Facile preparation of composites composed of high performance thermoplastic and difficult-to-process functional polymer

Abstract

A facile method is reported to prepare new polymer composites by introducing functional polymer (poly(3,4-ethylenedioxythiophene)) into a melt-processed film of a thermoplastic polymer matrix with a high melting-temperature (syndiotactic polystyrene), which benefits the applications of functional polymers which are difficult to process and vulnerable to high temperatures.

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Polymer composites consisting of engineering plastic as matrix and functional polymer as additive represent an important route to prepare new materials. As for most functional polymers, their practical applications are usually limited by the poor mechanical properties and the difficulty of processing. On the other hand, engineering plastics are widely used because of good mechanical properties, high thermal stability and easy processing, etc. Thus, mixing functional polymers with high performance thermoplastics has been regarded as an effective way to combine both advantages.

Syndiotactic polystyrene (sPS) has been extensively studied since the successful synthesis in 1986.^1–6 Its diverse excellent properties (such as high melting temperature of ∼270 °C, high crystallization rate along with high modulus, excellent resistance to a wide range of chemicals (e.g., acids, bases, and most organic solvents), hydrolytic stability, low dielectric constant, ease of injection molding and other melt processing) make it a major type of engineering plastics.^4,7,8 Recently, there has been an increasing interest in sPS/inorganic composites.^9–15 In sharp contrast, the preparation of sPS/functional polymer composites still remains a problem since the melt-processing at high temperatures will inevitably cause severe and irreversible damages to those functional polymers. Based on the fact that melt-processing is usually the final procedure to produce practical polymer products, raw sPS/functional polymer composites in any form can hardly survive during the melt-processing. Thus, to introduce functional polymers into the already melt-processed products of thermoplastics is highly desired to avoid the problem.

In order to successfully introduce the functional polymers into the melt-processed thermoplastic matrix, in situ polymerization of functional monomers within the matrix is expected. However, for most semicrystalline polymers, it is very difficult to diffuse the functional monomers into the thermoplastic polymer matrix due to the incompatibility and the hindrance of dense crystalline region. Fortunately, as for the polymers such as sPS, its unique nanoporous crystalline phases exhibit lower density and higher guest solubility, allowing the possibility to host various organic molecules.¹⁶ Among the most successful conjugated polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) has attracted much of current interests due to their low cost, convenience to be processed, flexibility and the low thermal conductivity.^17–20 However, it is insoluble and difficult to process. Moreover, high temperatures will cause its degradation.^21,22 Here, in this communication, we report a facile preparation of composites composed of nonpolar sPS matrix without chemical modification (e.g., sulfonation, grafting, and hydrogenation) and PEDOT via monomer diffusion and subsequent polymerization.

In the five main crystalline forms of sPS (α, β, γ, δ and ε), the δ and ε forms are nanoporous crystals, which have polymer chains with some empty space (isolated cavities and channels for δ and ε phase, respectively) regularly distributed, thus making it possible to absorb suitable organic guest molecules into the crystalline phase and become cocrystals.^23,24 Unfortunately, the report of polymerization inside the polymer with nanopores is very scarce. For both nanoporous crystalline phases of sPS, films with three different uniplanar orientations can be acquired by specific procedures.^25–27 Albunia et al.¹⁶ recently reported that high guest solubility was obtained only for the films exhibiting the nanoporous crystalline phases and the diffusivity reached maximum for channeled ε crystal films with C_⊥ uniplanar orientation (i.e. chain axes and channels preferentially perpendicular to the film plane).

Here, the unoriented films with γ form crystals were first obtained by immersing the amorphous sPS films in acetone for 72 h at room temperature. In Fig. 1A, the peaks at 2θ ≈ 9.3° and 16.0° indicate the acquirement of γ crystal.²⁵ Subsequently, the preliminary γ form samples were treated with liquid chloroform to achieve sPS/chloroform cocrystalline phase (Fig. 1B and 2A). Then, the sPS/CHCl₃ cocrystalline film samples were kept in FeCl₃/acetone solution for 72 h at 40 °C. This procedure was to introduce FeCl₃, the inorganic oxidant, into the matrix, and to extract CHCl₃ guest molecules with acetone to acquire films with ε form.²³ In Fig. 1C, the peaks at 2θ ≈ 6.9°, 8.1°, 16.2° and 20.2° proved the formation of ε form, and the peak at 2θ ≈ 22.5° ((002) reflection) is the most intense reflection for ε form films, which suggested that the ε form exhibited chain axes of crystalline phase being preferentially perpendicular to the film plane.^25,27 After this procedure, the films were translucent and light yellow. Alternatively, FeCl₃/acetonitrile solution was also applicable to this procedure and, as shown in Fig. S1, ESI,† both film samples showed the X-ray diffraction (XRD) pattern of the ε form. After the introduction of the oxidant, the films were immersed in EDOT/acetonitrile solution for only 15 min at 40 °C, and then kept in a vacuum oven at 45 °C for at least 12 h. The monomers of EDOT entered the nanopores of ε crystals of sPS. Finally, as shown in Fig. 2B, the films turned blue, indicating the successful polymerization of EDOT within the matrix (Scheme 1). On the contrary, by inversing the sequence of solution treatment after acquiring sPS/CHCl₃ cocrystalline films, i.e., the cocrystalline films were first immersed in EDOT/acetonitrile solution for 72 h at 40 °C, followed by soaking in FeCl₃/acetone or FeCl₃/acetonitrile solution for 15 min at 40 °C, no obvious blue color could be observed afterwards. In this case, even if the FeCl₃ solution treatment lasted for 72 h or longer, PEDOT existed mostly on the film surface as coating (inhibiting the diffusion of FeCl₃ into the matrix) and in the solution as precipitate.

Fig. 1 XRD patterns of sPS semicrystalline film samples presenting: (A) γ form, (B) sPS/CHCl₃ δ cocrystaline form (A after CHCl₃ treatment), (C) ε form (B after FeCl₃/acetone treatment), and (D) ε form (after the in situ polymerization of EDOT monomers within the matrix).

Fig. 2 Photographs of sPS crystalline film samples with (A) sPS/CHCl₃ cocrystalline form (corresponding to Fig. 1B) and (B) ε form after in situ polymerization of EDOT within the film (corresponding to Fig. 1D), and optical micrograph of a cross section of the film corresponding to Fig. 2B. The blue color indicates the existence of PEDOT.

Scheme 1 Polymerization reaction procedure of EDOT monomers.

The XRD pattern of the film sample after EDOT polymerization within the matrix is shown in Fig. 1D. Compared with Fig. 1C, the ε form was well preserved after the immersion of EDOT solution and the subsequent in situ polymerization. As no obvious peak-shift or crystal transformation can be observed, the influence of functional monomer/polymer on the crystalline phase needs to be investigated further. Fig. 2C explains the reason why no obvious changes in XRD pattern occurred. In the cross sectional view, a bright layer appeared in the middle of the section, and the blue color mostly concentrated in the layers close to the surface, suggesting that the core layer within the film was nearly free of functional monomer/polymer. Therefore, the XRD pattern revealed that the crystalline regions composed of the additives were not concentrated much, especially in the core layer. Compared to the film sample, the powder sample no doubt had larger specific surface area and showed more comprehensive XRD patterns due to the lack of specific chain orientation, which would help to reveal the variation of crystal lattice after the introduction of the monomers.

Because the amorphous sPS film sample was difficult to grind, the sPS pellets as received were directly dissolved in N-methyl-2-pyrrolidone (NMP) at 190 °C. After precipitation, filtration and drying under vacuum, the powders containing only γ crystal (Fig. 3A) could be obtained without involving acetone treatment on the amorphous sample. The subsequent procedures were similar to those film samples mentioned above. Thus, Fig. 3B and C correspond to Fig. 1B and C, respectively. The reflection region at 2θ < 10° for the powder sample was more obvious than that for the film sample. However, after the ε form powder was treated with EDOT solution, distinct crystal transformation appeared. As shown in Fig. 3D, the reflection peaks of (010) and (2̄10) indicated the acquirement of sPS/EDOT δ clathrate cocrystalline phase.^28–30 Given that the guest molecules in clathrate structure were diffused into isolated cavities, intermolecular guest reactions in cocrystalline phase were prohibited. Based on these results, it can be concluded that the in situ polymerization of the EDOT monomers mostly happened in the amorphous phase of both the powder and the film samples. Due to the large specific surface area of the powder samples, EDOT monomers were able to fully diffuse into tiny particles and thus δ cocrystalline form dominated. However, for thick film samples, most of the ε crystals (especially in the core layer) still remained unaffected after the introduction of EDOT. If the ε form film was immersed in EDOT solution for a long enough time, the δ cocrystallline film could be obtained as well (Fig. S2, ESI†).

Fig. 3 XRD patterns of sPS semicrystalline powder samples presenting: (A) γ form, (B) sPS/CHCl₃ δ cocrystalline form (A after CHCl₃ treatment), (C) ε form (B after FeCl₃/acetone treatment), and (D) sPS/EDOT δ cocrystalline form (C after treated with EDOT solution).

In Fig. 4A, on the film surface, a number of parallel straight grooves occurred in micron scale, which resulted from the indentations left by aluminium foil when being hot-pressed followed by rapid quenching. Originating from the amorphous films, the sPS/PEDOT films were finally obtained after complicated treatments. Most importantly, the surface textures of the original films were well preserved. In Fig. 4B, although PEDOT mostly concentrated in the surface layers, no obvious faults (representing stress defect) could be observed on the fracture surface. Thus, the present method makes it possible to introduce difficult-to-process functional polymers into the already melt-processed polymer products with high melting temperature while keeping the surface textures of the products nearly intact in micron scale.

Fig. 4 SEM images of sPS film sample after the in situ polymerization of EDOT within the film: (A) film surface, (B) fracture surface.

Besides introduction of functional polymers into the melt-processed matrix, coating on the matrix surface may be another option. However, the adhesion between the nonpolar substrate like sPS and the deposited functional polymer film is not so strong because of the low surface energy of the substrate.³¹ Even after the functional layer was coated on the surface, the properties such as anti-scratch and chemical resistance also remain problems to be resolved. Compared with coating, functional additives within the matrix should be more durable. Considering a large amount of the organic additives may act as plasticizers, thus being detrimental to the mechanical properties of the composites, keeping the balance between the amount of the functional polymers and the overall performance of the composites is of great importance. Since it is commonly necessary to keep sufficient concentration of the functional polymers and simultaneously minimize their overall amount, local concentration of the additives in the surface layers of the matrix offers a good option.

In summary, based on the high guest solubility for sPS nanoporous crystalline phases and maximum diffusivity for channeled ε crystal films with C_⊥ uniplanar orientation, EDOT monomers were introduced into the ε form film sample by quick solution treatment followed by in situ polymerization. XRD patterns of the powder sample further revealed that the absorption of the EDOT monomers caused crystal transformation from ε to δ clathrate cocrystal, so that the subsequent polymerization mostly happened in the amorphous phase. Here, we present a convenient method to prepare thermoplastic/functional polymer composites containing functional polymer in the surface layers. And the method offers a promising way to introduce functional polymers, which are difficult to process and vulnerable to high temperatures, into the nonpolar thermoplastic matrix without any chemical modification. The studies reported herein will deepen the understandings toward polymer composites, and have great potentials in the applications of functional polymers.

Experimental

The original sPS pellets were provided by Japan Idemitsu Kosan Co. Ltd., with a commercial name of XAREC SP130. Its syndiotacticity, [rr], is larger than 97%. 3,4-ethylenedioxythiophene (EDOT) was purchased from Adamas Reagent, Ltd, with purity >99.0%. For EDOT solution, the mass ratio of EDOT to solvent was around 0.19. The concentration of the FeCl₃ solution was 30 g L⁻¹.

X-ray diffraction (XRD) measurements were conducted on a Rigaku D/max 2500 diffractometer, using Cu Kα radiation (λ = 0.15418 nm). The scanning 2-theta angle range was between 5° and 35°, and the scanning rate was 2° min⁻¹. An Olympus BH-2 type optical microscope was used, mounted with YH-300 CCD camera at the top made by Wing Hang Shanghai optical instrument manufacturing Co., Ltd. HITACHI S-4800 scanning electron microscopy (SEM) was used to observe the film surface and fracture surface. The acceleration voltage was 10.0 kV, and Pt sputtering was applied before observation.

To introduce PEDOT into sPS films, first, the amorphous samples were obtained by pressing the molten sPS manually between two pieces of aluminium foils on a homemade heating stage followed by rapid quenching in cold water. The film thicknesses were 150–200 μm. Both FT-IR spectroscopic and XRD results confirmed no sign of crystalline phase. The γ form films were obtained by immersing the amorphous films in acetone for 72 h at room temperature. Then, the acquired γ form films were treated with liquid chloroform for 72 h, thus obtaining sPS/CHCl₃ cocrystalline form. After soaking the cocrystalline films in FeCl₃/acetone or FeCl₃/acetonitrile solution for another 72 h at 40 °C, the films were washed with deionized water to remove the residual solution on the surface, and then put into vacuum to get ε form. At last, the ε form films with inorganic oxidant were treated with EDOT/acetonitrile solution at 40 °C for 15 min, followed by drying in vacuum at 45 °C for at least 12 h.

Acknowledgements

We thank National Natural Science Foundation of China for financial support (no. 51343005). G. Chen acknowledges Youth Innovation Promotion Association, Chinese Academy of Sciences.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Part of the XRD results. See DOI: 10.1039/c4ra04468a

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