Investigation of optical properties of nanoparticle-dispersed polymer films

Composite films of polystyrene and polymethyl methacrylate with added ZnO and ZrO2 nanoparticles have been prepared. Effective medium approximations have been applied to calculate final refractive indices. Results are compared to the experimental values of films with different thickness. Transparency of neat polymer and nanocomposite layers is also studied.


Introduction
Optical polymer materials (PMs) are nowadays used not only in the design of consumer but also of precise optical systems and devices.They are low cost, light organic glasses that transmit well in the visible and near infrared regions [1].Polymers present, however, some challenges to the designers as high temperature sensitivity, restricted range of refractive index (RI) values, stress birefringence, hygroscopicity, etc. Better properties present hybrid plastic-glass optical elements [2].
Unique priority of polymers over glass is safety which determines their only choice in medical clinical applications.Optical polymers are applied in ophthalmology as well as in many surgical and diagnostic instruments as laparoscopes, arthroscopes, endoscopes, etc.Another field of important applications of polymers is communication and especially fiber optics [3].As known, optical losses of polymer waveguides are low in the three telecommunication windows around 850, 1310 and 1550 nm.
Optical applications of PMs are defined mainly by their index of refraction determined with sufficiently high accuracy in the visible (VIS) light where imaging instruments or free space optical devices are used and near-infrared (NIR) region for data transfer.Basic polymers and many trademarks have typical values of RI at d-line in the range from 1.47 to 1.59.Some polymer companies produce polymers with higher refraction up to n d = 1.74 [1].High RI materials are preferred in optical design giving a variety of aberration compensations, technological applications in waveguides, optical switches, antireflective coatings, etc.
One approach to increase refraction of polymers is incorporation of optical PMs with inorganic nanoparticles with high refractive index value.Many metal oxide coatings as ZnO, ZrO 2 , TiO 2 , etc. are reported to have refractive indices of 1.87, 1.96 and 2.28, respectively [4].Direct mixing of particles with the polymer matrix is the simplest method to obtain nanocomposites.Melt or solution blending techniques are usually used [5].Some disadvantages of the first method are degradation of polymer matrix as a result of the high melting temperature as well as the poor dispersion of particles in this state of the polymer.
In this study, ZnO and ZrO 2 nanoparticles (NPs) have been added to polystyrene (PS) and polymethyl methacrylate (PMMA) solutions.Among principal plastics PMMA and PS show highest transparency [6] which make them preferred materials for various optical applications.Casting and spin coating methods of blendеd solutions on glass substrates have been applied to obtain polymer films with different thicknesses.Prediction of final refractive properties of nanocomposites is possible applying some of the known theoretical models.Comparison among computed and experimental results is accomplished.Transmittance curves of different films are presented and discussed.

Experimental
All polymers and metal oxide nanoparticles have been purchased from Sigma-Aldrich Company and product numbers are included.Solvents are also reported.The list of the materials is as follows:  Poly(methyl methacrylate) (182230), chemical formula [CH 2 C(CH 3 )(CO 2 CH 3 )] n , average molecular weight MW ~120,000 g/mol and density 1.188 g/cm 3 at 25 C;  Poly(methyl methacrylate) (445746), average MW~350,000 g/mol and density 1.170 g/cm 3 ;  Polystyrene (430102), chemical formula [CH 2 CH(C 6 H 5 )] n and density 1.060 g/cm 3 ;  Zinc oxide (544906) nanopowder with particle size smaller than 100 nm, molecular weight 81.39 g/mol and density 5.68 g/cm 3 ;  Zirconium (IV) oxide (544760) nanopowder with particle size smaller than 100 nm, molecular weight 123.22 g/mol and density 5.89 g/cm 3 ;  Toluene, anhydrous with chemical formula C 7 H 8 , molecular weight 92.14 g/mol, density 0.865 g/mL and purity 99.8 %, product of Marvin Ltd.All materials were used as received without any further purification.

Preparation of pure PS and PMMA solutions
Amounts of 5 and 10 g of pure polystyrene were dissolved in 100 ml toluene and stirred overnight at rate 420 rpm at room temperature of about 25 C to complete dissolution of PS.According to the described procedure and the same polymer contents, both PMMA solutions were prepared.

Preparation of PS/PMMA solutions with ZnO or ZrO 2 nanoparticles
For each polymer solution 1, 3% ZnO and 5, 10% ZrO 2 nanoparticles were added to 30 ml of the previously prepared solutions of PS and PMMA in toluene in proportion to the polymer weight.The mixtures were stirred again until the nanoparticles were completely dispersed in the polymer solution and uniform solutions were obtained.Then they were further subjected to bath sonication with frequency of 28 kHz for 30 min to obtain homogeneous solutions.

Preparation of PS/PMMA films with ZnO or ZrO 2 nanoparticles
Thick polymer films have been obtained from the prepared mixtures with different concentrations of ZnO or ZrO 2 nanoparticles pouring the solutions into glass petri dishes (diameter: 50 mm) and stored for one week to remove solvent.Films with a thickness in the range of 0.1 ÷ 0.3 mm were obtained.
OSSILA spin coater has been used for preparation of thinner films.For comparison with previous results and optical properties of bulk materials homogeneous films with thickness of several to tens of microns were obtained.For each film an amount of 2 ml of the corresponding polymer solutions with nanoparticles was taken and dripped over microscopic object glass.Films were prepared at varying speed according to the introduced program of the spin coater with steps of 120 rpm for 5 s at the beginning, 200 rpm for the next 10 s and 600 rpm for the last 10 s.These parameters have been chosen as optimal as a result of many experiments in accordance to density and viscosity of the solutions.The samples were kept for 24 hrs to remove the excess solvent in the films.

Measuring
Thickness of polymer films was measured with a digital Mitutoyo micrometer model QuantoMike 0-25 mm with an accuracy of 1 µm.
RIs of polymer films have been measured at D-spectral line (589 nm) with a Kruss AR4 Abbe refractometer in accordance to the European RI standard at 20 C maintained by circulating thermostat PT80.Contact liquids have been used in case of solid-state samples: cinnamon oil (n D = 1.592) for lower refractive samples and diiodomethane (n D = 1.741).The contact liquids should have always higher RI than the solid sample.Obtained results were averaged for several sample measurements.
Transmission spectra have been measured by means of a UV-VIS-NIR Jasco spectrophotometer model V 770 in the diapason 300 -2700 nm.In case of thick films, which were free standing in the holder, no baseline was introduced.For the spin coated samples glass transmission was subtracted in the results by the software.

Refraction
Refractive index n of optical materials is related to their chemical structure, density and homogeneity.Some of the measured n values of polymer films are included in table 1. Results for the two types of pure polymers are close to our previous data of bulk samples [7].According to the group contribution theory, the PS plastic is more refractive material in comparison to PMMA, due to the high molar refractivity of the aromatic ring [8].Influence of density and molecular weight on the RI can be established from data of PMMA films.Since n values vary with thickness d, we consider the thick and thin samples separately.For example, n = 1.4961 for PMMA1 (MW350,000) film with d = 213 m and n = 1.4983 for PMMA2 (MW120,000) film with d = 215 m while the corresponding values for 3 m films are 1.4911 and 1.4964, respectively.Results for films with similar thickness show that RI increases when material with lower MW and greater density is used.
In case of composite materials, effective medium approximations [9] can be applied to estimate refractive index.Lorenz-Lorentz (L-L) model gives the following expression: where n p and n i are RIs of polymer and inclusions, respectively and f V is the volume fraction of nanoparticles (NPs).Other commonly used forms are Maxwell-Garnett (M-G) and Bruggeman (B) equations: These relations were applied to calculate RIs n cal of polymer nanocomposite films.Results for the measured and computed RIs are presented in table 1. Weights and densities of nanoparticles (m i ,  i ) and polymer (m p ,  p ) in the prepared solutions were used to determine volume fraction by the expression . The value of n i was 2.0028 for ZnO and 2.1745 in the case of ZrO 2 (https://refractiveindex.info).Measured RIs of pure polymer samples (f V = 0) were substituted as values of n p in equations ( 1) and ( 2).For our calculations, films with close thickness were selected.
Results show that measured RIs are usually greater than calculated values.It should be noticed that applied effective medium models do not consider particles' size, shape and interaction [9] and can be used only for approximate estimation of effective RIs of polymer nanocomposites.

Transmission
Typically, optical polymers are opaque in the ultraviolet (UV) and NIR region beyond 2600 nm.There are weak absorption bands at about 900 nm, 1150 nm, 1350 nm, 1675 nm, and greater around 2100 -2200 nm in respect to the structural groups [8].In figure 1(a) transmission spectra of neat free standing polymer films casted in petri dishes of 5% polymer solutions are presented.As seen, transmission depends on the polymer type as well as on its product specifications.Usually, bulk PMMA samples are most transparent in respect to other principal PMs.PMMA1 film with a thickness d = 228 µm shows highest transmission (T) of about 90 % in VIS and NIR region up to 1660 nm and T  87 % in the diapason 1800 -2100 nm.PS sample (d = 250 µm) is slightly less transparent in the same range with lowest transmission of 83 % at shortest wavelengths in the VIS spectrum.In case of PMMA2 (d = 227 µm), transmission in VIS light falls down to T = 30 ÷ 70 %, while in NIR increases and it reaches 80 % at 1600 nm.Absorption bands of both PMMA materials appear at the same wavelengths.Results show that PMMA2 is not suitable for optical applications in VIS region.
Transmission of composite polymer films are illustrated in figures 1(b), (c), (d).In figure 1(b) thick films of 5% PMMA1 and PS solution with 1 % ZnO are illustrated.Again, the polystyrene film is less transparent.Addition of ZnO nanoparticles dramatically decreases transmission of polymers in VIS and is acceptable (T  70÷80 %) in NIR spectra, respectively.The decrease in transmittance can be attributed to the differences in RIs of inorganic NPs and polymer matrix.Incorporation of NPs leads to increased light scattering losses depending on the particle size, shape and distribution, resulting in a higher RI of the nanocomposites [9].Another reason for reduced transparency is inhomogeneity of the thick films due to crystalline nature of added NPs.
In case of thin films in figure 1(c) transparency is comparable with neat polymers since inhomogeneity of the two phases is significantly loweredthere are fewer crystal grains to scatter light.It should be noted that thick films are opaque in UV light and the transmission extremely increases for thin films.Variation of transparency of composite films in respect to the weight percent of the filler ZrO 2 is demonstrated in figure 1(d).

Summary and conclusions
Investigation of polymer films with NPs of high refractive metal oxides has been carried out.Principal high transmissive PMs as PMMA and PS with different refraction have been blended to increase their RI.Possible optical applications were controlled by transmission measuring in VIS and NIR spectra.RI results of composites show higher refraction of about n  0.02 for PS films and  0.01 in case of PMMA layers (table 1).Attention should be drawn to the quiet different volume fractions of inclusions in both polymers.Results suggest that greater increase in RI is possible for the PS polymer.Close values of measured and calculated RIs are established with somewhat greater deviation for L-L approximation.Transmission of PS and PMMA films with additives is presented in figure 1 (b), (c), (d).Comparison among neat films 1 (a) and films with different percent concentration of the fillers is possible.Transmission depends on the polymer type as well as on its product specifications.Results show dramatic decrease of transparency of thick films in UV and especially VIS regions which is not noticed in case of thin films with the same additives.High transmission of composite materials in the most important NIR ranges in telecommunication applications is established.Conclusions for possible applications of composite PMs in medical vision devices are drawn.

23rd
International Summer School on Vacuum, Electron and Ion Technologies 2023 Journal of Physics: Conference Series 2710

Figure 1 .
Transmission spectra of: (a) thick polymer films; (b) thick films with ZnO NPs; (c) thin films with ZnO NPs; (d) composite films with different concentrations of ZrO 2 NPs.

Table 1 .
Refractive indices of polymer and composite films.