New insights into thermal conductivity of uniaxially stretched high density polyethylene films

Highlights

• The higher stretch ratio contributes the higher thermal conductivity.

• Stretching can induce the formation of interlocking shish-kebab structure.

• Our work helps understand the process-structure-property relationship.

Abstract

Thermal conductivity of drawn polyethylene (PE) fibers has been extensively studied, while thermal conductivity for the stretched PE sheets still remains unknown. In addition, no consensus conclusions have been reached on the thermal conduction mechanisms for drawn PE. In this work, melt processed PE sheets are subjected to uniaxial stretching under different stretch ratios. The in-plane and out-of-plane thermal conductivity of stretched PE has been measured and the crystalline structure has been analyzed through 2-dimensional wide-angle X-ray diffraction (2D-WAXD) and scanning electron microscopy (SEM). Results show that both in-plane and out-of-plane thermal conductivities of stretched PE increase with increasing stretch ratios. The enhancement of thermal conductivity has been ascribed to the formation of interlocking shish-kebab superstructures induced by the stretching process. A new model is proposed to demonstrate the thermal conductivity improvement seen for the uniaxial-stretched PE. To the best of our knowledge, it is the first time that the mechanisms of thermal conductive enhancement were revealed for uniaxially stretched semi-crystalline PE sheet along with the understanding of the associated process-structure-property relationship. This work may provide key information towards design and manufacturing of polymer-based materials for thermal management applications.

Introduction

Thermally conductive yet electrically insulating polymer materials are of great interest for various applications such as heat exchangers [1], circuit boards in power electronics, and other microelectronic appliances [2] due to their attractive multi-functionalities in addition to other advantages such as lightweight, corrosion resistance and ease of large scale processing [3]. Unfortunately, bulk polymers normally have very low thermal conductivity, typically in the order of 0.1 W m⁻¹ K⁻¹ [4]. The enhancement of polymer thermal conductivity has been mostly achieved through incorporation of thermally conductive fillers [[5], [6], [7], [8], [9]], to form a composite. However, most thermally conductive polymer composites contain high weight loading (>50 w.t. %) of fillers. This would inevitably lead to complexity in manufacturing and increased cost. The high content fillers are likely to agglomerate, which will not only compromise the thermal conduction performance but also severely deteriorated the mechanical properties of the composites [7,8]. Despite the addition of highly thermally conductive filler materials, the best thermal conductivity achieved by polymer composites is still in the range of 1–10 W m⁻¹ K⁻¹ [7,8], this is because the polymer matrix acts as a thermal barrier that limits the thermal transport efficiency of the composites. The conflicting requirements for high thermal conductivity and good mechanical properties make the design and manufacturing of thermally conductive polymer-based materials a long standing challenge. It is therefore of great interest to explore the approaches to enhance the thermal conductivity of neat polymers, as this would offer an effective solution to the existing challenges.

The low thermal conductivity of polymer is usually caused by the scattering of phonons due to the entangled molecular chains, presence of grain boundaries and defects etc [3,10]. Even for polymers with the highest degree of crystallinity, there is ∼40% of amorphous phase present in between the crystalline lamellae. Although single crystalline lamella possesses very high thermal conductivity (>100 W m⁻¹ K⁻¹) [10,11], the thermal transportation process is inevitably interrupted by the interfaces that present between the crystalline and amorphous phases, and crystallizable polymers normally exhibit thermal conductivity similar to that of its amorphous phases.

It is well accepted that thermal conductivity of polymer can be enhanced by improving the crystallites alignment and macromolecular chain orientation. Recent reports showed that ultra-drawn highly crystalline polyethylene fibres exhibit metal-like thermal conductivity (>100 W m⁻¹ K⁻¹), due to the formation of an ‘ideal’ single crystalline fibre [11]. A high thermal conductivity (up to 4.4 W m⁻¹ K⁻¹) for amorphous polythiophene nanofibers has also been achieved with molecular chain orientation along the fibre axis [12]. Moreover, a number of molecular dynamics simulation studies suggest that a single extended polymer chain could achieve thermal conductivity as high as ∼350 W m⁻¹ K⁻¹ [13,14]. Although past experimental and simulation results show that very high thermal conductivity can be achieved for neat polymers, the effort has been mostly focused on the ultra-drawn polymeric fibres, which have limited practical applications. Furthermore, no consensus conclusions have been drawn so far on thermal conduction mechanism for the polymers after being subjected to the external stretching force.

Here we report the enhanced thermal conductivity of bulk polyethylene (PE) sheet achieved by uniaxially stretching. The enhancement of thermal conductivity can be attributed to the combination of lamellae and chain alignments in a semi-crystallizable material induced by the stretching force. A new model has been proposed elucidating the mechanisms of thermal conductivity enhancement for uniaxially stretched bulk semi-crystalline PE through the analysis of the process-structure-property relationship. The new model may be generalized for a greater range of polymer-based materials and hence may provide key information towards design and manufacturing of high thermally conductive polymers and polymer-based composite materials.

Uniaxial drawing was carried out using an Instron 5564 Universal Tensile Tester fitted with an environmental chamber to enable uniaxial drawing of PE sheets at elevated temperatures to obtain tailored lamellar alignments and macromolecular orientation. Bulk PE sheet (76 mm × 76 mm × 1 mm) (S1) was first heated to 160 °C and hold for 3 min to erase any materials processing history. The sample was then cooled down to 110 °C at a cooling rate of 80 °C/min, followed by uniaxial drawing in a nitrogen atmosphere (S2). Four different stretch ratios (Λ = 1, 2, 3, 4, and 5, where Λ is the ratio between the final length l and the initial length L of the material, Λ = l/L. Λ = 1, denotes unstretched material) have been deployed. The in-plane and out-of-plane thermal conductivities (S3) as a function of stretch ratio has been measured using a laser flash method (LFA-447, Netzsch, Germany) based on xenon flash lamp source at room temperature. In order to understand the effect of structural change on the thermal conductivity of the bulk PE sheet during the uniaxial drawing process, a synchrotron 2-dimensional wide-angle X-ray diffraction (2D-WAXD) apparatus in the Hefei National Synchrotron Radiation Laborator (NSRL) was employed to detect the lamellar alignment of stretching PE samples (S4). In addition, the samples were etched using dimethylbenzene in order to reveal aligned lamellae and oriented macromolecular chains after the uniaxial stretching (S5). The etched samples were sputter-coated with gold and analyzed by scanning electron microscopy (SEM, FEI Quanta 2003D) at an accelerating voltage of 2.0 kV.

Fig. 1 shows the in-plane and out-of-plane thermal conductivities of bulk PE sheets as a function of stretch ratio. The in-plane thermal conductivity of PE increases with increasing stretch ratio. The in-plane thermal conductivity of the stretched PE is about ∼1.01 W m⁻¹ K⁻¹ under Λ = 5, almost 6 times greater than that of the unstretched PE sheet (0.18 W m⁻¹ K⁻¹). This finding is consistent with the previous reports on PE fibres, where the in plane thermal conductivity increased rapidly in the orientation direction with increasing strain or draw ratio [10,11,[15], [16], [17], [18], [19], [20], [21]]. Many other studies reported a slightly decreased thermal conductivity in the direction perpendicular to the drawing direction [22]. In contrast, the out-of-plane (perpendicular to the stretching direction) thermal conductivity of our stretched PE sheets shows a similar increasing trend for Λ = 2–5. The out-of-plane thermal conductivity values is generally lower than the in-plane values, indicating the anisotropic characteristics of the stretched material. The enhancement of both in-plane and out-of-plane thermal conductivities may be ascribed to the formation of interlocking shish-kebab structures [23] induced by the drawing force, as revealed from the SEM images.

Fig. 2 shows the typical high-resolution SEM images of the PE sheets (unstretched and stretched). Randomly oriented lamellar structures are clearly visible in unstretched PE sheets, see Fig. 2a (red arrows). Whereas the stretched PE features a shish-kebab structure, see Fig. 2b. The shish structures (dotted lines) consist of stretched chains [[24], [25], [26], [27], [28], [29]] along the stretching direction, whereas the kebab lamellae (solid lines) are perpendicular to the shish structure. The length of shish structure is typically in the range of microns, with the potential of extended-chains reaching dozens of microns. Interestingly, the connective kebab lamellae were found to link the shish structures forming the interlocking shish-kebabs [23]. Such interlocking shish-kebab structures create a free thermal transportation pathway, resulting in the enhancement of thermal conductivity, more detailed analysis can be found in thermal conduction mechanism section.

Fig. 3 shows selected 2D-WAXD patterns of stretched and unstretched bulk PE sheets. Two circle-like diffraction reflections for PE sheets (Λ = 1) can be clearly identified, which represent the (110) plane (the inner circle) and (200) plane (the outer circle) of PE orthorhombic crystals, respectively. The nearly perfect circle reflection indicates a random distribution of crystalline lamellae. When the bulk PE sheets are subjected to uniaxial stretching (Λ > 1), the circle-like diffractions gradually diminish and form arc-like diffractions. The length of the arcs decreases as the stretch ratio increases, indicating that the increasing lamellae alignment and greater degree of molecular chain orientation in the direction of stretching force [[30], [31], [32], [33], [34], [35]].

Typical WAXD patterns extracted from Fig. 3 are displayed in Fig. 4. All samples display distinct characteristic peaks at angles of 21.7° and 24.1°, indicating the stretching process did not have significant effect on the PE crystal structure. However, the orientation peaks seen in Fig. 5 are only clearly visible for stretch ratios 2–5. The degree of crystallinity and degree of orientation extracted from Fig. 4, Fig. 5 have been summarized in supporting documents (S6). With little variation in the degree of crystallinity, the degree of orientation has increased from 0.29 to 0.985 as the stretch ratio increases from 1 to 5, indicating increasingly aligned lamellae along the direction of stretching force.

There is a large body of literature reported that the thermal conductivity of semicrystalline polymers can be enhanced through simple shear [15], mechanical drawing [10], gel spinning [16,18,36], and superdrawing [19,20,37]. Various models, such as aligned crystalline structure [11,38], chains orientation [12,13,39], hydrogen bonding theory [21] and amorphous restructuring theory [40] etc., have been proposed for the thermal conduction mechanisms of drawn polymers. However, no consensus conclusions have been reached. The formation of shish-kebab structure is unique to semicrystalline polymers being subjected to external drawing force [[41], [42], [43], [44], [45]]. Herein we propose a new theory based on the interlocking shish-kebab structure (Fig. 6) to better elucidate the thermal conductive mechanism of uniaxially stretched PE sheet in our study.

Shish-kebab superstructure induced by the external force mainly consists of shish structure and kebab lamellae (Fig. 6), where the shish structure is composed of stretching molecular chains, and kebab lamellae are epitaxially grown folding chains attached to the shish chains. The kebab lamellae link the adjacent shish structures, leading to the formation of interlocking shish-kebab superstructure. In particular, the stretching chains in the shish structure is part of the kebab lamella, suppressing the phonon scattering during heat conduction. Thus, when the thermal energy is first transmitted to the surface of the stretched PE sheet, the atoms within the lamella and amorphous chains can gain and transfer the thermal energy between the neighboring structures. The thermal energy gain by the atoms in the amorphous chains mostly scatters due to the entangled molecular chains, while in the lamellae heat diffuses quickly (the thermal conductivity of polymer single lamella is about more than 100 W m⁻¹ K⁻¹). When the thermal energy passes both crystalline chains in a kebab-structure and stretched chains in a shish-structure, it first starts to diffuse along the shish direction (path of least resistance). The shish structure is composed of stretched chains and can be considered as a one-dimensional crystal since the molecular structure of stretched PE is simple, quasi-linear and well-ordered, resulting in the high thermal conductivity (more than 350 W m⁻¹ K⁻¹). Therefore, the thermal energy transports from the lamellae to the shish, then from the shish to the lamellae, and finally reaches the opposite side of the sample. In the interlocking shish-kebab structure, the highly thermal conductive lamellae in the stretched PE sample are connected through stretched chains (shish structure) to form a co-continuous phase, resulting in increased mean free path for phonon scattering.

The anisotropic thermal conduction seen in Fig. 1 can be attributed to the presence of crystalline interfaces between the kebab lamellae of interlocking shish-kebab structures and the lamellae which grows around the interlocking shish-kebab structures, resulting in greater scattering of thermal energy in the out-of-plane direction. Such anisotropy can be potentially reduced by suppressing the nucleation of lamellae around the interlocking shish-kebab structures, as suggested in our previous work [46,47].

In conclusion, we have demonstrated enhanced thermal conductivity of stretched PE sheets through uniaxial stretching. The greater thermal conductivity achieved has been explained by the theory based on the formation of interlocking shish-kebab structures induced by the stretching force. The highly thermally conductive lamellae and the stretched chains are connected to form a homogeneous structure for the more efficient diffusion of thermal energy, where the connective lamellae penetrate into the adjacent shish-kebab structures act as a bridging pathway for the phonons. Such structural evolution results in the focus of group velocity in all directions and the increase of mean free path of phonons, hence contributing to the enhancement of thermal conduction both in-plane and out-of-plane. It is expected that the thermal conductivity of stretched PE sheets can be further enhanced with the development of greater structural order and reduced defects. The in-depth understanding into polymer process-structure-property relationship from this work could provide key information towards design and manufacturing of future high thermally conductive polymer based materials.