Degradation of PCL-MPEG diblock copolymer in rat plasma

The poly(epsilon-caprolactone)-co-poly(ethylene glycol) (PCL-MPEG) amphiphilic diblock copolymer with molar ratio of epsilon-CL to MPEG 81:1 is synthesized via a ring-opening polymerization without a catalyst. The M(w) and M(n) molecular weights and the polydispersities are 18,000, 11,000 g/mole and 1.55, respectively. The pegylated amphiphilic copolymer forms micelles with a low critical micelle concentration 6.71 x 10(-8) mole/L, and the average particle size of copolymeric micelles is 62.3 +/- 12.9 nm. The degradation behavior of diblock copolymer was studied in rat plasma at 37 degrees C for 90 days. The changes of mass, composition, morphology, molecular weight, and thermal property of PCL-MPEG copolymer were investigated. The decrease of copolymer mass shows two phases with rate constants of 1.91 x 10(-1) day(-1) in the first-phase (1-24 h) and 1.77 x 10(-3) day(-1) in the second-phase (1-90 days). The degradation of labile ester linkage between PCL block and MPEG block accounts for continuous decrease of copolymer mass in plasma. The decrease of EG molar ratio from 1.30 to 0.67 and prominent reduction of enthalpy of fusion of remained copolymer from 116.5 to 85.2 J/g provide evidences of PCL-MPEG chain scission. On the other hand, the presence of partially degraded copolymers in the residuals results in its polydispersity increased from 1.55 to 2.24 at the end of 90 days. Nevertheless, the surface erosion of copolymer makes the molecular weight not quite different from its original value.     

Tissue anti-adhesion potential of ibuprofen-loaded PLLA-PEG diblock copolymer films

This study was designed to evaluate the effect of polyethylene glycol (PEG) and nonsteroidal anti-inflammatory drug (ibuprofen) on the prevention of postsurgical tissue adhesion. For this, poly(L-lactic acid) (PLLA)-PEG diblock copolymers were synthesized by ring opening polymerization of L-lactide and methoxy polyethylene glycol (Mw 5000) of different compositions. The synthesized copolymers were characterized by gel permeation chromatography and 1H-nuclear magnetic resonance spectroscopy. PLLA-PEG copolymer films were prepared by solvent casting. The prepared copolymer films were more flexible and hydrophilic than the control PLLA film, as investigated by the measurements of glass transition temperature, water absorption content, and water contact angle. The drug release behavior from the ibuprofen (10 wt%)-loaded copolymer films was examined by high performance liquid chromatography. It was observed that the drug was released gradually up to about 40% of total loading amount after 20 days, depending on PEG composition; more drug release from the films with higher PEG compositions. In vitro cell adhesions on the copolymer films with/without drug were compared by the culture of NIH/3T3 mouse embryo fibroblasts on the surfaces. For in vivo evaluation of tissue anti-adhesion potential, the copolymer films with/without drug were implanted between the cecum and peritoneal wall defects of rats and their tissue adhesion extents were compared. It was observed that the ibuprofen-containing PLLA-PEG films with high PEG composition (particularly PLLA113-PEG113 film with PEG composition, 50 mol%) were very effective in preventing cell or tissue adhesion on the film surfaces, probably owing to the synergistic effects of highly mobile, hydrophilic PEG and anti-inflammatory drug, ibuprofen.     

Effects of amphiphilic diblock copolymer on drug nanoparticle formation and stability

This study systematically compares the effects of amphiphilic diblock copolymer (di-BCP) on stabilizing hydrophobic drug nanoparticles formed by flash nanoprecipitation (FNP), and provides a guideline on choosing suitable di-BCPs. Four widely used di-BCPs, i.e., polystyrene-block-poly(ethylene glycol) (PS-b-PEG), polycaprolactone-block-poly(ethylene glycol) (PCL-b-PEG), polylactide-block-poly(ethylene glycol) (PLA-b-PEG), and poly(lactic-co-glycolic acid) (PLGA-b-PEG), and β-carotene as a model drug were used. The study showed that PLGA-b-PEG was the most suitable one, whose hydrophobic block was biodegradable and noncrystallizable as well as had relatively high glass transition temperature (T_g) and a right solubility parameter (δ). The molecular weight of PLGA block over the range from 5k to 15k showed an insignificant effect on controlling the particle size. Amorphous drug particles with a high drug loading of over 83 wt% can be achieved. Much remarkable evidence supported the nanoparticles with kinetically frozen and non-equilibrium packing structures of polymer chains rather than either the micelles or micellar nanoparticles with two well segregated polymer blocks. The thermodynamic effects of the drug and BCP on the particle stability, size and structures were discussed by using solubility parameters.     

Structure of polystyrene-block-poly(ethylene oxide) diblock copolymer micelles in water

Micellization in water of two homologous series of AB-type diblock copolymers, composed of polystyrene (PS) as the A block and poly(ethylene oxide) (PEO) as the B block, were investigated by small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS). The copolymers have molecular weights M _n in the range 2 000—34 800, and have in a given series, the same number of repeating units of the PS block, (N_PS = 10 and 38), and a variable number of repeating units of the PEO block (N_PEO values in the range 23–704). In order to avoid secondary association of micelles, a dialysis technique was used to prepare the micellar systems, in the case of copolymers having high M _n values of the PS block. The experimental micelle properties such as the core radius R_c and the aggregation number N of non-equilibrium structures, so called “frozen micelles”, obtained by dialysis, were found to be independent of the copolymer characteristics. However, for equilibrium structures, obtained by direct solubilization of the copolymers (N_PS = 10) in water, R_c and N were found to decrease with increasing N_PEO for the homologous series.     

Targeted siRNA delivery by anti-HER2 antibody-modified nanoparticles of mPEG-chitosan diblock copolymer

This study aims to determine the specificity of anti-human epidermal growth factor receptor antibody (anti-HER2) modified monomethoxy polyethylene glycol-chitosan (mPEG-CS) nanoparticles (anti-HER2/mPEG-CS NPs) in delivering small interfering RNA (siRNA) to the human epidermal growth factor receptor 2 (HER2) positive cancer cells. Physicochemical properties of the siRNA-loaded anti-HER2/mPEG-CS NPs (anti-HER2/mPEG-CS-siRNA NPs), including size, surface charge, siRNA encapsulation efficiency, and in vitro release profile of siRNA from NPs, were characterized by particle size and zeta potential analyzer, and ultraviolet–visible spectrophotometer. MTT assay was used to study the in vitro cytotoxicity of the NPs. Fluorescent microscope and flow cytometer analysis results showed that anti-HER2/mPEG-CS-siRNA NPs had much efficient delivery of siRNA than the siRNA alone, Lipofectamine-siRNA complexes and mPEG-CS-siRNA NPs. These results demonstrated that anti-HER2/mPEG-CS-siRNA NPs had great potential applications as a targeted strategy for siRNA delivery.     

Docetaxel-loaded nanoparticles based on star-shaped mannitol-core PLGA-TPGS diblock copolymer for breast cancer therapy

A star-shaped biodegradable polymer, mannitol-core poly(d,l-lactide-co-glycolide)-d-α-tocopheryl polyethylene glycol 1000 succinate (M-PLGA-TPGS), was synthesized in order to provide a novel nanoformulation for breast cancer chemotherapy. This novel copolymer was prepared by a core-first approach via three stages of chemical reaction, and was characterized by nuclear magnetic resonance, gel permeation chromatography and thermogravimetric analysis. The docetaxel-loaded M-PLGA-TPGS nanoparticles (NPs), prepared by a modified nanoprecipitation method, were observed to be near-spherical shape with narrow size distribution. Confocal laser scanning microscopy showed that the uptake level of M-PLGA-TPGS NPs was higher than that of PLGA NPs and PLGA-TPGS NPs in MCF-7 cells. A significantly higher level of cytotoxicity was achieved with docetaxel-loaded M-PLGA-TPGS NPs than with commercial Taxotere®, docetaxel-loaded PLGA-TPGS and PLGA NPs. Examination of the drug loading and encapsulation efficiency proved that star-shaped M-PLGA-TPGS could carry higher levels of drug than linear polymer. The in vivo experiment showed docetaxel-loaded M-PLGA-TPGS NPs to have the highest anti-tumor efficacy. In conclusion, the star-like M-PLGA-TPGS copolymer shows potential as a promising drug-loaded biomaterial that can be applied in developing novel nanoformulations for breast cancer therapy.     

Scattering properties of multicomponent polymer solutions: polyelectrolytes,homopolymer mixtures and diblock copolymer

This article reviews the understanding of static and dynamic scattering properties of multicomponent polymer systems in solution achieved during the past decade. We shall describe particularly ternary polymer systems which include polyelectrolyte solutions (polyion/counter-ions/water), mixture of homopolymers (A and B)/solvent and the case of diblock copolymer (A-B, linear or cyclic) solutions. The purpose of this paper is not an extensive survey of theoretical and experimental results obtained on the scattering behavior of these systems but rather an updating of recent results. For polyelectrolyte systems we shall focus, by means of scattering techniques, on the conformation of the polyelectrolyte chain and on the structure of the system induced by the dominant electrostatic interactions in solution involving polyions, counter-ions and solvent. As for the mixture of homopolymers in solution and diblock copolymer/solvent systems, we shall mainly discuss their dynamic behavior and show that using linear response theory and the Random Phase Approximation (RPA), two relaxation modes describe the autocorrelation functions as revealed using dynamic light scattering (DLS) or/and Neutron Spin Echo (NSE) techniques: the first mode characterizes the concentration fluctuations and the second one the composition fluctuations. We shall discuss the scattering properties of these systems on the basis of recent developments with emphasis on possible coiling of the polyelectrolyte chain at low charge density and also how important parameters such as the mobility of the chain (diffusion process) and the interaction parameter (compatibility), which control the dynamics and the thermodynamics in homopolymer mixtures and diblock copolymer systems, could be deduced from scattering experiments.     

Stable gene transfection mediated by polysulfobetaine/PDMAEMA diblock copolymer in salted medium

Cationic polyplexes would aggregate immediately after intravenous injection due to the plasma proteins and high ionic strength. A cationic polyplexes with long-term and salt stability was very important for a systemic gene therapy. In this research, a polysulfobetaine-b-polycation diblock copolymer composed of cationic block of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) and zwtterionic block of poly(propylsulfonate dimethylammonium ethylmethacrylate) (PSPE) was conveniently synthesized by atom transfer radical polymerization method to obtain a cationic polymers with long-term and salt stability. The results of agarose gel electrophoresis and transmission electron microscope indicated that copolymerization of PSPE did not compromise the DNA condensation ability of PDMAEMA, meanwhile exhibiting lower cytotoxicity. The effect of salt on the absorbance and particle size of PDMAEMA₁₀₀/DNA and PDMAEMA₁₀₀-PSPE_y/DNA complexes was investigated, which showed that PSPE block could increase the resistance of polyplexes against salt-induced aggregation owing to the antielectrolyte effect. In comparison with PDMAEMA homopolymer, PDMAEMA₁₀₀-PSPE_y retained more stable gene transfection in a certain range of salt concentration. The expression of red fluorescence protein (RFP) was evaluated by small animal in vivo fluorescence imaging system and the results showed that the expression of RFP was much higher in the mice injected with PDMAEMA₁₀₀-PSPE_20/pDNA-RFP than with PDMAEMA/pDNA-RFP. Both in vitro and in vivo results suggested that PDMAEMA-PSPE diblock copolymer may be potentially used as a vector for systemic gene therapy.     

Effect of PEG-PLLA diblock copolymer on macroporous PLLA scaffolds by thermally induced phase separation

A regular and highly interconnected macroporous poly(L-lactic acid) (PLLA) scaffold was fabricated from a PLLA-dioxane-water ternary system with added polyethylene glycol (PEG)-PLLA diblock using thermally induced phase separation (TIPS). The morphology of the scaffold was investigated in detail by controlling the following TIPS parameters: quenching temperature, aging time, polymer concentration, molecular structure, and diblock concentration. The phase diagram was assessed visually on the basis of the turbidity. The cloud-point curve shifted to higher temperatures with increasing PEG content in the additives (PEG-PLLA diblocks), due to a stronger interaction between PEG and water in solution. The addition of diblock series (0.5 wt% in solution) stabilized interconnections of pores at a later stage without segregation or sedimentation. The pore size of the scaffold could be easily controlled in the range 50-300 microm. A macroporous PLLA scaffold was used to study an MC3T3-E1 cell (an osteoblast-like cell) culture. The cells successfully proliferated in the PLLA scaffold in the presence of added PEG-PLLA diblock for 4 weeks.     

Camptothecin prodrug nanomicelle based on a boronate ester-linked diblock copolymer as the carrier of doxorubicin with enhanced cellular uptake

A novel pH-sensitive polymeric prodrug of camptothecin (CPT) by polymerizing γ-camptothecin-glutamate N-carboxyanhydride (Glu (CPT)-NCA) on boronate ester-linked poly (ethyleneglycol) (PEG) directly via the amine-initiated ring open polymerization (ROP) has been developed. The resulting amphiphilic prodrug (mPEG-BC-PGluCPT) could self-assemble into nanoparticles and encapsulate doxorubicin (Dox) simultaneously in aqueous solution for dual-drug delivery. The formation of polymeric prodrug micelles (mPEG-BC@PGluCPT) was confirmed by the measurements of critical aggregation concentration (CAC), particle size, and morphology observations. The mPEG-BC@PGluCPT micelles were colloidally stable in solutions for two weeks. Polymeric prodrug micelles mPEG-BC@PGluCPT and Dox-loaded micelles mPEG-BC@PGluCPT?Dox showed sustained drug release profiles over 48 h. As expected, drug release was accelerated by the decreasement of pH value from 7.4 to 6.0, which demonstrated pH-dependent manner of drug release. Additionally, it was found that cellular uptake of mPEG-BC@PGluCPT?Dox micelles on HepG2 cells was higher than that on HL-7702 cells, especially in culture medium at pH 6.0. The enhanced cellular uptake of mPEG-BC@PGluCPT?Dox micelles under acidic condition on HepG2 cells resulted in the higher cytotoxicity of mPEG-BC@PGluCPT?Dox micelles at acidic pH than that at pH 7.4.     

In vivo osteogenic differentiation of rat bone marrow stromal cells in thermosensitive MPEG-PCL diblock copolymer gels

Methoxy poly(ethylene glycol)-poly(epsilon-caprolactone) (MPEG-PCL) diblock copolymers were prepared by ring-opening polymerization and their phase transition behavior characterized as a function of temperature. The MPEG-PCL solutions formed a sol at room temperature, and underwent sol-to-gel followed by gel-to-sol phase transitions as the temperature was increased. The temperature range over which the solutions were in a gel state could be extended simply by increasing the PCL chain length in the diblock copolymer. Scanning electron microscopy (SEM) images of MPEG-PCL solutions in the sol and gel states revealed near-regular and irregular porous structures, respectively. in vitro culture of rat bone marrow stromal cells (rBMSCs) on gel surfaces exhibited mostly round cells after 1 day of incubation. SEM images of the attached cells clearly showed the cell body and anchoring filopodia. Injection of room-temperature diblock copolymer solutions into Sprague-Dawley rats produced a gel at body temperature. In situ gel-forming scaffolds in vivo were successfully fabricated by simple subcutaneous injection of MPEG-PCL diblock copolymer solutions. The gel implants retained their original shape for 4 weeks without in- flammation at the injection site. Gel implants removed after 4 weeks were found to be surrounded by a thin fibrous capsule consisting of fibroblasts and blood vessels cells. Hematoxylin and eosin (H&E) and von Kossa staining revealed bone formation in gel implants containing both rBMSCs and dexamethasone, with the degree of bone formation increasing markedly with increasing dexamethasone concentration. Thus, our results show that in situ gel scaffolds fabricated from MPEG-PCL diblock copolymer solutions containing dexamethasone enable multipotent rBMSCs to produce viable bone when injected into rats.     

Synthesis of a propene—methyl methacrylate diblock copolymer via “living” coordination polymerization

A new diblock copolymer of propene and methyl methacrylate (MMA) was prepared by adding MMA during the living coordination polymerization of propene together with the soluble catalyst V(acac)₃/Al(C₂H₅)₂Cl at ?78°C and subsequently by raising the polymerization temperature up to 25°C. The homopolymerization of MMA with this catalyst exhibits the characteristics of a living polymerization at the initial stage of polymerization. From the copolymerization of MMA with styrene, it was concluded that the polymerization of MMA occurs via a radical intermediate. It is suggested that a propene-MMA diblock copolymer is formed via the transformation of a living polypropylene end to a radical end.     

Terminal modification on mPEG-dendritic poly-(l)-lysine cationic diblock copolymer for efficient gene delivery

The development of new non-viral gene vectors with the advantages of low cytotoxicity and high gene transfection efficiency is a recent trend in gene therapy. In this work, we developed a series of termini-modified mPEG-dendritic poly-(l)-lysine cationic diblock copolymers (mPEG5k-DPL4-CG) by coupling various cationic groups to the dendritic skeleton. Their molecular structures were characterized by ¹H NMR, and the buffering capacities were measured by acid titration. The plasmid DNA (pDNA) binding affinities of the mPEG5k-DPL4-CG copolymers were investigated by EB displacement and agarose gel retardation assay, and the average particle size and surface charge of the polyplexes were analyzed by dynamic light scattering. Cytotoxicity and in vitro gene transfection were evaluated in several cell lines in the presence and absence of serum by the luciferase expression assay. The results indicated that the low molecular weight polyethylenimine (PEI800) termini-modified copolymer, mPEG5k-DPL4-PEI800, possessed high pDNA binding affinity, low cytotoxicity, and high gene transfection capability which were maintained in the presence of serum (10% FBS). It is worth noting that the gene delivery efficiency of the dendritic poly-(l)-lysine gene vector was enhanced by termini modification of suitable cationic blocks. The low cytotoxicity and serum-resistance properties of mPEG5k-DPL4-PEI800 make it a potential long-circulating gene vector in gene therapy applications.     

Quasi-elastic light scattering by polystyrene-poly(methyl methacrylate) diblock copolymer solutions in a non-selective solvent in the vicinity of the overlap concentration

The dynamic properties of a styrene (S)-methyl methacrylate (MMA) diblock copolymer in semidilute solutions of toluene have been investigated by quasi-elastic light scattering. One relaxation mode has been found in the low-concentration regime near the overlap concentration C^*. This mode is identified as the cooperative mode which is observed in semidilute solutions of the equivalent polystyrene (PS) homopolymer. A second mode appears simultaneously at higher concentrations. This slower diffusive mode is ascribed to the diffusion of the center of mass of the clusters formed by the copolymer molecules. The fast structural mode which is predicted by the theory^1–3 is not observed. It is suggested that this mode, which must have a small amplitude, is hidden by the other two relaxation modes.     

In vivo efficacy of an intratumorally injected in situ-forming doxorubicin/poly(ethylene glycol)-b-polycaprolactone diblock copolymer

The effectiveness of systemically administered anticancer treatments is limited by difficulties in achieving therapeutic doses within tumors, a problem that is complicated by dose-limiting side effects to normal tissue. This work examined injectable in situ-forming gels as a localized drug-delivery system. An MPEG-PCL (MP) solution containing doxorubicin (Dox) existed in an emulsion-sol state at room temperature and rapidly gelled in vitro and in vivo at body temperature. The release of Dox from Dox-loaded MP gels was sustained in vitro over 20 days after an initial burst, indicating that the MP gel acted as a drug depot. Dox-loaded MP gels exhibited remarkable in vitro anti-proliferative activities against B16F10 cancer cells. In vivo experiments employing B16F10 cancer cell xenograft-bearing mice showed that a single intratumoral injection of Dox-loaded MP gel inhibited the growth of tumors as effectively as repeated injections of free Dox, and more effectively than a single dose of free Dox, or saline or gel alone. Consistent with the observed suppression of tumor growth, intratumorally injected free Dox or Dox released from Dox-loaded MP gels caused apoptosis of tumor cells. The tumor biodistribution of free Dox after 1 day was ～90%, which dropped to ～15% after 4 days. The biodistribution of Dox following a single injection of Dox-loaded MP gel was also ～90% on day 1, but remained at ～13%, even after 15 days. Only a small amount of Dox was found in other organ tissues following intratumoral injection, implying fewer off-target side effects.     

Self-assembled star-shaped chlorin-core poly(epsilon-caprolactone)-poly(ethylene glycol) diblock copolymer micelles for dual chemo-photodynamic therapies

Amphiphilic 4-armed star-shaped chlorin-core diblock copolymers based on methoxy poly(ethylene glycol) (mPEG) and poly(varepsilon-caprolactone) (PCL) were synthesized and characterized in this study. The synthesized photosensitizer-centered amphiphilic star block copolymer that forms assembled micelle-like structures can be used in a photodynamic therapy (PDT)-functionalized drug delivery system. Moreover, the hydrophobic chemotherapeutic agent, paclitaxel, can be trapped in the hydrophobic inner core of micelles. In our results, the star-polymer-formed micelle exhibited efficient singlet oxygen generation, whereas the hydrophobic photosensitizer failed due to aggregation in aqueous solution. The chlorin-core micelle without paclitaxel loading exhibited obvious phototoxicity in MCF-7 breast cancer cells with 7J/cm2 or 14J/cm2 light irradiation at a chlorin concentration of 125microg/ml. After paclitaxel loading, the size of micelle increased from 71.4nm to 103.2nm. Surprisingly, these micelles were found to improve the cytotoxicity of paclitaxel significantly in MCF-7 cells after irradiation through a synergistic effect evaluated by median effect analysis. This functionalized micellar delivery system is a potential dual carrier for the synergistic combination of photodynamic therapy and chemotherapy for the treatment of cancer.     

Evaluation of cell affinity on poly(L-lactide) and poly(epsilon-caprolactone) blends and on PLLA-b-PCL diblock copolymer surfaces

An evaluation of cell proliferation and adhesion on biocompatible film supports was performed. A series of films were compression molded from commercially available poly (L-lactide), PLLA, and poly(epsilon-caprolactone), PCL, and from their melt mixed blends (PLLA/PCL blends). These were compared with compression molded films of PLLA-b-PCL model diblock copolymers. The samples were analyzed by differential scanning calorimetry (DSC), contact angle measurements, and scanning force microscopy (SFM). Cell adhesion and proliferation were performed with monkey derived fibroblasts (VERO) and with osteoblastic cells obtained either enzymatically or from explants cultures of Sprague-Dawley rat calvaria. Migration studies were performed with bone explants of the same origin. The results obtained indicate that although all materials tested were suitable for the support of cellular growth, a PLLA-b-PCL diblock copolymer sample with 93% PLLA was significantly more efficient. This sample exhibited a unique surface morphology with long range ordered domains (of the order of 2-3 mum) of edge-on PLLA lamellae that can promote "cell contact guidance." The influence of other factors such as chemical composition, degree of crystallinity, and surface roughness did not play a major role in determining cell preference toward a specific surface for the materials employed in this work.     

Poly(ethylene glycol)-poly(L-lactide) diblock copolymer prevents aggregation of poly(L-lactide) microspheres during ethylene oxide gas sterilization

Sterilization procedure is one of the most important obstacles in the clinical applications of biodegradable microspheres. The microspheres prepared with poly(alpha-hydroxy acid) were severely aggregated during ethylene oxide (EO) gas sterilization, and could not be used in clinical applications. In this study, the effects of EO gas sterilization on the poly(L-lactide) (PLLA) microspheres were analyzed by nuclear magnetic resonance spectroscopy (1H-NMR), differential scanning calorimetry (DSC), gel permeation chromatography (GPC), scanning electron microscope (SEM) and size fractionation. The aggregation between the microspheres might be stimulated by high mobility of amorphous regions of PLLA on the microsphere surfaces since both water vapor and gas mixture can reduce glass transition temperature (Tg) of PLLA below the sterilization temperature. During EO gas sterilization, there were no changes in the molecular structure and the molecular weight of PLLA in microspheres, but there were changes in the crystallinity of PLLA in microspheres. In this study, poly(L-lactide)-poly(ethylene glycol) diblock copolymers (PLE) were blended with PLLA homopolymers in various ratios to design the microsphere suitable for EO gas sterilization. Aggregation of PLLA microspheres was markedly prevented when more than 4wt% of PLE was blended in the microspheres. This inhibition effect on aggregation may be due to the increased initial crystallinity of the microspheres, which help to maintain the microsphere morphology during EO gas sterilization.     

Rheology of lamellar polystyrene-block-polyisoprene diblock copolymers

The linear viscoelastic properties of symmetric polystyrene-block-polyisoprene diblock copolymers (PS-b-PI) were studied over a range of molecular weights between 20 000 and 200 000 using dynamic mechanical spectroscopy. For all samples the low frequency rheological response can be divided into two frequency regimes in agreement with previous reports on this subject, indicating the onset of a third relaxation process at very low frequencies related to the microphase separated microstructure. Various methods to determine two characteristic frequencies, ω_c and ω_d, separating different orientation regimes of the lamellar microstructure observed under large amplitude oscillatory shear flow in the vicinity of T_ODT (T < T_ODT) are compared. It is further shown that for PS-b-PI diblock copolymers an apparent entanglement effect sets in at molecular weights which are much lower than those expected from the corresponding homopolymer data. A microscopic picture is proposed to account for the experimental results.     

Synthesis of a polyvinyl-polyamino acid graft copolymer

A copolymer having reactive primary amino groups was synthesized by the copolymerization of styrene and N-methyl-N-(4-vinylphenethyl)ethylenediamine ( 1 ). Polymerization of N-carboxy amino acid anhydrides (NCA's) of L -alanine, γ-benzyl-L -glutamate, or β-benzyl-L -aspartate was initiated by the primary amino groups of this copolymer to give a novel polyvinyl-polyamino acid graft copolymer.