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1.
Fourier transform mechanical spectroscopy technique (FTMS) is utilized as a powerful tool to study the sol-gel transition
of covalent bonded polymeric network. Winter and Chambon criteria resulting from the fractal-geometry characteristic of the
gel networks allow the determination of the gel point with only single experiment using this technique. The gelation behaviors
of low melt viscosity ternary systems of benzoxazine, epoxy, and phenolic resins are investigated and analyzed by the technique
in order to study the effect of epoxy diluent on the rheological property development before and after the gel points. The
gel time at 140 °C ranges from 5 min to 30 min and less than 5 min at 180 °C for all tested ternary system compositions. The
gelation of the ternary mixture shows an Arrhenius-type behavior and the gel time can be well-predicted by the Arrhenius equation.
Received: 23 November 1999 Accepted: 2 January 2001 相似文献
2.
Hideyuki Uematsu Yuji Aoki Masataka Sugimoto Takashi Taniguchi Kiyohito Koyama 《Rheologica Acta》2008,47(2):237-242
We investigated the dynamic viscoelasticity and elongational viscosity of polypropylene (PP) containing 0.5 wt% of 1,3:2,4-bis-O-(p-methylbenzylidene)-d-sorbitol (PDTS). The PP/PDTS system exhibited a sol–gel transition (T
gel) at 193 °C. The critical exponent n was nearly equal to 2/3, in agreement with the value predicted by a percolation theory. This critical gel is due to a three-dimensional
network structure of PDTS crystals. The elongational viscosity behavior of neat PP followed the linear viscosity growth function
3η
+
(t), where η
+
(t) is the shear stress growth function in the linear viscoelastic region. The elongational viscosity of the PP/PDTS system
also followed the 3η
+
(t) above T
gel but did not follow the 3η
+
(t) and exhibited strong strain-softening behavior below T
gel. This strain softening can be attributed to breakage of the network structure of PDTS with a critical stress (σ
c) of about 104 Pa. 相似文献
3.
Soumitra Choudhary Joseph C. White Whitney L. Stoppel Susan C. Roberts Surita R. Bhatia 《Rheologica Acta》2011,50(1):39-52
We report the preparation and rheological characterization of interpenetrating polymer network (IPN) hydrogels made from alginate
and hydrophobically modified ethyl hydroxyl ethyl cellulose (HMEHEC). To our knowledge, there have been no studies of the
gelation behavior of IPNs. We found that the rheology of these systems can be easily tuned, with the elastic modulus of the
IPN strongly dependent on the relative ratio of HMEHEC to alginate. The sol–gel transition of these systems was found to satisfy
the Winter–Chambon criterion for gelation at various crosslinker densities. From the power law relationship of the dynamic
moduli (G
′ ~G
″ ~ω
n), the exponent n appears to be dependent on both the crosslinker density and relative amount of two polymers. The value of n was found to be ~0.5 for all samples for stoichiometric amounts of crosslinker. The effect of molecular weight of HMEHEC
on the gel point and viscoelastic exponent has also been reported. Alginate seems to dominate the kinetics of the process
but the effect of high molecular weight HMEHEC on the gel point, especially at lower proportion was also evident. 相似文献
4.
In a previous work, we have shown that chitosan true physical gelation occurs in some organic and inorganic acids (Hamdine et al. 2004). Two systems presenting similar gelation mechanisms were characterized furthermore in order to investigate the sol–gel transition: the chitosan–phosphoric acid and the chitosan–oxalic acid systems. By performing rheological measurements in the framework of linear viscoelasticity, we have investigated the effect of time, temperature, and polymer concentration on the gelation evolution. For both acid-based systems, gelation occurred above a critical polymer concentration around 5% w/v (g/100 ml) of chitosan. Isothermal time sweep experiments showed that the gelation occurs in three stages: (i) incubation; (ii) rapid increase of G′; and (iii) a last stage where G′ slowly reached its equilibrium value due to slow molecular diffusion. At the gel point, G′ and G′′ scaled with ω
n
, with n=0.55 for both acid-based systems and a fractal dimension d
f of 1.9. Cooling–heating cycles revealed that the gels showed thermoreversibility after one sequence, but became permanent during subsequent cycles.This revised version was published online in October 2005 with corrections to the author's name. 相似文献
5.
A power law distribution of relaxation times, large normal stress differences, and physical rupture of molecular network strands dominate the shear behavior of polymers at the gel point (critical gels). This is shown in a series of well-defined experiments with increasing magnitude of shear on a model-network polymer system consisting of a linear, telechelic, vinyl-terminated poly-dimethylsiloxane (PDMS) and a four-functional siloxane crosslinker. Stable samples were prepared by stopping the crosslinking reaction at different extents of reaction in the vicinity of the gel point (GP). The Gel Equation has been shown to be valid up to strains of about 2 when using a finite strain tensor. Larger strains have been found to disrupt the network structure of the crosslinking polymer, and introduce a mechanical delay to the gel point. A sample that was crosslinked beyond the gel point (p>p
c
) can be reduced from the solid state to a critical gel, or even to a viscoelastic liquid, depending on the magnitude of shear strain. As a consequence, the relaxation exponent of a critical gel created under the influence of shear is less than that of a quiescently crosslinked critical gel. 相似文献
6.
Yuji Aoki Kentaro Hirayama Koji Kikuchi Masataka Sugimoto Kiyohito Koyama 《Rheologica Acta》2010,49(10):1071-1076
A poly(vinyl chloride) (PVC, Mw = 102×103)(\mbox{PVC,}\;{\rm M}_{\rm w} =102\times 10^3) di-octyl phthalate (DOP) gel with PVC content of 20 wt.% was prepared by a solvent evaporation method. The dynamic viscoelsticity
and elongational viscosity of the PVC/DOP gel were measured at various temperatures. The gel exhibited a typical sol–gel transition
behavior with elevating temperature. The critical gel temperature (Tgel) characterized with a power–law relationship between the storage and loss moduli, G′ and G″, and frequency ω, G¢=G¢¢/tan ( np/2 ) μ wn{G}^\prime={G}^{\prime\prime}{\rm /tan}\;\left( {{n}\pi {\rm /2}} \right)\propto \omega ^{n}, was observed to be 152°C. The elongational viscosity of the gel was measured below the Tgel. The gel exhibited strong strain hardening. Elongational viscosity against strain plot was independent of strain rate. This
finding is different from the elongational viscosity behavior of linear polymer solutions and melts. The stress–strain relations
were expressed by the neo-Hookean model at high temperature (135°C) near the Tgel. However, the stress–strain curves were deviated from the neo-Hookean model at smaller strain with decreasing temperature.
These results indicated that this physical gel behaves as the neo-Hookean model at low cross-linking point, and is deviated
from the neo-Hookean model with increasing of the PVC crystallites worked as the cross-linking junctions. 相似文献
7.
Electro-rheological suspensions (ERS) are known to undergo liquid-to-solid transition under the application of an electric
field. Long-range interaction between neighboring particles results in sample-spanning particulate structures which behave
as soft solids. Here, we studied the rheological expression of this field-induced transition which has many similarities with
chemical gelation. This similarity shows in mechanical spectroscopy on a suspension of monodisperse silica in PDMS as model
ERS. Upon application of the electric field, dynamic moduli G′, G′′ grow by orders of magnitude and evolve in a pattern which is otherwise typical for gelation of network polymers (random
chemical or physical gelation). At the gel point, the slow dynamics is governed by power-law relaxation behavior (frequency-independent
tan δ). A low field strength is sufficient to reach the gel point and, correspondingly, the percolating particle structure
at the gel point is still very fragile. It can be broken by the imposition of low stress. For inducing a finite yield stress,
the field strength needs to be increased further until the long-range electrostatic interaction generates string-like particle
alignments which become clearly visible under the optical microscope. The onset of fragile connectivity was defined experimentally
by the tan δ method. The ERS was probed dynamically at low frequencies where the transition is most pronounced, and also in
steady shear where the rate of structure formation equals the rate of internal breaking.
Received: 1 May 2001 Accepted: 11 August 2001 相似文献
8.
Loredana Elena Niţă Aurica P. Chiriac Maria Bercea Iordana Neamţu 《Rheologica Acta》2007,46(5):595-600
The effects of heating rate and shear stress on the sol–gel transition for a polyacrylamide gel during in situ preparation
were investigated by dynamic rheology. The gelation evolution was also studied through monitoring of the static gelation process
by means of temperature, pH, and conductivity variation. The gels were prepared by polymerization of acrylamide crosslinked
with N,N′-methylenebisacrylamide using a redox initiation based on potassium persulfate/ascorbic acid. In situ gelation process was
studied using oscillatory deformation tests at constant frequency of 0.1 Hz for different heating rate (from 0.5 to 5 °C/min)
and shear stress (in the range of 0.1 to 10 Pa).
The paper was presented at the third annual rheology conference, AERC 2006, Crete, Greece, 27–29 April 2006. 相似文献
9.
Linear viscoelastic properties of SiO2/(AP/EP) suspension with various SiO2 volume fractions (ϕ) in a blend of acrylic polymer (AP) and epoxy (EP) were investigated at various temperatures (T). The AP/EP contained 70 vol.% of EP. The SiO2 particles were treated with epoxy silane coupling agent. The effects of the SiO2 particles are more pronounced in the terminal zone: a transition from viscoelastic liquid (ϕ ≤ 30 vol.%) to viscoelastic solid (ϕ ≥ 40 vol.%) was observed which can be interpreted as a critical gelation occurring at a critical particle content and critical
gel temperature. The SiO2/(AP/EP) systems exhibited a critical gel behavior at ϕ ≅ 35 vol.% and T ≅ 100°C characterized with a power–law relationship between the storage and loss moduli (G
′ and G
″) and frequency (ω); G
′ = G
″/tan(nπ/2) ∝ ω
n
. The critical gel exponent (n) was estimated to be about 0.45. The gelation occurred with increasing T. 相似文献
10.
The crosslinking behaviour of a silicone resin which is interesting from a technical point of view was investigated by means
of rheology. In order to accelerate the crosslinking process, zinc acetylacetonate and aluminium acetylacetonate were applied
as latent catalysts. The effect of the type of catalyst, its concentration, and the temperature on crosslinking was determined
by isothermal dynamic-mechanical measurements. A radial gradient in crosslinking causes the gel point to be reached earlier
at the outer edge of the sample in the rheometer. This radial gradient is averaged when measuring G′ and G″. Therefore, since the physically well-defined state of critical gelation (gel point) could not be obtained from the data,
the time at which a distinct crosslinking state is reached was determined by the crossover of the moduli G′ and G″. For this distinguished point, the denotation gelation index GI is introduced. The gelation indices measured at different
temperatures follow an Arrhenius-type relationship. Activation energies between 89 and 126 kJ/mol were determined. They were
found to be dependent on the type of catalyst used but independent of its concentration. The activation energies of the crosslinking
processes enable the calculation of the gelation index at temperatures not measured directly. 相似文献
11.
We study the flow of yield stress fluids over a rotating surface when both the viscoelastic solid behavior below a critical
deformation (γ
c) and liquid properties beyond γ
c can play a significant role. We review the detailed characteristics of the flow in the solid regime in the specific case
of a pure elongational strain (large height to radius ratio). We, in particular, show that there exists a critical rotation
velocity (ω
c) associated with the transition from the solid to the liquid regime. We then consider the specific case of lubricational
regime (small height to radius ratio) in the liquid regime. In that case we describe the different possible evolutions of
the equilibrium shape of the material as a function of the rotation velocity (ω), from which we extrapolate the transient shape evolutions as ω increases. We show that for a sufficiently large rotation velocity the sample separates into two parts, one remaining at
rest around the rotation axis, the other going on moving radially. These predictions are then compared with systematic spin-coating
tests under increasing rotation velocity ramps followed by a plateau at ω
f with typical yield stress fluids. It appears that there exists a critical velocity below which the material undergoes a limited
elongation and beyond which it starts to spread significantly over the solid surface. For a larger ω
f value the sample forms a thick peripheral roll, leaving behind it a thin layer of fluid at rest relatively to the disc. These
characteristics are in qualitative agreement with the theoretical predictions. Beyond a sufficiently large ω
f value this roll eventually spreads radially in the form of thin fingers. Moreover, in agreement with the theory in the lubricational
regime, the different curves of deformation vs ω fall along a master curve when the rotation velocity is scaled by ω
c for different accelerations, different sample radii, or different material yield stress. The final thickness of the deposit
seems to be mainly governed by the displacement of the roll, the characteristics of which take their origin in the initial
stage of the spreading, including the solid–liquid transition. 相似文献
12.
A polystyrene-b-poly-4-vinypyridine (PS-b-P4VP) diblock copolymer is modified with a gold precursor to obtain an organic–inorganic (hybrid) block copolymer in bulk
with gold nanoparticles selectively incorporated in the P4VP block. In the linear viscoelastic regime, temperature sweep tests
over a series of these hybrid block copolymer systems revealed consistent shifts (ΔT) in the glass transition temperatures (both T
g\text-PS_{\rm g\text{-}PS} and T
g\text-P4VP_{\rm g\text{-}P4VP}) of the hybrid materials in comparison to the pristine polymers. Studying different volume fractions of the pyridine block,
a level-off point was found for block copolymers with f
P4VP > 0.26, where the shifts in T
g\text-P4VP_{\rm g\text{-}P4VP} consistently increased up to ΔT = 25°C. By artificially increasing the volume fraction of the pyridine block, the nanoparticles reduce the transition regime
determined in master curves. At higher volume fractions of the pyridine block, crossover frequencies were not detected after
the entanglement regime, indicating that the material does not relax from topological constraints (entanglements and nanoparticles)
into the terminal regime. Above a specific volume fraction of nanoparticles (Φ
P = 0.05), the flow behaviour of the hybrid materials becomes increasingly elastic, exhibiting wall-slip from the geometry
at lower strain values in comparison to the pristine material. In the non-linear viscoelastic regime, Fourier-transformed
rheology was used to analyse the raw signals from strain sweep experiments. It was clearly demonstrated the nanoparticle effect
by following the second and third harmonic (I
2/1, I
3/1) of the stress response. Comparing the behaviour of the third and second harmonics provided an unambiguous fingerprint for
the effect of the nanoparticles. 相似文献
13.
A linear elastic three-dimensional finite element analysis is made to analyze the near field stress behavior of an edge cracked rectangular bar simply supported and subjected to central impact at the back side of the crack. The material is made of 40 Cr steel. Determined numerically are the local time histories of the stress wave, displacement near load point, crack tip strain, and dynamic stress intensity factor K(d)1. The above quantities were also measured experimentally by performing impact tests; they agreed well with the analytical results and determine the load at fracture initiation and hence the critical dynamic stress intensity factor K(d)1c. The interaction effect between the loading bar and specimen appears to be negligible. 相似文献
14.
Oscillatory rheological experiments at different temperatures and over a wide range of frequencies have been used to investigate
the gelation process and, more particularly, the sol–gel transition of various poly(vinyl chloride) (PVC) plastisols. The
sol–gel transition process was found to be universal with respect to the temperature and solid volume fraction according to
the similarity of the fractal structure in PVC plastisols. The variation of the gel time (t
gel) with temperature for any composition of PVC plastisols was predicted from the Dickinson’s model (E. Dickinson, J Chem Soc
Faraday Trans, 93:111–114, 1997). Dynamic viscoelastic properties of PVC plastisols have also been studied as a function of
temperature that allowed us to follow the gelation process of various plastisols. Thus, the influence of the type and concentration
of PVC resins in gelation process was investigated. The variation of the complex shear modulus at a constant frequency was
depicted by a master curve regarding the dependence of the moduli on PVC concentrations. 相似文献
15.
Linear and nonlinear stability analyses were performed on a fluid layer with a concentration-based internal heat source. Clear
bimodal behaviour in the neutral curve (with stationary and oscillatory modes) is observed in the region of the onset of oscillatory
convection, which is a previously unobserved phenomenon in radiation-induced convection. The numerical results for the linear
instability analysis suggest a critical value γ
c
of γ, a measure for the strength of the internal heat source, for which oscillatory convection is inhibited when γ > γ
c
. Linear instability analyses on the effect of varying the ratio of the salt concentrations at the upper and lower boundaries
conclude that the ratio has a significant effect on the stability boundary. A nonlinear analysis using an energy approach
confirms that the linear theory describes the stability boundary most accurately when γ is such that the linear theory predicts the onset of mostly stationary convection. Nevertheless, the agreement between the
linear and nonlinear stability thresholds deteriorates for larger values of the solute Rayleigh number for any value of γ. 相似文献
16.
Masataka Sugimoto Hirokazu Hida Takashi Taniguchi Kiyohito Koyama Yuji Aoki 《Rheologica Acta》2007,46(7):957-964
Poly(vinyl chloride) (PVC)/di-isononyl phthalate systems with PVC content of 45.5 (PVC8) and 70.4 wt% (PVC6) were prepared
by a hot roller at 150 °C and press molded at 180 °C. The dynamic viscoelasticity and elongational viscosity of PVC8 and PVC6
were measured in the temperature range from 150 to 220 °C. We have found that the storage and loss shear moduli, G′ and G″, of PVC8 and PVC6 exhibited the power-law dependence on the angular frequency ω at 190 and 210 °C, respectively. Correspondingly, the tan δ values did not depend on ω. These temperatures indicate the critical gel temperature T
gel of each system. The critical relaxation exponent n obtained from these data was 0.75 irrespective of PVC content, which was in agreement with the n values reported previously for the low PVC concentration samples. These results suggest that the PVC gels of different plasticizer
content have a similar fractal structure. Below T
gel, the gradual melting of the PVC crystallites takes place with elevating temperature, and above T
gel, a densely connected network throughout the whole system disappears. Correspondingly, the elongational viscosity behavior
of PVC8 and PVC6 exhibited strong strain hardening below T
gel, although it did not show any strain hardening above T
gel. These changes in rheological behavior are attributed to the gradual melting of the PVC crystallites worked as the cross-linking
domains in this physical gel, thereby inapplicability of the of time–temperature superposition for PVC/plasticizer systems. 相似文献
17.
Etienne Ghiringhelli Denis Roux Didier Bleses Helene Galliard Francois Caton 《Rheologica Acta》2012,51(5):413-420
A new time-optimal rheometry technique is presented. It consists in applying to the material a continuous exponential frequency
sweep and analyzing its response by means of Fourier transforms. The properties of this method are that it takes the least
possible time to perform the linear viscoelastic measurement in a given frequency range, and also presents an optimal signal
to noise ratio in Fourier space. After validation against classical methods, it is used to characterize the gelation of a
dental alginate. Properly time-resolved measurements of the frequency dependent viscoelastic modulii are presented. This allows
to evaluate rigorously the gel point, using the Winter and Chambon criterion. Analysis of the data shows that a fractal networks
forms inside the material after a lag time of about 350s, with a subsequent fast evolution (less than two minutes) towards
the final structure. 相似文献
18.
Maik Nowak 《Rheologica Acta》2001,40(4):366-372
The first normal stress differences N
1 of a highly dilute cationic surfactant solution are investigated in a cone-and-plate rheometer. In continuation of a previous
paper (Nowak 1998), where the buildup of a shear induced structure in such a solution was attained after a reduced deformation,
the N
1 turned out to be in proportion to the square of the shear rate γ˙ reduced by a critical value γ˙
c
in a first range above γ˙
c
. At higher shear rates the N
1 tend to lower values than predicted by this relation.
Relaxation experiments were performed in the same geometry to determine the characteristic time scales of the shear induced
state's decay. In the lower range above &γdot;
c
the stress decay is a monoexponential process, while a second time constant has to be introduced to describe the relaxation
in that range, where the N
1 deviate from the parabolic dependence of the reduced shear rate.
Received: 10 May 1999 Accepted: 15 November 2000 相似文献
19.
Christian Sailer Martin Weber Helmut Steininger Ulrich A. Handge 《Rheologica Acta》2009,48(5):579-588
In this work, we studied the melt rheology of multigraft copolymers with a styrene–acrylonitrile maleic anhydride (SANMA)
terpolymer backbone and randomly grafted polyamide 6 (PA 6) chains. The multi-grafted chains were formed by interfacial reactions
between the maleic anhydride groups of SANMA and the amino end groups of PA 6 during melt blending. Because of the phase separation
of SANMA and PA 6, the grafted SANMA backbones formed nearly circular domains which were embedded in the PA 6 melt with a
diameter in the order of 20 to 40 nm. The linear viscoelastic behaviour of PA 6/SANMA blends at a sufficiently large SANMA
concentration displayed the characteristics of the critical gel state, i.e. the power relations G′ ∝ G′′ ∝ ω
0.5. In elongation, the PA 6/SANMA blend at the critical gel state showed a non-linear strain hardening behaviour already at
a very small Hencky strain. In contrast to neat PA 6, the elasticity of the PA 6/SANMA blends was strongly pronounced, which
was demonstrated by recovery experiments. Rheotens tests agreed with the linear viscoelastic shear oscillations and the measurements
using the elongational rheometer RME. Increasing the SANMA concentration led to a larger melt strength and a reduced drawability.
The occurrence of the critical gel state can be interpreted by the cooperative motion of molecules which develops between
the grafted PA 6 chains of neighbouring micelle-like SANMA domains. 相似文献
20.
Physical gelation can be induced in various organic and silicone-based liquids, as well as in polymeric melts, upon addition
of 1,3:2,4-dibenzylidene sorbitol (DBS). Such gels are stabilized by the formation of a percolated DBS network composed of
highly interconnected nanofibrils. In this study, we explore several factors affecting the rheological properties of poly(propylene
glycol) (PPG) gelled by DBS. To ascertain the effect of PPG molecular weight (MPPG) on gel formation and rheology, we have investigated three series of DBS-induced PPG gels in which MPPG varies from 425 g/mol to 4000 g/mol. Dynamic stress measurements reveal that the DBS concentration identifying the onset
of PPG gelation decreases with increasing MPPG. Since the solubility parameter (δs) of PPG decreases sharply as MPPG increases over this MPPG range, this observation suggests that DBS gelation is sensitive to δs of the matrix liquid, in agreement with previously reported data collected from DBS-gelled solvents. Moreover, the elastic
modulus and yield stress are found to increase with increasing (i) DBS concentration for the three series of PPG/DBS gels
examined here and (ii) recovery time after cessation of an introductory shear.
Received: 22 February 2000 Accepted: 9 August 2000 相似文献