Phase Boundaries and Crystallization of ... - ACS Publications

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Ind. Eng. Chem. Res. 2006, 45, 1478-1492

GENERAL RESEARCH Phase Boundaries and Crystallization of Polyethylene in n-Pentane and n-Pentane + Carbon Dioxide Fluid Mixtures Gerd Upper,† Wei Zhang,‡ Daniel Beckel,§ Seungman Sohn,| Kun Liu, and Erdogan Kiran* Department of Chemical Engineering, Virginia Polytechnic Institute and State UniVersity, Blacksburg, Virginia 24061

The liquid-liquid and fluid-solid phase boundaries and the densities of 5 wt % solutions of polyethylene in n-pentane and in n-pentane (85 wt %) + carbon dioxide (15 wt %) fluid mixtures were determined over a temperature range from ∼360 to 400 K at pressures up to 52 MPa. The measurements were conducted using a variable-volume view cell equipped with sensing elements to monitor the changes in the internal volume and in the transmitted light intensity as the pressure or the temperature of the system is changed. Polyethylene crystals were formed at selected temperatures below the fluid-solid phase boundary along a series of selected constant-pressure paths. They were analyzed by scanning electron microscopy (SEM) and by differential scanning calorimetry (DSC) at ambient conditions for differences in morphological features and thermal properties. Depending on the temperature, the pressure, and the crystallization time, crystallinity levels changed from 65 to 80 %. The crystals that are produced from these high-pressure solutions all showed multiple melting transitions. The majority of the DSC scans at 10 K/min heating rate show a small melting peak at ∼395 K and two additional, more-distinct peaks in the temperature range from 399 to 403 K. The multiple transitions were, however, observed to collapse to a single melting peak at 404 K in the second heating scans. Microscopic evaluations show that the morphologies are prevailingly dominated by ellipsoid-shaped folded-lamellar units 10-20 µm in longer dimension that agglomerate. Unique, long (50-100 µm) strands of stacked lamellar structures appear to form at ∼40 MPa. Introduction Polyethylene is perhaps the most important commodity polymer, with its production being the largest in the world. Its solubility and crystallization are naturally of great practical interest in the processing of polyethylene (PE). Studying highpressure compressed fluids is a more recent and growing activity. Several studies have already been carried out on the phase behavior of polyethylene and other polyolefins in nalkanes and n-alkane + carbon dioxide fluid mixtures.1-8 The majority of these studies have been on the liquid-liquid (LL) phase transition. The LL phase transition of polyolefin in these fluids is now well-known to display a lower critical solution temperature (LCST)-type behavior, which is shifted to higher pressures in the presence of carbon dioxide.1,2 The character of the phase boundary eventually transforms into an upper critical solution temperature (UCST)-type behavior with increasing level of carbon dioxide. Even though limited, some studies have also been carried out on the fluid-solid (FS) phase transition in polymer solutions in pressurized or supercritical fluids.9-12 These studies show that the fluid-solid phase boundary is * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: (540) 231-1375. Fax: (540) 231-5022. † Present address: Universita ¨ t Karlsruhe (TH), Institut fu¨r Technische Thermodynamik und Ka¨ltetechnik, Karlsruhe, Germany. ‡ Present address: Advanced and Applied Polymer Processing Institute (AAPPI), Danville, VA. § Present address: Nonmetallic Inorganic Materials, Swiss Federal Institute of Technology (ETH), Zurich, Switzerland. | Present address: E-Ink Corporation, Cambridge, MA.

usually very sharp, and the phase-separation temperature may display sensitivities to pressure. Earlier studies conducted in our laboratory on the FS phase boundary of polyethylene in n-pentane showed that the fluid-solid phase boundary is indeed sharp and confined to a relatively narrow temperature range. The influence of pressure on the morphology of polyethylene crystals has been of special interest, and studies have been reported up to ∼800 MPa.13-21 These studies have reported that the polyethylene crystals that are formed under pressure either from melt or from solutions may display different morphologies and/or display multiple melting transitions. The two common explanations for the multiple peaks that are given in the literature are based on the notions that (a) under high pressure, metastable crystal structures form which then undergo rearrangements during heating in differential thermal analysis (DTA) or differential scanning calorimetry (DSC) analysis, or that (b) foldedchain crystals undergo recrystallization. In the pressure range from 180 to 300 MPa, both folded-chain crystals (FCC) and extended-chain crystals (ECC) were observed. Miyata et. al.,14 who studied crystallization and melting of polyethylene from a xylene solution in a high-pressure DSC at pressures up to 800 MPa, observed that the formation of extended-chain crystals was decreased with increasing xylene concentration and that the characteristic DSC melting peak disappeared when the xylene content was >30%. The addition of xylene to polyethylene appeared to prevent the nucleation of the extended-chain crystals. Yasuniwa et. al.15 investigated the high-pressure crystallization of polyethylene samples of different molecular weights (in the range from 4 000 to 2.5 million) in a high-

10.1021/ie050620e CCC: $33.50 © 2006 American Chemical Society Published on Web 01/12/2006

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pressure DTA at pressures up to 590 MPa and temperatures up to 430 K. The DSC curves for samples with molecular weights of 4 000, 10 000, and 67 000 showed three peaks; the highest peak was associated with the thickest lamellae that is representative of the extended-chain crystals. The origins of the other two peaks were not elucidated. They noted that the morphology of the crystals shows a dependence on molecular weight and that there were significant differences for the higher-molecularweight samples. The heat of fusion of the pressure-crystallized samples decreased linearly with the logarithm of molecular weight. The melting temperature was higher than ∼420 K and increased slightly with the molecular weight in a molecular weight region >100 000. Yasuniwa and co-workers16 also reported on the crystallization of ultrahigh-molecular-weight polyethylene from 0.1 wt % decalin solutions. These studies were conducted at pressures up to 600 MPa and temperatures up to 550 K. Nakafuku and Sugiuchi17 investigated the behavior of n-alkanes with a carbon number up to 30 in a high-pressure DTA at pressures up to 500 MPa and temperatures up to 430 K. Nakafuku18 also investigated the crystallization of polyethylene and binary mixtures of low-molecular-weight polyethylene in a high-pressure DTA at pressures up to 500 MPa and temperatures up to 510 K. For a polyethylene sample with a molecular weight of 10 000, he reported the formation of extended-chain crystals from melt at pressures up to 400 MPa. However, mixing low-molecular-weight polyethylene impeded the formation of ECC. For example, ECC was not observed if lower-molecular-weight polyethylene content was >50%. Ho¨hne and Blankenhorn19 crystallized polyethylene in a high-pressure DSC at pressures up to 500 MPa and temperatures up to 500 K. At pressures above 300 MPa, they observed that the melting peak split into two, where the first peak was attributed to a transition into the hexagonal phase and the second represented the fusion peak. Shulgin et al.20 crystallized polyethylene and poly(diethylsiloxane) (PDES) in a high-pressure DTA at pressures up to 800 MPa and temperature up to 600 K. Their investigation had revealed that crystallization from mesomorphic states for polyethylene and PDES led to the formation of more perfect crystals. Ho¨hne and co-workers21 crystallized polyethylene under high pressure (up to 550 MPa) and temperature (up to 510 K); then they scanned these crystals both in an atmospheric-pressure DSC and a high-pressure DSC. They reported that the polymer crystallized at high pressure seemed to be in a metastable state. More recent studies include investigation of the dependence of the melting temperature for low- and high-density polyethylenes and polypropylene on pressure up to 330 MPa.22 It is reported that melting point increases with pressure by 11-17 K per 100 MPa. In studies conducted in our laboratory, solid-fluid phase boundary reflecting incipient crystallization of polyethylene in n-pentane showed a temperature minimum in the pressure range from 38 to 41 MPa.23 In a more recent work conducted in our laboratory on the kinetics of crystallization of polyethylene in dilute solutions in n-pentane, crystals with ellipsoidal morphologies that also display multiple melting peaks were obtained.24-26 In this paper, we report on the liquid-liquid and the fluidsolid phase boundaries as well as on density data of polyethylene in pure n-pentane and in a fluid mixture of CO2 (15%) + n-pentane (85%). We present the results of the crystallization of polyethylene from these solutions along constant-pressure paths in the range from 22 to 52 MPa, which is achieved by lowering the temperature to conditions below the fluid-solid phase boundary while continually adjusting the pressure during the process. The degree of crystallinity and the melting

Figure 1. Molecular-weight distribution of the polyethylene samples.

transitions of these samples were characterized using a DSC at ambient pressure. Morphological features were studied by electron microscopy. Experimental Section Materials. The majority of the measurements were performed using a high-density polyethylene sample with Mw ) 121 000, polydispersity index (PDI) ) 4.3, and F ) 0.96 gcm-3. A limited number of experiments were also conducted on the phase boundaries using a narrow molecular-weight-distribution hydrogenated polybutadiene sample with Mw ) 108 000 and PDI ) 1.3. The molecular weight distributions for these polymers are shown in Figure 1. The peak melting temperatures as determined by differential scanning calorimetry at a 10 K/min heating rate were 373 and 404 K for the narrow and broad molecular-weight-distribution samples, respectively. The solvent, n-pentane with a purity >99%, was purchased from SigmaAldrich. Carbon dioxide with a purity >99.99% was purchased from Air Products. The polymers and the fluids were used as received. Variable-Volume View Cell and Determination of Density and Phase Boundaries. A specially designed variable-volume view cell has been used for assessing the miscibility and phaseseparation conditions and for determination of densities. The view cell was also used to carry out the crystallization experiments. Figure 2 shows the details of the view cell3,23,24 and the methodology of determining the phase boundaries. The view cell is equipped with two sapphire windows. The internal volume of the cell is varied with the aid of a movable piston (MP). A linear-variable differential transformer (LVDT) is used to monitor the position of the piston. The piston is attached to a steel rod with a ferromagnetic metal piece, the position of which is sensed with the LVDT coil. To determine the exact position of the piston, the LVDT coil is moved until the digital signal from the unit shows zero. At this point, the position readout is set to zero. When the piston is moved to a new position (using the pressure generator, PGN), the LVDT coil is moved to sense its new location. A position readout unit attached to the LVDT coil gives the actual distance traveled by the piston with an accuracy of 0.013 mm. Four cartridge heating elements (HE) are used to heat the cell. Temperature is controlled and monitored with a resolution of (0.1 K. The pressure is measured to an accuracy of (0.06 MPa using a Dynisco flush-mounttype transducer and read with a resolution of (0.007 MPa. The temperature is measured with an accuracy of (0.5 K. Phase

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Figure 2. Schematic diagram of the view cell for determination of miscibility conditions and densities and illustration of the different paths followed to determine phase boundaries and the corresponding changes in temperature (T), pressure (P), and transmitted light intensity (Itr) with time. The lower part of this figure demonstrates the incipient demixing pressure (Pi) or temperature (Ti) along a (A) constant-temperature or (B) constant-pressure path (F-S ) fluid-solid-phase boundaries and L-L ) liquid-liquid phase boundaries).

boundaries are determined from the change in the transmitted light intensity (Itr). In a given experiment, the polymer is loaded first from the top by removing the variable-volume attachment. The cell is then closed, and the fluid is directly pumped into the cell using high-pressure liquid chromatography (HPLC)-type pumps (with cooled pump heads) from a preloaded transfer vessel. When working with mixtures of n-pentane and carbon dioxide, n-pentane is charged first. Then, using a second transfer vessel, CO2 is charged. The charged amounts are recorded by weighing each transfer vessel using a sensitive balance (Mettler 6100 with an accuracy of (0.01 g) before and after loading the view cell.

During the charging process, no pressure is applied on the pressure generator (PGN) to ensure that the experiments are started with the movable piston at its top position, i.e., the cell at its maximum volume. After the desired amount of the polymer and the solvent corresponding to a target composition are loaded, the system temperature is changed and the pressure is adjusted with the aid of the pressure generator until complete miscibility is achieved at that temperature. The miscibility is visually verified by observing the solution through the sapphire windows. A magnetic stirrer (MS) is used to facilitate the dissolution process. Phase-separation conditions are then determined by changing the pressure or the temperature of the system. The


Figure 3. Variation of temperature (T), pressure (P), and transmitted light intensity (Itr) in arbitrary units with time along (a) constant-pressure and (b) constant-temperature paths. The incipient demixing conditions at Itr are determined from the variation of the transmitted light intensity with pressure or temperature (lower curve in the figures). (a) Constant pressure path in 5 wt % polyethylene (121 K) in 15 wt % CO2 + 85 wt % n-pentane at 37.0 MPa; and (b) constant-temperature path in 4.99 wt % polyethylene in 15 wt % CO2 + 85 wt % n-pentane at 388.5 K.

density at any given pressure and temperature condition is determined from the total initial mass loading of the cell (M), which is known, and the volume (V) at the prevailing P and T, which is calculated from the initial cell volume and the volume change that occurs as a result of the piston movement that is monitored with the LVDT coil. Figure 2 demonstrates the methodology used in determining the solid-fluid- (SF) and the liquid-liquid (LL) phase boundaries. These determinations are made along either a constanttemperature path (path A) or a constant-pressure path (path B). When temperature is maintained constant while pressure is reduced, or when temperature is reduced while the pressure is held constant, a rapid decrease in the transmitted light intensity is observed upon phase separation. During an experiment, the temperature, the pressure, and the transmitted light intensity are recorded with a computer as a function of time. The data are then manipulated to generate a plot showing the change in transmitted light intensity with pressure or with temperature. The incipient demixing pressure Pi is identified as the departure point from the base transmitted light intensity for the homoge-

neous solution. Similarly, the incipient phase demixing temperature Ti is determined as the departure temperature from the base transmitted light intensity for this path, as illustrated in Figure 2. Experiments following isobaric paths such as B are employed to identify the solid-fluid boundaries, and experiments following isothermal paths such as A are used to identify the liquid-liquid boundaries. Parts a and b of Figure 3 show the actual computer outputs generated in determination of the SF and LL boundaries in a 5 wt % PE (121 K) solution in a fluid mixture of pentane (85 wt %) and CO2 (15 wt %). In this experiment, the SF boundary was crossed at a cooling rate of 0.02 Ks-1 while holding the pressure at 37 MPa, and the LL boundary was crossed with a pressure reduction rate of 1.09 MPa‚s-1 at 388.6 K. Crystallization at Constant Pressure. Figure 4 shows the details of isobaric crystallization pathways that were employed to identify the pressure dependence of the SF phase-separation conditions, where crystallization may begin. In some of the experiments, the temperature was lowered at constant pressure to a target crystallization temperature Tc. This leads to different

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Figure 5. Incipient demixing pressures and temperatures for 5 wt % polyethylene (121 K) solution in n-pentane; S-F ) solid-fluid phase boundary and L-L ) liquid-liquid phase boundary.

Morphological Characterizations. Morphological features of PE were characterized using a Leo 1550 field emission scanning electron microscope (FESEM). The samples were sputter-coated with a 6 nm gold-platinum layer to reduce the electron charging effects using a Cressington 208 HR sputter coater. Results and Discussion

Figure 4. Schematic diagram of the crystallization conditions along constant-pressure paths. Crystallization at constant Tc with different undercooling at different pressures (top), and crystallization with constant undercooling (bottom). F-S and L-L represent the fluid-solid and liquidliquid phase boundaries.

degrees of undercooling, that is, different values of (Tsf - Tc) at different pressures. To maintain a constant degree of undercooling, some experiments were carried out at different temperatures at different pressures. During cooling, the pressure in the view cell was continuously adjusted (by moving the piston with the pressure generator) to compensate for the volume reduction that accompanies cooling. The system was held at the crystallization temperature Tc for a selected time duration (typically 1 h) for crystallization to proceed further. Then the cell was depressurized by opening an exit valve. The cell content was purged with low-pressure (∼3 MPa) carbon dioxide to remove any residual n-pentane. Then the cell was cooled to ambient temperature, and the variable-volume attachment was removed to collect the polymer. Characterization with Differential Scanning Calorimetry. The polyethylene samples recovered from the view cell were characterized using differential scanning calorimetry (PerkinElmer DSC 7) at ambient pressures. All the samples were analyzed using crimped aluminum pans under a nitrogen atmosphere at a scanning rate of 10 K/min. For each sample, three consecutive scans were carried out. The first scan included heating to a temperature higher than the melting transitions, which was followed by immediate cooling to room temperature. This was followed by a second set of heating and cooling scans at identical rates, and then a final heating scan was carried out. There was no holding time between the heating and cooling scans.

Phase Boundaries. Figure 5 shows the fluid-solid phase boundary determined in the present study along with data on fluid-solid and liquid-liquid phase boundary data obtained in an earlier study23 in our laboratory for a 5 wt % solution of PE (121 K) in n-pentane. The new SF boundary data (filled symbols in the figure) were obtained by cooling while maintaining pressure constant at 52.1, 44.5, 40.8, 38.5, 37.0, 33.3, 29.5, and 22.0 MPa. Each data point corresponds to results obtained with a fresh loading of the cell. The demixing temperatures corresponding to these pressures were 379.5, 377.5, 377.3, 376.3, 377.3, 376.8, 375.3, and 376.3 K, respectively. As shown in the figure, the fluid-solid phase boundary is a sharp boundary, as expected. The demixing temperatures appear to show a local minimum (even though small), which is more noticeable in the pressure range from 37 to 41 MPa. These features are also observed in the data from our earlier studies.23 We recently conducted a new set of experiments with a polyethylene sample which has a narrower molecular-weight distribution (Mw ) 108 K, Mw/Mn ) 1.3; see Figure 1). The results are shown in Figure 6. Consistent with the DSC data for this polymer, SF boundaries are observed at lower temperatures. The LL boundary is at lower pressures. The SF boundary for this polymer sample also shows temperature minima at ∼42 MPa and at ∼18 MPa. Figure 7 shows the incipient demixing pressures and the incipient demixing temperatures for a 5 wt % polyethylene (Mw ) 121 K) solution in a CO2 (15 wt %) + n-pentane (85 wt %) fluid mixture. This FS boundary was determined at 52.1, 44.5, 40.8, 38.5, 37.0, and 33.3 MPa with corresponding incipient phase-separation temperatures of 379.3, 379.3, 379.2, 378, 378.2, and 377.2 K. As shown in the figure, the demixing temperatures appear to show a decrease in the pressure range from 37 to 41 MPa in this system also. Once again, each data point corresponds to a new loading of the cell with the target polymer concentration. With this system, with each loading, the liquidliquid phase boundary was also determined in the present study


Figure 6. Solid-fluid and liquid-liquid phase boundaries for 5 wt % solutions of a polyethylene sample with Mw ) 108 000 and PDI ) 1.3. Figure 8. Variation of density with temperature for 5 wt % polyethylene (121 K) in n-pentane at 44.5, 33.3, and 22.0 MPa.

Figure 7. Incipient demixing pressures and temperatures for 5 wt % polyethylene (121 K) solution in a 15 wt % CO2 + 85 wt % n-pentane fluid mixture. Each data point in the solid-fluid boundary (filled symbols) corresponds to a fresh loading. Each data set in the liquid-liquid boundary corresponds to a fresh loading.

in the temperature range from 380 to 400 K and is included in the figure. The liquid-liquid phase boundary shows a positive slope, with increasing demixing pressures from ∼29 to 31 MPa over the experimental temperature range of 383-398 K. It should be noted that the liquid-liquid phase boundary of the polymer-CO2-n-pentane system is shifted, by >17 MPa, to higher demixing pressures compared with the liquid-liquidphase boundary of the polymer-n-pentane system. The fluidsolid phase boundary, however, is not as sensitive to the presence of CO2 in the system. The SF boundary shows only a small shift to higher demixing temperatures compared to that of the polymer solution in the n-pentane system. Densities. The density data were generated along the constantpressure paths from the one-phase to the two-phase region during all fluid-solid phase boundary determination experiments. The density measurements were conducted in 2 K intervals. Figures 8 and 9 show the variation of the mixture density with temperature for 5 wt % polyethylene (121 K) in n-pentane and for a 5 wt % polyethylene in CO2 (15 wt %) + n-pentane (85 wt %) fluid mixture, respectively, along the constant-pressure paths from 390 to 370 K. No major changes in the density are noted while crossing the fluid-solid phase boundary at ∼378 K. Densities are in the range ∼0.56-0.64 g/cm3 for these solutions.

Figure 9. Variation of density with temperature for 5 wt % polyethylene (121 K) in 15 wt % CO2 and 85 wt % n-pentane fluid mixtures at 52.1, 45.5, and 33.3 MPa.

Thermal Behavior of Crystals. A number of crystallization experiments were conducted to study the effects of pressure, temperature, and crystallization time. During these experiments, the cooling rate in approaching the crystallization temperature from the one-phase solution was ∼0.02 K/s. The crystallization at the target temperature was carried out for 30, 60, and 120 min. In the majority of the experiments, the crystallization temperature was 363 K. However, experiments were also conducted at four additional temperatures: 368, 373, 377, and 376 K. In these experiments, the crystallization time was kept at 60 min. The effect of crystallization time was studied at 363 K. (a) Effect of Crystallization Pressure. Figure 10 shows the DSC scans for polyethylene (121 K) samples crystallized from n-pentane solutions at 38.5 MPa and 363 K for 60 min. The first scan shows the behavior of the polyethylene crystals as collected from the view cell. It displays multiple melting transitions. Around 395 K, there is a small shoulder followed by two additional endothermic peaks at 399 and 402 K. The sample was immediately cooled when the temperature reached 410 K at the end of the first scan. During cooling, a single exothermic crystallization peak is observed at 387 K. In the second heating scan, which is carried out immediately after cooling to 303 K, only one broad melting peak is observed at ∼403.5 K. The sample was cooled and heated again. The DSC output during the second cooling and third heating scans were

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Figure 10. DSC scans on polyethylene (121 K) crystallized from a 5 wt % solution in n-pentane at 38.5 MPa and 363 K for 60 min. The heating and cooling rates were 10 K/min.

Figure 11. Comparison of the first heating scans in DSC measurements for polyethylene (121 K) crystallized from an n-pentane solution at 363 K for 60 min at the pressures indicated with the original polyethylene sample.

Figure 12. Comparison of the first heating scans in DSC measurements for polyethylene (121 K) crystallized from a 15 wt % CO2 + 85 wt % n-pentane solution at 363 K for 60 min at the pressures indicated with the original polyethylene sample.


Figure 13. Comparison of the first heating scans in DSC measurements for polyethylene (121 K) crystallized from 5 wt % solution at different crystallization temperatures in n-pentane at 38.5 MPa for 60 min with the original polyethylene sample.

Figure 14. Comparison of the first heating scans in DSC measurements for polyethylene (121 K) crystallized from 5 wt % solution for different crystallization times in n-pentane at 38.5 MPa and 363 K with the original polyethylene sample.

identical to the first cooling and second heating scans. DSC analyses were conducted for all the samples in a similar way. Figure 11 compares the first heating scans of crystals formed at 363 K over 60 min at different pressures. The figure shows that all these polyethylene crystals formed in n-pentane under pressure show three melting transitions composed of a shoulder and two better resolved peaks. The scan for the original polyethylene sample is also included for comparison. Figure 12 shows the comparison of the first heating scans from all DSC measurements on polyethylene samples crystallized from the CO2 (15 wt %) + n-pentane (85 wt %) fluid mixture at 363 K for 60 min at the indicated crystallization pressures. These scans also display the general features observed with crystals formed in n-pentane. (b) Effect of Crystallization Temperature. Figure 13 compares the first heating scans of the DSC scans on the

polymers crystallized in n-pentane at 38.5 MPa for 60 min at different temperatures, namely, at 363, 368, 373, 376, and 377 K. The degrees of undercooling (Tsf - Tc) at these temperatures were 13, 8, 4, ∼1, and ∼0 K, respectively. Each crystallization run corresponds to a new loading of the cell, and therefore, the incipient phase-separation temperature shows small variations. The DSC scans of the crystals formed at 377 and 376 K, very close to the incipient fluid-solid demixing temperature, show only one major melting peak. With decreasing crystallization temperature and, thus, increasing undercooling, the multiple melting peaks become more distinct. (c) Effect of Crystallization Time. Figure 14 compares the first heating scans for polyethylene (121 K) samples crystallized in n-pentane solutions at 38.5 MPa and 363 K corresponding to ∼13 K undercooling, for different crystallization times, namely, 30, 60, and 120 min. No major changes in the

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Figure 15. Comparison of the intermediate melting peak in the first DSC scan (ambient pressure) of polyethylene (121 K) crystallized from 5 wt % solutions in n-pentane with the incipient fluid-solid demixing temperature in the view cell at the indicated pressures.

Figure 17. Comparison of the intermediate melting peak temperature from the first DSC scan with the melting peak temperature from the second DSC scan for polyethylene (121 K) crystallized from 5 wt % solution of polyethylene in n-pentane at 38.5 MPa for 60 min.

for the system to undergo liquid-liquid phase separation followed by crystallization of the polymer-lean and polymerrich phases that experience different degrees of undercooling. This could lead to the formation of crystals with different lamellar thickness and melting temperatures. For polymers, lamellar thickness is inversely proportional to the degree of undercooling according to

L)

Figure 16. Comparison of the intermediate melting peak in the first DSC scan (ambient pressure) of polyethylene (121 K) crystallized from 5 wt % solutions in 15 wt % CO2 + 85 wt % n-pentane fluid mixtures with the incipient fluid-solid demixing temperature in the view cell at the indicated pressures.

multiplicity of the melting peaks were observed for these crystallization times. However, the middle peak at 399 K becomes better defined with longer crystallization time. (d) Further Discussion on Multiplicity of DSC Melting Transitions. As already remarked in the Introduction, multiple melting transitions have been previously reported in highpressure crystallization of polyethylene melts. In those studies, these melting peaks were attributed to the presence of foldedchain crystals (FCC) and extended-chain crystals (ECC), with the ECC being promoted at high pressures. The present results describe crystals that form from solutions at high pressure. Multiplicity of melting peaks may also arise from the existence of crystals with distinct lamellar thicknesses. In the present experiments, the formation of crystals with different lamellar thicknesses is a strong possibility. During crystallization from these solutions at a given undercooling level, it is possible that a fraction, i.e., the lower-molecular-weight fractions of the polymer, remain in the dissolved state while the higher-molecular-weight chains phase separate and crystallize. The chains that remain in the solution are then forced to phase separate and rapidly crystallize in the second stage during depressurization of the cell. During this process, it is possible

2σT0m ∆H(T0m - Tc)

(1)

where T0m ) equilibrium melting temperature, σ ) fold surface free energy, Tc ) crystallization temperature, and ∆H ) latent heat of fusion. Examination of Figures 10-14 along with the data on the degree of undercooling suggest that the appearances of distinct melting peaks are promoted for a ∆T range of ∼12-13 K for the initial crystallization step in the view cell. For deeper quenches of 15-16 K or shallower quenches of 8 K or smaller, the melting transitions appear broad. The broadening of the DSC peak for crystals formed very close to the demixing temperature may be interpreted as implying that, in this case, the majority of the polymer actually undergoes the crystallization process in the depressurization stage. Figure 15 compares the melting temperature in terms of the intermediate melting temperature in the DSC scan with the fluid-solid boundary corresponding to the incipient demixing data from the view cell. The variation of these DSC melting peak temperatures with pressure is similar to the variation of the incipient demixing temperature with pressure. The remarkable similarity of these trends in every detail suggests that the crystals that are collected from the view cell are indeed representing the crystals that are formed at the incipient fluidsolid phase boundary or that the subtle changes in terms of localized minimum in the solid-fluid phase-separation temperatures with pressure are indeed real. The differences in the actual temperatures arise from the difference in the heating and cooling rates in these respective experiments. Figure 16 is a similar comparison of the DSC melting temperature data with the SF phase boundary data obtained in the view cell for the crystals obtained in the CO2 (15 wt %) + n-pentane (85 wt %) fluid mixtures. The examination of Figure 10 and similar runs on all other experiments shows that the melting transitions in the second

Ind. Eng. Chem. Res., Vol. 45, No. 4, 2006 1487 Table 1. Degree of Crystallinity for Polyethylene (121 K) (A) Crystallized from n-Pentane Solutions wc(Tc) (%) crystallization pressure (MPa)

first scan

second scan

52.1 44.5 40.8 38.5 37.0 33.3 29.5 22.0

74.0 77.2 76.1 75.9 74.8 71.9 71.7 74.8

64.5 69.2 66.4 66.4 66.3 62.7 63.3 65.0

(B) Crystallized from Fluid Mixture of 15 wt % CO2 + 85 wt % n-Pentane wc(Tc) (%) crystallization pressure (MPa)

first scan

second scan

52.1 44.5 40.8 38.5 37.0 33.3

72.7 77.6 75.0 75.2 75.1 72.8

65.0 68.5 65.1 64.6 64.8 63.9

Figure 18. Difference in the percent crystallinity of polyethylene (121 K) crystallized at different pressures as assessed from the first and second DSC scans of polyethylene. Filled symbols represent crystallization from n-pentane, and unfilled symbols represent crystallization from a 15 wt % CO2 + 85 wt % n-pentane fluid mixture.

(C) Crystallized from n-Pentane Solutions at Different Subcoolings wc (Tc) (%) crystallization temperature (K)

first scan

second scan

377.6 376.5 373.2 368.2 363.2

80.5 93.4 71.9 76.2 75.9

66.5 77.6 60.9 65.3 66.4

(D) Crystallized from n-Pentane Solutions at Different Crystallization Times wc(Tc) (%) crystallization time (min)

first scan

second scan

120 60 30

69.6 75.9 71.2

59.8 66.4 63.3

heating scans are at a higher temperature than the melting transitions observed in the first heating scans. We have analyzed the present data by comparing the differences between the intermediate melting peak temperatures in the first scan and the melting temperature observed in the second scan. Figure 17 compares the data for crystals formed in n-pentane at different levels of subcooling. With increasing undercooling or decreasing crystallization temperature, the difference in the DSC melting transition between the first and second scan increases from ∼1.8 K at the crystallization temperature of 377.6 K (which is very close to the incipient demixing temperature) to 4.5 K at 363 K. The crystallization carried out with a higher degree of undercooling results in polyethylene crystals that have lower melting temperatures, suggesting smaller lamellar thickness. (e) Degree of Crystallinity. The degree of crystallinity was calculated using the method of Mathot and Pijpers.27 In this method, the heat flux recorded at temperatures above the melting temperature, which is proportional to the heat capacity of the amorphous (liquid) polymer, is linearly extrapolated to the temperature T, at which point the degree of crystallinity wc(T) is to be calculated. The area under the melting peak bounded by the extrapolated baseline is then evaluated. The heat of fusion ∆h(T) corresponding to this area is then divided by a reference value of the heat of fusion ∆h*(T) corresponding to a completely crystalline sample at the same temperature. For linear polyeth-

Figure 19. Difference in the percent crystallinity at different crystallization temperatures between the first and second heating scan of polyethylene (121 K) crystallized from a n-pentane solution at 38.5 MPa for 60 min. The scanning rate was 10 K/min.

ylene, Mathot and Pijpers suggest the following equation for ∆h*(T) for temperatures T g 290 K:

∆h*(T) ) 293 - (0.3902 × 10-5)(414.6 - T)2(414.6 + 2T) (J/g) The degree of crystallinity wc(T) at a given temperature is then obtained from

wc(T) )

∆h(T) × 100% ∆h*(T)

For each sample, the heat of fusion ∆h(T) and the percent crystallinity wc(T) was calculated at the crystallization peak temperature Tc observed in the cooling cycle of the DSC scan. Calculations were carried out using the heat of fusion obtained from the first DSC scans and also from the second DSC scans, during which the multiple peaks collapse to one peak. The results are shown in Table 1. The average degree of crystallinity for all samples determined from the second DSC scans is ∼65.1 ( 0.4% at 386.5 ( 0.1 K. The degree of crystallinity of the original polyethylene was ∼64.3% at 385.8 K. This leads to the condition that the thermal history of the samples is completely erased after the first heating scans. The degree of crystallinity based on the first DSC scans are all higher than

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Figure 20. FESEM micrographs of polyethylene (121 K) samples that were crystallized at 363 K for 60 min in n-pentane at the nominal pressures of (a) 23, (b) 38, (c) 40, (d) 42, and (e) 53 MPa, and (f) repeated at 40 MPa.

the original polyethylene crystallinity. Figure 16 shows the difference in the degree of crystallinity of samples crystallized at different pressures in n-pentane and in CO2 + n-pentane mixtures compared to the crystallinity determined from the

second scans, which are taken as representative of the initial polyethylene sample. As shown in Figure 18, all the samples crystallized from n-pentane, show ∼8-11% higher crystallinity. Figure 19 and Table 1C show the difference in the degree of


Figure 22. Comparison of the micrographs of polethyelene crystals formed at 40 MPa and 363 K with different crystallization times of (a) 60 and (b) 120 min.

Figure 21. Micrographs of polyethylene (121 K) crystals formed at 40 MPa but at different temperatures, namely, (a) 363, (b) 368, and (c) 373 K.

crystallinity for samples prepared by crystallization in n-pentane at different temperatures corresponding to different subcoolings. These are for the samples crystallized from a n-pentane solution at 38.5 MPa for 60 min. The figure shows that the percent crystallinity increases in samples crystallized at temperatures closer to the incipient phase-separation temperatures. Polyeth-

ylene crystals formed at 377 K show nearly 15% higher crystallinity compared to the original polyethylene, whereas the samples crystallized at 363 K show a 9% increase in crystallinity. The degree of crystallinity for polyethylene, crystallized from n-pentane at 38.5 MPa (Table 1D) for different crystallization times, shows, even though small, an increase in the degree of crystallinity with crystallization time. The increase in percent crystallinity is ∼8 at 30 min, and ∼10 at 120 min crystallization time. The present results show that the degree of crystallinity is affected most by the degree of undercooling during crystallization in n-pentane. Morphology of the Crystals. (a) Effect of Pressure. Parts a-e of Figure 20 are the FESEM results for the PE (121 K) samples that were crystallized at 363 K for 60 min from 5 wt % solutions in n-pentane at the nominal pressures of 23, 38, 40, 42, and 53 MPa, respectively. The lamellar or stackedlamellar morphologies are the common features. At 23 MPa, the crystals are individual ellipsoid-shaped lamellae that are ∼20 µm in the long diameter. The crystals that formed at pressures of 38-40 MPa consist of two types of crystal morphologies that include lamellar ellipsoid structures along with very unique, long strings of stacked-lamellar structures. These are 50-100 µm long structures with ∼5-10 µm width. These small and longer lamellar structures remain to different levels at higher pressures of 45 and 52 MPa. Since these train-like morphologies

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Figure 23. Micrographs of polyethylene crystals formed in CO2-pentane mixtures at (a) 37, (b) 41, (c) 45, and (d) 52 MPa.

are unusual, we have explored their reproducibility. Figure 20f is a micrograph of sample crystallized at conditions similar to the conditions corresponding to crystals leading to Figure 20c but in a separate experiment with a fresh loading of polyethylene, which has also lead to similar morphologies. The different morphology that develops at ∼40 MPa is likely to be linked with the visible shift in the location of the solid-fluid phase boundary at these pressures, which is displayed in Figure 5. (b) Effect of Crystallization Temperature. In Figure 21, we show the micrographs of crystals formed at 40 MPa but at different temperatures, namely, 363, 368, and 373 K. The morphologies are somewhat different, with the difference being most apparent at 373 K. A range of different overall shapes from ellipsoidal to spherical to stacked-lamellar structures are observed. The DSC results for these samples, shown in Figure 13, display corresponding features in that the DSC scans from the 373 K sample display a broader melting transition. (c) Effect of Crystallization Time. Figure 22 compares the micrographs of crystals formed at 40 MPa and 363 K with different crystallization times, 60 and 120 min. The types of crystal shapes are visible in these systems. In the system with longer crystallization time, a larger fraction appears to be the ellipsoidal-lamellar structures. The DSC melting data, however, between the 60 and 120 min crystallization time do not show any major changes (see Figure 14).

(d) Morphology Generated in CO2 + n-Pentane Mixtures. Figure 23 shows the micrographs of crystals formed in CO2 + n-pentane mixtures at 37, 41, 45, and 52 MPa. Basic morphological features are similar to those crystals formed in n-pentane; however, some differences are visible. The lamellar-ellipsoidal structures are mostly ∼10 µm in the long axis instead of ∼20 µm, which was prevalent in n-pentane. Stacked-lamellar structures are also observed in these crystals. The morphology at 37 MPa is again a collection of different crystal types that could be the reason for the broadness or lesser definition of the melting peaks with this sample, as depicted by the DSC melting transition shown in Figure 12. (e) Further Discussion on the Formation of Crystals with Different Morphological Features. Possible mechanistic paths for the formation of different morphologies in crystallization from these high-pressure solutions likely involve a stage-wise process in which the folded-chain lamellar structures are formed first. These then stack together and transform into ellipsoidalor spherical-shaped units.25,26 How the long strings of trainlike structures are formed is not clear. Figure 24 is a schematic representation of the morphological evolution along the crystallization pathways and the recovery of the crystals from the view cell. The ellipsoidal-lamellar structures that are observed from all these crystallization experiments have been also observed in studies with dilute solutions (0.5-1.0 wt %) of polyethylene


Figure 24. Schematic representation of morphological evolution along the crystallization pathway, from the homogeneous solution state to the recovery of the crystals from the view cell (see also ref 25).

Figure 25. Interplay of the relative positions of the solid-fluid- (SF) and liquid-liquid (LL)-phase boundaries and consequences on the crystals that form. The lamellar thickness depends on the actual undercooling experienced and may be different for the initial homogeneous solution crossing the SF boundary (Tm1 - Tc1) and for the polymer-rich and the polymer-lean phase if crystallization proceeded by an LL-phase separation given by (Tm2 Tc2) and (Tm3 - Tc2), respectively (see also ref 25).

in n-pentane that were carried out in more confined geometries.25,26 These crystals also were formed to show multiple melting transitions that are representative of crystals with different lamellar thicknesses, suggesting that different fractions experience a different degree of undercooling in the process. The variation in the degree of undercooling can be rationalized if the initial state of the solution is analyzed with respect to the relative position of the liquid-liquid and solid-liquid phase boundaries. Figure 25 shows the schematic of the relative positions of the phase boundaries during the crystallization process. The homogeneous solution crosses the solid-fluid phase boundary

at constant pressure, generating crystals from the highermolecular-weight fractions of the polymer that cannot remain in the solution at the crystallization temperature and pressure. During this stage, the degree of undercooling that the system experiences is given by Tfs(m) - Tc1 (Figure 25 parts a and b (right figure)). Figure 25b (left figure) shows the fate of the upper phase during further cooling and depressurization. The pressure reduction is likely to cause the upper phase to undergo an LL-phase separation, leading to the formation of a polymerlean and a polymer-rich phase. Upon further cooling, these phases will cross the solid-fluid boundary and will experience different degrees of undercooling represented by (Tm2 - Tc2) for the polymer-rich phase and by (Tm3 - Tc2) for the polymerlean phase, thereby leading to different lamellar thicknesses and melting temperatures. Three different melting temperatures would not, therefore, be unusual to see in these systems. The second heating scans eliminate the multiple melting transitions, because during cooling, all fractions experience the same degree of undercooling from the melt in the DSC scans. These crystallization experiments inherently involve more than one step as the system is first brought to an initial phase separation (which may or may not involve crystallization, depending upon whether the FS or LL boundary is crossed first), and then crystallization proceeds in the next cooling or depressurization stages. Thermal fractionation at ambient pressures via step crystallizations from melts carried out in differential scanning calorimeters is a technique employed to assess chain heterogeneities in semicrystalline copolymers.28 The degree and distribution of short chain branches produced in copolymerizations of ethylene with alpha olefins are elucidated by the differences in their crystallization temperatures.28 The technique involves, starting from the melt state, holding the sample at progressively lower crystallization temperatures, which generates “fractionation windows”. The isothermal crystallization temperature range employed in these investigations is similar to the melting range of the polymer. When a sample is crystallized in such a step fashion, a heating DSC scan is then reported to lead to the observation of multiple melting peaks that reflect the different crystal lamellar thicknesses that are obtained at different crystallization temperatures. The present study with stage-wise crystallization from solutions brings a new dimension and can potentially lead to a new approach to polymer fractionation and elucidation of chain

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branching in copolymers, which we hope to explore in the future. (It should, however, be noted that step crystallization experiments similar to those described in the literature in DSC with the original HDPE sample investigated in the present study do not lead to multiplicity of peaks.) Conclusions Polyethylene samples crystallized from n-pentane or npentane + carbon dioxide fluid mixtures at high pressures all show multiple melting peaks in the DSC scans (run at ambient pressure), with a shoulder at ∼395 K and two more clearly defined melting peaks in the temperature range from 399 to 403 K under 10 K/min heating rate. After the initial heating cycle, the second DSC scans show only one melting peak at 404 K. It is shown that the degree of crystallinity in the polyethylene samples crystallized from n-pentane or carbon dioxide and n-pentane mixtures is higher by ∼7 to 11% compared to the original polyethylene sample. The percent crystallinity does not show much variation with crystallization pressure or fluid composition in the range evaluated but becomes higher for crystallizations carried out closer to the demixing temperature. The crystal morphology is predominately lamellarellipsoidal structures, or stacked-lamellar spherical as well as long train-like structures. The long string structures are mostly formed at ∼40 MPa. They are observed in crystals formed in both the n-pentane and n-pentane + carbon dioxide mixtures. Presence of CO2 shifts the liquid-liquid boundary to higher pressures and the solid-fluid boundary to higher temperatures, and it appears to lead to crystals with smaller overall dimension. Literature Cited (1) Kiran, E. Polymer formation, modification and processing in or with supercritical fluids. In Supercritical Fluids. Fundamentals for Application; Kiran, E., Levelt Sengers, J. M. H., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1994; pp 541-588. (2) Kiran, E. Polymer miscibility and kinetics of pressure-induced phase separation in near-critical and supercritical fluids. In Supercritical Fluids. Fundamentals for Application; Kiran, E., Debenedetti, P. G., Peters, C. J., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2000; pp 167-192. (3) Kiran, E.; Liu, K. The Miscibility and Phase Behavior of Polyethylene with Poly(dimethylsiloxane) in Near-Critical Pentane. Korean J. Chem. Eng. 2002, 19, 153. (4) Chan, A.; Adidharma, H.; Radosz, M. Fluid-Liquid Transitions of Poly(ethylene-co-octene-1) in Supercritical Ethylene Solutions. Ind. Eng. Chem. Res. 2000, 39, 4370. (5) Pan, C.; Radosz, M. Phase Behavior of Poly(ethylene-co-hexene-1) Solutions in Isobutane and Propane. Ind. Eng. Chem. Res. 1999, 38, 2842. (6) Chan, K.; Adidharma, H.; Radosz, M. Fluid-Liquid and FluidSolid Transitions of Poly(ethylene-co-octene-1) in Sub- and Supercritical Propane Solutions. Ind. Eng. Chem. Res. 2000, 39, 3069. (7) Chan, A.; Radosz, M. Fluid-Liquid and Fluid-Solid-Phase Behavior of Poly(ethylene-co-hexene-1) Solutions in Sub- and Supercritical Propane, Ethylene, and Ethylene + Hexene-1. Macromolecules 2000, 33, 6800. (8) Han, S. J.; Lohse, D. J.; Radosz, M.; Sperling, L. H. Short Chain Branching Effect on the Cloud-Point Pressures of Ethylene Copolymers in Subcritical and Supercritical Propane. Macromolecules 1998, 31, 2533.

(9) Chan, A.; Hemmingsen, P.; Radosz, M. Fluid-Liquid and FluidSolid Transitions of Tetracontane in Propane. J. Chem. Eng. Data 2000, 45, 362. (10) Weidner, E.; Wiesmet, V. Phase Equilibrium (solid-liquid-gas) in Binary Systems of Polyethylenglycols, Polyethylenglycoldimethylether with Carbon Dioxide, Propane and Nitrogen. CISF 99, 5th Conference on SCF and their Application 1999, 521. (11) Weidner, E.; Knez, Z.; Wiesmet, V.; Kokol, K. Phase Equilibrium (solid-liquid-gas) in the System of Polyethyleneglycols-Nitrogen. ITSSF 97, 4th Conference on SCF and their Application 1997, 409. (12) Stejny, J.; Whitfield, A. F.; Pritchard, G. M.; Hill, M. J. Crystallization of a linear aliphatic polyester from solution in supercritical carbon dioxide. Polymer 1998, 39, 4175. (13) Aulov, V. A. Structural Aspects of Influence of High Pressure on Polyethylene in High-Pressure Chemistry and Physics of Polymers; Kovarskii, A. L., Ed.; CRC Press: Boca Raton, FL, 1994; pp 23-57. (14) Miyata, S.; Arikawa, T.; Sakaoku, K. Crystallization of Polyethylene from Xylene Solutions under High Pressure. Anal. Calorim. 1974, 3, 603. (15) Yasuniwa, M.; Tsubakihara, S.; Nakafuku, C. Molecular Weight Effect on the High-Pressure Crystallization of Polyethylene. Polym. J. 1988, 20, 1075. (16) Yasuniwa, M.; Yamaguchi, M.; Nakamura, A.; Tsubakihara, S. Melting and Crystallization of Solution Crystallized Ultra-High Molecular Weight Polyethylene under High Pressure. Polym. J. 1990, 22, 411. (17) Nakafuku, C.; Sugiuchi, T. Effect of pressure on the phase diagram of binary mixtures of n-alkanes. Polymer 1993, 34, 4945. (18) Nakafuku, C. Pressure Change of the Phase Diagram of the Binary Mixture of Low Molecular Weight Polyethylenes. Polym. J. 1995, 27, 917. (19) Ho¨hne, G. W. H.; Blankenhorn, K. High-pressure DSC investigations on n-alkanes, n-alkane mixtures and polyethylene. Thermochim. Acta 1994, 238, 351. (20) Shulgin, A. I.; Godovsky, Yu. K. DTA measurement on polymers under high pressure, polyethylene and poly(diethylsiloxane). J. Therm. Anal. 1992, 38, 1243. (21) Ho¨hne, G.; Schawe, J. E. K.; Shulgin, A. I. The phase transition behavior of linear polyethylenes at high pressure. Thermochim. Acta 1997, 296, 1. (22) Seeger, A.; Freitag, D.; Friedel, F.; Luft, G. Melting point of polymers under high pressure. Part I: Influence of the polymer properties. Thermochmica Acta 2004, 424, 175-181. (23) Zhang, W.; Dindar, C.; Bayraktar, Z.; Kiran, E. Phase behavior, density, and crystallization of polyethylene in n-pentane and in n-pentane/ CO2 at high pressures. J. Appl. Polym. Sci. 2003, 89, 2201-2209. (24) Upper, G.; Beckel, D.; Zhang, W.; Kiran, E. High-Pressure Crystallization in Supercritical or Dense Fluids. In Proceedings of the 6th International Symposium on Supercritical Fluids, Versailles, France, April 28-30, 2003; Institut National Polytechnique de Lorraine: Vandeuvre, Cedex, France, 2003; Vol. 3sMaterials Processing, pp 1509-1514. (25) Zhang, W.; Kiran, E. High-Pressure Crystallization and Melting of Polyethylene in n-Pentane. J. Supercrit. Fluids, accepted for publication. (26) Zhang, W.; Upper, G.; Kiran, E. Thermal and Morphological Properties of Polyethylene Crystals Formed in n-Pentane Under High Pressure. Presented at 7th International Symposium on Supercritical Fluids, Orlando, Florida, May 1-4, 2005. (27) Mathot, V. B. F.; Pijpers, M. F. J. Heat capacity, enthalpy and crystallinity for a linear polyethylene obtained by DSC. J. Therm. Anal. 1983, 28, 349. (28) Mu¨ller, A. J.; Arnal, M. L. Thermal fractionation of polymers. Prog. Polym. Sci. 2005, 30, 559-603.

ReceiVed for reView May 26, 2005 ReVised manuscript receiVed November 18, 2005 Accepted December 5, 2005 IE050620E

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