Effects Of Stereoregularity Research Articles

Effects of stereo-regularity on the single-chain mechanics of polylactic acid and its implications on the physical properties of bulk materials.

The material properties of polylactic acid (PLA) are largely determined by its stereo-regularity (tacticity). To find out the origin at the molecular level, the single-chain mechanics of poly-l-lactic acid (PLLA) and poly-d,l-lactide (PDLLA) were comparatively investigated by single-molecule atomic force microscopy (AFM). At a low concentration, PLLA adopted a random-coil conformation in a good solvent. At a high concentration, however, the PLLA chain can be induced into a helix, which consumed additional energy during unfolding by further stretching. Due to the random arrangement of l- and d-repeating units in the PDLLA chain, PDLLA adopts a random-coil conformation at all concentrations. The difference in single-chain mechanics of PLLA and PDLLA at high concentrations may be the cause of their different macroscopic properties. This is the first report to reveal the stereo-regularity-dependent mechanics of a polymer at the single-molecule level, which may help to bridge the gap between understanding single-molecule and materials properties.

Syndiotactic- and heterotactic-specific radical polymerizations of N-n-propyl-α-fluoroacrylamide and phase-transition behaviors of aqueous solutions of poly(N-n-propyl-α-fluoroacrylamide)

Radical polymerization of N-n-propyl-α-fluoroacrylamide (NNPFAAm) was investigated in several solvents at low temperatures in the presence or absence of Lewis bases, Lewis acids, alkyl alcohols, silyl alcohols, or fluorinated alcohols. Different effects of solvents and additives on stereospecificity were observed in the radical polymerizations of NNPFAAm and its hydrocarbon analogs such as N-isopropylacrylamide (NIPAAm) and N-n-propylacrylamide (NNPAAm); for instance, syndiotactic (and heterotactic) specificities were induced in radical polymerization of NNPFAAm in polar solvents (and in toluene in the presence of alkyl and silyl alcohols), whereas isotactic (and syndiotactic) specificities were induced in radical polymerizations of the hydrocarbon analogs under the corresponding conditions. In contrast, heterotactic specificity induced by fluorinated alcohols was further enhanced in radical polymerization of NNPFAAm. The effects of stereoregularity on the phase-transition behaviors of aqueous solutions of poly(NNPFAAm) were also investigated. Different tendencies in stereoregularity were observed in aqueous solutions of poly(NNPFAAm)s from those in solutions of the hydrocarbon analogs such as poly(NIPAAm) and poly(NNPAAm). The polymerization behavior of NNPFAAm and the phase-transition behavior of aqueous poly(NNPFAAm) are discussed based on possible fluorine–fluorine repulsion between the monomer and propagating chain-end, and neighboring monomeric units.

Cooperativity length evolution during crystallization of poly(lactic acid)

Effects of stereoregularity and crystallization mode on the amorphous phase dynamics are investigated for poly(lactic acid) PLA. An isothermal crystallization from the melt and a cold crystallization are imposed. For each PLA, the cold crystallization leads to the appearance of a less perfect crystalline phase and to an important rigid amorphous fraction RAF content (35%), although only 10% of RAF is generated after crystallization from the melt. Temperature Modulated Differential Scanning Calorimetry is used to determine the Cooperative Rearranging Regions (CRR) size at the glass transition temperature in the mobile amorphous phase MAP. It is shown that the CRR size in the MAP is not modified by the appearance and the spherulite growth. For the intra-spherulite MAP, a confining effect is evidenced, causing an amorphous phase thickness decrease during crystallization, and inducing a drastic CRR size reduction.

Specific recognition of syndiotactic poly(methacrylic acid) in porous isotactic poly(methyl methacrylate) thin films based on the effects of stereoregularity, temperature, and solvent

AbstractThe effects of stereoregularity, temperature, and solvent on the specific recognition of syndiotactic (st)‐poly(methacrylic acid) (PMAA) in macromolecularly porous isotactic (it)‐poly(methyl methacrylate) films were investigated to give important insights into the regularity and stability of nanospaces in the it‐PMMA films as well as template polymerization. The porous it‐PMMA films were fabricated on quartz crystal microbalance (QCM) substrates via the layer‐by‐layer (LbL) assembly of it‐PMMA/st‐PMAA, plus the st‐PMAA extraction from the assembly. QCM analysis and infrared spectroscopy revealed the first case of stereocomplex formation using st‐PMAA with lower stereoregularity (rr = 73%) in the LbL films, while st‐PMAA obtained with conventional free radical polymerization (rr = 62%) was barely incorporated into the porous it‐PMMA films. The maximum st‐PMAA incorporation increased from 25 to 40 °C, but there were almost no difference between 40 and 55 °C, indicating that the it‐PMMA crystallization would also be accelerated with increasing temperature. The studies on st‐PMAA incorporation with various complexing solvents revealed that the host it‐PMMA in the porous films could only form the original stereocomplex with 2/1 unit‐molar stoichiometry (st‐PMAA/it‐PMMA) in acetonitrile/water or ethanol/water. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3651–3657, 2010

Impacts of stereoregularity and stereocomplex formation on physicochemical, protein adsorption and cell adhesion behaviors of star-shaped 8-arms poly(ethylene glycol)–poly(lactide) block copolymer films

Biodegradable stereocomplex film exhibiting soft and stretchy character was prepared by simply blending between enantiomeric 8-arms poly(ethylene glycol)- block-poly( l-lactide) (8-arms PEG- b-PLLA) and 8-arms PEG- b-PDLA copolymers with star-shaped structure. The stereocomplex film exhibited higher T g and PLA crystallinity than those of original copolymer films. Effects of stereoregularity and stereocomplexation on protein adsorption and L929 cells attachment/proliferation behaviors onto the films were analyzed from the viewpoint to design a new class of implantable soft biomaterial. The stereocomplex film was found to exhibit large amount of protein adsorption than original films. Furthermore, cell attachment efficiency and proliferation rate on the film were significantly enhanced by stereocomplexation. This stereocomplex material is expected to be applicable as degradable temporary scaffold for soft tissue regeneration. Consequently, it was indicated that the stereocomplex formation could be proposed to be a novel method to control the protein- and cell-adhesive properties of biodegradable matrix composed of PEG–PLA copolymer.

Effects of Stereoregularity on the Photodegradation of Poly (methyl Methacrylate) Exposed to Ultraviolet Irradiation

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Effects of Stereoregularity on Molecular Parameters of Polyacrylonitrile Polymerized by Gamma-Ray Irradiation

Effects of Stereoregularity on Molecular Parameters of Polyacrylonitrile Polymerized by Gamma-Ray Irradiation

The effects of stereoregularity on the pressure-volume-temperature and related properties of poly(methyl methacrylate)

Abstract A polymer of methyl methacrylate with a fractional content of isotactic triads exceeding 0.99, a viscosity-average molecular weight of about (5—6) 105 and a normal glass temperature of 47°C, is investigated at pressures up 2 kbar and in a temperature range of 9 to 190°C. The results are compared with those for corresponding conventional polymers with glass temperatures above 100°C. In the liquid state, the thermal expansivity a of iso-PMMA, the internal pressure and the reduction in internal energy upon compression are larger, whereas the compressibility K is slightly smaller at elevated temperatures. In the glassy state, both α and K are smaller. The results are in qualitative accord with the existence of a weaker molar cohesion and/or greater chain flexibility, and closer packing in the stereoregular polymer. The application of the hole theory to the liquid shows the good agreement observed for other polymers in respect to the thermal expansivity, compressibility factor, and energy change with ...

Side‐chain relaxation in stereoregular poly(isobutyl methacrylate)

AbstractThe effects of stereoregularity on the low‐temperature relaxation processes were studied by dynamic mechanical measurements on isotactic and syndiotactic polyisobutyl methacrylates (iso‐PiBMA and syn‐PiBMA). The α, β, and γ relaxation processes were observed in both stereoregular forms. Both the α, and β loss peaks were at lower temperatures for iso‐PiBMA than for syn‐PiBMA. The γ loss peak was observed at about −155°C at 30 Hz for both forms, and the apparent activation energy of this process was same for both samples within experimental error (6.7 ± 0.5 kcal/mole). It was reduced from these results that the α and β processes are both considerably influenced by the isotactic configuration but the γ process is not.