Chemically Interconnected Thermotropic Polymers for Transparency

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Chemically Interconnected Thermotropic Polymers for Transparency-Tunable and Impact-Resistant Windows Cheng Zhang, Heng Deng, Stuart Kenderes, Jheng-Wun Su, Alan Whittington, and Jian Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19740 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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ACS Applied Materials & Interfaces

Chemically Interconnected Thermotropic Polymers for Transparency-Tunable and Impact-Resistant Windows Cheng Zhang,† Heng Deng,† Stuart M. Kenderes,‡ Jheng-Wun Su,† Alan G. Whittington,‡ and Jian Lin*,† †Department

of Mechanical & Aerospace Engineering and ‡Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, United States

KEYWORDS: chemical interconnection, tough interfacial bonding, thermotropic polymers, impact-resistant, smart windows

ABSTRACT Thermotropic polymers with the capability of thermally tuning transparency are widely applied in smart windows and energy-saving windows, playing a critical role in enhancing comfort level and energy efficiency of indoor spaces. Usually, thermotropic polymer systems are constructed by physically dispersing phase transition materials in transparent hosting materials. However, bad interfaces universally exist in these systems, resulting in poor mechanical properties, weak interfaces to substrates, or bad long-term stability. Herein, we demonstrate a novel chemically interconnected thermotropic polymer, which is obtained by reacting dodecanedioic acid (DDA) with glycerol. In the system, some of DDA molecules were crosslinked to form a polyester network, poly(glycerol-dodecanoate) (PGD). Other grafted but non-crosslinked DDA molecules form semicrystalline domains which possess a solid-liquid phase transition within the PGD matrix. The phase transition offers the resulting hybrid materials with tunable optical transparency. The PGD1 ACS Paragon Plus Environment

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DDA system shows stable performance after 100 heating-cooling cycles. In addition, when applied for window coating, it results in tough interfacial bonding to glass substrates with toughness of > 6910 J m-2 below its transition temperature and > 135 J m-2 above its transition temperature. It increases the impact-resistance of the window by multiple times. KEYWORDS: chemical interconnection, tough interfacial bonding, thermotropic polymers, impact-resistant

1. INTRODUCTION A thermotropic polymer system, which is usually composed of a hosting matrix imbedded by a phase transition material (PTM), possesses a temperature-dependent light scattering property.1-2 The thermotropic effect resulting from such a system includes two modes.3 In the first mode (Mode 1), below the phase transition temperature (Ttrans) the refractive indices of the PTM and the matrix are almost identical; so the system exhibits a transparent state. When the temperature rises above Ttrans, the refractive index of the PTM is changed while the index of the matrix remains the same. The mismatch of the refractive indices causes light scattering. Thus, the system is transformed from a transparent state to a translucent state. In the second mode (Mode 2), the system exhibits a reverse transition from a translucent state to a transparent state as the temperature increases above Ttrans. Because below Ttrans the refractive indices of both materials are mismatched, leading to strong light scattering. Above Ttrans they become almost identical, thus minimizing the light scattering. Nevertheless, both modes have their widespread applications: the first mode has been applied as intelligent solar control coatings for saving energy and overheating protection;2-8 the second mode has been applied in privacy windows, displays, thermal sensors, and thermal memory all-optical devices.4, 9-12

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Traditional polymers based thermotropic systems include thermotropic hydrogels, thermotropic polymer blends, and thermotropic systems with fixed domains (TSFD). 2, 12-16 These thermotropic polymer systems are built by dispersing phase transition compounds (e.g., fatty acids or liquid crystals) or phase transition polymers [e.g., poly (N-isopropyl acryl amide) or hydroxypropyl cellulose] into polymer matrices or liquid systems (e.g., water), respectively. These physically dispersing systems usually have bad interfaces either between the dispersion and the matrix, or between the thermotropic system and the supporting substrates (e.g., glass). The aggregation of dispersed phases lowers the mechanical strengths of the matrix, such as impact toughness. While a high toughness is critical for practical applications (e.g. smart windows), it has been paid less attention in the past research.17 In addition, it induces stress concentration in the interfaces under loading, thus degrading interfacial bonding to the supporting substrates.2 Moreover, it also lowers the long-term stability of these systems because the physical dispersion cannot immobilize the possibly involved non-stable components.18 Herein, we report a novel thermotropic polymer system where PTMs and matrices are chemically interconnected. The chemical interconnection provides the system with robust mechanical properties, tough interfacial bonding to substrates, and long-term stability. Specifically, a fatty acid, dodecanedioic acid (DDA), is selected as the PTM which possesses the solid-liquid transition (i.e., melting and crystallization). First, the DDA will be mixed and reacted with glycerol through esterification reaction. The condensation polymerization of DDA and glycerol gradually occurs and produces a crosslinked polyester network, poly(glycerol-dodecanoate) (PGD). The crosslinked PGD networks form amorphous domains, while the grafted but un-crosslinked DDA molecules remain as the side chains of the PGD. They form semi-crystalline domains in the PGD matrix. These two chemically interconnected domains possess mismatched refractive indices



before the phase transition (i.e., melting) of the DDA, exhibiting a translucent state. After the phase transition the refractive index of DDA domains is very close to that of the PGD domains, thus exhibiting a transparent state. Therefore, this system owns Mode 2 thermotropic effect, i.e., it transforms from a translucent state to a transparent state when temperature rises above Ttrans of the system. Furthermore, as a coating material for windows the PGD-DDA system shows tunable optical transparency, long-term stability, tough bonding to the windows, and high resistance to impact. It shows a stable transmittance tunability in a 100-cycle test. The bonding toughness reaches > 6910 J m-2 below Ttrans, and > 135 J m-2 above Ttrans. As a research highlight, with only 0.2 mm thick coating, the impact resistance of a bare glass was improved by 4 times, while 1 mm thick PGD coating resulted in > 10 times improvement in the impact resistance without failure. The demonstrated PGD-DDA polymeric system presents a new strategy for fabricating thermotropic polymers, which would be able to be applied to other thermotropic systems to improve the mechanical strengths and interfacial bonding.


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Figure 1. Schematics of (a) esterification reaction and (b) condensation polymerization of glycerol and dodecanedioic acid (DDA): (i) raw materials; (ii) prepolymer; (iii) chemically interconnected PGD network domains and DDA domains.

2. RESULTS AND DISCUSSION 2.1. Polymer synthesis. The esterification reaction occurs between a hydroxyl group in the glycerol and a carboxyl group in the DDA, producing an ester bond and a water molecule (Figure 1a). As the glycerol is a trihydric alcohol and DDA is a dicarboxylic acid, any hydroxyl group on a glycerol molecule would have a chance to react with any carboxyl group on a DDA molecule to be polymerized into a PGD polymer network. As shown in Figure 1b, a two-step method was utilized to synthesize PGD. Specifically, the raw materials (glycerol and DDA) were reacted at 120 ℃ under nitrogen for 24 h to prepare the prepolymer (Figure 1b.i). In this step, the glycerol and DDA were esterified to form linear chains (Figure 1b.ii). Little or no crosslinked polymer network was formed in this step. In the following curing step, the prepolymer was cured at 120 ℃ under vacuum for different durations to make the linear chains be crosslinked to a polymer network (Figure 1b.iii). In the processes, different reaction durations and reaction temperatures were utilized to tune the properties of the resulting PGD-DDA thermotropic polymers.

2.2. Material characterizations. The morphology of a representative PGD film (cured at 120 ℃ for 48 h) was characterized by scanning electron microscopy (SEM, Figure S1). Both surface and cross-section areas of the film show uniform and compact morphologies without pores or aggregations. Structural evolution of the PGD during synthesis was characterized to confirm the proposed reaction mechanism. Fourier transform infrared (FTIR) spectra were obtained and



plotted in Figure 2a. Besides two peaks appearing in a range of 2850-2950 cm-1 representing the ubiquitous alkyl C-H stretch, there are three characteristic peaks: (i) O-H (3200-3600 cm-1) from hydroxyl groups, (ii) C=O (1690-1780 cm-1) from the carboxyl groups or ester bonds, and (iii) CO (1020-1250 cm-1) from the carboxyl groups or ester bonds.19 The raw DDA shows a C=O stretch (1692 cm-1) and a C-O stretch (1182 cm-1) arising from the carboxyl group. In the prepolymer, because of the addition of glycerol, a broad O-H stretch mode (3475 cm-1) from the hydroxyl group appears. The peaks of C=O and C-O are shifted from 1692 to 1732 cm-1 and from 1182 to 1168 cm-1, respectively, due to the formation of ester bonds. As the curing time increases from 24 to 72 h, the intensity of O-H decreases while the intensities of C=O and C-O increase, indicating that the hydroxyl groups are consumed to produce the ester bonds.

Figure 2. (a) FTIR spectra of DDA, prepolymer, and PGD samples cured for 24-72 hours. (b) DSC curves of a prepolymer and crosslinked PGD cured for 24 h. (c) Latent heat and transition temperature (Ttrans) of prepolymer (0 h) and PGD samples cured for 24-72 hours. (d) Stress-strain curves of PGD samples cured for 48-72 hours, measured at 55 ℃. (e) Young’s modulus and crosslinking density derived from (d). 6 ACS Paragon Plus Environment

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Differential scanning calorimetry (DSC) was used to study the thermal property of the obtained materials. A DSC curve of a representative prepolymer sample scanning from -20 to 80 ℃ is shown in Figure 2b. We can see that there is an endothermic peak appearing at 43.74 ℃ and no obvious upward shift of the baseline, indicating that the endothermic transition is melting rather than a glass transition.20 This suggests that the dominant structure of the prepolymer is semi-crystalline rather than amorphous. The peak position at 43.74 ℃ is defined as the melting temperature (Tmelt) which is also the transition temperature (Ttrans) for the phase transition.21 The DSC curves of PGD samples cured for 24 hours (Figure 2b) and samples cured for 48 to 72 hours (Figure S2) are similar to that for the prepolymer, illustrating that the major structures of PGD polymers are all semicrystalline even though cured for up to 72 hours. The Ttrans and the latent heat (the integral area of the peak) of the prepolymer (0 h) and PGD cured for 24-72 hours are plotted as function of the curing time (Figure 2c). Both of them decrease with the curing time, showing values reducing from 79.3 J/g (at 0 h) to 41.0 J/g (at 72 h) and from 43.7 ℃ (at 0 h) to 31.8 ℃ (at 72 h), respectively, which can be attributed to the reduction of semi-crystalline domains. This means that more DDA molecules are crosslinked to the PGD network and the areas of the DDA formed semi-crystalline domains are decreasing with the reaction,22 confirming our proposed reaction process. We also studied the crosslinking density of the polymers. The crosslinking density (n) represents the number of network chain segments per unit volume (mol m-3), which can be estimated from the theory of rubber elasticity:23-24 n = E(3RT)-1

(1)

where E is the Young’s modulus (Pa) at a rubber state, R is the universal gas constant (8.3144 J mol-1K-1), and T is the absolute temperature (K). First, we performed the tensile test at 55 ℃ (above



Ttrans) to obtain the modulus at the rubber state.25 It should be noted that the prepolymer and PGD samples cured for 24 h and 36 h were melted to fluids at 55 ℃ (Figure S3), indicating their very low crosslinking densities. The stress-strain curves of samples with curing time of 48 h, 60 h, and 72 h were plotted in Figure 2d. The curves only show elastic deformation without plastic deformation, suggesting rubber states of these three samples. As the curing time increases from 48 h to 72 h, the fracture strain decreases from ~120 % to ~45 %. Young’s moduli were calculated by linearly fitting the elastic deformation regions (Figure 2e). The modulus significantly increases with the curing time, from 0.12 MPa (at 48 h) to 2.9 MPa (at 72 h). As the crosslinking density is proportional to the modulus (Eq. 1), the crosslinking density also significantly increases from 14.7 mol m-3 to 354.5 mol m-3 as the curing time increases from 48 h to 72 h. We also measured stressstrain curves of these samples at 21 ℃ (below Ttrans) (Figure S4a). In contrast, the Young’s modulus decreases with the curing time from 60.4 MPa (at 48 h) to 1.53 MPa (at 72 h) (Figure S4b). The reason is that below Ttrans the DDA domains are in a semi-crystalline state and can act as resistance to external loadings. As curing time increases the number of the DDA domains decreases, leading to reduced mechanical strength.

2.3. Thermotropic properties. Besides understanding the structures of the PGD-DDA polymeric systems, we investigated their thermotropic properties by using the representative samples that are cured for 48 h (resulting Ttrans is 39.1 ℃). The PGD films are originally translucent at room temperature (21 ℃, below Ttrans) and become transparent after being heated to 55 ℃ (above Ttrans) on a hot plate (Figure 3a). As shown in Figure 3b, the mechanism is that below Ttrans the noncrosslinked semi-crystalline DDA is solid with a refractive index mismatch with the PGD network, resulting in light scattering. When the temperature is above Ttrans, the DDA is melted and the


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refractive index mismatch is lowered, thus alleviating the light scattering.26 Moreover, the transition is fully reversible. The transition and recovery processes are shown in Movie S1 and Movie S2, respectively. The transition process when heating temperature is 55 ℃ takes ~ 36 s, while the recovery process takes longer, reaching ~ 82 s. The characteristic transmittances and haze values at 550 nm for a 1 mm thick PGD film were measured by UV-vis spectroscopy (Figure S5). The transmittance-temperature curve (Figure S6) reveals that the PGD film keeps a constant transmittance (75%) below Ttrans. It is changed to 94% right above Ttrans (39.1 ℃). Then it maintains almost constant from 45 to 75 ℃. The haze value at 21 ℃ (below Ttrans) is 57.32% while the value at 55 ℃ (above Ttrans) is 0.68%. The negligible haze value at the transparent state is a merit for practical window applications,4 while the big difference of haze values in the translucent and transparent states indicates a large tunability in the optical property of the PGD system. A heating (55 ℃)-cooling (21 ℃) cycling test27 was performed to demonstrate the long-term stability of the thermotropic PGD-DDA system. As shown in Figure S7, the characteristic transmittances at 550 nm below Ttrans and above Ttrans are almost constant (75% and 94%, respectively) after 100 cycles, indicating a great cycling stability.



Figure 3. Thermotropic properties of PGD. (a) Photographs and (b) schematics of a PGD film below Ttrans (at 21 ℃) and above Ttrans (at 55 ℃). Scale bars: 1 cm. (c) UV-vis-NIR transmittance spectra of a 1 mm thick PGD sample cured for 48 h, measured below Ttrans and above Ttrans. (d) Optical transmittances measured at 550 nm wavelength for PGD samples with 1 to 5 mm thicknesses measured below Ttrans and above Ttrans and their corresponding contrasts. (e) Optical transmittances measured at 550 nm wavelength for 3 mm PGD samples cured for 24-72 hours measured below Ttrans and above Ttrans and their corresponding contrasts.

Figure 3c shows the UV-vis-NIR spectra (in the wavelength range of 200 to 1000 nm) of a representative PGD film with 1 mm in thickness below and above Ttrans (39.1 ℃). Below Ttrans, the transmittance of the sample increases with the wavelength and the characteristic transmittance at 550 nm is ~ 75%. Above Ttrans, the film becomes transparent and the characteristic transmittance at 550 nm is ~ 94%, resulting in a transparency to translucency contrast of 1.25 at 550 nm. Moreover, the transparency of the PGD films shows a wide tunability in the wavelength range 10 ACS Paragon Plus Environment

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from 300 nm to 1000 nm, which is a typical feature for thermotropic polymer systems.28-29 Their characteristic transmittances below Ttrans and above Ttrans vary with the film thickness changing from 1 mm to 5 mm (Figure S8, Figure 3d). Both of them decrease with increased thickness, while their contrast increases with thickness from 1.25 for 1 mm thick film to ~2 for 5 mm thick film. The transmittance is also changed with the curing time. The UV-vis spectra of the PGD films with variable curing time were measured below Ttrans and above Ttrans (Figure S9). The samples cured for 24 h and 36 h were excluded because they were melted to fluids at 55 ℃. Their characteristic transmittances of the films at both below Ttrans and above Ttrans increase with the curing time (Figure 3e). However, their corresponding contrasts decrease from 1.65 to 1.07 as the curing time increases from 48 h to 72 h. Because as the curing time increases the number of the DDA domains in the PGD network decreases, leading to reduced refractive index mismatch below and above Ttrans. We found that the thermotropic properties of the PGD film can be further tuned by changing the curing temperature (Tcuring). A higher Tcuring reduces semi-crystalline DDA content and enhances crosslinking density. To validate the hypothesis, we prepared a series of PGD samples with curing time of 48 h under varied Tcuring from 110 ℃ to 150 ℃. Figure 4a shows Ttrans and latent heat collected from DSC curves (Figure S10). Basically, a higher Tcuring results in a lower Ttrans and smaller specific latent heat capacity. Specifically, as Tcuring rises from 110 ℃ to 150 ℃, both Ttrans and latent heat drop, from 43.6 ℃ to 20.4 ℃ and from 63.5 J/g to 29.1 J/g, respectively. These phenomena confirm that semi-crystalline DDA domains decrease as Tcuring increases.



Figure 4. Thermotropic properties tuned by curing temperature (Tcuring). (a) Latent heat, transition temperature (Ttrans), (b) Young’s modulus (measured at 55 ℃ above Ttrans) and crosslinking density, and (c) optical transmittance and contrasts at wavelength of 550 nm for PGD samples cured at temperatures ranging from 110 to 150 ℃. The measurements were taken at 21 ℃ (below Ttrans) and 55 ℃ (above Ttrans).

Figure 4b is the Young’s modulus derived from stress-strain curves measured at 55 ℃ (Figure S11a) from which corresponding crosslinking densities can be determined by Eq. 1. Both the Young’s modulus and the crosslinking density increase exponentially with Tcuring. When Tcuring is 110 ℃, the PGD sample shows a very low modulus (0.002 MPa) and a very low crosslinking density (0.24 mol m-3). When it is increased to 150 ℃, the Young’s modulus is amplified by 2100 times to 4.2 MPa and the corresponding crosslinking density is magnified to 513.4 mol m-3. The tensile tests conducted at 21 ℃ also prove this hypothesis (Figure S11b-c), which show that Young’s modulus decreases from 109.1 MPa to 3.2 MPa as Tcuring increases from 110 ℃ to 150 ℃. Interestingly, 150 ℃-PGD shows only elastic deformation behaviors because it is already in the transition state at 21 ℃ due to a low Ttrans of 20.4 ℃ (Figure 4a). The reason for this change is that as Tcuring increases more DDA molecules are crosslinked to form PGD networks.


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The UV-vis spectra of these PGD samples with 3 mm thickness were measured at 21 ℃ and 55 ℃ (Figure S12a-b) and their characteristic transmittances as well as their corresponding contrasts were plotted in Figure 4c. The transmittance measured at 21 ℃ linearly increases from 37.9% (Tcuring =110 ℃) to 93.0% (Tcuring =150 ℃), while the transmittance at 55 ℃ slightly increases with Tcuring from 85.6% (Tcuring =110 ℃) to 93.6% (Tcuring =150 ℃). It can be attributed to the fact that a higher temperature accelerates the curing reaction, thus more DDA molecules are crosslinked into the PGD network, and fewer remain as light scattering centers. In addition, as Tcuring increases from 110 ℃ to 150 ℃, the contrast significantly decreases from 2.3 to 1.0. It is interesting that the transmittances of 150 ℃-PGD measured at 21 ℃ and 55 ℃ are identical. The reason is that the Ttrans of 150 ℃-PGD is 20.4 ℃ and it is already in the transition state at 21 ℃ (Figure S12c). In summary, these results indicate that the thermotropic properties of the PGD polymer systems can be finely tuned by changing the curing temperature.

2.4. Application in transparency-tunable windows with tough bonding. As a demonstration, PGD prepolymer was first coated on an air plasma-treated glass slide (1 mm thick). After cured for 48 h at 120 ℃, a 1 mm thick PGD layer (Ttrans = 39.09 ℃) was obtained. As shown in Figure 5a, at 21 ℃ (below Ttrans) the PGD coated glass is translucent. When it is heated to 55 ℃ (above Ttrans), the coated glass becomes transparent. Furthermore, the transparency can be controlled by electrothermally heating indium tin oxides (ITO) glass (Figure S13). At room temperature, it is translucent. After a 10 V voltage is applied for ~52 seconds, the coated ITO glass gradually becomes transparent. A thermal camera confirms that the temperature of the ITO glass is higher than the Ttrans of the coated PGD.



Figure 5. PGD coatings on glass. (a) Photographs of PGD-coated glass below Ttrans (at 21 ℃) and above Ttrans (at 55 ℃). (b) Curves of interfacial toughness below Ttrans (at 21 ℃) and above Ttrans (at 55 ℃). (c) Photographs of samples in the end of peeling tests below Ttrans (at 21 ℃) and above Ttrans (at 55 ℃). (d) Schematic showing mechanism of chemical bonding between PGD and glass.

One critical requirement for window coating is formation of a tough bonding to the substrate. We found that the PGD coatings have extremely high bonding toughness to the glass slides. A standard 180°-peeling test30 was performed to measure the interfacial toughness of 1 mm thick PGD films obtained by curing at 120 ℃ for 48 h on the glass substrates. The free surface of a PGD film was attached to a thin stiff polyimide film with ~125 µm in thickness to prevent the elongation of the PGD film along the peeling direction.31 The measured steady-state peeling force or maximum peeling force divided by width of the PGD layer is defined as the interfacial toughness of the hybrid.32-33 Movie S3 and S4 recorded the peeling tests below Ttrans (at 21 ℃) and above Ttrans (at 55 ℃), respectively. As the PGD films were straightened, the loading forces increased with the displacements (Figure 5b). However, before the PGD films started to be peeled off, the 14 ACS Paragon Plus Environment

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peeling tests were stopped due to the fracture of glass substrate (below Ttrans) or the fracture of the PGD films (below Ttrans) (Figure 5c). These results indicate that the mechanical strength of the interface is higher than those of the individual bonding components (PGD films and glass substrates). The nominal interfacial toughness under these fracture conditions was still very high, i.e., 6910 J m-2 below Ttrans and 135 J m-2 above Ttrans. The former was higher than the latter because the PGD film becomes soft after the phase transition. Compared with recent related thermotropic polymers-variable substrates33-35 and glass-variable polymers36-37 bonding systems (Table S1), the value of 6910 J m-2 is three times of the highest value of 2200 J m-2 reported in literature for the PVA hydrogel-glass interface.37 Two possible reasons contribute to the tough bonding. First, the chemical interconnection (ester bond) between the phase transition domains (non-crosslinked DDA) and polymer matrix (PGD network) eliminates the stress concentration in the interfaces which normally exist in traditional physically dispersed systems. Second, glass surface has abundant hydroxyl groups, especially after plasma treatment,38 which can react with the carboxyl groups in the PGD to form covalent bonds (Figure 5d).



Figure 6. Impact resistance of PGD-coated glass. (a) Schematic of the impact measurement setup. (b) Impact failure heights of a bare glass slide with 1 mm in thickness and PGD-coated glass below Ttrans (at 21 ℃) and above Ttrans (at 55 ℃) with 0.2 mm and 1 mm in thickness. Photographs of (c) a failed 0.2 mm PGD-coated glass slide (below Ttrans) and (d) a 1 mm PGD-coated glass (above Ttrans) after 110 cm height impact testing. Scar bars: 5 mm.

2.5. Impact resistance. Besides tough bonding, the chemically interconnected structure endows the thermotropic PGD-DDA coatings with an excellent impact-resistant property. To demonstrate this, PGD coated glass was subjected to an impact test26 by dropping a spherical steel ball (weight = 11.9 g, diameter = 14.2 mm) to the surfaces of the targets (Figure 6a). The minimum drop heights required for failure were measured, for a bare glass slide, and glass slides coated with 0.2 mm and


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1 mm thick PGD polymers (cured at 120 ℃ for 48 h, Ttrans = 39.09 ℃), and plotted in Figure 6b. The bare glass slide was broken into pieces at a drop height of only 10.5 cm (Movie S5). When it was coated with a 0.2 mm PGD film, the minimum height required increases to 41.5 cm (below Ttrans) or 45.7 cm (above Ttrans). No debris was observed when the slide was broken (Figure 6c & Figure S14a). When the thickness of the PGD films increase to 1 mm, the coated windows can successfully resist the impact from the steel ball at a height of > 110 cm, with the impact only leaving dents on the PGD coatings (Figure 6d & S14b). The testing process is shown in Movie S6. The impact-resistant property of the chemically interconnected PGD systems is much better than a recently reported hybrid system, wax/polydimethylsiloxane (PDMS),26 which endows a coated glass slide with only 2.5 times failure height compared to a bare glass slide. These results demonstrate that the PGD coatings can significantly absorb impact energy and protect the attached glass substrates.

3. CONCLUSION In summary, we demonstrate a new chemically interconnected thermotropic polymer system for transparency-tunable and impact-resistant coating on windows. The system includes grafted and non-crosslinked DDA semi-crystalline domains which act as the phase transition domains in the crosslinked PGD amorphous network. The melting/recrystallization of DDA eliminates/induces its refractive index mismatch to that of the PGD network, resulting in an optical transparency transition from a translucent state and a transparent state before and after the phase transition. In addition, the thermotropic PGD system forms a tough bonding to glass when applied for transparency-tunable windows. Another important attribute of the thermotropic PGD-DDA system is that the chemical interconnection results in long-term stability and strong interfacial bonding.



Such a unique structure empowers the coated glass substrates the high impact resistance. We envision that the demonstrated chemical interconnection strategy illustrated by this example would pave a new route to enhancing mechanical and interfacial properties of other thermotropic systems, leading to widespread applications in intelligent solar control coating, privacy windows, displays, and optical/thermal sensors.

4. EXPERIMENTAL SECTION 4.1. Polymer synthesis. The PGD was synthesized by a recipe modified from the previously reported one.25, 39-42 In details, dodecanedioic acid (DDA, 99%, Alfa Aesar) and glycerol (99+%, synthetic, ACROS Organics) were purchased from Thermo Fisher Scientific and used without further purification. They were weighed with a 1:1 molar ratio and put into a three-necked flask for reaction. The flask was heated to 120 ℃ by oil bath with magnetic stirring. After DDA was melted, nitrogen gas was bubbled into the solution. After 24 h reaction, the liquid prepolymer was casted to aluminum dishes or targeted glass slides. The dishes/glass slides with the prepolymer were then transferred into a vacuum oven. The prepolymer was then cured at temperatures ranging from 110 to 150 ℃ under vacuum for various durations from 24 h to 72 h. 4.2. PGD coating on glass slides. Glass slides (7.6 × 5.1 × 0.1 cm) or indium tin oxides (ITO) glass slides (5.1 × 5.1 × 0.1 cm, 10 Ω) were first cleaned by deionized (DI) water, acetone, and ethanol using sonication. Then, they were treated by air plasma (5 min, 50 W). After that, liquid PGD prepolymer was casted on the glass slides and transferred into a vacuum oven. In a standard method, the prepolymer was cured at 120 ℃ under vacuum for 48 h. 4.3. Polymer characterization. Fourier transform infrared (FTIR) spectra were measured by a Thermo Nicolet 380 FTIR Spectrometer with DIAMOND ATR. A Perkin Elmer DSC8500 was


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used to perform differential scanning calorimetry (DSC) measurement. The temperature program for all measurements included an isothermal hold for one minute at −20°C, followed by a temperature scan from −20 ℃ to 80 ℃ at a constant scanning rate of 20 ℃/min, and an isothermal hold at 80 ℃ for one minute. 4.4. Mechanical test. Tensile test (ASTM D638) and 180° peeling test (ASTM D3330) were conducted on a Mark-10 ESM303 tensile tester at a moving rate of 60 mm/min. The impactresistance property was characterized by dropping a steel spherical ball (weight = 11.9 g, diameter = 14.2 mm) to targeted surfaces. 4.5 Optical measurement. Optical transmittance and haze were measured by a Perkin Elmer Lambda 35 UV/vis spectrometer with an integrating sphere. The measurement setup is shown in Figure S5. The haze value is calculated by Haze = (T4/T2-T3/T1) × 100%, where T1 is the intensity of the incident light, T2 is the total transmittance including specular and diffuse light, T3 is the light scattering of the instrument, and T4 is the light scattering of the instrument and the sample.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Supplementary figures of DSC curves, stress-strain curves, UV-vis spectra, electrothermal heating and optical photos of PGD samples. (PDF) Supplementary table of comparison of interfacial toughness. (PDF) Supplementary movies of: Transition process of PGD being heated to 55 ℃ (5× speed) (AVI)



Recovery process of PGD being naturally cooled to 21 ℃ (10× speed) (AVI) Peeling test below Ttrans at 21 ℃ (real time) (AVI) Peeling test above Ttrans at 55 ℃ (real time) (AVI) Impact test of a bare glass slide (real time) (AVI) Impact test of a glass slide coated 1 mm thick PGD (real time) (AVI)

AUTHOR INFORMATION Corresponding Author *E-mail:

[email protected] (J. L.).

ORCID Jian Lin: 0000-0002-4675-2529 Conflict of interest: The authors have filed a patent based on this work.

ACKNOWLEDGEMENTS J.L. acknowledges financial support from University of Missouri-Columbia start-up fund, NASA Missouri Space Consortium (Project: 00049784), Department of Energy National Energy Technology Laboratory (Award Number: DE-FE0031645), and United States Department of Agriculture (Award number: 2018-67017-27880). The MU calorimetry lab was established by National Science Foundation (Award number: EAR 1220051) and NASA (Award number: NNX12AO44G).

REFERENCES 20 ACS Paragon Plus Environment

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Chemically Interconnected Thermotropic Polymers for Transparency

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