Article pubs.acs.org/Macromolecules
Molecular Self-Assembly Assisted Liquid−Liquid Phase Separation in Ultrahigh Molecular Weight Polyethylene/Liquid Paraffin/ Dibenzylidene Sorbitol Ternary Blends Sijun Liu, Wei Yu,* and Chixing Zhou Advanced Rheology Institute, Department of Polymer Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China ABSTRACT: Dibenzylidene sorbitol (DBS) was chosen as an in situ forming nucleating agent to study ultrahigh molecular weight polyethylene (UHMWPE)/liquid paraffin (LP) physical gel and complex phase separation. The experimental results indicated that DBS self-assembled into fibrils first and the solution became a physical gel before liquid−liquid phase separation (LLPS) and crystallization during thermally induced phase separation (TIPS) of UHMWPE/LP/DBS solution. The temperature of DBS selfassembly and viscoelasticity of UHMWPE/LP/DBS gel show a strong dependence on DBS concentration, temperature, and time. By controlling the relative quenching depth and annealing time, the grow rate of the characteristic length showed a crossover from LLPS to crystallization, which was further justified by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). With decreasing temperature further, crystallization occurred with the aid of DBS fibrils. Those interactions were affected mutually, showed complex phase separation behavior. We proposed a new mechanism of “self-assembly assisted liquid− liquid phase separation”, which explained excellently the relationship of DBS self-assembly and LLPS. On the basis of rheometer and optical microscopy (OM and POM), we obtained the phase diagram of UHMWPE/LP/DBS ternary blends. Meanwhile, DSC indicated the overwhelming changes of crystallization kinetics after the varying LLPS in the double quenching procedure, which was consistent with the “fluctuation assisted nucleation” mechanism. cause apparent shift of phase separation temperature.12−17 It is noticed that all these studies focus on the interactions between nanoparticles and polymers. The influence on the kinetics and morphology of phase separation is mainly attributed to the particle−polymer interactions. The specific interactions between nanoparticles are less studied. Then, in most cases, nanoparticles form small or large random aggregates. However, the slow aggregation process as well as the loose disordered structure of aggregates has little effect on the phase behavior. One can regard these kinds of nanoparticles as “inert”, where particle−polymer interactions are the only adjustable factor to affect the phase behavior of polymer blends. Dibenzylidene sorbitol (DBS) is an amphiphilic molecule derived from the sugar alcohol D-glucitol. As shown in Figure 1, due to the butterfly shape and propensity to undergo intermolecular hydrogen bonding between the terminal hydroxyl group and the acetal oxygens, DBS molecules can strongly interact in the presence of an organic solvent and form a physical gel through self-assembling into a fibril network with fibril diameter in nanoscale.18−20 The self-assembly of DBS
1. INTRODUCTION In recent years, there has been significant interest in the effect of nanoparticles on phase separation behavior of polymer blends.1−8 Usually, the addition of nanoparticles can reduce interfacial tension and improve miscibility between polymers. For example, Gao et al.8 investigated the effect of silica nanoparticles on the phase separation of poly(methyl methacrylate)/poly(styrene-co-acrylonitrile) (PMMA/SAN) blends and found that the miscibility of PMMA/SAN was enhanced significantly. Huang et al.9 found the same phase separation behavior for PMMA/SAN blends filled with untreated silica. However, the effect of nanoparticles on the miscibility of polymer blends depends greatly on the interactions among particles and polymers, which influence the location of nanoparticles in the phase-separated blends.10 For example, due to the selective location of SiO2 nanoparticles in the fast poly(vinyl methy ether) (PVME) component, which reduces the dynamic asymmetry between polystyrene (PS) and PVME, the characteristic transition from cocontinuous to network morphology in viscoelastic phase separation of PS/ PVME blends is replaced by the common spinodal decomposition with the transition from cocontinuous to droplet morphology in PS/PVME/SiO2 blends.11 Moreover, modification of the surface properties of nanoparticles can also © 2013 American Chemical Society
Received: May 8, 2013 Revised: July 8, 2013 Published: July 24, 2013 6309
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obtained by compressing UHMWPE/LP/DBS blends into a sheet with the thickness about 1.0 mm under 165 °C and 10 MPa. Strain sweeps with strain amplitude of 0.01−1.0 at frequencies of 0.1−2 Hz were carried out to determine the linear viscoelastic range of the materials at 165 °C. The annealing treatment was held at 165 °C for 10 min to eliminate the pristine crystalline structures, and the stability of the sample was checked through the measurement of G′ and G″ with 0.1 Hz in the linear viscoelastic region. Little change in G′ and G″ was found over a long period (120 min) at 165 °C with a strain amplitude of 1.0%, which illustrated that the effects of LP loss and thermal oxidation can be ignored. Three rheological experiments were adopted to monitor the rheological evolution of UHMWPE/LP/DBS samples under nonisothermal and isothermal procedures. (1) Temperature ramps from 165 to 100 °C on the ternary blends with different DBS concentrations were carried out with 2 °C/min cooling rate at a fixed frequency of 0.1 Hz and strain of 1.0%, from which the temperatures of DBS self-assembly and UHMWPE crystallization can be determined. (2) Frequency sweeps on blends with varying DBS concentration at different temperatures were conducted to determine the critical DBS concentration and critical temperature for the gel formation. (3) For 0.1 wt % DBS sample, cyclic frequency sweeps at a fixed temperature 125 °C were performed to study the kinetics of gel formation in ternary blends. 2.3. Differential Scanning Calorimeter. A differential scanning calorimeter (DSC) (Q2000, TA Instruments) was adopted to study crystallization behavior in the nonisothermal and isothermal process. The samples were sealed in an aluminum DSC pan, melted at 165 °C, and kept for 10 min to remove thermal history. Then they were cooled to 40 °C at different cooling rates. The onset of the exothermic peak during the cooling was taken as the nonisothermal crystallization temperature Tc. In order to investigate crystallization behavior in isothermal process, the sample was heated to 165 °C and kept for 10 min to remove thermal history and then was quenched to a certain temperature (115 °C for binary blend and 120 °C for ternary blend with 0.1% DBS) and annealed for certain time. Subsequently, the sample was heated to 160 °C and observed the change of melting peak. The onset time of crystallization can be estimated from the first appearance of melting peak. To investigate the effect of LLPS on crystallization, the sample was heated to 165 °C and kept for 10 min to erase thermal history and then cooled to the selected LLPS temperature (115 °C for binary blend and 120 °C for ternary blend with 0.1% DBS). After a certain period of annealing, the sample was cooled to 40 °C with 2 °C/min. Half-crystallization time was evaluated during the cooling process. All of these procedures were conducted under a nitrogen atmosphere. 2.4. Wide-Angle X-ray Diffraction. The crystalline structure of UHMWPE was studied by the two experimental procedures described herein. The first procedure to study the effect of DBS fibrils on crystalline structure at room temperature, wide-angle X-ray diffraction (WAXD), was taken using a D/max-2200/PC diffractometer (Rigaku Corporation, Japan) with Cu radiation. The WAXD profiles were recorded with a scanning speed of 4°/min between 5° and 45° in 2θ. The second procedure, the synchrotron radiation light source, was adopted to study isothermal crystallization behavior of UHMWPE/ LP/DBS blends with varying DBS concentrations. The sample was melted at 165 °C and kept for 10 min to remove thermal history and then cooled to a certain temperature (115 and 116 °C for binary blend, 120 °C for ternary blend with 0.1% DBS) for isothermal crystallization. The WAXD measurements were taken on a wide-angle X-ray diffraction state (BL16B1) with a long-slit collimation system in the Shanghai Synchrotron Radiation Facility (SSRF), Shanghai, China. The experiments were carried out with the radiation of X-ray with the wavelength of λ = 0.124 nm. The one-dimensional density profile was obtained by integrating the scattering intensity along the radial direction using Fit2D software. 2.5. Optical Microscopy. A small amount of the UHMWPE/LP/ DBS samples was sandwiched between two glass coverslips separated by a 50 μm thick polytetrafluoroethene (PTFE) spacer with a square opening in the center. The PTFE spacer allowed a uniform thickness
Figure 1. Chemical structure of dibenzylidene sorbitol (DBS).
depends on the solvent polarity, DBS concentration, and temperature21 and, most important of all, is a time process. The self-assembly of DBS can also happen in the melt of amorphous polymer like polystyrene or polycarbonate22,23 and crystalline polymer such as polypropylene.24,25 In the latter case, the nanofibrils formed from self-assembly can promote heterogeneous crystal nucleation.26 In contrast to the common “inert” nanoparticles, DBS can be regarded as a “living” one, where the formation of nanofibrils is a kinetic process, and the length and density of nanofibrils can be readily adjusted by temperature and time of self-assembly. In our previous work,27 we know that ultrahigh molecular polyethylene (UHMPWE) and liquid paraffin (LP) have excellent affinity due to their similar chemical structure and the close solubility parameter (δUHMWPE ≈ 16.12 J1/2/cm3/2 and δLP ≈ 15.96 J1/2/cm3/2). In contrast to most of the papers which cannot differentiate liquid−liquid phase separation (LLPS) from crystallization in this system,28−30 our experimental results showed that it is able to manipulate different phase separation behavior by control of the relative quenching depths and annealing time, which can offer more delicate control on the microporous structure of UHMWPE membrane made from the thermal induced phase separation.27 Although, it is well-known that DBS fibrils as a nucleating agent have an important influence on polymer crystallization, the effect of DBS fibrils on the liquid−liquid phase separation of polymer/ solvent blends has not been investigated yet. In this work, we will study the phase behavior of UHMWPE/LP/DBS ternary blends. The focus is about the influence of DBS self-assembly on the liquid−liquid phase separation and crystallization behavior. A new mechanism of “self-assembly assisted liquid− liquid phase separation” will be discussed.
2. EXPERIMENTAL SECTION 2.1. Materials and Samples Preparation. UHMWPE was kindly supplied by Shanghai Chemical Technology Institute, and the weightaverage molecular weight is approximately 2 000 000. DBS was obtained in powder form from Milliken Chemical. Liquid paraffin was purchased from Shanghai Chemical Reagent Factory. It is composed of short alkanes with average molecular weight of 150− 250 g/mol and used without further purification. First of all, the required quantities of DBS and LP were mixed in a glass vessel using magnetic stirring apparatus under a nitrogen atmosphere at 220 °C, where a clear solution was obtained. The sample was then cooled down to room temperature to obtain LP/DBS gel. Then the UHMWPE/LP/DBS ternary blends were obtained by melt mixing UHMWPE and LP/DBS gel in a Batch mixer (XSS-300, Shanghai Kechang Rubber & Plastic Equipment Co, China) at 165 °C for 12 min with the rotor speed 100 rpm. The concentration of UHMWPE was fixed at 10 wt %, while the DBS concentration varies from 0.05 to 0.5 wt %. 2.2. Rheological Measurements. The rheological measurements were performed on a rotational rheometer (Gemini 200HR, Bohlin Instruments, UK) with parallel plate geometry of 25 mm in diameter and a gap of 1.0 mm. The samples for rheological measurement were 6310
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of the sample in the observation using optical microscopy. Moreover, it could restrict LP evaporation during the experiment. Three optical microscopy measurements were carried out to monitor morphological changes of UHMWPE/LP/DBS samples. (1) In order to study the effect of DBS fibrils on UHMWPE crystallization, the samples were heated on a hot stage (LK-600PH, Linkam, UK) at 165 °C for 10 min to erase thermal history and then cooled to room temperature at a controlled rate 2 °C/min. The morphology was observed by optical microscopy (Leica DM LP, Leica Microsystems GmbH, Germany), and the images were captured using a Sony video camera. (2) Similar to the study of LLPS of UHMWPE/LP binary blends in the isothermal process, UHMWPE/LP/DBS ternary blends were heated to 165 °C for 10 min and then quenched to different temperatures; the changes in optical images were monitored with time. (3) The double quenching procedure was adopted to investigate the effect of DBS self-assembly on LLPS. First, 0.1 wt % DBS sample was heated at 165 °C for 10 min to erase thermal history and then quickly cooled to 125 °C for DBS self-assembly. Subsequently, the sample was quenched to LLPS temperature and observed the evolution of morphology. In order to characterize the length scale of phase separation, twodimensional fast Fourier transformation (2D-FFT) was carried out and radially averaged to get one-dimensional (1D) scattering intensity profile. The characteristic length lm is defined as lm = 2π/qm, where qm is the peak wave vector.
UHMWPE crystallization temperatures are increased as much as 2 °C when DBS content increases, which confirms the expectation that DBS fibrils can accelerate the UHMWPE crystallization through heterogeneous nucleation at temperature higher than Tc of the neat UHMWPE/LP blends. The transition from liquid-like state to solid-like state as DBS content increases can be seen more clearly from the frequency dependence of dynamic modulus. The dynamic modulus of ternary blends with different DBS concentrations at 122 °C are shown in Figure 3. It is obvious that the sample containing 0.05
3. RESULTS AND DISCUSSION 3.1. DBS Self-Assembly in UHMWPE/LP Matrix. It is well-known that DBS, as a classical thermoreversible gelator, can self-assemble into fibrils in a series of organic solvents or polymers via hydrogen bonds. The self-assembly can also happen in a mixture of solvent and polymer. Figure 2 shows the
Figure 3. Frequency dependence of G′ and G″ of UHMWPE/LP/ DBS blends with different DBS concentrations at 122 °C and strain of 1.0%.
wt % DBS exhibits a homogeneous behavior according to the typical terminal property at low frequencies, G′ ∼ ω2 and G″ ∼ ω2. In contrast, the significant increase in dynamic modulus can be found with blends containing 0.1 wt % DBS, especially in G′ at low frequency. Moreover, the decrease in the terminal slope and the appearance of a plateau in the low-frequency region manifests the formation of a certain network. As DBS concentration increases to 0.2 wt %, G′ becomes very close to G″ over wide frequencies. When blends contain 0.35 wt % or more DBS, DBS has fully self-assembled at 122 °C to give G′ is higher than G″ in the whole experimental frequency window, showing a pronounced elastic dominated behavior. Combining the rheological features from temperature ramps and frequency sweeps, we conclude that DBS can self-assemble into fibrils which forms network in UHMWPE/LP matrix with the critical DBS concentration as low as 0.1 wt %, which is similar to the self-assembly of DBS in a single solvent or polymer.21 It should be noted that the time of DBS self-assembly depends on temperature, and it becomes faster as temperature decreases. Figure 4 shows the evolution of the relative storage modulus (G′ − G0′)/(G∞′ − G0′) with annealing time at 0.1 rad/s during cyclic frequency sweep at 125 °C for UHMWPE/ LP/DBS ternary blends containing 0.1 wt % DBS, where G0′ and G∞′ denote the modulus at t = 0 and end of self-assembly, respectively. It is obvious that the relative storage modulus increases with annealing time, which indicates that DBS molecules take some time to self-assemble into DBS fibrils. Under other temperatures, the self-assembly and gel formation behavior are similar to that under 125 °C, but the time scales are different. 3.2. Crystallization of UHMWPE in Ternary Blends. For UHMWPE/LP/DBS ternary blends containing 0.1 wt % or more DBS, the experimental results shown in Figure 2 indicate
Figure 2. Temperature dependence of G′ and G″ as a function of DBS concentration during cooling at 2 °C/min and after annealing at 165 °C for 10 min.
variation of G′ and G″ with temperature for UHMWPE/LP/ DBS blends containing different amount of DBS at the cooling rate of 2 °C/min. In the case of low DBS concentration (0 and 0.05 wt %), G′ and G″ are seen to increase gradually when the temperature decreases down to about 110 °C, where they increase abruptly. Such sudden and dramatic increase in G′and G″ signifies the onset of UHMWPE crystallization. In contrast, G′ and G″ of sample containing 0.1 wt % DBS exhibits more complex temperature dependence. Before UHMWPE crystallization, G′ and G″ rise quickly at about 120 °C, which is associated with the self-assembly of DBS and the formation of DBS fibril network. With increasing DBS concentration further, the rise in G′ and G″ becomes more pronounced at higher temperature. The formation temperature of DBS gel in the UHMWPE/LP system is denoted as TSA. Note that the 6311
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temperature ramps. Tc almost keeps a constant value when the DBS amount is below 0.1 wt %. As DBS concentration increases, Tc increases sharply at 0.1 wt % DBS. This is ascribed to the substantial increase in the number of DBS fibrils after the gel formation, which can be effective heterogeneous nucleation sites for UHMWPE molecules. Moreover, Tc shows a slight decrease at higher DBS concentration. This phenomenon can be partly interpreted as the saturation of nucleation that has been often found in the case of excessive nucleating agent31,32 probably due to the aggregation of DBS fibrils at high DBS concentration,21 which has also been observed by Kristiansen33 in the iPP/DBS system. The other reason might come from the interplay between the self-assembly of DBS molecules and the phase transitions of UHMWPE/LP blends. It is noticed that TSA of 0.1 wt % DBS is between 120 and 130 °C, which coincides with the temperature range of LLPS and crystallization in UHMWPE/LP binary blends. While in ternary blends with higher DBS concentration, the self-assembly of DBS molecules has finished before the phase transition of polyethylene start. However, such possibility cannot be clarified just from nonisothermal crystallization. More evidence will be given in the next section. On the other hand, the effect of DBS self-assembly on UHMWPE crystallization can also be evaluated by the halfcrystallization time t1/2, which is defined as the time to attain the degree of crystallinity 50% as determined from DSC. It can be seen from Figure 5b that t1/2 decreases with increasing cooling rates, indicating the large undercooling accelerates UHMWPE crystallization. Moreover, the heterogeneous nucleation of DBS becomes important only at low cooling rate, and the effect of DBS concentration becomes more evident at lower cooling rate. For the effect of DBS concentration, t1/2 decreases sharply as DBS concentration increases from 0.05 to 0.1 wt % and then increases slightly with further increase of DBS concentration (0.2 and 0.5 wt %), which is similar to the variation of UHMWPE crystallization temperature. Crystallization of UHMWPE in ternary blends can also be viewed from POM. Figure 6 shows polarized optical micrographs for UHMWPE/LP/DBS samples containing different DBS concentrations, which undergo nonisothermal crystallization after melted at 165 °C for 10 min. The polarized optical micrographs of 0 and 0.05 wt % DBS show quite dark images. However, the great change in brightness can be seen as the DBS concentration becomes higher than 0.1 wt %. The change in the brightness can be seen more quantitatively from the average gray level of POM pictures obtained by Image-Pro 6.0 software. Please be noted that we are not discussing the absolute transparency of the sample. In contrast, the change of apparent transparency under the same input intensity of light is focused here. It is easy to see lots of crystals in binary blend (0 wt % DBS), which is not unexpected for UHMWPE with crystallinity of about 50%. Actually, the phenomenon in Figure 6 is very similar to the polypropylene system where DBS or its derivative acts as a clarifier.34 The clarity (or similarly the gray level in Figure 6) increases as the DBS content increases. The samples are almost transparent in the range of DBS content 0.3−1.0% for iPP. As DBS content increases, the clarity decreases. It is widely believed that the most important origins of the transparency are the absence of spherulite texture or significant depression of spherulite size.31,33,35 The former is owing to the epitaxial crystallization of polymer due to the nucleation effect of DBS nanofibrils (∼10 nm), which matches
Figure 4. Relationship of the relative storage modulus and annealing time at 0.1 rad/s during cyclic frequency sweep at 125 °C for UHMWPE/LP/DBS ternary blends containing 0.1 wt % DBS.
that DBS molecules self-assemble into fibrils and physical gel forms before UHMWPE crystallization. It is also found that DBS fibrils can act as nucleating agent to enhance UHMWPE crystallization. To elucidate the crystallization behavior of UHMWPE/LP/DBS ternary blends more clearly, the calorimetric approach DSC was adopted to estimate the effect of DBS fibrils on UHMWPE crystallization. Figure 5a shows the nonisothermal crystallization process of UHMWPE/LP/DBS samples containing different DBS concentrations. The onset of the exothermic peak during the cooling is taken as the crystallization temperature Tc, which is plotted as a function of DBS concentration in the inset of Figure 5a. The obvious increase in Tc is consistent with the observation in rheological
Figure 5. (a) Nonisothermal crystallization DSC thermograms of UHMWPE/LP/DBS samples (inset: Tc and crystallinity are plotted as a function of DBS concentration) and (b) plot of t1/2 versus cooling rate with different DBS concentrations in the nonisothermal process. 6312
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Figure 6. Polarized optical micrographs of UHMWPE/LP/DBS samples containing different DBS concentrations cooled to the room temperature (−1 °C/min) after annealing at 165 °C for 10 min.
the lamellar thickness of polymer crystals,36 while the latter is ascribed to the spatial restriction of spherulite growth by the crowded network that formed by self-assembling of DBS molecules. At high DBS concentration, the decrease in the transparency is related to the aggregation of DBS nanofibrils into microfibrils which lowers the nucleation efficiency. It is expected that highly transparent sample can only be obtained when DBS molecules self-assembly into nanofibrils but without large aggregates. Moreover, the change of brightness of POM images should not be related to the self-assembly of DBS itself since DBS fibrils can be seen in POM only at much higher DBS concentration (>1.5 wt % in LP).21 Before further discussing the complex phase behavior of ternary system, it is necessary to mention that the crystal structure of polyethylene does not change after adding DBS. This is illustrated from the XRD results in Figure 7. It is
above are ascribed to the interactions among three components instead of the change in the crystal structure. 3.3. Self-Assembly Assisted Liquid−Liquid Phase Separation. For the UHMWPE/LP binary system, our previous experiment results27 have indicated that it is difficult to differentiate LLPS from crystallization in the nonisothermal process due to the good affinity between UHMWPE and LP and rather fast crystallization process as compared to the slow LLPS. However, the cocontinuous structure associated with the late stage of spinodal decomposition can be observed from optical microscopy when the sample was annealed at suitable temperatures. Figure 8 shows the optical micrographs (bright) and polarized optical micrographs (black) which illustrates the effect of different DBS concentrations on LLPS and crystallization when the samples were annealed at different temperatures. The inset in optical micrograph corresponds to the image obtained by two-dimensional fast Fourier transform (2D-FFT). The pictures under cross-polarizer were also taken at different annealing time, and the picture with the first appearance of crystal is also shown in Figure 8. For UHMWPE/LP binary blends, no obvious morphology can be observed before 52 min at 116 °C. Later on, sparsely distributed domains appear in optical microscopy and grow up gradually and form cocontinuous structures, which is characteristic of spinodal decomposition in LLPS. Actually, the early stage of spinodal decomposition, where the intensity of scattering increases exponentially and the characteristic length scale is independent of time according to the classical Cahn’s linear theory, has not be observed due to the limit in the spatial resolution of optical microscopy. Meanwhile, the emergence and evolution of the scattered ring is obvious from the corresponding 2D-FFT images, which is more evident in the plot of the scatter intensity versus scatter vector. The peak of the scatter intensity moves to the smaller scatter vector, which implies the increase of the characteristic length in morphology. Moreover, as the quenching depth increases (T = 115 °C), the induction period of LLPS is hardly observed and UHMWPE crystals appear after annealed for 150 min. The effect of quenching depth on the phase separation can be seen more clearly in Figure 9, which shows the time evolution of the characteristic length lm. For UHMWPE/LP binary blends and 0.05 wt % DBS ternary system, lm increases linearly during the
Figure 7. XRD profiles of UHMWPE/LP/DBS ternary blends containing different DBS concentrations at room temperature.
obvious that all samples show the typical diffraction peaks of polyethylene, and the characteristic lattice planes of α crystal, i.e. (110) and (200) at 21.6° and 23.9°, are observed. This is different from other systems like poly(L-lactic acid)37 and isotactic polypropylene24 where DBS fibrils may have an important effect on polymer crystal structure. It means that all of effects of DBS on the crystallization of UHMWPE as shown 6313
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Figure 8. Optical micrographs and 2D-FFT images of the UHMWPE/LP/DBS blends containing different DBS during the samples were quenched to the different temperatures annealing.
characteristic length and the appearance of crystal under cross polarizer implies the growth of domain of LLPS is inhibited only when the crystal is large enough and comparable with domain size. For UHMWPE/LP/DBS ternary blends containing 0.1 wt % DBS, it is surprising that the similar cocontinuous structure is observed from optical microscopy when the sample was quenched to higher temperature (T = 120 °C) and lm decreases as compared to UHMWPE/LP binary blends. Such a phenomenon indicates that the presence of enough DBS fibrils induces LLPS and reduces the size of phase separation domain. It is important to see the formation of DBS fibril network is crucial since there is almost no difference in the coarsening behavior between UHMWPE/LP binary blends and UHMWPE/LP/DBS ternary blends of 0.05 wt % DBS. With increasing DBS concentration further (0.2 wt %), it is difficult to observe the cocontinuous structure and UHMWPE crystallization dominates the whole process at the same quenching temperature 120 °C. That is to say, the DBS selfassembly has an important influence on LLPS, and this influence shows a strong dependence on DBS concentration. Although optical microscopy helps to determine the LLPS and crystallization, due to the limit in the spatial resolution of optical microscopy, it is unable to confirm when the crystallization starts and whether the structures observed in optical microscopy are from LLPS or small crystals. To clarify this point, DSC and time-resolved WAXD experiments using synchrotron radiation facility (SSRF) were performed. For DSC tests, the samples were quenched to 115 °C (UHMWPE/ LP binary blend) or 120 °C (UHMWPE/LP/DBS ternary blends with 0.1 wt % DBS) and annealed for different times. Figure 10 exhibits the endothermic curves during heating after annealing. When the annealing time is short, there is no melting
Figure 9. Time evolution of the characteristic length for UHMWPE/ LP/DBS blends containing different DBS during the samples were quenched to the different temperatures annealing. The slope of solid line represents the growth rate of the domain.
tests when the sample was annealed at higher temperature (T = 116 °C), indicating LLPS is prominent with the whole process. At lower temperatures, lm increases linearly in the initial stage of annealing, followed by a slowdown of coarsening rate at longer times. For phase separation in partially miscible system, the thermodynamically driving force generally increases with quenching depth. So the domain growth rate at the initial stage is relatively higher at low temperature, which suggests UCST-type of phase diagram. However, with decrease of quenching temperature, the energy barrier of crystallization is readily overcome, and the crystallization process becomes predominant, which inhibits the further phase separation. The coincidence of transition time in the coarsening rate of 6314
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binary blends, the intensity of diffraction peak is more apparent, indicating that DBS fibrils as a heterogeneous nucleating agent improve crystal density and enhance UHMWPE crystallization, which is consistent with the measurements of DSC and POM. The crystallization times in binary and ternary blends from time-resolved WAXD are also marked in Figure 9. Obviously, the crystallization time of UHMWPE obtained through WAXD measurements is slightly shorter than that from DSC and much shorter than that from optical microscopy. This is because that POM only detects the UHMWPE crystals when it grows up to several micrometers. It is interesting to see from Figure 9 that the coarsening rate of domains does not change in the early stage of crystallization. Actually, Figure 9 exhibits the increase of characteristic length during annealing, which should be ascribed to the liquid−liquid phase separation as well as crystallization. Taking UHMWPE/ LP binary blends annealed at 115 °C as an example, the growth rate of characteristic length slowed down after 150 min due to UHMWPE crystallization. The slowdown of the growth rate is apparently related to the appearance of large crystals that are observable in POM. However, WAXD indicates that UHMWPE crystallization started at 60 min. The crystallization time obtained from WAXD lies in the range that the characteristic length increases linearly with time. It means that the coarsening process in the late stage of spinodal decomposition is not affected by the appearance of very small crystals probably due to the large difference in the length scale of LLPS domains and the size of crystals. The coarsening rate of the characteristic length during LLPS can be obtained from the slope is shown in Figure 9. Its dependence on the quenching depth (ΔT) is shown in Figure 12 for different blends. The quenching depth is defined as the
Figure 10. Endothermic curves of UHMWPE/LP/DBS blends containing different DBS after the samples were quenched to the different temperatures annealing.
peak. As the annealing time increases, the melting peak appears and the area of melting peak increases with further increasing annealing time. The endothermic peak first appearance after 120 min for UHMWPE/LP binary blends and 60 min for UHMWPE/LP/DBS ternary blends, which indicates that crystallization time obtained from DSC is earlier than POM. For time-resolved WAXD experiments, the samples were quenched to specified temperature undergoing liquid−liquid phase separation and crystallization. Figure 11 shows typical
Figure 11. Time evolution profiles of WAXD for UHMWPE/LP binary blends and UHMWPE/LP/DBS ternary blends containing 0.1 wt % DBS annealing at different temperatures. The red lines denote the onset of crystallization.
Figure 12. Coarsening rate of characteristic length at different quenching depths.
time-resolved WAXD profiles for UHMWPE/LP binary blends annealing at 116 and 115 °C and UHMWPE/LP/DBS ternary blends containing 0.1 wt % DBS annealing at 120 °C. For UHMWPE/LP binary blends, WAXD profiles show a diffuse background in the initial isothermal period and then the emergence and evolution of the typical diffraction peaks of α crystal after 180 min. With increase of quenching depth to 115 °C, the characteristic lattice planes (110) and (200) could be observed after 60 min, and the intensity of diffraction peak is stronger than that of 116 °C. For 0.1 wt % DBS sample, UHMWPE begin to crystallization after 30 min during the sample was quenched to 120 °C. Compared to UHMWPE/LP
difference between the liquid−liquid phase separation temperature (TLLPS) and the annealing temperature. The determination of TLLPS will be discussed in the next section. Obviously, the coarsening rate for all of blends increases with increasing quenching depth, indicating faster LLPS at lower quenching temperature due to larger thermodynamic driving force. For UHMWPE/LP binary blends and UHMWPE/LP/DBS ternary blends containing 0.05 wt % DBS, the dependence of coarsening rate on quenching depth is almost the same. However, for 0.1 wt % DBS ternary system, K becomes much larger, which is probably related to the self-assembly of DBS. It 6315
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than TLLPS in a UCST-type blend, DBS fibrils are formed in temperature range 160−135 °C during cooling, and selfassembly has little effect on the subsequent liquid−liquid phase separation. On the other hand, the fibrils formed from selfassembly are good nucleating agent for polyethylene. The competition effect between self-assembly assisted LLPS and crystallization depends on the extent of self-assembly. In the early stage of self-assembly, its effect on concentration fluctuation is significant and crystallization is less affected due to no sufficient fibrils and LLPS is accelerated as illustrated in Figure 14. In the late stage of self-assembly, DBS has fully self-
has been found that the self-assembly of DBS in the 0.1 wt % system can happen at temperature as high as 120 °C, which coincides with the temperature of liquid−liquid phase separation. While for blends with other DBS concentrations, either the DBS self-assembly has finished or does not happen during LLPS. In the former case (DBS concentration higher than 0.1 wt %), the fibril network of DBS accelerates the crystallization significantly due to the heterogeneous nucleation. In the latter case (DBS concentration lower than 0.1 wt %), DBS molecules have little effect on both LLPS and crystallization. Moreover, for blends with 0.1 wt % DBS, it usually takes tens of minutes to finish the self-assembly of DBS at 125 °C (Figure 4), and the process becomes faster at lower temperature. It is expected that longer self-assembly time would cause larger concentration fluctuation of UHMWPE, which can facilitate liquid−liquid phase separation. To justify the “self-assembly assisted liquid−liquid phase separation” mechanism, a direct evidence can be obtained if the rate of liquid−liquid phase separation changes as the extent of self-assembly changes. This can be done by controlling the selfassembly of DBS at high temperature without liquid−liquid phase separation (for example 125 °C) and then studying its effect on the phase separation at lower temperature (such as 120 °C). The self-assembly happens within 20 min, beyond which the viscoelasticity of the system does not change anymore (Figure 4). The effect of self-assembly time on subsequent liquid−liquid phase separation is shown in Figure 13. It is obvious that the growth rate of characteristic length
Figure 14. Scheme of liquid−liquid phase separation assisted by selfassembly of DBS in the early stage.
assembled and concentration fluctuation due to self-assembly becomes quite weak, and crystallization becomes dominant due to large amount of DBS fibrils. 3.4. Phase Diagram and Interplay between Multiple Phase Transitions. The apparent phase diagram of UHMWPE/LP/DBS ternary blends is shown in Figure 15. TSA was obtained from rheological frequency sweep at different temperatures and determined as the self-assembly temperature where the phase angle becomes frequency independent. TLLPS and Tc were obtained from OM and POM, respectively. When the samples were quenched to the different annealing
Figure 13. Time evolution of the characteristic length for 0.1 wt % DBS ternary blends isothermally phase separation at 120 °C after annealed at 125 °C for 0, 1, and 2 min. The slope of the solid line represents the growth rate of the domain.
increases as the time of DBS self-assembly increases. When the self-assembly time of DBS fibrils increases to 30 min, UHMWPE crystals appear shortly after the sample was quenched to 120 °C. The concentration fluctuation induced by the self-assembly of DBS is dominated by the nucleating agent. Therefore, the self-assembly of DBS has two effects on the phase transitions of UHMWPE/LP blends. The selfassembly can cause fluctuation of local concentration. If TSA is close to that of liquid−liquid phase separation, as the selfassembly of DBS happens, local concentration fluctuation increases near the end of DBS fibrils due to the difference between the DBS−LP interaction and DBS−UHMWPE interaction, such local concentration fluctuation could become unstable and develops into phase-separated structures. This is the case that observed in 0.1 wt % system. If TSA is much higher
Figure 15. Phase diagram of UHMWPE/LP/DBS ternary blends obtained using rheometer, OM, and POM. For DBS content higher than 0.1 wt %, LLPS is hardly observed, and the dashed line denotes the expected LLPS temperature instead of the real one. Four regions corresponding to the four different states can be identified: region I, homogeneous blends; region II, DBS self-assembly; region III, liquid− liquid phase separation; region IV, UHMWPE crystallization. 6316
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temperatures, the temperatures that sparsely distributed domains and crystal appear under OM and POM within 60 min are defined as TLLPS and Tc, respectively. These transition temperatures (TSA, TLLPS, Tc) are not thermodynamic transition temperatures and are affected by the time of test, i.e., the range of frequency to determine TSA and the observation time to determine TLLPS and Tc. Although named as apparent transition temperatures, they are instructive on the phase behavior when the observation time scale is the same. From the phase diagram, we can see four different regions corresponding to four different physical states. In region I, UHMWPE/DBS/LP form a homogeneous solution at high temperature. In region II, with decrease of temperature, DBS molecules self-assemble into fibrils and the blends become gel-like. In region III, LLPS dominates the whole process, and the region of phase separation decreases with increasing DBS concentration. In region IV, crystallization of UHMWPE dominates. It is obvious that TSA increases with increasing DBS concentration, indicating that more thermal energy is required to form gel networks in a densely packed arrangement. It is seen that the effects of DBS on LLPS and crystallization become obvious when its concentration is larger than 0.1 wt %. Shifting of TLLPS and Tc to higher temperatures denotes the acceleration of phase transitions due to the presence of DBS fibrils. However, its effect on crystallization is more significant as DBS concentration increases further. As a result, the temperature range with LLPS-dominated process becomes smaller. Therefore, the selfassembly assisted liquid−liquid phase separation is only obvious at DBS concentration around 0.1 wt %. The effect of liquid−liquid phase separation on the subsequent crystallization can be easily studied according to the phase diagram. Both UHMWPE/LP binary blends and UHMWPE/LP/DBS (0.1 wt %) ternary blends were quenched from homogeneous state to the annealing temperature: 115 °C for binary blends and 120 °C for ternary blends. After a certain time of annealing, the sample temperatures were decreased again to study the nonisothermal crystallization. Liquid−liquid phase separation happens during annealing according to the phase diagram. The phase separation time is kept within 60 min for binary blend and 30 min for ternary blend. It has been proved by synchrotron WAXD that no crystallization happens during annealing under such conditions. Then, the change in the subsequent crystallization would ascribe completely to LLPS. The dependence of nonisothermal half-crystallization time on the liquid−liquid phase separation time is shown in Figure 16. For binary blends, the half-crystallization time decreases first and then increases slightly with longer annealing time, which indicates that liquid−liquid phase separation accelerates UHMWPE crystallization in the early stage of spinodal decomposition, and this effect becomes weak in the later stage of spinodal decomposition. This is well consistent with the “fluctuation assisted nucleation” mechanism,38−40 where the aligned segment at the interface due to concentration fluctuation can act as nuclei or the precursor of nuclei for crystallization. Crystallization can proceed without overcoming the usual activation energy barrier of nucleation with the help of concentration fluctuation induced alignment. In contrast, for UHMWPE/LP/DBS ternary blends, the halfcrystallization time increases with increasing time of LLPS at 120 °C, which is completely different from that of binary blends. As compared to UHMWPE/LP binary blends, the halfcrystallization time is smaller in ternary blends after the same time of phase separation. It has been shown that DBS self-
Figure 16. Nonisothermal half-crystallization time of blends as a function of annealing time for UHMWPE/LP binary blends and UHMWPE/LP/DBS ternary blends containing 0.1 wt % DBS. The annealing temperature is 115 °C for binary blends and 120 °C for ternary blends.
assembly has two effects on phase transition: DBS self-assembly induces the local concentration fluctuation and assisted LLPS. Meanwhile, DBS fibrils as nucleating agent can accelerate the crystallization of UHMWPE. The self-assembly of DBS can promote the concentration fluctuation and bring the phase separation between UHMWPE and LP quickly into late stage. Then, as the annealing time increases, the concentration fluctuation assisted nucleation becomes weaker, and its accelerating effect on crystallization gradually vanishes, which makes the crystallization slow down. However, the overall speed of crystallization in ternary blend is higher due to the nucleation effect of DBS fibrils.
4. CONCLUSIONS In this paper, we studied a blend system with multiple phase transitions, i.e., self-assembly of DBS, liquid−liquid phase separation between UHMWPE and LP, and the crystallization of UHMWPE. A complex relationship among multiple phase transitions has been found. The temperature below which DBS starts to self-assembly depends on the concentration of DBS. With suitable DBS concentration in UHMWPE/LP blends, the self-assembly temperature can be very close to that of liquid− liquid phase separation. Under such condition, it is found the self-assembly of DBS can increase the apparent liquid−liquid phase separation temperature and accelerate the process of LLPS. This is ascribed to the local concentration fluctuation initiated by the self-assembly of DBS. Moreover, the fibrils formed after self-assembly of DBS can act as effective heterogeneous nucleating agent for the crystallization of UHMWPE, and the crystallization rate becomes faster. The competition between liquid−liquid phase separation and crystallization in UHMWPE/LP blend can then be tuned by the amount of DBS. When the DBS concentration is lower than the critical value to form a gel, both liquid−liquid phase separation and crystallization are not affected. When the DBS content is above the critical concentration, both the liquid− liquid phase separation and crystallization are accelerated, and crystallization is more affected as the DBS content increases further. The DBS self-assembly also affects the interplay between the liquid−liquid phase separation and crystallization. The 6317
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concentration fluctuation due to liquid−liquid phase separation in binary blend can assist the nucleation of crystallization and accelerate the crystallization process. However, in the ternary blend, DBS self-assembly accelerates the concentration fluctuation of LLPS and makes the LLPS quickly develop into the late stage, which has much weaker effect on the subsequent crystallization. On the other hand, in contrast to “inert” nanoparticles which cannot form ordered structures spontaneously, DBS can be regarded as a “living” nanomaterial which can self-assembly into nanofibrils and accelerate the liquid−liquid phase separation. This is different from the effect of most “inert” nanoparticles, whose retarding effect on the liquid−liquid phase separation depends on its selective location. The special properties of “living” nanomaterials offer a great opportunity in control of the liquid−liquid phase separation and crystallization of polymer blends.
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AUTHOR INFORMATION
Corresponding Author
*Tel +86 21 54743275; fax +86 21 54747159; e-mail wyu@ sjtu.edu.cn (W.Y.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
The authors thank the support from the National Natural Science Foundation of China (No. 50930002) and the National Basic Research Program of China (973 Program) 2011CB606005. W. Yu is supported by the Program for New Century Excellent Talents in University and the SMC project of Shanghai Jiao Tong University.
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