Microphase glassy

Table II shows Tgs obtained from DSC traces. (Footnotes a and b in Table II show T s values of three reference polymers two PIBs, whose Mns are similar to the Mns of MA-PIB-MA used in the network synthesis, and a PDMAAm the difference in the Tg for the Mn=4,000 and 9,300 PIBs is due to the dependence of Tg on Mn(72)). The DSC traces of the networks exhibited two Tgs, one in the range of -63 to -52 °C (PIB domains) and another in the range of 90 to 115 °C (PDMAAm domains) indicating microphase separated structures. The Tgs associated with the PIB phase in the PDMAAm-1-PIB networks were higher than those of the reference homoPIBs which may be due to PIB chain-ends embedded in the glassy PDMAAm phase restricting segmental mobility. The Tg of the PIB phase in the PDMAAm-1-PIB increases by increasing the PIB content which may be due to an increase in crosslink density. In contrast, the Tg for the PDMAAm phase in the network decreases upon increasing the PIB content. Interaction of the (-CH2-CH-) moiety of the PDMAAm with the flexible PIB and thus the formation of a more flexible structure may explain this phenomenon.

Table II shows Tg data obtained from DSC traces of the PHEMA-1 -PIB networks. The traces showed two Tgs indicating microphase separation into PHEMA and PIB domains. The presence of the PHEMA Tg at - 110°C indicates complete desilylation of all networks. The Tgs for the reference PIBs (see footnote a in Table II) are lower than the Tgs of the PIB incorporated into the network. This may be due to the flexible PIB chain-ends embedded in the glassy PHEMA matrix. The increase in the Tg of the PIB phase in the network with increasing % PIB is most likely due to an increase in crosslink density.

Weakly segregated systems, Todt > Tc < Tg with hard confinement. In this case, the crystallization of the semicrystalline block can overwhelm the microphase segregation of the MD structures even though the amorphous block is glassy at the crystallization temperature, because of the weak segregation strength [19]. 

Couchman and Karasz (11) recently have made some calculations indicating that spherical microphases should exhibit increased glass-transition temperatures because of an increased pressure inside such microphases attributed to the surface tension between microphases. Since there is some doubt about the existence of a surface of tension in the Gibbs sense (12) between chemically linked microphases, we shall simply note that these calculations are the only ones in existence that indicate a possible reason for an increase in the Tg of a glassy microphase and, in addition, that these calculations also postulate differences in Tg with differences in morphology. For example, this surface-tension-dependent effect would not be expected in samples with lamellar morphology, no matter how small the width of each lamella. 

Figure 2. Glass-transition temperatures of the glassy microphases of various block copolymers containing styrene blocks vs. the molecular weights of the styrene blocks as determined by DTA or DSC.

Table II shows the values obtained for the glassy phase of the styrene-dimethylsiloxane block copolymers used in this work in the case of the block copolymers, neither DSC nor DTA gave a consistently higher value of Tg. The only peculiar sample in the table is sample R14, which exhibited two Tgs on both instruments at least occasionally during the first heating cycle. Although the presence of two Tgs implies some kind of phase separation, possibly in addition to microphase separation, the GPC of this sample shows no double peak, shoulder, or other peculiarity which might explain its peculiar phase behavior. This sample is an anomaly unless one wishes to dismiss all first heating data.

We plan to continue studies on the glass-transition temperatures of the glassy microphases in block copolymers and, in the future, we also shall investigate the glass-transition temperatures of the rubbery microphases. 

Figure 13.2 Illustration of the network morphology of a microphase-separated triblock copolymer with the glassy end blocks in hard spherical domains bridged by the rubbery center blocks.

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