Thermoset-based organic-inorganic nanocomposites have attracted considerable attention as a method of enhancing polymer properties and extending their utility by using nanoscale reinforcements. The final properties of the polymer nanostructured materials are affected by several factors, such as intrinsic characteristics of each component, the contact, the shape and dimension of fillers, and the nature of the interface.
Herein, the effects of nano-sized inorganic fillers on the properties (i.e. mechanical, thermal, electrical properties, etc.) of thermoset polymer matrices were studied based on the various nano-characterization techniques. These characterizations were classified into structure analysis and property measurements, and the latter demonstrated how the final properties of nanocomposites were influenced by nano fillers. Thus, the property measurement characterizations are emphasized in this study. The factors of filler/polymer interaction and compatibility, and filler content are highlightened.
Additionally, the promise of this technology to deliver breakthrough coating performance in a number of areas (e.g., scratch and mar resistance, barrier properties such as corrosion resistance, and the mechanical properties) was also explored and demonstrated.
Key words:
Thermoset-based nanocomposites, inorganic fillers, structure analysis, property measurements, filler/polymer interaction, filler content, coating performance
Introduction:
A narrow and useful definition of a composite was given by Dr. A. Brent Strong [1] as "The combination of a reinforcement material (such as particle or fiber) in a matrix or a binder material". This simply definition implied that the materials in composite act in concert- one helping other-hence the term reinforcement. Generally, the matrix and reinforcement materials in composites could be the followings.
Matrices
Reinforcements
Polymer
Organic
Ceramic
Inorganic(Non-polymer)
Metal
Herein, this study is confined on Polymer Matrix Composites (PMCs) with non-polymer inclusions. Therefore, the non-polymer inclusions can be montmorillonite organoclays (MMT), glass fibers, carbon fibers, carbon/graphite tubes, and silica, aluminum oxide (Al2O3), titanium oxide (TiO2) particles, etc. The polymer binder matrices can be polyesters, epoxy resins, polyimides, bismaleimides (BMI), or others. Furthermore, PMCs can be classified based on the following factors:
Polymer Matrices
Shape of inclusions
Size of inclusions
Thermoplastics
Fibers
Micrometers
Thermosets
Particles
Nanometers
Elastomers
Whiskers
In general, polymer matrices can be classified into the three basic families of resins namely thermoplastics, thermesets, and elastomers [2]. Additionally, the hybrid properties of PMCs are also highly dependent on the size, shape and the degree of dispersion of inclusions in polymer matrix. Especially, concerning with the length scale of the inclusions, the polymer composites can be divided into macrocomposites (micrometers) and nanocomposites (nanometers), individually.
In the case of nanocomposites, they have ultra-large interfacial area per volume, and the distance between the polymer and inclusion components are extremely short; as a result, molecular interaction between the polymer and the nanoparticles will give polymer nanocomposites the unusual improved material properties that conventional macrocomposites do not process. Thus, polymer matrix composites filled with nano inclusions have a greater potential to be utilized in many applications than those of micro inclusions.
There are two main challenges that should be concerned, to developing real polymer nanocomposites after the desired inorganic filler has been selected for the polymer binder of interest [3].
Firstly, the choice of fillers (particles, or fibers) requires an interfacial interaction and/or compatibility with the polymer matrix.
Secondly, the fillers can be uniformly dispersed and distributed by some proper processing technique within the polymer matrix.
Definitely, in this study, the topic will be narrowed into the field of Polymer Nanocomposites (PNCs) with inorganic nano-sized fillers dispersed into the thermoset (highly crosslinked) polymer binders, with the assumption on the second challenge that the nano fillers are uniformly dispersed and distributed in the polymer matrix. The first challenge- interfacial interaction and compatibility between the matrix and inorganic fillers will be seriously considered as strong or weak to study its effect on the properties of polymer matrix.
Joseph H. Koo had already summarized: in most cases, polymer nanostructured materials exhibit the improved end-use properties as listed below [2]:
Thermal
Increased thermal resistance, higher glass transition temperature (Tg) or heat deflection temperature (HDT), reduced coefficient of thermal expansion
Mechanical
Increased modulus, strength, toughness, elongation (in some cases)
Chemical
Improved solvent resistance, improved moisture resistance
Electrical
Improved thermal conductivity, lower resistivity (depends on the nanoparticles)
Barrier
Reduced oxygen, moisture transmission
Optical
Clear, transparency provided in selective systems
Others
Abrasion resistance, reduced shrinkage
But, this study will not only talk about how the nano inorganic fillers in PNCs change the end-use properties from that of the polymer binders, but also why the end-use properties of polymer binders are changed in each characterization method.
The nano-sized fillers offer two key features- high interfacial material content and optical clarity- both of which are important in coating applications, especially for automotive coatings. The desired coating properties include modifications in stiffness, toughness, UV absorption, flame resistance, ionic conductivity, and biodegradability. In this study, a few of quantitative study of the effects of nano-sized particles (aluminum oxide and silica) on the surface mechanical properties of coatings were demonstrated, too.
Discussion:
To study the polymer nanocomposites, characterization involves two main processes: structure analysis and properties measurements. Actually, they are capable of answering two very necessary questions, individually:
Structure analysis: Why is it nanocomposite?
Properties measurements: What are the properties of the nanocomposite?
2.1 Structure analysis
Structure analysis is carried out using a variety of microscopic and spectroscopic techniques; it can be used to determine the size and shape of fillers, as well as the degree and level of dispersion and distribution of the fillers into the polymer matrix. Commonly, the structure analysis characterization techniques can be:
Wide-angle x-ray diffraction (WAXD)
Transmission electron microscopy (TEM) and spectroscopy
Scanning electron microscopy (SEM)
Small-angle x-ray diffraction (SAXD) , and others
Herein, the two most popular and useful nano structure analysis techniques- WAXD and TEM will be briefly discussed.
Wide-angle x-ray diffraction is the most commonly used technique to characterize the degree of nanodispersions of MMT organoclay in a polymer, which measures the spacing between the ordered crystalline layers of the organoclay by using Bragg's law: sinθ = nλ/2d where d is the spacing between atomic planes in the crystalline phase. The diffraction pattern is used to identify the specimen's crystalline phases and to measure its structural properties. Spacing change (increase or decrease) information can be used to determine the type of polymer nanocomposite formed, such as [2]:
Immiscible (no d-spacing change)
Decomposed/deintercalated (d-spacing decrease)
Intercalated (d-spacing increase)
Exfoliated (d-spacing outside of the WAXD, or disordered as to give a signal.)
Although some structural features can be revealed by x-ray and neutron diffraction, direct imaging of individual nanoparticles is only possible using TEM and scanning probe microscopy, which provides a real space image on the atom distribution in the nanocrystal and on its surface. TEM allows the observation of the overall organoclay dispersion in the polymer nanocomposite. Clay dispersion and the structure observed under the microscope can determine the nature of a clay nanocomposite as follows [2]:
Immiscible: usually large clay tactoids, undispersed clay particles
Intercalated: clay layers in ordered stacks can be observed
Exfoliated: single clay layers can be observed
2.2 Property analysis
Property characterization is rather diverse and depends on the individual application. Different application focuses on different properties.
Mechanical properties
The use of nano-sized fillers can lead to a significant improvement on the mechanical properties of the composite. In mechanical properties tests, a strong interfacial adhesion between polymer matrix and inorganic filler is needed to achieve high performance, because the load applied to the composite is mainly transferred to the fillers via the interface. Usually, the organic matrix is relatively incompatible with the inorganic phase.
Therefore, a lot of studies are interested in the surface modifications- more like "surface activation" on inorganic fillers, to increase its adhesion with polymer matrix. There are three concrete examples dealing with different nano fillers listed below:
Clays of Na+-montmorillonite, a natural clay mineral, has been modified to convert the surface from hydrophilic to organophilic via cation-exchange reaction with the surface sodium ion with organics producing organically modified layered silicates [4].
For carbon/graphite nanocomposites, the wettability and interfacial bond strength of carbon with matrix materials can be enhanced by controlled "oxidation" of the carbon surface with treatments such as acid etching [5]. Alternatively, there are some "gentle" methods without any damage on the carbon surface, such as plasma treatment [6], or even microwave plasma treatment [7].
To reduce the difference of surface interfacial energy between silica nano particles and polyimide matrix, a chemical surface modification which is the reaction of APTS with the hydroxyl groups of the silica substrate was managed [8].
Fig. 1 Schematic drawings of the chemical reactions: (a) modification of silica with APTS, (b) preparation of surface designed particle. (Ref. 8)
Thus, the following studies on the mechanical properties of thermoset-based nanocomposites will seriously consider the degree of interfacial interaction between the polymer matrix and the fillers.
Tensile modulus
Generally speaking, in most cases, the tensile modulus of nanocomposite should be increased, because of the intrinsic better mechanical properties provided by the inorganic fillers. Since the load applied to the composite is mainly transferred to the fillers via the interface, the stronger of the interfacial adhesion between polymer and fillers, the tensile modulus of nanocomposite will be enhanced greater.
But, if the interfacial adhesion was weak or even poor, there will be some possibility that the tensile modulus of nanocomposite is reduced because of the defects produced by nano fillers' insertion into polymer matrix. While, it still happens rarely, because it is really hard to find some paper talking about the tensile modulus of nanocomposites is lower than that of neat polymer matrix.
But, Zhou et al. discovered that the untreated or treated nano-silica fillers lead to a increase in tensile strength at low filler loading firstly, and then stat to embrittle the matrix at higher filler loading [9]. Therefore, the content of nano filler in nanocomposite is also a key point to the tensile strength.
Fig. 2 Tensile strength of PP (polypropylene) and its composites as a function of nano-silica content (Ref. 9)
Elongation at break
Elongation at break can be increased or decreased for nanocomposite, because it is highly dependent on the filler/matrix interaction. Sometimes, if the filler/matrix interaction was strong, the value of elongation at break for nanocomposites will be increased [4]; but sometimes, the nanocomposite will fail earlier than neat polymer does because of some possible serious defects resulting from the poor filler/matrix interaction.
It also highly influenced by the filler content. In assumption of good filler/matrix interaction, low filler loading tends to increase the elongation at break. But above some point of filler content, higher filler loading, might embrittle the matrix and reduce the value of elongation at break.
Izod Impact strength/ Notched fracture toughness
The impact strength of polymer based nanocomposite relies on two key factors- one is the nano filler content, the other is filler/matrix interaction. Zhou et al. [9] proved the effects of both two factors on the impact strength, using the following Fig. 3:
All the surface treated nano-silica epoxy composites have higher impact strength than the untreated nano-silica expoy composite.
The addition of nano-silica leads to an increase in impact strength at low filler loading and then embrittle the matrix at higher filler loading, for both treated and untreated nano-silica composites.
Fig. 3 Notched Charpy impact strength of PP (polypropylene) and its composites as a function of nano-silica content (Ref. 9)
Thus, the stronger of the filler/matrix interaction, impact strength of nanocomposite is higher. Above some point, the higher of the filler content, the impact strength is lower.
Kim et al. [10] studied the fracture toughness of the nano carbon black reinforced epoxy composite, and it was measured using the single edge notched bend (SENB) specimens with respect to the particle content. The fracture toughness enhancement provided by carbon black was clearly demonstrated in the following Fig. 4.
The fracture toughness of nanocomposite will be influenced by the two key factors as impact strength did, and followed the similar trend.
Fig. 4 The fracture toughness of neat epoxy and epoxy-based nanocomposite (Ref. 10)
Nanoindentation (Surface modulus)
Sung et al. [11] and Floryancic et al. [12] reported the impact of nano-alumina and nano-silica particles on the scratch and mar resistance behavior of polymeric coatings. The surface modulus of nanocomposites coatings determined from nanoindentation measurements. In that study, as the filler concentration increased, there was slight decrease in the surface mechanical modulus.
Thus, the lower surface modulus of nanocomposite polyurethane coating exhibited less scratch damage than the neat polyurethane coating in the same scratch test condition [11].
Atomic force microscopes (AFM)
The atomic force microscope (AFM) is a very high-resolution type of scanning probe microscope, with demonstrated resolution of fractions of a nanometer. Thus, AMF is capable of presenting the surface morphology of nanocomposite samples in nanometers, which can provide the information, such as the size and the size distribution of fillers, the level of dispersion of fillers in polymer on surface, and surface roughness, etc.
Especially, it can work with nanoindentation by sharing the same "tip", which effectively fixes the testing "area" and reduces the errors resulting from the tip-displacement. Richter et al. [13] reported nanoindentation studies of the heterogeneous carbonaceous film containing Ni nano-crystals, combining with AFM technique.
Brickweg et al. [14] used AFM to study the hear-induced 1-D alignment of alumina nanoparticles in polyurethane clear coating systems. It found out that the alignment was affected by the shear condition of the various coating application methods, spray or drawdown.
Thermal/dynamic mechanical properties
Nano-sized fillers offer a key feature- high interfacial material content to thermoset-based nanocomposites. Increased interfacial material content can cause significant shifts in properties of the overall composite. An example is the modification of glass transition temperature (Tg) of a polymer at an interface. Tg of the polymer would change due to the steric and enthalpic effects that alter the segmental mobility of the polymer molecules of the interfacial layer can also very.
Dynamic mechanical thermal analysis (DMA)
In the DMA technique an oscillatory force is applied to a sample and the response to that force is analyzed. Two different moduli are determined as a function of temperature, an elastic or storage modulus (E'), which is related to the ability of the material to return or store energy, and an imaginary or loss modulus (E"), which relates the ability of the polymer to disperse energy. The temperature dependence of the ratio E"/E', also called tan delta (tan δ), is related to the mechanical properties of the nanocomposites. The maximum in a tan δ vs temperature plot may be taken as an estimate of the glass transition temperature (Tg).
Triantafillidis et al. [15] studied the diamine modified montmorillonite thermoset epoxy-clay nanocomposites using DMA characterization. All the epoxy-clay nanocomposites showed a clear increase of the storage modulus, no matter of whether the clays in these samples were exfoliated or not. But surface-modified clay nanocomposites surly showed higher storage modulus than untreated clay nanocomposite. The similar discovery was also found in Abdalla's DMA studies [4].
Thus, the enhancement of storage modulus provided by the inorganic filler with intrinsic higher elastic property overwhelmed the factor of degree of dispersion and distribution.
Additionally, the relatively small changes in glass transition temperature were also observed for the nanocomposites in comparison to the pristine epoxy polymer [15].
Theoretically, there are two options for nano fillers contribute to the free volume for the segmental mobility of the polymer molecules- more or less. If the free volume for segmental mobility is bigger, the glass transition temperature of nanocomposite will be decreased. If the free volume for segmental mobility is smaller, the glass transition temperature of nanocomposite will be increased.
Generally speaking, the size of nano fillers (in assumption of narrow size distribution) is smaller than the length of segment between network joints in a highly crosslinked thermoset matrix. We consider the factors- glass transition temperature (Tg) of nanocomposites, crosslinking density (XLD) of matrix, free volume for segmental mobility of polymer molecules, and filler/polymer interaction, to explore the possible relations among them as the table below:
Tg of nanocomposites
Crosslinking density
(XLD)
Free volume polymer molecules
Filler/polymer interaction
Decreased
Decreased
Increased
Weak/poor
Increased
Decreased
Reduced
strong
Increased/decreased (slightly changed)
Decreased
Increased/decreased
(slightly changed)
Medium (most cases)
Thus, no accounting of the plasticizing effects provided by some possible additives, the glass transition temperature of thermoset-based nanocomposite is highly related to the free volume of polymer molecules, which is highly dependent on the level of filler/polymer interaction. The conclusion is based on two assumptions, which results in the crosslinking density always decreasing in any cases:
The size of nano fillers is smaller than the length of segment between network joints
Nano fillers are dispersed and distributed well in polymer matrix
Differential scanning calorimetry (DSC)
Differential scanning calorimetry is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference are measured as a function of temperature. The main application of DSC is in studying phase transitions, such as melting, glass transitions, or exothermic decompositions.
Glass transitions appear as a step in the baseline of the recorded DSC signal. This is due to the sample undergoing a change in heat capacity; no formal phase change occurs. Thus, in DSC characterization, the change of glass transition temperature provided by nano-filler is the change on the heat capacity of nanocomposites. Since the heat capacity of inorganic filler is lower than that of polymer, in most cases, Tg of nanocomposite is higher than that of pristine polymer via DSC characterization. Apparently, DSC and DMA focus on the different aspect on Tg measurement.
The melting process results in an endothermic peak in the DSC curve. Raghavan et al. [16] reported the small changes in the melting points of the nanocomposite menbrances filled by BaTiO3, SiO2 and Al2O3 nano particles, even though the earlier reported for polymer electrolytes containing BaTiO3 and Al2O3 barely had the changes on thermal properties. Interestingly, they explained the observation concerning the changes provided by nano fillers on the orientation of polymer molecules.
Fig. 5 Melting points (DSC) of electrospun P(VdF-HFP) membranes with (a) Al2O3, (b) SiO2, and (c) BaTiO3. The melting temperature of pure P(VdF-HFP) is 159 °C (Ref. 16)
Electrical properties (Ion conductivity)
Nanocomposite polymer electrolytes instead of traditional liquid electrolytes are currently used, because of their advantages in providing lighter and safer batteries with long shelf life, leak proof construction and easy fabrication into desired shape and size.
Raghavan et al. [16] prepared a series of nanocomposite polymer electrolytes (NCPEs) comprising nanoparticles of Al2O3, SiO2, and BaTiO3 using electrospinning technique and studied the absorbent ability of the liquid electrolyte, electrolyte retention capacity to determine the ionic conductivity and the electrochemical stability.
The following figures showed that the ionic conductivity and the electrochemical stability window of the electrospun P(VdF-HFP)-based polymer were enhanced by the presence of the fillers.
Ionic conductivity
Fig. 6 Temperature effect on the ionic conductivity of polymer electrolytes based on electrospun P(VdF-HFP) membranes with different ceramic fillers. (Ref. 16)
Electrochemical properties
The electrochemical stability window of electrospun P(VdFHFP)-based polymer electrolytes is shown in Fig. 7. The polymer electrolyte without ceramic filler exhibits an anodic stability up to 4.7 V. With the incorporation of filler particles in the polymer matrix, the electrochemical stability is enhanced.
Fig. 7 Anodic stability by LSV of polymer electrolytes based on electrospun P(VdFHFP) membranes with different ceramic fillers (Li/NCPE/SS cells, 1 mV/s, 2-5.5 V). (Ref. 16)
Conclusions:
For polymer-based organic-inorganic composites, nanoscale reinforcements have greater potential than microscale ones to enhance polymer properties and extend their utility.
To study the properties of thermoset-based organic-inorganic nanocomposites, a variety of characterization techniques were used. These characterizations were classified into structure analysis and property measurements, and the latter strongly demonstrated how the final properties of nanocomposites were influenced by nano fillers.
There are two main challenges to developing polymer nanocomposites. One is the well dispersion of nano fillers in polymer matrix, and the other is the interfacial interaction and/or compatibility between filler and polymer matrix. That is why a lot of surface design techniques are invented to "activate" filler's surface, in order to improve the disperse stability or polymer/filler interaction.
The mechanical properties of thermoset-based nanocomposites are highly dependent on the interfacial adhesion between the filler and matrix, as well as the filler content. Generally speaking, modulus is always increased, but tensile strength and elongation at break are closely related to the interface quality and how much fillers are loaded. Sometimes, poor interfacial adhesion and too much filler content can result in the tensile strength and elongation at break reduced. Surface mechanical modulus can be studied by nanoindentation, which can be strengthened by AFM technique.
The thermal properties of thermoset-based nanocomposites are also highly dependent on the polymer/filler. The glass transition temperature of the polymer would change due to the steric and enthalpic effects that alter the segmental mobility of the polymer molecules of the interfacial layer. Since thermoset polymer matrices are usually highly crosslinked and the size of nano particles are smaller than the length between network joints, crosslinking density will not be too much changed by filler insertion. Thus, the interfacial adhesion carried more weight than other factors to influence the Tg of nanocomposites. Usually, the storage modulus tested by DMA is always increased because of the intrinsic elastic property of inorganic fillers. Since Tg changes in DSC due to the change of heat capacity, but in DMA its changes due to the mechanical properties E"/E' (tan δ), DSC and DMA sometimes tell different results in Tg measurement on nanocomposites.
The ionic conductivity and the electrochemical stability of polymer-based nanocomposites were enhanced by the presence of the ceramic nano-sized fillers, such as Al2O3, SiO2, and BaTiO3.
