Yu Haifeng Group in the School of Materials Science and Engineering has made progress in liquid crystal block copolymers

Human skin has unique properties
that are both soft, elastic and malleable.
Recently, the development of soft elastomers that mimic human skin has led to new applications
in wearable electronics, flexible electronic packaging, and skin sensors.
Soft elastomers are a class of cross-linked materials whose Young's modulus (0.
1-1.
0 MPa) is much
smaller than that of traditional elastomers (1.
0-100 MPa).
This mechanical property can match the Young's modulus (on the order of KPa) of human skin, and can fit seamlessly with human skin, making it suitable for wearable applications
.
Recent studies have shown that the light-controlled stimulus response of the mechanical properties of soft materials has significant advantages
over other stimuli such as temperature, humidity, electric and magnetic fields.
Therefore, combining the elastic and photoresponsive properties of rubber can provide new opportunities to obtain soft elastomers
similar to human skin.

SBS widely used in industry is a class of styrene (PS)-butadiene (PB)-styrene (PS) triblock copolymers, usually with a two-phase structure, namely polybutadiene continuous phase and polystyrene dispersion phase
.
After heating, the material can flow and is easy to process and form
.
The PS segment cooled to room temperature is aggregated into a glassy microregion, which acts as a physical crosslink point for the PB segment, so that the material exhibits the characteristics of rubber, so it is called "thermoplastic elastomer"
.
Yu Haifeng's research group of the School of Materials Science and Engineering realized the mutual change
of light-controlled mechanical properties of polymers for the first time by imitating the molecular structure and phase structure of SBS, combined with the photoinduced phase transition characteristics of light-responsive liquid crystal polymers at room temperature.
The designed liquid crystal block copolymer has a triblock structure similar to SBS, except that the PB block is replaced
by a light-responsive liquid crystal polymer (PM11AZC4).
The material has strong rigidity at room temperature and shows the transformation properties to soft elastic light-controlled mechanical properties
in a non-thermal state.
Based on the reversible switching of light between rigidity and elasticity at room temperature, the researchers demonstrated the precise control of nanopatterns on non-planar substrates, while being particularly suitable for the fabrication of packaged perovskite solar cells, enabling a simple, convenient and controllable method for mechanically adaptable optical display and electronic device packaging applications, as shown
in Figure 1 and Video 1.

Figure 1.
Preparation process of responsive biomimetic soft elastomer with programmable mechanics and its application

Video 1.
Demonstration of mechanical properties of photosoft elastomers

Dynamic mechanical analysis (DMA) is used to characterize the phototunable mechanical behavior of triblock copolymers, and both the azobenzene-containing photoresponsive block PM11AZC4 and polystyrene (PS) block show glass transition temperatures above room temperature, so the polymer behaves as a hard plastic at
room temperature.
Under ultraviolet light, the glass transition temperature of PM11AZC4 drops below 0 °C, allowing it to act as a flexible segment that provides elasticity, while the light-inert PS part acts as a physical crosslinking point, making the triblock copolymer behave like the thermoplastic elastomer SBS, which the researchers call a "photoelastomer"
.
Moreover, this light-induced mechanical interchange occurs at room temperature, and the Young's modulus of the triblock copolymer after UV irradiation is 0.
2 MPa, which is a typical soft elastomer
that mimics human skin.

Using the light-controlled mechanical property transformation characteristics of the material, the researchers finally realized the preparation
of micro-nano patterns on the surface of complex geometries through pressure and illumination.
According to the difference between the two template cycles and patterns, different structural colors are produced, and finally the pattern of the Chinese character "Peking University" is printed on
the forearm and back of the hand (Figure 2C and Video 2).
Figures 2D and 2E show scanning electron microscope images
of polymer films after two-step imprinting, respectively.
Obviously, this method combines the advantages of photosoft elastomer and block copolymer microphase separation, and is very promising
in use cases for flexible displays, wearable technology and even fabrics in smartphones or computers.

Figure 2.
Precise programmable imprint printing process by irradiating triblock copolymer films with different wavelengths of light

Video 2.
Patterns obtained by nanoimprint knots combined with light

In addition, the researchers used liquid crystal block copolymer films to encapsulate perovskite solar cells on a flexible PDMS substrate, and it is clear that the obtained devices can be used on different curved surfaces, such as arms
.
This encapsulated perovskite solar energy studies its water resistance by immersing it in water
.
Unpackaged devices begin to decompose when immersed in water for 10 s and completely decompose at 180 s; When the packaged perovskite solar cell device was immersed in water for 1 h, no significant discoloration was observed (Video 3).

Subsequently, the stability
of unpackaged and packaged devices in water was quantitatively determined by UV-Vis absorption spectroscopy.
The absorption intensity of unpackaged devices begins to decrease significantly at 30 s and decreases to a minimum at 600 s
.
Even if the encapsulated perovskite solar cell is immersed in water for 3600 s, the absorption intensity remains basically unchanged, which indicates that the block copolymer with photocontrolled mechanical properties can effectively isolate the battery device from water and show good sealing effect
.

Video 3.
Test process of perovskite cells in water obtained by liquid crystal block copolymer packaging

The first author of the above research results was recently published in Science Advances, the first author of the paper is Cai Feng, a 2019 doctoral student at the School of Materials Science and Engineering of Peking University, and the corresponding authors are Yu Haifeng and Professor
Feng Wei of Tianjin University.
This work is supported by the National Natural Science Foundation of China, the Ministry of Science and Technology and the Key Laboratory of Polymer Chemistry and Physics of the Ministry of Education
.
The preparation of perovskite devices has been strongly supported
by Professor Zhou Huanping from the School of Materials Science and Engineering.

In addition, in the previous work, the research group used the two-block liquid crystal copolymer prepared by the same light-responsive liquid crystal polymer (PM11AZC4) to achieve efficient and high-density flexible storage of solar thermal dye cells, which can store solar energy and room temperature latent heat at the same time, and realize the controlled release
of heat energy.
The results were published in Chemistry of Materials and were chosen for the inner cover
.
The first author of the paper is Cai Feng, and the corresponding authors are Yu Haifeng and Feng Wei
.
This work is supported by the National Natural Science Foundation of China, the Ministry of Science and Technology and the Key Laboratory of Polymer Chemistry and Physics of the Ministry of Education
.

Figure 3.
The use of liquid crystal block copolymers realizes the efficient and high-density flexible storage of solar thermal dye cells

The results of the research group were selected as the inner cover of Chemistry of Materials

1）Cai, Feng; Yang, Bowen; Lv, Xuande; Feng, Wei*; and Yu, Haifeng*.
Mechanically mutable polymer enabled by light.
Science Advances, 2022, 8 (34), No.
eabo1626.

Full text link: style="text-indent: 2em;">2）Cai, Feng; Song, Tianfu; Yang, bowen; Lv, Xuande; Zhang, Liqun;  Yu, Haifeng*.
Enhancement of Solar Thermal Fuel by Microphase Separation and Nanoconfinement of Block Copolymer.
Chemistry of Materials 2021, 33 (24),9750-9759.

Full text link: https://pubs.
acs.
org/doi/full/10.
1021/acs.
chemmater.
1c03644