离子液体增塑二醋酸纤维素的红外及二维相关红外光谱研究

Studies on Plasticized Cellulose Diacetate with Ionic Liquid by Fourier Transform Infrared Spectroscopy(FTIR)and Two-Dimensional Correlation Spectroscopy

以离子液体1-丁基-3-甲基咪唑六氟磷酸盐(BMIMPF6)为增塑剂,采用傅里叶变换红外光谱(FTIR)和二维相关光谱分析技术,研究BMIMPF6增塑二醋酸纤维素(CDA)体系在室温(25℃)以及升温过程(35~210℃)中CDA与离子液体的相互作用,为离子液体增塑CDA熔融加工提供理论指导。结果表明,温度25℃时,增塑体系中CDA形成氢键的羟基、乙酰基基团、BMIMPF6中阴离子以及咪唑环上C-H相较于未增塑前均出现特征峰位置偏移或强度变弱的情况,且随着增塑剂含量增加,变化程度加剧,证实离子液体与CDA发生了相互作用,这种相互作用存在于离子液体咪唑环上活泼氢与CDA乙酰基、咪唑环上活泼氢与CDA骨架C-O以及阴离子与CDA乙酰基之间,有助于破坏和削弱CDA中原有氢键网络结构。在升温过程中,未增塑CDA形成氢键的羟基吸收峰呈现强度变小、高频偏移的规律,并一直存续于整个升温过程,乙酰基基团相应特征峰维持稳定的状态,而增塑后CDA中羟基吸收峰变化程度较之于增塑前明显加剧,且温度高于180℃后羟基完全消失,乙酰基中C=O,C-O以及骨架上C-O对应吸收峰变形、强度大幅度减小,说明增塑后CDA热稳定性下降,对热的抵抗力更弱。进一步对增塑CDA在35~170℃之间的二维红外相关技术分析表明,升温过程中增塑CDA体系经历离子液体咪唑环上孤立活泼氢和CDA中自由羰基先发生相互作用,随后离子液体阴阳离子解除相互作用,CDA中原本缔结氢键的乙酰基相应基团因氢键受热被破坏而释放出自由羰基、酯基C-O和甲基,并与离子液体阴阳离子形成了相互作用的过程,此后CDA骨架上的C-O也参与到与离子液体的相互作用中。CDA乙酰基与BMIMPF6阴阳离子相互作用比CDA原有氢键作用结合力和离子液体阴阳离子之间的相互作用更强,并随着温度升高进一步加强。

In this study,the interaction between cellulose diacetate(CDA)and ionic liquid(IL)was studied at room temperature(25℃)and in the temperature range from 35 to 210℃with 1-butyl-3-methylimidazolium hexafluorophosphate(BMIMPF6)as plasticizer,using Fourier transform infrared spectroscopy(FTIR)and two-dimensional correlation spectroscopy analyzer,guiding for melt process of plasticized CDA in theory.The results showed that at 25℃changes occurred in the positions or intensity of absorption bands assigned to hydroxyl forming hydrogen bonds,acetyl groups in CDA,the anion in BMIMPF6 and C-H in imidazole ring in plasticized CDA compared with CDA,in which absorption peaks’positions would shift or their intensity would become weaker,and the changes depended on the content of BMIMPF6 in CDA.It has proved that there were interactions between BMIMPF6 and CDA,and the interactions were formed between C-H in imidazole ring of BMIMPF6 and acetyl group in CDA,C-H in imidazole ring and C-O in the backbone of CDA,the anion of BMIMPF6 and acetyl group in CDA,which helped destroy and weaken the original hydrogen bonds network structure in CDA.In un-plasticized CDA,the absorption peaks of OH group forming intramolecular hydrogen bonds existing in the whole heating process from 35 to 210℃became gradually weaker and shifted towards the shorter wavelength side with the rise of temperature,and acetyl group absorption bands remained almost unchanged.However the changes of absorption peaks assigned to OH group in plasticized CDA had been accelerated,and the OH group absorption peaks vanished completely at a temperature above 180℃.Besides,the absorption bands assigned to C=O and C-O in acetyl group and C-O in the backbone of CDA had shifted and decreased in the intensity in plasticized CDA.It can be concluded that the thermal stability of CDA was reduced by BMIMPF6.Then a further investigation into plasticized CDA was carried on in the temperature range from 35 to 170℃by two-dimensional correlation spectroscopy,and the sequential order of the responses from different chemical groups in plasticized CDA towards the variation of temperature was illuminated.The results showed that the dominating changes of interactions in plasticized CDA during heating would start with the interaction between C-H in imidazole ring of BMIMPF6 and C=O in acetyl group in CDA,then remove the interaction between anions and cations in BMIMPF6.Subsequently isolated C=O,C-O and methyl groups were released fromassociated acetyl groups originally forming H-bonds as H-bonds were broken by heating,and then interacted with anions and cations in BMIMPF6 respectively,which was followed by the interactions between C-O in the backbone of CDA and BMIMPF6.The interaction between CDA and BMIMPF6 was stronger than original H-bonds interactions in CDA and interactions between anions and cations in BMIMPF6,and would be reinforced with the rise of temperature.