Random coil configurations of aromatic polyesters: temperature coefficient of the unperturbed dimensions of poly(diethylene glycol terephthalate) chains

Abstract

Hydroxyl-terminated poly(diethylene glycol terephthalate) chains having number-average molecular weights of 7200 were end-linked into noncrystallizable trifunctional networks using an aromatic triisocyanate. The networks thus obtained were studied with regard to their stress-strain isotherms at 62, 70, and 88 °C. For elongation ratios up to 400% none of the isotherms displayed an increase or upturn in the reduced forces [/*], indicating that the polymer does not crystallize under stress, probably due to very unfavorable kinetic conditions. Force-temperature measurements at constant length and pressure are also reported for these networks. The ratio of the energy contribution to the total tension, /"//, calculated from these measurements and the thermal expansion coefficient were used to calculate the temperature coefficient of the mean-square end-to-end distance. It was found that d In (r2)0/dT = (1.01 ± 0.14) X 10~3 K™1. A similar value was obtained for this coefficient from thermoelastic measurements carried out on swollen networks. The analysis of the temperature coefficient of poly(diethylene glycol terephthalate) in terms of the rotational isomeric state model confirms the results obtained from NMR studies, according to which the gauche states about CH2-CH2 bonds in the polymer chain have an energy significantly lower than these states about similar bonds in poly(oxyethylene).

Thermotropic properties and conformational studies on poly(triethylene glycol p,p'-bibenzoate) and poly(octamethylene p,p'-bibenzoate)

“…Rotational angles about O-CH2 bonds of the ester and ether groups were considered to be located at12 0, ±104°a nd13 0, ±100°, respectively, whereas for CH2-CH2 bonds these angles were assumed to be13 0, ±120°; furthermore, 0-C*0* bonds of the ester groups are restricted to a trans conformation.14 As for the conformational energies associated with the rotational states, they are described in detail elsewhere.13-18 Briefly, gauche states about O-CH2 bonds of the ester and ether groups have energies of 0.4 and 0.9 kcal mol-1, respectively, above those of the corresponding trans states,13-17 whereas the energy of gauche states about CH2-CH2 bonds adjacent to the ester groups is 0.8 kcal mol-1 below that of the corresponding trans. [14][15][16] Gauche states about the central CH2-CH2 bonds of the repeating unit were assumed to be preferred by only 0.5 kcal mol-1 over the alternative trans states. Second-order interactions of C*0*-0, CH2-0, and CH2-CH2 groups were considered to be 1.4, 0.6, and » 2 kcal mol-1, respectively.…”

Dipole moments and Kerr constants of dibenzoates of some diols with different numbers of methylene units as model compounds for polyesters

“…They indicate that the free rotation is not a good approximation for the calculation of Kerr constants; however, it is not = 120°(i.e., rotational isomers placed at 0, ±120°). 6 Set I, conformational energies taken from ref 11. c Set II, conformational energies of ref 13.…”

Birefringence and glass transition temperatures of poly(diethylene glycol terephthalate) networks

“…Recently, the conformational characteristics of PDET as well as the elastic behavior of model networks prepared from this polymer were examined. [11][12][13][14] It was found that although most of the skeletal bonds of the repeat unit are either restricted to trans states or show preference for these states, the high preference for gauche states of the two remaining skeletal C-C bonds confers on this polymer a high degree of flexibility, so that these molecules are more coiled than was suspected initia1ly.l Moreover, the evidence at hand indicates that the accessibility of PDET chains to different spatial arrangements should not be hindered by constraints such as might result from chain crystallization, at least for moderately high e10ngations.l~ Poly(diethy1ene glycol terephthalate) networks exhibit rubberlike behavior above room temperature, with essentially complete recovery even in the cases in which relatively large deformations can be accomplished. An important feature of these networks is that for elongation ratios a >5, an increase in the modulus or an upturn appears in the Mooney-Rivlin p10ts.l~ Since small crystallites usually act as reinforcers, preventing crack growth in the networks,15 the anomalous increase of the modulus was attributed to strain-induced crystallization.…”