Crystallizable Sequences Research Articles

Isotactic Polypropylene with (3,1) Chain-Walking Defects: Characterization, Crystallization, and Melting Behaviors

Using two living nickel α-diimine complexes (rac-1 and rac-4) activated with methylaluminoxane (MAO), isotactic polypropylene (iPP) samples containing five types of regiodefects were synthesized. Both isolated and successive groups of (2,1) and (3,1) enchainments were identified by solution-state 13C NMR spectroscopy. The extended monomer and bulky nature of these defects add some restraints to the crystallinity level that can be achieved in these polymer samples. The complex based on a cumyl-derived ligand, rac-4, produces higher molecular weight iPP with a higher content of bulky defects than rac-1. Melting temperatures and crystallinity levels of rac-4-derived iPP are accordingly lower than the polymers obtained from rac-1. Although the nature of the chain-walking defect is similar to the addition of the ethylene unit, the difference in polymer properties is profound. At equivalent point defects per total monomers (XB), iPP samples with chain-walking defects display lower melting temperatures and much lower degrees of crystallinity than random 1-alkene copolymers, including those with comonomers excluded from the crystal lattice such as the 1-hexene and 1-octene counits. Furthermore, iPP containing (3,1) regiodefects develops significantly higher contents of the γ-polymorph than any other iPP or random copolymer with a matched XB composition. The experimental evidence is consistent with shorter crystallizable sequences for iPP with chain-walking defects based on (3,1) enchainments. Taking crystallizable and noncrystallizable total units as the basis to compute point defects, the properties of (3,1) iPP adhere to the basis of exclusion equilibrium theory, indicating that the defects are random-bulky or generated in a random fashion but of a defined extended/multimonomer nature.

Use of the successive self-nucleation and annealing technique to characterize 60Co gamma irradiated HDPEs

The application of the Successive Self-nucleation and Annealing (SSA) thermal fractionation technique can yield detailed information of the structural changes induced in linear polyethylene by irradiation. The production of tertiary carbons during the crosslinking reactions can be equivalent to the structural heterogeneity present in branched polyethylenes since in both cases interruption of the linear crystallizable sequences occurs, and these are structural differences that can be easily detected by thermal fractionation. We demonstrate how correlations between melting point and short chain branching content employed for branched polymers can be useful to characterize the distribution of chain heterogeneity produced by crosslinking. As the radiation dose is increased and the crosslinking content also increases, the distribution of chain heterogeneity gets broader as detected by SSA. When the results are coupled with morphological observations made by transmission electron microscopy, valuable information on the morphological changes produced by crosslinking can also be ascertained, since the distribution of lamellar thicknesses substantially broadens with crosslinking. Such a broad distribution can also be predicted from SSA by simple calculations performed employing a modified version of the Gibbs–Thomson equation and is expected on the basis of random crosslinking reactions.

Investigation of UV aging influences on the crystallization of ethylene-vinyl acetate copolymer via successive self-nucleation and annealing treatment

Influences of UV aging on the crystallization of ethylene-vinyl acetate copolymer (EVA) were researched via successive self-nucleation and annealing (SSA) treatment. During the aging process, the polar vinyl acetate (VAc) units in the amorphous region were the most vulnerable structure. FTIR results demonstrated that, VAc units were initially attacked by the UV radiation, which further resulted in chain scission of molecules. Degradation expanded from the amorphous region to the crystal region gradually. Chain scission reaction freed crystallizable ethylene sequences from inter-/intra-molecular tanglement and confinement of neighboring VAc units. Although the crystallinity decreased after aging, newly freed crystallizable sequences preferentially arranged into more densely packed lamellae, leaving less residual fraction to arrange into loosely packed crystal region. WAXD patterns showed that the predominant orthorhombic crystal phase did not vary during aging. Simultaneously, re-arrangement in crystallization also resulted in the growth in lateral crystal size of EVA.

Deformation of elastomeric polyolefin spherulites

AbstractThe ethylene‐octene block copolymers in this study consist of long crystallizable sequences with low comonomer content alternating with rubbery amorphous blocks with high comonomer content. The crystallizable blocks form lamellae that organize into space‐filling spherulites even when the fraction of crystallizable block is so low that the crystallinity is only 7%. These unusual spherulites are highly elastic and recover from strains as high as 300%. This new class of thermoplastic elastomers is fundamentally different from conventional elastomeric olefin copolymers that depend on isolated, fringed micellar‐like crystals to provide the junctions for the elastomeric network. The elastomeric block copolymers are shown to be unique in that a hierarchical organization of space‐filling lamellar spherulites provides the junctions for the elastomeric network. The deformation of the elastic spherulites is readily studied with small angle light scattering, wide angle X‐ray diffractograms, and atomic force microscopy. At strains in excess of 300%, the spherulites break up into a fibrillar structure following lamellar deformation processes that are similar to those established for high density ethylenic polymers. The crystalline transformation produces a stiffer elastomer that exhibits complete recovery on subsequent loadings. Similar experiments on elastomeric random ethylene‐octene copolymers where fringed micellar crystals provide the physical crosslinks that connect the rubbery, amorphous chain segments reveal significant differences. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1313–1330, 2009

Characterization of the Microstructure of Bimodal HDPE Resin

In this work, two industrial bimodal high density polyethylene resins, resin A and resin B, having similar molecular weight (Mw), molecular weight distribution (MWD, and short-chain branching (SCB) content but different mechanical properties, were fractionated through cross-fractionation. The fractions were further characterized by GPC, 13C NMR, DSC and FT-IR techniques. These two resins were firstly fractionated into two fractions, i.e., high-temperature and low-temperature fractions, via preparative solution crystallization fractionation. Resin A with much better mechanical properties contains more high-temperature fractions with longer crystallizable sequences. The SCB content in the low-temperature fraction of resin A is lower than that of resin B. Both low-temperature fractions were then further fractionated using solvent gradient fractionation (SGF). The characterization of SGF fractions indicates that most of the branches fall into the high molecular weight chains in both low-temperature fractions. However, the high molecular weight chains in the low-temperature fraction of resin A contain less SCB than that of resin B.

Effect of comonomer type on the crystallization kinetics and crystalline structure of random isotactic propylene 1-alkene copolymers

Isothermal crystallization kinetics and properties related to the crystalline structure of four series of random propylene 1-alkene copolymers have been comparatively studied in this work. Comonomers studied include ethylene, 1-butene, 1-hexene and 1-octene in a concentration range up to 21mol%. All copolymers were synthesized with the same metallocene catalyst to provide an equivalent random distribution and a similar content of stereo and regio defects within the series. This has ensured that differences in crystallization kinetics and in crystalline properties of copolymers with matched compositions reflect the affinity of the comonomer type for co-crystallization with the propene units, and the effect of content and type of co-unit in the development of the crystalline structure. In the nucleation-driven crystallization range, that is for Tcs>Tc max, the values of the rate follow the sequence PB>PE>PH=PO for comonomer contents <13mol%, and PB>PE>PH>PO for >13mol% comonomer. These trends in overall crystallization are guided by differences in undercooling due to a similar progression of the degree of participation of the comonomer in the crystalline lattice. The variation of the rates at Tcs<Tc max follows the melt segmental dynamics driven by differences in Tg, especially at the highest co-unit contents, resulting in a reverse rate sequence for PHs and POs >15mol%, i.e., PB>PE∼PO>PH. In addition to crystallization kinetics, a comparative polymorphic analysis and unit cell expansion, crystalline morphology, and melting behavior have been instrumental in resolving the partitioning of the four types of co-units between crystalline and non-crystalline regions. 1-Butene units participate at the highest level followed by the ethylene units, as demonstrated by solid-state NMR. However, both units are defects that hinder crystallization, as given by the decreasing rates, decreased levels of crystallinity and lowered melting temperatures with increasing co-unit content. All crystalline properties of PHs and POs conform to a rejection model of the 1-octene units from the crystals in the whole compositional range, and rejection of the 1-hexene units for PH <13mol%, a conclusion also supported by NMR. The ability of PH >13mol% to pack comonomer-rich sequences into a stable trigonal lattice leads at Tcs>Tc max to an increased number of crystallizable sequences, and to faster crystallization rates than for matched PO copolymers.

Molecular Dynamics Study of the Solid-State Structures of Linear-Low-Density Polyethylenes with Blocky Branches

Solid-state structures formed in the early stage of crystallization of linear-low-density polyethylene models (branch content = 10 hexyl branches per 1000 backbone carbons) with all branches clustered near chain ends (end type) and chain middles (middle type) were studied using molecular dynamics (MD) simulation. Crystallization of a 50/50 mol/mol blend of the aforementioned two types of branched chains and of such chains with linear also 50/50 mol/mol, was also studied. The MD results showed that the end type chains tended to exhibit a slightly higher degree of order upon crystallization, as quantified by a higher trans/gauche (t/g) ratio than the middle type This seems to relate to the fact that the end type chains had longer crystallizable ethylene sequences than the middle type Mixing linear chains with branched chains enhanced the crystallizability of the branched regardless of the type. In fact, the t/g ratios of the branched chains in the blend models are significantly higher than those of the respective pure branched chain models. It was found that the linear chains crystallized at a higher rate than the branched chains and thereby acted as nucleating sites for the crystallization of the branched And it seems that both the linear and branched chains transversed through different ordered regions, thus forming the tie chains. Nonetheless, the most interesting observation was that the blend that contained both types of branched chains exhibited an unexpectably high degree of order, comparable to those branched chains in the models containing linear The finding seems to be attributed to the clustering of all the branches from both types of branched chains during the high-temperature annealing. In fact, the solid-state structure of such a blend contained essentially no tie chains, and all branched chains adopted an adjacent reentry mode to form their own ordered regions. Finally, it should be pointed out that the above conclusions are specific to the LLDPE models we used.

New insights into the multiple melting behaviors of the semicrystalline ethylene‐hexene copolymer: Origins of quintuple melting peaks

AbstractA semicrystalline ethylene‐hexene copolymer (PEH) was subjected to a simple thermal treatment procedure as follows: the sample was isothermally crystallized at a certain isothermal crystallization temperature from melt, and then was quenched in liquid nitrogen. Quintuple melting peaks could be observed in heating scan of the sample by using differential scanning calorimeter (DSC). Particularly, an intriguing endothermic peak (termed as Peak 0) was found to locate at about 45 °C. The multiple melting behaviors for this semicrystalline ethylene‐hexene copolymer were investigated in details by using DSC. Wide‐angle X‐ray diffraction (WAXD) technique was applied to examine the crystal forms to provide complementary information for interpreting the multiple melting behaviors. Convincing results indicated that Peak 0 was due to the melting of crystals formed at room temperature from the much highly branched ethylene sequences. Direct heating scans from isothermal crystallization temperature (Tc, 104–118 °C) were examined for comparison, which indicated that the multiple melting behaviors depended on isothermal crystallization temperature and time. A triple melting behavior could be observed after a relatively short isothermal crystallization time at a low Tc (104–112 °C), which could be attributed to a combination of melting of two coexistent lamellar stack populations with different lamellar thicknesses and the melting‐recrystallization‐remelting (mrr) event. A dual melting behavior could be observed for isothermal crystallization with both a long enough time at a low Tc and a short or long time at an intermediate Tc (114 °C), which was ascribed to two different crystal populations. At a high Tc (116–118 °C), crystallizable ethylene sequences were so few that only one single broad melting peak could be observed. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2100–2115, 2008

Effect of crystallization temperature and propylene sequence length on the crystalline structure of propylene-ethylene random copolymers

Crystallization behavior and resultant crystalline structure of a series of temperature-rising elution-fractionated specimen of a Ziegler-Natta catalyst-synthesized propylene-ethylene random copolymer were studied by DSC, WAXD and AFM. The experimental results indicate that both crystallization temperature and propylene sequence length exhibit great influence on the crystallization behavior and crystalline structure of the copolymer. It was found that the ethylene co-monomers acting as point defects inserted into the polypropylene chains play an important role in the formation of γ-iPP. As the co-monomer content increases, the crystallizable sequence length of iPP decreases, which produces an appropriate condition for its γ crystallization. At the same time, the existence of chain defects leads to a lower crystallinity of the copolymer and imperfection of the resultant crystals. For each individual sample with certain propylene sequence length or ethylene content, the increment of γ-iPP crystal content with increasing crystallization temperature demonstrates that higher crystallization temperature is in favor of the γ-iPP crystallization. Pure γ-iPP crystals have been got in samples with propylene sequence length lower than 21 under suitable crystallization conditions.

Random, Defect-Free Ethylene/Vinyl Halide Model Copolymers via Condensation Polymerization

The synthesis of ethylene vinyl halide (EVH) copolymers containing fluorine, chlorine, and bromine via the ADMET copolymerization of halogen containing α−ω-dienes with 1,9-decadiene is presented. The statistically random nature of the copolymers was established from their 13C NMR spectra. Thermal analysis via differential scanning calorimetry points to a distinct difference in the crystallization behavior of these random copolymers when compared to their compositionally matched precise analogues. In this case, sharp melting peaks typical of homopolymer-like crystallization of the precise analogues are no longer observed in Cl- and Br-based random copolymers and are replaced by broader peaks indicating a mechanism based on the selection of long crystallizable sequences. The results presented herein thus point to the utility of these random ADMET copolymers as suitable models for industrially relevant PE copolymers based on the perfectly linear, defect-free, and statistically random copolymer composition.

Fractionation and characterization of an ethylene–propylene copolymer produced with a MgCl2/SiO2/TiCl4/diester‐type ziegler–natta catalyst

AbstractAn ethylene–propylene copolymer synthesized with a Ziegler–Natta catalyst was fractionated by a combination of dissolution/precipitation and temperature‐gradient extraction fractionation. The fractions were characterized with 13C‐NMR, differential scanning calorimetry, and wide‐angle X‐ray diffraction. The fractionation was carried out mainly with respect to the content of ethylene, but the crystallizable propylene sequences could also exert an influence on the fractionation. The copolymer contained a series of components with wide variations in the compositions. With an increase in the ethylene content, the structure of the fractions became blockier and blockier, and the fraction extracted at 111°C had the blockiest structure. A further increase in the ethylene content led to a decrease in the length and number of the propylene sequences. Differential scanning calorimetry results showed that the composition distribution in single fractions was not homogeneous, and multiple melting peaks were observed. Wide‐angle X‐ray diffraction results revealed both polyethylene and polypropylene crystals in most of the fractions. Short propylene sequences could be included in the polyethylene crystals, and short ethylene sequences could also be incorporated into the polypropylene crystals. The incorporation of propylene sequences into polyethylene crystals strongly depended on the sequence distribution and crystallization conditions. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Oxidation of polycaprolactone to induce compatibility with other degradable polyesters

Chemical modification of poly(ɛ-caprolactone) PCL by oxidation with potassium permanganate in solution was investigated. According to the data obtained from Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance 1H NMR, after the oxidation reactions the PCL chains exhibited new functional groups (vinyl and hydroxyl) and possible intermolecular recombination, producing an oxidized-polycaprolactone (PCL-OX). Solution viscometry indicated that degradation also occurred during the oxidation reactions (∼30% drop in viscosity average molecular weight was detected). Differential scanning calorimetry (DSC) also indicated that PCL was chemically modified and degraded. The successive self-nucleation/annealing (SSA) treatment confirmed that a reduction (or interruption) in linear crystallizable sequences occurred. The immiscibility of blends of PCL with other degradable polyesters, such as poly( p-dioxanone) PPDX (PCL/PPDX 90:10 w/w), was shown by the invariability of the relevant thermal transitions as determined by DSC and FT-IR and NMR analysis. However, the blend prepared with oxidized PCL (PCL-OX/PPDX, in the same composition range) did not display signs of thermodynamic miscibility but showed an interesting thermal behaviour demonstrated by changes in the crystallization temperatures of both phases, and by the melting behaviour of the PCL-OX in the blend. These results together with spectroscopic analysis show that the oxidation of PCL induces physical interactions and/or compatibilisation among the phases of this blend.

Characterization on mixed-crystal structure and properties of poly(butylene adipate- co-terephthalate) biodegradable fibers

Biodegradable ideal random copolymer poly(butylene adipate- co-terephthalate) (PBAT), with 44 mol% butylene terephthalate (BT), was melt-spun into fibers with take-up velocity up to 5 km/min. The structure development and properties of the as-spun fibers were investigated through birefringence, WAXD, SAXS, DSC and tensile test. Despite of the ideal randomness and composition (1:1) of PBAT copolymer, PBAT fiber showed well-developed PBT-like crystal structure, while its melting temperature (ca. 121 °C) was over 100 °C lower than that of PBT. Based on the quantitative analyses on the lattice spacing, the crystallinity and the fraction of crystallizable BT sequences, the crystal structure of PBAT was characterized to be formed by mixed-crystallization of BT and BA units, where BA units were incorporated into BT lattice. This mixed-crystal structure was found to undergo PBT-like reversible crystal modification with the application and removal of tensile stress. This crystal modification was found to occur in a higher strain region compared with that of PBT fibers.

Studies on the intermolecular structural heterogeneity of a propylene‐ethylene random copolymer using preparative temperature rising elution fractionation

AbstractThe intermolecular structural heterogeneity of a propylene‐ethylene random copolymer was studied by preparative temperature rising elution fractionation combined with GPC, 13C‐NMR, differential scanning calorimetry, and wide‐angle X‐ray diffraction analysis of the obtained fractions. The isotacticity of fractions increased with increasing elution temperature, and the ethylene content decreased monotonously. Fitting of the obtained comonomer triad sequences by Bernoullian and first‐order Markovian statistical models indicated that lower isospecific active sites are more active toward ethylene. The isolated ethylene unit disrupted the crystallizable isotactic sequence and lowered the crystallizability of the polypropylene chain. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 232–239, 2005

Phase morphology and impact toughness of impact polypropylene copolymer

Factors influencing the impact toughness of two impact polypropylene copolymers (IPC) with almost the same ethylene content, molecular weight and molecular weight distribution were studied by temperature gradient extraction fractionation (TGEF), scanning electron microscopy (SEM), nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC). The results indicate that poor interfacial adhesion between the disperse phase and the continuous matrix, larger dimensions and non-uniform distribution of disperse phases are main reasons for the low impact toughness of IPC B that possesses of a low content of ethylene–propylene segmented copolymer with long crystallizable PE and PP sequences as a compatibilizer between the disperse phase and the matrix.

Non-linear Crystalline Spherulitic Growth Behavior for LLDPE

Abstract Nonlinear growth behavior was observed in two crystallization regimes, depending on the temperature. Non-linearity may be explained by the reduction of the concentration of crystallizable ethylene sequences (CES) in the melt phase. In the two regimes, the concentration of uncrystallizable ethylene sequences (UCES) increases, as the crystallization time increases, because UCES are continuously excluded from the crystal lattice into the melt phase. An empirical equation is proposed to describe the melting temperature of the crystal stem with the maximum possible length, T C, n * m , in nonlinear growth processes, assuming that the diffusion layer is negligible. A modified form of the Hoffman-Lauritzen equation (MHL) describes well the crystallization growth kinetics of LLDPE spherulites in the non-linear growth region.

Fractionation process in TREF systems: Validation of thermodynamic model and calculation procedure by Raman LAM studies

In this paper, possible sources for the unexpected distributions of crystalline sequence lengths calculated from temperature rising elution fractionation (TREF) calibration experiments, as reported in a previous work, are investigated. With this aim, chain folding and cocrystalization phenomena were explored in the conditions of crystallization as used for TREF or crystallization analysis fractionation (CRYSTAF). Slow crystallizations were performed from xylene solutions of model low molecular weight ethylene homopolymers with narrow molecular weight distributions. The same experiments were performed with homopolymers having narrow molecular weight distributions and with blends having wide molecular weight distributions. The resulting distributions of the lengths of crystalline methylene sequences were directly studied by Raman in the so-called longitudinal acoustic mode (LAM) and by DSC. For ethylene homopolymers with molecular weights below 2000 g/mol, the results from Raman LAM indicate that slow crystallization in TREF or CRYSTAF systems occurs in the extended-chain mode. For higher molecular weights, evidence of chain folding was found. In the case of blends, independent crystallization was observed for each molecular weight when the molecular weight ranges used for the blends are relatively narrow. Cocrystallization was observed when this range was increased. Overall, these results strongly support the inverse technique calculation procedure developed by our group for the calculation of distributions of lengths of crystallizable sequences from TREF spectra. In this context, the results confirm that the unexpected crystallizable sequence lengths found in our previous work really exist and can be associated to chain folding or cocrystallization phenomena. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3083–3092, 2005

Morphology and mechanical properties in the binary blends of isotactic polypropylene and novel propylene-co-olefin random copolymers with isotactic propylene sequence 1. Ethylene–propylene copolymers

The additive effects of the novel ethylene–propylene random (EP) copolymers with high isotacticity in propylene sequence on the morphology and mechanical properties of isotactic polypropylene (iPP) were investigated using polarized optical microscopy, transmission electron microscopy, dynamic mechanical analysis and tensile behavior. According to these results, the EP copolymers with a propylene content of more than 84mol% were miscible with iPP, in which the crystallizable PP sequences in these EP copolymers were incorporated in crystal lattice of iPP and the other portions in the EP chains were excluded to the amorphous phases. Consequently, they act as tie molecules linking between adjacent lamellae, leading to enhancement of yield toughness of iPP. On the other hand, the EP copolymers with a propylene-unit content of less than 77mol% were incompatible with iPP. The iPP/EP blends showed the phase-separated morphology.

Relationships between the molecular architecture, crystallization capacity, and miscibility in poly(butylene terephthalate)/polycarbonate blends: A comparison with poly(ethylene terephthalate)/polycarbonate blends

AbstractPoly(butylene terephthalate) (PBT)/polycarbonate (PC) samples, prepared via reactive blending in the presence of Ti‐ and Sm‐based catalysts, resulted in block copolymers whose block length decreased as the mixing time increased. A single homogeneous amorphous phase occurred when the blocks had monomeric sequences shorter than 10 units. Otherwise, a crystalline phase of PBT developed. Also, in poly(ethylene terephthalate) (PET)/PC blends previously studied, the miscibility was strictly correlated with the crystallizability of the system. Therefore, the miscibility of the PBT/PC and PET/PC blends was compared with respect to the tendency of the PBT and PET blocks to crystallize under isothermal conditions. The crystallization rate of the PBT/PC copolymers was faster than that of the PET/PC copolymers with similar block lengths. Accordingly, the minimum crystallizable sequence length of the PBT blocks was shorter than that of the PET blocks (18 vs 31 monomeric unit sequences). This behavior was interpreted as an effect of the more flexible PBT units, which had a greater tendency to fold and crystallize than the PET units. Therefore, PBT, the blocks of which tended to crystallize even if they were very short and phase‐separated, was characterized by a poorer compatibility with PC than that of PET. As a result, the block size had a fundamental role in determining the crystallizability and, therefore, phase behavior of the semicrystalline block copolymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2821–2832, 2004

The role of defect microstructure in the crystallization behavior of metallocene and MgCl2-supported Ziegler-Natta isotactic poly(propylenes)

The intermolecular defect distribution of poly(propylenes) of the Ziegler-Natta (ZN) and metallocene (M) types is assessed by classical fractionation and analysis of the fractions by GPC and 13C NMR. In addition, the linear growth rates, a property sensitive to differences in defects distribution in the poly(propylene) chain, are used to infer the stereoblock-like intramolecular distribution of defects in industrial type ZN iPPs and their fractions. The behavior of fractions from a matched metallocene iPP provide direct evidence of the single-site nature of the catalyst. Metallocene poly(propylenes) enable a quantitative assessment of the molecular, thermodynamic and kinetic factors that govern the formation and concentrations of the a and g polymorphs. Short continuous crystallizable sequences and high crystallization temperatures favor the formation of this polymorph in homo poly(propylenes) and random propylene copolymers. Differences in the partitioning of the comonomer between the crystalline and non-crystalline regions leads to contents of the gamma phase that differ among the copolymers at any given crystallization temperature. Qualitatively these differences can be used to assess the degree to which a counit participates in the crystallite.