A SANS study of the self-assembly in solution of syndiotactic polypropylene homopolymers, syndiotactic polypropylene-block-poly(ethylene-co-propylene) diblock copolymers, and an alternating atactic-isotactic multisegment polypropylene

We have studied by small-angle neutron scattering (SANS) the crystallization of syndiotactic polypropylene (sPP) homopolymer in dilute hydrocarbon solution in parallel with the self-assembling behavior of crystalline-amorphous block copolymers of (syndiotactic propylene)-block-poly(ethylene-copropylene) [sPP-P(E-co-P)] and atactic-isotactic multiblock polypropylene copolymers (aPP-iPP). Homopolymers and diblock copolymers of various molecular weights and volume fractions were investigated in decane over a wide temperature range (between 160 and -20 °C) while the temperature evolution of the aPP-iPP aggregates was studied in dodecane starting from 175 °C. The homopolymer crystallizes by forming platelets of a thickness of about 60-65 Å, independent of the molecular weight or polymer volume fraction in solution. The diblock copolymers aggregate by forming a complex morphology that shows a multilevel structure. This was investigated with SANS and ultra-SANS (USANS) over 3 orders of magnitude in size scale. At small scale (tens of angstroms) an overall 2-dimensional structure seems to be formed. It evolves at intermediate scale (hundreds of angstroms) into a rodlike structure. The rods associate in bundles that at a micrometric scale form supramolecular structures like networks. Such an association could be promoted by the PE sequences present within the P(E-co-P) block. The multiblock copolymer forms very open rodlike aggregates and demonstrates that the general morphology of self-assembling materials with multiple alternating crystalline-amorphous blocks appears to be 1-dimensional.