swelling and mechanical properties of crosslinked hydrogels containing n-vinylpyrrolidone

Polymer International 39 (1996) 121-127

Swelling and Mechanical Properties of Cross1 inked Hydrogels Containing

N-Vinyl pyrrol idone

Don 1. Perera* & Robert A. Shanks

Department of Applied Chemistry, Royal Melbourne Institute of Technology, Melbourne, Australia

(Received 19 June 1995; accepted 8 September 1995)

Abstract: Copolymers of 2-hydroxyethyl methacrylate/N-vinyl-2-pyrrolidone (HEMA/NVP) and methyl methacrylate (MMA)/NVP were prepared in the presence of varying amounts of ethylene glycol dimethacrylate (EGDMA) and methylene diacrylamide (MDA) as crosslinkers by photopolymerisation. The resultant solid polymers were swollen to equilibrium in water at 293K to produce hydrogels. These hydrogels were characterised by soluble fraction and equilibrium water content. The gels were also characterised by compression- strain measurements, which enabled the calculation of Young's modulus and effective crosslink density. The differences in these properties of HEMA/NVP and MMA/NVP polymer series and the effects of MDA versus EGDMA as a crosslinker were explained in terms of compositional drift of polymerisation, heterogeneous crosslinking and hydrophilicity/hydrophobicity of the components involved. In comparison with EGDMA, MDA was found to be more effective in reducing the soluble fraction of the polymers studied and to produce less rigid networks when swollen.

K e y words; 2-hydroxyethyl methacrylate, methyl methacrylate, N-vinylpyrrolidone, copolymers, hydrogels, crosslinking, crosslink density, modulus.

INTRODU CTlON in the material. Crosslinking with EGDMA does not solve this problem as EGDMA too undergoes a compositional -drift. The Q-e schemes have been used to Copolymers of N-vinyl-2-pyrrolidone (NVP) and 2- demonstrate the existence of heterogeneous crosslinking hydroxyethyl methacrylate (HEMA) or methyl meth- in terpolymer compositions."-12 The aim of this work acrylate (MMA) are used in many applications in the is to study the effect of methylene diacrylamide (MDA) biomedical field, especially contact lenses. '-' Copoly- as a crosslinker for HEMA/NVP and MMA/NVP mers of this type have been prepared by thermal, photo- systems. This paper reports the swelling and mechanical or irradiation polymerisation using ethylene glycol characteristics obtained from swelling measurements dimethacrylate (EGDMA) or other dimethacrylates as and compression-strain measurements performed on the c r~ss l inke r .~ -~ It is known that NVP and meth- swollen gels. A comparison is made of the properties of acrylates do not polymerise well due to compositional hydrogel networks produced with EGDMA and MDA drift during the course of polymerisation. 'O-" This as crosslinkers. effect is attributed to the differences in reactivities of

EXPERIMENTAL NVP and methacrylate comonomers. For example, the reactivity ratios for the copolymerisation of MMA and NVP are rl = 5 , r2 = 0-02." One problem with this is Po,ymerisation the presence of a considerable amount of water solubles

HEMA, MMA, NVP, EGDMA and MDA were used as * To whom correspondence should be addressed. supplied by Aldrich Chemicals for polymerisation.

121 Polymer International 0959-8103/96/$09.00 0 1996 SCI. Printed in Great Britain

122 D . I . Perera, R . A . Shanks

Polymerisation was carried out using a Rayonette RPRlOO photochemical reactor with twelve 300 nm lamps. A series of HEMA/NVP polymers were prepared at monomer ratio 30 : 70 w/w with varying amounts of EGDMA or MDA as crosslinker. The amounts of crosslinker used were 0.5, 1-0, 1.5 and 2.0% on the weight of monomer mixture. The crosslinker concentration can also be represented as a mole percentage, which is defined as moles of crosslinker per lOOg of monomer feed. A photo-initiator, benzoin methyl ether (0.5% w/w), was added to thoroughly mixed monomer feed and the mixture was UV-irradiated for 2h. Since polymerisation proceeded to completion, the polymer composition can be regarded to be the same as the monomer composition. No significant mass losses were observed on prolonged drying of polymer specimens in an oven at 373 K. Similarly, a series of MMA/NVP polymers were prepared using the same conditions.

Swelling and equilibrium water content

Thin strips (2mm x 1 cm x 2cm) were cut from cured materials using a lathe machine. Each sample was weighed and its dimensions were measured with a calliper. These specimens were swollen separately in deionised water at 25°C. The water was changed regu- larly to remove any material extracted during the swelling process. Equilibrium was attained generally in 4 weeks, which represents the time required for the swollen gels to attain constant weight. The dimensions of the swollen gels were also measured using a calliper. The degree of swelling was expressed as the equilibrium water content, EWC:

EWC(%) = 100(m2 - ml)/m2

where m, and m2 are the weights of dry and swollen polymer.

Soluble fraction

The soluble fraction was determined by

Soluble fraction(%) = 100(m0 - ml)/mo

where rn, is the weight of polymer before swelling.

Mechanical properties (compression test)

A compression rig fitted to an Instron tensile tester (model 1026) was used to obtain compression-strain data on swollen specimens (approximate dimensions 2cm x 2cm x 1.2cm) immersed in water (Fig. 1). A load cell having a range 5-50 kg was fitted to the instru- ment and the tests were run at a crosshead speed of 0.5 mmmin-’. The linear portion of the compression- strain curves was used to compute Young’s modulus.

AnACHED TO INSTRON MOVING CARAlAGf

t I

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WATER BATH

SAMPLE

1 AfIACHED TO INSTRON

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Effective crosslink density

The effective crosslink densities ve of the swollen gels were obtained from compression-strain measurements using the following relationship developed by Tobolsky :’

T = R T v , 4:”(1 - (1) where z is the applied stress, 42 is the polymer volume fraction, 1 is the ratio of the deformed length (1) and undeformed length ( lo) , and R T has its normal meaning. To obtain 42 for sol-free polymer in swollen gel at equilibrium, the volume of swollen polymer (after the soluble fraction has been removed) and densities of swelling agent (water) and dry polymers were used. From the values of v , , the molar mass between crosslinks, M , , was calculated using eqn (2):

Mc = P / v e (2) The theoretical crosslink density (v,) is given by eqn (3), where C is the concentration of crosslinking agent with functionality f :I4

v, = Cf/2 (3)

POLYMER INTERNATIONAL VOL. 39, NO. 2, 1996

Crosslinked hydrogels containing N-vinylpyrrolidone 123

In the present systems both EGDMA and MDA are tetrafunctional and hence eqn (3) reduces to

v, = 2 c (4)

The values of C were calculated from the densities of polymers and molar masses of the appropriate crosslinker. The densities of polymers were determined by measuring mass and volume of dry polymer samples. The volumes were obtained by weighing the specimens in air and in a non-solvent (n-heptane).

RESULTS AND DISCUSSION

All the polymer samples were clear, transparent and yellowish in colour after photopolymerisation. On absorption of water all the specimens except MMA/ NVPO (specimen without crosslinker) were clear and transparent. MMA/NVPO became white and opaque after absorption of water. HEMA/NVPO was very soft and difficult to handle. The swollen specimens with crosslinker ratios higher than 0.0075 mol% broke into several pieces due to their brittleness. Hence, HEMA/ NVP3, 4, 7 and 8 and MMA/NVP3, 4, 7 and 8 could not be used in compression tests.

Table 1 shows the variation of soluble fraction and EWC of the HEMA/NVP polymer series. The soluble fraction of the UV-cured polymer series seemed to be higher than values recorded previously for similar but y-irradiated terpolymer compositions.'.'' The EWCs are comparable, however. It can be seen that the soluble fraction of materials with no crosslinker is very high and it drops significantly as the crosslinker is intro- duced. However, it can be noted that even at a crosslinker concentration of 1-5%, the soluble fractions are as high as 31.2 and 25.8% for EGDMA and MDA crosslinked polymers, respectively. It has been reported previously that, because HEMA is more reactive than NVP, it enters the copolymer more quickly than NVP does.' This compositional drift has been substantiated by the use of Q-e schemes to obtain the polymer com-

position at different feed compositions. Hence, in the later stages of conversions, the product has a composition very close to PNVP homopolymer, which is soluble in water and is extracted out on swelling. It has also been shown that the addition of EGDMA as a crosslinker does not solve this problem fully, as EGDMA too undergoes a compositional drift, and both HEMA and EGDMA in the feed are consumed totally at a fraction conversion around 0.5.12 This explains adequately why NVP does not polymerise well with monomers such as HEMA and MMA even in the presence of a crosslinker.

As expected the soluble fraction reduces with the increase in amount of crosslinker for both EGDMA and MDA in this polymer series. A comparison of soluble fraction results at identical mol% of the two crosslinkers shows that MDA is somewhat more efi- cient in reducing the soluble fraction than EGDMA. This suggests that MDA possibly undergoes slightly less compositional drift than EGDMA does. This is not unexpected from the structural and hence compatibility differences between EGDMA and MDA. The presence of amide groups in MDA would allow it to enter a crosslinking reaction with NVP more readily than EGDMA.

Table 2 shows the variation of soluble fraction and EWC of the MMA/NVP polymer series. The variation of soluble fraction follows a similar pattern to the HEMA/NVP series. Here too the soluble fraction decreases with increase of crosslinker concentration. However, at 1.5% crosslinker concentration the soluble fractions for EGDMA and MDA crosslinked polymers are still 29.6 and 25.6% respectively. The factors affect- ing compositional drift in MMA/VP/EGDMA systems have also been studied previously." The findings are similar to HEMA/VP/EGDMA systems. The Q-e schemes have shown that MMA and EGDMA orig- inally in the feed are totally consumed at a fractional conversion of 0.63." Hence, at higher conversions the product obtained is almost PNVP. At complete conversion, the polymer is predicted to comprise crosslinked MMA/VP copolymer (relatively rich in MMA and

TABLE 1. Variation of soluble fraction and EWC of HEMA/NVP polymer series

Polymer EGDMA M DA Soluble fraction (YO) EWC (YO)

mol% wt% mol% wt%

H EM A/NV PO H EM A/NV P I H EMA/NVP2 HEMA/NVP3 H EM A/NV P4 H EM A/NV P5 H EMA/NVP6 H EMA/NVP7 H EMA/NVP8

0.00 0.0025 0.0050 0.0075 0.01 00 0.00 0.00 0.00 0.00

0.00 0.50 1 .oo 1.50 2.00 0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 0.0025 0.0050 0.0065 0.0097

0.00 0.00 0.00 0.00 0.00 0.39 0-78 1 .oo 1.50

51.8 33.8 31.8 31.2 30.0 30.1 28.5 26.8 25.8

90.3 80.3 76.2 73-4 70.0 74.9 73.5 72.6 68.2


124 D. I . Perera, R . A. Shanks

TABLE 2. Variation of soluble fraction and EWC of MMA/NVP polymer series

Polymer EGDMA M DA Soluble fraction (%) EWC (%)

mol% wt% mol% wt%

M MA/NVPO M MA/NVPI MMA/NVP2 MMA/NVP3 MMA/NVP4 MMA/NVP5 M MA/NVP6 MMA/NVP7 M MA/NVP8

0.00 0,0025 0.0050 0.0075 0.01 00 0.00 0.00 0.00 0.00

0.00 0.50 1 .oo 1.50 2.00 0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 0.0025 0.0050 0.0065 0.0097

0.00 0.00 0.00 0.00 0.00 0.39 0.78 1 .oo 1.50

44.5 35.2 34.7 29.6 29.7 34.0 31.8 28.4 25.6

EGDMA with respect to initial feed) and a considerable quantity of PNVP. As in the case of the HEMA/NVP polymer series, a comparison of soluble fraction results at identical mol% of the two crosslinkers shows that MDA is more effective in reducing soluble fraction than EGDMA. This again suggests that MDA is less sub- jected to compositional drift and more compatible with NVP. However, Q-e schemes need to be developed for MMA/NVP/MDA and HEMA/NVP/MDA systems to theoretically prove MDAs efficiency as a crosslinker.

The EWCs of HEMA/NVP polymers are higher than those of MMA/NVP polymers due to the relative hydrophilicity of HEMA and hydrophobicity of MMA. This is in agreement with the EWCs of similar systems reported previously. 11,12 The EWC drops significantly with the increase in crosslinker ratio in both polymer

77.9 65.5 61 .O 60.6 57.8 66.6 65.7 63.9 58.3

series and for both crosslinkers. One interesting observ- ation is that in the case of the MMA/NVP series, MDA crosslinked samples give slightly higher EWCs, whereas in the HEMA/NVP series, EGDMA crosslinked samples show a higher water absorptivity. In the MMA/NVP series, the reduction of the penetration of water into the network by hydrophobic MMA units is to a certain extent offset by inclusion of the hydrophilic difunctional monomer MDA. However, in the HEMA/ NVP polymer series, hydrophilicity is dominated by the presence of a relatively large amount of HEMA compared with crosslinker concentration.

Figures 2 and 3 show the variation of stress (compression) with strain for swollen HEMA/NVP and MMA/NVP polymers at equilibrium. The initial linear region of the curves was used to calculate Young’s

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0.02

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Strain Fig. 2. Compression-strain curves for HEMA/NVP polymers.

0.1



0.25

0.2

A N E I 1 0.15 U

0.05

0

0 0.02 0.04 0.06 0.08 0.1 0.1 2 0.1 4 0.1 6 Strain

Fig. 3. Compression-strain curves for MMA/NVP polymers.

modulus; the results are given in Tables 3 and 4. Plots deformation covered, these plots too are accurately of compression (7) versus (A - A-*) according to eqn (1) linear. The gradients were obtained by linear regression are shown in Figs 4 and 5 for HEMA/NVP and analysis and were used to calculate the effective cross- MMA/NVP polymers, respectively. Over the range of link density, v,. Values of v, relevant to the HEMA/

TABLE 3. Characteristics of HEMA/NVP hydrogels obtained from swelling and compression-strain measurements

Sample c x 1 0 3 v, x 103 x 103 M, x 103 Young's @2

(mol d ~ r - ~ ) (mol dm-3) (mol dm-3) (mol dm-3) modulus (MN IT-^)

H EMA/NVPO 00~000 00~000 1.6500 736.36 0.01 42 0-0856 H EMA/NVPI 30.375 60.750 10.6305 1 14.29 0.2800 0.1 761 H EMA/NVP2 60.750 121.500 14.71 92 82.55 0.4993 0.21 38

0.2253 H EMA/NVP5 30.375 60.750 12.6096 96.36 H EMA/NVPG 60.750 121.500 15.6237 77.77 0.2965 0.2390

0.1 806

TABLE 4. Characteristics of MMA/NVP hydrogels obtained from swelling and compression-strain measurements

Sample c x 103 (mol dm-3)

M MA/NVPO 00~000 M MA/NVPI 30.375 M MA/NVP2 60.750 M MA/NVP5 30-375 M MA/NVP6 60.750

v, x 103 x 103 (mot dm-3) (mol dm-3)

00~000 3.81 37 60.750 16.51 71

121.500 19.4891 60.750 21.5099

121.500 26.5239

M, x i 0 3 Young's @2

(mol dm-3) modulus (MN m-3)

~ ~_______

31 7.28 0.4422 0.1 981 73.25 0.6571 0.31 45 62.09 1.01 09 0.3577 56.25 0.5077 0.3040 45.62 0.8269 0.31 26


126

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D. I . Perera, R . A . Shanks

0.0002

0

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 -(h-l/h2)

Fig. 4. Plots of stress versus (A - l /A2) for HEMA/NVP polymers.

NVP and MMA/NVP polymer series are given in Tables 3 and 4. These tables also contain the crosslinker concentration. C, the theoretical crosslink density, v, , the molar mass between crosslinks, M , , and the polymer volume fraction in the swollen state, $ 2 .

It can be seen that even at zero crosslinker concentration the crosslink densities have finite values. This can be accountable to the presence of dimethacrylates as an impurity in methacrylates and also to physical, hydrophobic interactions which contribute to the effective crosslink den~ i ty . ' ~ A large discrepancy can be

0.0035

0.003

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found between Y, and v,, with v, being always smaller than v,, for the present systems obtained by photopolymerisation. Similar copolymer systems prepared by irradiation have shown the opposite trend (v, > v,) where irradiation contributed to additional chemical crosslinks. The efficiency of crosslinking of the present photopolymerised systems, therefore, is poor. This can be attributed to the formation of intramolecular crosslinks and the inhomogeneity of crosslinking due to compositional drift. The fact that the values of v , for MDA crosslinked networks are higher than those for

0

0 0.1 0.2 0.3 0.4 0.5 0.6 -(h- 1 /A2)

Fig. 5. Plots of stress versus (A - l / A z ) for MMA/NVP polymers.



EGDMA provides extra evidence for MDAs relatively less compositional drift compared with EGDMA.

As expected, the higher the amount of crosslinker the higher the modulus of the material and higher the v, in both polymer series. Both the moduli and v , of MMA/NVP polymers are much higher than those of HEMA/NVP polymers. The relatively more hydrophobic MMA compared with HEMA contributes to hydrophobic, physical interactions which can contribute to

According to one structure proposed from 13C NMR studies, MMA/NVP gels swollen in water contain MMA and NVP in separate domains.” It was suggested that MMA-rich domains might contain a rigid, disordered, glassy region that contributes to effective crosslinking. These physical interactions limit the penetration of water to allow elastic behaviour and impart a high modulus or rigidity to the gel.

Compared with MDA crosslinked materials, EGDMA crosslinked materials show a higher modulus in both HEMAfNVP and MMA/NVP series. MDA, being the more hydrophilic crosslinker of the two, would allow absorption of more water, which in turn makes it more elastic or less rigid.

CONCLUSIONS

The use of methylene diacrylamide as a crosslinker in place of ethylene glycol dimethacrylamide reduces the amount of soluble fraction in both polymer series inves- tigated. This could primarily be due to relatively homo- geneous crosslinking by MDA in comparison with EGDMA, which undergoes a compositional drift. However, a further reduction in soluble fraction is desired for intended applications. The MDA crosslinked polymers exhibit higher EWC in MMA/NVP polymers

and lower in HEMA/NVP polymers. The MDA crosslinked networks appear to be softer (low modulus) than EGDMAones.

ACKNOWLEDGEMENTS

This project was funded by GIRD, Eycon Lens Labor- atories Pty Ltd and Milpharma Pty Ltd. The authors wish to thank Mrs F. T. P. Choo for assistance in some experimental work.

REFERENCES

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9

10 11 12 13

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swelling and mechanical properties of crosslinked hydrogels containing n-vinylpyrrolidone

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