J.A. Semlyen

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).

Equilibrium ring concentrations and the statistical conformations of polymer chains: Part 11. Cyclics in poly(ethylene terephthalate)

Abstract Methods have been developed for extracting cyclic oligomers from poly(ethylene terephthalate) samples and for analysing the extracts by gel permeation chromatography. These methods have been used to measure the molar cyclization equilibrium constants Kx for cyclics (CO.C6H4.CO.O.CH2.CH2.O)t with x=3–9 in the undiluted polymer at 543K and in solution in 1-methyl naphthalene at 523K. The close agreement between the measured Kx values over the range x=3–9 shows that oligomeric ethylene terephthalate chains adopt similar conformations in the two environments (which contain 95% w/w and 6% w/w linear polymer respectively). Kx values were calculated by the Jacobson and Stockmayer theory by assuming that the corresponding open chain molecules obey Gaussian statistics and describing their statistical conformations by Williams and Florys rotational isomeric state model. These theoretical values were found to be lower than the experimental values by factors of at least two over the whole range of cyclics x=3–9. Poor agreement between experiment and theory was also obtained when K3 and K4 values were calculated by computing the end-to-end distances of acyclic trimeric and tetrameric ethylene terephthalate chains in all discrete conformations defined by the Williams and Flory model. It is suggested that these discrepancies might result, at least in part, from substantial correlations between the positions and directions of the termini of oligomeric ethylene terephthalate chains. Observed decreases in the concentrations of cyclics in commercial poly(ethylene terephthalate) samples resulting from heating the samples in the solid state are discussed briefly.

Equilibrium ring concentrations and the statistical conformations of polymer chains: Part 3. Substituent effects in polysiloxane systems

The molar cyclization equilibrium constants Kx for cyclics [R(CH3)SiO]x, where R = H (x = 4–15), R = CH3CH2 (x = 4–20), R = CH3CH2CH2 (x = 4–8) and R = CF3CH2CH2 (x = 4−20), have been measured in some undiluted and solution equilibrates with an accuracy of ±10%. K4 and K5 increase along the series R = H 10) decrease with increasing size of the substituent group R. The limiting proportionality between Kx and x−52 predicted for macrocyclics by the Jacobson and Stockmayer theory is found for values of x > ca 12 when R = H and for x > ca 15 when R = CH3CH2 but only for values of x > ca 25 when R = CF3CH2CH2. The weight fractions of cyclics in high molecular weight poly(hydrogenmethylsiloxane), poly(ethylmethylsiloxane) and poly(3,3,3-trifluoropropylmethylsiloxane) equilibrates increase with solvent dilution up to critical points corresponding to complete conversion of siloxane to ring molecules. The amounts of solvent required to attain these critical dilution points are ca 10% by volume of cyclohexanone at 383 K when R = CF3CH2CH2, ca 60% by volume of toluene at 383 K when R = CH3CH2 and ca 80% by volume of toluene at 273 K when R = H. As in the poly(dimethylsiloxane) system, Kx values for the low molecular weight cyclics in poly(hydrogenmethylsiloxane) and poly(ethylmethylsiloxane) equilibrates increase with solvent dilution, whereas those for larger cyclics remain constant.

Studies of cyclic and linear poly(dimethyl siloxanes): 6. Effect of heat

Abstract Cyclic and linear poly(dimethyl siloxanes) were heated under vacuum in the temperature range 623–693K for periods of hours or days in the absence of catalysts. The products were analysed by gas-liquid chromatography and gel permeation chromatography. The results for the linear poly(dimethyl siloxanes) were in full agreement with the published work of Thomas and Kendrick. The effect of heat on the cyclic poly(dimethyl siloxanes) was that predicted, assuming that similar siloxane bond interchange reactions take place to those believed to occur in the linear polymers. The cyclic poly(dimethyl siloxanes) produced mixtures of cyclic oligomers, together with polymeric products which have considerably higher molecular weights than the starting materials. It is proposed that these polymeric products consist of mixtures of ring molecules [(CH3)2SiO]x. Some of these cyclic polymers are estimated to contain (on average) more than 10 000 skeletal bonds. Similar mixtures of cyclic oligomers and high molecular weight polymeric products were obtained by heating hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane.

Studies of cyclic and linear poly(dimethylsiloxanes): 27. Bulk viscosities above the critical molar mass for entanglement

Low-shear bulk viscosity measurements are presented for sharp (narrow distribution) cyclic and linear poly(dimethylsiloxane) (PDMS) fractions in the number-average molar mass range 5 × 102 to 3.2 × 104 g mol−1. The results allow the first estimate to be made of the critical molar mass for entanglement, Mc, for PDMS rings. This value does not differ significantly from that of the linear polymer. The viscosity-molar mass relationships are reported here for ring and chain samples above and below Mc.

Cyclic polyesters: 6. Preparation and characterization of two series of cyclic oligomers from solution ring-chain reactions of poly(ethylene terephthalate)

Solution ring-chain reactions of poly(ethylene terephthalate) (PET) were carried out using a high boiling solvent, 1-methyl naphthalene and catalyst. At a dilution of 110 (w/w polymer/solvent) and catalyst concentration of 0.5% (w/w), a reaction with tetraisopropyl orthotitanate, gave cyclics at a yield of 3 wt%, with dibutyltin bis (2-ethyl-hexanoate) the yield was 11 wt%, and with zinc acetate it was 20 wt%. Increasing the dilution of the reaction to 130 (w/w polymer/solvent) using zinc acetate as catalyst, increased the yield of the cyclics to 30 wt%. The cyclics were analysed using PL-gel mixed-E gel permeation chromatographic columns, and by 1H nuclear magnetic resonance spectroscopy, fast atom bombardment mass spectrometry, and liquid chromatography tandem mass spectrometry. The cyclic compounds (CO.C6H4.CO.O.CH2.CH2.O)x, where x = 3–13, 30–130 skeletal bonds), were identified together with a series of ring compounds (CO.C6H4.CO.O.CH2.CH2.O)xCH2.CH2.O, (where x = 2–7) which were also found to be present. This latter series has not been previously reported.

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.