Mechanism of polymorph selection during crystallization of random butene-1/ethylene copolymer

Starting from an initial sample of butene-1/ethylene copolymer with stable form I’, we examined the nucleation of different crystalline polymorphs (here metastable form II and stable form I’) at different isothermal crystallization temperatures after being melted at different melt temperature (T _melt). When T _melt was just above the melting temperature (T _m) of the crystallites, self-seeding took place. There, residue crystallites served as nuclei leading to the crystallization of the same crystalline phase. When T _melt was a few degrees above the T _m, self-seeding was disabled due to complete melting of the initial crystals. Upon crystallization, the selection of the different polymorphs in this random copolymer was found to depend on an interplay between the domain size of segregated long crystallizable sequences and the size and energy barrier of the critical nucleus of the respective crystalline forms. Our results provide a clear understanding of the polymorphs selection during crystallization of a random copolymer as well as homo-polymers under confinement.     

Comparative study of the kinetics of non-isothermal melt solidification of random copolymers of butene-1 with either ethylene or propylene

The kinetics of non-isothermal melt solidification of random butene-1/propylene copolymers has been compared with that of random butene-1/ethylene copolymers. Analysis of the distance between neighbored chain segments in the crystal phase revealed inclusion of propylene chain defects into crystals, while ethylene co-units are excluded from crystallization. As a consequence of different acceptance of propylene and ethylene chain defects to participate in crystallization, the kinetics of the transition of the melt into ordered phase is significantly slower in random butene-1/ethylene copolymers. For samples of similar co-unit concentration, the decrease of the crystallization temperature and of the critical cooling rate to suppress ordering/crystallization is higher in random butene-1/ethylene copolymers than in butene-1/propylene copolymers. Due to the required rejection of ethylene co-units at the crystal growth front, ultimately, the maximum crystallinity is lower in butene-1/ethylene copolymers than in butene-1/propylene copolymers of similar amount of co-units.     

Comprehensive thermal analysis of random copolymers of ethylene and 1-olefins

Random copolymers of ethylene and propylene, butene-1 and hexene-1 were characterised by measurements of heat capacity in the temperature interval 140–480 K and specific volume of the melt in the temperature interval 330–490 K and in the range of pressures 27.8–100 MPa. Analysis of the composition dependences, of the degree of crystallinity, melting and glass transition temperatures, as well as of thermodynamic and thermophysical properties of the melt led to the conclusion about the microblock structure of macromolecules of all series at molar ethylene contentF ₁ > 0.8. In this range of compositions the properties of copolymers in the melt seem to be independent of the chemical nature of a comonomer, contrary to the solid state where at identical molar compositions, the degree of crystallinity diminishes and the melting temperature decreases, as the molecular structure of the comonomer becomes more complex. This effect becomes weaker asF ₁ decreases, so that in the composition rangeF ₁ < 0.8 the properties of copolymers of all series are additive.

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Zusammenfassung Durch Messungen der Wärmekapazität im Temperaturintervall 140–480 K und des spezifischen Volumens im Temperaturintervall 330–490 K und im Druckbereich 27.8–100 MPa wurden Random-Kopolymere von Ethylen und Propylen, Buten-1 und Hexen-1 charakterisiert. Die Analyse der Abhängigkeit des Kristallinitätsgrades, der Schmelz- und Glasumwandlungspunkte sowie der thermodynamischen und thermophysikalischen Eigenschaften der Schmelzen führte zu einem Schluß über die Mikroblock-Struktur von Makromolekülen aller Serien bei einem molaren Ethylengehalt vonF ₁>0.8. In diesem Zusammensetzungsintervall scheinen die Eigenschaften des Kopolymers unabhängig von der chemischen Natur des Komonomers zu sein, im Gegensatz zum festen Zustand, wo bei einer ähnlichen molaren Zusammensetzung sich der Kristallinitätsgrad verringert und die Schmelztemperatur sinkt, wenn die Molekülstruktur des Komonomers komplexer wird. Dieser Effekt wird kleiner, wennF ₁ abnimmt, so daß im Zusammensetzungsbereich mitF ₁<0.8 die Eigenschaften der Kopolymere aller Serien additiv sind.

     

Melting points of homogeneous random copolymers of ethylene and 1-alkenes

Melting points of copolymers of ethylene and 1-alkenes ranging from 1-butene to 1-octadecene have been determined. The copolymers were prepared by means of a homogeneous Et₃Al₂Cl₃/VOCl₃ initiating system so that in individual samples, comonomer contents do not vary with molecular weight. Evidence is presented for a random distribution of comonomer units in the copolymers. Melting points determined by differential scanning calorimetry are essentially independent of branch length at low comonomer contents. At higher comonomer contents (5–9 mol% 1-alkene), melting points decrease in the order 1-butene > 1-octene > 1-octadecene copolymers. The weight fraction of ethylene sequences drops to less than 60% in copolymers with 1-octadecene of high comonomer content and this results in a reduction in the crystallite thicknesses attained by these copolymers.     

Thermal and structural investigation of random ethylene/1-hexene copolymers with high 1-hexene content

Random ethylene/1-hexene copolymers with the 1-hexene content in the range from 2 to 28 mol% were produced with a novel post-metallocene catalyst and analyzed by three techniques, FTIR, ¹³C NMR, and DSC. The 1-hexene content and the sequence distribution in the copolymers were determined by means of FTIR-M and ¹³C NMR. The crystallization behavior of the copolymers was studied by DSC under dynamic and isothermal conditions; the Avrami model was used to analyze the crystallization kinetics. It was found that both the 1-hexene content and the crystallization temperature affect the relative crystallinity. The bulk crystallization rate decreases with the 1-hexene content and reduces exponentially with an increase of T _c. The melting behavior of isothermally crystallized samples was also investigated and it was found that the melting temperatures of the copolymers under equilibrium conditions were related to the composition.     

丙烯/1-丁烯和丙烯/乙烯无规共聚物的表征及性能研究

采用摩尔含量接近的两个单体乙烯和1-丁烯分别无规共聚聚丙烯样品,用三氯苯进行室温可溶物和不溶物的分离,采用凝胶渗透色谱、13C核磁共振波谱及热分析等方法对两种共聚聚合物及其分离物进行表征,探讨了乙烯和1-丁烯作为共聚单体对聚丙烯树脂结构和性能的影响.结果表明,与乙烯相比,1-丁烯更趋向于共聚在较长的聚丙烯分子链上,其结果导致丙烯/1-丁烯无规共聚聚丙烯的可溶物含量更低.同时,对两种无规共聚物结晶性能的差异以及对光学性能和动态力学性能的影响研究表明,如果共聚单体含量接近,丙烯/1-丁烯无规共聚物结晶度更高;透明制品雾度相同时,丙烯/1-丁烯无规共聚物的刚性更高.     

Dynamic mechanical properties of random ethylene/1-octene copolymers prepared by rapid cooling

The dynamic mechanical properties of a well-characterized series of homogeneous ethylene/1-octene copolymers with different random hexyl branch contents and prepared using different cooling conditions have been examined using dynamic mechanical analysis (DMA). It was confirmed that the relaxation behavior of copolymers varied continuously with the branch content: the magnitude of the β relaxation increased with branch content while the intensity of the α relaxation decreased with the branch content; both relaxation temperatures decreasing with increasing branch level in the copolymers. Copolymers prepared at different cooling conditions were further examined and strikingly continuous changes were found for the first time. The β relaxation was shown to correlate to the amorphous region, while the α₁ and α₂ relaxations can be clearly differentiated for some samples and are assumed to be associated with the interlamellar slip and intra-crystalline c-shear processes respectively. With increasing cooling rate, the relative intensity of α₁ relaxation to α₂ relaxation was found to decrease while the β relaxation did not change. The most informative data is determined from deconvolution of tan δ spectra. In higher crystallinity polymers the α₁ and α₂ relaxations are closely related in activation energy but have different temperature locations. For lower crystallinity systems, where the α₁ relaxation cannot be observed, the α₂ and β relaxations are closely linked, with activation energies approaching one another. These results show very clearly that, although the observed relaxation data can be separated through deconvolution into three separate peaks, the behaviors are closely linked. Presumably, this a clear reflection of the role of tie molecules in binding phases together and in influencing dynamic mechanical behavior. A clear change of behavior has also been observed in the β relaxation when a distinct amorphous phase exists outside of the spherulites, confirming the general belief that the crystalline phase influences the amorphous phase when it is confined within a spherulite. Again, this behavior is reflecting the role of tie molecules in binding together the nanocomposite structure of a spherulite.     

The stress relaxation behavior of isotactic poly(1-butene) and its ethylene copolymers

The tensile stress relaxation behavior subsequent to different imposed strains of poly(1-butene) was studied at room temperature. The plastic and elastic strains after a stress relaxation practically agree with those found for an immediate unloading. This indicates that the same decomposition of the total strain would also be found in an ideal quasi-stationary stretching experiment. Unloading curves as measured after a stress relaxation have a curved appearance indicative for a continuous change from forces of the crystal skeleton to forces of the entanglement network. Measured stress relaxation curves begin with an instantaneous drop, which can be associated with the cessation of a viscous flow, and then continue with a retarded relaxation process. The crystallite fragmentation process that sets in at the critical strain ε_H = 0.7 leads to a rising value of the instantaneous drop. The retarded component can be related to the transition of the crystalline blocks from positions that they adopt during the stretching to the actual equilibrium positions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2074–2080, 2004     

Structure formation of random isotactic copolymers of propylene and 1-hexene or 1-octene at rapid cooling

The effect of variation the cooling rate in a wide range between 10^?2 and 10³ K s^?1 on solidification the relaxed melt of random isotactic copolymers of propylene with low amount of 1-hexene or 1-octene has been studied. Emphasis has been placed on the structure formation at rapid cooling and an evaluation of the conditions required to permit crystallization, mesophase formation, or suppression of any ordering. The presence of low amount of either 1-hexene or 1-octene co-units in the propylene chain decreases drastically the critical cooling rate required for suppression of crystallization from about 150–200 K s^?1 in the homopolymer to about only 10 K s^?1 in the copolymers; increasing the cooling rate beyond these limits allowed mesophase formation or even generation of fully amorphous samples. The study of the kinetics of formation of specific structures is completed by a complementary analysis of the X-ray structure, morphology and superstructure of the ordered phase. The hindrance of non-isothermal crystallization and mesophase formation of random copolymers of propylene with 1-hexene or 1-octene is compared with that in propylene–1-butene copolymers; addition of only 2–3 mol% 1-hexene or 1-octene into the propylene chain leads to even larger hindrance of the ordering process than the addition of more than 10 mol% 1-butene.     

Coisotactic shift contributions in 13C NMR spectra of propylene/butene-1 copolymers

Coisotactic shift contributions caused by sterically crowded isotactic side-chain alkyl groups are proposed for peak assignments of isotactic propylene/butene-1 copolymers. Previously disputed analyses of methine triads and PB tetrads of backbone methylene carbons (α,α′-CH₂) have been verified using first-order Markovian distribution theory. Coisotactic shift contributions also account for the reverse order of the propylene-centered triads from that predicted by the Grant-Paul equation.     

Model copolymers of ethylene with butene-1 made by hydrogenation of polybutadiene: Chemical composition and selected physical properties

Hydrogenation of polybutadienes with from 8 to nearly 100% vinyl content was used to prepare a series of model copolymers of ethylene and butene-1 with uniform microstructures and narrow molecular weight distributions. They range from readily crystallizable to completely amorphous, depending on the frequency of ethyl side branches (2–50 per 100 skeletal carbons). Melting temperature, secondary transition temperature, density, plateau modulus for the melt, and elastic modulus for the solid were obtained as functions of branch content. The effect of crystallinity on the secondary transition and modulus of the solid is discussed.     

Effect of co-unit type in random propylene copolymers on the kinetics of mesophase formation and crystallization

Fast scanning chip calorimetry has been employed to study the effect of the type and concentration of co-units on the rate of mesophase formation and crystallization in random isotactic copolymers of propylene and 1-alkenes, including ethylene, 1-butene, 1-hexene, and 1-octene. The dependence of the rate of ordering on temperature of the propylene homopolymer shows two distinct maxima around 300 and 340–350 K which are related to mesophase formation and crystallization, respectively. Addition of 1-alkene co-units leads to a decrease of the maximum rate of both crystallization and mesophase formation. At comparable temperature and molar percentage of co-units in the propylene chain, ethylene, and 1-butene co-units cause less reduction of the maximum rate of ordering than 1-hexene or 1-octene co-units. The experimental observations are discussed in the context of possible incorporation of these chain defects into the ordered structures.     

Modeling random methyl branching in ethylene/ propylene copolymers using metathesis chemistry: synthesis and thermal behavior

The structure of random ethylene/propylene (EP) copolymers has been modeled using step polymerization chemistry. Six ethylene/propylene model copolymers have been prepared via acyclic diene metathesis (ADMET) polymerization and characterized for primary and higher level structure using in-depth NMR, IR, DSC, WAXD, and GPC analysis. These copolymers possess 1.5, 7.1, 13.6, 25.0, 43.3, and 55.6 methyl branches per 1000 carbons. Examination of these macromolecules by IR and WAXD analysis has demonstrated the first hexagonal phase in EP copolymers containing high ethylene content (90%) without the influence of sample manipulation (temperature, pressure, or radiation). Thermal behavior studies have shown that the melting point and heat of fusion decrease as the branch content increases. Further, comparisons have been made between these random ADMET EP copolymers, random EP copolymers made by typical chain addition techniques, and precisely branched ADMET EP copolymers.     

Physico-chemical study of the ternary system: isotactic polybutene-1-cyclohexane-n-propyl alcohol. Influence of molecular weight on the preferential adsorption phenomenon

This paper presents results on the influence of molecular weight on the preferential adsorption coefficient. λ, for the ternary system: isotactic polybutene-1-cyclohexane-n-propyl alcohol. It has been found that there are two different theta points.     

Dynamics in the crystalline polymorphic forms I and II and form III of isotactic poly‐1‐butene

Following an earlier study of the ¹H relaxation and NMR line shapes, we have carried out selective one‐dimensional and two‐dimensional ¹³C solid‐state NMR studies that yield to detailed interpretation of the dynamics in form I, II, and III polymorphs of isotactic poly‐1‐butene. A specific defect diffusion along the side group is proposed to account for the temperature dependence of the ¹³C spectra in form I. The backbone of the helix in forms II and III is shown to undergo large angle motions above the glass‐transition temperature. High‐resolution solid‐state ¹³C two‐dimensional exchange NMR under magic‐angle spinning with cross‐polarization techniques demonstrates the existence of slow rotational jumps of the helices in form III with typical jump rates of about 10 s⁻¹. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2611–2624, 2000     

Crystallization of side chains in copolymers of ethylene and 1-alkenes

The phase structure of random copolymers of ethylene and ethylene-d₄ with 1-octadecene and other 1-alkenes has been investigated. CPMAS ¹³C NMR spectra show that a fraction of the central sections of C₁₆H₃₃ side chains in ethylene-d₄ copolymers are in ordered environments at 298 K. They give rise to resonances from 32.9 ppm to 33.8 ppm, which show that they are in trans conformations; T_1C values for this group of resonances range from 1 s to 7 s. The remaining side chains are in an amorphous environment, the internal methylenes having a chemical shift of 30.8 ppm and a T_1C close to 0.4 s. A Raman band at 1062 cm⁻¹ in the spectrum of an ethylene-d₄-1-octadecene copolymer is consistent with partial crystallization of side chains. Some side-chain crystallization also occurs in a 1-tetradecene copolymer. X-ray diffraction studies suggest that smaller side chains do not crystallize to a significant extent at 298 K. © 1996 John Wiley & Sons, Inc.     

Polydispersity of ethylene sequence length in metallocene ethylene/α‐olefin copolymers. II. Influence on crystallization and melting behavior

In this work, crystallization and melting behavior of metallocene ethylene/α‐olefin copolymers were investigated by differential scanning calorimetry (DSC) and atomic force microscopy (AFM). The results indicated that the crystallization and melting temperatures for all the samples were directly related to the long ethylene sequences instead of the average sequence length (ASL), whereas the crystallization enthalpy and crystallinity were directly related to ASL, that is, both parameters decreased with a decreasing ASL. Multiple melting peaks were analyzed by thermal analysis. Three phenomena contributed to the multiple melting behaviors after isothermal crystallization, that is, the melting of crystals formed during quenching, the melting‐recrystallization process, and the coexistence of different crystal morphologies. Two types of crystal morphologies could coexist in samples having a high comonomer content after isothermal crystallization. They were the chain‐folded lamellae formed by long ethylene sequences and the bundlelike crystals formed by short ethylene sequences. The coexistence phenomenon was further proved by the AFM morphological observation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 822–830, 2002     

Interval kinetic gravimetric sorption of acetone in random copolymers of poly(ethylene terephthalate) and poly(ethylene 2,6-naphthalate)

Interval sorption kinetics of acetone in solvent cast films of random poly(ethylene terephthalate)-co-(ethylene 2,6-naphthalate) (PET-co-PEN) are reported at 35°C and at acetone pressures ranging from 0 to 7.3 cm Hg. Polymer composition is varied systematically from 0% to 50% poly(ethylene 2,6-naphthalate). Equilibrium sorption is well described by the dual-mode sorption model. Interval sorption kinetics are described using a two-stage model that incorporates both Fickian diffusion and protracted polymer structural relaxation. The incorporation of low levels of PEN into PET significantly reduces the excess free volume associated with the glassy state and, for these interval acetone sorption experiments in ∼ 5 μm-thick films, decreases the fraction of acetone uptake controlled by penetrant-induced polymer structural relaxation. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2973–2984, 1999