Transcrystalline structure

The crystallisation from melt in a temperature gradient or in contact with a surface acting as a nucleating substrate may result in a transcrystalline structure with the growth a.xis of the crystals, the h-axis in the case of polyethylene, the hydrogen bonds in the case of nylon parallel to the gradient or perpendicular to the substrate. 

A very similar orientation is obtained by annealing of roiled or drawn low density polyethylene sheets. The chain (c) axis is perpendicular to the sheet, the a-axis is oriented in the original draw direction and the b-axis is perpendicular to it in the plane of the eet. Very little is known about the orientation of the amorphous component and about the number of tautness of tie molecules. 

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Formation of the transcrystalline structure also depends on the geometry of the chain and the fiber surface. Carbon fibers and polyamides are a good match. This makes the chain arrangement on the surface of the fiber very precise and thus the resultant composite is very strong. ... 

The growth of a transcrystalline structure, for example, when the substrate acts as a nucleating agent [31]... 

Mucha and Krolikowski [39], studying the kinetics of isothermal crystallization of polypropylene, noticed that addition of a filler, e.g., wood flour, efficiently reduces the time of crystallization. This is desirable in the processing of composites as it reduces the injection-forming cycle and forms small spherulites improving the mechanical properties of composites. Transcrystalline structures are formed during polymer crystallization in the presence of lignocellulosic filler. 

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]. 

Atomistic simulation of an atactic polypropylene/graphite interface has shown that the local structure of the polymer in the vicinity of the surface is different in many ways from that of the corresponding bulk. Near the solid surface the density profile of the polymer displays a local maximum, the backbone bonds of the polymer chains develop considerable parallel orientation to the surface [52]. This parallel orientation due to adsorption can be one of the reasons for the transcrystallinity observed in the case of many anisotropic filler particles. 

Figure 7.18 shows how crystalline structure is affected by the presence of fiber. Here, bamboo fiber was used for polypropylene reinforcement. A nucleation occurs on the surfaces of fiber. Spherulites grow from the fiber surface. Such growth results in transcrystallinity. The maleation of polypropylene increases interaction because of reactivity with OH groups on the fiber surface. This organization contributes to the reinforcement. 

Crystallization rate, nucleation, size of crystalline units, crystalline structure, crystal modification, transcrystallinity, and crystal orientation are the most relevant characteristics of crystallization behavior in the presence of fillers. Here the discussion is focused on crystallization rate. The other topics are discussed in the following sub-chapters. 

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. 

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

Neat isotactic polypropylene (iPP) crystallized from melt exhibits spherulitic morphology of the crystalline phase (72,73). In some cases and under very specific conditions, cylindrites, axialites, quadrites, hedrites, and dendrites may be formed of iPP (74). In general, crystallization from quiescent melts results in spherulitic morphology, whereas crystallization fi-om melts subjected to mechanical loads results in cylindrites (75). Crystalline supermolecular structure caused by oriented crystal growth from heterogeneous surfaces is commonly termed transcrystallinity (76). 

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