Cross linking-dependent properties of PVK-MEH-PPV bi-block copolymer: Vibrational, thermal and optical properties

Accepted Manuscript Cross linking-dependent properties of PVK-MEH-PPV bi-block copolymer: Vibrational, thermal and optical properties Faten Abbassi, Mohamed Mbarek, David Kreher, Kamel Alimi PII:

S0022-3697(18)30099-4

DOI:

10.1016/j.jpcs.2018.08.004

Reference:

PCS 8680

To appear in:

Journal of Physics and Chemistry of Solids

Received Date: 11 January 2018 Revised Date:

9 July 2018

Accepted Date: 3 August 2018

Please cite this article as: F. Abbassi, M. Mbarek, D. Kreher, K. Alimi, Cross linking-dependent properties of PVK-MEH-PPV bi-block copolymer: Vibrational, thermal and optical properties, Journal of Physics and Chemistry of Solids (2018), doi: 10.1016/j.jpcs.2018.08.004. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical abstract

ACCEPTED MANUSCRIPT Cross linking-dependent properties of PVK-MEH-PPV bi-block copolymer: Vibrational, Thermal and Optical properties Faten Abbassia, Mohamed Mbareka, David Kreherb, Kamel Alimia* a

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Unité de Recherche (UR 11ES55), Matériaux Nouveaux et Dispositifs Electroniques Organiques, Faculté des Sciences, Université de Monastir, 5000, Tunisie. Institut Parisien de Chimie Moléculaire (IPCM), Chimie des Polymères, Sorbonne Université, UPMC Université Paris 06, UMR 8232, F-75005 Paris, France.

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*Corresponding author E-mail: [email protected] Tel: 216.73.500.274, Fax: 216.73.500.278

Abstract

Despite its interesting optoelectronic properties, the insolubility of poly(Nvinylcarbazole-poly[2-methoxy-5-2(ethylhexyloxy)-1,4-phenylenevinylene]

(PVK-MEH-

PPV) bi-block copolymer has caused some difficulties especially for the elaboration of new

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composites based on this copolymer and [6,6]-phenyl-C61-butanoate of methyl (PCBM). To solve the problem of insolubility, we first modify quantitatively a main parameter in the chemical synthesis, namely the oxidant agent and then replaced with a new one. Dramatic

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changes in the vibrational and optical properties are founded by varying the FeCl3 amount in the synthesis way due to the cross-linking of PVK units. This is induces the modulation of

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copolymers properties particularly theirs emissive proprieties due to the modulation of PVK and MEH-PPV contribution. A red shifting of the maximum of photoluminescence and a quenching was observed with increasing of the oxidation yields. Composite based PCBM and copolymer was elaborated due the solubility of one of copolymer by changing the oxidant agent. Keywords: A. Polymers; B. Chemical synthesis; C. Infrared Thermogravimetric analysis (TGA); D. Optical properties. 1

spectroscopy; C.

ACCEPTED MANUSCRIPT 1. Introduction Intensive efforts were deployed to synthesis new organic polymers involving different conjugated monomers [1-4]. The first aim is to obtain a copolymer with the optimum properties of each monomer. The second aim is to ameliorate their solubility [5-7] in order to

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make easy their elaboration in thin films with a controlled thickness [8-10].

Thus, it is interesting to synthesis new soluble copolymer with a well understand of its intimate relationship between its structure and its optoelectronic properties. For this reason,

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the synthesis methods have been diversified [11,12]; also, several parameters have been

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changed within the chemical reaction in order to obtain organic products possessing advantageous and important optical properties. In this context, in our previous work [13], a new copolymer has been synthesised based on i) poly(N-vinylcarbazole) (PVK) [14-17] and ii) poly[2-methoxy-5-2(ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) [18-20]. The first homopolymer is a semiconducting polymer widely used as an electronic and a

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luminescent material emitting in blue and the second is electroluminescent and has been widely studied for its optoelectronic properties. This copolymer was synthesised by oxidative

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route using anhydrous ferrochloride (FeCl3) as the oxidant agent. Based on different experimental analyses combined with calculations based on DFT the new copolymer present

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an interesting optical properties which make it suitable for organic electronic. However its insolubility makes its use in multilayer structures very difficult. In order to overcome this handicap, the new PVK-MEH-PPV copolymer has been re-synthesised by using different amounts of FeCl3. Furthermore, we have substituted the FeCl3 oxidant by tetrachloride (TiCl4). The effect of nature and quantity of oxidant agent on the properties of PVK-MEHPPV is investigated using different experimental measurements as well as those of composite PVK-MEH-PPV:PCBM. Varying oxidant agent amount can modulate the vibrational and photo-emissive properties of copolymers. It was found that changing FeCl3 by TiCl4 gives a 2

ACCEPTED MANUSCRIPT soluble copolymer, this is raise the handicap of insolubility and make possible the functionalization with PCBM particles in order to elaborate a nanocomposites based PVKMEHPPV.

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2. Experimental part 2.1. Materials

Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]

(MEH-PPV)

>99%

(Mn 40,000-70,000), poly(N-vinylcarbazole) (PVK) (Mn 25,000-50,000) with average

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molecular weight Mw = 1,100,000, [6,6]-phenyl-C61-butanoate of methyle (PCBM),

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ferrochloride (FeCl3), titanium tetrachloride (TiCl4), chloroform anhydrous ≥99%, methanol, ethanol and hydrazine were purchased from Sigma Aldrich. 2.2.

Synthesis of PVK-MEH-PPV copolymers

As previously recorded [13], the PVK-MEH-PPV statistic diblock copolymer

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(Copolymer (A)) was synthesized by a chemical oxidative reaction using FeCl3 as oxidant. We have resynthesized copolymers by the same method [13] by changing the FeCl3 amounts: 250 mg (Copolymer 1), 500 mg (Copolymer 2) and 750 mg (Copolymer 3). It should be

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noted that for these three compounds we have used 40 mg of PVK and 40 mg of MEH-PPV

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also. For the synthesis using TiCl4, the reaction was started with the amount of 120 mg of PVK dissolved in 50 ml of CHCl3, 120 mg of MEH-PPV dissolved in 50 ml of CHCl3 and 1g of TiCl4. The obtained mixture was further stirred for about three days. The reactant solution was precipitated into methanol. Then 5 to 8 ml of hydrazine was added in order to obtain the product in the neutral state. The resulting precipitate was collected by filtration. (We obtain a soluble compound in chloroform (Copolymer (B))).

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Characterization techniques The infrared spectra were obtained with a « Nicolet iS10 » infrared spectrometer related

to « OMNIC » software. Samples were prepared in pellets of KBr mixed with the organic

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compound. Thermal characterisations of all compounds were performed using TGAQ50 (TA instruments) controlled by a heat control program. For soluble compounds, the optical absorption measurements were realized using a Cary 300 Scan Uv-visible spectrometer at room temperature and the photoluminescence measurements (PL) were obtained using a Cary

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Eclipse Fluorescence spectrometer. For insoluble compounds, a Cary 5000 spectrometer and a

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Jobin-Yvon Fluorolog 3 spectrometer were used to determinate the optical absorption and photoluminescence (PL), respectively. The copolymers were prepared by inserting the powder into a hole on the small metal support. This hole is filled with the powder and then a quartz substrate is put on top. For all PL and optical absorption of soluble copolymer and the obtained composite the measurement are taken in chloroform solution phases (≈ 3 ml) using

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quartz cuvette using with a thickness of 1 cm. View the absence of chromatography equipment as well as mass spectroscopy in our physics laboratory both within our University;

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we are sorry not to give more chemical details of synthesized copolymers like (molar mass, chromatography method NMR measurements).

3.1.

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3. Results and discussion

Copolymers synthesised by FeCl3

3.1.1. Infrared analysis (IR) By comparing the different normalized IR spectra of the PVK-MEH-PPV copolymers synthesised with different amounts of FeCl3 (Figure 1), we can say that all curves shows almost the same vibrational bands, even if some modifications appear notably in the intensity of few of them. We can suggest so the reproducibility of copolymers. To try to elucidate this

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ACCEPTED MANUSCRIPT change, we compared the intensities of some common bands attributed to PVK and MEHPPV vibrations both. First, the band located at about 790 cm-1 attributed to the benzene ring vibration of dimer of PVK (bicarbazole) [21-22] was increased when the amount of FeCl3 increases, which indicates that the phenomena of cross linking of PVK chains increase

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proportionally with the amount of FeCl3 [23] (Figure II in Supplementary Materials). To estimate the degree of cross-linkage, we have computed the intensity ratio of two peaks C-C ring vibration of (PVK) at 1450 cm-1 and C-C (MEH-PPV) at 1506 cm-1 IMeh-PPV/IPVK

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bellowed in the aim of weighting contribution of two different segments of PVK and MEHPPV. The ratio value of the two peaks is nearly 0.23 for copolymer (c 3), 0.38 for the

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copolymer (b 2) and 0.63 in the case of copolymer (a 1) reveling that the increase of FeCl3 amount induces the increase of PVK and the decrease of MEH-PPV contributions. The same evolution is observed in the intensity ratio of peaks 1450 cm-1 C-C ring vibration of (PVK) and the new band located at about 790 cm-1 attributed to the crosslinking PVK. The ratio

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value of the two peaks I790-/I1450 was founded 1 for copolymer (c 3), 0.72 for the copolymer (b 2) and 0.56 in the case of copolymer (a 1). We examined the intensity ratio band located at 790 cm-1 attributed to the crosslinking PVK and band located at 720 and 745 cm-1 the ratio

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value I790-/I720 was found to be 1.3 for copolymer (3), 0.78 for the copolymer (2) and 0.43 in the case of copolymer (1). The same values are almost found for I790-/I745, 1.2 for copolymer

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(3), 0.8 for the copolymer (2) and 0.35 in the case of copolymer (1). We can note that the crosslinked yield is maxima when FeCl3 increase. The degree of crooslinking was estimated to be 100% for 750 mg of FeCl3, 76% for 500 mg and 44% for 250 mg. Thus, the contribution of MEH-PPV units is inversely proportional to the PVK cross-linking yield. Consequently the contents of PVK segments cross-linked probably prevent the grafting MEH-PPV segments on the PVK skeleton. This is also clear for the two PVK bands situated at about 720 and 745 cm-1 [21] which have experienced an increase in their intensity. Furthermore, some other PVK

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ACCEPTED MANUSCRIPT characteristic bands [21] have been extended by the increase of FeCl3 amount, like the band located at 1483 cm-1 assigned to the antisymmetric C=C stretching ,the band located at 1450 cm-1 assigned to the ring vibration and also the 1151 cm-1 band attributed to C-H in plane deformation of aromatic ring of PVK. In another hand, a reverse aspect has been noticed for

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the characteristic bands of MEH-PPV [24-25] especially the band localised at 1406 cm-1 assigned to CH2 deformation of vinylene group, we note also the decrease of the intensity of the MEH-PPV characteristic band located at 1506 cm-1 assigned to semi-circular phenyl

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stretch and as well its disappearance in the copolymer 3. Going from copolymer 3 to copolymer 1 (decreasing of FeCl3 amount), we notice a progressive smothering of the PVK

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vibration bands observed towards 921 cm-1, 1080 cm-1 and 1220 cm-1 corresponding respectively to C-C vibration, C-H deformation in plane of aromatic ring, and C-N stretching. This is can be related to the increase of MEH-PPV grafting to the PVK which limit the rotation and the vibration of PVK blocs. In the otherwise a increasing was noted in intensity

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of the band located at 1037 cm-1 assigned to stretching of alkoxyl groups of MEH-PPV. From all these modifications it can be noted that the change of FeCl3 amount involves the proportion of PVK and MEH-PPV in the copolymer skeleton. Indeed, counter to PVK, the

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increase of FeCl3 amount produces the decrease of MEH-PPV units; which was more illustrated by calculating the intensity ratio between the PVK band situated at 1483 cm-1 and

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the band at 1204 cm-1 attributed to phenyl-oxygen vibration of MEH-PPV. This ratio was increasing from 0.66 to 1.24 by going from copolymer 1 to copolymer 3. The thermal degradation analysis of the synthetic copolymers (Figure I) was discussed (see Supplementary Materials). 3.1.2. Optical properties

Figure 2 shows the optical absorption spectra of copolymers 1, 2 and 3. Despite the presence of some common bands in the three spectra, the first impression that can be noted is 6

ACCEPTED MANUSCRIPT the difference in their shape. Whereas this shape has been transformed by going from copolymer 1 to copolymer 3 to be similar to the PVK absorption spectrum. From this interpretation we can conclude that the increase of the FeCl3 amount steps up the PVK crosslinking phenomenon which prevents the grafting of the MEH-PPV units, for this reason the

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intensity of the MEH-PPV band situated at about 357 nm has been decreased almost disappeared in copolymer 3 and also the band attributed to the charge transfer [13] was less and less accentuated because of a decrease in the charge transfer between PVK and MEH-

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PPV segments. The charge transfer band has also undergone a bleu shift, where it was moved from the [600-700 nm] area for copolymer 1, to the [500-650 nm] area for copolymer 2, to

effective conjugation length.

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[450-600 nm] area for copolymer 3, the cause that allows finding that the FeCl3 reduce the

PL spectra of the copolymer 1, 2 and 3 presented in figure 3 range from 400 to 650 nm and there were recorded in the same experimental conditions at room temperature. Firstly we

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note the increase of PL intensity with the increase of FeCl3 amounts caused by the PVK crosslinking [23]. This phenomenon is more confirmed by PVK band situated at about 400 nm which is more accentuated by going from copolymer 1 to copolymer 3 which confirms the

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increase of PVK blocks contribution due to the PVK cross-linking. We can also note that the common maximum band is red shifted by 10 nm from copolymer 3 (482 nm) to copolymer

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1(492 nm). This shift is caused by the presence of MEH-PPV ramifications (metoxy and ethyl-hexyloxy) [13] which confirms the decrease of the contribution of MEH-PPV units with the increase of FeCl3 amounts which is in good agreement with infrared and optical absorption analyses. 3.2. Copolymer synthesised by TiCl4 (Copolymer (B)) The substitution of FeCl3 by TiCl4 has allowed improving the solubility of the PVK-MEHPPV copolymer. 7

ACCEPTED MANUSCRIPT 3.2.1. Infrared (IR) The normalized infrared IR spectra of the copolymer (A) and (B) are shown in figure 4. Practically, all vibrational bands of the previous copolymer (A) [13] are present in the new

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one (B). But some differences are clearly remarkable. Indeed, an increase of some MEH-PPV band intensity in copolymer (B) is observed especially the bands located at 1040 and 1505 cm-1. At the same time a decrease of PVK intensity bands [21-22] especially those located at 1600 and 1625 cm-1 as well as the benzene ring vibration of bicarbazole band situated at about

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790 cm-1. This can evince the decrease of cross-linking of PVK chains by substituting FeCl3

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by TiCl4. In fact, the decrease of the cross-linking, which means the decrease of the bicarbazole bridges, promotes the litheness of the grid of atoms of the polymer and therefore the enhancement of its solubility. In the whole, it can be noted that the two IR spectrums possess almost the same shape with same little changes in term of band intensity, but we can conclude that the two PVK-MEH-PPV copolymers synthesised with FeCl3 [13] and TiCl4

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seem to present the same molecular composition despite some difference in term of polymerisation degree of MEH-PPV and PVK.

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The thermal degradation analysis of the new synthetic copolymer (B) compared to copolymer

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(A) was elucidated in figure II (Supplementary Materials). 3.3.2. Optical properties

Figure 5, shows the UV-Vis and PL spectrum of PVK, MEH-PPV and copolymer (B) (solution). From figure 5-a, the absorption region of copolymer (B) seems to be the superposition of PVK and MEH-PPV absorption spectrum. Indeed, we note the presence of the five PVK characteristic bands located at 243, 262, 295, 330 and 344 nm. The UV-Vis spectra of the copolymer present also the MEH-PPV characteristic peak situated at about 500 nm (in native MEH-PPV) with a decrease in term of intensity and also it has been performed a 8

ACCEPTED MANUSCRIPT blue shift (440 nm for the copolymer) which marks the decrease in the number of MEH-PPV units in the copolymer by comparing it with the homopolymer. In another hand, a large band appears, located in the range of [570-750 nm] attributed to the charge transfer between homopolymers (PVK and MEH-PPV). This band correspond to an optical gap of 1.56 eV (1.7

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eV for the previous copolymer (A)) [13]). In other hand, the photoluminescence of the copolymer (Figure 5-b) show a maximum of photoluminescence situated at 538 nm which is blue shifted versus to the photoluminescence maximum of MEH-PPV (560 nm), which

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confirms the proposition suggested in the optical absorption study that the polymerization degree of the MEH-PPV segments has been increased. Moreover, the PL spectrum of

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copolymer (B) is extended in a range from 350 to 750 nm. It’s also clear that the spectrum of the copolymer is red shifted compared to PVK spectrum, which demonstrates that the effective conjugation length is higher in the case of copolymer. This can be related to conformational changes which increase the extent of conjugation in the copolymer backbone.

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A slight red-shift of absorption and PL spectrum is observed going from the solid state to copolymer (C) in chloroform solvent. This is can be related to dipole interaction between solvent molecules and the solute. The corresponding excitation energies show a red shift

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compared to the same insoluble copolymers previously synthesis. This is due to the charge transfer states caused by the solvent. This shift is more pronounced for the transition S1→ S0

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which is mainly due to the transition HOMO–LUMO PLmax from 495 nm to 538 nm in the solvent. Therefore, solvent clearly enhances the probability of this transition and then eases the charge transfer process. 3.3.3. Optical study of the (Copolymer (B) :PCBM) composites For the optical absorption spectra of the three PVK-MEH-PPV:PCBM composites prepared with different ratios: [1:0.5], [1:0.75] and [1:1] (Figure 6), we noticed that more the proportion of PCBM increase, more the intensity of PCBM characteristic bands located at 263 9

ACCEPTED MANUSCRIPT and 329 nm increase without any observed shift for the PVK characteristic bands. Whereas, for the characteristic bands of MEH-PPV and apart from the quenching phenomena, it have been undergone a red shift which indicates a change in the organization of the copolymer chains especially for the segments of MEH-PPV where its organization was disturbed by the

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addition of PCBM.

The emission spectra of copolymer and composites with different proportions of PCBM are shown in figure 7. The first observation is the quenching in PL intensity of the most

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intense peak of the copolymer situated at 539 nm by increasing the PCBM percentage.

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Furthermore, a modification is observed in the shape of the emission spectra especially for the 50 and 75 weight or percentage (Wt%) PCBM composites in the [400-500 nm] range. More precisely, the emission bands in this ranges for these composites was increasing compared to the copolymer (B) only. In another hand, two essential aspects are observed: i) a red shift is made in the emission spectra of composites with 50 and 75% of PCBM and ii) a bleu shift is

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made in the [1:1] composite. All these observations confirm the charge transfer between copolymer (B) and PCBM. The value of charge transfer efficiency of each composite (Table

ηT

= Id/a / Id

(1)

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1) was calculated using the following relation:

Where Id/a and Id are the integrated PL intensity of the donor (copolymer) in the presence and absence of the acceptor (PCBM), respectively. 4.

Conclusion

New copolymers based on PVK and MEH-PPV homopolymers were synthesized by oxidative way using FeCl3 and TiCl4 as oxidative agents. Acting on cross-linking of PVK in the synthesis pathway of PVK-MEHPPV copolymer, we have shown a dramatic change in

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ACCEPTED MANUSCRIPT vibrational and optical properties. Thus, a modification of the respective contribution of PVK and MEH-PPV segments in the photo-physical properties was proved by changing the oxidant agent amount. PVK cross-linking significantly affects the vibrational, the optical properties and the charge transfer. The contribution of PVK segments is accentuated when FeCl3

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increased causing a PL red-shifting. Changing FeCl3 by TiCl4 modulates the solubility of synthetic copolymer and giving the ability to elaborate new composite based PVKMEHPPV/PCBM. Thus, changing oxidant agent in the prepared copolymers can modulate the

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electronic and molecular structures of copolymers; this is also can vary their organic

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optoelectronic application.

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ACCEPTED MANUSCRIPT Table captions

Table 1: Charge transfer efficiency (%) of different composites.

λmax (nm)

Composite [1:1] 513

707

557

328

112

--

21

53

84

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Integrated PL intensity (a.u) Charge transfer efficiency (%)

Composite [1:0.75] 546

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Copolymer Composite [1:0.5] (B) 539 546

ACCEPTED MANUSCRIPT Figure Captions

Figure 1: Infrared spectra of copolymers synthesised using different amounts of FeCl3: Copolymer 1 (250 mg),

Figure 2: Optical absorption spectra of solid copolymers 1, 2 and 3. Figure 3: PL spectra of solid copolymers 1, 2 and 3.

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Figure 4: Infrared spectrum of copolymer (A) [13] and copolymer (B).

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Copolymer 2 (500 mg), Copolymer 3 (750 mg).

PVK, MEH-PPV and copolymer (B).

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Figure 5: (a) Optical absorption spectra of PVK, MEH-PPV and copolymer (B) in Chloroform, (b) PL spectra of

Figure 6: Optical absorption spectra of composites with different PCBM percentages in Chloroform.

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Figure 7: PL spectra of composites with different PCBM percentages in Chloroform.

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Normalized Infrared Intensity (a.u)

MEH-PPV

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Copolymer 3

792

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745 800

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Wavenumber (cm-1)

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Absorbance (a.u)

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Wavalength (nm) Figure 3

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PL Intensity (a.u)

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1505

1040

IR Intensity (a.u)

Figure 4

800

1000

M AN U

Copolymer A Copolymer B

1200 1400 -1 Wavenumber (cm )

1600

1800

(a)

PVK MEH-PPV Copolymer (B)

EP

1.0

AC C

0.6

0.4

0.2

(b)

1.0

Copolymer (B) MEH-PPV PVK

0.8

PL (a,u)

0.8

Absorbance (a.u)

TE D

Figure 5

0.6

0.4

0.2

0.0

0.0 250

300

350

400

450

500

550

600

Wavelength (nm)

650

700

750

400

500

600

Wavelength (nm)

700

800

ACCEPTED MANUSCRIPT Figure 6

Copolymer (B) Composite [1:0.5] Composite [1:0.75] Composite [1:1] PCBM

1.0

RI PT

0.6

0.4

SC

Absorbance (a.u)

0.8

M AN U

0.2

0.0 300

400

500

Wavelength (nm)

600

700

TE D

Figure 7

800 700

AC C

PL (a.u)

500

EP

600

Copolymer (B) Composite [1:0,5] Composite [1:0,75] Composite [1:1]

400 300 200 100 0

400

500

600 Wavelength (nm)

700

800

ACCEPTED MANUSCRIPT Highlights: • Vibrational and optical properties of copolymer are modulated in term of oxidant agent quantity. • PVK cross-linking phenomenon improves the PL intensity.

AC C

EP

TE D

M AN U

SC

RI PT

• The substitution of the oxidant agent enhances the solubility of the synthesised copolymer.