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corn starch blends by nanoclay intercalation
Accepted Manuscript Title: Enhanced mechanical and thermal properties of poly (vinyl alcohol)/corn starch blends by nanoclay intercalation Authors: Huafeng Tian, Kai Wang, Di Liu, Jiaan Yan, Aimin Xiang, A.Varada Rajulu PII: DOI: Reference:
S0141-8130(16)32785-4 http://dx.doi.org/doi:10.1016/j.ijbiomac.2017.03.111 BIOMAC 7277
To appear in:
International Journal of Biological Macromolecules
Received date: Revised date: Accepted date:
6-12-2016 8-3-2017 21-3-2017
Please cite this article as: Huafeng Tian, Kai Wang, Di Liu, Jiaan Yan, Aimin Xiang, A.Varada Rajulu, Enhanced mechanical and thermal properties of poly (vinyl alcohol)/corn starch blends by nanoclay intercalation, International Journal of Biological Macromoleculeshttp://dx.doi.org/10.1016/j.ijbiomac.2017.03.111 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced mechanical and thermal properties of poly (vinyl alcohol)/ corn starch blends by nanoclay intercalation Huafeng Tian1,2, Kai Wang1, Di Liu1, Jiaan Yan1, Aimin Xiang1 *, A.Varada Rajulu3 1 School of Material and Mechanical Engineering, Beijing Technology and Business University, Beijing 100048, China 2 Key Laboratory of Recycling and Eco-treatment of Waste Biomass of Zhejiang Province, China 3 Centre for Composite Materials, International Research Centre, Kalasalingam University, Anand Nagar, Krishnankoil-626 126, India. *
To whom correspondence should be addressed. Phone: +86-10-68988056.
E-mail: [email protected] (A. Xiang)
Highlights
PVA/starch/MMT nanocomposites were prepared by melt processing.
Highly exfoliated MMT layers were obtained containing MMT lower than 10wt%.
Intercalated structure was predominant with more than 10 wt% MMT.
Water resistant properties were improved with the incorporation of MMT.
Significant improvement in strength and flexibility were observed.
The thermal stability was improved by MMT addition.
1
Abstract: Poly (vinyl alcohol) (PVA) /corn starch blend films with enhanced properties were fabricated by melt processing and montmorillonite (MMT) reinforcing. It was revealed that strong hydrogen bonding occurred between the abundant –OH groups of the matrix and polar Si-O-Si and -OH groups of MMT. The highly exfoliated MMT nanolayers were randomly dispersed in the matrix containing MMT lower than 10wt%, whereas the intercalated structure was predominant with MMT content higher than 10 wt%. With the increase of MMT, the glass transition temperature as well as equilibrium torque increased. The water sorption decreased and water resistant properties were improved with the incorporation of MMT due to the restricted swelling of the matrix by MMT nanolayers. Significant improvement in strength and flexibility were observed due to the fine dispersion of the MMT layers and the strong interaction between MMT and the matrix. The thermal stability was also improved. The MMT nanolayers could act as the heat and mass transport barriers and retard the thermal decomposition of the composites. Keywords: starch; poly (vinyl alcohol); montmorillonite; nanocomposites; melt processing; mechanical properties
2
1. Introduction With the growing concerns on the environmental problems and resource limitation induced by the wide usage of petroleum based synthetic polymers, the development of biodegradable materials for substituting nonbiodegradable materials have attracted great interests [1-5]. Starch is one of the natural occurring biopolymers with inherent biodegradability, abundant availability, and low cost [6, 7]. However, the entire starch plastics with low molecular plasticizers possess poor mechanical properties and water resistance properties, which limit their wide applications [8]. Compared with chemical modifications such as grafting, esterification or etherization, the physical modification of starch would be a simple and green process without environmental pollution. Therefore, the blending of starch with other biodegradable synthetic polymers, such as polycaprolactone [9, 10], poly(lactic acid) [11, 12], polyurethane [13, 14], poly (vinyl alcohol) (PVA) [15-18], poly (butylene succinate) [19], have been carried out and reported in the literature. Among them, PVA/starch blends would have more potential use as disposable packaging articles for the abundance and low cost of the starting starch material as well as the eco-friendly and higher barrier character of PVA [20, 21]. Also to further improve the mechanical strength of the PVA/starch blends, nanoparticles such as silica [22], carbon nanotubes[23], graphene [24] were introduced into the blends to make nanocomposites. Natural or synthetic mineral layered silicates have gained great popularity as an effective reinforcement for nanocomposite systems [25-27]. The incorporation of layered
silicate
clays,
such
as
montmorillonite 3
(MMT,
general
formula
Mx(Al4−xMgx)Si8O20(OH)4 [28]), could dramatically improve polymer-based composite performance because of high aspect ratio of silicate nanolayers, high surface area, and high modulus [29-31]. With uniform dispersion of clays in polymer matrices with an intercalated or exfoliated structure, enhanced mechanical and barrier properties would be achieved [32-34]. In our previous works, PVA/starch blend films were prepared by melt processing instead of widely used solution casting method, which is energy consuming and inefficient [35]. The urea and formamide plasticizers dramatically improved the flexibility of the materials, however sacrificed the strength. And low content of MMT could form exfoliated structures and improve the mechanical properties [36]. In this study, the PVA/starch blends were further reinforced with higher amount of MMT nanoclays by melt intercalating, which is the most popular method to disperse clays into polymer matrix for its environmental friendliness with the absence of large amount of solvents. The influence of the MMT content as well as the different structures of both exfoliated and intercalated on the structure and properties of the resulting composites was investigated in detail. The physical properties were discussed depending on the MMT structures. It is aimed to provide an effective way to prepare the green materials with both good strength and flexibility.
2. Experimental 2.1. Materials
4
PVA (117) with the viscosity of 25.0-31.0×10-3 Pa·S, molecular weight of 83000 and alcoholysis degree of 98.0-99.0 mol/% was purchased from Kuraray Co., Ltd. (Japan). Corn starch was purchased from Quanfeng Co., Ltd. (Beijing). Formamide and urea were supplied by Sinopharm Chemical Reagent Co., Ltd (Beijing, China) and used as received. Na+-montmorillonite (MMT, GTP) was supplied by Nanocor Co., Ltd (USA). 2.2. Preparation of poly (vinyl alcohol)/starch/MMT nanocomposites Starch, PVA, water, mixed plasticizers (formamide and urea, weight ratio 2:1) and desired amount of MMT were mixed in a high speed mixer (GH-10, Zedao Mechanical Technology, Beijing, China) at room temperature for 3 min. The ratio of starch and PVA was fixed at 1:1. The content of the mixed plasticizers and water was fixed at 30 phr and 50 phr based on the solid materials (starch and PVA), respectively. Then the mixtures were charged into a HAAKE torque rheometer (TA Instruments, USA) and further mixed at 105 oC for 10min with a stirrer speed of 80 rpm. The mixtures were compression molded on plate vulcanizing machine (XLB-400, Huaqing Industry Group, Qingdao, China) to obtain the nanocomposite films with thickness of about 0.50.6 mm. The molding temperature, pressure and time maintained were 110 oC, 20 MPa and 10 min, respectively. Before testing, all samples were vacuum-dried at 80 oC for 2 days and kept at a relative humidity (RH) of 0% (conditioned by phosphorus pentoxide P2O5) for 2 weeks. 2.3. Characterization 5
The Fourier-transform infrared spectra (FTIR) of the samples were recorded on a Nicolet 5700 FTIR spectrometer (Thermo Electron Corporation, Waltham, MA, USA). Samples were prepared by mixing the fine powder with potassium bromide (KBr) and pressing. The spectra were obtained at a resolution of 4 cm-1 in the range 4000 cm-1 to 400 cm-1. X-ray diffractograms (XRD) were recorded using a DIP 2030 apparatus (MAC Science, Yokohama, Japan) using Cu–Kα radiation (0.154 nm) at 2θ values ranging from 5o to 10o at a scanning rate of 2o/min. The testing conditions were reflectance mode and 40kV, 30mA. The morphology of the nanocomposites were visualized by a transmission electron microscope (TEM F30, FEI TECNAI, Netherlands) at an accelerating voltage of 200 kV. Ultrathin sections of the composits were prepared using a Leica Ultracut UCT with EMFCS cryo-attachment at -120 °C. The ultrathin films of the samples were directly placed on the copper grids and observed after sputtering carbon. Differential scanning calorimetry (DSC) was carried out using a DSC Q100 calorimeter (TA Instruments, USA) under a nitrogen atmosphere. An empty pan was used as the reference. Samples were placed in an aluminum pan and were heated from 0 oC to 200 oC at a heating rate of 20 oC/min and held for 3 min. The initial sample weight was in the range of 5mg-10mg. The kinetics of water absorption at room temperature (25 C) was determined for all films. The specimens with dimensions around 10×10×0.2 mm3 were thin enough so 6
that the diffusion was supposed to be unidirectional. The samples were first vacuumdried overnight at 100 C. After being weighed using a four-digit balance, the sample was conditioned in a desiccator containing K2CO3 saturated solution to ensure a RH of 43%. The sample was removed at specific intervals and re-weighed. The water uptake (WU) was calculated by: WU (%)
mt m0 100 m0
(1)
where m0 and mt are the weights of the samples before and after a time t of conditioning, respectively. The mass of moisture absorbed at a short time t is expressed as [37]: 1/ 2
mt - m0 2 D m L
t1/ 2
(2)
where m∞ is the weight of the sample at equilibrium, 2L is the thickness of the film, and D is the diffusion coefficient. The plots of ((mt-m0)/m∞)2 as a function of (4t/(πL2)) were drawn for all samples with (mt-m0)/m∞ ≤0.5. The diffusion coefficients were calculated from the slope of the plots. For tensile test, specimens, with the shape of a dumb-bell according to the ASTM D 638-14 standard, were prepared. Tensile tests was performed on a universal testing machine (CMT 6104, SKYan, Shenzhen, China) equipped with 100 N load cell and operated at a cross-head speed of 50 mm/min. Five samples of each system were tested at room temperature and the average value is reported. Before testing, all specimens were conditioned at room temperature in a desiccator of 43% RH (conditioned by K2CO3 saturated solution) for two weeks. 7
Thermo-gravimetric analysis (TGA) was performed on a TGA Q5000 thermogravimetric analyzer (TA Instruments, USA) from 50 oC to 800 oC at a heating rate of 10 oC/min under nitrogen atmosphere. The sample weight was about 5mg - 10mg.
3. Results and discussion 3.1 FTIR The FTIR spectra of PVA/starch/MMT nanocomposites are shown in Figure 1. The peak at around 3300 cm-1 was assigned to the complex vibrational stretches associated with free, inter- and intramolecular bound hydroxyl groups. With the increase of MMT, the wavenumbers red shifted, suggesting the possible formation of hydrogen bonds. In the fingerprint region of the spectrum of pristine starch, three characteristic peaks appeared between 1000 and 1160 cm-1, which were attributed to C–O bond stretching. The band at 2930 cm-1 was characteristic of C–H stretches associated with the hydrogen atoms of methine ring [38]. Absorption peaks at about 1038 cm-1 and 525cm-1 were ascribed to the stretching vibration and bending vibration of Si-O, respectively. The Si-O-Si and -OH groups on the MMT gallery surface could act as hydrogen bonding sites for the guest molecules, and strong hydrogen bonding could occur between the abundant –OH groups and polar groups of MMT, which would be beneficial to the intercalating process and mechanical properties discussed in Section 3.7. 3.2 XRD
8
The XRD patterns of PVA/starch/MMT nanocomposites are shown in Figure 2. A sharp peak at 2θ = 9.1o appeared for pristine MMT, ascribing to the basal spacing of d001 = 0.96 nm calculated using Bragg equation. No diffraction peaks were observed in 2θ =2-10o for the unfilled PVA/starch materials, indicating no crystalline structure existed in this range. For nanocomposites with 5% MMT, the diffraction peak of (001) spacing disappeared, indicating a highly exfoliated structure. MMT was exfoliated into single nanolayers and dispersed in the matrix homogeneously and disorderly. The diffraction peaks reappeared and moved to 2θ = 4.7o and basal spacing was calculated to be about 1.87 nm with MMT content more than 10%. Also the peak intensity increased with the increase of MMT content. This suggested that the starch and PVA macromolecules had entered into the galleries of MMT and enlarged the spacing of MMT layers, which was the characteristic of intercalating structures. 3.3 TEM TEM images of the nanocomposites are shown in Figure 3. With 5% MMT, the MMT layers were well dispersed in the matrix as randomized nanolayers, indicating the exfoliating structure. While with 10% incorporated, MMT layers still retained their orientation to some degree indicating the intercalation structure. The results from TEM were in well agreement with that from XRD. On the basis of the evidence from XRD and TEM, the PVA/starch/MMT nanocomposites with an intercalated or exfoliated structure have been successfully prepared via a melt intercalation process without any special aid. Moreover, the results obviously suggest a high affinity between matrix and MMT fillers. 9
3.4 DSC Figure 4 shows the DSC curves of PVA/starch/MMT nanocomposites. Only one glass transition temperature (Tg) at about 102 oC was observed for PVA/starch blends, indicating fine compatibility between starch and PVA. It has been proven in our previous study that strong hydrogen bonding occurred between starch and PVA for the abundant of hydroxyl groups on the macromolecular chains [35]. Table 1 shows the influence of MMT content on the glass transition temperatures of the nanocomposites. With the increase of MMT, the glass temperature increased. The fine dispersion of MMT layers effectively restricted the segmental mobility of the starch and PVA macromolecules on the interface of the clay layers because of their high aspect ratio and reactive surface [39]. It is worth noting that with MMT content higher than 15%, the glass temperature did not increase and remained at about 105 oC. As indicated in the XRD section, the intercalating structures with high content of MMT would have a lower dispersing effect compared to exfoliating structure, which would limit the restricting effect of the matrix molecular motion. 3.5 Rheology The rheology was obtained by the HAAKE torque rheometer during the preparing process and Figure 5 shows the torque rheology of PVA/starch/MMT nanocomposites. The torque increased rapidly to a maximum value after feeding the starting materials and then decreased to obtain an equilibrium value at about 100s. Thus the starting materials fed in the mixer were changed to be thermoplastic. The plasticizing time (the mixing time by which the torque leveled off) increased with the increase of MMT, 10
indicating more time was needed to make the mixtures to be thermoplastic. Also the equilibrium torque increased with the increase of MMT. During the processing, PVA and starch macromolecules could enter into the galleries of MMT nanolayers and form exfoliating or intercalating structure. The strong interactions between the filler and matrix would inhibit the moving ability of macromolecular chains and decrease the melt flowability, leading to the increased torque and melt viscosity. The increased torque would also be beneficial to destroy the original structure of MMT and disperse them into the matrix. 3.6 Water uptake Figure 6a shows the water uptake kinetics of PVA/starch/MMT nanocomposites conditioned at 43% RH. The water uptake at equilibrium and water diffusion coefficient are shown in Figure 6b. The water absorption of the composite films increased quickly with time less than 50 hours and reached a plateau after 50h. With the increase of MMT, the water uptake at equilibrium decreased. The strong interaction between MMT nanolayers and PVA/starch macromolecules could restrict the swelling of the matrix, leading to the decreased water uptake. The water diffusion coefficient decreased quickly with increasing MMT and a turning point appeared at 10% MMT content. With MMT content higher than 10%, the coefficient decreased slightly. In fact, preparing polymer/clay nanocomposites with nano-scaled clay flakes dispersed in a pristine polymer membrane is an effective technique to improve the barrier properties of polymers [40]. According to the 11
tortuosity theory, clay flakes dispersed in polymers could act as impermeable domains, which would detour the diffusing molecules around the clay particles, resulting in a significant decrease in the diffusion coefficient and the associated permeability [41]. Therefore, the planar MMT layer could act as the transport barrier for water molecules, and the free path for the water molecules to pass through the nanocomposite would decrease, leading to the decrease of diffusion coefficient. With MMT content lower than 10%, MMT was exfoliated into single nanolayers and dispersed in the matrix disorderly causing a dramatic barrier effect and resulted in the quick decrease of the coefficient. With MMT content higher than 10%, the intercalated structure formed, with MMT maintained the original short-range order structure. The dispersive effect in this case would be not as well as in the exfoliated structure, therefore resulted in the slight decrease of the water diffusion coefficient. 3.7 Mechanical properties Figure 7 shows the mechanical properties of PVA/starch/MMT nanocomposites under 43% RH. The data were listed in Table 2. For unfilled PVA/starch films, with the increase of stain, the stress increased rapidly and linearly, and the yield point exhibited at about 100% strain. After yielding, the stress increased slowly with strain until fracture. As investigated in our previous studies [35], the urea and formamide plasticizers could dramatically improve the toughness of the materials and improve the flexibility, however with the sacrifice of the strength. With the increase of MMT, the yield strength increased significantly. The tensile strength increased first, and reached the maximum value at 15% MMT. With further increase of MMT, the tensile 12
strength decreased. The fracture energy, which was calculated from the area below the stress-strain curves and stand for the flexibility of the materials, exhibited similar trends with tensile strength, while reached the maximum value at 10% MMT. With MMT content lower than 15%, MMT could disperse into the matrix homogeneously with exfoliating and intercalating structure. The MMT tactoids could disperse in the matrix homogeneously. When imposed with external force, the stress could be transferred to the filler from the matrix through the interface effectively, which could improve the mechanical properties dramatically. The interaction between the filler and matrix could also restrict the macromolecular motions of the matrix, leading to the enhanced strength of the composites and decrease of the elongation at break. With MMT content more than 15%, phase separation phenomenon might occur and result in the decrease of the mechanical strength and flexibility. The Young’s modulus increased and elongation at break decreased with the increase of MMT. The Young’s modulus increased from 9.6 MPa of unfilled PVA/starch systems to 59.5 MPa of composites with 25% MMT. As a kind of rigid filler, MMT could disperse into the matrix and improve the modulus of the resulting composites significantly. 3.8 Thermal stability The TGA and DTG curves of PVA/starch/MMT nanocomposites are shown in Figure 8. All the samples exhibited fine thermal stability below 180 oC. The initial decomposition below 250 oC mainly involved the vaporization of volatiles including 13
urea and formamide. PVA exhibited a two-step degradation at about 300 oC and 450 o
C, which were attributed to the acetate group elimination at lower temperatures
followed by a breakdown of polymer backbone at higher temperatures [42]. The degradation of starch macromolecules mainly occurs at 290–370 oC. The degradation peaks of acetate group elimination of PVA and degradation of starch exhibited as one main peak with a shoulder peak on DTG curves for PVA/starch systems. With the increase of MMT, the thermal stability of the nanocomposites increased dramatically for the increased weight residue after thermal decomposition. Also the peak intensity value on the DTG curves decreased, and the area under the degradation peaks descended, especially during the acetate group elimination of PVA and degradation of starch, indicating the decreased weight loss rate and decreased weight loss amount during this temperature range. Generally, inorganic clay acts as a thermal barrier for the heat transfer into polymer chains, and prevents the heat to propagate quickly and limit the further degradation [43]. Also the inorganic fillers act as mass transport barrier to the volatile products generated during thermal decomposition, leading to the retardance of the thermal degradation action. In addition, hydrogen bonding interactions between MMT and PVA/starch matrix could decrease the quality of free –OH groups and stabilize the matrix. All of these could lead to the improved thermal stability of the nanocomposites.
Conclusions The PVA/starch/MMT nanocomposites were prepared by melt processing successfully in this work. The Si-O-Si and -OH groups on the MMT gallery surface 14
could act as hydrogen bonding sites for the guest molecules, and strong hydrogen bonding could occur between the abundant –OH groups of the matrix and polar groups of MMT. Exfoliated structure was formed for MMT content lower than 10%, while for MMT content higher than 10%, the intercalating structures were dominated. With the increase of MMT, the glass temperature as well as equilibrium torque increased, due to the restricted segmental motion of the starch and PVA macromolecules by MMT nanolayers. With the increase of MMT, the water uptake at equilibrium and water diffusion coefficient decreased. The strong interaction between MMT nanolayers and PVA/starch macromolecules could restrict the swelling of the matrix, leading to the decreased water uptake. Also the planar MMT layers could act as the transport barrier for water molecules, leading to the decrease of diffusion coefficient. Incorporation of MMT dramatically enhanced the mechanical properties of PVA/starch systems. The tensile strength, Young’s modulus as well as flexibility increased significantly compared with unfilled ones. The thermal stability was improved by MMT addition. The MMT nanolayers could act as the heat and mass transport barriers and retard the thermal decomposition of the composites. These nanocomposite films with improved performance would find wide applications in green packaging materials and biodegradable plastics.
Acknowledgement This work was supported by the National Natural Science Foundation of China (51373004), Beijing Top Young Innovative Talents Program (2014000026833ZK13)
15
and Open Funding of Key Laboratory of Recycling and Eco-treatment of Waste Biomass of Zhejiang Province (2016REWB18).
16
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MMT
25% 20% 15% 10% 5% 0%
4000
3500
3000
1500
1000
500
-1
Wavenumbers (cm )
Figure 1. FTIR spectra of PVA/starch/MMT nanocomposites.
18
25% 20% 15% 10% 5% 0% MMT
2
4
6
2 /
8
10
o
Figure 2. XRD patterns for PVA/starch/MMT nanocomposites.
Figure 3. TEM images for nanocomposites with 5 % (a) and 10 % (b) MMT.
19
25% 20% 15% 10% 5% 0%
0
50
100
150
Temperature (oC)
Figure 4. DSC curves of PVA/starch/MMT nanocomposites.
16
0% 5% 10% 15% 20% 25%
14
Torque (Nm)
12 10 8 6 4 2 0 0
100
200
300
400
500
600
Time (s)
Figure 5. Torque rheology of PVA/starch/MMT nanocomposites.
20
9
a 8
Water uptake (%)
7 6 5
0% 5% 10% 15% 20% 25%
4 3 2 1 0 0
20
40
60
80
100
120
Time (h)
9.0
b 1.0
0.6
7.5
7.0
0.4
6.5
0.2 0
5
10
15
20
10 2
0.8 8.0
D/ cm /s× 10
Water uptake /%
8.5
25
MMT content /%
Figure 6. Water uptake of PVA/starch/MMT composite films at 43% RH.
21
18 16
a
Stress（ MPa）
15%
20%
10%
25%
14
5% 12
0% 10 8 6 4 2 0 0
200
400
600
800
1000
1200
10000
b
1200
8000
1000 800
6000
600 400
4000
200 16
60
15
50 40
14
30
13
20
12
10
11
0 0
5
10
15
20
25
MMT content（ wt%)
Energy to fracture (J/m3) Young's modules (MPa)
Tensile strength (MPa) Elongation at break (%)
Strain（ %）
Figure 7. Mechanical properties of PVA/starch/MMT nanocomposites under 43% RH.
a
0% 5% 10% 15% 20% 25%
0% 5% 10% 15% 20% 25%
b
DTG
TG (%)
100
10 0
100
200
300
400
500
600
700
100
200
300
400
500
600
700
o
o
Temperature ( C)
Temperature ( C)
Figure 8. TGA and DTG curves of PVA/starch/MMT nanocomposite with different content of MMT.
22
Table 1. Glass transition temperatures of the composites with different MMT content. MMT content / %
0%
5%
10%
15%
20%
25%
Tg（oC）
102.6
102.9
103.2
105.9
105.6
105.7
Table 2. Mechanical data of PVA/starch/MMT nanocomposites. MMT content/%
Tensile strength/MPa
Elongation at Young’s break/% modulus/MPa
Energy to fracture/ J/m3
0
11.08
1111.02
9.65
7955
5
13.30
1007.15
23.76
9488
10
14.72
868.15
30.58
9681
15
16.17
666.62
39.17
8413
20
14.47
458.03
47.67
5427
25
13.81
260.57
59.46
3043
23
S0141-8130(16)32785-4 http://dx.doi.org/doi:10.1016/j.ijbiomac.2017.03.111 BIOMAC 7277
To appear in:
International Journal of Biological Macromolecules
Received date: Revised date: Accepted date:
6-12-2016 8-3-2017 21-3-2017
Please cite this article as: Huafeng Tian, Kai Wang, Di Liu, Jiaan Yan, Aimin Xiang, A.Varada Rajulu, Enhanced mechanical and thermal properties of poly (vinyl alcohol)/corn starch blends by nanoclay intercalation, International Journal of Biological Macromoleculeshttp://dx.doi.org/10.1016/j.ijbiomac.2017.03.111 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced mechanical and thermal properties of poly (vinyl alcohol)/ corn starch blends by nanoclay intercalation Huafeng Tian1,2, Kai Wang1, Di Liu1, Jiaan Yan1, Aimin Xiang1 *, A.Varada Rajulu3 1 School of Material and Mechanical Engineering, Beijing Technology and Business University, Beijing 100048, China 2 Key Laboratory of Recycling and Eco-treatment of Waste Biomass of Zhejiang Province, China 3 Centre for Composite Materials, International Research Centre, Kalasalingam University, Anand Nagar, Krishnankoil-626 126, India. *
To whom correspondence should be addressed. Phone: +86-10-68988056.
E-mail: [email protected] (A. Xiang)
Highlights
PVA/starch/MMT nanocomposites were prepared by melt processing.
Highly exfoliated MMT layers were obtained containing MMT lower than 10wt%.
Intercalated structure was predominant with more than 10 wt% MMT.
Water resistant properties were improved with the incorporation of MMT.
Significant improvement in strength and flexibility were observed.
The thermal stability was improved by MMT addition.
1
Abstract: Poly (vinyl alcohol) (PVA) /corn starch blend films with enhanced properties were fabricated by melt processing and montmorillonite (MMT) reinforcing. It was revealed that strong hydrogen bonding occurred between the abundant –OH groups of the matrix and polar Si-O-Si and -OH groups of MMT. The highly exfoliated MMT nanolayers were randomly dispersed in the matrix containing MMT lower than 10wt%, whereas the intercalated structure was predominant with MMT content higher than 10 wt%. With the increase of MMT, the glass transition temperature as well as equilibrium torque increased. The water sorption decreased and water resistant properties were improved with the incorporation of MMT due to the restricted swelling of the matrix by MMT nanolayers. Significant improvement in strength and flexibility were observed due to the fine dispersion of the MMT layers and the strong interaction between MMT and the matrix. The thermal stability was also improved. The MMT nanolayers could act as the heat and mass transport barriers and retard the thermal decomposition of the composites. Keywords: starch; poly (vinyl alcohol); montmorillonite; nanocomposites; melt processing; mechanical properties
2
1. Introduction With the growing concerns on the environmental problems and resource limitation induced by the wide usage of petroleum based synthetic polymers, the development of biodegradable materials for substituting nonbiodegradable materials have attracted great interests [1-5]. Starch is one of the natural occurring biopolymers with inherent biodegradability, abundant availability, and low cost [6, 7]. However, the entire starch plastics with low molecular plasticizers possess poor mechanical properties and water resistance properties, which limit their wide applications [8]. Compared with chemical modifications such as grafting, esterification or etherization, the physical modification of starch would be a simple and green process without environmental pollution. Therefore, the blending of starch with other biodegradable synthetic polymers, such as polycaprolactone [9, 10], poly(lactic acid) [11, 12], polyurethane [13, 14], poly (vinyl alcohol) (PVA) [15-18], poly (butylene succinate) [19], have been carried out and reported in the literature. Among them, PVA/starch blends would have more potential use as disposable packaging articles for the abundance and low cost of the starting starch material as well as the eco-friendly and higher barrier character of PVA [20, 21]. Also to further improve the mechanical strength of the PVA/starch blends, nanoparticles such as silica [22], carbon nanotubes[23], graphene [24] were introduced into the blends to make nanocomposites. Natural or synthetic mineral layered silicates have gained great popularity as an effective reinforcement for nanocomposite systems [25-27]. The incorporation of layered
silicate
clays,
such
as
montmorillonite 3
(MMT,
general
formula
Mx(Al4−xMgx)Si8O20(OH)4 [28]), could dramatically improve polymer-based composite performance because of high aspect ratio of silicate nanolayers, high surface area, and high modulus [29-31]. With uniform dispersion of clays in polymer matrices with an intercalated or exfoliated structure, enhanced mechanical and barrier properties would be achieved [32-34]. In our previous works, PVA/starch blend films were prepared by melt processing instead of widely used solution casting method, which is energy consuming and inefficient [35]. The urea and formamide plasticizers dramatically improved the flexibility of the materials, however sacrificed the strength. And low content of MMT could form exfoliated structures and improve the mechanical properties [36]. In this study, the PVA/starch blends were further reinforced with higher amount of MMT nanoclays by melt intercalating, which is the most popular method to disperse clays into polymer matrix for its environmental friendliness with the absence of large amount of solvents. The influence of the MMT content as well as the different structures of both exfoliated and intercalated on the structure and properties of the resulting composites was investigated in detail. The physical properties were discussed depending on the MMT structures. It is aimed to provide an effective way to prepare the green materials with both good strength and flexibility.
2. Experimental 2.1. Materials
4
PVA (117) with the viscosity of 25.0-31.0×10-3 Pa·S, molecular weight of 83000 and alcoholysis degree of 98.0-99.0 mol/% was purchased from Kuraray Co., Ltd. (Japan). Corn starch was purchased from Quanfeng Co., Ltd. (Beijing). Formamide and urea were supplied by Sinopharm Chemical Reagent Co., Ltd (Beijing, China) and used as received. Na+-montmorillonite (MMT, GTP) was supplied by Nanocor Co., Ltd (USA). 2.2. Preparation of poly (vinyl alcohol)/starch/MMT nanocomposites Starch, PVA, water, mixed plasticizers (formamide and urea, weight ratio 2:1) and desired amount of MMT were mixed in a high speed mixer (GH-10, Zedao Mechanical Technology, Beijing, China) at room temperature for 3 min. The ratio of starch and PVA was fixed at 1:1. The content of the mixed plasticizers and water was fixed at 30 phr and 50 phr based on the solid materials (starch and PVA), respectively. Then the mixtures were charged into a HAAKE torque rheometer (TA Instruments, USA) and further mixed at 105 oC for 10min with a stirrer speed of 80 rpm. The mixtures were compression molded on plate vulcanizing machine (XLB-400, Huaqing Industry Group, Qingdao, China) to obtain the nanocomposite films with thickness of about 0.50.6 mm. The molding temperature, pressure and time maintained were 110 oC, 20 MPa and 10 min, respectively. Before testing, all samples were vacuum-dried at 80 oC for 2 days and kept at a relative humidity (RH) of 0% (conditioned by phosphorus pentoxide P2O5) for 2 weeks. 2.3. Characterization 5
The Fourier-transform infrared spectra (FTIR) of the samples were recorded on a Nicolet 5700 FTIR spectrometer (Thermo Electron Corporation, Waltham, MA, USA). Samples were prepared by mixing the fine powder with potassium bromide (KBr) and pressing. The spectra were obtained at a resolution of 4 cm-1 in the range 4000 cm-1 to 400 cm-1. X-ray diffractograms (XRD) were recorded using a DIP 2030 apparatus (MAC Science, Yokohama, Japan) using Cu–Kα radiation (0.154 nm) at 2θ values ranging from 5o to 10o at a scanning rate of 2o/min. The testing conditions were reflectance mode and 40kV, 30mA. The morphology of the nanocomposites were visualized by a transmission electron microscope (TEM F30, FEI TECNAI, Netherlands) at an accelerating voltage of 200 kV. Ultrathin sections of the composits were prepared using a Leica Ultracut UCT with EMFCS cryo-attachment at -120 °C. The ultrathin films of the samples were directly placed on the copper grids and observed after sputtering carbon. Differential scanning calorimetry (DSC) was carried out using a DSC Q100 calorimeter (TA Instruments, USA) under a nitrogen atmosphere. An empty pan was used as the reference. Samples were placed in an aluminum pan and were heated from 0 oC to 200 oC at a heating rate of 20 oC/min and held for 3 min. The initial sample weight was in the range of 5mg-10mg. The kinetics of water absorption at room temperature (25 C) was determined for all films. The specimens with dimensions around 10×10×0.2 mm3 were thin enough so 6
that the diffusion was supposed to be unidirectional. The samples were first vacuumdried overnight at 100 C. After being weighed using a four-digit balance, the sample was conditioned in a desiccator containing K2CO3 saturated solution to ensure a RH of 43%. The sample was removed at specific intervals and re-weighed. The water uptake (WU) was calculated by: WU (%)
mt m0 100 m0
(1)
where m0 and mt are the weights of the samples before and after a time t of conditioning, respectively. The mass of moisture absorbed at a short time t is expressed as [37]: 1/ 2
mt - m0 2 D m L
t1/ 2
(2)
where m∞ is the weight of the sample at equilibrium, 2L is the thickness of the film, and D is the diffusion coefficient. The plots of ((mt-m0)/m∞)2 as a function of (4t/(πL2)) were drawn for all samples with (mt-m0)/m∞ ≤0.5. The diffusion coefficients were calculated from the slope of the plots. For tensile test, specimens, with the shape of a dumb-bell according to the ASTM D 638-14 standard, were prepared. Tensile tests was performed on a universal testing machine (CMT 6104, SKYan, Shenzhen, China) equipped with 100 N load cell and operated at a cross-head speed of 50 mm/min. Five samples of each system were tested at room temperature and the average value is reported. Before testing, all specimens were conditioned at room temperature in a desiccator of 43% RH (conditioned by K2CO3 saturated solution) for two weeks. 7
Thermo-gravimetric analysis (TGA) was performed on a TGA Q5000 thermogravimetric analyzer (TA Instruments, USA) from 50 oC to 800 oC at a heating rate of 10 oC/min under nitrogen atmosphere. The sample weight was about 5mg - 10mg.
3. Results and discussion 3.1 FTIR The FTIR spectra of PVA/starch/MMT nanocomposites are shown in Figure 1. The peak at around 3300 cm-1 was assigned to the complex vibrational stretches associated with free, inter- and intramolecular bound hydroxyl groups. With the increase of MMT, the wavenumbers red shifted, suggesting the possible formation of hydrogen bonds. In the fingerprint region of the spectrum of pristine starch, three characteristic peaks appeared between 1000 and 1160 cm-1, which were attributed to C–O bond stretching. The band at 2930 cm-1 was characteristic of C–H stretches associated with the hydrogen atoms of methine ring [38]. Absorption peaks at about 1038 cm-1 and 525cm-1 were ascribed to the stretching vibration and bending vibration of Si-O, respectively. The Si-O-Si and -OH groups on the MMT gallery surface could act as hydrogen bonding sites for the guest molecules, and strong hydrogen bonding could occur between the abundant –OH groups and polar groups of MMT, which would be beneficial to the intercalating process and mechanical properties discussed in Section 3.7. 3.2 XRD
8
The XRD patterns of PVA/starch/MMT nanocomposites are shown in Figure 2. A sharp peak at 2θ = 9.1o appeared for pristine MMT, ascribing to the basal spacing of d001 = 0.96 nm calculated using Bragg equation. No diffraction peaks were observed in 2θ =2-10o for the unfilled PVA/starch materials, indicating no crystalline structure existed in this range. For nanocomposites with 5% MMT, the diffraction peak of (001) spacing disappeared, indicating a highly exfoliated structure. MMT was exfoliated into single nanolayers and dispersed in the matrix homogeneously and disorderly. The diffraction peaks reappeared and moved to 2θ = 4.7o and basal spacing was calculated to be about 1.87 nm with MMT content more than 10%. Also the peak intensity increased with the increase of MMT content. This suggested that the starch and PVA macromolecules had entered into the galleries of MMT and enlarged the spacing of MMT layers, which was the characteristic of intercalating structures. 3.3 TEM TEM images of the nanocomposites are shown in Figure 3. With 5% MMT, the MMT layers were well dispersed in the matrix as randomized nanolayers, indicating the exfoliating structure. While with 10% incorporated, MMT layers still retained their orientation to some degree indicating the intercalation structure. The results from TEM were in well agreement with that from XRD. On the basis of the evidence from XRD and TEM, the PVA/starch/MMT nanocomposites with an intercalated or exfoliated structure have been successfully prepared via a melt intercalation process without any special aid. Moreover, the results obviously suggest a high affinity between matrix and MMT fillers. 9
3.4 DSC Figure 4 shows the DSC curves of PVA/starch/MMT nanocomposites. Only one glass transition temperature (Tg) at about 102 oC was observed for PVA/starch blends, indicating fine compatibility between starch and PVA. It has been proven in our previous study that strong hydrogen bonding occurred between starch and PVA for the abundant of hydroxyl groups on the macromolecular chains [35]. Table 1 shows the influence of MMT content on the glass transition temperatures of the nanocomposites. With the increase of MMT, the glass temperature increased. The fine dispersion of MMT layers effectively restricted the segmental mobility of the starch and PVA macromolecules on the interface of the clay layers because of their high aspect ratio and reactive surface [39]. It is worth noting that with MMT content higher than 15%, the glass temperature did not increase and remained at about 105 oC. As indicated in the XRD section, the intercalating structures with high content of MMT would have a lower dispersing effect compared to exfoliating structure, which would limit the restricting effect of the matrix molecular motion. 3.5 Rheology The rheology was obtained by the HAAKE torque rheometer during the preparing process and Figure 5 shows the torque rheology of PVA/starch/MMT nanocomposites. The torque increased rapidly to a maximum value after feeding the starting materials and then decreased to obtain an equilibrium value at about 100s. Thus the starting materials fed in the mixer were changed to be thermoplastic. The plasticizing time (the mixing time by which the torque leveled off) increased with the increase of MMT, 10
indicating more time was needed to make the mixtures to be thermoplastic. Also the equilibrium torque increased with the increase of MMT. During the processing, PVA and starch macromolecules could enter into the galleries of MMT nanolayers and form exfoliating or intercalating structure. The strong interactions between the filler and matrix would inhibit the moving ability of macromolecular chains and decrease the melt flowability, leading to the increased torque and melt viscosity. The increased torque would also be beneficial to destroy the original structure of MMT and disperse them into the matrix. 3.6 Water uptake Figure 6a shows the water uptake kinetics of PVA/starch/MMT nanocomposites conditioned at 43% RH. The water uptake at equilibrium and water diffusion coefficient are shown in Figure 6b. The water absorption of the composite films increased quickly with time less than 50 hours and reached a plateau after 50h. With the increase of MMT, the water uptake at equilibrium decreased. The strong interaction between MMT nanolayers and PVA/starch macromolecules could restrict the swelling of the matrix, leading to the decreased water uptake. The water diffusion coefficient decreased quickly with increasing MMT and a turning point appeared at 10% MMT content. With MMT content higher than 10%, the coefficient decreased slightly. In fact, preparing polymer/clay nanocomposites with nano-scaled clay flakes dispersed in a pristine polymer membrane is an effective technique to improve the barrier properties of polymers [40]. According to the 11
tortuosity theory, clay flakes dispersed in polymers could act as impermeable domains, which would detour the diffusing molecules around the clay particles, resulting in a significant decrease in the diffusion coefficient and the associated permeability [41]. Therefore, the planar MMT layer could act as the transport barrier for water molecules, and the free path for the water molecules to pass through the nanocomposite would decrease, leading to the decrease of diffusion coefficient. With MMT content lower than 10%, MMT was exfoliated into single nanolayers and dispersed in the matrix disorderly causing a dramatic barrier effect and resulted in the quick decrease of the coefficient. With MMT content higher than 10%, the intercalated structure formed, with MMT maintained the original short-range order structure. The dispersive effect in this case would be not as well as in the exfoliated structure, therefore resulted in the slight decrease of the water diffusion coefficient. 3.7 Mechanical properties Figure 7 shows the mechanical properties of PVA/starch/MMT nanocomposites under 43% RH. The data were listed in Table 2. For unfilled PVA/starch films, with the increase of stain, the stress increased rapidly and linearly, and the yield point exhibited at about 100% strain. After yielding, the stress increased slowly with strain until fracture. As investigated in our previous studies [35], the urea and formamide plasticizers could dramatically improve the toughness of the materials and improve the flexibility, however with the sacrifice of the strength. With the increase of MMT, the yield strength increased significantly. The tensile strength increased first, and reached the maximum value at 15% MMT. With further increase of MMT, the tensile 12
strength decreased. The fracture energy, which was calculated from the area below the stress-strain curves and stand for the flexibility of the materials, exhibited similar trends with tensile strength, while reached the maximum value at 10% MMT. With MMT content lower than 15%, MMT could disperse into the matrix homogeneously with exfoliating and intercalating structure. The MMT tactoids could disperse in the matrix homogeneously. When imposed with external force, the stress could be transferred to the filler from the matrix through the interface effectively, which could improve the mechanical properties dramatically. The interaction between the filler and matrix could also restrict the macromolecular motions of the matrix, leading to the enhanced strength of the composites and decrease of the elongation at break. With MMT content more than 15%, phase separation phenomenon might occur and result in the decrease of the mechanical strength and flexibility. The Young’s modulus increased and elongation at break decreased with the increase of MMT. The Young’s modulus increased from 9.6 MPa of unfilled PVA/starch systems to 59.5 MPa of composites with 25% MMT. As a kind of rigid filler, MMT could disperse into the matrix and improve the modulus of the resulting composites significantly. 3.8 Thermal stability The TGA and DTG curves of PVA/starch/MMT nanocomposites are shown in Figure 8. All the samples exhibited fine thermal stability below 180 oC. The initial decomposition below 250 oC mainly involved the vaporization of volatiles including 13
urea and formamide. PVA exhibited a two-step degradation at about 300 oC and 450 o
C, which were attributed to the acetate group elimination at lower temperatures
followed by a breakdown of polymer backbone at higher temperatures [42]. The degradation of starch macromolecules mainly occurs at 290–370 oC. The degradation peaks of acetate group elimination of PVA and degradation of starch exhibited as one main peak with a shoulder peak on DTG curves for PVA/starch systems. With the increase of MMT, the thermal stability of the nanocomposites increased dramatically for the increased weight residue after thermal decomposition. Also the peak intensity value on the DTG curves decreased, and the area under the degradation peaks descended, especially during the acetate group elimination of PVA and degradation of starch, indicating the decreased weight loss rate and decreased weight loss amount during this temperature range. Generally, inorganic clay acts as a thermal barrier for the heat transfer into polymer chains, and prevents the heat to propagate quickly and limit the further degradation [43]. Also the inorganic fillers act as mass transport barrier to the volatile products generated during thermal decomposition, leading to the retardance of the thermal degradation action. In addition, hydrogen bonding interactions between MMT and PVA/starch matrix could decrease the quality of free –OH groups and stabilize the matrix. All of these could lead to the improved thermal stability of the nanocomposites.
Conclusions The PVA/starch/MMT nanocomposites were prepared by melt processing successfully in this work. The Si-O-Si and -OH groups on the MMT gallery surface 14
could act as hydrogen bonding sites for the guest molecules, and strong hydrogen bonding could occur between the abundant –OH groups of the matrix and polar groups of MMT. Exfoliated structure was formed for MMT content lower than 10%, while for MMT content higher than 10%, the intercalating structures were dominated. With the increase of MMT, the glass temperature as well as equilibrium torque increased, due to the restricted segmental motion of the starch and PVA macromolecules by MMT nanolayers. With the increase of MMT, the water uptake at equilibrium and water diffusion coefficient decreased. The strong interaction between MMT nanolayers and PVA/starch macromolecules could restrict the swelling of the matrix, leading to the decreased water uptake. Also the planar MMT layers could act as the transport barrier for water molecules, leading to the decrease of diffusion coefficient. Incorporation of MMT dramatically enhanced the mechanical properties of PVA/starch systems. The tensile strength, Young’s modulus as well as flexibility increased significantly compared with unfilled ones. The thermal stability was improved by MMT addition. The MMT nanolayers could act as the heat and mass transport barriers and retard the thermal decomposition of the composites. These nanocomposite films with improved performance would find wide applications in green packaging materials and biodegradable plastics.
Acknowledgement This work was supported by the National Natural Science Foundation of China (51373004), Beijing Top Young Innovative Talents Program (2014000026833ZK13)
15
and Open Funding of Key Laboratory of Recycling and Eco-treatment of Waste Biomass of Zhejiang Province (2016REWB18).
16
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MMT
25% 20% 15% 10% 5% 0%
4000
3500
3000
1500
1000
500
-1
Wavenumbers (cm )
Figure 1. FTIR spectra of PVA/starch/MMT nanocomposites.
18
25% 20% 15% 10% 5% 0% MMT
2
4
6
2 /
8
10
o
Figure 2. XRD patterns for PVA/starch/MMT nanocomposites.
Figure 3. TEM images for nanocomposites with 5 % (a) and 10 % (b) MMT.
19
25% 20% 15% 10% 5% 0%
0
50
100
150
Temperature (oC)
Figure 4. DSC curves of PVA/starch/MMT nanocomposites.
16
0% 5% 10% 15% 20% 25%
14
Torque (Nm)
12 10 8 6 4 2 0 0
100
200
300
400
500
600
Time (s)
Figure 5. Torque rheology of PVA/starch/MMT nanocomposites.
20
9
a 8
Water uptake (%)
7 6 5
0% 5% 10% 15% 20% 25%
4 3 2 1 0 0
20
40
60
80
100
120
Time (h)
9.0
b 1.0
0.6
7.5
7.0
0.4
6.5
0.2 0
5
10
15
20
10 2
0.8 8.0
D/ cm /s× 10
Water uptake /%
8.5
25
MMT content /%
Figure 6. Water uptake of PVA/starch/MMT composite films at 43% RH.
21
18 16
a
Stress（ MPa）
15%
20%
10%
25%
14
5% 12
0% 10 8 6 4 2 0 0
200
400
600
800
1000
1200
10000
b
1200
8000
1000 800
6000
600 400
4000
200 16
60
15
50 40
14
30
13
20
12
10
11
0 0
5
10
15
20
25
MMT content（ wt%)
Energy to fracture (J/m3) Young's modules (MPa)
Tensile strength (MPa) Elongation at break (%)
Strain（ %）
Figure 7. Mechanical properties of PVA/starch/MMT nanocomposites under 43% RH.
a
0% 5% 10% 15% 20% 25%
0% 5% 10% 15% 20% 25%
b
DTG
TG (%)
100
10 0
100
200
300
400
500
600
700
100
200
300
400
500
600
700
o
o
Temperature ( C)
Temperature ( C)
Figure 8. TGA and DTG curves of PVA/starch/MMT nanocomposite with different content of MMT.
22
Table 1. Glass transition temperatures of the composites with different MMT content. MMT content / %
0%
5%
10%
15%
20%
25%
Tg（oC）
102.6
102.9
103.2
105.9
105.6
105.7
Table 2. Mechanical data of PVA/starch/MMT nanocomposites. MMT content/%
Tensile strength/MPa
Elongation at Young’s break/% modulus/MPa
Energy to fracture/ J/m3
0
11.08
1111.02
9.65
7955
5
13.30
1007.15
23.76
9488
10
14.72
868.15
30.58
9681
15
16.17
666.62
39.17
8413
20
14.47
458.03
47.67
5427
25
13.81
260.57
59.46
3043
23