Polyhedral Oligomeric Silsesquioxanes in Novel ... - ACS Publications

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The Nucleating and plasticization Effects of Low-loading OctavinylPolyhedral Oligomeric Silsesquioxanes in Novel Biodegradable Poly(ethylene succinate-co-diethylene glycol succinate) Based Nanocomposite Siqi Teng, and Zhaobin Qiu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04004 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 30, 2017

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Industrial & Engineering Chemistry Research

The

Nucleating

and

Octavinyl-Polyhedral

plasticization Oligomeric

Effects

of

Silsesquioxanes

Low-loading in

Novel

Biodegradable Poly(ethylene succinate-co-diethylene glycol succinate) Based Nanocomposite Siqi Teng and Zhaobin Qiu* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China E-mail: [email protected]

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ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract A novel biodegradable poly(ethylene succinate-co-diethylene glycol succinate) (PEDEGS)

based

nanocomposite

containing

low

loading

(0.5

wt%)

of

octavinyl-polyhedral oligomeric silsesquioxanes (ovi-POSS) was prepared via a solution and casting method. The scanning electron microscopy image illustrated a nice dispersion of ovi-POSS in PEDEGS matrix. PEDEGS crystallized faster in the nanocomposite under isothermal melt crystallization condition; moreover, the density of PEDEGS spherulites also increased, indicating ovi-POSS acted as a nucleating agent. Ovi-POSS crystallized separately in the PEDEGS matrix and did not alter the crystal structure of PEDEGS. The storage modulus of the nanocomposite decreased in the lower temperature range; moreover, the glass transition temperature of PEDEGS also decreased in the nanocomposite, indicating that ovi-POSS also acted as a plasticizer and lubricant.

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Introduction Biodegradable polymers have drawn much attention in recent years as a solution to white pollution caused by traditional plastics and as potential biomedical materials.1–3 Poly(ethylene succinate) (PES) is a typical biodegradable aliphatic polyester synthesized from the monomers that can be prepared from renewable sources.4,5 The crystallization, biodegradable and mechanical properties of PES were thoroughly researched.6–11 PES has been widely used in many fields, such as packaging, agricultural film and biomedical materials due to its biocompatibility and relatively good mechanical properties. Copolymerization is a common method to modify PES to improve its mechanical and biodegradable properties to expand its application. Usually, diacid or diol monomers are introduced into the PES chains. Diacid monomers, such as furanoate, adipate, sebacate, and suberate, as well as diol monomers, including diethylene glycol, decamethylene glycol, octamethylene glycol, and hexamethylene glycol, have already been copolymerized to prepare PES based copolyesters.12–22 In previous work, novel poly(ethylene succinate-co-diethylene glycol succinate) (PEDEGS) copolyesters have recently been prepared via a two-stage melt polycondensation method; furthermore, the basic thermal properties, crystal structure, isothermal crystallization kinetics, and spherulitic morphology of PEDEGS were systematically investigated and discussed.20 The diethylene glycol succinate (DEGS) unit with ether-linkages improved the chain mobility of PEDEGS copolyesters, leading to a slight reduction of the glass transition temperature values. However, after

3



copolymerization, the crystallization rates of PEDEGS became slower, while the degree of crystallinity was slightly reduced, resulting in lower melting point temperature and smaller heat of fusion values. The slow crystallization process usually limits the application of polymer due to the restriction during the manufacturing process. Therefore, it is necessary to speed up the crystallization process of PEDEGS copolymers from a practical application viewpoint. The easiest way to accelerate the crystallization process and increase the nucleation density of polymer is to introduce a nucleating agent into the system. In literature, graphene, thermal reduced graphene, SiO2, graphene oxide, carbon nanotubes, silver nanoparticles, and fullerene-like WS2 are frequently used as nucleating agents for PES and other biodegradable aliphatic polyesters.23–28 Polyhedral oligomeric silsesquioxanes (POSS) is a typical organic-inorganic hybrid molecule with a unique three-dimensional structure.29,30 The change in cage structure and end groups contributes to a wide variety of POSS. Octavinyl-POSS (ovi-POSS), as a member of POSS family, may behave as the effective nucleating agent for many biodegradable aliphatic polyesters, like poly(L-lactide) (PLLA), poly(ε-caprolactone) (PCL), poly(butylene adipate) (PBA), PES and so on.31–37 Tang et al. found that ovi-POSS could promote the crystallization behaviors of PES without changing its crystallization mechanism and enhance the nucleation density of PES spherulites in the PES/ovi-POSS nanocomposites at low ovi-POSS loadings.31 Chen et al. reported that a small amount of ovi-POSS could enhance the nonisothermal crystallization from the amorphous state, accelerate the overall isothermal crystallization process,

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and increase the nucleation density of poly(ethylene succinate-co-ethylene adipate) (PESA) in the nanocomposites.32 In brief, ovi-POSS has been proved to be an effective nucleating agent for both neat PES and some of its copolymers. Therefore, in this work, ovi-POSS was chosen as the nucleating agent to accelerate the crystallization process of PEDEGS. In this work, a new PEDEGS based nanocomposite with low-loading of ovi-POSS was prepared with the aims of enhancing the crystallization rate and modifying its mechanical properties of PEDEGS simultaneously. The fine distribution of ovi-POSS in PEDEGS was observed. The crystallization kinetics, spherulite morphology, crystal structure, and dynamic mechanical behavior of the nanocomposite were investigated and compared with those of neat PEDEGS. Ovi-POSS not only had a nucleating effect on the crystallization process of PEDEGS but also acted as a plasticizer and lubricant in the system. The results could be helpful to extend the application of PEDEGS. The novelties of this work were as follows. On one hand, to the best of our knowledge, PEDEGS was studied as the matrix in a polymer nanocomposite for the first time as a novel copolymer of PES. Through the fabrication of polymer nanocomposite, the physical properties of PEDEGS may be significantly improved by only a small amount of ovi-POSS, thereby extending its practical application. On the other hand, the slow crystallization of PEDEGS was obviously enhanced by ovi-POSS at low loading; moreover, more importantly, different from the common reinforcing effect in some polymers, the unusual plasticization effect of ovi-POSS was discovered in the present research.

5



Experimental Section Materials and Preparation of PEDEGS/ovi-POSS Nanocomposite The PEDEGS sample containing 11 mol% of DEGS (Mn = 2.6 × 104 g/mol, Mw = 7.9 × 104 g/mol, PDI = 3.0) was synthesized by our lab through a two-stage melt polycondensation method.20 Shenyang Amwest Technology Co. kindly supplied the Ovi-POSS sample. Through a solution and casting process, a PEDEGS/ovi-POSS nanocomposite was successfully prepared at 0.5 wt% of ovi-POSS loading. The reason why a low loading of 0.5 wt% of ovi-POSS was chosen was as follows. On one hand, the dispersion of nanofiller in polymer matrix should depend on its loadings. At high loading, ovi-POSS tended to agglomerate, thereby influencing the improvement of the physical properties of polymer matrix. At low loading, the dispersion of ovi-POSS should be fine; however, its nucleating agent and reinforcing effects on the polymer matrix would also usually decrease accordingly. Therefore, from a balanced viewpoint of a fine dispersion of nanofiller in polymer matrix and the enhanced effects on both the crystallization rate as nucleating agent and the mechanical properties of the matrix, a relatively low loading of 0.5 wt% of ovi-POSS was chosen in this work as a model. The detailed preparation procedure was similar to that in our previous work and described in the Supporting Information in detail.31 The nanocomposite was abbreviated as PEDEGS/ovi-POSS for brevity. Neat PEDEGS was treated through the similar process for comparison. Characterization The distribution and morphology of ovi-POSS in PEDEGS matrix were studied

6


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using a scanning electron microscope (SEM) (Hitachi, Model S–4700). The nanocomposite sample was fractured into liquid nitrogen and then coated with gold before examination. The crystallization kinetics was studied with a differential scanning calorimeter (DSC) (TA Instruments, Model Q100). All samples were first annealed at 130 oC for 3 min to erase any previous thermal history and then cooled at 60 oC/min to the chosen temperature and held until the crystallization was completed. The investigated crystallization temperatures were from 38 to 50 oC. The spherulitic morphologies of PEDEGS and its nanocomposite were studied by a polarized optical microscope (Olympus BX51) (POM) equipped with a hot stage (Linkam, Model THMS 600). The samples were held at 130 oC for 3 min to erase any thermal history before cooling at 60 oC/min to 50 oC. The crystal structures were studied with an X-ray diffractometer (Rigaku Model D/Max 2500) at 40 kV and 200 mA from 5° to 40° at a scanning rate of 5°/min. The samples were prepared on a hot stage at 130 °C into a film and then transferred into a vacuum oven at 50 °C for 12 h. Dynamic mechanical analysis (DMA) (Netzsch, Germany, Model DMA242C) was used to measure neat PEDEGS and its nanocomposite. Both samples were tested from – 60 to 60 oC at 3 oC/min and 1 Hz. Results and Discussion Dispersion and Morphology of Ovi-POSS in PEDEGS Matrix The fine dispersion of nanofillers in polymer matrix is critical to improve the

7



mechanical and other properties of nanocomposites. Therefore, the dispersion and distribution of ovi-POSS in PEDEGS matrix was observed with SEM first. Figure 1 shows the SEM images of the fractured surface of neat PEDEGS and PEDEGS/ovi-POSS. It is shown that ovi-POSS, represented by the white dots, dispersed homogeneously throughout the matrix. In literature, the PES/ovi-POSS nanocomposites displayed the similar images.31 The size of the aggregates was mostly around 150 to 400 nm. The result confirmed that the nice dispersion of ovi-POSS was accomplished throughout the PEDEGS matrix.

Figure 1. SEM images of the fractured surfaces of (a) neat PEDEGS and (b) PEDEGS/ovi-POSS. Isothermal Melt Crystallization Kinetics Study

8


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Page 9 of 27

The crystallization process directly determines the crystal structure and morphology and therefore affects the mechanical and other properties of polymer based nanocomposites. Hence, the effect of ovi-POSS on the isothermal melt crystallization kinetics of PEDEGS in the nanocomposite was investigated. Figure 2a shows the plots of relative crystallinity versus crystallization time at 38 oC for both samples. At the same isothermal melt crystallization temperature (Tc), it took neat PEDEGS

nearly

37

min

to

finish

the

crystallization

process,

while

PEDEGS/ovi-POSS completed crystallization within 12 min. The isothermal melt crystallization process of PEDEGS was accelerated by ovi-POSS dramatically. Figure 2b displays the plots of relative crystallinity against crystallization time for PEDEGS/ovi-POSS at different Tc values from 38 to 50 oC. With increasing Tc, crystallization time became longer, indicating a slow crystallization rate. Similar results were also found for neat PEDEGS and not shown here for brevity. 100 (a) 80 Relative crystallinity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20 neat PEDEGS PEDEGS/ovi-POSS 0 0

10

20

30

Crystallization time (min)

9


40


100 (b) 80 Relative crystallinity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 27

60

40 o

38 C o 42 C o 46 C o 50 C

20

0 0

10

20

Crystallization time (min)

Figure 2. Plots of relative crystallinity against crystallization time for (a) neat PEDEGS and PEDEGS/ovi-POSS at 38 oC and (b) PEDEGS/ovi-POSS at different Tc values. To better analyze the crystallization kinetics, the classical Avrami equation was applied for both neat PEDEGS and its nanocomposite. The Avrami equation describes that relative crystallinity (Xt) develops with crystallization time (t) as 1 – Xt = exp (–ktn)

(1)

where k is the crystallization rate constant related to both nucleation and growth rate parameters and n is the Avrami exponent depending on the nature of nucleation and growth geometry of the crystals.38, 39 Figure 3a displays the related Avrami plots for neat PEDEGS and PEDEGS/ovi-POSS at 38 oC. It is clearly shown that the two fitting lines were nearly parallel to each other, and both the linear correlation coefficients were above 0.99. The Avrami equation could describe the crystallization kinetics of neat PEDEGS and its nanocomposite very well. Figure 3b illustrates the

10


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Avrami plots of PEDEGS/ovi-POSS at different Tc values. Four almost parallel straight lines were found in Figure 3b, suggesting that the crystallization mechanism did not vary with Tc. The relevant results are summarized in Table 1. As clearly shown in Table 1, for both neat PEDEGS and PEDEGS/ovi-POSS, higher Tc led to longer crystallization time. For example, when Tc increased from 38 to 50 oC, crystallization half-time (t1/2), the time needed to achieve 50% of the final crystallinity of the samples, changed from 14.9 to 22.9 min for neat PEDEGS and from 4.3 to 8.6 min for PEDEGS/ovi-POSS. Comparing to neat PEDEGS, PEDEGS/ovi-POSS crystallized much faster. For instance, when Tc was 42 oC, the t1/2 values for neat PEDEGS and PEDEGS/ovi-POSS were 16.0 and 5.0 min, respectively. However, for both PEDEGS and its nanocomposite, the Avrami exponents were around 2.3 to 2.8, indicating that both samples may remain a three-dimensional truncated sphere growth with athermal nucleation.40 It must be careful to deduce the crystallization mechanism of semicrystalline polymers by only using the n values, as the same n values may correspond to different crystallization mechanisms in polymer crystallization. For instance, in the case of n = 3, the sample may crystallize through a two-dimensional circular growth with thermal nucleation mechanism or a three-dimensional spherical growth with athermal nucleation mechanism.40 Therefore, the crystallization mechanism of polymers can not be deduced convincingly only by the n values without the morphological observation. The following crystalline morphology study would support the proposed crystallization mechanism. Thus, the isothermal melt crystallization of PEDEGS was distinctly accelerated by the addition of ovi-POSS,

11



which may be due to its heterogeneous nucleation effect.

(a)

0.5

log(-ln(1-Xt))

0.0 -0.5 -1.0 -1.5 neat PEDEGS PEDEGS/ovi-POSS

-2.0 0.0

0.3

0.6

0.9

1.2

1.5

log t

0.5

(b)

0.0 log(-ln(1-Xt))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 27

-0.5 -1.0 o

38 C o 42 C o 46 C o 50 C

-1.5 -2.0 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

log t

Figure 3. Related Avrami plots for (a) neat PEDEGS and PEDEGS/ovi-POSS at 38 o

C and (b) PEDEGS/ovi-POSS at different Tc values.

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Table

1.

Summary

of

the

Avrami

parameters

for

neat

PEDEGS

and

PEDEGS/ovi-POSS. Samples

Tc (oC)

n

k (min−n)

t1/2 (min)

Neat PEDEGS

38

2.6

7.06 × 10−4

14.9

42

2.5

6.58 × 10−4

16.0

46

2.4

6.39 × 10−4

18.5

50

2.3

4.97 × 10

−4

22.9

38

2.7

1.27 × 10−2

4.3

42

2.8

7.19 × 10−3

5.0

46

2.8

4.13 × 10−3

6.3

50

2.8

1.78 × 10−3

8.6

PEDEGS/ovi-POSS

Spherulitic Morphology and Crystal Structure Studies To confirm the effect of ovi-POSS as a nucleating agent, both samples were studied with POM. Figure 4 illustrates the POM images of neat PEDEGS and PEDEGS/ovi-POSS crystallized at 50 oC. As shown in Figure 4, neat PEDEGS spherulites with clear Maltese cross were observed. As for PEDEGS/ovi-POSS, the spherulite density obviously increased, and the size of the spherulites decreased dramatically, compared with that of neat PEDEGS. The additional nucleation sites caused by ovi-POSS sharply increased the nucleation density of PEDEGS spherulites, which led to the increase in the crystallization rate of PEDEGS. Thus, ovi-POSS particles acted as heterogeneous nucleating agent during the crystallization process of PEDEGS. 13



Figure 4. POM images of (a) neat PEDEGS and (b) PEDEGS/ovi-POSS isothermally crystallized at 50 oC (same magnification). The crystal structures of neat PEDEGS, its nanocomposite, and ovi-POSS were further investigated with WAXD. The WAXD patterns are shown in Figure 5. As a copolymer of PES, PEDEGS has the same crystal structure as PES. Three characteristic diffraction peaks were present for neat PEDEGS at 2θ = 20.24o, 22.78o, and 23.34o, corresponding to (021), (121), and (200) planes, respectively.41 For PEDEGS/ovi-POSS, these three characteristic diffraction peaks appeared around the same positions. Therefore, as an effective heterogeneous nucleating agent for PEDEGS, ovi-POSS did not alter the crystal structure of the matrix. Additionally, for ovi-POSS, a strong peak appeared at 2θ = 9.66o. Interestingly, this small peak also appeared at the same position for the PEDEGS/ovi-POSS sample. This result indicated that ovi-POSS crystallized separately in PEDEGS matrix but did not modify the crystal structure of PEDEGS.

14


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(021) (200) (121) Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PEDEGS

PEDEGS/oviPOSS

oviPOSS 10

20

30

40

ο

2θ ( )

Figure 5. WAXD patterns of neat PEDEGS, ovi-POSS, and their nanocomposite. The above results confirmed that ovi-POSS was an efficient nucleating agent for PEDEGS. But the exact nucleation mechanism is still uncertain. One possible mechanism is epitaxial nucleation.42 Ueda reported the crystal structure of PES in 1971, and PES had an orthorhombic crystal structure of a = 0.760 nm, b = 1.075 nm, c = 0.833 nm.41 As confirmed by the WAXD patterns, PEDEGS had the same crystal structure as PES. Thus, the cell parameters of neat PEDEGS are the same as those of neat PES. Bonhomme et al. investigated the crystal structure of ovi-POSS and found that ovi-POSS had a trigonal crystal structure of a = 1.3533 nm, c = 1.4222 nm.43 The 5 times of the c axis of PEDEGS (4.165nm) is almost the same as 3 times of the c axis of ovi-POSS (4.2666 nm). The mismatching level was only about 2.4%. Therefore, a possible epitaxial nucleation may account for the accelerated crystallization process and increased the nucleation density of PEDEGS spherulites in the nanocomposite. Similar results were also reported in PLLA/cyanuric acid composite, poly(ethylene

15



terephthalate)/talc composite, and some other systems.44–47 Nevertheless, the exact nucleation mechanism still needs further investigation. Dynamic Mechanical Properties Study The dynamic mechanical properties of PEDEGS/ovi-POSS were investigated and compared with those of neat PEDEGS. Figure 6a displays the temperature dependence of storage modulus of neat PEDEGS and PEDEGS/ovi-POSS. From Figure 6a, in the range of lower temperature when both samples were in glassy states, the storage modulus of PEDGES/ovi-POSS was lower than that of neat PEDEGS. For instance, at –30 oC, the storage modulus of neat PEDEGS was about 5000 MPa while that of PEDEGS/ovi-POSS decreased to about 4620 MPa. Ovi-POSS reduced the storage modulus of PEDEGS. Additionally, Figure 6b also illustrates the temperature dependence of tan δ for both samples. As shown in Figure 6b, the glass transition temperature (Tg) was 9.7 oC for neat PEDEGS and decreased to be 4.9 oC for PEDEGS/ovi-POSS. In the nanocomposite, ovi-POSS lowered the Tg of PEDEGS. In conclusion, low-loading of ovi-POSS acted as plasticizer and lubricant for PEDEGS. Similar results were also found by Khonakdar et al. that low contents of trifluoropropyl-POSS acted as a plasticizer when it was dispersed near to molecular level in PLLA.48

16


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6000 neat PEDEGS PEDEGS/ovi-POSS

Storage modulus (MPa)

5000 4000 3000 2000 1000 0 -40

-20

0

20

40

60

o

Temperature ( C)

(a)

0.20 0.15 0.10 tan δ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.05 0.00 neat PEDEGS PEDEGS/ovi-POSS -0.05 -40

-20

0

20

40

60

o

Temperature ( C)

(b) Figure 6. Temperature dependence of (a) storage modulus and (b) tan δ for neat PEDEGS and its nanocomposites. The above results indicated the nucleating agent and plasticization effects of a small amount of ovi-POSS on the novel PEDEGS matrix. For better understand the structure and properties relationship in this novel polymer nanocomposite, the effects

17



of different ovi-POSS loadings and the use of temporal small and wide angle X-ray analysis of the crystallization processes to truly elucidate the crystallization process and morphology development should be further investigated. Such research results will be reported in the forthcoming work. Conclusions In this work, a novel PEDEGS based nanocomposite containing low loading (0.5 wt%) of ovi-POSS was prepared via a solution and casting method. The SEM image showed that a uniform distribution of ovi-POSS throughout the PEDEGS matrix was achieved and the dimensions of ovi-POSS aggregate were around 150 to 400 nm. The effects of low loading of ovi-POSS on the isothermal melt crystallization kinetics, crystalline morphology, and crystal structure of PEDEGS in the nanocomposite were studied systematically with DSC, POM, and WAXD. For both neat PEDEGS and its nanocomposite, the isothermal melt crystallization rates increased with decreasing crystallization temperature. PEDEGS/ovi-POSS crystallized faster than neat PEDEGS at the same crystallization temperature, while the crystallization mechanism remained unchanged. In the nanocomposite, the nucleation density of PEDEGS spherulites increased. Furthermore, the crystal structure study showed that ovi-POSS crystallized separately in PEDEGS and did not modify the crystal structure of the matrix. From these results, ovi-POSS was an effective nucleating agent for PEDEGS. One possible mechanism was epitaxial nucleation. Ovi-POSS reduced the storage modulus of PEDEGS at lower temperature, and the glass transition temperature of PEDEGS in the nanocomposite also decreased. In brief, in the case of the novel PEDEGS based

18


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nanocomposite, ovi-POSS acted as not only an efficient nucleating agent to enhance the crystallization rate but also a plasticizer and lubricant to reduce the storage modulus of PEDEGS. Supporting Information Preparation of PEDEGS/ovi-POSS nanocomposite. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements Part of this research was financially supported by the National Natural Science Foundation, China (51373020, 51573016 and 51521062).

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For TOC Use Only 6000 neat PEDEGS PEDEGS/ovi-POSS

5000 Storage modulus (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4000 3000 2000 1000 0 -40

-20

0

20

40

60

o

Temperature ( C)

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Polyhedral Oligomeric Silsesquioxanes in Novel ... - ACS Publications

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