Fluctuations of conformational mobility of macromolecules around the glass transition.

The heterogeneity of local dynamics in disordered systems is behind some key features of glass transition. In order to improve our understanding of the molecular dynamics in disordered systems in the vicinity of the glass transition, different parameters have been proposed to quantitatively describe dynamical heterogeneity. In the case of polymers, free volume models relate the macromolecular mobility to the free or accessible volume. The relationship between dynamic heterogeneity and fluctuations of accessible volume seems straightforward. In the present work, the heterogeneity of local dynamics in polymeric systems is analyzed by computer simulation with the bond fluctuation model. The value of the accessible volume around each polymer chain is evaluated from a snapshot or static structure at each system state, resulting in a distribution of accessible volume that reflects system heterogeneity. The relationship between the average value and the standard deviation of free volume distributions at different temperatures fits a master curve for different systems, regardless of the specific inter- and intramolecular interaction potentials that define each material. The dynamic slowdown around the glass transition is accompanied by a clear evolution of the mean value and shape of the accessible free volume distribution. The relative fluctuation of the dynamically accessible volume has been used as a parameter to quantitatively describe heterogeneity. The fluctuation varies with temperature with remarkable differences between the liquid and glassy states of the systems studied, presenting a peak at the glass transition temperature, which can be interpreted as a reflection of the distribution of local glass transition temperatures.

Surface stiffening and enhanced photoluminescence of ion implanted cellulose - polyvinyl alcohol - silica composite.

Novel Cellulose (Cel) reinforced polyvinyl alcohol (PVA)-Silica (Si) composite which has good stability and in vitro degradation was prepared by lyophilization technique and implanted using N(3+) ions of energy 24keV in the fluences of 1×10(15), 5×10(15) and 1×10(16)ions/cm(2). SEM analysis revealed the formation of microstructures, and improved the surface roughness on ion implantation. In addition to these structural changes, the implantation significantly modified the luminescent, thermal and mechanical properties of the samples. The elastic modulus of the implanted samples has increased by about 50 times compared to the pristine which confirms that the stiffness of the sample surface has increased remarkably on ion implantation. The photoluminescence of the native cellulose has improved greatly due to defect site, dangling bonds and hydrogen passivation. Electric conductivity of the ion implanted samples was improved by about 25%. Hence, low energy ion implantation tunes the mechanical property, surface roughness and further induces the formation of nano structures. MG63 cells seeded onto the scaffolds reveals that with the increase in implantation fluence, the cell attachment, viability and proliferation have improved greatly compared to pristine. The enhancement of cell growth of about 59% was observed in the implanted samples compared to pristine. These properties will enable the scaffolds to be ideal for bone tissue engineering and imaging applications.

Alkaline degradation study of linear and network poly(ε-caprolactone).

Alkaline hydrolysis of a polycaprolactone (PCL) network obtained by photopolymerization of a PCL macromer was investigated. The PCL macromer was obtained by the reaction of PCL diol with methacrylic anhydride. Degradation of PCL network is much faster than linear PCL; the weight loss rate is approximately constant until it reaches around 70%, which happens after approximately 60 h in PCL network and 600 h in linear PCL. Calorimetric results show no changes in crystallinity throughout degradation, suggesting that it takes place in the crystalline and amorphous phases simultaneously. Scanning electron microscopy microphotographs indicate that degradation is produced by a different erosion mechanism in both kinds of samples. The more hydrophilic network PCL would follow a bulk-erosion mechanism, whereas linear PCL would follow a surface-erosion mechanism. Mechanical testing of degraded samples shows a decline in mechanical properties due to changes in sample porosity as a consequence of the degradation process.

Bond fluctuation model to describe physical aging in polymeric materials.

The thermal behavior of a polymeric material during a cooling ramp was simulated by means of the bond fluctuation model. By introducing both an intramolecular and an intermolecular potential, if the cooling rate is fast enough, the glass transition occurs, and the states attained at low temperatures can be characterized as disordered glasses. The evolution of the resulting amorphous systems was then studied during isothermal periods both for systems starting as an amorphous liquid and as an amorphous glass. The results show that after a very long annealing time at temperatures above the glass transition, an excess of energy loss appears in the system when compared to the usual glass theory. The Monte Carlo method was used to simulate the physical aging phenomena at long time scales.

Dielectric relaxation spectrum of poly (epsilon-caprolactone) networks hydrophilized by copolymerization with 2-hydroxyethyl acrylate.

The dielectric relaxation spectrum of polycaprolactone (PCL) networks hydrophilized with different amounts of 2-hydroxyethyl acrylate (HEA) is investigated. PCL is a semicrystalline polyester with a complex relaxation spectrum that includes the main alpha relaxation and two secondary modes (beta, gamma) at lower temperatures. The overlapping of the different relaxational modes was split by using several Havriliak-Negami functions. Crosslinking the material modifies the dynamics of the main relaxation process as reflected by the parameters that characterize the Vogel behavior of the process and the dynamic fragility. The incorporation of HEA units in the network results in a material with microphase separation: two alpha processes are detected, the one corresponding to the PCL chains and the new one associated to nanometric regions that contain different amount of both comonomers. The incorporation of the HEA units in the system involves the presence of a new beta(sw) relaxation due to the link of two side chains by water molecules through hydrogen bonding.