Structure, Dynamics and Mechanical Properties of Polyimide-Grafted

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Structure, Dynamics and Mechanical Properties of Polyimide-Grafted Silica Nanocomposites Yu Lin, Shani Hu, and Guozhang Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12519 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 4, 2019

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The Journal of Physical Chemistry

Structure, Dynamics and Mechanical Properties of Polyimide-Grafted Silica Nanocomposites Yu Lin, Shani Hu, Guozhang Wu* Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China

*

Corresponding author. Tel.: +86-21-64251661, E-mail: [email protected] (G. Wu). 1

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ABSTRACT: A continuing challenge in polymer nanocomposites (PNCs) is to control nanoparticle (NP) dispersion and understand its role on property enhancements. Previous studies have been focused on the flexible polymer chain systems. In this study, we report the structure, glass transition behavior, segmental dynamics and mechanical properties of polyimide (PI) nanocomposites that consisted of either bare or PI-grafted silica NPs, to discriminate the role of grafted rigid chains on polymer-NP interactions and dynamic response of such hybrids. Silica NPs are well dispersed in the poly(amic acid) (the precursor of PI) nanocomposites. After thermal imidization, aggregate structure and self-assembled small clusters of NPs are observed in ungrafted and grafted silica NP-filled PI composites, respectively. The glass transition temperature (Tg) shifts to high temperature by the addition of silica NPs resulting from strong polymer-NP interactions and steric hindrance, and Tg deviation is more visible with increasing the molecular weight of grafted PI chains. The α-relaxation dynamics are suppressed in PI nanocomposites, but there is no interfacial layer relaxation detected because of the rigid chain characteristic of PI. The accelerated Maxwell-Wagner-Sillars polarization process is noted in the presence of PI-grafted silica NPs with high molecular weight. PI-grafted silica NPs are effective in improving polymer-NP interfacial adhesion, resulting in superior mechanical properties to those of bare silica NP-filled composites. Moreover, high molecular weight grafted PI chains are beneficial in mechanical enhancement of the resultant PI nanocomposites. These findings provide new insight into the fundamental understanding of chain packing and relaxation dynamics of PNCs with rigid polymer chains, and therefore provide guidance in designing such materials 2


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with desired macroscopic properties.

3



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 INTRODUCTION Polymer nanocomposites (PNCs) with tailorable performances have been applied in a diverse range of industrial fields because of their significantly improved physicochemical properties rendered by the introduction of nanoparticles (NPs) into polymer matrix.1, 2 While considerable effort has been focused on constructing the specific NP dispersion to optimize the desired macroscopic properties of PNCs, a recurring challenge is to control NP dispersion and understand its role on property enhancements.3 Importantly, the issues that determine interfacial interactions and NP spatial distribution, and how they affect dynamics and macroscale performance of such hybrids,4, 5 remain largely unresolved to date, which hinder the broader application of these materials. Multiple strategies have been developed to tailor polymer-NP interfacial interaction free energy and thereby control the NP dispersion state throughout a polymer matrix.4, 6-9

Grafting polymer chains on the NP surface is one facile strategy to achieve the

specific NP dispersion state. The guiding principle is well established that polymer-NP miscibility and hence spatial distribution is dependent on the grafting density of chains (σ) and the ratio of polymerization degrees of matrix (P) and grafted chains (N), verified by numerous experimental and simulations results.10-17 Recently, Kumar et al.18 reviewed the available literature results and created the composite morphology diagram as a function of σ and P/N (Figure S1). Different self-assembled anisotropic structures of NPs are observed by changing these two parameters. However, much effort has been focused on the case where both graft and matrix are flexible chains such as polystyrene 4


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(PS),19-23 poly(methyl methacrylate) (PMMA),15, 24 poly(ethylene oxide) (PEO)14, 17, 25, 26

and poly(butyl acrylate) (PBA),27 the use of rigid chains remains unexplored. In addition, structure evolution,13, 28-32 dynamics change24, 33-35 and enhancement in

mechanical properties36-40 are expected resulting from the presence of interface between grafted NPs and polymer matrices. In recent decades, intensive researches have been reported the incorporation of ungrafted NPs on the glass transition and segmental dynamics of PNCs.41-50 Due to different polymer-NP interfacial interactions, either an increased, decreased or unchanged glass transition temperature (Tg) can be observed,5155

and the relaxation dynamics will either slow down or speed up.50, 56-58 However, few

studies has been focused on the case of grafted NP-filled PNCs because the polymerNP interactions and glassy dynamics are complicated endowed by the grafted chains.3335, 59-61

Molecular dynamics simulations and experimental results reveal that we can

control the glass transition behavior and dynamics of polymer chains near grafted NPs by changing σ, P and N.33-35, 61 Importantly, the chain packing in rigid chain grafted NPfilled PNCs differs from that in flexible chain ones, and possibly influence the glass transition dynamics and the mechanisms responsible for mechanical enhancement of such hybrids, which remains to be explored. As one of the most excellent engineering polymers, polyimide (PI) has attracted increasing attention because of their combination of high Tg, excellent thermal stability, superior mechanical properties, outstanding radiation and chemical resistance.62,

63

Achieving a specific NP dispersion state and desirable properties by incorporation of NPs to PI matrix is still a great challenge. Fully aromatic polyimides containing imide 5



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structure have rigid chains, which result in different chain packing structure and relaxation dynamics than those of flexible chain polymers. A fundamental understanding of this issue is critical to optimize a desired property of PI composites. Nevertheless, very few studies have explored the relationships between interfacial chain packing and relaxation behavior of NP-filled PI composites,64 and how they affect the mechanical properties remains inadequately understood and largely unresolved. In this study, PI-grafted spherical silica NPs with different grafting density and chain lengths were introduced to PI matrix to investigate the effect of grafted rigid chain on the spatial distribution of NPs. Different structure, dynamics and property modifications are expected for ungrafted and PI-grafted NP-filled composites. The glass transition behavior, segmental dynamics and interfacial polarization process of PI nanocomposites are discussed in detail. Furthermore, mechanical properties of PIgrafted NP-filled composites are also compared with those of bare NP-filled ones to reveal the mechanisms responsible for the dynamics and mechanical reinforcement of such hybrids.

 EXPERIMENTAL SECTION Raw Materials. 4,4'-(Hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 4,4'-oxidianiline (ODA) as diamine were used to synthesize soluble PI and purchased from Changzhou Sunlight Pharmaceutical Co., Ltd. The coupling agent (3aminopropyl)triethoxysilane (APTES) was purchased from Aladdin Reagent Co., Ltd., China. Anhydrous N-methylpyrrolidinone (NMP) and N,N-dimethylformamide (DMF) 6


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were obtained from Shanghai Titan Chemical Co., Ltd. All the reagents were used without further purification. Preparation of APTES Modified Silica NPs. The SiO2 NPs with an average radius of 30 nm were synthesized in a suspension of ethanol/deionized water mixture according to the modified Stöber method.65 The pre-dried SiO2 NPs were modified using APTES in toluene at 323 K for 24 h under stirring. After the reaction, the resultant suspension was centrifuged and the precipitates were carefully washed with ethanol for at least three times to remove the unreacted APTES. The modified SiO2-APTES NPs were dried at 353 K in a vacuum oven for 24 h. Preparation of SiO2-g-PI Composite NPs. Firstly, soluble PI with different molecular weights were synthesized by changing the mole ratio of monomers and used for grafting reaction. Table S1 lists different formulations of PI made herein. As an example, the synthesis of sample 1 from Table S1 is as follows: ODA (0.1 mol) was completely dissolved in NMP solvent, and then 6FDA (0.1 mol) was gradually added to the mixture with stirring. The mixture was stirred for 6 h in an ice-water bath until dissolved to form viscous yellow polyamic acid (PAA, the precursor of PI) solution with a solid content of 10 wt%. The solution was diluted with NMP to form a concentration of 5 wt%, then cast on to flat glass Petri dishes. PAA films were prepared after solvent evaporation at 353, 373, and 423 K for 2 h, respectively. They were then thermally imidized at 473, 523, and 573 K for 1 h, respectively, to fabricate PI films. The PI-grafted silica (SiO2-g-PI) composite NPs were synthesized via “grafting to” method, as shown in Figure S2. Briefly, 0.3 g of PI films were completely dissolved in 7



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DMF solvent, and then 0.15 g of SiO2-APTES NPs were gradually added to the mixture with stirring. After reaction at 323 K for 24 h, the resultant product was centrifuged, and the precipitates were carefully washed with DMF for at least three times to remove the ungrafted PI. Finally, the obtained SiO2-g-PIx (the subscript x represents the average molecular weight of grafted chain) composite NPs with different grafting density and chain lengths were dried at 353 K in a vacuum oven for 24 h. Preparation of NP-filled PNCs. Different desired amounts of ungrafted or grafted silica NPs were dispersed in NMP and ultrasonicated in a water bath for 30 min. Then PAA1 (Table S1) was added to the solution at a concentration of 5 wt% under continuous stirring for 24 h at room temperature. The same as the procedure of preparing pure PAA and PI films described above, solution casting and thermal imidization were used to prepare PAA and PI composite films with various silica NP loadings. The thickness of resultant film samples was about 0.2 mm. Characterization. Fourier transform infrared (FTIR) spectra were conducted on a Nicolet 5700 FTIR spectroscope at a range of 400-4000 cm-1 measured at room temperature. X-ray photoelectron spectroscopy (XPS) was performed by using a thermo scientific ESCALAB 250Xi equipped with Al Ka X-ray source. Thermogravimetric analysis (TGA; Netzsch STA 409) were carried out under an air atmosphere from 303 K to 1073 K at a heating rate of 10 K/min. The number average molecular weight (Mn) and polydispersity index (PDI) of PI were characterized in DMF solution by using gel permeation chromatograph (GPC; Waters 1515) equipped with a 79911GP-MXC column and an RI detector at 298 K. The morphology of grafted NPs 8


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and the dispersion state of silica NPs in the PNCs were observed by using a Transmission electron microscope (TEM; JEM 1400, Electron, Japan). Tgs of grafted NPs and PNCs were determined by a differential scanning calorimeter (Q100, TA, USA). The samples were ﬁrst heated at the heating rate of 10 K/min from 323 to 623 K and held for 5 min to eliminate thermal history, then cooled down to 323 K at 10 K/min. The samples were reheated to 623 K at 10 K/min to obtain Tg values. Broadband dielectric spectroscopy (BDS) measurements were performed on a Novocontrol Alpha High-resolution Dielectric Analyzer (Novocontrol GmbH Concept 40; Novocontrol Technology, Germany). Isothermal frequency sweeps were carried out at the frequency range of 10−1-107 Hz over the temperature range of 573-623 K for pure PI and nanocomposites. The mechanical properties of the films were determined using a universal material testing machine (Instron 4320) according to the standard of GB 1302291. At least five samples were measured for each composite film to obtain the average values.

 RESULTS AND DISCUSSION Characterization of PI Grafted Silica NPs. FTIR spectra are first used to investigate the grafting structure of SiO2-g-PI composite NPs. As shown in Figure S3, compared to the spectra of ungrafted silica NPs, new absorption bands are observed at 2930 and 2850 cm-1, assigned to the vibrations of -CH2- and C-H groups of APTES, indicating the surface modification of silica NPs. As shown in Figure 1a, the characteristic absorption bands of PI can be noted from the spectra of SiO2-g-PI 9



composite particles: stretching vibration peak of imide group (-CONCO-) at 1376 cm1

, the absorption peak of p-substituted benzene at 1506 cm-1, and C=O vibration peak

at 1732 cm-1. Because the ungrafted PI chains were removed by washing with DMF solvent, FTIR spectra confirm that PI chains are successfully grafted onto the surface of APTES modified silica NPs. The color of silica NPs develop from white to light yellow (shown in Figure S4), also indicating the successful grafting reaction.

(a)

(b)

2000

1732 1506 1376

Intensity (a.u.)

C-N (400.5eV)

Absorbtance(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

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SiO2-g-PI SiO2

1600

1200

800 -1

400

Wavenumber(cm )

408

-NH- (399.7eV) -NH2 (401.5eV)

406

404

402

400

398

396

394

Binding Energy (eV)

Figure 1. (a) FTIR spectra of bare SiO2 and SiO2-g-PI composite particles. (b) XPS N1s peak of SiO2-g-PI composite particles.

In order to further validate the chemical bonds rather than physical adsorption on the surface of silica NPs, the XPS N1s spectra of SiO2-g-PI composite NPs are presented in Figure 1b. Obviously, peaks at 401.5, 400.5 and 399.7 eV are observed, which are assigned to –NH2, C-N and –NH- (the chemical structure of SiO2-g-PI shown in Figure S5), respectively. The XPS results further demonstrate that PI chains are successfully grafted onto the the silica NP surface, consistent with FTIR results. On the other hand, DSC results (Figure S6) show that Tg of SiO2-g-PI composite NPs is much higher (~ 40

10


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K) than that of pure PI, indicating the restricted mobility of grafted PI chains, which can be ascribed to the chemical bonding between silica NP surface and grafted chains. To quantitatively characterize the grafting ratio and grafting density, TGA curves of ungrafted, APTES modified and PI grafted silica NPs are shown in Figure 2. Due to the nature of hydrophilic, the weight loss of bare silica NPs is assigned to the volatilization of adsorbed water and dehydration of silanols. And that of SiO2-APTES NPs corresponds to the decomposition of coupling agent molecules. The evident weight loss can be observed at about 750-900 K in SiO2-g-PI composite NPs, which is attributed to the degradation of grafted PI chains. Therefore, the grafting ratio can be obtained by TGA residues, and σ, defined as the number of polymer chains per unit surface area of NPs, can be calculated as follows:10 σ = gdDρPNA/6Mg

(1)

where gd is the grafting degree, D is the diameter of NPs, ρP is the NP density, NA is Avogadro’s number, and Mg is the grafted chain molecular weight. Table 1 lists the parameters of the grafted chain information for SiO2-g-PI NPs. Obviously, the graft density is much lower than that of PS grafted NPs in our previous study.23, 60 Moreover, σ of SiO2-g-PI NPs decreases with increasing the molecular weight of grafted chains. It can be ascribed to the fact that steric repulsion between already attached polymer chains and the ones diffusing to the NP surface limits the available σ in the “grafting to” method.66 On the other hand, the rigid chain characteristic of PI also contributes to the low σ.

11



100

95

Weight (%)

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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90 SiO2 SiO2-APTES SiO2-g-PI61k

85

SiO2-g-PI37k SiO2-g-PI18k

80 300

400

500

600

700

800

900 1000

Temperature (K)

Figure 2. TGA curves of bare SiO2, APTES modified SiO2, and SiO2-g-PI composite particles with different degrees of polymerization of grafted chains. Table 1. The Parameters of Grafted Chains for SiO2-g-PI Composite Particles Sample

NP code

Mg (kg/mol)

gd(%)

σ

σ·N0.5

1/α (P/N)

1

SiO2-g-PI61k

61

3.39

0.0037

0.04

1.00

2

SiO2-g-PI37k

37

4.20

0.0075

0.06

1.65

3

SiO2-g-PI18k

18

5.36

0.0197

0.10

3.39

Dispersion Morphology of PI Grafted Silica NPs in PNCs. The spatial distribution of SiO2-g-PI NPs in PI nanocomposites might be different from those flexible chains grafted NPs in the polymer matrix, owing to the nature of rigid PI chains. Importantly, dispersion morphology evolution must be occurred during the thermal imidization process in fabrication of PI nanocomposites. Figure 3 shows the TEM images of bare and grafted silica NP-filled PAA composites. Generally uniform NP dispersion can be observed for both ungrafted and grafted silica NP-filled PAA composites. This observation indicates that the formation of NP aggregates does not take place during the solvent evaporation process to fabricate PAA composite films. However, the NP dispersion is not in the thermodynamic equilibrium state. The thermal imidization process involves changes in chain structure and thermal annealing, thus the NP 12


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dispersion state in PI composites is much different from that in PAA composites, as shown in Figure 4. In the case of bare silica-filled PNCs (Figure 4a), it is obvious that large aggregate structure of NPs is formed due to the large specific surface area, high interface energy and self-aggregation of NPs. However, small clusters of NPs can be observed in SiO2-g-PI filled composites (Figure 4b-d), independent of grafting chain length. It is generally accepted that the NP spatial distribution is closely determined by σ and P/N.7, 8, 11, 15, 25, 67 According to the related parameters of grafted NPs listed in Table 1 and the classical composite morphology diagram (Figure S1) created by Kumar et al.,18 they are expected to appear connected sheets and small clusters morphology, respectively. However, experimental observation only shows small clusters in PI/SiO2g-PI nanocomposites. It should be emphasized that good dispersion is never attained when the value of σ·N0.5 is below 2.10 The grafting density is very low due to the “grafting to” method discussed above, thus the values of σ·N0.5 are well below 2, as shown in Table 1. Grafted NPs assemble into anisotropic structures involving the exclusion of polymer chains from the interparticle space through diffusion. On the one hand, the PI chain diffusion is relative low in the composite melt due to the rigid chain structure. Hence, it is difficult for the grafted NPs to self-assemble into specific dispersion state such as phase separated morphology, strings and connected sheets. On the other hand, the grafted PI chains on the surface of NPs also limit the possibility of NP spatial distribution because of the rigidity. Furthermore, there is no obvious difference between the dispersion of SiO2-g-PI with various PI molecular weights, which can be attributed to the low grafting density and narrow molecular weight range 13



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of grafting chains (18 to 61 kg/mol). (a)

(b)

Figure 3. TEM images of (a) bare SiO2 and (b) SiO2-g-PI61k composite particle filled PAA nanocomposites. The silica NP concentration is 10 wt%. Scale bar = 1 µm. (a)

(c)

(b)

(d)

Figure 4. TEM images of (a) bare SiO2, (b) SiO2-g-PI61k, (c) SiO2-g-PI37k and (d) SiO2g-PI18k composite particle filled PI nanocomposites. The silica NP concentration is 10 wt%. Scale bar = 1 µm. It is well established that the spatial distribution of NPs is closely related to the viscosity of polymer melts and thermal annealing temperature.18 The viscosity of PAA is much lower than that of PI, thus the easier diffusion and motion of NPs are expected in the PAA nanocomposites. The well dispersion of silica NPs in PAA matrix can be attributed to the stabilizing effect of solvent and relative low drying temperature. The self-assembled structures of NPs in the resultant PI composites are ascribed to the high temperature annealing during the thermal imidization process.6, 18 Therefore, we can conclude that the spatial distribution of NPs in PI composites is dominated by high temperature rather than viscosity of PI. 14


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Glass Transition Behaviors of PNCs. To clarify the incorporation of grafted NPs on the glass transition behavior, Tg of ungrafted and grafted silica NP-filled PI nanocomposites are investigated. Figure 5 shows the DSC curves of bare and different grafted silica NP-filled PI nanocomposites with various NP contents. Compared to pure PI, an increased Tg can be observed in all the nanocomposites filled with either bare or grafted silica NPs. Moreover, Tg increases with increasing the NP loading. For PI/SiO2 nanocomposites (Figure 5a), the increase of Tg can be ascribed to the steric hindrance of NP loading, decreasing the polymer chain mobility. In addition, the hydrogen bonding interactions between hydroxyl groups on the silica NP surfaces and remaining amino groups of PI contribute to the Tg enhancement. These results are consistent with those of other PNC systems,5, 68, 69 but seem to contrast with the invariant Tg54, 55, 70-72 or appearance of another new Tg at a higher temperature originating from the restricted bound polymer chains.73-76 In the case of PI/SiO2-g-PI nanocomposites, such an increase in Tg can be attributed to the strong polymer-NP interactions between grafted chains and PI matrix, limiting the chain mobility. Ferreira and co-workers77 identified the scaling law for entropic effects in grafted NP-filled composites. They demonstrated the existence of diffusion and interpenetration between grafted chains and polymer matrix if σ·N0.5

Structure, Dynamics and Mechanical Properties of Polyimide-Grafted

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