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Enhancing the mechanical properties of an epoxy resin using polyhedral oligomeric silsesquioxane (POSS) as nano-reinforcement
Polymer Testing 62 (2017) 210e218
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Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Properties
Enhancing the mechanical properties of an epoxy resin using polyhedral oligomeric silsesquioxane (POSS) as nano-reinforcement Kunal Mishra a, Gajendra Pandey c, Raman P. Singh a, b, * a
School of Mechanical and Aerospace Engineering, College of Engineering, Architecture and Technology, Oklahoma State University, Stillwater, OK 74078, USA b School of Materials Science and Engineering, College of Engineering, Architecture and Technology, Oklahoma State University, Tulsa, OK 74106, USA c Center of Excellence for Advanced Modeling and Mimics (COE-AMM), Composites Solutions Business, Owens Corning Science Technology LLC, Granville, OH 43023, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 April 2017 Accepted 15 June 2017 Available online 5 July 2017
This paper reports on the mechanical and failure properties of epoxy reinforced by polyhedral oligomeric silsesquioxane (POSS), a hybrid organiceinorganic nanofiller. The unique properties of POSS-reinforced epoxy resin are exploited to overcome processing issues. POSS molecules with different organic substituents (methacryl, glycidyl, and trisilanol phenyl) were added to the epoxy resin by direct mixing at loadings between 0.5 and 8 wt%. It was observed that the addition of POSS significantly increases the fracture toughness and work of fracture of epoxy resin by an average of 130% and 400%, respectively, while no significant change was observed in the flexural modulus and strength. Scanning electron microscopy (SEM) was done on fractured surfaces to investigate evidence of extrinsic toughening mechanisms due to POSS inclusion. The chemical interaction of POSS with the epoxy resin was investigated using Fourier transform infrared (FTIR). Finally, differential scanning calorimetry (DSC) was used to measure the change in glass transition temperature (Tg ) resulting from inclusion of POSS. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Epoxy resins are widely used in various engineering applications due to their superior properties that include high glass transition temperature, high modulus, creep resistance and good resistance to chemicals [1,2]. These properties are due to the high degree of chemical cross-linking introduced by diglycidyl ether of bisphenols. Nonetheless, these high cross-linked densities also tend to make the resins brittle and vulnerable to fracture [3e7]. A considerable amount of work has been done to enhance the fracture properties of these resins by introducing a second phase as a reinforcement. Different types of filler, such as rubber particles [5,8], metal particles [4,9,10], layered silicates [11e13,15,16], carbon nanofibers [17,18], carbon nanotubes (CNT) [19], graphene [20] and block copolymers [21,22], and also a combination of multiple fillers [23], have been used with varying degrees of success.
* Corresponding author. School of Mechanical and Aerospace Engineering, College of Engineering, Architecture and Technology, Oklahoma State University, Stillwater, OK 74078, USA. E-mail address: [email protected] (R.P. Singh). http://dx.doi.org/10.1016/j.polymertesting.2017.06.031 0142-9418/© 2017 Elsevier Ltd. All rights reserved.
Conventionally, epoxy resins have been reinforced with micronsized fillers as they are most readily available. Nonetheless, micron sized fillers are challenging to use at low-volume fractions. They are more prone to settling which leads to non uniform distribution. They also tend to form agglomerates, especially at low percentage loading. To overcome these issues, there has been a great interest in recent years in the use of nanoparticles as reinforcements. Nanofillers, with very high surface-to-volume ratios, exhibit different properties from the bulk. This can lead to significant improvement in toughness even at lower weight fraction loadings. However, the dispersion and agglomeration of nanoscale particles is still a challenging issue. Fillers can be classified as organic or inorganic based on the type of material. In the case of toughening with organic materials, such as rubber particles and thermoplastics, energy is dissipated in the plastic zone near the crack tip through shear yielding [5,8]. Unfortunately, the use of such flexible organic materials decreases the modulus of the composite. When inorganic particles with high modulus are used as fillers, toughening is seen to occur via various mechanisms of energy dissipation without compromising the modulus. Examples of such mechanisms include crack pinning, crack bridging, crack path deflection and micro cracking [11e19].
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However, inorganic particles bundle up easily and lead to the formation of agglomerates at low loadings. This results in the deterioration of properties. In the past decade, due to the development of organic-inorganic hybrid nanoparticles, researchers have investigated the development of hybrid nanocomposites [27,32]. Such hybrid nanocomposites combine the advantages of inorganic materials (rigidity, high stability) and organic polymers (flexibility, reactivity and processability). Polyhedral oligomeric silsesquioxane (POSS) is one such hybrid material that possesses both organic and inorganic properties. The POSS molecule has a size of 1e3 nm and is made up of organiceinorganic monomer silsesquioxane (RSiO1.5). The POSS molecule possesses an inorganic rigid cage type structure containing silicon and oxygen, and selectable organic groups (R) attached to the silicon atoms. The degree of compatibility and reactivity of POSS can be tailored by designing the chemical structure of the organic groups. In addition, chemical structure of these organic groups can be either reactive or non-reactive. Therefore, POSS molecules can interact with an epoxy resin in two ways. If the organic group contains reactive functionalities, then the POSS reacts with the epoxy polymer chain forming covalent bonds. For non-reactive groups, POSS can show compatibility either by similarities in chemical structure or by specific polar interaction between the POSS molecule and the epoxy polymer chain. The dispersion of POSS in an epoxy resin is dependent on the interaction of the organic group (R) with the resin. This dispersion can occur at a molecular level (reaction between POSS and epoxy resin) or at micro level (non-reactive compatibility). This dual nature of POSS, along with fact that it is a nano-sized filler, motivates its selection as a novel reinforcement for epoxy resins. Various studies have been done on the characterization of POSS reinforced materials. These have included the study of the formation of nanoscale structures, the synthesis of novel POSS molecules with different functionalities, and the enhancement of physical properties such as thermal degradation and glass transition temperature [34e43]. However, limited work has been reported on the effects of POSS reinforcements on the mechanical properties and fracture toughness of polymers. Therefore, the objective of this paper is to investigate the reinforcement of epoxy by POSS and study the effects on mechanical properties and fracture toughness.
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compatibility and reactivity with epoxy resins. Mixing of the EPON 862 epoxy resin and POSS was carried out with a magnetic stirrer at 70 C and 200 rpm for 12 h. The mixture was then cooled to room temperature and a stoichiometric amount of an amine-based curing agent (EPIKURE 3274) was added to the epoxy resin. This was followed by mixing for 10 min. Subsequently, the mixture was placed in a vacuum chamber for 30 min for degassing. The degassed mixture was poured into a pre-prepared mold for casting and cured at room temperature for 24 h. The cast plate was then released from the mold and post cured at 121 C for 6 h. The same procedure was used for preparing neat resin samples for comparison. Materials were prepared with all three types of POSS at loading levels of 0.5%, 1%, 3%, 5% and 8%. 2.2. Characterization of mechanical properties Fracture toughness was determined using single edge notch bend testing as per ASTM De5045 [48]. Samples were machined from the cast plate with nominal dimensions of 54.0 12.7 6.3 mm. A 4.5 mm deep notch was cut using a diamond precision saw, and then the tip of the notch was tapped lightly with a razor blade to initiate a natural pre-crack. The nominal crack length was kept between 5.7 and 6.9 mm to maintain a a/W ratio of 0.45e0.55, as per the ASTM standard. The precracked single edge notch specimens were loaded under threepoint bending using a universal testing machine (Instron 5567, Norwood, MA). Tests were performed in displacement-controlled mode at a fixed crosshead speed of 0.5 mm/min until the point of specimen failure. Failure was identified by reaching a peak value of applied load. The fracture toughness of the nanocomposites was then measured in terms of the critical stress intensity factor (KIC) calculated from the peak load. Flexural experiments were performed with the specimen loaded under displacement controlled mode from loadedisplacement data. Samples were machined from the cast plate with nominal dimensions of 110.0 25.4 6.3 mm. Specimens were loaded until failure, the point of failure being identified from the peak load. Flexural modulus and flexural strength were determined using three-point bend testing according to ASTM De790 [49]. The flexural modulus was calculated from the initial linear portion of the load displacement curve and flexural strength determined from peak load.
2. Experimental 2.3. Characterization of physical and chemical properties 2.1. Sample preparation The polymer system used in this work consists of diglycidyl ether of bisphenol-F based resin (EPON 862®, Hexion Specialty Chemicals, Columbus, OH) that is cured with a low viscous aliphatic amine (EPIKURE 3274® Hexion Specialty Chemicals, Columbus, OH). Both the resin and the curing agent were purchased from Miller-Stephenson Chemical Company, (Danbury, CT). This epoxy resin system is a highly cross-linked network that results in superior mechanical properties and high chemical resistance. Three different functionalities of POSS were used in this work, namely, trisilanol phenyl, methacryl and glycidyl POSS. These were purchased from Hybrid Plastics (Hattiesburg, MS). The schematic diagrams of the three types of POSS molecules, as shown in Fig. 1, illustrate the different functionality. These specific variants of POSS were selected based on their interaction with the epoxy resin. Trisilanol phenyl POSS is compatible with epoxy resins due to hydrogen bonding and p p interactions. For methacryl POSS the methacrylate group can be incorporated into the network by reacting with epoxy resins forming covalent bonds. Glycidyl POSS can exhibit both
Thermal analysis of samples was carried out using a differential scanning calorimeter (Q 2000 DSC, TA Instruments, Inc., New Castle, DE). The instrument was calibrated by employing the temperature and heat of fusion of high purity Indium standard. Also, the thermal histories of all the nanocomposites were the same prior to each experiment. Approximately 5e10 mg of sample was loaded into an aluminum pan. To study the glass transition temperature (Tg ), the samples were first heated to 210 C at a scan rate of 10 C/ min, held at that temperature for 2 min to eliminate any previous thermal history, and then cooled to 25 C at a rate of 10 C/min. For the second scan, the samples were heated and cooled under the same conditions. The data was extracted from second heating cycle. The experiment was carried out in under constant nitrogen flow (flow rate 50 ml min1). The fractured surface was studied using field emission scanning electron microscopy (Hitachi S-4800 FESEM, Dallas, TX). Prior to imaging, the surfaces of the samples were coated with goldplatinum by an electro-deposition method using a sputter coater (Cressington Scientific Instruments Ltd, Redding, CA). This was done to make the sample conductive to prevent the accumulation
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Fig. 1. Molecular structure of the three types of POSS molecules used in this study (a) trisilanol phenyl, (b) methacryl and (c) glycidyl POSS.
of static electric charge during scanning electron microscopy. Transmission electron microscopy (TEM) was used to identify and characterize self-assembly phenomena in POSS-epoxy nanocomposites. All samples were ultramicrotomed (Reichert-Jung Ultracut E, Dickinson, TX) to 70 nm thicknesses using a diamond knife. Slices were mounted on amorphous-carbon-coated copper TEM grids. A JEOL JEM-100CX II (Peabody, MA) transmission electron microscope was used at 80e100 kV. Finally, Fourier transform infrared spectroscopy (FTIR) was performed to investigate chemical inter-actions between POSS and epoxy. The materials were powdered using a grinder under liquid nitrogen. These powdered samples were loaded into an FTIR spectrometer (Nicolet iS10, Waltham, MA) and subjected to 128 scans at a resolution of 4.0 cm1. The spectrum of interest was collected at room temperature between 4000 and 500 cm1. 3. Results and discussion 3.1. Mechanical properties Fig. 2 shows the variation of fracture toughness as a function of POSS loading. As described earlier, the fracture toughness was measured in terms of the value of the critical stress intensity factor at the initiation of crack growth. The error bars in the figure denote
Fig. 2. Fracture toughness value in terms of critical stress intensity factor (KIc) for different loading of POSS of different functional group.
the statistical spread of experimental data in terms of the standard deviation. From the graph it is apparent that the addition of POSS leads to an increase in the fracture toughness. In all three cases, after reaching a maximum value, the fracture toughness starts to decrease on further addition of POSS. This could be due to the agglomeration of POSS particles at higher loading that offset the positive effect of POSS reinforcement. There are two factors that govern the change in the fracture toughness. First, is the organic functionality present on the POSS molecule. Second, is the weight fraction of POSS added to the epoxy resin. Among the three types POSS used, glyicidyl POSSeepoxy nanocomposite showed maximum fracture toughness as compared to reinforcement by the other two types of POSS molecules. In the case of reinforcement by glycidyl POSS, the fracture toughness increased by a factor of 2.3 at 5 wt % loading, while for methacryl and trisilanol phenyl POSS at 5 wt % loading the fracture toughness increased by factors of 1.6 and 1.3, respectively. Fig. 3 shows the typical loadedisplacement curves obtained from fracture testing for the neat epoxy and POSSeepoxy nanocomposites (at 5 wt % loading). Neat epoxy, methacryl POSSe epoxy and trisilanol phenyl POSSeepoxy exhibited brittle failure, as shown by the sharp peak in the load-displacement curve. On the other hand, the glycidyl POSSeepoxy nanocomposite underwent ductile failure, as shown from the loadedisplacement graph. This difference in failure mode suggests that the addition of glycidyl POSS leads to stable crack
Fig. 3. Load displacement curve for neat resin and POSSeepoxy nanocomposites.
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growth, unlike neat epoxy or reinforcement by the other types of POSS. All the specimens selected for this analysis had similar initial crack lengths. In general, for similar crack lengths, POSS reinforced nanocomposite exhibited higher peak load. This observation is very well reflected in value of the critical stress intensity factor. Nonetheless, the glycidyl POSS reinforced nanocomposite exhibited slow crack growth which suggests that more energy was dissipated during fracture. To further quantify the toughening due to POSS inclusion, the work of fracture was determined. The total area under the loadedisplacement curve was normalized with the total fracture surface area created, and the results are shown in Fig. 4. Fig. 5 illustrates that more work was done to fracture POSSeepoxy nanocomposite as compared to neat epoxy. Among the three types of POSS used, the glycidyl POSS showed the highest value of the work of fracture. This observation correlates with the fracture toughness value obtained. This increase in the work of fracture suggests that reinforcement with POSS changes the basic toughening mechanisms. When POSS molecules are incorporated in an epoxy network, the rigid silica cage will provide toughening from crack-pinning, crack-bridging and shear-yielding. Also, the organic part of POSS molecules can interact with epoxy for better dispersion in the polymer network and also provide flexibility in polymer chain by increasing free volume in the network [43e45]. This increase in the flexibility of polymer chains can cause the failure mode to change from brittle to ductile. This change in the failure mode does not depend only on different functionality of POSS, as observed in Fig. 4. Increase in POSS loading in the epoxy resin also plays an important role in the change in failure mode. As observed from Fig. 5, the loadedisplacement the failure mode changes from brittle to ductile as a function of POSS loading. For 0.5 wt % and 1 wt % loading, the failure is brittle, and beyond 1 wt % loading the failure mode becomes ductile along with a significant increase in the peak failure load. Similar behavior was observed for the flexural response of POSSeepoxy nanocomposites. Flexural modulus was calculated from linear portion of loadedisplacement data from three-pointbend experiments. As shown in Fig. 6, the addition of methacryl or trisilanol phenyl POSS did not lead to significant changes in the flexural modulus. In addition, for the inclusion of glycidyl POSS,
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Fig. 5. Loadedisplacement curve for different percentages of glycidyl POSS reinforced epoxy resin.
Fig. 6. Flexural modulus of POSSeepoxy nanocomposites with different POSS content.
Fig. 4. Work of fracture of neat resin and POSSeepoxy nanocomposites.
there was no change in flexural modulus for 0.5 wt% and 1 wt% POSS loading. For higher loading by glycidyl POSS the flexural modulus decreased linearly. This change in transition of flexural properties of glycidyl POSS reinforced nanocomposites corroborates with results from fracture experiments. This change in the failure mode (and flexural response) with the inclusion of glycidyl POSS can be explained in terms of selfassembly of POSS due to POSSePOSS interaction at higher loadings. Various studies have showed that POSS tends to form selfassembly aggregates in the form of spherical, cylinders and vesicles morphologies [25e31]. Fig. 7 shows a schematic representation of self aggregation of POSS at higher loading. The dependence of these morphologies depends on the type of processing conditions. The formation of self-aggregates of POSS leads to a soft core that acts as a plasticizer, just like conventional organic filler. In the case of glycidyl POSS at lower loading, POSS molecules acts as rigid particles. Meanwhile, at higher loadings the formation of soft-shell domains causes an increase in flexibility in the polymer network
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Fig. 7. Dispersion of glycidyl POSS in the polymer at (a) low loading versus (b) at higher loading. POSS form soft-shell domains at high loading.
Fig. 8. TEM image of 1 wt% and 8 wt% glycidyl POSS reinforced nanocomposite showing the self assembly phenomenon.
chains. This causes the failure mode to change from brittle to ductile and the flexural modulus to decrease. The self assembly phenomenon is further supported by transmission electron microscopy images, as shown in Fig. 8 for epoxy reinforced with 1 wt % and 8 wt%. glycidyl POSS. The dark areas represent heavy silicon atoms while white areas represent epoxy resin. For low weight percent POSS reinforcement, no phase separation is observed, which suggests proper dispersion of POSS in the epoxy resin. However, at higher weight percent loading POSS tends to form self-assembled aggregates soft-core shell structures. Finally, Fig. 9 shows that there is not much change in the value of flexural strength with POSS loading. This implies that formation of
Table 1 Glass transition temperature of epoxy reinforces with different types of POSS at 5 wt % each.
Fig. 9. Flexural strength of POSSeepoxy nanocomposites with different POSS content.
Material
Glass transition temperature ( C)
Neat Resin Trisilanol phenyl POSSeepoxy Methacryl POSSeepoxy Glycidyl POSSeepoxy
81.1 87.9 80.2 72.5
K. Mishra et al. / Polymer Testing 62 (2017) 210e218 Table 2 Glass transition temperature of different wt.% of glycidyl POSS reinforced nanocomposites. Material
Glass transition temperature ( C)
Neat Resin 0.5 wt% POSSeepoxy 1 wt% POSSeepoxy 3 wt% POSSeepoxy 5 wt% POSSeepoxy 8 wt% POSSeepoxy
81.1 82.3 82.5 76.9 72.1 67.3
Fig. 10. Glass transition temperatures measure in DSC for glycidyl POSS-epoxy nanocomposites. The dotted line represents the prediction of the Fox equation.
softeshell domains, due to POSS self-assembly, does not affect the maximum load carrying capacity of the epoxyePOSS nanocomposite.
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3.2. DSC analysis Differential scanning calorimetry (DSC) was performed to further understand chain flexibility, crosslinking of chains and intermolecular interactions between POSS and epoxy. Soto et al. showed that, depending on nature of POSS and its degree of dispersion in a polymer, POSS molecules could either act as a plasticizer or an anti-plasticizer [32]. Similar observations were found in the current investigation for the three types of POSS investigated. Table 1 shows the variation of glass transition temperature (Tg ) of neat epoxy and POSS reinforced nanocomposite for 5 wt % POSS reinforcement. The inclusion of glycidyl and methacryl POSS decreased the Tg of the nanocomposite while the addition of trisilanol phenyl POSS increased the Tg . For trisilanol phenyl POSSeepoxy nanocomposite, Tg increased with the incorporation of POSS, Fu et al. [33] reported a similar result with trisilanol phenyl POSS. This might be due to the restriction of the chain mobility because of steric hindrance provided by the big phenyl groups. The decrease in Tg for glycidyl and methacryl POSS was due to plasticization effect cause by increase in the polymer chain flexibility. This increase in the chain flexibility is due to the increase in free volume with increase inclusion of POSS. A similar observation was also reported in other studies [43e45]. As observed in fracture and flexural results, the properties of POSS nanocomposites depend not only on the functionality of POSS but also on the POSS loading in the epoxy resin. Therefore, in order to understand effect of POSS loading, DSC experiments were also performed on glycidyl POSS-epoxy nanocomposites for different loading levels. Table 2 shows the DSC thermogram of glycidyl POSSeepoxy nanocomposites at different loading. It is observed that at lower loading of glycidyl POSS there is increase in the glass transition temperature but with increase in loading Tg decreases. This change in the trend of glass transition temperature with the inclusion of glycidyl POSS supports the self-assembly aggregation hypothesis.
Fig. 11. SEM images of fractured surface of (a) Neat resin (b) trisilanol phenyl POSSeepoxy (c) methacryl POSSeepoxy and (d) glycidyl POSSeepoxy nanocomposite.
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Glycidyl POSS acts as anti-plasticizer at low weight percentage. There is an increase in Tg and at higher loading of POSS due to the formation of soft core structure because of the self assembly of POSS molecules. Various studies have used the Fox equation to determine change in the glass transition temperature with inclusion of plasticizer such as conventional organic filler [46]. They found that addition of plasticizer increases the free volume in molecular network, and hence decreases Tg . A comparison of the change in the experimental value of Tg with inclusion of POSS with the theoretical Foxmodel, shown below, is plotted in Fig. 10.
1 4 ð1 4Þ ¼ þ Tg T ðglycidyl POSSÞ T ðepoxyÞ g g ðglycidyl POSSÞ
ðepoxyÞ
where Tg and Tg are the glass transition temperatures of glycidyl POSS (57 C) and epoxy resin (81.1 C), respectively. From Fig. 10, it can be observed that the experimental value of Tg for glycidyl POSSeepoxy nanocomposite does not follow the Fox equation. This suggests that glycidyl POSS can act as either a plasticizer or an anti-plasticizer depending on loading percentage in epoxy resin.
3.3. SEM analysis The fractured surfaces of neat epoxy and 5 wt% of POSSeepoxy nanocomposites were examined by scanning electron microscopy (SEM). Micrographs of the fractured surfaces are shown in Fig. 11(a)e(d). No phase separation was observed in Fig. 11(b), (c) and (d), indicating that good dispersion of POSS was achieved. The smooth and featureless surface, as shown in Fig. 11(a) for neat epoxy resin, gives evidence of its brittle failure and low fracture toughness. For the case of POSSeepoxy nanocomposites, the fractured surfaces were broken into irregular and rough surfaces. These rough surfaces indicate greater resistance against crack propagation that contributes towards the enhancement of fracture toughness. Toughness in the nanocomposites can due to extrinsic toughening mechanisms such as particle bridging, shear yielding and crack pinning. For trisilanol phenyl POSS reinforced nanocomposites (Fig. 11(b)), the fractured surface shows rougher surface compared to neat epoxy resin. For methacryl and glycidyl POSS reinforced nanocomposites (Fig. 11(c) and (d)), extensive shear yielding was observed. This shear was more prominent for glycidyl POSS epoxy nanocomposites, which supports the ductile failure as observed from fracture experiments. In addition to shear yielding,
Fig. 12. SEM images of the fracture surfaces of glycidyl POSSeepoxy nanocomposite at different wt.%.
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particle bridging was also observed on the fracture surface. Particle bridging caused the micron sized holes that might be due to the pull out of POSS particles from the epoxy matrix, as observed in SEM figure. Our previous results showed that both mechanical and thermal properties of POSSeepoxy nanocomposites, not only depend on the type of organic groups present in POSS, but also on the amount of loading in the epoxy resin. Similar observations were found in fractured images for glycidyl POSSeepoxy nanocomposites, as shown in Fig. 12. It is observed that, with increase in the POSS loading, the fracture behavior changes. For low wt.%, the surface shows brittle failure and, with increase in loading, shear yielding becomes predominant. 3.4. Fourier transform infrared analysis FTIR experiments were performed to study POSSeepoxy interactions. Fig. 13 shows the spectra of neat resin and 5 wt% of POSSeepoxy nanocomposite. The FTIR spectra indicated good dispersion. In addition, the broad peak of alcohol around 3500e3200 cm1 (1) suggests polymerization of the epoxy resin. Peaks around 1024 cm1 (3) represent epoxy groups and, from the spectra, it is observed that epoxy groups are well polymerized during curing. Epoxy reinforced with glycidyl POSS (that has epoxide groups attached to the silica cage) also exhibited complete polymerization as no peaks were observed around 1024 cm1 (3) to indicate uncured epoxy. The Si-O-Si stretching peak is masked by epoxide peak at 1024 cm1 and not showing any distinctive peak, which shows that POSS was very well consumed in the epoxy resin. For methacryl POSSeepoxy nanocomposite, a carbonyl peak around 1750 cm1 (2) is present, which corresponds to the stretching vibration of carboneoxygen double bond, suggesting that some of the carbonyl group present on silica cage is left unreacted. For trisilanol phenyl POSS-epoxy nanocomposite, a peak
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around 690 cm1 (4) evidenced existence of residual epoxy left after complete polymerization. 4. Conclusions Various functionalities of POSS were used as nanoparticle reinforcements to modify the mechanical properties of epoxy resin. Increase in fracture toughness was observed with the incorporation of POSS. For the 5 wt % glycidyl POSSeepoxy nanocomposite, fracture toughness increased by a factor of 2.3 as compared to the neat epoxy resin. Similar trends of increase in tough ness were observed for trisilanol phenyl POSS and methacryl POSS reinforced nanocomposites. Furthermore, from the loadedisplacement data, it was observed that, for the case of glycidyl POSS reinforcement, the failure mode changes from brittle to ductile as the POSS loading is increased beyond 1 wt%. This change in failure behavior is due to the formation of soft shell core structure as a result of self assembly of POSS molecules. The presence of these structures was identified using TEM imaging. Formation of soft shell core structures leads to decrease in the flexural modulus value of nanocomposites with increase in glycidyl POSS loading, whereas for methacryl and trisilanol phenyl POSS not much variation in the flexural modulus was observed. DSC experiments show that glass transition temperature of nanocomposite depends on two factor: first, the type of organic group present in POSS and second, amount of POSS loading in the epoxy resin. Depending on the type of organic group, Tg either increased or decreased by either obstructing or aiding the movement of polymer chain at higher temperature. We observed that, for glycidyl POSSeepoxy nanocomposite, Tg first increased at low loading then started to decrease at higher loading beyond 1 wt%. This change in glass transition temperature was compared with the Fox equation. The results support the formation of soft shell core structure at higher loading of glycidyl POSS. Finally, FTIR spectroscopy showed that there are good interactions between epoxy
Fig. 13. FTIR spectra of (a) Neat resin, and 5 wt% of (b) trisilanol phenyl POSSeepoxy, (c) methacryl POSSeepoxy and (d) glycidyl POSSeepoxy nanocomposite. Peaks indicated by (1) represents alcoholic group, (2) represents carbonyl group, (3) and (4) represents epoxide group.
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and POSS. Micrographs of fractured surfaces explained the increase in fracture toughness from various toughening mechanisms with the inclusion of POSS. Micrograph also showed evidence of change in the failure mode from brittle to ductile with increase in glycidyl POSS loading in epoxy resin. Acknowledgement We gratefully acknowledge that this work is funded in part or fully by a grant through the Oklahoma Nanotechnology Applications Project (ONAP) (Grant no. O9-20) and NASA Experimental Program to Stimulate Competitive Research (EPSCOR) (Grant no. NNXO9AP68A). References [1] B. Ellis, Chemistry and Technology of Epoxy Resin, Blackie academic professional, New York, NY, 1993. [2] L. In, Epoxy Resin and Composites II, No. 75, Adv. polymer science, Berlin, 1986. [3] Q. Chen, I. Chariots, C. Chen, A. Roy, Nanoscale and effective mechanical behavior and fracture of silica nanocomposites, Compos. Sci. Technol. 68 (2008) 3137e3144. [4] S.C. Zunjarrao, R.P. Singh, Characterization of the fracture behavior of epoxy reinforced with nanometer and micrometer sized aluminum particles, Compos. Sci. Technol. 66 (2006) 2296e2305. [5] R. Pearson, A. Yee, Toughening mechanisms in elastomer-modified epoxies, J. Mater. Sci. 24 (1989) 2571e2580. [6] D.S. Parker, H.J. Sue, J. Huang, A.F. Yee, Toughening mechanisms in core shell rubber modified polycarbonate, Polymer 31 (1990) 2267e2277. [7] J. Qian, R. Pearson, V. Dimonie, O. Shaffer, M. Elaasser, The role of dispersed phase morphology on toughening of epoxies, Polymer 38 (1997) 21e30. [8] R. Bagheri, R. Pearson, Role of particle cavitation in rubber-toughened epoxies, Polymer 37 (1996) 4529e4538. [9] B. Wetzel, P. Rosso, F. Haupert, K. Friedrich, Epoxy nanocomposites: fracture and toughening mechanisms, Eng. Fract. Mech. 73 (2006) 2375e2398. [10] R. Singh, M. Zhang, D. Chan, Toughening of a brittle thermosetting polymer: effects of reinforcement particle size and volume fraction, J. Mater. Sci. 37 (2002) 781e788. [11] B. Qi, Q.X. Zhang, M. Bannister, Y.W. Mai, Investigation of the mechanical properties of DGEBA-based epoxy resin with nanoclay additives, Compos. Struct. 75 (2006) 514e519. [12] G. Ragosta, M. Abbate, P. Musto, G. Scarinzi, L. Mascia, Epoxy-silica particulate nanocomposites: chemical interactions, reinforcement and fracture toughness, Polymer 46 (2005) 10506e10516. [13] M. Imanaka, Y. Takeuchi, Y. Nakamura, A. Nishimura, T. Iida, Fracture toughness of spherical silica-filled epoxy adhesives, Int. J. Adhes. Adhes. 21 (2001) 389e396. [14] Y. Nakamura, S. Okabe, T. Iida, Effects of particle shape, size and interfacial adhesion on the fracture strength of silica-filled epoxy resin, Polymer 7 (1999) 177e186. [15] B.T. Marouf, R. Bagheri, R.A. Pearson, Mechanical and thermal properties of montmorillonite-epoxy nanocomposite, Int. J. Mod. Phys. B 22 (2008) 3247e3253. [16] S.C. Zunjarrao, R. Sriraman, R.P. Singh, Effect of processing parameters and clay volume fraction on the mechanical properties of epoxy-clay nanocomposites, J. Mater. Sci. 41 (2006) 2219e2228. [17] A.T. Seyhan, Z. Sun, J. Deitzel, M. Tanoglu, D. Heider, Cure kinetics of vapor grown carbon nanofiber (vgcnf) modified epoxy resin suspensions and fracture toughness of their resulting nanocomposites, Mater. Chem. Phys. 118 (2009) 234e242. [18] Y.X. Zhou, F. Pervin, S. Jeelani, S.K. Mallick, Improvement in mechanical properties of carbon fabric-epoxy composite using carbon nanofibers, J. Mater. Process. Technol. 198 (2008) 445e453. [19] V.K. Rangari, M.S. Bhuyan, S. Jeelani, Microwave processing and characterization of epon 862/cnt nanocomposites, Mater. Sci. Eng. B-Adv. Funct. SolidState Mater. 168 (2010) 117e121. [20] M.A. Rafiee, J. Rafiee, Z. Wang, H.H. Song, Z.Z. Yu, N. Koratkar, Enhanced mechanical properties of nanocomposites at low graphene content, ACS Nano 3 (2009) 3884e3890. [21] L. Ruiz-Perez, G.J. Royston, J.P.A. Fairclough, A.J. Ryan, Toughening by nanostructure, Polymer 49 (2008) 4475e4488. [22] R.M. Hydro, R.A. Pearson, Epoxies toughened with triblock copolymers, J. ff
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Material Properties
Enhancing the mechanical properties of an epoxy resin using polyhedral oligomeric silsesquioxane (POSS) as nano-reinforcement Kunal Mishra a, Gajendra Pandey c, Raman P. Singh a, b, * a
School of Mechanical and Aerospace Engineering, College of Engineering, Architecture and Technology, Oklahoma State University, Stillwater, OK 74078, USA b School of Materials Science and Engineering, College of Engineering, Architecture and Technology, Oklahoma State University, Tulsa, OK 74106, USA c Center of Excellence for Advanced Modeling and Mimics (COE-AMM), Composites Solutions Business, Owens Corning Science Technology LLC, Granville, OH 43023, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 April 2017 Accepted 15 June 2017 Available online 5 July 2017
This paper reports on the mechanical and failure properties of epoxy reinforced by polyhedral oligomeric silsesquioxane (POSS), a hybrid organiceinorganic nanofiller. The unique properties of POSS-reinforced epoxy resin are exploited to overcome processing issues. POSS molecules with different organic substituents (methacryl, glycidyl, and trisilanol phenyl) were added to the epoxy resin by direct mixing at loadings between 0.5 and 8 wt%. It was observed that the addition of POSS significantly increases the fracture toughness and work of fracture of epoxy resin by an average of 130% and 400%, respectively, while no significant change was observed in the flexural modulus and strength. Scanning electron microscopy (SEM) was done on fractured surfaces to investigate evidence of extrinsic toughening mechanisms due to POSS inclusion. The chemical interaction of POSS with the epoxy resin was investigated using Fourier transform infrared (FTIR). Finally, differential scanning calorimetry (DSC) was used to measure the change in glass transition temperature (Tg ) resulting from inclusion of POSS. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Epoxy resins are widely used in various engineering applications due to their superior properties that include high glass transition temperature, high modulus, creep resistance and good resistance to chemicals [1,2]. These properties are due to the high degree of chemical cross-linking introduced by diglycidyl ether of bisphenols. Nonetheless, these high cross-linked densities also tend to make the resins brittle and vulnerable to fracture [3e7]. A considerable amount of work has been done to enhance the fracture properties of these resins by introducing a second phase as a reinforcement. Different types of filler, such as rubber particles [5,8], metal particles [4,9,10], layered silicates [11e13,15,16], carbon nanofibers [17,18], carbon nanotubes (CNT) [19], graphene [20] and block copolymers [21,22], and also a combination of multiple fillers [23], have been used with varying degrees of success.
* Corresponding author. School of Mechanical and Aerospace Engineering, College of Engineering, Architecture and Technology, Oklahoma State University, Stillwater, OK 74078, USA. E-mail address: [email protected] (R.P. Singh). http://dx.doi.org/10.1016/j.polymertesting.2017.06.031 0142-9418/© 2017 Elsevier Ltd. All rights reserved.
Conventionally, epoxy resins have been reinforced with micronsized fillers as they are most readily available. Nonetheless, micron sized fillers are challenging to use at low-volume fractions. They are more prone to settling which leads to non uniform distribution. They also tend to form agglomerates, especially at low percentage loading. To overcome these issues, there has been a great interest in recent years in the use of nanoparticles as reinforcements. Nanofillers, with very high surface-to-volume ratios, exhibit different properties from the bulk. This can lead to significant improvement in toughness even at lower weight fraction loadings. However, the dispersion and agglomeration of nanoscale particles is still a challenging issue. Fillers can be classified as organic or inorganic based on the type of material. In the case of toughening with organic materials, such as rubber particles and thermoplastics, energy is dissipated in the plastic zone near the crack tip through shear yielding [5,8]. Unfortunately, the use of such flexible organic materials decreases the modulus of the composite. When inorganic particles with high modulus are used as fillers, toughening is seen to occur via various mechanisms of energy dissipation without compromising the modulus. Examples of such mechanisms include crack pinning, crack bridging, crack path deflection and micro cracking [11e19].
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However, inorganic particles bundle up easily and lead to the formation of agglomerates at low loadings. This results in the deterioration of properties. In the past decade, due to the development of organic-inorganic hybrid nanoparticles, researchers have investigated the development of hybrid nanocomposites [27,32]. Such hybrid nanocomposites combine the advantages of inorganic materials (rigidity, high stability) and organic polymers (flexibility, reactivity and processability). Polyhedral oligomeric silsesquioxane (POSS) is one such hybrid material that possesses both organic and inorganic properties. The POSS molecule has a size of 1e3 nm and is made up of organiceinorganic monomer silsesquioxane (RSiO1.5). The POSS molecule possesses an inorganic rigid cage type structure containing silicon and oxygen, and selectable organic groups (R) attached to the silicon atoms. The degree of compatibility and reactivity of POSS can be tailored by designing the chemical structure of the organic groups. In addition, chemical structure of these organic groups can be either reactive or non-reactive. Therefore, POSS molecules can interact with an epoxy resin in two ways. If the organic group contains reactive functionalities, then the POSS reacts with the epoxy polymer chain forming covalent bonds. For non-reactive groups, POSS can show compatibility either by similarities in chemical structure or by specific polar interaction between the POSS molecule and the epoxy polymer chain. The dispersion of POSS in an epoxy resin is dependent on the interaction of the organic group (R) with the resin. This dispersion can occur at a molecular level (reaction between POSS and epoxy resin) or at micro level (non-reactive compatibility). This dual nature of POSS, along with fact that it is a nano-sized filler, motivates its selection as a novel reinforcement for epoxy resins. Various studies have been done on the characterization of POSS reinforced materials. These have included the study of the formation of nanoscale structures, the synthesis of novel POSS molecules with different functionalities, and the enhancement of physical properties such as thermal degradation and glass transition temperature [34e43]. However, limited work has been reported on the effects of POSS reinforcements on the mechanical properties and fracture toughness of polymers. Therefore, the objective of this paper is to investigate the reinforcement of epoxy by POSS and study the effects on mechanical properties and fracture toughness.
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compatibility and reactivity with epoxy resins. Mixing of the EPON 862 epoxy resin and POSS was carried out with a magnetic stirrer at 70 C and 200 rpm for 12 h. The mixture was then cooled to room temperature and a stoichiometric amount of an amine-based curing agent (EPIKURE 3274) was added to the epoxy resin. This was followed by mixing for 10 min. Subsequently, the mixture was placed in a vacuum chamber for 30 min for degassing. The degassed mixture was poured into a pre-prepared mold for casting and cured at room temperature for 24 h. The cast plate was then released from the mold and post cured at 121 C for 6 h. The same procedure was used for preparing neat resin samples for comparison. Materials were prepared with all three types of POSS at loading levels of 0.5%, 1%, 3%, 5% and 8%. 2.2. Characterization of mechanical properties Fracture toughness was determined using single edge notch bend testing as per ASTM De5045 [48]. Samples were machined from the cast plate with nominal dimensions of 54.0 12.7 6.3 mm. A 4.5 mm deep notch was cut using a diamond precision saw, and then the tip of the notch was tapped lightly with a razor blade to initiate a natural pre-crack. The nominal crack length was kept between 5.7 and 6.9 mm to maintain a a/W ratio of 0.45e0.55, as per the ASTM standard. The precracked single edge notch specimens were loaded under threepoint bending using a universal testing machine (Instron 5567, Norwood, MA). Tests were performed in displacement-controlled mode at a fixed crosshead speed of 0.5 mm/min until the point of specimen failure. Failure was identified by reaching a peak value of applied load. The fracture toughness of the nanocomposites was then measured in terms of the critical stress intensity factor (KIC) calculated from the peak load. Flexural experiments were performed with the specimen loaded under displacement controlled mode from loadedisplacement data. Samples were machined from the cast plate with nominal dimensions of 110.0 25.4 6.3 mm. Specimens were loaded until failure, the point of failure being identified from the peak load. Flexural modulus and flexural strength were determined using three-point bend testing according to ASTM De790 [49]. The flexural modulus was calculated from the initial linear portion of the load displacement curve and flexural strength determined from peak load.
2. Experimental 2.3. Characterization of physical and chemical properties 2.1. Sample preparation The polymer system used in this work consists of diglycidyl ether of bisphenol-F based resin (EPON 862®, Hexion Specialty Chemicals, Columbus, OH) that is cured with a low viscous aliphatic amine (EPIKURE 3274® Hexion Specialty Chemicals, Columbus, OH). Both the resin and the curing agent were purchased from Miller-Stephenson Chemical Company, (Danbury, CT). This epoxy resin system is a highly cross-linked network that results in superior mechanical properties and high chemical resistance. Three different functionalities of POSS were used in this work, namely, trisilanol phenyl, methacryl and glycidyl POSS. These were purchased from Hybrid Plastics (Hattiesburg, MS). The schematic diagrams of the three types of POSS molecules, as shown in Fig. 1, illustrate the different functionality. These specific variants of POSS were selected based on their interaction with the epoxy resin. Trisilanol phenyl POSS is compatible with epoxy resins due to hydrogen bonding and p p interactions. For methacryl POSS the methacrylate group can be incorporated into the network by reacting with epoxy resins forming covalent bonds. Glycidyl POSS can exhibit both
Thermal analysis of samples was carried out using a differential scanning calorimeter (Q 2000 DSC, TA Instruments, Inc., New Castle, DE). The instrument was calibrated by employing the temperature and heat of fusion of high purity Indium standard. Also, the thermal histories of all the nanocomposites were the same prior to each experiment. Approximately 5e10 mg of sample was loaded into an aluminum pan. To study the glass transition temperature (Tg ), the samples were first heated to 210 C at a scan rate of 10 C/ min, held at that temperature for 2 min to eliminate any previous thermal history, and then cooled to 25 C at a rate of 10 C/min. For the second scan, the samples were heated and cooled under the same conditions. The data was extracted from second heating cycle. The experiment was carried out in under constant nitrogen flow (flow rate 50 ml min1). The fractured surface was studied using field emission scanning electron microscopy (Hitachi S-4800 FESEM, Dallas, TX). Prior to imaging, the surfaces of the samples were coated with goldplatinum by an electro-deposition method using a sputter coater (Cressington Scientific Instruments Ltd, Redding, CA). This was done to make the sample conductive to prevent the accumulation
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Fig. 1. Molecular structure of the three types of POSS molecules used in this study (a) trisilanol phenyl, (b) methacryl and (c) glycidyl POSS.
of static electric charge during scanning electron microscopy. Transmission electron microscopy (TEM) was used to identify and characterize self-assembly phenomena in POSS-epoxy nanocomposites. All samples were ultramicrotomed (Reichert-Jung Ultracut E, Dickinson, TX) to 70 nm thicknesses using a diamond knife. Slices were mounted on amorphous-carbon-coated copper TEM grids. A JEOL JEM-100CX II (Peabody, MA) transmission electron microscope was used at 80e100 kV. Finally, Fourier transform infrared spectroscopy (FTIR) was performed to investigate chemical inter-actions between POSS and epoxy. The materials were powdered using a grinder under liquid nitrogen. These powdered samples were loaded into an FTIR spectrometer (Nicolet iS10, Waltham, MA) and subjected to 128 scans at a resolution of 4.0 cm1. The spectrum of interest was collected at room temperature between 4000 and 500 cm1. 3. Results and discussion 3.1. Mechanical properties Fig. 2 shows the variation of fracture toughness as a function of POSS loading. As described earlier, the fracture toughness was measured in terms of the value of the critical stress intensity factor at the initiation of crack growth. The error bars in the figure denote
Fig. 2. Fracture toughness value in terms of critical stress intensity factor (KIc) for different loading of POSS of different functional group.
the statistical spread of experimental data in terms of the standard deviation. From the graph it is apparent that the addition of POSS leads to an increase in the fracture toughness. In all three cases, after reaching a maximum value, the fracture toughness starts to decrease on further addition of POSS. This could be due to the agglomeration of POSS particles at higher loading that offset the positive effect of POSS reinforcement. There are two factors that govern the change in the fracture toughness. First, is the organic functionality present on the POSS molecule. Second, is the weight fraction of POSS added to the epoxy resin. Among the three types POSS used, glyicidyl POSSeepoxy nanocomposite showed maximum fracture toughness as compared to reinforcement by the other two types of POSS molecules. In the case of reinforcement by glycidyl POSS, the fracture toughness increased by a factor of 2.3 at 5 wt % loading, while for methacryl and trisilanol phenyl POSS at 5 wt % loading the fracture toughness increased by factors of 1.6 and 1.3, respectively. Fig. 3 shows the typical loadedisplacement curves obtained from fracture testing for the neat epoxy and POSSeepoxy nanocomposites (at 5 wt % loading). Neat epoxy, methacryl POSSe epoxy and trisilanol phenyl POSSeepoxy exhibited brittle failure, as shown by the sharp peak in the load-displacement curve. On the other hand, the glycidyl POSSeepoxy nanocomposite underwent ductile failure, as shown from the loadedisplacement graph. This difference in failure mode suggests that the addition of glycidyl POSS leads to stable crack
Fig. 3. Load displacement curve for neat resin and POSSeepoxy nanocomposites.
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growth, unlike neat epoxy or reinforcement by the other types of POSS. All the specimens selected for this analysis had similar initial crack lengths. In general, for similar crack lengths, POSS reinforced nanocomposite exhibited higher peak load. This observation is very well reflected in value of the critical stress intensity factor. Nonetheless, the glycidyl POSS reinforced nanocomposite exhibited slow crack growth which suggests that more energy was dissipated during fracture. To further quantify the toughening due to POSS inclusion, the work of fracture was determined. The total area under the loadedisplacement curve was normalized with the total fracture surface area created, and the results are shown in Fig. 4. Fig. 5 illustrates that more work was done to fracture POSSeepoxy nanocomposite as compared to neat epoxy. Among the three types of POSS used, the glycidyl POSS showed the highest value of the work of fracture. This observation correlates with the fracture toughness value obtained. This increase in the work of fracture suggests that reinforcement with POSS changes the basic toughening mechanisms. When POSS molecules are incorporated in an epoxy network, the rigid silica cage will provide toughening from crack-pinning, crack-bridging and shear-yielding. Also, the organic part of POSS molecules can interact with epoxy for better dispersion in the polymer network and also provide flexibility in polymer chain by increasing free volume in the network [43e45]. This increase in the flexibility of polymer chains can cause the failure mode to change from brittle to ductile. This change in the failure mode does not depend only on different functionality of POSS, as observed in Fig. 4. Increase in POSS loading in the epoxy resin also plays an important role in the change in failure mode. As observed from Fig. 5, the loadedisplacement the failure mode changes from brittle to ductile as a function of POSS loading. For 0.5 wt % and 1 wt % loading, the failure is brittle, and beyond 1 wt % loading the failure mode becomes ductile along with a significant increase in the peak failure load. Similar behavior was observed for the flexural response of POSSeepoxy nanocomposites. Flexural modulus was calculated from linear portion of loadedisplacement data from three-pointbend experiments. As shown in Fig. 6, the addition of methacryl or trisilanol phenyl POSS did not lead to significant changes in the flexural modulus. In addition, for the inclusion of glycidyl POSS,
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Fig. 5. Loadedisplacement curve for different percentages of glycidyl POSS reinforced epoxy resin.
Fig. 6. Flexural modulus of POSSeepoxy nanocomposites with different POSS content.
Fig. 4. Work of fracture of neat resin and POSSeepoxy nanocomposites.
there was no change in flexural modulus for 0.5 wt% and 1 wt% POSS loading. For higher loading by glycidyl POSS the flexural modulus decreased linearly. This change in transition of flexural properties of glycidyl POSS reinforced nanocomposites corroborates with results from fracture experiments. This change in the failure mode (and flexural response) with the inclusion of glycidyl POSS can be explained in terms of selfassembly of POSS due to POSSePOSS interaction at higher loadings. Various studies have showed that POSS tends to form selfassembly aggregates in the form of spherical, cylinders and vesicles morphologies [25e31]. Fig. 7 shows a schematic representation of self aggregation of POSS at higher loading. The dependence of these morphologies depends on the type of processing conditions. The formation of self-aggregates of POSS leads to a soft core that acts as a plasticizer, just like conventional organic filler. In the case of glycidyl POSS at lower loading, POSS molecules acts as rigid particles. Meanwhile, at higher loadings the formation of soft-shell domains causes an increase in flexibility in the polymer network
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Fig. 7. Dispersion of glycidyl POSS in the polymer at (a) low loading versus (b) at higher loading. POSS form soft-shell domains at high loading.
Fig. 8. TEM image of 1 wt% and 8 wt% glycidyl POSS reinforced nanocomposite showing the self assembly phenomenon.
chains. This causes the failure mode to change from brittle to ductile and the flexural modulus to decrease. The self assembly phenomenon is further supported by transmission electron microscopy images, as shown in Fig. 8 for epoxy reinforced with 1 wt % and 8 wt%. glycidyl POSS. The dark areas represent heavy silicon atoms while white areas represent epoxy resin. For low weight percent POSS reinforcement, no phase separation is observed, which suggests proper dispersion of POSS in the epoxy resin. However, at higher weight percent loading POSS tends to form self-assembled aggregates soft-core shell structures. Finally, Fig. 9 shows that there is not much change in the value of flexural strength with POSS loading. This implies that formation of
Table 1 Glass transition temperature of epoxy reinforces with different types of POSS at 5 wt % each.
Fig. 9. Flexural strength of POSSeepoxy nanocomposites with different POSS content.
Material
Glass transition temperature ( C)
Neat Resin Trisilanol phenyl POSSeepoxy Methacryl POSSeepoxy Glycidyl POSSeepoxy
81.1 87.9 80.2 72.5
K. Mishra et al. / Polymer Testing 62 (2017) 210e218 Table 2 Glass transition temperature of different wt.% of glycidyl POSS reinforced nanocomposites. Material
Glass transition temperature ( C)
Neat Resin 0.5 wt% POSSeepoxy 1 wt% POSSeepoxy 3 wt% POSSeepoxy 5 wt% POSSeepoxy 8 wt% POSSeepoxy
81.1 82.3 82.5 76.9 72.1 67.3
Fig. 10. Glass transition temperatures measure in DSC for glycidyl POSS-epoxy nanocomposites. The dotted line represents the prediction of the Fox equation.
softeshell domains, due to POSS self-assembly, does not affect the maximum load carrying capacity of the epoxyePOSS nanocomposite.
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3.2. DSC analysis Differential scanning calorimetry (DSC) was performed to further understand chain flexibility, crosslinking of chains and intermolecular interactions between POSS and epoxy. Soto et al. showed that, depending on nature of POSS and its degree of dispersion in a polymer, POSS molecules could either act as a plasticizer or an anti-plasticizer [32]. Similar observations were found in the current investigation for the three types of POSS investigated. Table 1 shows the variation of glass transition temperature (Tg ) of neat epoxy and POSS reinforced nanocomposite for 5 wt % POSS reinforcement. The inclusion of glycidyl and methacryl POSS decreased the Tg of the nanocomposite while the addition of trisilanol phenyl POSS increased the Tg . For trisilanol phenyl POSSeepoxy nanocomposite, Tg increased with the incorporation of POSS, Fu et al. [33] reported a similar result with trisilanol phenyl POSS. This might be due to the restriction of the chain mobility because of steric hindrance provided by the big phenyl groups. The decrease in Tg for glycidyl and methacryl POSS was due to plasticization effect cause by increase in the polymer chain flexibility. This increase in the chain flexibility is due to the increase in free volume with increase inclusion of POSS. A similar observation was also reported in other studies [43e45]. As observed in fracture and flexural results, the properties of POSS nanocomposites depend not only on the functionality of POSS but also on the POSS loading in the epoxy resin. Therefore, in order to understand effect of POSS loading, DSC experiments were also performed on glycidyl POSS-epoxy nanocomposites for different loading levels. Table 2 shows the DSC thermogram of glycidyl POSSeepoxy nanocomposites at different loading. It is observed that at lower loading of glycidyl POSS there is increase in the glass transition temperature but with increase in loading Tg decreases. This change in the trend of glass transition temperature with the inclusion of glycidyl POSS supports the self-assembly aggregation hypothesis.
Fig. 11. SEM images of fractured surface of (a) Neat resin (b) trisilanol phenyl POSSeepoxy (c) methacryl POSSeepoxy and (d) glycidyl POSSeepoxy nanocomposite.
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Glycidyl POSS acts as anti-plasticizer at low weight percentage. There is an increase in Tg and at higher loading of POSS due to the formation of soft core structure because of the self assembly of POSS molecules. Various studies have used the Fox equation to determine change in the glass transition temperature with inclusion of plasticizer such as conventional organic filler [46]. They found that addition of plasticizer increases the free volume in molecular network, and hence decreases Tg . A comparison of the change in the experimental value of Tg with inclusion of POSS with the theoretical Foxmodel, shown below, is plotted in Fig. 10.
1 4 ð1 4Þ ¼ þ Tg T ðglycidyl POSSÞ T ðepoxyÞ g g ðglycidyl POSSÞ
ðepoxyÞ
where Tg and Tg are the glass transition temperatures of glycidyl POSS (57 C) and epoxy resin (81.1 C), respectively. From Fig. 10, it can be observed that the experimental value of Tg for glycidyl POSSeepoxy nanocomposite does not follow the Fox equation. This suggests that glycidyl POSS can act as either a plasticizer or an anti-plasticizer depending on loading percentage in epoxy resin.
3.3. SEM analysis The fractured surfaces of neat epoxy and 5 wt% of POSSeepoxy nanocomposites were examined by scanning electron microscopy (SEM). Micrographs of the fractured surfaces are shown in Fig. 11(a)e(d). No phase separation was observed in Fig. 11(b), (c) and (d), indicating that good dispersion of POSS was achieved. The smooth and featureless surface, as shown in Fig. 11(a) for neat epoxy resin, gives evidence of its brittle failure and low fracture toughness. For the case of POSSeepoxy nanocomposites, the fractured surfaces were broken into irregular and rough surfaces. These rough surfaces indicate greater resistance against crack propagation that contributes towards the enhancement of fracture toughness. Toughness in the nanocomposites can due to extrinsic toughening mechanisms such as particle bridging, shear yielding and crack pinning. For trisilanol phenyl POSS reinforced nanocomposites (Fig. 11(b)), the fractured surface shows rougher surface compared to neat epoxy resin. For methacryl and glycidyl POSS reinforced nanocomposites (Fig. 11(c) and (d)), extensive shear yielding was observed. This shear was more prominent for glycidyl POSS epoxy nanocomposites, which supports the ductile failure as observed from fracture experiments. In addition to shear yielding,
Fig. 12. SEM images of the fracture surfaces of glycidyl POSSeepoxy nanocomposite at different wt.%.
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particle bridging was also observed on the fracture surface. Particle bridging caused the micron sized holes that might be due to the pull out of POSS particles from the epoxy matrix, as observed in SEM figure. Our previous results showed that both mechanical and thermal properties of POSSeepoxy nanocomposites, not only depend on the type of organic groups present in POSS, but also on the amount of loading in the epoxy resin. Similar observations were found in fractured images for glycidyl POSSeepoxy nanocomposites, as shown in Fig. 12. It is observed that, with increase in the POSS loading, the fracture behavior changes. For low wt.%, the surface shows brittle failure and, with increase in loading, shear yielding becomes predominant. 3.4. Fourier transform infrared analysis FTIR experiments were performed to study POSSeepoxy interactions. Fig. 13 shows the spectra of neat resin and 5 wt% of POSSeepoxy nanocomposite. The FTIR spectra indicated good dispersion. In addition, the broad peak of alcohol around 3500e3200 cm1 (1) suggests polymerization of the epoxy resin. Peaks around 1024 cm1 (3) represent epoxy groups and, from the spectra, it is observed that epoxy groups are well polymerized during curing. Epoxy reinforced with glycidyl POSS (that has epoxide groups attached to the silica cage) also exhibited complete polymerization as no peaks were observed around 1024 cm1 (3) to indicate uncured epoxy. The Si-O-Si stretching peak is masked by epoxide peak at 1024 cm1 and not showing any distinctive peak, which shows that POSS was very well consumed in the epoxy resin. For methacryl POSSeepoxy nanocomposite, a carbonyl peak around 1750 cm1 (2) is present, which corresponds to the stretching vibration of carboneoxygen double bond, suggesting that some of the carbonyl group present on silica cage is left unreacted. For trisilanol phenyl POSS-epoxy nanocomposite, a peak
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around 690 cm1 (4) evidenced existence of residual epoxy left after complete polymerization. 4. Conclusions Various functionalities of POSS were used as nanoparticle reinforcements to modify the mechanical properties of epoxy resin. Increase in fracture toughness was observed with the incorporation of POSS. For the 5 wt % glycidyl POSSeepoxy nanocomposite, fracture toughness increased by a factor of 2.3 as compared to the neat epoxy resin. Similar trends of increase in tough ness were observed for trisilanol phenyl POSS and methacryl POSS reinforced nanocomposites. Furthermore, from the loadedisplacement data, it was observed that, for the case of glycidyl POSS reinforcement, the failure mode changes from brittle to ductile as the POSS loading is increased beyond 1 wt%. This change in failure behavior is due to the formation of soft shell core structure as a result of self assembly of POSS molecules. The presence of these structures was identified using TEM imaging. Formation of soft shell core structures leads to decrease in the flexural modulus value of nanocomposites with increase in glycidyl POSS loading, whereas for methacryl and trisilanol phenyl POSS not much variation in the flexural modulus was observed. DSC experiments show that glass transition temperature of nanocomposite depends on two factor: first, the type of organic group present in POSS and second, amount of POSS loading in the epoxy resin. Depending on the type of organic group, Tg either increased or decreased by either obstructing or aiding the movement of polymer chain at higher temperature. We observed that, for glycidyl POSSeepoxy nanocomposite, Tg first increased at low loading then started to decrease at higher loading beyond 1 wt%. This change in glass transition temperature was compared with the Fox equation. The results support the formation of soft shell core structure at higher loading of glycidyl POSS. Finally, FTIR spectroscopy showed that there are good interactions between epoxy
Fig. 13. FTIR spectra of (a) Neat resin, and 5 wt% of (b) trisilanol phenyl POSSeepoxy, (c) methacryl POSSeepoxy and (d) glycidyl POSSeepoxy nanocomposite. Peaks indicated by (1) represents alcoholic group, (2) represents carbonyl group, (3) and (4) represents epoxide group.
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