Thermomechanical study of polyethylene porous membrane by coating silicon dioxide nanoparticles

2.1 Materials

A roll of PE membrane (with thickness of 8 μm) was used in this study. The membrane samples were all first washed in ethanol (EtOH) followed by two rinses in distilled water and then were dried at 40°C in an oven.

2.2 Preparation of SiO₂ sol

The SiO₂ sol was prepared by mixing precursors tetraethyl orthosilicate (TEOS; CAS:78-10-4) and EtOH (98%) at room temperature with high-speed stirring. HCl was then added into the mixture as a catalyst followed by deionized water and dimethylformamide (DMF), which were mixed into the system as buffers. The five liquid materials were mixed at a molar ratio of 1:8:0.04:3:1.6 (TEOS/EtOH/HCl/H₂O/DMF), and the reaction formula is as follows:

Formula (1) is the hydrolysis reaction using the HCl catalyst, and Formula (2) is the condensation of the hydrolysis products. The two-step reactions were going at the same time. The preparation of SiO₂ was carried out for approximately 6 h at 30°C with high-speed stirring. A kind of stable, thick, transparent liquid sol was then formed after the reaction.

2.3 Sol-gel coating on membrane

The samples of PE membrane were immersed in the as-prepared sol for 30 min. The wet PE membranes were then rolled on a very smooth surface plate by a glass bar. These steps were repeated twice. The extra liquid on the sample surface was absorbed by paper tissue. The membrane samples were then dried in an oven at 40°C for 1 h. The working principle is shown in Figure 1.

Figure 1:

Schematic illustration of SiO₂ sol preparation and PE membrane by sol-gel coating.

2.4 Surface characterization

2.5 Thermomechanical investigation

3.1 Analysis of thermal stability

As is known, the thermoplastic polymer has a nature of shrinking and melting when close to fire flame. This is owing to the enhanced mobility of long macromolecular chains and increased entropy for the unstable orientation of molecules under heat accumulation [18]. This shrinkage is obviously illustrated as shown in Figure 2A with black curly bead close to flame. Comparably, solid integrated SiO₂ gel blocks the curly shrinking of membrane, which fires directly after heat accumulation to the PE decomposed temperature shortly. Quantitatively, two 2×2 cm square frames were marked on the surface of two kinds of membranes, and then the membranes were heated up to 120°C under vacuum for 30 min. The two membranes experienced different size declines as the temperature increases, as shown in Figure 2B. Figure 2C shows that the area of marked frame on the PE membrane decreases from 4 to 1.77 cm² (44.25%), and the area of 3.66 cm² (91.5%) for shrinked PE/SiO₂ gel membrane under 120°C, indicating the significant improvement of thermal stability of PE membrane when coated with SiO₂ gel. The larger error bar of PE shrinkage variation discloses the uneven porous structure of PE membrane, which causes the anisotropic shrinkage under heating, whereas the filled solid SiO₂ gel prevents the anisotropic shrinkage of PE membrane significantly.

Figure 2:

Thermal stability of PE membranes and PE/SiO₂ gel composites: (A) comparison of close-to-flame reactions; (B) and (C) area variation of both membranes under 20°C and 120°C.

3.2 SEM, EDX, and AFM characterization

Figure 3A and B shows the surface morphology of PE and PE/SiO₂ gel composite membranes, where PE shows a porous structure due to the liquid-solid phase separation during membrane manufacture, and PE composite membrane displays a smooth surface of SiO₂ gel layer. The SiO₂ gel layer was analyzed by EDX with a point spectrum for confirming the Si-element existence (Figure 3D). AFM images (Figure 3C and D) show that the SiO₂ gel layer of PE membrane consists of gel nanoparticles with diameters from 5 to 50 nm using cross-section analysis for an arbitrary blue line. This diameter range of nanoparticles can definitely penetrate into the internal pores among microfibrils as shown in Figure 3A. However, it is noted that the densely stacked nanoparticles may not contribute to the modification of PE membrane when applied to battery separators as the significant decrease of membrane porosity. The effect of densely coated SiO₂ nanoparticles on PE membrane porosity was consistent with Jeong and Lee’s work [19].

Figure 3:

SEM images of (A) PE membrane and (B) PE/SiO₂ composite membrane and EDX observation of Si element on PE membrane and AFM images of (C) SiO₂ gel nanoparticles, (D) confirmation of Si element deposited on the PE membrane and (E) SiO₂ gel particle dimension distribution.

3.3 FTIR, XRD, and TGA analysis

In the XRD characterization of PE and PE/SiO₂ gel membranes (Figure 4A), characteristic peaks can be identified at 21.6° and 24.2° of 2θ. The sample crystallinity obtained from XRD was approximately 40% for PE and 15% for PE/SiO₂ gel membranes according to Eq. (3) [20]:

where A_T denotes the total area of the diffractogram and A_A denotes the area corresponding to the amorphous region. Thus, it is inferred that no crystalline phase exists inside the SiO₂ gel after coating based on the same abscissas of peaks of the two scanned curves.

Figure 4:

Testing results of PE membrane and PE/SiO₂ gel composites from (A) XRD, (B) FTIR, and (C) TGA.

The effect of coated SiO₂ gel on PE membrane was analyzed by FTIR, as presented in Figure 4B. The high peaks from 720 to 2916 cm^-1 demonstrated organic signals, such as a characteristic spectra of the asymmetric stretching vibration band of ⊥-C-C- at the peak of 2916 cm^-1 and symmetric stretching vibration band of ||-C-C- at the peak of 2850 cm^-1. The peaks at 1461 and 723 cm^-1 indicated the existence of symmetrical curve vibration of ⊥-C-C- at the PE crystal area and out-of-plane twist vibration of ⊥-C-C- and b-axis of CH₂ at the PE crystal areas. The characteristic peaks at 1651, 1066, and 973 cm^-1 represent the H-OH curve stretching vibration, the Si-OH curve vibration, and the Si-O-Si asymmetric/symmetric stretching vibrations, respectively. These peaks prove the SiO₂ gel on the PE membrane.

Figure 4C shows the measured weight percentage of PE and PE/SiO₂ gel membranes in the temperature range from 25°C to 700°C. It is noted that PE membrane shows weight balance until its decomposed temperature at 475°C, and the weight drops dramatically from this temperature. This is attributed to the porous structure and orientation disordering of PE macromolecular chains at decomposition and evaporation. However, the TGA curve of PE/SiO₂ gel membrane presents a slowly weight decrease with the increase of temperature and a dramatic weight drop at the same decomposed temperature of PE membrane, following with an almost constant line (residual weight) in the test temperature range. The slow reduction of weight indicates that the gel still contains sol-phase “liquid”, which is locked inside the gel network and escapes at high temperature. The final residual 60% of weight is composed of SiO₂ nanoparticles, which are not able to decompose in the test range of temperature.

3.4 Analysis of thermostretching properties

Figure 5 compares the surface morphologies of PE and PE/SiO₂ gel membranes under stretching at 120°C. There are a number of white lines in parallel alignment on the surface of pure PE membrane, which is attributed to the reorientation of -C-C- molecular chains under large elongation at high temperature. These white lines are covered by SiO₂ gel particles on the PE/SiO₂ gel membrane, which displays slight extrusion tendency of SiO₂ gel particles as shown in the red arrows in Figure 5, indicating the resistance of SiO₂ gel to the PE thermostretching behavior.

Figure 5:

SEM images of PE membrane and PE/SiO₂ gel composites under stretching at 120°C.

Figure 6 gives the results of DMA tests regarding the storage modulus (E′) and phase angle (D), which are the function of temperature (T) for the two membranes. Two plateaus appear as the glass- and rubbery-like states in Figure 6A, and the storage modulus reduces abruptly at 128°C for the PE and at 144°C for the PE/SiO₂ gel membranes, indicating that the coated SiO₂ gel can enhance the glass transition temperature and the modulus of PE porous membrane to some extent. Oppositely, the phase angle reflects the ratio of the viscous portion to the elastic portion of a polymer. The angle of PE membrane is observed larger than that of the PE/SiO₂ gel membrane from Figure 6B. This implies a good thermal protection of SiO₂ gel in preventing PE macromolecular chains from flowing above the glass transition temperature, owing to the stable thermal performance of SiO₂ gel nanoparticles in the porous PE membrane.

Figure 6:

DMA of PE membrane and PE/SiO₂ gel composites along heating duration.

During the Instron-5566 tensile tests, each sample dimension is a rectangle of 2.5×5 cm, and each one was performed under a series of constant temperatures. It is found that the maximum yield loads of both membranes decrease as the temperature increases from P30/PS30 (30°C) to P120/PS120 (120°C). The initial Young’s moduli of the tested membrane (slope of curve) have the same tendency along the increasing temperature, as shown in the histograms in Figure 7. The membrane strain achieves a higher value at the maximum yield load under a higher temperature as shown in the curves in Figure 7, owing to the increased mobility of macromolecular chains and less crystallinity of the membrane. The coated SiO₂ gel nanoparticles improve the Young’s modulus of membrane significantly by two times higher than the pristine value, indicating the sufficient enlarged stiffness of the modified membrane. This is coincident with the enlarged storage modulus of PE/SiO₂ gel composite membrane from the DMA test.

Figure 7:

Thermotensile of membrane composites by Instron under 30°C, 60°C, 90°C, and 120°C.