Flexible Poly(vinyl chloride) Nanocomposites Reinforced with

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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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10257 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 10, 2017

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ACS Applied Materials & Interfaces

Flexible Poly(vinyl chloride) Nanocomposites Reinforced with Hyperbranched Polyglycerol–Functionalized Graphene Oxide for Enhanced Gas Barrier Performance Kyu Won Leea, Jae Woo Chungb,*, Seung-Yeop Kwaka,*

a

Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea

b

Department of Organic Materials and Fiber Engineering, Soongsil University, 369 Sangdo-ro, Dongjak-gu, Seoul 06978, Korea

*

Corresponding Author: Seung-Yeop Kwak (E-mail: [email protected]) Tel.: +82-2-880-8365, Fax: +82-2-885-1748 Jae Woo Chung (E-mail: [email protected]) Tel: +82-2-828-7047, Fax: +82-2-817-8346;

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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 surface-initiated 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, 3



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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 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 matrices.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 5



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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 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, multi-hydroxyl functionality, and good 6


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solubility in polar solvents.25–27 Previously, we demonstrated that alkyl-terminal HPG (alkyl-HPG) is an efficient PVC-compatible plasticizer28,29 due to strong donor-acceptor interactions between its numerous polar groups and the main chain of PVC. Since 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 gas-impermeable flexible PVC nanocomposites for a wide range of applications.

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Experimental Materials Graphite (powder,

Flexible Poly(vinyl chloride) Nanocomposites Reinforced with

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