The interaction of various liquids with long-term denture soft lining materials

d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) e199–e206

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The interaction of various liquids with long-term denture soft lining materials Wen-Chien Liao a , Gavin J. Pearson b , Michael Braden c , Paul S. Wright b,∗ a

Department of Prosthetic Dentistry, Tri-Service General Hospital and School of Dentistry, National Defense Medical Center, Taipei, Taiwan b Formerly Queen Mary, University of London, London, UK c Queen Mary, University of London, Institute of Dentistry, Barts and The London School of Medicine and Dentistry, London, UK

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objectives. To study the uptake of liquids, representative of those encountered orally, by long-

Received 6 January 2011

term denture soft lining materials, and analyze the data in terms of appropriate theories.

Received in revised form

Methods. Four proprietary and one experimental soft lining material were investigated, and

24 April 2012

the weight change presented as a function of time in both aqueous and organic fluids over

Accepted 25 April 2012

the course of a year. A separate experiment determined the equilibrium swelling in ethanol of poly(ethyl methacrylate) and poly(methyl methacrylate). Results. Uptake date for the five soft lining materials in various aqueous solution, coconut oil

Keywords:

and HB307 are reported. The experimental value for the equilibrium swelling of poly(ethyl

Denture

methacrylate) and poly(methyl methacrylate) in ethanol was reported to indicate the solu-

Long-term soft-lining materials

bility parameter of the system.

Food simulating liquids

Significance. The results have been analyzed by relevant theoretical models, which have been

Theory

shown to explain the experimental data. © 2012 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The term soft lining refers to the lining of a denture with an elastomeric type material. Such materials, being easily deformable, will absorb energy during biting [1], and distribute the loads evenly over the whole denture bearing area, thus lessening the deformation of the oral mucosa. This reduces the discomfort when such loads exceed the ability of the tissues to support them and potentially reduces resorption of the residual bone [2]. Clearly, it is highly desirable that such materials do not degrade in the mouth, i.e. do not lose strength or compliance, nor become detached from the denture. The most obvious mechanism of compliance loss is with

the so-called soft acrylics, where compliance is achieved by incorporation of a plasticizer, and subsequently lost when the plasticizer leaches out [3]. Some experimental materials have been described which use a polymerisable plasticizer [4,5]. Another approach has been to make a preformed, heat curing dough, comprising a main elastomer from the rubber industry, e.g. polyisoprene, butadiene, styrene copolymers, doughed with a higher methacrylate monomer (n-butyl and above). In this way, it was hoped that the high strength of such elastomers would be of benefit. A number of studies have been made of these systems [6–15]. One disadvantage was that prolonged immersion in water at 37 ◦ C produced a marked deterioration in mechanical properties. This was attributed to peroxide catalyzed scission of the double-bonds

∗ Corresponding author at: Institute of Dentistry, Turner Street, London E1 2AD, UK. Tel.: +44 0020 7882 8656; fax: +44 0020 7377 7064. E-mail address: [email protected] (P.S. Wright). 0109-5641/$ – see front matter © 2012 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dental.2012.04.036

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in diene copolymers, the residual peroxide polymerization catalyst being the source. Hence brominated butyl rubber was investigated; butyl rubber is a copolymer of ∼98% isobutyl methacrylate and 2% isoprene [16]. The brominated version was used because it is easier to peroxide cross-link [15]. The loss of strength in water can also, a priori, be due either to certain aspects of water uptake, or the interaction of organic liquids in the diet such as products containing vegetable oils or ethanol. High water uptake can be experienced in otherwise hydrophobic elastomers, with consequent loss of strength, by the presence of water soluble components. Thus Braden and Wright observed water uptake values in a silicone rubber soft liner of >60% [17]. More recent studies are those of Mante et al. on the effect of aqueous solutions on hardness, and Leite et al. on the effect of various beverages on hardness [18,19]. The current study has involved representative proprietary materials from the soft acrylic and silicone types, and an experimental material based on brominated butyl rubber. The test liquids included aqueous, ethanol containing, and paraffinic liquids. However a major objective of this paper was the application of relevant theories to the results obtained.

2.

Materials and methods

2.1.

Materials

The materials used are listed in Table 1 and further details are listed in Table 2. Eversoft and Vertex are examples of the so-called soft acrylics. Ufi-gel and Molloplast b are silicone rubbers, the latter having dispersed polymethyl methacrylate (PMMA) domains, as indicated by infra-red spectroscopy. Various suggestions have been made as to its exact composition [20,21]. The brominated butyl rubber (BBR), doughed with n-butyl methacrylate and heat cured was an experimental material [15].

2.2.

Methods

2.2.1.

Sample preparation

A polyether impression material was used to make sheets nominally 1 mm thick, from which 20 mm diameter discs were cut using a cork borer. These discs were invested in dental stone using conventional dental techniques. On setting, the flask was separated and the discs removed to leave a mold ready for the preparation of specimens. Discs were prepared for each of the materials using the manufacturer’s instructions. In the case of the brominated butyl rubber, the monomer liquid was made up with 1% lauryl peroxide (w/w) as initiator and n-butyl methacrylate monomer containing 1% ethylene glycol dimethacrylate (wt/vol) as a cross linking agent. Lauryl peroxide was used instead of benzoyl peroxide, because the decomposition in the former case is lauric acid, and benzoic acid in the latter. Lauryl peroxide has a much lower solubility in water, and consequently will have much less influence on water uptake [20]. 100 g brominated butyl elastomer was doughed with 100 ml of the monomer liquid described above.

The curing cycle comprised 2 h at 74 ◦ C, followed by 30 min at 100 ◦ C.

2.2.2.

Water and fluid absorption characterization

All specimens were processed according to the manufacturers’ directions. A total of 42 specimens were constructed for each denture soft lining material. The specimens were then randomly divided into seven groups of six specimens. Specimens were preconditioned after manufacture by storing in a desiccator at 37 ± 1 ◦ C. The specimens were removed from the desiccator and then after immediately weighing, were weighed at regular intervals until a constant weight was achieved. All readings were taken to an accuracy of ±0.0002 g on an AE Mettler electronic balance (Metler-Toledo Ltd, Leicester, UK). This initial weight (W0 ) was noted. After weighing, each specimen was immediately transferred to a wide mouth, amber, screw topped glass jar containing 50 ml of a food simulating liquid conditioned to 37 ◦ C. The immersing liquids selected were distilled water (DW), artificial saliva (AS) (composition shown in Table 3) [22], 3% aqueous acetic acid (3AA) (EC Food Contact Legislation, 2000), 10% ethanol (10E), 50% ethanol (50E), coconut oil (CO) and HB307 (HB) (FDA, 2002). Each glass jar was then stored in an incubator (LABHEAT Model RLCH0400, Boro Labs Ltd, Berkshire, UK) at 37 ± 1 ◦ C. Each specimen was removed at predetermined time intervals using tweezers and carefully blotted to remove excess surface liquid using filter paper prior to weighing. The weights were then recorded. Initial intervals between weighing were short but subsequently were increased. The fluid was unchanged for the duration of the experiment but was topped up after each measurement to maintain a fixed volume. After a period of 52 weeks, specimens were removed from solution, weighed and then desorbed in an incubator (Gallenkamp Durastat Type 3, LTE Scientific Ltd, Oldham, UK) at 37 ± 1 ◦ C. Specimens were weighed at regular intervals until a minimum weight was reached (Wd ). Percentage weight change and percentage solubility were calculated as a percentage of the initial weight. Real percentage uptake was calculated as the sum of percentage weight change and percentage solubility, and desorption diffusion coefficients by the application of solutions of Fick’s equations [23]:

% Uptake =

W − W  t 0 W0

% Solubility =

× 100

 W −W  0 d W0

× 100

Real % Uptake = % Uptake + % Solubility

(1)

(2)

(3)

where W0 = initial weight, Wt = weight at time t and Wd = final minimum desorbed weight. The diffusion coefficient was calculated from the slope of the linear parts of the plot [24]:



Mt Dt =2 M∞ l2

1/2 (4)

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Table 1 – Denture soft lining materials investigated. Soft lining material

Code

TM

Vertex Soft (heat-cured acrylic resin) EverSoft® (methyl methacrylate-free acrylic resin) Molloplast B® (heat-cured silicone elastomer) Ufi Gel SC (self-cured silicone elastomer) Bromo butyl butyl elastomer (heat-cured experimental elastomer)

Manufacturers

Presentation

VX ES

Dentimex BV, Holland Myerson, Austenal Ltd, UK

Powder and liquid Powder, liquid and sealer

MB

Karl Huber GmbH & Co. Germany

Single component paste

UG BBR

Voco GmbH, Germany QMUL, UK

Cartridges (auto-mix) and glazer Elastomer and liquid

Table 2 – Denture soft lining materials composition. Code

Powder (paste, base, elastomer)

Liquid

VX

Polyethyl methacrylate

ES

Polyethyl methacrylate

MB UG BBR

␣-␻-Dihydroxy terminated poly(dimethyl siloxane) Vinyl dimethyl polysiloxane, hydrogen poly siloxane, silicone dioxide, fumed silica Bromo butyl elastomer

Code

Acetyl tributyl citrate (plasticizer, <80%) Methyl methacrylate (>15%) Crosslinker (<5%) Di-n-butyl phthalate (plasticizer) (60–90%) Ethyl acetate (5–15%) Ethyl alcohol (1–10%)

Initiator (catalyst)

VX ES MB UG BBR

Butyl methacrylate

Cross-linking agent

Sealer (glaze)

Benzoyl peroxide Methyl ethyl ketone Benzoyl peroxide Vinyl dimethyl polysiloxane, silicone dioxide 1% Lauryl peroxide

The same as base and catalyst 1% Ethylene glycol dimethacrylate

Table 3 – Food simulating liquids. Food simulating liquids

Code

Simulated food

Distilled water Artificial saliva 3% Acetic acid 10% Ethanol 50% Ethanol Coconut oil

DW AS 3AA 10E 50E CO

Aqueous foods (control) Saliva Aqueous and acidic foods (EC standard) Aqueous and low-alcoholic foods High-alcoholic foods Fatty foods

HB307

HB

Fatty foods

where Mt = mass uptake at time t; M∞ = mass uptake at equilibrium; D = diffusion coefficient; thickness = 2l. This predicts a linear plot with respect to t(1/2) of slope (S):

 D 1/2

S=2

(5)

l2

which rearranged gives the diffusion coefficient:

D=

(S2 4l2 ) 16

Manufacturers Queen Mary, University of London Fusuyama [22] formulation BDH Chemical Co. BDH Chemical Co. BDH Chemical Co. Coconut oil from Cocosnucifera, C1758, Sigma Chemical Co. USA NATEC GmbH, Hamburg, Germany

2.2.3. Solubility parameters of PMMA/ethanol and PEM/ethanol In the case of PMMA, a rectangular strip of “Perspex” (5.5 × 2 × 0.1 cm) was weighed, immersed in ethanol, and weighed until constant weight was reached, then desorbed to constant weight to give the equilibrium uptake C0 (g/cm3 ), from which v2 (Eq. (1)) was calculated. A similar sheet of PEM was made by making a dough of PEM powder and ethyl methacrylate monomer, which was processed in the normal way to give a heat cure product.

(6)

From which D is calculated from S. Eq. (4) is valid for the earlier stages of uptake; the relevant equation for the uptake up to equilibrium is [23]: Mt 8  1 =1− 2 exp 2 M∞  (2n + 1) ∞

0



2

−

(2n + 1) Dt 4l2

 (7)

3.

Results

Fig. 1 gives the uptake data for BBR in various aqueous solutions. Fig. 2 gives corresponding data in for coconut oil, and HB307, which is largely coconut oil based. Fig. 3 gives Shore hardness vs. storage time for BBR in coconut oil and HB307.

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Uptake in aqueous solutions by

Water uptake of Ufigel from various aqueous solutions

Brominated Butyl/n butyl methacrylate

1.2

systems 30

1

DW

20

AS

15

AS

AA 90/10

10

50/50

5 0

DW

0.8 Mt/M ∞

% Uptake

25

3%AA

0.6

10%eth 50% eth theory

0.4

0

200

400

600

t½ fit

800 0.2

½

t (mins½ )

Fig. 1 – Uptake in aqueous solutions by brominated butyl/n-butyl methacrylate systems.

0 0

200

400

600

800

t½ (mins½ )

Fig. 4 – Water uptake of Ufi-gel from various aqueous solutions.

Brominated butyl rubber in coconut oil and HB307

250

% uptake

200 150 100 50 0

0

200

400 ½

600

800

½

t (mins ) Fig. 2 – Brominated butyl rubber in oil and HB307.

Fig. 4 gives the uptake of Ufi-gel in various aqueous solutions, in the form of Mt /M∞ vs. t(1/2) , in order to check the validity of Eq. (7) for DW. The theoretical plot was obtained by substituting the value for D obtained from Eq. (6) in Eq. (7). Typically M∞ was ∼0.3% (w/w). Fig. 5 gives the uptake of Molloplast b in various aqueous solutions. Fig. 6 gives the weight change of Molloplast b and Ufi-gel in coconut oil. Note both graphs have a common plot up to ∼200 min(1/2) , with a maximum at ∼50 min(1/2) , with the two plots diverging beyond this point. Similar behavior is noted with HB307. Fig. 7 gives the uptake of Eversoft and Vertex Soft in various aqueous solutions.

Effect of immersion of brominated butyl rubber based lining on Hardness 45

Water uptake of Molloplast b from aqueous solutions

40

3.5

35

3 2.5

25

coc oil HB307

20 15 10 5

% Uptake

Shore hardness

30

DW AS

2

3%AA 1.5

10%eth.

1

50%eth.

0.5

0

0.1

1

10

100

1000

10000

time(hours)

Fig. 3 – Effect of immersion of brominated butyl rubber based lining on hardness.

0

0

200

400

600

800

t½ (mins½ )

Fig. 5 – Water uptake of Molloplast b from aqueous solutions.

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Absorption of Coconut Oli by

Table 4 – Comparison of the interaction between Ethanol and PMMA and PEMA.

Molloplast b and Ufi-gel

% Weight Change

1.8 1.6 1.4 1.2

MB

1 0.8

Ufi-gel

0.6 0.4 0.2 0 0

200

400 ½

600

800

½

t (mins ) Fig. 6 – Absorption of coconut oil by Molloplast b and Ufi-gel.

10

Eversoft and Vertex in 50/50 ethanol/distilled water

PMMA

% (w/w) uptake Volume fraction Interaction parameter

4.5 0.0027 2.66

0 0

200

400

600

800

-5

Eversoft Vertex

-10

Uptake from coconut oil

 = RT ln(1 − v2 ) + v2 + v22

-20 time (mins) Fig. 7 – Eversoft and Vertex in 50/50 ethanol/distilled water.

4.

Discussion

4.1.

Brominated butyl rubber

35.5 0.3350 0.6502

Fig. 2 shows the uptake from coconut oil to be diffusion controlled; from the linear t(1/2) region, the slope was determined and used to calculate the diffusion coefficient from (see Eq. (6) above), giving D = 1.07 × 10−13 m2 s−1 . The equilibrium concentration of coconut oil is ∼180%, which gives a volume fraction of polymer (vp ) of 0.357. The uptake of non-cross linked polymers is governed by the Flory Huggins equation [26,27]:



-15

PEMA

Muniandy and Thomas for natural rubber [24,25]. The fact that the uptake is essentially the same for artificial saliva indicates that the osmolarities of the impurities is very much greater than that of the artificial saliva. Clearly, the uptake is diffusion controlled, as the plot is linear with respect to t(1/2) ; the fact that the plot is linear over the whole time period of the experiment (∼340 days) indicates the uptake is less than half that at equilibrium [23]. The actual uptake at the end of the time period is greater for the solutions containing ethanol and acetic acid, particularly the latter. Whether this reflects a higher diffusion coefficient in these cases or an actual increase in the equilibrium uptake is impossible to discern.

4.1.2.

5 % Weight change

Ethanol



(8)

where  = chemical potential, R = universal gas constant, T = temperature (K), v2 = volume fraction of polymer,  = the solubility parameter. If  < 0.5, the polymer and solvent are completely miscible. For cross-linked polymers, a fourth term 1/3 is added between the square brackets, namely 2 V1 v2 /Mc . Braden et al. [28] have shown that Eq. (8) containing the cross-linking term can be written in terms of the Elastic Modulus (E) of the polymer, whence the  is given by Eq. (9): 1/3

In the current work, the brominated butyl polymer was mixed with n-butyl methacrylate to form a dough, which was subsequently heated to give a cured product. The fact that it formed a homogeneous dough suggests that the elastomer is dissolving, at least in part. However, when the product is cured, and the n-butyl methacrylate momomer polymerizes, phase separation is likely, because of the decrease in the entropy of mixing. Hence the interaction of the material with liquids needs to take this into account.

4.1.1.

=−

ln(1 − vp ) + vp + Ep (1 + C2 /C1 VL vp /3RT) v2p

(9)

where C1 and C2 are the parameters of the Mooney Rivlin Stored Energy function for the deformation of an elastomer [29]. In this case  has been calculated for the C2 /C1 ratio of 0, 0.5, and 1. E was calculated from the Shore hardness of the material [30]: E(MPa) =

0.0981(56 + 7.66s) 0.1375(254 − 2.54s)

(10)

Uptake from aqueous solutions (Fig. 1)

Since the structure of brominate butyl is largely paraffinic, it is hydrophobic. Hence a water uptake of all of the aqueous solutions of >10% is initially surprising – the water uptake of poly n-butyl methacrylate is ∼1%. However, the presence of water soluble moieties is the likely cause, and reflects the findings of

where E is Young’s modulus and s is the Shore hardness. The value of E so calculated from a Shore hardness of 44.6 is 2.00 MPA; this substituted in Eq. (9) gives the  values given in Table 4. Fig. 3 plots hardness against storage time for BBR. It is clear that the hardness decreases with time, in Youngs

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Modulus terms, when the Hardness is 20; this is equivalent to a Youngs Modulus of 0.73 MPa, i.e. a fall of 63.5%. All of these are <0.5, meaning that coconut oil is a solvent for brominated butyl rubber; this is hardly surprising given the paraffinic character of polyisobutene and coconut oil. It should also be noted that the parameter  for polyisobutene and octane is ∼0.4 [31]. Some circumspection is necessary with the application of the above theory to the actual system used, i.e. brominated butyl rubber doughed with n-butyl methacrylate. When this system is cured, it is not at this stage known whether the poly (n-butyl methacrylate) phase separates, or forms an interpenetrating network. However, with low TG of poly (n-butyl methacrylate) and the similarity of its solubility parameter (16.58 [J m−3 ](1/2) ) to that of n-dodecane, which is closely related to lauric acid (16.0 [J m−3 ](1/2) ) [31,32], the main constituent of coconut oil, suggests that it will dissolve rapidly in the coconut oil, and hence not limit the swelling of the brominated butyl system.

4.1.3.

Uptake from HB307

The uptake from HB307 is very much higher than that for coconut oil (240% c/f 180%), a major constituent of HB 307; this is reflected in the extreme physical degradation in HB307 beyond two months, to the point where Shore hardness values can no longer be determined. The constituents of HB307 are predominantly those of coconut oil, with two differences. HB307 contains 0.7% diglyceride and <0.4% monoglyceride. Clearly the question is whether these small quantities can cause the effects exhibited. Both glycerides are hydrophilic, and their solubility parameters indicate they will be immiscible with brominated butyl rubber, hence are water soluble impurities, and hence will promote absorption of water from the atmosphere. It is suggested the subsequent droplet growth around these droplets in an already highly swollen system, and the stresses generated therein, may lead to the mechanical breakdown noted above. These observations emphasize the importance of using clinically relevant solutions for evaluation purposes.

4.1.4.

Beyond the linear region, again the plots for the various solutions are similar, with the exception of the 50% ethanol in the case of Ufi-gel; the reason for this is not clear. If the diffusion coefficients obtained are substituted into Eq. (5), it can be seen that the experimental data lags behind the theoretically predicted values. This is symptomatic of the diffusion coefficient decreasing with increasing concentration and is a well known feature of the diffusion of water in silicone polymers [33]. Overall, the amount of water absorbed in the two materials is ∼1.5–3% (w/w). Considering that the solubility of water in silicone polymers is 30 mol m−3 , i.e. 0.054% [34] it is clear that the bulk of the water uptake is due to other causes, probably the presence of water soluble moieties in the filler. This feature could be important if higher levels of water soluble moieties are present. This could result in high enough osmotic pressures within droplets to cause failure. Indeed Amsden has utilized this phenomenon to produce drug delivery systems [35,36].

General observations

The substantial uptake noted in aqueous solutions and in coconut oil based systems, are clearly major disadvantages of brominated butyl rubber, which may well apply to other hydrocarbon based elastomers.

4.2.

Silicone polymers

4.2.1.

Uptake from aqueous solutions

Both Ufi-gel and Molloplast b (Figs. 4 and 5) show very similar plots, when the data is plotted as Mt /M∞ vs. t(1/2) ; for the linear t(1/2) region, the data for the various aqueous solutions fit a common straight line at the earlier values of time, from which the diffusion coefficients could be calculated from Eq. (6). These were 4.2 and 4.5 × 10−13 m2 s−1 for Molloplast b and Ufi-gel respectively. These near identical results indicate that the uptake of water is through the polymer matrix, even though Molloplast b contains poly(methyl methacrylate), presumably in discrete domains. M∞ values in the range 0.3–0.5% are low, although much greater then the solubility of water in polydimethyl siloxanes.

4.2.2.

Uptake of coconut oil based liquids (Fig. 6)

Both of the silicone materials give very similar and unusual plots for both coconut oil and HB307. A very rapid increase in weight is evident, but this rapidly reverses leaving ultimately a net uptake of 0.5–1.0%. This suggests the possibility that some soluble material is being extracted.

4.3.

Soft acrylics

The results for water/ethanol mixtures are drastically different from the behavior of both materials in distilled water itself, and artificial saliva (Fig. 7). In spite of the fact that that Eversoft is a visco-elastic gel, and Vertex is a heat cured system, the plots are remarkably similar in character. There are three distinct stages: (i) A rapid increase in uptake, suggestive of absorption of ethanol from the ethanol/water mixture. (ii) A subsequent reversal of the weight change process, indicative of an extraction process, presumably the extraction of the phthalate plasticizer, which is miscible with ethanol. (iii) A further reversal, indicative of an uptake process. This apparently complex behavior indicates that two processes are going on simultaneously, i.e. extraction of plasticizer and the absorption of ethanol by the PEM component. Considering first Eversoft, the weight change from the maximum weight loss to the final weight (+) change is ∼20%. However this is based on the original weight of the sample, whereas the remaining material is just the original PEM swollen by ethanol. Assuming a powder/liquid content of 2.5/1, this indicates that the PEM contains ∼30% ethanol. Separate experiments on a PEM polymer per se (see Materials and Methods) gave an equilibrium uptake of ethanol of 35.5% (Table 4), indicating the above estimate of ∼30% reasonable. Calculations of the Flory Huggins parameter (see Eq. (8)) for the PEM-ethanol system gave 0.6502. Parallel experiments on poly(methyl methacrylate) (PerspexTM ) gave a  value of 2.66, in reasonable agreement with published values

d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) e199–e206

[30,31]. This corresponds to an equilibrium uptake of ethanol by PMMA of 2.11%. This comparative data for the interaction of ethanol with PMMA and PEMA respectively, shows quantitatively why PEM is used in tissue conditioner type formulations. It also indicates that ethanol containing liquids could well have a deleterious effect on other materials based on poly(ethyl methacrylate), such as some denture relining and temporary crown and bridge materials.

5.

Conclusions

The uptake of water or organic liquids by the elastomers used as soft lining materials can be analyzed in terms of the Muniandy and Thomas theory for the effect of water soluble moieties on the water uptake of elastomers and the Flory Huggins equations for the interaction of organic liquids and polymers. The former is of particular interest, in that it explains the high water uptake of some essentially hydrophobic polymers.

Acknowledgements Based on a thesis submitted to the University of London in partial fulfillment of the requirements for the PhD degree. Dr Wen-Chien Liao was supported by the Tri-Service General Hospital and National Defence Medical Center, Taipei, Taiwan.

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d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) e199–e206

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