Polyolefin crystallization behavior

R.G. Alamo and C. Chi, Crystallization behavior and properties of polyolefins. In Y. Morishima, T. Norisuye and K. Tashiro (Eds.), Molecular Interactions and Time-Space Organization in Macromolecular Systems, Springer, New York, 1999, p. 29. 

With the details associated with ADMET chemistry reasonably well understood, we have embarked on a study of the synthesis of well-controlled polymer structures via metathesis polycondensation chemistry [37]. A series of well-defined polyolefins have been designed to model the crystallization behavior of polyethylene and its related copolymers, including new materials synthesized by metallocene-based catalysts. This synthesis concept has been reduced to practice, and polymers that will aid in the understanding of branching within polyethylene itself have been produced. 

Different ternary blends were prepared by mixing iPP with several polyolefins with the aim to modify the mechanical properties of the matrix (25-29). The effect of the composition on the morphology, the rheological properties, and the crystallization behavior was investigated. The nature of the components can act as a nucleant agent on iPP crystallization and can produce different effects on mechanical performance of the blends. 

CRYSTALLIZATION BEHAVIOR OF BLENDS OF POLYOLEFINS WITH A SEMIFLEXIBLE LIQUID CRYSTALLINE POLYMER... 

Concerning compatibilized blends, the interfacial behavior of the compatibilizer has an important effect upon the crystallization of the blended components as it was shown for crystaHine/crystalline polymer blends (60,65-67) and for crystalline polymer/LCP blends (32,37,38,68). For the latter blends, the enhanced phase interactions and improved interfacial adhesion could increase the above-mentioned nucleation activity of the LCP toward the crystallizable matrices. In the particular case of using polyolefin-g-LCP copolymer compatibilizer, the crystallization of the two blend phases might have a reverse effect upon the compatibilizing activity. Moreover, the miscibility (69,70) and/or cocrystallization (60) between the bulk homopolymers and corresponding segments of the copolymer could strongly influence the crystallization behavior of the blends. 

The crystallization behavior and kinetics under isothermal conditions of iPP/SBH and HDPE/SBH blends, compatibilized with PP-g-SBH and PE-g-SBH copolymers, respectively, have been investigated (71). It has been established that the LCP dispersed phase in the blends plays a nucleation role for the polyolefin matrix crystallization. This effect is more pronounced in the polypropylene matrix than in the polyethylene matrix, due to the lower crystallization rate of the former. The addition of PP-g-SBH copolymers (2.5-10 wt%) to 90/10 and 80/20 iPP/SBH blends provokes a drastic increase of the overall crystallization rate of the iPP matrix and of the degree of crystallinity. Table 17.4 collects the isothermal crystallization parameters for uncompatibilized and compatibilized iPP/SBH blends (71). On the contrary, the addition of PE-g-SBH copolymers (COP or COP 120) (2.5-8 wt%) to 80/20 HDPE/SBH blends almost does not change or only slightly decreases the PE overall crystallization rate (71). This is due to some difference in the compatibilization mechanism and efficiency of both types of graft copolymers (PP-g-SBH and PE-g-SBH). The two polyolefin-g-SBH copolymers migrate to blend interfaces and... 

Composites of isotactic PP with Hemp fibers with various compatibilizers (PP-g-GMA, SEES, SEBS-g-GMA) were studied. All modified composites showed improved fiber dispersion in the polyolefin matrix and higher interfacial adhesion when compared with the unmodified system (PP/Hemp) as a consequence of chemical bonding between fiber and polymer. The spherulitic morphology and crystallization behavior of PP were changed in the composites due to the nucleating effect of Hemp fibers. All composites displayed higher tensile modulus (about 2.9 GPa) and lower elongation at break compared with plain PP [41]. 

PB base polymers are semicrystalUne isotactic thermoplastic polyolefins. They are derived from the polymerization of butene-1 monomer with or without other alpha-olefin monomers utilizing a Ziegler-Natta type of catalyst. Their unique crystallization behavior means longer open times of adhesive and sealant formulations compared to other commonly used polymers such as polyethylene and ethylene-vinyl acetate copolymer (EVA). Polybutylene (PB), also called polybutene-1 or poly-1-butene, is different from polybutene or polyisobutylene (PIB). PIB is amorphous and rubbery, and comes in the form of a viscous liquid or big, hard block (6 in. in length and width or could be larger). PB base polymers are sup-pUed in the form of small pellets (about 0.25 in. in diameter) or nibs. 

In Section 10.4 the properties of immiscible blends, and in particular the types of morphology, the nucle-ation processes, and crystal growth are described. The crystallization behavior of some immiscible systems (with one or two crystallizable components) is reviewed. Section 10.5 deals with compatibilized blends. The main compatibilization methods, including addition of copolymers and reactive mixing methods, are reported. The peculiar crystallization phenomena occurring in compatibilized systems (fractionated crystallization) are examined for blends of polyamides and functionalized polyolefins. 

The presence of a mesomorphic component dispersed in a crystallizable polymer matrix, can induce noteable changes of the crystallization behavior, crystallinity degree, and crystal morphology of the matrix. For blends of PET with liquid-crystalline copolyesters the crystallization rate of PET matrix was found to increase with increasing the LCP content, indicating a nucleating effect of the LCP component [139]. Similar effects have been reported for blends with polyolefin matrix for blends of iPP with a smectic polyester (PTEB) compati-... 

In the debate about existence of pre-ordered states in the polymer melt, as advocated recently, polyolefins with chiral side chains may well become a major investigation tool. Indeed, the macromolecular amplification induces a pre-organization, or at least a preferred helical conformation in the polymer melt or solution. As such, these polymers display very precisely the behavior that is assumed by some of the recent crystallization schemes or scenarios. Furthermore, for the P4MH1 systems considered so far at least, the confor-mationally racemic character of the stable crystal structure implies that half of the stems must change their helical hands at some stage in the crystallization process - which may greatly delay the formation of this stable crystal structure, as illustrated by P(S)4MH1. 

Polyethylene Terephthalate Blends Wilfong et al. [1986] reported on the effects of blending low concentrations (1 to 10 wt%) polyolefin with PET on the crystallization and toughening behavior of the latter. The authors... 

This book is stmctured as follows Chapter 1 serves as a guide to polyolefin blends introducing this important class of materials, why they are important, typical systems studied, issues of fundamental and applied interest, and current trends. The contributed chapters are divided into two main categories polyolefin/polyolefin blends (Chapters 2-16) and polyolefin/nonpolyolefin blends (Chapters 17-21). Issues covered in these chapters include miscibility, phase behavior, functionalization, compatibilization, microstructure, crystallization, hierarchical morphology, and physical and mechanical properties. Most of the chapters are in the form of review articles. Some original articles are included to capture the latest development in polyolefin blends research. 

Wilfong et al. (1986) reported on the effects of blending low concentrations (1-10 wt%) polyolefin with PET on the crystallization and toughening behavior of the latter. The authors studied blends of PET with LLDPE, HDPE, PP, and poly (4-methylpentene-l), all of them having a lower melting point than PET (Table 3.15). Polyolefin melts did not enhance the nucleation of PET, although the spherulite size of the PET matrix was found to be 2.5-3 times larger than for the homopolymer, with a broader spherulite size distribution. Both the crystallization... 

Wood fiour is often added to thermoplastics as a low cost filler to alter mechanical performance, especially the stiffness of low melt temperature, commodity thermoplastics such as polypropylene and polyethylene without increasing density excessively. Wood is much stiffer than the commodity thermoplastics usually used as matrices. Additionally, vood and pulp fibers can nucleate crystal growth in polyolefins resulting in a transcrystalline layer that can infiuence mechanical behavior [33, 34[. 

It was shown that for most crystalline polymers, including polypropylene and other polyolefins, the tensile drawing proceeds at a much lower stress than kinematically similar channel die compression [10,17]. Lower stress in tension was always associated with cavitation of the material. Usually a cavitating polymer is characterized by larger and more perfect lamellar crystals and cavities are formed in the amorphous phase before plastic yielding of crystals. If the lamellar crystals are thin and defected then the critical shear stress for crystal plastic deformation is resolved at a stress lower than the stress needed for cavitation. Then voiding is not activated. An example of such behavior is low density polyethylene [10]. 

ADMET polyolefins with precisely placed halogen atoms provide an excellent system for studying crystallization and melting behavior of precision polyolefins. By synthesizing... 

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