Handbook of Modern Coating Technologies

Polymer nanocomposites

The film formation of polymer latexes has been adequately enlightened by many studies on theories and with trials [1—54]. An extent of knowledge is necessary to achieve higher perfor¬mance and innovative tools in latex coating. Polymer nanocomposites are possible to process in coating manufacturing, just like pure latex polymers used as protective layers. The dis¬persed latexes in at least two different types are usually mixed to produce polymer compo¬sites. After dehumidification, these are of the determinants of film formation in characteristics. The combined compounds allow new properties in the composite due to their immiscibility [55]. Typically the heterogeneous mixture has distinctive features in com-parison to the homogeneous substances, namely two pure components form a totally new one. Fig. 6—1 illustrates common application areas of polymeric composites. The complete homogeneity is expected to occur with the resulting properties determined mostly by the morphology and adhesion qualities in intermediate forms, rather than volume proportion for individual phases. This condition is rather subtle in composite latex film formation because of interphase interactions. In fact, there are many factors for the film formation including molecular weight and structure, synthetic methodology, latex morphology, annealing pro¬cess, stabilizers, surfactants, and other conditions [56—60].
Several experiments conducted on one- and multitype latex film formations have so far demonstrated that the properties of produced dry films were superior with two or more poly-mer components having different Tg values [61] both in mechanics and as barriers [62,63] 
than those with single one in only mechanics [1,21,35,39,64—67]. For this process, water-borne core/shell latexes are synthesized with a high-Tg polymer core and a film-forming shell. Two polymer latexes different in Tg are physically blended [61] to achieve a consistent film formation directly with soft latexes in the support of the filler hard particles having mechanical properties [62,63]. A significant trend in latex blends in the organic coating industry is zero-volatile organic compounds [68] because of the independence from the pol-lutant of volatile solvent plasticizers. The mixture has comparable properties with each com-ponent, and in some cases these are unique for the new composite [69]. Besides, physical entropy may help the blends of spherical particles in a limited size to “self-assemble” into super lattices, which makes it possible for latex films to form with controlled microstructures. There are sufficient evidence in literature on the effects of cross-linked fillers on melt rheol¬ogy studying easy dispersion in a set of matrices [70—73], incorporation into the matrix net¬work via covalent bonding, and addition to work of a shell for a higher filler—matrix compatibility [74—77]. In past decades, gelled rubber has become popular as a factor in rub¬ber formulations for process development and nonetheless with the lack of certain physical properties in the outputs. The solution has appeared as polymer mixtures of cross-linked latexes of colloidal size for reinforcement of various rubbers and plastics. This is an only practical way of blending to obtain an acceptable gel dispersion. Solid particles are mostly used to fill plastics and rubbers for better quality and cost efficiency [78]. The matrix—filler interactions are primary determinants of their properties rather than interparticle ones [79]. Developed using colloidal polymer fillers in water including inorganic natural and/or syn¬thetic compounds [80,81] for one decade, polymer nanocomposites take the advantages of rigidity and thermal stability of inorganic nanofillers and of flexibility, dielectric, ductility, and processability of organic polymers [82]. These materials can be defined by the vast inter¬facial area due to the smallest and so huge quantities of filler compared with known compo¬sites so that this significantly increases the volume proportion of interfacial polymer different from the bulk polymer regardless of dimension of loadings [83—87]. The materials including metal (e.g., Au, Ag, Cu, Fe, and Ge), semiconductor (e.g., PbS, CdS, and ZnO), clay mineral (e.g., montmorillonite and saponite) [88—90], metal oxide (e.g., TiO2, Al2O3, and SiO2) [91—93], graphite, and carbon nanofiber as well as carbon—based substances [e.g., carbon nanotube (CNT)] [94—97] can be used as inorganic nanoscale building blocks [98—110].
The wide variety of applications for polymer matrix ranges from the industry—based plas¬tics such as nylon 6, polyimide, and polypropylene [111a-d] to the conducting polymers such as polypyrrole and polyaniline (PANI) [93a,b,94] to the transparent polymers such as PMMA and PS [93e,94a,b]. Different applications of polymer/clay, polymer/CNT, and polymer/ metaloxide nanocomposites are presented in Tables 6—2—6—4, respectively. Moreover, the control processes related to the polymer matrix interfere with the role of the nanofillers on microstructure, even molecularly. Although the literature has many studies or researches simply to have a grasp of such effects, there is still limited evidence and a requirement for further discoveries. Background information on its characteristics is necessary to determine how the interfacial region between an organic or inorganic filler component and an organic polymer matrix influences the overall properties of the composite material.
Table 6-2 Application of polymer/clay nanocomposites.
Polymer/clay
composite Application References
Poly(ethylene
terephathalate)/clay Food packaging for fruit juice, cheese, processed meats, beer and carbonated drinks bottle, cereals, boil-in-the-bag foods, dairy products, confectionery; nanodielectrics for HV insulation systems [82b,85a,88a-c]

Polylactide/layered Lithium ions, battery development, edible, and biodegradable films [78d,88d,e]

silicate
Thermoplastic Beverage bottles and containers applications [89a-e]

polyolefin/clay
Nylon-6/layered
silicate Automotive and aerospace industries, protective coatings, selective barriers for the separation of gases, textiles, fire resistant, or abrasion resistant materials [85b,111ab]

Poly(ethylene oxide)/ layered silicate Support for immobilization of transition metals, nanocontainers for drug delivery systems, components in electrical and electronic parts, fuel tanks, brakes and tires, aeroplane interiors, solid state batteries, electrochemical devices, and sensors [85c,90a-e]

Polyamide/clay Automotive applications (timing-gear housing or Motors engine hood), multilayered containers for beer, juice, and single-served carbonated soft drinks [111c,d,85d,e]

Table 6-3 Application of polymer/CNT composites.
Polymer/CNT composites Applications References
Poly(methyl methacrylate)/SWCNT Biocatalytic films [94a,b]

Polyaniline/MWCNT and Polypyrrole/ Supercapacitor electrode materials [94c-e]

MWCNT
Polyamide/SWCNT and polyamide/MWCNT Automotive, electronics and industrial, and electrostatic discharge [95a-c]

Epoxy/MWCNT Electronics, flame retardant, radar-absorbing material, electromagnetic interface shielding, optical sensors from severe laser pulses, fuel cell, electronic packaging, defensive shielding of computers and consumer electronics, aircraft, and warship [78c,e,87e, d,95 d,e]

Epoxy/(MWCNT/SWCNT) Sporting goods, wing aircraft, reduce weight. Wind turbine blades, aerospace industry, aeronautics, military aircraft, and automobiles [82c,87c,96ab]

Polyethylene/MWCNT Automotive external body components, hot melt adhesives, yarn, and conductive plastic for surface resistivity [96c,d]

Polyurethane/SWCNT and polyurethane/ Wind turbine blade and flame retardant [97a,b]

MWCNT
Poly(vinyl alcohol)/MWCNT and poly(2- Sensors and actuators for biomedical applications [87b,97c]

acrylamido-2-methyl-1-propane sulfonic acid)/MWC NT
Poly(4-methyl-1-pentene)/SWCNT Space vehicles, space stations, and biomedical art [87a,97d]

CNT, Carbon nanotube; MWCNT, multiwalled CNT; SWCNT, single-walled CNT.

Table 6-4 Application of polymer/metaloxide composites.
Polymer/metal oxide composites Applications References
PVA/TiO2 UV-resistant material, chemical fiber production, printing ink, coatings, foods packing material, gas, and moisture sensor [86b,91a-c]

PMMA/Fe-oxide Lasers, optical isolators, solar cells, and LED [93e]

PVA-PEG-PVP/ZrO2,
PVA-PEG-PVP/TiO2 Sensors, medical devices, drug delivery, coatings, adhesives, optical integrated circuits, automobiles, microelectronic packaging, injection molded products, packaging materials, and aerospace [86a,91d]

PE/TiO2 Photocatalysts, sensor materials and photoelectric devices, antimicrobial coatings on textile, and solid-surface antimicrobial coatings [92a,b]

PVC/Al2O3 Grinding wheel/tool in clinical dentistry [82d,93c,d]

PANI/Cr2O, PANI/Al2O3, Active electrode materials in energy storage, optoelectronic devices, [86cd,92c-e,93a,

PANI/ZnO, and PANI/TiO2 short wavelength light emitting devices, sensors and solar cell, display devices, corrosion inhibitors, controller of electromagnetic radiations, and electrostatic charge b]

PP/TiO2 UV protection and antibacterial coating [78b,111e]

LED, Light-emitting diodes; PANI, polyaniline; PE, polyethylene; PEG, polyethylene glycol; PMMA, poly(methyl methacrylate); PVA, polyvinyl alcohol; PVC, polyvinyl chloride; PVP, polyvinyl pyrolidone.

6.1.3 Fluorescence technique
For past two decades a technique has come into prominence among researchers regarding the dissolution behavior of water-soluble polymers, called luminescence spectroscopy [112]. To meet the requirement for characterization of tangled and common systems in an indus¬try, the scientists aimed to expand the usage of this instrument getting inspired from the basic experiments and their positive results [113,114]. It is essential to determine the good¬ness of mixture of two polymers on a molecular basis, and therefore fluorescence technique is supportive in addition to real data of almost tiniest scale, that is a few nanometers, on the setting in which fluorophores exist. Having origin in Morawetz [115] FRET helps to analyze polymer blending. The film formation studies frequently applied this method using polymer latex dispersions [17,36,37]. Furthermore Winnik extensively studied interdiffusion alterna-tively within a wide variety of conditions [116]. FRET is used to characterize latex film forma¬tion in combination of both latex dispersions, one from donor (D) the other from acceptor (A), to ensure that the fluorescence and absorption spectra, respectively, will mutually match one-to-one. The typical way is use of dye comonomers to merge the dyes and the selected polymerizable compounds (e.g., methacrylate or vinyl substituents) into the polymer latexes. The transfer of the electronic energy to a nearby A, which is released during the radiation for the casted film with a generally absorbable wavelength by D, occurs via a resonance dipo¬le-dipole coupling mechanism.
At the Tg of the polymers, the energy on the dried film transfers across and when it is annealed at higher than Tg the polymer diffuses across the interfaces between the adjacent particles coded D and A, and then both polymer chains are blended, leading to productivity.
The diffusion phase is of key importance as a precondition for having stronger mechanics [117—119]. Our knowledge on A and D components and the morphology of the nanostruc-ture is upgraded depending on the imitating distribution of dyes to polymer components [120]. FRET suits the causality analysis of latex film formation including several contributing factors: temperature [121], composition [36], moisture [20], coalescing aids [122—124], polar groups at the latex surface [125], and the presence of filler particles [126—129].
The chromophores [naphthalene (N) and pyrene (P)] [130] attract the researchers in poly-meric dynamics: P is a fluorescence probe used in the successful trials of micellar [131] and phospholipid dispersion [132] primarily on the quenching dynamics of P monomer fluores-cence and excimer formation. The fluorescence spectrum of pyrene comprises two elements: a structured emission band within the range of 370 and 450 nm, where monomer molecules were excited and an amorphic, red shifted broad band. The excited dimers or excimers lead to the formation of blue emission band based on the interaction of excited and unexcited monomer molecules [130]. At higher concentrations of pyrene molecules, pyrene monomer intensity (IP) diminishes as the intensity of excimer (IE) escalates. Pyrene concentration is proportionally associated with the ratio of IE/IP [133]. The absorption spectrum exhibits char-acteristics of the monomer without dimers in the ground state, irrespective of pyrene con-centration. The investigation of the polymeric dynamics of end-to-end cyclization involves pyrene excimer formation [134,135] through which the nonaqueous latex morphology is studied including particles labeled with pyrene molecules [136]. In our previous studies behavioral analysis have been performed on film formation of different latex mixtures in Tg following after polymer diffusion across the adjacent cells in a blend film [137—140]. The fluorescence measurements are recorded to control polymer diffusion with one latex labeled with pyrene in terms of velocity and quantity.
This paper is focused on the application of fluorescence spectroscopy in characterization of film formation from fluorescent polymer latexes and polymer composites. The effects of annealing temperature and time, latex composition, organic/inorganic fillers, solvent composi¬tion, etc., based on our own research work, on the behavior of film formation and morphology of these films are particularly reviewed. In this paper, we use two polymer latexes (PMMA and PS) which have been treated with fluorescent labels (pyrene, fluorescein, and naphthalene) as the model systems to get information about the film formation behavior of these systems.