Fabrication And Studies Of Polyacryamie/Lignosulfonate Composite Gel And Properties

Polyacrylamide/lignosulfonates(PAAm/LGS) hydrogels were fabricated in aqueous solution of acrylamide with a small amount of cross-linking agent N, N'-methylenebisacrylamide and lignosulfonates, with varied cations, e.g. Na⁺, Ca²⁺ and Mg²⁺. The gelation process gel related structure, properties and application were studied.Before the fabrication of semi-IPN hydrogels, electric conductivity was adopted initially as a tool to monitor the free-radical crosslinking polymerization of polyacrylamide hydrogel to study the gelation process dynamically, and related models were deduced based on recorded dynamic curves in relation to the gelling reaction rate constant. Results showed that the concentration of cross-linker, initiator, catalyst and polymerization temperature have significant effects on the gel conductivity relating to the process. The increases of these factors are found all contributed to the reaction rate, especially before the gel point. The gelation process of polyacrylamide hydrogel is a complex reaction process, however, to be mainly controlled by the zero reaction rate constant. Of this process, the effects from polymerization temperature, the concentration of catalyst, initiator and cross-linker have been discussed and a comparison of those effects indicated that the best influence is from the cross-linker.Before the fabrication of PAAm/LGS gels, the surface properties, conductivity and viscosity of three lignosulfonates, e.g. Na⁺-LGS, Ca²⁺-LGS and Mg²⁺-LGS, were firstly studied using the column wicking technique, conductivity measurement and ubbelohde viscometer, respectively. Then the influence of these properties of lignosulfonates on the gelation process for polyacrylamide/lignosulfonate semi-IPN hydrogels were furthermore studied by taking electric conductivity and R/S-Plus Rheometer as tools and PAAm gel as a reference. The conclusion seems to be as that the higher the surface free energy of lignosulfonates, the lower the conductivity of related composite gel. It is also observed that the addition of lignosulfonates in polyacrylamide gel is indeed to cause the gel point delayed visibly.The network structure and morphology of PAAm/LGS semi-IPN gels were studied by FTIR spectroscopy, conductivity measurement, DSC, TGA and SEM, respectively. A comparison of FTIR spectra of PAAm, LGS and PAAm/LGS gels indicated that the main network structure of the composite gel was contributed by PAAm and LGS to form a semi-interpenetration network. Since the DSC curve of composite gels presented the compatibility behavior between the PAAm and LGS, it is considered that this phenomenon might be a support for understanding above mentioned semi-interpenetration structure for prepared gels, and the reason seem due to the chemical reaction without taken place. The TGA results suggest that the thermal stability of PAAm/LGS hydrogels is better than PAAm suggesting the good application possibility for this new gel. The experiment result of the LGS released from PAAm/LGS gels during the gel swelling by electric conductivity also supports the conclusion above. It is observed from SEM photographes that LGS, which has the hydrophilic group, to be acted as a porosity agent in the PAAm/LGS system to make or control the macroporous structure for hydrogels. Obviously, this aspect is favorable for applying this gel in some cases.The swelling behavior of composite gel was studied using the conductivity measurement and weight measurement, respectively. Results suggested that both these two methods are available for applying to study swelling consistently. However, the conductivity measurement may exactly provide results in dynamical and accurate. The swelling kinetics deduced-based on two theories suggested both the characteristic exponents of PAAm and PAAm/LGS gels are greater than 0.5 relating to the non-Fickian mode. While according to the Schott mode, the relationship between the swelling ratio and time has been found to follow the equation as: t/SR=A+ Bt. Additionally, both of the initial swelling rates and the equilibrium swelling ratio of PAAm/Na⁺-LGS gel are observed to be the biggest among the three LGSs-based gels.In the limited range, the increase of the temperature will enhance gel swelling, especially over 35℃. The relationship between the equilibrium swelling ratio and time has been found to be as: SR=E+FT+GT². Compared to the pure PAAm gel, it is noted that the equilibrium swelling ratio for PAAm/Ca²⁺-LGS gel could be improved about 12～15ï¼…, while the equilibrium swelling ratio for PAAm/Na⁺-LGS gel has been found to be increased of about 3.4 times.The role of LGS has been found probably to help gel swelling, especially in acid and base environment. For example, result showed that pH=11 is a critical condition for swelling of prepared gel because observed phenomena suggested no variety in small pH values related liquids, e.g. water, HCl and lower pH-based NaOH solution, while visible in the pH range over 11, e.g. in a NaOH solution. Additionally, the swelling behavior for referenced PAAm gel in salt solution has been found non-sensitivity indicating the addition of LGS to be to improve the sensitivity for gel.The hydrogels have been also found to presente different swelling behavior in Lewis acid-base interactions based liquids, e.g. in water, formamide and different volume ratios of ethanol/water. For instance, both the swelling ratios of PAAm and PAAm/LGS gels in water are higher, and the swelling ratio is almost zero in pure ethanol. During the swelling process, PAAm/Na⁺-LGS gel shows good swelling property than other gels.Meanwhile, the deswelling rates of PAAm/LGS gels are higher than PAAm gel. Three LGS-based gels are found unable deswelling in water. The swelling and deswelling behaviors of PAAm and PAAm/LGS gels in formamide-water and ethanol-water show good reversible responsibility, especially in ethanol-water. The addition of lignosulfonates to PAAm, it is observed to increase the response rates.The adsorption characteristics of heavy metal ions (Cu²⁺) by PAAm and PAAm/LGS gels were studied, and the increase of temperature seems to be favorable for the adsorption.