PE Crystals Research Articles

Nucleation‐controlled growth. Observations on (110) twinned polyethylene crystals

AbstractThe growth of (110) twinned crystals of a sharp fraction of linear polyethylene (Mw/Mn = 1.10) of moderate molecular weight (Mw = 17,000) is followed during crystallization by the isochronous decoration method. New morphological features are observed. The fast‐growing tip of our laths presents, in addition to the two (100) facets usually observed, a possibly stable small reentrant (110) corner. This is a situation intermediate between the facies described by Dawson and Keller. Moreover, the slow tip of our laths presents various degrees of asymmetry with respect to the junction plane. A new characteristic length Ln = j/i is introduced to explain our morphological observations on (110) twinned crystals: j is the nucleation rate at a reentrant corner and i the nucleation rate on a smooth facet. Three linear growth rates Ghkl are calculated as a function of the length L of the face (hkl): Ghkl and Gĥkl or G′hkl are respectively the growth rates of a face bordered by two salient corners and by a reentrant corner. A distinction between Gĥkl and G′hkl is introduced to take into account the relative sizes of the two faces of the reentrant dihedral angle. The major points of the discussion concern (i) the stability of the (110) reentrant corner of the fast tip of the lath, (ii) the nearly constant shape of the twinned crystals, (iii) the effects of dislocations incorporated in the fast edge of the laths, and (iv) the various asymmetries observed in the slow tip of our laths. Theories of surface nucleation‐controlled growth explain our various morphological observations on (110) twinned PE crystals, and growth usually proceeds in regime II.

Crystal Structure of Diphenylacetylene-Tetracyanobenzene 1:1 CT Complex and Spin Polarization of Triplet Exciton and of Trans-Stilbene Triplet Trap

Abstract The crystal structure of diphenylacetylene and 1, 2, 4, 5-tetracyanobenzene 1:1 charge transfer complex has been determined at room temperature. pe crystals belong to the monoclinic system with space group C2/m, a = 9.632(4) A, b = 12.403(6) A, c = 8.131(4) A, β = 101.59(4)° with two translationally equivalent complexes per unit cell. EPR spectra of photoexcited mobile and trapped triplet species were recorded. The nature of the trap has been investigated and attributed to the trans-Stilbene-TCNB 1:2 CT complex. The analysis of the orientation dependence of polarized spectra of exciton and trap has shown that spin polarization is provided by ISC in which the TCNB is involved. Excitation transfer to the trap takes place by preseming alignment of the spin states.

Compatibility of low‐density polyethylene–poly(ε‐caprolactone) blends

AbstractThe compatibility of low‐density polyethylene (LDPE)–poly(ε‐caprolactone) (PCL) was examined using the microscopic, the dynamic mechanical, and the DSC techniques. Morphological examination revealed that at low PCL concentrations, when specimens are severaly quenched, a uniform microspherulitic structure is formed. The dynamic mechanical spectra, at isochronous conditions (110 Hz), indicate very limited mixing of the amorphous phase of the two components at the intermediate compositions. More drastic relaxation shifts were observed at the extremes of the composition range. Ultimate strength and elongation at break are characteristic of a mechanically compatible system at high and low PCL contents. In the intermediate composition range their low value indicates phase separation. DSC measurements showed a nonlinear dependence of bulk crystallinity on blend composition; also, that the presence of PE enhances the degree of PCL crystallinity. Thermal history strongly affects bulk crystallinity. At elevated temperatures, near the mp of PE, thermal treatment affects considerably the melting point of the PE crystals.

Physical characterization of polyolefin blends. II. Polyethylene/ethylene-propylene-diene terpolymer

Abstract The properties of melt-mixed polyblends of high-density polyethylene (HDPE) and an amorphous ethylene-propylene-diene terpolymer (EPDM), containing 62 mol% of ethylene, were studied. The compositions covered the complete range. The techniques used were differential scanning calorimetry, dynamic mechanical spectroscopy, and phase contrast microscopy. The latter technique revealed a heterogeneous structure with a finer dispersion of the rubbery particles (at low rubber compositions), compared to a blend of a HDPE/ethylene-propylene copolymer (EPM) having a higher ethylene content and of low crystallinity. The dynamic mechanical technique at isochronous conditions (110 Hz), between–120° and 140°C, confirmed the heterogeneous nature of the polyblends and revealed mixed-phase formation at all rubber compositions. Appropriate mechanical models tested at -25°C revealed that the HDPE phase forms the matrix of the polyblends for volume fractions of EPDM, Φ (EPDM) < 0.8. More severe tests over an extended temperature range, using other models, led to the conclusion of a basically incompatible system of noninteracting phases with a highly parallel connectivity. This is consistent with a laminar distribution of phases. DSC studies showed that at Φ (EPDM) 8gt; 0.2, the PE crystallization mechanism changes, but there was no evidence of cocrystallization. In addition, the reorganization of the PE crystals during melting is significantly influenced by the viscous rubber surrounding it. This, together with the reduction of the α PE relaxation temperature, is taken as evidence that a limited mixing of the amorphous phases takes place within the interlamellar region, accompanied by a decrease of the crystal fold period.

Study of nascent polyethylenes of varied molecular weight by the microcalorimetric method

Using differential scanning microcalorimetry the authors make the first experimental study of the kinetics of melting and non-isothermal crystallization of PE with different MW. The higher enthalpy of melting of the high molecular weight PEs is explained for the first time by the extra quantity of energy required to produce a stressed network of communicating chains. The investigations confirm ideas on the structure of nascent PEs.

Physical characterization of polyolefin blends. I. Polyethylene/ethylene-propylene copolymer

Abstract Melt-mixed polyblends of high-density polyethylene (HDPE) and an ethylene-propylene copolymer (EPM) containing 78 mol% ethylene were studied in the complete composition range, using quenched and annealed samples. Phase contrast microscopy revealed that the polyblends are heterogeneous, the rubbery copolymer being dispersed as domains with sizes in the micron range, in the HDPE matrix. At intermediate compositions the blend structure was less uniform. DSC studies indicate that crystallization of PE proceeds independently of the rubbery component at all compositions with no evidence of cocrystallization to a significant extent. Dynamic mechanical studies, between –120° and 140°C at 110 Hz, of annealed and quenched specimens confirm that the system is incompatible; however, compound inclusions are formed at low EPM levels. At rubber-rich compositions EPM crystallinity is affected by the presence of the PE phase with the formation of a common crystalline supermolecular structure. Also, a small reduction of the crystallization temperature with an increase of EPM content and a melting point depression of PE at these compositions are evidence for the reduction of crystallite dimensions. This is further corroborated by a decrease of the α relaxation temperature.

Crystallinity of bulk polyacetylene

AbstractThe fine structure of bulk trans‐polyacetylene (PA) was investigated by x‐ray diffraction. The degree of crystallinity 0.81, determined by Ruland's method, was significantly higher than the value for linear polyethylene. The disorder parameter k was 5, which is much larger than the value for polyethylene. These results led us to propose that PA crystals are more disordered than PE crystals owing to the higher rigidity of PA chains. Small‐angle x‐ray diffraction indicated that the crystal morphology is of the extended‐chain type rather than lamellar. All results obtained in this work showed that the fine structure of PA bulk is such as to facilitate electrical conduction of bulk PA.

Heat of fusion of polyethylene crystal suspensions: Variation with crystallization temperature

Abstract A recent development of existing thermodynamic theory has been applied to the analysis of heat of fusion (△Hf) data from both “dry” and “suspension” polyethylene) - 1] were constructed for suspension samples assuming various fold models. Irrespective of the model chosen, the perfect crystal heat of fusion was found to be approximately 72 cal/g. From analysis of the slopes of these plots it was concluded that a 10-carbon-atom fold model best fits the suspension △AHf, data. Melting points for suspension crystals were also obtained. Analysis according to the Hoffman-Weeks equation reveals that the equilibrium melting point is somewhat lower than the usual range of accepted values. The discrepancy between the value determined here and that reported by other authors is discussed. The magnitude of the derived surface free energy from melting point data also indicates that PE crystals consist of folds containing approximately 10 methylene units on average. This conclusion is in excellent agreement with ...

Study of crystallization of industrial polyethylene and polyethylene mixtures

Depolarization of linear polarized light was used to examine isothermal crystallization of industrial PE distinguished by the method of synthesis. The temperature dependence of the half-life of crystallization was derived. A study was made of the structure of low and high density PE mixtures, according to their behaviour during crystallization and melting. It was shown that low density PE and linear PE in a mixture crystallize separately, forming general spherulites. Spherulite formation and increase in dimensions before collision take place during crystallization of high density PE, forming the skeleton of the spherulite. Joint extrusion of low density PE powder and high density, high molecular weight PE is not accompanied by homogenization of the alloy. No marked increase is observed in the fluidity of high density, high molecular weight PE on adding small amounts of low density PE.

Dielectric loss of high‐density polyethylene below 4.2°K

AbstractDrawing of oxidized high‐density polyethylene (HDPE) and subsequent annealing greatly reduce the low‐temperature dielectric loss when the electric field is applied perpendicular to the draw direction. This supports our model (J. Polym. Sci. Polym. Phys. Ed., 15, 43 (1977)) of the proton moving parallel to the c axis of the PE crystal. A particular antioxidant (Ionox 330) in unoxidized HDPE induces a dielectric loss with a frequency and temperature dependence which differs from that for the loss in oxidized HDPE. The antioxidant loss seems to be an overlapping of tunneling and a thermal activation process. The possibility that the hydroperoxide group in the PE crystal is the origin of the loss in oxidized HDPE was theoretically examined in a manner similar to that used for the hydroxyl group in the previous paper. Results suggest that the hydroperoxide group is less probable than hydroxyl as the origin of the low‐frequency loss in HDPE.

The existence of mosaic block structures in polymer single crystals

AbstractX‐ray line broadening measurements were used to determine the apparent “mosaic block” sizes of randomly oriented polyethylene and polyoxymethylene single crystals. Both dried‐down and uncollapsed crystals were examined. PE lamellae were grown at 80, 85, and 90°C by isothermal crystallization from a 0.1% solution in xylene. POM crystals were grown at 125°C by self seeding from an 0.05% solution of the polymer in o‐dichlorobenzene. A given preparation was split into two parts. One part was dried down in the usual manner, the other was exchanged to paraffin oil and the crystals never permitted to dry down. Previously reported studies used dried‐down crystals and gave crystallite sizes of approximately 300 Å. More recently, using electron microscopy, it has been postulated that PE single crystals are free of “mosaic block” over regions of several thousand angstroms. It is evident from this present study that crystallite sizes in uncollapsed lamellae are significantly larger than those observed for the same crystals dried down. In the case of uncollapsed lamellae, one can explain the observed crystallite sizes solely on the basis of chain obliquity rather than by invoking the “mosaic block” model. It has also been determined that there is an upper limit to the crystallite sizes that can be observed in PE and POM crystals using wide‐angle x‐ray techniques. This limit may account for discrepancies between x‐ray and electron diffraction.

Comparison of phosphorus in some standard rocks by X‐Ray fluorescence analyses using the Ge (111) and PE crystals

AbstractPhosphorus has proved to be one of the most difficult elements to determine precisely in rocks by X‐ray fluorescence analysis. This is partly because of its low concentration in rocks, and because of interference from Ca 2Kβ and its associated escape peak when using the common crystals PE or EDDT. The Ge (111) crystal does not give any second order reflection. A comparison of phosphorus determined in some geological materials using PE crystal and electronic discrimination, PE crystal and manual discrimination and Ge (111) crystal is given.

Molecular and supramolecular paracrystalline structure of linear synthetic high polymers

AbstractThe paracrystalline state is a quite common state for all kinds of matter. Paracrystalline distortions arise, for instance, in solid‐solution crystals. From x‐ray scattering they are experimentally observed in polyethylene and polyethylene oxide. Values of g of about 2% for the mosaic blocks in single crystals and of 3% for the ultrafibrils in bulk material are found. The ultrafibrils of Lupolen 6001 have a very uniform thickness of n X 90.7 A., where n = 1, 2, 3, and build up manyfold superstructures. The (hh0) “netplanes” at the boundaries of all observed mosaic blocks both in single PE crystals and in hot‐stretched material fluctuate statistically by 0.6 A., hence are no longer planes. The structure in the grain boundaries is discussed. Measurements of the diamagnetic susceptibility give evidence of three types of conformation, α, β, and γ, which play an important role in the different superstructures.