Constanta Ibanescu

Thermo-sensitive polymers based on graft polysiloxanes

The synthesis of novel polymers obtained by grafting poly(dimethyl siloxane) with NIPAM, N,N′-dimethyl acrylamide (DMA) and copolymers of NIPAM with DMA and butyl acrylate using SET-LRP technique is presented. The polymers were characterized by 1H NMR, fluorescence spectroscopy, and DSC. The thermo-sensitivity and the LCST as well as the aggregation phenomena during phase transition are evidenced by dynamic light scattering (DLS) and rheology coupled with small angle light scattering (SALS). Rheological and Rheo-SALS measurements proved to be useful tools to characterize the macroscopic behavior but also to evidence structural changes below and above the LCST for the analyzed systems. Good correlation was found between rheological, rheo-SALS and DLS data.

Amphiphilic azopolymers capable to generate photo-sensitive micelles

AbstractAmphiphilic macromolecular micelles are advantageous for drug delivery applications due to the decrease of side-effects, ease of screening drugs against degradation, long-term stability, targeted delivery and control of the amount of the released drug. A series of amphiphilic azo-polymers having a flexible or rigid main-chain were synthesized and characterized. The presence of chlorobenzyl side-groups allowed both the easy bonding of photo-sensitive or hydrophilic groups and good control of the degree of substitution. The chemical structure was confirmed by 1H-NMR. The critical concentration of aggregation (CCA) was calculated using the fluorescence emission spectrum of pyrene. The interest was focused on a preliminary study concerning the disaggregation capacity of micelles under UV irradiation. The presence of micellar aggregates was confirmed by DLS and SEM and different organization of the amphiphilic polymers was evidenced depending on polymers concentration and polymers structure. In low polymer concentrations in water predominantly globular aggregates were formed. The increase in concentration increased the polydispersity index due to the fusion of micelles and formation of associates of globular aggregates, inter-micellar associates (clusters) and vesicles.n

Photochromic Behaviour of Nano-Structurable Azo-Polysiloxanes with Potential Application in Biology

The miniaturization process involved in the devices of the future industry will require the use of functional molecules capable to response to external stimuli at nanoscale and to integrate into complex architectures. Nanotechnology has a significant impact in drug delivery, genomics, bioengineering, energy storage, electronics etc. The azobenzene-based polymers are one of the most interesting classes of external stimuli sensitive materials due to azo- chromophores trans-cis reversible photoisomerization capacity. This work presents a study of the photochromic behavior of a siloxanic main-chain polymer containing different azo-benzene groups in the side-chain and the correlations with the surface properties modification, using the contact angle method. As a function of the azobenzenic group dipole-moment value and substitution degree, the contact angle method is capable, or not, to evidence the surface properties modifications. The capacity to generate nano-structured surfaces in the case of azo-polysiloxanic films was demonstrated. The polymer chemical structure and the UV irradiation conditions allow controlling the relief topology. As an additional investigation method, molecular modeling studies were effectuated, in order to calculate the azobenzenic groups dipole- moment values and the supramolecular organization at the film surface.

Property peculiarities of the atelocollagen-hyaluronan conjugates crosslinked with a short chain di-oxirane compound.

Minimal amounts of a short-chain bifunctional crosslinker of about 1.3 nm length, the 1,4-butanediol-diglycidyl ether (BDDGE), were used to generate atelocollagen-hyaluronan conjugates in hydrogel state. Two a priori constraints were considered in recipe/procedure developing: (i) working in nondenaturing conditions, and (ii) ensuring a low cytotoxicity of the final product. Both atelocollagen (aK) and hyaluronan (NaHyal) were accurately purified to reduce their molecular-weight dispersity, in order to ensure the reproducibility of hydrogels characteristics. 1:5 aK:NaHyal weight ratios and 1:2.5 to 1:5 α-NH2:BDDGE molar ratios were found to be the most favorable recipe prescriptions that allow the obtaining of rheo-mechanically stable hydrogels, able to be manipulated during cell culturing protocols. Experiments revealed two unexpected effects due to the crosslinking reactions mediated by a short-chain molecule: (i) the occurrence of two thresholds in the rheological behavior of the hydrogels, related with the amount of added crosslinker, and (ii) a quasi-denaturation side-effect induced over the protein component by large or in excess amounts of crosslinker.

Azo-polymers photofluidisation – a transient state of matter emulated by molecular motors

The generation of nanostructured azo-polymer films upon interaction with light of a certain wavelength was reported for the first time more than two decades ago. Despite continuous efforts the mechanism governing structuration remains under debate. In the present paper we propose a new phenomenological model of inscription based on a particular state of matter induced by continuous laser irradiation. The continuous trans–cis–trans motion of azo-segments, resembling molecular motors, induces polymer chain displacement (flowing) even when the irradiated polymer has a very high viscosity. Film surface deformation is the result of two simultaneous processes, one or the other being dominant, as a function of the operational conditions and the azo-polymer chemical structure; the first process, is irreversible and based on the azobenzene molecular motor action leading to polymeric chain displacement (flow) and the second one, is reversible and an elastic deformation induced by the Weigert effect.

Synthesis and characterization of star and brush grafted polysiloxanes, obtained by atom transfer radical polymerization

Abstract Star- and brush-polysiloxanes were synthesized via living radical polymerization using two different catalytic systems. The new compounds were characterized by 1H-NMR, gel permeation chromatography (GPC) and differential scanning calorimetry (DSC) techniques. CuCl and 2,2’-bipyridyl proved to be the most efficient catalytic system. The type of catalytic system and the reaction conditions control the number of arms for star polymers or the grafted-chain density for brush polymers. The use of chloromethyl styrene as monomer or co-monomer yields homogeneous star- or brush-polysiloxanes, although chloromethyl styrene is an inimer system. The homogeneity of materials was verified by GPC and DSC (TOPEM) methods. For linear grafted polysiloxanes the molecular simulations reflect a tendency of chains to adopt globular conformation for high values of the branching density. The polysiloxanes grafted with styrene/chloromethyl styrene were modified with azobenzene in order to obtain systems able to react to the light stimuli. Rheological behavior of the polymer solutions were evaluated before and after irradiation with UV light. After the UV irradiation a significant viscosity increase was evidenced

Thermo-responsiveness of polysiloxanes grafted with poly(dimethyl acrylamide) segments

AbstractThermo-sensitive polymers are of outstanding importance due to their ability to undergo controlled major changes in their properties as a response to minor modifications in temperature. The syntheses of novel polymers by grafting polysiloxane containing chlorobenzyl groups in the side chain by the homopolymerization of N,N′-dimethyl acrylamide (DMA) or the copolymerization of DMA and methyl methacrylate (MMA) using SET-LRP technique are presented. The polymers were characterized by 1H-NMR, UV, and fluorescence spectroscopy, and DSC. The thermo-sensitivity and the lower critical solution temperature (LCST), as well as the aggregation phenomena during the phase transition are evidenced by dynamic light scattering (DLS) and rheology.n

Polysiloxane ionic liquids as good solvents for β-cyclodextrin-polydimethylsiloxane polyrotaxane structures

Summary An ionic liquid based on polydimethylsiloxane with imidazolium salt brushes was synthesized as a good solvent for β-cyclodextrin-polydimethylsiloxane rotaxane. As expected the PDMS-Im/Br ionic liquid had a liquid-like non-Newtonian behavior with rheological parameters dependent on frequency and temperature. The addition of rotaxane to the ionic liquid strengthened the non-Newtonian character of the sample and a type of stable liquid-like network was formed due to the contribution of weak ionic interactions. The structure is stable in the 20 to 80 °C domain as proved by the oscillatory and rotational rheological tests.

Biomacromolecular-based ionic-covalent hydrogels for cell encapsulation: The atelocollagen − Oxidized polysaccharides couples

Mixed crosslinked networks of ionic-covalent entanglement type were prepared starting from ternary mixtures of atelocollagen (aK; as fibrillary matrix generator), sodium hyaluronate (NaHyal; a microfibrillation assistant), and oxidized polysaccharides (OxPolys; as both cross-linkers and matrix fillers), and were tested as hydrogels for eukaryotic cell encapsulation. Either oxidized gellan (GellOx) or pullulan (PullOx) were used. An original procedure and optimal hydrogel recipes were developed to encapsulate fibroblasts and adipose-derived stem cells, while preserving their viability and proliferative ability during ex vivo temporarily storage. Physical-chemical, rheological, and biocompatibility properties of the prepared hydrogels were compared against the classic alginate hydrogel used for cell encapsulation. A larger range of material characteristics (from lax to stiff) and better laboratory maneuverability were demonstrated, which permit to design appropriate compositions for particular cell types. All hydrogels undergo fast liquefaction at temperatures between 42 and 50°C, permitting the cell release after a short innocuous thermal shock.