Khine Yi Mya

Polyimide/POSS nanocomposites: interfacial interaction, thermal properties and mechanical properties

Abstract A series of functional polyhedral oligomer silsesquioxnae (POSS)/polyimide (PI) nanocomposites were prepared using a two-step approach, first, the octa(aminophenyl)silsesquioxane (OAPS)/NMP solution was mixed with polyamic acid (PAA) solution prepared by reacting 4,4′-diaminodiphenylmethane and 3,3′,4,4′-benzophenonetetracarboxylic dianhydride in NMP, and second, the polycondensation solution was treated by thermal imidization. The well-defined ‘hard particles’ (POSS) and the strong covalent bonds between the PI and the ‘hard particles’ lead to a significant improvement in the thermal mechanical properties of the resulting nanocomposites. The glass transition temperature dramatically increases while the coefficient of thermal expansion (CTE) decreases, owing to the significant increase of the cross-linking density in the PI–POSS nanocomposites. The thermal stability and mechanical property of the nanocomposites were also improved.

Thermomechanical properties of polyimide-epoxy nanocomposites from cubic silsesquioxane epoxides

Novel polyimide-epoxy organic–inorganic nanocomposites are prepared by in situ curing of polyamic acid macromolecules with two POSS epoxides, octa[(epoxycyclohexylethyl)dimethylsilyloxy]silsesquioxane (OC) and octa[(epoxyhexyl)dimethylsilyloxy]silsesquioxane (OH). Comparison between two series of nanocomposites from OC and OH provides a typical example of controlling the macro-scale properties by adjusting the nano-tether structure of the POSS molecules. DMA studies conclude that the crosslink densities of the polyimide-epoxy nanocomposites increase significantly; the nanocomposite networks are predominantly formed by the linkage between the terminal amine groups of the polyimide molecules and the epoxide groups of the POSS molecules. Furthermore, it is interesting to observe that although the crosslink densities of nanocomposites increase significantly, the compressive moduli, hardness and coefficients of thermal expansion change only slightly.

Star-shaped POSS-polycaprolactone polyurethanes and their shape memory performance

Star-shaped polyhedral oligomeric silsesquioxane (SPOSS)-based multi-arm poly(e-caprolactones) (PCLs) with various arm lengths were synthesized by ring opening polymerization of e-caprolactone. Increasing the PCL arm lengths was observed to increase the melting temperature and also the melt crystallization temperature in SPOSS-PCLs. Star-shaped POSS-PCL polyurethanes (SPOSS-PUs) were then prepared by cross-linking SPOSS-PCLs with hexamethylene diisocyanate (HMDI) and their shape memory (SM) behaviour was determined. For this purpose two cyclic thermo-mechanical testing concepts with similar programming (deformation and shape fixing) but different recovery strategies (free strain versus fixed-strain constrained stress recovery) were used and the synthesized polymers were systematically investigated around the crystallization and melting temperatures of PCL. SPOSS-PU with the highest POSS content exhibited remarkable cycles-averaged (N = 2–5) shape fixities and strain recoverabilities of 98% and stress recoverabilities close to 100%. The main structural factors that were expected to reduce the SM performances with increasing PCL arm lengths were a lowering of cross-link density (content of octasubstituted POSS) and an increase in chain flexibility/molecular mobility.

Triple-shape properties of star-shaped POSS-polycaprolactone polyurethane networks

Here, we present an investigation of the triple-shape properties of star-shaped polyhedral oligomeric silsesquioxane-poly(e-caprolactone) polyurethanes (SPOSS-PUs), which have three-dimensional network structures. In a typical ‘triple-shape functionalization process’, mostly consisting of two tensile deformations at different temperatures, chain immobilization of the polymer network component poly(e-caprolactone) (PCL) was successfully realized first through crystallization and then through vitrification. Subsequently, large parts of the respective strains were released under stress-free recovery conditions. The two-fold fixed (‘programmed’) specimens responded to heating with two independent length contractions (switching steps); the first shape change was associated with PCL devitrification and the second one with the melting of hitherto crystalline PCL. It was revealed that the triple-shape properties of SPOSS-PU networks considerably depend on PCL network chain length. When applying exactly the same ‘triple-shape creation procedure’, larger strain releases were detected in the first transition for polymers with a higher PCL network chain length, whereas the second transition was more accentuated for SPOSS-PU networks with a shorter PCL chain length. In the course of thermo-mechanical cycling, the formation of a neck during the second tensile deformation was repeatedly detected for SPOSS-PUs with higher PCL network chain length; in the subsequent recovery process the specimens even exhibited the highest total strain recoverability. Finally, gradual strain release could be achieved at temperatures below the PCL melting transition through the selection of up to four temperature holding steps, at which every time stable shapes were formed.

Efficient gene delivery with paclitaxel-loaded DNA-hybrid polyplexes based on cationic polyhedral oligomeric silsesquioxanes

We present a novel hybrid cationic polymer based on polyhedral oligomeric silsesquioxanes (POSS). This is the first report of using a cationic POSS-based material for gene delivery. Due to its amphiphilic properties, hydrophobic drugs such as paclitaxel can be encapsulated within the hydrophobic core of this polymer. In this paper, it is shown that paclitaxel-encapsulated hybrid POSS-based polyplexes show superior gene transfection efficiency in human breast cancer cells than the non drug-loaded polyplexes.

Self-Assembly of Brush-Like Poly[poly(ethylene glycol) methyl ether methacrylate] Synthesized via Aqueous Atom Transfer Radical Polymerization

Self-assembly of brush-like well-defined poly[poly(ethylene glycol) methyl ether methacrylate] homopolymers, abbreviated as P(PEGMA-475) and P(PEGMA-1100) is investigated in aqueous solution by employing dynamic/static light scattering (DLS/SLS) and transmission electron microscopy (TEM), whereas 475 and 1100 is molar mass of the respective PEGMA macromonomer. The mentioned brush-like homopolymers are synthesized by aqueous ATRP at room temperature. The critical association concentration (CAC) of the synthesized polymers in water depends on the length of the PEG side chains but not on the overall molar mass of the polymer. Thus, approximately the same CAC of approximately 0.35 mg/mL is estimated for various P(PEGMA-1100) samples, and approximately 0.7 mg/mL is estimated for P(PEGMA-475) series. All the investigated P(PEGMA-1100) samples form multimolecular micelles in aqueous solution, where the hydrodynamic size (Rh) and the aggregation number (Nagg) of micelles decreases as the molecular weight of P(PEGMA-1100) increases. This can be attributed to the increased steric hindrances between the PEG side chains in corona of micelles formed by higher molar mass P(PEGMA-1100). The tendency of micelle formation by samples of P(PEGMA-475) series is significantly lower than that of P(PEGMA-1100) series, as demonstrated by their significantly higher CAC and micelles of lower Nagg. The Rh of micelles does not depend strongly on polymer concentration, which suggests that these micelles are formed via the closed association model. Micelles formed by P(PEGMA-1100) series slightly shrink with increase in temperature from 25 to 60 degrees C, while those of P(PEGMA-475) series are found to be insensitive to the same temperature variation. Finally, TEM is carried out to visualize the formed micelles after transferring the aqueous solution to carbon film.

Polyhedral oligomeric silsesquioxanes tethered with perfluoroalkylthioether corner groups: Facile synthesis and enhancement of hydrophobicity of their polymer blends

Octa(vinyl)-POSS or octa(vinyldimethylsilyloxy)-POSS reacts with perfluoroalkyl thiol in the presence of 2,2′-azobisisobutyronitrile to afford perfluoroalkylthioether substituted POSS compounds in high yields. 29Si NMR and MALDI-TOF mass spectroscopic analyses indicate that the eight perfluoroalkylthioether chains are attached to the POSS core structure. Studies on poly(methyl methacrylate) (PMMA) blends with 5 wt% fluorinated POSS (fluoroPOSS) reveal that the POSS molecules considerably enhance the hydrophobicity of polymer blend surfaces as evidenced by significant increases in water contact angles from 71° for pure PMMA to 114–124° for fluoroPOSS-blends, making this type of fluoroPOSS a promising new class of highly hydrophobic hybrid materials.

Nano-hybrid luminescent dot: synthesis, characterization and optical properties

A series of nano-hybrid light emitting dots with diameter range from 2 nm to 4 nm were synthesized through grafting organic conjugated chains directly onto an inorganic rigid cage polyhedral oligomeric silsesquioxanes (POSS). The effect of chain length, side groups, polarity of solvents on the property of light emitting dots were studied using FTIR, XRD, DSC, UV, PL, AFM and SEM. The unique structure of these nano-hybrid dots renders them with excellent PL properties which are much different from bulk organic molecules or conjugated polymers. The incorporation of conjugated molecules onto POSS transforms the oligo-conjugated arms from crystalline state to non-crystalline solid which can be solution processed. The PLQE (PL quantum efficiency) of the oligo-conjugated arms in condensed state increases significantly after grafting onto POSS. The light emitting dots are very sensitive to the polarity of organic solvents due to their nano-scaled size. PL spectrum of the nano-hybrid dots in solid film was blue shift from that in most of organic solvents.

Time-dependent polymerization kinetic study and the properties of hybrid polymers with functional silsesquioxanes.

Methacrylate-functionalized cubic silsesquioxane homopolymers [p(MA-CSSQ)] were synthesized by reversible addition-fragmentation chain transfer (RAFT)-mediated living radical polymerization in the presence of dodecyl(dimethylacetic acid)trithiocarbonate (DDTA) chain transfer agent, and their polymerization kinetics were studied. The DDTA-terminated p(MA-CSSQ) was then employed as a macro-RAFT agent in the polymerization of methylmethacrylate (MMA) for the synthesis of a brushlike p(MA-CSSQ)-b-PMMA block copolymer. The kinetics study of p(MA-CSSQ) showed that the monomer to polymer conversion, evaluated by (1)H NMR, was found to be approximately 80% with the maximum number average molecular weight (M(n)) of 24000 and 32300 Da, for the [MA-CSSQ]/[DDTA] ratios of 100 and 200, respectively, as determined by gel permeation chromatography (GPC). The broadening of molecular weight distributions in p(MA-CSSQ) homopolymer GPC traces was observed, presumably due to the presence of the radical-radical termination products. The resultant homopolymer and block copolymer exhibited excellent thermal stability as evidenced by thermogravimetric and differential scanning calorimetric analyses. The surface properties of p(MA-CSSQ) homopolymer and p(MA-CSSQ)-b-PMMA block copolymer, determined by water contact angle and atomic force microscopy (AFM) measurements, strongly indicated the surface enrichment of the hydrophobic silsesquioxane groups. The AFM images showed the microsized granular domains of p(MA-CSSQ) homopolymer, whereas the islandlike phase-separated domains were observed in p(MA-CSSQ)-b-PMMA block copolymer.

Self-Assembly of Block Copolymer Micelles: Synthesis via Reversible Addition−Fragmentation Chain Transfer Polymerization and Aqueous Solution Properties

Poly(hexafluorobutyl methacrylate) (PHFBMA) homopolymer was synthesized by reversible addition-fragmentation chain transfer (RAFT)-mediated living radical polymerization in the presence of cyano-2-propyl dithiobenzoate (CPDB) RAFT agent. A block copolymer of PHFBMA-poly(propylene glycol acrylate) (PHFBMA-b-PPGA) with dangling poly(propylene glycol) (PPG) side chains was then synthesized by using CPDB-terminated PHFBMA as a macro-RAFT agent. The amphiphilic properties and self-assembly of PHFBMA-b-PPGA block copolymer in aqueous solution were investigated by dynamic and static light scattering (DLS and SLS) studies, in combination with fluorescence spectroscopy and transmission electron microscopy (TEM). Although PPG shows moderately hydrophilic character, the formation of nanosize polymeric micelles was confirmed by fluorescence and TEM studies. The low value of the critical aggregation concentration exhibited that the tendency for the formation of copolymer aggregates in aqueous solution was very high due to the strong hydrophobicity of the PHFBMA(145)-b-PPGA(33) block copolymer. The combination of DLS and SLS measurements revealed the existence of micellar aggregates in aqueous solution with an association number of approximately 40 +/- 7 for block copolymer micelles. It was also found in TEM observation that there are 40-50 micelles accumulated into one aggregate and these micelles are loosely packed inside the aggregate.