Thermoreversible polymer gels are often considered to be a network fo
rmed through physical association of long-chain molecules.
1-5Further, one can mak
e the jump from the junction structure to the network properties, because no characteristic mesostructure is a consequence of the self-similarity of the network. Hence, The percolation model presents a"physically correct" picture for an understanding of the polymer gelation.1,2 The basic idea is to approach the formation of a topologically disordered network from the viewpoint of a randomly assigned two-state property (connected or disconnected between the neighboring polymer chain segments).6 This view explains why, except the rheology,7 the experiments8,9 over recent decades always remain on the microstructure in terms of the classification proposed by Flory10 and de Gennes.11
However, the vast observation of diverse, hierarchical gel morphologies, especially the gelation induced by a thermal driven phase transition, definitely contradict this.7,8,12-17 Recent researches on the colloid gels18 and the low-molecular gelators19-21 have suggested that as an experimental fact, the self-organization/assembly seems to be a reasonably realistic description.
Interestingly, some experimentalists have presumed somewhat similar mechanism in polymer systems in spite of no enough evidence.22 While all seem to question the universality of the percolation theory, there should not be any ambiguity in the answer. For variety gel systems, many theorists go a long way to give an exact description of their universality class in phase transitions.4,23,24 The idea behind the percolation theory has a very intrinsic problem. Namely, the gelation as a critical phenomenon, the theory expects a well-defined sol-gel “phase”
boundary in the equilibrium phase diagram.25,26 On the other hand, a new problem upraises: if the thermoreversible polymer gels are taken out from the percolation universality class, where will they belong? Hence, we contend that a more precise experiment, based on tracing the morphogenesis, is needed to identify the most important factor for the polymer gelation.
As early as 1979, de Gennes has distinguished between strong gelation and weak gelation:
a gelation is termed “strong” if it occurs by the percolation way; otherwise, it is a “weak”
gelation and corresponds to the progressive freezing of degrees of freedom of the system.11 The de Gennes' prediction is borne out by Kroy et al.'s theory and Lu et al.'s experiment in colloidal gels. Kroy et al. applied the mode-coupling theory to their "cluster glass" view of the weak gelation;27 Lu et al. emphasized the spinodal decomposition to generate the clusters that span the system and dynamically arrest.18 Indeed, the fragility under external stresses of
Viewed from this direction, unlike the past concern that the polymer gelation is the cross-links of a large number of microstructures (microcrystal, helical structure, molecular compound, etc.),7,8 now should be reinterpreted as the jamming transition induced by the self-organization/assembly of them.28 However, in conventional jamming systems, the objects which allow much more precise tracing of arrested dynamics are model colloids and granular materials. One must contend with the fact that the polymer can have a huge variety of structural states at different length scales. Lack of morphogenetic detail makes it difficult to investigate the dynamic evolution on the polymer weak gelation.
1.1. Polymer Gels with Fibrillar Network
Forming a fibrillar network is a very widespread phenomenon in the polymer gels and the low-molecular gelators. It is accepted for the gelators that the fibrillar structure is due to the crystalline fibril branching20,21 and may be depicted by the solidification.29 Naturally, this seems logical for polymer, because the crystallization does occur in most polymer gels.7-9 However, a similar observation does not automatically guarantee a similar origin and process. The solidification has no thought for the entanglement complexity of polymers, not to mention the fact that the mechanism does not by itself expect—how does the topological connectivity occur?
Obviously, the crux lies in how the fibril growth brings the topological connectivity on the mesoscale.
There are two mechanisms for the thermoreversible gelation with fibril structure. One is the bond percolation by fibril junctions (a typical phase transformation in polymer solutions);
another is the self-organization/assembly by fibril aggregation/evolution (an non-equilibrium phenomenon). However, the experiments based upon equilibrium thermodynamics are not possible to tell which one is most nearly in agreement with the fact. Thus, we proposed a judgment. If the gelation is characterized by the fibril nucleation covering over the entire transient network instantaneously (i.e., the bonding probability depends only on the nucleation kinetics), it is a percolation process; if not, it relies on when and how do the molecular topological entanglement pin the self-organizing/ assembling processes. Tracing the evolving fibrils by the depolarized small-angle light scattering (d-SALS) technique can provide an access to model the system in questions.17 In this report, we use a qualitatively reliable full-scattering-pattern analysis to perform a systematic stepwise survey of the gelation process.14,15 By reconstructing the whole process, the present work would not only answer how the fibrils self-organize into the network but also clarify what the nature of the morphogenesis in that process is.
1.2. Morphogenetic Transition in Polymer Gelation
In the thermoreversible gelation of crystallizable linear polymers, the two most frequently observed textures are fibrillar networks and spherulite assemblies, corresponding to the manner in which the packing of polymer chains was achieved in the crystallization, i.e., the fringed-micelle crystal (inter-chain packing) and the chain-folded lamellar crystal (intra-chain packing), respectively. Generally, expanding a polymer chain to increase the crystallization possible with other chains seems to be a most efficient way to gelation. However, from the crystallographic point of view, the direct inter-chain packing has been believed to forbid on kinetic grounds, and the metastability of a fringed-micelle crystal becomes an academic matter.30
Figure 1 Image plot of time-resolved depolarized scattering profiles at azimuthal angle φ = 45° for C = 7 gdL−1 solution after the temperature jump from 433 to 293 K. The three scattering patterns show the representatives in nucleation-growth (typical four-leaf-clover characterization), aggregation-coarsening (the four-crescent- moon shape), and ripening-gelation stage, respectively.14
In the first part of the report, we would identify the instability of the fringed-micelle formation (a process called "coherent evolution"—the nonlocalized, fibril nucleation via the fluctuation instability and fibril growth coupled with a cooperatively coarsening concentration wave); at this point, we further speculated that the fibrillar networks might represent the
“observable” spinodal crystallization of polymers.13 In contrast to the fibrillar networks, since the polymer chain folds into a crystalline grain, the self-assembling mechanism provides the only available means to form a “network” without the topological entanglement. By using the time-resolved depolarized small-angle light scattering (d-SALS) with the scattering modeling approach, the cartoon in Figure 1 vividly portrays the process, in which the crystalline grains self-assemble into compact spherical clusters (spherulites) and further pack these clusters into a jammed solid.14-18
Obviously, the above argument shows that the two textures effectively means a striking manifestation of the kinetics at the beginning. In this report, we also consider whether a morphogenetic transition may occur, if the morphogenetic differentiation results from the kinetic distinction between instability and metastability. We start with the spherulite assemblies, which we have made clear during these years and named “nucleation gel” according to its kinetics, 14-18 and focus our attention to what may happen in the early stage. All results offer valuable insights into the theoretical treatment and dynamic experiments of polymer weak gelation in the future.