Effects Of Block Lengths Research Articles

Formation of Asymmetric and Symmetric Hybrid Membranes of Lipids and Triblock Copolymers.

Hybrid membranes formed by co-assembly of AxByAx (hydrophilic-hydrophobic-hydrophilic) triblock copolymers into lipid bilayers are investigated by dissipative particle dynamics. Homogeneous hybrid membranes are developed as lipids and polymers are fully compatible. The polymer conformations can be simply classified into bridge- and loop-structures in the membranes. It is interesting to find that the long-time fraction of loop-conformation () of copolymers in the membrane depends significantly on the hydrophilic block length (x). As x is small, an equilibrium always results irrespective of the initial conformation distribution and its value depends on the hydrophobic block length (y). For large x, tends to be time-invariant because polymers are kinetically trapped in their initial structures. Our findings reveal that only symmetric hybrid membranes are formed for small x, while membranes with stable asymmetric leaflets can be constructed with large x. The effects of block lengths on the polymer conformations, such as transverse and lateral spans ( and ) of bridge- and loop-conformations, are discussed as well.

Bioresorbable hydrogels prepared by photo-initiated crosslinking of diacrylated PTMC-PEG-PTMC triblock copolymers as potential carrier of antitumor drugs

PTMC-PEG-PTMC triblock copolymers were prepared by ring-opening polymerization of trimethylene carbonate (TMC) in the presence of dihydroxylated poly(ethylene glycol) (PEG) with Mn of 6000 and 10,000 as macro-initiator. The copolymers with different PTMC block Lengths and the two PEGs were end functionalized with acryloyl chloride. The resulting diacrylated PEG-PTMC-DA and PEG-DA were characterized by using NMR, GPC and DSC. The degree of substitution of end groups varied from 50.0 to 65.1%. Hydrogels were prepared by photo-crosslinking PEG-PTMC-DA and PEG-DA in aqueous solution using a water soluble photo-initiator under visible light irradiation. The effects of PTMC and PEG block lengths and degree of substitution on the swelling and weight loss of hydrogels were determined. Higher degree of substitution leads to higher crosslinking density, and thus to lower degree of swelling and weight loss. Similarly, higher PTMC block length also leads to lower degree of swelling and weight loss. Freeze dried hydrogels exhibit a highly porous structure with pore sizes from 20 to 100 µm.The biocompatibility of hydrogels was evaluated by MTT assay, hemolysis test, and dynamic clotting time measurements. Results show that the various hydrogels present outstanding cyto- and hemo-compatibility. Doxorubicin was taken as a model drug to evaluate the potential of PEG-PTMC-DA and PEG-DA hydrogels as drug carrier. An initial burst release was observed in all cases, followed by slower release up to more than 90%. The release rate is strongly dependent on the degree of swelling. The higher the degree of swelling, the faster the release rate. Finally, the effect of drug loaded hydrogels on SKBR-3 tumor cells was evaluated in comparison with free drug. Similar cyto-toxicity was obtained for drug loaded hydrogels and free drug at comparable drug concentrations. Therefore, injectable PEG-PTMC-DA hydrogels with outstanding biocompatibility and drug release properties could be most promising as bioresorbable carrier of hydrophilic drugs.

Physical layer security over fading wiretap channels through classic coded transmissions with finite block length and discrete modulation

The chance to use existing coded transmission schemes for achieving some security at the physical layer besides reliability is of interest for many applications. In this paper, we assess the levels of physical layer security achievable by classic coding schemes over fading wiretap channels, taking into account the effects of finite block lengths and discrete modulations. In order to take these practical constraints into account, some previous works use the error rates experienced by legitimate receivers and eavesdroppers as reliability and security metrics, respectively. However, having a high error rate at the eavesdropper is a necessary but not a sufficient condition for security, thus we resort to more robust information theoretic security metrics for such a purpose. By focusing on mutual information security, we estimate the average number of attempts required by an attacker to recover the whole message in practical conditions and under outage constraints. Based on this metric, higher layer cryptographic protocols can be designed to achieve robust security built upon the physical layer. We obtain lower bounds on the wiretapper equivocation about the secret message, subject to some outage probability, and assess their tightness. We provide some examples considering classic coding and modulation techniques like extended Bose–Chaudhuri–Hocquenghem codes and convolutional codes with binary signaling.

Effects of block lengths and stacking policies on overall performance at non-transshipment container terminals

The ever-increasing worldwide container throughput and the intensified port competition have demanded better terminal operational performance, which is usually measured by the gross crane rate (GCR). In this paper, a multiagent-based simulation model is proposed to investigate how the block length of the storage yard and the tactical-level stacking policy affect the operational performance of non-transshipment container terminals over the long term. Experiments consider seven block lengths, two yard truck fleet sizes and two stacking policies. The results demonstrate that the best block length yielding the highest GCR is dependent on the stacking policy and the yard truck fleet size, and the separate stacking policy is essentially superior to the scattered stacking policy. Specifically, only when 9-yard trucks are deployed per quay crane under the separate stacking policy, can the typical 42-slot block length achieve the highest GCR. Although the experiments are not exhaustive, they do provide the first fundamental insights, with respect to the effects of block lengths and stacking policies, on the long-term performance at a manually controlled non-transshipment container terminal.

Increasing Stability of Biotin Functionalized Electrospun Fibers for Biosensor Applications.

This paper describes the effects of both solvent and copolymer block lengths on the stability of electrospun poly(lactic acid)/poly(lactic acid)-b-poly(ethylene glycol) (PLA/PLA-b-PEG) and PLA/PLA-b-PEG-Biotin fibers in water. By tailoring the block length of copolymers PLA-b-PEG, water stability of electrospun fibers is improved over fibers reported previously. The solvent used also influenced the stability and hydrophilicity of resulting fibers. Fibers formed using 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) have greater water stability, but less PEG at the surface of fibers than fibers spun from dimethylformamide (DMF). Attaching biotin to the end of PLA(3600)-b-PEG(2000) and spinning from DMF resulted, initially, in 7.6% of the total biotin incorporated into the fiber, assuming every PEG terminal had one biotin attached (1.1 mg of biotin per gram of fiber) available at the fibers' surface. In addition, PLA/PLA(3600)-b-PEG(2000)-Biotin spun from DMF hindered biotin migration to the aqueous phase, leaving 2% of the incorporated biotin remaining at the surface of fibers after 7 days of water exposure. The water wicking ability of DMF spun fibers also increased significantly with the biotin attachment to the PEG terminal. While HFIP spun fibers lost little biotin from fibers, there was no detectable surface available biotin, indicating biotin was at the interior. With biotin and PEG at the interior of the fibers spun from HFIP, the water wicking remained the same for PLA/PLA(3600)-b-PEG(2000) spun samples and decreased for PLA/PLA(5700)-b-PEG(1000). The dissimilarities observed in water wicking for HFIP spun samples are primarily the result of differences in fiber morphology.

Hydrolytic degradation behavior of stereo multiblock and diblock poly(lactic acid)s: Effects of block lengths

Stereo multiblock and diblock poly(lactic acid)s (PLAs), poly(DL-lactide) (PDLLA), and poly(DL-lactic acid), with block lengths comprising 3.8–61.9 lactyl units were synthesized. The effects of the block length sequence on the hydrolytic degradation behavior were investigated at 37 °C and pH 7.4 using amorphous (quenched) samples of these polymers, in comparison with quenched neat poly(l-lactide) (PLLA), poly(d-lactide) (PDLA), and PLLA/PDLA blend. 13C NMR spectra revealed that chain cleavage during the hydrolytic degradation occurred rapidly on the atactic segments connecting relatively long isotactic l-lactyl and d-lactyl segments. The rates of weight loss and hydrolytic degradation of stereoblock PLAs, PDLLA, and poly(DL-lactic acid) decreased with an increase in the average stereoblock length. The hydrolytic degradation of the stereo multiblock PLAs with average block lengths higher than 27 and 15 lactyl units was suppressed as compared to that of the PLLA/PDLA blend and the neat PLLA or PDLA, respectively. The stereocomplex crystallization occurred during the hydrolytic degradation of stereoblock PLAs and PDLLA with average stereoblock lengths higher than 7 lactyl units, in agreement with the results reported for the stereocomplex crystallization from the melt. The crystallization rate of stereoblock PLAs during the hydrolytic degradation increased on increasing the block length; however the rate of the hydrolytic degradation of the diblock PLA with the highest average stereoblock length was lower than that of the PLLA/PDLA blend.

Synthesis and Micellization of Triblock Copolymers Containing MePEG-b-PDMAEMA and Fluoropolymer: Effect of Block Lengths on Self-Assembly

A series of triblock copolymer consisting of a double-hydrophilic diblock and a fluorinated block, namely monomethyl end-capped poly(ethylene glycol)-b-poly[2-(dimethylamino)ethyl methacrylate]-b-poly(2, 2, 3, 3, 4, 4, 5, 5-octafluoropentyl methacrylate) (MePEG-b-PDMAEMA-b-POFPMA), was synthesized via oxyanion-initiated copolymerization. The hydroxyl group of poly(ethylene glycol) monomethyl ether (MePEG16, M n = 750 g mol−1 or MePEG45, M n = 2000 g mol−1) reacted with potassium methylsulfinyl carbanion (DMSO−K+) and converted into a potassium alcoholate macroinitiator (MePEGO−K+). The lengths of PDMAEMA and POFPMA blocks could be controlled by the original molar ratio of monomers to the macroinitiator. These triblock copolymers were characterized by the measurements of 1H-NMR, and gel permeation chromatography (GPC). The results indicated that they had relatively low polydispersities (M w/M n:1.15–1.36). A significant investigation in this work was the comparison of the effects of the different block lengths, especially for MePEG and PDMAEMA, on the mecellization of copolymers in aqueous solution by the measurements of surface activities and electrical conductivity. The lower critical solution temperatures (LCSTs) of the di- and triblock copolymers varied with different factors, including the chemical structure, the block lengths, and the concentration. The effects of PDMAEMA length on the two kinds of diblock copolymers MePEG-b-PDMAEMA, which were initiated respectively by MePEG-2000 or MePEG-750 oxyanions, were studied by turbidimetric measurement. Furthermore, the different morphologies of MePEG-b-PDMAEMA-b-POFPMA in aqueous solution at pH 7.0 were observed by transmission electron microscopy (TEM).

Synthesis and characterization of methoxy poly(ethylene glycol)-b-poly(DL-lactide-co-glycolide-co-ε-caprolactone) di block copolymers: Effects of block lengths and compositions

AbstractMethoxy poly(ethylene glycol)-b-poly(D,L-lactide-co-glycolide-co-ε- caprolactone) diblock copolymers, designated as MPEG-b-PDLLGCL, with different MPEG block lengths and DLL:G:CL ratios were synthesized via ring-opening polymerization in bulk using MPEG and stannous octoate as the initiating system. The diblock copolymers were characterized using a combination of FT-IR, 1HNMR, GPC and DSC. The percentage yields of the diblock copolymers were in excess of 90%. 1H-NMR analysis confirmed that the PDLLGCL blocks contained at least some random character. From their DSC curves, the diblock copolymers were amorphous in morphology. Their glass transition temperatures decreased as the MPEG block length increased and as G and CL units were incorporated.

Synthesis and Characterization of Thermo- and pH-Responsive Double-Hydrophilic Diblock Copolypeptides

Synthesis of novel double-hydrophilic diblock copolypeptides (BCPs), poly(l-glutamic acid)-block-poly(N-isopropylacrylamide) (PLGnPNm), and their thermoresponsive properties in aqueous solutions at different pH values are described. The diblock copolypeptides were synthesized by a combination of ring-opening polymerization (ROP) of gamma-benzyl-l-glutamate N-carboxyanhydrides (BLG-NCA) and reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide (NiPAM). A new class of RAFT agents (CTA-2 and CTA-3) with amino-functional groups was designed for this purpose. Two different strategies, i.e., macrochain transfer agent (CTA) and macroinitiator routes, were utilized and compared on the control of the chemical structures of the resulting BCPs. Their block ratios and lengths are broadly varied (n = 21-600 and m =180-442). Their thermally switchable aggregation behaviors in aqueous solutions were investigated at the microscopic level by 1H NMR spectroscopy and at the macroscopic level by turbidity measurements using UV/vis spectroscopy. The latter was also utilized for their lower critical aggregation temperature (LCAT) determination. The effects of block lengths and ratios as well as solution pH values on the collapse of NiPAM chain and aggregation process of BCPs were examined. This aggregation process was also followed by dynamic light scattering (DLS) measurements, and the thermally induced aggregate structures were investigated by transmission electron microscopy (TEM).

Novel Two‐Compartment Micelles Formed by Self‐Assembly of Linear Pentablock Copolymers in Selective Solvents

AbstractIn this work, the formation of two‐compartment micelles from symmetric pentablock copolymers in selective solvents was studied using the dissipative particle dynamics simulation technique, and the effects of block lengths and solvent quality were investigated. The simulations revealed several new morphologies and their formation mechanisms were elucidated at the molecular level, providing useful information that may contribute to the future rational design and synthesis of novel multicompartment micelles with tailored structures.magnified image

Apolipoprotein E genotypes do not influence the age of onset in Huntington’s disease

Objective: The ε4 allele of the apolipoprotein E (ApoE) gene has been defined as a critical factor for early onset neurodegeneration in Pick’s, Parkinson’s, and Alzheimer’s disease. Unexpectedly, the ε4...

Effects of block lengths on the validity of block resampling methods

Resampling methods for dependent variables typically involve selecting blocks of observations. In this paper, we investigate the effects of different block lengths on the consistency of approximations generated by block bootstrap and subsampling methods. A complete description is provided for the case of sample mean. It is shown that both methods are consistent if the block length grows at a rate slower than the sample size. When the growth rate of block lengths is comparable to the sample size, the resulting approximations are no longer consistent. In the latter case, we present a detailed account of the limit behavior of these resampling distribution approximations, considered as random elements in the space of all probability measures on the real line.

Effects of Block Lengths and Initial Water Content on the Swelling and Degradative Characteristics of an Injectable Poly(propylene Fumarate-Co-Ethylene Glycol) Block Copolymer Hydrogel

AbstractThe overall goal of this project is to develop an in-situ polymerizable, biodegradable material for use in cardiovascular applications that minimizes non-specific cell adhesion and contains functional moieties for the attachment of peptides to induce specific cell attachment. A novel block copolymer has been developed incorporating poly(propylene fumarate) (PPF) and poly(ethylene glycol) which satisfies these criteria. PPF is a new biodegradable polymer currently being investigated for orthopedic and cardiovascular applications while PEG is a hydrophilic polymer that has been extensively studied for biomedical applications. The copolymer is chemically crosslinked with PEG diacrylate using an ammonium persulfate-ascorbic acid redox initiator system to form the hydrogel. The PEG and PPF block lengths can be varied to modulate the properties of the hydrogel formed. In this study, the following three parameters were studied, (1) PPF block length, (2) PEG block length, and (3) initial water content, were varied to examine their effects on swelling, degradation and elastic modulus. A factorial experimental design was implemented to assess which of these three parameters had the greatest impact on swelling, degradation and elastic modulus. Swelling was found to be most affected by the initial water content followed by PEG block length and PPF block length. The swelling of the hydrogels ranged from 48% water uptake with low initial water content to up to 77% water uptake with the high initial water content. After three weeks, degradation of the hydrogels ranged from 4-13% mass fraction lost. Elastic modulus was determined by tensile testing of the various hydrogel formulations and ranged from 0.4 to 7.7 MPa.

Modelling of crystallization in stereoblock polypropylene. Idealized structures showing the effects of isotactic block lengths and their polydispersity

Characterizing partially crystalline isotactic polypropylene can be carried out by extractions which are based on decreases in solubility with increased crystallinity, which in turn increases with increased isotactic block length. The present investigation employs Monte Carlo techniques to model how crystallization in polypropylene might depend on increase in isotactic block length (with the overall isotacticity either increasing or being held constant), and on its distribution (polydispersity). While isotactic block length was found to have a strong effect on the characteristics of crystallization, it should have less effect on elastomeric moduli calculated from the number density of crystallites (when functioning as volume-less cross-links). In contrast, block-length polydispersity had little or no effect on the crystallization, but a rather unusual effect on mechanical properties. These dependences may be due to the assumption that the elastomeric moduli depended only on the number density of the crystallites, without account for size effects such as large crystallites also functioning as filler particles.

Effects of Block Lengths on the Association Numbers and Micellar Sizes of BnEmBn Type Triblock Copolymer Micelles in Aqueous Solution

Effects of Block Lengths on the Association Numbers and Micellar Sizes of B<i>_n</i>E<i>_m</i>B<i>_n</i> Type Triblock Copolymer Micelles in Aqueous Solution

Aqueous size-exclusion chromatography of micelles of poly(oxyethylene) methyl- n-alkyl ethers

Aqueous size-exclusion chromatography was used to determine hydrodynamic volumes of micelles formed by four series of poly(oxyethyelene) methyl- n-alkyl ethers, CH 3(OCH 2CH 2_ m O(CH 2) n H, with m having number-average values in the range 13–53 and n having values in the range 12–21. The surfactants in solution were injected at concentrations far above their critical micelle concentrations in order to avoid complexities due to the micelle-free-chain equilibrium. The hydrodynamic volumes of the micelles were found to increase with increase in n at fixed m and also with increase in m at fixed n. The effects of block lengths m and n on the aggregation numbers of the micelles was roughly estimated by combining their hydrodynamic volumes with measures of their intrinsic viscosities.

Small-angle x-ray scattering study of micelle formation in mixtures of butadiene homopolymer and styrene-butadiene block copolymer. 2. Effects of block lengths

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSmall-angle x-ray scattering study of micelle formation in mixtures of butadiene homopolymer and styrene-butadiene block copolymer. 2. Effects of block lengthsDavid Rigby and Ryong Joon RoeCite this: Macromolecules 1986, 19, 3, 721–728Publication Date (Print):March 1, 1986Publication History Published online1 May 2002Published inissue 1 March 1986https://doi.org/10.1021/ma00157a040RIGHTS & PERMISSIONSArticle Views283Altmetric-Citations41LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts