A Simple and Sensitive Method for an Important Physical Parameter

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A Simple and Sensitive Method for an Important Physical Parameter: Reliable Measurement of Glass Transition Temperature by AIEgens Zijie Qiu,†,‡ Eric K. K. Chu,†,‡ Meijuan Jiang,†,‡ Chen Gui,†,‡ Ni Xie,†,‡ Wei Qin,†,‡ Parvej Alam,†,‡ Ryan T. K. Kwok,†,‡ Jacky W. Y. Lam,†,‡ and Ben Zhong Tang*,†,‡,§ †

Guangdong Provincial Key Laboratory of Brain Science, Disease and Drug Development, HKUST-Shenzhen Research Institute, No. 9 Yuexing 1st RD, South Area, Hi-tech Park, Nanshan, Shenzhen 518057, China ‡ Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Division of Life Science, State Key Laboratory of Molecular Neuroscience, Institute for Advanced Study, Institute of Molecular Functional Materials, Division of Biomedical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China § Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China S Supporting Information *

ABSTRACT: The glass transition temperature (Tg) is an important physical parameter and signifies the reversible transition process between the glassy state and the rubbery state in polymeric materials. Herein, a simple yet reliable method, denoted as ADEtect, utilizing aggregation-induced emission luminogens (AIEgens) for Tg measurement was developed. The fluorescent images of AIEgen-doped polymer films taken at different temperatures were monitored by a camera under the same settings. By using a MATLAB program, the grayscale of the selected area in each image was calculated. This value decreased steadily with increasing the temperature, and a significant change was observed at Tg. By plotting a graph of the second derivative of relative grayscale against temperature, the Tg of a polymer can be unambiguously determined from the lowest point. The present method shows performance comparable to that of conventional DSC measurement in terms of sensitivity and reliability. Besides, it can measure multiple samples in parallel, suggesting that ADEtect is a versatile technique with high commercial value and promising for high-throughput Tg measurement.

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INTRODUCTION Materials science is a robust and interdisciplinary research field, which contributes numerous new materials for human being in the past decades. Besides different chemical compositions of materials, any subtle phase transition in the solid state can greatly affect their properties. On the other hand, the understanding and measurement of some phase transitions still remain intellectually challenging.1−4 In polymeric materials, the glass transition temperature (Tg) is the temperature for reversible transition from hard, stiff, and bristle glassy state to soft, elastic, and flexible rubbery state. However, unlike melting or vaporization, glass transition is not a first-order phase transition but is rather a laboratory phenomenon extending over a temperature range. Besides, Tg is one of the most crucial parameters in polymer industrial since it directly affects the properties and performance of polymers. For example, accurate measurement of Tg in quality control section is necessary to guarantee the elasticity and hardness of polymeric materials such as aircraft tire, biomaterials, and artificial heart.5−7 Reliable measurement of Tg is therefore of great value for fundamental understanding and practical application in polymer science. Based on the change of different physical © XXXX American Chemical Society

properties during glass transition, several techniques have been established to determine the Tg of polymers, including dilatometry, dynamic mechanical analysis (DMA), and differential scanning calorimetry (DSC). Dilatometry is a simple and facile method for Tg detection by measuring the change of sample length or volume as a function of temperature.8 DMA is applied mainly in engineering materials and determines the glass transition by investigating the mechanical properties of polymeric materials. However, massive samples in gram scale are required for each measurement.9 DSC is the most commonly used technique for studying phase transition.10−12 Compared with the aforementioned methods, DSC monitors the change of heat flow involved during the process. In the early stage of its development, the DSC results are sometimes ambiguous because of baseline fluctuation and inaccurate heat flow measurement. After its optimization in the past several decades, these problems still exist but become less severe. During glass transition, a step-shape signal was observed in the Received: May 22, 2017 Revised: September 17, 2017

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the T g value. Additionally, a photoluminescence (PL) spectrometer is required for the measurement, which is expensive, bulky, and time-consuming. To improve the previously developed technology, in this work, we developed a simple yet sensitive and reliable approach based on the grayscale determination of fluorescent images of AIEgen-doped polymer films. The fluorescence signals can either be detected as collection of emission photons by a PL machine or be resolved as a function of spatial coordinates in two or three dimensions from fluorescence images. In the field of image processing, digital information on a fluorescent image, such as RGB values and grayscale, can be obtained from computer program for every pixel. Among these digital data, grayscale is commonly used to represent the brightness or intensity of fluorescence signal.29−32 This technique is mainly used in the field of bioimaging but seldom used by material scientists to quantify the change of a physical process such as glass transition of polymeric materials. Thus, as a further development of our previous work, we achieved the first example of using grayscale as fluorescent intensity to detect the Tg of polymer films by a simple homemade device.

DSC thermogram, and Tg is determined by the half-height of two baselines. However, in some block copolymers, such as poly(styrene−butadiene−styrene) (SBS), when one of the phases is present in less than 15 wt %, the heat flow involved in the glass transition of the minor component will be small, making accurate determination of its Tg difficult.13 Although these methods have been well developed, scientists and engineers should never be constrained by the existing achievements. The development of a new approach with promising performance is always appealing. The fluorescencebased method is widely used for different analysis and process monitoring due to its fast response, high sensitivity, and easy detection.14−17 The utilization of fluorescent method for glass transition temperature detection can be dated back to Frank’s work in 1975.18 In more recent years, Torkelson and coworkers utilized conventional planar pyrene molecules as probes to detect Tg of ultrathin polymer film, which was difficult to be accomplished by the conventional approach.19−23 Unfortunately, the pyrene molecule suffers from sublimation at high temperature. Thus, the exploration of fluorescent molecules with high solid-state emission, high thermal stability, and high responsiveness is needed. Luminogens with aggregation-induced emission (AIE) characteristics is a class of novel chromophores which are weakly emissive in solution but emit intensely in the solid state.24−26 The restriction of intramolecular motion (RIM) is proposed as the working mechanism of the AIE phenomenon. This mechanism suggests that any external perturbation that restricts the motion of AIEgens will cause an emission change. Aggregation is one of the ways to activate the RIM process, but it is not a prerequisite for an AIEgen to show strong light emission. Thus, it is envisioned that when an AIEgen exists as isolated molecular species or aggregates in a solid polymer film, it will show strong light emission due to the RIM mechanism. However, the segmental movement of the polymer chain during the glass transition will lower the rigidity of the local microenvironment, making the AIEgen molecules to emit less intensely. Such possibility may pose a new opportunity for rapid and reliable Tg detection (Scheme 1).

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RESULTS AND DISCUSSION The equipment setup is shown in Scheme 2. As it is a detection method based on AIEgens, “ADEtect” is used as abbreviation Scheme 2. Equipment Setup of ADEtect Measurement

for the below discussion, which combines the meanings of both “AIE” and “detect”. The TPA-BMO-doped PS-1 thin film was selected as an example to illustrate the workflow of ADEtect. The polymer film with 1 wt % TPA-BMO was prepared according to the standard procedure described in the Supporting Information. The fluorescent images were captured by a Canon camera from 30 to 130 °C. Representative photos taken at 50−130 °C are shown in Figure 1A, which suggested that the PL intensity or brightness decreased with rising temperature. The red, green, and blue (RGB) values were obtained by the Matlab program, from which the brightness of an image was quantified as grayscale (G) (Figure 1B) according to eq 1:33

Scheme 1. Working Principle of AIEgen in Tg Detection

Grayscale (G) = 0.2989∗ Red + 0.5870∗ Green + 0.1140∗ Blue (1)

In 2015, we succeeded in utilizing AIEgens such as TPE and its derivatives as probes to measure Tg of different polymers.27 This work has proved the concept and the feasibility for such application. Herein, a recently designed AIEgen, TPA-BMO with multiple rotors,28 was selected to dope with, for example, polystyrene (PS-1, Mw = 2600), and the temperaturedependent fluorescence of the resulting dye-doped polymer film was measured (Figure S1A). The relative fluorescence intensity was plotted against temperature (Figure S1B). From the intersection of the two slops, the Tg of the polymer can be determined. However, the selection of the temperature range for the glassy state and the rubbery state is subjected and varies with different researchers. This unavoidably causes deviation in

The grayscale values at different temperatures were then obtained, and the relative value (G/G0) against temperature was plotted (Figure 1C). G0 is the grayscale at time of 0 s. Similar to the relative fluorescence intensity (I/I0), the G/G0 value also declines steadily with temperature. Again, the Tg is determined at a point where a slope change occurs. The slope change from G/G0 is less ambiguous than that from the I/I0 value. This is probably because the full spectrum is recorded in the fluorescent image and calculated as G/G0, while only the peak intensity at certain wavelength is analyzed in I/I0, unavoidably losing other spectral information. B

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Figure 1. (A) Photos of TPA-BMO-doped PS-1 films taken at different temperatures. (B) Illustration of the data processing procedure. (C) Change of relative grayscale (G/G0) of TPA-BMO-doped PS-1 film with temperature and the associated fitting curve as well as the second derivative of the fitting curve to reveal the change in G/G0 at different temperatures. Heating rate: 6 °C/min. (D) Reproducibility test for Tg detection in PS-1 and PS-2 by ADEtect. n

The detailed working mechanism is described below. Generally, upon heating, the TPA-BMO molecules will gain more energy to rotate and vibrate, leading to emission annihilation due to the higher probability of the excited state to relax through nonradiative channels.34 Before glass transition, the polymer chain is immobile and the polymer matrix is rigid enough to restrict the intramolecular motion of the dye molecules. Therefore, the emission of TPA-BMO decreases slower upon heating. When Tg is reached, the polymer matrix becomes soft and rubbery. This provides more freedom for the dye molecules to undergo intramolecular motion, thus causing a rapid emission decrease at temperature after Tg. Based on the AIE mechanism, the Tg of the polymer can be determined from the turning point of the grayscale value. To obtain Tg of PS-1 in an objective and automatic fashion, a computer program was designed and coded using R Studio. In mathematics, the spline simulation is a numeric function and is piecewise-defined by polynomial functions. It is widely adopted to generate optimized trade-off between the fidelity of the data and the roughness of the function estimate.35 Thus, spline smoothing is utilized for analyzing G/G0. After collecting the individual grayscale value of TPA-BMO-doped PS-1 film at different temperatures, let (Ti, Gi/G0); T1 < T2 < ... < Tn, i ∈ N be a sequence of the observed data points modeled by the relation Gi/G0 = f(Ti). The smoothing spline estimate f ̂ of the function f is defined to be the minimizer (over the class of twice differentiable functions) of μ(xi):

μ(xi) =

∑ (Yi − f ̂ (xi))2 + λ ∫ i=1

xn

x1

f ̂ ″(x)2 dx (2)

Afterward, the second derivative of the fitting curve Gi/G0 = f(Ti) is conducted to reveal the change of G/G0 at different temperatures (Figure 1C). The mathematical meaning of the lowest point in second derivative is the temperature where a sudden change in fluorescent intensity occurs, or the turning point of G/G0. The sudden change of the fluorescent intensity (G/G0) represents the occurrence of glass transition. Therefore, the Tg of PS-1 is identified at 71.8 °C. In the other point of view, the introduction of second derivative is a mathematical tool to assist the identification of Tg. In Figure 1C, the turning point is easy to be identified at ∼70 °C by our naked eyes, but the second derivative treatment helps accurate assignment of the Tg value as 71.8 °C. To verify the reproducibility of the result by ADEtect, TPABMO was used as probe to measure the Tg of polystyrenes with different molecular weight (PS-1, Mw = 2600; PS-2 Mw = 17 200) for ten trials (Figure 1D). Since all the measurement and data processing are controlled by computer program, the stable Tg values of 72.5 ± 2.0 and 100.3 ± 1.5 °C obtained for PS-1 and PS-2, respectively, suggest the high reproducibility of the measurements by ADEtect. The high reliability of ADEtect was also confirmed by the similar results obtained by DSC analysis (Figure S2A,B).36,37 It is noteworthy that besides the minimum point, which corresponds to the Tg of a polymer in the second derivative of the fitting curve (Figure 1C), several valleys are also observed at different temperatures. These peaks C

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Figure 2. Change of relative grayscale (G/G0) of (A) TPA-BMO, (B) DPA-IQ, (C) pyrene, and (D) perylene-doped PS-2 films with temperature and the associated fitting curves. Heating rate: 6 °C/min. Inset: chemical structures of dye molecules studied in this work and fluorescent photos of dye-doped PS-2 films taken at room temperature under 365 nm UV irradiation.

AIEgens cover the spectrum from blue to red, proving that G/ G0 is a universal parameter to process images with different colors since the information on the RGB values are all integrated into grayscale (eq 1). The performance of pyrene and perylene, two widely used conventional chromophores with planar structures, was also investigated using the same method. Pyrene was chosen since its fluorescence is sensitive to the local density of the surrounding nanoscale environment.19−23 As shown in Figure 2C and Figure S10, pyrene works also well as probe for Tg detection of PS-2. However, its doped polymer film shows weak emission as observed by the naked eye and detected by a CCD camera because only half of the emission spectrum falls in the visible range. Such weak emission leads to fluctuation in the G/G0 value (Figure 2C). Additionally, pyrene also shows low thermal stability and sublimes at high temperature. Therefore, pyrene is not a suitable dye for ADEtect. For perylene, the grayscale of its doped PS-2 film fails to show distinct change at temperature close to Tg (Figure 2D and Figure S11) because the perylene emission is not sensitive to the rigidity or polarity change of the surrounding environment. Interestingly, among all the AIEgens selected, DPA-IQ and Ir complex with positive charge provide more distinct G/G0 change at Tg. However, the Ir complex demonstrates poorer

may be derived from (1) the subtle polymer chain movements as observed in some thermoresponsive AIE polymers38 or (2) the amplification of the signal noise by mathematical treatment. To elucidate their origin, careful comparison between the results from the ten trials was made. Only the peak related to Tg of PS-1 is repeated for each time, while those small peaks at other temperatures are not repeatable at different trials, suggesting that they are pseudo signals resulted from the amplification of the signal noise by mathematic treatment (Figure S3). As discussed previously, AIEgens are a new class of luminogens whose emission will be greatly enhanced when their intramolecular motion is restricted. From the working mechanism, AIEgens are the most promising candidates for such application because they possess the advantages of high solid-state emission, high photostability, and sensitive emission change to the microenvironment variation. To demonstrate the novelty of AIEgens for Tg detection of polymers, molecules with different molecular structures such as DPA-IQ,39 BTPEPI,40 TTPAE,41 and Ir complex (structures shown in the inset of Figure 2 and Figure S4) were tested. These molecules were provided by AIEgen Biotech Co., Limited. As shown by the results in Figure 2A,B and Figures S4−S9, all the AIEgens perform as good as TPA-BMO. The emission colors of these D

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Figure 3. (A) Change of relative grayscale (G/G0) of DPA-IQ-doped SBS films with temperature and the associated fitting curves. (B) The second derivative of the fitting curve revealed the change of G/G0 at different temperatures. Heating rate: 6 °C/min. (C) DSC thermograms of SBS recorded during the second heating cycle under nitrogen at a heating rate of 30 °C/min.

Figure 4. (A−C) Change of relative grayscale (G/G0) of DPA-IQ-doped PMMA film with temperature at a heating rate of (A) 3, (B) 6, and (C) 12 °C/min and the associated fitting curve. (D−F) DSC thermograms of PMMA recorded during the second heating cycle under nitrogen at a heating rate of (D) 3, (E) 6, and (F) 12 °C/min.

PMMA and PVC films fades with increasing temperature (Figures S16 and S17), and a two-stage change in the relative G/G0 value was observed (Figure S18). After data processing, the Tg of PMMA and PVC was determined to be 118.2 and 88.2 °C, respectively, whose values were consistent with those of the DSC results (Figure S2C,D). SBS is a typical block copolymer with both rigid PS block and rubbery polybutadiene (PBD) segment. Thus, it possesses excellent mechanical strength but also shows high elasticity. This hard rubber is widely used in shoes, tire, and other applications where durability is required. It is worth noting that the Tg of the PS block in SBS is not easy to be determined by DSC13 because at room temperature the PS block forms glassy “islands” surrounded by the rubbery PBD “ocean”. The heat flow involved during the glass transition of the PS block will be likely to be absorbed by the surrounding PBD block, leading to a small heat flow change and an insignificant DSC signal. Since the PL spectrum of DPA-IQ overlaps little with the absorption

miscibility with polymeric materials than other small molecular weight organic dyes, probably due to the large-sized metallic Ir center. On the other hand, DPA-IQ possesses other advantages such as high quantum yield in the solid state (57%), excellent thermal stability (Figure S12), and photostability (Figure S13). Thus, AIEgens, especially DPA-IQ, prove to be an ideal candidate for ADEtect, and DPA-IQ is chosen as the standard AIEgen probe for ADEtect. The cooling cycle of DPA-IQdoped PS-1 was also conducted by ADEtect and result (Tg = 76.0 °C) similar to that of heating cycle was also obtained (Figures S14 and S15). The phase transition of other commercially available and commonly used polymers, such as poly(methyl methacrylate) (PMMA; Mw = 120 000), poly(vinyl chloride) (PVC; Mw = 160 000), and poly(styrene−butadiene−styrene) (SBS; 30 wt % styrene; Mw = 82 400), was also tested by ADEtect using DPA-IQ as fluorescent probe according to the standard procedure. As expected, the emission of the dye-doped E

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Figure 5. (A) Fluorescent image of two DPA-IQ-doped PMMA films at 60 °C. (B) Change of relative grayscale (G/G0) of left-sided film with temperature at a heating rate of 12 °C/min and the associated fitting curve. (C) Change of relative grayscale (G/G0) of right-sided film with temperature at heating rate of 12 °C/min and the associated fitting curve.

ically, faster heating rate will inevitably result in a larger variation from the ideal equilibrium phase transition process where the “static” Tg should be measured. However, the heat flow (mW) measured by DSC is a function of the measuring time. Thus, for example, when the heating rate is slow, the heat flow involved is also small. This will lead to less obvious baseline shift and hinder the accurate Tg measurement (Figure 4D−F). On the contrary, in ADEtect, the fluorescence intensity and grayscale are not dependent on the measuring time. Consequently, the detection sensitivities are all high at different heating rates in ADEtect, as supported by the similar spectral pattern in Figure 4A−C. Even at a slow heating rate of 3 °C/ min, a clear assignment of Tg can be performed. Additionally, the similar spectral shapes of the grayscale decay curves further verify the excellent reproducibility of ADEtect. The results obtained by ADEtect and DSC are summarized in Table S1. In ADEtect, the fluorescence signals of the dye-doped polymer films are recorded as two-dimensional fluorescence images with spatial coordinates in pixels. By using different pixel coordinates, different areas within one fluorescent image can be analyzed. As a result, ADEtect is promising for simultaneous Tg measurement of multiple polymer films. To prove the concept, two DPA-IQ-doped PMMA films were heated up simultaneously from 60 to 140 °C at a heating rate of 12 °C/min, and their fluorescent images were taken (Figure S22). Afterward, the G/G0 values of the left-sided and rightsided doped polymer films were analyzed, from which Tg of 121.2 and 120.2 °C were determined, respectively (Figure 5B,C). This result demonstrates that the Tg of multiple samples can be measured in parallel, which will greatly improve the measurement efficiency. On the contrary, conventional methods can only measure samples in a sequential fashion. Thus, a much longer measurement time is needed. Upon further engineering development and optimization, ADEtect is promising for high-throughput Tg measurement.

spectrum of SBS (Figure S19), the surrounding PBD block will exert little effect on the fluorescence of AIEgen during the glass transition of PS. Because of the higher structure similarity of DPA-IQ with polystyrene, DPA-IQ is expected to bind preferentially to the PS microdomains rather than PBD matrix. Indeed, ADEtect provides a clear and unambiguous change of grayscale when the dye-doped SBS film was heated up, suggesting a Tg of 93.6 °C for the PS segment (Figure 3A,B and Figure S20). For comparison, the same SBS sample was analyzed by DSC, but only a thermogram without obvious steplike Tg signal of the styrene block was obtained (Figure 3C). Thus, ADEtect can serve as an alternative approach for DSC if the heat flow involved during the glass transition is small. The dispersity of the organic dyes in polymer matrix was also studied by a transmission electron microscope (TEM) (Figure S21A,B), a scanning electron microscope (SEM) (Figure S21C), and a fluorescent microscope (Figure S21D) using DPA-IQ-doped SBS film as an example. The DPA-IQ-doped SBS sample was first stained with OsO4 before the TEM analysis to enhance the contrast of PS and PBD phases. As suggested by results, no aggregates of DPA-IQ were observed in the SBS matrix. This proves that DPA-IQ is monodispersed in the SBS matrix. Similar situations are expected to all the dyedoped polymer films in this study due to the low doping ratio (1.0 wt %). Segmental movement of the polymer chain is a widely accepted physical picture for glass transition, but it is sophisticated and will be affected by several parameters, such as thermal history of the sample and the measurement conditions of the instrument. Therefore, Tg is a heating or cooling rate dependent parameter because of the inherent response timelag between the relaxation of the polymer segment and the signal detection by the instrument. Would the Tg of a polymer detected by ADEtect change with the heating rate? Following are the results for Tg measurements of DPA-IQ-doped PMMA obtained at different heating rates using ADEtect. As shown in Figure 4A−C, the Tg of DPA-IQ-doped PMMA film is determined to be 118.2, 118.2, and 120.2 °C at heating rate of 3, 6, and 12 °C/min, respectively. These values are well matched with the DSC results under the same conditions (Figure 4D−F), further substantiating the high reliability of ADEtect. More information can be obtained by careful comparison between the shapes of curves obtained by ADEtect and DSC measurement. Glass transition is a kinetic process. Theoret-

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CONCLUSION In conclusion, a novel detection technique and device prototype utilizing AIEgens as probes for glass transition of polymers has been successfully developed and denoted as ADEtect. ADEtect has shown excellent performance in terms of easy operation, sensitivity, and reliability. Its size and cost are smaller and lower than other conventional techniques. What is more encouraging is that it can measure multiple samples in parallel, showing great potential for high-throughput Tg measurement. Combined with its low cost and small sample F

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Macromolecules loading, ADEtect has demonstrated its high value for both academia and industry. The high solid-state quantum yield, excellent thermal stability, photostability, and, most importantly, the RIM mechanism of the AIE phenomenon make DPA-IQ a perfect candidate for Tg detection of different polymers. More inspiringly, ADEtect is the first interdisciplinary example of using image processing technique for material science analysis, providing an intriguing method for detecting an important physical parameter in polymer science. Other subtle physical processes or transitions, such as phase transitions in liquid crystals or hydrogen bonding systems, may also be detected by ADEtect. Further studies are under investigation.

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Ryan T. K. Kwok: 0000-0002-6866-3877 Ben Zhong Tang: 0000-0002-0293-964X Author Contributions

Z.Q. and E.K.K.C. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the National Basic Research Program of China (973 Program; 2013CB834701 and 2013CB834702), the University Grants Committee of Hong Kong (AoE/P-03/08), the Research Grants Council of Hong Kong (16305015, 16308116, C2014-15G, A-HKUST605116), the Nissan Chemical Industries, Ltd., and the Innovation and Technology Commission (ITC-CNERC14SC01). B.Z.T. thanks the support of the Guangdong Innovative Research Team Program (201101C0105067115), Science and Technology Plan of Shenzhen (JCYJ20160229205601482), and the Shenzhen Peacock Plan.

EXPERIMENTAL SECTION

Materials. All the polymers were purchased from Sigma-Aldrich Co., Limited, and purified by precipitation in methanol. All the AIEgens were purchased from AIEgen Biotech Co., Limited, and used without further purification. The synthesis, structure characterization, and photophysical properties of AIEgens can be found in the corresponding references.28,39−41 Solvents such as tetrahydrofuran (THF) and 1,2-dichloroethane (DCE) were distilled immediately prior to use. Preparation of AIEgen-Doped Polymer Films. 50 mg of polymer and 1 wt % of AIEgen were dissolved in 10 mL of freshly distilled THF or DCE with sonication to yield a homogeneous solution. The resulting solution was spin-coated onto the surface of clean silicon wafers by a KW-4A spin coater purchased from the Institute of Microelectronics of the Chinese Academy of Science. The polymer films were annealed at 120 °C for 12 h under vacuum and then cooled to room temperature before measurement. Characterization. DSC measurement was carried out on a PerkinElmer DSC-7 Instrument under a nitrogen atmosphere. Temperature was calibrated with indium prior to the test. The heating rate was set at 10 °C/min unless specified. The Tg of the polymers was determined on the second heating cycle to prevent the interference from the thermal history of the samples. Transmission electron microscope images were obtained on a JEOL 2010 TEM. A scanning electron microscopy image was obtained on a SIRION-100 (FEI) SEM. The fluorescent image of Figure S10D was recorded on a fluorescence optical microscope (Nikon Eclipse 80i) taken under a 330−380 nm UV light irradiation. ADEtect Device Parameter. The photos were recorded on a Canon EOS 60D 18 MP CMOS digital SLR camera with Canon EF 100 mm f/2.8L Macro IS USM lens, and it was controlled by Canon EOS Utility 2 software. The photos were recorded at a constant rate of 1 photo per 10 s under the same ISO, aperture, and shutter speed. Sample heating was performed on a Linkam temperature-controlled stage with a T95-PE system controller. The 365 nm UV excitation was from a hand-held UV lamp.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01070. Selective representatives of raw images and additional characterization data (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*(B.Z.T.) E-mail [email protected]; phone +852-2358-7242 (7375); fax +852-2358-1594. ORCID

Zijie Qiu: 0000-0003-0728-1178 G

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Macromolecules

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DOI: 10.1021/acs.macromol.7b01070 Macromolecules XXXX, XXX, XXX−XXX