Polymethyl methacrylate relaxations

Triplet—triplet energy transfer from benzophenone to phenanthrene in polymethylmethacrylate at 77 and 298 K was studied by steady-state phosphorescence depolarisation techniques [182], They were unable to see any clear evidence for the orientational dependence of the transfer probability [eqn. (92)]. This may be due to the relative magnitude of the phosphorescence lifetime of benzophenone ( 5 ms) and the much shorter rotational relaxation time of benzophenone implied by the observation by Rice and Kenney-Wallace [250] that coumarin-2 and pyrene have rotational times of < 1 ns, and rhodamine 6G of 5.7 ns in polymethyl methacrylate at room temperature. Indeed, the latter system of rhodamine 6G in polymethyl methacrylate could provide an interesting donor (to rose bengal or some such acceptor) where the rotational time is comparable with the fluorescence time and hence to the dipole—dipole energy transfer time. In this case, the definition of R0 in eqn. (77) is incorrect, since k cannot now be averaged over all orientations. 

Rothwell, Martinson and Gorman have studied the formation of stress induced crazes in polymethyl methacrylate. The sample was initially stressed and held at a strain of 3.5 %. Upon relaxation, SAXS-patterns of 1.5 s were recorded using a linear photodiode detector array. A sequence of selected curves recorded in a plane parallel to the strain direction is shown in Fig. 49. The peak at h = 0.014 is probably due to an... 

As noted in the papers by Schaefer and coworkers and by Lyerla and coworkers, Tip data may be complicated by the fact that mechanisms other than spin-lattice interactions (namely spin-spin relaxation) are possible which don t map motional characteristics. In the case of polystyrene, Schaefer and coworkers conclude that the spin-spin contributions are negligible, whereas Lyerla and coworkers find that the levels of spin-spin and spin-lattice contributions for isotatic polypropylene and atatic polymethyl methacrylate were temperature dependent. 

Figure 5-13. Temperature dependence of tan 8 for polymethyl methacrylate showing the a and P relaxations. [After L. E. Nielsen, Mechanical Properties of Polymers, Reinhold, new York, 1962, p. 178.]...

In Chapter 5 we cited dynamic mechanical relaxation data for polymethyl methacrylate (PMMA). There it was shown that PMMA possesses two mechanical relaxation regions over the temperature range - 50° to 160°C at low frequencies. These were labeled a for the relaxation accompanying the glass transition and / for a secondary relaxation that has generally been associated with motions of the ester side group. PMMA has a predominantly nonpolar... 

Recent developments have been in the area of microthermal analysis using thermal conductivity with thermal diffiisivity signals or AFM to visualize specific areas or domains in the material and perform localized thermal analysis studies (183,184). Relaxational behavior over time and temperature is related to changes in free volume of the material. Positron annihilation lifetime spectroscopy (PALS) measurements of positron lifetimes and intensities are used to estimate both hole sizes and free volume within primarily amorphous phases of polymers. These data are used in measurement of thermal transitions (185,186) structural relaxation including molecular motions (187-189), and effects of additives (190), molecular weight variation (191), and degree of crystallinity (192). It has been used in combination with DSC to analyze the range of miscibility of polymethyl methacrylate poly(ethylene oxide) blends (193). 

The influence of molecular entanglements is illustrated by Figure 6.3, which shows the stress relaxation behaviour for two samples of polymethyl methacrylate. It is seen that the lower molecular mass sample does not show a rubbery plateau... 

Figure 6.3 Master stress-relaxation curves for low molecular mass (molecular mass 1.5 X 10 daltons, curve A) and high molecular mass (molecular mass 3.6 X 10 daltons, curve B) polymethyl methacrylate. (Reproduced with permission from McLoughlin and Toholsky, J. ColloidSci., 7, 555 (1952))...

It is customary to label relaxation transitions in polymers as a, P, y, d, etc. in alphabetical order with decreasing temperature. Three of the four transitions in polymethyl methacrylate (PMMA) ... 

Figure 12.20 Relaxation time for spread polymethyl methacrylate as a function of volume fraction of polymer in the spread film...

To test the validity of the free-volume restriction imposed by a polymer matrix on the dye internal relaxation, we studied the effect of temperature on the fluorescence yield of dye 2 and 3 in stereoregular polymethyl methacrylates. 

Plot of slope of the relaxation curve, n, against stiffness index, Ng, for several polymers. Circles are polyurethanes of nearly the same cross-link density, but varying catalyst to prepolymer ratios as indicated [23]. Other polymers and associated references areO - polyisoprene [13,21], - SBR [21,22], polyisobutylene [15,21], 7- polymethyl methacrylate [15,21] and - polystyrene [15,24]. 

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