Free volume modifications in chalcone chromophore doped PMMA films by electron irradiation: Positron annihilation study

Author’s Accepted Manuscript Free volume modifications in chalcone chromophore doped PMMA films by electron irradiation: positron annihilation study Ismayil, V. Ravindrachary, S.D. Praveena, M.G Mahesha www.elsevier.com/locate/radphyschem

PII: DOI: Reference:

S0969-806X(17)30898-8 http://dx.doi.org/10.1016/j.radphyschem.2017.08.015 RPC7620

To appear in: Radiation Physics and Chemistry Received date: 16 June 2017 Revised date: 25 July 2017 Accepted date: 2 August 2017 Cite this article as: Ismayil, V. Ravindrachary, S.D. Praveena and M.G Mahesha, Free volume modifications in chalcone chromophore doped PMMA films by electron irradiation: positron annihilation study, Radiation Physics and Chemistry, http://dx.doi.org/10.1016/j.radphyschem.2017.08.015 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 galley proof before it is published in its final citable 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.

Free volume modifications in chalcone chromophore doped PMMA films by electron irradiation: positron annihilation study Ismayila*, V. Ravindracharyb, S. D. Praveenac, Mahesha M. Ga a

Department of Physics, Manipal Institute of Technology, Manipal University, Manipal, Karnataka - 576 104, India b

c

Department of Physics, Mangalore University, Mangalagangotri - 574 199, India

Department of Physics, K.V.G College of Engineering, Kurunjibhag, Sullia, Karnataka - 574 327, India *Corresponding author E-mail address: [email protected] Tel: +91 820 2925624; Fax : +91 820 2571071

Abstract The free volume related fluorescence behaviour in electron beam irradiated chalcone chromophore doped Poly(methyl methacrylate) (PMMA) composite films have been studied using FTIR, UV-Visible, XRD and Positron Annihilation techniques. From the FTIR spectral study it is found that the formation of polarons and bipolaron takes place due to cross linking as well as chain scission processes at lower and higher doses respectively. It reveals that the formation of various polaronic defect levels upon irradiation is responsible for the creation of three optical energy band gaps within the polymer films as obtained from UV-Visible spectra. The crosslinking process at lower doses increases the distance between the pendant groups to reduce the interchain distance and chain scission process at higher doses decreases interchain separation to enhance the number of polarons in the polymer composites as suggested by XRD studies. The fluorescence studies show the enhancement of fluorescence emission at lower doses and reduction at higher doses under electron irradiation. The positron annihilation study suggests that the low radiation doses induce crosslinking which affect the free volume properties and in turn hinders the chalcone molecular rotation within the polymer composite. At higher doses chain scission process support polymer matrix relaxation and facilitates non-radiative transition of the chromophore upon excitation. This study shows that fluorescence enhancement and mobility of chromophore within the polymer matrix is directly related to the free volume around it.

Keywords: Poly(methyl methacrylate) (PMMA); Electron irradiation; Free volume; Crosslinking; Chain scission; Fluorescence.

1

1. Introduction The macroscopic properties of a polymer composite are mainly associated with the free volume related microstructure. Dispersing organic molecules in the solid polymer matrix is a simplest and economical method of tuning the property of the polymer. Particularly the fluorescence emission from a chromophore mainly depends upon the chemical structure of the chromophore and the free volume related microstructure of the polymer matrix in which it is embedded [1, 2]. The properties of the doped polymers can be further fine-tuned by electron irradiation, which is the feasible and fast method of tailoring the polymer properties in recent years. Hence it is remarkable to tune the fluorescence and structural properties of the chalcone chromophore [1-(4-methylphenyl)-3-(4- N, N, dimethylaminophenyl)-2-propen1-one (MPDMAPP)] doped Poly(methyl methacrylate) (PMMA) polymer composite by electron beam irradiation. The aim of this work is to understand the microstructural modification of fluorescent films with thermoplastic amorphous PMMA as a host matrix in which free volume content is more. The altered microstructural properties of electron beam irradiated PMMA/MPDMAPP films were studied by FTIR, UV-Visible, Fluorescence Spectroscopy, XRD, DTA, TGA and Positron Annihilation Lifetime Spectroscopy (PALS) techniques. The modifications in free volume properties were tried to link with the corresponding fluorescence behaviour.

2. Experimental The polymer PMMA used in this work was obtained from M/s Sigma-Aldrich Inc, USA. The NLO

chromophore

chalcone

derivative

1-(4-methylphenyl)-3-(4-N,N-dimethyl

aminophenyl)-2-propen-1-one (MPDMAPP) was synthesized using standard ClaiseneSchmidt condensation reaction method [3]. The 0.5wt% MPDMAPP doped PMMA composite films (thickness 120-180 μm) were prepared by solution casting method using dichloromethane as solvent. The films were irradiated for different doses (50 kGy, 100 kGy, 150 kGy, 200 kGy, 250 kGy, 300 kGy) using 8 MeV electron beam at Microtron Centre, Mangalore University, Mangalore, India. The Fourier transform infrared (FTIR) transmittance spectra for the prepared polymer films were obtained using NICOLET Model 5700 spectrophotometer in the wavenumber range 400-4000 cm-1. UV-Visible absorption spectra for the films were recorded using SHIMADZU UV-1800 spectrophotometer. The Fluorescence studies were carried out using Hitachi F-7000 Fluorescence Spectrophotometer. The Fluorescence images of the films were taken using OLYMPUS BX-51 optical microscope. The XRD studies were carried out using a Rigaku Miniflex-II benchtop X-ray 2

diffractometer with CuKα (λ = 1.5406 Å) radiation. Positron lifetime spectra for the prepared polymer films were recorded using the technique explained in our earlier communication [4]. Mean ortho-positronium (o-Ps) lifetimes and intensities were calculated with the PATFIT-88 program, while o-Ps lifetime distributions were obtained from evaluations made with the maximum entropy principle using MELT program [5]. The thermal studies are performed using Universal TA (SDT Q600) TG/DTA instrument from 30°C to 600°C at a heating rate of 10°C/min under Nitrogen atmosphere (flowing rate 100ml/min). 3. Results and Discussion 3.1. FTIR Studies The FTIR spectra of pristine as well as electron irradiated MPDMAPP doped PMMA films are shown in Figure 1 and corresponding band assignments presented in Table 1.

Figure 1: FTIR spectra of pristine and electron irradiated MPDMAPP/PMMA films.

3

From Figure 1 it is found that the IR peaks for pristine and electron irradiated films are the same for lower doses i.e. all spectra exhibit the characteristic peaks at 1483 cm-1 (CH2 bending vibration), 2848 cm-1 (CH2 stretching vibration), 3000 cm-1 (CH3 asymmetry stretching vibration) and 1728 cm-1 (C=O stretching in the ester carbonyl group) [6, 7]. The non-alteration of the bands in the irradiated film in 0 - 150kGy range indicates that there is no significant modification in the chemical structure of MPDMAPP doped PMMA upon electron irradiation. But for higher doses, a new peak around 1595 cm-1 is appeared and is ascribed to the characteristic C=C and C-N stretching vibration of MPDMAPP [8]. Nonexistence of this peak at lower doses confirms that this new peak is mainly due to C=C stretching. The origin of C=C bond within the polymer due to electron beam irradiation is understood using the reaction scheme as shown in Figure 2.

Table 1: FTIR peak assignments for unirradiated and electron beam irradiated MPDMAPP/PMMA films. Wavenumber (cm-1) Peak assignment 0kGy

300kGy

C─H asymmetric stretching [νa(C-H) of O-CH3]

3000

2995

C─H symmetric stretching [νs(C-H) of O-CH3]

2943

2950

CH2 stretching vibration

2848

2848

C═O stretching vibrations

1728

1728

----

1595

CH2 bending vibrations [δ(CH2)]

1483

1489

Bending vibrations δa(C-CH3)

1443

1438

C─H bending vibrations [δs(C-H) of CH3]

1386

1381

C─O stretching vibrations [ν(C-O)]

1242

1236

C─O asymmetric stretching [νa(C-O-C)]

1148

1148

O-CH3-rock

983

988

γ(CH2)-rock

843

843

γ(C-C) skeletal mode

753

750

C=C stretch (aromatic, s), C─N stretch

4

Figure 2:

Reaction Scheme of interaction of PMMA with MPDMAPP under electron

irradiation. When MPDMAPP is doped with PMMA, it forms hydrophobic interactions and produces a polymer composite as shown in the scheme III (Figure 2). Hence there is intramolecular interaction between the hydrocarbon portion of PMMA and the hydrocarbon portion of the chalcone (i.e., between -CH=CH-, -CH3, and -N(CH3)2 groups). The electron beam irradiation on these composite films further alters their chemical structure. Upon electron irradiation, a polaron (a quasiparticle composed of a charge and its accompanying polarization field) is

5

produced due to the extraction of a negative charge from a neutral segment of the chain (via partial oxidation) accompanied by lattice relaxation as shown in scheme IV (Figure 2). While electrons are removed from or donated to the polymer leads to chain scission and/or cross linking during irradiation, the polymer chain gets easily distorted around the injected charge leading to polaron formation. Successive removal of additional negative charges from the polymer composite that already comprises a polaron, may yield more polarons or the existing polaron may be ionized to form a bipolaron (two close-by polarons) via additional oxidation [9]. It is known that the conjugated systems are highly disordered and the size of the polarons depends on the extent of disorder [10]. The strong electron-lattice coupling is also responsible for the existence of the polarons and bipolaron in conjugated systems. In this particular electron irradiated PMMA/MPDMAPP polymer composite the polaron is the expected state at lower dose (due to cross linking) whereas the bipolaron is the expected state at higher doses (due to chain scission). 3.2. UV-Visible Studies The optical absorption spectra of electron beam irradiated PMMA/MPDMAPP films are taken and the observed optical absorption spectra is shown in Figure 3. The UV-Visible spectrum of unirradiated PMMA/MPDMAPP films show three absorption bands centered at 220nm, 265nm and 396nm. First band (220nm) is due to the carbonyl group segments and are assigned to localized π→π* transitions. The second band (265nm) is assigned to n→π* inter band transition and is attributed to the excitation in the aromatic rings and C═O groups as well as phenyl groups of MPDMAPP. The third band (396nm) is assigned to n→π* transition and may be created due to the charge transfer (CT) groups. Upon irradiation, the absorbance of the peaks at 265nm as well as at 396nm decreases up to 100kGy of dose and both peaks vanishes for higher doses. These variations may be due to the crosslinking and/or chain scission of the polymer matrix upon irradiation [11]. As seen in FTIR results, the electron irradiation gives both polarons and bipolarons and exhibit different absorption spectra: the polaron state yields three broad peaks, whereas the bipolaron state yields only a single broader peak. It is also observed the shift of the n→π* transition peak of the PMMA/MPDMAPP films from 265nm to about 257nm when increasing the electron dose implying the colour variation of fluorescence.

6

Figure 3: Optical absorption spectra of composite for various doses. The optical energy band gap and activation energies were calculated from UV-Visible spectra using the method explained in our earlier communication [7] and it is found that three energy band gap exists within this polymer composite. The appearance of three optical energy band gaps may be attributed to various polaronic and defect levels. The lowest energy band gap Eg1 seems to be related to PMMA matrix. The other two bandgaps (Eg2 & Eg3) evidences the presence of other type of induced states due to the dopant MPDMAPP. The Figure 4a shows that the optical band gap Eg1 varies from 4.48eV to 4.72eV, Eg2 varies from 3.26eV to 2.67eV and Eg3 varies from 2.58eV to 2.48eV. Here it is observed that the Eg1 slightly increases, but Eg2 and Eg3 decreases with irradiation dose. These variations of optical band gaps are attributed to the formation of polarons and/or bilpolarons as depicted from FTIR studies. The decrease in Eg with irradiation dose may be attributed to the creation of defects that existed within the band gap due to crosslinking upon irradiation. The presence of these defects might lead to the formation of lower-energy states [12]. With increasing dose, the process of chain scission dominates and produces free radicals. The formation of free radicals decreases the energy band-gap for electron irradiated polymer composite at higher doses.

7

Figure 4: Variation of a) optical energy band gap Eg and b) activation energy Ea with electron dose of MPDMAPP/PMMA films. Figure 4b shows the observed variations of three activation energies Ea1 (0.49eV0.24eV), Ea2 (0.53eV- 0.73eV) and Ea3 (0.07eV- 0.22eV). Here also Ea1 decreases whereas Ea2 and Ea3 increase with irradiation dose. The decrease in activation energy upon irradiation indicates the molecular disorderness within the composite due to cross linking and/or chain scission process within the MPDMAPP doped PMMA composites. 3.3. Fluorescence Studies Understanding the mechanisms leading to fluorescence is an essential prerequisite in the application of any fluorescent molecule to probe the structure and dynamics of macromolecules. Accordingly the fluorescence emission spectrum of PMMA/MPDMAPP composites at 265nm excitation was taken and the results are shown in Figure 5. From Figure 5(a), it is observed that, for 265 nm excitation, the composite films exhibits dual fluorescence peaks for all irradiation doses. The emission band can be resolved into two components and these bands can be attributed to an excimer at short-wavelength emission (~325 nm) and a dimer at long-wavelength emission (~480nm). The radiation in the second fluorescent band (the most shifted to the long-wavelength spectral region) is conventionally related to the electronic transition from the excited singlet state with a large intramolecular charge transfer (electron density). Further, the emission peak at 313nm increases, whereas the emission peak for 480nm decreases with electron irradiation dose as shown in Figure 5b.

8

Figure 5: a) Fluorescence emission spectra of composites at 265nm for different electron doses and b) variation of fluorescence emission peak intensities with irradiation dose for 265nm excitation. The fluorescence emission peak intensity values of MPDMAPP/PMMA composites irradiated with different dose for 396nm excitation are presented in Table 2. It is observed that for 396 nm excitation, an emission peak is observed around 482nm and this emission peak shifts towards lower wavelength region with increase in intensity up to 150kGy and then its intensity decreases for higher doses. Table 2: Fluorescence emission peak intensity values of MPDMAPP/PMMA composites for 265nm and 396nm excitations for different doses. λex = 265 nm Electron

Peak I

λex = 396 nm Peak II

Peak I

Dose (nm)

Emission Peak Intensity

(nm)

Emission Peak Intensity

(nm)

Emission Peak Intensity

0

313

21.66

483

473.8

482

546.3

50

317

29.22

482

333.3

481

601.3

100

323

33.9

481

292.3

477

689.8

150

326

39.84

478

241.4

475

767.1

200

327

47.31

475

174.2

472

705.1

300

329

50.04

472

158.8

470

655.3

(kGy)

λem

λem

9

λem

These observed special features of dual fluorescence in this composite are understood by invoking the rotation of twisted intramolecular charge transfer (TICT) state. The fluorescence in the organic NLO chromophore chalcone derivative MPDMAPP is due to the energy transformation of the excited molecule via internal rotation of the -N(CH3)2 group. This excited state corresponds to the twisted rotamer which has an important charge-transfer character and is called the twisted intramolecular charge transfer (TICT) state; the fluorescence from this state is less energetic than the normal fluorescence and is very dependent on its environment [13]. Thus the twisted intramolecular charge-transfer (TICT) fluorescence hypothesis suggests that the charge transfer is due to the rotation of a dimethylamino group from the plane of the molecule to the plane normal to the aromatic ring. In the present case of MPDMAPP doped system, a charge transfer (CT) complex in the excited state may result from the transfer of electron upon excitation from the donor ─N(CH3)2 to the acceptor –CH3 connected through benzene rings. That means a lone pair of nitrogen in the donor is transferred to the acceptor to form a highly polar CT state. As seen earlier in Figure 5, for 265 nm excitation the composite exhibits dual fluorescence; the weak high energy red shifted emission around 313 nm which arises from the Locally Excited (LE) state (excimer) and the blue shifted strong emission around 480 nm and arises due to the Charge Transfer (CT) state (dimer). That means when an electron-donating substituent and an electron accepting substituent are conjugated in a molecule and these are present in two molecular planes, one contains the donor group and another having the acceptor group, are coplanar in the ground state. In the excited state, these two planes are twisted perpendicular to each other so that the positive charge on the amino group and the negative charge on the carbonyl group are separated. In this case the polarity of the molecule is increased; an excited TICT-state becomes a lower excited state and is stabilized by the polar molecules [14, 15]. Although the twist occurs only in the excited state, a conformation already twisted in the ground state would be in favour of the TICT formation. Hence this type of fluorescence probe exhibits dual fluorescence: one from the non-charge transfer state appearing at high energy and another from the twisted charge-transfer state. The emission appearing at lower energy is sensitive to local polarity and segment mobility. The ratio of these two fluorescence peaks is therefore a measure of segment mobility. The reduction in the emission peak intensity (Figure 5(b)) at 480nm for 265nm excitation may be attributed to the fluorescence quenching upon irradiation and is mainly due to the strong π-interactions induced by excited-state planar conformation of the polymer. The much larger fluorescence quenching in the polymer films is from the fast energy migration 10

along and between the conjugated polymer backbones toward the lower energy π-aggregated sites [16, 17]. The tunable emission can also be observed from the evolution of fluorescence spectra of the composite films as a function of irradiation dose.

Figure

6:

Polarized

Fluorescence

emission

spectra

of

MPDMAPP/PMMA composite films for different irradiation doses.

11

pure

MPDMAPP

and

These results indicate that the efficient energy transfer from PMMA to MPDMAPP must occur in the present polymer composite and the emission at 480nm was partially quenched and the emission at 325nm was significantly enhanced by degradation of polymer chain upon electron irradiation. The gradual increase in fluorescence intensity of the PMMA/MPDMAPP composite upon irradiation can be attributed to the prevention of radiationless decay by stiffness of the polymer matrix due to crosslinking process. Here the local milieu around the chromophore molecules may confine the movement by suppressing the free volume to prevent the nonradiative decay motions to take place [18]. Hence the rotational motion of N(CH3)2 group of MPDMAPP chromophore is not viable under crosslinked conditions which points to the importance of free volume effects. The amount of free volume present in a polymer matrix signifies a driving force toward twisted intramolecular charge transfer (TICT) emission of MPDMAPP compound. The decrease in fluorescence intensity after 150 kGy for 396nm excitation is thus due to the fact that electron irradiation created sufficient free volume for the molecular rotation to occur. The increase in free volume with radiation dose is provided by a polymer chain translational relaxation mechanism.

Figure 7: Variation of luminescence polarization (P) and fluorescence anisotropy (r) of composite with electron dose (396nm excitation). As seen in our earlier communication [13], when a chalcone chromophore is embedded in a semicrystalline polymer PVA, a single fluorescence emission peak observed at 530nm for 263 nm excitation. But dual fluorescence emission peaks at 313 nm and 482 nm for 265 nm excitation is observed for amorphous polymer PMMA as a host matrix. This clearly

12

shows that the fluorescence emission from a chromophore mainly depends upon the chemical interaction of the chromophore with the host matrix and the free volume related microstructure of the polymer matrix in which it is embedded. The fluorescence anisotropy (polarized fluorescence) measurements were performed using the method explained in our earlier communication [13]. It is observed that the fluorescence intensities are different (Figure 6) when the angle between the orientated direction of the films and the direction of the polarizer changes. The measured luminescence polarization P and fluorescence anisotropy r of MPDMAPP/PMMA composite films and the variation of these parameters with electron dose for 396nm excitation are shown in Figure 7. From Figure 7, it is observed that both the luminescence polarization P and fluorescence anisotropy r increases up to 150kGy and then decreases for higher doses. As explained earlier the radiation affects the N(CH3)2 group of MPDMAPP which is responsible for the fluorescence emission in the matrix and the charge transfer is due to the rotation of a dimethylamino group from the plane of the molecule to the plane normal to the aromatic ring. As seen earlier the electron-donating substituent and an electron accepting substituent are present in two molecular planes and they are coplanar in the ground state. In the excited state, these two planes are twisted perpendicular to each other so that the positive charge on the amino group and the negative charge on the carbonyl group. These features are reflected in the variation of luminescence polarization P and fluorescence anisotropy r. More over the chalcone molecular motion is suppressed within the polymer matrix due to cross linking at lower doses, which favours fluorescence enhancement through TICT emission and fluorescence probe undergoes non-radiative decay at higher doses due to chain scission. These inferences are also reflected in positron annihilation data which will be discussed later. Correspondingly, the fluorescence microscopic imaging study for these samples shows interesting optical properties. As presented in our earlier communication [19], the yellow colored film emits sky blue colour under 350 nm excitation; green colour under 480 nm and Red colour on 530 nm excitation. This property of emission under excitation sustaining for long time, may be due to the aromatic C=C and phenyl groups present in the chalcone chromophore [20-22]. 3.4. XRD Studies: The

observed

X-ray

diffractogram

of

unirradiated

and

electron

irradiated

PMMA/MPDMAPP films are shown in Figure 8. The XRD pattern shows three broad diffraction halos at 2 = 14.04º, 29.85º and 41.26º for PMMA/MPDMAPP films. It is

13

observed that the intensity of first halo in the XRD pattern increases up to 150kGy and the intensity of the second halo dominate after 150kGy. In addition to this some sharp crystalline peaks appear upon irradiation, which confirms that the polymer matrix undergoes crosslinking process at lower doses. The observed increase in intensity of the first halo at lower doses can be attributed to the crosslinking of polymer chains. Further, the decrease in the first halo intensity and the slight raise in the intensity of the second halo may be due to the onset of chain scission process within the polymer matrix due to high irradiation dose.

Figure 8: X-ray diffractogram of pristine and irradiated composite films. According to polymer super chain model, the first broad halo is attributed to the intermolecular component and the second halo to the intramolecular separation [23, 24]. The d-spacing (first and second halo) calculated using Bragg’ gives the interchain distance between the polymer chains and the distance between pendant groups (COOCH3) respectively. This is mainly due to the fact that the MPDMAPP/PMMA matrix contains pendant group and three amorphous XRD halos which represent interchain distance and pendant group separations respectively.

14

For unirradiated MPDMAPP/PMMA, the interchain distance between polymer chains is 6.391Å (calculated from the first peak) and the distance between pendant groups is 2.852Å (calculated from the second peak). The variations of interchain distance between polymer chains and the distance between the pendant groups of MPDMAPP/PMMA with electron irradiation dose are shown in Figure 9. From the Figure 9 it is observed that interchain distance decreases continuously whereas the pendant group distance increases and then saturates with irradiation dose. This shows that the crosslinking process at lower doses increases the distance between the pendant groups and reduces the interchain distance.

Figure 9: Variations of interchain distance and pendant group distance as a function of irradiation dose in MPDMAPP/PMMA. The decrease in the pendant group as well as interchain distance may be attributed to the scission of polymer chain at higher doses. At higher doses, the decrease in interchain separation can significantly enhance the number of polarons in polymer composites. This increase in polaron formation results in a reduction in fluorescence intensity. These results also support the enhanced polaron formation with reduced fluorescence intensity and increased free volume size within the polymer film. 3.5. Differential Thermal Analysis (DTA) The thermal analysis of the composite was performed using DTA on unirradiated and electron beam irradiated PMMA/MPDMAPP composite. The obtained thermograms were characterized by the appearance of an endothermic peak due to melting. Values of these 15

melting temperatures were calculated using the DTA curves (Figure 10) and are given in Table 3.

Figure 10: DTA thermogram of pristine and irradiated composite films. Table 3: Glass transition temperature, α-relaxation temperature, melting temperatures and degradation temperature of unirradiated and electron irradiated MPDMAPP/PMMA. Electron Dose (kGy)

Tg ( C)

Tα ( C)

Tm (oC)

Td ( C)

0

114

200

297

374

50

108

202

296

374

100

105

208

295

375

150

104

214

292

377

200

105

217

294

379

300

108

218

297

381

o

o

16

o

It can be seen that the melting temperature decreases up to 150kGy and increases for higher doses. According to Nouh et al [25] the decrease in the melting temperature is attributed to the crosslinking process upon irradiation. In the present case also it is observed that melting temperature decreases with dose and hence it can be attributed to the crosslinking process which increases the degree of ordering within the polymer samples upon irradiation. On the other hand the observed increase in the melting temperature can be explained on the basis of chain scission at doses above 150kGy. Also, prolonged heating causes a random breaking of bonds and the formation of stable molecules with a lower molecular weight. Sometimes this also causes the detachment of low-molecular products because of the reactions of side groups without any appreciable change in initial molecular weight. 3.6. Thermogravimetric Analysis (TGA) The thermal stability of unirradiated and electron irradiated MPDMAPP/PMMA samples are studied by TGA and the observed profiles for various doses are shown in Figure 11.

Figure 11: TGA thermograms of pristine and irradiated PMMA/MPDMAPP composites. Using this TGA thermogram, the values of the onset temperature of decomposition T0 were calculated and are given in Table 4. It is observed that T0 remains almost constant up to 150kGy and decreases for higher doses. Peikai Miao et al [26] have observed similar type decrease in decomposition temperature for their sample upon irradiation and they attributed the same to the chain scission process. Hence in our sample the decrease in decomposition 17

temperature indicates the decrease in thermal stability of the MPDMAPP/PMMA due to degradation mechanism. Hence it can be seen that electron beam irradiation lowers the thermal stability of MPDMAPP/PMMA. On the other hand the decomposition temperature decreases with increasing dose because of the decrease in molecular weight.

Table 4: Weight loss (%) at different decomposition temperatures for MPDMAPP/PMMA composites for various doses. Irradiation Dose

Weight Loss (%) T0 (°C)

(kGy)

100°C

200°C

300°C

400°C

0

1.32

3.07

8.49

86.51

346

50

1.05

1.96

6.76

89.57

346

100

0.82

2.63

8.07

89.75

345

150

0.58

2.89

7.51

88.08

344

200

0.84

2.64

8.00

90.21

340

300

1.22

1.71

8.75

93.58

337

3.7. Positron Annihilation Studies The measured lifetime spectra were resolved into three components by fixing the first lifetime τ1 = 0.123 ns during the analysis. The observed three lifetimes are attributed to p-Ps and free annihilations (τ1, I1) the intermediate component (τ2, I2) to the positrons trapped in the defects present in the crystalline regions or trapped in the crystalline-amorphous interface regions. The longest lifetime component (τ3, I3) is due to o-Ps pick-off annihilation in the free volume sites present mainly in the amorphous regions of the polymer matrix. As observed from FTIR studies, when MPDMAPP is doped with PMMA, it forms hydrophobic interactions and produces a polymer composite. In this composite, the intramolecular interaction occurs between the hydrocarbon portion of PMMA and the hydrocarbon portion of the chalcone. Upon electron irradiation the formation of polarons may result in the weak intramolecular interaction between N+ and ester group of PMMA. It is expected that this interaction reduces the positron and positive free radicals trapping capacity of ester group of the polymer composite. As a result the built up electric field generation

18

probability is very less in this composite. Hence here in the present case the variation of τ3 & I3 can be understood on the basis of free volume model. Based on the free volume model by Nakanishi et al [27], the o-Ps is localized in the free volume hole with spherical potential well with radius R0 and the o-Ps lifetime (τ3) is directly related to the free volume radius (R). Assuming that the annihilation rate of the o-Ps inside the electron layer of width ΔR at the internal surface of the free volume is 2 ns-1, the o-Ps lifetime is given by the relation [28],

 2R  1 R 1   3  1   sin 2  R0 2 R  0 

1

where τ3 (o-Ps lifetime) and R (cavity radius) are expressed in units of ns and Å, respectively. R0 = R + ΔR, where ΔR=1.657 Å is the fitted empirical electron layer thickness. The average size of the free volume holes, Vf (in Å3), for spherical cavities are calculated using the relation,

4R 3 Vf  3 Table 5 : Positron lifetime, intensity values and free volume size of MPDMAPP/PMMA films for various doses. Lifetimes Electron Dose

Intensities

Free Fractional Free free volume volume volume size radius VfI3 Vf R (×10-3) (×10-4) 3 (Å) Å Å3

τ2

τ3

I1

I2

I3

(ns)

(ns)

(%)

(%)

(%)

0

0.367

1.920

15.679

58.791

25.529

0.278

90.19

37.9

50

0.355

1.859

16.095

59.657

24.247

0.272

84.57

33.8

100

0.354

1.841

15.786

60.042

24.170

0.270

82.91

33.0

150

0.350

1.790

17.635

58.980

23.384

0.265

78.39

30.2

200

0.351

1.801

20.325

57.902

21.772

0.266

79.37

28.5

250

0.360

1.834

20.654

57.867

21.477

0.269

82.35

29.1

300

0.360

1.892

20.208

58.653

21.138

0.275

87.59

30.5

(kGy)

19

Figure 12. Positron lifetime distributions in MPDMAPP/PMMA as a function of radiation dose. The distribution of positron lifetime in MPDMAPP/PMMA as a function of radiation dose is shown in Figure 12. The values of o-Ps lifetime τ3 and free volume size Vf calculated using PATFIT-88 program for different irradiation doses are presented in Table 5. It is observed that the o-Ps lifetime τ3 as well as free volume size Vf decreases with irradiation dose initially up to 150kGy and increases for higher doses. On the other hand, the o-Ps intensity I3 and fractional free volume (VfI3) decreases with irradiation dose. This variation is similar to that of interchain distance as observed from XRD studies. The variation in τ3 suggests that crosslinking of polymer chain at lower doses decreases the free volume size and at higher doses the process of chain scission increases the free volume size, similar to the results observed in FTIR and XRD studies. The decrease in I3 indicates that the free volume density decreases with irradiation dose. The intermediate lifetime τ2 decreases up to 150kGy and then increases for higher doses while its intensity I2 increases initially and decreases slowly for higher doses. The initial decrease in τ3 is mainly due to the fact that the irradiation leads to the formation of new bonds or cross-linking in the initial stage. Here the free volume holes are formed due to the irregular packing of the polymer segments, chain ends and free radicals [29]. Therefore the hole concentration and the size of the free volume may be changed during irradiation. A few chains may have ruptured owing to

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the high radiation dose, hence free chain ends and free radical can be produced. These lead to an increase of the dimension of the free volume holes at higher doses. From X-ray diffractograms data it is observed that the lattice dimensions decreases which is consistent with the variation of τ3. Here in the cross-linked state, the polymer chains are expected to have a closer packing, hence, a reduction in free volume is observed at this stage. These cross linking network also decreases the number of free volume holes (reflected in decrease in I3), which seems to fit quite well with the interpretation based on the process of cross-linking. It is found that average lifetime values obtained with MELT agree well with PATFIT-88 values. The fluorescence intensity is related to free volume size, following Meyer et al [15] relation here also a plot of ln(F) versus 1/Vf should give a straight line, the slope of which gives Vm/b and the intercept gives ln(F0) according to the equation,

F  F0 exp Vm bV f 

where F is the fluorescence intensity and F0 is the fluorescence intensity of the unhindered probe MPDMAPP. Vm is a critical volume required for radiationless de-excitation of the probe, b is a system dependant constant and Vf is the free volume of the polymer. It can be seen that the fluorescence intensity varies inversely with the free volume size. These observed results are understood based on the fact that, in polymeric media, free-volume availability becomes the controlling factor in the torsional relaxation of the molecular rotors within the composite [30, 31]. Restricting the internal motion of the molecular rotors leads to a decrease in radiationless decay, and consequently an increase in fluorescence at lower doses due to cross-linking process. At higher doses, because of chain scission process the polymer matrix gets relaxed and consequently the fluorescence intensity reduces due to nonradiative decay by free rotation of the fluoroprobes. Therefore, the fluorescence emission of this class of compounds can be correlated to the changes in free volume of polymers upon irradiation. Hence irradiation technique can be effectively used to tune the fluorescence and other microstructural properties of the polymer matrix. The lifetime data analysis along with FTIR spectroscopic studies reveals that the value of I2 is a measure of defect concentration in the polaronic and bipolaronic environment. Here it is understood that at lower doses positrons are trapped by polarons whereas at higher doses the positron localization takes place in the bipolaronic environment. The nature of variation of the relative intensity (I2) pertaining to the lifetime component 2 in the polymer composite

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also lends support to the above view. The dose at which bipolarons start to generate and polaron recombination commencement will depend upon the average polymer chain length. 4. Conclusions The effect of 8 MeV electron beam irradiation on the fluorescence and other microstructural properties of MPDMAPP/PMMA composite films are studied. The FTIR spectral study on the irradiated films suggests that at lower doses the process of cross linking leads to polarons formation whereas the bipolaron is the expected state at higher doses owing to chain scission. Using UV-Visible absorption spectra the optical parameters like optical energy bandgap and activation energy were determined. Here the appearance of three optical energy band gaps may be attributed to various polaronic defect levels. From fluorescence studies, the tunable emission can be observed through the evolution of fluorescence spectra of the composite films as a function of irradiation dose. XRD study suggests that the crosslinking process at lower doses increases the distance between the pendant groups and reduces the interchain distance. At higher doses, the decrease in interchain separation can significantly enhance the number of polarons in polymer composites. This increase in polaron formation results in a reduction in fluorescence intensity. PALS study reveals that the restriction of internal motion of the fluorescence probes leads to a decrease in radiationless decay, and consequently an increase in fluorescence at lower doses. On the basis of this description, the gradual increase in fluorescence intensity of MPDMAPP/PMMA with increase in radiation dose can be attributed to reduction of radiationless decay by rigidization of the probe by local environment within the polymer. Hence the variation in fluorescence emission intensity is in agreement with the variation of free volume size as measured from PALS studies. This clearly infers that the fluorescence emission from a chromophore mainly depends upon the chemical structure of the chromophore and the free volume related microstructure of the polymer matrix in which it is embedded. Acknowledgments The authors are thankful to the Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy, Government of India for their financial assistance in the form of Research Project No. 2005/34/30/BRNS/2821.

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Highlights



Microstructural modifications in electron irradiated PMMA/MPDMAPP are reported.



Formation of polarons takes place due to cross linking process at lower doses.



Bipolaron formation occurs via chain scission process at higher doses.



Free volume related parameters are studied using PALS technique.



Fluorescence enhancement is directly related to free volume around polymer matrix.

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