K. Dodgson

Studies of cyclic and linear poly(dimethylsiloxanes): 19. Glass transition temperatures and crystallization behaviour

Differential scanning calorimetry (d.s.c.) was used to investigate the thermal behaviour of cyclic and linear poly(dimethylsiloxanes) over the temperature range 103–298 K. Fractions of the polymers studied had number-average molar masses in the range 160 < Mn < 25 500 g mol−1 and heterogeneity indices MwMn < 1.1 in most cases. D.s.c. was applied to measure the glass transition temperatures Tg cold crystallization temperatures Tc and polymer crystalline melting temperatures Tm of the oligomer and polymer fractions. Cyclic siloxanes [(CH3)2SiO]x with number-average numbers of skeletal bonds nn in the range 24 ≦ nn ≦ 79 and linear siloxanes (CH3)SiO[(CH3)2SiO]ySi(CH3)3 with nn in the range 10 ≦ nn ≦ 40 were found not to crystallize. The Tg values of the linear siloxanes were found to be in agreement with values in the literature and they increased with increasing Mn. By contrast, the Tg values of the cyclics were found to decrease with increasing Mn.

Studies of cyclic and linear poly(dimethyl siloxanes): 3. Neutron scattering measurements of the dimensions of ring and chain polymers

Abstract The dimensions of both cyclic and linear poly(dimethyl siloxanes) in dilute solution in benzene- d 6 have been measured by small-angle neutron scattering. The mean-square radii of gyration of the linear polymers are consistent with values predicted from published data, including experimental molar cyclization equilibrium constants. The average dimensions of the cyclic poly(dimethyl siloxanes) in fractions containing z -average numbers of bonds n z in the range 130 n z , were found to be considerably smaller than those of the corresponding linear polymers. The neutron scattering results give a value for the ratio of the z -average radii of gyration for linear and ring poly(dimethyl siloxanes) (containing the same number of monomer units) 〈s 2 〉 z,l 2 〉 z,r = 1.9 ± 0.2 . This ratio may be compared with the value of 2.0 predicted theoretically for ‘flexible’ high molecular weight linear and cyclic polymers, unperturbed by excluded volume effects.

Studies of cyclic and linear poly(dimethyl siloxanes): 1. Limiting viscosity number-molecular weight relationships

Abstract The limiting viscosity numbers of ten cyclic and ten linear poly(dimethyl siloxane) fractions have been measured in a π-solvent (butanone at 293K) and in two ‘good’ solvents (toluene and cyclohexane at 298K). The dimethyl siloxane fractions studied were in the molecular weight range 800 M w . The data obtained are compared with related studies published in the literature. The ratio of the limiting viscosity numbers [η]r and [η]l of the cyclic and linear poly(dimethyl siloxanes) with M w > 2500 was found to be 0.67 in butanone at 293K. This value is identical (within experimental error) to the theoretical ratio [η] r [η] l = 0.66 predicted by Bloomfield and Zimm and others for ring and chain polymers in π-solvents. The ratio [η] r [η] l was found to be somewhat smaller for the higher molecular weight polymers in the ‘good’ solvents.

Studies of cyclic and linear poly(dimethyl siloxanes): 2. Preparative gel permeation chromatography

Abstract A preparative gel permeation chromatographic (g.p.c.) instrument has been constructed and used to separate broad fractions of cyclic poly(dimethyl siloxanes) into sharp fractions with heterogeneity indices M w M n = 1.05 ± 0.02 . The number-average molecular weights M n of the cyclic polymer fractions obtained were as high as 50 000, corresponding to number-average numbers of skeletal bonds n n up to 1300. The concentrations of linear poly(dimethyl siloxanes) in all but the highest molecular weight cyclic polymer fractions prepared are believed to be negligible. The preparative g.p.c. instrument was also used to obtain some sharp fractions of linear poly(dimethyl siloxanes).

Cyclic polysiloxanes: 2. Neutron scattering from poly(phenylmethylsiloxane)

The z-average radii of gyration 〈s2〉z of both cyclic (r) and linear (l) poly(phenylmethylsiloxanes) (PPMS) in dilute solution in benzene-d6 were measured by small-angle neutron scattering. The PPMS samples studied consisted of fractions with heterogeneity indices in the range 1.04 1 were also examined.

Studies of cyclic and linear poly(dimethyl siloxanes). 4. Bulk viscosities

Abstract The bulk viscosities η of over fifty sharp fractions of cyclic and linear poly(dimethyl siloxanes) in the weight-average molecular weight range 500 M 2 have been measured at 298 K using a cone- and-plate microviscometer. In the Iow molecular weight region M W ) the η values for the cyclics were found to be at least three times as large as the values for the corresponding chain molecules. By contrast, in the highest molecular weight region ( M W > 16 000 ), the η values for the cyclics were approximately one-half those for the corresponding linears. Cyclics and linears containing about one hundred skeletal bonds were found to have similar bulk viscosities. The temperature dependence of the bulk viscosities of eighteen of the cyclic and linear fractions were investigated, and the relationship η = A exp ( E visc RT ) was used to deduce values for the energies of activation for viscous flow Evisc and the constants A.

Studies of cyclic and linear poly(dimethyl siloxanes): 10. Calculations of radii of gyration

A Monte Carlo method has been devised for calculating the conformation-dependent properties of cyclic poly(dimethyl siloxanes) (PDMS), using Flory, Crescenzi and Marks rotational isomeric state model. Calculated values of the mean-square radii of gyration 〈s2r〉 of ring molecules unperturbed by excluded volume effects and containing 8–100 skeletal atoms are compared with the 〈s2l〉 values for the corresponding unperturbed chain molecules. Exact enumeration methods were also employed for rings [(CH3)2SiO]w2 with w ⩽ 24 and the results found to be in close agreement with those obtained by the Monte Carlo method. The ratio 〈s2l〉〈s2r〉 was found to attain limiting values close to 2.0 for w > 30, in agreement with theoretical predictions.

Studies of cyclic and linear poly(dimethyl siloxanes): 12. Observation of diffusion behaviour by quasielastic neutron scattering

Abstract High resolution neutron scattering experiments have been used to observe the diffusive motion of low molecular weight linear and cyclic poly(dimethyl siloxane) molecules in dilute solution in deuterated benzene. Diffusion coefficients (D) and hydrodynamic radii (RH) have been compared with values obtained by light scattering for higher molecular weight samples and with radii of gyration (Rg) obtained by small-angle neutron scattering. While the ratio D ring D chain is close to the predicted value of 0.85, the ratio R g R H falls below the theoretical value for both ring and chain molecules. The scattering curves show effects arising from both centre of mass diffusion and internal molecular motion, and the observed inverse correlation times are compared with calculated behaviour as a function of scattering vector, Q.

Cyclic polysiloxanes: 4. Glass transition temperatures of poly(phenylmethylsiloxanes)

Abstract The glass transition temperature (Tg) for cyclic and linear poly(phenylmethylsiloxanes) (PPMS) were determined using differential scanning calorimetry. The samples had number-average molar masses in the range 300 ⩽ Mn ⩽ 27 100 g mol−1, with heterogeneity indices 1.03 ⩽ M w M n ⩽ 1.16 ; they were all found to be amorphous materials, apart from the linear oligomer (CH3)3SiO[(C6H5)(CH3)SiO]Si(CH3)3. The Tg values for the linear PPMS fractions showed a linear dependence upon reciprocal number-average molar mass M−1n, in agreement with theoretical predictions for linear polymers. The Tg values for the cyclic PPMS fractions, by contrast, were found to show a non-linear dependence upon M−1n. The asymptotic values of the glass transition temperatures Tg(∞) for both cyclic PPMS and linear PPMS were found to be in close agreement with the literature value for high-molar-mass linear PPMS. High-resolution 1H nuclear magnetic resonance spectroscopy was used to demonstrate that the cyclic and linear PPMS fractions used in this investigation—and also in a variety of other studies in this series—are stereochemically atactic polymers.

Studies of cyclic and linear poly(dimethyl siloxanes): 14. Particle scattering functions

Particle scattering functions P(Q) (where Q represents the wave vector), have been calculated using a Monte Carlo method for cyclic and linear poly(dimethyl siloxanes) (PDMS) containing up to 100 skeletal bonds. A maximum is found in the Kratky plot at u (=Q〈s2〉12)≈2.0 for cyclic PDMS (with root-mean-square radii of gyration 〈s2〉12) and this is in satisfactory agreement with the analytical calculations of Casassa and of Burchard and Schmidt. In addition, other clearly-defined maxima are found at u≈5.0 for PDMS ring molecules with less than ≈40 skeletal atoms. These maxima are believed to be characteristic of small cyclic molecules as they are also predicted for small polymethylene rings. Comparisons are made with experimental small-angle neutron scattering (SANS) data for cyclic and linear PDMS. A single maximum in the experimental Kratky plot at u≈2.0 is found for PDMS ring molecules with an average of 550 skeletal atoms. The experimental data for cyclic PDMS are in better agreement with the Monte Carlo calculations of P(u) than with analytical predictions up to u≈2.0.