Branch length distribution in TREF fractionated polyethylene

Commercial polyethylene is typically heterogeneous in molecular weight as well as in molecular topology due to variability in catalyst systems and catalyst activity. Further, processing of polyethylene after polymerization may also result in changes to the structure. While quantification of molecular weight is routine using gel permeation chromatography (GPC); quantification of the heterogeneity in molecular topology and microstructure is more difficult. In this paper, a novel method is used to examine the structure and branch content of a linear low-density polyethylene (LLDPE). The method uses a scaling model to analyze small-angle neutron scattering (SANS) data from dilute solutions of a series of LLDPE fractions. The scaling approach quantifies short-chain and long-chain branch content in polymers concurrently, thereby illustrating the distribution of these branches in the polyethylene fractions. Additionally, new quantities such as the average long-chain branch length and hyperbranch content are measured to provide further insight into the structure of these polymers. LLDPE used in this study is fractionated using temperature rising elution fractionation (TREF). Results from the analysis of these fractions show evidence of long-chain branching in commercial LLDPE which could be partly attributed to post-synthesis processing conditions.     

Structural characterization of semicrystalline polymer blends by small-angle neutron scattering

A series of phase-separated polyethylene-polypropylene blends, which have undergone different thermal treatments, have been analysed by small-angle neutron scattering (SANS). The coherent scattering from normal hydrogenated blends is virtually zero, but strong contrast may be induced by partial or complete deuteration (labelling) of either phase. Here, the scattering from blends with complete labelling of the polyethylene phase was analysed to provide the domain dimensions by means of a theory due to Debye using an exponential correlation function. By this means the mean chord intercept lengths of both phases were shown to be in the range 1000–10000Å. The scattering from blends with partial labelling of the polyethylene was analysed to give the radius of gyration of the molecules in the polyethylene domains, which was found to be close to that measured in the homopolymer. For melt-quenched blends the deuterated polyethylene was shown to be statistically distributed in the polyethylene phase, whereas for slow-cooled blends, partial segregation of the labelled molecules occurred.     

Application of the optical transform to the electron diffraction pattern of polyethylene single crystal

The setting angle of the planar zigzag molecular chains in a polyethylene single crystal has been determined by optical transformation of the electron diffraction pattern, using the Patterson synthesis. The reproducibility of the original structure on optical transformation was confirmed by (1) the inverse optical transform of the optical diffraction pattern produced by a well defined two-dimensional model lattice and (2) the electron diffraction pattern calculated from the model crystal. The following points arising from the electron diffraction were examined: the dynamical scattering, multiple reflections and radiation damage of the specimen; the direction of the interatomic vector obtained by the present method of analysis and under these illumination conditions was found to be reliable. The optical results were confirmed by X-ray diffraction in which these effects were absent.     

X-ray low-angle scattering from oriented poly(ethylene terephthalate) films

Low-angle x-ray scattering data are used to deduce the morphology of oriented polymeric films. Generally, the structural models proposed to explain these patterns have been extrapolations of observations made from solution-grown polymer single crystals or from highly crystalline bulk polymers. These models and explanations may not be applicable broadly to oriented systems having only modest amounts of crystallinity or to those generated from precursor states that are grossly different. Poly(ethylene terephthalate) (PET) was chosen as a model polymer system for study. A systematic series of uniaxially and biaxially deformed films were produced from this polymer, made from the initially glassy or crystalline states. The low-angle x-ray scattering patterns generated from these films were studied as a function of (a) the sequence of deformation, (b) the precursor structure, (c) molecular orientation, and (d) the direction of observation. Optical diffraction and model structures were used to aid in the interpretation of the morphology produced. At least three different-sized domains are developed upon deformation, ranging from that of the unit cell (about 10 Å) to large laminar domains of average size 2,000 Å × 10,000 Å. This structure is shown to be substantially different from that developed in an oriented polyethylene film.     

Photocross-linking of low-density polyethylene. III. Supermolecular structure studied by SALS

The supermolecular structure of photocross-linked polyethylene (XLPE) has been studied by small-angle light scattering (SALS). The data show that the spherulitic structure of XLPE gradually deteriorates with increasing degree of cross-linking and increasing irradiation temperature from well-developed spherulites to rodlike aggregates and disordered lamellar structures. A photocross-linked sample of PE has lower crystallinity, smaller crystallites, and smaller spherulites than does the original sample. At high degrees of cross-linking, the SALS patterns show little or no spherulitic structure. Results with photocross-linked polyethylene demonstrate that the overall effect of cross-linking on the morphological structure is similar to that of an increase in molecular weight of the polymer. © 1993 John Wiley & Sons, Inc.     

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%).     

Small-angle neutron scattering of partially segregated blends of polyethylene and deuteropolyethylene

A previous paper described work in which polyethylene (PEH) blended with 4.3 vol% deuterated polyethylene (PED) was annealed and plastically deformed at different temperatures. The most prominent change resulting from the deformation is a significant reduction in the apparent molecular weight measured from the extrapolated small-angle neutron scattering (SANS) data. The model adopted in the data analysis was based on a heterogeneous distribution of the centres of mass of the labelled (PED) chains which form a two phase system of enriched and depleted regions described by a Debye-like correlation function. A comparison between this model and alternative approaches based on the correlation network and random phase approximation will be delineated. The results from these models lead to the conclusion that for typical melt crystallized samples the centres of mass of the labelled chains are only slightly perturbed from a random distribution. Plastic deformation of the blends tends to lessen the degree of segregation of the PED molecules and the results suggest that a portion of the specimen must undergo a melting and recrystallization mechanism during deformation.     

A review of small-angle scattering models for random segmented poly(ether-urethane) copolymers

Small-angle X-ray scattering data from several experimental and commercial poly(ether urethane) formulations was used to test various scattering models based on different morphologies. The best fits were generally found with ‘globular’ scattering models based on a distorted one-dimensional lattice or the Percus-Yevick model of liquid structure. Whilst this is not conclusive proof of the morphologies exhibited by the materials studied, these scattering models are roughly consistent with many AFM and TEM studies, which indicated discrete globular or elongated cylindrical microdomains. Moreover, reasons why meandering elongated or finite lamellar microdomains may behave more like globular scattering bodies are discussed. Hence, these models are proposed as a basis for interpreting scattering data from polyurethanes.Other models were unable to fit the observed scattering data adequately. A model based on the weak segregation of copolymers was rejected on the basis of deviations from the observed scattering behaviour in the Porod region. A model based on stacks of ideal (infinite, parallel, flat) lamellae was ruled out, since it was unable to reproduce the observed peak width. The Teubner-Strey model, based on microemulsion structure, was found to be incapable of fitting the data at low q, particularly for strained samples. However, this model appeared to be more successful at reproducing the scattering observed at elevated temperatures. One possible inference is that the morphologies became more akin to microemulsions during heating.     

Plasma-deposited polymer films. II. Transmission and scanning electron microscopy

A transmission electron microscopy (TEM) and scanning electron microscopy (SEM) study of plasma-formed polyethylene and polystyrene is reported. A two-phase structure of spheres embedded in a polymer binder is evident, supporting the predictions of earlier low-angle x-ray scattering data taken of these two plasma-deposited polymers.     

The effect of surface roughness and crystallinity on the light scattering of polyethylene tubular blown films

An experimental study of the effect of surface roughness and crystallinity on the light scattering of polyethylene tubular blown films is reported. Several films were prepared by varying the raw polyethylenes and the extrusion temperature. Other processing variables were maintained constant. We have made a statistical analysis of data and we have obtained multiple regressions between light scattering for a given wavelength of incident light and a given film thickness (30 μm, 50 μm, and 70 μm) and the independent variables average surface roughness and degree of crystallinity. It was demonstrated that, for the polymeric films studied, the surface texture evaluates better than crystallinity the variation of light scattering, but crystallinity has also some importance.     

Enabling controlled dispersion in polyethylene nanocomposites using small‐angle scattering

In this article, we present some key results that demonstrate the useful application of light scattering to characterize the dispersion of nanoparticles in polymer matrix, using polyethylene nanocomposite as an example. Our results show that the dispersions of nanoparticles in polyethylene nanocomposite, that influence the thermomechanical properties of polyethylene nanocomposites, can be characterized by using small angle light scattering. This proposition is motivated by recent synthetic and morphological characterization progress towards achieving ideally dispersed polyethylene nanocomposites. Our results are significant because it is shown that using light, size and shape features that correlate well with the scanning electron microscopy observations of nanoparticle dispersion can be obtained using our unique vertical light scattering set‐up. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Nanostructure evolution of oriented high-pressure injection-molded poly(ethylene) during heating

High-pressure injection-molded polyethylene (PE) rods are studied by ultra small-angle X-ray scattering from synchrotron during the heating of the polymer. Injection of a cool melt into a cold mold yields highly oriented PE rods with a core-shell structure. Samples from both the core and the shell material are studied. The two-dimensional scattering patterns are evaluated utilizing the multi-dimensional chord distribution function (CDF) analysis. From the obvious evolution of the nanostructure during successive crystallite melting, the sequence of processes occurring during crystallization is elucidated. First, nuclei form one-dimensional lattices with short-range order along the fiber axis. From this row structure, lamellae grow with wide lateral extension. An indication of an intermediate block structure is observed. Finally two steps of insertion crystallization result in two long period halvings. Increase of the mold pressure increases the lateral extension of the inserted lamellae in the shell material. In the core material a uniform row structure is absent. Extended primary lamellae form stacks with decreasing long periods before insertion crystallization takes over. But crystallites inserted in the core material do not form extended lamellae. Each of these steps leaves its footprint in the nanostructure and the corresponding scattering pattern. After CDF interpretation of the heating series, the room temperature pattern can be explained. The strong two-point pattern is associated with the primary lamellae and the intensity ridge extending along the meridian results from irregular insertion of lamellae. When the row structure is observed in the CDF, the fiber pattern exhibits equatorial scattering.Domain roughness generates a strong background scattering, which cannot be separated in one step. For the presented material it is shown that iterative background subtraction eliminates the scattering effects of the imperfect (i.e. inserted) lamellae.     

Light scattering studies on crystallization in polyethylene terephthalate

The light scattering from the spherulites of polyethylene terephthalate grown near the glass transition temperature has been investigated. The Hv scattering profiles can be reproduced by the sum of the ideal spherulite scattering with the distribution of spherulite radius and the isotropic scattering from randomly oriented crystallites. The ratio of optical anisotropies in the isotropic scattering to the ideal spherulite scattering is obtained by the method established to eliminate the effects of the number density of spherulites and the coefficient depending on the experimental conditions. It is found that the anisotropy ratio is almost independent of the crystallization time and of temperature above 106 °C, while it is larger at a crystallization temperature of 103 °C. The spherulitic structure is discussed in terms of the anisotropy ratio.     

Are there two temperature regions with different structure of N-alkane melts?

Summary From the comparison of measurements of viscosity, Brillouin scattering and depolarized Rayleigh scattering it appears that two types of structure exist in the n-alkane melts. The regions where these structures are dominant are separated by a relatively well defined temperature T_u. The difference in the activation energies of viscous flow in these two regions is E= 2.8 kJ/mol. From the molecular weight dependence of T_u we conclude that polyethylene always retains the low temperature structure which should not be much different from that of n-alkanes with n28.     

Plasma-deposited polymer films. I. Low-angle x-ray study

A low-angle x-ray scattering technique has been applied to determine the submicrostructure of plasma-formed polyethylene and polystyrene. The plasma-formed polymers appear to closely approximate a “filler—binder” structure where polymer spheres constitute the filler material and a lower-density polymer, the binder. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs confirm the spherical diameters predicted by the low-angle x-ray scattering data.     

On the molecular weight determination of bisphenol-A polycarbonate

In order to assess the reliability of absolute molecular weights of bisphenol-A polycarbonate samples measured by size exclusion chromatography, the results obtained from two experimental systems and two calibration methods are compared with each other as well as with data provided by light scattering determinations. The interest of coupling a light scattering detector to the chromatographic system is discussed.     

Gypsum dissolution: A comparison of laser-light scattering and conductance techniques

Laser-light scattering measurements can be used for investigations of solid dissolution in a liquid. In this paper the dissolution of gypsum in water is followed by scattering and conductance measurements. Both types of measurement are carried out simultaneously. The sensitivity of the scattering technique and its special applicability in solubility determinations is discussed.     

Uniaxial deformation of overstretched polyethylene: In-situ synchrotron small angle X-ray scattering study

Synchrotron small angle X-ray scattering was used to study the deformation mechanism of high-density polyethylene that was stretched beyond the natural draw ratio. New insight into the cooperative deformational behavior being mediated via slippage of micro-fibrils was gained. The scattering data confirm on the one hand the model proposed by Peterlin on the static structure of oriented polyethylene being composed of oriented fibrils, which are built by bundles of micro-fibrils. On the other hand it was found that deformation is mediated by the slippage of the micro-fibrils and not the slippage of the fibrils. In the micro-fibrils, the polymer chains are highly oriented both in the crystalline and in the amorphous regions. When stretching beyond the natural draw ratio mainly slippage of micro-fibrils past each other takes place. The thickness of the interlamellar amorphous layers increases only slightly. The coupling force between micro-fibrils increases during stretching due to inter-microfibrillar polymer segments being stretched taut thus increasingly impeding further sliding of the micro-fibrils leading finally to slippage of the fibrils.     

Extrudate swell of high density polyethylene. Part I: Aspects of molecular structure and rheological characterization methods

Two high density polyethylene (HDPE) resins–samples 801 and 802–both nominally the same material, as they are taken from successive batches of the same commercial grade, are characterized for their molecular structure and rheological properties. Gel permeation chromatography (GPC) and low angle laser light scattering (LALLS) results must be interpreted in combination with rheological data to show the presence of somewhat more high molecular weight material in 802 that in 801. Small amplitude oscillatory shear, steady shear, and capillary shear measurements performed in different laboratories show consistently higher shear viscosity values at low shear rates for sample 802. Extensional viscosity measurements show similar results. The interpretation of rheological data in terms of molecular structure could be complicated by the possible presence of long chain branching (LCB). The zero shear viscosity and discrete relaxation spectrum is estimated for both samples. The small rheological difference between 801 and 802 forms the basic information for understanding their time dependent extrudate swell behavior, as will be described in Part II.     

Orientation and crystalline morphology of blow molded polyethylene bottles

A study was carried out of the development of orientation during processing of conventional blow molded bottles made from low density polyethylene (LDPE), linear low density polyethylene (LLDPE), and high density polyethylene (HDPE). The level of molecular orientation was found to be relatively low in all of the polyethylene bottles examined; it increased with increasing inflation pressure and decreasing extrusion temperature. Pole figures for the HDPE bottles indicated a slight a-axis orientation toward the circumferential direction of the bottle. The b-axes, which correspond to the lamellar growth direction, tend to be parallel to the bottle thickness direction. A comparison was made of the structure developed in blow molded bottles and blown films. The results for both films and bottles indicate that crystallization during processing involves both an increase in level of molecular orientation and an increased tendency toward biaxiality. Based on pole figures and small angle X-ray scattering patterns, a morphological model for HDPE blow molded bottles is proposed.