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[EN] The aim of this study is to analyze the mobility of polymer chains in semicrystalline poly(vinylidene fluoride) (PVDF). PVDF crystallizes from the melt in the crystalline phase. The transformation from the phase to the electroactive phase can be induced by stretching at temperatures in the range between 80 and 140 ◦C. The spherulitic structure of the crystalline phase is deformed during stretching to form fibrils oriented in the direction of the strain. The amorphous phase confined among the crystalline lamellae is distorted as well and some degree of orientation of the polymer chains is expected. Dynamicmechanical and dielectric spectroscopy measurements were performed in PVDF films stretched to strain ratios up to 5 at temperatures between 80 and 140 ◦C. Dynamic-mechanical measurements were conducted between −60 ◦C and melting and in this temperature range the relaxation spectra show the main relaxation of the amorphous phase (called -relaxation) and at higher temperatures a relaxation related to crystallites motions (c-relaxation). Although the mean relaxation times of the -relaxation are nearly equal in PVDF before and after crystal phase transformation, a significant change of shape of the relaxation spectrum proves the effect of chain distortion due to crystal reorganization. In stretched PVDF the elastic modulus of the polymer in the direction of deformation is significantly higher than in the transversal one, as expected by chain and crystals fibril orientation. The recovery of the deformation when the sample is heated is related with the appearance of the c-relaxation. Dielectric spectroscopy spectrum shows the main relaxation of the amorphous phase and a secondary process (-relaxation) at lower temperatures. Stretching produces significant changes in the relaxation processes, mainly in the strength and shape of the main relaxation. The Havriliak-Negami function has been applied to analyze the dielectric response., This work is funded by FEDER funds through the "Programa Operacional Factores de Competitividade-COMPETE" and by national funds by FCT-Fundacao para a Ciencia e Tecnologia, project references NANO/NMed-SD/0156/2007, PTDC/CTM/69316/2006 and PTDC/CTM-NAN/112574/2009. V. S. thanks the FCT for the SFRH/BPD/63148/2009 grant. JLGR acknowledges the support of the Spanish Ministry of Science and Innovation through MAT2010-21611-C03-01 (including the FEDER financial support), and EUI2008-00126 projects and funding in the Centro de Investigacion Principe Felipe in the field of Regenerative Medicine through the collaboration agreement from the Conselleria de Sanidad (Generalitat Valenciana), and the Instituto de Salud Carlos III (Ministry of Science and Innovation). The authors also acknowledge support from the COST Action MP1003, 2010 "European Scientific Network for Artificial Muscles".

Glass transition during bulk polymerization was studied in free-radical cross-linking copolymerization (FCC) of methyl methacrylate (MMA) using the steady-state fluorescence (SSF) technique. Naphthalene (N) was used as a monomer and excimer forming probe. Changes in the viscosity of the pregel solutions due to gel formation dramatically enhance both monomer and excimer fluorescent yield of N molecules. The reaction time at which the monomer and excimer intensities exhibit a sudden increase corresponds to the reaction time at which the rate of polymerization becomes maximum resulting from the gel effect. This effect was used to study the gelation of MMA, as a function of time, in various N concentrations. The results were interpreted in the view of percolation theory. The gel fraction, beta, and weight average degree of polymerization, gamma, exponents beta=0.40 +/- 0.02 and gamma=1.70 +/- 0.07 were found in agreement with percolation results for both monomer and excimer measurements, respectively. Publisher's Version Q3 WOS:000230649700003