Polyethylene Unit Cell Research Articles

Melting and crystallization behavior of partially miscible high density polyethylene/ethylene vinyl acetate copolymer (HDPE/EVA) blends

• HDPE/EVA blends undergo phase separation, making it an interesting topic to investigate the relationships between miscibility and crystallization. • Influences from blending on the crystallization kinetics were successfully evaluated by Friedman's and Khanna's method. • X-ray diffraction studies revealed that blending with EVA the unit length of the unit cell of the HDPE increases. • Thermal fractionation method was successfully used to characterize the co-crystallization in HDPE/EVA blends. Crystallization studies on HDPE/EVA blends and the individual components were performed with differential scanning calorimetry (DSC) technique and wide angle X-ray scattering (WAXS). Influences of blending on the crystallization kinetics of each component in HDPE/EVA mixture were evaluated by Friedman's activation energy and Khanna's crystallization rate coefficient (CRC). The addition of more HDPE into the EVA matrix causes more heterogeneous nucleation while the addition of EVA would hinder the nucleation of HDPE at the beginning of cooling process. Inter-molecular interaction in the melt facilitated the crystallization of both EVA and HDPE components. X-ray diffraction studies revealed that HDPE and EVA have orthorhombic unit cell. Blending with EVA did not affect the crystalline structure of HDPE. In addition, a little shift of (1 1 0), (2 0 0) and (0 2 0) crystalline peaks toward lower 2 θ values of samples indicating a little increase of unit cell parameters of the orthorhombic unit cell of polyethylene. Thermal fractionation results showed that co-crystallization took place in the HDPE/EVA blend. All those results indicated that the polymer pair we choose was partially miscible.

Magnetic-field induced alignment of low molecular weight polyethylene

Magnetic-field induced alignment of polyethylene is firstly reported. Isothermal crystallization of low molecular weight polyethylene samples, high density polyethylene (HDPE) and linear low density polyethylene (LLDPE), from melting state was conducted under the magnetic-field of 2 or 10 T. Isothermal crystallization at the temperatures which induced effective self-seeding achieved alignment of polymer chains. The b axis of polyethylene unit cell was aligned parallel to the magnetic-field. Low viscosity and low crystallization rate of the polyethylene samples were favorable for high degree of the magnetic-field alignment.

Nonisothermal crystallisation, melting behavior and wide angle X-ray scattering investigations on linear low density polyethylene (LLDPE)/ethylene vinyl acetate (EVA) blends: effects of compatibilisation and dynamic crosslinking

Crystallisation studies on LLDPE/EVA blends and the individual components were performed with wide angle X-ray scattering (WAXS) technique and differential scanning calorimetry (DSC) DSC was used to characterise the quiescent crystallisation behavior. The heat of fusion and crystallinity of the blends were reduced by the addition of EVA. The experimental and theoretical values of crystallinity of the blends were found to be mutually agreeing. Crystallisation of LLDPE remains impeded to some extent due to the presence of amorphous EVA. Compatibilisation does not affect crystallinity whereas crosslinking decreases crystallinity. Crosslinking and compatibilisation make no significant change in the melting temperature of the blends. X-ray diffraction studies were carried out on un-crosslinked and crosslinked LLDPE/EVA blends with a view to study the effect of blend composition and crosslinking on crystallinity and lattice distance. Studies revealed that LLDPE and EVA have orthorhombic unit cell. Blending with EVA did not affect the crystalline structure of LLDPE, but the crystallinity decreases with EVA content. At low concentrations of EVA the lattice parameters remain unchanged. Above 30% EVA content however, a linear increase has been observed. Dicumyl peroxide (DCP) crosslinked samples show considerable shift of (1 1 0), (2 0 0) and (0 2 0) crystalline peaks towards lower 2 θ values indicating an increase of unit cell parameters of the orthorhombic unit cell of polyethylene. At lower EVA-concentrations (<50%) the crystalline structure remains unchanged. For EVA-contents of more than 70% however, increasing DCP-content reduces the crystallinity of the blends and increases the lattice distance. This indicates that DCP-crosslinking is more effective in EVA phase than in the LLDPE phase.

Polyethylene Unit Cell and Crystallinity Variations as a Consequence of Different Cross-Linking Processes

Changes of crystallinity and unit cell parameters for a low-density polyethylene as a consequence of cross-linking by dicumyl peroxide and by electron beam irradiations are compared. Polyethylene samples cross-linked by irradiation in the solid state present variations essentially only of the a unit cell parameter while those cross-linked in the melt state present similar variations of both a and b unit cell parameters. A strict analogy with unit cell variations observed for ethene random copolymers with propene and with bulkier comonomers, respectively, is shown. This suggests inclusion of cross-links for solid-state cross-linked samples, which would lead to a gradual transformation from an orthorhombic to a pseudohexagonal phase, and exclusion of cross-links for melt state cross-linked samples, which would generate tensions at the orthorhombic crystallite interphases.

The effect of molecular weight and crystallite structure on yielding in ethylene copolymers

Nominal stress-strain curves of a series of random ethylene-hexene copolymers having narrow composition and most probable molecular weight distributions were investigated. A series of such molecular weight copolymers with a constant concentration of branches were crystallized under a variety of conditions. In each molecular weight series the level of crystallinity was approximately constant. Particular attention was focused on the yield region and the nature of the yielding process. It was found, quite surprisingly, that the yield stress was not solely dependent on the crystallinity level. Moreover, the shape of the force-elongation curve in the yield region was very dependent on the molecular weight and the crystallization mode. These changes in yielding correlated quite well with the overall crystallite structure that was characterized by thin section transmission electron microscopy. The orthorhombic unit cell of polyethylene was maintained in all the samples despite the changes that occurred in the overall crystallite structure. © 1997 John Wiley & Sons, Inc.

An x‐ray pole figure analysis on biaxially deformed polyethylene film

AbstractThe orientational states induced upon two‐step biaxially stretching low‐density polyethylene at 25°C have been investigated. A pole figure analysis of the (200), (020), and (002) crystalline planes has been employed to elucidate the evolution of the molecular crystalline orientation as a function of biaxial stretching. The initial uniaxial‐like orientation induced along the extrusion direction of the films was gradually lost upon transverse stretching and, consequently, replaced by a biaxial orientation as suggested by the orientation functions. In these cases, the a crystallographic axis was observed to be strongly oriented along the film normal, thus confining the c and b axes to the film plane. The pole figures clearly indicate that the c and b axes are preferentially aligned 45° with respect to the stretching directions. This unique orientational state of the orthorhombic unit cell of polyethylene has been termed a biaxial‐double orientation. Birefringence measurements on the biaxial samples indicated that the amorphous and crystalline regions are simultaneously biaxially oriented. The evolution of the crystalline orientation as a function of stretching was conveniently followed on a White/Spruiell orientation triangle. Quantification was hindered, however, by the presence of different crystal populations in the biaxially stretched samples. © 1994 John Wiley &amp; Sons, Inc.

Study of crystalline orientation in drawn ultra‐high–molecular weight polyethylene films

AbstractA wide‐angle x‐ray diffraction (WAXD) study of the development of molecular orientation in the crystalline phase of ultra‐high–molecular weight polyethylene films prepared by the gelation–crystallization method is presented. WAXD scans of the undrawn films show that the lamellae are oriented in the plane of the films. Upon drawing at 130°C, the orientation of the molecular chains changes from the direction normal to the film surface (ND) to the elongation direction. The decrease of the 200/020 intensity ratio at low draw ration (λ &lt;10) indicates that double orientation develops during the transformation from the lamellar to the fibrillar morphology, with the a‐axis oriented parallel to ND. The orientation distributions of the 110, 200, 020, and 002 planes of the orthorhombic unit cell of polyethylene were studied and characterized by the coefficients of a Legendre polynomial series. At a draw ratio of 4.5, the second‐order coefficient, 〈P2(cos χ〉, already gets close to its limiting value, but it is shown that higher order coefficients of the polynomial series can be used to describe the evolution of the orentation, even up to λ = 50. The coefficients relative to the molecular chain orientation, 〈Pn(cos χ)〉c, can be calculated from different crystalline reflections. Curve‐fitting calculations were made in order to improve the correlation between the results obtained from the orientation distribution of the 110, 020, and 002 planes. A Person VII function was found to give a better fit of the experimental curves than Gaussian or Lorentzian equations. © 1993 John Wiley &amp; Sons, Inc.

Subcell-matched epitaxy of normal long chain compounds on polyethylene: I. on the (110) plane

Polyethylene films with surfaces bounded by {110} planes were prepared. On the film surfaces, various normal long chain compounds, such as normal paraffins (n-C n G 2 n+2 ) with n of 23 through 50, alcohols (n-C n H 2 n+1 OH) and carboxylic aci ds ( -C n -1 H 2 n-1 COOH) were epitaxially crystallized from solution, melt, and vapor phase. The molecular chains are lying down parallel to the chain axis of polyethylene when crystallized at low temperatures, exhibiting different crystalline modifications. Irrespective of the nature of the compound, the epitaxial relationships were explained in terms of a common lattice coincidence between orthorhombic subcells of the compounds and the unit cell of polyethylene: (110) sc∥(110) PE, [001] sc∥[001] PE, where sc a nd PE denote the subcells of the compounds and the polyethylene, respectively. By contrast, when crystallized at high temperatures from the vapor phase and the melt, the molecules stood normal on or tilted to the substrate, keeping the relations of (001) basal plane of the unit cell of the compounds parallel to (110) PE and the [110] direction parallel to the [001] direction of polyethylene.

Subcell-matched epitaxy of normal long chain compounds on polyethylene: II. On the (100) plane

Polythylene films with surfaces bounded with the (100) plane were prepared by high drawing of ultra-high molecular weight polyethylene mats made of solution-grown crystals. Normal long chain compounds, which are the same as examined in Paper I, were crystallized from solution by solvent evaporation and from the melt on the polyethylene films. Epitaxial crystallization was observed for those compounds that have the same orthorhombic subcell as the polyethylene unit cell. In the epitaxial overgrowth, the normal long chain compounds are arrayed with their axis parallel to the chain axis of polyethylene on the (100) polyethylene plane, performing the lattice coincidence of the (100) polyethylene plane with their corresponding subcell plane, irrespective of the chemical and crystal structures of the crystallized materials.

The location of comonomer units in crystallizable copolymers: Brominated polyethylene

Single crystals of polyethylene brominated in suspension contained 0% to 19.1% bromine (Br) on their fold surfaces. ‘As brominated’ crystals, crystals annealed at 388K and ‘melt-crystallized’ samples were all characterized using d.s.c. and WAXS. Melting points decreased with increasing levels of bromine for each set of samples. Heats of fusion for the ‘as brominated’ crystals were approximately constant up to 19.1% Br content. The annealed and ‘melt recrystallized’ brominated samples all possessed lower ΔH f values than the ‘as brominated’ crystals. Polyethylene unit cell dimensions for the ‘as brominated’ were constant to approximately 7% Br. At a concentration of 19.1% Br both the ‘ a’ and ‘ b’ dimensions of the unit cell increased, presumably because of distortions at the crystal/fold surface interface. The ‘ a’ and ‘ b’ dimensions of the annealed crystals increased at low levels of Br (<1 Br atom/fold then decreased at higher Br concentrations (>1 Br atom/fold). These dimensions increased continuously with Br concentration in the ‘melt crystallized’ samples. Both the observed WAXS and d.s.c. data on these materials are consistent with the inclusion of comonomer units in the polyethylene crystals.

Multiple deformation processes in polyethylene single crystals

Abstract Multiple deformation processes operate when polyethylene single crystals are strained in certain directions. The most complicated processes occur when the crystals are strained along the longest of the axes defining the orthorhombic unit cell of polyethylene. At large strains up to six twinning and martensitic transformation processes can operate, and the textures which result have been studied by selected-area electron diffraction. A description and an interpretation of the diffraction patterns are presented. The results are compared with those obtained in a recent study of deformation processes in bulk specimens of high-density polyethylene with a single-crystal texture (Young and Bowden 1974).

An analysis of limiting conditions on empirical nonbonded potential functions

The use of empirical potential functions for describing the interaction between molecules has suffered from the difficulty of finding suitable potential constants. In particular, the barrier to rotation about a C-C bond is open to the question of how to partition the energy between nonbonded through space interactions and “intrinsic” resistance to bond rotation. Using the equation for the cohesive energy and its derivative for each of the three lattice parameters,a, b, andθ of the unit cell of polyethylene, we obtain four equations which are linear in the six potential constants required to write these quantities in terms of aMie potential. Instead of making additional assumptions, we have applied a series of inequalities and used linear programming to assess the range of permissible values. Many of the reported potential function parameters do not fall within this permissible range. The results show that the total contribution of nonbonded interactions to the barrier to rotation must be in the vicinity of 10–25%. Permissible ranges for the potential constants were very sensitive to the value of the chain setting angle,θ with no possible solutions lying outside the range 44°<θ<49°.

Thermal Expansion of the Polyethylene Unit Cell

The 719–730-cm−1 doublet in the infrared spectrum of polyethylene (PE) is known to shift to higher frequencies with decreasing temperature. In this work we have measured these peak positions as a function of temperature down to 10°K for three bulk polyethylene samples of three different degrees of crystallinity, as well as solution-grown single polyethylene crystals both treated and untreated by fuming nitric acid. All these samples were found to behave similarly. X-ray diffraction measurements were performed for PE single crystals at four temperatures: 10°, 80°, 20°, and 300°K. The unit cell dimensions of polyethylene at these temperatures were found to correlate linearly with the infrared peak positions. On the basis of this correlation, a specific volume–temperature plot was made for the PE unit cell, from which thermal expansion coefficients were obtained as a function of temperature. The latter data can be fitted by the Debye–Grüneisen equation. Values of Grüneisen parameters were estimated and were found to increase monotonically with decreasing temperature.

Polyethylene unit cell variations with branching

AbstractIt is demonstrated by accurate x‐ray diffraction measurements that a variety of types of branches will affect the dimensions of the orthorhombic unit cell of polyethylene. In particular, the random placement of methyl branches on the molecular backbone by means of copolymerization of 12 mole‐% of propylene with ethylene will expand the a dimension of the unit cell by as much as 7%, and the b dimensions about 0.5%. In addition, alkyl branches up to five carbons in length, chlorine atoms, oxidation products, and probably crosslinks will also expand the cell dimensions. A qualitative explanation of the uniform, anisotropic expansion due to random inclusion of dilating centers is presented.

Polyethylene unit cell variations with temperature

AbstractThe unit cell dimensions of a linear polyethylene are reported as determined by x‐ray diffraction measurements at five‐degree intervals from −70°C. to +130°C. and at −196°C. The relative expansion coefficient of the a dimension of the orthorhombic unit cell was found to increase from 1.1 × 10−5/°C. at −196°C. to 22 × 10−5/°C. at 30°C. to 58 × 10−5/°C. at 138°C., the melting point of the crystal. The coefficient of the b dimension was found to vary from −2.8 × 10−5/°C. at −196°C. (questionable) to +3.8 × 10−5/°C. at 30°C., and then to −7.3 × 10−5/°C. at 138°C. These values were obtained from series expansion analytical fits to the experimental points, and are less accurate at the ends of the temperature range; the 30°C. values are probably correct to a few per cent.