Shear-induced crystallization of unimodal/bimodal polyethylene at high temperatures affected by C4 short-branching

Highlights

• C4 short-chain branched unimodal and bimodal polyethylenes were sheared at high temperatures.

• C4 short-chain branching promoted formation of shish crystals during shearing.

• High content C4 short-chain branching impeded the crystallization ability after shearing.

• Formation of shish crystals caused by coil-stretch transition mechanism.

Abstract

Generation of shish-kebab crystals of unimodal and bimodal polyethylenes (PEs) with different C4 short-branching sheared at high overmelt degree (the increment higher than the crystallization temperature) and a low shearing rate was investigated by using in-situ SAXS/WAXD techniques. Both PEs exhibiting SAXS streaks during the shearing indicate the shish crystals generated by shearing. The WAXD analysis indicates an orientation degree as high as 0.75–0.95 for both PEs. During the succeeding isothermal crystallization at 121 °C, kebab crystals and partially oriented stacked crystals in lamellae form, which elevate the crystallinity index but decrease the orientation degree. The shish crystal content of bimodal PE is higher than that of unimodal PE because the high molecular weight (MW) part facilitates the generation of shish crystals. The increase of the branching length is more favorable for the generation of shish crystals (comparing with C2). However, the high content of C4 short-chain branching makes shear-induced crystals less ordered. The generation mechanism of both PEs' shish crystals at high shearing temperatures can be ascribed to the coil-stretch transition (CST) mechanism.

Introduction

The melt flow of industrial polymer during the molding process may promote crystallization of polymer chains and induce oriented stacking of crystals during cooling, and in some cases, the flow may induce the generation of the shish-kebab crystals [[1], [2], [3], [4], [5]]. The materials with a shish-kebab structure usually have excellent mechanical properties [[6], [7], [8], [9]]. Thus, flow-induced crystallization, particularly the generation mechanism of shish-kebab crystals, has engaged tremendous research to achieve optimal performance over the last several decades [[10], [11], [12], [13], [14], [15], [16], [17], [18], [19]].

Dating back to the 1960s, Mitsuhashi [19] and Blackadder and Schleinitz separately [20] discovered a fibrous crystal structure in a dilute polyethylene solution. Later in 1965, Pennings and Kiel [21] isolated the shish-kebab crystals from the vigorously stirred polymer solution and further studied its hierarchical structure with an electron microscope. Keller [22] proposed that the fully-extended chains first composed the shish-like crystals and subsequently served as a supporter for the lateral growth of the folded-chain lamellar crystals (so-called kebabs) in solutions. De Gennes [23] initiated the coil-stretch transition (CST) model to depict the generation mechanism of shish-like crystals in the shearing flow of a dilute polymer solution. As a milestone, Keller and Kolnaar [24] and Mackley [25] provided direct evidence for the CST model under extensional flow in a dilute polymer solution. Keller et al. [26] found the analogical shish-kebab morphologies in the polymer melt and extended the CST concept to the polymer melt. Similar morphologies have been observed in other polymer melts under extensional flow [[27], [28], [29]]. Hsiao et al. [30] researched the shish-kebab crystals using a system of 2 wt% of ultrahigh molecular weight polyethylene (UHMWPE) and 98 wt% non-crystallized polyethylene (PE) matrix and discovered that a small number of chains with high molecular weight (MW) could significantly contribute to the generation of shish-kebab crystals, which is in line with the CST model. In addition, a small number of long chains was found to contribute to the generation of shish-kebab crystals in the polymer melt under the flow in different systems, which indirectly verifies the CST in the polymer melt [[31], [32], [33], [34], [35], [36]].

It can be inferred that the entanglements are playing a crucial role in the generation of the shish-kebab crystals in the polymer melt system. Hsiao et al. [31] thought that chains could be pulled out from the entangled polymer melt with low entanglement by melt flow, but only the chains with an MW larger than a critical value could compose shish. The shish-kebab crystals can form only when two conditions are satisfied based on the CST model. The first one is the mechanical work of polymer undertaking higher than a threshold. The other is the applied shearing rate higher than the inverse Rouse time of the polymer melt's longest chains [37,38].

Recent studies on entangled or crosslinked networks of polymer melt have led more researchers to trust that the primary nucleus of shishes stems from the stretched network of the polymer melt. As early as 1984, Pennings et al. [39] proposed that a stretched network should regulate the shish generation. Han et al. [[40], [41], [42]] studied the shear-induced crystallization of isotactic polypropylene (iPP) at a low shear rate and ascribed it to the stretched network. At low shear rates and temperatures, Han et al. [[43], [44], [45]] advanced the shear-induced crystallization further to different systems and stated that shish was still the resultant of oriented networks under shear. They also studied the shish generation mechanism under shearing flow using an in-situ study and proposed that the point-like nuclei were based firstly on the entanglement network's degeneration and secondly on the shish growth under shearing flow [46]. Li et al. [47] investigated the shish generation mechanism using a system of high-density polyethylene (HDPE) melt by using a combination of in-situ extensional rheological and small-angle X-ray scattering (SAXS) techniques and stated that a critical strain to induce the shish and kebab in polymer melt stemmed from the stretched network. Later, Li et al. [48] found the most convincing evidence for the stretched network model, confirming shish-kebab crystals generation in a permanently crosslinked HDPE system, revealing that the critical nucleus thickness led to a natural transition from lamellar to shish nuclei [49]. They further verified that the shish generation derived from the stretched network was a kinetic process from the initial chain conformation to final stable nuclei [50]. In summary, scientists have done much research and proposed two different notions on the formation of a shish crystal, but the formation mechanism is still an open question.

In addition to the global description of the CST or stretching network, the polymer chains' characteristics (chemical composition, MW, and molecular weight distribution (MWD)) should be considered. Many researchers have investigated the long-chain branching affecting on the relaxation time of stretched network and shish-kebab generation in the past time. Seki et al. [51] found that a long-chain branching lengthened the relaxation time, distinctly quickened the crystallization rate, and boosted the generation of shish-kebab crystals. Agarwal et al. [52] found that the long-chain branching widened the relaxation time of high MW components for branched iPP compared with linear iPP, resulting in higher orientation and changing crystallization kinetics more than a magnitude. Heeley et al. [53] also discovered that the long-chain branching in hydrogenated polybutadiene blended with high MW components facilitated the crystallization rate compared with those under quiescent circumstances. Fang et al. [54] investigated the long-chain branching on polylactide (PLA) crystallization under shearing by using polarized optical microscopy (POM), and stated that the long-chain branching increased the crystallization kinetics compared with the linear chains. Meantime, the nucleation density of the long-chain branched PLA was larger than that in the case of linear chains, which could be attributed to the widened and complex relaxation manners induced by long-chain branching. Kitade et al. [55] distinguished the shear-induced crystallization of long-chain hyper-branched PP and linear PP in the pre-crystallization region using SAXS and wide-angle X-ray diffraction (WAXD) methods, and their results indicated that the highly oriented shish-kebab crystals formed in long-chain hyper-branched PP preferentially because of the longer relaxation time and the point-like precursors boosted by the branching points. In summary, scientists have done much research about the long-chain branching affecting on the shish-kebab generation in the past time. However, the influence of the short-chain branching on the formation of shish-kebab crystals has seldom been reported.

In a recent study, by selecting the bimodal PEs with different contents of C2 short-chain branching, we found that the low-content short-chain branching presenting in the system favored the generation of shish-kebab crystals at shearing temperatures higher than the melting point and low shearing rates of 1 and 3 s⁻¹, which supported the CST mechanism [56,57]. In our more recent study, the same bimodal PE system was used to investigate the generation of shear-induced shish-kebab crystals at low shear temperatures [58,59]. The results illustrated that shish-kebab crystals also formed at the shear temperatures of 129, 127, and 124 °C and the low shear rate of 1 and 3 s⁻¹, which supported the stretched network mechanism. Therefore, it is acceptable that the shish generation in a mechanism of the CST or stretched network may be determined by the temperature of flow fields in the dedicated systems. The above studies clearly demonstrated the influence of C2 short-chain branching on the formation of the shish crystal. However, there is no report on the system with short-chain branching longer than C2 that would not only impede the crystallization but also increase the relaxation time, which is the subject of this study. Herein, to further confirm the shish crystals generation mechanism, two different PEs with different short-chain branching C4 contents and different MWD were studied with in-situ SAXS and WAXD techniques, during which the generation of shear-induced crystals was investigated at different overmelts (the increment higher than the crystallization temperature) of 22.5, 19.5 °C and isothermal crystallization temperature of 121 °C. Shish-kebab crystals and partially oriented lamellar crystals, which may be affected by the high C4 short-chain branching content, were observed in all samples. Long shear duration of 100 s and a low shear rate of 3 s⁻¹ were applied to investigate the influence of relaxation and stability for extended chains on shish-kebab crystals.