Effect of a reduced mobility layer on the interplay between molecular relaxations and diffusion-limited crystallization rate in ultrathin polymer films

In ultrathin polymer films, the coupling between the segmental mobility, precursor of the molecular diffusion, and the crystallization rate is broken down because of interfacial interactions. In particular, in the presence of a reduced mobility layer at the interface with the substrate, the crystallization kinetics slow down at a length scale bigger than the one connected with the deviation from bulk behavior of the structural relaxation. By modeling the influence of the substrate interactions on the parameters governing the temperature evolution of the main relaxation time, it was possible to reproduce the effect of geometrical confinement on the quantities connected to the diffusion-limited crystallization rate. Upon reduction of the thickness or increasing of the substrate interaction, the films show an apparent higher glass stability in terms of an increase of the cold crystallization temperature and of the crystallization time. The deviations from bulk behavior were found to vanish above a crossover temperature as already observed for the phenomena connected to the glass transition.status: publishe

Spirooxazine photoisomerization and relaxation in polymer matrices

9'-Hydroxy-1,3,3-trimethylspiro[indoline-2,3'[3H]naphtha[2,1-b]-1,4oxazine] (SPO-7OH) was used in studies of photochromic transformations in polymer matrices. Illumination with UV lamp caused opening the spirostructure of the oxazine with formation of open merocyanine species absorbing at ca. 610 nm. The kinetic studies of thermal relaxation of the open form showed that this process can be described with a biexponential function including both photochemical reaction and rheological behaviour of the polymeric environment. Basing on Arrhenius plot of the rate constant ascribed to the photochemical reaction, the activation energy was determined, which was 66.1 and 84.7 kJ/mole for poly(methyl methacrylate-co-butyl methacrylate) and poly(vinylpyrrolidone) matrix, respectively.status: publishe

Optical and dielectric characteristics of photochromic hybrid sol-gel materials

Photochromic silica based organic-inorganic hybrid materials containing covalently linked cyanoazobenzene chromophores were investigated by optical and dielectric spectroscopy. These materials, obtained via sol-gel process, were deposited onto glass substrates by spin coating technique to achieve thin transparent films. To investigate photoinduced alignment, the UV-Vis absorption spectra of the sol-gel films were recorded under illumination with linearly polarized blue light. Dielectric relaxation spectroscopy revealed a variety of relaxation processes: the alpha-process related to the dynamic glass transition temperature located around 150 degrees C, and an Arrhenius-type beta-relaxation (activation energy 58-60 kJ/mol) that was assigned to orientational fluctuations involving the azobenzene group. The correspondence between dielectric and photochemical behavior was discussed.status: publishe

Chromophore–zeolite composites: The organizing role of molecular sieves

Dyes incorporated in molecular sieve crystals can reveal information about the orientation of guests in these crystals through polarization microscopy, Raman scattering, and second-harmonic generation measurements. Pyroelectric studies of the dye molecule para-nitro-aniline inserted in the molecular sieve A1P04-5 are described that show which of two rival models explaining the ordering of guests in molecular sieves actually holds

Dielectric relaxations in ultrathin isotactic PMMA films and PS-PMMA-PS trilayer films

The local and cooperative dynamics of supported ultrathin films (L = 6.4 - 120 nm) of isotactic poly(methyl methacrylate) (i-PMMA, $\overline{M}_n = 118\times10^3$ g/mol) was studied using dielectric relaxation spectroscopy for a wide range of frequencies (0.1 Hz to 106 Hz) and temperatures (250 - 423 K). To assess the influence of the PMMA film surfaces on the glass transition dynamics, two different sample geometries were employed: a single layer PMMA film with the film surfaces in direct contact with aluminum films which act as attractive, hard boundaries; and a stacked polystyrene-PMMA-polystyrene trilayer film which contains diffuse PMMA-PS interfaces. For single layer films of i-PMMA, a decrease of the glass transition temperature T g by up to 10 K was observed for a film thickness L < 25 nm (comparable to R EE ), indicated by a decrease of the peak temperature T α in the loss ε ″(T) at low and high frequencies and by a decrease in the temperature corresponding to the maximum in the apparent activation energy E a (T) of the α-process. In contrast, measurements of i-PMMA sandwiched between PS-layers revealed a slight (up to 5 K) increase in T g for PMMA film thickness values less than 30 nm. The slowing down of the glass transition dynamics for the thinnest PMMA films is consistent with an increased contribution from the less mobile PMMA-PS interdiffusion regions

Observation of Permanent Polarization Distributions in Bio-organic Materials Using a Highly Sensitive Scanning Pyroelectric Microscope

Bio-organic materials such as bones, teeth and tendon generally show pyro- and piezoelectric activity that implies the existence of macroscopic polarity. The first discoveries date from 1957 1 in the bones and tendon of both humans and animals and has led to the suggestion that the presence of spontaneous polarity might play a crucial role during morphogenesis and could facilitate other biological functions. © 2011 IEEE.status: publishe

Dielectric properties of phase separated blends containing a microcapacitor network of carbon nanotubes : compatibilization by a random or block copolymer

The mechanisms governing the dielectric blend properties at different length scales for phase separating blends with multiwall carbon nanotubes (MWNTs) are unravelled by tuning the microstructure. Thereto, compatibilization by interfacially segregated block copolymers (bcp) and random copolymers (rcp) of poly(styrene-random/block-methyl methacrylate) (PS-r/b-PMMA) was achieved in phase-separating blends of poly[(α-methylstyrene)-co-acrylonitrile] and poly(methyl methacrylate) (PαMSAN/PMMA) undergoing spinodal decomposition. In our recent work, we elucidated the effects of copolymer architecture and molecular weight on the percolating network of selectively localized MWNTs. Only short bcp and long rcp/bcp improved the connectivity and refinement of the PαMSAN phase laden with MWNTs and the resulting conductivity. In the present work, we study the effects of copolymer type, architecture, and concentration on the dielectric properties. We demonstrate a concurrent increase of the interfacial capacitance and decrease of the interfacial resistance of MWNTs with entrapped PαMSAN upon effective compatibilization. This is attributed to the increasing amount of connected parallel microcapacitor RC elements formed by the network of adjacent MWNTs enclosing a thin dielectric layer of PαMSAN. At high frequencies (above 1 MHz) the electrons hop between the neighboring MWNTs, whereas at intermediate frequencies, the electrons of the MWNTs tunnel through the barriers imposed by the entrapped PαMSAN. The physical characteristics of the microcapacitor network, namely the thickness of the microcapacitors and the volume fraction of entrapped PαMSAN contributing to the microcapacitor network, are estimated by describing the dielectric relaxation time and strength using the fluctuation induced tunneling model and the interlayer model, respectively. Combining the knowledge of the aforementioned parameters allows to describe the evolution of the total interfacial capacitance of the microcapacitor assembly as a function of copolymer type and concentration. Our robust and simple procedure to tune the MWNT microcapacitor network in polymer blends via the efficiency of the compatibilizer can be used to achieve a synergistic increase in the dielectric properties at different length scales