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The structure of poly(vinyl alcohol) (PVA) hydrogels formed as a result of freeze/thaw treatments of aqueous solutions of the polymer (11 wt % PVA) in the freshly prepared state is analyzed through the combined use of small (SANS) and ultrasmall (USANS) angle neutron scattering techniques. The structure of these hydrogels may be described in terms of polymer rich regions, with dimensions of the order of 1−2 m, dispersed in a water rich phase, forming two bicontinuous phases. The PVA chains in the polymer rich phase form a network where the cross-linking points are mainly crystalline aggregates of PVA having average dimensions of ∼45 Å. The structural organization of freeze/thaw PVA hydrogel membranes does not change either after rehydration of dried gels or in the presence of a tensile force. Finally, addition of surfactant micelles inside the gel provides a formulation with both hydrophobic and hydrophilic regions, which demonstrates the potential of the system for drug delivery. Both SANS and EPR measurements show that sodium decylsulfate (C10OS) micelles do not significantly interact with the PVA gel. Variation of the gel structure by the number of freeze/thaw cycles should modulate the rate of release of an active constituent, for example, in a dermal patch. [ABSTRACT FROM AUTHOR]

The living, isoselective polymerization of 4-methyl-1,3-pentadiene, 4-methylstyrene, and 4-tert-butylstyrene was promoted by using the titanium complex 1activated by methylaluminoxane (MAO) at room temperature producing highly isotactic polymers. The synthesis of highly isotactic polystyrene-block-poly(4-methyl-1,3-pentadiene), poly(4-methylstyrene)-block-polystyrene, poly(4-methylstyrene)-block-1,4-trans-poly(1,3-butadiene), poly(4-methylstyrene)-block-polystyrene-block-poly(4-methylstyrene), poly(4-methylstyrene)-block-polystyrene-block-poly(4-methyl-1,3-pentadiene), and poly(4-methylstyrene)-block-polystyrene-block-poly(4-tert-butylstyrene) copolymers was also accomplished for the first time, by sequential monomer addition, in the presence of the system 1activated by MAO. The thermal features of these polymers clearly show that all copolymer blocks, except the 4-methylstyrene one, are crystalline with melting point values typical of the corresponding homopolymers. The synthesis of a low molecular weight polystyrene sample allowed us to detect the polymer chain ends, giving useful information about the polymerization mechanism. [ABSTRACT FROM AUTHOR]

The structural transformations occurring in initially homogeneous aqueous solutions of poly(vinyl alcohol) (PVA) through application of freezing (−13 °C) and thawing (20 °C) cycles is investigated by time resolving small-angle neutron scattering (SANS). These measurements indicate that formation of gels of complex hierarchical structure arises from occurrence of different elementary processes, involving different length and time scales. The fastest process that could be detected by our measurements during the first cryotropic treatment consists of the crystallization of the solvent. However, solvent crystallization is incomplete, and an unfrozen liquid microphase more concentrated in PVA than the initial solution is also formed. Crystallization of PVA takes place inside the unfrozen liquid microphase and is slowed down because of formation of a microgel fraction. Water crystallization takes place in the early 10 min of the treatment of the solution at subzero temperatures, and although below 0 °C the PVA solutions used for preparation of cryogels should be below the spinodal curve, occurrence of liquid−liquid phase separation could not be detected in our experiments. Upon thawing, ice crystals melt, and transparent gels are obtained that become opaque in ∼200 min, due to a slow and progressive increase of the size of microheterogeneities (dilute and dense regions) imprinted during the fast freezing by the crystallization of water. During the permanence of these gels at room temperature (for hours), the presence of a high content of water (higher than 85% by mass) prevents further crystallization of PVA. Crystallization of PVA, in turn, is resumed by freezing the gels at subzero temperatures, after water crystallization and consequent formation of an unfrozen microphase. The kinetic parameters of PVA crystallization during the permanence of these gels at subzero temperatures are the same shown by PVA during the first freezing step of the solutions. [ABSTRACT FROM AUTHOR]

The structure and thermal propertiesof a new class of isotacticcopolymers of ethylene (E)/4-methyl-1,3-pentadiene (4MPD) with 4MPDcontent in the range 40–73 mol % and of isotactic poly-1,2-(4-methyl-1,3-pentadiene)(iP4MPD) homopolymer have been analyzed. The copolymers have beensynthesized using 1,4-dithiabutanediyl-linked bis(phenolate) titaniumcomplexes as catalyst precursors, activated by methylaluminoxane.Solution 13C NMR analysis reveals the tendency of the catalystsystem to produce copolymers with alternating 4MPD/E constitution.All copolymers are crystalline and show melting temperatures in therange 90–100 °C. The samples with nearly equimolar concentrationof units crystallize in a new crystalline form because of the alternatingconstitution. Structural analysis indicates that in this new formthe alternating 4MPD-E sequences assume a nearly trans-planar conformation with periodicity c= 5.13 Åand are arranged in a monoclinic unit cell with parameters a= 5.70 Å, b(unique axis) = 14.95Å, c(chain axis) = 5.13 Å, and β= 114.4°, according to the space group symmetry Pn. [ABSTRACT FROM AUTHOR]