Chapter 12
Poly(dimethylsiloxane) Gelation Studies 1
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Downloaded by CALIFORNIA INST OF TECHNOLOGY on April 5, 2017 | http://pubs.acs.org Publication Date: May 4, 2000 | doi: 10.1021/bk-2000-0729.ch012
R. F. T . Stepto , D. J. R. Taylor , T . Partchuk , and M. Gottlieb
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1Polymer Science and Technology Group, Manchester Materials Science Centre, U M I S T and University of Manchester, Grosvenor Street, Manchester M1 7HS, United Kingdom Ben Gurion University, Chemical Engineering Department, 84105 Beer Sheva, Israel 2
Recent experimental gelation studies1 involving the endlinking of poly(dimethyl siloxane) (PDMS) chains of various lengths at various reactive-group concentrations (in a linear PDMS diluent) are interpreted using Ahmad-Rolfes-Stepto (A-R-S) theory of gelation2. These experimental systems have larger network-chain lengths, and cover wider ranges of initial dilutions of reactive-groups compared with polyester- and polyurethane-forming polymerisations analysed previously3. The PDMS data also include the effects of off-stoichiometry, a previously untested feature of A-R-S theory. The experimentally-measured delays in the gel-point, relative to the predictions of Flory-Stockmayer (F-S) theory , as functions of the initial dilutions of reactive groups, follow the trends predicted by A-R-S theory. Quantitative interpretation of the PDMS gel-points using A-R-S theory yields values for the effective bond length, b (a measure of chain stiffness), which are in the range expected from the known unperturbed dimensions of linear PDMS chains. 4
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© 2000 American Chemical Society
Clarson et al.; Silicones and Silicone-Modified Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2000.
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Experimental A series of PDMS networks was prepared* at 298K, using various initial dilutions of reactants, from the reaction of linear, vinyl-terminated PDMS chains (denoted R'B ) with an /-functional, hydrogen-terminated dimethyl siloxane endlinker (RA/). The diluent was inert, linear PDMS. The molar masses of the linear PDMS components used are listed in Table 1, along with their corresponding numbers of skeletal bonds, n. For a given reaction system, at each dilution, the reactive-group ratio was systematically adjusted, by increasing the initial concentration of the minority - H group (A) on the /-functional endlinker, until gelation was eventually observed. The critical reactive-group ratio, r required for gelation at complete reaction of A groups, was therefore determined in each case. The series of model networks may be denoted RA +R'B and RA4+R'B , where A represents an H atom on the tri- or tetra-functional endlinker species, and B a vinyl group (~CH=CH ) at either end of the linear PDMS pre-polymer. The structures of the reactants are shown in Figure 1, and the six reaction systems are listed in Table 2, where the linear (R'B ) components are coded according to their approximate molar masses. For example, 5K denotes the linear PDMS chain with M = 4900 ( « 5000 or "5K") g mol" , listed in Table 1.
Downloaded by CALIFORNIA INST OF TECHNOLOGY on April 5, 2017 | http://pubs.acs.org Publication Date: May 4, 2000 | doi: 10.1021/bk-2000-0729.ch012
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Theory of non-linear polymerisations and analysis of experimental data According to the classical, non-linear polymerisation theory of Flory^ and Stockmayer^ (F-S), a , the product of the critical extents of reaction at gelation,/?^ and of A and B reactive groups, respectively is related to the functionalities f and fb of the A and B units, respectively, as follows: c
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a c = P a c P b c
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U -W -r) a
( 1 )
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For non-stoichiometric polymerisation mixtures, a can be rewritten in terms of the reactant ratio, r (= Cao/cm = pdpa, where c o and Cm are the initial concentrations of A and B groups, respectively) as follows: c
fl
