Abstract

Molecularly ordered polymer is responsive to stimuli (thermal, photomechanical) and presents mechanical actuations based on the structural organization. The wireless control of molecularly ordered polymer by photochromic azobenzene has attracted considerable interest in industrial field and has been applied in soft muscles and robotic controllers. Here, we introduce the classification of molecularly ordered polymer and explain basic theory of thermal and photomechanical response. Advanced thermal and photomechanical actuations are presented followed by description of polymer characterizations. Current problem, study implication and future work are included in this literature review.

Keywords: Stimuli, azobenzene, thermal response, characterizations, actuations

1. Introduction

Molecularly ordered (liquid crystalline) polymer is a new type of material which can form ordered structure in liquid phase. The direction of the molecular can be aligned in one specific direction or domain. There are mainly three types of molecularly ordered polymer: mondomain LCP, twisted nematic LCP, splayed LCP. Fig.1 shows the basic structure of three LCP. (a) For monodomain type the direction of the director is uniform through the thickness of LCP. (b) For twisted nematic type the director rotates hierarchically through the thickness by left-hand or right-hand chirality along z axis. (c) For splayed nematic type the director rotates by Y axis from 0 to 90 degree.

  

(a)                                              (b)                                             (c)

Fig.1 Schematic of (a) monodomain (b) twisted nematic (c) splayed molecularly ordered polymer

Molecularly ordered/Liquid crystalline polymer embedded with anisotropy offers plenty of actuations to an array of stimuli, such as heat[1], [2], light[3], solvent[4], [5] and mechanical deformation. People mostly trigger the photo-response of molecularly ordered/liquid crystalline polymer remotely by trans – cis isomerization in order of 10ms. However, the relaxation process takes longer time by cis – trans, which influence recovery of LCP. So how to vary the structure of polymer to improve the efficiency of relaxation becomes important. Thermo effects can also be applied to LCE and LCN to show special response by geometrical dependent contraction/expansion. However, we need to discover better application of thermos – response which can be used in micro scales. The focus of my research is on triggering efficient actuation of molecularly ordered/liquid crystalline polymer (LCE and LCN) by thermo effect and UV light. This literature review will include what has been done by molecularly ordered polymer and the basic theory for light induced and thermos actuation by showing what kind of actuations have been done experimentally in macro and micro scales. Factors which influence the efficiency of actuations and thermo effect of LCP also will be included.

2. Introduction of molecularly ordered polymer

2.1. Classification of liquid crystalline polymer

The history of molecularly ordered polymer can be derived from 100 years ago. The discovery of intermediate liquid crystalline phase is by Friedrich Reinitzer. In 1956, Flory got the formulation of the effect of segmental rigidity in polymers[6], [7]. Based on the previous study, macromolecules of polymers contain flexible and rigid rod–like and disk–like fragments[8]. LCP exhibits long range orientational order in three dimensions and physical properties are governed by orientational molecularly anisotropy[9]. Nowadays, the technology of synthesizing LC polymers is mature and main-chain (Fig.1(a)[10]) or side-chain (Fig.1(b)[10]) LCP are synthesized by plenty of groups.

(a)                                                       (b)

Fig.1 (a) Main chain LCN  (b) Side chain LCE

Molecularly ordered polymer can be classified into two groups: glassy liquid crystalline networks (LCN) and liquid crystalline elastomers (LCE). The main difference between these two materials is that LCN is densely crosslinked by chains and backbones, which leads to a higher cross-linking density and less produced strain inside the networks. However, LCE is a subclass of chemically, crosslinked polymer, with lower cross-linking density and can be easily deformed to high strains with less stimuli[11]. LCE belongs to soft materials because of its low storage modulus. Fig.1 shows the basic structure of LCN (a) and LCE (b)[12].

  

Fig.1[12] Basic structure of LCN (a) and LCE (b)

As the LCN and LCE undergoes the nematic – isotropic transition, anisotropy of polymer chains or backbones will lose[13]. This transition changes the orientational order inside polymer, which induces internal stresses (strains)[14] and governs macroscopic shape change[15]. However, LCE can produce larger strain (more than 300%) and considerable work, with a smaller stress compared with LCN[15], [16]. Based on this advantage, LCE exhibits more complexed and intense response. For example, Dr. T.J. White’s group selects the curvature by designing the alignment of liquid crystalline elastomers[17]. By imprinted complex spatial patterns into LCE film, they vary the positive (fig.2 (a)) or negative (fig.2 (b)) Gaussian curvature through cooling/heating LCE film and quantify the final geometry.

(a)                                              (b)

Fig.2[17] (a) positive Gaussian curvature upon heating to 150 degree (b) negative Gaussian curvature upon cooling to 30 degree

Meanwhile, strain-induced director reorientation can be observed in LCE film. The external strain axis is not coincident with the original director axis[18] and this reorientation process has been illustrated experimentally and theoretically[14], [15]. When applying a strain perpendicular to the director axis of the LCE, reorientation of the molecular will happen. The external strain should over a threshold, which leads to this reorientation process irreversible. This kind of phenomena is not exhibited in LCN.

Both LCN and LCE exhibit different shape changes by varying the orientation of molecular or shape memory by external conditions. Twisted nematic LCN/LCE is one typical subclass of molecularly ordered polymer, which a spontaneous helical axis exists orthogonal to a preferred direction of monomers named local director[19], [20]. If cut the sample at different angle, thermos – response of twisted nematic liquid crystalline polymer will be different. In fig.10, if cut the TN LCP in 0 degree or 90 degree and heat it, only coiling actuation will be observed. If cut the sample in 45 degree, the sample will form spiral ribbon. Compressive strain will be produced along the direction of the director, which generates twisted spiral geometry at 45 degree. Shape change only happens when the temperature is above glassy temperature (Tg)[21].

Fig.10 Structure of twisted nematic liquid crystalline polymer

If we change the ratio of length and width of the strip, the shape of the strip will be strongly influenced. Fig.11 shows the difference of the shape change by varying the width[22]. With the increasing of the width, at elevated temperature, the TN LCN strip exhibits from a ribbon – like shape to a helicoidal – like shape[22].

Fig.11[22]  Spontaneous shape change in twisted nematic (TN) LCN

as a function of aspect ratio upon heating through the Tg of the material (130 °C).

Shape memory effect is observed both by LCN and LCE. This particular property can be controlled by modulating the external conditions, such as temperature and strain. Fig[23] illustrates the shape memory cycle by Smectic –C liquid crystalline elastomers. By adding constant load under constant temperature, LCE is stretched to 150% strain before cool down to low temperature at constant load. Then remove the force at low temperature and the shape of LCE will fix. Recovery of original shape can be realized by external stimuli, such as increased temperature and UV illumination[24]. This effect is expected to apply into biomedical field where low modulus soft materials are needed. Furthermore, the application of shape memory LCN and LCE is not limited to textile industry. Various applications range from surgical technology to artificial muscles are presented[25].

Fig. [23] Schematic of shape memory cycle of LCE

Both LCE and LCN have advantages and can be triggered by various stimuli based on their own properties. For glassy LCN, we can actuate ultrafast deformation such as snap through jumping by high cross linking density LCN. For LCE, repeatable and huge actuations are realized because of the soft property of LCE. Both LCE and LCN can be used in aerospace and medicine fields, both in macro and micro scales.

2.2. Characterization of liquid crystalline polymer

2.2.1. Polarized optical micrographs (POM):  Polarization is a property to transverse waves that describes the orientation of the oscillations. Polarized light can be produced by using a polarizing filter, which only allows waves of one specific direction to pass through. A pair of polarizing filter (polarizer) is always used to check the alignment of LCP. When polarization of the two filter is parallel, light will go through the second polarizer when the director orientation of LCP is parallel to both of the filters (Fig (a)). Opposite to Fig (a), if polarization of the two filters is perpendicular to each other, light will go through the first filter and LCP film and finally will be blocked by the second filter (Fig (b)).

   

(a)                                                                  (b)

Fig. Schematic of (a) parallel polarizer (b) cross polarizer

Under cross polarizer, if we rotate the film by some angles, partial light will go through the filters and we can observe some brightness through microscope. Fig. [26]shows the polarized optical micrographs (POM images) of aligned LCE (0.5 RM82), taken at 0° and 45° between the polarizer angles. By comparing the brightness through microscope, we can distinguish the level of alignment of liquid crystalline polymer.

Fig.[26] POM images of an aligned LCE (0.5 RM82) at 0° and 45° to the polarizer

2.2.2. Dynamic Mechanical Analysis (DMA):  Dynamic Mechanical Analysis (DMA) is a main technique that is widely applied to characterize LCP’s mechanical properties, such as storage modulus (Fig. [27]) as a function of temperature, time and other parameters. A small deformation is applied to a sample in a sinusoidal manner by a force motor during the test.

Fig. [27]Relationship between storage modulus and temperature for different compositions of LCN

The function of DMA test is trying to test the storage modulus and the glassy transition temperature (T_g). Based on the value of glassy temperature, we can calculate the cross-linking density of LCNs by equation[28]:

V_e = E’_high/(3RT_high)

V_e is the cross linking density, T_high is the temperature higher than T_g by 50 degree, E’_high is the value of storage modulus at T’_high.

2.2.3. Differential scanning calorimetry (DSC):  The differential scanning calorimetry is a fundamental thermal analysis technique, which is applied in many industries, especially for polymers. By heating the sample with known mass, the heat capacity (C_p) of polymer can be tracked by the variation of heat flow. The nematic temperature (from isotropic phase to nematic phase) and crystallized temperature (from nematic phase to crystallized phase) are detected by DSC. Fig. [29]shows is detection of glassy temperature with various curing modes for polymers.

Fig. [29] The glassy transition temperature of various light curing modes by DSC

 

2.2.4. Absorption test by microspectrophotometer:  Microspectrophotometer is an instrument which measures the transmittance, absorption, reflection from ultraviolet to visible wavelength in microscopic area. For the measurement of azo – LCP, we choose the absorption mode to measure the absorption coefficient to track the trans – cis isomerization. After irradiation by UV light, the absorption of trans peak decreases and cis peak increases. Fig. [30]shows the UV – Vis absorption spectra of azo – polyglutamate film before and after irradiating by linearly polarized UV light for 5 min, 10 min, 15 min, 25 min and 35 min. By microspectrophotometer we can detect the trans – cis and cis – trans isomerization visually and record absorption/transmittance ranging from UV to visible wavelength.

Fig. [30] UV – vis spectra of azo – polyglutamate film

2.2.5. Birefringence measurement: Birefringence spectroscopy is the optical technique of measuring orientation and order parameter in aligned LCP by measuring the retardation of polarized light going through the LCP[31]. In order to quantify the value of order parameter in LCP, a pair of polarizer is used. Fig. [32]shows the schematic illustration of setup for birefringence test. Randomly oriented light passes through the first polarizer and yields polarized light with a principle axis parallel to the polarization of the first polarizer. Then light keeps passing through the LCP and the second polarizer. By measuring the intensity of the light going through the second polarizer we can calculate the value of birefringence by equation:

I = I₀sin² (

πd

Δn/λ)

I represents for the intensity of light passing through the parallel polarizer. I₀ represents for the intensity of light passing through the cross polarizer. d is the thickness of the film and λ is the wavelength of the light. By calculating the value of birefringence we can detect the level of alignment inside the LCP film.

Fig.[32] Schematic illustration of experimental setup for birefringence measurement

3. Response theory of molecularly ordered polymer

Molecularly ordered polymer (LCE and LCN) can be widely applied to aerospace, medicine as artificial muscles of actuators because of their structures and advanced properties, such as low storage modulus and high mechanical flexibility and durability[13][33]. We can use plenty of stimuli to trigger the response of liquid crystalline polymer as actuators, such as UV, heat, electrical and magnetic field[28], [34], [35]. For the study of thermos effect to molecularly ordered polymer, silicon oil is the most preferred medium to provide uniform heat. Elastomers are much more sensitive to the heat compared with glassy liquid crystalline networks because of lower cross-linking density. Macroscopic shape change of LCE can be induced by heating the sample directly. Fig.2 (a) shows a demonstrative experiment by Finkelman and co-workers[11]. When the elastomers are heated gradually from ambient to elevated temperature, contraction is produced by phase transition (anisotropic phase to isotropic phase shown in Fig.2 (b)) which leads to the lifting of 10g weight attached at the bottom of the sample.

(a)                                                                            (b)

Fig.2[11]  (a) A demonstrative experiment done by Finkelmann and coworkers. (b) Phase transitions from anisotropic to isotropic phase

For the thermos-observation of glassy LCN, silicon oil is always applied to provide uniform heat. We can observe various deformations based on different types of LCN (monodomain LCN, twisted nematic LCN and splayed LCN), which are also triggered by phase transition. Fig.3 shows the reversible process of phase transition[36]. The structural change of LCN is caused by the variation of the order parameter. When heated by external stimuli, monomers will be reoriented and rotate. The chains become spherical which cause contraction from anisotropic phase to isotropic phase. When cool down to ambient temperature, relaxation of chains will happen, which leads to the recovery of the shape.

Fig.3[36]  A unit cube of rubber in the isotropic (I) state and nematic (N) state

Meanwhile, liquid crystalline polymers can also transduce light energy into mechanical work[37], which realizing the remotely control as actuators. To trigger the actuation by UV light, we add azobenzene as cross linkers into LCN and LCE. Azobenzene – functionalized polymers (azo-LCN and azo-LCE) are widely used in light controlled system by (1)trans – cis photoisomerization with UV irradiation (2) trans – cis – trans reorientation under green-blue light[38]. For trans – cis photoisomerization, the rod like trans – azobenzene will bend to cis state when absorbed the energy from 365nm UV light (Fig.6[39]).

Fig.6[39]  Molecular structures of trans isomer and cis isomer

This process is a isothermal order-disorder transition[28]. Usually we use order parameter S to represent the level of alignment inside the polymer. To calculate the value of the order parameter we firstly calculate the value of dichroic ratio R by equation[32]:

R =

A||A⊥                                                                                                                                             (1)

A|| and A⊥ are the absorbance measured with light polarized parallel and perpendicular to the director respectively[32]. Then we calculate the value of order parameter by equation[40]:

S =

R-1R+2                                                             (2)

Usually azo – LCN and azo – LCE will become disordered after irradiating by UV light, which means that the value of order parameter will decrease. The reversible process can be realized by irradiating visible light or by thermal effect (heating) which inducing the cis isomer to trans isomer.

For trans–cis–trans reorientation under 440 – 500nm light (Weigert effect[41]), we can control the polarization direction of light to trigger expansion/contraction of azo – LCN and azo – LCE. Fig.7[42] compares the effect of photoisomerization and Weigert effect, illustrating the effect of light polarization to the actuation of azo – LCN. When irradiated with 365nm UV light, azo – LCN bends towards the UV source no matter light polarization parallel or orthogonal to director (alignment). We observe different final curvature in photoisomerization (365nm) since the absorption coefficient of azobenzene in two situations (E||n and E⊥n) is different, which produces different generated strain through the thickness of LCN. In trans–cis–trans reorientation, contraction will be generated on the surface when E parallels to the alignment and expansion will be produced on the surface when E is perpendicular to the alignment. Based on the sensitivity to the UV and blue-green light, azo – LCN and azo – LCE can be applied to plenty of fields by remotely control.

Fig.7[42]  Photobending snapshots for 365 and 445 nm light irradiation: E⊥n for light polarization vertical to the alignment, and E∥n for light polarization parallel to the alignment

The actuation of LCN and LCE by photomechanical and thermos effect is widely applied in many industries. Photomechanical response of azo – LCN and azo – LCE are triggered by trans – cis isomerization which transduces light energy into mechanical work. For thermos response of LCP, the actuation is determined by the oriented structure of molecular and phase transition. The actuation level of LCN/LCE is dominated by many parameters, such as cross linking density, structure of azobenzene and length of spacer. Much more work in improving the molecularly structure of LCP needs to be done to trigger ultra–fast and significant deformation.

4. Actuation of molecularly ordered polymer

4.1. Thermos response of LCP

The thermo-mechanical response of LCN is observed by various group. For example, Dr. D.J.Broer’s group concentrates on the heat response of twisted nematic LCN and splayed LCN. Fig.4 shows the thermo-response of twisted nematic LCN and splayed LCN, with 20um and 40um thickness respectively[43]. In fig.4 (a), below 53 degree the contraction along the long axis of the strip dominates the deformation, which transfers a curved film to a flatted strip. Above 53 degree, the effect of orthogonal expansion becomes obvious and leads to a kinking process of the strip. At elevated temperature, the orthogonal expansion is maintained and changes the shape of the strip gradually. Reversible process can be triggered if cooling down the temperature. Fig.4 (b) shows the thermos-response of splayed molecular configuration as the function of temperature. From the figures we can see that there is no kinking and twisting response of splayed LCN. Only bending deformation can be observed and the curvature becomes smaller with increasing temperature. The different response between splayed and twisted nematic LCN is that there is only contraction/expansion along the long axis (director) in splayed LCN[44].

 

(a)                                                               (b)

Fig.4[43]  (a) Deformation as function of temperature with twisted nematic molecular configuration (b) Deformation as function of temperature with splayed molecular configuration

Furthermore, we can take advantage of these thermos actuations to trigger more advanced and complexed shape change. For example, Dr .Shankar’s group presented shape selection by using splayed glassy LCN. By curing the splayed LCN in a graded manner, the cross-linking density of planar side is higher than homeotropic side, which leads a natural curve. Fig. 5 (b) shows the natural state at room temperature when sample is excised from the substrates at different angles[45]. When the 90 degree splayed LCN strip was put into the silicon oil, an increasing magnitude of strain on planar side was observed just like fig.5 (c)[45]. With continued heating treatment, decreased magnitude of strain on planar side caused relaxation of the strip. This kind of phenomenon is also observed in 0 degree and 50 degree. Based on this test, we can select one shape based on our need by chooing specific condition (temperature and excised angle).

 

(a)                                                                           (b)

(c)

Fig.5[45]  (a) Deformation as function of temperature with twisted nematic molecular configuration (b) Deformation as function of temperature with splayed molecular configuration

 

4.2. Photomechanical response of LCP

Based on the special structure and advanced property of molecularly ordered polymers, people have triggered photo-response by light or induced actuation by thermos effect. For the response triggered by light, azobenzene moieties must be added and induce the actuation by trans – cis isomerization or by trans – cis – trans reorientation. In 2003, Dr. Tomiki Ikeda’s group discovered that by dominating the polarization direction of polarized light we can direct the bending actuation of polydomain polymer[3]. Unlike monodomain structure, polydomain LCP has varied director orientation and the direction of macroscopic alignment is not uniform[33]. The trans – cis isomerization happens only in the domain where azobenzene moieties align in the same direction of polarized light. Therefore, by controlling the polarization of light we can vary the bending direction of polydomain film.

Based on this theory, plenty of actuations are triggered by molecularly ordred polymers (monodomain, twisted nematic and splayed LCP). For example, an ultrafast snap – through actuation has been presented by designing a bistable arch geometry. A strip of LCN or LCE is excised from host sample. Two ends are clamped rigidly and then compressed slightly to make an arch geometry. Fig.8 shows the geometry of the buckled arch bistable actuator[46].

Fig.8[46]  A buckled arch geometry utilized as a bistable actuator

Light can be irradiated from the top and leads to a contraction on the surface of the strip by trans – cis isomerization or by trans – cis – trans reorientation. Fig.9 (a) illustrates the process of snap – through actuation when irradiated by 445nm laser in the middle of the strip from bottom[47]. The sample in this test is azo – polyimides (Fig.9 (b)). The polarization of 445nm laser is parallel to the y axis. After 8.4s irradiation compressive strains are accumulated at the bottom, which lead the buckled strip to an unstable state and reaches the threshold for snap – through actuation. The time for the ultrafast actuation is in the order of 10ms. After that the strip jumps upwards and keeps geometrically stable. We can take advantage of this ultrafast actuation to fabricate prototypical devices by integrating actuator arrays[46].

(a)                                                                               (b)

Fig.9[47]  (a) Progression of photomechanical snap-through when 445nm laser irradiates at the bottom (b) Molecular structure of an azobenzene-functionalized polyimide

Splayed LCN and LCE are also widely applied to large and reversible deformation, especially by light. Since the level of actuation is decided by the variation of molecular order, splayed liquid crystalline polymer can form larger deformation compared with monodomain and twisted nematic LCN, even in micro size. For example, Dr. Dirk J. Broer and his coworkers use inkjet printing technology to microstructure the actuators. Micro size actuators can be driven by light and applied as mixers or pumps in microfluidic systems[48]. As shown in fig. 12(a), two splayed LCN are synthesized by two kinds of azo dyes, which absorb energy from UV light and visible light respectively. When irradiated by UV light, first splayed LCN (yellow) responses to the stimuli and bends towards the source. The second splayed LCN (red) only responses to visible light. By inkjet printing, micro level size actuator can be fabricated and the same response can be controlled.

  

(a)                                                                                           (b)

Fig.12[48]  (a) Light-driven cilia motion controlled by the spectral composition of the light (b) Side view of the actuation polymer cilia with UV light in water

The efficiency of actuations triggered by thermo and photomechanical effect is controlled by many factors. One team has found that the maximum bending extent increased with increasing the cross – linking density of LCN, which also influence the bending speed of the free –standing films[32]. Meanwihle, the photoresponsiveness is found to be controlled by the free volume distribution around the photochromic moiety[49]. Spacer length also influences the speed of actuation, which related to the TNI and the melting temperature[50], [51]. And the irradiation time and the cross – linking density also control the relaxation of LCN[52]. In a word, complex thermos and ultrafast photo-response of molecularly ordered polymer can be discovered by varying the structure of molecular in the future.

5. Conclusion

Plenty of actuations by light and heat have been played by many groups. However, more complex thermos and ultrafast photo-response of molecularly ordered polymer is still needed to be discovered. Simulations of response are necessary since it will help us to deeply understand the mechanics behind the actuations so that we can improve the efficiency of molecularly ordered polymer. For future work, we will modulate the basic structure of molecular to realize faster actuation and relaxation of azo-LCN by figuring out relaxation dynamics. Then we can synthesized advanced azo–LCN (monodomain, twisted nematic, splayed geometry) to present more complexed actuations, such as kicking process or LCP origami. Also we will play with soft azo-LCE and in micro scales and trigger the same actuations by heat instead of light. Better application of thermos and photomechanical response in micro scales still needs to be discovered.

 

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