Study On The Photoluminescence Behavior Of Cellulose During The Change Of Crystallinity And Crystal Form

Because of its special photophysical properties,good biocompatibility,environmental friendliness and adjustable structure,nonconventional luminescent materials have great application prospects in anti-counterfeiting,encryption,biological imaging,and other fields.However,there are few studies on the regulation of the emission behavior of nonconventional luminescent materials,and the emission mechanism are still controversial.For different systems,researchers have proposed different luminescence mechanisms,such as oxidation,the introduction of hydrogen bonds,and the aggregation of carbonyl groups.However,these mechanisms are only for specific luminescence systems and are not universal.The clustering-triggered emission(CTE)mechanism proposed recently can reasonably explain the luminescence behavior of nonconventional luminescent materials,which has a certain universality.Further studies have shown that crystallization behavior can enhance the luminescence intensity of nonconventional luminescent compounds.Recently,cellulose can emit blue light and has room temperature phosphorescence(RTP)emission properties under UV irradiation.Cellulose is a natural polymer material with various crystal configurations and adjustable crystallinity.The crystallinity and crystal form can be adjusted by simple alkali treatment.This paper takes microcrystalline cellulose(MCC)with low polymerization degree and relatively simple structure and cotton fiber with high polymerization degree and relatively complex structure as the research objects.The luminescence behavior can be adjusted by adjusting their crystal form and crystallinity.The unique emission behavior of these materials,especially the persistent room temperature phosphorescence(p-RTP)emission properties,is explained by the single crystal structure analysis and theoretical calculation of the cellulose Ⅰ and cellulose Ⅱ,combined with the CTE mechanism.Simultaneously,the change of the luminescence behavior caused by the change of crystallinity of cotton fiber will provide theoretical guidance for the real-time detection of crystallinity in cotton fabric during mercerization.The above studies also further prove the rationality of CTE explaining the emission behavior of nonconventional luminescent materials.The main contents are as follows:1.Treat MCC with different concentrations of Na OH to obtain 7 kinds of MCC products with different crystallinities and crystal forms.The crystallinity of MCC decreases first and then increases with the increase of alkali concentration.When the alkali concentration is 15 wt%,the crystallinity of MCC is the smallest.When the alkali concentration is 17.5 wt%,the crystal form has been completely transformed from cellulose type I to cellulose type II.The photophysical properties of 7 kinds of MCC are studied,and the results show that the luminescence of excitation wavelength dependent under different wavelength UV light irradiation.The emission spectra of all MCCs under 272 nm excitation have two emission peaks.The intensity of the emission peak at422 nm is consistent with the change trend of its crystallinity.The intensity is deemed the emission from the MCC crystal region at 422 nm.The change trend of MCC photoluminescence quantum yield is also consistent with the change of its crystallinity.MCC also has p-RTP emission behavior,and has a weak excitation wavelength dependence.The above research shows that there are clusters with different emission capabilities in the MCC,that is,multiple emission centers,which is consistent with the related content of the CTE mechanism.In addition,the change in the p-RTP lifetime within a certain range also increases with the increase of crystallinity,which further shows that crystallization is beneficial to light emission.However,the p-RTP emission intensity changes of MCC with different crystallinities are not regular.It is inferred that the occurrence of this phenomenon may be related to the change of the crystal form of MCC.By adjusting the crystallinity and crystal shape of MCC,samples with different P-RTP lifetime are obtained,and the samples are successfully applied in the field of anti-counterfeiting and encryption.2.To explain the unique emission behavior of these MCCs,the single crystal structure of cellulose Ⅰ and cellulose Ⅱ were analyzed and theoretically calculated.The results show that there are a large number of O···O interactions around the single molecules of cellulose Ⅰ and Ⅱ,so that oxygen atoms form oxygen clusters,and different oxygen clusters lead to the formation of multiple emission centers.Cellulose Ⅰ and cellulose Ⅱ have different molecular accumulations.In addition,there is O···H short-range action between the two layers of cellobiose molecules,which further increases the structural rigidity of cellulose molecules.There are more O···H interactions between type Ⅰ molecules than type Ⅱ,indicating that type Ⅰ has better structural rigidity and is more conducive to the luminescence of cellulose Ⅰ.O···O interactions and O···H interactions together form an electronic communication network,which increases the delocalization of electrons.Theoretical calculations for the monomers,dimers,and trimers of the two crystal types of cellulose,as the number of aggregates increases,the delocalization range of electrons increase,which is beneficial to the luminescence of cellulose,especially p-RTP emission.Single crystal analysis and theoretical calculations further prove that the CTE mechanism can reasonably explain the luminescence behavior of cellulose during crystallinity and crystal form changes.3.To apply the pattern that the crystallinity of cellulose can be obtained by fluorescencemonitoring method to the alkali mercerization process of cotton fiber,the luminescence properties of cotton fiber are studied by the same method in this paper.The result shows that the crystallinity of cotton fiber decreases rapidly at first and then slowly increases with the increase of alkali concentration,and the crystallinity is the smallest when the alkali concentration is 17.5 wt%,and the crystallinity is completely transformed from cellulose type I to cellulose II when the alkali concentration is 25 wt%.Both the fluorescence and p-RTP emission of cotton fiber have excitation wavelength dependence,and the changes in its photoluminescence quantum yield and p-RTP lifetime increase with the increase of crystallinity.However,the fluorescence emission intensity and the p-RTP emission intensity change are opposite to the crystallinity change.The emission is strongest when the alkali concentration is 17.5 wt%,which is opposite to the emission of MCC.It is inferred that the complex physical structure of cotton fiber is related to the stacking state of cellulose multilayer molecules.The study proves that the crystallinity of cotton fiber can be obtained from the fluorescence-monitoring method,which provides a monitoring method for the degree of crystallinity change of cotton fiber in the mercerizing process.