Low Molecular Weight PS Research Articles

Preparation of low molecular weight cyclic polystyrenes with high purity via liquid chromatography at the critical condition

Cyclic polymers synthesized by ring-closure method from linear precursors contain some of linear contaminates. In this work, the origin of linear contaminates in cyclic polystyrenes (c-PS) is demonstrated by the coupling of liquid chromatography at the critical condition (LCCC) with matrix-assisted laser desorption/ionization time-of-flight mass spectra. The linear contaminates are revealed to be the “dead” chains during ATRP by chain termination, the unreacted linear polystyrene (l-PS) precursors, and the dimers by the imperfect ring-closure reaction. The c-PS are purified by LCCC fractionation, and the results show the LCCC fractionation at the critical adsorption point (CAP) of c-PS is more efficient than that at the CAP of linears for low molecular weight (<10,000) PS. A two-step LCCC method is presented for the preparation of c-PS with high purity (>99.6%) via the tandem-coupled LCCC fractionation at the CAP of l-PS and at the CAP of c-PS.

Chain Conformation near the Buried Interface in Nanoparticle-Stabilized Polymer Thin Films

It is known that when nanoparticles are added to polymer thin films, they often migrate to the film–substrate interface and form an “immobile interfacial layer”, which is believed to be the mechanism behind dewetting suppression. We here report a new mechanism of dewetting suppression from the structural aspect of polymer chains accommodated at the film–substrate interface. Dodecanethiol-functionalized gold (Au) nanoparticles embedded in relatively low molecular weight PS thin films prepared on silicon (Si) substrates were used as a model. We mimicked the previously reported conditions, where the nanoparticles preferentially migrate to the substrate, and successfully stabilized the PS thin films via thermal annealing. A suite of surface-sensitive techniques including atomic force microscopy, grazing incidence small-angle X-ray scattering, X-ray/neutron reflectivity, and sum frequency generation spectroscopy in conjunction with the established solvent leaching process enabled us to unveil the polymer chain...

Linear and nonlinear rheological behavior and crystallization of semicrystalline poly(styrene)–poly(l-lactide) block copolymers

The rheological behavior of a poly(styrene)–poly(l-lactide) (PS–PLLA) block copolymer was investigated in the viscoelastic linear and nonlinear regime. Large amplitude oscillatory shear (LAOS) experiments in the nonlinear regime led to an exponential decay of the G moduli and the nonlinearity parameter I3/1. During LAOS, the lamellar microstructure of a low molecular weight PS–PLLA copolymer was orientated parallel to the shear field to achieve a macroscopically ordered material. The degree of orientation was analyzed via small angle X-ray scattering (SAXS) measurements. The crystallization kinetics could be accurately described by the Avrami equation as determined by DSC and rheology. These experiments revealed that the crystallization process was slower for the PS–PLLA copolymers than for the PLLA homopolymers. SAXS was also used to monitor the crystallization of the PS–PLLA copolymers. When the temperature of the crystallization experiments, Tc, was lower than the glass transition temperature of the amorphous PS block, Tga (hard confinement), the lamellar microstructure of the low molecular weight PS–PLLA was maintained apart from a small increase in the domain spacing. When Tc was higher than Tga of PS (soft confinement), the block copolymer also retained its lamellar structure, but the increase in the domain spacing was unexpectedly increased compared to what occurred for crystallization under hard confinement conditions. In addition, two-dimensional (2-D) SAXS diffractograms indicated a loss of the structure periodicity during crystallization under soft confinement.

Effects of aggregation of poly(3-hexylthiophene) in solution on uniaxial alignment of nanofibers during zone casting

We report on the effects of aggregation of P3HT with ordered conformation in solution on improving the uniaxial alignment of the P3HT nanofibers by zone casting. Two approaches were employed to change the aggregation of P3HT: P3HT blending with coil insulating polymer and ultrasonic oscillating. The insulator polymer (i.e. PS) which has good solubility in the solution would disturb the aggregation of P3HT to prevent the chains entanglement. The ultrasonic oscillation can further improve the P3HT aggregation with ordered conformation in the solution. As a result, the P3HT nanofibers in the film grew much orientedly by zone casting the ultrasonic oscillating P3HT/PS polymer blends solution than the same solvent P3HT solution without ultrasonic oscillating and blending. The P3HT pi-pi stacking direction is parallel to the alignment direction of the nanofibers. Meanwhile, the P3HT/PS blend ratio and PS molecular weight have influence on the uniaxial alignment of P3HT nanofibers. Only P3HT/PS is 1:1, the P3HT nanofibers oriented well. The low molecular weight PS can make the P3HT nanofibers orient better than that of the high molecular weight. (C) 2013 Yan-Chun Han. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

Pattern Dimensions and Feature Shapes of Ternary Blends of Block Copolymer and Low Molecular Weight Homopolymers Directed To Assemble on Chemically Nanopatterned Surfaces

Ternary blends of cylinder-forming polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) and low molecular weight PS and PMMA were directed to assemble on chemically patterned surfaces with hexagonal symmetry. The chemical patterns consisted of strongly PMMA preferential spots, patterned by electron-beam lithography, in a matrix of PS. The spot-to-spot spacing of the chemical patterns (L(s)) was varied between 0.9L(0) and 1.1L(0), where L(0) is the cylinder-to-cylinder spacing of the pure block copolymer in bulk. The homopolymer volume fraction of the blends (ϕ(H)) was varied between 0 and 0.3. In addition, chemical patterns were formed with selected spots missing from the perfect hexagonal array, such that the interpolation of domains between patterned spots could be examined on patterns where the polymer/pattern feature density ranged from 1:1 to 4:1. The assemblies were analyzed with top-down SEM, from which orientational order parameter (OP(o)) values were determined. The SEM analysis was complemented by Monte Carlo simulations, which offered insights into the shapes of the assembled cylindrical domains. It was found that, in comparison to pure block copolymer, adding homopolymer increased the range of L(s) values over which assemblies with high OP(o) values could be achieved for 1:1 assemblies. However, the corresponding simulations showed that in the 1:1 assemblies the shape of the cylinders was more uniform for pure block copolymer than for blends. In the case of the 4:1 assemblies, the range of L(s) values over which assemblies with high OP(o) values could be achieved was the same for all values of ϕ(H) tested, but the domains of the pure block copolymer had a more uniform shape. Overall, the results provided insights into the blend composition to be used to meet technological requirements for directed assembly with density multiplication.

Connectivity of PS-b-PEO templated spherical pores in titanium oxide films

Titania films having relatively uniform spherical pores were successfully fabricated using polystyrene-block-poly(ethylene oxide) (PS(n)-b-PEO(m)) diblock copolymers. Depending on the molecular weight of PS(n)-b-PEO(m), the spherical pores were varied from large mesopores (n = 40,000, m = 53,000; ∼40 nm) to macropores (n = 58,600, m = 71,000; ∼60 nm, n = 100,000, m = 150,000; ∼100 nm) in diameter. It was confirmed that the porous structures were thermally stable with crystallization of the titania frameworks. Interconnectivity between the spherical pores was reliant on the micellar arrangement of hydrophobic PS cores, while the amount of isolated pores was increased in the films prepared using low molecular weight PS(n)-b-PEO(m). The presence of such isolated pores was confirmed on the basis of adsorption experiments by using a globular hemoprotein cytochrome c (ca. 3.1 nm). Highly interconnected spherical pores, which were preferentially fabricated using high molecular weight PS(n)-b-PEO(m), were quite helpful for smooth diffusion-adsorption of bulky proteins inside the porous films.

Order−Disorder and Order−Order Transitions in Mixtures of Highly Asymmetric Triblock Copolymer and Low Molecular Weight Homopolymers

The order−disorder and order−order transitions (ODT and OOT) of a series of mixtures of a compositionally asymmetric polystyrene-block-polyisoprene-block-polystyrene copolymer (SIS) (fPS = 0.131) and low molecular weight PS homopolymers with different molecular weights have been investigated by using transmission electron microscopy (TEM), synchrotron small-angle X-ray scattering (SAXS), and rheology. ODT temperatures determined from the discontinuous change in 1/Imax and σq2 vs 1/T plots were found to be in reasonable proximity to the ones determined from the discontinuous change in log G‘ vs T plots. Various OOT's were also identified for a few of V4113/PS mixtures, as evidenced from both SAXS and rheology. This was further confirmed by TEM for the quenched samples. Spherical microdomains in liquidlike packing (LLP) observed for pure V4113 at low temperature were found not to be an equilibrium phase, which would transform completely to spheres in body-centered-cubic array (BCC) with prolonged annealing ...

The Reactive Formation of Diblock Copolymer at a Polymer/Polymer Interface and its Effect on Interfacial Structure

ABSTRACTThe reaction of perdeuterated amino-terminal polystyrene (dPS-NH2) with anhydrideterminal poly(methyl methacrylate) (PMMA-anh) at a PS/PMMA interface has been observed with forward recoil spectrometry (FRES). Bilayer samples were constructed by placing thin films of PS containing ∼8.5 wt % dPS-NH2 on a PMMA-anh layer. Significant reaction was observed only after annealing the samples at 174°C for several hours, a time scale at least two orders of magnitude greater than the time required for the dPS-NH2 chains to diffuse through the bulk PS layer. The topography of the interfacial region as copolymer formed was measured using atomic force microscopy (AFM). Roughening of the PS/PMMA interface was observed to varying degrees in all annealed samples. Furthermore, the extent of this roughening was found to depend on the PS matrix molecular weight. Reaction in the samples with a high molecular weight PS matrix resulted in a root mean square roughness approximately equal to the radius of gyration Rg of the copolymer. However, approximately twice as much roughening was observed in the low molecular weight PS matrix. This study reveals how the molecular weight of one of the phases can affect the rate of reaction at a polymer/polymer interface.

Deformation and toughness of polymeric systems: 6. Critical thickness of diluted entanglement networks

Following Kramer et al. who compared the maximum local draw ratio inside craze fibrils or shear deformation zones for different types of network structures, here the critical thickness ( ID c), below which apparently brittle amorphous glassy polymers demonstrate a macroscopic draw ratio comparable with the high values determined on a microscopic scale, is studied for two — principally different — types of physical networks. The value of ID c is determined for network structures based on polystyrene-poly(2,6-dimethyl-1,4-phenylene ether) (PS-PPE) blends. Two types of networks are compared obtained via blending high molecular weight PPE with either (i) a (standard) high molecular weight PS (PS h) or (ii) an extremely low molecular weight PS (PS 1). In contrast to PS h, PS 1 cannot contribute to the entanglement network structure of the blend since the molecular weight falls well below the entanglement molecular weight of PPE and, consequently, it can be considered as a (glassy) solvent for PPE. The critical thickness of two network structures with the same network density but with different values of the natural draw ratio ( λ max) is experimentally determined via addition of (non-adhering) core-shell rubbers. ID c proves to be inversely proportional to the natural draw ratio of the network structure. The simple energy-based model for the prediction of ID c quantitatively explains the differences found.

Local compositional fluctuations in PPO/HIPSand PPO/SBS blends

AbstractSolid‐state NMR relaxation has been used to explore the distribution of components in poly(phenylene oxide) (PPO) high impact polystyrene (HIPS) and PPO/poly(styrene‐b‐butadiene‐b‐styrene) (SBS) blends. The nuclear relaxation of PPO in the former system is single exponential for all compositions, but the relaxation of PS in the blend is simple exponential only when the PPO content is low but is otherwise nonexponential. The nuclear magnetization decay curves were analyzed in terms of statistical compositional fluctuation at the scale of spin diffusion distances of several nm. Distribution functions for nuclear relaxation and for blend composition have been derived. Extraction of low molecular weight occluded PS from HIPS resulted in blends having reduced homogeneity. Addition of low molecular weight PS enhanced homogeneity in both the PPO/HIPS and PPO/SBS blends. © 1994 John Wiley &amp; Sons, Inc.

Effects Of Material Characteristics On Flame Spreading

Effects of initial molecular weight and thermal stability of polymer samples on horizontal flame spreading behavior and spread rate were studied by comparing results between two polystyrene, PS, samples with different initial molecular weights and between two poly(methyl methacrylate), PMMA, samples with different thermal stability and initial molecular weights. The flame spread rate of the higher molecular weight PS sample was about 25% larger than that for the low molecular weight PS sample and the flame spread rate of the higher molecular weight PMMA sample was about four times larger than that for the low molecular weight sample. The sample with high initial molecular weight does not form molten polymer near the flame front and the flame spreads steadily. However, the sample with low initial molecular weight forms molten polymer and the opposed slow fluid motion of molten polymer along the inclined vaporizing surface against the traveling flame significantly affects flame spreading behavior and its rate.

Approach to the composition dependence of the glass transition temperature of compatible polymer blends, 2 The effect of local chain orientation

AbstractIn a previous publication a new concept has been presented for the composition dependence of the glass transition in compatible polymer blends. The concept starts with the idea that besides conformational energy barriers, contacts due to interaction are responsible for both conformation and “free” volume distribution. In accordance with the Flory‐Huggins lattice theory, the contact probability has been related to the volume fraction of the components, corrected for the different volume expansion coefficients. The concept accounts for both chainspecific and interaction‐specific contributions. It is concluded that local chain orientation due to the increased hetero‐contact interaction may explain the observed molecular weight influences on the composition dependence of Tg of polymer blends. Using experimental data for poly(vinyl methyl ether)‐polystyrene (PVME‐PS) blends it is shown that this local chain orientation due to hetero‐interaction is extremely strong in blends of the low molecular weight PS with high MW PVME. Thus, apparently, the stiffer component is dominating the Tg behaviour of compatible polymer blends.