End Group Functionality of 95-99%: Epoxide Functionalization of Polystyryl-Lithium Evaluated via Solvent Gradient Interaction Chromatography.

End group functionality is a key parameter of functional polymer chains. The end-capping efficiency of living polystyryl lithium with various epoxides, namely ethylene oxide (EO), ethoxy ethyl glycidyl ether (EEGE) and isopropylidene glyceryl glycidyl ether (IGG), is investigated with solvent gradient interaction chromatography (SGIC). Generally, end-capping efficiencies >95% are observed. Hydroxy functional polystyrene (PS-OH, PS-EEGE-OH, and PS-IGG-OH) with molar masses ranging from 13.8 to 15.0 kg mol-1 are obtained, with dispersities of 1.05-1.06. Deprotection of the acetal (PS-EEGE-OH) and ketal protective group (PS-IGG-OH) is investigated. Nearly quantitative deprotection (>99%) resulting in the corresponding multihydroxy functional PS (PS-(OH)2 and PS-(OH)3 ) are observed via SGIC. Esterification of PS-OH with succinic anhydride shows a conversion of 98% to the corresponding ester. A detailed picture of side reactions during the carbanionic polymer synthesis subsequent epoxide termination is obtained, demonstrating 95-99% terminal functionality. Depending on the polarity of the end group, an elution order of PS-OH < PS-(OH)2  < PS-(OH)3  < PS-COOH is obtained in SGIC. The study demonstrates both the analytical power of SGIC and the exceptionally high terminal functionalization efficiency of anionic polymerization methods.

Molecular Weight Distribution of Two Types of Living Chains Formed during Nitroxide-Mediated Polymerization of Styrene.

Different types of polymer chains generated during the nitroxide-mediated polymerization of styrene are separated for the first time, and their molecular weight distribution (MWD) is investigated. Living and dead chains are monitored during the reaction; specifically, two types of living chains derived from the initiation of the alkoxyamine (RT) and the self-initiation of styrene and dead chains present in the as-prepared polystyrene (PS). To distinguish between each polymer species, different numbers of hydroxyl groups are introduced onto the T and R groups of the alkoxyamine (one and two groups, respectively). Each living and dead chains is resolved according to the distinct number of hydroxyl groups on its chain-end using high-performance liquid chromatography. Molecular structures of the fractionated PS are characterized using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and 1 H nuclear magnetic resonance spectroscopy, and the results of which show two distinct initiation paths: one originating from RT and the other from the self-initiation of styrene. Molecular weight and MWD are measured using size-exclusion chromatography and reveal a narrow MWD for the living chains derived from RT. Contrastingly, a broad and skewed MWD is observed for the other living chains derived from the self-initiation of styrene and the dead chains.

Nonlinear rheometry of entangled polymeric rings and ring-linear blends.

We present a comprehensive experimental rheological dataset for purified entangled ring polystyrenes and their blends with linear chains in nonlinear shear and elongation. In particular, data for shear stress growth coefficient, steady-state shear viscosity, and first and second normal stress differences are obtained and discussed as functions of shear rate as well as molecular parameters (molar mass, blend composition and decreasing molar mass of linear component in blend). Over the extended parameter range investigated, rings do not exhibit clear transient undershoot in shear, in contrast to their linear counterparts and ring-linear blends. For the latter, the size of the undershoot and respective strain appear to increase with shear rate. Universal scaling of strain at overshoot and fractional overshoot (ratio of maximum to steady-state shear stress growth coefficient) indicates subtle differences in the shear-rate dependence between rings and linear polymers or their blends. The shear thinning behaviour of pure rings yields a slope nearly identical to predictions (-4/7) of a recent shear slit model and molecular dynamics simulations. Data for the second normal stress difference are reported for rings and ring-linear blends. While N 2 is negative and its absolute value stays below that of N 1 , as for linear polymers, the ratio -N 2 /N 1 is unambiguously larger for rings compared to linear polymer solutions with the same number of entanglements (almost by factor of two), in agreement with recent non-equilibrium molecular dynamics simulations. Further, -N 2 exhibits slightly weaker shear rate dependence compared to N 1 at high rates, and the respective power-law exponents can be rationalized in view of the slit model (3/7) and simulations (0.6), although further work is needed to unravel the molecular original of the observed behaviour. The comparison of shear and elongational stress growth coefficients for blends reflects the effect of ring-linear threading which leads to significant viscosity enhancement in elongation. Along the same lines, the elongational stress is much larger than the first normal stress in shear, and their ratio is much larger for rings and ring-linear blends compared to linear polymers. This conforms the interlocking scenario of rings and their important role in mechanically reinforcing linear matrices.

Two-Dimensional Liquid Chromatography Analysis of Polystyrene/Polybutadiene Block Copolymers.

A detailed characterization of a commercial polystyrene/polybutadiene block copolymer material (Styrolux) was carried out using two-dimensional liquid chromatography (2D-LC). The Styrolux is prepared by statistical linking reaction of two different polystyrene- block-polybutadienyl anion precursors with a multivalent linking agent. Therefore, it is a mixture of a number of branched block copolymers different in molecular weight, composition, and chain architecture. While individual LC analysis, including size exclusion chromatography, interaction chromatography, or liquid chromatography at critical condition, is not good enough to resolve all the polymer species, 2D-LC separations coupling two chromatography methods were able to resolve all polymer species present in the sample; at least 13 block copolymer species and a homopolystyrene blended. Four different 2D-LC analyses combining a different pair of two LC methods provide their characteristic separation results. The separation characteristics of the 2D-LC separations are compared to elucidate the elution characteristics of the block copolymer species.

Strain hardening in startup shear of long-chain branched polymer solutions.

We show for the first time that entangled polymeric liquids containing long-chain branching can exhibit strain hardening upon startup shear. As the significant long-chain branching impedes chain disentanglement, Gaussian coils between entanglements can deform to reach the finite extensibility limit where the intrachain retraction force exceeds the value expected from the usual conformational entropy loss evaluated based on Gaussian chain statistics. The phenomenon is expected to lead to further theoretical understanding.

Elution behavior of shortened multiwalled carbon nanotubes in size exclusion chromatography.

We present a rigorous investigation on elution behaviors of ultrasonically shortened multiwalled carbon nanotubes in size-exclusion chromatography. The size separation of five carbon nanotube samples that underwent ultrasonic shortening for varying lengths of time revealed the existence of three kinds of carbon species: large nanotubes, small nanotubes, and amorphous carbon species. Separation of the three different carbon species was confirmed by SEM analyses on the fractionated eluates and also by light scattering/UV absorbance double detection. The chromatographic peak intensity ratio between the large and small nanotubes suggested an increased amount of small carbon nanotubes upon longer mechanical treatment time. The effect of the concentration of carbon nanotube dispersion on elution behavior was examined, and the elution volume of the shortened nanotubes was found to decrease upon dilution while that of the large nanotubes showed the opposite tendency. Unusual elution behaviors of the multiwalled carbon nanotubes were also observed by altering the flow rate, and these behaviors could be explained by the longer equilibration time taken for large nanotubes to access the pores of the packing materials and a possible morphology change of small carbon nanotubes.

2D-LC characterization of comb-shaped polymers using isotope effect.

A rigorous molecular characterization of comb-shaped polystyrene (PS) was carried out taking advantage of its molecular structure, a normal hydrogenous backbone, and deuterated side chains. Normal phase LC (NPLC) can separate the comb PS species well according to their molecular weight. Nonetheless, it cannot distinguish the backbone from the side chains and the differently structured polymers having a similar molecular weight, e.g, a single backbone comb and a coupled backbone comb with fewer side chains. In contrast to NPLC, the hydrogenous polymer is retained longer than the deuterated counterpart in reversed phase LC (RPLC). When the isotope sensitivity of RPLC is taken advantage of, the comb PS is cross fractionated by NPLC and RPLC, and a two-dimensional mapping with respect to the backbone chain length and the number of branches is fully established.

Isotopic effect in the separation of polystyrene by normal phase and reversed phase liquid chromatography.

The retention behavior of deuterated polystyrene (d-PS) and normal hydrogenous polystyrene (h-PS) was investigated in normal phase (NP) and reversed phase (RP) liquid chromatography (LC). It was found that d-PS was retained slightly longer than h-PS in NPLC (bare silica column and tetrahydrofuran (THF)/n-hexane mixed mobile phase) while in RPLC (C18-bonded silica column and THF/methanol mixed mobile phase) h-PS was retained longer than d-PS. The retention behaviors are interpreted in terms of the thermodynamic variables associated with LC separations. The different retention behavior of deuterated polymers from that of hydrogenous polymers could allow separation of partially deuterated polymers according to the deuterium content, such as for copolymers in LC characterization.

Temperature gradient interaction chromatography of polymers: A molecular statistical model.

A new model describing the retention in temperature gradient interaction chromatography of polymers is developed. The model predicts that polymers might elute in temperature gradient interaction chromatography in either an increasing or decreasing order or even nearly independent of molar mass, depending on the rate of the temperature increase relative to the flow rate. This is in contrast to solvent gradient elution, where polymers elute either in order of increasing molar mass or molar mass independent. The predictions of the newly developed model were verified with the literature data as well as new experimental data.

Stress relaxation in symmetric ring-linear polymer blends at low ring fractions.

We combine linear viscoelastic measurements and modelling in order to explore the dynamics of blends of the same-molecular-weight ring and linear polymers in the regime of the low volume fraction (0.3 or lower) of the ring component. The stress relaxation modulus is affected by the constraint release (CR) of both rings and linear components due to the motion of linear chains. We develop a CR-based model of ring-linear blends that predicts the stress relaxation function in the low fraction regime of ring component in excellent agreement with experiments. Rings trapped by their entanglements with linear chains can only relax by linear-chain-induced constraint release, resulting in much slower relaxation of rings than of linear chains. The relative viscosity η ( ϕ R * ) / η L of the blend with respect to the linear melt viscosity η L at ring overlap volume fraction ϕ R * increases proportionally to the square root of ring molecular weight M w , R . Our experimental results clearly demonstrate that it is possible to enhance the viscosity and simultaneously the structural relaxation time of linear polymer melts by adding a small fraction of ring polymers. These results not only provide fundamental insights into the physics of the CR process but also suggest ways to fine-tune the flow properties of linear polymers by means of adding rings.

Threading-Unthreading Transition of Linear-Ring Polymer Blends in Extensional Flow.

Adding small amounts of ring polymers to a matrix of their linear counterparts is known to increase the zero-shear-rate viscosity because of linear-ring threading. Uniaxial extensional rheology measurements show that, unlike its pure linear and ring constituents, the blend exhibits an overshoot in the stress growth coefficient. By combining these measurements with ex-situ small-angle neutron scattering and nonequilibrium molecular dynamics simulations, this overshoot is shown to be driven by a transient threading-unthreading transition of rings embedded within the linear entanglement network. Prior to unthreading, embedded rings deform affinely with the linear entanglement network and produce a measurably stronger elongation of the linear chains in the blend compared to the pure linear melt. Thus, rings uniquely alter the mechanisms of transient elongation in linear polymers.

New epitaxial phase transition between DG and HEX in PS-b-PI.

We investigated the epitaxial phase transition from double gyroid (DG) to hexagonally packed cylinder (HEX) phases of a polystyrene-b-polyisoprene diblock copolymer in a thin film on a Si wafer substrate. The internal structure of the thin film was investigated by grazing incidence small-angle X-ray scattering, transmission electron microscopy, and transmission electron microtomography (TEMT). TEMT allows observation of the 3D-image of coexisting DG and HEX directly. Good domain orientation in the thin film and direct 3D-imaging of TEMT made it possible to observe the coexisting phase of DG and HEX, and a new epitaxial transition path from DG to HEX was found.

Martin's rule for high-performance liquid chromatography retention of polystyrene oligomers.

The liquid chromatographic retention of polystyrene (PS) oligomer was examined taking the column hold-up volume as an adjustable parameter based on the assumption that Martin's rule is valid, i.e., ln k (retention factor) is proportional to the degree of polymerization (DP). In both bare silica and C18-bonded silica columns, thus determined column hold-up volume is systematically smaller than the elution volume of the injection solvent. When the column hold-up volume was taken as an adjustable parameter, the retention of PS oligomer in organic mobile phase follows Martin's rule (ln k proportional, variant DP) near perfectly. In addition, the enthalpy and entropy changes associated with the solute transfer, determined from the temperature dependence of the PS oligomer retention, also show excellent linear dependence on the degree of polymerization if the adjustable column hold-up volume was used. When we compare the retention behavior of PS and hydroxyl terminated PS, the repeating (styrene) unit exhibits the same enthalpy and entropy change while the end group shows the corresponding difference. All observations conform to Martin's rule regardless of the columns used, but the physical nature of the adjustable column hold-up volume remains to be elucidated.

Water-soluble porphyrin-polyethylene glycol conjugates with enhanced cellular uptake for photodynamic therapy.

Water soluble porphyrins were designed and prepared by Williamson ether synthesis reaction between tetrakis(p-bromomethylphenyl)porphyrin and polyethylene glycol (PEG) for photodynamic therapy. The quantum yields for the generation of singlet oxygen of tetra-polyethylene glycol branched porphyrin shows above 80% in D2O. Luminescence of singlet state oxygen was observed from D2O solution under the single-photon excitation at 514 nm. In vitro test, cellular uptake efficiency has been enhanced by simple modification of molecular structure through changing the number of PEG unit without any support such as polymer-encapsulated inorganic nanoparticles.

Toroidal micelles of uniform size from diblock copolymers.

Uniform nanodonuts: Stable toroidal micelles that have a highly uniform size and shape spontaneously self-assemble from a selective THF/ethanol solvent mixture (see 3D AFM image). The donut-shaped micelles can be used as a template to grow gold nanoparticles, which form along the ring surface.

Fast size-exclusion chromatography at high temperature.

We report on the size-exclusion chromatography (SEC) operated at high column temperature to reduce the analysis time. The column temperature was raised beyond the normal boiling point of the eluent and a sufficient column backpressure was applied to prevent the mobile phase from boiling by inserting a narrow bore tubing between the separation column and the detector. The narrow bore tubing also functions to cool the effluent down to the room temperature before it reaches the detector. Therefore, normal SEC detectors can be used without any modification. It was confirmed that the SEC analysis time could be shortened significantly by the high-temperature operation without serious deterioration in the resolution.

Molecular Weight Distribution of Living Chains in Polystyrene Prepared by Atom Transfer Radical Polymerization.

Living and dead chains of a polystyrene synthesized by atom transfer radical polymerization were separated and characterized by high performance liquid chromatography (HPLC), size exclusion chromatography (SEC), NMR, and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). The bromine end group in the living chain was quantitatively converted to a hydroxyl end group via first azidation and subsequent copper-catalyzed azide-alkyne cycloaddition (CuAAC) click reaction with propargyl alcohol. The living chains bearing a polar end group are fully resolved from the unmodified dead chains by HPLC separation using a bare silica stationary phase. Molecular weight distributions (MWD) of the living and dead chain are characterized by SEC and MALDI-MS. The MWD of the living chains is close to a Poisson distribution. Interestingly, the elution peak of the living chains in the HPLC separation split into two. The peak split is attributed to the diastereomeric structure of the chain end by NMR and MALDI-MS analyses.

Characterization of polydisperse poly(vinyl chloride) by temperature gradient interaction chromatography.

Temperature gradient interaction chromatography (TGIC) was employed to fractionate a commodity polymer, poly(vinyl chloride) (PVC) with wide molecular weight distribution (MWD). The TGIC fractionation was carried out with C18 bonded silica and dimethylformamide (DMF) as the stationary and mobile phase, respectively. TGIC exhibited a high resolution to fractionate the PVC into the fractions with a narrow MWD comparable to the anionically polymerized standards. In combination with light scattering detection, TGIC is able to characterize the polymers with wide MWD and shows a good potential to be further developed as a new preparative fractionation method of synthetic polymers.

Separation of branched polystyrene by comprehensive two-dimensional liquid chromatography.

Branched polystyrenes (PS) featuring a bivariate distribution in the molecular weight and in the number of branches were characterized by comprehensive two-dimensional liquid chromatography (2D-LC). The branched PS were prepared by anionic polymerization using n-butyl Li as an initiator and a subsequent linking reaction with p-(chlorodimethylsilyl)styrene (CDMSS). The n-butyl Li initiator yields polystyryl anions with broad molecular weight distribution (MWD) and the linking reaction with CDMSS yields branched PS with different number of branches. For the first dimension (1st-D) separation, reversed-phase temperature gradient interaction chromatography (RP-TGIC) was employed to separate the branched polymer according to mainly the molecular weight. In the second dimension (2nd-D) separation, the effluents from the RP-TGIC separation are subjected to liquid chromatography at chromatographic critical conditions (LCCC), in which the separation was carried out at the critical condition of linear homo-PS to separate the branched PS in terms of the number of branches. The 2D-LC resolution of RP-TGICxLCCC combination worked better than the common LCCCxsize-exclusion chromatography (SEC) configuration due to the higher resolution of RP-TGIC in molecular weight than SEC. Furthermore, by virtue of using the same eluent in RP-TGIC and LCCC (only the column temperature is different), RP-TGICxLCCC separation is free from possible 'break through' and large system peak problems. This type of 2D-LC separation could be utilized efficiently for the analysis of branched polymers with branching units distinguishable by LC separation.

X-ray scattering study of thermal nanopore templating in hybrid films of organosilicate precursor and reactive four-armed porogen.

The miscibility and the mechanism for thermal nanopore templating in films prepared from spin-coating and subsequent drying of homogenous solutions of curable polymethylsilsesquioxane dielectric precursor and thermally labile, reactive triethoxysilyl-terminated four-armed poly(epsilon-caprolactone) porogen were investigated in detail by in situ two-dimensional grazing incidence small-angle x-ray scattering analysis. The dielectric precursor and porogen components in the film were fully miscible. On heating, limited aggregations of the porogen, however, took place in only a small temperature range of 100-140 degrees C as a result of phase separation induced by the competition of the curing and hybridization reactions of the dielectric precursor and porogen; higher porogen loading resulted in relatively large porogen aggregates and a greater size distribution. The developed porogen aggregates underwent thermal firing above 300 degrees C without further growth and movement, and ultimately left their individual footprints in the film as spherical nanopores.