Molecular dynamics simulation for polyethylene crystallization: Effect of long chain branches (original) (raw)
Related papers
Polymer, 2015
We have performed molecular dynamics simulations to study the mechanism of crystallization from an undercooled polyethylene (C500) melt. We observe that crystal nucleation is initiated by the alignment of chain segments, which is followed by straightening of the chains and densification. Growth procedes via alignment of segments, which are in the vicinity of the growth front, with the chains in the crystalline lamella. Once chains are attached, the lamella thickens by sliding of the segments along the long axis of the chain from the amorphous regions into the crystalline regions. We do not observe the formation of any folded precursors.
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.
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.
Journal of Polymer Science Part B: Polymer Physics, 2015
The influence of short-chain branching on the formation of single crystals at constant supercooling is systematically studied in a series of metallocene catalyzed highmolecular-weight polyethylene samples. A strong effect of short-chain branching on the morphology and structure of single crystals is reported. An increase of the axial ratio with short-chain branching content, together with a characteristic curvature of the (110) crystal faces are observed. To the best of our knowledge, this is the first time that this observation is reported in high-molecular-weight polyethylene. The curvature can be explained by a continuous increase in the step initiation-step propagation rates ratio with short-chain branching, that is, nucleation events are favored against stem propagation by the presence of chain defects. Micro-diffraction and WAXS results clearly indicate that all samples crystallize in the ortho-rhombic form. An increase of the unit cell parameter a 0 is detected, an effect that is more pronounced than in the case of single crystals with ethyl and propyl branches. The changes observed are compatible with an expanded lattice due to the presence of branches at the surface folding. A decrease in crystal thickness with branching content is observed as determined from shadow measurements by TEM. The results are in agreement with additional SAXS results performed in single crystal mats and with indirect calorimetry measurements.
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.
Macromolecules, 1990
Molecular dynamics is used to study the melting on the surface of a polyethylene-like crystal. The rate constant for melting of a crystalline molecule without and with one to four folds is determined at several different temperatures and molecular lengths. The results show a strong dependence of the transition rate on the number of folds. For a constant lamellar thickness, the transition rate decreases with increasing number of folds for temperatures near the equilibrium melting temperature, as expected from analogy with experimental melting temperatures. In contrast, the transition rate increases with increasing number of folds for temperaturesthat exceed the equilibrium melting temperature by more than 100 K. Two melting paths are suggested to explain the simulation data. One pathway involves a competition between melting and crystallization. This pathway leads to a decreasing transition rate as a function of increasing folding. The second pathway exhibits dominating melting. In this case, the rate of transition tends to increase with increasing number of folds. Diffusion coefficients of segments at different locations along the chain show that motion of the ends of a polymer chain or of the folds is faster than in the center of the stem. The overall effect of increasing temperature is to increase the diffusion coefficients.
Molecular Dynamics of Nucleation and Crystallization of Polymers
Crystal Growth & Design, 2001
Nucleation and crystallization of isotactic polypropylene (iPP) has been simulated using molecular dynamics (MD) with and without nucleating agents. MD simulations were carried out with four iPP chains of 90, 180, 270, and 360 backbone carbons each in the presence of a number of derivatives of dibenzylidene sorbitol (DBS) as nucleators. A novel method was developed to analyze the results of molecular dynamics and to elucidate the molecular phenomenon of nucleation in the early stages of crystallization. This new method retrieves and compares the backbone conformation of iPP in the liquid state to that available in the crystalline form and enumerates the number of crystallizing sites (or nuclei) formed during the simulation. The comparison of the number of nuclei formed, during the simulation of iPP crystallization with and without nucleators, allows the comparison of the effectiveness of nucleators and therefore can be used as a screening tool to screen a large number of potential nucleators. Simulation results are compared to literature data and indicate the method can distinguish nucleator effectiveness.
e-Polymers, 2010
The isothermal crystallization behavior of two different high density polyethylene grades with monomodal and bimodal molar mass distribution was investigated by means of differential scanning calorimetry. The results indicate that extensive cocrystallization between linear short chains and long chains with short branches in bimodal polyethylene grade occurred. In contrast, polymer chains of different lengths in monomodal polyethylene exhibit different tendency to crystallize. This finding was explained qualitatively based on a general discussion of the effect of molar mass and branch content/length on crystallization tendency of polymeric chains.