Crystalline Morphology of Homopolymers and Block Copolymers

Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, Tokyo 152-8552, Japan 

In this chapter, we first summarize the crystalline morphology formed in homopolymers, which is usually explained in many textbooks of polymer science. Next, we describe more complicated morphology, that is, the crystalline morphology of block copolymers. This subject is relatively new as compared with that of homopolymers, so that it is not fully understood at present. 

In crystalline states, some parts of each polymer are regularly arranged with three-dimensional order to form crystals 

Polymer Morphology Principles, Characterization, and Processing, First Edition. Edited by Qipeng Guo. 2016 John Wiley Sons, Inc. Published 2016 by John Wiley Sons, Inc. 

The unit cell of a given crystal can be assigned to one of seven crystal systems shown in Table 10.1. If lattice points are only on vertexes of the unit cell, we call it the primitive lattice (P), whose total number is 7. If not, it is called the complex lattice (C base-centered, 7 body-centered, and F face-centered lattices). The introduction of complex lattices makes it easy to analyze the crystal structure because of higher symmetry as compared with corresponding primitive lattices. As a result. 

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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. 

The orientation of crystalline stems with respect to the interface of the microstructure in block copolymers depends on both morphology and the speed of chain diffusion, which is controlled by block copolymer molecular weight and the crystallization protocol (i.e. cooling rate). In contrast to homopolymers, where folding of chains occurs such that stems are always perpendicular to the lamellar interface, a parallel orientation was observed for block copolymers crystallized from a lamellar melt phase perpendicular folding was observed in a cylindrical microstructure. Both orientations are shown in Fig. 8. Chain orientation can be probed via combined SAXS and WAXS on specimens oriented by shear or compression. In PE, for example, the orientation of (110) and (200) WAXS reflections with respect to Bragg peaks from the microstructure in the SAXS pattern enables the unit cell orientation to be deduced. Since PE stems are known to be oriented along the c axis, the chain orientation with respect to the microstructure can be determined. 

Microphase separation and domain formation in block copolymers, which are the result of incompatibility of block chains, have been studied extensively (1,2). In addition to being incompatible, block chains in a copolymer generally have different thermal transition temperatures. The surface tensions of molten block chains also differ. When a crystalline block chain is incorporated into a block copolymer, it is expected that crystallization of the crystalline block chain causes considerable change in resultant morphology. Surface properties of a block copolymer and of its blend with a homopolymer should also be modified by the surface tension difference between block chains and the homopolymer. Since these factors determine the morphological features of a block copolymer both in bulk and at surface, a unified study of morphology, crystallization, and surface activity of any block copolymer is important to our understanding of its physical properties. 

Polypropylene is not one or even 100 products. Rather it is a multidimensional range of products with properties and characteristics interdependent on the type of polymer (homopolymers, random, or block copolymer), molecular weight and molecular weight distribution, morphology and crystalline structure, additives, fillers and reinforcing fillers, and fabrication techniques. 

Block copolymers, particularly of the A-B-A type, can exhibit properties that are quite different from those of random copolymers and even from mixtures of homopolymers. The physical behavior of block copolymers is related to their solid state morphology. Phase separation occurs often in such copolymers. This can result in dispersed phases consisting of one block dispersed in a continuous matrix from a second block. Such dispersed phases can be hard domains, either crystalline or glassy, while the matrices are soft and rubber-like. 

Silphenylene-Siloxane Copolymers.—The thermal properties vctsus structure for poly(tetramethyl-p-silphenylensesiloxane) and (tetramethyl-p-silphenylene/di-methylsiloxane) block copolymers have been compared. The homopolymer has a m.p. of 160°C, heat of fusion of 54.4 J/g and Tg of —20°C. The Tg of the copolymer varies monotonically with inojeased dimethylsiloxane content, from — 20 to —123 °C. Data have been reported on the crystallization kinetics and morphology of blends of fractionated poly(tetramethyl-p-silphenylenesiloxanes). The chemical degradation of poly(tetramethyl-/ -silphenylenesiloxane/dimethyl-siloxane) block copolymers by HF has been reported. In 48% HF at 30 °C, preferential attack occurs at the Si—O bond, particularly those of the MejSi—O non-crystalline components, in copolymers containing 15, 35, and 52% poly-dimethylsiloxane. - Further data have been reported on the crystal structure and fold conformation of poly(tetramethyl-/>-silphenylenesiloxane)s, obtained from X-ray diffraction studies. ... 

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