Uniquely broad glass transition temperatures of gradient copolymers relative to random and block copolymers containing repulsive comonomers
Uniquely broad glass transition temperatures of gradient copolymers relative to random and block copolymers containing repulsive comonomers
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DOI：
10.1021/ma061241f
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发表时间：
2006-09-05
期刊：
影响因子：
5.5
通讯作者：
Torkelson, John M.
中科院分区：
文献类型：
--
作者：
Kim, Jungki;Mok, Michelle M.;Torkelson, John M.
采用氮氧自由基聚合法合成了苯乙烯（S）和4-乙酰氧基苯乙烯（AS）的梯度和嵌段共聚物，采用常规自由基聚合法合成了S/AS无规共聚物。将S/AS共聚物水解以产生S/4-羟基苯乙烯（HS）共聚物。凝胶渗透色谱法和H-1 NMR的等分试样在聚合过程中产生的梯度共聚物结构的控制性质的证明。使用差示扫描量热法热曲线的导数比较玻璃化转变温度（T-g）响应，其中导数超过基准水平的温度范围等同于T-g宽度。在每个无规共聚物中获得单一的窄T-g，这与有限组成纳米异质性的单相一致。两个狭窄的T（g）是明显的，在每个嵌段共聚物，符合良好发展的纳米相含有几乎纯的S或几乎纯的AS或HS单元与一个非常窄的界面产生没有指示的中间Tg。相比之下，在许多S/HS梯度共聚物中观察到类似于65-80摄氏度的T-g宽度，这与其中晶胞组成正弦变化的有序纳米结构一致。讨论了在阻尼应用中利用梯度共聚物的宽Tg的可能性。
Nitroxide-mediated controlled radical polymerization is used to synthesize gradient and block copolymers of styrene ( S) and 4-acetoxystyrene ( AS), and conventional free radical polymerization is used to synthesize S/AS random copolymers. The S/AS copolymers are hydrolyzed to yield S/4-hydroxystyrene (HS) copolymers. Gel permeation chromatography and H-1 NMR of aliquots taken during polymerization yield proof of the controlled nature of the gradient copolymer structures. The glass transition temperature (T-g) responses are compared using the derivative of differential scanning calorimetry heat curves, with the temperature range over which the derivative exceeds a base level being equated to the T-g breadth. A single, narrow T-g is obtained in each random copolymer, consistent with a single phase of limited compositional nanoheterogeneity. Two narrow T(g)s are evident in each block copolymer, consistent with well-developed nanophases containing nearly pure S or nearly pure AS or HS units with a very narrow interphase yielding no indication of an intermediate Tg. In contrast, T-g breadths of similar to 65-80 degrees C are observed in many S/HS gradient copolymers, consistent with ordered nanostructures in which the unit cell composition varies sinusoidally. The possibility of capitalizing on the broad T-g of gradient copolymers in damping applications is discussed.