Preparation and Pyrolysis Behaviors of Poly(L-lactide)/Layered Double Hydroxide Nanocomposites

Abstract

本研究利用共沉澱法(coprecipitation)成功合成出具有良好層狀堆疊結構之鎂鋁型層狀雙氫氧化合物(layered double hydroxide, LDH)。由於LDH為一親水性層狀無機材料且高度的層間靜電力所引起之層間距離狹小問題，使其不易脫層分散於高分子基材中。為了改善LDH與親油性高分子基材間相容性與提升LDH層間距離，本研究首先利用具有生物相容性與生物降解特性之聚乳酸低聚合物[polylactide with carboxyl end group (PLA-COOH)]與聚麩胺酸[sodium γ-polyglutamate (γ-PGA)]進行LDH表面改質，以利LDH在與高分子製備成奈米複合材料時，高分子鏈能滲透或插層進入LDH層間，進而獲得具有較佳LDH分散性之高分子奈米複合材料。從廣角X光繞射儀(WAXD)之結果發現LDH經由PLA-COOH和γ-PGA改質後(p-LDH和γ-LDH)，其層間距離由7.98 A分別增加至14.03 A和15.29 A。元素分析儀(EA)與感應耦合電漿原子發射光譜分析儀(ICP-AES)之結果亦顯示p-LDH和γ-LDH層間之硝酸根離子於經過陰離子交換法改質後，分別被PLA-COOH和γ-PGA給取代。在研究LDH熱裂解行為對其結構影響中，由臨場廣角X光繞射儀(in situ WAXD)與臨場傅立葉紅外線光譜儀(in situ FTIR)之結果得知LDH於升溫過程中，層間水分子之逸失行為會使得LDH於氫氧金屬結構層、層間水分子與陰離子間之氫鍵作用受到破壞，進而導致LDH層間距離縮小與層狀堆疊結構產生改變。 研究中，同時藉由溶液混合法與熔融混煉法製備聚乳酸[poly(L-lactide), PLLA]/層狀雙氫氧化合物奈米複合材料，並探討LDH層狀無機材料之添加對於PLLA材料之機械性質、阻水氣性質與熱裂解行為之影響。由WAXD搭配穿透式電子顯微鏡(TEM)之結果證實LDH於未經過表面改質前，是以團聚型態存在於PLLA基材中；而利用p-LDH和γ-LDH所製備之PLLA/LDH高分子奈米複合材料結果顯示經表面改質之p-LDH和γ-LDH層板分別以剝離和插層型態存在於PLLA高分子基材。由動態熱機械分析儀(DMA)之量測結果發現，相較於未改質LDH與γ-LDH之添加，具有良好分散程度之p-LDH的添加，可使得PLLA儲存模數(storage modulus)顯著提升。當PLLA高分子添加3 wt% p-LDH時，可使得PLLA儲存模數提升120%。在利用熔融混煉法製備PLLA/LDH與PLLA/γ-LDH奈米複材薄膜之阻水氣性質的量測結果中，顯示當PLLA高分子中添加5 wt% γ-LDH後，使得PLLA之阻水氣性質提升116%。 然而，熱重量分析儀(TGA)結果顯示鎂鋁型LDH之添加可能於高溫環境下催化並加速PLLA之熱裂解行為，使得PLLA之熱穩定性隨著LDH含量增加而明顯下降。由PLLA高分子裂解動力學研究結果亦發現PLLA之熱裂解活化能於未改質或改質LDH添加後皆隨著含量增加產生下降之趨勢，顯示LDH的添加，於高溫環境下可能與PLLA產生反應並劣化了PLLA材料之熱穩定性。在利用熱裂解氣相層析質譜儀(Py-GC/MS)分析PLLA熱裂解揮發物質時，結果顯示LDH的添加改變PLLA熱裂解產物之組成比例，隱含著PLLA之熱裂解行為與機制於LDH添加後產生改變。相較於PLLA高分子是於較高熱裂解溫度下藉由分子間和分子內酯交換反應(inter- and intra-transesterification)生成丙交酯(lactide)和環狀低聚合物(cyclic oligomer)；PLLA/LDH奈米複合材料之熱裂解則是在含鎂和鋁之LDH催化下，利用解聚合反應(unzipping reaction)產生以lactide為主之熱裂解產物。此外，從熱裂解結果也發現層間陰離子之改變亦可能對PLLA/LDH之熱裂解行為與機制產生影響。

The present study aims to investigate the relationship between the structure and property of novel nanocomposites that are prepared with eco-friendly magnesium-aluminum (Mg-Al) layered double hydroxide (LDH) as a nanofiller and biodegradable poly(L-lactide) (PLLA). In addition, to clarify the influences of LDH on the thermal stability and/or thermal degradation behavior of PLLA, the pyrolysis process of these novel bionanocomposites was kinetically and mechanistically investigated. To enhance chemical compatibility between the hydrophilic LDH layers and the hydrophobic PLLA matrix, a [Mg0.68Al0.32(OH)2](CO3)0.06(NO3)0.20‧H2O was modified by a polylactide with carboxyl end group (PLA-COOH) and a sodium γ-polyglutamate (γ-PGA) using the anion exchange method, respectively. The composition of the organo-modified LDH (i.e., p-LDH and γ-LDH), identified by element analyzer (EA) and inductively coupled plasma atomic emission spectrometer (ICP-AES), exhibits that the nitrates are replaced by organomodifiers. After intercalation, both organo-modified LDHs, with the interlayer distance increased from 7.98 A to 14.03 A and 15.29 A, respectively, may be suitable for preparing PLLA/LDH nanocomposites. Moreover, the results suggested that both organomodifiers were arranged in the interlayer spacing of LDH in a tilted monolayer arrangement. In situ Fourier transform infrared spectroscopy (in situ FTIR) and in situ wide-angle X-ray diffraction (in situ WAXD) results indicated that the nature of intercalated anions and elimination of water molecules can influence the hydrogen bonding area among the intercalated anions, water molecules, and hydroxide sheets of LDH. Nanocomposites with various loadings of unmodified and organo-modified LDHs were prepared by a solution blending and a melt blending process. The results from WAXD and transmission electron microscopy (TEM) showed that unmodified LDH was unevenly dispersed throughout the PLLA matrix with the formation of large aggregates. Conversely, p-LDH and γ-LDH allow the formation of a randomly dispersed and an intercalated nanocomposite, respectively. The storage modulus determined by dynamic mechanical analysis showed that mechanical properties of exfoliated nanocomposites (3 wt% p-LDH) exhibit 120% enhancement when compared to that of the PLLA. The results can be attributed to the better dispersion of LDH platelets and the good interaction between the LDH layers and the PLLA matrix. Thus, the water vapor barrier property of the PLLA/LDH films was improved with increasing LDH concentration and dispersion of LDH. According to thermogravimetric analysis (TGA) results, the thermal stability of PLLA/LDH nanocomposites was significantly lower than that of a pure PLLA matrix, perhaps because the metallic compounds of LDH catalyzed the degradation of the PLLA. Moreover, the degradation activation energies of the PLLA/LDH samples, analyzed using the Kissinger method and the Ozawa-Flynn-Wall method, revealed that the decrease in thermal stability is in the following order: neat PLLA > intercalated PLLA-γL nanocomposites > PLLA-L microcomposites > exfoliated PLLA-pL nanocomposites. Finally, pyrolysis products of these PLLA/LDH nanocomposites were studied using pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The results indicated that the primary pyrolyzates of PLLA/LDH samples were L,L- and/or D,D-lactides, while the pyrolyzates of neat PLLA were composed of a large amount of cyclic oligomers as well as lactides. From these results, it follows that the metallic compounds of LDH significantly decreased the thermal degradation temperatures of PLLA and caused the formation of primary L,L- and/or D,D-lactides via unzipping depolymerization.