Transcrystallinity

The cooling of polymer melt in the presence of a foreign surface which can nucleate crystalline growth inhibits the lateral growth of spherulites. Crystallization occurs This is called transcrystallinity. It can im- 

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.  

Three processes of orientation occur simultaneously during the processing of filled materials. These are filler particle orientation (see Chapter 7), chain orientation (or conformation change) as related to filler particle, and the direction of crystallite growth. Often orientation is detrimental to the material produced. These processes are very difficult to study. Some information is available but more is needed. 

Talc is always an attractive subject of such studies due to its platelet structure. In thermoforming and compression molding processes of three resins (PP, HDPE, and PPS), each containing 20% talc, the talc particles were always parallel to the specimen surface, regardless of the resin used. Crystallites grew in a direction normal to the surface of talc particles and thus were perpendicular to the specimen surface. But in the case of unfilled HDPE, crystallites grew parallel to the specimen surface. There was no difference in crystallite growth direction in the case of polypropylene with and without talc. 

The orientation of PMMA chains on the surface of alumina was found to be affected by acid-base interactions. Due to these interactions, the trans conformation was more common at the interface than the gauche conformation which was prevalent in bulk.  

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Austenitic Steel weld has a well defined transcrystalline (oriented) macrostructure with continuously changing orientation of the crystal axis - from the periphery towards the centre the angle between the axis of the crystal and the axis of the weld is changed from 90 to 0 degrees. Weld metal eould be possible to approximate in the form of a discrete combination of crystals with parallel axes of the crystallites. 

Although hydrogen cyanide is a weak acid and is normally not corrosive, it has a corrosive effect under two special conditions (/) water solutions of hydrogen cyanide cause transcrystalline stress cracking of carbon steels under stress even at room temperature and in dilute solution and (2) water solutions of hydrogen cyanide containing sulfuric acid as a stabilizer severely corrode steel (qv) above 40°C and stainless steels above 80°C. 

Fig. 7. Different types of interphases. (a) Contact interphases, as produced for example by transcrystalline growth or enhanced adlayer crosslinking in the adhesive phase, (b) Diffusion interphases, as produced by interdigitation or interdiffusion of chains from either or both phases.

Only certain specific environments appear to produce stress corrosion of copper alloys, notably ammonia or ammonium compounds or related compounds such as amines. Mercury or solutions of mercury salts (which cause deposition of mercury) or other molten metals will also cause cracking, but the mechanism is undoubtedly differentCracks produced by mercury are always intercrystalline, but ammonia may produce cracks that are transcrystalline or intercrystalline, or a mixture of both, according to circumstances. As an illustration of this, Edmundsfound that mercury would not produce cracking in a stressed single crystal of brass, but ammonia did. 

The behaviour of a wide range of a, a-0 and /3 brasses in various corrosive environments was studied by Voce and Bailey and the results summarised by Whitaker . Penetration by mercury and by molten solder was intercrystalline in all three types of brass. In moist ammoniacal atmospheres the penetration of unstressed brasses of all types was intercrystalline. Internal or applied stresses accelerated the intercrystalline penetration of a brasses and initiated some transcrystalline cracking, and also caused severe transcrystalline cracking of /3 alloys and transcrystalline cracking across the 0 regions in the two-phase brasses. Immersion in ammonia solution, however, caused intercrystalline cracking of stressed 0 brasses. 

Where stress plays a part, the resultant metal failure may be present as either a transgranular (transcrystalline) or an intergranular ( intercrystalline) mechanism. 

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. 

Dilute cyanide-water solutions at ambient temperature can exhibit a unique destructive action on stressed carbon steel. Cracks can develop in the metal in a very short time. The characteristic feature of the cracks is their transcrystalline course without formation of slip planes254. 

Figure 3. The transcrystalline region generated at a high energy solid-polymer melt interface after solidification. The depth of the transcrystalline region is estimated to he about 25 ...

Transcrystalline region (TCR) polyethylene generated at 175°C. for V2 hour against etched aluminum (substrate removed by dissolution)... 

Extensive heterogeneous nucleation of polyethylene melts on high energy surfaces results in generation of transcrystallinity in the interfacial region [(S-L) (S-S)] (I, 3, 6, 12). Koutsky, Walton, and Baer (9)... 

Low energy surfaces—e.g., Teflon, Mylar, etc.—are apparently ineffective nucleating agents. When polymers are cooled in contact with these surfaces, nucleation is precluded at the S-L interface and is apparently initiated in the bulk. Sufficient supercooling has not occurred at the solid-liquid interface to nucleate the interfacial region before nucleation occurs in the bulk. Apparently, this is the reason for the lack of a well defined transcrystalline region when polyethylene is nucleated against a... 

Transcrystallinity occurs cellulose fiber maleic anhydride 79... 

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. 

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