Preparation And Characterization Of The Core-shell Structure Attapulgite-Polymer Nanocomposites

Attapulgite is a kind of rare natural clay minerals with fibrous structure.Owing to its properties,such as special crystal structure and superficial character,attapulgite possesses fine adsorptive,rheologic,catalytic and filling capabilities,which endow it with ion exchange,heat stabilization,salt resistance,coagulation,pulping and phase change(in high temperature)performance.Accordingly,attapulgite has high application value.Attapulgite was found in our country in early 80's.Our states have made it good application in many fields,such as absorption,decoloration, deodorization,salt-resistant slurry and so on.However,the in-depth processing and more abroad use of attapulgit capability remain in investigative stage,which is the important investigative way presently.The attapulgite nanorods are very good filler of the polymer.We introduce the preparation of ATP/polymer nanocomposites and the preparation of core-shell structure composites particles in this paper.1.Purification and surface-modification of ATP:The ATP was provided by Gansu ATP Co.(Linze Gansu).The ATP was purified as reported previously and dried in oven for the further experiments.The pure ATP was dispersed in water and cetyltrimethylammonium bromide(CTAB)was added into the mixture.The mixture was irradiated in ultrasonic bath for 8h.Then the residue settling at the bottom of the beaker was thrown away and the stable suspension was used for further experiments.After centrifugation,organo-ATP was got.The organo-ATP was washed by ethanol three times and water once.The CTAB chains of the outer shell were washed away and the organo-ATP can be dispersed into toluene with a good stability.The monolayer organo-ATP could be used for the preparation of the ATP/polymer nanocomposites.If the organo-ATP was not washed by any solvent, it has a double-layer CTAB shell on its surface and the outer of the organo-ATP is the CTAB's hydrophilic chain tail.So,it can be dispersed into water with a good stability and it can be used for emulsion polymerization or soapless emulsion polymerization.2.Preparation of the ATP/PS nanocomposite and core-shell structure ATP@Polymer naocomposites particles:2.1.Preparation of ATP/PS nanocomposite: CTAB-attapulgite for the polymerization was isolated by centrifugal precipitation and washed by ethanol for twice and distilled water for once,dried in an oven and then ground.A series of different grams styrene with the corresponding amount of CTAB-attapulgite and initiator were added into tubes in N₂ atmosphere,and the mixture was irradiated in ultrasonic bath for 15min.Then the tubes were put into microwave oven.The polymerization was initiated by microwave and the mixture became viscous quickly.The tubes were then put into oven 5h for further polymerization.Destroy the glass of the tube and we got the ATP/PS nanocomposite. TEM photographs show attapulgite is randomly oriented network of packed fibers. Thermal analysis shows that the decomposition temperature of the PS chains was increased a little and the interface properties had been improved by the adding of CTAB-ATP.2.2.Preparation of the core-shell structure ATP@PS naocomposites particles:The emulsion polymerization was carried out in the suspension directly.The org-ATP suspension was added to the reaction flask together with St so that St can be absorbed onto the hydrophobic ATP surface.Small droplets of St containing ATP needle as the core were formed under shear and in the presence of the surfactant.These small droplets containing a few needles might aggregate.Because the length of the ATP needles was different:the short ATP needles having a low length/diameter ratio can form a spherical core-shell structure;however,the long ATP needles that have a big length/diameter ratio cannot form spheres.The St can be absorbed into the needles. The shapes of the composite particles are different with the amount of monomer in the system with ultrasonic irradiation or electromagnetism stirring.With ultrasonic irradiation,the ATP needles were found to be in good dispersion,ATP was found adsorbed on the ball-shaped PS.The PS encapsulated completely the ATP needles with the smaller amount of charged monomer to form core-shell structure particles. Using electromagnetic stirring,with the same polymerizing condition,it was found that the ATP needles are encapsulated by PS.The PS on the needles in the form of spherical balls and free PS balls were also found.The organic phase around the attapulgite needles is not stable and the assembly could be destructed under violent stirring.Then the monomer droplets adsorbed the released CTAB molecules and formed PS balls.Then the PS balls were adsorbed onto the surface of org-ATP to form bead-string structure particles.The more monomer was added,the more PS balls were formed,and bigger composite particles were obtained.2.3.Preparation of the core-shell structure ATP@PMMA naocomposites particles:There are negative charges at the surface of the ATP needles.The CTAB chains can bond onto the surface via static forces.In the proposed method,the CTAB was used in high excess,and it is possible that the organic CTAB molecules formed a doublelayered structure at the ATP surfaces.And there were also some free CTAB molecules in the dispersion.The organo-ATP with double-layered CTAB molecules has polar groups(carboxyl groups)in the outer layer,so it could be dispersed well in water.It is expected to be used as seeds in the soapless emulsion polymerization.So the superfluous CTAB molecules in the organo-ATP dispersion were removed by washing with water.In the cases of encapsulation of ATP needles by the soapless polymerization,the general purpose of the pretreatment with CTAB is to introduce the organic chains onto the ATP needle surfaces so that they have more affinity for the monomers,facilitating encapsulation.So the MMA monomer was adsorbed onto the hydrophobic layer to form the oil phase in the latex.After the soapless emulsion polymerization of MMA in the organo-ATP suspension,the ATP nanoneedls were encapsulated by the PMMA.It is inclined to form a string-bead structure and only the ATP fibrillar single crystal with a small length/diameter ratio could lead to the core-shell structure.The superfluous monomer formed micelles by capturing the outer CTAB molecules of the organo-ATP with double-layered structures when more monomer was added.So some organo-ATP with monolayered structures were formed and aggregated together because of the hydrophobic surface.Therefore,some free PMMA particles and bare ATP needles coexisted with the ATP/PMMA particles after the soapless emulsion polymerization.2.4.Preparation of the core-shell structure ATP@DP naocomposites particles:Hundred milliliter of org-ATP suspension was centrifugated at 3000 rpm for 2 min at first.The sediment at the bottom was thrown away.And the org-ATP was separated by centrifugation at 13,000 rpm for 15 min from the stable suspension.After the wet org-ATP was dispersed into water with ultrasonic agitation,MMA was added into the flask as first monomer.The mixture was irradiated in ultrasonic was then subjected to electromagnetic stirring in flask at 70℃for 1h and the prepared ATP@ PMMA composite particles were used as a seed for the second stage of the seeded emulsion polymerization.St was added to the flask.Then the solution was maintained at room temperature for 24h under ultrasonic agitation,allowing the monomers to swell the seeds.After the emulsion polymerization again,ATP@DP composite particles were got.2.5.Preparation of the core-Shell ATP@PANI Composite ParticlesThere are negative centers on the ATP needles' surface.The aniline reacted with HCl and formed the anilinium chloride salts.The anilinium cations absorbed to the surfaces of ATP because of the negative centers introduced by the reaction of ATP with hydrochloric acid.Thus,the ATP surfaces can be more hydrophobic so that they have more affinity for the aniline monomers,facilitating encapsulation.So the anilinium chloride salts acted as both the monomer and the surface modifier in the oxidative polymerization.When the oxidant added,the polymerization occurred and the ATP needles were encapsulated by PANI to form the core-shell composite particles.TEM shows that the ATP nanoneedles were encapsulated very well like Chinese dates and the concentration of HCl had no effect on the morphologies of the ATP@PANI composite particles.Electrical conductivity shows that the ATP@PANI composite particles prepared with the more HCl amounts added exhibited the good electrical conductivities.