Thuy Chinh Nguyen*ab,
Thi Mai Tranb,
Anh Truc Trinhb,
Anh Hiep Nguyenb,
Xuan Thang Damc,
Quoc Trung Vud,
Dai Lam Tranab,
Duy Trinh Nguyene,
Truong Giang Lef and
Hoang Thai*ab
aGraduate University of Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Ha Noi, 100000, Vietnam
bInstitute for Tropical Technology, Vietnam Academy of Science and Technology, 18, Hoang Quoc Viet, Cau Giay, Ha Noi, 100000, Vietnam. E-mail: ntchinh@itt.vast.vn; hoangth@itt.vast.vn
cFaculty of Chemistry, Hanoi University of Industry, Minh Khai Commune, Bac Tu Liem, Ha Noi, 100000, Vietnam
dFaculty of Chemistry, Hanoi National University of Education, No. 136 Xuan Thuy Road, Cau Giay District, Ha Noi, 100000, Vietnam
eNTT Institute of High Technology, Nguyen Tat Thanh University, 300A Nguyen Tat Thanh, District 4, Ho Chi Minh City, 700000, Vietnam
fInstitute of Chemistry, Vietnam Academy of Science and Technology, 18, Hoang Quoc Viet, Cau Giay, Ha Noi, 100000, Vietnam
First published on 15th January 2020
Polyoxymethylene (POM) is a semicrystalline thermoplastic that displays high tensile strength, thermal stability, and chemical durability. However, its widespread application is limited by its low elongation at break and thermal durability. In the present study, nanosilica (NS) and polylactic acid-grafted polyethylene glycol (PELA) were used as enhancement additives to improve the performance of POM homopolymer. Specifically, the POM/PELA/NS nanocomposites with a fixed NS content and varying PELA contents were prepared by a melt mixing method. The influence of the additives on the processability, and dynamic thermo-mechanical and tensile properties of the nanocomposites was evaluated by comparing the torque, mixing energy at melt state, storage modulus, shear stress, loss modulus, tanδ, tensile strength, elongation at break and thermal degradation of the nanocomposites. The results showed that the combined addition of NS and PELA enhanced the thermal stability, tensile strength, elongation at break and chemical stability of the POM/PELA/NS nanocomposites owing to the good compatibility between PELA and the POM matrix. Furthermore, the morphology, and UV and ozone durability of POM and the nanocomposites were assessed and discussed.
Furthermore, to broaden the application scope of POM, nanoadditives have been mixed or blended with melting POM.4–9 The presence of such additives can also improve the mechanical properties and thermal stability of POM. Various nanofillers have been examined for the preparation of POM-based nanocomposites, including nanosilica (NS),4–6 carbon nanotubes,7–9 montmorillonite,10–14 CaCO3,15 graphite,16 Al2O3,17 ZnO,18 hydroxyapatite,19,20 boehmite alumina,14,21 polyhedral oligomeric silsesquioxane,22,23 and NS and carbon fibers.24 POM can also been combined with ethylene–octene copolymer to improve the relaxation properties of POM.25 These results show that the presence of nanoadditives in POM matrix can enhance its mechanical properties, hardness, and thermal stability.
Among the nanoadditives exemplified above, NS is a popular additive used in polymers, coatings, and rubber materials owing to its high strength, thermal stability, specific surface area, and UV reflectance, as well as its ease of dispersion in polymer matrices.26–29 It is known that, relative to POM, POM/silica nanocomposites display enhanced physical and mechanical properties such as impact toughness, tensile strength, and heat distortion resistance. However, to date, there are only a few studies on this particular nanocomposite. For instance, in a study by Fu et al.,24 NS (1–5 wt%) was combined with CF (5–25 wt%) as additives for POM. The resulting POM composites displayed enhanced Young modulus, hardness, and friction. NS in the nanocomposites influenced the adhesion, dispersion, and interaction of CF with POM. In another study,6 Xiang et al. synthesized POM/NS nanocomposites by melt mixing method. The addition of NS up to a concentration of 5 wt% in POM raised the degradation temperature of the nanocomposites in inert gas and natural air, i.e., 38.3 and 43.8 °C, respectively, relative to that of POM.6
In our previous work, POM/NS nanocomposites with varying NS concentrations of 0.5–2 wt% were prepared by melt mixing method.30 The presence of NS up to a concentration of 1.5 wt% improved the mechanical and thermal properties of POM. At concentrations higher than 1.5 wt%, agglomerates were obtained, which led to a reduction in the mechanical, thermal, and dielectric properties of the POM/NS nanocomposites. In another study,31 polymer additives, i.e., linear low density polyethylene, ethylene vinyl acetate copolymer, polylactic acid-grafted polyethylene glycol (PELA), and stearate zinc, were used in combination with NS in POM matrix. The concentrations of the polymer additive and NS in POM were 5 and 1.5 wt%, respectively. Among the polymer additives studied, PELA yielded the best improvements in processability and mechanical strength of POM.
From past literature findings, it can be inferred that the introduction of NS and PELA, as additives to POM, can improve the properties (such as mechanical, thermal, morphological, and electrical) of the resulting nanocomposites. However, studies on the effect of such additives on the dynamic thermo-mechanical property, and UV and ozone durability of these nanocomposites to potentially expand their application scope are lacking. Therefore, in the present work, the influence of both PELA and NS on the torque, mixing energy, mechanical, and dynamic thermo-mechanical properties, and ozone and UV durability of POM is investigated. The PELA content is varied in the range of 1–5 wt%, while the POM/NS ratio is fixed at 100/1.5. The sample nomenclature of the different nanocomposites prepared is presented in Table 1.
No. | POM weight (gram) | NS weight (gram) | PELA weight (gram) | Ratio or POM:NS:PELA | Abbreviation |
---|---|---|---|---|---|
1 | 68.10 | 0 | 0 | 100:0:0 | POM |
2 | 67.33 | 1.01 | 0 | 100:1.5:0 | 0PELA |
3 | 66.80 | 1.00 | 0.67 | 100:1.5:1 | 1PELA |
4 | 66.07 | 0.99 | 1.32 | 100:1.5:2 | 2PELA |
5 | 65.36 | 0.98 | 1.96 | 100:1.5:3 | 3PELA |
6 | 63.97 | 0.96 | 3.20 | 100:1.5:5 | 5PELA |
7 | 67.62 | 0.68 | 0 | 100:1:0 | PN1 |
8 | 65.67 | 0.66 | 1.97 | 100:1:3 | PN1P3 |
9 | 66.96 | 1.34 | 0 | 100:2:0 | PN2 |
10 | 65.05 | 1.30 | 1.95 | 100:2:3 | PN2P3 |
IR spectra of the nanocomposites as thin films were recorded on a Nicolet iS10 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) at room temperature by averaging of 32 scans with a resolution of 8 cm−1 in the wavenumber range of 400–4000 cm−1.
Tensile properties, i.e., tensile strength and elongation at break, of the nanocomposites were determined on a Zwick tensile 2.5 machine (Zwick Roell Group, Ulm, Germany) at room temperature according to ASTM D638 standard.
Morphology of the nanocomposites was analyzed on an S-4800 field-emission scanning electron microscope (Hitachi, Tokyo, Japan).
Dynamic mechanical thermal analysis of the nanocomposites was carried out on an MCR302 instrument (Anton Paar, Graz, Austria) within a temperature range of −120 to 200 °C in nitrogen, heating rate of 3 °C min−1, frequency of 1 Hz, and deformation of 0.1%. The size of the test sample was 50 × 10 × 0.65 mm3.
Thermal property of POM/PELA/NS nanocomposites was performed on the TGA209F1 (NETZSCH, Selb, Germany) under nitrogen atmosphere from room temperature to 600 °C with a heating rate of 10 °C min−1.
Ozone durability of the nanocomposites was assessed in an OTC-1 accelerated ozone test cabinet (In USA, Inc., Needham, MA, USA) according to ISO 1431 standard. The sample was placed in the chamber, which was set at an ozone concentration level of 2.5 ppm, temperature of 35 °C, and a testing time of 72 h.
UV durability of the nanocomposites was assessed in a UV test cabinet that was built at the Institute for Tropical Technology, VAST, Vietnam according to TCVN11608-3:2016. The sample was placed in the chamber before being irradiated continuously under UV-C lamps for 72 h and at a temperature of 35 °C.
The ozone- and UV-tested samples were stored for at least 48 h before assessing their tensile properties and recording their IR spectra.
Fig. 2 IR spectra of NS, POM, and the POM/PELA/NS nanocomposites prepared with different PELA contents. |
Fig. 3 (a) Torque and (b) mixing energy diagrams of the POM/PELA/NS nanocomposites prepared with different PELA contents. |
Sample | POM | 0PELA | 1PELA | 2PELA | 3PELA | 5PELA |
---|---|---|---|---|---|---|
Stable torque (nm) | 4.89 | 4.10 | 4.08 | 3.80 | 4.03 | 3.90 |
Total mixing energy (kJ) | 111079.8 | 89580.22 | 132161.10 | 66729.61 | 80499.96 | 65253.89 |
Fig. 4 Storage modulus diagrams of the POM/PELA/NS nanocomposites prepared with different PELA contents. |
From Fig. 4, it was also observed that the G′ value of the nanocomposites was higher than that of neat POM, regardless of the addition of PELA. This result indicates that the nanocomposites have better thermo-stability and experience reduced distortion at elevated temperature when compared with neat POM.
Variations in the sheer stress as a function of temperature of POM and the POM/PELA/NS nanocomposites were measured, and the results are shown in Fig. 5. The trends obtained were similar to the trends obtained for G′. The sheer stress value of the nanocomposites was higher than that of neat POM, which is also indicative of the higher thermo-stability of the nanocomposites relative to neat POM.
Fig. 5 Shear stress diagrams of the POM/PELA/NS nanocomposites prepared with different PELA contents. |
Fig. 6 shows changes in the loss modulus (G′′) of POM and the POM/PELA/NS nanocomposites. Two phase transition peaks were observed that corresponded to the phase transition from glass to the elastic region and from the elastic to the melting region of POM and the nanocomposites. The first peak appeared at −73.07, −74.21, −73.64, −74.26, −73.78, and −74.29 °C for POM, 0PELA, 1PELA, 2PELA, 3PELA, and 5PELA, respectively. The second peak was observed at 93.67, 101.89, 98.63, 99.21, 99.03, and 98.17 °C for POM, 0PELA, 1PELA, 2PELA, 3PELA, and 5PELA, respectively. The presence of NS and PELA caused a slight decrease in the glass transition temperature and an increase in the melting transition temperature of POM. In the nanocomposites, most NS particles inside were impacted by tension force and only some NS particles at the surface of the polymer–particle phase were deformed.24 In addition, it was observed that the small peak at 50 °C noted in the diagram of 5PELA, which corresponds to the glass transition of PELA, was absent in the diagrams of 1PELA, 2PELA, and 3PELA. At these lower PELA contents (1–3 wt%), PELA was compatible with the POM matrix. Accordingly, PELA was prone to deformation as POM was subject to deformation. Therefore, both NS and PELA displayed minimal influence on the flexibility of POM.34 These results confirm that the nanocomposites display higher thermo-stability and processability at low temperatures when compared with neat POM.
Fig. 6 Loss modulus diagrams of the POM/PELA/NS nanocomposites prepared with different PELA contents. |
The tanδ diagrams of POM and the POM/PELA/NS nanocomposites are shown in Fig. 7. The nanocomposites displayed tanδ values of less than 1, which is indicative that G′′ is smaller than G′. Using the tanδ value, the glass temperature values of POM, 0PELA, 1PELA, 2PELA, 3PELA, and 5PELA were determined to be −68.26, −71.40, −70.99, −71.47, −69.68, and −74.29 °C, respectively. The small reduction in the glass temperature observed for the nanocomposites (relative to that of POM) was attributed to the thermal conductive ability of NS and the lower number of chain ends and lower mobility of POM near the NS surface.20
Sample | Tensile strength (MPa) | Elongation at break (%) |
---|---|---|
POM | 59.25 | 14.52 |
0PELA | 61.37 | 14.74 |
1PELA | 67.81 | 12.49 |
2PELA | 68.67 | 12.69 |
3PELA | 65.07 | 15.47 |
5PELA | 61.46 | 12.93 |
PN1 | 64.95 | 13.90 |
PN1P3 | 65.36 | 14.85 |
PN2 | 59.40 | 12.48 |
PN2P3 | 59.26 | 11.57 |
In case of varying the NS content and fixing PELA content, it is clear that NS content at 2 wt%, the tensile strength and elongation at break was decreased for the nanocomposites with and without PELA. The increase of NS content can lead to the strong decrease in tensile properties of the nanocomposites due to the agglomeration of NS in polymer matrix. PELA exhibited the role of a compatibilizer in improvement the tensile strength and elongation at break of the nanocomposites at low NS contents.
Fig. 8 FESEM images of impact-fractured surfaces of POM and the nanocomposites at magnification of 5000× and 10000×: (a) POM, (b) 0PELA, (c) 1PELA, (d) 2PELA, (e) 3PELA, and (f) 5PELA. |
Fig. 9 FESEM images of impact-fractured surfaces of POM and the nanocomposites at a magnification of 50000×: (a) POM, (b) 0PELA, (c) 1PELA, (d) 2PELA, (e) 3PELA, and (f) 5PELA. |
To better evaluate the structure of POM and the dispersion of NS in the nanocomposites, FESEM images of impact-fractured surfaces of POM and the nanocomposites at a higher magnification (50000×) were captured (Fig. 9). The POM molecules are linked and arranged into continuous chains, which are intertwined to form POM sheets. The voids between the chains are believed to contribute to the rather low elongation at break of POM. In contrast, in the nanocomposites, the NS particles fill those voids as they tend to agglomerate in the presence of an applied stress. The introduction of PELA into the POM/NS nanocomposite led to the insertion of the PELA chains in the POM macromolecules. Phase separation between PELA and POM matrix was not observed owing to their good compatibility. In addition, it is likely that the interaction between PELA and NS with POM enhances the uniform dispersion of NS in the POM matrix. Therefore, NS serves as a barrier-endured stress for the nanocomposites. These led to increase in the tensile strength of POM using both NS and PELA.18
The FESEM images of fractured surfaces of POM/PELA/NS composites at different NS content and fixed PELA content are presented in Fig. 10. The agglomeration of NS can be observed for PN2 sample. Using PELA, the agglomeration of NS particles in the nanocomposites tends to decrease. The fractured surface of PN1P3 and PN2P3 is smoother than that of PN1 and PN2 corresponding to the better dispersion of NS in POM matrix in the presence of PELA compatibilizer.
Fig. 10 FESEM images of fractured surfaces of POM/PELA/NS composites: (a and b) PN1, (c and d) PN1P3, (e and f) PN2, and (g and h) PN2P3. |
POM and the POM/PELA/NS nanocomposites were examined under UV irradiation, using UV-C lamps, for 72 h at 35 °C and in an acceleration ozone chamber at an ozone concentration level of 2.5 ppm. The tensile strength and elongation at break, and changes in tensile strength (Δδ), elongation at break (Δε), and carbonyl index (ΔCI) of POM and the nanocomposites after UV and ozone testing are listed in Table 4. When compared with the data in Table 3, it can be seen that after UV and ozone testing, the tensile strength of POM and the nanocomposites increased and their elongation at break decreased. These results indicate that POM and the nanocomposites degrade during the UV and ozone tests.
Sample | After UV test | After ozone test | ||||||||
---|---|---|---|---|---|---|---|---|---|---|
δ (MPa) | Δδ (%) | ε (%) | Δε (%) | ΔCI | δ (MPa) | Δδ (%) | ε (%) | Δε (%) | ΔCI | |
POM | 68.91 | +16.30 | 12.06 | −16.94 | 0.47 | 64.64 | +9.10 | 8.63 | −40.56 | 0.59 |
0PELA | 64.86 | +5.69 | 7.25 | −50.81 | 0.29 | 61.10 | −0.44 | 7.01 | −52.44 | 0.35 |
3PELA | 66.58 | +2.32 | 7.51 | −51.45 | 0.02 | 65.83 | +1.17 | 9.32 | −39.75 | 0.54 |
5PELA | 68.48 | +11.42 | 8.40 | −35.03 | 0.22 | 62.70 | +2.02 | 8.67 | −32.95 | 1.12 |
It is noted that the UV and ozone durability of POM-based nanocomposites is scarcely reported in the literature. Therefore, the following is proposed as a rational for the results obtained for the POM/PELA/NS nanocomposites. Firstly, POM is a semi-crystal polymer, whose structure consists of polymer chains arranged in a continuous fashion. The sources of UV and ozone during the tests can instigate further cross-linking of the POM chains, leading to the re-arrangement of the crystal structure of POM. This leads to an increase in the tensile strength of POM and the nanocomposites. Secondly, the degradation of polymers typically occurs in the amorphous region of the polymer and proceeds to the crystal region of the polymer. During the UV and ozone tests, the increase in crystal degree also leads to an increase in the tensile strength and decrease in the elongation at break of POM and the nanocomposites. Thirdly, the introduction of NS and PELA into POM macromolecules can limit the extent of cross-linking of POM, and as a result, the tensile strength of the nanocomposites varied minimally relative to that of POM, whereas the elongation at break of the nanocomposites reduced considerably compared with that of POM. Finally, the OH groups in NS and CO groups in PELA are prone to attack by UV irradiation and ozone to form free radicals, which can catalyze the oxygen-induced photodegradation of POM macromolecules. Thus, the elongation at break of the nanocomposites was reduced compared with that of POM.
From the data in Table 4, it can be seen that the degradation of POM and the nanocomposites was influenced by UV irradiation to a greater extent than by ozone, as indicated from the larger variations noted for Δδ and Δε after UV testing. This may be due to the stronger cross-linking ability that the POM macromolecules and the nanocomposites possess in the presence of UV irradiation, which leads to higher tensile strength.
The degradation of POM and the nanocomposites after the UV and ozone tests was also characterized by CI, which was calculated from the ratio of the IR absorption band at 1755 cm−1 (A1755) (stretching vibration of CO group) and the IR absorption band at 1467 cm−1 (A1467) (bending vibration of the C–H group).35,37 The ΔCI of POM and the nanocomposites are displayed in Table 4. The increase in ΔCI observed for POM and the nanocomposites indicated that after the UV and ozone tests, the CO content in the samples was higher. This result confirmed the degradation of POM and the nanocomposites by oxy in natural air and UV irradiation to form low molecular weight substances containing CO groups.12,37
The UV and ozone testing results confirmed the degradation of POM and the nanocomposites by UV irradiation and ozone into low molecular weight substances containing CO groups. However, during the oxygen-induced photodegradation process, cross-linking of the POM chains occurred simultaneously with chain scission. Therefore, the tensile strength of POM and the nanocomposites increased after the UV and ozone tests. The introduction of PELA and NS into POM matrix can improve thermal stability of the nanocomposites in heating. These results have implications for evaluating the application of POM-based nanocomposites in automotive and electronic fields.
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