Main lectures

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ML01

SELF-ASSEMBLY OF HYDROGELS MEDIATED BY COILED-COIL PEPTIDE DOMAINS

J. KOPEČEK, J. YANG, C. XU, P. KOPEČKOVÁ

Traditional methods of biomaterial synthesis produced numerous materials with excellent properties. However, these synthetic pathways do not permit an exact control of chain length, sequence, and three-dimensional structure. Genetic engineering technology provides powerful tools for producing tailor-made biomaterials with predetermined three-dimensional structures. Exact control of the primary structure, composition and chain length of protein biomaterials can be achieved by manipulating the protein sequence encoding the protein structure.

One of the folding patterns of native proteins is called a coiled-coil, a super-helix of two or more α-helices. This motif has been often used in the design of "smart" materials, which may undergo a transition in response to external stimuli, such as temperature and/or pH changes. In this presentation several examples will be provided of macromolecules whose self-assembly into hydrogels is mediated by coiled-coil domains.

Hybrid hydrogels. Genetically engineered coiled-coil protein domains were used as physical crosslinkers of water-soluble synthetic polymers. Hybrid hydrogels assembled from water-soluble synthetic polymers and a coiled-coil motif were temperature-sensitive due to the cooperative conformational transition of the coiled-coil protein domain.

Self-assembly of graft copolymers. Graft copolymers composed of N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer backbone and peptide (coiled-coil) grafts were synthesized, and the hydrogels, resulting from self-assembly of graft copolymers mediated by coiled-coil domains, were characterized. The grafts were composed either from coiled-coils of different lengths (three to seven heptads) or the design involved two oppositely charged peptide grafts. The impact of parallel homodimer or antiparallel heterodimer formation on the self-assembly process was assessed. Microrheology was used to evaluate the gel formation.

Self-assembly of triblock copolymers. Triblock polypeptide copolymers containing two coiled-coil blocks (domains) flanking a random coil block were designed and synthesized. The structure of the coiled-coil domain was manipulated to produce copolymers with varying properties. It was found that the oligomerization states, thermal stability, and pH-sensitivity of the coiled-coil domains mediated the formation of the hydrogels and their physical properties.

The research was supported in part by NIH grant EB005288.

ML02

MY EARLY HISTORY WITH PNIPAAm, HOW KAREL DUSEK AND OTHERS INFLUENCED ME, AND OUR MORE RECENT WORK WITH PNIPAAm

A.S. HOFFMAN

I have been working with PNIPAAm for over twenty years. In this talk I will describe who helped me to discover this interesting polymer, and who influenced me along the way, especially in the early days. Karel Dusek was one of those people and I am especially happy to be able to join him and other colleagues in Prague to celebrate his 75^th birthday.

The talk will begin by describing my efforts to find a polymer that I could use to phase separate specific proteins from a complex mixture, using mild conditions. I will describe the reasons why I was trying to do that, and how that work led me to work with PNIPAAm hydrogels, and later with pH- and temperature-sensitive hydrogels. I will describe our application of those hydrogels to drug delivery applications and immobilized enzyme bioprocesses. I will finish with a brief review of our more recent work with site-specific PNIPAAm-protein conjugates, PNIPAAm-modified surfaces in microfluidic devices, and polymeric micelles from controlled molecular weight block copolymers of pH- and temperature-responsive copolymers.

ML03

NANOCOMPOSITES BASED ON SWOLLEN POLYMER GELS

A.R. KHOKHLOV

The lecture is devoted to nanocomposite materials prepared by embedding nanosized particles into polymer gels. In such nanocomposite materials the properties inherent to polymer gels are combined with the properties of filler component properly distributed throughout the gel volume. Three types of such materials are considered. In the first type of materials long one-dimensional (1-D) rods are embedded into the matrix of flexible polymer network. This system is promising for the use in different applications exploiting the superabsorbent properties of the gels, the rods of filler serving as a reinforcing agent for highly swollen gel. In the second type of materials flat 2-D clay particles are embedded into the gel matrix in order to impart to the gel ion-exchange properties combined with high absorption ability of the clay with respect to organic compounds. In the third type of structure 3-D nanovoids are introduced into the gel matrix. These cavities serve to concentrate small ions of salt having the same charge as the gel chains (co-ions). This property makes such gels very promising for new nanotechnologies as a medium with a specially designed set of microreactors.

ML04

MACROGELS, MICROGEL AND REVERSIBLE NETWORKS: WHAT IS THE DIFFERENCE?

W. BURCHARD

Interest in gels was mainly focussed on macroscopic material properties. In contrast the present contribution deals with structural properties in the mesoscopic scale. For a study of equilibrium gel properties non-invasive characterization techniques have to be applied which reduces the methods to opical spectroscopy and scattering procedures. Only light scattering covers the mesoscopic scale of interest between 3 and about 500 nm.

Macrogels: Best understood are macrogels prepared by random crosslinking of chains in solution. The heavily discussed difference between the Flory-Stockmayer (FS) theory and the empirical results of percolation theory from computer simulations on a lattice could be shown to have the same basis. Precolation theory includes excluded volume interaction. The perturbation of Gaussian chains in the randomly branched clusters also influences the angular dependence of scattered light which will be shown with an example. The segmental mobility of highly branched cluster develop deviating behavior from the Rouse-Zimm relaxation scheme. Clusters from the pre- and post-gel domains revealed non-expected differences in the branching density which gave evidence to crosslinking heterogeneities.

Microgels: Gelation is a critical phenomenon. The growing cluster formation diverges at the gelpoint, and the gelled system disproportionates into a gel- and a sol-fraction. These characteristics of a gel were also found when the crosslinking was conducted in monodisperse latex particles. A gelpoint was obtained when the growing clusters reached the size of the latex particle. The microgels were found to possess a monodisperse gel fraction and a sol fraction that was discovered in size exclusion chromatography (SEC). Soluble particles are also obtained if the monomer concentration in the solution was lower than needed for a macroscopically connected networks. Under these circumstances excessive ring formation occurs. Also these particles are commonly called microgels, but actually they are soluble highly branched samples which show no critical behavior and no separate sol fraction. The local properties of these two types of microgels may be similar but because of the small size they have to be studied by small angle neutron scattering.

Reversible networks: Truly reversible networks at thermodynamic equilibrium are rare. Mostly the gel formation by physical bonds and the return to solution (by changes in temperature, concentration or pH value) show hysteresis behavior. Apparently metastable structures are formed caused by cooperative interactions, for instance helix formation. In one case we were able to compare the end-linking of 3-arm star molecules prepared via associative bonds and covalent bonds. No gel was obtained via association. Probably the life time of an associative bond was too short.

ML05

DESIGN AND APPLICATIONS OF COLLOIDAL CORE/SHELL MICROGELS

L.A. LYON

This talk will focus on the fundamental aspects of core/shell hydrogel particle design and synthesis, as well as the potential use of such materials in biomedical applications. Hydrogel particles composed largely of poly-N-isopropylacrylamide display reversible volume phase transitions in response to temperature and thus form the basis of thermoresponsive materials. More complex materials, and hence more complex responsivities, can be created either through simple random copolymerization of other functional monomers, or through the creation of core/shell type structures in the nanoparticle. Both synthetic approaches will be discussed, along with the ramifications of each topology on the phase transition behavior of the particles. Opportunities for creating complex polymer architectures with a particular focus towards the polymer/biology interface will be discussed.

ML06

Block Copolymers / Thermoset Blends: keys to control processing and properties

J.P. Pascault

Nanostructured polymers present many interests. In classical Linear Polymer (LP)/Thermosets (TS) blends, the LP and the TS precursors are initially miscible. During the curing process of the TS precursor, the conformational entropy of mixing decreases as the molar mass increases, and thus the phase separation between the LP and the step-growing TS network generally occurs well before the gelation.

By using Block Copolymers, BCP instead of classical linear homopolymers, the macroscopic phase separation can be avoided. It was demonstrated that amphiphilic diblock or triblock copolymers, with one block miscible with the TS, can not only be dispersed in a reactive system but can also order themselves on the nanometer scale in both the unreacted and reacted mixtures. But, in most cases, because of these miscibility criteria, a given BCP is efficient in only one or few systems. Two epoxy systems have been used as model systems. They are constituted of the same epoxy prepolymer and two diamines with very different reactivities and solubility parameters as hardeners. The microstructure evolutions with the temperature and the composition of the solutions have been followed by rheology and small angle X-ray scattering.

In one epoxy system, a PMMA block is miscible up to the end of the reaction and allows stable nanostructured morphologies, while in the other model system, the same block is not miscible enough to stabilize the initial nanostructure, and a flocculation is observed during curing. In this case a random copolymer, poly (MMA-co-Y), where "Y" is a more polar monomer unit like poly (N,N-dimethylacrylamide), can be used as the nanostructuring block.

Finally, the use of AB, BAB, ABC and reactive ABC BCP is an elegant way of improving the mechanical properties of brittle epoxy TS polymers while keeping the transparency of the blend. Works are in progress with other TS.

ML07

PRESSURE EFFECTS ON THE PHASE BEHAVIOR AND DYNAMICS OF ENVIRONMENT-SENSITIVE POLYMER GELS

M. SHIBAYAMA^a, I. NASIMOVA^b, S. OKABE^a , T. KARINO^a , M. NAGAO^a

The phase behavior and microscopic structure of noncharged Poly(N-isopropylacrylamide) (PNIPA) gels and weakly-charged poly(N-isopropylacrylamide-co-acrylic acid) (PNIPA/AAc) gels were investigated by small-angle neutron scattering as a function of temperature, T, and hydrostatic pressure, P. The cloud point temperature curves of both types of gels were found to be concave functions with P, having a maximum at P = Pc, where Pc was in the range of 50 ≤ Pc ≤ 100 MPa dependent on the degree of ionization. At low temperatures below the LCST of PNIPA in water and at the atmospheric pressure, the structure of gels could be described by an Ornstein-Zernike (Lorentz) function, indicating that the structure factors of as-prepared gels are very similar to those of the corresponding semidilute polymer solutions. However, by increasing P, large inhomogeneities appeared in the noncharged gels, which were well described by squared-Lorentz functions.¹ On the other hand, a microphase separation, having a characteristic long spacing of the order of a few tens nanometer, took place in the weakly-charged gels.² Interestingly, the degree of inhomogeneities and the appearance of the microphase separation were reentrant with respect to P. It will be demonstrated that the reentrancy of the phase behavior is a common feature of water-soluble polymers carrying hydrophobic groups and of water soluble proteins. These reentrant behaviors are explained with pressure-dependence hydrophobic interaction.

1. M. Shibayama, K. Isono, S. Okabe, T. Karino, M. Nagao, Macromolecules, 37, 2909 (2004).

2. I.R. Nasimova, T. Karino, S. Okabe, S. Nagao, Macromolecules, 37, 8721 (2004).

3. I.R. Nasimova, T. Karino, S. Okabe, S. Nagao, J. Chem. Phys., 121, 9708 (2004).

ML08

NANOPARTICLES OF THERMALLY RESPONSIVE POLYMERS

H. TENHU

Novel thermally responsive materials, e.g. microgels and graft copolymers, based on N-vinylcaprolactam and an amphiphilic macromonomer have been synthesized. The role of the amphiphilic grafts on the colloidal stability of the microgels and the influence of the grafts on the solution properties of graft copolymers have been investigated. The synthesized amphiphilic macromonomer is highly surface-active and it can be utilized as a reactive surfactant in emulsion and microemulsion polymerizations. Poly(N-vinylcaprolactam) (PVCL) microgels with varying surface charge and structure were prepared by emulsion polymerization with the aid of the macromonomer. The PVCL microgels undergo a reversible volume change from swollen to collapsed state in water when the temperature is raised. Grafting the responsive particles with the macromonomer considerably increases their stability towards added electrolytes. Thermosensitive graft copolymers of N-vinylcaprolactam and the macromonomer have been synthesized with different grafting densities. The polymers are soluble in cold water and undergo a phase separation at TCP ~ 31 - 38 °C, depending on the molecular mass. Owing to the amphiphilicity of the grafts, the most densely grafted polymer forms intrapolymeric structures while the less grafted polymers build up mixtures of intra- and interpolymeric associates at temperatures below LCST. Upon heating, the graft copolymers, as well as the PVCL homopolymers, aggregate in water and form nano-sized particles. The particles are exceptionally stable against further aggregation, and dilution of the solutions has no effect on the particle size or shape.

ML09

INTELLIGENT GELS -AN APPROACH TO ARTIFICIAL MUSCLES

Y. OSADA, G.J. PING, A. KAKUGO

There are two basic differences between the motion in a man-made machine and in a biological motor. One is in their principles. The motion of a man-made machine, which is constructed from hard and dry materials such as metals, ceramics, or plastic, is realized by the relative displacement of the macroscopic constituent parts of the machine. In contrast to this, the motion of a living organism, which consists of soft and wet materials (protein and tissues), is caused by a molecular deformation that is integrated to a macroscopic level through its hierarchical structure. The other difference is in their energy sources. The man-made machine is fueled by electrical or thermal energy with an efficiency of around 30%, but a biological motor is driven by direct conversion from chemical energy with an efficiency as high as 80-90%.

We have been employing polymer gels to create biomimetic motility systems, focusing to their reversible size and shape change, thereby realizing motion by integrating the deformation on a molecular level. Over the past number of years, we have proposed several kinds of artificial soft machines constructed by synthetic polymer gels. Examples include Gelooper(gel-looper),gelf(gel golf), shape memory gel actuators, chemical motors, etc. Bio-Nanomachines built of actin and myosin fiber gels, which are able to move faster than native actin fiber, have also been made. The mechanism of the motion associated with hierarchical structure will be discussed.

ML10

SWELLING, ELASTICITY, AND INHOMOGENEITY OF GELS FORMED BY FREE-RADICAL CROSSLINKING MECHANISM

O. OKAY

The properties of polymer gels depend on their network structure whereas the latter is closely related to the reaction conditions under which the gels are formed. Considerable progress has been achieved in the past years in the prediction of the formation and growth of gels during free-radical cross-linking copolymerization (FCC). However, theories are still unable to predict the gel properties from their synthesis parameters. This is mainly due to the existence of several non-idealities (cyclization, multiple cross-linking, and diffusion controlled reactions) during FCC leading to nonrandom cross-linking between the polymer molecules and to micro-phase separations. A macro-phase separation may also occur during gelation leading to phase separated domains with fixed boundaries.

Because a network structure formed during gelation generally occupies the whole available reaction volume, the dilution degree of the monomers at the gel preparation significantly affects the gel properties. In the present lecture, we report the variation of the physical properties of acrylamide-based hydrogels depending on the polymer concentration after the gel preparation n₂^{0.The average network chain length, the linear deformation ratio of the network chains in swollen hydrogels, as well as the degree of spatial gel inhomogeneity are the parameters monitored as a function of} n₂^{0. Different gel regimes can be distinguished depending on the gel preparation concentration. Both the deformation ratio of the network chains and the apparent degree of spatial gel inhomogeneity are not monotonic functions of} n₂⁰ due to the intramolecular reactions occurring during FCC. We will also discuss our efforts to synthesize homogeneous hydrogels by FCC as well as to design inhomogeneous gels that improve their mechanical properties on their swelling.

ML11

FIFTY YEARS WITH POLYMER GELS AND NETWORKS AND BEYOND

K. DUŠEK

In early sixties,^1,2 the formation of porous structures by free-radical crosslinking polymerization induced by additives like linear polymer of the same composition as the network, good solvents, etc. was discovered¹. After a period of attempts of mechanistic explanations it had become clear that thermodynamics was the right key, and inspired by Flory's consideration of reference dimensions of network chains³, the conditions of phase separation were formulated^4-6. These observations were extended to hydrogels and it was established when phase separation can exist in the form of macro- or microsyneresis and when it is induced by increase in crosslink density and when by changes in thermodynamic interactions⁷. This thermodynamic excursion into the world of practical material determined my interest in structure of polymer networks and their interactions. Analysis of thermodynamics of swollen networks led to discovery of possibility of a phase transition at which two network phases having different degree of swelling can coexist. This type of transition was experimentally found by Toyoichi Tanaka⁸ which started development of a broad region of responsive gels contributed by many outstanding scientists. Present and future development in this field has been and will be affected by great potential of these systems in medicine where the specific interactions of the biopolymer motifs can induce the transition or couple transitions of different kind (cf., e.g., Ref. 9). Also, constructs from networks constraining each other seems to offer us new challenges in their behavior. When network formation by free-radical crosslinking polymerization was investigated in more detail it was found that it was very non-ideal strongly influenced by intramolecular reaction and formation of condensed intramolecularly crosslinked structures in intermediate states¹⁰. These ideas were developed and independently contributed in but also without connection with formation of porous and heterogeneous structures (cf., e.g., Refs.11-13). Perhaps the deepest (because most difficult at the beginning?) influence on me had the (cascade) branching theory I was taught by Manfred Gordon¹³ through his papers. Many crosslinked systems considered before intractable (e.g. epoxies, polyurethanes) could be elegantly described when correct, though sometimes complex, chemistry was coupled with efficient statistical treatment^14-16. An awareness of limited applicability of statistical methods in some cases lead to development of alternative approaches^17,18. The possibility to describe well structural changes occurring during network formation enables wide open the door of reaction induced phase separation¹⁹ and, for instance, enables to understand coating film formation by crosslinking and solvent evaporation²⁰. Many challenges are still ahead, like what happens when critical points - gelation and phase separation - merge, points where the polydispersity reaches its maximum coupled with discontinuity in composition of sol and gel.

1. Millar, J.R. Brit. Pat. 849122 (1960). 2. Kun, K.A.; Kunin, R. Polym. Letters 1964, 2, 587. 3. Flory, P.J. J. Am. Chem. Soc. 1956, 78, 5222. 4. Dušek, K. Polym. Letters 1965, 3, 209. 5. Seidl, J.; Malinský, J.;Dušek, K.; Heitz, W. Adv. Polym. Sci. 1967, 5 113. 6. Dušek, K.; Prins, W. Adv. Polym. Sci. 1969, 6, 1. 7. Dušek, K. in Polymer networks structure and mechanical properties, A. J. Chompff, S. Newman, Eds., Plenum Press. 1972, p. 245. 8. Tanaka ,T. Phys. Rev. Lett. 1978, 40, 820. 9. Dušek, K.; Dušková-Smrčková, M.; Ilavský, M.; Stewart, R.; Kopeček, J. Biomacromolecules 2003, 4, 1818. 10. Dušek, K., in Advances in polymerisation. III. R. N. Haward, Ed., Applied Science Publ., Barking (1982) 143-206. 11. Kloosterboer, J.G. Adv. Polym. Sci. 1988, 84, 1. 12. Okay, O. Progr. Polym. Sci. 2000, 25, 711. 13. Gordon, M. Proc. Roy. Soc., Ser. A, 1962, 268, 240. 14. Dušek, K. Adv. Polym. Sci. 1986, 78, 1 . 15. Dušek, K. in Telechelic polymers: Synthesis and applications, E. J. Goethals, Ed., CRC Press, Boca Raton, pp. 289-360 (1989). 16. Dušek, K., Dušková-Smrčková, M. Progr. Polym. Sci. 2000, 25, 1215.17. Stauffer, D., Coniglio, A., Adam, M. Adv. Polym. Sci. 1982, 44, 103. 18. Mikeš, J.; Dušek, K. Macromolecules 1982, 15, 93. 19. Pascault, J.-P., Sautereau, H.; Verdu, J.; Williams, R.J.J. Thermosetting polymers, M. Dekker 2002. 20. Dušková-Smrčková, M.; Dušek, K. Macromol. Symp. 2003 198, 259.