Improved mechanical and barrier properties of low-density polyethylene nanocomposite films by incorporating hydrophobic graphene oxide nanosheets

Abstract

The high hydrophilicity of graphene oxide nanosheets (GONSs), arising from their abundant oxygen-containing functional groups, gravely restricts their application in non-polar polymer nanocomposites. In the present study, alkylated GONSs were fabricated by facile refluxing of GONSs and octadecylamine (ODA), thus giving rise to the selective dispersion of ODA–GONSs in non-polar xylene rather than in polar water. Fourier-transform infrared spectroscopy, atomic force microscopy, and X-ray diffraction results demonstrated the occurrence of the nucleophilic substitution reaction between the primary amine groups of ODA and the epoxide groups of GONSs during the refluxing. In the low density polyethylene (LDPE) nanocomposites, ODA–GONSs were uniformly and randomly dispersed, exhibiting excellent compatibility with the LDPE matrix. As a result, when adding 4.0 wt% ODA–GONSs, the Young's modulus was improved by 58.9%; O₂ permeability was reduced by 37.0%; and initial decomposition temperature was elevated by 15.9 °C. Besides, the inclusion of ODA–GONSs could effectively block the transmission of UV light in the nanocomposite films and serve as heterogeneous nucleating agents for LDPE crystallization. These results confirm that such long alkane chain modification holds great value or potential to design and prepare LDPE nanocomposite films for packaging applications with excellent integrated performance.

---

1. Introduction

Polymer films are currently in great demand as packaging materials to protect perishable food, pharmaceuticals, and electrical devices, etc., originating from their lightweight, inexpensive, easy processing features.¹ However, single polymer films can hardly meet the demand for the various commodities due to their undesirable mechanical performances and poor gas barrier properties. Thus, it is highly attractive to develop high barrier polymer films with satisfactory mechanical properties. To this end, incorporation of a nanofiller in a polymer matrix has been demonstrated to be an efficient strategy.^2–4 Graphene oxide nanosheets (GONSs), a new class of two-dimensional carbon nanostructure, have emerged as promising nanoplatelets for the next generation high-performance polymer-based hybrid materials. The unique graphitized planar structure, extremely high specific surface area, and large aspect ratio make GONSs completely impermeable to liquids, vapors, and gases, including helium.^5–9 Furthermore, a large amount of oxygen-containing functional groups, such as the hydroxyl, epoxide, carbonyl and carboxyl groups, produce GONSs with excellent hydrophilicity, rendering GONSs easily exfoliated into individual nanosheets and uniformly dispersed in a polar polymer matrix.^10,11 As a consequence, the interfacial adhesion is enhanced significantly, which is beneficial for the interfacial transfer of stress. On the basis of these fact, the discovery of GONSs is viewed to open up a new avenue for simultaneously advancing mechanical and barrier performances of polar polymer nanocomposites.^5,9,12,13

Unfortunately, the highly hydrophilic GONSs frequently suffer from unsatisfactory compatibility with majority of non-polar polymer and hardly achieve the favorable dispersion quality even by solution mixing, which gravely restricts the development of such nanocomposites for high-performance packaging materials. Thus, it is of great significance for the hydrophilic GONS surface to be purposefully functionalized.¹¹ For instance, Ruoff et al. demonstrated that phenyl isocyanate-treated GONSs could be fully exfoliated into individual nanosheets that can form a stable dispersion in polar aprotic solvents.¹⁴ In this scenario, the modified GONSs effectively suppressed the irreversible agglomeration during the reduction process by N,N-dimethylhydrazine, thus achieving polystyrene-based nanocomposites with excellent electrical and barrier performances.^15,16 Macosko et al. also functionalized GONSs with isocyanate to dramatically optimize the gas barrier properties of polyurethane nanocomposites with the aid of solvent blending.¹³ Recently, the long alkyl chains have attracted tremendous attentions to lipophilically modify the surface of GONSs so that they can be uniformly dispersed in non-polar polymers.^17–27 Lee et al. decorated GONSs with dodecylamine and subsequently reduced GONSs with hydrazine. As a result, mechanical, electrical, and barrier properties of linear low density polyethylene nanocomposites could be improved to various extent by the functionalized graphene.^19,22 Simultaneous surface modification and reduction of GONSs was realized by facile refluxing of GONSs with octadecylamine (ODA), giving rise to significant enhancement on the electrical performances of polystyrene nanocomposites at rather low filler concentration by means of incidental thermal reduction during the compression molding.²⁰ In our previous work, ODA functionalized GONSs was utilized to achieve a dramatic improvement on mechanical properties of conductive polymer composites without compromising their electrical conductivity.²³ On the basis of these studies, it is clear that ODA is promising alternative candidate to make hydrophilic GONSs hydrophobic, promoting the application of GONSs in the non-polar polymers. Yet, to the best of our knowledge, effect of ODA modified GONSs without the use of any reducing agents on the integrated performances of non-polar polymers has not been investigated thoroughly.

In the current study, low density polyethylene (LDPE) was selected as a typical example of non-polar polymer owing to its excellent flexibility, durability and filmability. Surface functionalization of GONSs was achieved by facile refluxing of GONSs and ODA. Solution coagulation was utilized to prepare LDPE nanocomposite films, in which functionalized GONSs were uniformly dispersed, exhibiting excellent compatibility with LDPE matrix. Mechanical, barrier, and optical properties, thermal stability, and crystallization behavior of as-prepared nanocomposite films were investigated thoroughly. As a consequence, such long alkane chain modification is confirmed to be an effective strategy to fabricate excellent integrated performance of LDPE nanocomposite films.

2. Experimental

2.1 Materials

Commercially available LDPE (18D) was supplied by Daqing Petrochemical Company (China) with a weight-average molecular weight of 1.76 × 10⁵ g mol⁻¹, a density of 0.918 g cm⁻³ and a melt flow rate of 10 g/10 min. The modified “Hummers” method was employed to prepare GONSs from expandable graphite, which was purchased from Qingdao Haida Graphite Co., Ltd., China with an expansion rate of 200 ml g⁻¹. Details of preparation process was reported previously.²⁸ ODA, xylene, and anhydrous ethanol were provided by Chengdu Kelong Chemical Reagent Factory, Chengdu, China. Unless stated otherwise, all other reagents were of analytical-reagent grade and were used as received without further purification.

2.2 Preparation of ODA modified GONS (ODA–GONS)

ODA–GONS was prepared on the basis of our previous work.²³ Specifically, 0.6 g of graphite oxide obtained by the modified Hummers method was initially dispersed into 300 ml of deionized water with the aid of vigorous agitation and ultrasonic treatment for 0.5 h at room temperature. The resulting stable and uniform suspension was then mixed with the solution of ODA (0.9 g) in 90 ml anhydrous ethanol in a conical flask. The mixture was refluxed for chemical modification with intensified mechanical stirring for 24 h at 85 °C. Afterward, the reaction products were separated by filtrated, then repeatedly washed with excess ethanol to remove the physically absorbed ODA to the greatest extent. Finally, the ODA–GONS powders were dried in a vacuum oven overnight at 60 °C to evaporate the any residual solvent.

2.3 Preparation of ODA–GONS/LDPE nanocomposite films

Solution coagulation was utilized to prepare a series of LDPE nanocomposite films containing various ODA–GONS loadings of 0.1, 0.25, 0.5, 1.0, and 4.0 wt%. Taking the 0.1 wt% ODA–GONSs as an example, the detailed preparation procedures were as follows: 10 mg of ODA–GONSs were initially dispersed into a given amount of xylene with vigorous agitation and ultrasonic treatment for 2 h at room temperature. The transparent LDPE/xylene solution was obtained by adding 10 g of LDPE granules into about 150 ml of xylene with the assistance of mild stirring for 30 min at 140 °C, and then mixed with the above ODA–GONS/xylene suspension for 15 min at 140 °C. Upon completion, the resulting homogeneous ODA–GONS/LDPE solution was immediately added into a large amount of vigorously stirred anhydrous ethanol and the coagulated materials precipitated continuously. Subsequently, the coagulates composed of ODA–GONSs and LDPE were isolated by filtration, washed with anhydrous ethanol, left in a drying oven at 80 °C to remove bulk solvent, and dried in a vacuum oven overnight at 80 °C to evaporate the any residual solvent. Finally, the dried composite powders were shaped into a diameter of 100 mm and a thickness of about 180 μm films for barrier measurements by compression molding at 180 °C with a pressure of 10 MPa. For comparison, neat LDPE film was prepared according to the same procedures. For convenience, LDPE nanocomposite films were coded as LDPEx, and x is the weight concentration of ODA–GONSs.

2.4 Characterization and measurements

The thickness and surface morphology of ODA–GONSs were characterized by typical tapping-mode atomic force microscopy (AFM) measurement using Nanoscope Multimode & Explore atomic force microscope (Veeco Instruments, USA). Samples for AFM images were fabricated by depositing dilute suspension of ODA–GONSs in xylene on a fresh mica substrate and allowing them to dry in air. Fourier-transform infrared spectroscopy (FTIR) was performed on a Nicolet 6700 FTIR spectrometer with a resolution of 2 cm⁻¹ in transmission mode. The ODA, GONSs, and ODA–GONSs were ground with KBr to obtain fine powders and then pressed into KBr disks for FTIR measurements. The powder samples of expanded graphite, graphite oxide, and ODA–GONSs were directly used for X-ray diffraction (XRD) measurements on a DX-1000 diffractometer (40 kV, 25 mA) with Cu irradiation (λ = 0.154 nm) at a scanning rate of 0.06° s⁻¹ in the 2θ range of 2–30°. Light transmittance in the range of 250–850 nm through neat LDPE and its nanocomposite films was determined by a UV-3600 UV-vis spectrometer (Shimadzu, Japan). Thermogravimetric analysis (TGA) was carried out to evaluate the thermal stability of ODA, GONSs, ODA–GONSs, neat LDPE film and its nanocomposite films on a NETZSCH 209F1 at a heating rate of 10 °C min⁻¹ under nitrogen atmosphere. The field emission scanning electron microscopy (SEM) observation was performed using an FEI Inspect-F (Finland) with an acceleration voltage of 20 kV to study the morphology of ODA–GONSs in LDPE matrix. Samples for SEM images were cryo-fractured in liquid nitrogen and then coated with a thin layer of gold prior to being observed. Two-dimensional wide angle X-ray diffraction (2D-WAXD) patterns were collected at the beamline BL16B (λ = 0.124 nm) of Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China) with an X-ray detector (Model Mar165). The distance between the sample and detector was 108.5 mm. Linear 1D-WAXD profiles were obtained from circularly integrated intensities of the 2D-WAXD patterns. The crystallinity (χ_c-WAXD) of all samples obtained by a standard peak-fit procedure, can be estimated by the following equation:where A_cryst and A_amorp are the fitted areas of the crystal and amorphous phases, respectively. The crystallinity (χ_c-DSC) and non-isothermal crystallization behavior of LDPE nanocomposites were investigated by differential scanning calorimetry (DSC) using a TA Q2000 instrument. The experiments were carried out in nitrogen atmosphere using about 5 mg sample sealed in aluminum pans. The detailed temperature procedure was designed as follows: the sample was initially heated from 40 to 200 °C at a heating rate of 10 °C min⁻¹ (first heating), held at 200 °C for 5 min to eliminate thermal history, and then cooled to 40 °C at a cooling rate of 10 °C min⁻¹. The sample was further heated to 200 °C at a heating rate of 10 °C min⁻¹ (second heating) to study the subsequent melting behavior. The χ_c-DSC of LDPE nanocomposite films can be calculated from the first heating traces according to the following equation:where ΔH_o is the enthalpy of pure LDPE crystal (293 J g⁻¹),²⁹ ΔH_m is melting enthalpy measured via DSC. The mechanical properties of LDPE nanocomposite films were determined using a universal tensile instrument (Model 5576, Instron Instrument, USA) with a span length of 20 mm at a testing speed of 50 mm min⁻¹ according to ASTM standard D638. In all cases, more than five samples were tested to evaluate the average values and standard deviations of the mechanical properties. O₂ permeation analysis of neat LDPE and its nanocomposite films was conducted using a VAC-V2 film permeability testing machine (labthink instruments, Jinan, China) at room temperature with 50% relative humidity according to ISO2556:1974. The detailed experimental procedure has been previously reported elsewhere.^8,9

3. Results and discussion

3.1 Characterization of ODA functionalized GONSs

Fig. 1a shows the dispersion of GONSs and ODA–GONSs in the immiscible mixture of xylene/H₂O solvent after storing at least 48 h. It can be vividly seen that GONSs are completely dispersed in water due to their highly hydrophilic feature, whilst the ODA–GONSs display the solution-like suspension in xylene. The selective dispersion of ODA–GONSs in non-polar xylene rather than in polar water indicates the successful conversion from hydrophilic GONSs to hydrophobic ODA–GONSs by simply introducing the long alkyl chains of ODA. Interestingly, it is worth noting that an obvious color change happens from brown GONSs in water to black ODA–GONSs in xylene, probably originating from the mild thermal reduction during the refluxing of GONSs and ODA at 85 °C.^20,23 Fig. 1b depicts a typical AFM image of ODA–GONSs, exhibiting a thickness of about 1.5 nm. This value is expected to be a little thicker than the reported apparent thickness of single-layer GONSs (∼1.0 nm), which may be attributed to the graft of ODA chains to the surface of GONSs.

Fig. 1 (a) The dispersion of GONSs and ODA–GONSs in the immiscible mixture of xylene/water (v/v = 1/1) solvent (0.2 mg ml⁻¹). (b) A typical tapping-mode AFM image of ODA–GONSs.

In order to further clarify the mechanism of ODA functionalized GONSs, FTIR spectroscopy was carried out to examine the chemical changes that occurred during the refluxing of ODA and GONSs. As illustrated in Fig. 2, an intense and very broad band appeared at 3412 cm⁻¹ in the GONSs is attributed to the stretching mode of –OH band. The other strong bands at 1724, 1628, 1070 cm⁻¹ in the GONSs are assigned to carboxylic groups (C=O), unoxidized graphitic domains (C=C), and epoxide groups (C–O–C), respectively. After the chemical reaction with ODA, two noticeable new peaks at 2918, 2850 cm⁻¹ arising from the –CH₂ stretching of the octadecyl chain indicate the successful modification of GONSs by ODA. Furthermore, the weakened band of C–O–C groups and the emergence of a new peak at 1560 cm⁻¹ corresponding to N–H stretching vibration also imply the reaction between the epoxide group and the amine group. On the basis of the above results, we thus attribute the mechanism of ODA functionalized GONSs to the nucleophilic substitution reaction between the primary amine groups of ODA and the epoxide groups of GONSs, being in line with the reported in the literature.^18,20,23

Fig. 2 FTIR spectra of ODA, GONSs, and ODA–GONSs.

Fig. 3 presents XRD curves of expanded graphite, graphite oxide, and ODA–GONSs. The featured diffraction of expanded graphite appears at 2θ = 26.6° and that of graphite oxide is observed at 2θ = 10.9°. According to Bragg diffraction formula, the d-spacing of expanded graphite and graphite oxide can be estimated to ∼0.34 and 0.81 nm, respectively. As reported, the enlarged d-spacing after oxidation is due to the intercalating oxygen-containing functional groups and the amount of absorbed water.³⁰ Chemical modification further increases the d-spacing of ODA–GONSs to 1.80 nm with a lower Bragg angle of 4.9°. This is indicative of the intercalation of the octadecyl chain, supporting the reaction of ODA and GONSs. Additionally, a very broad diffraction peak of ODA–GONSs is observed at 2θ = 20.5° with a corresponding d-spacing of 0.43 nm. This reveals the removal of oxygen-containing functional groups (C–O–C) from the basal plane gallery and restoration of graphitic domains during the refluxing of GONSs and ODA at 85 °C, while the broadness of the diffraction peak shows the poor crystalline structure. The mild reduction of GONSs only by ODA is also confirmed by the electrically insulated LDPE nanocomposite film at an ODA–GONS loading of 4.0 wt%. Probably, the post-processing technique, such as thermal treatment, is necessary to further enhance the electrical properties of LDPE nanocomposite films.²⁰

Fig. 3 XRD patterns of expanded graphite (a), graphite oxide (b), and ODA–GONSs (c).

TGA analysis was performed to calculate the grafting ratio of ODA on the surface of GONSs. As displayed in Fig. 4, ODA exhibits a rapid weight loss starting at temperature as low as 120 °C, and is almost exhausted without any residue when the temperature reaches ∼300 °C. GONSs are also thermally unstable and show an ∼11% weight loss even near 100 °C, evidently owing to the evaporation of the trapped water molecules. Another significant weight loss (∼26%) is observed in the range of 180–230 °C, presumably due to pyrolysis of the labile oxygen-containing functional groups. Afterward, a slower, steady weight loss over the whole temperature range above 300 °C is assigned to the removal of the more stable oxygen-containing functional groups. On the contrary, ODA–GONSs are less sensitive to temperature and has nearly zero weight loss below 120 °C, indicating an enhanced hydrophobicity that minimizes the amount of absorbed water. Different from GONSs, ODA–GONSs show a gradual weight loss of about 9% in the temperature range of 160–210 °C, which could be ascribed to the desorption of physically absorbed ODA. These ODA molecules may be positively charged and electrostatically bonded with negatively charged surface of GONSs, consequently preventing them from being washed away by ethanol. At higher temperature, the higher weight loss of ODA–GONSs than GONSs is due to the decomposition of covalently bonded ODA, together with the decomposition of residual oxygen-containing functional groups of GONSs. Finally, according to the yields of residual carbon, the grafting ratio of ODA on the surface of GONSs is calculated to about 24%.

Fig. 4 TGA curves of ODA, GONSs, and ODA–GONSs.

3.2 Dispersion of ODA–GONSs in LDPE matrix

SEM observation for the fractured surface of neat LDPE and its nanocomposite films was used to assess the dispersion of ODA–GONSs. As depicted in Fig. 5a and b, ODA–GONSs are by and large individually exfoliated and randomly dispersed in the LDPE matrix without any visible aggregations. Compared with the smooth fractured surface of neat LDPE film, LDPE0.5 film shows a relatively rough fractured surface and ODA–GONSs are embedded in and tightly held the polymer matrix, exhibiting excellent compatibility or strong interfacial adhesion with LDPE matrix. This outcome accounts for the interpenetration of grafted octadecyl chains and LDPE chains, and is also an advocating factor for transferring the useful properties of ODA–GONSs to the LDPE matrix.

Fig. 5 Typical SEM images for the fractured surface of LDPE0.5 film (a and b) and neat LDPE film (c and d). (b) and (d) are magnified ones.

The dispersion of ODA–GONSs is further determined using 2D-WAXD. As shown in Fig. 6, it is clearly visible that there is no any signal of ODA–GONSs. The two uniform diffraction rings corresponding to (110) and (200) reflections of LDPE crystals suggest that the addition of ODA–GONSs has no impact on the crystalline structure and LDPE crystals are isotropically distributed in the nanocomposites. Fig. 7 illustrates the ID-WAXD intensity profiles of LDPE nanocomposite films, integrated in a circular manner from their corresponding 2D-WAXD. LDPE nanocomposite films with different ODA–GONS loadings exhibit the same 1D-WAXD profiles as the neat LDPE film with no characteristic diffraction peak of ODA–GONSs. This result clearly demonstrates that the regular and periodic structure of ODA–GONSs is destroyed and ODA–GONSs are fully exfoliated and uniformly dispersed in LDPE matrix, which is consistent with the SEM results. Besides, the value of χ_c-WAXD in the LDPE nanocomposite films determined by eqn (1) is almost identical to that of neat LDPE films (∼30%), varying slightly in the range of about 27.8–33.4% (Table 1). This result can be further confirmed by DSC measurement below. As displayed in Fig. 8, in all cases, only a melting peak of LDPE crystals located in the vicinity of 106.5 °C is observed. The χ_c-DSC calculated by eqn (2) fluctuates between 29.7–33.5%, yielding the consistent results shown by WAXD characterization.

Fig. 6 2D-WAXD patterns of LDPE nanocomposite films as a function of ODA–GONS loadings.

Fig. 7 ID-WAXD intensity profiles of LDPE nanocomposite films as a function of ODA–GONS loadings, integrated in a circular manner from their corresponding 2D-WAXD patterns shown in Fig. 6.

Table 1 Crystallinity obtained by 2D-WAXD and DSC, melting point and melting enthalpy of LDPE nanocomposite films as a function of ODA–GONS loadings

ODA–GONS loadings (wt%)	Tm (°C)	ΔHm (J g^−1)	χc-DSC (%)	χc-WAXD (%)
0	106.6	88.5	30.2	30.0
0.1	106.5	92.5	31.6	30.4
0.25	106.6	93.3	31.8	29.0
0.5	106.6	98.1	33.5	28.4
1.0	106.6	92.2	31.5	27.8
4.0	107.1	87.1	29.7	33.4

---

Fig. 8 Melting traces of LDPE nanocomposite films with different ODA–GONS loadings.

3.3 Mechanical and barrier properties of ODA–GONS/LDPE nanocomposite films

Fig. 9 illustrates the typical stress–strain curves of neat LDPE and its nanocomposite films. All the LDPE samples with and without ODA–GONSs exhibit a normal ductile fracture behavior with more than 400% elongation at break. The yield strength, Young's modulus, and elongation at break are summarized in Fig. 10. The neat LDPE film has the lowest yield strength and Young's modulus, with the values of 12.0 MPa and 269.1 MPa, respectively. The addition of ODA–GONSs leads to a limited improvement in the yield strength; that is, upon the ODA–GONS loading, yield strength is gently increased from 12.0 to 13.7 MPa at an ODA–GONS loading of 4.0 wt%. However, the incorporation of ODA–GONSs significantly boosts up the Young's modulus of LDPE nanocomposite films relative to pristine polymer. For instance, when only adding 1.0 wt% ODA–GONSs, a more than 35.9% increase in Young's modulus from 269.1 to 365.9 MPa is achieved. More remarkably, the incorporation of 4.0 wt% ODA–GONSs gives a Young's modulus of 427.6 MPa, corresponding to the increases of ∼58.9% relative to the neat LDPE film. Compared with other systems reinforced by alkylated graphene-based materials, the reinforcement efficiency of ODA–GONSs obtained in this work is very attractive.^{19,22,24–26} According to the results shown Fig. 7 and 8, eliminating the contribution of crystalline structure and crystallinity to the mechanical properties, we reasonably deduce that the efficiently enhanced mechanical performances of LDPE nanocomposite films can be ascribed to the fully exfoliated and uniformly dispersed ODA–GONSs, as well as strong interfacial adhesion between ODA–GONSs and LDPE matrix, which effectively facilitates the interfacial transfer of stress. In addition, the enhanced rigidity of LDPE nanocomposite films naturally gives rise to the declined ductility. With increasing the content of ODA–GONSs, elongation at break of LDPE nanocomposite films gradually decreased, but still keeping the excellent fracture ductility.

Fig. 9 Typical stress–strain curves of LDPE nanocomposite films with various ODA–GONS loadings.

Fig. 10 Yield strength, Young's modulus, and elongation at break of LDPE nanocomposite as a function of ODA–GONS content.

With the dispersion of layered ODA–GONSs throughout LDPE matrix, the gas barrier properties of nanocomposite films are expected to be enhanced. As shown in Fig. 11a, a typical time-lag data for O₂ penetration in LDPE nanocomposite films is seen, where O₂ diffusing rapidly establishes a steady flow after a transitory non-steady state. According to the slope of the steady-state permeating line, the permeability coefficient is calculated and summarized in Fig. 11b. It is clearly visible that the O₂ permeation in LDPE nanocomposite films is suppressed by the incorporation of ODA–GONSs. To be specific, upon the ODA–GONS loading, a more than 37% reduction in P_O2 from 1.749 to 1.101 × 10⁻¹³ cm³ cm cm⁻² s⁻¹ Pa⁻¹ is achieved by adding 4.0 wt% ODA–GONSs. Compared with our previous work,^7–9 the enhancement efficiency of ODA–GONSs for gas barrier properties is weakened to some extent, presumably owing to the gentle graphitization during the refluxing ODA and GONSs. Similarly, excluding the effect of crystalline structure and crystallinity on the barrier properties (Fig. 7 and 8), two aspects are responsible for the improved gas barrier performances. First, the impermeable ODA–GONSs are regarded as “nano-barrier wall”, giving rise to the decrease in available area for diffusing and the increase tortuous pathway for diffusing molecules. Second, the strong interfacial adhesion probably restricts the mobility of LDPE chains and increases the density of interfacial region, thus providing lower available free volume for diffusing molecules.

Fig. 11 (a) O₂ pressure variations as a function of the reduced time in the barrier measurements. (b) Permeability coefficient of O₂ (P_O2) for neat LDPE and its ODA–GONS nanocomposite films.

3.4 Crystallization behavior and thermal stability of ODA–GONS/LDPE nanocomposite films

It is well-known that crystalline morphology and crystallinity play a vital role in determining the ultimate properties of polymer nanocomposites. Herein, it is of great interest to investigate the addition of ODA–GONSs on the crystallization behavior of LDPE nanocomposites. As described in Fig. 12a, in the case of neat LDPE film, a crystallization peak temperature (T_p) is found to locate at around 93.5 °C with crystallization enthalpy (ΔH_c) of 72.9 J g⁻¹. However, in the case of LDPE nanocomposites, T_p shifts to higher-temperature range with increasing ODA–GONS loading. As listed in Table 2, T_p of LDPE1.0 and LDPE4.0 increases to 96.4 and 96.6 °C, respectively. This outcome can be explained in terms of the strong heterogeneous nucleation effect for LDPE crystallization in the presence of ODA–GONSs.^28,31 Fig. 12b depicts the subsequent melting behavior of neat LDPE and its nanocomposites after cooling from the melt at 10 °C min⁻¹ (second heating). It can be observed that all the samples display melting peak in the proximity of 107.0 °C with almost the same melting enthalpy and crystallinity (Table 2). This suggests that ODA–GONSs only serve as heterogeneous nucleating agents to accelerate the crystallization kinetics of LDPE, and have little impact on the crystalline structure and crystallinity of LDPE nanocomposites.

Fig. 12 Non-isothermal melt crystallization and subsequent melting traces of LDPE nanocomposite films as a function of ODA–GONS loadings: (a) first cooling and (b) second heating.

Table 2 DSC results of LDPE nanocomposite films as a function of ODA–GONS loadings, summarized from non-isothermal melt crystallization and subsequent melting traces shown Fig. 12

ODA–GONS loading (wt%)	Tp (°C)	ΔHc (J g^−1)	Tm (°C)	ΔHm (J g^−1)	χc (%)
0	93.5	72.9	106.6	92.6	31.6
0.1	95.8	72.6	106.7	94.2	32.2
0.25	95.9	71.3	107.0	91.8	31.3
0.5	96.4	71.1	106.8	95.3	32.5
1.0	96.4	71.5	107.0	94.3	32.2
4.0	96.6	69.1	107.2	88.9	30.3

---

TGA analysis was used to study the thermal stability of neat LDPE and its ODA–GONS nanocomposite films. Fig. 13 shows that the neat LDPE film is subjected to simple one-step degradation, in which the initial degradation temperature (T_i) at 5% weight loss takes place at 435.2 °C. As for the nanocomposites, T_i gradually shifts to higher temperature with increasing ODA–GONS loading. When adding 4.0 wt% ODA–GONSs, the value of T_i is elevated by ∼15.9 °C, effectively retarding the degradation of the LDPE host. One can also observe from Fig. 13 that the addition of ODA–GONSs slightly affect the maximum decomposition temperatures at 50% weight loss of LDPE nanocomposites, nearly comparable to that of the neat LDPE film. The markedly enhanced thermal stability of LDPE nanocomposites should be ascribed to the uniform dispersed ODA–GONSs with special layered structure and high aspect ratio, which has a potential to act as a natural barrier against the diffusion of heat to LDPE matrix.³² Meanwhile, the strong interfacial adhesion can also suppress the mobility of LDPE chains in the vicinity of ODA–GONS surface, giving rise to an excellent thermal stability of LDPE matrix.³³

Fig. 13 TGA curves of neat LDPE and its nanocomposite films with various ODA–GONSs loadings.

3.5 Optical property of ODA–GONS/LDPE nanocomposite films

UV-visible light transmittance spectra were performed to study the effect of ODA–GONSs on the optical property of LDPE nanocomposite films. As illustrated in Fig. 14, neat LDPE film is basically semitransparent due to its about 30.0% crystallinity, exhibiting the gradually reduced transmittance in the whole range of 250–850 nm. The inclusion of ODA–GONSs significantly blocks the light transmission throughout the UV-visible spectra. Interestingly, the reduction in light transmittance is more significant in the UV region. As the ODA–GONS loading is beyond 0.5 wt%, nearly no UV light can be penetrated through LDPE nanocomposite films, where the light transmittance curves of LDPE1.0 and LDPE4.0 are overlapped. The excellent UV shielding suggest the potential application of LDPE nanocomposite films as packaging materials for protecting perishable goods vulnerable to degradation from high-energy light.^8,16

Fig. 14 UV-visible light transmittance spectra in the range of 250–850 nm for neat LDPE and its nanocomposite films (∼180 μm) as a function of ODA–GONS loadings.

4. Conclusions

Surface functionalization of GONSs chains were realized by facile refluxing GONSs and ODA with the grafting ratio of ∼24%, wherein the nucleophilic substitution reaction between the primary amine groups of ODA and the epoxide groups of GONSs occurred. The presence of the long octadecyl chain made the hydrophilic GONSs hydrophobic, thus giving rise to uniform dispersion and excellent compatibility in non-polar LDPE matrix. With incorporation of 4.0 wt% ODA–GONSs, Young's modulus of LDPE nanocomposite films was improved by 58.9%; O₂ permeability of LDPE nanocomposite films was reduced by 37.0%; initial decomposition temperature of LDPE nanocomposite films was elevated by 15.9 °C. Additionally, ODA–GONSs could serve as the heterogeneous nucleating agents for LDPE crystallization and the inclusion of ODA–GONSs could effectively block the transmission of UV light in LDPE nanocomposite films. Such long alkane chain modification presented here is confirmed to be an effective strategy to promote the application of GONSs in the non-polar polymer.

Acknowledgements

The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant nos 51421061, 51473102 and 51273161), and the Innovation Team Program of Science & Technology Department of Sichuan Province (Grant no. 2014TD0002). We are also indebted to the Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China) for the kind help with the 2D-WAXD measurements.

Notes and references

1. J. Lange and Y. Wyser, Packag. Technol. Sci., 2003, 16, 149–158.
2. H. Kim, A. A. Abdala and C. W. Macosko, Macromolecules, 2010, 43, 6515–6530.
3. S. S. Ray and M. Okamoto, Prog. Polym. Sci., 2003, 28, 1539–1641.
4. Z. Spitalsky, D. Tasis, K. Papagelis and C. Galiotis, Prog. Polym. Sci., 2010, 35, 357–401.
5. Y. H. Yang, L. Bolling, M. A. Priolo and J. C. Grunlan, Adv. Mater., 2013, 25, 503–508.
6. R. Nair, H. Wu, P. Jayaram, I. Grigorieva and A. Geim, Science, 2012, 335, 442–444.
7. H.-D. Huang, P.-G. Ren, J. Chen, W.-Q. Zhang, X. Ji and Z.-M. Li, J. Membr. Sci., 2012, 409, 156–163.
8. H.-D. Huang, P.-G. Ren, J.-Z. Xu, L. Xu, G.-J. Zhong, B. S. Hsiao and Z.-M. Li, J. Membr. Sci., 2014, 464, 110–118.
9. H.-D. Huang, C.-Y. Liu, D. Li, Y.-H. Chen, G.-J. Zhong and Z.-M. Li, J. Mater. Chem. A, 2014, 2, 15853–15863.
10. D. R. Dreyer, S. Park, C. W. Bielawski and R. S. Ruoff, Chem. Soc. Rev., 2010, 39, 228–240.
11. D. R. Dreyer, A. D. Todd and C. W. Bielawski, Chem. Soc. Rev., 2014, 43, 5288–5301.
12. J. Liang, Y. Huang, L. Zhang, Y. Wang, Y. Ma, T. Guo and Y. Chen, Adv. Funct. Mater., 2009, 19, 2297–2302.
13. H. Kim, Y. Miura and C. W. Macosko, Chem. Mater., 2010, 22, 3441–3450.
14. S. Stankovich, R. D. Piner, S. T. Nguyen and R. S. Ruoff, Carbon, 2006, 44, 3342–3347.
15. S. Stankovich, D. A. Dikin, G. H. Dommett, K. M. Kohlhaas, E. J. Zimney, E. A. Stach, R. D. Piner, S. T. Nguyen and R. S. Ruoff, Nature, 2006, 442, 282–286.
16. O. C. Compton, S. Kim, C. Pierre, J. M. Torkelson and S. T. Nguyen, Adv. Mater., 2010, 22, 4759–4763.
17. G. Wang, X. Shen, B. Wang, J. Yao and J. Park, Carbon, 2009, 47, 1359–1364.
18. Z. Lin, Y. Liu and C.-P. Wong, Langmuir, 2010, 26, 16110–16114.
19. T. Kuila, S. Bose, C. E. Hong, M. E. Uddin, P. Khanra, N. H. Kim and J. H. Lee, Carbon, 2011, 49, 1033–1037.
20. W. Li, X.-Z. Tang, H.-B. Zhang, Z.-G. Jiang, Z.-Z. Yu, X.-S. Du and Y.-W. Mai, Carbon, 2011, 49, 4724–4730.
21. S. Choudhary, H. P. Mungse and O. P. Khatri, J. Mater. Chem., 2012, 22, 21032–21039.
22. T. Kuila, S. Bose, A. K. Mishra, P. Khanra, N. H. Kim and J. H. Lee, Polym. Test., 2012, 31, 31–38.
23. H. Pang, Y.-Y. Piao, C.-H. Cui, Y. Bao, J. Lei, G.-P. Yuan and C.-L. Zhang, J. Polym. Res., 2013, 20, 1–8.
24. P. G. Ren, H. Wang, H. D. Huang, D. X. Yan and Z. M. Li, J. Appl. Polym. Sci., 2014 DOI:10.1002/app.39803.
25. S. H. Ryu and A. Shanmugharaj, Chem. Eng. J., 2014, 244, 552–560.
26. X. Qian, L. Song, B. Yu, W. Yang, B. Wang, Y. Hu and R. K. Yuen, Chem. Eng. J., 2014, 236, 233–241.
27. Y. Cao, J. Feng and P. Wu, Carbon, 2010, 48, 1683–1685.
28. J.-Z. Xu, T. Chen, C.-L. Yang, Z.-M. Li, Y.-M. Mao, B.-Q. Zeng and B. S. Hsiao, Macromolecules, 2010, 43, 5000–5008.
29. B. Wunderlich and G. Czornyj, Macromolecules, 1977, 10, 906–913.
30. T. Szabó, O. Berkesi and I. Dékány, Carbon, 2005, 43, 3186–3189.
31. J.-Z. Xu, Y.-Y. Liang, H.-D. Huang, G.-J. Zhong, J. Lei, C. Chen and Z.-M. Li, J. Polym. Res., 2012, 19, 1–7.
32. P. A. Song, Y. Yu, T. Zhang, S. Fu, Z. Fang and Q. Wu, Ind. Eng. Chem. Res., 2012, 51, 7255–7263.
33. Y. Xu, W. Hong, H. Bai, C. Li and G. Shi, Carbon, 2009, 47, 3538–3543.

---