) into members of the serpentine mineral group, the clays or the chlorites. 

Crystalline polymers exhibit the following basic properties They are opaque as long as the size of the crystallites or spherulites, respectively, lies above the wavelength of light. Their solubility is restricted to few organic solvents at elevated temperature. The following crystalline polymers have attained technical importance as thermoplastic materials polyethylene, polypropylene, aliphatic polyamides, aliphatic/aromatic polyamides, aliphatic/aromatic polyesters, poly-oxymethylene, polytetrafluoroethylene, poly(phenylene sulfide), poly(arylene ether ketone)s. 

Figure 1.61 Schematic illustration of chain folding leading to lamellar crystallites (inset) and lamellar stacking to form spherulites.

In reality, the morphology of a polycrystalline thermoplastic consists of spherulites which holds for common PP, PE, PA 6, PA 6,6 and PEEK crystalHzed under common conditions. Some semicrystalhne polymers as weU as the above mentioned moderately filled ones may exhibit lameUar crystahine morphology without any spherulitic order. As a result of random orientation of individual crystallites in spheruhtes and the manner of their connectivity, the elastic modulus of about 10 GPa has been extrapolated for a hypothetical ideal polycrystalline PE containing no amorphous phase from the dependence of the elastic modulus of PE on the degree of crystallinity. The presence of an amorphous phase which reduces the content of the crystalline phase results in a further reduction of the overaU elastic modulus of the semicrystalhne polymers compared to ideal mono crystals. 

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