Download Solid State NMR Studies of Complex Two Dimensional Structures

Solid State NMR Studies of Complex Two Dimensional Structures:
Polyelectrolyte Multilayers and Layered Silicates
Robert Graf, Mark McCormick, Linda Reven*, Niklas Hedin+, Bradley F. Chmelka+ and Hans Wolfgang Spiess
Max-Planck-Institut für Polymerforschung, Postfach 3148, 55021 Mainz, Germany
* Dep. of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6
+ Dep. of Chemical Engineering, University of California, Santa Barbara, CA 93106, USA
Introduction
Structure of Surfactant Controlled Silicate Layers
The 29Si CPMAS spectrum of
the layered silicate sample
exhibits five different sites
(left). The chemical bonded
network was investigated
using INADEQUATE measurements [3]. Exploring the
CSA of these sites provides a
direct method to distinguish
the Q3 and Q4 sites present in
the sample.
The inherently low sensitivity of NMR hampers in many cases studies of low dimensional entities such as films or molecular layers. However, fast
MAS in combination with advanced recoupling techniques are able to increase the signal to noise ration sufficiently such that structure and
dynamics of polyelectrolyte multilayers (PEM) [1] or surfactant controlled silicate layers can investigated.
Due to the complex structure of the samples, the most robust recoupling sequences available had to be chosen to obtain artefact free results.
The double-quantum NMR experiments have been recorded using the Back-to-Back sequence [2] in the fast MAS limit, whereas the SUPERNMR scheme [4] has been applied for chemical shift anisotropy recoupling.
-95
Polyelectrolyte Multilayer
Anionic
Substrate
Polycation
Solution
Q3
PSS 500 MHz
Polycation
Solution
After Drying
4
H 3C
8
10
O
12
14
H
H
N
+
CH 3
OS
Double Quantum Dimension [ppm]
Double Quantum Dimension [ppm]
T1 [msec]
2
H
H
O
H
H
H
H
16
9
8
7
6
5
4
3
2
6
x -x
8
Spin-Lattice
Relaxation
14
1
2
3
16
4
5
6
8
7
6
5
4
3
2
1
7
8
9
10
C
110
550
100
O
350
300
250
3
4
5
6
7
Number of Layers
8
9
10
C
S
H
H
O
H
H
H
H
H 3C
H
H
N
+
-210
CH3
La yer 2
90
L aye r 3
80
La yer 4
70
L aye r 5
60
L aye r 6
50
L aye r 7
40
L aye r 8
1
2
3
4
5
6
7
8
9
10
C
L aye r 9
Number of Layers
Water adsorbed in PEMs, shows oscillatory relaxation times and chemical shifts depending on the material of the last layer. Its T1 relaxation is in the fast motion limit .
Lay er 10
8
7
1H
6
5
4
3
12
10
8
6
4
2
0
pp m
-200
5 .0
Wate r Chemi cal Shi ft [ppm ]
Linewidth at FWHH [Hz]
400
-220
O
L aye r 1
4 .8
4 .6
4 .4
b u lk
PDADM AC
4 .2
3 .8
b u lk PS S
3 .6
1
2
ppm
MAS NMR Spectra
1H
3
4 5 6 7 8
L a y e r Nu m b e r
9
10
-95
-100
-105
-110
II.
Q3
σiso= -100 ppm
δ = 44 ppm
η = 0.23
σiso= -96 ppm
δ = 60 ppm
η = 0.23
0
-50
-100
-150
ppm
single quantum dimension
Chemical Shift
Complexation:
(i) Polycation-polyanion complexation of polyelectrolyte multi-layers (PEM) is similar to that in the bulk polyelectrolyte complex (PEC).
(ii) Water-polymer association in PEMs is much stronger than in PEC.
Polymer Dynamics:
(i) Addition of water increases the polymer mobility in PEMs but not in the PEC.
(ii) Enhanced polymer mobility is observed for hydrated PDADMAC-capped films relative to PSS capped films.
(iii) This oscillation in the polymer mobility dampens and is superimposed on a gradient of decreasing mobility with film thickness. No
changes with layer number are observed for dry films.
III. Adsorbed Water:
(i) Mobility of adsorbed water is more restricted in PEMs than in the PEC.
(ii) Water mobility is lower and water content is higher in PSS capped films as compared to PDADMAC capped PEMs.
(iii) 1H NMR peak intensity increases monotonically and its chemical shift oscillates between the PEC bulk and the bulk PDADMAC value.
direct
neighbor
Next nearest
neighbor
-115 ppm
Combining short range spatial
information from NMR methods
29Si
I.
δ = 10 ppm
-190
b u lk PE C
4 .0
Si site
ppm
-230
T1 relaxation measurements of PDADMAC-PSS multilayers find the polymers to move in the slow motion regime
and exhibit an oscillatory behavior the relaxation times
approaching the value of the polyelectrolyte complex.
Linewidth (apparent 1/T2)
600
σiso=-102.5 ppm
pulse scheme of CSA recoupling experiment
Number of Layers
Single Quantum Dimension [ppm]
Spin-Lattice Relaxation
T1 [msec]
t1=n·τR
σiso= -113.5 ppm
200
12
1H double quantum NMR indicates that the layered structure of PDADMAC-PSS
multi-layers is strongly interdigitating and resembles the polyelectrolyte complex.
2
-x x
τR
10
Single Quantum Dimension [ppm]
1
DD
CP
600
400
9
1
Overlapping peaks and weak
signal intensities hamper the
acquisition of static powder
spectra.
CSA
recoupling
pulse program [4] (shown at
the left hand side) provides
access to the static line
shapes of the individual sites,
and thus to the complete set
of CSA parameters .
sites
18
18
200
and
Q4
Dipolar Decoupling
Polyanion
Solution
6
Q3
PSS 700 MHz
800
Polyelectrolyte Complex
Formation
4
ppm
PDADMAC 700MHz
Aqueous
Rinse
2
-115
Q4
sketch of CSA tensors in
PDADMAC 500MHz
1000
Polyanion
Solution
-110
σiso= -108 ppm
double quantum dimension
After 3 cycles
-105
CP MAS spectrum of surfactant controlled layered silicate
NMR Investigations of Polyelectrolyte Multi-Layers
Polyelectrolyte Complex
-100
Q4
double quantum spectra, recorded with various DQ
recoupling times, provide the nearest and next nearest
neighbor relation (given in the table) between the sites of
the sample, leading to a proposed structure shown right.
MD simulation and quantum chemical (QC) computations
have been used to verify the structure.
with MD simulations and QC
computations can unravel low
dimensional
structures
yet
inaccessible to common scattering methods.
References
[1]
M. McCormick, R.N. Smith, R. Graf, C.J. Barrett, L. Reven and H.W. Spiess, Macromolecules 36, (2003) 3616.
[2]
M. Feike, D.E. Demco, R. Graf, J. Gottwald, S. Hafner, and H.W. Spiess, J. Magn. Reson. A 122, (1996) 214.
[3]
S.C. Christiansen, D. Zhao, M.T. Janicke, C.C. Landry, G.D. Stucky, and B.F. Chmelka, J. Am. Chem. Soc. 123, (2001) 4519.
[4]
S-F. Liu, J-D. Mao, and K. Schmidt-Rohr, J. Magn. Reson. 155,(2002) 15.