(193j) Adjusting the Mechanical Properties of Polypropylene By Long Chain Branching Molecular Structure Designing

The change of the
microscopic chain architecture will affect the macro-properties of
polypropylene (PP). Recently, we reported a strategy for achieving long
chain branching polypropylene (LCBPP) by melt grafting reaction in the presence
of initiator benzoyl peroxide, macro-monomer vinyl polydimethylsiloxanes
and co-monomer styrene through one-step reactive extrusion^{[1, 2]}, and it was found that this LCBPP had not only excellent toughness but also desired
stiffness compared with linear PP.

To
uncover the relationship between the
molecular structure and the properties of LCBPP, LCBPPs (HMSPP1, HMSPP2,
HMSPP3) with different long chain branching (LCB) contents
were prepared, and the crystallization behaviors and mechanical properties was
compared. The results indicated that the LCB structure could decrease the nucleating activation energy to accelerate
the forming of nucleus but hinder the crystal growth, and eventually increased
the crystallization peak temperature (Fig.1), lamella thickness and lamella interaction
that have a positive correlation with the stiffness of LCBPPs. Moreover, with
the presence of LCB structure, beta-form
polypropylene was generated in LCBPPs under injection molding conditions, and
the content of beta-form increased
with the increasing of LCB structure content (Fig.2) that is good for the
toughness of LCBPPs. Benefiting from the combined effect of the thicker
lamella, stronger lamella interaction and forming of beta-form crystal caused by LCB, the LCBPPs revealed not only
outstanding stiffness but also desired toughness (Fig.3). The impact strength,
flexural modulus and tensile strength of HMSPP3 were increased by respectively
260.0%, 54.7% and 17.2% than those of linear isotactic polypropylene (iPP). This study provided guidance for designing
polymers with high mechanical performance through changing the molecular
structures.

Fig. 1 DSC cooling thermograms for all samples

  Fig. 2 WAXD patterns of the impact test splines for all samples

Fig. 3 Proposed illustrations of the different responses
of linear iPP and LCBPP samples to stress

1.    Zhou, S., Zhao S., Xin Z., Polymer Engineering & Science, 2015, 55(2):
251-259.

2.    Zhou, S., Zhao, S., Xin, Z.,Wang, W.,Journal of Macromolecular Science, Part B, 2014, 53(10):1695-1714.