Effects of ph on the Gelation Time in the Sol-Gel Polymerization of 1,6-Bis(trimethoxysiIy I)hexane

Transcription

1 Effects of ph on the Gelation Time in the Sol-Gel Polymerization of 1,6-Bis(trimethoxysiy )hexane ph 1-13 Buffered Constant onic Strength 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 Wan1gov TProperties of Organic Materials Department, Sandia National Laboratories, Albuquerque, NM $Department of Chemistry, University of California, mine, CA A LasAlamos

2 nfluence of ph on Gelation Time Gelation Times Long at soelectric point & High ph Gelation Time R-9-~ / n Substituent Effects: Sol-Gel Chemistry Silanol Acidity Gelation m m si21 n Cdtrain, 6 C; Melpolder, S M; W a, J M UhsttuctProcess A& Mater, [Pm nt Cant UhsttuctProcess Gram, Glasses Cbmpns], 4th 1992,69-76 So what happens when you replace one alkoxide group wi attached through a hydrolytically stable Si-C bond? r 2n H2

3 Sol-Gel Basics: Polymerization Hydrolysis / Rl--S\i-o~ + H2 / R'--S\~-OH + ROH Condensation RO- = M a, Et R4 R--S\i-o-Si-R + HOH Rd Water Producing PR RO \ R'--S\i-O-Si-R' Rd + ROH RO- = Me Alcohol Producing / n R'-Si- + 15n H2 b [R'SiOl& + 3n ROH \

4 Condensation Chemistry: Effect of ph Reaction Rates Minimum in Reaction Rate-pH Profile Dependent on SOH Acidity Silanol Acidity ncreases with Degree of Condensation (EWG: SiOSi >> OH > ) Reaction Rates Conventional Wisdom: Rate Profile shifts stepwise with each condensation to lower ph values until reaching isoelectric point

5 Sol-Gel Basics: Hydrolysis Kinetics ph Dependent Cross-Over from Specific Acid to General BaseCatalyzed Hydrolysis RSi( )3 PhSi( )3 -- PhSi(OH)( )2 1 Reaction RSi(OH)( )2 PhS(OH)( )2 Rate PhSi(OH)2(R ) Minimum at ph 7 Small difference between alkyl & aryl Slight shift to lower ph may be due to presence of silanols Moderate Substituent Effects K J AkNeilet al J Am Chem Soc 198, 12, 1859 Pohl E R; Osterholtz, F D Polymer Science and Technology (Plenum) (1985), 27(Mol Charact Compos nterfaces), a 9 1

6 Silsesquioxane ph-condensat ion Rate Profiles R-Si-OH OH pr-si-- br br cow Rate R Ostedwltz, F D; Pohl, E R J of Adhesion W and T& 1992, 6(1), e LosAlamos 9

7 Gelation of Polysilsesquioxanes Experimental Parameters: Measurable ph >5W% water in MeOH Gel across most of ph 1-12 Constant onic Strength & Buffered MeSi(OMe),: 2M conc 3nly gels at ph 1 & 12 ph 1-13 Buff e red Constant onic Strength MeOH 6 M 14 L' " 1 26 vol% water A

8 ph Effects on Gelation Time ph Butl#bd 1 1 Constant MmOH knic Strength 6 M 144 M Maximum ph E h 3 4 r Y f 2!

9 oh Effects on Gelation Time

10 ph Effects on Condensation Rates ph 15 T, T2 ph 12 T To 3min 6min 9min 24 hrs 1h Gel in 32 hrs Acid: Rapid Hydrolysis & lnfermediafes Observed r, ppn , -42, h , -56, Gel in 7 min Base: Hydrolysis RDS & Rapid Condensafion 1 ppn

11 ph Effects on Condensation Rates (ph 45) 96 M 4 M Tl min Gel in 1 year (72% Condensation) T2 4hrs 1day 1 month 2-36 ' (362%condensation) l ~ ~ l -62 ppm ~ l ~ ~ '

12 ph Effects on Xerogel Surface Areas Surface Area 81 Gel Time vs ph 7 E y5 f m n LosAlamos

13 Hexylene-Bridged Polysilsesquioxane Degree of Condensation vs ph CP MAS Si29NMR-Deconvoluted T1,T2 & T a 85 c, 8 ' H H t Q) w r H W H H H 65 \ 6,A LosAlamos

14 Hexylene-Bridged Polysilsesquioxane Degree of Condensation vs ph CP MAS 29SiNMR-Deconvoluted T1,T2 & T Nonporous A f 3 7 C - - B 65!Vi?sR A * r 8 6 Nonporous \ \ Phase Separated Materials /f+ LosAlamos A

15 Gelat ion of Polysilsesqu ioxanes 7 1 ph 1: Silica condenses fast Silsesquioxane fast ph 12: Silica condenses slow Silsesquioxane fast A

16 Silanol Acidities: Substituent Effect of HB? Common Belief: Acidity ncreases with Degree of Condensation -~a-cm HO HO ncreasing Acidity PKlP 775 Hydrogen Bonding: pka 7-8 PlGp 76 R OH HO PKlP 95 p16 99 R PKfP 9-7 PKlP 8-9 Djkstra, T W eta/ J Am Chem Soc m 2, 124,9856

17 Summary First ph-gel Time Study for Polysilsesquioxanes Slow Gelation ph NMR ndicates Slow Condensation Rate Phase Separation ph 7-1 Apparent Contradiction with Substituent Effect Model Calculated Standard Conditions ph 13 & ph 12 *Porous Xerogels when Degree of Condensation > 75% Future Efforts: Phenylene-Bridged System Systems Designed to Gauge Specific Condensation Rates

18 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-745-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- AC4-94AL85