Morphology and tensile properties of multigraft copolymers with regularly spaced tri-, tetra-, and hexafunctional junction points

The effect of chain architecture on the morphological and tensile properties of series of multigraft copolymers, with regularly spaced tri-, tetra-, and hexafunctional junction points, was investigated using transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and tensile testing. The materials were synthesized by coupling difunctional polyisoprene (PI) spacers and living polystyrene ( PS) branches, made by anionic polymerization, with chlorosilanes of different functionalities. Since the coupling process is a step-growth polymerization, yielding polydisperse products, fractionation was utilized to separate each material into three fractions ( high, middle, and low molecular weight), each of low polydispersity. All three fractions have the same chain architecture on a per junction point basis but differ in the number of junction point units per molecule. By applying the constituting block copolymer concept, the physical behavior of these molecules was compared with current theories. It was found that morphological behavior of these graft copolymers can be predicted using theoretical approaches and is independent on the number of junction points. The number of the junction points, however, greatly influences the long-range order of microphase separation. Additionally, two new parameters for adjusting mechanical properties of multigraft copolymers were found in this investigation: ( 1) functionality of the graft copolymerstri-, tetra-, or hexafunctionalsand ( 2) number of junction points per molecule. An increase in functionality causes a change in morphology, resulting in a high level of tensile strength for tetrafunctional ( cylindrical) and hexafunctional ( lamellae) multigraft copolymers, leading to about the twice the strength of the spherical trifunctional multigrafts of similar overall composition. Tetrafunctional multigraft copolymers show a surprisingly high strain at break, far exceeding that of commercial block copolymer thermoplastic elastomers (TPEs). Strain at break and tensile strength increase linearly with the number of junction points per molecule. Hysteresis experiments at about 300-900% deformation demonstrate that multifunctional multigraft copolymers have improved high elasticity as compared to commercial TPEs like Kraton or Styroflex.