Lamellar thickening of polyethylene under high pressure

Single crystal mat (SCM) samples of polyethylene (PE) were prepared from dilute solution of p-xylen, then they were annealed at pressures of 200 and 500 MPa. Lamellar thickness of the original and annealed SCM samples was measured by small-angle X-ray scattering method. Orientation of the molecular chain in those SCM samples was investigated by wide-angle X-ray diffraction pattern. From these X-ray measurements, annealing temperature dependence of the lamellar thickness, i.e., lamellar thickening, under high pressure was obtained. Melting process of the SCM samples was also investigated at 200 and 500 MPa by high pressure differential thermal analysis. Then correspondence between the lamellar thickening and the melting process was studied. The lamellar thickness increases markedly with approaching to the melting temperature of the orthorhombic crystal even in the high pressure region where the high pressure phase (hexagonal phase) appears. The annealing temperature dependence curve of the lamellar thickness at 200 MPa can be superimposed on the curve at 500 MPa by shifting the curve along the temperature scale by 47 K. Large scale lamellar thickening occurs in the orthorhombic crystal phase in the high pressure region. The formation process of extended-chain crystal is discussed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys, 35: 535–543, 1997     

DTA and x-ray studies on extended-chain crystals of polyethylene under high pressure

The relation between the thermal behavior of extended-chain crystals (ECCs) of polyethylene and the phase transitions, i.e., orthorhombic ? hexagonal ? melt, of polyethylene at high pressures above about 400 MPa has been studied by high-pressure differential thermal analysis (DTA), and with a high-pressure and high-temperature x-ray diffraction apparatus equipped with a position-sensitive proportional counter measuring system. The original sample used in this study consists mainly of two kinds of ECC, which we designate as “ordinary extended-chain” crystals (OECCs) and “highly-extended-chain” crystals (HECCs). Experimental results at pressures below 300 MPa substantiate the results previously reported: i.e., the phase diagram indicating the relation between the melting temperatures and pressure for the OECCs and HECCs can be determined for pressures up to 500 MPa. In heating at pressures above about 500 MPa, the peak intensity of the (100) reflection of the hexagonal structure decreases in two stages with increasing temperature. The phenomenon corresponds to the thermal behavior determined by high-pressure DTA in which two small endothermic peaks can be observed at temperatures above that of the crystal transition evidenced by the strong peak. This phenomenon suggests melting in two stages of hexagonal structures with different thermal stabilities, and that the change at higher temperature may be due to fusion of the hexagonal phase annealed either below or above the transition temperature.     

Lamellar structure in melt crystallized low density polyethylene

The investigation of representative details in the lamellar microstructure of LDPE observed after controlled chlorosulfonation using both EM and SAXD is reported. For this purpose melt crystallized PE with two branch contents (=0.28 and 2.53 branches per 10²CH²) has been prepared using Kanig's technique over a range of temperatures and treatment times. During the first treatment hours the selective incorporation of electron-dense atoms at the lamellar surface produces a macroscopic weight increase, swelling of the sample and a concurrent decrease of the SAXS intensity. The main result, however, is that the thickness of the lamellar structure does not vary with treatment time. After long chlorosulfonation times a saturation of electron-dense atoms within the surface layer and a reduction in the lateral dimensions of the lamellae take place. Optimum conditions for revealing the representative morphology are such as to lead to a weight increase of 50% for PE with=0.28 of branches and only to an increase of 10% for material of branch content represented by an value of 2.53.On leave from Inst. Estructura de la Materia, Madrid-6. Spain     

GPC analysis of degraded annealed single crystals

Annealed polyethylene single crystals have been degraded with fuming nitric acid, and the molecular weight distribution of the fragments determined by using a gel-permeation chromatograph. Peaks due to chain folding were observed in these distributions as for unannealed single crystals. The peaks moved to lower molecular weight with increasing degradation time. Comparison of the lowest molecular weight peak length after a given degradation time with the low-angle x-ray periodicity before degradation gave information about a disordered surface layer. The thickness of this layer at early states of degradation was dependent solely on annealing temperature, though changes in the layer must have occurred with annealing time, since there was an increase in reaction rate with annealing time. At higher degradation states, the thickness of the layer was dependent solely on the original low-angle periodicity. This has been related to the depth at which some folds are buried beneath the lamellar surfaces. The relevance of these observations to the structure of annealed single crystals is discussed.     

Crystallization kinetics of polyethylene under high pressure

The kinetics of isothermal crystallization of polyethylene under high pressures ranging from 840 to 5300 kg/cm² has been studied dilatometrically. The crystallization rate estimated from the half-time of the overall transformation increases markedly with pressure. The Avrami exponent n becomes smaller with increasing pressure. Values of n ≈ 2 for the crystallization at 840 and 1950 kg/cm², and n ≈ 1 at 5100 and 5300 kg/cm² were obtained. Differential scanning calorimetry and electron microscopy data are presented. It is concluded that extended-chain crystals grow rapidly, predominantly in one dimension, at high pressure. Relations between log k and T_m/T(ΔT) and T_m²/T(ΔT)² are nearly linear. Here, k is the crystallization rate constant from an Avrami equation, ΔT = T_m – T, T_m is the melting point, and T is the temperature of crystallization. From the dependence of the slope of the straight line on the crystallization pressure it is concluded that the surface energy of crystal nuclei decreases with increasing pressure.     

Differential thermal analysis of polyethylene under high pressure

The design of a differential thermal analysis apparatus for use at elevated pressure is described. Experiments on melting and crystallization of folded-chain crystals of polyethylene and poly(ethylene–butene-1) copolymer, and melting of extended-chain polyethylene crystals have been conducted at pressures up to 4200 bars. The precision in transition temperature measurement was ±1°C. The Clausius-Clapeyron equation predicts the melting point increase with pressure at atmospheric pressure to be 32.0°C/kb. The melting point depression due to copolymerization remained constant over the complete pressure range analyzed on the poly(ethylene–butene-1) used in this study. Crystallization of polyethylene is retarded at elevated pressures, and a 50% larger degree of supercooling is necessary at 5000 bars to give a crystallization rate equal to that observed at atmospheric pressure. The difference in melting point between folded-chain and extended-chain polyethylene increases from 8.4°C at 1 bar to 25.6°C at 3000 bars.     

Studies of low molecular weight polyethylene single crystals by differential scanning calorimetry

Fusion behaviour of solution-grown low molecular weight polyethylene single crystals was studied by differential scanning calorimetry at different heating rates. The results were correlated to the polymer chain conformation in the crystal. It was found that in the molecular weight range studied, crystals of shorter chain length and fewer foldings per chain are less stable and more susceptible to heat annealing. Melting endotherms of the crystals of the lowest molecular weight fraction grown at various temperatures indicate that during crystallization, a fractional stem at the end of a folding chain will be rejected outside the lamellae of the crystal.     

Extended-chain crystals. III. Size distribution of polyethylene crystals grown under elevated pressure

Extended-chain crystals of high molecular weight polymethylene, a polyethylene with a broad molecular weight distribution, and three fractions of polyethylene were grown from the melt under elevated pressure. Comparison of the crystal size distribution in the molecular chain direction (measured on fracture surfaces by electron microscopy) with the molecular weight distribution (measured by gel-permeation chromatography) gave the following results. Up to molecular weight 10,000 all samples showed eutectic separation into fully extended chain crystals of narrow molecular weight distribution. Above molecular weight 10,000 mixed crystals were formed. Under the chosen crystallization conditions larger chain extension was achieved with higher molecular weights. However, an increase in molecular weight by a factor of 1000 led only to a tenfold increase in chain extension. These facts are discussed in the light of a proposed mechanism of crystal growth.     

Extended-chain crystals. II. Crystallization of polyethylene under elevated pressure

A report on crystallization of polyethylene at elevated pressures to an extended-chain morphology is presented. The crystals have been characterized by electron microscopy and density determination. Pressure, supercooling (temperature), and crystallization time have been varied to find the best conditions for production of perfect crystals. At 10–30°C supercooling completely crystallized polyethylene was obtained between 4.5 and 7 kb crystallization pressure in 1–8 hr. Analysis of fracture surfaces of samples crystallized for different lengths of time shows an increase in size and number of crystal lamellae and an improvement of extended chain crystals in the early stages of crystallization. A further improvement of the less well crystallized material between the lamellae occurs after 15 min of crystallization time.     

NQR of hydrogen bonded crystals under high pressure

The work presents the results of NQR under high hydrostatic pressure for the following ferroelectrics: KDA, KD^xA, LHA, LDA and AHCA. Most of the below presented discussions concerns the behaviour of these ferroelectrics and the explanation of this behaviour on the basis of pseudo-spin-lattice coupling model.     

Lamellar thickness of polyethylene single crystal isothermally crystallized in dilute solution

Summary Polyethylene single crystals were formed isothermally from 0.1% solution in tetralin (at 77.2, 84.8, and 89.8°C) and n-hexadecane (106.6°C), and those lamellar thicknesses were investigated as a function of crystallization time. These crystals have revealed no change or, if any, a little change in the lamellar thickness. Even for the crystals formed at considerably high temperature, 106.6°C, at which the segmental motion in a cyrstal begins to become active, the increase in those lamellar thicknesses was of the order of 10 Å. When polyethylene fractions with different molecular weight were crystallized at the same temperature, those lamellar thicknesses have shown nearly the same values. The results observed in this study are in striking contrast to those from isothermal crystallization in bulk, as observed byHoffman andWeeks. The constancy of the thickness of solution-grown crystals is interpreted as that the longitudinal translation of chains and chain ends in the interior of a crystal, necessary to thickening, will have been restrained on account of a considerably lower crystallization temperature than that in bulk. The lamellar thickness observed is presumed to have nearly equal value to the thickness of the growth nucleus as derived from the kinetic theory byLauritzen andHoffman.

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Zusammenfassung Es wurden Polyäthylen-Einkristalle isotherm aus 0,1%iger Lösung in Tetralin bei 77,2, 84,8 und 89,8 °C und aus n-Hexadecane (106,6 °C) hergestellt, und es wurden die Lamellendicken als Funktion der Kristallisationszeit untersucht. Diese Kristalle haben keine Änderung gezeigt oder, wenn, dann nur eine sehr geringfügige Änderung der Dicke. Sogar für Kristalle bei beträchtlich höherer Temperatur, erzeugt bei 106,6 °C, bei der die Segmentbeweglichkeit in den Kristallen aktiv zu werden beginnt, war der Anstieg der Lamellendicken nur von der Größenordnung 10 Å. Wenn Polyäthylenfraktionen von verschiedenen Molekulargewichten bei derselben Temperatur kristallisiert wurden, zeigten die Lamellendicken nahezu dieselben Werte. Die beobachteten Ergebnisse sind in schlagendem Kontrast zu denjenigen isothermer Kristallisation aus der Masse, wie sie beiHoffman undWeeks beobachtet wurden. Die Konstanz der Dicke von lösungsgewachsenen Kristallen wird damit interpretiert, daß die longitudinale Translation der Ketten und Kettenenden im Inneren eines Kristalls, notwendig zum Dicken-Wachstum, infolge der beträchtlich langsameren, niedrigeren Kristallisationstemperaturen als bei Kristallisation in der Masse stark behindert ist. Die beobachteten Lamellendicken haben nahezu den gleichen Wert wie die Dicke des Wachstumskeims, wie er sich aus der kinetischen Theorie vonLauritzen undHoffman ableilet.

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With 1 table     

Extended-chain crystals. VI. Annealing of polyethylene under elevated pressure

Polyethylene crystals of different degrees of perfection were annealed at 5.1 kb pressure for 20 hr at various temperatures and analyzed by electron microscopy, thermal analysis, and density determination. No annealing took place until the temperature was close to the melting point of the starting material. Up to 235°C increasing solidstate annealing was observed. Mixed crystals of up to 0.989 g/cm³ density and 1500 Å thickness in the chain direction could be produced. At slightly higher temperature recrystallization to extended-chain crystals rather than annealing occurred. The annealing process at atmospheric pressure seems to be similar in nature, but takes much longer for comparable perfection. From a comparison of annealing and crystallization it is concluded that polymer crystallization goes through a stage of internally imperfect order during which most of the observed chain extension occurs. Estimates of this outer imperfect layer of a growing crystal place its depth at 30,000 Å.     

NMR study of polyethylene crystallization kinetics under high pressure

Crystallization of polyethylene under hydrostatic pressures of 1–4.5 kbar is directly observed using pulsed proton NMR. The rate of growth of extended-chain polyethylene crystals is measured over this pressure range and to a maximum temperature of 227°C. The observed crystallization isotherms are superimposable on a log time scale; this implies a consistent mechanism for extended-chain growth over this pressure range. Avrami coefficients for high-pressure extended-chain crystallization are determined to be 1.3–1.7. A decrease of crystal nucleus surface free energies with increasing pressure is indicated. Findings are consistent with Wunderlich's model of initial folded-chain crystallization followed immediately by chain extension. Future applications of this NMR technique are briefly considered.     

Crystallization and melting of polyethylene under high pressure. II. Effect of pressure on melting behavior of various types of crystals

The effect of pressure on the melting point and volume of fusion of polyethylene was studied by high-pressure dilatometry. Starting materials were crystallized slowly from the melt under pressures of 1500, 3500, 5130 kg/cm², and 1 atm. It has been shown that the unusual behavior observed at pressures above 4000 kg/cm² is due to crystallization and melting of two kinds of extended-chain crystals differing in thermal stability. These are designated as ordinary extended-chain and highly extended-chain crystals, respectively. The relation between pressure P and melting temperature T_m of folded-chain, ordinary extended-chain, and highly extended-chain polyethylene was determined precisely. At pressures up to about 3000 kg/cm², plots of P against T_m for the crystal forms have almost the same curvature and then become parallel. But at pressures above 4000 kg/cm², ordinary extended-chain crystals show a linear increase of T_m with a constant slope of about 70 atm/deg. Curve for the highly extended-chain crystals changes in slope from 70 to 50 atm/deg at pressures between 3500 and 4300 kg/cm², and then show a sharp increase of T_m with increasing pressure. Experiments show that the meltingpoint curve of the highly extended-chain crystals overlaps that of the ordinary extended-chain crystals at pressures below 4000 kg/cm². Annealing experiments with folded-chain and ordinary extended-chain crystals have been made under high pressure. It is suggested that the formation of highly extended-chain crystals occurs stepwise through the formation and reorganization of ordinary extended-chain crystals from the original folded-chain crystals by a mechanism of partial melting and recrystallization at pressures above 4000 kg/cm².     

Lamellar and whisker single crystals of poly(2,6-oxynaphthoate)

Electron diffraction from single crystal lamellae and whiskers of poly(2,6-oxynaphthoate) reveals the presence of at least 3 unit cells. The equatorial reflections in the patterns from the whiskers correspond to the dominant (phase I) hk0 diffraction pattern from the lamellae; phase I is monoclinic with 2 chemical repeats per physical repeat. The intensity distributions in the hk0 patterns of phase I and II resemble those of the same phases in poly(p-oxybenzoate). The hk0 reflections of phase III suggest a common internapthalene unit spacing, but variable lateral (and possibly axial) shifts; apparently related orthorhombic and monoclinic patterns, with variable γ^*, are observed. At elevated temperature, above the crystalliquid crystal transition (ca. 330°C), quadrant reflections are retained; the change in the hk0 pattern from any given crystal is gradual, extending over some 40°C. Above the liquid crystal-liquid crystal transition (ca. 460°C) the pattern can be interpreted in terms of nematic or possibly smectic A packing. © 1992 John Wiley & Sons, Inc.     

Crystallization and melting of polyethylene under high pressure. III. Mixed crystallization of two kinds of extended-chain crystals

Thermal behaviors of extended-chain crystals of polyethylene formed during various crystallization processes and conditions under about 5000kg/cm² were studied by high-pressure dilatometry and differential scanning calorimetry. The experiments indicate that by isothermal crystallization at small undercoolings for prolonged periods, the products show two endothermic peaks in the melting region of the usual extended-chain crystals. This means of the presence of a bimodal lamellar thickness distribution in the extended-chain crystals. A phase diagram has been made for pressures up to 5000 kg/cm². The experimental results confirm the existence of two kinds of extended-chain crystals, i.e., ordinary extended-chain and highly extended-chain crystals, as suggested previously by the authors.     

Observations of slip traces in polyethylene single crystals

Single crystals of polyethylene are investigated by transmission electron microscopy. A pattern of lines is observed in the crystals. The properties of these lines suggest that the lines represent slip traces (slip steps) generated by the movement of dislocations in the polyethylene crystals.