Papers by Dilru Ratnaweera
Bulletin of the American …, Jan 1, 2007
The dispersion and orientation of the nano particles within polymeric matrices determine their pr... more The dispersion and orientation of the nano particles within polymeric matrices determine their properties. They depend on inherent characteristics such as miscibility, entropic barriers, structure of the polymer etc., and on processing conditions under which the polymer and the particles are mixed. Chaotic flow patterns have been used to define the structure and orientation on a micron length scale. In efforts to derive quantitative correlations, we carried out structural studies using X-ray, AFM and TEM on Nylon 6/modified clay nano composites as a function of the strength of the chaotic advection in a smart blending process. On the micron length scale, the clay particles are dispersed into multi layers, where the thickness of these sheets and the orientation and the distribution of the clay particles are controlled by the strength of the chaotic advection. On the molecular level, increasing the strength results in increase of the gamma crystal form of the Nylon, a higher internal energy state, in which hydrogen bonds are formed between parallel chains. The structure on the micron level is attributed to the chaotic mixing which o the molecular level it is may be attributed to the fractal nature of the process.
Bulletin of the American …, Jan 1, 2012
Self-assembly of block copolymers in selective solvents was used to form a nanoparticle (NP)/poly... more Self-assembly of block copolymers in selective solvents was used to form a nanoparticle (NP)/polymer hybrid micelles. These micelles can be used as a cargo vehicle for other substances such as drug delivery, and as building blocks for polymer-nanocomposites with controlled ...
Bulletin of the American …, Jan 1, 2008
The structure of polymers within nanocomposites is strongly affected by the confinement of the po... more The structure of polymers within nanocomposites is strongly affected by the confinement of the polymer to the interface with the nanoparticles and the method of blending. In nanocomposites of Nylon 6 and clay particles are made by chaotic blending, the strength of the chaotic ...
Bulletin of the American Physical …, Jan 1, 2009
Solvent affinity drives the association of diblock co-polymers in selective solvents. The shape o... more Solvent affinity drives the association of diblock co-polymers in selective solvents. The shape of the micelles is affected by the size of the blocks and their interaction with the solvent. Most experimental and theoretical studies have investigated solutions of diblocks ...
Bulletin of the American …, Jan 1, 2010
Thin films of semifluorinated (SF) polymer-mediated nanoparticle (NP) assemblies have been invest... more Thin films of semifluorinated (SF) polymer-mediated nanoparticle (NP) assemblies have been investigated using neutron reflectometry. The inherent incompatibility between fluorinated (F) and protonated (H) domains within SF polymers coupled with modification of the ...
Biophysical Journal, Jan 1, 2010
The interaction of b-cyclodextrin (b-CD) with mixed bilayers composed of sphingomylein and choles... more The interaction of b-cyclodextrin (b-CD) with mixed bilayers composed of sphingomylein and cholesterol (Chol) above and below the accepted stable complexation ratio (67:33) was investigated. Membranes with the same (symmetric) and different (asymmetric) compositions in their inner and outer leaflets were deposited at surface pressures of 20, 30, and 40 mN/m at the solid-liquid interface. Using neutron reflectometry, membranes of various global molar ratios (defined as the sum of the molar ratios of the inner and outer leaflets), were characterized before and after b-CD was added to the subphase. The structure of bilayers with global molar ratios at or above the stable complexation ratio was unchanged by b-CD, indicating that b-CD is unable to remove sphingomyelin or complexed Chol. However, b-CD removed all uncomplexed Chol from bilayers composed of global molar ratios below the stable complexation ratio. The removal of Chol by b-CD was independent of the initial structure of the membranes as deposited, suggesting that asymmetric membranes homogenize by the exchange of molecules between leaflets. The interaction of b-CD with the aforementioned membranes was independent of the deposition surface pressure except for a symmetric 50:50 membrane deposited at 40 mN/m. The scattering from 50:50 bilayers with higher packing densities (deposited at 40 mN/m) was unaffected by b-CD, suggesting that the removal of Chol can depend on both the composition and packing density of the membrane.
Talks by Dilru Ratnaweera
Self-assembly of block copolymers in selective solvents was used to form a nanoparticle (NP)/poly... more Self-assembly of block copolymers in selective solvents was used to form a nanoparticle (NP)/polymer hybrid micelles. These micelles can be used as a cargo vehicle for other substances such as drug delivery, and as building blocks for polymer-nanocomposites with controlled NP distribution. Association of NPs into specific blocks of the copolymer depends on the compatibility between the NPs and the block as well as their preference to the solvent that micellization takes place. The current work introduces a small angle neutron scattering study of association of Polyhedral Oligomeric Silsesquioxane (POSS) NPs into micelles of a highly segregating random copolymer, Biphenyl Perfluorocyclobutane (BPh-PFCB), in toluene, which is a good solvent for BPh. Incompatibility between the blocks drives copolymer into micelles with PFCB in the core and BPh in swollen corona. Modification of NPs with polymer chains drives POSS cages into the micelle core and prevents the micelle dissociation at higher temperatures.
The permeation and distribution of solvents in polymer nanocomposites is governed by the way the ... more The permeation and distribution of solvents in polymer nanocomposites is governed by the way the nanoparticles (NP) associate within the matrix polymer. We have previously shown that in thin films interfacial effects affect the distribution of the NP. The current work focuses on the response of a semi-fluorinated random copolymer, Biphenyl Perfluorocyclobutane, and Polyhedral Oligomeric Silsesquioxane (POSS) NP modified with fluorinated or protonated side chains, to presence of D2O. POSS was introduced either as a free NP or tethered to a polymer chain. We found that the presence of POSS reduces the overall uptake of D2O. It also changes the distribution of water in the film as well. In the pristine polymer film the water mainly accumulated at the substrate/polymer interface. In the nanocomposite, the water distribution is correlated with the NP distribution, where NP at the air interface minimize water penetration.
Thin films of semifluorinated (SF) polymer-mediated nanoparticle (NP) assemblies have been invest... more Thin films of semifluorinated (SF) polymer-mediated nanoparticle (NP) assemblies have been investigated using neutron reflectometry. The inherent incompatibility between fluorinated (F) and protonated (H) domains within SF polymers coupled with modification of the surface of the NPs offer a means to control the interactions between the polymer and the NP and drive the structure and dynamics of the composite films. A random copolymer, Biphenyl Perfluorocyclobutane, has been used as the host for POSS (Polyhedral Oligomeric Silsesquioxanes) NPs whose surfaces were modified by either F or H side chains. Films of NPs, tethered to the polymer chains or blended, were spin cast from a mutual solvent, resulting in an initial homogenous distribution of the particles. With annealing as layering of the polymer takes place, the NPs have migrated to the F/H interfacial regions, with a distinct preference of the F-POSS to segregate into the F regions. Both tethers and blended NPs migrated to the film interfaces with the air interface enriched by the F one and the solid interface by the H one. The degree of enrichment of the interfaces was larger for the free NPs compared to tethered ones.
Solvent affinity drives the association of diblock co-polymers in selective solvents. The shape o... more Solvent affinity drives the association of diblock co-polymers in selective solvents. The shape of the micelles is affected by the size of the blocks and their interaction with the solvent. Most experimental and theoretical studies have investigated solutions of diblocks with a relatively low incompatibility, requiring relatively large blocks to associate. The current work introduces a small angle neutron scattering study of a highly segregated diblock-copolymer, a trifluoro propylmethyl siloxane - polystyrene (PTFPMS-PS) in solutions of d-toluene, a good solvent for the polystyrene. Studies were carried out over volume fractions of 0.1 to 0.5 of the fluorinated siloxane segment. The high degree of segregation results in association into star-like micelles with the fluorinated siloxane in the core and a swollen corona even at very low volume fractions of the fluorinated segments. The micelles exhibit unique temperature stability in comparison with aggregates formed by diblock-copolymers in a lower segregation regime. The detailed structure of these aggregates as a function of volume fraction and temperature will be discussed.
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Papers by Dilru Ratnaweera
Talks by Dilru Ratnaweera