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organo-vermiculite nanocomposites via direct melt intercalation
EUROPEAN POLYMER JOURNAL
European Polymer Journal 41 (2005) 881–888
www.elsevier.com/locate/europolj
Preparation of poly(propylene carbonate)/organo-vermiculite nanocomposites via direct melt intercalation J. Xu a, R.K.Y. Li b, Y. Xu a, L. Li c, Y.Z. Meng
a,c,*
a
b
Institute of Energy and Environment Materials, School of Physics and Engineering, Sun Yat-Sen University, 135 Xingang West, Guangzhou 510275, PR China Department of Physics and Materials Sciences, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong c School of Mechanical and Production Engineering, Nanyang Technology University, North Sprine 639798, Singapore Received 20 December 2003; received in revised form 22 October 2004; accepted 27 October 2004 Available online 12 January 2005
Abstract Intercalated nanocomposites comprised of poly(propylene carbonate) (PPC) and organo-vermiculite (OVMT) was first prepared via direct melt compounding of the alkali-vermiculite intercalated host with PPC in a twin rotary mixer. The dispersion and morphologies of OVMT within PPC were investigated by X-ray diffraction and transmission electron microscopic techniques. The results revealed the formation of intercalated-exfoliated vermiculite sheets in the PPC matrix. Because of the thermally sensitive nature of PPC, thermal degradation occurred during the melt compounding. The degradation led to a deterioration of the mechanical properties of the nanocomposites. Tensile test showed that the yield strength and modulus of the nanocomposites decrease with increasing vermiculite content. The degradation mechanism was discussed according to the results of GPC and TGA measurements. 2004 Elsevier Ltd. All rights reserved. Keywords: Nanocomposites; Melt intercalation; Vermiculite; Polycarbonate
1. Introduction Recently, polymer-layered silicate nanocomposites have received special attention because of the numerous advantages in comparison with traditional polymer macro composites. Macro polymer composites usually involve a high level of inorganic filler loadings for * Corresponding author. Address: Institute of Energy and Environment Materials, School of Physics and Engineering, Sun Yat-Sen University, 135 Xingang West, Guangzhou 510275, PR China. Tel.: +86 208 523 1343; fax: +86 208 411 4113. E-mail addresses: [email protected], stdpmeng@zsu. edu.cn (Y.Z. Meng).
imparting desired mechanical properties, such as CaCO3, carbon fiber, and glass fiber [1–6]. However, high loading levels of inorganic fillers may lead to deteriorated properties, such as increase in density and loss of tenacity due to the interfacial incompatibility between organic polymer and inorganic filler. Moreover, the processability becomes worse, as the torque level of the mixing equipment increases and dispersion of inorganic filler with the increase in filler content becomes poorer. On the other hand, nanocomposites may show improved mechanical properties, decreased thermal expansion coefficient [7–10]. Intercalation methods for nanocomposites includes in situ polymerization, melt polymer intercalation, exfoliation–adsorption, template synthesis, and direct
0014-3057/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.10.033
882
J. Xu et al. / European Polymer Journal 41 (2005) 881–888
solution intercalation [11–16]. Many commercially important non-polar polymers such as polyolefins [17], however, the in situ polymerization or solution techniques cannot be used for preparation of nanocomposites. In such cases, the direct melt intercalation method provides a convenient technique for preparation of the composites [18–22]. Melt intercalation involves mixing the layered silicates with the polymer matrix above its melting point. Recently, Vaia et al. suggested that polymer–clay nanocomposites could be fabricated via a direct route in the polymer melt in which the polymer chains are diffused into the space between the silicate layers or galleries [23–25]. Conventional polymer processing techniques such as extrusions can reduce the time for nanocomposites fabrication through its ability to break up the silicate layers. Vermiculite, like the well-known montmorillonite, belongs to the general family of 2:1 layered silicates. This clay contains either Al3+ or Mg2+ and Fe2+ as normal octahedral ions, and a tetrahedral sheet in which A3+ occurs as a substituted ion in place of some of the Si4+. Compared to montmorillonite, chlorite, kaolinite and hydrous mica, vermiculite is abundant and has a larger cation exchange content (CEC) as listed in Table 1 (100–150 cmol/kg). It is generally used as packaging material for anti-shocking. In previous works [26–29], we have successfully synthesized alternating poly(propylene carbonate) (PPC) from carbon dioxide and propylene oxide. The synthesized PPC exhibited very high molecular weight (generally >5 · 104 Da), however, the glass transition temperature (Tg) remained relatively lower due to its carbonate linkage in the backbone. The practical application of the PPC has been limited by the thermal stabilities [30]. In order to improve the stability, many organic and inorganic fillers have been compounded with the PPC to fabricate composites [31,32]. Accordingly, the mechanical properties and thermal stabilities were improved greatly. In this paper, we report the direct melt intercalation of PPC and organo-vermiculite. The focus is placed on further enhanc-
ing the mechanical properties of PPC to meet the need of practical applications. The morphology and the mechanism of formation of the resulting PPC/OVMT nanocomposites were investigated in this work.
2. Experimental 2.1. Materials Vermiculite (VMT) was purchased from Aldrich with grade number 3. It was heat treated at 300 C for 12 h before use. Poly(propylene carbonate) (PPC) was synthesized in our lab. The PPC has a very high glass transition temperature (Tg) (>40 C) and decomposition temperature (>250 C). The number average molar mass and molar mass polydispersity index of PPC are 50,000 Da and 4.0, respectively. Cetyltrimethyl ammonium bromide purchased from Shanghai, China, regent purity. 2.2. Preparation of organo-vermiculite The inorganic cations in the vermiculite crystals were first ion exchanged with sodium ions as follows: vermiculite crystals (250 mesh, 10 g) were refluxed in about 100 mL of aqueous solution containing 1.2 g NaCl for 48 h. The reaction temperature was controlled in the range of 50–60 C, and the slurry was washed with distilled water until no Cl was detected, which was performed using 0.1 mol/L AgNO3. The Na-exchanged vermiculite was then added to a 500 mL plastic flask, containing 12 g cetyltrimethyl ammonium bromide in 200 mL of deionized water. The mixture of vermiculite and alkyl ammonium salt was stirred for 3 days at 70 C. The resulting organo-vermiculite (OVMT) was washed with distilled water until no precipitate was observed upon titrating the filtrate with 0.1 mol/L AgNO3. The final OVMT in the form of fine powder was kept in a dryer for further use.
Table 1 Summary of clay mineral propertiesa Secondary mineral
Type
Interlayer condition/bonding
CEC [cmol/kg]
Swelling potential
Specific surface area [m2/g]
Basal spacing [nm]
Vermiculite Kaolinite
2:1 (expanding) 1:1 (non-expanding)
100–150 3–15
High Almost none
500–700 5–20
1.0–1.5+ 0.72
Montmorillonite Hydrous mica Chlorite
2:1 (expanding) 2:1 (non-expanding) 2:1:1 (non-expanding)
Weak bonding, great expansion Lack of interlayer surface, strong bonding Very weak bonding, great expansion Partial loss of K, strong bonding Moderate to strong bonding, non-expanding
80–150 10–40 10–40
High Low None
700–800 50–200 –
0.98–1.8+ 1.0 1.4
a
The data cited from the work of Sabine Grunwald, University of Florida, USA. See http://grunwald.ifas.ufl.edu.
J. Xu et al. / European Polymer Journal 41 (2005) 881–888
2.3. Nanocomposite preparation The direct melt intercalation for the preparation of polymer/clay nanocomposites has been extensively studied. PPC and organo-vermiculite was melt blended in a twin rotary mixer (Haake Co., German). Prior to mixing, the PPC and organo-vermiculite must be dried for >8 h to remove residual moisture. The compounding was carried out at 170 C with rotary speeds at 40 rpm and 80 rpm, respectively. The PPC was introduced into the chamber of the mixer as quickly as possible, followed by adding OVMT. The residence time inside the mixer was controlled ranging from 5 min to 10 min under the processing conditions. Upon completion of mixing, the nanocomposite samples were removed from the mixing chamber, and cooled for future use. 2.4. Instrumentation The above obtained PPC/OVMT nanocomposite pellets were hydro compressed into films with a thickness of 1 mm at 160 C. The pellets were dried for >8 h at 80 C before use. Wide-angle X-ray diffraction (XRD) measurement was performed using a Rigaku D/max-1200X diffractometer with 2h scan range of 1.5–20 at room temperature, at a scanning speed and step size of 5/min and 0.05, respectively. More direct evidence for the nanoscale dispersion of VMT within PPC matrix was provided by transmission electron microscope (TEM) observations. Ultra thin films of nanocomposites were sliced up using LKB-5 ultra microtome. The observations were carried out on the copper web after adhering the nanocomposite slices onto it. The TEM observation was performed using a JEOL100CX-II model operated at 100 kV. The samples for mechanical measurements were prepared according to the standard of ASTM D-638. Tensile tests were conducted using an Instron 1122 testing machine. Five specimens of each composition were tested, and the average values were reported. YoungÕs modulus yield strength and strain at yield point were carefully determined by using an extensometer at a crosshead speed of 5 mm/min. All Gel Permeation Chromatography (GPC) analyses were performed on a Waters 515 HPLC using three phenogel 7.8 · 300 mm columns arranged in series. Spectroscopic grade tetrahydrofuran was used as a fluent with a flow rate of 1 mL/min. Refractive index detector was used and polystyrene was selected as external calibration standard. Thermal properties of the nanocomposites were measured on a Thermogravimetric/Differential Thermal Analyzer (TG/DTA) instrument. The used temperature profile ranged from 30 to 600 C and experiments were
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performed at a scanning rate of 20 C/min. under the protection of N2 gas with a flow rate of 90 mL/min. Fourier transform infrared (FTIR) spectra were recorded using an Analect RFX-65A FTIR spectrometer.
3. Results and discussion In order to understand the physical and chemical process of the intercalation, simple theoretical analysis is done as below. VMT spacings between the closely packed sheets are on the order of 1 nm. Thus there is a large entropic barrier associated with the molten PPCs diffusing into the gap and hence, intercalation into the layered clay is hindered. One method of lowering this entropic barrier is to anchor short, surfactant-like chains in, or onto, the surface of the VMT sheets, thereby forming a socalled organically modified VMT (OVMT). For the appropriate choice of parameters, the energetic gains from the polymer-surfactant interactions can compensate for the entropic losses. Under these conditions, the polymers may penetrate into the galleries, separate the clay sheets, and finally disperse the sheets within the polymer melt. More recently, Vaia and Giannelis developed a lattice model for the interactions between polymers and organically modified clays [33]. DF ¼ F ðhÞ F ðh0 Þ ¼ DE T DS where DF is the total change in Helmholtz free energy; E is the internal energy change associated with the establishment of now inter molecular interactions; DS is an ideal combinatorial entropy change associated with configurational changes of the various constituents, and h0, h means the gallery height of unintercalated interlayer and the gallery height of polymer-intercalated interlayer, respectively. DF < 0 indicates that the layer separation is favorable, while DF > 0 implies that the initial unintercalated state is favorable. The major factors contributing to the free energy change may be identified as the relative confinement of the polymer establishment of new intermolecular interactions between PPC and OVMT during intercalation process (Fig. 1). 3.1. Delamination of VMT Fig. 2 shows XRD patterns for OVMT and PPC/ VMT nanocomposites. The OVMT, trace b of Fig. 2, gave three distinct peaks at 2h = 2.02, 4.58, and 7.36, corresponding to the d-spacing of 4.37, 1.93, 1.20 nm, respectively. It should be noted from the XRD pattern of OVMT that the d-spacing of 4.37 nm was significantly larger than that of the unmodified Na+ vermiculite, which had a d-spacing of 1.2 nm as
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cetyltrimethyl ammonium bromide 4.37nm
1.1nm PPC
Fig. 1. Schematic representation of the intercalation process of OVMT within PPC matrix.
served by FTIR measurements (Fig. 3). It can be seen that the C–H stretching vibration of alkylammonium chain appeared at 2929 cm1 and 2852 cm1, indicating the increase in hydrophobic property of resulting OVMT. From trace c of Fig. 2, the distinct diffraction peaks of OVMT were not observed and the intensity of the 0 0 1 basal peak decreased from 6500 to 200. This indicated that most of the OVMT sheets were dispersed uniformly in the polymer matrix in nanoscale.
a
Fig. 2. XRD patterns of (a) pristine VMT, (b) OVMT, (c) PPC/ VMT nanocomposite.
indicated in trace a of Fig. 2. The larger gallery spacing indicated the intercalation of alkyl ammonium in between the silicate layers. Depending on the packing density and chain length, the intercalating agents radiated away from the surface, forming bimolecular tilted arrangement as shown in Fig. 1. As the negative charge originated in the silicate layer, the cationic head groups of the alkylammonium salt preferentially resided on the layer surface, leaving the organic chain tail radiating away from the surface. The intercalation was also ob-
b
4000
3500
3000
2500
2000
1500
1000
Wavenumber cm-1
Fig. 3. FTIR spectra (a) pristine VMT, (b) OVMT.
500
J. Xu et al. / European Polymer Journal 41 (2005) 881–888
3.2. Morphologies of PPC/VMT nanocomposite TEM images of selected PPC/VMT nanocomposites are provided in Fig. 4. It can be seen that the stacks of VMT sheets were dispersed within the PPC matrix irregularly. The arrow a indicates that some of VMT layers were intercalated and dispersed perpendicularly to the sample surface within the PPC matrix, whilst arrow b shows the VMT sheets were parallel to the surface of the nanocomposites slice. The dark lines are the intersections of VMT layers with thickness of 2–5 nm, and length of 200 nm. Intercalated layers of OVMT were dispersed homogeneously in the PPC matrix, however, a small amount of unexfoliated OVMT layers existed as clusters as shown in Fig. 4A. In this sense, the PPC/
885
VMT nanocomposites is considered to be a mixed delaminated/intercalated system. The existence of the obstacle peaks in the XRD patterns of the nanocomposite (trace c of Fig. 2) was believed to be attributed these unexfoliated clusters. With increasing OVMT content, the dispersibility of VMT decreased. From Fig. 4B, it is apparent from the region around arrow c that the unexfoliated content of VMT increased. Presumably, as indicated by arrow d, the edges of OVMT are more accessible for the PPC chains than the center. Because of the shape of OVMT layers, PPC molecular chains must enter the gallery from the agglomerate polymer melt to the primary OVMT particles, and further to the edges of OVMT layers. VMT crystallites on the interior side of the front remained unintercalated. The
Fig. 4. TEM micrographs of (A) PPC/4% VMT; (B) PPC/8% VMT nanocomposites.
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Fig. 5. Solution photos of (a) pure PPC, (b) PPC/OVMT nanocomposite, (c) PPC/VMT blend.
schematic illustration of the intercalation process of OVMT within PPC melt is depicted in Fig. 1. In order to confirm the nanoscale dispersion of OVMT within PPC matrix, the nanocomposites and the solution mixed PPC/VMT specimens were dissolved in chloroform, respectively. It can be seen clearly from Fig. 5 that both pure PPC and the PPC/OVMT nanocomposite solution were transparent as indicated in Fig. 5a and b. However, the solution of simply mixed PPC/VMT specimen exhibited less transparence with some precipitates. This result demonstrated that the OVMT was well dispersed within the PPC matrix on the nanoscale (Fig. 6).
Fig. 6. Thermogravimetric traces of (a) pure PPC; (b) PPC/2% VMT; (c) PPC/4% VMT; (d) PPC/6% VMT; (e) PPC/8% VMT nanocomposites.
VMT nanocomposites, however, stronger interactions between PPC and VMT should be present due to the existence of polar groups (e.g. carbonyl) in PPC. The structure of PPC repeats unit is given as below: O O C O CH2 CH CH3 n
3.3. Properties of PPC/VMT nanocomposites The mechanical properties of PPC/VMT nanocomposites are listed in Table 2. We can see that upon the introduction of VMT into PPC, no obvious reinforcement was observed for the PPC/VMT nanocomposites containing VMT content up to 4 wt%. However, the strain at break decreased dramatically with the increase VMT content, which is a typical characteristic of polymer/clay nanocomposites [10,17]. The yield strength decreased stepwise with further increasing VMT content. The modulus change also followed this tendency. This phenomenon has been reported in the preparations of polystyrene–clay nonocomposites by M.W. Noh [21]. The authors attributed the poorer properties to the weak interactions between polystyrene and clay particles, due to the polar nature of the polystyrene matrix. For PPC/
Table 2 Static mechanical properties for PPC/VMT nanocomposites Composites
Yield strength, MPa
Strain at break, %
Engineering modulus, MPa
Pure PPC PPC/2% VMT PPC/4% VMT PPC/6% VMT PPC/8% VMT PPC/6% VMTa PPC/6% VMTb
39.93 37.57 40.23 28.93 23.30 30.18 16.50
17.67 7.13 7.69 16.05 27.66 36.38 559.5
1001 1184 927.0 671.7 611.4 630.8 304.7
a The mixing time was 10 min with other conditions unchanged. b The rotary speed was 80 rpm with other conditions unchanged.
J. Xu et al. / European Polymer Journal 41 (2005) 881–888 O O C O
Base
OH +
887
O HO C
Scheme 1.
According to previous work [26], PPC behave very thermally sensitive. It is believed that the thermal degradation occurred during the melt blending of PPC and OVMT. Furthermore, the cetyltrimethyl ammonium bromide (structure shown as below) remained in OVMT acted as an accelerator for the thermal degradation of PPC at elevated temperature. CH3 H3C
+
N
CH2
CH3
weight loss temperature for PPC/5% VMT was almost 50 C lower than pure PPC indicating the catalytic hydrolysis of PPC melts by ammonium quaternary salt. The direct proof of the thermal degradation can also be seen from the decrease in molecular weight of PPC matrix as shown in Table 3. The decrease in the polydispersity index also implied the decomposition of PPC matrix. This is due to that long molecular chains were disassembled into short units, leading to the poor thermal properties.
15
CH3
We have reported that aromatic polycarbonates (PC) suffered seriously thermal degradation during blending with K2Ti6O13 whiskers in the pressure of tetrabutyloxyl titanium [34]. Similarly, PPC contains carbonyl and esters groups like PC. Generally, cetyltrimethyl ammonium salt is unstable at temperatures <100 C, and can decompose to produce amines. The basic compounds can in turn promote the thermal hydrolysis of PPC. The mechanism is presented in Scheme 1. Also from Table 2, both increasing the mixing time and increasing the rotary speed can further lead to the deterioration of the mechanical strength of the composites. This can be considered as the further thermal hydrolysis of PPC matrix. In this connection, we can conclude that it is crucial to select suitable intercalation agents for layered silicates. The intercalation agent should be inert for the matrix of composites, or one can use high temperature ammonium quaternary salt as the intercalation agent. The thermal degradation behavior of PPC matrix was further studied by GPC and TGA measurements. It is evident that the decomposition temperature decreased with increasing VMT content (Fig. 6). The 5%
4. Conclusion Vermiculite can be readily intercalated by cetyltrimethyl ammonium bromide to yield organo-vermiculite (OVMT). Poly(propylene carbonate) (PPC) was direct melt intercalated into the OVMT based on its strong hydrophilicity. Accordingly, intercalated and partially exfoliated PPC/VMT nanocomposites were fabricated via direct melt compounding of OVMT and PPC. The nanoscale dispersion of VMT sheets was confirmed by both X-ray diffraction and TEM examinations. However, the mechanical strength of the PPC/VMT nanocomposites decreased with increasing VMT content. The phenomenon was investigated by molar mass measurements (GPC) and thermal degradation (TGA) methods. The experimental results were explained by thermal hydrolysis of PPC matrix in the presence of amine compounds produced from the intercalating agent at an elevated temperature. This indicated that the selected chemicals for VMT treatment should be thermally stable during the melt compounding, and not induce the thermal degradation of matrix.
Acknowledgement Table 3 Thermal degradation of PPC matrix during the melt blending Specimens
Mn
Mw
MDI
TGA-5%, C
Pure PPC PPC/2% VMT PPC/4% VMT PPC/6% VMT PPC/8% VMT PPC/6% VMTa PPC/6% VMTb
46,200 39,300 38,600 33,300 28,700 27,400 19,900
187,400 108,600 88,600 89,000 76,900 65,500 36,600
4.06 2.76 2.29 2.67 2.68 2.39 1.84
244.2 217.5 211.2 211.1 208.7 201.8 198.5
This work was supported by the Ministry of Science and Technology of China (Grant No. 2002BA653C), Natural Science Foundation of Guangdong Province (Team Project, Grant No. 015007), Key Strategic Project of Chinese Academy of Sciences (Grant No. KJCX2-206B) for financial support of this work.
References
a
The mixing time was 10 min with other conditions unchanged. b The rotary speed was 80 rpm with other conditions unchanged.
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European Polymer Journal 41 (2005) 881–888
www.elsevier.com/locate/europolj
Preparation of poly(propylene carbonate)/organo-vermiculite nanocomposites via direct melt intercalation J. Xu a, R.K.Y. Li b, Y. Xu a, L. Li c, Y.Z. Meng
a,c,*
a
b
Institute of Energy and Environment Materials, School of Physics and Engineering, Sun Yat-Sen University, 135 Xingang West, Guangzhou 510275, PR China Department of Physics and Materials Sciences, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong c School of Mechanical and Production Engineering, Nanyang Technology University, North Sprine 639798, Singapore Received 20 December 2003; received in revised form 22 October 2004; accepted 27 October 2004 Available online 12 January 2005
Abstract Intercalated nanocomposites comprised of poly(propylene carbonate) (PPC) and organo-vermiculite (OVMT) was first prepared via direct melt compounding of the alkali-vermiculite intercalated host with PPC in a twin rotary mixer. The dispersion and morphologies of OVMT within PPC were investigated by X-ray diffraction and transmission electron microscopic techniques. The results revealed the formation of intercalated-exfoliated vermiculite sheets in the PPC matrix. Because of the thermally sensitive nature of PPC, thermal degradation occurred during the melt compounding. The degradation led to a deterioration of the mechanical properties of the nanocomposites. Tensile test showed that the yield strength and modulus of the nanocomposites decrease with increasing vermiculite content. The degradation mechanism was discussed according to the results of GPC and TGA measurements. 2004 Elsevier Ltd. All rights reserved. Keywords: Nanocomposites; Melt intercalation; Vermiculite; Polycarbonate
1. Introduction Recently, polymer-layered silicate nanocomposites have received special attention because of the numerous advantages in comparison with traditional polymer macro composites. Macro polymer composites usually involve a high level of inorganic filler loadings for * Corresponding author. Address: Institute of Energy and Environment Materials, School of Physics and Engineering, Sun Yat-Sen University, 135 Xingang West, Guangzhou 510275, PR China. Tel.: +86 208 523 1343; fax: +86 208 411 4113. E-mail addresses: [email protected], stdpmeng@zsu. edu.cn (Y.Z. Meng).
imparting desired mechanical properties, such as CaCO3, carbon fiber, and glass fiber [1–6]. However, high loading levels of inorganic fillers may lead to deteriorated properties, such as increase in density and loss of tenacity due to the interfacial incompatibility between organic polymer and inorganic filler. Moreover, the processability becomes worse, as the torque level of the mixing equipment increases and dispersion of inorganic filler with the increase in filler content becomes poorer. On the other hand, nanocomposites may show improved mechanical properties, decreased thermal expansion coefficient [7–10]. Intercalation methods for nanocomposites includes in situ polymerization, melt polymer intercalation, exfoliation–adsorption, template synthesis, and direct
0014-3057/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.10.033
882
J. Xu et al. / European Polymer Journal 41 (2005) 881–888
solution intercalation [11–16]. Many commercially important non-polar polymers such as polyolefins [17], however, the in situ polymerization or solution techniques cannot be used for preparation of nanocomposites. In such cases, the direct melt intercalation method provides a convenient technique for preparation of the composites [18–22]. Melt intercalation involves mixing the layered silicates with the polymer matrix above its melting point. Recently, Vaia et al. suggested that polymer–clay nanocomposites could be fabricated via a direct route in the polymer melt in which the polymer chains are diffused into the space between the silicate layers or galleries [23–25]. Conventional polymer processing techniques such as extrusions can reduce the time for nanocomposites fabrication through its ability to break up the silicate layers. Vermiculite, like the well-known montmorillonite, belongs to the general family of 2:1 layered silicates. This clay contains either Al3+ or Mg2+ and Fe2+ as normal octahedral ions, and a tetrahedral sheet in which A3+ occurs as a substituted ion in place of some of the Si4+. Compared to montmorillonite, chlorite, kaolinite and hydrous mica, vermiculite is abundant and has a larger cation exchange content (CEC) as listed in Table 1 (100–150 cmol/kg). It is generally used as packaging material for anti-shocking. In previous works [26–29], we have successfully synthesized alternating poly(propylene carbonate) (PPC) from carbon dioxide and propylene oxide. The synthesized PPC exhibited very high molecular weight (generally >5 · 104 Da), however, the glass transition temperature (Tg) remained relatively lower due to its carbonate linkage in the backbone. The practical application of the PPC has been limited by the thermal stabilities [30]. In order to improve the stability, many organic and inorganic fillers have been compounded with the PPC to fabricate composites [31,32]. Accordingly, the mechanical properties and thermal stabilities were improved greatly. In this paper, we report the direct melt intercalation of PPC and organo-vermiculite. The focus is placed on further enhanc-
ing the mechanical properties of PPC to meet the need of practical applications. The morphology and the mechanism of formation of the resulting PPC/OVMT nanocomposites were investigated in this work.
2. Experimental 2.1. Materials Vermiculite (VMT) was purchased from Aldrich with grade number 3. It was heat treated at 300 C for 12 h before use. Poly(propylene carbonate) (PPC) was synthesized in our lab. The PPC has a very high glass transition temperature (Tg) (>40 C) and decomposition temperature (>250 C). The number average molar mass and molar mass polydispersity index of PPC are 50,000 Da and 4.0, respectively. Cetyltrimethyl ammonium bromide purchased from Shanghai, China, regent purity. 2.2. Preparation of organo-vermiculite The inorganic cations in the vermiculite crystals were first ion exchanged with sodium ions as follows: vermiculite crystals (250 mesh, 10 g) were refluxed in about 100 mL of aqueous solution containing 1.2 g NaCl for 48 h. The reaction temperature was controlled in the range of 50–60 C, and the slurry was washed with distilled water until no Cl was detected, which was performed using 0.1 mol/L AgNO3. The Na-exchanged vermiculite was then added to a 500 mL plastic flask, containing 12 g cetyltrimethyl ammonium bromide in 200 mL of deionized water. The mixture of vermiculite and alkyl ammonium salt was stirred for 3 days at 70 C. The resulting organo-vermiculite (OVMT) was washed with distilled water until no precipitate was observed upon titrating the filtrate with 0.1 mol/L AgNO3. The final OVMT in the form of fine powder was kept in a dryer for further use.
Table 1 Summary of clay mineral propertiesa Secondary mineral
Type
Interlayer condition/bonding
CEC [cmol/kg]
Swelling potential
Specific surface area [m2/g]
Basal spacing [nm]
Vermiculite Kaolinite
2:1 (expanding) 1:1 (non-expanding)
100–150 3–15
High Almost none
500–700 5–20
1.0–1.5+ 0.72
Montmorillonite Hydrous mica Chlorite
2:1 (expanding) 2:1 (non-expanding) 2:1:1 (non-expanding)
Weak bonding, great expansion Lack of interlayer surface, strong bonding Very weak bonding, great expansion Partial loss of K, strong bonding Moderate to strong bonding, non-expanding
80–150 10–40 10–40
High Low None
700–800 50–200 –
0.98–1.8+ 1.0 1.4
a
The data cited from the work of Sabine Grunwald, University of Florida, USA. See http://grunwald.ifas.ufl.edu.
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2.3. Nanocomposite preparation The direct melt intercalation for the preparation of polymer/clay nanocomposites has been extensively studied. PPC and organo-vermiculite was melt blended in a twin rotary mixer (Haake Co., German). Prior to mixing, the PPC and organo-vermiculite must be dried for >8 h to remove residual moisture. The compounding was carried out at 170 C with rotary speeds at 40 rpm and 80 rpm, respectively. The PPC was introduced into the chamber of the mixer as quickly as possible, followed by adding OVMT. The residence time inside the mixer was controlled ranging from 5 min to 10 min under the processing conditions. Upon completion of mixing, the nanocomposite samples were removed from the mixing chamber, and cooled for future use. 2.4. Instrumentation The above obtained PPC/OVMT nanocomposite pellets were hydro compressed into films with a thickness of 1 mm at 160 C. The pellets were dried for >8 h at 80 C before use. Wide-angle X-ray diffraction (XRD) measurement was performed using a Rigaku D/max-1200X diffractometer with 2h scan range of 1.5–20 at room temperature, at a scanning speed and step size of 5/min and 0.05, respectively. More direct evidence for the nanoscale dispersion of VMT within PPC matrix was provided by transmission electron microscope (TEM) observations. Ultra thin films of nanocomposites were sliced up using LKB-5 ultra microtome. The observations were carried out on the copper web after adhering the nanocomposite slices onto it. The TEM observation was performed using a JEOL100CX-II model operated at 100 kV. The samples for mechanical measurements were prepared according to the standard of ASTM D-638. Tensile tests were conducted using an Instron 1122 testing machine. Five specimens of each composition were tested, and the average values were reported. YoungÕs modulus yield strength and strain at yield point were carefully determined by using an extensometer at a crosshead speed of 5 mm/min. All Gel Permeation Chromatography (GPC) analyses were performed on a Waters 515 HPLC using three phenogel 7.8 · 300 mm columns arranged in series. Spectroscopic grade tetrahydrofuran was used as a fluent with a flow rate of 1 mL/min. Refractive index detector was used and polystyrene was selected as external calibration standard. Thermal properties of the nanocomposites were measured on a Thermogravimetric/Differential Thermal Analyzer (TG/DTA) instrument. The used temperature profile ranged from 30 to 600 C and experiments were
883
performed at a scanning rate of 20 C/min. under the protection of N2 gas with a flow rate of 90 mL/min. Fourier transform infrared (FTIR) spectra were recorded using an Analect RFX-65A FTIR spectrometer.
3. Results and discussion In order to understand the physical and chemical process of the intercalation, simple theoretical analysis is done as below. VMT spacings between the closely packed sheets are on the order of 1 nm. Thus there is a large entropic barrier associated with the molten PPCs diffusing into the gap and hence, intercalation into the layered clay is hindered. One method of lowering this entropic barrier is to anchor short, surfactant-like chains in, or onto, the surface of the VMT sheets, thereby forming a socalled organically modified VMT (OVMT). For the appropriate choice of parameters, the energetic gains from the polymer-surfactant interactions can compensate for the entropic losses. Under these conditions, the polymers may penetrate into the galleries, separate the clay sheets, and finally disperse the sheets within the polymer melt. More recently, Vaia and Giannelis developed a lattice model for the interactions between polymers and organically modified clays [33]. DF ¼ F ðhÞ F ðh0 Þ ¼ DE T DS where DF is the total change in Helmholtz free energy; E is the internal energy change associated with the establishment of now inter molecular interactions; DS is an ideal combinatorial entropy change associated with configurational changes of the various constituents, and h0, h means the gallery height of unintercalated interlayer and the gallery height of polymer-intercalated interlayer, respectively. DF < 0 indicates that the layer separation is favorable, while DF > 0 implies that the initial unintercalated state is favorable. The major factors contributing to the free energy change may be identified as the relative confinement of the polymer establishment of new intermolecular interactions between PPC and OVMT during intercalation process (Fig. 1). 3.1. Delamination of VMT Fig. 2 shows XRD patterns for OVMT and PPC/ VMT nanocomposites. The OVMT, trace b of Fig. 2, gave three distinct peaks at 2h = 2.02, 4.58, and 7.36, corresponding to the d-spacing of 4.37, 1.93, 1.20 nm, respectively. It should be noted from the XRD pattern of OVMT that the d-spacing of 4.37 nm was significantly larger than that of the unmodified Na+ vermiculite, which had a d-spacing of 1.2 nm as
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cetyltrimethyl ammonium bromide 4.37nm
1.1nm PPC
Fig. 1. Schematic representation of the intercalation process of OVMT within PPC matrix.
served by FTIR measurements (Fig. 3). It can be seen that the C–H stretching vibration of alkylammonium chain appeared at 2929 cm1 and 2852 cm1, indicating the increase in hydrophobic property of resulting OVMT. From trace c of Fig. 2, the distinct diffraction peaks of OVMT were not observed and the intensity of the 0 0 1 basal peak decreased from 6500 to 200. This indicated that most of the OVMT sheets were dispersed uniformly in the polymer matrix in nanoscale.
a
Fig. 2. XRD patterns of (a) pristine VMT, (b) OVMT, (c) PPC/ VMT nanocomposite.
indicated in trace a of Fig. 2. The larger gallery spacing indicated the intercalation of alkyl ammonium in between the silicate layers. Depending on the packing density and chain length, the intercalating agents radiated away from the surface, forming bimolecular tilted arrangement as shown in Fig. 1. As the negative charge originated in the silicate layer, the cationic head groups of the alkylammonium salt preferentially resided on the layer surface, leaving the organic chain tail radiating away from the surface. The intercalation was also ob-
b
4000
3500
3000
2500
2000
1500
1000
Wavenumber cm-1
Fig. 3. FTIR spectra (a) pristine VMT, (b) OVMT.
500
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3.2. Morphologies of PPC/VMT nanocomposite TEM images of selected PPC/VMT nanocomposites are provided in Fig. 4. It can be seen that the stacks of VMT sheets were dispersed within the PPC matrix irregularly. The arrow a indicates that some of VMT layers were intercalated and dispersed perpendicularly to the sample surface within the PPC matrix, whilst arrow b shows the VMT sheets were parallel to the surface of the nanocomposites slice. The dark lines are the intersections of VMT layers with thickness of 2–5 nm, and length of 200 nm. Intercalated layers of OVMT were dispersed homogeneously in the PPC matrix, however, a small amount of unexfoliated OVMT layers existed as clusters as shown in Fig. 4A. In this sense, the PPC/
885
VMT nanocomposites is considered to be a mixed delaminated/intercalated system. The existence of the obstacle peaks in the XRD patterns of the nanocomposite (trace c of Fig. 2) was believed to be attributed these unexfoliated clusters. With increasing OVMT content, the dispersibility of VMT decreased. From Fig. 4B, it is apparent from the region around arrow c that the unexfoliated content of VMT increased. Presumably, as indicated by arrow d, the edges of OVMT are more accessible for the PPC chains than the center. Because of the shape of OVMT layers, PPC molecular chains must enter the gallery from the agglomerate polymer melt to the primary OVMT particles, and further to the edges of OVMT layers. VMT crystallites on the interior side of the front remained unintercalated. The
Fig. 4. TEM micrographs of (A) PPC/4% VMT; (B) PPC/8% VMT nanocomposites.
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Fig. 5. Solution photos of (a) pure PPC, (b) PPC/OVMT nanocomposite, (c) PPC/VMT blend.
schematic illustration of the intercalation process of OVMT within PPC melt is depicted in Fig. 1. In order to confirm the nanoscale dispersion of OVMT within PPC matrix, the nanocomposites and the solution mixed PPC/VMT specimens were dissolved in chloroform, respectively. It can be seen clearly from Fig. 5 that both pure PPC and the PPC/OVMT nanocomposite solution were transparent as indicated in Fig. 5a and b. However, the solution of simply mixed PPC/VMT specimen exhibited less transparence with some precipitates. This result demonstrated that the OVMT was well dispersed within the PPC matrix on the nanoscale (Fig. 6).
Fig. 6. Thermogravimetric traces of (a) pure PPC; (b) PPC/2% VMT; (c) PPC/4% VMT; (d) PPC/6% VMT; (e) PPC/8% VMT nanocomposites.
VMT nanocomposites, however, stronger interactions between PPC and VMT should be present due to the existence of polar groups (e.g. carbonyl) in PPC. The structure of PPC repeats unit is given as below: O O C O CH2 CH CH3 n
3.3. Properties of PPC/VMT nanocomposites The mechanical properties of PPC/VMT nanocomposites are listed in Table 2. We can see that upon the introduction of VMT into PPC, no obvious reinforcement was observed for the PPC/VMT nanocomposites containing VMT content up to 4 wt%. However, the strain at break decreased dramatically with the increase VMT content, which is a typical characteristic of polymer/clay nanocomposites [10,17]. The yield strength decreased stepwise with further increasing VMT content. The modulus change also followed this tendency. This phenomenon has been reported in the preparations of polystyrene–clay nonocomposites by M.W. Noh [21]. The authors attributed the poorer properties to the weak interactions between polystyrene and clay particles, due to the polar nature of the polystyrene matrix. For PPC/
Table 2 Static mechanical properties for PPC/VMT nanocomposites Composites
Yield strength, MPa
Strain at break, %
Engineering modulus, MPa
Pure PPC PPC/2% VMT PPC/4% VMT PPC/6% VMT PPC/8% VMT PPC/6% VMTa PPC/6% VMTb
39.93 37.57 40.23 28.93 23.30 30.18 16.50
17.67 7.13 7.69 16.05 27.66 36.38 559.5
1001 1184 927.0 671.7 611.4 630.8 304.7
a The mixing time was 10 min with other conditions unchanged. b The rotary speed was 80 rpm with other conditions unchanged.
J. Xu et al. / European Polymer Journal 41 (2005) 881–888 O O C O
Base
OH +
887
O HO C
Scheme 1.
According to previous work [26], PPC behave very thermally sensitive. It is believed that the thermal degradation occurred during the melt blending of PPC and OVMT. Furthermore, the cetyltrimethyl ammonium bromide (structure shown as below) remained in OVMT acted as an accelerator for the thermal degradation of PPC at elevated temperature. CH3 H3C
+
N
CH2
CH3
weight loss temperature for PPC/5% VMT was almost 50 C lower than pure PPC indicating the catalytic hydrolysis of PPC melts by ammonium quaternary salt. The direct proof of the thermal degradation can also be seen from the decrease in molecular weight of PPC matrix as shown in Table 3. The decrease in the polydispersity index also implied the decomposition of PPC matrix. This is due to that long molecular chains were disassembled into short units, leading to the poor thermal properties.
15
CH3
We have reported that aromatic polycarbonates (PC) suffered seriously thermal degradation during blending with K2Ti6O13 whiskers in the pressure of tetrabutyloxyl titanium [34]. Similarly, PPC contains carbonyl and esters groups like PC. Generally, cetyltrimethyl ammonium salt is unstable at temperatures <100 C, and can decompose to produce amines. The basic compounds can in turn promote the thermal hydrolysis of PPC. The mechanism is presented in Scheme 1. Also from Table 2, both increasing the mixing time and increasing the rotary speed can further lead to the deterioration of the mechanical strength of the composites. This can be considered as the further thermal hydrolysis of PPC matrix. In this connection, we can conclude that it is crucial to select suitable intercalation agents for layered silicates. The intercalation agent should be inert for the matrix of composites, or one can use high temperature ammonium quaternary salt as the intercalation agent. The thermal degradation behavior of PPC matrix was further studied by GPC and TGA measurements. It is evident that the decomposition temperature decreased with increasing VMT content (Fig. 6). The 5%
4. Conclusion Vermiculite can be readily intercalated by cetyltrimethyl ammonium bromide to yield organo-vermiculite (OVMT). Poly(propylene carbonate) (PPC) was direct melt intercalated into the OVMT based on its strong hydrophilicity. Accordingly, intercalated and partially exfoliated PPC/VMT nanocomposites were fabricated via direct melt compounding of OVMT and PPC. The nanoscale dispersion of VMT sheets was confirmed by both X-ray diffraction and TEM examinations. However, the mechanical strength of the PPC/VMT nanocomposites decreased with increasing VMT content. The phenomenon was investigated by molar mass measurements (GPC) and thermal degradation (TGA) methods. The experimental results were explained by thermal hydrolysis of PPC matrix in the presence of amine compounds produced from the intercalating agent at an elevated temperature. This indicated that the selected chemicals for VMT treatment should be thermally stable during the melt compounding, and not induce the thermal degradation of matrix.
Acknowledgement Table 3 Thermal degradation of PPC matrix during the melt blending Specimens
Mn
Mw
MDI
TGA-5%, C
Pure PPC PPC/2% VMT PPC/4% VMT PPC/6% VMT PPC/8% VMT PPC/6% VMTa PPC/6% VMTb
46,200 39,300 38,600 33,300 28,700 27,400 19,900
187,400 108,600 88,600 89,000 76,900 65,500 36,600
4.06 2.76 2.29 2.67 2.68 2.39 1.84
244.2 217.5 211.2 211.1 208.7 201.8 198.5
This work was supported by the Ministry of Science and Technology of China (Grant No. 2002BA653C), Natural Science Foundation of Guangdong Province (Team Project, Grant No. 015007), Key Strategic Project of Chinese Academy of Sciences (Grant No. KJCX2-206B) for financial support of this work.
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The mixing time was 10 min with other conditions unchanged. b The rotary speed was 80 rpm with other conditions unchanged.
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