Solution-crystallized Polyethylene Research Articles

Overlapping of different rearrangement mechanisms upon annealing for solution-crystallized polyethylene

The lamellar rearrangement upon annealing has been studied by a combination of differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS) analysis, and transmission electron microscopy (TEM) observation for solution-crystallized samples of a linear polyethylene having a narrow molecular weight distribution. The initial unannealed sample showed the regular stacking of single crystal lamellae of ∼12nm thickness, but showed double melting peaks, independent of the measured heating rate (HR). Each of the endotherms corresponds to the melting of initial lamellae and that of the crystals reorganized during DSC heating, respectively. The annealing behavior of the sample was significantly affected by both the annealing temperature and the time. A low-temperature annealing at 115°C, which is below the peak temperature of the former endotherm, slowly thickened the whole lamellae without any melting including partial or local lamellae. Increasing the annealing temperature (Ta) induces the other type of lamellar rearrangement mechanism where the lamellae rapidly thicken with the cooperative partial melting upon annealing. This rearrangement causes lamellar doubling behavior, showing a thickening of about twice, which was confirmed on TEM observation of the annealed samples. Because these doubled lamellae increased at the higher Ta, the lateral continuity of the initial lamellar stacking was lost. The DSC results revealed that these two different rearrangement mechanisms competitively coexist upon annealing over 120°C. Their balance predominantly depends on Ta. Correspondingly, the SAXS results indicate that the initial periodicity of the crystal/amorphous arrangement within the lamellar stacking morphology is lost at the middle Ta range of 123°C, where these two types of lamellar reorganization overlap. At the higher Ta range of 126°C, only a doubly rearranged lamellar structure remains, which causes an increase in the SAXS peak intensity.

The investigation of solution-crystallized polyethylene by slow calorimetry

Single crystals of low molar mass polyethylene (PE) have been grown and dissolved in decalin. A C8O calorimeter was used to follow the heat flow during dissolution and crystallization. The aim of this study was to determine if new information on this well investigated subject could be obtained using exactly controlled conditions for the phase change. These conditions were: leaving the crystals in their mother solution for analysis, recording the dissolution traces in an unusually slow temperature ramp (0.2–12 K h −1), following the growth of the crystals continuously during the whole crystallization process and obtaining very accurate enthalpies for the phase change. Three new results have been found: a high-temperature phase change occurs in the solution on cooling and on heating, with a corresponding change in enthalpy of 40–50 J g −1 which gives new information on the phase composition of low molar mass PE and on the interlamellar phase; the change of growth rate during crystallization is found to be linear with time; and for crystals grown at high T c, two kinetics of dissolution/crystallization are observed. The first result is attributed to strain in chains, and the third to the dissolution/crystallization of the (100) face in larger crystals which have a truncated lozenge shape.

Polyethylene mixed crystal infra-red spectroscopy: The influence of matrix isotopic species on guest conformation

The chain conformation of solution-crystallized polyethylene has previously been confirmed by infra-red spectroscopy, using the CD 2 bending bandshape for samples with a minority of the deuterated species 1–3. The method of calculating the i.r. band profile from molecular conformational models is now developed to give a more accurate representation of doublet splittings and to include the singlet component. The CH 2 bending profile is calculated for samples with a deuterated matrix, and the good agreement with experimental data indicates that the same molecular model is applicable for samples with the deuterated polymer as either guest or matrix, for identical crystallization conditions. This result implies that isotopic fractionation is unimportant under these conditions.

Nuclear Magnetic and Mechanical Relaxations in the γ-Loss Band of Linear Polyethylene Treated with Fuming Nitric Acid

Measurements of the NMR second moment and mechanical loss tangent for solution-crystallized polyethylene treated with fuming nitric acid at 80°C for 20 h reveal a relaxiation region around –140°C. The relaxation (correlation-time) spectrum for this relaxation region is in good agreement with that for the γ3-relaxation, which is one of the component relaxations of the γ-loss band of untreated polyethylene crystallized from the melt or solution. The γ1- and γ2- relaxations existing in untreated polyethylene are greatly decreased by this treatment. The observed change in the second moment in the γ3- region (ca. 1.5 gauss2), is explained as being due to the rotational motion of part of the ciliary chain on the lamellar surface. It is found that absorbed water changes this relaxation into a new one similar to the γ2-relaxation.

Distribution of straight-chain lengths in unannealed and annealed solution-crystallized polyethylene by Raman spectroscopy

The effect of annealing on the morphology of solution-crystallized polyethylene has been studied by analysing the shape of the low-frequency Raman LAM-1 band. The distributions of lengths of straight-chain segments have been determined for samples annealed at different temperatures. Unannealed samples, which have distribution peaks L max near 100 Å, have halfwidths ΔL 1 2 less than 20 Å. However, this narrow distribution is drastically broadened when the sample is annealed. The broadening is less if the breadth of the distribution of the unannealed sample is initially less. For equilibrium crystallized samples, the observed halfwidth and peak position of LAM-1 are found to be related. This relation can be understood quantitatively if it is assumed that ΔL 1 2 and L −1 max are linearly related as is indeed found to be the case for solution-crystallized samples. As L −1 max becomes very large, ΔL 1 2 approaches a limiting value near 300 Å.

Mixed-crystal infrared studies of chain organization in solution-crystallized polyethylene

We have extended our earlier technique of mixed-crystal spectroscopy of solution-crystallized polyethylene (PEH) by obtaining low temperature infrared spectra. Band contours resolved by computer analysis showed that the CD2 bending crystal field doublet has a constant splitting from infinite dilution up to ≈ 10 PEH/ 1 PED. The value of this splitting at infinite dilution was compared with values expected from finite groups of oscillators. It can be concluded from this analysis, as was shown from our earlier work, that folding with adjacent re-entry along (110) planes predominates in these crystals. In addition, we now find that there is a high probability that a molecule will fold back along itself on the next adjacent (110) plane.

The annealing of solution-crystallized polyethylene mats: an X-ray diffraction study

Both low- and wide-angle X-ray diffraction measurements have been made on solution-crystallized mats of high density polyethylene which have been annealed for different periods at 125° C. The low-angle data, in addition to showing the rapid increase in both long period and density defect at each fold surface for the first stages of annealing, also indicate that after annealing for times in excess of 105 sec the density defect is reduced and approaches the value for the unannealed crystal. The wide-angle X-ray studies have centred around the Fourier analysis of the 0 0 2 diffraction peak. The results of this type of measurement on unannealed mats have already been reported [20]. In extending the work to annealed material evidence has been obtained for preferred chain stem lengths which are multiples of the unannealed length. The combination of these observations with the low-angle results has led to the formulation of a model for crystal thickening which invokes the unlooping mechanism proposed by Dreyfuss and Keller [15], but also envisages thickening occurring preferentially from one fold surface to give an asymmetric crystal profile in the [0 0 1] direction.

Cell dimensions of hydrocarbon crystals: Surface effects

The unit-cell dimensions of a given polyethylene have previously been shown to vary nearly linearly with the reciprocal of lamella thickness. Data obtained at 153.2 and 296.2°K are presented to show that the slope of this dependence is different for crystals of orthorhombic n -paraffins, melt-crystallized polyethylene, and solution-crystallized polyethylene. Within the limits of error, all extrapolate to the same basal area at infinite lamella thickness, and this agrees with the measured value for a sample crystallized from the melt under high pressure to yield a long period of about 3500 Å. Since the effect is a surface one, it is proposed that these differences result from the differences between methyl interactions, fold interactions, different fold planes, domains, etc. It is shown that the variation of cell dimension with lamella thickness leads to a quadratic term in the variation of macroscopic density. This term permits the separation of the thickness and density of a lower-density surface layer in a two-phase model.

The effect of crystallization conditions on radiation-induced crosslink formation in polyethylene

Abstract The decrease in solubility with radiation dose has been measured in bulk Marlex 50 polyethylene and in a series of solution-crystallized polyethylene prepared under different conditions. It was found that the solubility–dose curves were all of a similar shape and could be superimposed by multiplying the radiation doses given to each sample by a constant factor which is a measure of the ‘effectiveness’ of the radiation. The bulk material needed the lowest gelation dose while the gelation dose for the solution-crystallized specimens depended on the method of preparation. Measurements of hydrogen gas evolution and free radical formation indicated that these results could not be explained by differences in the overall number of crosslinks formed. They can be explained by assuming that there are ‘effective’ and ‘ineffective’ crosslinks and the relative numbers of these are a function of crystallization conditions. Our results are in accord with the hypothesis that crosslinks are formed preferentially at the chain folds at lamellar surfaces and that the ‘effective’ and ‘ineffective’ crosslinks are inter- and intra-lamellar crosslinks respectively. The amount of interlamellar contact is reflected by the degree of effectiveness of the radiation.

The effect of crystallization conditions on radiation-induced cross-link formation in polyethylene

Abstract The decrease in solubility with radiation dose has been measured in bulk Marlex 50 polyethylene and in a series of solution-crystallized polyethylene prepared under different conditions. It was found that the solubility–dose curves were all of a similar shape and could be superimposed by multiplying the radiation doses given to each sample by a constant factor which is a measure of the ‘effectiveness’ of the radiation. The bulk material needed the lowest gelation dose while the gelation dose for the solution-crystallized specimens depended on the method of preparation. Measurements of hydrogen gas evolution and free radical formation indicated that these results could not be explained by differences in the overall number of crosslinks formed. They can be explained by assuming that there are ‘effective’ and ‘ineffective’ crosslinks and the relative numbers of these are a function of crystallization conditions. Our results are in accord with the hypothesis that crosslinks are formed preferentially ...

Thermodynamic stability of solution-crystallized polyethylene

Thermodynamic stability of solution-crystallized polyethylene