Modulus Reduction Mechanism of Trimethylsiloxy Silicates in a

Chapter 16

Modulus Reduction Mechanism of Trimethylsiloxy Silicates in a Polyorganosiloxane

Downloaded by UNIV OF ROCHESTER on November 16, 2016 | http://pubs.acs.org Publication Date: March 10, 2003 | doi: 10.1021/bk-2003-0838.ch016

Randall G . Schmidt, Linda R. Badour, and Glenn V . Gordon Dow Corning Corporation, 2200 West Salzburg Road, Midland, MI 48686-0994

Trimethylsiloxy silicates can be added to network forming linear polyorganosiloxane-based formulations to reduce the elastic modulus of the resulting cross-linked network without the loss of ultimate strength or excessive bleeding that typically occurs when either linear oligomers or polymer additives are used. The linear viscoelastic properties of uncured blends of a polyorganosiloxane with different loadings of silicates were characterized using time-temperature superposition to investigate the physical factors responsible for the modulus drop. The reduction in the plateau modulus of the polymer was found to be consistent with the power-law relation observed for nonentanglement-forming diluents, and was accompanied by a decrease in thefrequencyrange of the plateau regime with increasing silicate content. Swelling and extraction studies on the corresponding cross-linked networks were conducted to help postulate the primary physical factors responsible for the modulus reduction. The modulus reduction enables low-durometer, high-elongation elastomeric materials useful in silicone rubbers, high-movement joint sealants and pressure-sensitive adhesives.

170

© 2003 American Chemical Society

Clarson et al.; Synthesis and Properties of Silicones and Silicone-Modified Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Introduction The interactions resulting from the addition of trimethylsiloxy silicates to linear polyorganosiloxanes has been exploited successfully in material technologies including silicone pressure-sensitive adhesives (PSA) (7, 2) and paper release coatings (5, 4). In the former, the appearance of a mechanical transition that is manifested by a maximum in the loss tangent can be controlled to tune tack and adhesive properties; in the latter, the increase in the viscous character of a coating of a cross-linked silicone network is used to control peel forces from organic PSAs. This paper focuses on the molecular mechanisms responsible for the reduction of the plateau modulus of a high molecular weight polyorganosiloxane gum that, when cross-linked, enables low-durometer, high-elongation elastomers useful as rubbers, mold-making materials, and high-movement joint sealants without loss of ultimate strength or excessive bleeding (5).

Experimental Materials Table I lists the materials in this study and some of their physical properties. A poly(dimethylsiloxane-co-phenylmethylsiloxane) (PDPMSi) was used to represent a linear polyorganosiloxane exhibiting rubbery behavior. The presence of 7.5-8.1 mol% phenylmethylsiloxane suppressed the crystalline phase observed in poly(dimethylsiloxane), which enabled the use of the time-temperature superposition technique (6) to investigate the different viscoelastic regimes of the polymer. PDPMSi also contained 0.15 mol% methylvinylsiloxane and was endcapped with a dimethylvinylsiloxy group. Three trimethylsiloxy silicates were investigated. A low molecular weight silicate (LS) was prepared from the acid-catalyzed hydrolysis and condensation of tetraethoxysilane and endcapped with hexamethyldisloxane. A medium (MS) and a high (HS) molecular weight silicate were obtainedfromthe acid-catalyzed polymerization of sodium silicate followed by a reaction with trimethylchlorosilane in a process described elsewhere (7). For comparison, a 50-mPa-s low molecular weight poly(dimethylsiloxane-co-phenylmethylsiloxane) (PDPMS ), with a phenylmethylsiloxane content of 9.7 mol% was used to function as a comparative oligomeric diluent for PDPMSi. The polymer was solvated in toluene to aid in mechanically mixing either a trimethylsiloxy silicate or the PDPMS at desired concentration levels of up to 50 wt%. The solvent was subsequently removed by placing the blend in a convection oven at 60 °C overnight followed by a 130 °C exposure for an hour. 2

2


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Table I. Physical Properties of Materials p, kg-m

Material

3

Dimethylsiloxane-cc~phenylmethylsiloxane PDPMS," PDPMS * 2

600 4

980 980

158 156

1060 1120 1180

220 320 >500


Trimethylsiloxy silicate

a

b

2 Low molecular weight (LS) 5 Medium molecular weight (MS) 14 High molecular weight (HS) 7.5-8.1 mol% phenylmethylsiloxane, 0.15%methylvinylsiloxane 9.7 mol% phenylmethylsiloxane

Cross-linked siloxane networks from blends containing PDPMSi and the nonreactive LS were prepared via a hydrosilylation reaction using a platinum complex catalyst, a diethylfumerate inhibitor, and a dimethylsiloxane-co-methylhydrogensiloxane copolymer as a cross-linker. Molds were fashioned out of fluorosilicone-coated release liners to obtain approximately 1-cm-thick slabs following a cure cycle (5 h at 22 °C; 16 h at 70 °C; and, 1 h at 130 °C) designed to remove the solvent before any cross-linking reaction occurred.

Characterization Linear viscoelastic properties were measured using a Rheometric Scientific™ RDAII. For uncross-linked blends, shear datafrom0.1 to 500 rad-s" were collected isothermally at several temperatures with 25-mm-diameter parallel plates used to collect data above -50 °C while 8-mm-diameter plates were required for colder temperatures. Thefrequencyrange was extended to 0.01 rad-s" at 25 °C, which represented the reference temperature. Strain sweeps were conducted at each isotherm to ensure that datafromthefrequencysweep were obtained in the linear viscoelastic region. Thermal profiles of the cross-linked siloxane networks were obtained in rectangular torsion at afrequencyof 1 rads" to compare the effect of either extracting the nonreactive LS silicate from the polymer network or swelling the network with LS on the shear storage modulus, G\ Extraction of the silicate from the siloxane network, with the requisite gravimetric analysis, was carried out using reagent-grade toluene; 30-mL vials of immersed specimens were placed on a rotary shaker for 24 h before being dried thoroughly. Gravimetric analysis was performed periodically while swelling the siloxane network with LS until the desired extent of swelling was attained. 1

1

1


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Results and Discussion Uncross-linked Blends Master curves for the linear viscoelastic properties of PDPMSi and its corre sponding blends with either a silicate or PDPMS were generated using timetemperature superposition (6). The temperature dependence of the modulus was taken into account from the kinetic theory of rubber elasticity. Figure 1 shows that the master curves for PDPMSi at a reference temperature of 25 °C illustrate the characteristic viscoelastic behaviors of linear polymers at different frequency regimes: glassy, transition, plateau, and terminal. The presence of a rubbery plateau is indicative of strong topological interactions that resemble the effects of covalent cross-links over a certain frequency interval, and is commonly referred to as entanglement effects. The plateau modulus, GjJ, is used to char acterize the rubbery plateau and define an average molecular weight between entanglements, M


2

t

^ = Τ Γ

(i)

where ρ is the density, R the gas constant, and Γ the absolute temperature.

«·-Terminal-)

Plateau

[-Transition -|—Glass —·»

ciju), rads

Figure 1. Master curves for PDPMSj at a reference temperature of25 °C.


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Figures 2 and 3 show the master curves for G'and the loss tangent, tan 5, at a reference temperature of 25 °C for blends of LS and HS in PDPMS respec tively. Using the approximation proposed by Wu (8), G° was determined from G ' at the tan δ minimum in the plateau region. b


N

Figure 2. Effect of volume fraction of a low molecular weight silicate, (l-φ), on the viscoelastic properties of PDPMSi at a reference temperature of 25 °C.

Figure 3. Effect of volumefractionof a high molecular weight silicate, (l-φ), on the viscoelastic properties of PDPMSj at a reference temperature of 25 °C.


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Table II summarizes the characteristics of the rubbery plateau region for the blends as a function of the polymer volume fraction, φ. In this concentration range, the introduction of a second component lowered the plateau modulus relative to the base polymer. If the blend density can be described by a linear mixing rule, the primary effect of the second component from eq 1 was to dilute the concentration of polymer chains per unit volume resulting in a reduced number of pair-wise contacts and, hence, a reduction in the number of entan glements in the base polymer. This effect is generic for the addition of any nonentanglement-forming diluent to a polymer (P). The reduction in G° scales as Downloaded by UNIV OF ROCHESTER on November 16, 2016 | http://pubs.acs.org Publication Date: March 10, 2003 | doi: 10.1021/bk-2003-0838.ch016

N

Glb)

= G l \ è ' \φ=\

(2)

where a is the concentration exponent and has a value of 2.0-2.3 for typical dilu ents (70). Theoretically, a ~ 2 if every contact between two chains has a constant

Table II. Blend Properties in the Viscoelastic Plateau Region Plateau region,

Blend

decades of ω

Φ"

G° , MPa

M kg mot

1.00 0.90 0.80 0.70 0.60 0.50

0.18 0.14 0.11 0.080 0.059 0.037

13.7 17.0 22.1 30.3 41.1 65.8

6.2 5.7 5.3 4.9 4.8 3.8

LS

0.91 0.81 0.72 0.62

0.15 0.10 0.076 0.056

16.7 24.1 32.7 44.7

5.2 2.9 2.1 2.3

MS

0.91 0.82 0.68 0.53

0.15 0.12 0.069 0.032

16.9 21.4 36.7 80.9

5.7 4.6 3.0 2.1

HS

0.92 0.83 0.69 0.64

0.14 0.10 0.065 0.049

17.9 24.1 39.6 53.0

6.0 4.8 3.3 3.3

Component PDPMS

a

1

2

N

0

Polymer (PDPMS0 volume fraction.


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probability of entanglement (P) whereas scaling laws applied to conditions for the overlap of random coils predict a = 9/4 (11). Figure 4 plots the effect of the low molecular weight blend component on the plateau modulus of the siloxane polymer. The concentration exponent a in eq 2 using PDPMS was 2.2 ± 0.07 as may be expected for a good solvent. However, blends based on the trimethylsiloxy silicate exhibited a values in excess of the typical range of 2.0 to 2.3, and increased from 2.5 ± 0.16 to 2.8 ± 0.13 with increasing silicate molecular weight. Values for a as high as 3.6 have been reported previously for poly(w-butylmethacrylate)/diethylphthalate systems (72). It was hypothesized that the silicate particles not only reduced the number of polymer pair-wise contacts but also induced a stiffening effect, which reduced the convolution of the polymer chains and resulted in enhanced a values. Further evidence for this stiffening effect, or restricted mobility of polymer chains, was from the observation that, unlike PDPMS , the silicates raised the temperature of the primary mechanical transition of the blend relative to the glass transition of PDPMSi (T = 158 K) as shown by the temperature depend ence at which tan δ exhibited a maximum for the series of MS/PDPMSi (top) and HS/PDPMSi (bottom) blends in Figure 5, for example. At a given silicate volume fraction, the higher its molecular weight, the more efficient it was in increasing the transition temperature and in reducing the plateau modulus of the base polymer.


2

2

g

Figure 4. Effect of the low molecular weight blend component on the plateau modulus ofPDPMSj, G° (φ=1), as a function ofPDPMSj volumefraction,φ. Also tabulated are the estimate and 95% confidence interval for the concentra tion exponent a in eq 2 obtained by the linear regression analysis of the slope. N



177

-100

-50

0

50

Temperature, °C

Figure 5. Effect of the medium (top) and high (bottom) molecular weight silicate on the primary mechanical transition ofPDPMSj as a function of polymer volumefraction,φ.

Table II also shows that a decrease in the extent of thefrequencyrange of the rubbery plateau region accompanied the reduction in G„ as thefractionof the low molecular weight component increased, where a more significant effect again was obtained using the silicates. This was consistent with the hypothesis that the addition of the trimethylsiloxy silicate reduced the number of effective polymer chain entanglements, thereby enabling the onset of long range translational flow atfrequencieshigher than that exhibited by the base polymer alone.


178

Organic tackifier resins used in organic PSA formulations are blended with a base polymer and, by definition, raise the T and lower the plateau modulus of the system (13). Tse (14) reported that the addition of a resin tackifier to styrene-isoprene block copolymers decreased thefrequencyrange of the plateau region and lowered G° with a concentration exponent a of approximately 2. The trimethylsiloxy silicate materials in this study have been shown previously to be effective tackifier resins for linear polyorganosiloxanes. g


N

Silicates in a Cross-linked Siloxane Network The formation of a covalently cross-linked network effectively locks into place polymer chain entanglements, and provides a scheme in which to further elucidate the effect of the silicates on a highly entangled linear polyorganosiloxane. Figure 6 shows the temperature dependence of G'at 1 rad-s" for the P D P M S based networks. The incorporation of 0.38 volumefractionnonreactive LS prior to cross-linking reduced considerably the network modulus (•) relative to the unfilled siloxane network ( · ) . After extracting more than 98 wt% LS from the network, the network modulus (O) remained essentially unchanged. It was hypothesized that the silicate effectively reduced the number of entanglements in PDPMSi prior to cross-linking—thus, reducing the plateau modulus—and that this effect was maintained even after the silicate was extracted from the crosslinked network. 1

r

Figure 6. Effect of the non-reactive low molecular weight silicate (LS) on a cross-linked PDPMSj network before and after extraction.



179

Another experiment to obtain additional insight into the modulus reduction mechanism involved swelling a cross-linked PDPMS! network with the silicate. Figure 7 shows that although a slight reduction was evident in the network modulus of PDPMSi when swollen with 0.42 volumefractionLS, the decrease was significantly less than if the non-reactive silicate was blended into the polymer prior to being cross-linked. Hence, the silicate was unable to alter the distribution of physical entanglements, which is the hypothesized mechanism for modulus reduction, if it is subsequently incorporated into a cross-linked network. This was consistent with the prediction that the reduction of the elastic modulus of a swollen network scales as φ in a θ solvent (75, 16) or the experimental observation φ in a good solvent (77), unlike the φ behavior when LS was blended into the precursor polymer. The utility of the modulus reduction mechanism can be exploited to enable low-durometer, high-elongation elastomers useful as rubbers, mold-making materials, and high-movement joint sealants. For example, trimethylsiloxy silicates can be added to standard silicone sealant formulations at loadings of 520 wt% to significantly reduce the modulus (> 30%) and enhance the elongation (> 30%) of the fully cured sealant without sacrificing strength. When a linear oligomer or polymer is utilized to reduce modulus, excessive bleeding of the diluent usually occurs that results in undesirable surface wetness and staining. The particulate nature of the silicates minimizes bleed and there is no surface wetness if solid silicates are employed. 1/3

7 / η

2 5

Figure 7. Effect of the non-reactive low molecular weight silicate on a PDPMSi network: PDPMSi cross-linked with 0.38 volume fraction LS (A); and, PDPMSi network swollen with with 0.42 volume fraction LS (Ώ) .


180


Conclusions Trimethylsiloxy silicates were shown to reduce the rubbery plateau modulus of a linear polyorganosiloxane and significantly reduce the extent of the plateau region in simple blends. The extent in the reduction of the polymer plateau modulus, as a function of the silicate concentration in the blend, exceeded that predicted and typically observed when using simple diluents. It was hypothe sized that the silicate particles not only reduced the number of polymer pair-wise contacts but also induced a stiffening effect, which reduced the convolution of the polymer chains and enhanced the modulus reduction. The ability of these trimethylsiloxy silicates to both increase the primary transition of the blend and lower the plateau modulus relative to the base polymer makes them well suited to function as tackifier resins in polyorganosiloxane systems.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Wengorvius, J. H.; Burnell, T. B.; Zumbrum, Μ. Α.; Krenceski, M . A. Polym. Preprints 1998, 39(1), 512. Lin, S. B.; Krenceski, M. A. Proc. Annu. Meet. Adhes. Soc. 1996, 363. Gordon, G. V., Tabler, R. L.; Perz, S. V.; Stasser, J. L.; Owen, M . J.; Tonge, J. S. Polym. Preprints 1998, 39(1), 537. Gordon, G. V.; Schmidt, R. G. J. Adhesion 2000, 72, 133. Juen, D. R.; Rapson, L. J.; Schmidt, R. G.; U. S. Patent 5,373,078, 1994 Dow Corning Corp. Williams, M.L.; Landel, R. F.; Ferry, J. D. J. Amer. Chem. Soc. 1955, 77, 3701. Daudt, W.; Tyler, L. U. S. Patent 2,676,182, 1954 Dow Corning Corp. Wu, S. J. Polym. Sci., Polym. Phys. 1987, 25, 557; 2511. Ferry, J. D. Viscoelastic Properties of Polymers; Wiley: New York, 1980. Graessley, W. W. Adv. Polym. Sci. 1974, 16, 1. DeGennes, P. G. Macromolecules 1976, 9, 587; 594. Stern, D. M.; Berge, J. W.; Kurath, S. F.; Sakoonkim, C.; Ferry, J. D. J. Colloid Sci. 1962, 17, 409. Chu, S. G. In Handbook of Pressure Sensitive Adhesive Technology; 2nd ed.; Satas, D., Ed.; Van Nostrand Reinhold: New York, 1989; ρ 176. Tse, M. F. J. Adhesion Sci. Technol. 1989, 3, 551. James, H.; Guth, E. J. Polym. Sci. 1949, 4, 153. Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. Obukhov, S. P.; Rubenstein, M.; Colby, R. H. Macromolecules 1994, 27, 3191.


Modulus Reduction Mechanism of Trimethylsiloxy Silicates in a

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