Preparation And Humidity Adaptive Behavior Of Alginate Based Polyelectrolyte Complex Fibers

When two polyelectrolytes that have oppositely charged ionic groups meet in solutions,complexation between them would emerge immediately,and the production is called polyelectrolyte complex(PEC).There are numerous ion-pairs composed by the charged groups in PEC which also make high physical crosslink density in it.At the same time,PEC materials have considerable sensitivity to the environment change,even could response to multi-stimulus simultaneously.The multi-responsiveness draws the attention of many researchers.Now the studies about the PEC materials are still concentrated on the thin films,capsules,and gels et al.Few reports are related to polyelectrolyte complex fiber which still need to be promoted forward.Firstly,existing preparation methods are hard to be applied in large scale production.Secondly,most of the research works are concerned about the application of PEC fiber.The formation mechanism of some methods is still needed to be identified.This polyelectrolyte complex process could be adjusted by many factors like p H,ionic strength and species,and temperatures.In this paper,ALG was selected as polyanion and PDDA and CHI were chosen as polycation.Two different methods just based on the controlled polyelectrolyte complexation were applied to produce polyelectrolyte complex fiber.One is inhibition and formation which is also a new preparation method.Spinnable polyelectrolyte complex solution was achieved by suppressing the interaction between oppositely charged polyelectrolytes,then injected into a coagulation bath to get ALG-PDDA complex fiber.The other one is in-situ polyelectrolyte complexation which means polyelectrolyte spinning solution is injected directly into an oppositely charged polyelectrolyte coagulation bath to fabricate ALG-CHI complex fiber.Not all oppositely charged polyelectrolyte pairs could be used in PEC fiber preparation through this method.PDDA was replaced by CHI due to the hydrophobicity of its backbone.The fiber formation mechanism was studied in detail.Meanwhile,the responsiveness of polyelectrolyte complex fiber to humidity was also studied.Through establishing the consistency between the time-humidity(water)superposition and time-temperature superposition,the assumption that humidity could replace temperature in the postprocessing and enhance the mechanical properties of polyelectrolyte complex fiber under a mild environment.(1)KBr was added into alginate(ALG)and poly(diallyl dimethyl ammonium chloride)(PDDA)complex solution to inhibit the interaction between them,and to get a spinnable solution.The first coagulation was an ethanol bath and the second was a distilled water washing bath.ALG-PDDA complex fiber was prepared through the wet-spinning process.The rheological properties of the spinning solution were studied by rheometer.The consequence implies that salt could be a key to tailor spinning solution which greatly influences the spinning parameter setting.The washing bath is essential to the morphology and mechanical properties of ALG-PDDA complex fiber.The nascent fiber without washing has a core-shell structure,and many pores distribute in the inner part.The pore size in nascent fiber increases from 80 nm to 800 nm when more KBr is added to the spinning solution.There are no pores in finished fiber with a washing step which mechanical properties also get improvement.Salt concentration and species are also important parameters to fiber properties.Through the ICP test,K⁺residue in finished fiber increased with more KBr added in the spinning solution.At the same time,breaking stress decreases from 60MPa to 40 MPa.The metal ions of added salt in the spinning solution were changed to Na⁺and Li⁺.With the ion's radius decrease,finished fiber's elongation decreases,and breaking stress could be enhanced from 60 MPa to 83 MPa.A larger hydrate radius achieves by a smaller ion radius,which makes water molecules in fiber are trapped,and leading to a poor water plasticization effect.Therefore,ALG-PDDA complex fiber could be strengthened.(2)Through the fibers'tensile test under different relative humidity,the plastic-rubbery transition was found during the increase of humidity.This transition also can be applied to the fiber shape memory which triggered by humidity.After drying ALG-PDDA complex fiber in a vacuum oven,the fiber could break in a brittle rupture behavior.An obvious yielding phenomenon could arise with humidity increase,meanwhile,the breaking stress decreases.After 80%RH,the yielding disappears,and the fiber breaks like a rubber.It's easy to find that the initial modulus of fiber decreases with humidity increase.Before65%RH,the decrement of modulus isn't obvious.And a dramatic drop of initial modulus would happen after 65%RH.This phenomenon is related to the water content change in fiber when humidity increases.Water in fiber increases slowly in low humidity area(0-65%RH),but become faster in high humidity area(65%-95%RH).Initial modulus could drop from a plastic platform(10⁹ Pa)to a rubbery platform(10⁶ Pa)when water content rising.A humidity-induced shape memory just is brought to ALG-PDDA fiber due to this transition.Comparison of stress relaxation behavior with or without vapor spray also indicates that ALG-PDDA complex fiber has fast responsiveness to humidity change.(3)ALG-CHI complex fiber was prepared by extruding ALG(CHI)spinning solution into CHI(ALG)coagulation solution which is called the in-situ polyelectrolyte complexation method.Fiber only forms when the spinning solution is thicker than coagulation.With the help of the viscosity test,an empirical rule could be summarized that ALG-CHI complex fiber could form continuously when the viscosity ratio that spinning solution to coagulation bath is larger than 10.Fiber formation using this method is also controlled by chain diffusion.The diffusion relied on factors like concentration,formation time and temperature were all essential to fiber composition and properties.Increasing the concentration of spinning solution and coagulation would make the chains diffusion slower.When applying CHI as coagulation,higher ALG and CHI concentration make the CHI content in fiber decrease from 4.6 wt.%to 2.7 wt.%.Oppositely,improvement of formation time or temperature is good for chain diffusion.The CHI content increases from 3.4 wt.%to 5.0 wt.%when the formation time is prolonged from 15 min to 24 h.And the formation temperature increasing from 25?to 65?also makes the CHI content grows from 3.7 wt.%to 9.4wt.%.Comparing the mechanical properties with ALG content or CHI content in fiber,a good linear relationship could be found between them.After CHI was labeled with FITC,the distribution of CHI was clearly showed via the laser scanning confocal microscope when using CHI as coagulation.The picture implies the fiber has a gradient core-shell structure that lots of CHI-FITC still stayed outside layer of ALG-CHI complex fiber.This fiber has good mechanical properties,biocompatibility,and biodegradability which make it a promising candidate for biomedical and healthcare materials.(4)The strain-stress curves of ALG-CHI complex fiber made by in-situ polyelectrolyte complexation in different humidity have a similar change tendency with ALG-PDDA complex fiber.Dynamic mechanical measurement with relative humidity-controlled accessory was applied to monitoring the storage modulus,loss modulus,and tan?variation when the water content is changing with humidity.In the creep experiment,fiber's elastic deformation and largest deformation are all decreased when the humidity is turned down from 80%to 40%or the temperature falls from 25?to 15?respectively.After conducting the frequency sweep test under different temperature and humidity through DMA,the equation between shift factor?_wand w-w_ref is found to has a similar form with the equation between?_T and T-T_ref established by time-humidity(water)superposition and time-temperature superposition.Based on this similarity,humidity could be utilized in postprocessing of polyelectrolyte complex fiber like the annealing operation in the traditional industry.The storage modulus and the loss modulus were all monitored during the ALG-CHI complex fiber annealing in20%RH three times.As the consequence,the storage modulus increases 500 MPa after annealing.Enhancing polyelectrolyte complex fiber can be processed in a mild environment which is indicated by these conclusions.