Thermal and Rheological Properties of Alkyl ... - ACS Publications

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Downloaded by MICHIGAN STATE UNIV on February 19, 2015 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0224.ch004

Thermal and Rheological Properties of Alkyl-Substituted Polysiloxanes Husam A. A. Rasoul, Steven M. Hurley, E. Bruce Orler, and Kevin M. O'Connor1 Louis Laboratory, S. C. Johnson and Son, Inc., 1525 Howe Street, Racine, WI 53403

The thermal and rheological properties of a series of poly(dimethylsiloxane-co-methylalkylsiloxane) (PDM-PMAS) copolymers containing 3.5 mol % methylalkylsiloxane units of various alkyl lengths were investigated. Calorimetric results show that the alkyl side chains are crystallizable. The side-chain melting temperatures and heats of fusion normalized for side-chain weight fraction increased with increasing side-chain length. The steady-shear melt viscosity of the polymers with C10, C , and C side chains decreased with increasing side-chain length. Low-strain oscillatory measurements indicated the formation of a network structure at room temperature for polymers with C and C side chains, which can be attributed to intermolecular crystallization of the paraffinic side chains. 12

16

14

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POLYSILOXANES WITH PENDANT SIDE CHAINS are interesting materials from

both the theoretical and practical points of view. A number of polysiloxanes with various side chains, such as liquid crystals (J, 2), carbazole groups (3), electron-donor and electron-acceptor groups (4), polystyrene (5), and functional groups (hydroxyl or carboxyl) (6), have been synthesized. Polysiloxanes are known for their useful properties, which includeflexibility,heat resistance, water repellence, and biological inertness. These properties, combined with the ease with which a tailored polymer structure can be prepared,

NY 14650-2116

Current address: Research Laboratories, Eastman Kodak Company, Building 82D,

J

0065-2393/90/0224-0091$06.00/0 © 1990 American Chemical Society

In Silicon-Based Polymer Science; Zeigler, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

Rochester,


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SILICON-BASED POLYMER SCIENCE: A COMPREHENSIVE RESOURCE

prompted investigators to consider these types of polymers in a number of applications, including their uses as drug carriers, mold-releasing agents, and photoconductive polymers. Our own particular interest has been the thermal and rheological behavior of polysiloxanes with crystallizable side chains. In a recent work (7), we reported that the alkyl side chains of poly(methylalkylsiloxane) (PMAS) are crystallizable and exist in the hexag onal unit cell. We also observed that the melt viscosity of PMAS increased with increasing side-chain length. In this chapter, the properties of a series of poly(dimethylsiloxane-comethylalkylsiloxane) (PDM-PMAS) of various alkyl side-chain lengths are discussed. These polymers contain 3.5 mol % methylalkylsiloxane units and are prepared from the same precursor; thus, any effects due to variations in the percentage of alkyl substitution and main-chain molecular weight dif ferences and effects due to block distribution of the alkyl substituents are eliminated.

Experimental Procedures Synthesis. Synthesis of the copolymers was performed by a hydrosilylation reaction of poly(dimethylsiloxane-co-methylhydrosiloxane) (Petrarch System, Inc.) and α-olefins of various lengths (Aldrich). A round-bottomedflaskequipped with a magnetic stirring bar, condenser, and calcium chloride tube was charged with a 50% solution of the reactants (up to 10% molar excess of α-olefin) in dry toluene. A solution of hydrogen hexachloroplatinate(IV) in diglyme-isopropyl alcohol (150 ppm Pt) was then added to the reaction mixture. The reaction mixture was stirred at 60 °C for 3 h. At the end of this period, the mixture was refluxed with activated charcoal for 1 h andfilteredwhile hot. Finally the solvent and excess α-olefins were removed under reduced pressure (67 Pa at 100 °C). The reaction proceeded to completion as evidenced by the absence of the Si-Η absorption at 2130 cm -1 in the IR spectra. Residual α-olefin in the purified polymers was determined by gas-liquid chroma tography. In all polymers, residual α-olefin was less than 1.5 wt %. Thermal Analysis. Differential scanning calorimetry (DSC) was performed with a DSC module (Du Pont 910) interfaced with a thermal analyzer (model 9900). The instrument was calibrated with indium and high-purity water. Samples ranging in size from 1 to 3 mg were initially heated to 100 °C to erase any prior thermal history. The samples were held at 100 °C for 1 min and then cooled to -20 °C at a rate of 10 °C/min. At -20 °C, the cooling rate was decreased to 2 °C/min until -100 °C and further to -150 °C at 10 °C/min. Each sample was scanned at a heating rate of 20 °C/min. The glass transition temperature (Tg) was determined as the midpoint of the change in heat capacity, and the melting temperature (Tm) was the temperature at the onset of the melting endotherm. Rheological Experiments. Melt viscosity and low-strain oscillatory experi ments were performed on a Rheometrics RDS-7700 dynamic spectrometer equipped with a 0.2-2.0-g-cm torque transducer. The samples were mounted on 25-mmdiameter parallel-platefixtureswith a gap of 0.5 mm. Prior to each scan, samples were heated to 50 °C and then cooled slowly to room temperature. Steady-shear


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Properties of Alkyl-Substituted

93

Polysiloxanes

measurements were obtained at shear rates from 0.03 to 1000 s_1. Dynamic meas urements were obtained by using frequencies between 0.01 to 100 rad/s at 75% strain, except for polymers with C i 6 and Ci 8 side chains, for which 2% strain was used.


Results and Discussion Thermal Analysis. As expected, PDM-PMAS polymers showed melting endotherms associated with the melting of the paraffinic side chains similar to those previously observed for PMAS polymers (7). Both side-chain melting temperatures (Tms) and heats of fusion (AHfs) normalized for sidechain weight fraction increased with increasing side-chain length. However, for a given side-chain length, T and A H f are depressed for PDM-PMAS polymers relative to those for PMAS (Figures 1 and 2). Table I lists the sidem

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π

1

1

1

1

1

1

1

1

60+ υ

40

ε

20+ 0+ Η -20 10

1

r~

Δ Ο Δ ο

fi

Δ ο 1

12

1

τ-

16

14

Η

18

1

1

20

h

22

Ne Figure 1. Side-chain melting point (T ) versus the number of side-chain carbon atoms (N ). Key: O, PDM-PMAS; and Δ, PMAS. The data for PMAS were obtained from reference 7. m

c

chain T m s for PDM-PMAS copolymers and the corresponding AH f s. The decrease in side-chain T and A H f for PDM-PMAS compared with PMAS (7) are being analyzed at present in terms of the increased spacing between alkyl groups in the PDM-PMAS polymers and its influence on crystal thick ness, lateral crystal size, and entropy of melting. The thermal transitions associated with the backbone were investigated also. The glass transition temperatures for all PDM-PMAS copolymers and their precursor were identical (-120 °C). Other thermal events associated m



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Nc Figure 2. Side-chain heat of fusion (ΔΗ/j versus the number of side-chain carbon atoms (N ). Key: O, PDM-PMAS; Δ, PMAS; and •, n-alkanes. The data for PMAS were obtained from reference 7. c

Table I. Thermal Events for PDM-PMAS Copolymers and Precursor Side Chain Material

Precursor Copolymers6 ^ 10

C12 C14

Ci6 Ci8 C20

Backbone

T m (°C)

ΔΗ/ (kj/mol)

8 14 29 42 50

3.0 5.5 14.2 17.6 32.5

a

T . , (°C) -58

ΔΗ/tf/g) 12.6

—

—

-68 -65 -62 -62

33.5 24.7 22.0 23.9

— means material did not exhibit thermal event. ^The PDM-PMAS copolymers differ in the length of the alkyl side chains. a

with the backbone of these polymers are summarized in Table I. The melting temperature of the backbone (Tmb) is not influenced by the length of the paraffinic side chain, except for polymers with C 1 0 and C 1 2 side chains, for which no backbone melting endotherm was observed, presumably because of the higher miscibility of shorter paraffinic chains with poly(dimethylsiloxane). The C 1 0 and C 1 2 side chains are probably too short to phaseseparate from the backbone, and thus they disrupt backbone crystallization. Neither PMAS polymers nor their precursor studied earlier exhibit backbone melting transitions. Figure 3 shows representative D S C traces of poly(methylhexadecylsiloxane) (a C 1 6 PMAS) and PDM-PMAS copolymers with C 1 6 and C 1 2 side chains. The precursor and most of the PDM-PMAS



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RASOUL E T AL.

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-120

-80

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Polysiloxanes

-40

0

40

80

Temperature (°C) Figure 3. Representative DSC traces: (1 ) poly(methylhexadecylsiloxane) (a C PMAS), (2) poly(dimethylsiloxane-co-methylhexadecylsiloxane) (PDM-PMAS with a C side chain), and (3) poly(dimethylsiloxane-co-methyldodecylsiloxane) (PDM-PMAS with a C side chain).

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m

12

copolymers show bimodal or multimodal backbone melting endotherms, when observed. A detailed study of the backbone melting behavior is reported elsewhere (8). Rheological Behavior. Figure 4 shows the room-temperature steadyshear viscosity as a function of shear rate for PDM-PMAS polymers and their precursors. Polymers with C 1 0 , C 1 2 , and C 1 4 side chains exhibit Newtonian behavior over the range of shear rates monitored. The order of decreasing viscosity is as follows: C 1 0 > C 1 2 > C 1 4 > precursor. This order is contrary to that of fully substituted PMAS polymers (7). The decrease in viscosity in this order probably results from the decrease in side-chain miscibility with poly(dimethylsiloxane). Longer side chains, such as C 1 4 side chains, are expected to phase-separate and form a moreordered polymer compared with short side chains (C10). However, C 1 2 and C 1 4 side chains are not long enough to crystallize above room temperature. Polymers with longer side-chains (C 1 6 and C 1 8 ) exhibit non-Newtonian viscosity at room temperature, and the viscosity decreases with increasing shear rate. Because the side-chain T m s for C 1 6 and C 1 8 polymers are above room temperature, network formation via intermolecular crystallization of the paraffinic side-chains is believed to be responsible for the unusually high


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10000

.01

100

o.io SHEAR RATE

1000

(1/sec)

Figure 4. Steady-shear viscosity versus shear rate for PDM-PMAS. Key: O, precursor; Δ, N = 10; O, N = 12; •, N = 14; A, N c = 16; and · , N c = 18. N is the number of side-chain carbon atoms. c

c

c

c

viscosity of these polymers at low shear rates. Indeed, low-strain oscillatory measurements indicate some type of network structure, as evident from the relatively constant value of the storage modulus G for polymers with C 1 6 and C 1 8 side chains at low frequencies (Figure 5). The higher level of G for the polymer with C 1 8 side chain can be due to lower molecular weight 100000

(VI Ε υ

ω ω c >

Ό

10000+ 1000 100+

CD

0.01

0.10 FREQ

1

10

100

(rad/sec)

Figure 5. Storage modulus (G') versus frequency of oscillation for PDM-PMAS copolymers. Key: • , N c = 14; N c = 16; and · , N c = 18. N c is the number of side-chain carbon atoms.


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Polysiloxanes

between cross-links. However, because all the polymers were prepared from the same precursor, the high level of G ' is more probably due to the fact that crystals of polymers with C 1 6 side chain are thermally labile (T m is close to room temperature). In Figure 6, the loss modulus (G") is plotted versus the frequency of oscillation. PDM-PMAS copolymers with C 1 0 , C 1 2 , and C 1 4 side chains and


100000

OJ Ε υ \ to ω c >

10000+ 1000+

TJ

100+

CD

10+

0.01

0.10 FREQ

1

10

100

(rad/sec)

Figure 6. Loss modulus (G") versus frequency of oscillation for PDM-PMAS polymers. Key: O , precursor; Δ, N c = 10; O, N c = 12; • , N c = 14; A, N c = 16; and 9, N — 18. Ν is the number of side-chain carbon atoms. c

c

the precursor exhibit a normal viscous liquid behavior, with G " directly proportional to frequency and increasing in the same order as the steadyshear melt viscosity. For polymers with C 1 6 and C 1 8 side chains, G ' is greater than G " at any frequency because of the elastic nature of the network. G" is closer to G' for polymers with C 1 6 side chains compared with polymers with C 1 8 side chains. This result indicates that PDM-PMAS copolymers with longer side chains are more structured. Figures 5 and 6 represent only part of the isothermal frequency response of these structured fluids, and we are currently investigating the use of time-temperature superposition to extend the range of frequency.

Conclusions 1. A DSC study shows that the alkyl side chains of PDM-PMAS copolymers are crystallizable and that both the melting point and the heat of fusion increase with increasing side-chain length. In Silicon-Based Polymer Science; Zeigler, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

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2. The steady-shear melt viscosity of the same materials with C^Q, ^ 1 2 » and C 1 4 side chains increase with decreasing side-chain length. 3. Low-strain oscillatory measurements show that PDM-PMAS copolymers with longer side-chains ( C 1 6 and C 1 8 ) form a network structure at tem peratures below the side-chain T m . Intermolecular side-chain crystalli zation may be responsible for this behavior.


References 1. Krueuder, W.; Ringsdorf, H . Makromol. Chem., Rapid Commun. 1983, 4, 807.

2. Mauzac, M . ; Hardouin, F.; Richard, H . ; Achard, M.F.; Sigaud, G.; Gasparoux, H . Eur. Poly. J. 1986, 22, 137. 3. Strohriegl, P. Makromol. Chem., Rapid Commun. 1986, 7, 771.

4. Zentel, R.; Wu, J.; Cantow, H . J. Makromol. Chem. 1985, 186, 1763. 5. Chujo, Y.; Murai, K.; Yamashita, Y.; Okumura, Y. Makromol. Chem. 1985, 186, 1203. 6. Katayama, Y.; Kato, T.; Ohyanagi, M . ; Ikeda, K.; Sekine, Y. Makromol. Chem., Rapid Commun. 1986, 7, 465.

7. Rim, P. B.; Rasoul, Η. Α. Α.; Hurley, S. M . ; Orler, Ε. B.; Scholsky, Κ. M . Macromolecules

1987, 20, 208.

8. O'Connor, Κ. M . ; Orler, E.B.; Rasoul, H.A.A.; Hurley, S. M . ; Submitted for publication. RECEIVED

for review May 27, 1988.

ACCEPTED

revised manuscript December 15,

1988.


Thermal and Rheological Properties of Alkyl ... - ACS Publications

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