Modification of the preformed polymer by the SCA before the sol—gel process is the frequently used approach [ 2 , 48 , 49 , 50 , 51 , 52 ], allowing polycondensation reactions between the trialkoxysilyl groups on the SCA bonded to the polymer and the metal alkoxide precursor, forming a covalent bond between the two phases.
In addition to SCAs, other coupling agents include carboxylic acids e. A comprehensive overview over all the inorganic—polymer nanocomposites prepared by an in situ sol—gel synthesis is beyond the scope of this review. Therefore, selected examples on the development of epoxy and polydimethylsiloxane PDMS nanocomposites prepared using sol—gel processes will be presented. The inorganic components of the nanocomposites from these examples are primarily transition metal oxides e. Table 1 shows a general overview of the various syntheses of nanoparticles in situ in the two different polymer systems.
It should be noted that in some of the works referenced, the authors do not specify the inorganic component in the hybrids as nanoparticles, but instead as nanodomains.
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This is most likely because the inorganic networks formed are so small and polymer-like that they may not qualify as particles with a defined shape e. In Table 1 , the inorganic components specified are based on the assumption that these inorganic networks will form nanoparticles if they grow to an appreciable size.
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The chemistry behind the synthesis routes, the effect of various parameters on the inorganic structures formed, as well as the resulting properties of the nanocomposites are reviewed afterwards. Selected examples of the precursors, surface modification and solvents used in the in situ synthesis of metal oxide nanoparticles via sol—gel processes in epoxy and polydimethylsiloxane PDMS nanocomposites. Epoxy is a thermosetting polymer and an excellent choice for high performance composite materials when reinforced with SiO 2 due to the resulting strength, toughness, good chemical and heat resistance, and high thermal stability [ 48 , 49 , 51 ].
Typically, epoxy composites are cured via a condensation reaction with an amine- or anhydride-based curing agent, forming a copolymer. Epoxy nanocomposites containing titania TiO 2 are also of interest due to the photocatalytic properties imparted to the polymer by the TiO 2 , as well as increases in the refractive index [ 15 , 60 , 61 ].
Due to the challenges with achieving a homogeneous dispersion of nanoparticles when employing a traditional ex situ blending route, there has been an increased focus on the use of in situ sol—gel techniques instead for nanocomposite synthesis. Diglycidyl ether of bisphenol A DGEBA is commonly used as the monomer, and poly oxypropylene diamine , also known as Jeffamine, is often used as the curing agent in these nanocomposites.
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For synthesizing nanoparticles in situ in epoxy, most researchers have attempted either a one-step or a two-step procedure, as shown in Figure 4. In the one-step procedure, the precursors and reaction components epoxy resin, coupling agent, inorganic oxide precursor, curing agent, solvent, catalysts, etc. There are several variations of the two-step procedure. The second step involves the polymerization of the organic components and the formation of the oxide network simultaneously when the pre-hydrolyzed precursor is mixed with the monomer and curing agent.
The inorganic oxide network in this case forms in a preformed organic network, as the epoxy is already cured. In the next step, the inorganic precursors alkoxide, water, catalyst, etc. Since the coupling agents provide a chemical bond between the organic and inorganic networks, this procedure results in the formation of Class II hybrids. One of the advantages with a two-step procedure is that it offers more control over specific reactions, depending on which variation of the procedure is used, since not all of the reactions are occurring simultaneously, as in the one-step procedure.
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The colors of the various solutions are described as follow: The pure epoxy resins are indicated either by light green monomer solution , dark green polymer solution , or orange monomers modified by silane coupling agents SCA , the inorganic oxide precursors are indicated by light purple, or dark purple pre-hydrolyzed , and the SCAs are indicated by red. Differences in the structure of the inorganic domains arose based on whether the reaction was carried out in a one-step or two-step procedure.
In the one-step procedure, large SiO 2 aggregates — nm were observed through scanning electron microscopy SEM [ 57 ], which was attributed to the reaction being catalyzed by the amine curing agent a base due to its molar excess over the acidic catalyst TSA. Base catalysis promotes the condensation reaction and the formation of colloidal spherical particles. In the two-step sequential process, the distribution of the inorganic phase was not uniform, with a higher SiO 2 concentration on the surface. This was due to the inhomogeneous swelling of the epoxy resin by the TEOS.
Dynamic mechanical analysis DMA showed a larger shear storage modulus for the in situ epoxy-SiO 2 nanocomposites compared to pure epoxy [ 57 ]. However, this reinforcement was dependent on the procedure used for preparation. Acid pre-hydrolysis of TEOS resulted in higher modulus in the nanocomposites, compared to those prepared without pre-hydrolysis e. The sequential two-step procedure with pre-hydrolyzed TEOS possessed the largest storage modulus. The observed reinforcement effects are attributed to increasing interphase interactions in the hybrid systems, resulting in a larger immobilized layer of polymer chains around the nanoparticles [ 57 ].
The nanocomposites were determined to have a bicontinuous morphology the SiO 2 forms a continuous phase in the organic matrix rather than a particulate composite with dispersed SiO 2 particles , based on agreement of the data with the two different models [ 57 ]. Bauer et al. This plateau was shifted to higher temperatures for the nanocomposites, with the inorganic network possibly acting as a barrier to the decomposition of the organics.
Figure 5 shows a schematic for a possible outline of the reactions occurring during this procedure between the DGEBA monomer, the coupling agent, and the precursor. The curing step is not shown in this schematic. Nazir et al. TGA also showed that the thermal stability, as well as the average energy of activation E a for the degradation, slightly increased for the nanocomposites when APTES was used [ 52 ].
However, there was no indication of char formation. Dynamic mechanical thermal analysis DMTA showed a higher storage modulus for the nanocomposites in the glassy region. Further increase in the SiO 2 content led to a decrease in the storage modulus. Reproduced with permission from Nazir et al. Progress in Organic Coatings ; published by Elsevier B. Afzal and Siddiqui [ 51 ] used a similar procedure to that used by Nazir et al. Atomic force microscopy AFM was used to investigate the microstructure and surface morphology of the nanocomposites Figure 7.
Inclusion of SiO 2 in epoxy led to increased roughness of the surface. The peaks in Figure 7 represent the SiO 2 nanoparticles and show a homogeneous distribution. This difference may be attributed to the SCA used, but could also be due to the differences in synthesis conditions. The use of lower pH and higher temperature will promote the hydrolysis of TEOS, resulting in a more network-like open structure, and therefore increased phase mixing.
The formation of the silica network was investigated by both Nazir et al. Afzal and Siddiqui attributed the increase in T g to the loss of mobility of the polymer chains around the SiO 2 nanoparticles, caused by the increased interactions at the interfaces [ 51 ].
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Changes in the glass transition temperature T g with silica content in epoxy nanocomposites. Guan et al. The refractive index at The tensile strength, impact strength, tensile and flexural moduli, and ductility are improved significantly in in situ prepared epoxy—SiO 2 nanocomposites, compared to pure epoxy [ 36 , 48 , 50 ]. This toughening of the nanocomposites is attributed to the strong covalent bonds formed at the interfaces between the organic and inorganic networks via the coupling agents, which can withstand external stresses and transfer them to the rigid nanoparticles.
However, agglomeration of the nanoparticles in the epoxy can compromise the mechanical properties. GPTMS was used as the coupling agent. For the epoxy—SiO 2 nanocomposites, they followed a one-step procedure by mixing all the reactants and adding an acid catalyst HCl dropwise while stirring.
A similar method was used for the epoxy—TiO 2 nanocomposites, but a mixture of tetraethylorthosilicate TEOT and acetylacetone was added dropwise instead of the acid. The images, however, show distinct TiO 2 nanoparticles, whereas the SiO 2 nanoparticles are less distinct in contrast and resemble the IPNs reported by Nazir et al.
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This could be attributed to the controlled hydrolysis of the TEOT, meaning that the condensation rate is higher, leading to the formation of colloidal TiO 2 nanoparticles. Meanwhile, TEOS hydrolysis is catalyzed by the acid, leading to the formation of more polymer-like SiO 2 networks, resulting in the less distinct phase in the TEM image. DSC results confirmed previous observations, with an increase in T g observed for both types of nanocomposite [ 59 ].
TGA results are consistent with other studies for the epoxy—SiO 2 nanocomposites, with an increase in thermal stability compared to the pure epoxy. However, the thermal stability of epoxy—TiO 2 nanocomposites is lower than that of pure epoxy, and is attributed to metal-catalyzed oxidative decomposition pathways [ 59 ].
This result is contrary to those reported by Guan et al. Recently, Donato et al. Methylimidazolium-based ILs organic salts with ionic—covalent crystal structures, e. Due to their selective interaction features and the ability to self-organize, they can act as molecular templates in the sol—gel synthesis of SiO 2 nanoparticles [ 63 , 81 ], with different ILs resulting in different matrix—filler interface characteristics [ 62 ].
As a result, the nanocomposites prepared with the MeS—anion IL showed strong interfacial interactions, with an increase in the shear storage modulus and decrease in the loss factor, while those prepared with the BF 4 —anion IL showed the opposite no mechanical reinforcement. This is attributed to the IL cation providing physical crosslinking, as the interfacial interaction is weak due to the immiscibility of the IL and the poly oxypropylene chains of the epoxy network [ 63 ].
SCAs can also be combined with ILs in the in situ synthesis of epoxy—SiO 2 nanocomposites to tune their mechanical properties. However, combination of the IL and GPTMS in the synthesis decreased the tensile moduli of the nanocomposites, which was attributed to a decrease in the crosslinking density in the organic network. Contrary to the findings reported previously in this review, Donato et al. The gray platelets represent SiO 2 nanoparticles. Reproduced with permission from Donato et al.
A non-hydrolytic sol—gel approach was also applied to prepare epoxy—SiO 2 nanocomposites using ILs, with boron trifluoride monoethylamine BF 3 MEA complex as the solvent [ 62 ]. The reaction in this approach is slower, allowing structure control and avoiding phase separation without the application of a co-solvent [ 62 ]. Similar to the hydrolytic approach, the use of ILs in the non-hydrolytic sol—gel process also resulted in an increase in the shear storage modulus, fracture strain, toughness, and tensile strength.
The non-hydrolytic sol—gel approach with ILs also appears to be better suited for glassy epoxy nanocomposites formed by using more basic amine curing agents, with lower amine equivalent weight than for rubbery nanocomposites formed by using less basic amine curing agents, with higher amine equivalent weight [ 62 ].
This is explained to be due to the catalytic effect of the IL which is slowed down in the non-hydrolytic approach and the resulting sensitivity to the basicity of the system when the curing agent is added, but further investigation into the effects of the reaction condition is required for these systems. Polysiloxanes silicones are quite prevalent in applications today, for example, in the textile, food, biomedical, aerospace, and electronics industries [ 82 , 83 , 84 ].
The use of transition metal oxide fillers for PDMS has recently attracted interest due to the improvements in the optical and mechanical properties of the nanocomposites, opening new possibilities for applications in optical devices [ 14 ]. The unique flexible and rubbery properties of PDMS, along with its thermal stability, have also made it suitable for application as thermally stable rubbers and hydrophobic coatings [ 66 , 68 , 72 ]. Unlike that of epoxy nanocomposites, the synthesis of most PDMS nanocomposites is done without the use of any coupling agents.