Weili Li*a,
Wei Yaoa,
Jun Wanga,
Zhenyu Qiua,
Jijun Tanga,
Shengyuan Yangb,
Meifang Zhu*b,
Zexiao Xuc,
Rong Hud,
Anjun Qind and
Ben Zhong Tang*de
aSchool of Material Science and Engineering, National Demonstration Center for Experimental Materials Science and Engineering Education, Jiangsu University of Science and Technology, Zhenjiang 212003, China. E-mail: just_liweili@163.com
bState Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai, 201620, China. E-mail: zmf@dhu.edu.cn
cSuzhou Jiren Hi-Tech Material Co., Ltd, Suzhou 215143, China
dGuangdong Innovative Research Team, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China
eDepartment of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China. E-mail: tangbenz@ust.hk
First published on 23rd August 2017
For polymer materials, both their compositions and preparation process greatly influence their service performance. Thus, the sound understanding of the relationship between materials' preparation processes and their properties is paramount. However, current research methods are partially limited due to the absence of a direct testing method to track the entire process, e.g. synthesizing, curing, ageing, and so on. With the ability for real-time sensitive characterization, fluorescence spectroscopy may be applied in testing polymer materials' performance. Here, we synthesized a novel aggregation induced emission (AIE) resin named TPE–EPOXY resin and prepared an AIE coating based on it. According to restriction of intramolecular rotation (RIR) mechanism, the preparation, curing, and aging processes for the AIE polymer resins & coatings could be studied with real-time observation. In addition, their properties could also be studied systematically. The results in this paper pave a good way to understand the relationship between the internal structure and the properties of polymer materials. Moreover, the prepared AIE polymer resins has a potential to expand the application fields of the AIE mechanism.
Fluorescence spectroscopy (also known as fluorometry or spectrofluorometry) is a type of electromagnetic spectroscopy that analyzes fluorescence from a sample. It involves using a beam of light, usually ultraviolet light, that excites the electrons in molecules of certain compounds and causes them to emit light; typically, but not necessarily, visible light. With real-time feature and sensitivity, it may be applied in studying the relationship between the structure and the properties of the polymer materials. Fluorescent organic molecules have been vigorously investigated due to the interesting photophysical properties accompanied by their intrinsic softness and lightness. However, they usually present prominent fluorescence in a solution, not in a solid state due to aggregation-caused quenching (ACQ) phenomenon,11,12 which limits its application in solid polymer materials. Aggregation-induced emission (AIE), such an abnormal emission behavior has drawn great research interest in the recent years, for it is exactly the opposite to the common belief that the emission of chromophores decreases in the aggregated state.13–16 For various AIEgens, tetraphenylethene (TPE) is one of the typical fluorescent molecule found to be non-emissive in dilute solution but highly luminescent when its molecules are aggregated in concentrated solutions or cast in solid films.17 In detail, in a dilute solution, TPE undergoes dynamic intramolecular rotations against its double bond and renders its molecule non-luminescent. On the other hand, in the aggregate state, the molecules cannot pack through a π–π stacking process due to its propeller shape, while the intramolecular rotations of its aryl rotors are greatly restricted owing to the physical constraint. This restriction blocks the non-radiative pathway and opens up the radiative channel.18,19 Besides when in aggregate state, AIEgens can also emit fluorescence when they are restricted with the help of surrounding polymer chains by covalent bond. Our group previously reported a facile approach to synthesize stimuli-responsive fluorescent elastomer by linking tetraphenylethylene (TPE) derivant to flexible polydimethylsiloxane (PDMS) polymer chains with covalent-bond. Beyond this, we also used TPE derivative as the fluorescent marker to study emulsion polymerization process and the properties of fluorescent emulsions.20,21
In the recent years, TPE derivative was frequently used as fluorescent marker to study the properties of the polymer based materials and various other applications in polymer science.1,22–29 In this line, Guan et al. designed and synthesized a unique fluorescent surfactant (TPE–DTAB), which combined both the properties of the aggregation-induced emission (AIE) and amphilicity which together were used to image the macrodispersion of the layered double hydroxide fillers (montmorillonite) in the polymer matrix.30 Wang's group also visualized the entire gelation process of chitosan (CS) by the synthesized AIE fluorogenic probe, that is, tetraphenylethene-labelled chitosan (TPE–CS).31
Inspired by these research findings, we synthesized and studied a new kind of fluorescent resin by modifying epoxy with AIEgen named dibenzyl amine tetraphenylethene (2CH2NH2–TPE). The preparation, curing, and aging process of fluorescent resin were trailed according to fluorescence spectrum. In addition, the interaction between polymer chains and knitted AIEgen was investigated systematically by combining the results of fluorescence spectrum with other characterization methods. Due to the labelling effect of AIEgen, the relationship between polymer materials structures and their properties could be studied systematically.
At first, p-bromide tetraphenylethylene (2CH2Br–TPE) was synthesized according to the reference in our lab.20 Then p-dimethylamino tetraphenylethylene (2CH2NH2–TPE) was prepared as follows, which was presented as Scheme 1. In a typical experiment, 40 ml of THF and 80 ml of NH3·H2O were added into a 250 ml two-necked round bottom flask, then 2.07 g 2CH2Br–TPE (0.004 mol) was dissolved in THF and added into the system drop-wise over a period of 4 h, and the mixture was then continued under stirring overnight. After that, THF was removed by rotoevaporation, and the product was extracted out with dichloromethane. 2CH2Br–TPE was crystallized out when its dichloromethane solution was dropped in petroleum ether. The solid was isolated by filtration, and then redissolved in dichloromethane. The process of dissolving and crystallization was repeated for 3–4 times. Pure 2CH2NH2–TPE was obtained (53% yield). 1H-NMR (400 MHz): δ 8.61 (m, 4H, NH2), 7.16–7.49 (m, 18H, Ar), 4.31 (s, 4H, CH2) (Fig. 1).
Sample | Reaction materials | Degree of labeling |
---|---|---|
TPE–EPOXY-1 | E-44, 2CH2NH2–TPE | 0.1 |
TPE–EPOXY-2 | E-44, 2CH2NH2–TPE | 0.02 |
TPE–EPOXY-3 | E-44, 2CH2NH2–TPE | 0.01 |
TPE–EPOXY-4 | E-44, 2CH2NH2–TPE | 0.002 |
TPE–EPOXY-5 | E-44, 2CH2NH2–TPE | 0.001 |
Comparing with 1HNMR spectra of TPE–EPOXY-1 resin and original epoxy resin (CDCl3 is used as the solution), the present peaks at around 7.000 (m, H-benzene) are attributed to the benzene ring of TPE, as observed in Fig. 1S.† The data indicate that the reaction successfully proceeded without damaging the structures, and AIEgen was linked onto the polymer chains of the epoxy resin.
2CH2NH2–TPE can be also be a cross-linking agent, the content of which may influence the property of the prepared TPE–EPOXY resin. The molecular weight of TPE–EPOXY resins varied with the content of 2CH2NH2–TPE which was characterized and presented in Table 2. With the relative content of 2CH2NH2–TPE increasing, the value of Mn and Mw of the prepared TPE–EPOXY increased gradually, which interfered with the property of the cured fluorescent coating. Fortunately, the results show that no gel was formed due to the controlled content of 2CH2NH2–TPE.
Sample | Mn | Mw | ρ |
---|---|---|---|
TPE–EPOXY-1 | 3168 | 5733 | 1.810 |
TPE–EPOXY-2 | 1247 | 1303 | 1.045 |
TPE–EPOXY-3 | 851 | 854 | 1.0043 |
TPE–EPOXY-4 | 676 | 678 | 1.0003 |
TPE–EPOXY-5 | 468 | 484 | 1.034 |
To cure the fluorescent resin, the curing agent diethylenetriamine (DETA), was added into the TPE–EPOXY resin, and the molar ratio between resin and curing agent was set at 5:2.4. After being laid aside for about 1 h, the pre-treated glass plates were dipped into the blending solution at a speed of 200 mm min−1 by using a specially designed apparatus. According to FTIR spectra (Fig. 2S†), the absorption peak for epoxy group, which is located at 915 cm−1, weakens obviously due to the cured AIE coating because it was consumed during the curing process. Fig. 6 presents the surface of the cured AIE coating based on TPE–EPOXY-1 resin observed using SEM measurement. The AIEgen was uniformly dispersed in the EPOXY resin matrix without the formation of large aggregates. A slight wrinkle could have been caused by the shrinkage of polymer chains during the curing process.
When the fluorescent resin was cured by DETA, the obtained solid fluorescent coating was tested using DSC measurement. TPE–EPOXY-1 resin was taken as the specimen. As observed from Fig. 4, the material presented an amorphous state with Tg value at around 55 °C. In addition, no melting peaks attributed to AIEgens means that it was dispersed in the epoxy resin effectively by chemical bond. This result is consistent with the XRD measurement.
Thermotolerance property is one of the important indexes for evaluating the performance of polymeric materials. Fig. 5 presents thermogravimetric results of pure epoxy resin, the fluorescent resin, and the cured fluorescent coating, respectively. This test was used to study the effect of knitted TPE on thermotolerance of resin and the cured coating. From the figure, TPE–EPOXY-1 resin starts to lose weight at 292 °C, which is higher than that of pure epoxy resin (271 °C). After being cured by DETA, the fluorescent coating starts to decompose at 316 °C although the unreacted small organic molecules constantly evaporate during being heated. The knitted AIEgen does not negatively effect the heat resistance of epoxy resin. On the contrary, slightly crosslinked structure is capable of decomposing TPE–EPOXY at a higher temperature.
Fig. 5 Thermogravimetric result of epoxy resin (E-44), TPE–EPOXY-1 resin, and the cured fluorescent coating based on TPE–EPOXY-1. |
At first, the impact of interaction between AIEgen and polymer chains on the photophysical property of the fluorescent resin was studied. The results are presented in Fig. 6. Comparing with the two samples, when the epoxy resin was blended with the AIEgen prior to dissolving in the THF, the blending showed a very weak fluorescence. However, when 2CH2NH2–TPE and epoxy were knitted together by a covalent bond, such as the sample TPE–EPOXY-1 resin, it showed a very strong fluorescence even when dissolved in the solution. We propose that the restriction of the intramolecular rotation (RIR) was the main cause for the AIE phenomenon.33,34 The linked polymer chains tended to restrict the intramolecular rotation of TPE molecular, and thus the absorbed UV light was released by fluorescent luminescence behavior for TPE–EPOXY samples.
Fig. 6 PL-spectra of the epoxy resin blended with 2CH2NH2–TPE (100:1) in THF solution TPE–EPOXY-1 resin (0.01 g ml−1). |
Various TPE–EPOXY resins were dissolved in THF at 0.01 g ml−1 in order to study the effect of DL on the fluorescence property of the synthesized TPE–EPOXY resin. Photoluminescence (PL) spectra of TPE–EPOXY resins presented a strong fluorescent property and concentration which was dependent on the intensity increase. We proposed that the restriction of intramolecular rotation (RIR) was the primary cause for the AIE phenomenon.35,36 When TPE deviant and polymer chains were knitted together by covalent bonds, the RIR process partially activated and thus made the resin somewhat emissive. According to Fig. 7, the intensity of PL spectra increased linearly with the concentration of TPE deviant. With the connection effect of covalent bonds and similar chemical environment, the intensity of PL spectra did not present an abrupt change. These data demonstrated that TPE deviant homogeneously dispersed in the elastomeric samples without forming macroscopic aggregates or domains.
Fig. 7 PL spectra of TPE–EPOXY resins varied with DL of TPE molecular in THF solution (0.01 g ml−1). |
In the next, TPE–EPOXY-1 resin was selected as example to study the resin's dissolution behavior. At first, it was dissolved into THF-water mixed solvents to study its dissolution behavior in good and poor solvents (Fig. 8). As discussed above, when AIEgen is knitted onto polymer chains, the synthesized TPE–EPOXY resin could still emit fluorescence (the emission peak emerged at round 480 nm) under UV light even when they are dissolved in good solution (THF) due to RIR effect. In the experiment, the emission became stronger progressively with increasing the content of water in the mixed solvents. When a large amount of water (60%) was added into the THF solution, an emission peak emerged at around 480 nm, demonstrating a typical AEE (aggregation enhanced emission) phenomenon. During the process, the solubility of TPE–EPOXY-1 resin got worse and the polymer chains changed from stretched conformation to curled conformations, which as the result enhance RIR effect of the knitted AIEgens.
Fig. 8 PL spectra of TPE–EPOXY-1 resin in THF/H2O mixed solution (0.01 g ml−1) varied with the content of H2O. |
Inspired by this result, TPE–EPOXY-1 resin was then dissolved in different solutions such as butanol, ethyl acetate (EA), THF, and toluene, to study their dissolution behavior according to PL spectra. Traditionally, this property is tracked by comparing the solubility parameter between the dissolved polymers and the solutions, which is presented in Table 3.
Sample | Solutions | Polymer | |||
---|---|---|---|---|---|
Toluene | Butanol | EA | THF | Epoxy resin | |
Solubility parameter | 8.9 | 23.32 | 9.1 | 9.2 | 8–13 |
For TPE–EPOXY-1 solutions with different solvents (Fig. 9), the emission peaks of PL spectra appeared in the 470 nm–480 nm range. However, their intensities varied with the dissolving solutions. For good solutions, such as toluene, ethyl acetate (EA), and THF, their solubility parameter are close to that of epoxy resin. Full stretching of polymer chains in these solutions are believed to have faded the bonding effect of the AIEgens. This lead to the decrease in the fluorescence intensity. However, when epoxy resin was dissolved in butanol, large differences of solubility parameter between them made the polymer chains of fluorescent resin to shrink and agglomerate in the solution. This restricted the intramolecular rotation of the knitted AIEgens. So the fluorescence intensity increased significantly (about four to five fold).
EA–TPE-1 resin was then dissolved in THF under varying concentrations in order to study the fluorescent resin dissolution behavior in its good solution. According to Fig. 10, the polymer solution did not present an obvious fluorescence when EA–TPE-1 resin concentration was extremely low (<1 × 10−4 g ml−1). At this state, the polymer chain segments stretched freely in the solvent and there existed little tangles between them. Therefore, the intramolecular rotation of the knitted AIEgens was not under restriction, and the absorbed energy from UV light was possibly consumed mostly by random molecular motion of AIEgens. The degree of interaction between polymer chains in solution increased linearly with the increase in the polymer concentration, which resulted in the restriction of intramolecular rotation of AIEgens.
Fig. 10 (A) PL spectra of EA–TPE-1 resin in THF varied with its concentration, (B) PL intensity of EA–TPE-1 in THF at 480 nm varied with its concentration. |
According to the taken pictures and the results of PL spectra, the interaction between polymer chains in solution varied with the type of solvents. Still, their concentrations could be trailed lively and accurately, which totally agreed with the perception of the past findings.
When the prepared fluorescent resin was cured with the help of the hardening agent, it formed a reticular conformation. Polymer chains were fixed to the hardening agent via covalent bonding. As AIEgens were knitted onto the polymer chains, the crosslinked polymer system would further restrict the intramolecular rotation of AIEgens, which led to the enhancement of PL intensity. According to Fig. 12, the peak positions locked were at around 475 nm during the curing process. There was a large increase in the PL intensity before 50 min. This is attributed to the simultaneously volatilizing solution at the beginning of the curing process. These two effects both limited the segmental motion, which made PL intensity to increase rapidly. However, when the solution was completely evaporated, there was only one way to further restrict the segmental motion. When the curing process completed, PL intensity showed less changes with the curing time.
Due to the molecular structure characteristics of epoxy resin, its anti-UV property is relatively poor. Previously, its weatherability property was checked using visual inspection. However, the method is known to be relative rough with relatively non precise judgment. In this paper, we tried to tell the whole aging process of epoxy resin according to PL spectra. In a typical testing procedure, the cured TPE–EPOXY-1 coating was placed under UV radiation (313 nm, 1.25 kW) constantly. Then the tested coating was taken out and checked by PL spectra at every 0.5 h. In contrast to the curing process, polymer chains of the cured fluorescent resin fractured constantly during the aging process, which is believed to have weakened the binding effect of AIEgens and lowered the PL intensity of the tested coatings. According to Fig. 13, the peak positions were locked at around 475 nm. However, the peak intensity changed with the aging time. From 0 min to 2000 min, it fluctuated to a certain range. With the extended aging time, there was a sudden weakening process for the peak intensity. This meant that the AIEgens started carrying the intramolecular rotation because of the loosening of the linked polymer chain segments. PL intensity weakened constantly as time went on.
p-Methyl benzophenone, N-bromobutanimide, zinc powder and titanium(IV) chloride (TiCl4) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). Ammonia and tetrahydrofuran (THF) were purchased from Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China). THF was distilled under normal pressure from sodium benzophenone ketyl under nitrogen, in addition, THF used in fluorescence spectra measurements was spectral purity grade.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra06527b |
This journal is © The Royal Society of Chemistry 2017 |