探討巨大體積官能基對於聚甲基丙烯酸酯共聚物及聚甲基丙烯醯胺衍生物其氫鍵作用行為之影響

Abstract

本論文旨在探討在側鏈具有巨大體積官能基對於高分子之氫鍵作用力行為的影響。在甲基丙烯酸酯及甲基丙烯醯胺的共聚物中，少量的巨大側鏈官能基不會影響分子內及分子間作用力，甚至可以使甲基丙烯醯胺之分子內氫鍵轉換為甲基丙烯醯胺與甲基丙烯酸酯之間的分子內氫鍵，進而提升共聚物之玻璃轉換溫度；但是大量的巨大側鏈官能基則會同時阻止分子內氫鍵以及分子間氫鍵的形成，而降低共聚物之玻璃轉換溫度。 在聚甲基丙烯醯胺(poly(methacrylamide))中，當氮上的巨大體積官能基具有共振結構，例如苯環，可以與甲基丙烯醯胺中的C=O鍵進行電子共振時，會進一步影響C=O做為氫鍵接受者(hydrogen-bond acceptor)及N-H鍵作為氫鍵供給者(hydrogen-bond donor)的能力，與具有相等體積之環己烷基(cyclohexyl group)相較，苯環使C=O鍵的氫鍵接受者的能力降低，卻提高N-H鍵的氫鍵供給者的能力，且氮上苯環與、C=O之□鍵以及吡啶苯環經由氮上孤電子對所構成的平面共振(planar delocalization)可以進一步增加聚甲基丙烯醯胺與聚(4-乙烯基吡啶) (poly(4-vinyl pyridine)，P4VP)之間的相容度。 當苯環上有誘導取代基時，會進一步影響此共振效應對於氫鍵作用行為的影響；若此誘導取代基為推電子基(electron donating group)，則會使N-H的氫鍵供給者能力下降，若此誘導取代基為拉電子基(electron withdrawing group)則使增加N-H的氫鍵供給者能力，增加與P4VP之間的相容度。 若氮上取代基為丁基(butyl group)，相較於normal-butyl group，tert-butyl group的體積較為巨大，且無法藉由轉動C-N鍵來消除結構中的立體張力(steric strain)；即使如此，butyl基之結構異構(constitutional isomeric)效應對於聚甲基丙烯醯胺的C=O氫鍵接受者能力以及N-H氫鍵供給者能力影響不大，由於巨大體積對於氫鍵的形成具有遮蔽效應(screening effect)，因此降低甲基丙烯醯胺中的自身氫鍵之平衡常數，而增加分子間氫鍵平衡常數/自身氫鍵常數比例(KA/KB)，而略為增加聚甲基丙烯醯胺與P4VP之間的相容度。 也由於巨大體積官能基的形式不同，聚甲基丙烯醯胺與P4VP之間的相容度受到溶劑影響的程度也會有所不同。若溶劑本身具有官能基可以與甲基丙烯醯胺產生氫鍵，例如N,N-dimethylformamide (DMF)，由於溶劑揮發的過程是一個動力學的過程(kinetic process)，甲基丙烯醯胺與DMF之間的氫鍵來不及轉換為甲基丙烯醯胺的自身氫鍵或與吡啶的分子間氫鍵，進而降低甲基丙烯醯胺與吡啶之間的相容度。因此使用TCE (tetrachloroethane)或DMF會有不同的相容度表現；但是對於氮上為tert-butyl group的聚甲基丙烯醯胺而言，溶劑效應的影響較小。

The main purpose of this study is to explore the steric bulky substituent effects on hydrogen-bond behaviors in poly(methyl methacrylate) terpolymers and poly(methacrylamides) derivatives. In poly(methyl methacrylate) terpolymers, small content of the bulky steric group into the PMMA-co-PMAA main chain does not sacrifice the fraction of hydrogen-bonded association in and does not cause Tg decrease. In addition, the steric bulky group plays the role of inert diluent to convert portion of the strong self-associated hydrogen bonded amide groups into inter-associated hydrogen bonding between carbonyl groups of ester units and amide unites. Excess amount of the bulky steric group can obstruct hydrogen-bond interaction, not only the inter-association between esters and amides but also the self-association of amides, and thus reduce Tg. In poly(methacrylamide)s, if the N-substitution possess resonance characteristic, i.e., aromatic group, the lone-paired electrons on nitrogen can delocalize over the π system of aromatic ring and carbonyl group simultaneously, and further, affect the hydrogen-bond-acceptor ability of C=O and hydrogen-bond-donor ability of N-H. Compared with cyclohexyl group, aromatic group as N-substitution can enhance the hydrogen-bond-donor ability of N-H and reduce the hydrogen-bond-acceptor ability of C=O through electron delocalization. Moreover, a planar delocalization, constructed by the aromatic-□ system of pyridine in P4VP, the aromatic-□ system and the carbonyl-□ system of amide group in poly(N-phenyl methacrylamide)s via the lone-paired electrons on nitrogen atom of PNPAA, is formed to be a driving force to replace the self-association of amide units with inter-association to pyridine units, and thus better miscibility with P4VP is obtained. The inductive substitution at para position of aromatic ring in poly(N-phenyl methacrylamide), regardless of electron-donating or electron-withdrawing group, can further affect the electron delocalization over the benzene ring and the carbonyl π system via the lone-paired electrons on nitrogen atom, changing the electron-cloud density of N－H or C＝O groups and the strength of self-association and inter-association with P4VP. Moreover, comparing to electron-donating group (-OCH3), electron-withdrawing group (-Br) can strengthening inter-associative hydrogen-bonding interaction with pyridine units of P4VP. If the N-substitution is butyl group, the constitution of butyl group can not affect the hydrogen-bond-acceptor ability of C=O and hydrogen-bond-donor ability of N-H in poly(methacrylamide). However, compared with n-butyl group，t-butyl group is more steric bulky, and the steric strain in poly(methacrylamide)s cannot be reduced no matter how the steric bulky t-butyl group rotates around the C-N bond in amide units when the N-substitution is t-butyl group. In that case, there is few the constitutional isomeric effects on the hydrogen-bond-acceptor of the carbonyl group and the hydrogen-bond-donor ability of N-H group in poly(methacrylamide)s. The screening effect of the bulky steric t-butyl group reduces the KB in poly(t-butyl methacrylamide) and enhance the ratio KA/KB, and thus slightly enhance the miscibility between poly(methacrylamide) and P4VP. Finally, to assess possible solvent effects on the heterogeneity of the blends, TCE, a polar solvent that may be capable of forming relatively weak hydrogen bonds with the amide, and DMF, which can form strong hydrogen bonds with the amide, are chosen as alternative casting solvents. The blends cast from TCE display homogeneous single-glass-transition behavior but two clear Tg in the blends cast from DMF, to speak nothing of significant dynamic heterogeneity in the blends cast from DMF when examined by solid-state NMR. When DMF is used as the casting solvent, the total amount of hydrogen-bonding interaction, not only self-association but also inter-association, is reduced.