Number Of Chain Ends Research Articles

Analysis of branched polymers by high resolution multidetector size exclusion chromatography: Separation of the effects of branching and molecular weight distribution

AbstractThis article presents the SEC analysis of branched polyisobutylene PIB and polystyrene PS with high molecular weight and broad multimodal molecular weight distribution. Both polymers were synthesized using an inimer technique, which results in long‐chain branched polymers with statistical branching and broad multimodal distributions. Using high resolution multidetector Size Exclusion Chromatography SEC the polymers were analyzed based on three branching factors: g = (Rz,br/Rz,lin)Mw; h = (〈Rh〉z,br/〈Rh〉z,lin)Mw ; and ρ = (R 1/2/〈Rh〉z). It is generally accepted that for monodisperse branched polymers g and h < 1. In the case of our polydisperse PIB and PS, it was seen that g and h > 1, and ρ increases with molar mass and the number of chain ends as predicted earlier. The multidetector SEC system allowed for the separation of branching and polydispersity, reported here for the first time experimentally. The g parameter as a function of DPi was compared to the theory developed by Zimm and Stockmayer. The plots followed a similar trend, but were shifted by a factor related to the average chain length between branching points. The ρ parameter decreased with increasing DPi, as predicted theoretically by Kajiwara. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Biodegradation of Physicochemically Treated Polycarbonate by Fungi

Two fungal strains isolated from soil and a commercial white-rot fungus, Phanerochaete chrysosporium NCIM 1170 (SF2), were tested for biodegradation of untreated, UV-, and thermal-treated bisphenol A polycarbonate (PC). The isolated strains based on 18S rDNA analysis were characterized as Engyodontium album MTP091 (SF1) and Pencillium spp. MTP093 (SF3). About 5.4% weight loss and 40% reduction in M(n) were observed for UV-treated polycarbonate in one year with SF2 strain. An increase in surface energy and oxygen content and a reduction in methyl index indicated oxidation of PC during this period. PC exposed to the SF1 strain showed a 15 degrees C decrease in glass transition temperature, indicating an increase in the number of chain ends and, hence, an increase in the free volume of polymer. No bisphenol A, the monomer of PC, was detected during the study. NMR and FTIR spectra showed the formation of methyl groups due to pretreatments. EDAX analysis exhibited surface oxidation of the PC. The current study advocates that biodegradation of PC can be enhanced by pretreatments.

The influence of polymer topology on pharmacokinetics: Differences between cyclic and linear PEGylated poly(acrylic acid) comb polymers

Water-soluble polymers for the delivery of chemotherapeutic drugs passively target solid tumors as a consequence of reduced renal clearance and the enhanced permeation and retention (EPR) effect. Elimination of the polymers in the kidney occurs due to filtration through biological nanopores with a hydrodynamic diameter comparable to the polymer. Therefore we have investigated chemical features that may broadly be grouped as “molecular architecture” such as: molecular weight, chain flexibility, number of chain ends and branching, to learn how they impact polymer elimination. In this report we describe the synthesis of four pairs of similar molecular weight cyclic and linear polyacrylic acid polymers grafted with polyethylene glycol (23, 32, 65, 114 kDa) with low polydispersities using ATRP and “click” chemistry. The polymers were radiolabeled with 125I and their pharmacokinetics and tissue distribution after intravenous injection were determined in normal and C26 adenocarcinoma tumored BALB/c mice. Cyclic polymers above the renal threshold of 30 kDa had a significantly longer elimination time (between 10 and 33% longer) than did the comparable linear polymer (for the 66 kDa cyclic polymer, t 1/2,β = 35 ± 2 h) and a greater area under the serum concentration versus time curve. This resulted in a greater tumor accumulation of the cyclic polymer than the linear polymer counterpart. Thus water-soluble cyclic comb polymers join a growing list of polymer topologies that show greatly extended circulation times compared to their linear counterparts and provide alternative polymer architecture for use as drug carriers.

Displacement Current of Nanodendrimer

In the Langmuir-Blodgett (LB) technique, a monolayer on the water surface is transferred onto a substrate, which is raised and dipped through the surface. From this, multilayers can be obtained in which constituent molecules are periodically arranged. The LB technique has attracted considerable interest in the fabrication of electrical and electronic devices. Many researchers have investigated the electrical properties of monolayer and multiplayer films. Dendrimers represent a new class of synthetic macromolecules characterized by a regularly branched tree-like structure. Multiple branching yields a large number of chain ends that distinguish dendrimers from conventional star-like polymers and microgels. The azobenzene dendrimer is one of the dendritic macromolecules that include the azo-group exhibiting a photochromic character. Due to the presence of the charge transfer element of the azo-group and its rod-shaped structure, these compounds are expected to have potential interest in electronics and photoelectronics, especially in nonlinear optics. In the present paper, we give pressure stimulation to organic thin films and detect the induced displacement current.

Investigation of the degradation mechanism of cross-linked polyethyleneimine by NMR spectroscopy

The degradation pathway of a cross-linked polyethyleneimine (c-PEI) suitable as a gene carrier was studied at different pH values by real-time 1H NMR and diffusion-weighted 1H NMR spectroscopy. The c-PEI was synthesized by cross-linking PEI segments with 2,4-pentanediol diacrylate (PDDA). The experimental results show that under basic, neutral and acidic conditions, the degradation of c-PEI occurs through hydrolysis of the ester moieties of the PDDA linkers. The degradation half-life of the polymer is 22, 48 and more than 720 h at pH 10.2, 7.4 and 4.6, respectively, showing that the degradation of c-PEI is highly pH sensitive. By using a modified version of diffusion-weighted 1H NMR experiment, the variation of the apparent molecular weight of c-PEI in the degradation process was monitored. Furthermore, a Monte Carlo simulation was applied to simulate the relation between the average molecular weight and the number of chain ends during the degradation process of a model system of c-PEI. By comparing the NMR results with those obtained from simulation, the mechanism of degradation under various pH conditions is discussed. The present work demonstrates that the combination of real-time 1H NMR, diffusion-weighted 1H NMR spectroscopy and Monte Carlo simulation is a useful strategy for characterizing the degradation process of degradable polymers.

Effects of Branch Points and Chain Ends on the Thermodynamic Interaction Parameter in Binary Blends of Regularly Branched and Linear Polymers

The architectural effects of branch points and chain ends on the bulk thermodynamic interaction parameter, χeff, in binary blends of branched polymers with their linear analogues have been investigated using small-angle neutron scattering (SANS) and a series of well-defined, regularly branched polystyrenes with the same molecular weight and differing only in the number of branch points or chain ends. The value of χeff increases as the number of branch points increases for constant number of chain ends or as the number of chain ends increases for constant number of branch points. A Gaussian field theory for an athermal blend predicts the general trend in bulk χeff with the number of chain ends but does not capture the changes in χeff with changes in the number of branch points. Differences in chemistry among the various branch points probably have to be included to quantitatively predict changes in χeff for shorter branches. Qualitative variations in the size of the branched molecules in solution compare favorably with a Gaussian model for the branched chain's radius of gyration. The variation in the average statistical segment length with chain architecture was also determined from the SANS data.

Ring-Opening Metathesis Polymerization of 7-tert-Butoxybicyclo[2.2.1]hepta-2,5-diene Initiated by Well-Defined Molybdenum and Ruthenium Carbene Complexes

The ROMP of 7-tert-butoxynorbonadiene initiated by Mo(=CHCMe 2 Ph)(=NC 6 H 3 -2,6-i-Pr 2 )(OR) 2 [1, OR = OCMe(CF 3 ) 2 ; 2, OR = OCMe 3 ] and Ru(=CHPh)Cl 2 (PCy 3 ) 2 (3) has been studied at ambient temperature by NMR in CDCl 3 , CD 2 Cl 2 and C 6 D 6 . The reaction with 1 as initiator is extremely fast, but with 2 and 3 reaction proceeds at measurable speed, being fastest in CDCl 3 and slowest in C 6 D 6 . The polymer formed in CD 2 Cl 2 using 1 contains 47% syn units and mainly cis double bonds; that formed using 2 contains 34% syn units and about 17% cis double bonds; and that formed using 3 contains less than 7% syn units and 32% cis double bonds. The carbene proton of the propagating carbenes derived from 1 and 2 give two groups of 1 H NMR doublets, attributed to species having syn and anti units adjacent to the Mo center, but that derived from 3 gives only a single group of signals in which the fine structure is independent of spectrometer frequency and therefore not due to spin coupling. With 3 as initiator, after polymerization is complete, the intiator is partiallu regenerated at the expense of the propagating species indicating the occurence of secondary metathesis reactions at the chains ends, leading to a decrease in the number of chain ends in the polymer. Under conditions where the chain length is low, the shorter chains may be detected by GPC and NMR. Such reactions occur intermolecurlarly resulting in an increase in the average molecular weight; they may also occur by ring-closing metathesis, though this has not been proved. k p /k i values were derived and in three cases individual values of k p and k i were determined. With 3 as initiator in CD 2 Cl 2 the presence of a five-fold excess of PCy 3 caused a marked diminution in the rate of ROMP showing that the reaction proceeds mainly via addition of monomer to monophosphine complexes, Ru(=CHR)Cl 2 (PCy 3 ).

Molecular-weight distribution of cellulose after ripening

As a result of studying ripening of alkali cellulose by the Monte Carlo method, it was found that the following model most closely fits the experimental data on the MWD of alkali cellulose after ripening: alkali cellulose is ripened by the parallel occurrence of chance oxidative degradation (in amorphous regions) and depolymerization from the chain end with significant predominance of the number of depolymerization events; the shape of the MWD curve after ripening of alkali cellulose is determined by the parallel occurrence of depolymerization from the chain end and chance degradation; chance degradation in amorphous regions of the cellulose accelerates depolymerization from the chain end due to an increase in the number of chain ends.

Evaluation of Chain Scission during Mixing of Filled Compounds

Abstract Whether chain scission takes place during mixing of black-filled compounds has been a debatable subject which is yet unsettled. The uncertainty originates from the material system which contains gel. A direct method to evaluate scission is to quantify the change in the number of chain ends, since two chain ends will be newly created on each event of scission. It requires determination of an average molecular weight of the linear components which constitute the gel-containing system. The degree of polymerization denned in the Charlesby-Pinner theory, derived for crosslinking study by means of sol-gel analysis, could be conveniently used for the purpose. The theory, however, cannot be directly applied for black-filled compounds because the composite structure in the compounds does not allow one to satisfy the assumptions made in the theory, i.e., an equal chance of crosslinking for every reactive site. A new technique is developed so that the assumption is satisfied and the sol-gel analysis can be carried out even with black-filled compounds. This technique is applied for numbers of compounds, with the same formulation but mixed under various conditions. A NR and an SBR formulations are used here. The results clearly indicate that chain scission takes place during mixing of both NR and SBR compounds.

Novel aspects of the microstructure of poly(ethylene oxide) as revealed by microhardness: Influence of chain ends

AbstractThe microhardness (H) of a series of melt crystallized samples of poly(ethylene oxide) (PEO) was investigated as a function of molecular weight and crystallization temperature (Tc). It is shown that for a given molecular weight, H and the crystal hardness (Hc) increase with Tc, following a thermodynamic approach which takes into account the dependence of Hc upon crystal thickness (ℓc). A combined analysis of H‐data and DSC results reveals that, both the surface free energy (σe) and the hardness‐derived parameter b of the samples of unaffected by Tc. The microhardness of the PEO samples was markedly affected by molecular weight. Furthermore, it is shown that the molecular weight variations produce changes in the surface free energy due to variations in the number of chain ends on the crystal surface, giving rise to a parallel variation in the b parameter. As a consequence, the melting temperature and the crystal hardness exhibit a similar dependence on crystal thickness.

Liquid crystalline polymers: self-organization and assembly

Liquid crystal forming monomers, typically rods, can be polymerized to form long mesogenic molecules. In contrast to simple rods, these polymers often have internal degrees of freedom so that they display the subtle behaviour of both high polymers and simple liquid crystals. They can have the rod elements either concatenated as a back-bone to give main chain (MC), or pendant to a back-bone to give side chain (SC) liquid crystals, or both. The physics unique to liquid crystalline polymers (LCPS) comes from their shape being dependent on the state of nematic order. Simple systems remain molecular rods (or disks) on ordering whereas a chain extends or flattens (depending on whether or not the nematic order is prolate or oblate). New phenomena as a result of this occur in situations as disparate as networks and, it is predicted, in dielectric response. We examine both SC and MC LCPS and the mechanisms by which they order lyotropically (in solution) and therm otropically (in the melt). Various types of models will be discussed in general and then restricted to the therm otropic case, lyotropic systems being discussed in Lekkerkerker & Vroege (this volume). The transition to the ordered state is first order as in simple nematics. The main characteristics of this state are modified chain conformations and, additionally for side chain polymers, transitions between various novel competing nematic states. A form of self-assembly that is a delicate function of the nematic order is observed in transesterifying LCPS. The number of chain ends is conserved but material exchanged between chains according to whether they are in the isotropic or nematic state. We review a model of this type of self-assembly.

Craze and fatigue resistance of glassy polymers

Abstract The fracture characteristics of several glassy polymers subject to monotonic tensile loading and to alternating tension-compression loading are described and compared. In both cases, a reflected light method is used to detect craze initiation at an early stage. Various methods have been investigated to enhance the craze and fatigue resistance of PS. It is shown that increases in mean fatigue life of 2 to 3 decades are possible by increase of molecular weight and reduction in number of chain ends but that no increases occur if molecular weight is raised by cross-linking and radiation. The use of surface treatments and surface coatings as additional methods for enhancing fatigue performance is explored. It is found that chemical treatments can be even more effective, in some cases, than mechanical polishing and that surface coatings of flexible compatible oligomers, as well as surface layers of viscous hydrogen-bonding liquids, can enhance fatigue lifetimes by inhibiting or delaying crazing. Severa...

Physical and chemical consequences of keratin crosslinking, with application to the determination of crosslink density.

The high levels of covalent disulfide crosslinking in keratins strongly affect (1) structural stability, (2) viscoelasticity, and (3) chemical reactivity. This paper briefly reviews recent work on these subjects, with critical emphasis on methods by which chemical and physical properties can be related to inter-and intra-molecular crosslink density in heterogeneous systems like keratins. Detailed attention is drawn to effects of crosslinking on the hydrolysis of keratin by acids or enzymes. Within the limits of reasonable assumptions, it is possible to account quantitatively for crosslink dependent variations in the hydrolysis rate of different keratins, and also to derive a formula for calculating the absolute intermolecular crosslink density from the amount of keratin dissolved after partial hydrolysis and the number of chain ends appearing in the soluble fraction.

Vulcanizate Structure, Relaxation, and Tensile Strength of Polyisoprenes

Abstract The tensile strength of lightly-filled sulfur vulcanizates of 1,4-cis polyisoprene is lower the more the specimen has undergone deformation after removal of a definite stress. With equal tensile strength, this deformation multiplied by the modulus is proportional to the square of the filler concentration. Along with this deformation the volume of the elongated specimen increases through the formation of internal stress cracks. Rupture originates even at lower stresses in the neighborhood of the filler particles. It is affected by the size of the filler particles as well as by the chemical nature of the filler and by the structure of the network. Encouraged by these results we checked the network structure. We examined the stresses at high elongations, the photoelaetic homogeneity and the turbidities and viscosities of solutions of the degraded vulcanizates, and found small local variations in crosslink density and differences in the number of elastically-ineffective chain ends. The number of these chain ends and the local variations of crosslink density may increase greatly during vulcanization, and substantially reduce the tensile strength.