Novel Functional Mesoporous Materials Obtained from Nanostructured Diblock Copolymers

Summary: Novel carboxy (COOH)-functionalized mesoporous polystyrene membranes were prepared from polystyrene-block-poly(D,L-lactide) (PS-b-PLA) diblock copolymers through the selective degradation of the PLA block. The combination of atom transfer radical polymerization (ATRP) and ring-opening polymerization (ROP) techniques enabled the synthesis of nanostructured diblock copolymers possessing carboxylic acid functionality at the junction between both blocks. Such copolymers were subjected to shear flow through the use of a channel die to align their nanodomains. Under mild alkaline conditions, the quantitative hydrolysis of the polyester nanodomains afforded mesoporous materials with COOH-coated pore walls. The PS-b-PLA precursors as well as the resulting porous systems were carefully analyzed by size exclusion chromatography (SEC), ¹H NMR, scanning electron microscopy (SEM), and two-dimensional small-angle X-ray scattering (2-D SAXS). Moreover, the specific surface area and pore size distribution were determined by nitrogen sorption porosimetry.     

Enzyme-catalyzed degradation of poly(l-lactide)/poly(?-caprolactone) diblock, triblock and four-armed copolymers

Poly(l-lactide)/poly(?-caprolactone) diblock, triblock and four-armed copolymers with the same monomer feed ratio (50/50) were synthesised by two step ring opening polymerisation of successively added ?-caprolactone and l-lactide, using isopropanol, ethylene glycol, or pentaerythritol as initiator and zinc lactate as co-initiator. The resulting copolymers were characterised by ¹H NMR, DSC, SEC, and FT-IR, which confirmed the blocky characteristic of the copolymers. Solution cast films were allowed to degrade at 37 °C in the presence of proteinase K, and the degradation was monitored by gravimetry, DSC, SEC, ¹H NMR and ESEM. The effects of chain structure, block length and crystallinity on the degradation are discussed. The four-armed block copolymer degrades the most rapidly, while the diblock copolymer exhibited the slowest degradation rate. The difference was related to the crystallinity depending on both the molecular structure and block length. Little compositional or molar mass changes were obtained during degradation, which strongly supports a surface erosion mechanism, in agreement with ESEM observations.     

Synthesis and characterization of Fe(II)-coordinated PS-b-P[NIPAM-co-(VBC-Fe-DMAP)] block copolymers

In this work,a new type of block polymers,polystyrene-b-poly[(N-isopropyl acrylamide)-co-(vinyl benzyl chloride)](PS-b-P(NIPAM-co-VBC)),was prepared via reversible addition fragmentation transfer polymerization,then pentacyano(4-(dimethylamino pyridine))ferrate(Fe-DMAP) was attached to VBC units through a quaternization process.The Fe(Ⅱ)-coordinated PS-b-P[NIPAM-co-(VBC-Fe-DMAP)]block copolymers were characterized by ~1H-NMR,FT-IR and TGA.The self-assembly behavior of the block copolymers was also investigated and the micelle morphology was characterized by TEM.It was found that the PS-b-P(NIPAM-co-VBC) block polymer and Fe-coordinated block copolymer could both form spherical micelles in DMF/MeOH mixed solvent.     

Synthesis and crystallization behaviors of poly(styrene-b-isoprene-b-ε-caprolactone) triblock copolymers

The triblock copolymers, poly(styrene-b-isoprene-b-ε-caprolactone)s (PS-b-PI-b-PCL) have been synthesized successfully by combination of anionic polymerization and ring-opening polymerization. Diblock copolymer capped with hydroxyl group, PS-b-PI-OH was synthesized by sequential anionic polymerization of styrene and isoprene and following end-capping reaction of EO, and then it was used as macro initiator in the ring-opening polymerization of CL. The results of DSC and WAXD show big effect of amorphous PS-b-PI on the thermal behaviors of PCL block in the triblock copolymers and the lower degree of crystalline in the triblock copolymer with higher molecular weight of PS-b-PI was observed. The real-time observation on the polarized optical microscopy shows the spherulite growth rates of PCL₂₇, PCL₃₂₈ and PS-b-PI-b-PCL₃₄₄ are 0.71, 0.46 and 0.07 μm s⁻¹, respectively. The atomic force microscopy (AFM) images of the PS₉₀-b-PI₆₆-b-PCL₂₈ show the columns morphology formed by it’s self-assembling.     

Aggregation phenomena of linear and miktoarm star copolymers of styrene and dimethylsiloxane: Influence of the architecture

The micellar behavior of PS-b-PDMS, PS-b-PDMS-b-PS linear block and (PS)₂(PDMS) miktoarm star copolymers of polystyrene (PS) and polydimethylsiloxane (PDMS) is investigated in DMF, a selective solvent for PS. The linear PS-b-PDMS and star (PS)₂(PDMS) copolymers exhibit different macromolecular architectures but similar compositions and total molecular weight, while the linear PS-b-PDMS-b-PS copolymer has the same composition as the diblock and miktoarm star but double their molecular weight. Static, dynamic light scattering and viscometry were used for the structural characterization of the micelles. Aggregation numbers were found to increase in the order PS-b-PDMS-b-PS < (PS)₂(PDMS) < PS-b-PDMS. The corona thickness was dependent on the molecular weight of the soluble PS chains. In the case of (PS)₂(PDMS), although the core area per PS chain, A_C, was significantly lower than that of the linear copolymers, the coronal chains were not significantly stretched. This can be attributed to the stiff nature of the PS chains, which maintains the elongated form of the chains.     

Synthesis and characterization of model diblock copolymers of poly(dimethylsiloxane) with poly(1,4‐butadiene) or poly(ethylene)

Model diblock copolymers of poly(1,4‐butadiene) (PB) and poly(dimethylsiloxane) (PDMS), PB‐b‐PDMS, were synthesized by the sequential anionic polymerization (high vacuum techniques) of butadiene and hexamethylciclotrisiloxane (D₃) in the presence of sec‐BuLi. By homogeneous hydrogenation of PB‐b‐PDMS, the corresponding poly(ethylene) and poly(dimethylsiloxane) block copolymers, PE‐b‐PDMS, were obtained. The synthesized block copolymers were characterized by nuclear magnetic resonance (¹H and ¹³C NMR), size‐exclusion chromatography (SEC), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and rheology. SEC combined with ¹H NMR analysis indicates that the polydispersity index of the samples (M_w/M_n) is low, and that the chemical composition of the copolymers varies from low to medium PDMS content. According to DSC and TGA experiments, the thermal stability of these block copolymers depends on the PDMS content, whereas TEM analysis reveals ordered arrangements of the microphases. The morphologies observed vary from spherical and cylindrical to lamellar domains. This ordered state (even at high temperatures) was further confirmed by small‐amplitude oscillatory shear flow tests. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1579–1590, 2006     

Synthesis of ABABA pentablock copolymers via copper mediated living radical polymerisation

In this work, the syntheses of poly(butyl methacrylate-b-methyl methacrylate-b-butyl methacrylate) triblock copolymer and poly(methyl methacrylate-b-butyl methacrylate-b-methyl methacrylate-b-butyl methacrylate-b-methyl methacrylate) pentablock copolymers using copper mediated living radical polymerisation are reported. Living radical polymerisations were performed using the system Cu^IBr/N-(n-propyl)-2-pyridylmethanimine as catalyst in conjunction with a difunctional initiator, the 1,4-(2^′-bromo-2^′-methylpropionoto)benzene (1). The syntheses of poly(MMA), poly(BMA-b-MMA-b-BMA) and poly(MMA-b-BMA-b-MMA-b-BMA-b-MMA) are described in detail using ¹H NMR spectroscopy and size exclusion chromatography. The living behaviour and the blocking efficiency of these polymerisations were investigated in each case. Difunctional initiator, 1, based on hydroquinone was synthesised and fully characterised and subsequently used to give difunctional poly(methyl methacrylate) macroinitiators with molecular weights up to 54,000 g mol⁻¹ and polydispersity between 1.07 and 1.32; molecular weights were close to the theoretical values. The difunctional macroinitiators were used to reinitiate butyl methacrylate to give triblock copolymers of M_n between 17,500 and 45,700 g mol⁻¹. Polydispersities remained narrow below 25,000 g mol⁻¹ but broadened at higher masses. The difunctional triblock macroinitiators were subsequently used to reinitiate methyl methacrylate to give ABABA pentablock copolymers with M_n up to 37,000 g mol⁻¹ with polydispersity=1.13. Under certain conditions radical-radical reaction led to a broadening of polydispersity index.     

Enzymatic degradation of block copolymers obtained by sequential ring opening polymerization of l-lactide and ?-caprolactone

Copolymers of ?-caprolactone and l-lactide with different molar ratios were prepared via sequential ring opening polymerization (ROP) of both monomers. The resulting PCL-PLLA-PCL triblock copolymers were characterized by using NMR, SEC, DSC and XRD. One melting peak corresponding to the PCL block was detected, but the presence of PLLA decreased the crystallinity of PCL. Enzyme-catalyzed biodegradation of solution cast films was investigated at 37 °C in the presence of Pseudomonas lipase. It was observed that the PLLA component retarded the degradation of the block copolymer as compared to the PCL homopolymer. Therefore, the enzymatic degradation rate can be adjusted by varying the composition of the copolymers. ¹H NMR and SEC data showed no significant chemical composition or molecular weight changes during degradation, indicating that the degradation proceeded according to a surface erosion mechanism. ESEM confirmed surface erosion with appearance of a rugged morphology.     

Studies on the self-assembly behavior of the amphiphilic block copolymer of PSt-b-PAA in apolar solvents with polar fluorescent probe

The block copolymer of polystyrene-b-poly(butyl acrylate) (PSt-b-PBA) with a well-defined structure was synthesized by atom transfer radical polymerization (ATRP); its structure was characterized, and the living polymerization was also validated by gel permeation chromatography, Fourier transform infrared, and ¹H NMR measurements. Then, the amphiphilic block copolymer of polystyrene-b-poly(acrylic acid) (PSt-b-PAA) has been prepared by hydrolysis of PSt-b-PBA, and copolymers of PSt-b-PAA with longer PSt blocks and shorter PAA blocks were obtained by controlling the conditions of ATRP polymerization. The reversed micelle solution of PSt-b-PAA in toluene was prepared by using the single-solvent dissolving method, and the reverse micellization behavior of PSt-b-PAA in toluene was mainly investigated in this paper. The fluorescent probe technique was used by using polar fluorescence compound N-(1-Naphthyl)ethylenediamine dihydrochloride (NEAH) as a polar fluorescent probe to study the reverse micellization behavior of PSt-b-PAA. It was found that the reverse micellization behaviors of PSt-b-PAA in toluene can be clearly revealed by using NEAH as a polar fluorescence probe, and the critical micelle concentrations (cmcs) can be well displayed. The experimental results showed that the self-assembling behavior of PSt-b-PAA in toluene depends apparently on the microstructure of the macromolecules and is also influenced by the temperature. For the copolymers of PSt-b-PAA with the same length of hydrophobic PSt blocks, the copolymer with a longer hydrophilic block PAA has lower cmc, and at higher temperature, the copolymer has lower cmc.     

SEC‐MR‐NMR: Online Coupling of Size Exclusion Chromatography and Medium Resolution NMR Spectroscopy

Online coupling of size exclusion chromatography together with medium resolution nuclear magnetic resonance (SEC‐MR‐NMR) might be one solution to the problem of chemically sensitive detection in liquid polymer chromatography. By use of a combination of SEC with a table‐top, specially designed 20 MHz NMR spectrometer, based on a permanent magnet, online ¹H NMR spectra of SEC fractions can be obtained. The integration of digital filters, mechanical shims and electronic shims led to substantially improved sensitivity and chemical selectivity compared to former TD (time domain) 20 MHz instruments. ¹H NMR spectra of PMMA and PS homopolymers as well as PS‐PMMA block copolymers were of sufficient quality to enable detection and de‐formulation of unknown polymer compounds. ¹H NMR spectra of acceptable resolution and S/N ratio were collected online during the chromatography. The SEC separation online with the NMR measurements performed well and resulted in the proof of principle of the SEC‐MR‐NMR combination.

     

Improved compositional analysis of block copolymers using Diffusion Ordered NMR Spectroscopy

Block copolymers constitute a fascinating class of polymeric materials that are used in a broad range of applications. The performance of these materials is highly coupled to the physical and chemical properties of the constituting block copolymers. Traditionally, the composition of block copolymers is obtained by ¹H NMR spectroscopy on purified copolymer fractions. Specifically, the integrals of a properly selected set of ¹H resonances are compared and used to infer the number average molecular weight (M_n) of one of the block from the (typically known) M_n value of the other. As a corollary, compositional determinations achieved on imperfectly purified samples lead to serious errors, especially when isolation of the block copolymer from the initial macro initiator is tedious. This investigation shows that Diffusion Ordered NMR Spectroscopy (DOSY) can be used to provide a way to assess the advancement degree of the copolymerization purification/reaction, in order to optimize it and hence contribute to an improved compositional analysis of the resulting copolymer. To this purpose, a series of amphiphilic polystyrene-b-poly(ethylene oxide) block copolymers, obtained by controlled free-radical nitroxide mediated polymerization, were analyzed and it is shown that, under proper experimental conditions, DOSY allows for an improved compositional analysis of these block copolymers.     

Synthesis and properties of amphiphilic vinyl acetate triblock copolymers prepared by copper mediated living radical polymerisation

Polymerisation of vinyl acetate by conventional free radical polymerisation using a diazo initiator followed by copper mediated living radical polymerisation with a range of monomers was studied. This method led to the synthesis of triblock copolymers. We have thus successfully prepared several new ABA triblock copolymers where B is poly(vinyl acetate) and A is (dimethylamino)ethyl methacrylate (DMAEMA), (polyethylene glycol) methyl ether methacrylate (MeO(PEG)MA) or solketal methacrylate (SMA). The sequential conventional/living radical polymerisation approach provided an efficient route to synthesis of new block copolymers. The properties of these amphiphilic polymers have been subsequently investigated by ¹H NMR, fluorescence spectroscopy, tensiometry and dynamic light scattering to investigate their behaviour as potential surfactants.     

Synthesis and characterization of model polybutadiene‐1,4‐b‐polydimethylsiloxane‐b‐polybutadiene‐1,4 copolymers

Model copolymers of poly(butadiene) (PB) and poly(dimethylsiloxane) (PDMS), PB‐b‐PDMS‐b‐PB, were synthesized by sequential anionic polymerization (high vacuum techniques) of 1,3‐butadiene and hexamethylciclotrisiloxane (D₃) on sec‐BuLi followed by chlorosilane‐coupling chemistry. The synthesized copolymers were characterized by nuclear magnetic resonance (¹H NMR), size‐exclusion chromatography (SEC), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). SEC and ¹H NMR results showed low polydispersity indexes (M_w/M_n) and variable siloxane compositions, whereas DSC and TGA experiments indicated that the thermal stability of the triblock copolymers depends on the PDMS composition. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2726–2733, 2007     

Reaction-induced microphase separation in epoxy resin containing polystyrene-block-poly(ethylene oxide) alternating multiblock copolymer

Polystyrene-block-poly(ethylene oxide) alternating multiblock copolymer (PS-alt-PEO) was synthesized with the combination of atom transfer radical polymerization (ATRP) and Huisgen 1,3-dipolar cycloaddition (i.e., click chemistry). The copolymer has been characterized by means of Fourier transform infrared spectroscopy (FTIR), ¹H nuclear magnetic resonance spectroscopy (NMR), gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). The PS-alt-PEO alternating multiblock copolymer was incorporated into epoxy resin to investigate the behavior of reaction-induced microphase separation, which has been compared to the case of the thermosets containing PS-b-PEO diblock copolymer. The morphology of epoxy thermosets containing PS-alt-PEO alternating multiblock copolymer were investigated by means of atomic force microscopy (AFM), and small-angle X-ray scattering (SAXS) and the nanostructures were detected in all the thermosetting blends investigated. In marked contrast to the case of the thermosets containing PS-b-PEO diblock copolymer, the thermosets containing PS-alt-PEO multiblock copolymer displayed disordered nanostructures, which have been interpreted on the basis of the restriction of the alternating multiblock topology of the block on the formation of the nanostructures via reaction-induced microphase separation.     

Synthesis and characterization of macrocyclic polystyrene block-copolymers

The synthesis of macrocyclic polystyrene- block-poly(2-vinylpyridine) and macrocyclic polystyrene- block-poly(dimethylsiloxane) was carried out by initiation of 2-vinylpyridine (2VP) and hexamethyl-cyclotrisiloxane (D3) by difunctional living polystyryllithium followed by coupling with 1,4-bis(bromethyl)benzene (1,4-DBX) and dichloro-dimethylsilane (Cl₂SiMe₂), respectively. A small portion of the living ABA precursors were protonated to serve as isobaric linear precursors. The linear and macrocyclic block copolymers were characterized by size-exclusion chromatography (SEC). The ratios of apparent cyclic/linear SEC molecular-weight maxima versus degree of polymerization (DP) show increases with decreasing DP varying from 0.70 ± 0.03 at high DP ≤ 200 to 0.78 ± 0.044 at low DP (≥60) whereas that of the linear ABA block copolymers decreased. Increases in glass transition temperature (Tg) were also observed for the cyclic PS-b-PDMS copolymers with respect to the isobaric linear precursors. The macrocycles were characterized by ¹H and ¹³C NMR and in the case of macrocyclic PS-b-PDMS by ²⁹Si NMR as well. Broadening in the NMR absorptions of the macrocyclic block copolymers is general and is similar to that observed for the homopolymers. Differemtial scanning calorimetry (DSC) analysis of the PS-b-P2VP macrocycles shows increases in Tg at lower molecular weight as was observed for the PS and P2VP macrocycles.     

Asymmetrical flow field-flow fractionation as a novel technique for the analysis of PS-b-PI copolymers

Asymmetrical flow field-flow fractionation (AF4) was used as a fractionation technique to investigate the molecular heterogeneity of poly(styrene-b-isoprene) diblock copolymers synthesized by either sequential living anionic polymerization or coupling of living precursor blocks. AF4 coupled to multi-angle laser light scattering (MALLS), refractive index (RI), and ultraviolet (UV) detectors was used to separate the diblock copolymers from the homopolymers and coupling products, and the molar masses of the different components were analyzed. In order to get more information about the separated block copolymers, homopolymers, and coupling products, fractions were collected directly after the AF4 channel. The collected fractions were analyzed offline by ¹H NMR to provide identification of the different species and additional information on the true chemical composition, and the microstructure of the diblock copolymer was obtained.

Some aspects of nitroxide-mediated living radical polymerization of N-(p-vinylbenzyl)phthalimide

The free radical polymerization of N-(p-vinylbenzyl)phthalimide (VBP) “initiated” with the adduct of 2-benzoyloxy-1-phenylethyl and TEMPO (BS-TEMPO) or TEMPO-terminated polystyrene (PS-TEMPO) in N,N-dimethylformamide (DMF) at 125 °C was found to proceed in a living fashion, providing low-polydispersity PVBP and block copolymers of the type PS-b-PVBA, where TEMPO is 2,2,6,6-tetramethylpiperidinyl-1-oxy. Unlike TEMPO-mediated styrene polymerization, the polymerization rate slightly but distinctly depended on the adduct concentration, which was interpretable as a pre-stationary behavior. The hydrolysis of those polymers gave poly(p-aminomethylstyrene) (PAMS) and PS-b-PAMS, and further treatment of the block copolymer with hydrogen chloride provided an amphiphilic block copolymer. The polymeric amphiphile was used as an emulsifier in emulsion polymerization to produce a positively charged polymeric microsphere.     

Chemical composition effects on the fracture of polystyrene-block-poly(methyl methacrylate)block copolymers

The crazing and fracture behaviors of glassy–glassy block copolymers were investigated for polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers that had similar overall molecular weights but different poly(methyl methacrylate) (PMMA) molar fractions. A liquid chromatography technique was applied to separate as-synthesized PS-b-PMMA [(1) weight-average molecular weight (M_w) = 94,000 g/mol and PMMA molar fraction = 0.35 and (2) M_w = 65,000 g/mol and PMMA molar fraction = 0.28] into three fractions with different chemical compositions. With a copper-grid technique, the fracture behaviors of 0.5-μm-thick PS-b-PMMA films were studied as a function of the applied strain. For the higher M_w PS-b-PMMA samples, the median strains at crazing and fibril breakdown increased with an increase in the PMMA molar fraction from 0.24 to 0.46, corresponding to an increase in the chain entanglements in the PMMA domains. In contrast, for the lower M_w samples, the two values were not significantly changed even when the PMMA molar fraction was varied from 0.16 to 0.35. M_w of the minor component in PS-b-PMMA played a critical role in controlling the fracture behaviors of the block copolymers. Specifically, M_w/M_e of the minor component (where M_e is the molecular weight between entanglements) had to be roughly larger than 2 for the block copolymers to sustain sufficient strains before fracture. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3612–3620, 2006     

Synthesis and characterization of aba-type block copolymer of poly(ϵ-caproplactone) with poly(ethylene glycol), by mean of activation end groups

Poly(?-caprolactone)-b-poly(ethylene glycol)-b-poly(?-caprolactone) (PCL-b-PEG-b-PCL) triblock copolymer were synthesized by mean anionic activation of the hydroxyl end groups of poly(ethylene glycol) in presence of diphenylmethylsodium. Copolymers were characterized by SEC, FT-IR and ¹H-NMR spectroscopy, TGA and DSC. Size exclusion chromatographic analysis of obtained copolymers indicated incorporation of CL monomer into PEG without formation of PCL homopolymer. Characterization by FT-IR and ¹H NMR spectroscopy of the resulting polymeric products, with respect to their structure, end-groups and composition, showed that they are best described as ester-ether-ester triblock copolymers, whose compositions can be adjusted changing the feeding molar ratio of PEG to CL. The thermal stability of triblock copolymers was less that PEG precursor, but higher that PCL homopolymer. Analysis by mean DSC showed that all copolymers were semi-crystalline and their thermal behavior depending on their composition.     

Poly(p‐phenylene methylene)‐based block copolymers by mechanistic transformation

The synthesis of poly(p‐phenylene methylene) (PPM)‐based block copolymers such as poly(p‐phenylene methylene)‐b‐poly(ε‐caprolactone) and poly(p‐phenylene methylene)‐b‐polytetrahydrofuran by mechanistic transformation was described. First, precursor PPM was synthesized by acid‐catalyzed polymerization of tribenzylborate at 16 °C. Then, this polymer was used as macroinitiators in either ring‐opening polymerization of ε‐caprolactone or cationic ring‐opening polymerization of tetrahydrofuran to yield respective block copolymers. The structures of the prepolymer and block copolymers were characterized by GPC and ¹H NMR investigations. The composition of block copolymers as determined by ¹H NMR and TGA analysis was found to be in very good agreement. The thermal behavior and surface morphology of the copolymers were also investigated, respectively, by differential scanning calorimetry and atomic force microscopy measurements, and the contribution of the major soft segment has been observed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011