142 Advances in Polymer Science Editorial Board: A. Abe. A.-C ...

Asymmetric Star Polymers Synthesis and Properties 123
PPO/PS phase. Scanning electron microscopy experiments showed that the different
domains were smaller in the blends containing miktoarm material. The
size of the domains decreased and their uniformity increased with increasing
miktoarm content in the blends. Miktoarm stars with short PS arms were less effective
in compatibilizing PPO and Nylon-6. Ternary blends were found to have
higher modulus, strength and elongation than binary PPO/miktoarm blends.
They also showed yield behavior. These improvements were attributed to the increased
adhesion at the PPO/Nylon-6 interface due to the presence of polystyrene-containing
copolymer.
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Applications
Although the synthesis of asymmetric star polymers was rather recently
achieved, several efforts have been made to use these polymers for specific applications.
An increased number of patents have appeared in recent years. Sometimes
ill-defined and not well characterized polymers were used in these studies.
Conventional star block copolymers bearing PS-b-PB or PS-b-PI arms have
been employed in adhesive compositions. These copolymers have the disadvantage
that the polydiene fraction degrades in processing and over time. The unsaturation
is also susceptible to possible grafting and crosslinking reactions
which lead to increased molecular weights, thus making the polymer ineffective
in applications such as pressure sensitive adhesives and removable tapes. These
applications require an adhesive that possesses a fourfold balance of tack, cohesive
strength and the proper balance between adhesion and resistance to low
stress peel. Asymmetric stars have been useful in the formulation of adhesive
compositions for pressure sensitive adhesives, removable tape applications etc.,
showing improved properties compared to conventional star block copolymers.
Miktoarm stars of the type (PS-b-PI) n (PI) m prepared using DVB as linking
agent were employed in adhesive compositions, showing a good balance of tack,
peel strength and hold power near room temperature due to their reduced cohesive
strength and viscosity without any reduction in the service temperature
[94].
Stars of the type [(PS-co-PI)-b-PI] n (PI-b-PS) m were also used in adhesives
showing high resistance to low-stress peel while maintaining moderate adhesion
[95]. This was achieved by controlling the T g of the end-block to relatively low
levels.
Tapes made from adhesives containing asymmetric star block copolymers of
the type (PS-b-PI) n does not represent an asymmetric starblock copolymer [96].
By using asymmetric rather than the conventional symmetric star block copolymers
the preparation of tapes with moderate adhesion and good resistance to
low stress peel which are easily removed from the substrate without leaving adhesive
residue was achieved.
High adhesive performance at low molecular weight and viscosity was
achieved using [PS-b-P(ethylene/butylene)] 2PI 2 and [PS-b-P(ethylene/buty-