A reinvestigation of the hypersonic properties and the specific heat capacity of PMMA around the quasi-static glass transition II. High performance caloric investigations
Abstract:To continue dynamic cp measurements in the range of smallest temperature rates and non-linear thermal relaxation investigations into the linear range, simultaneous ep and thermal relaxation measurements were carried out in an adiabatic vacuum calorimeter, using the pulse heating method. The rate-dependent % behaviour, known from dynamic measurements, does not continue at small temperature rates. This is confirmed by the relaxation process which is observed. The results suggest an extended interpretation of the… Show more
“…However, we cannot be sure whether the slowing of the decay at very long times discussed in relation with the Λ = 0.365 μm curve in Figure a is partly due to aging effects (see below) , mechanical relaxation, , and other techniques. ,− In polystyrene, the rotational diffusion of rubrene tracers showed a large slowing on aging at T g − 10 K. , 12 Effect of polymer aging on development of the FRS grating. The two experimental curves show the refractive index modulation Δ n (grating distance, Λ = 0.67 μm) as a function of time, determined in the same PMMA/PQ sample ( M w = 4260) within a few hours after melting (dotted line) and after annealing at 80 °C for two months (solid line).…”
Section: Resultsmentioning
confidence: 99%
“…However, we cannot be sure whether the slowing of the decay at very long times discussed in relation with the Λ ) 0.365 µm curve in Figure 4a is partly due to aging effects (see below). 40 Physical aging in PMMA has been studied previously by calorimetry, 41,42 mechanical relaxation, 43,44 and other techniques. 30,[45][46][47] In polystyrene, the rotational diffusion of rubrene tracers showed a large slowing on aging at T g -10 K. 30,47 The large translational enhancement of PQ diffusion exhibited in Figure 11 (see above) can be explained by the influence of dynamical heterogeneity which has also been observed in other polymers and glass-forming liquids close to the glass transition.…”
Section: Resultsmentioning
confidence: 99%
“…An alternative, more speculative, interpretation assumes that the large displacements are related with long time aging effects that may involve cooperative motion of large regions (∼20 nm), perhaps caused by spurious stress components, over the large distances (> 100 nm) detected in our experiments. Although it is known from calorimetric as well as thermal and mechanical relaxation experiments that molecular mobility (e.g., redistribution of free volume) is possible down to about 50 K below T g , [41][42][43][44]52,56 the large displacements indicated by our FRS results are rather unexpected and should be investigated by further experiments, in particular, by the neutron reflection technique where chain interdiffusion across an interface can be detected with high spatial resolution up to distances of about 10 nm. 53 With a particular FRS setup that allows for studies of translational and rotational diffusion with experimental times of many months, we have also determined translational and rotational diffusion coefficients of dye tracers and we have observed a small but distinct orientational anisotropy that persists in PMMA at 80 °C for times up to 1 year.…”
By dissolving tracer quantities of 9,10-phenanthrenequinone (PQ) in poly(methyl methacrylate) (PMMA) it was possible to study by forced Rayleigh scattering (FRS) the tracer diffusion of the unbleached PQ as well as the photoproduct which is covalently bound to the PMMA molecules. The PMMA samples with molecular weights, M w, of 10 3 -10 5 were intermittently annealed at 80 °C for diffusion times of up to one year. From the results we conclude that polymer chain diffusion may be possible at temperatures more than 20 K below the glass transition temperature Tg, but further experiments are necessary, in particular, since we cannot exclude that our results are due to yet unexplained long time aging effects in PMMA. The translational diffusion coefficients of the unbleached PQ are enhanced by several decades in comparison with predictions from rotational correlation times that were determined by using a modified FRS technique.
“…However, we cannot be sure whether the slowing of the decay at very long times discussed in relation with the Λ = 0.365 μm curve in Figure a is partly due to aging effects (see below) , mechanical relaxation, , and other techniques. ,− In polystyrene, the rotational diffusion of rubrene tracers showed a large slowing on aging at T g − 10 K. , 12 Effect of polymer aging on development of the FRS grating. The two experimental curves show the refractive index modulation Δ n (grating distance, Λ = 0.67 μm) as a function of time, determined in the same PMMA/PQ sample ( M w = 4260) within a few hours after melting (dotted line) and after annealing at 80 °C for two months (solid line).…”
Section: Resultsmentioning
confidence: 99%
“…However, we cannot be sure whether the slowing of the decay at very long times discussed in relation with the Λ ) 0.365 µm curve in Figure 4a is partly due to aging effects (see below). 40 Physical aging in PMMA has been studied previously by calorimetry, 41,42 mechanical relaxation, 43,44 and other techniques. 30,[45][46][47] In polystyrene, the rotational diffusion of rubrene tracers showed a large slowing on aging at T g -10 K. 30,47 The large translational enhancement of PQ diffusion exhibited in Figure 11 (see above) can be explained by the influence of dynamical heterogeneity which has also been observed in other polymers and glass-forming liquids close to the glass transition.…”
Section: Resultsmentioning
confidence: 99%
“…An alternative, more speculative, interpretation assumes that the large displacements are related with long time aging effects that may involve cooperative motion of large regions (∼20 nm), perhaps caused by spurious stress components, over the large distances (> 100 nm) detected in our experiments. Although it is known from calorimetric as well as thermal and mechanical relaxation experiments that molecular mobility (e.g., redistribution of free volume) is possible down to about 50 K below T g , [41][42][43][44]52,56 the large displacements indicated by our FRS results are rather unexpected and should be investigated by further experiments, in particular, by the neutron reflection technique where chain interdiffusion across an interface can be detected with high spatial resolution up to distances of about 10 nm. 53 With a particular FRS setup that allows for studies of translational and rotational diffusion with experimental times of many months, we have also determined translational and rotational diffusion coefficients of dye tracers and we have observed a small but distinct orientational anisotropy that persists in PMMA at 80 °C for times up to 1 year.…”
By dissolving tracer quantities of 9,10-phenanthrenequinone (PQ) in poly(methyl methacrylate) (PMMA) it was possible to study by forced Rayleigh scattering (FRS) the tracer diffusion of the unbleached PQ as well as the photoproduct which is covalently bound to the PMMA molecules. The PMMA samples with molecular weights, M w, of 10 3 -10 5 were intermittently annealed at 80 °C for diffusion times of up to one year. From the results we conclude that polymer chain diffusion may be possible at temperatures more than 20 K below the glass transition temperature Tg, but further experiments are necessary, in particular, since we cannot exclude that our results are due to yet unexplained long time aging effects in PMMA. The translational diffusion coefficients of the unbleached PQ are enhanced by several decades in comparison with predictions from rotational correlation times that were determined by using a modified FRS technique.
“…Because of its commercial and technological importance and apparent high purity, poly(methyl methacrylate) (PMMA) is the subject of a significant fraction of the literature on polymer relaxations and related studies. Certain aspects of its glassy state distinguish this polymer from others including a rather broad “main” glass transition − and other various properties related to tacticity. At low frequencies, the center of the glass transition ( T g ) is about 105 °C in “conventional atactic” PMMA.…”
The thermally stimulated current thermal sampling (TSC-TS) technique was used to study the cooperative glass transition like relaxations in different tacticity poly(methyl methacrylates) (PMMAs) including iso-and syndiotactic forms. We use the high sensitivity of the TSC-TS method to study these weak and sometimes overlapping relaxations at a frequency of about 10 -3 Hz where one can resolve cooperative relaxations well below the main glass transition (Tg), even if the species are a minor fraction of the overall relaxing species. The magnitudes of the values of Ea are compared with a theoretical prediction as a function of temperature, showing that the low-temperature tail of the glass transition in conventional atactic (e.g., 45% syndiotriads) extends down to 15 °C, while in our highest syndiocontent PMMA (80% syndiotriads), the glass-transition-like relaxations only extend down to about 80 °C. Results taken from the literature also substantiate the results obtained here for atactic PMMA. The results are contrasted to those for bisphenol-A polycarbonate and other polymers and suggest that compositional heterogeneity, more specifically the presence of low Tg "pockets" of predominantly isotactic sequences, contributes to the broad glass transition extending almost 90 °C below the main glass transition in atactic PMMA.
“…For the T g it is sometimes attempted to extrapolate to very slow heating rates. The relation of T g to heating rate turns out, however, not to be linear 125,126 and no further attempt is made in this work to obtain any extrapolated values for T g . For consistency and uniformity we used the values quoted in the Polymer Handbook.…”
This thesis would not have been possible without the help of many people in all three places where I have worked. I am indebted to my adviser, Dr. Bernhard Wunderlich, who has been my most important influence throughout these years. His help, always available when needed, has been a constant source of knowledge and fresh ideas. I am also grateful to all the
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