Phase Transition from Tetragonal Form II to Hexagonal Form I of

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Phase Transition from Tetragonal Form II to Hexagonal Form I of Butene-1/4-Methyl-1-pentene Random Copolymers: Molecular Factor versus Stretching Stimuli Lirong Zheng,† Long Liu,† Chunguang Shao,‡ Wei Wang,† Bin Wang,† Li Pan,† Yuesheng Li,†,§ and Zhe Ma*,†

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†

Tianjin Key Laboratory of Composite and Functional Materials, and School of Materials Science and Engineering, Tianjin University, Tianjin 300072, P. R. China ‡ National Engineering Research Center for Advanced Polymer Processing Technology, Zhengzhou University, Zhengzhou 450001, P. R. China § Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P. R. China S Supporting Information *

ABSTRACT: The phase transition from the kinetically favored tetragonal form II into the thermodynamically stable hexagonal form I is the general phenomenon and core issue in application of polybutene-1-based materials. It is known that the variation of molecular structure by copolymerizing counits and the imposition of external stretching both greatly affect the phase transition. In this work, a series of butene-1/4-methyl-1-pentene (4M1P) random copolymers were synthesized with the dimethylpyridylamidohafnium/organoboron catalyst, where the 4M1P incorporated is the counit type of depressing II−I phase transition. Mechanical tests were combined with the in-situ wide-angle X-ray diffraction (WAXD) method to study the competing effects of the presence of 4M1P counits and stretching on the II−I phase transition. First of all, the quiescent experiments reveal that addition of 4M1P counits not only slows down transition kinetics but also decreases the ultimate form I fraction in the transition plateau. The 4M1P concentration ≥3.40 mol % is high enough to completely impede the II−I phase transition even when the aging time is as long as 4 months. Second, the stretching-induced phase transition was explored with the combined structural and mechanical information from WAXD and mechanical characterizations, respectively. The influence of stretching stimuli in the phase transition varies with 4M1P concentration. For low 4M1P concentration ≤1.00 mol %, stretching significantly accelerates the transition kinetics and induces the complete transition of form II. For intermediate 4M1P concentration 3.40 mol %, stretching effectively triggers the occurrence of the II−I phase transition, which does not start under quiescent conditions but only induces partial transition until fracture. For high 4M1P concentration ranging from 7.80 to 30.1 mol %, stretching just orientates the form II crystallites without starting any phase transition to form I. Third, as the concentration of 4M1P counits is increased, the phase transition is accomplished with different orientations, which determines the microscopic stress applied to lamellae. Then, detailed kinetics of the II−I phase transition was correlated to the stretching stimuli of the total true stress, component stresses parallel and perpendicular to the c-axis in the crystal lattice. It was interesting to find that transition kinetics is dominated by the component stress perpendicular to the c-axis for the off-axis orientation pathway. For the molecular mechanism of the phase transition, this indicates that the activated chain lateral slip is the dominant process for nucleation of form I within original form II.

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INTRODUCTION Among diverse synthetic polymeric materials, more than twothirds are semicrystalline polymers. For these crystallizable substances, the ultimate mechanical properties are strongly dependent on their generated crystallite structures,1−7 of which the multiscale feature covers monomer composition, segmental conformation, stem packing within lattice, aggregation of unit cell, and so on. A polymer is usually able to orderly pack into various types of phase modifications; polymorphism becomes one crucial issue in polymer crystallization. Moreover, the formed crystallites often transform further into alternative © XXXX American Chemical Society

modifications to obtain the thermodynamic stability or adopt to the macroscopic deformation imposed. Thus, stretchinginduced phase transition becomes a rather general phenomenon in polymorphic polymers, for example, the orthorhombic to monoclinic phase transition in polyethylene,8 γ- to α-phase transition in isotactic polypropylene (iPP),9 and α′- to γ″phase transition in polyamide 12,10 to name a few. In addition Received: December 6, 2018 Revised: January 12, 2019

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DOI: 10.1021/acs.macromol.8b02600 Macromolecules XXXX, XXX, XXX−XXX

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phase transition depends on the concentration of butene counits, where α- and β-forms transform into the mesophase for butene concentration lower than 10−15 mol % and the αform transforms into the trigonal form for butene concentration around 50%.47 Concerning specific polybutene-1 copolymers, of great importance is the pioneering work done by Jones in the 1960s.48 He copolymerized butene-1 with different linear and branched α-olefin comonomers by the Ziegler−Natta catalyst and found that comonomer types of ethylene, propylene, and pentene accelerate the II−I phase transition, whereas other long linear α-olefins with more than five carbon atoms or branched α-olefins (like 3-methly-1butene, 4-methly-1-pentene, and 4,4-dimethly-1-pentene) tend to stabilize form II crystals, suppressing the phase transition. Clearly, modification of molecular architecture has both accelerating and retarding effects on the phase transition, different from the sole enhancement of external stretching. Then, here comes the question of whether the stretching can still induce the II−I phase transition in butene-1 copolymers incorporated with aforementioned counit types of retarding phase transition. In this work, a series of butene-1/4M1P random copolymers with different counit contents were synthesized using the postmetallocene hafnium catalyst, which allows a control of high regularity for the polybutene-1 main chain. It is known that 4M1P is the comonomer that suppresses the II−I phase transition.48 These butene-1/4M1P copolymers were chosen as the model system to diminish the intrinsic tendency to transform into form I. In this case, the quantitative relationship between stretching-induced transition kinetics and counit concentration was investigated to reveal the completion between molecular factor and external stimuli for the phase transition.

to being an intrinsic behavior, phase transition can in turn endow materials with unique mechanical properties. In syndiotactic polypropylene, the martensitic-like phase transition under deformation significantly improves the elasticity and the impact resistance.11,12 The contents of mesophase transformed from crystals in iPP,13,14 poly(ethylene naphthalate),15 and poly(ethylene terephthalate)16 determine the different degrees of toughness. Therefore, not only the initial crystallites prepared need to be fully characterized, but also detailed evolution of phase transition occurring during stretching should be uncovered to comprehensively understand the origin of material performance; especially the latter has not received enough attention as the former does. Polybutene-1 (PB-1), as the typical polymorphic polyolefin, is of great industrial importance due to its excellent mechanical properties,17−22 which is suitable for a broad range of applications like pressurized tanks, pumps, and hot water pipes. PB-1 has various crystal modifications23−25 of forms I/I′, II, and III. Form I consists of 3/1 helical conformations packing in the hexagonal modification26,27 and is the stable phase from a thermodynamic point of view. There are two variants of such hexagonal phase, termed forms I and I′, which are generated by transition of form II crystallites19,28 and by the direct crystallization of amorphous melt,23,29,30 respectively. Form II is the kinetically favored phase, which has the tetragonal modification with the 11/3 helix,27,31−33 and form III is an orthorhombic crystal possessing 4/1 helical conformation.17,34 Under deformation, stretching can induce a significantly accelerated phase transition from the metastable form II into form I.29,35−39 The II−I transition markedly improves the material’s impact and creep resistance, thermal stability, and hardness.17−20 Thus, the polybutene-1 structure− property relationship is attracting increasing attention from both the academy and industry. Miyoshi et al.40 utilized nuclear magnetic resonance (NMR) to study molecular dynamics of polybutene-1 crystalline stems within different modifications. It was found that with respect to form II, form I has rigid molecular stems up to the melting temperature, which endows transformed form I with higher strength. Liu et al.35 combined the mechanical test with synchrotron radiation X-ray diffraction to correlate the process of phase transition with the mechanical response. It was found that phase transition undergoes the solid−solid or melt−recrystallization routes depending on temperature. Considering the multiscale feature of crystallite structure, our recent work39 focuses on the orientation for the phase transition. The results show that the II−I phase transition can happen in the off-axis oriented or highly oriented crystallites, which further affects the transition kinetics. Moreover, Cavallo et al.36 worked on the transition kinetics and found that for various mechanical protocols of constant strain rate, constant stress, and constant stress rate the phase transition kinetics has an unexpected stress dependence for the transition range with a upper limit of 40−50% form I in the total crystallites. Even with these efforts, the mechanism of stretching-induced phase transition is still far from the complete understanding, for instance, why there exists the upper limit for the availability of the stress-dominant mechanism remains mysterious so far. On the other hand, it is worth emphasizing that modification of molecular architecture by copolymerizing counits into main chain is an effectively way to regulate the crystallization polymorphism and the related phase transition.24,41−46 In propylene−butene random copolymers, stretching-induced

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EXPERIMENTAL SECTION

Materials. To incorporate the 4-methyl-1-pentene (4M1P) counits into the polybutene-1 main chain, we used dimethylpyridylamidohafnium catalyst49 and [Ph3C][B(C6F5)4] and triisobutylaluminum Al(iBu)3 cocatalysts to synthesize a series of butene-1/ 4M1P random copolymers. The synthesis method is illustrated in Scheme 1.

Scheme 1. Butene-1/4M1P Copolymerization Using the Hafnium/Organoboron Catalyst

The synthesized butene-1/4M1P random copolymers are characterized by the 13C NMR spectrum to determine the concentration of 4M1P counits. The detailed molecular characterizations and physical properties of materials are summarized in Table 1. In this work, polybutene-1 homopolymer and butene-1/4M1P copolymers are denoted as “PB-1” and “4M1P0.18”, “4M1P1.00”, “4M1P3.40”, “4M1P7.80”, “4M1P12.4”, and “4M1P30.1” according to their 4M1P concentrations. Note that all polymers synthesized have the crystallization capability. The absence of 4M1P30.1 crystallization and melting temperatures in Table 1 is just due to the reason that the current rate of 5 °C/min employed is still too fast to start its crystallization during cooling. Our DSC isothermal experiments prove B


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Tensile experiments were performed at 30 °C using a Linkam TST350 tensile tester. In all tensile tests, the stretching speed was set as 10 μm/s. The time-resolved wide-angle X-ray diffraction (WAXD) method was employed to track the phase transition during stretching. The WAXD measurements were performed with the Bruker D8 Discovery X-ray setup. The wavelength of X-rays used was 1.54 Å, and the sample-to-detector distance was 56 mm. Two-dimensional (2D) images were recorded by a Mikrogap image plate detector with a resolution of 2048 × 2048 pixel of 68 μm × 68 μm. The WAXD acquisition time during tensile testing was 34.5 s per frame. During phase transition, the fraction of transformed form I in the total crystallites can be determined from the 1D WAXD curves using the equation

Table 1. Molecular Characterization and Physical Properties of Polybutene-1 Homopolymer and Butene-1/4M1P Random Copolymers

a

sample name

4M1P concna (mol %)

Mw (kg/mol)

Mw/Mnb

PB-1 4M1P0.18 4M1P1.00 4M1P3.40 4M1P7.80 4M1P12.4 4M1P30.1

0 0.18 1.00 3.40 7.80 12.4 30.1

112 324 272 218 262 386 371

2.9 2.8 2.9 2.9 2.9 2.8 2.9

b

Xc (%)

Tc (form II)d (°C)

Tm (form II)e (°C)

56.2 54.2 51.2 47.3 40.0 36.2 30.2

76.4 73.4 62.3 57.4 35.0 25.5 f

110.0 108.9 103.2 92.6 81.8 73.8 f

c

13

fI =

b

The 4M1P concentration was measured by C NMR. The molecular weight and distribution were measured by GPC. cThe crystallinity was determined from WAXD results, of which sample preparation protocol was given in the following section. dThe crystallization temperature was determined from the DSC cooling curves with the cooling rate of 5 °C/min. eThe melting temperature was determined from the DSC heating curves with heating rate of 5 °C/min. fThe 4M1P30.1 crystallization is too slow to start during the cooling at current rate of 5 °C/min, so there is no available Tc and Tm values measured for this copolymer. Note that actually 4M1P30.1 has the crystallization capability, which can be completed by the isothermal crystallization.

A(110)I A(110)I + R × A(200)II

where A(110) I and A(200) II are the integration areas of corresponding diffraction peaks. The correction parameter R is 0.36.50 Although alternative correction parameter of 0.67 has been reported,51,52 0.36 was used in present work to make the comparison with previous work using the identical value.36 The choice of parameter value does not influence the conclusion discussed in the present work.

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RESULTS AND DISCUSSION Retarding Effect of 4M1P Counits on Quiescent Phase Transition. The original pure form II samples should be prepared to carry out II−I phase transition experiments. It has been reported that the introduction of extra counits like ethylene, propylene, and methylene-1,3-cyclopentane to the polybutene-1 main chain may alter the crystallization polymorphism, where amorphous melt directly crystallizes into the thermodynamically stable hexagonal form I′.29,30,41 Thus, modifications of crystallites isothermally prepared in butene1/4M1P copolymers were examined by WAXD. The 1D results of Figure 1a show that the distinct diffraction peaks observed correspond to (200)II, (220)II, and (213)II crystallographic planes, which demonstrates that only tetragonal form II is generated without any appearance of hexagonal form I′. Moreover, it is interesting to distinguish that diffraction peaks of form II (200)II, (220)II, and (213)II crystallographic planes all shift to low 2θ direction with the increase of 4M1P concentration. According to the Bragg’s law, the decrease of the diffraction angle 2θ implies the increase of interplanar spacing. The shift of the (200) diffraction angle indicates the expansion of the a-axis. PB-1 form II has the tetragonal modification with identical a and b lengths of 14.85 Å for each unit cell.27,48 There are four chains in each unit cell of form II (NII = 4), so the cross-section dimension of individual chain (Achain) can be estimated from Achain = AII/NII with AII being the area of the a−b plane in the form II unit cell, which is around 55 Å2 per chain for the PB-1 11/3 helix in the form II lattice. In the same way, the cross-section area occupied by poly(4M1P) 7/2 helix is also calculated from the regular tetragonal unit cell with a = b = 18.66 Å to represent the size of 4M1P counits.53 The estimated cross section is 86 Å2 per chain, which is larger than that of the PB-1 11/3 helix. The above spacing expansion of crystallographic planes in butene1/4M1P copolymers may be attributed to incorporation of the large 4M1P counits into the PB-1 crystal lattice. Then, for the 4M1P copolymers with varying counit concentrations, the unit cell parameters of form II crystals generated are estimated with the tetragonal system equation:54

that 4M1P30.1 crystallization can actually happen, which requires a relatively long crystallization time. For the tensile test, polymers were compression-molded into plates with a thickness of 0.5 mm and cut into dumbbell-shaped samples. Before stretching, these dumbbell-shaped samples were treated thermally to prepare form II in initial samples. The thermal protocol is illustrated by Scheme 2 and was implemented using a Linkam

Scheme 2. Thermal Protocol of Form II Preparation

LTS420 hot stage. Samples were annealed at 180 °C for 5 min to erase thermal and mechanical history and then cooled to 30 °C at a cooling rate of 5 °C/min for crystallization. For homopolymer PB-1 and copolymers with low counit concentrations ≤3.4 mol %, i.e., 4M1P0.18, 4M1P1.00, and 4M1P3.40, crystallization of form II was accomplished during cooling. For the butene-1/4M1P copolymers ≥7.8 mol % counits, including 4M1P7.80, 4M1P12.4, and 4M1P30.1, isothermal crystallization was performed at 30 °C for 30, 60, and 600 min, respectively, to complete form II crystallization. Methods. The quiescent phase transition experiments were performed at room temperature. To assess kinetics of quiescent transition, ex-situ X-ray diffraction (XRD) measurements were performed on samples after aging for different durations. The XRD measurements were performed with a Rigaku Ultima IV diffractometer equipped with a Cu Kα X-ray source. The wavelength of Xray used was 1.54 Å, and the measurement time of each frame was 4 min. C


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Figure 2. Evolution of form I fraction during the quiescent aging at room temperature.

h and then develops slowly and ultimately reaches a plateau. However, as 4M1P concentration is raised to 1.00 mol %, transition kinetics of 4M1P1.00 is slowed down significantly, and form I fraction develops to only 27% even after aging for 700 h. What is more unexpected is that when 4M1P concentration reaches 3.4 mol % and higher, the II−I phase transition is completely inhibited for a long aging period up to 3000 h, as observed in copolymers 4M1P3.40−30.1. These results present that a threshold concentration of 3.4 mol % 4M1P exists to completely suspend the copolymer II−I phase transition for the specific 3000 h quiescent aging. This retarding effect of 4M1P counits on the II−I phase transition may be interpreted considering their behaviors in both crystalline and amorphous regions. To accomplish the II−I phase transition, macromolecular segments within crystal need to adjust their conformations from the 11/3 to 3/1 helix, and these stems should also change positional ordering from tetragonal to hexagonal modifications. This means that the II− I phase transition requires segmental movement for both conformational and positional orderings. The branched 4M1P counit is larger than the butene-1 repeating unit, and the former can pack into the crystal lattice of the latter. Considering the considerable rotational mobility in both PB13,40 and poly(4-methyl-1-pentene)55−57 for conformational change, the extra methyl group of 4M1P counits may reduce the mobility of their connected crystalline stems to the change packing position, suspending the II−I phase transition. At the same time, conformational and positional changes also need the connected segments that belong to the amorphous region to move cooperatively to adapt to these adjustments within the crystal. However, with respect to the butene-1 unit, the large 4M1P counits are also hard to move in the amorphous region. The glass transition temperatures (Tg) were measured for PB-1 and 4M1P copolymers. It was found that Tg is elevated with the increase of 4M1P content (see Figure S1 in the Supporting Information). In this case, the portion of 4M1P counits that are included in the amorphous domain reduce the mobility of amorphous segments, slowing down transition kinetics. The extreme is that if some 4M1P counits of amorphous segments are by chance located on the surfaces of crystalline lamellae, these large 4M1P units act as stoppers in the amorphous region to impede elongation of stems. The crystal distortion and mobility of amorphous segments both decrease with the increase of 4M1P concentration. Thus, 4M1P counits significantly slow down and even impede the II−I phase transition. In addition to the reduced mobility, the steric effect of 4M1P counits also contributes to the depression in the phase

Figure 1. (a) 1D WAXD curves of homopolymer PB-1 and 4M1P copolymers with different counit concentrations. (b) Unit cell parameters for PB-1 and 4M1P copolymers, where red and black symbols are practical values determined from experimental results of (a). For the measured practical a and b parameters, their theoretical reference c values for unchanged volume were calculated and are shown by gray symbols.

1 dhkl

2

=

h2 + k 2 l2 + 2 2 a c

where h, k, l are the Miller indices and a, b are lattice parameters. Figure 1b displays that with increasing 4M1P concentration both a and b increase in the copolymers, whereas c decreases. To know how crystal volume varies with counits, the reference c values are estimated assuming that 4M1P copolymers have the unchanged volume of unit cell compared with homopolymer (see gray symbols in Figure 1b). Obviously, the practical copolymer c parameters are much larger than their corresponding reference values for constant volume. In other words, the total volume of copolymer unit cell was actually expanded by incorporating 4M1P counits into the crystal. Moreover, it is worth noting that with increasing 4M1P concentration to 30.1 mol %, the total change in the caxis is 0.4 Å, which is much smaller than 1 Å of the a- and baxes. The WAXD results also imply that incorporation of 4M1P counits is mainly to expand the lateral dimensions, i.e., parameters a and b, perpendicular to the c-axis direction. The quiescent transition was studied for homopolymer and butene-1/4M1P copolymers to disclose the effect of 4M1P counits on the phase transition. Figure 2 displays the fractions of transformed form I after aging for different durations at room temperature. The 4M1P counit has a substantial retarding effect on the II−I phase transition. For the copolymer with no or only a few 4M1P counits, i.e., PB-1 and 4M1P0.18, the phase transition is a two-stage process. The fraction of transformed form I grows rapidly within the first 70 D


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Macromolecules transition. To be able to pack more densely into the transformed form I, segments decrease their lateral distance, and the corresponding helix change needs stems, i.e., lamellar thickness, to be elongated by 10% globally. However, Figure 1b presents that incorporation of 4M1P counits into polybutene-1 main chain expands the lateral dimension but reduces the length of the molecular chain. These two effects resulting from the existence of 4M1P counits are in contrast to the change of the phase transition and consequently impede the aforementioned structural adjustment necessary for the phase transition. It deserves noting that II−I phase transitions of PB-1, 4M1P0.18, and 4M1P1.00, although slowed down, are still possible to start by the intrinsic thermal stress due to the difference in thermal expansion coefficients between amorphous and crystallite regions. However, the triggered phase transition does not achieve the complete transition at the end. The fractions of form I in the transition plateau are 0.98, 0.94, and 0.89 in PB-1, 4M1P0.18, and 4M1P1.00, respectively. This incomplete transition with survived form II crystals is related to the low nucleation rate and scarce growth rate at a late stage.52 Qiao et al.52 recently proposed a reasonable mechanism for the incomplete phase transition. In that model, the phase transition that accomplished first causes the additional pressure in the normal direction and the tendency to expand in the lateral direction. These two effects suppress the elongation of the helix and the lateral shrinkage of the lattice, respectively, which are the two requisites to continue the II−I phase transition for residual form II. Similarly, incorporation of some 4M1P counits to the lattice exactly expands the lateral dimension and decreases the dimension of the unit cell along the chain direction for the butene-1/4M1P copolymers studied in this work, as shown in Figure 1b. Thus, conformational change and positional arrangement become more difficult as the increase of 4M1P counits, leading to the retarded phase transition. Stretching-Induced Phase Transition in 4M1P Copolymers. It has been well recognized that mechanical stretching can accelerate the process of II−I phase transition.29,35−38 This is contrast to the above retarding effect from 4M1P counits. Then, concerning copolymers containing such counits, the question arises about how mechanical stimulus affects the phase transition behavior of form II crystallites in the presence of molecular architecture retarding phase transition. To explore the mutual effects of 4M1P structural counit and mechanical deformation, stretchinginduced phase transitions were tracked combining in-situ WAXD characterizations with mechanical tests. Stretching Accelerates the II−I Phase Transition. Figure 3 depicts the stress−strain curve of copolymer 4M1P0.18 as the representative of low counit concentration copolymers. The corresponding WAXD patterns obtained during stretching are given together with the mechanical response. Before stretching, the presence of distinct (200)II, (220)II, and (213)II diffraction peaks confirms the preparation of pure form II crystal in the initial sample to be stretched. The homogeneous distribution of intensity along the azimuthal angle (see Figure 3A) further demonstrates that original form II crystallites are isotropic. As stretching was applied, no new characteristic diffraction signal of form I was observed in the elastic region (see Figure 3B). However, Figure 3C indeed presents that the II−I phase transition is triggered just beyond mechanical yielding. Clearly, mechanical yielding is required for phase transition, consistent with previous reports.58,59 For phase transition, segments

Figure 3. Engineering stress−engineering strain curve of copolymer 4M1P0.18 and the selected in-situ WAXD patterns obtained during stretching. The stretching direction is vertical.

belonging to form II crystallites adjust their conformation from 11/3 into 3/1 helix and change packing positions along the (110) planes.60 Form I has higher hardness and strength;17,21 the generated form I crystallites may act as physical cross-links, building up a new network, and significantly improve the tensile strength.61−63 Thus, the phase transition triggered causes the sample to enter the strain hardening region quickly. In other words, the onset of strain hardening after yielding also indicates the occurrence of the II−I phase transition, as shown by Figure 3. At the late stage of phase transition, what is interesting is that the sole diffraction signal of (110)I in Figure 3E shows that form II crystallites all transform into the thermodynamically stable modification of form I. This 100% transition is different from only partial conversion under quiescent condition, which has a plateau transition degree of f I = 94% for 4M1P0.18 (shown in Figure 2). As discussed above, the partial conversion is thought as the consequences of the additional external field for the lamellae, including a pressure in the normal direction and the expansion along the lateral direction.64 In this sense, macroscopic deformation applied can severely stretch the amorphous region located in between lamellae regions to eliminate appearance of this intrinsic local external fields for crystallite lamellae. In the course of phase transition, residual form II and transformed form I both rotate, and their c-axes tend to align along certain preferential directions. The ideal case is that crystallite c-axes are all parallel with stretching direction. However, II−I phase transition have already started before their original form II crystallites are well rotated to such high degree of orientation. Even when phase transition proceeds to f I = 77%, as indicated by Figure 3D (the estimation of form I fraction given in Figure S2), the red arrows point out that azimuthal distributions of (200)II and (110)I are both off the equatorial polarization, which presents some kind of nonperfect orientation. This will be referred as off-axis orientation in the following text of this paper to emphasize the deviation from the ideally high orientation with the c-axis oriented along stretching direction. Such off-axis orientation holds in forms II and I until completion of phase transition. This means that E


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is worth noting that the II−I transition finishes only partly until the sample breaks. Nonoccurrence of Phase Transition in High Counit Concentration Copolymers 4M1P7.80−30.1. The effect of mechanical stimuli stretching on triggering the II−I phase transition is not always available for high 4M1P concentration copolymers. As the 4M1P concentration is increased to 7.8 mol %, WAXD patterns obtained during stretching only exhibit the diffraction characters for form II crystallographic plane (200), and the structural change is mainly about the evolution of orientation (see Figure 5). The II−I phase transition is also

form II crystallites do rotate during stretching but cannot reach the ideally high orientation before they start transforming into form I. In other words, phase transition is completed with the off-axis oriented crystallites. After completion of the II−I phase transition, the transformed form I crystallites further rotate to allow c-axes completely parallel with stretching direction; see the equatorial diffraction signal of Figure 3F. This type of orientation is called as the highly oriented crystallites. The stretching-enhanced transition becomes more pronounced as increasing 4M1P concentration to 1 mol %. In 4M1P1.00, only 27% of form II crystallites transform into form I, even after aging for 1 month (see Figure 2). Surprisingly, stretching induces the 100% transformation, which needs only the stretching duration lasting less than 1 h (data given in Figure S3). With the accelerated transition, 4M1P1.00 behaves as a strong material just because of the increased amount of form I crystallites. Because of the reduced transition kinetics with respect to PB-1, 4M1P1.00 needs more deformation to finish the II−I phase transition and consequently exhibits the improved ductility. Stretching Triggers II−I Phase Transition in 4M1P3.40. In addition to accelerating kinetics of phase transition, stretching can also trigger the phase transition that was stopped under quiescent conditions. The results of Figure 2 already show that the quiescent II−I phase transition does not start for the aging time as long as 4 months for 4M1P copolymers with 3.4 mol % and more counits. Then, stretching was applied to those copolymers of 4M1P3.4−30.1. In Figure 4, it is interesting to

Figure 5. Engineering stress−engineering strain curve of copolymer 4M1P7.80 and selective in-situ WAXD patterns obtained during stretching. The stretching direction is vertical. Note that due to the limitation in maximum movement of clamps, the highest engineering strain applied is 500%, smaller than the breaking strain of copolymer 4M1P7.80.

prevented copolymers 4M1P12.4 and 4M1P30.1, though identical stretching was also applied. For the high 4M1P concentration copolymers, it is the molecular structure of branched 4M1P counits that dominates the II−I phase transition, which completely suppresses transition even with stretching (data given in Figures S4 and S5). When there is no phase transition, external stretching orientates crystallites continuously to the high degree of orientation with the c-axis parallel with the stretching direction. Now, it is suggested that molecular structure of 4M1P counits and mechanical stretching together determine the II−I phase transition, where the former suppresses but the latter favors the II−I phase transition. Figure 6 summarizes the mutual effects of 4M1P counits and stretching on kinetics of the II−I phase transition for the homopolymer and copolymers covering a broad 4M1P concentration range from 0.18 to 30.1 mol %. The influence of stretching in the phase transition depends on the 4M1P concentration. This counit dependence of stretching-induced phase transition is also different from that under quiescent conditions. For low 4M1P concentration ≤1.00 mol %, stretching significantly accelerates transition kinetics and induces the complete transition of original form II, where stretching dominates the phase transition. For intermediate 4M1P concentration = 3.40 mol %, stretching

Figure 4. Engineering stress−engineering strain curve of copolymer 4M1P3.40 and the selected in-situ WAXD patterns obtained during stretching. The stretching direction is vertical.

observe that the stretched copolymer 4M1P3.40 experiences a distinct II−I phase transition. The appearance of equatorial (110)I diffraction not only demonstrates the transformation into form I but also suggests that these transformed form I are highly oriented along the stretching direction. The molecular structures of 3.4 mol % 4M1P counits completely impede phase transition under quiescent conditions. However, stretching is also able to trigger the II−I phase transition. It F


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completely impedes phase transition, at least from a kinetic point of view. In Figure 6, copolymers 4M1P7.80−30.1 actually do not break at 500%. The presence of this maximum strain in the plot is just due to the clamp movement limitation in the special tensile tester equipped with the WAXD method. Then, someone may ask whether the II−I phase transition happens in the subsequent part of stretching. We also employed the standard mechanical tester and obtained the complete stress− strain curves until the sample breaks. The mechanical results show that copolymers 4M1P7.80−30.1 could be stretched to a much larger strain over 1000% (data given in Figure S6). Moreover, ex-situ WAXD examinations were performed on the broken samples, and the results showed that no phase transition was triggered in copolymers 4M1P7.80−30.1 (data given in Figure S7). Thus, the kinetic diagram shown in Figure 6 is applicable to the whole stretching process until sample fracture. Orientation for II−I Phase Transition. Simultaneously with the phase transition process that molecular segments adjust their helix conformation and positional ordering within the crystal lattice, crystallites as a whole are rotated by the external stimuli. If only orientation is considered, it is easy to

Figure 6. Kinetic diagram of II−I phase transitions in PB-1 and 4M1P copolymers.

effectively triggers the occurrence of the II−I phase transition that does not start under quiescent conditions, whereas still part of form II crystals survive after breaking. In this case, stretching and molecular factor both play important roles in the phase transition. For high 4M1P concentration range between 7.80 and 30.1 mol %, stretching just induces the orientation of form II without triggering any phase transition. When 4M1P concentration is high, it is 4M1P counit that

Figure 7. Azimuthal scans of (200)II and (110)I diffractions and the corresponding peak azimuth in copolymers (a−c) 4M1P0.18, (d−f) 4M1P1.00, and (g−i) 4M1P3.40 during stretching. The vertical dashed lines indicates the onset and finish boundaries of phase transition. 4M1P3.40 cannot completely transform into form I, so there is no finish boundary in (i). G


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crystallites. This is indeed observed in the stretching-induced phase transition of copolymer 4M1P3.40. As shown by Figure 7i, form II crystallites have already aligned their lamellar normals along the stretching direction in the very early stage transition period with f I below 20%. One reason is that the continuous increase of 4M1P counits to 3.40 mol % further retards the kinetics of phase transition in 4M1P3.40. The other is the significantly accelerated development of high orientation, which just needs 22% strain, as shown in Figure 7i. Of course, the extreme case is that when there is no phase transition happening at all, form II well develops into highly oriented crystallites until the sample breaks. This is exactly what is observed in copolymers 4M1P7.80−30.1. Our recent work on the butene-1/ethylene copolymer containing 1.5 mol % counits has revealed that above sequence between orientation evolution and phase transition varies with stretching speed and temperature. 39 Original form II crystallites always rotate to an off-axis orientation state, where the normal of lamellae has the preferential direction but the orientation direction does not reach the ideally parallel state with stretching direction. When stretching conditions are mild, such as low temperature of 30 °C or 5−10 μm/s at 50 °C, the II−I phase transition is accomplished within the offaxis oriented crystallites. The orientation pathway of the phase transition was varied by either increasing the stretching speed to 50 from 10 μm/s at 50 °C or elevating the temperature to 70 °C. In present work, stretching speed and temperature are both fixed. However, the transition kinetics in the copolymer was significantly suppressed by introducing the branched 4M1P counits into the polybutene-1 main chain. Thus, before triggering and development of II−I phase transition, original form II crystallites have sufficient time to rotate and even align their normal parallel with the stretching direction. The influence of orientation pathway on phase transition will be discussed in the next section. Kinetics of II−I Phase Transition. Once triggered, the II−I phase transition proceeds with stretching, so this section focuses on the kinetics of transition. To quantify the strength of deformation, instantaneous change of sample dimensions during stretching was recorded by a camera. The true strain εH can be determined from

infer that chain segments are severely stretched by the external deformation, and ultimately all c-axes of crystallites are oriented along the stretching direction. As soon as the phase transition is able to happen, the orientation evolution becomes more complex. The phase transition and orientation evolution are coupled with each other, although they represent structural ordering at various length scales. After yielding, coarse chain slip leads to the lamellar fragmentation, and the original lamellar aggregation cannot hold the crystallite skeleton as a whole. The amorphous region located between the neighboring lamellae is severely stretched. The microscopic stretching transmitting into the lamellar scale has two distinct effects. One is to rotate the fragmented crystallites, which are relatively small and are easier to move with respect to the original lamellae. The other is to stretch the crystal stems via their connected loops, entanglements, or interlamellar links located in amorphous region. Then, there will be three possibilities for occurrence sequences between orientation evolution and phase transition: (1) orientation first develops well to align all c-axes parallel with stretching direction before start of phase transition, (2) orientation develops after the completion of phase transition, and (3) orientation develops simultaneously with phase transition. The relative sequence between orientation and phase transition gives insight into the orientation pathway for phase transition. Figure 7 shows that all aforementioned three situations were observed in 4M1P copolymers, depending on the counit concentration. In PB-1 homopolymer and copolymers with no more than 1.00 mol % 4M1P counits, original form II crystallites are indeed rotated by stretching. However, if the oriented lamellae align their normal directions along stretching direction, the azimuthal (200)II diffraction signal should be located in the equatorial polarization direction, which has the azimuthal angle of 0 or 180° in 1D curves (as shown by Figure 7g). Differently, in Figures 7a and 7d, the azimuth peak of form II (200) diffraction is off the equatorial direction for polymers with 4M1P concentration no more than 1.00 mol % (results of homopolymer PB-1 given in the Supporting Information). This implies that although oriented, the normal direction of lamellae is not parallel with stretching direction but has some kind of misalignment. The detailed evolutions of peak azimuth, indicating variation of orientation degree during stretching, were determined and plotted together with course of phase transition (see Figures 7c,f,i). In 4M1P0.18, the off-axis orientation remains quite stable within a narrow azimuth range between around 146° and 153°, similar to the behavior of PB-1 (data given in Figure S8). Clearly, 4M1P0.18 crystallites rotate into the off-axis state for phase transition but do not achieve the high orientation with the c-axis parallel with the stretching direction. As the counit concentration was increased to 1.00 mol %, the orientation evolution is stable in the initial period of transition but rises significantly with the subsequent transition until original form II crystallites are all consumed. As revealed by above quiescent case, the II−I phase transition is retarded by incorporation of branched 4M1P counits. Then phase transition in 4M1P1.00 covers both stable off-axis and further development into high orientation steps due to the retarded transition kinetics. Then, it could be inferred that if II−I phase transition processes more slowly than orientation evolution, phase transition accomplishes with higher degree of orientation. As introduced in last section, the extreme type of c-axis parallel with stretching direction is referred to as highly oriented

εH = 2 ln

d0 d

where d0 and d are the widths of sample before and during stretching. The true stress σtrue is obtained as σtrue =

F d×b

where F is the measured force and b is the instantaneous thickness of the sample during stretching. Transition kinetics was plotted as a function of true strain and true stress in Figure 8. It was found that curves of form I fraction versus true strain diverge from each other, whereas those of form I fraction versus true stress are overlapped partially. True stress seems more suitable to correlate with the kinetics of the II−I phase transition, which, however, has a certain upper limit. Presuming that the kinetics is stress-determined, the deviation of this stress dependence appears at the transition degrees (i.e., fraction of transformed form I) of around 0.5−0.6 and 0.2−0.3 for 4M1P1.00 and 4M1P3.40, respectively. Recently, Cavallo et al.36 employed different mechanical protocols of true strain and true stress rates and also found that stress-dominated H


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Figure 8. Evolution of form I fraction plotted as a function of (a) true strain and (b) true stress.

transition kinetics is only available up to the form I fraction of 0.4−0.5, which is very comparable with results of the present work. It is known that in the II−I phase transition the ratedetermining step is form I nucleation within the original form II lamellae. Under quiescent conditions, nucleation is implemented by the internal stress resulting from different thermal expansion coefficients between crystalline and amorphous regions. In the present work with stretching, it is reasonable to correlate nucleation with microscopic stress applied to lamellae. Herein, one important information is that, with respect to the overlapped true stress dependences of PB-1 and 4M1P0.18, the kinetics of 4M1P1.00 deviates in the transition range f I = 0.5−0.6 (shown in Figure 8b), which corresponds to the evolution onset of off-axis orientation into the highly oriented crystallites (shown in Figure 7f). The orientation pathway affects transition kinetics. More specifically, orientation determines the decomposition of the total external stretching stress, σtotal, into two separated components of σ∥ and σ⊥, which are parallel with and perpendicular to the c-axis, respectively, as shown in Figure 9a. Based on the peak azimuth (ψ) of form II (200)II planes given in Figure 7 and the Polanyi equation,61,62,65−67 component stresses σ∥ = σtotal sin α and σ⊥ = σtotal cos α can be obtained with the deviation angle between the normal vector of lattice planes form II (200)II with respect to the stretching direction α = cos−1(cos(Ψ200 − 90°) × cos θ200)) and the Bragg diffraction angle 2θ200 of (200)II. Two component stresses enhance phase transition in different manners. The former σ∥ enhances the elongation of the conformational helix, and the latter σ⊥ helps the lateral slip of crystalline stems within the lattice. Then, the evolution of the phase transition is replotted in more detail as a function of corresponding σ∥ and σ⊥ to verify whether there exists any correlation of the transition kinetics with specific component stress. It is unexpected to observe that the deviated dependence of transition kinetics on total true stress (which was initially shown by the transition of f I = 0.5−1.0 in Figure

Figure 9. (a) Schematic of decomposing stretching stress into component stresses of σ∥ and σ⊥ parallel and perpendicular to the caxis, respectively. Replot of transition kinetics as a function of (b) component stress σ∥ and (c) component stress σ⊥.

8b) overlaps completely in the plot of transition kinetics as a function of component stress σ⊥ instead of σ∥ (see Figures 9b,c). This implies that in form I nucleation segmental elongation is relatively easy to actualize, and the lateral slip process of crystal stems becomes the rate-determining step. During phase changes from forms II to I, all of crystal stems are involved and have to move cooperatively within the a−b plane. In this case, incorporating only one branched 4M1P counit to each crystallite stem may even pull back the whole stem and slow down the corresponding lateral movement. In addition, the presence of 4M1P counits expands the lateral dimension of form II lattice. This means that the segmental traveling distance required to accomplish the II−I phase transition increases. Considering the decreased mobility and increased necessary distance with addition of 4M1P counits, it is reasonable to understand that chain lateral slip becomes the rate-determining step in nucleation for the II−I phase transition. Furthermore, the microscopic component stress is transmitted into the lamella though their connections including tie chains, entanglements, and even loops, of which parts are packed within the crystallite and the other parts are located within the amorphous region. Then, those corresponding crystallites stems that are connected with the amorphous I


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to 30.1 mol % were designed and synthesized to study the manual effects of the molecular factor of incorporated counits and external stretching on the phase transition from kinetically favored form II to the thermodynamically stable form I, of which the former and the latter have opposite inhibiting and enhancing influences, respectively. The quiescent experiments show that the presence of 4M1P counits in the polybutene-1 main chain not only significantly slows down kinetics but also decreases the ultimate form I fraction in the transition plateau. The high 4M1P concentrations ≥3.4 mol % is sufficient to completely impede the phase transition. Moreover, it is qualitatively known that the external stimulus of stretching enhances the II−I phase transition. Then the stretching-induced phase transition was in depth investigated by correlating the phase transition process with stretching strength. First of all, the molecular aspects of 4M1P counits and mechanical stretching together determine whether the stretching-induced phase transition can happen in copolymers. For polymers with no more than 1.00 mol % 4M1P counits, stretching largely accelerates transition kinetics and leads to the complete transition of the original form II. For copolymer 4M1P3.40, stretching can effectively trigger the II−I phase transition that does not happen under quiescent condition. For copolymers with 7.8−30.1 mol % 4M1P counits, stretching only induces the orientation of form II and cannot start the phase transition. Second, for the II−I phase transition, the orientation of form II lamellae may be off-axis or highly oriented with the stretching direction, depending on the completion between phase transition and orientation evolution. Third, the transition kinetics was correlated to mechanical responses, where the latter includes true strain, true stress, and component stresses parallel and perpendicular to the c-axis direction. These two component stresses parallel and perpendicular to the c-axis direction help the helical elongation and chain lateral slip, respectively. Interestingly, a master dependence on component stress σ⊥ was found for transition kinetics with off-axis orientation. This indicates that lateral chain slip along (110) plane is the rate-determining step in the nucleation process for the II−I phase transition.

region are all able to act as the initial sites for form I nucleation within form II. Also, the stresses acting on these stems within identical lamellae could cause local instability of the crystallite. Tashiro et al.27 proposed a translational lattice vibrational mode for phase transition. It was thought that the mutually opposite translational movements of the right- and left-handed chains occur along the (110) plane of the tetragonal lattice of form II. This translational lattice vibrational mode increases the amplitude and softens the original form II unit cell into a transient structure composed of the hexagonally packed chains, which is stabilized via conformation change to complete the transition into form I phase. In this case, the component stress σ⊥ enhances the translational lattice vibration mode of the whole lamellae or its local region and consequently accelerates nucleation and phase transition. On the other hand, when lamellae align their c-axes well along stretching direction, the total stress σtotal is equal to σ∥ (see Figure 9a), and there is no external σ⊥ (i.e., the component stress perpendicular to the c-axis direction), as shown by 4M1P3.40 of Figure 9c. The transmission of σ∥ elongates the segmental conformation from the 11/3 helix into the 3/1 helix, and the favored helical ordering still accelerates phase transition; otherwise, the quiescent phase transition does not happen even for the aging duration as long as 4 months. In this case, the necessary lateral movement of helix is accomplished by the thermally activated diffusion, similar to the quiescent condition. It has been well established that under quiescent conditions the number of intercrystalline links containing tie molecules and entangled loops is crucial to the rate of the II−I phase transition.68,69 This is due to the reason that in addition to introducing nucleation sites, tie chains transport the internal stress caused thermally to the crystalline stems and elongate them to change the segmental conformation. Although no pronounced perpendicular stress was considered for the quiescent transition, some tie chains might be obliquely connected with crystal lamellae and introduce the component stress perpendicular to the c-axis. Moreover, Qiao et al.70 studied a very low molecular weight polybutene-1 that has almost no intercrystalline links and found that nucleation of form I within form II can be overcome by the thermal fluctuation, as demonstrated by the occurrence of phase transition at the identical temperature with the crystallization temperature. This implies that even there is not any perpendicular stress, the lateral short-range movement of crystalline stems can be implemented by thermal fluctuation. This is also consistent with our results of 4M1P3.40 under stretching that after the very initial stretching stage the external stresses applied are parallel with the c-axis without the perpendicular component, but the II−I phase transition is still triggered. Of course, the lateral movement with internal thermal stress must be slower than that with external mechanical stretching, so the phase transition with sole σ∥ is slower than the case with both σ∥ and σ⊥. As shown in Figure 9b, the total true stress of 70 MPa causes the transition degree of f I = 0.51 in 4M1P3.40. When there are stresses acting along directions parallel and perpendicular to the stretching direction, 23.5 MPa is enough to lead to 100% transition into form I for PB-1, 4M1P0.18, and 4M1P1.00 (see Figure 9c).

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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.8b02600.

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Glass transition temperatures, WAXD results of stretching 4M1P1.00, 4M1P12.4, and 4M1P30.1, mechanical properties of PB-1 and 4M1P copolymers, WAXD results of 4M1P7.80−30.1 after breaking, stretching-induced orientation of homopolymer PB-1 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. ORCID

Lirong Zheng: 0000-0001-6803-5048 Chunguang Shao: 0000-0003-2316-4262 Bin Wang: 0000-0002-3130-3229 Li Pan: 0000-0002-9463-6856 Yuesheng Li: 0000-0003-4637-4254

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CONCLUSIONS In this work, a series of butene-1/4-methyl-1-pentene random copolymers with a broad range of counit concentration from 0 J


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Zhe Ma: 0000-0003-2458-4197 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (51573132 and 51633009) and the Tianjin Natural Science Foundation (16JCQNJC02700). We also thank Dr. Jinyou Lin and Prof. Fenggang Bian from beamline BL16B of Shanghai Synchrotron Radiation Facility (SSRF) for the help with synchrotron X-ray measurements.

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