Crystallization, Recrystallization, and Melting Lines in Syndiotactic

Oct 24, 2014 - Well-defined crystallization lines where the reciprocal lamellar thickness is linearly dependent on crystallization temperature were ob...
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Crystallization, Recrystallization, and Melting Lines in Syndiotactic Polypropylene Crystallized from Quiescent Melt and Semicrystalline State Due to Stress-Induced Localized Melting and Recrystallization Ying Lu, Yaotao Wang, Lianlian Fu, Zhiyong Jiang, and Yongfeng Men* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, University of Chinese Academy of Sciences, Renmin Street 5625, 130022 Changchun, P. R. China S Supporting Information *

ABSTRACT: Crystalline lamellar thickness in syndiotactic polypropylene (sPP) during crystallization from either isothermal molten or stretching induced localized melt states and during subsequent heating was investigated by means of temperature dependent small-angle X-ray scattering techniques. Well-defined crystallization lines where the reciprocal lamellar thickness is linearly dependent on crystallization temperature were observed. Unlike in the case of polybutene-1 where stretching crystallization line was shifted to direction of much smaller lamellar thickness (Macromolecules 2013, 46, 7874), the stretching induced crystallization line for sPP deviates from its corresponding isothermal crystallization line only slightly. Such phenomenon could be attributed to the fact that both crystallization processes from quiescent melt and stress induced localized melt are mediated in a mesomorphic phase in sPP. Subsequent heating of sPP after crystallization revealed the same melting behavior in both systems for the two kinds of crystallites obtained from either quiescent melt or stretching induced localized melt. Both of them underwent melting and recrystallization when the lamellar thickness was smaller than a critical value and melting directly without changing in thickness when the lamellar thickness was larger than the critical value. The melting behavior in sPP systems can be understood by considering the chain relaxation ability within crystalline phase and also can be used as evidence that the crystallization from molten state and stress-induced crystallization passed through the intermediate phase before forming crystallites. crystallization and final melting were termed as crystallization line and melting line. An equilibrium crystallization temper−1 ature T∞ = 0 was located many degrees above the mc at dc equilibrium melting temperature T∞ f . Therefore, a concept of different pathways followed by the crystallization and melting processes was suggested. The findings were interpreted as an indication that an intermediate phase of mesomorphic character participated in the formation of crystallites.5 Actually, the experiments carried out on polyethylene (PE) at an elevated pressure by Keller and his co-workers have provided the hint of the existence of such mesomoephic layer:9 a crystal formation was observed via a transient mesomorphic “hexagonal phase”. The multistage crystallization model proposed by Strobl is described as follows.10 The initial step is always the creation of a mesomorphic layer that spontaneously thickens, up to a critical value, where the core region solidifies under formation of blocklike crystallites (native crystallite). The second step is a structural relaxation process transferring native crystallites into the final lamellar form at a constant dc.11

1. INTRODUCTION Crystallization in polymer systems transferring the entangled melt into a semicrystalline state is a process of primary importance and has been studied for a long time. Conventional wisdom assumes that the layer-like crystallites grow in lateral direction by a direct attachment of chain sequences from the melt onto the growth face. For many years there were no experimental observations that would speak in opposition to this natural view. According to this view, Hoffman and Lauritzen1 and Salder2 respectively designed theories that were considered as a fundamental law that the crystallization should be controlled by the equilibrium melting temperature 3 T∞ f deduced from Gibbs−Thomsom equation, and the crystal thickness is related to the supercooling below this temperature. However, on the basis of many investigations implemented in semicrystalline polymers4−8 by Strobl’s group, a direct check of this relationship via time- and temperature-dependent smallangle X-ray scattering (SAXS) measurements did not confirm this law. These experiments provided a detailed, accurate view that both the crystallization temperature Tc and the melting temperature Tm were linearly dependent on the reciprocal crystalline layer thickness dc−1. These linear dependencies of d c −1 as a function of temperature during isothermal © 2014 American Chemical Society

Received: September 16, 2014 Revised: October 15, 2014 Published: October 24, 2014 13019

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2. EXPERIMENTAL SECTION sPP samples were purchased from Aldrich Polymer Products, with a molecular weight Mw = 174 000 g/mol, a polydispersity (Mw/Mn) of 2.3, and 93% syndiotactic regularity. The pellets were molded in a hot press at 180 °C, developing films of 0.5 mm in thickness, which were slowly cooled to room temperature (named as sPP-air) or rapidly transferred into a vacuum oven for isothermal crystallization at selected crystallization temperatures for 12 h. DSC measurements were carried out with a DSC1 Stare system (Mettler Toledo Swiss) under N2 atmosphere with a heating rate of 10 K/min. The isothermally crystallized samples were heated immediately after the completion of crystallization at the settled temperatures in DSC. The measurements for stretched samples were scanned from 25 to 200 °C. According to the literature, the ideal value of heat of fusion for 100% crystallinity of ΔHid = 183 J g−1 for sPP18 was chosen to calculate the weight crystallinity ϕw. SAXS experiments were conducted with a modified Xeuss system of Xenocs, France, at a sample-to-detector distance of 1063 mm. A multilayer focused Cu Kα X-ray source (GeniX3D Cu ULD, Xenocs SA, France, λ = 0.154 nm) and scatterless collimating slits were used during the experiments. SAXS images were recorded with a Pilatus 100 K detector of Dectris, Swiss. Samples were stretched to reach an engineer strain of over 250% at a constant crosshead speed of 20 μm/s at different deformation temperatures via a portable tensile testing machine (TST350, Linkam, U.K.) and then kept in tension during SAXS measurements. The isothermally crystallized samples were measured at the crystallization temperature to eliminate the effect of secondary crystallization occurring during cooling. In situ SAXS measurements were performed during heating of the isothermally crystallized and stressinduced crystallized samples from their isothermal crystallization or stretching temperatures to the molten state at a heating rate of 0.03 K/min. Each SAXS pattern was collected within 30 min which was then background corrected and normalized using the standard procedure. The one-dimensional scattering intensity distributions were first integrated within ±10° along stretching direction of 2D SAXS patterns, and then they were Lorentz-corrected by multiplying the intensity data by q2. The correlation function K(z) can be directly obtained by applying the inverse Fourier transformation,

Moreover, the recrystallization mechanism during a heating process of semicrystalline polymers is similar to the multistage route found upon initial crystallization of bulk polymers from an entangled melt, with the evidence of the crystallization and recrystallization lines extrapolating for dc−1 = 0 to the same 10,12 limiting temperature of T∞ mc and differing only in slope. Nevertheless, the recrystallization phenomenon during the heating process could not be observed for all samples. In general, the extent of recrystallization is determined by the following two factors:10 the stability of the initial crystallites developed at the crystallization temperature and the time scale of the experiments which should be comparable to the time required by the recrystallization process. Basically, the former impact is related to a special point in the dc‑1 vs T phase diagram of crystallization. This special point is defined as Xs and is the intersection point of the melting line and the recrystallization line at a certain temperature and a certain value of dc−1.13 When the initial lamellar thickness is larger than the thickness where Xs located, no recrystallization occurs and the sample just melts, whereas for an initial thickness below the critical value where Xs is located one always observes recrystallization before melting. Meanwhile, a slow heating rate prefers a melting process with the occurrence of recrystallization; however, a fast heating can suppress this behavior.14 Recently, we investigated stretching temperature dependency of the final structural parameters in polybutene-1 (PB-1) samples.15 As was found, the long spacing as well as the lamellar thickness of the samples after stretching relied only on the stretching temperature Ts independent of the original structure,16,17 and a linear relationship between Ts and dc−1 was obtained. Moreover, the stress-induced crystallization line and the isothermal crystallization line shared a similar slope but with very different limiting temperatures. An approach for understanding such case can be primarily associated with the initial melt states in the two cases. During stretching, localized stress induced melting occurred at positions of maximum shear stress followed by a recrystallization of those freed oriented polymeric chain segments. These localized oriented molten chain segments were obviously different from the random melts. Indeed, the disentangled melt presents an extremely high crystallization rate4,5 and possibly leads to a direct growth into the crystal phase.10 As a consequence, it was assumed that the chain sequences directly attached to the crystal phase from local ordered melt without passing a mesomorphic layer in the stress-induced crystallization process of PB-1.15 In this work, we investigated the crystallization and melting behaviors of syndiotactic polypropylene (sPP) from both quiescent molten state and localized molten state induced by stretching its semicrystalline structure by means of temperature-dependent SAXS and differential scanning calorimetry (DSC) techniques. As it turned out, the findings were in line with the view that the crystallization in bulk polymer from molten state passes through a route that includes a passage via a mesomorphic phase.5 However, unlike in the case of PB-1 stress-induced melting and recrystallization, in sPP it was found to proceed also with the participation of the mesomorphic phase. In addition, sPP systems crystallized from different routes show the same melting behavior. The results can be attributed to the chains mobility in the crystalline phase, which affects stabilization process during crystallization of the polymer.

∞

K (z ) =

∫0 I(q) q2 cos(qz) dq ∞

∫0 I(q) q2 dq

(1)

which was used for computing the thicknesses of crystalline lamellae (dc), amorphous layers (da), and the long period (dac) of the samples composed of two phase layer-like systems.19 These parameters were obtained in the way shown in the inset of Figure 2. The directly derived value in the correlation function is related to the dc when the sample possesses a crystallinity lower than 50% or to the da when its crystallinity is higher than 50%. The initial crystalline form of sPP was in form I. Only a tiny part of form I transformed to form III during stretching process at high temperatures.20

3. RESULTS AND DISCUSSION The DSC results of isothermally crystallized samples and stretched ones scanned in different ways are depicted in Figure 13020

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immediately followed by an exothermal peak, located at 79 °C because of a transition into helical form.21 This low temperature peak does not affect structural characterization during stretching, as it appears only after unloading and being cooled to room temperature before DSC measurement. Figure 2 displays the one-dimensional correlation functions of isothermally crystallized and stretched samples for sPP. The

1. For the isothermal crystallized sPP samples, melting just began at the crystallization temperature, revealing the existence

Figure 1. DSC melting curves of sPP measured after isothermal crystallization (top) and stretched (bottom) at the indicated temperatures in the plots. Samples slow-cooled in the air from the melt were used for stretching (undeformed melting curves are presented in the inset, heating rate of 10 K/min).

Figure 2. One-dimensional correlation function curves of isothermally crystallized sPP (top) and of sPP-air (bottom) after deformation at different temperatures. The inset presents the method of determining the thicknesses of crystalline lamellae (dc), amorphous layers (da), the long spacing (dac). (Stretching direction is horizontal.)

of crystallites in different stability.5 With an increase of the crystallization temperature, the main melting peak shifted to high temperature. Clearly, an increase in the crystallization temperature promotes the increase in the stability of the crystallites.5 On the other hand, for the samples crystallized at lower temperatures, i.e., 85 and 90 °C, the thermogram presented two peaks. The one at higher temperature associated with the final melting was constantly located at 130 °C, which was apparently due to a recrystallization process after first complete melting.4,5 For the stretched samples, their melting curves demonstrated the analogous phenomena as the isothermal samples, such as a higher deformation temperature resulting in the melting peak moving to a higher temperature. In addition, the final melting peak occurred in the samples stretched below 86 °C and remained unchanged at around 130 °C indicating a similar melting and recrystallization process during heating. Specially, the melting peak at 55 °C presented in DSC curve of sPP-air stretched at 70 °C was assigned to the melting of mesophase formed upon relaxing the stretched oriented sample from the metastable trans-planar form III.21 This small part of disordering trans-planar mesophase was

corresponding SAXS curves with Lorentz correction are provided in Figure S1 in Supporting Information. Detail information about the long period dac, the lamellar thickness dc, and the amorphous thickness da deduced from the correlation functions, linear crystallinity ϕl by using ϕl = dc/dac, and weight crystallinity derived from DSC is exhibited in Figure S2. In a short summary, the dc and dac gradually increase with elevating crystallization temperature; such general properties have been often observed and reported.16,17,22 As it appeared, the weight crystallinity of each sPP sample was below 50%. According to the results, the crystallization temperature Tc or the stretching temperature Ts versus inverse crystalline thicknesses dc−1 of sPP is sketched in Figure 3, and the related findings of PB-115 are also included in the figure. Clearly, the results for sPP are very different from that of PB-1. In PB-1, an obvious separation of the two crystallization lines with stretching induced crystallization line moving to much smaller lamellar thickness direction is observed, which has been attributed to a direct 13021

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of existence of mesomorphic phase in sPP during stretching have been reported extensively.21,24,25 The observed slight increase in the limiting temperature for stretching induced crystallization in sPP can be understood as a result of the influence of global orientation of polymer chains on the thermal dynamic parameters of the mesomorphic phase. In order to further verify the property of the crystallites obtained from both quiescent melt via isothermal crystallization and localized oriented melt via isothermal stretching, in situ SAXS measurements were conducted during heating of the corresponding samples. The one-dimensional SAXS intensity distribution profiles and correlation functions for such melting processes are given in Figures S3 and S4. Consequently, changes in lamellar thickness during subsequent heating process after isothermal crystallization and stretching for sPP samples are shown in Figure 4. Clearly, samples obtained from Figure 3. P1B and sPP: Relationships between the inverse crystalline lamellar thickness dc−1 and the crystallization temperature Tc for isothermally crystallized samples from the random molten state, and the stretching temperature Ts for the samples from the stress-induced melting and recrystallization.

arrangement of localized molten chain segments into crystallites without passing through mesomorphic phase in the case of stress induced crystallization. In the sPP system, the two crystallization lines nearly overlap with each other. The results indicate that stretching induced crystallization in sPP may take a different route compared to PB-1. In the case of isothermal crystallization from quiescent melt state, crystallization proceeds via passing through a mesomorphic phase. The crystallization line can be described as follows:13 ∞ Tmc −T≈

∞ (2σacn − 2σam)Tmc Δz dc ΔHcm

Figure 4. sPP: Relationships between the dc−1 and the Tc, and the Ts (crystallization lines) and Tm (melting line). The variations of dc−1 during the heating process of isothermally crystallized samples and stretched ones are all presented.

(2)

where ΔHcm is the heat of transition from mesomorphic phase to a crystalline phase, Δz is the stem length increment per structural unit, and σacn and σam denote the surface free energy of the native crystal layer and the mesomorphic layer, respectively. However, if the localized crystallization process from a stretching induced melt state proceeds via a direct route to crystalline phase without passing though the mesomorphic phase, a reduction in lamellar thickness is expected.15 In such a case, the equation of crystallization line is established for15 Tc∞

−T≈

both isothermal crystallization and stretching induced crystallization present same melting behavior. sPP samples obtained at low Tc or Ts showed a continuous thickening in crystallite thickness and eventually melted at 129 °C during heating. For the samples obtained at higher Tc or Ts, the lamellar thickness kept constant until melting. The above mentioned same melting behavior of both isothermally crystallized and stretching induced sPP samples should have its origin back to the crystallization process. Indeed, the crystallization line denotes a transition from mesomorphic phase to layer-like lamellar crystalline blocks of certain thickness depending on the actual temperature. This transition produces native crystallites of high surface energy. Upon stabilization via relaxation, the native crystallites might be finally transferred into stabilized crystals at a constant thickness.23 The stabilization process requires a high internal mobility of polymeric chain segments within crystallites. Clearly, such mobility is absent in sPP but exists in PB-1 with form II as was evidenced by nuclear magnetic resonance experiments that a sliding diffusion-like motion of PB-1 chains within crystalline region was found.26−28 Such absence of chains mobility is fully consistent with the absence of a mechanical α-relaxation due to chain sliding in sPP.29−31 Thus, in the case of sPP crystallized from the molten state, crystallites after isothermal crystallization may only be

2σacnTc∞ Δz ΔHca

dc

(3)

where ΔHca denotes the heat of transition from amorphous phase to the crystalline one. As was proven experimentally,23 the increase in the surface free energy term at the numerator is much smaller than the increase in ΔHca at the denominator of the equation, leading to a strong decrease in dc. Therefore, eqs 2 and 3 naturally explain the shift in crystallization lines in the case of PB-1 but not in the cases in sPP. The results for sPP strongly indicate that both crystallizations from quiescent isothermal molten state and from stretching induced local molten state proceed in the same manner via a mesomorphic phase. This is assigned to the fact that all thermodynamic parameters included in eq 2 that govern the crystalline lamellar thickness are related mostly to the mesomorphic and crystalline phase only independent of the state of amorphous phase. Hints 13022

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Notes

stabilized to a certain extent because of the absence of a chain relaxation process within crystallites. Although the exact molecular mechanism of the presence of such chain mobility within crystalline phase in PB-1 or without in sPP is still elusive, experimental evidence for the different relaxation behaviors in both systems is conclusive.26−31 Most probably it can originate from the packing mode of the chains in the crystalline phase, as in form II of PB-1 chains are rather loosely packed leaving enough freedom for such sliding diffusion motion to carry out, whereas chains in sPP crystallites are rather dense with interlocked structures. In the case of stretching induced structure, as the crystallization occurred within a localized space out of stress induced molten chain segments, such stabilization process cannot take place, yielding in the end mostly native crystallites in the system. Therefore, both crystallites generated isothermally from the molten state and from the stretching induced state end up with native crystallites of similar stability in sPP due to the lack of chain relaxation ability. As a consequence, they behave exactly the same with respect to the recrystallization and melting processes during heating. Evidently, it also can be assumed that the evolution of crystallites in sPP via the two crystallization methods passed the same route.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant 21134006).

4. CONCLUSIONS In summary, we have investigated crystallization, recrystallization, and melting lines of syndiotactic polypropylene from both quiescent and stress induced localized molten states. Unlike in the case of polybutene-1 where a significant shift toward much thinner lamellae of the crystallization line for the stress induced one was observed, the crystallization line for sPP due to stretching of the semicrystalline sample was close to its corresponding crystallization line from quiescent melts. The result indicates that in both cases crystallization of sPP proceeded via passing through a mesomorphic phase. Furthermore, subsequent heating of the systems revealed the same melting behaviors in sPP systems. This result has been attributed to the fact that crystalline lamellae in isothermal crystallized samples have commonly been further stabilized via a chain relaxation process in the crystalline phase. The ones crystallized from stress induced localized melt are much constrained and therefore end up at much reduced stability. In sPP, such chain relaxation process within crystalline phase is completely absent, and one therefore observed the same melting behavior for both systems isothermally crystallized from quiescent melt and stress induced crystallization from localized melt.

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ASSOCIATED CONTENT

S Supporting Information *

The scattering SAXS curves with Lorentz correction and onedimensional correlation functions of isothermal crystallization samples, of stretched samples, and of selected samples during heating process, the linear crystallinites ϕl and weight crystallinites ϕw of isothermally crystallized and stress-induced crystallized samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 13023

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