Search

The relaxation properties of polymethacrylates of the n-alkyl series with n = l, 2 and 4 (poly(methyl methacrylate) ( PMMA), poly(ethyl methacrylate) ( PEMA) and poly( n-butyl methacrylate) ( PnBMA)) have been measured and analyzed in order to relate their properties to the size of the lateral side chains. The n-alkyl series has been regarded as a model system and was used in this work to test a graphical data analysis method. Essentially, four relaxation processes were detected in the three polymers: the γ, β, α and αβ relaxations, in increasing order of temperature. It was found that the γ relaxation has a low activation energy, of around 36.3-38.5 kJ mol−1, independent of the side chain, exhibiting low entropy of activation values when referring to the Eyring description of the activation parameters. The β relaxation was found to be similar in PMMA and PEMA, showing an activation energy of 88.8 kJ mol−1, increasing to 112.8 kJ mol−1 in PnBMA. The activation entropy was also found to be low for this relaxation, although greater than that for the γ relaxation. In contrast, the α relaxation is quite different in these polymers. We observed a gradual shift in the glass transition temperature towards lower temperatures as the side chain increases in length. The manner in which the α transition makes its way into the dielectric spectra is more abrupt in PMMA than in PnBMA, denoting a higher fragility in the former polymer. Finally, there is a significant difference in the coalescence scenarios of the α and β relaxations for temperatures higher than the glass transition temperature, where they give rise to the so-called αβ relaxation. © 2013 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

The paper investigates the influenceof the crystalline structurein the dynamical behavior of semicrystalline Triton X-100 allowingenlightening the reason for the detection/nondetection of the α′-process.The work was preceded by the study of the full amorphous materialfor which dielectric relaxation spectroscopy (DRS) identified multiplerelaxations: the α-process associated with the dynamical glasstransition and two secondary relaxations (β- and γ- processes).To evaluate how crystallinity affects the detected relaxation processes,different crystallizations were induced under high and low undercoolingconditions. While the secondary relaxations are unaffected by crystallization,the mobility of the cooperative bulk α-process is sensitiveto the distinct morphologies. The distinct semicrystalline stateswere structurally characterized by X-ray diffraction and polarizedoptical microscopy (POM). Differential scanning calorimetry (DSC)was used as a complementary tool. Depending on the extension of undercooling,large and well-defined shperulites or grainy-like structure emerge,respectively, for low and high undercooling degrees, as monitoredby POM. In the two crystalline structures, X-ray diffraction patternsdetected the amorphous halo meaning that both are semicrystalline.However, no differences between the amorphous regions are indentifiedby this technique; the distinction was done by means of dielectricmeasurements probing different mobilities in each of those regions.When the large spherulites evolve, the bulk-like α-process nevergoes to extinction and slightly shifts to low frequencies increasingthe associated glass transition by 2–3 K, as confirmed by DSC;the slight change is an indication that the dimensions of the persistingamorphous regions become comparable to the length scale inherent tothe cooperative motion that determines the glass transition in thefull amorphous material. For the grainy-like structure, the α-processbecomes extinct and an α′-process evolves as revealedby isochronal plots of dielectric measurements, with the featuresof a glass transition as confirmed by temperature modulated differentialscanning calorimetry; both techniques indicate a 10–12 K displacementof the associated hindered glass transition toward higher temperaturesrelative to the amorphous glass transition. It is concluded that thedetection of the α′-process in Triton X-100 is greatlydetermined by the high degree of constraining of the amorphous regionsimposed by the grainy crystalline structure disabling the occurrenceof a bulk-like α-process. Triton X-100 can be taken as a modelfor understanding low molecular weight materials crystallization,allowing correlating the observed dynamical behavior with the achievedcrystalline morphology. [ABSTRACT FROM AUTHOR]