Effects of pH on the Gelation Time in the Sol-Gel Polymerization of 1 ...

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.