Melt Memory Effect beyond the Equilibrium Melting Point in

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Melt Memory Effect beyond Equilibrium Melting Point in Commercial Isotactic Polybutene-1 Peiru Liu, Yanhu Xue, and Yongfeng Men Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b06444 • Publication Date (Web): 14 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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Industrial & Engineering Chemistry Research

Melt Memory Effect beyond Equilibrium Melting Point in Commercial Isotactic Polybutene-1 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, 130022 Changchun, P.R. China 2. University of Science and Technology of China, Hefei, 230026, P.R. China * E-mail: [email protected]

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract：Memory effect denotes acceleration of crystallization kinetics in polymers when being cooled down from the molten state. Using the method of differential scanning calorimetry (DSC), it was found that a commercial isotactic polybutene-1 (iPB-1) sample showed a unique melt memory effect. Even though the iPB-1 sample was molten at the temperature (Tms) higher than the equilibrium melting point of iPB-1 (133 oC), the memory effect was still observed, which affected the subsequent crystallization behavior. Briefly speaking, the total crystallization rate increased greatly with the decrease of Tms. Moreover, the lower the preparation temperature, the faster it can crystallize. However, the melt memory effect beyond the equilibrium melting point disappeared after purifying this commercial sample. The results reveal that the increase of crystallization rate is directly related to the increased density of iPB-1 nuclei originated from some additives remaining in the melt. Only if Tms is higher than 165 oC, much higher than 133 oC, the influence of additives vanished and the crystallization rate of iPB-1 reaches a constant value.

Key Words ： Isotactic Polybutene-1; Melt Memory Effect; Additives; Half-time of Crystallization


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Introduction Crystallized polymers have no sharp melting peaks but broad melting ranges that correlate with given thermal histories because there are crystalline regions with different crystalline structure perfection.1 Flexible linear macromolecules crystallized from the mobile random state will always form into chain folded conformation.2 Chain folding, directly influenced by crystallization temperature, can influence melting point in turn, but the melting points of chain folded even chain extended polymers are still lower than equilibrium melting points.3,4 The aggregation structure of the polymer sample is possible to affect the state of melt, and then has an effect on the following crystallization process if the fusion temperature is not high enough to completely erase its former structure. This effect in polymers caused by original aggregation structure and thermal history usually leads to different crystallization processes5–32 and change of the as-formed polymorphic modiﬁcations33–37, which is so-called melt memory effect. Many processing conditions, such as the temperature to melt samples (Tms),5– 7,11,13,22,23,25,28,32,38,39

melting time,5,6,8,11,13,16,32,40 heating rate,13,19,21 and shearing of

melt,26,40–42 are able to impact melt memory effect. Higher Tms and longer holding time at Tms are the most common conditions and ordinarily beneficial to minimize this effect, resulting in slower crystallization rate and larger spherulites.5,6,11,13,16,22,23,25,32 Alfonso et al.40 found that the clusters originated from the flow of melt could also survive in the melt for a quite long time. These clusters played a role of predetermined athermal nuclei when the melt was cooled down below melting temperature. It is generally thought that the degree of entanglement is one of the most important factors that cause melt memory effect. Yamazaki and Hikosaka15–18 investigated the effect of disentanglement on crystallization of PE. Their study showed that polymer chains with disentanglement were easier to generate nuclei since the nucleation rate increased with the decrease of the number density of entanglements. The disentangled chains take quite a long time to be adequately entangled again even though the annealing temperature is higher than the equilibrium melting point. However, a result



opposite from that of Yamazaki and Hisosaka was presented by Psarski et al.14 that nucleation density decreased when PE crystallized from the chain-disentangled melt. This phenomenon was ascribed to the consequence of desorption of chains from heterogeneous nuclei. Recently, some researchers43,44 found that the interaction between the substrate and polymer matrix could stabilize the crystalline layer up to a temperature higher than the original melting point. The substrate was able to play the role of an excellent nucleating agent even better than the polymer nuclei. Most kinds of memory effect that have been studied up to date only took place when the melt temperature was below or about the equilibrium melting point. In this study, a very strong memory effect of a commercial isotactic polybutylene-1 (iPB-1) is observed even though samples are heated up to a temperature much higher than the equilibrium melting point. By means of differential scanning calorimetry (DSC), the half-time of isothermal crystallization was obtained which reflected the total crystallization rate. It shows a remarkable melt memory effect that crystallization rate is greatly related to Tms, crystallization temperature and crystalline structure of prepared samples. But this melt memory effect could be removed by purifying the sample, namely, there’s no longer melt memory effect beyond the equilibrium melting point in the purified iPB-1 sample. Therefore, it has indicated that the memory effect here comes from additives rather than the inherent property of the PB sample.

Experimental section The investigated iPB-1 was a commercial sample produced by Lyondell Basell Industries with a trade name of PB0400M. The weight-average molecular weight (Mw) is 237 kg/mol as measured by gel permeation chromatography (GPC) and the melt ﬂow rate (MFR) is 16.4 g/10 min (190 oC /2.16 kg). The purified sample, with Mw of 248 kg/mol and polydispersity index of 1.1, was separated from PB0400M by means of solvent gradient fractionation (SGF) technique according to molecular weight.45 In order to minimize the difference of property between these two samples,


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the fractionated ingredient we choose is the one with Mw and isotacticity similar to the original values. DSC measurements were performed using a DSC1 Stare system (Mettler Toledo Instruments, Swiss) under a nitrogen atmosphere (50 mL/min). The DSC instrument had been calibrated using indium as a standard before all the measurements. Polarized optical microscopy (POM) measurements were conducted with a 10× objective lens (Zeiss Axio Imager A2m, Carl Zeiss, Germany) equipped with a heating device (THMS 600, Linkam, UK) to control the temperatures. Figure 1 represents the main thermal protocol used in this research. The heating and cooling rates were 10 K/min and 50 K/min, respectively. The protocol consisted of two parts, which were preparation and testing. All samples were heated up to 180 oC (much higher than the equilibrium melting temperature, 133 oC) and kept for 10 min to ensure complete erasure of the preceding thermal history. Then, the melts were quickly cooled down to a designed temperature which was denoted as Tp to conduct the isothermal crystallization process. The samples undergone isothermally treatment at Tp were melted again at a specified temperature (Tms) for 5 min. Then, a second isothermal crystallization process at Tc (85 oC) was conducted, the half-times of which were recorded. It should be noted that the time to reach the summit of the exothermic peak during isothermal crystallization process was regarded as the half-time in this research. FTIR spectroscopy measurements were carried out on a Thermo-Nicolet 6700 spectrometer. The spectra were collected at 4 cm-1 resolution and with accumulation of 16 scans. The completely crystallized sample was measured at room temperature.



200 160

Tms,5min 180℃ ,10min -50℃ /min 0

Tm

120

T/℃

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80

Tp Tc

10℃ /min

40 0

t / min

Figure 1. Schematic representation of the temperature program used in the experiments.

Results and Discussion

Figure 2. Half-times of isothermal crystallization at 85 oC as a function of Tms. The samples were melted at 180 oC for 10 min, and then isothermally crystallizing at Tp before heating up to Tms.

Figure 2 illustrates the influences of Tp and Tms on the half-times of crystallization at 85 oC. When Tms is higher than a certain temperature (165 oC), half-times are about the same value for each Tc, which indicates similar crystallization process. But below


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this temperature, half-times become shorter and varies with different Tp, which is an obvious melt memory effect. The equilibrium melting point of form Ⅱ crystals in iPB-1, extrapolated from melting line through SAXS experiments, is about 133 oC which is lower than Tms in this experiment.46 Melt memory effect usually disappears once the temperature is higher than the equilibrium melting point.29 However, Figure 2 showed an evident memory effect when the sample was isothermally crystallized at 85 oC from the melt of 160 oC, a temperature much higher than 133 oC. The memory effect can be completely erased and the system becomes homogeneous only if the Tms is higher than 165 oC. The half-time of crystallization became shorter with decreasing Tms, which changed from 170 s (from the homogeneous state) to 71 s (Tp = 75 oC, Tms = 150 oC). Moreover, the dependence of half-time on Tp indicates that the preparing condition of the sample before heating to Tms can also impact the final crystallization behavior. Samples prepared at lower Tp showed stronger memory effect in the low Tms range. Effect on Nucleation density As known, crystallization temperature greatly affects the morphology and lamellar thickness of semicrystalline polymers. IPB-1 exhibits four crystal modifications.47,48 In this study, only formⅡwas obtained at Tp since all the samples were heated up to a specific Tms right after completely crystallizing at Tp which is high enough to avoid form II-to-I transformation during the isothermal crystallization process.49 It was also verified by the subsequent heating process after crystallization at Tp since only an endothermic peak of form II appeared. The degree of crystallinity is measured by the endothermic peak of form II and is about 64 %. For the iPB-1 samples prepared at a Tp ranging from 75 to 100 oC, the lamellar thickness changed from about 17.6 nm at 75 oC to 23.7 nm at 95 oC with correlative radius of gyration (Rg) equal to 21.9 nm on the basis of calculation.46,50 With the increase of Tp, longer segments comparable to Rg are needed to pack into lamellae so that the chains tend to be extended and disentanglement tends to happen in order to form thicker lamellae. Yamazaki et al.15–18 had performed a series of experiments to study the relationship between disentanglement and crystallization. They concluded



that the nucleation rate increased with the increase of the initial lamellar thickness of prepared samples. According to their deduction, the crystallization of sample initially with thicker lamellae was supposed to be faster because of its decreasing number density of entanglement resulting in accelerated nucleation rate. However, as shown in Figure 2, the half-times of crystallization for iPB-1 melt pretreated at high Tp (100 oC)

and low Tp (75 oC) in the case of Tms = 150 oC and Tc = 85 oC, are 135 s and 71 s,

respectively. It indicates the sample prepared at higher Tp has thicker lamellae before melting at Tms but it crystallizes slower at Tc. This kind of unusual phenomenon that quite opposite to the general expected one was observed at the Tms ranging from 140 oC

to 160 oC. Therefore, the entanglement density is not the main factor that leads to

the acceleration of crystallization here. The similar effect of pre-crystallization temperature on the bulk crystallization time was also found in a commercial sPP9, which showed that lower pre-crystallization temperature led to shorter crystallization time and larger growth power law exponent. Both the density of nuclei and the crystallite thickness would be responsible in the case of sPP.

Figure 3. Selected POM images of iPB-1 samples prepared at 100 oC and 85 oC before melting (left) and then crystallized at 85 oC for 65 s after melting at various Tms (right).

The half-time results exhibited in Figure 2 were obtained from the exothermic peak of DSC curves during isothermal crystallization, which stand for the total crystallization rate. Since nucleation and growth are two crucial aspects which contribute to the total


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crystallization rate of polymers, it is necessary to figure out which is the direct factor affecting the memory effect and determining the total crystallization rate. POM micrographs in Figure 3 provided evidence that the primary nucleation process is markedly impacted by melt memory effect. The left part of Figure 3 displays POM images of samples completely crystallized at 100 oC and 85 oC. Nucleation density of sample crystallized at 85 oC is larger and the size of spherulites is much smaller. The sample that completely crystallized at Tp was heated up again to a designed temperature Tms and then quickly cooled to Tc (85 oC) to crystallize. The spherulitic morphologies of samples crystallizing for 65 s are shown in the right part of Figure 3. The nucleation density varied with different preconditions that contained preparing temperature Tp and melt temperature Tms. The number of spherulites in images (c) and (f) are basically equal, which are also the smallest among all images from (a) to (f). This indicates that 180 oC is high enough to make the system homogeneous, at which melt memory effect cannot exist any longer. But with lower Tms, as shown in (a) (b) (d) and (e), nucleation density is larger, especially in (e). The nucleation density changes with Tms and is inversely proportional to half-time of crystallization. Generally, the growth rate of spherulite is mainly related to the isothermal crystallization temperature.51 On the basis of the POM images, Figure 4 shows the crystallization time dependence of spherulite radius of samples with designed pre-conditions and crystallized at 85 oC. Table 1 presents the spherulite radial growth rates derived from Figure 4. The spherulitic growth rates of samples with different preconditions are essentially the same. Consequently, melt memory effect makes difference on the primary nucleation rather than growth of iPB-1 spherulites.



Figure 4. The spherulite radius of samples crystallized at 85 oC vs crystallization time with designed thermal histories. Table 1. The spherulite radial growth rate at 85 oC from Figure 4.

Purify the sample Through solvent gradient fractionation (SGF) technique ， the original commercial PB0400M sample was fractionated into a series of components according to molecular weight. The major component with a molecular weight of about 248 kg/mol was picked out to determine its temperature range of melt memory effect.45 FTIR is a useful technique to identify the additives remained in the commercial sample.52 However, the infrared spectrums of these two samples show no apparent differences (Figure S1 in supporting information document) so that the content of additives might be quite low and cannot be identified by FTIR. The purified sample was firstly melted at 180 oC for 10 min to erase memory effect and then isothermally crystallized at 80 oC as the standard sample. The standard sample was heated up to


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Tms holding for 5 min and cooled down at a rate of 10 oC/min and Figure 5 shows the DSC curves during cooling. It is clear that the prepared sample does not melt below 110 oC since there is no exothermic peak during cooling. For the sample melted in the range of 115 oC to 120 oC, crystallization happens at a higher temperature because of the self-seeding effect, since Tms here is lower than the equilibrium melting point. However, the curves superpose well with the one cooling from 180 oC once increasing Tms, which indicates the melt memory effect disappeared when heated up to about 125 oC.

Using the same thermal protocol for PB0400M, the purified sample was firstly

melted at 180 oC and prepared at different Tp. After melting at Tms and quickly cooling to 85 oC, the half-times of crystallization were measured by DSC and showed in figure 6. The samples melted at 120 oC crystallized much faster than the others, owing to self-seeding effect. Being consistent with the nonisothermal results, memory effect disappears as long as the temperature is higher than 125 oC. Furthermore, there is no relationship between Tp and half-time above the equilibrium melting point. Therefore, in the commercial samples, it cannot be the original property of iPB that leads to melt memory effect, in other words, those extra nuclei formed in samples with melt memory effect are one kind of heterogeneous nuclei which are caused by foreign substances. Foreign surfaces of these substances are able to absorb polymer chains and act as potential nuclei. The temperature to desorb chains may be rather high and eventually leads to melt memory effect even though the sample is heated above the equilibrium melting temperature of the crystals.43,53 This melt memory effect is confirmed to have rather good repeatability in this commercial iPB sample so that additives in the production formulation play an important role in this case. We noticed that in other cases of melt memory effect beyond the equilibrium melting point,

such

as

isotactic

polybutene37,

isotactic

polystyrene54

and

Trans-1,4-Polyisoprene55, the used sample were all commercial samples. Therefore, it should be cautious when one investigates memory effect of the commercial samples.



8 o

Tms/ C

7

135 130 125 120 115 110 105 100 180

6 5 4 3 2 1 0 60

80

100

120

o

140

T/ C Figure 5. The cooling curves of the purified samples from different melt temperatures Tms. The samples were firstly isothermally crystallized at 80 oC before being heated up to Tms.

50 40 30

t1/2/s

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20

o

Tp/ C 85 90 95

10 0 120

130

140

150 o

160

170

180

Tms ( C) Figure 6. Half-times of isothermal crystallization at 85 oC as a function of Tms. The samples were melted at 180 oC for 10 min, and then isothermally crystallizing at Tp before heating up to Tms.

Effect of Additives on crystallization Although the crystallization rate varies with different preconditions including Tp and


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Tms, the thermogram curves of heating after crystallization at Tc overlap well. Therefore, only nucleation density changes and lamellar thicknesses do not show any obvious difference. But as shown in figure 2, the sample prepared at low Tp always has a stronger melt memory effect than the one prepared at high Tp when the Tms is under 165 oC on the basis of DSC and POM results. Melt memory effect caused by additives is the result of heterogeneous nucleation whose mechanism is similar to that of secondary nucleation.56 The spherulites originating from heterogeneous nuclei are generally of the same size unless the surface activity of the foreign substances varies and thereby leads to a different initiation time of nucleation. Nucleation induced by additives, the same as secondary nucleation, is affected by crystallization temperature and its critical nucleation barrier is proportional to the reciprocal degree of supercooling so that is less sensitive to temperature than homogeneous nucleation. During the preparation process, the sample is firstly heated up to 180 oC and holding for 10 min to erase thermal history including full desorption of additives. Therefore, most the effect of additives on nucleation is removed, but the following crystallization at Tp makes a difference. The sorption effect of additives is an interaction between additives and chains in the melt and is also influenced by temperature as we noted above so that there is a different number of primary nuclei with different Tp. More chains were likely to be absorbed by additives with lower Tp and a larger number of additives with sorption of chains might remain in the melt at Tms, leading to stronger melt memory effect.



o

Tms( C) tms(min)

140 160 140 160

Heat Flow

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0

100

200 t (s)

5 5 180 180

300

Figure 7. DSC curves of samples during isothermal crystallization processes at 85 oC. The samples were firstly prepared at a crystallization temperature of 85 oC before being heated up to Tms and hold for tms.

However, it should be noticed that in low Tms region (140 oC to 155 oC), half-time decreases with the increase of Tms but it turns to increase if Tms is continually raised above 160 oC. The tendency in low Tms region seems opposite to the explanation of desorption from additives. POM images in Figure 3 illuminates that the direct reason for Tms dependence of memory effect is still the change of nucleation density. It indicates a changed number of effective heterogeneous nuclei. Those additives which absorbed polymer chains in the crystallized sample have interaction with crystalline PB matrix similar to other systems.43,44 A local crystalline structure on those additives can be stabilized because of the interaction even though the sample is heated up to a temperature over the equilibrium melting point. Those additives with the absorption of chains act as clusters in the melt. The observed increase of number of heterogeneous nuclei in low Tms region suggests that the effective number of clusters that can be developed into stable nuclei increases with Tms. In low Tms region, some small clusters are rather unstable and would be erased with the increase of Tms while some big clusters are able to remain active and even grow more stable, which makes


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the increase of the number of the effective clusters. However, these big clusters cannot always keep their stability with further increase of Tms and there is a turning point of Tms at which the number of effective clusters begins to decrease. The turning point stands for the decrease of effective clusters as well as desorption of chains from additives, resulting in a slower crystallization rate. To prove this idea of occurrence of pre-nuclei clusters, we examined the crystallization behavior as a function of holding time at Tms. Here a sample with Tp=85 oC has been used. After holding at Tms of 140 oC

and 160 oC for 5 min and 3 hours respectively, the sample underwent isothermal

crystallization processes at 85 oC. The change of the heat flow during isothermal crystallization was recorded in Figure 7. The results remarkably supported our above discussion of the development of the pre-nuclei clusters. It shows a slower evolution of the number of cluster at 140 oC than at 160 oC. Eventually, after 3 hours of holding time, both Tms showed very close effect on isothermal crystallization behavior. This result indicates that the observed seemingly counter-intuitive melt temperature dependency of the crystallization rate is due to incomplete formation of stable clusters at each specific melt temperature. The results shown in this work could also be relevant to mechanical property studies as they provide ideal samples with well-defined structural features. Indeed, we have investigated structural evolution in PB-1 samples of same lamellar thickness but very different spherulite size prepared using the memory effect reported in this work. 57

Conclusions A melt memory effect of a commercial iPB-1 has been investigated using DSC and POM in this work. Although isothermally crystallized samples were heated up to a Tms, much higher than equilibrium melting point, the melt was not homogeneous and would crystallize much faster than the homogeneous one. There existed a specific Tms at which memory effect ultimately disappeared. Moreover, the crystal modification was not affected by this effect.



After being purified by SGF, all the melt memory effect above the equilibrium melting point disappeared, which indicated that the effect might come from additives rather than its original property. The POM images of crystallized samples from different thermal processes gave concrete evidence that nucleation density was greatly increased but the growth rate did not change at all, in other words, only primary nucleation was influenced by the memory effect. In fact, the melt memory effect gave assistance to overcome the barrier of primary nucleation. Except for Tms, the Tp was also a rather evident factor that influences memory effect. The iPB-1 sample with lower preparation temperatures always showed faster crystallization rate when they were cooled down to Tc for isothermal crystallization. Lower Tp brought about more additives with sorption of polymer chains and thus leading to stronger memory effect.

Acknowledgments This work is supported by National Natural Science Foundation of China (51525305).

Support Information FTIR spectra of crystallized PB0400M and the purified sample.

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For TOC use only

Melt Memory Effect beyond Equilibrium Melting Point in Commercial Isotactic Polybutene-1 Peiru Liu, Yanhu Xue and Yongfeng Men*


Melt Memory Effect beyond the Equilibrium Melting Point in

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