Refractive index and molar refraction of methacrylate monomers and polymers

643

Refractive index and molar refraction of methacrylate monomers and polymers M.P. Patel, K.W.M. Davy and M. Braden University of London Interdisciplinary Research Centre in Biomedical Hospital Medical College, Turner Street, London El 2AD, UK

Materials,

Dental School,

Royal London

The refractive indices of a number of methacrylate monomers have been measured and corresponding molar refraction data calculated. Similar determinations were made on a number of methacrylate polymers. The molar refraction values determined were in excellent agreement with the values calculated from the published molar refraction values of the chemical groups involved. There was little difference between the molar refraction values of monomers and that of the repeat unit in the corresponding polymers, in marked contrast to the molar volumes. The ratio of refractive index to molar volume was reasonably consistent for all methacrylates studied (0.25-0.30 approximately). Keywords:

Methacrylates,

dental

materials,

polymerization

Received 1 August 1991; revised 5 December 1991; accepted 16 December

The refractive index [II?] of polymers is relevant to their use in dentistry, or indeed in any other clinical application where aesthetics is of importance. With composite filling materials for example, filler and resin should be matched if translucency is required. The measurement of then;’ of organic liquids like monomers is particularly useful in synthesis work, because the II;’ is easy to measure quickly and accurately and the molar refraction (R) can be calculated from it. As the R of chemical groups is additive and such values are well documented’, the theoretical value for a given compound can be calculated and compared with the experimental value. Since a number of materials have been prepared from novel methacrylates, the I$’ and related properties have been determined. The R is further useful, because it is reported to be the molar volume (V,,,] of a given structure without its accompanying free space. Clearly this is of potential interest in studying the polymerization process, because it is well established that V, decreases; in the case of methacrylates’, this change is -20 cm3/mole.

MATERIALS

AND METHODS

Materials The methacrylate monomers studied are listed in Table 2. A smaller range of homopolymers, prepared as described previously, were also studied. Many of these were synthesized as described elsewhere3, 4. Correspondence

to

Dr M.P. Patel.

0 1992 Butterworth-Heinemann 0142-9612/92/090643-03

Ltd

1991

Methods Refractive indices (II:‘) of both monomers and polymers were measured with an Abbe refractometer, using a sodium light source. The refractometer was circulated with water from a thermostat bath. The measurement temperature was 20°C f O.l’C or 25°C It O.l”C.

RESULTS Tables 1 and 2 summarize the III:’ and R results. Figures 1 and 2 are plots of nk” and R versus the number of carbon atoms in the ester group (n) of n-alkyl methacrylates, respectively.

DISCUSSION The nh” of a material Mosotti equation:

is related to the R by the Clausius-

R =n2_ -lM n2+2p

(1)

where n is the refractive index of the material, M the molecular weight and p the density. Equation 1 is obtained from electromagnetic theory, and strictly applies only to dielectrics’; there are other empirical formulae’. This equation is extremely useful because the R value for a given molecule is made up of the contributions of each atom and other factors, such as double bonds and rings in the molecule. Such values are Biomaterials

1992, Vol. 13 No. 9

644

Methacrylate

Table 1

Refractive behaviour

Methacrylate

of methacrylate

monomer

index and molar refraction:

M.P.

Pate/ et al.

monomers

Refractive index (n$O)

Molar refraction, Experimental

Methyl Ethyl n-Butyl n-Octyl n-Nonyl Tridecyl 2,SEpoxypropyl Tetrahydrofu~u~l T~rahydropyranyl Tetrahydropyran-Bylmethyl lsobornyl Methoxyethyl 2-oxo-propyl P-hydroxyethyl 2-hydroxypropyl Shydroxypropyl Ethylene glycol dimer Triethylene glycol dimer Tetraethylene glycol dimer 1-(Gmethyl phenoxy) P-hydroxypropyl 3-methacrylyoxy P-hydroxypropyl benzoate 3-methac~lyo~-2-hydro~propyl (2-methyl benzoate) 3-methacrylyoxy-2-hydroxypropyl (3-methyl benzoate) 3-methac~lyoxy-2-hydroxypropyl (bmethyl benzoate) P-hydroxypropane diol dimethacrylate Dioctyl tin

refractive

R (cm3)

Molar volume cm3/mole

WV,

Density (g/ml)

0.9379 0.9090

Theoretical

1.4150 1.4145 1.4235 1.4380 1.4400 1.4465 1.4490 1.4760 1.4560 1.4602 1.4600 1.4302 1.4439 1.4515 1.4475 1.4429 1.4649 1.4608 1.4623 1.5327

26.700 31.412 40.773 58.954 63.779 82.080 35.377 46.077 45.091 49.195 61.915 37.430 35.930 33.890 37.240 36.840 51.100 72.900 83.700 69.690

26.519 31.166 40.460 59.048 63.695 82.263 36.014 45.011 45.011 49.658 64.547 37.460 33.290 32.690 37.340 37.340 56.980 72.860 79.150 70.770

106.621 125.571 159.952 224.581 240.900 307.842 131 .a93 163.363 165.878 179.548 226.057 144.850 135.310 125.730 139.730 139.450 188.390 265.800 304.200 224.660

0.250 0.250 0.255 0.263 0.265 0.267 0.266 0.262 0.272 0.294 0.274 0.258 0.266 0.270 0.287 0.264 0.271 0.274 0.275 0.310

0.8830 0.8800 0.8720 1.0777 1.0420 1.0261 1.026’1 0.9827 0.9941 1.0494 1.0340 1.0306 1.0326 1.0510 1.0760 i .oa47 1.1128

1.5184

68.040

68.020

224.410

0.303

1.1764

1.5195

73.510

72.920

242.030

0.320

1.1486

1.5162

73.050

73.200

241.760

0.302

1.1499

1.5192

73.360

73.230

241.630

0.304

1.1505

1.4672

56.730

57.17

204.41

0.278

1.1154

1.4814

167.781

448.60

0.374

1.1471

1.5047 1.5180

110.99 102.54

374.4 366.0

0.296 0.303

1.2018 1.2293

0.8890

Phthalic acid derived dimethacrylates ‘73

H P

W-4

=

C - CO&H - CH,O,C),

1,2 substituted 1.3 substituted

Table 2

Molar refraction for some methacrylate

polymers

Methacrylate polymer

Molar refraction (R) (cm3)

Molar volume (V,) (cm3/mole)

R/V,

Methyl Ethyl n-Butyl Tetrahydropyranyl51 Isopropyl Tertbutyl Cyclohexyl 1(4-methyl phenoxy) Bhydroxypropyf ~methac~loylo~P-hydroxypropyi(2-methyl benzoate) 3-methacryloyloxy-2hydroxypropyl-(3,5dimethyl benzoate)

25.0 29.19 38.44 46.07

(26.70) (29.30) (38.20) (46.08)

84.13 i 02.83 134.78 154.89

0.30 0.28 0.28 0.30

41.32 (36.44) 38.31 (36.85) 45.05 65.32

119.90 138.90 150.80 203.47

0.34 0.28 0.30 0.32

70.99

220.25

0.32

75.48

237.22

0.32

Figures in parentheses

Bioma~ri~s

are calculated

1992, Vol. 13 NO. 9

values.

101.46 101.75

publishedl, thus the R value for a given molecule can be calculated. Molar refraction is, in fact, a measure of polarizability of the atom or group and compared to experimental values, R has the dimensions cm3/mole. The plot of ng versus the number of carbon atoms in the n-alkyl side group (Figure 1) showed a minimum at C = 2. However, the same data plotted in the form of R versus the number of carbon atoms in the side group gave a straight line, of slope = 4.70 cm3 (Figure 2). This value compares well with the published theoretical value’ of 4.647 cm3 for the CH, group. As R is an approximate measure of the actual total volume (without free space) of the molecules in 1 g molecule’, it is interesting to compare the R value with the corresponding V,,,. Table I lists R, V,,, and with calculated R/V,,, values. These latter values were surprisingly consistent for a range of methacrylates. The limited WV, data for polymers are given in Table 2. Again there were consistent values for RN, similar to those of the corresponding monomers. Indeed

~ethac~late

refractive

index and molar refraction:

645

f&f.P. Pate/ et al. 90

r

SO-

0

I

I 5

I 10

I

15

n 1.411 0

I 5

I 10

I 15

n

Figure2 Plot of molar refraction (R) versus number of carbon atoms in side chain of n-alkyl methacrylate monomers.

Figure 1 Plot of refractive index (n$O) versus the number of carbon atoms in side chain ofn-alkyl methacrylate monomers.

there was little difference between the R values of monomers and their co~esponding polymers. RN, seems to be between 0.25 and 0.3. Since the van de Waal’s volume [VW) is -0.65 V,,,, R/V, is between 0.38 and 0.46.

Both Tables 1 and 2 show generally good agreement between theoretical and experimental R values. In the case of dioctyl tin dimethacrylate, by subtracting the values ascribed to the various organic groups, the R values for tin in this compound was calculated to be 49.69 cm3/mole. This is very close to the VW of tin, calculated from the corresponding radius5. This indicated both the reliability of the Clausius-Mosotti equation and the purity of the monomers and polymers used. CONCLUSIONS

R values, calculated from the ng and density of both monomers and polymers with the Clausius-Mosotti

equation, gave values in close agreement with those predicted theoretically. This indicates the value of nf and density measurement in synthesis of methac~lates. For a wide range of methacrylate monomers and polymers, the ratio of R/molar value was between 0.25 and 0.3 and R/VW between 0.38 and 0.46. REFERENCES

Weast, RX., CRC Handbook of Chemistry and Physics CRC Press, Cleveland, Ohio. USA, 1970 Patei, M., Braden, M. and Davy, K.W.M., Poiymerisation shrinkage

of ethacrylate

esters,

Homaterials

1967, 8,

53-56 Bhusate, M., The Preparation of Heterocyclic Methacrylate Esters, MSc Thesis, Council for National Academic Awards, 1983, UK Davy, K.W.M. and Braden, M., The Thermal Expansion of Glassy Polymers Bondi, A.J., van de Waal’s Volumes and Radii, J; Physics

Chem. 1964, 88,441~451

Biomaterials

1992, Vol. 13 No. 9