9214
Ind. Eng. Chem. Res. 2008, 47, 9214–9224
Synthesis and Characterization of Hyperbranched Polyurethane Hybrids Using Tetraethoxysilane (TEOS) As Cross-Linker Kishore K. Jena and K. V. S. N. Raju* Organic Coatings and Polymers DiVision, Indian Institute of Chemical Technology, Tarnaka, Hyderabad-500007, India
Organic-inorganic hybrid coatings were derived from a hydroxyl-terminated hyperbranched polyester, 3-isocyanatopropyl triethoxysilane (ISPTES), and then cross-linked with various percentages of SiO2. This system was able to cross-link by hydrolysis of a tetraethoxysilane group in room temperature and humid conditions and then at 120 °C for 12-14 h, forming a three-dimensional hybrid structure, where the organic and inorganic phases were bonded covalently. The main focus of this work was to study the change of properties of the coating with SiO2 modification. These hybrid coatings were also characterized by using NMR spectroscopy, Fourier transform infrared (FTIR) spectroscopy, dynamical mechanical thermal analyzer (DMTA), thermal analysis (TGA), and tensile strength. 1. Introduction Hyperbranched polymers can be prepared with commercially available raw materials with a wide range of physical and chemical properties.1-8 These polymers have received increased attention recently because they have low viscosity and a highly branched architecture can be produced at low cost for largescale application with improved properties.9-12 These can be tailored to meet the highly broadened demands of modern technologies such as coatings, adhesives, and thermosetting polymers. Cao et al. reported the synthesis of thermoplastic hyperbranched polyurethanes (HBPUs) by using Boltorn H30, as a precursor.13 Nasar et al. reported that the incorporation of 1% hyperbranched polyamide into the polyurethane chains has increased the tensile strength considerably.14 Fornof et al. synthesized the branched poly(ether urethane)s based on TMP Core and isocyanate endcapped polyethers by A2 + B3 methodology.15 Xu et al. studied the properties and morphologies of UV-cured epoxy acrylate blend films containing hyperbranched polyurethane acrylate and HB polyester16 and found that these polymers show good flexibility and ease of processing but are not thermally stable. Because of unique properties of “hybrid coatings”, the frontier research is carried out in this field in the development of coating materials.17-20 During the past several years, hybrid materials have attracted significant attention because of the efficient combination of organic and inorganic components by sol-gel techniques, which offer prospects of new and synergistic properties, such as mechanical and thermal properties.21,22 These systems can be produced starting from a wide variety of inorganic materials (sol-gel precursors) and polymers.23-25 Inorganic materials often strongly influence the properties of hybrid composites at lower weight percentage because of the interaction between inorganic particles and the polymer matrix, as well as the consequent change in morphology. Alkoxysilanes are particularly suitable for this purpose and are used to provide covalent bonding between inorganic materials and polymer matrices, enhanced interfacial adhesion, and improved mechanical properties of composite materials. Because of these interesting properties, a large number of research groups have done work on various types of polymer-silica hybrid coatings.26-30 * Corresponding author. Tel.: +914027193208. Fax: +914027193991. E-mail:
[email protected];
[email protected].
These types of materials are very much useful as organic coatings for various applications.31,32 Alkoxysilanes can be used to make a new class of silica-polymer hybrid coatings. There are different reactions involved in this process like hydrolysis, condensation, etc. The aim of the present work is to prepare and characterize the hyperbranched polyurethane hybrid polymer modified by silica to improve thermal and mechanical properties of these cross-linked hybrid coatings. The structural and thermomechanical performances of hybrid coatings are studied using different instrumental techniques. 2. Experimental Section 2.1. Materials. 2,2-bis(Hydroxymethyl) propionic acid (DMPA) and tetraethoxy orthosilicate (TEOS) were procured from Aldrich Chemicals (Milwaukee, WI). (3-Isocyanatopropyl) triethoxysilane (ISPTES) from Deguss (Germany) and titanium tetraisopropoxide (TTIP) from Fluka Chemical Corp. (Ronkonkoma, NY) were purchased. Spectroscopic-grade tetrahydrofuran (THF) and sulfur-free toluene were purchased from SD Fine Chemicals (Mumbai, India). Glycerol and dimethyl formamide (DMF) were purchased from Qualigens (Mumbai, India). All these reagents were used without any further purification. 2.2. Synthesis Procedure. Hydroxyl-terminated fourthgeneration hyperbranched polyester (HBP) was synthesized from glycerol and DMPA, using the method described in our previous work.33 After synthesis, the polyester was characterized by endgroup titration and also by gel-permeable chromatography (GPC) (Table 2). ISPTES was reacted with 75% of hydroxyl groups of these polyesters in a 250 mL flask under an inert atmosphere of nitrogen at 60-70 °C for 5-6 h under constant stirring. In order to reduce the viscosity of HBP, 50:50 wt % THF and DMF blend was homogeneously mixed. The reaction Table 1. Monomer Used for Preparation of Hybrid Coatings from Hyperbranched Polyester at OH/NCO Mol Ratio of 1:0.75 with varying SiO2 Content polyester HBP-OH/NCO-ISPTES HBP-OH glycerol + DMPA
OH/NCO ) 1:0.75 (75% modified)
10.1021/ie800884y CCC: $40.75 2008 American Chemical Society Published on Web 11/01/2008
sample code
SiO2 wt %
HBP-OH-ISPTES-SiO2 (2%) HBP-OH-ISPTES-SiO2 (5%)
2% 5%
HBP-OH-ISPTES-SiO2 (10%) 10%
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9215 Table 2. GPC Data of the Synthesized Hyperbranched Polyester and Hybrids HBP-OH-ISPTES-SiO2 sample code polyester generation Mtheoretical, g/mol Mn, g/mol Mw, g/mol PDI ) Mw/Mn
HBP-OH 4 5 401 1 568 3 122 1.9
0% 4 14 368 5 171 13 291 2.57
2%
5%
10%
4
4
4
4 974 14 562 2.92
5 678 17 773 3.13
4 878 18 880 3.87
was followed till the NCO peak disappears at 2200-2300 cm-1 in FTIR, and this was labeled as HBP-OH-ISPTES. 2.2.1. TEOS Sol Preparation. The sol was prepared by sol-gel technique followed by the hydrolysis and condensation process. A mixture of water (4 equiv of TEOS) and 0.01 mol HCL was added dropwise in the TEOS solution containing ethanol, and the reaction process was continued for 4-5 h. After hydrolysis, solvents (water + ethanol) were removed from the product by rotavapor to get SiO2 powder. Then the required weight percentage of SiO2 was dispersed in THF and then used for the preparation of hybrid coatings. 2.2.2. TEOS-Modified Hybrid Coatings. An amount of 2, 5, and 10 wt % of SiO2 powder with respect to the polymer (HBP-OH-ISPTES) was dispersed in THF and added drop by drop into the 250 mL round-bottomed flask containing HBPOH-ISPTES polymer with THF solvent for 1 h under vigorous stirring at room temperature. The reaction was continued again for another 2-3 h at 40 °C to obtain a desired hybrid product. The final hybrid solution was cast into tinfoil and aged at room temperature and humid conditions for 14-15 days. Then the hybrid samples were dried at 120 °C for 12-14 h. The details of hybrid-coating formulations are given in Table 1. 2.3. Characterization Techniques. The ESI mass spectra were recorded using a Quattro LC mass spectrometer (Micromass, Manchester, U.K.) using Masslynx software. The compounds were dissolved in methanol/water (1:1, v/v) and the [M+H]+ ions were generated by ESI. Sample concentrations of 1 ppm were used for the experiments. Samples were introduced into the source using a direct-infusion pump (Harvard Apparatus) at a flow rate of 10 µL/min. Capillary and cone voltages were kept at 3.75 kV and 30 V, respectively. The fullscan mass spectra were recorded by scanning. GPC measurements were carried out using Shimadzu LC 10ATVP series (Japan) with a refractive index detector to determine the molecular weight of the polymer solutions. THF was used as eluent at a nominal flow rate of 1 mL/min. 1H and 13C NMR were used to determine the structural conformation of the synthesized hyperbranched polyesters. Nuclear magnetic resonance (1H NMR and 13C NMR) spectra were recorded with a Varian VXR-Unity 200 MHz and Bruker UXNMR 300 MHz spectrometer at room temperature, using DMSO-d6 as a solvent and δ values relative to Me4Si (TMS). Contact angle measurements were done to determine the hydrophilicity of the films using a contact angle measuring instrument (KRUSS, GmbH, Hamburg) by sessile drop method, which involves placing a drop of water using a microsyringe on the dry films and then measuring the contact angle between the solid-liquid interface. The Fourier transform infrared (FTIR) spectra of the KBr coated samples were scanned on a Thermo Nicolet Nexus 670 spectrometer with resolution of 4 cm-1 and in the range of 400-4000 cm-1. To identify the complex bands in the O-H zone, the curve-fitting simulations were performed using Origin 6.0 software. The dynamical mechanical thermal analyzer (DMTA) IV scan (Rheometric Scientific, U.S.A.) was performed in tensile mode in the temperature range from 30 to 250 °C
with a heating rate of 3 °C/min using a sample of 15 × 10 × 0.15 mm3 at a frequency of 1 Hz. The E′ values in rubbery region at T > Tg are taken to calculate cross-link density, υe, by using the following equation: (1) υe ) E′ ⁄ 3RT where R is the universal gas constant and T is the temperature. The static mechanical properties of the samples of the cast films were measured with the help of Shimadzu Autograph 10kNG Universal Testing Machine (Kyoto, Japan) using a load cell of 10 kN at a crosshead speed of 10 mm/min. The gauge length of the specimens was fixed at 50 mm. Thermal analysis (TGA) of the hybrid samples has been conducted by using a thermal analyzer (Perkin ElmerTGA-7, Shimardzu, Japan) with 10-15 mg of the sample at the heating rate of 10 °C/min in N2 atmosphere from 30 to 600 °C. The morphology of the hybrid sample was found out using a Hitachi S520 scanning electron microscope instrument operating at 10 kV. 33,34 3. Results and Discussion 3.1. Interaction. The synthesized HBP was reacted with ISPTES in the mole ratio of 1:0.75 and further cross-linked with various amounts of SiO2, and the reaction scheme is given in Scheme 1. Then the prepared polymer was analyzed by 1H and 13 C NMR. Scheme 2 shows the schematic structure of hyperbranched polyester, hydrogen bonding interactions, and hybrid coatings. Then the synthesized SiO2 particles, through a condensation reaction, lead to the formation of the final inorganic network. The Si(OEt) groups of the modified HBP (HBP-OH-ISPTES) can be expected to involve the formation of the inorganic network, through condensation. The HBP-OH may also be interfering to form covalent bonds with Si-OH in the condensation reaction. This shows good compatibility between organic and inorganic phases. 3.2. Electrospray Ionization (ESI)-MS Spectrometry Analysis. ESI-MASS was used to distinguish between the molecular ions of acyclic and cyclic species during the analysis for the hyperbranched polymer. The molar mass of the acyclic species with and without a core molecule can be calculated using the following equations. Polymerization of acyclic species with core (desired product): (2) Mn)Mglycerol+ (MDMPA - MH2O)n+ MNa+ Polymerization of acyclic species without core: Mn)(MDMPA)n- (MH2O)n-1+ MNa+
(3)
The theoretical calculation of m/z is 116 amu, and the distribution possesses a distance of 116 amu between peaks, which corresponds to the repeat unit (MDMPA - MH2O), as shown in Figure 1. A series of ion peaks corresponding to [(MDMPA MH2O)n + Na]+ and [(MDMPA - MH2O)n + K]+, which has been generated even if no special cation salts are added, are also observed. This shows that HBP samples are ready to form both sodium and potassium adducts because of the high affinity of HBP with alkali salts as well as the presence of trace amount of sodium and potassium in the matrix. The mass spectrum shows that the intensity of the signals corresponding to cyclic species is very low and can be easily distinguished in the spectrum as a peak due to loss of one water molecule. In the above ESI-MASS study of HBP, we have observed a cyclic oligomer peak due to the loss of one water molecule. The endgroup analysis from the 13C NMR result shows that the synthesized polyester contained acid (terminal, linear, and dendritic) groups, which support the role of cyclization in the polyester.
9216 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 Scheme 1. Idealized Structure of Hybrid Coating Prepared from Hyperbranched Polyester (Glycerol + DMPA), ISPTES, and SiO2 Network
3.3. GPC Analysis. The molecular weight and polydispersity of polyester and hybrid samples were analyzed by GPC, and the data are given in Table 2. These values of hyperbranched polyester and hybrids are generally lower than the estimated values because of the higher branched nature, but they give accurate information about the polydispersity. These values show that the molecular weight increases and the polydispersity index becomes larger with increasing cross-linker concentration. 3.4. NMR Analysis. The synthesized HBP and hybrid samples were studied by 1H and 13C NMR spectroscopy. The 1 H and 13C NMR spectra of hyperbranched polyester in the range 0-14 ppm and 0-180 ppm are shown in Figure 2. The NMR peak resonates for hyperbranched polyester and hybrids are described below. For Figure 2: 1H NMR (glycerol + DMPA: DMSO-d6):33 δ ppm ) -CH3: T (1.05), L (1.11), and D (1.23); -CH2OH: (3.5-3.7); -COOCH2: (4.0-4.2); -OH: -OHT (4.43) and -OHL (4.87). 13C NMR (glycerol + DMPA: DMSO-d6): δ ppm ) -CH3: (15-20); -CH2OH: (62-68); quaternary carbon (qc): (45-55) and C)O: (173-178). For Figure 3: 1H NMR (HBP-OH + ISPTES: DMSO-d6): δ ppm ) -NH-: (5.98-6.10); Si-O-CH2 -CH3: (1.23);
Si-O-CH2 -CH3: (3.81); Si-CH2: (0.52); Si-CH2 -CH2: (1.55); and Si-CH2 -CH2 -CH2: (2.43). For Figure 4: 13C NMR (HBP-OH + ISPTES: DMSO-d6): δ ppm ) CH2-Si: (7.65); CH2-CH2-Si: (23.0); NH- CH2-CH2-CH2: (43.4); Si-OCH2-CH3: (18.27); and Si-O-CH2 -CH3: (58.4). The 1H NMR spectrum of polyester shows that the resonance at 4.4-5.0 ppm is due to the protons of the hydroxyl groups (OHT and OHL), whereas the methyl group of DMPA resonates at 0.95-1.02 ppm. The methylene groups attached to hydroxyl groups (CH2sOsCdO) and unreacted hydroxyl groups (CH2-OH) resonated at 4.0-4.2 ppm and 3.5-3.7 ppm. The expanded zones of -CH3 (1.04-1.25 ppm), quaternary carbon (46-55 ppm) and CdO (173-178 ppm) of polyester are shown in Figure 2 (inset). In the quaternary carbon zone of polyester (Figure 2) were detected carboxy end groups after complete reaction, and the acid value titration results showed the same observation of 13C NMR and observed that the conversion of COOH groups was 90%. The degree of branching (DB) and % of T, L, and D (terminal, linear, and dendritic units) were calculated from 1H NMR and 13C NMR, and results are reported in Table 3. A comparison is made among the 1H NMR spectra
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9217 Scheme 2. Systematic Structure of Hyperbranched Polyester, Hydrogen Bonding Interaction, and Hybrid Coating
shown in Figure 3; there is a clear difference in resonance area between 5.98 and 6.10 ppm, which is due to -NH protons of urethane linkage. The major difference in the spectra is marked with an arrow. A new peak has appeared in the resonance area of 5.98-6.10 ppm, which conforms the urethane bond from the reaction between HBP-OH and ISPTES.35
Figure 1. ESI-MASS spectrum of the hyperbranched polyester (HBP-OH).
3.5. Contact Angle Measurement. The contact angle data of HBP-OH-ISPTES-SiO2 hybrid coatings with different SiO2 concentrations reported in Table 4. This table shows that the coating hydrophilicity is improved by the addition of SiO2. This is due to the presence of hydroxyl groups in SiO2, which
9218 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008
Figure 2. 1H and
13
C NMR spectra of hyperbranched polyester in the range 0-14 ppm and 0-180 ppm.
are responsible for the hydrophilicity increase. However, the contact angle again increases when SiO2 concentration is 10%. This might be due to silica particles aggregate and decreasing hydroxyl groups in the hybrid. After curing for 40 days, the hydrophobic nature of the hybrids is increased at 2, 5, and 10% concentration of SiO2, due to the strong interaction between organic and inorganic phase. 3.6. FTIR Analysis of Hybrid Coatings. FTIR spectroscopy was used to follow the progress of the reaction. Initially, the completion of the reaction between glycerol and DMPA was observed by C-O-CdO, C-O-CdO, and CdO stretching of ester peaks at 1131, 1238, and 1734 cm-1, respectively, shown in Figure 5.33 Esteric CdO stretching zone in between 1600 and 1800 cm-1 is composed of free and hydrogen bonded CdO at 1734 and 1690 cm-1, respectively. The FTIR peak deconvolution of the O-H region of a representative sample HBP-OH is shown in Figure 5. The deconvoluted spectrum is composed of bands at 3195, 3338, 3485, and 3587 cm-1 corresponding to hydrogen bonding with esteric CdO · · · HsO, esteric OdCsO · · · HsO, inter/intramolecular OsH · · · OsH,
and free OsH, respectively. Similarly, the further reaction with ISPTES was conformed by the following characteristic bands, as shown in Figure 6: (i) a strong and broad band occurring in the range 3000-3800 cm-1, which corresponds to N-H stretching vibration (amide A + amide B); (ii) amide I, a welldefined, strong, and broad band occurring in the range 1600-1800 cm-1, which corresponds to CdO stretching vibration; (iii) amide II, at about 1520-1570 cm-1, corresponding to coupling between N-H bending and C-N stretching vibrations; (iv) amide III (1220-1330 cm-1), which contains a large percentage of N-H bending and C-N stretching; (v) amide IV-amide VII, observed at frequencies below 800 cm-1 and corresponding to out-of-plane vibrations of CO-NH group.36,37 The Si-O-C bonds, expected around 965 and 1120 cm-1, could not be verified due to the overlap with polyester signals. The absence of -NCO characteristics peak at about 2242 cm-1 is due to the complete reaction of -NCO group with the -OH groups of hyperbranched polyester. The characteristic absorption peaks at 1083 and 800 cm-1 wavenumber were due to the asymmetric and
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9219
Figure 3. 1H NMR spectra of HBP-OH-ISPTES hybrid coatings.
Figure 4.
13
C NMR spectrum of HBP-OH-ISPTES hybrid coatings.
Table 3. Quantification of Structural Units from 13
c
13
C and 1H NMR Intensity of Different Zone of Hyperbranched Polyester
C C ) Oa
13
C qcb
1
H -CH3c
DB %
sample name
T%
L%
D%
T%
L%
D%
Tacid %
Lacid %
Dacid %
T%
L%
D%
Frey, Frechet
HBP-OH
29.4
50
20.5
30.4
47.82
13.04
2.1
4.3
2.1
55
38.8
5.5
45.1a 50.0a 35.2b 47.61b 44.4c 61.1c
a Degree of branching calculated from carbonyl zone (13C NMR). Degree of branching calculated from -CH3 zone (1H NMR).
symmetric vibration modes of Si-O-Si formed by sol-gel reaction, respectively.37-41,34 Peaks at 755-830 cm-1 (Si-C stretching and Si-O-C deformation) confirm the formation
b
Degree of branching calculated from quaternary carbon zone (13C NMR).
of organic-inorganic hybrid composites. Enhancement of the peaks shows the increase in the amount of SiO2 in the agreement with the synthesis. It was expected that the peak
9220 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 Table 4. Contact Angle of HBP-OH-ISPTES-SiO2 Hybrid Coatings with Different SiO2 Concentration sample name HBP-OH-ISPTES-SiO2(0%) HBP-OH-ISPTES-SiO2(2%) HBP-OH-ISPTES-SiO2(5%) HBP-OH-ISPTES-SiO2(10%)
SiO2 contact angle (°) contact angle wt % (after 10 days) (after 40 days) 0% 2% 5% 10%
65.7 63.3 56.9 64.4
68.2 69.6 75.2 80.6
intensities of Si-O-Si came from the condensation of SiOH produced by the hydrolysis of TEOS and ISPTES. The broad peak Si-OH in between 3100-3800 cm-1 is formed in the hydrolysis reaction of the alkoxy groups of either TEOS or HBP-OH-ISPTES. The N-H peak from urethane overlaps with this Si-OH peak. The peaks at 3100-3800 cm-1 and 784 cm-1 indicated the concentrations of incompletely reacted groups, Si-OH, and unhydrolyzed Si-OC2H5 groups.42,37
The Si-OC2H5 groups appear in all the hybrid films, decreasing in intensity with respect to increasing the concentration of the SiO2. It could be suggested that the higher the SiO2 content, the lower the amount of unreacted groups that remained in the interior part of the silicon-oxo-cluster of modified polyester, suggesting the denser network formed and increased amounts of SiO2 group was introduced into the alkoxysilane-modified HBP matrix. This SiO2 group, which was cohydrolyzed and condensed to form the Si-O-Si, is responsible to form a strong network of hybrid matrix. The interaction between the silanol group of modified HBP silicon-oxo-clusters and the SiO2, such as hydrogen bonding and condensation of the hydroxyl groups, would also affect the properties of the hybrids. According to Huang et al.,43 the peak of CdO groups shifted to lower wavenumber for the hydrogen bonding. However, the shifts of the CdO
Figure 5. Representative full FTIR spectrum of HBP-OH and deconvulated O-H zone.
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9221 Table 5. E′, Tg, υe, and tan δ of HBP-OH-ISPTES-SiO2 Hybrid Coatings HBP-OH-ISPTES-SiO2 sample code E′ (Pa), at Tg ) 5 °C Tg, °C Tg, K υe, mol/cm3 × 10 2 tan δmax
0%
2%
5%
10%
2.93 × 108 3.9 × 108 4.19 × 108 4.56 × 108 116.63 132.19 135.42 150.96 389.63 405.19 408.42 423.96 2.97 3.81 4.06 4.26 0.737 0.688 0.571 0.530
Table 6. Tensile Properties Data of Different HBP-OH-ISPTES-SiO2 Hybrid Coatings HBP-OH-ISPTES-SiO2
Figure 6. Full FTIR spectra of HBP-OH-ISPTES and HBP-OH-ISPTESSiO2 in the zone of 400-4000 cm-1.
band at 1722 cm-1 could not be observed more, indicating that the hydrogen bonding interaction did not play a major role in the properties of hybrids due to high cross-liking density, but hydroxyl end groups of the modified polyester and hydroxyl end groups of silicon-oxo-clusters contribute to the network formation. 3.7. DMTA Analysis of Hybrid Coatings. The tan δ as a function of temperature for the HBP-OH-ISPTES and hybrid films is shown in Figure 7. The glass transition temperature (Tg) was related to the peak temperature of tan δ curve, and the relaxation strength corresponded to height of the tan δ peak. The cross-linking density was calculated using the equation reported earlier.44 Tg, storage modulus (E′), and cross-linking
sample code
0%
2%
5%
10%
max. stress, N/mm2 elongation, %
31.832 17.518
44.579 11.309
50.895 9.786
57.072 8.777
density of the hybrid films are reported in Table 5. As comparison of these results shows, the E′ value of SiO2 modified hybrid films is higher than that of the base polymer and E′ increased with increasing SiO2 content. Correspondingly, the tan δ peaks shifted higher and became broader. Table 5 shows that the Tg increased to 132.19, 135.42, and 150.96 °C, with SiO2 content of 2%, 5%, and 10%, respectively. The reactive groups of HBP-OH-ISPTES such as Si-OH, Si-OCH2CH3, and polyester-OH bonded with more hydroxyl groups in SiO2 should be responsible for the higher cross-linking density and higher Tg of the hybrid films, which agreed with the FTIR results. The lower relaxation strength of the hybrid films with higher SiO2 content also indicated that more HBP-OH-ISPTES segments were chemically bonded or entrapped within the silicon-oxo-cluster,45 and the inorganic particles strongly limit the mobility of the polymeric chains. 3.8. Mechanical Properties of Hybrid Coatings. The effects of the SiO2 content on the tensile strength and percent elongation of the hybrid films are displayed in the Table 6. The strength progressively increased and the elongation at break decreased
Figure 7. DMTA spectra of HBP-OH-ISPTES and HBP-OH-ISPTES-SiO2 with different concentrations.
9222 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008
Figure 8. Thermogravimetric curves of different hybrid coatings in N2 atmosphere at a heating rate of 10 °C/min. Table 7. Thermal Properties of Different HBP-OH-ISPTES-SiO2 Hybrid Coatings sample name
Ton
residue (%) at 550 °C
HBP-OH-ISPTES-SiO2 (0%) HBP-OH-ISPTES-SiO2(5%) HBP-OH-ISPTESSiO2 (10%)
268 273 282
10.98 13.72 16.34
with the increasing SiO2, indicating that the denser structure was favored. This phenomenon can be explained, as an interfacial tailor could be well-bonded to the polymer matrix and the surface of SiO2 because of its silanol groups. The SiO2 plays an important role in improving the mechanical properties of the hybrids. Overall, the tensile strength of the hybrid coatings depends on the cross-linking density, which is explained in DMTA section. 3.9. Thermal Analysis of Hybrid Coatings. Figure 8 shows the TGA curves of HBP-OH-ISPTES (0%), HBP-OH-ISPTESSiO2 (5%), and HBP-OH-ISPTES-SiO2 (10%). The order of the thermal decomposition temperature is HBP-OH-ISPTES-SiO2 (10%) > HBP-OH-ISPTES-SiO2 (5%) > HBP-OH-ISPTES. This suggests the enhancement of thermal stability by incorporating silica moiety. The TGA curve for each sample shows a significant difference above 250 °C. The first part of the curves at around 150 °C shows the evaporation of the residual solvent and unreacted monomer present in the coatings. The decomposition temperature of the HBP-OH-ISPTES (0%), HBP-OHISPTES-SiO2 (5%), and HBP-OH-ISPTES-SiO2 (10%) hybrid materials is in the range of 268-345, 273-375, and 282-389 °C, respectively. Table 7 shows thermal properties of different HBP-OH-ISPTES-SiO2 hybrid coatings. Therefore, considerable enhancement in the thermal stability at high temperature with the concentration of the SiO2 was achieved. The result shows that thermal resistance is enhanced with nanosilica, which might be due to the thermal insulation effect of nanosilica, as observed by Jeon et al.46,47 The strongest effect is obtained when the modified HBP (HBP-OH-ISPTES) is used as a matrix for 10 wt % SiO2. The temperature at which the coating HBP-OHISPTES-SiO2 (10%) obtained 55% weight loss is about 350 °C compared to 315 °C corresponding to HBP-OH-ISPTES (0%). The increasing residue at 550 °C with increasing silica content suggests the complete incorporation of the silica moiety in the modified HBP (HBP-OH-ISPTES) hybrid materials. The higher experimental residue is probably due to the trapping of the polymer matrix inside the inorganic moiety, giving higher residues at a higher temperature than expected.48
3.10. SEM Analysis of Hybrid Coatings. SEM observations showed the formation of a homogeneous, crackfree, and high compactibilization between organic and inorganic phases, which leads to good properties of coatings. Figure 9 (parts a, b, and c) indicated that a good interaction between organic and inorganic phases could be due to the formation of Si-O-C bond. We have observed that SiO2 particles were dispersed uniformly in HBP-OH-ISPTES-SiO2 (5%) when compared to HBP-OH-ISPTES- SiO2 (10%) films, where some aggregated silica particles were obtained. The aggregation of silica results in the formation of void in the hybrid and decreased the tensile strength. However, in our study, we observed a much lower amount of silica aggregated in HBP-OH-ISPTES-SiO2 (10%) films, so the properties were not affected much. The uniform dispersion might be due to HBP-OH forming covalent bonds with Si-OH groups of SiO2 in the condensation reaction, which prevents the agglomeration and shows good compactness between organic and inorganic phases. 4. Conclusions In the present study, transparent organic-inorganic hybrid coatings were prepared by hydroxyl-terminated hyperbranched polyester modified by ISPTES. These modified hybrid coatings were cross-linked with various amounts of SiO2. The synthesized modified polyester and hybrid coatings were characterized with NMR, FTIR, DMTA, UTM, and TGA. In these coatings, the interfacial interaction between inorganic particles and polymer matrix has a significant effect on the mechanical and thermal properties of the hybrid materials. The ESI-MASS spectrum analysis indicates the presence of cyclic products. FTIR suggests that the interaction between the organic and inorganic occurs through hydroxyl end groups of the modified hydroxylterminated polyester and the hydroxyl groups of the silanols. The observed data from DMTA analysis suggests that Tg increases with the SiO2 percentage variation for hybrid samples. The thermal decomposition profiles and the corresponding stability data in terms of characteristic decomposition temperature suggests that there are two stages of decomposition with increasing stability with increasing SiO2 content. TGA analysis also suggests that the residue content in the hybrid coatings at 550 °C increases with increasing SiO2 content. Overall, this study shows that the SiO2 content can have a significant effect on the mechanical and thermal properties of hybrid coatings. Tensile strength of the hybrid coating also demonstrated good
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9223
Figure 9. SEM results of HBP-OH-ISPTES-SiO2 (10%) and (5%) hybrid coatings: (a) 20 µm, (b) 10 µm, and (c) 50 µm.
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ReceiVed for reView June 5, 2008 ReVised manuscript receiVed September 23, 2008 Accepted September 23, 2008 IE800884Y
