Solution Properties of Poly(dimethyl siloxane)

Solution Properties of Poly(dimethyl siloxane) EBRU AYLİN BÜYÜKTANIR, ZUHAL KÜÇÜKYAVUZ Department of Chemistry, Middle East Technical University, Ankara 06531, Turkey Received 31 January 2000; revised 28 July 2000; accepted 2 August 2000 ABSTRACT: The solution properties of poly(dimethyl siloxane) (PDMS) were studied with light scattering (LS), gel permeation chromatography/light scattering (GPC/LS), and viscometry methods. PDMS samples were fractionated, and the weight-average molecular weights, second virial coefficient, and the z-average radius of gyration of each fraction were found according to the Zimm method with the LS technique. In this work, the molecular weight range studied was 7.5 10 4 to 8.0 10 5. Molecular weights and molecular weight distributions were determined by GPC/LS. The intrinsic viscosities of these fractions were studied in toluene at 30 C, in methyl ethyl ketone (MEK) at 20 C, and in bromocyclohexane (BCH) at 26 C and 28 C. The Mark Houwink Sakurada relationship showed that toluene was a good solvent, and MEK at 20 C and BCH at 28 C were solvents for PDMS. The unperturbed dimensions were calculated with LS and intrinsic viscosity data. The unperturbed dimensions, expressed in terms of the characteristic ratio, were found to be 6.66 with different extrapolation methods in toluene at 30 C. 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2678 2686, 2000 Keywords: poly(dimethyl siloxane); light scattering; gel permeation chromatography/light scattering; unperturbed dimensions INTRODUCTION The high molecular weight polymers consisting of an all silicon oxygen backbone with two alkyl and/or aryl substituents on each silicon atom are known collectively as polysiloxanes. One of the most important fundamental properties of all members of the polysiloxane [SiR 2 OOO] n family is the highly pronounced inherent conformational flexibility of their completely inorganic main-chain backbones, O[SiOO] n O, which enables the unusually high mobility of their segments and entire molecules. Poly- (dimethyl siloxane) ([Si(CH 3 ) 2 OOO] n ; PDMS) has some unique and very important physical properties, including elasticity, very low glass-transition temperature, high solubilities in nonpolar solvents, high gas permeability, resistance to oxidation and radiation effects, and stability at Correspondence to: Z. Küçükyavuz (E-mail: zuhal@metu. edu.tr) Journal of Polymer Science: Part B: Polymer Physics, Vol. 38, 2678 2686 (2000) 2000 John Wiley & Sons, Inc. high and low temperatures. 1,2 Because of its unique properties, PDMS has been the subject of various studies with respect to its solution properties. There are several studies in the literature 3 5 on the solution properties of PDMS. For the most part, intrinsic viscosity measurements have been carried out at 25 C in toluene. Dvornic et al. 1 and Zilliox et al. 3 studied PDMS in the molecular weight ranges 2000 1,380,000 and 20,000 900,000, respectively. The intrinsic viscosities of PDMS (M 5.5 10 5 to 1.2 10 6 ) have been determined at the point in two solvents: methyl ethyl ketone (MEK) at 20 C and a 1/2 mixture of C 8 F 18 and CCl 2 F CCl 2 Fat 22.5 C by Crescenzi and Flory. 6 The [ ] /M 1/2 ratios have been found to be 7.8 10 4 and 10.6 10 4, respectively. However, Dodgson and Semiyen 7 reported a Mark Houwink Sakurada (MHS) exponent, a, in MEK at 20 C as 0.58, but their molecular weight range was below 1.7 10 4. The other [ ] M relationships for PDMS were given by Haug and Meyerhoff: 8 2678

SOLUTION PROPERTIES OF POLY(DIMETHYL SILOXANE) 2679 8.28 10 5 M 0.72 w dl/g in toluene at 25 C (1) 7.80 10 4 M 0.50 w dl/g in BCH at 28 C (2) Haug and Meyerhoff obtained these results for molecular weights above 200,000. However, Schulz et al. 9,10 found the state at 29 C in BCH in the molecular weight range 3.3 10 4 to 1.06 10 6 : 7.45 10 4 M 0.50 w dl/g in BCH at 29 C (3) Lapp et al. 4 studied PDMS in the molecular weight range 2600 625,000 and found the following: 1.36 10 4 M 0.69 w dl/g in toluene at 25 C (4) Brzezinski et al. 11 studied the intrinsic viscosities of PDMS in the molecular weight range 3000 300,000. The [ ] M relationship for toluene at 25 C was 1.87 10 4 M w 0.658 dl/g (5) The unperturbed dimensions of the PDMS chain appeared to be affected by the solvent medium. In a fluorinated solvent with a low cohesive energy and a low dielectric constant, C was found to be 7.7 by Crescenzi and Flory. 6 Haug and Meyerhoff 8 found that C was 6.3 in BCH at 28 C. These results indicate that considerable differences existed between the data reported on the solution behavior of PDMS. In this study, we studied the solution properties and unperturbed dimensions of PDMS in the molecular weight range 7.5 10 4 to 8.0 10 5 in toluene at 30 C, MEK at 20 C, and BCH at 26 C and 28 C with different methods. The results were compared to those of related studies published in the literature. EXPERIMENTAL Fractionation of PDMS PDMS samples were produced by Aldrich Chemical Co., Ltd. and Polyscience, Inc. with molecular weight distributions [weight-average molecular weight/number-average molecular weight (M w / M n )] of 5.6 and 2.36, respectively. Broad molecular weight PDMS samples were separated into a series of fractions via liquid liquid precipitation. A solution of PDMS in toluene (10% w/v) was prepared and maintained at 25 C. Methanol, which was a nonsolvent for PDMS, was added slowly under stirring until a permanent turbidity was obtained. The mixture was warmed to 30 C until the precipitate dissolved; it was then allowed to cool to 25 C and was left to precipitate. This procedure was repeated for each fraction. The fractions were dried under vacuum at 50 C. dn/dc Measurement The differential refractive index increment (dn/ dc) was measured with an Atago DD5 differential refractometer at 589 nm and room temperature. Five concentrations (1 mg/ml c 8 mg/ml) of PDMS in toluene were used. The dn/dc value was determined with the following relationship: dn/dc kd d /dc (6) where k is an instrument calibration constant and d is the deviation of light. The k value was determined to be 1.533 10 3 with standard polystyrene samples. The dn/dc value of polystyrene in toluene was taken to be 0.1085 12 by the correction of data into 589 nm. The dn/dc value of PDMS in toluene was determined to be 0.0915 at 589 nm. It was corrected to the 632.8-nm wavelength at which light scattering (LS) measurements were carried out, and the dn/dc value of PDMS in toluene was determined to be 0.089 at 25 C (Fig. 1). LS LS measurements were performed in toluene at room temperature at 632.8 nm with a Dawn B laser photometer produced by Wyatt Technology Corp. Solution concentrations varied from 4 10 3 to 1 10 3 g/ml. Solvent and solutions were clarified by filtration through Millipore or Gelman filters with the proper pore sizes 0.2 m and 0.45 m, respectively. The LS data were treated according to the Zimm method. 13 The Aurora program was used to determine the molecular weights and sizes. M w, the z-mean-square radius of gyration (R g ), and

2680 BÜYÜKTANIR AND KÜÇÜKYAVUZ sp /c k 1 2 c.. (7) and according to the Kraemer equation: ln r /c k 2 2 c.. (8) From the double extrapolation of sp /c versus the concentration and a plot of (ln r )/c versus concentration, [ ] values were found. Figure 1. Plot of d versus concentration (g/ml) for PDMS in toluene. the second virial coefficient, A 2, were obtained with a Zimm plot. In all measurements, the dn/dc value was taken to be 0.089. Gel Permeation Chromatography/Light Scattering (GPC/LS) This analytic technique was used for determining the molecular weight and molecular weight distributions of polymers. A size exclusion chromatography modular system consisted of a Waters 590 pump, a Waters automatic sample injector, sets of three size exclusion columns (7.8 50 mm Phenogel and 7.8 300 mm Phenogel-2 columns), and two detectors (Dawn DSP multiangle LS and Waters 410 differential refractometer). The mobile phase was toluene. A 100- L sample of a 0.5 6 mg/ml solution was injected for each analysis. The flow rate was 0.9 ml/min. The mobile phase was kept under nitrogen to prevent moisturizing, was degassed by helium being blown through it for several hours before the experiment, and was filtered through a 0.2- m Whatman filter prior to use. Viscometry The viscosity measurements of the PDMS samples were determined with an Ubbelohde viscometer for which the solvent flow times were higher than 100 s. The intrinsic viscosity measurements were carried out in toluene at 30 C, BCH at 26 C and 28 C, and MEK at 20 C. All concentrations in this study are expressed in grams per 100 ml (g/dl). Intrinsic viscosities ([ ]) were obtained according to the Huggins equation: RESULTS AND DISCUSSION LS M w, A 2, and R g of the PDMS fractions were determined by LS measurements in toluene at room temperature. Table I summarizes these results. A typical Zimm plot for one fraction can be seen in Figure 2. Measurements were recorded at angle intervals of 24.3 129.5. The dependence of A 2 on M w is illustrated in Figure 3 as log A 2 versus log M w. As expected, log A 2 decreased with increasing molecular weight b according to the relationship A 2 BM w (A 2 0.0013M 0.10 w ), where B is a constant that depends on the polymer solvent system. Krigbaum and Flory 14 predicted a value of b between 0.05 and 0.25 for random coils. In this study, the value of b was found to be 0.10 in toluene for PDMS. The dependence of the radius of gyration on the molecular weight for PDMS was R g 0.0808M 0.45 w ; this is shown in Figure 4 as log R g versus log M w. Table I. Data Obtained from LS Measurements for PDMS Fractions Fraction M w (10 3, g/mol) R g (nm) A 2 (10 4, mol cm 3 /g 2 ) 1 777 0.7 35.6 2.0 4.21 0.3 2 706 0.2 40.2 2.3 3.27 0.1 3 688 0.3 39.5 2.3 3.01 0.2 4 595 0.1 32.3 2.0 3.01 0.1 5 335 0.2 20.6 5.2 3.29 0.2 6 304 0.1 27.0 3.2 3.35 0.2 7 269 0.3 30.1 2.5 3.43 0.7 8 196 0.1 13.4 6.7 5.14 0.1 9 156 0.1 31.0 6.6 3.73 0.4 10 136 0.8 3.3 0.1 3.11 0.9 11 128 0.7 16.3 3.6 6.85 0.5 12 93 0.3 26.4 2.5 3.35 1.0 13 79 0.2 25.6 0.3 3.83 0.7

SOLUTION PROPERTIES OF POLY(DIMETHYL SILOXANE) 2681 Figure 2. Zimm plot of PDMS (595,000). The exponent value was expected to be between 0.5 and 0.6 for random coils, whereas a value of 1 was expected for rods. 15 This result was also indicative of a random-coil conformation for PDMS in toluene. Determination of the Molecular Weight Distribution of PDMS by GPC/LS We obtained the average molecular weights and molecular weight distributions of PDMS samples by GPC/LS. The results obtained in toluene with three Phenogel columns at 25 C are shown in Table II. All the samples had M w /M n values smaller than 2, which was required for the solution studies. Viscometry The results obtained from the intrinsic viscosity measurements are tabulated in Table III. The intrinsic viscosity [ ] (deciliters per gram) and M w data yielded the usual MHS relationship under both ideal ( state) and nonideal conditions: KM w a (9) On the basis of Table III, the following equations were obtained for each system by the plotting of log [ ] versus log M w : 1.65 10 4 M w 0.67 in toluene at 30 C (10) Figure 3. Relationship between log A 2 and log M w. Figure 4. Relationship between log R g and log M w.

2682 BÜYÜKTANIR AND KÜÇÜKYAVUZ Table II. LS M w (10 3 ) GPC/LS Data for PDMS Fractions GPC/LS M w (10 3 ) 6.79 10 4 M w 0.50 1.11 10 3 M w 0.43 7.11 10 4 M w 0.50 GPC/LS M n (10 3 ) GPC/LS M w /M n 777 844.4 571.8 1.48 706 722.8 512.8 1.41 688 749.5 539.6 1.39 595 601.7 465.9 1.29 269 401.7 238.2 1.68 196 176.2 126.5 1.39 156 162.9 87.87 1.85 136 126.2 81.01 1.56 128 121.8 75.42 1.61 93 97.09 51.45 1.88 79 75.37 48.17 1.56 in BCH at 28 C (11) in BCH at 26 C (12) in MEK at 20 C (13) The MHS exponent a indicated that toluene was a good solvent for PDMS. BCH at 26 C was a poor solvent, and MEK at 20 C and BCH at 28 C were the solvents for PDMS. Examining the results of Dvornic et al. 1 and Zilliox et al. 3 as well (Table IV), we concluded that as the temperature increased, a increased because of the expansion of coil. Unperturbed Dimensions Fox and Flory 16 suggested that because the intrinsic viscosity of a polymer solution depends on the volume occupied by the polymer chain, it should be feasible to relate coil size and [ ]. The intrinsic viscosity for random-coiled polymers of high molecular weight M is 17 r 2 3/2 /M (14) where is a universal constant and is equal to 2.5 ( 0.1) 10 21 (c g s), 18,19 with r in centimeters and [ ] in deciliters per gram. If the measurements are carried out at the point, this expression is written as follows: 18 r 2 0 /M 3/2 M 1/2 (15) K M 1/2 (16) K r 2 0 /M 3/2 (17) The characteristic ratio depends on the chain configuration. r 2 o is the unperturbed mean-square end-to-end length of the chain, n is the number of bonds, and l is the bond length of the polymers. The characteristic ratio, C, is a measure of the departure from freely jointed chain. This dimensionless ratio is given as follows: C r 2 0 /nl 2 (18a) The characteristic ratio is generally preferred over r 2 o /M as a basis for comparing the average dimensions of various random-coil chains: r 2 0 /M K/ 2/3 (18b) Table III. Intrinsic Viscosity Results of PDMS Fraction [ ] (dl/g, Toluene, 30 C) [ ] (dl/g, BCH, 26 C) [ ] (dl/g, BCH, 28 C) [ ] (dl/g, MEK, 20 C) 1 1.47 0.380 0.692 0.676 2 1.44 0.372 0.676 0.646 3 1.43 0.437 0.661 0.589 4 1.28 0.347 0.537 0.575 5 1.24 0.407 0.501 0.537 6 1.14 0.389 0.479 0.380 7 0.66 0.324 0.398 0.407 8 0.53 0.234 0.347 0.295 9 0.48 0.195 0.288 0.282 10 0.44 0.186 0.282 0.275 11 0.40 0.170 0.245 0.214 12 0.33 0.148 0.224 0.219 13 0.31 0.148 0.186 0.209

SOLUTION PROPERTIES OF POLY(DIMETHYL SILOXANE) 2683 Table IV. Comparison of the MHS Equations for PDMS in Toluene at Different Temperatures Temperature ( C) K (10 4 ) a 35 (Zilliox et al.) 3 1.25 0.703 30 (This work) 1.65 0.67 25 (Dvornic et al.) 1 2.15 0.65 Thus, in the limit for long chains, C r 2 0 /M M b /l 2 (19) where M b is the mean molecular weight per skeletal bond or C K/ 2/3 M b /l 2 (20) For the PDMS chain, M b is equal to 37.05 and l is equal to 1.64 10 8 cm. 6 For random coils, r 2 0 6 s 2 0 (21) The obtained values of C for PDMS are shown in Table V. The largest value, 6.45, was in toluene at 30 C. In MEK and BCH at 28 C, it was 5.90; in BCH at 26 C, it was 5.32. The small difference between these values could be attributed to a specific solvent effect. 6,20 Mark and Flory suggested that because of the inequality of the bond angles ( OSiO 110 and SiOSi 143 ), r 2 o /nl 2 increases. 2,3 The larger value observed for the ratio in a less polar medium is predicted for enhanced electrostatic interaction within a chain of partially ionic SiOO bonds. 21 The oxygen atom alternating along the chain must be vulnerable to interaction with the solvent. The length and polarity of the SiOO bond should also be expected to enhance its susceptibility to the nature of the medium. 6 Stockmayer Fixman Plots Unperturbed dimensions can be estimated from the intrinsic viscosity measurements with the Stockmayer Fixman equation: M w 1/2 K 0.51 BM w 1/2 (22) where B is a parameter characterizing the longrange interactions. Through the term K,[ ] is connected to the short-range interactions of chains. K is related to the unperturbed dimensions, which are calculated from the Fox Flory equation as s 2 o /M. The results of the Stockmayer Fixman plots (Fig. 5) are tabulated in Table V. The excluded volume parameter, B, for PDMS in BCH at 26 C, below the temperature, was negative as expected, 7.84 10 29. When we compared their s 2 o /M values, we saw that the lowest value was obtained in BCH at 26 C, as it was expected that in poor solvents the radius of gyration would decrease. K was 7.00 10 4 for both MEK and BCH (Table V). However, the Stockmayer Fixman plot for PDMS revealed that a higher K value in toluene due to expansion from the excluded volume could be relevant even at the lower molecular weights. 22 For PDMS in MEK at 20 C, Crescenzi and Flory 6 found [ ] /M 1/2 to be 7.8 10 4, and the ratio r 2 o /nl 2 deduced from this study was 6.3. 20 In BCH at 28 C, Haug and Meyerhoff 8 found the same result. However, our result was slightly lower than this value (7.0 10 4 ). Different Extrapolation Methods With the M w and intrinsic viscosities, values of K, B, s 2 o /M, and C were obtained by different extrapolation methods: the Berry plot, 23 the Fox Flory plot, 24 the Kurata Stockmayer Roig plot, and the Inagaki Suzuki Kurata plot. 25 These methods eliminated the excluded volume effect through extrapolation to zero molecular weights. Table V. Results of the Stockmayer Fixman Plots for PDMS Solvents B (cm 3 g 2 mol 2 ) K (10 4, dl/g) C s 2 0 /M (10 18,cm 2 ) Toluene, 30 C 7.06 8.0 6.45 7.80 MEK, 20 C 1.57 7.0 5.90 7.13 BCH, 28 C 2.35 7.0 5.90 7.13 BCH, 26 C 0.785 6.0 5.32 6.44

2684 BÜYÜKTANIR AND KÜÇÜKYAVUZ Figure 5. Comparison of the Stockmayer Fixman plots of PDMS: (Œ) in toluene at 30 C, (E) in BCH at 28 C, ( ) in BCH at 26 C, and (F) in MEK at 20 C. Figure 7. 30 C. Fox Flory plot of PDMS in toluene at An analysis of the plots showed that the different relations were practically valid in the entire molecular weight range. These plots were drawn according to the intrinsic viscosity of PDMS in toluene at 30 C. Kurata Stockmayer Roig Plot In Figure 8, the Kurata Stockmayer Roig plot has been drawn with the following relations: Berry Plot 2/3 M w 1/3 K 2/3 0.363 Bg n M w 2/3 1/3 25 According to eq 23, [ ] 1/2 M w 1/4 values were plotted versus M w [ ] 1 (Fig. 6): 1/2 M w 1/4 K 1/2 0.42K 1/2 BM w 1 (23) Fox Flory Plot This plot can be seen in Figure 7. It was drawn according to eq 24: 2/3 M w 1/3 K 2/3 constant M w 1 (24) g n 8 n 3 / 3 n 2 1 3/2 (26) The chain expansion factor or linear expansion coefficient, 3, gave information about the expansion due to long-range interactions or the excluded volume effect. 17 In this research, the linear expansion coefficients were [ ] toluene /[ ] (Table VI). Inagaki Suzuki Kurata Plot The Inagaki Suzuki Kurata plot (eq 27) can be seen in Figure 9: Figure 6. Berry plot of PDMS in toluene at 30 C. Figure 8. Kurata Stockmayer Roig plot of PDMS in toluene at 30 C.

SOLUTION PROPERTIES OF POLY(DIMETHYL SILOXANE) 2685 Table VI. Results of the Linear Expansion Coefficients of the PDMS Fractions Fraction 4/5 M w 2/5 0.786K 4/5 0.454K 2/5 2/3 B 2/3 M w 1/3 (27) The average characteristic ratio was 6.66 in toluene from the different extrapolation methods (Table VII). A comparison of our results of C with literature values is given in Table VIII. From this table, one can conclude that on average our results were consistent with the existing values, except that C was found in a fluorinated solvent medium. 6 We also found C to be higher in a good solvent medium than in solvents. This could be attributed to the specific solvent effect. CONCLUSIONS [ ] toluene /[ ] MEK 1 2.18 2 2.23 3 1.43 4 2.27 5 2.31 6 3.00 7 1.62 8 1.80 9 1.70 10 1.60 11 1.87 12 1.51 13 1.48 The solution properties of PDMS were studied with LS and viscometry methods. The average Table VII. Results of K, C, and B According to the Different Extrapolation Methods in Toluene at 30 C Methods K (10 4, dl/g) B (10 28,cm 3 g 2 mol 2 ) dimensions of PDMS were affected by solvent and temperature changes. The dependence of both A 2 and R g on M w indicated that PDMS was a random-coil polymer. The intrinsic viscosity and molecular weight relations were found according to the MHS equation. The exponent value showed that the polymer solvent interaction was more favorable in toluene, so it was a good solvent. However, BCH at 26 C was a poor solvent, and MEK at 20 C and BCH at 28 C were the solvents for PDMS. The characteristic ratio of PDMS was 6.66 in toluene according to different extrapolation methods. A comparison of this value with that of polystyrene (10.0) 26 showed that the PDMS chain was more flexible because of the presence of oxygen atoms in the backbone and no bulky side groups. The hindrance to the internal rotation in the polymer chain increased with the increasing size of the substituent. C Stockmayer Fixman 8.00 7.06 6.45 Berry 8.24 6.64 6.57 Kurata Stockmayer Roig 8.97 6.61 6.96 Fox Flory 8.11 6.50 Inagaki Suzuki Kurata 8.70 2.27 6.82 This work was supported by Middle East Technical University Research Funds (AFP). The authors thank Table VIII. Characteristic Ratio for PDMS in Different Solvents Medium K (10 4, dl/g) C Figure 9. Inagaki Suzuki Kurata plot of PDMS in toluene at 30 C. MEK, 20 C (Crescenzi et al. 6 ) 7.80 6.3 1 2 8F 18 and CCl 2 F CCl 2 F, 22.5 C (Crescenzi et al. 6 ) 10.6 7.7 BCH, 28 C (Haug et al. 8 ) 7.80 6.3 BCH, 29 C (Schulz 9 ) 7.45 6.1 Toluene, 25 C (Lapp 4 ) 7.50 6.2 BCH, 28 C (This study) 7.00 5.9 Toluene, 30 C (This study) 8.00 6.7 MEK, 20 C (This study) 7.00 5.9

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