Melt-crystallized polymers lamellar thickness

Melt Crystallized.—Spherulites are observed to consist of radiating fibrils or lamellae which in melt-crystallized polymers are 100—500 A thick, depending on crystallization temperature, and /um in breadth. The c axis, i.e, the chain direction, coincides with the thickness of the lamellae and, by analogy to single crystal morphology, the molecular chain folds backwards into the lamellae. Amorphous polymer fills the regions between lamellae and can account for a substantial amount of the content. Painter et al. have shown that these inter-lamellar regions are very similar to melt-crystallized polymer in their i.r. spectrum. 

Hikosaka M, Okada H, Toda A, Rastogi S, Keller A (1995) Dependence of the lamellar thickness of an extended-chain single crystal of polyethylene on the degree of supercooling and the pressure. J Chem Soc Faraday Trans 91(16) 2573-2579 Hoffman JD (1983) Regime III crystallization in melt-crystallized polymers the variable cluster model of chain folding. Polymer 24(l) 3-26... 

Independent of crystallization conditions, whether from solutions or melt, the polymer molecules crystallize into thin lamellae. The lamellar thickness is about 10 nm, about two orders of magnitude smaller than values allowed by existing equilibrium considerations. This is in contrast to the case of crystallized short alkanes, where the lamellar thickness is proportional to the length of the molecules. Clearly the chains in the case of polymers should fold back and forth in the lamellae to support the experimentally observed lamellar thickness. It is believed in the literature [3-9] that the lamellar thickness is kinetically selected and that if enough time is permissible, the lamella would thicken to extended chain crystal dimension. What determines the spontaneous selection of lamellar thickness ... 

Semicrystalline polymers may crystallize from solution, as well as from the melt, in the form of chain folded lamellar crystals. The high spatial resolution of ATM enables one to assess lamellar thicknesses from images of these lamellar crystals in edge-on and flat-on orientation. As discussed in this section, images of flat-on oriented lamellae are particularly suitable for a quantitative determination of lamellar thicknesses. 

The morphology of crystalline isotactic polystyrene, i-PS, has been investigated by others, and they have concluded that i-PS normally crystallizes as stacks of folded chain lamellae which are arranged in volume filling spherulites. The melting point of lamellar polymer crystals depends on the lamella thickness, L, as follows (28 )... 

It is well known (66) that the a-relaxation process of crystalline polymers consists of at least two processes, referred to as ai and U2 in the order of lower temperature, respectively. The ai-process (67-77) is pronounced in melt crystallized samples and is associated with the relaxation of grain boundaries, such as dislocation of lamellae with a frictional resistance related to disordered interface layers. The magnitude of the ai-process increases with the increase in the crystal defects. The o 2-process (71,73,78-83) is pronounced in single crystal mats and is ascribed to incoherent oscillations of the chains about their equilibrium positions in the crystal lattice in which intermolecular potential suffers smearing out. The magnitude of the Q 2-process increases with the increase in the lamellar thickness and/or the degree of crystallization (39). 

Polymer crystallization has been described in the framework of a phase field free energy pertaining to a crystal order parameter in which = 0 defines the melt and assumes finite values close to unity in the metastable crystal phase, but = 1 at the equilibrium limit (23-25). The crystal phase order parameter (xj/) may be defined as the ratio of the lamellar thickness (f) to the lamellar thickness of a perfect polymer crystal (P), i.e., xlr = l/P, and thus it represents the linear crystallinity, that is, the crystallinity in one dimension. The free energy density of a polymer blend containing one crystalline component may be expressed as... 

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