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Damping elastomer with broad temperature range based on irregular networks formed by end-linking of hydroxyl-terminated poly(dimethylsiloxane).


Noise reduction and attenuation of vibration have become important issues in modern industry. The requirement of damping materials with high shock absorbing ability has grown rapidly. Elastomers are widely used as damping materials in many fields for their excellent shock absorbing capacity. This property arises from the viscoelastic movement of segments in polymer chains [1]. The hysteresis of strain arising from the viscoelasticity of polymers under alternating stress, leads to the absorption of energies. The ratio of E"/E', named as tan[delta] (loss tangent), is used as an assessment of the ability to dissipate energy by elastomers, where E" and E' are the loss modulus and storage modulus, respectively [2, 3]. High-performance damping materials should meet the requirement of tan[delta] > 0.3 over a broad temperature range of at least 60-80[degrees]C [4].

However, elastomers are generally characterized by high damping properties around their glass transition temperature. This temperature range is far below room temperature and covers only the range between 20[degrees]C and 40[degrees]C [5]. This fact restricts the application of elastomers as damping material. Some methods have been introduced to broaden the temperature range by introducing large tan[delta] in order to improve the damping properties of elastomers at room temperature and above [6, 7]. These methods include blending elastomers with other components or preparing interpenetrating elastomeric networks with polymers having different glass transition temperatures (Tg). However, since the damping properties depend on the glass transition, the effective damping temperature region (tan[delta]) > 0.3) by these methods was not sufficiently broad. In recent years, a novel damping elastomer, based on irregular networks with dangling chains, has attracted much attention [8-15]. As only one end of the pendant chains was linked to the permanent networks, the irregular networks have many relaxation modes with different relaxation times [12]. As a result, these irregular networks exhibit high damping properties over a broad temperature range above the Tg. End-linking of precursor chains is widely used in the study of irregular networks because it is easy to determine their structure. Some end-linked irregular networks have been prepared and their damping properties were found to improve as the number of end-links was increased [13-15]. For example, Urayama et al. reported the damping properties of irregular networks with dangling chains made from hydrosilylation reaction between vinyl-terminated poly(dimethylsiloxane) (PDMS) and triskis(dimethylsiloxy)silane [13]. The results showed that the elastomers had the lowest tan[delta] when the networks had the fewest pendant chains coming from irregular cross-linking. The network structure (the cross-linking density, the content of pendant chains, and elastic chains) of the elastomer was speculated in Macosko and Miller's calculation [16]. As the application condition of the calculation is quite strict, the reaction used in the experiment is elaborate, which restricts the application of irregular networks. Furthermore, the high cost of triskis(dimethylsiloxy)silane and the difficult reaction conditions restrict the use of these elastomers.

The condensation reaction between siloxane and PDMS is widely used in industry and daily life for its simple to implement technology and environmentally friendly character. In the present study, the condensation reaction of hydroxyl-terminated PDMS with cross-linker was used to prepare end-linked irregular networks with dangling chains. By controlling the ratio of the precursor and the cross-linker, novel silicone elastomers with irregular networks were obtained. Unlike previous reports, the content of the pendant chains (Wpen) and elastic chains (Wela) of the networks was determined by Nuclear Magnetic Resonance Cross-link Density Spectroscopy. Furthermore, the loss modulus and the transverse relaxation time of the irregular networks were studied to permit discussion of the effects of the pendant chains on the damping properties of the networks. To get quantified information about the structure of the irregular networks, the content of the pendant chains (Wpen), and elastic chains (Wela) were determined. This work aims at a deeper understanding of the dynamic properties and determination of the structure of irregular networks containing many pendant chains.



Three kinds of bi-functional PDMS were employed as end-reactive precursor chains. The hydroxyl-terminated PDMS were supplied by Dow Coming, USA .The number-average molecular weight (Mn) and the molecular weight distribution (Mw/Mn) of the PDMS were measured by gel permeation chromatography (Viscotek TDA305 Malvern) using tetrahydrofuran as the mobile phase (Table 1).

Tetraethoxysilane (TEOS), methyltrimethoxysilane (MTMS), and n-octyltrimethoxysilane (OTMS), as cross-linkers, and dibutyltin dilaurate, as catalyst, were all supplied by Guangzhou Chemical Reagent Factory, China.

Sample Preparation and Test

Sample Preparation. The networks of PDMS were prepared using the condensation reaction [17], as shown in Scheme 1.

PDMS precursor were dry at 100[degrees]C in a vacuum for 2 h. In a typical experiment a known amounts of the precursor was first mixed with the cross-linker and the catalyst (dibutyltin dilaurate). The cross-linker was tetra-functional alkoxysilane (TEOS) or tri-functional alkoxysilane (OTMS and MTMS). The network was prepared using nonstoichiometric ratio (r) of the crosslinker to the precursor. The mass ratio between the precursor and the catalyst was 100:0.5. After stirring for about half an hour at room temperature, the reactant mixture was poured into an aluminum mold of interior dimensions 12 x 7 x 5 mm. The reaction was allowed to proceed at 150[degrees]C in a vacuum oven for three days. After cross-linking, each of the samples was weighed, and then washed in toluene for 1 week to remove the unreacted precursor. In order to ensure complete removal of all the unreacted precursor, the sample was repeatedly washed with fresh toluene daily. After extraction of the unreacted precursor, the elastomer was equilibrated in toluene and alcohol mixtures at different proportions. The content of the alcohol in the solvent mixture was increased gradually until finally the sample was washed in pure alcohol then dried in vacuum at 120[degrees]C until constant weight.

Dynamic Mechanical Property Testing. The E', E", and tan[delta] were measured on NETZSCH Dynamic Mechanical Thermal Analyzer model DMA 242C in compression mode. The dynamic force on sample was 5 N, and static force was 1 N. The maximum amplitude was 50 pm. The test was operated from -60[degrees]C to 200[degrees]C with a heating rate of 5[degrees]C/min at a frequency of 1 Hz.


Magnetic Resonance Cross-Link Density Spectrometer Analysis. The weight fraction of pendant and elastic chains were determined in Magnetic Resonance Cross-link Density Spectrometer of IIC model XLDS-15. The frequency used was 15 MHz, the Magnetic induction was 315 A/m and the temperature was 60[degrees]C. The transverse relaxation time of the irregular networks was tested too.


Influence of Different Functional Group Ratios on Properties of the Irregular Networks

The precursor used in this part was precursor 1 in Table 1 and cross-linker used was OTMS. Figure 1 shows the values of tan[delta] as a function of temperature for the irregular networks made from different functional group ratios (r). It is apparent that the magnitude of tan<5 is inversely proportional to r. The networks have highest damping properties when r is 1.4. The loss tangent is more than 0.1 in a temperature range of -60[degrees]C to 180[degrees]C. According to Urayama [13] pendant chains appear to improve the damping properties of

the networks. It has been proved that Pendant chains are obtained by either controlling the ratio of the functional groups of the precursor and the crosslinkers or by introducing mono-functional precursors [8, 13], The control of r in the present system is the main reason for having excess of pendant chains.

Nuclear Magnetic Resonance Cross-link Density Spectroscopy was introduced to ensure the existence and estimate the contents of the pendant chains. The mobility of elastic chains is different than pendant chains because the elastic chains are fixed at both ends while the pendant chains are fixed at only one end. The fractional contributions of elastic chains and pendant chains to the NMR signal density can be deduced according to the BPP and Anderson-Weiss theories of relaxation [18]. The weight fraction data obtained from the NMR experiments are shown in Fig. 2. The weight fraction of the elastic chains and the pendant chains is denoted as Wela and Wpen, respectively. The results show that as r is increased, the contents of pendant chains decrease. The changes in Wpen and Wela are not obvious for r < 2.4 as the amount of the cross-linker is very small. Unlike previous reports, the molar ratio with the lowest pendant chains content is not 1 [13]. That is a result of intermolecular reactions between the cross-linkers.

The signal decay in the NMR (Fig. 3) shows the effect of the pendant chains on the relaxation properties of the elastomers. The velocity of the signal decay in irregular networks increases in proportion to the value of r. This phenomenon indicates that the relaxation movement in networks with low r was easier and the mobility was higher.

The transverse relaxation time ([T.sub.2]) of the networks is shown in Fig. 4. [T.sub.2] reflects the movement of small molecules, pendant chains, elastic chains and the whole networks. As all these movements impact the relaxation of elastomers, [T.sub.2] can be used as a measure of the overall relaxation in the networks. It is observed that [T.sub.2] decreases as r increases and the relaxation time in networks with more pendant chains becomes longer.

Elastomers prepared from low r contain more pendant chains and the pendant chains increase the relaxation time. Longer relaxation time of the networks is believed to be the reason for the improved damping.

The storage moduli and loss moduli are shown in Figs. 5 and 6, respectively. It is apparent that as r is increased, the loss modulus gradually decreases while the storage modulus slightly increases. These results are in agreement with previous reports [13]. As pendant chains have higher mobility in relaxation movements, they will increase the loss modulus of elastomers. However, because of the larger number of pendant chains and the lower cross-link density, the storage modulus of network is lower resulting in poor mechanical properties (Fig. 7).

Influence of the Cross-Linker on the Damping Properties of Irregular Networks

The precursor used in this part was precursor 1 in Table 1.

The effects of the side groups of the tri-functional cross-linkers on the damping properties of the irregular networks were discussed. The values of r were 1.9 and 2.9. The cross-linkers used in this part were MTMS and OTMS (OTMS). The purpose of this part was to explore the effects of side groups of the cross-linkers.

The characteristics of the irregular networks cross-linked by MTMS and OTMS were listed in Table 2. The differences between OTMS and MTMS lie in the length of the side group with no reactivity. MTMS has a shorter side group than OTMS suggesting OTMS has lower reactivity [19, 20].

Figure 8 shows the loss tangent (tan[delta]) as a function of temperature for the elastomers cross-linked by MTMS and OTMS, respectively. Networks cross-linked by OTMS had higher tan[delta] than those cross-linked by MTMS in the temperature range studied. The reason is the same as discussed above. Irregular networks cross-linked by OTMS have more pendant chains and exhibit better damping properties. The long unreactive side group in the OTMS cross-linker could increase the damping properties of the irregular networks.

Figure 9 shows the comparison of E' of the elastomers cross-linked by OTMS and MTMS at the same r. It can be seen that the elastomers cross-linked by OTMS have lower E' than those cross-linked by MTMS in the temperature range studied because of the close relationship between E' and [W.sub.el]: E' decreases with decreasing [W.sub.el]. Networks cross-linked by OTMS have fewer elastic chains than those cross-linked by MTMS, so their storage Young's modulus (E') would decrease under the same conditions.

The influence of the number of functional groups in cross-linkers on the damping properties of the irregular networks also was discussed. Figure 10 shows that the elastomers cross-linked by TEOS had higher damping properties than the elastomers cross-linked by MTMS and OTMS. It is deduced that TEOS has four functional groups and the steric hindrance in the elasomters is higher than the other two kinds of elastomers cross-linked by MTMS and OTMS. As a result, the reactivity of the TEOS is lower and it would produce more pendant chains than MTMS and OTMS.

Influence of the Length of the Precursor Chains on the Damping Properties of the Networks

The irregular networks made from PDMS with different number-average molecular weights were explored in this part. The cross-linker and r used in this part was TEOS and 1.45, respectively.

Figure 11 shows the tan[delta]-T curves of the irregular networks made from PDMS with different number-average molecular weights. At all temperatures, the damping properties improved as the precursor chains are longer. There are two reasons for this phenomenon. On the one hand, the increase of the molecular weight means an increase of weight fraction of pendant chains. The contents of pendant chains and elastic chains of the elastomers made from the different precursors are shown in Fig. 12. Pendant chains can increase the viscosity and damping properties of ealstomers. On the other hand, as the main chains of the precursors were the most important part of the pendant chains, networks made from long precursors have longer pendant chains, and that will have influence on the modulus. The ultimate result is that networks made from longer chains have better damping properties.

From what has been discussed above, it is obvious that elastomers made from tri-functional cross-linkers (MTMS and OTMS) and precursors with low molecular weight exhibited low damping properties (tan[delta] < 0.3). However, when the cross-linker used was tetra-functional TEOS and the molecular weight of the precursor was 1.7 X [10.sup.4], the elastomers exhibited the desired damping properties. In order to further study, the damping property of elastomers made from the precursor with molecular weight of 1.7 X [10.sup.4] and TEOS, the restults for different values of r were obtained. Figure 13 shows that elastomers made from tetra-functional TEOS and the precursors with molecular weight of 1.7 X [10.sup.4] (for values of r were 1.30 and 1.45) exhibited excellent damping properties (tan[delta] [greater than or equal to] 0.3) in a broad temperature range. Especially when r was 1.45, the elastomer exhibited the best damping property (tan[delta] [greater than or equal to] 0.3), from -60[degrees]C to 190[degrees]C.


A series of irregular networks were prepared in this study. Nuclear Magnetic Resonance Cross-link Density Spectroscopy was used to explore the network structure and relaxation properties of the elastomers. The results indicated that with the decrease of the functional group ratio between cross-linkers and precursors, the weight fraction of elastic chains decreased. The transverse relaxation time of the elastomers increased as the weight fraction of elastic chains decreased. DMA analysis showed that elastomers with fewer elastic chains exhibited better damping properties. The difference in transverse relaxation time was predicted as the reason for that. Furthermore, the influence of the structure of cross-linkers and the molecular weight of the precursors on the damping properties of elatomers was explored. Elastomers cross-linked by tetra-functional cross-linker (TEOS) had higher damping properties than the elastomers cross-linked by tri-functional cross-linkers (MTMS and OTMS). Elastomers made from precursors with molecular weight of 1.7 X [10.sup.4] and TEOS exhibited excellent damping property (tan[delta]>0.3), from -60[degrees]C to 190[degrees]C.


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Zhipeng Li, Xun Lu, Gang Tao, Jianhua Guo, Hongwei Jiang

Department of Polymer Materials Science and Engineering, School of Materials Science and Engineering, South China University of Technology, NO.381 Wushan RD., Tianhe District, Guangzhou, 510641, People's Republic of China

Correspondence to: X. Lu; e-mail:

DOI 10.1002/pen.24196

Published online in Wiley Online Library (

TABLE 1. Characteristics of hydroxyl of PDMS precursors.

Precursor          Mn          Mw/Mn

1           1.9 x [10.sup.3]   1.376
2           1.2 x [10.sup.4]   2.149
3           1.7 x [10.sup.4]   2.030

TABLE 2. Weight fraction of elastic and pendant chains of elastomers
cross-linked by cross-linkers with different structure measured in NMR.

                      r = 1.9             r = 2.9

Cross-linker   Wpen(%)   Wela(%)   Wpen(%)   Wela(%)

MTMS            14.84     79.92     14.03     78.68
OTMS            44.98     52.14     32.72     66.62
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Author:Li, Zhipeng; Lu, Xun; Tao, Gang; Guo, Jianhua; Jiang, Hongwei
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:1USA
Date:Jan 1, 2016
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