Multicolor recordable and erasable photonic crystals based on on-off thermoswitchable mechanochromism toward inkless rewritable paper

Mechanochromic photonic crystals are attractive due to their force-dependent structural colors; however, showing unrecordable color and unsatisfied performances, which significantly limits

their development and expansion toward advanced applications. Here, a thermal-responsive mechanochromic photonic crystal with a multicolor recordability-erasability was fabricated by

combining non-close-packing mechanochromic photonic crystals and phase-change materials. Multicolor recordability is realized by pressing thermal-responsive mechanochromic photonic crystals

to obtain target colors over the phase-change temperature followed by fixing the target colors and deformed configuration at room temperature. The stable recorded color can be erased and

reconfigured by simply heating and similar color-recording procedures respectively due to the thermoswitchable on-off mechanochromism of thermal-responsive mechanochromic photonic crystals

along with solid-gel phase transition. These thermal-responsive mechanochromic photonic crystals are ideal rewritable papers for ink-freely achieving multicolor patterns with high

resolution, difficult for conventional photonic papers. This work offers a perspective for designing color-recordable/erasable and other stimulus-switchable materials with advanced

applications.

Responsive photonic crystals (PCs) that can change their optical performances under diverse stimuli have attracted extensive attention due to their wide potential applications in

displays1,2,3,4, sensing5,6,7,8,9,10, printing11,12,13,14,15, anti-counterfeiting16,17,18, and optical devices19,20,21. Among them, mechanochromic photonic crystals (MPCs) have attracted

particular interest because they possess non-close-packing structures and hence can change their structural colors in response to forces, similar to the working mechanism of chameleon

skin22,23,24. Moreover, forces possess the merits of being easily available, low-power consumption, and applicable in large areas with high resolution. So far, various MPCs have been

prepared based on self-assembly, shear-induced assembly, two-step filling, and swelling close-packing or non-close-packing structures25,26,27. These works mostly focus on improving the

sensitivity and tuning range of structural colors of MPCs, while little attention was paid to expanding their advanced applications due to the lack of structure and composition design.

Several key challenges must be addressed to promote the rapid practical applications of MPCs. First, multicolor recordability, that is recording the changed colors of an MPC, is strongly

desired to fulfill the increasing low-power demand for colors. However, nearly all state-of-the-art MPCs need external forces to retain the changed structural colors which will return to the

original ones once the load is removed. This is majorly because the deformed MPC configuration cannot be fixed. Second, erasing recorded colors by easily operational stimuli without

environmental pollution is needed to realize good recyclability and green chemical industry. In addition, the color recordability and erasability should be repeatable. Third, multicolor

recording and fast color switching speed are required to achieve high efficiency and good tunability. Fourth, excellent stability of MPCs themselves under normal conditions, color recording,

and color releasing states is necessary to meet the needs of the practical application. The combination of all these intriguing features will lead to high-performance,

multicolor-recordable, fast-responsive, and long-term stable MPCs toward advanced applications.

To date, considerable methods and efforts have been devoted to realizing multicolor recordability on MPCs with inverse opal structures. Jiang et al. have prepared vapor-responsive

shape-memory polymers (SMPs)-based MPCs that enable unusual cold-programming and instantaneous shape recovery at the nanoscale28,29,30,31. The MPC can switch from brilliant color to a

recordable colorless state under pressure owing to the collapse of macropores, leading to the recording of monotonous colors. Although the monochrome color recordability and the use of

organic vapors will limit their practical applications, these works lay an important foundation for exploring color recordability. Wu and co-workers reported color recordability of

monochrome retroreflective color using an SMP-based micrometer-sized dome monolayer array32. Unfortunately, the unsatisfied color purity and on-off colors may limit their practical

applications. Wang’s group prepared mechanochromic and thermochromic PCs with close-packing structures based on the SMP-based core-shell particles33. Nevertheless, their applications might

be difficult because of the unsatisfied color tunability and the significant decrease in color saturation. Du’s group developed a colorless SMPs-based MPC with compression-induced collapsed

micropores and a heat-programming shape recovery degree34. Multicolor can be generated and recorded by region-selectively heating the MPCs to different temperatures (T) using lasers and

integrating a pre-printed photothermal layer. However, the unwanted diffusion colors caused by the inevitable thermo-delivery at boundaries between heated and unheated regions will restrict

the color purity. Yang and co-workers realize multicolor recording based on the elastoplastic deformation of the uncrosslinked photoresist film with inverse opal structures35. Nevertheless,

the recorded colors cannot be erased due to the permanent change of the structure, unfavorable for green and economic materials. Therefore, it remains a big challenge for reported MPCs to

simultaneously possess multicolor recordability, erasability, and good color purity.

Here, we report a thermal-responsive MPC (TRMPC, Fig. 1) showing untraditional multicolor recordability, erasability, and high color purity, prepared by the combination of the specific

phase-change material (PCM) of polyoxyethylene (40) nonylphenyl ether (PNE) and MPCs with both swellable non-closely packed structures and small refractive index contrast (Δn, Eq. 1,

Supplementary Note 1) between silica particles and surroundings. The uniform introduction of PCM into MPCs’ lattice leads to an on-off themoswitchable mechnanochromism and a small influence

on the color output, which is the key to realizing the above characteristics of TRMPCs. Different from the reported strategies, color recordability is accomplished in two processes: color

generation by pressing the TRMPC at T higher than the melting point (Tm) of the PCM and subsequently color fixation through a cooling down process. Multicolor can be recorded by adjusting

pressure with similar procedures. Interestingly, recorded colors exhibit excellent stability (>1 year) under normal conditions but can be easily erased in 2 s by simply heating the TRMPC to

a T higher than Tm to recover the TRMPC’s configuration. Compared to other stimuli, the use of forces and T for recording and erasing colors is efficient and convenient, which will

facilitate their practical applications. We demonstrate an inkless way to construct multicolor, high resolution, reconfigurable patterns by alternate region-selectively pressing the TRMPC

with hot stamps to obtain patterns and erasing patterns through heating. The ink-free writable TRMPC paper shows advantages in reducing the usage of conventional disposable papers and inks

and environmental pollution. This work offers a perspective for designing color-recordable and erasable MPCs and other stimulus-switchable materials and will promote their applications in

green printing, smart sensing, and low-power displays.

TRMPC transforms from solid to gel when T > Tm and its color changes (such as red to blue) when pressure is applied. After cooling to T  Tm.

TRMPCs (Fig. 2a) with thermoswitchable on-off mechanochromism were fabricated based on swelling MPC with non-closely packed structures and appropriately small Δn by the PCM with a high Tm.

Briefly, silica particles (167 nm, Supplementary Fig. 1) were dispersed in ethanol, di(ethylene glycol) ethyl ether acrylate (DEGEEA), and polyethylene glycol mono-phenyl ester acrylate

(PEGPEA) by sonication. After evaporating ethanol at 100 °C for 1 h, a liquid silica/DEGEEA/PEGPEA PC showing iridescent colors was obtained, suggesting the ordered packing of silica

particles in the mixture of DEGEEA and PEGPEA. This liquid PC was converted into solid MPC through UV-triggered polymerization. The volume fraction of silica particles (φs: 15–40%) is

designed much smaller than that (74%) of PC with a closely packed structure to construct a non-close-packing structure. The high content of elastic DEGEEA and PEGPEA combined with the

non-closely packed structure enables the mechanochromic properties. For easy discussion, we use fd and fp to represent the volume fraction of DEGEEA (φd) and PEGPEA (φp) relative to the

PEGPEA/DEGEEA mixture, respectively. Before optimizing the fd and fp, the effect of Δn (fp) on reflectivity was investigated without the effect of fp (Δn). Correspondingly, the results

demonstrated that reflectance is proportional to Δn but inversely proportional to fp (Supplementary Note 2). A large Δn and small fp will result in a high reflectance.

a Schematic illustration of the fabrication of the TRMPC. b–d The SEM images, microscope images, and reflection spectra of the b MPC, c TRMPC gel, and d TRMPC solid, respectively. e, f The

reflective signal switches of the TRMPC under alternative heating and cooling. g The stress-strain curves of the TRMPC solid and gel.

Then, MPCs (φs: 15%, Supplementary Fig. 2) with different fp were investigated to determine the optimized value. When fp = 0, the reflectance of MPC is only 19% because of the ultrasmall Δn

(0.010) between silica particles and DEGEEA. With fp increases from 0 to 0.3, Δn increases from 0.010 to 0.025 (Supplementary Fig. 3), leading to an increase in reflectance from 19% to 47%.

However, when fp further increases (>0.3), the reflectance decreases dramatically despite the increase in Δn, which can be attributed to the break of long-range order by the high fp.

In the aspect of colloidal assembly in acrylates, there is the lowest value (φth) of φs, over which silica particles can self-assemble into uniform long-range ordered structures. MPC shows

high and low order degree and thus reflectance when φth is lower and higher than φs, respectively. The φth of DEGEEA (φth-d) and PEGPEA (φth-p) is 10% and 25%, and the φth of the

PEGPEA/DEGEEA mixture (φth-mix) can be calculated by Eqs. 2–3. Supplementary Fig. 4 shows that the φth-mix increases as fp increases and φth-mix = 15% when fp = 0.345. These results mean

high reflectance can be obtained when fp  φs (15%). Thus, fp = 0.3 should be the optimized value for MPC possessing both high order degree and appropriate Δn.

Under the scanning electron microscope (SEM, Fig. 2b), the MPC (φs: 30%) shows long-range ordered and non-close-packing structures. The interparticle distance (Did) is measured to be 207 nm.

The surface-to-surface distance (Ds-s) between neighboring particles is 39 nm, exceeding the effective distance of van der Waals attraction. These results indicate the strong electrostatic

repulsion between silica particles is the major driving force for assembling silica particles into the non-closely packed structure36. The ζ-potential of silica particles in the

DEGEEA/PEGPEA solution is −43 mV, giving evidence of the above hypothesis. By introducing a trace amount of the ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide)

into the liquid silica/DEGEEA/PEGPEA PC solution to selectively screen the electrostatic force, the structural color disappears and only amorphous structures were obtained (Supplementary

Fig. 5), demonstrating the critical role of electrostatic force for ordered structure. The highly ordered structure of the MPC can efficiently and selectively diffract incident light,

resulting in a bright green MPC with an intense reflectance at 506 nm. The reflection wavelength (λ) can be calculated by Bragg’s law (Eq. 4), where m is the diffraction order. θ is the

angle between the reflected beam and the normal. ni and φi are the refractive index and volume fraction of each material (silica particles and acrylates) of the PC, respectively. The λ is

calculated to be 502 nm, in line with the value from reflection spectra. The MPC exhibits a good monochromaticity as demonstrated by the ultranarrow full width at half-maximum (FWHM, 10.0 

nm), much smaller than those (30–70 nm) of conventional MPCs37,38,39. We attribute this to the small Δn (0.025, calculated by Eq. 1) between silica particles and polymers, which

significantly suppresses incoherent scattering of light.

The non-close-packing structure enables us to adjust MPCs’ λ and colors covering most visible ranges by simply altering φs with the same particle size, more convenient than the conventional

way of regulating particle sizes. As presented in Supplementary Fig. 6, λ decreases from 622 to 476 nm and the corresponding color varies from red to blue when φs increases from 15 to 40%.

The increase in φs will cause a decrease in Did and thus λ according to Bragg’s law. These MPCs show high reflectance, brilliant colors, and small FWHM (7.2–12.2 nm), demonstrating their

remarkable color purity. However, further decreasing φs (5% and 10%) lead to negligible reflectance due to the lack of long-range order (Supplementary Fig. 7). A small φs will lead to a

too-large Did, which causes a dramatic decrease in electrostatic repulsion, giving rise to the disordered structures and thus neglectable reflectance. Therefore, φs of 15–40% should be a

good choice for fabricating MPC.

TRMPCs were fabricated by swelling the MPC in the PCM at a T > Tm. Here, MPC with φs of 30% and a λ of 506 nm is used for swelling. When soaked in the liquid PCM preheated to 80 °C, the PCM

swelled into the polymer network of the MPC, leading to the increase in Did (Fig. 2c) and thus λ over time (Supplementary Fig. 8). After reaching the swelling balance, the λ remains

constant, resulting in a red TRMPC organic gel with a λ located at 632 nm. The ratio of λ between TRMPC and MPC is 1.25, comparable to the ratio (1.22, Supplementary Fig. 9) of the

corresponding thicknesses, firmly proving that the increase in Did is the key to the λ redshift. The φs, φp, and φPCM (PCM’s volume fraction) of the TRMPC are 13.2%, 30.9%, and 55.9%,

respectively. The FWHM of the TRMPC gel is 15.9 nm, slighter larger than that of MPC due to the slight increase in Δn (0.038, Supplementary Note 1) caused by the PCM. After cooling down, the

TRMPC undergoes a phase transition from gel to solid state with negligible change on λ, while the reflectance is decreased by 35%. Despite the slight increase in Δn, the TRMPC exhibits no

obvious enhancement in incoherent light scattering compared to the MPC (Supplementary Fig. 10). This can be explained by that the TRMPC’s Δn (0.038) is still small, which can efficiently

suppress the incoherent scattering of light. Supplementary Fig. 11 shows that MPC and TRMPC show highly ordered structures without significant differences in order degree, demonstrating that

the order degree is well retained with negligible influence by the PCM.

Under SEM (Fig. 2d), a coarse layer of PCM was observed on the surface of the TRMPC solid, slightly hindering the output of reflective light, thereby resulting in a decrease in reflectance

and an increase in FWHM (21.1 nm). Despite these, both the TRMPC gel and solid exhibit brilliant colors and good monochromaticity. This characteristic is quite different from conventional

PCM-PCs showing on-off colors under different temperatures40,41,42,43. The movement of the PCM in the TRMPC is supposed to be limited by the good affinity between PCM and MPC (Supplementary

Fig. 12), leading to the slow and uniform crystallization of the PCM across the whole TRMPC and thus the bright color of the TRMPC solid. The thermal switching between the gel and solid

states is highly reversible (Fig. 2e). It takes ≈1 s for TRMPC to finish the phase transition (Fig. 2f), suggesting the high thermoswitchable speed. The λ and color of the TRMPC can be

adjusted by altering the φs of the MPC. As shown in Supplementary Fig. 13, the λ of TRMPC decreases from 764 to 576 nm and the corresponding color changes from invisible to red and yellow

when φs increases from 15% to 40%.

The TRMPC exhibits significantly different mechanical properties in gel and solid states (Fig. 2g). The TRMPC gel is elastic and easily stretchable under low stress, showing a maximal

stretching strain of 58% corresponding to a stress of 51.4 kPa. In dramatic contrast, during the stretching process, the TRMPC solid first experiences (1) reversibly elastic deformation

(strain: 0–12%, maximal stress: 847.7 kPa), and (2) then irreversibly plastic deformations (strain: 12–118%, stress: 792–848 kPa). The Young’s modulus of the TRMPC solid is 22.86 MPa, nearly

260 times higher than that of the TRMPC gel, firmly demonstrating that the deformability of the non-close-packing structure is greatly limited by the solid PCM at room temperature.

TRMPCs also can be fabricated by using long-chain-based PCMs. Here, other PCMs (Supplementary Fig. 14), including dodecanoic acid, myristic acid, palmitic acid, polyoxyethylene (23) lauryl

ether (PLE), polyoxyethylene (20) cetyl ether (PCE), and polyoxyethylene (100) stearyl ether (PSE) were used to fabricate TRMPCs based on similar fabrication procedures. Compared to acids,

TRMPCs were successfully fabricated when PLE, PCE, and PSE were used (Supplementary Fig. 15), which can be attributed to the different swelling behavior induced by their structure

difference.

Like the typical PCM (named PNE), the PLE, PCE, and PSE also possess long chains and can be swelled into the MPC’s polymer network with excellent swelling controllability. The swelling will

be self-stopped after reaching the balance state, resulting in TRMPCs with the desired optical performance. Compared to PLE and PCE, the longer chain of PSE restricts the swelling, causing a

smaller λ and tuning range of wavelength (Δλ = 112 nm, Supplementary Fig. 16). Different from PLE, PCE, and PSE, the acid-based PCMs with short molecular lengths can be easily and rapidly

swelled into the MPC’s polymer network, leading to uncontrollable swelling and redshift of λ. Even worse, reflectance decreases quickly to ~0 during the swelling process, probably due to the

break of long-range order by over-swelling. After cooling down, only white originating from the incoherent scattering of light by plenty of solid acids can be observed. These results

suggest that PCMs with long-chain are suitable candidates for preparing TRMPCs and the responsive temperature can be tailored by simply using PCMs with different melting points.

Compared to conventional MPCs, the TRMPC possesses a thermoswitchable on-off mechanochormism. When T > Tm, PCM melts, resulting in a TRMPC gel with highly sensitive mechanochromism (Fig. 

3a). The pristine TRMPC gel displays a red color with a λ located at 632 nm. The structural color gradually changes from red to blue (Fig. 3b) and the corresponding λ decreases from 632 to

440 nm (Fig. 3c, d) when the pressure increases from 0 to 13.9 kPa. The tuning range of the wavelength (Δλ) is 192 nm, similar to those of the best MPCs44. The sensitivity defined by

Δλ/pressure (nm/kPa) is calculated to be 13.8 nm/kPa, higher than the most sensitive MPC36,45,46. The large Δλ and high sensitivity can be attributed to increased Did caused by swelling,

which leaves more room for deformation. The continuous decrease in λ under pressure can be attributed to the gradual decrease in lattice distance (Fig. 3e). As presented in Fig. 3f, with a

13.9 kPa pressure loaded, the lattice distance decreases from 216 to 137 nm, which leads to the blueshift of λ according to Bragg’s law. The decrease in reflectance is due to the inevitable

disturbance of the order degree by mechanical forces. After releasing pressure, the configuration and structural color of TRMPC gel return to the pristine state instantly. The switching of λ

under 100 cycles of alternate 0 and 13.9 kPa (Fig. 3g) demonstrates the excellent reversibility of the TRMPC gel. These results verify the on-state mechanochromism of the TRMPC gel.

a Schematic illustration of the on-state mechanochromism of the TRMPC gel. b The microscope images and c reflection spectra of the TRMPC gel under different pressures. d The λ of the TRMPC

gel as a function of pressure. e The cross-sectional SEM images of TRMPC gel under the pressures of 0, 6.2, and 13.9 kPa, respectively. f The cross-sectional SEM images show the variation in

lattice distance. g The λ of the TRMPC gel as a function of alternate pressure and release. h Schematic illustration of the off-state mechanochromism of the TRMPC solid. i The microscope

images and j reflection spectra of the TRMPC solid under different pressures. k The λ of TRMPC solid as a function of pressure. l The cross-sectional SEM images of the TRMPC solid under the

pressures of 0, 6.2, and 13.9 kPa, respectively. m The cross-sectional SEM images show the variation in lattice distance of the TRMPC solid. n The wavelength of the TRMPC solid as a function

of time (under a constant pressure of 13.9 kPa).

Different from the gel, the TRMPC solid shows an off-state mechanochromism (Fig. 3h). When the T  Tm, the color turns to blue, and the corresponding λ changes to 482 nm due to the on-state

mechanochromism. The blue color was then fixed by cooling down along with the gel-solid phase transition once T  Tm. By heating, the blue TRMPC solid returns to the TRMPC gel displaying red

color and λ, like the pristine one. It takes only 1.8 s to erase the recorded color (Fig. 4c), substantiating the fast speed of color erasing. Owing to the high Tm (46 °C) of the PCM, the

recorded color exhibits excellent stability under a wide range of temperatures (Fig. 4d).

a Schematic illustration of the color recording and erasing of the TRMPC. b Recording a blue color on the red TRMPC gel by pressing (13.9 kPa) on and cooling down, and erasing the blue color

by heating (T > Tm). c 3D reflection spectra show the variation of the reflection signal of the TRMPC during the heating process. d The λ of the pressed TRMPC solid under different

temperatures. e The reflection spectra and f the corresponding microscope images of the TRMPC gel and solid under different pressures. g The switch of reflective signals of the TRMPC by

programmable color recordability and erasability.

The recorded color can be precisely adjusted by altering the pressure during the color generation process. As presented in Fig. 4e, f, desired λ and colors ranging from red to yellow, green,

and blue can be easily recorded by simply adjusting the pressure. The colors and corresponding λ of these recorded colors at the solid state are similar to those at the gel state and can be

retained for a long time (Supplementary Fig. 17), indicating the successful recording of diverse colors. Moreover, these recorded colors show constant hue and λ under diverse pressure

(Supplementary Fig. 18), further demonstrating that the TRMPC solid can resist external force by turning off the mechanochromic switch. These recorded colors can be easily erased by heating

the TRMPC solid to a T > Tm. In addition, these colors can be switched repeatedly or in a programmable way (Fig. 4g) by taking advantage of repeatable color recording and erasing. These

results proved that TRMPCs possess repeatable multicolor recordability and erasability. The thermoswitchable on-off mechanochromism of the TRMPC is the key to these interesting

characteristics; however, which is exceedingly difficult for conventional MPCs.

Further experimental results suggest that the percentage of the PCM plays an important role in color recordability. Here, the weight percentage of the PCM (WPPCM) can be easily adjusted by

altering the swelling time. As shown in Supplementary Fig. 19, WPPCM increases from 6.5% to 40.7% and the corresponding TRMPC’s λ increases from 509 to 630 nm can be easily obtained when

adjusting the swelling time from 1 to 25 min, respectively. Then, these TRMPCs were pressed to a desired wavelength (λ1 = 450 nm) at a T > Tm, followed by a cooling down process to record

the blue color. Afterward, the pressure was removed and the corresponding wavelength (λ2) was collected. In the absence of PCM, the MPC returns to its pristine λ instantly after releasing

the pressure because of its elastic structure. When the WPPCM is low (6.5 and 12.0%, Supplementary Fig. 20a, b), the insufficient solid PCM cannot fully fix the deformed configuration of the

TRMPC, leading to the partial recovery of λ (λ1  Tm, followed by cooling down till T  Tm. These results proved that multicolor recordability can be realized by stretching during the color

change process.

It should be noted that the multicolor recordability and erasability of the TRMPC are quite different from SMP-based structural color materials in the aspect of material fabrication,

structure design, color-generating mechanism, optical properties, color recordability-erasability, and printing patterns (Supplementary Note 3 and Supplementary Table 1).

Conventional papers and prints have been widely used in our daily lives, especially in communication and information storage and spreading. Nevertheless, large-scale paper and ink

consumption leads to significant environmental pollution, involving deforestation, toxic gases, water, and solids. Developing rewritable papers that can record information without using ink

should be an ideal solution to address the above issues. In this work, the repeatable color recordability and erasability inspire us that the as-prepared TRMPC is indeed an excellent

rewritable paper capable of constructing patterns in an inkless way.

In our design, the pattern was printed on a TRMPC by region-selective pressing the TRMPC gel using a stamp, followed by a cooling down process (Fig. 5a). At pristine, the TRMPC gel (T > Tm)

shows a uniform red color (Fig. 5b) with a corresponding λ located at 632 nm (Supplementary Fig. 23). Afterward, a stamp with a bird pattern was pressed on the TRMPC and a pressure of 12.3 

kPa was applied to obtain a blue-colored pattern. The stamp was removed after cooling down (T  Tm owing to the on-off switch of mechanochromism. The recovered TRMPC exhibits a similar red

color and λ to the pristine state, indicating a patterning-erasing cycle (Supplementary Movies 1–2) has a negligible effect on the optical properties of the TRMPC. The color recording and

erasing can be repeatedly realized more than 100 times (Supplementary Fig. 25). Using similar procedures, a variety of multicolor (Fig. 5e) and high-resolution (Fig. 5f) patterns can be

repeatedly reconfigured on the same TRMPC. Supplementary Fig. 26 suggests that the minimum pressure for generating an effective pattern should be around 2.2 kPa. The line and point

resolutions that we can obtain are measured to be 7.2 and 5.4 μm (Supplementary Fig. 27), respectively. We believe that better resolution can be realized with more elaborate stamps. In

addition, the pattern can be selectively erased through heating the desired region. As presented in Fig. 5g, a crystal pattern was firstly printed on the TRMPC. The l of the crystal was

selectively erased by simply covering a preheated copper metal (T > Tm) on it. Similarly, each letter can be removed in a programmable way.

a Schematic illustration of the processes of constructing and then reconfiguring patterns in an inkless way. b–e Digital photos of the b pristine, c patterned, d pattern-erased, and e

pattern-reconfigured TRMPC papers. f The reconfiguration of high-resolution patterns. g Schematic illustration of erasing pattern selectively and the corresponding digital photos.

Compared to conventional inkless rewritable papers (CIRPs) based on the oxide and redox of organic dyes47,48,49,50, TRMPC paper possesses various advantages: (1) multicolor patterns can be

easily achieved using TRMPC paper, while only mono-color patterns can be obtained based on CIRPs; (2) TRMPC paper-based pattern exhibits anti-photobleaching and stable colors, in dramatic

contrast to the CIRP-based pattern suffering from photobleaching; and (3) the printing of pattern is considerably efficient and convenient without requiring complex instrument. The utility

of TRMPC paper instead of conventional papers will facilitate not only the reduction of environmental pollution but also the applications in green printing, sensors, optical devices, and so

forth.

In summary, TRMPCs capable of recording and erasing multicolor were fabricated by (1) self-assembling silica particles into acrylates, followed by a photopolymerization process to achieve

MPC with non-close-packing structures and subsequently (2) swelling the MPC into the liquid PCM with a high Tm. The loading of PCM into the polymer network of MPC endows TRMPC T-dependent

on-off mechanochormism along with the solid-gel phase changing, contributing to the color recordability and erasability, respectively. The color recording was realized by two processes:

color generation by pressing or stretching the TRMPC with on and highly sensitive mechanochromism at T > Tm and color fixation by cooling down the pressed TRMPC to turn off the

mechanochromism till T  Tm that allows the recovery of the TRMPC to the pristine (relax) state. The color recordability and erasability make TRMPC an ideal candidate as an inkless rewritable

paper, on which multicolor color and high-resolution patterns have been repeatedly printed. Compared to conventional papers, the usage of TRMPC paper will significantly reduce deforestation

and environmental pollution thanks to no use of inks and disposable papers. The on-off thermoswitchable mechanochromism will facilitate the applications of TRMPC in green printing,

low-power display, and next-generation sensors.

Di(ethylene glycol) ethyl ether acrylate (DEGEEA), polyethylene glycol mono-phenyl ester acrylate (PEGPEA), poly(propylene glycol) acrylate (PPGA), poly(ethylene glycol) diacrylate (PEGDA,

Mn: 250), pentaerythritol tetraacrylate (PETA), polyoxyethylene (40) nonylphenyl ether (PNE, the typical PCM, C15H24O(C2H4O)n, Mn: 1982), polyoxyethylene (23) lauryl ether (PLE,

(C2H4O)nC12H26O, Mn: 1198), polyoxyethylene (20) cetyl ether (PCE, HO(CH2CH2O)20C16H33, Mn: 1124), polyoxyethylene (100) stearyl ether (PSE, C18H37(OCH2CH2)nOH, Mn: 4670), dodecanoic acid

(CH3(CH2)10COOH), myristic acid (CH3(CH2)12COOH), palmitic acid (CH3(CH2)14COOH), and 2-hydroxy-2-methylpropiophenone (photo-initiator, 96%) were obtained from Sigma-Aldrich.

1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMBTF, 97%) was purchased from Aladdin. All the chemicals were used as received without further purification.

Briefly, silica particle powders (167 nm, 0.03 cm3) were dispersed in a mixture of ethanol (0.5 mL), PEGPEA (0.021 mL), and DEGEEA (0.049 mL) containing 5% photo-initiator by sonication. The

mixed solution was heated in an oven at 100 °C for 1 h and approximately 0.1 mL precursor solution showing iridescent color was obtained. This precursor solution (0.03 mL) was then

sandwiched between two substrates separated by 0.09 mm and converted into a green MPC after irradiating by UV light (365 nm, 4.8 mW cm−2) for 3 min. The distance between the UV light source

and precursor solution is about 15 cm.

Typically, the red TRMPC was achieved by swelling the green MPC with excessive PCM (5 mL) at 80 °C for 1 h to reach the swelling balance state and wiping off the redundant PCM. The TRMPC is

an organic gel at T > Tm but solid at T