Styrene monomer was mixed with quaternary ammonium montmorillonites (org-MMTs) and polymerized in the presence of radical initiator. The intercalated and exfoliated structures of polymerized polystyrene/org-MMT nanocomposites were identified by XRD and TEM. TEM was employed to examine the microstructures of ordered intercalates as well as exfoliated MMT layers. The observed microstructures were compared with those concluded from XRD results.
The radical polymerization of the styrene penetrated between the interlayers of org-MMT has resulted in intercalated PS/org-MMT hybrid. The intercalated nanocomposites exhibited the microstructures which is very similar to that of the unintercalated organically modified layered silicate (org-MMT). The X-ray diffraction study on PS/org-MMT nanocomposite revealed the distance of MMT interlayers increased from 11.8Å for Na-MMT and to 34Å for PS/org-MMT nanocomposite. This finite layer expansion associated with polymer intercalation resulted in the appearance of a new basal reflection corresponding to the larger gallery height. This result was in good agreement with the interlayer spacing determined by transmission electron microscope (TEM). Fourier transform infrared (FTIR) spectra further confirmed the formation of polystyrene in the MMT gallery. The ratio of intercalated PS to MMT ranged from 0.62 to 0.94 g-PS / 1g-MMT depending on the structure of organic group in org-MMT and on the amount of MMT.
These intercalated nanocomposites exhibited higher thermal stability compared with the virgin polystyrene (PS) or PS/pristine-MMT microcomposites. PS/Bz-MMT, containing benzyl-unit similar to styrene in org-MMT, exhibited higher decomposition temperature even for MMT loading content as low as 0.3 wt.% compared with other PS/org-MMT nanocomposites. Therefore, intercalation of the MMT particles in the polymer matrix is dependent on the accessibility of the monomers to the MMT galleries. It was found that the structural affinity between styrene monomer and the organic group of org-MMT played an important role in the structure and the properties of nanocomposite.
The exfoliated nanocomposites, on the other hand, exhibited microstructures with increased layer disorder and layer spacing. In these exfoliated nanocomposites, the extensive layer separation associated with delamination of the original silicate structure in the polymer matrix resulted in the eventual disappearance of any coherent X-ray scattering from the layers. We have also polymerized the exfoliated PS/Bz-MMT nanocomposites by copolymerization of styrene with proper comonomers, such as macromonomer, maleic anhydride. As the amount of these comonomers increases, a transition from intercalated to exfoliated nanocomposite is observed, as judged by XRD. In addition to the copolymerization method, the additional shear force to the intercalated nanocomposites affected the formation of nanocomposites, resulted in more disordered or exfoliated nanocomposite structures.
These intercalated and exfoliated PS/Bz-MMT exhibited the improved properties compared to that of polystyrene at a very small concentration level of MMT. The reinforcement effect observed in the Ps/Bz-MMT nanocomposites should be attributed to their nanostructures in which the silicate layers are dispersed in PS matrix. In the comparison of the mechanical properties of the exfoliated and intercalated nanocomposites, we found that the exfoliated nanocomposite showed improved performance in the tensile strength, elongation, and impact strength at low MMT loadings, but similar properties in the modulus and thermal expansion coefficient, relative to the intercalated nanocomposite. Therefore, the exfoliated PS/Bz-MMT nanocomposite was favored over intercalated analogues for improving the mechanical performance of a polymer.
여러 종류의 4가 유기암모늄이온으로 처리된 유기화된 몬모릴로나이트를 스티렌 단량체에 충분히 용해시킨 후 라디칼성 개시제를 이용하여 나노복합체를 합성하고 그 특성을 연구하였다. 나노복합체 합성시에 적절한 단량체의 조합으로, 가능한 두가지 형태의 나노복합체를 제조하였다. 즉, 몬모릴로나이트 층사이에 폴리스티렌이 존재하는 형태와 각 층이 매트릭스 수지내에 나노-크기로 완전히 분산된 형태의 나노복합체를 각각 제조하였다. 유기화-몬모릴로나이트의 층간으로 침투된 스티렌단량체의 단일중합은 폴리스티렌이 층간에 삽입된 형태의 나노복합체를 형성하였다. 층사이에 존재하는 폴리스티렌에 의해 층간의 거리는 나트륨-몬모릴로나이트의 층간거리 11.8Å 보다 상당히 증가된 약 34Å 정도를 나타내었다. 이러한 결과는 투과전자현미경으로 분석된 결과와 일치하였다. 형성된 나노복합체의 몬모릴로나이트 층사이에 존재하는 폴리스티렌의 함량은 0.62~0.94 (g-폴리스티렌/1g-몬모릴로나이트) 수준이었다.
제조된 나노복합체는 폴리스티렌 자체 및 나트륨-몬모릴로나이트의 복합체보다 향상된 열안정성을 나타내었으며, 상당히 적은 량(0.3중량%)의 첨가로도 높은 열분해온도를 나타내었다. 특히 스티렌과 유사한 구조의 암모늄이온으로 처리된 유기화-몬모릴로나이트의 경우가 가장 높은 열안정성을 나타내었다. 이 결과로, 단량체와 사용된 유기암모늄의 유기성분간의 친화성이 나노복합체의 구조와 성질에 중요한 작용을 하는 인자임을 알 수 있었다. 또한, 분자크기가 크거나 친수성기를 가지는 단량체를 스티렌과 공중합하거나, 중합후에 적절한 혼련을 통하여 몬모릴로나이트의 층을 나노-크기로 분산된 나노복합체를 제조하였다. 이때 공단량체의 함량이 증가할수록 층의 분산이 용이하였다. 제조된 두 가지 형태의 나노복합체 모두 폴리스티렌보다 우수한 물성을 나타내었다. 또한 몬모릴로나이트의 층이 완전히 분산된 형태의 나노복합체가 더 우수한 물성을 나타내었다.
