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Sensitive phase separation behavior of ultra-high molecular weight polyethylene in polybutene
Polymer Testing 81 (2020) 106243
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Material Behaviour
Sensitive phase separation behavior of ultra-high molecular weight polyethylene in polybutene Changlin Cao a, b, 1, Wei Jiang b, 1, Yu Lin b, Xiaochuan Chen a, Qingrong Qian b, ***, Qinghua Chen b, c, Dingshan Yu a, *, Xudong Chen a, ** a
Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education and Key Laboratory of High Performance Polymer-based Composites of Guangdong Province, School of Chemistry, Sun Yat-Sen University, Guangzhou, 510275, China College of Environmental Science and Engineering, Fujian Key Laboratory of Pollution Control & Resource Reuse and Engineering Research Center of Polymer Green Recycling of Ministry of Education, Fujian Normal University, Fuzhou, 350007, China c Fuqing Branch of Fujian Normal University, Fuqing, Fujian Province, 350300, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Thermally induced phase separation (TIPS) Rheological properties Crystallization rate Ultra-high molecular weight polyethylene Polybutene
Thermally induced phase separation (TIPS) has prompted a great deal of interest, especially as an effective approach to fabricate ultra-high molecular weight polyethylene (UHMWPE) microporous membranes. However, the existing utilized diluents for the TIPS process of UHMWPE suffer from environmental and health issues. Herein, we utilized low molecular weight polybutene (PB) bearing similar structure with liquid paraffin (LP) but inferior miscibility with UHMWPE relative to UHMWPE/LP blend, as a diluent for the TIPS process of UHMWPE. The phase separation behavior of UHMWPE/PB blends were investigated by the combination of rheological measurements, optical microscopy as well as differential scanning calorimeter (DSC). The results suggest that PB is fully miscible with UHMWPE at elevated temperature, but yielding a more sensitive phase separation behavior in respect to LP in TIPS process, because PB has weaker interaction with UHMWPE. The Jeziorny method analysis indicates that the crystallization mechanism of UHMWPE/LP blends is in line with that of UHMWPE/PB blends, which includes nucleation and growth as well as their dynamic competition. Moreover, compared to those of UHMWPE/LP blends, UHMWPE/PB blends display higher TIPS temperature and faster TIPS rate along with faster overall crystallization rate, further demonstrating that PB can accelerate phase separation rates and enhance the efficiency of TIPS process.
1. Introduction Ultra-high molecular weight polyethylene (UHMWPE) is a unique polymer with outstanding physical and mechanical properties [1]. Most notable are its chemical inertness, lubricity, impact resistance, and abrasion resistance [2,3]. However, UHMWPE shows a rubbery state at melt owing to its ultra-high molecular weight which is usually over 1.0 � 106 g/mol [4]. Thus, the extremely high viscosity and poor process ability of UHMWPE make it hard to fabricate membrane by traditional methods. Thermally induced phase separation (TIPS), a novel technique for various polymers that could not be processed in routine methods [5],
has been applied to fabricate UHMWPE microporous membranes for lithium-ion battery separators [6], microfiltration [7] and some other fields [8]. On the principle of phase separation, TIPS process can be divided into liquid-liquid phase separation (LLPS) and solid-liquid phase separation (SLPS) [5,9]. The deviation of solubility parameter between polymer and diluent is the major factor to determine the occurrence of SLPS or LLPS for a polymer/diluent binary system [9,10]. Small devia tion of solubility parameter between polymer and diluent gives rise to strong polymer/diluent interaction and occurrence of SLPS via polymer crystallization at low temperature. Thus, the crystallization kinetics of the polymer plays essential roles in determining the structure of mem brane. Conversely, large deviation between the solubility parameters
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (Q. Qian), [email protected] (D. Yu), [email protected] (X. Chen). 1 Changlin Cao and Wei Jiang contributed equally to this work. https://doi.org/10.1016/j.polymertesting.2019.106243 Received 26 August 2019; Received in revised form 17 November 2019; Accepted 19 November 2019 Available online 23 November 2019 0142-9418/© 2019 Published by Elsevier Ltd.
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blends were performed with 10 � C min 1 cooling rate at a constant frequency of 0.1 Hz and strain of 1.0%. Frequency sweeps on UHMWPE/ diluent blends at a constant temperature of 150 � C and strain of 1.0% were carried out to study the viscoelasticity of samples with 20 wt% UHMWPE. The melt flow rate (MFR) of all UHMWPE/diluent blends were measured on a Melt Flow Indexer (XNR-400C, Chengde Jinjian Testing Instrument Co., Ltd, China) at 190 � C under a 2.16 kg load ac cording to GB/T 3682-2000. A differential scanning calorimeter (DSC, Q20, TA Instruments, USA) was adopted to study the crystallization behavior under the nitrogen atmosphere. The sample was first heated at 200 � C and maintained at this temperature for 5 min, and then subsequently cooled to 30 � C at a rate of 10 � C min 1. The onset of the exothermic peak during the cooling was taken as the nonisothermal crystallization temperature Tc, which can reveal the temperatures of the phase separation of UHMWPE/ diluent blends. A small amount of the UHMWPE/diluent blend was placed between a pair of microscope cover slips. To prevent the loss of diluent, the sample was heated (THMSE 600, Linkam, USA) to 160 � C to form a homoge neous solution and cooled to 80 � C with a rate of 10 � C min 1. The temperature of the cloud point (Tcloud) was determined visually based on the appearance of turbidity under an optical microscope (DMRX, Leica, GER), which can be taken as the temperatures of phase separation of UHMWPE/diluent blends.
results in weak polymer/diluent interaction and occurrence of LLPS with an upper critical solution temperature (UCST) behavior in the cooling process. At this point, droplets of diluent rich phase form within a continuous matrix of polymer rich phase. The size of droplets is strongly related to the pore size of membrane. Accordingly, an appropriate solvent is essential for the TIPS process of UHMWPE. Various kinds of solvent including dodecanol, p-xylene, toluene and camphene have been employed to dissolve UHMWPE; however, a satisfied effect is still not obtained [11]. Among the diluents used in the TIPS process, liquid paraffin (LP) and decahydronaphthalene (decalin) are the most successful for manufacturing UHMWPE micro porous membranes in industry. The evaporability of decalin is in favor of generating microporous structures without extractions, but its inflam mability and toxicity bring about health hazards [12]. On the contrary, LP is more stable than decalin at high temperature; however, it has to be extracted from the UHMWPE/LP blend membranes to form porosity in TIPS process owing to its nonvolatile. The strong interaction between LP and UHMWPE originates from their analogical chemical repeat units, leading to a long extraction time that also means environmental and health issues [13]. As a result, it is still necessary to find a new diluent with good processability and controllable phase separation behaviors for the demanding UHMWPE microporous materials. In this context, polybutene (PB), which presents an approximate solubility parameter to that of polyethylene, was selected as a diluent for the TIPS process of UHMWPE. Besides, PB also possesses high boiling point (above 300 � C) and nontoxicity due to its stable chemical struc ture. Although PB has been used as a solvent for gel spinning of UHMWPE resins, the phase separation behavior of UHMWPE in PB has not been explored. In the present investigation, the phase separation of UHMWPE in PB and LP were probed by means of melt flow indexer (MFR), optical microscopy, and rotational rheometer as well as differ ential scanning calorimeter (DSC), respectively. Furthermore, the real phase separation temperature was also obtained via rheological method, and the practical phase separation kinetics was studied with the help of the Jeziorny method.
3. Results and discussion LP mainly consists of some short chain alkane, and its chemical repeat unit is identical with UHMWPE, which often results in a strong interaction between LP and UHMWPE. In contrast, the chemical repeat unit of PB is nearly made of isobutene [–CH2–C(CH3)2-], i.e. the chain of PB exists in numerous methyl side group, PB tends to afford a relatively weak interaction with UHMWPE compared to LP. As shown in Table S1, both LP and PB exhibit an approximate solubility parameter with rela tively high boiling point and low toxicity comparable to that of poly ethylene. The compatibility and homogeneity of polymer blends were characterized by the following rheological measurements. The dependence of storage modulus G0 on the frequency at 150 � C for all the blends with 20 wt% UHMWPE are shown in Fig. 1. It is generally suggested that G0 is a measure of the energy stored and recovered per cycle, and regarded as the indication of the entropy elasticity for poly mer chains [14]. The values of G0 are higher for SLL-6/LP and SLL-2/LP blends than those of SLL-6/PB and SLL-2/PB blends. This suggests that the rheological behaviors of UHMWPE/diluent mixture exhibit strong dependence on the diluent type and the molecular weight of UHMWPE.
2. Experimental 2.1. Materials UHMWPE nascent powders with molecular weight of 2.0 � 106 and 6.0 � 106 g mol 1 were supported by Shanghai Lianle Chemical Industry Science and Technology Co., Ltd, China. LP was purchased from Tianjin Fuchen Chemical Reagent Co. Ltd. PB with a number average molecular mass of 400 g mol 1 was supplied by Daelin Corporation, South Korea. All these obtained materials were utilized as received without further refinement. 2.2. Preparation The predetermined amounts of UHMPE and LP or PB with polymer concentrations of 10, 20, 30, 40, 50 and 60 wt% were manually stirred with a glass bar for several minutes followed by heating at 60 � C for 12 h. Then the obtained compounds were prepared respectively in the mixing chamber of a rheometer (RM-200B, Harbin Hapro Electric Technology Co., Ltd, China) at 200 � C for 10 min with the rotor speed of 40 rpm. 2.3. Characterization The rheological measurements were performed on a rotational rheometer (DHR-2, TA Instruments, USA) with parallel plate geometry of 25 mm in diameter and a gap of 1.0 mm. Strain sweeps with strain amplitude from 0.01 to 100% at frequency of 0.1 Hz were carried out at 160 � C to determine the linear viscoelastic range of the samples. Time sweep was carried out at 160 � C with a strain amplitude of 2.0% in 60 min. Temperature ramps from 160 to 100 � C for UHMWPE/diluent
Fig. 1. Variation of G0 with frequency for the UHMWPE/LP and UHMWPE/PB blends with 20 wt% UHMWPE during the frequency sweeps at 150 � C. 2
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Low molecular weight of UHMWPE can reduce elasticity of the blends, consequently improving the plasticity. In addition, the value of G’ at low frequency is related to the confined effect caused by the chain entan glement of UHMWPE, which can be reflected by the stress relaxation results in the following part. The relaxation modulus H(τ) of the UHMWPE/PB and the UHMWPE/LP blends with 20 wt% UHMWPE during the frequency sweeps at 150 � C are presented in Fig. 2, which was converted by dy namic data using the method presented by Ninomiya and Ferry [15]. The transition and the termination zone of the relaxation modulus curve are associated with the average spacing between entanglement coupling points [16]. According to the relaxation mechanism, the slopes of relaxation modulus curves at long time region decrease with the increase of molecular chain entanglement. As depicted in Fig. 2, the slope of blends decreases in an order of SLL-2/PB, SLL-6/PB, SLL-2/LP and SLL-6/LP blends. This obtain result suggest that UHMWPE with higher molecular weight (Mη ¼ 6.0 � 106) exhibits higher chain entanglement, resulting in decreasing the fluidity of blends. Moreover, the higher slope for UHMWPE in PB also indicates the worse compatibility with respect to UHMWPE in LP that molecular chains are more stretched in LP. This supports the results of dependence of storage modulus on the MFR an alyses (Fig. S1). At low concentration (<20 wt%), UHMWPE molecular chains get good diffusion in diluent, resulting in separating the original entangled molecular chains and high MFR. However, UHMWPE mo lecular chains hardly diffuse in diluent at high concentration (>20 wt %), leading to the dramatically decrease of MFR. It is also found that the MFR values of UHMWPE/PB blends is always higher than that of UHMWPE/LP blends at same polymer concentration, also suggesting that UHMWPE molecular chains possess lower entanglements in PB. In the case of membrane industry, the UHMWPE membranes are usually produced by passing the blends through a heated extruder, then calendared. After cooling, a part of the oil is extracted from the mem brane by the use of solvent. The polymer blends with MFR values less than 0.1 g/10min can hardly be processed by extruder, higher MFR values of polymer blends enhance the processing efficiency of extruding [17]. For the SLL-2/PB blend with 20 wt% polymer concentration, its MFR value is 15.8 g/10min, which is 10 times than that of SLL-2/LP blend (MFR ¼ 1.52 g/10min) with the same concentration. In addi tion, lower storage modulus and shorter relaxation time can reduce elasticity of the blends, improving the plasticity of the blends, which the designed thickness of membranes by calendaring process are needed. The Cole-Cole plots for the UHMWPE/LP and the UHMWPE/PB blends with 20 wt% UHMWPE during the frequency sweeps at 150 � C are depicted in Fig. 3, where η’’ and η’ are the imaginary part of complex viscosity and the real part of complex viscosity, respectively [14]. It is
well known that the plot is often used to characterize the compatibility and homogeneity of polymer blends [18]. In Fig. 3, all curves display a single smooth semicircle, indicating that LP and PB are both good sol vents for UHMWPE at 150 � C. Obviously, the slopes of UHMWPE/PB blends are always less than those of UHMWPE/LP blends, signifying a better dispersibility of UHMWPE in LP than in PB, which is contributed to the better compatibility between UHMWPE and LP. It is known that Tanδ is defined as the value of G’’/G’, which has been proved to be more sensitive to network formation than the storage modulus [14]. The variations of loss tangent (tanδ) with frequency for UHMWPE/LP and UHMWPE/PB blends with 20 wt% UHMWPE during the frequency sweeps at 150 � C are given in Fig. 4. Notably, SLL-2/PB blend gives a dramatic decrease in tanδ with the frequency, presenting a state of typical viscoelastic liquid, whereas the other three blends show little dependence on frequency. These illustrate that high molecular weight causes the formation of the critical entanglement network of UHMWPE more easily in LP than in PB. We also found that the deviation of tanδ between the SLL-2/LP and SLL-6/LP blends is very small, whereas it is evident between the SLL-2/PB and SLL-6/PB blends, sug gesting that the effect of molecular weight on entanglement network of UHMWPE in PB is more obvious than in LP. The effect of molecular weight on the entanglement network is weakened due to the good compatibility of UHMWPE and LP. The phase diagram of the UHMWPE/diluent blend was drawn with the changes of cloud points and crystallization temperatures with UHMWPE concentration. The TIPS mechanism of crystalline polymer/ diluent system is determined by the competition between LLPS and crystallization i.e. SLPS. In addition, crystallization is the prominent process at temperature under the melt point without nucleation barrier [19,20]. The phase separation of various UHMWPE/diluent bends was investigated. The phase separation temperatures (obtained by optical microscopy), the dynamic crystallization temperatures (obtained by DSC) and the critical transition storage modulus temperatures (obtained by rheology) of various UHMWPE/diluent blends as function of UHMWPE concentrations were given in Fig. 5. Both the phase separation temperature and the crystallization temperature of all the system in crease with the UHMWPE concentration. The cloud points arise from the crystallization of UHMWPE, and actually a SLPS rather than a LLPS during the cooling process. The crystallization temperature is higher than cloud points, which probably due to systematic error where hot stage temperature lags behind the true temperature of the UHMWPE/ diluent blends in cooling process. In practical processing environment, the UHMWPE/diluent blends are affected not only by heat, but also by force during the whole TIPS. There are many kinds of rheological models for the determination of
Fig. 2. Relaxation moduli of the UHMWPE/PB and UHMWPE/LP blends with 20 wt% UHMWPE during the frequency sweeps at 150 � C.
Fig. 3. Cole–Cole plot for the UHMWPE/LP and the UHMWPE/PB blends with 20 wt% UHMWPE during the frequency sweeps at 150 � C. 3
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[22]. Fig. 6 shows the variations of G0 with polymer concentration for SLL-2/PB blends during the dynamic temperature ramp. In the homo geneous region above the critical temperature, storage modulus gradu ally increases with temperature, which is due to the slowing down of chain segment movement or increasing of intermolecular friction. As the temperature further decreases, the phase boundary is approached, the value of G0 dramatically increases. An obvious slope change near the phase separation region is observed in the plot, which is boiled down to the synergistic effect of the decreased molecular motion and enhanced elasticity driven by interfacial tension caused by crystallization of UHMWPE. As shown in Fig. 6, TG’ obtained by rheometer is higher than Tc by DSC in all blends, indicating that phase separation temperature obtained by rheometry is indeed more sensitive [23]. Because rheolog ical measurements are more approaching to actual processing environ ment, critical transition storage modulus temperatures can be regarded as the real phase separation temperature relative to the theoretical phase separation temperature (namely dynamic crystallization temperature obtained by DSC). Fig. 7 exhibits the real phase diagram of the UHMWPE/diluent blends based on dynamic temperature ramp. Three important clues can be acquired: (1) Phase separation temperatures increase with the in crease of UHMWPE content, because the reduction of diluent decreases the interaction distance of molecular chains, which can improve the interaction, heighten the chemical potential of the blends and raise the equilibrium melt point of polymer (Table 1) [24]. (2) The separation temperature in UHMWPE/PB is obviously higher than that in UHMW PE/LP blends, which is possibly related with the compatibility of
Fig. 4. Variations of tanδ with frequency for UHMWPE/LP and UHMWPE/PB blends with 20 wt% UHMWPE during the frequency sweeps at 150 � C.
phase separation. With phase separation progressing, the G0 shows monotonic increase or decrease depending on the viscoelasticity of mixed system [21]. If the frequency of oscillation is sufficiently small, modulus response at the early stage of phase separation can be detected. Therefore, the transition of G0 with temperatures can be used as a sen sitive mode to investigate the critical phase separation temperature
Fig. 5. The influence of UHMWPE concentration on temperature of phase separation by different test methods: (a) SLL-2/PB blends, (b) SLL-6/PB blends, (c) SLL-2/ LP blends, and (d) SLL-6/LP blends. 4
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different cooling rates [28,29], the Jeziorny method (Eq. (1)), an extended Avrami equation [30], was used to explore the nonisothermal crystallization kinetics of UHMWPE/LP and UHMWPE/PB blends in this work. log½
lnð1
XðtÞÞ� ¼ log Z þ n log t
(1)
Where the Avrami exponent n, is a constant of crystallization mecha nism depending on the nucleation type and growth process, Z is the Avrami rate constant involving nucleation and growth parameters, t is the crystallization time, and X(t) is the relative degree of crystallization at time t. In view of nonisothermal process, Jeziorny considered that the rate parameter Z should be corrected by the influence of cooling rate Φ of polymer [30]. The modified form of the parameter characterizing the kinetics of nonisothermal crystallization Zc is given as follows: logZc ¼
Where dHc ðtÞ=dt is the rate of crystallization heat flow, ΔHc ðtÞ is the heat produced at time t, and ΔH∞ ðtÞ is the total heat generated during the whole crystallization process. Fig. 8 shows the Avrami plots of log[-ln(1-X(t))] vs. logt for all blends. It is found that each curve almost displays a commendable linear type, only a small deviation from linearity occurs at high logt value. This implies that the primary crystallization dominates the whole process without the secondary crystallization [31]. The kinetic parameters for the UHMWPE/diluent blends based on Jeziorny analysis are shown in Table 2. The Avrami exponent n of all samples are not integers due to the complexity of nonisothermal crystallization, and consequently values of n lies between 2 and 3. The values of n for PE reported in literatures are generally ranged from 2 to 4 [32]. As listed in Table 2, the values of n is about 2.4, and hardly fluctuate with UHMWPE concentration, molecular weight and type of diluent, indicating that all blends possess almost the same crystallization mechanism where the crystallization process is attributed to heterogeneous nucleation, and a strong tendency of instantaneous two dimensional and three dimensional growth. In addi tion, the reviewed rate constant of crystallization Zc decreases with increasing polymer concentration in the UHMWPE/diluent blends, demonstrating that the overall crystallization rate is faster at a low concentration of UHMWPE. Variation of the overall crystallization rate with polymer concen tration for all UHMWPE/diluent blends is exhibited in Fig. 9. Crystalli zation half-time t1/2 was calculated from Eq. (3) and presented in Table 2. We can find that all curves are the same in two plots, suggesting nonisothermal crystallization kinetics can be perfectly described by the Jeziorny method. Therefore, values of Zc can be used to analyze non isothermal crystallization kinetics for the blends as conveniently and directly as values of t1/2. As seen in Fig. 9(b), diluent plays an important role in nonisothermal crystallization, namely, PB promotes the crystal lization rate of UHMWPE compared to LP with the same polymer con tent and molecular weight. The phenomenon is most likely resulted from a weaker miscibility between UHMWPE and PB, and the molecule mo tion becomes more active. In other words, the miscibility between UHMWPE and diluent is moderately reduced, facilitating the overall crystallization rate of UHMWPE and thereby accelerating phase separation.
Fig. 7. Phase diagram of the UHMWPE/diluent blends based on dynamic temperature ramp.
UHMWPE and diluent. The weak interaction between UHMWPE and PB causes the phase separation obviously in the UHMWPE/PB blends. (3) Molecular weight has an influence on the phase separation temperature. The phase separation temperature is higher in SLL-6/PB blends than in SLL-2/PB blends under the identical conditions where diluent and polymer concentration are uniform, and the same situation can be observed in two kinds of molecular weight UHMWPE/LP blends. On one hand, the increasing molecular weight enhances the number of crystal lattice occupied by UHMWPE molecule, which reduces the contribution of mixed entropy to mixed free energy at the same temperature [25]. On the other hand, the increase of molecular weight reduces the miscibility between UHMWPE and diluent [26,27]. To investigate the nonisothermal crystallization of the blends under Table 1 Melt point of different blends system based on UHMWPE concentration.
SLL-2/PB SLL-6/PB SLL-2/LP SLL-6/LP
Tm (� C) based on concentration of UHMWPE 10 wt%
20 wt%
30 wt%
40 wt%
50 wt%
60 wt%
120.64 120.42 117.28 115.60
120.88 121.00 119.23 119.25
123.41 123.57 120.99 120.55
124.61 124.77 123.05 122.70
125.61 126.66 125.83 126.08
129.24 127.80 127.22 126.61
(2)
Where Φ is equal to 10 � C min 1 in this experiment. If Eq. (1) adequately describes the nonisothermal crystallization behavior of a polymer, the straight-line relationship of log[-ln(1-X (t))] vs. logt would give the values of n and Z or Zc from the slopes and the intercepts, respectively. In addition, the relative crystallinity X(t) is defined as, Rt fdHc ðtÞ=dtgdt ΔHc ðtÞ 0 XðtÞ ¼ R ∞ (3) ¼ fdHc ðtÞ=dtgdt ΔH∞ ðtÞ 0
Fig. 6. Variations of G0 with polymer concentration for the SLL-2/PB blends during the dynamic temperature ramp.
Constituent
log Z Φ
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Fig. 8. Avrami plots of log[-ln(1-X(t))] vs. log t for nonisothermal crystallization with various UHMWPE concentrations. (a) SLL-2/PB blends, (b) SLL-6/PB blends, (c) SLL-2/LP blends, and (d) SLL-6/LP blends. Table 2 Kinetic Parameters for the UHMWPE/diluent blends system based on Jeziorny Analysis. Diluent
UHMWPE Content
PB
10 wt% 20 wt% 30 wt% 40 wt% 50 wt% 60 wt% 10 wt% 20 wt% 30 wt% 40 wt% 50 wt% 60 wt% 100 wt%
LP
None
SLL-2 (Mη ¼ 2.0 � 106)
SLL-6 (Mη ¼ 6.0 � 106)
n
Z
Zc
t1/2 (min)
n
Z
Zc
t1/2 (min)
2.43 2.40 2.36 2.37 2.32 2.32 2.44 2.39 2.38 2.34 2.26 2.38 2.35
16.776 14.591 8.555 5.840 4.871 3.709 12.194 7.203 4.960 4.454 3.739 3.747 2.404
1.326 1.307 1.239 1.193 1.172 1.140 1.284 1.218 1.174 1.161 1.141 1.141 1.092
0.283 0.293 0.357 0.427 0.450 0.503 0.323 0.393 0.457 0.473 0.493 0.513 0.613
2.41 2.39 2.34 2.30 2.29 2.25 2.40 2.36 2.35 2.27 2.33 2.30 2.35
14.656 11.400 9.151 7.695 6.039 4.772 8.788 6.663 5.439 4.829 3.977 3.823 2.318
1.308 1.276 1.248 1.226 1.197 1.169 1.243 1.209 1.185 1.171 1.148 1.143 1.088
0.293 0.323 0.343 0.363 0.403 0.440 0.363 0.407 0.437 0.440 0.490 0.493 0.630
Another interesting finding is that an evident rate intersection (here named crossover rate Rx) appears with increasing polymer concentra tion between two types of molecular weight UHMWPE/PB blends (or UHMWPE/LP blends). The concentration corresponded Rx is defined as crossover concentration Cx. It is generally accepted that a combination of nucleation and growth controls the whole crystallization process ac cording to the classical theory [33]. Hence, here comes a hypothesis that a dynamic competition between nucleation and growth occurs in the overall crystallization rate under the influence of polymer concentration
and molecular weight. The molecular weight dependence of nucleation rate (Rn) and growth rate (Rg) all abide by their respective power law, which are expressed as followed: Rn ∝M α
(4)
Rg ∝M β
(5)
where M is molecular weight; the exponent α and β are dependent on materials and crystallization [34,35]. 6
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Fig. 9. Variations of the overall crystallization rate with polymer concentration for all UHMWPE/diluent blends: (a) obtained by the reciprocal of crystallization halftime V, and (b) obtained by the crystallization rate constant.
Although the exponent α and β concerning polyethylene has been reported, surprisingly little attention has been devoted to UHMWPE. As for the common polyethylene like high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE) etc, α and β are both negative i.e. α < 0, β < 0. And α is nearly equal to 1 [36]; β usually lies in the range from 1.8 to 1.3 at rela tively small supercooling, or reaches 0.5 at relatively large super cooling [37]. As shown in Fig. 9, UHMWPE with low molecular weight (Mη ¼ 2.0 � 106) always displays a faster overall crystallization rate when polymer concentration is less than Cx. This can be attributed to the dominant effect of Rg on the overall rate. At this moment, nucleation density of UHMWPE declines and molecule chains is more stretched rather than curly with the addition of diluent, crystal has a sufficient free volume to develop. Nevertheless, an opposite consequence occurs in high polymer concentration region i.e. when UHMWPE concentration exceeding Cx, UHMWPE with high molecular weight (Mη ¼ 6.0 � 106) shows high rate. The result is not identical with the former, which is against Eq (4). This indicates that this factor affects the overall rate and the mechanism for molecular weight dependence of Rn change due to complicated nonisothermal crystallization influenced by polymer con centration in UHMWPE/diluent blends. When UHMWPE content ex ceeds Cx, the free volume decreases sharply and the distance among molecule chains are shortened, resulting in increase of the entanglement concentration that generate plenty of entanglement points [38]. More over, the increased entanglement points accelerate the sites of self-nucleation, resulting in increase of the nucleation density. Conse quently, the effect of nucleation on the overall rate becomes dominator in the whole process due to lack of enough free volume, and its mech anism for molecular weight dependence of Rn transforms into that Rn is proportional to molecular weight [39]. According to the analysis of the overall rate, we can determine the appropriate polymer concentration and molecular weight via roughly estimating the crossover concentra tion of UHMWPE. Thus, the optimum conditions for the TIPS process is the combination of SLL-2 (Mη ¼ 2.0 � 106) and concentration ranging from 20 to 30 wt% in this work. Besides, the SEM morphologies of the obtained membranes for 20 wt % UHMWPE concentration with different diluents are shown in Fig. S4. Apparently, the surface pore size is smaller as compared to that in the bulk, usually contributed to the different cooling rate near the surface and in the bulk. Meanwhile, the cross-sectional structure also shows a similar leafy structure. For the microporous membrane obtained from UHMPWE/LP blends exhibits smaller pore size with respective to that of UHMPWE/LP blend. This is because PB can improve phase separation rates and the efficiency of TIPS process. In addition, the coarsening
process is also limited by the higher viscosity of the UHMWPE/LP solution. 4. Conclusions In summary, the phase separation behavior of UHMWPE in LP and PB during TIPS process is explored. Compared to the behaviors of UHMWPE in LP, UHMWPE in PB shows better rheological properties with higher MFR, lower storage modulus, shorter relaxation time, slightly poorer slope in Cole-Cole curve and more sensitive loss tangent due to the poorer miscibility with UHMWPE. The larger phase separa tion significantly enhances the efficiency of TIPS process. The phase separation mechanism of UHMWPE/PB and UHMWPE/LP blends are contributed to the crystallization of UHMWPE induced soild-liquid phase separation, and the phase separation kinetics within the real processing environment. The overall crystallization rate of UHMWPE/ PB and UHMWPE/LP blends exhibits a strong dependence on UHMWPE molecular weight and concentration, with a crossover rate and crossover concentration approximately ranging from 20 to 30 wt%. As for UHMWPE/PB blends, the overall crystallization rate is faster than that of UHMWPE/LP blends, also demonstrating that PB can accelerate phase separation rates and enhance the efficiency of TIPS process. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Changlin Cao: Conceptualization, Writing - review & editing, Data curation. Wei Jiang: Investigation, Writing - original draft. Yu Lin: Data curation, Investigation. Xiaochuan Chen: Investigation. Qingrong Qian: Supervision, Validation. Qinghua Chen: Supervision, Validation. Dingshan Yu: Writing - review & editing, Validation. Xudong Chen: Writing - review & editing, Funding acquisition.
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Acknowledgements
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Financial support from National Key R&D Program of China (2016YFB0302300) and the Science and Technology planning project of Guangzhou (201704020008) are gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.polymertesting.2019.106243. References [1] T. Liu, A. Huang, L.H. Geng, X.H. Lian, B.Y. Chen, B.S. Hsiao, T.R. Kuang, X. F. Peng, Ultra-strong, tough and high wear resistance high-density polyethylene for structural engineering application: a facile strategy towards using the combination of extensional dynamic oscillatory shear flow and ultra-high-molecular-weight polyethylene, Compos. Sci. Technol. 167 (2018) 301–312. [2] C. Cao, X. Chen, J. Wang, L. Yu, Y. Guo, Q. Qian, Q. Chen, Y. Feng, D. Yu, X. Chen, Structure and properties of ultrahigh molecular weight polyethylene processed under a consecutive elongational flow, J. Polym. Res. 25 (2018) 1–9. [3] F.P. Zaribaf, Medical-grade ultra-high molecular weight polyethylene: past, current and future, Mater. Sci. Technol. 34 (2018) 1940–1953. [4] J. Cheng, X. Yang, L. Dong, Z. Yuan, W. Wang, S. Wu, S. Chen, G. Zheng, W. Zhang, D. Zhang, Effective nondestructive evaluations on UHMWPE/Recycled-PA6 blends using FTIR imaging and dynamic mechanical analysis, Polym. Test. 59 (2017) 371–376. [5] D.R. Lloyd, S.S. Kim, K.E. Kinzer, Microporous membrane formation via thermally induced phase separation. I. Solid-liquid phase separation, J. Membr. Sci. 52 (1990) 239–261. [6] H. Yoneda, Y. Nishimura, Y. Doi, M. Fukuda, M. Kohno, Development of microporous PE films to improve lithium ion batteries, Polym. J. 42 (2010) 425–437. [7] Z.Y. Cui, X.X. Tang, W. Li, H.N. Liu, J. Zhang, H. Wang, J.X. Li, EVOH in situ fibrillation and its effect of strengthening, toughening and hydrophilic modification on PVDF hollow fiber microfiltration membrane via TIPS process, J. Mater. Sci. 54 (2019) 5971–5987. [8] J.M. Holzwarth, P.X. Ma, Biomimetic nanofibrous scaffolds for bone tissue engineering, Biomaterials 32 (2011) 9622–9629. [9] D.R. Lloyd, S.S. Kim, K.E. Kinzer, Microporous membrane formation via thermallyinduced phase separation. II. Liquid—liquid phase separation, J. Membr. Sci. 64 (1991) 1–11. [10] R. Liu, L. Li, S. Liu, S. Li, X. Liao, Structure and properties of wool keratin/poly (vinyl alcohol) blended fiber, Adv. Polym. Technol. 37 (2018) 2756–2762. [11] M. Liu, S.H. Liu, Z.L. Xu, Y.M. Wei, H. Yang, Formation of microporous polymeric membranes via thermally induced phase separation: a review, Front. Chem. Sci. Eng. 10 (2016) 57–75. [12] R. Schaller, K. Feldman, P. Smith, T.A. Tervoort, High-performance polyethylene fibers “Al Dente”: improved gel-spinning of ultrahigh molecular weight polyethylene using vegetable oils, Macromolecules 48 (2015) 8877–8884. [13] X. Fang, T. Wyatt, Y. Hong, D. Yao, Gel spinning of UHMWPE fibers with polybutene as a new spin solvent, Polym. Eng. Sci. 56 (2016) 697–706. [14] Y.M. Li, Y. Wang, L. Bai, H.L.Z. Zhou, W. Yang, M.B. Yang, Dynamic rheological behavior of HDPE/UHMWPE blends, J. Macromol. Sci. Part B-Physics 50 (2011) 1249–1259. [15] J.D. Ferry, Viscoelastic Properties of Polymer, Wiley, New York, 1980. [16] M. Nofar, A. Maani, H. Sojoudi, M.C. Heuzey, P.J. Carreau, Interfacial and rheological properties of PLA/PBAT and PLA/PBSA blends and their morphological stability under shear flow, J. Rheol. 59 (2015) 317–333. [17] C.L. Rohn, Analytical Polymer Rheology: Structure-Processing-Property Relationships, Hanser, 1995.
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Material Behaviour
Sensitive phase separation behavior of ultra-high molecular weight polyethylene in polybutene Changlin Cao a, b, 1, Wei Jiang b, 1, Yu Lin b, Xiaochuan Chen a, Qingrong Qian b, ***, Qinghua Chen b, c, Dingshan Yu a, *, Xudong Chen a, ** a
Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education and Key Laboratory of High Performance Polymer-based Composites of Guangdong Province, School of Chemistry, Sun Yat-Sen University, Guangzhou, 510275, China College of Environmental Science and Engineering, Fujian Key Laboratory of Pollution Control & Resource Reuse and Engineering Research Center of Polymer Green Recycling of Ministry of Education, Fujian Normal University, Fuzhou, 350007, China c Fuqing Branch of Fujian Normal University, Fuqing, Fujian Province, 350300, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Thermally induced phase separation (TIPS) Rheological properties Crystallization rate Ultra-high molecular weight polyethylene Polybutene
Thermally induced phase separation (TIPS) has prompted a great deal of interest, especially as an effective approach to fabricate ultra-high molecular weight polyethylene (UHMWPE) microporous membranes. However, the existing utilized diluents for the TIPS process of UHMWPE suffer from environmental and health issues. Herein, we utilized low molecular weight polybutene (PB) bearing similar structure with liquid paraffin (LP) but inferior miscibility with UHMWPE relative to UHMWPE/LP blend, as a diluent for the TIPS process of UHMWPE. The phase separation behavior of UHMWPE/PB blends were investigated by the combination of rheological measurements, optical microscopy as well as differential scanning calorimeter (DSC). The results suggest that PB is fully miscible with UHMWPE at elevated temperature, but yielding a more sensitive phase separation behavior in respect to LP in TIPS process, because PB has weaker interaction with UHMWPE. The Jeziorny method analysis indicates that the crystallization mechanism of UHMWPE/LP blends is in line with that of UHMWPE/PB blends, which includes nucleation and growth as well as their dynamic competition. Moreover, compared to those of UHMWPE/LP blends, UHMWPE/PB blends display higher TIPS temperature and faster TIPS rate along with faster overall crystallization rate, further demonstrating that PB can accelerate phase separation rates and enhance the efficiency of TIPS process.
1. Introduction Ultra-high molecular weight polyethylene (UHMWPE) is a unique polymer with outstanding physical and mechanical properties [1]. Most notable are its chemical inertness, lubricity, impact resistance, and abrasion resistance [2,3]. However, UHMWPE shows a rubbery state at melt owing to its ultra-high molecular weight which is usually over 1.0 � 106 g/mol [4]. Thus, the extremely high viscosity and poor process ability of UHMWPE make it hard to fabricate membrane by traditional methods. Thermally induced phase separation (TIPS), a novel technique for various polymers that could not be processed in routine methods [5],
has been applied to fabricate UHMWPE microporous membranes for lithium-ion battery separators [6], microfiltration [7] and some other fields [8]. On the principle of phase separation, TIPS process can be divided into liquid-liquid phase separation (LLPS) and solid-liquid phase separation (SLPS) [5,9]. The deviation of solubility parameter between polymer and diluent is the major factor to determine the occurrence of SLPS or LLPS for a polymer/diluent binary system [9,10]. Small devia tion of solubility parameter between polymer and diluent gives rise to strong polymer/diluent interaction and occurrence of SLPS via polymer crystallization at low temperature. Thus, the crystallization kinetics of the polymer plays essential roles in determining the structure of mem brane. Conversely, large deviation between the solubility parameters
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (Q. Qian), [email protected] (D. Yu), [email protected] (X. Chen). 1 Changlin Cao and Wei Jiang contributed equally to this work. https://doi.org/10.1016/j.polymertesting.2019.106243 Received 26 August 2019; Received in revised form 17 November 2019; Accepted 19 November 2019 Available online 23 November 2019 0142-9418/© 2019 Published by Elsevier Ltd.
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blends were performed with 10 � C min 1 cooling rate at a constant frequency of 0.1 Hz and strain of 1.0%. Frequency sweeps on UHMWPE/ diluent blends at a constant temperature of 150 � C and strain of 1.0% were carried out to study the viscoelasticity of samples with 20 wt% UHMWPE. The melt flow rate (MFR) of all UHMWPE/diluent blends were measured on a Melt Flow Indexer (XNR-400C, Chengde Jinjian Testing Instrument Co., Ltd, China) at 190 � C under a 2.16 kg load ac cording to GB/T 3682-2000. A differential scanning calorimeter (DSC, Q20, TA Instruments, USA) was adopted to study the crystallization behavior under the nitrogen atmosphere. The sample was first heated at 200 � C and maintained at this temperature for 5 min, and then subsequently cooled to 30 � C at a rate of 10 � C min 1. The onset of the exothermic peak during the cooling was taken as the nonisothermal crystallization temperature Tc, which can reveal the temperatures of the phase separation of UHMWPE/ diluent blends. A small amount of the UHMWPE/diluent blend was placed between a pair of microscope cover slips. To prevent the loss of diluent, the sample was heated (THMSE 600, Linkam, USA) to 160 � C to form a homoge neous solution and cooled to 80 � C with a rate of 10 � C min 1. The temperature of the cloud point (Tcloud) was determined visually based on the appearance of turbidity under an optical microscope (DMRX, Leica, GER), which can be taken as the temperatures of phase separation of UHMWPE/diluent blends.
results in weak polymer/diluent interaction and occurrence of LLPS with an upper critical solution temperature (UCST) behavior in the cooling process. At this point, droplets of diluent rich phase form within a continuous matrix of polymer rich phase. The size of droplets is strongly related to the pore size of membrane. Accordingly, an appropriate solvent is essential for the TIPS process of UHMWPE. Various kinds of solvent including dodecanol, p-xylene, toluene and camphene have been employed to dissolve UHMWPE; however, a satisfied effect is still not obtained [11]. Among the diluents used in the TIPS process, liquid paraffin (LP) and decahydronaphthalene (decalin) are the most successful for manufacturing UHMWPE micro porous membranes in industry. The evaporability of decalin is in favor of generating microporous structures without extractions, but its inflam mability and toxicity bring about health hazards [12]. On the contrary, LP is more stable than decalin at high temperature; however, it has to be extracted from the UHMWPE/LP blend membranes to form porosity in TIPS process owing to its nonvolatile. The strong interaction between LP and UHMWPE originates from their analogical chemical repeat units, leading to a long extraction time that also means environmental and health issues [13]. As a result, it is still necessary to find a new diluent with good processability and controllable phase separation behaviors for the demanding UHMWPE microporous materials. In this context, polybutene (PB), which presents an approximate solubility parameter to that of polyethylene, was selected as a diluent for the TIPS process of UHMWPE. Besides, PB also possesses high boiling point (above 300 � C) and nontoxicity due to its stable chemical struc ture. Although PB has been used as a solvent for gel spinning of UHMWPE resins, the phase separation behavior of UHMWPE in PB has not been explored. In the present investigation, the phase separation of UHMWPE in PB and LP were probed by means of melt flow indexer (MFR), optical microscopy, and rotational rheometer as well as differ ential scanning calorimeter (DSC), respectively. Furthermore, the real phase separation temperature was also obtained via rheological method, and the practical phase separation kinetics was studied with the help of the Jeziorny method.
3. Results and discussion LP mainly consists of some short chain alkane, and its chemical repeat unit is identical with UHMWPE, which often results in a strong interaction between LP and UHMWPE. In contrast, the chemical repeat unit of PB is nearly made of isobutene [–CH2–C(CH3)2-], i.e. the chain of PB exists in numerous methyl side group, PB tends to afford a relatively weak interaction with UHMWPE compared to LP. As shown in Table S1, both LP and PB exhibit an approximate solubility parameter with rela tively high boiling point and low toxicity comparable to that of poly ethylene. The compatibility and homogeneity of polymer blends were characterized by the following rheological measurements. The dependence of storage modulus G0 on the frequency at 150 � C for all the blends with 20 wt% UHMWPE are shown in Fig. 1. It is generally suggested that G0 is a measure of the energy stored and recovered per cycle, and regarded as the indication of the entropy elasticity for poly mer chains [14]. The values of G0 are higher for SLL-6/LP and SLL-2/LP blends than those of SLL-6/PB and SLL-2/PB blends. This suggests that the rheological behaviors of UHMWPE/diluent mixture exhibit strong dependence on the diluent type and the molecular weight of UHMWPE.
2. Experimental 2.1. Materials UHMWPE nascent powders with molecular weight of 2.0 � 106 and 6.0 � 106 g mol 1 were supported by Shanghai Lianle Chemical Industry Science and Technology Co., Ltd, China. LP was purchased from Tianjin Fuchen Chemical Reagent Co. Ltd. PB with a number average molecular mass of 400 g mol 1 was supplied by Daelin Corporation, South Korea. All these obtained materials were utilized as received without further refinement. 2.2. Preparation The predetermined amounts of UHMPE and LP or PB with polymer concentrations of 10, 20, 30, 40, 50 and 60 wt% were manually stirred with a glass bar for several minutes followed by heating at 60 � C for 12 h. Then the obtained compounds were prepared respectively in the mixing chamber of a rheometer (RM-200B, Harbin Hapro Electric Technology Co., Ltd, China) at 200 � C for 10 min with the rotor speed of 40 rpm. 2.3. Characterization The rheological measurements were performed on a rotational rheometer (DHR-2, TA Instruments, USA) with parallel plate geometry of 25 mm in diameter and a gap of 1.0 mm. Strain sweeps with strain amplitude from 0.01 to 100% at frequency of 0.1 Hz were carried out at 160 � C to determine the linear viscoelastic range of the samples. Time sweep was carried out at 160 � C with a strain amplitude of 2.0% in 60 min. Temperature ramps from 160 to 100 � C for UHMWPE/diluent
Fig. 1. Variation of G0 with frequency for the UHMWPE/LP and UHMWPE/PB blends with 20 wt% UHMWPE during the frequency sweeps at 150 � C. 2
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Low molecular weight of UHMWPE can reduce elasticity of the blends, consequently improving the plasticity. In addition, the value of G’ at low frequency is related to the confined effect caused by the chain entan glement of UHMWPE, which can be reflected by the stress relaxation results in the following part. The relaxation modulus H(τ) of the UHMWPE/PB and the UHMWPE/LP blends with 20 wt% UHMWPE during the frequency sweeps at 150 � C are presented in Fig. 2, which was converted by dy namic data using the method presented by Ninomiya and Ferry [15]. The transition and the termination zone of the relaxation modulus curve are associated with the average spacing between entanglement coupling points [16]. According to the relaxation mechanism, the slopes of relaxation modulus curves at long time region decrease with the increase of molecular chain entanglement. As depicted in Fig. 2, the slope of blends decreases in an order of SLL-2/PB, SLL-6/PB, SLL-2/LP and SLL-6/LP blends. This obtain result suggest that UHMWPE with higher molecular weight (Mη ¼ 6.0 � 106) exhibits higher chain entanglement, resulting in decreasing the fluidity of blends. Moreover, the higher slope for UHMWPE in PB also indicates the worse compatibility with respect to UHMWPE in LP that molecular chains are more stretched in LP. This supports the results of dependence of storage modulus on the MFR an alyses (Fig. S1). At low concentration (<20 wt%), UHMWPE molecular chains get good diffusion in diluent, resulting in separating the original entangled molecular chains and high MFR. However, UHMWPE mo lecular chains hardly diffuse in diluent at high concentration (>20 wt %), leading to the dramatically decrease of MFR. It is also found that the MFR values of UHMWPE/PB blends is always higher than that of UHMWPE/LP blends at same polymer concentration, also suggesting that UHMWPE molecular chains possess lower entanglements in PB. In the case of membrane industry, the UHMWPE membranes are usually produced by passing the blends through a heated extruder, then calendared. After cooling, a part of the oil is extracted from the mem brane by the use of solvent. The polymer blends with MFR values less than 0.1 g/10min can hardly be processed by extruder, higher MFR values of polymer blends enhance the processing efficiency of extruding [17]. For the SLL-2/PB blend with 20 wt% polymer concentration, its MFR value is 15.8 g/10min, which is 10 times than that of SLL-2/LP blend (MFR ¼ 1.52 g/10min) with the same concentration. In addi tion, lower storage modulus and shorter relaxation time can reduce elasticity of the blends, improving the plasticity of the blends, which the designed thickness of membranes by calendaring process are needed. The Cole-Cole plots for the UHMWPE/LP and the UHMWPE/PB blends with 20 wt% UHMWPE during the frequency sweeps at 150 � C are depicted in Fig. 3, where η’’ and η’ are the imaginary part of complex viscosity and the real part of complex viscosity, respectively [14]. It is
well known that the plot is often used to characterize the compatibility and homogeneity of polymer blends [18]. In Fig. 3, all curves display a single smooth semicircle, indicating that LP and PB are both good sol vents for UHMWPE at 150 � C. Obviously, the slopes of UHMWPE/PB blends are always less than those of UHMWPE/LP blends, signifying a better dispersibility of UHMWPE in LP than in PB, which is contributed to the better compatibility between UHMWPE and LP. It is known that Tanδ is defined as the value of G’’/G’, which has been proved to be more sensitive to network formation than the storage modulus [14]. The variations of loss tangent (tanδ) with frequency for UHMWPE/LP and UHMWPE/PB blends with 20 wt% UHMWPE during the frequency sweeps at 150 � C are given in Fig. 4. Notably, SLL-2/PB blend gives a dramatic decrease in tanδ with the frequency, presenting a state of typical viscoelastic liquid, whereas the other three blends show little dependence on frequency. These illustrate that high molecular weight causes the formation of the critical entanglement network of UHMWPE more easily in LP than in PB. We also found that the deviation of tanδ between the SLL-2/LP and SLL-6/LP blends is very small, whereas it is evident between the SLL-2/PB and SLL-6/PB blends, sug gesting that the effect of molecular weight on entanglement network of UHMWPE in PB is more obvious than in LP. The effect of molecular weight on the entanglement network is weakened due to the good compatibility of UHMWPE and LP. The phase diagram of the UHMWPE/diluent blend was drawn with the changes of cloud points and crystallization temperatures with UHMWPE concentration. The TIPS mechanism of crystalline polymer/ diluent system is determined by the competition between LLPS and crystallization i.e. SLPS. In addition, crystallization is the prominent process at temperature under the melt point without nucleation barrier [19,20]. The phase separation of various UHMWPE/diluent bends was investigated. The phase separation temperatures (obtained by optical microscopy), the dynamic crystallization temperatures (obtained by DSC) and the critical transition storage modulus temperatures (obtained by rheology) of various UHMWPE/diluent blends as function of UHMWPE concentrations were given in Fig. 5. Both the phase separation temperature and the crystallization temperature of all the system in crease with the UHMWPE concentration. The cloud points arise from the crystallization of UHMWPE, and actually a SLPS rather than a LLPS during the cooling process. The crystallization temperature is higher than cloud points, which probably due to systematic error where hot stage temperature lags behind the true temperature of the UHMWPE/ diluent blends in cooling process. In practical processing environment, the UHMWPE/diluent blends are affected not only by heat, but also by force during the whole TIPS. There are many kinds of rheological models for the determination of
Fig. 2. Relaxation moduli of the UHMWPE/PB and UHMWPE/LP blends with 20 wt% UHMWPE during the frequency sweeps at 150 � C.
Fig. 3. Cole–Cole plot for the UHMWPE/LP and the UHMWPE/PB blends with 20 wt% UHMWPE during the frequency sweeps at 150 � C. 3
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[22]. Fig. 6 shows the variations of G0 with polymer concentration for SLL-2/PB blends during the dynamic temperature ramp. In the homo geneous region above the critical temperature, storage modulus gradu ally increases with temperature, which is due to the slowing down of chain segment movement or increasing of intermolecular friction. As the temperature further decreases, the phase boundary is approached, the value of G0 dramatically increases. An obvious slope change near the phase separation region is observed in the plot, which is boiled down to the synergistic effect of the decreased molecular motion and enhanced elasticity driven by interfacial tension caused by crystallization of UHMWPE. As shown in Fig. 6, TG’ obtained by rheometer is higher than Tc by DSC in all blends, indicating that phase separation temperature obtained by rheometry is indeed more sensitive [23]. Because rheolog ical measurements are more approaching to actual processing environ ment, critical transition storage modulus temperatures can be regarded as the real phase separation temperature relative to the theoretical phase separation temperature (namely dynamic crystallization temperature obtained by DSC). Fig. 7 exhibits the real phase diagram of the UHMWPE/diluent blends based on dynamic temperature ramp. Three important clues can be acquired: (1) Phase separation temperatures increase with the in crease of UHMWPE content, because the reduction of diluent decreases the interaction distance of molecular chains, which can improve the interaction, heighten the chemical potential of the blends and raise the equilibrium melt point of polymer (Table 1) [24]. (2) The separation temperature in UHMWPE/PB is obviously higher than that in UHMW PE/LP blends, which is possibly related with the compatibility of
Fig. 4. Variations of tanδ with frequency for UHMWPE/LP and UHMWPE/PB blends with 20 wt% UHMWPE during the frequency sweeps at 150 � C.
phase separation. With phase separation progressing, the G0 shows monotonic increase or decrease depending on the viscoelasticity of mixed system [21]. If the frequency of oscillation is sufficiently small, modulus response at the early stage of phase separation can be detected. Therefore, the transition of G0 with temperatures can be used as a sen sitive mode to investigate the critical phase separation temperature
Fig. 5. The influence of UHMWPE concentration on temperature of phase separation by different test methods: (a) SLL-2/PB blends, (b) SLL-6/PB blends, (c) SLL-2/ LP blends, and (d) SLL-6/LP blends. 4
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different cooling rates [28,29], the Jeziorny method (Eq. (1)), an extended Avrami equation [30], was used to explore the nonisothermal crystallization kinetics of UHMWPE/LP and UHMWPE/PB blends in this work. log½
lnð1
XðtÞÞ� ¼ log Z þ n log t
(1)
Where the Avrami exponent n, is a constant of crystallization mecha nism depending on the nucleation type and growth process, Z is the Avrami rate constant involving nucleation and growth parameters, t is the crystallization time, and X(t) is the relative degree of crystallization at time t. In view of nonisothermal process, Jeziorny considered that the rate parameter Z should be corrected by the influence of cooling rate Φ of polymer [30]. The modified form of the parameter characterizing the kinetics of nonisothermal crystallization Zc is given as follows: logZc ¼
Where dHc ðtÞ=dt is the rate of crystallization heat flow, ΔHc ðtÞ is the heat produced at time t, and ΔH∞ ðtÞ is the total heat generated during the whole crystallization process. Fig. 8 shows the Avrami plots of log[-ln(1-X(t))] vs. logt for all blends. It is found that each curve almost displays a commendable linear type, only a small deviation from linearity occurs at high logt value. This implies that the primary crystallization dominates the whole process without the secondary crystallization [31]. The kinetic parameters for the UHMWPE/diluent blends based on Jeziorny analysis are shown in Table 2. The Avrami exponent n of all samples are not integers due to the complexity of nonisothermal crystallization, and consequently values of n lies between 2 and 3. The values of n for PE reported in literatures are generally ranged from 2 to 4 [32]. As listed in Table 2, the values of n is about 2.4, and hardly fluctuate with UHMWPE concentration, molecular weight and type of diluent, indicating that all blends possess almost the same crystallization mechanism where the crystallization process is attributed to heterogeneous nucleation, and a strong tendency of instantaneous two dimensional and three dimensional growth. In addi tion, the reviewed rate constant of crystallization Zc decreases with increasing polymer concentration in the UHMWPE/diluent blends, demonstrating that the overall crystallization rate is faster at a low concentration of UHMWPE. Variation of the overall crystallization rate with polymer concen tration for all UHMWPE/diluent blends is exhibited in Fig. 9. Crystalli zation half-time t1/2 was calculated from Eq. (3) and presented in Table 2. We can find that all curves are the same in two plots, suggesting nonisothermal crystallization kinetics can be perfectly described by the Jeziorny method. Therefore, values of Zc can be used to analyze non isothermal crystallization kinetics for the blends as conveniently and directly as values of t1/2. As seen in Fig. 9(b), diluent plays an important role in nonisothermal crystallization, namely, PB promotes the crystal lization rate of UHMWPE compared to LP with the same polymer con tent and molecular weight. The phenomenon is most likely resulted from a weaker miscibility between UHMWPE and PB, and the molecule mo tion becomes more active. In other words, the miscibility between UHMWPE and diluent is moderately reduced, facilitating the overall crystallization rate of UHMWPE and thereby accelerating phase separation.
Fig. 7. Phase diagram of the UHMWPE/diluent blends based on dynamic temperature ramp.
UHMWPE and diluent. The weak interaction between UHMWPE and PB causes the phase separation obviously in the UHMWPE/PB blends. (3) Molecular weight has an influence on the phase separation temperature. The phase separation temperature is higher in SLL-6/PB blends than in SLL-2/PB blends under the identical conditions where diluent and polymer concentration are uniform, and the same situation can be observed in two kinds of molecular weight UHMWPE/LP blends. On one hand, the increasing molecular weight enhances the number of crystal lattice occupied by UHMWPE molecule, which reduces the contribution of mixed entropy to mixed free energy at the same temperature [25]. On the other hand, the increase of molecular weight reduces the miscibility between UHMWPE and diluent [26,27]. To investigate the nonisothermal crystallization of the blends under Table 1 Melt point of different blends system based on UHMWPE concentration.
SLL-2/PB SLL-6/PB SLL-2/LP SLL-6/LP
Tm (� C) based on concentration of UHMWPE 10 wt%
20 wt%
30 wt%
40 wt%
50 wt%
60 wt%
120.64 120.42 117.28 115.60
120.88 121.00 119.23 119.25
123.41 123.57 120.99 120.55
124.61 124.77 123.05 122.70
125.61 126.66 125.83 126.08
129.24 127.80 127.22 126.61
(2)
Where Φ is equal to 10 � C min 1 in this experiment. If Eq. (1) adequately describes the nonisothermal crystallization behavior of a polymer, the straight-line relationship of log[-ln(1-X (t))] vs. logt would give the values of n and Z or Zc from the slopes and the intercepts, respectively. In addition, the relative crystallinity X(t) is defined as, Rt fdHc ðtÞ=dtgdt ΔHc ðtÞ 0 XðtÞ ¼ R ∞ (3) ¼ fdHc ðtÞ=dtgdt ΔH∞ ðtÞ 0
Fig. 6. Variations of G0 with polymer concentration for the SLL-2/PB blends during the dynamic temperature ramp.
Constituent
log Z Φ
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Fig. 8. Avrami plots of log[-ln(1-X(t))] vs. log t for nonisothermal crystallization with various UHMWPE concentrations. (a) SLL-2/PB blends, (b) SLL-6/PB blends, (c) SLL-2/LP blends, and (d) SLL-6/LP blends. Table 2 Kinetic Parameters for the UHMWPE/diluent blends system based on Jeziorny Analysis. Diluent
UHMWPE Content
PB
10 wt% 20 wt% 30 wt% 40 wt% 50 wt% 60 wt% 10 wt% 20 wt% 30 wt% 40 wt% 50 wt% 60 wt% 100 wt%
LP
None
SLL-2 (Mη ¼ 2.0 � 106)
SLL-6 (Mη ¼ 6.0 � 106)
n
Z
Zc
t1/2 (min)
n
Z
Zc
t1/2 (min)
2.43 2.40 2.36 2.37 2.32 2.32 2.44 2.39 2.38 2.34 2.26 2.38 2.35
16.776 14.591 8.555 5.840 4.871 3.709 12.194 7.203 4.960 4.454 3.739 3.747 2.404
1.326 1.307 1.239 1.193 1.172 1.140 1.284 1.218 1.174 1.161 1.141 1.141 1.092
0.283 0.293 0.357 0.427 0.450 0.503 0.323 0.393 0.457 0.473 0.493 0.513 0.613
2.41 2.39 2.34 2.30 2.29 2.25 2.40 2.36 2.35 2.27 2.33 2.30 2.35
14.656 11.400 9.151 7.695 6.039 4.772 8.788 6.663 5.439 4.829 3.977 3.823 2.318
1.308 1.276 1.248 1.226 1.197 1.169 1.243 1.209 1.185 1.171 1.148 1.143 1.088
0.293 0.323 0.343 0.363 0.403 0.440 0.363 0.407 0.437 0.440 0.490 0.493 0.630
Another interesting finding is that an evident rate intersection (here named crossover rate Rx) appears with increasing polymer concentra tion between two types of molecular weight UHMWPE/PB blends (or UHMWPE/LP blends). The concentration corresponded Rx is defined as crossover concentration Cx. It is generally accepted that a combination of nucleation and growth controls the whole crystallization process ac cording to the classical theory [33]. Hence, here comes a hypothesis that a dynamic competition between nucleation and growth occurs in the overall crystallization rate under the influence of polymer concentration
and molecular weight. The molecular weight dependence of nucleation rate (Rn) and growth rate (Rg) all abide by their respective power law, which are expressed as followed: Rn ∝M α
(4)
Rg ∝M β
(5)
where M is molecular weight; the exponent α and β are dependent on materials and crystallization [34,35]. 6
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Fig. 9. Variations of the overall crystallization rate with polymer concentration for all UHMWPE/diluent blends: (a) obtained by the reciprocal of crystallization halftime V, and (b) obtained by the crystallization rate constant.
Although the exponent α and β concerning polyethylene has been reported, surprisingly little attention has been devoted to UHMWPE. As for the common polyethylene like high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE) etc, α and β are both negative i.e. α < 0, β < 0. And α is nearly equal to 1 [36]; β usually lies in the range from 1.8 to 1.3 at rela tively small supercooling, or reaches 0.5 at relatively large super cooling [37]. As shown in Fig. 9, UHMWPE with low molecular weight (Mη ¼ 2.0 � 106) always displays a faster overall crystallization rate when polymer concentration is less than Cx. This can be attributed to the dominant effect of Rg on the overall rate. At this moment, nucleation density of UHMWPE declines and molecule chains is more stretched rather than curly with the addition of diluent, crystal has a sufficient free volume to develop. Nevertheless, an opposite consequence occurs in high polymer concentration region i.e. when UHMWPE concentration exceeding Cx, UHMWPE with high molecular weight (Mη ¼ 6.0 � 106) shows high rate. The result is not identical with the former, which is against Eq (4). This indicates that this factor affects the overall rate and the mechanism for molecular weight dependence of Rn change due to complicated nonisothermal crystallization influenced by polymer con centration in UHMWPE/diluent blends. When UHMWPE content ex ceeds Cx, the free volume decreases sharply and the distance among molecule chains are shortened, resulting in increase of the entanglement concentration that generate plenty of entanglement points [38]. More over, the increased entanglement points accelerate the sites of self-nucleation, resulting in increase of the nucleation density. Conse quently, the effect of nucleation on the overall rate becomes dominator in the whole process due to lack of enough free volume, and its mech anism for molecular weight dependence of Rn transforms into that Rn is proportional to molecular weight [39]. According to the analysis of the overall rate, we can determine the appropriate polymer concentration and molecular weight via roughly estimating the crossover concentra tion of UHMWPE. Thus, the optimum conditions for the TIPS process is the combination of SLL-2 (Mη ¼ 2.0 � 106) and concentration ranging from 20 to 30 wt% in this work. Besides, the SEM morphologies of the obtained membranes for 20 wt % UHMWPE concentration with different diluents are shown in Fig. S4. Apparently, the surface pore size is smaller as compared to that in the bulk, usually contributed to the different cooling rate near the surface and in the bulk. Meanwhile, the cross-sectional structure also shows a similar leafy structure. For the microporous membrane obtained from UHMPWE/LP blends exhibits smaller pore size with respective to that of UHMPWE/LP blend. This is because PB can improve phase separation rates and the efficiency of TIPS process. In addition, the coarsening
process is also limited by the higher viscosity of the UHMWPE/LP solution. 4. Conclusions In summary, the phase separation behavior of UHMWPE in LP and PB during TIPS process is explored. Compared to the behaviors of UHMWPE in LP, UHMWPE in PB shows better rheological properties with higher MFR, lower storage modulus, shorter relaxation time, slightly poorer slope in Cole-Cole curve and more sensitive loss tangent due to the poorer miscibility with UHMWPE. The larger phase separa tion significantly enhances the efficiency of TIPS process. The phase separation mechanism of UHMWPE/PB and UHMWPE/LP blends are contributed to the crystallization of UHMWPE induced soild-liquid phase separation, and the phase separation kinetics within the real processing environment. The overall crystallization rate of UHMWPE/ PB and UHMWPE/LP blends exhibits a strong dependence on UHMWPE molecular weight and concentration, with a crossover rate and crossover concentration approximately ranging from 20 to 30 wt%. As for UHMWPE/PB blends, the overall crystallization rate is faster than that of UHMWPE/LP blends, also demonstrating that PB can accelerate phase separation rates and enhance the efficiency of TIPS process. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Changlin Cao: Conceptualization, Writing - review & editing, Data curation. Wei Jiang: Investigation, Writing - original draft. Yu Lin: Data curation, Investigation. Xiaochuan Chen: Investigation. Qingrong Qian: Supervision, Validation. Qinghua Chen: Supervision, Validation. Dingshan Yu: Writing - review & editing, Validation. Xudong Chen: Writing - review & editing, Funding acquisition.
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Acknowledgements
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