Optical and structural dynamical behavior of Crystal Violet Lactone – Phenolphthalein binary thermochromic systems

Dyes and Pigments 134 (2016) 69e76

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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig

Optical and structural dynamical behavior of Crystal Violet Lactone e Phenolphthalein binary thermochromic systems Alina Raditoiu, Valentin Raditoiu*, Cristian Andi Nicolae, Monica Florentina Raduly, Viorica Amariutei, Luminita Eugenia Wagner National Research and Development Institute for Chemistry and Petrochemistry, ICECHIM, 202 Splaiul Independentei, 060021, Bucharest, Romania

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 May 2016 Received in revised form 26 June 2016 Accepted 27 June 2016 Available online 29 June 2016

Several binary composites based on Crystal Violet Lactone (CVL) color former and Phenolphthalein (PPht) developing agent dissolved in 1-tetradecanol (TD) were studied in order to determine how different ratios between components influence the thermochromic behavior of the composites. The color dependence on temperature was investigated in direct relationship with structural changes and interactions between components. The stoichiometry of the colored complex was determined from diffuse reflectance data and confirmed by means of steady state fluorescence measurements and temperature modulated differential scanning calorimetry. The color former: developing agent molar ratio equal to 1:2.5 was determined to ensure the reversibility of the process in optimal conditions. The characteristics of the color path and the influence of thermal history during thermochromic transition were analyzed from the variations of CIELAB color parameters during successive heating/cooling cycles. Dynamic colorimetric properties of the thermochromic binary composite containing the components in optimum ratio are described by four characteristic temperatures in the hysteresis. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Thermochromic composite Lactone dyes Hysteresis Color measurements Fluorescence Thermal analysis

1. Introduction Thermochromism is well represented by both inorganic and organic compounds, many of them exhibiting reversible color changing in the solid state or in solution, following different mechanisms depending on their structure or the presence of other compounds that can interact with them. The most important inorganic thermochromic systems are: hydrated cobalt compounds [1], metallic chelates with different ligands [2] or mercury iodide double salts [3]. Organic thermochromic systems are represented by: thermotropic and lyotropic liquid crystal molecules [4], overcrowded ethylene or acetylene derivatives [5] and chromogenic systems in which the color switching is dependent on molecular rearrangement arising from tautomeric equilibrium [6]. However, the most important drawback of such systems in commercial applications is poor fastness to light and important efforts were made in order to improve this property [7e10]. The color forming composites based on the reaction of leuco dyes with weak acids represents the main class of thermochromic

* Corresponding author. E-mail address: [email protected] (V. Raditoiu). http://dx.doi.org/10.1016/j.dyepig.2016.06.046 0143-7208/© 2016 Elsevier Ltd. All rights reserved.

materials, intensively studied during the last decades [11e13]. This type of thermochromic composites previously encapsulated in suitable polymers are increasingly gaining importance for a diverse range of applications e heat sensitive recording media and indicator stripes [14], printed textiles [15e17], plastics [18e20], smart coating materials [21,22] or core-shell nanofibers [23]. Multi-component thermochromic systems consists of a color former, a color developer and a solvent and the thermochromic transition takes place around the melting point of the solvent. Some developments in the field, based on this type of systems, claimed the usage of intrinsically thermochromic fluorans [24] or composites of cyanine dyes and carboxylic fatty acids in polylactic acid [25]. Interactions between the color former and the developer and the competition between different equilibria established in this type of composites determine some of their critical properties such as: the discoloration rate, the color contrast and stability, hysteresis, etc [26e28]. However, the most important parameter to be studied in any of the thermochromic composites, which could be developed on this basis, is the optimal ratio between the color former and the developer [29]. As already established, the formation of supramolecular complexes of weak acids with leuco dyes followed by their subsequent segregation are the main causes of the color formation in these systems [30,31]. Generally, it was determined that as the

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mole fraction of color developer increases, the formation of a more intense colored composite could be observed [32]. The hydrogen bond is the most important interaction of the non-covalent type found in chemical and biological systems and was intensively investigated. Thermochromic systems of donatingaccepting type are based on intermolecular interactions by hydrogen bonds with effect on the formation or dissociation of some complexes as a function of the temperature [33]. As already mentioned in the literature, in the case of reversible thermochromic systems made of a color forming compound and a developing agent dissolved in an organic, hydrophobic and nonvolatile solvent, on heating, the solvent suffers a phase transition leading to a color change, while on cooling the system reverts to the original color due to variations recorded in forming/breaking hydrogen bonds between components [34]. Our work deals with thermochromic behavior of composites formed by Crystal Violet Lactone (CVL) and Phenolphthalein (PPht) dissolved in 1-tetradecanol (TD), based on investigations by means of visible and infrared spectroscopy correlated with thermal analysis. CVL and PPht were respectively chosen as color former and developing agent, because both of the main components are phthalein dyes and it can be presumed that dynamical properties of such systems could be slightly influenced by this particularity compared to the other systems studied until now. In our experiments, among C12eC18 alcohols, TD was chosen as a solvent because it possessed an alkyl chain of intermediate length, which seemed to be more suitable in order to provide the required solubility of the components at the melting point and the greatest color development during crystallization, in accordance to the phase separation mechanism of thermochromism [29]. The influence of the molar ratio of the components on the color intensity and optical properties together with structural modifications and interactions between components were studied according to the response of the thermochromic system upon successive heating-cooling cycles.

successive cycles, in 1
C intervals, at a heating/cooling rate of about 0.5
C and with an accuracy of ±0.1
C. Color measurements express differences perceived visually when the thermochromic transition occurs. Color modifications are quantified by its attributes: hue (H*), chroma (C*) and lightness (L*). The total color difference is qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 calculated from the equation: DE* ¼ ðDL* Þ þ ðDa* Þ þ ðDb* Þ , where: L* is the lightness, a*,b* are red-green, respectively yellowblue color components and calculated differences indicate how much the two states of a thermochromic sample differ from one another, while the composition is evaluated using: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 * * * * DH ¼ ðDE Þ  ðDL Þ  ðDC Þ , where: H* is hue and C* is chroma. Fluorescence spectra were recorded with a JASCO FP 6500 spectrofluorimeter, at 25
C, using the solid-state attachment, at an excitation wavelength of 365 nm. Temperature modulated differential scanning calorimetry (TMDSC) was conducted with a Q 2000 instrument from TA Instruments, LNCS assisted, in the temperature range 5e50
C, modulate ± 1
C every 30s, at a heating rate of 2
C/min, using helium as purge gas at a flow rate of 30 mL/min and T zero aluminum pans. 3. Results and discussion This study is intended to be an investigation about optical and structural behavior of some binary composites and establishing optimal molar ratio between CVL and PPht in order to obtain thermochromic complexes with high color intensity, maximum contrast, good compatibility between components and total solubility in the molten solvent. The study is important to find out how the components of the binary mixture will interact with each-other and with solvent molecules and which will be the influence on the thermochromic properties of the final composites.

2. Experimental 3.1. Color intensity and molar ratio between components Crystal Violet Lactone (CVL), Phenolphthalein (PPht) and 1tetradecanol (TD) were of laboratory reagent grade and were obtained from Merck, Germany and Aldrich e USA. Preparation of each of the thermochromic composites was performed by melting the solvent (TD), stirring and heating it at about 80e90
C, while CVL and PPht, in an appropriate molar ratio, are added in small portions in order to obtain, at the end, a clear solution. For spectroscopic measurements, the transparent and homogeneous composites were immediately placed into quartz cuvettes and were allowed to cool at room temperature. FTIR spectra were recorded in the range 400e4000 cm1 (32 accumulations at a resolution of 4 cm1), on a JASCO FT-IR 6300 spectrometer equipped with a Pike GladiATR, resistively heated up to 300
C. Total color differences in CIELAB system, using a 10
standard observer and illuminant D65 and diffuse reflectance spectra of solid samples were measured with a JASCO V570 UV-VIS-NIR spectrophotometer equipped with a JASCO ILN e 472 (150 mm) integrating sphere (specular component included), using spectralon as reference. The samples were heated/cooled on aluminum modular heating block for square cuvettes from Carl Zeiss Jena e Germany. The measurements of bulk thermochromic composites were performed using a quartz cuvette with the path length of 0.5 cm and the bottom coated with a Teflon plate. The temperature of the module was varied by circulation of thermostatically controlled water using a MGW Lauda thermostat coupled with a VEB MLW cryostat e Germany and a TOHO TTM-i4N-P-AB digital temperature controller e Japan. The reflectance spectra were measured in three

The mechanism of color formation is decisively determined by the equilibrium of the proton transfer between the hydroxyphthalein dye (PPht) and the amino-phthalein dye (CVL) as a function of the solvent aggregation state and constitutes the base of the thermochromic effect in this type of composite (Fig. 1). In order to study the influence of the components in the thermochromic composites, several samples obtained using different molar ratios between CVL and PPht and with various CVL loadings were prepared and investigated. Therefore, after the reflectance spectrum of each component was recorded, a diminished reflectance was observed as the CVL content increases. Analyzing the variation of the reflectance values, measured at l ¼ 608 nm, as a function of CVL: PPht molar ratio and CVL loading it is concluded that the maximum CVL content in the thermochromic composites is determined by the solubility of the components in molted TD at the transition temperature. In our case a maximum concentration of 2% by weight ensured total solubility of the components in the molten solvent and the highest color intensity of the solid composite. Over this concentration the discoloration process during the heating is seriously disturbed, the thermochromic complex formed between CVL and PPht remains partially un-dissociated even at temperatures above the melting point of the solvent and color changing during heating takes place from a fully to a slightly colored state, instead of a totally discolored state. Simultaneously, as can be seen from Fig. 2, when the CVL: PPht molar ratio is situated around 1:2e3, the reflectance value becomes minimum and remains unchanged on further increasing the amount of

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Fig. 1. Equilibrium of the proton transfer between CVL and PPht and molecular structures of colored complex (a) and colorless components (b).

Fig. 3. Reflectance of binary thermochromic composites as a function of CVL mole fraction. Fig. 2. Reflectance of different thermochromic composites measured at 608 nm as a function of CVL:PPht molar ratio and CVL loading (%) by weight.

developing agent which indicates that a saturation of the color takes place. Therefore, samples prepared and further analyzed by us have a CVL loading of 2% by weight which provide high intensity and contrast during transition. At this loading, the formation of the colored complex completely takes place during the cooling cycle, while on heating the dissociation is complete in the molten state. The method of continuous variation was used in another work to predict the stoichiometry in the case of thermochromic composites containing Bisphenol A and 20 -di(phenylmethyl)-amino-60 (diethylamino)spiro(isobenziofuran-1(3H)-90 -(9H)xanthenes-3one [35]. The same method was used in the present work to evaluate the number of PPht donating molecules which are necessary to interact with CVL, as accepting molecule, in order to form the highest degree of saturation in the thermochromic complex. In this regard, the reflectance spectra of several composites containing different molar ratios between the components were recorded during successive heating-cooling cycles. As it can be observed from Fig. 3, the reflectance of the thermochromic composites shows a minimum, as a function of the molar ratio between the color former and the developing agent. This value corresponds to the optimum ratio between components which provide maximum color intensity due to the stabilization of the CVL ionic structure engaged in the complex with PPht. The minimum value of the reflectance measured at 25
C on solid samples, in which the mole fraction of the components is varied in such a way to maintain constant CVL loading at 2% by weight, was recorded when the CVL mole fraction xCVL ¼ 0.29. At this value it is ensured the maximum coordination number around the color former and corresponds to the optimum molar ratio CVL:PPht ¼ 1:2.5. The thermochromic

composition with these characteristics shows optimal color properties, high contrast and total solubility of the components in the molten state of the solvent, as it will be demonstrated further. Differences in the reflectance as a function of temperature was further investigated in order to determine the profile of hysteresis loop during successive heating-cooling cycles for this thermochromic composite. Temperature-dependent curves differ significantly in symmetry and steepness with a relatively wide hysteresis loop, as it can be seen from Fig. 4. The analysis of the diagram shows that the transition temperature (melting of TD) is situated between the end of the discoloration process (during heating) and before the beginning of the coloration process (during cooling) and doesn’t represent a characteristic of the color switching. Therefore, it is important to define other parameters which will better describe

Fig. 4. Reflectance spectra of thermochromic CVL:PPht ¼ 1:2.5 as a function of temperature.

composite

with

molar

ratio

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the color switching of this type of thermochromic system. Due to the hysteresis loop, during heating, the decolorization process is described by the initial (Tai) and final (Taf) achromic temperatures while on cooling are found the initial (Tci) and final (Tcf) chromic temperatures, respectively. Further analysis consists of the CIELAB parameter measurement during heating-cooling cycles, which describe better color changes during the thermochromic transition. The chroma (C*) shows a large dependence on temperature variations and was correlated with the molar ratio between the components of the binary system, as shown in Fig. 5. Moreover, it was observed that when the amount of developer increased, the color became darker in the solid state and slightly higher transition temperatures were recorded. The parameter is useful to estimate the optimum ratio between the color former and developer in thermochromic systems due to the saturation effect observed when the developer loading increases above a certain limit. It is obvious that these systems which in the solid state are colored closer to black, during the thermochromic transition, changes the color to a white-blue color due to the maintaining of un-dissociated colored complex even after the melting of the solvent as previously showed. This is caused by a poor solubility of the components at high developer loadings and leads to poorer contrast attributed to scattering of light on developer and colored complex crystallites still existing after solvent melts. The lightness L*(T) curve loop, recorded for the thermochromic composite with CVL mole fraction XCVL ¼ 0.29, shows that in this case the discoloration is almost complete and CIELAB lightness parameter become higher during the heating and then remains unchanged with further increasing of temperature. In the reverse process coloration takes place at a lower temperature and consequently, the color changing cannot be defined only by variations of this parameter without knowing the path followed by the thermochromic system before the current state is achieved. A more complete description of the color changing during thermochromic transition is made by analyzing the evolution of a* and b* parameters. During the heating, the CIELAB b* factor shifts to yellow on the yellow-blue chromatic axis which is in accordance with the observed increasing in yellowing during the heating, as shown in Fig. 6b. If b* parameter varies within a single sense on heating and on cooling, respectively, a* parameter reaches a minimum value during thermochromic transition on each of the heating/cooling cycle. It is worth to be mentioned that the temperature corresponding to the minimum value of a* on the heating curve is the same with Taf, while on the cooling curve corresponds to Tci. Besides the hysteresis phenomenon which is recorded also in this case, a* is shifted to red on the red-green chromatic axis after a complete heating/cooling cycle. Therefore, a* of the final state do

Fig. 5. CIELAB chroma value variations during heating of binary thermochromic composites with different molar ratios and a CVL loading of 2% by weight.

Fig. 6. The temperature dependence of CIE lightness (L*) (a) and a*,b* values (b) for binary thermochromic composite with molar ratio CVL:PPht ¼ 1:2.5 and a CVL loading of 2% by weight.

not match the value of the initial state due to the effect of the new sample thermal history. This is due to physical transformations and chemical interactions established during the melting of the solvent, in a system dominated by competitive processes. Finally, it should be mentioned that during the color analysis it was obvious that all the CIELAB parameters together with reflectance values follow a hysteresis loop which is largely influenced by the composition, ratio between components and thermal history. Total color differences between heated and cooled states of thermochromic composites could be more appropriate in order to characterize the color switching during thermochromic transition. If we analyze the diagram containing total color differences as a function of temperature we can observe a peak situated at the same temperature independently of the molar ratio between components. The peak is relatively symmetrical and its width is in direct relationship with the rate of color change during transition. A comparative analysis of the total color differences shows larger color changes expressed by an increase of the peak intensity, when the developer loading increases. However, in the case of the binary composite with CVL:PPht ¼ 1:3 molar ratio, the width of the peak is the largest which means the lowest rate of color change during the transition. It must be mentioned that in the case studied by us, the temperature of the peak maximum is situated at 36
C, irrespective of the molar ratio between the components, as can be seen from Fig. 7. The temperature of the peak maximum is situated below the transition temperature and between Tci and Taf. However,

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Fig. 8. Steady state fluorescence intensity of binary thermochromic composites measured as a function of CVL mole fraction. Fig. 7. Total color difference between heated and cooled states of binary thermochromic composites with different molar ratios as a function of temperature.

the temperature corresponds to the crystallization point recorded by DSC measurements during cooling of the same sample, as it will further show.

this theory. Alcohols and weak acids quenched the singlet excited state as the H-bonding increases, due to efficient energy dissipation via intersystem crossing. The results confirm stoichiometry determined from reflectance spectra for this type of thermochromic composites.

3.2. Steady state fluorescence measurements

3.3. Structural changes by dynamic ATR-FTIR

Fluorescence spectra of CVL show a large dependence on environments polarity and protic character [36]. The fluorescence of CVL in alcohols is very weak and depends on the length of the aliphatic chain. One broad band is characteristic in these solvents and it was suggested that emission originates from different electronic structures than in aprotic solvents. A rapid deactivation process takes place, suggesting a different mechanism of excited state processes. Two excited singlet states are characteristic in such systems, a charge transfer (CT) state localized in the cycle containing the lactone ring and a highly polar CT with electron density transferred from one auxochromic group to the aromatic cycle next to the lactone ring. The hydrogen bonding ability of the alcohols shows the important role of the proton transfer and possibility to relax via hydrogen bonds with the solvent. Possibility of the opening of lactone ring due to the electronic transfer sustains two deactivation pathways originating from two excited states [37e39]. PPht, essentially a phenol with a lower oxidation potential than an alcohol (TD) quenches both singlet and triplet states [40]. In this view, the stoichiometry of the colored CVL-PPht complex can be studied applying Job’s method for fluorescence spectra because fluorescence intensity is very sensitive to the existence of intermolecular hydrogen bonds. CVL shows fluorescence emission properties in solution and in solid state when it is excited in ultraviolet light, as already reported in the literature [39,41]. In order to determine the optimum wavelength of the excitation light, the fluorescence intensity of solid solutions of CVL in TD was recorded simultaneously as a function of emission and excitation wavelengths. From the experiments it could be observed that a good excitation wavelength is situated at 365 nm. Plotting emission intensity against CVL mole fraction for different thermochromic composites, it was found that when CVL mole fraction XCVL ¼ 0.29 the value recorded for fluorescence intensity is minimum, as it can be observed from Fig. 8. This can be explained by the formation of the maximum number of hydrogen bonds in CVL-PPht complex. Therefore, fluorescence quenching is due to the formation of different deactivation pathways of excited states in supramolecular complexes. The hydrogen bond strengthening in excited states describe spectral features of CVL in TD and deactivation by internal conversion can be explained using

The FTIR spectra provide important information not only regarding the structure of the components in the thermochromic composites but are an important tool to identify structural changes during the thermochromic transition [42]. The study of structural modifications that takes place during thermochromic transition establishes if changes in the vibration bands can be correlated to the formation of supramolecular complexes between components in such a binary composite. It is obvious that the changes recorded in the signal of carbonyl stretching vibration are in direct relationship with the degree of involvement in hydrogen bonding of the CVL carbonyl group and the opening of the lactone ring. A particularity of the system studied by us is the existence of a lactone ring in both of the components. Far from being an inconvenience, this characteristic facilitates systematic examinations of structural modifications which take place during transition. As it was concluded from the reflectance spectra in the visible domain, for all composites investigated it was found that as the mole fraction of PPht increases, the formation of a more intense colored complex can be observed. In addition, from the FTIR spectra it was found that the peak at 1740 cm1 corresponding to the lactone carbonyl vibration of CVL decreases in intensity. This behavior is due to the formation, in the solid state, of a supramolecular complex accompanied by the opening of the lactone ring of CVL. It is important to note that in the IR spectra can be seen a band, at about 1735 cm1, which correspond to the lactone carbonyl stretching vibration of PPht. This evidence supports that in supramolecular complexes PPht interact with CVL exclusively through phenolic groups. Next to this band, at 1722 cm1 appears a peak assigned to the C]O stretching vibration of carboxylic group belonging to CVL ring-opened form, when the spectrum is recorded at normal temperature. When the temperature is increased, the band at 1740 cm1 increases together with the formation of a supplemental band at 1770 cm1 which is characteristic to ɣ-lactones such as CVL and is due to a Fermi resonance effect exhibited when TD melted [43]. The existence of interactions between components is clearly observed when we analyze comparatively, on one side the FTIR spectra of each of the components dissolved in TD and on the other side the thermochromic composite. As it can be seen from Fig. 9 in

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values, while for methyl groups the band is shifted to lower wavenumbers. In the region where are active bands assigned to deformation vibrations bands are merged into a broadband located next to them at higher wavenumbers. The presence of a strong band at 1611 cm1 confirms the existence of iminium form of CVL, at normal temperature, while its intensity diminishes when the temperature increases. At 1580 cm1 is present a band which can be attributed to the asymmetric stretching vibration, while the band at 1425 cm1 is attributed to the symmetric stretching vibration of COO group, respectively. Both bands disappeared from the spectra when the transition occurs confirming the ring closure of CVL when TD melted and no specific CVL-PPht interactions in the discolored state. Regarding the influence of the temperature on the evolution of FTIR spectra, it must be emphasized that during the heating only small differences are recorded, beginning with 37
C, while at 38
C the spectra changed significantly, as it could be observed from Table 1. It is very clear that major structural changes are observed in the FTIR spectra above the melting point of the solvent. Bands assigned to the asymmetric C]O and to the symmetric C]O stretching vibration of COO are situated at 1580 and 1425 cm1, respectively as weak bands only under 36
C. Over this temperature, these bands diminish in intensity and completely disappear from the spectra when the composites are heated to 38
C. Based on this finding we may conclude that at this temperature, which corresponds to Taf determined from visible reflectance hysteresis loop, CVL lactone ring is closed and the colored supramolecular complex is totally dissociated. Fig. 9. ATR-FTIR spectra of thermochromic composites (a) and shifting of the carbonyl stretching vibration bands as a function of temperature (b).

the spectra of the colored complex are found some supplementary bands compared to those in the spectra of the components. This is strong evidence of the existence of some interactions between components in the composite. Therefore, in the spectrum of the composite with the mole fraction XCVL ¼ 0.29, besides the bands corresponding to carbonyl stretching vibration as already mentioned, are observed bands which confirm the opening of CVL lactone ring accompanied by the formation of COO characteristic bands and also the appearance of the iminium group. Methylene stretching modes are in concordance with the order of polymethylene chain and the shifts of these bands are due to conformational order [44]. During melting of TD, conformational disorder will modify the FTIR spectra in the long-wavenumber region by shifting the peaks assigned to methylene groups to higher

3.4. Thermal analysis Thermal analysis shows the appearance of a large endothermic peak during heating and an exothermal peak, usually split in two, during cooling. Analysis made on different samples reveal that the shape of the curves is largely influenced by the components of the thermochromic composites and it is quite evident that interactions between the components determine the profile of the thermal curve. The addition of different quantities of color former and developing agent determined only a small variation of phase transition temperatures during the heating cycles, as can be observed in Table 2. The total enthalpy is determined by several reversible and irreversible processes, which take place simultaneously or successively during thermochromic transition. As it was already demonstrated, on solidification, the phase separation of the colored complex, formed between color former

Table 1 Assignments of FTIR peaks influenced by the temperature. Peak position (cm1) and intensity


36 C 3326 3241 2960 2916 2847

38 C (s) (s) (s) (s) (s)

3326 e 2954 2921 2852 1770

1740 (s)

C¼O stretching vibration in CVL lactone ring

(s) (s) (m) (m) (m) (s) (s)

1735 (s) e e e e e e

C¼O stretching vibration in PPht lactone ring C¼O stretching vibration in carboxyl groups CO asymmetric stretching in COO CO symmetric stretching in COO; CH2 scissoring vibration CH2 wagging vibration CH3 rocking vibration CH2 rocking vibration

e e 1735 1722 1580 1425 1331 1026 1003

Peak assignment (s-strong; m-medium; w-weak; sh-shoulder; br-broad)


(s,br) (sh) (s) (s) (s)

OH stretching vibration, H-bonded OH stretching vibration, H-bonded CH symmetric stretching vibration in CH3 groups CH asymmetric stretching vibration in CH2 groups CH symmetric stretching vibration in CH2 groups C¼O Fermi resonance in ɣ-lactones

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Table 2 Enthalpies of thermochromic binary composites with different CVL mole fractions during successive heating and cooling cycles. XCVL

0 0.18 0.2 0.25 0.29 0.36 0.43 0.50 0.57 1

Transition temperature (
C)

37.3 37.4 37.2 37.2 37.1 37.2 37.1 37.2 37.3 37.2

Total enthalpy (J/g)

Enthalpy of reversible processes (J/g)

Enthalpy of irreversible processes (J/g)

Heating

Cooling

Heating

Cooling

Heating

Cooling

210.2 206.6 206.7 205.7 204.9 210.3 210.0 207.5 205.8 217.8

225.1 211.4 202.0 207.7 214.3 216.9 220.4 211.1 209.4 221.6

89.7 98.4 90.5 81.7 104.4 93.3 88.3 83.8 72.5 106.4

55.5 54.3 47.7 63.1 51.2 56.6 59.9 53.4 58.3 56.6

120.5 108.2 116.2 123.9 100.4 117.0 121.7 123.7 133.3 111.4

169.6 157.2 154.3 144.6 163.1 160.3 160.6 157.7 151.1 165.0

and developer, takes place. Moreover, several authors observed an important effect of ageing of the solid composites [29]. This phenomenon is related to the compatibility between the color former and the developing agent and depends on the solubility of each component in the solvent and associations established between components. Therefore, mass transport due to the concentration gradient becomes very important in this type of composites and is the base of the thermal effect due to irreversible processes measured by TMDSC. Reversible processes are associated with physical interactions between components (hydrogen bonds established between components and p-p stacking between aromatics, van der Waals forces between hydrocarbon tails of TD, used as solvent), phase transitions, dissolving of the components in TD, polymorphic transitions (the existence of different crystalline structures), cyclization reaction and aromaticity. It is well known that when an irreversible process takes place in a certain range of temperature belonging to a reversible transition, a standard analysis of the reversible process by DSC is incorrect. Therefore, data regarding equilibrium and kinetic processes are extracted by successive scanning at different rates, as it can be observed from Fig. 10. The uniform distribution of the components in composite materials due to the maximum coordination number attained in the thermochromic complex when CVL mole fraction xCVL ¼ 0.29 lead to a minimum value of the total enthalpy. From the experiments, for the same composite with xCVL ¼ 0.29, result a maximum value of the heat attributed to reversible processes, while for irreversible processes a minimum value is recorded, as it can be seen from Table 2. On the one side, when the optimal ratio is reached, the maximum number of hydrogen bonds is achieved and it is necessary for the maximum amount of energy to break them down during melting, explaining this behavior. On the contrary, viscosity diminished while the temperature increased and depends on the molar fraction of the components. It is presumed that the minimum energy necessary to diminish viscosity for the mass transport and for the uniform distribution of the components in the melted solvent is recorded when the optimum molar ratio between components is reached. Finally, it should be noted that the differences between reversible and nonreversible processes are well detected only from the melting endotherms because irreversible processes are creating entropy and consuming thermal energy. The study on the enthalpies determined by TMDSC revealed that the composition with CVL mole fraction XCVL ¼ 0.29 could be considered the optimum composite and confirm the results obtained from visible reflectance spectroscopy and fluorescence measurements.

4. Conclusions The thermochromic composites of the type described in this work change from blue to colorless. The color change occurred around the melting temperature of the solvent and intensity of the coloration is determined by the molar ratio between the color former (CVL) and the developing agent (PPht). It is important to ensure a sufficient amount of solvent, which allows the full dissolution of the components in the molten state of the thermochromic system. Otherwise, due to the hydrogen bonds established between the components in the solid state maintain the blue color of the composites even after the transition temperature (melting of the solvent).

Fig. 10. Modulated dynamic scanning calorimetry on thermochromic binary composite with CVL mole fraction XCVL ¼ 0.29 on heating (a) and on cooling (b).

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The optimum ratio between the components could be determined by Job’s method from UVeVis diffuse reflectance spectra, fluorescence intensity or by calorimetric measurements. In this case the optimum molar ratio between CVL and PPht is 1:2.5 and was verified using all the methods previously mentioned. This ratio allows the development of the color intensity when the temperature is situated bellow the melting point of the solvent. The area of the surface described by L*a*b* parameters in the CIELAB color space is not the same on heating with that obtained on cooling cycle. The hysteresis loop describes temperature dependence of composites color by means of initial and final temperatures of color switching on heating and cooling cycles. Measurement of total color differences by CIELAB method is more appropriate to evaluate color switching properties of this type of thermochromic composites since it depends on all the other color parameters on the entire visible region of the spectrum. The fluorescence intensity of CVL is totally quenched by the involvement in hydrogen bonds with PPht when the maximum number of hydrogen bonds is reached and the optimum molar ratio between components is attained. It is obvious that processes, which take place at the transition temperature, are complex and involve besides the melting of the solvent, polymorphic transitions, dissolution of each component in the melted solvent, breaking up the hydrogen bonds established between components in the solid state and absolute values for each off the processes are hard to evaluate. However, from variations of the heat for irreversible and reversible processes during the heating of thermochromic composites containing different molar ratios between components it is possible to estimate the optimum value of the molar ratio in binary thermochromic composites. Acknowledgement This work was performed through Partnerships in priority areas Program e PNII-PCCA-2013-4 e developed with the support of MENe UEFISCDI, project number 0864.

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