Effect of small amount of ultra high molecular weight component on the crystallization behaviors of bimodal high density polyethylene

In order to clarify the effect of high molecular weight component on the crystallization of bimodal high density polyethylene (HDPE), a commercial PE-100 pipe resin was blended with small loading of ultra high molecular weight polyethylene (UHMWPE). The isothermal crystallization kinetics and crystal morphology of HDPE/UHMWPE composites were studied by differential scanning calorimetry (DSC) and polarized optical microscopy (POM), respectively. The presence of UHMWPE results in elevated initial crystallization temperature of HDPE and an accelerating effect on isothermal crystallization. Analysis of growth rate using Lauritzen-Hoffman model shows that the fold surface free energy (σ_e) of polymer chains in HDPE/UHMWPE composites was lower than that in neat HDPE. Morphological development during isothermal crystallization shows that UHMWPE can obviously promote the nucleation rate of HDPE. It should be reasonable to conclude that UHMWPE appeared as an effective nucleating agent in HDPE matrix. Rheological measurements were also performed and it is shown that HDPE/UHMWPE composites are easy to process and own higher melt viscosity at low shear rate. Combining with their faster solidification, gravity-induced sag in practical pipe production is expected to be effectively avoided.     

A different approach to morphological diversity and surface nucleation in linear polyethylene

The classical Kossel-Stranski crystal model is adapted to a critical re-examination of surface nucleation in polyethylene. Several aspects of the diverse morphologies displayed by this polymer are well accounted for. The suggestion is made that curved {200} faces in lenticular crystals arise as a consequence of normal growth rather than nucleated growth on these faces. Processes generally involved in surface nucleation and subsequent layer spreading are discussed with emphasis upon entropic considerations. This leads to the view that, although nucleation kinetics are followed, molecular conformations can become more tightly clumped than is commonly thought to follow inevitably from layer spreading.     

Effects of high molecular weight component on crystallization of polyethylene under shear flow

Crystallization of polyethylene (PE) blends of low and high molecular weight components under shear flow was studied using time-resolved depolarized light scattering (DPLS), focusing on effects of the high molecular weight component on the shish-kebab structure formation. Anisotropic two-dimensional scattering pattern due to shish-like structure formation was observed above a certain concentration of the high molecular weight PE. The threshold was about 2.5-3 times larger than the chain overlap concentration, suggesting an important role of entanglements of the high molecular weight component. On the basis of these results a gel-spinning-like mechanism for the shish-like structure formation has been proposed. The DPLS results also implied that the shish-like structure was mainly formed from the high molecular weight PE. This was confirmed by small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) measurements on an elongated PE blend of low molecular weight deuterated PE and high molecular weight hydrogenated PE (3 wt%).     

Roles of cyclic units in copolyethylene crystallization: a molecular dynamics simulation

Molecular dynamics (MD) simulations of several polyethylene copolymer chains containing 1,2-, 1,3- or 1,4-disubstituted cyclopentane or hexamethylene structures in the main chain (with 500 CH₂) are performed to investigate the influence of cyclic units on the crystallization properties of polyethylene (PE). From the isothermal relaxation process it is found that they generally collapse to a globule via a local collapse process. The copolymer chains containing 1,2-disubstituted cycloparaffin structures form more kinks and take shorter time to totally collapse into a single globule than the others. Moreover, from the morphology of the crystal structures after annealing it is found that the copolymer chains containing 1,2-disubstituted cycloparaffin structures can yield more ordered structures with cyclic units rejected to the fold surface. For the copolymer chains containing 1,3- or 1,4-disubstituted cycloparaffin, the lamellar structures are not perfect and some cyclic units are always incorporated in the crystalline phase.     

Molecular dynamics simulation of orientation and crystallization of polyethylene during uniaxial extension

Molecular dynamics simulations of realistic, united atom models of polyethylene undergoing uniaxial extension are described. Systems composed of chains ranging from 25 to 400 carbons have been studied, under a variety of processing histories, including isothermal deformation at constant applied stress below the melt temperature T_m, isothermal deformation below T_m followed by annealing, isothermal deformation above T_m followed by crystallization at a quench temperature below T_m, and non-isothermal crystallization during simultaneous deformation and cooling through T_m. Extension and orientation of large segments of flexible chains by uniaxial deformation accelerates the primary nucleation rate to a time scale accessible by molecular dynamics simulation. Entanglements operative during active deformation promote extension and orientation without nucleation of a crystal phase, while relaxation of stress at constant strain is sufficient to allow slippage of chains past pinning points and rapid nucleation and growth of crystallites as neighboring oriented chains come into registry. Isothermal crystallization of pre-oriented systems shows an apparent increase in nucleation density at lower temperatures; the resulting ordered regions are smaller and more closely aligned in the direction of orientation. During non-isothermal deformation, where stretching and cooling occur simultaneously, a first order transition is observed, with discontinuities in the volume and global order parameter, when the system crystallizes.     

The crystallization of low-density polyethylene: a molecular dynamics simulation

Three models (star-shaped, H-shaped, and comb-shaped polyethylenes) are used to study the crystallization behavior of low-density polyethylene at the molecular level by means of molecular dynamics simulation. It is shown that, for the three types of polyethylene corresponding to the models, the neighboring sequences of trans bonds firstly aggregate together to form local ordered domains, and then they coalesce to a lamellar structure. In the process, the branching sites are rejected to the fold surface gradually. The driving force for the relaxation process is the attractive van der Waals interaction between the chain segments. Furthermore, it is found that the number of the branch sites and the length of the branch play an important role in determining the formation of the lamellar structure. The longer the length of the branch and the fewer the number of the branch sites, the more perfect lamellar structure can be formed.     

Influence of molecular composition on the development of microstructure from sheared polyethylene melts: Molecular and lamellar templating

A combination of in situ and ex situ X-ray scattering techniques and transmission electron microscopy has been used to study the crystallization behaviour of polyethylene, following the imposition of melt shear. In the case of a branched material, the imposition of shear flow up to a rate of 30 s⁻¹ was found to induce no anisotropy. Although shearing the linear material only ever induced a very small degree of anisotropy in the melt, for shear rates >0.15 s⁻¹, subsequent crystallization resulted in increasing anisotropy. Blends of the above two polyethylenes were produced, in which the linear material constituted the minority fraction (∼10%). Isothermal crystallization at temperatures where extensive crystallization of the branched material does not occur demonstrated that the behaviour of the linear component of the sheared blend mirrored that of the linear polyethylene alone. However, in addition, it was found that when crystallized in the presence of an oriented morphology, the branched polymer also formed anisotropic structures. We have termed the process templating, in which the crystallization behaviour of the bulk of the system (∼90% branched material) is completely altered (spherulitic to oriented lamellar) by mapping it onto a pre-existing minority structure (∼10% linear polymer).     

Crystallization regimes and reptation in polypropylene molecular weight fractions

Crystal growth rate data based on the kinetic nucleation theory of chain folding and the effect of reptation, have been used to predict the rate of crystal growth at moderate to high supercoolings in iPP molecular weight fractions. Growth rate data obtained for the fractions seem to be in agreement with the theoretical predictions of the regime theory. However, an extension of the gambler ruin treatment to the iPP data has not been successful with regard to the dominant morphology in regime II. The variable cluster model suggested as the morphology for polyethylene in regime II does not appear to be evident from this study. The effect of polydispersity, molecular weight, and tacticity on the crystallization behavior of iPP fractions have also been studied and correlated with the structure of polymer samples investigated. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 579–584, 1999     

Crystallization of linear low density polyethylene under two-dimensional confinement in high barrier blend systems

Blends of linear low density polyethylene (LLDPE) and ethylene vinyl alcohol (EVOH) with different weight fractions are extruded to fabricate thin films. The extruded blend film morphology is investigated by atomic force microscopy (AFM). The extruded blend films have shown extended morphology along the extrusion direction (ED) and dispersed morphology along the transverse direction (TD). We report that due to this morphology, a two-dimensional (2-D) confined crystallization occurs. The EVOH has successfully confined the LLDPE from both film normal direction (ND) and transverse direction (TD) in this study. The confinement from ND results in an on-edge orientation of LLDPE, while the confinement from TD forces the on-edge oriented LLDPE crystals to further elongate and extend along the extrusion direction (ED). This specific crystal orientation is different from one-dimensional (1-D) confined crystallization observed in multilayered films. Both wide angle X-ray scattering (WAXS) and small angle X-ray scattering (SAXS) are utilized to investigate the crystal orientations of LLDPE in the extruded blend films. Moreover, due to the morphology, the extruded blend films have shown high oxygen barrier properties, which make this material valuable in packaging applications.     

Effect of molecular weight on the morphology and drawing behaviour of melt crystallized linear polyethylene

The effect of molecular weight on the cold drawing behaviour of melt crystallized linear polyethylene has been studied. It is shown that the draw ratio achieved under comparable conditions rises with decreasing $\text{M}\text{?}{}_{\mathrm{w}}$, very high draw ratios (～36) being possible for optimum morphology of the undrawn polymer. The yield behaviour was also examined, and it is shown that the yield stress is affected in a complex fashion by both crystallization conditions and molecular weight. These results are discussed in terms of the crystallization and morphology of the melt crystallized polymer.     

On the time for fold surfaces to order during the crystallization of polyethylene from the melt and its dependence on molecular parameters

New experiments underpin the interpretation of the basic division in crystallization behaviour of polyethylene in terms of whether or not there is time for the fold surface to order before the next molecular layer is added at the growth front. For typical growth rates, in Régime II, polyethylene lamellae form with disordered {001} fold surfaces then transform, with lamellar thickening and twisting, towards the more-ordered condition found for slower crystallization in Régime I, in which lamellae form with and retain {201} fold surfaces. Several linear and linear-low-density polyethylenes have been used to show that, for the same polymer crystallized alone or in a blend, the growth rate at which the change in initial lamellar condition occurs is reasonably constant thereby supporting the concept of a specific time for surfaces to attain the ordered {201} state. This specific time, in the range from milliseconds to seconds, increases with molecular length, and in linear-low-density polymer, for higher branch contents.     

A colorimetric method for the molecular weight determination of polyethylene glycol using gold nanoparticles

A gold nanoparticle (AuNP)-based colorimetric method was developed for the molecular weight (MW) determination of polyethylene glycol (PEG), a commonly used hydrophilic polymer. Addition of a salt solution to PEG-coated AuNP solutions helps in screening the electrostatic repulsion between nanoparticles and generating a color change of the solutions from wine red to blue in 10 min in accordance with the MW of PEG, which illustrates the different stability degrees (SDs) of the AuNPs. The SDs are calculated by the absorbance ratios of the stable to the aggregated AuNPs in the solution. The root mean square end-to-end length (〈h²〉^1/2) of PEG molecules shows a linear fit to the SDs of the PEG-coated AuNPs in a range of 1.938 ± 0.156 to 10.151 ± 0.176 nm. According to the Derjaguin-Landau-Verwey-Overbeek theory, the reason for this linear relationship is that the thickness of the PEG adlayer is roughly equivalent to the 〈h²〉^1/2 of the PEG molecules in solution, which determines the SDs of the AuNPs. Subsequently, the MW of the PEG can be obtained from its 〈h²〉^1/2 using a mathematical relationship between 〈h²〉^1/2 and MW of PEG molecule. Applying this approach, we determined the 〈h²〉^1/2 and the MW of four PEG samples according to their absorbance values from the ordinary ultraviolet–visible spectrophotometric measurements. Therefore, the MW of PEG can be distinguished straightforwardly by visual inspection and determined by spectrophotometry. This novel approach is simple, rapid, and sensitive.     

Molecular weight dependence of growth and morphology of it-poly(butene-1) spherulites

Molecular weight dependence of growth and morphology of spherulites of isotactic poly(butene-1), iPB-1, and those of the mixtures with atactic poly(butene-1), aPB-1, were examined by atomic force microscopy (AFM) and polarizing optical microscopy (POM) in order to examine the mechanism of the structural evolution by the branching and re-orientation of lamellar crystals at the growth front. The width of lamellar crystals and the characteristic size of the inner structure of spherulites decreased with increasing molecular weight. The result suggests that the mobility of the melt determines the sizes in spherulites and supports the growth front instability induced by a gradient triggering the branching. The sizes in the mixtures also decreased with increasing weight-averaged molecular weight, M_w. The size dependence in low M_w region, however, was too strong and that in high M_w was too weak in comparison with the predicted dependence for the prepared M_w. It has been concluded that the peculiar behaviors should be discussed with effective M_w influenced by the occurrence of separation and exclusion of non-crystallizing aPB-1 at the growth front.     

Ultradrawing behavior of one- and two-stage drawn gel films of ultrahigh molecular weight polyethylene and low molecular weight polyethylene blends

The ultradrawing behavior of gel films of plain ultrahigh molecular weight polyethylene (UHMWPE) and UHMWPE/low molecular weight polyethylene (LMWPE) blends was investigated using one- and two-stage drawing processes. The drawability of these gel films were found to depend significantly on the temperatures used in the one- and two-stage drawing processes. The critical draw ratio (λ_c) of each gel film prepared near its critical concentration was found to approach a maximum value, when the gel film was drawn at an “optimum” temperature ranging from 95 to 105°C. At each drawing temperature, the one-stage drawn gel films exhibited an abrupt change in their birefringence and thermal properties as their draw ratios reached about 40. In contrast, the critical draw ratios of the two-stage drawn gel films can be further improved to be higher than those of the corresponding single-stage drawn gel films, in which the two-stage drawn gel films were drawn at another “optimum” temperature in the second drawing stage after they had been drawn at 95°C to a draw ratio of 40 in the first drawing stage. These interesting phenomena were investigated in terms of the reduced viscosities of the solutions, thermal analysis, birefringence, and tensile properties of the drawn and undrawn gel films. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 149–159, 1998     

UHMWPE/CNTs复合纤维的结晶行为研究

分别用DSC、X衍射、热台偏光显微镜对超高分子质量聚乙烯（UHMWPE）和UHMWPE／CNTs（碳纳米管）复合纤维的结晶行为进行了研究。结果表明：碳纳米管的加入使得复合材料的熔点较UHMWPE有所提高,碳纳米管起到了成核剂的作用。晶片厚度较UHMWPE增加。     

Molecular weight scaling of the spherulite growth rate in isothermally melt crystallized polyethylene nanocomposites

The spherulite growth rate, G_II, was measured for three monodisperse linear polyethylenes filled with up to 4 vol. % of SiO₂ nanoparticles in the crystallization regime II of small undercooling, ΔT. The fumed SiO₂ used did not exhibit any measurable nucleation activity. The G_II scaled with the number average molecular weight, M_n, as M_n^ν with the scaling exponent, ν, equal to (2.2 ± 0.1). This corresponds to the reptation controlled surface self-diffusion of loop-train adsorbed chains with the contour length fluctuation (CLF) and the chain constraint release (CR) contributions. In order to verify the hypothesis of the chain reptation as the molecular mechanism responsible for the chain transport, logG_II was plotted against the logarithm of the number of effective entanglements per chain, logN_eff. The N_eff was the sum of the number of “true” entanglements in the neat resin of a given M_n and the number of apparent “temporary” entanglements due to adsorption/desorption of segments of PE chains onto SiO₂ nanoparticles with their inter-particle distance equal or shorter than the average entanglement length. Adding 2 vol % and 4 vol. % SiO₂, respectively, resulted in an increase of the N_eff by 40% and 80% of apparent “temporary” entanglements, respectively. When plotted against logN_eff, all the experimental logG_II data for a given undercooling, ΔT, collapsed to a single line. The slope of the logG_II vs. logN_eff dependence was independent of ΔT and varied from −2.13 to −2.24, similarly to the slope of the logG_II vs. logM_n dependence. This supported the conclusion that the effects of increasing the M_n and/or adding the non-nucleating nanometer sized SiO₂ on the spherulite growth rate were additive in nature and their effect can be superimposed. The retarded reptation of the chains to the growing crystal front was identified as the primary molecular mechanism of chain transport controlling the reduction of the spherulite growth rate in the model PE/SiO₂ nanocomposites investigated.     

Isothermal crystallization of intercalated and exfoliated polyethylene/montmorillonite nanocomposites prepared by in situ polymerization

Polyethylene/montmorillonite (PE/MMT) nanocomposites with different dispersion states of MMT were prepared by in situ polymerization. Isothermal crystallization of the intercalated nanocomposite, in which the PE chains were confined in the MMT layers, was studied and was compared with that of the exfoliated nanocomposite. It is observed that the intercalated sample has longer induction period, longer crystallization half time and larger crystallization activation energy than the exfoliated sample, showing that crystallization of PE is retarded due to confinement of the MMT layers. Analysis of crystallization kinetics shows that Avrami exponent (n) increases gradually with crystallization temperature. However, the maximal value of n is 2.0 for the intercalated sample, but it can reach 3.0 for the exfoliated sample. It is inferred that the stems of the PE crystals confined in the MMT layers are parallel to the MMT layers. The Hoffman-Weeks extrapolation method cannot be applied in the intercalated sample because of the small lateral surface of the PE crystals. Based on the depression of the melting temperature, the specific free energy of the PE/MMT interface was estimated, which is about 1.0 mJ/cm², much smaller than the free energy of the lateral surface of PE crystals. This is attributed to the origin of the strong nucleation effect of MMT.     

Correlation between crystallization kinetics and melt phase behavior of crystalline-amorphous block copolymer/homopolymer blends

We show that the phase behavior of the strongly segregated blend consisting of a crystalline-amorphous diblock copolymer (C-b-A) and an amorphous homopolymer (h-A), which depends on the degree of wetting of A blocks by h-A, can be probed by the crystallization kinetics of the C block. A lamellae-forming poly(ethylene oxide)-block-polybutadiene (PEO-b-PB) was blended with PB homopolymers (h-PB) of different molecular weights to yield the blends exhibiting ‘wet brush’, ‘partially dry brush’, and ‘dry brush’ phase behavior in the melt state. The crystallization rate of the PEO blocks upon subsequent cooling, as manifested by the freezing (crystallization) temperature (T_f), was highly sensitive to the morphology and spatial connectivity of the microdomains governed by the degree of wetting of PB blocks. As the weight fraction of h-PB reached 0.48, for instance, T_f experienced an abrupt rise as the system entered from the wet-brush to the dry-brush regime, because the crystallization in the PEO cylindrical domains in the former required very large undercooling due to a homogeneous nucleation-controlled mechanism while the process could occur at the normal undercooling in the latter since PEO domains retained lamellar identity with extended spatial connectivity. Our results demonstrate that as long as the C block is present as the minor constituent the melt phase behavior of C-b-A/h-A blends can also be probed using a simple cooling experiment operated under differential scanning calorimetry (DSC).     

Role of reptation in the rate of crystallization of polyethylene fractions from the melt

The theory of polymer crystallization with chain folding is extended to include the effect of reptation in the melt on the rates of crystallization G_I and G_II in régimes I and II. The result is that the pre-exponential factors for G_I and G_II contain a factor $\text{1}\text{n}$, Where n is the number of monomer units in the pendant chain being reeled onto the substrate by the force of crystallization; n is proportional to the molecular weight. The predicted fall in growth rate with increasing molecular weight is found experimentally in nine polyethylene fractions M_z=2.65 × 10⁴ to M_z=2.04 × 10⁵, corresponding to n_z=1.90 × 10³ to 1.45 × 10⁴. The data on these fractions are analysed to find the reptation or ‘reeling’ rate r and the substrate completion rate g. The values g_nuc～0.5/n_z cm s^?1 and r_nuc～21/n_z cm s^?1 at 400K are obtained from the data in conjunction with nucleation theory adapted to account for reptation assuming a substantial degree of regular folding. These results are consistent with a melting point in the range of ～142° to ～145°C. (The analysis using T°_m(∞)=145°C gives values of such quantities as σ σ_e and α that are quite similar to those deduced in earlier studies.) An estimate of g (denoted g_expt) that is independent of the molecular details of nucleation theory gives g_expt～0.4/n_z cm s^?1 and r～17/n_z cm s^?1 at 400K. Calculations of the reptation rate from r_1,2 = (force of crystallization ÷ friction coefficient for reptation in melt), where the friction coefficient is determined from diffusion data on polyethylene melts, leads to r_1,2～17/n_z to 34/n_z cm s^?1 at at 400K, or g_1,2～0.4/n_z to 0.8/n_z cm s^?1. The conclusion is that the reptation rate characteristic of the melt is fast enough to allow a significant degree of adjacent re-entry or ‘regular’ folding during substrate completion at the temperature cited, and that the substrate completion process is governed jointly by the activation energy for reptation $\text{Q1}{}_{\mathrm{D}}$ and the work of chain folding q. The nucleation theory and the friction coefficient theory approaches are compared, and the formulations found to be essentially equivalent; the ‘reeling’ rate r is found to be proportional to $\text{(}\text{1}\text{n}\text{)A}{}_0\text{(Δf)v}{}_0\text{exp}\text{[?}\text{(Q1}{}_{\mathrm{D}}\text{+q)}\text{RT}\text{]}$, where v₀ is a frequency factor, and A₀(Δf) is the force of crystallization on the pendant chain. The data analysis on the fractions confirms the detailed applicability of régime theory. The growth rate theory presented allows the possibility that the growth front may be microfaceted in régime I.