Elsevier

Polymer

Volume 195, 8 May 2020, 122425
Polymer

Hierarchical structure of polybutene-1 in crystal blocks resulting from the form II to I solid-to-solid transition as revealed by small-angle X-ray scattering

https://doi.org/10.1016/j.polymer.2020.122425Get rights and content

Highlights

  • The semicrystalline morphology before and after the form II to I transition in PB-1 was studied using SAXS techniques.

  • Fine structures much smaller than lamellar long spacing were formed after form II to I transition in PB-1.

  • Lamellar thickness increases a bit after phase transition, but not to full extent of the elongation of the crystal lattice.

Abstract

Polybutene-1 (PB-1) exhibits excellent mechanical properties, while an unavoidable polymorphic transition from a metastable form II to the stable form I occurs after solidification from the molten state. This transition goes along with a deformation of the crystal lattice corresponding to a lateral shrinkage and normal elongation of about 20% and 12%, respectively. Here we study the semicrystalline morphology before and after the transition using small-angle X-ray scattering (SAXS) and differential scanning calorimeter (DSC). Scattering curves of samples with form II crystals could be well described by a model of layer-like stacked lamellar crystals. However, the scattering signal of form I samples showed additional scattering contributions at higher scattering vectors, indicating the generation of fine structures during the transition, much smaller than lamellar long spacing. The lamellar thickness increased less than the factor of 1.12, expected from the elongation of the lattice. The results indicate that the large lamellar crystals split up into blocks during the transition creating new interstitial spaces within the lamellae. This process is caused by the fact that the laterally extended lamella of form II cannot shrink as a whole due to the connectivity of the chains emanating from the crystals and due to possible overcrowding of the chain segments at the crystalline/amorphous interfaces.

Introduction

Polymers containing regular chemical constitution and linear architecture usually crystallize at lower temperatures and form a semicrystalline morphology consisting of alternating, nanoscopic crystalline and amorphous layers. Within the crystalline lamellae the chains assume a helical conformation and pack parallel to each other in a regular manner [1]. In many cases, the crystallites show polymorphism. For the different polymorphic states, the crystal structures differ in helical conformation and lateral packing. They can be either stable in different temperature ranges, or they show up as metastable states during crystallization. Ostwald's stage rule formulated that the transformation of an unstable state into a stable state prefers to pass through a metastable transition stage being closer to the original state, instead of reaching the most stable conformation directly [2]. The metastable states are likely to transform into more stable states and can exist only for a certain time because they are thermodynamically stable but not at the lowest free energy for a certain temperature and pressure [3]. Metastable states often occur in polymers with multiple polymorphs, as e.g. in isotactic polypropylene [[4], [5], [6], [7], [8]], syndiotactic polystyrene [[9], [10], [11], [12], [13], [14], [15], [16], [17]], poly(vinylidene fluoride) [[18], [19], [20], [21], [22], [23], [24]], or isotactic polybutene-1 (PB-1) [[25], [26], [27], [28]].

Industrial products of PB-1 in form I show very high resistance towards creep at high temperatures but the material is not widely-used in the industry or our daily life [29]. It passes through a metastable form II during the transformation from the melt into stable form I, because the nucleation barrier of form II crystals for crystallization from the melt is much lower than that of form I [29,30]. The crystal-to-crystal transition from form II to form I occurs spontaneously and irreversibly at room temperature, going along with deformation of the samples and significant improvement of physical properties [[31], [32], [33]]. It takes weeks to complete, thus the production cost is significantly increased due to the long-time storage. This transition is of special interest as it was suggested to be responsible for the excellent mechanical properties of PB-1 [34]. Chains in form II crystal display a fast intracrystalline relaxation process, the so-called αc-relaxation, which is absent in form I [34,35]. Polymer chains in both crystalline and amorphous phase are significantly immobilized by the form II to I transition [36]. Much attention has been paid to the acceleration of this phase transition [[37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49]] and the direct formation of form I [30,[50], [51], [52], [53], [54], [55], [56], [57]]. However, there is a relative dearth of research on changes of the semicrystalline morphology during the transition. Gohil et al. observed the morphology of thin PB-1 film using transmission electron microscopy, and found that metastable form II has a high degree of crystal perfection but form I crystals are full of defects [58]. The form II to I transition was suggested to be a solid-to-solid transition as the (110) crystal plane of the two forms were parallel and the helical hands were preserved during transition [[59], [60], [61]]. The chain conformation changes from a 11/3 helix in the tetragonal form II to a 3/1 helix in the hexagonal form I [25,32,62]. As detailed below, at the same time the lattice shrinks by about 20% laterally and elongates by about 12% in the c-direction. It is not known how the lamellar crystals can accommodate these relatively large changes, which should lead to larger changes of the shape of the crystals and also deformations of the connected amorphous layers. To follow the changes in semicrystalline morphology during the transition by small angle X-ray scattering (SAXS) is not straightforward, as the scattering signal for from II is very low at room temperature due to a small density contrast with the amorphous phase [62,63]. Recent studies on the other hand revealed that the SAXS signal is sufficiently strong around the typical crystallization temperatures of PB-1 [48]. By performing SAXS measurements at elevated temperatures we were therefore able to make a detailed comparison of the semicrystalline morphology before and after the transition from form II to form I using in addition a recently developed extended approach for the quantitative analysis of SAXS data.

As we will show, during crystallization in form II, the typical crystal-mobile morphology of a polymer with fast intracrystalline chain dynamics develops, which is characterized by a high crystallinity [34,64]. Furthermore, during the transformation to form I, an additional scattering signal in the region of large scattering vectors appears, which we interpret as indication for a small lateral structures caused by breakup of the original lamellar crystals. The presence of a blocky structure of the final form I crystals together with the high crystallinity resulting from the crystallization in the crystal-mobile form II, with subsequent transformation to the crystal-fixed form I [34] makes PB-1 a rather special polymer.

Section snippets

Sample

In this study, we used the commercial isotactic PB-1 sample PB0800 from LyondellBasell, whose weight-averaged molecular weight is 77 kg/mol and whose polydispersity (Mw/Mn) is 3.0. It has a melt flow rate (MFR) of 200 g/10 min (190 °C/2.16 kg). The PB-1 samples were melted at 180 °C to erase the thermal history and compressed into 0.5 mm slices. After about 10 min at 180 °C, they were transferred into water baths with different preset temperatures for isothermal crystallization of form II as

Theoretical estimation of structure change from form II to I in lamella dimension

The crystal structures of form II and form I of PB-1 were refined by Tashiro and coworkers [32]. The unit cell parameters measured at room temperature and the physical properties of the two crystalline phases are listed in Table 1 [32]. One can calculate the deformation of the unit cell induced by the transition from form II to form I from the different chain packing modes [73]. In lateral direction of lamella, the area occupied by one unit cell in ab-plane can be obtained:Sab=a0b0sinγ

The

Conclusions

In this work, we performed a direct comparison of the semicrystalline morphology in Polybutene-1 before and after the transformation from form II to form I by a combination of SAXS and DSC experiments. The detailed analysis of the structural parameters obtained from SAXS confirms that the large lamellar thickness of PB1 in form I is a consequence of crystallization in the crystal-mobile form II and the subsequent transformation. The increase in crystal thickness during the transition is smaller

Notes

The authors declare no competing financial interests.

CRediT authorship contribution statement

Yongna Qiao: Conceptualization, Validation, Investigation, Data curation, Writing - original draft. Martha Schulz: Conceptualization, Validation, Investigation, Data curation, Writing - original draft. Hai Wang: Formal analysis, Investigation. Ran Chen: Formal analysis, Investigation. Mareen Schäfer: Data curation, Formal analysis, Investigation. Thomas Thurn-Albrecht: Conceptualization, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition. Yongfeng

Declaration of competing interest

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.

Acknowledgement

This work is supported by the National Natural Science Foundation of China (51525305). We thank Dr. Jinyou Lin for his supports during SAXS measurements at beamline BL16B1, Shanghai Synchrotron Radiation Facilities (SSRF). The work is further funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – project number 189853844 – SFB TRR 102.

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