No abstract

A series of A−B−A triblock copolymers was prepared by a Sn(II) 2-ethylhexanoate catalyzed chain extension of various telechelic soft segments, with l,l-lactide (LLA). Poly(ε-caprolactone)s, PεCL, poly(ethylene glycol)s, PEG, and a poly(dimethyl siloxane), PDMS, served as soft segments. The lengths of both soft and hard segments (poly-LLA, PLLA, or poly-d,d-lactide, PDLA) were varied. For the lactide blocks, average lengths of 25, 50, and 100 lactic acid units were selected. An analogous series of A−B−A triblock copolymers was prepared with DLA. Compositions and block lengths of these triblock copolymers were characterized by 1H NMR spectroscopy. Furthermore, stereocomplexes (racemates) of PLLA- and PDLA-based triblock copolymers were prepared to find out if the attractive forces between PLLA and PDLA blocks or the repulsive forces between incompatible soft segments dominate the morphology. Both neat triblock copolymers and stereocomplexes were characterized by differential scanning calorimtery measurements by wide-angle X-ray diffraction. The influence of block lengths on glass transition (T g) and melting temperatures (T m) of soft and hard segments was studied in detail. Stereocomplexes of all triblock combinations were obtained, even when the soft segments were incompatible.

The quadrupolar splittings of deuteriated para-and ortho-dichlorobenzene ͑1,4-DCB and 1,2-DCB, respectively͒ are measured by nuclear magnetic resonance ͑NMR͒ in the nematic solvents hexyland pentyloxy-substituted diphenyl diacetylene ͑DPDA-C6 and DPDA-OC5, respectively͒. Measurements are taken for all four combinations of the nominally apolar ͑1,4-DCB͒ and polar ͑1,2-DCB͒ solutes in the apolar ͑DPDA-C6͒ and polar ͑DPDA-OC5͒ solvents, and throughout the entire nematic temperature range of the solutions. The temperature dependence of the second-rank orientational order parameters of the solutes are obtained from these measurements and the respective order parameters of the mesogenic cores of solvent molecules are obtained independently from carbon-13 NMR measurements. The order parameter profiles of the two solutes are found to be very different but show little variation from one solvent to the other. The results are analyzed and interpreted in terms of the underlying molecular interactions using atomistic solvent-solute potentials. The influence of electrostatic interactions on solute ordering is directly evaluated by computing the order parameters with and without the electrostatic component of the atomistic potential. It is observed to be small. It is also found that the important interactions in these solventsolute systems are operative over short intermolecular distances for which the representation of the partial charge distributions in terms of overall molecular dipole and quadrupole moments is not valid.

Two different equations are presented describing the frequency distribution (fl) of linear chains in step‐growth (SG) polymerizations. Two related equations for the weight distribution (wl) are formulated as well. All equations contain either an exponent α that represents the reaction rates (or probabilities) of chain growth versus cyclization reactions or they contain a cyclization factor β′. Plots of fl and wl versus conversion or degree of polymerization are presented and discussed. Distribution curves of individual oligomers versus conversion and versus β′ are described. Furthermore, the consequences for hyperbranched polymers resulting from polycondensations of abn monomers are discussed. Analogous to Flory's theory all equations concern kinetically controlled SG polymerizations. magnified image

We are indebted to three very conscientious referees for valuable suggestions and critiques. Both M.B. and E.T.S. thank NSF for support. E.T.S. is grateful for enlightening conversations with Michael Rubinstein and support from the Cary Boshamer Professorship.