Solvent Effect on Junction Size in Syndiotactic Polystyrene Physical Gel

Abstract

ABSTRACT:In order to estimate the junction point size of syndiotactic polystyrene (sPS) physical gels formed in chloroform, toluene, benzene, and o-dichlorobenzene, gel-melting temperatures were measured as a function of polymer concentration. The size in number of styrene units was estimated with the aid of the Tanaka and Stockmayer theory (TS theory), which is based upon the conjecture that junction points of a gel are aggregates comprised of units in length and sequences in cross section. According to measurement of in situ FT-IR, the junction points of sPS/ organic solvent gels were suggested to be aggregates of sPS chains with helical structure of TTGG conformation regardless of the difference in solvents. Quantitative analysis by the TS theory led to the results that the size was little dependent upon the sPS molecular weight for each sPS/organic solvent system. On the other hand, the size was strongly affected by solvents, i.e., in sPS/o-dichlorobenzene gel was the largest among the solvents used in the present study, while that in sPS/chloroform gel was the smallest. This solvent effect on the junction point size probably arises from the difference in magnitude of polymer/solvent interaction. [DOI 10.1295/polymj.37.294] KEY WORDS Syndiotactic Polystyrene / Polystyrene / Thermoreversible Gel / Junction Size / Gel-melting Temperature / Time-resolved FT-IR / Syndiotactic polystyrene (sPS) is of considerable scientific and industrial interest as a rigid high performance semicrystalline material. It is known that sPS polymer has two types of molecular chain conformations in its crystalline region: one is the all-trans planar zigzag structure (TT), and the other is the helical conformation (TTGG). 1-5Like other crystalline or semicrystalline polymers, sPS polymer also converts to thermoreversible gel in suitable organic solvents such as toluene or chloroform. Kobayashi et al. investigated the gelation process of sPS with time-resolved Fourier transform infrared (FT-IR) spectroscopy and showed that highly ordered TTGG conformations are formed on gelation. 6 Recent studies in the temperature-concentration phase diagrams of sPS gels have suggested that solvent molecules are intercalated between sPS chains in the TTGG conformation, and the number of solvent molecules involved is dependent on the gelation solvent.7-11 The ordered TT conformation was observed on gelation of sPS in big size solvent like octadecyl benzoate.12 Time-resolved small-angle neutron scattering (SANS) experiments also showed that the gelnetwork structure formation occurred in parallel to the conformational ordering of sPS chains. 13The thermoreversible gelation of highly stereoregular polymers such as sPS and syndiotactic poly-(methyl methacrylate) is considered to proceed in two steps:14-16 A first step is an intramolecular coil-tohelix transition; This change in molecular conformation is then followed by an intermolecular association of multiple helix segments, which leads to the formation of network structure. This gelation mechanism is ...

Syndiotactic Polystyrene Physical Gels: Guest Influence on Structural Order in Molecular Complex Domains and Gel Transparency

“…In several organic solvents, sPS can form thermoreversible gels where the polymer rich-junctions are formed by crystalline domains. Depending on solvent-type and/or thermal history, two kinds of gel can be obtained: past-like opaque gels for which the crystalline domains present the trans-planar β crystalline phase and elastic gels , for which the crystalline domains present s(2/1)2 helical molecular complex (clathrate 7 or intercalate 8 ) phases.…”

High-Resolution Comonomer Sequencing of Blocky Brominated Syndiotactic Polystyrene Copolymers Using 13C NMR Spectroscopy and Computer Simulations

“…The low q scattering feature is strongly suggestive of a large-scale mass fractal-type structure and is attributed to clusters or aggregates of lamellar crystallites, similar to that observed in other semicrystalline gels. , Thus, the presence of these two scattering features is strongly suggestive of an sPS/CCl 4 gel morphology consisting of clusters of crystallites that act as physical cross-links bound together by a percolating network of solvent-swollen amorphous chains. The hierarchical three-dimensional (3D) network defined by these USAXS/SAXS data constitutes the reaction media for the heterogeneous gel-state functionalization reactions. − …”

“…This work reveals that long segments of unfunctionalized monomer units in copolymers prepared by a gel-state, postpolymerization functionalization method originate from the precise lamellar structure within the heterogeneous gel morphology (i.e., block length is correlated with lamellar thickness). We anticipate that the ability to control the precise morphology of semicrystalline gel networks, specifically through changing the polymer concentration, gelation solvent, ,,, and/or gelation conditions, will provide avenues of further investigation into tailoring the degree of blockiness in copolymers prepared by gel-state, postpolymerization functionalization processes. Future efforts will also focus on developing a robust method to simulate the gel-state, postpolymerization functionalization using the gel morphology as an input parameter, to ultimately be able to predict the copolymer microstructure and degree of blockiness of copolymers prepared from semicrystalline homopolymers that form thermoreversible gels.…”

“…Recently, we introduced a new postpolymerization functionalization method carried out in the heterogeneous gel state that is capable of preparing copolymers with a blocky microstructure that preserves crystallizability at a relatively high functional group content (e.g., 29 mol % functionalized units) . Gels of crystallizable polymers (e.g., sPS) are composed of tightly packed chain segments in lamellar crystallites that act as physical cross-links bound together by a percolating network of solvent-swollen amorphous chains. − In our gel-state functionalization process, the functionalizing reagent is believed to be excluded from the crystalline component of the gel network and thus accessible only to monomers in the amorphous component between crystallites. ,, This preferential functionalization thus preserves long, crystallizable runs of consecutive monomer units that were inaccessible to the functionalizing reagents within the tightly packed crystalline lamella of the heterogeneous reaction medium. Copolymers formed by this new postpolymerization process exhibit significantly greater crystallizability and crystallize faster than their random copolymer analogues prepared by a conventional, homogeneous, solution-state functionalization method. , This enhanced crystallization behavior is attributed to a significant degree of nonrandomness in the distribution of comonomers along the copolymer chain, which herein will be described as a copolymer with a “blocky” microstructure.…”

Research on the crosslinking mechanism of polyacrylamide/resol using molecular simulation and X-ray photoelectron spectroscopy

“…On the contrary, organic crosslinkers can gel slowly and bear relatively high temperatures, prompting them to be candidates for chemical flooding in enhanced oil recovery. The different types of crosslinkers lead to discrepancies in rheological properties, salt resistance and temperature resistance; moreover, crosslinking density and junction size also have an effect on these properties, [13][14][15] hence comprehension of the structure-function relation has become increasingly important. However, it is difficult to characterize the structure and composition of the crosslinked products with poor solubility and meltability, hence the crosslinked structure and mechanism are not clearly understood.…”