Motion Of Main Chain Research Articles

Effects of introducing methacrylic and acrylic acid groups on the molecular relaxation of poly(isopropyl methacrylate)

Abstract Dielectric and IR measurements were carried out for copolymers of isopropyl methacrylate with methacrylic or acrylic acid at various content units. The acid units introduced into isopropyl methacrylate polymer chain formed acid dimers and free acid units in the neighborhood of isopropyl methacrylate units. According to the segmental mode relaxation of main chains (α process), the motion of main chains was found to be restricted by acid dimers formed among acid groups, and reinforced by the free acid units. The acid dimers and free acid units acted even on the local mode relaxation of isopropyl methacrylate side chains. The relaxation strength of the α process (Δα) for copolymer samples increased with temperature. The increase in Δα was attributed to an increase in the number of motional units caused by breakdown of the acid dimmers. The dissociation energy (ΔH0) of the acid dimmer could then be estimated from the temperature dependence of Δeα.

Photoisomerization of polymer azoaromatic polyimide in concentrated sulfuric acid

To analyze the mechanism of main-chain motion of a semi-stiff chain polymer in fluid solution, the decay curves of absorption intensity of azoaromatic polyimides have been measured in concentrated sulfuric acid using a pulsed nitrogen laser. The rate constants of cis → trans isomerization differ between the structure isomers of azoaromatic polyimides.

ESR study of the pressure and temperature dependence of free radical decay in γ-irradiated polyvinylpyrrolidone

The decay of free radicals in γ-irradiated polyvinylpyrrolidone has been studied as a function of pressure and temperature. Rate constants and activation volumes for free radical decay have been determined. High pressure reduces the free volume and segmental motions of main chains and thus the transport of the radical centre is suppressed. The ESR spectra have been analyzed throughout the temperature region. They reflect the presence of three types of radicals. The contribution of a nitrogen centered radical prevails above room temperature.

Effect of temperature in region of γ and β relaxations on photo-oxidation of polystyrene

Atactic polystyrene films were irradiated at 254 nm in oxygen. The influence of the polystyrene mobility on chain scission and acetophenone formation in the temperature range 233–387 K has been studied. The increase of quantum yield for chain scission in the γ relaxation region was attributed to phenyl group rotation and increase of the probability of energy transfer from this chromophore to induce C-H bond scission. The considerable increase of acetophenone formation at the β relaxation was attributed to increase of oxygen interaction with the chromophore because of cooperative motion of main chain and benzene rings.

Damped orientational diffusion model of polymer local main-chain motion. 5. Comparison with three alternative models

Damped orientational diffusion model of polymer local main-chain motion. 5. Comparison with three alternative models

Molecular motions of non-crystalline poly(aryl ether-ether-ketone) PEEK and influence of elecron beam irradiation

The dynamic mechanical relaxation of non-crystalline poly(aryl ether-ether-ketone) PEEK and the one irradiated with electron beam were studied. The three distinct γ, β, α′ relaxation maxima were observed in unirradiated PEEK from low to high temperature. It was revealed from the study on the irradiation effects that three different molecular processes are overlapped in γ relaxation peak, i.e., molecular motion of water bound to main chain (peak temperature; at −100°C), local motion of main chain (at −80°C), and local mode of the aligned and/or oriented moiety (at −40°C). The β relaxation connected with the glass transition occurred at 150°C and it shifted to higher temperature by irradiation. The α′ relaxation which can be attributed to rearrangement of molecular chain due to crystallization was observed in unirradiated PEEK ∼ 180°C and its magnitude decreased with the increase in irradiation dose. This effect indicates the formation of structures inhibiting crystallization such as crosslinking and/or short branching during irradiation. A new relaxation, β′, appeared in the temperature range of 40° to 100°C by irradiation and its magnitude increased with dose. This relaxation was attributed to rearrangement of molecular chain from loosened packing around chain ends, which were introduced into the non-crystalline region by chain scission under irradiation, to more rigid molecular packing, From these observations, we proposed that deterioration in mechanical properties of non-crystalline PEEK by high energy electron beam was brought about not only by chain scission but structural changes such as crosslinking and/or branching in the main chain.

Damped orientational diffusion model of polymer local, main chain motion. 4. Effects of probes and side-chains

The complex damped orientational diffusion model of polymer local, main chain motion in moderately dilute solution is extended to include the effects of attached probes and periodic side chains on the dynamics. In the case of an appended probe attached to the main chain, a prescription is given for extracting the contribution of the probe to the local, main chain dynamics and thereby allows one to determine when the probe essentially exerts a minor influence on the character of the local motions and when it does not. In the case of a polymer containing a periodic array of side chains (as is normally the case), we demonstrate that to an excellent approximation in the context of the complex damped orientational diffusion model, side chains merely modify the effective damping constant of an equivalent polymer lacking any side chains. This is an important conclusion that requires experimental verification. Finally, we demonstrate that the complex damped diffusion model in general remains invariant to the particular kind of short-wavelength cutoff employed (either soft or hard) to represent the fact that the fundamental motional unit is of finite extent. In conjunction with the previous conclusion (Macromolecules 1982,15,1041) that the model is insensitive to the particular form of the long-wavelength cutoff, this implies a certain robustness of the basic orientational diffusion picture of polymer local motion. In a series of we have developed a damped orientational diffusion model of the local, main chain motion of polymers in solution and have shown that NMR and dielectric relaxation processes can be adequately de- scribed by such models. We have also extended the ori- entational diffusion model to include chain-chain inter- actions in an averaged way through the use of a complex damping constant? thus enabling it to be applied to more concentrated polymer solutions. In addition, the model has been extended to the case of restricted rotational diff~sion.~ Recently, Viovy, Monnerie, and Brochon5 presented a detailed comparison of the ability of a variety of diffusional models of polymer local, main chain motion to fit their measured fluorescence depolarization mea- surements on polystyrene and found that these models fit the measured autocorrelation function very well. It has become increasingly evident as more experimental results are reported and compared to theory that local polymer motion as probed by typical relaxation measurements can be well described by an orientational diffusion picture. Encouraged by the above, we shall extend the range of applicability of the damped diffusion model of polymer motion to include the effect of spatial inhomogeneties. In particular two cases are explicitly treated. In the first case a polymer chain contains a probe molecule attached to it. Examples include spin-labels required in ESR measure- ments and dyes required in fluorescence depolarization measurements. Here, we present a procedure for deter- mining whether the probe moiety is essentially benign, that is, it does not change the intrinsic relaxation characteristics of the polymer local motion, and if so, a method is given for extracting the intrinsic motional parameters of the polymer chain. The second case we shall treat is the be- havior of a polymer that contains a periodic array of side chains appended to the main chain. Here we shall examine how the power spectrum of the polymer (and hence its relaxation properties) is modified by the presence of the side chains. In the damped diffusional model, orientational diffusion down the polymer chain is treated as a superposition of damped diffusional waves. In the context of this model an inhomogeneity is treated as an isolated change in the

Mechanical and gas transport properties of poly-ε-caprolactone model networks

The glass transition temperature increases with increasing crosslink densities in model networks formed by endlinking poly-ϵ-caprolactone with a triisocyanate crosslinking agent. In the noncrystalline networks, the gas permeability decreases with increasing crosslink density. These results are consistent with an interpretation that the crosslinks reduce the main-chain molecular motions which are important to these processes. At the lowest crosslink density, where poly-ϵ-caprolactone networks are crystalline, the gas permeability is lower than would be expected based on the volume fraction of amorphous polymer. The excess reduction in permeability is attributed to crystallization-induced enrichment of crosslink junction points in the amorphous fraction of the network. This reduces the permeability by creating an artificially high crosslink density in those regions of the network responsible for gas transport. Since crosslinking increases the stiffness and reduces the flexibility of the network polymer chains, it affects large penetrants more strongly than small ones. Therefore, increasing the crosslink density proves to be a useful method for increasing gas separation factors.

Effect of molecular weight on rate of enthalpy relaxation of poly (methyl methacrylate) and its oligomers

AbstractThe enthalpy relaxation process of methyl methacrylate oligomers and poly (methyl methacrylate) was analyzed by use of the excess enthalpy (ΔHt), which was obtained from a differential scanning calorimetric measurement. The apparent relaxation time (τ½) was estimated by the assumption of the single relaxation time response. The Tg of oligomers increased with an increase of molecular weight, and approached a constant value above 1 × 104. The apparent relaxation time showed a similar molecular‐weight dependency as Tg, and approached a constant value above 1 × 104. It was expected that the molecular motion which caused the enthalpy relaxation was based on the segmental motion of main chains.

Slow stable crack growth in high density polyethylenes

The phenomenon of slow stable crack growth in polyethylene is investigated using notched specimens subject to constant load and the concepts of fracture mechanics. The effect of specimen geometry and dimension, the loading and the mode of loading on the applied stress intensity factor versus crack speed ( K c - a ̇ ) curves has been studied to demonstrate that K c is the controlling stress parameter for crack growth under suitable conditions. K c - a ̇ curves are obtained for a high density polyethylene homopolymer in distilled water and in a diluted detergent solution at four different temperatures. Results are also obtained for a much tougher medium density polyethylene copolymer whenever possible. Several mechanisms can be identified from the form of the K c - a ̇ curves. Two, in particular, have been observed but not explained before: (i) crack growth with a time dependence of 0.25, and (ii) the high K c - a ̇ slopes for crack growth in a tough copolymer. With the help of scanning electron microscopic studies of the fracture surfaces, (i) is postulated to be due to diffusion controlled void growth process and (ii) is considered to be the result of crack tip blunting effects. From the temperature dependence of crack growth, the activation energy of the diffusion controlled crack growth process is found to coincide with that of the x-relaxation process in polyethylene implying that diffusion controlled crack growth may be related to the motion of main chains in the polymer.

Damped orientational diffusion model of polymer local main-chain motion. 3. Inclusion of chain-chain interactions

The damped orientational model of polymer local main-chain motion is generalized to include chain-chain interactions using multiple scattering theory in the mean field approximation and an optical potential. The resulting dynamical equation, which is in the form of a complex damped diffusion equation, still permits analytic solutions for the bond orientational correlation function and the spectral density. Thus by incorporating chain-chain interactions into an orientational diffusion model, the theory should be capable of describing local motions for concentrations outside the dilute solution regime. The resulting three-parameter analytic expression for the spectral density is fit to the low-temperature-phase lH NMR relaxation times of poly(viny1 acetate) (PVA) in toluene. In this phase PVA is a compact random coil. Both the real and complex dampled diffusion models are found to fit the data equally well. The real damped diffusion model parameters fit to the low-temperature phase are considerably different in magnitude from those for the high-temperature phase. Therefore the low-temperature parameters presumably are effective parameters and reflect the role of intrachain interactions. The complex and real damped diffusion models also are capable of fitting the high-temperature proton relaxation data equally well. Thus, based on goodness of fit and given the uncertainty of some of the NMR relaxation data, the real and complex damped diffusion models cannot be distinguished a t this time.

The mobility of calcium-trigger proteins and its function.

Trigger activity implies the transfer of the energy of a signal to some amplified (energy) response. Actions in cells, from calcium concentration changes to major protein reorganization are discussed here. The changes must be fast, so mobile polymers must be involved. The first step is the calcium on/off binding to its receptor, calmodulin, troponin C or a comparable protein. Calcium binding is to a loop, EF-hand, between helices. The structures and internal mobilities of these proteins are described using nuclear magnetic resonance and the temperature dependence of NMR shifts. It is suggested that these proteins illustrate a general working hypothesis that proteins made from interacting helices as opposed to beta-sheet proteins will have relatively easy internal main chain motions. Loops connecting the helices then provide particularly obvious read-out points, for example of the initial message of calcium binding. These and other regions of loose structure appear to be associated with highly charged sequences. The further transfer of the trigger message is to highly mobile sequences in troponin I, troponin T and tropomyosin.

Effects of binding of S-peptide and 2′-cytidine monophosphate on hydrogen exchange from the S-protein component of ribonuclease S: The amide protons of serine 123 and valine 124

The hydrogen exchange rates of amide protons from many chain segments in S-protein, previously shown to be dramatically affected on association with S-peptide, are essentially unchanged, over a period of 150 hours at least, on binding the inhibitor 2′CMP to RNAase S. An exception was the C-terminal tetrapeptide (121–124), which is close to, but not in actual contact with, substrates or inhibitors bound at the active site of the enzyme. Due to the effect of the intrinsic exchange rates within the fragment during analysis only two protons are observed, those of serine 123 and valine 124. The intrinsic rates of these two sites are also sufficiently different that their rates of exchange in the intact protein can be separately monitored. Both rates decrease on binding S-peptide and decrease by an additional factor of ten on binding 2′CMP. The amide of Vall24 is hydrogen bonded as part of a β-sheet region in the native structure. The amide of Ser123 is not hydrogen-bonded, is directed towards the solvent, but is not accessible to it in the time average structure of the intact enzyme. The main-chain motion required to permit exchange is such that both amides are affected in the same way in spite of their different environments.

Damped orientational diffusion model of polymer local main-chain motion. 2. Application to poly(vinyl acetate)

The damped diffusion model presented in the previous paper is applied to the 'H NMR relaxation rates and dielectric loss data of poly(viny1 acetate) in toluene. The results are compared to those obtained from the cutoff diffusion model of Bendler and Yaris and the model of Valeur, Jarry, GBny, and Monerie applied to the same system. The damped and cutoff diffusion models give identical fits to experiment. The model of Valeur et al. also fits the experimental results but only at the expense of nonphysical values for the parameters. A tentative interpretation of the physical origin of the molecular motion is presented.

Liquid crystalline side chain polymers and their behaviour in the electric field

AbstractThe electro‐optical properties of liquid crystalline polymers with polar mesogenic units in the side chain were investigated. It could be shown, that in an electric field, they show all effects known from low molecular weight liquid crystals, e.g. the Freedericksz‐transition, the DAP‐effect, the formation of William's domain, and the dynamic scattering. The optical response times are slightly longer than for low molecular weight liquid crystals, but in some cases at higher temperatures as short as 200 ms. By comparison of different polymers and some corresponding liquid crystalline monomers, it is found that: (1) the difference between the measuring temperature and the glass transition temperature mainly determines the optical response times, (2) the fixation of mesogenic groups to a polymer main chain via spacers influences the threshold voltage. Low values are found for long spacers. These results allow a discussion of the influence of polymer fixation of the mesogenic groups on their liquid crystalline properties, pointing to a partial decoupling of the motions of main chains and side chains.

Proton relaxation studies of dynamics of proteins in the solid state

Dynamical information concerning proteins and other macromole— cules in the solid state may be obtained from investigations of proton magnetic relaxation. Measurements have been made of T1 for protons at 18, 30 and 60 MHz on polycrystalline ct—chymotrypsin, insulin, lysozyme and ribonuclease A in the temperature range 10 to 300 K. Analysis identifies the predominant source of relaxation between 70 and 250 K as methyl group reorientation in amino acid sidechains. Activation parameters character— izing a distribution of methyl rotors are deduced. Deuteration gives information concerning the relaxation contribution of water and other exchangeable protons. Investigations were extended to solid polymers devoid of methyl groups such as polyglycine and poly—L—proline to give information concerning relaxation contributions from main chain motions and ring puckering. The measurement of dipolar relaxation enables slower motions to be investigated. Measurements of T1D in solid a—chymotrypsin and solid lysozyme exhibit behaviour similar to T1 but displaced about 100 K lower in temperature, and enable other dynamical effects to be obser— ved at higher temperatures.

Separation of the motion around cross-link points from main chain motion in network samples

High-resolution neutron scattering experiments on deuterium-labelled model trifunctional networks show that the junction points move more slowly than the free chain centres by a factor of about two.

E.s.r. study of annealing and melting effects on the molecular motion of spin trapped chain-end radicals located on the polyethylene surface

E.s.r. spectra of chain-end spin labels on the surface of polyethylene powder were obtained for high and low density polyethylene samples at various stages of heat treatment process. The subsequent crystalline structures of the samples were examined with wide angle and small angle X-ray diffraction spectra. The effects of annealing on the motion of the labels were dependent on annealing temperature and the type of the samples. Higher temperature annealing decreased the rotational correlation times ( τ c ) and their activation energy in high density polyethylene but did not affect those of low density polyethylene. Melting decreases the values of τ c and ΔE for both high an low density polyethylene samples. The cause of these decreases were attributed to the changing of the sites of the chain-end labels. The motion of the labels appeared to be influenced by the local segmental motion of the polymer main chains.

Dielectric Study on Cooperative Motion of Side Chain of Copoly(γ-methyl L-glutamate, γ-p-chlorobenzyl L-glutamate) in the Solid State

Dielectric measurements on copoly(γ-methyl L-glutamate, γ-p-chlorobenzyl L-glutamate) [copoly(MLG, pCIBLG)] were performed in the frequency range of 0.03 to 300 kHz and over the temperature range from −150 to +130°C in order to study the side-chain motion. Two side-chain relaxations related respectively to MLG and pClBLG residues were observed. The results imply that there exists a cooperative motion of the side chains of poly(α-amino acid), though not as much as the main-chain motion of conventional amorphous polymers. The relaxation related to MLG residues is caused by clusters of MLG residues that are not affected by interactions between pC1BLG residues.

Mechanical relaxations in polybutene-1 and poly-4-methylpentene-1

Dynamic mechanical measurements between — 180°C and 180°C were made on both isotropic and drawn samples of polybutene-1 (PB-1) and poly-4-methylpentene-1 (P4MB1) over a wide frequency range by the use of a torsional pendulum (0.3–3 Hz), a viscoelastic spectrometer (5–90 Hz) and ultrasonic technique (3 MHz). The relaxation peaks were identified and the associated activation energies determined from Arrhenius plots. For PB-1 it was observed that orientation reduces the height and shifts up the temperature of the α a -peak associated with large scale main-chain motion in the amorphous regions, but has little effect on the β-peak associated with side-group motion. In addition to the α a and β relaxations a high-temperature crystalline relaxation ( α c ) is also observed in P4MP1. For both the α c and β relaxations the mechanical loss at 45° to the draw direction is much larger than that at 90°, which indicates that shear processes are involved in these relaxations.