Layers, transcrystalline

In semi-crystalline polymers the interaction of the matrix and the tiller changes both the structure and the crystallinity of the interphase. The changes induced by the interaction in bulk properties are reflected by increased nucleation or by the formation of a transcrystalline layer on the surface of anisotropic particles [48]. The structure of the interphase, however, differs drastically from that of the matrix polymer [49,50]. Because of the preferred adsorption of large molecules, the dimensions of crystalline units can change, and usually decrease. Preferential adsorption of large molecules has also been proved by GPC measurements after separation of adsorbed and non-attached molecules of the matrix [49,50]. Decreased mobility of the chains affects also the kinetics of crystallization. Kinetic hindrance leads to the development of small, imperfect crystallites, forming a crystalline phase of low heat of fusion [51]. 

The strength of the material and the adhesion between fiber and matrix depend on the thickness of the transcrystalline layer. Figure 10.14 shows the effect of some process conditions (in this case temperature) on the thickness of the transcrystalline layer. °... 

Figure 10.14. The thickness of transcrystalline layer versus time at different melt temperatures in cellulose reinforced PP. [Adapted, by permission, from Gatenhom P, Hedenberg P, Karlsson J, Fehx J, Antec 96. Volume II. Conference proceedings, Indianapohs, 5th-10th May 1996, 2302-4.]...

The two-component system—crystal lamellae or blocks alternating with amorphous layers which are reinforced by tie molecules— results in a mechanism of mechanical properties which is drastically different from that of low molecular weight solids. In the latter case it is based on crystal defects and grain boundaries. In the former case it depends primarily on the properties and defects of the supercrystalline lattice of lamellae alternating with amorphous surface layers (in spherulitic, transcrystalline or cylindritic structure) or of microfibrils in fibrous structure, and on the presence, number, conformation and spatial distribution of tie molecules. It matters how taut they are, how well they are fixed in the crystal core of the lamellae or in the crystalline blocks of the microfibrils and how easily they can be pulled out of them. In oriented material the orientation of the amorphous component (/,) is a good indicator of the amount of taut tie molecules present and hence an excellent parameter for the description of mechanical properties. In fibrous structure it directly measures the fraction and strength of microfibrils present and therefore turns out to be almost proportional to elastic modulus and strength in the fibre direction. 

Klein N, Marom G, Wachtel E, Microstructure of nylon 6,6 transcrystalline layers in earbon and aramid fibre reinforced composites. Polymer, 37(24), 5493-5498, 1996. 

Keywords Composites Mechanical properties Natural fibers Supermolecular structure Transcrystalline layer... 

Effect of Chemical Modification on Transcrystalline Layer Formation. .. 276... 

A transcrystalline layer (TCL) is the supermolecular crystalline stmcture, induced by an oriented growth in the presence of the foreign surface. Transcrystallization occurs when the nucleation density of a solid filler that is in contact with melted... 

Fig. 10.6 Transcrystalline layer formed as a result of crystallization of iPP with lignocellulosic...

During the cooling of crystallizing Hot melt adhesives, the presence of the snbstrate can lead to surface nucleation, giving a colnmnal transcrystalline polymer morphology in the interfacial region. There have been reports that such layers influence measured adhesion, but conclusive demonstration has proved elusive. ... 

Wood fiour is often added to thermoplastics as a low cost filler to alter mechanical performance, especially the stiffness of low melt temperature, commodity thermoplastics such as polypropylene and polyethylene without increasing density excessively. Wood is much stiffer than the commodity thermoplastics usually used as matrices. Additionally, vood and pulp fibers can nucleate crystal growth in polyolefins resulting in a transcrystalline layer that can infiuence mechanical behavior [33, 34[. 

Carbon and aramid fiber produced a uniform transcrystalline layer in polyamide-66, the transcrystallization depended strongly on thermodynamic conditions such as ciystalli-zation temperature and cooling rate. ... 

Transcrystalline morphology is formed when crystallization takes place on the solid surface of fillers or reinforcements. Transcrystallizafion takes place when the density of the crystal nuclei is substantially greater on the surface of solid inclusions than in the melt bulk (77). Because polyhedral sphemlites cannot develop due to restricted lateral growth on the solid surface, crystallites are allowed to grow only in stacks perpendicularly to the surface plane (78). In the case when only one crystal form occurs in a polymer, Keller (79) confirmed that the microstructure of transcrystalline layer and bulk crystalline phase is identical. For PP, however, the situation is more complicated by the polymorphism so that one crystal form can exist in the transcrystalline layer and another in the polymer bulk. The nature of nucleation of the transcrystalline layer is still somewhat con-... 

A transcrystalline layer possesses different mechanical properties, and, moreover, unlike PP in bulk, this layer is strongly anisotropic. This is expected to be extremely important for determining the mechanical properties and hydrolytic stability, especially in the case of glass fiber reinforced PP. 

Figure 1 a transCrystalline layer induced by a polyethylene terephthalate (PET) fiber in a quiescent P-nudeated FP melt under isothermal conditions (crystallization temperature and time T = 124°C and = 30 min, respectively). 

Klein N, Marom G and Wachtel E (1996) Microstructure of nylon 66 transcrystalline layers in carbon and aramid fiber reinforced composites. Polymer 37 .5493-5498. 

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