Article pubs.acs.org/IECR
Filler-Reinforced Elastomers Based on Functional Polyolefin Prepolymers Ning Ren,† Megan E. Matta,‡ Henry Martinez,‡ Kim L. Walton,§ Jeffrey C. Munro,§ Deborah K. Schneiderman,‡ and Marc A. Hillmyer*,‡ †
Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455-0132, United States ‡ Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455-0431, United States § The Dow Chemical Company, Freeport, Texas 77541, United States S Supporting Information *
ABSTRACT: Carboxy-telechelic polyolefin prepolymers were synthesized via ring-opening metathesis polymerization of 3-ethyl-1cyclooctene in the presence of maleic acid. Following hydrogenation these difunctional prepolymers were cross-linked with a trifunctional aziridine in the presence of fumed silica or carbon black fillers to yield polyolefin elastomers with significantly enhanced mechanical properties as compared to the unfilled versions. The tensile strength, modulus, and tear strength of the elastomers can be tuned by changing the filler type and the total filler content. Further study on the reinforcing mechanism suggests that both the polymer−filler interactions and the filler−filler interactions play key roles in the mechanical property enhancements.
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INTRODUCTION Liquid silicon rubbers (LSRs), such as cross-linked polydimethylsiloxane (PDMS), are a class of amorphous polymers with glass transition temperature values (Tg) as low as −123 °C.1 Two component LSRs, comprising PDMS containing pendent vinyl groups and PDMS with silane (Si−H) groups along the polymer backbone, can be cross-linked to prepare silicone elastomers. For example, telechelic α,ω-divinyl polydimethylsiloxane can be obtained by the anionic polymerization of cyclic siloxane where the vinyl end-groups can then react with silicone polymers containing multiple Si−H groups.2 Compared with the conventional cross-linking methods like sulfur vulcanization and irradiation, an end-linking method can minimize the formation of dangling chains and loops that will reduce the number of effective elastic chains and compromise the mechanical behavior of final elastomers.3 LSRs have excellent hydrophobicity, a wide operating temperature range, and a high elongation at break, and are used for a wide range of applications, including sealing and joint technologies.4,5 Previously we synthesized a telechelic polyolefin prepolymer with carboxylic end-groups that could be cross-linked to form a polyolefin elastomer with LSR-like properties.6 To achieve this, we first synthesized an unsaturated telechelic polymer by ringopening metathesis polymerization (ROMP) of alkyl substituted cyclooctene derivatives in the presence of a difunctional chain transfer agent (CTA) to control the molar mass and endgroup functionality.7−11 The unsaturated polymer was hydrogenated to obtain a difunctional polyolefin prepolymer that © XXXX American Chemical Society
could be cross-linked using a trifunctional cross-linker trisaziridine (TAz). The uniaxial tensile properties of the polyolefin elastomers, however, were poor when compared to commercially available LSRs. One effective way to reinforce and mechanically enhance elastomers is to add filler in the polymer matrix. Fillers are important additives in polymeric materials that not only have the potential to alter the mechanical, electrical, thermal, and magnetic properties of elastomers, but also may lead to cost reductions.12 The reinforcing mechanism of the filler involves the hydrodynamic effect, polymer−filler interactions, and filler−filler interactions.13 The basic reinforcing factor of the filler is the hydrodynamic effect, which can be attributed to the addition of rigid filler particles into the polymer matrix. This reinforcing effect can be described with the following equation:
G = Gm(1 + 2.5ϕ)
(1)
Equation 1 is known as the Einstein−Smallwood formula, in which Gm is the modulus of the elastomer without fillers and ϕ is the volume fraction of the filler. G is the modulus of the filled elastomer. The equation was first used by Einstein to describe the hydrodynamic effect of spherical particles in a fluid and was later adapted to the field of filled elastomers by Smallwood in Received: January 29, 2016 Revised: April 28, 2016 Accepted: May 2, 2016
A
DOI: 10.1021/acs.iecr.6b00426 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research 1944.14,15 The equation requires freely dispersed solid spherical particles inside a rubbery matrix, where the constant 2.5 is associated with a geometric factor for spherical particles. On the basis of the Smallwood equation, the Guth model also takes the interactions between particle pairs into consideration in the Smallwood−Guth−Einstein equation:16 G = Gm(1 + 2.5ϕ + 14.1ϕ2)
Figure 1. Filler-reinforced cross-linked elastomers from telechelic polyolefin prepolymers.
(2)
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Polymer−filler interactions involve interfacial van der Waals force between polymer chains and filler, as well as hydrogen bonds or covalent bonds between polymers and fillers if applicable.17,18 By connecting polymer chains and filler particles, a network between fillers and polymers is created that enhances the mechanical properties of elastomers. Filler− filler interaction mainly refers to the interaction between filler particles/aggregates that can form a filler−filler network at high enough concentrations. The reinforcing effect of filler−filler interaction becomes more significant with increased filler loading.19−21 Carbon black, fumed silica, precipitated silica, and clay are commonly used fillers for elastomer reinforcement.22,23 Recently, carbon nanotubes and graphene have also been used as fillers to enhance the properties of elastomers.24,25 Carbon black is one of the first studied reinforcing agents for elastomers and is still extensively used. A typical carbon black contains spherical particles that range from 5 to 100 nm in size but can aggregate into 100−500 nm particles.13,26,27 Fumed silica with small particle sizes (and consequently larger surface area) is also widely recognized as an effective filler and has been studied extensively.13,26−29 However, since the siloxane and silanol groups on the surface of the silica particles are hydrophilic in nature, interactions with a hydrophobic polymer backbone tend to be relatively weak. Conversely, attractive filler−filler interactions are strong due to the hydrogen bonds between silica particles. Thus, silica particles often form larger agglomerates that will lead to inhomogeneous filler distribution in polyolefins, making the dispersion of silica particles more difficult than the dispersion of carbon black particles.13,30 Both carbon black and silica have been used to enhance the mechanical properties of a wide variety of elastomers.31−34 In the case of polyolefin elastomers, Baer et al. studied the mechanical properties of carbon black filled ethylene-octene elastomers as a function of filler content. An increase in modulus was observed with increasing filler content while the elongation at break remained the same for elastomers with different filler content.31 In another example, Morselli et al. studied a series of silica filled ethylene-propylene-dienemonomer (EPDM) rubber and found that the modulus of a sample with 30 wt % of silica filler was 2.5 times higher than the unfilled sample and the elongation at break for the filled sample was about 2 times higher than the unfilled one.32 These studies motivated our work in exploring filled elastomers based on our telechelic polyolefin prepolymer approach. In the present work we report two significant advances: (i) a modified and simplified synthesis of carboxy-telechelic polyolefin prepolymers based on our previous work and (ii) a straightforward approach to enhance significantly the mechanical properties of the materials through the addition of fumed silica and carbon black fillers (Figure 1). To further understand the reinforcing mechanism, we explored the thermal, mechanical, and rheological properties of the filled elastomers.
RESULTS AND DISCUSSION 1. Prepolymer Synthesis. Scheme 1 shows the synthetic route for the carboxy-telechelic prepolymers by homopolymerScheme 1. Synthesis of Carboxy-Telechelic Prepolymers via Ring-Opening Metathesis Polymerization (ROMP) Followed by Hydrogenation and Cross-Linking of the Hydrogenated Prepolymers Using Tris-aziridine (TAz) with Either Silica or Carbon Black as a Filler
ization of 3-ethyl-1-cyclooctene, followed by hydrogenation. Similar to our previous report, maleic acid was used as CTA to control the molar mass and install carboxylic acid end-groups.6 The target molar mass was 4 kg/mol to keep the viscosity of the prepolymer low and to provide a high cross-link density (i.e., a high concentration of cross-linkable end-groups) in the final elastomer. The prepolymer was hydrogenated using a SiO2 supported platinum catalyst, and the obtained saturated prepolymer (PHEt) is amorphous (Tg ≈ −70 °C). Compared to our previous report, in which the prepolymer was obtained by statistical copolymerization of cis-cyclooctene and 3-hexyl-1cyclooctene, the homopolymerization of 3-ethyl-1-cyclooctene is more straightforward synthetically. The PHEt prepolymer B
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prepolymers are highly cross-linked by the tris-aziridine. As for the physical appearance, samples with silica particles were colorless and transparent while samples with carbon black were black and opaque. 3. Morphology Study. Scanning electron microscopy (HITACHI 4700 FE-SEM) was used to study the morphology of the filled elastomers. As shown in low magnification images in Figure 3a,c, both the silica filler and carbon black filler are
synthesized in this manner has a regioregular polymer chain structure (>95% head-to-tail content determined by 1H NMR spectroscopy6) with a lower Tg than our previously described copolymers. The 1H NMR spectrum of the prepolymer before and after hydrogenation confirms successful incorporation of the CTA and hydrogenation of ≥98% of the double bonds (Figure 2). The molar mass of the prepolymer was calculated to
Figure 3. Representative SEM images of neat fumed silica (a, b) and neat carbon black (c, d) fillers. Black scale bars = 50 μm. White scale bars = 10 μm.
present as aggregates. The aggregates of silica have a larger size distribution when compared to the aggregates of carbon black. In general, the size of fumed silica aggregates ranged from 2 to 15 μm while those on V7H carbon black ranged from 2 to 8 μm. At higher magnification (Figure 3b,d) fine silica particles were observed around the silica aggregates while the carbon black aggregates appeared to have a more compact structure. The silica particles around the aggregates may agglomerate since the interaction between silica particles is likely stronger than the interaction between the silica and the polyolefin backbone. For polyolefin samples with the carbon black filler (Figure 4), the size of the filler−filler aggregates increased with filler
Figure 2. 1H NMR spectrum of the prepolymer before (top) and after (bottom) hydrogenation.
be 4.3 kg/mol by integration of the 1H NMR spectrum, assuming that each chain has exactly two end-groups. The copolymerization of 3-hexyl-1-cyclooctene with cis-cyclooctene was also performed and the corresponding hydrogenated prepolymer utilized in this work (see the Supporting Information for details). 2. Filler Incorporation and Cross-Linking of Prepolymers. Two types of fillers, Aerosil R-812 (hydrophobic silica, surface area 260 m2/g, produced by Evonik) and V7H (low structure carbon black, surface area 112 m2/g, produced by Cabot) were used in this study. The silica filler used in this work is typically named “fumed silica” since it is obtained from the reaction between silica tetrachloride and water by a vapor process at high temperature.12 To prepare the filled elastomer, the prepolymer was first mechanically mixed with different quantities of filler to prepare samples that contained 5, 10, 20, and 30 wt % of fumed silica/ carbon black filler. The samples were further homogenized with a speed-mixer (FlackTek SpeedMixer, DAC 150.1 FVZ-K) at 2800 rpm for 30 min to ensure that filler particles were welldispersed into the polymer matrix. Viscosities of the mixtures increased dramatically with the addition of filler, which made the processing of the mixture and the dispersion of the filler more difficult (Figure S1). After the initial mixing, the trifunctional cross-linker, tris-aziridine, was added into the homogeneous prepolymer−filler mixture (with 3:2 prepolymer to cross-linker mole ratio). The speed-mixer was used again to mix the tris-aziridine into the prepolymer. The ternary mixtures of prepolymer, tris-aziridine, and filler were then press-molded into rectangular films (8.3 cm × 8.3 cm × 1 mm, ≈ 6.7 g) at 150 °C under 3000 psi load for 20 min. After press-molding, samples were left in an oven at 100 °C for 6 h to drive the cross-linking reaction to completion. Half-life for the crosslinking reaction (i.e., the time required for half of the crosslinkers to react at least once) was calculated as it was in our previous study, and was determined to be about 10 min at 100 °C (assuming no significant influence on the kinetics due to the presence of the fillers6). The critical extent of reaction (pc) is calculated to be 0.71 for this cross-linking reaction.1 Gel fraction tests for the crosslinked elastomers demonstrate values >90%, indicating that the
Figure 4. SEM images of carbon black filled polyolefin elastomers: (a) xPHEt-CB5, (b) xPHEt-CB10, (c) xPHEt-CB20, (d) xPHEt-CB30. Scale bars = 10 μm.
content; at 5 wt % (2.1% volume fraction, see Table 1) filler content, the carbon black particles were well separated and there was no obvious filler−filler network, while at 30 wt % (14.7% volume fraction) filler content, large aggregates were observed, suggesting the possible presence of a filler−filler network. For the polyolefin samples with the fumed silica filler (Figure 5), however, large aggregates appeared in samples with 10 wt % (4.2% volume fraction) of filler. This aggregation is consistent with previous work and suggests that the filler−filler interaction is likely stronger than the polymer−filler interaction.13,30 4. Thermal Properties. The Tg values for both fumed silica and carbon black filled samples were essentially the same as those for the unfilled samples, indicating that the interactions between the polyolefin prepolymer and the fillers were not significant enough to modify the glass transition temperature, even at a high level of filler loading (Table 1). The degradation temperature (Td, defined here as the temperature at 5% mass loss) of the filled elastomers increased with the addition of the filler. Although the shift in Td suggests an enhancement in the C
DOI: 10.1021/acs.iecr.6b00426 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Table 1. Filler Content and Thermal Properties of Filled Elastomers sample ID xPHEt xPHEtSi5 xPHEtSi10 xPHEtSi20 xPHEtSi30 xPHEtCB5 xPHEtCB10 xPHEtCB20 xPHEtCB30
filler content (wt %)
Tga (°C)
Tdb (°C)
volume fraction (%)c
5
−68 −68
317 344
0 2.1
10
−68
360
4.2
20
−70
369
9.1
30
−71
375
14.2
5
−69
357
2.1
V7H
10
−70
356
4.3
V7H
20
−67
356
9.3
V7H
30
−70
375
14.7
filler type Aerosil R812 Aerosil R812 Aerosil R812 Aerosil R812 V7H
Figure 6. Dynamic mechanical thermal analysis of silica filled elastomers. Heating at 5 °C/min, 6.28 rad/s, strain 0.05 %.
a Determined by DSC (second heating cycle) at 10 °C min−1 (Figure S3). bTemperature at 5% mass loss determined by TGA at 10 °C min−1 in N2 (Figure S4). cCalculated using the density of silica filler (2.2 g/cm3) or carbon black filler (2.1 g/cm3).
Figure 5. SEM images of fumed silica filled polyolefin elastomers: (a) xPHEt-Si5, (b) xPHEt-Si10, (c) xPHEt-Si20, (d) xPHEt-Si30. Scale bars = 10 μm.
thermal stability of the filled elastomer,29,35 this enhancement was likely due to the fact that neither filler forms volatile degradation products under the temperatures tested during the thermal gravimetric analysis experiment. Since the observed mass loss was due to polymer matrix degradation, it is somewhat misleading to report Td as the temperature at 5% mass loss; at the reported Td the actual mass loss of the polymer component equates to 5.25%, 5.55%, 6.25%, 7.1% for the samples with 5%, 10%, 20%, and 30% filler, respectively. 5. Mechanical Properties. 5.1. Dynamic Mechanical Thermal Analysis. To better understand the mechanical behavior of the filled elastomers, dynamic mechanical thermal analysis (DMTA) was performed. Figures 6 and 7 show the temperature−storage modulus relationship of filled elastomers from −90 to 200 °C at 6.28 rad/s. At temperatures lower than −70 °C, the samples were glassy with G′ values between 0.5 and 2 GPa. At temperature above the Tg, the storage modulus dropped about 2 orders of magnitude. Above −50 °C, samples were rubbery, and the storage modulus of the cross-linked samples was essentially constant. For both silica and carbon black filled elastomers, the storage modulus at the rubbery plateau was higher with increasing filler content, consistent with the elastic modulus obtained by tensile testing (see below). The addition of filler had little influence on the DMTA-determined glass transition temperature of the elastomer, consistent with the DSC results. 5.2. Tensile Properties. The tensile strengths for both fumed silica and carbon black filled elastomers were significantly higher than those of the unfilled elastomers. For example, the
Figure 7. Dynamic mechanical thermal analysis of carbon black filled elastomers. Heating at 5 °C/min, 6.28 rad/s, strain 0.05 %.
tensile strength of sample with 30 wt % of silica filler was an order of magnitude larger than that of the sample with no filler (Table 2). Moreover, for samples with 30 wt % filler, the moduli increased by 2.5−5 times as compared to those of the unfilled samples. At lower filler content, however, the moduli for both fumed silica and carbon black filled elastomers were only slightly higher than that of the parent elastomer. For fumed silica filled elastomers, the elongation at break values were about 2 times higher than the elastomer without filler, while for carbon black filled elastomers the elongation at break values were about the same as the sample without filler. These data imply that, with high filler content, the formation of some percolating filler structures contribute to the modulus of the elastomer. To further compare the mechanical properties of our materials with cured silicone elastomers, we prepared a baseline material from commercially available LSRs. These LSRs contain between 20 and 30 wt % of fumed silica according to the manufacturer, though detailed information about the characteristics of this filler was not provided. As shown in Table 2, compared to the tensile properties of the cross-linked LSR, the synthesized polyolefin elastomers with 30 wt % of either fumed silica or carbon black filler had a higher elastic modulus, and the D
DOI: 10.1021/acs.iecr.6b00426 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Table 2. Mechanical Properties of Filler-Reinforced Prepolymers and LSRa tensileb
hysteresisd
sample
εb (%)
σTS (Mpa)
E (Mpa)
tear N/mm
xPHEt xPHEt-Si5 xPHEt-Si10 xPHEt-Si20 xPHEt-Si30 xPHEt-CB5 xPHEt-CB10 xPHEt-CB20 xPHEt-CB30 LSRe
94 ± 4 250 ± 30 250 ± 20 270 ± 30 170 ± 30 100 ± 10 110 ± 10 130 ± 10 90 ± 10 455
1.3 ± 0.1 3.5 ± 0.5 3.6 ± 0.4 10.0 ± 1.0 12.0 ± 1.0 2.1 ± 0.2 2.6 ± 0.2 3.7 ± 0.4 5.1 ± 0.7 9.5
2.6 ± 0.2 3.0 ± 0.2 2.6 ± 0.1 7.0 ± 1.0 17.0 ± 2.0 4.1 ± 0.1 4.3 ± 0.3 5.5 ± 0.1 10.0 ± 0.3 3.0
0.68, 0.57 1.06, 1.01 1.20, 1.18 1.49, 1.63 1.82, 1.76 0.59, 0.67 0.87, 1.65 2.34, 1.85 2.32, 2.65 f
c
cycle 1 (%) 18.0, 15.2, 19.5, 41.5, 50.3, 6.9, 11.0, 23.8, 35.2, 26.6,
17.5 16.0 13.4 40.4 46.3 6.5 10.5 25.9 35.0 24.1
cycle 2 (%) 12.1, 7.4, 8.5, 15.6, 23.2, 2.3, 3.9, 14.2, 20.9, 12.9,
11.7 7.4 8.5 15.6 21.9 3.2 3.6 12.8 21.6 12.0
a
Mechanical properties measured on ASTM D1708 microtensile bars. bMeasured at 69 mm/min, at least 4 tensile bars were tested for each sample. Measured at 100 mm/min, 2 bars were tested for each sample (2 in. × 0.5 in. geometry samples with a 1 in. cut). dMeasured at 60 mm/min, strains no greater than 50% of the elongation at break, 2 bars were tested for each sample. eThe cross-linked LSR was prepared by two component LSRs. The two LSRs, Elastosil LR 3003/60/A and 3003/60/B, were provided by Wacker Chemie AG; component A is PDMS functionalized with vinyl pendent groups, while component B is functionalized with silane pendent groups. These two components were mixed in a 1:1 wt % ratio and cured by press molding using conditions similar to those for the polyolefin elastomers. fThe tear strength of LSR cannot be obtained accurately for this experiment since the tensile force causes the stretching of the sample instead of tearing. c
Table 3. Characterization of Filler-Reinforced Cross-Linked Prepolymers sample
density (g/cm3)a
volume fraction (%)
G′@25 °C (MPa)b
Me (kg/mol)c
xPHEt xPHEt-Si5 xPHEt-Si10 xPHEt-Si20 xPHEt-Si30 xPHEt-CB5 xPHEt-CB10 xPHEt-CB20 xPHEt-CB30
0.87 0.91 0.93 1.01 1.04 0.88 0.90 0.98 1.03
0 2.1 4.2 9.1 14.2 2.1 4.3 9.3 14.7
2.4 3.1 3.4 3.8 14.3 3.7 3.5 5.3 12.0
0.89 0.72 0.67 0.66 0.18 0.59 0.64 0.46 0.21
Determined by an isopropanol/ethylene glycol density gradient column at 25 °C. bDMTA (Figures 6 and 7) at 25 °C, 6.28 rad/s, using torsion on a rectangular geometry. cApparent average effective molar mass between cross-links Me = ρRT/G at 25 °C. a
sample with 30 wt % of silica filler had an even higher tensile strength. The LSR, however, had a higher elongation at break value than the filled xPHEt elastomers. 5.3. Tear Strength. As shown in Table 2, the tear strength also underwent a significant increase upon the addition of the filler. The xPHEt with no filler has a low tear strength value of 0.6 N/mm, while the tear strength value of the sample with 30 wt % of carbon black was about 3 times higher on average. According to the literature, the tear strength of a LSR with about 30 wt % of silica filler is around 15 N/mm, much higher than the filled polyolefin samples we obtained.4 The tear strength of an elastomer is related to the fracture energy that is required to tear through a unit area of the elastomer. The fracture energy (GC) is correlated to the individual chain properties and the network density of the elastomer, and can be expressed as the following relationship: GC = KM 0.5
higher bond strength (464 kJ/mol) than the C−C bond (347 kJ/mol),39 and (3) differences in the level of chain extension versus cross-linking. 5.4. Tensile Hysteresis. The hysteresis study summarized in Table 2 shows that both fumed silica and carbon black filled elastomers demonstrated higher energy loss with increasing filler content during the elongation cycles. For the sample with 30 wt % silica filler, the average energy loss during the two testing cycles was twice as high as the sample with 5 wt % silica filler, and similar behavior was observed for the carbon black filled elastomers. The increasing energy loss with higher filler content during the hysteresis experiment is a typical stress softening behavior of filled elastomers. This behavior (called the Mullins effect) is caused by the detachment and slippage of chain segments that have reached their limit of extensibility on the filler surface.13 The observed Mullins effect in our samples indicates that there are polymer−filler interactions in both the fumed silica and carbon black filled elastomers. 6. Reinforcing Mechanism. Table 3 shows the density and filler volume fraction of the cross-linked prepolymers, their modulus, and apparent molar mass between cross-links. Since the densities of two fillers are quite close, samples that had the same wt % filler were also similar in density and volume fraction of filler. The room temperature storage modulus rose with the addition of the filler, and the corresponding apparent molar mass between cross-links declined. Since the molar mass
(3)
Here M is the molar mass between cross-linking sites,36and K is a prefactor that describes the chain properties of the elastomer which varies with chemical nature of the polymer.37,38 Therefore, the fact that LSR has a higher tear strength could be explained by (1) the higher network density of LSR, controlled by the amount of silane groups along the PDMS side chain, (2) the inherent strength of PDMS chain being higher than the polyethylene chains, because the Si−O bond has a E
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between chemical cross-link sites remained the same (all the samples are from the same prepolymer), this suggests that the filler aggregates serve as cross-linking sites inside the polymer matrix that contribute to the modulus of the filled elastomers. Figure 8 compares the storage modulus of fumed silica filled cross-linked prepolymers with that predicted by the Small-
Article
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00426. Materials, characterization, rheology discussion, Figures S1−S14, Scheme S1, and Tables S1 and S2 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank The Dow Chemical Company for financial support of this research and PolyAziridine LLC for providing us with a sample of the tris-aziridine. Parts of this work were carried out in the Institute of Technology Characterization Facility (CharFac) at the University of Minnesota, a member of the NSFfunded Materials Research Facilities Network, via the MRSEC program. We also thank Dr. David Giles for helpful input on rheology experiments and Dr. Madalyn Radlauer for assistance in the preparation of this manuscript.
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Figure 8. Volume fraction−storage modulus relationship of xPHEt with silica and carbon black filler. The predicted curve is plotted based on the Smallwood−Guth−Einstein equation: G = Gm(1 + 2.5ϕ + 14.1ϕ2). Storage modulus taken from DMTA (Figures 6 and 7) at 25 °C, 6.28 rad/s, strain 0.05 %.
wood−Guth−Einstein equation, eq 2, which describes the hydrodynamic reinforcement of the filler.13 At low filler volume fraction the experimental result was quite close to the predicted value. However, as the filler content increased, the experimental modulus became much higher than the predicted value. These data suggest that, in addition to hydrodynamic reinforcement, both filler−filler interaction and polymer−filler interaction contribute to the enhancement of mechanical properties of filled elastomers, as discussed in the above sections.
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REFERENCES
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CONCLUSION
Saturated carboxy-telechelic polyolefin prepolymers were successfully synthesized and cross-linked with the addition of fumed silica and carbon black fillers. The mechanical properties of the filled elastomers were enhanced by the addition of fillers. Compared with a commercially available liquid silicon rubber, the filled elastomers demonstrated higher tensile strength and modulus, while the elongation at break and tear strength of the filled elastomers were still much lower than the LSR. Mechanical and morphological studies suggest that the hydrodynamic effect, filler−filler interaction, and polymer− filler interaction all contribute to the reinforcement of elastomers by filler addition. F
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DOI: 10.1021/acs.iecr.6b00426 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
