Spherulitic crystallization and structure

Morphological features, kinetics of growth, formation of structure and melting behavior of iPP spherulites were discovered [1, 2 and references therein]. During the crystallization of iPP, being a polymorphic material with several modifications [5], different types of spherulites may develop, which imply crystallites of the a-, p- and v-modification [1, 2, 5-7]. All these polymorphs consist of right-, and/or left-handed threefold helices with 0.65 nm chain axis repeat distance. The molecular 

During the crystallization of commercial iPP grades, essentially the a-modification is formed, sometimes accompanied by a lower or higher amount of -modification. The a-modification of iPP (a-iPP) seems to be the thermodynamically stable form (but, see a critical analysis about the types of iPP polymorphism in [1]). The crystal cell of a-iPP is monoclinic with parameters a = 0.665 nm, b = 2.096 nm, c = 0.65 nm and p = 99°80, which consists of alternating right- and left-handed helices [5, 7]. 

The experimental melting point of a-spherulites crystallized under usual thermal condition is 165°C. Literature data for the equilibrium melting temperature of a-iPP (T (a)) are very contradictory. The reported values faU into the range 174-220°C [1], the author accepts T (a)) = 208°C as the most reliable one [2]. 

P phase rich or even pure p-phase with spherulitic structure can be prepared by using a selective p-nucleating agent [1]. The experimental data indicated a possible upper (TO ) = 140°C) and lower temperature (T(ap) = 100-110 C) limit of the formation of pure 3-iPP [1, 2, 9]. The p-modification of iPP (p-iPP) has a trigonal cell with parameters a = b = 1.101 nm, c = 0.65nm containing three isochiral helices. This cell is frustrated the three helices do not have similar orientation [7]. 

The 7-modification of iPP (y-iPP) may form in degraded, low molecular weight iPP or in samples crystallized under high pressure [5, 6]. Certain propylene copolymers with low comonomer content (4-10 wt.%) crystallize preferentially in y-form, as well. y-iPP has a face-centred ortho-rombic unit cell with parameters a = 0.85 run, b = 0.993 nm and c = 4.241 nm containing isochiral helices. The cell structure proposed by Bruckner et al. [5] is unique in polymer crystallography the chain axes in adjacent crystal layers are not parallel. The angle between the chain stems is about 80°. y-iPP is not usually observed as an independent phase, but crystallizes with and within the a-spherulites. According to Lotz et al. [7], the positive spherulites observed in samples with mixed polymorphic composition of a- and y-iPP are probably made of a 

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A further increase in extension leads to irreversible changes which immediately precede the transition of the polymer into the oriented state. During this transition, the spherulites undergo considerable structural changes and are thus converted qualitatively into different structural elements i.e. macrofibrils4). After a certain critical elongation has been attained, the initial crystallites collapse and melt and a new oriented structure is formed in which the c axes of crystals are oriented in the direction of extension. 

Recent developments have allowed atomic force microscopic (AFM) studies to follow the course of spherulite development and the internal lamellar structures as the spherulite evolves [206-209]. The major steps in spherulite formation were followed by AFM for poly(bisphenol) A octane ether [210,211] and more recently, as seen in the example of Figure 12 for a propylene 1-hexene copolymer [212] with 20 mol% comonomer. Accommodation of significant content of 1-hexene in the lattice allows formation and propagation of sheaf-like lamellar structure in this copolymer. The onset of sheave formation is clearly discerned in the micrographs of Figure 12 after crystallization for 10 h. Branching and development of the sheave are shown at later times. The direct observation of sheave and spherulitic formation by AFM supports the major features that have been deduced from transmission electron and optical microscopy. The fibrous internal spherulite structure could be directly observed by AFM. 

Amorphous polymers, as the name implies, are structureless except at the molecular level where we shall propose a suitable RVE. Semicrystaliine polymers exhibit a wide variety of structures depending upon their chemical nature, the degree of polymerization, the form and size of crystals and their assembly into spherulites, lamellae, fibrils etc. 

Shish-kebab. In addition to spherulitic crystals, which are formed by plate- and ribbonlike structures, there are also shish-kebab crystals which are formed by circular plates and whiskers. Shish-kebab structures are generated when the melt undergoes a shear deformation during solidification. A typical example of a shish-kebab crystal is shown in Fig. 1.17. 

In polymers crystallized from the melt, in most cases spherulitic structures are observed spherical agglomerates of crystals and amorphous regions, grown from a primary nucleus via successive secondary nucleation (Figure 4.18). The dimensions of the spherulites are commonly between 5 pm and 1 mm. When spherulites grow during the crystallization process, they touch each other and are separated by planes. In a microtome slice they show a very attractive coloured appearance in polarized light. 

The spherulite formed from the sample with the longer stereosequence has a denser structure with a greater frequency of fibrillar branching than does the other spherulite from the sample with shorter stereosequence. This is in accord with the mechanism of spherulitic crystallization recently proposed by Keith and Padden (15). 

Most polymers fall in the class of translucent resins. These include acetal, polyamide, polybutylene terephthalate (PBT), polyethylene, and polypropylene as examples. There are very few neat polymers that are truly opaque (this depends on thickness as well). Liquid crystal polymer (LCP) is an example of a typically opaque polymer. It is theorized that these semicrystalline and crystalline resins will scatter some portion of incident light due to spherulitic crystal structure and the amorphous-crystalline region interfaces themselves. 

Figure 20.3 Spherulite growth rate (G) for sPS/PPE and sPS/PVME blends as a function of the crystallization temperature Tci ( ) sPS ( ) sPS/PPE 90 10 ( ) sPS/ PPE 80 20 (A) sPS/PVME 80 20 ( ) sPS/PVME 70 30 ( ) sPS/PVME 50 50. Reprinted from Polymer, vol. 34, Cimmino S., Di Pace E., Martuscelli E., Silvestre C., sPS based blends crystallization and phase structure , p. 2799, Copyright 1993, with permission from Elsevier Science.

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