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Deformation of Ultrahigh Molecular Weight Polyethylene Precursor Fiber: Crystal Slip with or without Melting Fei Lv, Xiaowei Chen, Caixia Wan, Fengmei Su, Youxin Ji,* Yuanfei Lin, Xueyu Li, and Liangbin Li* National Synchrotron Radiation Lab and CAS Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei 230029, China S Supporting Information *

ABSTRACT: Temperature effects on deformation behaviors of extracted ultrahigh molecular weight polyethylene (UHMWPE) precursor fibers are studied with the in situ synchrotron radiation wide-angle X-ray scattering technique (WAXS) during tensile deformation at temperatures from 25 to 130 °C. The structural and mechanical evolution behaviors during tensile deformation can be divided into four temperature regions with boundaries located at temperatures of αI and αII relaxations and the onset of melting, respectively, which reveal that the deformation behaviors of polymer crystals are determined by the interplay between intrinsic structural dynamic or chains mobility and external stress field. Irrespective of temperature, yield and strain-softening proceed via partial melting while crystal slip via cutting crystal planes occurs in the strain-hardening zone. Finally we construct morphological diagrams containing crystallinity, crystal size, and orientation in temperature−strain space, which may serve as a roadmap for UHMWPE fibers processing.

1. INTRODUCTION Ultrahigh molecular weight polyethylene (UHMWPE) fibers, due to their outstanding mechanical performance properties, self-lubrication, low-temperature resistance, and biocompatibility, have been widely applied in various fields.1−3 The wellknown gel-spinning method of UHMWPE fibers covers three main manufacturing steps: (i) UHMWPE solution extrusion, (ii) solution spinning including cooling of the solution to form UHMWPE gelation and solvent extraction or evaporation, and (iii) poststretch or ultradrawing of fibers at high temperature.4,5 Before the poststretch in the third step, the precursor fibers are composed of folded-chain crystals, while the final high performance UHMWPE fibers contain microfibrils or extended-chain crystals.6−11 Therefore, understanding how folded-chain lamellar crystals transform into microfibrils or extended-chain crystals is of prime importance for the manufacture of UHMWPE fibers. 12−14 In fact, tensile deformation-induced crystal morphological transformation is a universal challenge in the poststretch step of high performance films and fibers,15,16 which is also the core science of nonlinear mechanics of semicrystalline polymeric materials. Here the morphological transformation is a nonequilibrium process driven by external stress and temperature, during which either stress or temperature may play their own roles. As the deformation behaviors of UHMPE precursor fibers are rarely studied systematically in wide temperature−strain space, how stress and temperature couple with each other and drive morphological transformation is not fully understood yet. UHMWPE precursor fibers after solvent extraction are composed of pores and randomly oriented lamellae or shish © 2017 American Chemical Society

kebabs, which are determined by spinning speed and collecting rate.17 For decades, studies on the relationship between morphologies and mechanical properties of UHMWPE fibers reveal that the extremely high strength and modulus of fibers may originate from extended-chain crystals structure with molecular chains oriented along the fiber axis.18−20 Thus, many studies have been devoted to explore how such extended-chain crystals structure form during the hot drawing process. Tian et al.8 reported that the formation of shish-kebab structure during hot drawing of extracted UHMWPE precursor fibers is induced by the break−reorganization of original lamella. Hashimoto et al.7 conducted a series of TEM observation of fibers after hot drawing to different ratios and found the transformation of folded-chain in the kebab structure into extended-chain structure through the interchain slippage. By a combination of SEM and SAXS investigation of fibers at different hot drawing ratios, Pennings et al.6 suggested that the individual lamellae first transforms into fibrils at a moderate ratio, followed by a sliding displacement of fibrils at large draw ratio. Temperature plays an important role in the deformation mechanisms of UHMWPE precursor fibers for crystal chains mobility increases due to thermal expansion of crystalline unit cell at elevated temperature, which enhances the ease of chain slip or crystal shear.21−23 Miyoshi et al.24 for the first time proved that the unfolding of the folded iPP chains in crystals induced by uniaxial stretching at 100 °C with 13C−13C double Received: June 2, 2017 Revised: August 21, 2017 Published: August 29, 2017 6385

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Figure 1. (a) Temperature dependence of the storage modulus (G′), the loss modulus (G″), and the loss factor (tan δ). (b) DSC melting curve.

quantum (DQ) NMR, demonstrating the importance of αrelaxation for large deformability of crystal polymer. Kajiyama et al.25 had mentioned that crystallites decompose into smaller fragments during drawing around temperature of αI-process (T αI ) and above higher α II-process temperature (T αII) crystallites tend to break into larger blocks. Men et al.26 clarified the existence of two α-processes and that the assignment is made up by intralamellar block grain boundary slip for the αI-process and diffusion-like motion of chain segments in crystal for the αII-process, which were also postulated by Ohta et al.22 and Matsuo et al.27 Increasing temperature enhances the mobility of chains in the slip plane and weakens the constraint of entangled amorphous chains between lamellae, which promotes crystallographic slip and chain slippage.28,29 According to the study of Litvinov et al.,30 morphological structure of drawn fibers can be regarded as a mixture of crystalline fibrils and extended-chain crystals with trapped defects. The increase of temperature is beneficial to homogeneous deformation and to reduce local stress concentration, accounting for smoother fibrils transformation and less fragmentation of fibrils. Besides, rising up temperature narrows the thermodynamic difference between amorphous and crystal phases,28 which may increase the final crystallinity and obtain fibers with higher mechanical performance. Irrespective of temperature, crystal slip is commonly taken in interpreting the morphological transformation from foldedchain to microfibrils or extended-chain crystals as well as nonlinear mechanical behaviors like yield, strain-softening, and strain-hardening. However, how crystal slip takes place at different conditions in wide temperature−strain space is still a matter of debate, where models like fine slip, block slip, and melting−recrystallization are widely discussed.31−33 In this work, deformation behaviors of extracted UHMWPE precursor fibers are studied with in situ synchrotron radiation WAXS during uniaxial deformation at temperatures from 25 to 130 °C. The structural and mechanical evolution behaviors during tensile deformation can be divided into four temperature regions with boundaries located at temperatures of αI and αII relaxations and the onset of melting, respectively. This assignment is nicely coincident with the temperature dependence of chains mobility measured with dynamic mechanical analysis (DMA), suggesting that the deformation behaviors of polymer crystals are determined by the interplay between intrinsic structural dynamic and external stress field.

Industrial Technology (Produced by Beijing Dongfang Petrochemical Co., Ltd.). The fibers were prepared by the following procedure. The UHMWPE powder was first dissolved in paraffin oil at a concentration of 5 wt % together with 0.5 wt % antioxidant. Gel-spun fibers were extruded at 270 °C with a TSE-35 corotating twin-screw extruder. After spinning from 0.1 mm spinneret, fibers were immediately quenched in cold water. Both the spinning and collecting rate were 1 m/min. The paraffin oil was first extracted carefully with n-hexane by method of ultrasonic extraction at 30 °C. Then, UHMWPE precursor fibers were dried soon afterward in a vacuum oven at 45 °C. During extraction and drying processes, the two ends of fibers were kept fixed to avoid shrinkage in length. The amount of residual paraffinic oil after extraction was 3.0 wt % (Figure S1), and the surface and crosssectional morphologies of extracted UHMWPE precursor fibers are characterized by SEM (Figure S2). 2.2. DMA and DSC Measurements. DMA measurement was carried out in a DMA Q800 instrument. The sample (a bundle of extracted UHMWPE precursor fibers with 70 filaments and average diameter of 190 μm) was heated from −40 to 120 °C at a heating rate of 3 °C/min. The frequency was fixed at 1 Hz. DSC measurement was performed with an apparatus (DSC CQ2000, TA Instruments) calibrated for temperature and melting enthalpy by using high-purity indium. The heating rate was 10 °C/min. As shown in Figure 1a, three different thermally activated processes can be distinguished from DMA measurement, namely β-process corresponding to the onset of upturn of G″, αI-process characterized by the peak of G″ located at about 51 °C, and αII-process demonstrated by the inflection point of G″ at around 80 °C.25,26 In Figure 1b, the onset of melting temperature (Tonset = 115 °C) and peak temperature of melting curve (Tpeak = 138 °C) can be obtained. Thus, the temperature space can be divided into four temperature regions, i.e., region I (Tβ < T < TαI), region II (TαI < T < TαII), region III (TαII < T < Tonset), and region IV (T > Tonset). Generally speaking, G′ mainly depends on crystalline region properties (such as crystallinity, lamellar thickness, chains mobility in crystalline), and G″ mainly reflects chains mobility in the amorphous region, which however is also influenced by the coupling effect between amorphous and crystalline regions as the latter imposes constraint on the former.34−36 On the whole, DMA can provide a hint of temperature effect on chains mobility in crystalline and amorphous regions, which helps to interpret temperature effect on deformation behaviors of UHMWPE fibers as will be shown in the Results and Discussion sections. 2.3. In Situ Synchrotron Radiation WAXS Measurements during Tensile Deformation. A homemade two-drum stretching instrument was used to conduct tensile deformation at different temperatures, during which stress−strain curves and WAXS measurements were recorded simultaneously. Its design is similar to the Sentmenat extensional rheometer, which can impose Hencky strain on samples as schematically shown in Figure S3.37,38 Nitrogen gas flow was used to homogenize temperature, and the temperature fluctuation was within ±0.5 °C. Sample secured on two geared drums by means of clamping was placed in heating cabinet after reaching the setting temperature. Step extensions with constant strain rate of 1 s−1 were

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. UHMWPE powder with a weight-average molecular weight of 4000 kg/mol (Mw) and polydispersity index of 5.8 was kindly supplied by Ningbo Institute of 6386

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Figure 2. (a) Selected in situ 2D WAXS patterns acquired during 15 min temperature stabilization at 100 °C. The fiber axis is horizontal. The horizontal axis demonstrates the annealing time increasing direction. (b, c) One-dimensional diffraction intensity profiles of 2D WAXS scattering patterns and azimuthal intensity distribution of (200) diffraction plane at different annealing times for 100 °C, respectively. (d, e) Crystallinity and crystal size evolutions as a function of annealing time at temperatures from 100 to 130 °C.

Figure 3. (a) A representative engineering stress−strain curve is acquired at 60 °C; the first and the second black dashed lines indicate the yield (εy, σy) and hardening (εh, σh) points, respectively. (b) Engineering stress−strain curves acquired varying from 25 to 130 °C. (c, d) Plots of εh and σy, εh and σh vs deformation temperature. imposed on the samples after 15 min temperature stabilization. Samples were stretched until fracture strain at temperatures varying from 25 to 130 °C. In situ synchrotron radiation WAXS characterizations during tensile deformation were carried out in BL16B1 beamline of Shanghai Synchrotron Radiation Facility (SSRF). The wavelength is 0.124 nm, and the exposure time is 90 ms with an additional 10 ms for signal reading and cleaning. Two-dimensional (2D) WAXS patterns were collected with a Pilatus 300K detector (2048 × 2048 pixels and pixel size of 172 μm). The sample-to-detector distance calibrated for WAXS

was 114.8 mm. FIT2D software was used to analyze 2D WAXS patterns and subtract air background. The apparent crystallinity (Xc) is calculated by the following equation:39,40

Xc =

Ac Ac + A a

(1)

where Ac and Aa represent the sum of areas under the crystalline and amorphous peaks in 1D integrated curves of WAXS patterns, respectively. 6387

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Macromolecules 2.4. Effect of Annealing at Temperatures above 100 °C by in Situ WAXD Experiments. The effect of annealing at temperatures from 100 to 130 °C during 15 min temperature stabilization was studied by in situ WAXS experiments (Figure S4). Figure 2a shows selected in situ 2D WAXS patterns acquired during 15 min temperature stabilization at 100 °C, which hardly changes with annealing time increase. In Figure 2b, two diffraction peaks of (200) and (110) can be distinguished. It is clear that nearly no diffraction intensity decrease of these peaks and continuous shift of these peaks position appear as annealing time increases. In Figure 2c, the initially broad peak located in meridian shows a tendency of broadening with annealing time increase. In Figure 2d, crystallinity (Xc) nearly remains constant during 15 min temperature stabilization for 100 and 110 °C, which follows a continuous decrease tendency above 115 °C. Moreover, as the temperature increases, the residual crystallinity after 15 min temperature stabilization drops obviously. Figure 2e depicts the evolution of crystal sizes (L200) in the direction perpendicular to (200) plane during 15 min temperature stabilization, which are calculated by Scherrer’s equation41−43 (Supporting Information 5). It is clear that crystal size increases slightly below 115 °C, while the obvious thickening of crystal size for higher temperature appears. Moreover, the shrinkage of unconstrained fibers at elevated temperatures after 15 min temperature stabilization is given in Figure S5.

Figure 4. Engineering stress−strain curve and selected in situ 2D WAXS patterns acquired during uniaxial tensile deformation at 60 °C. The white numbers in the bottom-left corner of each pattern represent the strain. (200) and (110) diffraction planes are indicated by the red arrows. The stretching direction (SD) is horizontal as shown by the black double-headed arrow.

For quantitative analysis, one-dimensional (1D) diffraction intensity profiles of 2D WAXS scattering patterns and azimuthal intensity distribution of (200) diffraction plane are presented in Figures 5a and 5b, respectively. In Figure 5a, one

3. RESULTS 3.1. Mechanical Properties at Different Temperatures. A representative engineering stress−strain curve acquired at 60 °C is shown in Figure 3a, which can be roughly divided into three zones, i.e., linear elastic, strain-softening or plastic deformation, strain-hardening zones with boundaries located at strains of about 1.0 (εy) and 1.8 (εh), respectively. Stress first increases nearly linearly as a function of strain up to about 15.5 MPa (σy) at εy, then drops to about 13 MPa (σh) at εh, and follows a continuous increase in the strain-hardening zone. Figure 3b presents engineering stress−strain curves at different temperatures, from which we extract strains and stresses at yield and hardening points, respectively. We plot εy and σy as well as εh and σh vs temperature in Figures 3c and 3d, respectively. Coincidently, the mechanical properties summarized in Figures 3c and 3d can also be divided into four temperature regions, which is nicely coincident with the four temperature regions defined in Figure 1 as obtained with DMA measurement. In region I (Tβ < T < TαI), εy and σy show a slight downtrend with temperature increase, where samples fracture at low strain without the occurrence of strain hardening. In region II (TαI < T < TαII), strain-hardening appears after strain-softening, where both (εy, σy) and (εh, σh) drop sharply with temperature increase. From TαII of about 80 °C to Tonset of about 115 °C (region III), (εy, σy) keep almost constant, while εh shows an obvious uptrend with slight increase of σh. In region IV (T > Tonset), (εy, σy) and (εh, σh) follow a reduction trend with the increase of temperature. 3.2. Uniaxial Tensile Deformation at 60 °C. Selected 2D WAXS patterns with corresponding stress−strain curve obtained at 60 °C are depicted in Figure 4, which is taken as an example for data analysis. In linear elastic zone, 2D WAXS patterns hardly change. Both (200) and (110) diffraction planes of PE orthorhombic crystal move toward the equator and form bright diffraction dots on top of broaden diffraction arcs in the plastic deformation zone. As deformation proceeds after εh, brightened diffraction dots for both planes are observed to be located in the equator, and the azimuthal widths of diffraction arcs gradually decrease.

Figure 5. (a, b) One-dimensional diffraction intensity profiles of 2D WAXS scattering patterns and azimuthal intensity distribution of the (200) diffraction plane at different strains during uniaxial deformation, respectively. (c) Gaussian fit result of azimuthal intensity distribution of the (200) diffraction plane at a strain of 2.0. The deformation temperature is 60 °C, and the red solid triangle represents strain closest to εy.

can find slight diffraction intensity decrease of these peaks in the linear elastic zone. Beyond εy of about 1.0, deformation results in a sharp drop of diffraction intensity of these peaks. The azimuthal intensity distribution of the (200) plane in Figure 5b shows a broad peak initially located in the meridian, which nearly vanishes after εy. Afterward, the intensity maximum of the (200) plane shifts to the equator. Moreover, the azimuthal intensity distribution of the (200) plane can be clearly regarded as a superposition of two peaks in the equator. Indeed, Figure 5c shows that the azimuthal intensity 6388

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Figure 6. (a) Evolutions of Xc and OC. (b) Evolution of L200. The deformation temperature is 60 °C, and the two black dashed lines indicate εy and εh.

distribution of the (200) plane at strain of 2.0 during uniaxial deformation at 60 °C can be fitted with two Gaussian peaks, implying that orientation of crystallites along drawing direction is not homogeneous. During the deformation process, only part of the crystals are oriented along the drawing direction. As the two fitted peaks illustrated in Figure 5c superpose on each other and both their widths and intensities change simultaneously during deformation, it is difficult to quantitatively analyze the content of highly oriented crystals (OC). Thus, semiquantitative OC is calculated by the ratio between the intensity within azimuthal angle from 257° to 281° (indicated by the two black dashed lines in Figure 5c) and the intensity in azimuthal angle from 180° to 360°. Figure 6a presents Xc and OC during uniaxial stretch at 60 °C. Before εy, Xc keeps almost constant. Beyond εy, Xc follows a notable drop to about 42% at εh, which remains nearly constant in the later deformation until fracture. To quantitatively analyze deformation-induced structural evolution, we calculate the average slopes within specific strain windows, which express the decrease or increase rate per unit strain and may help us to correlate nonlinear mechanical properties with structural evolutions. From εy to εh, the average slope of Xc reduction (kXc) is about −0.28 per unit strain, during which OC increases nearly linearly with an average slope (kOC) of about 0.34 per unit strain. Exceeding εh, the increasing tendency weakens down with average kOC of 0.12 per unit strain, while an obvious upturn of OC occurs near fracture strain. Figure 6b depicts the evolution of L200 in the direction perpendicular to the (200) plane, which is calculated by the Gaussian approximation after removal of lattice distortion broadening due to microstrain and instrumental broadening43,44 Supporting Information 7). In the linear elastic zone, L200 drops slightly. After εy, L200 follows a continuous decrease with an average slope (kL) of −4.7 nm per unit strain until fracture. 3.3. Uniaxial Tensile Deformation at Selected Temperatures. Selected in situ 2D WAXS patterns at 25, 60, 100, and 130 °C are displayed in Figure 7, which are taken as representatives for the four temperature regions. In the linear elastic zone, 2D WAXS patterns show little change at all temperatures, while after εy higher temperature promotes (200) and (110) planes to be oriented in the equator easily. Beyond εh, dotlike (200) and (110) diffraction points concentrate in the equator of 2D WAXS patterns at 100 and 130 °C, which differ greatly from WAXS patterns containing bright diffraction dots on top of broaden diffraction arcs at lower temperatures. Similar to the case of 60 °C, the azimuthal intensity distribution of the (200) plane for 25 °C can also be clearly regarded as a superposition of two peaks in the equator (Figure S6c), while for 100 and 130 °C, the azimuthal intensity distribution of the

Figure 7. Selected in situ 2D WAXS patterns acquired during uniaxial stretching at 25, 60, 100, and 130 °C. The SD is horizontal. The vertical axis stands for the augment of drawing temperature, and the horizontal axis demonstrates strain increasing direction. Numbers in red represent strains closest to εy, εh, and εf.

(200) plane can nearly be fitted with one Gaussian peak (Figures S7c and S8c). This suggests that higher temperature favors uniform deformation or structural transformation. For conciseness, 2D WAXS patterns as well as integrated 1D curves for the rest temperatures are omitted here. Instead, we present the analyzed results with the same method as 60 °C in the following. 3.4. Temperature Effect on Structural Evolutions. In an effort to elucidate temperature effect on deformation behaviors of extracted UHMWPE precursor fibers, we quantitatively calculate Xc during uniaxial tensile deformation at all temperatures, which are summarized in Figure 8. The evolution behaviors of Xc are discrepant in different temperature regions but similar in the same temperature region. In region I (Tβ < T < TαI), Xc stays about 60% in linear elastic zone, which drops dramatically with average kXc of −0.22 from εy to εh and reaches about 35% before fracture. In region II (TαI < T < TαII), from εy to εh, Xc follows a continuous decrease with kXc of −0.24, which then keeps almost constant after εh (kXc = 0). The residual Xc increases from about 37% at 50 °C to 43% at 70 °C. Here the enhanced chains mobility may significantly promote crystal slip and reduce crystal destruction, which may be responsible for the rise of residual Xc compared to that in region I. Different from that in region II, in region III (TαII < T < Tonset) an increase of Xc is observed around εh, although a similar drop of Xc also occurs from εy to εh. This nonmonotonic evolution results in a low Xc valley and indicates the occurrence of recrystallization. Further increasing strain away from εh until 6389

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The evolutions of OC during tensile deformation in different temperature regions are illustrated in Figure 10. In region I,

Figure 8. Evolutions of Xc during tensile deformation in different temperature regions. Figure 10. Evolutions of OC during tensile deformation in different temperature regions.

fracture, Xc keeps relatively constant, and the residual Xc increases with temperature rising up. The increase of Xc or recrystallization is also observed in region IV (T > Tonset) after εh. Nevertheless, different from the plateau occurring in region III, the increase of Xc in region IV proceeds until sample fracture. Figure 9 presents the evolutions of L200 during tensile deformation in different temperature regions. In region I, L200

after εy, OC first increases slightly with kOC of 0.14 at 25 °C to 0.20 at 40 °C and shows steeper uptrend at larger strain with kOC of 0.55−0.35, which reaches about 42% before sample fracture. In region II, the increase of OC is almost linear with kOC of 0.31−0.39 from εy to εh. After εh the increasing tendency of OC slows down (kOC of 0.11−0.19), and OC at the same strain is larger at higher temperature. In regions III and IV, OC shows similar increase trend as that in region II during deformation from εy to εh, while differently a plateau of OC appears in the two high-temperature regions, which increases with temperature. Note that when temperature is higher than 100 °C (above TαII), the orientation of crystals is homogeneous (Figures S7c and S8c). In order to have an overview of mechanical behaviors and structural evolutions in wide temperature−strain space, visualized contour maps of stress−strain curve, Xc, OC, and L200 during uniaxial deformation from 25 to 130 °C are summarized in Figure 11a−d, respectively. The denser contour lines represent larger slope of mechanical and structural transitions. We will not describe the evolution trends of these data in details as they are already analyzed in the above sections. With contour plots, it is easier to define the four temperature regions as well as the three mechanical zones. The bold red and blue lines in Figure 11 separate the mechanical behaviors into linear elastic, softening, or plateau and hardening zones, while three dashed lines define the four temperature regions. To correlate the structural evolutions with mechanical behaviors, we summarize the average slopes (kXc, kL, kOC, and kE) sequentially in different temperature regions and mechanical zones in Figure 11e. Note in regions III and IV the area for recrystallization is specifically highlighted with two light black lines (blue fill area). Figure 11e is actually a table of average slopes of structural and mechanical evolutions per unit strain in different temperature regions and mechanical zones, where all the values are averaged within each temperature region. Correlating the slopes of different structural parameters may help us to unveil the microstructural evolution mechanism during tensile deformation and establish the relationship between structural evolutions and nonlinear mechanical properties, as discussed in the following section.

Figure 9. Evolutions of L200 during tensile deformation in different temperature regions.

shows a weak decrease in linear elastic zone, while after εy a fast drop of L200 occurs, which differs greatly at different temperatures with average slopes kL of about −3.2 nm at 25 °C to −5.6 nm at 40 °C. In region II, L200 follows decrease tendency with kL of −4.6 to −3.7 nm before εh, which drops further after εh until fracture with kL of −4.0 to −3.8 nm. In region III, L200 keeps nearly constant before εy, which decreases with kL of −4.2 to −6.4 nm from εy to εh. Beyond εh, the decrease trend of L200 is reduced and kL of −1.5 nm at 115 °C is observed. In region IV, the initial L200 is bigger and increases with temperature. From εy to εh, L200 decreases nearly linearly with kL of −5.1 to −3.6 nm, which remains nearly constant after εh. 6390

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the discussions, we will consider the temperature effect on structural and mechanical behaviors in each mechanical zone separately. As summarized in Figure 11, in the linear elastic zone little structural change occurs, while structures change largely in strain-softening and -hardening zones. Thus, we will focus our discussions in the latter two zones. 4.1. Structural and Mechanical Evolutions in Plastic Deformation Zone. Yield and strain-softening of semicrystalline polymer materials are generally attributed to two mechanisms, namely crystal slip45−51 and melting−recrystallization.52−56 Considering the relative stabilities of melt and crystal, melting−recrystallization seems to prefer higher temperature,57 while crystal slip may occur at lower temperature.58 However, as shown in Figures 8 and 11, after εy, Xc shows a sharp drop at all temperature regions. In fact, from εy to εh in region I, the average kXc is about −0.22, while within the same strain window kXc is −0.14 and −0.20 in regions III and IV, respectively, indicating lower temperature even leads to a stronger reduction of Xc. These results show that crystal break or fracture always involves portion of lamellar crystal transforming into amorphous in strain-softening zone, which can be named as stress-induced amorphization or melting.59 To quantify this, we propose a simple schematic model in Figure 12a to show the evolutions of Xc and L200, which essentially incorporates crystal slip and melting two mechanisms into one model. Note here crystal slip or plastic deformation occurs through destroying a small volume of crystal ΔV into amorphous rather than simply cutting a crystal through a plane. We assume that the volume of sample, crystallinity, and the volume of crystal at εy are V, X0, and V0, respectively. Thus, V0 can be formulated as follows: V0 = lcL0 2 = X 0V

(2)

Here, lc and L0 are the thickness of crystal and crystal size at εy, respectively. After deformation of Δε, crystallinity becomes Xε, and the crystal breaks into n + 1 pieces with each ΔV of crystal transforming into amorphous regions. L is the thickness of the amorphous region. Hence, the crystallinity can be expressed as follows:

Figure 11. (a, b, c, d) Contour maps of stress−strain curve, Xc, OC, and L200 during uniaxial deformation from 25 to 130 °C, respectively. (e) Diagram of structural evolution slopes of kXc, kL, kOC, and kE during uniaxial deformation from 25 to 130 °C. The bold red, blue, and orange lines indicate εy, εh, and εf, respectively. The green, red, and orange fill areas cover linear elastic, plastic deformation, and strainhardening zones, respectively. The blue fill area indicates the recrystallization zone.

Xε =

4. DISCUSSION On the basis of the above results, we will discuss the structural evolutions as well as their correlations with mechanical behaviors. The nonlinear mechanical behaviors can be roughly defined into three zones, namely linear elastic, strain-softening or plastic deformation, and strain-hardening zones. To simplify

V0 − nΔV X nL = X0 − 0 V L0

(3)

And the average crystal size is Lε, which can be formulated as follows: Lε =

V0 − nΔV L − nL = 0 (n + 1)lcL0 n+1

(4)

Figure 12. (a) Schematic illustration of the break of crystal. (b) Plot of kXc/kL vs temperature during deformation. 6391

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value of kXc/kL can only be attributed to large drop of nucleus size L, as suggested by eq 5. Small L is supported by the enhanced crystal chains diffusion motion in region III, as enhanced crystal chains diffusion motion should correspond to easier pulling out of chains by external stress, which leads to low nucleation barrier and correspondingly smaller L in the crystal−amorphous transition process. Therefore, the physical picture is that initial lamellar crystals are cut by a large number n but fine sized (small L) amorphous domains (as evidenced by the fact that many compact and thin filaments are oriented parallel to each other and that oriented structures are rather uniform in Figure S9c). Region IV (T > Tonset). Instead of mechanical stress, thermal fluctuation is the dominant contributor to induce melting, which increases nucleation density n dramatically and consequently results in the increase of kXc/kL back to 4.4%/ nm. Note the simple model presented in Figure 12a is a “mean field” type of model and does not consider the heterogeneous distribution of nucleation of amorphization, while the distribution of nuclei of melt in lamellar crystal may also affect the value of kXc/kL. Currently we are still not able to construct quantitative correlations between stress-induced nucleation (n and L) and external field (stress and temperature), and how crystal chains mobility influences nucleation in crystal− amorphous transition is only intepreted at the qualitative level. Nevertheless, the global trend of deformation behaviors in yield and stress-softening zones can be understood reasonably enough with the schematic model presented in Figure 12, where the interplay between the chain mobilities of internal strucutres and external fields (mechancial stress and temperature) determines the density, size, and spatial distribution of nucleation in the crystal−amorphous transition process under tensile deformation. The tensile modulus is generaly correlated with crystallinity and orienation positively. In other words, increase in either Xc or OC can lead to modulus increase, and vice versa. Indeed, the evolution rates of apparent tensile modulus (kE) (Figure S10) can be understood by linking slopes of evolutions of crystallinity (kXc) and orientation (kOC) in different temperature regions. In region I, the average kE of −4.3 corresponds to kXc and kOC of −0.22 and 0.17, respectively, while a comparable kXc (−0.24) but nearly doubled kOC (0.32) in region II results in kE of −0.29, which is 1 order smaller than that in region I. This clearly demonstrates that orientation of crystal plays an important role in the tensile modulus. The same mechanism is also observed in high-temperature regions III and IV, where the fast increase of OC prevents tensile modulus from strongly decreasing and eventually leads to stress plateau rather than softening. 4.2. Structural and Mechanical Evolutions in StrainHardening Zone. In the strain-hardening zone (filled with orange) of regions II and III, OC keeps constant and the average kXc are 0, while L200 does show reduction tendency. kL is about −3.9 in the strain-hardening zone of region II, which is smaller than that in the strain-softening zone (−4.3), while kL is nearly the same (−1.0) in the hardening zone of region III. Continuous reduction of L200 but without accompanied decrease of Xc suggests that crystal slip occurs through cutting crystal plane rather than destroying a volume of crystal as that in the strain-softening zone discussed above. In other words, here crystal slip occurs without melting or crystal−amorphous transition. Higher temperature regions have smaller kL, indicating that temperature-promoted chain mobility leads to

With this simple model, we can obtain kXc/kL as follows: kX c/kL =

(n + 1)LX 0 X ≈ 20 (n + 1)L ∝ (n + 1)L (L 0 + L ) L 0 L0

(5)

From eq 5, kXc/kL is proportional to the number and size of amorphous domains transforming from crystals under deformation. Equivalently, kXc/kL is determined by nucleation density of amorphization (n) and nucleus size (L) if we neglect the influence of crystal growth. Figure 12b plots kXc/kL vs deformation temperature, which clearly divides the temperature space into four temperature regions as discussed above. The average kXc/kL for the four temperature regions are 5.0, 5.7, 2.7 and 4.4%/nm, respectively, which can be explained in nucleation density (n) and critical nucleus size (L) by considering the chain mobility as well as external driven forces in different temperature regions. While considering external driven forces, mechanical stress is a nonequilibrium force and inhomogeneous in nature, while temperature or thermal fluctuation activates system homogeneously, which is considered as an equilibrium force. Therefore, deformation-induced structural evolutions are essentially determined by the interplay between mobility of internal structures and external driven forces, which result in the variations of n and L, as reflected by the evolutions of kXc/kL in different temperature regions. Region I (Tβ < T < TαI). The deformation process is mainly driven by inhomogeneous mechancial stress rather than temperature. Indeed, in region I the deformation is inhomogeneous as demonstrated by two groups of crystal orientations during tensile deformation (Figures S5c and S9a). As temperature in region I is far below the melting temperature of PE crystal, here in the stress-induced crystal−amorphous transition process nucleation barrier and nucleus size L are large, which is mainly responsible for the large average kXc/kL of 5.0%/nm. Within region I, increasing temperature leads to a decrease of kXc/kL, which may be attributed to the reduction of L. In other words, with the increase of temperature in region I, the decrease of L is more significant than the increase of n. Region II (TαI < T < TαII).The crystal chains mobility increases dramatically due to thermal expansion of crystalline unit cell at elevated temperature, which enhances the ease of chain slip and deformation of crystal.21,28 Taking the continuous decrease trend in region I as reference, kXc/kL shows an abrupt kickback to high level with average value of 5.7%/nm. The high values of kXc/kL in region II essentially stem from the thermal expansion of the unit cell, which consequently enhances nucleation rate or density (n) in the stress-induced crystal−amorphous transition process sharply. Naturally, the combination of sharp n increase and continuous decrease of L leads to the kickback of kXc/kL as temperature enters in region II. Region III (TαII < T < Tonset). Here the rates of the 180 jumps of the chain stems or chains diffusion motion in the polyethylene crystallites are greatly enhanced.60,61 Moreover, chains may be pulled out from crystalline structure by external stress,62,63 and the average kXc/kL decreases largely to 2.7%/nm compared with the former two regions. In this region mechanical stress still plays an important role, but the effect of thermal fluctuation is greatly enhanced and gradually shifts to be the major contributor to induce melting. As nucleation density n in the crystal−amorphous transition process is expected to increase continuously with temperature, the small 6392

DOI: 10.1021/acs.macromol.7b01153 Macromolecules 2017, 50, 6385−6395

Article

Macromolecules less splitting of crystals. In region IV, kL is only −0.6, which may be partially due to recrystallization though the increase of Xc is rather small in this region. In summary, crystal slip proceeds through cutting crystal plane without melting is the dominant mechanism for structural evolution in the strainhardening zone, though recrystallization may continue in high temperature (region IV) and orientation of crystal goes on weakly in low temperature (region II). The correlation between mechanical and structural evolutions is rather peculiar in strain-hardening zone. In strainhardening zone of region II, Xc keeps unchanged while a weak increase of OC occurs, where the large increase of tensile modulus (kE of 3.1) may still be attributed to the increase of OC (kOC of 0.12). However, in region III, both kXc and kOC are 0 (both crystallinity and orientation remain nearly constant) in the strain-hardening zone, while L200 only shows continuous decrease with kL of −1.0, which may contribute to toughening rather than modulus increasing. Then what is the structural origin for kE of 3.6 in the stain-hardening zone of region III? As it seems that no obvious change from crystal side occurs, we tentatively attribute strain hardening to the nonlinear elastic property of amorphous or rubber chains, which seems to agree with general observation of rubbers under large strain. After crystals break during plastic deformation zone, thermally activation of chains mobility in crystals (above TαII) permits further deformation with the transformation of folded chains into oriented tie-chains connecting crystal blocks.63−66 These oriented tie-chains are beneficial to enhance tensile modulus of samples, which may be responsible for kE of 3.6 in the strainhardening zone even without increment of Xc and OC. Note the nonlinear elastic properties of amorphous chains should also contribute their roles in strain-hardening zones of regions II and IV, although orientation or recrystallization still continues there. 4.3. Structural and Mechanical Evolutions in Recrystallization Zone. The high-temperature regions III and IV have a special recrystallization zone (filled with blue), which we would like to discuss separately. Recrystallization leads to increase of Xc with positive average kXc of 0.1 and 0.05 in regions III and IV, respectively. Note recrystallization in this zone slows down the decrease of L200, as the lower kL of −1.6 appears in this zone of region III compared with the values in strain-softening zone (kL of −5.3). Additionally, OC shows continuous increase with kOC of about 0.12, which is smaller than that in the strain-softening zone (kOC of 0.37) but larger than 0 in the strain-hardening zone. Correlated with mechanical behaviors, recrystallization may be the major player to initiate strain-hardening here.

strain space. We propose a model of crystal slip via melting to account for plastic deformation behaviors at yield and strainsoftening zone for all temperature regions. The different deformation behaviors in different temperature regions are ascribed to stress-induced nucleation density n and nucleus size L, which are essentially determined by the interplay between the mobilities of internal structures and external fields (stress and temperature). In the strain-hardening zone, crystal slip without melting occurs via splitting crystal planes, where the formation of highly oriented fibrillar crystals and the stretch of amorphous chains are responsible for strain-hardening in hightemperature regions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01153. Amount of residual paraffinic oil after extraction, SEM of extracted UHMWPE precursor fibers, schematic illustration of homemade two-drum extensional rheometer, the effect of annealing by in situ WAXS experiments, Scherrer equation for calculation of crystal size (L200), the shrinkage of unconstrained fibers at elevated temperatures, Gaussian approximation for calculation of crystal size (L200), integrated one-dimensional curves of 2D WAXS patterns and Gaussian fit results, morphologies of deformed samples after stress release by SEM, definition of the average slope of dσ/dε evolution (kE) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (Y.J.). *E-mail [email protected] (L.L.). ORCID

Liangbin Li: 0000-0002-1887-9856 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Prof. Peng Chen’s group (Ningbo Institute of Industrial Technology, Chinese Academy of Sciences) for supplying UHMWPE gel spun fibers. This work is supported by the National Natural Science Foundation of China (51325301, 51633009). The experiment is partially carried out in National Synchrotron Radiation Lab (NSRL) and Shanghai Synchrotron Radiation Lab (SSRL).

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5. CONCLUSION The temperature effect on deformation behaviors of extracted UHMWPE precursor fibers is studied with in situ synchrotron radiation WAXS during uniaxial tensile stretch at temperatures from 25 to 130 °C. The structural and mechanical evolution behaviors during tensile deformation can be divided into four temperature regions with boundaries located at temperatures of αI and αII relaxations and the onset of melting, respectively. Based on visualized contour maps of stress−strain curve, Xc, OC, and L200 during uniaxial deformation, a morphological diagram composed of average slopes of structural and mechanical evolutions per unit strain in different temperature regions and mechanical zones is constructed in temperature−

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