The influence of short-chain branching on the morphology and structure of polyethylene single crystals (original) (raw)
Related papers
New habits in branched polyethylene single crystals
European Polymer Journal, 2016
Faceted polymer single crystals obtained from solution have been examined for decades. Most of the studies have been performed with linear polyethylene. It is well known that solution-grown linear polyethylene single crystals may exhibit different and often complex morphologies and habits. However, the effect of molecular architecture (i.e., short chain branching) in the morphology of the single crystals remains practically unexplored. At the highest crystallization temperature investigated, the shape of the crystals is lozenge-like, but with curved and slightly truncated {110} faces. As the crystallization temperature decreases, strong changes in the width-to-length ratio and, consequently in the characteristic angles of the single crystal occur. Interestingly, the single crystals obtained at the lowest crystallization temperature explored exhibit a nearly square shape. This phenomenon has not yet been observed in linear high molecular weight polyethylene, for which the characteristic lozenge habit with straight {110} faces is retained as crystallization temperature decreases. The application of the Shcherbina and Ungar approach to fit the unusual shapes of the branched PE single crystals obtained in this work requires a strong decrease in the ratio of the rates of propagation to the right and to the left of the growth edge. This result is probably linked to different steric conditions of chain attachments into non-equivalent niches induced by the presence of branches.
1991
Works on strictly uniform ultra-long n-alkanes enabled the exploration of the onset of chain folding with increasing molecular length. It was established that folding sets in beyond a certain chain length, more specifically dependant on crystallization temperature (T,), starting in an initially irregular chain deposition, reorganizing subsequently into a strictly quantized conformation of integral fractional fold lengths through either thickening or thinning of the crystal. In the final reorganized stage the folds are regular and sharp. The isothermal crystallization rates for extended chain crystals were found to go through a maximum followed by a minimum with decreasing T, where chain folding takes over. This remarkable rate inversion, observed for c 2 4 6 H494 and c198 H398, so far, occurs both for solution and melt crystallization and could be verified for both primary nucleation and crystal growth. We interpret it in terms of a "self poisoning" phenomenon, where chain depositions, which occur transiently in the "wrong" conformation, are blocking the nucleation and growth of the crystal, a phenomenon also reflected in the reversal of the temperature dependence of isothermal refolding on crystallization from solution. Rate reversals of all these kinds promise to be basic to our understanding and are giving rise to recent alternative explanations from elsewhere which are being quoted and discussed.
Size distribution of folded chain crystal nuclei of polyethylene on active centers
The Journal of Chemical Physics, 2011
Kinetic equations describing temporal evolution of the size distribution of crystalline nuclei of folded chain polyethylene on active centers are solved numerically. Basic characteristics of nucleation processes (the total number of supercritical nuclei and the size distribution of nuclei) are determined and compared with the experimental data. It is shown that even though the total number of supercritical nuclei coincides with the experimental data, the size distribution prediction fails. This is caused by the fact that the total number of nuclei (usually used in analysis of the experimental data), in contrast to the size distribution of nuclei, represents an integral quantity. Using the experimental data of the steady state size distribution of nuclei enables us to determine thermodynamic parameters (especially interfacial energies) of the studied system more precisely and consequently to correct kinetic parameters to get coincidence of kinetic model with the experimental data in ...
Polymer Science Series B, 2014
In this research, the effect of long chain branching (LCB) and polymerization conditions on ther mal, mechanical, and rheological properties of polyethylene synthesized via a metallocene polymerization was studied. The LCB was varied in the range of 0.64-1.14 per 10 4 atoms of C. 13 C NMR spectra showed that the distributions of both short as well as long chain branches in the polymer backbone chain are influenced by polymerization conditions. The increase in ethylene pressure leads to rise in polymer yield, catalyst activ ity, molecular weight, and narrowing of molecular weight distribution. In contrast, the increase of polymer ization duration results in broadening of MWD and a decrease in catalyst activity. In addition, the influence of frequency and LCB on dynamic shear and extensional melt rheology has been reported. The polymer crys tallization was discussed in light of Avrami model and modified Hoffman Lauritzen theory. LCB promoted the transport of chain segments but retarded the nucleation in polyethylene crystallization. The tensile strength decreased with the increase in LCB content. The different macroscopic properties were correlated to LCB content.
Linear low-density polyethylene (LLDPE) chains with different levels of branch content (BC), ranging from 10 to 80 branches/1000 C, distributed uniformly along the chain are simulated in vacuum at a temperature of 350K. The influence of BC on the relaxation and crystallization of LLDPE chains is studied. The collapse of the branched chains is found to occur via a local followed by a global collapse mechanism with branches acting as nucleation points for the collapse of the molecule leading to faster collapse of chains with higher BC. Branches are observed to selfassemble away from the backbone at high BC. Radius of gyration correlates to M w by a power-law relationship: R g =K s α w M with α=0.35. The trans population is found to be dominant at all branch contents; however, it decreases with increasing BC. Increasing BC is found to decrease order and to strongly influence chain conformation. Chain conformation undergoes a transition from lamellar to a more random coil-like structure near a critical BC of 50 branches/1000 C. Branches are observed to be excluded from the lamella and to self assemble at high BC. This work also provides insight into the conformation adopted during the coil-globule transition experienced by a single chain in an infinitely dilute solution much below the θ temperature.
Materials Science, 2011
A bimodal high density polyethylene (HDPE) has been successfully fractionated by analytical size exclusion chromatography into molar mass fractions with M w 's ranging from 3.6 kg/mol to 8 000 kg/mol, and subsequently deposited on germanium disks using the Lab Connections Transform method. After removal of the fractions from the disks, having masses in between 10 µg-150 µg, differential scanning calorimetry has been successful in measuring the (re)crystallization and melting behavior of these fractions. Comparing the crystallization and melting peak temperatures of the fractions with those of narrow molar mass linear polyethylenes points to the HDPE being linear below and short chain branched above 100 kg/mol respectively. This value coincides roughly with the 'split' between the molar mass distributions resulting from the first and the second polymerization reactor-confirming the addition of 1-butene in the second reactor.
Molecular dynamics simulation for polyethylene crystallization: Effect of long chain branches
Polyolefins Journal, 2021
The influence of long branches on crystallization behavior has been studied by means of molecular dynamics simulations. Using two systems: polyethylene (PE) with long branches (LCB-PE) and PE without long branches (linear-PE) with the same molecular weight, we have examined the crystallization behavior of the two systems by molecular dynamics simulation. This paper explains the influence of long branches on the isothermal crystallization process and the non-isothermal crystallization process with similar initial interchain contact fraction (ICF) in terms of final ICF, crystal regions, crystallinity, concentration of tie chains and energy. It is found that the crystallization process is classified as two stages: the nucleation stage and the crystal growth stage. The existence of long branches is favorable for the first stage while unfavorable for the second stage. Knots that act as crystalline defects are excluded from the lamella, resulting in decreasing in regularity and crystallinity of molecular chains. From the perspective of potential energy and non-bond energy, LCB-PE has lower energy than linear-PE in the nucleation stage while the energy of linear-PE is lower than that of LCB-PE in the second stage. In short, the long branched chains inhibit the crystallization process.
New insights into the characteristics of early stage crystallization of a polyethylene
Polymer, 2007
Small angle light scattering has been used to probe structure formation during isothermal crystallization of an ethylene-1-hexene copolymer (EH064, M w ¼ 70,000 g/mol, r ¼ 0.900 g/cm 3 , M w /M n w 2, 6.4 mol% hexene). It is shown that clear structural information on size scales ranging from hundreds of nanometers to several micrometers during early stage crystallization can be obtained by this method when crystallizing the polyethylenes at the high temperatures (above the peak melting temperature of a rapidly crystallized polymer sample) required for resolving early stage crystallization without the influence of the crystal growth. The results show that the early stage crystallization is characterized by large scale orientation fluctuations that precede the formation of local crystalline order manifest in X-ray scattering and the initial collapse of these large scale anisotropic/ordered domains. The scattering intensity increases exponentially with time initially, and the wave vector dependence of the growth rate of fluctuations is consistent with predictions for initial stages of a phase transformation process. However, the detailed mechanism cannot be described by existing models. The implications of our results are discussed within the context of proposed models for early stage crystallization.
Journal of Polymer Science Part B: Polymer Physics, 2008
Effect of molar mass distribution (MMD) and composition distribution (CD) on crystallization behavior of linear low-density polyethylene materials at moderate and high supercooling was studied using differential scanning calorimetry, hotstage polarized light microscopy, small-angle light scattering, and chip nanocalorimetry methods. A set of uni-and bimodal materials having small variation in average molar mass, density, and melt flow rate, but large differences in MMD and CD, was investigated. The results indicate a clear effect of structural heterogeneity on morphology and crystallization behavior of the materials. Broader MMD and CD increased in average radius of superstructures, melting, crystallization temperatures, and isothermal crystallization rate at different supercoolings. Origin of such behavior is discussed.
Crystal nucleation kinetics of polyethylene on active centers
Journal of Crystal Growth, 2014
Model of nucleation on active centers is applied to formation of folded chain crystals of polyethylene at low supercooling. Nucleation rate and the total number of nuclei are determined by numerical solution of kinetic equations. Thermodynamic and kinetic parameters were chosen to get the coincidence with experimental data for the total number of nuclei and the size distribution of nuclei as a function of time. Nucleation rates for various cluster sizes and the exhaustion of active centers due to the phase transition process were determined. It is shown that the number of exhausted active centers, on which crystalline nuclei are formed, increases with time.