Mobility Of Chain Ends Research Articles

Optimized End Functionality of Silane-Terminated Liquid Butadiene Rubber for Silica-Filled Rubber Compounds.

As the world is shifting from internal combustion engine vehicles to electric vehicles in response to environmental pollution, the tire industry has been conducting research on tire performance to meet the requirements of electric vehicles. In this experiment, functionalized liquid butadiene rubber (F-LqBR) with triethoxysilyl groups at both ends was introduced into a silica-filled rubber compound as a substitute for treated distillate aromatic extract (TDAE) oil, and comparative evaluation was conducted according to the number of triethoxysilyl groups. The results showed that F-LqBRs improved silica dispersion in the rubber matrix through the formation of chemical bonds between silanol groups and the base rubber, and reduced rolling resistance by limiting chain end mobility and improving filler-rubber interaction. However, when the number of triethoxysilyl groups in F-LqBR was increased from two to four, self-condensation increased, the reactivity of the silanol groups decreased, and the improvement of properties was reduced. As a result, the optimized end functionality of triethoxysilyl groups for F-LqBR in silica-filled rubber compound was two. The 2-Azo-LqBR with the optimized functionality showed an improvement of 10% in rolling resistance, 16% in snow traction, and 17% in abrasion resistance when 10 phr of TDAE oil was substituted.

Chain dynamics in foamed polysiloxane materials: Influence of the tin catalyst phase on material properties

We report on our underpinning science activities investigating the through life performance properties of foamed polysiloxanes. These materials are manufactured through tin octanoate based condensation cure chemistry. The tin catalyst plays an important role, not only in determining crosslink density and microstructure, but also potentially influencing properties (compression set and load retention) important to through life performance. In this study, Mossbauer Spectroscopy and X-ray fluorescence has been used to assess the nature and distribution of the tin species and show evidence for highly localized catalyst regions, including a mobile tin phase. In addition, testing was carried out using a NMR MOUSE (Mobile Unit for Surface Exploration) together with a CPMG (Carr Purcell Meiboom Gill) spin echo sequence to probe chain dynamics through the measurement of spin-spin or transverse relaxation (T2) responses. We have used model polysiloxanes to demonstrate that the NMR MOUSE capability is relatively sensitive to crosslink density. In general, the more resticted the environment around a given chain structure, the more rapidly it is likely to relax, and potential chain confinement effects of the tin phase may be assessed through the measurement of T2 parameters. Our work shows that the tin catalyst has a binding action and affects the dynamics of the relatively more rigid environments (such as crosslinks) as well as the mobility of chain ends and mid chain components (long T2). Load retention properties are reduced with increased tin catalyst loading, and this ties in with the influence of the tin particles on chain dynamics. The observations reported here support our initiative to develop predictive models representative of the through life performance of foamed polysiloxanes.

Solid-State Polymerization of Semiaromatic Copolyamides of Nylon-4,T and Nylon-4,6: Composition Ratio Effect and Thermal Properties

Solid-state polymerization (SSP) of semiaromatic copolyamides composed of nylon-4,T and nylon-4,6 in different molar ratios were investigated using a mixture of nitrogen and steam as the sweep fluid at reaction temperatures in the range of 220–260 °C. Prepolymers having different nylon-4,T contents (28.2, 37.9, and 47.8 mol %) were synthesized by melt polyamidation with stoichiometric ratios of diamine and diacid. The SSP conditions were carefully chosen to eliminate the influences of both internal and external diffusion of the reaction byproducts. Under these conditions, a high-molecular-weight semiaromatic copolyamide with an intrinsic viscosity of 2.469 dL/g was obtained upon SSP at 260 °C for 48 h. As the nylon-4,T content increased, the SSP rate decreased owing to the inherently low reactivity of the aromatic diacid and inhibition of chain-end mobility. The glass transition, melting, and decomposition temperatures of the polymers synthesized from the higher-nylon-4,T-content prepolymer were always higher than those of the polymers synthesized from the lower-nylon-4,T-content prepolymer.

Solid-State Polymerization of Poly(trimethylene terephthalate): Reaction Kinetics and Prepolymer Molecular Weight Effects

The reaction kinetics and the effects of prepolymer molecular weight on the solid-state polymerization (SSP) of poly(trimethylene terephthalate) (PTT) were investigated using nitrogen as the sweep fluid. The synthetic conditions were carefully chosen to eliminate the influences of both internal and external diffusion of the reaction byproducts (1,3-propanediol and water), so that the reaction kinetics were controlled by the forward chain extension reaction. Higher forward reaction rate constants were consistently obtained for the SSP of the high-molecular-weight prepolymer, compared to that of the low-molecular-weight prepolymer. The activation energy of the polymerization reaction was determined to be 15–26 kcal/mol, depending on the prepolymer molecular weight. The slower reaction rate for polymerization of the low-molecular-weight prepolymer may be attributed to the inhibition of the chain-end mobility due to the higher crystallinity and larger lamellar thickness of the obtained polymers. In addition, the high concentration of carboxylic end groups in the low-molecular-weight prepolymer may also decrease the reaction rate by preferential transesterification between 3-hydroxyl propyl end groups over the esterification reaction between 3-hydroxyl propyl and carbonyl end groups. High-molecular-weight PTT with an intrinsic viscosity of 2.05 dL/g (that corresponds to a number average molecular weight of 57600 g/mol) can be obtained via the SSP of the high-molecular-weight prepolymer at a relatively low temperature of 190 °C under the conditions in which byproduct diffusion resistance is eliminated.

The effect of prepolymer crystallinity on solid-state polymerization of poly(bisphenol A carbonate)

The effect of prepolymer crystallinity on the solid-state polymerization (SSP) of poly(bisphenol A carbonate) was examined using nitrogen as a sweep fluid. A low-molecular-weight prepolymer was synthesized by melt transesterification and prepolymers with different crystallinities (11.7%, 23.3%, 33.7%) were prepared with supercritical carbon dioxide treatment. SSP of the three prepolymers was then carried out at reaction temperatures in the range of 150–190 °C, with a prepolymer particle size of 75 μm and a N2 flow rate of 1600 ml/min. The glass-transition temperature (Tg), absolute weight-average molecular weight (Mn), and percent crystallinity were measured at various times during each SSP. At each reaction temperature, SSP of the lower crystallinity prepolymer (11.7%) always resulted in higher-molecular-weight polymers, compared with the polymers synthesized using the higher crystallinity prepolymer (23.3% and 33.7%). The crystallinity of the polymers synthesized from the high crystallinity prepolymer was significantly higher than for those synthesized from the low crystallinity prepolymer. Higher crystallinity of the prepolymer and the synthesized polymers may lower the reaction rate by reducing chain-end mobility or/and by inhibiting byproduct diffusion.

Self-Assembly of Pluronic Block Copolymers in Aqueous Dispersions of Single-Wall Carbon Nanotubes as Observed by Spin Probe EPR

The self-assembly of Pluronic block copolymers in dispersions of single-wall carbon nanotubes (SWNT) was investigated by spin probe electron paramagnetic resonance (EPR) spectroscopy. Nitroxide spin labeled block copolymers derived from Pluronic L62 and P123 were introduced in minute amounts into the dispersions. X-band EPR spectra of the SWNT dispersions and of native polymer solutions were measured as a function of temperature. All spectra, below and above the critical micelle temperature (CMT), were characteristic of the fast limit motional regime. The temperature dependence of the 14N isotropic hyperfine coupling, aiso, and the rotational correlation time, tauc, were determined. It was observed that, below the CMT, EPR does not distinguish between chains adsorbed on SWNT and free chains. Above CMT, substantial differences were observed: in the native solution, the Pluronics spin labels experience only one environment, Sm, assigned to spin labels in the corona of the Pluronic micelle, whereas in the SWNT dispersions, in addition to Sm, a second population of nonaggregated, individual chains, Si, is observed. The relative amounts of Sm and Si were found to depend on the relative concentrations of the Pluronic and SWNT. Furthermore, the aggregates formed in the SWNT dispersions do not show the typical increase in chain-end mobility as a function of temperature, observed in the post-CMT regime of the native Pluronic solutions. This suggests a larger dynamical coupling among aggregated chains in the presence of the SWNT as compared to the native micelles. The overall findings are consistent with the formation of a new type of aggregates, composed of a SWNT-polymer hybrid.

ジブロック共重合体が形成するミクロ相分離構造中のマトリックスブロックの自由末端セグメントの分子運動特性

ジブロック共重合体が形成するミクロ相分離構造中のマトリックスブロックの自由末端セグメントの分子運動特性

Bone-Tissue-Engineering Material Poly(propylene fumarate): Correlation between Molecular Weight, Chain Dimensions, and Physical Properties

Poly(propylene fumarate) (PPF) is an important biodegradable and cross-linkable polymer designed for bone-tissue-engineering applications. For the first time we report the extensive characterization of this biomaterial including molecular weight dependences of physical properties such as glass transition temperature Tg, thermal degradation temperature Td, density rho, melt viscosity eta0, hydrodynamic radius RH, and intrinsic viscosity [eta]. The temperature dependence of eta0 changes progressively with molecular weight, whereas it can be unified when the temperature is normalized to Tg. The plateau modulus and entanglement molecular weight Me have been obtained from the rheological master curves. A variety of chain microstructure parameters such as the Mark-Houwink-Sakurada constants K and alpha, characteristic ratio Cinfinity, unperturbed chain dimension r0(2)/M, packing lengthp, Kuhn length b, and tube diameter a have been deduced. Further correlation between the microstructure and macroscopic physical properties has been discussed in light of recent progress in polymer dynamics to supply a better understanding about this unsaturated polyester to advance its biomedical uses. The molecular weight dependence of Tg for six polymer species including PPF has been summarized to support that Me is irrelevant for the finite length effect on the glass transition, whereas surprisingly these polymers can be divided into two groups when their normalized Tg is plotted simply against Mw to indicate the deciding roles of inherent chain properties such as chain fragility, intermolecular cooperativity, and chain end mobility.

Chain Length Dependent Termination in Free Radical Copolymerization. 1. The Copolymerization System Styrene−Methyl Methacrylate in Bulk at 25 °C

The termination behavior in the stationary free radical copolymerization system styrene (ST)-methyl methacrylate (MMA) at 25 °C was reexamined by means of two newly developed methods. In essence, these are adaptations of the now already familiar methods of determining k t from rates and degrees of polymerization, originally developed for analysis of pulsed laser polymerization (PLP) experiments, for steady-state experiments conducted by photoinitiation using a continuous light source. Like the earlier methods they allow for the determination of the termination coefficient k t as a function of chain length. The extent of the chain length dependence of k t , characterized by the parameter b in the power law connecting k t and radical chain length v', k t = A(v') -b , is found to be on the order of 0.1 over the full composition range (including the two monomers ST and MMA) which is slightly smaller than the data obtained from PLP for the two pure monomers (0.16-0.18). However, there are no large variations as a function of feed composition. The absolute values of k t either obtained via extrapolation to unity chain lengths (k 1.1 t) or calculated for a fixed chain length of 500 (k 500 t), however, pass through a maximum. In view of the diffusion-controlled character of the termination reaction this result is interpreted by assuming the mobility of the chain ends to be increased when a monomer unit different from the terminal unit is incorporated into the penultimate (or penpenultimate) position of the growing chain, especially if MMA is replaced by ST in a MMA terminated chain. Assisted by the alternation tendency of the copolymer system ST-MMA this might finally lead to an overall chain end mobility in excess of that of either homopolymeric radical for intermediate monomer compositions.

Voronoi space division of a polymer: Topological effects, free volume, and surface end segregation

In order to investigate the topological effects of chain molecules, united-atom molecular dynamics simulations of a 500-mer polyethylene linked by 50 hexyl groups (a grafted polymer having 52 ends) are carried out and analyzed in terms of Voronoi space division. We find that the volume of a Voronoi polyhedron for a chain end is larger than that for an internal or junction atom, and that it is the most sensitive to temperature, both of which suggest higher mobility of chain ends. Moreover, chain ends dominantly localize at the surface of the globule: The striking evidence is that while the ratio of surface atoms is only 24% of all atoms, the ratio of ends at the surface is 91% out of all ends. The shape of Voronoi polyhedra for internal atoms is prolate even in the bulk, and near the surface it becomes more prolate. We propose the concept of bonding faces, which play a significant role in the Voronoi space division of covalently bonding polymers. Two bonding faces occupy 38% of the total surface area of a Voronoi polyhedron and determine the prolate shape.

Spin-label study of immiscible polymers: 4. Location of chain ends in blends of poly(methyl methacrylate) and poly(2-ethylhexyl methacrylate)

The electron spin resonance (e.s.r.) spectra of nitroxide end-labelled poly(methyl methacrylate) (PMMA) were recorded over a temperature range for the pure, bulk polymer and for 1:1 (w/w) immiscible blends with poly(2-ethylhexyl methacrylate) (PEHMA) and polystyrene (PS). The changes in the e.s.r. spectra brought about by blending and varying the temperature show that chain ends tend to congregate in the interphase region. In the blends of PMMA and PEHMA the chain ends of the PMMA in the interphase are mobilized through plasticization by the rubbery PEHMA. This phenomenon was observed previously in blends of end-labelled PS with unlabelled polyisoprene. Our present results, particularly from the PMMA-PS blends, reveal that an additional enhancement of mobility of chain ends results from the accumulation of free volume in the interphase.

Synthesis and characterization of Tg for pyrene end-labelled polystyrene having no ester linkages

A series of pyrene end-labelled polystyrene (PS) samples having molecular weights ranging from 2000 to 40 000 have been synthesized anionically. Complete characterization data are provided. The end-labelling reaction is performed by coupling the living polystyryl anion with 1-bromobutylpyrene, with coupling efficiencies ranging from 72 to 90%. The resulting pyrene end-labelled PS has no ester linkage in the backbone, a common structure resulting from alternative end-labelling reactions proceeding via terminal hydroxyl units. This leads to a potential enhancement in thermal and hydrolytic stability; in fact, the samples have the same thermal stability as unlabelled PS. The glass transition temperatures ( T g) of our samples vary linearly with M ̄ −1 n , consistent with the Fox-Flory equation. However, the T g values of the labelled PS samples are higher than the unlabelled ones with the difference widening as M ̄ n decreases. This difference is explained by a decrease of chain end mobility and hence of free volume caused by bulky end groups.

A model for the onset of entanglements of transient hydrogen-bonded intermolecular structures in oligomeric poly(propylene glycol)

This paper presents a model for the hypothesis that entanglement effects between transient intermolecular structures in poly(propylene glycol) (PPG) can lead to an additional (to segment modes) low-frequency relaxation above a quasi-critical molecular weight far below the critical molecular weight of the bulk. The model describes the formation of end-to-end hydrogen-bonded intermolecular structures and explains the fact that tube-like topological restrictions occur when the quasi-critical molecular weight of the primary chains M′ is reached. By comparison with the experimental value M′ ≈ 1300 g mol −1 for PPG at room temperature, it is concluded, from a kinetic point of view, that the timescale for the relevant hydrogen-bond kinetics is comparable with the timescale of chain-end mobility on the spatial scale of the molecular radius for the primary oligomer chain in the crossover regime.

Modeling of diffusion‐controlled free‐radical polymerization reactions

AbstractThe present paper introduces a new fundamental approach to the modeling of diffusion‐controlled free‐radical polymerization reactions. Our analysis follows the original work of Chiu, Carratt, and Soong (CCS), according to which the termination and propagation rate constants are expressed in terms of both a purely reaction‐limited term and a diffusion‐limited one. The contribution of the latter term to the apparent rate constants is described in terms of the polymer and monomer effective diffusion coefficients and an effective reaction radius. It is shown that all parameters appearing in the original CCS model can be calculated from first principles using available data on the physical and transport properties of a particular monomer–polymer binary system. The generalized free volume theory of Vrentas and Duda and the theory of excess chain end mobility are invoked for the calculation of the effective diffusion coefficients and the reaction radius, respectively. The approach followed in this study is general and needs only the specification of one unknown parameter with a clear physical meaning. All other parameters can be readily calculated from available data. The ability of the new model to predict molecular weight developments and monomer conversion in diffusion‐controlled reactions is demonstrated by application of the proposed model equations to the bulk polymerization of MMA.

Chain end mobility and lamellar crystallization in polymer melts: Discussion of a new model

AbstractThe following model relates to exceptionally high crystal growth rates in polymers: The chain end concentration near the crystal growth face is locally so high that the folding chain can be disentangled by tube destruction owing to the high mobility of the foreign chain ends.

Termination‐rate coefficients in methyl methacrylate polymerizations

AbstractIt is shown that, by observing the kinetics of the seeded emulsion polymerization of methyl methacrylate with and without added chain‐transfer agent, and using γ‐radiolytic initiation, it is possible to examine separately the effects of the length of the growing chain and of the weight fraction of polymer on the termination rate coefficient. Studies using this technique confirm predictions that chain entanglement causes translational diffusion to be the rate‐determining termination event for growing chains of lower molecular weight, while the rate‐determining termination event for growing chains of higher molecular weight is “residual,” the annihilation encounter being caused by excess chain end mobility of two growing chains in a viscous medium. The data show unambiguously that free‐volume theories which do not incorporate the molecular weight of the growing chain are inapplicable to this system.

Crystallization behaviour of pmma-b-peo-b-pmma triblock copolymers

Abstract New synthesized ABA type triblock copolymers of poly /methyl methacrylate/ /PMMA/ and poly /ethylene oxide/ /PEO/ with constant PEO sequence length and varying PMMA sequence length were examined by DSC to study crystalization and melting phenomena. Well defined dependence of melting temperature T m and crystallinity α of PEO on PMMA sequence size were found in contrast to most of the literature data on other PEO block copolymers. Crystallization of the PEO chains was strongly hampered by the PMMA sequences. From our results we concluded that the crystallization is essentially determined by the chain end's mobility of the crystallizable component.

Spectroscopic markers of the conformational mobility of chain ends in molecules containing n-alkane residues

Spectroscopic markers of the conformational mobility of chain ends in molecules containing n-alkane residues

Chemical reactivity and conformational properties of growing chains. Part 2.—Effect of the reaction medium on the termination kinetics in styrene + methyl methacrylate solution copolymerization

The solution copolymerization of styrene and methyl methacrylate in acetone, in dimethylformamide and in dioxan has been interpreted on the basis of a proposed model which includes chain end mobility and chemical reactivity. For all systems the derived equation giving the overall rate of copolymerization is in agreement with the experimental data. The effect of the reaction medium on bimolecular termination can be represented by the dependence of the rate-conditioning segmental rearrangement on medium fluidity.There is a marked solvent effect on the propagation rate coefficients. It is more pronounced for styrene than for methyl methacrylate and it shows an opposite trend for the two monomers: compared to the value for bulk polymerization kp for styrene is decreased, whereas kp for methyl methacrylate is increased in solution.