Crystallization from oriented melts

An appropriate procedure is the application of a short shear pulse on a sample confined by two parallel plates, which is then followed by an observation of the evolving structure. The pulse time can be kept short compared to the crystallization time. Experiments then extract the effect of flow on the 

Other experiments show that effects not only relate to the magnitude of the shearing stress but also to the pulse duration, i.e., to the applied mechanical work. In the experiment of Fig. 5.56 the kinetics of crystallization of i-polypropylene initiated by pulses with various shearing times was monitored 

The nonlinearity is also seen in the experimental results presented in Fig. 5.57. Nucleation densities c uc in i-polypropylene were again determined after shearing pulses of different lengths producing different amounts of shear Aej x, now also at various temperatures. The dependence of c uc on the mechanical work can be represented by a power law with an exponent 

The lamellar crystallites grow according to the same laws that are valid for the crystals in the spherulites. Since the nucleation sites are all along a microfibril a row of crystallites with a uniform chain orientation results. The splaying typical for growing spherulites is suppressed there exists a stack of parallel lamellae from the very beginning. The new phenomenon are the microfibrils and they form in peculiar manner. If the degree of chain orientation in 

Allegra (Ed.) Interfaces and Mesophases in Polymer Crystallization, Advances in Polymer Science 180, 181, 191, Springer, 2005 

---

Leugering, H. J. Kirsch, G.(1973). Effect of crystallization from oriented melts on crystal-structure of isotactic polypropylene. Angew. Makromol. Chem., Vol.33, Issue OCT,... 

When highly branched polyethylene samples, either (dendritic) low density polyethylene or (comblike) very low density polyethylene, crystallize from oriented melts they do not form cylindrites because they contain insufficient linear chain segments to generate microfibrillar nuclei. In such cases the relatively slow crystallization kinetics and low crystallization temperature permit the molecules a relatively long time to relax prior to solidification. The lamellae that form under these circumstances are well separated from one another and do not share a common axis. The resultant semicrystalline morphology is similar to that of low density samples crystallized from an isotropic melt. 

Main Features of Crystallization from Oriented Solutions and Melts. 214... 

A characteristic feature of the structure of samples obtained under the conditions of molecular orientation is the presence of folded-chain crystals in addition to ECC. Kawai22 has emphasized that the process of crystallization from the melt under the conditions of molecular orientation can be regarded as a bicomponent crystallization in which, just as in the case of fibrous structures in the crystallization from solutions, the formation of crystals of the packet type (ECC) occurs in the initial stage followed by the crystallization with folding . 

Finally, we were led to the last stage of research where we treated the crystallization from the melt in multiple chain systems [22-24]. In most cases, we considered relatively short chains made of 100 beads they were designed to be mobile and slightly stiff to accelerate crystallization. We could then observe the steady-state growth of chain-folded lamellae, and we discussed the growth rate vs. crystallization temperature. We also examined the molecular trajectories at the growth front. In addition, we also studied the spontaneous formation of fiber structures from an oriented amorphous state [25]. In this chapter of the book, we review our researches, which have been performed over the last seven years. We want to emphasize the potential power of the molecular simulation in the studies of polymer crystallization. 

Additional neutron scattering studies on different polymer systems could prove very important. Strobl [31,32,47,103] provides evidence that, for some polymers, lamellar crystallization is preceded by pre-ordering of the melt followed by formation of planar arrays of blocks. Investigating crystallization from the melt, Kaji and coworkers [25] find pre-ordering phenomena relating to orientational fluctuations of stiff polymer segments which, under appropriate conditions, determine phase separation prior to crystallization. 

Ueda et al. [26] recently investigated a flow-oriented PE-fr-aPP diblock copolymer with Mw = 113 000 (Mn/Mw = 1.1) and a PE volume fraction of 0.48. This diblock copolymer is in the strong segregation regime (i.e., estimated xN = 10.5 and Todt = 290 °C) and has a lamellar morphology in the melt. They found a breakout phenomenon with the formation of spherulites in an intermediate crystallization temperature range 95 < Tc < 101 °C. At crystallization temperatures above 101 °C or below 95 °C spherulites were not formed and the crystallization was confined within the lamellar MD. Ueda et al. report that lamellar MD and spherulites do not co-exist when the material crystallizes from the melt which is separated in lamellar MDs. In other words, in this particular case, breakout or confined crystallization within lamellar MDs depends on the crystallization conditions. 

Crystallization from the melt often leads to a distinct (usually lamellar) structure, with a different periodicity from the melt. Crystallization from solution can lead to non-lamellar crystalline structures, although these may often be trapped non-equilibrium morphologies. In addition to the formation of extended or folded chains, crystallization may also lead to gross orientational changes of chains. For example, chain folding with stems parallel to the lamellar interface has been observed for block copolymers containing poly(ethylene), whilst tilted structures may be formed by other crystalline block copolymers. The kinetics of crystallization have been studied in some detail, and appear to be largely similar to the crystallization dynamics of homopolymers. 

A preferentially and a sheaf-like aggregation with random in-plane orientation are observed for the thinner films (thicknesses of 0.1, 0.2 and 0.4 pm in panels a-c). By contrast thick films (0.6 pm and thicker, panel d) show a morphology that resembles the well known (bulk) spherulitic form with a banded structure, characteristic of linear polyethylene crystallized from the melt at moderately high undercooling. 

Crystallization of an oriented mesophase> The crystallization process of oriented nematic melts can lead in certain cases to highly oriented crystalline fibers while, in other cases, the crystallite orientation is totally or partly disrupted by the process of crystalliza-tion% Thus, DDA-8, MAA-8 and its copolymers MAA-DDA-8 lead to strongly oriented crystalline fibers on crystallization from nematic melts oriented in a magnetic field of 12 Tesla On the other hand, DDA-9 and its copolymers are much less oriented under the same circumstances. 

Another concept in synthesis is epitaxy. Epitaxy is the continuation of the crystal orientation of the monocrystalline substrate in the deposited crystalline product, which may be the same compound as the substrate or a different solid that has the same crystal orientation as the monocrystalline solid. Epitaxial layers are essential for microlithography in the electronic industry carefully formed epitaxial layers do not have localized electronic interface states, which are deleterious for the functioning of the device. The process conditions for epitaxy by molecular beam epitaxy (MBE) are very low process pressure, comparatively high temperatures, and a low growth rate. MBE is a form of CVD, which was described in Chapter 6. Liquid phase epitaxy (LPE) is a form of growth of single crystals from a melt. 

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). 

---