Lamellar Thickness Distribution Research Articles

Effect of short chain branching on the blown film properties of linear low density polyethylene

Three linear low density polyethylene (LLDPE) resins of similar melt index and density were synthesized with different comonomers in the Unipol pilot-plant scale reactor. The molecular structure, blown film morphology, and film strength properties of the resins have been comprehensively characterized. The film dart drop impact strength of the LLDPEs increases in the order of ethylene/1-butene, ethylene/1-octene, and ethylene/1-hexene copolymers; whereas the Elmendorf tear strength of them increases in the order of ethylene/1-butene, ethylene/1-hexene, and ethylene/1-octene copolymers. The mechanical properties seem to be highly associated with the length and distribution of short chain branches and, consequently, the lamellar thickness distribution of the resins. © 1996 John Wiley & Sons, Inc.

Effect of short chain branching on the blown film properties of linear low density polyethylene

Three linear low density polyethylene (LLDPE) resins of similar melt index and density were synthesized with different comonomers in the Unipol pilot-plant scale reactor. The molecular structure, blown film morphology, and film strength properties of the resins have been comprehensively characterized. The film dart drop impact strength of the LLDPEs increases in the order of ethylene/1-butene, ethylene/1-octene, and ethylene/1-hexene copolymers; whereas the Elmendorf tear strength of them increases in the order of ethylene/1-butene, ethylene/1-hexene, and ethylene/1-octene copolymers. The mechanical properties seem to be highly associated with the length and distribution of short chain branches and, consequently, the lamellar thickness distribution of the resins. © 1996 John Wiley & Sons, Inc.

Comparative study of size and distribution of lamellar thicknesses and long periods in polyethylene with a shish-kebab structure

This study contains a combined application of three different techniques for the study of injection moulded polyethylene (PE), showing an oriented shish-kebab structure: small angle X-ray scattering (SAXS), low frequency Raman spectroscopy (LAM) and transmission electron microscopy (TEM), A series of linear PEs and molecular weights in the range 51000–478000 has been investigated and two injection temperatures have been used (Tm=144 and 210 °C). SAXS patterns from the highly oriented regions show the presence of either one axial long period (L1) or two (L1 and L2) depending on molecular weight (¯Mw) and Tm. It is shown that L1 and L2 increase with ¯Mw up to a given critical molecular weight ¯Mc. Above ¯Mc, L1 and L2 remain constant. Raman results qualitatively confirm the existence of two separate distributions of straight-length chain segments for those samples having molecular weights above the critical value. Shorter segments are shown to be more abundant than the longer ones. In the lowest molecular weight sample, results from SAXS, TEM and Raman spectroscopy seem to be consistent with each other, although in some cases a tilted molecular arrangement within the lamellae has to be invoked. On the other hand, in case of the highest molecular weight sample, the length of the short straight-chain segments derived from Raman spectroscopy agrees well with the double periodicity obtained from SAXS. On the contrary, long periods measured from TEM only correspond to the shorter SAXS periodicity. This result is discussed by assuming the occurrence of crystalline bridges among adjacent lamellae.

Studies of the Comonomer Distributions in Low Density Polyethylenes Using Temperature Rising Elution Fractionation and Stepwise Crystallization by DSC

The comonomer distributions of commercial linear low density polyethylenes (LLDPE) and linear very low density polyethylenes (VLDPE) produced with traditional high activity Ziegler–Natta (Z–N) catalysts were characterized by temperature rising elution fractionation (TREF). In order to develop faster characterization methods the polymers were also characterized using a segregation fractionation technique (SFT) based on a stepwise crystallization by differential scanning calorimetry (DSC). Comparative studies of SFT and TREF demonstrated that SFT provides an alternative tool for the relative qualitative analysis of the chemical composition distribution (CCD) and the technique is useful to characterize the heterogeneity in comonomer unit distribution. Lamellar thickness distributions can be calculated from the DSC endo-therms by applying the Thomson–Gibbs equation. The SFT technique was also applied to commercial single-site (metaliocene) LLDPE and VLDPE polymers. In spite of their more homogeneous structure compared with the Z–N copolymers, which contain many active sites, these single site copolymers also gave thermograms resolved into several peaks.

Dissolution mechanism of semicrystalline poly(vinyl alcohol) in water

Changes occurring in the degree of crystallinity and lamellar thickness distribution of poly(vinyl alcohol) (PVA) samples during dissolution in water were investigated. PVA samples of three different molecular weights were crystallized by annealing at 90, 110, and 120°C. The initial degrees of crystallinity measured by differential scanning calorimetry (DSC) and by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) varied from 43 to 60% and the average lamellar thicknesses measured by DSC ranged from 50 to 400 A. PVA dissolution was followed at 25, 35, and 45°C from 30 s up to 195 min. Lamellar thicknesses were determined as a function of dissolution time using DSC. There was an initial drastic decrease in the degree of crystallinity, which leveled off to a fairly constant value before reaching zero by the time the polymer dissolved completely. Increase in molecular weight led to lesser number of crystals, but with larger average lamellar thickness, which were more stable in the presence of water. Increase in crystallization temperature or decrease in dissolution temperature led to larger average lamellar thickness. Based on these findings, a dissolution mechanism involving unfolding of the polymer chains of the crystal was proposed.

Changes and distribution of lamellae in the spherulites of poly(ether ether ketone) upon stepwise crystallization

Abstract Coexistence of multiple lamellar thicknesses and lamellar thickness changes in poly(ether ether ketone) (PEEK) have been supported by the results obtained from investigations using differential scanning calorimetry (d.s.c.), X-ray and scanning electron microscopy. The melting behaviour, crystallization kinetics and morphology of PEEK subjected to multiple-step crystallization have been studied. During stepwise crystallization of the polymer, a major lamella crystal develops first, which possesses a melting peak temperature close to the equilibrium melting temperature; however, a small fraction of the polymer is packed into minor crystal aggregates of smaller lamellar thicknesses which, upon d.s.c. display the observed phenomenon of multiple minor melting peaks located further below the main melting peak. It is this multiple distribution of lamellar thicknesses that causes multiple melting behaviour. A descriptive crystallization kinetic model is used to help explain this phenomenon by correlating the crystallization and thermal behaviour in PEEK.

Small-angle X-ray scattering from high-density polyethylene: lamellar thickness distributions

Small-angle X-ray diffraction patterns were recorded for a number of high-density polyethylene samples and were successively analysed by a fit with the calculated patterns corresponding to certain theoretical models. The parameters determined by the above fits were as follows: long-period, crystallinity, mean dimensions of the crystalline lamellae, amorphous thickness, crystallinity and lamellar dimension distributions. Regarding the latter, two mathematical functions were used for the fits, i.e. a symmetrical and an asymmetrical type, and for all of the samples examined the function was determined which gives the best results. Finally, a correlation is suggested between the polymer molecular weight and lamellar dimension distributions.

Changes in the properties of HDPE fibers upon radiation grafting with acrylic acid

AbstractThe morphology and the physical and mechanical properties of graft‐modified polyethylene fibers have been studied. Two types of fibers, with the diameters of 10 μm (1.1 dtex) and 40 μm (7.5 dtex), were modified by radiation‐induced grafting with acrylic acid. The extent of grafting was determined gravimetrically. Confirmation of gravimetrically obtained values was achieved using conductometric titration. The fibers were hydrated at pH 2 and pH 7. The degree of swelling was 120% at pH 2 and 200% at pH 7. The transversal distribution of polyacrylic acid in the fibers was determined. Fibers were stained and observed with an optical microscope. The diffusion of the monomer into the bulk was found to be rather fast. The changes in the total crystalline content and the lamellar thickness distributions in consequence of irradiation and grafting were determined by differential scanning calorimetry analysis. The measurements showed no effects of irradiation on the crystallinity in either type of fiber, whereas a decreasing crystallinity caused by grafting was noticed in the 40 μm fibers. The lamellar thickness distributions narrowed upon irradiation, indicating recrystallization as a result of chain scission. Wide angle x‐ray scattering and Raman analysis of dry and hydrated fibers were conducted to study the behavior of the fibers in an aqueous environment. These results both showed a decreasing crystalline content caused by fiber hydration. Tensile tests were carried out to evaluate how grafting, hydration and Ca2+‐crosslinking of grafts affected the fiber strength. Grafting and Ca2+‐crosslinking, as well as hydration, resulted in a decreasing E‐modulus for the 40 μm fibers, whereas no significant change could be noticed in the 10 μm fibers. © 1995 John Wiley & Sons, Inc.

Rheological Properties of a Partially Molten Polypropylene Random Copolymer during Annealing

Semicrystalline polymers exhibit a broad melting transition. This effect is due to the lamellar thickness distribution in the materials. The melting temperature T m (1) of a given lamella increases with its thickness 1. When heating to a temperature T a in the melting range, the crystalline lamellae with T m (1)< T a melt. This process is accompanied by the thickening of the nonmolten lamellae which suck in molten polymer chains. We used a polypropylene random copolymer as a convenient material to investigate rheological and structural aspects of this reorganizational process. The temperature and time dependence of the dynamic moduli G' and G″ in oscillatory shear flow and for the first time of the transient uniaxial elongational viscosity were measured during the reorganization of the partially molten material. Data from differential scanning calorimetry (DSO runs on the annealed samples are correlated with the respective data from the rheological experiments. With increasing amount of crystalline material the shear viscosity function increases at small shear rates. At the same time marked strain hardening shows up in uniaxial elongation. The rheological phenomena observed during annealing at temperatures in the melting range are interpreted by assuming a network of nonmolten, growing lamellae which are connected by tie chains

Polymer melting: Simulation and experimental results from a single melting peak model system

Experimental polymer melting as observed in differential scanning calorimetry (DSC) is generally complicated by the influence of lamellar thickness distributions and/or reorganizational effects. Idealized polymer melting in the absence of such effects is critical both for a basic understanding of the DSC results and for decoupling instrumental from sample size broadening effects. A model system which does not have the mentioned complicating effects was developed, consisting of a polyethylene-indium powder mixture. The melting behavior of this model system was experimentally investigated and simulated by a “shell model” that treats melting in a DSC sample as the melting of a series of shells. All of the parameters in the model are obtained experimentally; none is adjustable. The simulation was generally quite satisfying, although a somewhat greater deviation was observed for extreme heating rates (1°C min −1 and 40°c min −1). In addition to the “shell model” simulation, a single Gray type melting peak was also found useful in approximating the experimental curve.

Lamellar Thickness Distribution in Linear Polyethylene and Ethylene Copolymers

Crystallite thickness distributions have been calculated using the Thomson-Gibbs equation from the endotherms obtained by DSC. These distributions were then compared with those obtained by transmission electron microscopy or by Raman spectroscopy (LAM). For linear polyethylene, the experimentally measured distribution can only be reproduced by choosing appropriate combinations of the interfacial free energy and the heating rate. The distributions calculated from endotherms of random ethylene copolymers are very different from those that are obtained directly. The nature of the fusion process of random copolymers, where the free energy of the melt is not invariant with temperature, restrains the applicability of the simple Thomson-Gibbs equation

The melting of polymers — a three-phase approach

Information on the morphology of semi-crystalline polymers can be obtained from their melting behaviour. Due to the lamellae thickness distribution, a very broad melting region is observed. From a comparison of the glass transition intensity and the crystallinity, e.g. from X-ray diffractometry, it is known that there are rigid amorphous regions inside semi-crystalline polymers with no contribution to the glass transition or to the melting. Both the broad melting region and the deviation from the normally used two-phases model often result in incorrect crystallinities and other morphological parameters. Therefore, the analysis of the melting behaviour, taking into account the broad melting region and the rigid amorphous fraction, should result in a better, more detailed description of the morphology. A procedure to do this on the basis of a separation of the measured heat flux into the baseline specific heat capacity and the excess portion is suggested here. Using this procedure, it should be possible to obtain the temperature dependence of the crystalline, rigid amorphous, and liquid amorphous fractions, as well as the lamellae thickness distribution, the thickness of the interfaces of the lamellae, and the specific inner surface of the crystalline fraction.

Infra-red spectroscopic investigation of bulk-crystallized trans-1,4-polyisoprene

Crystallization from the melt of trans-1,4-polyisoprene was followed using Fourier-transform infra-red spectroscopy. The spectral subtraction factor versus time was obtained at the crystallization temperature and after cooling to 25°C for fractions and for the parent material. Crystallization of the β crystal form was followed at 36 and 43°C and of the α form at 43 and 51°C. Samples crystallized at 25°C and subsequently annealed at 43°C were also monitored. The amorphous fraction for prenucleated melt-crystallized α-form-containing samples increased with molecular weight at the crystallization temperature and after cooling to 25°C. Samples containing the β crystal form showed a molecular-weight dependence at a crystallization/annealing temperature of 43°C that disappears upon cooling the sample to 25°C. The results are discussed in terms of lamellar surface disorder and lamellar thickness distribution.

Mechanical γ and β relaxations in polyethylene—I. Glass transitions of polyethylene

The mechanical γ relaxation of a linear polyethylene has been extensively analysed by means of high resolution mechanical spectrometry. This analysis was completed by quantitative measurements of the morphological parameters governing the characteristics of the γ relaxation, such as the crystallinity index and the crystalline lamellar thickness distribution determined from differential scanning calorimetry. The γ relaxation was found to have the characteristics of mechanical relaxation due to glass transition: there is a high apparent activation energy; linear polyethylene exhibits structural recovery induced by annealing at a temperature below the γ peak, whereas the γ relaxation has a WLF temperature dependence. As for the γ relaxation, it was found that the β relaxation displayed by a branched polyethylene has the characteristics of a glass transition (high value of apparent activation energy; structural recovery during ageing at T < T β ). Recalling definitions of the glass temperature, it was proposed that the lower glass transition (γ region) is characteristic of Van der Waals interactions between CH 2 groups while the upper glass transition (β region) is specific of Van der Waals interactions between CH 2 and CH 3 groups.

The influence of crystallinity distribution on small‐angle x‐ray scattering from semicrystalline polymers

AbstractTheoretical x‐ray scattering curves have been developed for the lamellar structure in semicrystalline polymers in which there are present distributions of lamellar thickness and crystallinity. The models have been tested against samples of linear low‐density, low‐density and crosslinked polyethylenes. When variation in crystallinity is present in a material, a major effect is an increase in the magnitude of near‐zero angle scattering. The Bragg maximum can appear as an ill‐defined hump on an apparently high level of background scattering. The shape of the Bragg peak is influenced more by crystallinity distribution than by lamellar thickness distribution. Of the polymers we have studied so far only linear low density polyethylene shows significant crystallinity distribution effects. A “rule‐of‐thumb” method for rapid estimation of crystallinity distribution effects has been developed, obviating the need for lengthy simulation.

Time-temperature-transformation diagram of polyethylene: origin of the double population of lamellae

The factors leading to the appearance of two lamellar populations in linear polyethylene were investigated using differential scanning calorimetry and mechanical spectrometry. Two systems which underwent identical thermal treatments but which differed by their polydispersity degree show similar lamellar thickness distribution curves. It was shown that the primary factor governing the breadth of the crystallite size distribution is not the dispersity degree of the molecular weight of polyethylene. A comparison of the dynamic mechanical behaviour and the lamellar thickness distribution curves of two systems which have different sheet thicknesses indicates the existence of a single population of lamellae for the thinner specimen and two populations of crystallites for the thicker film. Thus, the appearance of a broad distribution size of crystallites depends on the existence of a distribution of the cooling rate within the sample which induces a morphological gradient because of the well known low thermal conductivity of polyethylene. In addition, based on analysis of the isothermal crystallization behaviour of linear polyethylene carried out by means of a mechanical spectrometer, a time-temperature-transformation (T.T.T diagram) was established. Thus, by drawing on physical metallurgy concepts, an original interpretation for the appearance of two populations of lamellae based on different cooling rates within the sample is found.

Dynamics of ferroelectric phase transition in vinylidene fluoride/trifluoroethylene (VF2/F3E) copolymers. I. Acoustic study

We report the behavior of ultrasonic absorption and velocity in the ferroelectric copolymers of vinylidene fluoride (VF2) and trifluoroethylene (F3E) near the Curie point. The ferroelectric transition of VF2/F3E is strictly of the first order, however, it has the second-order character especially in the copolymers with low VF2 content. A large critical absorption due to cooperative fluctuation has been observed near the Curie point. From the temperature dependence of the critical absorption, we have found that the critical anomaly of the absorption is weak and the specific heat has logarithmic divergence at the Curie point if we ignore the effect of the distribution of the Curie point which may come from a distribution of lamellar thickness. This weak critical anomaly can be ascribed to the long-range nature of the bare interaction between dipoles due to connectivity of polymer chain or the electric dipolar interaction, the randomness of the interaction due to random copolymerization, and/or the dimensionality of the order parameter. The rotation of dipole is always accompanied by cooperative conformational change of the backbone, which is unique to a polymer chain. We have also found that a type of the dominant interaction between acoustic phonon and the order parameter depends on VF2 content.

Numerical analysis of lamellar thickness distributions

AbstractA comparison of crystallite thickness distributions, as obtained from thin section electron microscopy, small‐angle x‐ray scattering, and low‐frequency Raman acoustical model (LAM), is discussed in detail. It is shown with the aid of a large number of examples that a reliable numerical estimate of crystal thicknesses and long spacings is a complex problem. The values that are obtained by the different experimental techniques depend not only on the crystallite morphology, such as curvature and lateral extension, but very specifically, and crucially, on the nature of the thickness distribution. A series of widely different types of size distributions have been developed by controlling the molecular weight and crystallization temperature. Studies of these distributions, by the aforementioned experimental techniques, have revealed the reasons when and why agreement or disagreement is obtained in the determination of the crystallite thickness distribution.

Hierarchical structure of the intervertebral disc.

Optical microscope techniques are used to characterize the hierarchical structure of the collagenous components of the human intervertebral disc. In the anterior annulus fibrosus, the thickness of lamellae increases abruptly 2 mm inward from the edge of the disc, dividing the annulus into peripheral and transitional regions. Lamellae in the lateral and posterior aspects of the disc have a broad distribution of lamellar thicknesses throughout the annulus. In alternating lamellae, fibers are inclined with respect to the vertical axis of the spine in a layup structure. From the edge of the disc inward to the nucleus, this interlamellar angle decreases from +62 to +45 degrees. Within lamellae, the collagen fibers exhibit a planar crimped morphology. The plane of the waveform is inclined with respect to the vertical axis by the interlamellar angle. From the edge of the disc inward, the crimp angle increases from 20 to 45 degrees and the crimp period decreases from 26 to 20 um. A hierarchical model of the intervertebral disc has been developed that incorporates these morphological gradients.

Structural Distribution of Linear Low-Density Polyethylenes

Molecular and crystalline structures of linear low-density polyethylenes (LLDPE) were investigated by a series of characterization techniques. Molecular structural characteristics were elucidated by temperature-rising elution fractionation (TREF) and solvent-gradient elution fractionation (SGEF). A bird’s eye view and a contour map of LLDPE obtained by a combination of TREF and size exclusion chromatography exhibited a broad and multimodal chemical composition distribution (CCD), in contrast to a sharp and single CCD of conventional high-pressure low-density polyethylene (HP-LDPE). Short chain branching (SCB) was found to decrease with increase of molecular weight by SGEF technique. Thermal analysis of cross-fractions proved that a characteristic broad endothermic curve of LLDPE is attributable to its broad and multimodal CCD. Then, using DSC results, an indicative index (DI) which expresses the degree of the distribution of lamellar crystal thickness is proposed. DI was found to be sensitive both to CCD and to a kind of SCB. The crystallinity and melting temperature of cross-fractions having comparable molecular weights decrease with increasing comonomer content in the order of octene-1≍4-methyl-pentene-1>hexene-1>butene-1. From a statistical approach to the relationship between crystallinity and degree of SCB, the probability of exclusion of a bulky branching such as isobutyl from a crystalline lattice is considered to be twice as large as than that of ethyl branching.