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nematic polymer liquid crystals
M. Warner, D. Williams
To cite this version:
M. Warner, D. Williams. NMR spin-lattice relaxation from molecular defects in nematic polymer liquid crystals. Journal de Physique II, EDP Sciences, 1992, 2 (3), pp.471-486. �10.1051/jp2:1992144�.
�jpa-00247644�
Clmsification Physics Abstracts
36.20 61.30 03.20 77.00
NMR spin-lattice relaxation from molecular defects in nematic
polymer liquid crystals
M. Warner and D.R.M. Willianw(°)
Cavendish Laboratory, Madingley Road, Cambridge CB3 ORE, G-B-
(RecHved 26 July1991, re~ised 25 October1991, accepted 21 November 1991)
Abstract. We calculate the contributions Ji Iwo) and J2(2wo) to the NMR relaxation rate
Tp~(wo) caused by the motion of proposed molecular defects in polymer liquid crystals. The results are Ji c~ w(/~ and J2 oc wj~/~ for low Lamor frequencies wo. The scaring of the J functions with chain length is also given. It is hoped that NMR may offer a way of deterrn%ing
the contribution of molecular defects to dynamical processes in well ordered polymer liquid crystal melts.
1 Introduction.
In a series of papers [1-3] etc., we described the molecular dynamics of polymer liquid crystals (PLCS) in their well-ordered phases by the motion of their molecular defects. For comb polymer Iiquid crystals these defects were torsional [I] a region of upward pointing teeth along the backbone being separated from a downward region by a twist through 180°. For main chain
polymers the defects were [2, 3] de Gennes' hairpins [4], that is, changes in direction from up to down of the chain backbone. In each case the constraint of a nematic field makes the defect-mediated dynamics of nematic polymers radically dilserent from that met in conventional isotropic flexible polymers. Our purpose here is to calculate the signature in dynamical NMR
experiments of these dynamical processes in PLCS. NMR offers possibly the most direct test of how realistic these models are and we hope to stimulate with this paper tests of the foregoing theory.
The dynamics of teeth in a comb or side chain PLC (SC PLC) is relatively straightforward
and we thus start with this example. In contrast, main chain polymers (MC PLCS) have
exceptionally complicated motion, the inextensibility of a worm making the equations of motion
highly non-linear and non-local- A model that would appear to capture the essence of this (* Pmsent addmss: Matefials Department, College of Engineering, University of Cafifomia, Santa Barbara, CA 93106 Santa Barbara, U-S-A-
motion is the "diffusing pulley" [2, 3]. In both the SC and MC cases we give expressions for the correlation functions Ji and J2 that determine Tp~(wo), the longitudinal relaxation rate in NMR.
We shall find that Tp~ ci up~/~ holds for times ci wp~ long compared with that needed for a defect to diffuse through its own length and for times short compared with that needed for a defect to diffuse completely along a chain. These times will be quantified below.
This result is that expected for a simple diffuser. The complicated diffusional mechanism for MC PLC defects does not seem to influence the characteristic exponent of -1/2. It instead manifests itself in the length dependence (L) of the prefactors of the wp~/~ result for the rate.
The -1/2 exponent is also that which one expects from the director fluctuation modes in 3-D [S] and hence the molecular mechanism we discuss must be distinguished from the director
effects by supplementary experiments possibly by using the L dependence or by varying the
angle between the NMR field and the director. In a concluding section we discuss experiments already carried out by Kothe et al. [6].
The w[~/~ result h that for the dominant relaxation mechanism at low frequencies. We shall find, as a consequence of orientational localisation to the director that there is another
contribution of the form w(/~ arising from the Ji term. By the use of appropriate pulse
sequences [7], these two relaxation mechanisms are experimentally separately discernible [8].
2. Nature of the defects and NMR correlation fianctions.
For concreteness imagine a pair of interacting protons with a separation vector anchored rigidly
in a mesogenic unit of the chain, that is, either in a tooth of a comb polymer or in
a rigid
section of the backbone of a MC polymer. Corrections for the separation vector not being parallel to the chain axis may easily be applied. Equally one can imagine a deuteron in the electric field gradient of a chemical bond, that is a system with an electric quadrupole moment
instead of a proton pair.
In a nematic enviroment the dipolar interaction between the proton spins gives a splitting proportional to the nematic order and the coupling strength see for example de Gennes [9]. The nematic environment is locally very mobile and these lines suffer motional narrowing associated with the dipole pairs seeing an averaged environment. However non secular (oil diagonal) parts of the interaction induce transitions between the states of dilserent z-spin and hence, since the lifetime is now finite, a line broadening is produced. Such transitions contribute to the spin lattice relaxation rate Tp~(wo) thus:
j
= WI/~ dtlexP(iwot)lfi(t)Fi(°)) + exP(2iWot)lF2lt)F11°))1, Ii)
where wD
" h7~/d~, 7 is the proton gyromagnetic ratio and d the distance between the protons
in a pair. As we note above, this is a homonuclear dipole-dipole relaxation mechanism. The first part of (I) is conventionally called Ji(wo) and the second part J2(2wo). Here the F
functions are given by:
~ ~~~ 2~§ ~~~ ~~~~ ~°~ ~~~~~~~~~~~~~~' ~~~
F2(t) = sin~@(tj expj2i#(t)j. (3)
The averages of the F functions are taken over all proton pairs and all chains. Using the fact that the correlation functions are stationary we have <g(t)g°(0)) = (g°(-t)g(0)). It is easy to
show that Ti is real and that an alternative expression is:
~
= 2%~(WIj~ dtlexP(iwt)lfi(t)Fi(°)) + exPl2iWt)lF2(t)Fi(°))1) (4)
where %~ denotes the real part. See [5] for a discussion and [10] for a derivation. These results
are discussed in the context of monomeric liquid crystals in [11].
and # are the polar and azimuthal angles of the chain element containing the proton pair.
They are measured with respect to the director which we take as being parallel to the NMR field. In highly ordered phases ci 0 or ~, except in the region of a defect. For uniaxial phases
# is random. The defects we envisage are sketched in figure I. In each case one can show 11, 2, 4] that for long chains:
tam@(S)/21 = exP(S/1) (5)
where s is the arc length along the chain and where the defect is located at s
= 0. The
characteristic length I is given by I
=
@@ where
e is the torsional (for combs) or bend
(for MC polymers) stiffness, a is the nematic coupling constant, and S is the order parameter.
The latter must be close to I for well defined hairpins to exist. The form (5) and the geometric quotient I are a consequence of the compromise between two competing effects, elastic and nematic. The former requires that the transition up to down be as slow as possible, the latter
as fast as possible, as one changes arc position long the chain.
We now introduce two simplifying assumptions: (a) we are in a regime of temperature T and
length L such that there is only one hairpin per chain (we later see how this can be relaxed).
(b) The nematic melt is dense enough that # motion of the portion of the chain with a hairpin
is supressed by the steric obstruction of the neighbouring chains, at least on timescales where motion is still important. For MC hairpins this may be a good assumption: # values are relevant whenever # 0, that is when the protons pass over the hairpin. What is in question is whether or not the # direction of the chain at a particular proton pair changes between visits of the hairpins. For # to change the chain has to rotate about an axis through the hairpin and parallel to i1. The smaller scale thermal undulations (shown in Fig. lb but supressed in
Eq. (5) see [12] for a discussion of why (5) is still usefill for discussing thermal effects) in fact give the chain a transverse dimension greater than I. This means that the arms of the
hairpin are hindered from rotating because they are in conflict with the arms of many other chains. We expect this hinderance to make # rotational motion of the hairpin extremely slow.
If the # motion were totally free then it would provide a rapid relaxation process which would totally dominate the motion(~). For side chain polymers the static nature of # is much more
questionable since the backbone can move at least locally, and hence the angle, #, of the plane
of @-rotation can change. In the simplified picture of figure la this plane swept out by the
motion is perpendicular to the backbone axis and hence it (and #) move as the backbone does
Having, by assumption (b), frozen # it drops out of (I) and can hence be neglected in (2), (3) and (4). Consider a hairpinned chain which has one arm of length si(t), that is si(t) is the position at time t of the hairpin measured from the end of the chain, as shown in figure
(~) For an unobstructed hairpinned chain -one
can see that the two titnescales
are comparable.
The time for a proton pair to diffuse through a hairpin length I is rA
oJ
l~ID
oJ A~L/Do where the curvilinear diffusion constant D scales like Do/L. For Add angular diffusion the drag is also
proportional to L in the absence of wiggles that increase the lateral extent. Hence (#~)
oJ I
Djrj - rj =
oJ I/Dj
oJ L as in the curvilinear case.
Proton pulp ~teeth
~ ~~
la)
/defect
~~ ~~~
fi-
' 16) (c)
~ l
I
~
-
i~
"
proton pair
e
Fig. 1. (a) A typical side chain polymer liquid crystal in the oblate phase NI. The teeth point up
or down with respect to the director fi. A torsional defect between up and down teeth is shown. lb
A main chain Iiquid crystal polymer with one hairpin defect. The proton pair at arc position s has its principal axis parallel to the chain axis. The extent I (the hairpin length) is also shown. (c) The
angles 8 and # of
a tooth or backbone with respect to the director fi.
2. sp is the distance of the proton pair from the end of the chain. Then the angle
@p that the proton pair axis has with respect to the director (also taken to be the NMR field direction) is:
Spit) = 2tan~~lexPl~~ )~~~~
)l, 16)
that is, it depends on the distance between the pair and the current position of the hairpin.
The F's are then functions of y = (sp si(t)) II since we put @p(t) from (6) into (2) and (3) respectively:
~~~~ 2~§~s~~ ~~~
proton pal r
Id
w
s~
Fig. 2. The coordinate system used in the evaluation of ii(t) and f2(t) for a hairpin defect in a main chain PLC via the diffusing pulley model. The length si is the length of one of the arms at time
t, and sp (a constant in time) is the arc length position of the proton pair in question. The transverse
size d of the hairpin is set equal to zero for the purposes of dynamics but is shown as finite here for
clarity.
and
~~~~ ~
4cosh~ y ~~~
The two contributions to Tp~ are then obtained by Fourier transforming time correlation functions f(t):
fjt) dSlG(~1' ~0',f)fir(SP ~l ~~Sp So~j ~g~
~ ~ ~p,SO
where G(si, soil) is the probability that if the defect is at so at t = 0 then it is at si at a time t. This Green's function thus contains the information on the molecular dynamics. In addition
one should average over starting points so of the defect and over choices sp for the proton pair
so that one obtains:
f(1) ~ /
~~P / ~~0 / dSlG(Sl>SO>t)F(~~
~
~~)F(~~
~
~~). (10)
~ ~ ~
In proceeding from (9) to (10) we have assumed that the proton pairs are distributed uniformly along the chain and that the distribution of arm lengths of the chain is uniform. It now remains
to calculate f(t) for various dynamical evolutions G, that is for the various models applicable
to SC and MC PLCS.
3 Molecular dynamics.
3.I SII~E CHAW PLCS. In [I] a stereo regular comb polymer was envisaged. This meant that in addition to being nematically ordered (Ni in the nomenclature of [13]) the teeth are positioned on the same side of the chain, unless a torsional defect (costing torsional and nematic
energy) intervenes. Such a defect can move without the cost of further energy by the flipping
of teeth adjacent to the defect [I]. We re-adopt thin model, with the relaxation of the sterec- chemical requirement that for NMR we are not concerned with a regularity of attachment.
In dielectric response ii] we wanted a defect to delineate regions of up and down teeth and hence a reversal of dipole direction to be associated with a defect. Here it is a llamiltonian of
quadrupolar symmetry that gives the broadening and hence no distinction is drawn between up and down. Hence from the point of view of NMR dynamics, the regularity with which side
teeth
are attached to the backbone is unimportant. Of the essence here is that to turn
a tooth
over requires torsional energy, except where the tooth lies next to a defect between sequences of teeth otherwise in their natural position. Flipping the tooth in question then causes the
defect to move I unit (in the direction of the tooth). Thus for this aspect of SC dynamics
careful stereoregular synthesis is not vital and we hope that our present model will be relevant for currently available SC systems in nematic and smectic phases.
In this situation a defect takes « steps of length (of random direction along the chain) per
unit time. Standard random walk statistics yields (h~) = «l~t, whence by identification the diffusion constant for such motion is D = «l~. One can immediately write down the propagator G as the Gaussian form since we are dealing with a simple random walker:
~~~~ ~°'~~ ~2~Dt~
~~
~~~
~~2ji~~~
~' ~~~~
This is for times short enough that a defect does not explore the whole chain, I-e- t < L~ID.
A qualitative analysis of (10) is most informative. The functions Fi and F2 are highly
localised (to within I) of sp, and G is finite for values of si within Dt of so (see Fig. 3). For
appreciable contributions to (10) one must evidently have so and si both within I of sp. If Dt » l~ (that is
a defect has time to diffuse through a distance much greater than its own
length), then G is broad compared with I and doesn't distinguish between different values of
(si so) in the range (-I, I) and one can efsectively set si
# so in G. The result is just the
i 1/2
return probability G(0; t) = (-) There remains fdsiF2((sp si)/1]
= 31/2 and then
2xDt
similarly the f dso whence:
The same analysis for fi(t) requires more care as Fi is anti-symmetric about the origin.
Then f dzfi(z) = 0 and the above method yields zero. More accurate analysis may be found in appendix A. The result is:
~~~~
~~~~~j~~~)~~~
(13)
The difference between the fi and f2 results arises from the different symmetry of the F functions and the rigid orientational constraint of the nematic field which has F
= 0 everywhere except in the defect.
From the definitions (7) and (8) one sees that F2 is smaller than Fi when ci 0, but since the main contributions to Tp~ comes when the proton pairs are in a defect, then the » 0
values dominate. The fact that the two halves of the Fi function cancel and those of the F2
do not, means F2 is much more efficient in broadening the lines.
The final result for Tp~(wo) for SC PLCS is, at the most divergent order,
Tii(wo) m i iii ~'~
+ i f
+ (14)
F or
6(s s .t>
~(18~ -s~ I/J~ ' ° ~
F~ (is~ -s~ I/J~ )
/
Dt
s
h ~
~p ~0
Fig. 3. The functions Fi F2 and G which enter equation lo). Both Fi and F2 have a characteristic width A. The propagator G has a width @. Since in the graph we have (sp sol > this particular
situation only contributes an exponentially small amount to (10).
3.2 MAIN CHAIN DYNAMICS. After considerable analysis we have suggested in previous
work [2, 3] that a reasonable and tractible model for hairpin dynamics is that of the "diffusing pulley~. A hairpin can move by the diffusional motion of either one or both of its arnw. The
drag on an arm is proportional to its length. Its mobility is therefore proportional to the inverse of its length. It turns out [2] that one must add the mobilities of the two arrns to
get the overall mobility for a hairpin. The probability function for the length, lV(si>t) then satisfies the diffusion equation:
~ ~~ ~i i
~ L si ~~~' ~~~~
where p is the drag per unit length. Considering diffusion along the line (0,L) inspection of equation (IS) shows that the hairpin has a position-dependent diffusion constant. The diffusional rate is large if si or L si is small. That is, when one the arnw is short and thus little drag is exerted, it moves quickly and hence contributes efsectively to the motion of the
hairpin. It is clear that although a hairpin is a localised defect its motion is highly non-local and requires the rest of the chain to move.
There are two main timescales in this problem. The longest is the time it takes a hairpin to diffuse along the length of the chain and be destroyed at the ends. This is roughly [3]
~~3
TL " q. (16)
One can see this by scaling si by L in (15) and extracting L~ on the right hand side which then sets the scale oft on the left hand side as is explicitly done in appendix B. More qualitatively
one sees that most of the time the hairpin arms have length scaling like L, actually being
~- L/2,
and hence the diffusion constant is effectively D
~- kBT/pL. A time can then be extracted from the diffusion law L~
= DTL. Thus the characteristic time scale is TL
'~
L~ID
~- L~p/kBT.
The second timescale is that taken by a hairpin to diffuse through its own characteristic width I, and is approximately:
~ TX "
~( (17)
These timescales are widely separated because L > I. We are interested in times t such that TX < t < TL. At shorter times than this one is looking at molecular details and the motion of
thermal fluctuations or wiggles on the chain. At times longer than TL one needs to account for
hairpin creation and destruction.
The diffusion equation (15) allows us in principle to calculate the propagator for hairpin motion:
~
G(si>soil) = £ fltn(si)lvn(so)e~~*~, (18)
n=o
where en and lvn are the eigenvalues and eigenfunctions of:
-enlvn(si) = ~(~£(( +
~
~"
si E (0,L), (19)
P Si Si Si Si
and the eigenfunctions are orthonormal:
f~ds~v~(s)~v~(s) = J~~. (20)
Putting the G of (18) into equation (10) for f(t) we obtain:
f(f) = f e~~"~/~
dspq((Sp) +
2 L ~~~~~~~~~ ~~~~
L~
~_~ 0 ~ ~0
where the functions qn(s) are given by
qn(sp) = f~ dsotvn(so)F(~P j ~° (22)
(23)
and p(~) ~Y
p~(n) " j j~
~~~~~~~~~
For the F2 case one can make straightforward progress. One expects that the lvn(so) for small n and for L » I not to vary rapidly on the length scale I. Under these conditions, over
the range of so for which F2 is finite (I.e, within roughly I of s) one can take tvn(so) to be constant, namely lvn(sp), and qn(sp) then becomes:
qn(sp) = ~vn(sp) f dsof~(~P j ~°)
= 2>tvn(sp). (24)
~
The the @n) part of the summand in (21) becomes p(n) = 41~f)dsp~((sp) which by orthonormality can then be evaluated trivially and there remains
~~~f~ = (3)~(2fl~ f
e~~"~ (25)
~ ~
n=0
