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Flexible Poly(vinyl chloride) Nanocomposites Reinforced with Hyperbranched Polyglycerol−Functionalized Graphene Oxide for Enhanced Gas Barrier Performance Kyu Won Lee,† Jae Woo Chung,*,‡ and Seung-Yeop Kwak*,† †
Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea Department of Organic Materials and Fiber Engineering, Soongsil University, 369 Sangdo-ro, Dongjak-gu, Seoul 06978, Korea
‡
S Supporting Information *
ABSTRACT: Herein, we describe the preparation of flexible poly(vinyl chloride) (PVC) containing hyperbranched polyglycerol (HPG)functionalized graphene oxide (HGO) as a reinforcing filler and reveal that the obtained composites exhibit greatly improved gas barrier properties. Moreover, we show that HGO, synthesized by surfaceinitiated ring-opening polymerization of glycidol followed by esterification with butyric anhydride, exists as individual exfoliated nanosheets possessing abundant functional groups capable of interacting with PVC. A comparative study of butyl-terminated graphene oxide (BGO) reveals that functionalization with HPG is of key importance for achieving a uniform dispersion of HGO in the PVC matrix and results in strong interfacial interactions between HGO and PVC. As a result, flexible PVC/HGO nanocomposite films exhibit significantly enhanced tensile strength and toughness compared to those of neat plasticized PVC while maintaining its inherent stretchability. Furthermore, the two-dimensional planar structure and homogeneous distribution of HGO in PVC/HGO nanocomposites make gas molecules follow a highly tortuous path, resulting in remarkably reduced oxygen permeability, which is more than 60% lower than that of neat plasticized PVC. Consequently, HGO is demonstrated to be promising component of flexible and gas-impermeable PVC films for a wide range of applications. KEYWORDS: graphene oxide, hyperbranched polyglycerol, flexible PVC, gas barrier property, interfacial interaction
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most graphene fillers are not very effective in achieving this goal, mainly due to the aggregation/restacking of graphene sheets and the unfavorable interfacial compatibility between graphene and polymer matrixes.15−17 For example, graphene-reinforced poly(vinyl alcohol) (PVA) composites with greatly increased tensile strength and Young’s modulus developed by Zhang et al. showed a sharply decreased elongation at break as a result of irreversible graphene sheet aggregation.15 A similar phenomenon was reported by Valiyaveettil et al., i.e., the use of surfactant-wrapped graphene increased the tensile strength and Young’s modulus of PVC/graphene composites by 130 and 58%, respectively, whereas the elongation at break was decreased by more than 65% due to the weakness of interfacial interactions between graphene and PVC.16 Graphene oxide (GO) is commonly used as an alternative to the poorly performing graphene,18 possessing a large number of oxygen-containing functional groups (hydroxyl, epoxy, carboxyl, etc.) and having the potential to be modified for imparting desirable functionalities to its backbone.19,20 Jo et al. reported that alkyl-functionalized GO could be
INTRODUCTION Poly(vinyl chloride) (PVC) is one of the most versatile synthetic polymers due to its excellent flame retardancy, good mechanical properties, safety, low density, and low cost.1,2 Unlike other common polymers, PVC is easily formulated as either rigid or flexible via the addition of plasticizers. In particular, flexible PVC finds use in fields such as packaging, flooring, wallcovering, tubing, wires, cables, containers, gloves, toys, and medical applications due to its high processability, stretchability, and durability.3−5 In addition to the above advantages, good gas barrier properties would further expand the potential applicability of flexible PVC. However, plasticizers increase the free volume of the PVC matrix and weaken intermolecular cohesion, inevitably deteriorating the mechanical strength, thermal stability, and gas barrier properties of flexible PVC.6−8 Thus, researchers are currently challenged with improving gas barrier and other physical properties of flexible PVC while maintaining its intrinsic flexibility. Graphene is a two-dimensional carbon-based nanomaterial, attracting increased attention owing to its high surface area and aspect ratio, outstanding mechanical properties, and excellent thermal and electrical conductivities.9−12 Despite being used for improving the physical properties of polymer composites,13,14 © 2017 American Chemical Society
Received: July 14, 2017 Accepted: September 7, 2017 Published: September 7, 2017 33149
DOI: 10.1021/acsami.7b10257 ACS Appl. Mater. Interfaces 2017, 9, 33149−33158
Research Article
ACS Applied Materials & Interfaces
mixture was sonicated for 1 h. The resulting suspension was magnetically stirred at 100 °C for 20 h under argon and allowed to cool to room temperature. The crude product was separated by filtration and repeatedly washed with methanol to yield HPG-grafted GO, the hydroxyl groups of which were subsequently esterified using butyric anhydride. For esterification, the HPG-grafted GO (1.5 g) was mixed with butyric anhydride (20 mL), and the mixture was sonicated for 1 h and magnetically stirred at 100 °C for 20 h under argon. The solid product was filtered, repeatedly washed with methanol, and dried for 24 h at 60 °C under vacuum to furnish HGO as a brownish powder. Preparation of Plasticized PVC/HGO Nanocomposite Films. Plasticized PVC nanocomposite films containing 0−5 wt % HGO were prepared by solution blending. Alkyl-HPG was employed as a plasticizer with the total amount of HGO and alkyl-HPG fixed at 40 wt %. A typical preparation of PVC/HGO containing 1 wt % HGO (denoted as PVC/ HGO 1 wt %) is given below. HGO (50 mg) and alkyl-HPG (1.95 g) were added to tetrahydrofuran (THF, 30 mL), and the mixture was sonicated for 1 h, followed by the addition of PVC resin (3 g) and vigorous stirring for 1 h at room temperature. The obtained mixture was subsequently poured into a glass Petri dish and thoroughly dried in an oven at 40 °C for 48 h to completely remove the residual solvent, affording a PVC/HGO 1 wt % nanocomposite film with a thickness of ∼0.2 mm. The thickness of all PVC nanocomposite films with 0−5 wt % HGO was approximately equal to 0.2 mm. Characterization. Fourier transform infrared (FT-IR) spectra were acquired using a Thermo Scientific Nicolet 6700 spectrometer. 13C NMR spectrum was recorded in D2O at 600 MHz in a Bruker Avance 600 spectrometer. Thermogravimetric analysis (TGA; Q500, TA Instruments) was carried out in a flow of nitrogen at temperatures of up to 600 °C and a heating rate of 10 °C min−1. X-ray diffraction (XRD; New D8 Advance, Bruker)) patterns were collected at room temperature using Cu Kα radiation (λ = 1.541 Å), a voltage of 40 kV, a current of 40 mA, and Bragg angles (2θ) of 5−30° with scan rate of 2° min−1. Raman spectra (T64000, HORIBA, FR) were recorded using a 640 nm laser as an excitation source. X-ray photoelectron spectroscopy (XPS; Kratos AXIS photoelectron spectrometer) analyses were performed using a monochromatic Mg Kα X-ray source. Transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS; JEM-2100F, JEOL) was performed at an acceleration voltage of 200 kV. The TEM samples were prepared by dispersing the GO nanosheets in ethanol and then coating the dispersed nanosheets onto carbon coated TEM copper grid by simply dropping. Ultraviolet−visible (UV−vis, PerkinElmer Lambda25) spectra were recorded at ambient temperature in a wavelength range of 400−800 nm. The mechanical properties of films were measured using a universal testing machine (UTM; Instron-5543) with a 50-N static load cell at a strain rate of 20 mm min−1. The tensile test was carried out according to the American Standard Testing Method (ASTM) D638, and the specimens were conditioned at 30 ± 2% relative humidity and 25 ± 2 °C temperature for 48 h before testing. Surface morphologies were observed by field-emission scanning electron microscopy (FE-SEM; JSM-7600F, JEOL) with an accelerating voltage of 10 kV. Platinum coating was carried out by sputtering at 10 mA for 100 s prior to FE-SEM imaging. The cross-section samples were obtained by breaking films in liquid nitrogen. Dynamic mechanical analysis (DMA; DMA 2980, TA Instruments) was conducted in tension mode at a frequency of 1 Hz at temperatures of −80 to 100 °C and a heating rate of 3 °C min−1 with the oscillatory amplitude and static force equaling 15 μm and 0.01 N, respectively. Differential scanning calorimetry (DSC; DSC 200 F3, Netzsch) measurements were performed under nitrogen at temperatures of −80 to 100 °C and a heating rate of 10 °C min−1. Gas barrier properties were evaluated using a gas permeation analyzer (MOCON, OX-TRAN 2/21), and the oxygen permeability coefficient (P) was determined as P = (OTRt)/Δp, where OTR is the equilibrium oxygen transmission rate, t is the film thickness, and Δp is the partial pressure of O 2.
homogeneously dispersed in poly(ethylene terephthalate) (PET), with the obtained composites showing remarkably improved gas barrier and mechanical properties.21 Park et al. successfully prepared poly(dopamine)-treated GO/PVA composite films showing 39, 100, and 89% increases in Young’s modulus, ultimate tensile strength, and elongation at break, respectively, due to the strong adhesion of poly(dopamine) at the interface of PVA and GO sheets.22 The above studies reveal that functionalization of GO can maximize the structural integrity of polymer/GO composites, allowing their physical properties to be considerably improved. Therefore, the design of functionalized GO capable of providing specific interfacial interactions with neighboring polymer chains is crucial to the fabrication of high-performance GO-based composites. Hyperbranched polyglycerol (HPG) is an aliphatic polyether characterized by a globular dendritic structure and abundant terminal hydroxyl groups,23,24 showing the advantages of excellent biocompatibility, ease of synthesis, multihydroxyl functionality, and good solubility in polar solvents.25−27 Previously, we demonstrated that alkyl-terminal HPG (alkylHPG) is an efficient PVC-compatible plasticizer28,29 due to strong donor−acceptor interactions between its numerous polar groups and the main chain of PVC. Because bulky globular hyperbranched polymers can be incorporated into the interlayer spaces of GO and promote its efficient exfoliation into individual GO nanosheets,30,31 we anticipated that HPG-modified GO can be uniformly dispersed in PVC and strongly interact with PVC chains to afford flexible PVC nanocomposites with significantly improved physical properties. Herein, we used HPG-functionalized GO (HGO) as a reinforcing filler to prepare flexible PVC with greatly improved gas barrier properties, with HPG moieties synthesized directly on the surface of GO sheets via one-pot anionic ring-opening polymerization of glycidol. The thus prepared HGO was completely exfoliated into single nanosheets and could be homogeneously dispersed in the PVC matrix to afford PVC/ HGO nanocomposite films exhibiting remarkably enhanced Young’s modulus, tensile strength, and toughness without any loss of inherent stretchability. Furthermore, the gas barrier properties and thermal stability of PVC/HGO were significantly improved compared to those of nanocomposites with an identical content of butyl-terminated GO (BGO). Thus, HGO was expected to be a good candidate for developing gasimpermeable flexible PVC nanocomposites for a wide range of applications.
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EXPERIMENTAL SECTION
Materials. Graphite (powder,
