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1.
Two distinct single-chain particles of poly(N-isopropylacrylamide) (PNIPAM) in the state of loose coil and compact globule, have been prepared successfully below and above the lower critical solution temperature (LCST) in extreme dilute aqueous solution by the freeze-drying method, respectively. During the preparation of the compact globular single-chain sample, the surfactant of sodium n-dodecyl sulfate (SDS) was added into the system to prevent aggregation of globular single chains formed at a temperature above the LCST. After all the coil has been transformed into the compact globular particle, the SDS molecules were removed by dialysis. The glass transition temperature (Tg) of the two single-chain samples has been measured by differential scanning calorimetery (DSC) in comparison with that of bulk polymer. It was found that the Tg of the single-chain sample in compact-globule state was very near to that of the bulk polymer, whereas the Tg of the single-chain sample in loose-coil state was approximately 6 K lower than that of the bulk polymer. After treating the sample with repeated DSC cycles, the Tg of the single-chain sample in loose-coil state rose up successively near to that of the bulk polymer. These results have been explained in terms of the effect of entanglement on the mobility of the polymer segments in the two distinct single-chain samples. 相似文献
2.
Small-angle X-ray scattering was used to study the mixture of C12E5 (pentaethylene glycol monododecyl ether)/H2O/n-decane microemulsion and polyethylene glycol (PEG). The size, shape and the structure factor of the microemulsion were investigated by adding the polymer (PEG) to the mixture. Attractive depletion potential was induced between the microemulsion droplets by the non-adsorb polymer. The range and strength of the attractive potential were changed by varying the molecular weight and concentration of PEG. The forward scattering, S(0), of the spherical microemulsion, declined gradually as the polymer concentration decreased. For PEG with the molecular weight of Mn = 285?315, the microemulsion morphology remained spherical, but the main peak of the structure factor moved towards a bigger q. When PEG with molecular weights of Mn = 2200 and Mn = 6000 were used, a shape transition from spherical to cylindrical was induced in line with increasing polymer concentration. 相似文献
3.
Erichsen J Shiferaw T Zaporojtchenko V Faupel F 《The European physical journal. E, Soft matter》2007,24(3):243-246
Using the cluster-embedding method of V. Zaporojchenko et al. (Macromolecules 34, 1125 (2000)), we measured the glass transition temperature T
g at the polystyrene/vacuum interface of bimodal mixtures of monodisperse polystyrenes of 3.5k and 1000k. Embedding of ≈ 1
nm Au clusters was monitored in situ by X-ray photoelectron spectroscopy (XPS). The clusters were formed by evaporation of Au onto the polymer surface. Only one
glass transition was observed in the mixtures. The surface glass transition temperatures are correlated to but are below the
bulk values of the mixtures and obey the Gordon-Taylor equation. The results suggest that the earlier reported molecular-weight
dependence of the surface glass transition is not due to segregation of short chains to the surface. 相似文献
4.
Living polymers are formed by reversible association of primary units (unimers). Generally the chain statistical weight involves
a factor σ < 1 suppressing short chains in comparison with free unimers. Living polymerization is a sharp thermodynamic transition
for σ ≪ 1 which is typically the case. We show that this sharpness has an important effect on the kinetics of living polymerization
(one-dimensional association). The kinetic model involves i) the unimer activation step (a transition to an assembly-competent
state); ii) the scission/recombination processes providing growth of polymer chains and relaxation of their length distribution.
Analyzing the polymerization with no chains but unimers at t = 0 , with initial concentration of unimers M ≳ M
* (M* is the critical polymerization concentration), we determine the time evolution of the chain length distribution and find
that: 1) for M
* ≪ M ≪ M
*/σ the kinetics is characterized by 5 distinct time stages demarcated by 4 characteristic times t1, t2, t3 and t*; 2) there are transient regimes (t
1 ≲ t ≲ t
3) when the molecular-weight distribution is strongly non-exponential; 3) the chain scissions are negligible at times shorter
than t2. The chain growth is auto-accelerated for t
1 ≲ t ≲ t
2 : the cut-off chain length (= polymerization degree 〈n〉w
N
1 ∝ t
2 in this regime. 4) For t
2 < t < t
3 the length distribution is characterized by essentially 2 non-linear modes; the shorter cut-off length N1 is decreasing with time in this regime, while the length scale N2 of the second mode is increasing. (5) The terminal relaxation time of the polymer length distribution, t*, shows a sharp maximum in the vicinity of M*; the effective exponent
is as high as ∼ σ-1/3 just above M*. 相似文献
5.
Free energy of semiflexible polymers and structure of interfaces 总被引:1,自引:0,他引:1
The free energy of semiflexible polymers is calculated as a functional of the compositional scalar order parameter and the orientational order parameter of second-rank tensor Sij on the basis of a microscopic model of wormlike chains with variable segment lengths. We use a density functional theory
and a gradient expansion to evaluate the entropic part of the free energy, which is given in a power series of .The interaction term of the free energy is derived with a random phase approximation. For the rigid rod limit, the nematic-isotropic
transition point is given by , N and w being the degree of polymerization and the anisotropic interaction parameter, respectively, and the degree of ordering at
the transition point is 0.33448. We also find that the contour length of polymer chains becomes larger in a nematic phase
than in an isotropic phase. Interface profiles are obtained numerically for some typical cases. In the neighborhood of isotropic-isotropic
interfaces, polymer chains tend to align parallel to the interface on the polymer-rich side and perpendicular on the poor
side. When an isotropic region and a nematic region coexist, orientational order parallel to the interface is preferred in
the nematic region.
Received: 28 May 1998 / Revised: 12 August 1998 / Accepted: 8 September 1998 相似文献
6.
Koga T 《The European physical journal. E, Soft matter》2005,17(3):381-388
The formation of intramolecular micelles in copolymers with periodic sequence, where hydrophobic units (stickers) are periodically placed along the chain, is studied by using multicanonical Monte Carlo computer simulations for an off-lattice bead-rod model in three dimensions. With decreasing the temperature, a transition from random-coil conformations to micelles occurs and flower-type micelles are formed via the transition. The number of stickers forming a micelle core is limited by the excluded-volume effect of loop chains around micelle cores. By this effect, two intramolecular micelles are formed for long polymer chains with 60 bonds via the coil-to-micelle transition. By further decreasing the temperature, we find that another transition, i.e., a micelle-to-micelle transition, takes place. At this transition point, the two intramolecular micelles merge into one micelle. Furthermore, we extend the multicanonical MC method to study elastic properties of single polymer chains with strong attractive interactions under external force fields, and study how the intramolecular micellization affects the elastic property of single polymer chains. 相似文献
7.
We re-examine here the theoretical study of the phase separation between phospholipids and grafted long polymer chains onto
a fluid membrane. The polymer chains are assumed to be anchored to the membrane by one extremity (anchor). The anchors are big amphiphile lipid molecules. The anchors and phospholipids forming the bilayer phase separate under
the variation of a suitable parameter (temperature, pressure, membrane environment, ...). To investigate the demixtion transition,
we elaborate a new approach that takes into account the membrane undulations. We show that these undulations have the tendency
to induce additional attractive forces between anchors, and consequently, the separation transition is accentuated and occurs
at high temperature. Quantitatively, we show that the membrane undulations contribute with an extra positive segregation parameter
χm > 0 , which scales as χm
κ-2 , where κ is the bending rigidity constant. Therefore, the attraction phenomenon between species of the same kind is significant
only for those membranes of small bending rigidity constant. Finally, the study is extended to the case where the lengths
of the anchored polymer chains are randomly distributed. To achieve calculations, we choose a length distribution of fractal form. The essential conclusion is that the polydispersity increases the size of domains alternatively rich in phospholipids
and anchors. 相似文献
8.
V. Fourmaux-Demange A. Brûlet F. Boué P. Davidson P. Keller J.P. Cotton 《The European physical journal. E, Soft matter》2000,1(4):301-317
We have studied the rheology and the conformation of stretched comb-like liquid-crystalline polymers. Both the influence of
the comb-like structure and the specific effect of the nematic interaction on the dynamics are investigated. For this purpose,
two isomers of a comb-like polymetacrylate polymer, of well-defined molecular weights, were synthesized: one displays a nematic
phase over a wide range of temperature, the other one has only an isotropic phase. Even with high degrees of polymerization
N, between 40 and 1000, the polymer chains studied were not entangled. The stress-strain curves during the stretching and relaxation
processes show differences between the isotropic and nematic comb-like polymers. They suggest that, in the nematic phase,
the chain dynamics is more cooperative than for a usual linear polymer. Small-angle neutron scattering has been used in order
to determine the evolution of the chain conformation after stretching, as a function of the duration of relaxation t
r. The conformation can be described with two parameters only: , the global deformation of the polymer chain, and p, the number of statistical units of locally relaxed sub-chains. For the comb-like polymer, the chain deformation is pseudo-affine:
is always smaller than (the deformation ratio of the whole sample). In the isotropic phase, has a constant value, while pincreases as tr. This latter behavior is not that expected for non-entangled chains, in which p varies as t
r
1/2 (Rouse model). In the nematic phase, decreases as a stretched exponential function of t
r, while p remains constant. The dynamics of the comb-like polymers is discussed in terms of living clusters from which junctions are
produced by interactions between side chains. The nematic interaction increases the lifetime of these junctions and, strikingly,
the relaxation is the same at all scales of the whole polymer chain.
Received 5 May 1999 and Received in final form 18 October 1999 相似文献
9.
Kostakis K Mourmouris S Charalabidis D Pitsikalis M 《The European physical journal. E, Soft matter》2003,10(1):55-63
Linear polyisoprenes having dimethylamine end groups were prepared by high vacuum anionic polymerization techniques using
3-dimethylaminopropyllithium as the initiator. The amine group was reacted with 2-cholesteryl-2-oxo-1,3,2-dioxaphospholane
to provide polymer chains having end zwitterionic groups chemically connected with cholesterol. The association behavior of
these end-functionalized polymers was studied in cyclohexane by low angle laser light scattering, dynamic light scattering,
and viscometry. The aggregation numbers, N
w were found to decrease by increasing the molecular weight of the precursor polymer, due to excluded volume repulsions. The
ability of cholesterol to form liquid crystal mesophases facilitated the association process leading to higher N
w values. The hydrodynamic behavior of the aggregates was similar to that of star polymers. The dependence of the N
w values on the molecular weight of the base polymer, the polydispersity of the associates and the absence of critical micelle
concentration, cmc are compatible with the linear head-packing model.
Received 29 April 2002 and Received in final form 13 November 2002 Published online: 11 March 2003 相似文献
10.
Petit M Daoudi A Ismaili M Buisine JM 《The European physical journal. E, Soft matter》2006,20(3):327-333
We report the effect of an anisotropic polymer network formed from an achiral photoreactive monomer in a short-pitch chiral
SmC* phase on the distortion and the unwinding of the helical structure of the ferroelectric phase. The electro-optical behaviour
and ferroelectric properties were experimentally determined for films containing various polymer concentrations. The critical
field, Eu, for the transition from the distorted structure to the homogeneous state was measured as a function of polymer concentration.
A linear increase of Eu
versus polymer concentration was observed, showing that the helical structure of the short-pitch SmC* phase was stabilized by the
polymer network. This behaviour was expected to be a consequence of the increase of the apparent elastic constants of the
ferroelectric liquid crystal stabilized by the anisotropic polymer network films. The polymer network morphology was investigated
using atomic-force microscopy, revealing a twisted structure of the polymer fibers. This twisted structure was transferred
onto a polymer network during the polymerization process within a short-pitch SmC* phase. The increase of the apparent elasticity
can then be interpreted by a strong interaction between polymer network and the liquid-crystal molecules. From our experimental
data, the coupling coefficient, Wp, characterizing this interaction was evaluated for all studied polymer concentrations. 相似文献
11.
Hartmann L Gorbatschow W Hauwede J Kremer F 《The European physical journal. E, Soft matter》2002,8(2):145-154
The molecular dynamics in thin films (18 nm-137 nm) of isotactic poly(methyl methacrylate) (i-PMMA) of two molecular weights
embedded between aluminium electrodes are measured by means of dielectric spectroscopy in the frequency range from 50 mHz
to 10 MHz at temperatures between 273 K and 392 K. The observed dynamics is characterized by two relaxation processes: the
dynamic glass transition (α-relaxation) and a (local) secondary β-relaxation. While the latter does not depend on the dimensions
of the sample, the dynamic glass transition becomes faster (≤2 decades) with decreasing film thickness. This results in a
shift of the glass transition temperature T
g to lower values compared to the bulk. With decreasing film thickness a broadening of the relaxation time distribution and
a decrease of the dielectric strength is observed for the α-relaxation. This enables to deduce a model based on immobilized
boundary layers and on a region displaying a dynamics faster than in the bulk. Additionally, T
g was determined by temperature-dependent ellipsometric measurements of the thickness of films prepared on silica. These measurements
yield a gradual increase of T
g with decreasing film thickness. The findings concerning the different thickness dependences of T
g are explained by changes of the interaction between the polymer and the substrates. A quantitative analysis of the T
g shifts incorporates recently developed models to describe the glass transition in thin polymer films.
Received 12 August 2001 and Received in final form 16 November 2001 相似文献
12.
We study isotropic-isotropic and isotropic-nematic phase transitions of fluid mixtures containing hard spherocylinders (HSC)
and added non-adsorbing ideal polymer chains using scaled particle theory (SPT). First, we investigate isotropic-nematic (I -N phase coexistence using SPT in the absence of polymer. We compare the results obtained using a Gaussian form of the orientational
distribution function (ODF) to minimize the free energy versus minimizing numerically. We find that formal numerical minimization gives results that are much closer to computer simulation
results. In order to describe mixtures of HSC plus ideal chains we studied the depletion of ideal chains around a HSC. We
analyze the density profiles of ideal chains near a hard cylinder and find the depletion thickness δ is a function of the
ratio of the polymer's radius of gyration Rg and the cylinder radius Rc. Our results are compared with a common approximation in which the depletion thickness is taken equal to the radius of gyration
of the polymer chain. We incorporate the correct depletion thickness into SPT and find that for R
g/R
c < 1.56 using ideal chains gives phase transitions at smaller polymer concentrations, whereas for R
g/R
c > 1.56 , which is a common experimental situation, the phase transitions are found at larger polymer concentrations with
respect to δ = R
g . The differences are significant, especially for R
g ≫ R
c , so we can conclude it is essential to take into account the properties of ideal polymer chains and the resulting depletion
near a cylinder. Finally, we present phase diagrams for rod-polymer mixtures which could be realized under experimental conditions. 相似文献
13.
E. Magyari 《Zeitschrift für Physik B Condensed Matter》1987,68(2-3):363-368
Heisenberg spin chains with ferromagnetic nearest-neighbor coupling and an easy axis of magnetisation are considered in the classical limit. The critical ratioa
c between the anisotropy coefficient and the exchange integral for which the 180°-Blochtype soliton undergoes a phase transition to an Ising-type soliton is calculated by an exact analytical algorithm. For chains including 2N=4,6,8,... spins, one finds a sequencea
c (2N) which converges rapidly to the valuea
c()=2/3 (e.g.a
c (12)=0.66664) marking the phase transition from a Bloch- to an Ising-structure of the soliton in an infinite chain.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday 相似文献
14.
Zhulina EB Birshtein TM Borisov OV 《The European physical journal. E, Soft matter》2006,20(3):243-256
We revise the classical Daoud-Cotton (DC) model to describe conformations of polymer and polyelectrolyte chains end-grafted to convex spherical and cylindrical surfaces.
In the framework of the DC model, local stretching of chains in the brush does not depend on the degree of polymerization of grafted chains, and the
polymer density profile follows a single-exponent power law. This model, however, does not correspond to a minimum in free
energy of the curved brush. The nonlocal (NL) approximation exploited in the present paper implies the minimization of the overall free energy of the brush and predicts
that the polymer density profile does not follow a single-exponent power law. In the limit of large surface curvature the
NL approximation provides the same scaling laws for brush thickness and free energy as the local DC model. Numerical prefactors are however different. Extra extension of chains in the brush interior region leads to larger
equilibrium brush thickness and lower free energy per chain. A significant difference between outcomes of the two models is
found for brushes formed by ionic polymers, particularly for weakly dissociating (p H-sensitive) polyelectrolytes at low solution salinity. 相似文献
15.
Théneau C Salmerón Sánchez M Rodríguez Hernández JC Monleón Pradas M Saiter JM Gómez Ribelles JL 《The European physical journal. E, Soft matter》2007,24(1):69-77
The enthalpy relaxation of polymer-silica nanocomposites prepared by simultaneous polymerization of poly(2-hydroxyethyl methacrylate)
(PHEMA) and tetraethyloxysilane, TEOS, a silica precursor, is investigated. Both the glass transition temperature, Tg, and the temperature interval of the glass transition, ΔT
g , increase as the silica content in the sample does. Structural relaxation experiments show that the temperature interval
in which conformational motions take place broadens as the silica content in the hybrid increases. A phenomenological model
based on the evolution of the configurational entropy during the structural relaxation process, the SC model, has been used
for determining the temperature dependence of the relaxation times during the process. The results show an increase of the
fragility of the polymer as the silica content increases, a feature that can be related to the broadening of the distribution
of relaxation times characterized by the β parameter of the stretched exponential distribution. On another hand the silica
content increase produces a significant change of the relaxation times in the glassy state. 相似文献
16.
This contribution demonstrates a synthesis of comb polymer consisting of a poly(epichlorohydrin) (PECH) backbone and poly(oxyethylene
methacrylate) side chains. Atom transfer radical polymerization (ATRP) was used to directly initiate the chlorine atoms of
PECH macroinitiator. The structure of comb polymer was characterized by nuclear magnetic resonance (1H nuclear magnetic resonance) and Fourier transform infrared (FT-IR) spectroscopy, presenting the successful “grafting from”
method using ATRP. The comb polymer was used as a polymer matrix for dissolving potassium iodide (KI) to prepare solid polymer
electrolyte. FT-IR spectroscopy indicates that the potassium salts are dissolved in the polymeric matrix due to coordination
interaction with the ether oxygens of graft copolymer. Differential scanning calorimetry showed that glass transition temperature
(T
g) of polymer electrolytes continuously increased with increasing salt concentration up to 15 wt.%, mostly due to coordinative
interactions between the potassium ions and the ether oxygens of polymer matrix. Ionic conductivity at room temperature increased
with increasing salt concentrations up to 5 wt.% (maximum ionic conductivity ~3.7 × 10−5 S/cm), after which it gradually decreased. 相似文献
17.
Emanuela Fioravanti Frédéric M. D. Vellieux Patricia Amara Dominique Madern Martin Weik 《Journal of synchrotron radiation》2007,14(1):84-91
Intense synchrotron radiation produces specific structural and chemical damage to crystalline proteins even at 100 K. Carboxyl groups of acidic residues (Glu, Asp) losing their definition is one of the major effects observed. Here, the susceptibilities to X‐ray damage of acidic residues in tetrameric malate dehydrogenase from Haloarcula marismortui are investigated. The marked excess of acidic residues in this halophilic enzyme makes it an ideal target to determine how specific damage to acidic residues is related to their structural and chemical environment. Four conclusions are drawn. (i) Acidic residues interacting with the side‐chains of lysine and arginine residues are less affected by radiation damage than those interacting with serine, threonine and tyrosine side‐chains. This suggests that residues with higher pKa values are more vulnerable to damage than those with a lower pKa. However, such a correlation was not found when calculated pKa values were inspected. (ii) Acidic side‐chains located in the enzymatic active site are the most radiation‐sensitive ones. (iii) Acidic residues in the internal cavity formed by the four monomers and those involved in crystal contacts appear to be particularly susceptible. (iv) No correlation was found between radiation susceptibility and solvent accessibility. 相似文献
18.
采用气相扩散方法将C60分子填充到单壁碳纳米管(SWNTs)中,制备出高填充比率的豆荚形纳米材料C60@SWNT,又称为peapod.用金刚石对顶砧(DAC)装置获得高压,在高压下同时利用紫外激光处理样品,通过激光和压力的共同作用研究了C60分子在碳管内的聚合相变.在21.5GPa高压下,同时紫外激光(325nm)照射30min后,拉曼光谱表明C60分子在碳管内发生了聚合,形成一维链状O相聚合结构,且该相变是不可逆的.
关键词:
60 peapod')" href="#">C60 peapod
紫外激光
高压
拉曼光谱 相似文献
19.
Polyacrylamide (PAAm) doped by κ-carrageenan (κC) gels were prepared with various amounts of κC varying in the range between
0 wt.% and 3 wt.%. Steady-state fluorescence (SSF) technique was employed for studying sol-gel transition and swelling of
PAAm-κC composite gels which were prepared by free-radical crosslinking copolymerization. Pyranine was introduced as a fluorescence
probe. Pyranine molecules start to bind to acrylamide polymer chains upon the initiation of the polymerization, thus the spectra
of the bonded pyranines shift to the shorter wavelengths. Fluorescence spectra from the bonded pyranines allow one to monitor
the sol-gel transition and to test the universality of the sol-gel transition as a function of some kinetic parameters like
polymer concentration. Observations around the gel point, t
c
for PAAm-κC composite gels showed that the gel fraction exponent β obeyed the percolation result for low κC (<2.0 wt. %) however classical results were produced at higher κC (>2.0 wt.%). On
the other hand, fluorescence intensity of pyranine was measured during in situ swelling process at various amounts of κC and
it was observed that fluorescence intensity values decreased as swelling is proceeded. Li-Tanaka equation was used to determine
the swelling time constants, τ and cooperative diffusion coefficients, D. 相似文献
20.
Abraham G. Alvarado Jorge Cortés Lourdes A. Pérez-Carrillo Martin Rabelero Jesús Arellano Juan Carlos Sánchez-Díaz 《Journal of Macromolecular Science: Physics》2016,55(11):1086-1098
The synthesis, by two sequential inverse microemulsion polymerizations, of interpenetrating polymer networks (IPN) formed by polyacrylamide (PAM) and poly(acrylic acid) (PAA) and their response to changes in pH and temperature are reported here. The temperature and pH responses of the IPN nanoparticles are compared with those of polyacrylamide and random copolymers of polyacrylamide and poly(acrylic acid) P(AM-co-AA) nanoparticles also made by inverse microemulsion polymerization. We found that only the IPN nanogels exhibited a sharp swelling increase with temperature associated with its Upper Consolute Solution Temperature, driven by hydrogen bonding interactions, and with pH, driven by electrostatic repulsions of the PAA carboxylic groups, especially at pHs larger than the pKa of the PAA. The ?-potentials of the PAM, P(AM-co-AA) and IPN nanogels were measured as a function of pH and temperature, to determine the effects of these two variables, which in turn, affected the swelling of the nanogels. Field emission scanning electron microscopy revealed that the IPN nanogels were spheroidal with sizes similar to those determined by dynamic light scattering. 相似文献