Photon Factory Activity Report 2014 #32 (2015) B

BL-10C/2013G544

Inclusion of Chain Ends in Long Period Lamellae of Main-Chain
Polyester Smectic Liquid Crystals
Masatoshi Tokita1,*1
Tokyo Institute of Technology, Tokyo 152-8552, Japan
1 Introduction
The degree of polymerization (DP) of liquid crystal
polymers (LCPs) affects liquid crystal (LC) temperature
range especially in oligomeric range. Even if a monomer
does not exhibit LC phase, the polymer can exhibit LC
phase to show crystal-to-LC and LC-to-isotropic liquidtransitions.[1–3] While both transition temperatureincreases with DP, the increment of LC isotropization
temperature (Ti) is steeper than that of crystal melting
temperature (Tm) or the glass transition temperature (Tg)and the LC temperature region enlarges with DP and thetransition temperatures and the LC temperature range
become constant when DP is larger than 10. A monotropic
LCP can alter to enantiotropic one with increasing DP.[4]These DP dependences of LC transition behaviors havebeen understood thermodynamically by depicting
temperature dependence of the free energies of the threephases for different DP with assuming that the entropies
of isotropic liquid and LC phases decrease asymptoticallywith DP.[5]In this study we examine the LC phase transition
behaviors for a series of main-chain LC PB-10 polyesterwith different DPs. The PB-10 polyester consisting of
4,4′-biphenol and 1,10-dodecanoic acid forms crystal,smectic I (SmI) LC, and isotropic liquid phases in orderof increasing temperature.[6] The morphological featureof the PB-10 SmI LC is 40 nm thick lamellae stacking ata spacing of 47 nm along the chain axis and being
separated by isotropic liquid. The isotropization behaviorof the lamellar LC should be influenced by lamellar
thickness like as the melting of lamellar crystal
polymers.7 Here we investigated the correspondence ofthe lamellar morphology with the phase transitions forthree PB-10 polyesters with different DPs. While thelamellar thickness was hardly dependent on DP, Ti
decreased by 12 °C with decreasing DP from 50 to 12.
The chain contour length of PB-10 polyester with DP =12 was 1.5 times smaller than the lamella thickness. SuchDP dependence of Ti is attributed to the inclusion of chain
ends into the lamellar smectic LC.

2 Experiment
Materials: Three PB-10 polyesters with different
molecular weights were synthesized by melt condensationof 4,4′-biphenol diacetate and 1,10-dodecanoic acid. The
melt condensation was followed by solid statepolymerization for preparation of the highest molecularweight sample.[6] The number-average molecular weight(Mn) and polydispersity index (PDI) of the polymers were
estimated by gel permeation chromatography (GPC)
using universal calibration and listed in Table 1. The
polymers are identified as PB-10-xn, where xn is thenumber average degree of polymerization.
Table 1: Characterization of PB-10 Polyesters.

TibMn/°C
Lc
PDIΔHi/ΔHi°/g mol−1ΔHib/nm−1/kJ mol
PB-104,500 1.7
246
27 0.51
1212.5
PB-108,600 1.8
253
52 0.802319.7
PB-1018,900 5.9
258
115 0.92
5022.7
adetermined by GPC. bdetermined by DSC. cchain contour
length calculated from xn and the lattice constant c = 2.31 nm.
a

Measurements: Differential scanning calorimetry (DSC)
was performed using a PerkinElmer DSC7 calorimeter
under a flow of dry nitrogen. The synchrotron radiation(SR)-SAXS measurement was performed at a BL-10C
beamline in Photon Factory, Tsukuba, Japan. The
scattering intensity was corrected by transmission andsubtraction of background scattering and plotted against
the scattering vector q = 4π sin θ/λ.
3 Results and Discussion
PB-10 polyesters form crystal, smectic I (SmI),
isotropic liquid phases in order of increasing temperature.
The temperature on the isotropization of SmI LC (Ti) andthe corresponding enthalpy change (ΔHi) were measured
by DSC at heating rate of 10 °C min−1 and listed in thefifth column of Table 1. Before the measurement, thesample was cooled from the isotropic phase to ambient
temperature at a rate of 10 °C min−1 to erase any priorhistory. With increasing xn, Ti and ΔHi increase
remarkably.
The SmI phase has been identified by the WAXD
pattern of a PB-10-50 fiber which includes the reflections
attributed to the monoclinic unit cell with a = 0.64 nm, b= 0.90 nm, c = 2.31 nm, and β = 126° with the c-axis
parallel to the fiber axis.[6] In the C-centered monoclinicunit cells, mesogenic moieties are packed with their longaxis parallel to the c-axis. Two mesogens neighboringalong the c-axis at a spacing of 2.31 nm are connected byan alkyl spacer assuming most extended configuration.
Thus most extended polymer chains lie along the fiberPhoton Factory Activity Report 2014 #32 (2015) B

axis. On the other hand, the same fiber exhibited SAXS
pattern including several reflections which are attributed
lamellae stacking along the fiber axis at a spacing of 47
nm. By combining the WAXD pattern with SAXS one,
we can depict polymer chains are parallel to the fiber axis
and form lamellae stacking along the fiber axis at aspacing of 47 nm and SmI structures with the smecticlayers stacking in the direction tilted by 36° from the fiber
axis at a spacing of 1.87 nm.

thickness (σLC and σam) are listed in Table 2. Here dLC is
assumed to be larger than dam because a sufficient numberof smectic layers stack within each lamella to produce thesmectic layer reflection in the WAXS pattern.
These structural parameters, however, are notconsistent with Ti of SmI LC. Ti of the lamellar SmI LC
should be depend on dLC as expected by the GibbsThomson relationship and such a trend has been observed
for the homologue PB-14 polyester.[8] Because dLC ofthese three PB-10 polyesters are almost constant of 40 nm,
Ti is expected to be almost constant according to theGibbs-Thomson relationship. However, Ti increases withxn and Ti of PB-10-50 becomes 12 °C higher than PB-1012. Thus Ti of lamellar SmI LC cannot be explained bythe lamellar structural parameters.
Table 2: Structural Parameters Determined by theSimulation Method.
PB-10-12
PB-10-23
PB-10-50

Fig. 1: SR-SAXS intensity profile (dots) of (a) PB-1012, (b) PB-10-23, and (c) PB-10-50. The samples were
initially prepared by cooling the isotropic melt at a rate of10 °C min−1 and heated to a LC temperature of 210 °C.
The solid curve shows the calculated intensity based on
the paracrystal theory.
The lamella structure as well as the SmI structure iscommon in PB-10 polyesters with different molecularweights. Figure 1 shows the SAXS profile measured for
PB-10 polyesters at 220 °C. The sample was cooled fromthe isotropic liquid phase to room temperature at a rate of
10 °C min−1, and then the temperature was jumped to
220 °C just moments before the X-ray irradiation. To
prevent the morphology change such as lamellar
thickening, the SAXS data were obtained by 5 minsynchrotron radiation X-ray illumination. These profiles
include three or four maxima. The ratios of the scatteringvectors (q) at the peaks are 1, 2, 3, and 4, indicating theexistence of lamellae staking. These lamellae consist of
the smectic LC and isotropic liquid phases.[8] The
thickness of each phase lamella can be estimated bycomparing the observed intensity with the scatteringprofile calculated numerically using the paracrystal
theory.[9,10] The solid curves in Figure 1 represent thebest-fitted calculated profiles. While the third-order peaks
are well-fitted to the calculated profile, the first- andsecond-order peaks are deviated from the calculated
profiles of PB-10-12 and PB-10-23, suggesting that thecorrelation length of lamellae decreases with decrease in
xn. The thicknesses of the LC and isotropic phase lamellae(dLC and dam) and the standard deviations of each lamellar

d0/nm
48.0
46.9
42.0

dLC/nm
40.0
40.2
37.0

dam
/nm
8.0
6.7
5.0

σLC/nm5.0
3.8
3.5

σam/nm2.0
2.3
2.3

dLC/d0
0.83
0.86
0.88

The inconsistency between the variations of Ti and dLC
can be attributed to incorporation of chain ends in LC
lamellae. We can sketch the accommodation of polymer
chain in LC lamella by comparing dLC with the averagecontour length of polymer chain, L calculated from xn by
the equation of L (nm) = 2.31xn, where 2.31 is the repeat
length along the chain axis (c-axis) in the SmI phase inunit of nm. L of PB-10-50 is 115 nm, which is three times
larger than dLC. The polymer chains assume moststretched conformation in the lamella inside and fold andassemble chain ends at the lamella boundaries. In contrast,
L of PB-10-12 is 27 nm, 1.5 times smaller than dLC = 40
nm. When the one side of end is located at a lamellaboundary, the other side of end should be in the lamella.
Thus many chain ends can be included in LC lamellae inPB-10-12. The probability of finding chain ends in LC
lamellae decreases with increasing xn.
Assuming such inclusion of chain ends in LC lamellae,
the dependence of Ti on xn can be explained. In Figure 2,
the reciprocal of Ti is plotted against the reciprocal of xn.
The data are consistent with a straight line as shown in
Figure 2. Indeed, such a relationship is similar to thatbetween the melting point of polymer crystals and DP
where the end groups of polymer chains are assumed tobe included in the crystal lattice and regarded as a foreigncomponent. Thus Ti can be described by a following
equation in the same way as the melting point of polymer
crystals:[11,12]
11
R 1
  (1)Ti Ti
ΔH i x n
where R is gas constant; Ti°, Ti of the SmI LC formed byinfinite length polymer chains, and ΔHi°, the enthalpy offusion per structural unit. The data is extrapolated to Ti°of 535 K (262 °C) and the slope of the line yields ΔHi° =24.7 kJ mol−1. These estimated values seem reasonablePhoton Factory Activity Report 2014 #32 (2015) B

because they are close to the values observed for PB-1050 (Ti = 258 °C, ΔHi = 22.7 kJ mol−1).

Fig 2: Reciprocal isotropization temperature of PB-10
polyester as a function of the reciprocal of the number
average number of repeat units in PB-10 polyesters.
Such an inclusion of polymer chain ends into LC
lamellae decreases the LC fraction even in the LC
lamellae. The LC fraction can be estimated by thefollowing two ways. First is the volume fraction fv of LC
lamellae in sample which is equal to the ratio of dLC to d0.
The other is the weight fraction fw which can bedetermined as fw = ΔHi/ΔHi°. The values of fv and fw
should be comparable to each other when the LC andamorphous phases are completely segregated into eachlamella. Such a complete segregation may be achieved inPB-10-23 and PB-10-50 which show certain agreement
between the values of f v and fw. On the other hand, PB-1012 exhibited fw whose value is significantly lower thanthe value of f v, suggesting that the amorphous phase is notaccommodated only in the amorphous lamellae but in theLC lamellae.
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* mtokita@polymer.titech.ac.jp