Stress Transfer Within Microphase‐Separated Structures: A Post‐Synthetic Strategy to Impart Mechanoresponsiveness to Block Copolymer Materials

ABSTRACT

Mechanoresponsive polymers, which exhibit changes in properties such as color in response to mechanical stimuli, have attracted increasing attention for smart materials. Among various design strategies, mechanophore‐incorporated systems are widely used approaches that enable precise molecular‐level design and control; however, this approach often suffers from complicated synthesis and limited applicability to existing materials such as block copolymers, which have found widespread industrial use as high‐performance materials. Here, we present a post‐synthetic strategy enabled by exploiting microphase‐separated structures to impart mechanochromism to existing polymer systems while retaining the advantages of mechanophore‐based approaches. By exploiting the microphase‐separated structure of styrene–butadiene–styrene (SBS) block copolymers, blending with polystyrene bearing a tetraarylsuccinonitrile (TASN) mechanophore enables domain‐selective localization of the mechanophore within the rigid domains without chemical modification of the host polymer. The resulting materials exhibit fluorescence upon tensile deformation originating from force‐induced TASN cleavage, demonstrating mechanoactivation even under minimal deformation of the hard domains. The mechanoresponse is governed by the molecular weight and content of the mechanophore‐containing polystyrene. Importantly, bulk mechanical properties are preserved and can even be enhanced. This strategy establishes a scalable platform for integrating mechanochemical functionality into existing polymer systems.

A domain‐engineered post‐functionalization strategy is presented to impart mechanochromism to styrene–butadiene–styrene (SBS) block copolymers by simple blending with a tetraarylsuccinonitrile (TASN) mechanophore‐containing polystyrene (PS‐TASN‐PS). The resulting materials exhibit reversible fluorescence under tensile deformation while preserving, or even enhancing, intrinsic mechanical properties, offering a scalable platform for stress visualization in microphase‐separated polymer systems.

1. Introduction

Mechanically responsive polymers [1, 2, 3, 4, 5, 6, 7, 8] have emerged as promising candidates for next‐generation smart materials. Among these, mechanochromic polymers, whose optical properties change in response to applied mechanical stimuli [3, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33], have been intensely investigated in recent years. Because regions experiencing tensile, compressive, or other mechanical stresses exhibit changes in color or fluorescence, such polymers enable the visualization of damage and stress distribution in materials, making them highly attractive for stress‐sensing and damage‐detection applications.

The most widely employed strategy for both mechanochromic and other mechanically responsive polymers is the incorporation of a mechanically reactive chemical structure called a mechanophore directly into polymer backbones [3, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33]. Mechanophore‐incorporated systems represent a powerful design strategy that enables precise molecular‐level control, which in turn affords superior structural and storage stability compared to structurally colored systems [10] and those based on dispersed dyes [9] or microcapsules [11], owing to the covalent integration of the mechanophore within polymer chains. Nonetheless, this strategy requires complex polymer synthesis, which complicates manufacturing processes and increases production costs. To overcome these limitations, alternative strategies that introduce mechanoresponsive functionality without covalent modification of the host polymer have recently begun to emerge. However, these approaches remain very limited [34]. The need for such simplified strategies is particularly evident for block copolymers, whose synthesis typically demands precise, multistep polymerization. Given that block copolymers exhibit unique physical properties arising from their microphase‑separated structures [35, 36, 37], and are well established in industry as functional materials such as thermoplastic elastomers, the development of simplified methods to impart mechanical responsiveness to block copolymers is important for expanding the scope of mechanically responsive polymers and for promoting their deployment in practical applications.

Here, we propose a new design strategy that preserves the advantages of mechanophore‑based polymers while enabling facile post‑synthetic functionalization of common block copolymer materials. Styrenic block copolymers such as styrene–butadiene–styrene (SBS) and styrene–isoprene–styrene (SIS), which are often used as thermoplastic elastomers, exhibit microphase‑separated structures in the film state consisting of hard polystyrene (PS) domains and soft polyolefin domains. When these films are subjected to tensile deformation, not only the soft domains but also the hard PS domains undergo deformation [35, 36, 37]. Similar microphase‑separated structures are also formed when PS is blended with SBS, and these PS domains deform in a comparable manner during stretching [38, 39]. We hypothesized that blending SBS with mechanophore‑containing PS having the same chemical composition as the hard domain would allow the mechanophore to localize preferentially within the hard domains and thereby transmit stress from the block copolymer to the mechanophore. Additionally, blending PS with SBS or styrene–butadiene rubber has been reported to increase the stress at a given strain [38, 40], suggesting that mechanical reinforcement may also be possible. In this study, we synthesized PS bearing a centrally located tetraarylsuccinonitrile (TASN) unit, a mechanophore that is highly sensitive to mechanical force and generates highly fluorescent diarylacetonitrile (DAAN) radicals upon activation, thereby enabling both facile visual detection and quantitative analysis by electron paramagnetic resonance (EPR) spectroscopy (Figure 1a) [41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51]. The resulting polymer (PS‑TASN‑PS) was blended into commercially available SBS to investigate the post‑synthetic introduction of mechanochromic behavior arising from deformation of the hard domains (Figure 1b).

FIGURE 1.

Conceptual design of mechanochromic SBS materials. (a) Scheme of force‐induced homolytic cleavage of TASN, generating fluorescent DAAN radicals. (b) Illustration of the post‐synthetic strategy based on microphase‐separated structures: blending PS‐TASN‐PS with SBS block copolymers drives selective localization of the mechanophore within rigid polystyrene domains.

2. Results and Discussion

2.1. Preparation and Characterization of Blend Films

Following the previously reported procedure [41], we synthesized PS‑TASN‑PS_9k (M _n = 8630 g mol⁻¹, M _w/M _n = 1.09), PS‑TASN‑PS_18k (M _n = 17 760 g mol⁻¹, M _w/M _n = 1.08), and PS‑TASN‑PS_30k (M _n = 30 250 g mol⁻¹, M _w/M _n = 1.07), which are polystyrenes containing a TASN mechanophore at the chain center (Table S1). To systematically elucidate the correlations between the blend ratio and molecular weight of PS‑TASN‑PS and the resulting mechanochromic behavior of the blend films, we prepared a series of films by blending each PS‑TASN‑PS into SBS (M _n = 73 650 g mol⁻¹, M _w/M _n = 1.16, PS = 32 wt%) at weight ratios ranging from 99/1 to 70/30 (Films 1–15, Table 1). A film of SBS alone (Film 0) was also prepared as a control sample. All films were annealed at 100°C under reduced pressure for 24 h to promote phase separation, followed by storage at room temperature for more than 1 week prior to characterization.

TABLE 1.

Compositions and thermal properties of Films 0–15.

Film	Mixing ratio		PS	TASN	T g1 a	T g2 a
No.	wt/wt		wt%	wt%	°C	°C
0	SBS / PS‐TASN‐PS	= 100 / 0	32.0	0	−90	96
1	SBS / PS‐TASN‐PS9k	= 99 / 1	32.7	0.055	−91	96
2	SBS / PS‐TASN‐PS9k	= 95 / 5	35.4	0.27	−91	98
3	SBS / PS‐TASN‐PS9k	= 90 / 10	38.8	0.55	−88	101
4	SBS / PS‐TASN‐PS9k	= 80 / 20	45.6	1.1	−88	102
5	SBS / PS‐TASN‐PS9k	= 70 / 30	52.4	1.6	−88	102
6	SBS / PS‐TASN‐PS18k	= 99 / 1	32.7	0.027	−89	98
7	SBS / PS‐TASN‐PS18k	= 95 / 5	35.4	0.13	−89	100
8	SBS / PS‐TASN‐PS18k	= 90 / 10	38.8	0.27	−85	103
9	SBS / PS‐TASN‐PS18k	= 80 / 20	45.6	0.53	−84	108
10	SBS / PS‐TASN‐PS18k	= 70 / 30	52.4	0.80	−81	112
11	SBS / PS‐TASN‐PS30k	= 99 / 1	32.7	0.016	−89	96
12	SBS / PS‐TASN‐PS30k	= 95 / 5	35.4	0.078	−87	105
13	SBS / PS‐TASN‐PS30k	= 90 / 10	38.8	0.16	−84	106
14	SBS / PS‐TASN‐PS30k	= 80 / 20	45.6	0.31	−82	108
15	SBS / PS‐TASN‐PS30k	= 70 / 30	52.4	0.47	−84	117

^a Determined by DMA.

Dynamic mechanical analysis (DMA) revealed two relaxation processes for all films, appearing at −91°C to −81°C and 96°C to 117°C, corresponding to the glass transitions of the polybutadiene (PB) and PS domains, respectively (Figures S7–S22), indicating the formation of phase‑separated structures. Transmission electron microscopy (TEM) further confirmed the presence of microphase‑separated morphologies in all samples (Figures S23 and S24). In addition, macrophase‑separated structures that can be attributed to aggregates of PS‑TASN‑PS were observed for Films 9, 10, 12, 13, 14, and 15 (Figure S25). Such macrophase separation was observed when the molecular weight of the added PS‑TASN‑PS was 17 760 g mol⁻¹ at blend ratios of ≥20 wt%, and when it was 30 250 g mol⁻¹ at blend ratios of ≥5 wt%. These results indicate that aggregation among PS‑TASN‑PS chains becomes significant when the molecular weight and/or the blend content exceed certain thresholds.

Upon irradiation of the films with 365 nm UV light, faint fluorescence was detected in all blend films (Films 1–15) (Figure S26). EPR spectroscopy revealed a signal with a g‑value of 2.003, consistent with carbon‑centered radicals (Figure S27), suggesting the presence of DAAN radicals generated through TASN scission. Fluorescence spectroscopy further confirmed emission spectra characteristic of DAAN radicals (Figure S28) [41]. These observations are attributed to partial thermally induced TASN cleavage during annealing at 100°C [41], followed by restricted diffusion and incomplete recombination of the DAAN radicals attached to the polymer chain end due to vitrification of the PS chains during cooling.

2.2. Mechanochromic Behavior of SBS/PS‐TASN‐PS Blend Films

To evaluate the mechanochromic behavior of the blend films, we performed tensile testing on Films 0–15 and subsequently analyzed the dissociation of the TASN units using fluorescence measurement and EPR spectroscopy. Stress–strain curves revealed a pronounced stress upturn at high strain, likely attributable to strain‑induced crystallization, indicating that sufficient stress was transmitted through the polymer chains. Accordingly, fluorescence and EPR measurements were conducted using stretched samples. To ensure a consistent comparison of the mechanochemical response among all films, a constant strain was applied for all samples. For the fluorescence measurement, a strain of 1000% was selected to provide sufficient deformation while remaining below the fracture strain of all films, thereby ensuring reproducible measurements under identical conditions. For the EPR spectroscopy during tensile deformation, however, the experimental setup limited the maximum strain to 400%; therefore, the radical concentration was measured immediately after the samples reached 400% strain. Films containing 1 wt% of any PS‑TASN‑PS (Films 1, 6, and 11) exhibited tensile properties comparable to those of neat SBS (Film 0), whereas films containing ≥5 wt% PS‑TASN‑PS consistently showed higher stress at a given strain than Film 0 (Figure S29).

Observation of the films after tensile testing demonstrated that all blend films (Films 1–15), except for SBS alone (Film 0, Figure S30), displayed distinct yellow‐green fluorescence under 365 nm UV irradiation (Figure 2a). Fluorescence spectra of the stretched films exhibited emission profiles characteristic of DAAN radicals generated via TASN scission [41], confirming that mechanical stress triggered the cleavage of the TASN units (Figure S31).

FIGURE 2.

Mechanochromic response of SBS/PS‑TASN‑PS blend films. (a) Photographs of Films 1–15 under 365 nm UV irradiation after stretching to 1000% strain. (b) Representative TEM images (reproduced from Figure S25) illustrating films with and without observable PS aggregates, together with the dependence of DAAN radical generation on PS‐TASN‐PS content after stretching to 400% strain. (c) Dependence of DAAN radical generation on TASN content after stretching to 400% strain. (d) Dependence of TASN dissociation on PS‐TASN‐PS content after stretching to 400% strain. (e) Dependence of TASN dissociation on TASN content after stretching to 400% strain for films showing no observable PS aggregation. Error bars in (b–e) represent the standard deviation of three independent samples (n = 3).

EPR measurements conducted immediately after stretching to 400% strain further verified the presence of radicals in all films showing mechanochromism (Films 1–15) (Figure S32). The g‑value of 2.003 indicated that the observed signal originated from DAAN radicals formed through TASN cleavage. To quantitatively assess the mechanochromic response, we examined how the blend ratio and molecular weight of PS‑TASN‑PS influenced radical generation. The amount of DAAN radicals generated per unit mass of film was calculated after subtracting radical species pre‑existing before deformation. In general, higher blend ratios yielded increased radical concentrations regardless of the PS‑TASN‑PS molecular weight. However, radical formation became less efficient for PS‑TASN‑PS_18k at blend ratios of ≥20 wt% (Films 9 and 10) and for PS‑TASN‑PS_30k at ≥10 wt% (Films 13–15) (Figure 2b). Similar trends were observed when the data were normalized by the amount of incorporated TASN (Figure 2c). Notably, macrophase‑separated aggregates attributed to PS‑TASN‑PS were detected in the films (Figure S25) exhibiting the above behavior, suggesting that mechanical stress was not efficiently transmitted to the TASN units within aggregated PS‑TASN‑PS domains, leading to reduced radical formation.

When normalized to the total TASN content, PS‑TASN‑PS_18k produced a greater quantity of DAAN radicals than PS‑TASN‑PS_9k, consistent with the previous findings indicating that higher molecular weight polymers exhibit more efficient force transmission to centrally located mechanophores [52]. In contrast, PS‑TASN‑PS_30k showed reduced mechanochemical activation due to macrophase separation, highlighting the necessity of selecting appropriate molecular weights and blend ratios to achieve efficient mechanochromism in SBS‑based systems.

The correlation between TASN dissociation efficiency and PS‑TASN‑PS content was further examined for each molecular weight (Figure 2d). In all cases, increasing the amount of PS‐TASN‐PS led to decreased TASN dissociation efficiency, likely because the applied stress within the hard domains was distributed across a larger number of TASN units. When comparing films only within blend ratios that did not exhibit PS‑TASN‑PS aggregation, TASN dissociation efficiency increased with the molecular weight of PS‑TASN‑PS (Figure 2e), reflecting more efficient force transduction to centrally located mechanophores in longer polymer chains [52]. This molecular weight dependence was most evident at low blend ratios. Because films containing 1 wt% PS‑TASN‑PS retained T _g values (Table 1) and tensile properties (Figure S29) nearly identical to those of SBS alone, the use of higher molecular weight PS‑TASN‑PS is advantageous for imparting mechanochromism while minimizing alterations to the intrinsic physical properties of SBS. However, lower blend ratios inevitably yielded weaker fluorescence; therefore, when stronger emission is required, appropriate selection of both the molecular weight and blend ratio of PS‑TASN‑PS is required.

2.3. Effect of Polystyrene Composition on Mechanochemical Activation

In the experiments using Films 1–15, the amount of generated radicals increased with increasing PS‑TASN‑PS content. However, it remained unclear whether this trend originated simply from a larger absolute amount of TASN units, or from enhanced stress applied to the TASN units due to an increased PS domain fraction. To distinguish between these possibilities, we fixed the PS‑TASN‑PS content at 10 wt% and varied the PS domain content by additionally incorporating 10 or 20 wt% of PS lacking a mechanophore. As a control, PS without a mechanophore, polystyrene containing bisphenol A (BPA) instead of TASN at the center of the polymer chain (PS‑BPA‑PS; M _n = 16 890 g mol⁻¹, M _w/M _n = 1.09) was synthesized and used.

Films were prepared by blending SBS, PS‑TASN‑PS, and PS‑BPA‑PS at weight ratios of 80/10/10 or 70/10/20 using PS‑TASN‑PS samples of different molecular weights (Films 16–21, Table 2). DMA of the films annealed at 100°C under reduced pressure for 24 h revealed two relaxation processes in all samples, appearing at −89°C to −85°C and 100°C to 116°C (Figures S33–S38). These relaxations are attributed to the glass transitions of the PB and PS domains, respectively, indicating the presence of phase‑separated structures. TEM further confirmed microphase separation in all films (Figure S39). Moreover, macrophase‑separated structures, likely originating from aggregates of PS‑TASN‑PS and PS‑BPA‑PS, were observed in all samples (Figure S40). This aggregation is presumed to result from the relatively high molecular weight of PS‑BPA‑PS combined with the large total loading of PS‑TASN‑PS and PS‑BPA‑PS (≥20 wt%).

TABLE 2.

Compositions and thermal properties of Films 16–21.

Film	Mixing ratio		PS	TASN	T g1 a	T g2 a
No.	wt/wt/wt		wt%	wt%	°C	°C
16	SBS / PS‐TASN‐PS9k / PS‐BPA‐PS	= 80 / 10 / 10	45.6	0.55	−89	100
17	SBS / PS‐TASN‐PS9k / PS‐BPA‐PS	= 70 / 10 / 20	52.4	0.55	−89	101
18	SBS / PS‐TASN‐PS18k / PS‐BPA‐PS	= 80 / 10 / 10	45.6	0.27	−88	106
19	SBS / PS‐TASN‐PS18k / PS‐BPA‐PS	= 70 / 10 / 20	52.4	0.27	−86	107
20	SBS / PS‐TASN‐PS30k / PS‐BPA‐PS	= 80 / 10 / 10	45.6	0.16	−86	112
21	SBS / PS‐TASN‐PS30k / PS‐BPA‐PS	= 70 / 10 / 20	52.4	0.16	−85	116

^a Determined by DMA.

Tensile testing of Films 16–21 revealed that, regardless of the molecular weight of the incorporated PS‑TASN‑PS, films containing PS‑BPA‑PS exhibited higher stress at a given strain compared to films containing only 10 wt% PS‑TASN‑PS (Films 3, 8, and 13) (Figure S41). This trend is consistent with the behavior observed with increasing PS‑TASN‑PS content. Visual observation of the fluorescence of the stretched films showed no noticeable difference in emission intensity between films containing only 10 wt% PS‑TASN‑PS (Films 3, 8, and 13) and those containing additional PS‑BPA‑PS (Figure 3a). Fluorescence spectra collected after stretching confirmed the presence of DAAN radicals and exhibited emission profiles characteristic of TASN scission (Figure S42) [41].

FIGURE 3.

Effect of polystyrene domain fraction on mechanochemical activation. (a) Photographs of Films 3, 8, 13, and 16–21 under 365 nm UV irradiation after stretching to 1000% strain. (b) Dependence of DAAN radical generation on PS‐BPA‐PS content at a fixed PS‐TASN‐PS content of 10 wt% after stretching to 400% strain. (c, d) Comparison of DAAN radical generation after stretching to 400% strain between films containing only PS‐TASN‐PS and films in which a portion of PS‐TASN‐PS was replaced by PS‐BPA‐PS, while maintaining a constant total PS content (20 wt% for (c) and 30 wt% for (d)). Error bars in (b–d) represent the standard deviation of three independent samples (n = 3).

EPR measurements of Films 16–21 performed immediately after stretching to 400% strain also revealed signals with a g‑value of 2.003, consistent with the formation of DAAN radicals (Figure S43). To quantitatively assess the influence of PS‑BPA‑PS on the mechanochromic response, the radical concentration generated upon stretching was analyzed after subtracting the radicals present prior to deformation. When plotting the total PS content (sum of PS‑TASN‑PS and PS‑BPA‑PS) against the amount of DAAN radicals generated per unit mass of film, only a slight increase in radical concentration was observed with increasing PS content (Figure 3b). This result indicates that the radical increase observed in Figure 2b upon increasing PS‑TASN‑PS content is primarily attributable to the increased number of TASN units, rather than enhanced stress transfer resulting from increased PS domain content. In other words, the incorporation of PS‑BPA‑PS to expand the PS domains did not significantly alter the strain experienced by PS‑TASN‑PS.

Furthermore, the radical concentrations generated immediately after stretching were compared between films containing 20 or 30 wt% PS‑TASN‑PS alone and those with the same total PS content composed of PS‑TASN‑PS and PS‑BPA‑PS. The results demonstrate that films containing only PS‑TASN‑PS consistently produced larger amounts of radicals (Figure 3c,d). These results strongly support the conclusion that the increased radical generation observed with higher PS‑TASN‑PS loadings arises from the greater amount of TASN present in the films, rather than from increased macroscopic stress applied to the films.

2.4. Dynamics and Reversibility of Mechanochemical Activation

The fading behavior of radicals generated from TASN units within the PS domains was evaluated by tracing the fluorescence after tensile deformation of the SBS/PS‑TASN‑PS blend films. First, the mechanochromic response of a film containing 10 wt% PS‑TASN‑PS_18k (Film 8) was monitored after stretching to 1000% strain at 100 mm min⁻¹ and subsequently storing the sample under ambient conditions for 24 h. The fluorescence intensity rapidly decreased within 10 min and nearly disappeared after 24 h (Figure 4a), suggesting recombination of mechanically generated DAAN radicals within the PS domains. In contrast, rapid cooling of Film 8 in liquid nitrogen immediately after stretching largely suppressed fluorescence decay, and the yellow‐green emission persisted even after 24 h (Figure 4b). This result suggests that radical recombination is governed by chain mobility within the PS domains.

FIGURE 4.

Dynamics and reversibility of the mechanochromic response. (a) Photographs of Film 8 under 365 nm UV irradiation, showing the fluorescence decay after stretching to 1000% strain and recovery upon re‐stretching after a 24 h relaxation period. (b) A photograph of Film 8 under 365 nm UV irradiation after stretching to 1000% strain and subsequent storage in liquid nitrogen for 24 h. (c–e) Normalized fluorescence decay profiles monitored at 554 nm for (c) Films 1–5, (d) Films 6–10, and (e) Films 11–15. (f) Temporal evolution of radical concentration in Film 10 measured by EPR after stretching to 400% strain. (g) Fluorescence spectra of Film 8 immediately after stretching to 1000% strain and after storage in liquid nitrogen for 24 h. (h) Stress–strain curves of Film 8 during repeated stretching cycles.

To quantitatively evaluate this fading behavior, Films 1–15 were stretched to 1000% strain at 100 mm min⁻¹, and the temporal decrease in fluorescence intensity at 554 nm was monitored. Fluorescence measurements were initiated within 2 min after stretching, and intensities were normalized to the value at the start of measurement. In all films, fluorescence decayed rapidly within several minutes, and in most cases, decreased to less than one‑tenth of the initial value within 2 h (Figure 4c–e). Film 1 exhibited a slower apparent decay, likely because its initial fluorescence was low, making the relative decrease appear smaller.

To directly assess radical decay, EPR measurements were performed on a film containing 30 wt% PS‑TASN‑PS_18k (Film 10) after stretching to 400% strain. Consistent with the fluorescence decay results, a rapid decrease in radical concentration was observed (Figure 4f). Although DAAN radicals are relatively stable toward oxygen and exhibit low hydrogen‑abstraction capability, they are known to recombine rapidly when their mobility is high [41]. Thus, the disappearance of yellow‐green fluorescence after stretching is attributed primarily to radical recombination. Typically, radicals bound to PS chains recombine extremely slowly at room temperature due to restricted mobility below the PS T _g (≈ 100°C) [16, 41, 52]. Nevertheless, the rapid recombination observed here suggests that the DAAN radicals generated upon TASN scission remain in close proximity within the PS domains of the blend films. Because the T _g of the PS domains is well above room temperature, molecular diffusion within the domains is highly restricted. Nevertheless, the DAAN radicals generated through TASN scission are expected to remain sufficiently close to each other that local segmental motions can still induce recombination. The α‑relaxation of PS occurs near the T _g (101–113°C), whereas γ‑relaxation occurs around –73°C [53, 54]. To assess the effect of suppressed γ‑relaxation on recombination behavior, Film 8 was rapidly cooled after stretching and stored in liquid nitrogen for 24 h. Under these conditions, yellow‐green fluorescence was retained (Figure 4b), and its spectrum closely matched that obtained immediately after stretching (Figure 4g), confirming that DAAN radicals persisted. These observations indicate that the DAAN radicals generated in the glassy PS domains remain sufficiently close that even local segmental motions can drive recombination.

To examine the repeatability of the mechanochromic response, Film 8 was stretched to 1000% strain at 100 mm min⁻¹, allowed to rest for 24 h, and then stretched again under identical conditions. After the fluorescence produced by the first stretching had fully faded over 24 h, yellow‐green fluorescence reappeared upon the second stretching cycle (Figure 4a). Moreover, the stress–strain curves of the first and second cycles were nearly identical (Figure 4h). These results demonstrate that the SBS films containing PS‑TASN‑PS exhibit reversible mechanochromic behavior with retained mechanical performance.

2.5. Stress Transfer and Mechanochemical Activation During Tensile Deformation

To clarify how stress transfer to the PS hard domains evolves during tensile deformation, the strain dependence of mechanochemical activation was investigated using Film 8 (10 wt% PS‐TASN‐PS_18k), which exhibited the highest radical generation among the blend films without observable PS‐TASN‐PS aggregation.

Figure 5a shows representative fluorescence images extracted from a video recorded during tensile deformation up to 1000% strain under 365 nm UV irradiation. Images corresponding to strains of 0%, 5%, 10%, 15%, 30%, 50%, 100%, 200%, 400%, 700%, and 1000% were selected. Negligible fluorescence was observed below the yield point, whereas the fluorescence intensity increased markedly after yielding and continued to increase with strain, indicating that TASN dissociation was progressively promoted.

FIGURE 5.

Strain‐dependent mechanochemical activation of Film 8 and proposed stress‐transfer mechanism in SBS/PS‐TASN‐PS blend films. (a) Photographs of Film 8 under 365 nm UV irradiation during stretching to different strains. (b) Stress–strain curve of Film 8 with the corresponding DAAN radical concentrations measured at each strain after subtracting the radical concentration present at 0% strain. The inset shows an enlarged view of the low‐strain region (0%–60%). Error bars represent the standard deviation of three independent samples (n = 3). (c) Schematic illustration of the proposed activation mechanism: enhanced stress transfer to the PS hard domains after yielding induces mechanochemical activation of localized TASN mechanophores.

To quantitatively evaluate this behavior, the amount of DAAN radicals generated at each strain was measured by EPR spectroscopy (Figure 5b). Because the in situ tensile‐EPR apparatus limits the maximum strain to 400%, films were instead stretched to the prescribed strain using a tensile testing machine, immediately immersed in liquid nitrogen after unloading to suppress radical recombination, and the gauge sections between the grips were subsequently excised for EPR analysis. To exclude contributions from radicals generated during sample preparation, the radical concentration present at 0% strain was regarded as originating from thermally generated DAAN radicals formed during annealing and was subtracted from all reported radical concentrations. EPR analysis revealed that the radical concentration remained low before the yield point but increased markedly after yielding and continued to increase with strain, suggesting that stress is increasingly transferred to the PS hard domains as deformation proceeds. This trend was consistent with the strain‐dependent increase in fluorescence intensity observed in Figure 5a. These results demonstrate that mechanochemical activation is strongly correlated with the onset of yielding.

Based on these findings, a schematic illustration of the proposed activation mechanism is shown in Figure 5c. At low strains, deformation is primarily accommodated by the PB soft domains, and only a limited amount of stress is transmitted to the PS hard domains. Consequently, the PS‐TASN‐PS chains localized in the PS hard domains experience insufficient force transfer to induce significant TASN dissociation. After yielding, however, the marked increase in mechanochemical activation suggests that applied stress is more effectively transferred to the PS hard domains, resulting in local deformation of the PS domains. Consequently, force is efficiently propagated along the PS‐TASN‐PS chains, leading to activation of the localized TASN mechanophores. These results demonstrate that efficient mechanochemical activation is triggered predominantly after yielding, when stress is effectively transferred from the PB soft domains to the PS hard domains. Consequently, deformation of the PB soft domains alone does not generate sufficient force to activate the TASN mechanophores.

3. Conclusion

In this work, we developed a versatile and facile post‑synthetic strategy for imparting mechanical responsiveness to existing block copolymers and elucidated the mechanochromic behavior of styrenic block copolymers blended with PS containing a TASN mechanophore. Tensile tests on SBS blended with PS‑TASN‑PS with various molecular weights and blend ratios demonstrated mechanochromism in all films. Detailed analysis revealed that, under the conditions employed in this study, the most efficient mechanochromic response was achieved when 10 wt% of PS‑TASN‑PS with an intermediate molecular weight (M _n = 17 760) was blended into SBS. Conversely, when maintaining the intrinsic mechanical properties of SBS is required, using higher molecular weight PS‑TASN‑PS at lower loadings was found to be advantageous.

The fluorescence generated upon tensile deformation decayed rapidly over time, suggesting fast recombination of DAAN radicals generated within the hard PS domains due to their close spatial proximity. Notably, after fluorescence had vanished, re‑stretching the films regenerated yellow‐green emission, confirming that SBS films blended with PS‑TASN‑PS possess reversible mechanochromic behavior.

Furthermore, strain‐dependent fluorescence observations and quantitative EPR measurements revealed that mechanochemical activation was negligible before yielding but increased markedly after the yield point. These results indicate that efficient activation of the localized TASN mechanophores is induced by stress transfer to the PS hard domains during large deformation.

Overall, this approach provides a general method for endowing block copolymer materials containing hard and soft domains with mechanochromic functionality in a post‑synthetic and facile manner. Importantly, the strategy enables the introduction of mechanical responsiveness without compromising the intrinsic properties of the host material, offering a practical pathway for expanding mechanoresponsive functionality across a broad range of polymer systems.

Author Contributions

Akira Takahashi: methodology, writing – review and editing, validation. Akira Kodaka: methodology, investigation, formal analysis. Hideyuki Otsuka: conceptualization, funding acquisition, supervision, resources, writing – review and editing, project administration. Kuniaki Ishizuki: conceptualization, methodology, investigation, writing – review and editing, writing – original draft, formal analysis, visualization.

Funding

JST CREST grant JPMJCR1991 from the Japan Science and Technology Agency (JST) and KAKENHI grant 26H02290 from the Japan Society for the Promotion of Science (JSPS).

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: adma74610‐sup‐0001‐SuppMat.pdf.

Data Availability Statement

The data that support the findings of this study are available in the Supporting Information of this article.