Photon Factory Activity Report 2017 #35 (2018)

BL-6A/2016G612

Liquid Crystalline Block Copolymer Elastomers Comprising Main-Chain Liquid
Crystal Polyesters Attached to Cross-Linked Amorphous Segments at Both Ends
1Masatoshi TOKITA1,*, Kohei ABE1 and Maito KOGA1
Department of Chemical Science and Enginnering, Tokyo Institute of Technology, 2-12-1
Ookayama, Meguro-ku, Tokyo 152-8552, Japan

1 Introduction
Liquid crystal elastomers (LCEs) have attracted muchattention because of the coupling between the backbone
conformation and the LC orientational order can give riseto spontaneous and reversible elongation along thedirector axis on the isotropic–LC transition.[1] The mainchain LCEs with mesogenic moieties incorporated in thepolymer backbone elongate larger than the side-chain
LCEs due to the stronger coupling between the chain
conformation and the ordering of the mesogens.[2–9]Such an anisotropic elongation is particularly pronounced
for monodomain LCEs; however, the preparation ofmonodomain LCEs remains difficult because it requires
polymer chains to be cross-linked in a macroscopicallyoriented state. Another puzzling phenomenon connectedto the cross-linking LC polymers is the decreases in theisotropization temperature and enthalpy change.[7–11] A
cross-linked main-chain smectic BB-5 polyester formedby the incorporation of 2 mol% of 1,3,5-benzene
tricarboxylic acid exhibited the isotropization temperature(Ti) and enthalpy change (ΔHi) of 182 °C and 2.8 kJ mol−1,respectively, fairly lower than the corresponding values
for the BB-5 homopolyester (i.e, Ti = 213 °C and ΔHi =5.9 kJ mol−1).[9]The difficulties in preparing monodomain LC
elastomers may be avoided using rubber–nematic–rubber(RNR) triblock copolymers because, in this case, thecross-links are not included in the LC segment but rather
in the rubber (R) segment. The R blocks segregate fromthe nematic (N) block to link the network of N segments.
Although the nematic block is a main-chain nematic
polymer in the model proposed by de Gennes,[12] thefirst RNR copolymer was prepared using a side-on
nematic polyacrylate with a narrow molecular-weight
distribution in order to form long-range-ordered lamellar
microdomains.[13] The monodomain triblock copolymer
was obtained using a magnetic-field orientation followed
by a cross-linking in the R block induced by UV
irradiation. The director of the N block was perpendicular
to the lamellae, and the film contracted reversibly on theisotropization and elongated by a factor 1.23 on the liquid
crystallization.
In this study, we investigated triblock B5Cin
copolymers that comprise a main-chain smectic BB-5(3Me) polyester where photo-cross-linkable poly(2cinnamoyloxyethyl methacrylate) (PCEMA) blocks areconnected at both ends (Chart 1). BB-5(3-Me) wassynthesized by the polycondensation of 4,4′-biphenyl
dicarboxylic acid and 3-methyl-1,5-pentanediol andforms a SmCA LC.[14–18] We use the main-chain
smectic LC polymer as the middle block for the following
two reasons. First, copolymers comprising smectic mainchain LC polyesters connected with polymethacrylates atbothendsform
long-range-orderedlamellarmicrodomains in a large range of compositions eventhough the LC segments have polydispersity indices ashigh as 2.[19–22] Second, the lamellae and LC director
can be arranged perpendicular to the fiber axis (or thefilm drawing direction) and perpendicular to the lamellae,
respectively. Such lamellae and LC orientations havebeen characteristically observed for the main-chain
smectic triblock copolymers and are similar to the RNR
copolymers.[12,13] In contrast, a main-chain nematic
triblock copolymer formed lamellar microdomains, butboth the lamellae and LC director were arranged parallelto the drawing direction.[23]Similar to the smectic LC triblock copolymers, theB5Cin copolymers formed lamellar microdomains, inwhich the BB-5(3-Me) segments formed a SmCA LCarranging the LC director perpendicular to the lamellae.
Melt drawing and successive annealing yieldedmonodomain films with the lamellar normal and the LCdirector lying along the film drawing direction. ThePCEMA block lamellae were cross-linked by UV
irradiation up to 20% conversion. Similar to monodomain
LCEs, the UV-irradiated monodomain films contracted
reversibly along the direction of the LC director on theLC isotropization. This film contraction coincided withthe decrease in the lamellar spacing. The film elongatedreversibly on the liquid crystallization by a factor ranging
from 1.07 to 1.24, which is explained by the ratio oflamellar spacing in the LC phase to that in the isotropicphase.
Chart 1OOBrOOOOOnOOOO

BrOmOO nOO

Photon Factory Activity Report 2017 #35 (2018)

2 Experiment
Materials. A B5Cin-235-41 copolymer was preparedusing a BB-5(3-Me) polyester with number-averagemolecular weight (Mn,LC) of 23500. The numbers of 235
and 41 in the code of the copolymer correspond to Mn,LC
divided by 100 and the volume percentage (φ) of thePCEMA segment, respectively. The values of Mn,LC andMn of amorphous PCEMA segment (Mn,am) weredetermined by 1H NMR spectroscopy. φ values werecalculated from Mn,LC, Mn,am, and the densities of eachsegment (i.e., BB-5(3-Me): 1.23 g cm−3, PCEMA: 1.23 gcm−3). The polydispersity indices (PDIs) of thecopolymers measured for the BB-5(3-Me) polyester andB5Cin-235-41 were 2.17 and 1.66 by size-exclusion
chromatography (SEC).
The LC block of B5Cin235-41 exhibited the glasstransition temperature (Tg) at 31°C, and the isotropization
at 139 °C involving ΔHi = 2.82 kJ mol−1 whereas that inthe photo-cross-linked B5Cin235-41 exhibited Tg = 31 °Cand Ti = 144 °C (ΔHi = 2.47 kJ mol−1). (cf. OHterminated BB-5(3-Me)-235: Tg = 28 °C, Ti = 151 °C, and
ΔHi = 3.86 kJ mol−1)
Methods. Wide-angle and small-angle X-ray scattering
patterns were measured using Cu Kα radiation with
Bruker D8 DISCOVER and NanoSTAR-U, respectively,
equipped with Vantec 500 and Vantec 2000 detectors,respectively. The synchrotron radiation (SR) small-angle
X-ray scattering (SAXS) measurement was performed at
the BL-10C and BL-6A beamlines in Photon Factory,
Tsukuba, Japan, equipped with a Dectris PILATUS 300KW detector with a camera length of approximately 2 m.
The X-ray wavelength was 0.1488 nm. The temperaturedependences of the strain for the B5Cin films wereobtained using a Hitachi TMA7100. The elastomer in thefilm form with a thickness of 0.1 mm was cut into
rectangular strips with lengths of 5 mm (in the elongationdirection) and widths of 2 mm and clamped in the tensile
testing machine. The sample was heated and cooled at arate of 2 °C min−1 under a dry nitrogen atmosphere andthe film length (L) was measured.
3 Results and Discussion
Morphology and Photo-Cross-Linking of B5Cin235-41 Monodomain Films. The BB-5(3-Me) segment
was segregated from the PCEMA segment to form alamellar microdomain. Both the smectic LC and lamellar
microdomain could be well oriented by drawing thecopolymer melt at 160 °C into 100-μm-thick films andannealing the films for 12 h at 120 °C leading to theformation of the SmCA LC by the LC block. Figure 1
shows the typical wide-angle X-ray scattering (WAXS)
and SAXS patterns measured for the B5Cin-235-41 film.
The smectic layer reflections and scattering maxima
ascribed to the stacked lamellae are observed along thefilm drawing direction in the WAXS and SAXS patterns,
respectively, demonstrating that the smectic layers andmicrodomain lamellae stack along the film stretchingdirection. Thus, both the lamellae and smectic LC can beoriented macroscopically by first drawing the copolymer
melt and then annealing at the LC temperature. Suchlamellar and smectic LC orientations are a characteristicof ABA triblock copolymers with main-chain smectic LC
polyesters as the middle blocks.[19–22] The lamellar
spacing D and smectic layer spacing were 34.3 and 1.63
nm, respectively. These values were not altered for thephoto-crosslinking of the amorphous segments byirradiating UV light (λmax = 365 nm, 50 mW cm−2) oneach film face for 90 min. The value of the orientational
order parameter, S, estimated from the azimuthal intensity
distribution of the lamellar reflection was 0.86.[24]Fig. 1: (a) WAXS and (b) SAXS patterns for theB5Cin-235-41 film. The drawing direction is vertical.
Reversible Contraction of Photo-Cross-Linked
B5Cin-235-41 Monodomain Films on the LC-toIsotropic Transition. The reversible contraction along
the film drawing direction on the LC–isotropic transitionof the BB-5(3-Me) block is typical of the photo-crosslinked B5Cin copolymer films. Such reversible
contraction was observed in the cross-linked copolymers
although it has not been observed for the non-cross-linkedcopolymers. The temperature dependence of the film
length (L) along the lamellar normal direction is shown in
the upper part of Figure 2, where the ratio of the L to theL in the isotropic phase (Liso) is plotted versus
temperature. The measurements were performed byapplying a stress of 1.5 kPa to the 100-μm-thick film
sample, and the temperatures were scanned at a rate of
2 °C min−1. It was found that the cross-linked film
contracted upon heating to temperature beyond the Ti of
the LC block. The shape change was fully reversible witha variation of 18%. The temperatures at which L/Liso
decreased and increased on heating and cooling,respectively, corresponded to the endothermic andexothermic peaks in the heating and cooling DSC
thermograms, respectively, as shown in the lower part ofFigure 2.

Fig. 2: Relative film lengths L/Liso (upper) and DSC
thermograms (lower) measured for the cross-linked

Photon Factory Activity Report 2017 #35 (2018)

B5Cin-235-41 film during first cooling and then heatingprocesses at a rate of 2 °C min−1.
The spontaneous LC orientation can be attributed to theconservation of lamellar microdomain at T > Ti. SAXS
patterns obtained for the heating of the film at 170 °C andthe subsequent cooling to 120 °C include lamellar
scatting maxima on the meridian showing that thelamellae stacking along the film drawing direction wasconserved in the entire range of investigated temperatures.
Although the lamellae were conserved, the spacing
decreased at T > Ti. Figure 3a shows the SR-SAXS
profiles measured for the cross-linked B5Cin-235-41 filmon heating up to 170 °C followed by cooling at a rate of2 °C min−1. Here, the scattering intensities were obtained
from the 2D scattering patterns by averaging the intensity
over azimuthal sectors of 10° on each side of the film
drawing direction and plotting against the scatteringvector q. The as-UV irradiated film showed scattering
maxima at q with ratios of 1:2:3:4:5 that were ascribed tolamellar microdomains stacking along the film drawing
direction at a spacing (D) of 34.5 nm. The scattering
intensity began to decrease at a temperature of 142 °C,
and the scattering maxima attributed to lamellae at asmaller D (i.e., 26.7 nm) that began to appear at q withratio of 1:2. Upon subsequent cooling, the scattering
maxima ascribed to the original spacing recovered at146 °C. Such a reversible decrease in D on theisotropization of the LC segment has been observed forsimilar ABA LC triblock copolymers and was explainedby conformation changes in the LC block on the phasetransition.[20,21]

Fig. 3: (a) SR-SAXS profiles for the cross-linked
B5Cin-235-41 film collected during heating and coolingat a rate of 2 °C min−1. The arrows indicate the positionsof scattering maxima. The profiles at an interval of 16 °C
are shown by thick lines. (b) Temperature dependences ofthe lamellar spacing of the cross-linked B5Cin-235-41
film. The lamellar spacings collected through heating andcooling processes are shown with square and triangle
marks, respectively.

The reversible contraction of the film length can beconnected to the reversible decrease in D. In Figure 3b, Dvalues measured by SR-SAXS for a cross-linked B5Cin235-41 film upon heating or cooling at a rate of 2 °C
min−1 were plotted versus the temperature. When the twosets of lamellae with different spacings appeared, theseries of lamellae exhibiting the stronger scattering wereselected. The lamellar spacing at T < Ti was constant at
DLC = 34.5 nm and decreased to Diso = 26.7 nm at T > Ti.
The comparison of the temperature dependence of D withthe temperature dependence of L/Liso allows us toconclude that the film contraction coincides with thedecrease in D. When the lamellae are tilted with respect
to the film draw direction by an average angle of β,
LLC/Liso is expected to correspond to the value of <cos β>
DLC/Diso. <cos β> can be calculated from the value of S =[3<cos2 β> − 1]/2 for the lamellae. For B5Cin-235-41, thevalue of LLC/Liso is 1.18, which fairly corresponds to thevalue of <cos β> DLC/Diso = 0.95 × 34.5/26.7 = 1.23.
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* mtokita@polymer.titech.ac.jp