Block Copolymer Side Chains Research Articles

Synthesis and Morphology of Molecular Brushes with Polyacrylonitrile Block Copolymer Side Chains and Their Conversion into Nanostructured Carbons

Densely grafted brushes with polyacrylonitrile (PAN) di- (AB) and triblock (ABC) copolymer side chains were synthesized first time by ATRP using the “grafting from” technique. Tapping-mode atomic force microscopy showed that these brushes displayed wormlike shapes on mica surface. A necklacelike structure was observed for brushes with poly(n-butyl acrylate)-b-polyacrylonitrile-b-poly(tert-butyl acrylate) triblock copolymer side chains. After conversion of poly(tert-butyl acrylate) side chains to poly(acrylic acid), they were cross-linked with diamine to fix the extended 3-dimensional structure. Such shell-cross-linked molecular brushes showed improved stability during transferring from solution to flat substrates and have been successfully converted into nanostructured carbons through pyrolysis. The pyrolyzed materials exhibited characteristics of partially graphitic carbon.

Surface Induced Tilt Propagation in Thin Films of Semifluorinated Liquid Crystalline Side Chain Block Copolymers

The supramolecular self-assembly within a liquid crystalline block copolymer thin film with mesogenic semifluorinated alkyl side groups was studied using both grazing incidence X-ray diffraction and small angle scattering. Previous studies employing near-edge X-ray absorption fine structure spectroscopy indicated that mesogens at the film surface are tilted relative to the surface-normal. This tilt may arise from the interplay between the low surface energy of perfluoromethyl groups and the entropic degrees of freedom of each side chain (constrained by neighboring mesogens in the smectic layer and the alkyl linker attachment to the polymer backbone). Here, we show that the mesogen tilt at the surface propagates over several smectic layers into the film interior. We propose that the high grafting density of side chains in combination with a head-to-head arrangement of side chain free ends plays a significant role in transmitting mesogen tilt from the surface to the adjacent smectic layers within the film interior. In addition to scattering from individual mesogens, we observed characteristic scattering patterns in the plane of incidence produced by the horizontal smectic layers. The amorphous polystyrene block formed vertical domains interspersed in the liquid crystalline matrix, resulting in out-of-plane Bragg rods.

Cylindrical Core−Shell Brushes Prepared by a Combination of ROP and ATRP

A molecular brush with block copolymer side chains, i.e., a crystalline poly(ε-caprolactone) (PCL) core with an amorphous poly(n-butyl acrylate) (PBA) shell, was synthesized by the “grafting from” approach using a combination of atom transfer radical polymerization (ATRP) and ring-opening polymerization (ROP). The brush was prepared in the following steps. First, a well-defined poly(2-hydroxyethyl methacrylate) (PHEMA) was synthesized by ATRP in a methanol/anisole cosolvent. In the second step, PCL was grafted from the PHEMA macroinitiators by ring-opening polymerization (ROP) in the presence of tin(II) 2-ethylhexanoate (Sn(EH)2) catalyst, and then the pendent hydroxyl groups in the resulting PCL brush were transformed to bromopropionyl ATRP initiating groups. Finally, the PBA chains were grafted from the PCL brush macroinitiators by ATRP. Gel permeation chromatography (GPC) analysis provided evidence for formation of a well-defined block copolymer side chain brush. The melting point and the heat of fusion of the homo-PCL brush and the PCL−PBA block copolymer side chain brush were measured by differential scanning calorimetry (DSC). According to the DSC results, the melting point of the PCL segments in the PCL−PBA block copolymer side chain brush was depressed when compared with a homo-PCL brush. This was due to the PCL crystalline domain being disturbed by the presence of the second PBA block. The crystalline core−amorphous shell structure of the brush was directly visualized by atomic force microscopy (AFM). On the basis of the AFM results, the polymer molecules obtained from the combined methods of ATRP and ROP were indeed well-defined brush macromolecules, indicating successful polymerization at each step.

Stimulus Responsive Behavior of Elastin-Based Side Chain Polymers

Linear poly(VPGVG) has been extensively studied over the years as a model for the structural protein elastin. Elastin is characterized by a lower critical solution temperature (LCST). This LCST can be influenced by several factors, mainly molecular weight, concentration, and pH. An ABA type block copolymer was synthesized using atom transfer radical polymerization (ATRP), in which the VPGVG sequence is in the side chain of the A block and the B block is poly(ethylene glycol) (PEG). The LCST behavior of this series of elastin based side chain block copolymers was investigated by changing the degree of polymerization, polymer concentration, and pH. The effects of these parameters on the LCST of the side chain block copolymers were similar when compared with linear poly(VPGVG). The aggregates which were formed above the transition temperature were investigated using light scattering and cryo-SEM techniques.

Nitroxide‐Controlled Grafting by an Insertion Mechanism using a Polymerizable N‐Oxyl

AbstractSummary: The polymerizable nitroxide 4‐acryloyloxy‐2,2,6,6‐tetramethylpiperidine‐N‐oxyl (AOTEMPO) was utilized in a two step polymerization to prepare alkoxyamine‐functionalized backbone polymers. These backbones were then used as initiators for stable free‐radical graft polymerizations in bulk at 125 °C. The products were cleaved into their linear components by reaction with L‐(+)‐ascorbic acid allowing a reproducible analysis of the molecular weights. The influences of the backbone concentration and the graft density on the polymerization were investigated. Graft polymers purely consisting of styrene were produced as well as polymers with a backbone based on 2‐ethoxyethyl acrylate (EOEA). In addition a graft polymer with block copolymer side chains was prepared by extending a preformed graft polymer. In all cases the controlled course of the reaction was confirmed by the linear increase of the molecular weights over conversion and the low polydispersities of the products.Structure of an extended graft copolymer.imageStructure of an extended graft copolymer.

Amphiphilic cylindrical brushes with poly(acrylic acid) core and poly(n-butyl acrylate) shell and narrow length distribution

Core–shell cylindrical polymer brushes with poly(t-butyl acrylate)-b-poly(n-butyl acrylate) (PtBA-b-PnBA) diblock copolymer side chains were synthesized via 'grafting from’ technique using atom transfer radical polymerization (ATRP). The formation of well-defined brushes was confirmed by GPC and 1H NMR. Multi-angle light scattering (MALS) measurements on brushes with 240 arms show that the radius of gyration scales with the degree of polymerization of the side chains with an exponent of 0.57±0.05. The hydrolysis of the PtBA block of the side chains resulted amphiphilic cylindrical core–shell nanoparticles. In order to obtain a narrow length distribution of the brushes, the backbone, poly(2-hydroxyethyl methacrylate), was synthesized by anionic polymerization in addition to ATRP. The characteristic core–shell cylindrical structure of the brush was directly visualized on mica by scanning force microscopy (SFM). Brushes with 1500 block copolymer side chains and a length distribution of lw/ln=1.04 at a total length ln=179nm were obtained. By choosing the proper solvent in the dip-coating process on mica, the core and the shell can be visualized independently by SFM.

Selective Alteration of Polymer Surfaces by Thermal Cleavage of Fluorinated Side Chains

We have synthesized a series of fluorinated side chain block copolymers based on either a hydroborated polystyrene-b-polyisoprene or a poly(methyl methacrylate)-b-poly(2-hydroxyethyl methacrylate) backbone. Through polymer-analogous reactions, perfluorinated ester, fluorinated urethane, and fluorinated carbonate side chains were attached to the pendant hydroxyl groups. Their selective thermal degradation behavior was investigated using thermogravimetric analysis (TGA) and thermal desorption mass spectrometry (TDMS). In addition, we investigated the heat- and laser-induced selective alteration of polymer film surfaces by chemically highly selective cleavage of the fluorinated side chains using X-ray photoelectron spectroscopy (XPS), FT-infrared spectroscopy (FT-IR), and contact angle measurements. Finally, we demonstrate the potential to create a laterally patterned surface with different regions of chemically well-defined surface moieties from our fluorinated block copolymers by a “dry” laser process.

Microphase Separation Transition and Rheology of Side-Chain Liquid-Crystalline Block Copolymers

Microphase separation was induced from homogeneous polystyrene-block-polyisoprene (SI diblock) copolymer by grafting of a liquid-crystalline (LC) monomer onto the polyisoprene (PI) block. The origin of the formation of microdomain structure in the side-chain LC block copolymer from a homogeneous SI diblock copolymer is explained in terms of a dramatic increase in repulsive segmental interactions between the constituent blocks. For the investigation, three homogeneous SI diblock copolymers with varying block length ratios (SI-5/6, SI-10/6, and SI-14/3) were synthesized via anionic polymerization in tetrahydrofuran as solvent. This resulted in PI blocks having the following microstructures: 34% 1,2-addition, 59% 3,4-addition, and 7% 1,4-addition. Subsequently, the PI blocks in each of the SI diblock copolymers were first hydroxylated via hydroboration/oxidation reactions to yield hydroxylated SI diblock copolymers (SI-5/6-OH, SI-10/6-OH, and SI-14/3-OH), and then a liquid-crystalline (LC) monomer, 6-[(4-cyano-4'-biphenyl)oxy]hexanoic acid (5CN-COOH), synthesized in our laboratory, was grafted onto the hydroxylated PI blocks (PI-OH), yielding side-chain LC diblock copolymers (SI-5/6-5CN, SI-10/6-5CN, and SI-14/3-5CN). The disappearance of the -COOH and -OH groups after coupling reaction was confirmed using 1 H nuclear magnetic resonance spectroscopy and infrared spectroscopy. Transmission electron microscopy revealed that at room temperature SI-5/6-5CN has spherical microdomains, SI-10/ 6-5CN has hexagonally packed cylindrical microdomains, and SI-14/3-5CN has lamellar microdomains. The LC phase of the side-chain block copolymers was identified using polarizing optical microscopy. The clearing temperature (T cl ), determined by differential scanning calorimetry, of the side-chain LC block copolymers is found to be ca. 84 °C, whereas the T cl of LC monomer 5CN-COOH is 170 °C. Oscillatory shear rheometry indicates that the sphere-forming block copolymer SI-5/6-5CN undergoes a lattice disordering/ordering transition at 140 °C and a demicellization/micellization transition at 155 °C, while both the cylinder-forming block copolymer SI-10/6-5CN and lamella-forming block copolymer SI-14/3-5CN undergo microphase separation transition at temperatures much higher than 240 °C, the highest experimental temperature employed. Binodal curves exhibiting upper critical solution temperature were constructed, via cloud point measurement, for four pairs of polystyrene (PS) and PI grafted with 5CN-COOH (PI-5CN). An expression for the temperature dependence of the Flory-Huggins interaction parameter X for the PS/(PI-5CN) pair was determined and compared with the X expression for the PS/PI pair. The origin of the formation of microdomain structure in the side-chain LC block copolymers SI-5CN from a homogeneous SI diblock copolymer is explained in terms of molecular weight, block composition, and X for the PS/(PI-5CN) pair.

Synthesis of Molecular Brushes with Block Copolymer Side Chains Using Atom Transfer Radical Polymerization

Brush macromolecules having poly(n-butyl acrylate-block-styrene) and poly(styrene-block-n-butyl acrylate) side chains have been synthesized by the “grafting from” approach using atom transfer radical polymerization (ATRP). The molecular weights of the resulting polymers were characterized by gel permeation chromatography (GPC) using refractive index and multiangle light scattering detection. The block copolymer side chains were cleaved from the backbone and analyzed by GPC, confirming the synthesis of well-defined copolymer brushes. Visualization of individual molecules by atomic force microscopy (AFM) enabled analysis of the conformation and microstructure of the brush macromolecules on mica surface. The brushes with the pnBuA core were almost fully stretched, while the inverted structure with the pS core exhibited longitudinal contraction compared to the contour length of the main chain. In addition, the poly(n-butyl acrylate-block-styrene) brushes demonstrated a characteristic necklace morphology which...

Selectively Thermally Cleavable Fluorinated Side Chain Block Copolymers: Surface Chemistry and Surface Properties

A series of monodisperse poly(styrene-b-isoprene[-g-perfluoroacyl]) block copolymers was synthesized by sequential anionic polymerization of styrene and isoprene. The perfluorinated ester side chains were attached to the remaining double bonds of the isoprene block via oxidative hydroboration and esterification of the resulting alcohol by the corresponding perfluorinated acid chloride. Surfaces of the pure polymers or blends in polystyrene exhibit a notable fractional surface excess of the perfluorinated side chains within the uppermost 100 Å depending on the surface preparation and the block copolymer composition. Thus, these surfaces are highly hydrophobic having advancing water contact angles of up to 122°. When samples are heated to 340 °C, the perfluorinated side chains can be selectively cleaved off by a thermal ester cleavage, leaving the backbone of the polymer completely intact. As a result, the hydrophobicity of the resulting poly(styrene-b-isoprene) polymer surface is reversed to an advancing water contact angle < 90°. The selective thermal ester cleavage in combination with a subsequent chemical surface derivatization may have the potential to selectively change the surface functionality. If the thermal cleavage is done locally, patterning of the surface functionality seems possible.

Synthesis and assembly of novel chitin derivatives having amphiphilic polyoxazoline block copolymer as a side chain

The first synthesis of chitin derivatives with well-defined block copolymer side chains, i. e., chitin-graft-[poly(2-methyl-2-oxazoline)-block-poly(2-phenyl-2-oxazoline)] (5), chitin-graft-[poly(2-methyl-2-oxazoline)-block-poly(2-butyl-2-oxazoline)] (6), and chitin-graft-[poly(2-methyl-2-oxazoline)-block-poly(2-tert-butyl-2-oxazoline)] (7), was achieved by the reaction of partially deacetylated chitin (1) with living polyoxazoline block copolymers 2–4. The graft copolymers 5–7 are associated into micelles above the critical micelle concentration (CMC). CMCs of 5 (0.01–0.02 wt.-%) are smaller than those (0.32–0.50 wt.-%) of ω-hydroxyl-terminated poly(2-phenyl-2-oxazoline)-block-poly(2-methyl-2-oxazoline) (2-OH), which is a model block copolymer of the side chain segment of 5. The self-aggregates of 5–7 are capable of forming a complex with hydrophobic low molecular weight substances such as pyrene and magnesium 1-anilinonaphthalene-8-sulfonate (ANS). Cryo-transmission electron microscopy showed that the graft copolymer 5 forms globular particles (diameter: 40 nm) and larger cylindrical aggregates (diameter: 40 nm, length: 80–200 nm). The average radius of gyration of the particles of 5 from the SANS analysis is 36 nm.

Phase Behavior of New Side Chain Smectic C* Liquid Crystalline Block Copolymers

The liquid crystalline and microphase behavior of new chiral side chain liquid crystalline diblock copolymers is described. The materials, which contain a smectic C* mesogen, are block copolymers of polystyrene and methacrylates containing (s)-2-methyl-1-butyl-4‘-(((4-hydroxyphenyl)carbonyl)oxy)-1,1‘-biphenyl-4-carboxylate mesogens. The influences of the liquid crystalline (LC) mesophase on block copolymer ordering and of the block copolymer morphology on LC properties as a function of molecular weight and block copolymer composition are discussed. Microphase segregated structures were observed for blcok copolymers ranging from 24 to 63% w/w LC block. The lamellar domains of the block copolymers stabilize the layered smectic mesophase, particularly the smectic C* phase. The length of the polystyrene block, as well as the LC block, affected LC phase transition temperatures, probably due to phase mixing and the nature of the block copolymer interface. When the molecular weight is low, and the liquid crystal block content is high, the order−disorder transition temperature, at which block copolymer phase segregation is observed, and the LC clearing point occur at the same temperature; this correlation between LC order and the ODT has not been reported before for LC side chain block copolymers. The block copolymer morphology in these cases is arranged to accommodate the focal conic superstructure seen in optical micrographs of this LC block copolymer.

On the Influence of the Morphological Structure on the Liquid Crystalline Behavior of Liquid Crystalline Side Chain Block Copolymers

The influence of the morphological structure on the phase behavior of a liquid crystalline side chain block copolymer has been investigated using SAXS, DSC, TEM, and low-angle electron diffraction. All samples of poly[styrene-block-2-((3-cholesteryloxycarbonyl)oxy)ethyl methacrylate] (PS-b-PChEMA) show a phase separation between the two blocks. An influence on the phase structure of the liquid crystalline subphase exerted by the morphological structure was observed. It was found that in the case of those samples where the liquid crystalline subphase is not continuous (spheres), only a nematic phase is seen, whereas in all samples in which there is a continuous liquid crystalline subphase, the smectic A phase of the homopolymer is formed