Formation and stabilization of crystal nuclei in isotactic polybutene-1 aged below glass transition temperature

Journal Pre-proof Formation and Stabilization of Crystal Nuclei in Isotactic Polybutene-1 Aged below Glass Transition Temperature

Peiru Liu, Yanhu Xue, Yongfeng Men PII:

S0032-3861(20)30132-4

DOI:

https://doi.org/10.1016/j.polymer.2020.122293

Reference:

JPOL 122293

To appear in:

Polymer

Received Date:

28 November 2019

Accepted Date:

13 February 2020

Please cite this article as: Peiru Liu, Yanhu Xue, Yongfeng Men, Formation and Stabilization of Crystal Nuclei in Isotactic Polybutene-1 Aged below Glass Transition Temperature, Polymer (2020), https://doi.org/10.1016/j.polymer.2020.122293

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Journal Pre-proof TOC graphic Formation and Stabilization of Crystal Nuclei in Isotactic Polybutene-1 Aged below Glass Transition Temperature Peiru Liu, Yanhu Xue and Yongfeng Men*

Journal Pre-proof Formation and Stabilization of Crystal Nuclei in Isotactic Polybutene-1 Aged below Glass Transition Temperature Peiru Liu1,2, Yanhu Xue1 and Yongfeng Men1,2,*

1. State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Renmin Street 5625, Changchun, 130022, P.R. China 2. University of Science and Technology of China, Hefei, 230026, P.R. China * E-mail: [email protected]

Journal Pre-proof Abstract：Aging of glassy isotactic polybutene-1 (iPB-1) below the glass transition temperature (Tg) leads to remarkable acceleration of the subsequent isothermal crystallization due to the promotion of crystal nuclei. The formation and development of crystal nuclei during aging below Tg and during heating to the crystallization temperature have been probed by fast scanning chip calorimetry. The size and stability of the crystal nuclei change with the aging time. A short aging below Tg can only induce nuclei with small sizes which can grow into bigger stable one during the heating process above Tg if the heating rate is not high enough. With a prolonged aging time below Tg, bigger stable nuclei can also be developed, and its number increases till reaches a maximum value. These results suggest that noncooperative local motions rather than cooperative segmental motions in glassy iPB-1 are responsible to the formation of crystal nuclei during aging below Tg.

Key Words ： Isotactic Polybutene-1; Cold crystallization; Nuclei; Noncooperative Local Motions

Journal Pre-proof Introduction Polymers are able to crystallize from either the molten state during cooling or the glassy state during heating, known as the melt crystallization and cold crystallization, respectively. Melt crystallization is the most common one and generally consists of two steps, namely, firstly primary nucleation and then growth. Polymers must crystallize at a temperature lower than the equilibrium melting point (Tm0) because the formation of nuclei has to overcome free enthalpy barrier associated with the specific surface free energy.1 Usually, the difference between the equilibrium melting point and crystallization temperature is called supercooling.2 Nucleation rate and grow rate always change with supercooling and have their respective optimum temperatures.3–5 Therefore, supercooling is important for both nucleation and growth. Cold crystallization always shows an obvious advantage of nucleation comparing with the melt crystallization and its overall crystallization rate is usually much faster as well.6–10 Ordinarily, the advantage of nucleation is the main reason for the acceleration of crystallization. The optimum temperature of nucleation is lower than that of growth and a much faster cooling rate is required to avoid nucleation before cooling below Tg.3 Moreover, the influence of heating rate on nucleation can be more complicated. Then, the wider temperature range of cold crystallization provides an advantage for nucleation. The cooling and heating rate to avoid crystallization, which includes nucleation and growth, are such fast that the investigation of cold crystallization is inadequate until the fast scanning calorimetry was widely used. At present, the scanning rate is able to reach as high as 106 K/s and can be well controlled by a single thin film chip sensor.11,12 The cold crystallization of many polymers5,6,13 such as isotactic polypropylene (iPP)14–17, isotactic polybutene-1 (iPB-1),18 poly(lactic acid) (PLA)19–26, poly(ethylene terephthalate) (PET)8,27,28, poly(butylene terephthalate) (PBT)29,30, poly(butylene succinate) (PBS)31,32, polyamide 6 (PA 6)33,34 and poly(ε-caprolactone) (PCL)3,12 have been investigated in detail. It shows that the formation of homogeneous nucleation in the glass of polymers is able to take place after completion of enthalpy relaxation. The cold crystallization rate was obviously faster than the melt crystallization one when

Journal Pre-proof iPB-1 samples were isothermally crystallized at the same temperature, especially throughout at the higher temperature range. Through analyzing the enthalpy from heating curves of nonisothermal cold crystallized samples, it was found that cold crystallization during heating was promoted by aging the glass of polymers below Tg and arose only if enthalpy relaxation/densification of the glass is accomplished. Hence, it is suggested that nuclei of polymer crystals are able to form below Tg without the cooperative segmental motions. In some cases such as PA 6, iPP, PBS and PCL, these nuclei can even grow into small crystals when aging below Tg.3,12,14,31,33 However, it was observed that in iPB-1, there was no crystal growing below Tg.18 As a typical polymorphic polymer, iPB-1 has four crystal modifications.35,36 The melt crystallization and cold crystallization always lead to form II that is kinetically superior37,38 but it will spontaneously transform into form I, which is thermodynamically stable.39–42 Form I’ is the one with the same crystal structure of form I but crystallized directly from melt or solution rather than transformed from form II. The lamella of form I’ is much thinner than that of form I and melts at a lower temperature.43 Boor41 tried to bypass the transformation process through quench the iPB-1 melt to a lower temperature to crystallize but finally failed because the growth rate of form I’ is too slow to compete with form II.37 Only in iPB-1 prepared with metallocene catalysts44,45 and iPB-1 copolymers46,47 can form I’ crystals directly develop from the melt, where rr defects and co-units do play an irreplaceable role of manipulating surface free energy of the crystalline lamellae so that the generation of form II crystals is suppressed47. Therefore, melt crystallization and cold crystallization of iPB-1 homopolymer can hardly bypass the formation of form II, except in the case with the self-seeding effect.18,48–50 In order to search for the effect of aging the as-quenched glassy iPB-1 below Tg on the formation of crystal nuclei in iPB-1 and the stabilization of such nuclei during cold crystallization, in this work, we performed systematic fast scanning calorimetry measurements. Our results indicate that crystal nuclei of less stability could be formed after a short period aging at a temperature below Tg which could be stabilized during heating up above Tg if the heating rate is not high enough. Furthermore, stable nuclei

Journal Pre-proof could also form after long time aging process below Tg.

Experimental section The iPB-1 used in the experiment was a purified sample separated from a commercial material,51 using a method of solvent gradient fractionation (SGF) technique according to molecular weight.52 Its weight-average molecular weight (Mw) is 248 kg/mol and the polydispersity index 1.1. All the fast scanning chip calorimetry (FSC) measurements were performed in a powercompensation Flash DSC1 (Mettler Toledo Instruments, Switzerland), which was equipped with a Huber Intracooler TC100. Dry nitrogen purge gas with a flow rate of 50 mL/min was used to prevent water condensation. A grain of the powder sample was picked out and placed in the center of the sample cell under a microscope (Leica AE5). The sample was pre-melted by a slow heating rate of 1 K/s and was smeared by a thin copper wire after reaching 200 oC to ensure good thermal contact with the chip sensor and a proper thickness to reduce the thermal lag as far as possible.5 In order to erase the shear induced orientation due to smearing, the smeared sample was heated up to 200 oC

for 60 s to erase the internal stress as well as the thermal history. This holding time

of only 60 s has been sufficient because the time to erase thermal history for FSC is 2~3 orders of magnitude shorter than that for Standard DSC.53 The weight of the sample on the chip sensor in this research is about 60 ng, estimated by analyzing heat capacity.5 The cooling and heating rate was generally 2000 K/s but might change in some particular measurements that were pointed out in the paper.

Results and Discussion Cold Crystallization Figure 1 represents the effect of aging glassy iPB-1 when the heating rate is 2000 K/s. The equilibrium melting point of iPB-1 form II crystal is 133 oC and it must be higher for form I crystals54 so that the sample was firstly melted at a relatively high temperature of 180 oC for 0.2 s to erase the thermal history and then quenched to -30 oC which was

Journal Pre-proof lower than Tg. The sample was cooled to -90 oC after aging for a period of time at -30 oC

and then reheated to 180 oC. The corresponding heating curves of the aged sample

are shown in figure 1. Clearly, even aging at -30 oC for 1000 s, there is neither endothermic nor exothermic peak appears on these heating curves. It means that no crystals could form under these aging conditions and the heating rate is also fast enough to avoid nonisothermal cold crystallization during heating. Form II of iPB-1 is a very special modification since the mobility of chains in form II crystals is high and the heat of fusion is rather low.55 The lamellar thickness of iPB-1 is usually about 20 nm which is rather large for polyolefin. However, in the case of polymers which can form crystals below Tg, the aging temperature is slightly lower than Tg and the formed crystals are very small (5–10 nm).56–58 Thus, formation of relatively stable form II crystals requires larger range of cooperative segmental motions and then is much more difficult for iPB-1 below Tg.

Figure 1. The thermal protocol (top) and heating curves of iPB-1 after annealing for different times at -30 oC (bottom).

The endothermic peaks near Tg are the enthalpy recovery peaks which are the result of the change from the unstable as-quenched glassy state to the nearly equilibrium liquid

Journal Pre-proof state at -30 oC. By aging, the as-quenched glass relaxed with lowering of enthalpy and the degree of relaxation is reflected by the area of enthalpy recovery peak.3,13,18,34 Therefore, the area of the endothermic peak increases with the aging time (ta) until it reaches the theoretical maximum enthalpy value of relaxation.59 As shown in figure 1, the iPB-1 sample relaxes very fast at -30 oC since the area of recovery peak reaches a constant value after aging for only 2 s and changes little even after aging for a much longer time of 1000 s.

Figure 2. The thermal protocol (top), the heating curves (middle) and the melting enthalpies of iPB1 cooled from 180 oC and crystallized at 45 oC for different times (bottom).

If the iPB-1 sample is melted at 180 oC and then crystallizes at 45 oC, it shows a

Journal Pre-proof common crystallization behavior in figure 2. During the first 2 s, the melting enthalpy is almost zero, which means there are little stable crystals. The degree of crystallization increased rapidly after 2 s for nucleation and the crystallization rate dropped to a rather low value when crystallizing for a period longer than 10 s. For the purpose of analyzing the influence of aging on cold crystallization, we chose 3 s as the fixed crystallization time and the melting enthalpy of the melt crystallized sample at 45 oC for 3 s is named as ΔHm,ori and marked in figure 2. Then the thermal protocol of cold crystallization in figure 3 was carried out. The aged sample was heated at 2000 K/s to the crystallization temperature (45 oC) and was reheated to 180 oC after isothermal crystallization for 3 s. It should be noted that the crystallized sample was cooled down to -90 oC and hold for 0.2 s before reheating so as to obtain a relatively complete heating curve. As shown in figure 3, the melting enthalpy (ΔHm) of cold-crystallized sample (at 45 oC for 3 s) is equal toΔH m,ori only if ta is less than 2 s, the time for the complete enthalpy

relaxation. Therefore, for the incompletely relaxed amorphous sample, there seems to be no difference between cold and melt crystallization. However, a longer aging at -30 oC

makes a difference since ΔHm increases with ta up to an ultimate value (ΔHm,max),

which indicates the gross crystallization rate is greatly accelerated by aging glassy iPB1 sample before crystallization. Because of the identical crystallization temperature, the growth rate is not likely to change4,51 with ta so that the acceleration of crystallization must come from the improved primary nucleation process. There must be an extra number of primary nuclei compared with the melt crystallized one.

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Figure 3. The thermal protocol (top), the heating curves (middle) and the melting enthalpies of iPB1 crystallized at 45 oC for 3 s with different annealing times at -30 oC (bottom).

The Nucleation during Heating It is logical to deduce that the extra nuclei must be related to the aging process and some structures have formed during aging at -30 oC or heating from -30 oC to 45 oC. Although many polymers3,12,14,31,33 have been proven to form nuclei below Tg and even grow into small crystals under some conditions, the possibility of forming nuclei during heating could not be ruled out.60 It is impossible to directly prove whether the nuclei begin to develop during aging below Tg. One easy way is to exclude the influence of heating process first, that is, to figure out the influence of heating process on cold crystallization.

Journal Pre-proof It is known that cooperative segmental motions in the amorphous phase are frozen below Tg, hence the molecular mobility is quite different on the two sides of Tg. During the heating process, Tg is the turning point for cooperative segmental motions so that the influence of heating rate on nucleation can be divided into two parts by glass transition. The high temperature end of the glass transition region (-5 oC) is selected as the turning point in the thermal protocols. Figure 4 shows the result of changing the heating rate β1 in the low temperature range from -30 oC (the aging temperature) to -5 oC (the high temperature end of the glass transition region). The heating and cooling rate in this thermal protocol are 2000 K/s except for β1 which varies from 100 to 20000 K/s. The result in figure 4 demonstrates that more nuclei appear with a slower heating rate but 1000 K/s is fast enough to avoid the formation of nuclei in the low temperature range. The minimum enthalpy under quite fast β1, however, is still much larger thanΔHm,ori of the melt crystallized one, meaning that there are more primary nuclei in cold crystallization. The extra nuclei must come from the effect of either the 10 seconds aging at -30 oC or the heating process above -5 oC. It needs further investigation to figure out the actual process which influences cold crystallization of iPB-1.

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Figure 4. The thermal protocol (top), the heating curves (middle) and the respective melting enthalpies of iPB-1 with different β1 (bottom).

On the other side, results in figure 5 show that increasing the heating rate β2 in the high temperature range from -5 to 45 oC (the crystallization temperature) up to 12000 K/s led to a continual decrease of melting enthalpy, which actually means enormous decrease of the number of nuclei with the increase of β2. The melting enthalpy, which corresponds to the crystallinity as well as the number of primary nuclei, decreases to the minimum which is equal toΔHm,ori. Therefore, it is clear that the acceleration of crystallization due to aging glassy iPB-1 can be almost wiped out through faster heating during the heating process above Tg, not during the heating process below Tg. That is to

Journal Pre-proof say, with the heating rate of 2000 K/s, a great number of nuclei were stabilized during the heating process above Tg.

Figure 5. The thermal protocol (top), the heating curves (middle) and the respective melting enthalpies of iPB-1 with different β2 (bottom).

The Nucleation during Aging below Tg At the beginning of aging, cooperative segmental motions are able to proceed in the asquenched glassy iPB-1 sample because the enthalpy difference between as-quenched glass and supercooled liquid of iPB-1 acts as a driving force.59 Under this thermodynamic driving force, the as-quenched sample with a highly heterogeneous structure61,62 undergoes the equalization of density. As a result of aging, both the

Journal Pre-proof enthalpy and volume decrease. It is proven that only 2 s is needed to complete relaxation/densification of this glassy iPB-1 at -30 oC, while the promotion of nucleation through aging glass does not show up until ta is longer than 5 s. The nucleation advantage of aging glassy iPB-1 occurred after complete densification, which was similar to the conclusions in some other researches.3,18,33 It has been advanced that nucleation can occur in many polymers13 at the glassy state, just after the enthalpy relaxation. Therefore, one cannot entirely exclude the possibility that stable nuclei arise just by aging below Tg because the aging time in figure 4 and 5 is not long enough. For the iPB-1 sample with ta of only 10 s, all the extra nuclei can even be avoided by fast heating according to the decrease in the melting enthalpies, as shown in figure 5. This indicates that there might be some small or early stage nuclei formed by aging but the frozen chains cannot directly be arranged into stable nuclei within the short period of aging. Once the aged sample was heated up to a temperature higher than Tg, cooperative segmental motions recovered and it is hard to suppress the stabilization and growth of those small or early stage nuclei during heating. If the aging time is prolonged, however, noncooperative local motions associated with β relaxation persistently proceeded regardless of the stopped cooperative segmental motions, which might leads to formation of stable nuclei.63 In figure 6, the sample with designed ta was heated from -30 to 45 oC by different heating rates (β) and the corresponding ΔHm was shown as well. A short aging less than 20 s is still not long enough to form stable nuclei for crystallization at 45 oC since the nucleation advantage can be erased by increasing the heating rate. But for the one aging for longer than 50 s, there seems to be a part of stable nuclei remained even under a very fast heating rate of 20000 K/s. Moreover, as soon as ta is longer than 500 s, ΔHm reaches the maximum value and heating rate cannot affect the number of stable nuclei any more. One might notice that although the longer ta results in a maximum and constant value ( Δ Hm,max), the sample is still not completely crystallized because the Δ Hm of the complete melt crystallized one is larger thanΔHm,max, as shown in figure 2. That is to say, the number of stable nuclei formed below Tg reaches the maximum and the

Journal Pre-proof isothermal crystallization at 45 oC for only 3 s is not enough to crystallize completely even with these stable nuclei.

Figure 6. The thermal protocol (top) of FSC experiment and the melting enthalpies of iPB-1 crystallized at 45 oC for 3 s with different ta and β (bottom).

Using the method shown in figure 2, the half-time of crystallization at different temperatures can be measured easily and are collected in figure 7. It shows a typical inverted bell shape and the overall crystallization rate is fastest at a temperature between 35 and 45 oC. The critical size of nuclei for spontaneous growth varies with crystallization temperatures, therefore there is a possibility that the cold crystallization at 45 oC is influenced by the size distribution of stable nuclei which change with aging time at -30 oC. In other words, only the nuclei with certain sizes can act as extra nuclei when crystallizing at a fixed temperature. In order to figure out the influence of nuclei size distribution on cold crystallization, we chose another two crystallization temperatures respectively higher and lower than 45 oC and repeated the thermal protocol in figure 6. For the one crystallizes at 20 oC for 5 s, where homogeneous nucleation begins to dominate, the acceleration of crystallization rate by aging the iPB-1 glass is still very strong. However, the heating rate faster than 2000 K/s does not affect

Journal Pre-proof the number of extra nuclei formed before isothermal crystallization even with a short aging time. It indicates that the nuclei indeed form during aging below Tg and they are stable and large enough to act as primary nuclei at 20 oC, while cannot satisfy that at 45 oC. For the one crystallizes at 70 oC for 20 s, the size of nuclei needed for primary nucleation is much bigger and a much longer aging time below Tg is also needed to form larger stable nuclei then. In figure 7, the one crystallizes at 70 oC seems more sensitive to the heating rate because it requires a larger size of nuclei since the growth or stabilization of nuclei becomes more detectable under a slower heating rate. In summary, aging of glassy iPB-1 leads to formation of nuclei and the sizes of these nuclei change with time as well.

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Figure 7. The half-times of melt crystallization with different crystallization temperatures. (top) The melting enthalpies of iPB-1 crystallized at 20 oC for 5 s (middle) or at 70 oC for 20 s (bottom) with different ta and β.

Then, for the as-quenched sample, cooperative segmental motions proceed but become slower and slower as the enthalpy difference between the as-quenched glassy phase and the hypothetic liquid phase reduces with aging.18,59 This is actually a relaxation/densification process and seems to hinder nucleation of the glass. The cooperative segmental motions stop after enthalpy relaxation but there is still some remaining molecular mobility from noncooperative local motions which needs much lower temperature to be frozen.63 The segments in the relaxed glass tend to arrange into

Journal Pre-proof nuclei through noncooperative local motions during aging. A short aging leads to some small nuclei which are easy to arrange into bigger stable nuclei during heating, especially above Tg with the help of cooperative segmental motions. A long aging leads to bigger stable nuclei and the number increases with aging until reaches the maximum.

Conclusions The influence of aging on the development of nuclei in glassy iPB-1 was investigated by an isothermal crystallization method. The glassy iPB-1 sample was quenched from a temperature much higher than the equilibrium melting point and aged for a period of time prior to isothermal crystallization. Aging below Tg leads to the formation of nuclei with different stabilities depending on the aging time before isothermal process at the crystallization temperature so that the rate of cold crystallization is much higher than that of melt crystallization. After enthalpy relaxation with cooperative segmental motions, the glassy sample begins to form small nuclei with the help of noncooperative local motions. With an aging time less than 20 s, stable nuclei can form during heating once the temperature is above Tg even though the heating rate is as high as 2000 K/s. But a faster heating is able to avoid the formation of stable nuclei during heating and the crystallization rate makes no difference then. If aging for a longer time, stable nuclei can be produced and the number of which increases until it reaches a maximum value. Besides, the bigger the stable nuclei are, the less the cold crystallization can be affected by the faster heating rate. It shows that noncooperative local motions rather than cooperative segmental motions play a major role in forming crystal nuclei when the glassy iPB-1 is aged below Tg.

Acknowledgments This work is supported by National Natural Science Foundation of China (51525305). We thank Prof. Christoph Schik, Prof. Günter Reiter and Porf. Wenbing Hu for fruitful discussions during the IDMPC 2019.

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Journal Pre-proof CRediT author statement Peiru Liu: Conceptualization, Validation, Investigation, Data Curation, Writing- Original Draft. Yanhu Xue: Polymer Purification. Yongfeng Men: Conceptualization, Resources, Writing- Review & Editing, Supervision, Project administration, Funding acquisition.

Journal Pre-proof

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Journal Pre-proof Highlights: 1. Nucleation of isotactic polybutene-1 below glass transition temperature has been investigated. 2. Early stage nuclei can be stabilized during heating up to crystallization temperature when the heating rate is below a critical value. 3. Nuclei can also be stabilized below glass transition temperature via prolonged annealing time.