Progress 09/01/07 to 02/28/10
Outputs
OUTPUTS: Broadband dielectric relaxation spectroscopy (DRS) experiments were conducted on a number of modified starches in order to determine their conductivity and to explore the molecular level motions connected to ion conduction. In one effort, a comprehensive investigation of dynamics of a carboxylated starch was conducted. In order to minimize the possible effect of moisture on the findings, all samples were soaked at 120 oC under dry nitrogen before starting the measurements. The assignment of relaxation processes was made based on a comparison between conduction behavior and dielectric relaxations. We have also investigated the dynamics of soluble starch/glycerol hybrids in detail by DRS. In addition, we have also studied the role of ionic liquids, in particular 1-ethyl-3-methylimidazolium tetrafluoroborate, on starch conduction using DRS. The ionic liquid was added to soluble starch in various ratios, and some materials were additionally plasticized with glycerol. All samples were prepared in film form at ambient conditions and dried for 24 hours before measurement. PARTICIPANTS: Dr HyungKi Lee, post-doctoral fellow TARGET AUDIENCES: Not relevant to this project. PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
The effect of glycerol on the dynamics of soluble starch was examined first. The alpha process (dynamic Tg) is barely visible when the glycerol content is increased to above 20 w/w% due to the high conductivity contribution of glycerol. The alpha process for the starch-glycerol hybrids exhibits clear VFT temperature dependence indicating cooperative motion of starch with glycerol. We observe a gradual decrease of the dynamic glass transition temperature with increasing glycerol content due to the strong interaction between OH groups in glycerol and hydrophilic starch molecules. As glycerol increases and reaches 33 w/w%, the intensity of peak abruptly increases and the shape becomes narrower. Furthermore the frequency maximum shifts in the reverse direction (higher frequencies) due to phase separation. The relaxation strength rises gradually from 2.6 for neat starch but jumps from 5 to 21.1 when the glycerol content increases from 29 to 33 w/w%. Chemically modified starches in which hydroxy methyl groups are replaced by carboxylic salts have been examined to investigate the ionic conduction and chain relaxation behavior under well-controlled laboratory conditions. The principal objective of this work was to explore the role of the carboxylic salt as well as that of glycerol on the conductivity and chain mobility. The alpha process becomes Debye-like and narrower as the degree of carboxylation increases. There is no clear evidence of decoupling between conductivity and the α-process, indicating that the charge transport is highly coupled to segmental chain dynamics. The segmental motion is accelerated further with increasing carboxylation. The dynamic glass transition temperature gradually decreases with increasing carboxylation. Even though the increase in conductivity with increasing degree of carboxylation itself was not remarkable, the synergistic effect of glycerol together with the carboxylic salt is outstanding, indicating that the ionic conduction is presumably accelerated when glycerol is present. The dc conductivity of carboxylated starch as well as starch/glycerol hybrids exhibits VFT behavior, indicating that the conduction is dominated by segmental motion of neat starch as well as glycerol. Finally, the effect of the ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate) and glycerol on the conductivity and dynamics of soluble starch has been examined using dielectric spectroscopy. As ionic liquid content increases, the segmental relaxation shifts to higher frequencies, perhaps due to coordination with charged species. The effect of glycerol on the dc conductivity of starch/IL hybrids was investigated. Before phase separation between glycerol and the hybrid, the temperature dependence of the conductivity exhibits Arrhenius behavior. However, when the glycerol content exceeds 29 wt%, conductivity exhibits VFT temperature dependence. This indicates that the conduction is dominated by segmental motion of the neat glycerol. Moreover dc conductivity levels off as the glycerol content reaches 29 wt%.
Publications
- No publications reported this period
|
Progress 09/01/08 to 08/31/09
Outputs
OUTPUTS: Broadband dielectric relaxation spectroscopy (DRS) experiments were conducted on a number of modified starches in order to determine their conductivity and to explore the molecular level motions connected to ion conduction. In one effort, a comprehensive investigation of dynamics of a carboxylated starch was conducted. In order to minimize the possible effect of moisture on the findings, all samples were conditioned at 120 deg C under dry nitrogen before starting the measurements. The assignment of relaxation processes was made based on a comparison between conduction behavior and dielectric relaxations. The effect of glycerol has been addressed. We have also investigated the role of ionic liquids, in particular 1-ethyl-3-methylimidazolium tetrafluoroborate, on starch conduction using DRS. The ionic liquid was added to soluble starch in various ratios, and some materials were additionally plasticized with glycerol. All samples were prepared in film form at ambient conditions and dried for 24 hours before measurement. PARTICIPANTS: James P Runt, PD Gregory Ziegler, co-PD HyungKi Lee, post-doctoral fellow TARGET AUDIENCES: The technical communities in food and agricultural sciences, as well as materials science. PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
The observed low frequency process of soluble starch is believed to be a segmental (dynamic Tg) motion of hydrogen bonded glucosidic chain segments. We observed that the segmental process systematically shifts to higher frequencies and becomes Debye-like as the degree of carboxylation increased, indicating that the bulk relaxation is connected with ionic conduction, and ion transport becomes prevalent with increasing carboxylation. The dc conductivity of the neat starch exhibits Arrhenius behavior but the carboxylated one follows a VFT form, likely due to conduction arising from a different origins. The addition of glycerol to modified starches leads to a faster segmental relaxation. This result demonstrates the role of glycerol in the dynamics of starch is pertinent to the hydrogen bonding interaction between the plasticizer and matrix polymer. As ionic liquid content increases, the segmental relaxation shifts to higher frequencies, perhaps due to coordination with charged species. There is no effect of glycerol addition to the starch/IL hybrid on the crystallinity obtained by wide-angle X-ray diffraction. The degree of B-type crystallinity of soluble starch (of potato origin) remains the same regardless of glycerol content. The effect of glycerol on the dc conductivity of starch/IL hybrids was investigated. Before phase separation between glycerol and the hybrid, the temperature dependence of the conductivity exhibits Arrhenius behavior. However, when the glycerol content exceeds 29 wt%, conductivity exhibits VFT temperature dependence. This indicates that the conduction is dominated by segmental motion of the neat glycerol. Moreover dc conductivity levels off as the glycerol content reaches 29 wt%.
Publications
- No publications reported this period
|
Progress 09/01/07 to 08/31/08
Outputs
OUTPUTS: Broadband dielectric relaxation spectroscopy (DRS) experiments were conducted on a number of modified starches in order to determine their conductivity and to explore polymer motions connected to ion conduction. In one effort, sodium single ion conductors were prepared using amylose-rich soluble starch by a TEMPO mediated oxidation process, where the primary hydroxyl groups in alpha-D-glucose units were replaced by carboxylic salts. The stoichiometric oxidation procedure was applied to efficiently control the content of C6 carboxylic acid sodium salt along starch chains. The sodium ionomer was chosen for study since no ion exchange is necessary, and we found previously that sodium and lithium ionomers behave similarly. Additional material characterization was conducted with Fourier transform infrared spectroscopy, X-ray diffraction and differential scanning calorimetry. We are also in the process of evaluating the role of ionic liquids, in particular the water soluble ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate), on starch conduction using DRS. A soluble starch was dissolved in distilled water with the ionic liquid in various ratios. All samples were prepared in film form at ambient condition and dried for 24 hours before measurements. Finally, we are also exploring conduction and its correlation with starch polymer motions in a hydroxypropylated (HP) starch using DRS. The hydroxypropyl content of this material is approximately 8.4 weight percent as measured by NMR. Materials with various water contents were prepared by placing films into desiccators with drierite and several saturated salt solutions at room temperature. The final water content of the films was determined gravimetrically. In addition, the influence of glycerol and glycerol + lithium perchlorate on ion conductivity is also being evaluated. PARTICIPANTS: Prof James Runt, co-investigator Prof Gregory Ziegler, co-investigator Dr. HyungKi Lee, post-doctoral research associate Dr. Mingfu Lu, post-doctoral research associate National Starch - collaborators TARGET AUDIENCES: The target audience for the project is broad, and includes the academic/government/industry community interested in renewable agricultural starch-based materials for non-food applications. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
For the sodium single ion starch conductors, dielectric spectroscopy measurements showed that ionic conductivity increases significantly with increasing carboxylic acid sodium salt content on starch chains (e.g. 78 percent carboxylated starch with 25 weight percent added glycerol as a plasticizer exhibited around three orders of magnitude higher ionic conductivity compared to a 25 percent carboxylated version with the same glycerol content). Four processes (termed beta, gamma, delta and sigma) were observed in DRS measurements for the original unmodified starch. The origin of the delta process is still unclear although one possibility is that it originates from water. The sigma process shows a different temperature dependence depending upon the sample properties. The extent of starch functionalization as well as the amount of added glycerol is observed to influence these relaxation processes. Initial DRS results on the ionic liquid containing starch show that ion conductivity is enhanced and proportional to the amount of added ionic liquid. There is also an effect of the ionic liquid on the crystallinity of the starch - ionic liquid system. When glycerol is also involved in the system, the effect of this plasticizer on the increase of ionic conductivity appears to be larger at lower ionic liquid concentration than at higher contents. When the weight percent of ionic liquid is over 40, a different crystal structure is observed in X-ray diffraction patterns, i.e., Vh-type. The results indicate that at higher ionic liquid contents, the ionic liquid is no longer soluble and precipitates, resulting in lower conductivity. Finally, for HP starch containing between 3 to 22 percent water, two dielectric relaxations connected with the polymer dynamics were observed: beta and gamma relaxations. With increasing water content, these relaxations move to lower temperatures. With increasing glycerol content at a fixed water concentration, both the beta and gamma processes move to lower temperatures. Both the dielectric constant and loss increase greatly with increasing glycerol content at room temperature. The addition of glycerol had a significant effect on conductivity, reaching 1E-8 S/cm at 303K when glycerol content is 100 phr (at this temperature the conductivity of the film without glycerol was under 1E-14 S/cm).
Publications
- No publications reported this period
|
|