摘要
以玻璃化转变的热力学理论为基础,根据相邻C原子的不同取代情况对主链化学键进行分类,假定化学键的刚性能具有加和性,从而对Gibbs-DiMarzio方程进行了修订,提出了新的Tg-序列结构-共聚物组成关系方程.新方程中包含不同三元组序列的摩尔分率和相关均聚物、周期共聚物的Tg,方程中的所有参数可以通过实验测定或计算得到,没有纯粹的拟合参数,在共聚物的结构与Tg之间建立了直接而唯一的关系,因而此方程可以方便地解释各种共聚物Tg-组成关系曲线.如果二元共聚体系的周期共聚物或交替共聚物尚未合成,还可以通过该方程由无规共聚物的Tg-组成曲线对其Tg进行预测.将该方程应用到MMA-St、E-VAc无规共聚物,得到很好的结果.
Glass transition temperature （Tg） is an important intrinsic parameter of polymers,which greatly affects their properties and application.Tg of a polymer could be changed through copolymerization.It is well known that Tg of a copolymer usually deviates from the additivity of that of the homopolymers.Tg of a copolymer was affected not only by the composition but also by the sequence distribution of the copolymer.
Many researchers have proposed various equations to describe the copolymer Tg-composition relationship based on free volume theory as well as thermodynamic theory of glass transition.Among them,Johnston and Barton considered the diad effect,and better fitting result between the theory predicted and experimental data was obtained.Whereas they failed when the Tg-composition curve is asymmetric or even S-shaped.When triad sequence effect was taken into consideration,the fitting was greatly improved.Unfortunately,there are 6 intractable parameters in the equation which greatly holdback its application.Ham and Uematsu proposed their methods to reduce the number of intractable parameters to 1,which increased the facility of the triad sequence equation.Whereas the manners they proposed were unreasonable.
It is found that all the equations proposed by now were based on the additivity of contributions of different monomers or sequences.In this paper,we propose an equation based on the additivity of contributions of different bonds concerning the substitution on the adjacent C atoms.For the case of a head-to-tail binary copolymer composed of mono-substituted vinyl monomers,the bonds were classified into 8 kinds when the substitutions on C1,C2 and C3 were concerned.Based on the thermodynamic theory of glass transition,the equation is described as below under the assumption of steady-state concentration.
Tg=nAAA TgA ＋nBBB T gB ＋2（n ABA -n AAB ）T g[AB] ＋3n AAB T g[AAB] ＋3n BBA T g[BBA]
Where nijk represents the mole fraction of triad sequence ijk,Tg[x] is Tg of periodic copolymer poly[x].nijk could be obtained experimentally or calculated from the monomer feed compositions and reactivity ratios.The equation could be applied to investigate the relation between copolymer Tgs and compositions conveniently because that it contains no mere fitting parameter.If the periodic copolymers poly[AAB] and poly[BBA] cannot be acquired,there are only two intractable parameters in the equation.Even if the alternating copolymer poly[AB] also cannot be acquired,there are only three adjustable parameters,far less than the original six in the Ham equation.More over,Tgs of them could be predicted by using the equation and Tg data of random copolymers for this case.
The new equation was applied to MMA-St and E-VAc random copolymers,and excellent fitting was obtained.Whereas large deviations pronounced when only diad sequence effect was taken into consideration.Especially for the case of E-VAc copolymers,the predicted Tgs show opposite derivation compared with the experimental results.Tgs of corresponding periodic copolymers were also evaluated.
This equation may be extended to describe multi-copolymers or block,graft copolymers only if no phase separation appears in their bulk.Also the fact of Tg changed with tacticity,such as PMMA,may be explained by using this equation.
出处
《高分子学报》
SCIE
CAS
CSCD
北大核心
2008年第6期550-554,共5页
Acta Polymerica Sinica
基金
河北省自然科学基金(基金号B2007000019)资助项目
关键词
二元共聚物
玻璃化温度
组成
化学键
序列结构
Binary copolymer, Glass transition temperature, Composition, Bond, Sequence
