Light scattering and crossover critical phenomena in polymer solutions

Jacob, J.; Анисимов, М. А.; Sengers, J. V.; Dechabo, V. A.; Yudin, I. K.; Gammon, Robert W.

doi:10.1364/ao.40.004160

Cited by 27 publications

(30 citation statements)

References 33 publications

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“…Experimental technique, sample preparation, and experimental procedure have been described in detail elsewhere [18]. Five polystyrene samples (obtained from Polymer Laboratories Inc.) with a polydispersity in-…”

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confidence: 99%

Competition of mesoscales and crossover to tricriticality in polymer solutions

2002

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We show that the approach to asymptotic fluctuation-induced critical behavior in polymer solutions is governed by a competition between a correlation length diverging at the critical point and an additional mesoscopic length-scale, the radius of gyration. Accurate light-scattering experiments on polystyrene solutions in cyclohexane with polymer molecular weights ranging from 200,000 up to 11.4 million clearly demonstrate a crossover between two universal regimes: a regime with Ising asymptotic critical behavior, where the correlation length prevails, and a regime with tricritical theta-point behavior determined by a mesoscopic polymer-chain length. PACS: 64.75.+g; 61.25.Hg; 05.70.Jk Close enough to the critical point, the correlation length ξ of the fluctuations of the order parameter has grown so large that the microscopic and even the mesoscopic structure of fluids become unimportant: complex fluids become "simple". This feature is known as criticalpoint universality [1]. Within a universality class, determined by the nature of the order parameter, properly chosen physical properties of different systems exhibit the same near-critical behavior. All critical phase-separation transitions in fluids belong to the 3-dimensional Isingmodel universality class, as the order parameter (associated with density or/and concentration) is a scalar. However, in practice, the pure asymptotic regime is often hardly accessible. Even in simple fluids, like xenon and helium, the physical properties in the critical region show a tendency to crossover from Ising asymptotic behavior to mean-field behavior [2,3]. This crossover depends on the microscopic structure of the system, namely, on the range of interaction and on a molecular-size "cutoff". In simple fluids, crossover to mean-field critical behavior is never completed within the critical domain (which can be defined roughly as within 10% of the critical temperature): the "cutoff" length and the range of interactions are too short. In complex fluids, regardless of the range of interaction, the role of the cutoff is played by a mesoscopic characteristic length scale ξ D that is associated with a particular mesoscopic structure [4]. If the cutoff length is mesoscopic, it can compete with the correlation length ξ within the critical domain. The temperature at which the correlation length becomes equal to the structural length can be naturally defined as a crossover temperature between two regimes, namely, an Ising asymptotic critical regime and a regime determined by the nature of the mesoscopic structure of the complex fluid. In some complex fluids, like polymer solutions, it is possible to tune the structural length-scale and make it very large. If both lengths, the correlation length of the critical fluctuations associated with the fluid-fluid separation and the structural correlation length, diverge at the same point, this point will be a multicritical point. A perfect example of such a multicritical phenomenon appears in a polymer solution near the theta point. The theta p...

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Competition of mesoscales and crossover to tricriticality in polymer solutions

2002

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“…The correlation function for PS1 was found to be a single exponential and no additional mode was detected [11]. For this sample the values of qR g for the two scattering angles of 30…”

Section: 41+ementioning

confidence: 69%

“…5 , M w /M n = 1.02, φ c = 0.066, T c = 296.47 K) have been described in a previous publication [11].…”

Section: 41+ementioning

confidence: 93%

“…This fact, also reported in ref. [11], requires further investigation. The phenomenon of coupling of soft modes has also been observed in polymer blends [23] and in sheared polymer solutions [24].…”

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confidence: 98%

“…Our experiments reveal how the coupled critical dynamics in polymer solutions varies with increasing molecular weight. Moreover, making use of theoretical predictions previously developed for the static and dynamic crossover behavior from Ising to mean-field (tricritical) behavior, we are able to present a quantitative description of the phenomenon.The original light-scattering setup as well as measurements for a moderate-molecular-weight sample PS1 (M w =1.96·105 , M w /M n = 1.02, φ c = 0.066, T c = 296.47 K) have been described in a previous publication [11]. …”

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Dynamic crossover to tricriticality and anomalous slowdown of critical fluctuations by entanglements in polymer solutions

2002

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We have performed accurate dynamic light-scattering measurements near critical demixing points of solutions of polystyrene in cyclohexane with polymer molecular weight ranging from 200,000 to 11.4 million. Two dynamic modes have been observed, "slow" and "fast", that result from a coupling between diffusive relaxation of critical fluctuations of the concentration and visco-elastic relaxation associated with the entanglement network of the polymer chains. The coupling with the visco-elastic mode causes an additional slowdown of the critical mode on top of the uncoupled diffusion mode. By implementing crossover from critical to theta-point tricritical behavior for both static and dynamic properties, we are able to present a quantitative description of the phenomenon and to obtain a scaling of the visco-elastic parameters as a function of the molecular weight. 61.41.+eCritical phenomena in high-molecular-weight polymer solutions differ from critical phenomena in simple fluids. In polymer solutions, the thermodynamic properties and the static correlations near the critical point of mixing are determined by a competition of two mesoscale lengths, namely, the correlation length of critical fluctuations of the concentration (tuned by the distance to the critical point) and the radius of gyration of the polymer molecules (tuned by the molecular weight) [1,2]. In the asymptotic vicinity of the critical point, the correlation length becomes much larger than the radius of gyration and the polymer solution exhibits Ising critical behavior. With increase of the polymer molecular weight and, hence, of the radius of gyration, the range of asymptotic Ising critical behavior shrinks, ultimately yielding to theta-point tricritical behavior [2]. This competition of two mesoscales arises from a coupling between two different order parameters belonging to two different static universality classes, namely, one associated with phase separation and another one with self-avoiding-walk singularities of long polymer chains [3]. Such a competition of two mesoscales and, consequently, crossover from critical to multicritical behavior are expected to be a general feature of phase transitions with coupled order parameters [4]. Hence, it is natural to expect that the dynamic critical behavior near the theta point will be affected by a coupling between two soft dynamic modes associated with the two order parameters [5].In the present communication we report a study of dynamic correlations in near-critical solutions of polystyrene in cyclohexane at the critical volume fraction φ c as a function of temperature and of polymer molecular weight, ranging from 200,000 to 11.4 million. Specifically, we have found that starting with a molecular weight of about one million, two effective dynamic modes emerge, that result from a coupling between two soft critical modes, a diffusion mode (association with the decay of critical fluctuations) and a visco-elastic mode (associated with entanglements of long polymer chains). Very close to the critical point, as...

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State‐of‐the‐Art Analytical Methods for Assessing Dynamic Bonding Soft Matter Materials

et al. 2014

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Dynamic bonding materials are of high interest in a variety of fields in material science. The reversible nature of certain reaction classes is frequently employed for introducing key material properties such as the capability to self-heal. In addition to the synthetic effort required for designing such materials, their analysis is a highly complex--yet important--endeavor. Herein, we critically review the current state of the art analytical methods and their application in the context of reversible bonding on demand soft matter material characterization for an in-depth performance assessment. The main analytical focus lies on the characterization at the molecular level.

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Light scattering and crossover critical phenomena in polymer solutions

Cited by 27 publications

References 33 publications

Competition of mesoscales and crossover to tricriticality in polymer solutions

Competition of mesoscales and crossover to tricriticality in polymer solutions

Dynamic crossover to tricriticality and anomalous slowdown of critical fluctuations by entanglements in polymer solutions

State‐of‐the‐Art Analytical Methods for Assessing Dynamic Bonding Soft Matter Materials

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