A nucleation efficiency scale for isotactic poly(L‐lactide) (PLLA) was obtained with self‐nucleation and nonisothermal differential scanning calorimetry experiments. The maximum nucleation efficiency occurred at the highest concentration of self‐nucleating sites, and the minimum efficiency occurred in the absence of these sites (pure PLLA polymer melt). Blends of PLLA and isotactic poly(D‐lactide) (PDLA) led to the formation of a 1/1 stereocomplex. In comparison with the homopolymer (PLLA), the stereocomplex had a higher melting temperature and crystallized at higher temperatures from the melt. Small stereocomplex crystallites were formed in PLLA/PDLA blends containing low concentrations of PDLA. These crystallites acted as heterogeneous nucleation sites for subsequent PLLA crystallization. Using the PLLA nucleation efficiency scale, we evaluated a series of PLLA/PDLA blends (0.25–15 wt % PDLA). A maximum nucleation efficiency of 66% was observed at 15 wt % PDLA. The nucleation efficiency was largely dependent on the thermal treatment of the sample. The nucleating ability of the stereocomplex was most efficient when it was formed well before PLLA crystallization. According to the efficiency scale, the stereocomplex was far superior to talc, a common nucleating agent for PLLA, in its ability to enhance the rate of PLLA crystallization. In comparison with the PLLA homopolymer, the addition of PDLA led to reduced spherulite sizes and a reduction in the overall extent of PLLA crystallization. The decreased extent of crystallization was attributed to the hindered mobility of the PLLA chains due to tethering by the stereocomplex. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 300–313, 2001

A nucleation efficiency scale for isotactic poly(L-lactide) (PLLA) was obtained with self-nucleation and nonisothermal differential scanning calorimetry experiments. The maximum nucleation efficiency occurred at the highest concentration of self-nucleating sites, and the minimum efficiency occurred in the absence of these sites (pure PLLA polymer melt). Blends of PLLA and isotactic poly(D-lactide) (PDLA) led to the formation of a 1/1 stereocomplex. In comparison with the homopolymer (PLLA), the stereocomplex had a higher melting temperature and crystallized at higher temperatures from the melt. Small stereocomplex crystallites were formed in PLLA/PDLA blends containing low concentrations of PDLA. These crystallites acted as heterogeneous nucleation sites for subsequent PLLA crystallization. Using the PLLA nucleation efficiency scale, we evaluated a series of PLLA/PDLA blends (0.25-15 wt % PDLA). A maximum nucleation efficiency of 66% was observed at 15 wt % PDLA. The nucleation efficiency was largely dependent on the thermal treatment of the sample. The nucleating ability of the stereocomplex was most efficient when it was formed well before PLLA crystallization. According to the efficiency scale, the stereocomplex was far superior to talc, a common nucleating agent for PLLA, in its ability to enhance the rate of PLLA crystallization. In comparison with the PLLA homopolymer, the addition of PDLA led to reduced spherulite sizes and a reduction in the overall extent of PLLA crystallization. The decreased extent of crystallization was attributed to the hindered mobility of the PLLA chains due to tethering by the stereocomplex.

Model polyisoprene−polylactide (PI−PLA) diblock copolymers were synthesized by a combination of living anionic polymerization and controlled coordination-insertion polymerization. Living anionic polymerization of isoprene followed by end-capping with ethylene oxide yielded hydroxyl-terminated polyisoprenes (PI−OH) with narrow molecular weight distributions. In a second step, an aluminum alkoxide macroinitiator was formed from the equimolar reaction of triethylaluminum with the PI−OH prepolymer and subsequently utilized for the ring-opening polymerization of lactide to produce the desired PI−PLA diblock copolymer. The final molecular weight of each block was controlled through manipulation of the monomer-to-initiator ratio in both polymerizations. Well-defined blocks were obtained as evidenced by the narrow molecular weight distributions and the absence of homopolymer as characterized by GPC analysis. Molecular characterization of the block copolymers by spectroscopy (1H NMR, 13C NMR, and IR) and elemental analysis confirmed the relative compositions of the component blocks. We characterized the morphology of a representative PI−PLA diblock copolymer using DSC and SAXS. Both analyses indicated a microphase-separated structure characteristic of an ordered diblock copolymer. These model diblock copolymers are ideal materials for fundamental phase behavior and mechanical property studies.

A set of well‐defined poly(ethylene‐alt‐propylene)‐b‐polylactide (PEP‐PLA) diblock copolymers containing volume fractions of PLA (fPLA) ranging between 0.08 and 0.91 were synthesized by a combination of living anionic polymerization, catalytic hydrogenation, and controlled coordination‐insertion ring‐opening polymerization. The morphological behavior of these relatively low‐molecular‐weight PEP‐PLA diblock copolymers was investigated with a combination of rheology, small‐angle X‐ray scattering, and differential scanning calorimetry. The ordered microstructures observed were lamellae (L), hexagonally packed cylinders (C), spheres (S), and gyroid (G), a bicontinous cubic morphology having Ia3d space group symmetry. The G morphology existed in only a small region between the L‐C morphologies in close proximity to the order–disorder transition (ODT). Transformations from L to G were observed upon heating in several samples. The efficacy of the reverse G to L transition in one sample was cooling rate dependent. The PEP‐PLA Flory–Huggins interaction parameter as a function of temperature χPEP‐PLA(T) was estimated from TODT's by mean‐field theory and subsequently used in the construction of the experimental PEP‐PLA morphology diagram (χN versus fPLA). The resultant morphology diagram was symmetric there were the well‐defined L‐C morphology boundaries. The low molecular weight of the materials imparted no significant deviation from previously documented diblock systems. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2364–2376, 2002

Styrene/acrylic acid (S/AA) mixtures were copolymerized in concentrated 50 wt % 1,4-dioxane solutions at 120 °C at two ratios of additional free nitroxide mediator, N,N-tert-butyl-N-[1′-diethylphosphono-2,2′-dimethylpropyl] nitroxide (SG1), relative to 2-[N-tert-butyl-2,2-(dimethylpropyl)aminooxy]propionic acid (BlocBuilder, Arkema) alkoxyamine unimolecular initiator (4.5 and 9 mol % [SG1]/[BlocBuilder]). Without SG1 at initial acrylic acid monomer feed concentrations f AA,0 > 40 mol %, the apparent rate constants increased sharply, noticeable exotherms were observed, and polydispersities increased from 1.20 at f AA,0 ) 0 mol % to 1.48 at f AA,0 ) 80 mol %. With 4.5 mol % [SG1]/[BlocBuilder], polymerization rates were slower and not as strongly affected by f AA,0 although exotherms were still noticeable at high f AA,0 . Polydispersities remained ∼1.3 and only increased to >1.4 at high conversions for f AA,0 ) 80 mol %. Exotherms were rendered nearly negligible when 9 mol % [SG1]/[BlocBuilder] was used, leading to copolymers with much narrower molecular weight distributions and k p K values for comonomer mixtures bracketed between those of styrene and acrylic acid homopolymerizations at 120 °C (k p ) propagation rate constant, K ) equilibrium constant). Copolymer reactivity ratios estimated for samples produced using 9 mol % [SG1]/[BlocBuilder] by nonlinear least-squares minimization were r AA ) 0.25 ( 0.11 and r S ) 0.93 ( 0.37, in agreement with previous literature.