Structure and mechanical properties of elastomeric block copolymers.
2010-12
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Structure and mechanical properties of elastomeric block copolymers.
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2010-12
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Abstract
This research presents the synthesis (by anionic polymerization and catalytic
hydrogenation) and characterization of two types of block copolymers: CMC and
XPX. In CMC, C is glassy poly(cyclohexylethylene) and M, the matrix, can be
semicrystalline poly(ethylene) E, rubbery poly(ethylene-alt-propylene) P, or rubbery
poly(ethylethylene) EE, or a combination to yield: CPC, CEEC, CEC, CPEEC and
CEPC, with fC ≈ 0.18 – 0.30. XPX materials have X = CEC, fC ≈ fE, and fP ≈ 0.40 –
0.60. Block copolymer phase behavior and morphology were examined through a
combination of DSC, rheology, SAXS, WAXS and TEM. CMC materials are meltordered
due to block thermodynamic incompatibility with TODT > Tg (C) ≈ 147 °C
and show lamellar or C cylinder morphologies. The design of XPX yields melt
disordered materials up to high Mn with microphase segregation induced by E
crystallization. Two high Mn XPX polymers are melt ordered above Tm(E) and show
two correlation lengths in SAXS assigned to the C – E and X – P length scales. TEM
images indicate that all XPX materials, irrespective of melt segregation, are
characterized by composite glassy and crystalline hard domains dispersed in rubbery P at room temperature. Tensile and recovery testing at room temperature show that CMC and XPX
materials, with the exception of plastic CEC, behave as thermoplastic elastomers with
tunable properties. Interestingly, melt disordered XPX materials have competitive
mechanical properties comparable to the strongest CMC polymers, but with
advantageous processing. For melt ordered CMC, Tprocess > TODT, which is dependent on Mn, while for melt disordered XPX, Tprocess > Tm(E) ≈ 100 °C independent of Mn.
The deformation of melt disordered XPX materials, probed by recovery studies and
WAXS, suggests that deformation is first taken by P, then E and finally C, which
causes ultimate failure, as agreed in the literature for conventional SBS and SIS
thermoplastic elastomers. This implies that strain recovery in XPX materials can be
comparable to that of CPC if materials contain low hard block content or are stretched
to strains below the onset of E deformation.
Finally, a collection of data of mechanical properties, namely modulus E,
strain at break εb, tensile strength σTS and tension set εs, obtained from CMC, XPX
and previously reported materials were examined. Most notably, E and εs were found
to be strongly correlated with the volume fractions of C and E, as properties increase
with (fC + fE)δ, where δ = 1 – 2.4. Ultimate properties such as σTS and εs are unaffected
by changes in composition as failure is dictated by that of the hard domains and
values are similar above a minimum amount of hard block. In addition, E, σTS, and εb
are inversely correlated to rubber entanglement molecular weight Me, which implies
that modulus and ultimate properties are affected by the ability of the rubber network
to redistribute stress by entanglement slippage. However, εs is unresponsive to Me
variations, which indicates that irrecoverable deformation in these materials results
from deformation of the hard domains.
Description
University of Minnesota Ph.D. dissertation. December 2010. Major: Material Science and Engineering. Advisor: Frank S. Bates. 1 computer file (PDF); xix, 234 pages, appendices A-D.
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Alfonzo, Carlos Guillermo. (2010). Structure and mechanical properties of elastomeric block copolymers.. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/109347.
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