Patent Application: US-201113010020-A

Abstract:
nanocrystalline cellulose is employed as the cross - linker and reinforcement domain for developing nanocomposite hydrogels possessing high strength and improved diffusion property ; the resulting nanocomposite hydrogels are shown to have high mechanical properties , reversible swelling ability , and are biodegradable and biocompatible ; the approach relies on free radical polymerization to form the hydrogels using a variety of hydrophilic vinyl monomers . these hydrogels are suitable for developing highly absorbent hygiene products , as well as for applications in medicine , engineering materials and sensors .

Description:
the nanocomposite hydrogel comprises a hydrophilic polymer , in particular a hydrophilic polymer derived from polymerization of a hydrophilic vinyl monomer , which polymer is crosslinked by a crosslinker comprising nanocrystalline cellulose ( ncc ). nanocrystalline cellulose ( ncc ) is extracted as a colloidal suspension by acid hydrolysis of typically chemical wood pulps , but other cellulosic materials , such as bacteria , cellulose - containing sea animals ( e . g . tunicate ), or cotton can be used . ncc is constituted of cellulose , a linear polymer of β ( 1 → 4 ) linked d - glucose units , the chains of which arrange themselves to form crystalline and amorphous domains . ncc obtained via hydrolytic extraction has a degree of polymerization ( dp ) in the range 90 ≦ dp ≦ 110 , and 3 . 7 - 6 . 7 sulphate groups per 100 anhydroglucose units . ncc comprise crystallites whose physical dimension ranges between 5 - 10 nm in cross - section and 20 - 100 nm in length , depending on the raw material used in the extraction . these charged crystallites can be suspended in water , or other solvents if appropriately derivatized , or self assemble to form solid materials via air , spray - or freeze - drying . when dried , ncc forms an agglomeration of parallelepiped rod - like structures , which possess cross - sections in the nanometer range ( 5 - 20 nm ), while their lengths are orders of magnitude larger ( 100 - 1000 nm ) resulting in high aspect ratios . ncc is also characterized by high crystallinity (& gt ; 80 %, and most likely between 85 and 97 %) approaching the theoretical limit of the cellulose chains . hydrogen bonding between cellulose chains can stabilize the local structure in ncc , and plays a key role in the formation of crystalline domains . crystallinity , defined as the crystalline fraction of the sample , strongly influences the physical and chemical behaviour of ncc . for example , the crystallinity of ncc directly influences the accessibility for chemical derivatization , swelling and water - binding properties . in particular , the crosslinker comprises a modified ncc in which a modifier has a first functional group coupled with a hydroxyl group of the ncc , and a second functional group coupled to the polymer . the modifier is suitably a vinyl monomer having a functional group , by way of example there may be mentioned glycidyl methacrylate . based on their molecular structures , a group of chemicals can appropriately be selected to function as modifiers for the ncc . some examples are : trimethylsilyl methacrylate , 2 -( trimethylsilyloxy ) ethyl methacrylate , 2 - aminoethyl methacrylate , 2 - isocyanatoethyl methacrylate , 2 - chloroethyl acrylate , 3 -( trimethoxysilyl ) propyl acrylate , glycidyl acrylate , vinyl isocyanate , 2 - aminoethyl vinyl ether , and vinyltrimethoxysilane . suitable hydrophilic monomers include , by way of example , acrylic acid , acrylamide ( am ), vinyl - 2 - pyrrolidinone . n - isopropylacrylamide ( nipam ), and n - vinylformamide ( nvf ). the following group of hydrophilic monomers can also be identified for polymer formation . α - ethylacrylic acid , methacrylic acid , 2 - acrylamido - 2 - methyl - 1 - propanesulfonic acid , hydroxypolyethoxy ( 10 ) allyl ether , 2 - hydroxyethyl acrylate , 3 - sulfopropyl acrylate potassium salt , poly ( ethylene glycol ) acrylate , tetrahydrofurfuryl acrylate , and methacrylamide . furthermore , in principle all of the aforementioned monomers can also be chosen as comonomers , so that the hydrophilic polymer is a hydrophilic copolymer , and hence as suitable combinations for the copolymerization reaction . suitable comonomer combinations for copolymerization to produce the hydrophilic polymer are for example , acrylamide and acrylic acid , acrylamide and methacrylic acid , n - isopropylacrylamide and acrylic acid , n - isopropylacrylamide and poly ( ethylene glycol ) acrylate , and poly ( ethylene glycol ) acrylate and acrylamide . thus the polymerization of the hydrophilic vinyl monomer can be a homopolymerization of a vinyl monomer or a homopolymerization of a vinyl monomer and a comonomer . in the case of a copolymerization , in order to make a good hydrogel , hydrophilic monomers and hydrophilic comonomers are employed to form the crosslinked network which should be able to swell in aqueous media . it will be understood that the first functional group may be any group that will react or couple with a group on the ncc such as a hydroxyl group , to couple the modifier to the ncc ; and the second functional group may be any group that will react or couple with a group on the polymer or the monomer forming the polymer . persons in the art will be able to select modifiers with suitable functional groups for coupling with the ncc and hydrophilic polymer or monomer for forming such polymer . similarly persons in the art will be able to select monomers for the polymer formation having suitable functional groups for coupling with the second functional group of a modifier . it will be understood that while reference is made to a monomer for polymer formation , it is within the invention to additionally employ comonomers so as to produce crosslinked copolymers rather than crosslinked homopolymers ; in such case the comonomers need not participate in the crosslinking although this is also possible . in this way hydrogels of a variety of desired characteristics can be produced by selection of hydrophilic monomers , modifiers and optional comonomers . furthermore a method for the preparation of nanocomposite hydrogels with nanocrystalline cellulose ( ncc ) as the crosslinker and reinforcement domain is provided . each ncc nanoparticle attaches to at least two units of the modifier . the ncc is expected to be completely randomly distributed within the hydrogel structure . the prepared hydrogel has high mechanical strength while maintaining the original properties . compared to clay - based nanocomposite hydrogels , ncc is biodegradable and biocompatible . this makes the ncc - based hydrogels more promising for the medical related applications since biodegradability and biocompatibility are critical . the preparation of the ncc nanocomposite hydrogel involves a two step process . the first step is ncc modification to render it a suitable crosslinker , and the second is in situ polymerization of hydrophilic vinyl monomers with the modified ncc to make nanocomposite hydrogels . hydrogels of the invention possess excellent mechanical strength while retaining their original properties , such as stimuli responsiveness and fast diffusion , and as such can be developed in a great number of hydrogel - based new applications . in specific embodiments of this invention , free radical polymerization is used for forming nanocrystalline ( ncc ) based nanocomposite hydrogels . a variety of hydrophilic vinyl monomers are suitable , such as acrylic acid , acrylamide ( am ), vinylpyrrolidone , n - isopropylacrylamide ( nipam ), n - vinylformamide ( nvf ) etc . since there is no crosslinking spot on ncc during the polymerization , ncc must first be modified to function as a crosslinker . the modification is performed by following a procedure whereby a vinyl monomer with functional groups , for example glycidyl methacrylate ( gma ), is used to react with the hydroxyl groups on ncc , which results in ncc coupled with the methacrylate group . then , the nanocomposite hydrogel is prepared by using the modified ncc and hydrophilic monomers , such as acrylamide , initiated with free radical initiators , such as potassium persulfate . following the procedure of this invention , nanocomposite hydrogels prepared from the modified ncc are much stronger than the hydrogels prepared from a regular organic crosslinker , for instance , n , n ′- methylenebisacrylamide ( bis ). 11 the water swelling ability of the resulting nanocomposite hydrogel is reversible , which indicates that the diffusion property of the hydrogel is retained while the mechanical properties are enhanced . the molecular weight or polymer chain length between ncc particles can be controlled by two principal factors : ( i ) ncc concentration , and ( ii ) the density of grafted modifiers on the ncc surface . the higher the ncc concentration and the density of grafted modifier on ncc surface are , the shorter the polymer chains will be . conversely , the lower the ncc concentration and the density of grafted modifier on ncc surface are , the longer the polymer chains become . however , the ncc concentration can also affect the mechanical strength of the resulting hydrogel . the above described method is not limited to the use of acrylamide as the monomer . the choice of different monomers is dependent on the final application . for different monomer systems , the nanocomposite hydrogels can be ph responsive , temperature sensitive , electrochemically sensitive , etc . enhancing the mechanical strength will widen the potential application for hydrogels into a wide range of engineering materials and medical systems . detailed description of the specific steps which may be employed for the preparation of ncc - based nanocomposite hydrogels is described below . surface modification of ncc with a vinyl monomer possessing suitable functional groups : ncc suspension in water with a certain concentration , for example 4 . 38 % wt , is used . the ph of the ncc suspension is adjusted to neutral , and the ncc suspension is then dried using an appropriate technique , for instance spray or freeze drying . several grains of freeze dried ncc , typically 1 to 10 g , for example 5 g is resuspended in 50 to 500 ml of an appropriate solvent , for example dimethyl sulfoxide ( dmso ), preferably 100 ml , with stirring for 10 mins to 4 hours until ncc is fully suspended . the choice of solvent is only limited by the fact that the ncc suspension must be fully dispersed . the ncc suspension is then ultrasonicated for 10 to 60 mins . desired amount of a catalyst , for example 1 g of 4 - dimethylaminopyridine ( dmap ), is added into the suspension , and the suspension is further bubbled with nitrogen for 10 to 60 mins to get rid of dissolved oxygen . the reaction is started by introducing the required amount , for instance 0 . 73 g , of a suitable vinyl monomer with functional groups , for example glycidyl methacrylate ( gma ), into the suspension . the reaction can be performed at any temperature ranging from room temperature up to the degradation temperature of ncc ; 50 ° c . is used in this case . the reaction time can range from 4 hours to several days , and 48 hours is applied here . the reaction time , temperature , ratio of gma to ncc will determine the modification degree . after the reaction , some deionized ( di ) water , 100 ml in this case , is introduced into the suspension and the ph of the system is adjusted to 7 - 8 . the obtained suspension is dialyzed against di water for 3 to 10 days , 7 days in this case . after dialysis , the suspension is ultrasonicated for 30 mins and filtered through , for example , whatman no . 42 filter paper . finally , the ph of the modified ncc suspension is adjusted to 7 - 8 and freeze dried . preparation of ncc - based nanocomposite hydrogels : the calculated amount of modified ncc and acrylamide are dissolved in di water while stirring until fully dissolved , typically 10 mins to 60 mins , and then ultrasonicated for 10 to 30 mins . the suspension is then filtered through a 0 . 45 μm needle filter . the desired amount of a suitable catalyst , for instance n , n , n ′, n ′- tetramethylethylenediamine ( temed ), to assist with the generation of free radicals from the initiator is added to the ncc suspension , and the suspension is bubbled with nitrogen for at least 30 mins . the calculated amount of a suitable initiator , for example potassium persulfate ( kps ), is dissolved in di water and bubbled with nitrogen at the same time . to start the reaction , the kps solution is introduced to the ncc suspension in an ice - water bath . the in situ free - radical polymerization is allowed to proceed at room temperature for 1 to 5 days , for example 2 days , until the reaction is finished in nitrogen atmosphere . the molar ratio of the monomer ( acrylamide ), initiator ( kps ), and catalyst ( temed ) is kept constant in this specific sample at 381 : 1 : 2 . 35 . different ncc to acrylamide ratios can used to optimize the effect of ncc concentration on nanocomposite hydrogel mechanical properties . the molar ratio of other reactants can also be adjusted within a reasonable range to control the properties of the prepared ncc hydrogels . fig1 : is a schematic of how ncc functions as a crosslinker in the described nanocomposite hydrogels . each nanoparticle attaches to at least two units of the modifier . the ncc is expected to be completely randomly distributed within the hydrogel structure . fig2 : is an ft - ir spectra of modified ncc according to the conditions described in table 1 : fig3 : is a plot showing the tensile behaviour of ncc - based nanocomposite ( open triangles ) and bis - based ( solid line ) hydrogels ; fig4 : is a plot showing the tensile strength response to increasing nanoreinforcement / crosslinker loading for ncc - based nanocomposite ( solid triangle ) and bis - based ( solid square ) hydrogels ; and fig5 : is a plot showing the compression modulus as a function of nanoreinforcement / crosslinker loading for ncc - based nanocomposite ( solid triangle ) and bis - based ( solid square ) hydrogels . the modification of ncc by gma is controlled using different reaction conditions . it is illustrated in table 1 that the surface charge of the modified ncc is decreased compared to the original ncc . moreover , the modified ncc becomes smaller in size than the original ncc . this is likely because the catalyst , dmap , is a base , which will possibly hydrolyze the ncc to make it smaller and decrease the surface charge . to confirm the grafting of gma onto ncc , fourier transform infra - red ( ft - ir ) examination is carried out to check the modified ncc samples . as demonstrated in fig2 , the peak at about 1720 cm − 1 represents the vibration of carbonyl group on methacrylate , which indicates that the methacrylate group is grafted onto ncc . the mechanical properties of the hydrogels can be analyzed using an instron , or any other , tensile testing machine at an extension rate of 100 mm / min . the gauge length was set to 30 mm , and the test specimen is a solid cylinder with a diameter of 11 . 46 mm . the initial cross section is used to calculate the tensile strength and modulus . fig3 displays typical tensile responses of ( 1 ) ncc - based nanocomposite hydrogel prepared according to this invention , and ( 2 ) hydrogel prepared using an organic crosslinker , bis . the ncc - based nanocomposite hydrogel exhibits a characteristically different behaviour to that of hydrogels produced using organic crosslinkers . the ncc - based nanocomposite hydrogel shows a clear inflexion point at about 100 % strain , indicating transition from linear elastic to plastic response . this elastic - plastic behaviour indicates that the ncc - based nanocomposite hydrogels ( 1 ) have the ability to absorb a significant amount of energy ( highly tough material ), ( 2 ) are stiff ( high modulus ), and ( 3 ) have high tensile strength in the plastic range ( over 7 times higher than the elastic range ). this significant plastic response for ncc - based nanocomposite hydrogels is a unique feature made possible by the ncc reinforcing mechanism . fig4 depicts the tensile strength response as a function of increasing ncc ( or bis ) contents in ncc - based nanocomposite hydrogels , and hydrogels prepared using an organic crosslinker , respectively . it is evident that the tensile strength increases with increasing bis content ( solid squares ), reaches a peak at ˜ 0 . 015 mpa , then decreases . the tensile strength for the ncc - based nanocomposite hydrogel , however , increases steadily with increasing ncc content ( solid triangles ). for instance , at 50 % w / w ncc loading on the monomer , the tensile strength reaches 0 . 126 mpa , more than 8 times higher than that for the regular hydrogel . compression tests are carried out using a thermal mechanical analyzer ( tma q 400 ) under an expansion probe . the height of the sample is about 6 mm and the diameter of the probe is 2 . 795 mm . the measurement is done by applying an initial force of 0 . 01 n and a force ramp 0 . 1 n / min to 1 . 2 n . compressions between 0 . 5 to 2 . 5 mm are used to calculate the compressive modulus . fig5 depicts the compression modulus of the hydrogels as a function of crosslinking densities or ncc loading as weight percentage onto the monomer . for example , for the bis hydrogel ( solid squares ) the compression modulus increases with increasing crosslinker concentration and the data follow a logarithm response levelling off at ˜ 9 kpa . the compression modulus for the ncc - 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