States of Water in Hydrogels Containing with Glyceryl Methacrylate

CHINESE J. BIOMED. ENG. VOL.23 N0.1, MAR.2014States of Water in Hydrogels Containing with GlyceryI MethacrylateLI Qin-hua', LIU Li, HUANG Zhi- -rong', LIN Dong- _qing2lI.Institute of Biomedical Engineering, Jinan University, Guangzhou 510632, China2.Conmaster Eyeglasses Co.,LId , Guangzhou 510600 , ChinaAbstract. Hydrogel materials were prepared by thermopolymerization witldifferent content of glyceryl methacrylate and hydroxyethyl methacrylate. The differentstates of water in swelling hydrogels were described and studied by differential scanningcalorimetry (DSC). It was found that the hydrophilicity of GMA was stronger than HEMA,the water content and bound water of GMA hydrogel are higher than HEMA hydrogel.With the increase of GMA content, the content of free water in hydrogel increased. WhenGMA content was lower than 50%, the increase of GMA content also increased thecontent of bound water; but when GMA content was higher than 50% , the increase ofGMA content decreased the content of bound water, which was caused by the chainhydrogen bond formed on the GMA chain with hydroxyl group each other.Key words: glyceryl methacrylate; states of water; bound water; free water; chainhydrogen bondINTRODUCTIONIn the hydrogel, water retention and permeability of soft contact lenses are related to the state ofwater; the transmission of oxygen and electrolyte through soft contact lenses is driven by free water inthe hydrogel while water evaporating from the surface of materials, which is an important factor incausing corneal drying due to the volatilization of free water or water loss caused by osmotic pressure.Therefore, the research on hydrophilia, water retention and permeability of soft contact lenses hasalways been a hot spot.Water has three states in the hydrogel material: 1) non-freezing bond water: non -freezing bondwater combined with the polar hydrophilic group in polymer network through hydrogen bonding withoutphase transition in the temperature range of 170-320 K; 2) freezing bond water (also known aintermediate state water): freezing bond water characterized by the phase transition temperature lowerthan pure water or free water and limited in a sorting state in the network; 3) free water: thethermodynamic behavior is similar to pure water, existing in a complete free state.中国煤化工CLC number: TQ 225.2 Document code:A Article ID: 1004-0552(2014)01-0020-09MHCNMHGCorresponding author: LIN Dong -qing. E- -mail:15986358779@ 163.comReceived 06 March 2014; revised 16 March 2014-20-Chinese Journal of Biomedical Engineering (English Edition) Volume 23 Number 1, March 2014Bound water has a plasticization effect on hydrogel while free water has a promotion effect on thetransmission of oxygen,electrolytes,and glucose. At the same moisture content, the material that hasmore free water is more likely to cause corneal drying than that containing more bound water. W henhydrogel is placed in the natural air, the evaporation firstly begins on its surface, because free waterhas the same properties with the natural state of water. With the continuous evaporation of free water inthe surface of hydrogel, free water within the network of hydrogel permeates into the surface throughcapillary structure which is formed by cross -linking of the hydrogel network, and evaporated in thesurface, thus dehydration occurs, thereby causing corneal drying. Ideal hydrogel contact lens hasability to make bound water (or intermediate water) coexist with free water at a certain proportion toensure the best water retention and oxygenl-2l.MATERIALS AND EQUIPMENTSMate rials : hydroxyethyl methacrylate (HEMA),glyceryl methacrylate (GMA)[Germany DEGUSSA],methyl methacrylate (MMA), azobisisobutyronitrile (AIBN)， (bis) ethylene glycol methacrylate ester[Japan JNUSEI, TCI].Equipments:AmericanTAQ -200 DSC; Fourier Transform Infrared Spectrometer BRUKEREQUINOX 55.METHODSSample preparationProportionally compound a mixed monomer solution, make it well-combined by magnetic stirring,and pour it into a mold (yin and yang set of mold, the male die is made with the material of PBT andhas smooth surface with uniform design arc, the female mold material is PP), then put the mold in aprecisely programmed heating oven for polymerization. The progress of polymerization is as follows: 1 h559C,1 h 659C, 30 min 90C, 15 min 105C. When polymerization is finished, separate the molds andpolymer material is attached to the surface of male mold. After turning and grinding by CNC lathe, drymaterials with same size and thickness are peeled from the male mold. Then put the dry material irnormal saline for hydration reaction at room temperature. The equilibrium time for hydration reactionis at least 24 h. After hydration, the materials turn into hydrogel lenses sample.The measurement of water content of hydrogelThe hydrogel samples were placed in normal saline for 24 h with fully swelling. After swelling,use filter paper to dry out the surface water of the hydrogel, weigh the sample W wet (g); then place the :sample in an oven at 1059C and dry it to constant weight, Wary (g). Calculate the total water content ofthe hydrogel by Equation (1):Woul= (Wwen-Wany)1 WwX 100%(1)The measurement by DSCIn the different GMA content of hydrogel materials, the cc中国煤化Ius stats wasmeasured by DSC. The sample was sealed in an aluminum sampleMHCNMHGiofnitrogen.Then it was placed in Q200 differential scanning calorimeter, and the temperature increased from -609C-21-CHINESE J. BIOMED. ENG. VOL.23 N0.1, MAR.2014to 209C, at the rate of 5C/min with the nitrogen flow rate at 40 ml/min to get the DSC chart.The free water content of the hydrogel material (W ) can be caleculated by the enthalpy change ofthe hydrogel samples near 0C as Equation (2):W. =(OH/△H) x100%(2The intermediate water content of the hydrogel material (W:m) can be calculated by the enthalpychange of the hydrogel sample in the temperature among -209C-0C as Equation (3):Wm=(O H/△H)x100%(3)Where,△H is enthalpy change of unit mass hydrogel measured by DSC endothermic peak area,the unit isJ'g';△Ho is the corresponding enthalpy change of per unit mass of pure water, the unit isJ'g.Non-freezing bond water content is as Equation (4):WwmE=Wun-W1. W.m(4)RESULTS AND DISCUSSIONThe result of DSC and discussionAccording to hydrogel alignment state of water in polymer proposed by Starkweather (Table 1)(Fig.1),water has three states, the first state is layer A with an orderly arrangement, the second one is layer Bwith relative disordered state, and layer C scattered in random state. The process of hydrogel hydrationis as follows: firstly the water molecules enter the hydrogel network, then they adhere to hydrophilicgroup in the molecule (hydroxy) closely by hydrogen bonds. As they were fixed to the network, theirflowing ability were restrained. Therefore， they acted as non -freezing bond water with littleendothermic melting below - 209C，which illustrated that no phase transition occurred. After the .formation of hydrogen bonds between hydroxyl group in the molecule and water was complete, thwater molecules, in their surrounding and the internal network of hydrogel, made up a layer B ooriented arrangement on the effect of weak hydrogen bond, which was known as freezing intermediatewater (freezing bond water), a state of aggregation of water caused by the tendency of water moleculesforming as more as hydrogen bond under the conditions of limited space. Since there was still weakinteractions between the intermediate water and the hydrogel, crystallization or melting might generatemore heat than pure water did. While bonding between the group of macromolecular chain and watermolecules is complete, the condensation of water on the surface leads to a concentration differencebetween inside and outside part of the network due to the three -dimensional network structure ofhydrogel. Therefore, the water molecules, under the effect of osmotic pressure, diffused in all spacebetween hydrogel network until the water in hydrogel reached saturation point and kept swellingequilibrium. As water is filled in the voids of network structure, its fluidity is quite large and itsproperties are similar to those of pure water, therefore, it is called free water. The endothermic meltingoccured near 0C in the DSC was caused by free waterl-3.中国煤化工MHCNMHG-22一Chinese Journal of Biomedical Engineering (English Edition) Volume 23 Number 1, March 2014Table 1 The DSC result of hydrogel sampleNumbersSamplesFree water Intermediate water Bond waterTotalContent%1HEMA20.228.930.993.53GAM104.2.932.438.584.163GMA204..13.543.580.46GMA308.5.050.575.255GMA4013.93.438.155.468.776GMA5017.1.636.957.664.06GMA6015.93.136.857.364.22GMA7021.2.336.56060.83GMA8027.6.735.164.454.501(GMA9030.110.9935.366.453.16GMA10036.11.733.571.247.05 .①0_8①日③θ①θ oo①free waterfreezing bondnon-freezingwatercopolymer hydrogelsFig.1 States of water in polymerThe content of water in hydrogel containing with GMAUtilize GMA content as the horizontal axis to plot four figures, the vertical axis are respectively totalwater content, the bound water, free water,and intermediate water quantity in hydrogel (Fig.2-Fig.5),which shows that the total water content and the free water content of the hydrogel increase with theincreasing of GMA content, but the content of the bond water and the intermediate water contentincreases first and decreases later with the increasing of GMA content. Because of many side groups init, CMA caused more steric hindrance than the material HEMA, which resulted in the tendency thatthe more GMA content,the larger space in the polymer network, and the more water molecules allowedinto the network. As a consequence, free water content increased (Fig.3, Fig.4). Due to the contributionof the large free water, even though the content of bond water and intermediate water decreased, thetotal water content still increased (Fig.2). According to the results of Table 2, the GMA -containedhydrogel material had more free water than pure HEMA hydrogel中国煤化Iise the sterichindrance of the polymer network formed by GMA is larger thanMHCNM H G, which canaccommodate more free water.- 23-CHINESE J. BIOMED. ENG. VOL.23 N0.1, MAR.20148t 60i 5021205000|GMA eotent %Fig.2 Trend graph of water content in hydrogeli260B00GMA conmertsFig.3 Trend graph of bond water content in hydrogel。:40100GMA conteret”Fig.4 Trend of intermediate water content in hydrogel目3025 .有154中国煤化工GIUA content售MYHCNM HGFig.5 Trend of free water content in hydrogel一24-Chinese Journal of Biomedical Engineering (English Edition) Volume 23 Number 1, March 2014Every molecule GMA had one more hydroxyl group than the same amount of HEMA (Fig.1), So,as for the structure, GMA bound water content should be twice as much as HEMA. According to thedata about H-NMR and DSC studies on PHEMA hydrogel carried by G. Smyth, each structural unitbonded two water molecules (bound water) on average. While Barrie's research on water absorptionshowed that when water was saturated (259C), each hydroxyl (-OH) group could absorb two watermolecules, and some carbonyl (-C=0) can also form hydrogen bonds with water molecule in eachgroup adsorbing 0.2 molecules of water on average at water saturated state. As a result, the theoreticalbond water content of HEMA was (2.2x18)/130, which was approximately 30%. But for GMA with twohydroxyl groups, if the calculation followed the same rule that each hydroxyl group absorbed twomolecules of water, the theoretical bond water content should be (4.2 x18)/160, near 473 %. Thecomparison of theoretical and experimental values is shown in Table 2.Table 2 Theoretical value and experimental value of the bond water contentSampleTheoretical arithmeticTheoretical valueExperimental valueHEMA0.3040.289GAM100.9*0.304+0.1*0.4730.3210.324 .GMA200.8*0.304+0.2*0.4730.3380.350GMA300.7*0.304+0.3*0.4730.3550.380GMA400.6*0.304+0.4*0.4730.3720.381GMA500.5*0.304+0.5*0.4730.3890.369GMA600.4*0.304+0.6*0.4730.4050.368GMA700.3*0.304+0.7*0.4730.422GMA800.2*0.304+0.8*0.4730.4390.351GMA900.1*0.304+0.9*0.4730.4560.353GMA1000.4730.335Fig.6 shows that when GMA content is higher than 50%，the bond water content in the hydrogel .deceases significantly as the GMA content increases, and the results deviate more significantly fromthe theoretically calculate value. Especially, PGMA (GMA100%) bond water content is only 33.5% ,which indicates that not all of the hydroxyl groups (polar groups)in the hydrogel macromolecular chainsformed hydrogen bonds with water molecule, and produced bond water.中国煤化工MHCNMHG一25-CHINESE J. BIOMED. ENG. VOL.23 N0.1, MAR.201450E 40T台30 -■Theoretical val20。Experimental va10lue0406080 100GIA contentFig.6 The trend of the theoretical value and the experimental value of bond waterThe ability of polar group absorbing water relies not only on the number of groups, but also onthe degree of crystallinity and accessibility of the polar group. Also there is a more important factorhat polar groups formed hydrogen bonds inner-molecule or inter-molecule. Since the groups in thehydrate polymer chain formed a weaker hydrogen bond with water molecule compared to that with thehydrophilic group, the hydrophile group within the hydrogel has a higher possibility to form in -tramolecular hydrogen or intermolecular hydrogen bonds.As for pure HEMA materials, inner/intermolecular hydrogen bonds are also formed as theirmolecules arranged in neat order, so the actual bond water content is lower (Table 1) than the theoreti-cal value. When a quantity of CMA material (50% or less) was added in HEMA, the polymer networkwas stretched due to the large steric hindrance of CMA side groups, so that the bonding force betweenthe GMA and HEMA hydroxyl groups became weak which led to the rise of accessiblity of hydroxylgroups and more hydrogen bonds formed with water, thus the amount of bond water increased withGMA content increasing. The intermediate water surrounding the bond water also increased.But as GMA content increased, the force between polymer chains became stronger, more inter-molecular and inner-molecular hydrogen bonds were formed in polymer chain, thus the outside watermolecules cannot destroy the inner hydrogen bond formed by polar groups to combine with more polargroups and for me more bond water, and as a result the amount of bond water reduced (Fig.3). Further-more, the existence of inner hydrogen bonds resulted in the formation of microcrystalline region nearthe hydrogen bonds, which made it diffcult for water molecules to access the space near the cluster, in aconsequence intermediate water content also declined (Fig.4). This is similar with PVA (polyvinyl alco-hol) hydrogels water absorption. The study about swelling of PVA showed that PVA with a high molarmass had better ability of water absorption, but PVA with excessive molar mass would cause hydroxy tocombine each other by hydrogen bond making an orderly arranged中国煤化工ter moleculesis difficult to combine with hydroxyl groups, resulting in the declineMHCN MH GVA.And in the hydrogels formed by high content of GMA, macromolecular chain has a large number- 26-Chinese Journal of Biomedical Engineering (English Edition) Volume 23 Number 1, March 2014of regular hydroxyl groups, and the strong hydrogen bond formed between the hydroxyl groups maycause local crystallization. And the large number of methyl contained in polymer would emerge elec-trostatic repulsion each other. Because of the effect of local microcrystalline, electrostatic repulsion andthe side groups with large steric hindrance, the space between the polymer chains has extended, and alarger void formed in network, therefore, a massive amount of water still can access its network, result-ing in the formation of large amount of free water.In the hydrogels formed by high content of GMA, although the content of bond water decreased,massive amount of free water formed, so the total water content still remained increasing (Fig.2). But inthe hydrogel formed by pure HEMA, the network structure is very regular and close with lttle free wa-ter, so the bond water constitute the majority of total water. And when GMA was gradually added tothe hydrogel, the network structure of the hydrogel expanded, and free water content increased gradu-ally. In the end, free water became the majority of the water content in the hydrogel, and the propor-tion of bond water gradually decreased!-4.CONCLUSIONApplying DSC method in this experiment, we got the data of different water contents under vari-ous states in hydrogel with different GMA contents. The conclusions are shown as flllows:1) In this copolymer research system, water molecules exist in three states in the hydrogel: boundwater connected with molecular groups in hydrogen bonds, aggregative intermediate water clusteraround bonds and free water which can get through polymer network without restriction. Moreover, freewater and bond water remain dominat states in the polymer.2) The hydrogel formed by material GMA contains more bond water and free water compared tothose formed by traditional material HEMA, material GMA shows stronger hydrophilic than HEMA.3) When GMA contents in hydrogel is below 50%, the contents of all the three water states, free/bond/intermediate water increase with GMA content increasing; on the contrary, bond water and inter-mediate water content have a opposite trends with GMA content increasing when GMA content isabove 50%, while free water content is rised.4) The bound water is generated by polar groups of macromolecular chains bonding with watermolecules in a hydrogen bond. When polymer contains extremely high GMA contents (100% GMA),due to the abundant neat and orderly hydroxy polar groups, it is highly possible to form hydrogen bondin each molecule and between different molecules, so that the possibility of bonding with watermolecules decreases, which leads to bond water content decreasing.5) The amount of GMA pendant groups generates big three -dimensional network structure poly-mer, as a result, the free water content increases with the GMA content increasing. When GMA con-tent is extremely high (100%), there are a lot of gaps between macromolecular chains, where the freewater can exist compatibly; these gaps are led by part crystallization in the polymer, which was bene-fited by strong hydrogen bonds between amounts of structured hydr中国煤化工6) The free water contents increase dramatically when theMYHC N M H Gbesides, freewater content is much larger than bond water content; therefore, although bond water content goes lower,- 27-CHINESE J. BIOMED. ENG. VOL.23 N0.1, MAR.2014the total water content of hydrogel material increase.7) Bond water contents in hydrogel material rely on not only the quantity of hydrophilic groupsbut also on other comonomer categories, contents and polymer cross-linking rate. Therefore, the maxi-mum bond water contents can be achieved by changing the cross-linking rate or network regularity inpractical produces.REFERENCES1] Capitani D, Mensitieri G, Porro F, et al. NMR and calorimetric investigation of water in a super absorbingcross-linked network based on cellulose derivatives[J]. Polymer , 2003, 44 (21): 6589-6598.[2] Muta H, Ishida K, Tamaki E, et al. An IR study on ion- -pecific and solvent- -specific swelling of poly(N-Vinyl-2-Pyrrolidone ) gel[J]. Polymer , 2002, 43 (1):103- -110[3] Takayuki Baba, Ryosaku Sakamoto, Masami Shibukawa, et al. Solute retention and the states of water in polyethy-lene glycol and poly (vinyl alcohol) gels[J]. Jourmal of Chromatography A,2004, 1040 (1):45-51.[4] Li Wenbo, Xue Feng, Cheng Rongshi. States of water in partially swollen poly(vinyl alcohol) hydrogels[J]. Polymer,2005, 46: 12026-12031.NEWS●Print 3-D Fingerprints for Better BiometricsTo test the accuracy of a new fingerprint scanner, researchers typically run millions of knownfingerprint images through the system's matching sofware. But this testing procedure can't quite mimicreal operating conditions, as a 2-D image fed into a program is fundamentally different than a 3-Dfinger pressed to a sensor.To get around that problem, researchers have come up with the first 3-D-printed fingerprint. 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