Perkin-Elmer Luminescence spectrometer LS55 controlled by FL Winlab software. The fluorescence quantum yield (Φf) in solutions and polymer films.
AGGREGATION INDUCED LUMINESCENCE OF POLY(ISOBUTENE) SUCCINIC ANHYDRIDES AND IMIDES Andrea Pucci,a Riccardo Rausa,b and Francesco Ciardellia,c a
Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via Risorgimento 35, I-56126 Pisa, Italy b EniTecnologie SpA, Via F. Maritano 26, 20097 San Donato Milanese, Italy c PolyLab-CNR e Dipartimento di Chimica e Chimica Industriale, Università di Pisa, largo Pontecorvo 3, I-56127 Pisa, Italy
Introduction Polyisobutene (PIB) derivatives with molecular weight range of 500-5000 find various important technological applications in many fields. For example, polyisobutene succinic anhydrides (PIBSA) and succinimides (PIBSI) derivatives are widely employed as additives in lubricants or fuels formulations.1 PIBSA derivatives are prepared by the reaction between highly reactive PIB and maleic anhydride (MAH), according to the thermal induced Alder-Ene mechanism.2 The functionalization degree, i.e. the amount of succinic anhydride (SA) grafted to the polymer backbone, and the nature of the PIBSA derivative (Scheme 1) depend on the reaction parameters such as temperature (higher than 160 °C) and reaction time. PIBSI are generally obtained through a condensation mechanism involved PIBSA and the selected polyamine, as triethylenetetramine (TETA) used in this study. In particular, PIBSA mono and PIBSA bis were alternatively obtained by selecting the stechiometric amount of TETA. O CH3
CH3 CH3 C CH2 C CH2 CH3 CH3 n
O
O O O
CH3 CH3 H CH3 C CH2 C CH CH3 CH3 n
O
(a)
150 - 250°C
O
O
CH3 CH3 CH2 C CH2 CH3 C CH3 CH3 n
O H
O
2 - 10 h CH3
CH3
CH3 CH3 C CH2 C CH CH3 CH3 n
O O
CH3
(b)
studied as a function of the nature (succinic anhydrides or succinimides) and the number of interacting groups. In particular, the results were discussed in terms of the application of the fluorescence technique as a effective spectroscopic tool for the determination of the functionalization degree of PIB. Experimental Materials. Highly reactive polyisobutene (PIB) 1000 was supplied by BASF Italia S.p.A. and was employed without further purification. This polymer contains 91.6 % by mol of α-olefin units. Heptane, spectrophotometric grade (Aldrich, 99 %), was used without further purification. Instrumentation. UV-Vis absorption spectra of polymer solutions were recorded at room temperature with a Perkin-Elmer Lambda 650. Steady-state fluorescence spectra were acquired at room temperature with the help of a Perkin-Elmer Luminescence spectrometer LS55 controlled by FL Winlab software. The fluorescence quantum yield (Φf) in solutions and polymer films was determined relative to quinine sulphate (Φf = 0.54 in 0.1 M H2SO4) using the following relation:5 ∞
φ f = φ fs
∫0 ∞
∫0
I F (υ )dυ 1 − 10− A s n 2 −A 2 I S (υ )dυ 1 − 10 ns F
φ sf
where is the quantum yield of standard and the integrals are the area under fluorescence peaks. A and AS are the absorbances of the dye and standard, respectively, at the excitation wavelength (350 nm). n is the refractive index of the medium. The refractive index of heptane is 1.39. Synthesis of PIBSA and PIBSI derivatives. PIBSA derivatives were prepared according to previously reported procedure. PIBSI was prepared by reacting PIBSA and the opportune amount of TETA in a glass cylindrical reactor equipped with a mechanical stirrer a bottom drain valve and a reflux condenser, swept by nitrogen flux and heated at a temperature higher than 150 °C. After 2 hours of stirring the residual water was stripped off and the crude product was purified by filtration at high temperature. The reaction conversion and functionalization degrees were evaluated according to previously reported procedures.
PIBSA O O
PIBSA (a)
200 - 250°C 2 - 10 h
CH3 CH3 CH3 C CH2 C CH CH3 CH3 n
O O O
(c)
O O O
CH3
CH3 CH3 C CH2 C CH2 CH3 CH3 n
O O O
O
PIBSA
TETA 150 - 180°C
H N
N O
(d)
O
Results and Discussion PIBSA derivatives obtained respectively after 4, 7, 10 and 14 hours of reaction and purified from the unreacted MAH were analysed by UV-vis and fluorescence spectroscopy after solubilization in heptane (1.45·10-3 M). The polymer obtained after 4 hours of functionalization showed a very weak fluorescence signal indicating that molecularly dissolved functionalized derivatives are practically non-luminescent (Figure 1). On the contrary, by increasing the reaction time, from 7 to 14 hours, a well-defined emission band compares in the visible region and centred at about 415 nm.
NH2
N H
PIBSI mono O O
H N
N O
N H
N O
PIBSI bis
Scheme 1. Alder-Ene reaction between PIB and MAH and PIBSI preparation scheme. PIBSA and PIBSI appear as dark brown viscous fluids and surprisingly blue-green when excited by a long-range UV lamp at 366 nm. This property, recently reported by other authors for different molecular systems, is called aggregation-induced emission (AIE):3,4 this phenomenon is due to the luminescence generated from the aggregation of non-emissive molecule or chromophoric groups (as carbonyls for PIBSA or PIBSI derivatives) causing restricted vibro-rotational motions with reduced non-radiative relaxations and increased fluorescence quantum efficiency. Since the luminescence quantum yield is directly related to the aggregation extent and therefore to the concentration of interacting groups, in this work the AIE phenomenon was
Figure 1. Emission and absorption (inset) spectra of 1.45·10-3 M heptane solutions of PIBSA derivatives at different reaction time (hours) This phenomenon is attributed to the formation of aggregates between the chromophoric units of succinic anhydride (SA) moieties grafted to the PIB backbone. Increasing the SA content per PIB chain, the number of inter(macro-
Polymer Preprints 2007, 48(2), 229
)molecular interactions becomes so large to strongly reduce the non-radiative relaxations of the chromophores thus producing an intense signal of fluorescence. The formation of aggregates among PIBSA derivatives in heptane solution was also confirmed by UV-vis spectra which display the formation of aggregation bands in the region comprised between 320 and 400 nm (inset figure 1). While the fluorescence signal suggests the considerable aggregation extent, no precipitates were formed indicating the nanostructured supramolecular structure of PIBSA assemblies. The calculated photoluminescent quantum yield (Фf) is reported in figure 2 and compared with the functionalization degree (FD) of PIBSA derivatives at different reaction time.
Conclusions In this work, the aggregation behaviour of PIBSA and PIBSI derivatives were studied in heptane solution by means of UV-vis and emission spectroscopies. We demonstrated that the aggregation-induced emission (AEI) phenomenon was strongly dependent on the number of interacting carbonyl chromophores grafted on the polymer backbone and the associated quantum photoluminescent efficiency mostly fits the calculated functionalization degree. In addition, by increasing the associative strength between aggregates on passing from PIBSA to PIBSI derivatives through the introduction of polyamine moieties, the luminescent response greatly enhances its contribution. In conclusion, fluorescence spectroscopy may act as an effective tool for the characterization of the aggregation nature and extent between functionalized polyisobutenes and for the potential determination of their functionalization degree. References (1) Won, Y.-Y.; Meeker, S. P.; Trappe, V.; Weitz, D. A.; Diggs, N. Z.; Emert, J. I. Langmuir 2005, 21, 924-932. (2) Pucci, A.; Barsocchi, C.; Rausa, R.; D'Elia, L.; Ciardelli, F. Polymer 2005, 46, 1497-1505. (3) Li, Y.; Li, F.; Zhang, H.; Xie, Z.; Xie, W.; Xu, H.; Li, B.; Shen, F.; Ye, L.; Hanif, M.; Ma, D.; Ma, Y. Chemical Communications 2007, 231-233. (4) Tong, H.; Hong, Y.; Dong, Y.; Haeussler, M.; Lam, J. W. Y.; Li, Z.; Guo, Z.; Guo, Z.; Tang, B. Z. Chemical Communications 2006, 3705-3707. (5) Kaholek, M.; Hardlovic, P.; Bartos, J. Polymer 2000, 41, 991-1001.
Figure 2. Comparison between Фf and FD of PIBSA derivatives as a function of reaction time In particular, as the number of grafted SA per PIB unit increases, the Фf value rises as a function of time and, interestingly, with a growing behaviour that accurately fits the FD progression. Moreover, the capability to form aggregates between SA chromophores when grafted on PIB backbone was analysed even in PIBSI mono and in PIBSI bis derivatives obtained from the reaction between PIBSA (we selected one with a low FD, i.e. = 1.05) and TETA and characterized by the presence of N-H linkages potentially able to generate hydrogen bondings. Actually, as revealed by the emission spectra reported in figure 3, on passing from PIBSA to PIBSI, the presence of the polyamine moiety allows the formation of hydrogen bonding between the functionalized polymer chains promoting the formation of effective intermolecular aggregates with a high quantum photoluminescent response even at low functionalization degree.
Figure 3. Emission and absorption (inset) spectra of 1.45·10-3 M heptane solutions of PIBSA and PIBSI derivatives In particular, the increment in Фf obtained on passing from PIBSA to PIBSI mono heptane solution was about 84 %. Similar increment (80 %) was recorded for PIBSI bis derivative: probably, the increment in system rigidity achieved through the double TETA functionalization was reduced with the number of potential hydrogen bonds due to the disappearance of the free –NH2 group.
Polymer Preprints 2007, 48(2), 230
