Influence Of End Groups Research Articles

Excess heat capacity for low-molecular-weight amorphous polystyrene below the glass-transition temperature: influence of end groups

In this work, we analyze the absolute heat capacity below the glass-transition temperature Tg for two types of amorphous polystyrene with differing low-molecular weights. Both amorphous polystyrenes have excess heat capacities that cannot be reproduced by skeletal and group vibrations from 70 K to Tg, which differs from the results for amorphous polystyrene with large molecular weights. The excess heat capacity is analyzed using the Schottky model. The ratio of the degrees of freedom of the Schottky model NSC for PS-A300 and PS-A500 (NSC,PS-A300/NSC,PS-A500) is 1.6, which reflects the ratio of the end groups per molecule ((i.e., the number of monomer units of PS-A500/the number of monomer units of PS-A300 = 5/3) = 1.7), and the resulting excitation energy (=31 meV) is consistent with that determined using inelastic neutron scattering (=30 meV). The results of this investigation indicate that, compared with the high-molecular-weight amorphous polystyrene, the two low-molecular-weight amorphous polystyrenes studied herein have excess heat capacity below the glass-transition temperature. These results were obtained by analyzing the molecular vibrations of the high-molecular-weight polystyrene. The excitation energy obtained by regression analysis of the excess heat capacity determined by using the Schottky model is consistent with the excitation energy obtained by inelastic neutron scattering. This evidence strongly suggests that the excess heat capacity is due to vibrations of methyl end groups.

Separation of Branched Poly(bisphenol A carbonate) Structures by Solvent Gradient at Near-Critical Conditions and Two-Dimensional Liquid Chromatography.

Branching is a molecular metric that strongly influences the application properties of polymers. Consequently, detailed information on the microstructure is required to gain a deeper understanding of structure-property relationships. In the present case, we employ high-performance liquid chromatography to characterize the branching in a poly(bisphenol A carbonate) (PC). To this end, a method was developed based on a mobile phase gradient in a very narrow range (±1.4 vol %) around the point of adsorption (98.9/1.1 vol % chloroform/methyl tert-butyl ether), which we refer to as solvent gradient at near-critical conditions. Application of such gentle gradient enabled separation of PC according to end-groups. The separation mechanism was confirmed by collecting fractions of a separated sample and subsequently analyzing these by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Hyphenating the developed gradient method with size-exclusion chromatography as the second dimension (2D-LC) enabled separation of linear and branched PC chains and determination of the molar mass distribution of the fractions. A reversed elution order was observed for branched species in 2D-LC, meaning that low molar mass chains exhibited higher elution volumes in the first dimension than higher molar masses. This finding was explained by influences of end-groups as well as the architecture of the branched polymer chains.

Influence of End Groups in Photoinduced Reorientation of Liquid Crystalline Polymer Films Based on Axis-Selective Photo-Fries Rearrangement

Photoreactive liquid crystalline polymethacrylates with phenylbenzoate side groups with different molecular terminal were synthesized and their photoresponsive behavior was investigated. All polymer films underwent axis-selective photo-Fries rearrangement and generated photoinduced optical anisotropy upon exposure to linearly polarized ultraviolet (LPUV) light. In addition, annealing the photoreacted films enhanced molecular reorientation perpendicular to polarization direction of LPUV light, and the molecular terminal groups strongly affected the reorientation behavior.

Influence of end groups in hyperbranched polyesters used as modifiers in the characteristics of epoxy thermosets cured by adipic dihydrazide

Mixtures of diglycidylether of bisphenol A (DGEBA) resin and different ratios of aliphatic-aromatic hyper- branched polyester (HBP) were cured by a latent curing agent, adipic dihydrazide (AH). The HBPs prepared have hydroxyl groups or 10-undecenoyl or allyl groups as chain ends. The curing mixtures were investigated by differential scanning calorimetry (DSC) to study the curing process and to evaluate the kinetic parameters of the different formulations. These studies suggest that HBPs decrease the curing rate of epoxy/AH in the case of vinyl terminated HPB, whereas OH termi- nated HBP accelerates the first stages and delays the lasts. The thermosets obtained showed an improvement in microhardness and impact strength without any reduction of the Tg and thermal parameters. Microparticle phase separation was observed with the undecenoyl HBP derivatives or when a 10% of allyl HBP derivative was in the formulation.

Synthesis, metal ion complexation and antibacterial activities of some thiapodands with lipophilic amide and ester end groups

A series of acyclic analogues of thiacrown ethers (podands) 7–12 with lipophilic amide and ester end groups were synthesized in high yield and in a simple way. Their transition metal ions complexation was studied using a conductometric method in acetonitrile at 25°C. Podands 7 and 11 showed a continuous decrease in the molar conductances in their complexation with Ag+, Cu2+, Cd2+, Hg2+, Zn2+ and Pb2+ which begins to level off at a mole ratio of 1:1 podand to metal indicating the formation of a stable 1:1 complexes. On the other hand, podand 9 also showed the formation of 1:1 complexes with above metal cations except with Hg2+ ion, which formed a 1:2 podand-to-metal ratio complex. An influence of end groups on metal ion selectivity is evident. Podands having ethoxy end groups (podands 8, 10 and 12 exhibit pronounced metal ion selectivity over podands having amino end groups (podands 7, 9 and 11). Compounds 10 and 12 with dithiaethylene units and ethoxy end groups provide the best selectivity for Hg2+ and Ag+ ions. These results suggest that podands 10 and 12 could be useful for the selective removal of Hg2+ and Ag+ ions from industrial waste that may contain a variety of toxic heavy and transition metal ions. The in vitro antibacterial activity of the investigated compounds was tested against several microorganisms such as Bacillus subtilis (ATCC 6633), Micrococcus luteus (ATCC 10240), Staphylococcus aureus (ATCC 43300), Escherichia coli (ATCC 25922) and Enterobacter aerogenes (ATCC 13048). The antibacterial activity of podand 10 is significant for M. luteus and B. subtilis compared with other podands under investigation.

Effects of End-Groups on Properties of Hyperbranched Polyether

In order to investigate influence of end-groups on thermodynamic property and viscosity of hyperbranched polyether. Three series of hyperbranched polyethers with three different end-groups were studied. These three groups are: hydroxyl, acetate and stearic acid ester. The result confirms the complicated effect of end-groups on properties of hyperbranched polymer. When end-groups change from polar groups of hydroxyl to non-polar groups of acetate, glass transition temperature reduced about 50°C, viscosity reduced sharply too. Glass transition temperature and viscosity all increase with generation rise. When end-groups change from acetate to stearic acid ester, corresponding hyperbranched polyether change from amorphous to crystalline. The crystallization stemmed from long chain structure of end-group, which is easy to form crystalline, its crystalline region molten become main thermodynamic transformation, and obscure the glass transition.

Influence of End Groups on the Stimulus-Responsive Behavior of Poly[oligo(ethylene glycol) methacrylate] in Water

The influence of the chemical structure of both end groups onto the lower critical solution temperature (LCST) of poly[oligo(ethylene glycol) monomethyl ether methacrylate] (POEGMA) in water was systematically investigated. POEGMA of Mn = 3550 g/mol and Mw/Mn = 1.14 prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization was equipped with two different functional end groups in a one-step postpolymerization reaction combining activated esters, functional amines, and functional methane thiosulfonates. As end groups, n-propyl, n-hexadecyl, di(n-octadecyl), poly(ethylene glycol)-550 (PEG), 1H,1H-perfluorononyl, azobenzene, and trimethylethylammonium groups were systematically combined with methyl, n-hexadecyl, and 1H,1H,2H,2H-perfluorooctyl groups. Polymers were characterized by gel permeation chromatography, dynamic light scattering, and turbidimetry. Hydrophobic end groups at either end of the polymer chain decreased the LCST. For hydrophobic groups at both ends of the chain their influence was additive. Two large hydrophobic end groups allowed micelle formation below the LCST and an LCST higher than to be expected from nonaggregated polymers. The strongest hydrophobic effect was found for rigid aromatic end groups, which was attributed to their incompatibility with the flexible polymer chain. Charged end groups increased the LCST and could compensate for the effect of hydrophobic end groups at the opposite end group. PEG end groups could mask a hydrophobic influence of the opposite end group and stabilized the LCST.

Influence of End Group and Surface Structure on the Current−Voltage Characteristics of Alkanethiol Monolayers on Au(111)

We present density functional theory calculations of the electronic structure and tunneling characteristics of alkanethiolate monolayers on Au(111). We systematically analyze radical3 x radical3 full coverage monolayers of SC6H12X molecules with different terminal groups, X=CH3, NH2, SH, OH, COOH, OCH3, on defect-free ("perfect") Au(111). We also study the influence of the surface-molecule bonding structure by comparing the properties of monolayers of SC6H12CH3 molecules on the perfect surface and on Au(111) surfaces with vacancies or adatoms. The tunneling currents (I) through the adsorbed monolayers with a single chemical contact have been calculated within the Tersoff-Hamann approach for voltages between -1 and +1 V. Computed currents are found to depend linearly on V at low voltage, with typical values of approximately 60 and 150 pA/molecule at 0.2 and 0.5 V, respectively, in good agreement with several experimental data. Computed tunneling currents show also a significant dependence on both the terminal group X and the surface structure. In particular, in order of decreasing intensities, currents for the different end groups are NH2 approximately SH>CH3>OH>OCH3>COOH. The relationships between the tunneling current, the work function of the surface+SAM, and the lineup of the HOMO with respect to the Fermi energy of the metal surface are examined.

Influence of end groups on in vitro release and biological activity of lysozyme from a phase-sensitive smart polymer-based in situ gel forming controlled release drug delivery system

Phase-sensitive in situ gel forming controlled release formulations of lysozyme were prepared using poly lactic acid (PLA) and/or poly glycolic acid (PGA) based polymers differing in end groups in addition to composition, and a solvent system consisting of various ratios of benzyl benzoate (BB) and benzyl alcohol (BA). The amount of lysozyme in the released samples was determined by measuring absorbance at 280 nm using suitable controls to nullify the effect of absorption of formulation degradation products. Biological activity of lysozyme was studied by an enzyme activity assay using Micrococcus lysodeikticus as substrate. Polymers bearing carboxylic acid end group were not soluble in 100% BB but polymers having ester end groups were soluble up to 27% (w/v) except polymer 4. A biphasic release profile consisting of slower first phase followed by faster second phase was observed. Formulations prepared from polymer with carboxylic acid groups showed significantly ( p < 0.05) lower burst release (4%) than those containing ester end groups (20–30%). However, formulations consisting of polymer with carboxylic acid end groups showed significantly ( p < 0.05) faster release rate of incorporated lysozyme, although the total amount released was less in comparison to the total amount released from formulations prepared using polymers containing ester end groups. The mean percentage specific enzyme activity (MPSEA) data were supported by the release profiles. In conclusion, polymer end groups may influence the release profiles of a protein from an in situ gel depot forming controlled release formulations.

Molecular Organization of Alkoxy-Substituted Oligo(phenylene-ethynylene)s Studied by Scanning Tunneling Microscopy

Morphologies and structures of self-assembled monolayers of two conjugated oligo(phenylene-ethynylene)s are observed by scanning tunneling microscopy at submolecular resolution. The cooperation of intermolecular and molecule/substrate interactions contributes to the long-range order. The influence of endgroups' hydrogen bonding and steric hindrance on the surface patterning is discussed. The different arrangement configuration of the two molecules could be due to the changed endgroups. This control of the two-dimensional self-assembly is promising for future studies on the electronic properties of these molecules.

Influences of end-groups and thermal effect on the red-shifted emission in the poly (9, 9-dioctylfluorene) light emitting diodes

Blue polymer light emitting diode based on poly (9,9-dioctylfluorene) (PFO) has been demonstrated. The effects of end-group of PFO on the optoelectronic characteristics of the devices are explored. Our results suggest that end-group functions as both the carrier trap and the luminescence quenching center. Both the end-group and the thermal effect contribute to the red-shifted emission.

Influence of End Group and Molecular Weight on Polybutadiene Fingerprint Secondary Ion Mass Spectra

Polybutadiene samples of different molecular weight have been synthesized by anionic polymerization as initiated by sec-butyllithium with low polydispersity and a major content of 1,2-vinyl units. They have been analyzed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) in order to investigate the sensitivity of this method toward the sec-butyl end group and toward the molecular weight. The SIMS spectra show the characteristic fragment of the end group, C4H9+ at m/z = 57, whose the peak intensity is strongly dependent on the polymer molecular weight, as is the case for almost all the fragment intensities. A model consistent with the peak intensity variations is used to give some new insights into the fragmentation mechanism at the end groups and within the main chain. Moreover, the analysis of the end group fragment allows Mn to be readily determined up to Mn = 4 × 104 from, for example, the Y(53)/Y(57) intensity ratio where Y(53) is the intensity of the deprotonated repeat unit ([M − H]+). Other Mn calibration methods have also been used and are discussed in terms of their accuracy and physical meaning.

The glass temperature of dendritic polymers

AbstractA theoretical treatment of the glass temperature of dendritic polymers is presented. The influences of polymer backbone, end group, initiator core, branching unit, composition and functionality are discussed. In dendritic polymers the glass temperature is dependent only on the generation number of dendritic growth and thus only on the molecular weight of a dendron, but not on the molecular weight of the whole molecule. It is governed primarily by the backbone glass temperature and depends little on branching functionality. Only minor differences between linear polymer and dendrite are obtained, since the influences of end groups and branching compensate each other to a large extent. © 1995 John Wiley &amp; Sons, Inc.

Relation between characteristics and melting point of oligomers in poly(ethylene terephthalate) manufacturing

AbstractThe applicable range of the regression equation proposed by Yamada, et al. expressing oligomer melting point by oligomer characteristics and relations derivable from the equation correlating each oligomer characteristic and melting point were studied. It was confirmed that the equation is applicable not only to poly(ethylene terephthalate) (PET) oligomer but also to the extent of polymerization. As number‐average degree of polymerization (Pn) increases, the influence of end groups on melting point becomes weak, and when Pn exceeds 50, the influence of end groups becomes negligibly small. On the other hand, when Pn is below 2, It was confirmed that the influence of end groups on melting point is very strong.

The influence of end groups on the stabilization of poly(vinyl chloride)

The influence of end groups on the stabilization of poly(vinyl chloride)