effects of ph on the gelation time in the sol-gel polymerization of 1
TRANSCRIPT
Effects of pH on the Gelation Time in the Sol-Gel Polymerization of 1 ,6- B i s (t r i met hoxys i I y I) hexane
pH 1-13 Buffered Constant Ionic Strength
0.6 M
Douglas A. b y * , Brian Mather?, Andrei R. Straumanisf, Colleen Baugher f, Duane A. Schneider f, Arturo Sanchez?,
and Kenneth J . Shea$
*Polymers and Coatings Group, MST-7, Los Alamos National Laboratory, Los Alamos, NM 87545, daloy Wan1 .gov TProperties of Organic Materials Department, Sandia National Laboratories, Albuquerque, NM 871 85-0888 $Department of Chemistry, University of California, Imine, CA 9271 7
.A LasAlamos
r
Gelation Time
Influence of pH on Gelation Time
0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2
PH
Gelation Times Long at Isoelectric point & High pH
2n H20 OR /
R 0 - 9 - 0 ~ m
OR si021 n
Cdtrain, 6. IC; Melpolder, S. M.; W a , J. M. Uhsttuct. Process. A&. Mater., [Pm. Int. Cant Uhsttuct. Process. Gram., Glasses Cbmpns.], 4th 1992,69-76.
n So what happens when you replace one alkoxide group wi attached through a hydrolytically stable Si-C bond?
Substituent Effects: Sol-Gel Chemistry Silanol Acidity Gelation
m
Sol-Gel Basics: Polymerization Hydrolysis
OR /
Rl--S\i-o~ + H20
OR
Condensation
RO- = M a , Et0
RO- = Me0
OR /
R'--S\~-OH
OR
+ ROH
OR R 4 I
OR R d RI--S\i-o-Si-RI + HOH
P R RO \
R'--S\i-O-Si-R'
OR R d
Water Producing
+ ROH
Alcohol Producing
OR /
\ OR
n R'-Si-OR + 1.5n H20 b [R'SiOl.& + 3n ROH
Condensation Chemistry: Effect of pH
I
Reaction Rates
1 1 1 1 1 1 1 1 1
2 3 4 5 6 7 8 9 10
PH
Minimum in Reaction Rate-pH Profile Dependent on SOH Acidity Silanol Acidity Increases with Degree of Condensation
Reaction Rates
(EWG: SiOSi >> OH > OR)
1 1 1 1 1 1 1 1 I 1 1 1 I
0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2
PH
Conventional Wisdom: Rate Profile shifts stepwise with each condensation to lower pH values until reaching isoelectric point
Sol-Gel Basics: Hydrolysis Kinetics
pH Dependent Cross-Over from Specific Acid to General Base- Catalyzed Hydrolysis
Reaction
RSi(OH)(OR’)2 - RSi(OR’)3 - -------- PhSi(OR’)3 - PhSI(OH)(OR’)2 Rate 1
PhSi(OH)(OR’)2 - PhSi(OH)2(0R’)
Minimum at pH 7 Small difference between
alkyl & aryl Slight shift to lower pH may be
due to presence of silanols
Moderate Substituent Effects
I I I I I I I I 3 4 5 6 7 a 9 10
PH
K. J. AkNeilet. al. J. Am. Chem. Soc. 1980, 102, 1859
Pohl. E. R.; Osterholtz, F. D. Polymer Science and Technology (Plenum) (1985), 27(Mol. Charact. Compos. Interfaces), 157-70.
cow. Rate
Si lsesq u ioxane p H -Co nde nsa t i on Rate Prof i les
I I I I I I I I I 0 1 2 3 4 5 6 7 8
’ PH
OR O H I OR I
R-Si-OH p R-Si-0-
b R b R
0
R
0
-e 9 LosAlamos
Ostedwltz, F. D.; Pohl, E. R.. J. of Adhesion W. and T&. 1992, 6(1), 127-49.
Gelation of Polysilsesquioxanes
Experimental Parameters: Measurable pH
>5W% water in MeOH Gel across most of pH 1-12 Constant Ionic Strength
& Buffered MeSi (OMe),: 2M conc. 3nly gels at pH 1 & 12
pH 1-13 Buff e red Constant Ionic Strength MeOH
0.6 M 14 L' " 1
26 vol% water A
pH Effects on Gelation Time
700
1 600
500 h
E 3 400 0 r Y
200
100
0
1 0.6 M 14.4 M
pH 113 Butl#bd Constant MmOH knic Strength
Maximum pH 4.5-5.2
.5
0 2 4 6 8 10 12 14
f ! + PH
oH Effects on Gelation Time
pH Effects on Condensation Rates
pH 1.5 T, T2
24 hrs -36 -38 -40 -42 - 4 4 -16 -18 -50 -52 - 5 4 - 5 6 -58 -60 -62 ppn
Gel in 32 hrs
Acid: Rapid Hydrolysis & lnfermediafes Observed
To pH 12
TI
3min
6min I 9min
1 h r h , . I , . , . I - 1 - I - I . I , . , . I 8 . 1 ’
-36 -38 -40 -42 -44 -46 -48 -50 -52 -54 -56 -58 -60 -62 p p n
Gel in 70 min
Base: Hydrolysis RDS & Rapid Condensa fion
pH Effects on Condensation Rates (pH 4.5)
10 min
4hrs
1day
1 month 2
0.4 M
0
9.6 M
Tl
4 T2
4 Gel in 1 year (72% Condensation)
(36.2% condensation) I ' I . I . I . I - I l ~ I ~ l ~ l ~ ~ ' 1 ~ ~ ~ ~ ~ ~ ~
-36 -38 -40 -42 -44 -46 -48 -50 -52 -54 -56 -58 -60 -62 ppm
pH Effects on Xerogel Surface Areas
700
E y500
f
Surface Area 81 Gel Time vs pH
200
100
0 0 2 4 6 8
PH
10 12 14
n m LosAlamos
Hexylene-Bridged Polysilsesquioxane Degree of Condensation vs pH
H
CP MAS Si29 NMR-Deconvoluted T1, T2 & T3
II
H
95
90
.I 85
80
r 75 ' 70 8 65
c, a
Q) w
t
60
W H H
H H
I
\
PH ,A
LosAlamos
Hexylene-Bridged Polysilsesquioxane Degree of Condensation vs pH
Nonporous II
* A r
+ 1
Nonporous
CP MAS 29Si NMR-Deconvoluted T1, T2 & T3
I
f
95
- - - _ _ -
A 3
I C I I I
90
85
80
B .
75
70
65 \
\ 60 0 2 4 6
PH Phase Separated
12
A
Materials /f+ LosAlamos !Vi?sR
Gelat ion of Polysilsesqu ioxanes
700 1
pH 1: Silica condenses fast Silsesquioxane fast
pH 12: Silica condenses slow Silsesquioxane fast
0 2 4 6 8 10 12 14
PH A
Silanol Acidities: Substituent Effect of HB? Common Belief: Acidity Increases with Degree of Condensation
-~a-cm HO’ I
HO
Increasing Acidity
Hydrogen Bonding: pKa 7-8 PKlP 7.75
PlGp 7.6
R OH
HO
PKlP 9.5 p16 9.9
R
PKfP 9-7 PKlP 8-9
Djkstra, T. W. eta/. J. Am. Chem. Soc. m 2 , 124,9856.
Summary
.First pH-Gel Time Study for Polysilsesquioxanes
.Slow Gelation pH 4.5-5.2
.NMR Indicates Slow Condensation Rate Phase Separation pH 7-1 I
.Apparent Contradiction with Substituent Effect Model
.Calculated Standard Conditions pH 1.3 & pH 12 *Porous Xerogels when Degree of Condensation > 75%
Future Efforts: Phenylene-Bridged System Systems Designed to Gauge Specific Condensation Rates
Acknowledgments
Todd Alam (SNL) for NMR
Los Alamos National Laboratory, an affirmative actiodequal opportunity employer, is operated by the University of California for the United States Department of Energy under contract W-7405-ENG-36. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE- AC04-94AL85000.