Effect Of End Groups Research Articles

Thermotropic and barotropic phase transition on bilayer membranes of phospholipids with varying acyl chain-lengths

The bilayer phase-transitions of a series of 1,2-diacylphosphatidylcholines containing linear saturated acyl chain of even- and odd-number carbons (C=12, 13, 14, 15, 16, 17 and 18) were observed by two kinds of optical methods. One is the observation of isothermal barotropic phase transition and the other is the isobaric thermotropic phase transition. The temperature of the main transition from the ripple gel phase to the liquid crystal phase for each lipid was elevated linearly by pressure in the range of 150 MPa. The slope of the temperature–pressure diagram, d T/d p, was in the range 0.20–0.23 K MPa −1 depending on the acyl chain-length. The chain length dependence of the main transition temperature under ambient pressure described a smooth curve with no evidence of odd/even discontinuities. The phase transition enthalpy, Δ H, which was determined by the differential scanning calorimetry (DSC), increased with an increase in the acyl chain-length. The Δ H vs. chain-length curve was non-linear and convex upward. The volume change, Δ V, associated with the transition was calculated from the values of Δ H and d T/d p by means of the Clapeyron–Clausius equation. The values of Δ V increased with an increase in the acyl chain-length, which were best described by a smooth curve and not a linear function. The increment of the transition volume tends to be moderated as the length of the hydrocarbon chain is increased and amounts to 1.4 cm 3 mol −1 per one methylene group. Non linear properties of thermodynamic quantities with respect to the acyl chain-length seem to be attributable to the end-group effects of the fatty acyl chains.

Lattice theory of polymer solutions with endgroup effects

We present a unified lattice theory for a binary solution where endgroups are treated differently from middle groups. This is a simple example of a triblock and the present study provides a starting point for studying a general triblock system. We replace the original homogeneous lattice by a Bethe lattice of the same coordination number as the original lattice. The model is solved exactly on the Bethe lattice. The resulting solution goes beyond the random mixing approximation and provides us with an approximate theory of the model on the regular lattice. The contributions of endgroups on various thermodynamic properties of a binary solution are investigated in a quantitative way using the theory. In particular, our theory predicts that contributions to the energy are more important than to the entropy.

Synthesis and characterization of α, ω- and α-functionalized hydrogenated polybutadienes: telechelic and semi-telechelic amine and phosphite terminated polymers

Polymers with terminal amine or phosphite groups (X) were synthesized from tosylated derivatives of hydrogenated α-hydroxy- and α,ω-dihydroxy-poly-1,3-butadienes, produced by anionic techniques. Products are semi-telechelic and telechelic polymers with X = NH 2 ( 5), NHPh ( 6), NHPr″ ( 7), NH(CH 2) 3NMe 2 ( 8), O(CH 2) 2NH 2 ( 9), O(CH 2) 2NH(CH 2) 2NH 2 ( 10), NMe 2 ( 13), OPO 2C 6H 4 ( 14) of varying ethyl side-group content, controlled molar mass ( M ̄ n = 600–3000 ) and low polydispersity. Characterization of polymers by n.m.r. and i.r. spectroscopy is reported, with comparative studies on short chain model systems. Thermal properties and viscosities have been investigated in relation to molar mass, backbone structure and terminal functionality; comparisons are made with related saturated and unsaturated systems, and with polymers containing terminal ionic functionalities, formed by alkylation, protonation, or coordination with Cu(II), Ni(II) and Fe(III) chlorides. Effects of end-groups on variations in T g, higher thermal transitions and on melt viscosities are discussed. Oxidation of phosphite end-groups is also observed.

Effect of molecular weight and chain end groups on crystal forms of poly(vinylidene fluoride) oligomers

The micro-structure of poly(vinylidene fluoride) oligomers was characterized by 19F and 13C n.m.r. and the effects of molecular weight and the chain end groups on the crystal forms were studied by i.r. and X-ray methods. One of the oligomers terminated with -CH 3 and the fragment of an initiator at the chain ends. The other oligomers were terminated with -Cl and CCl 3- at their chain ends. The larger chain end groups tended to produce the conformational disorder in the molecules yielding the γ form. The smaller molecular weight of the oligomer gave rise to the β form.

Characterization of Poly(dimethylsiloxane)s by Time-of-Flight Secondary Ion Mass Spectrometry

A series of poly(dimethylsiloxane)s (PDMS) terminated by trimethylsilyl groups, and having molecular weights from 600 to 20 000, were characterized by time-of-flight secondary ion mass spectrometry. The fragmentation patterns of the PDMS were examined. The predominant fragmentation pathway involves formation of cyclic fragments, probably via an intermediate with a four-membered ring. The relative intensities of the clusters in the PDMS fragmentation patterns were investigated as a function of sample molecular weight. The relative intensities of cyclic fragments increase with increasing polymer molecular weight. Poly(dimethylsiloxane)s containing different end groups were also investigated to determine the effect of the terminal groups on fragmentation. The terminal groups in PDMS were identified by analysis of low mass range spectra. Effects of nitrogen-containing end groups were found on the fragmentation of PDMS. Large fragments with intrinsic positive charge were observed in the spectra of PDMS terminated by (aminopropyl)dimethylsiloxy and (amidopropyl)dimethylsiloxy groups.

The influence of basic filler materials on the degradation of amorphous D- and L-lactide copolymer

Composite films made of poly L/D copolymer and hydroxylapatite (HA) as filler material were made in two different volume percentages. As references pure copolymer films and composite films with magnesium oxide (MgO) were prepared to serve as strongly basic model filler material. All these materials were immersed in phosphate buffered saline to study the effect of filler materials on the degradation rate of the copolymer. Filled films showed less molecular weight decrease than the unfilled films, while lactate release was higher for the filled films than for the unfilled films. This effect implies that the filler materials influence the degradation mechanism by preventing the occurrence of the autocatalytic effect of the acidic endgroups, resulting from hydrolysis of the polymer chains. The polymer chains at the surface of the material are less protected by the filler materials, which causes a more rapid degradation of these chains and a higher lactate release. At the same time composites seem to show an erosion type of degradation rather than bulk degradation in unfilled materials.

Influence of structure of polymeric fuels on the combustion behaviour of composite solid propellants

This is a first report in which the influence of the structure of polymeric binders, such as molecular weight ( M), end-group functionality and cis-trans proportion, on the combustion behaviour of composite solid propellants (CSP) has been examined. In admixture with ammonium perchlorate (AP), dicarboxylic acids were used to simulate the commonly used telechelic polymers such as carboxy-terminated polybutadiene (CTPB) fuels. The burning rate ( r ̇ ) varies exponentially with the M of the acids and embraces the values for CTPB. The end-group effect diminishes at higher M due to dilution of the acid groups. The dependence of r ̇ on M is expressed as a newly defined molecular weight sensitivity of burning rate ( σ m). The σ m values can be divided into three regimes: I (118 < M < 150), II (150 < M < 200) and III ( M > 200) for polymeric fuels. The σ m values in regimes I and II are explained by the selective tendency for anhydride formation; in regime III, fragmentation of the polymeric fuel occurs. In regimes I and II, volatilization of the acids occurs at lower temperatures than for their neat form and in regime II this volatilization is limited by the phase transition of AP. The volatilization rate decreases with increase in M, but the ratio of the condensed-phase to gas-phase heat release increases with M.

Characterization of Polystyrenes (1 kDa) Functionalized With and Without a Perfluorinated Endgroup by TOF-SIMS: Observation of Intact Oligomers and Endgroup Effects

TOF-SIMS measurements of 1 kDa polystyrene functionalized with a perfluoroalkyl endgroup are presented. Intact oligomer signals in the high mass range and fragment signals in the low mass range are observed (monolayer sample preparation on a silver substrate). The successful functionalization of the polystyrene with the perfluoroalkyl endgroup can be confirmed by the masses at which the intact oligomer signals arise. The high mass region of the spectrum is compared to TOF-SIMS measurements of polystyrene with a proton endgroup. The signal for a polystyrene oligomer with a proton endgroup with a specific degree of polymerization (n) arises at a lower mass because of the absence of the perfluoroalkyl endgroup. The effect of endgroup on the ion transformation probability of intact oligomers is demonstrated in a TOF-SIMS spectrum of a physical mixture (1:1 mole ratio) of polystyrene with a proton endgroup and polystyrene with a perfluoroalkyl endgroup.

Decomposition of Poly(perfluorinated ethers): Effect of End Groups on Main-Chain Degradation and Product Formation

The decomposition of polyperfluorinated ether copolymers containing perfluoroethylene oxide, −CF2CF2O−, and perfluoromethylene oxide units, −CF2O−, capped by different end groups, −OCF3, −CH2OH, and −O(OC)CH3, are investigated. Thus, the polymers are scissioned via an electron beam, and the subsequent chemistry is investigated. In all cases, the polymers lose mass, R−CFO on the ends of scissioned chains are produced, and volatile gases such as COF2 are evolved. The G values for polymer decomposition are 0.2−0.8; for gas evolution are 6−7; and for R−CFO end groups are ≃1.5. The end group −CH2OH has a significant effect on the chemistry after irradiation. The oligomeric R−CFO groups produced upon irradiation react with the −CH2OH end groups to produce ester and HF via alcoholysis. The aldehyde −CHO is also produced as a result of direct electron beam irradiation.

Effect of Temperature on NMR Self-Diffusion in Aqueous Associative Polymer Solutions

The temperature dependence of polymer self-diffusion rates in aqueous solutions of C12EO200C12 (AP9, Mw = 9300) and C12EO90C12 (AP4, Mw = 4600) and nonmodified analogues PEO10 and PEO4 (Mw = 10 000 and 3400) has been studied. The effect of the hydrophobic end-groups on the self-diffusion of the associative polymer (AP) was found to be proportional to the polymer content and inversely related to the temperature. The variation of the AP self-diffusion coefficient follows an Arrhenius behavior. The resulting apparent activation energies, Ea, increase with polymer content from 15 to 55 kJ/mol in the range 0.5−50 wt % for AP9, whereas the parent PEO10 shows an almost constant Ea of about 25 kJ/mol in the same concentration range. Activation energies derived from self-diffusion and low shear viscosity measurements were found to be quite similar. The distribution of self-diffusion coefficients often observed in AP systems is discussed in terms of distribution of aggregate sizes at low AP content and homogeneity ...

End Group Effects on the Phase Behavior of Polymer Blends: Poly(dimethylsiloxane) and Poly(methylphenylsiloxane) Blend

A computational method, using the Flory−Huggins lattice model, the extended random copolymer theory, and a molecular modeling method to account for the end group effect on the χ interaction parameter and on the value of the temperature where the phase separation of a polymer blend occurs, is proposed. The overall χ interaction parameters are calculated based on the individual segmental interaction parameters of the end groups and the monomeric units in the polymer. The individual segmental interaction parameters are calculated from a combination of the Flory−Huggins lattice model and molecular simulation techniques. The χ interaction parameters of poly(dimethylsiloxane) (PDMS) and poly(methylphenylsiloxane) (PMPS) blends are calculated, and the results are compared with those from the experiment.1,2 The end groups on PDMS and PMPS are α,ω-trimethylsilyl and α,ω-dimethylsilanol. The simulation results indicate that the end groups and their fractions in polymers at low molecular weights can significantly affect the χ interaction parameters and the phase separation temperatures. For example, in the case of blended polymers having 14 repeat units, the critical solution temperature of α,ω-trimethylsilyl-terminated PDMS and α,ω-dimethylsilanol-terminated PMPS is found to be 192 °C lower than that of α,ω-dimethylsilanol-terminated PDMS and α,ω-trimethylsilyl-terminated PMPS.

The effects of diluent molecular weight on the structure of thermally-induced phase separation membrane

The effects of molecular weight of a homologous series of diluent on the droplet size of the thermally-induced phase separation membranes were investigated for the mixtures of poly(ethylene glycol) used as a diluent and nylon 12 as a matrix. All the phase diagrams obtained for different molecular weights of the diluent ranging from 200 to 600 showed the upper critical solution temperature type phase boundaries. When molecular weight of PEG was higher than 1000, nylon 12 did not form miscible blends with PEG below the thermal degradation temperature. Interaction energy denssty for each pair was evaluated from the phase separation temperature by the Flory-Huggins theory. The chain end group effects on the interaction energy were also considered using a binary interaction model and a Massa plot. The equilibrium domain size increased as the molecular weight of the diluent increased and then approached an asymptotic value. The increase of the average domain size with the increase of the diluent molecular weight was suppressed to a great extent for a higher content of nylon 12 in the mixture suggesting that there exist specific interactions between PEG and nylon 12.

Highly Ag +-selective podans: thermodynamics of complexation and membrane transport

ΔH values for the 1:1 interaction of some sulfur-containing podands with Ag +, K +, Cu 2+, Cd 2+ and Pb 2+ have been measured by calorimetry in methanol. For the Ag + complexation, log K values were determined by potentiometry using an Ag +-ISE and, in this case, TΔS could be obtained. The complexations were enthalpy driven and Ag + exhibited remarkably higher complexation selectivity due to the increase of covalent binding between soft acid and soft base. The reaction enthalpy and entropy are discussed in terms of donor atom, end group and donor site effects, respectively. In the membrane transport study, efficient and highly selective Ag + transfer was observed both in single and competitive cation experiments.

Spectroscopic Analysis of Chain Conformation of Poly(propylene oxide)-Based Polymer Electrolytes

Fourier transform Raman spectroscopy has been utilized, in combination with normal coordinate analysis, to characterize chain conformational changes of amorphous poly(propylene oxide) (PPO) when used as a matrix for polymer electrolytes. Our data indicate that in the presence of salt, poly(propylene oxide) displays a characteristic Raman band at 810 cm -1 increasing in intensity with an increase in LiClO 4 concentration. 1,2-Dimethoxypropane (DMP) was found to be an excellent model for data interpretation. Both the intensity and frequency ofRaman active 1,2-dimethoxypropane vibrations were calculated. The C-C or C-O bands in the 700-900 cm -1 region were found to be particularly sensitive to chain conformational changes. The bands at 810 and 836 cm -1 were assigned to TG α T and TTT conformations of the -O-C-C-O- sequence along the backbone, respectively. Therefore an increase in intensity at 810 cm -1 directly correlates to an increase in the TG α T conformation with an increase in salt concentration. The effects of end groups on chain conformational changes were also studied since Li + can interact with both -OH end groups and ether oxygens. Poly(propylene oxide)s having hydroxy and methoxy end groups were compared. Our analyses indicate the interaction between hydroxy end groups and the lithium cation does not contribute as significantly to formation of the TG α T conformation as does the interaction between the ether oxygen and lithium cation.

Phase Behavior and Transreaction Studies of Model Polyester/Bisphenol-A Polycarbonate Blends. 2. Molecular Weight, Composition, and End-Group Effects

The equilibrium phase behavior of poly(2-ethyl-2-methylpropylene terephthalate) (PEMPT)/Bisphenol-A polycarbonate (PC) blends is monitored as a function of polyester and polycarbonate molecular weight, composition ratio, and end-group type. Six polyesters, number-average molecular weights (M n s) varying from 4100 to 37 500, and two PCs, M n s of 11 000 and 21 000, are studied at PEMPT/PC composition ratios of 15/85, 30/70, 50/50, 70/30, and 85/15 (w/w). PEMPT/PC blends are found to be two phase at all but the lowest molecular weight pairs examined. Composition effects are most apparent in blends containing components with low molecular weight. Hydroxyl- and benzoate-terminated PEMPT/PC 50/50 blends exhibit similar phase behavior and qualitatively follow Flory-Huggins theory with enhancement in miscibility as molecular weight is decreased. Blends containing polyesters with heptafluorobutyrate end groups show substantially different phase behavior. Virtually no enhancement in partial miscibility of the components is observed as the molecular weight of the polyester is lowered. These results are explained in terms of a multicomponent interaction parameter that accounts for both midchain/midchain interactions and midchain/end-group interactions. The interaction parameter of the blend, as calculated from critical point data, is 0.044.

Low ‐ frequency raman modes in crystalline chain molecules

AbstractLow ‐ frequency Raman modes of substituted derivatives of n ‐ alkanes and oligo(oxyethylene)s are discussed. The major features in these modes arise from whole molecule longitudinal vibrations. The effects of end group, chain length and temperature on the frequencies of these modes are described. The frequencies of the longitudinal vibrations are interpreted in terms of chain models.

Number‐average molecular weight and functionality of poly(tetramethylene glycol) by multidetector SEC

AbstractMultidetector size exclusion chromatography (SEC) is used to simultaneously determine molecular weight and number of reactive end groups per chain (functionality) of poly(tetramethylene glycol)s. Hydroxyl groups are first quantitatively derivatized with phenyl isocyanate, providing an end‐group‐selective UV‐absorbing tag. The number of end groups per chain is then determined from the SEC chromatogram using a UV detector. Molecular weight at each retention volume and the number‐average molecular weight of the whole polymer are calculated by four methods involving (1) a concentration detector and a narrow standard log M calibration curve, (2) the UV detector and a narrow standard log M calibration, (3) a viscometry detector and a universal calibration curve, and (4) combined differential viscometry and concentration detectors using a universal calibration curve. The multidetector experiment provides a unique opportunity to assess the accuracy and limitations of each approach on low‐molecular‐weight polymers. In particular, the effect of end groups on the concentration detector response and the application of universal calibration principles at small molecular sizes are important factors. It is shown that the concentration response can be corrected by a simple relationship between detector response and reciprocal molecular weight. Also, the quality of calibration curves is critical to the calculation of accurate molecular weight. In general, log M calibration curves provide superior results to universal calibration methods. © 1995 John Wiley &amp; Sons, Inc.

Kinetic study on simultaneous interpenetrating polymer network formation of epoxy resin and unsaturated polyester

AbstractSimultaneous interpenetrating polymer networks (SIN) based on diglycidyl ether of bisphenol‐A (DGEBA) and unsaturated polyester (UP) in weight ratios of 100/0, 50/50, and 0/100 were blended and cured simultaneously by using cumene hydroperoxide (CHP) and m‐xylenediamine (MXDA) as curing agents. A kinetic study during SIN formation was carried out at 43, 53, 58, and 64°C. Concentration changes for both epoxide and CC bond were monitored with Fourier transform infrared (FTIR). Rate expressions for epoxide polymerization were established with model reactions for phenyl glycidyl ether (PGE) and benzylamine in dichloroethane containing benzyl alcohol. Experimental results revealed that a lower cure rate constant for the CC bond during SIN formation was found, compared with pure UP network formation. A model of network interlock is proposed to account for this phenomenon. During simultaneous cure of DGEBA and UP, the network interlock should provide a sterically hindered environment, which subsequently retards cure rate for UP. On the other hand, epoxide cure during SIN formation indicates higher rate constants for both uncatalyzed and catalyzed reactions, compared with those of pure DGEBA cure. Presumably the catalytic effect of hydroxyl end groups in UP overcomes the effect of network interlock. Kinetic parameters were calculated and are discussed. © 1995 John Wiley &amp; Sons, Inc.

The role of a single end group in poly(ethylene oxide) adsorption on colloidal and film polystyrene: complimentary sedimentation and total internal reflectance fluorescence studies

This work examines “apparent equilibrium” and kinetic features of adsorbing homopolymers with a modification at one end of each chain to enhance chain-end adsorption. The end groups combined fluorescent and hydrophobic units so that polymers could be easily assayed and label-induced artifacts avoided. A series of end-modified poly(ethylene oxide)s (PEOs) of constant backbone length (molecular weight 100 000) but with varied end-group hydrophobicity were synthesized and their adsorption on polystyrene (PS) latex and films examined. Isotherms generated by the centrifugation of PS latex revealed that hydrophobically modified PEO adsorbed up to five times more than unmodified PEO. Total internal reflectance fluorescence (TIRF) spectroscopy revealed faster accumulation of hydrophobe-containing PEO compared with less hydrophobic samples. All layers, however, appeared equally stable to washing in pure solvent. Hydrophobe-containing PEOs could incorporate into or displace chains from preadsorbed layers of less hydrophobic PEO, but a preadsorbed layer of end-hydrophobic PEO excluded chains from a less hydrophobic sample. The impact of the small hydrophobic end groups on surface coverages and kinetic features was greater than expected for brush-forming polymers and PEO-surfactant systems. The enhanced end-group effects were attributed to the adsorption of the main backbone, a feature lacking in many previous polymer brush or surfactant studies. A comparison of surface coverages measured by TIRF spectroscopy and centrifugation methods revealed inaccuracies in the internal TIRF calibrations. The centrifugation method was thought to give a more accurate determination of the surface excess, while TIRF spectroscopy is better suited to kinetic and dynamic studies.

Coordination and conformation in PEO, PEGM and PEG systems containing lithium or lanthanum triflate

The effect of chain length, cation type and polymer end group on the coordination and conformation in polymer electrolytes has been studied using high molecular weight poly(ethylene oxide) (PEO), poly(ethylene glycol) (PEG(400)) and poly(ethylene glycol)dimethyl ether (PEGM(400)) as polymers, with LiCF 3SO 3 and La(CF 3SO 3) 3 as salts. The experimental technique used has been Fourier transform infra-red spectroscopy. A coordination of nine to ten ether oxygens to the lanthanum ions has been found for the PEGM system by increasing the concentration of lanthanum triflate in the polymers and following the disappearance of the COC stretching vibration of non-coordinated ether oxygens. The same coordination seems to be present also for the PEG and PEO systems. Conformational changes have been observed on addition of salt to the polymer, by following the disappearance of an absorption band at about 990 cm −1, indicative of a trans conformation around the CH 2CH 2 bond. For all three polymer types the gauche conformation is dominant at high salt concentrations. The OH end groups present in the PEG samples are important for the coordination in this system and both the cations and the triflate anions are coordinated by the OH groups to some extent.