Dynamic mechanical analysis of high pressure polymerized urethane dimethacrylate

Abstract

Objectives

The aim of this study was to compare the viscoelastic properties of high pressure (HP) polymerized urethane dimethacrylate (UDMA) with those of control, ambient pressure thermo-polymerized and photo-polymerized, UDMA and to assess the effect of varying polymerization parameters (protocol, temperature, and initiator) on the viscoelastic properties of HP polymerized UDMA.

Methods

The viscoelastic properties of the two control polymers, polymerized under atmospheric pressure, and four experimental polymers, polymerized under HP, were determined via dynamic mechanical analysis (DMA), in three point bending configuration. Atomic force microscopy (AFM) was used to characterize fractured polymer surface morphologies.

Results

The results showed that: HP-polymerization lead to a polymer with significantly higher T g and E rub , indicative of a higher crosslink density; modifying the polymerization protocol resulted in a significant increase in tan δ ; increasing the polymerization temperature lead to a significant decrease in E rub and T g ; and that the polymer with no initiator had the lowest E ′, E ″, T g , and E rub and the highest tan δ , suggesting that under this conditions a polymer with significantly reduced crosslink density had been obtained. A characteristic nodular appearance was seen for the two control polymers under AFM, while a modified surface morphology was present in the case of HP polymerized materials.

Significance

The DMA results suggest that polymerization under HP resulted in polymers with an increased crosslink density and that the higher polymerization temperature or the lack of initiator was detrimental to the viscoelastic properties determined. Changes in polymer network morphology were identified by AFM characterization.

Introduction

The fabrication of crowns and fixed partial dentures (FPDs) using dental computer aided design/computer aided machining (CAD/CAM) systems is nowadays current practice due to the standardization and the efficiency of the manufacturing process . Esthetic appearance is quintessential in modern clinical practice and lead to an explosion in available cosmetic materials. Biocompatibility, good mechanical and excellent esthetic properties have rendered dental ceramics as a first choice among them. However, their manufacturing through milling is difficult and intraoral repair remains a challenge. Dental resin composites offer a viable alternative approach and several CAD/CAM composite blocks are currently available on the market. Their attractiveness lies in good esthetic properties, significantly easier machinability, and, purportedly, easier repair. Their mechanical properties have been improved by optimizing the fillers, the filler volume fraction, and their integration into the matrix by coupling agents . However, poor long term stability, poor fatigue performance, loss of esthetic properties are mainly due to the organic phase. Thus, the lack of chemical stability of polymers coupled with their water sorption properties result in deterioration of optical properties of resin composites. Moreover, the release of unpolymerized monomers, which increases in an inverse proportional manner with the degree of conversion (DC) , and/or degradation products leads to biocompatibility concerns. Dental resin composites matrix is generally composed of a mixture of high molecular weight dimethacrylate monomers (such as bisphenol A glycidyl methacrylate – Bis-GMA and urethane dimethacrylate – UDMA) and lower molecular weight monomers (such as triethyleneglycol dimethacrylate – TEGDMA), which are used in order to reduce viscosity and facilitate the incorporation of fillers . It has been demonstrated that low molecular weight monomers are more toxic to pulpal cells and are released at higher rates than high molecular weight ones . Among the high molecular weight monomers, UDMA, a monomer with a highly flexible structure, has been gaining attention since it is the least sensitive to release and it does not contain bisphenol A, a substance which is widely discussed due to potential health related issues .

Studies on mechanical properties of resin composites have established that UDMA-based materials had superior flexural strength, in the 140 MPa range , compared to that Bis-GMA-based materials, in the 86 MPa to 110 MPa range. Even if the individual contribution of a specific component (matrix, filler, or coupling agent) to the properties of a composite is difficult to assess, it has been established that matrix properties influence the thermo-mechanical properties of dental resin composites , which are strongly linked to their mechanical properties . Viscoelastic properties of UDMA/TEGDMA copolymers , as well as those of other copolymers have already been studied and characterized. However, as far as these authors are aware, the characterization a pure UDMA-based matrix has not been reported.

Thermo-polymerization is currently the most common method of fabrication of CAD/CAM composite blocks. Shrinkage induced during thermo-polymerization leads to the presence of defects and high internal stress . A possible alternative, which could minimize this, is high pressure (HP) polymerization. It has been shown that polymerization under HP limits internal stress by reducing free volume . Furthermore, it has been reported that HP polymerization increases the density and elastic modulus of polymers and leads to dental composites with improved mechanical properties .

The aims of this study were: to compare the viscoelastic properties of HP polymerized UDMA with those of both ambient pressure thermo-polymerized and photo-polymerized UDMA and to assess the effect of varying polymerization parameters (protocol, temperature, and initiator) on the viscoelastic properties of HP polymerized UDMA; to assess the effect of the different polymerization protocols on the surface morphology of the polymers using atomic force microscopy (AFM). The null hypotheses tested were: (1) there is no difference in viscoelastic properties between ambient pressure polymerized and HP polymerized UDMA; (2) polymerization protocol does not influence the viscoelastic properties of HP polymerized UDMA; (3) polymerization temperature does not influence the viscoelastic properties of HP polymerized UDMA; (4) the initiator does not influence the viscoelastic properties of HP polymerized UDMA; (5) there is no difference in the surface morphology of the different polymers at the AFM level.

Materials and methods

Polymers

The UDMA monomer used in this study was 7,7,9(Or 7,9,9)-trimethyl-4,13-dioxo-3,14-dioxa-5,12-diazahexadecane-1,16-diyl bismethacrylate (MW = 470.56; CAS 72869-86-4; Esstech, Essington, USA):

For all the experimental groups, polymerization under HP was achieved by placing ∼100 g monomer into a silicone tube (25 mm internal diameter, 1 mm thickness) that was then introduced into a custom-built autoclave with pressure and temperature control (LabVIEW version 8.2, National Instruments, USA) . A thermocouple was placed in the proximity of samples to enable accurate monitoring and, via feed-back, control of the temperature. In the first stage, the pressure within the autoclave was increased from 0.1 MPa to 300 MPa at a rate of 0.1 MPa/s at ambient temperature. In the second stage, the temperature was increased to the desired temperature at a rate of 2 °C/min. The sample was maintained at 300 MPa and final temperature for determined time before being cooled off and the pressure released at temperature inferior to 60 °C.

Two control polymers, polymerized under atmospheric pressure, and four experimental polymers, polymerized under HP, were made according to the details provided below and summarized in Table 1 .

Table 1
Materials and polymerization protocol.
Polymer a Composition b (wt%) Polymerization protocol
UDMA BPO CQ TA Pressure (MPa) T (°C) Time
CI90 99.5 0.5 0.1 90 12 h
CIIPhoto c 98.5 0.5 1 0.1 Ambient 2 × (2 × 40) s
HP90 99.5 0.5 300 90 12 h
HP90MP 99.5 0.5 300 90 1 h
0.1 90 12 h
HP180MP 99.5 0.5 300 180 1 h
0.1 90 12 h
HP180MPNI 100 300 180 1 h
0.1 90 12 h

a CI90 is control, thermo-polymerized; CIIPhoto is control, photo-polymerized; HP-polymers are high pressure polymerized experimental groups.

b UDMA is urethane dimethacrylate; BPO is benzoyl peroxide; CQ is camphoroquinone; TA is tertiary amine.

c Also contained 100 ppm of hydroquinone monomethyl ether, as inhibitor.

Control group one – thermo-polymerized at 90 °C under atmospheric pressure (CI90)

The monomer was mixed with 0.5% (wt.) benzoyl peroxide (BPO), at room temperature, for 10 min, in a planetary mixer (Thinky ARE-250, Thinky Corporation, Tokyo, Japan), and allowed to degas overnight at room temperature. Polymerization was conducted at 90 °C under atmospheric pressure (0.1 MPa) in an oven (Memmert, Schwabach, Germany) for 12 h.

Control group two – photo-polymerized under atmospheric pressure (CIIPhoto)

The monomer was mixed with 100 ppm hydroquinone monomethyl ether (MEHQ, Fluka, France) as inhibitor, 0.5% (wt.) camphoroquinone (CQ, Aldrich, Germany), and 1% (wt.) 4,N,N-trimethylanilin (Aldrich, Germany), at room temperature, for 10 min, in a planetary mixer and allowed to degas overnight, in the dark, at room temperature. The mix was then cast into 5 mm × 5 mm × 20 mm silicone molds and light cured for 2 × 40 s from top and bottom with a LED curing unit (440–480 nm; Radii Plus, SDI, Victoria, Australia), having an irradiance of 1200 mW/cm 2 (measured with a Curing radiometer, Dentsply Caulk, Milford, USA).

Experimental group one – thermo-polymerized at 90 °C under HP (HP90)

The monomer was mixed with 0.5% (wt.) benzoyl peroxide (BPO), at room temperature, for 10 min, in a planetary mixer, and allowed to degas overnight at room temperature. The mix was then introduced into a silicone tube and polymerization was conducted at 90 °C under high pressure (300 MPa), for 12 h.

Experimental group two – thermo-polymerized at 90 °C under HP, modified protocol (HP90MP)

The monomer was mixed with 0.5% (wt.) BPO, at room temperature, for 10 min, in a planetary mixer, and allowed to degas overnight at room temperature. The mix was then introduced into a silicone tube and polymerization was achieved in two stages: the first stage was conducted in the custom-built autoclave at 90 °C under HP (300 MPa) for 1 h; the second stage was conducted in an oven at 90 °C under 0.1 MPa for 12 h.

Experimental group three – thermo-polymerized at 180 °C under HP, modified protocol (HP180MP)

The monomer was mixed with 0.5% (wt.) BPO, at room temperature, for 10 min, in a planetary mixer, and allowed to degas overnight at room temperature. The mix was then introduced into a silicone tube and polymerization was achieved in two stages: the first stage was conducted in the custom-built autoclave at 180 °C under HP (300 MPa) for 1 h; the second stage was conducted in an oven at 90 °C under 0.1 MPa for 12 h.

Experimental group four – thermo-polymerized at 180 °C under HP, modified protocol, no initiator (HP180MPNI)

The monomer, with no initiator, was introduced into a silicone tube and polymerization was achieved in two stages: the first stage was conducted in the custom built autoclave at 180 °C under HP (300 MPa) for 1 h; the second stage was conducted in an oven at 90 °C under 0.1 MPa for 12 h.

Samples preparation

One part of each polymerized polymer block obtained was cut with a low-speed Isomet saw (Buehler, Lake Bluff, Il, USA) under water irrigation, into 20 rectangular bars [1 mm × 4 mm × 20 mm] from which seven were randomly selected. Each sample was polished on 500 grit silicon carbide paper on a water-irrigated wheel and stored at room temperature in dry condition.

Dynamic mechanical analysis

The viscoelastic properties of the polymers were determined via dynamic mechanical analysis (DMA) , operating in three point bending mode, using a RSA3 instrument (TA Instrument, New Castle, DE, USA). The dimensions of each sample were measured with a digital caliper (Mituyo Co, Kawasaki, Japan) before testing. Measurements were carried out each two degrees from 30 °C to 180 °C at a frequency of 1 Hz. The properties determined under this oscillating loading conditions were storage modulus ( E ′), loss modulus ( E ″) and damping factor (tan δ ). In DMA, E ′ is a measure of stored energy without phase difference between stress and strain and represents the elastic component of a viscoelastic material; E ″ is a measure of the energy lost as heat and represents the viscous component of a viscoelastic material; δ represents the phase lag between the applied stress and the corresponding strain in a viscoelastic material, while tan δ is the ratio of loss modulus to storage modulus ( E ″/ E ′) and is a measure of the energy dissipation, or damping. Tan δ reaches a maximum as the polymer undergoes transition from the glassy to the rubbery state at the glass transition temperature, T g . From the DMA results, T g was determined as the position of the maximum on the tan δ vs. temperature plot. The value of T g is dependent on the degree of polymerization and crosslinking present in the analyzed polymer. In the rubbery plateau, the area above T g and before the melting temperature, at 170 °C, the storage modulus, E rub , was determined for each polymer. The value of E rub is dependent on the molecular weight between entanglements or crosslinks.

Statistical analysis

The normality of the distribution of the results was assessed using Shapiro–Wilks test. Levene test was used to compare variance across the groups. The results were analyzed by one-way ANOVA followed, if warranted, by Scheffé multiple means comparisons ( α = 0.05). Statistical analyses were performed using PASW Statistics 18 (IBM, USA).

Atomic force microscopy

Freshly fractured polymer surfaces were characterized with a NANO-R2™ AFM apparatus (Pacific Nanotechnology, Santa Clara, CA, USA) operated in contact mode. On each sample, five randomly selected locations of 25 μm 2 were scanned at a scan rate of 0.5 Hz with a contact probe (P-MAN-SICT-0 Contact Mode Mounted Cantilever, Pacific Nanotechnology, Santa Clara, CA, USA). The related height- and phase-contrast images were captured and processed with Nanorule+ Software (Nanorule+ v2.5.05, Pacific Nanotechnology, Santa Clara, CA, USA).

Materials and methods

Polymers

The UDMA monomer used in this study was 7,7,9(Or 7,9,9)-trimethyl-4,13-dioxo-3,14-dioxa-5,12-diazahexadecane-1,16-diyl bismethacrylate (MW = 470.56; CAS 72869-86-4; Esstech, Essington, USA):

For all the experimental groups, polymerization under HP was achieved by placing ∼100 g monomer into a silicone tube (25 mm internal diameter, 1 mm thickness) that was then introduced into a custom-built autoclave with pressure and temperature control (LabVIEW version 8.2, National Instruments, USA) . A thermocouple was placed in the proximity of samples to enable accurate monitoring and, via feed-back, control of the temperature. In the first stage, the pressure within the autoclave was increased from 0.1 MPa to 300 MPa at a rate of 0.1 MPa/s at ambient temperature. In the second stage, the temperature was increased to the desired temperature at a rate of 2 °C/min. The sample was maintained at 300 MPa and final temperature for determined time before being cooled off and the pressure released at temperature inferior to 60 °C.

Two control polymers, polymerized under atmospheric pressure, and four experimental polymers, polymerized under HP, were made according to the details provided below and summarized in Table 1 .

Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Dynamic mechanical analysis of high pressure polymerized urethane dimethacrylate
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