Min Liu,
Ming Kang*,
Yongren Mou,
Kexu Chen and
Rong Sun
College of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China. E-mail: mkang311@126.com; Tel: +86 13547133979
First published on 21st November 2017
Most of the present studies of filler network structure basically stay in some phenomenological descriptions and classical electron microscopes are restricted by conventional 2-D imaging. This paper completed the visualization of filler networks in silica-reinforced silicone rubber (MVQ). Using a special network visualization technique based on fluorescent markers embedded in silica and Confocal Laser-Scanning Microscopy with an optical lateral resolution of about 200 nm, insights into the precise 3-D morphology and detailed micro-structure of filler networks were gained. A kind of sensitized-silica phosphor was prepared, with the excitation wavelength of 405 nm, which is consistent with the laser wavelength of Confocal Laser-Scanning Microscopy. Combining Image J software and tensile measurements, shapes, network structures, size distributions of aggregates in the silicone rubber matrix with silica loading from 10 to 60 phr were directly observed and the loading threshold for the formation of filler network in this experiment was obtained.
• However, due to the complexity of the micro-structure of filled-rubber and the restriction of research methods and technology, it is very difficult to observe the formation of the filler network structure and its contribution to the mechanical behavior of rubber directly. Although the synchrotron radiation X-ray nano-computed tomography technique has a great advantage in the study of filler network structure, this technology is based on large-scale scientific devices with limited resources, which limits its widely use in filler network research. Thus, the present studies of filler network structure basically stay in some phenomenological description and lack relatively ideal technology for detecting the filler network structure. On the one hand, a number of theories and models about Payne effect have been proposed to interpret the importance of the filler aggregates and networks, such as the Kraus model, the links–nodes–blobs (LNB) model,6,7 the chain slippage models, the bound rubber/entanglement network model,8 the localized glassy layer model9 and so on. The result that the Payne effect of CB gel (CBG) is frequency-independent and does not exhibit hysteresis during loading–unloading cycle indicates that the CBG network rather than the entanglement network dominates the instant recovery of the highly filled compounds,10 supporting the deduction of Satoh et al. about the quick healing behavior of filler network.11
• On the other hand, in general for the purpose of detailed microscopic characterization and fine-scale identification, other conventional microscopic techniques or additional high-resolution imaging tools such as scanning electron microscopy (SEM), environmental scanning electron microscope (ESEM), transmission electron microscopy (TEM), or atomic force microscopy (AFM) have been applied to observe the filler network structure in rubber. Atitaya Tohsan used TEM and AFM to observe the morphological features of peroxide cross-linked in situ silica-filled nanocomposites.12 It can be seen that spherical aggregates with an average size of 500 nm were detected and a filler network structure was also generated. Yong Lin13 and Jun Yang14 also used TEM to describe filler network structures generated from silica/reduced graphene oxide and silica, respectively. Jingyi Wang used SEM to observe the fracture surfaces of NR vulcanizates with different fillers.15 It can be seen that pristine filler agglomerated highly with large-size agglomerates in NR matrix. However, the agglomerates dealt with Si69 were smaller and dispersed well in NR matrix. However, SEM can only detect the morphology and distribution of filler in the rubber surface but can not observe the spatial distribution of filler inside the rubber. For AFM, although three-dimensional images of fillers in the surface can be obtained through the different value of the force between micro-probe and rubber matrix, micro-probe and fillers, it requires extreme level of roughness on the surface of the sample for the reason that its maximum scanning height is 2 μm. Besides, the soft nature of the silicone rubber also has a great influence on the accuracy of scan probe. For TEM, as a result of weak electronic penetration ability, the sample is required to be thin enough, which results in that only partially spatial distribution information of filler inside rubber can be obtained. Besides, because of the co-existence of rubber phase and the various kinds of filler aggregation, a pure filler phase that is covered by rubber matrix could not be achieved by SEM, TEM or AFM, which results in that it is difficult to define a so-called “percolation threshold” for forming model filler network in the rubber matrix.
Here, fluorescent markers and Confocal Laser-Scanning Microscopy (CLSM) with optical lateral resolution about 200 nm are employed to study the filler network structure of silica in a large volume of silicone rubber matrix. A kind of sensitized, spherical, and mono-disperse silica phosphor (SiO2@SiO2:Eu(DBM)3phen) was prepared by the sol–gel method, implementing the fluorescent markers of silica. What's more, the particle size and luminescence properties of this kind of filler are controllable, by adjusting the reaction conditions, therefore it is feasible to prepare the filler that is suitable for fluorescent markers. For CLSM, it is a general technique to draw a series of sliced images of a specimen, which can be reconstructed as 3-D images. The non-invasive and non-destructive laser-raster technique is applied to characterize fluorescent silica fillers and the in depth examinations along Z-axis in XY, XZ, or YZ scanning modes at increments of 150 nm are capable to reconstruct 3-D internal structure of them.16,17 It demonstrated that CLSM is more advantageous in (1) detecting the micro-structure of fluorescent silica fillers at micron to sub-micron scale resolution, (2) obtaining pure filler phase in the composites, (3) reconstructing 3-D morphology of fluorescent silica fillers, (4) observing the morphology of silica filler network structure and the aggregates size through fluorescence distribution, (5) in situ observation of samples without serious damages.18–23
The samples silica phosphors filled MVQ coded as MVQx, in which x denotes the silica phosphors loading in phr (parts per hundred rubber) of the original compounds.
The resulting optical section contains only information from one focus plane. To ensure the fluorescence of the silica phosphors filled silicone rubber and to enable a high-quality analysis of the filler network structures, during image acquisition in XY, XZ, YZ, and XYZ modes, line average was 2 and images were captured as 2-D images in digital image resolution of up to 1024 × 1024 pixel. The confocal images were captured in jpg format. The 200 consecutive confocal images, obtained at increments of up to 0.15 μm, were stacked as optical sections in z-direction. The CLSM settings for the evaluation of the experiments were presented in Table 2. Finally, Image J software was used to complete 3-D reconstruction.
Parameter [unit] | Laser wavelength [nm] | Magnification [—] | Line average [—] | Channel 1 type [nm] | Channel 2 type [nm] | Image size [μm2] | Pixel size [nm2] | Z-Step size [μm] | Nr of steps [—] | Format [—] |
---|---|---|---|---|---|---|---|---|---|---|
405 | 20× | 2 | PMT (417–517) | PMT (588–800) | 581.25 × 581.25 | 568.18 × 568.18 | 0.15 | 200 | 1024 × 1024 |
Since the wavelength of four independent solid-state lasers in CLSM we used are 405 nm, 488 nm, 552 nm and 638 nm, the control of peak position of emission spectra is necessary. The excitation and emission spectra of SiO2@SiO2:Eu(DBM)3phen phosphors were shown in Fig. 4a and b. The excitation spectrum was obtained by monitoring the emission of the 5D0–7F2 transition of the Eu3+ ions at 610 nm (Fig. 4a). The excitation spectrum of the SiO2@SiO2:Eu(DBM)3phen phosphors was composed of a broad band centered at 405 nm, which was consistent with the laser wavelength of CLSM. Upon excitation at 405 nm, the strongest red emission peaking at 610 nm arose (Fig. 4b), which was consistent with the testing results of CLSM. Fig. 4a1 and b1 present the excitation and emission spectra of MVQ10-60. They all showed excellent fluorescence performance, and no shift of characteristic peak position when compared with silica phosphors, indicating that the fluorescent marker of filler network has been completed.
Fig. 4 The excitation spectra and emission spectra of (a), (b) silica phosphors, and (a1), (b1) MVQ10-60. |
Image J software was used to realize the establishment of 3-D reconstruction. Briefly, 200 consecutive confocal images were imported through image sequence program and 3-D images were obtained from 3D plugins, from which the structure characteristics of the sample could be observed easily at any angle. Fig. 5 showed the process of 3-D reconstruction of silica phosphors network obtained from MVQ60 and a representative front view 3-D image of silica phosphors dispersing was shown at the bottom right corner of Fig. 5, where the rubber matrix was made transparent. As it showed, the aggregates were dispersed throughout and could be found with various morphologies, such as rod-like, semilunar, elliptical, and spherical and with size of a few micrometers.5 In order to describe the filler network structure more intuitively, the silica phosphors networks were constructed along the backbone of their aggregates from reconstruction imaging Briefly, 3-D images of silica phosphors networks were converted to binary imaging and skeletoned using Image J software, and a full scale of filler network associating with the aggregates dispersing were obtained (as shown in Fig. 6).
Fig. 6 Step of filler network construction. A small zone of the filler network was constructed along the backbone of silica phosphors aggregates by Image J software. |
Fig. 7 SEM images, CLSM images and skeletonization of silica phosphors networks obtained from MVQ10-60. |
As shown in SEM images, the white and spherical parts showed the silica phosphors, which fused into aggregates of a few micrometers and there were no distinct large-scale agglomerates. Because of the limitations on SEM testing principle, it could only observe the distribution in the surface of sample, therefore the 3-D structure of filler network still could not be observed clearly. It could be seen from CLSM images and skeletonization results that filler networks were generated at the loading of filler over 30 phr and some different characteristics were reasonably detected. Notably, the filler network of sample MVQ60 seemed to be larger and clearer than that of other samples. Adding more silica phosphors resulted in a denser and more connected network and the size of their aggregates became larger. It could be explained by the reason that hydrogen bonds between the abundant silanol groups on its surface were significantly increased with the increase of the silica content, so it can be reasonably concluded that the silica phosphors with a high content tended to undergo severe aggregation. In order to verify the feasibility of this method, the experimental value, theoretical value of filler volume fraction and the related visualization rate were calculated and shown in Fig. 8. The theoretical value of filler volume fraction (υf) were calculated by the following eqn (1):
(1) |
The theoretical value of MVQ10-60 were 4.1%, 7.4%, 10.3%, 12.8%, 15.0% and 17.0%, while the experimental value were 3.3%, 6.1%, 8.8%, 11.9%, 14.2% and 16.5%. The related visualization rate were 80.5%, 82.4%, 85.4%, 93.0%, 94.7% and 97.1% (the ratio of the experimental value with the theoretical value). The deviation of the volume fraction could be explained by the reason that a small number of mono-disperse silica in the rubber matrix with diameters less than 200 nm could not be observed by CLSM. However, based on the fact that visualization rate attained 90% above with filler content over 40 phr, it was feasible to conduct the thorough research to the filler network structure through this new method, including the statistics of aggregates size, the calculation of the network connectivity, the strain-induced deformation, destruction, and reconstruction. With the increase of filler content, the visualization rate increased rapidly, which would be explained later. So, it provided a new method to detect the structural evolution under loading and a new direction for revealing the reinforcement mechanism of filler network.
Moreover, based on CLSM imaging, the size distributions of silica phosphors aggregates in the silicone rubber matrix could be counted directly. The reconstruction imaging was analyzed by Image J software with the assumption that aggregates were spherical, and then the diameter distributions of aggregates were obtained. Fig. 9a–f were the size distribution of silica phosphors aggregates obtained from MVQ10-60. For all samples, the sizes of aggregates were mainly in the range of 2.5–7 μm and occupied about half of the total counted results, while large ones with sizes more than 7 μm and small ones with sizes less than 2.5 μm took a minor fraction. Thus, the contents of aggregates with sizes smaller than 2.5 μm and larger than 7 μm (denoted as PS and PL) were selected to evaluate the aggregation degree of aggregates. Fig. 9g depicted PS and PL at different content of silica phosphors. Upon increasing the content of fillers, PS showed a monotonic decrease. For MVQ10, PS was about 64%, which reached to 8% at content of 60 phr (MVQ60). Meanwhile, PL increased from 1% at content of 10 phr to 39% at content of 60 phr. The synchronous changing trends of PS and PL revealed silica phosphors with a high content tended to undergo severe aggregation,27 which was the reason why CLSM was more applicable to high filler content rubber.
The network connectivity (Tnet) is defined as an indicator to explore the network structures of the samples with various silica phosphors amounts. With the aid of imaging analysis from CLSM and the numerical statistical analysis from Image J software, the effective inter-particle distance of adjacent silica phosphors aggregates (dc) and the percentage of the filler network connectivity Tnet could be calculated by the following equations:28
dc = (0.86υ−1/3f − 1)dave | (2) |
Tnet = N(dl)/N(di) | (3) |
(4) |
Fig. 10 (a) Engineering stress–strain curves obtained from MVQ10-60, (b) fracture energy as a function of network connectivity. The solid line shows a linear fitting of experimental data. |
Based on eqn (4), the fracture energy Γ of MVQ0-60 samples were calculated to be from 72.37 to 216.92 J m−2, indicating continuous reinforcement and toughening effects. Fig. 10b showed the fracture energy as a function of network connectivity and the solid line showed a linear fitting of experimental data. At the loading of silica below 30 phr, filler network was not formed and the reinforcing performance was mainly affected by the rubber cross-linked chains, resulting in deviation from the linear relationship in Fig. 10b. At the loading of filler over 30 phr (the network connectivity over 37%), the excessive stress resulted in fracture of bundles or phase morphology of filler network, so the fracture energy was consistent with the network connectivity and the correlation coefficient R2 was 0.997. It also demonstrated that the loading threshold for the formation of filler network in this experiment was 30 phr, over which the filler network was crucial for rubber reinforcing.
This journal is © The Royal Society of Chemistry 2017 |