Search

The crystallization behavior of butene-ethylene random isotactic copolymers prepared with a Ziegler-Natta catalyst, in particular the surprising crystallization of form II of isotactic polybutene in samples with a high ethylene concentration, provides novel evidence of the long-debated blocky molecular structure of these "random" copolymers.

The crystallization and thermal properties of copolymers of isotactic poly(1-butene) (iPB) with ethylene (iPBEt) synthesized with a Ziegler-Natta catalyst and ethylene (E) content between 7 and 45 mol % are presented. The crystallization properties provide evidence of a multiblock molecular structure of these copolymers. The samples have been separated in two or three fractions by standard fractionation in boiling solvents. Samples with ethylene concentration lower than 16 mol % surprisingly crystallize in form II of iPB, and crystallization of polyethylene also occurs for the maximum ethylene content of 45 mol %. Even more surprising is the result that the melting temperature of crystals of any polymorphic form of iPB that crystallize in these samples do not depend on the ethylene concentration. This suggests that long crystallizable butene sequences are present even at high ethylene concentration. In all fractions, the chains are composed of multiblocks where the comonomers are segregated, with blocks characterized by long crystallizable butene sequences and blocks with shorter butene sequences. In the different fractions, the average length of butene sequences is nearly the same. The different fractions are featured by different amounts of the constituting blocks.

The mechanical properties and the structural transformations occurring during deformation of some commercial grades of ethylene/1-octene multiblock copolymers (OBCs) obtained from chain shuttling technology are analyzed. The samples are characterized by a statistical multiblock architecture, where soft and amorphous blocks with high octene concentration (approximate to 18.9 mol %) alternate with hard and crystalline blocks with low octene concentration (approximate to 0.5 mol %). The length of blocks (BL) and the number of blocks/chain (NB) change from chain to chain according to a statistical distribution. The selected samples have molecular mass in the range 85-130 kg/mol, percentage of hard segments in the range 15-27%, and a melting temperature of approximate to 120 degrees C. The average molecular masses of the hard blocks M-H and soft blocks M-S are in the ranges 2-3 kg/mol and 6-15 kg/mol, respectively, whereas the number of blocks/chain is in between 2 and 17. Even though the samples are characterized by similar octene concentration, small difference in molecular mass, and fractional content of hard blocks, they show remarkable differences of mechanical properties, depending on the average BL and NB values, encompassing from those of strong elastomers, in the case of samples with low block length and high number of blocks/chain, to those of soft elastomers, in the case of samples with high block length and low number of blocks/chain. The differences in the mechanical properties of OBC samples are amplified by stretching at high temperatures. Not previously stretched films obtained by compression molding show only partial recovery of the initial dimensions in mechanical cycles of stretching and releasing the tension, with values of recovered strain higher than 50% at 25 degrees C. However, the resultant specimens obtained by release of the tension show good elastomeric properties in a wide deformation range at 25 degrees C and, in the case of the samples with high strength, also at 60 degrees C. Fiber diffraction analysis reveals that by stretching at high deformation the orientation of the crystals is accomplished by mechanical melting and formation of an oriented amorphous phase, namely, involving the hard segments extracted from the crystals. Upon releasing the tension recrystallization occurs, and the high degree of orientation achieved by the crystals and amorphous phase is lost.

Syndiotactic propene–hexene (sPPC6) and propene–octene (sPPC8) copolymers have been synthesized in a wide range of compositions with a syndiospecific metallocene catalyst. The samples are crystallized up to a comonomer content of nearly 20 mol% with crystallinity, melting and glass transition temperatures that decrease with increasing concentrations of comonomers. The copolymers show elastic properties that depend on the crystallization behavior. The as-prepared samples with a comonomer content lower than 2 mol% crystallize in mixtures of the helical form I and form II of syndiotactic polypropylene (sPP), the latter containing conformational kink band defects and the former containing a b/4 shift disorder. Both sPPC6 and sPPC8 copolymers crystallize from the melt in disordered modifications of form I characterized by disorder in the alternation of right- and left-handed two-fold helical chains along the axes of the orthorhombic unit cell of form I of sPP. At high crystallization temperatures, more ordered modifications of form I develop for samples with a low comonomer concentration and form II surprisingly crystallizes in samples with a high comonomer content. The crystallization of form II rather than the ordered form I in melt-crystallized samples is a valuable result because it indicates that it is possible to tune the polymorphic transformations and the elastic properties of these copolymers through tuning of the branches concentrations.

The mechanical properties of copolymers of syndiotactic polypropylene (sPP) with 1-hexene and 1-octene have been studied and correlated with the structural transformations occurring during stretching and relaxation. These copolymers crystallize in disordered modifications of form I of sPP, with crystallinity, melting, and glass-transition temperatures that decrease with increasing comonomer content. All copolymers show remarkable mechanical properties and elastic behavior, with great improvement of flexibility and ductility with respect to the sPP homopolymer already at small comonomer concentrations. For low comonomer content up to 4-5 mol %, the elastic recovery is associated to a reversible conformational phase transition between the trans-planar form III or the trans-planar mesophase and the helical form II, which gives an enthalpic contribution to the elasticity. As the comonomer content increases, the trans-planar conformation is progressively destabilized and the observed elastic properties are not associated with any conformational transformation during stretching and relaxation. In these cases, copolymers behave as conventional thermoplastic elastomers where elasticity is purely entropic and crystals only act as topological constraints of the elastomeric network. These copolymers represent an exemplary case of the possibility to combine in the same materials desired and apparently incompatible physical properties of elasticity, crystallinity, and strength by controlling the crystallization behavior, achieved by tailoring the chemical structure in the synthetic process.

The mechanical properties and elastic behavior of Ziegler-Natta isotactic 1-butene-ethylene (BuEt-ZN) copolymers, having a broad composition from 1.7 to 45.5 mol% of ethylene units, are studied. The multi-site nature of the catalytic system determines a non-uniform distribution of the comonomers along the polymer chains; the copolymers present a blocky microstructure and composed of fractions of chains made of long and short butene sequences, which can be separated by solvent fractionation procedures. Incorporation of ethylene induces improving of drawability and decrease of rigidity with respect to homopolymer, associated with the lower crystallinity, and outstanding elastomeric behavior when the ethylene amount exceeds 5.5 mol%. In these copolymers the presence of irregular fractions of chains with short butene sequences, showing crystallization of the defective and low-melting temperature form I′ upon aging of the amorphous at room temperature, is responsible for the good elasticity of the materials.

A study of the crystallization behavior and kinetics of form II-form I transition in isotactic butene-ethylene copolymers (iPBEt) prepared with Ziegler-Natta catalyst with ethylene concentration in the range 9–44 mol% is presented. The samples have been separated with boiling solvents in different fractions containing chains with different ethylene concentration. Crystallization of form II from the melt is observed in each fraction even at high ethylene concentration, up to nearly 16 mol%. For higher ethylene content the samples do not crystallize from the melt but still crystallize during aging at room temperature in form I′. The comparison with the crystallization behavior of iPBEt copolymers prepared with homogeneous metallocene catalysts, characterized by a random distribution of ethylene units, has shown that the form II-form I transformation in Ziegler-Natta copolymers is much slower than in metallocene copolymers having the same ethylene concentrations and for the metallocene copolymers a low concentration of ethylene units of nearly 6 mol% is sufficient to prevent crystallization from the melt of form II. This indicates that in the chains of Ziegler-Natta iPBEt copolymers the regular butene sequences interrupted by the ethylene units are much longer than those of the chains of copolymers prepared with metallocene catalysts of the same ethylene concentration, giving further evidence of the multiblock structure of Ziegler-Natta iPBEt copolymers.

Isotactic propylene-butene copolymers [i(P-co-B)] with precise and controlled molecular structures were synthesized with various organometallic catalysts having different stereoselectivities. Stereoregular and stereodefective samples of i(P-co-B) with 1-butene (B) content variable in the whole range of composition were synthesized. All samples crystallize regardless of composition, indicating cocrystallization of propene and 1-butene units, which are incorporated in the unit cells of polymorphic forms of isotactic poly(propylene) (iPP) and isotactic poly(1-butene) (iPB). The copolymers show a continuum change of crystal morphology with the composition, transforming from big spherulites to bundle-like and needle-like crystals, to granular crystals. The cocrystallization allows maintaining high crystallinity of copolymers for any composition and provides an opportunity to develop outstanding mechanical properties that can be tailored by changing the isotacticity and composition. This allows, ideally, combining in the same material the different properties of stiffness of iPP and flexibility of iPB. These copolymers show, indeed, mechanical properties intermediate between iPP and iPB, ranging from stiffness/brittleness and ductility/flexibility depending on the composition and isotacticity, with high strength and Young's modulus that may be regulated by the stereoregularity of the iPP and iPB sequences, which is, in turn, dictated by the catalyst structure.

The phase behavior of ethylene/propylene(/diene) (EP(D)M) co- (and ter-)polymers characterized by ethylene content in the range 64–78 wt%, and comparatively low level of crystallinity, in the range 1–15%, is analyzed during stretching, through collection of wide angle X-ray scattering data. All samples experience crystallization by effect of stretching, but whereas for the samples with ethylene content ≥71% crystallinity starts increasing already after yielding, the nearly amorphous sample, with 64 wt% ethylene content, experiences crystallization only at strain above a threshold (≈170%). In all cases, a high degree of orientation of the amorphous phase is achieved already at low deformations. It is shown that for the samples with high ethylene content, the pre-existing crystals undergo initial fragmentation and/or mechanical melting at low deformations, and successive oriented recrystallization at higher deformations. The role of oriented amorphous phase, along with the role of pre-existing crystals as nuclei able to foster further crystallization is highlighted.

In this paper the morphologies and crystallization properties of crystalline block copolymers are reviewed. The equilibrium morphologies expected in crystalline-amorphous diblock copolymers and the different modes of crystallization, such as, breakout, templated crystallization, confined crystallization and pass through crystallization are described. The effects of confinement on crystallization mode and crystallization kinetics are also analyzed in detail. The paper is focused on the morphologies and nanostructures produced in crystalline block copolymers based on polyolefins. Early studies performed on block copolymers containing highly defective polyethylene block, synthesized by hydrogenation of block copolymers containing 1,4-polybutadiene blocks prepared by classic anionic living polymerization, are described in the first part of the paper. Then, the paper features recent advances in the synthesis and characterization of crystalline block copolymers containing stereoregular polyolefins, such as stereoregular polypropylene and linear polyethylene prepared using novel organometallic catalysts able to promote the living and stereoselective polymerization of α-olefins. Strategies for creating nanoscale patters exploiting the ability to induce desired orientations of the crystalline phase through epitaxial crystallization of the crystallizable blocks on crystalline substrates are described. According to this strategy, the desired orientations of the crystalline phase may be suitably designed and act as templates to create the desired final ordered nanostructure.

The synthesis of a poly(3,5-tricyclo[2.2.1.0(2,6)]heptylene) [poly(norbornadiene) (PNBD)] mediated by the TiCl4/Et2AlCl catalytic system is reported. This polymer is characterized by a rigid nortricyclene repeating structure and unique 3,5-enchainment and is obtained through a transannular cationic polymerization of norbornadiene involving both double bonds. The new PNBD is crystalline with high glass transition and melting temperatures due to the rigid tricyclic repeating structure. C-13 NMR spectra indicate that the chains of PNBD contain only exo/exo (xx) and exo/endo (xn) nortricyclene repeating monomeric units. Models of the chain conformation and crystal structure of PNBD are also reported. Models of low conformational energy of isotactic and syndiotactic chains of PNBD have been obtained from the regular enchainment of xx and xn units. The model of the isotactic (xx)(m) chain with s(2/1) helical symmetry assumes a nearly trans-planar conformation, whereas the models of the syndiotactic (xn-xn)(m) and (nx-xn)(m) chains with tc and ti symmetries, respectively, assume the conformations (TG(-)G(-)TG(+)G(+))(n) and (G(-)TTTG(+)T)(n), respectively. Ideal limit ordered models of the crystal structure have been obtained from the X-ray powder diffraction data and the conformational energy analysis. A good agreement between experimental and calculated powder diffraction profiles has been obtained for a limit ordered model of packing of syndiotactic chains (nx-xn)(m) of PNBD with ti symmetry in an orthorhombic unit cell with axes a = 6.6 angstrom, b = 11.0 angstrom, and c = 6.84 angstrom according to the monoclinic space group P2(1)/c. The real crystal structure of PNBD is disordered because of the possible presence of configurational disorder in the chains due to possible random enchainment of nortricyclene monomeric units in the xx, xn and nx configurations that produces conformational and packing disorder.

The crystallization behavior and the morphology of blends obtained by mixing a highly crystalline and isotactic poly(1-butene) (iPB) and a copolymer of isotactic polypropylene (iPP) with 26 mol% of 1-hexene (C3C6 copolymer) have been studied. The C3C6 copolymer does not crystallize from the melt but the amorphous sample crystallizes into the trigonal form of iPP by aging at room temperature. All the blends crystallize from the melt in form II of iPB and the aging at room temperature induces transformation into form I and crystallization of the C3C6 copolymer. Both the melt crystallization of form II and its transformation into form I of iPB are retarded in the blends by the presence of the C3C6 component, whereas the crystallization from amorphous phase of the C3C6 copolymer is accelerated thanks to the nucleating effect of the preformed iPB crystals. A good inter-mixing of the two components occurs by solution blending. However, the initial state of the blend obtained by precipitation from solution is not at equilibrium and at 200 °C phase separation takes place by effect of spinodal decomposition, and the degree of demixing increases with increasing the residence time at 200 °C. The final morphology of the iPB/C3C6 blends depends on the composition and can be controlled and modulated by changing time and temperature of melt treatment.

The yield behavior during uniaxial drawing of isotactic random copolymers of propene with ethylene (iPPEt), 1-butene (iPPBu), 1-pentene (iPPPe), 1-hexene (iPPHe), and 1-octadecene (iPPOc) is analyzed within the framework of our current understanding of deformation properties of semicrystalline polymers, that is, the intrinsic stability of lamellar crystals and related polymorphism phenomena, along with the ability of entangled amorphous chains to transmit stress. The samples selected for analysis were synthesized using single-site metalorganic catalysts, are highly stereoregular, and contain small amounts of regiodefects caused by secondary 2,1 erythro units. Moreover, the interchain and intrachain distribution of comonomeric units is uniform. In the case of iPPEt copolymers, samples containing ≈3.5 mol% stereodefects were also studied. The yield behavior of these samples depends on the kind and concentration of defects, and is directly related to the level of inclusion in and exclusion from crystals of the comonomeric units. Apart from iPPBu copolymer samples with high butene content, the yield stress of all samples increases with the thickness of lamellar crystals according to a common trend, regardless of comonomer. In the case of iPPBu copolymers containing a high concentration of butene units, the yield stress decreases with increasing lamellar thickness. The increase in yield stress with lamellar thickness is rationalized in terms of the micromechanical model of crystallographic slips, based on thermal activation of screw dislocations. The parameters of the model describing the yield behavior are the critical free energy required to form a screw dislocation and the shear modulus associated with the slip planes of the dislocations. These were set as identical to those deduced for isotactic polypropylene homopolymer samples (iPP) crystallized under different conditions. Study of the yield behavior of these copolymers extends the use of the dislocation model to a set of samples crystallized under similar conditions but characterized by differences in comonomeric unit, degree of crystallinity, lamellar thickness, polymorphism, and intrinsic flexibility of the chain backbone. The results indicate that for a homogeneous class of propene-based copolymers, namely crystallized in the α-form of iPP under similar conditions, lamellar thickness controls the level of plastic resistance provided that the concentration of structural irregularities in the crystals is not too high. iPPBu copolymers with high comonomer concentration do not obey this rule because of the high level of inclusion of comonomers in the crystals, which induces an increase in lamellar thickness but also a decrease in crystal stability.