ANNUAL REPORT 2011 - Instituto de Estructura de la Materia

educed. X-ray scattering results suggested that the exfoliated nanoclay acted as stress concentrators, encounteringmore difficulties in orienting under the action of tensile stress, thus promoting loss of adhesion between thenanoclay and polymer matrix and leading to rapid failure.RECRYSTALIZATION PROCESSES INPET: INTERPLAY BETWEEN STRUCTURE EVOLUTIONAND CONFORMATIONAL RELAXATIONThe changes induced in the amorphous regions of poly (ethylene terephthalate) as a consequence of recrystallizationprocesses, taking place after cold-crystallization at 100 ºC, were analyzed by means of isothermal dynamicalmechanical spectroscopy and microindentation hardness. Overall, a recrystallization process at either 115 ºC or 125ºC was found to induce an increase of the rigidity within the amorphous domains confined by the crystals.Microhardness measurements carried out at room temperature revealed that recrystallization lead to an enhancedmechanical performance of the amorphous regions. The analysis of isothermal segmental relaxation patternsrecorded in a frequency interval of 10 -3 - 60 Hz indicated the appearance of two distinct contributions, which foundcorrespondence with observations by broad band dielectric spectroscopy on the same systems. The faster one wasascribed to segmental relaxation within the amorphous domains where the confinement by crystals is relativelyweak. The slow relaxation mode was associated with regions where the conformational dynamics is stronglyrestricted by the crystals. A relative increase of the slow process was detected upon recrystallization. A recentlydeveloped relaxation function model was employed to estimate the size of the static cooperatively rearrangingregions for both, the slow and the fast modes. It was found that this size increased either upon decreasing thetemperature or as an effect of recrystallization. In addition, the number of monomers involved in a conformationalrearrangement turned out to be significantly larger in the regions associated to the slow mode process.HARDNESSAn update of “Hardness”, a review chapter in the Encyclopedia of Polymer Science and Technology published in2003, has been carried out. The article includes the the basic principles of the hardness technique and a descriptionof the various methods employed in the hardness determination with special emphasis on the new generation ofdepth-sensing instrumentation. The article also includes the latest advances in the application of indentationtechniques to Polymer Science and provides a general overview of the correlations between the mechanisms of localdeformation and the polymer basic nanostructural entities.From the experimental point of view, the production by means of spin-coating of a series of amorphous polymericthin films with varying thickness down to a few nanometers has been carried out. A method to precisely determinethe film thicknes from nanohardness measurements is under development in order to correlate such values to theviscoelastic properties of the materials.Group of DYNAMICS AND STRUCTURE OF SOFT AND POLYMER MATTERSTRUCTURE AND DYNAMICS OF SOFT AND POLYMERIC MATTER NANOSTRUCTRED INTHREE DIMENSIONSSemicrystalline Polymers. Following our research topic aiming to elucidate the interrelation between the intrinsicbulk nanostructure of semicrystalline polymers and the segmental dynamics of the amorphous phase we have shownfor the first time that the combination of dielectric with neutron spin echo measurements performed in a modeldeuterated polymer as poly(ethylene terephthalate) reveals that the dynamics of semicrystalline polymers occurs inan homogeneous scenario similar to that valid to describe the dynamics of amorphous polymers. Accordingly, theintermolecular cooperativity is expected to be rather similar in both amorphous and semicrystalline polymers. Thereduced segmental mobility of the semicrystalline polymer is restricted to well differentiate regions, probably in thecrystal-amorphous interface.The significant broadening of the dielectric segmental relaxation in semicrystallinepolymers can be attributed to the averaging effect of measuring a homogeneous relaxation with similar stretchingexponents over an inhomogeneous environment.Within a similar context, the effects of strain-induced crystallization on the segmental dynamics of vulcanizednatural rubber (NR) have been studied by combining dielectric relaxation spectroscopy and wide-angle X-rayscattering. We have shown that segmental dynamics is clearly affected by uniaxial stretching. At low strain levelsstretching takes place without crystallization and a dramatic increase of the dielectric strength is observed. Thiseffect is accompanied by an increment of the dynamical fragility since as free volume reduces by stretching morecooperativity is needed in order to accomplish segmental motions. At higher strains crystals develop and theinclusion of segments into the crystalline phase counteracts the dielectric increment. Our results support a73