Nanofiller materials for transparent polymer composites: Influences on the properties and on the transparency

Abstract

Filler materials are widely used in combination with polymer materials. Conventional filler particles generally cause light scattering and absorption because of their optical characteristic or refractive index difference. With nanoparticles (NPs) as a filler material, it is theoretically possible to manufacture transparent compounds due to their small particle dimensions reducing the interaction with light. Nevertheless, the particles tend to build agglomerates and aggregates which reduce the composite’s transparency considerably. This review gives an overview of the effect different particle materials have on the properties of transparent polymer composites with consideration of the composite’s transparency. There are very few reports on highly transparent and thick (>1 mm) polymer nanocomposites with such an amount of particles that affect other properties of the polymer significantly. In the majority of cases, NPs lead to a significant lower transparency. This indicates that the homogeneous dispersion of the particles is still a major difficulty in producing transparent nanocomposites with enhanced properties.

Keywords

Nanocomposites transparent polymers transparent composites nanofillers transparent fillers

Introduction

Nanoparticles (NPs) as a filler for polymers have gained a great deal of attention since 1993 when researchers increased the yield and tensile strength of nylon-6 by an amount of about 2.5 by adding only 7 wt% clay platelets.¹ The main difference in NPs is the enormous specific surface area which is one to two orders of magnitude higher than for conventional fillers.^2,3 The amount of surface atoms is 20% for a 10 nm particle and increases to a percentage of 80% of all atoms for a particle with a diameter of 2 nm.⁴ A larger surface area leads to a higher surface reactivity.⁵ Therefore, it is possible to improve the polymer’s properties with considerably low filler loadings or even to integrate new properties.^6
–8 Moreover, low filler loadings and a filler size far smaller than the wavelength of visible light enable the manufacture of transparent composites.

Nevertheless, scattering and absorption of light by NPs impair the transmittance of a composite. The absorption and scattering cross-section express the efficiency absorption and scattering. In standard spectroscopy measurements, the intensity losses represent the extinction, which is composed of the losses due to absorption and scattering.⁹ The scattering intensity I_S of a particle depends on the following correlation^10
–12

I_{S} \sim \frac{d^{6}}{λ^{4}}

where d is the particle diameter and λ is the wavelength of light. Therefore, a proper dispersion of the particles in the matrix is very important to reduce their effective diameter. However, homogeneous particle dispersion is one of the main challenges in manufacturing nanocomposites,^7,13,14 because the particles’ high surface energy causes strong interactions between them.¹⁵ There exist different approaches to the dispersion of NPs and the preparation of composites with a polymer matrix. Several methods for the preparation of transparent composites are described by Demir and Wegner¹⁴ and Althues et al.,¹⁶ whereas in situ polymerization and in situ particle processing are mentioned as an advantageous way to avoid particle aggregation. Other routes are the incorporation of NPs into the polymer melt by means of shear mixing with an extruder^17
–19 or the injection of a NP solution into the polymer melt during extrusion.²⁰ Chemically more demanding processes include the improvement of the particle dispersion and stabilization with grafted polymer chains.²¹

The aim of this work is to give an overview of different NP materials for transparent composites and their effect on the composite’s properties. Another focus lies on the transparency of these composites, which is critically evaluated with respect to the part thickness.

Following the “Introduction” section, this article summarizes the influence of NPs on transparent polymers sorted by particle material. A table offers an overview of the effects the fillers have on the properties and on the transparency. The different filler materials and the effect on the properties, as well as the contradiction between the composites’ enhanced properties and maintained transparency, is discussed in the subsequent section.

Particle materials and their effects on polymer properties

Gold and silver

Silver (Ag) NPs are often used because of their antibacterial properties in different matrix materials.^22
–24 Apart from this, an increase in the degradation temperature of 26 K (thermogravimetric analysis measurements) was observed with a filler content of 2 wt% by Vodnik et al.²⁵ Caseri²⁶ and Zimmermann et al.²⁷ demonstrate that it is possible to decrease the refractive index (RI) of gelatin by the incorporation of gold (Au) particles. At very high contents of 92.9 wt%, the RI is reduced from about 1.54 to 0.96 at 632.8 nm. The absorption and scattering of single metal particles is described in more detail in the study by Van Dijk et al.²⁸ and Nie and Emory.²⁹ This behavior can be applied to the coloring of polymers¹⁴ with Au NPs, whereas the resultative color of the nanocomposite depends on the particle size and geometry.³⁰

Titan oxide

Titan oxide (TiO₂) on the microscale is often used as a white colorant for polymers because it scatters all visible wavelengths.³¹ Nano-TiO₂ in transparent polymers causes absorption primarily in the ultraviolet (UV) region and is therefore often used as an UV absorber for wavelengths below 400 nm.^19,32
–34 Additionally, influences on the RI and the mechanical and thermal properties have been reported. Nussbaumer et al. observed an increase in the RI in poly(vinyl alcohol)^32,34 and Tao et al. raised the RI of PGMA (poly(glycidyl methacrylate)) and epoxy which also increased the Abbe number of the material.³⁵ Han and Yu³³ reported reduced elongation at break values in poly(ethylene phthalate), whereas Chatterjee¹⁹ showed an increase in the tensile modulus, decomposition temperature, and thermal stability of poly(methyl methacrylate) (PMMA).

Zinc oxide and cerium oxide

These kinds of NPs are utilized to produce luminescent nanocomposites.^36

–41 For cerium ions, the relationships among blueshift, the valence state of ions, and particle size are described by Tsunekawa et al.⁴⁰ Regarding other effects on the composite’s properties, an increase of 26°C in the glass transition temperature,³⁹ an increase in the thermal stability,⁴² and an increase in the RI⁴³ were observed with a PMMA matrix. The transmittance of rather thin specimen (0.4 mm thickness, 0.5 wt% zinc oxide (ZnO) quantum dot (QD)) is high.³⁷ On the other hand, thicker specimens have a low transmittance in the visual range (thickness: 3.5 mm, 1 wt% ZnO NP)⁴⁴ or simply a “transparent appearance” (thickness: 3 mm, ZnO QD).³⁸

A remarkable effect of the transmission of composites made of transparent polystyrene (PS) and cerium oxide (CeO₂) NPs was observed by Parlak and Demir.⁴⁵ The transmittance of the thin films showed a first-order exponential decay up to a particle concentration of 20 wt%, as the Rayleigh scattering theory proposes. Higher filling degrees do not impair further the transmission. This fact is explained as a result of interference in the multiple instances of light scattering by the quasi-ordered internal microstructure, developing at high filling degrees.

Modified tin oxide (ATO and ITO)

The conductivity and antistatic behavior of antimony-doped tin oxide (ATO) NPs dispersed in transparent polymers is described by Wakabayashi et al.⁴⁶ and Sun et al.⁴⁷ These examined films exhibit antistatic properties above a threshold value of only 0.2 vol%.⁴⁶ This is explained by the forming of chain-like aggregates in the NPs. Modified indium tin oxide (ITO) NPs as processed by Zhou et al.⁴⁸ affect the transparency such that both a UV and infrared (IR) absorption occur in poly(urethane acrylate) (PUA) while showing high transmittance values between 500 and 1000 nm (thickness: 0.1 mm).

Iron oxide and cobalt

Iron oxide (Fe₃O₄) NPs^49
–51 and cobalt (Co) NPs⁵² can lead to a ferromagnetic behavior in a composite. Peluso et al.⁵⁰ show that composites starting as iron(II)mercaptides show higher transmittance values in the range from 400 to 900 nm than composites starting as iron(III)mercaptides. High transparency in the composites is observed for wavelengths over 550⁵² or 700 nm.⁵¹ Potential applications mentioned by Barnakov et al.⁵³ can be found in the magneto-optic field or in electromagnetic interference blocking substrates.

Aluminum oxide

Aluminum oxide (Al₂O₃) NPs have a primarily positive effect on the mechanical properties of the composite. They increase the elastic modulus,^54,55 the yield stress,⁵⁵ and the impact strength.^56,57 According to Sarwar et al., the optimum loading for the best mechanical properties is 2.5 wt%.⁵⁵

Zirconium oxide

Zirconium oxide (ZrO₂) NPs diversely affect the composite properties. They can lead to improvements to the hardness and scratch resistance (for which a high filling degree of 15 wt% is needed),⁵⁸ enhancements to the mechanical properties,⁵⁹ adjustments to RI,^59,60 and improvements to thermal properties.^59,61 The different filler loading of 20 wt% for a variation in the RI between 1.475 to 1.625⁵⁹ and only 1 wt% for a variation between 1.515 and 1.659⁶⁰ proves most remarkable.

Silicon dioxide

Rahman and Padavettan⁶² offer a good summary of property changes in composites with silicon dioxide (SiO₂) particles. The mostly attained property changes are enhanced mechanical properties^63,64 and influences on the glass transition temperature in both directions.

Cadmium

Composites with NPs containing cadmium (Cd) are mainly prepared to achieve luminescent properties.^{16,65

–70} The luminescent properties increase with a good dispersion of the NPs⁶⁸ and primarily depend on the particle concentration^66,67 and the particle size.^65,69 The transmittance decreases strongly below a wavelength of 550 nm, whereas the composites have a good transmittance above.^68,70 Mohan et al.⁷⁰ also investigated the mechanical properties and observed an increase in the tensile modulus.

Zinc sulfide

The main purpose behind using zinc sulfide (ZnS) particles is also luminescent properties^71,72 or high RI materials.^73
–75 Zhang et al.⁷³ increased the RI of poly(vinylpyrrolidone) linearly from 1.5061 to 1.7523 with an increasing particle content up to 80 wt% while reducing the Abbe number. The transparency of thin films (thickness: 25 µm) above 500 nm was maintained.

Lead sulfide

As with ZnS, lead sulfide (PbS) is used to increase the RI.^76
–78 As such, Zimmermann et al.⁷⁶ and Weibel et al.⁷⁸ claim to have achieved the highest RIs of 2.5 and 2.9 (600 nm), respectively, known for polymeric composite materials. Lü et al.⁷⁷ offered insights into the transparence, indicating their nanocomposite films containing PbS particles exhibit good transparency (>90%) above 600 nm, although the PbS particles exhibit strong absorption below 600 nm in the UV–visible region.

Carbon nanotubes

A great deal of research has been done on transparent polymer composites with CNTs, whose production is described by Sun and Wang.⁷⁹ The authors manufactured electrically conductive transparent films,^80

–84 a transparent composite with enhanced mechanical properties⁸⁵ or a combination of mechanical, electrical, and transparent properties.⁸⁶ Another application reported by Yu et al. lies in advanced materials for packaging with increased mechanical and barrier properties as well as an enhanced decomposition temperature.⁸⁷

Table 1 offers a summary of the affected properties according to particle material with information about the transparency and specimen thickness.

Table 1.

Summary of the effects on the composite properties of different filler materials with information about thickness and transparency.

Property		Change	Transmittance	Specimen thickness	Filling degree	Matrix	Filler (size)	References
	RIUV absorption	1.52 (polymer) up to 1.61 (34 wt%)New property	Absorbance: 4.1% (11 wt%, 400 nm)	100 µm	Up to 34 wt%	PVAL	TiO₂ (2.5 nm)	³²
	RIHardnessThermal stability	1.475 up to 1.625+100%+50°C	97% (550 nm)	100 µm	20 wt%	PU	ZrO₂ (3.8 nm)	⁵⁹
	RI	1.54 up to 1.68 (4.6 vol%)	90% (550 nm, 3.8 vol%)	50–70 µm (transm.)70–500 nm (RI)	Up to 7.4 vol%	Siloxane-based	ZnS (1–5 nm)	⁷⁵
	RI	1.5–2.49 (632.8 nm)	—	40 nm–2 µm	Up to 82 wt%	Gelatin	PbS (2–5 nm)	⁷⁶
RI	RI	1.57–2.06 (632.8 nm)	90% (37 wt%, 600 nm)	1 µm	0–41.8 wt%	PTU	PbS (<10 nm)	⁷⁷
	RI	Up to 2.9 (632.8 nm)	—	—	88 wt%	PEO	PbS (2–40 nm)	⁷⁸
	RI	1.515–1.659 (acrylate)1.589–1.679 (epoxy), 633 nm	>90% (86 wt%)	2–5 µm	Up to 90 wt%	Acrylate-based/epoxy-based	ZrO₂ (3–4 nm)	⁶⁰
	RIUV absorption	1.42–1.475New property	70% (5 mm thickness, 600 nm)	50 µm–5 mm	20 wt%	Silicone	TiO₂ (5 nm)	⁸⁸
	RIThermal properties	1.506–1.752 (80 wt%, 590 nm)Thermal stability of the coupling agent of NPs increases with filler content	85% (70 wt%, 600 nm)	25 µm	0–80 wt%	PVP	ZnS (3 nm)	⁷³
	RI Scratch resistance	1.54 (polymer) to 1.92 (95 wt%, 550 nm)Increases	High (antireflectance coating)	52–80 nm	65–95 wt%	Acrylic	CeO₂ (10–20 nm)	⁴³
	RI Glass temperature	1.542 (polymer) up to 1.568 (47 vol%)Increases with NP content	80% (600 nm, 15 wt%)	4 mm	0–15 wt%	PDMAA/St/DVB	ZnS (3 nm)	⁷⁴
	Thermal stability	+35°C (Al₂O₃)+40°C (ZnO)	—	—	1 vol%	PMMA	Al₂O₃ (sphere, 20–45 nm) and ZnO (elongated, 20–35 nm)	⁴²
	Thermal stability	In N₂: +40°CIn air: +95°C	95% (600 nm, 20 wt%)	Thin	0–20 wt%	PMMA	ZrO₂/SiO₂ (<100 nm)	⁶¹
Thermal properties	Thermal stabilityUV absorption	+25°C (1 wt%)New property	2% (1 wt%)50% (0.01 wt%)<90% (0.01 wt%, prepolymer procedure, 600 nm)	3.5 mm	Up to 1 wt%	PMMA	ZnO (75 nm)	⁴⁴
Thermal properties	Glass temperaturePhotoluminescence	+26°CNew property	—	—	3.3 wt%	PMMA	ZnO (6–7 nm)	³⁹
	Tensile modulusdecomposition temperatureGlass temperatureUV absorption	+95% +11%+23%New property	—	0.3 mm	0–15 wt%	PMMA	TiO₂ (5 nm)	¹⁹
	Young modulusTensile strengthUV absorption	+30%+100%New property	80% (2 wt%, 600 nm)	0.15 mm	2 wt%	PC	AlO₂ (whisker: diameter = 3 nm, length = 2800 nm)	⁵⁴
Mechanical properties	Young modulusGlass temperatureTensile strength	+41% (5 wt%)+37% (15 wt%)+41% (2.5 wt%)	87% (600 nm, 15 wt%)	0.12–0.29 mm	0–15 wt%	PA	Al₂O₃	⁵⁵
	StrengthElongation at break	+21%+100%	71% (600 nm)	50 µm	0.5 wt%	PAEK	MWNT (diameter: 10–20 nm, length: 5–20 µm)	⁸⁵
	Impact strength	+126% (1 wt%)	52% (1 wt%, 600 nm)	2 mm	1 or 2 wt%	PC	Al₂O₃ (96 nm)	⁵⁶
	Impact resistance	Increases with the number of layers and addition of NP in the layer bonding adhesive	88% (2 layers)65% (10 layers, 600 nm)	2 mm pure PMMA layers, bonded with NP adhesive	2 wt% in adhesive	PMMA, styrene, SAN	Alumina (35 nm)	⁵⁷
	HardnessElastic modulus	+40%+25%	—	—	3 wt%	PMMA	SiO₂ (63 nm)	⁶³
	Tensile strengthGlass temperatureScratch resistanceCoefficient of friction	+10% (3 wt%)+6% (3 wt%)HigherLower	82% (600 nm, 3 wt%)	Films	1–5 wt%	COC	SiO₂ (8 nm)	⁶⁴
Mechanical properties	HardnessScratch resistance	IncreasesIncreases	80% (600 nm, 15 wt%)	—	0–15 wt%	PMMA	ZrO₂ (4 nm)	⁵⁸
	Tensile modulusLuminescence	+52%New property	High	—	—	Epoxy	CdSe-CdS-ZnS core–multi-shell (3–10 nm)	⁷⁰
	Elastic modulusYield stressFracture strain	+52%+14%+257%	—	100 nm	0–15 wt%	PS	SiO₂ (14 nm)	⁸⁹
	LuminescenceUV absorption	New propertyNew property	90% (600 nm)	4 mm	0.5 wt%	PMMA	ZnO (2.3 nm)	³⁷
	Luminescence	New property	30% (0.1 ppm, 600 nm)	5 mm	0.02–0.1 ppm	PSO	CdS (6 nm)	⁶⁶
	Luminescence	New property	65% (1.2 wt%, 600 nm)	4 mm	0–1.2 wt%	Epoxy	CdSe (4 nm)	⁶⁸
Luminescence	PhotoluminescenceEmitting layer of LED	New propertyNew property	High	50 nm (LED)	Zn: 8.2 wt%Cu: 0.12 wt%	Sulphonated PS	ZnS: Cu (3 nm)	⁷¹
	Conductivity	10⁻² S/cm (50 vol%)	75% (50 vol%, 1.3 µm thick), 17% (50 vol%, 3.4 µm thick), 600 nm	1–16 µm (coating)	0–50 vol%	Poly(vinyl acetate-acrylic) (PVAc-co-acrylic)	ATO (15 nm)	⁴⁷
	Permittivity	+46%	>85% above 400 nm	0.127 mm	0.26 wt%	PMMA	SWNT	⁸⁰
	Antistatic properties	Threshold value: 0.2 vol%	91% (0.8 vol%, total luminous transmittance)	2 µm	0–0.8 vol%	Acrylate	ATO (8 nm)	⁴⁶
	Electrical conductivityPower conversion efficiency of an organic solar cell	Sheet resist.: −60% +30%	94% (600 nm)	Thin films	—	PEDOT:PPS	SWNT	⁸¹
Electrical properties	Conversion efficiency of an organic solar cell	+20%	High (>600 nm)	20 nm	—	PEDOT:PSS	Ag (13 nm)	⁹⁰
	Magnetic properties	New property	“Good, but brown colored” (>550 nm)	1 mm	3–15 wt%	PS	Fe₃O₄ (acicular, 10 × 50 nm and spherical, 6 nm)	⁵⁰
	Magnetic properties	New property	“Quite” transparent (>550 nm)	100 µm	5–15 wt%	PS	Fe₃O₄ (10 × 50 nm)	⁵¹
	Magneto-optical properties: Faraday rotation spectra	Red shift with larger particles	Increasing absorption >600 nm	100–300 µm	—	PMMA	Fe₃O₄ (8–200 nm)	⁵³
Magnetic properties	Magneto-optical properties: Faraday rotation spectra	Red shift with larger particles	Increasing absorption >600 nm	100–300 µm	—	PMMA	Fe₃O₄ (8–200 nm)	⁵³
	Transparency at high loadings	—	Decreases to 75% (20 wt%), then stays constant (up to 95 wt%); 550 nm	2.5 µm	Up to 95 wt%	PS	CeO₂ (20 nm)	⁴⁵
	UV and IR absorption	New property	85% (9 wt%, 600 nm)	100 µm	0–9 wt%	PUA	ITO (163 nm)	⁴⁸
Optical properties	UV absorptionTensile strength	New property−11%	—	Fibers and fabric	0–2 wt%	PET	TiO₂	³³
Other	Oxygen barrier	New property	93% (77 wt%, 135 nm thickness, 565 nm)	1.2–230 nm (several layers)	77–84 wt%	PEI	MMT	⁹¹

PET: poly(ethylene phthalate); PUA: poly(urethane acrylate); PMMA: poly(methyl methacrylate); ZNO: zinc oxide; CeO₂: cerium oxide; ATO: antimony-doped tin oxide; ITO: indium tin oxide; IR: infrared; UV: ultraviolet; Fe₃O₄: iron oxide; CO: cobalt; Al₂O₃: aluminum oxide; ZrO₂: zirconium oxide; SiO₂: silicon dioxide; ZnS: zinc sulfide; PbS: lead sulfide; PU: polyurethane; PTU: polythiourethane; PEO: poly(ethyleneoxide); PDMAA/St/DVB: poly(N,N-dimethylacrylamide)/styrene/divinylbenzene; PC: polycarbonate; PA: polyamide; PAEK: poly(aryl ether ketone); SAN: styrene acrylonitrile; PSO: polysiloxane; PEDOT:PPS: poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate); PEI: polyethylenimine; PVA: poly(vinyl alcohol); PVP: poly(vinylpyrrolidone); MMT: sodium montmorillonite; MWNT: multiwalled carbon nanotube; SWNT: single-walled carbon nanotube; Ag: silver; LED: light-emitting diode; Al2O3 : aluminium oxide; CdSe: cadmium selenide; CdS: cadmium sulfide.

Discussion

Filler materials and properties

As described above, NPs affect the composite properties in a broad range. Additional to the materials of filler and matrix, the achievable properties depend on the filling degree, particle size and shape, processing method, particle dispersion, matrix–particle adhesion and other factors. Nevertheless, Figure 1 gives a comparative overview over the materials listed in Table 1 and their effects on the nanocomposites properties. The figure shows that specific materials diversely affect the properties due to the numerous additional influences mentioned above. However, some materials are particularly suitable to improve specific properties, like Al₂O₃ and SiO₂ for better mechanical properties or PbS for a higher RI.

Figure 1.

Percentage changes of properties in dependence of the filler material including the references.

Influence of the particles on the transparency

According to Rayleigh scattering, the scattering section C_s of a particle merely depends on its diameter d, the RI relation m between particle and matrix and the wavelength λ¹²

C_{s} = \frac{2 π^{5} d^{6}}{3 λ^{4}} {(\frac{m^{2} - 1}{m^{2} + 1})}^{2}

Consequently, the only possibilities to reduce scattering in the visual range are to reduce the particle size (improving the particle dispersion has the same effect) or matching the RI between particles and matrix. Promising results with RI matching were achieved by synthesizing core–shell particles^92
–94 or polymer–graft modifications of fillers, both with the matrix’ RI is in between the RIs of core and shell or of particles and grafted polymer chains.⁹⁵ Effective medium theory describes the RI of core–shell particles⁹⁵ and polymer–graft modificated particles.⁹⁴ The theory enables the calculation of an effective RI out of the different materials’ properties of the particle to match it’s RI to the matrix’ RI. Nevertheless, limitations have to be considered if the particles size is comparable to the wavelength of the incident light, due to additional Mie scatterings by the core.⁹⁶

Furthermore, some materials absorb shares of the incident light. Only dielectric particles with zero electrical conductivity are completely nonabsorbing.¹² The absorption of a material is expressed by the material’s complex RI. Nevertheless, almost dielectric materials like glass have a negligible dielectric loss in the visual range and therefore hardly absorb the light. On the contrary, metal particles, even with small diameters, have a strong wavelength-dependent absorption in the visual range due to the oscillation of free surface electrons.^97
–99 The absorption cross-section of particles varies as d³. Therefore, the absorption of very small particles exceeds their total scattering.^28,100,101

Transparency and enhanced properties

Regarding the specimen from the listed references in this article, most authors analyze very thin specimen or give no exact information about the thickness. However, the thickness is crucial for classifying the transparency gains, as transmission decreases strongly with it. Investigations of thin films have several advantages: The specimens enable easy preparation from a polymer solution via drying or melt spinning and thin films require only a small amount of NPs. In addition, a high rate of transparency can be easily achieved.

In few of the analyzed papers, specimens with a thickness of ≥1 mm are described and information about the transparency is given.^{37,44,50,56,66,68,74,88,94} These specimens mostly have very low NP content if they exhibit a relatively high amount of transparency. With this low particle loading, hardly any influence on properties other than the specimen’s optical characteristics (luminescence and UV absorption) is ascertainable. For example, Anžlovar et al.⁴⁴ reach high transparency values with a filler content of 0.01 wt%, but for a significant change in thermal stability, a filler content of 1 wt% is needed. However, with this filler content, the transparency at 500 nm is almost zero. Magnetic behavior is reported by Peluso et al.⁵⁰ who claim that the 1-mm thick specimens have perfect transparency above a wavelength of 550 nm, but there is no specific transparency value given and the specimens are simply “brown colored”. Chandra et al.⁵⁶ attained an increased impact strength but also have a high transmission loss (transmittance: 40% at 500 nm and 1 wt% of Al₂O₃ NPs). There is little information about transparent thick specimens with NP filling that enhances the properties: Guan et al.⁷⁴ describe 4-mm thick, highly transparent composites with a filling degree of up to 15 wt% ZnS NPs which had been prepared through one-pot in situ bulk polymerization. The particles cause an increase in RI and in glass transition temperature of the transparent composite. Also thick (5 mm), highly filled (20 wt%) specimens with enhanced RI are described by Li et al.⁸⁸ They achieved a good dispersion in silicone by using bimodal-poly(dimethyl siloxane)-brush-grafted TiO₂ particles.

On the other hand, optical properties can be affected by very low filling degrees, which maintain the transparency in the visual range. Luminescent properties were integrated by Zou et al. with very low filling degrees of 0.3 wt% and by maintaining the transparency (73% for 530 nm).⁶⁸ As shown by Anžlovar et al.⁴⁴ UV absorption is possible with a very low ZnO content of 0.01 wt% while exhibiting high transparency values in the visual range.

As mentioned above, more recent approaches focus on core–shell particles and polymer–graft modification of particles. Li et al.⁹⁴ achieved high rates of transparency (70% at 500 nm) with a filling degree of 1 wt% of SiO₂/TiO₂ core–shell particles and a specimen thickness of 4 mm. It was possible to control the RI of the total particle with the ratio of core to shell. If the RI does not match the matrix, the specimen has a completely opaque appearance. Incel et al.⁹² manufactured highly transparent PS films, filled with ceria/silica hybrid particles. The particles have the same RI as the matrix due to their controlled core–shell ratio. The approach of RI matching by grafting polymer chains which are different to the polymer matrix is described by Parlak and Demir.¹⁰² and Dang et al.⁹⁵ Methods of synthesis, analysis, and some properties of nanocomposites with polymer-grafted NPs are summarized by Kumar et al.²¹ An increase in the mechanical properties observed by Maillard et al.⁸⁹ was attributed to the uniform dispersion and strong interfacial binding due to the polymer-grafted NPs. The achieved transparency values are promising. However, in combination with transparent polymers, most authors focus on the transparency only and the impact on other properties is scarcely investigated. Furthermore, these particles are more challenging to synthesize, so they are produced in very small quantities only. To investigate the role of film thickness and to validate these results on a macroscopic scale, it would be necessary to produce larger quantities of these particles.²¹

Conclusion

NPs are widely used in research to enhance material properties and to functionalize transparent polymers. For a significant impact on the mechanical and thermal properties, for example, the glass transition temperature or decomposition temperature, very low filler loadings compared to conventional fillers are necessary. However, there are considerable shortcomings in the transparency after integrating NPs into the transparent polymers. Despite several approaches to dispersing the particles homogeneously, achieving transparency for thicker parts while simultaneously enhancing other properties significantly has largely remained an insurmountable hurdle. Also, thin films mostly show a significant decrease in the transmission. Only optical properties such as luminescence and UV absorption can be included with very low filler contents enabling to maintain the polymer’s transparency. These shortcomings in the transmission of transparent nanocomposites indicate that the homogeneous dispersion of the particles remains a major difficulty, which has not yet been satisfactorily solved on a macroscopic level. However, there are promising approaches in NP chemistry to improve their dispersion and/or to adapt their RI to the matrix in order to achieve high transparency values. However, until now, there is sparse information available about the effect on other properties. Consequently, future research should focus on the upscaling of the particle production and on a comprehensive analysis of the macroscopic nanocomposites properties to identify and exploit their potential in practical applications.

Footnotes

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was financially supported by the Deutsche Forschungsgemeinschaft (DFG) through the Cluster of Excellence Engineering of Advanced Materials.

ORCID iD

Wolfgang Wildner

References

Kojima

Usuki

Kawasumi

, et al. Mechanical properties of nylon 6-clay hybrid. J Mater Res 1993; 8: 1185–1189.

Rothon

. Particulate-filled polymer composites. Akron: Rapra Technology, 2003.

Siegel

. Nanostructure science and technology: R & D status and trends in nanoparticles, nanostructured materials and nanodevices. Dordrecht: Springer Netherlands, 2013.

Ichinose

Ozaki

Kashu

. Superfine particle technology. London: Springer Verlag, 1992.

Gilbertson

Albalghiti

Fishman

, et al. Shape-Dependent surface reactivity and antimicrobial activity of nano-cupric oxide. Environ Sci Technol 2016; 50: 3975–3984.

Bonanni

Bonetti

Pakizeh

, et al. Designer magnetoplasmonics with nickel nanoferromagnets. Nano Lett 2011; 11: 5333–5338.

Ajayan

Schadler

Braun

. Nanocomposite science and technology. Weinheim: Wiley VCH, 2003.

Friedrich

Fakirov

Zhang

. Polymer composites: From nano- to macro-scale. New York: Springer US, 2005.

Evanoff

Jr Chumanov

. Size-controlled synthesis of nanoparticles. 2. Measurement of extinction, scattering, and absorption cross sections. J Phys Chem B 2004; 108: 13957–13962.

10.

Meeten

. Optical properties of polymers. New York: Elsevier Applied Science Publishers, 1986.

11.

Schärtl

. Light scattering from polymer solutions and nanoparticle dispersions. Berlin, Heidelberg: Springer-Verlag Berlin Heidelberg, 2007.

12.

Hulst

. Light scattering by small particles. New York: Dover-Publications, 1981.

13.

Uddin

Sun

. Effect of nanoparticle dispersion on mechanical behavior of polymer nanocomposites. In: 50th AIAA/ASME/ASCE/AHS/ASC Structures, structural dynamics and materials conference. Palm Springs, California, 4-7 May 2009. USA.

14.

Demir

Wegner

. Challenges in the preparation of optical polymer composites with nanosized pigment particles: a review on recent efforts. Macromolecular Materials and Engineering 2012; 297: 838–863.

15.

Liu

Gao

Cao

, et al. Nanoparticle dispersion and aggregation in polymer nanocomposites: insights from molecular dynamics simulation. Langmuir 2011; 27: 7926–7933.

16.

Althues

Henle

Kaskel

. Functional inorganic nanofillers for transparent polymers. Chem Soc Rev 2007; 36: 1454–1465.

17.

Shah

Paul

. Nylon 6 nanocomposites prepared by a melt mixing masterbatch process. Polymer 2004; 45: 2991–3000.

18.

Zhao

RKY

. A study on the photo-degradation of zinc oxide (ZnO) filled polypropylene nanocomposites. Polymer 2006; 47: 3207–3217.

19.

Chatterjee

. Properties improvement of PMMA using nano TiO₂ . J Appl Polym Sci 2010; 118: 2890–2897.

20.

Bacher

Berkei

Drude

, et al. Direktprozess zur herstellung von nanosuspensionen und deren zudosierung in thermoplastische matrices zur herstellung von nanocomposites. Pfinztal: Fraunhofer Verlag, 2011.

21.

Kumar

Jouault

Benicewicz

, et al. Nanocomposites with polymer grafted nanoparticles. Macromolecules 2013; 46: 3199–3214.

22.

Pinto

RJB

Fernandes

SCM

Freire

CSR

, et al. Antibacterial activity of optically transparent nanocomposite films based on chitosan or its derivatives and silver nanoparticles. Carbohyd Res 2012; 348: 77–83.

23.

Akhavan

. Lasting antibacterial activities of Ag-TiO₂/Ag/a-TiO₂ nanocomposite thin film photocatalysts under solar light irradiation. J Colloid Interf Sci 2009; 336: 117–124.

24.

Chan

CMN

Cheng

Djurišić

, et al. Multicomponent antimicrobial transparent polymer coatings. J Appl Polym Sci 2011; 122: 1572–1578.

25.

Vodnik

Vuković

Nedeljković

. Synthesis and characterization of silver-poly(methylmethacrylate) nanocomposites. Colloid Polym Sci 2009; 287: 847–851.

26.

Caseri

. Nanocomposites of polymers and metals or semiconductors: historical background and optical properties. Macromol Rapid Comm 2000; 21: 705–722.

27.

Zimmermann

Weibel

Caseri

, et al. Polymer nanocomposites with “ultralow” refractive index. Polym Advan Technol 1993; 4: 1–7.

28.

Van Dijk

Tchebotareva

Orrit

, et al. Absorption and scattering microscopy of single metal nanoparticles. Phys Chem Chem Phys 2006; 8: 3486–3495.

29.

Nie

Emory

. Probing single molecules and single nanoparticles by surface-enhanced Raman scattering. Science 1997; 275: 1102–1106.

30.

Liu

Mills

Composto

. Tuning optical properties of gold nanorods in polymer films through thermal reshaping. J Mater Chem 2009; 19: 2704–2709.

31.

Murphy

. Additives for plastics handbook. New York: Elsevier Science, 2001.

32.

Nussbaumer

Caseri

Smith

, et al. Polymer-TiO₂ nanocomposites: a route towards visually transparent broadband UV filters and high refractive index materials. Macromol Mater Eng 2003; 288: 44–49.

33.

Han

. Study of the preparation and properties of UV-blocking fabrics of a PET/TiO₂ nanocomposite prepared by in situ polycondensation. J Appl Polym Sci 2006; 100: 1588–1593.

34.

Nussbaumer

. Synthese und charakterisierung von rutilnanopartikeln und deren einbau in transparente polymer-nanoverbundwerkstoffe. Zürich: Eidgenössische Technische Hochschule, 2004.

35.

Tao

Rungta

, et al. TiO₂ nanocomposites with high refractive index and transparency. J Mater Chem 2011; 21: 18623–18629.

36.

Althues

. Lumineszierende, transparente Nanokomposite—Synthese und Charakterisierung. Fakultät Mathematik und Naturwissenschaften. Dresden: Technischen Universität Dresden, 2007.

37.

Zhang

Wang

Liu

, et al. Highly transparent bulk PMMA/ZnO nanocomposites with bright visible luminescence and efficient UV-shielding capability. J Mater Chem 2012; 22: 11971–11977.

38.

Matsuyama

Mishima

Kato

, et al. Transparent polymeric hybrid film of ZnO nanoparticle quantum dots and PMMA with high luminescence and tunable emission color. J Colloid Interf Sci 2012; 367: 171–177.

39.

Sato

Kawata

Morito

, et al. Preparation and properties of polymer/zinc oxide nanocomposites using functionalized zinc oxide quantum dots. Eur Polym J 2008; 44: 3430–3438.

40.

Tsunekawa

Wang

Kawazoe

, et al. Blueshifts in the ultraviolet absorption spectra of cerium oxide nanocrystallites. Jpn J Appl Phys 2003; 94: 3654–3656.

41.

Sarapulova

Sherstiuk

Shvalagin

, et al. Photonics and nanophotonics and information and communication technologies in modern food packaging. Nanoscale Res Lett 2015; 10: 229.

42.

Viratyaporn

Lehman

. Effect of nanoparticles on the thermal stability of PMMA nanocomposites prepared by in situ bulk polymerization. J Therm Anal Calorim 2011; 103: 267–273.

43.

Krogman

Druffel

Sunkara

. Anti-reflective optical coatings incorporating nanoparticles. Nanotechnology 2005; 16: S338–S343.

44.

Anžlovar

Orel

Žigon

. Sub micrometer and nano ZnO as filler in PMMA materials. Mater Tehnol 2011; 45: 269–274.

45.

Parlak

Demir

. Anomalous transmittance of polystyrene-ceria nanocomposites at high particle loadings. J Mater Chem C 2013; 1: 290–298.

46.

Wakabayashi

Sasakawa

Dobashi

, et al. Optically transparent conductive network formation induced by solvent evaporation from tin-oxide-nanoparticle suspensions. Langmuir 2007; 23: 7990–7994.

47.

Sun

Gerberich

Francis

. Electrical and optical properties of ceramic-polymer nanocomposite coatings. J Polym Sci, Part B-Polym Phys 2003; 41: 1744–1761.

48.

Zhou

Wang

Tian

, et al. Preparation of UV-curable transparent poly(urethane acrylate) nanocomposites with excellent UV/IR shielding properties. Compos Sci Technol 2014; 94: 105–110.

49.

Gass

Poddar

Almand

, et al. Superparamagnetic polymer nanocomposites with uniform Fe₃O₄ nanoparticle dispersions. Adv Funct Mater 2006; 16: 71–75.

50.

Peluso

Pagliarulo

Carotenuto

, et al. Synthesis and characterization of polymer embedded iron oxide nanocomposites. Microw Opt Techn Let 2009; 51: 2774–2777.

51.

Carotenuto

Pepe

Davino

, et al. Transparent-ferromagnetic thermoplastic polymers for optical components. Microw Opt Techn Let 2006; 48: 2505–2508.

52.

Horikawa

Yamaguchi

Inoue

, et al. Magneto-optical effect of films with nano-clustered cobalt particles dispersed in PMMA plastics. Mat Sci Eng A 1996; 217–218: 348–352.

53.

Barnakov

Scott

Golub

, et al. Spectral dependence of Faraday rotation in magnetite-polymer nanocomposites. J Phys Chem Solids 2004; 65: 1005–1010.

54.

Hakimelahi

Rupp

, et al. Synthesis and characterization of transparent alumina reinforced polycarbonate nanocomposite. Polymer 2010; 51: 2494–2502.

55.

Sarwar

Zulfiqar

Ahmad

. Investigating the property profile of polyamide-alumina nanocomposite materials. Scripta Mater 2009; 60: 988–991.

56.

Chandra

Turng

, et al. Fracture behavior and optical properties of melt compounded semi-transparent polycarbonate (PC)/alumina nanocomposites. Compos Part A-Appl S 2011; 42: 1903–1909.

57.

Rai

Singh

. Impact resistance behavior of polymer nanocomposite transparent panels. J Compos Mater 2009; 43: 139–151.

58.

Zhou

. Surface mechanical properties of transparent poly(methyl methacrylate)/zirconia nanocomposites prepared by in situ bulk polymerization. Polymer 2009; 50: 3609–3616.

59.

. Fabrication of transparent Pu/ZrO₂ nanocomposite coatings with high refractive index. Chinese J Polym Sci (English Edition) 2010; 28: 13–20.

60.

Kudo

Nagase

Kubo

, et al. Optically transparent and refractive index-tunable ZrO₂/photopolymer composites designed for ultraviolet nanoimprinting. Jpn J Appl Phys 2011; 50, 06GK12-1 to 06GK12-7.

61.

Wang

Zhong

, et al. Transparent poly(methyl methacrylate)/silica/zirconia nanocomposites with excellent thermal stabilities. Polym Degrad Stabil 2005; 87: 319–327.

62.

Rahman

Padavettan

. Synthesis of Silica nanoparticles by sol-gel: size-dependent properties, surface modification, and applications in silica-polymer nanocomposites—a review. J Nanomater 2012; 1–15.

63.

Stojanovic

Orlovic

Markovic

, et al. Nanosilica/PMMA composites obtained by the modification of silica nanoparticles in a supercritical carbon dioxide-ethanol mixture. J Mater Sci 2009; 44: 6223–6232.

64.

Roy

Das

Yue

, et al. Transparent cyclic olefin copolymer/silica nanocomposites. Polym Int 2014; 63: 327–332.

65.

Zhang

Cui

Wang

, et al. From water-soluble CdTe nanocrystals to fluorescent nanocrystal-polymer transparent composites using polymerizable surfactants. Adv Mater 2003; 15: 777–780.

66.

Shen

Liu

, et al. Transparent luminescent bulk nanocomposites of polysiloxane embedded with CdS nanocrystallines by a direct dispersion process. Nanoscale 2012; 4: 1652–1657.

67.

Zhang

Jiang

Wang

, et al. In situ synthesis of transparent fluorescent cadmium sulfide-poly(arylene ether ketone) nanocomposite hybrids. Adv Polym Sci 2013; 25: 879–885.

68.

Zou

, et al. A transparent and luminescent epoxy nanocomposite containing CdSe QDs with amido group-functionalized ligands. J Mater Chem 2011; 21: 13276–13282.

69.

Lee

Sundar

Heine

, et al. Full color emission from II-VI semiconductor quantum dot-polymer composites. Adv Mater 2000; 12: 1102–1105.

70.

Mohan

Oluwafemi

Songca

, et al. Facile synthesis of transparent and fluorescent epoxy-CdSe-CdS-ZnS core-multi shell polymer nanocomposites. New J Chem 2014; 38: 155–162.

71.

Huang

Yang

Xue

, et al. Photoluminescence and electroluminescence of ZnS: Cu nanocrystals in polymeric networks. Appl Phys Lett 1997; 70: 2335–2337.

72.

Dai Prè

Martucci

Leoni

. Synthesis and characterization of ZnS: Mn nanoparticles. Proceedings of SPIE—The International Society for Optical Engineering, Photonics for Solar Energy Systems III, Brussels, Belgium, 13–15 April 2010.

73.

Zhang

Goh

ESM

Beuerman

, et al. Development of optically transparent ZnS/poly(vinylpyrrolidone) nanocomposite films with high refractive indices and high Abbe numbers. J Appl Polym Sci 2013; 129: 1793–1798.

74.

Guan

Lü

Cheng

, et al. A facile one-pot route to transparent polymer nanocomposites with high ZnS nanophase contents via in situ bulk polymerization. J Mater Chem 2009; 19: 617–621.

75.

Sergienko

Godovsky

Zavin

, et al. Nanocomposites of ZnS and poly-(dimethyl)-block-(phenyl)siloxane as a new high-refractive index polymer media. Nanoscale Res Lett 2012; 7: 1–7.

76.

Zimmermann

Weibel

Caseri

, et al. High refractive index films of polymer nanocomposites. J Mater Res 1993; 8: 1742–1748.

77.

Lü

Guan

Liu

, et al. PbS/polymer nanocomposite optical materials with high refractive index. Chem Mater 2005; 17: 2448–2454.

78.

Weibel

Caseri

Suter

, et al. Preparation of polymer nanocomposites with “Ultrahigh” refractive index. Polymers for Advanced Technologies 1991; 2: 75–80.

79.

Sun

Wang

. Syntheses and applications of carbon nanotubes and their composites. Rijeka: INTECH, 2013.

80.

Clayton

Sikder

Kumar

, et al. Transparent poly(methyl methacrylate)/single-walled carbon nanotube (PMMA/SWNT) composite films with increased dielectric constants. Adv Funct Mater 2005; 15: 101–106.

81.

Jin

Cha

Jun

, et al. Non-covalently functionalized single walled carbon nanotube/poly(3, 4ethylenedioxythiophene): poly(styrenesulfonate) nanocomposites for organic photovoltaic cell. Synthetic Met 2013; 181: 92–97.

82.

Huang

Yang

, et al. A new and general fabrication of an aligned carbon nanotube/polymer film for electrode applications. Adv Mater 2011; 23: 4707–4710.

83.

Zhang

, et al. Effect of carbon nanotubes shape on the properties of multiwall carbon nanotubes/polyethylene flexible transparent conductive films. J Mater Sci-Mater El 2014; 25: 2692–2696.

84.

Kim

Park

. In-situ preparation of multi-walled carbon nanotube (MWNT)/cellulose nanocomposites and their physical properties. Fiber Polym 2013; 14: 566–570.

85.

Zhou

, et al. Dispersion of multi-walled carbon nanotubes in poly(aryl ether ketone) obtained by in situ polymerization. Polym Int 2009; 58: 832–837.

86.

Park

Ounaies

Watson

, et al. Dispersion of single wall carbon nanotubes by in situ polymerization under sonication. Chem Phys Lett 2002; 364: 303–308.

87.

Qin

Sun

, et al. Reinforcement of transparent poly(3-hydroxybutyrate-co-3-hydroxyvalerate) by incorporation of functionalized carbon nanotubes as a novel bionanocomposite for food packaging. Compos Sci Technol 2014; 94: 96–104.

88.

Tao

Viswanath

, et al. Bimodal surface ligand engineering: the key to tunable nanocomposites. Langmuir 2013; 29: 1211–1220.

89.

Maillard

Kumar

Fragneaud

, et al. Mechanical properties of thin glassy polymer films filled with spherical polymer-grafted nanoparticles. Nano Lett 2012; 12: 3909–3914.

90.

Kim

, et al. Plasmon enhanced performance of organic solar cells using electrodeposited Ag nanoparticles. Appl Phys Lett 2008; 93: 073307-1 to 073307-3.

91.

Priolo

Gamboa

Grunlan

. Transparent clay-polymer nano brick wall assemblies with tailorable oxygen barrier. ACS Appl Mater Inter 2010; 2: 312–320.

92.

Incel

Güner

Parlak

, et al. Null extinction of Ceria@ silica hybrid particles: transparent polystyrene composites. ACS Appl Mater Inter 2015; 7: 27539–27546.

93.

Small

Hong

Pine

. Scattering properties of core-shell particles in plastic matrices. J Polym Sci Part B-Pol Phys 2005; 43: 3534–3548.

94.

Yang

, et al. Facile synthesis of highly transparent polymer nanocomposites by introduction of core-shell structured nanoparticles. Chem Mater 2008; 20: 2637–2643.

95.

Dang

Ojha

Hui

, et al. High-transparency polymer nanocomposites enabled by polymer-graft modification of particle fillers. Langmuir 2014; 30: 14434–14442.

96.

Zhang

Zhu

, et al. Effective medium theory for random media composed of two-layered spheres. J Opt Soc Am A 2011; 28: 2292–2297.

97.

Jain

Lee

El-Sayed

, et al. Calculated absorption and scattering properties of gold nanoparticles of different size, shape, and composition: Applications in biological imaging and biomedicine. J Phys Chem B 2006; 110: 7238–7248.

98.

El-Sayed

. Some interesting properties of metals confined in time and nanometer space of different shapes. Acc Chem Res 2001; 34: 257–264.

99.

Bohren

Huffman

. Absorption and scattering of light by small particles. Hoboken: Wiley, 1998.

100.

Fan

Zheng

Singh

. Light scattering and surface plasmons on small spherical particles. Light Sci Appl. 2014; 3: e179.

101.

van de Hulst

. Light scattering by small particles. New York: Dover Publications, 1981.

102.

Parlak

Demir

. Toward transparent nanocomposites based on polystyrene matrix and PMMA-grafted CeO₂ nanoparticles. ACS Appl Mater Inter 2011; 3: 4306–4314.

Nanofiller materials for transparent polymer composites: Influences on the properties and on the transparency—A review

Abstract

Keywords

Introduction

Particle materials and their effects on polymer properties

Gold and silver

Titan oxide

Zinc oxide and cerium oxide

Modified tin oxide (ATO and ITO)

Iron oxide and cobalt

Aluminum oxide

Zirconium oxide

Silicon dioxide

Cadmium

Zinc sulfide

Lead sulfide

Carbon nanotubes

Discussion

Filler materials and properties

Influence of the particles on the transparency

Transparency and enhanced properties

Conclusion

Footnotes

Funding

ORCID iD

References