Three-Dimensional Visualization for Early-Stage Evolution of Polymer Aging.

Monitoring the evolution of polymer aging, especially early-stage aging, over both time and dimensionality can provide in-depth insight into aging-induced material invalidation and even disastrous accidents. However, it remains a great challenge because currently available methods for polymer aging only provide statistic results at a macroscopic scale. Herein, we report the first three-dimensional early-stage visualization (ESV) technique of polymer aging by using the fluorophore-bonded boronic acid to specifically target aging-induced hydroxyl groups through the B-O click reaction. This method can identify the initial aging of polypropylene (PP) as early as 20.0 min. In contrast, no signals can be detected by conventional infrared spectroscopy even after 21 days of thermal treatment. More importantly, the three-dimensional evolution for early-stage polymer aging was demonstrated: faster aggression in the horizontal plane (4.1 × 10-4 s-1) than in the vertical direction (2.6 × 10-9 m s-1) for PP films. The approach could undoubtedly provide valuable information in elucidating mechanistic details of polymer aging in three-dimensional scale and assessing the utility of advanced antiaging materials.

Introduction

Polymers have been used in a wide range of applications spanning from daily routines to biomedical science and military applications.[1−3] The worldwide annual production of polymers has ramped up from 1.5 million tons in 1950 to 360 million tons in 2018.[4] The striking development is highly dependent on the sustainable and reliable commitment of polymers.[5−7] However, polymer aging occurs inevitably when polymer-based products are exposed to natural factors, such as oxygen, humidity, UV irradiation, and thermal gradients.[8−11] A progressive aging process could lead to structural deterioration, morphological decomposition, and functional invalidation of polymers.[12,13] In addition, polymer aging could occasionally induce horrible disasters. For example, an aged vinyl polymer container has caused the well-known nuclear leakage in Japan in 2017,[14] and the explosion of the famous Challenger shuttle happened after its launch because of the invalidation of an O-shaped seal ring in the rocket booster in 1986.[15] These disasters happened due to the neglect of polymer aging, ringing an alarm for an effective monitoring of polymer aging, especially for an early stage and overall checkup.

In general, the polymer aging comes along with an induction period and an acceleration period, as illustrated by a heterogeneous oxidation model.[16,17] At the induction stage, the chain scission and cross-linking occur between adjacent molecules through the slow chemical changes in the backbone of polymers. Significantly, abundant free radicals are generated in the induction period, accelerating the polymer chain reaction and pushing the polymer aging into the acceleration period.[18] These native features in the aging evolution of polymers make the monitoring of the induction period vital. On the other hand, polymer aging is a three-dimensional aggression process, which could diffuse throughout the surface and permeate into the interior of the polymer simultaneously.[19,20] Therefore, many efforts should be made on the evaluation on the polymer aging process over both time and dimensionality.

Currently, chemiluminescence, thermogravimetry analysis, and oxygen consumption experiments have been implemented to study the polymer aging evolution.[21−24] These investigations are based on the simulation of the aging process for polymers through a heating progress, and the signal changes are recorded as a function of time.[25−28] However, these techniques can only provide statistic results for polymer aging at a macroscopic scale.[29−32] In addition, Fourier transform infrared spectroscopy (FT-IR) has been regarded as the most conventional technique to detect the variations of functional groups during the polymer aging, such as hydroxyl, carbonyl, and ketone groups.[33−36] However, the sensitivity of the FT-IR technique is not competent for ultratrace detection of these functional groups in early-stage aging.[37−39] More importantly, all of the present techniques failed to give a three-dimensional evaluation on polymer aging.[40−42] Therefore, it is highly desired to develop a sensitive and multidimensional evaluation strategy for the early-stage identification for polymer aging.

The confocal laser scanning microscopy (CLSM) technique has drawn great attention due to its high sensitivity and three-dimensional and real-time observation characteristics.[43] These advantages of CLSM imaging have made it a promising technique in exploring the micro-/macrostructure of materials and monitoring the dynamic process of molecular self-assembly and biological events.[44−46] In this contribution, we present an early-stage visualization (ESV) technique for the aging process of polypropylene (PP) by the CLSM technique (Figure ). Hydroxyl groups from the oxidation of the C–C bond in polymer backbones were taken as an indicator for aging evolution. The fluorophore-bonded boronic acid molecules could target the produced hydroxyl groups through a facile and specific B–O click reaction.[47,48] It is noteworthy that the proposed approach has realized an early-stage identification of PP aging after 20.0 min of thermal treatment at 60 °C. In contrast, no signal can be detected by conventional FT-IR for PP thermally treated (60 °C) for even 21 days. Moreover, the dimension-dependent dynamic rate constants have been acquired. The results showed that more aggressive invasion happened on the surface than in the depth during the aging evolution for PP. The universality of the proposed method was further verified by monitoring the aging evolution of other polymer materials, including polyethylene (PE), ethylene vinyl acetate copolymer (EVA), and polydimethylsiloxane (PDMS). Our findings not only realized an early-stage monitoring for polymer aging but also intensified our understanding of aging dynamics in three dimensions. It is believed that the developed strategy can be widely applied for the whole community dedicated to polymer materials.

Figure 1

Schematic representation for fluorescent early-stage identification and evolutionary visualization for polymer aging.

Results and Discussion

Specific Fluorescence Recognition of Hydroxyl Groups

Early studies demonstrated that a facile reaction could occur between boronic acid and hydroxyl groups to form a cyclic boronic ester with structural rigidity, contributing to the wide applications in the molecular recognition and sensing.[49−51] To realize the specific recognition toward hydroxyl groups generated during the polymer aging, a commercially available fluorescent probe, 3-(10-phenyl-9-anthracenyl)phenyl boronic acid (DPBA), was employed in this work. The excitation and emission spectra of DPBA solution were recorded: strong blue emission at 415 and 430 nm under the excitation of 375 nm (Figure A). The fluorescent targeting toward hydroxyl groups was first validated by adding a hydroxyl-rich poly(vinyl alcohol) (PVA) into the DPBA molecules. The B–O linkage was specifically formed between boronic acid groups in DPBA and hydroxyl groups in PVA within a few minutes under an ambient environment. As a result, the fluorescent signals of DPBA-PVA were enhanced through the inhibited molecular motions of DPBA. The quantitative relationship between the concentration of hydroxyl groups and fluorescent intensity of DPBA-PVA was studied. The quantity of hydroxyl groups in the PVA chain was calculated through the polymerization degree and molecular weight of PVA molecules. Good linearity can be obtained from 0.19 to 1.53 mM for hydroxyl groups (Figure B), according to the equation y = 1.109 × 102x – 1.290 (R = 0.9973), where y is the relative fluorescent intensity (ΔI = I – I0), and x is the concentration of hydroxyl groups. This phenomenon indicated that the DPBA molecules can be used as a specific fluorescent probe to monitor the hydroxyl groups in polymers.

Figure 2

(A) Excitation (λem = 430 nm) and emission (λex = 375 nm) spectra of DPBA solution (100 μM). (B) Fluorescent intensity variations of DPBA (10 μM) in the presence of different concentrations of hydroxyl groups in PVA and the linear fitting equation; fluorescence confocal microscopy images (1167 × 1167 μm2) for PP films after thermal treatment at 60 °C for (C) 0.0 min, (D) 30.0 min, (E) 60.0 min, (F) 90.0 min, (G) 120.0 min, and (H) 150.0 min.

Polyolefin is a class of high-demand plastic materials, and it takes up a share of 90% in the market of thermoplastic polymers, such as PP and PE. The previous studies have demonstrated that PP aging under external irradiation is mostly initiated from oxidation of tertiary carbon in backbones, and it is accompanied by the generation of hydroxyl groups.[52,53] In our work, PP films were thermally treated in an aging chamber under atmospheric conditions: the heating temperature is 60 °C, the oxygen content ∼21%, and the humidity around 15–20%. After the aging process, the DPBA molecules would specifically target the sites with hydroxyl groups, and these aged sites would exhibit fluorescent emissions. The fluorescence images for the aged PP films were captured by CLSM, and the data were processed through Leica Application Suite X. When the fluorescent volumes of the generated aged sites were larger than 2.2 μm3, a cyan block was defined in the visualization. As shown in two-dimensional images, PP film without thermal treatment showed a clear background without fluorescence emission (Figure C). In contrast, time-dependent fluorescent changes can be observed for PP films after thermal treatment. With the prolonged treatment time from 30.0 to 150.0 min, the PP films exhibited brightened fluorescence with an increasing number of fluorescent blocks (Figure D–H). To confirm the attribution of the fluorescence, physical damages in PP films were carried out as controlled experiments. It can be concluded that neither the scratching nor poking could induce the fluorescent emissions in PP films (Figure S1). In other words, the morphological variations in the absence of hydroxyl groups would not trap DPBA molecules nor show emission in the PP films. We concluded that the fluorescent emissions in PP films were attributed to the DPBA labeling of the generated hydroxyl groups during the aging. Therefore, the microstructural variations in the PP aging process have been successfully realized through fluorescence identification.

Early-Stage Identification of Polymer Aging

We have implemented the fluorescent labeling approach with the time interval of 5.0 min (Figure S2) to investigate the sensitivity of the proposed strategy for polymer aging. It is noteworthy that weak fluorescence appeared by CLSM when the aging time of PP approached 20.0 min (Figure A), indicating a success in the identification of early-stage aging. In comparison, the conventional FT-IR technique in combination with attenuated total reflection (ATR) was implemented to confirm the generation of hydroxyl groups in PP. Disappointingly, no peaks at 3731 cm–1, attributed to the hydroxyl groups, can be observed for PP films when the time of thermal treatment varied from 5.0 to 150.0 min, even to 21 days (Figure S3). In addition, a microinfrared mapping technique recording the absorbance of hydroxyl groups in PP films was carried out. Similarly, no appearance of hydroxyl groups can be observed for the early-stage identification at 20.0 min (Figures B). These results demonstrated the advantage of our proposed strategy for the ultratrace detection of hydroxyl groups in the early stage of polymer aging.

Figure 3

(A, C) Two-dimensional fluorescent images (90 × 90 μm2) and (B, D) micro-FT-IR mapping images (90 × 90 μm2) of PP films after thermal treatment at 60 °C for (A, B) 20.0 min and (C, D) 14 days. (E) Fluorescence and (F) IR absorbance comparison for aged PP films at 20.0 min (black line) and 14 days (red line).

To confirm the accuracy of our proposed approach, the PP film was thermally treated for 14 days at 60 °C and applied for both the fluorescence imaging and the microinfrared mapping techniques at the same location. The fluorescent images in Figure C showed that the cyan blocks could spread across the film. In comparison, the absorbance variation at 3731 cm–1, attributed to the hydroxyl groups generated in the aged PP, can also be detected by microinfrared mapping technique. The distinguished color from blue to red indicated the increased hydroxyl groups (Figure D). The good agreement between the fluorescence and microinfrared mapping images demonstrated the accuracy of our proposed strategy in the identification of hydroxyl groups. Moreover, the bigger changes in fluorescent variations (Figure E), in comparison with FT-IR measurements (Figure F), indicated that our method was highly sensitive for early-stage identification of polymer aging.

Evolutionary Visualization of Polymer Aging

The dynamics is of great significance to understand the polymeric aging mechanism and predict the aging behaviors for polymers. However, the previous dynamic studies were generally carried out through thermogravimetry analysis which records the mass variation in a heating progress to simulate polymer aging behaviors.[52−54] In this work, we tried to explore the ESV technique for the dynamics of PP films by monitoring hydroxyl groups. In order to rule out the diffusion of dye being the limiting factor for the analysis, we have studied the penetration capacity of DPBA into the polymer matrix by immersing the blank PP films in DPBA solution for 1.0 min under ultrasonic treatment. It can be seen in Figure S4A–C that 2D images of PP films taken at different depths showed uniformly distributed fluorescent emissions. This phenomenon demonstrated that DPBA molecules can easily permeate into the polymer films. Moreover, the DPBA molecules were free in the blank films and can be removed by ultrasonic treatment in methanol (Figure S4D). Three-dimensional images for aged PP films were captured. The statistical data from three-dimensional images showed that the total volume of cyan blocks in PP films boomed from 115.9 to 19 815.2 μm3 with an increased treatment time from 30.0 to 150.0 min (Table S1). The varied block volume takes up only 0.002–0.291‰ of the whole volume in the imaging window (1167 × 1167 × 50 μm3), verifying the high sensitivity of our proposed strategy in the aging evaluation. Significantly, it can be seen from the volume evolution that the aging proceeded slowly at the first 90.0 min (Figure S5A); afterward, the aging developed much more rapidly. This evolution can be well explained through the established heterogeneous oxidation model.[16−18] The aging was initially developed in the amorphous phase with the molding defect or initiating species. Oxidizable material accumulated in these oxidizing centers exhibited a high degree of freedom. However, the rate-determining step in the oxidative spread was the diffusion of oxygen. With the diffusion of oxygen, the oxidative infection was physically spread to the adjacent sites in a heterogeneous model, resulting in the fast exponential increase. The variation trend agrees well with that by the thermogravimetry analysis.[54−56] Notably, it is based on the high sensitivity of our three-dimensional fluorescent imaging method that the imperceptible changes in the microstructural variations during the aging process can be shown. Moreover, the evolution achieved through the volume (defined as V) as a function of time (defined as t) complied with a first-order reaction, with good linearity (R = 0.9954) between ln V and t (Figure S5B):

The confidence intervals of the fitting slope and intercept are (6.9 × 10–4, 7.6 × 10–4) and (−38.1, −37.7), respectively. The rate constant k can be determined as 7.2 × 10–4 s–1 by the slope in the expression through a linear regression method in mathematical statistics. The results showed the invasive aging process of PP film through the time-dependent volume changes. Therefore, it is proven that the proposed strategy can provide valid information for three-dimensional dynamics of the PP aging.

Two aging spots were selected and tracked through three-dimensional imaging in order to offer the delicate and accurate profiles for PP films (Figure A). The cross-sectional view and sliced images at different depths were displayed according to the schematic representation in Figure B. Cyan fragments gradually appeared at the initiation stage of aging, and they sprouted along the z axis after 60.0 min of treatment in cross-sectional images (Figure C). With the treatment time prolonged to 150.0 min, the cyan blocks grew up gradually and stretched to 25.1 μm in the vertical direction. Moreover, the sliced images were acquired by splitting the three-dimensional images every 5 μm along the z axis. The images in rows showed the extended area distribution in the x–y plane with prolonged treatment time, while the images in columns demonstrated the aggressive penetration at different depths (Figure D). These results suggested that the thermal aging was not primarily a surface reaction but also could intrude down through the polymer.

Figure 4

(A) Three-dimensional fluorescence confocal microscopy image (45 × 700 × 700 μm3), (B) schematic representation, (C) side-view fluorescent images (28 × 80 μm2), and (D) cross-section fluorescent images for PP films (45 × 70 μm2) after the thermal treatment (60 °C). Dynamic calculations of aging variation in (E) depth, (F) area, and (G) volume, respectively.

Dynamic calculations for these two spots during the aging process were performed taking depth (D, along the z axis) and area (S, in the x–y plane) as variations with time (Table S2). Significantly, the relations between D, S, and t varied with the dimensions. An excellent linear relationship can be drawn between one-dimensional depth (D) and time (t), conforming to the law of zero-order reaction (Figure E):

The confidence intervals of the fitting slope and intercept are (2.5 × 10–9, 2.7 × 10–9) and (7.6 × 10–7, 1.8 × 10–6), respectively. The expression suggested that the depth variation in the vertical direction is dependent on time. In contrast, the two-dimensional horizontal area followed a first-order reaction (Figure F), and the logarithm value of area (S) was proportional to time (t):

The confidence intervals of the fitting slope and intercept are (3.7 × 10–4, 4.5 × 10–4) and (−26.2, −25.7), respectively. According to the dynamic model, the variation in the horizontal plane is affected by the quantities of reactants. The comparison between rate constants, 2.6 × 10–9 m s–1 for vertical invasion and 4.1 × 10–4 s–1 for horizontal aggression, demonstrated that the aging occurred in the horizontal plane preferentially, rather than the penetration in the vertical direction. Moreover, the spatial integration was analyzed as volume V:

The confidence intervals of the fitting slope and intercept are (6.8 × 10–4, 7.4 × 10–4) and (−39.6, −39.2), respectively. It can be noted that the relationship between ln V and t followed the similar expression in the previous part, suggesting the accuracy of this calculation (Figure G). This three-dimensional monitoring provided the informative profiles for aging evolutionary study and lifetime prediction of polymers.

Universality of the Evolutionary Visualization of Polymer Aging

The external conditions (e.g., temperature and heating rate) could influence the aging behaviors of polymer variably.[57] In this work, a series of PP films were thermally treated at different temperatures from 60 to 75, 90, 105, 120, and 130 °C for 40.0 min, respectively. Two-dimensional images showed that higher temperatures could result in a more severe aging degree in PP films with prosperously distributed blocks (Figure S6). Three-dimensional topographies through depth coding analysis further confirmed the depth invasion upon temperature. As shown in Figure A, the blue color labeled blocks reached a depth of 7.1 μm under the thermal treatment of 60 °C. In comparison, a depth of 39.9 μm was observed for the blocks in PP film at 130 °C. Note that the coded color turned red from Figures A to 5F, demonstrating the trend of increasing depth as a result of elevated aging temperature (Figure G). In addition, the temperature-programmed aging experiments were further implemented with varied heating rates set according to Figure S7 and Table S3. Both two-dimensional images and three-dimensional depth coding showed that the rapid heating rates accelerated the aging process for PP films (Figures S8 and S9). These findings demonstrated that the developed ESV technique could effectively distinguish the varied aging degree for polymers under different external conditions through both horizontal and vertical analysis in multidimensions.

Figure 5

(A–F) Three-dimensional depth coding analysis (800 × 800 × 40 μm3) and (G) depth analysis (47 × 94 μm2) of PP films after thermal treatment at different temperatures: (A) 60 °C, (B) 75 °C, (C) 90 °C, (D) 105 °C, (E) 120 °C, and (F) 130 °C.

The universality of the ESV technique was further verified by other polymer materials, including PE, EVA, and PDMS. First, the free diffusion of DPBA molecules into these films was validated by analyzing the fluorescent distribution at different depths after dyeing by DPBA molecules (Figure S10). These films were then thermally treated at 60 °C and labeled by DPBA molecules for ESV. For PE films, two-dimensional fluorescent images showed gradually emerged cyan blocks after thermal treatment. With the prolonged aging time from 2 to 10 h, the number of blocks increased; the blocks grew larger (Figure S11), and the aging depth was deepened from 4.1 μm for 2 h to 12.3 μm for 10 h (Figure S12). The dynamics of PE achieved through the aging volume (V) and time (t) complied with a first-order reaction (Figure S13):

This fitting equation showed the rate constant of 8.9 × 10–5 s–1 for PE films. It should be noted that PE showed the lower rate constant (8.9 × 10–5 s–1) as compared to PP (7.2 × 10–4 s–1) in the aging evolution. The poorer thermal stability of PP may be attributed to the unstable alkyl chains in the main backbone.[58,59] Furthermore, the early-stage aging of EVA and PDMS can be monitored through the fluorescent labeling by DPBA and CLSM imaging. Both 2D images and depth measurements showed the aggressive aging evolution as a function of time (Figures S14–S17). These results demonstrated the applicability and feasibility of our strategy in the early-stage identification and three-dimensional monitoring of polymer aging, facilitating a systematic and intensive exploration in the polymer.

Conclusions

In conclusion, we have established an ESV technique for the identification and evolutionary monitoring of polymer aging. To the best of our knowledge, this is the first example of the monitoring of aging dynamics in both horizontal and vertical directions. The proposed strategy exhibited high sensitivity, good accuracy, and wide applicability, and thus, it would gain deeper insight into the mechanism for polymer aging. This approach has the potential to predict the lifetime of polymers exposed to environments and provide a viable assessment for decisions on repair or replacements of polymer products. Next, we will focus on the design of advanced antiaging materials with controllable degradability.