Retardance of Form II to Form I Transition in Polybutene-1 at Late

Mar 7, 2018 - *E-mail: [email protected] (Y.M.). ... Form II samples with different lamellar thickness and constituent in amorphous phase were generated ...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Retardance of Form II to Form I Transition in Polybutene‑1 at Late Stage: A Proposal of a New Mechanism Yongna Qiao,†,‡ Hai Wang,† and Yongfeng Men*,†,‡ †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Renmin Street 5625, 130022 Changchun, P. R. China ‡ University of Science and Technology of China, Hefei 230026, P. R. China ABSTRACT: The retardation of polybutene-1 form II to form I transition in the late stage has been investigated by means of time-resolved wide-angle X-ray diffraction. Form II samples with different lamellar thickness and constituent in amorphous phase were generated via varying crystallization temperature and molecular weight and aged at room temperature for the form II to form I transition. The II to I polymorphic transition in polybutene-1 undergoes two stages, where slow nucleation and rapid growth proceed in the first stage and extremely slow secondary nucleation and growth take place in the late stage. The degree of transition reaches a plateau value in the late stage of transition, which depends highly on crystallization temperature and molecular weight that low molecular weight and high crystallization temperature always contribute to a greater degree of transition, but is less affected by the kinetics in the first stage. According to the crystal unit cell parameters of form II and form I, there should be an extension in the normal direction of lamellae and a shrinkage in the lateral direction during phase transition, which thus would induce an additional pressure in the normal direction and the tendency to expansion in the lateral direction on the residual from II crystallites. As a result, the nucleation and further growth of the II to I transition are retarded. Low molecular weight and high crystallization temperature lead to larger content of chain ends and higher chain mobility in the amorphous phase; hence, the samples are more likely to release the unfavorable factors restraining generation of form I nuclei to some extent. The transition degree in late stage is thus an intrinsic property of PB-1 samples determined by their microstructures and would not be influenced by the transition behavior in the first stage.

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and isotactic polypropylene.9,10 Recently, we observed a melt temperature dependent polymorphous selection during crystallization of butene-1/ethylene random copolymer11 and clarified such behavior considering nucleation barriers and critical sizes of metastable form II and stable form I′ crystals, respectively.12 It is thus clear that a direct formation of stable form I′ in PB-1 homopolymer under ordinary conditions is not feasible. Therefore, potential ways of accelerating the phase transition and thus stabilizing the shape of products have been extensively investigated, such as copolymerization with random 1-alkane counits fewer than five carbon atoms13 and applying high pressure,14,15 pressured CO2,16−18 solvent annealing,19 and external or thermal stress.20−25 The evolution of fraction of form I or form II during phase transition has been studied by time-resolved observations with the in situ Fourier transform infrared spectroscopic imaging technique,26−28 the microindentation hardness technique,29 and wide-angle X-ray scattering techniques.25,30,31 Although the crystal fraction of form I increased with the aging time, it is often still below 100%

INTRODUCTION Isotactic polybutene-1 (PB-1) exhibits a complex polymorphism and crystallizes into hexagonal form I/I′, tetragonal form II, or orthorhombic form III depending on the crystallization conditions.1 Industrial products of PB-1 show attractive properties, such as stiffness, puncture resistance in films, temperature resistance, environmental stress cracking resistance, low creep, and good abrasion resistance.1 The outstanding PB-1 products are mainly in form I modification. However, crystallization from the melt usually leads to the kinetically favored form II, which is metastable, and would transform into the thermodynamically stable form I spontaneously during the storage at room temperature. It needs several weeks to complete such solid-to-solid phase transition, and the transformation results in deformation and volume change due to the difference between unit cell sizes of form II and form I,2,3 thus increasing the production costs. In order to overcome such a practical problem, many efforts were concentrated on seeking ways to directly crystallize the system into stable form I′.4,5 However, up to now, PB-1 homopolymers can only be crystallized into stable form I′ under specific conditions, such as ultrathin film,6 presence of both constitutional chain defects and stereodefects,7,8 and blends of PB-1 © XXXX American Chemical Society

Received: November 23, 2017 Revised: January 16, 2018

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DOI: 10.1021/acs.macromol.7b02481 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules after a long time aging, indicating a significant slowdown of the phase transition rate in the late stage. There is little amount form II crystals left even after several months of aging. Understanding the mechanism of slowdown of phase transition rate and the amount of residual form II crystals in the late stage of transition is of not only scientific importance but also great practical significance, while there is a relative dearth of research on this problem. Chau et al. suggested that the nucleation of the stable form I is a rate-determining step in the late stage.32 They suggested that the majority of tetragonal form II crystallites transform in the initial stage where the nucleation of form I is sporadic in time and place, and growth of stable phase is the ratedetermining step. The very slow rate in secondary transformation stage is due to that the form II crystals were not “nucleated” at the beginning where there was no initial stress. Besides this proposal, there is rarely discussion on the residual form II crystals which are difficult to transform into the stable form I in late stage of phase transition. In our previous study, we found similar but slightly different transformation kinetics compared to Chau’s proposal25 that the nucleation is found to be the main process in the beginning of aging the metastable form II crystals, but once the nuclei of form I are generated in form II crystal matrix, the growth of from I occurs immediately, which we called the first stage. The late stage of transition corresponds to the so-called secondary transformation stage in Chau’s proposal, where the transition rate becomes very slow. Thus, we can deduce that nucleation is the rate-determining process in both the initial stage and late stage of transformation. In this study, the evolution of the content of the two crystalline modifications during the form II to form I phase transition in PB-1 at room temperature was recorded by means of in situ wide-angle X-ray diffraction (WAXD). The mechanism of the sudden slowdown of the transition rate in the late stage was ascribed to the additional pressure of amorphous phase in the normal direction of lamellae and the tendency to expansion in the lateral direction, which resulted from the difference between unit cell size of form II and form I crystals. The results give a thorough knowledge of the mechanism of the slow phase transition in late stage and are of guiding significance because the slow transition in late stage is the chief factor for the long process cycle of PB-1 materials in the industry.

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Figure 1. Schematic of thermal protocols applied to the three PB-1 samples.

molded at 180 °C and held for 10 min to erase the thermal history and develop films of 0.5 mm in thickness. Then, the molten films were transferred into the isothermal water bath at different preset temperatures (Tc = 50, 60, 70, 80, 85, 90, and 95 °C) and kept for 3 h to complete the crystallization of form II. The PB-1 samples in form II were moved to sample holder of WAXD experiments at room temperature immediately after the isothermal crystallization to determine the crystalline structure change from form II to form I. Time-resolved WAXD experiments were conducted with NanoInXider vertical SAXS/WAXS system of Xenocs SA, France, with the aid of a hybrid pixel detector (Pilatus3 100K, Dectris, Swiss). A multilayer mirror focused the Cu Kα X-ray source (Genix3D Cu ULD, Xenocs SA, France), generated at 50 kV and 0.6 mA, and scatterless collimating slits were used during experiments. The wavelength of the X-ray radiation is 0.154 nm, and the sample-to-detector distance is 75.3 mm. The beam size at the sample position was 800 × 800 μm2. Each pattern was collected in 600 s in an all vacuum environment, which was then background corrected using the standard procedure. All samples show diffraction rings with homogeneous intensity distribution (a quarter of which was measured by the detector due to the configuration of the setup).This allows us to use onedimensional diffraction intensity distributions to determine relative fractions of different crystalline modifications precisely. The main diffraction peaks observed at 2θ of about 10.0°, 17.5°, and 20.4° correspond to crystallographic planes of (110), (300), and (220 + 211) of form I. Besides, a sequence of reflections at 11.9°, 16.9°, and 18.5° can be assigned to the (200), (220), and (213) lattice planes of form II, respectively. Previously, the fraction of form I is obtained by the equation8,34

EXPERIMENTAL SECTION

Three isotactic PB-1 samples with different molecular weight used in this study were produced by LyondellBasell. A summary of their physical properties is listed in Table 1.33 Figure 1 illustrates the thermal protocols used for isothermal crystallization and phase transition. The pellets were compression-

XI =

melt flow rate (MFR) (190 °C/2.16 kg)

Mw (kg/mol)

crystallinity Φwa (%)

Tm (form II)a (°C)

PDI

PB0110 PB0400 PB0800

0.4 16.4 200.0

711 188 77

61 64 71

115.9 115.4 115.1

3.5 2.7 3.0

(1)

where I(110)I and I(200)II are the integrated intensities of the (110)I and (200)II reflections, respectively, and corrected by the parameter R = 0.36 taking into account of structure factors and Lorentz polarization factors.8 The structure factors of (110) reflection in form I and that of (200) reflection in form II were calculated based on the space group R3c ̅ and P4̅, respectively.8 However, the crystalline structure of polybutene-1 has been recently refined by Tashiro et al.3 showing different symmetries compared to such original proposal. The space groups of form I and form II crystals have been determined as P3̅ and P4̅b2, respectively. It is thus not precise to continue our discussion using the R value of 0.36. A method allowing experimental estimation of the R value has been reported by Tashiro et al.31 as follows. One can assume linear relations between the observed WAXD intensities and the content of the corresponding crystalline forms as follows:

Table 1. Characterization of PB-1 Samples trade name

I(110)I I(110)I + RI(200)II

a The weight crystallinity (Φw) of form II crystals and the corresponding melting temperature (Tm) were derived from the DSC cooling process from the molten state at a rate of 10 K/min and the following heating scan at a rate of 10 K/min, respectively.

XI = aII(110)I and XII = aIII(200)II B

(2) DOI: 10.1021/acs.macromol.7b02481 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules where aI and aII are constant coefficients for a certain sample. From the equation

XI + XII = 1

(3)

we can obtain the following equation: aII(110)I + aIII(200)II = 1 or I(110)I = −(aII /aI)I(200)II + 1/aI

(4) Clearly, the ratio aII/aI in eq 4 is equal to R in eq 1. This ratio can be easily obtained from the slope of plots of I(110)I vs I(200)II. Figure 2

Figure 2. Plots of the integrated (110) diffraction intensity of form I, I(110)I, against that of (200) of form II, I(200)II, obtained during the phase transition at room temperature in isothermally crystallized PB0800 samples. shows typical plots the integrated diffraction intensity of form I, I(110)I, against that of form II, I(200)II, obtained during the phase transition at room temperature in isothermally crystallized PB0800 samples. The resultant straight lines with the almost same negative slope indicate that the ratio aII/aI is a constant with a value of 0.67. Similar analysis performed for other samples used in this investigation showed the same result of negative slope with a value of 0.67. Therefore, the fraction of form I can be calculated by

XI =

I(110)I I(110)I + 0.67I(200)II

(5) Figure 3. Selected in situ WAXD curves of PB0800 obtained during annealing at room temperature after the isothermal crystallization at 85 °C (top), the corresponding fraction of form I crystals obtained from analysis of intensity distribution of (110)I and (200)II diffraction peaks in WAXD measurements taken during annealing at different conditions (middle), and the Avrami plot of XI present in the middle figure (bottom).

The real-time fraction of form I can be considered to be the transition degree (XI).

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RESULTS AND DISCUSSION The phase transition from form II to form I in PB-1 has been confirmed to be a two-step process containing nucleation and growth, and the formation of form I nuclei is the ratedetermining step which shows the highest rate at about −10 °C.25 The optimal growth temperature was found to be 40 °C. As a result of the cooperative effects of temperature on nucleation and growth, the overall phase transition rate exhibits a maximum value at room temperature when the system is aged at single temperature.30,35−42 The evolution of content of the transformed form I of PB0800 during annealing at room temperature after isothermal crystallization at 85 °C is shown in Figure 3. In the top part, the diminishment of diffraction intensity of the crystallographic plane in form II and the increasing intensity in form I peaks are indicated by the red and black vertical arrows, respectively. The corresponding content of form I during the course of phase transition is illustrated in the middle of Figure 3. One finds clearly that the phase transition goes through two stages that exhibit different kinetics. In early time of the initial stage, generation of form

I nuclei in the form II matrix is the dominant process, together with slow incipient growth of the newly formed stable nuclei. After nucleation, it goes into the rapid growing process of form I nuclei in the first stage; a little secondary nucleation is also included in the growth surface. With the development of form I crystals, phase transition comes into the late stage where the transition rate is reduced and the transition degree reaches a plateau value close to but always less than 1. The kinetics of phase transition can be studied by Avrami analysis which reads XI = 1 − exp[−kt n]

(6)

where k is the transition rate coefficient and n is the Avrami index, and eq 6 can be rewritten as follows: log[− ln(1 − XI)] = log k + n log t C

(7)

DOI: 10.1021/acs.macromol.7b02481 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

form I in the three samples of different molecular weights, estimated from the WAXD data analysis, as a function of annealing time is plotted in Figure 5. In accordance with our

The bottom of Figure 3 shows the Avrami plots of data present in middle part according to eq 7. It shows that the phase transition kinetics in the first stage is in accord with Avrami equation while kinetics in late stage deviate from the equation. The deviation part with extremely slow kinetics was defined as the late stage of phase transition. The plateau value of transition degree in late stage varied with the molecular weight of PB-1, as shown in Figure 4. The

Figure 5. Evolution of content of the transformed form I in PB-1 samples as a function of time during annealing at room temperature.

previous work, low molecular weight PB-1 would obtain more form I under the same annealing temperature and time for the 50 °C crystallized samples over the two stages of phase transition for their high chain mobility. Interestingly, for 95 °C isothermally crystallized samples, the high molecular weight one shows fast kinetics in the first stage but less plateau value of transition degree in the late stage. The different molecular weight dependency in transition rate during the early stage but similar in transition degree in late stage between samples isothermally crystallized at 50 and 95 °C indicates that the late stage phase transition behavior would not be affected by the kinetics in the first stage. The adjustment of annealing condition changes the kinetics in first stage43 but is less effective to the late stage of transition. The influence of crystallization temperature on phase transition of a PB-1 sample was investigated, and the evolution of form I fraction as a function of time in PB0800 samples derived from WAXD experiments is illustrated at the top of Figure 6. At a certain aging conditions, it showed different phase transition kinetics as the crystallization temperature of form II changed. The late stage part was enlarged in the inset, where transition degree showed very little changes with time but increased monotonically with crystallization temperature. Though the difference of transition degree between samples

Figure 4. WAXD curves of PB-1 samples after annealing at room temperature for 4 days after crystallized at 50 °C (top) and 95 °C (bottom).

top part of Figure 4 exhibits the WAXD curves of the three PB1 samples crystallized at 50 °C in late stage of the form II to I phase transition after aging at room temperature for 4 days. The intensity of the diffraction peaks of form I crystals increased as the molecular weight decreased from 711 kg/mol of PB0110 to 77 kg/mol of PB0800, and the diffraction peaks of form II are stronger in high molecular weight samples accordingly. When crystallized at 50 °C, all the three samples were able to form intercrystalline links and develop foldedchain crystals which facilitate the nucleation of form I within form II crystallites. However, when crystallized at 95 °C, PB0400 and PB0800 samples possess less intercrystalline links due to their short chain length and large lamellar long spacing at such high crystallization temperature.33,43 Nevertheless, in this situation, the transition degree in late stage still shows a negative relationship with molecular weight as confirmed by the WAXD results in the bottom part of Figure 4. The situation can be better viewed when a time-dependent fraction of form I for these samples is shown. The evolution of the content of crystal D

DOI: 10.1021/acs.macromol.7b02481 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 7. Plateau value of XI after annealing for 9 days at room temperature of PB-1 samples as a function of the crystallization temperature of form II crystals.

helix;44−46 thus, the c-axis kept unchanged during the phase transition. The crystal unit cell parameters of form II and form I are illustrated in Table 2.3 The distance between two adjacent repeating units in the chains along the c-axis could be estimated by c lc = cell Nmono (8) where Nmono is the number of monomer unit in one unit cell. In form II crystals, lc,II = 21.2 Å/11 = 1.93 Å, while in form I, lc,I = 6.5 Å/3 = 2.17 Å. After polymorphic transition into form I, the distance between repeating units increases from 1.93 to 2.71 Å, which leads to an elongation of the chain. In this study, the Nano-inXider system gave both SAXS and WAXD results simultaneously. However, the phase transition experiments were carried out at room temperature that the SAXS results of form II cannot be determined due to the similar electron density between the crystalline and amorphous phase.31 Though it would become detectable via raising testing temperature, the thicknesses of amorphous phase and lamellae increase to different extent due to the difference in their thermal expansion coefficients compared with the value at room temperature. Meanwhile, the rate of form II to I transformation would be very slow at elevated temperatures. Therefore, lamellar thickness of three PB-1 samples in form I are illustrated in the Table 3 after aging at room temperature for about 95 h. The results were in accord with the previous study that the long spacing and lamellar thickness increase with crystallization temperature.43 The average area occupied by one chain in the ab-plane could be calculated as

Figure 6. Evolution of content of the transformed form I in PB0800 samples crystallized at different temperatures as indicated in the figure as a function of time during annealing at room temperature (top) and the integrated WAXD curves after aging at room temperature for about 95 h corresponding to the last point in top figure (bottom).

here is small, one can see clearly from the 1-dimentional WAXD curves of samples after aging at room temperature for about 95 h in the bottom of Figure 6 that the diffraction peak intensity of form I crystals increased and that of form II decreased with crystallization temperature. The WAXD curves after 9 days aging at room temperature of three PB-1 samples crystallized at a series of temperatures were collected to derive the transition degree in late stage. The thusobtained transition degrees in the late stage are illustrated in Figure 7. For all the crystallization temperatures studied in this work, the low molecular weight sample exhibited a more complete phase transition after same aging time, even though the relative value of transition rate of three samples varied with crystallization temperature. The transition degree in late stage for all the three samples shows a positive relationship with crystallization temperatures. Note that the common point between the two favorable factors, low molecular weight and high crystallization temperature, leading to a larger transition degree in late stage is that there were larger content of chain ends and thus higher chain mobility in amorphous phase and the interface between lamellae and amorphous region. It could be assumed that there is a connection between chain mobility and the plateau value of transition degree in late stage. The form II to form I phase transition in PB-1 was constrained by preservation of helical hands in which the (110) planes of the resultant trigonal form I of 3/1 helix are parallel to (110) planes of parent tetragonal form II crystals of 11/3

S0, ab =

Scell, ab Nchain

(9)

where Scell,ab is the area of the ab-plane in a unit cell and Nchain is the number of chains per unit cell. The average area occupied by a chain in the ab-plane in form II, 53.3 Å2, was larger than that in form I, 44.2 Å2. One can deduce that there would be a shrinkage in lateral direction of crystallites. Therefore, in late stage of phase transition, most of crystals change into form I modification with an extension in normal direction of lamellae and a shrinkage in lateral direction. The extension of crystallites in normal direction leads to a looser intercrystalline links between the adjacent newly transformed form I crystallites and E

DOI: 10.1021/acs.macromol.7b02481 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 2. Physical Properties of PB-1 Samples unit cell form II form I

space group

a0 [Å]

b0 [Å]

c0 [Å]

γ [deg]

no. of chains

density [g/cm3]

chain conformation

P4b̅ 2 P3̅

14.6 17.5

14.6 17.5

21.2 6.5

90 120

4 6

0.89 0.96

11/3 3/1

in PB-1. Chains in the metastable form II have mobile and disordered conformation, while those in the stable form I crystals have ordered and well rigid conformation according to the NMR data.48−51 Therefore, in the late stage of the form II to I phase transition, the increase of available volume of the remnant form II, resulting from the shrinkage of adjacent crystallites in lateral direction, would lead to a relatively higher side-chain conformational entropy. As a result, the free energy barrier of nucleation increases due to the increase of the loss of conformational entropy of a side chain. A new mechanism of form II to I transformation was proposed by Tashiro3 based on a kind of soft mode concept, 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 pairs of right- and left-handed chains. Then, this transient structure is stabilized to the crystal form I, when the chain conformation changes cooperatively from 11/3 to 3/1 helical conformation by a slight conformational change. Moreover, Nozaki42 emphasized that the phase transition process needs the diffusion of chain segments in the amorphous region because the intercrystalline links pass through both the crystalline and amorphous phases alike, and molecular rearrangement in the crystalline lamellae must be accompanied the molecular motion in the amorphous region. With the development of phase transition, the movement of chain segments in amorphous phase would be regulated by the fixed form I crystals. Thus, the cooperative translational movements of segments in form II crystals and amorphous phase was weakened, and the further transformation of form II was retarded. The above discussions considered local situation of residual form II crystallites only. It is thus necessary to discuss the effect of global internal stresses generated by the form II to I phase transition of crystallites of different orientations. To clarify this point, we need to consider stress transmission mechanism in semicrystalline polymers. On the basis of extensive tensile deformation and microscopic structural studies, we concluded that semicrystalline polymers can be effectively regarded as two interpenetrated networks of hard crystalline skeleton and soft amorphous entangled network.51 Therefore, at small deformation, the load is mainly transmitted by the crystalline skeleton. In the case of PB-1 with form II, amorphous entangled network also contributed to the force transmission at very small deformation because of the ability of intensive translational motion of the chains within crystalline phase.49 As such, at the early stage of form II to I transition, internal stress generated by the shrinkage of crystallites with random orientation can be effectively transmitted to different places, leading to an acceleration of the overall transition. However, at the late stage of transition, the chains within the already transformed form I crystals are completely fixed, leading to a less effective transmission of the internal stress. Those remaining form II crystallites are thus under such a circumstance that they are not

Table 3. Values of Lamellar Thickness (dc) and Long Spacing (dac) of Form I Crystals in Three PB-1 Samples after Isothermally Crystallized at Different Temperatures Followed by Aging at Room Temperature for about 95 h PB0110

PB0400

PB0800

Tc [°C]

dc [nm]

dac [nm]

dc [nm]

dac [nm]

dc [nm]

dac [nm]

50 60 70 80 85 90 95

16.8 16.9 18.2 20.1 21.0 22.4 24.4

28.1 28.4 31.0 34.4 36.1 38.6 42.2

14.0 15.0 16.2 17.8 18.7 19.9 21.5

23.8 25.3 27.6 30.3 31.9 34.1 37.1

12.2 13.1 14.6 16.6 17.7 19.3 21.3

20.8 22.4 24.9 28.4 30.3 33.0 36.6

a temporary and local increase in density of amorphous phase. Compared with the initial state in form II modification, the residual form II crystals underwent an additional pressure of amorphous phase. The shrinkage of crystallites in lateral direction after phase transition results in an increase of available volume for the remnant form II; thus, there is a tendency to expansion in this direction. Both of the additional pressure in normal direction and the tendency to expansion in lateral direction are opposite to the conditions necessary for phase transition. These unfavorable factors retards the nucleation process and further slow down the overall phase transition rate; thus, it shows a plateau in late stage of XI−t profile. The above proposed mechanism of slowdown of the form II to I transition due to local unfavored stress field can be further verified by considering more quantitative models proposed in the literature. The equation of overall form II to I phase transition rate referring to the classical theory of nucleation and growth of crystals and ameliorated by Tashiro31 is expressed as ⎡ ⎤ ⎛ 3ΔE ⎞ K1 0 ⎟ exp⎢ − ⎥ Goverall = Goverall exp⎜ − 0 2 ⎝ RT ⎠ ⎣ T (Tt − T ) ⎦ ⎡ ⎤ K2 × exp⎢ − ⎥ ⎣ T (T0 − T ) ⎦

(10)

(

where the exponential term exp −

3ΔE RT

) corresponds to the

translational migration of molecular chains in form II domains, and ΔE is the energy barrier for this motion. K1 and K2 are parameters proportional to the surface free energy of the side and bottom parts, T0 is the equilibrium temperature between form II and I, and T is the phase transition temperature. At a certain annealing temperature, conditions which affect the translational migration of molecular chains and surface free energies would influence the transition rate. It has been reported that the lateral surface free energy of the trigonal form I phase is about 7 times as large as that of the form II phase, which is roughly in agreement with the value calculated according to Hoffmann’s equation.47 The difference between the values of the two forms can be attributed to the loss of conformational entropy of the ethyl side chains of form I F

DOI: 10.1021/acs.macromol.7b02481 Macromolecules XXXX, XXX, XXX−XXX

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affected by internal stresses away from them (these stresses are shielded by the hard crystalline skeleton) but more affected by their local environments. The idea of uneven distribution of stress within the samples finds support from experimental results reported by Cavallo et al. where they reported that there were always residual form II crystallites remaining even after large macroscopic strain and stress deformation.24 It is known that long spacing and lamellar thickness increased as the isothermal crystallization temperature was increased. Therefore, the density of chain ends increased because the number of chain folding was reduced in order to develop thicker lamellae at high crystallization temperatures. There would be less entangled loops and more chain ends in the amorphous phase. The higher mobility of chain ends was likely to release the unfavorable factors restraining generation of form I nuclei to some extent. As a result, samples with low molecular weight or crystallized at high temperature show a relatively high plateau value of XI in late stage of phase transition, benefiting from its adequate chain ends.

REFERENCES

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CONCLUSIONS In the present research the late stage behavior of the form II to form I phase transition in PB-1 has been studied by means of in situ WAXD. By altering the isothermal crystallization temperature and molecular weight, we obtained samples with different lamellar thickness, long spacing, and number of intercrystalline links in amorphous phase in form II modification. The form II to I phase transition in PB-1 undergoes two stages, of which the main processes are the main nucleation and growth and extremely slow nucleation and scarcely growth, respectively. When it proceeds into the late stage, the value of the transition degree reaches a plateau value, and the plateau value varies with the isothermal crystallization temperature and molecular weight. After partial phase transition, the extension of form I crystals in the normal direction of lamellae leads to a temporary additional pressure, and the shrinkage in lateral direction results in the tendency to expansion in the residual form II crystallites. These two variations strongly retard the generation of form I nuclei embodying a hyperslow phase transition rate. PB-1 samples with low molecular weight or high crystallization temperature exhibit a larger plateau value of transition degree in the late stage because of the larger content of chain ends or higher chain mobility to release the unfavorable stress. The molecular weight/crystallization temperature dependencies of transition degree in the late stage would not be influenced by the relative transition rate in the first stage but affected by its own structure and morphology.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.M.). ORCID

Hai Wang: 0000-0001-8902-4014 Yongfeng Men: 0000-0003-3277-2227 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 (51525305 and 21134006). G

DOI: 10.1021/acs.macromol.7b02481 Macromolecules XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.macromol.7b02481 Macromolecules XXXX, XXX, XXX−XXX