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Functional Nanostructured Materials (including low-D carbon)

Dyed poly (styrene-methyl methacrylate-acrylic acid) photonic nanocrystals for enhanced structural color Gonul Yavuz, Helena P. Felgueiras, Ana Isabel Ribeiro, Necdet Seventekin, Andrea Zille, and Antonio Pedro Souto ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03003 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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ACS Applied Materials & Interfaces

Dyed poly (styrene-methyl methacrylate-acrylic acid) photonic nanocrystals for enhanced structural color

Gonul Yavuz†, Helena P. Felgueiras◊, Ana Isabel Ribeiro◊, Necdet Seventekin‡, Andrea Zille◊* and Antonio Pedro Souto◊

†

Textile and Apparel Research and Application Center, Ege University, Izmir, Turkey

◊

2C2T - Centro de Ciência e Tecnologia Têxtil, Universidade do Minho, Campus de

Azurém, 4800-058 Guimarães, Portugal ‡

Ege University, Faculty of Engineering, Textile Engineering Department, 35100 Izmir,

Turkey.

KEYWORDS: photonic crystal; textile fabric; iridescence; dyestuff; polyamide coating

1 ACS Paragon Plus Environment

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ABSTRACT In the present work, we investigated the combined effect of poly (styrene-methyl methacrylate-acrylic acid) [P(St-MMA-AA)] PCs with the disperse dye C.I. Disperse Red 343, on the photonic crystals (PCs) shape, distribution, organization, iridescence, chemical structure, thermal stability and reflectance. PCs were successfully produced in the form of highly spherical, monodisperse colloidal structures. Presence of dye in the PCs inner core-shell structure was confirmed via Fourier-transformed infrared spectroscopy. The PCs brightness and iridescent effect was enhanced by the presence of the dyestuff, which also promoted the self-assembly of the colloidal nanospheres in the form of arrays. The P(St-MMA-AA) PCs thermal stability did not alter with the introduction of the dye. In a side experiment, dyed PCs were also coated onto dyed polyamide fabrics. Data reported successful coating of the textile fabric and an improvement of its reflectance. Fabric immobilization fostered the self-assembling of the dyed colloidal nanospheres in the form of well-organized face-centered cubic, closed-packed arrays. This is the simplest and most energy favorable organization for PCs. The combination of disperse dyes with PCs is a very recent and challenging idea and could open new ways to understand the influence the PCs photonic-band structure may exert on the photoluminescence properties of the dyes embedded in the PCs inner space, and vice-versa.


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1. INTRODUCTION In nature, color production is accomplished by means of pigments or structural media. Pigments result from the absorption of incoherently scattered light in a restricted wavelength range that produces a spectrally selective reflection.1 Structural coloration involves materials that are themselves colorless but possess nanometer-sized structures, with periodically changing refractive indices that can create colors by coherent light scattering.2-3 Structural colors are usually iridescent, very bright, deeply saturated, free from photo fading and long-term resistant to discoloration. Also, their coloring effect cannot be mimic by pigments or synthetic dyes since they are of physical origin rather than chemical.4 Structural colors are very frequent in animals, namely beetles,5 cephalopods,2 morpho-butterflies,6 peacock feathers,7 etc., and are originated from five fundamental optical processes and their combinations: (I) thin-film interference, (II) multi-layer interference, (III) diffraction grating effect, (IV) light scattering, and (V) photonic crystals (PCs).8-11 PCs are periodic dielectric structures capable of manipulating photons in much the same way an atomic crystal lattice control electrons, meaning that light scattered from each particle interferes and radiates secondary emission in regular directions.4, 12 PCs can create a range of forbidden frequencies, the so called band gap, that cannot be propagated through medium. They can be regarded as a special case of composite in which two materials, with inherent refractive indices, are mesh together to generate a structure with an invariant refractive index that allows the virtual modification of the propagated light in a controllable manner.8, 13 PCs are spatially free, giving rise to periodic structures of one-, two- or three-dimensions.12 PCs unique optical properties make them widely desirable for applications in photonic integrated circuits,14



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optical waveguides,15 telecommunications,16 sensors,17 solar cells,18 etc. PCs have also fostered the application of structural colors in textile fabrics' coloration.19 In the past decade, different approaches have been researched to produce PCs in the form of three-dimensional colloidal crystals. The colloidal self-assembly method is considered, between many, the most simple, reliable and cost-effective route to generate monodisperse highly charged colloidal particles of adjustable size and shape, that can spontaneously self-assemble into face-centered cubic or body-centered cubic crystalline colloidal arrays (the simplest form of PCs).19-20 In general, the process to obtain structural color from PCs on solid surfaces starts with the synthesis of monodisperse colloidal nano or microspheres, followed by self-assembling by sedimentation (i.e gravitational) on the substrate, and ends with a thermal treatment to induce solvent evaporation.21 An important example of three-dimensional PCs in the visible frequency range is the artificial opal, which cubic packing arrangement combines colloidal crystals of several hundred nanometers in diameter.22 Loading of opals with dyes has become a subject of interest since it allows to investigate the influence of the photonic band structure on the photoluminescence properties of the dyes embedded in the opals inner space. This combined effect may either decrease or enhance their emission. Fluorescent dyes possessing sufficient water-solubility can infiltrate through the opal voids during emulsion polymerization process, and be directly incorporated into the colloidal particles.23 This is the easiest method to load opals with dyes. Other methods include, swelling and de-swelling processes,24 diffusion,22 charge attraction,25 etc. In the present work, we report PCs prepared from poly-(styrene-methyl methacrylate-acrylic acid) [P(St-AA-MMA)] by soap free emulsion copolymerization, and dyed with Disperse Red 343 dyestuff. The selection of the polymer P(St-AAMMA) was based on its multiple choices of chemical composition, tunable particle size


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and polymer shape, and low cost.19 Dyed and undyed P(St-AA-MMA) PCs were characterized in the free form or after immobilization onto polyamide woven fabrics using: scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), optical microscopy, dynamic light scattering (DLS), Fourier transform infrared spectroscopy (ATR-FTIR), thermogravimetric analysis (TGA), dynamic calorimetric analysis (DSC), diffuse reflectance spectroscopy and glowmeter analysis. The combination of disperse dyes with PCs take advantage of both traditional coloration and structural color iridescence. Studying the mutual influence dyes and PCs exert on each other and its inherent properties are the novelty portrayed in the present research. By controlling the PCs structural colors and the polyamide base color, correlations between the PCs organization on dyed textile fabrics and the changes introduced in color were uncovered. Compared to the surface color of dyed textiles, structural color by means of dye-loaded PCs allows for vivid and intense colors, a variety of tunable coloring effects, great resistance to fading and reduced toxicity.26-28 The textile fabrics flexibility, texture, structure of the fibers/yarns, presence of gaps and roughness, which confers a fluctuant and complicated appearance to the fabrics' surface, makes the production of structural colors on these materials much more complex than on smooth, compact, uniform surfaces.29 Because of these limitations very little reports have been published about the self-assembly of dyed PCs and their application on flexible textiles.



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2. EXPERIMENTAL SECTION

2.1. Materials. Commercial white polyamide fabric with a warp density of warp density of 50 threads cm-1, a weft density of 32 threads cm-1 and an areal density of 110 g m-2 was used in this study. Samples were pre-washed with a 1 g L−1 of non-ionic detergent solution at 30 ºC for 30 min and then rinsed with water for another 15 min to minimize contaminations. Disperse Dianix Red CC (C.I. Disperse Red 343) dyestuff was supplied by Dystar Textil Farben, Frankfurt, Germany. Dianix Red CC is a mixture of two dyestuffs i.e. C.I. Disperse Red 343 (ethyl) and C.I. Disperse Red 343 (propyl) (Figure S1 in Supporting Information).30 Styrene (St), methyl methacrylate (MMA) and acrylic acid (AA) were distilled before use. The remainder analytical grade reagents were purchased from Sigma–Aldrich, St. Louis, MO, USA, and used without further purification. 2.2. Preparation of Monodisperse P(St-MMA-AA) Composite Nanospheres. Monodisperse composite latex nanospheres of P(St-MMA-AA) were synthesized by soap free emulsion copolymerization as previously described.31 Briefly, St (20 g), MMA (1 g), AA (1 g), distilled water (dH2O, 100 g), sodium dodecyl sulfate (0-0.004 g), and ammonium bicarbonate (0.5 g) were added sequentially to a three-necked flask equipped with an N2 inlet, a reflux condenser and a mechanical stirrer moving at 300 rpm. Reaction of the mixture was initiated at 70 ºC for 30 min. Following the addition of an aqueous solution of sodium persulfate (Na2S2O8, 0.48 g dissolved in 15 g dH2O), polymerization was carried out at 80 ºC for 5 h to obtain a homogeneous latex with monodispersed particles. The resulting P(St-MMA-AA) composite latex nanospheres, hereafter referred to as PCs, were used without further purification.32


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2.3. Dyeing of P(St-MMA-AA) Nanospheres (PCs). Disperse Red 343 at dyestuff concentrations of 1, 3, 5 and 10 wt% was used to dye the PCs. Dyeing tests were carried out in a "Data Color Ahiba Spectra Dye" machine equipped with infrared heating, liquor ratio at 1:20, and stainless steel dye pots of 150 cm3 capacity each. The program started at 20 ºC and was raised at a rate of 2 ºC min-1 up to 130 ºC, and kept at this temperature for 115 min. Then, the system was cooled at a rate of 2 ºC min-1 until 60 ºC. Using a 0.2 µm cellulose acetate filter of 47 mm diameter, the dyeing solution containing the PCs was filtered and the dyed PCs were collected. Pictures of the dyed PCs from different visual perspectives were taken. For comparison purposes dyestuff solution at 1 wt% concentration was also filtered (control). 2.4. Production of Structural Color on Dyed Polyamide Fabrics. Polyamide fabrics were dyed at dyestuff concentration of 1 wt%, following the same procedure described in section 2.3. Dyed PCs were then coated onto the previously dyed fabrics via gravitational sedimentation. Briefly, a piece of dyed polyamide woven fabric was placed in a petri dish filled with 5 mL of dyed PCs suspension (Figure S6 in Supporting Information). The polyamide fabric coated with the dyed P(St-MMA-AA) colloidal nanospheres, combined by self-assembly, was placed in a oven at 60 ºC. This allowed water to evaporate, obtaining the dyed PCs structural color on the dyed polyamide fabrics. Uncoated dyed fabrics were used as control. 2.5. Photographs. Optical photos of PCs and the fabrics coated with PCs were taken with a Nikon CoolPix4300 digital camera. The pictures were acquired in a light chamber under a D65 light source. In the case of the multi-angle photographs the camera was disposed at different angles using a goniometer maintaining the same distance and using the same sample. No adjustment of pixels, color, brightness and contrast was applied to



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the images. The presented pictures are parts of larger images selected without obscure, eliminate, or misrepresent any information present in the original. 2.6. Microscopic Observation of Filter and Fabric Cross Sections. Images of the cellulose acetate filter, after dye and dyed PCs filtration, and the dyed PCs uncoated and coated polyamide fabric cross sections were captured using a reflection optical microscope Olympus BH (Japan) coupled to a JVC TK1280E (Japan) camera and a Micron Measurement video recorder associated with the Leica Quantimet 500 (Germany) software. Samples were visualized separately for a period of 5 min using a magnification of 40 × 65. Images were optimized in terms of contrast and brightness using the function “auto level” of the Graphic Converter 9.7.5 program of the Lemke Software GmbH, Germany. The adjustment applied to every pixel in the image was done without obscuring, eliminating, or misrepresenting any information given by the original images. 2.7. Dynamic Light Scattering (DLS) and Zeta Potential Measurements. The size distribution, polydispersity index and zeta potential of the PCs and dyed PCs were measured by DLS and Electrophoretic Light Scattering (ELS) using a Zeta Sizer-Nano (Malvern Instruments). Data was collected after 30 scans at constant temperature of 25 ± 1 ºC. Zeta potentials were measured in solution at a moderate electrolytic concentration. Each value was obtained by averaging measurements of three samples. 2.8. Scanning Electron Microscopy (SEM) and Scanning Transmission Electron Microscope (STEM). Morphological analyses of the PCs, dyed PCs and dyed PCs coated fabrics were carried out by SEM and STEM with an Ultra-high resolution Field Emission Gun SEM (FEG-SEM), NOVA 200 Nano SEM, FEI Company. Secondary electron images were obtained with an acceleration voltage from 5 kV to 17.5 KV, while backscattering electron images were acquired with an acceleration voltage of 15 kV.


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Samples were covered with a thin film of Au-Pd (80-20 weight %) in a high-resolution sputter coater, 208HR Cressington Company, coupled to a MTM-20 Cressington High Resolution Thickness Controller. 2.9. Fourier Transformed Infrared Spectroscopy with Attenuated Total Reflection (ATR-FTIR). A Nicolet Avatar 360 FTIR (Madison, USA) with a horizontal ATR accessory, composed of a Zn/Se crystal, was used to record the ATR-FTIR spectra of the dye, the PCs, and the dyed PCs. 45 scans at a spectral resolution of 4 cm-1 over the range 700–4000 cm-1 were conducted. All measurements were conducted in triplicate. 2.10. Thermal Gravimetric Analysis (TGA). TGA was performed on a Modulated TGA Q500 from TA Instruments. Dye, PCs and dyed PCs were dried at 60 ºC for 1 h and placed in a porcelain sample pan. The TGA trace was obtained in the range 40-700 ºC under nitrogen atmosphere, flow rate of 20 mL min-1 and temperature rise of 10 ºC min-1. Results were plotted as percentage of weight loss vs. temperature. 2.11. Differential Scanning Calorimeter (DSC). DSC analyses were carried out in a Mettler-Toledo DSC822 instrument (Giessen, Germany). Dye, PCs and dyed PCs were dried at 60 ºC for 1 h and placed in an aluminum sample pan. Analyses were carried out in nitrogen atmosphere with a flow rate of 20 mL min-1 and heating rate of 10 ºC min-1. The DSC trace was obtained in the range 30-500 ºC. The graph was plotted as heat flow vs. temperature. 2.12. Diffuse Reflectance Spectroscopy. The PCs coated and uncoated dyed polyamide fabrics coloration was evaluated using a Spectraﬂash 600 (Datacolor) diffuse reflectance spectrophotometer at standard illuminant D65 (LAV/Spec. Incl., d/8, D65/10°). Spectral reflectance values for each sample were obtained at 10 nm intervals within the visible spectrum, which ranged between 360 and 700 nm. The color characteristics analyzed were: K/S, L*, a*, b*. K/S is the color strength calculated using



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the Kubelka-Munk's equation (K/S= (1-R)2/2R, where R is the reflectance). L*, a*, and b* are the coordinates of the color in the cylindrical color space, based on the theory that color is perceived by black-white (L*, lightness), red-green (a*), and yellow-blue (b*) sensations. Five areas on each sample were measured in various positions. All measurements were conducted in triplicate, with results representing average values with up to 1% variation. 2.13. Glowmeter Analysis. The specular gloss observed from a sample is the relative luminous reflectance factor in the mirror direction. The specular gloss of the dyed polyamide fabric uncoated and coated with PCs and dyed PCs was measured with a BYK-Gardner micro-TRI-gloss meter (model GB-4520) at the incident angles of 20º, 60º and 85º. The measuring fields for 20º, 60º and 85º angles were 9 x 9 mm, 9 x 18 mm, and 7 x 42 mm, respectively. The multi-angle glossmeter was calibrated with a black gloss standard (GB-4522; 20º: 92; 60º: 95; 85º: 99) built in the holder. For each angle and sample, the specular gloss was recorded.

3. RESULTS AND DISCUSSION 3.1. PCs Size and Distribution. Monodisperse composite nanospheres of P(StMMA-AA), also referred to as PCs, were successfully synthesized by soap free emulsion polymerization and dyed with the Disperse Red 343. SEM micrographs of the PCs were taken before (Figure 1a) and after dyeing (Figure 1b) with the purpose of evaluating the size, shape and distribution of the colloids. These properties are critical to the resulting optical features of the PCs.33 In both cases, PCs were found highly spherical in shape, smooth in morphology, and uniform in size and distribution. High degree of sphericity is vital for the PCs optical performance since their ability to self-assembled and generate


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homogeneous crystalline arrays, with large domain sizes, relies on this feature.34 DLS technique, which is used to determine the size distribution profile of small particles in suspension, established the average diameter of PCs at 258.0 ± 6.1 nm and of dyed PCs at 291.6 ± 6.9 nm. The impregnation and coating of the colloidal nanospheres core-shell structure with Disperse Red 343 promoted the growing of the PCs size. This may also affect the PDI, which is used to evaluate the width of the particle size distribution.35 The PDI of dyed PCs was determined at 0.1, while the PDI for PCs was found at 0.2, meaning the dyed PCs were more monodisperse than the undyed. This observation is corroborated by the DLS spectrum that relates colloids size with volume (Figure S2 in Supporting Information). PCs DLS spectrum displayed two peaks, while dyed PCs only exhibited one. This confirms its superior monodispersity, uniformity and propensity for generating highly ordered structures. SEM analysis at higher magnification (200.000x) confirms that the dyed PCs have a more ordered distribution and an even surface than undyed PCs (Figure 1d). When the dye is not present some agglomeration occurs among PCs with pseudo-sphericity and collapsed structures (Figure 1c). The STEM images of the P(St-MMA-AA) nanospheres confirm that the latex spheres are composed by a shell rich in PAA and PMMA covering the PS domains in the core as previously observed (Figure 1e).36 Moreover, they also confirm the complete and even penetration of the dye to the core of the PCs (Figure 1f).



(a)

(b)

(c)

(d)

(e)

(f)

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Figure 1. SEM micrographs of the PCs before (a, c) and after (b, d) dyeing with Disperse Red 343 dyestuff at 1 wt% concentration captured at 50,000x and 200,000x magnifications, respectively. STEM micrographs of the PCs before (e) and after (f) dyeing captured at 200,000x magnification. 12 ACS Paragon Plus Environment

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3.2. PCs Dyeing. On a parallel experiment, with the purpose of confirming the PCs capacity to retain color and to evaluate the Disperse Red 343 dyestuff diffusion ability within the nanospheres core-shell structure, the dyeing solutions containing the PCs were filtered and the dyed PCs were collected. Successful PCs dyeing and retention of color were confirmed in the tested concentrations 1, 3, 5 and 10 wt%. After filtering, the recovered dyeing solution were colorless, particularly the one at 1 wt% concentration, meaning the dyestuff was diffused within the nanospheres and not in the dyebath (Figure S3 in Supporting Information). This concentration was used in all subsequent experiments. The Disperse Red 343 is a disperse aminoazobenzene dye with high substantivity for hydrophobic materials. Its molecules are small, planar and non-ionic, with attached -CN polar functional groups to confer a minimum solubility in water. A dispersing agent is already present in this commercial dye formulation to maintain the dye particles in dispersion. Once the preparation is added to water, it forms micelles above critical, but low, concentration. The hydrophobic tails of the dispersing agent molecules coordinate the dye inside the micelle providing a higher apparent solubility. The dyeing was performed at temperatures of 130ºC. At this temperature, thermal agitation causes the PCs polymer's structure to become looser and less crystalline, opening gaps for the dye molecules to enter.37 Dye molecules that have been adsorbed on the surface diffuse into the interior of the PCs by a relatively simple mechanism that obeys to the Fick’s equation. The dye molecule simple shape allows it to slide between the tightly packed polymer chains of PCs, while the polar groups improve the water solubility and favors Van-der-Waals and dipole forces interactions with the PCs polymer structure.38



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Microscopic visualization of cross sections of the filters used to filter dyestuff (control) and dyed PCs demonstrated the capacity of the PCs to retain the dye within their structure (Figure S4 in Supporting Information). While dyestuff alone passed through the cellulose filter and stained it, dyed PCs got retained instead. Disperse dyes are very small in molecular size and possess limited solubility in water at room temperature, and tend to diffuse within polymeric fibers (i.e. cellulose). This explains the staining of the filter by the single dyestuff and the clear presence of dye in the recovered dyebath.34, 38 Due to their bigger size (291.6 ± 6.9 nm) compared to the filter pores (≈ 200 nm), dyed PCs were not capable of crossing the cellulose membrane becoming trapped on its surface. These were then collected and pictures were taken at specific visual angles (Figure 2). PCs are capable of exhibiting bright, brilliant monochromatic colors, as long as the photonic band gap falls into the visible region.34 As the angle of incidence of the light varied so did the brightness of the color diffracted by the colloidal PCs. This phenomenon indicates a range of ordering between the arrays of colloids. That ordering can even be depicted at Figure 1b where combinations/agglomerates of nanospheres occurred in a cubic-like form. Another fact confirming this arrangement is the saturation of color and iridescent effect when the observation angle altered.19,

39

Iridescence is

commonly described as the change of hue with the angle of observation and occurs over a wide range of wavelengths, including the range visible to human. This effect is an unique feature of structural colors,40 attesting once again the success of the PCs production.


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90º

70º

45º

0º

Figure 2. Dyed PCs recovered after filtering process. Images were captured at specific observation angles.

3.3. Chemical Characteristics. ATR-FTIR spectra of dye, PCs and dyed PCs were collected (Figure 3). In the Disperse Red 343 curve an important peak, characteristic of the dye skeleton, was detected at 1601 cm-1, the C=C stretching vibration band of the benzene ring. The N=N stretching band was observed at 1350 cm-1, while C-N stretching aliphatic amine was found at 1140 cm-1. These adsorption bands are associated with the induced electric dipole oscillation along the long molecular axis of the dye structure.41 15 ACS Paragon Plus Environment


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At 1520 cm-1 and 1320 cm-1 the NO2 asymmetric and symmetric stretching bands were detected, respectively. At 820 cm-1 a small peak was observed and attributed to C-H outof-plane bending of the aromatic ring. Two very small peaks were also observed at the bands 2220 cm-1 and 3260 cm-1 of Disperse Red 343 spectra. They are attributed to CΞN and N-H stretching vibration bands.42 The IR spectra of the P(St-MMA-AA) PCs was consistent with previous reports.43-44 At 3027 cm-1, the sp3 C-H stretching vibration of the benzene ring was detected. Stretching vibrations of the hydrogens present at the ring were also seen at 3040 cm-1 and 3080 cm-1. The peaks detected at 2850 and 2922 cm-1 were attributed to the aliphatic C-H stretching vibration of saturated -C-H.45 The very small peak observed at 1726 cm-1 can be related to C=O, which is an indicator of the presence of carboxyl groups on the surface of the P(St-MMA-AA) PCs.46 The peak at 1601 cm-1 is associated to the C=C stretching vibrations of the benzene ring of the styrene. The peaks at 1495 cm-1 and 1454 cm-1 may be related to the CH2 scissoring and CH3 asymmetric stretching or deformation the C-H bonds in the methyl groups. In the fingerprint region, at 1028 cm-1, C-O-C symmetric stretching vibrations of the MMA were detected. Finally, the band at 758 cm-1 can be assigned to the flexural vibrations of the C-H groups of the benzene ring, namely the CH2 rocking in-plane bending of MMA.44 The combination of the Disperse Red 343 with the P(St-MMA-AA) PCs introduced some alterations to the PCs spectra. While most of the characteristic groups of the PCs remained, bands characteristic of the dye were also detected, confirming the incorporation of the dye within the inner structure of the colloidal nanocrystals. A new peak at 1560 cm-1, which is commonly assigned to the bending of the colloids CH2 groups, may also be an indicative of the NO2 asymmetric stretching bands of the dye. NO2 symmetric stretching bands were also observed at 1370 cm-1. In both cases, the binding of groups from dye and PCs may have shifted the peaks from their original


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position suggesting interactions between dye molecules and polymer. It was previously observed that the latex polymers and azo dyes could interact through hydrogen bonding and crosslinking occurrences.47 The attractive force between hydrogen atoms of PCs molecule and more electronegative atoms such as nitrogen or oxygen molecules of the dye can shift the wavenumbers. At 1140 cm-1 the peak attributed to the C-N stretching aliphatic amine was also found.

(a)



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(b)

Figure 3. ATR-FTIR spectra from dye, PCs and dyed PCs samples in the regions between (a) 4000-700 cm-1 and (b) 1700-700 cm-1.

3.4. Thermal Properties. Degradation steps associated with temperature rising were identified on dye, PCs and dyed PCs via TGA (Figure 4). The first step at ≈ 51 ºC (T peaks of 1st derivative on Table 1) refers to the initial weight loss resultant from the evaporation of physically adsorbed water and dehydration of hydrated cations as confirmed in DSC (Figure 5). The subsequent degradation step for the Disperse Red 343 dyestuff was registered at 270 ºC. At this point, the mass loss may be related to the degradation of the azo groups (N=N), which are notoriously know to degrade at temperatures above 200ºC.48 The third and fourth degradation steps for the dyestuff were registered at the DTG peaks of ≈ 324 ºC and 480 ºC and were associated with the degradation of different benzyl rings residues. After 480 ºC the gradual carbonization of the dye organic structure starts, leaving a high residual mass of ≈ 50% at 700 ºC. It is 18 ACS Paragon Plus Environment

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likely the presence of the various double bound interactions and aromatic rings to increased the material rigidity and thus to slow the degradation process. Because of its entrapment within the colloidal crystals and small concentration (1 wt%) compared to the P(St-MMA-AA) shell structure, the presence and the degradation steps of the Disperse Red 343 dyestuff were not detected on the dyed PCs spectra. In fact, the degradation profile between PCs and dyed PCs suffered very little changes. In both cases, the colloidal crystals decomposition peak was at ≈ 430ºC, indicating that until this point the newly synthesized PCs maintained a good thermal stability. This temperature was superior to the individual decomposition temperatures of PS (≈ 250ºC), PMMA (≈ 140ºC),44 and PAA (≈ 80ºC),49 the polymers composing the P(St-MMA-AA) composite. It is likely this increase in the entire composite thermal stability to be a result of internal rearrangements between the polymeric structures and the formation of new hydrogen bonds between chemical groups.50 From the DTG peak and until 700 ºC degradation continued, resulting in a total mass loss of 93% for PCs and 96% for dyed PCs.



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Figure 4. TGA of dye, PCs and dyed PCs samples from 40 to 700 ºC, performed at a heating rate of 10 °C min-1 in a nitrogen atmosphere.

DSC measurements were conducted in the range 30 to 500 ºC for dye, PCs and dyed PCs (Figure 5). Generally, DSC data was consistent with the TGA spectra. For both PCs and dyed PCs a well-defined peak at ≈ 430 ºC, representative of the melting point, was detected. Prior, a small peak located between 70 and 120 ºC was attributed to water evaporation.51 The same was seen at 110 ºC for the Disperse Red 343. Dyeing of the PCs reduced slightly (but not significantly) the melting point and the enthalpy of the reaction (Table 1). This is to be expected since the dye displays a low crystalline structure and its gradual carbonization is most important after 268 ºC, almost 200 ºC before the polymer.

Figure 5. DSC thermograms of the dye, PCs and dyed PCs in a temperature range of 30 to 500 °C, performed at a heating rate of 10 °C min-1 in a nitrogen atmosphere.


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Table 1. Main DSC thermal transitions, TGA weight loss temperature peaks and residual weight of samples (n=3; S.D.

Dyed Poly(styrene-methyl Methacrylate-acrylic ... - ACS Publications

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