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Pure and Applied Chemistry

The Scientific Journal of IUPAC

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Volume 87, Issue 8

Issues

Preparation and intermolecular interaction of bio-based elastomer/hindered phenol hybrid with tunable damping properties

Xinxin Zhou
  • State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China
  • Key Laboratory of Beijing City for Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, China
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/ Lesi Cai
  • Key Laboratory of Beijing City for Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, China
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/ Weiwei Lei
  • State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China
  • Key Laboratory of Beijing City for Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, China
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/ He Qiao
  • State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China
  • Key Laboratory of Beijing City for Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, China
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/ Chaohao Liu
  • State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China
  • Key Laboratory of Beijing City for Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, China
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/ Xiuying Zhao
  • State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China
  • Key Laboratory of Beijing City for Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, China
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/ Jianfeng Chen
  • State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China
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/ Runguo Wang
  • Corresponding author
  • State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China
  • Key Laboratory of Beijing City for Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, China
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/ Liqun Zhang
  • Corresponding author
  • State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China
  • Key Laboratory of Beijing City for Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, China
  • Beijing Laboratory of Biomedical Materials, Beijing University of Chemical Technology, Beijing 100029, China
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Published Online: 2015-07-02 | DOI: https://doi.org/10.1515/pac-2014-1207

Abstract

In this research, crosslinked hybrids of a newly invented bio-based elastomer poly(di-isoamyl itaconate-co-isoprene) (PDII) and 3,9-bis[1,1-dimethyl-2{β-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy}ethyl]-2,4,8,10-tetraoxaspiro[5,5]-undecane (AO-80) were designed and prepared by the mechanical kneading of the PDII/AO-80 hybrids at a temperature higher than the melting point of AO-80, followed by the crosslinking of PDII during the subsequent hot-pressing/vulcanization process. The microstructure, morphology, and mechanical properties of the hybrids were systematically investigated in each preparation stage by using DSC, FTIR, XRD, SEM, DMTA, and tensile testing. Part of the AO-80 molecules formed an AO-80-rich phase, but most of them dissolved in the PDII to form a very fine dispersion in amorphous form. The results of FTIR and DSC indicated that strong intermolecular interactions were formed between the PDII and the AO-80 molecules. Each PDII/AO-80 crosslinked hybrid showed a single transition with a higher glass transition temperature and significantly higher loss value (tan δ) than the neat PDII because of intermolecular interactions between the PDII and the AO-80 molecules. For instance, tan δ of PDII/AO-80 consisting of 100 phr AO-80 achieved 2.6 times as neat PDII. The PDII/AO-80 crosslinked hybrids with applicability at room temperature are potential bio-based damping materials for the future.

Keywords: bio-based material; biomaterials; damping properties; elastomer; hybrid; nanocomposites; NICE-2014; polymers

Article note:

A collection of invited papers based on presentations at the 2nd International Conference on Bioinspired and Biobased Chemistry and Materials: Nature Inspires Chemical Engineers (NICE-2014), Nice, France, 15–17 October 2014.

Introduction

Vibration and noise often lead to undesirable consequences such as unpleasant noise, fatigue and failure of structures, decreased reliability, and degraded performance [1]. Rubber is commonly used in controlling noise and vibration because of its high damping property. The viscoelastic properties of rubbers make them ideally suited for use as effective damping materials because of their ability to dissipate mechanical energy [2, 3]. Based on the damping theory [4–6], the damping properties can be determined by dynamic mechanical testing. The loss tangent (tan δ) and the loss modulus (G′) are measures of damping, which requires the transformation of mechanical energy into heat. Thus, the damping characteristics of rubbers are dependent upon the intensity and breadth of the tan δ or the G′ peaks at the applicable temperature. The damping properties of rubbers are dominated by the glass transition. Typically, the temperature range for efficient damping of known rubbers is about 20–30 °C around the glass transition temperature. The glass transition can be broadened or shifted by the use of plasticizers or fillers, blending, grafting, copolymerization, crosslinking, or the formation of interpenetrating polymer networks (IPNs) [7–14]. But it is hard to obtain a damping material with a high tan δ value and a wide temperature range for damping properties at the same time.

Hybrid materials are a type of multiphase materials in which the dimension of at least one phase is on the nano or even molecular scale. The nanophase interacts with other phases through various nanoscale chemical bonding and physical absorption phenomena [15]. The hindered phenol 3,9-bis[1,1-dim ethyl-2-{b-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy}ethyl]-2,4,8,10-tetraoxaspiro-[5,5]-undecane (AO-80) was introduced into various polymeric matrices to prepare hybrids that exhibited damping properties. Figure 1 shows the chemical structure of AO-80, which includes many hydroxyl and ester groups. Compared with neat chlorinated polyethylene (CPE), the CPE/AO-80 hybrid exhibited a much higher dynamic mechanical loss [16, 17]. Furthermore, AO-80 also can be used to improve the damping properties of rubbers. The binary hybrid damping materials of acrylic rubber (ACM) and nitrile butadiene rubber (NBR) with AO-80 were prepared by Wu [18] and Zhao et al. [19], respectively. The ACM/AO-80 hybrid showed only one relaxation peak, which is larger than that of pure ACM [18]. Interestingly, the damping properties and the static mechanical performance of pure NBR were both improved by blending NBR with AO-80 because of the strong intermolecular interactions between the AO-80 and the NBR molecules [19].

Molecular structures of (a) AO-80 and (b) PDII. (c) Possible H-bonds(a) and H-bonds(b) in hybrids, with black thick lines, blue short lines, and red dashed lines denoting PDII molecules, AO-80 small molecules, and H-bonds, respectively.
Fig. 1:

Molecular structures of (a) AO-80 and (b) PDII. (c) Possible H-bonds(a) and H-bonds(b) in hybrids, with black thick lines, blue short lines, and red dashed lines denoting PDII molecules, AO-80 small molecules, and H-bonds, respectively.

In recent years, many researchers focused on the transformation of biomass into chemicals [20–23] and the synthesis of bio-based monomers into polymers [24–29]. Monomers that come from biomass directly and indirectly, such as vegetable oils, lactic acid, and itaconic acid, have been used to synthesize various polymers. These polymers are commonly known as bio-based materials and are expected to replace polymers based exclusively on petrochemical feedstock. Recently, we have reported on a series of new bio-based engineering elastomers (BEE) based on bio-based monomers [30–37]. Four primary criteria for BEE have been introduced first by our group [30]. According to the criteria, large-scaled bio-based monomers, such as sebacic acid, itaconic acid, succinate acid, 1,3-propanediol, 1,4-butanediol, and vegetable oils, were chosen to generate polyester-typed BEE (PE-BEE) [30], poly(di-alkyl itaconate-co-isoprene)-typed BEE (PDII) [33], and poly(epoxidized soybean oil-co-decamethylene diamine)-typed BEE (PESD) [36], all of which are linear and noncrystalline elastomers with low glass transition temperatures (Tg) and crosslinkable groups. These new elastomers exhibit physical and mechanical properties that are comparable with those of commercially available elastomers, and may replace petroleum-based rubbers in many applications.

In our previous studies, a novel crosslinkable, high molecular weight poly(diisoamyl itaconate-co-isoprene) (PDII) elastomer was prepared from itaconic acid, isoamyl alcohol, and isoprene by redox emulsion polymerization [33]. Carbon black (CB) and silica were used as fillers to reinforce the elastomer, and the dispersion of silica in the silica/PDII composites was more homogenous than that of CB in the CB/PDII composites because of the formation of hydrogen bonds between the silica silanols and ester groups attached to the PDII backbone [34]. The ester groups directly attached to the polymer backbone have already been found to make a significant contribution to the high damping of these polymeric materials [38]. Thus, the PDII elastomers with appropriate compositions are expected to exhibit good damping abilities.

The aim of this study was to prepare, characterize, and evaluate PDII/AO-80 crosslinked hybrids with expected damping performance. We supposed that the ester groups in the PDII chains can form intermolecular interactions (possibly hydrogen bonds) with the phenolic hydroxyl groups in the AO-80 molecules. In addition, we believed that sporadic AO-80-rich phases exist in the PDII/AO-80 crosslinked hybrids because of the intermolecular interactions of AO-80 with itself. Figure 1c shows two kinds of potential H-bonds and the AO-80-rich phases in the PDII/AO-80 ceosslinked hybrids. The first kind refers to the H-bonds(a) between the phenolic hydroxyl groups –OH of AO-80 (denoted by 1 and 2 in Fig. 1a) and the carbonyl groups –C=O of PDII (denoted by 5 and 6 in Fig. 1b). The second kind refers to the H-bonds(b) between the phenolic hydroxyl groups –OH of AO-80 (denoted by 1 and 2 in Fig. 1a) and the carbonyl groups –C=O of AO-80 (denoted by 3 and 4 in Fig. 1a). These two kinds of H-bonds formed a network, as shown in Fig. 1c. The intermolecular interactions will enhance the intermolecular friction and result in large energy consumption during dynamic deformations. Thus, the PDII/AO-80 crosslinked hybrids will exhibit excellent damping performance. Four stages were adopted in the preparation of the NBR/AO-80 crosslinked hybrids [19]. We improved the second stage of the preparation by blending the PDII/AO-80 uncrosslinked hybrids in a Haake mixer at 135 °C instead of kneading the hybrids on a two-roll mill at 135 °C. The PDII/AO-80 uncrosslinked hybrids become viscous fluids at 135 °C and cannot be effectively kneaded on the two-roll mill. However, the PDII/AO-80 uncrosslinked hybrids, even in the state of viscous fluid, can be kneaded in the sealed chamber of the Haake mixer. During the kneading of PDII/AO-80 uncrosslinked hybrids at 135 °C, the AO-80 molecules were not only melted but also dispersed well in the PDII. The structure and morphology of the PDII/AO-80 hybrids in each preparation stage were systematically investigated, and the static and dynamic mechanical properties of the crosslinked hybrids were evaluated. The results indicated that PDII/AO-80 crosslinked hybrids were successfully prepared and exhibited high dynamic mechanical loss values. The PDII/AO-80 crosslinked hybrids with applicability at room temperature are potential bio-based damping materials for the future and are expected to replace the petroleum-based damping materials.

Experimental

Materials

The bio-based PDII elastomer, which was synthesized by redox emulsion copolymerization in our lab, consisted of 80 wt.% di-isoamyl itaconate and 20 wt.% isoprene [33]. AO-80 (ADK-ATAB-AO-80) in the form of crystalline powder was provided by Asahi Denka Co. Ltd. (Tokyo, Japan) and was used without further purification. All other chemicals and ingredients were purchased in China and were used without further purification

Preparation of PDII/AO-80 hybrids

PDII/AO-80 crosslinked hybrids were prepared according to the following procedure: (1) After the prepared PDII was kneaded on a 6-inch two-roll mill at room temperature for 2 min, AO-80 (crystalline powder) was added in the PDII/AO-80 mass ratios of 100/0, 100/20, 100/40, 100/60, 100/80, and 100/100. Each of these mixtures was kneaded at room temperature for 5 min to form the first-stage PDII/AO-80a hybrids. (2) The PDII/AO-80a hybrids were then kneaded in a Haake mixer (Rheomix 600p, Thermal Electron Corp., USA) at 50 rpm for 5 min at 135 °C and then gradually cooled to room temperature to form the PDII/AO-80b hybrids. (3) The PDII/AO-80b hybrids were then blended with compounding and crosslinking additives, including 5.0 phr of ZnO, 2.0 phr of stearic acid, 0.2 phr of tetramethylthiuram disulfide, 0.5 phr of diphenyl guanidine, 0.5 phr of dibenzothiazole disulfide, and 1.0 phr of sulfur. The hybrids were then kneaded on the two-roll mill at room temperature for 10 min. (4) Finally, the hybrids were hot-pressed and vulcanized at 150 °C under the pressure of 15 MPa for various periods of time, and then naturally cooled down to room temperature to form the PDII/AO-80 crosslinked hybrids (PDII/AO-80c hybrids). The optimum vulcanization time for each crosslinked hybrid was determined by using a disc rheometer (P355C2, Huanfeng Chemical Technology and Experimental Machine Co., Beijing, China).

Measurements and characterization

Infrared (IR) measurements were conducted on a Bruker Tensor 27 spectrometer. The FTIR spectra were acquired by scanning the specimens in the wavenumber range 400 cm−1 to 4000 cm−1 for 128 times with a resolution of 2 cm−1. The FTIR spectra of AO-80 (both as-received and quenched AO-80) were acquired from ultra-thin disk specimens pressed from AO-80 and KBr. The FTIR spectra of PDII/AO-80 hybrids were acquired from film specimens (prepared by hot-pressing and crosslinking) with a thickness of approximately 1 mm by using the attenuated total reflection (ATR) technique. Differential scanning calorimetry (DSC) analysis was performed on a DSC 204F1 calorimeter (Netzsch Co., Germany). The DSC curves were recorded from −80 °C to 200 °C at a heating rate of 10 °C/min and under a nitrogen purge of 50 cm3/min. X-ray diffraction (XRD) measurements were performed on a Rigaku D/Max 2500 VBZt/PC X-ray diffractometer (Rigaku Co., Japan). The XRD data were recorded in the scattering angle range 3°–90°. The surface morphologies of the PDII/AO-80 hybrids were observed under a Hitachi S-4800 scanning electron microscope (SEM) (Hitachi, Ltd., Japan). The SEM specimens were prepared by fracturing the hybrids in liquid nitrogen. Dynamic viscoelasticity measurements were performed on a VA3000 dynamic mechanical thermal analyzer (DMTA) (01 dB-Metravib Co., France). The DMTA specimens were 10 mm long, 10 mm wide, and 1 mm thick, and were prepared by hot-pressing and crosslinking. The temperature dependence of the dynamic tensile modulus was measured in the range −40 °C–100 °C at a frequency of 10 Hz and a heating rate of 3 °C/min. Tensile tests of the PDII/AO-80 hybrids were conducted according to ASTM standard (D412: dumbbell-shaped), and the specimens were tested on an LRX Plus Tensile Tester (Lloyd Instruments, Ltd., UK).

Results and discussion

Thermal behavior and crystallization of PDII/AO-80 hybrids

We chose the PDII/AO-80 (100/60) hybrids to investigate the structures and the intermolecular interactions in PDII/AO-80 hybrids. The DSC curves for AO-80, neat PDII, and the PDII/AO-80 (100/60) hybrids are shown in Fig. 2a. As shown in Fig. 2a, the as-received AO-80 powder is crystalline and has a melting temperature at around 123 °C. After the as-received AO-80 was heated to 200 °C and quickly quenched to room temperature, amorphous AO-80 with a Tg at around 45 °C is obtained. It also can be seen that the neat PDII is amorphous, with a Tg at around −21 °C, and no crystallization melting peaks are found. Additionally, the DSC curve of the PDII/AO-80a (100/60) hybrid shows both the Tg of PDII and the melting temperature of AO-80. These results indicate that the intermolecular interactions between the PDII and the AO-80 molecules were weak in the PDII/AO-80a (100/60) hybrid, and a phase separation occurred, consistent with the SEM results (Fig. 3). Furthermore, the early start of melting and the blunting of the melting peak of AO-80 in the PDII/AO-80a (100/60) hybrid suggest that the crystal structure of AO-80 is destroyed because of the heat and the shear force in the blending process.

(a) DSC curves and (b) XRD traces of AO-80 (as-received and quenched), neat PDII, and PDII/AO-80 (100/60) hybrids in different preparation stages.
Fig. 2:

(a) DSC curves and (b) XRD traces of AO-80 (as-received and quenched), neat PDII, and PDII/AO-80 (100/60) hybrids in different preparation stages.

SEM images of representative fracture surfaces: (a) PDII/AO-80a (100/60), (b and c) PDII/AO-80b (100/60), (d and e) PDII/AO-80c (100/60) hybrids, and (f) AO-80-rich phase.
Fig. 3:

SEM images of representative fracture surfaces: (a) PDII/AO-80a (100/60), (b and c) PDII/AO-80b (100/60), (d and e) PDII/AO-80c (100/60) hybrids, and (f) AO-80-rich phase.

The DSC curve of the PDII/AO-80b (100/60) hybrid shows neither the Tg nor the melting temperature of AO-80, and the Tg of PDII is shifted from −21.3 °C to −6.5 °C, as shown in Fig. 2a. The AO-80 in the PDII/AO-80b hybrid might exist mainly as tiny nanoparticles or even as molecules after the PDII/AO-80a hybrid was mechanically kneaded above the melting temperature of AO-80. The extensive shearing associated with the high temperature mechanical kneading process could significantly promote the molecular-level mixing of the rubbery-state PDII molecules and the liquid-state AO-80 molecules. Interestingly, the subsequent cooling did not cause any evident phase separation, probably because of the strong intermolecular interactions between the PDII and the AO-80 molecules. The strong intermolecular interactions act as physical crosslinking points and remarkably restrict the mobility of the PDII macromolecules. The restriction greatly increases the glass transition temperature of PDII. Therefore, the PDII/AO-80b hybrids could be considered as molecular composites. This conclusion is further supported by the FTIR, XRD, SEM, and DMA results.

The DSC curve of the PDII/AO-80c (100/60) hybrid shows little difference from the PDII/AO-80b (100/60) hybrid, except that the Tg of PDII was further increased by 4 °C because of the chemical crosslinking. The AO-80 molecules in PDII/AO-80c (100/60) as well as in PDII/AO-80b (100/60) could not reaggregate and crystallize during the subsequent cooling, mainly because of the strong intermolecular interactions and the fast cooling rate. As a result, the AO-80 melting peak disappear.

Figure 2b shows the X-ray diffraction (XRD) traces of AO-80 (both as-received and quenched), neat PDII, and the PDII/AO-80 (100/60) hybrid in different preparation stages. As shown in Fig. 2b, the as-received and the quenched AO-80 demonstrate crystalline and amorphous characteristics, respectively. The XRD trace of the PDII/AO-80a (100/60) hybrid is similar to that of the as-received AO-80, indicating that the AO-80 in the hybrid is crystalline. However, the position (2θ) of the crystallization peak of the AO-80 in PDII/AO-80a (100/60) is larger than that of the as-received AO-80, indicating that the thickness of the AO-80 crystals in PDII/AO-80a (100/60) is smaller than that of the as-received AO-80, based on the Bragg equation, because the crystal structure of AO-80 may be destroyed by the heat and the shear force in the blending process, consistent with the DSC results. The XRD traces of the PDII/AO-80b (100/60) and PDII/AO-80c (100/60) hybrids, however, indicate that AO-80 molecules in these hybrids are amorphous, consistent with the DSC results.

Morphology of PDII/AO-80 hybrids

Phase morphology studies can provide the relationship between the microstructure and mechanical properties. Therefore, representative fracture surfaces of the prepared PDII/AO-80 (100/60) hybrids were studied by SEM. In Fig. 3a, a rough surface with large particles is observed, indicating that the normal mechanical blending of PDII and AO-80 at room temperature will lead to a poor dispersion of AO-80 in the PDII. Besides, the interfacial adhesion between AO-80 and the PDII was very weak, as indicated by the numerous holes left behind by the removal of AO-80 particles from the fracture surface.

Figure 3b and c show the microstructure of the PDII/AO-80b (100/60) hybrid. The representative fracture surface of the PDII/AO-80b (100/60) hybrid is uneven and distorted, mainly because the PDII/AO-80b (100/60) hybrid was not hot-pressed and crosslinked. But AO-80 particles cannot be clearly observed and very few voids can be identified by the microscope. These results suggest that the mechanical kneading of the PDII/AO-80a (100/60) hybrid above the melting temperature of AO-80 improves the dispersion of AO-80 and intermolecular interactions between then PDII and the AO-80 in the hybrid. Additionally, the subsequent cooling did not cause any obvious phase separation, probably because of the strong intermolecular interactions between PDII and the AO-80 molecules. The SEM results are in accordance with the DSC results.

Representative fracture surface of the PDII/AO-80c (100/60) hybrid are shown in Fig. 3d and e. The fracture surface of the hybrid is very smooth, and no AO-80 particles are observed on the surface. The particles in Fig. 3d and e were confirmed to be ZnO by using the energy dispersive spectrometer (EDS) attached to the SEM. In addition, sporadic aggregations of AO-80 molecules are observed in the PDII/AO-80c (100/60) hybrid, as shown in Fig. 5f. The excess AO-80 molecules readily come into contact with one another to form aggregates because of the intermolecular interactions (H-bonds(b)) between the AO-80 molecules. Thus, the AO-80-rich phases that were supposed in Fig. 1 are demonstrated by the sporadic aggregations of AO-80 molecules in Fig. 5f. However, crosslinked PDII network was developed during the hot-pressing and vulcanization process, and the three-dimensional rubbery network could block the AO-80 molecules and effectively prevent them from aggregating into large particles. Even more important, the intermolecular interactions between the PDII and the AO-80 molecules restrict the free movement of the AO-80 and the PDII. Thus, the PDII/AO-80c hybrid is indeed a molecular composite.

FTIR spectra of AO-80 and PDII/AO-80 hybrids

Figure 4 shows the FTIR spectra of AO-80 and the PDII/AO-80 hybrids. The telescopic vibration of –OH is absorbed usually in the range 3125–3704 cm−1. In Fig. 4a, there is an absorption peak near 3552 cm−1 associated with free hydroxyl groups and another absorption peak near 3467 cm−1 associated with the hydroxyl groups with hydrogen bonds in the AO-80 crystals. After the crystalline AO-80 powder was melted and then quenched to room temperature, amorphous AO-80 with hydrogen bonds was obtained, as indicated by the single infrared absorption peak near 3494 cm−1 in Fig. 4a. After the crystalline AO-80 was added into the PDII, the same change of AO-80 was observed, as shown in Fig. 4b. Furthermore, the infrared absorption peak near 3502 cm−1 in Fig. 4b suggests that there are molecular interactions (H-bonds(a)) between the PDII and the AO-80 molecules in the PDII/AO-80 hybrids.

FTIR spectra of (a) AO-80 (as-received and quenched) and (b) PDII/AO-80 hybrids in different preparation stages.
Fig. 4:

FTIR spectra of (a) AO-80 (as-received and quenched) and (b) PDII/AO-80 hybrids in different preparation stages.

Dynamic mechanical properties of PDII/AO-80c hybrids

Of the DMTA results, the loss factor tan δ reflects the internal and the external friction and expresses the ratio of energy dispersed in one deformation cycle to energy accumulated during the deformation cycle. A high value of tan δ means good damping performance of the material.

Figure 5a shows the temperature dependence of the tan δ values of the prepared PDII/AO-80c hybrids with various mass ratios of PDII to AO-80. Every PDII/AO-80c hybrid has only one tan δ peak, as shown in Fig. 5a, indicating that the AO-80 molecules have a good compatibility with the PDII molecules. The H-bonds(a) formed between the PDII and the AO-80 molecules also improve the compatibility. Specifically, as the AO-80 content in the PDII/AO-80c hybrids increases from 0 to 100 phr, the tan δ value increases from 1.42 to 3.66. Such an uncommon but favorable increase in the tan δ value is very remarkable. Different from other materials, a polymeric material gives the damping performance mainly by its viscoelastic behavior in the transitional region between the rubbery and the glassy state (near the glass transition temperature). In this region, macromolecular chain segments, but not entire macromolecules, essentially transform from the “frozen” state to the “free” state and tend to vibrate in phase with an external vibration. However, the molecular conformational changes usually cannot keep up with the imposed vibration, resulting in internal friction and energy dissipation. The stronger the internal friction, the higher the tan δ value and the better the damping performance of the material will be. Generally speaking, the introduction of inorganic fillers (such as carbon black, silica, and metal oxide/hydroxide) and organic molecules (such as plasticizers) into a polymer typically leads to a decrease of the tan δ value because the interactions between the fillers and the matrix are often weak. For the PDII/AO-80c hybrids, however, the increase in tan δ value can be attributed to the strong intermolecular interactions between the finely dispersed AO-80 molecules and the PDII molecules, which effectively restrict the motion of PDII molecules and increase the intermolecular friction during dynamic deformation. The high energy dissipation of intermolecular interaction under dynamic deformation is responsible for the remarkable increase of tan δ. A high tan δ in the glass transition region suggests that the PDII/AO-80c hybrids can be a good damping material in the corresponding working temperature range.

Temperature dependence of (a) loss tangent (tan δ) and (b) storage modulus (G’) for PDII and PDII/AO-80c hybrids with various mass ratios of PDII to AO-80.
Fig. 5:

Temperature dependence of (a) loss tangent (tan δ) and (b) storage modulus (G’) for PDII and PDII/AO-80c hybrids with various mass ratios of PDII to AO-80.

Figure 5b shows the temperature dependence of the storage moduli (G′) of the prepared PDII/AO-80c hybrids. Every storage modulus curve displays only one transition, and the curve gradually moves toward high temperatures with increasing amount of AO-80. Previous studies [39–42] showed that the G′ values of rubber nanocomposites increased with increasing amount of inorganic filler. For the PDII/AO-80c hybrids, the G′ values in the glassy region do not vary significantly, but those in the rubbery region significantly decrease as the AO-80 content increases. Because the stiffness of the amorphous AO-80 particles is similar to that of the crosslinked matrix, the AO-80 particles have little effect on the G′ value of the matrix in the glassy state. However, the AO-80 particles become soft at temperatures higher than the glass transition temperature of AO-80 (45.3 °C), leading to the decrease of G′ in the rubber region of the PDII/AO-80c hybrids.

Crosslink density can also affect the strength of tan δ and the G′ at high temperature. The crosslink densities of the PDII/AO-80c hybrids were measured by swelling in toluene using the Flory–Rehner theory [43, 44]. The Flory–Huggins interaction parameter χ was calculated in previous study, and the value is 0.12 [33]. The crosslink densities of the PDII/AO-80c hybrids are listed in Table 1. As expected, crosslink density of PDII/AO-80c hybrids decreased with the increase of AO-80 content. The decrease of the crosslink density helps partly to increase the tan δ values and decrease the G′ values at high temperature.

Table 1

Crosslink density of PDII/AO-80c hybrids.

Static mechanical properties of PDII/AO-80 hybrids

Figure 6 shows the characteristic curing curves of neat PDII and the PDII/AO-80 hybrids with various mass ratios of PDII to AO-80 at 150 °C. The torque of the vulcanizates decreases with the increase of AO-80 content, an indication that the AO-80 acted as a plasticizer in the PDII/AO-80 hybrids at 150 °C. Actually, the AO-80 molecules melted in the PDII/AO-80 hybrids at a temperature (150 °C) higher than the melting point of AO-80 (123.6 °C). The liquid AO-80 plasticized the PDII and decreased the torque of the hybrids. Table 2 shows the static mechanical properties of the PDII/AO-80c hybrids with various mass ratios of PDII to AO-80.

Curing curves of neat PDII and PDII/AO-80 hybrids with various mass ratios of PDII to AO-80.
Fig. 6:

Curing curves of neat PDII and PDII/AO-80 hybrids with various mass ratios of PDII to AO-80.

Table 2

Mechanical properties of PDII/AO-80c hybrids.

Unexpectedly, the PDII/AO-80c hybrids are not reinforced by the organic filler AO-80, although it is well dispersed in the PDII matrix and the interfacial bonding is strong. As the AO-80 content increases, the elongation at break of PDII/AO-80c increases from 199 % to 278 %, but the tensile strength decreases from 5.64 to 3.70 MPa. The decrease of the PDII/AO-80c hybrid crosslink density is responsible for the increase of the elongation at break and the decrease of the tensile strength. These results indicate that the PDII/AO-80c hybrids are more suitable to be used as functional damping materials rather than engineering materials. Table 2 also shows that the hybrids have very small permanent residual deformations, which are important for damping applications.

Conclusions

A bio-based elastomer (PDII) with numerous ester groups was synthesized by a redox system and designed to form a bio-based damping material with a hindered phenol (AO-80). PDII/AO-80 crosslinked hybrids were successfully prepared by the melt blending technique. Most of the AO-80 dissolved in the PDII at the molecular level and formed strong intermolecular interactions with the PDII molecules. In addition, a sporadic aggregation of AO-80 molecules was observed in the PDII/AO-80c (100/60) hybrid. The structures and morphologies of the PDII/AO-80 hybrids in each preparation stage were systematically investigated. The mechanical properties of the PDII/AO-80 crosslinked hybrids with different mass ratios of PDII to AO-80 were studied. The results indicated that AO-80 was homogenously dispersed in the PDII/AO-80 crosslinked hybrids.The PDII/AO-80 crosslinked hybrids exhibited a single relaxation transition, high dynamic mechanical loss values, and decreased tensile strengths. As expected, the PDII/AO-80 crosslinked hybrids had strong intemolecular interactions and tunable damping properties. The tunable damping properties make these hybrids potential candidates for bio-based damping material of the future.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (50933001, 51221102), the National Outstanding Youth Science Fund (50725310), the National Basic Research Program (973 Program) of China (2011 CB606003), the Beijing Nova Program (Z131102000413015), and the Beijing Municipal Training Program Foundation for the Talents (2013D00303400041).

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About the article

Corresponding authors: Runguo Wang, State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China; and Key Laboratory of Beijing City for Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, China, e-mail: wangrunguo@126.com; and Liqun Zhang, State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China; Key Laboratory of Beijing City for Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, China; and Beijing Laboratory of Biomedical Materials, Beijing University of Chemical Technology, Beijing 100029, China, e-mail: zhanglq@mail.buct.edu.cn


Published Online: 2015-07-02

Published in Print: 2015-08-01


Citation Information: Pure and Applied Chemistry, Volume 87, Issue 8, Pages 767–777, ISSN (Online) 1365-3075, ISSN (Print) 0033-4545, DOI: https://doi.org/10.1515/pac-2014-1207.

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