Mitsuhiro Shibayama

Volume phase transition and related phenomena of polymer gels

This review covers the recent advances in studies of the volume phase transition and critical phenomena of polymer gels mostly carried out in our group from 1973 to the present. We aimed here to discuss intensively (i) the basic understanding of the transition from the viewpoints of structure, dynamics, kinetics, and equilibrium thermodynamics, (ii) technological applications of the volume transition, and (iii) the relation between the phase transition and biological interactions.

Spatial inhomogeneity and dynamic fluctuations of polymer gels

Polymer gels have two types of concentration fluctuations; liquidlike and solidlike fluctuations. The former are time-dependent (thermal) fluctuations and the latter are time-independent spatial inhomogeneity. The static structure factor, obtained by small-angle X-ray (SAXS) or neutron scattering (SANS), consists of both contributions. Several methods which allow to decompose the static structure factor into the individual contributions are reviewed. In practice, however, the decomposition is not as easy as that predicted by the theory due to the lack of information about dynamic fluctuations. The dynamic light scattering technique (DLS), on the other hand, provides the complimentary information of the dynamic fluctuations. It will be shown that DLS becomes a powerful tool to distinguish the two contributions if it is coupled with ensemble averaging. In this article, recent developments of the structure investigation of polymer gels, particularly those of environmental sensitive polymer gels, are reviewed from both experimental and theoretical points of view. The advantage to use the environmental sensitive gels is that the structure of a gel can be examined as a function of its environmental parameters, e.g., temperature, concentration, and pH. Furthermore, the characteristic features of these environmental sensitive gels, such as a volume phase transition and phase separation, are extensively discussed in terms of the thermal fluctuations and the spatial inhomogeneity.

Small angle neutron scattering study on poly(N-isopropyl acrylamide) gels near their volume-phase transition temperature

The small angle neutron scattering experiments were conducted on N‐isopropyl acrylamide (NIPA) gels in D2O and on the corresponding NIPA solutions. The NIPA gels underwent a sharp, but a continuous volume phase transition at 34.6 °C from a swollen state to a shrunken state with increasing temperature. In the case of the gels, an excess scattering due to the presence of crosslinks was observed at low q region (q≤0.02 A−1), where q is the magnitude of the scattering vector. The scattered intensity function for the gel was well described with a combination of Gauss and Lorentz‐type functions, i.e., I(q)=IG(0)exp[−Ξ2q2]+[IL(0)/(1+ξ2q2)] as proposed by Geissler et al. IG(0) and IL(0) are the intensities at q=0 for the contributions of Gaussian and Lorentzian functions, respectively. The Gaussian part results from solidlike inhomogeneity, having a characteristic size of Ξ, which is due to the introduction of crosslinks into the system. The Lorentzian part is originated from the liquid nature of the local concen...

Molecular Basis of the Shish-Kebab Morphology in Polymer Crystallization

In the rich and long-standing literature on the flow-induced formation of oriented precursors to polymer crystallization, it is often asserted that the longest, most extended chains are the dominant molecular species in the “shish” of the “shish-kebab” formation. We performed a critical examination of this widely held view, using deuterium labeling to distinguish different chain lengths within an overall distribution. Small-angle neutron-scattering patterns of the differently labeled materials showed that long chains are not overrepresented in the shish relative to their concentration in the material as a whole. We observed that the longest chains play a catalytic role, recruiting other chains adjacent to them into formation of the shish.

Small-angle neutron scattering study on weakly charged temperature sensitive polymer gels

The static structure factor for poly(N‐isopropylacrylamide‐co‐acrylic acid) (NIPA/AAc) gels was investigated in terms of small‐angle neutron scattering (SANS). The NIPA/AAc gels underwent a discrete volume phase transition at 50.8 °C from swollen to shrunken states as temperature increased. The static structure factors for NIPA/AAc gels were well described by a Lorentz‐type scattered intensity function at temperatures below 34 °C which was near the so‐called Θ temperature of NIPA in D2O. At higher temperatures, the static structure factor had a distinct scattering maximum although the gel was still in the highly swollen state. The scattering maximum was located around q=0.02 A and was temperature and concentration dependent, where q is the magnitude of the scattering vector. The appearance of the peak indicates the strong concentration fluctuations that created polymer rich and poor regions in the system. The contrast in the concentrations appeared to result from the competing interactions in the network,...

Experimental evidences for molecular origin of low-Q peak in neutron/x-ray scattering of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ionic liquids.

Short- and long-range liquid structures of [C(n)mIm(+)][TFSA(-)] with n = 2, 4, 6, 8, 10, and 12 have been studied by high-energy x-ray diffraction (HEXRD) and small-angle neutron scattering (SANS) experiments with the aid of MD simulations. Observed x-ray structure factor, S(Q), for the ionic liquids with the alkyl-chain length n > 6 exhibited a characteristic peak in the low-Q range of 0.2-0.4 Å(-1), indicating the heterogeneity of their ionic liquids. SANS profiles I(H)(Q) and I(D)(Q) for the normal and the alkyl group deuterated ionic liquids, respectively, showed significant peaks for n = 10 and 12 without no form factor component for large spherical or spheroidal aggregates like micelles in solution. The peaks for n = 10 and 12 evidently disappeared in the difference SANS profiles ΔI(Q) [=I(D)(Q) - I(H)(Q)], although that for n = 12 slightly remained. This suggests that the long-range correlations originated from the alkyl groups hardly contribute to the low-Q peak intensity in SANS. To reveal molecular origin of the low-Q peak, we introduce here a new function; x-ray structure factor intensity at a given Q as a function of r, S(Q) (peak)(r). The S(Q) (peak)(r) function suggests that the observed low-Q peak intensity depending on n is originated from liquid structures at two r-region of 5-8 and 8-15 Å for all ionic liquids examined except for n = 12. Atomistic MD simulations are consistent with the HEXRD and SANS experiments, and then we discussed the relationship between both variations of low-Q peak and real-space structure with lengthening the alkyl group of the C(n)mIm.

11B n.m.r. study on the reaction of poly(vinyl alcohol) with boric acid

Abstract The crosslinking mechanism of an aqueous alkaline solution of poly(vinyl alcohol) (PVA) in the presence of boric acid was investigated by means of 11B nuclear magnetic resonance (n.m.r.). 2-Propanol and 2,4-pentanediol were chosen as monomeric and dimeric model samples of PVA, respectively. In the case of 2-propanol, only one strong signal was observed, which was the same as in the case without alcohol. By contrast, 2,4-pentanediol and PVA had two distinct signals, one of which was unique to 1,3-diols, i.e. pentanediol and PVA. It is concluded that monohydroxyl alcohol does not react with boric acid, whereas diand polyhydroxyl alcohols having the structure of 1,3-diol do. The temperature dependence of the chemical shift of the signals shows that the product is a so-called monodiol type but not a didiol type, which is not consistent with the conventional concept of PVA and boric acid reaction.

Structure-mechanical property relationship of tough hydrogels

Recent developments of tough hydrogels have changed the concept of hydrogels. These gels have an extraordinarily high modulus, high tenacity, and high deformability. In this review, it is demonstrated that structural analyses with small-angle neutron scattering (SANS) have elucidated the origin of the toughness and the mechanisms of deformation of tough hydrogels. Particularly, we focus on Tetra-PEG gels because of their simple and uniform architecture, i.e., unimodal network free from trapped entanglements and with negligible network defects, and investigate the structure-mechanical property relationship.

Highly Elastic and Deformable Hydrogel Formed from Tetra-arm Polymers

After decades of efforts by many researchers, we have succeeded in realizing a near-ideal polymer network. This network, the Tetra network, is made by cross-end-coupling of tetra-arm polymer modules. The mechanical energy dissipation was extremely low (tan δ ≈ 10(-4) ). The macroscopic stress-strain relationship of the Tetra network was in good agreement with that of microscopic elastic blobs. The maximum breaking strength was extremely high (≥27 MPa). These results indicate that the Tetra network is closer to an ideal polymer network than any other conventional model networks. Because the Tetra network can be treated as uniformly packed elastic blobs, it should help apply the knowledge of single polymer chains seamlessly to the design of polymer materials and help further develop the theory of rubber elasticity.

pH and salt concentration dependence of the microstructure of poly(N‐isopropylacrylamide‐co‐acrylic acid) gels

The structure of environment‐sensitive gels consisting of poly(N‐isopropylacrylamide‐co‐acrylic acid) (NIPA/AAc) hydrogels was investigated by small‐angle neutron scattering (SANS). Effects of the variation of ionic circumstance were observed both on the volume phase transition and on the structure factors: (1) The volume phase transition temperature, Tc, increases with increasing pH from 4 to 9 and then decreases with further increasing pH. (2) the volume gap between the shrunken and swollen gels at Tc is highest around pH 7 to 9. (3) Though the structure factor, I(q), has a maximum above the Tc of the corresponding NIPA homopolymer gels, the peak intensity decreases with increasing pH, where q is the magnitude of the scattering vector. When NaCl is added, a similar scattering maximum appears for T≳Tc. However, strong ionic screening leads to a lower swelling power and a decrease in Tc, resulting in precipitation at a lower temperature than for the gel at a moderate pH. The roles of pH and salt are discu...