Lamellar Thickness Distribution Research Articles

Effect of lamellae thickness on bubble dissolution in rotomoulding

AbstractThis paper is devoted to the study of bubble dissolution of the melted polymer in rotomoulding process. Two types of linear low‐density polyethylene (LLDPE) having the same properties, that is, melt index, melting point, crystallinity, viscosity, comonomer type, but different lamellae thickness were investigated. The results showed that bubbles in rotomoulded parts impaired the low‐temperature impact and LLDPE with thicker lamellae showed better bubble dissolution. Furthermore, as the annealing treatment was applied to the LLDPE powders, lamellae thickness distribution derived from Thomson–Gibbs equation coincided with bubble observation, suggesting that the improvement in bubble dissolution is reasonably attributable to annealing which makes the lamellae of treated LLDPE thick. In addition, the rheological measurements showed that modulus was built‐up with time indicating intermediate disentanglements are formed. Correlation between lamellae thickness distribution derived from differential scanning calorimetry data and experimental rheological result suggests that thicker lamellae is related to enhancing bubble dissolution. Consequently, it is reasonable to consider that the thicker lamellae is quite benefit for bubble dissolution, which could be well explained by the free volume according to free volume theory. LLDPE having thicker lamellae contains more free volume resulting in better bubble dissolution.

The nanostructuring of ultra-high-molecular-weight polyethylene in thermal and mechanical fields as revealed by DSC

The ultra-high-molecular-weight polyethylene reactor powders are widely used for the actively developing solvent-free method for producing high-strength high-modulus PE filaments, which includes the compaction and sintering of a powder followed by orientational hardening. To find an appropriate regime of the technological process, it is important to know how the nanostructure changes when transforming from a powder to a precursor for hardening. Nanocrystalline lamellae are characteristics of the powder structure. For the first time, the DSC technique was used to follow changes in the thickness distribution of lamellae in ultra-high-molecular-weight polyethylene reactor powder on its way to a precursor for orientation hardening. It was found that the percentage of thick (>15 nm) and thin (10 nm) lamellae in compacted samples and those sintered at temperatures lower than the melting temperature of PE (140°C) remains nearly the same. However, significant changes in the content of lamellae of different thicknesses were observed in the samples sintered at 145°C with subsequent cooling under different conditions. The influence of the lamellae thickness distribution in precursors on the mechanical characteristics of oriented filaments was discussed.

Lummus Novolen Technology awarded major polypropylene contract

Kinetic model of development of lamellar thickness distribution and average lamellar thickness during melt crystallization is proposed. The model bases on the Avrami–Evans and Hoffman–Lauritzen approaches to the crystallization and crystal growth kinetics and involves a thermal thickening model proposed in this paper which accounts for two processes – lamellar doubling transformation and logarithmic thickening of the doubled crystals. In the thickening model, continuity equation for the lamellar thickness distribution is proposed which includes a term responsible for fast creation of doubled crystals during a doubling time introduced as phenomenological parameter and a term which accounts for slow, logarithmic with time thickening subsequent to the doubling transformation. The model concerns melt crystallization in systems where the undercooling in the crystallization and melting processes refers to the same thermodynamic point – the thermodynamic equilibrium melting temperature.Analytical formulae for time-dependent lamellar thickness distribution vs. crystallization time are obtained by solving the continuity equation for the case of isothermal crystallization, as well as for the development of the average lamellar thickness of the doubled and un-doubled crystals during the crystallization time.The model predictions are illustrated by example computations for isothermal crystallization of PE grades characterized by high differences in the crystallization half-time resulting in high differences in the development of lamellar thickness distribution. Relative contributions of doubled and un-doubled crystals are predicted during the conversion and influence of the doubling time and logarithmic thickening parameters is discussed.

Modeling Crystallization Kinetics and Resulting Properties of Polyamide 6

This paper provides a model of the crystallization kinetics of polyamide 6 (PA6), including primary and secondary crystallization, lamellar thickness distribution, and the evolution of the mobile a...

Glass-Transition-Temperature-Independent Form II to I Phase Transition of Low-Molar-Mass Isotactic Polybutene-1

Temperature-dependent form II to I transition in isothermally crystallized low-molar-mass polybutene-1 has been investigated by means of fast scanning calorimetry. The as-crystallized sample possesses metastable form II crystals with a wide distribution of lamellar thickness and thus a wide distribution of the density of intercrystalline links between adjacent lamellae due to the high mobility of the chain segments within form II crystals and a limited chain length (low molar mass). Upon annealing the as-crystallized sample at different temperatures, the form II to I transition was found to occur in a wide temperature range irrespective of the glass transition temperature. The form I crystals transformed below glass transition possess a higher melting point than those transformed at a temperature close to the crystallization temperature. The results indicate that those lamellae with less or no intercrystalline links are not restricted by the vitrified amorphous phase. The mobility of chain segments in those well-crystallized form II crystals remains even at a temperature few tens of kelvin below the glass transition.

Effect of annealing on the microvoid formation and evolution during biaxial stretching of β nucleated isotactic polypropylene

ABSTRACT Cast films, based on β-iPP, were prepared via melt extrusion, and, then annealed at different temperature. The effect of annealing on the properties of the cast films, as well as on microvoid formation and evolution was investigated. After annealing, the initial lamellar structure was improved, and more secondary crystals were formed which decreased the numbers of tie chains, which was beneficial to cavitation. And the stability of β-crystal and β lamellar thickness distribution was improved. Finally, the membrane obtained by unannealed film had inferior pore-size distribution, while the annealed sample had uniform pore-size distribution.

Photooxidative degradation of graphene‐reinforced high‐density polyethylene nanocomposites

ABSTRACTNatural weathering of high‐density polyethylene (HDPE) and its graphene nanocomposites was studied. The nanocomposites, prepared by in situ polymerization of ethylene, were exposed to the natural environment for 30, 60, and 90 days. The effects of weathering on the crystallinity, melting temperature, and lamellar thickness distribution were studied using differential scanning calorimetry. The changes in chain microstructure and molecular weight were determined by crystallization analysis fractionation and gel permeation chromatography, respectively. The effects of weathering on the dynamic mechanical properties and the morphology of the nanocomposite were also determined. The parent HDPE suffered severe degradation as expected. However, graphene‐reinforced HDPE nanocomposites show resistance to degradation. The changes in the crystallinity, lamellar thickness, molecular weight, and dynamic mechanical properties of graphene nanocomposites were less pronounced as compared to the parent HDPE. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47030.

A General Approach for Estimating Lamella-Thickness Distribution in Polymers with Low-Frequency Raman Spectroscopy: Application to Lamella Formation in Crystallizing Polyethylene.

A general method for estimating lamella-thickness distribution in semicrystalline polymers has been developed and applied to polyethylene (PE). The longitudinal acoustic mode (LAM) of PE appears at very low frequencies (i.e., ν˜ 8-20 cm-1 ) in the Raman spectrum. It represents a distribution of lamellae of varying thicknesses. We propose a distribution function that converts a low-frequency LAM Raman band into the corresponding lamellae-thickness distribution. By using this distribution function, we can study lamella formation in crystallizing PE to elucidate the influence of supercooling and determine critical lamella thickness, the minimum chain length at which folding occurs, and the associated thermodynamic parameters accurately. This method has a general applicability toward the examination of polymer crystallization in an accurate and straightforward manner. Understanding the molecular details of polymer crystallization has applications, particularly in polymer thin-film photovoltaics and polymer processing, beyond its fundamental academic significance.

Polypropylene copolymer films: Heat sealing and oligomer/additive migration

Ease of heat sealing and oligomer/additive migration are two important and often interrelated phenomena that affect the engineering performance of high clarity polypropylene random copolymer (RCP) films. From a fundamental standpoint, this work investigates the non‐standard microstructural aspects that play a critical role in the heat sealing process of semicrystalline polymer films. Two different propylene–ethylene copolymers having widely different seal initiation temperatures (SITs) but similar melting points were chosen for this study. The contribution of both the amorphous phase and the crystalline phase to heat sealing was investigated utilizing dynamic mechanical analysis and thermal fractionation differential scanning calorimetry, respectively. Results indicate that the melting distribution of the thermally fractionated material plays a critical role in determining SIT while the effect of the mobility of the amorphous phase was found to be insignificant. In order to deeply understand the molecular origin of this difference in melting distribution or lamellar thickness distribution, the composition distribution of the co‐monomer was measured using Gel Permeation Chromatography‐Infra‐Red (GPC‐IR) and found to be fundamentally different. In addition, this work systematically studies the effect of oligomer/additive migration on optics, co‐efficient of friction (CoF), and most importantly heat sealing behavior. Migration is found to dramatically increase the SIT and haze while decrease the CoF of PP RCP films. POLYM. ENG. SCI., 58:1479–1484, 2018. © 2017 Society of Plastics Engineers

Investigation of deformation and pore formation in isotactic polypropylene containing active nano-CaCO3

In order to improve the β-lamellae distribution and properties of β-iPP membrane, 5% and 10% active nano-CaCO3 were added into β-iPP. The DSC, XRD and SEM results show that nano-CaCO3 would not reduce the content of β-crystal, but the thickness, lamellae thickness distribution and stability of β-lamellae decrease apparently. Then the tensile test was conducted at 25 °C and 90 °C. The results manifested that the second yield point, which has a strong negative correlation with lamellae thickness distribution, delays monotonously with adding nano-CaCO3 when stretched at 25 °C, indicating that nano-CaCO3 could narrow down the lamellae thickness distribution of β-iPP effectively. Furthermore, when stretched at 90 °C, the subdued yield peak, retarded necking down phenomenon and enhanced strain-hardening modulus demonstrated that the deformation stability improves gradually with introduction of nano-CaCO3, which has completely opposite law to the β-lamellae stability. Through further detailed characterization of morphological evolutions during stretching, we found that the interfacial debonding between nano-CaCO3 and β-iPP is triggered and abundant microviods can be formed, which can retard the rotation and slip of β-lamellae and make β-α transformation slow down in the initial stage of stretching, consequently make a better isotropic deformation. Moreover, nano-CaCO3 could restrain the formation of coarse fibrils efficiently, leading to more uniform pore size distribution within biaxial stretching microporous membrane. However, excessive nano-CaCO3 (10%) would cause the aggregation within the β-iPP cast film and finally resulted in larger pores and poor pore distribution in the membrane.

UHMW Ziegler–Natta polyethylene: Synthesis, crystallization, and melt behavior

Abstract The fabrication of normal and UHMW PE end-products involves melting and crystallization of the polymer. Therefore, the melt behavior and crystallization of as-synthesized UHMW PE, and NMW PE and E-1-hexene copolymer have been studied using a new nonisothermal crystallization model, Flory's equilibrium theory and ethylene sequence length distribution concept (SLD), Gibbs–Thompson equation, and DSC experiments. By using this approach, the effects of MW, 1-hexene incorporation, ethylene SLD, the level of undercooling θ , and crystal surface free energy D on crystallite stability, relative crystallinity α , instantaneous crystallinity χ , the crystallization kinetic triplet, crystallization entropy, and lamellar thickness distribution (LTD) have been evaluated. Consequently, this study reports insightful new results, interpretations, and explanations regarding the melting and crystallization of the aforementioned polymers. The UHMW PE results significantly differ from the NMW PE and E-1-hexene copolymer ones. Ethylene sequences shorter than the so called minimum crystallizable ethylene sequence length , irrespective of E-1-hexene copolymer MW, can also crystallize. Additionally, the polymer preparation shows that the catalyst coordination environment and symmetry, as well as achiral ethylene versus prochiral α -olefin steric encumbrance and competitive diffusion affect the synthesis of UHMW PE, particularly the corresponding UHMW copolymers.

An improved approach for evaluating the semicrystalline lamellae of starch granules by synchrotron SAXS

A fitting method combined with a linear correlation function was developed as an improved approach for the SAXS analysis of the semicrystalline lamellae of starch granules. Using a power-law function with two Gaussian plus Lorentz functions, the SAXS pattern was resolved into sub-patterns of the net lamellar peak and the power-law scattering plus scattering background (PL+B). The ratio of the net lamellar peak area (Apeak) to the total scattering area (Atotal) was proposed equal to the proportion of the lamellae within the starch granule (PSL). Along with this fitting method, we obtained a better profile of linear correlation function, with the elimination of the interference of non-lamellar amorphous starch (i.e., amorphous growth rings). Then, we could accurately calculate the lamellar parameters, e.g., PSL, the thicknesses of semicrystalline (d), crystalline (dc) and amorphous (da) lamellae, and the volume fraction (φc) of crystalline lamellae within semicrystalline lamellae. Quantitative analysis revealed that PSL was positively correlated with the crystallinity (Xc) of starch. It was confirmed that the distribution of lamellar thickness was more important than the starch botanical origin in affecting the validity of the developed fitting method. We also proposed a criterion to test the validity of the proposed method. Specifically, the total SAXS pattern should be mostly tangent to the profile of PL+B at a high q tail (close to 0.2Å−1).

Polyoxymethylene/Polyhedral Oligomeric Silsesquioxane Composites: Processing, Crystallization, Morphology and Thermo-Mechanical Behavior

Abstract Polyoxymethylene composites with octakis (dimethylsiloxy, ethyl epoxycyclohexy)octasilsesquioxanes and octakis(dimethylsiloxy, vinyl)octasilsesquioxanes contents (0.5 and 1 wt.%) were studied. The effect of silsesquioxanes (POSS) on crystallization and melting behaviour of polyoxymethylene (POM) was investigated by polarized light microscope (PLM), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM), with the aim to determine the structural properties like crystallization temperature, degree of crystallinity and lamellar thickness distributions. It was found that POSS as nucleating agent affects the crystallization process of POM, leading to a homogeneous structure, improving the thermo-mechanical properties of polyoxymethylene composites. Based on SEM observations it was found that the POSS particles in a concentration of 0.5 wt.% are uniformly dispersed in polyoxymethylene matrix. The storage modulus and relaxations temperatures of POM with POM/0.5 wt.% ehPOSS and vinPOSS are slightly increased.

Crystallization behaviour and lamellar thickness distribution of metallocene‐catalyzed polymer: Effect of 1‐alkene comonomer and branch length

AbstractThe effect of comonomer and branching on the melt crystallization and lamellar thickness distribution was studied for ethylene and 1‐alkene copolymers. The comonomers used in this study are 1‐hexene, 1‐octene, and 1‐decene. A notable influence of the comonomer ratio in the feed was observed on the crystallization and melting behaviour. The Ozawa and Mo models were found suitable for these copolymers. However, variation of relative crystallinity at different heating rates preferred the Mo method over the Ozawa method. The melting behaviour and lamellar thickness distribution of the copolymers were analyzed by the help of the modified Gibbs‐ Thomson equation. The activation energies (EA) for the melt crystallization were calculated using the Kissinger method. It was observed that 1‐hexene comonomer exhibits lower EA, indicating an easier crystallization process as compared to other comonomers used. Overall, crystallization was found to be more influenced by the degree of branching rather than the comonomer type.

Spherulitic formation and characterization of partially fluorinated copolymers and their nanohybrids with functional fillers

Poly[vinylidenefluoride-co-(tetrafluoroethylene)] (P(VDF-TeFE)) exhibited clear spherulitic texture with negative birefringence. The number and growth rates of the spherulites decreased at high crystallization temperature than at low crystallization temperature. Nonetheless, overall larger spherulites were found at high crystallization temperature and the brightness of the spherulites increased very fast at low crystallization temperature, thereafter it seemed as diminution of birefringence. AFM was used to investigate the impact of organo modified nanodiamond(ND) on spherulitic textures, lamellar thickness, and thickness distribution of P(VDF-TeFE) copolymer. Poly[ethylene-co-(tetrafluoroethylene)] (ETFE) also confirmed spherulites structure and the boundaries could be clearly observed. By incorporation of the organo modified nanodiamond (ND) and organo-modified montmorillonite (MMT) in fluropolymer matrix, it was found that spherulitic texture was seriously disordered and their nanohybrids was found only to have poorly developed spherulite structure. Both of the nanohybrids samples show better crystallization temperature as compared to their neat copolymer samples. Furthermore, the incorporation of nanoparticles decreased the size of the spherulites. POLYM. ENG. SCI., 2016. © 2016 Society of Plastics Engineers

Effects of stereo-defect distribution on the crystalline morphology and tensile behavior of isotactic polypropylene prepared by compression molding process

ABSTRACTIn this study, the aggregation morphology, tensile behavior, and morphology evolution during the tensile test of two isotactic polypropylene (iPP) samples with similar molecular weight and average isotacticity but different uniformities of stereo-defect distribution are investigated by differential scanning calorimetry (DSC), two-dimensional wide angle X-ray diffraction (2D-WAXD), and scanning electronic microscopy (SEM). The results revealed that the uniformity of stereo-defect distribution of iPP determines the crystalline structure and aggregation morphology, and further influences the tensile behavior and morphology evolution during the tensile test. For PP-A with less uniform stereo-defect distribution, its ability of crystallization is stronger compared with PP-B, resulting in smaller spherulite sizes, higher melting point and degree of crystallinity, and narrower distribution of lamellar thickness of the compression molding specimens. During the tensile test, mainly the inter-spherulite deformation takes place at the early stage for deformation, which further results in drastic deformation of lamellar and high degree of reorientation at the strain increases, exhibiting higher yield strength and elastic modulus, and lower elongation at break compared with PP-B; for PP-B with more uniform stereo-defect distribution, larger spherulite sizes, lower melting point and degree of crystallinity in its compression molding sample are observed. During the tensile test, intra-spherulite deformation mainly takes place, which can disperse the tensile stress more uniformly. As the strain increases, lower degree of crystalline destruction and reorientation of the crystallites take place. The yield strength and elastic modulus of PP-B is lower than PP-A, and its elongation at break is higher.

The effect of temperature on the physicochemical properties and lamellar structure of canna starch subjected to enzymatic degradation

Electron density distribution function of paracrystalline model for starch lamellar structure (ρc: crystal lamellar density; ρa: amorphous lamellar density; Δρ: density differences; d: long period; da: amorphous thickness; dc: crystal thickness).

The prediction of mechanical performance of isotactic polypropylene on the basis of processing conditions

A strategy is presented to predict the yield kinetics following from different thermomechanical histories experienced during processing in non-isothermal quiescent conditions. This strategy deals with three main parts, i.e. processing, structure and properties. In the first part the applied cooling conditions are combined with the crystallization kinetics and the cooling history of the material is calculated. From this history the lamellar thickness distributions are predicted in the second part. Finally, in the third part these distributions are used to predict yield stresses. Experimental validation is carried out for all the different parts of the strategy. In situ temperature measurements, lamellar thickness distributions from SAXS experiments and yield stresses measured in uniaxial tensile deformation are performed for validation purposes. The versatility is investigated by applying this procedure on two different iPP grades. The yield stress predictions show good agreement with the experimentally obtained results in two separate deformation mechanisms, and only a few parameters are dependent on the specific iPP grades that were used here. Moreover, it is shown that the average lamellar thickness is sufficient to predict the yield stress, and that the width of lamellar thickness distributions does not have to be taken into account.

Method for Calculation of the Lamellar Thickness Distribution of Not-Reorganized Linear Polyethylene Using Fast Scanning Calorimetry in Heating

The lamellar thickness distribution (LTD) of not-reorganized linear polyethylene was calculated on the basis of the melting point distribution using fast scanning calorimetry (FSC) by applying the following precautions. First, by taking sufficiently small sample mass and thickness, the influence of thermal lag during fast-heating is thought to be negligible. Second, by fast-heating, reorganization and cold crystallization are strongly hindered or even suppressed. Third, the influence of superheating has been accounted for and corrected by deconvolution of the FSC curve using the calculated melting kinetics of single-sized lamellae consisting of folded-chain crystal. Under such precautions, the resulting FSC heating curve reflects the melting point distribution of metastable-but-not-reorganized folded-chain crystals. Finally, the Gibbs–Thomson equation was applied to calculate the LTD from the melting point distribution. The average lamellar thickness calculated was in good agreement with that determined by small-angle X-ray scattering and low-frequency Raman spectroscopy.

Kinetic model of polymer crystallization with the lamellar thickness distribution

Kinetic model of development of lamellar thickness distribution and average lamellar thickness during melt crystallization is proposed. The model bases on the Avrami–Evans and Hoffman–Lauritzen approaches to the crystallization and crystal growth kinetics and involves a thermal thickening model proposed in this paper which accounts for two processes – lamellar doubling transformation and logarithmic thickening of the doubled crystals. In the thickening model, continuity equation for the lamellar thickness distribution is proposed which includes a term responsible for fast creation of doubled crystals during a doubling time introduced as phenomenological parameter and a term which accounts for slow, logarithmic with time thickening subsequent to the doubling transformation. The model concerns melt crystallization in systems where the undercooling in the crystallization and melting processes refers to the same thermodynamic point – the thermodynamic equilibrium melting temperature.Analytical formulae for time-dependent lamellar thickness distribution vs. crystallization time are obtained by solving the continuity equation for the case of isothermal crystallization, as well as for the development of the average lamellar thickness of the doubled and un-doubled crystals during the crystallization time.The model predictions are illustrated by example computations for isothermal crystallization of PE grades characterized by high differences in the crystallization half-time resulting in high differences in the development of lamellar thickness distribution. Relative contributions of doubled and un-doubled crystals are predicted during the conversion and influence of the doubling time and logarithmic thickening parameters is discussed.