The Molecular Stiffening of Polymers Ionically Grafted on the Surface of Molecular Clusters

Abstract

Polymer nanocomposites (PNCs) have been broadly applied in various industries for the synergy of nanoparticles (NPs)’ functions and polymers’ flexibility; nevertheless, the study of NP/polymer interfacial structures is still rare. Herein, the cation-terminated polystyrene (PS) is ionically associated with a 2.9 nm anionic molecular cluster, {Mo₁₃₂}, and the obtained hairy NPs are applied as model systems for the studies of structures and relaxation dynamics of the absorbed polymer layers. The M _w of PS is systematically varied from 1.1 to 9.8 k, while structural analysis from X-ray and light scattering confirms the morphologies of the fabricated hairy NPs. The grafted polymers show significantly slower segment dynamics up to two orders of magnitude compared to pristine ones due to the surface nanoconfinement effect, and the PNCs demonstrated enhanced glass transition temperatures and mechanical strengths. Moreover, dynamics heterogeneity of the grafted polymers can be probed in broadband dielectric spectrum measurements: The segments close to the {Mo₁₃₂} core exhibit slower relaxation dynamics compared to the segments far from the surface of {Mo₁₃₂} due to the varied confinement condition. This work provides microscopic understanding of the interface structure and dynamics in PNCs and sheds lights on the unique role of ultrasmall particles in mechanical strengthening.

Introduction

Polymer nanocomposites (PNCs) are prepared by dispersing nanoparticles (NPs) into the polymer matrix for the expectation to synergy NPs’ broad functionalities and polymers’ mechanical flexibility and processability. , The relaxation dynamics of polymer chains will be altered with the introduction of NPs, − resulting in a wide range of tunability of their macroscopic properties, including mechanical properties, ionic conduction, and gas transport. − The deep understanding of the microscopic mechanism regulating PNCs’ macroscopic properties is crucial to the rational design of functional materials with expected performance. , However, the vague definition of surface functionality and the poly-dispersed sizes and morphologies of NPs impose a great barrier for the study of the structure–property relationship of PNCs. , Specifically, the NPs with well-defined structures are urgently required for the construction of a model system of PNCs for further mechanism study.

Polyoxometalates (POMs) are a class of nanoscale anionic metal oxide clusters with monodispersed, well-defined structures while they can deliver strong hydrogen bond and/or electrostatic interaction with polymers due to their highly charged feature and enriched surface oxo or hydroxy ligands. − Benefiting from their unique coordination structure and ultrasmall sizes (< 5 nm), POMs have extremely high specific surface areas, which leads to their admirable catalytic efficiencies, ion conductivities, electromagnetic properties, and excellent stabilities. − The ultrasmall size of POM shall bring intense impacts on polymers in PNCs, such as notably varied glass transition temperature and fragility due to their high specific surface areas. In this work, the cation-terminated polystyrene (PS) is ionically associated with 2.9 nm anionic POMs, {Mo₁₃₂} (formula: [Mo^VI ₇₂Mo^V ₆₀O₃₇₂ (CH₃COO)₃₀(H₂O)₇₂]^42–), and the obtained hairy NPs are applied as model systems for the studies of structures and relaxation dynamics of the absorbed polymer layers (Figure ). Light scattering and small angle X-ray scattering (SAXS) are applied to analyze the structure thoroughly, while the relaxation dynamics of grafted PS are investigated using broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). The macroscopic structural stiffening of the PNCs is evaluated by typical nanoindentation technology to confirm the correlation among PNC structures, relaxation dynamics, and physico-chemical properties.

1.

Synthesis of {Mo₁₃₂}-PS by the “grafting to” method and schematic diagram of the concentrated polymer brush (CPB) region and the semidilute polymer brush (SDPB) region in {Mo₁₃₂}-PS.

Experimental Section

Materials

Chemical reagents used in this work were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. All chemicals were used without further purification. DMF was purified by vacuum distillation under a nitrogen atmosphere; other solvents were purchased from commercial sources and used without further purification. Deionized water was obtained from the ultrapure water system (Water Purifier, WP-RO-10B).

Synthesis of CH₃(CH₂)₃SCS₂C­(CH₃)₂COOH (Compound a)

10 g of 1-butanethiol and 23.55 g of potassium phosphate were added to a 500 mL beaker containing 100 mL of acetone. The mixed solution at 0 °C under N₂ atmosphere was stirred for 10 min. 8.44 g of carbon disulfide was added to a mixed solution and stirred under N₂ atmosphere for 1 h. After stirring, ice bath was removed, and 18.37 g of 2-bromoisobutyric acid was added while stirring; it was stirred under N₂ atmosphere for 24 h. The solution was filtered, the filtrate was collected, and the solvent was removed by a rotary evaporator to obtain the crude product. The crude product was redissolved in n-pentane and washed three times with 10 wt % HCl and distilled water and filtered again, and the solvent was removed through a rotary evaporator. The raw residue was purified by column chromatography on silica gel with PE/EA = 20:1 as eluent to get pure compound a.

Synthesis of CH₃(CH₂)₃SCS₂C­(CH₃)₂COO­(CH₂)₁₁Br (Compound b)

0.98 g of a and 1.00 g of 11-bromo-1-ol were weighed and dissolved in 38 mL of DCM and stirred in an ice bath for 30 min to obtain a clear solution. 0.83 g of N,N′-dicyclohexylcarbodiimide dissolved in 8 mL of methylene chloride was added to the mixed solution and stirred for 5 min. Then, 12.3 mg of 4-dimethylaminopyridine dissolved in 5 mL of methylene chloride was added and stirred for 30 min. The ice bath was removed and stirred at room temperature for 12 h, and the color of the solution gradually changed from red to yellow. The solution was filtered to remove insoluble solids. The filtrate was concentrated with a rotary evaporator and dropped into PE dropwise to obtain the crude product. The crude product was purified by column chromatography on silica gel with PE/EA=20:1 as eluent to get pure compound b.

Synthesis of Br-N⁺(CH₃)₃(CH₂)₁₁OOCC­(CH₃)₂SCSS­(CH₂)₃CH₃ (Compound c)

4.4 g of b was weighed and dissolved in 40 mL of ethanol, and 8.06 g of 33 wt % trimethylamine solution was added under stirring; it was stirred at room temperature without light for 5 days. The solution was concentrated with a rotary evaporator and precipitated with ether to get the crude product. The crude product was dissolved in chloroform and precipitated three times with deionized water to obtain the pure compound c (N⁺-CTA).

Synthesis of PS-CTA-N⁺

10 g of styrene, 0.92 g of c, 0.043 g of azodiisobutyronitrile, and 10 mL of DMF were added to a 50 mL Schlenck flask under N₂ atmosphere. Degasification of the reaction mixture was performed via freeze–pump–thaw cycling three times. Polymerization was performed at 65 °C for 48 h. The solution was concentrated with a rotary evaporator and precipitated with methanol to get the crude product. The crude product was dissolved in tetrahydrofuran, precipitated three times with methanol, and dried under vacuum to obtain a pure product. PS-CTA-N⁺ with five different molecular weights (M _w = 1.1, 3.0, 5.7, 7.3, 9.8 kg/mol) was prepared by the same method.

Synthesis of {Mo₁₃₂}-PS

PS-CTA-N⁺ was dissolved in 5 mL of chloroform; {Mo₁₃₂} in 5 mL of deionized water. The two solutions were mixed and stirred at room temperature for 48 h. n({Mo₁₃₂}):n(PS-CTA-N⁺) = 1:42 was controlled, depending on the charge ratio of the two reactants. The chloroform phase was separated and washed with deionized water and methanol three times. Chloroform was removed by a rotary evaporator to get the crude product. The crude product was purified by fractional precipitation with toluene and methanol in the temperature range from room temperature to 50 °C. The purification was repeated three times to obtain pure product {Mo₁₃₂}-PS.

Characterization

Nuclear magnetic resonance (NMR) experiments were performed on a Bruker AVANCE II 500 spectrometer at 298 K. Fourier transform infrared spectroscopy (FT-IR) experiments are performed on a Nicolet iS50 built by Thermo Fisher with an attenuated total refraction setup. Gel permeation chromatography (GPC) experiments were performed on a HLC-8420GPC built by Tosoh. Small-angle X-ray scattering (SAXS) experiments were performed on the HomeLab built by Rigaku. Dynamic light scattering (DLS) experiments were performed on an LLS built by Brookhaven. Thermal gravimetric analysis (TGA) experiments were conducted on a TGA 5500 built by TA. Differential scanning calorimetry (DSC) experiments were performed on a DSC 3 built by Mettler Toledo. Broadband dielectric spectroscopy (BDS) experiments were conducted on Novocontrol Concept 80. AFM was conducted on Cypher S built by Oxford. The transmission electron microscopy (TEM) experiments were conducted on a JEM-F200 built by Japan Electron Optics Laboratory.

Results and Discussion

Synthesis and Structural Characterization of {Mo₁₃₂}-PS

{Mo₁₃₂}-PS is synthesized by the typical “grafting to” method. , Polystyrene with ammonium as the end group (PS-CTA-N⁺) is synthesized, and the {Mo₁₃₂}-PS complex is prepared from its ionic interaction with {Mo₁₃₂}. {Mo₁₃₂} is prepared by the method reported in the literature, while PS-CTA-N⁺ with different molecular weights (M _w) are prepared through the reversible addition–fragmentation chain transfer (RAFT) polymerization method. The cationic PS-CTA-N⁺ is grafted onto {Mo₁₃₂} through phase transfer reaction, and {Mo₁₃₂}-PS with different M _w can be obtained, noted as {Mo₁₃₂}-PS­(n), where n is the M _w of the grafted PS.

The successful synthesis of {Mo₁₃₂}, N⁺-CTA, and PS-CTA-N⁺ is characterized by nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FT-IR) (Figures S1–S7). The integration of polymers after complexation with {Mo₁₃₂} can be confirmed from the inheriting of characteristic information in NMR and FT-IR. Meanwhile, the M _w and distribution of PS-CTA-N⁺ are determined by gel permeation chromatography (GPC) (Figure a). The M _w of these five cationic PSs are 1.1, 3.0, 5.7, 7.3, and 9.8 kDa, respectively, and the PDIs are all controlled to be less than 1.2 to ensure the credibility of extensive polymer physics studies. Specifically, the exact values of the PDIs of these five cationic PSs are 1.08, 1.18, 1.15, 1.18, and 1.08, respectively. Based on the principle of ionic interaction, {Mo₁₃₂}-PS composites are prepared by phase transfer reaction between water and chloroform. According to the charge ratios of {Mo₁₃₂} and PS-CTA-N⁺, the molar ratio of the reaction is n(PS-CTA-N⁺):n({Mo₁₃₂}) = 42:1. The successful synthesis of {Mo₁₃₂}-PS is confirmed by the ¹H NMR spectrum and FT-IR (Figures S8 and S9).

2.

(a) GPC spectra of PS-CTA-N⁺. (b) SAXS profiles of {Mo₁₃₂}-PS with different M _w. (c) CONTIN results of {Mo₁₃₂}-PS with different M _w. (d) TGA results of {Mo₁₃₂} and {Mo₁₃₂}-PS with different M _w.

X-ray is sensitive to metal centers, and therefore, the SAXS technique is utilized to characterize the microstructures of PNCs of {Mo₁₃₂}-PS. As shown in Figure b, both of the form factor P(q) reflecting the morphology of {Mo₁₃₂} and the structure factor S(q) related to the inter-particle correlation can be observed without diffraction peaks, verifying the structural integrity of {Mo₁₃₂} and the amorphous nature of the PNCs. With the increase of M _w of PS, the structure factor shifts to the low q region, representing the increase of the spacing of {Mo₁₃₂}. The spacing of {Mo₁₃₂} can be calculated as 6.1, 6.8, 8.0, 8.6, and 8.9 nm, respectively, based on the equation d = 2π/q, which are larger than the diameter of {Mo₁₃₂} (d = 2.9 nm). Transmission electron microscopy (TEM) provided us with images of the monodisperse state of {Mo₁₃₂}-PS in the limited microscopic areas that are under focus (Figure S15), which are consistent with SAXS results. Dynamic light scattering (DLS) is applied to characterize the size of {Mo₁₃₂}-PS, and the composites are monodispersed in dilute solution with the particle sizes increasing upon the increasement of the PS M _w (Figure c). This result further demonstrates the hairy NP morphology of the title composites. Thermogravimetric analysis (TGA) is used to confirm the grafting densities of {Mo₁₃₂}-PS with different M _w (Figure d). For {Mo₁₃₂}, the weight loss can be divided into three stages, and the first stage (from 25 to 200 °C) corresponds to the loss of both lattice and coordinated water. The second stage (from 200 to 500 °C) corresponds to degradation of ammonium acetate fragments and the fallen of the {Mo₁₃₂} cluster, while the third stage (over 700 °C) corresponds to the evaporation of MoO₃. The TGA curves of {Mo₁₃₂} exhibit a plateau once the temperature exceeds 550 °C, while PS-N⁺-CTA is completely decomposed at 550 °C, whose mass residue is zero. Therefore, the grafting density can be calculated by the mass residue of {Mo₁₃₂}-PS and {Mo₁₃₂} at 550 °C. From {Mo₁₃₂}-PS (1.1k) to {Mo₁₃₂}-PS (9.8k); the grafting densities calculated are 38, 36, 22, 19, and 16 per {Mo₁₃₂}. With the increase of the M _w of PS, the grafting density gradually decreases, which is due to the steric hindrance and entropic penalty resulting from the grafted PS chains.

Chain Relaxation Dynamics of the Grafted PS

The polymers under spatial confinement always exhibit varied relaxation dynamics from the pristine polymers, and DSC and BDS are applied to quantify the chain relaxation dynamics {Mo₁₃₂}-PS and pristine PS for comparative studies. As shown in Figure a, the incorporation of {Mo₁₃₂} leads to an increase of the glass transition temperature (T _g) for all the {Mo₁₃₂}-PS composites compared to that of pure PS with equivalent M _w. This can be attributed to the restricted segmental relaxation resulting from the ionic attraction between {Mo₁₃₂} and PS. Due to the unique confinement exerted by spherical geometry of {Mo₁₃₂}, the PS chain segments can be divided into two regions: the concentrated polymer brush (CPB) region that is close to the {Mo₁₃₂} core and the semidilute polymer brush (SDPB) region that is far away from the {Mo₁₃₂} core (Fig. ). The chain segments in the CPB region suffer from stronger confinement compared to that in the SDPB region, and thus, the mobility of the chain segment in the CPB region is much more reduced. Therefore, as the M _w increases, the disparity in T _g between {Mo₁₃₂}-PS and pure PS decreases (Figure b). Such a phenomenon can be attributed to the reduced proportion of PS in the CPB region with the increase of M _w.

3.

(a) DSC curves of PS and {Mo₁₃₂}-PS with different M _w. (b) Variation of the glass transition temperature of PS and {Mo₁₃₂}-PS with different M _w. (c) HN fitting of PS (9.8 k) and {Mo₁₃₂}-PS (9.8 k) at 180°C. (d) Temperature dependence of α relaxation of PS and {Mo₁₃₂}-PS with different M _w.

The relaxation time for polymer chain dynamics is probed using BDS, and their temperature dependence is indexed as fingerprint information to differentiate their structural origin. For pristine PS, a single α relaxation process can be observed within the test temporal window of BDS that corresponds to the cooperative chain relaxation mode. Interestingly, the {Mo₁₃₂}-PS composites exhibit two distinct relaxation processes, originating from the structural heterogeneity of the co-existence of CPB and SDPB regions (Figure c). The relaxation peak observed at the high frequency range corresponds to the relaxation of PS segments in the SDPB region (α relaxation) while the peak at low frequency is ascribed to the relaxation of PS segments in the CPB region (α′ relaxation). The temperature dependence of α relaxation of pure PS and {Mo₁₃₂}-PS can be well fitted by the Vogel–Fulcher–Tammann (VFT) equation (Figure d). Since α′ relaxation only appears at high temperature, there are not enough points to analyze the temperature dependence of α′ relaxation. In general, the α relaxation of {Mo₁₃₂}-PS is slower than that of pure PS with the same M _w. With the increase of M _w of PS, the α relaxation of both pure PS and {Mo₁₃₂}-PS becomes slower and the characteristic relaxation time of α relaxation of these two becomes closer, which is consistent with the results of DSC.

Mechanical Properties of {Mo₁₃₂}-PS Composites

The mechanical strengths of {Mo₁₃₂}-PS are assessed using atomic force microscopy (AFM). Taking {Mo₁₃₂}-PS (9.8 k) as an example, as illustrated in Figure a, the height variation across the sample surface is limited to 20 nm. Furthermore, the modulus values measured at different locations on the sample are consistent, suggesting that the surface is flat and the distribution of modulus is homogeneous (Figure b). The average modulus for each sample is determined through fitting with a Gaussian distribution function (Figure S10). Figure c presents the correlation between modulus and M _w for both PS and {Mo₁₃₂}-PS, and the modulus of both of PS and {Mo₁₃₂}-PS increase with higher M _w. For samples with the same M _w, the incorporation of {Mo₁₃₂} will obviously enhance the modulus due to the rigidity of the cluster. Furthermore, the slowdown of segmental relaxation makes the deformation difficult, which further contribute to the increase of modulus. Figure d illustrates the enhancement in the modulus of {Mo₁₃₂}-PS compared to that of PS with equivalent M _w. Notably, when M _w is 3.0k or 5.7k, the modulus increase exceeds 120%. However, at higher M _w, the difference in modulus falls below 100%. This trend may be attributed to average effect, as M _w increases, the proportion of the CPB region diminishes, causing the properties of {Mo₁₃₂}-PS to be more similar to pure PS. At very low M _w (1.1k), the low M _w may limit the modulus enhancement, resulting in a lower modulus increase for {Mo₁₃₂}-PS (1.1k) compared to {Mo₁₃₂}-PS (3.0k).

4.

(a) Height plot (left) and modulus plot (right) of {Mo₁₃₂}-PS (9.8 k). (b) Modulus Gaussian distribution of {Mo₁₃₂}-PS (9.8 k) and (c) modulus of PS and {Mo₁₃₂}-PS with different M _w. (d) Modulus increase of {Mo₁₃₂}-PS compared with PS with the same M _w.

Conclusions

In this work, we have successfully synthesized monodispersed PNCs of {Mo₁₃₂}-PS based on electrostatic interactions using a ″grafting to″ strategy. The structure of {Mo₁₃₂}-PS was comprehensively characterized by NMR, FT-IR, and SAXS. Subsequently, the effect of electrostatic confinement on the dynamics of {Mo₁₃₂}-PS was studied and both of DSC and BDS exhibit a consistent conclusion that the confinement of {Mo₁₃₂} will slow down the motion of the PS chain segment, manifested as higher T _g in DSC and longer α relaxation time in BDS. In addition, after the incorporation of {Mo₁₃₂}, there are two kinds of PS chain segments in {Mo₁₃₂}-PS: one is PS segments in the CPB region, which is close to {Mo₁₃₂} and exhibits severely restricted dynamics; the other one is PS segments in the SDPB region, which is far away from {Mo₁₃₂} and has relatively free relaxations. Therefore, with the increase in M _w, the confinement effect attenuates. AFM verified that the addition of {Mo₁₃₂} enhances the mechanical properties due to the slowdown of the dynamics and the rigidity of the molecular clusters. This work provides a reference for explaining the dynamic of electrostatic POM-based PNCs.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/polymscitech.5c00130.

• Supplementary figures (Figures S1–S15); and references (PDF)

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.